Investigating the role and mechanism of methionine in different types of skeletal fluorosis based on Siglec-15 methylation

Skeletal fluorosis has a complex pathogenic mechanism and diverse phenotypes, which mainly manifests as osteosclerosis, osteoporosis, and osteomalacia. Differences in nutrient levels exert a vital effect on skeletal fluorosis development. Aberrant DNA methylation modification is related to skeletal fluorosis pathogenesis and progression, and there are nutritional factors significantly impacting DNA methylation. Methionine, as the essential amino acid, is the only direct precursor of methyl donor S-adenosylmethionine. Based on our previous research, the present work first explored the effect of methionine on bone turnover abnormalities in rats with different types of skeletal fluorosis. Our results showed that osteoclast-mediated bone resorption had a predominant role in osteoporotic/osteomalacic skeletal fluorosis, while methionine supplementation primarily attenuated this process. Then, we examined the methylation levels of Siglec-15 in fluoride-exposed osteoblasts and osteoclasts under different nutritional conditions, and explored the regulatory role of methionine in Siglec-15 methylation. The results indicated that the Siglec-15 showed aberrant methylation and expression in fluoride-exposed osteoblasts and osteoclasts under different nutritional conditions. Notably, Siglec-15 up-regulation specifically promoted fluoride-exposed osteoclasts differentiation through the TYROBP-SYK pathway, while it was not associated with osteoblast differentiation. Under low nutritional conditions, methionine supplementation inhibited fluoride-exposed osteoclast differentiation by modulating Siglec-15 methylation. Finally, we explored the potential mechanism underlying the effect of methionine on Siglec-15 methylation. From our findings, the up-regulation of TET2 promoted Siglec-15 hypomethylation and induced its high expression, therefore increasing the fluoride-exposed osteoclast differentiation under different nutritional conditions. Under low nutritional conditions, methionine deficiency reduced the SAM/SAH ratio and activated TET2 to induce Siglec-15 hypomethylation in fluoride-exposed osteoclasts. Our findings elucidate the logical relationships among methionine, DNA methylation, osteoclast differentiation and different skeletal fluorosis types.