Gut Microbiota Manipulation Promotes Bone Homeostasis in Obese Mice Through Regulatory T-Cell Differentiation

A better understanding of the role of the gut microbiome in regulating skeletal metabolism has recently been reported. Manipulation of the gut microbiome with probiotics can prevent pathologic bone loss. However, the effects of probiotics/gut microbiota on immunomodulatory effect and skeletal homeostasis in the high fat diet (HFD)-linked obesity remains to be explored. Here, we examined the impact of supplementation with probiotics (VSL#3) on immune function and bone homeostasis in HFD mice. µ-CT and mechanical testing revealed that probiotics induced a significant increase of trabecular bone volume and bone mechanical strength in obese mice. Butyrate produced in the gut following probiotics treatment, or butyrate injection directly to obese mice, prevented gut inflammation, improved gut-barrier integrity, and induced the expansion of bone marrow (BM) regulatory T (Treg) cells. Mechanistically, probiotics treatment promoted the Kdm6b/Jmjd3 histone demethylase dependent Tfam promoter demethylation by inhibiting the H3K27me3 epigenetic mark in Treg cells, which increases Tfam expression. Reduced Tfam expression in Treg cells, were found to secrete greater concentration of mtDNA and modulate paracrine signaling. BMMSCs expressed DEC205 that bound to mtDNA and contributed to BMMSCs inflammation. Further, Tfam-transgenic mice, fed with HFD, prevented the obesity-linked reduction in Treg cells and mtDNA-DEC205 dependent BMMSC inflammation. Moreover, fecal transplantation from healthy, probiotic-treated donor mice is sufficient to improve gut barrier permeability and bone loss, suggesting that the probiotics mediated changes in the gut microbiome and may be a novel agent to regulate bone anabolism via the gut-immune axis.