Use of monoclonal antibodies in the analysis of myoblast development

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

H36-alpha 7 is a novel integrin alpha chain that is developmentally regulated during skeletal myogenesis

H36 is a 120,000-D membrane glycoprotein that is expressed during the differentiation of skeletal muscle. H36 cDNA clones were isolated from a lambda UniZapXR rat myotube cDNA library and sequenced. The deduced amino acid sequence demonstrates that H36 is a novel integrin alpha chain that shares extensive homology with other alpha integrins that includes: (a) the GFFKR sequence found in all alpha integrins; (b) a single membrane spanning region; (c) conservation of 18 of 22 cysteines; and (d) a protease cleavage site found in the non-I region integrin alpha chains. The cytoplasmic domain of H36 is unique and additional regions of nonhomology further indicate H36 is distinct from all other alpha chains. In keeping with current nomenclature we designate this alpha chain alpha 7. Northern blots demonstrate that expression of H36-alpha 7 mRNA is regulated both early in the development of the myogenic lineage and later, during terminal differentiation. Detection of H36-alpha 7 mRNA coincides with conversion of H36- myogenic precursor cells to H36+ cells. H36-alpha 7 mRNA is present in replicating myoblasts: expression increases upon terminal differentiation and is markedly reduced in developmentally defective myoblasts. In addition, H36-alpha 7 mRNA is not detected in C3H10T1/2 cells. It is in myotubes derived from myoblasts obtained by treatment of 10T1/2 cells with azacytidine or transfection with MRF4. Immunoblots and immunofluorescence demonstrate that the H36-alpha 7 chain is associated with integrin beta 1. Affinity chromatography demonstrates that H36-alpha 7 beta 1 selectively binds to laminin. The expression of H36-alpha 7 on secondary myoblasts during the development of the limb in vivo corresponds with the appearance of laminin in the limb, with the responsiveness of secondary myoblast proliferation to laminin, and with the onset of increased muscle mass, suggesting that H36-alpha 7 modulates this stage in limb development. We conclude that H36-alpha 7 is a novel alpha integrin laminin binding protein whose expression is developmentally regulated during skeletal myogenesis.

Thy-1-like immunoreactivity in developing chicken skeletal muscle: identification of a cross-reactive slow-fiber specific molecule that is not Thy-1

We investigated the Thy-1-like immunoreactivity during the development of chicken skeletal muscle using a group of monoclonal antibodies raised and characterised against purified chicken brain Thy-1. The immunoreactivity attributable to bona fide Thy-1 in muscle was present in nerves, connective tissue associated with the intrafusal capsule and blood vessels, and the extracellular matrix of the muscle fibres. During development there was no change in the staining of nerves, blood vessels or intrafusal capsules. However, the extracellular staining of muscle first appeared around hatching and gradually increased in intensity reaching maximal levels in the adult. The intensity of staining varied within and between the muscles examined. One of the antibodies (SB1 20.11) recognised an additional molecule that is not Thy-1 and that was localised in the cytoplasm of slow muscle fibres. This immunoreactivity was first detectable at E10 in all myotubes that contained both alkali and acid stable myosin ATPase activity (presumptive slow), but not in those myotubes with only alkali-stable myosin ATPase activity (presumptive fast). Thereafter, the staining increased to a maximum in the newly hatched animal and then decreased until reactivity was undetectable in the adult (greater than 25 weeks). All positive fibres initially stained with a uniform intensity but the time of commencement and the rate of loss of staining was variable. Those fibres that contained both acid stable and acid labile myosin ATPase activity lost the antigen much faster than the fibres containing only acid-stable myosin ATPase activity, which also tended to increase in intensity for a longer period. These may represent, respectively, the slow tonic type III fibres and the slow twitch type I fibres classified by Barnard et al. (1982).

Membrane-cytoskeleton associations during myogenesis deviate from traditional definitions

Plasma membrane-cytoskeleton associations involving four membrane proteins (A5, H58, H36, and I20) were studied in developing L8E63 rat skeletal muscle cells using immunofluorescence microscopy and photometry on the basis of three criteria: Triton-insolubility, colocalization with cytoskeletal components, and sensitivity to cytoskeleton-directed drugs. The results presented demonstrate that there are developmental stage-specific associations between membrane proteins and the cytoskeleton during skeletal myogenesis. Several inconsistencies were found with traditional expectations of membrane-cytoskeleton associations. For example, although A5 is Triton-insoluble and sensitive to cytochalasin, its distribution generally does not correspond with any known cytoskeletal structure. Furthermore, the topography of A5 is dependent on the integrity of the plasma membrane. H36 and I20 are completely soluble in Triton and therefore by accepted definitions would not be expected to be associated with any cytoskeletal component. Yet H36 and actin codisrupt in the presence of cytochalasin, while I20, whose distribution does not correspond with microtubules, is uniquely sensitive to their disruption. These results demonstrate that (i) neither Triton-solubility nor colocalization alone predicts all membrane-cytoskeleton associations; some associations between the membrane and cytoskeleton are unstable in nonionic detergent; (ii) the native distribution of proteins in the membrane may not reflect their cytoskeletal associations; and (iii) the topography of some membrane proteins with no apparent association with the cytoskeleton may be greatly influenced by the cell cytoskeleton.

Structural characterization of chick embryonic skeletal muscle chondroitin sulfate proteoglycan

Embryonic chick skeletal muscle has been shown to synthesize a distinct proteoglycan of large size with relatively large, highly 6-sulfated chondroitin sulfate glycosaminoglycans. Further analysis of these proteoglycans indicates that tryptic digestion gives rise to fragments with an average of two chondroitin sulfate chains per peptide. The skeletal muscle chondroitin sulfate proteoglycan also contains oligosaccharides whose characteristics suggest the presence of both O-linked and N-linked oligosaccharides. These characteristics include the average hydrodynamic size of the oligosaccharides as well as their localization. Approximately 10% of the putative O-linked oligosaccharides reside on the same tryptic fragments which contain the chondroitin sulfate chains, while the presumptive N-linked oligosaccharides appear to be present at sites distant from the chondroitin sulfate. Further support for this identification comes from radioisotopic labeling with [3H]mannose, which is incorporated exclusively into the putative N-linked oligosaccharides. Some of the O-linked oligosaccharides which are not in close apposition to the chondroitin sulfate seem to occur in clusters. The skeletal muscle chondroitin sulfate proteoglycan has the ability to interact in a link protein-stabilized fashion with hyaluronic acid. This ability as well as the estimated number of chondroitin sulfate chains per cluster and the estimated number of oligosaccharides per chondroitin sulfate chain have implications about the structure of the core protein of the skeletal muscle proteoglycan. The information presented is used to construct a model of these molecules; with this detailed model, attention can now be directed at other aspects of the skeletal muscle chondroitin sulfate proteoglycan, such as its role in myogenesis.

Combinations of monoclonal antibodies distinguish mesenchymal, myogenic, and chondrogenic precursors of the developing chick embryo

Monoclonal antibodies (MAbs) were used as probes for molecular differences in the surfaces of nonterminally differentiated cells of the developing chick limb. The specificity of the MAbs was determined by immunofluorescent localization performed on cultured breast muscle and limb bud cells and cryosections of a variety of embryonic (stages 15-37) and neonatal tissues. Subpopulations of MAb-positive and -negative cells were isolated by fluorescence-activated cell sorting and their developmental potential was assessed in vitro. Cells of the compacted somite, lateral plate mesoderm, and early limb bud were labeled with the CSAT MAb. Myogenic precursors of the dermatome and limb bud were labeled with the CSAT and L4 MAbs. Chondrogenic precursors of the sclerotome and limb bud were labeled with the CSAT, L4, and C5 MAbs. These precursors were distinguished from fibroblasts which were labeled with the CSAT and C1 MAbs. The differentiation and maturation of muscle and cartilage were accompanied by alterations in the labeling patterns of the MAbs. These results indicate that combinations of these MAbs can be used to distinguish mesenchymal, myogenic, and chondrogenic precursors, identify their site of origin during development, and isolate subpopulations of embryonic cells.

Identification of sarcolemma-associated antigens with differential distributions on fast and slow skeletal muscle fibers

We have identified three sarcolemma-associated antigens, including two antigens that are differentially distributed on skeletal muscle fibers of the fast, fast/slow, and slow types. Monoclonal antibodies were prepared using partially purified membranes of adult chicken skeletal muscles as immunogens and were used to characterize three antigens associated with the sarcolemma of muscle fibers. Immunofluorescence staining of cryosections of adult and embryonic chicken muscles showed that two of the three antigens differed in expression by fibers depending on developmental age and whether the fibers were of the fast, fast/slow, or slow type. Fiber type was assigned by determining the content of fast and slow myosin heavy chain. MSA-55 was expressed equally by fibers of all types. In contrast, MSA-slow and MSA-140 differed in their expression by muscle fibers depending on fiber type. MSA-slow was detected exclusively at the periphery of fast/slow and slow fibers, but was not detected on fast fibers. MSA-140 was detected on all fibers but fast/slow and slow fibers stained more intensely suggesting that these fiber types contain more MSA-140 than fast fibers. These sarcolemma-associated antigens were developmentally regulated in ovo and in vitro. MSA-55 and MSA-140 were detected on all primary muscle fibers by day 8 in ovo of embryonic development, whereas MSA-slow was first detected on muscle fibers just before hatching. Those antigens expressed by fast fibers (MSA-55 and MSA-140) were expressed only after myoblasts differentiated into myotubes, but were not expressed by fibroblasts in cell culture. Each antigen was also detected in one or more nonskeletal muscle cell types: MSA-55 and MSA-slow in cardiac myocytes and smooth muscle of gizzard (but not vascular structures) and MSA-140 in cardiac myocytes and smooth muscle of vascular structures. MSA-55 was identified as an Mr 55,000, nonglycosylated, detergent-soluble protein, and MSA-140 was an Mr 140,000, cell surface protein. The Mr of MSA-slow could not be determined by immunoblotting or immunoprecipitation techniques. These findings indicate that muscle fibers of different physiological function differ in the components associated with the sarcolemma. While the function of these sarcolemma-associated antigens is unknown, their regulated appearance during development in ovo and as myoblasts differentiate in culture suggests that they may be important in the formation, maturation, and function of fast, fast/slow, and slow muscle fibers.

Calpain-calpastatin and fusion. Fusibility of erythrocytes is determined by a protease-protease inhibitor [calpain-calpastatin] balance

Rat erythrocytes fuse when treated with the membrane mobility agent, 2-(2-methoxyethoxy)ethyl-cis-8-(2-octylcyclopropyl)octanoate (A2C) and Ca2+, whereas human cells do not. Membrane proteolysis promoted by calpain is required for rat cell fusion [(1986) Eur. J. Biochem., in press]. Human calpain induced a selective proteolysis in both the human and rat erythrocyte ghosts (mainly band 4.1 in the human, band 4.1 and band 3 in the rat cell) and rendered them fusible. Calpastatin (calpain inhibitor) prevented A2C-induced fusion in both ghosts, via inhibition of proteolysis. The human erythrocyte has excess calpastatin and resists A2C-promoted fusion. A regulatory role of calpastatin in membrane fusion is thus indicated.

Remodeling of the myoblast membrane accompanies development

We have prepared a library of cloned hybridomas that produce monoclonal antibodies reactive with the surface of E63 rat myoblasts. Using immunofluorescence analysis of antigens on single cells we have studied the expression of determinants at distinct stages of development. Conditions were established for quantitative photometry and were used to confirm the diversity in stage-specific expression that accompanies development. The remodeling of the myoblast membrane also involves stage-specific and transient changes in topography and aggregation of many antigens, and the period surrounding fusion is one of particular activity. The localization of antigens on the upper and attached surfaces of myogenic cells was often distinct, and quantitative and spatial stage-specific reorganizations of antigens differed with respect to these two surfaces. This polarity represents an additional level of complexity in the continuous remodeling of the muscle cell membrane. Comparisons of quantitative and topographic analyses of antigens on E63 cells with Rat-1 fibroblasts and developmentally defective (fu-) myoblasts indicate that the outer membranes of these nonmyogenic cells are distinct from differentiating myoblasts. One determinant, H36, is absent on Rat-1 cells and on all fu- lines tested, and undergoes interesting stage-specific changes in expression and topography.

Expression of a developmentally regulated antigen on the surface of skeletal and cardiac muscle cells

H36 is a species-specific, cell-surface antigen on differentiating newborn rat skeletal myoblasts and myogenic lines. This membrane antigen has been defined by a monoclonal antibody raised by the fusion of SP 2/0-Ag14 myeloma cells with spleen cells from mice immunized with myotubes derived from the myogenic E63 line. H36 antigen, isolated by immunoaffinity chromatography, is comprised of two polypeptides with apparent molecular weights of 98,000 and 117,000. Fluorescence photometry and radioimmunoassays have been used to follow quantitative and topographic changes in the H36 determinant during myogenesis. H36 is present at a basal level on replicating myoblasts; it increases on prefusion myoblasts and persists on myotubes. At or near the time of prefusion, it becomes concentrated between adjacent aligned myoblasts and localized on membrane "blebs". H36 is present on both skeletal and cardiac cells but absent from a variety of cells that include fibroblasts, neuronal cells, and smooth muscle. There are approximately 4 x 10(5) determinants per myoblast, and the Ka of the antibody is 3.8 x 10(8) liters/mol. The distributions of H36 on the top and attached surfaces of myoblasts and myotubes are distinct, which suggests localized specialization of these surfaces. H36 is an integral membrane component and upon cross-linking, it associates with the detergent-insoluble cytoskeletal framework. Inhibition of myogenesis by 5-bromodeoxyuridine or by calcium deprivation prevents the developmentally associated changes in the expression of H36. H36 is also absent or markedly reduced on the fu- and Ama102 developmentally defective mutant myoblast lines. We conclude that H36 is a muscle-specific, developmentally regulated cell-surface antigen that may have a role in myoblast differentiation and that can be used to determine the embryonic lineages of skeletal and cardiac muscle.

New immunological approaches to studying the odontoblast

The use of specific polyclonal and monoclonal antibodies as probes to study odontoblast morphology, function, and differentiation has received relatively little attention. The extent of the odontoblast processes in human and rat teeth is one question that we have approached recently by utilizing antibodies specific for intracellular elements, i.e., the cytoskeleton. Indirect immunofluorescence on both paraffin-embedded thin sections and surface-demineralized collagenase-digested whole mounts has indicated that the odontoblast process does extend to the dentino-enamel junction. By using other antibodies which recognize intra- and extracellular components, or antibodies which recognize unique antigens expressed at the cell surface of the odontoblast or its precursor cells, many more precise molecular details of odontoblast form and function would become accessible for analysis.

Expression of myoblast and myocyte antigens in relation to differentiation and the cell cycle

Cell cycle parameters and expression of myoblast and myocyte antigens were investigated during exponential growth and during the differentiation phase of rat L8( E63 ) myoblasts by an integrated approach involving microspectrophotometry with DNA fluorochromes, [3H]thymidine autoradiography, and immunofluorescent staining with monoclonal antibodies. In addition to the majority of cells which are recruited into myotubes, two distinct populations of mononucleate cells were resolved in cultures of rat myoblasts undergoing differentiation. These mononucleate cells consist of (1) a population of proliferating cells with a prolonged G1 transit time; (2) a population of non-proliferating cells which remain arrested in G1 for more than 72 h. The latter group was examined with respect to the expression of two marker antigens recognized by two monoclonal antibodies: antibody B58 reacts with a macromolecular component present in undifferentiated myoblasts but not in mature myotubes, and antibody XMlb reacts with a muscle-specific isoform of myosin. All four possible combinations of expression of these antigens by single cells were found: B58 +XM1b -, B58 +XM1b +, B58 - XM1b -, and B58 - XMlb +. The implication of these findings with respect to the transition from the proliferative to the differentiative phase of myogenesis is discussed.

A chick neural antigen identified by monoclonal antibodies

A monoclonal antibody technique has been used to locate a neural antigen which appears to be involved in developmental processes. Hybridomas were prepared using chick embryo sympathetic neurons as an immunogen and one clone, H3, was found to secrete antibodies which bound to neurons of the peripheral and central nervous system. The antibodies bound to both membrane and cytoplasmic sites of neurons but only to cytoplasmic sites of glial cells. When added to newly prepared cultures of embryonic sympathetic neurons the H3 antibodies impaired both neurite outgrowth and long-term neuronal viability. No such effect was seen when the antibodies were added to established, differentiated neurons.

Changes in cell surface antigens during in vitro lizard myogenesis

Indirect immunofluorescence has been used to examine surface antigens of lizard myogenic cells during in vitro differentiation. At least two developmental stage-specific surface alterations have been identified. One of these is a compositional change and involves the appearance of a cell-surface antigen(s) as the cells differentiate. This antigen(s) (Ag1422) is muscle specific and is characteristic of some rounded-up G0 myosin-positive myocytes, all stretched-back, G0 myosin-positive myocytes, and all identifiable myotubes. The antigen is not found on proliferating myoblasts, extended G1 (myosin-negative) cell-cycle-competent myoblasts or newly differentiated rounded-up, G0 myosin-positive myocytes. Pretreatment of cells with neuraminidase, trypsin, or proteinase K indicates the antigen is not present in "masked" form on normally nonreactive cells. Proteinase K is effective in the removal or destruction of the antigen, indicating it is at least partially protein in nature. The antigen is expressed in a similar developmental stage-specific fashion on early-passage myogenic cells taken from both adult lizard tail regenerates and embryonic muscle. The antibodies identifying Ag1422 can be removed by adsorption with homogenates of mature skeletal muscle. Therefore, Ag1422 is not an artifact due to in vitro conditions or the expression of a transformation antigen unique to the continuous culture line. The second alteration is an apparent restriction in the mobility of surface components (antigens and lectin receptors). Upon treatment with multivalent ligands, undifferentiated myosin-negative myoblasts exhibit rapid patching and capping of cell surface components while well-differentiated myocytes and myotubes do not. This mobility restriction is evident after the appearance of Ag1422. Treatment with cytochalasin B (15 micrograms/ml) and/or colchicine (100 microM) does not alter the restricted mobility of surface components seen on differentiated cells. Therefore, neither microfilaments nor microtubules seem to be involved in the mobility restriction. These observations are discussed in relation to current views of myogenesis.

New approaches to the study of human dystrophic muscle cells in culture

Tissue culture provides a system for studying the growth and differentiation of muscle cells in a controlled environment. Several studies have been carried out on diseased muscle cells in culture in attempts to elucidate the aetiology of Duchenne muscular dystrophy (DMD) but the results were equivocal. Work in our laboratory in recent years has yielded an improved method for preparing primary muscle cell cultures from dissociated biopsies which permits the morphological and biochemical evaluation of these cultures at all stages of growth and development. Our results have shown abnormalities in cell behaviour, ultrastructure and creatine kinase synthesis. The background to these studies is reviewed. Recently we have developed a cell cloning procedure that allows the accumulation of a large number of cells from a single selected cell. We can with this technique monitor quantitative and qualitative cellular and cytochemical differences between individual cell types without the ambiguities inherent in the use of mixed cell populations. The results obtained with 4 different clonal preparations derived from dystrophic muscle have shown that a number of specific features were expressed by each of the 4 clones with respect to their growth pattern, ultrastructure, synthesis of muscle specific protein and cell surface antigen. These findings clearly illustrate the potential of these cloning procedures for studying the genetic expression of homogeneous cell populations derived from normal adult human muscle and patients with X-linked muscle disease.

Analysis of myogenesis by somatic cell hybridization. II. Retention of myogenic competence and suppression of transformed properties in hybrids between differentiation competent and incompetent rat L6 myoblasts

This study describes the characteristics of hybrids between two closely related rat myoblast lines, which differ both in the ability to express their program of differentiation and in the expression of neoplastic properties. Myogenic, nonneoplastic L6J1-S cells were hybridized with nonmyogenic, neoplastic L6J1-N1 cells. Six hybrid clones were isolated and expanded for analysis of myogenic competence, and four of these clones were also evaluated for parameters of transformation, including tumorigenicity, ability to clone in agar, and surface fibronectin. In addition to our analysis of isolated clones, we also assessed myogenic differentiation in colonies representing 226 early hybrid clones. Results of all these analyses demonstrate that the myogenic phenotype is retained and that the tumorigenic/transformed phenotype is suppressed in the hybrids. Furthermore, our results indicate that when the programs for myogenesis and neoplastic transformation are confronted within a single cell, they are expressed as mutually exclusive alternatives. In contrast to these results on myogenic X nonmyogenic L6 hybrids, it has been reported that isolated clones of A9 X L6 exhibited extinction of myogenic competence and retention of transformed properties. We have evaluated myotube formation in over 300 early hybrid clones between A9 and either diploid or subtetraploid L8 rat myoblasts. Our results demonstrate that all of these hybrid clones exhibit extinction regardless of the ploidy of the myoblast parent, and they further indicate that extinction is not a consequence of chromosome loss. These results support the conclusion that in A9 X L6 hybrids, the nonmyogenic, transformed phenotype is dominant.

Hybridomas: a new dimension in biological analyses

Since the first report of hybridomas producing monoclonal antibodies by Kohler and Milstein in 1975, this technique has spread to nearly all areas of biological, biochemical, and biomedical research. Watching the use of these methods spread from immunologists to cell biologists, developmental biologists, biochemists and to other biological disciplines and observing the nearly logarithmic increase in publications using these reagents has been in itself fascinating and informative. An overview of the development of this technology and its applications is presented including the use of monoclonal antibodies to study cell surface molecules, differentiation antigens, receptors, and histocompatibility antigens. The use of these antibodies to analyze microorganisms and parasitic antigens as well as their use in the genetic analysis of human cell surface antigens and the detection of polymorphic variation in enzymes and other proteins is discussed. Examples of the application of monoclonal reagents to the study of tumor cell biology including the labeling of metastatic tumor cells and the detection of cell surface molecules implicated in the regulation of growth control and cell division are provided.

Quantitation of changes in cell surface determinants during skeletal muscle cell differentiation using monospecific antibody

The differentiation of skeletal muscle is characterized by recognition, alignment, and subsequent fusion of myoblast cells at their surfaces to form large, multinucleated myotubes. Monoclonal antibodies were used to investigate antigenic changes in the cell surface membrane specific for various stages of myogenesis. Chick embryonic skeletal muscle cells were cultured in vitro to the desired stage of differentiation and then injected into BALB/c mice. Spleen cells from the immunized mice were hybridized with NS-1 or P3 8653 mouse myeloma cells. Hybrid cell clones were selected in HAT medium and screened using an indirect radioimmunoassay for the production of monoclonal antibodies specific to myogenic cell surfaces. Target cells for the radioimmunoassay included three stages of myogenesis (myoblasts, midfusion, myoblasts, and myotubes) and chick lung cells as a control for polymorphic antigens. Sixty-one clones were obtained which produced antibodies specific for myogenic cells. Thirty-five of these clones were generated from mice immunized with midfusion myoblast stages of myogenesis and 26 were obtained from mice immunized with the later myotube stage of myogenesis. Quantitative measurements by RIA of myogenic determinants per cell surface area on each target cell type revealed that most of the determinants decrease during myogenesis when midfusion myoblasts are used as the immunogen. When myotube stages are used as the immunogen, more determinants increase with cell differentiation. Therefore, the most common pattern of determinant change is for them to be present at all stages of myogenesis but to vary quantitatively through development. There are determinants unique to each stage of myogenesis and marked quantitative differences within a cell stage for each determinant.