Early-onset sleep defects in Drosophila models of Huntington's disease reflect alterations of PKA/CREB signaling.

Huntington's disease (HD) is a progressive neurological disorder whose non-motor symptoms include sleep disturbances. Whether sleep and activity abnormalities are primary molecular disruptions of mutant Huntingtin (mutHtt) expression or result from neurodegeneration is unclear. Here, we report Drosophila models of HD exhibit sleep and activity disruptions very early in adulthood, as soon as sleep patterns have developed. Pan-neuronal expression of full-length or N-terminally truncated mutHtt recapitulates sleep phenotypes of HD patients: impaired sleep initiation, fragmented and diminished sleep, and nighttime hyperactivity. Sleep deprivation of HD model flies results in exacerbated sleep deficits, indicating that homeostatic regulation of sleep is impaired. Elevated PKA/CREB activity in healthy flies produces patterns of sleep and activity similar to those in our HD models. We were curious whether aberrations in PKA/CREB signaling were responsible for our early-onset sleep/activity phenotypes. Decreasing signaling through the cAMP/PKA pathway suppresses mutHtt-induced developmental lethality. Genetically reducing PKA abolishes sleep/activity deficits in HD model flies, restores the homeostatic response and extends median lifespan. In vivo reporters, however, show dCREB2 activity is unchanged, or decreased when sleep/activity patterns are abnormal, suggesting dissociation of PKA and dCREB2 occurs early in pathogenesis. Collectively, our data suggest that sleep defects may reflect a primary pathological process in HD, and that measurements of sleep and cAMP/PKA could be prodromal indicators of disease, and serve as therapeutic targets for intervention.

Spatio-temporal in vivo recording of dCREB2 dynamics in Drosophila long-term memory processing.

CREB (cAMP response element-binding protein) is an evolutionarily conserved transcription factor, playing key roles in synaptic plasticity, intrinsic excitability and long-term memory (LTM) formation. The Drosophila homologue of mammalian CREB, dCREB2, is also important for LTM. However, the spatio-temporal nature of dCREB2 activity during memory consolidation is poorly understood. Using an in vivo reporter system, we examined dCREB2 activity continuously in specific brain regions during LTM processing. Two brain regions that have been shown to be important for Drosophila LTM are the ellipsoid body (EB) and the mushroom body (MB). We found that dCREB2 reporter activity is persistently elevated in EB R2/R4m neurons, but not neighboring R3/R4d neurons, following LTM-inducing training. In multiple subsets of MB neurons, dCREB2 reporter activity is suppressed immediately following LTM-specific training, and elevated during late windows. In addition, we observed heterogeneous responses across different subsets of neurons in MB αß lobe during LTM processing. All of these changes suggest that dCREB2 functions in both the EB and MB for LTM formation, and that this activity contributes to the process of systems consolidation.

Time of day influences memory formation and dCREB2 proteins in Drosophila.

Many biological phenomena oscillate under the control of the circadian system, exhibiting peaks and troughs of activity across the day/night cycle. In most animal models, memory formation also exhibits this property, but the underlying neuronal and molecular mechanisms remain unclear. The dCREB2 transcription factor shows circadian regulated oscillations in its activity, and has been shown to be important for both circadian biology and memory formation. We show that the time-of-day (TOD) of behavioral training affects Drosophila memory formation. dCREB2 exhibits complex changes in protein levels across the daytime and nighttime, and these changes in protein abundance are likely to contribute to oscillations in dCREB2 activity and TOD effects on memory formation.

In vivo circadian oscillation of dCREB2 and NF-κB activity in the Drosophila nervous system.

cAMP response element-binding protein (CREB) and nuclear factor kappa-B (NF-κB) are two ubiquitous transcription factors involved in a wide number of cellular processes, including the circadian system. Many previous studies on these factors use cellular assays that provide limited information on circadian activity or anatomical specificity. The ability to study transcription factors in defined tissue within intact animals will help to bridge the gap between cellular and in vivo data. We have used the GAL4-UAS and FLP-FRT systems to gain spatial control over reporter gene expression. Using a luciferase-based reporter, we show in vivo that Drosophila dCREB2- and NF-κB-mediated transcription oscillates in neuronal cells, glia, and in the mushroom body, a higher-order brain center in flies. This oscillation is under circadian control, cycling with a 24-hour rhythm, under both light-dark and dark-dark conditions. In light-light conditions, dCREB2 and NF-κB reporter flies exhibit a suppression of rhythmic activity. Furthermore, neuronal cycling of dCREB2 and NF-κB activity are modulated in period mutant flies, indicating these oscillations are controlled through the central clock. This study shows for the first time region-specific circadian oscillation of dCREB2/NF-κB activity in the Drosophila nervous system.

Autoreceptor control of peptide/neurotransmitter corelease from PDF neurons determines allocation of circadian activity in drosophila.

Drosophila melanogaster flies concentrate behavioral activity around dawn and dusk. This organization of daily activity is controlled by central circadian clock neurons, including the lateral-ventral pacemaker neurons (LN(v)s) that secrete the neuropeptide PDF (pigment dispersing factor). Previous studies have demonstrated the requirement for PDF signaling to PDF receptor (PDFR)-expressing dorsal clock neurons in organizing circadian activity. Although LN(v)s also express functional PDFR, the role of these autoreceptors has remained enigmatic. Here, we show that (1) PDFR activation in LN(v)s shifts the balance of circadian activity from evening to morning, similar to behavioral responses to summer-like environmental conditions, and (2) this shift is mediated by stimulation of the Gα,s-cAMP pathway and a consequent change in PDF/neurotransmitter corelease from the LN(v)s. These results suggest another mechanism for environmental control of the allocation of circadian activity and provide new general insight into the role of neuropeptide autoreceptors in behavioral control circuits.

Memory formation: filling in the gaps in flies.

Research in Drosophila has many advantages for the study of complex behavior. Two studies identify a new role for chemical and electrical signaling in the anterior paired lateral neurons during memory formation.