Cathepsin B overexpression and lysosomal leakage in inflammatory microglia promote neuroinflammation in olfactory...

Olfactory dysfunction (OD) is a common sensory disorder with age-related prevalence and serves as an early clinical manifestation for neurodegenerative and inflammatory diseases. Microglia in the olfactory bulb (OB) rapidly respond to olfactory injury and initiate immune responses, but the cell state dynamics and pathways driving OD remain poorly understood. Here, we performed single-cell RNA sequencing of mouse OBs at 0, 7, and 30 days post olfactory injury, and identified 3 distinct microglial states including inflammatory, negative regulatory, and homeostatic.The inflammatory microglia exacerbated neuroinflammation by secreting cytokines and chemokines that recruited immune cells and amplify local immune responses. Cathepsin B was identified as a key regulator of this inflammatory microglial activity. In vitro studies using BV2 and primary microglia demonstrated that both pharmacological inhibition and genetic deletion of CTSB attenuated lipopolysaccharide (LPS)-induced mitochondrial dysfunction, NLRP3 activation, and pro-inflammatory cytokine release. In mice with OD, pharmacological inhibition of CTSB with CA074me promoted olfactory function recovery and modulated microglial pro-inflammatory responses. Our findings uncover inflammation-associated microglial subpopulations enriched in OD and unveiled a deleterious role for CTSB-mediated neuroinflammatory signaling in OD pathogenesis. Targeting CTSB may therefore serve as a promising therapeutic strategy to mitigate microglia-mediated neuroinflammation and facilitate olfactory recovery in OD.