Microtubule disruption modulates Ca(2+) signaling in rat cardiac myocytes

Anti-tumor and cardiotoxic effects of microtubule polymerization inhibitors: The mechanisms and management strategies

Microtubule polymerization inhibitors (MPIs) have long been used as anticancer agents because they inhibit mitosis. Microtubules are thought to play an important role in the migration of tumor cells and the formation of tumor blood vessels, and new MPIs are being developed. Many clinical trials of novel MPIs have been conducted in humans, while some clinical studies in dogs have also been reported. More attempts to apply MPIs not only in humans but also in the veterinary field are expected to be made in the future. Meanwhile, MPIs have a risk of cardiotoxicity. In this paper, we review findings on the pharmacological effects and cardiotoxicity of MPIs, as well as the mechanisms of their cardiotoxicity. Cardiotoxicity of MPIs involves not only the direct effects of MPIs on cardiomyocytes but also their effects on vascular function. For example, hypertension induced by impaired vascular function also contributes to the exacerbation of myocardial damage, and blood pressure control may be useful in reducing cardiotoxicity. By combined administration of MPIs and other anticancer agents, MPI efficacy may be enhanced, thereby potentially allowing to keep MPI dosage low. Measurement of myocardial injury markers in blood and echocardiography may be useful for monitoring cardiotoxicity. In particular, two-dimensional speckle tracking may have high sensitivity for the early detection of MPI-induced cardiac dysfunction. The exploration of the potential of new MPIs while understanding their toxicity and how to deal with them will lead to the further development of cancer chemotherapy.

A maladaptive feedback mechanism between the extracellular matrix and cytoskeleton contributes to hypertrophic cardiomyopathy pathophysiology

Hypertrophic cardiomyopathy is an inherited disorder due to mutations in contractile proteins that results in a stiff, hypercontractile myocardium. To understand the role of cardiac stiffness in disease progression, here we create an in vitro model of hypertrophic cardiomyopathy utilizing hydrogel technology. Culturing wild-type cardiac myocytes on hydrogels with a Young's Moduli (stiffness) mimicking hypertrophic cardiomyopathy myocardium is sufficient to induce a hypermetabolic mitochondrial state versus myocytes plated on hydrogels simulating healthy myocardium. Significantly, these data mirror that of myocytes isolated from a murine model of human hypertrophic cardiomyopathy (cTnI-G203S). Conversely, cTnI-G203S myocyte mitochondrial function is completely restored when plated on hydrogels mimicking healthy myocardium. We identify a mechanosensing feedback mechanism between the extracellular matrix and cytoskeletal network that regulates mitochondrial function under healthy conditions, but participates in the progression of hypertrophic cardiomyopathy pathophysiology resulting from sarcomeric gene mutations. Importantly, we pinpoint key 'linker' sites in this schema that may represent potential therapeutic targets.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Elevated myocardial SORBS2 and the underlying implications in left ventricular noncompaction cardiomyopathy

Background

Left ventricular noncompaction cardiomyopathy (LVNC) is a hereditary heart disease characterized by an excessive trabecular meshwork of deep intertrabecular recesses within the ventricular myocardium. The guidelines for management of LVNC patients aim to improve quality of life by preventing cardiac heart failure. However, the mechanism underlying LVNC-associated heart failure remains poorly understood.

Methods

Using protein mass spectrometry analysis, we established that Sorbin And SH3 Domain Containing 2 (SORBS2) is up-regulated in LVNC hearts without changes to structure proteins. We conducted in vivo experiments wherein the heart tissues of wild-type mice were injected with an AAV9 vector to overexpress SORBS2, followed by analysis using echocardiography, T-tubule analysis and Ca²⁺ imaging to identify functional and morphological changes. In addition, we analyzed the function and structure of SORBS2 overexpressing human embryonic stem cell (hESC) derived cardiomyocytes (hESC-CM) via immunoblotting, immunohistochemistry, immunofluorescence, and confocal Ca²⁺ imaging.

Findings

LVNC myocardial tissues feature strongly elevated expression of SORBS2, microtubule densification and redistribution of Junctophilin 2 (JP2). SORBS2 interacts with β-tubulin, promoting its polymerization in 293T cells and hESC-derived CMs. In vivo, cardiac dysfunction, β-tubulin densification, JP2 translocation, T-tubule disorganization and Ca²⁺ handling dysfunction were observed in mice overexpressing SORBS2.

Interpretation

We identified a novel mechanism through which SORBS2 interacts with β-tubulin and promotes microtubule densification, eventually effecting JP2 distribution and T-tubule, potentially contributing to heart failure in LVNC disease.

Fund

This work was supported by a CAMS Initiative for Innovative Medicine grant (CAMS-I2M, 2016-I2M-1-015 to Y.J.Wei)

Microtubules Provide a Viscoelastic Resistance to Myocyte Motion

BACKGROUND

Microtubules (MTs) buckle and bear load during myocyte contraction, a behavior enhanced by post-translational detyrosination. This buckling suggests a spring-like resistance against myocyte shortening, which could store energy and aid myocyte relaxation. Despite this visual suggestion of elastic behavior, the precise mechanical contribution of the cardiac MT network remains to be defined.

METHODS

Here we experimentally and computationally probe the mechanical contribution of stable MTs and their influence on myocyte function. We use multiple approaches to interrogate viscoelasticity and cell shortening in primary murine myocytes in which either MTs are depolymerized or detyrosination is suppressed and use the results to inform a mathematical model of myocyte viscoelasticity.

RESULTS

MT ablation by colchicine concurrently enhances both the degree of shortening and speed of relaxation, a finding inconsistent with simple spring-like MT behavior and suggestive of a viscoelastic mechanism. Axial stretch and transverse indentation confirm that MTs increase myocyte viscoelasticity. Specifically, increasing the rate of strain amplifies the MT contribution to myocyte stiffness. Suppressing MT detyrosination with parthenolide or via overexpression of tubulin tyrosine ligase has mechanical consequences that closely resemble colchicine, suggesting that the mechanical impact of MTs relies on a detyrosination-dependent linkage with the myocyte cytoskeleton. Mathematical modeling affirms that alterations in cell shortening conferred by either MT destabilization or tyrosination can be attributed to internal changes in myocyte viscoelasticity.

CONCLUSIONS

The results suggest that the cardiac MT network regulates contractile amplitudes and kinetics by acting as a cytoskeletal shock-absorber, whereby MTs provide breakable cross-links between the sarcomeric and nonsarcomeric cytoskeleton that resist rapid length changes during both shortening and stretch.

Suppression of detyrosinated microtubules improves cardiomyocyte function in human heart failure

Detyrosinated microtubules provide mechanical resistance that can impede the motion of contracting cardiomyocytes. However, the functional effects of microtubule detyrosination in heart failure or in human hearts have not previously been studied. Here, we utilize mass spectrometry and single-myocyte mechanical assays to characterize changes to the cardiomyocyte cytoskeleton and their functional consequences in human heart failure. Proteomic analysis of left ventricle tissue reveals a consistent upregulation and stabilization of intermediate filaments and microtubules in failing human hearts. As revealed by super-resolution imaging, failing cardiomyocytes are characterized by a dense, heavily detyrosinated microtubule network, which is associated with increased myocyte stiffness and impaired contractility. Pharmacological suppression of detyrosinated microtubules lowers the viscoelasticity of failing myocytes and restores 40-50% of lost contractile function; reduction of microtubule detyrosination using a genetic approach also softens cardiomyocytes and improves contractile kinetics. Together, these data demonstrate that a modified cytoskeletal network impedes contractile function in cardiomyocytes from failing human hearts and that targeting detyrosinated microtubules could represent a new inotropic strategy for improving cardiac function.

Highly variable contractile performance correlates with myocyte content in trabeculae from failing human hearts

Heart failure (HF) is defined by compromised contractile function and is associated with changes in excitation-contraction (EC) coupling and cardiomyocyte organisation. Tissue level changes often include fibrosis, while changes within cardiomyocytes often affect structures critical to EC coupling, including the ryanodine receptor (RyR), the associated protein junctophilin-2 (JPH2) and the transverse tubular system architecture. Using a novel approach, we aimed to directly correlate the influence of structural alterations with force development in ventricular trabeculae from failing human hearts. Trabeculae were excised from explanted human hearts in end-stage failure and immediately subjected to force measurements. Following functional experiments, each trabecula was fixed, sectioned and immuno-stained for structural investigations. Peak stress was highly variable between trabeculae from both within and between failing hearts and was strongly correlated with the cross-sectional area occupied by myocytes (MCSA), rather than total trabecula cross-sectional area. At the cellular level, myocytes exhibited extensive microtubule densification which was linked via JPH2 to time-to-peak stress. Trabeculae fractional MCSA variability was much higher than that in adjacent free wall samples. Together, these findings identify several structural parameters implicated in functional impairment in human HF and highlight the structural variability of ventricular trabeculae which should be considered when interpreting functional data.

In vitro model to study the effects of matrix stiffening on Ca2+ handling and myofilament function in isolated adult rat cardiomyocytes

Key points

This paper describes a novel model that allows exploration of matrix‐induced cardiomyocyte adaptations independent of the passive effect of matrix rigidity on cardiomyocyte function.

Detachment of adult cardiomyocytes from the matrix enables the study of matrix effects on cell shortening, Ca²⁺ handling and myofilament function.

Cell shortening and Ca²⁺ handling are altered in cardiomyocytes cultured for 24 h on a stiff matrix.

Matrix stiffness‐impaired cardiomyocyte contractility is reversed upon normalization of extracellular stiffness.

Matrix stiffness‐induced reduction in unloaded shortening is more pronounced in cardiomyocytes isolated from obese ZSF1 rats with heart failure with preserved ejection fraction compared to lean ZSF1 rats.

Abstract

Extracellular matrix (ECM) stiffening is a key element of cardiac disease. Increased rigidity of the ECM passively inhibits cardiac contraction, but if and how matrix stiffening also actively alters cardiomyocyte contractility is incompletely understood. In vitro models designed to study cardiomyocyte–matrix interaction lack the possibility to separate passive inhibition by a stiff matrix from active matrix‐induced alterations of cardiomyocyte properties. Here we introduce a novel experimental model that allows exploration of cardiomyocyte functional alterations in response to matrix stiffening. Adult rat cardiomyocytes were cultured for 24 h on matrices of tuneable stiffness representing the healthy and the diseased heart and detached from their matrix before functional measurements. We demonstrate that matrix stiffening, independent of passive inhibition, reduces cell shortening and Ca²⁺ handling but does not alter myofilament‐generated force. Additionally, detachment of adult cultured cardiomyocytes allowed the transfer of cells from one matrix to another. This revealed that stiffness‐induced cardiomyocyte changes are reversed when matrix stiffness is normalized. These matrix stiffness‐induced changes in cardiomyocyte function could not be explained by adaptation in the microtubules. Additionally, cardiomyocytes isolated from stiff hearts of the obese ZSF1 rat model of heart failure with preserved ejection fraction show more pronounced reduction in unloaded shortening in response to matrix stiffening. Taken together, we introduce a method that allows evaluation of the influence of ECM properties on cardiomyocyte function separate from the passive inhibitory component of a stiff matrix. As such, it adds an important and physiologically relevant tool to investigate the functional consequences of cardiomyocyte–matrix interactions.

This paper describes a novel model that allows exploration of matrix‐induced cardiomyocyte adaptations independent of the passive effect of matrix rigidity on cardiomyocyte function.

Detachment of adult cardiomyocytes from the matrix enables the study of matrix effects on cell shortening, Ca²⁺ handling and myofilament function.

Cell shortening and Ca²⁺ handling are altered in cardiomyocytes cultured for 24 h on a stiff matrix.

Matrix stiffness‐impaired cardiomyocyte contractility is reversed upon normalization of extracellular stiffness.

Matrix stiffness‐induced reduction in unloaded shortening is more pronounced in cardiomyocytes isolated from obese ZSF1 rats with heart failure with preserved ejection fraction compared to lean ZSF1 rats.

Expression of tubulin folding cofactor B in mouse hepatic ischemia-reperfusion injury

The aim of the present study was to investigate the association between tubulin folding cofactor B (TBCB) expression and ischemia-reperfusion injury (IRI) in mice. A total of 48 C57BL/6 mice were randomly divided into a control group (Sham, n=6) and an ischemia-reperfusion group (n=42). The ischemia-reperfusion group was further divided into 6 subgroups as per different times after reperfusion (2, 4, 6, 8, 12 and 24 h), with 7 mice per subgroup. A hepatic IRI model was established in mice by clamping the hepatic hilum. Morphology, serum levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), interleukin 6 (IL-6) and tumor necrosis factor-α (TNF-α), and the expression level of TBCB were detected. Compared with the control group, the livers from the ischemia-reperfusion group were significantly changed, particularly at 12 h following ischemia-reperfusion, with obvious hepatic cell degeneration and necrosis. The ALT, AST, IL-6 and TNF-α levels in the sera of the mice in the hepatic ischemia-reperfusion group were increased at all time points following ischemia-reperfusion, and were the highest at 12 h, demonstrating statistically significant differences when compared with the control group (P<0.05). Furthermore, the expression levels of TBCB, TNF-α and IL-6 were significantly increased at all time-points following ischemia-reperfusion, and were the most significant at 12 h. At 24 h following ischemia-reperfusion, the expression levels had decreased. The present study indicated that TBCB expression is associated with TNF-α and IL-6 expression levels in mice with hepatic ischemia-reperfusion, and may be key in the development of liver injury during ischemia-reperfusion in mice.

Microtubule mechanics in the working myocyte

The mechanical role of cardiac microtubules (MTs) has been a topic of some controversy. Early studies, which relied largely on pharmacological interventions that altered the MT cytoskeleton as a whole, presented no consistent role. Recent advances in the ability to observe and manipulate specific properties of the cytoskeleton have strengthened our understanding. Direct observation of MTs in working myocytes suggests a spring-like function, one that is surprisingly tunable by post-translational modification (PTM). Specifically, detyrosination of MTs facilitates an interaction with intermediate filaments that complex with the sarcomere, altering myocyte stiffness, contractility, and mechanosignalling. Such results support a paradigm of cytoskeletal regulation based on not only polymerization, but also associations with binding partners and PTMs that divide the MT cytoskeleton into functionally distinct subsets. The evolutionary costs and benefits of tuning cytoskeletal mechanics remain an open question, one that we discuss herein. Nevertheless, mechanically distinct MT subsets provide a rich new source of therapeutic targets for a variety of phenomena in the heart.

Combretastatin A4 disodium phosphate-induced myocardial injury

Histopathological and electrocardiographic features of myocardial lesions induced by combretastatin A4 disodium phosphate (CA4DP) were evaluated, and the relation between myocardial lesions and vascular changes and the direct toxic effect of CA4DP on cardiomyocytes were discussed. We induced myocardial lesions by administration of CA4DP to rats and evaluated myocardial damage by histopathologic examination and electrocardiography. We evaluated blood pressure (BP) of CA4DP-treated rats and effects of CA4DP on cellular impedance-based contractility of human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs). The results revealed multifocal myocardial necrosis with a predilection for the interventricular septum and subendocardial regions of the apex of the left ventricular wall, injury of capillaries, morphological change of the ST junction, and QT interval prolongation. The histopathological profile of myocardial lesions suggested that CA4DP induced a lack of myocardial blood flow. CA4DP increased the diastolic BP and showed direct effects on hiPS-CMs. These results suggest that CA4DP induces dysfunction of small arteries and capillaries and has direct toxicity in cardiomyocytes. Therefore, it is thought that CA4DP induced capillary and myocardial injury due to collapse of the microcirculation in the myocardium. Moreover, the direct toxic effect of CA4DP on cardiomyocytes induced myocardial lesions in a coordinated manner.

Differential Effects of Colchicine on Cardiac Cell Viability in an in vitro Model Simulating Myocardial Infarction

OBJECTIVES

We aimed to examine the effects of colchicine, currently in clinical trials for acute myocardial infarction (AMI), on the viability of cardiac cells using a cell line model of AMI.

METHODS

HL-1, a murine cardiomyocyte cell line, and H9C2, a rat cardiomyoblast cell line, were incubated with TNFα or sera derived from rats that underwent AMI or sham operation followed by addition of colchicine. In another experiment, HL-1/H9C2 cells were exposed to anoxia with or without subsequent addition of colchicine. Cell morphology and viability were assessed by light microscopy, flow cytometry and Western blot analyses for apoptotic markers.

RESULTS

Cellular viability was similar in both sera; however, exposing both cell lines to anoxia reduced their viability. Adding colchicine to anoxic H9C2, but not to anoxic HL-1, further increased their mortality, at least in part via enhanced apoptosis. Under any condition, colchicine induced detachment of H9C2 cells from their culture plates. This phenomenon did not apply to HL-1 cells.

CONCLUSIONS

Colchicine enhanced cardiomyoblast mortality under in vitro conditions mimicking AMI and reduced their adherence capability. HL-1 was not affected by colchicine; nevertheless, no salvage effect was observed. We thus conclude that colchicine may not inhibit myocardial apoptosis following AMI.

The Critical Roles of Zinc: Beyond Impact on Myocardial Signaling

Zinc has been considered as a vital constituent of proteins, including enzymes. Mobile reactive zinc (Zn(2+)) is the key form of zinc involved in signal transductions, which are mainly driven by its binding to proteins or the release of zinc from proteins, possibly via a redox switch. There has been growing evidence of zinc's critical role in cell signaling, due to its flexible coordination geometry and rapid shifts in protein conformation to perform biological reactions. The importance and complexity of Zn(2+) activity has been presumed to parallel the degree of calcium's participation in cellular processes. Whole body and cellular Zn(2+) levels are largely regulated by metallothioneins (MTs), Zn(2+) importers (ZIPs), and Zn(2+) transporters (ZnTs). Numerous proteins involved in signaling pathways, mitochondrial metabolism, and ion channels that play a pivotal role in controlling cardiac contractility are common targets of Zn(2+). However, these regulatory actions of Zn(2+) are not limited to the function of the heart, but also extend to numerous other organ systems, such as the central nervous system, immune system, cardiovascular tissue, and secretory glands, such as the pancreas, prostate, and mammary glands. In this review, the regulation of cellular Zn(2+) levels, Zn(2+)-mediated signal transduction, impacts of Zn(2+) on ion channels and mitochondrial metabolism, and finally, the implications of Zn(2+) in health and disease development were outlined to help widen the current understanding of the versatile and complex roles of Zn(2+).

Cardiotoxic changes of colchicine intoxication in rats: electrocardiographic, histopathological and blood chemical analysis

The microtubule inhibitor colchicine is cardiotoxic and is suggested to impair impulse formation and conduction. However, little is known about the electrocardiographic (ECG) changes induced by colchicine in experimental animals and the detailed pathogenesis of its cardiotoxicity. Therefore, we analyzed cardiotoxicity in colchicine-treated rats using electrocardiographic, histopathological and blood-chemistry approaches. A telemetry device for transmitting ECG data was implanted into male Crl:CD(SD) rats, and ECG tracings were obtained. At 6 weeks of age, 1.25 mg/kg colchicine was injected intravenously once daily for 2 consecutive days, and ECG waveforms and heart rate variability were analyzed. Furthermore, 1.25 mg/kg colchicine or vehicle was injected for 1 or 2 consecutive days in other rats at 6 weeks of age. One day after the final dosing, heart and blood samples were taken for histopathological and bloodchemical examination. ECG analysis revealed a prolonged RR interval, QRS duration, PR interval and QT interval. Heart rate variability analysis showed an increase in high frequency (HF) components as an index of parasympathetic nervous activity. In blood chemical examinations, colchicine induced high levels of parameters of cardiac injury and low levels and/or variations in Ca, inorganic phosphorus, potassium and chloride. Histopathologically, colchicine-treated rats showed eosinophilic granular degeneration and cytoplasmic vacuolation of ventricular myocardial cells but no remarkable change in the atrioventricular node. Not only blood chemical and histopathological changes but also ECG changes were induced in colchicine-treated rats, which indicated a decrease in myocardium excitability and conductivity, and these changes might be related to increased parasympathetic nervous activity and low blood Ca levels.

Microtubule-mediated defects in junctophilin-2 trafficking contribute to myocyte transverse-tubule remodeling and Ca2+ handling dysfunction in heart failure

BACKGROUND

Cardiac dysfunction in failing hearts of human patients and animal models is associated with both microtubule densification and transverse-tubule (T-tubule) remodeling. Our objective was to investigate whether microtubule densification contributes to T-tubule remodeling and excitation-contraction coupling dysfunction in heart disease.

METHODS AND RESULTS

In a mouse model of pressure overload-induced cardiomyopathy by transaortic banding, colchicine, a microtubule depolymerizer, significantly ameliorated T-tubule remodeling and cardiac dysfunction. In cultured cardiomyocytes, microtubule depolymerization with nocodazole or colchicine profoundly attenuated T-tubule impairment, whereas microtubule polymerization/stabilization with taxol accelerated T-tubule remodeling. In situ immunofluorescence of heart tissue sections demonstrated significant disorganization of junctophilin-2 (JP2), a protein that bridges the T-tubule and sarcoplasmic reticulum membranes, in transaortic banded hearts as well as in human failing hearts, whereas colchicine injection significantly preserved the distribution of JP2 in transaortic banded hearts. In isolated mouse cardiomyocytes, prolonged culture or treatment with taxol resulted in pronounced redistribution of JP2 from T-tubules to the peripheral plasma membrane, without changing total JP2 expression. Nocodazole treatment antagonized JP2 redistribution. Moreover, overexpression of a dominant-negative mutant of kinesin 1, a microtubule motor protein responsible for anterograde trafficking of proteins, protected against JP2 redistribution and T-tubule remodeling in culture. Finally, nocodazole treatment improved Ca(2+) handling in cultured myocytes by increasing the amplitude of Ca(2+) transients and reducing the frequency of Ca(2+) sparks.

CONCLUSION

Our data identify a mechanistic link between microtubule densification and T-tubule remodeling and reveal microtubule-mediated JP2 redistribution as a novel mechanism for T-tubule disruption, loss of excitation-contraction coupling, and heart failure.

Role of triadin in the organization of reticulum membrane at the muscle triad

The terminal cisternae represent one of the functional domains of the skeletal muscle sarcoplasmic reticulum (SR). They are closely apposed to plasma membrane invaginations, the T-tubules, with which they form structures called triads. In triads, the physical interaction between the T-tubule-anchored voltage-sensing channel DHPR and the SR calcium channel RyR1 is essential because it allows the depolarization-induced calcium release that triggers muscle contraction. This interaction between DHPR and RyR1 is based on the peculiar membrane structures of both T-tubules and SR terminal cisternae. However, little is known about the molecular mechanisms governing the formation of SR terminal cisternae. We have previously shown that ablation of triadins, a family of SR transmembrane proteins that interact with RyR1, induced skeletal muscle weakness in knockout mice as well as a modification of the shape of triads. Here we explore the intrinsic molecular properties of the longest triadin isoform Trisk 95. We show that when ectopically expressed, Trisk 95 can modulate reticulum membrane morphology. The membrane deformations induced by Trisk 95 are accompanied by modifications of the microtubule network organization. We show that multimerization of Trisk 95 by disulfide bridges, together with interaction with microtubules, are responsible for the ability of Trisk 95 to structure reticulum membrane. When domains responsible for these molecular properties are deleted, anchoring of Trisk 95 to the triads in muscle cells is strongly decreased, suggesting that oligomers of Trisk 95 and microtubules contribute to the organization of the SR terminal cisternae in a triad.

Dynamic changes in sarcoplasmic reticulum structure in ventricular myocytes

The fidelity of excitation-contraction (EC) coupling in ventricular myocytes is remarkable, with each action potential evoking a [Ca²⁺]_i transient. The prevalent model is that the consistency in EC coupling in ventricular myocytes is due to the formation of fixed, tight junctions between the sarcoplasmic reticulum (SR) and the sarcolemma where Ca²⁺ release is activated. Here, we tested the hypothesis that the SR is a structurally inert organelle in ventricular myocytes. Our data suggest that rather than being static, the SR undergoes frequent dynamic structural changes. SR boutons expressing functional ryanodine receptors moved throughout the cell, approaching or moving away from the sarcolemma of ventricular myocytes. These changes in SR structure occurred in the absence of changes in [Ca²⁺]_i during EC coupling. Microtubules and the molecular motors dynein and kinesin 1(Kif5b) were important regulators of SR motility. These findings support a model in which the SR is a motile organelle capable of molecular motor protein-driven structural changes.

Stabilizing Microtubules Decreases Myocardial Ischaemia-Reperfusion Injury

Microtubules are important components of the cytoskeleton that forms the backbone of myocardial architecture, sustaining its form and size. This study investigated whether stabilizing microtubules with paclitaxel (0.1 or 1 μM) could decrease myocardial ischaemia-reperfusion injury, and reduce myocardial infarct size and the incidence of ischaemic ventricular arrhythmia. Isolated rat hearts were used, with arrhythmia induced by regional ischaemia and myocardial infarcts induced by ischaemia-reperfusion. In these ex vivo rat models, paclitaxel decreased myocardial ischaemia-reperfusion injury, significantly reducing the incidence and severity of ischaemic ventricular arrhythmia and significantly decreasing infarct size.

Taxol, a microtubule stabilizer, prevents ischemic ventricular arrhythmias in rats

Microtubule integrity is important in cardio-protection, and microtubule disruption has been implicated in the response to ischemia in cardiac myocytes. However, the effects of Taxol, a common microtubule stabilizer, are still unknown in ischemic ventricular arrhythmias. The arrhythmia model was established in isolated rat hearts by regional ischemia, and myocardial infarction model by ischemia/reperfusion. Microtubule structure was immunohistochemically measured. The potential mechanisms were studied by measuring reactive oxygen species (ROS), activities of oxidative enzymes, intracellular calcium concentration ([Ca(2+) ](i) ) and Ca(2+) transients by using fluorometric determination, spectrophotometric assays and Fura-2-AM and Fluo-3-AM, respectively. The expression and activity of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) was also examined using real-time polymerase chain reaction, Western blot and pyruvate/Nicotinamide adenine dinucleotide-coupled reaction. Our data showed that Taxol (0.1, 0.3 and 1 μM) effectively reduced the number of ventricular premature beats and the incidence and duration of ventricular tachycardia. The infarct size was also significantly reduced by Taxol (1 μM). At the same time, Taxol preserved the microtubule structure, increased the activity of mitochondrial electron transport chain complexes I and III, reduced ROS levels, decreased the rise in [Ca(2+)](i) and preserved the amplitude and decay times of Ca(2+) transients during ischemia. In addition, SERCA2a activity was preserved by Taxol during ischemia. In summary, Taxol prevents ischemic ventricular arrhythmias likely through ameliorating abnormal calcium homeostasis and decreasing the level of ROS. This study presents evidence that Taxol may be a potential novel therapy for ischemic ventricular arrhythmias.

Model of ionic currents through microtubule nanopores and the lumen

It has been suggested that microtubules and other cytoskeletal filaments may act as electrical transmission lines. An electrical circuit model of the microtubule is constructed incorporating features of its cylindrical structure with nanopores in its walls. This model is used to study how ionic conductance along the lumen is affected by flux through the nanopores, both with and without an external potential applied across its two ends. Based on the results of Brownian dynamics simulations, the nanopores were found to have asymmetric inner and outer conductances, manifested as nonlinear IV curves. Our simulations indicate that a combination of this asymmetry and an internal voltage source arising from the motion of the C-terminal tails causes cations to be pumped across the microtubule wall and propagate in both directions down the microtubule through the lumen, returning to the bulk solution through its open ends. This effect is demonstrated to add directly to the longitudinal current through the lumen resulting from an external voltage source applied across the two ends of the microtubule. The predicted persistent currents directed through the microtubule wall and along the lumen could be significant in directing the dissipation of weak, endogenous potential gradients toward one end of the microtubule within the cellular environment.

The p38/MAPK pathway regulates microtubule polymerization through phosphorylation of MAP4 and Op18 in hypoxic cells

In both cardiomyocytes and HeLa cells, hypoxia (1% O(2)) quickly leads to microtubule disruption, but little is known about how microtubule dynamics change during the early stages of hypoxia. We demonstrate that microtubule associated protein 4 (MAP4) phosphorylation increases while oncoprotein 18/stathmin (Op18) phosphorylation decreases after hypoxia, but their protein levels do not change. p38/MAPK activity increases quickly after hypoxia concomitant with MAP4 phosphorylation, and the activated p38/MAPK signaling leads to MAP4 phosphorylation and to Op18 dephosphorylation, both of which induce microtubule disruption. We confirmed the interaction between phospho-p38 and MAP4 using immunoprecipitation and found that SB203580, a p38/MAPK inhibitor, increases and MKK6(Glu) overexpression decreases hypoxic cell viability. Our results demonstrate that hypoxia induces microtubule depolymerization and decreased cell viability via the activation of the p38/MAPK signaling pathway and changes the phosphorylation levels of its downstream effectors, MAP4 and Op18.

Tubulin polymerization modifies cardiac sodium channel expression and gating

AIMS

Treatment with the anticancer drug taxol (TXL), which polymerizes the cytoskeleton protein tubulin, may evoke cardiac arrhythmias based on reduced human cardiac sodium channel (Na(v)1.5) function. Therefore, we investigated whether enhanced tubulin polymerization by TXL affects Na(v)1.5 function and expression and whether these effects are beta1-subunit-mediated.

METHODS AND RESULTS

Human embryonic kidney (HEK293) cells, transfected with SCN5A cDNA alone (Na(v)1.5) or together with SCN1B cDNA (Na(v)1.5 + beta1), and neonatal rat cardiomyocytes (NRCs) were incubated in the presence and in the absence of 100 microM TXL. Sodium current (I(Na)) characteristics were studied using patch-clamp techniques. Na(v)1.5 membrane expression was determined by immunocytochemistry and confocal microscopy. Pre-treatment with TXL reduced peak I(Na) amplitude nearly two-fold in both Na(v)1.5 and Na(v)1.5 + beta1, as well as in NRCs, compared with untreated cells. Accordingly, HEK293 cells and NRCs stained with anti-Na(v)1.5 antibody revealed a reduced membrane-labelling intensity in the TXL-treated groups. In addition, TXL accelerated I(Na) decay of Na(v)1.5 + beta1, whereas I(Na) decay of Na(v)1.5 remained unaltered. Finally, TXL reduced the fraction of channels that slow inactivated from 31% to 18%, and increased the time constant of slow inactivation by two-fold in Na(v)1.5. Conversely, slow inactivation properties of Na(v)1.5 + beta1 were unchanged by TXL.

CONCLUSION

Enhanced tubulin polymerization reduces sarcolemmal Na(v)1.5 expression and I(Na) amplitude in a beta1-subunit-independent fashion and causes I(Na) fast and slow inactivation impairment in a beta1-subunit-dependent way. These changes may underlie conduction-slowing-dependent cardiac arrhythmias under conditions of enhanced tubulin polymerization, e.g. TXL treatment and heart failure.

Hypersensitivity of excitation-contraction coupling in dystrophic cardiomyocytes

Duchenne muscular dystrophy represents a severe inherited disease of striated muscle. It is caused by a mutation of the dystrophin gene and characterized by a progressive loss of skeletal muscle function. Most patients also develop a dystrophic cardiomyopathy, resulting in dilated hypertrophy and heart failure, but the cellular mechanisms leading to the deterioration of cardiac function remain elusive. In the present study, we tested whether defective excitation-contraction (E-C) coupling contributes to impaired cardiac performance. "E-C coupling gain" was determined in cardiomyocytes from control and dystrophin-deficient mdx mice. To this end, L-type Ca2+ currents (ICaL) were measured with the whole cell patch-clamp technique, whereas Ca2+ transients were simultaneously recorded with confocal imaging of fluo-3. Initial findings indicated subtle changes of E-C coupling in mdx cells despite matched Ca2+ loading of the sarcoplasmic reticulum (SR). However, lowering the extracellular Ca2+ concentration, a maneuver used to unmask latent E-C coupling problems, was surprisingly much better tolerated by mdx myocytes, suggesting a hypersensitive E-C coupling mechanism. Challenging the SR Ca2+ release by slow elevations of the intracellular Ca2+ concentration resulted in Ca2+ oscillations after a much shorter delay in mdx cells. This is consistent with an enhanced Ca2+ sensitivity of the SR Ca2+-release channels [ryanodine receptors (RyRs)]. The hypersensitivity could be normalized by the introduction of reducing agents, indicating that the elevated cellular ROS generation in dystrophy underlies the abnormal RyR sensitivity and hypersensitive E-C coupling. Our data suggest that in dystrophin-deficient cardiomyocytes, E-C coupling is altered due to potentially arrhythmogenic changes in the Ca2+ sensitivity of redox-modified RyRs.

L-type Ca(2+) current in ventricular cardiomyocytes

L-type Ca(2+) channels are mediators of Ca(2+) influx and the regulatory events accompanying it and are pivotal in the function and dysfunction of ventricular cardiac myocytes. L-type Ca(2+) channels are located in sarcolemma, including the T-tubules facing the sarcoplasmic reticulum junction, and are activated by membrane depolarization, but intracellular Ca(2+)-dependent inactivation limits Ca(2+) influx during action potential. I(CaL) is important in heart function because it triggers excitation-contraction coupling, modulates action potential shape and is involved in cardiac arrhythmia. L-type Ca(2+) channels are multi-subunit complexes that interact with several molecules involved in their regulations, notably by beta-adrenergic signaling. The present review highlights some of the recent findings on L-type Ca(2+) channel function, regulation, and alteration in acquired pathologies such as cardiac hypertrophy, heart failure and diabetic cardiomyopathy, as well as in inherited arrhythmic cardiac diseases such as Timothy and Brugada syndromes.

Axial stretch of rat single ventricular cardiomyocytes causes an acute and transient increase in Ca2+ spark rate

We investigate acute effects of axial stretch, applied by carbon fibers (CFs), on diastolic Ca2+ spark rate in rat isolated cardiomyocytes. CFs were attached either to both cell ends (to maximize the stretched region), or to the center and one end of the cell (to compare responses in stretched and nonstretched half-cells). Sarcomere length was increased by 8.01+/-0.94% in the stretched cell fraction, and time series of XY confocal images were recorded to monitor diastolic Ca2+ spark frequency and dynamics. Whole-cell stretch causes an acute increase of Ca2+ spark rate (to 130.7+/-6.4%) within 5 seconds, followed by a return to near background levels (to 104.4+/-5.1%) within 1 minute of sustained distension. Spark rate increased only in the stretched cell region, without significant differences in spark amplitude, time to peak, and decay time constants of sparks in stretched and nonstretched areas. Block of stretch-activated ion channels (2 micromol/L GsMTx-4), perfusion with Na+/Ca2+-free solution, and block of nitric oxide synthesis (1 mmol/L L-NAME) all had no effect on the stretch-induced acute increase in Ca2+ spark rate. Conversely, interference with cytoskeletal integrity (2 hours of 10 micromol/L colchicine) abolished the response. Subsequent electron microscopic tomography confirmed the close approximation of microtubules with the T-tubular-sarcoplasmic reticulum complex (to within approximately 10(-8)m). In conclusion, axial stretch of rat cardiomyocytes acutely and transiently increases sarcoplasmic reticulum Ca2+ spark rate via a mechanism that is independent of sarcolemmal stretch-activated ion channels, nitric oxide synthesis, or availability of extracellular calcium but that requires cytoskeletal integrity. The potential of microtubule-mediated modulation of ryanodine receptor function warrants further investigation.

Plasma membrane tubulin

The association of tubulin with the plasma membrane comprises multiple levels of penetration into the bilayer: from integral membrane protein, to attachment via palmitoylation, to surface binding, and to microtubules attached by linker proteins to proteins in the membrane. Here we discuss the soundness and weaknesses of the chemical and biochemical evidence marshaled to support these associations, as well as the mechanisms by which tubulin or microtubules may regulate functions at the plasma membrane.

Cell membrane stretch and chest blow-induced ventricular fibrillation: commotio cordis

INTRODUCTION

Commotio cordis, sudden cardiac death secondary to blunt nonpenetrating chest blows in sports, is reported with increasing frequency. In a swine model, ventricular fibrillation (VF) is induced by a baseball blow to the chest, and the initiation of VF is related to the peak left ventricular (LV) pressure produced by the blow. LV pressure changes likely result in cell membrane stretch and mechanical activation of ion channels. Disruption of cell cytoskeleton that anchors the cell membrane prior to precordial blows offers the opportunity to explore whether cell membrane deformation is critical to commotio cordis.

METHODS AND RESULTS

Twelve juvenile swine (mean 12.7 +/- 1.6 kg) were randomized to intravenous normal saline (control, n = 6) or 10 mg of intravenous colchicine (n = 6), which is known to depolymerize microtubules. Animals were given up to six blows timed to the vulnerable portion of the cardiac cycle with a 30 mph baseball on the chest directly over the cardiac silhouette. VF was initiated by 14 of the 29 (48%) impacts in the colchicine-treated animals compared with only 3 of 28 (11%) in the controls (P = 0.002). The peak generated LV pressure did not differ between colchicine animals (405 +/- 61 mmHg) and controls (387 +/- 115) (P = 0.47). However, animals administered colchicine were more likely to have VF generated by the chest blow at all pressures.

CONCLUSION

The initiation of VF by chest blows is significantly increased by selective disruption of the cytoskeleton, suggesting that mechanical deformation of the cell membrane is fundamental to the activation of ion channels and underlies the mechanism of VF in commotio cordis.

Cardiac dysfunction in aging conscious rats: altered cardiac cytoskeletal proteins as a potential mechanism

The objective of this study was to test the hypothesis that the mechanism mediating left ventricular (LV) dysfunction in the aging rat heart involves, in part, changes in cardiac cytoskeletal components. Our results show that there were no significant differences in heart rate, LV pressure, or LV diameter between conscious, instrumented young [5.9 +/- 0.3 mo (n = 9)] and old rats [30.6 +/- 0.1 mo (n = 10)]. However, the first derivative of LV pressure (LV dP/dt) was reduced (8,309 +/- 790 vs. 11,106 +/- 555 mmHg/s, P < 0.05) and isovolumic relaxation time (tau) was increased (8.7 +/- 0.7 vs. 6.3 +/- 0.6 ms, P < 0.05) in old vs. young rats, respectively. The differences in baseline LV function in young and old rats, which were modest, were accentuated after beta-adrenergic receptor stimulation with dobutamine (20 mug/kg), which increased LV dP/dt by 170 +/- 9% in young rats, significantly more (P < 0.05) than observed in old rats (115 +/- 5%). Volume loading in anesthetized rats demonstrated significantly impaired LV compliance in old rats, as measured by the LV end-diastolic pressure and dimension relationship. In old rat hearts, there was a significant (P < 0.05) increase in the percentage of LV collagen (2.4 +/- 0.2 vs. 1.3 +/- 0.2%), alpha-tubulin (92%), and beta-tubulin (2.3-fold), whereas intact desmin decreased by 51%. Thus the cardiomyopathy of aging in old, conscious rats may be due not only to increases in collagen but also to alterations in cytoskeletal proteins.

IKs response to protein kinase A-dependent KCNQ1 phosphorylation requires direct interaction with microtubules

AIMS

KCNQ1 (alias KvLQT1 or Kv7.1) and KCNE1 (alias IsK or minK) co-assemble to form the voltage-activated K(+) channel responsible for I(Ks)-a major repolarizing current in the human heart-and their dysfunction promotes cardiac arrhythmias. The channel is a component of larger macromolecular complexes containing known and undefined regulatory proteins. Thus, identification of proteins that modulate its biosynthesis, localization, activity, and/or degradation is of great interest from both a physiological and pathological point of view.

METHODS AND RESULTS

Using a yeast two-hybrid screening, we detected a direct interaction between beta-tubulin and the KCNQ1 N-terminus. The interaction was confirmed by co-immunoprecipitation of beta-tubulin and KCNQ1 in transfected COS-7 cells and in guinea pig cardiomyocytes. Using immunocytochemistry, we also found that they co-localized in cardiomyocytes. We tested the effects of microtubule-disrupting and -stabilizing agents (colchicine and taxol, respectively) on the KCNQ1-KCNE1 channel activity in COS-7 cells by means of the permeabilized-patch configuration of the patch-clamp technique. None of these agents altered I(Ks). In addition, colchicine did not modify the current response to osmotic challenge. On the other hand, the I(Ks) response to protein kinase A (PKA)-mediated stimulation depended on microtubule polymerization in COS-7 cells and in cardiomyocytes. Strikingly, KCNQ1 channel and Yotiao phosphorylation by PKA-detected by phospho-specific antibodies-was maintained, as was the association of the two partners.

CONCLUSION

We propose that the KCNQ1-KCNE1 channel directly interacts with microtubules and that this interaction plays a major role in coupling PKA-dependent phosphorylation of KCNQ1 with I(Ks) activation.

Microtubule-dependent distribution of mRNA in adult cardiocytes

Synthesis of myofibrillar proteins in the diffusion-restricted adult cardiocyte requires microtubule-based active transport of mRNAs as part of messenger ribonucleoprotein particles (mRNPs) to translation sites adjacent to nascent myofibrils. This is especially important for compensatory hypertrophy in response to hemodynamic overloading. The hypothesis tested here is that excessive microtubule decoration by microtubule-associated protein 4 (MAP4) after cardiac pressure overloading could disrupt mRNP transport and thus hypertrophic growth. MAP4-overexpressing and pressure-overload hypertrophied adult feline cardiocytes were infected with an adenovirus encoding zipcode-binding protein 1-enhanced yellow fluorescent protein fusion protein, which is incorporated into mRNPs, to allow imaging of these particles. Speed and distance of particle movement were measured via time-lapse microscopy. Microtubule depolymerization was used to study microtubule-based transport and distribution of mRNPs. Protein synthesis was assessed as radioautographic incorporation of [3H]phenylalanine. After microtubule depolymerization, mRNPs persist only perinuclearly and apparent mRNP production and protein synthesis decrease. Reestablishing microtubules restores mRNP production and transport as well as protein synthesis. MAP4 overdecoration of microtubules via adenovirus infection in vitro or following pressure overloading in vivo reduces the speed and average distance of mRNP movement. Thus cardiocyte microtubules are required for mRNP transport and structural protein synthesis, and MAP4 decoration of microtubules, whether directly imposed or accompanying pressure-overload hypertrophy, causes disruption of mRNP transport and protein synthesis. The dense, highly MAP4-decorated microtubule network seen in severe pressure-overload hypertrophy both may cause contractile dysfunction and, perhaps even more importantly, may prevent a fully compensatory growth response to hemodynamic overloading.

Colchicine, a microtubule depolymerizing agent, inhibits myocardial apoptosis in rats

Heart failure is the most common cardiovascular disease with high mortality and morbidity. Both enhanced microtubule polymerization and cardiomyocyte apoptosis are involved in the pathogenesis of heart failure. However, the link between the two mechanisms remains to be elucidated. In this study, we thus address this important issue in cultured cardiomyocytes from Wistar rats in vitro and in angiotensin II (ATII)-infused rats in vivo. Confocal microscopy examination showed that in cultured rat cardiomyocytes, micrographic density of microtubules was increased by paclitaxel, a microtubule-polymerizing agent, and decreased by colchicine, a microtubule-depolymerizing agent, but not affected by ATII, isoproterenol, or tumor necrosis factor-alpha alone. Immunoblotting analysis showed that Bax/Bcl-2 ratio, which is associated with the activation of caspase-3, was significantly increased in ATII-stimulated cultured cardiomyocytes in vitro and in ATII-infused rats in vivo, both of which were inhibited by co-treatment with colchicine. Caspase-3 and TUNEL assay to detect apoptosis in vitro demonstrated that paclitaxel or ATII alone significantly enhanced and their combination further accelerated cardiomyocyte apoptosis, which was again significantly inhibited by colchicine. Caspase-3 and TUNEL assay in vivo also demonstrated that ATII infusion significantly increased myocardial apoptosis and that co-treatment with colchicine significantly suppressed the apoptosis. In conclusion, these results indicate that a microtubule-depolymerizing agent could be a potential therapeutic strategy for treatment of heart failure.

Decreased L-type Ca2+ current in cardiac myocytes of type 1 diabetic Akita mice due to reduced phosphatidylinositol 3-kinase signaling

OBJECTIVE

Contraction of cardiac myocytes is initiated by Ca(2+) entry through the voltage-dependent L-type Ca(2+) channel (LTCC). Previous studies have shown that phosphatidylinositol (PI) 3-kinase signaling modulates LTCC function. Because PI 3-kinases are key mediators of insulin action, we investigated whether LTCC function is affected in diabetic animals due to reduced PI 3-kinase signaling.

RESEARCH DESIGN AND METHODS

We used whole-cell patch clamping and biochemical assays to compare cardiac LTCC function and PI 3-kinase signaling in insulin-deficient diabetic mice heterozygous for the Ins2(Akita) mutation versus nondiabetic littermates.

RESULTS

Diabetic mice had a cardiac contractility defect, reduced PI 3-kinase signaling in the heart, and decreased L-type Ca(2+) current (I(Ca,L)) density in myocytes compared with control nondiabetic littermates. The lower I(Ca,L) density in myocytes from diabetic mice is due at least in part to reduced cell surface expression of the LTCC. I(Ca,L) density in myocytes from diabetic mice was increased to control levels by insulin treatment or intracellular infusion of PI 3,4,5-trisphosphate [PI(3,4,5)P(3)]. This stimulatory effect was blocked by taxol, suggesting that PI(3,4,5)P(3) stimulates microtubule-dependent trafficking of the LTCC to the cell surface. The voltage dependence of steady-state activation and inactivation of I(Ca,L) was also shifted to more positive potentials in myocytes from diabetic versus nondiabetic animals. PI(3,4,5)P(3) infusion eliminated only the difference in voltage dependence of steady-state inactivation of I(Ca,L).

CONCLUSIONS

Decreased PI 3-kinase signaling in myocytes from type 1 diabetic mice leads to reduced Ca(2+) entry through the LTCC, which might contribute to the negative effect of diabetes on cardiac contractility.

Cytoskeletal networks and the regulation of cardiac contractility: microtubules, hypertrophy, and cardiac dysfunction

The cytoskeleton as classically defined for eukaryotic cells consists of three systems of protein filaments: the microtubules, the intermediate filaments, and the microfilaments. In mature striated muscle such as the heart of the adult mammal, these three types of cytoskeletal filaments are superimposed spatially on the myofilaments, a specialized system of contractile protein filaments. Each of these systems of protein filaments has the potential to respond in an adaptive or maladaptive manner during load-induced hypertrophic cardiac growth. However, the extent to which such hypertrophy is compensatory is also critically dependent on the type of hemodynamic overload that serves as the hypertrophic stimulus. Thus cardiac hypertrophy is not intrinsically maladaptive; rather, it is the nature of the inducing load rather than hypertrophy itself that is responsible, through effects on structural and/or regulatory proteins, for the frequent deterioration of initially compensatory hypertrophy into the congestive heart failure state. As one example reviewed here of this load specificity of maladaptation, increased microtubule network density is a persistent feature of severely pressure-overloaded, hypertrophied, and failing myocardium that imposes a primarily viscous load on active myofilaments during contraction.

Mechanisms of [Ca2+]i transient decrease in cardiomyopathy of db/db type 2 diabetic mice

Cardiovascular disease is the leading cause of death in the diabetic population. However, molecular mechanisms underlying diabetic cardiomyopathy remain unclear. We analyzed Ca2+-induced Ca2+ release and excitation-contraction coupling in db/db obese type 2 diabetic mice and their control littermates. Echocardiography showed a systolic dysfunction in db/db mice. Two-photon microscopy identified intracellular calcium concentration ([Ca2+]i) transient decrease in cardiomyocytes within the whole heart, which was also found in isolated myocytes by confocal microscopy. Global [Ca2+]i transients are constituted of individual Ca2+ sparks. Ca2+ sparks in db/db cardiomyocytes were less frequent than in +/+ myocytes, partly because of a depression in sarcoplasmic reticulum Ca2+ load but also because of a reduced expression of ryanodine receptor Ca2+ channels (RyRs), revealed by [3H]ryanodine binding assay. Ca2+ efflux through Na+/Ca2+ exchanger was increased in db/db myocytes. Calcium current, I(Ca), triggers sarcoplasmic reticulum Ca2+ release and is also involved in sarcoplasmic reticulum Ca2+ refilling. Macroscopic I(Ca) was reduced in db/db cells, but single Ca2+ channel activity was similar, suggesting that diabetic myocytes express fewer functional Ca2+ channels, which was confirmed by Western blots. These results demonstrate that db/db mice show depressed cardiac function, at least in part, because of a general reduction in the membrane permeability to Ca2+. As less Ca2+ enters the cell through I(Ca), less Ca2+ is released through RyRs.

Functional properties of the titin/connectin-associated proteins, the muscle-specific RING finger proteins (MURFs), in striated muscle

The efficient functioning of striated muscle is dependent upon the proper alignment and coordinated activities of several cytoskeletal networks including myofibrils, microtubules, and intermediate filaments. However, the exact molecular mechanisms dictating their cooperation and contributions during muscle differentiation and maintenance remain unknown. Recently, the muscle specific RING finger (MURF) family members have established themselves as excellent candidates for linking myofibril components (including the giant, multi-functional protein, titin/connectin), with microtubules, intermediate filaments, and nuclear factors. MURF-1, the only family member expressed throughout development, has been implicated in several studies as an ubiquitin ligase that is upregulated in response to multiple stimuli during muscle atrophy. Cell culture studies suggest that MURF-1 specifically has a role in maintaining titin M-line integrity and yeast two-hybrid studies point toward its participation in muscle stress response pathways and gene expression. MURF-2 is developmentally down-regulated and is assembled at the M-line region of the sarcomere and with microtubules. Functionally, its expression is critical for maintenance of the sarcomeric M-line region, specific populations of stable microtubules, desmin and vimentin intermediate filaments, as well as for myoblast fusion and differentiation. A recent study also links MURF-2 to a titin kinase-based protein complex that is reportedly activated upon mechanical signaling. Finally, MURF-3 is developmentally upregulated, associates with microtubules, the sarcomeric M-line (this report) and Z-line, and is required for microtubule stability and myogenesis. Here, we focus on the biochemical and functional properties of this intriguing family of muscle proteins, and discuss how they may tie together titin-mediated myofibril signaling pathways (perhaps involving the titin kinase domain), biomechanical signaling, the muscle stress response, and gene expression.

Effect of cytoskeleton disruptors on L-type Ca channel distribution in rat ventricular myocytes

The cytoskeleton plays an important role in many aspects of cardiac cell function, including protein trafficking. However, the role of the cytoskeleton in determining Ca channel location in cardiac myocytes is unknown. In the present study we therefore investigated the effect of the cytoskeletal disruptors cytochalasin D, latrunculin, nocadazole and colchicine on the distribution of Ca channels in rat ventricular myocytes during culture for up to 96 h. During culture in the absence of these agents, cell edges became rounded, t-tubule density decreased, and the normal transverse distribution of the alpha1 (pore-forming) subunit of the L-type Ca channel became more punctate and peri-nuclear; these changes were associated with loss of synchronous Ca release in response to electrical stimulation. Disruption of tubulin using nocadazole or colchicine or sequestration of monomeric actin by latrunculin had no effect on these changes. In contrast, cytochalasin D inhibited these changes: cell shape, t-tubule density, transverse Ca channel staining and synchronous Ca release were maintained during culture. The protein synthesis inhibitor cycloheximide had similar effects to cytochalasin. These data suggest that cytochalasin stabilizes actin in adult ventricular myocytes in culture, thus stabilizing cell structure and function, and that actin is important in trafficking L-type Ca channels from the peri-nuclear region to the t-tubules, where they are normally located and provide the trigger for Ca release.

Cardiocyte cytoskeleton in hypertrophied myocardium

The Frank-Starling mechanism, by which load directly regulates muscle length and thus performance is the means by which the mechanics and energetics of cardiac muscle are regulated on a beat-to-beat basis. When this short-term compensation for increased load is insufficient, the long-term compensation of cardiac hypertrophy ensues. The simplest and most direct mechanism for load regulation of cardiac mass would obtain if an analog of the short-term Frank-Starling mechanism of functional regulation operated in the long-term time domain of mass regulation; that is, if heart muscle were able to directly transduce increased load into growth. It is now clear that load does indeed serve as a direct regulator of cardiac mass in the adult. Cardiac hypertrophy, at the levels of intact animal, isolated tissue, and cultured cells, is a direct response of the adult mammalian cardiocyte to increased load, modified by but without the requisite involvement of factors external to the cell. The extent to which such hypertrophy is compensatory is critically dependent on the type of hemodynamic overload that serves as the hypertrophic stimulus. Thus, cardiac hypertrophy is not intrinsically maladaptive; rather, it is the nature of the inducing load rather than hypertrophy itself that is responsible for the frequent deterioration of initially compensatory hypertrophy into the congestive heart failure state. As one example reviewed here of this load specificity of maladaptation, increased microtubule network density is a persistent feature of severely pressure overloaded, hypertrophied and failing myocardium which imposes a viscous load on active myofilaments during contraction.

The extracellular matrix and the cytoskeleton in heart hypertrophy and failure

Cell characteristics and phenotype depend on the nature of the extracellular matrix, the type and organization of integrins and cytoskeleton. The interactions between these components are poorly known at the myocyte level and during cardiac remodeling associated with cardiac hypertrophy and heart failure. We analyze here the nature and organization of extracellular matrix (ECM) proteins, cytoskeleton and integrins and their regulation by growth factors, such as angiotensin II, in normal myocyte growth and in pathological growth (hypertrophy) of the myocardium and heart failure.

Unloaded shortening velocity in single permeabilized vascular smooth muscle cells is independent of microtubule status

Microtubules may influence smooth muscle contraction either via involvement in signal transduction processes or by serving as an internal load that opposes contraction. To test the latter hypothesis, microtubule distribution and the unloaded shortening velocity were investigated in freshly isolated single vascular smooth muscle cells (VSMCs) treated with microtubule modulating drugs. Immunocytochemical studies showed that microtubules run mainly longitudinally in relaxed VSMCs. They are oriented more obliquely, almost transversely to the long axis of the cells after contraction, suggesting that microtubules are compressed during shortening, and thus might impart an internal passive load. Quantitative immunocytochemical analysis revealed that, relative to the control group, colchicine (15 microM) decreased the microtubule density by 40% while taxol (10 microM) increased the microtubule density by 46%. However, alteration of microtubule polymerization status by these microtubule-modulating drugs did not have a significant effect on unloaded shortening velocity in alpha-toxin permeabilized VSMCs under maximal activating conditions or submaximal activating conditions (about 36% of maximal velocity). These results suggest that microtubules do not present an appreciable internal load to dampen single VSMCs shortening in the present experimental system, and that their influence on smooth muscle contraction is primarily via signal transduction mechanisms.

Microtubule alteration is an early cellular reaction to the metabolic challenge in ischemic cardiomyocytes

Cytoskeleton damage, particularly microtubule (MT) alterations, may play an important role in the pathogenesis of ischemia-induced myocardial injury. However, this disorganization has been scarcely confirmed in the cellular context. We evaluated MT network disassembly in myoblast cell line H9c2 and in neonatal rat cardiomyocytes in an in vitro substrate-free hypoxia model of simulated ischemia (SI). After different duration of SI from 30 up to 180 min, the cells were fixed and the microtubule network was revealed by immunocytochemistry. The microtubule alterations were quantified using a house-developed image analysis program. Additionally, the tubulin fraction were extracted and quantified by Western blotting. The cell respiration, the release of cellular LDH and the cell viability were evaluated at the same periods. An early MT disassembly was observed after 60 min of SI. The decrease in MT fluorescence intensity at 60 and 90 min was correlated with a microtubule disassembly. Conversely, SI-induced significant LDH release (35%) and decrease in cell viability (34%) occurred after 120 min only. These results suggest that the simulated ischemia-induced changes in MT network should not be considered as an ultrastructural hallmark of the cell injury and could rather be an early ultrastructural correlate of the cellular reaction to the metabolic challenge.

Post-translational modifications of tubulin and microtubule stability in adult rat ventricular myocytes and immortalized HL-1 cardiomyocytes

Little is known about the subcellular distribution and the dynamics of tubulins in adult cardiac myocytes although both are modified during cardiac hypertrophy and heart failure. Using confocal microscopy, we examined post-translational modifications of tubulin in fully differentiated ventricular myocytes isolated from adult rat hearts, as well as in immortalized and dividing HL-1 cardiomyocytes. Detyrosinated Glu-alpha-tubulin was the most abundant post-translationally modified tubulin found in ventricular myocytes, while acetylated- and delta2-alpha-tubulins were found in lower amounts or absent. In contrast, dividing HL-1 cardiomyocytes exhibited high levels of tyrosinated or acetylated alpha-tubulins. A mild nocodazole treatment (0.1 microM, 1 h) disrupted microtubules in HL-1 myocytes, but not in adult ventricular myocytes. A stronger treatment (10 microM, 2 h) was required to disassemble tubulins in adult myocytes. Glu-alpha-tubulin containing microtubules were more resistant to nocodazole treatment in HL-1 cardiomyocytes than in ventricular myocytes. Endogenous activation of the cAMP pathway with the forskolin analog L858051 (20 microM) or the beta-adrenergic agonist isoprenaline (10 microM) disrupted the most labile microtubules in HL-1 cardiomyocytes. In contrast, isoprenaline (10 microM), cholera toxin (200 ng/ml, a G(S)-protein activator), L858051 (20 microM) or forskolin (10 microM) had no effect on the microtubule network in ventricular myocytes. In addition, intracellular Ca2+ accumulation induced either by thapsigargin (2 microM) or caffeine (10 mM) did not modify microtubule stability in ventricular myocytes. Our data demonstrate the unique stability of the microtubule network in adult cardiac myocytes. We speculate that microtubule stability is required to support cellular integrity during cardiac contraction.

Calstabin deficiency, ryanodine receptors, and sudden cardiac death

Altered cardiac ryanodine receptor (RyR2) function has an important role in heart failure and genetic forms of arrhythmias. RyR2 constitutes the major intracellular Ca2+ release channel in the cardiac sarcoplasmic reticulum (SR). The peptidyl-prolyl isomerase calstabin2 (FKBP12.6) is a component of the RyR2 macromolecular signaling complex. Calstabin2 binding to RyR2 is regulated by PKA phosphorylation of Ser2809 in RyR2. PKA phosphorylation of RyR2 decreases the binding affinity for calstabin2 and increases RyR2 open probability and sensitivity to Ca2+-dependent activation. In heart failure, a majority of studies have found that RyR2 becomes chronically PKA hyper-phosphorylated which depletes calstabin2 from the channel complex. Calstabin2 dissociation causes a diastolic SR Ca2+ leak contributing to depressed intracellular Ca2+ cycling and decreased cardiac contractility. Missense mutations linked to genetic forms of exercise-induced arrhythmias and sudden cardiac death also cause decreased calstabin2-binding affinity and leaky RyR2 channels. We review the importance of calstabin2 for RyR2 function and excitation-contraction coupling, and discuss new observations that implicate dysregulation of calstabin2 binding as a central mechanism for abnormal calcium cycling in heart failure and triggered arrhythmias.

Cytoskeletal structure and recovery in single human cardiac myocytes

BACKGROUND

Mechanical support of the failing human heart with a left ventricular assist device (LVAD) normalizes many components of myocyte structure and function. We hypothesized that recovery of the cytoskeleton, a major site of mechanotransduction in cardiac myocytes, is crucial for sustained improvement of myocardial function. We therefore measured the effects of LVAD support on 4 cytoskeletal proteins in single human heart cells.

METHODS

Myocytes were isolated from non-failing (NF), hypertrophied (H), failing (F) and LVAD-supported failing (L) human hearts. Protein quantitation was performed using Western blot analysis and cellular distribution was determined by immunolabeling and confocal microscopy.

RESULTS

alpha-actinin did not differ in cells from H or F as compared with NF, and L had no effect. Vinculin was not quantitatively different in H or F vs NF, but localization at the intercalated disks was significantly decreased in H and absent in F, and this pattern was consistently reversed in L. Desmin protein was significantly increased in F vs NF, both in quantity and distribution, and these increases were reversed in L. beta-tubulin was increasingly polymerized in H and F, and the hyperpolymerization was reversed in L.

CONCLUSIONS

On the level of the single cardiomyocyte, major proteins of the cytoskeleton are significantly altered in hypertrophied and failing human hearts. These alterations are reversed by mechanical unloading with an LVAD, suggesting that the cytoskeleton is not the limiting factor in determining full cardiac recovery.