Synapses are asymmetric cell junctions with precisely juxtaposed presynaptic and postsynaptic sides. Transsynaptic adhesion complexes are thought to organize developing synapses. The molecular composition of these complexes, however, remains incompletely understood, precluding us from understanding how adhesion across the synaptic cleft guides synapse development. Here, we define two immunoglobulin superfamily members, SynCAM 1 and 2, that are expressed in neurons in the developing brain and localize to excitatory and inhibitory synapses. They function as cell adhesion molecules and assemble with each other across the synaptic cleft into a specific, transsynaptic SynCAM 1/2 complex. Additionally, SynCAM 1 and 2 promote functional synapses as they increase the number of active presynaptic terminals and enhance excitatory neurotransmission. The interaction of SynCAM 1 and 2 is affected by glycosylation, indicating regulation of this adhesion complex by posttranslational modification. The SynCAM 1/2 complex is representative for the highly defined adhesive patterns of this protein family, the four members of which are expressed in neurons in divergent expression profiles. SynCAMs 1, 2, and 3 each can bind themselves, yet preferentially assemble into specific, heterophilic complexes as shown for the synaptic SynCAM 1/2 interaction and a second complex comprising SynCAM 3 and 4. Our results define SynCAM proteins as components of novel heterophilic transsynaptic adhesion complexes that set up asymmetric interactions, with SynCAM proteins contributing to synapse organization and function.

Summary Synaptogenesis is required for wiring neuronal circuits in the developing brain and continues to remodel adult networks. However, the molecules organizing synapse development and maintenance in vivo remain incompletely understood. We now demonstrate that the immunoglobulin adhesion molecule SynCAM 1 dynamically alters synapse number and plasticity. Overexpression of SynCAM 1 in transgenic mice promotes excitatory synapse number, while loss of SynCAM 1 results in fewer excitatory synapses. By turning off SynCAM 1 overexpression in transgenic brains, we show that it maintains the newly induced synapses. SynCAM 1 also functions at mature synapses to alter their plasticity by regulating long-term depression. Consistent with these effects on neuronal connectivity, SynCAM 1 expression affects spatial learning, with knock-out mice learning better. The reciprocal effects of increased SynCAM 1 expression and loss reveal that this adhesion molecule contributes to the regulation of synapse number and plasticity, and impacts how neuronal networks undergo activity-dependent changes.

SUMMARY The cleft is an integral part of synapses, yet its macromolecular organization remains unclear. We here show that the cleft of excitatory synapses exhibits a distinct density profile as measured by cryo-electron tomography (cryo-ET). Aiming for molecular insights, we analyzed the synapse-organizing proteins SynCAM 1 and EphB2. Cryo-ET of SynCAM 1 knock-out and overexpressor synapses showed that this immunoglobulin protein shapes the cleft’s edge. In agreement, SynCAM 1 delineates the postsynaptic perimeter as determined by immuno-electron microscopy (EM) and superresolution imaging. In contrast, the EphB2 receptor tyrosine kinase is enriched deeper within the postsynaptic area. Unexpectedly, SynCAM 1 can form ensembles proximal to postsynaptic densities, and synapses containing these ensembles were larger. Postsynaptic SynCAM 1 surface puncta were not static but became enlarged after a long-term depression paradigm. These results support that the synaptic cleft is organized on a nanoscale into sub-compartments marked by distinct trans-synaptic complexes.

The survival of adult-born dentate gyrus granule cells critically depends on their synaptic integration into the existing neuronal network. Excitatory inputs are thought to increase the survival rate of adult born neurons. Therefore, whether enhancing the stability of newly formed excitatory synapses by overexpressing the synaptic cell adhesion molecule SynCAM 1 improves the survival of adult-born neurons was tested. Here it is shown that overexpression of SynCAM 1 improves survival of adult-born neurons, but has no effect on the proliferation rate of precursor cells. As expected, overexpression of SynCAM 1 increased the synapse density in adult-born granule neurons. While adult-born granule neurons have very few functional synapses 15 days after birth, it was found that at this age adult-born neurons in SynCAM 1 overexpressing mice exhibited around three times more excitatory synapses, which were stronger than synapses of adult-born neurons of control littermates. In summary, the data indicated that additional SynCAM 1 accelerated synapse maturation, which improved the stability of newly formed synapses and in turn increased the likelihood of survival of adult-born neurons.

Synapses allow neurons to communicate with each other, and the function as well as the asymmetric structure of mature synapses is well described. In contrast, the molecular signals that differentiate nascent synapses remain poorly understood. A select group of synaptic adhesion molecules is now known to function in synapse development. We here define SynCAM proteins as a family of four synaptic cell adhesion molecules that organize synapses.SynCAMs are specific for vertebrates and comprise of four membrane proteins belonging to the immunoglobulin super family. To place SynCAMs within the context of brain development, we analyzed their expression patterns and found that all SynCAMs are predominantly expressed in neurons throughout the developing and adult central nervous system. They are expressed in the majority of neurons, indicating that they are of general relevance for neuronal differentiation, and can be differentially expressed within brain regions. Within neurons, SynCAMs are present in synaptic plasma membranes and localize to excitatory and to a lesser extent also to inhibitory synapses. To understand their extracellular interactions, we analyzed their adhesive binding and found that SynCAMs engage each other in specific heterophilic adhesive interactions that complement their differential expression patterns in brain. These heterophilic interactions are preferred over the homophilic binding that SynCAMs 1, 2 and 3 can exert. The SynCAM 1/2 adhesion complex is representative of heterophilic SynCAM interactions in synaptic membranes. SynCAM 1 and 2 engage each other into a highly specific and stable trans-synaptic adhesion complex. The assembly of this trans-synaptic SynCAM 1/2 complex is accompanied by synaptic membrane differentiation in cultured neurons. Notably, SynCAM 1 and 2 promote synaptogenesis and synaptic function as they increase the number of active presynaptic terminals and enhance excitatory neurotransmission. These trans interactions of SynCAMs are affected by glycosylation, indicating regulation of this novel synaptic adhesion complex by post-translational modification.Together, we define SynCAM proteins as components of novel heterophilic trans-synaptic adhesion complexes. Our results elucidate the composition of trans-synaptic adhesion complexes and provide insight into the mechanisms guiding synaptic membrane organization across the synaptic cleft.