Neuronal nitric oxide synthase mediates insulin- and oxidative stress-induced glucose uptake in skeletal muscle myotubes

Nitric oxide in modulating oxidative stress mediated skeletal muscle insulin resistance

Insulin resistance (IR) being the major cause behind different metabolic disorders, has attracted a lot of attention. Epidemiological data shows marked rise in the cases over a period of time. Nitric oxide (NO), produced from nitric oxide synthases (NOS), is involved in a variety of biological functions, alteration in which causes various disorders like hypertension, atherosclerosis, and angiogenesis-associated disorders. IR has been found to be a contributing factor, which is associated with abnormal NO signalling. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in metabolic disease. In this article, we give an overview of the significance of NO in oxidative stress (OS) mediated IR, describing its role in different conditions that are associated with skeletal muscle IR. NO is found to be involved in the activation of insulin receptor downstream pathway, which suggests absence of NO could lead to reduced glucose uptake, and may ultimately result in IR.

Anti-obesity and anti-diabetic effects of L-citrulline are sex-dependent

AIMS

Anti-diabetic and anti-obesity effects of L-citrulline (Cit) have been reported in male rats. This study determined sex differences in response to Cit in Wistar rats.

MAIN METHODS

Type 2 diabetes (T2D) was induced using a high-fat diet followed by low-dose of streptozotocin (30 mg/kg) injection. Male and female Wistar rats were divided into 4 groups (n = 6/group): Control, control+Cit, T2D, and T2D + Cit. Cit (4 g/L in drinking water) was administered for 8 weeks. Obesity indices were recorded, serum fasting glucose and lipid profile were measured, and glucose and pyruvate tolerance tests were performed during the Cit intervention. White (WAT) and brown (BAT) adipose tissues were weighted, and the adiposity index was calculated at the end of the study.

KEY FINDINGS

Cit was more effective in decreasing fasting glucose (18 % vs. 11 %, P = 0.0100), triglyceride (20 % vs. 14 %, P = 0.0173), and total cholesterol (16 % vs. 11 %, P = 0.0200) as well as decreasing gluconeogenesis and improving glucose tolerance, in females compared to male rats with T2D. Following Cit administration, decreases in WAT weight (16 % vs. 14 % for gonadal, 21 % vs. 16 % for inguinal, and 18 % vs. 13 % for retroperitoneal weight, all P < 0.0001) and increases in BAT weight (58 % vs. 19 %, for interscapular and 10 % vs. 7 % for axillary, all P < 0.0001) were higher in females than male rats with T2D. The decrease in adiposity index was also higher (11 % vs. 9 %, P = 0.0007) in females.

SIGNIFICANCE

The anti-obesity and anti-diabetic effects of Cit in rats are sex-dependent, with Cit being more effective in female than male rats.

Diabetic Encephalopathy: Role of Oxidative and Nitrosative Factors in Type 2 Diabetes

Diabetes mellitus is a set of complex metabolic disorders characterized by chronic hyperglycaemic condition due to defective insulin secretion (Type 1) and action (Type 2), which leads to serious micro and macro-vascular damage, inflammation, oxidative and nitrosative stress and a deranged energy homeostasis due to imbalance in the glucose and lipid metabolism. Moreover, patient with diabetes mellitus often showed the nervous system disorders known as diabetic encephalopathy. The precise pathological mechanism of diabetic encephalopathy by which it effects the central nervous system directly or indirectly causing the cognitive and motor impairment, is not completely understood. However, it has been speculated that like other extracerebellar tissues, oxidative and nitrosative stress may play significant role in the pathogenesis of diabetic encephalopathy. Therefore, the present review aimed to explain the possible association of the oxidative and nitrosative stress caused by the chronic hyperglycaemic condition with the central nervous system complications of the type 2 diabetes mellitus induced diabetic encephalopathy.

Curcumin Administration Improves Force of mdx Dystrophic Diaphragm by Acting on Fiber-Type Composition, Myosin Nitrotyrosination and SERCA1 Protein Levels

The vegetal polyphenol curcumin displays beneficial effects against skeletal muscle derangement induced by oxidative stress, disuse or aging. Since oxidative stress and inflammation are involved in the progression of muscle dystrophy, the effects of curcumin administration were investigated in the diaphragm of mdx mice injected intraperitoneally or subcutaneously with curcumin for 4-12-24 weeks. Curcumin treatment independently of the way and duration of administration (i) ameliorated myofiber maturation index without affecting myofiber necrosis, inflammation and degree of fibrosis; (ii) counteracted the decrease in type 2X and 2B fiber percentage; (iii) increased about 30% both twitch and tetanic tensions of diaphragm strips; (iv) reduced myosin nitrotyrosination and tropomyosin oxidation; (v) acted on two opposite nNOS regulators by decreasing active AMP-Kinase and increasing SERCA1 protein levels, the latter effect being detectable also in myotube cultures from mdx satellite cells. Interestingly, increased contractility, decreased myosin nitrotyrosination and SERCA1 upregulation were also detectable in the mdx diaphragm after a 4-week administration of the NOS inhibitor 7-Nitroindazole, and were not improved further by a combined treatment. In conclusion, curcumin has beneficial effects on the dystrophic muscle, mechanistically acting for the containment of a deregulated nNOS activity.

Metabolic effects of L-citrulline in type 2 diabetes

The prevalence of type 2 diabetes (T2D) is increasing worldwide. Decreased nitric oxide (NO) bioavailability is involved in the pathophysiology of T2D and its complications. L-citrulline (Cit), a precursor of NO production, has been suggested as a novel therapeutic agent for T2D. Available data from human and animal studies indicate that Cit supplementation in T2D increases circulating levels of Cit and L-arginine while decreasing circulating glucose and free fatty acids and improving dyslipidemia. The underlying mechanisms for these beneficial effects of Cit include increased insulin secretion from the pancreatic β cells, increased glucose uptake by the skeletal muscle, as well as increased lipolysis and β-oxidation, and decreased glyceroneogenesis in the adipose tissue. Thus, Cit has antihyperglycemic, antidyslipidemic, and antioxidant effects and has the potential to be used as a new therapeutic agent in the management of T2D. This review summarizes available literature from human and animal studies to explore the effects of Cit on metabolic parameters in T2D. It also discusses the possible mechanisms underlying Cit-induced improved metabolic parameters in T2D.

Nitric oxide regulation of cellular metabolism: Adaptive tuning of cellular energy

Nitric oxide can interact with a wide range of proteins including many that are involved in metabolism. In this review we have summarized the effects of NO on glycolysis, fatty acid metabolism, the TCA cycle, and oxidative phosphorylation with reference to skeletal muscle. Low to moderate NO concentrations upregulate glucose and fatty acid oxidation, while higher NO concentrations shift cellular reliance toward a fully glycolytic phenotype. Moderate NO production directly inhibits pyruvate dehydrogenase activity, reducing glucose-derived carbon entry into the TCA cycle and subsequently increasing anaploretic reactions. NO directly inhibits aconitase activity, increasing reliance on glutamine for continued energy production. At higher or prolonged NO exposure, citrate accumulation can inhibit multiple ATP-producing pathways. Reduced TCA flux slows NADH/FADH entry into the ETC. NO can also inhibit the ETC directly, further limiting oxidative phosphorylation. Moderate NO production improves mitochondrial efficiency while improving O2 utilization increasing whole-body energy production. Long-term bioenergetic capacity may be increased because of NO-derived ROS, which participate in adaptive cellular redox signaling through AMPK, PCG1-α, HIF-1, and NF-κB. However, prolonged exposure or high concentrations of NO can result in membrane depolarization and opening of the MPT. In this way NO may serve as a biochemical rheostat matching energy supply with demand for optimal respiratory function.

Epigenetic Changes Induced by High Glucose in Human Pancreatic Beta Cells

Epigenetic changes in pancreatic beta cells caused by sustained high blood glucose levels, as seen in prediabetic conditions, may contribute to the etiology of diabetes. To delineate a direct cause and effect relationship between high glucose and epigenetic changes, we cultured human pancreatic beta cells derived from induced pluripotent stem cells and treated them with either high or low glucose, for 14 days. We then used the Arraystar 4x180K HG19 RefSeq Promoter Array to perform whole-genome DNA methylation analysis. A total of 478 gene promoters, out of a total of 23,148 present on the array (2.06%), showed substantial differences in methylation (p < 0.01). Out of these, 285 were hypomethylated, and 193 were hypermethylated in experimental vs. control. Ingenuity Pathway Analysis revealed that the main pathways and networks that were differentially methylated include those involved in many systems, including those related to development, cellular growth, and proliferation. Genes implicated in the etiology of diabetes, including networks involving glucose metabolism, insulin secretion and regulation, and cell cycle regulation, were notably altered. Influence of upstream regulators such as MRTFA, AREG, and NOTCH3 was predicted based on the altered methylation of their downstream targets. The study validated that high glucose levels can directly cause many epigenetic changes in pancreatic beta cells, suggesting that this indeed may be a mechanism involved in the etiology of diabetes.

miR-378 affects metabolic disturbances in the mdx model of Duchenne muscular dystrophy

Although Duchenne muscular dystrophy (DMD) primarily affects muscle tissues, the alterations to systemic metabolism manifested in DMD patients contribute to the severe phenotype of this fatal disorder. We propose that microRNA-378a (miR-378) alters carbohydrate and lipid metabolism in dystrophic mdx mice. In our study, we utilized double knockout animals which lacked both dystrophin and miR-378 (mdx/miR-378^-/-). RNA sequencing of the liver identified 561 and 194 differentially expressed genes that distinguished mdx versus wild-type (WT) and mdx/miR-378^-/- versus mdx counterparts, respectively. Bioinformatics analysis predicted, among others, carbohydrate metabolism disorder in dystrophic mice, as functionally proven by impaired glucose tolerance and insulin sensitivity. The lack of miR-378 in mdx animals mitigated those effects with a faster glucose clearance in a glucose tolerance test (GTT) and normalization of liver glycogen levels. The absence of miR-378 also restored the expression of genes regulating lipid homeostasis, such as Acly, Fasn, Gpam, Pnpla3, and Scd1. In conclusion, we report for the first time that miR-378 loss results in increased systemic metabolism of mdx mice. Together with our previous finding, demonstrating alleviation of the muscle-related symptoms of DMD, we propose that the inhibition of miR-378 may represent a new strategy to attenuate the multifaceted symptoms of DMD.

Oxidative Stress, Inflammation and Connexin Hemichannels in Muscular Dystrophies

Muscular dystrophies (MDs) are a heterogeneous group of congenital neuromuscular disorders whose clinical signs include myalgia, skeletal muscle weakness, hypotonia, and atrophy that leads to progressive muscle disability and loss of ambulation. MDs can also affect cardiac and respiratory muscles, impairing life-expectancy. MDs in clude Duchenne muscular dystrophy, Emery-Dreifuss muscular dystrophy, facioscapulohumeral muscular dystrophy and limb-girdle muscular dystrophy. These and other MDs are caused by mutations in genes that encode proteins responsible for the structure and function of skeletal muscles, such as components of the dystrophin-glycoprotein-complex that connect the sarcomeric-actin with the extracellular matrix, allowing contractile force transmission and providing stability during muscle contraction. Consequently, in dystrophic conditions in which such proteins are affected, muscle integrity is disrupted, leading to local inflammatory responses, oxidative stress, Ca²⁺-dyshomeostasis and muscle degeneration. In this scenario, dysregulation of connexin hemichannels seem to be an early disruptor of the homeostasis that further plays a relevant role in these processes. The interaction between all these elements constitutes a positive feedback loop that contributes to the worsening of the diseases. Thus, we discuss here the interplay between inflammation, oxidative stress and connexin hemichannels in the progression of MDs and their potential as therapeutic targets.

Dietary Cocoa Flavanols Enhance Mitochondrial Function in Skeletal Muscle and Modify Whole-Body Metabolism in Healthy Mice

Mitochondrial dysfunction is widely reported in various diseases and contributes to their pathogenesis. We assessed the effect of cocoa flavanols supplementation on mitochondrial function and whole metabolism, and we explored whether the mitochondrial deacetylase sirtuin-3 (Sirt3) is involved or not. We explored the effects of 15 days of CF supplementation in wild type and Sirt3^-/- mice. Whole-body metabolism was assessed by indirect calorimetry, and an oral glucose tolerance test was performed to assess glucose metabolism. Mitochondrial respiratory function was assessed in permeabilised fibres and the pyridine nucleotides content (NAD⁺ and NADH) were quantified. In the wild type, CF supplementation significantly modified whole-body metabolism by promoting carbohydrate use and improved glucose tolerance. CF supplementation induced a significant increase of mitochondrial mass, while significant qualitative adaptation occurred to maintain H₂O₂ production and cellular oxidative stress. CF supplementation induced a significant increase in NAD⁺ and NADH content. All the effects mentioned above were blunted in Sirt3^-/- mice. Collectively, CF supplementation boosted the NAD metabolism that stimulates sirtuins metabolism and improved mitochondrial function, which likely contributed to the observed whole-body metabolism adaptation, with a greater ability to use carbohydrates, at least partially through Sirt3.

Nitric oxide modulates bone anabolism through regulation of osteoblast glycolysis and differentiation

Previous studies have shown that nitric oxide (NO) supplements may prevent bone loss and fractures in preclinical models of estrogen deficiency. However, the mechanisms by which NO modulates bone anabolism remain largely unclear. Argininosuccinate lyase (ASL) is the only mammalian enzyme capable of synthesizing arginine, the sole precursor for nitric oxide synthase-dependent (NOS-dependent) NO synthesis. Moreover, ASL is also required for channeling extracellular arginine to NOS for NO production. ASL deficiency (ASLD) is thus a model to study cell-autonomous, NOS-dependent NO deficiency. Here, we report that loss of ASL led to decreased NO production and impairment of osteoblast differentiation. Mechanistically, the bone phenotype was at least in part driven by the loss of NO-mediated activation of the glycolysis pathway in osteoblasts that led to decreased osteoblast differentiation and function. Heterozygous deletion of caveolin 1, a negative regulator of NO synthesis, restored NO production, osteoblast differentiation, glycolysis, and bone mass in a hypomorphic mouse model of ASLD. The translational significance of these preclinical studies was further reiterated by studies conducted in induced pluripotent stem cells from an individual with ASLD. Taken together, our findings suggest that ASLD is a unique genetic model for studying NO-dependent osteoblast function and that the NO/glycolysis pathway may be a new target to modulate bone anabolism.

Involvement of nitric oxide synthase/nitric oxide pathway in the regulation of SIRT1-AMPK crosstalk in podocytes: Impact on glucose uptake

The protein deacetylase sirtuin 1 (SIRT1) and adenosine monophosphate-dependent protein kinase (AMPK) play important roles in the development of insulin resistance. In glomerular podocytes, crosstalk between these two enzymes may be altered under hyperglycemic conditions. SIRT1 protein levels and activity and AMPK phosphorylation decrease under hyperglycemic conditions, with concomitant inhibition of the effect of insulin on glucose uptake into these cells. Nitric oxide (NO)-dependent regulatory signaling pathways have been shown to be downregulated under diabetic conditions. The present study examined the involvement of the NO synthase (NOS)/NO pathway in the regulation of SIRT1-AMPK signaling and glucose uptake in podocytes. We examined the effects of NOS/NO pathway alterations on SIRT1/AMPK signaling and glucose uptake using pharmacological tools and a small-interfering transfection approach. We also examined the ability of the NOS/NO pathway to protect podocytes against high glucose-induced alterations of SIRT1/AMPK signaling and insulin-dependent glucose uptake. Inhibition of the NOS/NO pathway reduced SIRT1 protein levels and activity, leading to a decrease in AMPK phosphorylation and blockade of the effect of insulin on glucose uptake. Treatment with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) prevented high glucose-induced decreases in SIRT1 and AMPK activity and increased GLUT4 protein expression, thereby improving glucose uptake in podocytes. These findings suggest that inhibition of the NOS/NO pathway may result in alterations of the effects of insulin on glucose uptake in podocytes. In turn, the enhancement of NOS/NO pathway activity may prevent these deleterious effects of high glucose concentrations, thus bidirectionally stimulating the SIRT1-AMPK reciprocal activation loop.

Re-Evaluating the Oxidative Phenotype: Can Endurance Exercise Save the Western World?

Mitochondria are popularly called the "powerhouses" of the cell. They promote energy metabolism through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation, which in contrast to cytosolic glycolysis are oxygen-dependent and significantly more substrate efficient. That is, mitochondrial metabolism provides substantially more cellular energy currency (ATP) per macronutrient metabolised. Enhancement of mitochondrial density and metabolism are associated with endurance training, which allows for the attainment of high relative VO2 max values. However, the sedentary lifestyle and diet currently predominant in the Western world lead to mitochondrial dysfunction. Underdeveloped mitochondrial metabolism leads to nutrient-induced reducing pressure caused by energy surplus, as reduced nicotinamide adenine dinucleotide (NADH)-mediated high electron flow at rest leads to "electron leak" and a chronic generation of superoxide radicals (O2-). Chronic overload of these reactive oxygen species (ROS) damages cell components such as DNA, cell membranes, and proteins. Counterintuitively, transiently generated ROS during exercise contributes to adaptive reduction-oxidation (REDOX) signalling through the process of cellular hormesis or "oxidative eustress" defined by Helmut Sies. However, the unaccustomed, chronic oxidative stress is central to the leading causes of mortality in the 21st century-metabolic syndrome and the associated cardiovascular comorbidities. The endurance exercise training that improves mitochondrial capacity and the protective antioxidant cellular system emerges as a universal intervention for mitochondrial dysfunction and resultant comorbidities. Furthermore, exercise might also be a solution to prevent ageing-related degenerative diseases, which are caused by impaired mitochondrial recycling. This review aims to break down the metabolic components of exercise and how they translate to athletic versus metabolically diseased phenotypes. We outline a reciprocal relationship between oxidative metabolism and inflammation, as well as hypoxia. We highlight the importance of oxidative stress for metabolic and antioxidant adaptation. We discuss the relevance of lactate as an indicator of critical exercise intensity, and inferring from its relationship with hypoxia, we suggest the most appropriate mode of exercise for the case of a lost oxidative identity in metabolically inflexible patients. Finally, we propose a reciprocal signalling model that establishes a healthy balance between the glycolytic/proliferative and oxidative/prolonged-ageing phenotypes. This model is malleable to adaptation with oxidative stress in exercise but is also susceptible to maladaptation associated with chronic oxidative stress in disease. Furthermore, mutations of components involved in the transcriptional regulatory mechanisms of mitochondrial metabolism may lead to the development of a cancerous phenotype, which progressively presents as one of the main causes of death, alongside the metabolic syndrome.

A historical perspective on the interactions of insulin at the blood-brain barrier

Subsequent to the discovery of insulin in 1921, the role of insulin in the brain has been investigated throughly. The ability of insulin to act within the brain to regulate peripheral glucose levels helped evolve the research surrounding the ability of insulin to be transported into the brain. Investigations aiming to determine the transport of insulin into the brain from the circulation soon followed. Once it was established that insulin could enter the brain, the ability of insulin to bind brain microvessels and regulators of this process were determined. As technology advanced, quantitative measurements to specify the transport rate of insulin across the blood-brain barrier (BBB) and the impact of physiological conditions and diseases were the logical next steps. Lastly, with the advent of genetic mouse models and high-specificity antagonists, the specific role of the insulin receptor in mediating insulin transport could begin to be explored. In this review, we summarise the main findings throughout the decades regarding the interactions of insulin at the BBB.

Uric Acid Is Elevated in Children With Obesity and Decreases After Weight Loss

Introduction: Childhood obesity is an increasing condition associated with continuous obesity into adulthood and development of comorbidities. Adult studies show an association between serum uric acid (SUA) levels and body mass index (BMI). The aim of this retro perspective exploratory study was to investigate SUA in obese children and adolescents and the effects of a subsequent weight reduction. Materials and Methods: One hundred and seventy-one children (age 4-18), with obesity (i.e. BMI-SDS of +2 or higher) were included in a multifactorial lifestyle intervention. The children participating were annually measured for anthropometrics, blood samples and DEXA-scans for up to 3 years. Eighty-nine children were included for follow-up analysis. Results: After a follow-up of 20.7 ± 9.4 months a reduction in BMI-SDS of -0.34 ± 0.53 (p < 0.01) was observed. SUA was found to be positively associated with changes in BMI-SDS. SUA levels decreased in the 65 children who lost weight during the trial, conversely, SUA increased in the 23 children who gained weight during the trial (p < 0.01 between groups). Conclusion: SUA was found to correlate with measures of obesity and for the first time, this intervention demonstrates a positive relationship between SUA and weight reduction in children with obesity.

Potential roles of neuronal nitric oxide synthase and the PTEN-induced kinase 1 (PINK1)/Parkin pathway for mitochondrial protein degradation in disuse-induced soleus muscle atrophy in adult rats

Excessive nitric oxide (NO) production and mitochondrial dysfunction can activate protein degradation in disuse-induced skeletal muscle atrophy. However, the increase in NO production in atrophied muscles remains controversial. In addition, although several studies have investigated the PTEN-induced kinase 1 (PINK1)/Parkin pathway, a mitophagy pathway, in atrophied muscle, the involvement of this pathway in soleus muscle atrophy is unclear. In this study, we investigated the involvement of neuronal nitric oxide synthase (nNOS) and the PINK1/Parkin pathway in soleus muscle atrophy induced by 14 days of hindlimb unloading (HU) in adult rats. HU lowered the weight of the soleus muscles. nNOS expression showed an increase in atrophied soleus muscles. Although HU increased malondialdehyde as oxidative modification of the protein, it decreased 6-nitrotryptophan, a marker of protein nitration. Additionally, the nitrosocysteine content and S-nitrosylated Parkin were not altered, suggesting the absence of excessive nitrosative stress after HU. The expression of PINK1 and Parkin was also unchanged, whereas the expression of heat shock protein 70 (HSP70), which is required for Parkin activity, was reduced in atrophied soleus muscles. Moreover, we observed accumulation and reduced ubiquitination of high molecular weight mitofusin 2, which is a target of Parkin, in atrophied soleus muscles. These results indicate that excessive NO is not produced in atrophied soleus muscles despite nNOS accumulation, suggesting that excessive NO dose not mediate in soleus muscle atrophy at least after 14 days of HU. Furthermore, the PINK1/Parkin pathway may not play a role in mitophagy at this time point. In contrast, the activity of Parkin may be downregulated because of reduced HSP70 expression, which may contribute to attenuated degradation of target proteins in the atrophied soleus muscles after 14 days of HU. The present study provides new insights into the roles of nNOS and a protein degradation pathway in soleus muscle atrophy.

Heat shock protein 90α increases superoxide generation from neuronal nitric oxide synthases

Neuronal nitric oxide synthase (nNOS) generates superoxide, particularly at sub-optimal l-arginine (l-Arg) substrate concentrations. Heat shock protein 90 (Hsp90) was reported to inhibit superoxide generation from nNOS protein. However, commercially available Hsp90 product from bovine brain tissues with unspecified Hsp90α and Hsp90β contents and an undefined Hsp90 protein oligomeric state was utilized. These two Hsp90s can have opposite effect on superoxide production by NOS. Importantly, emerging evidence indicates that nNOS splice variants are involved in different biological functions by functioning distinctly in redox signaling. In the present work, purified recombinant human Hsp90α, in its native dimeric state, was used in electron paramagnetic resonance (EPR) spin trapping experiments to study the effects of Hsp90α on superoxide generation from nNOS splice variants nNOSμ and nNOSα. Human Hsp90α was found to significantly increase superoxide generation from nNOSμ and nNOSα proteins under l-Arg-depleted conditions and Hsp90α influenced superoxide production by nNOSμ and nNOSα at varying degrees. Imidazole suppressed the spin adduct signal, indicating that superoxide was produced at the heme site of nNOS in the presence of Hsp90α, whereas l-Arg repletion diminished superoxide production by the nNOS-Hsp90α. Moreover, NADPH consumption rate values exhibited a similar trend/difference as a function of Hsp90α and l-Arg. Together, these EPR spin trapping and NADPH oxidation kinetics results demonstrated noticeable Hsp90α-induced increases in superoxide production by nNOS and a distinguishable effect of Hsp90α on nNOSμ and nNOSα proteins.

Recent advances in understanding glucose transport and glucose disposal

Deficient glucose transport and glucose disposal are key pathologies leading to impaired glucose tolerance and risk of type 2 diabetes.  The cloning and identification of the family of facilitative glucose transporters have helped to identify that underlying mechanisms behind impaired glucose disposal, particularly in muscle and adipose tissue.  There is much more than just transporter protein concentration that is needed to regulate whole body glucose uptake and disposal.  The purpose of this review is to discuss recent findings in whole body glucose disposal.  We hypothesize that impaired glucose uptake and disposal is a consequence of mismatched energy input and energy output.  Decreasing the former while increasing the latter is key to normalizing glucose homeostasis.

Nox4 - RyR1 - Nox2: Regulators of micro-domain signaling in skeletal muscle

The ability for skeletal muscle to perform optimally can be affected by the regulation of Ca²⁺ within the triadic junctional space at rest. Reactive oxygen species impact muscle performance due to changes in oxidative stress, damage and redox regulation of signaling cascades. The interplay between ROS and Ca²⁺ signaling at the triad of skeletal muscle is therefore important to understand as it can impact the performance of healthy and diseased muscle. Here, we aimed to examine how changes in Ca²⁺ and redox signaling within the junctional space micro-domain of the mouse skeletal muscle fibre alters the homeostasis of these complexes. The dystrophic mdx mouse model displays increased RyR1 Ca²⁺ leak and increased NAD(P)H Oxidase 2 ROS. These alterations make the mdx mouse an ideal model for understanding how ROS and Ca²⁺ handling impact each other. We hypothesised that elevated t-tubular Nox2 ROS increases RyR1 Ca²⁺ leak contributing to an increase in cytoplasmic Ca²⁺, which could then initiate protein degradation and impaired cellular functions such as autophagy and ER stress. We found that inhibiting Nox2 ROS did not decrease RyR1 Ca²⁺ leak observed in dystrophin-deficient skeletal muscle. Intriguingly, another NAD(P)H isoform, Nox4, is upregulated in mice unable to produce Nox2 ROS and when inhibited reduced RyR1 Ca²⁺ leak. Our findings support a model in which Nox4 ROS induces RyR1 Ca²⁺ leak and the increased junctional space [Ca²⁺] exacerbates Nox2 ROS; with the cumulative effect of disruption of downstream cellular processes that would ultimately contribute to reduced muscle or cellular performance.

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Nox2 ROS does not influence RyR1 Ca²⁺ leak in skeletal muscle.

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Lack of Nox2 ROS increases Nox4 expression.

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Nox4 ROS induces RyR1 Ca²⁺ leak via S-nitrosylation.

Role of Nitric Oxide in Insulin Secretion and Glucose Metabolism

Nitric oxide (NO) contributes to carbohydrate metabolism and decreased NO bioavailability is involved in the development of type 2 diabetes mellitus (T2DM). NO donors may improve insulin signaling and glucose homeostasis in T2DM and insulin resistance (IR), suggesting the potential clinical importance of NO-based interventions. In this review, site-specific roles of the NO synthase (NOS)-NO pathway in carbohydrate metabolism are discussed. In addition, the metabolic effects of physiological low levels of NO produced by constitutive NOS (cNOS) versus pathological high levels of NO produced by inducible NOS (iNOS) in pancreatic β-cells, adipocytes, hepatocytes, and skeletal muscle cells are summarized. A better understanding of the NOS-NO system in the regulation of glucose homeostasis can hopefully facilitate the development of new treatments for T2DM.

Exaggerated exercise pressor reflex in type 2 diabetes: Potential role of oxidative stress

Type 2 diabetes mellitus (T2DM) leads to exaggerated cardiovascular responses to exercise, in part due to an exaggerated exercise pressor reflex. Accumulating data suggest excessive oxidative stress contributes to an exaggerated exercise pressor reflex in cardiovascular-related diseases. Excessive oxidative stress is also a primary underlying mechanism for the development and progression of T2DM. However, whether oxidative stress plays a role in mediating the exaggerated exercise pressor reflex in T2DM is not known. Therefore, this review explores the potential role of oxidative stress leading to increased activation of the afferent arm of the exercise pressor reflex. Several lines of evidence support direct and indirect effects of oxidative stress on the exercise pressor reflex. For example, intramuscular ROS may directly and indirectly (by attenuating contracting muscle blood flow) increase group III and IV afferent activity. Oxidative stress is a primary underlying mechanism for the development of neuropathic pain, which in turn is associated with increased group III and IV afferent activity. These are the same type of afferents that evoke muscle pain and the exercise pressor reflex. Furthermore, oxidative stress-induced release of inflammatory mediators may modulate afferent activity. Collectively, these alterations may result in a positive feedback loop that further amplifies the exercise pressor reflex. An exaggerated reflex increases the risk of adverse cardiovascular events. Thus, identifying the contribution of oxidative stress could provide a potential therapeutic target to reduce this risk in T2DM.

High intensity interval training and molecular adaptive response of skeletal muscle

Increased cardiovascular fitness, V˙O_2max, is associated with enhanced endurance capacity and a decreased rate of mortality. High intensity interval training (HIIT) is one of the best methods to increase V˙O_2max and endurance capacity for top athletes and for the general public as well. Because of the high intensity of this type of training, the adaptive response is not restricted to Type I fibers, as found for moderate intensity exercise of long duration. Even with a short exercise duration, HIIT can induce activation of AMPK, PGC-1α, SIRT1 and ROS pathway as well as by the modulation of Ca²⁺ homeostasis, leading to enhanced mitochondrial biogenesis, and angiogenesis. The present review summarizes the current knowledge of the adaptive response of HIIT.

Lifestyle Modification for Enhancing Autonomic Cardiac Regulation in Children: The Role of Exercise

Decreased physical activity (PA) is a global concern contributing to the rise in cardiometabolic diseases. One potential mechanism linking insufficient PA and poor health is dysregulated autonomic nervous system (ANS) activity. This relationship is established in adults and PA recommendations, with specific exercise prescription guidelines, have been proposed to overcome this societal health burden. However, research on the benefits and underlying mechanisms of exercise on ANS activity in children <18 years old is limited. This review aimed to describe the optimal exercise “dose” and potential mechanisms of action that exercise may pose on enhancing child ANS activity, represented by heart rate variability (HRV). PubMed, Web of Science and Google Scholar were searched for articles examining the influence of exercise on child HRV. Various exercise duration and frequency combinations appear to improve HRV indices, primarily those representing parasympathetic influence. Furthermore, both aerobic and resistance training benefit HRV through potentially different mechanisms with intensity proposed to be important for exercise prescription. Findings indicate that exercise is a crucial lifestyle modification with protective and therapeutic effects on cardiometabolic health associated with improvements in child ANS activity. Exercise programming must consider the various components including mode, intensity and population characteristics to optimize ANS health.

Anxiety-like behaviors and hippocampal nNOS in response to diet-induced obesity combined with exercise

A high-fat diet (HFD) and overweight status can induce hippocampal dysfunction, leading to depression and anxiety. Exercise has beneficial effects on emotional behaviors. We previously reported that exercise training rescues HFD-induced excess hippocampal neuronal nitric oxide synthase (nNOS) expression, which is a key regulator of anxiety. Here, we investigated anxiety-like behaviors and hippocampal nNOS expression in response to HFD combined with exercise. Mice were assigned to standard diet, HFD, or HFD with exercise groups for 12 weeks. We found that exercise during the final 6 weeks of the HFD regime improved 12 weeks of HFD-induced defecation, accompanied by rescue of excess nNOS expression. However, anxiety indicators in the elevated plus maze were unchanged. These effects were not apparent after only 1 week of exercise. In conclusion, 6 weeks of exercise training reduced HFD-related anxiety according to one of our measures (defecation), and reversed changes in the hippocampal nNOS/NO pathway.

Role of neuronal nitric oxide synthase (nNOS) in Duchenne and Becker muscular dystrophies - Still a possible treatment modality?

Neuronal nitric oxide synthase (nNOS) is involved in nitric oxide (NO) production and suggested to play a crucial role in blood flow regulation of skeletal muscle. During activation of the muscle, NO helps attenuate the sympathetic vasoconstriction to accommodate increased metabolic demands, a phenomenon known as functional sympatholysis. In inherited myopathies such as the dystrophinopathies Duchenne and Becker muscle dystrophies (DMD and BMD), nNOS is lost from the sarcolemma. The loss of nNOS may cause functional ischemia contributing to skeletal and cardiac muscle cell injury. Effects of NO is augmented by inhibiting degradation of the second messenger cyclic guanosine monophosphate (cGMP) using sildenafil and tadalafil, both of which inhibit the enzyme phosphodiesterase 5 (PDE5). In animal models of DMD, PDE5-inhibitors prevent functional ischemia, reduce post-exercise skeletal muscle pathology and fatigue, show amelioration of cardiac muscle cell damage and increase cardiac performance. However, effect on clinical outcomes in DMD and BMD patients have been disappointing with minor effects on upper limb performance and none on ambulation. This review aims to summarize the current knowledge of nNOS function related to functional sympatholysis in skeletal muscle and studies on PDE5-inhibitor treatment in nNOS-deficient animal models and patients.

Oxidative Stress, Intrauterine Growth Restriction, and Developmental Programming of Type 2 Diabetes

Intrauterine growth restriction (IUGR) leads to reduced birth weight and the development of metabolic diseases such as Type 2 diabetes in adulthood. Mitochondria dysfunction and oxidative stress are commonly found in key tissues (pancreatic islets, liver, and skeletal muscle) of IUGR individuals. In this review, we explore the role of oxidative stress in IUGR-associated diabetes etiology.

Reactive Oxygen and Nitrogen Species Regulate Key Metabolic, Anabolic, and Catabolic Pathways in Skeletal Muscle

Reactive oxygen and nitrogen species (RONS) are important cellular regulators of key physiological processes in skeletal muscle. In this review, we explain how RONS regulate muscle contraction and signaling, and why they are important for membrane remodeling, protein turnover, gene expression, and epigenetic adaptation. We discuss how RONS regulate carbohydrate uptake and metabolism of skeletal muscle, and how they indirectly regulate fat metabolism through silent mating type information regulation 2 homolog 3 (SIRT3). RONS are causative/associative signaling molecules, which cause sarcopenia or muscle hypertrophy. Regular exercise influences redox biology, metabolism, and anabolic/catabolic pathways in skeletal muscle in an intensity dependent manner.