Soluble urokinase plasminogen activator receptor and cardiotoxicity in doxorubicin-treated breast cancer patients: a prospective exploratory study

BACKGROUND

Soluble urokinase plasminogen activator receptor is an inflammatory biomarker that may prognosticate cardiovascular outcomes. We sought to determine the associations between soluble urokinase plasminogen activator receptor and established markers of cardiotoxicity in breast cancer patients receiving doxorubicin.

METHODS

We conducted a prospective cohort study of women with newly diagnosed breast cancer receiving standard-dose doxorubicin (240 mg/m2) at Rush University Medical Center and Rush Oak Park Hospital (Chicago, IL) between January 2017 and May 2019. Left ventricular ejection fraction, global longitudinal strain, and cardiac biomarkers (N-terminal prohormone B-type natriuretic peptide, troponin-I, and high-sensitivity C-reactive protein) were measured at baseline and at intervals up to 12-month follow-up after end of treatment. The associations between soluble urokinase plasminogen activator receptor and these endpoints were evaluated using multivariable mixed effects linear regression.

RESULTS

Our study included 37 women (mean age 47.0 ± 9.3 years, 60% white) with a median baseline soluble urokinase plasminogen activator receptor level of 2.83 ng/dL. No participant developed cardiomyopathy based on serial echocardiography by one-year follow-up. The median percent change in left ventricular strain was -4.3% at 6-month follow-up and absolute changes in cardiac biomarkers were clinically insignificant. There were no significant associations between soluble urokinase plasminogen activator receptor and these markers of cardiotoxicity (all p > 0.05).

CONCLUSIONS

In this breast cancer cohort, doxorubicin treatment was associated with a very low risk for cardiotoxicity. Across this narrow range of clinical endpoints, soluble urokinase plasminogen activator receptor was not associated with markers of subclinical cardiotoxicity. Further studies are needed to clarify the prognostic utility of soluble urokinase plasminogen activator receptor in doxorubicin-associated cardiomyopathy and should include a larger cohort of leukemia and lymphoma patients who receive higher doses of doxorubicin.

The role of P21-activated kinase (Pak1) in sinus node function

Sinoatrial node (SAN) dysfunction (SND) and atrial arrhythmia frequently occur simultaneously with a hazard ratio of 4.2 for new onset atrial fibrillation (AF) in SND patients. In the atrial muscle attenuated activity of p21-activated kinase 1 (Pak1) increases the risk for AF by enhancing NADPH oxidase 2 dependent production of reactive oxygen species (ROS). However, the role of Pak1 dependent ROS regulation in SAN function has not yet been determined. We hypothesize that Pak1 activity maintains SAN activity by regulating the expression of the hyperpolarization activated cyclic nucleotide gated cation channel (HCN). To determine Pak1 dependent changes in heart rate (HR) regulation we quantified the intrinsic sinus rhythm in wild type (WT) and Pak1 deficient (Pak1-/-) mice of both sexes in vivo and in isolated Langendorff perfused hearts. Pak1-/- hearts displayed an attenuated HR in vivo after autonomic blockage and in isolated hearts. The contribution of the Ca2+ clock to pacemaker activity remained unchanged, but Ivabradine (3 μM), a blocker of HCN channels that are a membrane clock component, eliminated the differences in SAN activity between WT and Pak1-/- hearts. Reduced HCN4 expression was confirmed in Pak1-/- right atria. The reduced HCN activity in Pak1-/- could be rescued by class II HDAC inhibition (LMK235), ROS scavenging (TEMPOL) or attenuation of Extracellular Signal-Regulated Kinase (ERK) 1/2 activity (SCH772984). No sex specific differences in Pak1 dependent SAN regulation were determined. Our results establish Pak1 as a class II HDAC regulator and a potential therapeutic target to attenuate SAN bradycardia and AF susceptibility.

The 'Reverse FDUF' Mechanism of Atrial Excitation-Contraction Coupling Sustains Calcium Alternans-A Hypothesis

Cardiac calcium alternans is defined as beat-to-beat alternations of Ca transient (CaT) amplitude and has been linked to cardiac arrhythmia, including atrial fibrillation. We investigated the mechanism of atrial alternans in isolated rabbit atrial myocytes using high-resolution line scan confocal Ca imaging. Alternans was induced by increasing the pacing frequency until stable alternans was observed (1.6-2.5 Hz at room temperature). In atrial myocytes, action potential-induced Ca release is initiated in the cell periphery and subsequently propagates towards the cell center by Ca-induced Ca release (CICR) in a Ca wave-like fashion, driven by the newly identified 'fire-diffuse-uptake-fire' (FDUF) mechanism. The development of CaT alternans was accompanied by characteristic changes of the spatio-temporal organization of the CaT. During the later phase of the CaT, central [Ca]_i exceeded peripheral [Ca]_i that was indicative of a reversal of the subcellular [Ca]_i gradient from centripetal to centrifugal. This gradient reversal resulted in a reversal of CICR propagation, causing a secondary Ca release during the large-amplitude alternans CaT, thereby prolonging the CaT, enhancing Ca-release refractoriness and reducing Ca release on the subsequent beat, thus enhancing the degree of CaT alternans. Here, we propose the 'reverse FDUF' mechanism as a novel cellular mechanism of atrial CaT alternans, which explains how the uncoupling of central from peripheral Ca release leads to the reversal of propagating CICR and to alternans.

Abstract P1094: Sexual Dimorphism In Histone Deacetylases Activity Regulates Ventricular Calcium Handling Properties

Introduction: There is a sexual dimorphism in cardiac excitation contraction coupling (ECC) with female ventricular myocytes (F VM ) exhibiting an attenuated basal calcium concentration ([Ca] i ), Ca transient (CaT) amplitude, and a prolonged CaT decay constant (τ). Increased expression levels of the sodium-calcium exchanger (NCX) have been described in F VM however, the role NCX expression plays in sexual dimorphism of physiological and pathophysiological Ca handling properties, and how NCX expression is regulated in a sex specific manner remains to be determined. Here we tested the hypothesis, that a sexual dimorphism in HDAC activity regulates NCX expression and NCXs differential contribution to the CaT adds to the sexual dimorphism in ventricular ECC.

Method: [Ca] i was quantified in field stimulated, freshly isolated ventricular myocytes (VMs) from male (M VM ) and female (F VM ) FVB/N mice loaded with the Ca sensitive fluorescent dye Fluo-4/AM. Changes in NCX expression were quantified by qPCR and western blotting.

Results: Under basal conditions F VM exhibited an attenuated CaT amplitude (ΔF/F 0 ; M VM :1.8±0.08, F VM :1.4±0.06, p=0.038) and prolonged CaT decay constant (τ: M VM :0.37±0.01ms, F VM :0.46±0.02ms, p<0.01). The caffeine induced Ca transient (CaT caf ) amplitude was comparable in F VM and M VM , but the CaT caf decay constant (τ caf: M VM : 3.34±0.22s ; F VM :4.81±0.36s) was longer in F VM although increased NCX expression was determined on the protein and mRNA level. Consistent with increased NCX expression, F VM showed an attenuated increase in basal [Ca] i (F/F 0 ; M= 60%, F=44%, p=0.0033) and τ (M=0.37±0.01s, F=0.46±0.02s, p<0.0001) when SERCA activity was attenuated (CPA: 10μM). Also during conditions favoring NCX reverse mode ([Na] o =93, 46 mM) F VM exhibited a larger increase in CaT amplitude (ΔF/F 0 ; M=81%, F=133%, p<0.01) and an increased propensity for spontaneous Ca release events. Inhibition of HDAC class II (LMK: 5mg/kg/day) did not change Ca handling properties in F VM but slowed τ (M LMK : 0.44±0.03ms) and τ caf in M VM (M LMK =5.1±0.33s) making them comparable to F VM .

Conclusion: The sexual dimorphism in ventricular ECC is mediated by attenuated HDAC activity in F VM s and facilitates NCX Ca overload.

Abstract P1087: P21-activated Kinase 1 Regulates Intrinsic Pacemaker Activity Through HCN4 Expression

Introduction: Cardiac pacemaker activity generated in the sinoatrial node (SAN) depends on a coupled clock mechanism that consists of a calcium (Ca-) and a voltage clock (V m -clock). Attenuation of either clock results in bradycardia which increases the risk for atrial fibrillation (AF). We have previously demonstrated that p21-activated kinase 1 (Pak1) is a negative regulator of NADPH oxidase 2 dependent production of reactive oxygen species (ROS). Here we hypothesized that attenuated Pak1 activity increases the susceptibility for AF by attenuating SAN function.

Methods: In wild type (WT) and Pak1 knock-out (Pak1 -/- ) mice (FVBN), ECGs and atrial electrograms were recorded in vivo and in isolated Langendorff (LD)-perfused hearts.

Results: Pak1 -/- compared to WT mice exhibited an attenuated intrinsic HR in vivo after autonomic blockage with atropine (1 mg/kg) and propranolol (1 mg/kg) (WT: 399 ± 8.05 bpm, n=16; Pak1 -/- : 365 ± 9.6 bpm, n=13, p<0.05) and in the isolated heart (WT: 343 ± 8.5 bpm, n=24; Pak1 -/- : 280 ± 6.03 bpm, n=28, p<0.01), suggesting attenuated SAN impulse generation. Block of the Ca-clock (cyclopiazonic acid (CPA): 5 μM) did not eliminate the difference in HR. Block of the V m -clock with the hyperpolarization activated cyclic nucleotide gated channel (HCN4) blocker ivabradine (IVA: 3 μM) however, had an attenuated effect on HR in Pak1 -/- (WT IVA : -45 ± 2.5 %, n=6; Pak1 -/- IVA : -22 ± 3.2 %, n=7, p<0.001) and eliminated the difference in SAN frequency (WT IVA : 186 ± 17 bpm, n=6; Pak1 -/- IVA : 234 ± 15 bpm, n=7) supporting a decreased V m -clock function. Attenuated HCN4 expression in Pak1 -/- could be recovered by suppression of Histone deacetylases class II activity (LMK235: 5 mg/kg/day, 3 days); WT LMK : 322 ± 9.4 bpm, n = 5; Pak1 -/- LMK 314 ± 3.4 bpm, n= 5;) as well as by scavenging of ROS (TEMPOL supplemented drinking water, 2 mmol/L, 3 days; WT Tem : 332 ± 12.7 bpm, n = 5; Pak1 -/- Tem : 318 ± 22 bpm, n= 5). Both treatments, restored the contribution of V m -clock to pacemaker activity in Pak1 -/- animals (WT LMK : -38 ± 8.3%, n = 5; Pak1 -/- LMK : -38 ± 1.2%, n= 4; WT Tem : -46 ± 3.7 %, n = 5; Pak1 -/- Tem : -41 ± 2.5 %, n= 5) and attenuated AF inducibility.

Conclusion: In both sexes attenuated Pak1 signaling results in SAN bradycardia by ROS/HDAC dependent suppression of HCN4 expression.

Abstract P1105: Inflammatory Bowel Disease Induced Autonomic Dysregulation Increases The Risk For Atrial Fibrillation

Introduction: Ulcerative colitis causes inflammation of the innermost lining of the colon and rectum. The disease has a relapsing and remitting course and is accompanied by extraintestinal manifestations such as an increased risk for atrial fibrillation (AF). Here we will test the hypothesis that during active colitis, dysregulation of parasympathetic signaling increases the propensity for atrial arrhythmic events.

Methods: Dextran Sulfate Sodium (DSS: 3%, 7days) treated mice (C57BL/6) exhibit disrupted intestinal barrier function that mimics the disease phenotype of active colitis. Changes in atrial electrophysiology was quantified in-vivo (EKG) and in the Langendorff (LD)-perfused heart (atrial electrograms) during peak inflammation (DSS A ) and remission (DSS R ).

Results: During peak inflammation, structural remodeling was reflected in a decreased heart weight to tibia length ratio (Control (CTL A ): 9.9±0.4 mg/mm, n=10; DSS A : 8.8±0.3 mg/mm, n=12; p<0.05) whereas electrophysiological atrial remodeling was supported by a prolonged P-wave duration (CTL A : 25.5±0.6 ms, n=10; DSS A : 31.2±0.6 ms, n=24; p<0.0001) and shortened atrial effective refractory period (CTL A : 27.3±1.7 ms, n=4; DSS A : 19.3±0.6 ms, n=4; p<0.01). In comparison to CTL A , DSS A mice exhibited autonomic dysregulation reflected in an attenuated heart rate (HR) variability and a decreased change in HR in response to the muscarinic receptor blocker atropine (CTL A : 7.6±1.8 %, n=5; DSS A : 2.2±0.7 %, n=7; p<0.01). The decreased vagal tone in DSS A coincided with an exaggerated response to the parasympathomimetic carbachol (CCh, 150ng/g) that resulted in a larger decrease in HR (in vivo: CTL A : -12.3±3.1 %, n=10; DSS A : -46.0±5.6 %, n=10; p<0.0001) and increased number of spontaneous arrhythmic events (CTL A : 1.7±0.6 events/min, n=10; DSS A : 9.8±3.8 events/min, n=10; p<0.05) in vivo and in the LD-configuration. The susceptibility for pacing induced AF trended to be increased in DSS A animals. Structural and electrophysiological changes were reversible upon remission.

Conclusion: Active colitis induces transient structural and electrophysiological remodeling. We propose that dysregulation of cholinergic signaling drives the increased propensity for atrial arrhythmia.

Transthyretin deposition alters cardiomyocyte sarcomeric architecture, calcium transients, and contractile force

Age-related wild-type transthyretin amyloidosis (wtATTR) is characterized by systemic deposition of amyloidogenic fibrils of misfolded transthyretin (TTR) in the connective tissue of many organs. In the heart, this leads to age-related heart failure with preserved ejection fraction (HFpEF). The hypothesis tested is that TTR deposited in vitro disrupts cardiac myocyte cell-to-cell and cell-to-matrix adhesion complexes, resulting in altered calcium handling, force generation, and sarcomeric disorganization. Human iPSC-derived cardiomyocytes and neonatal rat ventricular myocytes (NRVMs), when grown on TTR-coated polymeric substrata mimicking the stiffness of the healthy human myocardium (10 kPa), had decreased contraction and relaxation velocities as well as decreased force production measured using traction force microscopy. Both NRVMs and adult mouse atrial cardiomyocytes had altered calcium kinetics with prolonged transients when cultured on TTR fibril-coated substrates. Furthermore, NRVMs grown on stiff (~GPa), flat or microgrooved substrates coated with TTR fibrils exhibited significantly decreased intercellular electrical coupling as shown by FRAP dynamics of cells loaded with the gap junction-permeable dye calcein-AM, along with decreased gap junction content as determined by quantitative connexin 43 staining. Significant sarcomeric disorganization and loss of sarcomere content, with increased ubiquitin localization to the sarcomere, were seen in NRVMs on various TTR fibril-coated substrata. TTR presence decreased intercellular mechanical junctions as evidenced by quantitative immunofluorescence staining of N-cadherin and vinculin. Current therapies for wtATTR are cost-prohibitive and only slow the disease progression; therefore, better understanding of cardiomyocyte maladaptation induced by TTR amyloid may identify novel therapeutic targets.

Inositol 1,4,5-trisphosphate receptor - reactive oxygen signaling domain regulates excitation-contraction coupling in atrial myocytes

The inositol 1,4,5-trisphosphate receptor (InsP₃R) is up-regulated in patients with atrial fibrillation (AF) and InsP₃-induced Ca²⁺ release (IICR) is linked to pro-arrhythmic spontaneous Ca²⁺ release events. Nevertheless, knowledge of the physiological relevance and regulation of InsP₃Rs in atrial muscle is still limited. We hypothesize that InsP₃R and NADPH oxidase 2 (NOX2) form a functional signaling domain where NOX2 derived reactive oxygen species (ROS) regulate InsP₃R agonist affinity and thereby Ca²⁺ release. To quantitate the contribution of IICR to atrial excitation-contraction coupling (ECC) atrial myocytes (AMs) were isolated from wild type and NOX2 deficient (Nox2^-/-) mice and changes in the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i; fluo-4/AM, indo-1) or ROS (2',7'-dichlorofluorescein, DCF) were monitored by fluorescence microscopy. Superfusion of AMs with Angiotensin II (AngII: 1 μmol/L) significantly increased diastolic [Ca²⁺]_i (F/F₀, Ctrl: 1.00 ± 0.01, AngII: 1.20 ± 0.03; n = 7; p < 0.05), the field stimulation induced Ca²⁺ transient (CaT) amplitude (ΔF/F₀, Ctrl: 2.00 ± 0.17, AngII: 2.39 ± 0.22, n = 7; p < 0.05), and let to the occurrence of spontaneous increases in [Ca²⁺]_i. These changes in [Ca²⁺]_i were suppressed by the InsP₃R blocker 2-aminoethoxydiphenyl-borate (2-APB; 1 μmol/L). Concomitantly, AngII induced an increase in ROS production that was sensitive to the NOX2 specific inhibitor gp91ds-tat (1 μmol/L). In NOX2^-/- AMs, AngII failed to increase diastolic [Ca²⁺]_i, CaT amplitude, and the frequency of spontaneous Ca²⁺ increases. Furthermore, the enhancement of CaTs by exposure to membrane permeant InsP₃ was abolished by NOX inhibition with apocynin (1 μM). AngII induced IICR in Nox2^-/- AMs could be restored by addition of exogenous ROS (tert-butyl hydroperoxide, tBHP: 5 μmol/L). In saponin permeabilized AMs InsP₃ (5 μmol/L) induced Ca²⁺ sparks that increased in frequency in the presence of ROS (InsP₃: 9.65 ± 1.44 sparks*s^-1*(100μm)^-1; InsP₃ + tBHP: 10.77 ± 1.5 sparks*s^-1*(100μm)^-1; n = 5; p < 0.05). The combined effect of InsP₃ + tBHP was entirely suppressed by 2-APB and Xestospongine C (XeC). Changes in IICR due to InsP₃R glutathionylation induced by diamide could be reversed by the reducing agent dithiothreitol (DTT: 1 mmol/L) and prevented by pretreatment with 2-APB, supporting that the ROS-dependent post-translational modification of the InsP₃R plays a role in the regulation of ECC. Our data demonstrate that in AMs the InsP₃R is under dual control of agonist induced InsP₃ and ROS formation and suggest that InsP₃ and NOX2-derived ROS co-regulate atrial IICR and ECC in a defined InsP₃R/NOX2 signaling domain.

Excitation-contraction coupling and calcium release in atrial muscle

In cardiac muscle, the process of excitation-contraction coupling (ECC) describes the chain of events that links action potential induced myocyte membrane depolarization, surface membrane ion channel activation, triggering of Ca²⁺ induced Ca²⁺ release from the sarcoplasmic reticulum (SR) Ca²⁺ store to activation of the contractile machinery that is ultimately responsible for the pump function of the heart. Here we review similarities and differences of structural and functional attributes of ECC between atrial and ventricular tissue. We explore a novel "fire-diffuse-uptake-fire" paradigm of atrial ECC and Ca²⁺ release that assigns a novel role to the SR SERCA pump and involves a concerted "tandem" activation of the ryanodine receptor Ca²⁺ release channel by cytosolic and luminal Ca²⁺. We discuss the contribution of the inositol 1,4,5-trisphosphate (IP₃) receptor Ca²⁺ release channel as an auxiliary pathway to Ca²⁺ signaling, and we review IP₃ receptor-induced Ca²⁺ release involvement in beat-to-beat ECC, nuclear Ca²⁺ signaling, and arrhythmogenesis. Finally, we explore the topic of electromechanical and Ca²⁺ alternans and its ramifications for atrial arrhythmia.

Abstract 410: Transthyretin Amyloid Fibrils Deposited in the Microenvironment Alter Cardiac Myocyte and Fibroblast Function and Cytoskeletal Structure

Age-related wild type transthyretin amyloidosis (ATTRwt) is a systemic condition that is characterized by deposition of amyloidogenic fibrils of misfolded transthyretin (TTR) in various organ systems. This disease manifests as an infiltrative cardiomyopathy hallmarked by extracellular deposition of misfolded TTR fibrils in the cardiac tissue, leading to cardiac dysfunction and heart failure with preserved ejection fraction (HFpEF); this disease is underdiagnosed and may affect 25% of elderly men and women. We hypothesize that deposition of amyloidogenic TTR fibrils alters cardiomyocyte contractility and sarcomere structure via modulation of cell growth due to altered microenvironmental features such as substrate stiffness. Recombinant TTR fibrils were deposited on glass substrates or substrates mimicking the stiffness of the healthy myocardium (10kPa). Neonatal rat ventricular myocytes exhibited decreased rate of linear contractility via line scan kymography and altered cytoskeletal structure when exposed to deposited TTR fibrils, with greater maladaptive effects seen on 10kPa substrates. Isolated adult mouse atrial cardiomyocytes plated on TTR fibril-containing substrates had attenuated calcium transient amplitude and a prolonged decay constant, consistent with lower amplitude. We also hypothesize that deposited TTR fibrils induce cardiac fibroblast activation and promote progression of fibrosis. Isolated cardiac fibroblasts grown on TTR-coated glass showed a reorganization of the F-actin cytoskeleton, with preferential distribution to the cell periphery. Barrier removal tests after confluence resulted in significantly faster migration velocity of fibroblasts cultured on TTR fibrils. Increased reduction of MTS tetrazolium by a colorimetric assay suggests a significant TTR-related increase in the proliferation rate of cells. Together, results suggest that amyloidogenic TTR fibrils may affect cardiac myocyte and fibroblast structure and function through both chemical and mechanical pathways. As recently approved therapies for ATTRwt are cost-prohibitive and are intended to slow disease progression only, better understanding of disease mechanisms may elucidate novel therapeutic targets.

Spatially defined InsP3-mediated signaling in embryonic stem cell-derived cardiomyocytes

The functional role of inositol 1,4,5-trisphosphate (InsP3) signaling in cardiomyocytes is not entirely understood but it was linked to an increased propensity for triggered activity. The aim of this study was to determine how InsP3 receptors can translate Ca(2+) release into a depolarization of the plasma membrane and consequently arrhythmic activity. We used embryonic stem cell-derived cardiomyocytes (ESdCs) as a model system since their spontaneous electrical activity depends on InsP3-mediated Ca(2+) release. [InsP3]i was monitored with the FRET-based InsP3-biosensor FIRE-1 (Fluorescent InsP3 Responsive Element) and heterogeneity in sub-cellular [InsP3]i was achieved by targeted expression of FIRE-1 in the nucleus (FIRE-1nuc) or expression of InsP3 5-phosphatase (m43) localized to the plasma membrane. Spontaneous activity of ESdCs was monitored simultaneously as cytosolic Ca(2+) transients (Fluo-4/AM) and action potentials (current clamp). During diastole, the diastolic depolarization was paralleled by an increase of [Ca(2+)]i and spontaneous activity was modulated by [InsP3]i. A 3.7% and 1.7% increase of FIRE-1 FRET ratio and 3.0 and 1.5 fold increase in beating frequency was recorded upon stimulation with endothelin-1 (ET-1, 100 nmol/L) or phenylephrine (PE, 10 µmol/L), respectively. Buffering of InsP3 by FIRE-1nuc had no effect on the basal frequency while attenuation of InsP3 signaling throughout the cell (FIRE-1), or at the plasma membrane (m43) resulted in a 53.7% and 54.0% decrease in beating frequency. In m43 expressing cells the response to ET-1 was completely suppressed. Ca(2+) released from InsP3Rs is more effective than Ca(2+) released from RyRs to enhance INCX. The results support the hypothesis that in ESdCs InsP3Rs form a functional signaling domain with NCX that translates Ca(2+) release efficiently into a depolarization of the membrane potential.

p21-Activated kinase1 (Pak1) is a negative regulator of NADPH-oxidase 2 in ventricular myocytes

Ischemic conditions reduce the activity of the p21-activated kinase (Pak1) resulting in increased arrhythmic activity. Triggered arrhythmic activity during ischemia is based on changes in cellular ionic balance and the cells Ca(2+) handling properties. In the current study we used isolated mouse ventricular myocytes (VMs) deficient for the expression of Pak1 (Pak1(-/-)) to determine the mechanism by which Pak1 influences the generation of arrhythmic activity during simulated ischemia. The Ca(2+) transient amplitude and kinetics did not significantly change in wild type (WT) and Pak1(-/-) VMs during 15 min of simulated ischemia. However, Pak1(-/-) VMs exhibited an exaggerated increase in [Ca(2+)]i, which resulted in spontaneous Ca(2+) release events and waves. The Ca(2+) overload in Pak1(-/-) VMs could be suppressed with a reverse mode blocker (KB-R7943) of the sodium calcium exchanger (NCX), a cytoplasmic scavenger of reactive oxygen species (ROS; TEMPOL) or a RAC1 inhibitor (NSC23766). Measurements of the cytoplasmic ROS levels revealed that decreased Pak1 activity in Pak1(-/-) VMs or VMs treated with the Pak1 inhibitor (IPA3) enhanced cellular ROS production. The Pak1 dependent increase in ROS was attenuated in VMs deficient for NADPH oxidase 2 (NOX2; p47(phox-/-)) or in VMs where NOX2 was inhibited (gp91ds-tat). Voltage clamp recordings showed increased NCX activity in Pak1(-/-) VMs that depended on enhanced NOX2 induced ROS production. The exaggerated Ca(2+) overload in Pak1(-/-) VMs could be mimicked by low concentrations of ouabain. Overall our data show that Pak1 is a critical negative regulator of NOX2 dependent ROS production and that a latent ROS dependent stimulation of NCX activity can predispose VMs to Ca(2+) overload under conditions where no significant changes in excitation-contraction coupling are yet evident.

The C-terminus of the long AKAP13 isoform (AKAP-Lbc) is critical for development of compensatory cardiac hypertrophy

The objective of this study was to determine the role of A-Kinase Anchoring Protein (AKAP)-Lbc in the development of heart failure, by investigating AKAP-Lbc-protein kinase D1 (PKD1) signaling in vivo in cardiac hypertrophy. Using a gene-trap mouse expressing a truncated version of AKAP-Lbc (due to disruption of the endogenous AKAP-Lbc gene), that abolishes PKD1 interaction with AKAP-Lbc (AKAP-Lbc-ΔPKD), we studied two mouse models of pathological hypertrophy: i) angiotensin (AT-II) and phenylephrine (PE) infusion and ii) transverse aortic constriction (TAC)-induced pressure overload. Our results indicate that AKAP-Lbc-ΔPKD mice exhibit an accelerated progression to cardiac dysfunction in response to AT-II/PE treatment and TAC. AKAP-Lbc-ΔPKD mice display attenuated compensatory cardiac hypertrophy, increased collagen deposition and apoptosis, compared to wild-type (WT) control littermates. Mechanistically, reduced levels of PKD1 activation are observed in AKAP-Lbc-ΔPKD mice compared to WT mice, resulting in diminished phosphorylation of histone deacetylase 5 (HDAC5) and decreased hypertrophic gene expression. This is consistent with a reduced compensatory hypertrophy phenotype leading to progression of heart failure in AKAP-Lbc-ΔPKD mice. Overall, our data demonstrates a critical in vivo role for AKAP-Lbc-PKD1 signaling in the development of compensatory hypertrophy to enhance cardiac performance in response to TAC-induced pressure overload and neurohumoral stimulation by AT-II/PE treatment.

Ischemia/Reperfusion injury protection by mesenchymal stem cell derived antioxidant capacity

Mesenchymal stem cell (MSC) transplantation after ischemia/reperfusion (I/R) injury reduces infarct size and improves cardiac function. We used mouse ventricular myocytes (VMs) in an in vitro model of I/R to determine the mechanism by which MSCs prevent reperfusion injury by paracrine signaling. Exposure of mouse VMs to an ischemic challenge depolarized their mitochondrial membrane potential (Ψmito), increased their diastolic Ca(2+), and significantly attenuated cell shortening. Reperfusion of VMs with Ctrl tyrode or MSC-conditioned tyrode (ConT) resulted in a transient increase of the Ca(2+) transient amplitudes in all cells. ConT-reperfused cells exhibited a decreased number early after depolarization (EADs) (ConT: 6.3% vs. Ctrl: 28.4%) and prolonged survival (ConT: 58% vs. Ctrl: 33%). Ψmito rapidly recovered in Ctrl as well as ConT-treated VMs on reperfusion; however, in Ctrl solution, an exaggerated hyperpolarization of Ψmito was determined that preceded the collapse of Ψmito. The ability of ConT to attenuate the hyperpolarization of Ψmito was suppressed on inhibition of the PI3K/Akt signaling pathway or IK,ATP. However, protection of Ψmito was best mimicked by the reactive oxygen species (ROS) scavenger mitoTEMPO. Analysis of ConT revealed a significant antioxidant capacity that was linked to the presence of extracellular superoxide dismutase (SOD3) in ConT. In conclusion, MSC ConT protects VMs from simulated I/R injury by its SOD3-mediated antioxidant capacity and by delaying the recovery of Ψmito through Akt-mediated opening of IK,ATP. These changes attenuate reperfusion-induced ROS production and prevent the opening of the permeability transition pore and arrhythmic Ca(2+) release.

Functional integrity of the T-tubular system in cardiomyocytes depends on p21-activated kinase 1

p21-activated kinase (Pak1), a serine-threonine protein kinase, regulates cytoskeletal dynamics and cell motility. Recent experiments further demonstrate that loss of Pak1 results in exaggerated hypertrophic growth in response to pathophysiological stimuli. Calcium (Ca) signaling plays an important role in the regulation of transcription factors involved in hypertrophic remodeling. Here we aimed to determine the role of Pak1 in cardiac excitation-contraction coupling (ECC). Ca transients were recorded in isolated, ventricular myocytes (VMs) from WT and Pak1(-/-) mice. Pak1(-/-) Ca transients had a decreased amplitude, prolonged rise time and delayed recovery time. Di-8-ANNEPS staining revealed a decreased T-tubular density in Pak1(-/-) VMs that coincided with decreased cell capacitance and increased dis-synchrony of Ca induced Ca release (CICR) at individual release units. These changes were not observed in atrial myocytes of Pak1(-/-) mice where the T-tubular system is only sparsely developed. Experiments in cultured rabbit VMs supported a role of Pak1 in the maintenance of the T-tubular structure. T-tubular density in rabbit VMs significantly decreased within 24h of culture. This was accompanied by a decrease of the Ca transient amplitude and a prolongation of its rise time. However, overexpression of constitutively active Pak1 in VMs attenuated the structural remodeling as well as changes in ECC. The results provide significant support for a prominent role of Pak1 activity not only in the functional regulation of ECC but for the structural maintenance of the T-tubular system whose remodeling is an integral feature of hypertrophic remodeling.

Mesenchymal stem cells improve cardiac conduction by upregulation of connexin 43 through paracrine signaling

Mesenchymal stem cells (MSCs) were shown to improve cell survival and alleviate cardiac arrhythmias when transplanted into cardiac tissue; however, little is known about the mechanism by which MSCs modify the electrophysiological properties of cardiac tissue. We aimed to distinguish the influence of cell-cell coupling between myocytes and MSCs from that of MSC-derived paracrine factors on the spontaneous activity and conduction velocity (θ) of multicellular cardiomyocyte preparations. HL-1 cells were plated on microelectrode arrays and their spontaneous activity and θ was determined from field potential recordings. In heterocellular cultures of MSCs and HL-1 cells the beating frequency was attenuated (t(0h): 2.26 ± 0.18 Hz; t(4h): 1.98 ± 0.26 Hz; P < 0.01) concomitant to the intercellular coupling between MSCs and cardiomyocytes. In HL-1 monolayers supplemented with MSC conditioned media (ConM) or tyrode (ConT) θ significantly increased in a time-dependent manner (ConT: t(0h): 2.4 cm/s ± 0.2; t(4h): 3.1 ± 0.4 cm/s), whereas the beating frequency remained constant. Connexin (Cx)43 mRNA and protein expression levels also increased after ConM or ConT treatment over the same time period. Enhanced low-density lipoprotein receptor-related protein 6 (LRP6) phosphorylation after ConT treatment implicates the Wnt signaling pathway. Suppression of Wnt secretion from MSCs (IWP-2; 5 μmol/l) reduced the efficacy of ConT to induce phospho-LRP6 and to increase θ. Inhibition of β-catenin (cardamonin; 10 μmol/l) or GSK3-α/β (LiCl; 5 mmol/l) also suppressed changes in θ, further supporting the hypothesis that MSC-mediated Cx43 upregulation occurs in part through secreted Wnt ligands and activation of the canonical Wnt signaling pathway.

Isoform- and tissue-specific regulation of the Ca(2+)-sensitive transcription factor NFAT in cardiac myocytes and heart failure

Nuclear factors of activated T cells (NFATs) are Ca(2+)-sensitive transcription factors that have been implicated in hypertrophy, heart failure (HF), and arrhythmias. Cytosolic NFAT is activated by dephosphorylation by the Ca(2+)-sensitive phosphatase calcineurin, resulting in translocation to the nucleus, which is opposed by kinase activity, rephosphorylation, and nuclear export. Four different NFAT isoforms are expressed in the heart. The activation and regulation of NFAT in adult cardiac myocytes, which may depend on the NFAT isoform and cell type, are not fully understood. This study compared basal localization, import, and export of NFATc1 and NFATc3 in adult atrial and ventricular myocytes to identify isoform- and tissue-specific regulatory mechanisms of NFAT activation under physiological conditions and in HF. NFAT-green fluorescent protein fusion proteins and NFAT immunocytochemistry were used to analyze NFAT regulation in adult cat and rabbit myocytes. NFATc1 displayed basal nuclear localization in atrial and ventricular myocytes, an effect that was attenuated by reducing intracellular Ca(2+) concentration and inhibiting calcineurin, and enhanced by the inhibition of nuclear export. In contrast, NFATc3 was localized to the cytoplasm but could be driven to the nucleus by angiotensin II and endothelin-1 stimulation in atrial, but not ventricular, cells. Inhibition of nuclear export (by leptomycin B) facilitated nuclear localization in both cell types. Ventricular myocytes from HF rabbits showed increased basal nuclear localization of endogenous NFATc3 and reduced responsiveness of NFAT translocation to phenylephrine stimulation. In control myocytes, Ca(2+) overload, leading to spontaneous Ca(2+) waves, induced substantial translocation of NFATc3 to the nucleus. We conclude that the activation of NFAT in adult cardiomyocytes is isoform and tissue specific and is tightly controlled by nuclear export. NFAT is activated in myocytes from HF animals and may be secondary to Ca(2+) overload.

Delayed enrichment of mesenchymal cells promotes cardiac lineage and calcium transient development

Bone marrow-derived mesenchymal stem cells (BM-MSCs) can be induced to differentiate into myogenic cells. Despite their potential, previous studies have not been successful in producing a high percentage of cardiac-like cells with a muscle phenotype. We hypothesized that cardiac lineage development in BM-MSC is related to cell passage, culture milieu, and enrichment for specific cell subtypes before and during differentiation. Our study demonstrated that Lin(-) BM-MSC at an intermediate passage (IP; P8-P12) expressed cardiac troponin T (cTnT) after 21 days in culture. Cardiac TnT expression was similar whether IP cells were differentiated in media containing 5-azacytidine+2% FBS (AZA; 14%) or 2% FBS alone (LS; 12%) and both were significantly higher than AZA+5% FBS. This expression was potentiated by first enriching for CD117/Sca-1 cells followed by differentiation (AZA, 39% and LS, 28%). A second sequential enrichment for the dihydropyridine receptor subunit alpha2delta1 (DHPR-alpha2) resulted in cardiac TnT expressed in 54% of cultured cells compared to 28% of cells after CD117/Sca-1(+) enrichment. Cells enriched for CD117/Sca-1 and subjected to differentiation displayed spontaneous intracellular Ca(2+) transients with an increase in transient frequency and a 60% decrease in the transient duration amplitude between days 14 and 29. In conclusion, IP CD117/Sca-1(+) murine BM-MSCs display robust cardiac muscle lineage development that can be induced independent of AZA but is diminished under higher serum concentrations. Furthermore, temporal changes in calcium kinetics commensurate with increased cTnT expression suggest progressive maturation of a cardiac muscle lineage. Enrichment with CD117/Sca-1 to establish lineage commitment followed by DHPR-alpha2 in lineage developing cells may enhance the therapeutic potential of these cells for transplantation.

Regulation of nuclear factor of activated T cells (NFAT) in vascular endothelial cells

Proteins of the NFAT family (nuclear factor of activated T cells) are Ca(2+)-sensitive transcription factors, which are involved in hypertrophic cardiovascular remodeling. Activation and nuclear translocation is mediated by dephosphorylation by the Ca(2+)-sensitive phosphatase calcineurin (CaN). We identified Ca(2+) signals that induced nuclear translocation of NFAT in cultured calf pulmonary artery endothelial (CPAE) cells using confocal fluorescence microscopy to measure simultaneously [Ca(2+)](i) and subcellular localization of NFAT-GFP (isoforms NFATc1 and NFATc3). The vasoactive agonists ATP (5 microM) or bradykinin (20 microM) in the presence of 2 mM extracellular Ca(2+) induced Ca(2+) release from the endoplasmic reticulum (ER) and activated capacitative Ca(2+) entry (CCE), which caused robust translocation of NFAT to the nucleus. This effect was sensitive to the CaN-inhibitor cyclosporin A (1 microM). Influx of extracellular Ca(2+) via CCE, but not ER Ca(2+) release was identified as the activating Ca(2+) source. NFAT was also activated by Ca(2+) influx induced by cell swelling, reverse mode Na/Ca exchange or ionomycin treatment. NFAT regulation was isoform-specific. Whereas activation of NFATc1-GFP by ATP resulted in persistent nuclear localization, NFATc3-GFP was only transiently imported into the nucleus, followed by rapid export back to the cytoplasm. Inhibition of nuclear kinases, which mediate export of NFAT via phosphorylation, or direct block of nuclear export (Leptomycin B) resulted in stable nuclear localization of NFATc3. These data demonstrate that extracellular Ca(2+) entry mediates NFAT activation. Furthermore, the regulation of nuclear localization of NFAT is isoform-specific and dependent on nuclear export processes.

Decreased intercellular coupling improves the function of cardiac pacemakers derived from mouse embryonic stem cells

The aim of this study was to determine if embryonic stem cell derived cardiomyocyte aggregates (ESdCs) can act as pacemakers in spontaneously active cardiomyocyte preparations when their connexin isoform expression is tuned toward a more sinus nodal phenotype. Using microelectrode array recordings (MEAs), we demonstrate that mouse ESdCs establish electrical coupling with spontaneously active cardiomyocyte preparations (HL-1 monolayer) and obtain pacemaker dominance. WT- and Cx43(-/-)-ESdCs comparably established intercellular coupling with cardiac host tissue (Cx43(-/-): 86% vs. WT: 91%). Although both aggregates had a 100% success rate in pacing quiescent cardiac preparations, Cx43(-/-)-ESdCs had an increased likelihood of gaining pacemaker dominance (Cx43(-/-): 40% vs. WT: 13%) in spontaneously active preparations. No differences in size, beating frequency, V(m), or differentiation were detected between WT- and Cx43(-/-)-ESdCs but the intercellular coupling resistance in Cx43(-/-)-ESdCs was significantly increased (Cx43(-/-): 1.2nS vs. WT: 14.8nS). Lack of Cx43 prolonged the time until Cx43(-/-)-ESdCs established frequency synchronization with the host tissue. It further hampered the excitation spread from the cardiomyocyte preparation into the ESdC. However rectifying excitation spread in these co-cultures could not be unequivocally identified. In summary, ESdCs can function as dominant biological pacemakers and Cx43 expression is not a prerequisite for their electrical integration. Maintenance of pacemaker dominance depends critically on the pacemaker's gap junction expression benefiting those with increased intercellular coupling resistances. Our results provide important insight into the design of biological pacemakers that will benefit the use of cardiomyocytes for cell replacement therapy.

Phosphatidylinositol-bisphosphate regulates intercellular coupling in cardiac myocytes

Changes in the lipid composition of cardiac myocytes have been reported during cardiac hypertrophy, cardiomyopathy, and infarction. Because a recent study indicates a relation between low phosphatidylinositol-bisphosphate (PIP(2)) levels and reduced intercellular coupling, we tested the hypothesis that agonist-induced changes in PIP(2) can result in a reduction of the functional coupling of cardiomyocytes and, consequently, in changes in conduction velocity. Intercellular coupling was measured by Lucifer Yellow dye transfer in cultured neonatal rat cardiomyocytes. Conduction velocity was measured in cardiomyocytes grown on microelectrode arrays. Intercellular coupling was reduced by angiotensin II (43.7 +/- 9.3%, N = 11) and noradrenaline (58.0 +/- 10.7%, N = 11). To test if reduced intercellular coupling after agonist stimulation was caused by PIP(2)-depletion, myocytes were stimulated by angiotensin II (57.3 +/- 5.7%, N = 14) and then allowed to recover in medium with or without wortmannin (an inhibitor of PIP(2) synthesis). Intercellular coupling fully recovered in control medium (102.1 +/- 8.9%, N = 10), whereas no recovery occurred in the presence of wortmannin (69.3 +/- 7.8%, N = 12). Inhibition of PKC, calmodulin, or arachidonic acid production did not affect the response to either angiotensin II or noradrenaline. Furthermore, decreasing or increasing PIP(2) also decreased and increased intercellular coupling, respectively. This supports the role of PIP(2) in the regulation of intercellular coupling. In beating myocytes, conduction velocity was reduced by angiotensin II stimulation, and recovery after wash out was prevented by inhibition of PIP(2) production. Reductions in PIP(2) inhibit intercellular coupling in cardiomyocytes, and stimulation by physiologically relevant agonists reduces intercellular coupling by this mechanism. The reduction in intercellular coupling lowered conduction velocity.

The relevance of non-excitable cells for cardiac pacemaker function

Age-dependent changes in the architecture of the sinus node comprise an increasing ratio between fibroblasts and cardiomyocytes. This change is discussed as a potential mechanism for sinus node disease. The goal of this study was to determine the mechanism through which non-excitable cells influence the spontaneous activity of multicellular cardiomyocyte preparations. Cardiomyocyte monolayers (HL-1 cells) or embryonic stem cell-derived cardiomyocytes were used as two- and three-dimensional cardiac pacemaker models. Spontaneous activity and conduction velocity (theta) were monitored by field potential measurements with microelectrode arrays (MEAs). The influence of fibroblasts (WT-fibs) was determined in heterocellular cultures of different cardiomyocyte and fibroblast ratios. The relevance of heterocellular gap junctional coupling was evaluated by the use of fibroblasts deficient for the expression of Cx43 (Cx43(-/-)-fibs). The beating frequency and of heterocellular cultures depended negatively on the fibroblast concentration. Interspersion of fibroblasts in cardiomyocyte monolayers increased the coefficient of the interbeat interval variability. Whereas Cx43(-/-)-fibs decreased theta significantly less than WT-fibs, their effect on the beating frequency and the beat-to-beat variability seemed largely independent of their ability to establish intercellular coupling. These results suggest that electrically integrated, non-excitable cells modulate the excitability of cardiac pacemaker preparations by two distinct mechanisms, one dependent and the other independent of the heterocellular coupling established. Whereas heterocellular coupling enables the fibroblast to depolarize the cardiomyocytes or to act as a current sink, the mere physical separation of the cardiomyocytes by fibroblasts induces bradycardia through a reduction in frequency entrainment.

Human heart failure is associated with abnormal C-terminal splicing variants in the cardiac sodium channel

Heart failure (HF) is associated with reduced cardiac Na+ channel (SCN5A) current. We hypothesized that abnormal transcriptional regulation of this ion channel during HF could help explain the reduced current. Using human hearts explanted at the transplantation, we have identified 3 human C-terminal SCN5A mRNA splicing variants predicted to result in truncated, nonfunctional channels. As compared with normal hearts, the explanted ventricles showed an upregulation of 2 of the variants and a downregulation of the full-length mRNA transcript such that the E28A transcript represented only 48.5% (P<0.01) of the total SCN5A mRNA. This correlated with a 62.8% (P<0.01) reduction in Na+ channel protein. Lymphoblasts and skeletal muscle expressing SCN5A also showed identical C-terminal splicing variants. Variants showed reduced membrane protein and no functional current. Transfection of truncation variants into a cell line stably transfected with the full-length Na+ channel resulted in dose-dependent reductions in channel mRNA and current. Introduction of a premature truncation in the C-terminal region in a single allele of the mouse SCN5A resulted in embryonic lethality. Embryonic stem cell-derived cardiomyocytes expressing the construct showed reductions in Na+ channel-dependent electrophysiological parameters, suggesting that the presence of truncated Na+ channel mRNA at levels seen in HF is likely to be physiologically significant. In summary, chronic HF was associated with an increase in 2 truncated SCN5A variants and a decrease in the native mRNA. These splice variations may help explain a loss of Na+ channel protein and may contribute to the increased arrhythmic risk in clinical HF.

Cell cycle-dependent calcium oscillations in mouse embryonic stem cells

During cell cycle progression, somatic cells exhibit different patterns of intracellular Ca2+signals during the G0phase, the transition from G1to S, and from G2to M. Because pluripotent embryonic stem (ES) cells progress through cell cycle without the gap phases G1and G2, we aimed to determine whether mouse ES (mES) cells still exhibit characteristic changes of intracellular Ca2+concentration during cell cycle progression. With confocal imaging of the Ca2+-sensitive dye fluo-4 AM, we identified that undifferentiated mES cells exhibit spontaneous Ca2+oscillations. In control cultures where 50.4% of the cells reside in the S phase of the cell cycle, oscillations appeared in 36% of the cells within a colony. Oscillations were not initiated by Ca2+influx but depended on inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+release and the refilling of intracellular stores by a store-operated Ca2+influx (SOC) mechanism. Using cell cycle synchronization, we determined that Ca2+oscillations were confined to the G1/S phase (∼70% oscillating cells vs. G2/M with ∼15% oscillating cells) of the cell cycle. ATP induced Ca2+oscillations, and activation of SOC could be induced in G1/S and G2/M synchronized cells. Intracellular Ca2+stores were not depleted, and all three IP3receptor isoforms were present throughout the cell cycle. Cell cycle analysis after EGTA, BAPTA-AM, 2-aminoethoxydiphenyl borate, thapsigargin, or U-73122 treatment emphasized that IP3-mediated Ca2+release is necessary for cell cycle progression through G1/S. Because the IP3receptor sensitizer thimerosal induced Ca2+oscillations only in G1/S, we propose that changes in IP3receptor sensitivity or basal levels of IP3could be the basis for the G1/S-confined Ca2+oscillations.

Inositol-1,4,5-trisphosphate-mediated spontaneous activity in mouse embryonic stem cell-derived cardiomyocytes

Embryonic stem cell-derived cardiomyocytes (ESdCs) have been proposed as a source for cardiac cell-replacement therapy. The aim of this study was to determine the Ca2+-handling mechanisms that determine the frequency and duration of spontaneous Ca2+ transients in single ESdCs. With laser scanning confocal microscopy using the Ca2+-sensitive dye Fluo-4/AM, we determined that spontaneous Ca2+ transients in ESdCs at the onset of beating (day 9) depend on Ca2+ entry across the plasma membrane (50%) whereas Ca2+-induced Ca2+ release is the major contributor to Ca2+ transients in ESdCs after 16 days (72%). Likewise, Ca2+ extrusion in 9-day-old ESdCs depends on Na+-Ca2+ exchange (50.0+/-8%) whereas Ca2+ reuptake by the sarco(endo)plasmic Ca2+ ATPase (72+/-5%) dominates in further differentiated cells. Spontaneous Ca2+ transients were suppressed by the inositol-1,4,5-trisphosphate (IP3) receptor (IP3R) blocker 2-aminoethoxydiphenyl borate (2-APB) and the phospholipase C blocker U73122 but continued in the presence of caffeine. Stimulation of IP3 production by phenylephrine or endothelin-1 had a positive chronotropic effect that could be reversed by U73122 and 2-APB. The presence of Ca2+-free solution and block of L-type Ca2+ channels by nifedipine also resulted in a cessation of spontaneous activity. Overall, IP3R-mediated Ca2+ release in ESdCs is translated into a depolarization of the plasma membrane and a whole-cell Ca2+ transient is subsequently induced by voltage-dependent Ca2+ influx. Although ryanodine receptor-mediated Ca2+ release amplifies the IP3R-induced trigger for the Ca2+ transients and modulates its frequencies, it is not a prerequisite for spontaneous activity. The results of this study offer important insight into the role of IP3R-mediated Ca2+ release for pacemaker activity in differentiating cardiomyocytes.

A possible mechanism of halocarbon-induced cardiac sensitization arrhythmias

Cardiac sensitization is the term used for malignant ventricular arrhythmias associated with exposure to inhaled halocarbons in the presence of catecholamines. We investigated the electrophysiological changes associated with cardiomyocyte exposure to epinephrine and a halocarbon known to be associated with cardiac sensitization (halon 1301, CF3Br). Cardiomyocytes (CMs) were isolated from neonatal rats and grown on multielectrode arrays (MEAs). Upon exposure to epinephrine, the CM inter-spike interval (ISI) was decreased 14% at 10 microg/L (P<0.05) and 27% at 100 microg/L (P<0.05) as compared to baseline. Halon alone (50 mg/L) mildly prolonged the field potential (FP) duration (7%). CMs exposed to combinations of epinephrine (100 microg/L) and halon (50 mg/L) for 15 min showed a blunted increase in the ISI (35+/-12%) and a 38% decrease in conduction velocity (P<0.05) when compared to epinephrine alone. There was no change in field potential properties, but dephosphorylated connexin 43 (Cx43) was increased 60+/-16% with the combination as compared to epinephrine alone (P<0.05). Treatment with okadaic acid, a phosphatase inhibitor, prevented the Cx43 dephosphorylation and the reduction in conduction velocity upon exposure to halon and epinephrine. Moreover, the electrophysiological changes induced by epinephrine and halon were indistinguishable from those seen with the gap junction inhibitor heptanol. In conclusion, the combination of a halocarbon and epinephrine results in a unique electrophysiological signature including slow conduction that may explain, in part, the basis for cardiac sensitization. The slowing of conduction is most likely related to changes in the phosphorylation state of Cx43.

Estimation of action potential changes from field potential recordings in multicellular mouse cardiac myocyte cultures

BACKGROUND

Extracellular recordings of electrical activity with substrate-integrated microelectrode arrays (MEAs) enable non-invasive long-term monitoring of contracting multicellular cardiac preparations. However, to characterize not only the spread of excitation and the conduction velocity from field potential (FP) recordings, a more rigorous analysis of FPs is necessary. Therefore in this study we aim to characterize intrinsic action potential (AP) parameters by simultaneous recording of APs and FPs.

METHODS

A MEA consisting of 60 substrate-integrated electrodes is used to record the FP-waveform from multicellular preparations of isolated embryonic mouse cardiomyocytes. Simultaneous current clamp recordings in the vicinity of individual microelectrodes and pharmacological interventions allowed us to correlate FP and AP components and their time course.

RESULTS

The experiments revealed a linear relationship between AP rise time and FP rise time as well as a linear relationship between AP duration and FP duration. Furthermore a direct contribution of the voltage dependent Na(+)- and Ca(2+)-current to the FP could be identified.

CONCLUSION

The characterization of the FP allows us for the first time to estimate AP changes and the contribution of individual current components to the AP by the help of non-invasive recording within a multicellular cardiac preparation during long-term culture.

Development of electrical activity in cardiac myocyte aggregates derived from mouse embryonic stem cells

Embryonic stem cells differentiate into cardiac myocytes, repeating in vitro the structural and molecular changes associated with cardiac development. Currently, it is not clear whether the electrophysiological properties of the multicellular cardiac structure follow cardiac maturation as well. In long-term recordings of extracellular field potentials with microelectrode arrays consisting of 60 substrate-integrated electrodes, we examined the electrophysiological properties during the ongoing differentiation process. The beating frequency of the growing preparations increased from 1 to 5 Hz concomitant to a decrease of the action potential duration and action potential rise time. A developmental increase of the conduction velocity could be attributed to an increased expression of connexin43 gap junction channels. Whereas isoprenalin elicited a positive chronotropic response from the first day of spontaneous beating onward, a concentration-dependent negative chronotropic effect of carbachol only developed after approximately 4 days. The in vitro development of the three-dimensional cardiac preparation thus closely follows the development described for the mouse embryonic heart, making it an ideal model to monitor the differentiation of electrical activity in embryonic cardiomyocytes.

Intracellular domains of mouse connexin26 and -30 affect diffusional and electrical properties of gap junction channels

To evaluate the influence of intracellular domains of connexin (Cx) on channel transfer properties, we analyzed mouse connexin (Cx) Cx26 and Cx30, which show the most similar amino acid sequence identities within the family of gap junction proteins. These connexin genes are tightly linked on mouse chromosome 14. Functional studies were performed on transfected HeLa cells stably expressing both mouse connexins. When we examined homotypic intercellular transfer of microinjected neurobiotin and Lucifer yellow, we found that gap junctions in Cx30-transfected cells, in contrast to Cx26 cells, were impermeable to Lucifer yellow. Furthermore, we observed heterotypic transfer of neurobiotin between Cx30-transfectants and HeLa cells expressing mouse Cx30.3, Cx40, Cx43 or Cx45, but not between Cx26 transfectants and HeLa cells of the latter group. The main differences in amino acid sequence between Cx26 and Cx30 are located in the presumptive cytoplasmic loop and C-terminal region of these integral membrane proteins. By exchanging one or both of these domains, using PCR-based mutagenesis, we constructed Cx26/30 chimeric cDNAs, which were also expressed in HeLa cells after transfection. Homotypic intercellular transfer of injected Lucifer yellow was observed exclusively with those chimeric constructs that coded for both cytoplasmic domains of Cx26 in the Cx30 backbone polypeptide chain. In contrast, cells transfected with a construct that coded for the Cx26 backbone with the Cx30 cytoplasmic loop and C-terminal region did not show transfer of Lucifer yellow. Thus, Lucifer yellow transfer can be conferred onto chimeric Cx30 channels by exchanging the cytoplasmic loop and the C-terminal region of these connexins. In turn, the cytoplasmic loop and C-terminal domain of Cx30 prevent Lucifer yellow transfer when swapped with the corresponding domains of Cx26. In chimeric Cx30/Cx26 channels where the cytoplasmic loop and C-terminal domains had been exchanged, the unitary channel conductance was intermediate between those of the parental channels. Moreover, the voltage sensitivity was slightly reduced. This suggests that these cytoplasmic domains interfere directly or indirectly with the diffusivity, the conductance and voltage gating of the channels.

Modulation of intercellular communication by differential regulation and heteromeric mixing of co-expressed connexins

Intercellular communication may be regulated by the differential expression of subunit gap junction proteins (connexins) which form channels with differing gating and permeability properties. Endothelial cells express three different connexins (connexin37, connexin40, and connexin43) in vivo. To study the differential regulation of expression and synthesis of connexin37 and connexin43, we used cultured bovine aortic endothelial cells which contain these two connexins in vitro. RNA blots demonstrated discordant expression of these two connexins during growth to confluency. RNA blots and immunoblots showed that levels of these connexins were modulated by treatment of cultures with transforming growth factor-ss1. To examine the potential ability of these connexins to form heteromeric channels (containing different connexins within the same hemi-channel), we stably transfected connexin43-containing normal rat kidney (NRK) cells with connexin37 or connexin40. In the transfected cells, both connexin proteins were abundantly produced and localized in identical distributions as detected by immunofluorescence. Double whole-cell patch-clamp studies showed that co-expressing cells exhibited unitary channel conductances and gating characteristics that could not be explained by hemi-channels formed of either connexin alone. These observations suggest that these connexins can readily mix with connexin43 to form heteromeric channels and that the intercellular communication between cells is determined not only by the properties of individual connexins, but also by the interactions of those connexins to form heteromeric channels with novel properties. Furthermore, modulation of levels of the co-expressed connexins during cell proliferation or by cytokines may alter the relative abundance of different heteromeric combinations.

The influence of surface charges on the conductance of the human connexin37 gap junction channel

The single-channel conductance of the hCx37 homotypic gap junction channel does not saturate with transjunctional voltages up to +/-75 mV, nor does it depend linearly on the intracellular electrolyte concentration. The average maximum unitary conductances measured in KCl were 175 pS (30 mM), 236 pS (55 mM), 343 pS (110 mM), and 588 pS (270 mM) in the presence of 0.1 mM MgCl(2). The unexpectedly high unitary conductance at low salt concentrations can be explained by fixed charge groups within or near the channel orifice. Fixed cytoplasmic surface charges (3.4 e) positioned adjacent (15 A) to the channel pore adequately model the data (surface charge density of 0.24 e/(nm)(2)). In other experiments, high Mg(2+) reduced the unitary conductance of hCx37 homotypic gap junction channels more than predicted by screening alone, consistent with specific effects of Mg(2+) on the channel.

Voltage gating of Cx43 gap junction channels involves fast and slow current transitions

RIN cells transfected with mouse cDNA coding for connexin43 (Cx43) were used to further examine the electrical properties of single gap junction channels. The experiments involved measuring intercellular currents from cell pairs using dual whole-cell recording with the patch-clamp method. We found that the single-channel currents exhibit two types of transitions and several conductance states. Besides fast transitions between the main open state and the residual state, the channels underwent slow transitions between an open state (i.e. main open state or residual state) and a closed state. The fast transitions lasted less than 2 ms, the slow ones ranged from 3.5 to 145 ms. The incidence of slow transitions increased with increasing transjunctional voltage. These observations are consistent with the notion that Cx43 gap junction channels possess more than one mechanism of voltage gating.

A three-state model for connexin37 gating kinetics

The gating behavior of human connexin 37 (hCx37) is unaffected by the nature of the bathing monovalent (for Na, K, Rb). It is modified by [Mg] in the millimolar range. For fitting the kinetics, we propose a simple extension to three states of the canonical 2-state model of the hemichannel. The extra closed state allows for some immobilization of a hemichannel at high transjunctional voltages. The model is reasonably efficient at fitting data at various voltage protocols. Interpreting the fits of the data at different [Mg] is consistent with a binding site for Mg.

Evidence for heteromeric gap junction channels formed from rat connexin43 and human connexin37

Homomeric gap junction channels are composed solely of one connexin type, whereas heterotypic forms contain two homomeric hemichannels but the six identical connexins of each are different from each other. A heteromeric gap junction channel is one that contains different connexins within either or both hemichannels. The existence of heteromeric forms has been suggested, and many cell types are known to coexpress connexins. To determine if coexpressed connexins would form heteromers, we cotransfected rat connexin43 (rCx43) and human connexin37 (hCx37) into a cell line normally devoid of any connexin expression and used dual whole cell patch clamp to compare the observed gap junction channel activity with that seen in cells transfected only with rCx43 or hCx37. We also cocultured cells transfected with hCx37 or rCx43, in which one population was tagged with a fluorescent marker to monitor heterotypic channel activity. The cotransfected cells possessed channel types unlike the homotypic forms of rCx43 or hCx37 or the heterotypic forms. In addition, the noninstantaneous transjunctional conductance-transjunctional voltage (Gj/Vj) relationship for cotransfected cell pairs showed a large range of variability that was unlike that of the homotypic or heterotypic form. The heterotypic cell pairs displayed asymmetric voltage dependence. The results from the heteromeric cell pairs are inconsistent with summed behavior of two independent homotypic populations or mixed populations of homotypic and heterotypic channels types. The Gj/Vj data imply that the connexin-to-connexin interactions are significantly altered in cotransfected cell pairs relative to the homotypic and heterotypic forms. Heteromeric channels are a population of channels whose characteristics could well impact differently from their homotypic counterparts with regard to multicellular coordinated responses.

Connexin43 gap junctions exhibit asymmetrical gating properties

A communication-deficient cell line (RIN cells, derived from a rat islet tumour), stably transfected with cDNA coding for rat connexin43 (Cx43), was chosen to further assess the mechanism of voltage gating of Cx43 gap junction channels. The experiments were carried out on preformed cell pairs using a dual whole-cell, voltage-clamp method. The junctional current, Ij, revealed a time- and voltage-dependent inactivation at transjunctional voltages Vj>+/-40 mV. When an asymmetrical pulse protocol was used (in cell 1 the holding potential was maintained, in cell 2 it was altered to establish a variable Vj), the channels exhibited an asymmetrical gating behaviour: Vj,0=-73.7 mV and 65.1 mV for negative and positive Vj, respectively (Vj at which Ij is half-maximally inactivated); gj(min)=0.34 and 0.29 (normalized minimal conductance); tau = 350 ms and 80 ms at Vj=100 mV (time constant of Ij inactivation). Hence, these parameters were more sensitive to positive Vj values. When a symmetrical pulse protocol was used (the holding potentials in cell 1 and cell 2 were altered simultaneously in steps of equal amplitude but of opposite polarity), the Vj -dependent asymmetries were absent: Vj,0=-60.5 and 59.5; gj (min)=0.27 and 0.29; tau =64 ms and 47 ms at 100mV. Putative explanations for these observations are discussed. A possibility is that the number of channels alters with the polarity of Vj.

Serum contains a potent factor that decreases beta-adrenergic receptor-stimulated L-type Ca2+ current in cardiac myocytes

L-type Ca2+ current (ICa) was measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp. Potentiation of ICa induced by beta-adrenergic stimulation (isoprenaline 2.10(-7) M) could be completely antagonized by diluted sera (1:100 v/v). Half-maximal inhibition of beta-receptor-stimulated ICa occurred at about 1:1000. Basal ICa was not affected by serum. Atropine in a concentration (10(-6) M) that completely antagonized the anti-adrenergic effect of acetylcholine (ACh, 2.10(-6) M) did not interfere with the effect of serum. In cells dialysed with cyclic adenosine monophosphate (cAMP)-containing (10(-4) M) pipette solution, potentiated ICa was insensitive to both ACh and serum. Preincubation of the myocytes with pertussis toxin almost completely abolished the anti-adrenergic effects of both ACh and serum. The potency of serum was not reduced by dialysis. It is concluded that serum contains a factor which, like ACh, inhibits beta-receptor-stimulated adenylyl cyclase via Gi-protein.

Activation of muscarinic K+ current in guinea-pig atrial myocytes by a serum factor

1. Atrial myocytes obtained by enzymatic perfusion of hearts from adult guinea-pigs and cultured for 0-14 days were studied using the whole-cell voltage-clamp technique. 2. Superfusion of the myocytes with diluted sera (1:100 to 1:10,000) from different species (human, horse, guinea-pig) evoked an inward rectifying K+ current. The voltage-dependent properties of this current were identical to those of the K+ current activated by acetylcholine (IK(ACh)). Current density in the presence of horse serum (1:100) approximately corresponded to the non-desensitizing fraction of IK(ACh) during superfusion with 1-2 x 10(-6) M ACh. 3. During a maximal serum-evoked current, application of ACh (10(-6) M) failed to evoke additional K+ current. After switching superfusion from serum-containing to serum-free solution, the K+ current decayed 1-2 orders of magnitude slower than ACh-activated IK(ACh). During the decay of the serum-evoked current, a proportional increase in responsiveness to ACh was recorded. During submaximal activation of K+ current by serum, a saturating concentration of ACh resulted in a total current that was identical to the current evoked by ACh alone minus the desensitizing component. Thus, activation of K+ current by serum caused desensitization of IK(ACh). From these results it is concluded that sera contain a factor that activates the same population of K+ channels as ACh. 4. Irreversible activation of IK(ACh) by ACh in myocytes dialysed with the GTP-analogue GTP-gamma-S abolished sensitivity to serum and vice versa. 5. The effect of serum was not modified by atropine (10(-6) M) which completely blocked the response to 2 x 10(-6) M ACh. Furthermore, theophylline (1 mM), which completely inhibited IK(ACh) activation by adenosine (100 microM), failed to inhibit the effect of serum. Thus, neither muscarinic nor purinergic (A1) receptors are involved. 6. The peptide somatostatin (10(-6) M) and the alpha 1-agonist phenylephrine (1 microM) which previously have been shown to cause activation of IK(ACh) channels, in the present study failed to evoke any measurable current, which excludes the involvement of the corresponding receptors. 7. Pre-incubation of the cells with pertussis toxin completely abolished IK(ACh) evoked by ACh, adenosine and serum, suggesting that the activating factor, like the classical agonists, causes opening of IK(ACh) channels via a G protein (Gi, GK). 8. The potency of serum to activate IK(ACh) was not reduced by dialysis, suggesting the molecular mass of the unknown factor to be > or = 5 kDa. No activating potency was found in the dialysing solutions.(ABSTRACT TRUNCATED AT 400 WORDS)