Artikel
Global disruption of the circadian clock impairs bone regeneration in a standardized murine fracture model
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Veröffentlicht: | 25. Oktober 2022 |
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Gliederung
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Objectives: The circadian clock controls fundamental physiological processes, such as body temperature, metabolic activity, and hormonal balance. In many tissues, more than 10 percent of genes are expressed in a circadian manner, indicating a pivotal role in both physiological and pathological processes. In fact, strong evidence has emerged that bone metabolism is controlled by circadian mechanisms. Disruption of the biological clock, as observed in rotational shift-work or sleep restriction, strongly impairs bone structure and remodeling not only in affected individuals but also in animal models. In the case of regeneration after trauma, however, these mechanisms have been widely neglected in the past and first insights, especially in regard to wound healing, have been gained only recently. As the function of the circadian clock during fracture healing remains so far unknown, we aimed to elucidate this topic in an experimental setup using genetically modified mice with a femoral osteotomy.
Methods: We employed female mice lacking a functional circadian clock by genetic deletion of the core clock gene Bmal1 in a C57Bl6/J background. We induced a femoral osteotomy stabilized with an external fixator (RISystem). N=8 mice were euthanized at 7, 14 and 21 days following fracture and the callus structure was analyzed using micro-CT and histology. Wild-type littermates served as controls. All animals received a subcutaneous chip (RFID implant) to track circadian rhythms of locomotor activity. Mice were kept under 12/12 hrs light/dark conditions for one week before release into constant darkness for the time of the experiment to assess the effect of the endogenous clock and to exclude any effects of exogenous light-dark cycles.
Results and conclusion: Wild-type mice showed rhythmic locomotor activity under light-dark conditions, as well as in constant darkness. Their Bmal1-deficient littermates were behaviorally arrhythmic with moderate activity throughout the day, confirming the disrupted circadian rhythm. The analyses of the fracture callus showed a significantly reduced bone volume and bone surface in Bmal1-deficient mice. In addition, the histological examination revealed major alterations in tissue composition and cell content of the callus. These results show for the first time that global disruption of the circadian clock impairs fracture healing in long bones. Further studies are necessary for a more profound and mechanistic understanding of these findings, as they bear the potential to improve current treatment strategies, e.g. through the optimized timing of orthopedic surgeries or drug applications.