An aggregation-removal model for the formation and size determination of post-synaptic scaffold domains

The formation and stability of synapses are key questions in neuroscience. Post-synaptic domains have been classically conceived as resulting from local insertion and turnover of proteins at the synapse. However, insertion is likely to occur outside the post-synaptic domains and advances in single-molecule imaging have shown that proteins diffuse in the plane of the membrane prior to their accumulation at synapses. We quantitatively investigated this scenario using computer simulations and mathematical analysis, taking for definiteness the specific case of inhibitory synapse components, i.e., the glycine receptor (GlyR) and the associated gephyrin scaffolding protein. The observed domain sizes of scaffold clusters can be explained by a dynamic balance between the aggregation of gephyrin proteins diffusing while bound to GlyR and their turnover at the neuron membrane. We also predict the existence of extrasynaptic clusters with a characteristic size distribution that significantly contribute to the size fluctuations of synaptic domains. New super-resolution data for gephyrin proteins established the existence of extrasynaptic clusters the sizes of which are consistent with the model predictions in a range of model parameters. At a general level, our results highlight aggregation with removal as a non-equilibrium phase separation which produces structures of tunable size.

突触的形成与稳定性是神经科学领域的核心研究问题。传统观点认为，突触后域（post-synaptic domains）源自突触处蛋白质的局部插入与周转。然而，蛋白质的插入大概率发生在突触后域之外，且单分子成像技术的进展表明，蛋白质在突触处聚集前会在细胞膜平面内扩散。本研究通过计算机模拟与数学分析对该场景开展定量研究，为明确起见，选取抑制性突触组分——即甘氨酸受体（glycine receptor, GlyR）及其结合的gephyrin支架蛋白作为具体研究对象。观测到的支架蛋白簇结构域尺寸，可通过结合甘氨酸受体的gephyrin蛋白在扩散过程中的聚集，与神经元细胞膜上蛋白质周转之间的动态平衡得到合理解释。本研究还预测了具有特征尺寸分布的突触外簇的存在，这类簇会显著影响突触结构域的尺寸波动。针对gephyrin蛋白的新型超分辨率成像数据证实了突触外簇的存在，其尺寸在一系列模型参数范围内与本模型的预测结果相符。从一般性层面而言，本研究结果表明，伴随移除的聚集过程属于一种非平衡相分离机制，可生成尺寸可调的结构。