Function of Cell Adhesion Complexes
in Synapse Assembly
The specificity of synaptic connectivity is likely to emerge as a consequence of several sequential programs of molecular recognition and consolidation. As envisioned by Sperry, surface proteins engaging in a key-lock mechanism might underlie the initial selectivity of neuronal target interactions. In the vertebrate nervous system most neurons appear to engage in somewhat promiscuous interactions that are subsequently refined. Therefore, specificity factors most likely act at multiple steps of vertebrate synaptogenesis, some that precede or initiate the process of synapse assembly and others that direct synapse maturation through the selective stabilization of appropriate connections and elimination of inappropriate ones. Such a refinement of an imprecise initial connectivity pattern permits further regulation of synaptic connectivity by neuronal activity, which has emerged as a second key mechanism for selective neuronal wiring during development.
Using in vitro assay systems we identified a synapse-organizing activity for the Neuroligin-Neurexin adhesion complex. These studies demonstrated that Neuroligins and neurexins have bi-directional signaling capabilities that organize pre- and postsynaptic specializations. Neuroligins and neurexins are each encoded by multiple genes and alternative splicing generates further molecular diversity of this adhesion system. We are currently testing a model in which neuroligin and neurexin variants form a selective adhesive code that is differentially employed at specific GABAergic and glutamatergic synapses. For these studies we are using the mouse cerebellum as a model system where we can genetically mark and molecularly identify specific synapses in vivo.

 

Reconstitution of synapse formation between cerebellar granule neurons and pontine neurons in a mixed culture assay. Synaptic vesicles in the pontine axons, labeled for the protein synapsin (red) accumulate in punctate structures at contact with the postsynaptic granule cell (green, stained for dendritic marker MAP2).

Contact of pontine axons with a neuroligin-1 expressing non-neuronal cell (green) leads to synaptic vesicle clustering (synapsin, red) and the formation of apparent presynaptic elements. This experiment provided the first insight that adhesion complexes formed with neuroligins are sufficient to induce presynaptic differentiation.

Confocal microscope view of a beta-neurexin / neuroligin-1 complex in cultured cerebellar granule cells. Neuroligin-1 (green) and beta-Neurexin (red) concentrate at cell-cell contacts (co-localizing signal is yellow).