Purinergic receptors expressed in human skeletal muscle fibres

Next Generation Sequencing and Electromyography Reveal the Involvement of the P2RX6 Gene in Myopathy

Ion channelopathies result from impaired ion channel protein function, due to mutations affecting ion transport across cell membranes. Over 40 diseases, including neuropathy, pain, migraine, epilepsy, and ataxia, are associated with ion channelopathies, impacting electrically excitable tissues and significantly affecting skeletal muscle. Gene mutations affecting transmembrane ionic flow are strongly linked to skeletal muscle disorders, particularly myopathies, disrupting muscle excitability and contraction. Electromyography (EMG) analysis performed on a patient who complained of weakness and fatigue revealed the presence of primary muscular damage, suggesting an early-stage myopathy. Whole exome sequencing (WES) did not detect potentially causative variants in known myopathy-associated genes but revealed a novel homozygous deletion of the P2RX6 gene likely disrupting protein function. The P2RX6 gene, predominantly expressed in skeletal muscle, is an ATP-gated ion channel receptor belonging to the purinergic receptors (P2RX) family. In addition, STRING pathways suggested a correlation with more proteins having a plausible role in myopathy. No previous studies have reported the implication of this gene in myopathy. Further studies are needed on patients with a defective ion channel pathway, and the use of in vitro functional assays in suppressing P2RX6 gene expression will be required to validate its functional role.

Energy Regulation in Inflammatory Sarcopenia by the Purinergic System

The purinergic system has a dual role: the maintenance of energy balance and signaling within cells. Adenosine and adenosine triphosphate (ATP) are essential for maintaining these functions. Sarcopenia is characterized by alterations in the control of energy and signaling in favor of catabolic pathways. This review details the association between the purinergic system and muscle and adipose tissue homeostasis, discussing recent findings in the involvement of purinergic receptors in muscle wasting and advances in the use of the purinergic system as a novel therapeutic target in the management of sarcopenia.

Dipyridamole activates adenosine A2B receptor and AMPK/cAMP signaling and promotes myogenic differentiation of myoblastic C2C12 cells

Introduction: Sarcopenia is defined as a loss of muscle mass and strength. ATP homeostasis is crucial during myogenesis. We determined how the purinergic system modulates myogenesis using dipyridamole (blocks adenosine taken up by the cells) and tenofovir (inhibits ATP release) in a myoblast cell line. Methods: C2C12 cells were differentiated in the presence/absence of tenofovir/dipyridamole, with/without the A2B selective inhibitor PSB-603. Extra-/intracellular nucleotides were examined via HPLC. The expression of muscle differentiation proteins (Pax7, Mif5, MyoD, MyoG, and MHC), PKA/CREB, adenosine receptors (A1, A2A, A2B, and A3), ATP-channel pannexin-1 and the P2X7 receptor was analyzed via WB and RT-PCR. cAMP and AMPK activation was measured. Results: Tenofovir increased intracellular ATP and reduced extracellular adenosine, decreasing Pax7 expression and increasing MHC expression prematurely. Dipyridamole increased intracellular AMP and extracellular adenosine, counteracting the premature myogenesis promoted by tenofovir. All adenosine receptors were expressed during differentiation with dipyridamole, increasing A2B expression. Tenofovir maintained inactive AMPK and decreased cAMP levels, as well as PKAα and pCREB expression, which were recovered with dipyridamole. Discussion: Adenosine and ATP act as mediators in muscle myogenesis. The blockade of ATP release by tenofovir promotes premature myogenesis, with dipyridamole counteracting the premature differentiation promoted by tenofovir via the adenosine A2B receptor and cAMP/AMPK pathways. Therefore, dipyridamole might be of interest as a therapeutic approach in sarcopenia.

L-leucine stimulation of glucose uptake and utilization involves modulation of glucose - lipid metabolic switch and improved bioenergetic homeostasis in isolated rat psoas muscle ex vivo

The antidiabetic effect of l-leucine has been attributed to its modulatory effect on glucose uptake and lipid metabolism in muscles. However, there is a dearth on its effect on glucose metabolism in muscles. Thus, the present study investigated the effect of l-leucine - stimulated glucose uptake on glucose metabolism, dysregulated lipid metabolic pathways, redox and bioenergetic homeostasis, and proteolysis in isolated psoas muscle from Sprague Dawley male rats. Isolated psoas muscles were incubated with l-leucine (30-240 μg/mL) in the presence of 11.1 mMol glucose at 37 ˚C for 2 h. Muscles incubated in only glucose served as the control, while muscles not incubated in l-leucine and/or glucose served as the normal control. Metformin (6.04 mM) was used as the standard antidiabetic drug. Incubation with l-leucine caused a significant increase in muscle glucose uptake, with an elevation of glutathione levels, superoxide dismutase, catalase, E-NTPDase and 5'nucleotidase activities. It also led to the depletion of malondialdehyde and nitric oxide levels, ATPase, chymotrypsin, acetylcholinesterase, glycogen phosphorylase, glucose-6-phosphatase, fructose-1,6-bisphosphatase and lipase activities. There was an alteration in lipid metabolites, with concomitant activation of glycerolipid metabolism, fatty acid metabolism, and fatty acid elongation in mitochondria in the glucose-incubated muscle (negative control). Incubation with l-leucine reversed these alterations, and concomitantly deactivated the pathways. These results indicate that l-leucine-enhanced muscle glucose uptake involves improved redox and bioenergetic homeostasis, with concomitant suppressed proteolytic, glycogenolytic and gluconeogenetic activities, while modulating glucose - lipid metabolic switch.

High extracellular ATP levels released through pannexin-1 channels mediate inflammation and insulin resistance in skeletal muscle fibres of diet-induced obese mice

AIMS/HYPOTHESIS

Skeletal muscle is a key target organ for insulin's actions and is the main regulator of blood glucose. In obese individuals and animal models, there is a chronic low-grade inflammatory state affecting highly metabolic organs, leading to insulin resistance. We have described that adult skeletal muscle fibres can release ATP to the extracellular medium through pannexin-1 (PANX1) channels. Besides, it is known that high extracellular ATP concentrations can act as an inflammatory signal. Here, we propose that skeletal muscle fibres from obese mice release high levels of ATP, through PANX1 channels, promoting inflammation and insulin resistance in muscle cells.

METHODS

C57BL/6J mice were fed with normal control diet (NCD) or high-fat diet (HFD) for 8 weeks. Muscle fibres were isolated from flexor digitorum brevis (FDB) muscle. PANX1-knockdown FDB fibres were obtained by in vivo electroporation of a short hairpin RNA Panx1 plasmid. We analysed extracellular ATP levels in a luciferin/luciferase assay. Gene expression was studied with quantitative real-time PCR (qPCR). Protein levels were evaluated by immunoblots, ELISA and immunofluorescence. Insulin sensitivity was analysed in a 2-NBDG (fluorescent glucose analogue) uptake assay, immunoblots and IPGTT.

RESULTS

HFD-fed mice showed significant weight gain and insulin resistance compared with NCD-fed mice. IL-6, IL-1β and TNF-α protein levels were increased in FDB muscle from obese mice. We observed high levels of extracellular ATP in muscle fibres from obese mice (197 ± 55 pmol ATP/μg RNA) compared with controls (32 ± 10 pmol ATP/μg RNA). ATP release in obese mice fibres was reduced by application of 100 μmol/l oleamide (OLE) and 5 μmol/l carbenoxolone (CBX), both PANX1 blockers. mRNA levels of genes linked to inflammation were reduced using OLE, CBX or 2 U/ml ATPase apyrase in muscle fibres from HFD-fed mice. In fibres from mice with pannexin-1 knockdown, we observed diminished extracellular ATP levels (78 ± 10 pmol ATP/μg RNA vs 252 ± 37 pmol ATP/μg RNA in control mice) and a lower expression of inflammatory markers. Moreover, a single pulse of 300 μmol/l ATP to fibres from control mice reduced insulin-mediated 2-NBDG uptake and promoted an elevation in mRNA levels of inflammatory markers. PANX-1 protein levels were increased two- to threefold in skeletal muscle from obese mice compared with control mice. Incubation with CBX increased Akt activation and 2-NBDG uptake in HFD fibres after insulin stimulation, rescuing the insulin resistance condition. Finally, in vivo treatment of HFD-fed mice with CBX (i.p. injection of 10 mg/kg each day) for 14 days, compared with PBS, reduced extracellular ATP levels in skeletal muscle fibres (51 ± 10 pmol ATP/μg RNA vs 222 ± 28 pmol ATP/μg RNA in PBS-treated mice), diminished inflammation and improved glycaemic management.

CONCLUSIONS/INTERPRETATION

In this work, we propose a novel mechanism for the development of inflammation and insulin resistance in the skeletal muscle of obese mice. We found that high extracellular ATP levels, released by overexpressed PANX1 channels, lead to an inflammatory state and insulin resistance in skeletal muscle fibres of obese mice.

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

Key genes and co-expression network analysis in the livers of type 2 diabetes patients

AIMS/INTRODUCTION

The incidence of type 2 diabetes is increasing worldwide. Hepatic insulin resistance and liver lipid accumulation contributes to type 2 diabetes development. The aim of the present study was to investigate the key gene pathways and co-expression networks in the livers of type 2 diabetes patients.

MATERIALS AND METHODS

Dataset GSE15653 containing nine healthy individuals and nine type 2 diabetes patients was downloaded from the National Center for Biotechnology Information Gene Expression Omnibus database. Differentially expressed genes were obtained from the livers of type 2 diabetes patients, annotated pathway enrichment and protein-protein interaction network analysis. Next, functional modules and transcription factor networks were constructed. Gene co-expression networks were analyzed by weighted correlation network analysis to identify key modules related to clinical traits, and the candidate key genes were validated in hepatic insulin resistance models in vitro.

RESULTS

A total of 778 differentially expressed genes were filtered in the livers of type 2 diabetes patients, pathway enrichment analysis identified ke y pathways, such as the mitogen-activated protein kinase signaling pathway, Hippo signaling pathway and hypoxia-inducible factor-1 signaling pathway, that were associated with type 2 diabetes. Several transcription factors of three functional modules identified from protein-protein interaction networks are likely to be implicated in type 2 diabetes. Furthermore, weighted correlation network analysis identified five modules that were shown to be highly correlated with type 2 diabetes and other clinical traits. Functional annotation showed that these modules were mainly enriched in pathways such as metabolic pathways, phosphoinositide 3-kinase-protein kinase B signaling pathway and natural killer cell-mediated cytotoxicity. UBE2M and GPER were upregulated in L02 and HepG2 models, whereas P2RY11 only upregulated in L02 model, and UBE2N only downregulated in HepG2 model at a significant level.

CONCLUSIONS

These results would offer new insights into hepatic insulin resistance, type 2 diabetes pathogenesis, development and drug discovery.

Association between allopurinol use and hip fracture in older patients

BACKGROUND

Allopurinol reduces oxidative stress and interacts with purinergic signalling systems important in bone metabolism and muscle function. We assessed whether allopurinol use was associated with a reduced incidence of hip fracture in older people.

METHODS

Analysis of prospective, routinely-collected health and social care data on patients undergoing health and social work assessment in a single geographical area over a 12year period. Exposure to allopurinol was derived from linked community prescribing data, and hospitalisation for hip fracture and comorbid disease was derived from linked hospitalisation data. Fine and Gray modelling was used to model time to hip fracture accounting for the competing risk of death, incorporating previous use of allopurinol, cumulative exposure to allopurinol as a time dependent variable, and covariate adjustments.

RESULTS

17,308 patients were alive at the time of first social work assessment without previous hip fracture; the mean age was 73years. 10,171 (59%) were female, and 1155 (8%) had at least one exposure to allopurinol. 618 (3.6%) sustained a hip fracture, and 4226 (24%) died during a mean follow-up of 7.2years. In fully-adjusted analyses, each year of allopurinol exposure conferred a hazard ratio of 1.01 (95% CI 0.99, 1.02; p=0.37) for hip fracture and 1.00 (0.99, 1.01; p=0.47) for death. Previous use of allopurinol conferred a hazard ratio of 0.76 (0.45, 1.26; p=0.28) for hip fracture and 1.13 (0.99, 1.29; p=0.07) for death.

CONCLUSION

Greater cumulative use of allopurinol was not associated with a reduced risk of hip fracture or death in this cohort.

Exaggerated increases in blood pressure during isometric muscle contraction in hypertension: role for purinergic receptors

Physical activity is a cornerstone therapy for the primary prevention and treatment of hypertension, which is becoming increasingly prevalent in modern societies. During exercise, heart rate and blood pressure (BP) increase in order to acutely meet the metabolic demands of the working skeletal muscle. In hypertensive adults, isometric exercise-induced increases in BP are excessive, potentially increasing the risk of an acute cardiovascular event during or after physical activity. Recently, the skeletal muscle metaboreflex has emerged as a significant contributor to the development of aberrant cardiovascular control during isometric exercise in this clinical population. Our laboratory has conducted a series of studies characterizing the skeletal muscle metaboreflex in hypertensive humans. We and others have demonstrated that hypertension is characterized by greater increases in muscle sympathetic nerve activity and BP during selective activation of the metaboreflex during post-exercise muscle ischemia compared to the increases noted in healthy age-matched normotensive adults, suggesting that the skeletal muscle metaboreflex is exaggerated in human hypertension. The focus of this review is the skeletal muscle metaboreflex (i.e., the metabolic component of the exercise pressor reflex) in hypertension, with particular emphasis on the potential role of purinergic receptors in mediating the exaggerated responses to muscle metaboreflex activation.

New insights into the relationship between mIGF-1-induced hypertrophy and Ca2+ handling in differentiated satellite cells

Muscle regeneration involves the activation of satellite cells, is regulated at the genetic and epigenetic levels, and is strongly influenced by gene activation and environmental conditions. The aim of this study was to determine whether the overexpression of mIGF-1 can modify functional features of satellite cells during the differentiation process, particularly in relation to modifications of intracellular Ca²⁺ handling.

Satellite cells were isolated from wild-type and MLC/mIGF-1 transgenic mice. The cells were differentiated in vitro, and morphological analyses, intracellular Ca²⁺ measurements, and ionic current recordings were performed.

mIGF-1 overexpression accelerates satellite cell differentiation and promotes myotube hypertrophy. In addition, mIGF-1 overexpression-induced potentiation of myogenesis triggers both quantitative and qualitative changes to the control of intracellular Ca²⁺ handling. In particular, the differentiated MLC/mIGF-1 transgenic myotubes have reduced velocity and amplitude of intracellular Ca²⁺ increases after stimulation with caffeine, KCl and acetylcholine. This appears to be due, at least in part, to changes in the physico-chemical state of the sarcolemma (increased membrane lipid oxidation, increased output currents) and to increased expression of dihydropyridine voltage-operated Ca²⁺ channels. Interestingly, extracellular ATP and GTP evoke intracellular Ca²⁺ mobilization to greater extents in the MLC/mIGF-1 transgenic satellite cells, compared to the wild-type cells.

These data suggest that these MLC/mIGF-1 transgenic satellite cells are more sensitive to trophic stimuli, which can potentiate the effects of mIGF-1 on the myogenic programme.

Regulation of the skeletal muscle blood flow in humans

In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation.

Purinergic effects on Na,K-ATPase activity differ in rat and human skeletal muscle

Background

P2Y receptor activation may link the effect of purines to increased maximal in vitro activity of the Na,K-ATPase in rat muscle. The hypothesis that a similar mechanism is present in human skeletal muscle was investigated with membranes from rat and human skeletal muscle.

Results

Membranes purified from rat and human muscles were used in the Na,K-ATPase assay. Incubation with ADP, the stable ADP analogue MeS-ADP and UDP increased the Na⁺ dependent Na,K-ATPase activity in rat muscle membranes, whereas similar treatments of human muscle membranes lowered the Na,K-ATPase activity. UTP incubation resulted in unchanged Na,K-ATPase activity in humans, but pre-incubation with the antagonist suramin resulted in inhibition with UTP, suggesting that P2Y receptors are involved. The Na,K-ATPase in membranes from both rat and human could be stimulated by protein kinase A and C activation. Thus, protein kinase A and C activation can increase Na,K-ATPase activity in human muscle but not via P2Y receptor stimulation.

Conclusion

The inhibitory effects of most purines (with the exception of UTP) in human muscle membranes are probably due to mass law inhibition of ATP hydrolysis. This inhibition could be blurred in rat due to receptor mediated activation of the Na,K-ATPase. The different effects could be related to a high density of ADP sensitive P2Y₁ and P2Y₁₃ receptors in rat, whereas the UTP sensitive P2Y₁₁ could be more abundant in human. Alternatively, rat could possesses a mechanism for protein-protein interaction between P2Y receptors and the Na,K-ATPase, and this mechanism could be absent in human skeletal muscle (perhaps with the exception of the UTP sensitive P2Y₁₁ receptor).

Perspective

Rat muscle is not a reliable model for purinergic effects on Na,K-ATPase in human skeletal muscle.

Purinergic signalling and diabetes

The pancreas is an organ with a central role in nutrient breakdown, nutrient sensing and release of hormones regulating whole body nutrient homeostasis. In diabetes mellitus, the balance is broken-cells can be starving in the midst of plenty. There are indications that the incidence of diabetes type 1 and 2, and possibly pancreatogenic diabetes, is rising globally. Events leading to insulin secretion and action are complex, but there is emerging evidence that intracellular nucleotides and nucleotides are not only important as intracellular energy molecules but also as extracellular signalling molecules in purinergic signalling cascades. This signalling takes place at the level of the pancreas, where the close apposition of various cells-endocrine, exocrine, stromal and immune cells-contributes to the integrated function. Following an introduction to diabetes, the pancreas and purinergic signalling, we will focus on the role of purinergic signalling and its changes associated with diabetes in the pancreas and selected tissues/organ systems affected by hyperglycaemia and other stress molecules of diabetes. Since this is the first review of this kind, a comprehensive historical angle is taken, and common and divergent roles of receptors for nucleotides and nucleosides in different organ systems will be given. This integrated picture will aid our understanding of the challenges of the potential and currently used drugs targeted to specific organ/cells or disorders associated with diabetes.

Effect of extraluminal ATP application on vascular tone and blood flow in skeletal muscle: implications for exercise hyperemia

During skeletal muscle contractions, the concentration of ATP increases in muscle interstitial fluid as measured by microdialysis probes. This increase is associated with the magnitude of blood flow, suggesting that interstitial ATP may be important for contraction-induced vasodilation. However, interstitial ATP has solely been described to induce vasoconstriction in skeletal muscle. To examine whether interstitial ATP induces vasodilation in skeletal muscle and to what extent this vasoactive effect is mediated by formation of nitric oxide (NO) and prostanoids, three different experimental models were studied. The rat gluteus maximus skeletal muscle model was used to study changes in local skeletal muscle hemodynamics. Superfused ATP at concentrations found during muscle contractions (1-10 μM) increased blood flow by up to 400%. In this model, the underlying mechanism was also examined by inhibition of NO and prostanoid formation. Inhibition of these systems abolished the vasodilator effect of ATP. Cell-culture experiments verified ATP-induced formation of NO and prostacyclin in rat skeletal muscle microvascular endothelial cells, and ATP-induced formation of NO in rat skeletal muscle cells. To confirm these findings in humans, ATP was infused into skeletal muscle interstitium of healthy subjects via microdialysis probes and found to increase muscle interstitial concentrations of NO and prostacyclin by ~60% and ~40%, respectively. Collectively, these data suggest that a physiologically relevant elevation in interstitial ATP concentrations increases muscle blood flow, indicating that the contraction-induced increase in skeletal muscle interstitial [ATP] is important for exercise hyperemia. The vasodilator effect of ATP application is mediated by NO and prostanoid formation.