Cholecystokinin-A receptors regulate photic input pathways to the circadian clock

Cholecystokinin receptor type A are involved in the circadian rhythm of the mouse retina

The retina is the only organ projecting external light to the suprachiasmatic nucleus. Cholecystokinin receptor type A (Cckar/Cckar) is one of the essential factors for light reception in retinal cells. As there was a lack of literature on the matter, we aimed to elucidate the cause of the time-dependent phase change in clock gene expression. We found that Cckar mRNA expression in retinal cells exhibited diurnal variations. The rhythm of expression of the clock gene Per1/Per2 in retinal cells was altered in Cckar -/- mice. The light sensitivity of retinal cells was evaluated in wild-type mice, which showed c-Fos was activated in the ganglion cell layer more than in the inner granular layer. This increase in the number of c-Fos-positive cells was suppressed by lorglumide, a Cckar antagonist. Treatment of rat retina primary cells with lorglumide suppressed Per2 transcription, which was altered in a time-dependent manner relative to the Per2 expression. Light irradiation studies in Cckar -/- mice did not exhibit an increase in Period expression in the suprachiasmatic nucleus. These results indicate that Cckar is among the factors that regulate the cycle of clock genes on the retina. Cckar knockout attenuates the light responsiveness of suprachiasmatic nucleus and reduces the expression amplitude of Period genes in the retina. Thus, Cckar may contribute to entrainment of the light environment and maintenance of the expression cycle of Period gene, which is one of the core clock genes.

Chronobiotics, satiety signaling, and clock gene expression interplay

The biological clock regulates the way our body works throughout the day, including releasing hormones and food intake. Disruption of the biological clock (chronodisruption) may deregulate satiety, which is strictly regulated by hormones and neurotransmitters, leading to health problems like obesity. Nowadays, using bioactive compounds as a coadjutant for several pathologies is a common practice. Phenolic compounds and short-chain fatty acids, called "chronobiotics," can modulate diverse mechanisms along the body to exert beneficial effects, including satiety regulation and circadian clock resynchronization; however, the evidence of the interplay between those processes is limited. This review compiles the evidence of natural chronobiotics, mainly polyphenols and short-chain fatty acids that affect the circadian clock mechanism and process modifications in genes or proteins resulting in a signaling chain that modulates satiety hormones or hunger pathways.

The Efemp1R345W Macular Dystrophy Mutation Causes Amplified Circadian and Photophobic Responses to Light in Mice

Purpose

The R345W mutation in EFEMP1 causes malattia leventinese, an autosomal dominant eye disease with pathogenesis similar to an early-onset age-related macular degeneration. In mice, Efemp1^R345W does not cause detectable degeneration but small subretinal deposits do accumulate. The purpose of this study was to determine whether there were abnormal responses to light at this presymptomatic stage in Efemp1^R345W mice.

Methods

Responses to light were assessed by visual water task, circadian phase shifting, and negative masking behavior. The mechanism of abnormal responses was investigated by anterior eye exam, electroretinogram, melanopsin cell quantification, and multielectrode recording of retinal ganglion cell activity.

Results

Visual acuity was not different in Efemp1^R345W mice. However, amplitudes of circadian phase shifting (P = 0.016) and negative masking (P < 0.0001) were increased in Efemp1^R345W mice. This phenotype was not explained by anterior eye defects or amplified outer retina responses. Instead, we identified increased melanopsin-generated responses to light in the ganglion cell layer of the retina (P < 0.01).

Conclusions

Efemp1^R345W increases the sensitivity to light of behavioral responses driven by detection of irradiance. An amplified response to light in melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) is consistent with this phenotype. The major concern with this effect of the malattia leventinese mutation is the potential for abnormal regulation of physiology by light to negatively affect health.

Circadian clock-gastrointestinal peptide interaction in peripheral tissues and the brain

Food intake and sleep are two mutually exclusive behaviors and both are normally confined to opposing phases of the diurnal cycle. The temporal coordination of behavior and physiology along the 24-h day-night cycle is organized by a network of circadian clocks that orchestrate transcriptional programs controlling cellular physiology. Many of the peptide hormones of the gastrointestinal tract are not only secreted in a circadian fashion, they can also affect circadian clock function in peripheral metabolic tissues and the brain, thus providing metabolic feedback to metabolic and neurobehavioral circuits. In this review, we summarize the current knowledge on this gastrointestinal peptide crosstalk and its potential role in the coordination of nutrition and the maintenance of metabolic homeostasis.

Regulation of molecular clock oscillations and phagocytic activity via muscarinic Ca2+ signaling in human retinal pigment epithelial cells

Vertebrate eyes are known to contain circadian clocks, however, the intracellular mechanisms regulating the retinal clockwork remain largely unknown. To address this, we generated a cell line (hRPE-YC) from human retinal pigmental epithelium, which stably co-expressed reporters for molecular clock oscillations (Bmal1-luciferase) and intracellular Ca²⁺ concentrations (YC3.6). The hRPE-YC cells demonstrated circadian rhythms in Bmal1 transcription. Also, these cells represented circadian rhythms in Ca²⁺-spiking frequencies, which were canceled by dominant-negative Bmal1 transfections. The muscarinic agonist carbachol, but not photic stimulation, phase-shifted Bmal1 transcriptional rhythms with a type-1 phase response curve. This is consistent with significant M3 muscarinic receptor expression and little photo-sensor (Cry2 and Opn4) expression in these cells. Moreover, forskolin phase-shifted Bmal1 transcriptional rhythm with a type-0 phase response curve, in accordance with long-lasting CREB phosphorylation levels after forskolin exposure. Interestingly, the hRPE-YC cells demonstrated apparent circadian rhythms in phagocytic activities, which were abolished by carbachol or dominant-negative Bmal1 transfection. Because phagocytosis in RPE cells determines photoreceptor disc shedding, molecular clock oscillations and cytosolic Ca²⁺ signaling may be the driving forces for disc-shedding rhythms known in various vertebrates. In conclusion, the present study provides a cellular model to understand molecular and intracellular signaling mechanisms underlying human retinal circadian clocks.

Cholecystokinin receptors regulate sperm protein tyrosine phosphorylation via uptake of HCO3-

Cholecystokinin (CCK), a peptide hormone and a neurotransmitter, was detected in mature sperm two decades ago. However, the exact role of CCK and the types of CCK receptors (now termed CCK1 and CCK2) in sperm have not been identified. Here, we find that CCK1 and CCK2 receptors are immunolocalized to the acrosomal region of mature sperm. The antagonist of CCK1 or CCK2 receptor strongly activated the soluble adenylyl cyclase/cAMP/protein kinase A signaling pathway that drives sperm capacitation-associated protein tyrosine phosphorylation in dose- and time-dependent manners. But these actions of stimulation were abolished when sperm were incubated in the medium in the absence of HCO3-. Further investigation demonstrated that the inhibitor of CCK1 or CCK2 receptor could accelerate the uptake of HCO3- and significantly elevate the intracellular pH of sperm. Interestingly, the synthetic octapeptide of CCK (CCK8) showed the same action and mechanism as antagonists of CCK receptors. Moreover, CCK8 and the antagonist of CCK1 or CCK2 receptor were also able to accelerate human sperm capacitation-associated protein tyrosine phosphorylation by stimulating the influx of HCO3-. Thus, the present results suggest that CCK and its receptors may regulate sperm capacitation-associated protein tyrosine phosphorylation by modulating the uptake of HCO3-.

Fish Oil Accelerates Diet-Induced Entrainment of the Mouse Peripheral Clock via GPR120

The circadian peripheral clock is entrained by restricted feeding (RF) at a fixed time of day, and　insulin secretion regulates RF-induced entrainment of the peripheral clock in mice. Thus, carbohydrate-rich food may be ideal for facilitating RF-induced entrainment, although the role of dietary oils in insulin secretion and RF-induced entrainment has not been described. The soybean oil component of standard mouse chow was substituted with fish or soybean oil containing docosahexaenoic acid (DHA) and/or eicosapentaenoic acid (EPA). Tuna oil (high DHA/EPA), menhaden oil (standard), and DHA/EPA dissolved in soybean oil increased insulin secretion and facilitated RF-induced phase shifts of the liver clock as represented by the bioluminescence rhythms of PER2::LUCIFERASE knock-in mice. In this model, insulin depletion blocked the effect of tuna oil and fish oil had no effect on mice deficient for GPR120, a polyunsaturated fatty acid receptor. These results suggest food containing fish oil or DHA/EPA is ideal for adjusting the peripheral clock.

Infant satiety depends on transient expression of cholecystokinin-1 receptors on ependymal cells lining the third ventricle in mice

Cholecystokinin (CCK) is a hypothetical controller for suckling and infancy body weight, although the underlying mechanisms remain unclear. Therefore, the present study analysed the mechanisms using mice lacking the CCK-1 receptor (CCK1R-/-). Although CCK1R-/- mice displayed normal weights at birth and adulthood, CCK1R-/- pups had enlarged adipocytes and were overweight from the first to second week after birth, regardless of maternal genotype. The lacZ reporter gene assay and/or calcium imaging analysis demonstrated that CCK-1 receptors were abundant in satiety-controlling regions such as the hypothalamus, brainstem, nodose ganglion and pylorus in adults, whereas these signals were few to lacking at pre-weanling stages. At postnatal day (PD) 6, the increase in cFos expression in the medullary nucleus tractus solitarius was similarly triggered by gastrointestinal milk- or saline filling in both genotypes, further indicating immature CCK-1 receptor function in an ascending satiety-controlling system during infancy. Conversely, third ventricle ependymal tanycyte-like cells expressed CCK-1 receptors with expression peaking at PD6. At PD6, wild-type but not CCK1R-/- mice had increased cFos immunoreactivity in ependymal cells following gastrointestinal milk filling whereas the response became negligible at PD12. In addition, ependymal cFos was not increased by saline filling, indicating that these responses are dependent on CCK-1 receptors, developmental stage and nutrients. Furthermore, body weights of wild-type pups were transiently increased by blocking ependymal CCK receptor function with microinjection of a CCK-1 antagonist, but not a CCK-2 antagonist. Hence, we demonstrate de novo functions of ependymal CCK-1 receptors and reveal a new aspect of infant satiety-controlling mechanisms.

Functional compensation between cholecystokinin-1 and -2 receptors in murine paraventricular nucleus neurons

Cholecystokinin (CCK) and its receptor subtypes CCK-1 and -2 have diverse homeostatic functions. CCK-1 and -2 receptors share a common phosphatidylinositol signaling pathway, yet little is known regarding their possible functional coupling. We focused on CCK-mediated Ca(2+) signaling in parvocellular paraventricular nucleus (PVN) cells, which control satiety and other autonomic functions. Analysis of mouse hypothalamic slices demonstrated that the general CCK receptor agonist CCK-8s (10 nM) triggered Ca(2+) transients most significantly in the posterior subregion of the PVN (PaPo). This 10 nM CCK-8s-induced response was absent in CCK-1 receptor knock-out (CCK1R(-/-)) slices, showing that the response is mediated by CCK-1 receptors. CCK-8s concentrations higher than 30 nM triggered a Ca(2+) rise similarly in wild-type and CCK1R(-/-) slices. The large CCK-8s (100 nM)-induced Ca(2+) responses in CCK1R(-/-) slices were blocked by a CCK-2 receptor antagonist (CI-988), whereas those in wild-type slices required a mixture of CI-988 and lorglumide (a CCK-1 receptor antagonist) for complete antagonism. Therefore, CCK-1 and -2 receptors may function synergistically in single PaPo neurons and deletion of CCK-1 receptors may facilitate CCK-2 receptor signaling. This hypothesis was supported by results of real-time RT-PCR, immunofluorescence double labeling and Western blotting assays, which indicated CCK-2 receptor overexpression in PaPo neurons of CCK1R(-/-) mice. Furthermore, behavioral studies showed that intraperitoneal injections of lorglumide up-regulated food accesses in wild-type but not in CCK1R(-/-) mice, whereas CI-988 injections up-regulated food accesses in CCK1R(-/-) but not in wild-type mice. Compensatory CCK signaling via CCK-2 receptors in CCK1R(-/-) mice shed light on currently controversial satiety-controlling mechanisms.

Evaluating the links between schizophrenia and sleep and circadian rhythm disruption

Sleep and circadian rhythm disruption (SCRD) and schizophrenia are often co-morbid. Here, we propose that the co-morbidity of these disorders stems from the involvement of common brain mechanisms. We summarise recent clinical evidence that supports this hypothesis, including the observation that the treatment of SCRD leads to improvements in both the sleep quality and psychiatric symptoms of schizophrenia patients. Moreover, many SCRD-associated pathologies, such as impaired cognitive performance, are routinely observed in schizophrenia. We suggest that these associations can be explored at a mechanistic level by using animal models. Specifically, we predict that SCRD should be observed in schizophrenia-relevant mouse models. There is a rapidly accumulating body of evidence which supports this prediction, as summarised in this review. In light of these emerging data, we highlight other models which warrant investigation, and address the potential challenges associated with modelling schizophrenia and SCRD in rodents. Our view is that an understanding of the mechanistic overlap between SCRD and schizophrenia will ultimately lead to novel treatment approaches, which will not only ameliorate SCRD in schizophrenia patients, but also will improve their broader health problems and overall quality of life.

Cholecystokinin (CCK)-expressing neurons in the suprachiasmatic nucleus: innervation, light responsiveness and entrainment in CCK-deficient mice

The suprachiasmatic nucleus (SCN) is the principal pacemaker driving circadian rhythms of physiology and behaviour. Neurons within the SCN express both classical and neuropeptide transmitters which regulate clock functions. Cholecyctokinin (CCK) is a potent neurotransmitter expressed in neurons of the mammalian SCN, but its role in circadian timing is not known. In the present study, CCK was demonstrated in a distinct population of neurons located in the shell region of the SCN and in a few cells in the core region. The CCK neurons did not express vasopressin or vasoactive intestinal peptide. However, CCK-containing processes make synaptic contacts with both groups of neurons and some CCK cell bodies were innervated by VIPergic neurons. The CCK neurons received no direct input from the three major pathways to the SCN, and the CCK neurons were not light-responsive as evaluated by induction of cFOS, and did not express the core clock protein PER1. Accordingly, CCK-deficient mice showed normal entrainment and had similar τ, light-induced phase shift and negative masking behaviour as wild-type animals. In conclusion, CCK signalling seems not to be involved directly in light-induced resetting of the clock or in regulating core clock function. The expression of CCK in a subpopulation of neurons, which do not belonging to either the VIP or AVP cells but which have synaptic contacts to both cell types and reverse innervation of CCK neurons from VIP neurons, suggests that the CCK neurons may act in non-photic regulation within the clock and/or, via CCK projections, mediate clock information to hypothalamic nuclei.

The adjustment and manipulation of biological rhythms by light, nutrition, and abused drugs

Daily restricted feeding entrains the circadian rhythm of mouse clock gene expression in the central nervous system, excluding the suprachiasmatic nucleus (SCN), as well as in the peripheral tissues such as the liver, lung, and heart. In addition to entrainment of the clock genes, daily restricted feeding induces a locomotor activity increase 2-3h before the restricted feeding time initiates. The increase in activity is called the food-anticipatory activity (FAA). In addition to FAA, daily restricted feeding can also entrain peripheral circadian clocks in other organs such as liver, lung, and heart. This type of oscillator is called the food-entrainable peripheral oscillator (FEPO). At present, the mechanisms for restricted feeding-induced entrainment of locomotor activity (FAA) and/or peripheral clock (FEPO) are still unknown. In this review, we describe the role of the central nervous system and peripheral tissues in FAA performance and also in the entrainment of clock gene expression. In addition, the mechanism for entrainment of circadian oscillators by the abuse of drugs, such as methamphetamine, is discussed.

Distribution and postnatal development of Gpr54 gene expression in mouse brain and gonadotropin-releasing hormone neurons

Kisspeptin and G protein-coupled receptor 54 (GPR54) are now acknowledged to play essential roles in the neural regulation of fertility. Using a transgenic Gpr54 LacZ knock-in mouse model, this study aimed to provide 1) a detailed map of cells expressing Gpr54 in the mouse brain and 2) an analysis of Gpr54 expression in GnRH neurons across postnatal development. The highest density of Gpr54-expressing cells in the mouse central nervous system was found in the dentate gyrus of the hippocampus beginning on postnatal d 6 (P6). Abundant Gpr54 expression was also noted in the septum, rostral preoptic area (rPOA), anteroventral nucleus of the thalamus, posterior hypothalamus, periaqueductal grey, supramammillary and pontine nuclei, and dorsal cochlear nucleus. No Gpr54 expression was detected in the arcuate and rostral periventricular nuclei of the hypothalamus. Dual-labeling experiments showed that essentially all Gpr54-expressing cells in the rPOA were GnRH neurons. Analyses of mice at birth, P1, P5, P20, and P30 and as adults revealed a gradual increase in the percentage of GnRH neurons expressing Gpr54 from approximately 40% at birth through to approximately 70% from P20 onward. Whereas GnRH neurons located in the septum displayed a consistent increase across this time, GnRH neurons in the rPOA showed a sharp reduction in Gpr54 expression after birth (to approximately 10% at P5) before increasing to the 70% expression levels by P20. Together these findings provide an anatomical basis for the exploration of Gpr54 actions outside the reproductive axis and reveal a complex temporal and spatial pattern of Gpr54 gene expression in developing GnRH neurons.

A balanced diet is necessary for proper entrainment signals of the mouse liver clock

Background

The peripheral circadian clock in mice is entrained not only by light-dark cycles but also by daily restricted feeding schedules. Behavioral and cell culture experiments suggest an increase in glucose level as a factor in such feeding-induced entrainment. For application of feeding-induced entrainment in humans, nutrient content and dietary variations should be considered.

Principal Finding

To elucidate the food composition necessary for dietary entrainment, we examined whether complete or partial substitution of dietary nutrients affected phase shifts in liver clocks of mice. Compared with fasting mice or ad libitum fed mice, the liver bioluminescence rhythm advanced by 3–4 h on the middle day in Per2::luciferase knock-in mice that were administered a standard mouse diet, i.e. AIN-93M formula [0.6–0.85 g/10 g mouse BW] (composition: 14% casein, 47% cornstarch, 15% gelatinized cornstarch, 10% sugar, 4% soybean oil, and 10% other [fiber, vitamins, minerals, etc.]), for 2 days. When each nutrient was tested alone (100% nutrient), an insignificant weak phase advance was found to be induced by cornstarch and soybean oil, but almost no phase advance was induced by gelatinized cornstarch, high-amylose cornstarch, glucose, sucrose, or casein. A combination of glucose and casein without oil, vitamin, or fiber caused a significant phase advance. When cornstarch in AIN-93M was substituted with glucose, sucrose, fructose, polydextrose, high-amylose cornstarch, or gelatinized cornstarch, the amplitude of phase advance paralleled the increase in blood glucose concentration.

Conclusions

Our results strongly suggest the following: (1) balanced diets containing carbohydrates/sugars and proteins are good for restricted feeding-induced entrainment of the peripheral circadian clock and (2) a balanced diet that increases blood glucose, but not by sugar alone, is suitable for entrainment. These findings may assist in the development of dietary recommendations for on-board meals served to air travelers and shift workers to reduce jet lag-like symptoms.