Prosaposin (PSAP) is a lysosomal protein that serves as a precursor for saposins, which are non-enzymic cofactors essential for sphingolipid degradation in lysosomes 1. The protein is proteolytically cleaved into four saposins that facilitate the activity of hydrolytic enzymes in lysosomal degradation pathways 2. Beyond canonical lysosomal function, PSAP plays critical roles in neuronal stress responses and immune regulation. In neurons, PSAP knockdown sensitizes cells to oxidative stress by triggering lipofuscin accumulation and ferroptosis 3. PSAP mediates dendritic cell processing of apoptotic bodies for antigen presentation; however, TGF-β-induced hyperglycosylation impairs this function, promoting cancer immune escape 4. In Parkinson's disease, secreted PSAP activates GPR37 on oligodendrocytes, inducing IL-6 production that drives neuroinflammation and dopamine neuron degeneration 5. In Alzheimer's disease, PSAP signaling alters cell communications and induces astrogliosis 6. Mutations in PSAP cause rare lysosomal storage diseases including combined saposin deficiency, metachromatic leukodystrophy, and atypical Gaucher disease variants 21. PSAP-targeting approaches show therapeutic promise in tauopathies 7.