Gq-initiated cardiomyocyte hypertrophy is mediated by phospholipase Cbeta1b

Emerging Roles of Phospholipase C Beta Isozymes as Potential Biomarkers in Cardiac Disorders

Phospholipase C (PLC) enzymes represent crucial participants in the plasma membrane of mammalian cells, including the cardiac sarcolemmal (SL) membrane of cardiomyocytes. They are responsible for the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) into 1,2-diacylglycerol (DAG) and inositol (1,4,5) trisphosphate (Ins(1,4,5)P3), both essential lipid mediators. These second messengers regulate the intracellular calcium (Ca2+) concentration, which activates signal transduction cascades involved in the regulation of cardiomyocyte activity. Of note, emerging evidence suggests that changes in cardiomyocytes' phospholipid profiles are associated with an increased occurrence of cardiovascular diseases, but the underlying mechanisms are still poorly understood. This review aims to provide a comprehensive overview of the significant impact of PLC on the cardiovascular system, encompassing both physiological and pathological conditions. Specifically, it focuses on the relevance of PLCβ isoforms as potential cardiac biomarkers, due to their implications for pathological disorders, such as cardiac hypertrophy, diabetic cardiomyopathy, and myocardial ischemia/reperfusion injury. Gaining a deeper understanding of the mechanisms underlying PLCβ activation and regulation is crucial for unraveling the complex signaling networks involved in healthy and diseased myocardium. Ultimately, this knowledge holds significant promise for advancing the development of potential therapeutic strategies that can effectively target and address cardiac disorders by focusing on the PLCβ subfamily.

Internalized β2-Adrenergic Receptors Inhibit Subcellular Phospholipase C-Dependent Cardiac Hypertrophic Signaling

Chronically elevated neurohumoral drive, and particularly elevated adrenergic tone leading to β-adrenergic receptor (β-AR) overstimulation in cardiac myocytes, is a key mechanism involved in the progression of heart failure. β1-AR and β2-ARs are the two major subtypes of β-ARs present in the human heart, however, they elicit different or even opposite effects on cardiac function and hypertrophy. For example, chronic activation of β1ARs drives detrimental cardiac remodeling while β2AR signaling is protective. The underlying molecular mechanisms for cardiac protection through β2ARs remain unclear. Here we show that β2-AR protects against hypertrophy through inhibition of PLCε signaling at the Golgi apparatus. The mechanism for β2AR-mediated PLC inhibition requires internalization of β2AR, activation of Gi and Gβγ subunit signaling at endosomes and ERK activation. This pathway inhibits both angiotensin II and Golgi-β1-AR-mediated stimulation of phosphoinositide hydrolysis at the Golgi apparatus ultimately resulting in decreased PKD and HDAC5 phosphorylation and protection against cardiac hypertrophy. This reveals a mechanism for β2-AR antagonism of the PLCε pathway that may contribute to the known protective effects of β2-AR signaling on the development of heart failure.

The emerging roles of Shank3 in cardiac function and dysfunction

Shank3 is a member of the Shank family proteins (Shank1-3), which are abundantly present in the postsynaptic density (PSD) of neuronal excitatory synapses. As a core scaffold in the PSD, Shank3 plays a critical role in organizing the macromolecular complex, ensuring proper synaptic development and function. Clinically, various mutations of the SHANK3 gene are causally associated with brain disorders such as autism spectrum disorders and schizophrenia. However, recent in vitro and in vivo functional studies and expression profiling in various tissues and cell types suggest that Shank3 also plays a role in cardiac function and dysfunction. For example, Shank3 interacts with phospholipase Cβ1b (PLCβ1b) in cardiomyocytes, regulating its localization to the sarcolemma and its role in mediating Gq-induced signaling. In addition, changes in cardiac morphology and function associated with myocardial infarction and aging have been investigated in a few Shank3 mutant mouse models. This review highlights these results and potential underlying mechanisms, and predicts additional molecular functions of Shank3 based on its protein interactors in the PSD, which are also highly expressed and function in the heart. Finally, we provide perspectives and possible directions for future studies to better understand the roles of Shank3 in the heart.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

Upregulation of Phospholipase C Gene Expression Due to Norepinephrine-Induced Hypertrophic Response

The activation of phospholipase C (PLC) is thought to have a key role in the cardiomyocyte response to several different hypertrophic agents such as norepinephrine, angiotensin II and endothelin-1. PLC activity results in the generation of diacylglycerol and inositol trisphosphate, which are downstream signal transducers for the expression of fetal genes, increased protein synthesis, and subsequent cardiomyocyte growth. In this article, we describe the signal transduction elements that regulate PLC gene expression. The discussion is focused on the norepinephrine- α₁-adrenoceptor signaling pathway and downstream signaling processes that mediate an upregulation of PLC isozyme gene expression. Evidence is also indicated to demonstrate that PLC activities self-regulate the expression of PLC isozymes with the suggestion that PLC activities may be part of a coordinated signaling process for the perpetuation of cardiac hypertrophy. Accordingly, from the information provided, it is plausible that specific PLC isozymes could be targeted for the mitigation of cardiac hypertrophy.

Role of Phospholipase C in Catecholamine-induced Increase in Myocardial Protein Synthesis

The activation of the α1-adrenoceptor-(α1-AR) by norepinephrine results in the G-protein (Gqα) mediated increase in the phosphoinositide-specific phospholipase C (PLC) activity. The byproducts of PLC hydrolytic activity, namely, 1,2-diacylglycerol and inositol-1,4,5-trisphosphate, are important downstream signal transducers for increased protein synthesis in the cardiomyocyte and the subsequent hypertrophic response. In this article, evidence is outlined to demonstrate the role of cardiomyocyte PLC isozymes in the catecholamine-induced increase in protein synthesis by using a blocker of α1-AR and an inhibitor of PLC. The discussion will be focused on the α1-AR-Gqα-PLC-mediated hypertrophic signaling pathway from the viewpoint that it may compliment the other β1-AR-Gs protein-adenylyl cyclase signal transduction mechanisms in the early stages of cardiac hypertrophy development, but may become more relevant at the late stage of cardiac hypertrophy. From the information provided here, it is suggested that some specific PLC isozymes may potentially serve as important targets for the attenuation of cardiac hypertrophy in the vulnerable patient population at-risk for heart failure.

Involvement of phospholipase C in the norepinephrine-induced hypertrophic response in Cardiomyocytes

Norepinephrine (NE) is known to mediate cardiomyocyte hypertrophy through the G protein coupled a1 -adrenoceptor (a1 -AR) and the activation of the phosphoinositide-specific phospholipase C (PLC). Since the by-products of PLC activity are important downstream signal transducers for cardiac hypertrophy, the role of and the regulatory mechanisms involved in the activation of PLC isozymes in cardiac hypertrophy are highlighted in this review. The discussion is focused to underscore PLC in different experimental models of cardiac hypertrophy, as well as in isolated adult and neonatal cardiomyocytes treated with NE. Particular emphasis is laid concerning the a1 -AR-PLC-mediated hypertrophic signalling pathway. From the information provided, it is evident that the specific activation of PLC isozymes is a primary signalling event in the a1 -AR mediated response to NE as well as initiation and progression of cardiac hypertrophy. Furthermore, the possibility of PLC involvement in the perpetuation of cardiac hypertrophy is also described. It is suggested that specific PLC isozymes may serve as viable targets for the prevention of cardiac hypertrophy in patient population at-risk for the development of heart failure.

Stimulation of phospholipase Cβ1 by Gαq promotes the assembly of stress granule proteins

[Figure: see text].

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Activation of Phospholipase C β by Gβγ and Gαq Involves C-Terminal Rearrangement to Release Autoinhibition

Phospholipase C (PLC) enzymes hydrolyze phosphoinositide lipids to inositol phosphates and diacylglycerol. Direct activation of PLCβ by Gα_q and/or Gβγ subunits mediates signaling by Gq and some Gi coupled G-protein-coupled receptors (GPCRs), respectively. PLCβ isoforms contain a unique C-terminal extension, consisting of proximal and distal C-terminal domains (CTDs) separated by a flexible linker. The structure of PLCβ3 bound to Gα_q is known, however, for both Gα_q and Gβγ; the mechanism for PLCβ activation on membranes is unknown. We examined PLCβ2 dynamics on membranes using hydrogen-deuterium exchange mass spectrometry (HDX-MS). Gβγ caused a robust increase in dynamics of the distal C-terminal domain (CTD). Gα_q showed decreased deuterium incorporation at the Gα_q binding site on PLCβ. In vitro Gβγ-dependent activation of PLC is inhibited by the distal CTD. The results suggest that disruption of autoinhibitory interactions with the CTD leads to increased PLCβ hydrolase activity.

MicroRNAs in Cardiac Hypertrophy

Like other organs, the heart undergoes normal adaptive remodeling, such as cardiac hypertrophy, with age. This remodeling, however, is intensified under stress and pathological conditions. Cardiac remodeling could be beneficial for a short period of time, to maintain a normal cardiac output in times of need; however, chronic cardiac hypertrophy may lead to heart failure and death. MicroRNAs (miRNAs) are known to have a role in the regulation of cardiac hypertrophy. This paper reviews recent advances in the field of miRNAs and cardiac hypertrophy, highlighting the latest findings for targeted genes and involved signaling pathways. By targeting pro-hypertrophic genes and signaling pathways, some of these miRNAs alleviate cardiac hypertrophy, while others enhance it. Therefore, miRNAs represent very promising potential pharmacotherapeutic targets for the management and treatment of cardiac hypertrophy.

Golgi localized β1-adrenergic receptors stimulate Golgi PI4P hydrolysis by PLCε to regulate cardiac hypertrophy

Increased adrenergic tone resulting from cardiovascular stress leads to development of heart failure, in part, through chronic stimulation of β1 adrenergic receptors (βARs) on cardiac myocytes. Blocking these receptors is part of the basis for β-blocker therapy for heart failure. Recent data demonstrate that G protein-coupled receptors (GPCRs), including βARs, are activated intracellularly, although the biological significance is unclear. Here we investigated the functional role of Golgi βARs in rat cardiac myocytes and found they activate Golgi localized, prohypertrophic, phosphoinositide hydrolysis, that is not accessed by cell surface βAR stimulation. This pathway is accessed by the physiological neurotransmitter norepinephrine (NE) via an Oct3 organic cation transporter. Blockade of Oct3 or specific blockade of Golgi resident β1ARs prevents NE dependent cardiac myocyte hypertrophy. This clearly defines a pathway activated by internal GPCRs in a biologically relevant cell type and has implications for development of more efficacious β-blocker therapies.

Gαq and the Phospholipase Cβ3 X-Y Linker Regulate Adsorption and Activity on Compressed Lipid Monolayers

Phospholipase Cβ (PLCβ) enzymes are peripheral membrane proteins required for normal cardiovascular function. PLCβ hydrolyzes phosphatidylinositol 4,5-bisphosphate, producing second messengers that increase intracellular Ca²⁺ level and activate protein kinase C. Under basal conditions, PLCβ is autoinhibited by its C-terminal domains and by the X-Y linker, which contains a stretch of conserved acidic residues required for interfacial activation. Following stimulation of G protein-coupled receptors, the heterotrimeric G protein subunit Gα_q allosterically activates PLCβ and helps orient the activated complex at the membrane for efficient lipid hydrolysis. However, the molecular basis for how the PLCβ X-Y linker, its C-terminal domains, Gα_q, and the membrane coordinately regulate activity is not well understood. Using compressed lipid monolayers and atomic force microscopy, we found that a highly conserved acidic region of the X-Y linker is sufficient to regulate adsorption. Regulation of adsorption and activity by the X-Y linker also occurs independently of the C-terminal domains. We next investigated whether Gα_q-dependent activation of PLCβ altered interactions with the model membrane. Gα_q increased PLCβ adsorption in a manner that was independent of the PLCβ regulatory elements and targeted adsorption to specific regions of the monolayer in the absence of the C-terminal domains. Thus, the mechanism of Gα_q-dependent activation likely includes a spatial component.

Intramolecular electrostatic interactions contribute to phospholipase Cβ3 autoinhibition

Phospholipase Cβ (PLCβ) enzymes regulate second messenger production following the activation of G protein-coupled receptors (GPCRs). Under basal conditions, these enzymes are maintained in an autoinhibited state by multiple elements, including an insertion within the catalytic domain known as the X-Y linker. Although the PLCβ X-Y linker is variable in sequence and length, its C-terminus is conserved and features an acidic stretch, followed by a short helix. This helix interacts with residues near the active site, acting as a lid to sterically prevent substrate binding. However, deletions that remove the acidic stretch of the X-Y linker increase basal activity to the same extent as deletion of the entire X-Y linker. Thus, the acidic stretch may be the linchpin in autoinhibition mediated by the X-Y linker. We used site-directed mutagenesis and biochemical assays to investigate the importance of this acidic charge in mediating PLCβ3 autoinhibition. Loss of the acidic charge in the X-Y linker increases basal activity and decreases stability, consistent with loss of autoinhibition. However, introduction of compensatory electrostatic mutations on the surface of the PLCβ3 catalytic domain restore activity to basal levels. Thus, intramolecular electrostatics modulate autoinhibition by the X-Y linker.

The Role of Reactive Oxygen Species in In Vitro Cardiac Maturation

Recent advances in developmental biology and biomedical engineering have significantly improved the efficiency and purity of cardiomyocytes (CMs) generated from pluripotent stem cells (PSCs). Regardless of the protocol used to derive CMs, these cells exhibit hallmarks of functional immaturity. In this Opinion, we focus on reactive oxygen species (ROS), signaling molecules that can potentially modulate cardiac maturation. We outline how ROS impacts nearly every aspect associated with cardiac maturation, including contractility, calcium handling, metabolism, and hypertrophy. Though the precise role of ROS in cardiac maturation has yet to be elucidated, ROS may provide a valuable perspective for understanding the molecular mechanisms for cardiac maturation under various conditions.

Microarray analysis of long non-coding RNA and mRNA expression profiles in diabetic cardiomyopathy using human induced pluripotent stem cell-derived cardiomyocytes

AIM

This study aims to investigate the altered expression signature of long non-coding RNAs, mRNAs and deregulated pathways related to diabetic cardiomyopathy disease pathogenesis.

METHOD

We utilize the previously established in vitro diabetic cardiomyopathy model of human induced pluripotent stem cell-derived human cardiomyocytes to perform long non-coding RNA and mRNA expression analysis on glucose (11 mM), endothelin-1 (10 nM) and cortisol (1 µM) stimulated human induced pluripotent stem cell-derived human cardiomyocytes to interrogate diabetic cardiomyopathy associated RNA expression profile.

RESULT

Out of 20,730 mRNAs and 40,173 long non-coding RNAs being screened, 2046 long non-coding RNAs and 1582 mRNAs were differentially regulated (fold change > 2, p < 0.05) between diabetic cardiomyopathy and control group, of which more than half were intergenic and antisense long non-coding RNAs. Most of the coding transcripts were associated with processes like inflammation, structural reorganization, metabolism, smooth muscle contraction, focal adhesion and repair contributing towards the development of diabetic cardiomyopathy. The subgroup analysis further revealed 411 long non-coding RNAs being co-expressed with neighbouring genes. However, our coding-non-coding co-expression analysis showed an overall 48,155 co-expression network connections. In addition to that, the long non-coding RNAs with highest network connections were profoundly enriched for focal adhesion, cell-matrix adhesion and muscle contraction.

CONCLUSION

These results provide comprehensive data about the pathways and regulatory mechanisms associated with diabetic cardiomyopathy and indicate that long non-coding RNAs may play a crucial role in diabetic cardiomyopathy.

Direct observation of conformational dynamics of the PH domain in phospholipases Cϵ and β may contribute to subfamily-specific roles in regulation

Phospholipase C (PLC) enzymes produce second messengers that increase the intracellular Ca2+ concentration and activate protein kinase C (PKC). These enzymes also share a highly conserved arrangement of core domains. However, the contributions of the individual domains to regulation are poorly understood, particularly in isoforms lacking high-resolution information, such as PLCϵ. Here, we used small-angle X-ray scattering (SAXS), EM, and functional assays to gain insights into the molecular architecture of PLCϵ, revealing that its PH domain is conformationally dynamic and essential for activity. We further demonstrate that the PH domain of PLCβ exhibits similar dynamics in solution that are substantially different from its conformation observed in multiple previously reported crystal structures. We propose that this conformational heterogeneity contributes to subfamily-specific differences in activity and regulation by extracellular signals.

Compartmentalized cyclic nucleotides have opposing effects on regulation of hypertrophic phospholipase Cε signaling in cardiac myocytes

In cardiac myocytes activation of an exchange factor activated by cAMP (Epac) leads to activation of phospholipase Cε (PLCε)-dependent hydrolysis of phosphatidylinositol 4-phosphate (PI4P) in the Golgi apparatus a process critical for development of cardiac hypertrophy. Here we show that β-adrenergic receptor (βAR) stimulation does not stimulate this pathway in the presence of the broad spectrum phosphodiesterase (PDE) inhibitor IBMX, but selective PDE3 inhibition revealed βAR-dependent PI4P depletion. On the other hand, selective inhibition of PDE2 or PDE9A blocked endothelin-1 (ET-1) and cAMP-dependent PI4P hydrolysis by PLCε. Direct activation of protein kinase A (PKA), protein kinase G (PKG), or the atrial natriuretic factor (ANF) receptor abolished PI4P hydrolysis in response to multiple upstream stimuli. These results reveal distinct pools of cyclic nucleotides that either inhibit PLCε at the Golgi through PKA/PKG, or activate PLCε at the Golgi through Epac. These data together reveal a new mechanism by which ANF and selective PDE inhibitors can protect against cardiac hypertrophy.

Screening key candidate genes and pathways involved in insulinoma by microarray analysis

Insulinoma is a rare type tumor and its genetic features remain largely unknown. This study aimed to search for potential key genes and relevant enriched pathways of insulinoma.The gene expression data from GSE73338 were downloaded from Gene Expression Omnibus database. Differentially expressed genes (DEGs) were identified between insulinoma tissues and normal pancreas tissues, followed by pathway enrichment analysis, protein-protein interaction (PPI) network construction, and module analysis. The expressions of candidate key genes were validated by quantitative real-time polymerase chain reaction (RT-PCR) in insulinoma tissues.A total of 1632 DEGs were obtained, including 1117 upregulated genes and 514 downregulated genes. Pathway enrichment results showed that upregulated DEGs were significantly implicated in insulin secretion, and downregulated DEGs were mainly enriched in pancreatic secretion. PPI network analysis revealed 7 hub genes with degrees more than 10, including GCG (glucagon), GCGR (glucagon receptor), PLCB1 (phospholipase C, beta 1), CASR (calcium sensing receptor), F2R (coagulation factor II thrombin receptor), GRM1 (glutamate metabotropic receptor 1), and GRM5 (glutamate metabotropic receptor 5). DEGs involved in the significant modules were enriched in calcium signaling pathway, protein ubiquitination, and platelet degranulation. Quantitative RT-PCR data confirmed that the expression trends of these hub genes were similar to the results of bioinformatic analysis.The present study demonstrated that candidate DEGs and enriched pathways were the potential critical molecule events involved in the development of insulinoma, and these findings were useful for better understanding of insulinoma genesis.

Phospholipase Cβ3 Membrane Adsorption and Activation Are Regulated by Its C-Terminal Domains and Phosphatidylinositol 4,5-Bisphosphate

Phospholipase Cβ (PLCβ) enzymes hydrolyze phosphatidylinositol 4,5-bisphosphate to produce second messengers that regulate intracellular Ca²⁺, cell proliferation, and survival. Their activity is dependent upon interfacial activation that occurs upon localization to cell membranes. However, the molecular basis for how these enzymes productively interact with the membrane is poorly understood. Herein, atomic force microscopy demonstrates that the ∼300-residue C-terminal domain promotes adsorption to monolayers and is required for spatial organization of the protein on the monolayer surface. PLCβ variants lacking this C-terminal domain display differences in their distribution on the surface. In addition, a previously identified autoinhibitory helix that binds to the PLCβ catalytic core negatively impacts membrane binding, providing an additional level of regulation for membrane adsorption. Lastly, defects in phosphatidylinositol 4,5-bisphosphate hydrolysis also alter monolayer adsorption, reflecting a role for the active site in this process. Together, these findings support a model in which multiple elements of PLCβ modulate adsorption, distribution, and catalysis at the cell membrane.

microRNA-133a attenuates cardiomyocyte hypertrophy by targeting PKCδ and Gq

During the past decade, microRNAs have continuously been suggested as a promising therapeutic tool due to their beneficial effects, such as their multi-targets and multi-functions in pathologic conditions. As a pathologic phenotype is generally regulated by multiple signaling pathways, in this study we identified a microRNA regulating multiple target genes within cardiac hypertrophic signaling pathways. microRNA-133a is known to play a crucial role in cardiac hypertrophy. However, the role of microRNA-133a, which may regulate several signaling pathways in norepinephrine-induced cardiac hypertrophy via multi-targeting, has not been investigated. In the current study, we showed that microRNA-133a can protect cardiomyocyte hypertrophy against norepinephrine stimulation in neonatal rat ventricular cardiomyocytes via new targets, PKCδ and Gq, all of which are related to downstream signaling pathways of the α₁-adrenergic receptor. Taken together, these results suggest the advantages of the therapeutic use of microRNAs as an effective potential drug regulating multiple signaling pathways under pathologic conditions.

Selective coupling of the S1P3 receptor subtype to S1P-mediated RhoA activation and cardioprotection

Sphingosine-1-phosphate (S1P), a bioactive lysophospholipid, is generated and released at sites of tissue injury in the heart and can act on S1P₁, S1P₂, and S1P₃ receptor subtypes to affect cardiovascular responses. We established that S1P causes little phosphoinositide hydrolysis and does not induce hypertrophy indicating that it does not cause receptor coupling to G_q. We previously demonstrated that S1P confers cardioprotection against ischemia/reperfusion by activating RhoA and its downstream effector PKD. The S1P receptor subtypes and G proteins that regulate RhoA activation and downstream responses in the heart have not been determined. Using siRNA or pertussis toxin to inhibit different G proteins in NRVMs we established that S1P regulates RhoA activation through Gα₁₃ but not Gα₁₂, Gα_q, or Gα_i. Knockdown of the three major S1P receptors using siRNA demonstrated a requirement for S1P₃ in RhoA activation and subsequent phosphorylation of PKD, and this was confirmed in studies using isolated hearts from S1P₃ knockout (KO) mice. S1P treatment reduced infarct size induced by ischemia/reperfusion in Langendorff perfused wild-type (WT) hearts and this protection was abolished in the S1P₃ KO mouse heart. CYM-51736, an S1P₃-specific agonist, also decreased infarct size after ischemia/reperfusion to a degree similar to that achieved by S1P. The finding that S1P₃ receptor- and Gα₁₃-mediated RhoA activation is responsible for protection against ischemia/reperfusion suggests that selective targeting of S1P₃ receptors could provide therapeutic benefits in ischemic heart disease.

Chronic Contractile Dysfunction without Hypertrophy Does Not Provoke a Compensatory Transcriptional Response in Mouse Hearts

Diseased myocardium from humans and experimental animal models shows heightened expression and activity of a specific subtype of phospholipase C (PLC), the splice variant PLCβ1b. Previous studies from our group showed that increasing PLCβ1b expression in adult mouse hearts by viral transduction was sufficient to cause sustained contractile dysfunction of rapid onset, which was maintained indefinitely in the absence of other pathological changes in the myocardium. We hypothesized that impaired contractility alone would be sufficient to induce a compensatory transcriptional response. Unbiased, comprehensive mRNA-sequencing was performed on 6 biological replicates of rAAV6-treated blank, PLCβ1b and PLCβ1a (closely related but inactive splice variant) hearts 8 weeks after injection, when reduced contractility was manifest in PLCβ1b hearts without evidence of induced hypertrophy. Expression of PLCβ1b resulted in expression changes in only 9 genes at FDR<0.1 when compared with control and these genes appeared unrelated to contractility. Importantly, PLCβ1a caused similar mild expression changes to PLCβ1b, despite a complete lack of effect of this isoform on cardiac contractility. We conclude that contractile depression caused by PLCβ1b activation is largely independent of changes in the transcriptome, and thus that lowered contractility is not sufficient in itself to provoke measurable transcriptomic alterations. In addition, our data stress the importance of a stringent control group to filter out transcriptional changes unrelated to cardiac function.

Common miR-590 Variant rs6971711 Present Only in African Americans Reduces miR-590 Biogenesis

MicroRNAs (miRNAs) are recognized as important regulators of cardiac development, hypertrophy and fibrosis. Recent studies have demonstrated that genetic variations which cause alterations in miRNA:target interactions can lead to disease. We hypothesized that genetic variations in miRNAs that regulate cardiac hypertrophy/fibrosis might be involved in generation of the cardiac phenotype in patients diagnosed with hypertrophic cardiomyopathy (HCM). To investigate this question, we Sanger sequenced 18 miRNA genes previously implicated in myocyte hypertrophy/fibrosis and apoptosis, using genomic DNA isolated from the leukocytes of 199 HCM patients. We identified a single nucleotide polymorphism (rs6971711, C57T SNP) at the 17th position of mature miR-590-3p (= 57th position of pre-miR-590) that is common in individuals of African ancestry. SNP frequency was higher in African American HCM patients (n = 55) than ethnically-matched controls (n = 100), but the difference was not statistically significant (8.2% vs. 6.5%; p = 0.5). Using a cell culture system, we discovered that presence of this SNP resulted in markedly lower levels of mature miR-590-5p (39 ± 16%, p<0.003) and miR-590-3p (20 ± 2%, p<0.003), when compared with wild-type (WT) miR-590, without affecting levels of pri-miR-590 and pre-miR-590. Consistent with this finding, the SNP resulted in reduced target suppression when compared to WT miR-590 (71% suppression by WT vs 60% suppression by SNP, p<0.03). Since miR-590 can regulate TGF-β, Activin A and Akt signaling, SNP-induced reduction in miR-590 biogenesis could influence cardiac phenotype by de-repression of these signaling pathways. Since the SNP is only present in African Americans, population studies in this patient population would be valuable to investigate effects of this SNP on myocyte function and cardiac physiology.

Expressing an inhibitor of PLCβ1b sustains contractile function following pressure overload

The activity of phospholipase Cβ1b (PLCβ1b) is selectively elevated in failing myocardium and cardiac expression of PLCβ1b causes contractile dysfunction. PLCβ1b can be selectively inhibited by expressing a peptide inhibitor that prevents sarcolemmal localization. The inhibitory peptide, PLCβ1b-CT was expressed in heart from a mini-gene using adeno-associated virus (rAAV6-PLCβ1b-CT). rAAV6-PLCβ1b-CT, or blank virus, was delivered IV (4×10(9)vg/g body weight) and trans-aortic-constriction (TAC) or sham-operation was performed 8weeks later. Expression of PLCβ1b-CT prevented the loss of contractile function, eliminated lung congestion and improved survival following TAC with either a 'moderate' or 'severe' pressure gradient. Hypertrophy was attenuated but not eliminated. Expression of the PLCβ1b-CT peptide 2-3weeks after TAC reduced contractile dysfunction and lung congestion, without limiting hypertrophy. PLCβ1b inhibition ameliorates pathological responses following acute pressure overload. The targeting of PLCβ1b to the sarcolemma provides the basis for the development of a new class of inotropic agent.

Molecular mechanisms of phospholipase C β3 autoinhibition

Phospholipase C β (PLCβ) enzymes are dramatically activated by heterotrimeric G proteins. Central to this response is the robust autoinhibition of PLCβ by the X-Y linker region within its catalytic core and by the Hα2' helix in the C-terminal extension of the enzyme. The molecular mechanism of each and their mutual dependence are poorly understood. Herein, it is shown that distinct regions within the X-Y linker have specific roles in regulating activity. Most important,an acidic stretch within the linker stabilizes a lid that occludes the active site, consistent with crystal structures of variants lacking this region. Inhibition by the Hα2' helix is independent of the X-Y linker and likely regulates activity by limiting membrane interaction of the catalytic core. Full activation of PLCβ thus requires multiple independent molecular events induced by membrane association of the catalytic core and by the binding of regulatory proteins.

The atypical 'b' splice variant of phospholipase Cβ1 promotes cardiac contractile dysfunction

The activity of the early signaling enzyme, phospholipase Cβ1b (PLCβ1b), is selectively elevated in diseased myocardium and activity increases with disease progression. We aimed to establish the contribution of heightened PLCβ1b activity to cardiac pathology. PLCβ1b, the alternative splice variant, PLCβ1a, and a blank virus were expressed in mouse hearts using adeno-associated viral vectors (rAAV6-FLAG-PLCβ1b, rAAV6-FLAG-PLCβ1a, or rAAV6-blank) delivered intravenously (IV). Following viral delivery, FLAG-PLCβ1b was expressed in all of the chambers of the mouse heart and was localized to the sarcolemma. Heightened PLCβ1b expression caused a rapid loss of contractility, 4-6 weeks, that was fully reversed, within 5 days, by inhibition of protein kinase Cα (PKCα). PLCβ1a did not localize to the sarcolemma and did not affect contractile function. Expression of PLCβ1b, but not PLCβ1a, caused downstream dephosphorylation of phospholamban and depletion of the Ca(2+) stores of the sarcoplasmic reticulum. We conclude that heightened PLCβ1b activity observed in diseased myocardium contributes to pathology by PKCα-mediated contractile dysfunction. PLCβ1b is a cardiac-specific signaling system, and thus provides a potential therapeutic target for the development of well-tolerated inotropic agents for use in failing myocardium.

G protein βγ subunits regulate cardiomyocyte hypertrophy through a perinuclear Golgi phosphatidylinositol 4-phosphate hydrolysis pathway

Gβγ regulation of the perinuclear Golgi PI4P pathway and a separate pathway at the PM is required for ET-1–stimulated hypertrophy, and the efficacy of Gβγ inhibition in preventing heart failure may be due, in part, to its blocking both of these pathways.

We recently identified a novel GPCR-dependent pathway for regulation of cardiac hypertrophy that depends on Golgi phosphatidylinositol 4-phosphate (PI4P) hydrolysis by a specific isoform of phospholipase C (PLC), PLCε, at the nuclear envelope. How stimuli are transmitted from cell surface GPCRs to activation of perinuclear PLCε is not clear. Here we tested the role of G protein βγ subunits. Gβγ inhibition blocked ET-1–stimulated Golgi PI4P depletion in neonatal and adult ventricular myocytes. Blocking Gβγ at the Golgi inhibited ET-1–dependent PI4P depletion and nuclear PKD activation. Translocation of Gβγ to the Golgi stimulated perinuclear Golgi PI4P depletion and nuclear PKD activation. Finally, blocking Gβγ at the Golgi or PM blocked ET-1–dependent cardiomyocyte hypertrophy. These data indicate that Gβγ regulation of the perinuclear Golgi PI4P pathway and a separate pathway at the PM is required for ET-1–stimulated hypertrophy, and the efficacy of Gβγ inhibition in preventing heart failure maybe due in part to its blocking both these pathways.

RNA expression profiling of human iPSC-derived cardiomyocytes in a cardiac hypertrophy model

Cardiac hypertrophy is an independent risk factor for cardiovascular disease and heart failure. There is increasing evidence that microRNAs (miRNAs) play an important role in the regulation of messenger RNA (mRNA) and the pathogenesis of various cardiovascular diseases. However, the ability to comprehensively study cardiac hypertrophy on a gene regulatory level is impacted by the limited availability of human cardiomyocytes. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer the opportunity for disease modeling. Here we utilize a previously established invitro model of cardiac hypertrophy to interrogate the regulatory mechanism associated with the cardiac disease process. We perform miRNA sequencing and mRNA expression analysis on endothelin 1 (ET-1) stimulated hiPSC-CMs to describe associated RNA expression profiles. MicroRNA sequencing revealed over 250 known and 34 predicted novel miRNAs to be differentially expressed between ET-1 stimulated and unstimulated control hiPSC-CMs. Messenger RNA expression analysis identified 731 probe sets with significant differential expression. Computational target prediction on significant differentially expressed miRNAs and mRNAs identified nearly 2000 target pairs. A principal component analysis approach comparing the invitro data with human myocardial biopsies detected overlapping expression changes between the invitro samples and myocardial biopsies with Left Ventricular Hypertrophy. These results provide further insights into the complex RNA regulatory mechanism associated with cardiac hypertrophy.

Characterization of the PLCB1 promoter and regulation by early growth response transcription factor EGR-1

The Gαq/-Gα11-PLCβ1 pathway is important for intracellular signalling and associated with pathological conditions, such as cardiac hypertrophy. The GNAQ and GNA11 promoters (encoding for Gαq and Gα11) have already been characterized and are both regulated by the transcription factor early growth response 1 (Egr-1). In contrast, the PLCB1 promoter (encoding for the direct downstream effector PLCβ1) has neither been cloned nor characterized. Therefore, the purpose of this study was to 1) characterize the PLCB1 promoter, and 2) assess its potential regulation by Egr-1. By means of 5'- Rapid Amplification of 5'-cDNA ends analysis in human heart tissue we found an initiation of transcription from multiple starting points, the main transcription starting point being located at nt-235 relative to the translation start point. The PLCB1 promoter was cloned and deletion constructs were generated. Luciferase assays were performed in three different cell lines and regulatory regions were identified between nt-595/nt-313 (Hek293: P=0.013; HASMC: P=0.019; H9c2: P=0.005). In electrophoretic mobility shift assays one specific Egr-1 binding site was identified at nt-451/-419 and PLCB1 promoter activity was increased more than 5-fold (Hek293: P=0.0008) and 1,6- fold (H9c2: P=0.0499) following overexpression of Egr-1. Thus, the PLCB1 promoter was characterized for the first time and a specific interaction with the transcription factor Egr-1 was shown. Our data provide a potential molecular mechanism relating to pathophysiological conditions such as cardiac hypertrophy where activation by Egr-1 of Gαq/Gα11-PLCβ1 plays an important role.

The phosphatidylinositol(4,5)bisphosphate-binding sequence of transient receptor potential channel canonical 4α is critical for its contribution to cardiomyocyte hypertrophy

Cardiomyocyte hypertrophy requires a source of Ca(2+) distinct from the Ca(2+) that regulates contraction. The canonical transient receptor potential channel (TrpC) family, a family of cation channels regulated by activation of phospholipase C (PLC), has been implicated in this response. Cardiomyocyte hypertrophy downstream of Gq-coupled receptors is mediated specifically by PLCβ1b that is scaffolded onto a SH3 and ankyrin repeat protein 3 (Shank3) complex at the sarcolemma. TrpC4 exists as two splice variants (TrpC4α and TrpC4β) that differ only in an 84-residue sequence that binds to phosphatidylinositol(4,5)bisphosphate (PIP2), the substrate of PLCβ1b. In neonatal rat cardiomyocytes, TrpC4α, but not TrpC4β, coimmunoprecipitated with both PLCβ1b and Shank3. Heightened PLCβ1b expression caused TrpC4α, but not TrpC4β, translocation to the sarcolemma, where it colocalized with PLCβ1b. When overexpressed in cardiomyocytes, TrpC4α, but not TrpC4β, increased cell area (893 ± 18 to 1497 ± 29 mm(2), P < 0.01) and marker gene expression (atrial natriuretic peptide increased by 409 ± 32%, and modulatory calcineurin inhibitory protein 1 by 315 ± 28%, P < 0.01). Dominant-negative TrpC4 reduced hypertrophy initiated by PLCβ1b, or PLCβ1b-coupled receptor activation, by 72 ± 8% and 39 ± 5 %, respectively. We conclude that TrpC4α is selectively involved in mechanisms downstream of PLCβ1b culminating in cardiomyocyte hypertrophy, and that the hypertrophic response is dependent on the TrpC4α splice variant-specific sequence that binds to PIP2.

Enhanced expression of Gqα and PLC-β1 proteins contributes to vascular smooth muscle cell hypertrophy in SHR: role of endogenous angiotensin II and endothelin-1

Vascular Gqα signaling has been shown to contribute to cardiac hypertrophy. In addition, angiotensin II (ANG II) was shown to induce vascular smooth muscle cell (VSMC) hypertrophy through Gqα signaling; however, the studies on the role of Gqα and PLC-β1 proteins in VSMC hypertrophy in animal model are lacking. The present study was therefore undertaken to examine the role of Gqα/PLC-β1 proteins and the signaling pathways in VSMC hypertrophy using spontaneously hypertensive rats (SHR). VSMC from 16-wk-old SHR and not from 12-wk-old SHR exhibited enhanced levels of Gqα/PLC-β1 proteins compared with age-matched Wistar-Kyoto (WKY) rats as determined by Western blotting. However, protein synthesis as determined by [3H]leucine incorporation was significantly enhanced in VSMC from both 12- and 16-wk-old SHR compared with VSMC from age-matched WKY rats. Furthermore, the knockdown of Gqα/PLC-β1 in VSMC from 16-wk-old SHR by antisense and small interfering RNA resulted in attenuation of protein synthesis. In addition, the enhanced expression of Gqα/PLC-β1 proteins, enhanced phosphorylation of ERK1/2, and enhanced protein synthesis in VSMC from SHR were attenuated by the ANG II AT1 and endothelin-1 (ET-1) ETA receptor antagonists losartan and BQ123, respectively, but not by the ETB receptor antagonist BQ788. In addition, PD98059 decreased the enhanced expression of Gqα/PLC-β1 and protein synthesis in VSMC from SHR. These results suggest that the enhanced levels of endogenous ANG II and ET-1 through the activation of AT1 and ETA receptors, respectively, and MAP kinase signaling, enhanced the expression of Gqα/PLC-β1 proteins in VSMC from 16-wk-old SHR and result in VSMC hypertrophy.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

Hydrolysis rates of different small interfering RNAs (siRNAs) by the RNA silencing promoter complex, C3PO, determines their regulation by phospholipase Cβ

C3PO plays a key role in promoting RNA-induced gene silencing. C3PO consists of two subunits of the endonuclease translin-associated factor X (TRAX) and six subunits of the nucleotide-binding protein translin. We have found that TRAX binds strongly to phospholipase Cβ (PLCβ), which transmits G protein signals from many hormones and sensory inputs. The association between PLCβ and TRAX is thought to underlie the ability of PLCβ to reverse gene silencing by small interfering RNAs. However, this reversal only occurs for some genes (e.g. GAPDH and LDH) but not others (e.g. Hsp90 and cyclophilin A). To understand this specificity, we carried out studies using fluorescence-based methods. In cells, we find that PLCβ, TRAX, and their complexes are identically distributed through the cytosol suggesting that selectivity is not due to large scale sequestration of either the free or complexed proteins. Using purified proteins, we find that PLCβ binds ∼5-fold more weakly to translin than to TRAX but ∼2-fold more strongly to C3PO. PLCβ does not alter TRAX-translin assembly to C3PO, and brightness studies suggest one PLCβ binds to one C3PO octamer without a change in the number of TRAX/translin molecules suggesting that PLCβ binds to an external site. Functionally, we find that C3PO hydrolyzes siRNA(GAPDH) at a faster rate than siRNA(Hsp90). However, when PLCβ is bound to C3PO, the hydrolysis rate of siRNA(GAPDH) becomes comparable with siRNA(Hsp90). Our results show that the selectivity of PLCβ toward certain genes lies in the rate at which the RNA is hydrolyzed by C3PO.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

The role of the paracrine/autocrine mediator endothelin-1 in regulation of cardiac contractility and growth

Endothelin-1 (ET-1) is a critical autocrine and paracrine regulator of cardiac physiology and pathology. Produced locally within the myocardium in response to diverse mechanical and neurohormonal stimuli, ET-1 acutely modulates cardiac contractility. During pathological cardiovascular conditions such as ischaemia, left ventricular hypertrophy and heart failure, myocyte expression and activity of the entire ET-1 system is enhanced, allowing the peptide to both initiate and maintain maladaptive cellular responses. Both the acute and chronic effects of ET-1 are dependent on the activation of intracellular signalling pathways, regulated by the inositol-trisphosphate and diacylglycerol produced upon activation of the ET(A) receptor. Subsequent stimulation of protein kinases C and D, calmodulin-dependent kinase II, calcineurin and MAPKs modifies the systolic calcium transient, myofibril function and the activity of transcription factors that coordinate cellular remodelling. The precise nature of the cellular response to ET-1 is governed by the timing, localization and context of such signals, allowing the peptide to regulate both cardiomyocyte physiology and instigate disease.

LINKED ARTICLES

This article is part of a themed section on Endothelin. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.168.issue-1.

Phospholipase Cε hydrolyzes perinuclear phosphatidylinositol 4-phosphate to regulate cardiac hypertrophy

Phospholipase Cε (PLCε) is a multifunctional enzyme implicated in cardiovascular, pancreatic, and inflammatory functions. Here we show that conditional deletion of PLCε in mouse cardiac myocytes protects from stress-induced pathological hypertrophy. PLCε small interfering RNA (siRNA) in ventricular myocytes decreases endothelin-1 (ET-1)-dependent elevation of nuclear calcium and activation of nuclear protein kinase D (PKD). PLCε scaffolded to muscle-specific A kinase-anchoring protein (mAKAP), along with PKCε and PKD, localizes these components at or near the nuclear envelope, and this complex is required for nuclear PKD activation. Phosphatidylinositol 4-phosphate (PI4P) is identified as a perinuclear substrate in the Golgi apparatus for mAKAP-scaffolded PLCε. We conclude that perinuclear PLCε, scaffolded to mAKAP in cardiac myocytes, responds to hypertrophic stimuli to generate diacylglycerol (DAG) from PI4P in the Golgi apparatus, in close proximity to the nuclear envelope, to regulate activation of nuclear PKD and hypertrophic signaling pathways.

Full-length Gα(q)-phospholipase C-β3 structure reveals interfaces of the C-terminal coiled-coil domain

Phospholipase C-β (PLCβ) is directly activated by Gα_q, but the molecular basis for how its distal C-terminal domain (CTD) contributes to maximal activity is poorly understood. Herein we present both the crystal structure and cryo-EM 3D reconstructions of human full-length PLCβ3 in complex with murine Gα_q. The distal CTD forms an extended, monomeric helical bundle consisting of three anti-parallel segments with structural similarity to membrane-binding bin–amphiphysin–Rvs (BAR) domains. Sequence conservation of the distal CTD identifies putative membrane and protein interaction sites, the latter of which bind the N-terminal helix of Gα_q in both the crystal structure and cryo-EM reconstructions. Functional analysis suggests the distal CTD plays roles in membrane targeting and in optimizing the orientation of the catalytic core at the membrane for maximal rates of lipid hydrolysis.

Duplication of 20p12.3 associated with familial Wolff-Parkinson-White syndrome

Wolff-Parkinson-White (WPW) syndrome is caused by preexcitation of the ventricular myocardium via an accessory pathway which increases the risk for paroxysmal supraventricular tachycardia. The condition is often sporadic and of unknown etiology in the majority of cases. Autosomal dominant inheritance and association with congenital heart defects or ventricular hypertrophy were described. Microdeletions of 20p12.3 have been associated with WPW syndrome with either cognitive dysfunction or Alagille syndrome. Here, we describe the association of 20p12.3 duplication with WPW syndrome in a patient who presented with non-immune hydrops. Her paternal uncle carries the duplication and has attention-deficit hyperactivity disorder and electrocardiographic findings consistent with WPW. The 769 kb duplication was detected by the Affymetrix Whole Genome-Human SNP Array 6.0 and encompasses two genes and the first two exons of a third gene. We discuss the potential role of the genes in the duplicated region in the pathogenesis of WPW and possible neurobehavioral abnormalities. Our data provide additional support for a significant role of 20p12.3 chromosomal rearrangements in the etiology of WPW syndrome.

An introduction to murine models of atrial fibrillation

Understanding the mechanism of re-entrant arrhythmias in the past 30 years has allowed the development of almost curative therapies for many rhythm disturbances. The complex, polymorphic arrhythmias of atrial fibrillation (AF) and sudden death are, unfortunately, not yet well understood, and hence still in need of adequate therapy. AF contributes markedly to morbidity and mortality in aging Western populations. In the past decade, many genetically altered murine models have been described and characterized. Here, we review genetically altered murine models of AF; powerful tools that will enable a better understanding of the mechanisms of AF and the assessment of novel therapeutic interventions.

Inositol 1,4,5-trisphosphate receptors and pacemaker rhythms

Intracellular Ca(2+) plays an important role in the control of the heart rate through the interaction between Ca(2+) release by ryanodine receptors in the sarcoplasmic reticulum (SR) and the extrusion of Ca(2+) by the sodium-calcium exchanger which generates an inward current. A second type of SR Ca(2+) release channel, the inositol 1,4,5-trisphosphate receptor (IP(3)R), can release Ca(2+) from SR stores in many cell types, including cardiac myocytes. However, it is still uncertain whether IP(3)Rs play any functional role in regulating the heart rate. Accumulated evidence shows that IP(3) and IP(3)R are involved in rhythm control in non-cardiac pacemaker tissues and in the embryonic heart. In this review we focus on intracellular Ca(2+) oscillations generated by Ca(2+) release from IP(3)R that initiates membrane depolarization and provides a common mechanism producing spontaneous activity in a range of cells with pacemaker function. Emerging new evidence also suggests that IP(3)/IP(3)Rs play a functional role in normal and diseased hearts and in cardiac rhythm control. Several membrane currents, including a store-operated Ca(2+) current, might be activated by Ca(2+) release from IP(3)Rs. IP(3)/IP(3)R may thus add another dimension to the complex regulation of heart rate.

Role of phospholipase Cε in physiological phosphoinositide signaling networks

Receptor-initiated phospholipase C activation and generation of IP(3) and DAG are important common triggers for a diversity of signal transduction processes in many cell types. Contributing to this diversity is the existence and differential cellular and subcellular distribution of distinct phospholipase C isoforms with distinct regulatory properties. The recently identified PLCε enzyme is an isoform that is uniquely regulated by multiple upstream signals including ras-family GTP binding proteins as well as heterotrimeric G-proteins. In this review we will consider the well documented biochemical regulation of this isoform in the context of cell and whole animal physiology and in the context of other G protein-regulated PLC isoforms. These studies together reveal a surprisingly wide range of unexpected functions for PLCε in cellular signaling, physiology and disease.

Cardiovascular catecholamine receptors in children: their significance in cardiac disease

Adrenoceptors and dopamine receptors are grouped together under the name 'catecholamine receptors.' Catecholamines and catecholaminergic drugs act on catecholamine receptors located on or near the cardiovascular system. The physiological effects of catecholamine receptor stimulation are only partly understood. The catecholaminergic drugs used in critical care medicine today are not selective, or are, at best, in part selective for the various catecholamine receptor subtypes. Many patients, however, depend on them. A variety of animal models has been developed to unravel catecholamine distribution and function. However, the identification of species heterogeneity makes it imperative to determine catecholamine receptor distribution and function in humans. In addition, age-related alterations in catecholamine receptor distribution and function have been identified in human adults. This might have implications for our understanding of the effect of catecholamines in pediatric patients. This article will focus on the pediatric population and will review currently available in vitro data on the distribution and the function of catecholamine receptors in the cardiovascular system of fetuses and children. Also discussed are relevant young animal models and in vivo hemodynamic effects of cardiotonic drugs acting on the catecholamine receptor in children requiring major cardiac surgery. A better understanding of these topics might provide clues for new, receptor subtype-selective, therapeutic approaches in newborns and children with cardiac disease.