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Amita Sehgal

Publications and source records attributed to Amita Sehgal.

At least 19 recordsLinked to original sources
PubMed2006

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.

Aging↗
PubMed2006

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.

Amino Acid Sequence↗
PubMed2006

Sleep in Drosophila is regulated by adult mushroom bodies.

Sleep is one of the few major whole-organ phenomena for which no function and no underlying mechanism have been conclusively demonstrated. Sleep could result from global changes in the brain during wakefulness or it could be regulated by specific loci that recruit the rest of the brain into the electrical and metabolic states characteristic of sleep. Here we address this issue by exploiting the genetic tractability of the fruitfly, Drosophila melanogaster, which exhibits the hallmarks of vertebrate sleep. We show that large changes in sleep are achieved by spatial and temporal enhancement of cyclic-AMP-dependent protein kinase (PKA) activity specifically in the adult mushroom bodies of Drosophila. Other manipulations of the mushroom bodies, such as electrical silencing, increasing excitation or ablation, also alter sleep. These results link sleep regulation to an anatomical locus known to be involved in learning and memory.

Aging↗
PubMed2006

A sleep-promoting role for the Drosophila serotonin receptor 1A.

BACKGROUND: Although sleep is an important process essential for life, its regulation is poorly understood. The recently developed Drosophila model for sleep provides a powerful system to genetically and pharmacologically identify molecules that regulate sleep. Serotonin is an important neurotransmitter known to affect many behaviors, but its role in sleep remains controversial. RESULTS: We generated or obtained flies with genetically altered expression of each of three Drosophila serotonin receptor subtypes (d5-HT1A, d5-HT1B, and d5-HT2) and assayed them for baseline sleep phenotypes. The data indicated a sleep-regulating role for the d5-HT1A receptor. d5-HT1A mutant flies had short and fragmented sleep, which was rescued by expressing the receptor in adult mushroom bodies, a structure associated with learning and memory in Drosophila. Neither the d5-HT2 receptor nor the d5-HT1B receptor, which was previously implicated in circadian regulation, had any effect on baseline sleep, indicating that serotonin affects sleep and circadian rhythms through distinct receptors. Elevating serotonin levels, either pharmacologically or genetically, enhanced sleep in wild-type flies. In addition, serotonin promoted sleep in some short-sleep mutants, suggesting that it can compensate for some sleep deficits. CONCLUSIONS: These data show that serotonin promotes baseline sleep in Drosophila. They also link the regulation of sleep behavior by serotonin to a specific receptor in a distinct region of the fly brain.

Animals↗
PubMed2005

Serotonin modulates circadian entrainment in Drosophila.

Entrainment of the Drosophila circadian clock to light involves the light-induced degradation of the clock protein timeless (TIM). We show here that this entrainment mechanism is inhibited by serotonin, acting through the Drosophila serotonin receptor 1B (d5-HT1B). d5-HT1B is expressed in clock neurons, and alterations of its levels affect molecular and behavioral responses of the clock to light. Effects of d5-HT1B are synergistic with a mutation in the circadian photoreceptor cryptochrome (CRY) and are mediated by SHAGGY (SGG), Drosophila glycogen synthase kinase 3beta (GSK3beta), which phosphorylates TIM. Levels of serotonin are decreased in flies maintained in extended constant darkness, suggesting that modulation of the clock by serotonin may vary under different environmental conditions. These data identify a molecular connection between serotonin signaling and the central clock component TIM and suggest a homeostatic mechanism for the regulation of circadian photosensitivity in Drosophila.

5-Hydroxytryptophan↗
PubMed2005

Pharmacological rescue of synaptic plasticity, courtship behavior, and mushroom body defects in a Drosophila model of fragile X syndrome.

Fragile X syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. A Drosophila model for Fragile X syndrome, based on the loss of dfmr1 activity, exhibits phenotypes that bear similarity to Fragile X-related symptoms. Herein, we demonstrate that treatment with metabotropic glutamate receptor (mGluR) antagonists or lithium can rescue courtship and mushroom body defects observed in these flies. Furthermore, we demonstrate that dfmr1 mutants display cognitive deficits in experience-dependent modification of courtship behavior, and treatment with mGluR antagonists or lithium restores these memory defects. These findings implicate enhanced mGluR signaling as the underlying cause of the cognitive, as well as some of the behavioral and neuronal, phenotypes observed in the Drosophila Fragile X model. They also raise the possibility that compounds having similar effects on metabotropic glutamate receptors may ameliorate cognitive and behavioral defects observed in Fragile X patients.

Animals↗
PubMed2005

Drosophila melanogaster: an insect model for fundamental studies of sleep.

In 2000, Drosophila melanogaster joined the ranks of vertebrates and invertebrates with a defined behavioral sleep state. The characterization of this sleep state revealed striking similarities to sleep in humans: sleep in flies has both circadian and homeostatic components, it is influenced by sex and age, and it is affected by pharmacological agents such as caffeine and antihistamines. As in mammals, arousal thresholds in flies increase with sleep deprivation. Furthermore, changes in brain electrical activity accompany the change from wake to sleep states. Not only do flies and vertebrates share these behavioral and physiological traits of sleep, but they are likely to share at least some genetic mechanisms underlying the regulation of sleep as well. This article reviews the methods currently used to identify and characterize the Drosophila sleep state. As these methods become more refined and our understanding of Drosophila sleep more detailed, the powerful techniques afforded by this organism are likely to unveil deep insights into the function(s) and regulatory mechanisms of sleep.

Animals↗
PubMed2005

The thymus is similar to the testis in its pattern of circadian clock gene expression.

The molecular basis for the circadian clock in mammals consists of a number of genes and proteins that form transcription-translation feedback loops. These loops result in a 24-h rhythm in the expression of mRNA and protein levels. Although the anatomical site of the central circadian clock is the SCN of the hypothalamus, all of the circadian clock genes are expressed in tissues other than the brain. Moreover, cyclic gene and protein expression occurs in most of these tissues. The best known exception to this rule is the testis, which shows constant rather than cyclic expression of circadian clock genes. Indeed, the testis of multiple animal species displays constant circadian clock gene expression. In recent work, the authors showed that the thymus is similar to the testis in that expression of circadian clock genes is either constant over a 24-h period or cycles with a dampened amplitude, depending on which gene is examined. In the current study, they extend and confirm their findings regarding noncyclic circadian clock gene and protein expression in the testis and the thymus. More important, they also show that expression of these genes in both testis and thymus does not depend on the transcriptional activator, CLOCK, which is necessary for cyclic gene expression in the SCN and in other tissues. These results extend the molecular similarities between the thymus and the testis and suggest that similar mechanisms are at work for regulating expression of circadian clock genes in both tissues. One commonality between these 2 organs is that they are composed primarily of differentiating cells. The authors hypothesize that the circadian clock is not operational in immature, differentiating cells. Possibly, the clock starts in mature cells upon receipt of an initiating signal.

Animals↗
PubMed2004

Why a fly? Using Drosophila to understand the genetics of circadian rhythms and sleep.

Among simple model systems, Drosophila has specific advantages for neurobehavioral investigations. It has been particularly useful for understanding the molecular basis of circadian rhythms. In addition, the genetics of fruit-fly sleep are beginning to develop. This review summarizes the current state of understanding of circadian rhythms and sleep in the fruit fly for the readers of Sleep. We note where information is available in mammals, for comparison with findings in fruit flies, to provide an evolutionary perspective, and we focus on recent findings and new questions. We propose that sleep-specific neural activity may alter cellular function and thus accomplish the restorative function or functions of sleep. In conclusion, we sound some cautionary notes about some of the complexities of working with this "simple" organism.

Animals↗
PubMed2004

Posttranslational regulation of Drosophila PERIOD protein by protein phosphatase 2A.

The posttranscriptional mechanisms that control the cycling of circadian clock protein levels are not known. Here we demonstrate a role for protein phosphatase 2A (PP2A) in the cyclic expression of the PER protein. PP2A regulatory subunits TWS and WDB target PER and stabilize it in S2 cells. In adult fly heads, expression of tws cycles robustly under control of the circadian clock. Hypomorphic tws mutants show delayed accumulation of PER, while overexpression of tws in clock neurons produces shorter, weaker rhythms. Reduction of PP2A activity reduces PER expression in central clock neurons and results in long periods and arrhythmia. In addition, overexpression of the PP2A catalytic subunit results in loss of behavioral rhythms and constitutive nuclear expression of PER. PP2A also affects PER phosphorylation in vitro and in vivo. We propose that the posttranslational mechanisms that drive cycling of PER require the rhythmic expression of PP2A.

Animals↗
PubMed2004

Pattern of distribution and cycling of SLOB, Slowpoke channel binding protein, in Drosophila.

BACKGROUND: SLOB binds to and modulates the activity of the Drosophila Slowpoke (dSlo) calcium activated potassium channel. Recent microarray analyses demonstrated circadian cycling of slob mRNA. RESULTS: We report the mRNA and protein expression pattern of slob in Drosophila heads. slob transcript is present in the photoreceptors, optic lobe, pars intercerebralis (PI) neurons and surrounding brain cortex. SLOB protein exhibits a similar distribution pattern, and we show that it cycles in Drosophila heads, in photoreceptor cells and in neurosecretory cells of the PI. The cycling of SLOB is altered in various clock gene mutants, and SLOB is expressed in ectopic locations in tim01 flies. We also demonstrate that SLOB no longer cycles in the PI neurons of Clkjrk flies, and that SLOB expression is reduced in the PI neurons of flies that lack pigment dispersing factor (PDF), a neuropeptide secreted by clock cells. CONCLUSIONS: These data are consistent with the idea that SLOB may participate in one or more circadian pathways in Drosophila.

Animals↗
PubMed2003

Novel insights into the regulation of the timeless protein.

In the Drosophila circadian clock, period (per) and its partner, timeless (tim), play a central role in the negative limb of an autoregulatory feedback loop. Unlike per, the dosage of which affects the frequency (tau) of the circadian cycle, we found that increasing copies of the tim gene has no effect on clock period length. The use of the tim promoter to express per results in a shortening of circadian period, also indicating that the regulation of tim is different from that of per. Drosophila TIM is similar to the mammalian circadian protein mPER2 in that it shuttles independently between the nucleus and cytoplasm both in vivo and in vitro. Contrary to the current model that PER and TIM heterodimerization is a prerequisite for their nuclear entry, PER is not required to transport TIM into nuclei, although it influences TIM localization and vice versa. Blocking nuclear export led to increased nuclear expression of TIM in S2 cells and in wild-type and per01 larvae, suggesting that PER may be required for nuclear retention of TIM. Unlike PER, nuclear TIM alone has no ability to repress transcription. We propose that TIM drives cycles of PER expression by regulating its stability, and in turn, PER retains TIM in the nucleus, either for the regulation of its own stability or for a novel nuclear role of TIM.

ARNTL Transcription Factors↗
PubMed2003

An abrupt shift in the day/night cycle causes desynchrony in the mammalian circadian center.

The suprachiasmatic nucleus (SCN) is the neuroanatomical locus of the mammalian circadian pacemaker. Here we demonstrate that an abrupt shift in the light/dark (LD) cycle disrupts the synchronous oscillation of circadian components in the rat SCN. The phases of the RNA cycles of the period genes Per1 and Per2 and the cryptochrome gene Cry1 shifted rapidly in the ventrolateral, photoreceptive region of the SCN, but were relatively slow to shift in the dorsomedial region. During the period of desynchrony, the animals displayed increased nighttime rest, the timing of which was inversely correlated with the expression of Per1 mRNA in the dorsomedial SCN. Molecular resynchrony required approximately 6 d after a 10 hr delay and 9 approximately 13 d after a 6 hr advance of the LD cycle and was accompanied by the reemergence of normal rest-activity patterns. This dissociation and slow resynchronization of endogenous oscillators within the SCN after an LD cycle shift suggests a mechanism for the physiological symptoms that constitute jet lag.

Animals↗
PubMed2003

Circadian control of eclosion: interaction between a central and peripheral clock in Drosophila melanogaster.

Drosophila melanogaster display overt circadian rhythms in rest:activity behavior and eclosion. These rhythms have an endogenous period of approximately 24 hr and can adjust or "entrain" to environmental inputs such as light. Circadian rhythms depend upon a functioning molecular clock that includes the core clock genes period and timeless (reviewed in and ). Although we know that a clock in the lateral neurons (LNs) of the brain controls rest:activity rhythms, the cellular basis of eclosion rhythms is less well understood. We show that the LN clock is insufficient to drive eclosion rhythms. We establish that the prothoracic gland (PG), a tissue required for fly development, contains a functional clock at the time of eclosion. This clock is required for normal eclosion rhythms. However, both the PG clock function and eclosion rhythms require the presence of LNs. In addition, we demonstrate that pigment-dispersing factor (PDF), a neuropeptide secreted from LNs, is necessary for the PG clock and eclosion rhythms. Unlike other clocks in the fly periphery, the PG is similar to mammalian peripheral oscillators because it depends upon input, including PDF, from central pacemaker cells. This is the first report of a peripheral clock necessary for a circadian event.

Animals↗
PubMed2003

Non-cyclic and developmental stage-specific expression of circadian clock proteins during murine spermatogenesis.

The central circadian clock in mammals is housed in the brain and is based on cyclic transcription and translation of clock proteins. How the central clock regulates peripheral organ function is unclear. However, cyclic expression of circadian genes in peripheral tissues is well established, suggesting that these tissues have their own endogenous oscillators. Reproduction is a process influenced by circadian rhythms in many organisms, thus making the testis an attractive model for studying clock function in peripheral organs. However, results addressing cyclic expression of clock genes in the mammalian testis are inconsistent. To resolve this issue, RNA was extracted from testes of mice at various times of day. Expression of the circadian clock genes mPer1, mPer2, Bmal1, Clock, mCry1, and Npas2 was constant at all times. Immunohistochemical localization of mPER1 and CLOCK proteins revealed restricted expression only in cells at specific developmental stages of spermatogenesis. For mPER1, these stages are the spermatogonia and the condensing spermatids. In contrast, CLOCK expression was restricted to round spermatids, specifically within the developing acrosome. Expression of mPER1 and CLOCK was constant at all times of day. These results suggest that clock proteins have noncircadian functions in spermatogenesis. Noncircadian expression of clock genes was also found in the thymus, which, like the testis, is composed primarily of differentiating cells. We propose that cyclic expression of clock genes is suspended during cellular differentiation.

Acrosome↗
PubMed2003

Gender dimorphism in the role of cycle (BMAL1) in rest, rest regulation, and longevity in Drosophila melanogaster.

The central clock is generally thought to provide timing information for rest/activity but not to otherwise participate in regulation of these states. To test the hypothesis that genes that are components of the molecular clock also regulate rest, the authors quantified the duration and intensity of consolidated rest and activity for the four viable Drosophila mutations of the central clock that lead to arrhythmic locomotor behavior and for the pdf mutant that lacks pigment-dispersing factor, an output neuropeptide. Only the cycle (cyc01) and Clock (Clk(Jrk)) mutants had abnormalities that mapped to the mutant locus, namely, decreased consolidated rest and grossly extended periods of activity. All mutants with the exception of the cyc01 fly exhibited a qualitatively normal compensatory rebound after rest deprivation. This abnormal response in cyc01 was sexually dimorphic, being reduced or absent in males and exaggerated in females. Finally, the cyc01 mutation shortened the life span of male flies. These data indicate that cycle regulates rest and life span in male Drosophila.

ARNTL Transcription Factors↗
PubMed2003

A fly's eye view of circadian entrainment.

The Drosophila circadian clock is an ideal model system for teasing out the molecular mechanisms of circadian behavior and the means by which animals synchronize to day-night cycles. The clock that drives behavioral rhythms, located in the lateral neurons in the central brain, consists of a feedback loop of the circadian genes period (per) and timeless (tim). The molecular cycle, roughly 24 h long, is constantly reset by the environment. This review focuses on the main input pathways of the dominant circadian zeitgeber, light. Light acts directly on the clock primarily through cryptochrome (cry), a deep brain blue-light photoreceptor. CRY activation causes rapid TIM degradation, which is a predicted means of resetting the clock both on a daily basis at dawn and on an acute basis following an entraining light pulse during the night hours. In the absence of cry, the clock can still be driven by photic input through the visual system, though the mechanisms underlying this entrainment are unclear. Temperature can also entrain the clock, although the mechanisms by which this occurs are also unclear.

Animals↗