Regulation of FGF/MAPK signaling at the primary cilium base controls the proliferation-neurogenesis balance in human cerebellar organoids

The primary cilium acts as a specialized signaling compartment that coordinates multiple developmental pathways, yet its role in regulating signaling activity during human brain development remains poorly understood. Biallelic mutations in ciliary genes cause neurodevelopmental ciliopathies such as Joubert syndrome, which are characterized by cerebellar hypoplasia and dysplasia. Here, we generated human cerebellar organoids carrying null or patient-derived mutations in RPGRIP1L, a ciliary transition zone gene associated with Joubert syndrome, to investigate how ciliary dysfunction alters human cerebellar neurogenesis. While control organoids robustly express markers of cerebellar glutamatergic and GABAergic lineages including Purkinje cells, RPGRIP1L-deficient organoids display a consistent and severe reduction in Purkinje cell markers, accompanied by impaired neurogenesis and increased progenitor proliferation. These defects coincide with prolonged overactivation of the FGF/MAPK signaling pathway. We further show that the MAPK effector pMEK1/2 localizes at the base of primary cilia, where its levels are significantly increased in RPGRIP1L-deficient cerebellar progenitors. Pharmacological inhibition of FGF receptors reduces pMEK1/2 activation at cilia base and rescues both the proliferation-neurogenesis imbalance and Purkinje lineage defects, without restoring the underlying ciliary abnormalities. Together, our findings identify the primary cilium as a compartment that refines FGF/MAPK signaling in human cerebellar progenitors, and support a model in which RPGRIP1L restrains FGF pathway activity to promote neurogenesis. These results provide a mechanism linking ciliary dysfunction to altered cerebellar development and provide new insight into the developmental origin of cerebellar impairment in neurodevelopmental ciliopathies