Activity-dependent regulation of clustered gamma protocadherins (c-γPCDH) expression during circuit assembly in Purkinje cells

Neuronal circuits assemble in a non-random fashion, achieving specificity beyond cell-cell apposition. Two mechanisms have been classically proposed to fine-tune brain circuits: activity or genetics-dependent processes. Experimental evidence supports both options, suggesting a balanced contribution of each. In this study, we explored the interplay between synaptic activity and gene expression during developmental critical periods in olivo-cerebellar circuitry. Early postnatally, cerebellar Purkinje cells (PC) are connected by multiple climbing fibers (CF) emerging from the inferior olive in the brainstem. CFs engage in an activity-dependent competition process after which only the most active fiber is strengthened and retained, achieving CF-PC mono-innervation. We previously reported that CF competition leaves an epigenetic trace in PCs that retain the winning fiber while preventing them from reconnecting multiple afferents. This trace may represent a “molecular code for connectivity”, referring to the combined expression of genes regulating and stabilizing specific connections. Since cell-adhesion molecules (CAMs) have been proposed as main actors of this code, we used Patch-Seq ex vivo to investigate the regulation of CAMs during CF competition. Here, we show that Clustered Gamma Protocadherins (c-γPCDH) expression appears under a specific regulated pattern correlating CF maturation. Furthermore, PCs that develop in the absence of CF innervation retain an immature pattern of c-γPCDH, resembling early development PCs, and in agreement with their ability to accept multiple afferents. Therefore, c-γPCDH may allow multiple CF connections during developmental critical periods, while their regulation may be contributing to the retainment of a single afferent and prevention of non-specific connectivity.