Molecular neurobiology and genetics of circadian clocks

The long-term objective of the research in our laboratory is to understand the cellular and molecular mechanisms that regulate circadian rhythms. We have initiated a genetic approach to study the mechanism of circadian rhythms in mammals using the mouse as a model organism. In a deliberate chemical mutagenesis screen for circadian rhythm mutants, we isolated the first circadian mutation in mice which is name Clock. The Clock mutation is semidominant and lengthens circadian period by 1 hr in heterozygots and by 4 hr in homozygotes. Importantly, Clock homozygotes lose persistent circadian rhythms in constant darkness. The Clock mutation maps onto the midportion of chromosome 5 in a region with conserved synteny with human chromosome 4. In 1997, we identified the gene encoding the Clock mutation by the method of positional cloning and by functional rescue of the Clock mutant phenotype by transgenic expression in mice of large-insert genomic BAC clones. Clock encodes a novel member of the basic-helix-loop-helix (bHLH)~PAS protein domain family of transcription factors. In the ENU-induced Clock mutant allele, we identified a single A to T nucleotide transversion in a splice donor site which causes exon skipping and the deletion of 51 amino acids in the C-terminal region of the CLOCK protein. Recently, we have found that the CLOCK protein dimerizes with another bHLH~PAS protein known as BMAL1 (aka, MOP3, JAP3). The CLOCK-BMAL1 heterodimer binds to and transactivates through an E-box motif (CACGTG)) found in the period gene promoters of both Drosophila and mice. The Drosophila orthologs of Clock and BMAL1 have also been identified and play the same role. Thus, the CLOCK protein and its partner are positive regulators of period (and timeless) gene transcription. In Drosophila we have found that the PERIOD and TIMELESS proteins subsequently inhibit their own transcription viz the CLOCK-BMAL1 complex. These four genes define a basic framework for a transcriptional autoregulatory loop that appears to compose the circadian oscillator mechanism in animals. The delineation of this circadian gene pathway should eventually lead to an understanding of how circadian clocks function, how they are regulated by environmental inputs, and how clocks regulate their various outputs.

N. Gekakis, D. Staknis, H. B. Nguyen, F. C. Davis, L. D. Wilsbacher, D. P. King, J. S. Takahashi and C. J. Weitz (1998). "Role of the CLOCK protein in the mammalian circadian mechanism." Science 280:1564-1569.

T. K. Darlington, K. Wager-Smith, M.F. Ceriani, D. Staknis, N. Gekakis, T.D.L. Steeves, C.J. Weitz, J.S. Takahashi and S.A. Kay (1998). "Closing the circadian loop: CLOCK-induced transcription of its own inhibitors per and tim." Science 280:1599-1603.

D. P. King, Y. Zhao, A.M. Sangoram, L.D. Wilsbacher, M. Tanaka, M.P. Antoch, T.D.L. Steeves, M.H. Vitaterna, J.M. Kornhauser, P.L. Lowrey, F.W. Turek and J.S. Takahashi (1997). "Positional cloning of the mouse circadian Clock gene." Cell 89:641-653.

M. P. Antoch, E. J. Song, A.-M. Chang, M.H. Vitaterna, Y. Zhao, L.D. Wilsbacher, A.M. Sangoram, D.P. King, L.H. Pinto and J.S. Takahashi (1997). "Functional identification of the mouse circadian Clock gene by transgenic BAC rescue." Cell 89:655-667.

View all publications by publications by Joseph S. Takahashi listed in the National Library of Medicine (PubMed).