RhoA GTPase regulates L-type Ca2+ currents in cardiac myocytes

RhoA as a Signaling Hub Controlling Glucagon Secretion From Pancreatic α-Cells

Glucagon hypersecretion from pancreatic islet α-cells exacerbates hyperglycemia in type 1 diabetes (T1D) and type 2 diabetes. Still, the underlying mechanistic pathways that regulate glucagon secretion remain controversial. Among the three complementary main mechanisms (intrinsic, paracrine, and juxtacrine) proposed to regulate glucagon release from α-cells, juxtacrine interactions are the least studied. It is known that tonic stimulation of α-cell EphA receptors by ephrin-A ligands (EphA forward signaling) inhibits glucagon secretion in mouse and human islets and restores glucose inhibition of glucagon secretion in sorted mouse α-cells, and these effects correlate with increased F-actin density. Here, we elucidate the downstream target of EphA signaling in α-cells. We demonstrate that RhoA, a Rho family GTPase, plays a key role in this pathway. Pharmacological inhibition of RhoA disrupts glucose inhibition of glucagon secretion in islets and decreases cortical F-actin density in dispersed α-cells and α-cells in intact islets. Quantitative FRET biosensor imaging shows that increased RhoA activity follows directly from EphA stimulation. We show that in addition to modulating F-actin density, EphA forward signaling and RhoA activity affect α-cell Ca2+ activity in a novel mechanistic pathway. Finally, we show that stimulating EphA forward signaling restores glucose inhibition of glucagon secretion from human T1D donor islets.

Small G-protein RhoA is a potential inhibitor of cardiac fast sodium current

Small G-proteins of Rho family modulate the activity of several classes of ion channels, including K⁺ channels Kv1.2, Kir2.1, and ERG; Ca²⁺ channels; and epithelial Na⁺ channels. The present study was aimed to check the RhoA potential regulatory effects on Na⁺ current (I_Na) transferred by Na⁺ channel cardiac isoform Na_V1.5 in heterologous expression system and in native rat cardiomyocytes. Whole-cell patch-clamp experiments showed that coexpression of Na_V1.5 with the wild-type RhoA in CHO-K1 cell line caused 2.7-fold decrease of I_Na density with minimal influence on steady-state activation and inactivation. This effect was reproduced by the coexpression with a constitutively active RhoA, but not with a dominant negative RhoA. In isolated ventricular rat cardiomyocytes, a 5-h incubation with the RhoA activator narciclasine (5 × 10^-6 M) reduced the maximal I_Na density by 38.8%. The RhoA-selective inhibitor rhosin (10^-5 M) increased the maximal I_Na density by 25.3%. Experiments with sharp microelectrode recordings in isolated right ventricular wall preparations showed that 5 × 10^-6 M narciclasine induced a significant reduction of action potential upstroke velocity after 2 h of incubation. Thus, RhoA might be considered as a potential negative regulator of sodium channels cardiac isoform Na_V1.5.

Regulation of NaV1.5 Sodium Channels by Small G-Proteins of the Rho Family in a Heterologous Expression System

The effect of small G-proteins of the Rho family on sodium current conducted by cardiac isoform Na_V1.5 of voltage-gated sodium channels was studied in heterologous expression system, CHO-K1 cell line transfected with a plasmid containing the Na_V1.5 gene. The influence of cotransfection with genes of wild-type, constitutively-active, and dominant-negative small G-proteins RhoA, Rac1, and Cdc2 on the parameters of sodium current and its noninactivating component (I_Na,late) was estimated. Among three studied small G-proteins, only RhoA (wild-type and constitutively-active type) strongly affected sodium current reducing its peak amplitude, but not the value of I_Na,late. Cotransfection with wild-type Rac1 resulted in a minor decrease in sodium current. Thus, small G-protein RhoA has potential capability for suppression of sodium current, although physiological relevance of this property has to be verified.

Mechanism of H2S-mediated ROCK inhibition of total flavones of Rhododendra against myocardial ischemia injury

Our previous studies have indicated that pretreatment with total flavones of Rhododendra flower (TFR) may protect against myocardial ischemic injuries in rats and mice. The cystathionine γ-lyase/hydrogen sulfide (CSE/H₂S) pathway have been associated with several cardiovascular diseases, but the effect of TFR on the Rho-associated protein kinase (ROCK) and CSE/H₂S signaling pathways remains unknown. In the present study, the protective effects of TFR as a ROCK inhibitor in a mice model of myocardial infarction induced by isoproterenol (ISO) were investigated, and the hearts from the wild type and CSE knockout (KO) mice were examined. It was identified that the CSE KO mice exhibited decreased levels of ST segment elevation following anoxia/reoxygenation damage, increased LDH and CK-MB levels, aggravated pathological damage, and increased ROCK1, ROCK2 and MLC1 protein levels. In the CSE KO mice, there were no marked changes of the above experimental results between the TFR group and the model group. These results suggested that TFR-based inhibition of the RhoA/ROCK signal pathway may be mediated by the CSE-H₂S signalling pathway and may be a novel therapeutic target for myocardial ischemia injury.

Rho kinase inhibitors reduce voltage-dependent Ca2+ channel signaling in aortic and renal microvascular smooth muscle cells

Voltage-dependent L-type Ca²⁺ channels (L-VDCCs) and the RhoA/Rho kinase pathway are two predominant intracellular signaling pathways that regulate renal microvascular reactivity. Traditionally, these two pathways have been thought to act independently; however, recent evidence suggests that these pathways could be convergent. We hypothesized that Rho kinase inhibitors can influence L-VDCC signaling. The effects of Rho kinase inhibitors Y-27632 or RKI-1447 on KCl-induced depolarization or the L-VDCC agonist Bay K8644 were assessed in afferent arterioles using an in vitro blood-perfused rat juxtamedullary nephron preparation. Superfusion of KCl (30-90 mM) led to concentration-dependent vasoconstriction of afferent arterioles. Administration of Y-27632 (1, 5, and 10 µM) or RKI-1447 (0.1, 1, and 10 µM) significantly increased the starting diameter by 16-65%. KCl-induced vasoconstriction was markedly attenuated with 5 and 10 µM Y-27632 and with 10 µM RKI-1447 (P < 0.05 vs. KCl alone). Y-27632 (5 µM) also significantly attenuated Bay K8644-induced vasoconstriction (P < 0.05). Changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i) were estimated by fura-2 fluorescence during KCl-induced depolarization in cultured A7r5 cells and in freshly isolated preglomerular microvascular smooth muscle cells. Administration of 90 mM KCl significantly increased fura-2 fluorescence in both cell types. KCl-mediated elevation of [Ca²⁺]_i in A7r5 cells was suppressed by 1-10 µM Y-27632 (P < 0.05), but 10 µM Y-27632 was required to suppress Ca²⁺ responses in preglomerular microvascular smooth muscle cells. RKI-1447, however, significantly attenuated KCl-mediated elevation of [Ca²⁺]_i. Y-27632 markedly inhibited Bay K8644-induced elevation of [Ca²⁺]_i in both cell types. The results of the present study indicate that the Rho kinase inhibitors Y-27632 and RKI-1447 can partially inhibit L-VDCC function and participate in L-VDCC signaling.

Druggable targets in the Rho pathway and their promise for therapeutic control of blood pressure

The prevalence of high blood pressure (also known as hypertension) has steadily increased over the last few decades. Known as a silent killer, hypertension increases the risk for cardiovascular disease and can lead to stroke, heart attack, kidney failure and associated sequela. While numerous hypertensive therapies are currently available, it is estimated that only half of medicated patients exhibit blood pressure control. This signifies the need for a better understanding of the underlying cause of disease and for more effective therapies. While blood pressure homeostasis is very complex and involves the integrated control of multiple body systems, smooth muscle contractility and arterial resistance are important contributors. Strong evidence from pre-clinical animal models and genome-wide association studies indicate that smooth muscle contraction and BP homeostasis are governed by the small GTPase RhoA and its downstream target, Rho kinase. In this review, we summarize the signaling pathways and regulators that impart tight spatial-temporal control of RhoA activity in smooth muscle cells and discuss current therapeutic strategies to target these RhoA pathway components. We also discuss known allelic variations in the RhoA pathway and consider how these polymorphisms may affect genetic risk for hypertension and its clinical manifestations.

Class II PI3Ks α and β Are Required for Rho-Dependent Uterine Smooth Muscle Contraction and Parturition in Mice

Class II phosphoinositide 3-kinases (PI3Ks), PI3K-C2α and PI3K-C2β, are highly homologous and distinct from class I and class III PI3Ks in catalytic products and domain structures. In contrast to class I and class III PI3Ks, physiological roles of PI3K-C2α and PI3K-C2β are not fully understood. Because we previously demonstrated that PI3K-C2α is involved in vascular smooth muscle contraction, we studied the phenotypes of smooth muscle-specific knockout (KO) mice of PI3K-C2α and PI3K-C2β. The pup numbers born from single PI3K-C2α-KO and single PI3K-C2β-KO mothers were similar to those of control mothers, but those from double KO (DKO) mothers were smaller compared with control mice. However, the number of intrauterine fetuses in pregnant DKO mothers was similar to that in control mice. Both spontaneous and oxytocin-induced contraction of isolated uterine smooth muscle (USM) strips was diminished in DKO mice but not in either of the single KO mice, compared with control mice. Furthermore, contraction of USM of DKO mice was less sensitive to a Rho kinase inhibitor. Mechanistically, the extent of oxytocin-induced myosin light chain phosphorylation was greatly reduced in USM from DKO mice compared with control mice. The oxytocin-induced rise in the intracellular Ca2+ concentration in USM was similar in DKO and control mice. However, Rho activation in the intracellular compartment was substantially attenuated in DKO mice compared with control mice, as evaluated by fluorescence resonance energy transfer imaging technique. These data indicate that both PI3K-C2α and PI3K-C2β are required for normal USM contraction and parturition mainly through their involvement in Rho activation.

RHOA-ROCK signalling is necessary for lateralization and differentiation of the developing sinoatrial node

Aims

RHOA-ROCK signalling regulates cell migration, proliferation, differentiation, and transcription. RHOA is expressed in the developing cardiac conduction system in chicken and mice. In early development, the entire sinus venosus myocardium, including both the transient left-sided and the definitive sinoatrial node (SAN), has pacemaker potential. Later, pacemaker potential is restricted to the right-sided SAN. Disruption of RHOA expression in adult mice causes arrhythmias including bradycardia and atrial fibrillation, the mechanism of which is unknown but presumed to affect the SAN. The aim of this study is to assess the role of RHOA-ROCK signalling in SAN development in the chicken heart.

Methods and results

ROCK signalling was inhibited chemically in embryonic chicken hearts using Y-27632. This prolonged the immature state of the sinus venosus myocardium, evidenced by up-regulation of the transcription factor ISL1, wide distribution of pacemaker potential, and significantly reduced heart rate. Furthermore ROCK inhibition caused aberrant expression of typical SAN genes: ROCK1, ROCK2, SHOX2, TBX3, TBX5, ISL1, HCN4, CX40, CAV3.1, and NKX2.5 and left-right asymmetry genes: PITX2C and NODAL. Anatomical abnormalities in pulmonary vein development were also observed. Patch clamp electrophysiology confirmed the immature phenotype of the SAN cells and a residual left-sided sinus venosus myocardium pacemaker-like potential.

Conclusions

RHOA-ROCK signalling is involved in establishing the right-sided SAN as the definitive pacemaker of the heart and restricts typical pacemaker gene expression to the right side of the sinus venosus myocardium.

Inhibition of mevalonate pathway prevents ischemia-induced cardiac dysfunction in rats via RhoA-independent signaling pathway

AIM

We previously demonstrated that anoxia-mediated Ca²⁺ handling dysfunction could be ameliorated through inhibition of mevalonate pathway via RhoA- and Ras-related mechanisms in H9c2 cells. In this study, we further explored whether inhibition of mevalonate pathway is associated with cardiac remodeling and dysfunction in ischemic cardiomyopathy, and discussed the possible role of Ras, Rac and RhoA in cardiac dysfunction.

METHODS

We investigated the role of mevalonate pathway in cardiac remodeling and cardiomyocyte Ca²⁺ handling proteins expression in a rat model of cardiac dysfunction due to myocardial infarction (MI). After MI, adult male Sprague-Dawley rats were treated with drugs that antagonize key components in mevalonate pathway, including 3-hydroxy-3-methylglutaryl-CoA reductase, farnesyl pyrophosphate synthase, and Rho-kinase for 10 weeks. The protein expression of ryanodine receptor 2 (RyR2), sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) 2a, phospholamban (PLB), phospho-PLB at serine-16 (PSer16-PLB), FKBP12.6, and RhoA as well as RyR2 and FKBP12.6 mRNA levels was evaluated.

RESULTS

Rosuvastatin and alendronate treatment prevented myocardial remodeling, improved cardiac function and reduced infarct size. Furthermore, rosuvastatin and alendronate promoted an increase in the protein expression of SERCA2a and PSer16-PLB/PLB ratio as well as partially restored the RyR2 and FKBP12.6 gene and protein expression. Fasudil failed to exert these beneficial effects.

CONCLUSIONS

These findings indicate that mevalonate pathway inhibition by rosuvastatin and alendronate prevents cardiac remodeling and dysfunction possibly through RhoA-independent mechanisms.

Mechanisms of sphingosine-1-phosphate-mediated vasoconstriction of rat afferent arterioles

AIM

Sphingosine-1-phosphate (S1P) influences resistance vessel function and is implicated in renal pathological processes. Previous studies revealed that S1P evoked potent vasoconstriction of the pre-glomerular microvasculature, but the underlying mechanisms remain incompletely defined. We postulated that S1P-mediated pre-glomerular microvascular vasoconstriction involves activation of voltage-dependent L-type calcium channels (L-VDCC) and the rho/rho kinase pathway.

METHODS

Afferent arteriolar reactivity was assessed in vitro using the blood-perfused rat juxtamedullary nephron preparation, and diameter was measured during exposure to physiological and pharmacological agents.

RESULTS

Exogenous S1P (10^-9 -10^-5 mol L^-1 ) evoked concentration-dependent vasoconstriction of afferent arterioles. Superfusion with nifedipine, a L-VDCC blocker, increased arteriolar diameter by 39 ± 18% of baseline and significantly attenuated the S1P-induced vasoconstriction. Superfusion with the rho kinase inhibitor, Y-27632, increased diameter by 60 ± 12% of baseline and also significantly blunted vasoconstriction by S1P. Combined nifedipine and Y-27632 treatment significantly inhibited S1P-induced vasoconstriction over the entire concentration range tested. In contrast, depletion of intracellular Ca²⁺ stores with the Ca²⁺ -ATPase inhibitors, thapsigargin or cyclopiazonic acid, did not alter the S1P-mediated vasoconstrictor profile. Scavenging reactive oxygen species (ROS) or inhibition of nicotinamide adenine dinucleotide phosphate oxidase activity significantly attenuated S1P-mediated vasoconstriction.

CONCLUSION

Exogenous S1P elicits potent vasoconstriction of rat afferent arterioles. These data also demonstrate that S1P-mediated pre-glomerular vasoconstriction involves activation of L-VDCC, the rho/rho kinase pathway and ROS. Mobilization of Ca²⁺ from intracellular stores is not required for S1P-mediated vasoconstriction. These studies reveal a potential role for S1P in the modulation of renal microvascular tone.

RhoA signaling and blood pressure: The consequence of failing to “Tone it Down”

Uncontrolled high blood pressure is a major risk factor for heart attack, stroke, and kidney failure and contributes to an estimated 25% of deaths worldwide. Despite numerous treatment options, estimates project that reasonable blood pressure (BP) control is achieved in only about half of hypertensive patients. Improvements in the detection and management of hypertension will undoubtedly be accomplished through a better understanding of the complex etiology of this disease and a more comprehensive inventory of the genes and genetic variants that influence BP regulation. Recent studies (primarily in pre-clinical models) indicate that the small GTPase RhoA and its downstream target, Rho kinase, play an important role in regulating BP homeostasis. Herein, we summarize the underlying mechanisms and highlight signaling pathways and regulators that impart tight spatial-temporal control of RhoA activity. We also discuss known allelic variations in the RhoA pathway and consider how these polymorphisms may affect genetic risk for hypertension and its clinical manifestations. Finally, we summarize the current (albeit limited) clinical data on the efficacy of targeting the RhoA pathway in hypertensive patients.

Regulation of neuronal high-voltage activated Ca(V)2 Ca(2+) channels by the small GTPase RhoA

High-Voltage-Activated (HVA) Ca(2+) channels are known regulators of synapse formation and transmission and play fundamental roles in neuronal pathophysiology. Small GTPases of Rho and RGK families, via their action on both cytoskeleton and Ca(2+) channels are key molecules for these processes. While the effects of RGK GTPases on neuronal HVA Ca(2+) channels have been widely studied, the effects of RhoA on the HVA channels remains however elusive. Using heterologous expression in Xenopus laevis oocytes, we show that RhoA activity reduces Ba(2+) currents through CaV2.1, CaV2.2 and CaV2.3 Ca(2+) channels independently of CaVβ subunit. This inhibition occurs independently of RGKs activity and without modification of biophysical properties and global level of expression of the channel subunit. Instead, we observed a marked decrease in the number of active channels at the plasma membrane. Pharmacological and expression studies suggest that channel expression at the plasma membrane is impaired via a ROCK-sensitive pathway. Expression of constitutively active RhoA in primary culture of spinal motoneurons also drastically reduced HVA Ca(2+) current amplitude. Altogether our data revealed that HVA Ca(2+) channels regulation by RhoA might govern synaptic transmission during development and potentially contribute to pathophysiological processes when axon regeneration and growth cone kinetics are impaired.

Protective Effect and Mechanism of Total Flavones from Rhododendron simsii Planch Flower on Cultured Rat Cardiomyocytes with Anoxia and Reoxygenation

Many flavonoids have cardioprotection against myocardial ischemia/reperfusion (I/R) injury. Total flavones from Rhododendron simsii Planch flower (TFR) can protect myocardial ischemic injuries. However, its protective mechanism is still unknown. The present study was designed to investigate the mechanism of TFR on myocardial I/R and anoxia/reoxygenation (A/R) injuries. Rat model of myocardial I/R injury was made, and myocardial infarction was determined. A/R injury was induced in cultured rat cardiomyocytes; cellular damage was evaluated by measuring cell viability, LDH and cTnT releases, and MDA content. Expressions of ROCK₁ and ROCK₂ protein were examined by Western blot analysis, and K⁺ currents were recorded by using whole-cell patch clamp technique. TFR 20~80 mg/kg markedly reduced I/R-induced myocardial infarction. TFR 3.7~300 mg/L significantly inhibited A/R-induced reduction of cell viability, LDH and cTnT releases, and MDA production. Exposure to A/R significantly increased ROCK₁ and ROCK₂ expressions in rat cardiomyocytes, but TFR 33.3~300 mg/L obviously inhibited this increase. 300 mg/L TFR significantly augmented inward rectifier K⁺ current and other K⁺ currents in rat cardiomyocytes. These results indicate that TFR has a protective effect on rat cardiomyocytes A/R damage, and the protective mechanism may be engaged with the inhibition of ROCK₁ and ROCK₂ and activation of K⁺ channels.

Mechanosensing and Regulation of Cardiac Function

The role of mechanical force as an important regulator of structure and function of mammalian cells, tissues, and organs has recently been recognized. However, mechanical overload is a pathogenesis or comorbidity existing in a variety of heart diseases, such as hypertension, aortic regurgitation and myocardial infarction. Physical stimuli sensed by cells are transmitted through intracellular signal transduction pathways resulting in altered physiological responses or pathological conditions. Emerging evidence from experimental studies indicate that β1-integrin and the angiotensin II type I (AT1) receptor play critical roles as mechanosensors in the regulation of heart contraction, growth and leading to heart failure. Integrin link the extracellular matrix and the intracellular cytoskeleton to initiate the mechanical signalling, whereas, the AT1 receptor could be activated by mechanical stress through an angiotensin-II-independent mechanism. Recent studies show that both Integrin and AT1 receptor and their downstream signalling factors including MAPKs, AKT, FAK, ILK and GTPase regulate heart function in cardiac myocytes. In this review we describe the role of mechanical sensors residing within the plasma membrane, mechanical sensor induced downstream signalling factors and its potential roles in cardiac contraction and growth.

The small GTPase Rac1 is required for smooth muscle contraction

The role of the small GTP-binding protein Rac1 in smooth muscle contraction was examined using small molecule inhibitors (EHT1864, NSC23766) and a novel smooth muscle-specific, conditional, Rac1 knockout mouse strain. EHT1864, which affects nucleotide binding and inhibits Rac1 activity, concentration-dependently inhibited the contractile responses induced by several different modes of activation (high-K+, phenylephrine, carbachol and protein kinase C activation by phorbol-12,13-dibutyrate) in several different visceral (urinary bladder, ileum) and vascular (mesenteric artery, saphenous artery, aorta) smooth muscle tissues. This contractile inhibition was associated with inhibition of the Ca2+ transient. Knockout of Rac1 (with a 50% loss of Rac1 protein) lowered active stress in the urinary bladder and the saphenous artery consistent with a role of Rac1 in facilitating smooth muscle contraction. NSC23766, which blocks interaction between Rac1 and some guanine nucleotide exchange factors, specifically inhibited the α1 receptor responses (phenylephrine) in vascular tissues and potentiated prostaglandin F2α and thromboxane (U46619) receptor responses. The latter potentiating effect occurred at lowered intracellular [Ca2+]. These results show that Rac1 activity is required for active contraction in smooth muscle, probably via enabling an adequate Ca2+ transient. At the same time, specific agonists recruit Rac1 signalling via upstream modulators, resulting in either a potentiation of contraction via Ca2+ mobilization (α1 receptor stimulation) or an attenuated contraction via inhibition of Ca2+ sensitization (prostaglandin and thromboxane receptors).

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.

Kinase-dependent activation of voltage-gated Ca2+ channels by ET-1 in pulmonary arterial myocytes during chronic hypoxia

Exposure to chronic hypoxia (CH) causes pulmonary hypertension. The vasoconstrictor endothelin-1 (ET-1) is thought to play a role in the development of hypoxic pulmonary hypertension. In pulmonary arterial smooth muscle cells (PASMCs) from chronically hypoxic rats, ET-1 signaling is altered, with the ET-1-induced change in intracellular calcium concentration (Δ[Ca(2+)](i)) occurring through activation of voltage-dependent Ca(2+) channels (VDCC) even though ET-1-induced depolarization via inhibition of K(+) channels is lost. The mechanism underlying this response is unclear. We hypothesized that activation of VDCCs by ET-1 following CH might be mediated by protein kinase C (PKC) and/or Rho kinase, both of which have been shown to phosphorylate and activate VDCCs. To test this hypothesis, we examined the effects of PKC and Rho kinase inhibitors on the ET-1-induced Δ[Ca(2+)](i) in PASMCs from rats exposed to CH (10% O(2), 3 wk) using the Ca(2+)-sensitive dye fura 2-AM and fluorescent microscopy techniques. We found that staurosporine and GF109203X, inhibitors of PKC, and Y-27632 and HA 1077, Rho kinase inhibitors, reduced the ET-1-induced Δ[Ca(2+)](i) by >70%. Inhibition of tyrosine kinases (TKs) with genistein or tyrphostin A23, or combined inhibition of PKC, TKs, and Rho kinase, reduced the Δ[Ca(2+)](i) to a similar extent as inhibition of either PKC or Rho kinase alone. The ability of PKC or Rho kinase to activate VDCCs in our cells was verified using phorbol 12-myristate 13-acetate and GTP-γ-S. These results suggest that following CH, the ET-1-induced Δ[Ca(2+)](i) in PASMCs occurs via Ca(2+) influx through VDCCs mediated primarily by PKC, TKs, and Rho kinase.

Fasudil hydrochloride hydrate, a Rho-kinase inhibitor, suppresses isoproterenol-induced heart failure in rats via JNK and ERK1/2 pathways

The Rho-kinase (ROCK) plays an important role in the pathogenesis of heart injury. Recent cellular and molecular biology studies indicated a pivotal role of the RhoA/ROCK cascade in many aspects of cardiovascular function such as heart failure, cardiac hypertrophy, and ventricular remodeling following myocardial infarction. However, the signal transduction of RhoA/ROCK and its down-stream signaling pathways remains elusive, and the mechanism of ROCK-mediated isoproterenol (ISO)-induced heart failure is still not thoroughly understood. In the present study, we investigated the effect of the ROCK inhibitor, fasudil hydrochloride hydrate, on ISO-induced heart failure and the potential relationship of RhoA/ROCK to the extracellular signal-regulated kinases (ERK) and the c-jun NH 2-terminal kinase (JNK) pathways. Male Sprague-Dawley (SD) rats, maintained on a normal diet, were randomly divided into four groups given control, ISO alone, ISO with low-dose fasudil, or ISO with high-dose fasudil treatments. Fasudil effectively inhibited ISO-induced heart failure, as evaluated by biometric, hemodynamic, and histological examinations. Consistently, ISO-induced ROCK-1 mRNA expression and myosin phosphatase target subunit-1 (MYPT-1) phosphorylation were markedly suppressed by fasudil. In addition, fasudil significantly decreased ISO-induced JNK activation, ERK translocation to the nucleus and subsequent c-fos, c-jun expression and upregulated c-FLIP(L) expression. Taken together, these results indicate that the RhoA/ROCK pathway is essential for ISO induced heart failure, which can be effectively suppressed by fasudil.

RhoA inactivation prevents photoreceptor axon retraction in an in vitro model of acute retinal detachment

PURPOSE

An early injury response to retinal detachment is disruption of synaptic connectivity between photoreceptors and second-order neurons. Most dramatic is the retraction of rod cell axons and their terminals away from the outer synaptic layer and toward their cell bodies. This study tested whether axonal retraction in detached retina was due to the activation of the small GTPase RhoA and was preventable using RhoA antagonists.

METHODS

Retinal detachments were created in in vitro preparations of porcine eyecups. RhoA activation was determined with a Rhotekin binding assay. To block axon retraction, drugs were applied to neural retinal explants either before or after detachment from the retinal pigment epithelium. Presynaptic movement was quantified by image analysis of double-labeled retinas examined with confocal microscopy.

RESULTS

Active RhoA increases transiently after detachment followed by morphologic evidence of axonal retraction over the next 24 hours. Pretreating the retina with a RhoA antagonist, CT-04, or a Rho kinase inhibitor, Y27632, at multiple concentrations significantly inhibited axonal retraction. Reducing calcium influx through L-type calcium channels with nicardipine also blocked retraction. To create a more plausible therapeutic scenario, drug treatments were delayed and applied after retinal detachment. The Rho kinase inhibitor, but not nicardipine, significantly blocked rod axonal retraction when applied up to 6 hours after detachment.

CONCLUSIONS

Thus, RhoA and downstream Rho kinase activity constitute part of the mechanism that produces rod axonal retraction in retinal explants. Treatments that manipulate RhoA signaling may promote synaptic stability after retinal detachment.

Ephrin-A5 and EphA5 interaction induces synaptogenesis during early hippocampal development

BACKGROUND

Synaptogenesis is a fundamental step in neuronal development. For spiny glutamatergic synapses in hippocampus and cortex, synaptogenesis involves adhesion of pre and postsynaptic membranes, delivery and anchorage of pre and postsynaptic structures including scaffolds such as PSD-95 and NMDA and AMPA receptors, which are glutamate-gated ion channels, as well as the morphological maturation of spines. Although electrical activity-dependent mechanisms are established regulators of these processes, the mechanisms that function during early development, prior to the onset of electrical activity, are unclear. The Eph receptors and ephrins provide cell contact-dependent pathways that regulate axonal and dendritic development. Members of the ephrin-A family are glycosyl-phosphatidylinositol-anchored to the cell surface and activate EphA receptors, which are receptor tyrosine kinases.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that ephrin-A5 interaction with the EphA5 receptor following neuron-neuron contact during early development of hippocampus induces a complex program of synaptogenic events, including expression of functional synaptic NMDA receptor-PSD-95 complexes plus morphological spine maturation and the emergence of electrical activity. The program depends upon voltage-sensitive calcium channel Ca2+ fluxes that activate PKA, CaMKII and PI3 kinase, leading to CREB phosphorylation and a synaptogenic program of gene expression. AMPA receptor subunits, their scaffolds and electrical activity are not induced. Strikingly, in contrast to wild type, stimulation of hippocampal slices from P6 EphA5 receptor functional knockout mice yielded no NMDA receptor currents.

CONCLUSIONS/SIGNIFICANCE

These studies suggest that ephrin-A5 and EphA5 signals play a necessary, activity-independent role in the initiation of the early phases of synaptogenesis. The coordinated expression of the NMDAR and PSD-95 induced by eprhin-A5 interaction with EphA5 receptors may be the developmental switch that induces expression of AMPAR and their interacting proteins and the transition to activity-dependent synaptic regulation.

Activation of PKN mediates survival of cardiac myocytes in the heart during ischemia/reperfusion

RATIONALE

The function of PKN, a stress-activated protein kinase, in the heart is poorly understood.

OBJECTIVE

We investigated the functional role of PKN during myocardial ischemia/reperfusion (I/R).

METHODS AND RESULTS

PKN is phosphorylated at Thr774 in hearts subjected to ischemia and reperfusion. Myocardial infarction/area at risk (MI/AAR) produced by 45 minutes of ischemia and 24 hours of reperfusion was significantly smaller in transgenic mice with cardiac-specific overexpression of constitutively active (CA) PKN (Tg-CAPKN) than in nontransgenic (NTg) mice (15+/-5 versus 38+/-5%, P<0.01). The number of TUNEL-positive nuclei was significantly lower in Tg-CAPKN (0.3+/-0.2 versus 1.0+/-0.2%, P<0.05). Both MI/AAR (63+/-9 versus 45+/-8%, P<0.05) and the number of TUNEL-positive cells (7.9+/-1.0 versus 1.3+/-0.9%, P<0.05) were greater in transgenic mice with cardiac-specific overexpression of dominant negative PKN (Tg-DNPKN) than in NTg mice. Thr774 phosphorylation of PKN was also observed in response to H(2)O(2) in cultured cardiac myocytes. Stimulation of PKN prevented, whereas inhibition of PKN aggravated, cell death induced by H(2)O(2), suggesting that the cell-protective effect of PKN is cell-autonomous in cardiac myocytes. PKN induced phosphorylation of alpha B crystallin and increased cardiac proteasome activity. The infarct reducing effect in Tg-CAPKN mice was partially inhibited by epoxomicin, a proteasome inhibitor.

CONCLUSIONS

PKN is activated by I/R and inhibits apoptosis of cardiac myocytes, thereby protecting the heart from I/R injury. PKN mediates phosphorylation of alpha B crystallin and stimulation of proteasome activity, which, in part, mediates the protective effect of PKN in the heart.

RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.

Ion channel regulation by Ras, Rho, and Rab small GTPases

Regulation of ion channels by heterotrimeric guanosine triphosphatases (GTPases), activated by heptathelical membrane receptors, has been the focus of several recent reviews. In comparison, regulation of ion channels by small monomeric G proteins, activated by cytoplasmic guanine nucleotide exchange factors, has been less well reviewed. Small G proteins, molecular switches that control the activity of cellular and membrane proteins, regulate a wide variety of cell functions. Many upstream regulators and downstream effectors of small G proteins now have been isolated. Their modes of activation and action are understood. Recently, ion channels were recognized as physiologically important effectors of small GTPases. Recent advances in understanding how small G proteins regulate the intracellular trafficking and activity of ion channels are discussed here. We aim to provide critical insight into physiological control of ion channel function and the biological consequences of regulation of these important proteins by small, monomeric G proteins.

Small GTP-binding proteins and their regulators in cardiac hypertrophy

Small GTP-binding proteins (small G proteins) act as GDP-GTP-regulated molecular switches and are activated by guanine nucleotide exchange factors (GEFs) in response to diverse extracellular stimuli. During this last decade, numerous molecular and cellular studies, as well as genetically-modified animal models, have highlighted the role of small G proteins in the regulation of cardiac hypertrophy. The growing interest in small G protein signalling comes from the fact that chronic hypertrophic response is considered maladaptive and predisposes individuals to heart failure. Although some of the hypertrophic signalling pathways involving small G proteins have now been identified, a central question deals with the identity of the GEFs that modulate small G protein activation in the context of cardiac hypertrophy. Here, we discuss the precise regulation of Ras and Rho subfamilies of GTPases by GEFs and other regulatory proteins during cardiac hypertrophy. In addition, we summarize recent published data, mainly those describing the role of small G proteins in the development of myocardial hypertrophy and we further present the importance of their downstream effectors in myocardial remodelling.

Osmotic stress and the cytoskeleton: the R(h)ole of Rho GTPases

Hyperosmotic stress initiates a variety of compensatory and adaptive responses, which either serve to restore near-normal volume or remodel and reinforce the cell structure to withstand the physical challenge. The latter response is brought about by the reorganization of the cytoskeleton; however, the underlying mechanisms are not well understood. Recent research has provided major breakthroughs in our knowledge about the link between message and structure, i.e. between signalling and cytoskeletal remodelling, predominantly in the context of cell migration. The major components of this progress are the in-depth characterization of Rho family small GTPases, master regulators of the cytoskeleton, and the discovery of the actin-related protein 2/3 complex, a signalling-sensitive structural element of the actin polymerization machinery. The primary aim of this review is to find the place of these novel and crucial players in osmotically induced (volume-dependent) remodelling of the cytoskeleton. We aim to address three questions: (1) What are the major structural changes in the cytoskeleton under hyperosmotic conditions? (2) Are the Rho family small GTPases (Rho, Rac and Cdc42) regulated by osmotic stress, and if so, by what mechanisms? (3) Are Rho GTPases involved, as mediators, in major adaptive responses, including cytoskeleton rearrangement, changes in ion transport and genetic reprogramming? Our answers will show how fragmentary our current knowledge is in these areas. Therefore, this overview has been written with the hardly disguised intention that it might foster further research in this field by highlighting some intriguing questions.

The Rac and Rho hall of fame: a decade of hypertrophic signaling hits

Over the last decade, the Rho family GTPases have gained considerable recognition as powerful regulators of actin cytoskeletal organization. As with many high profile signal transducers, these molecules soon attracted the attention of the cardiovascular research community. Shortly thereafter, two prominent members known as RhoA and Rac1 were linked to agonist-induced gene expression and myofilament organization using the isolated cardiomyocyte cell model. Subsequent creation of transgenic mouse lines provided evidence for more complex roles of RhoA and Rac1 signaling. Clues from in vitro and in vivo studies suggest the involvement of numerous downstream targets of RhoA and Rac1 signaling including serum response factor, NF-kappaB, and other transcription factors, myofilament proteins, ion channels, and reactive oxygen species generation. Which of these contribute to the observed phenotypic effects of enhanced RhoA and Rac activation in vivo remain to be determined. Current research efforts with a more translational focus have used statins or Rho kinase blockers to assess RhoA and Rac1 as targets for interventional approaches to blunt hypertrophy or heart failure. Generally, salutary effects on remodeling and ischemic damage are observed, but the broad specificity and multiple cellular targets for these drugs within the myocardium demands caution in interpretation. In this review, we assess the evolution of knowledge related to Rac1 and RhoA in the context of hypertrophy and heart failure and highlight the direction that future exploration will lead.

Regulation of a family of inwardly rectifying potassium channels (Kir2) by the m1 muscarinic receptor and the small GTPase Rho

Inwardly rectifying potassium channels Kir2.1-Kir2.3 are important regulators of membrane potential and, thus, control cellular excitability. However, little is known about the regulation of these channels. Therefore, we studied the mechanisms mediating the regulation of Kir2.1-Kir2.3 by the G-protein-coupled m1 muscarinic receptor using the whole-cell patch-clamp technique and recombinant expression in the tsA201 mammalian cell line. Stimulation of the m1 muscarinic receptor inhibited all subtypes of inward rectifier tested, Kir2.1-Kir2.3. The inhibition of each channel subtype was reversible and was attenuated by the muscarinic receptor antagonist, atropine. The protein kinase C activator phorbol 12-myristate 13-acetate (PMA) mimicked the effects of m1 receptor activation by inhibiting Kir2.1 currents. However, PMA had no effect on Kir2.2 or Kir2.3. Inclusion of 200-microM guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) in the patch pipette solution prevented the effects of m1 muscarinic receptor stimulation on all three of the channel subtypes tested, confirming the mediation of the responses by G-proteins. Cotransfection with the activated mutant of the small GTPase Rho reduced current density, while C3 exoenzyme, a selective inhibitor of Rho, attenuated the m1 muscarinic receptor-induced inhibition of Kir2.1-Kir2.3. Also, buffering the intracellular calcium concentration with a high concentration of EGTA abolished the m1 receptor-induced inhibition of Kir2.1-Kir2.3, implicating a role for calcium in these responses. These results indicate that all three of the Kir2 channels are similarly inhibited by m1 muscarinic receptor stimulation through calcium-dependent activation of the small GTPase Rho.