Osteoporosis is one of the most common metabolic disorders in the aging population, leading to increased risk of fragility fractures that can result in pain, loss of mobility, and even death. Current strategies for preventing and treating osteoporosis primarily focus on increasing bone quantity. However, whole bone failure is determined not only by bone mass but also by the quality of the bone matrix. Emerging evidence suggests that the gut microbiome plays a role in regulating bone matrix quality, yet the mechanisms through which microbiome alterations influence bone remain poorly understood. Elucidating how the gut microbiome impacts bone strength could pave the way for therapies that improve bone matrix quality.

Previous research has shown that lifelong alteration of the gut microbiome in young male mice leads to a significant reduction in bone tissue strength. However, since the bone matrix formed entirely under dysbiosis, it remained unclear whether this effect occurred through traditional osteoclast- and osteoblast-mediated remodeling. Moreover, it was unknown whether similar effects occur in young females or in aged mice.

In the second chapter of this thesis, we investigated the effects of microbiome alteration before and after skeletal maturity in both male and female mice. Because bone remodeling slows significantly after skeletal maturity, this model allowed us to determine whether late-life microbiome changes can influence pre-existing bone matrix properties. We found that such alterations primarily affected males, suggesting a sex-specific response. Importantly, changes in bone matrix strength occurred even without active remodeling, indicating that the microbiome can directly affect matrix properties. Notably, restoring the gut microbiome partially rescued bone strength in males.

The third chapter explores the relationship between the gut microbiome and the musculoskeletal system. We performed comprehensive profiling, including fecal metagenomics, plasma metabolomics, vitamin K levels in feces, liver, and kidney, and mechanical and structural assessments of the femur, spine, muscle, and serum. These data revealed correlations between gut microbial composition, circulating metabolites, and musculoskeletal health.

In the fourth chapter, we developed a novel method to directly assess bone matrix fracture toughness without relying on assumptions about bone geometry. We found that microbiome alteration led to reduced fracture toughness in bone matrix formed prior to the intervention, providing direct evidence that the gut microbiome can alter bone material properties independently of remodeling processes.

Together, these findings underscore a previously underappreciated role of the gut microbiome in regulating bone matrix quality. This work establishes a foundation for future research into microbiome-targeted interventions to enhance bone strength and develop new therapeutic strategies for osteoporosis that focus on improving bone quality.