Neuron-derived SPP1 instructs microglia to limit degeneration.

Neurons actively shape immune responses that maintain central nervous system integrity. We identify SPP1 (secreted phosphoprotein 1) as a neuron-derived signal that reprograms microglia into a neuroprotective, homeostatic state after injury and during neurodegeneration. In mouse models of glaucoma and optic nerve damage, neuronal SPP1 enhances microglial autophagy, debris clearance, and anti-inflammatory activity, preserving neuronal survival and visual function. SPP1 is elevated in neurons of human and primate glaucomatous retinas, where SPP1+ cells show increased resilience. In Alzheimer's disease brain, neuronal SPP1 correlates with neuronal survival, while microglia around Aβ plaques display defective autophagy. In human iPSC co-cultures, SPP1 enhances microglial Aβ clearance and prevents neurodegeneration. Thus, SPP1 defines a protective neuron-microglia axis in glaucoma and possibly other neurodegenerative diseases.