Cadherins and synaptic specificity.
Cadherins are a family of cell-cell adhesion molecules that regulate morphogenesis in a variety of organs during development. In this review, we summarize recent evidence that cadherins may be involved in synaptogenesis in the vertebrate central nervous system. The first cadherin identified in synapses was N-cadherin, which is a major glycoprotein in postsynaptic density preparations. Electron microscopic studies have shown that this molecule is present at the synaptic cleft, bordering the transmitter release zone. To date, several other cadherins have also been found in synaptic junctions. Some cadherins have been observed in distinct subsets of synapses. The homophilic binding properties of cadherins may provide a molecular basis for the adhesive interactions between opposing synaptic membranes, and cadherins may promote a stable locking-in of pre- and postsynaptic membranes. Thus, cadherins may play a role in the formation and maintenance of synapses. Cadherin expression in synapses has been studied during development, regeneration, and activity-dependent plasticity. Moreover, it has been shown that each cadherin is expressed in specific neural circuits. In this context, we discuss the possibility that the differential expression of cadherins in the nervous system provides an adhesive framework for synaptic specificity.