Loss of Atp8a2 drives neurodegeneration through the dysregulation of spatiotemporal phosphatidylserine externalization in mature neurons.

Phosphatidylserine (PS) asymmetry in plasma membranes is critical for cellular functions and serves as an apoptotic signal in many cell types. However, in mature neurons, the molecular mechanisms governing PS distribution, its precise regulation, and its functional significance beyond apoptosis and development remain poorly understood - particularly in the context of neurodegeneration. Here, we mapped the spatiotemporal dynamics of PS exposure in mature hippocampal neurons under physiological and pathological conditions using time-lapse imaging, revealing specific PS externalization hotspots at dendritic branching points. Using multiple in vitro and in vivo neurodegeneration models combined with molecular modeling, RNA interference, pharmacological interventions, and biochemical assays, we identified Atp8a2 as the primary regulator of PS asymmetry in mature neurons beyond its known roles in development. Notably, Atp8a2 expression levels - rather than its flippase activity alone - were essential for maintaining neuronal structural integrity and viability. Atp8a2 expression was significantly altered by neurotoxic stimuli and in multiple mouse models of neurodegeneration. Reduced Atp8a2 expression led to increased PS exposure, compromised neuronal architecture, and heightened susceptibility to degeneration, whereas Atp8a2 overexpression conferred substantial neuroprotection. The distinction between Atp8a2's enzymatic activity and expression level reveals a mechanism of neuronal homeostasis linking PS regulation to structural integrity and survival, possibly through association with cytoskeletal protein networks. Thus, Atp8a2 expression is a critical determinant of mature neuronal viability, presenting a potential target for neuroprotective strategies in neurodegeneration.