Trimethylamine-N-oxide accelerates osteoporosis by PERK activation of ATF5 unfolding.

Imbalances in gut microbiota and their metabolites have been implicated in osteoporotic disorders. Trimethylamine-n-oxide (TMAO), a metabolite of L-carnitine produced by gut microorganisms and flavin-containing monooxygenase-3, is known to accelerate tissue metabolism and remodeling; however, its role in bone loss remained unexplored. This study investigates the relationship between gut microbiota dysbiosis, TMAO production, and osteoporosis development. We further demonstrate that the loss of beneficial gut microbiota is associated with the development of murine osteoporosis and alterations in the serum metabolome, particularly affecting L-carnitine metabolism. TMAO emerges as a functional metabolite detrimental to bone homeostasis. Notably, transplantation of mouse gut microbiota counteracts obesity- or estrogen deficiency-induced TMAO overproduction and mitigates key features of osteoporosis. Mechanistically, excessive TMAO intake augments bone mass loss by inhibiting bone mineral acquisition and osteogenic differentiation. TMAO activates the PERK and ATF4-dependent disruption of endoplasmic reticulum autophagy and suppresses the folding of ATF5, hindering mitochondrial unfolding protein response (UPR^mt) in osteoblasts. Importantly, UPR^mt activation by nicotinamide riboside mitigates TMAO-induced inhibition of mineralized matrix biosynthesis by preserving mitochondrial oxidative phosphorylation and mitophagy. Collectively, our findings revealed that gut microbiota dysbiosis leads to TMAO overproduction, impairing ER homeostasis and UPR^mt, thereby aggravating osteoblast dysfunction and development of osteoporosis. Our study elucidates the catabolic role of gut microflora-derived TMAO in bone integrity and highlights the therapeutic potential of healthy donor gut microbiota transplantation to alter the progression of osteoporosis.