Kyunghee Koh

Identification of SLEEPLESS, a Sleep-Promoting Factor

Sleep is an essential process conserved from flies to humans. The importance of sleep is underscored by its tight homeostatic control. Through a forward genetic screen, we identified a gene, sleepless, required for sleep in Drosophila. The sleepless gene encodes a brain-enriched, glycosylphosphatidylinositol-anchored protein. Loss of SLEEPLESS protein caused an extreme (>80%) reduction in sleep; a moderate reduction in SLEEPLESS had minimal effects on baseline sleep but markedly reduced the amount of recovery sleep after sleep deprivation. Genetic and molecular analyses revealed that quiver, a mutation that impairs Shaker-dependent potassium current, is an allele of sleepless. Consistent with this finding, Shaker protein levels were reduced in sleepless mutants. We propose that SLEEPLESS is a signaling molecule that connects sleep drive to lowered membrane excitability.

A Drosophila model for age-associated changes in sleep:wake cycles.

One of the most consistent behavioral changes that occurs with age in humans is the loss of sleep consolidation. This can be quite disruptive and yet little is known about its underlying basis. To better understand the effects of aging on sleep:wake cycles, we sought to study this problem in Drosophila melanogaster, a powerful system for research on aging and behavior. By assaying flies of different ages as well as monitoring individual flies constantly over the course of their lifetime, we found that the strength of sleep:wake cycles decreased and that sleep became more fragmented with age in Drosophila. These changes in sleep:wake cycles became faster or slower with manipulations of ambient temperature that decreased or increased lifespan, respectively, demonstrating that they are a function of physiological rather than chronological age. The effect of temperature on lifespan was not mediated by changes in overall activity level or sleep amount. Flies treated with the oxidative stress-producing reagent paraquat showed a breakdown of sleep:wake cycles similar to that seen with aging, leading us to propose that the accumulation of oxidative damage with age contributes to the changes in rhythm and sleep. Together, these findings establish Drosophila as a valuable model for studying age-associated sleep fragmentation and breakdown of rhythm strength, and indicate that these changes in sleep:wake cycles are an integral part of the physiological aging process.

JETLAG Resets the Drosophila Circadian Clock by Promoting Light-Induced Degradation of TIMELESS

Organisms ranging from bacteria to humans synchronize their internal clocks to daily cycles of light and dark. Photic entrainment of the Drosophila clock is mediated by proteasomal degradation of the clock protein TIMELESS (TIM). We have identified mutations in jetlag—a gene coding for an F-box protein with leucine-rich repeats—that result in reduced light sensitivity of the circadian clock. Mutant flies show rhythmic behavior in constant light, reduced phase shifts in response to light pulses, and reduced light-dependent degradation of TIM. Expression of JET along with the circadian photoreceptor cryptochrome (CRY) in cultured S2R cells confers light-dependent degradation onto TIM, thereby reconstituting the acute response + of the circadian clock to light in a cell culture system. Our results suggest that JET is essential for resetting the clock by transmitting light signals from CRY to TIM.

The Effects of Caffeine on Sleep in Drosophila Require PKA Activity, But Not the Adenosine Receptor

Caffeine is one of the most widely consumed stimulants in the world and has been proposed to promote wakefulness by antagonizing function of the adenosine A2A receptor. Here, we show that chronic administration of caffeine reduces and fragments sleep in Drosophila and also lengthens circadian period. To identify the mechanisms underlying these effects of caffeine, we first generated mutants of the only known adenosine receptor in flies (dAdoR), which by sequence is most similar to the mammalian A2A receptor. Mutants lacking dAdoR have normal amounts of baseline sleep, as well as normal homeostatic responses to sleep deprivation. Surprisingly, these mutants respond normally to caffeine. On the other hand, the effects of caffeine on sleep and circadian rhythms are mimicked by a potent phosphodiesterase inhibitor, IBMX (3-isobutyl-1-methylxanthine). Using in vivo fluorescence resonance energy transfer imaging, we find that caffeine induces widespread increase in cAMP levels throughout the brain. Finally, the effects of caffeine on sleep are blocked in flies that have reduced neuronal PKA activity. We suggest that chronic administration of caffeine promotes wakefulness in Drosophila, at least in part, by inhibiting cAMP phosphodiesterase activity.

SLEEPLESS, a Ly-6/neurotoxin family member, regulates the levels, localization and activity of Shaker

Sleep is a whole-organism phenomenon accompanied by global changes in neural activity. We previously identified SLEEPLESS (SSS) as a glycosylphosphatidyl inositol–anchored protein required for sleep in Drosophila. Here we found that SSS is critical for regulating the sleep-modulating potassium channel Shaker. SSS and Shaker shared similar expression patterns in the brain and specifically affected each others expression levels. sleepless (sss) loss-of-function mutants exhibited altered Shaker localization, reduced Shaker current density and slower Shaker current kinetics. Transgenic expression of sss in sss mutants rescued defects in Shaker expression and activity cell-autonomously and suggested that SSS functions in wake-promoting, cholinergic neurons. In heterologous cells, SSS accelerated the kinetics of Shaker currents and was co-immunoprecipitated with Shaker, suggesting that SSS modulates Shaker activity via a direct interaction. SSS is predicted to belong to the Ly-6/neurotoxin superfamily, suggesting a mechanism for regulation of neuronal excitability by endogenous toxin-like molecules.

WIDE AWAKE Mediates the Circadian Timing of Sleep Onset

How the circadian clock regulates the timing of sleep is poorly understood. Here, we identify a Drosophila mutant, wide awake (wake), that exhibits a marked delay in sleep onset at dusk. Loss of WAKE in a set of arousal-promoting clock neurons, the large ventrolateral neurons (l-LNvs), impairs sleep onset. WAKE levels cycle, peaking near dusk, and the expression of WAKE in l-LNvs is Clock dependent. Strikingly, Clock and cycle mutants also exhibit a profound delay in sleep onset, which can be rescued by restoring WAKE expression in LNvs. WAKE interacts with the GABAA receptor Resistant to Dieldrin (RDL), upregulating its levels and promoting its localization to the plasma membrane. In wake mutant l-LNvs, GABA sensitivity is decreased and excitability is increased at dusk. We propose that WAKE acts as a clock output molecule specifically for sleep, inhibiting LNvs at dusk to promote the transition from wake to sleep.

Genetic and Anatomical Basis of the Barrier Separating Wakefulness and Anesthetic-Induced Unresponsiveness

A robust, bistable switch regulates the fluctuations between wakefulness and natural sleep as well as those between wakefulness and anesthetic-induced unresponsiveness. We previously provided experimental evidence for the existence of a behavioral barrier to transitions between these states of arousal, which we call neural inertia. Here we show that neural inertia is controlled by processes that contribute to sleep homeostasis and requires four genes involved in electrical excitability: Sh, sss, na and unc79. Although loss of function mutations in these genes can increase or decrease sensitivity to anesthesia induction, surprisingly, they all collapse neural inertia. These effects are genetically selective: neural inertia is not perturbed by loss-of-function mutations in all genes required for the sleep/wake cycle. These effects are also anatomically selective: sss acts in different neurons to influence arousal-promoting and arousal-suppressing processes underlying neural inertia. Supporting the idea that anesthesia and sleep share some, but not all, genetic and anatomical arousal-regulating pathways, we demonstrate that increasing homeostatic sleep drive widens the neural inertial barrier. We propose that processes selectively contributing to sleep homeostasis and neural inertia may be impaired in pathophysiological conditions such as coma and persistent vegetative states.

An Isoform-Specific Mutant Reveals a Role of PDP1ε in the Circadian Oscillator

The Drosophila PAR domain protein 1 (Pdp1) gene encodes a transcription factor with multiple functions. One isoform, PDP1ε, was proposed to be an essential activator of the core clock gene, Clock (Clk). However, a central clock function for PDP1ε was recently disputed, and genetic analysis has been difficult due to developmental lethality of Pdp1-null mutants. Here we report the discovery of a mutation that specifically disrupts the Pdp1ε isoform. Homozygous Pdp1ε mutants are viable and exhibit arrhythmic circadian behavior in constant darkness and also in the presence of light:dark cycles. Importantly, the mutants show diminished expression of CLK and PERIOD (PER) in the central clock cells. In addition, expression of PDF (pigment-dispersing factor) is reduced in a subset of the central clock cells. Loss of Pdp1ε also alters the phosphorylation status of the CLK protein and disrupts cyclic expression of a per-luciferase reporter in peripheral clocks under free-running conditions. Transgenic expression of PDP1ε in clock neurons of Pdp1ε mutants can restore rhythmic circadian behavior. However, transgenic expression of CLK in these mutants rescues the expression of PER in the central clock, but fails to restore behavioral rhythms, suggesting that PDP1ε has effects outside the core molecular clock. Together, these data support a model in which PDP1ε functions in the central circadian oscillator as well as in the output pathway.

The COP9 Signalosome Is Required for Light-Dependent Timeless Degradation and Drosophila Clock Resetting

The ubiquitin–proteasome system plays a major role in the rhythmic accumulation and turnover of molecular clock components. In turn, these ∼24 h molecular rhythms drive circadian rhythms of behavior and physiology. In Drosophila, the ubiquitin–proteasome system also plays a critical role in light-dependent degradation of the clock protein Timeless (TIM), a key step in the entrainment of the molecular clocks to light–dark cycles. Here, we investigated the role of the COP9 signalosome (CSN), a general regulator of protein degradation, in fly circadian rhythms. We found that null mutations in the genes encoding the CSN4 and CSN5 subunits prevent normal TIM degradation by light in the pacemaker lateral neurons (LNs) as does LN-specific expression of a dominant-negative CSN5 transgene. These defects are accompanied by strong reductions in behavioral phase shifts of adult flies lacking normal CSN5 activity in LNs. Defects in TIM degradation and resetting of behavioral phases were rescued by overexpression of Jetlag (JET), the F-box protein required for light-mediated TIM degradation. Flies lacking normal CSN activity in all clock neurons are rhythmic in constant light, a phenotype previously associated with jet mutants. Together, these data indicate that JET and the CSN lie in a common pathway leading to light-dependent TIM degradation. Surprisingly, we found that manipulations that strongly inhibit CSN activity had minimal effects on circadian rhythms in constant darkness, indicating a specific role for the CSN in light-mediated TIM degradation.

Molecular Analysis of Sleep: Wake Cycles in Drosophila

Sleep is controlled by two major regulatory systems: a circadian system that drives it with a 24-hour periodicity and a home-ostatic system that ensures that adequate amounts of sleep are obtained. We are using the fruit fly Drosophila melanogaster to understand both types of regulation. With respect to circadian control, we have identified molecular mechanisms that are critical for the generation of a clock. Our recent efforts have focused on the analysis of posttranslational mechanisms, specifically the action of different phosphatases that control the phosphorylation and thereby the stability and/or nuclear localization of circadian clock proteins period (PER) and timeless (TIM). Resetting the clock in response to light is also mediated through posttranslational events that target TIM for degradation by the proteasome pathway; a recently identified ubiquitin ligase, jet lag (JET), is required for this response. Our understanding of the homeostatic control of sleep is in its early stages. We have found that mushroom bodies, which are a site of synaptic plasticity in the fly brain, are important for the regulation of sleep. In addition, through analysis of genes expressed under different behavioral states, we have identified some that are up-regulated during sleep deprivation. Thus, the Drosophila model allows the use of cellular and molecular approaches that should ultimately lead to a better understanding of sleep biology.