Gut Microbiota-Targeted Photobiomodulation Ameliorates Alzheimer’s Pathology via the Gut–Brain Axis: Comparable Efficacy to Transcranial Irradiation

Alzheimer's disease (AD) is a major neurodegenerative disorder with limited effective and affordable therapies. Photobiomodulation (PBM) offers a safe, non-invasive treatment strategy, yet conventional transcranial PBM (tc-PBM) is restricted by low skull penetration. To overcome this limitation, gut microbiota-targeted PBM (gm-PBM) has been proposed to modulate the gut-brain axis, though its efficacy and mechanisms remain unclear. Here, six-month-old APPswe/PS1dE9 mice received gm-PBM or tc-PBM (810 nm, 25 mW/cm², 20 min/day for 4 weeks). Behavioral testing revealed that both treatments improved spatial learning and memory, while histological analyses showed reduced amyloid-β deposition and microglial shift toward an anti-inflammatory phenotype. Notably, gm-PBM specifically enriched short-chain fatty acid-producing bacteria, elevated propionate, butyrate, and secondary bile acids, and restored intestinal barrier integrity, whereas tc-PBM induced minimal microbiota changes. These findings suggest that gm-PBM confers neuroprotective effects comparable to or exceeding tc-PBM through modulation of the gut microbiota-metabolism-immune axis, highlighting its potential as a non-invasive and cost-effective therapeutic approach for AD.