The suprachiasmatic nucleus, circadian clocks, and the liver

Brain Dopamine-Clock Interactions Regulate Cardiometabolic Physiology: Mechanisms of the Observed Cardioprotective Effects of Circadian-Timed Bromocriptine-QR Therapy in Type 2 Diabetes Subjects

Despite enormous global efforts within clinical research and medical practice to reduce cardiovascular disease(s) (CVD), it still remains the leading cause of death worldwide. While genetic factors clearly contribute to CVD etiology, the preponderance of epidemiological data indicate that a major common denominator among diverse ethnic populations from around the world contributing to CVD is the composite of Western lifestyle cofactors, particularly Western diets (high saturated fat/simple sugar [particularly high fructose and sucrose and to a lesser extent glucose] diets), psychosocial stress, depression, and altered sleep/wake architecture. Such Western lifestyle cofactors are potent drivers for the increased risk of metabolic syndrome and its attendant downstream CVD. The central nervous system (CNS) evolved to respond to and anticipate changes in the external (and internal) environment to adapt survival mechanisms to perceived stresses (challenges to normal biological function), including the aforementioned Western lifestyle cofactors. Within the CNS of vertebrates in the wild, the biological clock circuitry surveils the environment and has evolved mechanisms for the induction of the obese, insulin-resistant state as a survival mechanism against an anticipated ensuing season of low/no food availability. The peripheral tissues utilize fat as an energy source under muscle insulin resistance, while increased hepatic insulin resistance more readily supplies glucose to the brain. This neural clock function also orchestrates the reversal of the obese, insulin-resistant condition when the low food availability season ends. The circadian neural network that produces these seasonal shifts in metabolism is also responsive to Western lifestyle stressors that drive the CNS clock into survival mode. A major component of this natural or Western lifestyle stressor-induced CNS clock neurophysiological shift potentiating the obese, insulin-resistant state is a diminution of the circadian peak of dopaminergic input activity to the pacemaker clock center, suprachiasmatic nucleus. Pharmacologically preventing this loss of circadian peak dopaminergic activity both prevents and reverses existing metabolic syndrome in a wide variety of animal models of the disorder, including high fat-fed animals. Clinically, across a variety of different study designs, circadian-timed bromocriptine-QR (quick release) (a unique formulation of micronized bromocriptine-a dopamine D2 receptor agonist) therapy of type 2 diabetes subjects improved hyperglycemia, hyperlipidemia, hypertension, immune sterile inflammation, and/or adverse cardiovascular event rate. The present review details the seminal circadian science investigations delineating important roles for CNS circadian peak dopaminergic activity in the regulation of peripheral fuel metabolism and cardiovascular biology and also summarizes the clinical study findings of bromocriptine-QR therapy on cardiometabolic outcomes in type 2 diabetes subjects.

Photobiomodulation at molecular, cellular, and systemic levels

Since the reporting of Endre Mester's results, researchers have investigated the biological effects induced by non-ionizing radiation emitted from low-power lasers. Recently, owing to the use of light-emitting diodes (LEDs), the term photobiomodulation (PBM) has been used. However, the molecular, cellular, and systemic effects involved in PBM are still under investigation, and a better understanding of these effects could improve clinical safety and efficacy. Our aim was to review the molecular, cellular, and systemic effects involved in PBM to elucidate the levels of biological complexity. PBM occurs as a consequence of photon-photoacceptor interactions, which lead to the production of trigger molecules capable of inducing signaling, effector molecules, and transcription factors, which feature it at the molecular level. These molecules and factors are responsible for cellular effects, such as cell proliferation, migration, differentiation, and apoptosis, which feature PBM at the cellular level. Finally, molecular and cellular effects are responsible for systemic effects, such as modulation of the inflammatory process, promotion of tissue repair and wound healing, reduction of edema and pain, and improvement of muscle performance, which features PBM at the systemic level.

Photobiomodulation for Alzheimer's disease: photoelectric coupling effect on attenuating Aβ neurotoxicity

Alzheimer's disease (AD) and dementia are the most worrying health problems faced by people globally today. Although the pathological features of AD consisting of amyloid-beta (Aβ) plaques in the extracellular space (ECS) and intracellular tau tangles are well established, the developed medicines targeting these two proteins have not obtained the expected clinical effects. Photobiomodulation (PBM) describes the therapeutic use of red light (RL) or near-infrared light (NIR) to serve as a noninvasive neuroprotective strategy for brain diseases. The present review discusses the mechanisms of the photoelectric coupling effect (light energy-induced special electronic transition-related alterations in protein structure) of PBM on reducing Aβ toxicity. On the one hand, RL or NIR can directly disassemble Aβ in vitro and in vivo. On the other hand, formaldehyde (FA)-inhibited catalase (CAT) and H2O2-inactived formaldehyde dehydrogenase (FDH) are formed a vicious circle in AD; however, light energy not only activates FDH to degrade excessive FA (which crosslinks Aβ monomer to form Aβ oligomers and senile plaques) but also sensitizes CAT to reduce hydrogen peroxide levels (H2O2, which can facilitate Aβ aggregation and enhance FA generation). In addition, it also activates mitochondrial cytochrome-c to produce ATP in the neurons. Clinical trials of phototherapeutics or oral coenzyme Q10 have shown positive effects in AD patients. Hence, a promising strategy combined PBM with nanopacked Q10 has been proposed to apply for treating AD.

Skipping breakfast regimen induces an increase in body weight and a decrease in muscle weight with a shifted circadian rhythm in peripheral tissues of mice

Meal timing is a key factor in synchronising the circadian clock in peripheral tissues. Circadian disorders are associated with the metabolic syndrome. Previously, we demonstrated that a skipping breakfast regimen (SBR) with a high-fat diet increased body weight gain in rats. In this study, we investigated whether SBR with a normal diet led to abnormal lipid metabolism and muscle metabolism in mice. Male C57BL/6 mice were fed during zeitgeber time (ZT) 12-24 in the control group and ZT 16-24 in the SBR group for 2 weeks. SBR mice showed increased body weight gain and perirenal adipose tissue weight. The plantar muscle weight was decreased in the SBR group compared with that in the control group. Furthermore, SBR delayed the circadian oscillations in clock gene expression in peripheral tissues, such as the liver, adipose tissue and muscle, as well as the oscillations in the expression of lipid metabolism-related genes in the liver and adipose tissue. These results suggest that skipping breakfast over a long period of time is associated with a risk of obesity, the metabolic syndrome and muscle loss, such as sarcopenia.

The Change in Circadian Rhythms in P301S Transgenic Mice is Linked to Variability in Hsp70-related Tau Disaggregation

Circadian disruption often involves a neurodegenerative disorder, such as Alzheimer's disease or frontotemporal dementia, which are characterized by intraneuronal tau accumulations. The altered sleep pattern and diurnal rhythms in these disorders are the results of tau pathology. The circadian disturbance in reverse is thought to develop and potentially aggravate the condition. However, the underlying mechanism is not fully understood. In this study, perturbed oscillations in BMAL1 , the core clock gene, were observed in P301S tau transgenic mice. Tau fractionation analysis of the hippocampus revealed profound fluctuations in soluble and insoluble tau protein levels that were in opposite directions to each other according to zeitgeber time. Interestingly, a diurnal oscillation was detected in the heat shock 70 kDa protein 1A (Hsp70) chaperone that was in-phase with soluble tau but out-of-phase with insoluble tau. Tau protein levels decreased in the soluble and insoluble fractions when Hsp70 was overexpressed in HEK293T cells. Transfection of the BMAL1 carrying vector was continual with the increase in Hsp70 expression and diminished tau protein levels, and it was effectively attenuated by the knockdown of Hsp70, suggesting that Bmal1 could modulate tau protein by Hsp70. Our results suggest that altered circadian oscillations affect tau status and solubility by modulating Hsp70 expression in an experimental model of tau pathology. These findings suggest Hsp70 as a possible pathogenic link between circadian disruption and aggravations of tau pathology.

Satiety Associated with Calorie Restriction and Time-Restricted Feeding: Central Neuroendocrine Integration

This review focuses on summarizing current knowledge on how time-restricted feeding (TRF) and continuous caloric restriction (CR) affect central neuroendocrine systems involved in regulating satiety. Several interconnected regions of the hypothalamus, brainstem, and cortical areas of the brain are involved in the regulation of satiety. Following CR and TRF, the increase in hunger and reduction in satiety signals of the melanocortin system [neuropeptide Y (NPY), proopiomelanocortin (POMC), and agouti-related peptide (AgRP)] appear similar between CR and TRF protocols, as do the dopaminergic responses in the mesocorticolimbic circuit. However, ghrelin and leptin signaling via the melanocortin system appears to improve energy balance signals and reduce hyperphagia following TRF, which has not been reported in CR. In addition to satiety systems, CR and TRF also influence circadian rhythms. CR influences the suprachiasmatic nucleus (SCN) or the primary circadian clock as seen by increased clock gene expression. In contrast, TRF appears to affect both the SCN and the peripheral clocks, as seen by phasic changes in the non-SCN (potentially the elusive food entrainable oscillator) and metabolic clocks. The peripheral clocks are influenced by the primary circadian clock but are also entrained by food timing, sleep timing, and other lifestyle parameters, which can supersede the metabolic processes that are regulated by the primary circadian clock. Taken together, TRF influences hunger/satiety, energy balance systems, and circadian rhythms, suggesting a role for adherence to CR in the long run if implemented using the TRF approach. However, these suggestions are based on only a few studies, and future investigations that use standardized protocols for the evaluation of the effect of these diet patterns (time, duration, meal composition, sufficiently powered) are necessary to verify these preliminary observations.

Reference Gene Optimization for Circadian Gene Expression Analysis in Human Adipose Tissue

A hallmark of biology is the cyclical nature of organismal physiology driven by networks of biological, including circadian, rhythms. Unsurprisingly, disruptions of the circadian rhythms through sleep curtailment or shift work have been connected through numerous studies to positive associations with obesity, insulin resistance, and diabetes. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) measures oscillation in messenger RNA expression, an essential foundation for the study of the physiological circadian regulatory network. Primarily, measured oscillations have involved the use of reference gene normalization. However, the validation and identification of suitable reference genes is a significant challenge across different biological systems. This study focuses on adipose tissue of premenopausal, otherwise healthy, morbidly obese women voluntarily enrolled after being scheduled for laparoscopic sleeve gastrectomy surgery. Acquisition of tissue was accomplished by aspiratory needle biopsies of subcutaneous adipose tissue 1 to 2 weeks prior to surgery and 12 to 13 weeks following surgery and an in-surgery scalpel-assisted excision of mesenteric adipose tissue. Each biopsy was sterile cultured ex vivo and serially collected every 4 h over approximately 36 h. The candidate reference genes that were tested were 18S rRNA, GAPDH, HPRT1, RPII, RPL13α, and YWHAZ. Three analytic tools were used to test suitability, and the candidate reference genes were used to measure oscillation in expression of a known circadian clock element (Dbp). No gene was deemed suitable as an individual reference gene control, which indicated that the optimal reference gene set was the geometrically averaged 3-gene panel composed of YWHAZ, RPL13α, and GAPDH. These methods can be employed to identify optimal reference genes in other systems.

Circadian Regulation of Cochlear Sensitivity to Noise by Circulating Glucocorticoids

The cochlea possesses a robust circadian clock machinery that regulates auditory function. How the cochlear clock is influenced by the circadian system remains unknown. Here, we show that cochlear rhythms are system driven and require local Bmal1 as well as central input from the suprachiasmatic nuclei (SCN). SCN ablations disrupted the circadian expression of the core clock genes in the cochlea. Because the circadian secretion of glucocorticoids (GCs) is controlled by the SCN and GCs are known to modulate auditory function, we assessed their influence on circadian gene expression. Removal of circulating GCs by adrenalectomy (ADX) did not have a major impact on core clock gene expression in the cochlea. Rather it abolished the transcription of clock-controlled genes involved in inflammation. ADX abolished the known differential auditory sensitivity to day and night noise trauma and prevented the induction of GABA-ergic and glutamate receptors mRNA transcripts. However, these improvements were unrelated to changes at the synaptic level, suggesting other cochlear functions may be involved. Due to this circadian regulation of noise sensitivity by GCs, we evaluated the actions of the synthetic glucocorticoid dexamethasone (DEX) at different times of the day. DEX was effective in protecting from acute noise trauma only when administered during daytime, when circulating glucocorticoids are low, indicating that chronopharmacological approaches are important for obtaining optimal treatment strategies for hearing loss. GCs appear as a major regulator of the differential sensitivity to day or night noise trauma, a mechanism likely involving the circadian control of inflammatory responses.

Circadian integration of inflammation and glucocorticoid actions: Implications for the cochlea

Auditory function has been shown to be influenced by the circadian system. Increasing evidence point towards the regulation of inflammation and glucocorticoid actions by circadian rhythms in the cochlea. Yet, how these three systems (circadian, immune and endocrine) converge to control auditory function remains to be established. Here we review the knowledge on immune and glucocorticoid actions, and how they interact with the circadian and the auditory system, with a particular emphasis on cochlear responses to noise trauma. We propose a multimodal approach to understand the mechanisms of noise-induced hearing loss by integrating the circadian, immune and endocrine systems into the bearings of the cochlea. Considering the well-established positive impact of chronotherapeutic approaches in the treatment of cardiovascular, asthma and cancer, an increased knowledge on the mechanisms where circadian, immune and glucocorticoids meet in the cochlea may improve current treatments against hearing disorders.

Microbiome diurnal rhythmicity and its impact on host physiology and disease risk

Host-microbiome interactions constitute key determinants of host physiology, while their dysregulation is implicated in a wide range of human diseases. The microbiome undergoes diurnal variation in composition and function, and this in turn drives oscillations in host gene expression and functions. In this review, we discuss the newest developments in understanding circadian host-microbiome interplays, and how they may be relevant in health and disease contexts. We summarize the molecular mechanisms by which the microbiome influences host function in a diurnal manner, and inversely describe how the host orchestrates circadian rhythmicity of the microbiome. Furthermore, we highlight the future perspectives and challenges in studying this new and exciting facet of host-microbiome interactions. Finally, we illustrate how the elucidation of the microbiome chronobiology may pave the way for novel therapeutic approaches.

Paradigm Shifts in Nocturnal Glucose Control in Type 2 Diabetes

CONTEXT

A better understanding of nocturnal regulation of glucose homeostasis will provide the framework for designing rational therapeutic strategies to improve the management of overnight glucose in patients with type 2 diabetes (T2D).

OBJECTIVE

To establish the nocturnal pattern and regulation of glucose production (EGP) in humans and to determine whether the pattern is dysregulated in people with T2D.

DESIGN

Subjects were infused with [3-3H] glucose overnight. Arterial blood samples were drawn for hormones and analytes to estimate EGP throughout the night. Deuterium-labeled water was provided to measure gluconeogenesis (GNG) using the hexamethylenetetramine method of Landau.

SETTING

Mayo Clinic Clinical Research Trials Unit, Rochester, MN, USA.

PARTICIPANTS AND INTERVENTIONS

A total of 43 subjects [23 subjects with T2D and 20 nondiabetic (ND) subjects comparable for age and body mass index] were included in this study.

MAIN OUTCOME(S) MEASURE(S)

Glucose and EGP.

RESULTS

Plasma glucose, C-peptide, and glucagon concentrations were higher throughout the night, whereas insulin concentrations were higher in subjects with T2D vs ND subjects at 1:00 and 4:00 am but similar at 7:00 am. EGP was higher in the subjects with T2D than in the ND subjects throughout the night (P < 0.001). Glycogenolysis (GGL) fell and GNG rose, resulting in significantly higher (P < 0.001) rates of GNG at 4:00 and 7:00 am and significantly (P < 0.001) higher rates of GGL at 1:00, 4:00, and 7:00 am in T2D as compared with ND.

CONCLUSIONS

These data imply that optimal therapies for T2D for nocturnal/fasting glucose control should target not only the absolute rates of EGP but also the contributing pathways of GGL and GNG sequentially.

Insights into the Role of Circadian Rhythms in Bone Metabolism: A Promising Intervention Target?

Numerous physiological processes of mammals, including bone metabolism, are regulated by the circadian clock system, which consists of a central regulator, the suprachiasmatic nucleus (SCN), and the peripheral oscillators of the BMAL1/CLOCK-PERs/CRYs system. Various bone turnover markers and bone metabolism-regulating hormones such as melatonin and parathyroid hormone (PTH) display diurnal rhythmicity. According to previous research, disruption of the circadian clock due to shift work, sleep restriction, or clock gene knockout is associated with osteoporosis or other abnormal bone metabolism, showing the importance of the circadian clock system for maintaining homeostasis of bone metabolism. Moreover, common causes of osteoporosis, including postmenopausal status and aging, are associated with changes in the circadian clock. In our previous research, we found that agonism of the circadian regulators REV-ERBs inhibits osteoclast differentiation and ameliorates ovariectomy-induced bone loss in mice, suggesting that clock genes may be promising intervention targets for abnormal bone metabolism. Moreover, osteoporosis interventions at different time points can provide varying degrees of bone protection, showing the importance of accounting for circadian rhythms for optimal curative effects in clinical treatment of osteoporosis. In this review, we summarize current knowledge about circadian rhythms and bone metabolism.

Hypothalamic glucagon signaling in fasting hypoglycemia

AIMS

Sustained glucagon infusion increases hepatic glucose production, but this effect is transient due to hypothalamic glucagon signaling. In hypoglycemia, glucagon acts as a major defense to sustain the blood glucose level and this raises the question regarding glucagon signaling associated glucose production in prolonged fasting hypoglycemia. In this study, we investigated the proteins associated with hypothalamic glucagon signaling and liver gluconeogenesis during fasting hypoglycemia.

MAIN METHODS

8-9week old, male C57BL6/J mice were fasted for 4, 8, 12, 18, 24, 30, 36 or 42h. In the hypothalamus, we investigated glucagon signaling by analyzing the glucagon receptor and its downstream protein, peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC-1) expression. In the liver, we investigated gluconeogenesis by analyzing p-protein kinase A (PKA)(Ser/Thr) substrate and phosphoenolpyruvate carboxykinase - cytosolic (PEPCK-C) expression using the western blotting technique.

KEY FINDINGS

The elevated or trended higher hypothalamic glucagon receptor and PGC-1 expressions at 18 and 42h were correlated with the attenuated liver p-PKA(Ser/Thr) substrate expression. The attenuated hypothalamic glucagon receptor and PGC-1 expressions at 12, 24, 30 and 36h were correlated with the elevated or trended higher liver p-PKA(Ser/Thr) substrate expression.

SIGNIFICANCE

The hypothalamic glucagon signaling during fasting hypoglycemia might have been modulated by circadian rhythm and this possibly attenuates the liver p-PKA(Ser/Thr) substrate to modify the gluconeogenesis pathway. This mechanism will help to understand the hyperglucagonemia associated complications in diabetes.

Comparison of a carbohydrate-free diet vs. fasting on plasma glucose, insulin and glucagon in type 2 diabetes

OBJECTIVE

Hyperglycemia improves when patients with type 2 diabetes are placed on a weight-loss diet. Improvement typically occurs soon after diet implementation. This rapid response could result from low fuel supply (calories), lower carbohydrate content of the weight-loss diet, and/or weight loss per se. To differentiate these effects, glucose, insulin, C-peptide and glucagon were determined during the last 24 h of a 3-day period without food (severe calorie restriction) and a calorie-sufficient, carbohydrate-free diet.

RESEARCH DESIGN

Seven subjects with untreated type 2 diabetes were studied. A randomized-crossover design with a 4-week washout period between arms was used.

METHODS

Results from both the calorie-sufficient, carbohydrate-free diet and the 3-day fast were compared with the initial standard diet consisting of 55% carbohydrate, 15% protein and 30% fat.

RESULTS

The overnight fasting glucose concentration decreased from 196 (standard diet) to 160 (carbohydrate-free diet) to 127 mg/dl (fasting). The 24 h glucose and insulin area responses decreased by 35% and 48% on day 3 of the carbohydrate-free diet, and by 49% and 69% after fasting. Overnight basal insulin and glucagon remained unchanged.

CONCLUSIONS

Short-term fasting dramatically lowered overnight fasting and 24 h integrated glucose concentrations. Carbohydrate restriction per se could account for 71% of the reduction. Insulin could not entirely explain the glucose responses. In the absence of carbohydrate, the net insulin response was 28% of the standard diet. Glucagon did not contribute to the metabolic adaptations observed.