im钱包下载链接|eph
Ephrin 家族 Ephrin/Eph Family - 知乎
Ephrin 家族 Ephrin/Eph Family - 知乎切换模式写文章登录/注册Ephrin 家族 Ephrin/Eph FamilyMCE-haoyuan Ephrins是一类与 Eph 受体结合的配体,Eph 受体是酪氨酸激酶受体中最大的一个家族。Ephrins和Eph家族是膜结合分子,通过细胞-细胞接触介导短期轴突导向。Ephrins 可分为两大类,一种是通过糖基磷脂酰肌醇 (GPI) 部分连接到细胞膜上的ephrinAs (A1-A6),另一种是跨越细胞膜并拥有细胞质信号域的ephrinBs (B1-B3)。在人类基因组中,共有9个EphA 受体和5个EphB受体。EphA 受体混杂地结合5个Ephrin-A配体,EphB 受体混杂地结合3个Ephrin-B配体。Eph/Ephrin信号转导是双向的,在 Eph 受体表达细胞中称为正向信号转导,在 Ephrin 配体表达细胞中称为反向信号转导。Eph/Ephrin信号通路在发育过程、成体组织稳态和各种疾病中发挥重要作用,它的异常功能与包括癌症在内的各种疾病有关。鉴于Eph受体及其配体在人类恶性肿瘤中常过表达并与不良预后相关,Eph受体和Ephrins被认为是非常有前途的药物靶点。 重组蛋白是应用了重组 DNA 或重组 RNA 的技术而获得的蛋白质。重组蛋白已被广泛应用于蛋白结构研究、细胞功能试验、免疫检测试剂、重组蛋白药物、诊断试剂开发等众多领域。近十年来,随着药物蛋白的快速发展,越来越多的高纯度高活性蛋白应用于生命科学和医学等研究领域,如:抗体制备、ELISA、药物研究、免疫实验、细胞培养、晶体结构分析等。 MCE 拥有经验丰富的技术团队和专业高效的重组蛋白表达技术,可提供毫克至克级高纯度高活性蛋白定制服务。MCE 拥有多种蛋白表达系统,包括原核蛋白表达系统、酵母蛋白表达系统、昆虫细胞蛋白表达系统和哺乳动物细胞蛋白表达系统,具备多种融合技术,可以提供在蛋白表达与纯化方面的多种选择。MCE 提供从方案设计、基因优化、表达条件优化到纯化的全流程技术服务体系。我们专注于提供高质量的蛋白,助力您的研究。 MCE重组蛋白产品种类丰富,涵盖细胞因子、病毒蛋白、免疫检查点蛋白、CAR-T蛋白、CD抗原、受体蛋白、生物素标记蛋白、GMP蛋白、酶等多种分类,可用于人、病毒、小鼠、大鼠、猴、猪、狗、牛、羊、兔、猫、非洲爪蟾等多种不同研究领域的生物学研究,包括蛋白结构研究、免疫检测试剂、重组蛋白药物、诊断试剂开发、抗体药物靶点、CAR-T细胞治疗靶点、Fc受体、流感病毒蛋白和细胞因子等热门研究领域。MCE因高品质的产品和优质的售前售后服务,成为全球数以百万计的科学家和技术人员值得信赖的实验伙伴。 产品种类丰富:细胞因子和生长因子, 免疫检查点蛋白, CAR-T相关蛋白, CD抗原, 受体蛋白, 酶&调节子, 补体系统蛋白, 病毒类蛋白, 生物素标记蛋白, 荧光标记蛋白, GMP蛋白, 泛素相关蛋白等。原文参考:Ephrin 家族 (Ephrin/Eph Family) | 重组蛋白 | MCE发布于 2022-05-26 17:25欧洲大学赞同 1添加评论分享喜欢收藏申请
肿瘤治疗靶点:EphA2 - 知乎
肿瘤治疗靶点:EphA2 - 知乎首发于生物医药知识聚合社区切换模式写文章登录/注册肿瘤治疗靶点:EphA2佰傲谷肾上腺素受体(Eph)是受体酪氨酸激酶(RTK)中很重要的一类。EphA1是在1987年筛选RTK时在肝癌细胞中首次发现的Eph受体。目前,已有14种Eph受体和8种相关配体(肾上腺素)。Eph受体根据其胞外结构域分为A和B两类,它们决定了与配体(Eph受体相互作用蛋白或肾上腺素)的结合亲和力。在人类中发现了9个EphA和5个EphB受体。Eph受体信号参与多种生物事件,主要引起细胞间的排斥或粘附。因此,Eph受体和相应的配体在胚胎的组织构型、神经元靶向和血管发育中具有重要功能。同时,在多种恶性肿瘤中发现高水平的Eph蛋白,这种过度表达极大地促进了癌症的发生。一些Eph受体,特别是EphA2,因为已经证实对癌症发生和肿瘤进展的调控过程有重要作用,所以引起了广泛的关注(图1)。本文就EphA2靶点以及其与癌症相关性以及治疗做一个介绍。图1. 肿瘤靶向EphA2的历史发展与突破EphA2信号通路EphA2受体是一个130 kDa的跨膜糖蛋白,含有976个氨基酸。EphA2可与8种不同的Ephrin A家族配体中的任何一种相互作用,优先与Ephrin A1结合。Ephrin A1是一种GPI锚定蛋白,与 EphA2相互作用,在细胞间接触后诱导不同的信号网络。由于EphA2受体和Ephrin A1配体都是膜结合的,配体依赖的激活触发了一种独特的双向信号机制,在EphA2受体表达的细胞中有“正向信号”,在Ephrin A1配体表达的细胞中有“反向信号”(图2)。正向信号通常是细胞排斥的,促进EphA2寡聚和磷酸化,从而增强激酶活性。EphA2磷酸化的直接生物学后果包括细胞-细胞外基质(ECM)附着减少。Ephrin A1:EphA2 诱导抑制粘着斑激酶(FAK)、细胞外调节蛋白激酶(ERK)和Akt磷酸化,从而调节多种恶性肿瘤细胞系的运动、活力和增殖。而反向信号更有可能是粘附性的,因为Ephrin A1缺乏酶活性通常反向信号被认为是不依赖于激酶的,目前Ephrin A1发出的反向信号还知之甚少。图2. EphA2受体和Ephrin A1配体的双向信号机制此外,EphA2在癌细胞中具有配体非依赖性的激酶活性,例如,EphA2已被证明与E-钙粘附素、EGFR、HER2和整合素二聚化,并以一种非规范的、配体无关的方式改变下游信号,导致了它在非磷酸化状态下的恶性作用(图3)。图3. EphA2 配体非依赖性的激酶活性因此,EphA2与Ephrin A1的相互作用或EphA2配体非依赖的激酶活性可能通过多种因素共同作用发挥作用,调节胚胎发育、血管生成和肿瘤发生中的多个细胞过程(增殖、存活、迁移、形态、细胞间排斥和粘附)。破译EphA2和Ephin A1配体在生理和病理过程中的许多不同机制仍然是一个挑战,但这为确定哪些靶向策略最适合于EphA2起促癌作用的特定类型的癌症提供了机会。EphA2与癌症与大多数在发育过程中合成的Eph激酶不同,EphA2主要局限于成人增生的上皮细胞。成人的EphA2只有在具有高度增殖的上皮细胞时才会在正常组织中表达,而EphA2的重要性和功能还不是很清楚。然而,越来越多的证据表明,人类EphA2在前列腺癌、肺癌、食管癌、结直肠癌、宫颈癌、卵巢癌、乳腺癌和皮肤癌中大量表达。EphA2的表达与肿瘤患者预后不良、转移潜能增加和生存期降低相关。此外,EphA2不仅是恶性特征的生物标志物,也是恶性进展的积极参与者。因此,EphA2的转录模式和在恶性肿瘤中的功能相关性使该蛋白成为癌症治疗的一个有吸引力的靶点。靶向EphA2的癌症治疗EphA2/Ephrin A1系统至少可以通过两种机制被作为癌症治疗的靶点(图4)。首先,EphA2的致癌功能可以被抑制,如降低EphA2的表达,促进EphA2的降解,阻断内源性EphA2的激活。或者,EphA2受体可用于向癌细胞和相关血管输送治疗药物(外源性药物或内源性免疫细胞)。图4. 靶向癌症中的EphA2作用机制EphA2靶向治疗已经出现在多种类型和阶段的恶性肿瘤的临床试验中。临床上抑制EphA2的策略包括EphA2靶向抗体-细胞毒药物结合物(ADC)或肽-药物结合物(PDC);酪氨酸激酶抑制剂(TKI),如已批准上市的达沙替尼;识别和靶向EphA2抗原的CAR-T细胞;以及设计用于向肿瘤细胞运送靶向EphA2的siRNAs的纳米载体。未来潜在的抑制非规范信号的策略可能还包括EphA2激动剂,如可溶性EphA2激动剂(A1-Fc),或其他小分子抑制剂,以阻断S897处的EphA2磷酸化(Akti/rski/PKAi)(图5)。图5. EphA2治疗靶向策略尽管有大量数据支持EphA2药物用于癌症治疗,但仍存在一些挑战。EphA2在多种细胞和组织类型中的表达既代表着机遇,也代表着挑战。在正常组织的表达可能导致意外的毒副作用。EphA2受体的多种信号模式也使靶向策略变得复杂,最合适的方法可能会因环境而异。此外,由于EphA2可能是非激酶依赖性的,传统的RTK小分子抑制剂阻断受体的激酶活性可能不能抑制EphA2配体非依赖性致癌效应。尽管存在这些挑战,利用达沙替尼等再利用药物以及正在开发的多肽-毒素结合物的下一代靶向治疗都在积极进行中。小编总结总体而言,EphA2在肿瘤生物学中的重要作用使其成为一个很有前途的治疗靶点,临床上正在进行的针对EphA2的努力可能会为抗击癌症提供一种有价值的新武器。未来的努力包括继续深入了解EphA2-Ephrin A1信号,并精确阐明与其他致癌途径的串扰。已完成的临床研究和新的生物学发现为开发下一代EphA2靶向疗法提供了线索:(I)应将重点放在提高疗效和选择性,同时防止非靶向副作用;(II)与其他疗法联合使用可能会有所帮助。FYI:总结生物医药更多知识点,敬请关注公众平台「佰傲谷BioValley」部分图源网络,侵权联删参考文献1.Oncogenic functions and therapeutic targeting of EphA2 in cancer.Targeting EphA2 in cancer.2.Emerging and diverse functions of the EphA2 noncanonical pathway in cancer progression.3.Eph receptor signalling: from catalytic to non-catalytic functions.4.Emerging strategies for EphA2 receptor targeting for cancer therapeutics.发布于 2021-03-16 13:57肿瘤分子生物学肾上腺素赞同 152 条评论分享喜欢收藏申请转载文章被以下专栏收录生物医药知识聚合社区垂直生物科技领域,依托媒体矩阵,为生物制药助
EPF?EPH?这是什么牛马? - 知乎
EPF?EPH?这是什么牛马? - 知乎切换模式写文章登录/注册EPF?EPH?这是什么牛马?有点迷糊各位大佬,路人朋友们,这是一篇求助贴,由于自己对这方面的知识知道的不是很多,所以在这个问题上很纠结,所以在这求助各位。我是一名在校学生,今年大二,五一刚放假,一回来,我爸就在跟我说,说他在别人介绍下投资了一个什么原始股,说什么几个月后就直接涨几万块钱,而且每个月都有几百或者过千的利息,我当时也在想什么东西这么好,不可能吧,但由于他跟朋友喝了酒一时半会儿也说不出个所以然,但我还是心里存着质疑。他跟朋友分别的时候,他朋友说,现在十二点了,正好可以提点出来了。说着我爸拿出手机打开了一个软件,我撇了一眼,看到几个合约挖矿,什么EFP,USDT......我在想,这不是最近闹的比较厉害的虚拟货币吗?然后他在手机上点了几下,完了后我存着疑惑在车上问了我爸怎么开始挖矿炒虚拟币了,他说这就是他朋友介绍给他的什么原始股,然后有利息之类的,然后我就跟他解释了一番,明明软件里的挖矿,换币写的明明白白的,哪里什么原始股,这不扯吗?!然后我在用我仅有的不多的关于诸如比特币此类的认知来跟他解释了一下,他还是抱着原始股的概念不放,但有点松动了...有时候大多数时候是会听取我的建议的(因为我爸学历不高,对金融方面的知识更是一干二净),即使我还在读书。因为他之所以弄这些都是为我准备的,所以我希望把这个弄明白给他讲明白,希望爸爸的苦心不会受到恶意的蹂躏。回到家我就把他的那个软件弄来看了一遍,真的就是一个虚拟币交易的一个软件,我看到上面有很多功能,但很多功能都没有开放(之后上图),最主要的是什么,它上面没有直接账户,购买软件上面的那个币,是通过微信转账给客服,然后通过他那边,让你的软件上有一个资金到账可以用来购买的显示。然后他上面的赎回限制,一个月之类赎回本金扣除25%,两个月15%,至少三个月以后赎回,不扣除费用。这就是这个软件的大概。我觉得最离谱的也是这个最像骗局的是什么,就是他有微信群,群里有所谓的专员,然后是很多注册的用户,一群人在群里发什么要感恩什么的,还有一些关于这个东西的宣传资料,最最离谱的是什么,也让我最不敢相信的,是宣传资料里提到他们这个叫什么所谓的“云上联盟”,“打通什么线上线下零售”,“和京东,阿里,腾讯”这几个巨头进行的合作,各种夸张宣传这个epf有多好有多好,然后有专员在讲解他们这个东西怎么怎么样,如何如何好,为什么要买,我听了其中几个语音,他是这样说的,我也终于明白别人跟我爸说的原始股是什么玩意儿,他提到,其中EPF是一级市场放出的货币,EPH是二级市场的,也就是现在是一级市场的放开中,其中EPF的保有量只有两百万个好像,然后时间截止了,这个就不再发放了,过段时间就是开放EPH,这就是所谓的二级市场,(不知道后面还有没有什么三四五六级的,不知道他们的毅力能让他们坚持到哪里不卷款跑路),当EPH出来之后,之前的那个EPF的价格就等于15个USDT(我专门查了这个USDT是泰达币,基本和美元一比一)按照他们的时间也就九月份就能涨到那个地步了。按照现在他软件上显示的,目前一个EFP=五点几USDT,中间利润之大,不可谓不让人心痒。我在网上找了一些资料,根本找不到什么所谓云上联盟,什么ecpay(就那个软件),公司名字叫重庆巨星耀科技有限公司,公司到是有,有1000万的注册,其他的就什么都查不到,更别说和几个巨头合作的新闻动态,他那视频里还说要上中央新闻,还要港交所上市,我当场心里就开始骂骂咧咧了,我越听越扯淡,越看这些个越假,但我又拿不定,我家不富,几万对于我们来说也是巨款了,真不想这么白白被骗,所以这篇文章出来了。如果有权威,有懂行的朋友,麻烦掌掌眼,帮我家一个小忙,我在这里谢谢大家!后面是我拍的图发布于 2021-05-02 04:37赞同 16 条评论分享喜欢收藏申请
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什么是EphB6/Eph Receptor B6)? - 知乎
什么是EphB6/Eph Receptor B6)? - 知乎首页知乎知学堂发现等你来答切换模式登录/注册阿片受体抗促甲状腺素受体抗体(trab)受体G蛋白偶联受体H2受体什么是EphB6/Eph Receptor B6)?关注者1被浏览757关注问题写回答邀请回答好问题添加评论分享1 个回答默认排序联科生物高品质ELISA和流式荧光抗体产品国家高新技术企业和领导品牌 关注EphB6/Ephrin Receptor B6:根据结构和序列关系,Ephrins可分为ephrin-A (EFNA)类和ephrin-B (EFNB)类。属于受体酪氨酸激酶(RTK)家族。EphB6是一个不寻常的Eph受体,由于其激酶结构域的改变导致它缺少催化能力。EphB6可正向、负向地调节细胞粘附和迁移。另外,Ephrin-B2可能是EphB6受体的一个生理配体。在一些Tumour类型中,转移活性的增加与EphB6受体表达量降低有关。EphB6通过c-Cbl依赖的信号、形态变化和细胞附着来抑制Malignant tumor的侵袭,可能是一个有用的预后指标和治疗靶点。发布于 2021-04-25 11:37赞同 1添加评论分享收藏喜欢收起
Eph及配体ephrin在恶性胶质瘤的研究进展_神经胶质瘤_酪氨酸激酶_神经外科_医脉通
Eph及配体ephrin在恶性胶质瘤的研究进展_神经胶质瘤_酪氨酸激酶_神经外科_医脉通
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Eph及配体ephrin在恶性胶质瘤的研究进展
2017-12-11
来源:中国微侵袭神经外科杂志
关键词:
神经胶质瘤
酪氨酸激酶
神经外科
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作者:刘展闻,陈晓丰,滕雷,哈尔滨医科大学附属第一医院神经外科 胶质瘤是最常见原发性中枢神经系统肿瘤,约占中枢神经系统肿瘤40%,其中多形性胶质母细胞瘤(glioblastoma multiforme,GBM)恶性程度最高。尽管现代抗肿瘤治疗已取得显著进步,但GBM中位生存时间仅为14.6个月。顽固侵袭特性导致治疗失败和肿瘤复发是GBM治疗面临的主要问题。本研究总结有关促红细胞生成素衍生人肝细胞癌受体(erythropoietin-producing human hepatocellular carcinoma receptors,Eph)及其相应配体ephrin的近年文献,旨在综述其在GBM发生、发展中的作用,并讨论其成为GBM临床治疗新策略的可能。 1.Eph及配体ephrin的概述 目前,在胶质瘤增殖和侵袭方面,实验已证明酪氨酸激酶在病理及生物学中显现双向信号调节作用。酪氨酸激酶是能够将靶蛋白上某一酪氨酸残基磷酸化的磷酸化酶。酪氨酸激酶受体(receptor tyrosine kinase,RTK)是指位于细胞膜上、可被其配体激活的酪氨酸激酶胞外结构域。酪氨酸激酶的磷酸化作用,是许多重要细胞活性调控途径开关和实现细胞内通讯机制之一。人类基因组中发现90种不同的酪氨酸激酶基因(包括58RTK),广泛参与调节细胞增殖、存活、分化以及细胞能动性。酪氨酸激酶在正常细胞发育和许多肿瘤演进中,均起关键性作用。因此,酪氨酸激酶是肿瘤化疗的重要靶点。Eph以其14个成员成为最大类酪氨酸激酶受体家族。 Eph可分为2个亚群:EphA(A1-A8)和EphB(B1-B6),Eph同时具有同源氨基酸序列和特异性配体结合位点。这些受体位于细胞表面,并可转导与邻近细胞表面配体结合后产生的信号。 ephrin配体分为2类:与糖基磷脂酰肌醇(glycosylphosphatidylinositol,GPI)相连、且与EphA结合的ephrin-A配体(ephrinA1-A6);具有跨膜和胞内结构域、与EphB结合的ephrin-B配体(ephrinB1-B3)。像其他RTK一样,Eph通过配体-受体结合后产生的自磷酸化作用,启动的转导信号被认为是“正向信号”;相对于其他可溶性RTK配体而言,膜结合的ephrin可激活“反向信号”,这形成双向细胞通讯过程。一般而言,配体ephrin-A和ephrin-B,分别与EphA和EphB相结合,但也有例外(如ephrin-A5可与EphB2结合、ephrin-A和ephrin-B均可与EphA4结合)。这种双向信号通路是细胞间相互联系的重要通讯形式。 2.Eph及配体ephrin在胶质瘤的作用 Eph及配体ephrin在胚胎发生和发育中是一个重要角色,通常高表达于神经和内皮细胞,且其信号主要调节胚胎发育过程中神经嵴细胞迁徙、轴突导向以及神经血管生成。实际上Eph及配体ephrin目前几乎在所有组织中,调节复杂的发育过程(如细胞黏附、轴突导向、细胞迁移、细胞分类、血小板聚集等)。例如,Eph及配体ephrin参与建立听觉系统的神经元发育,并引导皮质层树突延伸和成熟。但Eph及配体ephrin表达在病理状态下、损伤和成年期恶性肿瘤中亦有改变。 在胶质瘤发生和发展中,Eph及配体ephrin蛋白异常表达水平,往往与较高肿瘤分级和预后不良相关。Eph及配体ephrin定位在肿瘤细胞、肿瘤血管、胶质瘤细胞浸润的脑组织和免疫细胞浸润的肿瘤表面。由于相邻肿瘤细胞和肿瘤微环境可产生互相作用,Eph及配体ephrin可以通过促进或抑制肿瘤发生而发挥作用,这取决于下游产生的是正向或反向信号。上述特点使得Eph/ephrin系统在胶质瘤治疗新策略中成为一个有吸引力的候选对象。 2.1 EphA/ephrin-A 相关研究 在EphA/ ephrin-A蛋白中,EphA2和ephrin-A1与胶质瘤生物学间的关系报道最多。EphA2在不同肿瘤中高度表达,包括乳腺癌、卵巢癌、胃癌、胰腺癌和前列腺癌,这提示EphA2是许多肿瘤的共同癌蛋白。EphA2在约60%胶质母细胞瘤中过度表达,也在星形细胞瘤中高度表达且随病理分级增高而表达明显增强,其过度表达与病人不良预后直接相关,与病人存活率成反比。 EphA2过度表达不仅可出现在肿瘤细胞中,还可出现在具有血管高密度的GBM区域,以及内皮细胞包绕的GBM区域,这提示EphA2参与肿瘤血管形成;虽然EphA2过度表达,但在胶质瘤组织中很少有ephrin-A1表达。 由于GBM中ephrin-A1配体低水平表达,EphA2尽管过度表达但仍处于非生物活性状态。与此相矛盾的是,在胶质瘤中起抑制增殖、侵袭作用的EphA2激酶是由ephrin-A1激活。因此,ephrin-A1有肿瘤抑制作用。然而,如果长期给予外源性ephrin-A1刺激,可导致EphA2受体和点状黏附激酶(FAK)下调,从而导致增殖和迁徙能力下降。Ephrin-A1所致EphA2下调是由EphA2/ephrin-A1复合体的内吞作用造成。Ephrin-A1在GBM低表达部分解释EphA2活性缺乏和过度表达。在体外和体内实验中,发现EphA2/ephrin-A1相矛盾的表达情况,提示ephrin-A1与EphA2间存在相互抑制的环路。在检测其他EphA与胶质瘤病人预后的关系中,EphA7可染色肿瘤细胞血管内皮细胞,但不染色周围结缔组织。其可作为一种新的胶质母细胞瘤预后标志物。在约45%GBM样本中,EphA7过度表达并预示GBM病人不良预后。此外,通过半定量PCR,检测到EphA5在正常脑组织中表达,但在低级别胶质瘤标本中表达降低,在高级别胶质瘤中进一步降低。这个结果表明:EphA5表达下降程度可作为胶质瘤进展情况的标志物,也同时突出EphA5作为肿瘤抑制基因的作用。 2.2EphB/ephrin-B相关研究 在EphB/ephrin-B信号途径中,ephrin-B2被看作是恶性星形细胞瘤短期存活的一个预后指标,ephrin-B2高表达的肿瘤病人存活率显著低于低表达者,EphB4/ephrin-B2表达增加与胶质瘤病理分级呈现依赖性,且表达水平与胶质母细胞瘤病人无进展生存期相关。胶质瘤中EphB2、B3和B4表达水平均显著高于正常脑组织,EphB1表达不随胶质瘤病理分级而变化。通过Kaplan-Meier生存曲线分析,EphB1高水平表达者相比于EphB1低水平表达者,有着显著延长的存活期,这提示EphB1高表达与良好预后相关。 研究表明:EphB1以配体依赖的方式抑制胶质瘤细胞侵袭,且EphB1可作为胶质瘤预后的阳性指标。 3.Eph/ephrin在胶质瘤治疗中的作用 在胶质瘤病理分级中,低级别胶质瘤(WHO分级Ⅰ~Ⅱ级,如星形细胞瘤及室管膜瘤),多数显示良性特征,预后较好,而进展性胶质瘤(WHOⅢ~Ⅳ级,如少突胶质细胞瘤或胶质母细胞瘤),通常表现为未分化的特点和预后。目前胶质瘤标准治疗,包括外科手术、术后放疗和以替莫唑胺为基础的细胞毒素化疗,但这些方式只能暂时推迟不良预后的时间,无法从根本上达到治愈。 目前Eph/ephrin在胶质瘤的靶向治疗包括:①基于配体ephrinA1可以通过激活含有融合蛋白ephrinA1-Fc的肿瘤细胞,抑制EphA2磷酸化,诱导其退化,从而减小癌蛋白影响。ephrinA1激活可抑制EphA2过表达,因此,目前认为EphA2抑制剂可能通过上调ephrinA1表达,抑制胶质瘤恶性进展。因此,针对胶质瘤靶向治疗,使用EphA2-Fc可能阻断EphA2-ephrinA1间的应答反应。②IL-13Rα2、EphA2以及Fra-1是GBM靶向治疗中具有代表性的靶点,这三种蛋白在胶质瘤的表达显著高于正常脑组织及低级别星形细胞瘤,在超过95%胶质瘤细胞中过表达且至少有一种蛋白表达。细胞毒素IL-13.E13K.PE38QQR和ephrinA1-PE38QQR可分别特异性杀灭IL-13Rα2和EphA2高表达的GBM转移细胞,这为靶向药物治疗的合理组合及临床诊断提供分子学基础。 其他可能的潜在靶点及分子标记物包括:①胰岛素样生长因子结合蛋白-2(IGFBP-2)。通过整合素β1-ERK途径刺激胶质瘤细胞增殖、侵袭和对替硝唑胺的耐受,与胶质瘤病人生存期呈负相关。②上皮水解素(MMP-28)的表达是胶质瘤病人总生存期的独立危险因素,与MGMT启动子甲基化、IDH1/2突变,共同参与恶性胶质瘤病人预后评价和临床结局。 4.讨论 由于胶质瘤侵袭性造成病人预后较差,研究显示近60%GBM病人对替莫唑胺治疗疗效甚微或无效。而位于脑组织周边的侵袭性胶质瘤细胞则可能是对化疗和放疗产生抵抗的原因,现在仍无有效抗胶质瘤侵袭的方法。临床病理分析显示:失控的细胞增殖和异常的细胞侵袭是GBM形成过程中存在的两个显著特征,具体表现为在肿瘤中心区的细胞拥有高增殖特性,而边缘区细胞呈现高侵袭性和低增殖性。因此,针对GBM侵袭性机制的研究,有助于开发新的有效抗侵袭治疗方案。Eph/ephrin应是RTK家族及相应配体中最受瞩目且最有希望成为恶性胶质瘤治疗靶点的候选对象。 Eph/ephrin蛋白在生长发育过程中经常表达,在肿瘤细胞异常表达亦就不意外,其存在和功能对干细胞或肿瘤干细胞增殖是必要的。Eph/ephrin调控作用在胶质瘤进展中有诸多看似相互矛盾的作用机制和明显差异,这使Eph受体既可作为癌基因也可作为抑癌基因。明确Eph/ephrin调节胶质瘤侵袭的机制,将有助于制备新型靶向制剂并服务于临床。目前一些Eph/ephrin特异性靶向新疗法显示初步结果。朝着这个方向继续研究将有助于我们加深理解Eph/ephrin与恶性胶质瘤病理学之间的关系,并可能提供更有效的胶质瘤治疗策略。 来源:刘展闻, 陈晓丰, 滕雷. Eph及配体ephrin在恶性胶质瘤的研究进展[J]. 中国微侵袭神经外科杂志, 2017, 22(8).
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Eph receptor signalling: from catalytic to non-catalytic functions | Oncogene
Eph receptor signalling: from catalytic to non-catalytic functions | Oncogene
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Eph receptor signalling: from catalytic to non-catalytic functions
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Published: 12 August 2019
Eph receptor signalling: from catalytic to non-catalytic functions
Lung-Yu Liang1,2, Onisha Patel1,2, Peter W. Janes3, James M. Murphy
ORCID: orcid.org/0000-0003-0195-39491,2 & …Isabelle S. Lucet
ORCID: orcid.org/0000-0002-8563-87531,2 Show authors
Oncogene
volume 38, pages 6567–6584 (2019)Cite this article
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Cell biologyCell signalling
AbstractEph receptors, the largest subfamily of receptor tyrosine kinases, are linked with proliferative disease, such as cancer, as a result of their deregulated expression or mutation. Unlike other tyrosine kinases that have been clinically targeted, the development of therapeutics against Eph receptors remains at a relatively early stage. The major reason is the limited understanding on the Eph receptor regulatory mechanisms at a molecular level. The complexity in understanding Eph signalling in cells arises due to following reasons: (1) Eph receptors comprise 14 members, two of which are pseudokinases, EphA10 and EphB6, with relatively uncharacterised function; (2) activation of Eph receptors results in dimerisation, oligomerisation and formation of clustered signalling centres at the plasma membrane, which can comprise different combinations of Eph receptors, leading to diverse downstream signalling outputs; (3) the non-catalytic functions of Eph receptors have been overlooked. This review provides a structural perspective of the intricate molecular mechanisms that drive Eph receptor signalling, and investigates the contribution of intra- and inter-molecular interactions between Eph receptors intracellular domains and their major binding partners. We focus on the non-catalytic functions of Eph receptors with relevance to cancer, which are further substantiated by exploring the role of the two pseudokinase Eph receptors, EphA10 and EphB6. Throughout this review, we carefully analyse and reconcile the existing/conflicting data in the field, to allow researchers to further the current understanding of Eph receptor signalling.
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Eph receptors and ephrin ligands: an overviewReceptor tyrosine kinases (RTKs) are a major type of membrane receptors, which govern cell proliferation, differentiation and mobility [1]. Deregulation of RTK signalling pathways leads to many diseases, such as cancers and developmental disorders [2]. The erythropoietin-producing hepatoma (Eph) receptor subfamily is the largest amongst the RTKs with 14 members classified into type A and type B. Compared with other RTKs, Eph receptors share common functions in some disease states, as shown by their roles in cancer progression [3]. In addition, Eph receptors can govern tissue patterning and cell differentiation [4]. Their activation relies on the binding of their cognate membrane-tethered ligands, known as ephrins (Fig. 1). Human EphA receptors (EphA1–A8 and EphA10) preferentially bind to the ephrin A ligands, whereas human EphB receptors (EphB1–B4 and EphB6) bind to the ephrin B ligands. However, promiscuous binding of EphA receptors to ephrin-Bs, or EphB receptors to ephrin-As have been shown [5,6,7]. The specificity of the Eph receptors appears to be dictated by their ectodomains, as shown in a chimera experiment [8]. Thus, the promiscuous binding of type-A and type-B ephrin ligands can likely provide a broader range of signalling functions downstream of an ephrin-ligated Eph receptor.Fig. 1The architecture of Eph receptors and ephrins. Class A and B Eph receptors share a very similar architecture. The ligand-binding domains (LBD) of Eph receptors can bind to the receptor-binding domain (RBD) of the ephrin ligands. C-terminal to the LBD is the cysteine-rich domain (CRD) and two fibronectin III domains in tandem. Intracellularly, the Eph receptors consist of a juxtamembrane (JM) region, a kinase domain (KD), a SAM domain and a C-terminal PDZ domain-binding motif (PBM). Class A ephrins harbour a glycosylphosphatidylinositol (GPI) anchor tethering the RBD, whereas class B ephrins have a transmembrane domain and a PBM intracellularlyFull size imageBoth type A and type B ephrins have an extracellular receptor-binding domain (RBD) in their N-terminus. The primary difference between ephrin-As and ephrin-Bs is how their RBDs are tethered to the plasma membrane: ephrin-As are linked to the plasma membrane by a glycosylphosphatidylinositol (GPI) linker, whereas ephrin-Bs harbour a transmembrane domain and a C-terminal PDZ (an acronym from three proteins: PSD-95, Dlg1 and ZO-1) domain-binding motif (Fig. 1).Due to their membrane-tethered nature, ephrin ligands account for the ability of the Eph receptors to initiate intracellular signalling events upon cell–cell contact. Although the signalling events mediated by Eph receptors are substantially dependent on cell types, common signalling pathways (e.g. Rho family GTPases-mediated cytoskeletal reorganisation) that principally govern developmental processes, including cell sorting, tissue patterning and cell migration, have been mapped [5].Research has so far focused on studying how Eph receptors drive signalling via their catalytic activity. However, recent studies have highlighted that the Eph receptor family proteins also display non-catalytic functions. Two members of the family, EphA10 and EphB6, are classified as pseudokinases due to the absence of key amino acids known to catalyse phosphoryl transfer from ATP in conventional protein kinases (Fig. 2b) [9, 10]. The presence of these two catalytically-dead Eph receptors suggests a role for non-catalytic functions in regulating kinase-active Eph receptors. Emerging evidence suggests that EphB6 can be phosphorylated by other Eph receptors, such as EphB1 and EphB4, potentially leading to a reciprocal regulation of EphB6 through direct interaction with its kinase-active counterparts [11, 12]. Such a regulatory mechanism has been demonstrated for the receptor tyrosine pseudokinase ErbB3/HER3, where ErbB3 acts as an activator of EGFR upon heterodimerisation [13]. As per ErbB3, EphB6 and EphA10 have retained an intact ATP binding site [14]. The ability of EphB6 and EphA10 to bind nucleotide suggests that they may function as molecular switches, modulating the kinase activity of other kinase-active Eph receptors [15]. Importantly, recent studies have revealed that deregulated expression of EphA10 and EphB6 is associated with cancers, raising the prospect that, like other pseudokinases, these proteins may contribute to disease states [9, 16].Fig. 2Sequence alignment of the juxtamembrane region and the kinase domain of Eph receptors. a Sequence alignment of the juxtamembrane (JM) region and the kinase domain (KD) of all Eph receptors. Grey shading indicates the conserved residues critical for kinase catalytic activity, and/or highlights residues that serve as docking sites for SH2 domain-containing proteins when phosphorylated. Bold highlights the catalytically critical residues that have diverged in EphB6 and EphA10. The predicted secondary structures are based on the EphB2 kinase domain crystal structure (PDB: 1JPA) and are annotated as follows: bars represent helix structures, arrows represent β-strands. b Summary of the key catalytic motifs necessary for kinase activity compared with the motifs found in EphA10 and EphB6Full size imageIn this review, we carefully examine the current state of knowledge on the Eph receptor signalling by dissecting their structural features and molecular mechanisms of regulation. We focus on the importance of the non-catalytic functions of the kinase-active Eph receptors, and the roles of EphA10 and EphB6 pseudokinases in cancers.Overall organisation and regulation of the Eph receptor intracellular domainsThe Eph N-terminal ectodomain is connected by a single transmembrane α-helix, which is extended intracellularly to a juxtamembrane (JM) region that tethers a tyrosine kinase domain. The tyrosine kinase domain is connected by a linker to a sterile-alpha motif (SAM) domain and a PDZ domain-binding motif (Fig. 1) [1]. These additional protein–protein interaction domains imply that Eph receptors coordinate complex intracellular signalling pathways and we detail below the structural and functional characteristics of each of these intracellular domains.The juxtamembrane region and the kinase domainStructurally, a JM region is a 35–40 amino acid long peptide linker located N-terminal to the tyrosine kinase domain (Figs. 2 and 3). The role of the JM region is two-fold. One role is to regulate the intrinsic kinase activity of its adjacent kinase domain by locking the protein in an inactive conformation and therefore blocking the substrate and nucleotide access [1]. Mutations of the two conserved tyrosine residues in the JM region (termed JX1 and JX2) to phenylalanine has been shown to completely abolish the kinase activity of EphA4 [17], suggesting that phosphorylation of the JM region is required to unleash an active conformation. The second role is to provide binding sites to SH2 domain-containing proteins upon autophosphorylation, thereby stabilising the kinase domain in an active conformation and allowing the propagation of downstream signals.Fig. 3Structural features of the Eph receptor kinase domain. a Key structural features of the Eph receptor kinase domain are highlighted in the EphA3 kinase domain crystal structure (PDB: 2QO2). Note that this kinase domain adopted an inactive conformation. b Superposition of the C-lobes of EphA3 (PDB: 2QO2, in cyan) and EphB2 (PDB: 1JPA, in brown) kinase domain structures shows a very similar alignment of the juxtamembrane regions. The distortion of the αC helix, coordinated by the unphosphorylated juxtamembrane region, leads to an inactive form of the kinase domain. c Superposition of the C-lobes from the inactive (PDB: 2QO2, in cyan) and the active (PDB: 2QO9, in pink) EphA3 kinase domain structures reveals the impact of unphosphorylated juxtamembrane region on the activation loop. The unphosphorylated juxtamembrane region (in cyan) of 2QO2 causes misalignment of the activation loop (in red), giving rise to an inactive conformation of the kinase domain. The phosphorylated juxtamembrane region of 2QO9 dislodges from the kinase domain and cannot be seen in the crystal structure. This results in a more ordered activation loop (in pink), which stabilises the kinase domain in its active conformationFull size imageThe structure of EphB2 kinase domain (PDB: 1JPA) in the presence of the JM region, harbouring phenylalanine mutations in both JX1 and JX2 sites, clearly demonstrated the role that the JM region plays in stabilising an inactive conformation of the kinase domain (Fig. 3b). The JM region adopts a helix-turn-helix conformation and wraps around the αC helix, preventing its correct alignment to allow phosphotransfer activity [18]. The JX1 (Y→F) residue sits in close proximity to the αC helix in this structure, pointing toward the catalytic site, while the JX2 is located at the hinge of the helix-turn-helix and is solvent exposed (Fig. 3b), implying that it can serve as a binding site, once phosphorylated, for phosphotyrosine binding proteins. The structure of the EphA3 kinase domain connected to the JM region (PDB: 2QO2) also highlighted a similar positioning of the unphosphorylated JX1 (Fig. 3b) [19]. In addition to locking the αC helix conformation, the unphosphorylated JM region also plays a role in preventing the activation loop from fully adopting an active conformation (Fig. 3c) [19]. In agreement with this, Wiesner et al. used NMR spectroscopy to demonstrate that phosphorylation of JX1 and JX2 unleashed the JM region from the kinase domain, leading to an active form of the EphB2 kinase domain [17]. Similarly, in vitro kinase assays suggested that the JM region autophosphorylation is a sequential event, whereby autophosphorylation of JX2 preceded autophosphorylation of JX1 [20, 21]. The JM region therefore provides the first layer of the kinase activity regulation. Once phosphorylated and dislodged, Eph kinase activity relies on the phosphorylation of the conserved tyrosine residue in the activation loop (Fig. 2), a feature conserved in many RTKs.The SAM domain linker and the SAM domainC-terminal to the Eph receptor kinase domain is a protein–protein interaction domain called the SAM domain (Figs. 1c and 4). The modular SAM domain is highly conserved, comprising five helices that govern homo-/hetero-dimerisation or oligomerisation [22, 23] (Fig. 4b–d). The crystal structure of homo-dimeric EphA4 SAM (PDB: 1B0X) has identified the α1, α3 and the C-terminal segment of the α5 helices are the major structural elements engaged at the dimerisation interface (Fig. 4b). Mutagenesis studies revealed that substitution of the key residues at this interface, such as L940, M972 and M976 that are relatively conserved in other Eph receptors, disrupted SAM domain dimerisation (Fig. 4a) [22]. On the other hand, the crystal structure of EphB2 SAM domain (PDB: 1B4F) has identified a possible oligomerisation mechanism through two additional interaction interfaces (Fig. 4a, c). One oligomerisation interface, defined by the authors [23] as the “b-region interface”, is primarily composed of the loop connecting α3 and α4 helices, and the C-terminal segment of the α5 helix. The other oligomerisation interface, called the “arm-exchange interface”, consists of multiple conserved residues on the α1, α3 and the C-terminal segment of the α5 helices. Therefore, it appears that α1, α3 and the C-terminal α5 helices of the Eph receptor SAM domains are responsible for both homo-dimerisation and homo-oligomerisation.Fig. 4Sequence alignment of the SAM domain linker and the SAM domain of Eph receptors. a The predicted secondary structures based on the EphB2 SAM domain crystal structure (PDB: 1B4F) are annotated: bars represent helix structures. The key amino acid residues for potential homo-dimerisation/-oligomerisation are highlighted in dark green. The key amino acid residues critical for interaction with downstream interactors are highlighted in yellow. Once phosphorylated, the conserved tyrosine residue highlighted in grey is a potential docking site of SH2 domain-containing proteins. The four C-terminal amino acids highlighted in green are predicted to be the PDZ-domain binding motif (PBM). Note that the protein sequence of EphB2 is based on the EphB2 isoform 2 from Uniprot, as it contains a conserved C-terminal PBM. b The key structural features mediating homo-dimerisation of the EphA4 SAM domain (PDB: 1B0X). c The key structural features facilitating homo-oligomerisation of the EphB2 SAM domain (PDB: 1B4F). The oligomeric EphB2 SAM domains harbour two interaction interfaces and the critical amino acid residues mediating oligomerisation are labelled. d Superposition of the Eph SAM domains of the heterodimeric structures (PDB: 5ZRX, 5ZRY and 5ZRZ, respectively) of the EphA2/SHIP2, EphA6/Odin and EphA5/SAMD5 SAM domains reveal that the key dimerising interface is mediated by the N-terminal residues of Eph receptor α5 helix. e The crystal structure of the EphA5 SAM domain (PDB: 5ZRZ, in purple) were colour coded to represent the interaction interfaces of homo- (the α1, α3 and the C-terminal segment of α5 helices, in pink) and hetero- (the N-terminal segment of the α5 helix, in green) dimerisation/oligomerisation of the Eph receptor SAM domainsFull size imageIn addition to acting as a direct protein–protein interaction domain, the Eph SAM domain can modulate the activity of its adjacent kinase domain and can facilitate recruitment of SH2 domain-containing proteins. Phosphorylation of the conserved tyrosine residue (Y928 of EphB1 and Y921 of EphA2) located in the α2 helix (Fig. 4a) of the SAM domain is able to recruit SH2 domain-containing proteins, such as the adapter proteins Grb7, Grb10 and low-molecular-weight protein tyrosine phosphatase [23,25,26,27]. Truncation of the SAM domain was shown to promote EphA2 and EphB2 homo-dimerisation and clustering at the plasma membrane, respectively [27,29,30], although opposing findings suggested that the presence of the SAM domain enhanced EphA3 dimerisation in cells [31]. Relevant to this, the truncation of the SAM domain induced autophosphorylation in the activation loop of EphA2 in cells [29, 32], whereas the truncation of the SAM domain in EphA3 exhibited decreased activation loop autophosphorylation [31], suggesting an intramolecular regulatory role of the Eph SAM domain. The increased receptor clustering and phosphorylation observed upon deletion of the SAM domain on EphA2 and EphB2 dimerisation [27,29,30] seems to be counter-intuitive as the SAM domain is a known dimerisation domain predicted to favour Eph receptor dimerisation/oligomerisation. One possibility to reconcile this discrepancy is that the SAM domain may impose steric hindrance on the kinase domain and the kinase domain could also be a major intracellular dimerisation determinant. The core kinase domain in other proteins has previously been demonstrated to act as a scaffold [15]. While we cannot rule out a role for the SAM domain in mediating dimerisation/oligomerisation, Wimmer-Kleikamp et al. clearly demonstrated that in addition to the ectodomains that contribute to Eph receptor clustering, the lateral homotypic recruitment of EphA3 is independent of its kinase activity [33], and the intracellular domains of the Eph receptors were also reported to contribute to the receptor hetero-clustering [34] Thus, the Eph tyrosine kinase domain may exhibit non-catalytic functions, including aiding the recruitment of other Eph receptors. Further investigation is required to consolidate this potential non-catalytic function of Eph receptors.Attempts to determine the structure of the tandem EphA3 kinase and SAM domains by Davies et al. failed due to protein degradation and hence the structural understanding of how SAM domain modulates kinase activity remains unresolved [19]. Using bioinformatics, molecular dynamics simulation in conjunction with biochemical analyses, Kwon et al. recently demonstrated that the linker connecting the SAM domain and the kinase domain (termed the SAM domain linker) plays a major role in modulating the intrinsic tyrosine kinase activity (Figs. 3a and 4a). The interaction between the SAM domain linker and the kinase domain αF–αG loop, located away from the active site, was shown to impact on the autophosphorylation of the JM region. In addition, the SAM domain linker and the JM region have been demonstrated to collaboratively regulate the autophosphorylation of the conserved tyrosine residue in the activation loop (Y779 in EphA3) [20]. The JM region and the SAM domain linker therefore seem to be critical allosteric regulatory elements, with their spatial organisation dictating Eph receptor tyrosine kinase activation. In addition to the regulatory role of the SAM domain linker, it is very likely that the SAM domain itself directly impacts on the conformation of the kinase domain and hence regulates its kinase activity [28].While the structural role of the SAM domain in fine tuning Eph receptor kinase activity is unclear and awaits the determination of the three-dimensional structure of a construct encompassing the JM, the kinase and the SAM domains, the interaction between the SAM domain of Eph receptors with SAM domains of other proteins has been clearly demonstrated. Disrupting such interaction abrogated the normal cell retraction response upon ephrin-A1 treatment [35]. The heterodimeric SAM domain crystal structures (PDB: 5ZRX, 5ZRY and 5ZRZ, respectively) have been solved for SHIP2, Odin and SAMD5 with EphA2, EphA6 and EphA5, respectively (Fig. 4d). This has provided fruitful structural insights, such as the identification of the key residues responsible for hetero-dimerisation, and a greater understanding of how downstream signalling effectors are recruited (Fig. 4d). Specifically, the residues of the Eph SAM domain responsible for downstream interactor hetero-dimerisation are predominantly concentrated on the N-terminus of the α5 helix, revealing a distinct interaction mechanism compared with the one driving Eph SAM homo-dimerisation/-oligomerisation (Fig. 4a, e).Unique features of EphA10 and EphB6 pseudokinasesIn contrast to the extensive studies of the kinase-active Eph receptors, very little is known about their pseudokinase counterparts, EphA10 and EphB6. To date, there are no three-dimensional protein structures solved for any domains of EphB6 and EphA10. Based on protein sequence alignments of kinase and pseudokinase domains (Fig. 2a), EphA10 and EphB6 have relatively high sequence conservation and the same domain organisation compared with other Eph receptors. Specifically, the closest homologue of EphA10 is EphA7, with a 53.80% identity in protein sequence, whereas EphB6 shares 49.17% identity with its closest homologue, EphB1 (Fig. 2). This close protein sequence similarity implies that EphA10 and EphB6 originated from gene duplication events [16]. Both EphA10 and EphB6 likely harbour non-catalytic regulatory functions (Fig. 2a), although the exact role they play in regulating Eph receptor signalling is unknown. Interestingly, the JM region of EphB6 retains the two conserved tyrosine residues (JX1 and JX2), whereas in EphA10 the corresponding residues are phenylalanine and cysteine, respectively (Fig. 2). This suggests a distinct regulation of the JM region for each of these two Eph receptor pseudokinases. Notably, half of the predicted activation loop of EphB6 is missing, as well as the conserved activation loop tyrosine residue (Fig. 2a), implying a distinct activation loop regulatory mechanism compared with other Eph receptors. By contrast, the activation loop of EphA10 has retained the conserved tyrosine residue, suggesting that EphA10 could adopt various conformational states dependent on its activation loop phosphorylation state. These unique features between EphB6 and EphA10 clearly underscore their mechanistic specificity in the Eph receptor signalling pathways. Future biochemical and structural studies of these two kinase-dead Eph receptors will shed light on the role of non-catalytic functions in Eph receptor signalling.Eph receptor forward signallingBy the virtue that Eph receptors and ephrin ligands are tethered to the presenting cells, their predominant roles are in cell communication. Upon cell–cell contact, both Eph receptors and ephrin can initiate signal transduction in each of the ligand- and receptor-presenting cells. The signalling initiated by the ephrin-ligated Eph receptors is called “forward signalling”, whereas the signalling initiated by the Eph-bound ephrins is called “reverse signalling” (Fig. 5a). The Eph forward signalling has drawn greater attention as it is driven by the “canonical receptors” whose role is to transduce signals from ligand stimulation.Fig. 5Dimerisation and oligomerisation of Eph receptors activate forward (and reverse) signaling. a Ephrin-induced dimerisation of Eph receptors triggers autophosphorylation of the Eph receptor tyrosine kinase domain. Autophosphory lation of the juxtamembrane region, the kinase domain and the SAM domain provides binding sites for recruitment of SH2 domain-containing proteins, which themselves may be substrates for phosphorylation by the Eph receptor tyrosine kinase domain. In cells expressing ephrins ligated to the Eph receptors, Src family kinases (SFKs)-mediated phosphorylation of the C-terminal tail of class B ephrins induces reverse signalling pathways. b Ephrin-induced dimerised Eph receptors can further oligomerise to form clustering signaling centres, from which more Eph receptors and downstream signalling proteins can be recruited, phosphorylated or activated. This clustered signalling centre amplifies forward signalling, and presumably also reverse signalling in the opposed cell. Both forward signalling and reverse signalling substantially rely on Src family kinases (SFKs), which further phosphorylate Eph receptor-interacting proteins or even Eph receptors themselves. The extensive number of phosphorylation sites from the clustered signalling centre facilitates recruitment of effectors for diverse signalling pathways, including the PI3K-Akt/PKB signalling axisFull size imageThe role that Eph receptors play in regulating major pathways, such as via Rho/Rac GTPases, which control actin organisation, and Ras/MAPK, which controls proliferation, have been extensively reviewed elsewhere [36]. Thus, we will focus on the PI3K-Akt/PKB signalling axis downstream of Eph receptors as an example of Eph forward signalling. We will also illustrate how phosphorylation of the Eph receptors is able to relay the signals via the Src-homology 2 (SH2)-containing proteins, which are critical in both Eph forward and reverse signalling.The Eph-mediated signalling pathways are largely receptor, cell type and context dependent, as suggested by a report describing that some Eph receptors, such as EphA2 [5], can promote both tumour progression and suppression. The tyrosine kinase domain of Eph receptors plays a central role in forward signalling, such that upon ligand stimulation, the ephrin-bound Eph receptors undergo dimerisation, which results in transphosphorylation and activation of the receptor kinase domains (Fig. 5a). The activated Eph kinase domain then phosphorylates downstream substrates, such as adaptor protein Nck1/2 [37]. The two most frequent autophosphorylation sites are located in the JM region, JX1 and JX2, as demonstrated by in vitro kinase assays and in cell proteomic mapping [38]. The phosphorylation of JX1 and JX2 is a sequential event, which further leads to the autophosphorylation of the activation loop, resulting in full kinase activity [20]. Once phosphorylated, these tyrosine residues also become potential docking sites for SH2 domain-containing signalling or adaptor proteins (Fig. 5a, b). The best characterised SH2 domain-containing proteins that interact with Eph receptors include the adaptor proteins Nck and CrkII [39] and Src family kinases (SFKs). For example, Src was demonstrated to bind to the phosphorylated tyrosine residue JX2 in the JM region of EphB2 [40]. SFKs recruited to the Eph receptors are thought to then phosphorylate downstream substrates, relaying signals from the Eph receptors. In these cases, once phosphorylated, Eph receptors execute their non-catalytic functions by acting as scaffold proteins.The fact that Eph receptors can act as scaffold proteins has been well exemplified by the interaction with several guanine nucleotide exchange factors (GEFs). Sahin et al. demonstrated that, in fibroblasts, the activation of an upstream GEF of Rho small GTPases, ephexin1 requires EphA4 kinase activity, implying ephexin1 is an EphA4 substrate. Phosphorylated ephexin1 promoted RhoA activity, leading to the formation of stress fibres. However, the protein kinase Src rather than EphA4, appears to be the direct upstream kinase of ephexin1. This suggests that phosphorylated EphA4, presumably via its JM region, recruits Src kinase, which in turn phosphorylates the downstream substrates [41]. Similarly, other SH2 domain-containing proteins such as the GEFs Vav2 and Vav3 were shown to interact with the JM region phosphorylated tyrosine residues of EphA2, suggesting that an active kinase conformation is required to allow interactions with their binding partners. In addition, the interaction of SH2 domain-containing proteins with Eph receptors is not restricted to the JM region. In the same study, the interaction of the p85 subunit of phosphoinositide 3-kinase (PI3K) with EphA2 was disrupted, when tyrosine Y745 (the conserved tyrosine residue preceding the catalytic loop) of the kinase domain and Y930 of the SAM domain were mutated [42]. Together, these studies clearly highlight the scaffolding function of Eph receptors, a function dependent on their phosphorylation states.PI3K has been known to interact with RTKs, such as platelet-derived growth factor receptor, via its SH2 domain(s) in the p85 subunit. Fang et al. demonstrated that the p85 subunit of PI3K interacts with EphA2 by immunoprecipitation [42]. Multiple studies have demonstrated that the Eph receptors modulate the PI3K-Akt/PKB signalling axis in cell migration. For example, Akt/PKB is phosphorylated and activated upon EphB2 binding to ephrin-B1 in HEK293 cells stably expressing the microtubule-associated protein tau. In addition, the sole overexpression of EphB2 without ephrin-B1 induction triggered PI3K-Akt/PKB signalling to a lesser extent [43].On the other hand, conflicting evidence suggested that in non-small cell lung cancer (NSCLC) cell lines, ephrin-B1 activated EphB3 led to downregulated phosphorylation of the Ser/Thr residues on Akt/PKB that are required for activation, a mechanism mediated by phosphatase PP2A [44]. Similarly, dephosphorylation of the same Ser/Thr residues in Akt/PKB was observed downstream of EphA2 activation in prostate cancer cells [45]. In a glioblastoma (GBM) cell line, EphA2 can act both upstream (as a regulator) and downstream (as a substrate) of Akt/PKB in a ligand-dependent or -independent mode, respectively [46]. The crosstalk between Eph receptors and Akt/PKB was further demonstrated by Stallaert et al., who showed that activation of EphAs inhibited Akt/PKB activity, which hindered the endosomal trafficking of EGFR and reduced EGFR recycling to the plasma membrane. In line with this, EGF-induced cell migration was also suppressed upon EphA receptor activation [47]. These studies collectively suggest that distinct Eph receptors can elicit opposite signalling events under different cellular contexts.Taken together, the Eph receptor-mediated forward signalling relies on autophosphorylation and activation of the tyrosine kinase domain, in turn creating binding sites for downstream adaptors or signalling proteins. Surprisingly, other than the Eph receptor itself, very little is known about the direct protein substrates of the Eph receptor tyrosine kinase domain. The main difficulty in identifying the Eph receptor kinase domain direct downstream substrates is due to the associated SH2 domain-containing SFKs, as both of them are tyrosine kinases with potential overlapping substrates. Nck1/2, an adaptor protein that is involved in cytoskeletal organisation, was recently identified to be a direct substrate of EphA4 in vitro and in cells [37]. The binding between the Nck SH3 domain and its interacting partners was abolished once the conserved tyrosine residue in the Nck SH3 domain is phosphorylated by EphA4. Nck was also reported to bind to the phosphorylated JM region of EphB1 via its SH2 domain [48, 49]. In addition, the adapter protein Caskin recruited by Nck was shown to be phosphorylated by EphB1 [49]. These results combined suggests that Eph receptor phosphorylation sites within the JM region, activation loop, the SAM domain can recruit adaptors or effectors, which then become potential substrates for Eph tyrosine kinase domain. Deciphering the Eph interactome will therefore be critical in order to fully appreciate the signalling network of Eph receptors.Ephrin-independent Eph receptor activationThe Eph receptor-mediated signalling events are reliant on the phosphorylation status of the Eph receptors its intracellular domains. The activation mechanism of Eph receptors upon membrane-tethered ephrin ligation resembles those used by other RTKs: receptors undergo dimerisation upon binding to soluble ligands for activation. Normal Eph signalling initiated upon binding of ephrins to Eph receptors induces dimerisation, however, the ephrin-ligated Eph receptor can further oligomerise to form clusters, from which downstream signalling can be magnified (Fig. 5b) [50]. Interestingly, the cellular signalling events/phenotypic changes triggered by activation of dimeric or oligomeric Eph receptors appeared to be different [25]. On the other hand, aberrant expression of Eph receptors is commonly observed in cancers. This leads to the hypothesis that upregulated expression of Eph receptors can achieve Eph receptor-mediated signalling by dimerisation and higher order oligomerisation, independent of ephrin ligation. Moreover, the signalling pathway outputs resulting from ligand binding or from ephrin-independent Eph receptor dimerisation/oligomerisation can be distinct or even opposite as demonstrated in the case of EphA2—an apparent oncogene in the absence of ephrins, but a tumour suppressor when interacting with its ephrin ligands [45, 46, 51, 52].Structurally, the ephrin induces an extensive dimerisation interface upon binding to the ectodomain of the Eph receptors via a highly conserved interface [53]. For example, a conserved polar bulky amino acid (Gln) at position 109 of ephrin-B2 forms a hydrogen bond to the conserved Thr38 in the ectodomain of EphB2, and very likely to those of other EphB receptors. The crystal structure of ephrin-B2 binding to the ligand-binding domain (LBD) of EphB2 revealed a tetrameric complex (Fig. 6a) [53]. However, as suggested by the crystal packing, the LBD of EphB2 seems to be able to tetramerise without the involvement of ephrins in the tetrameric complex structure (Fig. 6a) [53]. A similar Eph LBD–LBD interface is also seen in the crystal structures of heterotetrameric EphA2/ephrin-A5 and EphA4/ephrin-A5 [54,55,56,57]. In these EphA receptor structures, a second Eph–Eph interaction interface dictated by the cysteine-rich domain (CRD) was found, suggesting that in addition to the LBD, the CRD is another dimerising determinant allowing Eph receptors to assemble via Eph–Eph interaction (Fig. 6b) [54,55,56,57]. Both Eph LBD–LBD and CRD–CRD interfaces do not engage in ephrin binding (Fig. 6b, c). In the absence of ephrins, the EphA2 ectodomain crystal structure showed the presence of the same Eph CRD–CRD interface (Fig. 6c), suggesting a conserved mechanism of dimerisation driven by Eph receptor ectodomains. Mutations introduced to the Eph CRD–CRD interface amino acids (L223, L254 and V255) perturbed EphA2 dimerisation [58], which is in agreement with the argument that Eph receptors can dimerise/oligomerise in the absence of ephrin. Taken together, these data show that Eph receptors do not have to rely on ephrins to form dimers and potentially cluster to high-order oligomers if Eph receptor concentration in the plasma membrane is sufficient (Fig. 6d). Indeed, once the initial EphA3 and ephrin-A5 nucleating centre is formed, more EphA3 receptors can be laterally recruited without being directly associated with ephrin-A5 in cells [33]. Moreover, using fluorescence resonance energy transfer, EphA2 or EphA3 was shown to form dimers in the plasma membrane without binding to the ephrin ligands [29, 31]. These dimerised Eph receptors are constitutively active, as signified by autophosphorylation of the kinase domain activation loop [31], suggesting that when Eph receptors are highly expressed, they can exhibit basal activity at the plasma membrane even in the absence of ephrin stimulation.Fig. 6Ephrin-independent Eph receptor dimerisation/oligomerisation. Dimerisation/oligomerisation of the Eph receptors can be ephrin independent, as Eph receptors harbour Eph–Eph interacting interfaces, as shown from both class A and class B Eph receptors. a The ephrin-B2 (shown in blue) ligated EphB2 LBD (shown in green) crystal structure has Eph–Eph crystal-packing interface indicated by black arrows (PDB: 1KGY). b The crystal structure comprising two heterotetramers of ephrin-A5 ligated the EphA2 ectodomains (LBD + CRD + N-terminal fibronectin domain III) reveals two Eph–Eph interacting interfaces (PDB: 3MX0). Highlighted in the purple circle is the Eph LBD–LBD interface (key residues including K116, T144 and P147, etc.), whereas highlighted in the green circle is the Eph CRD–CRD interface (key residues such as L223, L254 and V255, etc.). The G131 residue lies at the ephrin-induced Eph LBD–LBD interface, as circled in black. c In the absence of ephrin ligation, very similar Eph LBD–LBD and Eph CRD–CRD interfaces also exist, as shown in another EphA2 crystal structure (PDB: 3FL7). d The Eph receptor dimerisation/oligomerisation is hypothesized to be also regulated by the intracellular domains. The lateral extension of the Eph receptor to the clustered signalling centre can be potentially mediated by the non-catalytic functions of the kinase domain, and by the SAM domainFull size imageAlthough capable of functioning independent of ephrins, under physiological conditions, it is rare that Eph receptors exclude the engagement of ephrins. As illustrated in Fig. 6b, c, in the presence of ephrins, a simplified Eph receptor oligomer can be regarded as repeats of ephrin-dependent Eph receptor dimers (with an ephrin-induced Eph LBD–LBD interface, circled in black) and ephrin-independent Eph receptor dimers (with an Eph LBD–LBD interface, circled in purple, and an Eph CRD–CRD interface, circled in green). Interestingly, the major residues clustering on the Eph LBD–LBD interface (e.g. D104, K116, E117 and T144 of EphA2) and those clustering on the Eph CRD–CRD interface (e.g. L223, V255 of EphA2) are highly conserved across EphA and EphB receptors (Fig. 6b, c) [54,55,56,57], suggesting that both types of Eph receptors are able to undergo similar ephrin-independent dimerisation. This implies that dimerisation of the interclass Eph receptors can occur. In contrast, the EphA2 G131 residue located at the ephrin-induced Eph LBD–LBD interface is specifically conserved in EphA receptors, except for EphA4 (Fig. 6b). The G131Y EphA2 variant exhibited a reduced clustering propensity in a ephrin-dependent manner [57]. G131, however, appeared to be not essential for Eph receptor dimerisation/oligomerisation in the absence of ephrins [58]. Collectively, the ephrin-independent Eph receptor dimers/oligomers are predicted to be promiscuously composed of type A and B Eph receptors, whereas ephrin binding to Eph receptors is likely a mechanism to selectively produce homotypic dimers/oligomers of EphA, or EphB receptors.Ephrin reverse signallingThe ligands of the Eph receptors, ephrins, are membrane tethered and are able to initiate downstream signalling in their expressing cells. To this end, they can also act as “receptors”. The reverse signalling mediated by ephrins is induced by their ligation to the Eph receptors and additional mechanisms [59]. Similar to Eph receptors, the intracellular tail of the B-type ephrins harbours conserved multiple tyrosine residues that can be phosphorylated. For example, ephrin-B1 can be phosphorylated following incubation with the ectodomain of EphB2 [59,60,61], which can be mediated by SFKs [62]. Similar to Eph receptors, tyrosine phosphorylation on the C-terminal tail of ephrin-Bs provides binding sites to SH2 domains of SFKs, thereby inducing subsequent signalling events [62]. Intriguingly, the cytosolic domain of ephrin-Bs was also reported to affect the Eph forward signalling events in trans in a phosphoproteomics study [63], but the molecular details remain unclear. In the same study, Jorgensen et al. demonstrated that co-incubation of EphB2- and ephrin-B1-expressing HEK293T cells led to asymmetric intracellular signalling with differential levels of substrate tyrosine phosphorylation [63]. This bias likely arises from the distinct intracellular architectures of Eph receptors and ephrins.Fewer studies have examined ephrin-A mediated reverse signalling pathways compared with those driven by ephrin-Bs. Ephrin-As are tethered to cells solely via a GPI linker embedded in the plasma membrane. This precludes reverse signalling directly, although it was proposed that ephrin-As interact with other transmembrane proteins upon binding to Eph receptors [5].The recombinant soluble ectodomain of ephrin is not sufficient to elicit the Eph forward signalling events, as it is required to be either membrane bound in the expressing cells or pre-clustered by antibodies [64]. A possible explanation is that ephrins exert both inhibitory and activating effects on Eph receptors. In trans, membrane-bound or pre-clustered ephrins can activate Eph receptors, but, when expressed in the same cell (in cis), ephrins bind and block Eph receptors from initiating forward signalling. Therefore, the soluble ectodomain of ephrins can potentially act in cis, failing to activate the Eph forward signalling [65]. Yin et al. showed that the cis-expression of ephrin-A2 inhibited the trans-binding of ephrin-A5 to the EphA4 receptor in HEK293 cells [66]. The competition of cis and trans binding of ephrin-As was also reflected in cis binding reducing EphA receptor phosphorylation levels, consistent with ephrin-As regulating Eph-mediated forward signalling. In agreement, multiple studies have independently confirmed the attenuated activation of trans-EphA receptor by interacting with ephrin-As in cis from both in vitro and in cells neuron growth cone models [67, 68].To add to the complexity of this system, each EphA and EphB receptor has multiple ephrin-As and ephrin-Bs as ligands, respectively, resulting in additional cross-reactivity among EphA/ephrin-Bs and EphB/ephrin-As. Potentially, the overall expression pattern of ephrins and Eph receptors in one cell dictates which Eph receptors can be activated by an adjacent cell upon cell–cell contact. Nevertheless, such pleiotropy necessitates caution in interpreting studies, in which a single type of ephrin is used to activate Eph receptors, as several ephrins and Eph receptors usually coexist in each cell type.The non-catalytic functions of alternative Eph receptor isoformsWhile the tyrosine kinase activity of Eph receptors is central to many well-characterised signalling pathways, their kinase-independent functions have proved to be important in cancer development. Strikingly, almost all the Eph receptor genes yield at least one isoform that does not have kinase activity due to partial or complete truncation of the intracellular domains from alternative splicing. In addition to the roles in cancer, these Eph isoforms possess a wide array of functions in neuronal development [69, 70], cell reprogramming [71] and cell adhesion and repulsion [72]. Holmberg et al. discovered that expression of an EphA7 isoform that lacks the entire intracellular domain is able to shift cell repulsion to adhesion. The intracellular domain-truncated EphA7 isoform was able to block the full-length EphA7 autophosphorylation [72], which might be partially due to a dominant-negative effect, where co-clustering of the truncated form dilutes the cross-phosphorylation of Eph receptor intracellular domains [34]. The EphA7 isoform was later found to be able to inhibit the MAPK and SFK signalling pathways, likely through a mechanism in which EphA7 forms an inhibitory dimer with EphA2 that antagonises lymphoma development [73]. Similarly, EphA10 is a proposed oncogenic Eph receptot but the secreted EphA10 isoform, comprising only the LBD abd rge partial CBD, was shown to suppress breast tumour growth abdmetastasis in mice [74].In addition to isoforms derived from alternative splicing, the proteolytic products of the full-length Eph receptors appear to serve additional functions. For example, the cleavage of EphA2 by membrane type-1 matrix metalloproteinase induced an ephrin-independent EphA2 activation. The resulting activation of small Rho GTPases led to enhanced cancer cell invasion in vivo [75, 76]. More detailed consequences of the proteolysis of the Eph receptors and ephrins have been reviewed elsewhere [77]. Collectively, the physiological and pathological roles of these truncated Eph isoforms and proteolytically cleaved forms provide strong evidence for the importance of the non-catalytic and ephrin-independent functions of Eph receptors.Eph compositions in signalling clusters and their non-catalytic functionsOne of the major difficulties of studying Eph signalling is to precisely define the genuine functions of each Eph receptor family member in cells. This is mainly due to the forward signalling output representing the integrated input of all the stimulated Eph receptors in a clustered signalling centre. While phosphatase activity driven by the protein tyrosine phosphatase receptor type O (Ptpro) and other protein tyrosine phosphatases can negate Eph receptor activation [78, 79], co-expressed ephrins may also exert potential cis-inhibitory effects on Eph receptors. Not surprisingly, one outstanding question is how the interaction among Eph receptors dictates the overall downstream signalling events. Another outstanding question is: to what extent can the Eph non-catalytic functions govern the signalling pathways in an Eph receptor clustering centre? As reviewed elsewhere, different pools of Eph receptors/ephrins in cells can confer opposing functions on a given Eph receptor [36]. The non-catalytic functions of the Eph receptors are likely to be responsible for this observation.The preferential binding of Eph receptors to their corresponding type of ephrins provides a useful avenue to demonstrate that a cell expresses a mixture of Eph receptors, and is able to respond distinctly upon binding to type A or type B ephrins. Astin et al. observed that treatment with ephrin-A1 or ephrin-A5 rendered contact inhibition of locomotion (CIL) in the PC3 prostate cancer cell line [80]. On the contrary, ephrin-B2 promoted PC3 cells to migrate. The authors further demonstrated that PC3 cells exhibited homotypic CIL among themselves, but failed to undergo CIL, when contacting normal cells, such as fibroblasts and endothelial cells. By comparing the Eph/ephrin expression level vs. the corresponding phenotypic changes, they concluded that the differential expression of Eph receptors/ephrins in PC3 cells, fibroblasts and endothelial cells was responsible for the distinct CIL responses. The abundance of ephrins in the activating cells (fibroblasts and endothelial cells) dictate the composition of the Eph receptors in the signalling cluster of the receiving cells (PC3 cells), thereby evoking cell retraction or invasiveness. More importantly, in prostate cancer patient specimens, the surrounding stroma cells of the prostate cancer cells expressed relatively high levels of ephrin-B2, implicating Eph-ephrin signalling in the cancer microenvironment, potentially facilitating cancer metastasis [80].An Eph receptor cluster does not have to be homotypic, adding to the complexity of the signalling platform. EphA receptors can intermingle with EphB receptors in a clustered signalling centre as exemplified by EphA3 and EphB2, regardless of the EphA3 kinase activity [34]. This heterotypic binding was mainly mediated by their ectodomains, although the intracellular domains also partially accounted for the interaction. Importantly, expression of wild-type EphA3 elevated the EphB2 kinase activity and collaboratively induced cell rounding. In addition, aberrant cell segregation due to the truncation of the intracellular domains of EphB2 was able to be restored by wild-type EphA3 expression. This observation has two implications: [1] EphA3 can compensate for EphB2 kinase activity and/or [2] EphA3 is able to reconstitute the potential EphB2 non-catalytic functions by recruiting EphB2 interacting partners. Surprisingly, a kinase-dead EphA3 mutant suppressed EphB2 kinase activity, reinforcing the idea that Eph receptors harbour non-catalytic regulatory functions. This is thought to work through a dominant-negative effect, where co-clustering of the truncated form effectively dilutes the cross-phosphorylation of intracellular domains. Thus, Eph receptor signalling clusters can contain a wide array of different Eph receptor species, with the combination of species dictating the forward signalling output following ephrin stimulation.Accumulating evidence suggests that Eph receptors exhibit non-catalytic functions. The kinase-inactive EphA2 mutant can induce similar chemotaxis compared with wild-type EphA2, when overexpressed in HEK293 cells [46]. In another study using breast cancer cell lines, the kinase-dead EphA2 mutant exerted a dominant-negative effect that mitigated ephrin-A1 mediated EphA2 phosphorylation and contributed to tumour suppression [81]. EphA3 was reported to regulate the protease activity of the transmembrane A-Disintegrin-And-Metalloprotease 10 (ADAM10), independently of the EphA3 intrinsic tyrosine kinase activity [82]. The proximity of the inactive EphA3 kinase domain to the plasma membrane prevented ADAM10-mediated cleavage through steric hindrance, until binding of ephrin-A5 and ensuing receptor clustering and phosphorylation reorganised the EphA3 JM region and the kinase domain, and enabled access of ADAM10. This example illustrates one of the Eph receptor non-catalytic functions, in which EphA3 can block ADAM10 activity, and is reminiscent of the functions that pseudoenzymes perform as protein–protein interaction domains to regulate signalling [15]. More broadly, it is unlikely that all Eph receptors within a cluster solely rely on their kinase activity to transduce signals. Instead, some of the Eph receptors primarily perform their non-catalytic functions, exerting regulatory properties via scaffolding, recruiting or competing with other Eph receptors. The best evidence supporting this argument is the existence of the two kinase-deficient Eph receptor members, EphA10 and EphB6, and their emerging roles in cancers.The roles of EphA10 and EphB6 in cancersEndocrinologically, EphB6 expression is correlated with catecholamine biosynthesis and secretion [83,84,85]. Its role in cancer, however, has attracted more attention. For example, EphB6 was reported to modulate cell death by anoikis in cancer cells by interacting with another Eph receptor, EphA2, via their ectodomains [86]. Nonetheless, the molecular mechanisms underpinning EphB6 functions remain to be fully determined. Expression of EphB6 in MDA-MB-231 triple-negative breast cancer cells, which reportedly do not express the receptor endogenously, lowered cadherin 17 protein expression and altered MEK2 and β-catenin expression [87]. Using HEK293T cells, overexpression of EphB6 allowed Matsuoka et al. to identify SFKs as constitutive interactors of EphB6 [88]. In addition, upon binding to ephrin-B2, tyrosine phosphorylation of EphB6 was increased. Nonetheless, it is inconclusive whether the phosphorylation was catalysed by the associated SFKs, or it was carried out by other kinase-active Eph receptors, such as EphB4 and EphB1 (Fig. 7) [12, 13].Fig. 7The potential roles of EphA10 and EphB6 pseudokinases. While very little is known about the EphA10 interactors, EphB1 and EphB4 were reported to phosphorylate EphB6. EphB6 phosphorylation may potentially perturb the binding of constitutive EphB6 interactors, such as Fyn kinase [88] and Cbl E3 ligase [12]. It is postulated that EphB6 can regulate the kinase activity of EphB1 and EphB4 via heterodimerisation, resulting in changes to downstream phospho-signalling eventsFull size imageDownregulation of EphB6 mRNA resulting from promoter hypermethylation has been found in NSCLC and breast cancer cells with invasive characteristics or tendency [89, 90]. The protein expression level of EphB6 was also decreased in multiple cancers, including breast cancer [89], NSCLC [90] and colorectal cancer [91]. Moreover, downregulation of EphB6 promoted cancer metastasis [92,93,94], whereas restoration of EphB6 expression suppressed metastasis [90, 94, 95]. Thus, EphB6 has been proposed as a tumour suppressor. Interestingly, while playing a role as a potential metastasis suppressor, EphB6 was recently reported to accelerate cell proliferation in triple-negative breast cancer (TNBC) cell lines [95]. These studies imply that at different stages of cancer progression, EphB6 can perform distinct functions, although other factors, such as cell types or receptor expression level, cannot be excluded.Very little is known about EphA10, but upregulated expression of EphA10 is associated with multiple cancers [96,97,98]. EphA10 expression is rarely seen in normal tissues except for testes [99]. Owing to the prevalence of EphA10 expression in cancers, researchers have proposed to use EphA10 as a marker for identifying cancers.Eph receptors: oncogenes or tumour suppressorsIn 2010, the landmark review article by Elena Pasquale clearly described Eph receptors as (paradoxical) tumour suppressors or oncogenes, depending on their different expression pattern and the cellular contexts [3]. The versatile activation mechanisms displayed by the well-characterised Eph receptor, EphA2, is an excellent example that illustrates the biphasic role of Eph receptors in cancer [3]. The seemingly controversial cancer-agonising and -antagonising actions downstream of EphA2 receptor activation are due to different degrees of contribution from other Eph receptors and ephrin ligands, and the ensuing signalling events. In cancer biology, the tumour microenvironment (TME) has proved to be essential for the development of cancer cells. Studying the functions of Eph receptors using cancer cells in culture can be restricted by the lack of interaction with Eph receptor-expressing stromal and endothelial cells. Thus, below, we carefully examine the recent studies on understanding the Eph receptor functions in cancer in mouse models. In particular, we emphasise the Eph receptor non-catalytic functions in vivo. The sequences of the human and mouse Eph receptors share very high similarity, implying a resemblance of the functionalities, and making mouse models more likely to recapitulate the human Eph receptor functions. In spite of the complexity of Eph receptor regulation, certain Eph receptors have been categorised as either oncogenes or tumour suppressors based on their predominant function shown in the mouse model.EphA7 was proposed to be a tumour suppressor in follicular lymphoma, in which downregulated expression due to hypermethylation of the gene promoter was found. The survival rate of the follicular lymphoma mouse model significantly decreased, when EphA7 expression was suppressed, equivalent to the effect exhibited by knocking down the tumour suppressor, p53. The EphA7 tumour-antagonising effect is independent of its intracellular domains, as restoration of the EphA7 ectodomains in the human lymphoma cells xenografted into mice was sufficient to exhibit a profound antiproliferative and apoptosis-inducing effects [73]. In another study, EphA7 was also shown to be a tumour suppressor in prostate cancer. Nonetheless, only the full-length EphA7 with intact kinase activity was able to contribute to tumour shrinkage in prostate tumour xenografted mice [100].The kinase-dependent and kinase-independent functions of Eph receptors can sometimes determine whether an Eph receptor is a tumour suppressor or an oncogene. For example, the tumour growth was significantly suppressed in EphA3 knockout mice, where a primary human GBM cell line was injected. The orthotopic xenograft model further confirmed that the survival rate of mice transplanted with high EphA3-expressing patient sample was much lower than the low expression counterparts [101]. Interestingly, the oncogenic effect upon EphA3 expression appeared to be not kinase activity dependent, as the EphA3 from human GBM specimens and primary cell lines remained constitutively unphosphorylated. Similarly, the oncogenic properties of EphA3 were observed in a prostatic tumour xenograft mouse setting: expression of EphA3 in the stromal and vascular regions benefited tumour formation [102]. These effects could be countered by an activating EphA3-specific antibody, which reduced the tumour burden in of both GMB and prostatic cancer mouse models by increasing apoptosis [101, 102]. It therefore appears that EphA3 promotes oncogenesis in a kinase-independent manner in some contexts, and exerts a kinase-dependent tumour suppressive effect in others.The epigenetic silencing of Eph receptors such as EphA5, EphA7, EphB6 and others that contribute to downregulated protein expression has been linked to cancer development [3, 73, 103,104,105], suggesting their potential roles as tumour suppressors. Interestingly, in different cancers, they can somehow function in an opposite manner. For example, overexpression of EphA5 has been found in lung cancers [106]. Neutralisation and degradation of EphA5 proteins by an EphA5-specific antibody sensitised the lung tumour bearing in vivo models to irradiation. Also, inhibition of EphA5 kinase activity was able to suppress the progression of hepatocellular carcinoma in mice [107]. Although more detailed studies are required, it appears that the oncogenic property of EphA5 is dependent on its catalytic activity.Depletion of EphA2 was shown to exhibit an antiproliferative effect in NSCLC and TNBC in in vivo models [108, 109]. Consistently, in an oncogenic KRasK12D mutant-driven NSCLC mouse model, the tumour cells grown in the EphA2−/− mice underwent significant apoptosis. In addition, the administration of a relatively selective EphA2 kinase inhibitor was able to reduce the tumour size in the NSCLC xenografted mice, by inducing apoptosis, suggesting that EphA2 is oncogenic in NSCLC [110]. Interestingly, Yeddula et al. reported that targeting EphA2 in a tissue-specific manner can lead to an opposite outcome: knockout of EphA2 from the lung adenocarcinoma tissue driven by KRasK12D in vivo deteriorated cancer development. The increased tumour burden shown in lung tissue-specific EphA2-deficient mice suggested that EphA2 is a tumour suppressor. The EphA2 tumour suppressive effect was likely due to an inhibition of the MAPK pathway downstream of EphA2 activation by binding to its ephrin ligand, ephrinA1, as shown in analysis at the cellular level [111]. These results collectively suggested that whether EphA2, and other Eph receptors, are oncogenic or tumour suppressive in a cancer cell, is determined by its surrounding environment, namely, the tumour microenvironment.Concluding remarksThe Eph receptors have been identified as critical players that control tissue development. Aberrant expression or mutation of Eph receptors is a hallmark of many diseases, including cancers. In contrast to other RTKs, the uniqueness of the Eph receptor-mediated signalling pathways includes: [1] ephrin ligand-dependent or -independent Eph receptor activation; [2] Eph receptor oligomerisation, a mechanism allowing the amplification of the downstream signal transduction; [3] the presence of the two kinase-dead members, EphA10 and EphB6, whose roles remain to be determined. To further advance the understanding of how Eph receptors function, in-depth investigation of their structural details, the spatiotemporal mechanism of oligomerisation in the plasma membrane and their intracellular signalling pathways is required.Owing to the nature of membrane proteins, full-length three-dimensional structures of the Eph receptors will not likely be solved by the traditional techniques, such as NMR and X-ray crystallography. The emerging state-of-the-art cryo-electron microscopy may provide a feasible solution to examine how ephrin binding to the ectodomains of the Eph receptor can convey extracellular cues to intracellular effectors. Recent advances in microscopy techniques, such as total internal reflection fluorescence (TIRF), are likely to cast light on the dynamic oligomerisation process of the Eph receptors in the plasma membrane. Two recent studies used TIRF to study the kinetics of ephrin-induced Eph receptor oligomerisation in the plasma membrane [50, 112]. Ephrin-bound Eph receptor oligomerisation was thus described as a process of nucleation, polymerisation and condensation [50]. Furthermore, the amplitude of the Eph receptor oligomerisation correlated with their intracellular autophosphorylation [50], indicating the oligomerisation state is a key parameter in controlling Eph receptor forward signalling. Most unknowns about Eph receptor functions lie in their downstream signalling pathways. Specifically, how EphA10 and EphB6, the two Eph receptors without tyrosine kinase activity, are able to modulate the Eph receptor signalling is an intriguing question, and is illustrative of the importance of catalysis-independent functions more broadly within the Eph family. An enhanced molecular level understanding of the non-catalytic family members, EphA10 and EphB6, and how they act as molecular switches to regulate their kinase-active Eph receptor counterparts is essential to target the non-catalytic functions of Eph receptors therapeutically, including in cancers.
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Download referencesAcknowledgementsThis work is supported by the Australian Cancer Research Foundation (to LYL, OP and IL), and the Cancer Council of Victoria (to PWJ). IL acknowledges support from the Walter and Eliza Hall Institute. LYL is supported by Melbourne Research Scholarship. JMM is grateful to the NHMRC for fellowship support (1105754). We acknowledge the NHMRC IRIISS and the Victorian State Government Operational Infrastructure Support Scheme.Author informationAuthors and AffiliationsThe Walter and Eliza Hall Institute of Medical Research, Parkville, VIC, 3052, AustraliaLung-Yu Liang, Onisha Patel, James M. Murphy & Isabelle S. LucetDepartment of Medical Biology, University of Melbourne, Parkville, VIC, 3052, AustraliaLung-Yu Liang, Onisha Patel, James M. Murphy & Isabelle S. LucetOlivia Newton-John Cancer Research Institute, 145 Studley Road, Heidelberg, VIC, 3084, AustraliaPeter W. JanesAuthorsLung-Yu LiangView author publicationsYou can also search for this author in
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Reprints and permissionsAbout this articleCite this articleLiang, LY., Patel, O., Janes, P.W. et al. Eph receptor signalling: from catalytic to non-catalytic functions.
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Mechanisms and functions of eph and ephrin signalling | Nature Reviews Molecular Cell Biology
Mechanisms and functions of eph and ephrin signalling | Nature Reviews Molecular Cell Biology
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nature reviews molecular cell biology
review articles
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Review Article
Published: 01 July 2002
Mechanisms and functions of eph and ephrin signalling
Klas Kullander1 & Rüdiger Klein2
Nature Reviews Molecular Cell Biology
volume 3, pages 475–486 (2002)Cite this article
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Key Points
Biological functions for Eph receptors and ephrins include vascular development, tissue-border formation, cell migration, axon guidance and synaptic plasticity.
Eph receptors can act as a ligand in the same way that an ephrin ligand can act as a receptor. Ephrins are also able to signal into their host cell, which is referred to as 'reverse signalling'
Recent examples of Eph-receptor signalling mechanisms are: (1) Ephexin and Abl and their role in regulating the actin cytoskeleton; (2) Eph receptors as negative regulators of the extracellular-signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathway; and (3) Eph receptors as regulators of cell adhesion through integrin cell–substrate interactions.
Recent examples of ephrin signalling mechanisms are: (1) Src family kinases (SFK) as positive regulators of ephrinB phosphorylation; (2) the recruitment of PDZ-domain-containing protein-tyrosine phosphatase PTP-BL to ephrinB membrane clusters; (3) the SH2–SH3 domain adaptor protein Grb4 as a downstream effector of ephrinB ligands; and (4) phosphorylation-independent signalling by ephrinB ligands through the new cytoplasmic protein PDZ-RGS3.
Some of the functions that are dependent on receptor-mediated signalling are: (1) kinase-dependent functions in formation of the corticospinal tract mediated by EphA4; (2) EphB2 forward signalling in the regulation of fluid homeostasis in the inner ear by EphB2; and (3) EphrinB–EphB2–NMDA-receptor influenced signalling clusters at the synapse and modulation of synapse functions, although some of these events are dependent on Eph receptor kinase signalling, whereas others are not.
Functions that require the ephrinB cytoplasmic domain are: (1) the establishment of boundaries between segments of the vertebrate hindbrain; and (2) remodelling of the embryonic vasculature.
A function that does not require the ephrinB cytoplasmic domain is neural-crest-cell migration, and is instead probably mediated by forward signalling of Eph receptors.
AbstractEph receptors constitute the largest family of tyrosine kinase receptors and, together with their plasma-membrane-bound ephrin ligands, have many important functions during development and adulthood. In contrast with most receptor tyrosine kinases, unidirectional signalling can originate from the ephrin ligands as well as from the Eph receptors. Furthermore, the concept of bidirectional signalling has emerged as an important mechanism by which Ephs and ephrins control the output signal in processes of cell–cell communication.
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Figure 1: General features of Eph receptors and ephrins.Figure 2: Regulation of Eph receptor catalytic activity.Figure 3: Summary of adaptor interactions described in this review.Figure 4: Molecular mechanisms of ephrinB reverse signalling.Figure 5: Corticospinal-tract-fibre guidance.Figure 6: Long-term potentiation.Figure 7: Boundary sorting.Figure 8: Angiogenesis and vasculogenesis.Figure 9: Migration of neural-crest cells.
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Download referencesAcknowledgementsWe thank I. C. Grunwald, A. Palmer and G. A. Wilkinson for critically reading the manuscript and helpful discussions. Work in the laboratory was supported by grants from the Human Frontiers Science Programme Organization, the Deutsche Forschungsgemeinschaft and the Max Planck Society. K. K is a Marie Curie Fellow.Author informationAuthors and AffiliationsAstraZeneca Transgenics & Comparative Genomics (ATCG), Mölndal, S-431 83, SwedenKlas KullanderDepartment of Molecular Neurobiology, Max Planck Institute of Neurobiology, Am Klopferspitz 18A, Martinsried, D-82152, GermanyRüdiger KleinAuthorsKlas KullanderView author publicationsYou can also search for this author in
PubMed Google ScholarRüdiger KleinView author publicationsYou can also search for this author in
PubMed Google ScholarRelated linksRelated linksDATABASESFlybase
dock
InterPro
Dbl
PDZ-binding motif
pleckstrin homology
SAM
SH2
SH3 domains
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Abi-1
axin
BDNF
Cdc42
CXCR4
EGF
EphA1
EphA2
EphA3
EphA4
EphA8
EphB1
EphB2
EphB3
EphB4
EphB6
Grb4
GRIP1
GRIP2
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R-Ras
SDF-1
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syndecan-2
syntenin
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Arg
Ephexin
ephrinA1
ephrinA3
ephrinA4
ephrinA5
ephrinB1
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TrkB receptor
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GlossaryGPI ANCHOR
The function of this post-translational modification is to attach proteins to the exoplasmic leaflet of membranes, and possibly to specific domains therein. The anchor is made of one molecule of phosphatidylinositol to which a carbohydrate chain is linked through the C-6 hydroxyl of the inositol, and is linked to the protein through an ethanolamine phosphate moiety.
FIBRONECTIN TYPE III REPEAT
A 90-amino-acid-long stretch that is repeated 15–17 times in the fibronectin molecule. It is a common motif in many cell-surface proteins.
STERILE α-MOTIF
(SAM). A domain of ∼70 amino acids that is roughly conserved in many proteins and thought to participate in protein–protein interactions.
PDZ-DOMAIN
(PSD-95, Dlg and ZO-1/2). A protein–protein interaction domain of around 90 amino acids that binds particularly to carboxy-terminal polypeptides.
SYNAPTIC PLASTICITY
A change in the functional properties of a synapse as a result of use.
AUTOPHOSPHORYLATION
The phosphorylation by a protein of one of its own residues.
SH2 DOMAIN
(Src-homology-2 domain). A protein motif that recognizes and binds tyrosine-phosphorylated sequences, and thereby has a key role in relaying cascades of signal transduction.
SH3 DOMAIN
(Src-homology-3 domain). A protein sequence of around 50 amino acids that recognizes and binds sequences that are rich in proline.
NEURAL TUBE
A hollow, dorsal tube of embryonic nerve tissue that is formed by the rolling up of the neural plate. At the front, the tube expands to form the brain, whereas the posterior part narrows to form the spinal cord.
RHO FAMILY GTPASES
Ras-related GTPases that are involved in controlling the polymerization of actin.
GROWTH CONE
A motile, exploratory tip of the axon or dendrite of a growing nerve cell, which spreads out into a large cone-shaped appendage.
FILOPODIA
Long, thin protrusions at the periphery of cells and growth cones. They are composed of F-actin bundles.
LAMELLIPODIA
Flattened, sheet-like projections of crosslinked F-actin from the surface of a cell, which are often associated with cell migration.
DOMINANT-NEGATIVE
A defective protein that retains interaction capabilities and so distorts or competes with normal proteins.
RETINAL GANGLION CELLS
Cells that form the third and last layer of the retina. They are a type of interneuron that convey information from the retinal bipolar, horizontal and amacrine cells of the eye to the brain. The axons of ganglion cells form the fibers of the optic nerve.
VOMERONASAL SYSTEM
A cluster of sensory neurons in the nasal arch that detects pheromones and transmits this information to higher cortical centres.
ENDOTHELIAL CELLS
Thin, flattened cells of mesoblastic origin that are arranged in a single layer that lines the blood vessels and some body cavities; for example, those of the heart.
FASCICULATION
The bundling of axonal processes of neurons.
FOCAL ADHESIONS
Cellular structures that link the extracellular matrix on the outside of the cell, through integrin receptors, to the actin cytoskeleton inside the cell.
STRESS FIBRES
Axial bundles of F-actin underlying the cell bodies.
OPTIC DISC
The exit point of retinal axons from the eye into the optic nerve (also called the optic nerve head).
VENTRAL ENCLOSURE
The ability of epithelial sheets with free edges to join together and fuse at the ventral midline.
NEOCORTEX
The 'newer cortex', which refers to the telencephalic cortex as opposed to the evolutionarily older primitive cortex (piriform cortex). It is found in higher vertebrates and is the site of higher mental processes.
SEMICIRCULAR CANAL
Liquid-filled archformed canal that is part of the inner-ear balance organ.
VESTIBULAR SYSTEM
The inner-ear balance organ that keeps track of the position and motion of the head in space. It consists of three perpendicularly oriented semicircular canals, which detect angular acceleration, and the utricle and saccule, which detect linear acceleration.
PROTEOGLYCANS
A class of acidic glycoproteins that are found in the extracellular matrix, especially in connective tissues. They contain more carbohydrate than protein.
LONG-TERM POTENTIATION
A long-lasting increase in the efficacy of synaptic transmission, which is commonly elicited by high-frequency neuron stimulation.
NEURAL-CREST CELL
An embryonic cell that separates from the embryonic neural plate and migrates, giving rise to the spinal and autonomic ganglia, peripheral glia, chromaffin cells, melanocytes and some haematopoietic cells.
BRANCHIAL ARCH
In the higher vertebrate embryo, this is one of a series of arches — populated by neural-crest cells — that develop into structures of the ear, neck and face. The corresponding structures in fish and amphibians, sometimes referred to as gill arches, are made of bone or cartilage and are located on either side of the pharynx.
AORTIC ARCHES
Paired arches in vertebrate embryos that connect the ventral aorta with dorsal aorta(e) by running up between gill slits or gill pouches on each side.
PROTEOME
The complete set of (predicted) proteins in an organism. The term is analogous to the genome (the complete genetic material).
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Nature Reviews Molecular Cell Biology (Nat Rev Mol Cell Biol)
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