Eph/ephrin对树突棘的调控与中枢神经系统疾病

树突棘是神经元树突上的功能性突起结构,通常作为突触后成份与投射来的轴突共同构成完整的突触连接。树突棘的形态与结构具有明显的可塑性,其变化通常会引起突触功能的改变。Eph受体酪氨酸激酶家族分子与其配体ephrin都是重要的神经导向因子,同时对树突棘结构也有直接的调控作用。Eph受体的活化可以促进树突棘的发生并影响树突棘的形态及内部结构;而Eph受体的异常也往往会损害正常的突触功能,甚至导致许多与树突棘结构异常相关的神经系统病变的发生。     

药物滥用与神经元结构可塑性改变的关系

药物成瘾所导致的行为和生理方面的长时程改变可能与相关脑区突触连接的重构有关。安非他命、可卡因、吗啡和尼古丁滥用时,精神依赖和奖赏效应相关脑区神经元的树突和树突棘的结构发生改变,这反映药物滥用致相关神经回路突触连接方式的改变。这种改变足长时程的、脑区特异性的,是多种因素调节的结果。     

活性依赖的突触抑制和突触稳态的分子机制

常规RNA干涉或基因敲除的功能缺失手段仅仅只是简单地移除某个基因或蛋白,而这个过程常常会掩盖磷酸化对某个特定蛋白的调节。在树突发育和突触功能活性依赖的调节过程中,突触后致密蛋白磷酸化的机制仍然是未知的领域。突触后Rap GTP酶激活蛋白SPAR与PSD95结合,可以促进树突棘的生长并加强突触。Plk2（polo-like kinase2,也称为Snk）是一种受突触活性诱导表达的蛋白激酶,它可以磷酸化SPAR,磷酸化的SPAR通过泛素化．蛋白酶体途径降解,从而导致树突棘和突触的减少。Plk2的诱导表达和随后SPAR的降解是长时间神经活性增强过程中突触强度的稳态抑制（突触剥落）所必需的。有趣的是,SPAR需要被另外一种激酶cDK5磷酸化后才能被Plk2所降解。这种机制通过CDK5对一部分突触进行标记,为由Plk2-SPAR通路抑制或去除这些突触提供了可能的途径,但其分子机制在神经退行性疾病突触丢失中的作用仍需进一步探讨。     

树突棘的动态变化与调控机制

树突棘是兴奋性突触的主要突触后结构基础,其数量与形态受神经电活动调控,并在整个生命过程中呈现复杂且有序的动态变化。树突棘的动态变化在神经环路的形成和精确化修剪中扮演重要的角色,该过程的异常可导致孤独症谱系障碍、精神分裂症等神经系统疾病。主要综述了近年来关于树突棘形态与数量动态变化的研究工作,包括发育早期的树突棘发生和青春期的树突棘修剪。在此基础上,还简要阐述了介导树突棘动态变化的信号分子,讨论了其与神经系统疾病的关联,并提出了该领域尚未解决的一些问题。     

Drebrin与认知功能障碍

树突棘和突触的病理改变在认知功能障碍发病机制中具有十分重要的作用,研究表明大脑发育调节蛋白（developmentregulationbrainprotein,Drebrin）能够调节树突棘和突触的形态和重塑。Drebrin的减少可能通过树突棘内细胞骨架变化,使树突棘的形态结构受到影响,导致突触功能和结构的变化。但目前阿尔茨海默病（Alzheimer’Sdisease,AD）脑内突触病理变化的具体机制及Drebrin和突触之间的关系仍不明确。探讨Drebrin与认知功能的关系及其机制,对临床上早期干预认知功能障碍、寻找AD的有效诊断治疗措施具有重要意义。     

谷氨酸受体调控树突棘形态可塑性的研究进展

树突棘是神经元之间产生直接联系的部位,其形态可塑性是记忆的结构基础。谷氨酸信息传递是中枢神经信息传递的主要方式,能产生突触传递效率的可塑性,由此引起树突棘形态的可塑性变化。本文从谷氨酸受体途径的角度对树突棘形态可塑性的调控机制做一综述。谷氨酸受体主要通过其下游信号分子调节棘内肌动蛋白动力学蛋白,参与树突棘的形态发生和稳定。该作用在局部受到不同的蛋白、信号分子、激素、mi RNAs的调节,从而参与生理及病理过程。最后,提出展望,研究脑区特异的局部微环境变化对记忆相关疾病病因及治疗探讨有参考价值。     

竞争机制介导树突棘的协同修剪与成熟

树突棘是中枢神经系统中绝大多数兴奋性突触的突触后位点。在出生后早期,脑内树突棘大量形成;当个体进入青少年期,脑内树突棘总数逐渐减少,这一过程被称为树突棘修剪,并被认为是神经环路精确化的重要过程。在孤独症谱系障碍、精神分裂症等发育性神经系统疾病中被报道存在树突棘修剪的异常。虽然树突棘修剪的现象已被广泛描述,然而介导该过程的分子机制尚待进一步研究。该研究组近期工作发现,在小鼠触须所对应的感觉皮层,树突棘的修剪与成熟是协同发生的,并且受感觉经验的双向调控。进一步研究发现,神经电活动可以引起相邻树突棘对cadherin/catenin细胞黏附复合物的竞争,导致该复合物的重新分布,并使这两个树突棘的命运产生分化:得到cadherin/catenin复合物的树突棘变得更加成熟而相邻失去这些分子的树突棘变小或被修剪。这一cadherin/catenin复合物依赖的竞争机制为树突棘的协同成熟与修剪提供了特异性,对于理解介导神经环路精确化的机制至关重要。     

NMDA受体参与皮层内突触结构和功能的调节

在中枢神经系统内神经细胞的树突棘是突触信息传递的重要部位,树突棘的体积和密度影响神经环路的功能。2007年美国加利福尼亚大学的SilaK．Ultanir等人在皮层NRl亚基（是NMDA受体的必要组分）基因敲除的小鼠上发现NMDA受体对树突棘的发育有重要影响。急性分离出生后三周内小鼠的脑片,用电压钳全细胞记录的方法,发现在皮层2／3层的锥体细胞中,AMPA受体介导的微小兴奋性突触后电流（mEP-SC）的幅度和频率均明显增大。     

神经递质释放的分子机制

大脑中的神经细胞主要依赖神经突触进行细胞间信息传递。神经递质从突触前释放到突触间隙中,将电信号转换为化学信号。释放的递质与突触后的相应受体结合,引起受体通道的打开再将化学信号转换为突触后电信号。到目前为止,对SNARE复合体介导的钙离子触发的神经递质释放分子机制已经有了深入理解,囊泡融合的基本模型也得到了广泛认可,但仍有问题没有解决。该文对近年来与神经递质释放分子机制相关的研究作一综述,以期为递质释放过程中重要分子的深入解析提供理论依据。     

阿尔茨海默病理条件下突触系统病变的研究进展

阿尔茨海默病(Alzheimer’s disease,AD)是一种起病隐匿的进行性发展的神经系统退行性疾病,主要病理特征表现为淀粉样蛋白沉积形成的老年斑、tau蛋白过度磷酸化导致的神经纤维缠结以及神经元丢失和突触损伤。其中,淀粉样蛋白沉积和tau蛋白过度磷酸化均可导致突触和神经棘缺失,严重影响神经递质系统功能,最终造成大脑学习记忆和认知能力损伤。从突触损伤角度出发,总结AD病理条件下,突触和神经递质及其受体的变化,为后期开展生物活性物质对AD病理中神经细胞信号通路的研究提供思路和理论基础。     

EphA signaling promotes actin-based dendritic spine remodeling through slingshot phosphatase

Actin cytoskeletal remodeling plays a critical role in transforming the morphology of subcellular structures across various cell types. In the brain, restructuring of dendritic spines through actin cytoskeleletal reorganization is implicated in the regulation of synaptic efficacy and the storage of information in neural circuits. However, the upstream pathways that provoke actin-based spine changes remain only partly understood. Here we show that EphA receptor signaling remodels spines by triggering a sequence of events involving actin filament rearrangement and synapse/spine reorganization. Rapid EphA signaling over minutes activates the actin filament depolymerizing/severing factor cofilin, alters F-actin distribution in spines, and causes transient spine elongation through the phosphatases slingshot 1 (SSH1) and calcineurin/protein phosphatase 2B (PP2B). This early phase of spine extension is followed by synaptic reorganization events that take place over minutes to hours and involve the relocation of pre/postsynaptic components and ultimately spine retraction. Thus, EphA receptors utilize discrete cellular and molecular pathways to promote actin-based structural plasticity of excitatory synapses.     

The EphA4 receptor regulates dendritic spine remodeling by affecting beta1-integrin signaling pathways

Remodeling of dendritic spines is believed to modulate the function of excitatory synapses. We previously reported that the EphA4 receptor tyrosine kinase regulates spine morphology in hippocampal pyramidal neurons, but the signaling pathways involved were not characterized (Murai, K.K., L.N. Nguyen, F. Irie, Y. Yamaguchi, and E.B. Pasquale. 2003. Nat. Neurosci. 6:153-160). In this study, we show that EphA4 activation by ephrin-A3 in hippocampal slices inhibits integrin downstream signaling pathways. EphA4 activation decreases tyrosine phosphorylation of the scaffolding protein Crk-associated substrate (Cas) and the tyrosine kinases focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (Pyk2) and also reduces the association of Cas with the Src family kinase Fyn and the adaptor Crk. Consistent with this, EphA4 inhibits beta1-integrin activity in neuronal cells. Supporting a functional role for beta1 integrin and Cas inactivation downstream of EphA4, the inhibition of integrin or Cas function induces spine morphological changes similar to those associated with EphA4 activation. Furthermore, preventing beta1-integrin inactivation blocks the effects of EphA4 on spines. Our results support a model in which EphA4 interferes with integrin signaling pathways that stabilize dendritic spines, thus modulating synaptic interactions with the extracellular environment.     

EphA5 and EphA6: regulation of neuronal and spine morphology

Background

The Eph family of receptor tyrosine kinases plays important roles in neural development. Previous studies have implicated Eph receptors and their ligands, the ephrins, in neuronal migration, axon bundling and guidance to specific targets, dendritic spine formation and neural plasticity. However, specific contributions of EphA5 and EphA6 receptors to the regulation of neuronal cell morphology have not been well studied.

Results

Here we show that deletion of EphA5 and EphA6 results in abnormal Golgi staining patterns of cells in the brain, and abnormal spine morphology.

Conclusion

These observations suggest novel functions of these Eph receptors in the regulation of neuronal and spine structure in brain development and function.

     

Ion fluxes and neurotransmitters signaling in neural development

The brain develops and functions in a complex ionic milieu, which is a prerequisite for neurotransmitter function and neuronal signaling. Neurotransmitters and ion fluxes are, however, important not only in neuronal signaling, but also in the control of neural differentiation, and in this review, we highlight the recent advances in our understanding of how the gamma-amino butyric acid (GABA) neurotransmitter and ion fluxes are relevant for cell cycle control and neural differentiation. Conversely, proteins previously associated with ion transport across membranes have been endowed with novel ion-independent functions, and we discuss this in the context of gap junctions in cell adhesion and of the neuron-specific K(+)-Cl(-) cotransporter KCC2 in dendritic spine development. Collectively, these findings provide a richer and more complex picture of when ion fluxes are needed in neural development and when they are not.     

Coordinated Nuclear and Synaptic Shuttling of Afadin Promotes Spine Plasticity and Histone Modifications

The ability of a neuron to transduce extracellular signals into long lasting changes in neuronal morphology is central to its normal function. Increasing evidence shows that coordinated regulation of synaptic and nuclear signaling in response to NMDA receptor activation is crucial for long term memory, synaptic tagging, and epigenetic signaling. Although mechanisms have been proposed for synapse-to-nuclear communication, it is unclear how signaling is coordinated at both subcompartments. Here, we show that activation of NMDA receptors induces the bi-directional and concomitant shuttling of the scaffold protein afadin from the cytosol to the nucleus and synapses. Activity-dependent afadin nuclear translocation peaked 2 h post-stimulation, was independent of protein synthesis, and occurred concurrently with dendritic spine remodeling. Moreover, activity-dependent afadin nuclear translocation coincides with phosphorylation of histone H3 at serine 10 (H3S10p), a marker of epigenetic modification. Critically, blocking afadin nuclear accumulation attenuated activity-dependent dendritic spine remodeling and H3 phosphorylation. Collectively, these data support a novel model of neuronal nuclear signaling whereby dual-residency proteins undergo activity-dependent bi-directional shuttling from the cytosol to synapses and the nucleus, coordinately regulating dendritic spine remodeling and histone modifications.     

Eph receptors are involved in the activity-dependent synaptic wiring in the mouse cerebellar cortex

Eph receptor tyrosine kinases are involved in many cellular processes. In the developing brain, they act as migratory and cell adhesive cues while in the adult brain they regulate dendritic spine plasticity. Here we show a new role for Eph receptor signalling in the cerebellar cortex. Cerebellar Purkinje cells are innervated by two different excitatory inputs. The climbing fibres contact the proximal dendritic domain of Purkinje cells, where synapse and spine density is low; the parallel fibres contact the distal dendritic domain, where synapse and spine density is high. Interestingly, Purkinje cells have the intrinsic ability to generate a high number of spines over their entire dendritic arborisations, which can be innervated by the parallel fibres. However, the climbing fibre input continuously exerts an activity-dependent repression on parallel fibre synapses, thus confining them to the distal Purkinje cell dendritic domain. Such repression persists after Eph receptor activation, but is overridden by Eph receptor inhibition with EphA4/Fc in neonatal cultured cerebellar slices as well as mature acute cerebellar slices, following in vivo infusion of the EphA4/Fc inhibitor and in EphB receptor-deficient mice. When electrical activity is blocked in vivo by tetrodotoxin leading to a high spine density in Purkinje cell proximal dendrites, stimulation of Eph receptor activation recapitulates the spine repressive effects of climbing fibres. These results suggest that Eph receptor signalling mediates the repression of spine proliferation induced by climbing fibre activity in Purkinje cell proximal dendrites. Such repression is necessary to maintain the correct architecture of the cerebellar cortex.     

Synaptic activity prompts γ-secretase–mediated cleavage of EphA4 and dendritic spine formation

Alzheimer''s disease is an age-dependent neurodegenerative disorder that is characterized by a progressive decline in cognitive function. γ-secretase dysfunction is evident in many cases of early onset familial Alzheimer''s disease. However, the mechanism by which γ-secretase dysfunction results in memory loss and neurodegeneration is not fully understood. Here, we demonstrate that γ-secretase is localized at synapses and regulates spine formation. We identify EphA4, one of the Ephrin receptor family members, as a substrate of γ-secretase, and find that EphA4 processing is enhanced by synaptic activity. Moreover, overexpression of EphA4 intracellular domain increases the number of dendritic spines by activating the Rac signaling pathway. These findings reveal a function for EphA4-mediated intracellular signaling in the morphogenesis of dendritic spines and suggest that the processing of EphA4 by γ-secretase affects the pathogenesis of Alzheimer''s disease.     

Cyclin-Dependent Kinase 5 Links Extracellular Cues to Actin Cytoskeleton During Dendritic Spine Development

Emerging evidence has indicated a regulatory role of cyclin-dependent kinase 5 (Cdk5) in synaptic plasticity as well as in higher brain functions, such as learning and memory. However, the molecular and cellular mechanisms underlying the actions of Cdk5 at synapses remain unclear. Recent findings demonstrate that Cdk5 regulates dendritic spine morphogenesis through modulating actin dynamics. Ephexin1 and WAVE-1, two important regulators of the actin cytoskeleton, have both been recently identified as substrates for Cdk5. Importantly, phosphorylation of these proteins by Cdk5 leads to dendritic spine loss, revealing a potential mechanism by which Cdk5 regulates synapse remodeling. Furthermore, Cdk5-dependent phosphorylation of ephexin1 is required for the ephrin-A1 mediated spine retraction, pointing to a critical role of Cdk5 in conveying signals from extracellular cues to actin cytoskeleton at synapses. Taken together, understanding the precise regulation of Cdk5 and its downstream targets at synapses would provide important insights into the multi-regulatory roles of Cdk5 in actin remodeling during dendritic spine development.Excitatory synaptic transmission occurs primarily at dendritic spines, small protrusions that extend from dendritic shafts. Emerging studies have shown that dendritic spines are dynamic structures which undergo changes in size, shape and number during development, and remain plastic in adult brain.¹ Regulation of spine morphology has been implicated to associate with changes of synaptic strength.² For example, enlargement and shrinkage of spines was reported to associate with certain forms of synaptic plasticity, i.e., long-term potentiation and long time depression, respectively.³ Thus, understanding the molecular mechanisms underlying the regulation of spine morphogenesis would provide insights into synapse development and plasticity. Receptor tyrosine kinases (RTKs) such as the Ephs are known to play critical roles in regulating spine morphogenesis. Eph receptors are comprised of 14 members, which are classified into EphAs and EphBs according to their sequence homology and ligand binding specificity. With a few exceptions, EphAs typically bind to A-type ligands, whereas EphBs bind to B-type ligands. During development of the central nervous system (CNS), ephrin-Eph interactions exert repulsive/attractive signaling, leading to regulation of axon guidance, topographic mapping and neural patterning.⁴ Activated Ephs trigger intracellular signaling cascades, which subsequently lead to remodeling of actin cytoskeleton through tyrosine phosphorylation of its target proteins or interaction with various cytoplasmic signaling proteins. Intriguingly, emerging studies have revealed novel functions of Ephs in synapse formation and synaptic plasticity.⁵ Specific Ephs expressed in dendritic spines of adult brain are implicated in regulating spine morphogenesis, i.e., EphBs promote spine formation and maturation, while EphA4 induces spine retraction.^6,7In the adult hippocampus, EphA4 is localized to the dendritic spines.^7,8 Activation of EphA4 at the astrocyte-neuron contacts, triggered by astrocytic ephrin-A3, leads to spine retraction and results in a reduction of spine density.⁷ It has been well established that actin cytoskeletal rearrangement is critical for spine morphogenesis, and is controlled by a tight regulation of Rho GTPases including Rac1/Cdc42 and RhoA. Antagonistic regulation of Rac1/Cdc42 and RhoA has been observed to precede changes in spine morphogenesis, i.e., activation of Rac1/Cdc42 and inhibition of RhoA is involved in spine formation, and vice versa in spine retraction.⁹ Rho GTPases function as molecular switches that cycle between an inactive GDP-bound state and an active GTP-bound state. The activation status of GTPase is regulated by an antagonistic action of guanine-nucleotide exchange factors (GEFs) which enhance the exchange of bound GDP for GTP, and GTPase-activating proteins (GAPs) which increase the intrinsic rate of hydrolysis of bound GTP.¹⁰ Previous studies have implicated that Rho GTPases provides a direct link between Eph and actin cytoskeleton in diverse cellular processes including spine morphogenesis.¹¹ In particular, EphBs regulate spine morphology by modulating the activity of Rho GTPases, thereby leading to rearrangement of actin networks.^12–14 Although EphA4 activation results in spine shrinkage, the molecular mechanisms that underlie the action of EphA4 at dendritic spines remain largely unclear.Work from our laboratory recently demonstrated a critical role of cyclin-dependent kinase 5 (Cdk5) in mediating the action of EphA4 in spine morphogenesis through regulation of RhoA GTPase.¹⁵ Cdk5 is a proline-directed serine/threonine kinase initially identified to be a key regulator of neuronal differentiation, and has been implicated in actin dynamics through regulating the activity of Pak1, a Rac effector, during growth cone collapse and neurite outgrowth.¹⁶ We found that EphA4 stimulation by ephrin-A ligand enhances Cdk5 activity through phosphorylation of Cdk5 at Tyr15. More importantly, we demonstrated that ephexin1, a Rho GEF, is phosphorylated by Cdk5 in vivo. Ephexin1 was reported to transduce signals from activated EphA4 to RhoA, resulting in growth cone collapse during axon guidance.^17,18 Interestingly, we found that ephexin1 is highly expressed at the post-synaptic densities (PSDs) of adult brains.¹⁵ Loss of ephexin1 in cultured hippocampal neurons or in vivo perturbs the ability of ephrin-A to induce EphA4-dependent spine retraction. The loss of ephexin1 function in spine morphology can be rescued by reexpression of wild-type ephexin1, but not by expression of its phosphorylation-deficient mutant. Our findings therefore provide important evidence that phosphorylation of ephexin1 by Cdk5 is required for the EphA4-dependent spine retraction.Molecular mechanisms underlying the action of Cdk5/ephexin1 on actin networks in EphA4-mediated spine retraction is just beginning to be unraveled. It was reported that activation of EphA4-signaling induces tyrosine phosphorylation of ephexin1 through Src family kinases (SFKs), and promotes its exchange activity towards RhoA.¹⁷ Interestingly, mutation of the Cdk5 phosphorylation sites of ephexin1 attenuates the Src-dependent tyrosine phosphorylation of ephexin1 at Tyr⁸⁷ upon EphA4 activation. These findings suggest that Cdk5 is the “priming” kinase for ephexin1. We propose that EphA4 activation by ephrin-A ligand increases Cdk5 activity, leading to phosphorylation and priming of ephexin1 for the subsequent phosphorylation of ephexin1 by Src kinase at Tyr⁸⁷, resulting in an increase of its exchange activity towards RhoA. Thus, regulation of Cdk5 activity might indirectly control the phosphorylation of ephexin1 by Src. It is tempting to speculate that phosphorylation of ephexin1 by Cdk5 at the amino-terminal region leads to a conformational change of protein, thus facilitating the access of Tyr⁸⁷ site on ephexin1 to Src kinase. Whereas accumulating evidence have pointed to a pivotal role of various GEFs including Tiam1, intersectin and kalirin in regulating spine morphogenesis, the involvement of GAPs is not clear. For example, oligophrenin-1, a Rho GAP, is implicated in maintaining the spine length through repressing RhoA activity.¹⁹ Thus, it is conceivable that a specific GAP is involved in EphA4-dependent spine retraction. Recently, we found that α2-chimaerin, a Rac GAP, regulates EphA4-dependent signaling in hippocampal neurons (Shi and Ip, unpublished observations). Taken into consideration that α2-chimaerin is enriched in the PSDs, α2-chimaerin is a likely candidate that cooperates with ephexin1 during EphA4-dependent spine retraction.In addition to stimulation of the RTK signaling cascade following EphA4 receptor activation, clustering of EphA4 signaling complex is required for eliciting maximal EphA4 function.²⁰ It is tempting to speculate that Cdk5 also regulates the formation of EphA4-containing clusters in neurons. Indeed, Cdk5^-/- neurons show reduced size of EphA4 clusters upon ephrin-A treatment, suggesting that Cdk5 regulates the recruitment of downstream signaling proteins to activate EphA4. Moreover, since ephrinA-EphA4 interaction stimulates the activity of Cdk5 at synaptic contacts, it is possible that Cdk5 might play additional roles at the post-synaptic regions through phosphorylation of its substrates. For example, PSD-95, the major scaffold protein in the PSDs, and NMDA receptor subunit NR2A are both substrates for Cdk5. Interestingly, phosphorylation of these proteins by Cdk5 has been implicated in regulating the clustering of neurotransmitter receptors as well as synaptic transmission.^21,22 Consistent with these observations, spatial distribution of neurotransmitter receptors at neuromuscular synapses is altered and abnormal neurotransmission is observed in Cdk5^-/- mice.²³ Thus, further analysis to delineate the precise roles of Cdk5 in EphA4-dependent synapse development, including regulation of neurotransmitter receptor clustering, is required.Recently, Cdk5 was shown to regulate dendritic spine density and shape through controlling the phosphorylation status of Wiskott-Aldrich syndrome protein-family verprolin homologous protein 1 (WAVE-1), a critical component of actin cytoskeletal network.²⁴ In particular, phosphorylation of WAVE-1 by Cdk5 prevents actin from Arp2/3 complex-dependent polymerization and leads to a loss of dendritic spines at basal state, while reduced Cdk5-dependent phosphorylation of WAVE-1 through cAMP-dependent dephosphorylation leads to an enhanced actin polymerization and increased number of spines. It is interesting to note that phosphorylation of ephexin1 and WAVE-1 by Cdk5 both results in a reduction of spine density. Whether a concerted phosphorylation of these proteins at synapses by Cdk5 plays a role in synaptic plasticity awaits further studies. Precise regulation of Cdk5 activity is unequivocally important to maintain its proper functions at synaptic contacts. Activation of Cdk5 is mainly dependent on its binding to two neuronal-specific activators, p35 or p39, and its activity can be enhanced upon phosphorylation at Tyr¹⁵.While the signals that lie upstream of Cdk5 have barely begun to be unraveled, Cdk5 has been demonstrated to be a key downstream regulator of signaling pathways activated by extracellular cues such as neuregulin, BDNF and semaphorin. To the best of our knowledge, ephrin-EphA4 signaling is the first extracellular cue that has been identified to phosphorylate Cdk5 and promote its activity at CNS synapses.^15,25 Since BDNF-TrkB and semaphorin3A-fyn signaling have also been implicated in synapse/ spine development, it is of importance to examine whether Cdk5 is the downstream integrator of these signaling events at synapses during spine morphogenesis.^26,27Although accumulating evidence highlights a role of Cdk5 in spatial learning and synaptic plasticity, the molecular mechanisms underlying the action of Cdk5 are largely unclear.^28,29 With the recent findings that reveal the critical involvement of Cdk5 in the regulation of Rho GTPases to affect spine morphology, it can be anticipated that precise regulation of actin dynamics by Cdk5 at synapses will be an important mechanism underlying synaptic plasticity in the adult brain.? Open in a separate windowFigure 1Phosphorylation of actin regulators by Cdk5 during dendritic spine morphogenesis. (A) In striatal and hippocampal neurons, phosphorylation of WAVE-1 by Cdk5 at basal condition prevents WAVE-1-mediated actin polymerization and leads to a loss of dendritic spines. However, activation of cyclic AMP-dependent signaling by neurotransmitter such as dopamine, reduces the Cdk5-dependent phosphorylation of WAVE-1 in these neurons. Dephosphorylation of WAVE-1 promotes actin polymerization and results in an increased number of mature dendritic spines. (B) In mature hippocampal neurons, activation of EphA4 by ephrin-A increases Cdk5-dependent of ephexin1. The phosphorylation of ephexin1 by Cdk5 facilitates its EphA4-stimulated GEF activity towards RhoA activation and leads to spine retraction.     

Activation of NMDA receptors promotes dendritic spine development through MMP-mediated ICAM-5 cleavage

Matrix metalloproteinase (MMP)-2 and -9 are pivotal in remodeling many tissues. However, their functions and candidate substrates for brain development are poorly characterized. Intercellular adhesion molecule-5 (ICAM-5; Telencephalin) is a neuronal adhesion molecule that regulates dendritic elongation and spine maturation. We find that ICAM-5 is cleaved from hippocampal neurons when the cells are treated with N-methyl-d-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA). The cleavage is blocked by MMP-2 and -9 inhibitors and small interfering RNAs. Newborn MMP-2- and MMP-9-deficient mice brains contain more full-length ICAM-5 than wild-type mice. NMDA receptor activation disrupts the actin cytoskeletal association of ICAM-5, which promotes its cleavage. ICAM-5 is mainly located in dendritic filopodia and immature thin spines. MMP inhibitors block the NMDA-induced cleavage of ICAM-5 more efficiently in dendritic shafts than in thin spines. ICAM-5 deficiency causes retraction of thin spine heads in response to NMDA stimulation. Soluble ICAM-5 promotes elongation of dendritic filopodia from wild-type neurons, but not from ICAM-5-deficient neurons. Thus, MMPs are important for ICAM-5-mediated dendritic spine development.     

alpha2-Chimaerin is an essential EphA4 effector in the assembly of neuronal locomotor circuits

The assembly of neuronal networks during development requires tightly controlled cell-cell interactions. Multiple cell surface receptors that control axon guidance and synapse maturation have been identified. However, the signaling mechanisms downstream of these receptors have remained unclear. Receptor signals might be transmitted through dedicated signaling lines defined by specific effector proteins. Alternatively, a single cell surface receptor might couple to multiple effectors with overlapping functions. We identified the neuronal RacGAP alpha2-chimaerin as an effector for the receptor tyrosine kinase EphA4. alpha2-Chimaerin interacts with activated EphA4 and is required for ephrin-induced growth cone collapse in cortical neurons. alpha2-Chimaerin mutant mice exhibit a rabbit-like hopping gait with synchronous hindlimb movements that phenocopies mice lacking EphA4 kinase activity. Anatomical and functional analyses of corticospinal and spinal interneuron projections reveal that loss of alpha2-chimaerin results in impairment of EphA4 signaling in vivo. These findings identify alpha2-chimaerin as an indispensable effector for EphA4 in cortical and spinal motor circuits.