Cell Adhesion by SynCAM 1 Dynamically Regulates Synapse Organization and Plasticity

Synaptogenesis is a critical process in the initial formation of the central nervous system (CNS) and in the ability of the CNS to continually reshape itself in response to stimuli throughout the life of an organism. Cell adhesion molecules (CAMs) are able to coordinate diverse molecular interactions for the organization of synapses. The abundance of recently discovered CAMs has presented a challenge for understanding the discrete actions of individual CAMs at synapses. Using knockout and transgenic strategies, we investigated the in vivo function of synaptic cell adhesion molecule 1 (SynCAM 1) to identify distinct requirements for SynCAM 1 over multiple developmental stages. Previous in vitro experiments have indicated that SynCAM 1 promotes excitatory synapse formation. We first characterize a newly generated transgenic mouse line, the SynCAM 1 conditional overexpressor, which uses the Tet-Off system to temporally control SynCAM 1 overexpression. Exogenous SynCAM 1 in overexpressing mice is found to recapitulate both the spatial and temporal expression patterns of endogenous SynCAM 1, and to localize to excitatory synapses. Electron microscopy in the stratum radiatum layer of hippocampal region CA1 reveals that synapse numbers are significantly altered by SynCAM 1 expression. Excitatory synapses are increased in overexpressing mice and decreased in knockouts, while inhibitory synapses are unaffected in both mouse models. Loss of SynCAM 1 also results in shortened pre- and postsynaptic membrane appositions. Electrophysiological recordings performed in collaboration support the complimentary effects of SynCAM 1 expression on excitatory synapse number, as mEPSC frequency in CA1 is elevated in overexpressors and reduced in knockouts. To address the developmental role of SynCAM 1, we ask whether SynCAM 1 acts exclusively in the early process of synapse formation or later in synapse maintenance. By repressing SynCAM 1 overexpression either before or after peak developmental synaptogenesis, we show that continual SynCAM 1 expression is required in order to maintain the increase in excitatory synapses induced by SynCAM 1. Additionally, SynCAM 1 is sufficient to increase synapses even after developmental synaptogenesis is largely complete, and appears to specifically affect mushroom-type and thin, filopodia-like postsynaptic spine structures. Synaptic plasticity involves structural remodeling of excitatory synapses that is akin to mechanisms required during synaptogenesis, and is considered the molecular basis of learning and memory. So we next examine two forms of synaptic plasticity, long-term potentiation (LTP) and depression (LTD) in the hippocampus, and learning and memory behaviors of SynCAM 1 mice in the hippocampus-dependent Morris water maze task. Interestingly, SynCAM 1 overexpressing mice which have more excitatory synapses perform poorly in this task, whereas spatial memory in the water maze is improved in SynCAM 1 knockouts that have fewer synapses. When synaptic plasticity is measured by recording for LTP and LTD, we observe that LTD is abolished in SynCAM 1 overexpressors and enhanced in knockouts, in parallel with the reciprocal behavioral consequences of SynCAM 1 expression for spatial memory. Shutdown of SynCAM 1 overexpression restored normal LTD in transgenic mice. No effects of SynCAM 1 expression were observed on LTP, proving SynCAM 1 to be the first adhesion molecule capable of selectively regulating LTD. Activity-dependent mechanisms relating to SynCAM 1 expression are furthermore examined. We identify SynCAM 1 expression to induce the immediate early gene c-Fos, a marker of neuronal activity. As well, we show that NeuroD2 is an activity-dependent transcription factor that developmentally regulates the expression of multiple SynCAMs. Together, these data establish novel biological functions of SynCAM 1 in driving synapse organization through its dynamic regulation of 1) excitatory synapse number and morphology, 2) LTD, a form of activity-dependent plasticity, and 3) cognitive function, as measured by its effect on spatial memory.;We also describe similar experiments used to probe the effects of mutations in the cell adhesion molecule Caspr2 that have been linked to autism spectrum disorders (ASD). The cytosolic tail of Caspr2 closely resembles that of SynCAMs. Though Caspr2 is primarily understood as an organizer of myelinated axons, we show Caspr2 and its binding partner TAG-1 are localized to excitatory synapses. The organization of hippocampal neurons expressing mutant Caspr2 is investigated in vitro. We test the results of Caspr2 mutation on dendritic branching, spine density, and morphology. Upon expression of a particularly deleterious Caspr2 mutation, we note changes in postsynaptic spine morphology that may relate to changes in synapse formation and synaptic transmission observed in ASD patients, underscoring the functional importance of cell adhesion on neural development and pathophysiology.