Endocytic adaptor AP-2 maintains Purkinje cell function by balancing cerebellar parallel and climbing fiber synapses

The selective loss of cerebellar Purkinje cells is a hallmark of various neurodegenerative movement disorders, yet the precise mechanism driving their degeneration remains enigmatic. Here, we show that the endocytic adaptor protein complex 2 (AP-2) is essential for the survival of Purkinje cells. Employing a multidisciplinary approach encompassing mouse genetics, viral tracing, ex vivo calcium imaging, and kinematic analysis, we demonstrate that mice lacking the {micro}-subunit of AP-2 in cerebellar Purkinje cells exhibit early-onset ataxia associated with progressive Purkinje cell degeneration. Importantly, we uncover that synaptic input dysfunctions, characterized by a predominance of parallel fiber (PF) over climbing fiber (CF) synapses, precede Purkinje cell loss. Mechanistically, we find that AP-2 localizes to Purkinje cell dendrites, where it interacts with the PF synapse-enriched protein GRID2IP. The loss of AP-2 results in proteasome-dependent degradation of GRID2IP and accumulation of the glutamate {delta}2 receptor (GLUR{delta}2) in distal Purkinje cell dendrites, leading to an excess of PF synapses while CF synapses are drastically reduced. The overrepresentation of PF synaptic input induces Purkinje cell hyperexcitation, which can be alleviated by enhancing synaptic glutamate clearance using the antibiotic ceftriaxone. Our findings demonstrate the critical role of AP-2 in preventing motor gait dysfunctions by regulating GRID2IP levels in Purkinje cells, thereby preserving the equilibrium of PF and CF synaptic inputs in a cell-autonomous manner.