Gut Lachnospiraceae improves white matter injury-related cognitive decline by increasing L-arginine

White matter injury (WMI) is the most prevalent lesion in cerebral small vessel disease and a major contributor to cognitive decline. Recent studies have highlighted the critical role of gut microbiota in regulating brain disorders. However, the role of gut microbiota in WMI-related cognitive decline remains unclear. A bilateral carotid artery stenosis (BCAS) mouse model was established to mimic WMI and related cognitive decline. Fecal microbiota transplantation was employed to verify the causal relationship between gut microbiota dysbiosis and WMI. 16 S rRNA gene sequencing was used to analyze gut microbiota and its potential functions. Untargeted metabolomics was applied to identify differential metabolites. Cognitive function was assessed through Y-maze, novel object recognition, and fear conditioning tests. WMI was assessed using in vivo imaging, immunostaining, and electron microscopy. The changes in oligodendrocyte lineage cells, microglia, and blood-brain barrier were investigated using immunofluorescence staining, EdU cell proliferation assays, and Western blotting. Patients with ischemic WMI were included to examine the correlation between serum L-arginine (L-Arg) levels, brain imaging, and cognition. We discovered that BCAS mice exhibited gut microbiota dysbiosis and reduced arginine biosynthesis, with decreased L-Arg levels in serum and white matter. Fecal microbiota from BCAS mice resulted in WMI and related cognitive decline in normal mice. Serum L-Arg levels were reduced in patients with ischemic WMI and were closely associated with WMI and cognitive decline. Importantly, L-Arg supplementation improved WMI-related cognitive decline in BCAS mice. Mechanistically, L-Arg promoted oligodendrocyte precursor cell proliferation and differentiation, enhanced the anti-inflammatory activity of microglia, and reduced blood-brain barrier leakage, thereby mitigating WMI-related cognitive decline. Furthermore, Lachnospiraceae was identified as the main source of gut-to-brain L-Arg. Supplementation with Lachnospiraceae alleviates WMI-related cognitive decline. Overall, our study revealed the critical role of gut microbiota, particularly Lachnospiraceae, and L-Arg in improving WMI-related cognitive decline, providing novel strategies for understanding and treating WMI-related cognitive decline.