. Synaptic activity prompts gamma-secretase-mediated cleavage of EphA4 and dendritic spine formation. J Cell Biol. 2009 May 4;185(3):551-64. PubMed.

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  1. This work by Inoue et al. presents exciting evidence that the PS1/γ-secretase system regulates synaptic physiology through the cleavage of EphA4 receptor. The Eph receptors and the Ephrin ligands are very important multimember families of proteins known to play critical roles in synaptic function and integrity. The regulated cleavage of EphA4 receptor by PS1/γ-secretase is very interesting since this receptor participates in the morphological shaping of dendritic spines, induction of LTP and LTD, and the regulation of structural plasticity of synaptic connections in the mature brain.

    By downregulating PS in neurons and using γ-secretase inhibitors the authors show that presenilin/γ-secretase regulates the morphogenesis of dendritic spines and more specifically regulates the number and shape of synapses as well as the clustering of AMPA receptors. The molecular mechanism through which PS1/γ-secretase exerts this function involves the activation of Rac1 by the product of γ-secretase cleavage of EphA4 (peptide EphA4 ICD), which interacts with and activates Rac1, leading to cytoskeletal rearrangements. Inhibition of these functions in disease may thus lead to impaired cytoskeletal dynamics in neurons and eventually loss of synaptic connections in the brain. Interestingly, the authors show that mutations of PS1 found in familial Alzheimer disease inhibit the processing of EphA4, as it has been previously shown for cadherins, EphrinB ligands, and EphB receptors, confirming the observation that these mutations cause loss of function, potentially leading to neurodegeneration in AD (Marambaud et al., 2003).

    The regulation of EphA4 by PS1/γ-secretase is of particular interest since EphA4 receptors play a critical role in the development and function of neural connections in the brain and synaptic abnormalities correlate more closely with the degree of dementia than any other histopathological hallmark of AD.

    View all comments by Anastasios Georgakopoulos
  2. Presenilin is the catalytic subunit of γ-secretase, which is responsible for the cleavage of amyloid-precursor protein (APP), leading to the generation of the amyloidogenic 42-residue β-amyloid peptide (Aβ) and formation of senile plaques that are hallmark features in Alzheimer disease (AD). Familial Alzheimer disease (FAD) is associated with mutations of the presenilin (PS) gene, but it is not clear how the dysfunction of presenilin accounts for the AD pathophysiology. One possibility is a toxic gain-of-function mechanism that involves increase in Aβ production and subsequent cell death. Alternatively, increasing evidence suggests that PS may have a crucial role at synapses under normal physiological condition, and loss-of-function mutations in PS1 gene may account for the impaired synaptic plasticity and memory formation associated with AD (Saura et al., 2004). This later hypothesis is supported by the recent paper by Inoue et al., which reveals a novel function of PS in promoting the clustering of AMPA receptors and formation of dendritic spines in hippocampal neurons via processing of the receptor tyrosine kinase EphA4.

    In this study, PS1 was shown to be enriched in the synaptic lipid raft of hippocampal neurons. Inhibition of γ-secretase reduces both the clustering of AMPA receptors and the density of dendritic spines. To understand the mechanisms underlying the reduction of synapse number upon inhibition of γ-secretase, the authors went on to identify its substrates by performing mass spectrometry of the synaptic lipid raft fraction. Among the ~300 proteins identified in the lipid raft is EphA4, which is cleaved by matrix metalloproteases (MMP) followed by γ-secretase to generate the intracellular domain (EphA4-ICD). The processing of EphA4 to generate the ICD is induced by synaptic activity (treatment with forskolin, rolipram, and bicuculline). Importantly, expression of EphA4-ICD in dissociated hippocampal neurons promotes the formation of dendritic spines through activation of the Rho family of GTPase Rac1. Moreover, the inhibitory effect of γ-secretase inhibitor on spine formation and AMPA receptor clustering is abolished either by the expression of EphA4-ICD or knockdown of EphA4 expression. Together these findings suggest that EphA4 processing and generation of ICD by γ-secretase may be crucial for activity-dependent spine morphogenesis during synaptic plasticity. In addition, the PS1 construct that harbors FAD-linked PS1 mutation fails to cleave EphA4, raising the possibility that impaired processing of EphA4 may underlie the memory formation deficit in AD patients.

    Studies from my laboratory and others have found that activation of EphA4 by its cognate ligand Ephrin results in spine retraction through activation of another Rho GTPase, RhoA, and inhibition of integrin signaling pathway (Murai et al., 2003; Fu et al., 2007; Bourgin et al., 2007). In contrast, the processing of EphA4 to produce ICD by γ-secretase and the subsequent activation of Rac1 do not depend on ligand binding, but instead can be enhanced by synaptic activity. The study by Inoue et al., therefore, raises an interesting notion that EphA4 can be differentially coupled to activation of different Rho GTPases in response to distinct stimuli (synaptic activity vs. Ephrin-binding), leading to opposing consequences in spine density. One important missing link in their hypothesis is the direct demonstration that increasing synaptic activity in either hippocampal slices or dissociated culture can indeed promote spine formation in a PS1- or EphA4-dependent manner. Further examination on EphA4 processing and spine morphogenesis in conditional PS1/2 double knockout mice (Saura et al., 2004) will also provide in vivo evidence to strengthen this interesting hypothesis. Nonetheless, this and other studies that reveal potential physiological roles of PS and γ-secretase in normal functioning of synapses might raise concerns about potential side effects of using γ-secretase inhibitors as a therapeutic strategy for AD.

    View all comments by Nancy Ip
  3. The paper by Inoue et al. adds important new evidence to the synaptic function of EphA4, a receptor involved in the development and function of the CNS. In addition, it verifies reports that PS1 is localized at synaptic contacts (Georgakopoulos et al., 1999) and indicates that this protein may promote formation of dendritic spines and synapses. Interestingly, PS1 is reported to specifically affect clustering and localization of AMPA receptors, but it seems to have little or no effect on other synaptic markers like synaptophysin and PSD95. The lack of effects on these synaptic markers may be explained by the suggestion of specific effect on "silent" synapses. Interestingly, the effects of γ-secretase inhibitors on the size of synapses and PSD95 clusters may indicate involvement of synaptic cadherins, a class of proteins processed by γ-secretase and known to regulate synaptic structure and function.

    Importantly, the paper also reports inhibitory effects of PS1 FAD mutations on the processing of EphA4 protein. This result is in excellent agreement with earlier reports that the γ-secretase processing of many synaptic proteins, including cadherins, EphB receptors, and EphrinB ligand proteins, is inhibited by FAD mutations. Thus, the report by Inoue et al. adds to accumulating evidence that loss of PS and γ-secretase functions caused by FAD mutations may be involved in the mechanism by which these mutations promote neurodegeneration and AD (see also Marambaud et al., 2003).

    View all comments by Nikolaos K. Robakis

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