A conformational switch in collybistin determines the differentiation of inhibitory postsynapses

Abstract

The formation of neuronal synapses and the dynamic regulation of their efficacy depend on the assembly of the postsynaptic neurotransmitter receptor apparatus. Receptor recruitment to inhibitory GABAergic and glycinergic synapses is controlled by the scaffold protein gephyrin and the adaptor protein collybistin. We derived new insights into the structure of collybistin and used these to design biochemical, cell biological, and genetic analyses of collybistin function. Our data define a collybistin-based protein interaction network that controls the gephyrin content of inhibitory postsynapses. Within this network, collybistin can adopt open/ active and closed/inactive conformations to act as a switchable adaptor that links gephyrin to plasma membrane phosphoinositides. This function of collybistin is regulated by binding of the adhesion protein neuroligin-2, which stabilizes the open/active conformation of collybistin at the postsynaptic plasma membrane by competing with an intramolecular interaction in collybistin that favors the closed/inactive conformation. By linking trans-synaptic neuroligin-dependent adhesion and phosphoinositide signaling with gephyrin recruitment, the collybistin-based regulatory switch mechanism represents an integrating regulatory node in the formation and function of inhibitory postsynapses.

Gephyrin: a central GABAergic synapse organizer

“…76,77 Furthermore, a recent structural study demonstrated that the collybistins can adopt open and closed conformations that act as switchable adaptors, and that NL-2 favors the open conformation by competing with a closed-conformation-favoring intramolecular interaction in collybistin. 78 The proposed functions of collybistin were demonstrated in vivo: collybistin-deficient mice displayed region-specific reductions in synaptic gephyrin clustering, reduced clustering of the GABA A receptor γ2-subunit, decreased GABAergic synaptic transmission, impaired hippocampal synaptic plasticity (i.e., increased long-term potentiation and decreased long-term depression) and impaired spatial learning. 79 Similar functional alterations were reproduced in conditional collybistin-knockout mice.…”

Analysis of splice variants for the C. elegans orthologue of human neuroligin reveals a developmentally regulated transcript

“…In particular the major variants are predicted to encode proteins that will lack or contain a candidate polyproline SH3 binding domain and the homologous region in the mammalian neuroligins harbours the binding site for gephyrin. This is pertinent because this kind of protein scaffolding domain has been implicated in dynamic regulation of neuroligin dependent synaptic differentiation of inhibitory synapses in mammals (Soykan et al, 2014). Interestingly, we observed dynamic expression of these nlg-1 variants during the development of C. elegans.…”

“…The latter harbours a Dbl domain which has an intrinsic guanine nucleotide exchange factor activity (GEF). This activity is otherwise inhibited by an intramolecular interaction with an N terminal inhibitory SH3 domain found in most collybistin splice variants (Mayer et al, 2013;Soykan et al, 2014). This inhibition is released by ancillary binding to polyproline containing proteins, which, in the case of the gephyrin recruited collybistin, can be encoded by the SH3 binding domain that is selectively expressed by the NLGN-2 intracellular domain (Soykan et al, 2014).…”

“…This activity is otherwise inhibited by an intramolecular interaction with an N terminal inhibitory SH3 domain found in most collybistin splice variants (Mayer et al, 2013;Soykan et al, 2014). This inhibition is released by ancillary binding to polyproline containing proteins, which, in the case of the gephyrin recruited collybistin, can be encoded by the SH3 binding domain that is selectively expressed by the NLGN-2 intracellular domain (Soykan et al, 2014). This co-ordinated gephyrin binding and collybistin activation mediates maturation of overlying inhibitory synapses (Papadopoulos and Soykan, 2011;Soykan et al, 2014).…”

“…This inhibition is released by ancillary binding to polyproline containing proteins, which, in the case of the gephyrin recruited collybistin, can be encoded by the SH3 binding domain that is selectively expressed by the NLGN-2 intracellular domain (Soykan et al, 2014). This co-ordinated gephyrin binding and collybistin activation mediates maturation of overlying inhibitory synapses (Papadopoulos and Soykan, 2011;Soykan et al, 2014). In this scenario dual recruitment and activation is achieved by distinct protein interaction motifs singularly encoded by the neuroligin (Levinson et al, 2010;Shipman et al, 2011).…”