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The mechanisms of sleep have been studied in Drosophila melanogaster, which show behaviors reminiscent of sleep in vertebrates. A new study identifies a growth factor signaling pathway involved in sleep regulation and consolidation in this model. Inhibiting this pathway causes a sleep pattern that is similar to insomnia in humans.
A good night’s sleep is hard to come by. All too often, my nights are spent looking up at the ceiling thinking about questions that I’m not sure I want answered. Will my grant application finally be funded? Will my graduate student ever finish her thesis? Will that last experiment satisfy the reviewer? I know that I am not alone. Increasingly large numbers of people do not get enough sleep at night and subsequently suffer the consequences. We try to make up for this lack of sleep by drinking coffee during the day and taking sleeping pills at night. Even if we do manage to fall asleep, staying asleep for the duration of the night becomes a challenge. As many will attest, this problem only gets worse with age. I suspect that the main solution for our sleepless society will involve lifestyle changes, but there is also a large and growing market for new classes of sleep aids.
The development of new therapeutic targets for sleep disorders will require a better understanding of both the circuitry and the signaling molecules that are involved in the control of sleep. Given the magnitude of our modern society’s problem staying asleep, new research examining these topics is particularly welcome. A new study in this issue by Foltenyi and colleagues1 reports a newly identified role for EGFR and ERK signaling in sleep regulation and consolidation in Drosophila, not only providing valuable new insights into the molecular control of sleep, but also continuing to validate Drosophila as a powerful tool for this type of research.
Researchers have made much progress toward developing a mechanistic understanding of sleep. For example, our understanding of the underlying circuitry in mammals has steadily improved2. Discovering a role for the hypocretinorexin neuropeptide in the regulation of sleep and narcolepsy has been an important step forward3,4. That being said, we are still a long way from a molecular or genetic understanding of sleep. It is not clear that many sleep researchers would even accept the premise that sleep is a process that can be understood at this level. The use of Drosophila as a model for understanding the molecular basis of behavior has proven fruitful, with the usefulness of this organism in circadian rhythms research providing a particularly compelling example. Of course, for flies to become an effective model system in this research area, we have to be convinced that flies sleep, or at least undergo a process that is biologically similar to sleep in humans.
For clinically oriented researchers, the sleep state is defined by EEG recordings of cortical activity coupled with measurements of muscle tone. These electrophysiological correlates of sleep provide more quantitative measures than behavioral markers. They allow for the discrimination of quiet inactivity from sleep, as well as a measurement of the different stages of sleep (rapid eye movement, slow-wave sleep). These objective criteria have proven to be extremely useful for the study of sleep in humans and other mammals. However, even among vertebrates, EEG correlates of sleep and wakefulness have to be considered in the context of the ecological niche of a particular species5. From a biological point of view, it seems unnecessarily narrow to define sleep based solely on patterns of cortical activity.
Behavioral analysis provides considerable evidence that fruitflies show canonical features of sleep. Drosophila undergo a consolidated state of behavioral inactivity, an increased arousal threshold during these inactive periods, and the ability to rapidly reverse the inactivity: that is, to wake up6,7. Importantly, if deprived of sleep, the flies respond by sleeping more, experiencing a ‘sleep rebound’. This homeostatic drive to recover sleep after deprivation is one of the hallmark features of sleep. Fruitflies even respond to the same pharmacological agents that modulate arousal in humans, including caffeine6,7 and methamphetamine8. Older flies show fragmentation in sleep episodes, similar to many aging humans9. Accepting the premise that shared features exist between the sleep states in insects and mammals enables us to apply the power of Drosophila genetics to the problem of uncovering the basic biological mechanisms controlling sleep.
Taking advantage of this strategy, the authors of the present study1 examined the transforming growth factor-α (TGF-α) signaling cascade. TGF-α is rhythmically transcribed and secreted by cells within the main mammalian circadian oscillator, the suprachiasmatic nucleus10. In the fruitfly, members of the TGF-α family (such as Spitz) bind the epidermal growth factor receptor (EGFR). Activation of EGFR requires the processing proteins Star and Rhomboid family (Rho), which are integral membrane proteases that cleave membrane bound TGF-α ligands to produce a soluble form of the signaling molecule. Triggering the EGFR pathway, in turn, activates extracellular signal-regulated kinase (ERK). ERK is familiar to neuroscientists as a kinase implicated in the regulation of plasticity in the adult nervous system, including the photic regulation of circadian timing11.
In the fruitfly, EGFR is widely expressed in the nervous system. The new study by Foltenyi and colleagues1 demonstrates that overexpression of the EGFR signaling components Rho and Star causes an acute, reversible and dose-dependent increase in sleep that tightly parallels an increase in phosphorylated ERK (pERK). Unfortunately, the authors were unable to measure endogenous levels of pERK (due to a lack of sensitivity in the assay) to confirm that sleeping flies have more pERK than awake flies. The ability of a dominant-negative EGFR to block activation of ERK argues that the manipulation is specific to the EGFR pathway. In contrast to the increase in sleep observed after Rho overexpression, inhibiting Rho expression leads to a significant decrease in sleep. Importantly, this decrease in sleep was due to a dramatic shortening of the duration of sleep episodes accompanied by an elevation in the number of sleep bouts. This observation suggests that the mutant flies have an increased need for sleep, but are unable to stay asleep. This is a sleep pattern that is similar to insomnia in humans (Fig. 1).
Part of the significance of this work1 is that the authors were able to demonstrate anatomical specificity in their manipulations of the signaling pathway. The brain regions involved in the influence of this signaling cascade on sleep are the pars intercerebralis (PI) and tritocerebrum (TriC). The cells of the PI contain Rho and generate the EGFR ligands that activate ERK in the receiving cells within the TriC. The authors identified the PI as the region responsible for EGFR ligand secretion by demonstrating that the cells in that region express endogenous Rho, and that inhibiting Rho in this region resulted in decreased sleep. In insects, the PI contains neurosecretory cells that have been compared to the vertebrate hypothalamus. The TriC region was identified by the robust pERK expression that was stimulated in response to the overexpression of the EGFR processing components Rho and Star. The authors argue that Rho and Star overexpression only enhances EGFR signaling in cells that endogenously express the ligand. Future studies will determine the effects of electrically silencing neurons in the PI or TriC regions on sleep.
Surprisingly, given the role of TGF-α in the SCN10, EGFR-ERK signaling did not alter the circadian timing of sleep. Circadian rhythms in Drosophila are driven in large part by a population of ventral lateral neurons (LNv) that express the neuropeptide pigment dispersing factor (PDF). Basic circadian properties were not altered in flies with Rho knocked down or in flies overexpressing Rho or Star. Inhibiting Rho expression in the LNv cells with a Pdf-Gal4 driver did not change sleep patterns. Therefore, the effects observed on sleep regulation by EGFR-ERK signaling in the PI are most likely driven by a signal coming from a region of the brain that lies downstream of circadian control.
What are the identities of the downstream targets for EGFR-ERG signaling in the TriC region of the fly brain? ERK directly phosphorylates the potassium channel Kv4.2 (ref. 12). Phosphorylated ERK seems to be expressed in the processes, but not in the soma of the TriC neurons. Thus, the final target of this signaling pathway may well be changes in electrical activity or synaptic transmission in these neurons. This suggestion fits nicely with work in Drosophila indicating that a mutation in the potassium channel Kv1.4 also produces abnormalities in sleep maintenance13.
Researchers using Drosophila to study the regulation of sleep have progressed to the point where they have begun to identify the regions and signaling pathways involved in the control of sleep and daily rhythms. However, much work remains to be done before they develop a circuit level understanding for the control of sleep. We will need to determine the relationship between the neurons in the mushroom bodies14,15, the PI and the LNv, which are all implicated in the control of daily rhythms in sleep. With the improvements in the ability of this field to manipulate specific cell populations, we are likely to see continued progress in this area. This is good news for the prospect of developing future therapeutic targets for sleep disorders. In time, this research may help us all get a good night’s sleep.
COMPETING INTERESTS STATEMENT
The author declares no competing financial interests.