Rhythmic expression of BMAL1 mRNA is altered in Clock mutant mice: differential regulation in the suprachiasmatic nucleus and peripheral tissues.

BMAL1 is a putative clock gene which encodes a basic helix-loop-helix (bHLH)-PAS transcription factor. To examine whether the CLOCK protein is required for the circadian expression of BMAL1 mRNA, in situ hybridization and Northern blot analysis were performed in the suprachiasmatic nucleus (SCN) and peripheral tissues of homozygous Clock mutant mice. In the SCN of Clock mutants, BMAL1 mRNA did not oscillate significantly but apparently expressed with low levels, while in wild-type mice the mRNA was robustly oscillated in a circadian manner. The peak-trough amplitudes of BMAL1 mRNA levels were 6.5-, 8.6-, and 6.7-fold in liver, heart, and kidney of wild-type mice, respectively. In Clock mutants, the amplitudes were extremely damped to 1.2-, 2.1-, and 1.4-fold, respectively. Furthermore, expressions of BMAL1 mRNA in the peripheral of Clock mutant mice were close to the peak level in wild-type mice, whereas mPer2 mRNA levels were severely blunted at trough values. Daily expression of albumin site D-binding protein (DBP), a clock controlled output gene (CCG), was also abolished at trough values by the Clock mutation in all tissues examined. These observations suggest that the circadian expression of BMAL1 mRNA is affected by the CLOCK-induced transcriptional feedback loop in the SCN and peripheral tissues in a different way and that the regulation mechanism appeared to be different from those in mPer2 and DBP expressions in vivo.