Immunocytochemical studies of spectrin in hamster cardiac tissue

Riding the waves of the intercalated disc of the heart

Cardiomyocytes interact with each other at their ends through the specialised membrane complex, the intercalated disck (ID). It is a fascinating structure. It allows cardiomyocytes to interact with several neighbouring cells, thereby allowing the complex structure of the heart to develop. It acts as tension transducer, structural prop, and multi signalling domain as well as a regulator of growth. It achieves its many functions through a number of specialised domains and intercellular junctions associated with its complex folded membrane. This review outlines the results of some 20 years of fascination with the ups and downs of the ID. These include locating the spectrin-associated membrane cytoskeleton in the ID and investigating the role of Protein 4.1R in calcium signalling; structural studies of the relationship of the ID to myofibrils, sarcoplasmic reticulum and mitochondria and, finally, consideration of the role of the ID in cardiomyocyte growth and heart disease.

Characterization and expression of a heart-selective alternatively spliced variant of alpha II-spectrin, cardi+, during development in the rat

Spectrin is a large, flexible protein that stabilizes membranes and organizes proteins and lipids into microdomains in intracellular organelles and at the plasma membrane. Alternative splicing occurs in spectrins, but it is not yet clear if these small variations in structure alter spectrin's functions. Three alternative splice sites have been identified previously for alpha II-spectrin. Here we describe a new alternative splice site, a 21-amino acid sequence in the 21st spectrin repeat that is only expressed in significant amounts in cardiac muscle (GenBank GQ502182). The insert, which we term alpha II-cardi+, results in an insertion within the high affinity nucleation site for binding of alpha-spectrins to beta-spectrins. To assess the developmental regulation of the alpha II-cardi+ isoform, we used qRT-PCR and quantitative immunoblotting methods to measure the levels of this form and the alpha II-cardi- form in the cardiac muscles of rats, from embryonic day 16 (E16) through adulthood. The alpha II-cardi+ isoform constituted approximately 26% of the total alpha II-spectrin in E16 hearts but decreased to approximately 6% of the total after 3 weeks of age. We used long-range RT-PCR and Southern blot hybridization to examine possible linkage of the alpha II-cardi+ alternatively spliced sequence with alternatively spliced sequences of alpha II-spectrin that had been previously reported. We identified two new isoforms of alpha II-spectrin containing the cardi+ insert. These were named alpha II Sigma 9 and alpha II Sigma 10 in accordance with the spectrin naming conventions. In vitro studies of recombinant alpha II-spectrin polypeptides representing the two splice variants of alpha II-spectrin, alpha II-cardi+ and alpha II-cardi-, revealed that the alpha II-cardi+ subunit has lower affinity for the complementary site in repeats 1-4 of betaII-spectrin, with a K(D) value of approximately 1 nM, as measured by surface plasmon resonance (SPR). In addition, the alpha II-cardi+ form showed 1.8-fold lower levels of binding to its site on beta II-spectrin than the alpha II-cardi- form, both by SPR and blot overlay. This suggests that the 21-amino acid insert prevented some of the alpha II-cardi+ form from interacting with beta II-spectrin. Fusion proteins expressing the alpha II-cardi+ sequence within the two terminal spectrin repeats of alpha II-spectrin were insoluble in solution and aggregated in neonatal myocytes, consistent with the possibility that this insert removes a significant portion of the protein from the population that can bind beta subunits. Neonatal rat cardiomyocytes infected with adenovirus encoding GFP-fusion proteins of repeats 18-21 of alpha II-spectrin with the cardi+ insert formed many new processes. These processes were only rarely seen in myocytes expressing the fusion protein lacking the insert or in controls expressing only GFP. Our results suggest that the embryonic mammalian heart expresses a significant amount of alpha II-spectrin with a reduced avidity for beta-spectrin and the ability to promote myocyte growth.

Diverse roles of the actin cytoskeleton in striated muscle

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

The transitional junction: a new functional subcellular domain at the intercalated disc

We define here a previously unrecognized structural element close to the heart muscle plasma membrane at the intercalated disc where the myofibrils lead into the adherens junction. At this location, the plasma membrane is extensively folded. Immunofluorescence and immunogold electron microscopy reveal a spectrin-rich domain at the apex of the folds. These domains occur at the axial level of what would be the final Z-disc of the terminal sarcomere in the myofibril, although there is no Z-disc-like structure there. However, a sharp transitional boundary lies between the myofibrillar I-band and intercalated disc thin filaments, identifiable by the presence of Z-disc proteins, alpha-actinin, and N-terminal titin. This allows for the usual elastic positioning of the A-band in the final sarcomere, whereas the transduction of the contractile force normally associated with the Z-disc is transferred to the adherens junctions at the plasma membrane. The axial conjunction of the transitional junction with the spectrin-rich domains suggests a mechanism for direct communication between intercalated disc and contractile apparatus. In particular, it provides a means for sarcomeres to be added to the ends of the cells during growth. This is of particular relevance to understanding myocyte elongation in dilated cardiomyopathy.

High-resolution en-face visualization of the cardiomyocyte plasma membrane reveals distinctive distributions of spectrin and dystrophin

The actin-binding proteins, spectrin and dystrophin, are key components of the plasma membrane-associated cytoskeleton of the cardiac muscle cell. From confocal immunofluorescence studies, the distribution of spectrin appears to overlap with that of dystrophin, but the precise functional differentiation, molecular distributions and spatial relationship of these two cytoskeletal systems remain unclear. Freeze-fracture replica immuno-electron microscopy, in parallel with immunofluorescence/confocal microscopy, were applied to examine at high resolution the spatial relationships between the spectrin and dystrophin membrane-associated cytoskeleton systems in cardiac muscle. Application of freeze-fracture replica cytochemistry, with single and double immunogold labeling, permitted simultaneous examination of the organization of spectrin and dystrophin in en-face views of the plasma membrane at high resolution. In contrast to the close spatial relationship previously demonstrated for dystrophin and beta-dystroglycan, no association between the gold label marking dystrophin and that marking spectrin was observed. Our freeze-fracture cytochemical results suggest that the two membrane skeletal networks formed by dystrophin and spectrin in cardiac muscle are independently organized, implying that whatever overlap of function (e.g., in structural support to the plasma membrane) may exist between them, the two systems may each have additional distinctive roles.

Not just a plasma membrane protein: in cardiac muscle cells alpha-II spectrin also shows a close association with myofibrils

Spectrin and its associated proteins are essential for the integrity of muscle cells and there is increasing evidence for their involvement in signalling pathways as well as having a structural function in mediating stress. Spectrin is a multigene family and it is essential to determine which isoforms are present and their location in the cell. In heart muscle, we have found that one spectrin isoform, alphaII-spectrin, is strongly represented and, using immunofluorescence, we show that it lies within the contractile fibres near the Z-disc as well as on the cardiomyocyte plasma membrane. Electron microscopy of immunogold-labelled cryosections reveals statistically significant clustering of gold particles near the Z-disc, within and close to the edge of myofibrils. betaII-spectrin and ankyrin-R and G are both known to occupy this region. We suggest that alphaIIbetaII spectrin tetramers with ankyrin organise and/or stabilise cardiac muscle cell membrane components relative to the contractile apparatus.

Cytochalasin D reduces Ca2+ sensitivity and maximum tension via interactions with myofilaments in skinned rat cardiac myocytes

The F-actin disrupter cytochalasin D depresses cardiac contractility, an effect previously ascribed to the interaction of cytochalasin D with cytoskeletal actin. We have investigated the possibility that this negative inotropic effect is due to the interaction of cytochalasin D with sarcomeric actin of the thin filament. Confocal images of Triton X-100-skinned myocytes incubated with a fluorescent conjugate of cytochalasin D revealed a longitudinally striated pattern of binding, consistent with a myofibrillar rather than cytoskeletal structure.Tension-pCa relationships were determined at sarcomere lengths (SLs) of 2.0 and 2.3 [mu]m following 2 min incubation with 1 [mu]M cytochalasin D. Cytochalasin D significantly reduced the pCa for half-maximal activation (pCa50) at both SLs. The shift in pCa50 was significantly greater at a SL of 2.3 [mu]m compared with that at a SL of 2.0 [mu]m. Cytochalasin D had no effect on the Hill co-efficient at either SL. Cytochalasin D significantly reduced the maximum tension at both SLs. We suggest that the length-dependent decrease in myofilament Ca2+ sensitivity in response to cytochalasin D is due to a decrease in the affinity of troponin C for Ca2+. Cytochalasin D has been used for many years as the agent of choice for disruption of cytoskeletal actin. However, we have demonstrated for the first time an interaction of cytochalasin D with sarcomeric actin of the thin filament, which can account for the effects of cytochalasin D on cardiac contractility.

Two populations of beta-spectrin in rat skeletal muscle

We use immunoblotting, immunoprecipitation, and centrifugation in sucrose density gradients to show that the product of the erythrocyte beta-spectrin gene in rat skeletal muscle (muscle beta-spectrin) is present in two states, one associated with fodrin, and another that is not associated with any identifiable spectrin or fodrin subunit. Immunofluorescence studies indicate that a significant amount of beta-spectrin without alpha-fodrin is present in the myoplasm of some muscle fibers, and, more strikingly, at distinct regions of the sarcolemma. These results suggest that alpha-fodrin and muscle beta-spectrin associate in muscle in situ, but that some muscle beta-spectrin without a paired alpha-subunit forms distinct domains at the sarcolemma.

Expression of intermediate filament desmin and vimentin in the human fetal heart

BACKGROUND

The intermediate filament (IF) desmin provides support for contractile machinery in muscle cells, and vimentin plays an important role in maintaining the stability of mesenchymal cells and in signal transduction. However, development of IFs in heart tissue during intrauterine life in human is not well established.

METHODS

In the present study, development of desmin and vimentin in human fetal hearts aged 9-28 weeks of gestation (n = 41) were investigated by immunohistochemistry with monoclonal antibodies against desmin and vimentin. Relative density of fluorescence of each sample was determined by densitometry. Left ventricle (LV) tissues from a 1-year-old child (n = 1) were examined by immunohistochemistry for postnatal comparison. Western blot analyses were done with only a few randomly selected LV tissues from fetuses of 9, 20, and 28 weeks gestation to assess trends of desmin and vimentin expression.

RESULTS

By Western blot analyses, 53-kDa desmin and 54-kDa vimentin were present in all fetal heart tissues examined. Desmin intensity was progressively increased with increasing fetal age, whereas vimentin intensity decreased. Desmin was present only in cardiomyocytes. In the earlier period (10-14 weeks gestation), desmin was localized along the cardiomyocyte membrane and/or Z lines in regular intervals, and later (25-28 weeks gestation) it was structurally well integrated; however, its network was incomplete. Only cardiomyocytes from a 1-year-old child revealed highly developed and integrated desmin lattices. However, vimentin was present in the mesenchymal tissue including fibroblasts and surrounding blood vessels. In part, some cardiomyocytes showed a weakly positive reaction with monoclonal antibody against vimentin in 9-14 weeks gestation. Vimentin-positive areas, however, were progressively diminished with increasing fetal age. Vimentin was present only in the connective tissue and coverings of the 1-year-old child's heart. Relative density of fluorescence of desmin was increased with increasing fetal age, whereas that of vimentin decreased.

CONCLUSIONS

These results indicate that there is a fetal age (or gestation)-dependent expression of IFs in human fetal heart: desmin increases with increasing fetal age, whereas vimentin decreases.

Inhomogeneous disappearance of myofilament-related cytoskeletal proteins in stunned myocardium of guinea pig

The decrease in Ca2+ responsiveness of myofilaments in stunned myocardium implies that there may be structural changes in proteins composing the contractile machinery. To elucidate the lesion in stunned myocardium, isolated guinea pig hearts were subjected to global ischemia at 37 degrees C and reperfused. SDS-PAGE revealed that the contents of desmin, alpha-actinin, and spectrin decreased in the myofibrillar fraction isolated from hearts reperfused after 60-minute ischemia compared with nonischemic control hearts. To examine the change of cytoskeletal proteins in stunned myocardium, immunohistochemical studies with antibodies against these proteins were performed after 15 minutes of ischemia. In stunned myocardium, the staining was largely intact, but there were some lesions where desmin was not stained and alpha-actinin and spectrin were only weakly identified. The percentage of normally stained areas in the myocardium (percent stained area), quantified by image processing, was significantly lower in stunned myocardium (79.6 +/- 3.6%, mean +/- SEM) than in nonischemic control myocardium (96.5 +/- 0.7%). Percent recovery of developed pressure significantly correlated with percent stained area (r = .82, P < .001). In hearts subjected to 15-minute ischemia but not reperfused, or in hearts reperfused with Ca(2+)-free solution after 15-minute ischemia, staining by the antibodies remained intact, suggesting that the change of the cytoskeletal proteins is mediated by Ca2+ overload during reperfusion. In hearts treated with the protease inhibitor leupeptin (50 mumol/L) or calpain inhibitor I (100 mumol/L), both developed pressure and staining were well preserved. These results indicate that contractile dysfunction in stunned myocardium has a strong correlation with the disappearance of cytoskeletal proteins that may be mediated by a Ca(2+)-dependent intracellular protease activated during reperfusion. The disruption of cytoskeletal proteins is a possible mechanism for stunning, although it may be a secondary effect of protease activation.

Reperfusion of rat heart after brief ischemia induces proteolysis of calspectin (nonerythroid spectrin or fodrin) by calpain

Rat myocardium expresses the 240- and 235-kD polypeptides antigenically related to alpha- and beta-subunits of brain calspectin (nonerythroid spectrin or fodrin), respectively. In the subcellular fractions of the myocardium, alpha-calspectin was found in the 600g, 10,000g, and 100,000g pellets, whereas beta-calspectin was localized to the 10,000g pellet. On the basis of the Na+,K(+)-ATPase activity and the contents of a gap junction protein, the sarcolemma was distributed to the 10,000g and 100,000g pellets, and the intercalated disks were enriched in the 10,000g pellet. Both alpha- and beta-calspectin were proteolyzed by calpain in vitro. The two subunits were also proteolyzed in vivo, when the rat hearts underwent 10 to 60 minutes of global ischemia followed by 30 minutes of reperfusion. The reperfusion following the ischemia induced the proteolysis of alpha-calspectin in the 10,000g and 100,000g pellets, producing the 150-kD fragment. A synthetic calpain inhibitor, calpain inhibitor-1, suppressed the degradation of calspectin in vivo, which indicates that calpain is responsible for the reperfusion-induced proteolysis of calspectin. The inhibitor also improved myocardial stunning. Immunohistochemical study revealed that the proteolysis of alpha-calspectin occurs at the intercalated disks and the sarcolemma after postischemic reperfusion, in accord with the biochemical data. These results suggest that degradation of calspectin partly accounts for the contractile failure of the myocardium after postischemic reperfusion by disrupting the membrane skeleton and the intercalated disks.

Immunocytochemistry of the muscle cell cytoskeleton

The muscle cell cytoskeleton is defined for this review as any structure or protein primarily involved in linking or connecting protein filaments to each other or to anchoring sites. In striated muscle, the M line connects thick filaments at their centers to adjacent thick filaments. Titin forms elastic filaments that extend from the M line to the Z line and may contribute to the resting tension properties of striated muscle. Nebulin forms inextensible filaments in skeletal muscle that are closely associated with thin filaments and that may provide a length template for thin filaments. Z lines anchor thin filaments from adjacent sarcomeres via the actin-binding function of alpha-actinin. Other proteins located at the Z line include Cap Z, Z-nin, Z protein, and zeugmatin. Intermediate filaments connect myofibrils to each other at the level of the Z line and to the sarcolemma at the Z- and possibly the M-line levels. Immunolocalization has identified the adhesion plaque proteins spectrin, vinculin, dystrophin, ankyrin, and talin at subsarcolemmal sites where they may be involved with filament attachment. Smooth muscle cell cytoskeletons are believed to include membrane associated dense bodies (MADBs), intermediate filaments, cytoplasmic dense bodies (CDBs), and perhaps a subset of actin filaments. MADBs contain a menu of attachment plaque proteins and anchor both thin filaments and intermediate filaments to the sarcolemma. CDBs are intracellular analogs of striated muscle Z lines and anchor thin filaments and intermediate filaments.

Developmental studies of dystrophin and other cytoskeletal proteins in cultured muscle cells

We studied the developmental changes of localization of dystrophin and other cytoskeletal proteins, especially actin, spectrin and dystrophin related protein (DRP) using immunocytochemistry and quick-freezing and deep-etching (QF-DE) method. In developmental studies of mouse and human muscle cultures, some myoblasts had positive-reactions to spectrin, DRP, and F-actin, but not dystrophin. In aneurally cultured myotubes, dystrophin, DRP, and spectrin were localized diffusely in the cytoplasm and later in discontinuous patterns on the plasma membrane, when myotubes became mature. Spectrin and DRP had more positive reactions in immature myotubes, compared with those of dystrophin. In some areas of myotubes, dystrophin/spectrin and spectrin/actin were localized reciprocally. In innervated cultured human muscle cells, dystrophin and DRP were localized in neuro-muscular junctions, which were co-localized with clusters of acetylcholine receptors. By using the QF-DE method, dystrophin was localized just underneath the plasma membrane, and closely linked to actin-like filaments (8-10 nm in diameter), most of which were decorated with myosin subfragment 1. In actin-poor regions, spectrin was detected as well-organized filamentous structures in highly interconnected networks with various diameters. DRP was distributed irregularly with granular appearance inside the cytoplasm and also under the plasma membrane in immature mouse myotubes. Our present studies show that dystrophin, spectrin, and DRP are localized differently at the developmental stages of myotubes. These results suggest that dystrophin, spectrin, and DRP are organized independently in developing myotubes and these cytoskeletal proteins might play different functions in the preservation of plasma membrane stability in developing myotubes.

Immunohistochemical localization of gap junction protein channels in hamster sinoatrial node in correlation with electrophysiologic mapping of the pacemaker region

INTRODUCTION

Gap junction proteins are thought to form the low resistance pathways that connect neighboring cells within the sinoatrial node, and to mediate pacemaker synchronization.

METHODS AND RESULTS

We have carried out microelectrode mapping experiments of the hamster sinoatrial region to localize the primary pacemaker area for subsequent light, electron, and immunofluorescence microscopic studies aimed at testing the hypothesis that the major cardiac gap junction protein (connexin43) is present in such an area. The site of earliest activation is unifocal and the pattern of activation, obtained by multiple sequential microelectrode recordings of the sinoatrial region, is qualitatively similar to that previously described for other species. However, quantitatively, the impulse transmission time from the primary pacemaker area to the crista (sulcus) terminalis in the hamster sinoatrial node is about 50% briefer than that of the guinea pig and five times faster than that of the rabbit. Immunolocalization studies in the hamster sinoatrial node using anti-connexin43 antisera demonstrated specific staining at the areas of cell-to-cell apposition and suggested that the apparently high degree of electrical coupling in this tissue is the result of abundant connexin43 expression. The immunofluorescence data were supported by light microscopic studies, which demonstrated the typical morphologic characteristics of sinus nodal cells in the pacemaker area. In addition, an electron microscopic study of the sinoatrial region revealed the presence of gap junctions in the junctional complex at areas of cell-to-cell contact.

CONCLUSION

Our results demonstrate that cells in the sinoatrial region of the hamster heart are electrically well coupled and strongly suggest that such coupling is mediated by gap junctional channels formed by connexin43.

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.

Localization of spectrin isoforms in the adult mouse heart

The distribution of two isoforms of spectrin in the adult mouse heart was investigated by Western blotting and immunocytochemistry by use of monospecific antibodies to erythrocyte spectrin and nonerythroid brain spectrin (240/235). Western blotting revealed proteins analogous to both isoforms of alpha-spectrin in adult heart. Light-microscopic immunocytochemistry indicated that erythroid spectrin was distributed throughout the myocardium, with immunofluorescence localized to plasma membranes, Z-lines, and intercalated discs. Antibodies to brain spectrin (240/235) exhibited staining throughout the heart, with a generally diffuse distribution except for the prominent immunoreactivity associated with the intercalated discs. Nonerythroid spectrin immunofluorescence was detected in the endothelial cells of the endocardium and the mesothelial cell lining of the epicardium. Erythrocyte spectrin was not detected in the endocardium or the epicardium. The identification and localization of spectrin isoforms in the mammalian heart suggest the importance of spectrin proteins in the structural integrity and proper function of cardiac cells and tissues. This is the first demonstration of two different alpha-spectrin subunits in the mammalian heart.

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.

Gap junctional channels in adult mammalian sinus nodal cells. Immunolocalization and electrophysiology

The subcellular mechanism of cell-to-cell communication in the natural pacemaker region of the mammalian heart was studied using electrophysiological and immunofluorescence techniques in isolated pairs of rabbit sinus nodal cells. By measuring whole-cell currents using a double patch-clamp approach, it was demonstrated that communication in the sinus node is mediated through gap junctional channels similar to those in other types of adult cardiac cell pairs. Macroscopic sinus nodal junctional resistance had a mean value of 387.9 +/- 97.1 M omega (mean +/- SEM, n = 10) and was greatly increased by superfusion with alkanols. Single-channel junctional conductance could be resolved in three cell pairs. Given their high membrane resistance (1.16 +/- 0.32 G omega, n = 12), the electrical coupling provided by as few as three gap junctional channels between nodal cells will allow for pacemaker synchronization. Further evidence for the presence of the channels was obtained from immunofluorescent double-labeling of desmin and the gap junction protein (connexin43) in sinus nodal tissue as well as in cultured sinus nodal cells. Using antisera against residues 243-257 of the connexin43 protein, a specific staining at the site of cell-to-cell apposition was demonstrated. These data provide direct evidence in favor of electronic coupling as the means for achieving pacemaker synchronization in the rabbit sinus node.

Protein p67. A calcium-binding protein localized at the sarcolemma of secretory atrial myocytes

Bovine heart 67-kd protein (p67) was coisolated with calpactin I complex by cycles of Ca(2+)-dependent precipitation followed by solubilization with EGTA-containing buffer. Using affinity-purified anti-p67 antibody and anti-p36 (36-kd subunit of calpactin I) antibody, we examined the localization of the two proteins in secretory atrial myocytes and other endocrine tissues of adult rats. Immunofluorescence microscopy showed that p67 was expressed both in the atrial myocytes in situ and in cultured atrial myocytes in which we failed to detect p36 and that p67 appeared to be closely associated with the cell surface. We also found that p67 was colocalized with p36 in the thyroid follicle epithelium and zona reticularis of the adrenal gland. On the other hand, neither p67 nor p36 was detectable in pancreas islet cells. Immunoelectron microscopy revealed that p67 was localized at the sarcolemma in the atrial myocytes in situ. The p67, which was shown to be a globular molecule with a diameter of 18-25 nm by a low-angle rotary shadowing method, bound radioactive Ca2+ on a nitrocellulose membrane. The results suggest that Ca(2+)-binding proteins expressed in endocrine cells seem to vary from tissue to tissue and that p67 may function in Ca(2+)-mediated events at the plasma membrane of secretory atrial myocytes and some types of endocrine cells expressing this protein.

Human cardiac and skeletal muscle spectrins: differential expression and localization

We describe multiple human cardiac and skeletal muscle spectrin isoforms. Cardiac muscle expresses five erythroid alpha,beta spectrin-reactive isoforms with estimated MR's of 280, 274, 270, 255, and 246 kD, respectively. At least one nonerythroid alpha-spectrin of MR 284 kD is expressed in heart. While skeletal muscle shares the 280, 270, and 246 kD erythroid spectrins, it expresses an immunologically distinct 284 kD nonerythroid alpha-spectrin isoform. The 255 kD erythroid beta-spectrin isoform is specific for cardiac tissue. By immunocytochemistry, both erythroid beta- and nonerythroid alpha-spectrins are localized to costameres, the plasma membrane, and the neuromuscular junctional region.

Studies of hamster cardiac myofibrillogenesis in vivo with antibodies to spectrin, desmin, and alpha-actinin

The spectrins are a family of cytoskeletal-membrane proteins that have generated much interest in the past decade. In the present study, we utilized immunohistochemical, morphological, and electrophoretic techniques to assess the possible function(s) of spectrin in mammalian cardiac tissue during development. Antibodies generated against alpha-actinin and desmin were also employed to identify myofibrils and intermediate filaments in relation to changes in the distribution of spectrin. Spectrin is localized along the sarcolemma of pre-myofibrillar hamster cardiac myocytes (day 8, postcoitum) and remains associated with the cell membrane throughout development. The staining pattern is somewhat diffuse at first, but eventually the cell margin becomes clearly defined by day 13 postcoitum. A second, more profound change in the distribution of spectrin occurs during the newborn stage, when spectrin begins to appear in the sarcoplasm. It appears as regularly spaced invaginations that are diffuse at first, eventually attaining a position around the Z-bands of adult muscle. The change in the distribution of spectrin coincides temporally with the appearance of T-tubules, which are sarcolemmal invaginations that reside at the Z-bands of adult heart. Thus, spectrin may act as a guidance mechanism for the proper positioning of T-tubules around the Z-discs of mammalian cardiac tissue. Although spectrin does not appear to interact directly with early myofibrils it may assist in the proper alignment of T-tubules and, in doing so, act to stabilize the entire contractile apparatus by enveloping it and attaching it to the sarcolemma.