Role of mTORC2 in biphasic regulation of brown fat metabolism in response to mild and severe cold.

Nonshivering thermogenesis is essential for mammals to maintain body temperature. According to the canonical view, temperature is sensed by cutaneous thermoreceptors and nerve impulses transmitted to the hypothalamus, which generates sympathetic signals to ß-adrenergic receptors in brown adipocytes. The energy for heat generation is primarily provided by the oxidation of fatty acids derived from triglyceride hydrolysis and cellular uptake. Fatty acids also activate the uncoupling protein, UCP1, which creates a proton leak that uncouples mitochondrial oxidative phosphorylation from ATP production, resulting in energy dissipation as heat. Recent evidence supports the idea that in response to mild cold, ß-adrenergic signals stimulate not only lipolysis and fatty acid oxidation, but also act through the mTORC2-Akt signaling module to stimulate de novo lipogenesis. This opposing anabolic effect is thought to maintain lipid fuel stores during increased catabolism. We show here, using brown fat-specific Gs-alpha knockout mice and cultured adipocytes that, unlike mild cold, severe cold directly cools brown fat and bypasses ß-adrenergic signaling to inhibit mTORC2. This cell-autonomous effect both inhibits lipogenesis and augments UCP1 expression to enhance thermogenesis. These findings suggest a novel mechanism for overriding ß-adrenergic-stimulated anabolic activities while augmenting catabolic activities to resolve the homeostatic crisis presented by severe cold.

ATAC-Seq Reveals an Isl1 Enhancer That Regulates Sinoatrial Node Development and Function.

RATIONALE: Cardiac pacemaker cells (PCs) in the sinoatrial node (SAN) have a distinct gene expression program that allows them to fire automatically and initiate the heartbeat. Although critical SAN transcription factors, including Isl1 (Islet-1), Tbx3 (T-box transcription factor 3), and Shox2 (short-stature homeobox protein 2), have been identified, the cis-regulatory architecture that governs PC-specific gene expression is not understood, and discrete enhancers required for gene regulation in the SAN have not been identified. OBJECTIVE: To define the epigenetic profile of PCs using comparative ATAC-seq (assay for transposase-accessible chromatin with sequencing) and to identify novel enhancers involved in SAN gene regulation, development, and function. METHODS AND RESULTS: We used ATAC-seq on sorted neonatal mouse SAN to compare regions of accessible chromatin in PCs and right atrial cardiomyocytes. PC-enriched assay for transposase-accessible chromatin peaks, representing candidate SAN regulatory elements, were located near established SAN genes and were enriched for distinct sets of TF (transcription factor) binding sites. Among several novel SAN enhancers that were experimentally validated using transgenic mice, we identified a 2.9-kb regulatory element at the Isl1 locus that was active specifically in the cardiac inflow at embryonic day 8.5 and throughout later SAN development and maturation. Deletion of this enhancer from the genome of mice resulted in SAN hypoplasia and sinus arrhythmias. The mouse SAN enhancer also directed reporter activity to the inflow tract in developing zebrafish hearts, demonstrating deep conservation of its upstream regulatory network. Finally, single nucleotide polymorphisms in the human genome that occur near the region syntenic to the mouse enhancer exhibit significant associations with resting heart rate in human populations. CONCLUSIONS: (1) PCs have distinct regions of accessible chromatin that correlate with their gene expression profile and contain novel SAN enhancers, (2) cis-regulation of Isl1 specifically in the SAN depends upon a conserved SAN enhancer that regulates PC development and SAN function, and (3) a corresponding human ISL1 enhancer may regulate human SAN function.

A haplotype variant of the human chromogranin A gene (CHGA) promoter increases CHGA expression and the risk for cardiometabolic disorders.

The acidic glycoprotein chromogranin A (CHGA) is co-stored/co-secreted with catecholamines and crucial for secretory vesicle biogenesis in neuronal/neuroendocrine cells. CHGA is dysregulated in several cardiovascular diseases, but the underlying mechanisms are not well established. Here, we sought to identify common polymorphisms in the CHGA promoter and to explore the mechanistic basis of their plausible contribution to regulating CHGA protein levels in circulation. Resequencing of the CHGA promoter in an Indian population (n = 769) yielded nine single-nucleotide polymorphisms (SNPs): G-1106A, A-1018T, T-1014C, T-988G, G-513A, G-462A, T-415C, C-89A, and C-57T. Linkage disequilibrium (LD) analysis indicated strong LD among SNPs at the -1014, -988, -462, and -89 bp positions and between the -1018 and -57 bp positions. Haplotype analysis predicted five major promoter haplotypes that displayed differential promoter activities in neuronal cells; specifically, haplotype 2 (containing variant T alleles at -1018 and -57 bp) exhibited the highest promoter activity. Systematic computational and experimental analyses revealed that transcription factor c-Rel has a role in activating the CHGA promoter haplotype 2 under basal and pathophysiological conditions (viz. inflammation and hypoxia). Consistent with the higher in vitro CHGA promoter activity of haplotype 2, individuals carrying this haplotype had higher plasma CHGA levels, plasma glucose levels, diastolic blood pressure, and body mass index. In conclusion, these results suggest a functional role of the CHGA promoter haplotype 2 (occurring in a large proportion of the world population) in enhancing CHGA expression in haplotype 2 carriers who may be at higher risk for cardiovascular/metabolic disorders.

Detection of localized changes in the metabolism of hyperpolarized gluconeogenic precursors 13 C-lactate and 13 C-pyruvate in kidney and liver.

PURPOSE: The purpose of this study was to characterize tissue-specific alterations in metabolism of hyperpolarized (HP) gluconeogenic precursors 13 C-lactate and 13 C-pyruvate by rat liver and kidneys under conditions of fasting or insulin-deprived diabetes. METHODS: Seven normal rats were studied by MR spectroscopic imaging of both HP 13 C-lactate and 13 C-pyruvate in both normal fed and 24 h fasting states, and seven additional rats were scanned after induction of diabetes by streptozotocin (STZ) with insulin withdrawal. Phosphoenolpyruvate carboxykinase (PEPCK) expression levels were also measured in liver and kidney tissues of the STZ-treated rats. RESULTS: Multiple sets of significant signal modulations were detected, with graded intensity in general between fasting and diabetic states. An approximate two-fold reduction in the ratio of 13 C-bicarbonate to total 13 C signal was observed in both organs in fasting. The ratio of HP lactate-to-alanine was markedly altered, ranging from a liver-specific 54% increase in fasting, to increases of 69% and 92% in liver and kidney, respectively, in diabetes. Diabetes resulted in a 40% increase in renal lactate signal. STZ resulted in 5.86-fold and 2.73-fold increases in PEPCK expression in liver and kidney, respectively. CONCLUSION: MRI of HP 13 C gluconeogenic precursors may advance diabetes research by clarifying organ-specific roles in abnormal diabetic metabolism. Magn Reson Med 77:1429-1437, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Naturally occurring variants of the dysglycemic peptide pancreastatin: differential potencies for multiple cellular functions and structure-function correlation.

Pancreastatin (PST), a chromogranin A-derived peptide, is a potent physiological inhibitor of glucose-induced insulin secretion. PST also triggers glycogenolysis in liver and reduces glucose uptake in adipocytes and hepatocytes. Here, we probed for genetic variations in PST sequence and identified two variants within its functionally important carboxyl terminus domain: E287K and G297S. To understand functional implications of these amino acid substitutions, we tested the effects of wild-type (PST-WT), PST-287K, and PST-297S peptides on various cellular processes/events. The rank order of efficacy to inhibit insulin-stimulated glucose uptake was: PST-297S > PST-287K > PST-WT. The PST peptides also displayed the same order of efficacy for enhancing intracellular nitric oxide and Ca(2+) levels in various cell types. In addition, PST peptides activated gluconeogenic genes in the following order: PST-297S ≈ PST-287K > PST-WT. Consistent with these in vitro results, the common PST variant allele Ser-297 was associated with significantly higher (by ∼17 mg/dl, as compared with the wild-type Gly-297 allele) plasma glucose level in our study population (n = 410). Molecular modeling and molecular dynamics simulations predicted the following rank order of α-helical content: PST-297S > PST-287K > PST-WT. Corroboratively, circular dichroism analysis of PST peptides revealed significant differences in global structures (e.g. the order of propensity to form α-helix was: PST-297S ≈ PST-287K > PST-WT). This study provides a molecular basis for enhanced potencies/efficacies of human PST variants (likely to occur in ∼300 million people worldwide) and has quantitative implications for inter-individual variations in glucose/insulin homeostasis.

A common genetic variant of the chromogranin A-derived peptide catestatin is associated with atherogenesis and hypertension in a Japanese population.

Chromogranin A (CHGA) is a major protein in the secretory granules of chromaffin cells. CHGA also gives rise to cardiovascular/metabolism regulatory peptides, such as catestatin (CST) and pancreastatin (PST). While CST is a potent inhibitor of catecholamine secretion, PST is a potent physiological inhibitor of glucose-induced insulin secretion. Recently, several SNPs were identified in the CST and PST domains of CHGA locus in different populations. Among the discovered SNPs, CST variant allele Ser-364 was associated with blood pressure alteration and PST variant allele Ser-297 was associated with significantly higher plasma glucose level. In this study, we examined whether these CST and PST variant alleles exist and influence cardiovascular and metabolic phenotypes in Japanese population. Our study comprised of 343 Japanese subjects aged 45-85 years (143 men and 200 women, mean age 66 ± 8 years). We determined the genotypes of CST and PST by PCR-direct sequencing method and carried out genotype-phenotype association analysis. In 343 participants, the minor allele frequency of CST variant Ser-364 was 6.10%. On the other hand, we did not detect the PST variant Ser-297 in this entire study population. The presence of Ser-364 allele was associated with increased in baPWV (an index of systemic arterial stiffness) that suggests an initiation and/or progression atherogenesis and hypertension. The Ser-364 allele was also associated with elevated systolic blood pressure and pulse pressure, consistent with increased baPWV. In conclusion, the CST Ser-364 allele may increase the risk for cardiovascular diseases in Japanese population.

Molecular interactions of the physiological anti-hypertensive peptide catestatin with the neuronal nicotinic acetylcholine receptor.

Catestatin (CST), a chromogranin-A-derived peptide, is a potent endogenous inhibitor of the neuronal nicotinic acetylcholine receptor (nAChR). It exerts an anti-hypertensive effect by acting as a 'physiological brake' on transmitter release into the circulation. However, the mechanism of interaction of CST with nAChR is only partially understood. To unravel molecular interactions of the wild-type human CST (CST-WT) as well as its naturally occurring variants (CST-364S and CST-370L, which have Gly→Ser and Pro→Leu substitutions, respectively) with the human α3ß4 nAChR, we generated a homology-modeled human α3ß4 nAChR structure and solution structures of CST peptides. Docking and molecular dynamics simulations showed that ~90% of interacting residues were within 15 N-terminal residues of CST peptides. The rank order of binding affinity of these peptides with nAChR was: CST-370L>CST-WT>CST-364S; the extent of occlusion of the receptor pore by these peptides was also in the same order. In corroboration with computational predictions, circular dichroism analysis revealed significant differences in global structures of CST peptides (e.g. the order of α-helical content was: CST-370L>CST-WT>CST-364S). Consistently, CST peptides blocked various stages of nAChR signal transduction, such as nicotine- or acetylcholine-evoked inward current, rise in intracellular Ca(2+) and catecholamine secretion in or from neuron-differentiated PC12 cells, in the same rank order. Taken together, this study shows molecular interactions between human CST peptides and human α3ß4 nAChR, and demonstrates that alterations in the CST secondary structure lead to the gain of potency for CST-370L and loss of potency for CST-364S. These findings have implications for understanding the nicotinic cholinergic signaling in humans.

Functional genetic variants of the catecholamine-release-inhibitory peptide catestatin in an Indian population: allele-specific effects on metabolic traits.

Catestatin (CST), a chromogranin A (CHGA)-derived peptide, is a potent inhibitor of catecholamine release from adrenal chromaffin cells and postganglionic sympathetic axons. We re-sequenced the CST region of CHGA in an Indian population (n = 1010) and detected two amino acid substitution variants: G364S and G367V. Synthesized CST variant peptides (viz. CST-Ser-364 and CST-Val-367) were significantly less potent than the wild type peptide (CST-WT) to inhibit nicotine-stimulated catecholamine secretion from PC12 cells. Consistently, the rank-order of blockade of nicotinic acetylcholine receptor (nAChR)-stimulated inward current and intracellular Ca(2+) rise by these peptides in PC12 cells was: CST-WT > CST-Ser-364 > CST-Val-367. Structural analysis by CD spectroscopy coupled with molecular dynamics simulations revealed the following order of α-helical content: CST-WT > CST-Ser-364 > CST-Val-367; docking of CST peptides onto a major human nAChR subtype and molecular dynamics simulations also predicted the above rank order for their binding affinity with nAChR and the extent of occlusion of the receptor pore, providing a mechanistic basis for differential potencies. The G364S polymorphism was in strong linkage disequilibrium with several common CHGA genetic variations. Interestingly, the Ser-364 allele (detected in ∼15% subjects) was strongly associated with profound reduction (up to ∼2.1-fold) in plasma norepinephrine/epinephrine levels consistent with the diminished nAChR desensitization-blocking effect of CST-Ser-364 as compared with CST-WT. Additionally, the Ser-364 allele showed strong associations with elevated levels of plasma triglyceride and glucose levels. In conclusion, a common CHGA variant in an Indian population influences several biochemical parameters relevant to cardiovascular/metabolic disorders.

FoxK1 associated gene regulatory network in hepatic insulin action and its relationship to FoxO1 and insulin receptor mediated transcriptional regulation.

OBJECTIVE: Insulin acts on the liver via changes in gene expression to maintain glucose and lipid homeostasis. This study aimed to the Forkhead box protein K1 (FOXK1) associated gene regulatory network as a transcriptional regulator of hepatic insulin action and to determine its role versus FoxO1 and possible actions of the insulin receptor at the DNA level. METHODS: Genome-wide analysis of FoxK1 binding were studied by chromatin immunoprecipitation sequencing and compared to those for IR and FoxO1. These were validated by knockdown experiments and gene expression analysis. RESULTS: Chromatin immunoprecipitation (ChIP) sequencing shows that FoxK1 binds to the proximal promoters and enhancers of over 4000 genes, and insulin enhances this interaction for about 75% of them. These include genes involved in cell cycle, senescence, steroid biosynthesis, autophagy, and metabolic regulation, including glucose metabolism and mitochondrial function and are enriched in a TGTTTAC consensus motif. Some of these genes are also bound by FoxO1. Comparing this FoxK1 ChIP-seq data to that of the insulin receptor (IR) reveals that FoxK1 may act as the transcription factor partner for some of the previously reported roles of IR in gene regulation, including for LARS1 and TIMM22, which are involved in rRNA processing and cell cycle. CONCLUSION: These data demonstrate that FoxK1 is an important regulator of gene expression in response to insulin in liver and may act in concert with FoxO1 and IR in regulation of genes in metabolism and other important biological pathways.

Functional Gly297Ser Variant of the Physiological Dysglycemic Peptide Pancreastatin Is a Novel Risk Factor for Cardiometabolic Disorders.

Pancreastatin (PST), a chromogranin A-derived potent physiological dysglycemic peptide, regulates glucose/insulin homeostasis. We have identified a nonsynonymous functional PST variant (p.Gly297Ser; rs9658664) that occurs in a large section of human populations. Association analysis of this single nucleotide polymorphism with cardiovascular/metabolic disease states in Indian populations (n = 4,300 subjects) displays elevated plasma glucose, glycosylated hemoglobin, diastolic blood pressure, and catecholamines in Gly/Ser subjects as compared with wild-type individuals (Gly/Gly). Consistently, the 297Ser allele confers an increased risk (∼1.3-1.6-fold) for type 2 diabetes/hypertension/coronary artery disease/metabolic syndrome. In corroboration, the variant peptide (PST-297S) displays gain-of-potency in several cellular events relevant for cardiometabolic disorders (e.g., increased expression of gluconeogenic genes, increased catecholamine secretion, and greater inhibition of insulin-stimulated glucose uptake) than the wild-type peptide. Computational docking analysis and molecular dynamics simulations show higher affinity binding of PST-297S peptide with glucose-regulated protein 78 (GRP78) and insulin receptor than the wild-type peptide, providing a mechanistic basis for the enhanced activity of the variant peptide. In vitro binding assays validate these in silico predictions of PST peptides binding to GRP78 and insulin receptor. In conclusion, the PST 297Ser allele influences cardiovascular/metabolic phenotypes and emerges as a novel risk factor for type 2 diabetes/hypertension/coronary artery disease in human populations.

Hypotheses about sub-optimal hydration in the weeks before coronavirus disease (COVID-19) as a risk factor for dying from COVID-19.

To address urgent need for strategies to limit mortality from coronavirus disease 2019 (COVID-19), this review describes experimental, clinical and epidemiological evidence that suggests that chronic sub-optimal hydration in the weeks before infection might increase risk of COVID-19 mortality in multiple ways. Sub-optimal hydration is associated with key risk factors for COVID-19 mortality, including older age, male sex, race-ethnicity and chronic disease. Chronic hypertonicity, total body water deficit and/or hypovolemia cause multiple intracellular and/or physiologic adaptations that preferentially retain body water and favor positive total body water balance when challenged by infection. Via effects on serum/glucocorticoid-regulated kinase 1 (SGK1) signaling, aldosterone, tumor necrosis factor-alpha (TNF-alpha), vascular endothelial growth factor (VEGF), aquaporin 5 (AQP5) and/or Na+/K+-ATPase, chronic sub-optimal hydration in the weeks before exposure to COVID-19 may conceivably result in: greater abundance of angiotensin converting enzyme 2 (ACE2) receptors in the lung, which increases likelihood of COVID-19 infection, lung epithelial cells which are pre-set for exaggerated immune response, increased capacity for capillary leakage of fluid into the airway space, and/or reduced capacity for both passive and active transport of fluid out of the airways. The hypothesized hydration effects suggest hypotheses regarding strategies for COVID-19 risk reduction, such as public health recommendations to increase intake of drinking water, hydration screening alongside COVID-19 testing, and treatment tailored to the pre-infection hydration condition. Hydration may link risk factors and pathways in a unified mechanism for COVID-19 mortality. Attention to hydration holds potential to reduce COVID-19 mortality and disparities via at least 5 pathways simultaneously.

Non-invasive detection of divergent metabolic signals in insulin deficiency vs. insulin resistance in vivo.

The type 2 diabetic phenotype results from mixed effects of insulin deficiency and insulin resistance, but the relative contributions of these two distinct factors remain poorly characterized, as do the respective roles of the gluconeogenic organs. The purpose of this study was to investigate localized in vivo metabolic changes in liver and kidneys of contrasting models of diabetes mellitus (DM): streptozotocin (STZ)-treated wild-type Zucker rats (T1DM) and Zucker diabetic fatty (ZDF) rats (T2DM). Intermediary metabolism was probed using hyperpolarized (HP) [1-13C]pyruvate MRI of the liver and kidneys. These data were correlated with gene expression data for key mediators, assessed using rtPCR. Increased HP [1-13C]lactate was detected in both models, in association with elevated gluconeogenesis as reflected by increased expression of phosphoenolpyruvate carboxykinase. In contrast, HP [1-13C]alanine diverged between the two models, increasing in ZDF rats, while decreasing in the STZ-treated rats. The differences in liver alanine paralleled differences in key lipogenic mediators. Thus, HP [1-13C]alanine is a marker that can identify phenotypic differences in kidneys and liver of rats with T1DM vs. T2DM, non-invasively in vivo. This approach could provide a powerful diagnostic tool for characterizing tissue metabolic defects and responses to treatment in diabetic patients with ambiguous systemic manifestations.

Catestatin Gly364Ser Variant Alters Systemic Blood Pressure and the Risk for Hypertension in Human Populations via Endothelial Nitric Oxide Pathway.

Catestatin (CST), an endogenous antihypertensive/antiadrenergic peptide, is a novel regulator of cardiovascular physiology. Here, we report case-control studies in 2 geographically/ethnically distinct Indian populations (n≈4000) that showed association of the naturally-occurring human CST-Gly364Ser variant with increased risk for hypertension (age-adjusted odds ratios: 1.483; P=0.009 and 2.951; P=0.005). Consistently, 364Ser allele carriers displayed elevated systolic (up to ≈8 mm Hg; P=0.004) and diastolic (up to ≈6 mm Hg; P=0.001) blood pressure. The variant allele was also found to be in linkage disequilibrium with other functional single-nucleotide polymorphisms in the CHGA promoter and nearby coding region. Functional characterization of the Gly364Ser variant was performed using cellular/molecular biological experiments (viz peptide-receptor binding assays, nitric oxide [NO], phosphorylated extracellular regulated kinase, and phosphorylated endothelial NO synthase estimations) and computational approaches (molecular dynamics simulations for structural analysis of wild-type [CST-WT] and variant [CST-364Ser] peptides and docking of peptide/ligand with ß-adrenergic receptors [ADRB1/2]). CST-WT and CST-364Ser peptides differed profoundly in their secondary structures and showed differential interactions with ADRB2; although CST-WT displaced the ligand bound to ADRB2, CST-364Ser failed to do the same. Furthermore, CST-WT significantly inhibited ADRB2-stimulated extracellular regulated kinase activation, suggesting an antagonistic role towards ADRB2 unlike CST-364Ser. Consequently, CST-WT was more potent in NO production in human umbilical vein endothelial cells as compared with CST-364Ser. This NO-producing ability of CST-WT was abrogated by ADRB2 antagonist ICI 118551. In conclusion, CST-364Ser allele enhanced the risk for hypertension in human populations, possibly via diminished endothelial NO production because of altered interactions of CST-364Ser peptide with ADRB2 as compared with CST-WT.