IMPAIRED GLUCOSE METABOLISM IN THE ABSENCE OF SKELETAL MUSCLE BRAIN AND MUSCLE ARNT-LIKE-PROTEIN 1 (\u3cem\u3eBMAL1\u3c/em\u3e)

Metabolism is a critical physiological function that works to generate energy for cells, store substrates and maintain homoeostasis. Alterations in normal metabolism can have a severe effect on physiology, leading to metabolic disease. Skeletal muscle is a key metabolic tissue, taking up ~80% of postprandial glucose. Therefore it contributes considerably to glucose metabolism: glucose uptake, oxidation and homeostasis. To address the role of the skeletal muscle clock in insulin sensitivity and glucose tolerance, our lab generated an inducible skeletal muscle specific Bmal1-/- mouse (iMSBmal1-/-). 5 weeks post-recombination we observed impairment in both insulin- and AICAR-stimulated skeletal muscle glucose uptake. RT-PCR and western blot analysis demonstrated a significant decrease in mRNA expression and protein content of the skeletal muscle glucose transporter, Glut4. Glucose uptake may be affected by glucose utilization so we examined aspects of glycolysis in the skeletal muscle. Both mRNA expression and activity of rate limiting enzymes hexokinase 2 (Hk2) and phosphofructokinase 1 (Pfk1) were significantly reduced. Additionally, metabolomics illustrated a reduction in metabolites of the glycolytic pathway further supporting a decrease in glycolytic flux. These changes in skeletal muscle glucose metabolism led to altered overall body metabolic health. iMSBmal1-/- mice presented with glucose intolerance and non-fasting hyperglycemia. Furthermore, changes in body composition were seen from 5-12 weeks post-recombination. These data propose a critical role for skeletal muscle Bmal1 in both skeletal muscle glucose metabolism and overall body metabolic health. The presented findings also illuminate skeletal muscle Bmal1 and circadian rhythms as potential targets for metabolic disease

Muscle-Specific Loss of \u3cem\u3eBmal1\u3c/em\u3e Leads to Disrupted Tissue Glucose Metabolism and Systemic Glucose Homeostasis

Background: Diabetes is the seventh leading cause of death in the USA, and disruption of circadian rhythms is gaining recognition as a contributing factor to disease prevalence. This disease is characterized by hyperglycemia and glucose intolerance and symptoms caused by failure to produce and/or respond to insulin. The skeletal muscle is a key insulin-sensitive metabolic tissue, taking up ~80 % of postprandial glucose. To address the role of the skeletal muscle molecular clock to insulin sensitivity and glucose tolerance, we generated an inducible skeletal muscle-specific Bmal1 −/− mouse (iMSBmal1 −/−). Results: Progressive changes in body composition (decreases in percent fat) were seen in the iMSBmal1 −/− mice from 3 to 12 weeks post-treatment as well as glucose intolerance and non-fasting hyperglycemia. Ex vivo analysis of glucose uptake revealed that the extensor digitorum longus (EDL) muscles did not respond to either insulin or 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) stimulation. RT-PCR and Western blot analyses demonstrated a significant decrease in mRNA expression and protein content of the muscle glucose transporter (Glut4). We also found that both mRNA expression and activity of two key rate-limiting enzymes of glycolysis, hexokinase 2 (Hk2) and phosphofructokinase 1 (Pfk1), were significantly reduced in the iMSBmal1 −/− muscle. Lastly, results from metabolomics analyses provided evidence of decreased glycolytic flux and uncovered decreases in some tricarboxylic acid (TCA) intermediates with increases in amino acid levels in the iMSBmal1 −/− muscle. These findings suggest that the muscle is relying predominantly on fat as a fuel with increased protein breakdown to support the TCA cycle. Conclusions: These data support a fundamental role for Bmal1, the endogenous circadian clock, in glucose metabolism in the skeletal muscle. Our findings have implicated altered molecular clock dictating significant changes in altered substrate metabolism in the absence of feeding or activity changes. The changes in body composition in our model also highlight the important role that changes in skeletal muscle carbohydrate, and fat metabolism can play in systemic metabolism

Temperature as a Circadian Marker in Older Human Subjects: Relationship to Metabolic Syndrome and Diabetes

Background: Circadian rhythms are characterized by approximate 24-hour oscillations in physiological and behavioral processes. Disruptions in these endogenous rhythms, most commonly associated with shift work and/or lifestyle, are recognized to be detrimental to health. Several studies have demonstrated a high correlation between disrupted circadian rhythms and metabolic disease. The aim of this study was to determine which metabolic parameters correlate with physiological measures of circadian temperature amplitude (TempAmp) and stability (TempStab). Methods: Wrist skin temperature was measured in 34 subjects (ages 50 to 70, including lean, obese, and diabetic subjects) every 10 minutes for 7 consecutive days. Anthropometric measures and fasting blood draws were conducted to obtain data on metabolic parameters: body mass index, hemoglobin A1C, triglycerides, cholesterol, high-density lipoprotein, and low-density lipoprotein. A history of hypertension and current blood pressure was noted. Results: Analysis of the data indicated a substantial reduction in TempAmp and TempStab in subjects with metabolic syndrome (three or more risk factors). To determine the impact of individual interdependent metabolic factors on temperature rhythms, stepwise multilinear regression analysis was conducted using metabolic syndrome measurements. Interestingly, only triglyceride level was consistently correlated by the analysis. Triglyceride level was shown to contribute to 33% of the variability in TempAmp and 23% of the variability in TempStab. Conclusion: Our results demonstrate that elevated triglycerides are associated with diminished TempAmp and TempStab in human subjects, and triglycerides may serve as a primary metabolic predictor of circadian parameters

Effect of Chronotype on Sleep Quality in a Laboratory Setting

International Journal of Exercise Science 13(3): 1283-1294, 2020. Sleep is undoubtedly important for human health as insufficient sleep has been associated with a plethora of diseases. Adequate sleep assessment is critical in clinical and research settings, however current sleep assessment protocols fail to account for circadian rhythms, despite the fact that sleep is a well-recognized circadian process. Purpose: The purpose of this study was to determine if circadian parameters, such as chronotype, influence sleep quality in a sleep laboratory setting. Methods: In order to investigate this, twenty participants (10 men and 10 women) aged 18-31 years old had their sleep recorded by electroencephalography in a sleep lab. Participants also complete surveys which provided data on chronotype, social jet lag and subjective sleep quality. Participants were allowed to self-select sleep time for the study, and sleep discrepancy, defined as the difference between reported and experienced mid-sleep, was determined. Results: Interestingly, results indicated a significant correlation between self-reported sleep quality and social jet lag, with those who typically experience more social jet lag being more satisfied with their sleep during the study (r = 0.549, p = 0.012). In addition, when participants were separated into groups based on chronotype, sleep discrepancy and social jet lag, sizeable differences were noted for parameters such as sleep onset latency, number of awakenings, and percent of time spent in REM sleep. Conclusion: These results suggest circadian parameters serve as predictors of both subjective and objective sleep quality, and thus illuminates a necessity for these parameters to be taken into account in the assessment and research of sleep

9th Annual Michigan Physiological Society Meeting: June 17-18, 2022

In this Meeting Report, we describe the 9th Annual Michigan Physiological Society Meeting

The endogenous molecular clock orchestrates the temporal separation of substrate metabolism in skeletal muscle

BACKGROUND: Skeletal muscle is a major contributor to whole-body metabolism as it serves as a depot for both glucose and amino acids, and is a highly metabolically active tissue. Within skeletal muscle exists an intrinsic molecular clock mechanism that regulates the timing of physiological processes. A key function of the clock is to regulate the timing of metabolic processes to anticipate time of day changes in environmental conditions. The purpose of this study was to identify metabolic genes that are expressed in a circadian manner and determine if these genes are regulated downstream of the intrinsic molecular clock by assaying gene expression in an inducible skeletal muscle-specific Bmal1 knockout mouse model (iMS-Bmal1-/- ). METHODS: We used circadian statistics to analyze a publicly available, high-resolution time-course skeletal muscle expression dataset. Gene ontology analysis was utilized to identify enriched biological processes in the skeletal muscle circadian transcriptome. We generated a tamoxifen-inducible skeletal muscle-specific Bmal1 knockout mouse model and performed a time-course microarray experiment to identify gene expression changes downstream of the molecular clock. Wheel activity monitoring was used to assess circadian behavioral rhythms in iMS-Bmal1-/- and control iMS-Bmal1+/+ mice. RESULTS: The skeletal muscle circadian transcriptome was highly enriched for metabolic processes. Acrophase analysis of circadian metabolic genes revealed a temporal separation of genes involved in substrate utilization and storage over a 24-h period. A number of circadian metabolic genes were differentially expressed in the skeletal muscle of the iMS-Bmal1-/- mice. The iMS-Bmal1-/- mice displayed circadian behavioral rhythms indistinguishable from iMS-Bmal1+/+ mice. We also observed a gene signature indicative of a fast to slow fiber-type shift and a more oxidative skeletal muscle in the iMS-Bmal1-/- model. CONCLUSIONS: These data provide evidence that the intrinsic molecular clock in skeletal muscle temporally regulates genes involved in the utilization and storage of substrates independent of circadian activity. Disruption of this mechanism caused by phase shifts (that is, social jetlag) or night eating may ultimately diminish skeletal muscle\u27s ability to efficiently maintain metabolic homeostasis over a 24-h period