S-SCAM is essential for synapse formation

Synapse formation is critical for the wiring of neural circuits in the developing brain. The synaptic scaffolding protein S-SCAM/MAGI-2 has important roles in the assembly of signaling complexes at postsynaptic densities. However, the role of S-SCAM in establishing the entire synapse is not known. Here, we report significant effects of RNAi-induced S-SCAM knockdown on the number of synapses in early stages of network development in vitro. In vivo knockdown during the first three postnatal weeks reduced the number of dendritic spines in the rat brain neocortex. Knockdown of S-SCAM in cultured hippocampal neurons severely reduced the clustering of both pre- and postsynaptic components. This included synaptic vesicle proteins, pre- and postsynaptic scaffolding proteins, and cell adhesion molecules, suggesting that entire synapses fail to form. Correspondingly, functional and morphological characteristics of developing neurons were affected by reducing S-SCAM protein levels: neurons displayed severely impaired synaptic transmission and reduced dendritic arborization. A next generation sequencing approach showed normal expression of housekeeping genes, but changes of expression levels in 39 synaptic signaling molecules in cultured neurons. These results indicate that S-SCAM mediates the recruitment of all key classes of synaptic molecules during synapse assembly and is critical for the development of neural circuits in the developing brain. Overall design: Comparative gene expression profiling analysis of RNA-seq data for hippocampal neuronal cultures from rats (Rattus norvergicus) in wild-type condition and knock-down of S-SCAM.