A dynamic splicing program modulated by Sam68 insures proper synaptic connections in the developing cerebellum

Sam68 (Src-Associated substrate in Mitosis of 68 kDa) belongs to the STAR (signal transduction and activation of RNA metabolism) family of RNA binding proteins. Sam68 is involved in the regulation of alternative splicing of pre-mRNAs, acting as repressor or enhancer depending on the specific target. Although expressed almost ubiquitously, Sam68 levels are particularly high in the developing cerebellum at birth and steadily decline over the first three weeks. Notably, many splicing targets of Sam68 encode for synaptic proteins with potential role in the establishment of neuronal circuits. Starting from these observations, we investigated the role of Sam68 during cerebellar development and in mature cerebellar functions. At postnatal days 10, the cerebellum of Sam68 knockout (Sam68-/- ) mice displays abnormal foliation in the central zone (lobes VI-VII), accompanied by mislocalization of Bergmann glia cells and Purkinje cells in the molecular layer of the developing cortex. Notably, we also found that Sam68 orchestrates a timely regulated splicing program of genes encoding synaptic proteins during cerebellar development, whose dysregulation in Sam68-/- mice leads to functional defects in adult neurons. Indeed, electrophysiological recording revealed reduced frequency and amplitude of spontaneous excitatory postsynaptic current (sESPSCs) in mature Purkinje cells, suggesting dysregulation of synaptic contacts. Accordingly, adult Sam68-/- mice exhibit an ataxic phenotype and motor coordination deficit. Thus, our study uncovers a Sam68-dependent splicing program that is required to guarantee the establishment of the correct spatial/temporal neuronal circuitry during cerebellar development.