Clock gene Bmal1 is dispensable for intrinsic properties of murine hematopoietic stem cells

BACKGROUND: Circadian rhythms are known to influence a variety of biological phenomena such as cell cycle, sleep-wake rhythm, hormone release and other important physiological functions. Given that cell cycle entry of hibernating hematopoietic stem cells (HSCs) plays a critical role in controlling hematopoiesis, we asked functional significance of the clock gene Bmal1, which plays a central role in regulating circadian rhythms as a transcription factor. Here we investigated the necessity of Bmal1 for HSC functions using Bmal1 deficient (Bmal1(−/−)) mice. FINDINGS: Using colony-forming assays in vitro, we found that the frequency of mixed colony formation between Bmal1(+/+) and Bmal1(−/−) CD34(−)KSL cells does not differ significantly. Competitive bone marrow assays also revealed that Bmal1(−/−) bone marrow cells competed normally with wild-type cells and displayed long-term multi-hematopoietic lineage reconstitution. In addition, there were no significant differences in the frequencies and hibernation state of bone marrow HSCs between Bmal1(+/+) and Bmal1(−/−) mice, suggesting that they are independent of circadian rhythms. CONCLUSIONS: This paper discusses the necessity of circadian rhythms for HSC functions. Our data clearly shows that a key circadian clock gene Bmal1 is dispensable for intrinsic functions of HSCs, such as differentiation, proliferation and repopulating ability.