Department of Molecular Biology, Division of Biological Sciences, Nagoya University Graduate School of Science, Nagoya, 464-8602, Japan.
Department of Anatomy, Hokkaido University Graduate School of Medicine, Sapporo, 060-8638, Japan.
Neurochem Int. 2018 Oct;119:190-198. doi: 10.1016/j.neuint.2018.01.003. Epub 2018 Jan 9.
Configuration of tripartite synapses, comprising the pre-, post-, and peri-synaptic components (axon terminal or bouton, dendritic spine, and astroglial terminal process), is a critical determinant of neurotransmitter kinetics and hence synaptic transmission. However, little is known about molecular basis for the regulation of tripartite synapse morphology. Previous studies showed that CDC42EP4, an effector protein of a cell morphogenesis regulator CDC42, is expressed exclusively in Bergmann glia in the cerebellar cortex, that it forms tight complex with the septin heterooligomer, and that it interacts indirectly with the glutamate transporter GLAST and MYH10/nonmuscle myosin ΙΙB. Scrutiny of Cdc42ep4 mice had revealed that the CDC42EP4-septins-GLAST interaction facilitates glutamate clearance, while the role for CDC42EP4-septins-MYH10 interaction has remained unsolved. Here, we find anomalous configuration of the tripartite synapses comprising the parallel fiber boutons, dendritic spines of Purkinje cells, and Bergmann glial processes in Cdc42ep4 mice. The complex anomalies include 1) recession of Bergmann glial membranes from the nearest active zones, and 2) extension of nonactive synaptic contact around active zone. In line with the recession of Bergmann glial membranes by the loss of CDC42EP4, overexpression of CDC42EP4 in heterologous cells promotes cell spreading and partitioning of MYH10 to insoluble (i.e., active) fraction. Paradoxically, however, Cdc42ep4 cerebellum contained significantly more MYH10 and N-cadherin, which is attributed to secondary neuronal response mainly in Purkinje cells. Given cooperative actions of N-cadherin and MYH10 for adhesion between neurons, we speculate that their augmentation may reflect the extension of nonactive synaptic contacts in Cdc42ep4 cerebellum. Transcellular mechanism that links the absence of CDC42EP4 in Bergmann glia to the augmentation of N-cadherin and MYH10 in neurons is currently unknown, but the phenotypic similarity to GLAST-null mice indicates involvement of the glutamate intolerance. Together, the unique phenotype of Cdc42ep4 mice provides a clue to novel molecular network underlying tripartite synapse configuration.
三分突触的结构，包括前、后突触成分（轴突末梢或末梢球、树突棘和星形胶质终末过程），是神经递质动力学和突触传递的关键决定因素。然而，对于三分突触形态的调节的分子基础知之甚少。先前的研究表明，CDC42EP4 是细胞形态发生调节剂 CDC42 的效应蛋白，仅在小脑皮质的 Bergmann 胶质细胞中表达，它与 septin 异源寡聚体形成紧密复合物，并与谷氨酸转运体 GLAST 和 MYH10/非肌肉肌球蛋白 IIB 间接相互作用。对 Cdc42ep4 小鼠的研究表明，CDC42EP4-septin-GLAST 相互作用促进谷氨酸清除，而 CDC42EP4-septin-MYH10 相互作用的作用仍未解决。在这里，我们发现 Cdc42ep4 小鼠的三分突触包括平行纤维末梢球、浦肯野细胞的树突棘和 Bergmann 胶质细胞过程的异常构型。复杂的异常包括 1）Bergmann 胶质细胞膜从最近的活性区退缩，以及 2）非活性突触接触围绕活性区延伸。与 Bergmann 胶质细胞膜因 CDC42EP4 的缺失而退缩一致，CDC42EP4 在异源细胞中的过表达促进细胞扩散和 MYH10 向不溶性（即活性）部分的分配。然而，矛盾的是，Cdc42ep4 小脑包含明显更多的 MYH10 和 N-钙黏蛋白，这归因于主要在浦肯野细胞中的神经元的次级反应。鉴于 N-钙黏蛋白和 MYH10 对神经元之间粘附的协同作用，我们推测其增加可能反映了 Cdc42ep4 小脑中非活性突触接触的延伸。将 Bergmann 胶质细胞中 CDC42EP4 的缺失与神经元中 N-钙黏蛋白和 MYH10 的增加联系起来的细胞间机制目前尚不清楚，但与 GLAST 缺失小鼠的表型相似表明涉及谷氨酸不耐受。总之，Cdc42ep4 小鼠的独特表型为三分突触结构的新型分子网络提供了线索。
