Circadian disruption of memory consolidation in .

The role of the circadian system in memory formation is an important question in neurobiology. Despite this hypothesis being intuitively appealing, the existing data is confusing. Recent work in has helped to clarify certain aspects of the problem, but the emerging sense is that the likely mechanisms are more complex than originally conceptualized.

TRITHORAX-dependent arginine methylation of HSP68 mediates circadian repression by PERIOD in the monarch butterfly.

Transcriptional repression drives feedback loops that are central to the generation of circadian (∼24-h) rhythms. In mammals, circadian repression of circadian locomotor output cycles kaput, and brain and muscle ARNT-like 1 (CLOCK:BMAL1)-mediated transcription is provided by a complex formed by PERIOD (PER) and CRYPTOCHROME (CRY) proteins. PER initiates transcriptional repression by binding CLK:BMAL1, which ultimately results in their removal from DNA.

CLOCKWORK ORANGE promotes CLOCK-CYCLE activation via the putative Drosophila ortholog of CLOCK INTERACTING PROTEIN CIRCADIAN.

The Drosophila circadian clock is driven by a transcriptional feedback loop in which CLOCK-CYCLE (CLK-CYC) binds E-boxes to transcribe genes encoding the PERIOD-TIMELESS (PER-TIM) repressor, which releases CLK-CYC from E-boxes to inhibit transcription. CLOCKWORK ORANGE (CWO) reinforces PER-TIM repression by binding E-boxes to maintain PER-TIM bound CLK-CYC off DNA, but also promotes CLK-CYC transcription through an unknown mechanism. To determine how CWO activates CLK-CYC transcription, we identified CWO target genes that are upregulated in the absence of CWO repression, conserved in mammals, and preferentially expressed in brain pacemaker neurons.

Crosstalk between transcripts, proteins, and regulatory elements controlling circadian rhythms and development in .

The () gene encodes a transcriptional repressor required for development as well as circadian behavior in adults. Alternate first exons produce transcripts predicted to produce a short VRI isoform during development and long VRI in adults. A mutant ( ) lacking long VRI transcripts is viable, confirming that short VRI is sufficient for developmental functions, yet behavioral rhythms in flies persist, showing that short VRI is sufficient for clock output.

Proteomic analysis of Drosophila CLOCK complexes identifies rhythmic interactions with SAGA and Tip60 complex component NIPPED-A.

Circadian clocks keep time via ~ 24 h transcriptional feedback loops. In Drosophila, CLOCK-CYCLE (CLK-CYC) activators and PERIOD-TIMELESS (PER-TIM) repressors are feedback loop components whose transcriptional status varies over a circadian cycle. Although changes in the state of activators and repressors has been characterized, how their status is translated to transcriptional activity is not understood.

Reflections on contributing to "big discoveries" about the fly clock: Our fortunate paths as post-docs with 2017 Nobel laureates Jeff Hall, Michael Rosbash, and Mike Young.

In the early 1980s Jeff Hall and Michael Rosbash at Brandeis University and Mike Young at Rockefeller University set out to isolate the () gene, which was recovered in a revolutionary genetic screen by Ron Konopka and Seymour Benzer for mutants that altered circadian behavioral rhythms. Over the next 15 years the Hall, Rosbash and Young labs made a series of groundbreaking discoveries that defined the molecular timekeeping mechanism and formed the basis for them being awarded the 2017 Nobel Prize in Physiology or Medicine. Here the authors recount their experiences as post-docs in the Hall, Rosbash and Young labs from the mid-1980s to the mid-1990s, and provide a perspective of how basic research conducted on a simple model system during that era profoundly influenced the direction of the clocks field and established novel approaches that are now standard operating procedure for studying complex behavior.

VRILLE Controls PDF Neuropeptide Accumulation and Arborization Rhythms in Small Ventrolateral Neurons to Drive Rhythmic Behavior in Drosophila.

In Drosophila, the circadian clock is comprised of transcriptional feedback loops that control rhythmic gene expression responsible for daily rhythms in physiology, metabolism, and behavior. The core feedback loop, which employs CLOCK-CYCLE (CLK-CYC) activators and PERIOD-TIMELESS (PER-TIM) repressors to drive rhythmic transcription peaking at dusk, is required for circadian timekeeping and overt behavioral rhythms. CLK-CYC also activates an interlocked feedback loop, which uses the PAR DOMAIN PROTEIN 1ε (PDP1ε) activator and the VRILLE (VRI) repressor to drive rhythmic transcription peaking at dawn.

CLOCK stabilizes CYCLE to initiate clock function in .

The circadian clock keeps time via transcriptional feedback loops. These feedback loops are initiated by CLOCK-CYCLE (CLK-CYC) heterodimers, which activate transcription of genes encoding the feedback repressors PERIOD and TIMELESS. Circadian clocks normally operate in ∼150 brain pacemaker neurons and in many peripheral tissues in the head and body, but can also be induced by expressing CLK in nonclock cells.

Vertebrate-like CRYPTOCHROME 2 from monarch regulates circadian transcription via independent repression of CLOCK and BMAL1 activity.

Circadian repression of CLOCK-BMAL1 by PERIOD and CRYPTOCHROME (CRY) in mammals lies at the core of the circadian timekeeping mechanism. CRY repression of CLOCK-BMAL1 and regulation of circadian period are proposed to rely primarily on competition for binding with coactivators on an α-helix located within the transactivation domain (TAD) of the BMAL1 C terminus. This model has, however, not been tested in vivo.

Drosophila CRY Entrains Clocks in Body Tissues to Light and Maintains Passive Membrane Properties in a Non-clock Body Tissue Independent of Light.

Circadian (∼24 hr) clocks regulate daily rhythms in physiology, metabolism, and behavior via cell-autonomous transcriptional feedback loops. In Drosophila, the blue-light photoreceptor CRYPTOCHROME (CRY) synchronizes these feedback loops to light:dark cycles by binding to and degrading TIMELESS (TIM) protein. CRY also acts independently of TIM in Drosophila to alter potassium channel conductance in arousal neurons after light exposure, and in many animals CRY acts independently of light to repress rhythmic transcription.

CLOCKWORK ORANGE Enhances PERIOD Mediated Rhythms in Transcriptional Repression by Antagonizing E-box Binding by CLOCK-CYCLE.

The Drosophila circadian oscillator controls daily rhythms in physiology, metabolism and behavior via transcriptional feedback loops. CLOCK-CYCLE (CLK-CYC) heterodimers initiate feedback loop function by binding E-box elements to activate per and tim transcription. PER-TIM heterodimers then accumulate, bind CLK-CYC to inhibit transcription, and are ultimately degraded to enable the next round of transcription.

An RNAi Screen To Identify Protein Phosphatases That Function Within the Drosophila Circadian Clock.

Circadian clocks in eukaryotes keep time via cell-autonomous transcriptional feedback loops. A well-characterized example of such a transcriptional feedback loop is in Drosophila, where CLOCK-CYCLE (CLK-CYC) complexes activate transcription of period (per) and timeless (tim) genes, rising levels of PER-TIM complexes feed-back to repress CLK-CYC activity, and degradation of PER and TIM permits the next cycle of CLK-CYC transcription. The timing of CLK-CYC activation and PER-TIM repression is regulated posttranslationally, in part through rhythmic phosphorylation of CLK, PER, and TIM.

The Drosophila Receptor Protein Tyrosine Phosphatase LAR Is Required for Development of Circadian Pacemaker Neuron Processes That Support Rhythmic Activity in Constant Darkness But Not during Light/Dark Cycles.

Unlabelled: InDrosophila, a transcriptional feedback loop that is activated by CLOCK-CYCLE (CLK-CYC) complexes and repressed by PERIOD-TIMELESS (PER-TIM) complexes keeps circadian time. The timing of CLK-CYC activation and PER-TIM repression is regulated post-translationally, in part through rhythmic phosphorylation of CLK, PER, and TIM. Although kinases that control PER, TIM, and CLK levels, activity, and/or subcellular localization have been identified, less is known about phosphatases that control clock protein dephosphorylation.

Circadian Activators Are Expressed Days before They Initiate Clock Function in Late Pacemaker Neurons from Drosophila.

Circadian pacemaker neurons in the Drosophila brain control daily rhythms in locomotor activity. These pacemaker neurons can be subdivided into early or late groups depending on whether rhythms in period (per) and timeless (tim) expression are initiated at the first instar (L1) larval stage or during metamorphosis, respectively. Because CLOCK-CYCLE (CLK-CYC) heterodimers initiate circadian oscillator function by activating per and tim transcription, a Clk-GFP transgene was used to mark when late pacemaker neurons begin to develop.

ChIPping away at the Drosophila clock.

In eukaryotes, the circadian clock controls 24h rhythms in physiology, metabolism, and behavior via cell autonomous transcriptional feedback loops. These feedback loops keep circadian time and control rhythmic outputs by driving rhythms in transcription; thus, it is important to determine when clock transcription factors bind their target sequences in vivo to promote or repress transcription. Interactions between proteins and DNA can be measured in cells, tissue, or whole organisms using a technique called chromatin immunoprecipitation (ChIP).

Phosphorylation of a central clock transcription factor is required for thermal but not photic entrainment.

Transcriptional/translational feedback loops drive daily cycles of expression in clock genes and clock-controlled genes, which ultimately underlie many of the overt circadian rhythms manifested by organisms. Moreover, phosphorylation of clock proteins plays crucial roles in the temporal regulation of clock protein activity, stability and subcellular localization. dCLOCK (dCLK), the master transcription factor driving cyclical gene expression and the rate-limiting component in the Drosophila circadian clock, undergoes daily changes in phosphorylation.

Effects of TWIN-OF-EYELESS on Clock Gene Expression and Central-Pacemaker Neuron Development in Drosophila.

Circadian oscillators are autonomous molecular rhythms that reside in cells to align whole-organism physiology and behavior to the 24-h day. In flies, as in mammals, the oscillator operates in cells that coexpress CLOCK (CLK) and CYCLE (CYC). Recent work in Drosophila has shown that CLK is unique in its ability to generate heterologous oscillators, indicating that Clk gene expression defines the circadian cell fate.

Phosphorylation of the transcription activator CLOCK regulates progression through a ∼ 24-h feedback loop to influence the circadian period in Drosophila.

Circadian (≅ 24 h) clocks control daily rhythms in metabolism, physiology, and behavior in animals, plants, and microbes. In Drosophila, these clocks keep circadian time via transcriptional feedback loops in which clock-cycle (CLK-CYC) initiates transcription of period (per) and timeless (tim), accumulating levels of PER and TIM proteins feed back to inhibit CLK-CYC, and degradation of PER and TIM allows CLK-CYC to initiate the next cycle of transcription. The timing of key events in this feedback loop are controlled by, or coincide with, rhythms in PER and CLK phosphorylation, where PER and CLK phosphorylation is high during transcriptional repression.

Circadian clocks: the tissue is the issue.

The circadian clock uses a widely expressed pair of clock activators to drive tissue-specific rhythms in target gene expression. A new study sheds light on this tissue specificity by showing that binding of clock activators and tissue-specific transcription factors to closely associated target sites enables cooperative activation of target genes in different tissues.

Circadian timekeeping and output mechanisms in animals.

Daily rhythms in animal behavior, physiology and metabolism are driven by cell-autonomous clocks that are synchronized by environmental cycles, but maintain ∼24 hours rhythms even in the absence of environmental cues. These clocks keep time and control overt rhythms via interlocked transcriptional feedback loops, making it imperative to define the mechanisms that drive rhythmic transcription within these loops and on a genome-wide scale. Recent work identifies novel post-transcriptional and post-translational mechanisms that govern progression through these feedback loops to maintain a period of ∼24 hours.

Circadian rhythm of temperature preference and its neural control in Drosophila.

A daily body temperature rhythm (BTR) is critical for the maintenance of homeostasis in mammals. Whereas mammals use internal energy to regulate body temperature, ectotherms typically regulate body temperature behaviorally [1]. Some ectotherms maintain homeostasis via a daily temperature preference rhythm (TPR) [2], but the underlying mechanisms are largely unknown.

Molecular genetic analysis of circadian timekeeping in Drosophila.

A genetic screen for mutants that alter circadian rhythms in Drosophila identified the first clock gene-the period (per) gene. The per gene is a central player within a transcriptional feedback loop that represents the core mechanism for keeping circadian time in Drosophila and other animals. The per feedback loop, or core loop, is interlocked with the Clock (Clk) feedback loop, but whether the Clk feedback loop contributes to circadian timekeeping is not known.

NEMO kinase contributes to core period determination by slowing the pace of the Drosophila circadian oscillator.

The Drosophila circadian oscillator is comprised of transcriptional feedback loops that are activated by CLOCK (CLK) and CYCLE (CYC) and repressed by PERIOD (PER) and TIMELESS (TIM) [1]. The timing of CLK-CYC activation and PER-TIM repression is regulated posttranslationally, in part through rhythmic phosphorylation of CLK, PER, and TIM [2-4]. Although kinases that control PER and TIM levels and subcellular localization have been identified [5-10], additional kinases are predicted to target PER, TIM, and/or CLK to promote time-specific transcriptional repression.

Time to taste: circadian clock function in the Drosophila gustatory system.

Circadian clocks keep time in the digestive, circulatory, reproductive, excretory and nervous systems even in absence of external cues. Central oscillators in the brain control locomotor activity of organisms ranging from fruit flies to man, but the functions of the clocks in peripheral nervous system are not well understood. The presence of autonomous peripheral oscillators in the major taste organ of Drosophila, the proboscis, prompted us to test whether gustatory responses are under control of the circadian clock.

DN1(p) circadian neurons coordinate acute light and PDF inputs to produce robust daily behavior in Drosophila.

Background: Daily behaviors in animals are determined by the interplay between internal timing signals from circadian clocks and environmental stimuli such as light. How these signals are integrated to produce timely and adaptive behavior is unclear. The fruit fly Drosophila exhibits clock-driven activity increases that anticipate dawn and dusk and free-running rhythms under constant conditions.