Incorporation of fluorescently labeled contractile proteins into freshly isolated living adult cardiac myocytes

Assembly and Maintenance of Myofibrils in Striated Muscle

In this chapter, we present the current knowledge on de novo assembly, growth, and dynamics of striated myofibrils, the functional architectural elements developed in skeletal and cardiac muscle. The data were obtained in studies of myofibrils formed in cultures of mouse skeletal and quail myotubes, in the somites of living zebrafish embryos, and in mouse neonatal and quail embryonic cardiac cells. The comparative view obtained revealed that the assembly of striated myofibrils is a three-step process progressing from premyofibrils to nascent myofibrils to mature myofibrils. This process is specified by the addition of new structural proteins, the arrangement of myofibrillar components like actin and myosin filaments with their companions into so-called sarcomeres, and in their precise alignment. Accompanying the formation of mature myofibrils is a decrease in the dynamic behavior of the assembling proteins. Proteins are most dynamic in the premyofibrils during the early phase and least dynamic in mature myofibrils in the final stage of myofibrillogenesis. This is probably due to increased interactions between proteins during the maturation process. The dynamic properties of myofibrillar proteins provide a mechanism for the exchange of older proteins or a change in isoforms to take place without disassembling the structural integrity needed for myofibril function. An important aspect of myofibril assembly is the role of actin-nucleating proteins in the formation, maintenance, and sarcomeric arrangement of the myofibrillar actin filaments. This is a very active field of research. We also report on several actin mutations that result in human muscle diseases.

Elevated Ca2+ transients and increased myofibrillar power generation cause cardiac hypercontractility in a model of Noonan syndrome with multiple lentigines

Noonan syndrome with multiple lentigines (NSML) is primarily caused by mutations in the nonreceptor protein tyrosine phosphatase SHP2 and associated with congenital heart disease in the form of pulmonary valve stenosis and hypertrophic cardiomyopathy (HCM). Our goal was to elucidate the cellular mechanisms underlying the development of HCM caused by the Q510E mutation in SHP2. NSML patients carrying this mutation suffer from a particularly severe form of HCM. Drawing parallels to other, more common forms of HCM, we hypothesized that altered Ca(2+) homeostasis and/or sarcomeric mechanical properties play key roles in the pathomechanism. We used transgenic mice with cardiomyocyte-specific expression of Q510E-SHP2 starting before birth. Mice develop neonatal onset HCM with increased ejection fraction and fractional shortening at 4-6 wk of age. To assess Ca(2+) handling, isolated cardiomyocytes were loaded with fluo-4. Q510E-SHP2 expression increased Ca(2+) transient amplitudes during excitation-contraction coupling and increased sarcoplasmic reticulum Ca(2+) content concurrent with increased expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase. In skinned cardiomyocyte preparations from Q510E-SHP2 mice, force-velocity relationships and power-load curves were shifted upward. The peak power-generating capacity was increased approximately twofold. Transmission electron microscopy revealed that the relative intracellular area occupied by sarcomeres was increased in Q510E-SHP2 cardiomyocytes. Triton X-100-based myofiber purification showed that Q510E-SHP2 increased the amount of sarcomeric proteins assembled into myofibers. In summary, Q510E-SHP2 expression leads to enhanced contractile performance early in disease progression by augmenting intracellular Ca(2+) cycling and increasing the number of power-generating sarcomeres. This gives important new insights into the cellular pathomechanisms of Q510E-SHP2-associated HCM.

Jasplakinolide reduces actin and tropomyosin dynamics during myofibrillogenesis

The premyofibril model proposes a three-stage process for the de novo assembly of myofibrils in cardiac and skeletal muscles: premyofibrils to nascent myofibrils to mature myofibrils. FRAP experiments and jasplakinolide, a drug that stabilizes F-actin, permitted us to determine how decreasing the dynamics of actin filaments affected the dynamics of tropomyosin, troponin-T, troponin-C, and two Z-Band proteins (alpha-actinin, FATZ) in premyofibrils versus mature myofibrils. Jasplakinolide reduced markedly the dynamics of actin in premyofibrils and in mature myofibrils in skeletal muscles. Two isoforms of tropomyosin-1 (TPM1α, TPM1κ) are more dynamic in premyofibrils than in mature myofibrils in control skeletal muscles. Jasplakinolide reduced the exchange rates of tropomyosins in premyofibrils but not in mature myofibrils. The reduced tropomyosin recoveries did not match the YFP-actin recoveries in premyofibrils in jasplakinolide. There were no significant differences in the effects of jasplakinolide on the dynamics of troponins in the thin filaments or of two Z-band proteins in premyofibrils or skeletal mature myofibrils. Cardiac control mature myofibrils lack nebulin, and small decreases in actin (∼5%) and two tropomyosin isoforms (∼10-15%) dynamics are detected in premyofibril to mature myofibril transformations compared with skeletal muscle. In contrast to skeletal muscle, jasplakinolide lowered the dynamics of actin and tropomyosin isoforms in the cardiac mature myofibrils. These results suggest that the dynamics of tropomyosins in control muscle cells are related to actin exchange. These results also suggest a stabilizing role for nebulin, an actin and tropomyosin-binding protein, present in mature myofibrils but not in premyofibrils of skeletal muscles.

Actin in striated muscle: recent insights into assembly and maintenance

Striated muscle cells are characterised by a para-crystalline arrangement of their contractile proteins actin and myosin in sarcomeres, the basic unit of the myofibrils. A multitude of proteins is required to build and maintain the structure of this regular arrangement as well as to ensure regulation of contraction and to respond to alterations in demand. This review focuses on the actin filaments (also called thin filaments) of the sarcomere and will discuss how they are assembled during myofibrillogenesis and in hypertrophy and how their integrity is maintained in the working myocardium.

Fatty acid (FFA) transport in cardiomyocytes revealed by imaging unbound FFA is mediated by an FFA pump modulated by the CD36 protein

Free fatty acid (FFA) transport across the cardiomyocyte plasma membrane is essential to proper cardiac function, but the role of membrane proteins and FFA metabolism in FFA transport remains unclear. Metabolism is thought to maintain intracellular FFA at low levels, providing the driving force for FFA transport, but intracellular FFA levels have not been measured directly. We report the first measurements of the intracellular unbound FFA concentrations (FFA(i)) in cardiomyocytes. The fluorescent indicator of FFA, ADIFAB (acrylodan-labeled rat intestinal fatty acid-binding protein), was microinjected into isolated cardiomyocytes from wild type (WT) and FAT/CD36 null C57B1/6 mice. Quantitative imaging of ADIFAB fluorescence revealed the time courses of FFA influx and efflux. For WT mice, rate constants for efflux (∼0.02 s(-1)) were twice influx, and steady state FFA(i) were more than 3-fold larger than extracellular unbound FFA (FFA(o)). The concentration gradient and the initial rate of FFA influx saturated with increasing FFA(o). Similar characteristics were observed for oleate, palmitate, and arachidonate. FAT/CD36 null cells revealed similar characteristics, except that efflux was 2-3-fold slower than WT cells. Rate constants determined with intracellular ADIFAB were confirmed by measurements of intracellular pH. FFA uptake by suspensions of cardiomyocytes determined by monitoring FFA(o) using extracellular ADIFAB confirmed the influx rate constants determined from FFA(i) measurements and demonstrated that rates of FFA transport and etomoxir-sensitive metabolism are regulated independently. We conclude that FFA influx in cardiac myocytes is mediated by a membrane pump whose transport rate constants may be modulated by FAT/CD36.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

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.

Tenascin-C modulates adhesion of cardiomyocytes to extracellular matrix during tissue remodeling after myocardial infarction

Tenascin-C (TNC), an extracellular matrix glycoprotein, plays important roles in tissue remodeling. TNC is not normally expressed in adults but reappears under pathologic conditions. The present study was designed to clarify the contribution of TNC to ventricular remodeling after myocardial infarction. We examined the expression of TNC after experimental myocardial infarction in the rat by immunohistochemistry and in situ hybridization. Within 24 hours of permanent coronary ligation, interstitial fibroblasts in the border zone started to express TNC mRNA. The expression of TNC was down-regulated on Day 7 and was no longer apparent by Day 14 after infarction. During the healing process, TNC protein and TNC-producing cells were found at the edges of the residual myocardium. Some of the TNC-producing cells were immunoreactive for alpha-smooth muscle actin. In culture, TNC increased the number of cardiomyocytes attached to laminin but inhibited the formation of focal contacts at costameres. The results indicate that during the acute phase after myocardial infarction, interstitial cells in the border zone synthesize TNC, which may loosen the strong adhesion of surviving cardiomyocytes to connective tissue and thereby facilitate tissue reorganization.

Vinculin, Talin, Integrin alpha6beta1 and laminin can serve as components of attachment complex mediating contraction force transmission from cardiomyocytes to extracellular matrix

Recently, we reported that cardiomyocytes adhere to extracellular matrix at costameres, the striated distribution of vinculin between Z-lines and the sarcolemma, where transmission of contraction forces from myofibrils to the extracellular matrix occurs. To identify other molecules involved in force transmission at costameres, we examined adult rat and embryonic chick cardiomyocytes cultured on coverslips or flexible thin silicone rubber substrata. Immunolocalization of talin showed a costameric, striated distribution, which corresponded to dark contacts with interference reflection microscopy. The molecules involved in substrate adhesion were cross-linked with the non-penetrating cross-linking agent Bis(sulfosuccinimidyl)-suberate and detected by immunohistochemical staining with anti-alpha6, alpha3, alphav, or beta1 integrin antibodies. Both alpha6 and beta1 showed costameric distributions, but alpha3 and alpha(v) did not. The distribution of laminin after cross-linking and extraction also showed a costameric distribution. When anti-integrin beta1 antibody was added to live cardiomyocytes grown on the silicone rubber substratum, the transmission of contraction forces was inhibited. These findings suggest that vinculin, talin, integrin alpha6beta1 and laminin system can be involved in transmission of contraction force to the extracellular matrix.

Premyofibrils in spreading adult cardiomyocytes in tissue culture: evidence for reexpression of the embryonic program for myofibrillogenesis in adult cells

Do adult cardiomyocytes use the same pathways hypothesized for the formation of myofibrils in embryonic cardiomyocytes in tissue culture. [Rhee, et al., Cell Motil. Cytoskeleton 28:1-24, 1994]? Premyofibrils in embryonic cardiomyocytes are composed of short sarcomeric units of alpha-actinin (Z-bodies) and actin filaments held together by short nonmuscle myosin IIB filaments. Premyofibrils are believed to be transformed into nascent myofibrils by their capture of muscle-specific myosin II filaments aligned in aperiodic arrays. Nascent myofibrils are thought to transform into mature myofibrils by the loss of nonmuscle myosin IIB, the fusion of the Z-bodies into Z-bands, and the periodic alignment of muscle myosin II filaments into A-bands. Freshly isolated cat and rat adult cardiomyocytes placed in tissue culture lack premyofibrils and nascent myofibrils. Adult cardiomyocytes spreading in culture reinitiate the synthesis of nonmuscle myosin IIB. Moreover, patterns similar to the proposed embryonic myofibrillar program first detected in spreading chick embryonic hearts were also detected in these spreading adult mammalian cardiomyocytes. The isolated adult cardiomyocytes begin to spread after 1 day in culture by sending out lamellipodia. When these cells are injected with fluorescently labeled alpha-actinin, linear arrays of short spacings of beaded alpha-actinin bodies are detected in the spreading edges of the adult cardiomyocytes. These dense bodies (Z-bodies) stain positively for the same sarcomeric-specific isoform of alpha-actinin that is in the Z-bands of mature sarcomeres. These linear arrays of alpha-actinin-containing Z-bodies have other characteristics of premyofibrils and are detected only in the spreading regions of the cells. Thus, these premyofibrils at the edges of the spreading adult cardiomyocytes stain positively for nonmuscle myosin IIB but negatively for muscle-specific myosin II. Initially, no vinculin is associated with any parts of the premyofibrils in the spreading regions of the early spreading cardiomyocytes. However, later, vinculin is found to be associated with the ends of the premyofibrils. Fibers that stain solidly for muscle-specific myosin II (i.e., nascent myofibrils) are localized between the peripheral premyofibrils and the centrally positioned, mature myofibrils. It is suggested that the puzzling ability of cardiomyocytes in hypertrophic hearts to reinitiate the synthesis of fetal sarcomeric proteins may be related to the reinitiation of the embryonic premyofibril program for myofibrillogenesis.

N-cadherin is required for the differentiation and initial myofibrillogenesis of chick cardiomyocytes

To investigate initial stages of cardiac myofibrillogenesis, heart-forming mesoderm was excised from stage 6 chick embryos and explanted on fibronectin-coated coverglasses. The explants were fixed at various times and immunofluorescently stained with antibodies to N-cadherin, alpha-catenin, beta-catenin, sarcomeric myosin, pan and sarcomeric alpha-actinins, or rhodamine phalloidin. After 7 hours in culture the cells appeared epithelial. N-cadherin, alpha- and beta-catenin, pan alpha-actinin, and F-actin showed circumferential localization at cell borders. No cells in the explant were positive for sarcomeric alpha-actinin or sarcomeric myosin at this stage. Sarcomeric alpha-actinin and sarcomeric myosin were detected around 10 hours after plating. Sarcomeric alpha-actinin initially appeared as small beads along thin actin filaments. Mature Z-lines began to be organized at 20 hours, at the same time the cells started to contract. When the rat monoclonal antibody NCD-2, which inhibits N-cadherin function, was added to the culture at early time-points, cells lost cell-cell contacts, became spherical in shape, and contained tangled actin fibers. The expression of sarcomeric alpha-actinin and sarcomeric myosin was suppressed. These results indicate that 1) the precardiac mesoderm explant cells differentiate and form well-organized myofibrils in culture, 2) N-cadherin-mediated cell-cell interactions are necessary for early differentiation of cardiomyocytes and organization of myofibrils.

Sites of monomeric actin incorporation in living PtK2 and REF-52 cells

The purpose of this study was to analyze where monomeric actin first becomes incorporated into the sarcomeric units of the stress fibers. We microinjected fluorescently labeled actin monomers into two cell lines that differ in the sarcomeric spacings of alpha-actinin and nonmuscle myosin II along their stress fibers: REF-52, a fibroblast cell line, and PtK2, an epithelial cell line. The cells were fixed at selected times after microinjection (30 s and longer) and then stained with an alpha-actinin antibody. Localization of the labeled actin and alpha-actinin antibody were recorded with low level light cameras. In both cell types, the initial sites of incorporation were in focal contacts, lamellipodia and in punctate regions of the stress fibers that corresponded to the alpha-actinin rich dense bodies. The adherent junctions between the epithelial PtK2 cells were also initial sites of incorporation. At longer times of incorporation, the actin fluorescence extended along the stress fibers and became almost uniform. We saw no difference in the pattern of incorporation in peripheral and perinuclear regions of the stress fibers. We propose that rapid incorporation of monomeric actin occurs at the cellular sites where the barbed ends of actin filaments are concentrated: at the edges of lamellipodia, the adherens junctions, the attachment plaques and in the dense bodies that mark out the sarcomeric subunits of the stress fibers.

Relation of nebulin and connectin (titin) to dynamics of actin in nascent myofibrils of cultured skeletal muscle cells

Cultured embryonic chicken skeletal muscle cells microinjected with rhodamine (rh)-labeled actin were stained with antibodies against nebulin and connectin (titin). In premyofibril areas, nebulin was observed as dotted structures, many of which were arranged in a linear fashion. These structures were associated with injected rh-actin. Among these linearly arranged dots of nebulin and rh-actin, numerous small nebulin dots without rh-actin incorporation were scattered. It is probable that the dots of nebulin and/or its associated protein(s) represent a preformed scaffold upon which actin monomers accumulate; exogenously introduced actin associates initially with small nebulin dots, which in turn coalesce to form rh-actin dots and are arranged linearly. In developing myofibrils, two patterns of nebulin distribution were found: "singlets" and "doublets." Recovery of rh-actin's fluorescence after photobleaching was slowest in the nonstriated dotted portions, followed by the striated myofibrillar portions with nebulin singlets and those with doublets, in that order. Thus, the distribution patterns of nebulin seem to be related to the accessibility/exchangeability of actin into nascent myofibrils. It is possible that early nebulin filaments exhibiting singlets are not tightly associated with actin filaments and that this loose association allows myofibrils to exchange nonadult isoforms of actin and other proteins into adult types. Connectin formed a striated pattern before the formation of rh-actin/nebulin striations. It appears that connectin does not have any significant role in the accessibility of actin into nascent myofibrils.

Thin filament protein dynamics in fully differentiated adult cardiac myocytes: toward a model of sarcomere maintenance

Sarcomere maintenance, the continual process of replacement of contractile proteins of the myofilament lattice with newly synthesized proteins, in fully differentiated contractile cells is not well understood. Adenoviral-mediated gene transfer of epitope-tagged tropomyosin (Tm) and troponin I (TnI) into adult cardiac myocytes in vitro along with confocal microscopy was used to examine the incorporation of these newly synthesized proteins into myofilaments of a fully differentiated contractile cell. The expression of epitope-tagged TnI resulted in greater replacement of the endogenous TnI than the replacement of the endogenous Tm with the expressed epitope-tagged Tm suggesting that the rates of myofilament replacement are limited by the turnover of the myofilament bound protein. Interestingly, while TnI was first detected in cardiac sarcomeres along the entire length of the thin filament, the epitope-tagged Tm preferentially replaced Tm at the pointed end of the thin filament. These results support a model for sarcomeric maintenance in fully differentiated cardiac myocytes where (a) as myofilament proteins turnover within the cell they are rapidly exchanged with newly synthesized proteins, and (b) the nature of replacement of myofilament proteins (ordered or stochastic) is protein specific, primarily affected by the structural properties of the myofilament proteins, and may have important functional consequences.

Transfected muscle and non-muscle actins are differentially sorted by cultured smooth muscle and non-muscle cells

We have analyzed by immunolabeling the fate of exogenous epitope-tagged actin isoforms introduced into cultured smooth muscle and non-muscle (i.e. endothelial and epithelial) cells by transfecting the corresponding cDNAs in transient expression assays. Exogenous muscle actins did not produce obvious shape changes in transfected cells. In smooth muscle cells, transfected striated and smooth muscle actins were preferentially recruited into stress fibers. In non-muscle cells, exogenous striated muscle actins were rarely incorporated into stress fibers but remained scattered within the cytoplasm and frequently appeared organized in long crystal-like inclusions. Transfected smooth muscle actins were incorporated into stress fibers of epithelial cells but not of endothelial cells. Exogenous non-muscle actins induced alterations of cell architecture and shape. All cell types transfected by non-muscle actin cDNAs showed an irregular shape and a poorly developed network of stress fibers. beta- and gamma-cytoplasmic actins transfected into muscle and non-muscle cells were dispersed throughout the cytoplasm, often accumulated at the cell periphery and rarely incorporated into stress fibers. These results show that isoactins are differently sorted: not only muscle and non-muscle actins are differentially distributed within the cell but also, according to the cell type, striated and smooth muscle actins can be discriminated for. Our observations support the assumption of isoactin functional diversity.

Myosin heavy chain turnover in cultured neonatal rat heart cells: effects of [Ca2+]i and contractile activity

Blockade of L-type Ca2+ channels in spontaneously contracting cultured neonatal rat ventricular myocytes causes contractile arrest, myofibrillar disassembly, and accelerated myofibrillar protein turnover. To determine whether myofibrillar protein turnover. To determine whether myofibrillar atrophy results indirectly from loss of mechanical signals or directly from alterations in intracellular Ca2+ concentration ([Ca2+]i), contractile activity was inhibited with verapamil (10 microM) or 2,3-butanedione monoxime (BDM), and their effects on cell shortening, [Ca2+]i, and myosin heavy chain (MHC) turnover were assessed. Control cells demonstrated spontaneous [Ca2+]i transients (peak amplitude 232 +/- 15 nM, 1-2 Hz) and vigorous contractile activity. Verapamil inhibited shortening by eliminating spontaneous [Ca2+]i transients. Low concentrations of BDM (5.0-7.5 mM) had no effect on basal or peak [Ca2+]i transient amplitude but reduced cell shortening, whereas 10 mM BDM reduced both [Ca2+]i transient amplitude and shortening. Both agents inhibited MHC synthesis, but only verapamil accelerated MHC degradation. Thus MHC half-life does not change in parallel with contractile activity but rather more closely follows changes in [Ca2+]i. [Ca2+]i transients appear critical in maintaining myofibrillar assembly and preventing accelerated MHC proteolysis.

Living adult rat cardiomyocytes in culture: evidence for dissociation of costameric distribution of vinculin from costameric distributions of attachments

Adult rat cardiomyocytes were placed in tissue culture to determine the relationships of their vinculin positive costameres, their attachments associated with the costameres, the fate of their myofibrils. The costameric structures were detected using interference contrast microscopy and the visualization of the fluorescently labeled vinculin and alpha-actinin molecules. The cardiomyocytes isolated from the heart retained their myofibrils upon attachment to the cell surfaces. One group of cells then rounded up, only to respread after 6 days in culture. These cells initially demonstrated costameric distributions of attachments and vinculin. These relationships were lost during the rounding-up process only to be regained as the cells respread. The second group of freshly isolated cardiomyocytes did not round up but began to spread on the substratum by sending out lamellipodia from their rectangularly shaped body. These newly cultured cardiomyocytes initially exhibited costameric distributions of close attachments detected by interference microscopy. Over the next 3 days although the cells remain attached to the substratum, the costameric attachments were gradually lost. Nevertheless, when similar cells were injected with fluorescently labeled vinculin, costameric distributions of vinculin could be detected in the absence of costameric attachments. Cardiomyocytes, injected with fluorescent alpha-actinin, revealed that during the first few days in culture the existing myofibrils disassembled from the edges of the cell towards the middle. The center group of myofibrils was retained as the cells began to spread. Our observations of living cells support the hypothesis that proteins in addition to vinculin are needed for cardiomyocytes to generate costameric attachments to the cell surfaces. We speculate that the ability of the vinculin-attached Z-lines of adult cardiomyocytes to dissociate from the extracellular matrix may aid in the remodeling of the adult heart in the repair process after myocardial infarction and also in stress induced hypertrophic growth.

Reversible vacuolation of the transverse tubules of frog skeletal muscle: a confocal fluorescence microscopy study

A confocal microscope was used to investigate the reversible vacuolation of frog skeletal muscle fibres produced by the efflux and entry of glycerol (80-100 mM). The formation, development and disappearance of vacuoles was observed in the fibres by staining simultaneously with two fluorescent membrane probes, RH414 and DiOC6(3). The styryl dye, RH414, stains only the plasmalemma and the membranes of the transverse tubules. In normal and glycerol-loaded fibres, RH414 revealed regular, narrow dotted bands located at the position of the Z-lines. Glycerol removal produced, within 2-10 min, many empty round vacuoles (0.4-1.5 microns in diameter) that were apparently anchored to the stained bands. Later on, individual vacuoles tended to enlarge and align into longitudinal chains of vacuoles. Neighbouring vacuoles that contacted each other fused to form large vacuoles up to several sarcomeres in length. Neither the T-tubules, nor the vacuoles, were stained by DiOC6(3). However, glycerol efflux was also accompanied by a redistribution of sarcoplasmic reticulum membranes and by changes in mitochondria that were revealed on staining the same fibres with the carbocyanine dye, DiOC6(3). The alterations in staining patterns revealed by RH414 and DiOC6(3) were completely reversible. Within 5-10 min after a second application of glycerol, the pattern of staining returned to normal with the exception of very bright, spots stained with RH414, which appeared in place of many but not all of the vacuoles, and probably correspond to the irregular nets of T-tubules observed under the electron microscope in such fibres. They are considered to be defects in regeneration of the T-system after vacuolation. The vacuolation/devacuolation cycle could be repeated several times following glycerol efflux and entry. The development and disappearance of vacuoles then mainly involved conversion of bright spots to large vacuoles and vice versa. Some possible mechanisms of vacuole formation and disappearance are discussed, and it is suggested that vacuolation of the T-system may be important in relation to regulating the volume of skeletal muscle cells.

Three-dimensional analysis and visualization of myofibrillogenesis in adult cardiomyocytes by confocal microscopy

Confocal light microscopy has found its place among the standard analytical tools in cell and molecular biology. When combined with techniques such as immunofluorescence or fluorescent in situ hybridization, the spatial distribution of individual biological components can be traced within cells and tissues and, under certain circumstances, even with living samples. In this article, advanced 3D visualization techniques have been applied to analyze the distribution of myofibrillar proteins in cultured adult rat cardiomyocytes. By combining confocal immunofluorescence microscopy with specially designed three-dimensional visualization, we have obtained images which are similar to those obtained with the scanning electron microscope. The subcellular distribution of proteins expressed after transfection of cDNA is monitored in the cultured heart cells. The expressed proteins are distinguished from their endogenous counterparts by the use of an epitope tagging technique. The described methods are suitable to specifically monitor the behavior of several closely related isoprotein mutants in cell or tissue preparations.

The premyofibril: evidence for its role in myofibrillogenesis

When cardiac muscle cells are isolated from embryonic chicks and grown in culture they attach to the substrate as spherical cells with disrupted myofibrils, and over several days in culture, they spread and extend lamellae. Based on antibody localizations of various cytoskeletal proteins within the spreading cardiomyocyte, three types of myofibrils have been identified: 1) fully formed mature myofibrils that are centrally positioned in the cell, 2) premyofibrils that are closest to the cell periphery, and 3) nascent myofibrils located between the premyofibrils and the mature myofibrils. Muscle-specific myosin is localized in the A-bands in the mature, contractile myofibrils, and along the nascent myofibrils in a continuous pattern, but it is absent from the premyofibrils. Antibodies to non-muscle isoforms of myosin IIB react with the premyofibrils at the cell periphery and with the nascent myofibrils, revealing short bands of myosin between closely spaced bands of alpha-actinin. In the areas where the nascent myofibrils border on the mature myofibrils, the bands of non-muscle myosin II reach lengths matching the lengths of the mature A-bands. With the exception of a small transition zone consisting of one myofibril, or sometimes several sarcomeres, bordering the nascent myofibrils, there is no reaction of these non-muscle myosin IIB antibodies with the mature myofibrils in spreading myocytes. C-protein is found only in the mature myofibrils, and its presence there may prevent co-polymerization of non-muscle and muscle myosins. Antibodies directed against the non-muscle myosin isoforms, IIA, do not stain the cardiomyocytes. In contrast to the cardiomyocytes, the fibroblasts in these cultures stain with antibodies to both non-muscle myosin IIA and IIB. The premyofibrils near the leading edge of the lamellae show no reaction with antibodies to either titin or zeugmatin, whereas the nascent myofibrils and mature myofibrils do. The spacings of the banded alpha-actinin staining range from 0.3 to 1.4 microns in the pre- and nascent myofibrils and reach full spacings (1.8-2.5 microns) in the mature myofibrils. Based on these observations, we propose a premyofibril model in which non-muscle myosin IIB, titin, and zeugmatin play key roles in myofibrillogenesis. This model proposes that pre- and nascent myofibrils are composed of minisarcomeres that increase in length, presumably by the concurrent elongation of actin filaments, the loss of the non-muscle myosin II filaments, the fusion of dense bodies or Z-bodies to form wide Z-bands, and the capture and alignment of muscle myosin II filaments to form the full spacings of mature myofibrils.

Cytoskeletal alterations in cultured cardiomyocytes following exposure to the lipid peroxidation product, 4-hydroxynonenal

Damage to the cardiac myocyte sarcolemma following any of several pathological insults such as ischemia (anoxia) alone or followed by reperfusion (reoxygenation), is most apparent as progressive sarcolemmal blebbing, an event attributed by many investigators to a disruption in the underlying cytoskeletal scaffolding. Scanning electron microscopic observation of tissue cultured rat neonatal cardiomyocytes indicates that exposure of these cells to the toxic aldehyde 4-hydroxynonenal (4-HNE), a free radical-induced, lipid peroxidation product, results in the appearance of sarcolemmal blebs, whose ultimate rupture leads to cell death. Indirect immunofluorescent localization of a number of cytoskeletal components following exposure to 4-HNE reveals damage to several, but not all, key cytoskeletal elements, most notably microtubules, vinculin-containing costameres, and intermediate filaments. The exact mechanism underlying the selective disruption of these proteins cannot be ascertained at this time. Colocalization of actin indicated that whereas elements of the cytoskeleton were disrupted by increasing length of exposure to 4-HNE, neither the striated appearance of the myofibrils nor the lateral register of neighboring myofibrils was altered. Monitoring systolic and diastolic levels of intracellular calcium ([Ca2+]i) indicated that increases in [Ca2+]i occurred after considerable cytoskeletal changes had already taken place, suggesting that damage to the cytoskeleton, at least in early phases of exposure to 4-HNE, does not involve Ca(2+)-dependent proteases. However, 4-HNE-induced cytoskeletal alterations coincide with the appearance of, and therefore suggest linkage to, sarcolemmal blebs in cardiac myocytes. Although free radicals produced by reperfusion or reoxygenation of ischemic tissue have been implicated in cellular damage, these studies represent the first evidence linking cardiomyocyte sarcolemmal damage to cytoskeletal disruption produced by a free radical product.

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.

Remodelling of cardiomyocyte cytoarchitecture visualized by three-dimensional (3D) confocal microscopy

The break-down and reassembly of myofibrils in long-term cultures of adult rat cardiomyocytes was investigated by a novel combination of confocal laser scanning microscopy and three-dimensional image reconstruction, referred to as FTCS, to visualize the morphological changes these cells undergo in culture. FTCS is discussed as an alternative imaging mode to low-magnification scanning electron microscopy. The three-dimensional shape of the cells are correlated with the assembly state of myofibrils in different stages. Based on immunofluorescence and confocal laser scanning microscopy it was shown that myofibrils are degraded within a few days after plating and that newly assembled myofibrils are predominantly confined to the continuous area in the perinuclear region close to the membrane in contact with the substratum. The localization of myofibrils along the cell's vertical axis has been investigated both by optical sectioning using confocal light microscopy and by physical sectioning followed by transmission electron microscopy. Based on the distribution of myofibrillar proteins we propose a model of myofibrillar growth locating the putative assembly sites to a region concentric around the nuclei. We provide evidence that the cell shape is dominated by the myofibrillar apparatus.

Probing actin incorporation into myofibrils using Asp11 and His73 actin mutants

We used a cell free system Bouché et al.: J. Cell Biol. 107:587-596, 1988] to study the incorporation of actin into myofibrils. We used alpha-skeletal muscle actin and actins with substitutions of either His73 [Solomon and Rubenstein: J. Biol.Chem. 262:11382, 1987], or Asp11 [Solomon et al.: J. Biol. Chem. 263:19662, 1988]. Actins were translated in reticulocyte lysate and incubated with myofibrils. The incorporated wild type actin could be cross-linked into dimers using N,N'-1,4-phenylenebismaleimide (PBM), indicating that the incorporated actin is actually inserted into the thin filaments of the myofibril. The His73 mutants incorporated to the same extent as wild type actin and was also cross-linked with PBM. Although some of the Asp11 mutants co-assembled with carrier actin, only 1-3% of the Asp11 mutant actins incorporated after 2 min and did not increase after 2 hr. Roughly 17% of wild type actin incorporated after 2 min and 31% after 2 hr. ATP increased the release of wild type actin from myofibrils, but did not increase the release of Asp11 mutants. We suggest that (1) the incorporation of wild type and His73 mutant actins was due to a physiological process whereas association of Asp11 mutants with myofibrils was non-specific, (2) the incorporation of wild type actin involved a rapid initial phase, followed by a slower phase, and (3) since some of the Asp11 mutants can co-assemble with wild type actin, the ability to self-assemble was not sufficient for incorporation into myofibrils. Thus, incorporation probably includes interaction between actin and a thin filament associated protein. We also showed that incorporation occurred at actin concentrations which would cause disassembly of F-actin. Since the myofibrils did not show large scale disassembly but incorporated actin, filament stability and monomer incorporation are likely to be mediated by actin associated proteins of the myofibril.

Contractile protein dynamics of myofibrils in paired adult rat cardiomyocytes

The purpose of this study was to determine how quickly contractile proteins are incorporated into the myofibrils of freshly isolated cardiomyocytes and to determine whether there are regions of the cells that are more dynamic than others in their ability to incorporate the proteins. Paired cardiomyocytes joined at intercalated discs and single cells were isolated from adult rats, and microinjected 3 hours later with fluorescently labeled actin, alpha-actinin, myosin light chains and vinculin. The cells were fixed and permeabilized at various period, 5 seconds and longer, after microinjection. Actin became incorporated throughout the I-Bands in as short a time as 5 seconds. The free edges of the cells, which were formerly intercalated discs, exhibited concentrations of actin greater than that incorporated in the I-Bands. This extra concentration of actin was not detected, however, at intact intercalated discs connecting paired cells. Alpha-actinin was incorporated immediately into Z-Bands and intercalated discs. Vinculin, also, was localized at the Z-Bands and at intercalated discs, but in contrast to alpha-actinin, there was a higher concentration of vinculin in the region of the intact intercalated discs. Both alpha-actinin and vinculin were concentrated at the free ends of the cells that were formerly parts of intercalated discs. Myosin light chains were observed to incorporate into the A-Bands in periods as short as 5 seconds. These results suggest that the myofibrils of adult cardiomyocytes may be capable of rapid isoform transitions along the length of the myofibrils. The rapid accumulation of fluorescent actin, alpha-actinin, and vinculin in membrane sites that were previously parts of intercalated discs, may reflect the response to locomotory activity that is initiated in these areas as cells spread in culture. A similar response after an injury in the intact heart could allow repair to occur.

Costameres are sites of force transmission to the substratum in adult rat cardiomyocytes

Costameres, the vinculin-rich, sub-membranous transverse ribs found in many skeletal and cardiac muscle cells (Pardo, J. V., J. D. Siciliano, and S. W. Craig. 1983. Proc. Natl. Acad. Sci. USA. 80:363-367.) are thought to anchor the Z-lines of the myofibrils to the sarcolemma. In addition, it has been postulated that costameres provide mechanical linkage between the cells' internal contractile machinery and the extracellular matrix, but direct evidence for this supposition has been lacking. By combining the flexible silicone rubber substratum technique (Harris, A. K., P. Wild, and D. Stopak. 1980. Science (Wash. DC). 208:177-179.) with the microinjection of fluorescently labeled vinculin and alpha-actinin, we have been able to correlate the distribution of costameres in adult rat cardiac myocytes with the pattern of forces these cells exert on the flexible substratum. In addition, we used interference reflection microscopy to identify areas of the cells which are in close contact to the underlying substratum. Our results indicate that, in older cell cultures, costameres can transmit forces to the extracellular environment. We base this conclusion on the following observations: (a) adult rat heart cells, cultured on the silicone rubber substratum for 8 or more days, produce pleat-like wrinkles during contraction, which diminish or disappear during relaxation; (b) the pleat-like wrinkles form between adjacent alpha-actinin-positive Z-lines; (c) the presence of pleat-like wrinkles is always associated with a periodic, "costameric" distribution of vinculin in the areas where the pleats form; and (d) a banded or periodic pattern of dark gray or close contacts (as determined by interference reflection microscopy) has been observed in many cells which have been in culture for eight or more days, and these close contacts contain vinculin. A surprising finding is that vinculin can be found in a costameric pattern in cells which are contracting, but not producing pleat-like wrinkles in the substratum. This suggests that additional proteins or posttranslational modifications of known costamere proteins are necessary to form a continuous linkage between the myofibrils and the extracellular matrix. These results confirm the hypothesis that costameres mechanically link the myofibrils to the extracellular matrix. We put forth the hypothesis that costameres are composite structures, made up of many protein components; some of these components function primarily to anchor myofibrils to the sarcolemma, while others form transmembrane linkages to the extracellular matrix.