Contractile activity and passive stretch regulate tubulin mRNA and protein content in cardiac myocytes

Childhood-Onset Neurodegeneration with Cerebellar Atrophy Syndrome: Severe Neuronal Degeneration and Cardiomyopathy with Loss of Tubulin Deglutamylase Cytosolic Carboxypeptidase 1

The cytoskeleton is a dynamic filamentous network with various cellular and developmental functions. The loss of cytosolic carboxypeptidase 1 (CCP1) causes neuronal death. Childhood-onset neurodegeneration with cerebellar atrophy (CONDCA, OMIM no.: 618276) is an extremely rare disease caused by ATP/GTP binding protein 1 (AGTPBP1) gene-related CCP1 dysfunction of microtubules affecting the cerebellum, spinal motor neurons, and peripheral nerves. Also, possible problems are expected in tissues where the cytoskeleton plays a dynamic role, such as cardiomyocytes. In the present study, we report a novel homozygous missense (NM_015239: c.2447A > C, p. Gln816Pro) variant in the AGTPBP1 gene that c.2447A > C variant has never been reported in a homozygous state in the Genome Aggregation (gnomAD; v2.1.1) database, identified by whole-exome sequencing in a patient with a seizure, dystonia, dilated cardiomyopathy (DCM)-accompanying atrophy of caudate nuclei, putamen, and cerebellum. Unlike other cases in the literature, we expand the phenotype associated with AGTPBP1 variants to include dysmorphic features, idiopathic DCM which could be reversed with supportive treatments, seizure patterns, and radiological findings. These findings expanded the spectrum of the AGTPBP1 gene mutations and associated possible manifestations. Our study may help establish appropriate genetic counseling and prenatal diagnosis for undiagnosed neurodegenerative patients.

Cell stretchers and the LINC complex in mechanotransduction

How cells respond to mechanical forces from the surrounding environment is critical for cell survival and function. The LINC complex is a central component in the mechanotransduction pathway that transmits mechanical information from the cell surface to the nucleus. Through LINC complex functionality, the nucleus is able to respond to mechanical stress by altering nuclear structure, chromatin organization, and gene expression. The use of specialized devices that apply mechanical strain to cells have been central to investigating how mechanotransduction occurs, how cells respond to mechanical stress, and the role of the LINC complexes in these processes. A large variety of designs have been reported for these devices, with the most common type being cell stretchers. Here we highlight some of the salient features of cell stretchers and suggest some key parameters that should be considered when using these devices. We provide a brief overview of how the LINC complexes contribute to the cellular responses to mechanical strain. And finally, we suggest that stretchers may be a useful tool to study aging.

Accelerated cardiac remodeling in desmoplakin transgenic mice in response to endurance exercise is associated with perturbed Wnt/β-catenin signaling

Arrhythmogenic ventricular cardiomyopathy (AVC) is a frequent underlying cause for arrhythmias and sudden cardiac death especially during intense exercise. The mechanisms involved remain largely unknown. The purpose of this study was to investigate how chronic endurance exercise contributes to desmoplakin (DSP) mutation-induced AVC pathogenesis. Transgenic mice with overexpression of desmoplakin, wild-type (Tg-DSP(WT)), or the R2834H mutant (Tg-DSP(R2834H)) along with control nontransgenic (NTg) littermates were kept sedentary or exposed to a daily running regimen for 12 wk. Cardiac function and morphology were analyzed using echocardiography, electrocardiography, histology, immunohistochemistry, RNA, and protein analysis. At baseline, 4-wk-old mice from all groups displayed normal cardiac function. When subjected to exercise, all mice retained normal cardiac function and left ventricular morphology; however, Tg-DSP(R2834H) mutants displayed right ventricular (RV) dilation and wall thinning, unlike NTg and Tg-DSP(WT). The Tg-DSP(R2834H) hearts demonstrated focal fat infiltrations in RV and cytoplasmic aggregations consisting of desmoplakin, plakoglobin, and connexin 43. These aggregates coincided with disruption of the intercalated disks, intermediate filaments, and microtubules. Although Tg-DSP(R2834H) mice already displayed high levels of p-GSK3-β(Ser9) and p-AKT1(Ser473) under sedentary conditions, decrease of nuclear GSK3-β and AKT1 levels with reduced p-GSK3-β(Ser9), p-AKT1(Ser473), and p-AKT1(Ser308) and loss of nuclear junctional plakoglobin was apparent after exercise. In contrast, Tg-DSP(WT) showed upregulation of p-AKT1(Ser473), p-AKT1(Ser308), and p-GSK3-β(Ser9) in response to exercise. Our data suggest that endurance exercise accelerates AVC pathogenesis in Tg-DSP(R2834H) mice and this event is associated with perturbed AKT1 and GSK3-β signaling. Our study suggests a potential mechanism-based approach to exercise management in patients with AVC.

The physiological role of cardiac cytoskeleton and its alterations in heart failure

Cardiac muscle cells are equipped with specialized biochemical machineries for the rapid generation of force and movement central to the work generated by the heart. During each heart beat cardiac muscle cells perceive and experience changes in length and load, which reflect one of the fundamental principles of physiology known as the Frank-Starling law of the heart. Cardiac muscle cells are unique mechanical stretch sensors that allow the heart to increase cardiac output, and adjust it to new physiological and pathological situations. In the present review we discuss the mechano-sensory role of the cytoskeletal proteins with respect to their tight interaction with the sarcolemma and extracellular matrix. The role of contractile thick and thin filament proteins, the elastic protein titin, and their anchorage at the Z-disc and M-band, with associated proteins are reviewed in physiologic and pathologic conditions leading to heart failure. 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

X-ROS signaling in the heart and skeletal muscle: stretch-dependent local ROS regulates [Ca²⁺]i

X-ROS signaling is a novel redox signaling pathway that links mechanical stress to changes in [Ca(2+)]i. This pathway is activated rapidly and locally within a muscle cell under physiological conditions, but can also contribute to Ca(2+)-dependent arrhythmia in the heart and to the dystrophic phenotype in the heart and skeletal muscle. Upon physiologic cellular stretch, microtubules serve as mechanotransducers to activate NADPH oxidase 2 in the transverse tubules and sarcolemmal membranes to produce reactive oxygen species (ROS). In the heart, the ROS acts locally to activate ryanodine receptor Ca(2+) release channels in the junctional sarcoplasmic reticulum, increasing the Ca(2+) spark rate and "tuning" excitation-contraction coupling. In the skeletal muscle, where Ca(2+) sparks are not normally observed, the X-ROS signaling process is muted. However in muscular dystrophies, such as Duchenne Muscular Dystrophy and dysferlinopathy, X-ROS signaling operates at a high level and contributes to myopathy. Importantly, Ca(2+) permeable stretch-activated channels are activated by X-ROS and contribute to skeletal muscle pathology. Here we review X-ROS signaling and mechanotransduction in striated muscle, and highlight important questions to drive future work on stretch-dependent signaling. We conclude that X-ROS provides an exciting mechanism for the mechanical control of redox and Ca(2+) signaling, but much work is needed to establish its contribution to physiologic and pathophysiologic processes in diverse cell systems.

A direct test of the hypothesis that increased microtubule network density contributes to contractile dysfunction of the hypertrophied heart

Contractile dysfunction in pressure overload-hypertrophied myocardium has been attributed in part to the increased density of a stabilized cardiocyte microtubule network. The present study, the first to employ wild-type and mutant tubulin transgenes in a living animal, directly addresses this microtubule hypothesis by defining the contractile mechanics of the normal and hypertrophied left ventricle (LV) and its constituent cardiocytes from transgenic mice having cardiac-restricted replacement of native β4-tubulin with β1-tubulin mutants that had been selected for their effects on microtubule stability and thus microtubule network density. In each case, the replacement of cardiac β4-tubulin with mutant hemagglutinin-tagged β1-tubulin was well tolerated in vivo. When LVs in intact mice and cardiocytes from these same LVs were examined in terms of contractile mechanics, baseline function was reduced in mice with genetically hyperstabilized microtubules, and hypertrophy-related contractile dysfunction was exacerbated. However, in mice with genetically hypostabilized cardiac microtubules, hypertrophy-related contractile dysfunction was ameliorated. Thus, in direct support of the microtubule hypothesis, we show here that cardiocyte microtubule network density, as an isolated variable, is inversely related to contractile function in vivo and in vitro, and microtubule instability rescues most of the contractile dysfunction seen in pressure overload-hypertrophied myocardium.

The preterm parturition syndrome

The implicit paradigm that has governed the study and clinical management of preterm labour is that term and preterm parturition are the same processes, except for the gestational age at which they occur. Indeed, both share a common pathway composed of uterine contractility, cervical dilatation and activation of the membranes/decidua. This review explores the concept that while term labour results from physiological activation of the components of the common pathway, preterm labour arises from pathological signalling and activation of one or more components of the common pathway of parturition. The term "great obstetrical syndromes" has been coined to reframe the concept of obstetrical disease. Such syndromes are characterised by: (1) multiple aetiology; (2) long preclinical stage; (3) frequent fetal involvement; (4) clinical manifestations that are often adaptive in nature; and (5) gene-environment interactions that may predispose to the syndromes. This article reviews the evidence indicating that the pathological processes implicated in the preterm parturition syndrome include: (1) intrauterine infection/inflammation; (2) uterine ischaemia; (3) uterine overdistension; (4) abnormal allograft reaction; (5) allergy; (6) cervical insufficiency; and (7) hormonal disorders (progesterone related and corticotrophin-releasing factor related). The implications of this conceptual framework for the prevention, diagnosis, and treatment of preterm labour are discussed.

Expression of Cytoskeletal, Linkage and Extracellular Proteins in Failing Dog Myocardium

In the setting of chronic heart failure (HF), progressive left ventricular (LV) dysfunction and chamber remodeling may be due, in part, to altered expression and disorganization of cytoskeletal, linkage and extracellular proteins. This brief review describes changes in expression of cytoskeletal, linkage and extracellular protein using LV tissue obtained from dogs with progressive HF produced by multiple sequential intracoronary microembolizations. LV tissue samples from 6 untreated HF dogs (LV ejection fraction 20% to 25%) and 3 normal dogs were used. Sections from freshly frozen tissue were prepared, immunostained for specific proteins and studies by confocal microscopy. In failing hearts, confocal microscopy showed disorganization of key cytoskeletal proteins that, when combined with the loss of myofilaments and sarcomeric skeleton, suggest substantial cardiomyocyte remodeling. Cardiomyocytes in areas bordering old infarcts invariably exhibited disorganization of alpha-actinin. The cytoskeleton protein desmin showed increased expression in areas of extensive fibrosis. Staining for pancadherin showed interruptions of intercalated disks in areas of intensive interstitial fibrosis. Observation of increased fibronectin and increased interstitial cellularity based on vimentin labeling is suggestive of ongoing fibrosis. Based on these findings, we conclude that the structural changes observed in failing LV myocardium of dogs with intracoronary microembolizations-induced HF are extensive and typical of those seen and previously described in LV myocardium of explanted failed human hearts. The observed structural changes in this experimental model of HF also support the notion that these cytoskeletal, linkage and extracellular disorganization of structural proteins may be important maladaptations that contribute, albeit in part, to the progression of LV dysfunction and remodeling characteristic of the HF state.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

Differential effects of stretch and compression on membrane currents and [Na+]c in ventricular myocytes

Mechano-electrical feedback was studied in the single ventricular myocytes. A small fraction (approximately 10%) of the cell surface could be stretched or compressed by a glass stylus. Stretch depolarised, shortened the action potential and induced extra systoles. Stretch activated non-selective cation currents (I(ns)) showed a linear voltage dependence, a reversal potential of 0 mV, a pure cation selectivity, and were blocked by 8 microM Gd(3+) or 30 microM streptomycin. Stretch reduced Ca(2+) and K(+) (I(K)) currents. Local compression of broadwise attached cells activated I(K) but not I(ns). Cytochalasin D or colchicin, thought to disrupt the cytoskeleton, suppressed the mechanosensitivity of I(ns) and I(K). During stretch, the cytosolic sodium concentration increased with spatial heterogeneities, local hotspots with [Na(+)](c)>24 mM appeared close to surface membrane and t-tubules (pseudoratiometric imaging using Sodium Green fluorescence). Electronprobe microanalysis confirmed this result and indicated that stretch increased total sodium [Na] in cell compartments such as mitochondria, nuclear envelope and nucleus. Our results obtained by local stretch differ from those obtained by end-to-end stretch (literature). We speculate that channels may be activated not only by axial but also by shear stress, and, that stretch can activate channels outside the deformed sarcomeres via second messenger.

Post-translational modifications of cardiac tubulin during chronic heart failure in the rat

Cytoskeletal reorganization has been shown to participate in cellular remodeling and in the alterations of mechanical function of isolated cardiomyocytes during pressure overload hypertrophy. Post-translational modifications of tubulin towards stabilization of microtubules have also been described in animal models of compensatory hypertrophy, but the status of the microtubules network in end stage heart failure is not clearly established. Using a rat model of congestive heart failure (CHF) induced by aortic banding, we studied the expression of alpha- and beta-tubulin, as well as their post-translational modification and distribution in the soluble and polymerized fraction by immunoblotting. We found an accumulation of alpha- and beta-tubulin protein content specifically in the soluble fraction with no change in the polymerized fraction. Amongst the several variants of alpha-tubulin examined, only detyrosinated Glu-tubulin and deglutamylated delta2-tubulin levels were selectively increased during heart failure. Glu-tubulin accumulated in the polymerized fraction while delta2-tubulin levels were increased in the soluble fraction in CHF hearts. These results show that a profound remodeling of the microtubule network occurs in heart failure. This remodeling suggests an increase in the stability of the microtubule network which is discussed in terms of possible functional consequences.

Rapid electrical stimulation of contraction reduces the density of beta-adrenergic receptors and responsiveness of cultured neonatal rat cardiomyocytes. Possible involvement of microtubule disassembly secondary to mechanical stress

BACKGROUND

Although tachycardia is commonly present in patients with congestive heart failure, its role in the development of congestive heart failure remains unclear. We studied the effect of rapid electrical stimulation of contraction on beta-adrenergic receptor (beta-AR) signal pathway in cultured cardiomyocytes of neonatal rats.

METHODS AND RESULTS

Contraction of cardiomyocytes was induced by electrical stimulation at 50 V with twice the threshold pulse width. beta-ARs were identified by [(3)H]CGP-12177 and [(3)H]dihydroalprenolol. Electrical stimulation reduced cell-surface but not total beta-AR density; the effect was dependent on pacing frequency (a reduction of 11%, 28%, and 18% in cells paced at 2.5, 3. 0, and 3.3 Hz, respectively). This reduction was apparent at 3 hours, in contrast to reduced beta-AR density after exposure to isoproterenol (ISP) for 1 hour. The fraction and inhibition constant of beta-AR binding agonist with high affinity were not affected by rapid electrical stimulation. In cardiomyocytes paced at 3.0 Hz for 24 hours, the response to ISP decreased compared with unpaced cells, 142% versus 204% of baseline with 1 micromol/L ISP, whereas the responses to forskolin or acetylcholine were not different. Treatment of cardiomyocytes with 2,3-butanedione monoxime (10 mmol/L) or taxol (10 micromol/L) inhibited the rapid pacing-induced reduction in beta-AR density.

CONCLUSIONS

Our results suggest that contractile activity is involved in regulation of cardiac function by modulating the beta-AR system independently of hemodynamic and neurohormonal factors. This may help to elucidate the role of mechanical stress in the development of heart failure.

Increased expression of cytoskeletal, linkage, and extracellular proteins in failing human myocardium

Experimental studies have shown that in hypertrophy and heart failure, accumulation of microtubules occurs that impedes sarcomere motion and contributes to decreased ventricular compliance. We tested the hypothesis that these changes are present in the failing human heart and that an entire complex of structural components, including cytoskeletal, linkage, and extracellular proteins, are involved in causing functional deterioration. In explanted human hearts failing because of dilated cardiomyopathy (ejection fraction </=20%), expression of alpha- and beta-tubulin, desmin, vinculin, fibronectin, and vimentin was determined by Northern and Western blot analysis and compared with normal myocardium from explants not used for transplantation. The mRNA for alpha- and beta-tubulin was increased to 2.4-fold (P<0.01) and 1.25-fold (NS), respectively; for desmin, 1.2-fold (P<0.05); for fibronectin, 5-fold (P<0.001); and for vimentin, 1.7-fold (P<0.05). Protein levels for alpha-tubulin increased 2.6-fold (P<0.02); for beta-tubulin, 1.2-fold (P<0.005); for desmin, 2.1-fold (P<0.001); for vinculin, 1.2-fold (P<0.005); for fibronectin, 2.9-fold (P<0.001); and for vimentin, 1.5-fold (P<0. 005). Confocal microscopy showed augmentation and disorganization of all proteins studied. In combination with the loss of myofilaments and sarcomeric skeleton previously reported, these changes suggest cardiomyocyte remodeling. Increased fibronectin and elevated interstitial cellularity (vimentin labeling) indicate progressive fibrosis. The present results suggest a causative role of cytoskeletal abnormalities and myofilament loss for intrinsic contractile and diastolic dysfunction in failing hearts.

Microtubule assembly is regulated by externally applied strain in cultured smooth muscle cells

Mechanical forces clearly regulate the development and phenotype of a variety of tissues and cultured cells. However, it is not clear how mechanical information is transduced intracellularly to alter cellular function. Thermodynamic modeling predicts that mechanical forces influence microtubule assembly, and hence suggest microtubules as one potential cytoskeletal target for mechanical signals. In this study, the assembly of microtubules was analyzed in rat aortic smooth muscle cells cultured on silicon rubber substrates exposed to step increases in applied strain. Cytoskeletal and total cellular protein fractions were extracted from the cells following application of the external strain, and tubulin levels were quantified biochemically via a competitive ELISA and western blotting using bovine brain tubulin as a standard. In the first set of experiments, smooth muscle cells were subjected to a step-increase in strain and the distribution of tubulin between monomeric, polymeric, and total cellular pools was followed with time. Microtubule mass increased rapidly following application of the strain, with a statistically significant increase (P<0.05) in microtubule mass from 373+/-32 pg/cell (t=0) to 514+/-30 pg/cell (t=15 minutes). In parallel, the amount of soluble tubulin decreased approximately fivefold. The microtubule mass decreased after 1 hour to a value of 437+/-24 pg/cell. In the second set of experiments, smooth muscle cells were subjected to increasing doses of externally applied strain using a custom-built strain device. Monomeric, polymeric, and total tubulin fractions were extracted after 15 minutes of applied strain and quantified as for the earlier experiments. Microtubule mass increased with increasing strain while total cellular tubulin levels remained essentially constant at all strain levels. These findings are consistent with a thermodynamic model which predicts that microtubule assembly is promoted as a cell is stretched and compressional loads on the microtubules are presumably relieved. Furthermore, these data suggest microtubules are a potential target for translating changes in externally applied mechanical stimuli to alterations in cellular phenotype.