Formation of myofibrils in spreading chick cardiac myocytes

Independent regulation of Z-lines and M-lines during sarcomere assembly in cardiac myocytes revealed by the automatic image analysis software sarcApp

Sarcomeres are the basic contractile units within cardiac myocytes, and the collective shortening of sarcomeres aligned along myofibrils generates the force driving the heartbeat. The alignment of the individual sarcomeres is important for proper force generation, and misaligned sarcomeres are associated with diseases, including cardiomyopathies and COVID-19. The actin bundling protein, α-actinin-2, localizes to the 'Z-Bodies" of sarcomere precursors and the 'Z-Lines' of sarcomeres, and has been used previously to assess sarcomere assembly and maintenance. Previous measurements of α-actinin-2 organization have been largely accomplished manually, which is time-consuming and has hampered research progress. Here, we introduce sarcApp, an image analysis tool that quantifies several components of the cardiac sarcomere and their alignment in muscle cells and tissue. We first developed sarcApp to utilize deep learning-based segmentation and real space quantification to measure α-actinin-2 structures and determine the organization of both precursors and sarcomeres/myofibrils. We then expanded sarcApp to analyze 'M-Lines' using the localization of myomesin and a protein that connects the Z-Lines to the M-Line (titin). sarcApp produces 33 distinct measurements per cell and 24 per myofibril that allow for precise quantification of changes in sarcomeres, myofibrils, and their precursors. We validated this system with perturbations to sarcomere assembly. We found perturbations that affected Z-Lines and M-Lines differently, suggesting that they may be regulated independently during sarcomere assembly.

A-Band assembly in avian skeletal muscles observed with super-resolution microscopy

Myofibrils in vertebrate skeletal muscle are organized in aligned arrays of filaments formed from multiple protein components. Despite considerable information describing individual proteins, how they assemble de novo into mature myofibrils is still a challenge. Studies in our lab of sarcomeric protein localization during myofibril assembly led us to propose a three-step progression: premyofibrils to nascent myofibrils, culminating in mature myofibrils. Premyofibrils, forming at the spreading edges of muscle cells, are composed of minisarcomeres containing small bands of non-muscle myosin II filaments alternating with muscle-specific α-actinin Z-Bodies attached to barbed ends of actin filaments, establishing bipolar F-actin arrangements in sarcomeres. Assembly of nascent myofibrils occurs with addition of muscle-specific myosin II, F-actin, titin, and the alignment of Z-Bodies in adjacent fibrils to form beaded Z-Bands. Muscle-specific myosin II filaments in nascent myofibrils appear in an overlapping arrangement when viewed with wide-field and confocal microscopes. In mature myofibrils, non-muscle myosin II is absent, and M-Band proteins localize to the muscle myosin II filaments, aiding their alignment by cross-linking them into A-Bands. Super-resolution microscopy (SIM and STED) revealed muscle myosin II in mini-A-Bands in nascent myofibrils. In contrast to previous reports that vertebrate muscle myosin thick filaments form at their final 1.6 μm lengths, mini-A-Bands are first detected at a length of about 0.4 μm, and gradually increase four-fold in length to 1.6 μm in mature myofibrils. These new discoveries in avian skeletal muscle cells share a common characteristic with invertebrate muscles where some A-Bands can grow to lengths reaching 25 μm.

Independent regulation of Z-lines and M-lines during sarcomere assembly in cardiac myocytes revealed by the automatic image analysis software sarcApp

Sarcomeres are the basic contractile units within cardiac myocytes, and the collective shortening of sarcomeres aligned along myofibrils generates the force driving the heartbeat. The alignment of the individual sarcomeres is important for proper force generation, and misaligned sarcomeres are associated with diseases including cardiomyopathies and COVID-19. The actin bundling protein, α-actinin-2, localizes to the "Z-Bodies" of sarcomere precursors and the "Z-Lines" of sarcomeres, and has been used previously to assess sarcomere assembly and maintenance. Previous measurements of α-actinin-2 organization have been largely accomplished manually, which is time-consuming and has hampered research progress. Here, we introduce sarcApp, an image analysis tool that quantifies several components of the cardiac sarcomere and their alignment in muscle cells and tissue. We first developed sarcApp to utilize deep learning-based segmentation and real space quantification to measure α-actinin-2 structures and determine the organization of both precursors and sarcomeres/myofibrils. We then expanded sarcApp to analyze "M-Lines" using the localization of myomesin and a protein that connects the Z-Lines to the M-Line (titin). sarcApp produces 33 distinct measurements per cell and 24 per myofibril that allow for precise quantification of changes in sarcomeres, myofibrils, and their precursors. We validated this system with perturbations to sarcomere assembly. We found perturbations that affected Z-Lines and M-Lines differently, suggesting that they may be regulated independently during sarcomere assembly.

Inhibitors of the Ubiquitin Proteasome System block myofibril assembly in cardiomyocytes derived from chick embryos and human pluripotent stem cells

Details of sarcomeric protein assembly during de novo myofibril formation closely resemble myofibrillogenesis in skeletal and cardiac myocytes in birds, rodents and zebrafish. The arrangement of proteins during myofibrillogenesis follows a three-step process: beginning with premyofibrils, followed by nascent myofibrils, and concluding with mature myofibrils (reviewed in Sanger et al., 2017). Our aim is to determine if the same pathway is followed in human cardiomyocytes derived from human inducible pluripotent stem cells. We found that the human cardiomyocytes developed patterns of protein organization identical to the three-step series seen in the model organisms cited above. Further experiments showed that myofibril assembly can be blocked at the nascent myofibril by five different inhibitors of the Ubiquitin Proteasome System (UPS) stage in both avian and human cardiomyocytes. With the exception of Carfilzomib, removal of the UPS inhibitors allows nascent myofibrils to proceed to mature myofibrils. Some proteasomal inhibitors, such as Bortezomib and Carfilzomib, used to treat multiple myeloma patients, have off-target effects of damage to hearts in three to six percent of these patients. These cardiovascular adverse events may result from prevention of mature myofibril formation in the cardiomyocytes. In summary, our results support a common three-step model for the formation of myofibrils ranging from avian to human cardiomyocytes. The Ubiquitin Proteasome System is required for progression from nascent myofibrils to mature myofibrils. Our experiments suggest a possible explanation for the cardiac and skeletal muscle off-target effects reported in multiple myeloma patients treated with proteasome inhibitors. This article is protected by copyright. All rights reserved.

Effect of MG-132 on myofibrillogenesis and the ubiquitination of GAPDH in quail myotubes

In the three-step myofibrillogenesis model, mature myofibrils are formed through two intermediate structures: premyofibrils and nascent myofibrils. We have recently reported that several inhibitors of the Ubiquitin Proteosome System, for example, MG-132, and DBeQ, reversibly block progression of nascent myofibrils to mature myofibrils. In this investigation, we studied the effects of MG132 and DBeQ on the expression of various myofibrillar proteins including actin, myosin light and heavy chains, tropomyosin, myomesin, and myosin binding protein-C in cultured embryonic quail myotubes by western blotting using two loading controls-α-tubulin and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Surprisingly, we found that MG-132 affected the level of expression of GAPDH but DBeQ did not. Reverse transcription polymerase chain reaction (RT-PCR) and quantitative reverse transcription-PCR (qRT-PCR) showed no significant effect of MG-132 on GAPDH transcription. Two-dimensional (2D) western blot analyses with extracts of control and MG-132-treated cells using anti-ubiquitin antibody indicated that MG132-treated myotubes show a stronger emitter-coupled logic signal. However, Spot% and Spot volume calculations for all spots from both western blot film signals and matched Coomassie-stained 2D polyacrylamide gel electrophoresis showed that the intensity of staining in a spot of ~39 kDa protein is 3.5-fold lower in the gel of MG-132-treated extracts. Mass spectrometry analyses identified the ~39 kDa protein as quail GAPDH. Immunohistochemical analysis of fixed MG-132-treated myotubes with anti-GAPDH antibody showed extensive clump formation, which may be analogous to granule formation by stress response factors in MG132-treated cells. This is the first report on in vivo ubiquitination of GAPDH. This may be essential for the moonlighting (Jeffery, 1999) activity of GAPDH for tailoring stress in myotubes.

Myofibril assembly and the roles of the ubiquitin proteasome system

De novo assembly of myofibrils in vertebrate cross-striated muscles progresses in three distinct steps, first from a minisarcomeric alignment of several nonmuscle and muscle proteins in premyofibrils, followed by insertions of additional proteins and increased organization in nascent myofibrils, ending with mature contractile myofibrils. In a search for controls of the process of myofibril assembly, we discovered that the transition from nascent to mature myofibrils could be halted by inhibitors of three distinct functions of the ubiquitin proteasome system (UPS). First, inhibition of pathway to E3 Cullin ligases that ubiquitinate proteins led to an arrest of myofibrillogenesis at the nascent myofibril stage. Second, inhibition of p97 protein extractions of ubiquitinated proteins led to a similar arrest of myofibrillogenesis at the nascent myofibril stage. Third, inhibitors of proteolytic action by proteasomes also blocked nascent myofibrils from transitioning to mature myofibrils. In contrast, inhibitors of autophagy or lysosomes did not affect myofibrillogenesis. To probe for differences in the effects of UPS inhibitors during myofibrillogenesis, we analyzed by fluorescence recovery after photobleaching the exchange rates of two selected sarcomeric proteins (muscle myosin II heavy chains and light chains). In the presence of p97 and proteasomal inhibitors, the dynamics of each of these two myosin proteins decreased in the nascent myofibril stage, but were unaffected in the mature myofibril stage. The increased stability of myofibrils occurring in the transition from nascent to mature myofibril assembly indicates the importance of dynamics and selective destruction in the muscle myosin II proteins for the remodeling of nascent to mature myofibrils.

Nonmuscle myosin II in cardiac and skeletal muscle cells

De novo assembly of contractile myofibrils begins with the formation of premyofibrils where filaments of non-muscle myosin (NM II), and actin organize in sarcomeric patterns with Z-Bodies containing muscle-specific alpha-actinin. Interactions of muscle specific myosin (MM II) with NM II occur in a nascent myofibril stage that precedes the assembly of mature myofibrils. By the final stage of myofibrillogenesis, the only myosin II present in the mature myofibrils is MM II. In this current study of myofibril assembly, the three vertebrate isoforms of NM II (A, B, and C) and sarcomeric alpha-actinin, ligated to GFP family proteins, were coexpressed in avian embryonic skeletal and cardiac muscle cells. Each isoform of NM II localized only in the mini-A-Bands of premyofibrils and nascent myofibrils. There was no evidence of localization of NM II in Z-Bodies of premyofibrils and nascent myofibrils or in Z-Bands of mature myofibrils. Fluorescence Recovery After Photobleaching (FRAP) experiments indicated similar exchange rates in premyofibrils for NM II isoforms A and B, whereas the IIC isoform was significantly less dynamic. Fluorescence Resonance Energy Transfer (FRET) measurements of colocalized fluorescent pairs of different NM II isoforms yielded signals similar to identical pairs, indicating copolymerization of the different NM II pairs. The role of NM II may reside in establishing the future sarcomere pattern in mature myofibrils by binding to the oppositely polarized actin filaments that extend between pairs of Z-Bodies along premyofibrils prior to their transformation into mature myofibrils.

Fhod1, an actin-organizing formin family protein, is dispensable for cardiac development and function in mice

The formin family proteins have the ability to regulate actin filament assembly, thereby functioning in diverse cytoskeletal processes. Fhod3, a cardiac member of the family, plays a crucial role in development and functional maintenance of the heart. Although Fhod1, a protein closely-related to Fhod3, has been reported to be expressed in cardiomyocytes, the role of Fhod1 in the heart has still remained elusive. To know the physiological role of Fhod1 in the heart, we disrupted the Fhod1 gene in mice by replacement of exon 1 with a lacZ reporter gene. Histological lacZ staining unexpectedly revealed no detectable expression of Fhod1 in the heart, in contrast to intensive staining in the lung, a Fhod1-containing organ. Consistent with this, expression level of the Fhod1 protein in the heart was below the lower limit of detection of the present immunoblot analysis with three independent anti-Fhod1 antibodies. Homozygous Fhod1-null mice did not show any defects in gross and histological appearance of the heart or upregulate fetal cardiac genes that are induced under stress conditions. Furthermore, Fhod1 ablation did not elicit compensatory increase in expression of other formins. Thus, Fhod1 appears to be dispensable for normal development and function of the mouse heart, even if a marginal amount of Fhod1 is expressed in the heart.

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.

Combretastatin A-4 derived 5-(1-methyl-4-phenyl-imidazol-5-yl)indoles with superior cytotoxic and anti-vascular effects on chemoresistant cancer cells and tumors

5-(1-Methyl-4-phenyl-imidazol-5-yl)indoles 5 were prepared and tested as analogs of the natural vascular-disrupting agent combretastatin A-4 (CA-4). The 3-bromo-4,5-dimethoxyphenyl derivative 5c was far more active than CA-4 with low nanomolar IC50 concentrations against multidrug-resistant KB-V1/Vbl cervix and MCF-7/Topo mamma carcinoma cells, and also against CA-4-resistant HT-29 colon carcinoma cells. While not interfering markedly with the polymerization of tubulin in vitro, indole 5c completely disrupted the microtubule cytoskeleton of cancer cells at low concentrations. It also destroyed real blood vessels, both in the chorioallantoic membrane (CAM) of fertilized chicken eggs and within tumor xenografts in mice, without harming embryo or mouse, respectively. Indole 5c was less toxic than CA-4 to endothelial cells, fibroblasts, and cardiomyocytes. In highly vascularized xenograft tumors 5c induced distinct discolorations and histological features typical of vascular-disrupting agents, such as disrupted vessel structures, hemorrhages, and extensive necrosis. In a first preliminary therapy trial, indole 5c retarded the growth of resistant xenograft tumors in mice. © 2016 Elsevier Science Ltd. All rights reserved.

Creating living cellular machines

Development of increasingly complex integrated cellular systems will be a major challenge for the next decade and beyond, as we apply the knowledge gained from the sub-disciplines of regenerative medicine, synthetic biology, micro-fabrication and nanotechnology, systems biology, and developmental biology. In this prospective, we describe the current state-of-the-art in the assembly of source cells, derived from pluripotent cells, into populations of a single cell type to produce the components or building blocks of higher order systems and finally, combining multiple cell types, possibly in combination with scaffolds possessing specific physical or chemical properties, to produce higher level functionality. We also introduce the issue, questions and ample research opportunities to be explored by others in the field. As these "living machines" increase in capabilities, exhibit emergent behavior and potentially reveal the ability for self-assembly, self-repair, and even self-replication, questions arise regarding the ethical implications of this work. Future prospects as well as ways of addressing these complex ethical questions will be discussed.

Expression and subcellular localization of mammalian formin Fhod3 in the embryonic and adult heart

The formin family proteins play pivotal roles in actin filament assembly via the FH2 domain. The mammalian formin Fhod3 is highly expressed in the heart, and its mRNA in the adult heart contains exons 11, 12, and 25, which are absent from non-muscle Fhod3 isoforms. In cultured neonatal cardiomyocytes, Fhod3 localizes to the middle of the sarcomere and appears to function in its organization, although it is suggested that Fhod3 localizes differently in the adult heart. Here we show, using immunohistochemical analysis with three different antibodies, each recognizing distinct regions of Fhod3, that Fhod3 localizes as two closely spaced bands in middle of the sarcomere in both embryonic and adult hearts. The bands are adjacent to the M-line that crosslinks thick myosin filaments at the center of a sarcomere but distant from the Z-line that forms the boundary of the sarcomere, which localization is the same as that observed in cultured cardiomyocytes. Detailed immunohistochemical and immuno-electron microscopic analyses reveal that Fhod3 localizes not at the pointed ends of thin actin filaments but to a more peripheral zone, where thin filaments overlap with thick myosin filaments. We also demonstrate that the embryonic heart of mice specifically expresses the Fhod3 mRNA isoform harboring the three alternative exons, and that the characteristic localization of Fhod3 in the sarcomere does not require a region encoded by exon 25, in contrast to an essential role of exons 11 and 12. Furthermore, the exon 25-encoded region appears to be dispensable for actin-organizing activities both in vivo and in vitro, albeit it is inserted in the catalytic FH2 domain.

Arg/Abl-binding protein, a Z-body and Z-band protein, binds sarcomeric, costameric, and signaling molecules

ArgBP2 (Arg/Abl-Binding Protein) is expressed at high levels in the heart and is localized in the Z-bands of mature myofibrils. ArgBP2 is a member of a small family of proteins that also includes vinexin and CAP (c-Cbl-associated protein), all characterized by having one sorbin homology (SOHO) domain and three C-terminal SH3 domains. Antibodies directed against ArgBP2 also react with the Z-bodies of myofibril precursors: premyofibrils and nascent myofibrils. Expression in cardiomyocytes of plasmids encoding Yellow Fluorescent Protein (YFP) fused to either full length ArgBP2, the SOHO, mid-ArgBP or the SH3 domains of ArgBP2 led to Z-band targeting of the fusion proteins, whereas an N-terminal fragment lacking these domains did not target to Z-bands. Although ArgBP2 is not found in skeletal muscle cells, YFP-ArgBP2 did target to Z-bodies and Z-bands in cultured myotubes. GST-ArgBP2-SH3 bound actin, α-actinin and vinculin proteins in blot overlays, cosedimentation assays, and EM negative staining techniques. Over-expression of ArgBP2 and ArgBP2-SH3 domains, but not YFP alone, led to loss of myofibrils in cardiomyocytes. Fluorescence recovery after photobleaching was used to measure the rapid dynamics of both the full length and some truncated versions of ArgBP2. Our results indicate that ArgBP2 may play an important role in the assembly and maintenance of myofibrils in cardiomyocytes.

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.

Assembly and dynamics of myofibrils

We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric proteins moving between the sarcomere and the cytoplasm. We also address in this personal review the recent evidence that indicates that the assembly and dynamics of myofibrils are conserved whether the cells are analyzed in situ or in tissue culture conditions. We suggest that myofibrillogenesis is a fundamentally conserved process, comparable to protein synthesis, mitosis, or cytokinesis, whether examined in situ or in vitro.

Myofibrillogenesis in skeletal muscle cells in zebrafish

The "premyofibril" model of myofibrillogenesis, based on observations in cultured avian muscle cells, proposes that mature myofibrils are preceded by two intermediary structures: premyofibrils and nascent myofibrils. To determine if this model applies to zebrafish skeletal muscle development, we stained developing embryos with antibodies to sarcomeric alpha-actinin and myosin II. In the youngest muscle cells, sarcomeric alpha-actinin and non-muscle myosin II were each localized in linear arrays of small bands that resembled the premyofibrils in avian myocytes. The distribution of muscle-specific myosin II began as scattered short filaments followed in time by overlapping bundles of filaments and organized A-bands in the older somites. Alpha-actinin organization changed from small z-bodies to beaded Z-bands and ordered Z-bands in myofibrils that extended the length of the elongating somites. In older somites with mature myofibrils, premyofibrils were also present at the ends of the mature myofibrils, suggesting that as the cells and somites grew longer, premyofibrils were involved in the elongation of existing mature myofibrils. Fluorescence Recovery After Photobleaching showed that the exchange of proteins (actin, alpha-actinin, FATZ, myotilin and telethonin) between sarcoplasm and the Z-bands of mature myofibrils in zebrafish resembled that seen for the same proteins in cultured avian myotubes, suggesting that myofibril assembly and maintenance in zebrafish share common properties with avian muscle. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Myofibrillogenesis in the developing zebrafish heart: A functional study of tnnt2

Various hypotheses have been proposed to explain the molecule processes of sarcomere assembly, partially due to the lack of systematic genetic studies of sarcomeric genes in an in vivo model. Towards the goal of developing zebrafish as a vertebrate model for this purpose, we characterized myofibrillogenesis in a developing zebrafish heart and went on to examine the functions of cardiac troponin T (tnnt2). We found that sarcomere assembly in zebrafish heart was initiated from a non-striated actin filament network at the perimembrane region, whereas sarcomeric myosin is independently assembled into thick filaments of variable length before integrating into the thin filament network. Compared to Z-discs that are initially aligned to form shorter periodic dots and expanded longitudinally at a later time, M-lines assemble later and have a constant length. Depletion of full-length tnnt2 disrupted the striation of thin filaments and Z-bodies, which sequentially affects the striation of thick filaments and M-lines. Conversely, truncation of a C-terminal troponin complex-binding domain did not affect the striation of these sarcomere sub-structures, but resulted in reduced cardiomyocyte size. In summary, our data indicates that zebrafish are a valuable in vivo model for studying both myofibrillogenesis and sarcomere-based cardiac diseases.

How to build a myofibril

Building a myofibril from its component proteins requires the interactions of many different proteins in a process whose details are not understood. Several models have been proposed to provide a framework for understanding the increasing data on new myofibrillar proteins and their localizations during muscle development. In this article we discuss four current models that seek to explain how the assembly occurs in vertebrate cross-striated muscles. The models hypothesize: (a) stress fiber-like structures as templates for the assembly of myofibrils, (b) assembly in which the actin filaments and Z-bands form subunits independently from A-band subunits, with the two subsequently joined together to form a myofibril, (c) premyofibrils as precursors of myofibrils, or (d) assembly occurring without any intermediary structures. The premyofibril model, proposed by the authors, is discussed in more detail as it could explain myofibrillogenesis under a variety of different conditions: in ovo, in explants, and in tissue culture studies on cardiac and skeletal muscles.

Differential effects of Latrunculin-A on myofibrils in cultures of skeletal muscle cells: insights into mechanisms of myofibrillogenesis

To test different models of myofibrillogenesis, we followed live cells expressing Green Fluorescent Proteins ligated to either actin or alpha-actinin and analyzed stress fibers, premyofibrils, and myofibrils in quail myotube cultures. Actin filaments in the three types of fibers were compared by analyzing the effects of Latrunculin-A (Lat-A), a monomeric actin binding macrolide drug (M.W. = 422 Daltons), on stress fibers in fibroblasts and on myofibrils in skeletal myotubes in the same culture. Lat-A, at low concentrations (0.2 microM), induced the loss of stress fibers in fibroblasts within a few hours and within 10 min when Lat-A was increased to 1.0 microM. The effect was reversible with reformation of the stress fibers when the drug was removed. In contrast to the Lat-A induced disassembly of stress fibers in fibroblasts, assembling myofibrils in the skeletal muscle cells were not affected by 1.0-microM concentrations of Lat-A. With increasing concentrations of Lat-A (up to 5 microM), and increasing incubation times, however, the drug induced premyofibrils, the precursors of mature myofibrils, to disassemble and the accumulation of mature myofibrils to be halted. Removal of the drug led to the reformation of premyofibrils and the resumption of myofibrillogenesis in the spreading edges of the myotubes. In contrast, the mature myofibrils in the central shaft of the myotubes were stable in doses of Lat-A as high as 50 microM. The newly assembled mature myofibrils located adjacent to the premyofibrils at the ends and sides of the myotube were intermediate in sensitivity to Lat-A, disassembling when exposed to 10 microM Lat-A for one hour. To determine how a change in the actin filaments during myofibrillogenesis might confer greater resistance to depolymerization by Lat-A, we stained the myotubes with an antibody directed against CapZ, a protein that blocks the release of monomer actin from the barbed ends of actin filaments. CapZ was absent from premyofibrils. It was distributed uniformly along nascent myofibrils where F-actin was unstriated, and was localized in a clearly striated Z-band pattern in the mature myofibrils where F-actin patterns were fully striated. These Lat-A and CapZ results are discussed in the context of various models of myofibrillogenesis.

Changes in myofibrils and cytoskeleton of neonatal hamster myocardial cells in culture: an immunofluorescence study

Myocardial cells in culture offer many possibilities for studying cellular and molecular biology of cardiac muscles. However, it is important to know how long these cells can be maintained in vitro without significant structural and biochemical changes. In this study, we have investigated the morphological changes of myofibril proteins and cytoskeletons by using immunofluorescent techniques in cultured neonatal hamster myocardial cells at different culture durations. Our results have demonstrated that these cultured cells still contain intact myofibrils and cytoskeletal proteins after 6 days in vitro incubation, however, the organization of some of these proteins is altered. The proteins most sensitive to these in vitro conditions are: myosin heavy chain, actin and desmin. The data indicate that the duration of the culture and the contractile activity of the myocardial cells in culture can influence organization of their contractile apparatus and cytoskeleton.

Use of green fluorescent proteins linked to cytoskeletal proteins to analyze myofibrillogenesis in living cells

Once the appropriate site has been selected for the attachment of GFP to the sarcomeric protein, it is quite remarkable that the large size of the GFP molecule does not appear to interfere with the localization of the fluorescent sarcomeric proteins into the sarcomeric regions of the myofibrils. A similar approach using truncated parts of sarcomeric proteins linked to GFP should allow studies of the targeting properties of other sarcomeric domains for localization and assembly studies.

Comparative characteristic of Mytilus muscle cells developed in vitro and in vivo

The mussel cells from premyogenic larval stages are capable of differentiation into smooth muscle cells in vitro. However, the behavior and protein composition of these cells are not completely identical to those of smooth muscle cells of adult mussels. In this study we compared some properties of mussel muscle cells forming from cells of trochophore (premyogenic larval stage) in vitro with those of muscle cells of veliger and adult mussel. We found a substantial difference between the contractile apparatus protein composition of veliger muscle and cultivated cells. Myorod, one of the molecular markers of the phenotype of mollusc smooth muscle cells (Shelud'ko et al., 1999, Comp Biochem Physiol 122:277-285), is not a constituent of the contractile apparatus of veliger muscle. At the same time the protein composition of contractile apparatus in cultivated cells was similar to that of adult Mytilus muscles. There were only few quantitative differences between them. The contractile activity of cultivated cells was changing in time. The kinetic parameters of first spontaneous contractions were similar to those of phasic contractions, while their period was close to that of tonic contractions. After 50-55 hrs cultivation the cells produced both phasic and tonic contractions, but the character of contractile activity of cultivated cells was regulated after six days of cultivation only. However, there were no muscle cells in vitro, whose contractile activity was similar to that of veliger muscle cells. So, we concluded that properties of muscle cells forming from premyogenic larval mussel cells in culture are similar to those of muscle cells of the adult mussel, but not of veliger.

Myofibrillogenesis in the first cardiomyocytes formed from isolated quail precardiac mesoderm

De novo assembly of myofibrils was investigated in explants of precardiac mesoderm from quail embryos to address a controversy about different models of myofibrillogenesis. The sequential expression of sarcomeric components was visualized in double- and triple-stained explants before, during, and just after the first cardiomyocytes began to beat. In explants from stage 6 embryos, cultured for 10 h, ectoderm, endoderm, and the precardiac mesoderm displayed arrays of stress fibers with alternating bands of the nonmuscle isoforms of alpha-actinin and myosin IIB. With increasing time in culture, mesoderm cells contained fibrils composed of actin, nonmuscle myosin IIB, and sarcomeric alpha-actinin. Several hours later, before beating occurred, both nonmuscle and muscle myosin II localized in some of the fibrils in the cells. Concentrations of muscle myosin began as thin bundles, dispersed in the cytoplasm, often overlapping one another, and progressed to small, aligned A-band-sized aggregates. The amount of nonmuscle myosin decreased dramatically when Z-bands formed, the muscle myosin became organized into A-bands, and the cells began beating. The sequential changes in protein composition of the fibrils in the developing muscle cells supports the model of myofibrillogenesis in which assembly begins with premyofibrils and progresses through nascent myofibrils to mature myofibrils.

p0071, a member of the armadillo multigene family, is a constituent of sarcomeric I-bands in human skeletal muscle

p0071 is a member of the armadillo gene family that is expressed in a wide variety of mammalian tissues and cell types with a prominent cell-cell contact association in epithelial cells. Here, we report the expression and localization patterns of p0071 in differentiating human skeletal muscle cells and in normal and diseased human skeletal muscle tissues. Northern blots revealed expression of p0071 mRNA in adult skeletal muscle tissue. RT-PCR analysis and Western blotting experiments identified two differentially spliced isoforms of p0071. The balance between these isoforms shifted during in vitro differentiation of isolated muscle cells from predominant expression of the short variant to a preponderance of the larger variant from day 6 onwards. Immunolocalization studies in mature skeletal muscle tissue revealed that p0071 is a constituent of myofibrils with a distinct localization at the level of sarcomeric N2-lines. During myofibrillogenesis, p0071 was not detected in non-striated nascent myofibrils, but became apparent shortly after the development of compact Z-discs in early myotubes. Furthermore, we studied the expression of p0071 in a wide variety of neuromuscular disorders by indirect immunofluorescence. Here, the myofibrillar staining of p0071 was preserved in all the disease entities included in our study. Our results provide the first evidence that a member of the armadillo multigene family is a constituent of the contractile apparatus in human skeletal muscle. The localization of p0071 at the level of I-bands and the timepoint of its integration into developing myofibrils suggest a possible role in the organization of thin filaments.

Assembly of myofibrils in cardiac muscle cells

How do myofibrils assemble in cardiac muscle cells? When does titin first assemble into myofibrils? What is the role of titin in the formation of myofibrils in cardiac muscle cells? This chapter reviews when titin is first detected in cultured cardiomyocytes that have been freshly isolated from embryonic avian hearts. Our results support a model for myofibrillogenesis that involves three stages of assembly: premyofibrils, nascent myofibrils and mature myofibrils. Titin and muscle thick filaments were first detected associated with the nascent myofibrils. The Z-band targeting site for titin is localized in the N-terminus of titin. This region of titin binds alpha-actinin and less avidly vinculin. Thus the N-terminus of titin via its binding to alpha-actinin, and vinculin could also help mediate the costameric attachment of the Z-bands of mature myofibrils to the nearest cell surfaces.

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.

The muscle regulatory and structural protein MLP is a cytoskeletal binding partner of betaI-spectrin

Muscle LIM protein (MLP) is a striated muscle-specific factor that enhances myogenic differentiation and is critical to maintaining the structural integrity of the contractile apparatus. The ability of MLP to regulate myogenesis is particularly interesting since it exhibits multiple subcellular localizations, being found in both nuclear and cytoplasmic compartments. Despite extensive biochemical analyses on MLP, the mechanism(s) by which it influences the myogenic program remains largely undefined. To further examine the role of MLP as a positive myogenic regulator, a yeast two-hybrid screen was employed to identify cytoplasmic-associated MLP binding partners. From this screen, the cytoskeletal protein betaI-spectrin was isolated. Protein interaction assays demonstrate that MLP and betaI-spectrin associate with one another in vivo as well as when tested under several in vitro binding conditions. betaI-spectrin binds specifically to MLP but not to the MLP related proteins CRP1 and CRP2 or to other LIM domain containing proteins. The MLP:beta-spectrin interaction is mediated by the second LIM motif of MLP and by repeat 7 of beta-spectrin. Confocal microscopy studies also reveal that MLP co-localizes with beta-spectrin at the sarcolemma overlying the Z- and M-lines of myofibrils in both cardiac and skeletal muscle tissue. Given that beta-spectrin is a known costamere protein, we propose that sarcolemma-associated MLP also serves as a key costamere protein, stabilizing the association of the contractile apparatus with the sarcolemma by linking the beta-spectrin network to the alpha-actinin crosslinked actin filaments of the myofibril.

Role of desmin filaments in chicken cardiac myofibrillogenesis

Desmin filaments are muscle-specific intermediate filaments located at the periphery of the Z-discs, and they have been postulated to play a critical role in the lateral registration of myofibrils. Previous studies suggest that intermediate filaments may be involved in titin assembly during the early stages of myofibrillogenesis. In order to investigate the putative function of desmin filaments in myofibrillogenesis, rabbit anti-desmin antibodies were introduced into cultured cardiomyocytes by electroporation to perturb the normal function of desmin filaments. Changes in the assembly of several sarcomeric proteins were examined by immunofluorescence. In cardiomyocytes incorporated with normal rabbit serum, staining for alpha-actinin and muscle actin displayed the typical Z-line and I-band patterns, respectively, while staining for titin with monoclonal anti-titin A12 antibody, which labels a titin epitope at the A-I junction, showed the periodic doublet staining pattern. Staining for C-protein gave an amorphous pattern in early cultures and identified A-band doublets in older cultures. In contrast, in cardiomyocytes incorporated with anti-desmin antibodies, alpha-actinin was found in disoriented Z-discs and the myofibrils became fragmented, forming mini-sarcomeres. In addition, titin was not organized into the typical A-band doublet, but appeared to be aggregated. Muscle actin staining was especially weak and appeared in tiny clusters. Moreover, in all ages of cardiomyocytes tested, C-protein remained in the disassembled form. The present data suggest the essential role of desmin in myofibril assembly.

Characterization of muscle filamin isoforms suggests a possible role of gamma-filamin/ABP-L in sarcomeric Z-disc formation

Filamin, also called actin binding protein-280, is a dimeric protein that cross-links actin filaments in the cortical cytoplasm. In addition to this ubiquitously expressed isoform (FLN1), a second isoform (ABP-L/gamma-filamin) was recently identified that is highly expressed in mammalian striated muscles. A monoclonal antibody was developed, that enabled us to identify filamin as a Z-disc protein in mammalian striated muscles by immunocytochemistry and immunoelectron microscopy. In addition, filamin was identified as a component of intercalated discs in mammalian cardiac muscle and of myotendinous junctions in skeletal muscle. Northern and Western blots showed that both, ABP-L/gamma-filamin mRNA and protein, are absent from proliferating cultured human skeletal muscle cells. This muscle specific filamin isoform is, however, up-regulated immediately after the induction of differentiation. In cultured myotubes, ABP-L/gamma-filamin localises in Z-discs already at the first stages of Z-disc formation, suggesting that ABP-L/gamma-filamin might play a role in Z-disc assembly.

Targeting of cardiac muscle titin fragments to the Z-bands and dense bodies of living muscle and non-muscle cells

A 6.5-kb N-terminal region of embryonic chick cardiac titin, including the region previously reported as part of the protein zeugmatin, has been sequenced, further demonstrating that zeugmatin is part of the N-terminal region of titin, and not a separate Z-band protein. This Z-band region of cardiac titin, from both 7- and 19-day embryos as well as from adult animals, was found to contain six different small motifs, termed z-repeats [Gautel et al., 1996: J. Cell Sci. 109:2747-2754], of approximately 45 amino acids each sandwiched between flanking regions containing Ig domains. Fragments of Z-band titin, linked to GFP, were expressed in cultured cardiomyocytes to determine which regions were responsible for Z-band targeting. Transfections of primary cultures of embryonic chick cardiomyocytes demonstrated that the z-repeats play the major role in targeting titin fragments to the Z-band. Similar transfections of skeletal myotubes and non-muscle cells lead to the localization of these cardiac z-repeats in the Z-bands of the myofibrils and the dense bodies of the stress fibers. Over-expression of these z-repeat constructs in either muscle or non-muscle cells lead to the loss of the myofibrils or stress fibers, respectively. The transfection experiments also indicated that small domains of a protein, 40 to 50 amino acids, can be studied for their localization properties in living cells if a suitable linker is placed between these small domains and the much larger 28 kDa GFP protein.

The cardiac mutant Mexican axolotl is a unique animal model for evaluation of cardiac myofibrillogenesis

Hearts from cardiac mutant Mexican axolotl, Ambystoma mexicanum, do not form organized myofibrils and fail to beat. Though previous biochemical and immunohistochemical experiments showed a possible reduction of cardiac tropomyosin it was not clear that this caused the lack of organized myofibrils in mutant hearts. We used cationic liposomes to introduce both rabbit and chicken tropomyosin protein into whole hearts of embryonic axolotls in whole heart organ cultures. The mutant hearts had a striking increase in the number of well-organized sarcomeric myofibrils when treated with rabbit or chicken tropomyosin. FITC-labeled rabbit tropomyosin was used to examine the kinetics of incorporation of the exogenous protein into mutant hearts and confirmed the uptake of exogenous protein by the cells of live hearts in culture. By 4 h of transfection, both normal and mutant hearts were found to incorporate FITC-labeled tropomyosin into myofibrils. We also delivered an anti-tropomyosin antibody (CH 1) into normal hearts to disrupt the existing cardiac myofibrils which also resulted in reduced heartbeat rates. CH1 antibody was detected within the hearts and disorganization of the myofibrils was apparent when compared to normal controls. Introduction of a C-protein monoclonal antibody (ALD 66) did not result in a disruption of organized myofibrils. The results show clearly that chicken or rabbit tropomyosin could be incorporated by the mutant hearts and that it was sufficient to overcome the factors causing a lack of myofibril formation in the mutant. This finding also suggests that a lack of organized myofibrils is caused primarily by either inadequate levels of tropomyosin or endogenous tropomyosin in mutant hearts is unsuitable for myofibril formation, which we were able to duplicate with the introduction of tropomyosin antibody. Furthermore, incorporation of a specific exogenous protein or antibody into normal and mutant hearts of the Mexican axolotl in whole heart organ culture offers an unique model to evaluate functionalroles of contractile proteins necessary for cardiac development and differentiation.

Myofibrillogenesis visualized in living embryonic cardiomyocytes

Myofibril formation was visualized in cultured live cardiomyocytes that were transfected with plasmids expressing green fluorescent protein (GFP) linked to the Z-band protein, alpha-actinin. The expression of this fluorescent protein provided an in vivo label for structures containing alpha-actinin. The GFP-alpha-actinin fusion protein was incorporated into Z-bands, intercalated discs, and attachment plaques, as well as into the punctate aggregates, or Z-bodies, that are thought to be the precursors of Z-bands. Observations of live cells over several days in culture permitted us to test aspects of several theories of myofibril assembly that had been proposed previously based on the study of fixed cells. Fine fibrils, called premyofibrils, that formed de novo at the spreading edges of cardiomyocytes, contained punctate concentrations of alpha-actinin, termed Z-bodies. The punctate Z-bodies grew and aligned with Z-bodies in adjacent fibrils. With increasing time, adjacent fibrils and Z-bodies appeared to fuse and form mature myofibrils and Z-bands in cytoplasmic regions where the linear arrays of Z-bodies had been. These new myofibrils became aligned with existing myofibrils at their Z-bands to form myofibrils that spanned the length of the spread cell. These results are consistent with a model that postulates that the fibrils that form de novo near the cell membrane are premyofibrils-i.e., the precursors of mature myofibrils.

An N-terminal fragment of titin coupled to green fluorescent protein localizes to the Z-bands in living muscle cells: overexpression leads to myofibril disassembly

Cultures of nonmuscle cells, skeletal myotubes, and cardiomyocytes were transfected with a fusion construct (Z1.1GFP) consisting of a 1.1-kb cDNA (Z1.1) fragment from the Z-band region of titin linked to the cDNA for green fluorescent protein (GFP). The Z1.1 cDNA encodes only 362 amino acids of the approximately 2000 amino acids that make up the Z-band region of titin; nevertheless, the Z1.1GFP fusion protein targets the alpha-actinin-rich Z-bands of contracting myofibrils in vivo. This fluorescent fusion protein also localizes in the nascent and premyofibrils at the edges of spreading cardiomyocytes. Similarly, in transfected nonmuscle cells, the Z1.1GFP fusion protein localizes to the alpha-actinin-containing dense bodies of the stress fibers in vivo. A dominant negative phenotype was also observed in living cells expressing high levels of this Z1.1GFP fusion protein, with myofibril disassembly occurring as titin-GFP fragments accumulated. These data indicate that the Z-band region of titin plays an important role in maintaining and organizing the structure of the myofibril. The Z1.1 cDNA was derived from a chicken cardiac lambda gt11 expression library, screened with a zeugmatin antibody. Recent work has suggested that zeugmatin is actually part of the N-terminal region of the 81-kb titin cDNA. A reverse transcriptase polymerase chain reaction using a primer from the distal end (5' end) of the Z1.1 zeugmatin cDNA and a primer from the nearest known proximal (3' end) chicken titin (also called connectin) cDNA resulted in a predicted 0.3-kb polymerase chain reaction product linking the two known chicken titin cDNAs to each other. The linking region had a 79% identity at the amino acid level to human cardiac titin. This result and a Southern blot analysis of chicken genomic DNA hybridized with Z1.1 add further support to our original suggestion that zeugmatin is a proteolytic fragment from the N-terminal region of titin.

Partial characterization of zeugmatin indicates that it is part of the Z-band region of titin

Zeugmatin is a muscle specific protein discovered by Maher et al. [1985: J. Cell Biol. 101:1871-1883] to be in Z-Bands of muscle and in the dense bodies of smooth muscle. Maher et al. [1985] generated a zeugmatin specific monoclonal antibody, McAb20, and then used immunoaffinity chromatography to isolate a 600-800 kD protein. During myofibrillogenesis of embryonic cardiac muscle, zeugmatin is detected in fully formed Z-bands in the mature myofibrils but not in the Z-bodies of premyofibrils [Rhee et al., 1994: Cell Motil. Cytoskeleton 28:1-24]. Rhee et al. [1994] have postulated that zeugmatin may be responsible for the fusion of the alpha-actinin containing Z-bodies to form the solid Z-Bands of the mature myofibrils. The current studies were undertaken to characterize the properties of zeugmatin. The McAb20 was used to probe a chicken heart lamba gt11 expression library, and three unique positive clones of 1.1, 1.4, and 1.7 kB were isolated. These were inserted into pcDNA3, sequenced, and assembled into a 1.8 kB ORF. A 60% identity with N-terminal region of the human cardiac titin sequence was revealed at the amino acid level. This region of the 1.8 kB zeugmatin sequence is located entirely in the Z-band region of the human cardiac titin molecule. The 1.1 kB clone of zeugmatin was subcloned into pTrcHisC and expressed in bacteria. Bacterial lysates were prepared and run over nickel columns to isolate a 46 kD fusion protein. This fusion protein formed a complex with purified alpha-actinin that could be immunoprecipitated with the zeugmatin specific antibody, McAb 20. The 1.1 kB sequence was transfected into non-muscle cell lines, PtK2 and REF. Twenty-four hours after transfection, the 46 kD zeugmatin peptide, not present in control non-muscle cells, was localized in focal adhesions and in a punctate pattern along the stress fibers. Double immunofluorescence staining revealed that zeugmatin colocalized with the alpha-actinin in the dense bodies and focal contacts of the stress fibers. At longer time points, as the transfected cells accumulated more truncated zeugmatin molecules, the cells lost adhesion plaques and stress fibers, and became detached from the substratum. Our results indicate the zeugmatin is part of the titin molecule that is located within the Z-band and that this section of the titin molecule anchors the actin crosslinking alpha-actinin molecules.

A new model for the research into rhythmic contraction activity of cardiomyocytes in vitro

Heart cells continue to contract rhythmically after isolation and in culture in vitro. We describe a model of heart preparation in vitro that permits quantitative research on the frequency of contractions of cardiomyocytes. The chick embryo heart explants placed on a network of elastic glass fibers continued beating for months, recorded and analyzed with the methods of computer-assisted image analysis. The efficacy of this experimental model for the screening of effects of various agents on the frequency of contractions was examined by following the effects of nifedipine, caffeine, ethanol, and benzamide. The reversibility of the effects and the reproducibility of results were demonstrated quantitatively. The significance of a mechanical elastic load provided by glass fibers for the preservation of long-lasting contractile activity of cardiomyocytes is discussed and the common occurrence of oscillatory contraction processes in various eucaryotic cells is noted.

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.

Teratogenic effects of ethanol during hyperplastic growth in cardiac myocyte cultures

Ethanol alters cellular growth and maturation, teratologic factors that are recognized as contributing to abnormal phenotypic expression. Cultured neonatal Sprague-Dawley rat cardiac myocytes were utilized to determine how ethanol alters growth and development. Two ethanol exposure paradigms were studied: (1) constant, to cultures in closed chambers for 7 days at low (10 mM) and high (50 mM) concentrations; and (2) periodic (24-hr) to cells during hyperplastic growth. In constantly exposed cultures, 10 and 50 mM ethanol concentrations depressed the rate of leucine incorporation and the rate of thymidine uptake during early hyperplastic growth (log phase growth). A resultant slower expansion of cell populations was noted. Although the period of maximum vulnerability appeared to be the hyperplastic growth phase, a second set of experiments using 10 and 50 mM ethanol were performed to assess the effects of short (24-hr) exposures. DNA synthesis was depressed during early hyperplastic growth compared with controls (days 2-4), reflected as a decrease in thymidine incorporation and smaller cell population. This study demonstrates that ethanol depresses both DNA and protein synthesis during hyperplastic growth resulting in an insufficient, protein-deficient cell mass, incapable of participating in normal embryogenesis.

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.

Immunofluorescent studies on Z-line-associated protein in cultured cardiomyocytes from neonatal hamsters

The organization of the cytoskeletal proteins, alpha-actinin, vinculin and desmin, was studied in newborn hamster cardiomyocytes in vitro by immunofluorescent microscopy. Since there have been indications that the in vitro organization of certain cytoskeletal elements of cardiomyocytes is not the same as in vivo, the studies were designed to examine the reorganization of these proteins in cultured cells. The observations concentrated on three proteins that are known to be associated in vivo with myofibrillar Z-lines. Beginning at 2 days in culture, and during subsequent days, the proteins examined underwent substantial redistributions before they reorganized back to their associations with the myofibrillar Z-lines. The pattern and time course for these redistributions were characteristic for each protein. Alpha-actinin was the first to return to its typical location at the level of the Z-lines during the second day in culture, followed by desmin at 4 days. Vinculin usually did not become associated with the Z-lines until 6 days in vitro. In the present study, analyses of the distributions and redistributions of particular proteins in the cultured cardiomyocytes have been useful for helping to identify changes in the myocyte as a result of isolation and culture conditions. In addition, a better understanding of the temporal and spatial relationships between cytoskeletal proteins assembling into the Z-line area has been gained.

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.

Spatial relationship of nebulin relative to other myofibrillar proteins during myogenesis in embryonic chick skeletal muscle cells in vitro

The developmental expression of nebulin was studied in embryonic chick skeletal muscle cells in vitro by means of immunofluorescence microscopy. Initially nebulin appeared homogeneously or in a punctate form in the cytoplasm, and then it was assembled into I-Z-I-like complexes containing actin and alpha-actinin but not myosin and connectin (titin). Striated patterns of nebulin ('singlets') in myofibrils appeared simultaneously with those of alpha-actinin (Z-bands), myosin (A-bands) and connectin ('doublets'), but earlier than those of actin. After actin striations were formed as myofibrils matured, each nebulin band started to exhibit 'droplets'. The delayed development of nebulin compared to the I-Z-I brush formation and the myofibril maturation seems to indicate that this giant myofibrillar protein is unnecessary for both the initial (formation of I-Z-I-like structures) and the subsequent (regular alignment of myofibrils) phases of myofibrillogenesis.

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

When fluorescently labeled contractile proteins are injected into embryonic muscle cells, they become incorporated into the cells' myofibrils. In order to determine if this exchange of proteins is unique to the embryonic stage of development, we isolated adult cardiac myocytes and microinjected them with fluorescently labeled actin, myosin light chains, alpha-actinin, and vinculin. Each of these proteins was incorporated into the adult cardiomyocytes and was colocalized with the cells' native proteins, despite the fact that the labeled proteins were prepared from noncardiac tissues. Within 10 min of injection, alpha-actinin was incorporated into Z-bands surrounding the site of injection. Similarly, 30 sec after injection, actin was incorporated into the entire I-bands at the site of injection. Following a 3-h incubation, increased actin fluorescence was noted at the intercalated disc. Vinculin exchange was seen in the intercalated discs, as well as in the Z-bands throughout the cells. Myosin light chains required 4-6 h after injection to become incorporated into the A-bands of the adult muscle. Nonspecific proteins, such as fluorescent BSA, showed no association with the myofibrils or the former intercalated discs. When adult cells were maintained in culture for 10 days, they retain the ability to incorporate these contractile proteins into their myofibrils. T-tubules and the sarcoplasmic reticulum could be detected in periodic arrays in the freshly isolated cells using the membrane dye WW781 and DiOC6[3], respectively. In conclusion, the myofibrils in adult, as in embryonic, muscle cells are dynamic structures, permitting isoform transitions without dismantling of the myofibrils.

The fub-1 mutation blocks initial myofibril formation in zebrafish muscle pioneer cells

The earliest muscle in zebrafish arises from iterated sets of two to six cells in each somite, the muscle pioneers (MP). MP develop synchronously in young trunk myotomes adjacent to the notochord, precisely where the horizontal myoseptum will form. They elongate without cell fusion and differentiate hours earlier than surrounding cells, thus providing a simple and accessible system for in vivo study of myogenesis and muscle patterning. Before the MP form definitive myofibrils they assemble long bundles of actin-containing filaments, similar to "stress-fiber-like structures" reported by others. In fub-1 mutants, in which myofibrils are disorganized in all skeletal muscle cells, the MP appear and elongate normally, but ordered actin filament bundles are not seen. This defect could underlie the later myofibrillar ones, consistent with the proposal that actin filament bundles are essential for proper formation of the muscle contractile apparatus.

Assembly of connectin (titin) in relation to myosin and alpha-actinin in cultured cardiac myocytes

By using polyclonal and monoclonal antibodies against connectin (titin) which stain the A-I junctional area and the A-band domain (polyclonal anti-connectin and monoclonal 4C9) and the I-band domain (monoclonal SM1), the developmental relationship of this elastic protein with sarcomeric proteins, especially and alpha-actin, was examined in embryonic chick cardiac myocytes in vitro under fluorescence microscopy. During premyofibril stages, I-Z-I proteins were detected first (alpha-actinin dots and diffuse actin [phalloidin and anti-troponin C] staining), and later in these areas connectin and myosin dots appeared with nearly identical distribution. Somewhat later, phalloidin-positive nonstriated fibrils were observed in a straight course. They were always reactive with antibodies against alpha-actinin and troponin C, but unreactive or only weakly reactive with anticonnectin and anti-myosin. Initially, alpha-actinin dots were aligned along these fibrils but did not form striations. As they aggregated to form Z-bands, connectin and myosin started to exhibit typical striation ('doublets' and A-bands, respectively). No difference in the staining pattern was observed with two kinds of monoclonal antibodies against different domains of connectin filaments (4C9 and SM1) at early phases. As myosin staining began to show clear A-bands, connectin epitopes became arranged in polarized positions. We conclude that primitive I-Z-I complexes appear prior to the assembly of connectin and myosin filaments and then connectin filaments, developing intimately and coordinately with myosin, become associated with the alpha-actinin lines. Thus it appears that the putative elastic protein connectin plays some role in integrating myosin filaments with the preexisting I-Z-I brushes. The occasional absence of connectin and A-bands between two Z-bands, beyond both of which clear sarcomeres have been formed, indicates that connectin is not a preformed scaffold of myofibrils on which sarcomeric proteins accumulate.

Disruption of microfilament organization in living nonmuscle cells by microinjection of plasma vitamin D-binding protein or DNase I

Plasma vitamin D-binding protein (DBP), which binds to monomeric actin, causes the breakdown of stress fibers when it is microinjected into nonmuscle cells. Disruption of the stress fiber network is also accompanied by shape changes in the cell that resemble those seen after cytochalasin treatment. When DBP was coinjected with fluorescently labeled alpha-actinin, no fluorescent stress fibers or attachment plaques were visible 30 min after injection. Twelve hours later the cells regained their flattened shape and their stress fibers. Fluorescently labeled DBP causes the same reversible changes in cell shape as the unlabeled protein. Upon injection, the labeled DBP diffuses throughout the cytoplasm, becoming localized by 12 hr in a punctate pattern, presumably due to lysozomal sequestration. Similar injections of DBP into skeletal myotubes and cardiac myocytes did not lead to shape changes or breakdown of nascent and/or fully formed myofibrils, even though DBP has a 2-fold higher binding affinity for muscle actin over that of the nonmuscle isoactins. Similar differential effects in nonmuscle cells were also observed after the microinjection of DNase I, another protein capable of binding monomer actin. The effects of these microinjected monomer actin-binding proteins imply that an accessible pool of monomer actin is needed to maintain stress fiber integrity in nonmuscle cells but not the integrity of the nascent or fully formed myofibrils in muscle cells.

Posttranslational incorporation of contractile proteins into myofibrils in a cell-free system

The incorporation of newly synthesized protein into myofibrils has been examined in a cell-free system. Myofibrils were added to a reticulocyte lysate after the in vitro translation of muscle-specific poly(A)+RNA. Only a small number of the many synthesized proteins were found to associate with the exogenously added myofibrils. These proteins were all identified as sarcomeric components and had subunit mobilities (Mr) of 200, 140, 95, 86, 43, 38, 35, 25, 23, 20, and 18 kD. The association was rapid (t1/2 less than 15 min) and, for most of the proteins, relatively temperature insensitive. Except for a 43-kD polypeptide, tentatively identified as beta-actin, none of the proteins encoded by brain poly(A)+RNA associated with the myofibrils. When filaments made from purified myosin or actin were used as the "capture" substrates, only thick or thin filament proteins, respectively, were incorporated. Incorporation was substantially reduced when cross-linked myosin filaments were used. These results are compatible with a model in which proteins of the sarcomere are in kinetic equilibrium with homologous proteins in a soluble pool.

Incorporation of fluorescently labeled actin and tropomyosin into muscle cells

The two major proteins in the I-bands of skeletal muscle, actin and tropomyosin, were each labeled with fluorescent dyes and microinjected into cultured cardiac myocytes and skeletal muscle myotubes. Actin was incorporated along the entire length of the I-band in both types of muscle cells. In the myotubes, the incorporation was uniform, whereas in cardiac myocytes twice as much actin was incorporated in the Z-bands as in any other area of the I-band. Labeled tropomyosin that had been prepared from skeletal or smooth muscle was incorporated in a doublet in the I-band with an absence of incorporation in the Z-band. Tropomyosin prepared from brain was incorporated in a similar pattern in the I-bands of cardiac myocytes but was not incorporated in myotubes. These results in living muscle cells contrast with the patterns obtained when labeled actin and tropomyosin are added to isolated myofibrils. Labeled tropomyosins do not bind to any region of the isolated myofibrils, and labeled actin binds to A-bands. Thus, only living skeletal and cardiac muscle cells incorporate exogenous actin and tropomyosin in patterns expected from their known myofibrillar localization. These experiments demonstrate that in contrast to the isolated myofibrils, myofibrils in living cells are dynamic structures that are able to exchange actin and tropomyosin molecules for corresponding labeled molecules. The known overlap of actin filaments in cardiac Z-bands but not in skeletal muscle Z-bands accounts for the different patterns of actin incorporation in these cells. The ability of cardiac myocytes and non-muscle cells but not skeletal myotubes to incorporate brain tropomyosin may reflect differences in the relative actin-binding affinities of non-muscle tropomyosin and the respective native tropomyosins. The implications of these results for myofibrillogenesis are presented.