BIRTH AND EXCITABILITY-DEPENDENT MATURATION OF PARALLEL FIBER/PURKINJE CELL SYNAPSES

Granule cells (GCs) of the cerebellum are the most numerous neuronal population in the mammalian brain, constituting more than 50% of the total number. Their proper differentiation and integration in the circuit of the cerebellar cortex is essential for cerebellar computation. GCs receive diverse sensorimotor inputs from mossy fibers and transmit information to Purkinje cells (PCs) via synapses made by their axons, the parallel fibers (PFs). While the mechanisms controlling GC generation, differentiation, and migration have been extensively studied, the timing of PF/PC synapse formation and the mechanisms regulating its maturation remain surprisingly poorly described.
Here we show that immature synaptic contacts form between PFs and PCs when differentiating GCs in the external granule cell layer reaches the surface of PC dendrites. Spatial transcriptomics analysis identifies molecularly distinct GC precursor subpopulations at different differentiation stages. In particular one stage of GC differentiation is defined by the start of expression of synaptic markers when the precursors reach the surface of the PC dendrites in the external granule layer, in agreement with their nascent capacity to form synapses on their target PCs. Using chemogenetics, we show that excitability of GC precursors promotes coordinated differentiation of immature GCs when they reach the molecular layer and the molecular maturation of their PF synapses on PCs. These results reveal a previously unrecognized role for excitability in early cerebellar differentiation and the establishment of the first PF/PC contacts.