Gamma-Protocadherins regulate filopodia self-recognition and dynamics to drive dendrite self-avoidance

Abstract

Neurons form cell type-specific morphologies that are shaped by molecular cues and their cellular events governing dendrite growth. One growth rule is distributing dendrites uniformly within a neuronal territory by avoiding sibling branches. In mammalian neurons, dendrite self-avoidance is regulated by the clustered Protocadherins (cPcdhs), a large family of recognition molecules. Genetic and molecular studies suggest that the cPcdhs mediate homophilic recognition and repulsion between self-dendrites but this model has not been tested through direct investigation of self-avoidance during development. Here we performed live imaging and 4D quantifications of dendrite morphogenesis to define the cPcdh-dependent mechanisms of self-avoidance. We focused on the mouse retinal starburst amacrine cell (SAC), which requires the gamma-Pcdhs (Pcdhgs) and self/non-self recognition to establish a stereotypic radial morphology while permitting dendritic interactions with neighboring SACs. Through morphogenesis, SACs extend a transient population of dynamic filopodia that fill the growing arbor and contact nearby self-dendrites. Compared to non-self-contacting filopodia, self-contacting events have longer lifetimes and a subset persists as filopodia bridges. In the absence of the Pcdhgs, non-self-contacting filopodia dynamics are unaffected but self-contact-induced retractions are significantly diminished. Filopodia bridges accumulate, leading to the bundling of dendritic processes and disruption to the arbor shape. By tracking dendrite self-avoidance in real-time, our findings demonstrate that the γ-Pcdhs selectively mediate contact-induced retractions upon filopodia self-recognition. Our results also illustrate how self-avoidance shapes the stochastic and space-filling behaviors of filopodia for robust dendritic pattern formation in mammalian neurons.

Visualization of trans-interactions of a protocadherin-α between processes originating from single neurons

“…Recently, the detailed process of the dendritic self-avoidance in starburst amacrine cells has been visualized using sophisticated sample preparation and time-lapse imaging. 56 , 57 Since Pcdhα has been reported to be cooperatively involved with Pcdhγ in the dendritic self-avoidance of starburst amacrine cells, 17 we expect that the combination of cIPAD with the previous sophisticated methods may provide spatiotemporal information on α4 trans -interactions in the self-avoidance of starburst amacrine cells in the future. In addition, it is necessary to approach the significance in homophilic interactions of Pcdh by confirming biological functions using IPADs as future effort.…”