Expression of a Gi-coupled receptor in the heart causes impaired Ca2+ handling, myofilament injury, and dilated cardiomyopathy

The GPR17 Receptor-A Promising Goal for Therapy and a Potential Marker of the Neurodegenerative Process in Multiple Sclerosis

One of the most important goals in the treatment of demyelinating diseases such as multiple sclerosis (MS) is, in addition to immunomodulation, reconstruction of the lost myelin sheath. The modulator of the central nervous system myelination is the metabotropic receptor coupled to the G-protein: GPR17. GPR17 receptors are considered to be sensors of local damage to the myelin sheath, and play a role in the reconstruction and repair of demyelinating plaques caused by ongoing inflammatory processes. GPR17 receptors are present on nerve cells and precursor oligodendrocyte cells. Under physiological conditions, they are responsible for the differentiation and subsequent maturation of oligodendrocytes, while under pathological conditions (during damage to nerve cells), their expression increases to become mediators in the demyelinating processes. Moreover, they are essential not only in both the processes of inducing damage and the death of neurons, but also in the local repair of the damaged myelin sheath. Therefore, GPR17 receptors may be recognized as the potential goal in creating innovative therapies for the treatment of the neurodegenerative process in MS, based on the acceleration of the remyelination processes. This review examines the role of GRP17 in pathomechanisms of MS development.

Mathematical model for β1-adrenergic regulation of the mouse ventricular myocyte contraction

The β1-adrenergic regulation of cardiac myocyte contraction plays an important role in regulating heart function. Activation of this system leads to an increased heart rate and stronger myocyte contraction. However, chronic stimulation of the β1-adrenergic signaling system can lead to cardiac hypertrophy and heart failure. To understand the mechanisms of action of β1-adrenoceptors, a mathematical model of cardiac myocyte contraction that includes the β1-adrenergic system was developed and studied. The model was able to simulate major experimental protocols for measurements of steady-state force-calcium relationships, cross-bridge release rate and force development rate, force-velocity relationship, and force redevelopment rate. It also reproduced quite well frequency and isoproterenol dependencies for intracellular Ca2+ concentration ([Ca2+]i) transients, total contraction force, and sarcomere shortening. The mathematical model suggested the mechanisms of increased contraction force and myocyte shortening on stimulation of β1-adrenergic receptors is due to phosphorylation of troponin I and myosin-binding protein C and increased [Ca2+]i transient resulting from activation of the β1-adrenergic signaling system. The model was used to simulate work-loop contractions and estimate the power during the cardiac cycle as well as the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The developed mathematical model can be used further for simulations of contraction of ventricular myocytes from genetically modified mice and myocytes from mice with chronic cardiac diseases.

NEW & NOTEWORTHY A new mathematical model of mouse ventricular myocyte contraction that includes the β1-adrenergic system was developed. The model simulated major experimental protocols for myocyte contraction and predicted the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The model also allowed for simulations of work-loop contractions and estimation of the power during the cardiac cycle.

Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology

G protein-coupled receptors (GPCRs) are a family of cell-surface proteins that play critical roles in regulating a variety of pathophysiological processes and thus are targeted by almost a third of currently available therapeutics. It was originally thought that GPCRs convert extracellular stimuli into intracellular signals through activating G proteins, whereas β-arrestins have important roles in internalization and desensitization of the receptor. Over the past decade, several novel functional aspects of β-arrestins in regulating GPCR signaling have been discovered. These previously unanticipated roles of β-arrestins to act as signal transducers and mediators of G protein-independent signaling have led to the concept of biased agonism. Biased GPCR ligands are able to engage with their target receptors in a manner that preferentially activates only G protein- or β-arrestin-mediated downstream signaling. This offers the potential for next generation drugs with high selectivity to therapeutically relevant GPCR signaling pathways. In this review, we provide a summary of the recent studies highlighting G protein- or β-arrestin-biased GPCR signaling and the effects of biased ligands on disease pathogenesis and regulation.

Integrated Left Ventricular Global Transcriptome and Proteome Profiling in Human End-Stage Dilated Cardiomyopathy

Aims

The disease pathways leading to idiopathic dilated cardiomyopathy (DCM) are still elusive. The present study investigated integrated global transcriptional and translational changes in human DCM for disease biomarker discovery.

Methods

We used identical myocardial tissues from five DCM hearts compared to five non-failing (NF) donor hearts for both transcriptome profiling using the ABI high-density oligonucleotide microarrays and proteome expression with One-Dimensional Nano Acquity liquid chromatography coupled with tandem mass spectrometry on the Synapt G2 system.

Results

We identified 1262 differentially expressed genes (DEGs) and 269 proteins (DEPs) between DCM cases and healthy controls. Among the most significantly upregulated (>5-fold) proteins were GRK5, APOA2, IGHG3, ANXA6, HSP90AA1, and ATP5C1 (p< 0.01). On the other hand, the most significantly downregulated proteins were GSTM5, COX17, CAV1 and ANXA3. At least ten entities were concomitantly upregulated on the two analysis platforms: GOT1, ALDH4A1, PDHB, BDH1, SLC2A11, HSP90AA1, HSP90AB1, H2AFV, HSPA5 and NDUFV1. Gene ontology analyses of DEGs and DEPs revealed significant overlap with enrichment of genes/proteins related to metabolic process, biosynthetic process, cellular component organization, oxidative phosphorylation, alterations in glycolysis and ATP synthesis, Alzheimer’s disease, chemokine-mediated inflammation and cytokine signalling pathways.

Conclusion

The concomitant use of transcriptome and proteome expression to evaluate global changes in DCM has led to the identification of sixteen commonly altered entities as well as novel genes, proteins and pathways whose cardiac functions have yet to be deciphered. This data should contribute towards better management of the disease.

Biased β2-adrenoceptor signalling in heart failure: pathophysiology and drug discovery

The body is constantly faced with a dynamic requirement for blood flow. The heart is able to respond to these changing needs by adjusting cardiac output based on cues emitted by circulating catecholamine levels. Cardiac β-adrenoceptors transduce the signal produced by catecholamine stimulation via Gs proteins to their downstream effectors to increase heart contractility. During heart failure, cardiac output is insufficient to meet the needs of the body; catecholamine levels are high and β-adrenoceptors become hyperstimulated. The hyperstimulated β1-adrenoceptors induce a cardiotoxic effect, which could be counteracted by the cardioprotective effect of β2-adrenoceptor-mediated Gi signalling. However, β2-adrenoceptor-Gi signalling negates the stimulatory effect of the Gs signalling on cardiomyocyte contraction and further exacerbates cardiodepression. Here, further to the localization of β1- and β2-adrenoceptors and β2-adrenoceptor-mediated β-arrestin signalling in cardiomyocytes, we discuss features of the dysregulation of β-adrenoceptor subtype signalling in the failing heart, and conclude that Gi-biased β2-adrenoceptor signalling is a pathogenic pathway in heart failure that plays a crucial role in cardiac remodelling. In contrast, β2-adrenoceptor-Gs signalling increases cardiomyocyte contractility without causing cardiotoxicity. Finally, we discuss a novel therapeutic approach for heart failure using a Gs-biased β2-adrenoceptor agonist and a β1-adrenoceptor antagonist in combination. This combination treatment normalizes the β-adrenoceptor subtype signalling in the failing heart and produces therapeutic effects that outperform traditional heart failure therapies in animal models. The present review illustrates how the concept of biased signalling can be applied to increase our understanding of the pathophysiology of diseases and in the development of novel therapies.

A mathematical model of the mouse ventricular myocyte contraction

Mathematical models of cardiac function at the cellular level include three major components, such as electrical activity, Ca²⁺ dynamics, and cellular shortening. We developed a model for mouse ventricular myocyte contraction which is based on our previously published comprehensive models of action potential and Ca²⁺ handling mechanisms. The model was verified with extensive experimental data on mouse myocyte contraction at room temperature. In the model, we implemented variable sarcomere length and indirect modulation of the tropomyosin transition rates by Ca²⁺ and troponin. The resulting model described well steady-state force-calcium relationships, dependence of the contraction force on the sarcomere length, time course of the contraction force and myocyte shortening, frequency dependence of the contraction force and cellular contraction, and experimentally measured derivatives of the myocyte length variation. We emphasized the importance of the inclusion of variable sarcomere length into a model for ventricular myocyte contraction. Differences in contraction force and cell shortening for epicardial and endocardial ventricular myocytes were investigated. Model applicability for the experimental studies and model limitations were discussed.

Remote control of neuronal signaling

A significant challenge for neuroscientists is to determine how both electrical and chemical signals affect the activity of cells and circuits and how the nervous system subsequently translates that activity into behavior. Remote, bidirectional manipulation of those signals with high spatiotemporal precision is an ideal approach to addressing that challenge. Neuroscientists have recently developed a diverse set of tools that permit such experimental manipulation with varying degrees of spatial, temporal, and directional control. These tools use light, peptides, and small molecules to primarily activate ion channels and G protein-coupled receptors (GPCRs) that in turn activate or inhibit neuronal firing. By monitoring the electrophysiological, biochemical, and behavioral effects of such activation/inhibition, researchers can better understand the links between brain activity and behavior. Here, we review the tools that are available for this type of experimentation. We describe the development of the tools and highlight exciting in vivo data. We focus primarily on designer GPCRs (receptors activated solely by synthetic ligands, designer receptors exclusively activated by designer drugs) and microbial opsins (e.g., channelrhodopsin-2, halorhodopsin, Volvox carteri channelrhodopsin) but also describe other novel techniques that use orthogonal receptors, caged ligands, allosteric modulators, and other approaches. These tools differ in the direction of their effect (activation/inhibition, hyperpolarization/depolarization), their onset and offset kinetics (milliseconds/minutes/hours), the degree of spatial resolution they afford, and their invasiveness. Although none of these tools is perfect, each has advantages and disadvantages, which we describe, and they are all still works in progress. We conclude with suggestions for improving upon the existing tools.

Engineered G-protein Coupled Receptors are Powerful Tools to Investigate Biological Processes and Behaviors

Understanding how discreet tissues and neuronal circuits function in relation to the whole organism to regulate physiological processes and behaviors is a fundamental goal of modern biological science. Powerful and important new tools in this discovery process are modified G-protein coupled receptors (GPCRs) known as ‘Receptors Activated Solely by Synthetic Ligands (RASSLs),’ and ‘Designer Receptors Exclusively Activated by a Designer Drug (DREADDs).’ Collectively, these are GPCRs modified either through rational design or directed molecular evolution, that do not respond to native ligand, but functionally respond only to synthetic ligands. Importantly, the utility of these receptors is not limited to examination of the role of GPCR-coupled effector signal transduction pathways. Due to the near ubiquitous expression of GPCRs throughout an organism, this technology, combined with whole animal transgenics to selectively target expression, has the ability to regulate activity of discreet tissues and neuronal circuits through effector pathway modulation to study function and behavior throughout the organism. Advantages over other systems currently used to modify in vivo function include the ability to rapidly, selectively and reversibly manipulate defined signal transduction pathways both in short term and long term studies, and no need for specialized equipment due to convenient systemic treatment with activating ligand.

Left ventricular global transcriptional profiling in human end-stage dilated cardiomyopathy

We employed ABI high-density oligonucleotide microarrays containing 31,700 sixty-mer probes (representing 27,868 annotated human genes) to determine differential gene expression in idiopathic dilated cardiomyopathy (DCM). We identified 626 up-regulated and 636 down-regulated genes in DCM compared to controls. Most significant changes occurred in the tricarboxylic acid cycle, angiogenesis, and apoptotic signaling pathways, among which 32 apoptosis- and 13 MAPK activity-related genes were altered. Inorganic cation transporter, catalytic activities, energy metabolism and electron transport-related processes were among the most critically influenced pathways. Among the up-regulated genes were HTRA1 (6.9-fold), PDCD8(AIFM1) (5.2) and PRDX2 (4.4) and the down-regulated genes were NR4A2 (4.8), MX1 (4.3), LGALS9 (4), IFNA13 (4), UNC5D (3.6) and HDAC2 (3) (p<0.05), all of which have no clearly defined cardiac-related function yet. Gene ontology and enrichment analysis also revealed significant alterations in mitochondrial oxidative phosphorylation, metabolism and Alzheimer's disease pathways. Concordance was also confirmed for a significant number of genes and pathways in an independent validation microarray dataset. Furthermore, verification by real-time RT-PCR showed a high degree of consistency with the microarray results. Our data demonstrate an association of DCM with alterations in various cellular events and multiple yet undeciphered genes that may contribute to heart muscle disease pathways.

Engineered GPCRs as tools to modulate signal transduction

Different families of G-protein-coupled receptors (GPCRs) have been engineered to provide exclusive control over the activation of these receptors and thus to understand better the consequences of their signaling in vitro and in vivo. These engineered receptors, named RASSLs (receptors activated solely by synthetic ligands) and DREADDs (designer receptors exclusively activated by designer drugs), are insensitive to their endogenous ligands but can be activated by synthetic drug-like compounds. Currently, the existing RASSLs and DREADDs cover the Gi, Gq, and Gs signaling pathways. These modified GPCRs can be utilized as ideal tools to study GPCR functions selectively in specific cellular populations.

Endothelin downregulates SERCA2 gene and protein expression in adult rat ventricular myocytes: regulation by pertussis toxin-sensitive Gi protein and cAMP

Downregulation of the sarcoplasmic reticulum calcium ATPase (SERCA2) is associated with diastolic dysfunction in the failing heart. Elevated plasma endothelin-1 (ET) levels are correlated with congestive heart failure suggesting that ET may play a pathophysiological role. We have investigated the ability of ET to regulate SERCA2 gene expression in isolated adult rat ventricular myocytes. We find that ET enhances net protein synthesis by approximately 40% but significantly downregulates SERCA2 mRNA expression, time dependently, by approximately 30-50%, and the expression of SERCA2 protein by approximately 50%. In myoyctes, ET binds to ET(A) receptor that couples to G(q) and G(i) proteins. Inhibition of G(q)-PLC-induced phosphoinositide (PI) hydrolysis with U73122 (1 muM) or inhibition of G(i) protein with pertussis toxin (PTX) abolishes the ability of ET to downregulate SERCA2 mRNA gene expression. Further investigation suggests that ET coupling to PTX-sensitive G(i) with consequent lowering of cAMP is required for downregulation of SERCA2 mRNA levels. Increasing intracellular cAMP quantity using cAMP-specific PDE inhibitor Ro20-1724 or cAMP analog dibutyryl-cAMP reverses ET-induced downregulation of SERCA2 mRNA levels. The data indicate that, in adult myocytes, ET downregulates SERCA2 mRNA and protein levels, and the effect requires cross-talk between G(q) and PTX-sensitive G(i) pathways.