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.突触形成对于发育中大脑的神经环路构建至关重要。突触支架蛋白S-SCAM/MAGI-2在突触后致密区（postsynaptic densities）的信号复合物组装中发挥重要作用。然而，S-SCAM在完整突触建立过程中的具体功能仍未明确。本研究发现，RNA干扰（RNAi）介导的S-SCAM敲低对体外神经网络发育早期的突触数量产生显著影响。在大鼠出生后的前三周进行体内敲低，可降低其大脑新皮层内的树突棘数量。在培养的海马神经元中敲低S-SCAM，会严重削弱突触前与突触后组分的聚集，这涵盖了突触囊泡蛋白、突触前后支架蛋白以及细胞黏附分子，提示完整突触无法正常形成。相应地，降低S-SCAM蛋白水平会影响发育中神经元的功能与形态特征：神经元的突触传递功能严重受损，树突分支也出现明显减少。通过下一代测序（next generation sequencing）技术分析，培养神经元中的管家基因（housekeeping genes）表达水平保持正常，但39种突触信号分子的表达量发生了显著改变。上述结果表明，S-SCAM在突触组装过程中介导了所有关键类别突触分子的招募，对于发育中大脑的神经环路发育至关重要。