Differential regulation of circadian pacemaker output by separate clock genes in Drosophila

Circadian rhythms in a nutshell

Living organisms on this planet have adapted to the daily rotation of the earth on its axis. By means of endogenous circadian clocks that can be synchronized to the daily and seasonal changes in external time cues, most notably light and temperature, life forms anticipate environmental transitions, perform activities at biologically advantageous times during the day, and undergo characteristic seasonal responses. The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create "cities that never sleep" we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior. Moreover, malfunctions in the human circadian timing system are implicated in several disorders, including chronic sleep disorders in the elderly, manic-depression, and seasonal affective disorders (SAD or winter depression). Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable. In its most basic form, circadian clocks are comprised of a set of proteins that, by virtue of the design principles involved, generate a self-sustaining transcriptional-translational feedback loop with a free-running period of about 24 h. One or more of the clock components is acutely sensitive to light, resulting in an oscillator that can be synchronized to local time. This review provides an overview of the roles circadian clocks play in nature, how they might have arisen, human health concerns related to clock dysfunction, and mainly focuses on the clockworks found in Drosophila and mice, the two best studied animal model systems for understanding the biochemical and cellular bases of circadian rhythms.

Neuroanatomy of cells expressing clock genes in Drosophila: transgenic manipulation of the period and timeless genes to mark the perikarya of circadian pacemaker neurons and their projections

Subsets of brain neurons expressing the clock genes period (per) and timeless (tim) are involved in the generation of circadian behavioral rhythms. However, current knowledge of projection patterns of these neurons is limited to those immunoreactive to an antibody against a crustacean neuropeptide. The GAL4-expression system was utilized to visualize neuronal processes from all per and tim-expressing neurons in the central nervous system. Each of two types of GAL4-driver fusion genes, per-gal4 or tim-gal4, was combined in transgenic flies with marker genes-lacZ, and sequences encoding green fluorescent protein or TAU protein-under the control of the GAL4-responsive element UAS. This allowed visualization of the cytoplasm of GAL4-expressing cells. Thus, neurites of clock neurons in the adult brain as well as those of larvae and pupae were revealed. Among the anatomical patterns revealed by per-gal4- or tim-gal4-driven marker expression were a previously unknown, dorsally located neuronal cluster, along with the projections of these cells and of other dorsal neurons characterized in earlier studies only by the location of their perikarya. The similarity of projections from PER- or TIM-containing neurons during development to those in the adult implies that these features of mature clock neurons are established by the larval stages. Neurons that have never been identified as PER- or TIM-immunoreactive were also visualized in this assay system, indicating promoter activity of the clock genes in these cells and suggesting that their products cannot accumulate to detectable levels in certain neurons.

The Drosophila takeout gene is a novel molecular link between circadian rhythms and feeding behavior

We report the characterization of a novel Drosophila circadian clock-regulated output gene, takeout (to). The to amino acid sequence shows similarity to two ligand binding proteins, including juvenile hormone binding protein. to mRNA is expressed in the head and the cardia, crop, and antennae-structures related to feeding. to expression is induced by starvation, which is blocked in all arrhythmic central clock mutants, suggesting a direct molecular link between the circadian clock and the feeding/starvation response. A to mutant has aberrant locomotor activity and dies rapidly in response to starvation, indicating a link between locomotor activity, survival, and food status. We propose that to participates in a novel circadian output pathway that conveys temporal and food status information to feeding-relevant metabolisms and activities.

Disruption of synaptic transmission or clock-gene-product oscillations in circadian pacemaker cells of Drosophila cause abnormal behavioral rhythms

To study the function of clock-gene-expressing neurons, the tetanus-toxin light chain (TeTxLC), which blocks chemical synaptic transmission, was expressed under the control of promoters of the clock genes period (per) and timeless (tim), each fused to GAL4-encoding sequences. Although TeTxLC did not affect cycling of a clock-gene product at the gross level, it disrupted the rhythmic behavior of adult Drosophila. In constant darkness, the proportion of rhythmic flies was reduced in flies expressing active TeTxLC compared to controls, including those expressing inactive toxin. The behavior of TeTxLC-expressing flies was less synchronized to light:dark cycles than that of controls. To determine which neurons are responsible for these effects on behavior, the toxin was also expressed in restricted subsets of per/tim-expressing, laterally located pacemaker neurons by expressing TeTxLC under the control of a driver in which GAL4-encoding sequences are fused to the promoter of the pigment dispersing factor (pdf) gene. pdf-gal4-driven TeTxLC expression had relatively little effect on behavioral rhythms, implying that per/tim neurons other than pdf-expressing lateral neurons participate in the generation of rhythmic behavior. In another set of experiments, period gene products were expressed under the control of per-gal4 or tim-gal4. This resulted in an increased level of PER protein in many brain cells and reduction of bioluminescence cycling reported by a per-luciferase transgene, especially in the case of per expression affected by tim-gal4. This indicates a disruption of the transcriptional feedback loop that is a part of the oscillatory mechanism underlying Drosophila's circadian rhythms. Consistent with this molecular defect, the proportion of rhythmic individuals in constant darkness was subnormal in flies expressing PER under the control of tim-gal4, and their behavior in light:dark cycles was abnormal.

Drosophila CRY is a deep brain circadian photoreceptor

cry (cryptochrome) is an important clock gene, and recent data indicate that it encodes a critical circadian photoreceptor in Drosophila. A mutant allele, cry(b), inhibits circadian photoresponses. Restricting CRY expression to specific fly tissues shows that CRY expression is needed in a cell-autonomous fashion for oscillators present in different locations. CRY overexpression in brain pacemaker cells increases behavioral photosensitivity, and this restricted CRY expression also rescues all circadian defects of cry(b) behavior. As wild-type pacemaker neurons express CRY, the results indicate that they make a striking contribution to all aspects of behavioral circadian rhythms and are directly light responsive. These brain neurons therefore contain an identified deep brain photoreceptor, as well as the other circadian elements: a central pace-maker and a behavioral output system.