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

Differential Regulation of Hemichannels and Gap Junction Channels by RhoA GTPase and Actin Cytoskeleton: A Comparative Analysis of Cx43 and Cx26

Connexins (Cxs) are transmembrane proteins that assemble into gap junction channels (GJCs) and hemichannels (HCs). Previous researches support the involvement of Rho GTPases and actin microfilaments in the trafficking of Cxs, formation of GJCs plaques, and regulation of channel activity. Nonetheless, it remains uncertain whether distinct types of Cxs HCs and GJCs respond differently to Rho GTPases or changes in actin polymerization/depolymerization dynamics. Our investigation revealed that inhibiting RhoA, a small GTPase that controls actin polymerization, or disrupting actin microfilaments with cytochalasin B (Cyto-B), resulted in reduced GJCs plaque size at appositional membranes and increased transport of HCs to non-appositional plasma membrane regions. Notably, these effects were consistent across different Cx types, since Cx26 and Cx43 exhibited similar responses, despite having distinct trafficking routes to the plasma membrane. Functional assessments showed that RhoA inhibition and actin depolymerization decreased the activity of Cx43 GJCs while significantly increasing HC activity. However, the functional status of GJCs and HCs composed of Cx26 remained unaffected. These results support the hypothesis that RhoA, through its control of the actin cytoskeleton, facilitates the transport of HCs to appositional cell membranes for GJCs formation while simultaneously limiting the positioning of free HCs at non-appositional cell membranes, independently of Cx type. This dynamic regulation promotes intercellular communications and reduces non-selective plasma membrane permeability through a Cx-type dependent mechanism, whereby the activity of Cx43 HCs and GJCs are differentially affected but Cx26 channels remain unchanged.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

Preventative therapeutic approaches for hypertrophic cardiomyopathy

Sarcomeric gene mutations are associated with the development of hypertrophic cardiomyopathy (HCM). Current drug therapeutics for HCM patients are effective in relieving symptoms, but do not prevent or reverse disease progression. Moreover, due to heterogeneity in the clinical manifestations of the disease, patients experience variable outcomes in response to therapeutics. Mechanistically, alterations in calcium handling, sarcomeric disorganization, energy metabolism and contractility participate in HCM disease progression. While some similarities exist, each mutation appears to lead to mutation‐specific pathophysiology. Furthermore, these alterations may precede or proceed development of the pathology. This review assesses the efficacy of HCM therapeutics from studies performed in animal models of HCM and human clinical trials. Evidence suggests that a preventative rather than corrective therapeutic approach may be more efficacious in the treatment of HCM. In addition, a clear understanding of mutation‐specific mechanisms may assist in informing the most effective therapeutic mode of action.

Alterations of Ca2+ signaling and Ca2+ release sites in cultured ventricular myocytes with intact internal Ca2+ storage

Although cultured adult cardiac myocytes in combination with cell-level genetic modifications have been adopted for the study of protein function, the cellular alterations caused by the culture conditions themselves need to be clarified before we can interpret the effects of genetically altered proteins. We systematically compared the cellular morphology, global Ca²⁺ signaling, elementary Ca²⁺ release (sparks), and arrangement of ryanodine receptor (RyR) clusters in short-term (2 days)-cultured adult rat ventricular myocytes with those of freshly isolated myocytes. The transverse (t)-tubules were remarkably decreased (to ∼25%) by culture, and whole-cell capacitance was reduced by ∼35%. The magnitude of depolarization-induced Ca²⁺ transients decreased to ∼50%, and Ca²⁺ transient decay was slowed by culture. The culture did not affect sarcoplasmic reticulum (SR) Ca²⁺ loading. Therefore, fractional Ca²⁺ release was attenuated by culture. In the cultured cells, the L-type Ca²⁺ current (I_Ca) was smaller (∼50% of controls) and its inactivation was slower. In cultured myocytes, there were significantly fewer (∼50% of control) Ca²⁺ sparks, the local Ca²⁺ releases through RyR clusters, compared with in freshly isolated cells. Amplitude and kinetics (duration and time-to-peak) of individual sparks were similar, but they showed greater width in cultured cells. Immunolocalization analysis revealed that the cross-striation of RyRs distribution became weaker and less organized, and that the density of RyR clusters decreased in cultured myocytes. Our data suggest that the loss of t-tubules and generation of compromised Ca²⁺ transients and I_Ca in short-term adult ventricular cell culture are independent of SR Ca²⁺ loading status. In addition, the deteriorated arrangement of the RyR-clusters and their decreased density after short-term culture may be partly responsible for fewer Ca²⁺ sparks and a decrease in global Ca²⁺ release.

Rapid Turnover of the Cardiac L-Type CaV1.2 Channel by Endocytic Recycling Regulates Its Cell Surface Availability

Calcium entry through Ca_V1.2 L-type calcium channels regulates cardiac contractility. Here, we study the impact of exocytic and post-endocytic trafficking on cell surface channel abundance in cardiomyocytes. Single-molecule localization and confocal microscopy reveal an intracellular Ca_V1.2 pool tightly associated with microtubules from the perinuclear region to the cell periphery, and with actin filaments at the cell cortex. Channels newly inserted into the plasma membrane become internalized with an average time constant of 7.5 min and are sorted out to the Rab11a-recycling compartment. Ca_V1.2 recycling suffices for maintaining stable L-type current amplitudes over 20 hr independent of de novo channel transport along microtubules. Disruption of the actin cytoskeleton re-routes Ca_V1.2 from recycling toward lysosomal degradation. We identify endocytic recycling as essential for the homeostatic regulation of voltage-dependent calcium influx into cardiomyocytes. This mechanism provides the basis for a dynamic adjustment of the channel's surface availability and thus, of heart's contraction.

A1603P and K1617del, Mutations in β-Cardiac Myosin Heavy Chain that Cause Laing Early-Onset Distal Myopathy, Affect Secondary Structure and Filament Formation In Vitro and In Vivo

Over 20 mutations in β-cardiac myosin heavy chain (β-MHC), expressed in cardiac and slow muscle fibers, cause Laing early-onset distal myopathy (MPD-1), a skeletal muscle myopathy. Most of these mutations are in the coiled-coil tail and commonly involve a mutation to a proline or a single-residue deletion, both of which are predicted to strongly affect the secondary structure of the coiled coil. To test this, we characterized the effects of two MPD-1 causing mutations: A1603P and K1617del in vitro and in cells. Both mutations affected secondary structure, decreasing the helical content of 15 heptad and light meromyosin constructs. Both mutations also severely disrupted the ability of glutathione S-transferase–light meromyosin fusion proteins to form minifilaments in vitro, as demonstrated by negative stain electron microscopy. Mutant eGFP-tagged β-MHC accumulated abnormally into the M-line of sarcomeres in cultured skeletal muscle myotubes. Incorporation of eGFP-tagged β-MHC into sarcomeres in adult rat cardiomyocytes was reduced. Molecular dynamics simulations using a composite structure of part of the coiled coil demonstrated that both mutations affected the structure, with the mutation to proline (A1603P) having a smaller effect compared to K1617del. Taken together, it seems likely that the MPD-1 mutations destabilize the coiled coil, resulting in aberrant myosin packing in thick filaments in muscle sarcomeres, providing a potential mechanism for the disease.

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It is unclear how mutations in the coiled coil of β-myosin heavy chain cause distal myopathy.

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A1603P and K1617del mutations reduce helicity and affect filament formation in vitro.

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eGFP-tagged β-myosin heavy chain abnormally accumulates at the M-line of sarcomeres in skeletal myotubes.

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Molecular dynamics simulations provide a molecular understanding for these experiments.

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Effects on structure and packing into the thick filament provide a molecular basis for the disease.

Transient activation of PKC results in long-lasting detrimental effects on systolic [Ca2+]i in cardiomyocytes by altering actin cytoskeletal dynamics and T-tubule integrity

AIMS

Protein kinase C (PKC) isozymes contribute to the development of heart failure through dysregulation of Ca²⁺ handling properties and disruption of contractile function in cardiomyocytes. However, the mechanisms by which PKC activation leads to Ca²⁺ dysfunction are incompletely understood.

METHODS AND RESULTS

Shortly upon ventricular pressure overload in mice, we detected transient PKC activation that was associated with pulsed actin cytoskeletal rearrangement. In cultured cardiomyocytes, transient activation of PKC promoted long-term deleterious effects on the integrity of the transverse (T)- tubule system, resulting in a significant decrease in the amplitude and increase in the rising kinetics of Ca²⁺ transients. Treatment with a PKCα/β inhibitor restored the synchronization of Ca²⁺ transients and maintained T-tubule integrity in cultured cardiomyocytes. Supporting these data, PKCα/β inhibition protected against T-tubule remodeling and cardiac dysfunction in a mouse model of pressure overload-induced heart failure. Mechanistically, transient activation of PKC resulted in biphasic actin cytoskeletal rearrangement, consistent with in vivo observations in the pressure overloaded mouse model. Transient inhibition of actin polymerization or depolymerization resulted in severe T-tubule damage, recapitulating the T-tubule damage induced by PKC activation. Moreover, inhibition of stretch activated channels (SAC) protected against T-tubule remodeling and E-C coupling dysfunction induced by transient PKC activation and actin cytoskeletal rearrangement.

CONCLUSIONS

These data identify a key mechanistic link between transient PKC activation and long-term Ca²⁺ handling defects through PKC-induced actin cytoskeletal rearrangement and resultant T-tubule damage.

The ESCRT-III pathway facilitates cardiomyocyte release of cBIN1-containing microparticles

Microparticles (MPs) are cell–cell communication vesicles derived from the cell surface plasma membrane, although they are not known to originate from cardiac ventricular muscle. In ventricular cardiomyocytes, the membrane deformation protein cardiac bridging integrator 1 (cBIN1 or BIN1+13+17) creates transverse-tubule (t-tubule) membrane microfolds, which facilitate ion channel trafficking and modulate local ionic concentrations. The microfold-generated microdomains continuously reorganize, adapting in response to stress to modulate the calcium signaling apparatus. We explored the possibility that cBIN1-microfolds are externally released from cardiomyocytes. Using electron microscopy imaging with immunogold labeling, we found in mouse plasma that cBIN1 exists in membrane vesicles about 200 nm in size, which is consistent with the size of MPs. In mice with cardiac-specific heterozygous Bin1 deletion, flow cytometry identified 47% less cBIN1-MPs in plasma, supporting cardiac origin. Cardiac release was also evidenced by the detection of cBIN1-MPs in medium bathing a pure population of isolated adult mouse cardiomyocytes. In human plasma, osmotic shock increased cBIN1 detection by enzyme-linked immunosorbent assay (ELISA), and cBIN1 level decreased in humans with heart failure, a condition with reduced cardiac muscle cBIN1, both of which support cBIN1 release in MPs from human hearts. Exploring putative mechanisms of MP release, we found that the membrane fission complex endosomal sorting complexes required for transport (ESCRT)-III subunit charged multivesicular body protein 4B (CHMP4B) colocalizes and coimmunoprecipitates with cBIN1, an interaction enhanced by actin stabilization. In HeLa cells with cBIN1 overexpression, knockdown of CHMP4B reduced the release of cBIN1-MPs. Using truncation mutants, we identified that the N-terminal BAR (N-BAR) domain in cBIN1 is required for CHMP4B binding and MP release. This study links the BAR protein superfamily to the ESCRT pathway for MP biogenesis in mammalian cardiac ventricular cells, identifying elements of a pathway by which cytoplasmic cBIN1 is released into blood.

Microparticles are small vesicles generated from the cell surface membrane and externally released for communication with other cells. We now show that heart ventricular muscle cells, which form the main pumping chambers of the heart, release microparticles in both mouse and human. Ventricular microparticles arise from surface membrane microdomains organized by cardiac bridging integrator 1 (cBIN1), a membrane deformation protein that has been shown to be reduced during human heart failure. Here we identify microparticles containing cBIN1 in blood, which were reduced in mutant mice lacking heart cBIN1 expression. Furthermore, the process leading to microparticle release involves the recruitment of CHMP4B protein to snip the cBIN1 membrane. In humans, cBIN1 is present in blood and within microparticles. Upon osmotic shock, human microparticles burst, allowing for the quantification of cBIN1 in plasma by enzyme-linked immunosorbent assay (ELISA). The measured cBIN1 level was greatly reduced in patients with heart failure. Thus, we introduce the biology for a new blood-based diagnostic tool that can assess cardiac muscle health and identify failing heart in human patients.

Cardiac T-Tubule Microanatomy and Function

Unique to striated muscle cells, transverse tubules (t-tubules) are membrane organelles that consist of sarcolemma penetrating into the myocyte interior, forming a highly branched and interconnected network. Mature t-tubule networks are found in mammalian ventricular cardiomyocytes, with the transverse components of t-tubules occurring near sarcomeric z-discs. Cardiac t-tubules contain membrane microdomains enriched with ion channels and signaling molecules. The microdomains serve as key signaling hubs in regulation of cardiomyocyte function. Dyad microdomains formed at the junctional contact between t-tubule membrane and neighboring sarcoplasmic reticulum are critical in calcium signaling and excitation-contraction coupling necessary for beat-to-beat heart contraction. In this review, we provide an overview of the current knowledge in gross morphology and structure, membrane and protein composition, and function of the cardiac t-tubule network. We also review in detail current knowledge on the formation of functional membrane subdomains within t-tubules, with a particular focus on the cardiac dyad microdomain. Lastly, we discuss the dynamic nature of t-tubules including membrane turnover, trafficking of transmembrane proteins, and the life cycles of membrane subdomains such as the cardiac BIN1-microdomain, as well as t-tubule remodeling and alteration in diseased hearts. Understanding cardiac t-tubule biology in normal and failing hearts is providing novel diagnostic and therapeutic opportunities to better treat patients with failing hearts.

A Simplified, Langendorff-Free Method for Concomitant Isolation of Viable Cardiac Myocytes and Nonmyocytes From the Adult Mouse Heart

RATIONALE

Cardiovascular disease represents a global pandemic. The advent of and recent advances in mouse genomics, epigenomics, and transgenics offer ever-greater potential for powerful avenues of research. However, progress is often constrained by unique complexities associated with the isolation of viable myocytes from the adult mouse heart. Current protocols rely on retrograde aortic perfusion using specialized Langendorff apparatus, which poses considerable logistical and technical barriers to researchers and demands extensive training investment.

OBJECTIVE

To identify and optimize a convenient, alternative approach, allowing the robust isolation and culture of adult mouse cardiac myocytes using only common surgical and laboratory equipment.

METHODS AND RESULTS

Cardiac myocytes were isolated with yields comparable to those in published Langendorff-based methods, using direct needle perfusion of the LV ex vivo and without requirement for heparin injection. Isolated myocytes can be cultured antibiotic free, with retained organized contractile and mitochondrial morphology, transcriptional signatures, calcium handling, responses to hypoxia, neurohormonal stimulation, and electric pacing, and are amenable to patch clamp and adenoviral gene transfer techniques. Furthermore, the methodology permits concurrent isolation, separation, and coculture of myocyte and nonmyocyte cardiac populations.

CONCLUSIONS

We present a novel, simplified method, demonstrating concomitant isolation of viable cardiac myocytes and nonmyocytes from the same adult mouse heart. We anticipate that this new approach will expand and accelerate innovative research in the field of cardiac biology.

Electrical consequences of cardiac myocyte: fibroblast coupling

Gap junctions are channels which allow electrical signals to propagate through the heart from the sinoatrial node and through the atria, conduction system and onwards to the ventricles, and hence are essential for co-ordinated cardiac contraction. Twelve connexin (Cx) proteins make up one gap junction channel, of which there are three main subtypes in the heart; Cx40, Cx43 and Cx45. In the cardiac myocyte, gap junctions are present mainly at the intercalated discs between neighbouring myocytes, and assist in rapid electrical conduction throughout the ventricular myocardium. Fibroblasts provide the structural skeleton of the myocardium and fibroblast numbers significantly increase in heart disease. Fibroblasts also express connexins and this may facilitate heterocellular electrical coupling between myocytes and fibroblasts in the setting of cardiac disease. Interestingly, cardiac fibroblasts have been demonstrated to increase Cx43 expression in experimental models of myocardial infarction and functional gap junctions between myocytes and fibroblasts have been reported. Therefore, in the setting of heart disease enhanced cardiac myocyte: fibroblast coupling may influence the electrical activity of the myocyte and contribute to arrhythmias.

Keeping up the balance: role of HDACs in cardiac proteostasis and therapeutic implications for atrial fibrillation

Cardiomyocytes are long-lived post-mitotic cells with limited regenerative capacity. Proper cardiomyocyte function depends critically on the maintenance of a healthy homeostasis of protein expression, folding, assembly, trafficking, function, and degradation, together commonly referred to as proteostasis. Impairment of proteostasis has a prominent role in the pathophysiology of ageing-related neurodegenerative diseases including Huntington's, Parkinson's, and Alzheimer's disease. Emerging evidence reveals also a role for impaired proteostasis in the pathophysiology of common human cardiac diseases such as cardiac hypertrophy, dilated and ischaemic cardiomyopathies, and atrial fibrillation (AF). Histone deacetylases (HDACs) have recently been recognized as key modulators which control cardiac proteostasis by deacetylating various proteins. By deacetylating chromatin proteins, including histones, HDACs modulate epigenetic regulation of pathological gene expression. Also, HDACs exert a broad range of functions outside the nucleus by deacetylating structural and contractile proteins. The cytosolic actions of HDACs result in changed protein function through post-translational modifications and/or modulation of their degradation. This review describes the mechanisms underlying the derailment of proteostasis in AF and subsequently focuses on the role of HDACs herein. In addition, the therapeutic potential of HDAC inhibition to maintain a healthy proteostasis resulting in a delay in AF onset and progression is discussed.

BIN1 regulates dynamic t-tubule membrane

Cardiac transverse tubules (t-tubules) are specific membrane organelles critical in calcium signaling and excitation-contraction coupling required for beat-to-beat heart contraction. T-tubules are highly branched and form an interconnected network that penetrates the myocyte interior to form junctions with the sarcoplasmic reticulum. T-tubules are selectively enriched with specific ion channels and proteins crucial in calcium transient development necessary in excitation-contraction coupling, thus t-tubules are a key component of cardiac myocyte function. In this review, we focus primarily on two proteins concentrated within the t-tubular network, the L-type calcium channel (LTCC) and associated membrane anchor protein, bridging integrator 1 (BIN1). Here, we provide an overview of current knowledge in t-tubule morphology, composition, microdomains, as well as the dynamics of the t-tubule network. Secondly, we highlight multiple aspects of BIN1-dependent t-tubule function, which includes forward trafficking of LTCCs to t-tubules, LTCC clustering at t-tubule surface, microdomain organization and regulation at t-tubule membrane, and the formation of a slow diffusion barrier within t-tubules. Lastly, we describe progress in characterizing how acquired human heart failure can be attributed to abnormal BIN1 transcription and associated t-tubule remodeling. Understanding BIN1-regulated cardiac t-tubule biology in human heart failure management has the dual benefit of promoting progress in both biomarker development and therapeutic target identification. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Forskolin Regulates L-Type Calcium Channel through Interaction between Actinin 4 and β3 Subunit in Osteoblasts

Voltage-dependent L-type calcium channels that permit cellular calcium influx are essential in calcium-mediated modulation of cellular signaling. Although the regulation of voltage-dependent L-type calcium channels is linked to many factors including cAMP-dependent protein kinase A (PKA) activity and actin cytoskeleton, little is known about the detailed mechanisms underlying the regulation in osteoblasts. Our present study investigated the modulation of L-type calcium channel activities through the effects of forskolin on actin reorganization and on its functional interaction with actin binding protein actinin 4. The results showed that forskolin did not significantly affect the trafficking of pore forming α_1c subunit and its interaction with actin binding protein actinin 4, whereas it significantly increased the expression of β₃ subunit and its interaction with actinin 4 in osteoblast cells as assessed by co-immunoprecipitation, pull-down assay, and immunostaining. Further mapping showed that the ABD and EF domains of actinin 4 were interaction sites. This interaction is independent of PKA phosphorylation. Knockdown of actinin 4 significantly decreased the activities of L-type calcium channels. Our study revealed a new aspect of the mechanisms by which the forskolin activation of adenylyl cyclase - cAMP cascade regulates the L-type calcium channel in osteoblast cells, besides the PKA mediated phosphorylation of the channel subunits. These data provide insight into the important role of interconnection among adenylyl cyclase, cAMP, PKA, the actin cytoskeleton, and the channel proteins in the regulation of voltage-dependent L-type calcium channels in osteoblast cells.

Cardiomyocyte protein trafficking: Relevance to heart disease and opportunities for therapeutic intervention

Cardiomyocytes, the individual contractile units of heart muscle, are long-lived and robust. Given the longevity of these cells, it can be easy to overlook their dynamic intracellular environment that contain rapid protein movements and frequent protein turnover. Critical gene transcription and protein translation occur continuously, as well as trafficking and localization of proteins to specific functional zones of cell membrane. As heart failure becomes an increasingly important clinical entity, growing numbers of investigative teams are examining the cell biology of healthy and diseased cardiomyocytes. In this review, we introduce the major architectural structures and types of protein movements within cardiac cells, and then review recent studies that explore the regulation of such movements. We conclude by introducing current translational directions of the basic studies with a focus on novel areas of therapeutic development.

Caveolin contributes to the modulation of basal and β-adrenoceptor stimulated function of the adult rat ventricular myocyte by simvastatin: a novel pleiotropic effect

The number of people taking statins is increasing across the globe, highlighting the importance of fully understanding statins' effects on the cardiovascular system. The beneficial impact of statins extends well beyond regression of atherosclerosis to include direct effects on tissues of the cardiovascular system ('pleiotropic effects'). Pleiotropic effects on the cardiac myocyte are often overlooked. Here we consider the contribution of the caveolin protein, whose expression and cellular distribution is dependent on cholesterol, to statin effects on the cardiac myocyte. Caveolin is a structural and regulatory component of caveolae, and is a key regulator of cardiac contractile function and adrenergic responsiveness. We employed an experimental model in which inhibition of myocyte HMG CoA reductase could be studied in the absence of paracrine influences from non-myocyte cells. Adult rat ventricular myocytes were treated with 10 µM simvastatin for 2 days. Simvastatin treatment reduced myocyte cholesterol, caveolin 3 and caveolar density. Negative inotropic and positive lusitropic effects (with corresponding changes in [Ca2+]i) were seen in statin-treated cells. Simvastatin significantly potentiated the inotropic response to β2-, but not β1-, adrenoceptor stimulation. Under conditions of β2-adrenoceptor stimulation, phosphorylation of phospholamban at Ser16 and troponin I at Ser23/24 was enhanced with statin treatment. Simvastatin increased NO production without significant effects on eNOS expression or phosphorylation (Ser1177), consistent with the reduced expression of caveolin 3, its constitutive inhibitor. In conclusion, statin treatment can reduce caveolin 3 expression, with functional consequences consistent with the known role of caveolae in the cardiac cell. These data are likely to be of significance, particularly during the early phases of statin treatment, and in patients with heart failure who have altered β-adrenoceptor signalling. In addition, as caveolin is ubiquitously expressed and has myriad tissue-specific functions, the impact of statin-dependent changes in caveolin is likely to have many other functional sequelae.

Spectrum of Cav1.4 dysfunction in congenital stationary night blindness type 2

Defective retinal synaptic transmission in patients affected with congenital stationary night blindness type 2 (CSNB2) can result from different dysfunction phenotypes in Cav1.4 L-type calcium channels. Here we investigated two prototypical Cav1.4 variants from either end of the functional spectrum. Using whole-cell and single-channel patch-clamp techniques, we provide analysis of the biophysical characteristics of the point mutation L860P and the C-terminal truncating mutation R1827X. L860P showed a typical loss-of-function phenotype attributed to a reduced number of functional channels expressed at the plasma membrane as implied by gating current and non-stationary noise analyses. This phenotype can be rationalized, because the inserted proline is predicted to break an amphipatic helix close to the transmembrane segment IIIS1 and thus to reduce channel stability and promote misfolding. In fact, L860P was subject to an increased turnover. In contrast, R1827X displayed an apparent gain-of-function phenotype, i.e., due to a hyperpolarizing shift of the IV-curve and increased single-channel activity. However, truncation also resulted in the loss of functional C-terminal modulation and thus unmasked calcium-dependent inactivation. Thus R1827X failed to support continuous calcium influx. Current inactivation curtails the dynamic range of photoreceptors (e.g., when adapting to variation in illumination). Taken together, the analysis of two representative mutations that occur in CSNB2 patients revealed fundamental differences in the underlying defect. These may explain subtle variations in the clinical manifestation and must be taken into account, if channel function is to be restored by pharmacochaperones or related approaches.

Cardiac BIN1 folds T-tubule membrane, controlling ion flux and limiting arrhythmia

Cardiomyocyte T tubules are important for regulating ion flux. Bridging integrator 1 (BIN1) is a T-tubule protein associated with calcium channel trafficking that is downregulated in failing hearts. Here we find that cardiac T tubules normally contain dense protective inner membrane folds that are formed by a cardiac isoform of BIN1. In mice with cardiac Bin1 deletion, T-tubule folding is decreased, which does not change overall cardiomyocyte morphology but leads to free diffusion of local extracellular calcium and potassium ions, prolonging action-potential duration and increasing susceptibility to ventricular arrhythmias. We also found that T-tubule inner folds are rescued by expression of the BIN1 isoform BIN1+13+17, which promotes N-WASP-dependent actin polymerization to stabilize the T-tubule membrane at cardiac Z discs. BIN1+13+17 recruits actin to fold the T-tubule membrane, creating a 'fuzzy space' that protectively restricts ion flux. When the amount of the BIN1+13+17 isoform is decreased, as occurs in acquired cardiomyopathy, T-tubule morphology is altered, and arrhythmia can result.

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.

Activation of histone deacetylase-6 induces contractile dysfunction through derailment of α-tubulin proteostasis in experimental and human atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is characterized by structural remodeling, contractile dysfunction, and AF progression. Histone deacetylases (HDACs) influence acetylation of both histones and cytosolic proteins, thereby mediating epigenetic regulation and influencing cell proteostasis. Because the exact function of HDACs in AF is unknown, we investigated their role in experimental and clinical AF models.

METHODS AND RESULTS

Tachypacing of HL-1 atrial cardiomyocytes and Drosophila pupae hearts significantly impaired contractile function (amplitude of Ca(2+) transients and heart wall contractions). This dysfunction was prevented by inhibition of HDAC6 (tubacin) and sirtuins (nicotinamide). Tachypacing induced specific activation of HDAC6, resulting in α-tubulin deacetylation, depolymerization, and degradation by calpain. Tachypacing-induced contractile dysfunction was completely rescued by dominant-negative HDAC6 mutants with loss of deacetylase activity in the second catalytic domain, which bears α-tubulin deacetylase activity. Furthermore, in vivo treatment with the HDAC6 inhibitor tubastatin A protected atrial tachypaced dogs from electric remodeling (action potential duration shortening, L-type Ca(2+) current reduction, AF promotion) and cellular Ca(2+)-handling/contractile dysfunction (loss of Ca(2+) transient amplitude, sarcomere contractility). Finally, atrial tissue from patients with AF also showed a significant increase in HDAC6 activity and reduction in the expression of both acetylated and total α-tubulin.

CONCLUSIONS

AF induces remodeling and loss of contractile function, at least in part through HDAC6 activation and subsequent derailment of α-tubulin proteostasis and disruption of the cardiomyocyte microtubule structure. In vivo inhibition of HDAC6 protects against AF-related atrial remodeling, disclosing the potential of HDAC6 as a therapeutic target in clinical AF.

Cardiomyopathy mutations in the tail of β-cardiac myosin modify the coiled-coil structure and affect integration into thick filaments in muscle sarcomeres in adult cardiomyocytes

It is unclear why mutations in the filament-forming tail of myosin heavy chain (MHC) cause hypertrophic or dilated cardiomyopathy as these mutations should not directly affect contraction. To investigate this, we first investigated the impact of five hypertrophic cardiomyopathy-causing (N1327K, E1356K, R1382W, E1555K, and R1768K) and one dilated cardiomyopathy-causing (R1500W) tail mutations on their ability to incorporate into muscle sarcomeres in vivo. We used adenoviral delivery to express full-length wild type or mutant enhanced GFP-MHC in isolated adult cardiomyocytes. Three mutations (N1327K, E1356K, and E1555K) reduced enhanced GFP-MHC incorporation into muscle sarcomeres, whereas the remainder had no effect. No mutations significantly affected contraction. Fluorescence recovery after photobleaching showed that fluorescence recovery for the mutation that incorporated least well (N1327K) was significantly faster than that of WT with half-times of 25.1 ± 1.8 and 32.2 ± 2.5 min (mean ± S.E.), respectively. Next, we determined the effects of each mutation on the helical properties of wild type and seven mutant peptides (7, 11, or 15 heptads long) from the myosin tail by circular dichroism. R1382W and E1768K slightly increased the α-helical nature of peptides. The remaining mutations reduced α-helical content, with N1327K showing the greatest reduction. Only peptides containing residues 1301-1329 were highly α-helical suggesting that this region helps in initiation of coiled coil. These results suggest that small effects of mutations on helicity translate into a reduced ability to incorporate into sarcomeres, which may elicit compensatory hypertrophy.

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.

Cytoskeletal roles in cardiac ion channel expression

The cytoskeleton and cardiac ion channel expression are closely linked. From the time that newly synthesized channels exit the endoplasmic reticulum, they are either traveling along the microtubule or actin cytoskeletons or likely anchored in the plasma membrane or in internal vesicular pools by those scaffolds. Molecular motors, small GTPases and even the dynamics of the cytoskeletons themselves influence the trafficking and expression of the channels. In some cases, the functioning of the channels themselves has profound influences on the cytoskeleton. Here we provide an overview of the current state of knowledge on the involvement of the actin and microtubule cytoskeletons in the trafficking, targeting and expression of cardiac ion channels and a few channels expressed elsewhere. We highlight, also, some of the many questions that remain about these processes. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

L-type calcium channel targeting and local signalling in cardiac myocytes

In the heart, Ca(2+) influx via Ca(V)1.2 L-type calcium channels (LTCCs) is a multi-functional signal that triggers muscle contraction, controls action potential duration, and regulates gene expression. The use of LTCC Ca(2+) as a multi-dimensional signalling molecule in the heart is complicated by several aspects of cardiac physiology. Cytosolic Ca(2+) continuously cycles between ~100 nM and ~1 μM with each heartbeat due to Ca(2+) linked signalling from LTCCs to ryanodine receptors. This rapid cycling raises the question as to how cardiac myocytes distinguish the Ca(2+) fluxes originating through L-type channels that are dedicated to contraction from Ca(2+) fluxes originating from other L-type channels that are used for non-contraction-related signalling. In general, disparate Ca(2+) sources in cardiac myocytes such as current through differently localized LTCCs as well as from IP3 receptors can signal selectively to Ca(2+)-dependent effectors in local microdomains that can be impervious to the cytoplasmic Ca(2+) transients that drive contraction. A particular challenge for diversified signalling via cardiac LTCCs is that they are voltage-gated and, therefore, open and presumably flood their microdomains with Ca(2+) with each action potential. Thus spatial localization of Cav1.2 channels to different types of microdomains of the ventricular cardiomyocyte membrane as well as the existence of particular macromolecular complexes in each Cav1.2 microdomain are important to effect different types of Cav1.2 signalling. In this review we examine aspects of Cav1.2 structure, targeting and signalling in two specialized membrane microdomains--transverse tubules and caveolae.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

BIN1 is reduced and Cav1.2 trafficking is impaired in human failing cardiomyocytes

BACKGROUND

Heart failure is a growing epidemic, and a typical aspect of heart failure pathophysiology is altered calcium transients. Normal cardiac calcium transients are initiated by Cav1.2 channels at cardiac T tubules. Bridging integrator 1 (BIN1) is a membrane scaffolding protein that causes Cav1.2 to traffic to T tubules in healthy hearts. The mechanisms of Cav1.2 trafficking in heart failure are not known.

OBJECTIVE

To study BIN1 expression and its effect on Cav1.2 trafficking in failing hearts.

METHODS

Intact myocardium and freshly isolated cardiomyocytes from nonfailing and end-stage failing human hearts were used to study BIN1 expression and Cav1.2 localization. To confirm Cav1.2 surface expression dependence on BIN1, patch-clamp recordings were performed of Cav1.2 current in cell lines with and without trafficking-competent BIN1. Also, in adult mouse cardiomyocytes, surface Cav1.2 and calcium transients were studied after small hairpin RNA-mediated knockdown of BIN1. For a functional readout in intact heart, calcium transients and cardiac contractility were analyzed in a zebrafish model with morpholino-mediated knockdown of BIN1.

RESULTS

BIN1 expression is significantly decreased in failing cardiomyocytes at both mRNA (30% down) and protein (36% down) levels. Peripheral Cav1.2 is reduced to 42% by imaging, and a biochemical T-tubule fraction of Cav1.2 is reduced to 68%. The total calcium current is reduced to 41% in a cell line expressing a nontrafficking BIN1 mutant. In mouse cardiomyocytes, BIN1 knockdown decreases surface Cav1.2 and impairs calcium transients. In zebrafish hearts, BIN1 knockdown causes a 75% reduction in calcium transients and severe ventricular contractile dysfunction.

CONCLUSIONS

The data indicate that BIN1 is significantly reduced in human heart failure, and this reduction impairs Cav1.2 trafficking, calcium transients, and contractility.

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.

Functional and morphological preservation of adult ventricular myocytes in culture by sub-micromolar cytochalasin D supplement

In cardiac myocytes, cytochalasin D (CytoD) was reported to act as an actin disruptor and mechanical uncoupler. Using confocal and super-resolution STED microscopy, we show that CytoD preserves the actin filament architecture of adult rat ventricular myocytes in culture. Five hundred nanomolar CytoD was the optimal concentration to achieve both preservation of the T-tubular structure during culture periods of 3 days and conservation of major functional characteristics such as action potentials, calcium transients and, importantly, the contractile properties of single myocytes. Therefore, we conclude that the addition of CytoD to the culture of adult cardiac myocytes can indeed be used to generate a solid single-cell model that preserves both morphology and function of freshly isolated cells. Moreover, we reveal a putative link between cytoskeletal and T-tubular remodeling. In the absence of CytoD, we observed a loss of T-tubules that led to significant dyssynchronous Ca(2+)-induced Ca(2+) release (CICR), while in the presence of 0.5 μM CytoD, T-tubules and homogeneous CICR were majorly preserved. Such data suggested a possible link between the actin cytoskeleton, T-tubules and synchronous, reliable excitation-contraction-coupling. Thus, T-tubular re-organization in cell culture sheds some additional light onto similar processes found during many cardiac diseases and might link cytoskeletal alterations to changes in subcellular Ca(2+) signaling revealed under such pathophysiological conditions.

L-type calcium channel activity in osteoblast cells is regulated by the actin cytoskeleton independent of protein trafficking

Voltage-dependent L-type calcium channels (VDCC) play important roles in many cellular processes. The interaction of the actin cytoskeleton with the channel in nonexcitable cells is less well understood. We performed whole-cell patch-clamp surface biotinylation and calcium imaging on different osteoblast cells to determine channel kinetics, amplitude, surface abundance, and intracellular calcium, respectively. Patch-clamp studies showed that actin polymerization by phalloidin increased the peak current density of I (Ca), whereas actin depolymerization by cytochalasin D (CD) significantly decreased the current amplitude. This result is consistent with calcium imaging, which showed that CD significantly decreased Bay K8644-induced intracellular calcium increase. Surface biotinylation studies showed that CD is not able to affect the surface expression of the pore-forming subunit α(1C). Interestingly, application of CD caused a significantly negative shift in the steady-state inactivation kinetics of I (Ca). There were decreases in the voltage at half-maximal inactivation that changed in a dose-dependent manner. CD also reduced the effect of activated vitamin D(3) (1α,25-D3) on VDCC and intracellular calcium. We conclude that in osteoblasts the actin cytoskeleton affects α(1C) by altering the channel kinetic properties, instead of changing the surface expression, and it is able to regulate 1α,25-D3 signaling through VDCC. Our study provides a new insight into calcium regulation in osteoblasts, which are essential in many physiological functions of this cell.

Methods in cardiomyocyte isolation, culture, and gene transfer

Since techniques for cardiomyocyte isolation were first developed 35 years ago, experiments on single myocytes have yielded great insight into their cellular and sub-cellular physiology. These studies have employed a broad range of techniques including electrophysiology, calcium imaging, cell mechanics, immunohistochemistry and protein biochemistry. More recently, techniques for cardiomyocyte culture have gained additional importance with the advent of gene transfer technology. While such studies require a high quality cardiomyocyte population, successful cell isolation and maintenance during culture remain challenging. In this review, we describe methods for the isolation of adult and neonatal ventricular myocytes from rat and mouse heart. This discussion outlines general principles for the beginner, but also provides detailed specific protocols and advice for common caveats. We additionally review methods for short-term myocyte culture, with particular attention given to the importance of substrate and media selection, and describe time-dependent alterations in myocyte physiology that should be anticipated. Gene transfer techniques for neonatal and adult cardiomyocytes are also reviewed, including methods for transfection (liposome, electroporation) and viral-based gene delivery.

HSPB1, HSPB6, HSPB7 and HSPB8 protect against RhoA GTPase-induced remodeling in tachypaced atrial myocytes

Background

We previously demonstrated the small heat shock protein, HSPB1, to prevent tachycardia remodeling in in vitro and in vivo models for Atrial Fibrillation (AF). To gain insight into its mechanism of action, we examined the protective effect of all 10 members of the HSPB family on tachycardia remodeling. Furthermore, modulating effects of HSPB on RhoA GTPase activity and F-actin stress fiber formation were examined, as this pathway was found of prime importance in tachycardia remodeling events and the initiation of AF.

Methods and Results

Tachypacing (4 Hz) of HL-1 atrial myocytes significantly and progressively reduced the amplitude of Ca²⁺ transients (CaT). In addition to HSPB1, also overexpression of HSPB6, HSPB7 and HSPB8 protected against tachypacing-induced CaT reduction. The protective effect was independent of HSPB1. Moreover, tachypacing induced RhoA GTPase activity and caused F-actin stress fiber formation. The ROCK inhibitor Y27632 significantly prevented tachypacing-induced F-actin formation and CaT reductions, showing that RhoA activation is required for remodeling. Although all protective HSPB members prevented the formation of F-actin stress fibers, their mode of action differs. Whilst HSPB1, HSPB6 and HSPB7 acted via direct prevention of F-actin formation, HSPB8-protection was mediated via inhibition of RhoA GTPase activity.

Conclusion

Overexpression of HSPB1, as well as HSPB6, HSPB7 and HSPB8 independently protect against tachycardia remodeling by attenuation of the RhoA GTPase pathway at different levels. The cardioprotective role for multiple HSPB members indicate a possible therapeutic benefit of compounds able to boost the expression of single or multiple members of the HSPB family.

Divide and conquer: the application of organelle proteomics to heart failure

Chronic heart failure is a worldwide cause of mortality and morbidity and is the final outcome of a number of different etiologies. This reflects both the complexity of the disease and our incomplete understanding of its underlying molecular mechanisms. One experimental approach to address this is to study subcellular organelles and how their functions are activated and synchronized under physiological and pathological conditions. In this review, we discuss the application of proteomic technologies to organelles and how this has deepened our perception of the cellular proteome and its alterations with heart failure. The use of proteomics to monitor protein quantity and posttranslational modifications has revealed a highly intricate and sophisticated level of protein regulation. Posttranslational modifications have the potential to regulate organelle function and interplay most likely by targeting both structural and signaling proteins throughout the cell, ultimately coordinating their responses. The potentials and limitations of existing proteomic technologies are also discussed emphasizing that the development of novel methods will enhance our ability to further investigate organelles and decode intracellular communication.

Numerical analysis of Ca2+ signaling in rat ventricular myocytes with realistic transverse-axial tubular geometry and inhibited sarcoplasmic reticulum

The t-tubules of mammalian ventricular myocytes are invaginations of the cell membrane that occur at each Z-line. These invaginations branch within the cell to form a complex network that allows rapid propagation of the electrical signal, and hence synchronous rise of intracellular calcium (Ca²⁺). To investigate how the t-tubule microanatomy and the distribution of membrane Ca²⁺ flux affect cardiac excitation-contraction coupling we developed a 3-D continuum model of Ca²⁺ signaling, buffering and diffusion in rat ventricular myocytes. The transverse-axial t-tubule geometry was derived from light microscopy structural data. To solve the nonlinear reaction-diffusion system we extended SMOL software tool (http://mccammon.ucsd.edu/smol/). The analysis suggests that the quantitative understanding of the Ca²⁺ signaling requires more accurate knowledge of the t-tubule ultra-structure and Ca²⁺ flux distribution along the sarcolemma. The results reveal the important role for mobile and stationary Ca²⁺ buffers, including the Ca²⁺ indicator dye. In agreement with experiment, in the presence of fluorescence dye and inhibited sarcoplasmic reticulum, the lack of detectible differences in the depolarization-evoked Ca²⁺ transients was found when the Ca²⁺ flux was heterogeneously distributed along the sarcolemma. In the absence of fluorescence dye, strongly non-uniform Ca²⁺ signals are predicted. Even at modest elevation of Ca²⁺, reached during Ca²⁺ influx, large and steep Ca²⁺ gradients are found in the narrow sub-sarcolemmal space. The model predicts that the branched t-tubule structure and changes in the normal Ca²⁺ flux density along the cell membrane support initiation and propagation of Ca²⁺ waves in rat myocytes.

In cardiac muscle cells, calcium (Ca²⁺) is best known for its role in contraction activation. A remarkable amount of quantitative data on cardiac cell structure, ion-transporting protein distributions and intracellular Ca²⁺ dynamics has been accumulated. Various alterations in the protein distributions or cell ultra-structure are now recognized to be the primary mechanisms of cardiac dysfunction in a diverse range of common pathologies including cardiac arrhythmias and hypertrophy. Using a 3-D computational model, incorporating more realistic transverse-axial t-tubule geometry and considering geometric irregularities and inhomogeneities in the distribution of ion-transporting proteins, we analyze several important spatial and temporal features of Ca²⁺ signaling in rat ventricular myocytes. This study demonstrates that the computational models could serve as powerful tools for prediction and analyses of how the Ca²⁺ dynamics and cardiac excitation-contraction coupling are regulated under normal conditions or certain pathologies. The use of computational and mathematical approaches will help also to better understand aspects of cell functions that are not currently amenable to experimental investigation.

Remodelling of Ca2+ handling organelles in adult rat ventricular myocytes during long term culture

It is well known that for cardiomyocytes, isolation and culturing induce largely unknown remodelling processes. We analysed changes in the structure of cell compartments with optical techniques such as confocal microscopy and fluorescence redistribution after photobleaching employing adenoviral-mediated transduction of targeted fluorescent proteins and small molecule dyes. We identified characteristic remodelling processes: the T-tubular membrane system was gradually lost by a process referred to as "sequential pinching off", in an outward direction. Mitochondria fell in one of three classes, very small (0.9 microm length), medium long (1.8 microm) or extended shape (3.6 microm) organelles. Over the culturing time mitochondria gradually fused. Bleaching of individual mitochondria revealed association between apparently separated mitochondria by "tunnelling" via sub-resolution organelle-tubes. This tunnelling process was increasing over the culturing time. A gradual loss of the cross-striation arrangement in the endoplasmic/sarcoplasmic reticulum was visualised. Analysis of large populations of Ca(2+) sparks by video-rate confocal 2D-scanning revealed significant albeit small changes of these elementary SR-Ca(2+) release events in adult cardiomyocytes that could be related to changes in SR-Ca(2+) content rather than resting Ca(2+) concentration. In conclusion, primary isolated cardiomyocytes from adult hearts undergo a well-defined, but reproducible subcellular remodelling during optimised long term culture.

There goes the neighborhood: pathological alterations in T-tubule morphology and consequences for cardiomyocyte Ca2+ handling

T-tubules are invaginations of the cardiomyocyte membrane into the cell interior which form a tortuous network. T-tubules provide proximity between the electrically excitable cell membrane and the sarcoplasmic reticulum, the main intracellular Ca²⁺ store. Tight coupling between the rapidly spreading action potential and Ca²⁺ release units in the SR membrane ensures synchronous Ca²⁺ release throughout the cardiomyocyte. This is a requirement for rapid and powerful contraction. In recent years, it has become clear that T-tubule structure and composition are altered in several pathological states which may importantly contribute to contractile defects in these conditions. In this review, we describe the “neighborhood” of proteins in the dyadic cleft which locally controls cardiomyocyte Ca²⁺ homeostasis and how alterations in T-tubule structure and composition may alter this neighborhood during heart failure, atrial fibrillation, and diabetic cardiomyopathy. Based on this evidence, we propose that T-tubules have the potential to serve as novel therapeutic targets.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.

Ezrin mediates tethering of the gamma-aminobutyric acid transporter GAT1 to actin filaments via a C-terminal PDZ-interacting domain

A high density of neurotransmitter transporters on axons and presynaptic boutons is required for the efficient clearance of neurotransmitters from the synapse. Therefore, regulators of transporter trafficking (insertion, retrieval, and confinement) can play an important role in maintaining the transporter density necessary for effective function. We determined the interactions that confine GAT1 at the membrane by investigating the lateral mobility of GAT1-yellow fluorescent protein-8 (YFP8) expressed in neuroblastoma 2a cells. Through fluorescence recovery after photobleaching, we found that a significant fraction ( approximately 50%) of membrane-localized GAT1 is immobile on the time scale investigated ( approximately 150 s). The mobility of the transporter can be increased by depolymerizing actin or by interrupting the GAT1 postsynaptic density 95/Discs large/zona occludens 1 (PDZ)-interacting domain. Microtubule depolymerization, in contrast, does not affect GAT1 membrane mobility. We also identified ezrin as a major GAT1 adaptor to actin. Förster resonance energy transfer suggests that GAT1-YFP8 and cyan fluorescent (CFP) tagged ezrin (ezrin-CFP) exist within a complex that has a Förster resonance energy transfer efficiency of 19% +/- 2%. This interaction can be diminished by disrupting the actin cytoskeleton. In addition, the disruption of actin results in a >3-fold increase in gamma-aminobutyric acid uptake, apparently via a mechanism distinct from the PDZ-interacting protein. Our data reveal that actin confines GAT1 to the plasma membrane via ezrin, and this interaction is mediated through the PDZ-interacting domain of GAT1.

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.

Spontaneous frequency of rabbit atrioventricular node myocytes depends on SR function

Spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) appears to play an important role in cardiac sinoatrial node pacemaking. However, comparatively little is known about the role of intracellular Ca(2+) in the atrioventricular node (AVN). Intracellular Ca(2+) was therefore monitored in cells isolated from the rabbit AVN, using fluo-3 in conjunction with confocal microscopy. These cells displayed spontaneous Ca(2+) transients and action potentials. Ca(2+) transients were normally preceded by a small, slow increase (ramp) of intracellular Ca(2+) which was sometimes, but not always, accompanied by Ca(2+) sparks. During the Ca(2+) transient, intracellular [Ca(2+)] increased initially at the cell periphery and propagated inhomogeneously to the cell centre. The rate of spontaneous activity was decreased by ryanodine (1muM) and increased by isoprenaline (500nM); these changes were accompanied by a decrease and increase, respectively, in the slope of the preceding Ca(2+) ramp, with no significant change in Ca(2+) spark characteristics. Rapidly reducing bathing [Na(+)] inhibited spontaneous activity. These findings provide the first information on Ca(2+) handling at the sub-cellular level and link cellular Ca(2+) cycling to the genesis of spontaneous activity in the AVN.

Contractile regulation by overexpressed ETArequires intact T tubules in adult rat ventricular myocytes

Endothelin (ET)-1 regulates the contractility and growth of the heart by binding G protein-coupled receptors of the ET type A receptor (ETA)/ET type B (ETB) receptor family. ETA, the predominant ET-1 receptor subtype in myocardium, is thought to localize preferentially within cardiac T tubules, but the consequences of mislocalization are not fully understood. Here we examined the effects of the overexpression of ETAin conjunction with T-tubule loss in cultured adult rat ventricular myocytes. In adult myocytes cultured for 3 to 4 days, the normally robust positive inotropic effect (PIE) of ET-1 was lost in parallel with T-tubule degeneration and a decline in ETAprotein levels. In these T tubule-compromised myocytes, an overexpression of ETAusing an adenoviral vector did not rescue the responsiveness to ET-1, despite the robust expression in the surface sarcolemma. The inclusion of the actin polymerization inhibitor cytochalasin D (CD) during culture prevented gross morphological changes including a loss of T tubules and a rounding of intercalated discs, but CD alone did not rescue the responsiveness to ET-1 or prevent ETAdownregulation. The rescue of a normal PIE in 3- to 4-day cultured myocytes required both an increased expression of ETAand intact T tubules (preserved with CD). Therefore, the activation of ETAlocalized in T tubules was associated with a strong PIE, whereas the activation of ETAin surface sarcolemma was not. The results provide insight into the pathological cardiac conditions in which ETAis upregulated and T-tubule morphology is altered.

Functional vitamin D receptor (VDR) in the t-tubules of cardiac myocytes: VDR knockout cardiomyocyte contractility

We have previously shown that the active form of vitamin D, 1,25 dihydroxyvitamin D3 [1,25(OH)(2)D(3)], has both genomic and rapid nongenomic effects in heart cells; however, the subcellular localization of the vitamin D receptor (VDR) in heart has not been studied. Here we show that in adult rat cardiac myocytes the VDR is primarily localized to the t-tubule. Using immunofluorescence and Western blot analysis, we show that the VDR is closely associated with known t-tubule proteins. Radioligand binding assays using (3)H-labeled 1,25(OH)(2)D(3) demonstrate that a t-tubule membrane fraction isolated from homogenized rat ventricles contains a 1,25(OH)(2)D(3)-binding activity similar to the classic VDR. For the first time, we show that cardiac myocytes isolated from VDR knockout mice show accelerated rates of contraction and relaxation as compared with wild type and that 1,25(OH)(2)D(3) directly affects contractility in the wild-type but not the knockout cardiac myocyte. Moreover, we observed that acute (5 min) exposure to 1,25(OH)(2)D(3) altered the rate of relaxation. A receptor localized to t-tubules in the heart is ideally positioned to exert an immediate effect on signal transduction mediators and ion channels. This novel discovery is fundamentally important in understanding 1,25(OH)(2)D(3) signal transduction in heart cells and provides further evidence that the VDR plays a role in heart structure and function.

Transformation of adult rat cardiac myocytes in primary culture

We characterized the morphological, electrical and mechanical alterations of cardiomyocytes in long-term cell culture. Morphometric parameters, sarcomere length, T-tubule density, cell capacitance, L-type calcium current (I(Ca,L)), inward rectifier potassium current (I(K1)), cytosolic calcium transients, action potential and contractile parameters of adult rat ventricular myocytes were determined on each day of 5 days in culture. We also analysed the health of the myocytes using an apoptotic/necrotic viability assay. The data show that myocytes undergo profound morphological and functional changes during culture. We observed a progressive reduction in the cell area (from 2502 +/- 70 microm(2) on day 0 to 1432 +/- 50 microm(2) on day 5), T-tubule density, systolic shortening (from 0.11 +/- 0.02 to 0.05 +/- 0.01 microm) and amplitude of calcium transients (from 1.54 +/- 0.19 to 0.67 +/- 0.19) over 5 days of culture. The negative force-frequency relationship, characteristic of rat myocardium, was maintained during the first 2 days but diminished thereafter. Cell capacitance (from 156 +/- 8 to 105 +/- 11 pF) and membrane currents were also reduced (I(Ca,L), from 3.98 +/- 0.39 to 2.12 +/- 0.37 pA pF; and I(K1), from 34.34p +/- 2.31 to 18.00 +/- 5.97 pA pF(-1)). We observed progressive depolarization of the resting membrane potential during culture (from 77.3 +/- 2.5 to 34.2 +/- 5.9 mV) and, consequently, action potential morphology was profoundly altered as well. The results of the viability assays indicate that these alterations could not be attributed to either apoptosis or necrosis but are rather an adaptation to the culture conditions over time.

Interactive effects of surface topography and pulsatile electrical field stimulation on orientation and elongation of fibroblasts and cardiomyocytes

In contractile tissues such as myocardium, functional properties are directly related to the cellular orientation and elongation. Thus, tissue engineering of functional cardiac patches critically depends on our understanding of the interaction between multiple guidance cues such as topographical, adhesive or electrical. The main objective of this study was to determine the interactive effects of contact guidance and electrical field stimulation on elongation and orientation of fibroblasts and cardiomyocytes, major cell populations of the myocardium. Polyvinyl surfaces were abraded using lapping paper with grain size 1-80 microm, resulting in V-shaped abrasions with the average abrasion peak-to-peak width in the range from 3 to 13 microm, and the average depth in the range from 140 to 700 nm (AFM). The surfaces with abrasions 13 microm wide and 700 nm deep, exhibited the strongest effect on neonatal rat cardiomyocyte elongation and orientation as well as statistically significant effect on orientation of fibroblasts, thus they were utilized for electrical field stimulation. Electrical field stimulation was performed using a regime of relevance for heart tissue in vivo as well as for cardiac tissue engineering. Stimulation (square pulses, 1 ms duration, 1 Hz, 2.3 or 4.6 V/cm) was initiated 24 h after cell seeding and maintained for additional 72 h. The cover slips were positioned between the carbon rod electrodes such that the abrasions were either parallel or perpendicular to the field lines. Non-abraded surfaces were utilized as controls. Field stimulation did not affect cell viability. The presence of a well-developed contractile apparatus in neonatal rat cardiomyocytes (staining for cardiac Troponin I and actin filaments) was identified in the groups cultivated on abraded surfaces in the presence of field stimulation. Overall we observed that (i) fibroblast and cardiomyocyte elongation on non-abraded surfaces was significantly enhanced by electrical field stimulation, (ii) electrical field stimulation promoted orientation of fibroblasts in the direction perpendicular to the field lines when the abrasions were also placed perpendicular to the field lines and (iii) topographical cues were a significantly stronger determinant of cardiomyocyte orientation than the electrical field stimulation. The orientation and elongation response of cardiomyocytes was completely abolished by inhibition of actin polymerization (Cytochalasin D) and only partially by inhibition of phosphatidyl-inositol 3 kinase (PI3K) pathway (LY294002).

Molecular target of the antischistosomal drug praziquantel

Evaluation of: Pica-Mattoccia L, Valle C, Basso A et al.: Cytochalasin D abolishes the schistosomicidal activity of praziquantel. Exp. Parasitol. 115(4), 344–351 (2007). Parasitic flatworms of the genus Schistosoma are the causative agents of schistosomiasis, a widespread tropical disease that affects hundreds of millions of people worldwide. The current drug of choice against schistosomiasis is praziquantel. For some time, it has been known that praziquantel disrupts calcium homeostasis within the parasite. However, in the three decades since its introduction, the precise mode of praziquantel action has remained undefined. In this report, Pica-Mattoccia and colleagues use pharmacological agents to help further dissect the molecular target of this drug.