Reviewing What is Known of How the Aging Gut Microbiome May Contribute to Osteoporosis
The gut microbiome is made up of thousands of distinct microbial species, interacting with one another to process food and generate a broad range of metabolite molecules that are varying degrees of necessary, helpful, and harmful to the tissues of the body. The balance of these microbial populations changes with age in detrimental ways; microbes that provoke chronic inflammation grow in number at the expense of microbes that manufacture beneficial metabolites. In recent years, researchers have mapped the microbiome in human populations, finding correlations with age-related diseases and specific microbial metabolites that contribute to specific forms of age-related dysfunction. We might expect near every common age-related disease to have many different meaningful connections to aspects of the composition of the gut microbiome and how that composition changes with age.
In today's open access paper, researchers discuss what is known of the ways in which the aging of the gut microbiome contributes to the characteristic age-related loss of bone mineral density, leading to osteoporosis. The extracellular matrix of bone is continually remodeled throughout life, created by osteoblast cells and broken down by osteoclast cells. The proximate cause of osteoporosis is a growing imbalance between these two cell populations that favors osteoclasts. Why this imbalance happens, which of the many known contributing mechanisms are more versus less important than the others, and whether all of the important mechanisms are in fact known, is a topic of ongoing debate and research. Here, researchers focus on inflammatory effects resulting from the influence of the aging gut microbiome upon the immune system, which in turn disrupts the balance to favor destruction of bone extracellular matrix.
Age-related osteoporosis has traditionally been attributed to impaired bone formation, excessive bone resorption, senescence of bone marrow mesenchymal stem/stromal cells (BMSCs), and deterioration of the bone marrow microenvironment. However, these local alterations do not fully explain the progressive, systemic, and heterogeneous nature of age-related bone loss. The bone marrow is not only a site of bone remodeling but also a critical niche for hematopoietic stem cell maintenance and for the generation, differentiation, and functional regulation of immune cells. Aging is accompanied by profound remodeling of the immune system. T cells, B cells, macrophages, and dendritic cells undergo age-related changes in phenotype and function. Together, immunosenescence and inflammaging disrupt the RANKL/OPG axis, increase exposure to pro-inflammatory cytokines, and enhance osteoclastogenic signaling. Consequently, bone homeostasis gradually shifts from protective immunoregulation toward a resorption-dominant state.
Gut microbiota dysbiosis has recently emerged as an important upstream regulator of bone homeostasis. During aging, alterations in microbial composition and metabolism, together with impaired intestinal barrier integrity, may promote the systemic dissemination of gut-derived inflammatory signals. These signals may reach the bone marrow and contribute to remodeling of its immune microenvironment. In addition, gut microbiota and their metabolites may influence autonomic nervous system activity through vagal and sympathetic pathways. Through these routes, intestinal signals may modulate the activation state of bone marrow immune cells and the immune networks that regulate bone remodeling. Although gut microbiota dysbiosis, inflammaging, and neural regulation have each been implicated in disordered bone metabolism, their interactions in age-related osteoporosis remain incompletely understood. In particular, the mechanistic links among gut microbiota dysbiosis, neuroimmune dysregulation, remodeling of the bone marrow immune microenvironment, and impaired bone remodeling have not been systematically clarified.
Therefore, this review summarizes the pathological basis of age-related osteoporosis, focusing on how aging-associated gut microbiota dysbiosis, intestinal barrier dysfunction, and gut-derived inflammatory signals reshape the bone marrow immune microenvironment. We also discuss the potential roles of impaired vagal anti-inflammatory signaling and abnormal sympathetic outflow in promoting osteoimmune imbalance within the bone marrow. By positioning the bone marrow neuroimmune microenvironment as a key intersection between gut-derived signals, neural regulation, and osteoimmune remodeling, this review provides an immunological framework for understanding disease mechanisms and for developing risk-stratification strategies and precision interventions in age-related osteoporosis.