Uncoupling protein-1 is protective of bone mass under mild cold stress conditions

Brown adipose tissue (BAT), largely controlled by the sympathetic nervous system (SNS), has the ability to dissipate energy in the form of heat through the actions of uncoupling protein-1 (UCP-1), thereby critically influencing energy expenditure. Besides BAT, the SNS also strongly influences bone, and recent studies have demonstrated a positive correlation between BAT activity and bone mass, albeit the interactions between BAT and bone remain unclear. Here we show that UCP-1 is critical for protecting bone mass in mice under conditions of permanent mild cold stress for this species (22 °C). UCP-1^−/− mice housed at 22 °C showed significantly lower cancellous bone mass, with lower trabecular number and thickness, a lower bone formation rate and mineralising surface, but unaltered osteoclast number, compared to wild type mice housed at the same temperature. UCP-1^−/− mice also displayed shorter femurs than wild types, with smaller cortical periosteal and endocortical perimeters. Importantly, these altered bone phenotypes were not observed when UCP-1^−/− and wild type mice were housed in thermo-neutral conditions (29 °C), indicating a UCP-1 dependent support of bone mass and bone formation at the lower temperature. Furthermore, at 22 °C UCP-1^−/− mice showed elevated hypothalamic expression of neuropeptide Y (NPY) relative to wild type, which is consistent with the lower bone formation and mass of UCP-1^−/− mice at 22 °C caused by the catabolic effects of hypothalamic NPY-induced SNS modulation. The results from this study suggest that during mild cold stress, when BAT-dependent thermogenesis is required, UCP-1 activity exerts a protective effect on bone mass possibly through alterations in central NPY pathways known to regulate SNS activity.