CNP Promotes Antiarrhythmic Effects via Phosphodiesterase 2.

BACKGROUND: Ventricular arrhythmia and sudden cardiac death are the most common lethal complications after myocardial infarction. Antiarrhythmic pharmacotherapy remains a clinical challenge and novel concepts are highly desired. Here, we focus on the cardioprotective CNP (C-type natriuretic peptide) as a novel antiarrhythmic principle. We hypothesize that antiarrhythmic effects of CNP are mediated by PDE2 (phosphodiesterase 2), which has the unique property to be stimulated by cGMP to primarily hydrolyze cAMP. Thus, CNP might promote beneficial effects of PDE2-mediated negative crosstalk between cAMP and cGMP signaling pathways. METHODS: To determine antiarrhythmic effects of cGMP-mediated PDE2 stimulation by CNP, we analyzed arrhythmic events and intracellular trigger mechanisms in mice in vivo, at organ level and in isolated cardiomyocytes as well as in human-induced pluripotent stem cell-derived cardiomyocytes. RESULTS: In ex vivo perfused mouse hearts, CNP abrogated arrhythmia after ischemia/reperfusion injury. Upon high-dose catecholamine injections in mice, PDE2 inhibition prevented the antiarrhythmic effect of CNP. In mouse ventricular cardiomyocytes, CNP blunted the catecholamine-mediated increase in arrhythmogenic events as well as in ICaL, INaL, and Ca2+ spark frequency. Mechanistically, this was driven by reduced cellular cAMP levels and decreased phosphorylation of Ca2+ handling proteins. Key experiments were confirmed in human iPSC-derived cardiomyocytes. Accordingly, the protective CNP effects were reversed by either specific pharmacological PDE2 inhibition or cardiomyocyte-specific PDE2 deletion. CONCLUSIONS: CNP shows strong PDE2-dependent antiarrhythmic effects. Consequently, the CNP-PDE2 axis represents a novel and attractive target for future antiarrhythmic strategies.

Chronic isoprenaline/phenylephrine vs. exclusive isoprenaline stimulation in mice: critical contribution of alpha1-adrenoceptors to early cardiac stress responses.

Hyperactivity of the sympathetic nervous system is a major driver of cardiac remodeling, exerting its effects through both α-, and ß-adrenoceptors (α-, ß-ARs). As the relative contribution of subtype α1-AR to cardiac stress responses remains poorly investigated, we subjected mice to either subcutaneous perfusion with the ß-AR agonist isoprenaline (ISO, 30 mg/kg × day) or to a combination of ISO and the stable α1-AR agonist phenylephrine (ISO/PE, 30 mg/kg × day each). Telemetry analysis revealed similar hemodynamic responses under both ISO and ISO/PE treatment i.e., permanently increased heart rates and only transient decreases in mean blood pressure during the first 24 h. Echocardiography and single cell analysis after 1 week of exposure showed that ISO/PE-, but not ISO-treated animals established α1-AR-mediated inotropic responsiveness to acute adrenergic stimulation. Morphologically, additional PE perfusion limited concentric cardiomyocyte growth and enhanced cardiac collagen deposition during 7 days of treatment. Time-course analysis demonstrated a diverging development in transcriptional patterns at day 4 of treatment i.e., increased expression of selected marker genes Xirp2, Nppa, Tgfb1, Col1a1, Postn under chronic ISO/PE treatment which was either less pronounced or absent in the ISO group. Transcriptome analyses at day 4 via RNA sequencing demonstrated that additional PE treatment caused a marked upregulation of genes allocated to extracellular matrix and fiber organization along with a more pronounced downregulation of genes involved in metabolic processes, muscle adaptation and cardiac electrophysiology. Consistently, transcriptome changes under ISO/PE challenge more effectively recapitulated early transcriptional alterations in pressure overload-induced experimental heart failure and in human hypertrophic cardiomyopathy.

Inflammation leads through PGE/EP3 signaling to HDAC5/MEF2-dependent transcription in cardiac myocytes.

The myocyte enhancer factor 2 (MEF2) regulates transcription in cardiac myocytes and adverse remodeling of adult hearts. Activators of G protein-coupled receptors (GPCRs) have been reported to activate MEF2, but a comprehensive analysis of GPCR activators that regulate MEF2 has to our knowledge not been performed. Here, we tested several GPCR agonists regarding their ability to activate a MEF2 reporter in neonatal rat ventricular myocytes. The inflammatory mediator prostaglandin E2 (PGE2) strongly activated MEF2. Using pharmacological and protein-based inhibitors, we demonstrated that PGE2 regulates MEF2 via the EP3 receptor, the ßγ subunit of Gi/o protein and two concomitantly activated downstream pathways. The first consists of Tiam1, Rac1, and its effector p21-activated kinase 2, the second of protein kinase D. Both pathways converge on and inactivate histone deacetylase 5 (HDAC5) and thereby de-repress MEF2. In vivo, endotoxemia in MEF2-reporter mice induced upregulation of PGE2 and MEF2 activation. Our findings provide an unexpected new link between inflammation and cardiac remodeling by de-repression of MEF2 through HDAC5 inactivation, which has potential implications for new strategies to treat inflammatory cardiomyopathies.

Regulation of heterotrimeric G-protein signaling by NDPK/NME proteins and caveolins: an update.

Heterotrimeric G proteins are pivotal mediators of cellular signal transduction in eukaryotic cells and abnormal G-protein signaling plays an important role in numerous diseases. During the last two decades it has become evident that the activation status of heterotrimeric G proteins is both highly localized and strongly regulated by a number of factors, including a receptor-independent activation pathway of heterotrimeric G proteins that does not involve the classical GDP/GTP exchange and relies on nucleoside diphosphate kinases (NDPKs). NDPKs are NTP/NDP transphosphorylases encoded by the nme/nm23 genes that are involved in a variety of cellular events such as proliferation, migration, and apoptosis. They therefore contribute, for example, to tumor metastasis, angiogenesis, retinopathy, and heart failure. Interestingly, NDPKs are translocated and/or upregulated in human heart failure. Here we describe recent advances in the current understanding of NDPK functions and how they have an impact on local regulation of G-protein signaling.

Atropine augments cardiac contractility by inhibiting cAMP-specific phosphodiesterase type 4.

Atropine is a clinically relevant anticholinergic drug, which blocks inhibitory effects of the parasympathetic neurotransmitter acetylcholine on heart rate leading to tachycardia. However, many cardiac effects of atropine cannot be adequately explained solely by its antagonism at muscarinic receptors. In isolated mouse ventricular cardiomyocytes expressing a Förster resonance energy transfer (FRET)-based cAMP biosensor, we confirmed that atropine inhibited acetylcholine-induced decreases in cAMP. Unexpectedly, even in the absence of acetylcholine, after G-protein inactivation with pertussis toxin or in myocytes from M2- or M1/3-muscarinic receptor knockout mice, atropine increased cAMP levels that were pre-elevated with the ß-adrenergic agonist isoproterenol. Using the FRET approach and in vitro phosphodiesterase (PDE) activity assays, we show that atropine acts as an allosteric PDE type 4 (PDE4) inhibitor. In human atrial myocardium and in both intact wildtype and M2 or M1/3-receptor knockout mouse Langendorff hearts, atropine led to increased contractility and heart rates, respectively. In vivo, the atropine-dependent prolongation of heart rate increase was blunted in PDE4D but not in wildtype or PDE4B knockout mice. We propose that inhibition of PDE4 by atropine accounts, at least in part, for the induction of tachycardia and the arrhythmogenic potency of this drug.

Calcium/Calmodulin-Dependent Protein Kinase II Activity Persists During Chronic ß-Adrenoceptor Blockade in Experimental and Human Heart Failure.

BACKGROUND: Considerable evidence suggests that calcium/calmodulin-dependent protein kinase II (CaMKII) overactivity plays a crucial role in the pathophysiology of heart failure (HF), a condition characterized by excessive ß-adrenoceptor (ß-AR) stimulation. Recent studies indicate a significant cross talk between ß-AR signaling and CaMKII activation presenting CaMKII as a possible downstream mediator of detrimental ß-AR signaling in HF. In this study, we investigated the effect of chronic ß-AR blocker treatment on CaMKII activity in human and experimental HF. METHODS AND RESULTS: Immunoblot analysis of myocardium from end-stage HF patients (n=12) and non-HF subjects undergoing cardiac surgery (n=12) treated with ß-AR blockers revealed no difference in CaMKII activity when compared with non-ß-AR blocker-treated patients. CaMKII activity was judged by analysis of CaMKII expression, autophosphorylation, and oxidation and by investigating the phosphorylation status of CaMKII downstream targets. To further evaluate these findings, CaMKIIδC transgenic mice were treated with the ß1-AR blocker metoprolol (270 mg/kg*d). Metoprolol significantly reduced transgene-associated mortality (n≥29; P<0.001), attenuated the development of cardiac hypertrophy (-14±6% heart weight/tibia length; P<0.05), and strongly reduced ventricular arrhythmias (-70±22% premature ventricular contractions; P<0.05). On a molecular level, metoprolol expectedly decreased protein kinase A-dependent phospholamban and ryanodine receptor 2 phosphorylation (-42±9% for P-phospholamban-S16 and -22±7% for P-ryanodine receptor 2-S2808; P<0.05). However, this was paralled neither by a reduction in CaMKII autophosphorylation, oxidation, and substrate binding nor a change in the phosphorylation of CaMKII downstream target proteins (n≥11). The lack of CaMKII modulation by ß-AR blocker treatment was confirmed in healthy wild-type mice receiving metoprolol. CONCLUSIONS: Chronic ß-AR blocker therapy in patients and in a mouse model of CaMKII-induced HF is not associated with a change in CaMKII activity. Thus, our data suggest that the molecular effects of ß-AR blockers are not based on a modulation of CaMKII. Directly targeting CaMKII may, therefore, further improve HF therapy in addition to ß-AR blockade.

Nucleoside Diphosphate Kinase-C Suppresses cAMP Formation in Human Heart Failure.

BACKGROUND: Chronic heart failure (HF) is associated with altered signal transduction via ß-adrenoceptors and G proteins and with reduced cAMP formation. Nucleoside diphosphate kinases (NDPKs) are enriched at the plasma membrane of patients with end-stage HF, but the functional consequences of this are largely unknown, particularly for NDPK-C. Here, we investigated the potential role of NDPK-C in cardiac cAMP formation and contractility. METHODS: Real-time polymerase chain reaction, (far) Western blot, immunoprecipitation, and immunocytochemistry were used to study the expression, interaction with G proteins, and localization of NDPKs. cAMP levels were determined with immunoassays or fluorescent resonance energy transfer, and contractility was determined in cardiomyocytes (cell shortening) and in vivo (fractional shortening). RESULTS: NDPK-C was essential for the formation of an NDPK-B/G protein complex. Protein and mRNA levels of NDPK-C were upregulated in end-stage human HF, in rats after long-term isoprenaline stimulation through osmotic minipumps, and after incubation of rat neonatal cardiomyocytes with isoprenaline. Isoprenaline also promoted translocation of NDPK-C to the plasma membrane. Overexpression of NDPK-C in cardiomyocytes increased cAMP levels and sensitized cardiomyocytes to isoprenaline-induced augmentation of contractility, whereas NDPK-C knockdown decreased cAMP levels. In vivo, depletion of NDPK-C in zebrafish embryos caused cardiac edema and ventricular dysfunction. NDPK-B knockout mice had unaltered NDPK-C expression but showed contractile dysfunction and exacerbated cardiac remodeling during long-term isoprenaline stimulation. In human end-stage HF, the complex formation between NDPK-C and Gαi2 was increased whereas the NDPK-C/Gαs interaction was decreased, producing a switch that may contribute to an NDPK-C-dependent cAMP reduction in HF. CONCLUSIONS: Our findings identify NDPK-C as an essential requirement for both the interaction between NDPK isoforms and between NDPK isoforms and G proteins. NDPK-C is a novel critical regulator of ß-adrenoceptor/cAMP signaling and cardiac contractility. By switching from Gαs to Gαi2 activation, NDPK-C may contribute to lower cAMP levels and the related contractile dysfunction in HF.

Phosphodiesterase 2 Protects Against Catecholamine-Induced Arrhythmia and Preserves Contractile Function After Myocardial Infarction.

RATIONALE: Phosphodiesterase 2 is a dual substrate esterase, which has the unique property to be stimulated by cGMP, but primarily hydrolyzes cAMP. Myocardial phosphodiesterase 2 is upregulated in human heart failure, but its role in the heart is unknown. OBJECTIVE: To explore the role of phosphodiesterase 2 in cardiac function, propensity to arrhythmia, and myocardial infarction. METHODS AND RESULTS: Pharmacological inhibition of phosphodiesterase 2 (BAY 60-7550, BAY) led to a significant positive chronotropic effect on top of maximal ß-adrenoceptor activation in healthy mice. Under pathological conditions induced by chronic catecholamine infusions, BAY reversed both the attenuated ß-adrenoceptor-mediated inotropy and chronotropy. Conversely, ECG telemetry in heart-specific phosphodiesterase 2-transgenic (TG) mice showed a marked reduction in resting and in maximal heart rate, whereas cardiac output was completely preserved because of greater cardiac contraction. This well-tolerated phenotype persisted in elderly TG with no indications of cardiac pathology or premature death. During arrhythmia provocation induced by catecholamine injections, TG animals were resistant to triggered ventricular arrhythmias. Accordingly, Ca2+-spark analysis in isolated TG cardiomyocytes revealed remarkably reduced Ca2+ leakage and lower basal phosphorylation levels of Ca2+-cycling proteins including ryanodine receptor type 2. Moreover, TG demonstrated improved cardiac function after myocardial infarction. CONCLUSIONS: Endogenous phosphodiesterase 2 contributes to heart rate regulation. Greater phosphodiesterase 2 abundance protects against arrhythmias and improves contraction force after severe ischemic insult. Activating myocardial phosphodiesterase 2 may, thus, represent a novel intracellular antiadrenergic therapeutic strategy protecting the heart from arrhythmia and contractile dysfunction.

Redox Imaging Using Cardiac Myocyte-Specific Transgenic Biosensor Mice.

RATIONALE: Changes in redox potentials of cardiac myocytes are linked to several cardiovascular diseases. Redox alterations are currently mostly described qualitatively using chemical sensors, which however do not allow quantifying redox potentials, lack specificity, and the possibility to analyze subcellular domains. Recent advances to quantitatively describe defined redox changes include the application of genetically encoded redox biosensors. OBJECTIVE: Establishment of mouse models, which allow the quantification of the glutathione redox potential (EGSH) in the cytoplasm and the mitochondrial matrix of isolated cardiac myocytes and in Langendorff-perfused hearts based on the use of the redox-sensitive green fluorescent protein 2, coupled to the glutaredoxin 1 (Grx1-roGFP2). METHODS AND RESULTS: We generated transgenic mice with cardiac myocyte-restricted expression of Grx1-roGFP2 targeted either to the mitochondrial matrix or to the cytoplasm. The response of the roGFP2 toward H2O2, diamide, and dithiothreitol was titrated and used to determine the EGSH in isolated cardiac myocytes and in Langendorff-perfused hearts. Distinct EGSH were observed in the cytoplasm and the mitochondrial matrix. Stimulation of the cardiac myocytes with isoprenaline, angiotensin II, or exposure to hypoxia/reoxygenation additionally underscored that these compartments responded independently. A compartment-specific response was also observed 3 to 14 days after myocardial infarction. CONCLUSIONS: We introduce redox biosensor mice as a new tool, which allows quantification of defined alterations of EGSH in the cytoplasm and the mitochondrial matrix in cardiac myocytes and can be exploited to answer questions in basic and translational cardiovascular research.

p63RhoGEF regulates auto- and paracrine signaling in cardiac fibroblasts.

Cardiac remodeling, a hallmark of heart disease, is associated with intense auto- and paracrine signaling leading to cardiac fibrosis. We hypothesized that the specific mediator of Gq/11-dependent RhoA activation p63RhoGEF, which is expressed in cardiac fibroblasts, plays a role in the underlying processes. We could show that p63RhoGEF is up-regulated in mouse hearts subjected to transverse aortic constriction (TAC). In an engineered heart muscle model (EHM), p63RhoGEF expression in cardiac fibroblasts increased resting and twitch tensions, and the dominant negative p63ΔN decreased both. In an engineered connective tissue model (ECT), p63RhoGEF increased tissue stiffness and its knockdown as well as p63ΔN reduced stiffness. In 2D cultures of neonatal rat cardiac fibroblasts, p63RhoGEF regulated the angiotensin II (Ang II)-dependent RhoA activation, the activation of the serum response factor, and the expression and secretion of the connective tissue growth factor (CTGF). All these processes were inhibited by the knockdown of p63RhoGEF or by p63ΔN likely based on their negative influence on the actin cytoskeleton. Moreover, we show that p63RhoGEF also regulates CTGF in engineered tissues and correlates with it in the TAC model. Finally, confocal studies revealed a closely related localization of p63RhoGEF and CTGF in the trans-Golgi network.

A systemic Pasteurella multocida toxin aggravates cardiac hypertrophy and fibrosis in mice.

Pasteurella multocida toxin (PMT) persistently activates heterotrimeric G proteins of the Gαq/11 , Gα12/13 and Gαi family without interaction with G protein-coupled receptors (GPCRs). We show that PMT acts on heart tissue in vivo and on cardiomyocytes and cardiac fibroblasts in vitro by deamidation of heterotrimeric G proteins. Increased normalized ventricle weights and fibrosis were detected after intraperitoneal administration of PMT in combination with the GPCR agonist phenylephrine. In neonatal rat cardiomyocytes, PMT stimulated the mitogen-activated protein kinase pathway, which is crucial for the development of cellular hypertrophy. The toxin induced phosphorylation of the canonical phosphorylation sites of the extracellular-regulated kinase 1/2 and, additionally, caused phosphorylation of the recently recognized autophosphorylation site, which appears to be important for the development of cellular hypertrophy. Moreover, PMT stimulated the small GTPases Rac1 and RhoA. Both switch proteins are involved in cardiomyocyte hypertrophy. In addition, PMT stimulated RhoA and Rac1 in neonatal rat cardiac fibroblasts. RhoA and Rac1 have been implicated in the regulation of connective tissue growth factor (CTGF) secretion and expression. Accordingly, we show that PMT treatment increased secretion and expression of CTGF in cardiac fibroblasts. Altogether, the data indicate that PMT is an inducer of pathological remodelling of cardiac cells and identifies the toxin as a promising tool for studying heterotrimeric G protein-dependent signalling in cardiac cells.

Dopamine and lipophilic derivates protect cardiomyocytes against cold preservation injury.

Donor heart allografts are extremely susceptible to prolonged static cold storage. Because donor treatment with low-dose dopamine improves clinical outcome after heart transplantation, we tested the hypothesis that dopamine and its lipophilic derivate, N-octanoyl dopamine (NOD), protect cardiomyocytes from cold storage injury. Neonatal rat cardiomyocytes were treated with dopamine or NOD or left untreated and subsequently subjected to static cold storage (8-12 hours). Dopamine- and NOD-treated cardiomyocytes displayed a better viability compared with untreated cells after hypothermia. In untreated cardiomyocytes, cell damage was reflected by lactate dehydrogenase (LDH) release and a decrease in intracellular ATP. NOD was approximately 20-fold more potent than dopamine. Similarly to cardiomyocytes in vitro, rat hearts perfused with NOD before explantation showed significantly lower LDH release after static cold storage. ATP regeneration and spontaneous contractions after cold storage and rewarming only occurred in treated cardiomyocytes. Hypothermia severely attenuated isoprenaline-induced cAMP formation in control but not in dopamine- or NOD-treated cells. Esterified derivates of NOD with redox potential and lipophilic side chains reduced cell damage during cold storage similarly to NOD. In contrast to dopamine, neither NOD nor its derivates induced a significant ß-adrenoceptor-mediated elevation of cellular cAMP levels. The ß1-adrenoceptor antagonist atenolol and D1/D2 receptor antagonist fluphenazine had no impact on the protective effect of NOD or dopamine. We conclude that dopamine as well as NOD treatment mitigates cold preservation injury to cardiomyocytes. The beneficial effects are independent of ß-adrenoceptor or dopaminergic receptor stimulation but correlate with redox potential and lipophilic properties.

The activation of RhoC in vascular endothelial cells is required for the S1P receptor type 2-induced inhibition of angiogenesis.

Sphingosine-1-phosphate (S1P) is a multifunctional phospholipid inducing a variety of cellular responses in endothelial cells (EC). S1P responses are mediated by five G protein coupled receptors of which three types (S1P1R-S1P3R) have been described to be of importance in vascular endothelial cells (EC). Whereas the S1P1R regulates endothelial barrier function by coupling to Gαi and the monomeric GTPase Rac1, the signaling pathways involved in the S1P-induced regulation of angiogenesis are ill defined. We therefore studied the sprouting of human umbilical vein EC (HUVEC) in vitro and analyzed the activation of the RhoGTPases RhoA and RhoC. Physiological relevant concentrations of S1P (100-300nM) induce a moderate activation of RhoA and RhoC. Inhibition or siRNA-mediated depletion of the S1P2R preferentially decreased the activation of RhoC. Both manipulations caused an increase of sprouting in a spheroid based in vitro sprouting assay. Interestingly, a similar increase in sprouting was detected after effective siRNA-mediated knockdown of RhoC. In contrast, the depletion of RhoA had no influence on sprouting. Furthermore, suppression of the activity of G proteins of the Gα12/13 subfamily by adenoviral overexpression of the regulator of G protein signaling domain of LSC as well as siRNA-mediated knockdown of the Rho specific guanine nucleotide exchange factor leukemia associated RhoGEF (LARG) inhibited the S1P-induced activation of RhoC and concomitantly increased sprouting of HUVEC with similar efficacy. We conclude that the angiogenic sprouting of EC is suppressed via the S1P2R subtype. Thus, the increase in basal sprouting can be attributed to blocking of the inhibitory action of autocrine S1P stimulating the S1P2R. This inhibitory pathway involves the activation of RhoC via Gα12/13 and LARG, while the simultaneously occurring activation of RhoA is apparently dispensable here.

Phosphodiesterase-2 is up-regulated in human failing hearts and blunts ß-adrenergic responses in cardiomyocytes.

OBJECTIVES: This study investigated whether myocardial phosphodiesterase-2 (PDE2) is altered in heart failure (HF) and determined PDE2-mediated effects on beta-adrenergic receptor (ß-AR) signaling in healthy and diseased cardiomyocytes. BACKGROUND: Diminished cyclic adenosine monophosphate (cAMP) and augmented cyclic guanosine monophosphate (cGMP) signaling is characteristic for failing hearts. Among the PDE superfamily, PDE2 has the unique property of being able to be stimulated by cGMP, thus leading to a remarkable increase in cAMP hydrolysis mediating a negative cross talk between cGMP and cAMP signaling. However, the role of PDE2 in HF is poorly understood. METHODS: Immunoblotting, radioenzymatic- and fluorescence resonance energy transfer-based assays, video edge detection, epifluorescence microscopy, and L-type Ca2(+) current measurements were performed in myocardial tissues and/or isolated cardiomyocytes from human and/or experimental HF, respectively. RESULTS: Myocardial PDE2 expression and activity were ~2-fold higher in advanced human HF. Chronic ß-AR stimulation via catecholamine infusions in rats enhanced PDE2 expression ~2-fold and cAMP hydrolytic activity ~4-fold, which correlated with blunted cardiac ß-AR responsiveness. In diseased cardiomyocytes, higher PDE2 activity could be further enhanced by stimulation of cGMP synthesis via nitric oxide donors, whereas specific PDE2 inhibition partially restored ß-AR responsiveness. Accordingly, PDE2 overexpression in healthy cardiomyocytes reduced the rise in cAMP levels and L-type Ca2(+) current amplitude, and abolished the inotropic effect following acute ß-AR stimulation, without affecting basal contractility. Importantly, PDE2-overexpressing cardiomyocytes showed marked protection from norepinephrine-induced hypertrophic responses. CONCLUSIONS: PDE2 is markedly up-regulated in failing hearts and desensitizes against acute ß-AR stimulation. This may constitute an important defense mechanism during cardiac stress, for example, by antagonizing excessive ß-AR drive. Thus, activating myocardial PDE2 may represent a novel intracellular antiadrenergic therapeutic strategy in HF.

Angiopoietin-2 differentially regulates angiogenesis through TIE2 and integrin signaling.

Angiopoietin-2 (ANG-2) is a key regulator of angiogenesis that exerts context-dependent effects on ECs. ANG-2 binds the endothelial-specific receptor tyrosine kinase 2 (TIE2) and acts as a negative regulator of ANG-1/TIE2 signaling during angiogenesis, thereby controlling the responsiveness of ECs to exogenous cytokines. Recent data from tumors indicate that under certain conditions ANG-2 can also promote angiogenesis. However, the molecular mechanisms of dual ANG-2 functions are poorly understood. Here, we identify a model for the opposing roles of ANG-2 in angiogenesis. We found that angiogenesis-activated endothelium harbored a subpopulation of TIE2-negative ECs (TIE2lo). TIE2 expression was downregulated in angiogenic ECs, which abundantly expressed several integrins. ANG-2 bound to these integrins in TIE2lo ECs, subsequently inducing, in a TIE2-independent manner, phosphorylation of the integrin adaptor protein FAK, resulting in RAC1 activation, migration, and sprouting angiogenesis. Correspondingly, in vivo ANG-2 blockade interfered with integrin signaling and inhibited FAK phosphorylation and sprouting angiogenesis of TIE2lo ECs. These data establish a contextual model whereby differential TIE2 and integrin expression, binding, and activation control the role of ANG-2 in angiogenesis. The results of this study have immediate translational implications for the therapeutic exploitation of angiopoietin signaling.

A novel player in cellular hypertrophy: Gißγ/PI3K-dependent activation of the RacGEF TIAM-1 is required for α1-adrenoceptor induced hypertrophy in neonatal rat cardiomyocytes.

Activation of α(1)-adrenoceptors (α(1)-AR) by high catecholamine levels, e.g. in heart failure, is thought to be a driving force of cardiac hypertrophy. In this context several downstream mediators and cascades have been identified to potentially play a role in cardiomyocyte hypertrophy. One of these proteins is the monomeric G protein Rac1. However, until now it is unclear how this essential G protein is activated by α(1)-AR agonists and what are the downstream targets inducing cellular growth. By using protein-based as well as pharmacological inhibitors and the shRNA technique, we demonstrate that in neonatal rat cardiomyocytes (NRCM) Rac1 is activated via a cascade involving the α(1A)-AR subtype, G(i)ßγ, the phosphoinositide-3'-kinase and the guanine nucleotide exchange factor Tiam1. We further demonstrate that this signaling induces an increase in protein synthesis, cell size and atrial natriuretic peptide expression. We identified the p21-activated kinase 2 (PAK2) as a downstream effector of Rac1 and were able to link this cascade to the activation of the pro-hypertrophic kinases ERK1/2 and p90RSK. Our data thus reveal a prominent role of the α(1A)-AR/G(i)ßγ/Tiam1-mediated activation of Rac1 and its effector PAK2 in the induction of hypertrophy in NRCM.

p63RhoGEF--a key mediator of angiotensin II-dependent signaling and processes in vascular smooth muscle cells.

The purpose of our study was to investigate the role of endogenous p63RhoGEF in G(q/11)-dependent RhoA activation and signaling in rat aortic smooth muscle cells (RASMCs). Therefore, we studied the expression and subcellular localization in freshly isolated RASMCs and performed loss of function experiments to analyze its contribution to RhoGTPase activation and functional responses such as proliferation and contraction. By this, we could show that p63RhoGEF is endogenously expressed in RASMCs and acts there as the dominant mediator of the fast angiotensin II (ANG II)-dependent but not of the sphingosine-1-phosphate (S(1)P)-dependent RhoA activation. p63RhoGEF is not an activator of the concomitant Rac1 activation and functions independently of caveolae. The knockdown of endogenous p63RhoGEF significantly reduced the mitogenic response of ANG II, abolished ANG II-induced stress fiber formation and cell elongation in 2-D culture, and impaired the ANG II-driven contraction in a collagen-based 3-D model. In conclusion, our data provide for the first time evidence that p63RhoGEF is an important mediator of ANG II-dependent RhoA activation in RASMCs and therewith a leading actor in the subsequently triggered cellular processes, such as proliferation and contraction.

Galpha q allosterically activates and relieves autoinhibition of p63RhoGEF.

Galpha(q) directly activates p63RhoGEF and closely related catalytic domains found in Trio and Kalirin, thereby linking G(q)-coupled receptors to the activation of RhoA. Although the crystal structure of G alpha(q) in complex with the catalytic domains of p63RhoGEF is available, the molecular mechanism of activation has not yet been defined. In this study, we show that membrane translocation does not appear to play a role in G alpha(q)-mediated activation of p63RhoGEF, as it does in some other RhoGEFs. G alpha(q) instead must act allosterically. We next identify specific structural elements in the PH domain that inhibit basal nucleotide exchange activity, and provide evidence that G alpha(q) overcomes this inhibition by altering the conformation of the alpha 6-alpha N linker that joins the DH and PH domains, a region that forms direct contacts with RhoA. We also identify residues in G alpha(q) that are important for the activation of p63RhoGEF and that contribute to G alpha subfamily selectivity, including a critical residue in the G alpha(q) C-terminal helix, and demonstrate the importance of these residues for RhoA activation in living cells.