Microglial Mitophagy Integrates the Microbiota-gut-brain Axis to Restrain Neuroinflammation During Neurotropic Herpesvirus Infection

Herpes simplex encephalitis (HSE), mainly caused by herpes simplex virus type 1 (HSV-1), is a severe central nervous system disease commonly followed by cognitive impairment, behavioral changes, and focal neurological signs. Although increasing evidence implicates the central role of microglia in HSE progression, the intrinsic restrictors or the acquired environmental factors that balance the beneficial or detrimental immune responses in microglia remain unclear. In a recent study, we find that a gut microbial metabolite activates mitophagy to regulate microglia-mediated neuroinflammation and to mitigate HSE progression. HSV-1 neurotropic infection causes gut microbiota dysbiosis and microglial antiviral immune response, whereas depletion of gut microbiota by oral antibiotics treatment further results in hyperactivated microglia and exacerbated HSE pathology. Notably, exogenous administration of nicotinamide n-oxide (NAMO), an oxidative product of nicotinamide mainly produced by intestinal neomycin-sensitive bacteria, especially Lactobacillus gasseri and Lactobacillus reuteri, can significantly suppress HSE progression. Mechanistically, HSV-1 infection causes mitochondrial dysfunction and impairs mitophagy to activate microglia and promote proinflammatory cytokine production, whereas NAMO restores NAD+-dependent mitophagy to restrain microglial over-activation and to prevent HSV-1 early infection in neuronal cells. This work reveals a novel function of gut microbial metabolites as intrinsic regulators of microglia homeostasis and neuroinflammation via mitophagy. AD: Alzheimer disease; ABX: antibiotics; HSE: herpes simplex encephalitis; HSV-1: herpes simplex virus type 1; NAD: nicotinamide adenine dinucleotide; NAMO: nicotinamide n-oxide; SCFAs: short-chain fatty acids.