The pacemaker role of the suprachiasmatic nucleus in a mammalian circadian system was tested by neural transplantation by using a mutant strain of hamster that shows a short circadian period. Small neural grafts from the suprachiasmatic region restored circadian rhythms to arrhythmic animals whose own nucleus had been ablated. The restored rhythms always exhibited the period of the donor genotype regardless of the direction of the transplant or genotype of the host. The basic period of the overt circadian rhythm therefore is determined by cells of the suprachiasmatic region.

The tau mutation is a semidominant autosomal allele that dramatically shortens period length of circadian rhythms in Syrian hamsters. We report the molecular identification of the tau locus using genetically directed representational difference analysis to define a region of conserved synteny in hamsters with both the mouse and human genomes. The tau locus is encoded by casein kinase I epsilon (CKIɛ), a homolog of the Drosophila circadian gene double-time. In vitro expression and functional studies of wild-type and tau mutant CKIɛ enzyme reveal that the mutant enzyme has a markedly reduced maximal velocity and autophosphorylation state. In addition, in vitro CKIɛ can interact with mammalian PERIOD proteins, and the mutant enzyme is deficient in its ability to phosphorylate PERIOD. We conclude that tau is an allele of hamster CKIɛ and propose a mechanism by which the mutation leads to the observed aberrant circadian phenotype in mutant animals.Daily rhythms in biochemical, physiological, and behavioral processes are regulated by biological clocks (1, 2). In natural conditions, the endogenous circadian rhythms that are generated by these clocks are synchronized (entrained) to the 24-hour cycles of the external world by time cues such as the daily light/dark cycle (2). While these clocks are found in organisms as divergent as cyanobacteria, plants, fruit flies, and mammals, there is an extraordinary degree of evolutionary conservation of the underlying generative molecular mechanisms (3-9). The first mammalian circadian gene, Clock, was cloned and characterized in mouse (10-12). Clock encodes a novel member of the basic helix-loop-helix (bHLH) PER-ARNT-SIM (PAS) family of transcription factors. Shortly following the cloning of Clock, three mouse Period orthologs, denoted mPer1 (13,14), and mPer3 (18, 19), as well as a Timeless (mTim) ortholog, were cloned (20)(21)(22)(23)(24). At the same time, another gene, BMAL1 (25), was found to encode the protein dimerization partner for CLOCK (26), and together the CLOCK/BMAL complex was shown to transactivate mPer1 via conserved E-box elements found in the promoters of Drosophila and mouse period genes (26-28). Finally, in Drosophila and mammals, the negative feedback effects of PER and TIM were shown to act at the level of the CLOCK/BMAL complex (20, 28) in a surprisingly direct manner (29, 30).

A mutation has been found that dramatically shortens the period of the circadian locomotor rhythm of golden hamsters. The pattern of inheritance of this mutation suggests that it occurred at a single, autosomal locus (tau). Wild-type animals have rhythms with free-running periods averaging about 24 hours; animals heterozygous for the mutation have periods of about 22 hours, whereas homozygous animals have rhythms with periods close to 20 hours. Animals that carry the mutant alleles exhibit abnormal entrainment to 24-hour light:dark cycles or are unable to entrain.

Computational modeling and experimentation explain how intercellular coupling and intracellular noise can generate oscillations in a mammalian neuronal network even in the absence of cell-autonomous oscillators.

Sleep deprivation, shift work, and jet lag all disrupt normal biological rhythms and have major impacts on health; however, circadian disorganization has never been shown as a causal risk factor in organ disease. We now demonstrate devastating effects of rhythm disorganization on cardiovascular and renal integrity and that interventions based on circadian principles prevent disease pathology caused by a short-period mutation (tau) of the circadian system in hamsters. The point mutation in the circadian regulatory gene, casein kinase-1epsilon, produces early onset circadian entrainment with fragmented patterns of behavior in +/tau heterozygotes. Animals die at a younger age with cardiomyopathy, extensive fibrosis, and severely impaired contractility; they also have severe renal disease with proteinuria, tubular dilation, and cellular apoptosis. On light cycles appropriate for their genotype (22 h), cyclic behavioral patterns are normalized, cardiorenal phenotype is reversed, and hearts and kidneys show normal structure and function. Moreover, hypertrophy does not develop in animals whose suprachiasmatic nucleus was ablated as young adults. Circadian organization therefore is critical for normal health and longevity, whereas chronic global asynchrony is implicated in the etiology of cardiac and renal disease.