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 require d for the circadian expression of BMAL1 mRNA, in situ hybridization and Nor thern blot analysis were performed in the suprachiasmatic nucleus (SCN) and peripheral tissues of homozygous Clock mutant mice. In the SCN of Clock mu tants, BMAL1 mRNA did not oscillate significantly but apparently expressed with low levels, while in wild-type mice the mRNA was robustly oscillated i n 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, res pectively. In Clock. mutants, the amplitudes were extremely damped to 1.2-, 2.1-, and 1.4-fold, respectively. Furthermore, expressions of BMAL1 mRNA i n 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 value s. Daily expression of albumin site D-binding protein (DBP), a clock contro lled output gene (CCG), was also abolished at trough values by the Clock mu tation in all tissues examined. These observations suggest that the circadi an expression of BMAL1 mRNA is affected by the CLOCK-induced transcriptiona l feedback loop in the SCN and peripheral tissues in a different way and th at the regulation mechanism appeared to be different from those in mPer2 an d DBP expressions in vivo. (C) 2000 Academic Press.