Denosumab is a monoclonal anti-RANKL antibody that inhibits bone resorption, increases bone mass, and reduces fracture risk. Denosumab discontinuation causes an extensive wave of rebound resorption, but the cellular mechanisms remain poorly characterized. We utilized in situ hybridization (ISH) as a direct approach to identify the cells that activate osteoclastogenesis through the RANKL/OPG pathway. ISH was performed across species, skeletal sites, and following recombinant OPG (OPG:Fc) and parathyroid hormone 1–34 (PTH) treatment of mice. OPG:Fc treatment in mice induced an increased expression of RANKL mRNA mainly in trabecular, but not endocortical bone surface cells. Additionally, a decreased expression of OPG mRNA was detected in bone surface cells and osteocytes of both compartments. A similar but more pronounced effect on RANKL and OPG expression was seen one hour after PTH treatment. These findings suggest that bone surface cells and osteocytes conjointly regulate the activation of osteoclastogenesis, and that OPG:Fc treatment induces a local accumulation of osteoclastogenic activation sites, ready to recruit and activate osteoclasts upon treatment discontinuation. Analysis of publicly available single-cell RNA sequencing (scRNAseq) data from murine bone marrow stromal cells revealed that Tnfsf11⁺ cells expressed high levels of Mmp13, Limch1, and Wif1, confirming their osteoprogenitor status. ISH confirmed co-expression of Mmp13 and Tnfsf11 in bone surface cells of both vehicle- and OPG:Fc-treated mice. Under physiological conditions of human/mouse bone, RANKL is expressed mainly by osteoprogenitors proximate to the osteoclasts, while OPG is expressed mainly by osteocytes and bone-forming osteoblasts.

Osteoporosis is a common metabolic bone disease that affects about 40% of postmenopausal women. Treatment options for osteoporosis are limited, however. Icariin is an herbal substance that has been shown to improve bone mass, but the mechanisms are largely unknown. Using bioinformatics analysis, we have identified the hub genes and KEGG pathways shared between icariin-targeted genes and osteoporosis. The top five shared KEGG pathways were the Toll-like receptor signaling pathway, adipocytokine pathway, neurotrophin signaling pathway, NOD-like receptor signaling, and B cell receptor signaling pathway; the hub genes were RELA, NFKBIA, and IKBKB, belonging to the NF-κB family. The identified icariin-targeted genes are involved in inflammation, insulin resistance, apoptosis, and immune responses, and regulate the PI3K-Akt, NF-κB, MAPK, and JNK signaling pathways. Our in vitro data show that icariin inhibits apoptosis in human mesenchymal stem cells by suppressing JNK/c-Jun signaling pathway. Together, these findings indicate that icariin exerts its anti-osteoporotic function by inhibiting JNK/c-Jun signaling pathway, and suggest that icariin may be a promising treatment option for osteoporosis.

Aging-associated microbial alteration is hypothesized to play an important role in host health and longevity. However, investigations regarding specific gut microbes or microbial functional alterations associated with aging have had inconsistent results.

Human milk guides the structure and function of microbial commensal communities that colonize the nursing infant gut. Indigestible molecules dissolved in human milk establish a microbiome often dominated by bifidobacteria capable of utilizing these substrates. Interestingly, urea accounts for ~15% of total human milk nitrogen, representing a potential reservoir for microbiota that may be salvaged for critical metabolic operations during lactation and neonatal development. Accordingly, B. infantis strains are competent for urea nitrogen utilization, constituting a previously hypothetical phenotype in commensal bacteria hosted by humans. Urease gene expression, downstream nitrogen metabolic pathways, and enzymatic activity are induced during urea utilization to yield elevated ammonia concentrations. Moreover, biosynthetic networks relevant to infant nutrition and development are transcriptionally responsive to urea utilization including branched chain and other essential amino acids. Importantly, isotopically labeled urea nitrogen is broadly distributed throughout the expressed B. infantis proteome. This incisively demonstrates that the previously inaccessible urea nitrogen is incorporated into microbial products available for infant host utilization. In aggregate, B. infantis possesses the requisite phenotypic foundation to participate in human milk urea nitrogen recycling within its infant host and thus may be a key contributor to nitrogen homeostasis early in life.

Bifidobacterium longum subsp. infantis (B. infantis) utilizes oligosaccharides secreted in human milk as a carbohydrate source. These human milk oligosaccharides (HMOs) integrate the nitrogenous residue N-acetylglucosamine (NAG), although HMO nitrogen utilization has not been described to date. Herein, we characterize the B. infantis nitrogen utilization phenotype on two NAG-containing HMO species, LNT and LNnT. This was characterized through in vitro growth kinetics, incorporation of isotopically labeled NAG nitrogen into the proteome, as well as modulation of intracellular 2-oxoglutarate levels while utilizing HMO nitrogen. Further support is provided by comparative transcriptomics and proteomics that identified global regulatory networks deployed during HMO nitrogen utilization. The aggregate data demonstrate that B. infantis strains utilize HMO nitrogen with the potential to significantly impact fundamental and clinical studies, as well as enable applications.

Emerging evidence suggests a significant role of gut microbiome in bone health. Aging is well recognized as a crucial factor influencing the gut microbiome. In this study, we investigated whether age-dependent microbial change contributes to age-related bone loss in CB6F1 mice. The bone phenotype of 24-month-old germ-free (GF) mice was indistinguishable compared to their littermates colonized by fecal transplant at 1-month-old. Moreover, bone loss from 3 to 24-month-old was comparable between GF and specific pathogen-free (SPF) mice. Thus, GF mice were not protected from age-related bone loss. 16S rRNA gene sequencing of fecal samples from 3-month and 24-month-old SPF males indicated an age-dependent microbial shift with an alteration in energy and nutrient metabolism potential. An integrative analysis of 16S predicted metagenome function and LC-MS fecal metabolome revealed an enrichment of protein and amino acid biosynthesis pathways in aged mice. Microbial S-adenosyl methionine metabolism was increased in the aged mice, which has previously been associated with the host aging process. Collectively, aging caused microbial taxonomic and functional alteration in mice. To demonstrate the functional importance of young and old microbiome to bone, we colonized GF mice with fecal microbiome from 3-month or 24-month-old SPF donor mice for 1 and 8 months. The effect of microbial colonization on bone phenotypes was independent of the microbiome donors’ age. In conclusion, our study indicates age-related bone loss occurs independent of gut microbiome.