A group of specialized genes has been defined to govern the molecular mechanisms controlling the circadian clock in mammals. Their expression and the interactions among their products dictate circadian rhythmicity. Three genes homologous to Drosophila period exist in the mouse and are thought to be major players in the biological clock. Here we present the generation of mice in which the founding member of the family, Per1, has been inactivated by homologous recombination. These mice present rhythmicity in locomotor activity, but with a period almost 1 h shorter than wild-type littermates. Moreover, the expression of clock genes in peripheral tissues appears to be delayed in Per1 mutant animals. Importantly, light-induced phase shifting appears conserved. The oscillatory expression of clock genes and the induction of immediate-early genes in response to light in the master clock structure, the suprachiasmatic nucleus, are unaffected. Altogether, these data demonstrate that Per1 plays a distinct role within the Per family, as it may be involved predominantly in peripheral clocks and/or in the output pathways of the circadian clock.

The primary hallmark of circadian clocks is their ability to entrain to environmental stimuli. The dominant, and therefore most physiologically important, entraining stimulus comes from environmental light cycles. Here we describe the establishment and characterization of a new cell line, designated Z3, which derives from zebrafish embryos and contains an independent, light-entrainable circadian oscillator. Using this system, we show distinct and differential light-dependent gene activation for several central clock components. In particular, activation of Per2 expression is shown to be strictly regulated and dependent on light. Furthermore, we demonstrate that Per1, Per2, and Per3 all have distinct responses to light-dark (LD) cycles and light-pulse treatments. We also show that Clock, Bmal1, and Bmal2 all oscillate under LD and dark-dark conditions with similar kinetics, but only Clock is significantly induced while initiating a light-induced circadian oscillation in Z3 cells that have never been exposed to a LD cycle. Finally, our results suggest that Per2 is responsible for establishing the phase of a circadian rhythm entraining to an alternate LD cycle. These findings not only underscore the complexity by which central clock genes are regulated, but also establishes the Z3 cells as an invaluable system for investigating the links between light-dependent gene activation and the signaling pathways responsible for vertebrate circadian rhythms.I n recent years, a new and exciting dimension has been added to our knowledge of the vertebrate circadian clock system. The classical view of the circadian system describes it as diverse physiological rhythms, which are regulated by a centralized clock structure (1, 2). Over the past few years, the idea that the clock consists exclusively of a few centralized structures has been challenged. Data coming from both vertebrate and invertebrate systems have demonstrated that the circadian timing system is dispersed throughout the animal (3-7), and that possibly every cell contains a functional circadian clock (8). In these studies, it has been revealed that a variety of tissues and cells contain functional autonomous clocks. These clocks are able to maintain an oscillation when placed in vitro and removed from any external cues or signals that originate from the classical clock structures and͞or the environment.The discovery of a number of genes involved in the generation and maintenance of circadian oscillations (9, 10) and the recent realization that the circadian system consists of a complex network of independent clocks, which are somehow synchronized to properly regulate all physiological rhythms (5,11,12), has necessitated the development of new tools and methodologies for deciphering the circadian system. An ideal tool is an in vitro cell-based system that displays robust circadian rhythms. Cultured cells may be used to fully understand the complex molecular mechanisms, signal coupling, and regulatory feedback loops that are responsible for the proper timing of a circa...