Increase in expression of P2X1 receptors in the atria of patients suffering from dilated cardiomyopathy

Purine Nucleotides Metabolism and Signaling in Huntington's Disease: Search for a Target for Novel Therapies

Huntington’s disease (HD) is a multi-system disorder that is caused by expanded CAG repeats within the exon-1 of the huntingtin (HTT) gene that translate to the polyglutamine stretch in the HTT protein. HTT interacts with the proteins involved in gene transcription, endocytosis, and metabolism. HTT may also directly or indirectly affect purine metabolism and signaling. We aimed to review existing data and discuss the modulation of the purinergic system as a new therapeutic target in HD. Impaired intracellular nucleotide metabolism in the HD affected system (CNS, skeletal muscle and heart) may lead to extracellular accumulation of purine metabolites, its unusual catabolism, and modulation of purinergic signaling. The mechanisms of observed changes might be different in affected systems. Based on collected findings, compounds leading to purine and ATP pool reconstruction as well as purinergic receptor activity modulators, i.e., P2X7 receptor antagonists, may be applied for HD treatment.

Cardiac Connexin-43 Hemichannels and Pannexin1 Channels: Provocative Antiarrhythmic Targets

Cardiac connexin-43 (Cx43) creates gap junction channels (GJCs) at intercellular contacts and hemi-channels (HCs) at the peri-junctional plasma membrane and sarcolemmal caveolae/rafts compartments. GJCs are fundamental for the direct cardiac cell-to-cell transmission of electrical and molecular signals which ensures synchronous myocardial contraction. The HCs and structurally similar pannexin1 (Panx1) channels are active in stressful conditions. These channels are essential for paracrine and autocrine communication through the release of ions and signaling molecules to the extracellular environment, or for uptake from it. The HCs and Panx1 channel-opening profoundly affects intracellular ionic homeostasis and redox status and facilitates via purinergic signaling pro-inflammatory and pro-fibrotic processes. These conditions promote cardiac arrhythmogenesis due to the impairment of the GJCs and selective ion channel function. Crosstalk between GJCs and HCs/Panx1 channels could be crucial in the development of arrhythmogenic substrates, including fibrosis. Despite the knowledge gap in the regulation of these channels, current evidence indicates that HCs and Panx1 channel activation can enhance the risk of cardiac arrhythmias. It is extremely challenging to target HCs and Panx1 channels by inhibitory agents to hamper development of cardiac rhythm disorders. Progress in this field may contribute to novel therapeutic approaches for patients prone to develop atrial or ventricular fibrillation.

Biophysics of human anatomy and physiology-a Special Issue in honor of Prof. Cristobal dos Remedios on the occasion of his 80th birthday

In 2001, Cristobal dos Remedios was made Professor of Anatomy (now emeritus) within Australia’s highest-ranked university (University of Sydney). For the majority of his career, he has examined the biomechanics and biophysics of human muscle contraction. To coincide with the occasion of his 80^th birthday, this Special Issue has commissioned a collection of review articles from experts exploring biophysical subjects within the general areas of human anatomy and physiology. After introducing the scope and contents of the Issue, we provide a short scientific biography, placing his scientific achievements within the context of the course of his life’s developments.

Myocardial metabolism in heart failure: Purinergic signalling and other metabolic concepts

Despite significant therapeutic advances in heart failure (HF) therapy, the morbidity and mortality associated with this disease remains unacceptably high. The concept of metabolic dysfunction as an important underlying mechanism in HF is well established. Cardiac function is inextricably linked to metabolism, with dysregulation of cardiac metabolism pathways implicated in a range of cardiac complications, including HF. Modulation of cardiac metabolism has therefore become an attractive clinical target. Cardiac metabolism is based on the integration of adenosine triphosphate (ATP) production and utilization pathways. ATP itself impacts the heart not only by providing energy, but also represents a central element in the purinergic signaling pathway, which has received considerable attention in recent years. Furthermore, novel drugs that have received interest in HF include angiotensin receptor blocker-neprilysin inhibitor (ARNi) and sodium glucose cotransporter 2 (SGLT-2) inhibitors, whose favorable cardiovascular profile has been at least partly attributed to their effects on metabolism. This review, describes the major metabolic pathways and concepts of the healthy heart (including fatty acid oxidation, glycolysis, Krebs cycle, Randle cycle, and purinergic signaling) and their dysregulation in the progression to HF (including ketone and amino acid metabolism). The cardiac implications of HF comorbidities, including metabolic syndrome, diabetes mellitus and cachexia are also discussed. Finally, the impact of current HF and diabetes therapies on cardiac metabolism pathways and the relevance of this knowledge for current clinical practice is discussed. Targeting cardiac metabolism may have utility for the future treatment of patients with HF, complementing current approaches.

The Sydney Heart Bank: improving translational research while eliminating or reducing the use of animal models of human heart disease

The Sydney Heart Bank (SHB) is one of the largest human heart tissue banks in existence. Its mission is to provide high-quality human heart tissue for research into the molecular basis of human heart failure by working collaboratively with experts in this field. We argue that, by comparing tissues from failing human hearts with age-matched non-failing healthy donor hearts, the results will be more relevant than research using animal models, particularly if their physiology is very different from humans. Tissue from heart surgery must generally be used soon after collection or it significantly deteriorates. Freezing is an option but it raises concerns that freezing causes substantial damage at the cellular and molecular level. The SHB contains failing samples from heart transplant patients and others who provided informed consent for the use of their tissue for research. All samples are cryopreserved in liquid nitrogen within 40 min of their removal from the patient, and in less than 5-10 min in the case of coronary arteries and left ventricle samples. To date, the SHB has collected tissue from about 450 failing hearts (>15,000 samples) from patients with a wide range of etiologies as well as increasing numbers of cardiomyectomy samples from patients with hypertrophic cardiomyopathy. The Bank also has hearts from over 120 healthy organ donors whose hearts, for a variety of reasons (mainly tissue-type incompatibility with waiting heart transplant recipients), could not be used for transplantation. Donor hearts were collected by the St Vincent's Hospital Heart and Lung transplantation team from local hospitals or within a 4-h jet flight from Sydney. They were flushed with chilled cardioplegic solution and transported to Sydney where they were quickly cryopreserved in small samples. Failing and/or donor samples have been used by more than 60 research teams around the world, and have resulted in more than 100 research papers. The tissues most commonly requested are from donor left ventricles, but right ventricles, atria, interventricular system, and coronary arteries vessels have also been reported. All tissues are stored for long-term use in liquid N or vapor (170-180 °C), and are shipped under nitrogen vapor to avoid degradation of sensitive molecules such as RNAs and giant proteins. We present evidence that the availability of these human heart samples has contributed to a reduction in the use of animal models of human heart failure.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Transcript analysis of P2X receptors in PBMCs of chronic HCV patients: an insight into antiviral treatment response and HCV-induced pathogenesis

BACKGROUND

After invasion of hepatocytes and immune cells, hepatitis C virus has the ability to escape from the host immune system, leading to the progression of disease into chronic infection with associated liver morbidities. Adenosine 5'triphosphate (ATP) is released in most of the pathological events from the affected cells and acts as a signaling molecule by binding to P2X receptors expressed on the host's immune cells and activates the immune system for pro-inflammatory response. Therefore, the present study was designed to analyze the transcript expression of the ionotropic purinergic P2X receptors on peripheral blood mononuclear cells (PBMCs) of chronic HCV patients to have study the immune responses mediated by P2X receptors in chronic HCV infections.

METHODS

PBMCs were isolated from the collected blood samples. Transcript analysis of P2X receptors in PBMCs was done. The identity of amplified product was confirmed by sequencing PCR, while the quantification of the transcript expression was done by real time PCR. The relative expression of the P2X receptors was analyzed by unpaired Student's t test using GraphPad Prims 5 software.

RESULTS

We found that out of seven isoforms of P2X receptors, P2X1, P2X4, P2X5, and P2X7 receptors are expressed on the PBMCs. P2X1 and P2X7 are significantly upregulated in treatment-naïve chronic HCV patients by 2.2- and 2.5-fold, respectively. However, only P2X7 expression is found increased by 2.7-fold in patients achieving sustained virological response (SVR) after antiviral treatment compared to healthy controls. The expression of P2X receptors remained unaltered in chronic HCV patients not responding to the treatment.

CONCLUSION

The present study confirms the significant involvement of P2X receptors in the immune responses mediated by the PBMCs in the chronic HCV infection, which should be further investigated to devise strategies to augment the immune system against this chronic viral disease.

Treatment of heart failure by a methanocarba derivative of adenosine monophosphate: implication for a role of cardiac purinergic P2X receptors

Evidence is accumulating to support a potentially important role for purinergic (P2X) receptors in heart failure (HF). We tested the hypothesis that a hydrolysis-resistant nucleotide analog with agonist activity at myocardial P2X receptors (P2XRs) improves the systolic HF phenotype in mouse and dog models. We developed a hydrolysis-resistant adenosine monophosphate derivative, (1'S,2R,3S,4'R,5'S)-4-(6-amino-2-chloro-9H-purin-9-yl)-1-[phosphoryloxymethyl] bicycle[3.1.0]hexane-2,3-diol) (MRS2339), with agonist activity at native cardiac P2XRs. Chronic MRS2339 infusion in postinfarct and calsequestrin (CSQ) mice with HF resulted in higher rates of pressure change (+dP/dt), left ventricle (LV)-developed pressure, and cardiac output in an in vitro working heart model. Heart function in vivo, as determined by echocardiography-derived fractional shortening, was also improved in MRS2339-infused mice. The beneficial effect of MRS2339 was dose-dependent and was identical to that produced by cardiac myocyte-specific overexpression of the P2X(4) receptor. The HF improvement was associated with the preservation of LV wall thickness in both systole and diastole in postinfarct and CSQ mice. In dogs with pacing-induced HF, MRS2339 infusion reduced left ventricular end-diastolic pressure, improved arterial oxygenation, and increased +dP/dt. MRS2339 treatment also decreased LV chamber size in mice and dogs with HF. In murine and canine models of systolic HF, in vivo administration of a P2X nucleotide agonist improved contractile function and cardiac performance. These actions were associated with preserved LV wall thickness and decreased LV remodeling. The data are consistent with a role of cardiac P2XRs in mediating the beneficial effect of this agonist.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Disruption of lipid rafts inhibits P2X1 receptor-mediated currents and arterial vasoconstriction

P2X1 receptors for ATP are ligand-gated cation channels expressed on a range of smooth muscle preparations and blood platelets. The receptors appear to be clustered close to sympathetic nerve varicosities and mediate the underlying membrane potential changes and constriction following nerve stimulation in a range of arteries and resistance arterioles. In this study we have used discontinuous sucrose density gradients, Western blot analysis, and cholesterol measurements to show that recombinant and smooth muscle (rat tail artery, vas deferens, and bladder) P2X1 receptors are present in cholesterol-rich lipid rafts and co-localize with the lipid raft markers flotillin-1 and -2. Lipid rafts are specialized lipid membrane microdomains involved in signaling and trafficking. To determine whether lipid raft association was essential for P2X1 receptor channel function we used the cholesterol-depleting agent methyl-beta-cyclodextrin (10 mm for 1 h). This led to a redistribution of the P2X1 receptor throughout the sucrose gradient and reduced P2X1 receptor-mediated (alpha,beta-methylene ATP, 10 microm) currents in HEK293 cells by >90% and contractions of the rat tail artery by approximately 50%. However contractions evoked by potassium chloride (60 mm) were unaffected by methyl-beta-cyclodextrin and the inactive analogue alpha-cyclodextrin had no effect on P2X1 receptor-mediated currents or contractions. P2X1 receptors are subject to ongoing regulation by receptors and kinases, and the present results suggest that lipid rafts are an essential component in the maintenance of these localized signaling domains and play an important role in P2X1 receptor-mediated control of arteries.

Localized expression of specific P2X receptors in dystrophin-deficient DMD and mdx muscle

Using a combination of molecular and immunohistochemical methods, we have obtained evidence for a distinctive change in the expression patterns of ATP-gated (P2X) receptor subunits in dystrophic muscle from both Duchenne muscular dystrophy (DMD) patients and the mdx mouse model. In control myofibres there was no staining for any P2X subtype studied here, although P2X1 stained the smooth muscle of the blood vessels and P2X6 nerves and the tunica intima in small arteries. In contrast, P2X1 and P2X6 were co-expressed strongly in small regenerating muscle fibres in the dystrophic muscles, whereas this expression decreased in fully regenerated fibres. Moreover, immunoreactivity for the P2X2 receptor re-appeared in dystrophic muscle, where it co-localised with the Type 1 fibres. There is, thus, a burst of production of several P2X receptor subtypes in regenerating dystrophic muscle, which may have implications for drug targets for this muscle pathology.

P2X1 receptors are closely associated with connexin 43 in human ventricular myocardium

BACKGROUND

It has been suggested that gap-junctional conductance between cardiomyocytes is regulated through a specific ligand-receptor interaction between ATP and connexins. In this study we examined the localization of P2X1 ionotropic receptors and their relation to connexin43 in gap junctions in human left ventricles.

METHODS AND RESULTS

Using immunohistochemistry, we detected P2X1 expression predominantly in the intercalated discs. Labelling of the P2X1 receptor and the gap junction protein connexin43 showed close association in some gap junctions, while in others the two proteins often appeared to be spatially discrete. Western blotting detected four major bands at 45, 60, 95 and 120 kDa in the protein extracts from human left ventricles corresponding to equivalent bands from rat vas deferens. The most prominent band in human left ventricles was at 95 kDa, possibly a dimer of the native P2X1 receptor, whereas in rat vas deferens it was at 60 kDa. After preincubation of the antibody with its epitope peptide, the 45 and 60 kDa bands almost disappeared and the 95 and 120 kDa bands were significantly attenuated.

CONCLUSIONS

P2X1 receptors in human myocardium are densely localized in gap junctions at intercalated discs between muscle cells. Close association of P2X1 receptors and connexin 43 occurred in some regions of some gap junctions, but in others they were spatially separate. Little difference in the pattern of distribution of P2X1 receptors was found in failing left ventricles of patients with dilated cardiomyopathy, although Western blots showed an enhancement of P2X1 receptor protein.

Determination of P2X1alpha-sarcoglycan (adhalin) expression levels in failing human dilated cardiomyopathic left ventricles

This study is concerned with the molecular basis of human idiopathic dilated cardiomyopathy (DCM). This disorder affects the entire heart including both atria and ventricles. It is characterized by a progressive dilatation of the ventricles and loss of contractile power that results in an impaired cardiac output. Changes in cellular levels of dystrophin have been reported in patients with muscular dystrophies (Beckers and Duchenne) which manifest as DCM. However, previous studies using Western blots dos Remedios et al., Electrophoresis 1996, 17, 235-238) of samples of left ventricles from DCM patients showed no abnormalities in dystrophin content. P2X receptors are ATP-gated cation channels located in the sarcolemma. They are upregulated by a factor of about two in the atria of DCM patients compared with nondiseased control samples. A dystrophin-associated protein, alpha-sarcoglycan, has recently been shown to be an ecto-ATPase (an extracellular ATPase) capable of regulating ATP concentrations in the space between the cardiomyocytes. In this report we examine the relationship between changes in P2X1 receptors in left ventricle samples from DCM patients and the concentration of alpha-sarcoglycan. We found no evidence for upregulation of P2X1 receptors nor was the expression of alpha-sarcoglycan significantly altered.