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.

骨质疏松症是老年人群中最常见的代谢性疾病之一，会显著增加脆性骨折的风险，而脆性骨折可能导致疼痛、行动能力丧失，甚至死亡。目前预防和治疗骨质疏松症的策略主要集中在增加骨量。然而，骨整体的失效不仅由骨量决定，还取决于骨基质的质量。新兴证据表明，肠道微生物群在调节骨基质质量方面发挥一定作用，但微生物群变化如何影响骨骼的机制仍不清楚。阐明肠道微生物群如何影响骨强度，有望为改善骨基质质量的治疗方法奠定基础。 此前的研究表明，在年轻雄性小鼠中，终生改变肠道微生物群会显著降低骨组织强度。然而，由于这些骨基质完全在菌群失调的环境中形成，因此尚不清楚该效应是否通过传统的破骨细胞和成骨细胞介导的重塑过程发生。此外，也不清楚这一现象是否同样出现在年轻雌性或老年小鼠中。 本论文的第二章研究了在骨骼成熟前后改变肠道微生物群对雌雄性小鼠的影响。由于骨重塑在骨骼成熟后显著减缓，该模型使我们能够探究晚年微生物群改变是否会影响既有的骨基质特性。研究发现，这种改变主要影响雄性小鼠，提示存在性别特异性的响应。重要的是，即使在缺乏活跃重塑的情况下，骨基质强度仍发生变化，说明肠道微生物群可直接影响骨基质的材料特性。值得注意的是，恢复肠道微生物群在雄性中可部分挽救骨强度。 第三章探讨了肠道微生物群与肌肉骨骼系统之间的关系。我们进行了综合分析，包括粪便宏基因组测序、血浆代谢组学分析、粪便、肝脏和肾脏中的维生素K水平检测，以及股骨、脊柱、肌肉和血清的结构与力学评估。结果显示，肠道微生物组成、循环代谢物与肌肉骨骼健康之间存在相关性。 第四章中，我们开发了一种新方法，可在不依赖骨几何假设的情况下，直接测量骨基质的断裂韧性。研究发现，微生物群的改变会降低干预前形成的骨基质的断裂韧性，直接证明肠道微生物群可以在不依赖重塑过程的前提下改变骨材料特性。 综上所述，本研究强调了肠道微生物群在调控骨基质质量方面一种先前被低估的重要作用。本研究为未来开发靶向肠道微生物群的干预策略以增强骨强度、并以改善骨质量为核心的骨质疏松新疗法提供了理论基础。