Niacin promotes motor function recovery after spinal cord injury via Hcar2‐dependent microglia immunometabolic regulation

BACKGROUND: Traumatic spinal cord injury (SCI) induces a robust local inflammatory response that can both facilitate repair and exacerbate pathology. Hydroxycarboxylic acid receptor 2 (Hcar2) is known to exert immunomodulatory effects; however, its role in SCI and its potential for targeting Hcar2 to alleviate motor deficits remain unclear. METHODS: The spinal cord transcriptome following SCI, with a focus on Hcar2, was analysed via publicly available single-cell RNA sequencing datasets from mice and rhesus macaques. Additionally, an in vivo SCI mouse model with Hcar2 knockout and an in vitro LPS-induced BV2 microglial model were established to assess Hcar2 gene and protein expression, microglial activation and inflammatory responses via bulk RNA sequencing, immunofluorescence staining, Western blotting, and real-time polymerase chain reaction. To evaluate the protective effects of Hcar2 activation, niacin, a known Hcar2 agonist, was administered to mice or BV2 cells, followed by assessments of the inflammatory response and motor function. RESULTS: Hcar2 gene expression, which was enriched predominantly in spinal cord microglia, was upregulated following SCl, peaking at 7 days post-SCl. Genetic knockout of Hcar2 decreased the percentage of impaired anti-inflammatory polarized microglia and increased the inflammatory response. In contrast, Hcar2 activation with niacin in LPS-stimulated microglia BV cell models reversed mitochondrial dysfunction, increased the oxygen consumption rate and reduced the expression of the cytokines IL-6 and IL-1β. The administration of niacin to SCl mice upregulated anti-inflammatory microglia, reduced the expression of multiple proinflammatory cytokines, increased the number of motor neurons and improved motor function recovery. Notably, all these protective effects were abolished by genetic loss of Hcar2. CONCLUSIONS: Hcar2 serves as a critical regulator of microglial polarization, promoting the switch from a proinflammatory phenotype to an anti-inflammatory phenotype through immunometabolic reprogramming. Targeting Hcar2 with niacin may offer a translatable therapeutic strategy to improve functional recovery after SCl. KEY POINTS: Hcar2 is identified as a conserved, injury-induced metabolic checkpoint specifically enriched in microglia following spinal cord injury. Hcar2 activation reprogrammes microglial metabolism from glycolysis to oxidative phosphorylation to drive reparative anti-inflammatory polarization. Pharmacological targeting of Hcar2 with niacin resolves neuroinflammation and promotes functional motor recovery in an Hcar2-dependent manner.