Using old male mice, we found that the load-induced bone anabolic response was significantly higher when applied on a background of disuse. Comparisons of our new data from old male mice with our previous studies in old female mice and young mice of both sexes show site specific changes in response to load that were sex and age specific. This emphasises that caution should be taken when extrapolating loading responses at specific parts of the bone to other parts and to different sexes or ages.
Loading results in site-specific increases in the number of bone forming osteoblast cells. Interestingly, it has not been possible to detect any increase in cell proliferation at the sites of bone formation before the increase in osteoblast cell number can be detected after a period of loading. It is thus unknown from where these new osteoblasts come. Using mice that overexpress fluorescent proteins in specific cells of interest, we have now identified the origin of some of the osteoblasts at the most actively bone forming regions at the bone surface. Interestingly, these cells cannot account for all the cells found on these surfaces, so the mechanosensitive bone forming osteoblasts must also originate from other sites than the periosteum.
Parathyroid hormone (PTH) is, when given intermittently (iPTH) the only bone anabolic drug currently in clinical use. Wefound that in aged, as in young mice, iPTH is osteogenic in cortical bone but, unlike in young mice, it acts independently of the mechanostat (the ability of bone to sense mechanical load). In trabecular bone, iPTH does not have the dramatic osteogenic effect seen in young mice and appears to reduce the positive effects of mechanical loading on trabecular number. The mechanism of this age-related dissociation of the effects of iPTH and mechanical loading needs to be determined.
There are several putative osteoporosis treatments currently in clinical trials. One of them, an antibody against sclerostin, is believed to be bone anabolic in humans as it is in rodents. Sclerostin inhibits Wnt16, an important contributor to bone formation, controlling bone mass and fracture risk, and is downregulated by loading. Taken together this led us to investigate whether Wnt16 expression is regulated by age, sex, loading/unloading and oestrogen treatment (previously a common treatment for osteoporosis). Our findings suggest that while Wnt16 is not an obligatory contributor to regulation of bone mass per se, it potentially plays a role in influencing pathways associated with regulation of bone mass during ageing and oestrogen withdrawal.
Bisphosphonates are the first line treatments for osteoporosis world-wide today. We have studied old female mice and can show that, as in young mice, the osteogenic effects of loading in aged mice is primarily via bone modelling (only bone formation altering bone shape and architecture), rather than remodelling (both bone formation and resorption, not affecting shape or architecture).
We extended this study to investigate the effect of risedronate on disuse mediated rescue of the blunted loading response in old female mice. There was a site specific response to the combined treatment with risedronate, disuse and load, and we conclude that the rescue of the anabolic response to load in old mice can be further improved by co-treatment with risedronate. To the extent that this could be extrapolated to the human situation, exercise management in combination with bisphosphonate treatment could improve bone formation in the old.
Within RESCBONE, we have also investigated the combination of iPTH-treatment and loading, which are anabolic, with risedronate which is an anti-resorptive agent. Our preliminary data show that iPTH and risedronate site-specifically further enhance bone mass in response to load. Thus, the bone anabolic effects of loading seem to be stronger when it is combined with the two different drugs.
