Skeletal muscle Nur77 expression enhances oxidative metabolism and substrate utilization

Nuclear receptor Nur77 and Yin-Yang 1 synergistically increase mitochondrial abundance and activity in macrophages

Mitochondrial biogenesis requires precise regulation of both mitochondrial-encoded and nuclear-encoded genes. Nuclear receptor Nur77 is known to regulate mitochondrial metabolism in macrophages and skeletal muscle. Here, we compared genome-wide Nur77 binding site and target gene expression in these two cell types, which revealed conserved regulation of mitochondrial genes and enrichment of motifs for the transcription factor Yin-Yang 1 (YY1). We show that Nur77 and YY1 interact, that YY1 increases Nur77 activity, and that their binding sites are co-enriched at mitochondrial ribosomal protein gene loci in macrophages. Nur77 and YY1 co-expression synergistically increases Mrpl1 expression as well as mitochondrial abundance and activity in macrophages but not skeletal muscle. As such, we identify a macrophage-specific Nur77-YY1 interaction that enhances mitochondrial metabolism.

The Molecular Mechanisms of Fuel Utilization during Exercise

Exercise is widely recognized for its positive impact on human health and well-being. The process of utilizing substrates in skeletal muscle during exercise is intricate and governed by complex mechanisms. Carbohydrates and lipids serve as the primary fuel sources for skeletal muscle during exercise. It is now understood that fuel selection during exercise is not solely determined by physical activity itself but is also influenced by the overall metabolic state of the body. The balance between lipid and carbohydrate utilization significantly affects exercise capacity, including endurance, fatigue, and overall performance. Therefore, comprehensively understanding the regulation of substrate utilization during exercise is of utmost importance. The aim of this review is to provide an extensive overview of the current knowledge regarding the pathways involved in the regulation of substrate utilization during exercise. By synthesizing existing research, we can gain a holistic perspective on the intricate relationship between exercise, metabolism, and fuel selection. This advanced understanding has the potential to drive advancements in the field of exercise science and contribute to the development of personalized exercise strategies for individuals looking to optimize their performance and overall health.

Nuclear receptors as potential therapeutic targets in peripheral arterial disease and related myopathy

Peripheral arterial disease (PAD) is a prevalent cardiovascular complication of limb vascular insufficiency, causing ischemic injury, mitochondrial metabolic damage and functional impairment in the skeletal muscle, and ultimately leading to immobility and mortality. While potential therapies have been mostly focussed on revascularization, none of the currently available pharmacological treatments are fully effective in PAD, often leading to amputations, particularly in chronic metabolic diseases. One major limitation of focussed angiogenesis and revascularization as a therapeutic strategy is a limited effect on metabolic restoration and muscle regeneration in the affected limb. Therefore, additional preclinical investigations are needed to discover novel treatment options for PAD preferably targeting multiple aspects of muscle recovery. In this review, we propose nuclear receptors expressed in the skeletal muscle as potential candidates for ischemic muscle repair in PAD. We review classic steroid and orphan receptors that have been reported to be involved in the regulation of paracrine muscle angiogenesis, oxidative metabolism, mitochondrial biogenesis and muscle regeneration, and discuss how these receptors could be critical for recovery from ischemic muscle damage. Furthermore, we identify existing gaps in our understanding of nuclear receptor signalling in the skeletal muscle and propose future areas of research that could be instrumental in exploring nuclear receptors as therapeutic candidates for treating PAD.

Exosomes from hepatitis B virus-infected hepatocytes activate hepatic stellate cells and aggravate liver fibrosis through the miR-506-3p/Nur77 pathway

Cumulative evidence indicates the important role of Nur77 in organ fibrogenesis. However, the role of Nur77 in hepatitis B virus (HBV)-related liver fibrosis (LF) remains unclear. Cells were transfected with the microRNA mimic miRNA-506-3p or inhibitor, and pcDNA3.1-Nur77 or Nur77 guide RNA. Exosomes were isolated from HBV-infected HepG2-sodium taurocholate cotransporting polypeptide cells. The levels of miR-506-3p, Nur77, and LF-related genes and proteins were detected by quantitative polymerase chain reaction (qPCR) and western blot analysis, respectively. The pathology of the liver from HBV-infected patients was examined using hematoxylin-eosin and Masson's staining. The expression of Nur77 in liver tissue was determined by immunohistochemistry, and the LF score was assessed using the METAVIR system. The relationship between miR-506-3p/Nur77 and LF score was analyzed by correlation analysis. HBV infection downregulated miR-506-3p expression and upregulated Nur77 levels in hepatocytes. Exosomes from HBV-infected hepatocytes also displayed decreased gene expression of miR-506-3p and increased expressions of Nur77- and LF-related genes in stellate cells compared with exosomes from hepatocytes with mock infection. These changes were reversed by Nur77 guide RNA. Nur77 expression in liver tissue was strongly correlated with LF, whereas serum miR-506-3p was strongly negatively correlated with LF. Exosomes from HBV-infected hepatocytes activate stellate cells and aggravate LF through the miR-506-3p/Nur77 pathway. These exosomes may be the basis of a promising therapeutic strategy.

Exercise metabolism and adaptation in skeletal muscle

Viewing metabolism through the lens of exercise biology has proven an accessible and practical strategy to gain new insights into local and systemic metabolic regulation. Recent methodological developments have advanced understanding of the central role of skeletal muscle in many exercise-associated health benefits and have uncovered the molecular underpinnings driving adaptive responses to training regimens. In this Review, we provide a contemporary view of the metabolic flexibility and functional plasticity of skeletal muscle in response to exercise. First, we provide background on the macrostructure and ultrastructure of skeletal muscle fibres, highlighting the current understanding of sarcomeric networks and mitochondrial subpopulations. Next, we discuss acute exercise skeletal muscle metabolism and the signalling, transcriptional and epigenetic regulation of adaptations to exercise training. We address knowledge gaps throughout and propose future directions for the field. This Review contextualizes recent research of skeletal muscle exercise metabolism, framing further advances and translation into practice.

Roles of Estrogen, Estrogen Receptors, and Estrogen-Related Receptors in Skeletal Muscle: Regulation of Mitochondrial Function

Estrogen is an essential sex steroid hormone that functions primarily in female reproductive system, as well as in a variety of tissues and organs with pleiotropic effects, such as in cardiovascular, nervous, immune, and musculoskeletal systems. Women with low estrogen, as exemplified by those in postmenopause, are therefore prone to suffer from various disorders, i.e., cardiovascular disease, dementia, metabolic syndrome, osteoporosis, sarcopenia, frailty, and so on. Estrogen regulates the expression of its target genes by binding to its cognate receptors, estrogen receptors (ERs) α and β. Notably, the estrogen-related receptors (ERRs) α, β, and γ are originally identified as orphan receptors that share substantial structural homology and common transcriptional targets with ERs. Accumulating evidence suggests that ERs and ERRs play crucial roles in skeletal muscles, such as muscle mass maintenance, muscle exercise physiology, and muscle regeneration. In this article, we review potential regulatory roles of ERs and ERRs in muscle physiology, particularly with regard to mitochondrial function and metabolism.

Yiqi Huoxue Tongluo recipe regulates NR4A1 to improve renal mitochondrial function in unilateral ureteral obstruction (UUO) rats

CONTEXT

Yiqi Huoxue Tongluo recipe (YHTR) is a traditional Chinese medicine for the treatment of chronic kidney disease, but its exact mechanism is not clear.

OBJECTIVES

To monitor the potential improvement of renal mitochondrial function in unilateral ureteral obstruction (UUO) rats by regulating NR4A1 using the YHTR.

MATERIALS AND METHODS

Wistar rats were randomly divided into four groups: sham, UUO (left ureteral ligation for 14 days), eplerenone (EPL) (UUO + EPL), and YHTR (UUO + YHTR). UUO rats were established and intragastrically administered EPL (100 mg/day/kg) or YHTR (11.7 g/day/kg) for 14 days. The expression of related proteins in kidneys was detected by immunohistochemistry, western blot, RT-PCR, and chemical colorimetric assay, respectively.

RESULTS

In vivo, YHTR treatment reduced the levels of BUN and Scr (by 17.9% and 23.5%) in UUO rats. Moreover, YHTR improved the renal mitochondrial function via increasing key enzymes of the tricarboxylic acid (TCA) cycle (p < 0.05) and activity of the mitochondrial complex (I-V) (by 30.8%, 29.1%, 19.7%, 35.9%, and 22.4%) in UUO rats. Compared with the UUO group, the expression of NR4A1 and Bcl-2 were significantly increased (p < 0.05), the expression of caspase-3 and caspase-9 were significantly decreased (p < 0.05) in the YHTR group. YHTR could upregulate key enzymes of the TCA cycle via promoting NR4A1 expression in HK2 cells, leading to inhibition of TGF-β1 induced cell apoptosis.

CONCLUSIONS

YHTR significantly improved the development of CKD; this study may provide new ideas for the pathogenesis of CKD and new strategies for the development of new drugs against CKD.

Considerations for Sex-Cognizant Research in Exercise Biology and Medicine

As the fields of kinesiology, exercise science, and human movement developed, the majority of the research focused on male physiology and extrapolated findings to females. In the medical sphere, basing practice on data developed in only males resulted in the removal of drugs from the market in the late 1990s due to severe side effects (some life-threatening) in females that were not observed in males. In response to substantial evidence demonstrating exercise-induced health benefits, exercise is often promoted as a key modality in disease prevention, management, and rehabilitation. However, much like the early days of drug development, a historical literature knowledge base of predominantly male studies may leave the exercise field vulnerable to overlooking potentially key biological differences in males and females that may be important to consider in prescribing exercise (e.g., how exercise responses may differ between sexes and whether there are optimal approaches to consider for females that differ from conventional approaches that are based on male physiology). Thus, this review will discuss anatomical, physiological, and skeletal muscle molecular differences that may contribute to sex differences in exercise responses, as well as clinical considerations based on this knowledge in athletic and general populations over the continuum of age. Finally, this review summarizes the current gaps in knowledge, highlights the areas ripe for future research, and considerations for sex-cognizant research in exercise fields.

NR4A Family Genes: A Review of Comprehensive Prognostic and Gene Expression Profile Analysis in Breast Cancer

This report analyzes nuclear receptor (NR) subfamily 4A’s potential role in treating those diagnosed with breast cancer. Here we reviewed the current literature on NR4 family members. We also examined the relative gene expression of the NR4A receptor subfamily in the basal, HER2 (human epidermal growth factor receptor 2) positive, luminal A, and luminal B subtypes using data from tumor samples in The Cancer Genome Atlas (TCGA) and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC). These data showed a positive link between NR4A1-NR4A3 expression and increased overall survival and relapse-free survival in breast cancer patients. In addition, we observed that high expression of NR4A1, NR4A2, and NR4A3 led to better survival. Furthermore, NR4A family genes seem to play an essential regulatory role in glycolysis and oxidative phosphorylation in breast cancer. The novel prognostic role of the NR4A1–NR4A3 receptors implicates these receptors as important mediators controlling breast cancer metabolic reprograming and its progression. The review establishes a strong clinical basis for the investigation of the cellular, molecular, and physiological roles of NR4A genes in breast cancer.

Age-associated changes in microglia and astrocytes ameliorate blood-brain barrier dysfunction

Blood-brain barrier (BBB) dysfunction is associated with an accumulation of neurotoxic molecules and increased infiltration of peripheral cells within the brain parenchyma. Accruing evidence suggests that microglia and astrocytes play a crucial role in the recovery of BBB integrity and the corralling of infiltrating cells into clusters after brain damage, but the mechanisms involved remain unclear. Intriguingly, the results of flow cytometry and immunofluorescence analyses have shown that BBB permeability to peripheral cells is substantially enhanced during normal aging at 12 months in mice. Thus, we used the SMART-seq2 method to perform RNA sequencing of microglia and astrocytes at five time points before and immediately after the BBB permeability change. Our comprehensive analyses revealed that microglia are characterized by marked alterations in the negative regulation of protein phosphorylation and phagocytic vesicles, whereas astrocytes show elevated enzyme or peptidase-inhibitor activity in the recovery of BBB function. Moreover, we identified a cassette of key genes that might ameliorate the insults of pathophysiological events in aging and neurodegenerative disease.

Transcriptome features of striated muscle aging and predictability of protein level changes

We performed total RNA sequencing and multi-omics analysis comparing skeletal muscle and cardiac muscle in young adult (4 months) vs. early aging (20 months) mice to examine the molecular mechanisms of striated muscle aging. We observed that aging cardiac and skeletal muscles both invoke transcriptomic changes in innate immune system and mitochondria pathways but diverge in extracellular matrix processes. On an individual gene level, we identified 611 age-associated signatures in skeletal and cardiac muscles, including a number of myokine and cardiokine encoding genes. Because RNA and protein levels correlate only partially, we reason that differentially expressed transcripts that accurately reflect their protein counterparts will be more valuable proxies for proteomic changes and by extension physiological states. We applied a computational data analysis workflow to estimate which transcriptomic changes are more likely relevant to protein-level regulation using large proteogenomics data sets. We estimate about 48% of the aging-associated transcripts predict protein levels well (r ≥ 0.5). In parallel, a comparison of the identified aging-regulated genes with public human transcriptomics data showed that only 35-45% of the identified genes show an age-dependent expression in corresponding human tissues. Thus, integrating both RNA-protein correlation and human conservation across data sources, we nominate 134 prioritized aging striated muscle signatures that are predicted to correlate strongly with protein levels and that show age-dependent expression in humans. The results here reveal new details into how aging reshapes gene expression in striated muscles at the transcript and protein levels.

Matrisome, innervation and oxidative metabolism affected in older compared with younger males with similar physical activity

BACKGROUND

Due to the interaction between skeletal muscle ageing and lifestyle factors, it is often challenging to attribute the decline in muscle mass and quality to either changes in lifestyle or to advancing age itself. Because many of the physiological factors affecting muscle mass and quality are modulated by physical activity and physical activity declines with age, the aim of this study is to better understand the effects of early ageing on muscle function by comparing a population of healthy older and young males with similar physical activity patterns.

METHODS

Eighteen older (69 ± 2.0 years) and 20 young (22 ± 2.0 years) males were recruited based on similar self-reported physical activity, which was verified using accelerometry measurements. Gene expression profiles of vastus lateralis biopsies obtained by RNA sequencing were compared, and key results were validated using quantitative polymerase chain reaction and western blot.

RESULTS

Total physical activity energy expenditure was similar between the young and old group (404 ± 215 vs. 411 ± 189 kcal/day, P = 0.11). Three thousand seven hundred ninety-seven differentially expressed coding genes (DEGs) were identified (adjusted P-value cut-off of <0.05), of which 1891 were higher and 1906 were lower expressed in the older muscle. The matrisome, innervation and inflammation were the main upregulated processes, and oxidative metabolism was the main downregulated process in old compared with young muscle. Lower protein levels of mitochondrial transcription factor A (TFAM, P = 0.030) and mitochondrial respiratory Complexes IV and II (P = 0.011 and P = 0.0009, respectively) were observed, whereas a trend was observed for Complex I (P = 0.062), in older compared with young muscle. Protein expression of Complexes I and IV was significantly correlated to mitochondrial capacity in the vastus lateralis as measured in vivo (P = 0.017, R2  = 0.42 and P = 0.030, R2  = 0.36). A trend for higher muscle-specific receptor kinase (MUSK) protein levels in the older group was observed (P = 0.08).

CONCLUSIONS

There are clear differences in the transcriptome signatures of the vastus lateralis muscle of healthy older and young males with similar physical activity levels, including significant differences at the protein level. By disentangling physical activity and ageing, we appoint early skeletal muscle ageing processes that occur despite similar physical activity. Improved understanding of these processes will be key to design targeted anti-ageing therapies.

The Comparative Methylome and Transcriptome After Change of Direction Compared to Straight Line Running Exercise in Human Skeletal Muscle

The methylome and transcriptome signatures following exercise that are physiologically and metabolically relevant to sporting contexts such as team sports or health prescription scenarios (e.g., high intensity interval training/HIIT) has not been investigated. To explore this, we performed two different sport/exercise relevant high-intensity running protocols in five male sport team members using a repeated measures design of: (1) change of direction (COD) versus; (2) straight line (ST) running exercise with a wash-out period of at least 2 weeks between trials. Skeletal muscle biopsies collected from the vastus lateralis 30 min and 24 h post exercise, were assayed using 850K methylation arrays and a comparative analysis with recent (subject-unmatched) sprint and acute aerobic exercise meta-analysis transcriptomes was performed. Despite COD and ST exercise being matched for classically defined intensity measures (speed × distance and number of accelerations/decelerations), COD exercise elicited greater movement (GPS-Playerload), physiological (HR), metabolic (lactate) as well as central and peripheral (differential RPE) exertion measures compared with ST exercise, suggesting COD exercise evoked a higher exercise intensity. The exercise response alone across both conditions evoked extensive alterations in the methylome 30 min and 24 h post exercise, particularly in MAPK, AMPK and axon guidance pathways. COD evoked a considerably greater hypomethylated signature across the genome compared with ST exercise, particularly at 30 min post exercise, enriched in: Protein binding, MAPK, AMPK, insulin, and axon guidance pathways. Comparative methylome analysis with sprint running transcriptomes identified considerable overlap, with 49% of genes that were altered at the expression level also differentially methylated after COD exercise. After differential methylated region analysis, we observed that VEGFA and its downstream nuclear transcription factor, NR4A1 had enriched hypomethylation within their promoter regions. VEGFA and NR4A1 were also significantly upregulated in the sprint transcriptome and meta-analysis of exercise transcriptomes. We also confirmed increased gene expression of VEGFA, and considerably larger increases in the expression of canonical metabolic genes PPARGC1A (that encodes PGC1-α) and NR4A3 in COD vs. ST exercise. Overall, we demonstrate that increased physiological/metabolic load via COD exercise in human skeletal muscle evokes considerable epigenetic modifications that are associated with changes in expression of genes responsible for adaptation to exercise.

Nuclear Receptor Nur77 Controls Cardiac Fibrosis through Distinct Actions on Fibroblasts and Cardiomyocytes

Fibrosis is a hallmark of adverse cardiac remodeling, which promotes heart failure, but it is also an essential repair mechanism to prevent cardiac rupture, signifying the importance of appropriate regulation of this process. In the remodeling heart, cardiac fibroblasts (CFs) differentiate into myofibroblasts (MyoFB), which are the key mediators of the fibrotic response. Additionally, cardiomyocytes are involved by providing pro-fibrotic cues. Nuclear receptor Nur77 is known to reduce cardiac hypertrophy and associated fibrosis; however, the exact function of Nur77 in the fibrotic response is yet unknown. Here, we show that Nur77-deficient mice exhibit severe myocardial wall thinning, rupture and reduced collagen fiber density after myocardial infarction and chronic isoproterenol (ISO) infusion. Upon Nur77 knockdown in cultured rat CFs, expression of MyoFB markers and extracellular matrix proteins is reduced after stimulation with ISO or transforming growth factor–β (TGF-β). Accordingly, Nur77-depleted CFs produce less collagen and exhibit diminished proliferation and wound closure capacity. Interestingly, Nur77 knockdown in neonatal rat cardiomyocytes results in increased paracrine induction of MyoFB differentiation, which was blocked by TGF-β receptor antagonism. Taken together, Nur77-mediated regulation involves CF-intrinsic promotion of CF-to-MyoFB transition and inhibition of cardiomyocyte-driven paracrine TGF-β-mediated MyoFB differentiation. As such, Nur77 provides distinct, cell-specific regulation of cardiac fibrosis.

Targeting Orphan Nuclear Receptors NR4As for Energy Homeostasis and Diabetes

Orphan nuclear receptors are important members of the nuclear receptor family and may regulate cell proliferation, metabolism, differentiation, and apoptosis. NR4As, a subfamily of orphan nuclear receptors, have been reported to play key roles in carbohydrate and lipid metabolism and energy homeostasis. Popularity of obesity has resulted in a series of metabolic diseases such as diabetes and its complications. While imbalance of energy intake and expenditure is the main cause of obesity, the concrete mechanism of obesity has not been fully understood. It has been reported that NR4As have significant regulatory effects on energy homeostasis and diabetes and are expected to become new targets for discovering drugs for metabolic syndrome. A number of studies have demonstrated that abnormalities in metabolism induced by altered levels of NR4As may contribute to numerous diseases, such as chronic inflammation, tumorigenesis, diabetes and its complications, atherosclerosis, and other cardiovascular diseases. However, systematic reviews focusing on the roles of NR4As in mediating energy homeostasis and diabetes remain limited. Therefore, this article reviews the structure and regulation of NR4As and their critical function in energy homeostasis and diabetes, as well as small molecules that may regulate NR4As. Our work is aimed at providing valuable support for the research and development of drugs targeting NR4As for the treatment of obesity and related metabolic diseases.

Simvastatin Improves Behavioral Disorders and Hippocampal Inflammatory Reaction by NMDA-Mediated Anti-inflammatory Function in MPTP-Treated Mice

The cognitive function impairment may be related to the inflammation of the hippocampus in Parkinson's disease. Simvastatin can play a positive role in Parkinson's disease. The purpose of this study was to investigate whether simvastatin could improve behavioral disorders, especially depression, anxiety and cognitive function in mouse PD models, and further explore the molecular mechanism. In the present study, C57BL-6 mice underwent intraperitoneal injection of MPTP (30 mg/kg) once a day for 5 consecutive days. At the same time, simvastatin (10 mg/kg) was pretreated for 2 days before the Parkinson's disease model was established, and then continued for 5 days, and the control group underwent intraperitoneal injection of MK801 (dizocilpine, 0.2 mg/kg) and saline solution. Depression status was tested by a tail suspension test and a sucrose splash test, followed by an open-field test and an elevated plus maze test to determine anxiety levels. Spatial behavior and muscle status were measured with a water maze and a rotarod test. The expression of RNA and protein of N-methyl-D-aspartate receptor subtype 2B (NMDAR2B), nerve growth factor IB (Nur77), cyclooxygenase-2 (COX-2), and tumor necrosis factor (TNF) α were assayed by real-time polymerase chain reaction and Western blot. Our results showed that simvastatin can improve the cognitive function, anxiety, and depression of PD mice with MPTP injury. Simvastatin reversed the NMDAR2B increase, restored Nur77 downward, and reduced the expression of COX-2 and TNF-α in MPTP-treated mice. This role of simvastatin was consistent with MK801 in increasing the expression of Nur77 and inhibiting NMDAR2B and cytokines in MPTP-lesioned PD mice. These findings suggest that reversed the NMDAR2B increase, restored Nur77 downward, and reduced the expression of COX-2 and TNF-α in MPTP-treated mice may be one of the mechanisms that simvastatin improves cognitive functions, depression, and anxiety in MPTP-lesioned mice.

Nur77 protects against adverse cardiac remodelling by limiting neuropeptide Y signalling in the sympathoadrenal-cardiac axis

Aims

Cardiac remodelling and heart failure are promoted by persistent sympathetic activity. We recently reported that nuclear receptor Nur77 may protect against sympathetic agonist-induced cardiac remodelling in mice. The sympathetic co-transmitter neuropeptide Y (NPY) is co-released with catecholamines and is a known cardiac modulator and predictor of heart failure mortality. Recently, transcriptome analyses revealed NPY as a putative target of Nur77. In this study, we assess whether Nur77 modulates adverse cardiac remodelling via NPY signalling.

Methods and results

Nur77 represses NPY expression in the PC12 adrenal chromaffin cell line. Accordingly, NPY levels are higher in adrenal gland, plasma, and heart from Nur77-KO compared to wild-type mice. Conditioned medium from Nur77-silenced chromaffin cells and serum from Nur77-KO mice induce marked hypertrophy in cultured neonatal rat cardiomyocytes, which is inhibited by the NPY type 1 receptor (NPY1R) antagonist BIBO3304. In cardiomyocytes from Nur77-KO mice, intracellular Ca2+ is increased partially via the NPY1R. This is independent from elevated circulating NPY since cardiomyocyte-specific Nur77-deficient mice (CM-KO) do not have elevated circulating NPY, but do exhibit BIBO3304-sensitive, increased cardiomyocyte intracellular Ca2+. In vivo, this translates to NPY1R antagonism attenuating cardiac calcineurin activity and isoproterenol-induced cardiomyocyte hypertrophy and fibrosis in full-body Nur77-KO mice, but not in CM-KO mice.

Conclusions

The cardioprotective action of Nur77 can be ascribed to both inhibition of circulating NPY levels and to cardiomyocyte-specific modulation of NPY-NPY1R signalling. These results reveal the underlying mechanism of Nur77 as a promising modifier gene in heart failure.

Function of Nr4a Orphan Nuclear Receptors in Proliferation, Apoptosis and Fuel Utilization Across Tissues

The Nr4a family of nuclear hormone receptors is composed of three members-Nr4a1/Nur77, Nr4a2/Nurr1 and Nr4a3/Nor1. While currently defined as ligandless, these transcription factors have been shown to regulate varied processes across a host of tissues. Of particular interest, the Nr4a family impinge, in a tissue dependent fashion, on cellular proliferation, apoptosis and fuel utilization. The regulation of these processes occurs through both nuclear and non-genomic pathways. The purpose of this review is to provide a balanced perspective of the tissue specific and Nr4a family member specific, effects on cellular proliferation, apoptosis and fuel utilization.

Muscle-generated BDNF is a sexually dimorphic myokine that controls metabolic flexibility

The ability of skeletal muscle to switch between lipid and glucose oxidation for ATP production during metabolic stress is pivotal for maintaining systemic energy homeostasis, and dysregulation of this metabolic flexibility is a dominant cause of several metabolic disorders. However, the molecular mechanism that governs fuel selection in muscle is not well understood. Here, we report that brain-derived neurotrophic factor (BDNF) is a fasting-induced myokine that controls metabolic reprograming through the AMPK/CREB/PGC-1α pathway in female mice. Female mice with a muscle-specific deficiency in BDNF (MBKO mice) were unable to switch the predominant fuel source from carbohydrates to fatty acids during fasting, which reduced ATP production in muscle. Fasting-induced muscle atrophy was also compromised in female MBKO mice, likely a result of autophagy inhibition. These mutant mice displayed myofiber necrosis, weaker muscle strength, reduced locomotion, and muscle-specific insulin resistance. Together, our results show that muscle-derived BDNF facilitates metabolic adaption during nutrient scarcity in a gender-specific manner and that insufficient BDNF production in skeletal muscle promotes the development of metabolic myopathies and insulin resistance.

Functions of estrogen and estrogen receptor signaling on skeletal muscle

Activity of estrogen, a sex steroid hormone, is not only limited to the reproductive organs but also involves other organs and tissues, including skeletal muscle. In postmenopausal women, estrogen decline causes endocrine and metabolic dysfunction, leading to a predisposition to osteoporosis, metabolic syndrome, and decreased muscle mass and strength. The decline in skeletal muscle mass often associates with sarcopenia, a popular condition observed in fragile elder people. In addition, varying estrogen levels associated with the menstrual phases may modulate exercise performance in women. Estrogen is thus considered to play a crucial role in skeletal muscle homeostasis and exercise capacity, although its precise mechanisms remain to be elucidated. In this article, we review the role of estrogen in the skeletal muscle, outlining the proposed molecular mechanisms. We especially focus on the current understanding of estrogen actions on mitochondria metabolism in skeletal muscle.

Transcriptome Changes of Skeletal Muscle RNA-Seq Speculates the Mechanism of Postprandial Hyperglycemia in Diabetic Goto-Kakizaki Rats During the Early Stage of T2D

To address how skeletal muscle contributes to postprandial hyperglycemia, we performed skeletal muscle transcriptome analysis of diabetic Goto-Kakizaki (GK) and control Wistar rats by RNA sequencing (RNA-Seq). We obtained 600 and 1785 differentially expressed genes in GK rats compared to those Wistar rats at three and four weeks of age, respectively. Specifically, Tbc1d4, involved in glucose uptake, was significantly downregulated in the skeletal muscle of GK aged both three and four weeks compared to those of age-matched Wistar rats. Pdk4, related to glucose uptake and oxidation, was significantly upregulated in the skeletal muscle of GK aged both three and four weeks compared to that of age-matched Wistar rats. Genes (Acadl, Acsl1 and Fabp4) implicated in fatty acid oxidation were significantly upregulated in the skeletal muscle of GK aged four weeks compared to those of age-matched Wistar rats. The overexpression or knockout of Tbc1d4, Pdk4, Acadl, Acsl1 and Fabp4 has been reported to change glucose uptake and fatty acid oxidation directly in rodents. By taking the results of previous studies into consideration, we speculated that dysregulation of key dysregulated genes (Tbc1d4, Pdk4, Acadl, Acsl1 and Fabp4) may lead to a decrease in glucose uptake and oxidation, and an increase in fatty acid oxidation in GK skeletal muscle at three and four weeks, which may, in turn, contribute to postprandial hyperglycemia. Our research revealed transcriptome changes in GK skeletal muscle at three and four weeks. Tbc1d4, Acadl, Acsl1 and Fabp4 were found to be associated with early diabetes in GK rats for the first time, which may provide a new scope for pathogenesis of postprandial hyperglycemia.

NURR1 activation in skeletal muscle controls systemic energy homeostasis

Skeletal muscle plays a central role in the control of metabolism and exercise tolerance. Analysis of muscle enhancers activated after exercise in mice revealed the orphan nuclear receptor NURR1/NR4A2 as a prominent component of exercise-responsive enhancers. We show that exercise enhances the expression of NURR1, and transgenic overexpression of NURR1 in skeletal muscle enhances physical performance in mice. NURR1 expression in skeletal muscle is also sufficient to prevent hyperglycemia and hepatic steatosis, by enhancing muscle glucose uptake and storage as glycogen. Furthermore, treatment of obese mice with putative NURR1 agonists increases energy expenditure, improves glucose tolerance, and confers a lean phenotype, mimicking the effects of exercise. These findings identify a key role for NURR1 in governance of skeletal muscle glucose metabolism, and reveal a transcriptional link between exercise and metabolism. Our findings also identify NURR1 agonists as possible exercise mimetics with the potential to ameliorate obesity and other metabolic abnormalities.

Skeletal muscle Nur77 and NOR1 insulin responsiveness is blunted in obesity and type 2 diabetes but improved after exercise training

Obesity and type 2 diabetes (T2DM) are characterized by a blunted metabolic response to insulin, and strongly manifests in skeletal muscle insulin resistance. The orphan nuclear receptors, Nur77 and NOR1, regulate insulin-stimulated nutrient metabolism where Nur77 and NOR1 gene expression is increased with acute aerobic exercise and acute insulin stimulation. Whether Nur77 or NOR1 are associated with the insulin-sensitizing effects of chronic aerobic exercise training has yet to be elucidated. Fourteen lean healthy controls (LHC), 12 obese (OB), and 10 T2DM individuals (T2DM) underwent hyperinsulinemic-euglycemic clamps with skeletal muscle biopsies. Muscle was analyzed for Nur77 and NOR1 gene and protein expression at basal and insulin-stimulated conditions. Furthermore, a subcohort of 18 participants (OB, n = 12; T2DM, n = 6) underwent a 12-week aerobic exercise intervention (85% HRmax , 60 min/day, 5 days/week). In response to insulin infusion, LHC increased protein expression of Nur77 (8.7 ± 3.2-fold) and NOR1 (3.6 ± 1.1-fold), whereas OB and T2DM remained unaffected. Clamp-derived glucose disposal rates correlated with Nur77 (r2  = 0.14) and NOR1 (r2  = 0.12) protein expression responses to insulin, whereas age (Nur77: r2  = 0.22; NOR1: r2  = 0.25) and BMI (Nur77: r2  = 0.22; NOR1: r2  = 0.42) showed inverse correlations, corroborating preclinical data. In the intervention cohort, exercise improved Nur77 protein expression in response to insulin (PRE: -1.2 ± 0.3%, POST: 6.2 ± 1.5%). Also, insulin treatment of primary human skeletal muscle cells increased Nur77 and NOR1 protein. These findings highlight the multifactorial nature of insulin resistance in human obesity and T2DM. Understanding the regulation of Nur77 and NOR1 in skeletal muscle and other insulin-sensitive tissues will create opportunities to advance therapies for T2DM.

Perm1 regulates CaMKII activation and shapes skeletal muscle responses to endurance exercise training

Objective

Endurance exercise training remodels skeletal muscle, leading to increased mitochondrial content and oxidative capacity. How exercise entrains skeletal muscle signaling pathways to induce adaptive responses remains unclear. In past studies, we identified Perm1 (PGC-1 and ERR induced regulator, muscle 1) as an exercise-induced gene and showed that Perm1 overexpression elicits similar muscle adaptations as endurance exercise training. The mechanism of action and the role of Perm1 in exercise-induced responses are not known. In this study, we aimed to determine the pathway by which Perm1 acts as well as the importance of Perm1 for acute and long-term responses to exercise.

Methods

We performed immunoprecipitation and mass spectrometry to identify Perm1 associated proteins, and validated Perm1 interactions with the Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). We also knocked down Perm1 expression in gastrocnemius muscles of mice via AAV-mediated delivery of shRNA and assessed the impact of reduced Perm1 expression on both acute molecular responses to a single treadmill exercise bout and long-term adaptive responses to four weeks of voluntary wheel running training. Finally, we asked whether Perm1 levels are modulated by diet or diseases affecting skeletal muscle function.

Results

We show that Perm1 associates with skeletal muscle CaMKII and promotes CaMKII activation. In response to an acute exercise bout, muscles with a knock down of Perm1 showed defects in the activation of CaMKII and p38 MAPK and blunted induction of regulators of oxidative metabolism. Following four weeks of voluntary training, Perm1 knockdown muscles had attenuated mitochondrial biogenesis. Finally, we found that Perm1 expression is reduced in diet-induced obese mice and in muscular dystrophy patients and mouse models.

Conclusions

Our findings identify Perm1 as a muscle-specific regulator of exercise-induced signaling and Perm1 levels as tuners of the skeletal muscle response to exercise. The decreased Perm1 levels in states of obesity or muscle disease suggest that Perm1 may link pathological states to inefficient exercise responses.

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Perm1 interacts with CaMKII and activates the CaMKII-MEF2 pathway.

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Perm1 is important for CaMKII activation and PGC-1α induction by an exercise bout.

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In endurance training, Perm1 impacts muscle oxidative metabolism pathway responses.

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Skeletal muscle levels of Perm1 are reduced in obesity and muscular dystrophy.

The orphan nuclear receptor Nor1/Nr4a3 is a negative regulator of β-cell mass

The Nr4a subfamily of nuclear receptor comprises three members in mammalian cells: Nur77/Nr4a1, Nurr1/Nr4a2, and Nor1/Nr4a3. Nr4a proteins play key roles in the regulation of glucose homeostasis in peripheral metabolic tissues. However, their biological functions in β-cells remain relatively uncharacterized. Here we sought to investigate the potential role of Nor1 in the regulation of β-cell mass and, in particular, β-cell survival/apoptosis. We used histological analysis to examine the consequences of genetic deletion of either Nur77 and Nor1 on β-cell mass, investigated the expression patterns of Nr4as in human islets and INS cells and performed gain- and loss-of-function experiments to further characterize the role of Nor1 in β-cell apoptosis. Surprisingly, Nor1 knockout mice displayed increased β-cell mass, whereas mice with genetic deletion of Nur77 did not exhibit any significant differences compared with their WT littermates. The increase in β-cell mass in Nor1 knockout mice was accompanied by improved glucose tolerance. A gene expression study performed in both human islets and INS cells revealed that Nor1 expression is significantly increased by pro-inflammatory cytokines and, to a lesser extent, by elevated concentrations of glucose. Nor1 overexpression in both INS and human islet cells caused apoptosis, whereas siRNA-mediated Nor1 knockdown prevented cytokine-induced β-cell death. Finally, Nor1 expression was up-regulated in islets of individuals with type 2 diabetes. Altogether, our results uncover that Nor1 negatively regulates β-cell mass. Nor1 represents a promising molecular target in diabetes treatment to prevent β-cell destruction.

Estrogen signaling increases nuclear receptor subfamily 4 group A member 1 expression and energy production in skeletal muscle cells

Estrogen deficiency has been known to associate with musculoskeletal diseases in women, based on the clinical observations of frequent susceptibility to osteoporosis and sarcopenia among postmenopausal women. In skeletal muscles, estrogen has been assumed to play physiological roles in maintaining muscle mass and strength, although its precise molecular mechanism remains to be elucidated. We have previously shown that estrogen regulates energy metabolism through the downregulation of mitochondrial uncoupling protein 3 (UCP3) in skeletal muscles, which may contribute to the prolonged exercise endurance in female mice. In the present study, we investigated the effects of estrogen on the expression levels of all members of the nuclear receptor superfamily. Microarray analysis showed that the mRNA level of nuclear receptor subfamily 4 group A member 1 (Nr4a1) was upregulated by the transduction of a recombinant adenovirus expressing constitutively active estrogen receptor α (caERα) in differentiated myoblastic C2C12 cells. Thus we assumed that NR4A1 may be an estrogen-inducible gene in myoblastic cells. We also demonstrated that caERα increases the cellular ATP content along with an increase in mitochondrial DNA content in differentiated myoblastic C2C12 cells. In contrast, the knockdown of Nr4a1 using siRNA exhibited reduced ATP generation as well as a decrease in mitochondrial DNA content. Overall, the present study indicates a crosstalk between estrogen signaling and NR4A1 in skeletal muscle cells. We consider that estrogen-dependent NR4A1 upregulation could increase efficient ATP generation in skeletal muscle cells partly through enhancing mitochondrial functions.

Energy and metabolic pathways in trefoil factor family member 2 (Tff2) KO mice beyond the protection from high-fat diet-induced obesity

AIMS

Trefoil factor family member 2 (TFF2) is a small gut peptide. We have previously shown that Tff2 knock out (KO) mice are protected from high-fat (HF) diet-induced obesity (De Giorgio et al., 2013a). Thus, exploring Tff2 KO-related pathways of mice at the genomic, proteinic and biochemical levels would allow us to elucidate the processes behind this protection from obesity.

MAIN METHODS

To explore the metabolic and energetic effects related to Tff2 deficiency, we used sampled blood from the previous study to measure levels of free fatty acids, glucose, glycerol and triglycerides in serum. Expression levels of selected genes and proteins related to energy metabolism in the skeletal muscle, liver and adipose tissue were also studied.

KEY FINDINGS

Following the 12-wk challenging of Tff2 KO and WT mice with both HF and low-fat diet, Tff2 KO mice had lower levels of serum glucose, triglycerides and glycerol. Importantly, western blotting and Q_RT-PCR revealed that the expression levels of selected genes and proteins are toward less fat storage and increased energy expenditure by enhancing lipid and glucose utilization via oxidative phosphorylation.

SIGNIFICANCE

We mapped a part of the metabolic and biochemical pathways of lipids and glucose involving the adipose tissue, liver, skeletal muscle and sympathetic nervous system that protect Tff2 KO mice from the HF diet-induced obesity. Our data highlight Tff2-related pathways as potential targets for obesity therapies.

The Nuclear Receptor Nor-1 Is a Pleiotropic Regulator of Exercise-Induced Adaptations

Exercise induces various physical and metabolic changes in skeletal muscle that adaptively reprograms this tissue to current physiological and environmental demands. Underlying these changes are broad modifications to gene expression. We postulate that the nuclear hormone receptor, Nor-1, is activated after exercise, and this transcription factor modifies gene expression to drive the molecular and cellular adaptations associated with contractile reorganization.

Nur77 serves as a molecular brake of the metabolic switch during T cell activation to restrict autoimmunity

T cells critically depend on reprogramming of metabolic signatures to meet the bioenergetic demands during activation and clonal expansion. Here we identify the transcription factor Nur77 as a cell-intrinsic modulator of T cell activation. Nur77-deficient T cells are highly proliferative, and lack of Nur77 is associated with enhanced T cell activation and increased susceptibility for T cell-mediated inflammatory diseases, such as CNS autoimmunity, allergic contact dermatitis and collagen-induced arthritis. Importantly, Nur77 serves as key regulator of energy metabolism in T cells, restricting mitochondrial respiration and glycolysis and controlling switching between different energy pathways. Transcriptional network analysis revealed that Nur77 modulates the expression of metabolic genes, most likely in close interaction with other transcription factors, especially estrogen-related receptor α. In summary, we identify Nur77 as a transcriptional regulator of T cell metabolism, which elevates the threshold for T cell activation and confers protection in different T cell-mediated inflammatory diseases.

The Orphan Nuclear Receptor 4A1: A Potential New Therapeutic Target for Metabolic Diseases

Orphan nuclear receptor 4A1 (NR4A1) is a transcriptional factor of the nuclear orphan receptor (NR4A) superfamily that has sparked interest across different research fields in recent years. Several studies have demonstrated that ligand-independent NR4A1 is an immediate-early response gene and the protein product is rapidly induced by a variety of stimuli. Hyperfunction or dysfunction of NR4A1 is implicated in various metabolic processes, including carbohydrate metabolism, lipid metabolism, and energy balance, in major metabolic tissues, such as liver, skeletal muscle, pancreatic tissues, and adipose tissues. No endogenous ligands for NR4A1 have been identified, but numerous compounds that bind and activate or inactivate nuclear NR4A1 or induce cytoplasmic localization of NR4A1 have been identified. This review summarizes recent advances in our understanding of the molecular biology and physiological functions of NR4A1. And we focus on the physiological functions of NR4A1 receptor to the development of the metabolic diseases, with a special focus on the impact on carbohydrate and lipid metabolism in skeletal muscle, liver, adipose tissue, and islet.

Role of Nuclear Receptors in Exercise-Induced Muscle Adaptations

Skeletal muscle is not only one of the largest, but also one of the most dynamic organs. For example, plasticity elicited by endurance or resistance exercise entails complex transcriptional programs that are still poorly understood. Various signaling pathways are engaged in the contracting muscle fiber and collectively culminate in the modulation of the activity of numerous transcription factors (TFs) and coregulators. Because exercise confers many benefits for the prevention and treatment of a wide variety of pathologies, pharmacological activation of signaling pathways and TFs is an attractive avenue to elicit therapeutic effects. Members of the nuclear receptor (NR) superfamily are of particular interest owing to the presence of well-defined DNA- and ligand-binding domains. In this review, we summarize the current understanding of the involvement of NRs in muscle biology and exercise adaptation.

Cardiac nuclear receptors: architects of mitochondrial structure and function

The adult heart is uniquely designed and equipped to provide a continuous supply of energy in the form of ATP to support persistent contractile function. This high-capacity energy transduction system is the result of a remarkable surge in mitochondrial biogenesis and maturation during the fetal-to-adult transition in cardiac development. Substantial evidence indicates that nuclear receptor signaling is integral to dynamic changes in the cardiac mitochondrial phenotype in response to developmental cues, in response to diverse postnatal physiologic conditions, and in disease states such as heart failure. A subset of cardiac-enriched nuclear receptors serve to match mitochondrial fuel preferences and capacity for ATP production with changing energy demands of the heart. In this Review, we describe the role of specific nuclear receptors and their coregulators in the dynamic control of mitochondrial biogenesis and energy metabolism in the normal and diseased heart.

Nur77 deletion impairs muscle growth during developmental myogenesis and muscle regeneration in mice

Muscle atrophy is a prevalent condition in illness and aging. Identifying novel pathways that control muscle mass may lead to therapeutic advancement. We previously identified Nur77 as a transcriptional regulator of glycolysis in skeletal muscle. More recently, we showed that Nur77 expression also controls myofiber size in mice. It was unknown, however, whether Nur77’s regulation of muscle size begins during developmental myogenesis or only in adulthood. To determine the importance of Nur77 throughout muscle growth, we examined myofiber size at E18.5, 3 weeks postnatal age, and in young adult mice. Using the global Nur77^-/- mice, we showed that Nur77 deficiency reduced myofiber size as early as E18.5. The reduction in myofiber size became more pronounced by 3 weeks of age. We observed comparable reduction in myofiber size in young myofiber-specific Nur77-knockout mice. These findings suggest that Nur77’s effect on muscle growth is intrinsic to its expression in differentiating myofibers, and not dependent on its expression in myogenic stem cells. To determine the importance of Nur77 expression in muscle accretion in mature mice, we generated an inducible-, muscle-specific, Nur77-deficient mouse model. We demonstrated that tamoxifen-induced deletion of Nur77 in 3-month-old mice reduced myofiber size. This change was accompanied by increased activity of Smad2 and FoxO3, two negative regulators of muscle mass. The role of Nur77 in muscle growth was further elaborated in the cardiotoxin-induced muscle regeneration model. Compared to wildtype mice, regenerated myofibers were smaller in Nur77^-/- mice. However, when normalized to saline-injected muscle, the recovery of sarcoplasmic area was comparable between Nur77^-/- and wildtype mice. These findings suggest that Nur77 deficiency compromises myofiber growth, but not the regenerative capacity of myogenic progenitor cells. Collectively, the findings presented here demonstrate Nur77 as an important regulator of muscle growth both during prenatal and postnatal myogenesis.

Transcriptomic Analysis Shows Decreased Cortical Expression of NR4A1, NR4A2 and RXRB in Schizophrenia and Provides Evidence for Nuclear Receptor Dysregulation

Many genes are differentially expressed in the cortex of people with schizophrenia, implicating factors that control transcription more generally. Hormone nuclear receptors dimerize to coordinate context-dependent changes in gene expression. We hypothesized that members of two families of nuclear receptors (NR4As), and retinoid receptors (RARs and RXRs), are altered in the dorsal lateral prefrontal cortex (DLPFC) of people with schizophrenia. We used next generation sequencing and then qPCR analysis to test for changes in mRNA levels for transcripts encoding nuclear receptors: orphan nuclear receptors (3 in the NR4A, 3 in the RAR, 3 in the RXR families and KLF4) in total RNA extracted from the DLPFC from people with schizophrenia compared to controls (n = 74). We also correlated mRNA levels with demographic factors and with estimates of antipsychotic drug exposure (schizophrenia group only). We tested for correlations between levels of transcription factor family members and levels of genes putatively regulated by these transcription factors. We found significantly down regulated expression of NR4A1 (Nurr 77) and KLF4 mRNAs in people with schizophrenia compared to controls, by both NGS and qPCR (p = or <0.01). We also detected decreases in NR4A2 (Nurr1) and RXRB mRNAs by using qPCR in the larger cohort (p<0.05 and p<0.01, respectively). We detected decreased expression of RARG and NR4A2 mRNAs in females with schizophrenia (p<0.05). The mRNA levels of NR4A1, NR4A2 and NR4A3 were all negative correlated with lifetime estimates of antipsychotic exposure. These novel findings, which may be influenced by antipsychotic drug exposure, implicate the orphan and retinoid nuclear receptors in the cortical pathology found in schizophrenia. Genes down stream of these receptors can be dysregulated as well, but the direction of change is not immediately predictable based on the putative transcription factor changes.

Sarcolipin: A Key Thermogenic and Metabolic Regulator in Skeletal Muscle

Skeletal muscle constitutes ∼40% of body mass and has the capacity to play a major role as thermogenic, metabolic, and endocrine organ. In addition to shivering, muscle also contributes to nonshivering thermogenesis via futile sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity. Sarcolipin (SLN), a regulator of SERCA activity in muscle, plays an important role in regulating muscle thermogenesis and metabolism. Uncoupling of SERCA by SLN increases ATP hydrolysis and heat production, and contributes to temperature homeostasis. SLN also affects whole-body metabolism and weight gain in mice, and is upregulated in various muscle diseases including muscular dystrophy, suggesting a role for SLN during increased metabolic demand. In this review we also highlight the physiological roles of skeletal muscle beyond contraction.

Role of protein phosphatase inhibitor-1 in cardiac beta adrenergic pathway

Phosphoproteomic studies have shown that about one third of all cardiac proteins are reversibly phosphorylated, affecting virtually every cellular signaling pathway. The reversibility of this process is orchestrated by the opposing enzymatic activity of kinases and phosphatases. Conversely, imbalances in subcellular protein phosphorylation patterns are a hallmark of many cardiovascular diseases including heart failure and cardiac arrhythmias. While numerous studies have revealed excessive beta-adrenergic signaling followed by deregulated kinase expression or activity as a major driver of the latter cardiac pathologies, far less is known about the beta-adrenergic regulation of their phosphatase counterparts. In fact, most of the limited knowledge stems from the detailed analysis of the endogenous inhibitor of the protein phosphatase 1 (I-1) in cellular and animal models. I-1 acts as a nodal point between adrenergic and putatively non-adrenergic cardiac signaling pathways and is able to influence widespread cellular functions of protein phosphatase 1 which are contributing to cardiac health and disease, e.g. Ca²⁺ handling, sarcomere contractility and glucose metabolism. Finally, nearly all of these studies agree that I-1 is a promising drug target on the one hand but the outcome of its pharmacological regulation maybe extremely context-dependent on the other hand, thus warranting for careful interpretation of past and future experimental results. In this respect we will: 1) comprehensively review the current knowledge about structural, functional and regulatory properties of I-1 within the heart 2) highlight current working hypothesis and potential I-1 mediated disease mechanisms 3) discuss state-of-the-art knowledge and future prospects of a potential therapeutic strategy targeting I-1 by restoring the balance of cardiac protein phosphorylation.

β-Cell deletion of Nr4a1 and Nr4a3 nuclear receptors impedes mitochondrial respiration and insulin secretion

β-Cell insulin secretion is dependent on proper mitochondrial function. Various studies have clearly shown that the Nr4a family of orphan nuclear receptors is essential for fuel utilization and mitochondrial function in liver, muscle, and adipose. Previously, we have demonstrated that overexpression of Nr4a1 or Nr4a3 is sufficient to induce proliferation of pancreatic β-cells. In this study, we examined whether Nr4a expression impacts pancreatic β-cell mitochondrial function. Here, we show that β-cell mitochondrial respiration is dependent on the nuclear receptors Nr4a1 and Nr4a3. Mitochondrial respiration in permeabilized cells was significantly decreased in β-cells lacking Nr4a1 or Nr4a3. Furthermore, respiration rates of intact cells deficient for Nr4a1 or Nr4a3 in the presence of 16 mM glucose resulted in decreased glucose mediated oxygen consumption. Consistent with this reduction in respiration, a significant decrease in glucose-stimulated insulin secretion rates is observed with deletion of Nr4a1 or Nr4a3. Interestingly, the changes in respiration and insulin secretion occur without a reduction in mitochondrial content, suggesting decreased mitochondrial function. We establish that knockdown of Nr4a1 and Nr4a3 results in decreased expression of the mitochondrial dehydrogenase subunits Idh3g and Sdhb. We demonstrate that loss of Nr4a1 and Nr4a3 impedes production of ATP and ultimately inhibits glucose-stimulated insulin secretion. These data demonstrate for the first time that the orphan nuclear receptors Nr4a1 and Nr4a3 are critical for β-cell mitochondrial function and insulin secretion.

Adaptive muscle plasticity of a remaining agonist following denervation of its close synergists in a model of complete spinal cord injury

Complete spinal cord injury (SCI) alters the contractile properties of skeletal muscle, and although exercise can induce positive changes, it is unclear whether the remaining motor system can produce adaptive muscle plasticity in response to a subsequent peripheral nerve injury. To address this, the nerve supplying the lateral gastrocnemius (LG) and soleus muscles was sectioned unilaterally in four cats that had recovered hindlimb locomotion after spinal transection. In these spinal cats, kinematics and electromyography (EMG) were collected before and for 8 wk after denervation. Muscle histology was performed on LG and medial gastrocnemius (MG) bilaterally in four spinal and four intact cats. In spinal cats, cycle duration for the hindlimb ipsilateral or contralateral to the denervation could be significantly increased or decreased compared with predenervation values. Stance duration was generally increased and decreased for the contralateral and ipsilateral hindlimbs, respectively. The EMG amplitude of MG was significantly increased bilaterally after denervation and remained elevated 8 wk after denervation. In spinal cats the ipsilateral LG was significantly smaller than the contralateral LG, whereas the ipsilateral MG weighed significantly more than the contralateral MG. Histological characterizations revealed significantly larger fiber areas for type IIa fibers of the ipsilateral MG in three of four spinal cats. Microvascular density in the ipsilateral MG was significantly higher than in the contralateral MG. In intact cats, no differences were found for muscle weight, fiber area, or microvascular density between homologous muscles. Therefore, the remaining motor system after complete SCI retains the ability to produce adaptive muscle plasticity.

A MED13-dependent skeletal muscle gene program controls systemic glucose homeostasis and hepatic metabolism

Amoasii et al. found that skeletal muscle-specific deletion of the Mediator subunit MED13 in mice conferred resistance to hepatic steatosis by activating a metabolic gene program that enhances muscle glucose uptake and storage as glycogen. MED13 suppressed expression of genes involved in glucose uptake and metabolism in skeletal muscle by inhibiting the nuclear receptor NURR1 and the MEF2 transcription factor.

The Mediator complex governs gene expression by linking upstream signaling pathways with the basal transcriptional machinery. However, how individual Mediator subunits may function in different tissues remains to be investigated. Through skeletal muscle-specific deletion of the Mediator subunit MED13 in mice, we discovered a gene regulatory mechanism by which skeletal muscle modulates the response of the liver to a high-fat diet. Skeletal muscle-specific deletion of MED13 in mice conferred resistance to hepatic steatosis by activating a metabolic gene program that enhances muscle glucose uptake and storage as glycogen. The consequent insulin-sensitizing effect within skeletal muscle lowered systemic glucose and insulin levels independently of weight gain and adiposity and prevented hepatic lipid accumulation. MED13 suppressed the expression of genes involved in glucose uptake and metabolism in skeletal muscle by inhibiting the nuclear receptor NURR1 and the MEF2 transcription factor. These findings reveal a fundamental molecular mechanism for the governance of glucose metabolism and the control of hepatic lipid accumulation by skeletal muscle. Intriguingly, MED13 exerts opposing metabolic actions in skeletal muscle and the heart, highlighting the customized, tissue-specific functions of the Mediator complex.

The Nuclear Receptor, Nor-1, Induces the Physiological Responses Associated With Exercise

Skeletal muscle remodels metabolic capacity, contractile and exercise phenotype in response to physiological demands. This adaptive remodeling response to physical activity can ameliorate/prevent diseases associated with poor diet and lifestyle. Our previous work demonstrated that skeletal muscle-specific transgenic expression of the neuron-derived orphan nuclear receptor, Nor-1 drives muscle reprogramming, improves exercise endurance, and oxidative metabolism. The current manuscript investigates the association between exercise, Nor-1 expression and the role of Nor-1 in adaptive remodeling. We demonstrate that Nor-1 expression is induced by exercise and is dependent on calcium/calcineurin signaling (in vitro and in vivo). Analysis of fatigue-resistant transgenic mice that express Nor-1 in skeletal muscle revealed increased hypertrophy and vascularization of muscle tissue. Moreover, we demonstrate that transgenic Nor-1 expression is associated with increased intracellular recycling, ie, autophagy, involving 1) increased expression of light chain 3A or LC3A-II, autophagy protein 5, and autophagy protein 12 in quadriceps femoris muscle extracts from Tg-Nor-1 (relative to Wild-type (WT) littermates); 2) decreased p62 expression indicative of increased autophagolysosome assembly; and 3) decreased mammalian target of rapamycin complex 1 activity. Transfection of LC3A-GFP-RFP chimeric plasmid demonstrated that autophagolysosome formation was significantly increased by Nor-1 expression. Furthermore, we demonstrated a single bout of exercise induced LC3A-II expression in skeletal muscle from C57BL/6 WT mice. This study, when combined with our previous studies, demonstrates that Nor-1 expression drives multiple physiological changes/pathways that are critical to the beneficial responses of muscle to exercise and provides insights into potential pharmacological manipulation of muscle reprogramming for the treatment of lifestyle induced chronic diseases.

Perm1 enhances mitochondrial biogenesis, oxidative capacity, and fatigue resistance in adult skeletal muscle

Skeletal muscle mitochondrial content and oxidative capacity are important determinants of muscle function and whole-body health. Mitochondrial content and function are enhanced by endurance exercise and impaired in states or diseases where muscle function is compromised, such as myopathies, muscular dystrophies, neuromuscular diseases, and age-related muscle atrophy. Hence, elucidating the mechanisms that control muscle mitochondrial content and oxidative function can provide new insights into states and diseases that affect muscle health. In past studies, we identified Perm1 (PPARGC1- and ESRR-induced regulator, muscle 1) as a gene induced by endurance exercise in skeletal muscle, and regulating mitochondrial oxidative function in cultured myotubes. The capacity of Perm1 to regulate muscle mitochondrial content and function in vivo is not yet known. In this study, we use adeno-associated viral (AAV) vectors to increase Perm1 expression in skeletal muscles of 4-wk-old mice. Compared to control vector, AAV1-Perm1 leads to significant increases in mitochondrial content and oxidative capacity (by 40-80%). Moreover, AAV1-Perm1-transduced muscles show increased capillary density and resistance to fatigue (by 33 and 31%, respectively), without prominent changes in fiber-type composition. These findings suggest that Perm1 selectively regulates mitochondrial biogenesis and oxidative function, and implicate Perm1 in muscle adaptations that also occur in response to endurance exercise.

Regulation of skeletal muscle mitochondrial function by nuclear receptors: implications for health and disease

Skeletal muscle metabolism is highly dependent on mitochondrial function, with impaired mitochondrial biogenesis associated with the development of metabolic diseases such as insulin resistance and type 2 diabetes. Mitochondria display substantial plasticity in skeletal muscle, and are highly sensitive to levels of physical activity. It is thought that physical activity promotes mitochondrial biogenesis in skeletal muscle through increased expression of genes encoded in both the nuclear and the mitochondrial genome; however, how this process is co-ordinated at the cellular level is poorly understood. Nuclear receptors (NRs) are key signalling proteins capable of integrating environmental factors and mitochondrial function, thereby providing a potential link between exercise and mitochondrial biogenesis. The aim of this review is to highlight the function of NRs in skeletal muscle mitochondrial biogenesis and discuss the therapeutic potential of NRs for the management and treatment of chronic metabolic disease.

Constitutive activities of estrogen-related receptors: Transcriptional regulation of metabolism by the ERR pathways in health and disease

The estrogen-related receptors (ERRs) comprise a small group of orphan nuclear receptor transcription factors. The ERRα and ERRγ isoforms play a central role in the regulation of metabolic genes and cellular energy metabolism. Although less is known about ERRβ, recent studies have revealed the importance of this isoform in the maintenance of embryonic stem cell pluripotency. Thus, ERRs are essential to many biological processes. The development of several ERR knockout and overexpression models and the application of advanced functional genomics have allowed rapid advancement of our understanding of the physiology regulated by ERR pathways. Moreover, it has enabled us to begin to delineate the distinct programs regulated by ERRα and ERRγ that have overlapping effects on metabolism and growth. The current review primarily focuses on the physiologic roles of ERR isoforms related to their metabolic regulation; therefore, the ERRα and ERRγ are discussed in the greatest detail. We emphasize findings from gain- and loss-of-function models developed to characterize ERR control of skeletal muscle, heart and musculoskeletal physiology. These models have revealed that coordinating metabolic capacity with energy demand is essential for seemingly disparate processes such as muscle differentiation and hypertrophy, innate immune function, thermogenesis, and bone remodeling. Furthermore, the models have revealed that ERRα- and ERRγ-deficiency in mice accelerates progression of pathologic processes and implicates ERRs as etiologic factors in disease. We highlight the human diseases in which ERRs and their downstream metabolic pathways are perturbed, including heart failure and diabetes. While no natural ligand has been identified for any of the ERR isoforms, the potential for using synthetic small molecules to modulate their activity has been demonstrated. Based on our current understanding of their transcriptional mechanisms and physiologic relevance, the ERRs have emerged as potential therapeutic targets for treatment of osteoporosis, muscle atrophy, insulin resistance and heart failure in humans.

Amelioration of palmitate-induced metabolic dysfunction in L6 muscle cells expressing low levels of receptor-interacting protein 140

We have shown that reduced expression of receptor-interacting protein 140 (RIP140) alters the regulation of fatty-acid (FA) oxidation in muscle. To determine whether a high level of FA availability alters the effects of RIP140 on metabolic regulation, L6 myotubes were transfected with or without RNA interference oligonucleotide sequences to reduce RIP140 expression, and then incubated with high levels of palmitic acid, with or without insulin. High levels of palmitate reduced basal (53%-58%) and insulin-treated (24%-44%) FA uptake and oxidation, and increased basal glucose uptake (88%). In cells incubated with high levels of palmitate, low RIP140 increased basal FA uptake and insulin-treated FA oxidation and glucose uptake, and decreased basal glucose uptake and insulin-treated FA uptake. Under basal conditions, low RIP140 increased the mRNA content of FAT/CD36 (159%) and COX4 (61%), as well as the protein content of Nur77 (68%), whereas the mRNA expression of FGF21 (50%) was decreased, as was the protein content of CPT1b (35%) and FGF21 (44%). Under insulin-treated conditions, low RIP140 expression increased the mRNA content of MCAD (84%) and Nur77 (84%), as well as the protein content of Nur77 (23%). Thus, a low level of RIP140 restores the rates of FA uptake in the basal state, in part via a reduction in upstream insulin signaling. Our data also indicate that the protein expression of Nur77 may be modulated by RIP140 when muscle cells are metabolically challenged by high levels of palmitate.

Muscle as a "mediator" of systemic metabolism

Skeletal and cardiac muscles play key roles in the regulation of systemic energy homeostasis and display remarkable plasticity in their metabolic responses to caloric availability and physical activity. In this Perspective we discuss recent studies highlighting transcriptional mechanisms that govern systemic metabolism by striated muscles. We focus on the participation of the Mediator complex in this process, and suggest that tissue-specific regulation of Mediator subunits impacts metabolic homeostasis.

The orphan nuclear receptor Nur77 is a determinant of myofiber size and muscle mass in mice

We previously showed that the orphan nuclear receptor Nur77 (Nr4a1) plays an important role in the regulation of glucose homeostasis and oxidative metabolism in skeletal muscle. Here, we show using both gain- and loss-of-function models that Nur77 is also a regulator of muscle growth in mice. Transgenic expression of Nur77 in skeletal muscle in mice led to increases in myofiber size. Conversely, mice with global or muscle-specific deficiency in Nur77 exhibited reduced muscle mass and myofiber size. In contrast to Nur77 deficiency, deletion of the highly related nuclear receptor NOR1 (Nr4a3) had minimal effect on muscle mass and myofiber size. We further show that Nur77 mediates its effects on muscle size by orchestrating transcriptional programs that favor muscle growth, including the induction of insulin-like growth factor 1 (IGF1), as well as concomitant downregulation of growth-inhibitory genes, including myostatin, Fbxo32 (MAFbx), and Trim63 (MuRF1). Nur77-mediated increase in IGF1 led to activation of the Akt-mTOR-S6K cascade and the inhibition of FoxO3a activity. The dependence of Nur77 on IGF1 was recapitulated in primary myoblasts, establishing this as a cell-autonomous effect. Collectively, our findings identify Nur77 as a novel regulator of myofiber size and a potential transcriptional link between cellular metabolism and muscle growth.

The NR4A nuclear receptors as potential targets for anti-aging interventions

The development of innovative anti-aging strategy is urgently needed to promote healthy aging and overcome the occurrence of age-related diseases such as cancer, diabetes, cardiovascular and neurodegenerative diseases. Genomic instability, deregulated nutrient sensing and mitochondrial dysfunction are established hallmark of aging. Interestingly, the orphan nuclear receptors NR4A subfamily (NR4A1, NR4A2 and NR4A3) are nutrient sensors that trigger mitochondria biogenesis and improve intrinsic mitochondrial function. In addition, NR4A receptors are components of DNA repair machinery and promote DNA repair. Members of the NR4A subfamily should also be involved in anti-aging properties of hormesis since these receptors are induced by various form of cellular stress and stimulate protective cells response such as anti-oxidative activity and DNA repair. Previous studies reported that NR4A nuclear receptors subfamily is potential therapeutic targets for the treatment of age related disorders (e.g. metabolic syndromes, diabetes and neurodegenerative diseases). Consequently, we propose that targeting NR4A receptors might constitute a new approach to delay aging and the onset of diseases affecting our aging population.

The NR4A orphan nuclear receptors: mediators in metabolism and diseases

The NR4A subfamily is orphan nuclear receptors that belong to the larger nuclear receptors (NRs) superfamily of eukaryotic transcription factors. The NR4A subfamily includes three members, namely Nur77 (NR4A1), Nurr1 (NR4A2) and Nor1 (NR4A3) which are gene regulators and participate in diverse biological functions. Though the ligands for these receptors are presently unidentified, they are thought to be constitutively active. NR4A acts as molecular switches in gene regulation and their action is increasingly seen to be modulated by complex network of cellular signaling pathways. Members of the NR4A are expressed in tissue-specific fashion which indicates their selective control of various biological processes. Data reveal a host of functions governed by the NR4A subfamily members including general metabolism, immunity, cellular stress, memory, insulin sensitivity and cardiac homeostasis by regulating specific target genes whose products participates in such processes. Moreover, these receptors have a role in the onset and progression of various diseases such as various types of cancer, inflammation, atherosclerosis and obesity. In this review, a concise overview of the current understanding of the important metabolic roles governed by NR4A members including their participation in a number of diseases shall be provided.