A Novel Bmal1 Mutant Mouse Reveals Essential Roles of the C-Terminal Domain on Circadian Rhythms

The mammalian circadian clock is an endogenous biological timer comprised of transcriptional/translational feedback loops of clock genes. Bmal1 encodes an indispensable transcription factor for the generation of circadian rhythms. Here, we report a new circadian mutant mouse from gene-trapped embryonic stem cells harboring a C-terminus truncated Bmal1 (Bmal1 (GTΔC)) allele. The homozygous mutant (Bmal1 (GTΔC/GTΔC)) mice immediately lost circadian behavioral rhythms under constant darkness. The heterozygous (Bmal1 (+/GTΔC)) mice displayed a gradual loss of rhythms, in contrast to Bmal1 (+/-) mice where rhythms were sustained. Bmal1 (GTΔC/GTΔC) mice also showed arrhythmic mRNA and protein expression in the SCN and liver. Lack of circadian reporter oscillation was also observed in cultured fibroblast cells, indicating that the arrhythmicity of Bmal1 (GTΔC/GTΔC) mice resulted from impaired molecular clock machinery. Expression of clock genes exhibited distinct responses to the mutant allele in Bmal1 (+/GTΔC) and Bmal1 (GTΔC/GTΔC) mice. Despite normal cellular localization and heterodimerization with CLOCK, overexpressed BMAL1(GTΔC) was unable to activate transcription of Per1 promoter and BMAL1-dependent CLOCK degradation. These results indicate that the C-terminal region of Bmal1 has pivotal roles in the regulation of circadian rhythms and the Bmal1 (GTΔC) mice constitute a novel model system to evaluate circadian functional mechanism of BMAL1.