Microtubules in mammalian heart muscle

Cardiomyocyte Proliferation from Fetal- to Adult- and from Normal- to Hypertrophy and Failing Hearts

Simple Summary

Death from injury to the heart from a variety of causes remains a major cause of mortality worldwide. The cardiomyocyte, the major contracting cell of the heart, is responsible for pumping blood to the rest of the body. During fetal development, these immature cardiomyocytes are small and rapidly divide to complete development of the heart by birth when they develop structural and functional characteristics of mature cells which prevent further division. All further growth of the heart after birth is due to an increase in the size of cardiomyocytes, hypertrophy. Following the loss of functional cardiomyocytes due to coronary artery occlusion or other causes, the heart is unable to replace the lost cells. One of the significant research goals has been to induce adult cardiomyocytes to reactivate the cell cycle and repair cardiac injury. This review explores the developmental, structural, and functional changes of the growing cardiomyocyte, and particularly the sarcomere, responsible for force generation, from the early fetal period of reproductive cell growth through the neonatal period and on to adulthood, as well as during pathological response to different forms of myocardial diseases or injury. Multiple issues relative to cardiomyocyte cell-cycle regulation in normal or diseased conditions are discussed.

Abstract

The cardiomyocyte undergoes dramatic changes in structure, metabolism, and function from the early fetal stage of hyperplastic cell growth, through birth and the conversion to hypertrophic cell growth, continuing to the adult stage and responding to various forms of stress on the myocardium, often leading to myocardial failure. The fetal cell with incompletely formed sarcomeres and other cellular and extracellular components is actively undergoing mitosis, organelle dispersion, and formation of daughter cells. In the first few days of neonatal life, the heart is able to repair fully from injury, but not after conversion to hypertrophic growth. Structural and metabolic changes occur following conversion to hypertrophic growth which forms a barrier to further cardiomyocyte division, though interstitial components continue dividing to keep pace with cardiac growth. Both intra- and extracellular structural changes occur in the stressed myocardium which together with hemodynamic alterations lead to metabolic and functional alterations of myocardial failure. This review probes some of the questions regarding conditions that regulate normal and pathologic growth of the heart.

Targeting Microtubules for the Treatment of Heart Disease

Heart disease remains the leading cause of morbidity and mortality worldwide. With the advancement of modern technology, the role(s) of microtubules in the pathogenesis of heart disease has become increasingly apparent, though currently there are limited treatments targeting microtubule-relevant mechanisms. Here, we review the functions of microtubules in the cardiovascular system and their specific adaptive and pathological phenotypes in cardiac disorders. We further explore the use of microtubule-targeting drugs and highlight promising druggable therapeutic targets for the future treatment of heart diseases.

Myocardial Cytoskeletal Adaptations in Advanced Kidney Disease

Background The myocardial cytoskeleton functions as the fundamental framework critical for organelle function, bioenergetics and myocardial remodeling. To date, impairment of the myocardial cytoskeleton occurring in the failing heart in patients with advanced chronic kidney disease has been largely undescribed. Methods and Results We conducted a 3-arm cross-sectional cohort study of explanted human heart tissues from patients who are dependent on hemodialysis (n=19), hypertension (n=10) with preserved renal function, and healthy controls (n=21). Left ventricular tissues were subjected to pathologic examination and next-generation RNA sequencing. Mechanistic and interference RNA studies utilizing in vitro human cardiac fibroblast models were performed. Left ventricular tissues from patients undergoing hemodialysis exhibited increased myocardial wall thickness and significantly greater fibrosis compared with hypertension patients (P<0.05) and control (P<0.01). Transcriptomic analysis revealed that the focal adhesion pathway was significantly enriched in hearts from patients undergoing hemodialysis. Hearts from patients undergoing hemodialysis exhibited dysregulated components of the focal adhesion pathway including reduced β-actin (P<0.01), β-tubulin (P<0.01), vimentin (P<0.05), and increased expression of vinculin (P<0.05) compared with controls. Cytoskeletal adaptations in hearts from the hemodialysis group were associated with impaired mitochondrial bioenergetics, including dysregulated mitochondrial dynamics and fusion, and loss of cell survival pathways. Mechanistic studies revealed that cytoskeletal changes can be driven by uremic and metabolic abnormalities of chronic kidney disease, in vitro. Furthermore, focal adhesion kinase silencing via interference RNA suppressed major cytoskeletal proteins synergistically with mineral stressors found in chronic kidney disease in vitro. Conclusions Myocardial failure in advanced chronic kidney disease is characterized by impairment of the cytoskeleton involving disruption of the focal adhesion pathway, mitochondrial failure, and loss of cell survival pathways.

Cardiomyocyte Microtubules: Control of Mechanics, Transport, and Remodeling

Microtubules are essential cytoskeletal elements found in all eukaryotic cells. The structure and composition of microtubules regulate their function, and the dynamic remodeling of the network by posttranslational modifications and microtubule-associated proteins generates diverse populations of microtubules adapted for various contexts. In the cardiomyocyte, the microtubules must accommodate the unique challenges faced by a highly contractile, rigidly structured, and long-lasting cell. Through their canonical trafficking role and positioning of mRNA, proteins, and organelles, microtubules regulate essential cardiomyocyte functions such as electrical activity, calcium handling, protein translation, and growth. In a more specialized role, posttranslationally modified microtubules form load-bearing structures that regulate myocyte mechanics and mechanotransduction. Modified microtubules proliferate in cardiovascular diseases, creating stabilized resistive elements that impede cardiomyocyte contractility and contribute to contractile dysfunction. In this review, we highlight the most exciting new concepts emerging from recent studies into canonical and noncanonical roles of cardiomyocyte microtubules.

Mitochondrial Arrest on the Microtubule Highway-A Feature of Heart Failure and Diabetic Cardiomyopathy?

A pathophysiological consequence of both type 1 and 2 diabetes is remodelling of the myocardium leading to the loss of left ventricular pump function and ultimately heart failure (HF). Abnormal cardiac bioenergetics associated with mitochondrial dysfunction occurs in the early stages of HF. Key factors influencing mitochondrial function are the shape, size and organisation of mitochondria within cardiomyocytes, with reports identifying small, fragmented mitochondria in the myocardium of diabetic patients. Cardiac mitochondria are now known to be dynamic organelles (with various functions beyond energy production); however, the mechanisms that underpin their dynamism are complex and links to motility are yet to be fully understood, particularly within the context of HF. This review will consider how the outer mitochondrial membrane protein Miro1 (Rhot1) mediates mitochondrial movement along microtubules via crosstalk with kinesin motors and explore the evidence for molecular level changes in the setting of diabetic cardiomyopathy. As HF and diabetes are recognised inflammatory conditions, with reports of enhanced activation of the NLRP3 inflammasome, we will also consider evidence linking microtubule organisation, inflammation and the association to mitochondrial motility. Diabetes is a global pandemic but with limited treatment options for diabetic cardiomyopathy, therefore we also discuss potential therapeutic approaches to target the mitochondrial-microtubule-inflammatory axis.

Cardiac microtubules in health and heart disease

Cardiomyocytes are large (∼40,000 µm3), rod-shaped muscle cells that provide the working force behind each heartbeat. These highly structured cells are packed with dense cytoskeletal networks that can be divided into two groups—the contractile (i.e. sarcomeric) cytoskeleton that consists of filamentous actin-myosin arrays organized into myofibrils, and the non-sarcomeric cytoskeleton, which is composed of β- and γ-actin, microtubules, and intermediate filaments. Together, microtubules and intermediate filaments form a cross-linked scaffold, and these networks are responsible for the delivery of intracellular cargo, the transmission of mechanical signals, the shaping of membrane systems, and the organization of myofibrils and organelles. Microtubules are extensively altered as part of both adaptive and pathological cardiac remodeling, which has diverse ramifications for the structure and function of the cardiomyocyte. In heart failure, the proliferation and post-translational modification of the microtubule network is linked to a number of maladaptive processes, including the mechanical impediment of cardiomyocyte contraction and relaxation. This raises the possibility that reversing microtubule alterations could improve cardiac performance, yet therapeutic efforts will strongly benefit from a deeper understanding of basic microtubule biology in the heart. The aim of this review is to summarize the known physiological roles of the cardiomyocyte microtubule network, the consequences of its pathological remodeling, and to highlight the open and intriguing questions regarding cardiac microtubules.

Impact statement

Advancements in cell biological and biophysical approaches and super-resolution imaging have greatly broadened our view of tubulin biology over the last decade. In the heart, microtubules and microtubule-based transport help to organize and maintain key structures within the cardiomyocyte, including the sarcomere, intercalated disc, protein clearance machinery and transverse-tubule and sarcoplasmic reticulum membranes. It has become increasingly clear that post translational regulation of microtubules is a key determinant of their sub-cellular functionality. Alterations in microtubule network density, stability, and post-translational modifications are hallmarks of pathological cardiac remodeling, and modified microtubules can directly impede cardiomyocyte contractile function in various forms of heart disease. This review summarizes the functional roles and multi-leveled regulation of the cardiac microtubule cytoskeleton and highlights how refined experimental techniques are shedding mechanistic clarity on the regionally specified roles of microtubules in cardiac physiology and pathophysiology.

Electron tomography of rabbit cardiomyocyte three-dimensional ultrastructure

The field of cardiovascular research has benefitted from rapid developments in imaging technology over the last few decades. Accordingly, an ever growing number of large, multidimensional data sets have begun to appear, often challenging existing pre-conceptions about structure and function of biological systems. For tissue and cell structure imaging, the move from 2D section-based microscopy to true 3D data collection has been a major driver of new insight. In the sub-cellular domain, electron tomography is a powerful technique for exploration of cellular structures in 3D with unparalleled fidelity at nanometer resolution.

Electron tomography is particularly advantageous for studying highly compartmentalised cells such as cardiomyocytes, where elaborate sub-cellular structures play crucial roles in electrophysiology and mechanics. Although the anatomy of specific ultra-structures, such as dyadic couplons, has been extensively explored using 2D electron microscopy of thin sections, we still lack accurate, quantitative knowledge of true individual shape, volume and surface area of sub-cellular domains, as well as their 3D spatial interrelations; let alone of how these are reshaped during the cycle of contraction and relaxation. Here we discuss and illustrate the utility of ET for identification, visualisation, and analysis of 3D cardiomyocyte ultrastructures such as the T-tubular system, sarcoplasmic reticulum, mitochondria and microtubules.

Dynamic Myofibrillar Remodeling in Live Cardiomyocytes under Static Stretch

An increase in mechanical load in the heart causes cardiac hypertrophy, either physiologically (heart development, exercise and pregnancy) or pathologically (high blood pressure and heart-valve regurgitation). Understanding cardiac hypertrophy is critical to comprehending the mechanisms of heart development and treatment of heart disease. However, the major molecular event that occurs during physiological or pathological hypertrophy is the dynamic process of sarcomeric addition, and it has not been observed. In this study, a custom-built second harmonic generation (SHG) confocal microscope was used to study dynamic sarcomeric addition in single neonatal CMs in a 3D culture system under acute, uniaxial, static, sustained stretch. Here we report, for the first time, live-cell observations of various modes of dynamic sarcomeric addition (and how these real-time images compare to static images from hypertrophic hearts reported in the literature): 1) Insertion in the mid-region or addition at the end of a myofibril; 2) Sequential addition with an existing myofibril as a template; and 3) Longitudinal splitting of an existing myofibril. The 3D cell culture system developed on a deformable substrate affixed to a stretcher and the SHG live-cell imaging technique are unique tools for real-time analysis of cultured models of hypertrophy.

UNC-89 (obscurin) binds to MEL-26, a BTB-domain protein, and affects the function of MEI-1 (katanin) in striated muscle of Caenorhabditis elegans

The ubiquitin proteasome system is involved in degradation of old or damaged sarcomeric proteins. Most E3 ubiquitin ligases are associated with cullins, which function as scaffolds for assembly of the protein degradation machinery. Cullin 3 uses an adaptor to link to substrates; in Caenorhabditis elegans, one of these adaptors is the BTB-domain protein MEL-26 (maternal effect lethal). Here we show that MEL-26 interacts with the giant sarcomeric protein UNC-89 (obscurin). MEL-26 and UNC-89 partially colocalize at sarcomeric M-lines. Loss of function or gain of function of mel-26 results in disorganization of myosin thick filaments similar to that found in unc-89 mutants. It had been reported that in early C. elegans embryos, a target of the CUL-3/MEL-26 ubiquitylation complex is the microtubule-severing enzyme katanin (MEI-1). Loss of function or gain of function of mei-1 also results in disorganization of thick filaments similar to unc-89 mutants. Genetic data indicate that at least some of the mel-26 loss-of-function phenotype in muscle can be attributed to increased microtubule-severing activity of MEI-1. The level of MEI-1 protein is reduced in an unc-89 mutant, suggesting that the normal role of UNC-89 is to inhibit the CUL-3/MEL-26 complex toward MEI-1.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

Inhibition of ErbB2/neuregulin signaling augments paclitaxel-induced cardiotoxicity in adult ventricular myocytes

Paclitaxel (Taxol) has been successfully combined with the monoclonal antibody trastuzumab (Herceptin) in the treatment of ErbB2 overexpressing cancers. However, this combination therapy showed an unexpected synergistic increase in cardiac dysfunction. We have studied the mechanisms of paclitaxel/anti-ErbB2 cardiotoxicity in adult rat ventricular myocytes (ARVM). Myofibrillar organization was assessed by immunofluorescence microscopy and cell viability was tested by the TUNEL-, LDH- and MTT-assay. Oxidative stress was measured by DCF-fluorescence and myocyte contractile function by video edge-detection and fura-2 fluorescence. Treatment of ARVM with paclitaxel or antibodies to ErbB2 caused a significant increase in myofilament degradation, similarly as observed with an inhibitor of MAPK-signaling, but not apoptosis, necrosis or changes in mitochondrial activity. Paclitaxel-treatment and anti-ErbB2 reduced Erk1/2 phosphorylation. Paclitaxel increased diastolic calcium, shortened relaxation time and reduced fractional shortening in combination with anti-ErbB2. A minor increase in oxidative stress by paclitaxel or anti-ErbB2 was found. We conclude, that concomitant inhibition of ErbB2 receptors and paclitaxel treatment has an additive worsening effect on adult cardiomyocytes, mainly discernible in changes of myofibrillar structure and function, but in the absence of cell death. A potential mechanism is the modulation of the MAPK/Erk1/2 signaling by both drugs.

Stable microtubules contribute to cardiac dysfunction in the streptozotocin-induced model of type 1 diabetes in the rat

Cardiac microtubule stability is increased in the streptozotocin (STZ) model of type 1 diabetes. Here, we investigate the reason for increased microtubule stability, and the functional consequences of stable microtubule disruption. Ventricular myocytes were isolated from rats at 8-12 weeks after injection of STZ. A 10% increase in microtubule density, but no difference in the ratio of microtubule-associated protein 4 (MAP4) to tubulin was seen in myocytes from STZ rats. Functionally, STZ myocytes showed a tendency for reduced shortening and intracellular Ca2+ ([Ca2+]i) transient amplitude, and a significant prolongation of time to peak (ttp) shortening and [Ca2+]i. Although microtubules in STZ myocytes were less sensitive to the microtubule disruptor nocodazole (NOC; 33 microM) than control myocytes, we only saw marked functional consequences of microtubule disruption by NOC in myocytes from diabetic animals. NOC increased shortening and [Ca2+]i transient amplitude in STZ myocytes by 45 and 24%, respectively (compared with 4 and 6% in controls). Likewise, NOC decreased ttp shortening and [Ca2+]i only in STZ myocytes, such that these parameters were no longer different between the two groups. In conclusion, stable microtubules in diabetes are not associated with an increase in MAP4, but are functionally relevant to cardiac dysfunction in diabetes, regulating both [Ca2+]i and shortening.

Cardiocyte cytoskeleton in hypertrophied myocardium

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

Microtubule-dependent transport and organization of sarcomeric myosin during skeletal muscle differentiation

It has been proposed that microtubules (MTs) participate in skeletal muscle cell differentiation. However, it is still unclear how this happens. To examine whether MTs could participate directly in the organization of thick and thin filaments into sarcomeres, we observed the concomitant reorganization and dynamics of MTs with the behavior of sarcomeric actin and myosin by time-lapse confocal microscopy. Using green fluorescent protein (GFP)-EB1 protein to label MT plus ends, we determined that MTs become organized into antiparallel arrays along fusing myotubes. Their dynamics and orientation was found to be different across the thickness of the myotubes. We observed fast movements of Dsred-myosin along GFP-MTs. Comparison of GFP-EB1 and Dsred-myosin dynamics revealed that myosin moved toward MT plus ends. Immuno-electron microscopy experiments confirmed that myosin was actually associated with MTs in myotubes. Finally, we confirmed that MTs were required for the stabilization of myosin-containing elements prior to incorporation into mature sarcomeres. Collectively, our results strongly suggest that MTs become organized into a scaffold that provides directional cues for the positioning and organization of myosin filaments during sarcomere formation.

Myofibrillogenesis in skeletal muscle cells in the presence of taxol

We address the controversy of whether mature myofibrils can form in the presence of taxol, a microtubule-stabilizing compound. Previous electron microscopic studies reported the absence of actin filaments and Z-bands in taxol-treated myocytes [Antin et al., 1981: J Cell Biol 90:300-308; Toyoma et al., 1982: Proc Natl Acad Sci USA 79:6556-6560]. Quail skeletal myoblasts were isolated from 10-day-old embryos and grown in the presence or absence of taxol. Taxol inhibited the formation of multinucleated elongated myotubes. Myocytes cultured in the continual presence of taxol progressed from rounded to stellate shapes. Groups of myocytes that were clustered together after the isolation procedure fused in the presence of taxol but did not form elongated myotubes. Actin filaments and actin-binding proteins were detected with several different fluorescent probes in all myofibrils that formed in the presence of taxol. The Z-bands contained both alpha-actinin and titin, and the typical arrays of A-Bands were always associated with actin filaments in the myofibrils. Myofibril formation was followed by fixing cells each day in culture and staining with probes for actin, muscle-specific alpha-actinin, myosin II, nebulin, troponin, tropomyosin, and non-muscle myosin II. Small linear aggregates of alpha-actinin or Z-bodies, premyofibrils, were detected at the edges of the myocytes and in the arms of the taxol-treated cells and were always associated with actin filaments. Non-muscle myosin II was detected at the edges of the taxol-treated cells. Removal of the taxol drug led to the cells assuming a normal compact elongated shape. During the recovery process, additional myofibrils formed at the spreading edges of these elongated and thicker myotubes. Staining of these taxol-recovering cells with specific fluorescent reagents reveals three different classes of actin fibers. These results are consistent with a model of myofibrillogenesis that involves the transition of premyofibrils to mature myofibrils.

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.

Transient association of titin and myosin with microtubules in nascent myofibrils directed by the MURF2 RING-finger protein

Assembly of muscle sarcomeres is a complex dynamic process and involves a large number of proteins. A growing number of these have regulatory functions and are transiently present in the myofibril. We show here that the novel tubulin-associated RING/B-box protein MURF2 associates transiently with microtubules, myosin and titin during sarcomere assembly. During sarcomere assembly, MURF2 first associates with microtubules at the exclusion of tyrosinated tubulin. Then, MURF2-labelled microtubules associate transiently with sarcomeric myosin and later with A-band titin when non-striated myofibrils differentiate into mature sarcomeres. Finally, MURF2 labelled microtubules disappear from the sarcomere after the incorporation of myosin filaments and the elongation of titin. This suggests that the incorporation of myosin into nascent sarcomeres and the elongation of titin require an active, microtubule-dependent transport process and that MURF2-associated microtubules play a role in the alignment and extension of nascent sarcomeres. MURF2 is expressed in at least four isoforms, of which a 27 kDa isoform is cardiac specific. A C-terminal isoform is generated by alternative reading frame use, a novelty in muscle proteins. In mature cardiac sarcomeres, endogenous MURF2 can associate with the M-band, and is translocated to the nucleus. MURF2 can therefore act as a transient adaptor between microtubules, titin and nascent myosin filaments, as well as being involved in signalling from the sarcomere to the nucleus.

HSC73-tubulin complex formation during low-flow ischemia in the canine myocardium

Canine myocardium was exposed to bouts of low-flow ischemia to identify the interactions that develop between the microtubule-based cytoskeleton and the heat shock protein 70 (HSP70) family of heat shock proteins in viable cardiomyocytes. "Moderate" or "severe" low-flow ischemia was produced in chronically instrumented dogs by reducing circumflex coronary flow by 50% for 2 h or by 75% for 5 h followed by reperfusion for 2 and 24 h, respectively. Electron and immunofluorescence microscopy demonstrated either partial or nearly complete depolymerization of the intermyofibrillar microtubules in areas of myofibril disruption and partial dissolution of the perinuclear microtubule girdle. In contrast, centrosomal tubulin arrays appeared to remain intact following low-flow ischemia. In cardiomyocytes displaying myofibril disruption, constitutively expressed HSP73 (HSC73) colocalized with intact but not disrupted microtubules and with perinuclear and centrosomal tubulin following moderate ischemia. Microtubule depolymerization and high molecular weight tubulin-HSC73 complexes were present in more severely ischemic tissue. These results suggest that HSC73 directly interacts with tubulin and may protect selected elements of the microtubule network and limit myofibril disruption during reversible low-flow ischemia.

Effect of the microtubule polymerizing agent taxol on contraction, Ca2+ transient and L-type Ca2+ current in rat ventricular myocytes

1. Microtubules form part of the cytoskeleton. Their role in adult ventricular myocytes is not well understood although microtubule proliferation has previously been linked with reduced contractile function. 2. We investigated the effect of the anti-tumour drug taxol, a known microtubule polymerizing agent, on Ca2+ handling in adult rat ventricular myocytes. 3. Treatment of cells with taxol caused proliferation of microtubules. 4. In taxol-treated cells there was a reduction in the amplitude of contraction, no significant effect on the amplitude of L-type Ca2+ current, but a significant reduction in the amplitude of the Ca2+ transient. 5. Caffeine was used to release Ca2+ from the sarcoplasmic reticulum (SR). There was a significant reduction in the ratio of electrically stimulated : caffeine-induced Ca2+ transients in taxol-treated cells. This observation is consistent with the hypothesis that taxol reduces fractional SR Ca2+ release. 6. We suggest that the negative inotropic effect of taxol may, at least in part, be the result of reduced release of Ca2+ from the SR. Microtubules may be important regulators of Ca2+ handling in the heart.

Terminal warm blood cardioplegia improves cardiac function through microtubule repolymerization

BACKGROUND

To elucidate the mechanisms responsible for the beneficial effects of terminal warm blood cardioplegia, we studied dynamic change in microtubules induced by cold cardioplegia followed by rewarming. Further, we investigated the relationship between cardiac function and morphologic changes in microtubules caused by hyperkalemic, hypocalcemic warm cardioplegia during initial reperfusion.

METHODS

In protocol 1 isolated rat hearts were perfused at 37 degrees C with Krebs-Henseleit buffer (KHB). After 3 hours of hypothermic cardiac arrest at 10 degrees C, hearts were reperfused at 37 degrees C with one of two buffers: group C, 60-minute reperfusion with KHB (K+, 5.9 mmol/L; Ca2+, 2.5 mmol/L); and group TC, 10-minute initial reperfusion with modified KHB (K+, 15 mmol/L; Ca2+, 0.25 mmol/L), followed by 50 minutes of reperfusion with KHB. Cardiac function after reperfusion was determined as a percentage of the prearrest value. In protocol 2 hearts were perfused at 37 degrees C with KHB containing colchicine (10(-5) mol/L) for 60 minutes.

RESULTS

There was spontaneous contractile recovery after 10 minutes of initial reperfusion in hearts from group TC as well as improved cardiac function after 15, 30, and 60 minutes of reperfusion compared with that in group C. Immunohistochemical staining and immunoblot analysis demonstrated microtubule depolymerization during hypothermic cardiac arrest and complete repolymerization after 10 minutes of reperfusion with warm buffers in both groups. Colchicine-induced microtubule depolymerization is associated with deterioration of cardiac function.

CONCLUSIONS

One mechanism responsible for improved cardiac function mediated by terminal warm blood cardioplegia is the restart of contraction after complete microtubule repolymerization.

Cytoskeletal mechanics in pressure-overload cardiac hypertrophy

We have shown that the cellular contractile dysfunction characteristic of pressure-overload cardiac hypertrophy results not from an abnormality intrinsic to the myofilament portion of the cardiocyte cytoskeleton but rather from an increased density of the microtubule component of the extramyofilament portion of the cardiocyte cytoskeleton. To determine how, in physical terms, this increased microtubule density mechanically overloads the contractile apparatus at the cellular level, we measured cytoskeletal stiffness and apparent viscosity in isolated cardiocytes via magnetic twisting cytometry, a technique by which magnetically induced force is applied directly to the cytoskeleton through integrin-coupled ferromagnetic beads coated with Arg-Gly-Asp (RGD) peptide. Measurements were made in two groups of cardiocytes from cats with right ventricular (RV) hypertrophy induced by pulmonary artery banding: (1) those from the pressure-overloaded RV and (2) those from the normally loaded same-animal control left ventricle (LV). Cytoskeletal stiffness increased almost twofold, from 8.53 +/- 0.77 dyne/cm2 in the normally loaded LV cardiocytes to 16.46 +/- 1.32 dyne/cm2 in the hypertrophied RV cardiocytes. Cytoskeletal apparent viscosity increased almost fourfold, from 20.97 +/- 1.92 poise in the normally loaded LV cardiocytes to 87.85 +/- 6.95 poise in the hypertrophied RV cardiocytes. In addition to these baseline data showing differing stiffness and, especially, apparent viscosity in the two groups of cardiocytes, microtubule depolymerization by colchicine was found to return both the stiffness and the apparent viscosity of the pressure overload-hypertrophied RV cells fully to normal. Conversely, microtubule hyperpolymerization by taxol increased the stiffness and apparent viscosity values of normally loaded LV cardiocytes to the abnormal values given above for pressure-hypertrophied RV cardiocytes. Thus, increased microtubule density constitutes primarily a viscous load on the cardiocyte contractile apparatus in pressure-overload cardiac hypertrophy.

Effects of fluoro-doxorubicin (ME2303) on microtubules: influence of different classes of microtubule-associated proteins

Anthracyclines are among the most useful agents for the treatment of neoplastic disease, but their clinical use is limited by progressive cardiomyopathy. A few studies have suggested the role of microtubules for the understanding of this toxicity. By using kinetic and structural studies, we demonstrate the disorganizing action of fluoro-doxorubicin, a novel anthracycline, on the microtubule system. Microtubules have a rich and complex composition in relation to their numerous functions in cells. In the present study, we investigate the role of two major microtubule-associated protein (MAP) families, Tau and MAP2. Both MAP families are responsible for the properties of different classes of microtubules. We show the differential effect of fluoro-doxorubicin on these two classes of microtubules. Furthermore, we show that fluoro-doxorubicin is able to affect the capacity of purified tubulin to form normal microtubules. This study confirms that anthracyclines may interfer with the microtubule organization. We suggest that some classes of microtubules, with regard to their MAP composition, may be affected more specifically in cardiac myocytes.

Beta-adrenergic stimulation disassembles microtubules in neonatal rat cultured cardiomyocytes through intracellular Ca2+ overload

Catecholamine cardiotoxicity is attributable in part to Ca2+ overload. To test whether the cytoskeletal structures of microtubules in cardiomyocytes are reversibly injured by catecholamine through excessive Ca2+ influx, morphological changes in the microtubules of neonatal rat myocytes were studied by immunohistochemical technique during exposure to norepinephrine (NE). In intact myocytes, microtubules appeared as a filamentous network throughout the cytoplasm and around the nucleus. NE exposure (10 mumol/L) for > 30 minutes elicited microtubular disassembly in a duration-dependent fashion without any irreversible change in sarcomere structure, and this abnormality recovered within 24 hours after cessation of stimulation. Microtubular disruption scores obtained by semiquantitative assessment were significantly increased in a dose-dependent manner (10.8 +/- 4.0 in the control condition, 23.4 +/- 4.7 at 60 minutes with 10 mumol/L NE), whereas they were significantly attenuated by pretreatment with propranolol (100 mumol/L; score, 11.8 +/- 3.3) but not with phentolamine (100 mumol/L; score, 26.4 +/- 4.8). Isoproterenol (1 mumol/L) and denopamine (10 mumol/L) mimicked the effects of NE, but phenylephrine did not, indicating that NE-induced microtubular disassembly is mediated by beta 1-adrenergic receptor stimulation. This beta-adrenergic receptor-mediated insult was significantly attenuated by a decrease in Ca2+ concentration in the medium from 2 to 0.5 mmol/L and by pretreatment with diltiazem (1 mumol/L). In contrast, microtubular disassembly was induced by an increase in Ca2+ concentration in the medium and an administration of the Ca2+ ionophore A23187, even without beta-adrenergic receptor stimulation. Involvement of intracellular hypoxia and activation of Ca(2+)-calmodulin-dependent kinase or Ca(2+)-dependent neutral protease were excluded from possible mechanisms; however, inhibition of tubulin polymerization by excessive Ca2+ influx during beta-adrenergic receptor stimulation may be primarily involved. We conclude that microtubular structures that support cellular integrity are reversibly injured by beta-adrenergic receptor stimulation through excessive Ca2+ influx.

Role of microtubules in contractile dysfunction of hypertrophied cardiocytes

Cardiac hypertrophy in response to systolic pressure overloading frequently results in contractile dysfunction, the cause for which has been unknown. Since, in contrast, the same degree and duration of hypertrophy in response to systolic volume overloading does not result in contractile dysfunction, we postulated that the contractile dysfunction of pressure hypertrophied myocardium might result from a direct effect of stress as opposed to strain loading on an intracellular structure of the hypertrophied cardiocyte. The specific hypothesis tested here is that the microtubule component of the cytoskeleton is such an intracellular structure, which, forming in excess, impedes sarcomere motion. The feline right ventricle was either pressure overloaded by pulmonary artery banding or volume overloaded by atrial septotomy. The quantity of microtubules was estimated from immunoblots and immunofluorescent micrographs, and their mechanical effects were assessed by measuring sarcomere motion during microtubule depolymerization. We show here that stress loading increases the microtubule component of the cardiac muscle cell cytoskeleton; this apparently is responsible for the entirety of the cellular contractile dysfunction seen in our model of pressure-hypertrophied myocardium. No such effects were seen in right ventricular cardiocytes from normal or volume-overloaded cats or in left ventricular cardiocytes from any group of cats. Importantly, the linked microtubule and contractile abnormalities are persistent and thus may be found to have significance for the deterioration of initially compensatory cardiac hypertrophy into the congestive heart failure state.

Organization of calsequestrin-positive sarcoplasmic reticulum in rat cardiomyocytes in culture

The sarcoplasmic reticulum (SR) regulates the levels of cytoplasmic free Ca2+ ions in muscle cells. Calsequestrin is a major Ca(2+)-storing protein and is localized at special sites in the SR. To investigate the development of calsequestrin-positive SR and its interaction with the cytoskeleton, we examined the distribution of calsequestrin in cultured cardiomyocytes from newborn rats by immunofluorescence with anticalsequestrin and antitubulin antibodies and rhodamine-phalloidin. In frozen sections of neonatal rat heart, anticalsequestrin immunostaining was apparent as cross-striations at Z-lines. When newborn cardiomyocytes were isolated, calsequestrin-positive SR was disorganized and was apparent as small vesicles beneath the sarcolemma, whereas myofibrils accumulated in the center of the cells. As the cells spread in culture, calsequestrin-positive vesicles spread to the periphery of the cytoplasm, becoming associated with the developing myofibrils. In mature cells, calsequestrin was closely associated with myofibrils, showing cross-striations at the Z-lines. Double-labeling using anticalsequestrin and antitubulin antibodies demonstrated that the distribution of calsequestrin-positive structures was similar to that of the microtubular arrays. When the microtubules were depolymerized by nocodazole at an early stage, the extension of the SR to the cell periphery was inhibited. In mature cardiomyocytes, nocodazole appeared not to affect the distribution of the SR. These results indicate that the calsequestrin-positive SR in cardiomyocytes is organized at the proper sites of myofibrils during myofibrillogenesis and that the microtubules might serve as tracts for the transport of components of the SR.

Cytoskeletal role in the contractile dysfunction of hypertrophied myocardium

Cardiac hypertrophy in response to systolic pressure loading frequently results in contractile dysfunction of unknown cause. In the present study, pressure loading increased the microtubule component of the cardiac muscle cell cytoskeleton, which was responsible for the cellular contractile dysfunction observed. The linked microtubule and contractile abnormalities were persistent and thus may have significance for the deterioration of initially compensatory cardiac hypertrophy into congestive heart failure.

Reperfusion after brief ischemia disrupts the microtubule network in canine hearts

Histological changes in the stunned myocardium are believed to be minimal. This study examined whether cytoskeletal structures of microtubules are disrupted in the stunned myocardium. In 38 dogs, the left anterior descending coronary artery was occluded for 15 minutes and reperfused to produce the stunned myocardium. Microtubules were stained immunohistochemically. In intact myocardium, microtubules appeared as a filamentous network throughout the cytoplasm and encircled the nucleus. This pattern was not affected by 15 minutes of ischemia. One hour of reperfusion, however, disrupted microtubular structure substantially (disruption score in the endocardium, 53.4 +/- 6.0%) although actin filaments remained intact. Microtubular structures were reconstituted 1-3 days after reperfusion, showing supernormal immunoreactivity. Five days after reperfusion, the pattern of microtubular staining was normal. In another protocol, the role of Ca2+ during reperfusion in microtubular disruption was examined. When intracoronary infusion of EDTA (1.67 mumol/kg body wt per minute) was performed during the initial 10 minutes of reperfusion, myocardial stunning was attenuated. The fractional shortening in the perfused area after 1 hour of reperfusion was 20.1 +/- 1.2% versus 11.5 +/- 0.5% in the control condition (p < 0.05), and the microtubular disruption score was lower (12.6 +/- 1.4%). Although intracoronary infusion of calcium chloride (9 mumol/kg body wt per minute) for 10 minutes in nonischemic hearts increased contractile function (fractional shortening, 25.3 +/- 2.0%), it severely disrupted microtubular networks (microtubular disruption score, 64.0 +/- 10.6%). We conclude that microtubules supporting the structural integrity of myofibrils and other organelles are reversibly disrupted by reperfusion after brief ischemia probably through calcium overload.

Detrimental effects of beta-adrenergic stimulation on beta-adrenoceptors and microtubules in the heart

Increased plasma catecholamines - in particular, excessive beta-adrenoceptor activation in chronic heart failure - may easily desensitize the beta-adrenoceptors as well as the postreceptor signal transductions. Since these detrimental changes in the failing heart could be reversible, administration of low-dose beta-blocker, which minimizes the negative inotropic effects, may be effective in attenuating the harmful effects of sympathetic nerve activation. Beta-adrenoceptor stimulation may also produce microtubule disruptions of the cell either through direct action or through an increase in heart rate. Treatment with beta-blockers could attenuate Ca overload by slowing the heart rate and may be useful as a protection from the structural disintegration of the cell. Thus, to clarify the underlying mechanisms of beta-blocker therapy for chronic heart failure, we have to consider not only to the functional aspects but also to the structural changes of the cells.

Impairment of the myocardial ultrastructure and changes of the cytoskeleton in dilated cardiomyopathy

This study was designed to determine the morphological correlate of chronic heart failure. Myocardial tissue from eight patients undergoing transplantation surgery because of end-stage dilated cardiomyopathy was investigated by electron microscopy and immunocytochemistry using monoclonal antibodies against elements of the cytoskeleton: desmin, tubulin, vinculin, and vimentin. The tissue showed hypertrophy, atrophy of myocytes, and an increased amount of fibrosis. Ultrastructural changes consisted of enlargement and varying shape of nuclei, numerous very small mitochondria, proliferation of T tubules, and accumulation of lipid droplets and glycogen. The most obvious ultrastructural alteration was the decrease of myofilaments, ranging from rarefication to complete absence of sarcomeres in cells filled with unspecified cytoplasm. Immunocytochemistry showed that desmin was localized at the Z lines. In diseased myocardium, the amount of desmin was increased, but it was disorderly arranged. Tubulin formed a fine network throughout the myocytes and was significantly increased in cardiomyopathic hearts. Vinculin, a protein closely associated with the cytoskeleton, occurred not only at the sarcolemma and the intercalated disc but also within the myocardial cells. Ultrastructural changes and alterations of the cytoskeleton were severe in about one third of all cells. About one third of all cells showed moderately severe changes, and the remaining cells were normal. Vimentin was present in the interstitial cells and was increased in relation to the increase of fibrosis. We conclude that the increase of fibrosis, the degeneration of hypertrophied myocardial cells, and the alterations of the cytoskeleton are the morphological correlates of reduced myocardial function in chronic heart failure.

Disruption of microtubules as an early sign of irreversible ischemic injury. Immunohistochemical study of in situ canine hearts

Structural disruption of the cytoskeleton may be involved in irreversible ischemic injury. In the present study, ischemic changes in microtubules during various periods of myocardial ischemia were studied with an immunohistochemical technique in open-chest dogs. In intact myocardium, microtubules were stained as a filamentous network throughout cytoplasm and a circular network around the nucleus, which disappeared with colchicine treatment. In brief ischemia of less than 15 minutes, microtubule patterns were unaltered. After 20 minutes, however, characteristic microtubule stains were partly lost in patchy lesions. As an increase in ischemic period, lesions of loss of microtubule stains were increased in number and size. After 120 minutes of reperfusion following 60 minutes of ischemia, the lesions with intact actin filaments but with disrupted microtubules were replaced by the severely injured cells in which the regular myofibrillar registrations were distinctly disrupted. After 24 hours of reperfusion following 40 minutes of occlusion of the left circumflex artery, the percent area of disrupted microtubules at 40 minutes of ischemia was replaced by that of irreversibly injured lesions in the posterior papillary muscle. These results indicate that disruption of microtubules during ischemia heralds irreversible ischemic injury. However, in in vitro study, the myocardium incubated in hypoxic solution for 60-120 minutes demonstrated earlier disruption of the microtubules than the vinculin. Electron microscopic study also showed minimal irreversible changes in the lesions with disrupted microtubules. Thus, taken together, we conclude that microtubules that support the structural integration of myofibrils and other organelles are disrupted in severe myocardial ischemia before the irreversible injury, promoting the irreversible change after reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Early development and myogenesis of the posterior anuran lymph hearts

The early development of anuran posterior lymph hearts studied by light- and electron-microscopy in frog larval stages 25-29 (Gosner 1960) can be subdivided into three phases. In phase I, mesenchymal myogenic cells are found, each possessing a single 9 + 0 cilium and numerous filopode-like processes aggregated near the vena caudalis lateralis, forming up to three metameric organ anlagen arranged like a cuff around the preexisting lymphatics (stages 26/27). In phase II, cell proliferation starts at stage 28 within the lymph heart wall as does the formation of primarily polynuclear myofibres by fusion of several myoblasts. At this stage immature myofibres show a vast sarcoplasm, a poorly developed SR and only few myofibrils with not yet distinguishable A- and I-bands. In phase III, the afferent and efferent valves are formed at the onset of pulsation in stage 29. Contractile myofibres contain large glycogen fields and a considerable amount of myofibrils which frequently branch and show distinct Z-lines, A-, I-, H- and M-bands; 1-3 cilia were found lying within a channel-like cell invagination. The peculiarities of organogenesis and myofibre development are discussed.

Associations between beta-tubulin and mitochondria in adult isolated heart myocytes as shown by immunofluorescence and immunoelectron microscopy

We have investigated the associations between beta-tubulin and mitochondria in freshly isolated cardiac myocytes from the rat. Beta-tubulin was identified by using monoclonal antibodies for immunofluorescence and high resolution immunogold electron microscopy. In addition, conventional transmission and scanning electron microscopic studies were performed. After chemical stabilization in a formaldehyde solution, the myocytes were shock-frozen at -150 degrees C, cryosectioned at -70 degrees C and subsequently processed for immunohistochemical and immunocytochemical microscopy. A characteristic of the rod shaped myocytes is the presence of a dense network of microtubules in the cytoplasm displaying a pattern of strong anti-beta-tubulin reaction. The complexity of this network however varies considerably among the myocytes reflecting microtubule dynamic instability. Further, our findings demonstrate that the beta-tubulin label in rod cells is confined to the perinuclear and interfibrillar spaces and, therefore, is largely colocalized with the cytoplasmic organelles. In myocytes undergoing severe contracture the distribution of beta-tubulin is entirely restricted to the outer mitochondrial-containing domain. This implies that, in a cell model with marked segregation of the contractile filaments and organelles, mitochondria are codistributed with microtubules in the total absence of desmin intermediate filaments. Moreover, our immunogold preparations demonstrate anti-beta-tubulin labelling in the outer mitochondrial membrane as well as of fibres in close apposition to this membrane. These results indicate the presence of a specific beta-tubulin binding to the outer mitochondrial membrane that probably also involves microtubule based translocators and/or MAPs.

Structural diversity and dynamics of microtubules and polymorphic tubulin assemblies

Tubulin, the main protein of microtubules (MTs), has the potency of forming a variety of other assembly products in vitro: rings, ring-crystals, C- and S-shaped ribbons, 10 nm fibres, hoops, sheets, heaped sheets, MT doublets, MT triplets, double-wall MTs, microtubules, curled ribbons, and paracrystals. The supramolecular subunits of all of them are the protofilaments which might be arranged either parallel to the axis (e.g., in MTs, ribbons) or curved (e.g., in hoops, microtubules). There is strong evidence that in the second case the protofilaments have an inside-out orientation compared to MTs. All assembly products mentioned are described structurally and their relevance to the in vivo situation is considered. Moreover, MTs and the other assemblies undergo permanent changes. These dynamics occurring in both individual assemblies and assembly populations are discussed from the structural point of view.

Effect of taxol on the heparin-induced secretion of lipoprotein lipase from cardiac myocytes

The heparin-induced secretion of LPL into the incubation medium of cardiac myocytes occurred in two phases: a rapid release (5-10 min), followed by a slower rate of release (10-60 min). Reducing the incubation temperature from 37 degrees C to 23 degrees C inhibited the slow phase of secretion, but had no effect on the rapid phase. Similarly, taxol, a microtubule-stabilizing drug, selectively reduced the slow phase of LPL release, without influencing the rapid release of LPL into the medium or cellular LPL activity. The rapid heparin-induced release of LPL probably occurs from sites that are at or near the cell surface, and so microtubules must participate in the intracellular transport of LPL from sites of synthesis and glycosylation to the surface binding sites. Heparin-releasable LPL could be resolved into two fractions by chromatography on con A-Sepharose; this pattern of elution was not affected by the prior treatment of cardiac myocytes with taxol.

Dynamic behavior of endoplasmic reticulum in living cells

Endoplasmic reticulum (ER) was studied by fluorescence microscopy of living CV-1 cells treated with the fluorescent carbocyanine dye DiOC6(3). Using video recording and image processing techniques, several distinct forms of highly localized movements of ER were documented, categorized, and analyzed in terms of mechanism and structural implications. These include tubule branching, ring closure, and sliding. These localized movements have been observed to generate the basic elements of ER: linear tubules, polygonal reticulum, and triple junctions. We propose that as such they act as the mechanism for constructing the polygonal lattice of interconnected membrane tubules that constitutes ER. The nature of these movements suggests possible involvement of the cytoskeleton, and, in view of the close correlations in the distributions of ER and microtubules, and the accompanying paper (Dabora and Sheetz), it is possible that microtubules may play a role in generating ER motility and in constructing and maintaining the ER network in living cells.

Microtubules and desmin filaments during onset of heart hypertrophy in rat: a double immunoelectron microscope study

The distribution of tubulin and desmin, the constituent proteins of microtubules and intermediate filaments, respectively, were studied in normal and hypertrophied rat myocardium by high-resolution immunofluorescence and immunoelectron microscopy. Cardiac hypertrophy was induced in 25-day-old rats by aortic stenosis. In the normal heart, double immunolabelling of ultrathin frozen sections of papillary muscle using gold-labelled probes for tubulin and desmin showed that microtubules ran primarily in a longitudinal direction through the intermyofibrillar spaces, perpendicularly to the desmin filaments. Microtubules were present near nuclei, mitochondria, and plasma membranes, while desmin filaments formed transverse connections between adjacent Z disks. No tubulin was observed near the intercalated disks, which were rich in desmin filaments. In hypertrophied hearts, myocytes exhibited the typical morphological features of developing hypertrophy. While there was little difference in the distribution of the microtubules around mitochondria and at the plasma membrane, considerable increases were seen near the nuclei and along the myofibrils. Desmin labelling was distributed transversely as in the controls; however, sometimes it was longitudinally oriented either in the intermyofibrillar space linking 2 Z disks out of register or along digitations of the intercalated disks connecting neighboring desmosomes. The unique rearrangement of desmin and tubulin filaments in hypertrophied cardiac myocytes emphasizes their distinct role in myocyte organization. We suggest that, during the development of cardiac hypertrophy, desmin filaments are mainly involved in maintaining the myofibrils in register, whereas the degree of assembly of microtubules is correlated with the rate of protein synthesis and with myofibrillogenesis.

Some ultrastructural features of the myocardial cells in the hypertrophied human papillary muscle

An ultrastructural study using various electron microscopical techniques has been conducted on biopsy material from the hypertrophied papillary muscle of the human heart. About 75% of the myocardial cells were classified as hypertrophic with diameters ranging from 15 micron to 53 micron. The increased cell diameter appeared to be the result of an elevated amount of mitochondria and contractile material. The hypertrophied myocytes displayed a general ultrastructural organization in many ways similar to that of the normal sized myocytes. However, the former cells were characterized by focal deposits of excess laminar coat material and abnormal Z-band patterns as well as of multiple intercalated discs. The preferential sites for the production of new sarcomere elements appeared to be in the subsarcolemmal and intercalated disc regions. Adjacent myocardial cells were interconnected by collagen bundles, and, by an elaborate collagen-fibril-microthread-granule lattice. The surface folds were linked to each other by surface cables, which probably constituted a separate category of extracellular material of unknown function. Intramembranous particles were abundant in the sarcolemma proper but scarce in the membranes of the sarcoplasmic vesicles. Such particles were also observed in the lipofuscin granular membrane and in the membranes surrounding the lipid droplets. A framework of transverse cytoskeletal filaments interconnected the Z-bands of adjacent myofibrils and anchored the contractile material to the sarcolemma as well as to the nucleus. A large and lobulated nucleus containing well developed nucleoli together with an abundance of sarcoplasmic free and membrane-attached ribosomes, were interpreted as morphological signs of enhanced synthetic activity in the hypertrophied cell. Degenerative phenomena on the other hand were confined to lysosomal degeneration of worn-out cell constituents that were manifested by the numerous lysosomes and aggregates of lipofuscin granules. Abnormal Z-band patterns as seen in the present material were interpreted as an initial stage in the formation of new contractile elements.

Microtubules and the endoplasmic reticulum are highly interdependent structures

The interrelationships of the endoplasmic reticulum (ER), microtubules, and intermediate filaments were studied in the peripheral regions of thin, spread fibroblasts, epithelial, and vascular endothelial cells in culture. We combined a fluorescent dye staining technique to localize the ER with immunofluorescence to localize microtubules or intermediate filaments in the same cell. Microtubules and the ER are sparse in the lamellipodia, but intermediate filaments are usually completely absent. These relationships indicate that microtubules and the ER advance into the lamellipodia before intermediate filaments. We observed that microtubules and tubules of the ER have nearly identical distributions in lamellipodia, where new extensions of both are taking place. We perturbed microtubules by nocodazole, cold temperature, or hypotonic shock, and observed the effects on the ER distribution. On the basis of our observations in untreated cells and our experiments with microtubule perturbation, we conclude that microtubules and the ER are highly interdependent in two ways: (a) polymerization of individual microtubules and extension of individual ER tubules occur together at the level of resolution of the fluorescence microscope, and (b) depolymerization of microtubules does not disrupt the ER network in the short term (15 min), but prolonged absence of microtubules (2 h) leads to a slow retraction of the ER network towards the cell center, indicating that over longer periods of time, the extended state of the entire ER network requires the microtubule system.

Rearrangement of tubulin, actin, and myosin in cultured ventricular cardiomyocytes of the adult rat

Antitubulin, phalloidin, and antimyosin were used to study the distribution of microtubules, microfilaments, and myofibrils in cultured adult cardiomyocytes. These cells undergo a stereotypic sequence of morphological change in which myotypic features are lost and then reconstructed during a period of polymorphic growth. Microtubules, though rearranged during these events in culture, are always present in an organized network. Myosin and actin structures, on the other hand, initially degenerate. This initial degeneration is reversed when a cell attaches to the culture substratum. Upon attachment, new microtubules are laid down as a cortical network adjacent to the sarcolemma and, subsequently, as a network in the basal part of the cell. Actin and then myosin filament bundles appear next, in a pattern corresponding to the pattern of the microtubules. Finally, striated myofibrils are formed, first in the central part of the cell, and subsequently in the outgrowing processes of the cell. A mechanism is suggested by which the eventual polymorphic shape of a cell is related to the shape of its initial area of contact with the culture substratum. Finally, a model of myofibrillogenesis is proposed in which microtubules participate in the insertion of myosin among previously formed actin filament bundles to produce myofibrils.

Fate of microtubule-organizing centers during myogenesis in vitro

Microtubule organization and nucleation were studied during in vitro human myogenesis by immunocytology that used monoclonal and polyclonal antitubulin antibodies and a rabbit nonimmune serum that reacts with human centrosomes. In myoblasts, we observed a classical microtubule network centered on juxtanuclear centrosomes. Myotubes possessed numerous microtubules organized in parallel without any apparent nucleation centers. Centrosomes in these cells were not associated one to each nucleus but were often clustered in the vicinity of nuclei groups. They were significantly smaller than those of the mononucleated cells. The periphery of each nucleus in myotubes was labeled with the serum that labels centrosomes suggesting a profound reorganization of microtubule-nucleating material. Regrowth experiments after Nocodazole treatment established that microtubules were growing from the periphery of the nuclei. The redistribution of nucleating material was shown to take place early after myoblast fusion. Such a phenomenon appears to be specific to myogenic differentiation in that artificially induced polykaryons behaved differently: the centrosomes aggregated to form only one or a few giant nucleating centers and the nuclei did not participate directly in the nucleation of microtubules. The significance of these results is discussed in relation to the possible role of the centrosome in establishing cell polarity.

Microtubules in the heart muscle of the postnatal and adult rat

In the postnatal rat heart, muscle cells continue to divide as well as increase in size. At the same time the cells in the soleus muscle (a slow skeletal muscle) do not divide, although they continue to grow in size. Since microtubules may have a role in orienting intracellular structures in muscle, we determined the numbers of microtubules/micron2 cross-sectional area in the rat heart papillary muscle during development. We have previously determined that in the soleus muscle, microtubule number/micron2 increases to a maximum at five to nine days of age, after which there is an abrupt decrease to a steady level characteristic of the adult [2]. The numbers of microtubules/micron2 in the heart were similar to those in the soleus muscle at the same age. The numbers of microtubules/micron2 increased from birth to a maximum at nine days, then decreased to a steady state. This decrease in microtubule number in heart muscle occurred at 9 to 11 days as in the soleus muscle. The distributions of microtubules are thus similar for cardiac and slow skeletal muscle, suggesting similar function(s) in these different muscle types.

MAP 4: a microtubule-associated protein specific for a subset of tissue microtubules

The cytological distribution of microtubule-associated protein 4 (MAP 4) (L. M. Parysek, C. F. Asnes, J. B. Olmsted, 1984, J. Cell Biol., 99:1309-1315) in mouse tissues has been examined. Adjacent 0.5-0.9-micron sections of polyethylene glycol-embedded tissues were incubated with affinity-purified MAP 4 or tubulin antibodies, and the immunofluorescent images were compared. Tubulin antibody labeling showed distinct microtubules in all tissues examined. MAP 4 antibody also labeled microtubule-like patterns, but the extent of MAP 4 reactivity was cell type-specific within each tissue. MAP 4 antibody labeled microtubules in vascular elements of all tissues and in other cells considered to have supportive functions, including Sertoli cells in the testis and glial elements in the nervous system. Microtubule patterns were also observed in cardiac, smooth, and skeletal (eye) muscle, podocytes in kidney, Kuppfer cells in liver, and spermatid manchettes. The only MAP 4-positive cells in which the pattern was not microtubule-like were the principal cells of the collecting ducts in kidney cortex, in which diffuse fluorescence was seen. MAP 4 antibody did not react with microtubule-rich neuronal elements of the central and peripheral nervous system, skeletal muscle from anterior thigh, liver parenchymal cells, columnar epithelial cells of the small intestine, and absorptive cells of the tubular component of the nephron. These observations indicate that MAP 4 may be associated with only certain kinds of cell functions as demonstrated by the preferential distribution with microtubules of defined cell types.

Effects of hormones, amino acids and specific inhibitors on rat heart heparin-releasable lipoprotein lipase and tissue neutral lipase activities during long-term perfusion

Rat hearts were perfused for long periods in the presence of 14C-labeled amino acids. From these hearts, postheparin-effluent and a tissue homogenate containing lipoprotein lipase and neutral lipase, respectively, were derived. Lipolytic activity and 14C-labeled protein in both preparations were characterized by affinity chromatography, immunoprecipitation and SDS-polyacrylamide gel electrophoresis. Lipase activity and 14C-labeled protein co-eluted from heparin-Sepharose 4B at 1.2 M NaCl and were inhibited and precipitated by preincubation with anti-lipoprotein lipase gamma-globulins. Gel electrophoresis of both preparations showed the presence of 14C-labeled protein with a molecular weight of 35 000. These data strongly suggest similarity between lipoprotein lipase and neutral lipase and their possible precursor-product relationship and indicate that during perfusion continuous synthesis, secretion and vascular binding of lipase molecules occur. Cycloheximide perfusion induced a dramatic decrease of lipoprotein lipase and neutral lipase activity, indicating a half-life of less than 90 min for both enzymes. Tunicamycin present during perfusion also induced a drop in lipoprotein lipase and tissue neutral lipase activity, indicating that glycosylation is necessary for secretion of lipoprotein lipase. Long-term perfusion of rat hearts in the presence of norepinephrine, glucagon or tyrosine leads to reciprocal alterations in lipoprotein lipase and neutral lipase activities, i.e., lipoprotein lipase activity increased and neutral lipase activity decreased, whereas total lipase activity (lipoprotein lipase + neutral lipase) remained unaltered. During perfusion in the presence of insulin, no net change in lipase activities was observed. Also, insulin did not affect the glucagon-induced inverse effects on either lipase activity. The reciprocal changes in lipase activities occurring during norepinephrine perfusion were hampered by colchicine and propranolol, pointing towards beta-receptor and microtubular mediation of tissue lipase processing and endothelial binding. Our data suggest that the tissue flux and vascular binding of lipase protein may be important sites of hormonal regulation of lipoprotein lipase homeostasis.