ROLE OF Γ-PROTOCADHERINS IN THE MOLECULAR ENCODING OF OLIVO-CEREBELLAR CONNECTIONS

During development, millions of neurons in the brain must choose their synaptic partners and undergo deep functional and molecular changes to stabilize specific connections. The olivo-cerebellar circuit presents a highly stereotyped development, with synapse selection happening during a critical period, making it a perfect model to study these phenomena. Early after birth, multiple climbing fibers (CF) emerging from the inferior olivary nucleus innervate each Purkinje cell (PC) in the cerebellum. Over the three first postnatal weeks, supernumerary CF are pruned until each PC is mono-innervated by a single CF. Beyond the role of activity in afferent selection, the molecular mechanisms underlying cell-cell recognition in this circuit remain unknown. Here, we employed ex vivo and in vitro approaches with hindbrain explants to study a molecular code for cerebellar connectivity. The highly diverse family of gamma Protocadherins (γPCDH) has been proposed as part of this code, although experimental evidence remains scarce. Here, using PatchSeq we show that during CF elimination, the diversity of γPCDH isoforms expressed by individual PCs gradually decreases. In addition, overexpressing single isoforms to mimic a premature decrease in γPCDH diversity in single PCs significantly accelerates CF selection. Furthermore, we detected a higher similarity in γPCDH repertoire expressed by PCs connected by the same CF, suggesting that γPCDH expression in PCs is determined by the presynaptic neuron identity. Our results support the role of γPCDHs as part of the cerebellar molecular code of connectivity with a direct effect in CF-to-PC synapse specificity, and a not random γPCDH isoform expression.