Abstract
The pathogenesis of falling bone mineral density (BMD) as a universal feature of advancing age is poorly understood1. Frequently culminating in the development of osteoporosis, the process is attributable to more than 500,000 fragility fractures occurring every year in the UK Such injuries are associated with great levels of morbidity, mortality and a £3.5 billion cost to the healthcare economy2.
With age, humans are known to accumulate somatic mitochondrial DNA (mtDNA) mutations in mitotic and post mitotic tissue, and stem cell precursors3. Compelling evidence in recent years, particularly that provided by animal models suggests that these mutations are intrinsic to the ageing process4–6. We provide evidence for the first time that mitochondrial dysfunction contributes significantly to the failure of bone homeostasis and falling BMD.
We have utilised a mouse model that accumulates mtDNA mutations at 3–5 times the rate of normal mice, consequently ageing and developing osteoporosis prematurely7, to clearly demonstrate that osteoblasts are vulnerable to mtDNA mutations. We have developed a new quadruple immunofluorescent assay to show that mitochondrial respiratory chain dysfunction occurs in osteoblasts as a consequence (p < 0.0001). We show that this mitochondrial dysfunction is associated with reduced BMD in female and male mice by 7 (p = 0.003) and 11 (p = 0.0003) months of age respectively. Using osteoblasts derived from mesenchymal stem cells extracted from male and female mice with mitochondrial dysfunction aged 4, 7 and 11 months, we demonstrate a vastly reduced capacity to produce new mineralised bone in vitro when compared to wild type cell lines (p < 0.0001). Exercise was found to have no beneficial effect on osteoblast and whole bone phenotype in this mouse model. It is likely that mtDNA mutations accumulating over a longer time period in human ageing have significantly detrimental effects on bone biology and diminishing BMD.
