Rhythmic control of activity and sleep by class B1 GPCRs

Loss of neuropeptide signalling alters temporal expression of mouse suprachiasmatic neuronal state and excitability

Individual neurons of the hypothalamic suprachiasmatic nuclei (SCN) contain an intracellular molecular clock that drives these neurons to exhibit day-night variation in excitability. The neuropeptide vasoactive intestinal polypeptide (VIP) and its cognate receptor, VPAC2, are synthesized by SCN neurons and this intercellular VIP-VPAC2 receptor signal facilitates coordination of SCN neuronal activity and timekeeping. How the loss of VPAC2 receptor signalling affects the electrophysiological properties and states of SCN neurons as well as their responses to excitatory inputs is unclear. Here we used patch-clamp electrophysiology and made recordings of SCN neurons in brain slices prepared from transgenic animals that do not express VPAC2 receptors (Vipr2-/- mice) as well as animals that do (Vipr2+/+ mice). We report that while Vipr2+/+ neurons exhibit coordinated day-night variation in their electrical state, Vipr2-/- neurons lack this and instead manifest a range of states during both day and night. Further, at the population level, Vipr2+/+ neurons vary the membrane threshold potential at which they start to fire action potentials from day to night, while Vipr2-/- neurons do not. We provide evidence that Vipr2-/- neurons lack a component of voltage-gated sodium currents that contribute to SCN neuronal excitability. Moreover, we determine that this aberrant temporal control of neuronal state and excitability alters neuronal responses to a neurochemical mimic of the light-input pathway to the SCN. These results highlight the critical role VIP-VPAC2 receptor signalling plays in the temporal expression of individual neuronal states as well as appropriate ensemble activity and input gating of the SCN neural network.

Insights into the structure and activation mechanism of some class B1 GPCR family members

In humans, 15 genes encode the class B1 family of GPCRs, which are polypeptide hormone receptors characterized by having a large N-terminal extracellular domain (ECD) and receive signals from outside the cell to activate cellular response. For example, the insulinotropic polypeptide (GIP) stimulates the glucose-dependent insulinotropic polypeptide receptor (GIPR), while the glucagon receptor (GCGR) responds to glucagon by increasing blood glucose levels and promoting the breakdown of liver glycogen to induce the production of insulin. The glucagon-like peptides 1 and 2 (GLP-1 and GLP-2) elicit a response from glucagon-like peptide receptor types 1 and 2 (GLP1R and GLP2R), respectively. Since these receptors are implicated in the pathogenesis of diabetes, studying their activation is crucial for the development of effective therapies for the condition. With more structural information being revealed by experimental methods such as X-ray crystallography, cryo-EM, and NMR, the activation mechanism of class B1 GPCRs becomes unraveled. The available crystal and cryo-EM structures reveal that class B1 GPCRs follow a two-step model for peptide binding and receptor activation. The regions close to the C-termini of hormones interact with the N-terminal ECD of the receptor while the regions close to the N-terminus of the peptide interact with the TM domain and transmit signals. This review highlights the structural details of class B1 GPCRs and their conformational changes following activation. The roles of MD simulation in characterizing those conformational changes are briefly discussed, providing insights into the potential structural exploration for future ligand designs.

Molecular and Neural Mechanisms of Temperature Preference Rhythm in Drosophila melanogaster

Temperature influences animal physiology and behavior. Animals must set an appropriate body temperature to maintain homeostasis and maximize survival. Mammals set their body temperatures using metabolic and behavioral strategies. The daily fluctuation in body temperature is called the body temperature rhythm (BTR). For example, human body temperature increases during wakefulness and decreases during sleep. BTR is controlled by the circadian clock, is closely linked with metabolism and sleep, and entrains peripheral clocks located in the liver and lungs. However, the underlying mechanisms of BTR are largely unclear. In contrast to mammals, small ectotherms, such as Drosophila, control their body temperatures by choosing appropriate environmental temperatures. The preferred temperature of Drosophila increases during the day and decreases at night; this pattern is referred to as the temperature preference rhythm (TPR). As flies are small ectotherms, their body temperature is close to that of the surrounding environment. Thus, Drosophila TPR produces BTR, which exhibits a pattern similar to that of human BTR. In this review, we summarize the regulatory mechanisms of TPR, including recent studies that describe neuronal circuits relaying ambient temperature information to dorsal neurons (DNs). The neuropeptide diuretic hormone 31 (DH31) and its receptor (DH31R) regulate TPR, and a mammalian homolog of DH31R, the calcitonin receptor (CALCR), also plays an important role in mouse BTR regulation. In addition, both fly TPR and mammalian BTR are separately regulated from another clock output, locomotor activity rhythms. These findings suggest that the fundamental mechanisms of BTR regulation may be conserved between mammals and flies. Furthermore, we discuss the relationships between TPR and other physiological functions, such as sleep. The dissection of the regulatory mechanisms of Drosophila TPR could facilitate an understanding of mammalian BTR and the interaction between BTR and sleep regulation.

The Photoperiod-Driven Cyclical Secretion of Pineal Melatonin Regulates Seasonal Reproduction in Geese (Anser cygnoides)

The photoperiod is the predominant environmental factor that governs seasonal reproduction in animals; however, the underlying molecular regulatory mechanism has yet to be fully elucidated. Herein, Yangzhou geese (Anser cygnoides) were selected at the spring equinox (SE), summer solstice (SS), autumn equinox (AE), and winter solstice (WS), and the regulation of seasonal reproduction via the light-driven cyclical secretion of pineal melatonin was investigated. We show that there were seasonal variations in the laying rate and GSI, while the ovarian area decreased 1.5-fold from the SS to the AE. Moreover, not only did the weight and volume of the pineal gland increase with a shortened photoperiod, but the secretory activity was also enhanced. Notably, tissue distribution further revealed seasonal oscillations in melatonin receptors (Mtnrs) in the pineal gland and the hypothalamus-pituitary-gonadal (HPG) axis. The immunohistochemical staining indicated higher Mtnr levels due to the shortened photoperiod. Furthermore, the upregulation of aralkylamine N-acetyltransferase (Aanat) was observed from the SS to the AE, concurrently resulting in a downregulation of the gonadotrophin-releasing hormone (GnRH) and gonadotropins (GtHs). This trend was also evident in the secretion of hormones. These data indicate that melatonin secretion during specific seasons is indicative of alterations in the photoperiod, thereby allowing for insight into the neuroendocrine regulation of reproduction via an intrinsic molecular depiction of external photoperiodic variations.

Gene co-expression analysis identifies modules related to insufficient sleep in humans

BACKGROUND

Insufficient sleep and circadian rhythm disruption may cause cancer, obesity, cardiovascular disease, and cognitive impairment. The underlying mechanisms need to be elucidated.

METHOD

Weighted gene co-expression network analysis (WGCNA) was used to identify co-expressed modules. Connectivity Map tool was used to identify candidate drugs based on top connected genes. R ptestg package was utilized to detected module rhythmicity alteration. A hypergeometric test was used to test the enrichment of insomnia SNP signals in modules. Google Scholar was used to validate the modules and hub genes by literature.

RESULTS

We identified a total of 45 co-expressed modules. These modules were stable and preserved. Eight modules were correlated with sleep restriction duration. Module rhythmicity was disrupted in sleep restriction subjects. Hub genes that involve in insufficient sleep also play important roles in sleep disorders. Insomnia GWAS signals were enriched in six modules. Finally, eight drugs associated with sleep disorders were identified.

CONCLUSION

Systems biology method was used to identify sleep-related modules, hub genes, and candidate drugs. Module rhythmicity was altered in sleep insufficient subjects. Thiamphenicol, lisuride, timolol, and piretanide are novel candidates for sleep disorders.

Molecular Mechanisms Underlying Reciprocal Interactions Between Sleep Disorders and Parkinson's Disease

Sleep-wake disruptions are among the most prevalent and burdensome non-motor symptoms of Parkinson's disease (PD). Clinical studies have demonstrated that these disturbances can precede the onset of typical motor symptoms by years, indicating that they may play a primary function in the pathogenesis of PD. Animal studies suggest that sleep facilitates the removal of metabolic wastes through the glymphatic system via convective flow from the periarterial space to the perivenous space, upregulates antioxidative defenses, and promotes the maintenance of neuronal protein homeostasis. Therefore, disruptions to the sleep-wake cycle have been associated with inefficient metabolic clearance and increased oxidative stress in the central nervous system (CNS). This leads to excessive accumulation of alpha-synuclein and the induction of neuronal loss, both of which have been proposed to be contributing factors to the pathogenesis and progression of PD. Additionally, recent studies have suggested that PD-related pathophysiological alterations during the prodromal phase disrupt sleep and circadian rhythms. Taken together, these findings indicate potential mechanistic interactions between sleep-wake disorders and PD progression as proposed in this review. Further research into the hypothetical mechanisms underlying these interactions would be valuable, as positive findings may provide promising insights into novel therapeutic interventions for PD.

Circadian rhythms of sorting nexin 25 in the mouse suprachiasmatic nucleus

Entrainment of mammalian circadian rhythms requires receptor-mediated signaling in the hypothalamic suprachiasmatic nucleus (SCN), the site of the master circadian pacemaker. Receptor-mediated signaling is regulated by endocytosis, indicating that endocytosis-related proteins contribute to SCN pacemaking. Sorting nexin 25 (SNX25) belongs to the sorting nexin superfamily, whose members are responsible for membrane attachment to organelles of the endocytic system. In this study, we showed that Snx25 mRNA and SNX25 protein are highly expressed and exhibit remarkable circadian rhythms in the SCN of adult mice. Expression was maximal at about zeitgeber time (ZT) 16 in the subjective night and minimal at ZT8 in the subjective day. Prominent SNX25 immunoreactivity was found in the arginine vasopressin-positive neurons of the SCN. These findings suggest that SNX25 is a new actor in endocytic signaling, perhaps contributing to the circadian pacemaking system.

Drosophila Temperature Preference Rhythms: An Innovative Model to Understand Body Temperature Rhythms

Human body temperature increases during wakefulness and decreases during sleep. The body temperature rhythm (BTR) is a robust output of the circadian clock and is fundamental for maintaining homeostasis, such as generating metabolic energy and sleep, as well as entraining peripheral clocks in mammals. However, the mechanisms that regulate BTR are largely unknown. Drosophila are ectotherms, and their body temperatures are close to ambient temperature; therefore, flies select a preferred environmental temperature to set their body temperature. We identified a novel circadian output, the temperature preference rhythm (TPR), in which the preferred temperature in flies increases during the day and decreases at night. TPR, thereby, produces a daily BTR. We found that fly TPR shares many features with mammalian BTR. We demonstrated that diuretic hormone 31 receptor (DH31R) mediates Drosophila TPR and that the closest mouse homolog of DH31R, calcitonin receptor (Calcr), is essential for mice BTR. Importantly, both TPR and BTR are regulated in a distinct manner from locomotor activity rhythms, and neither DH31R nor Calcr regulates locomotor activity rhythms. Our findings suggest that DH31R/Calcr is an ancient and specific mediator of BTR. Thus, understanding fly TPR will provide fundamental insights into the molecular and neural mechanisms that control BTR in mammals.

Wakefulness Is Promoted during Day Time by PDFR Signalling to Dopaminergic Neurons in Drosophila melanogaster

Circadian clocks modulate timing of sleep/wake cycles in animals; however, the underlying mechanisms remain poorly understood. In Drosophila melanogaster, large ventral lateral neurons (l-LN_v) are known to promote wakefulness through the action of the neuropeptide pigment dispersing factor (PDF), but the downstream targets of PDF signalling remain elusive. In a screen using downregulation or overexpression (OEX) of the gene encoding PDF receptor (pdfr), we found that a subset of dopaminergic neurons responds to PDF to promote wakefulness during the day. Moreover, we found that small LN_v (s-LN_v) and dopaminergic neurons form synaptic contacts, and PDFR signalling inhibited dopaminergic neurons specifically during day time. We propose that these dopaminergic neurons that respond to PDFR signalling are sleep-promoting and that during the day when PDF levels are high, they are inhibited, thereby promoting wakefulness. Thus, we identify a novel circadian clock pathway that mediates wake promotion specifically during day time.

ER Lipid Defects in Neuropeptidergic Neurons Impair Sleep Patterns in Parkinson's Disease

Parkinson's disease patients report disturbed sleep patterns long before motor dysfunction. Here, in parkin and pink1 models, we identify circadian rhythm and sleep pattern defects and map these to specific neuropeptidergic neurons in fly models and in hypothalamic neurons differentiated from patient induced pluripotent stem cells (iPSCs). Parkin and Pink1 control the clearance of mitochondria by protein ubiquitination. Although we do not observe major defects in mitochondria of mutant neuropeptidergic neurons, we do find an excess of endoplasmic reticulum-mitochondrial contacts. These excessive contact sites cause abnormal lipid trafficking that depletes phosphatidylserine from the endoplasmic reticulum (ER) and disrupts the production of neuropeptide-containing vesicles. Feeding mutant animals phosphatidylserine rescues neuropeptidergic vesicle production and acutely restores normal sleep patterns in mutant animals. Hence, sleep patterns and circadian disturbances in Parkinson's disease models are explained by excessive ER-mitochondrial contacts, and blocking their formation or increasing phosphatidylserine levels rescues the defects in vivo.

Calcitonin receptors are ancient modulators for rhythms of preferential temperature in insects and body temperature in mammals

Goda et al. provide molecular evidence that body temperature rhythm (BTR) is regulated distinctly from locomotor activity rhythms and show that diuretic hormone 31 receptor/calcitonin receptor is an ancient specific mediator of BTR during the active phase in organisms ranging from ectotherms to endotherms.

Daily body temperature rhythm (BTR) is essential for maintaining homeostasis. BTR is regulated separately from locomotor activity rhythms, but its molecular basis is largely unknown. While mammals internally regulate BTR, ectotherms, including Drosophila, exhibit temperature preference rhythm (TPR) behavior to regulate BTR. Here, we demonstrate that the diuretic hormone 31 receptor (DH31R) mediates TPR during the active phase in Drosophila. DH31R is expressed in clock cells, and its ligand, DH31, acts on clock cells to regulate TPR during the active phase. Surprisingly, the mouse homolog of DH31R, calcitonin receptor (Calcr), is expressed in the suprachiasmatic nucleus (SCN) and mediates body temperature fluctuations during the active phase in mice. Importantly, DH31R and Calcr are not required for coordinating locomotor activity rhythms. Our results represent the first molecular evidence that BTR is regulated distinctly from locomotor activity rhythms and show that DH31R/Calcr is an ancient specific mediator of BTR during the active phase in organisms ranging from ectotherms to endotherms.

Habituation as an adaptive shift in response strategy mediated by neuropeptides

Habituation is a non-associative form of learning characterized by a decremented response to repeated stimulation. It is typically framed as a process of selective attention, allowing animals to ignore irrelevant stimuli in order to free up limited cognitive resources. However, habituation can also occur to threatening and toxic stimuli, suggesting that habituation may serve other functions. Here we took advantage of a high-throughput Caenorhabditis elegans learning assay to investigate habituation to noxious stimuli. Using real-time computer vision software for automated behavioral tracking and optogenetics for controlled activation of a polymodal nociceptor, ASH, we found that neuropeptides mediated habituation and performed an RNAi screen to identify candidate receptors. Through subsequent mutant analysis and cell-type-specific gene expression, we found that pigment-dispersing factor (PDF) neuropeptides function redundantly to promote habituation via PDFR-1-mediated cAMP signaling in both neurons and muscles. Behavioral analysis during learning acquisition suggests that response habituation and sensitization of locomotion are parts of a shifting behavioral strategy orchestrated by pigment dispersing factor signaling to promote dispersal away from repeated aversive stimuli.

Allatostatin A Signalling in Drosophila Regulates Feeding and Sleep and Is Modulated by PDF

Feeding and sleep are fundamental behaviours with significant interconnections and cross-modulations. The circadian system and peptidergic signals are important components of this modulation, but still little is known about the mechanisms and networks by which they interact to regulate feeding and sleep. We show that specific thermogenetic activation of peptidergic Allatostatin A (AstA)-expressing PLP neurons and enteroendocrine cells reduces feeding and promotes sleep in the fruit fly Drosophila. The effects of AstA cell activation are mediated by AstA peptides with receptors homolog to galanin receptors subserving similar and apparently conserved functions in vertebrates. We further identify the PLP neurons as a downstream target of the neuropeptide pigment-dispersing factor (PDF), an output factor of the circadian clock. PLP neurons are contacted by PDF-expressing clock neurons, and express a functional PDF receptor demonstrated by cAMP imaging. Silencing of AstA signalling and continuous input to AstA cells by tethered PDF changes the sleep/activity ratio in opposite directions but does not affect rhythmicity. Taken together, our results suggest that pleiotropic AstA signalling by a distinct neuronal and enteroendocrine AstA cell subset adapts the fly to a digestive energy-saving state which can be modulated by PDF.

Distinct Firing Properties of Vasoactive Intestinal Peptide-Expressing Neurons in the Suprachiasmatic Nucleus

The suprachiasmatic nucleus (SCN) regulates daily rhythms in physiology and behavior. Previous studies suggest a critical role for neurons expressing vasoactive intestinal peptide (VIP) in coordinating rhythmicity and synchronization in the SCN. Here we examined the firing properties of VIP-expressing SCN neurons in acute brain slices. Active and passive membrane properties were measured in VIP and in non-VIP neurons during the day and at night. Current-clamp recordings revealed that both VIP and non-VIP neurons were spontaneously active, with higher firing rates during the day than at night. Average firing frequencies, however, were higher in VIP neurons (3.1 ± 0.2 Hz, day and 2.4 ± 0.2 Hz, night) than in non-VIP neurons (1.8 ± 0.2 Hz, day and 0.9 ± 0.2 Hz, night), both day and night. The waveforms of individual action potentials in VIP and non-VIP neurons were also distinct. Action potential durations (APD50) were shorter in VIP neurons (3.6 ± 0.1 ms, day and 2.9 ± 0.1 ms, night) than in non-VIP neurons (4.4 ± 0.3 ms, day and 3.5 ± 0.2 ms, night) throughout the light-dark cycle. In addition, afterhyperpolarization (AHP) amplitudes were larger in VIP neurons (21 ± 0.8 mV, day and 24.9 ± 0.9 mV, night) than in non-VIP neurons (17.2 ± 1.1 mV, day and 20.5 ± 1.2 mV, night) during the day and at night. Furthermore, significant day/night differences were observed in APD50 and AHP amplitudes in both VIP and non-VIP SCN neurons, consistent with rhythmic changes in ionic conductances that contribute to shaping the firing properties of both cell types. The higher day and night firing rates of VIP neurons likely contribute to synchronizing electrical activity in the SCN.