Multiple CB splice variants exist that differ in their C-terminal structures and by the presence or absence of an N-terminal SH3 domain (Kins et al., 2000 and Harvey et al., 2004). Intriguingly, the predominant CB isoforms detected in vivo contain an SH3 domain, which inhibits the aforementioned CB-dependent formation of submembrane gephyrin clusters, indicating that CB is negatively regulated by its SH3 domain (Kins et al., 2000 and Harvey et al., 2004). However, cotransfection of CBSH3+ and gephyrin with NL2 negates the inhibitory effect SCR7 of the SH3 domain (Poulopoulos

et al., 2009). CB splice variants invariably contain a pleckstrin homology (PH) domain that is required for its interaction with plasma-membrane-restricted

phosphoinositides and for clustering of gephyrin at inhibitory synapses (Harvey et al., 2004 and Reddy-Alla et al., 2010). selleck chemical The data are consistent with a heterotrimeric membrane-associated complex that consists of NL2, CBSH3+, and gephyrin and that enables the selective deposition of gephyrin at NL2-containing inhibitory synapses. Experiments in heterologous cells indicate that NL1 can potentially substitute for NL2 and similarly induce submembrane gephyrin clusters but only with constitutively active CB isoforms that lack an SH3 domain. In addition, preliminary evidence suggests that the α2 subunit can substitute for NL2 and activate the gephyrin-clustering function of CBSH3+ (Saiepour et al., 2010). This GABAAR-dependent function of CB is specific for α2-containing receptors and abolished by a naturally occurring missense mutation (CBG55A) that disrupts the clustering of α2-containing GABAARs and gephyrin in cultured neurons and is associated with mental retardation, epilepsy, and hyperekplexia in a patient (Harvey et al., 2004 and Saiepour et al., 2010). NL1- and α2 subunit-mediated activation of CB might contribute to residual clustering of gephyrin seen in NL2 KO mice (Jedlicka et al., 2011). Other gephyrin binding proteins cAMP that are concentrated at synapses include the Ser/Thr kinase mTor (mammalian target of rapamycin,

also known as RAFT1 and FRAP1) (Sabatini et al., 1999) and the dynein light chains (DLC) 1 and 2 (Fuhrmann et al., 2002). mTor functions as an important regulator of mRNA translation, allowing for speculation that gephyrin might contribute to translational control of postsynaptic protein synthesis. This idea is supported by recent evidence that both gephyrin and collybistin are part of the eukaryotic translation initiation factor 3 complex (Sertie et al., 2010). However, whether gephyrin and collybistin play a role in translational control in dendrites remains to be elucidated. The interaction between gephyrin and the DLC is implicated in retrograde vesicular transport of gephyrin-glycine receptor complexes from glycinergic synapses (Maas et al., 2009).

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