CHIT1-positive microglia drive motor neuron aging in the primate spinal cord.

Aging is a critical factor in spinal cord-associated disorders 1 , yet aging-specific mechanisms underlying this relationship remain poorly understood. To address this knowledge gap, we combined single-nucleus RNA sequencing with behavioral and neurophysiological analysis in non-human primates (NHPs). We identified motor neuron senescence and neuroinflammation with microglial hyperactivation as intertwined hallmarks of spinal cord aging. As an underlying mechanism, we identified a previously unreported neurotoxic microglial state demarcated by elevated expression of CHIT1 (a secreted mammalian chitinase) specific to the aged spinal cords in NHP and human biopsies. In the aged spinal cord, CHIT1-positive microglia preferentially localize around motor neurons (MNs), and they are capable of triggering senescence, partly by activating SMAD signaling. We further validated the driving role of secreted CHIT1 on MN senescence by multi-modal experiments both in vivo, utilizing the NHP spinal cord as a model and in vitro, employing a sophisticated human MN-and-microenvironment interplay modeling system. Moreover, we demonstrated that ascorbic acid, a geroprotective compound, counteracted the pro-senescent effect of CHIT1 and mitigated motor neuron senescence in aged monkeys. Our findings provide the first single-cell resolution cellular and molecular landscape of the aged primate spinal cord and identify a new biomarker and intervention target for spinal cord degeneration.