Expression of myosin heavy chain isoforms in stimulated fast and slow rat muscles

The expression of 4 myosin heavy chain (MHC) isoforms was analyzed in the rat soleus (SOL) and extensor digitorum longus (EDL) muscles after denervation and chronic electric stimulation. The stimulation frequencies used were 20 and 150 Hz and the amount of stimulation was either large (20 Hz), intermediate (150 Hz), or small (150 Hz). These patterns resemble some features of normal motor unit activity in SOL and EDL of freely moving rats (Hennig and Lømo, 1985). The relative expression of each MHC isoform depended strongly on the stimulation pattern. Furthermore, for any particular stimulation pattern, fibers in SOL and EDL expressed different MHCs. Coexistence of different MHC types in the same fiber was frequently observed in stimulated muscles. 20-Hz stimulation preserved normal expression of type 1-MHC in SOL but failed to induce type 1-MHC in type 2 fibers of the EDL, where type 2A- and 2X-MHC expression dominated and type 2B-MHC expression was completely suppressed. 150-Hz low-amount stimulation preserved nearly normal 2B-MHC expression in many type 2 fibers of the EDL but failed to induce type 2B-MHC expression in the SOL, where 2X-MHC became predominant. 150-Hz high-amount stimulation differed from 150-Hz small amount stimulation by suppressing almost all type 2B-MHC expression in EDL and by inducing considerable type 2A-MHC expression in the SOL. Scattered fibers in EDL that were probably the original type 1 fibers responded differently from both type 2 fibers in the EDL and from type 1 fibers in the SOL to stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Troponin I switching in the developing heart

Monoclonal antibodies identify two distinct isoforms of troponin I in rat cardiac muscle, one predominant in the embryonic and fetal heart and one predominant in the adult heart. The two isoforms can be resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with apparent molecular weights of 27,000 and 31,500, respectively. The adult isoform is specifically recognized by a monoclonal antibody that is unreactive with the embryonic variant, while two other monoclonal antibodies recognize both isoforms. A monoclonal antibody to cardiac troponin T was used to isolate by affinity chromatography the troponin complex from adult and neonatal rat heart. Affinity purified troponin from neonatal heart was found to contain both the embryonic and adult isoforms of troponin I. Comparative immunoblotting analysis with different muscle tissues shows that embryonic troponin I is identical with respect to electrophoretic mobility and pattern of immunoreactivity to the major troponin I isoform found in adult slow skeletal muscle. Troponin I switching may be implicated in developmental changes involving Ca2+ and pH sensitivity of the contractile system and response to beta-adrenergic stimulation.

Three myosin heavy chain isoforms in type 2 skeletal muscle fibres

Mammalian skeletal muscles consist of three main fibre types, type 1,2A and 2B fibres, with different myosin heavy chain (MHC) composition. We have now identified another fibre type, called type 2X fibre, characterized by a specific MHC isoform. Type 2X fibres, which are widely distributed in rat skeletal muscles, can be distinguished from 2A and 2B fibres by histochemical ATPase activity and by their unique staining pattern with seven anti-MHC monoclonal antibodies. The existence of the 2X-MHC isoform was confirmed by immunoblotting analysis using muscles containing 2X fibres as a major component, such as the normal and hyperthyroid diaphragm, and the soleus muscle after high frequency chronic stimulation. 2X-MHC contains one determinant common to 2B-MHC and another common to all type 2-MHCs, but lacks epitopes specific for 2A- and 2B-MHCs, as well as an epitope present on all other MHCs. By SDS-polyacrylamide gel electrophoresis 2X-MHC shows a lower mobility compared to 2B-MHC and appears to comigrate with 2A-MHC. Muscles containing predominantly 2X-MHC display a velocity of shortening intermediate between that of slow muscles and that of fast muscles composed predominantly of 2B fibres.

Nonsynchronous accumulation of alpha-skeletal actin and beta-myosin heavy chain mRNAs during early stages of pressure-overload--induced cardiac hypertrophy demonstrated by in situ hybridization

The development of cardiac hypertrophy secondary to pressure overload is accompanied by isoformic changes of contractile proteins such as myosin and actin. 35S-Labeled complementary RNA (cRNA) probes and in situ hybridization procedures were used for analysis of the regional distribution of newly formed transcripts from alpha-skeletal actin (alpha-sk-actin) and beta-myosin heavy chain (beta-MHC) genes during the early stages of pressure overload. The study was performed in 25-day-old rats submitted to a thoracic aortic stenosis and killed after surgery at times ranging from 4 hours to 3 days. Neither alpha-sk-actin nor beta-MHC messenger RNA (mRNA) was detected in the hearts of normal and sham-operated animals. However, alpha-sk-actin mRNA accumulated throughout the entire left ventricle as early as 4 hours after aortic stenosis, and by 12 hours was also detected in the left atrium. In contrast, beta-MHC mRNA was hardly detectable before day 1, and by days 2-3 was mainly restricted to the inner part of the left ventricle and around the coronary arteries. The absence of spatial and temporal coordination in the accumulation of alpha-sk-actin and beta-MHC mRNAs indicates that different signals and/or regulatory mechanisms are implicated in the induction of the two genes in response to hemodynamic overload.

Myosin heavy-chain isoforms in human smooth muscle

The myosin heavy-chain composition of human smooth muscle has been investigated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, enzyme immunoassay, and enzyme-immunoblotting procedures. A polyclonal and a monoclonal antibody specific for smooth muscle myosin heavy chains were used in this study. The two antibodies were unreactive with sarcomeric myosin heavy chains and with platelet myosin heavy chain on enzyme immunoassay and immunoblots, and stained smooth muscle cells but not non-muscle cells in cryosections and cultures processed for indirect immunofluorescence. Two myosin heavy-chain isoforms, designated MHC-1 and MHC-2 (205 kDa and 200 kDa, respectively) were reactive with both antibodies on immunoblots of pyrophosphate extracts from different smooth muscles (arteries, veins, intestinal wall, myometrium) electrophoresed in 4% polyacrylamide gels. In the pulmonary artery, a third myosin heavy-chain isoform (MHC-3, 190 kDa) electrophoretically and antigenically distinguishable from human platelet myosin heavy chain, was specifically recognized by the monoclonal antibody. Analysis of muscle samples, directly solubilized in a sodium dodecyl sulfate solution, and degradation experiments performed on pyrophosphate extracts ruled out the possibility that MHC-3 is a proteolytic artefact. Polypeptides of identical electrophoretic mobility were also present in the other smooth muscle preparations, but were unreactive with this antibody. The presence of three myosin heavy-chain isoforms in the pulmonary artery may be related to the unique physiological properties displayed by the smooth muscle of this artery.

A combined histochemical and immunohistochemical study on the dynamics of fast-to-slow fiber transformation in chronically stimulated rabbit muscle

Chronically stimulated fast-twitch muscles of the rabbit were histochemically and immunohistochemically analyzed in serial cross sections (1) for percentages of fiber types, and (2) for the presence of myosin heavy chain isoforms during fast-to-slow transformation. By four weeks of stimulation the number of type-I fibers had increased more than fourfold, while only about 6% of the original IIB fibers remained. Type-IC and -IIC fibers transiently rose to 20% of the total fiber population. After 16 weeks, the number of type-I fibers had increased to 42%. With prolonged stimulation fewer fibers reacted with antibodies against embryonic and neonatal myosins and more with the antibody against slow myosin. The reaction for embryonic myosin was most often detected in the C fibers (IC, IIC). Immunohistochemical subtypes were observed for each fiber type in the stimulated muscles. The greatest number was seen in type-IIC fibers, which, in addition to their reaction for fast/neonatal and slow myosins, might also react with the antibodies against neonatal/embryonic and embryonic myosins. These findings indicated that the transforming fibers temporarily expressed myosin heavy chain isoforms normally not detectable in adult skeletal muscle. Myotubes reacted strongly with the antibodies against fast/neonatal and embryonic myosins, and some of them also with the antibody against slow myosin. Thus, it appears that under the influence of the low frequency stimulus pattern some of the newly formed myotubes developed into type-I fibers.

Heart conduction system: a neural crest derivative?

Using the anti-neurofilament monoclonal antibody iC8 we report here that muscle fibers of the conduction system of the adult and developing rabbit heart express a cytoskeletal protein antigenically and electrophoretically similar to the middle subunit of neurofilaments (NF-M). In the 11-day embryo a number of cardiac muscle cells also express a neural crest surface marker recognized by the monoclonal antibody HNK-1. Both markers are found in many cells of the 3rd and 4th branchial arches, which are populated by cells of neural crest origin. In the 11-day embryo cells of the 4th branchial arch are in close proximity to and intermingled with the atrial myocardium: cells co-expressing sarcomeric myosin heavy chain with iC8 and HNK-1 immunoreactivity are seen at these sites. The findings suggest that conduction tissue cells of the rabbit heart originate from a population of neural crest-derived cells migrating from the branchial arches into the developing heart.

Slow-to-fast transformation of denervated soleus muscles by chronic high-frequency stimulation in the rat

1. Adult soleus muscles were denervated and stimulated directly for 2-130 days with 'fast' (short pulse trains at 100 Hz) or 'slow' (continuously at 10 Hz, or long pulse trains at 15 Hz) stimulus patterns. 2. At the end of the period of stimulation isometric twitches and tetani and isotonic shortening velocities were measured. Frozen cross-sections were later examined with antibodies against myosin heavy chains specific for adult fast, adult slow and fetal myosin. 3. Isometric twitch duration (twitch time-to-peak and half-relaxation time) decreased during intermittent 100 Hz stimulation to values that were almost as fast as in the normal extensor digitorum longus (EDL) (95 and 94% transformation). The major part of the decrease occurred between 2 and 21 days after the onset of stimulation, and was accompanied by post-tetanic potentiation of the twitch, 'sag' in tension during an unfused tetanus, lower twitch/tetanus ratio and marked shifts to the right (higher frequencies) of the tension-frequency curve of the muscle. In contrast, during 10 or 15 Hz stimulation the isometric twitch duration remained slow, the twitch continued to show post-tetanic depression and absence of 'sag', while the twitch/tetanus ratio increased. 4. Denervation per se led to a slight increase and, then, after about a month, to a moderate and gradual decrease in twitch duration. The twitch/tetanus ratio increased markedly and post-tetanic depression became less pronounced or disappeared. Muscle weight and particularly tetanic tension were markedly reduced and these reductions were to a large extent counteracted by electrical stimulation. 5. Implantation of sham electrodes had no effect on twitch duration of denervated or innervated control muscles, but reduced tetanic tension in the innervated control muscles. 6. Maximum isotonic shortening velocity of the whole muscle (mm/s) increased during intermittent 100 Hz stimulation to a value as fast as in the normal EDL (110% transformation). Since the muscle fibres also increased in length (35%) maximum intrinsic shortening velocity (fibre lengths/s) was only incompletely transformed (55%). The increase in Vmax occurred between 7 and 14 days after the onset of stimulation. 7. All the fibres stimulated intermittently at 100 Hz were strongly labelled with anti-fast myosin and more than 90% were in addition weakly labelled by anti-slow myosin. Weak and variable labelling with anti-fast myosin was first detected 7 days after the onset of stimulation. In contrast, essentially all the fibres stimulated at 10 or 15 Hz showed no binding of anti-fast but strong binding of anti-slow myosin.(ABSTRACT TRUNCATED AT 400 WORDS)

Nodal myosin distribution in the bovine heart during prenatal development: an immunohistochemical study

A novel type of cardiac myosin heavy chain, immunologically related to the myosin isoforms expressed during skeletal muscle development, has recently been described in sinoatrial and atrioventricular nodal fibers of the adult bovine heart (Gorza et al, J Cell Biol 1986; 102:1758-1766). The tissue-specific expression of this myosin type has been utilized in the present study to investigate the differentiation of nodal fibers during cardiac development. In 4-6-week-old bovine embryos, reactivity for nodal myosin was observed in a cluster of cardiac fibers in the sinus venosus wall, corresponding to the sinoatrial node primordium and in a number of fibers localized in the left atrial wall, especially in proximity to vascular orifices, possibly corresponding to the postulated left-sided sinoatrial node. In contrast, reactivity for nodal myosin was not detected in the atrioventricular node until 12 weeks of gestation. Before this stage, fibers reactive for nodal myosin were also seen scattered in the left atrial wall and interatrial septum, raising the possibility that atrioventricular nodal fibers may derive from the left-sided sinoatrial node. Reactivity for nodal myosin was never seen in normal atrial and ventricular myocardium, nor in the ventricular conduction tissue, indicating that nodal myosin does not represent a primordial myosin form, but is rather a specific marker of a distinct muscle cell lineage.

Embryonic and neonatal myosin heavy chain in denervated and paralyzed rat skeletal muscle

Using immunofluorescence procedures with specific polyclonal and monoclonal antimyosin antibodies we have found that embryonic and neonatal myosin heavy chains (MHCs), which in rat skeletal muscle disappear during the first weeks after birth, are reexpressed in adult muscle after denervation. Reactivity for embryonic and neonatal MHCs was detected in some fibers as early as 3 days after denervation, became more evident by 7 days, and occurred exclusively in the type 2A fiber population. Paralysis of innervated muscles by tetrodotoxin block of the sciatic nerve also resulted in the reappearance of embryonic and neonatal MHCs in type 2A fibers. Significant variation in the degree of immunoreactivity was observed in different segments of the same muscle fiber, suggesting that coordination of muscle fiber nuclei in the control of myosin heavy chain gene expression is partially lost following denervation.

Coronary flow as a determinant of c-myc and c-fos proto-oncogene expression in an isolated adult rat heart

Chronic cardiac overload induces quantitative and qualitative changes of the phenotype which finally adapt the myocardium to its new functional requirements (Swynghedauw, 1986). It has been proposed that both stretch and enhanced isometric tension could trigger these modifications (Peterson and Lesch, 1972), but until now no real messenger has been found. In a search for signals which may account for these changes, we decided to investigate the expression of two proto-oncogenes, c-fos and c-myc, coding for nuclear proteins (review in Adamson, 1987) because several of their properties are consistent with their possessing a role in the transduction of extracellular growth signals to the cell interior. We report here that, in adult rat heart, expression of the c-fos and c-myc proto-oncogene was both sequentially and transitorily increased when, in a beating heart but not in an arrested heart, the coronary flow (and/or pressure) was augmented. This was studied in an isolated, coronary perfused heart preparation, a model in which the initial conditions of a cardiac overload may be mimicked in such a way that protein synthesis is stimulated by increasing the coronary perfusion pressure (Kira et al., 1984).

Maximum velocity of shortening related to myosin isoform composition in frog skeletal muscle fibres

1. The velocity of unloaded shortening (V0), the myofibrillar ATPase activity and the immunoreactivity to two monoclonal antibodies (A1 and A2) that were raised against the myosin heavy chains were studied in single fibres of the anterior tibialis muscle of Rana temporaria. V0 was recorded for the fibre as a whole using the slack-test method. Myofibrillar ATPase activity was determined by means of a quantitative histochemical technique. 2. A highly significant, direct relationship was found to exist between V0 and the myofibrillar ATPase activity recorded in the same single fibres. Both V0 and the myofibrillar ATPase activity changed in proportion to the cross-sectional area of the fibres. 3. Muscle fibres that had first been characterized with respect to V0 and myofibrillar ATPase activity were exposed to monoclonal antibodies A1 and A2. Thin fibres, having relatively low V0 and low myofibrillar ATPase activity, reacted preferentially with A1. Thick fibres, on the other hand, exhibiting relatively high V0 and high myofibrillar ATPase activity, were preferentially stained by A2. A third category of fibres reacted with both A1 and A2. The results support the view that the variability in shortening velocity and myofibrillar ATPase activity that exists among twitch fibres in frog skeletal muscle is based on differences in myosin heavy-chain composition. 4. Attempts were made to elucidate further the previous observation (Edman, Reggiani & te Kronnie, 1985) that the velocity of unloaded shortening (V0) differs along the length of individual muscle fibres. To this end discrete segments (0.5-0.7 mm in length) of intact fibres were delineated by opaque markers of hair that were placed on the fibre surface. The change in length between two adjacent markers (one segment) was recorded photo-electrically while the fibre was released to shorten against a very small load between 2.2 and 2.0 micron sarcomere lengths. In the majority of fibres (eight out of eleven preparations), V0 and myofibrillar ATPase activity exhibited similar patterns of variation along the fibre. Pooled data from thirty-three segments of twelve fibres showed a positive correlation between V0 and myofibrillar ATPase activity (P less than 0.05). 5. The possibility was explored that the myosin isoform composition might vary along the length of an individual muscle fibre. For this purpose bundles of fibres were cross-sectioned at 0.5-1 mm intervals along their entire length and the reactivity to monoclonal antibody A2 was tested at each location.(ABSTRACT TRUNCATED AT 400 WORDS)

A developmentally regulated isoform of 150,000 molecular weight neurofilament protein specifically expressed in autonomic and small sensory neurons

Neurofilament heterogeneity has been demonstrated using a monoclonal antibody (CH1) specific for the 150,000 molecular weight neurofilament subunit. In the peripheral nervous system of adult rats CH1 stained selectively sympathetic and parasympathetic neurons and a subpopulation of small neurons in the sensory dorsal root ganglia. Somatic motor neurons and large neurons in dorsal root ganglia were completely unreactive. In contrast, the anti-neurofilament antibody iC8, directed against the 150,000 molecular weight subunit, labelled all peripheral nervous system neurons. The immunostaining pattern with both antibodies was unchanged by phosphatase treatment. These data indicate that two antigenically distinct variants of the 150,000 molecular weight neurofilament subunit exist in somatic and autonomic neurons of adult animals. In addition, the phosphatase treatment suggests that the antigen recognized by CH1 is not masked by phosphorylation. In contrast, all neurons were labelled by this antibody in the peripheral nervous system of newborn rats. It is suggested that CH1 identifies a fetal 150,000 molecular weight neurofilament polypeptide isoform whose expression is prevented by the growth of somatic neurons and is selectively maintained in autonomic and small sensory neurons.

Myosin isoform expression in rat rhabdomyosarcoma induced by Moloney murine sarcoma virus

Myosin isoform expression was analyzed in experimental rhabdomyosarcoma (RMS) using monoclonal antibodies (mAbs) and immunofluorescence techniques. Tumors induced by inoculating newborn rats with Moloney murine sarcoma virus (Mo-MSV) were examined 30-90 days after birth. Nine tumors and two lymph node metastases were studied by direct, indirect, and double immunofluorescence assays using a panel of five anti-myosin mAbs. The mAb BF-45 was specifically reactive with embryonic myosin heavy chain (MHC), mAb BF-34 was specific for a neonatal MHC epitope, mAb BF-B6 was directed against an epitope present in both embryonic and neonatal MHC, and mAbs BF-F3 and BF-32 detected epitopes present in adult MHC isoforms. Anti-desmin antibodies were also used for comparison. The results of this study show that: (1) the majority of neoplastic cells stained for desmin while only a minority of neoplastic cells were labeled by anti-myosin antibodies; (2) myosin positive tumor cells contained predominantly embryonic and neonatal MHC types but rare RMS cells reacted exclusively with anti-adult myosin antibodies; and (3) adult and embryonic MHC phenotypes were occasionally detected within the same tumor cell especially in RMS with the longest latencies. Together these results would suggest that the mechanism(s) regulating MHC gene expression in skeletal muscle cells can be altered by the transforming activity of Mo-MSV.

Fibre types in extraocular muscles: a new myosin isoform in the fast fibres

We report on the existence of a myosin heavy chain (MHC) isoform with unique structural properties in extraocular (EO) muscles. Differences in MHC composition are apparent using a polyclonal antibody prepared against myosin isolated from bovine EO muscle myosin. In enzyme immunoassays and western blotting experiments, this anti-EO myosin antibody reacted specifically with the heavy chains of EO muscle myosin and not with the heavy chains of other myosins. The distribution of this new MHC isoform in the globe rotating muscles from different mammalian species was analysed using a panel of specific anti-myosin antibodies and comparing the histochemical myosin ATPase profile of muscle fibres with their isomyosin content. Most fibres which display a type 2 ATPase reaction pattern were selectively labelled by anti-EO antibodies. A few type 2 fibres were found to react with both anti-EO and anti-2A myosin antibodies and others, located almost exclusively in the orbital layers, reacted with anti-foetals as well as anti-EO antibodies. The presence of a distinct form of myosin in EO muscle fibres is probably related to the particular functional characteristics of these muscles, which are known to be exceptionally fast-contracting but display a very low tension output.

RMZ: a new cell line from a human alveolar rhabdomyosarcoma. In vitro expression of embryonic myosin

The RMZ cell line was established from a bone marrow metastasis of a human alveolar rhabdomyosarcoma. Since the beginning of the in vitro culture, RMZ cells showed differentiation-related morphological heterogeneity: actively proliferating polygonal or spindle-shaped cells were observed along with a few multinucleated myotube-like structures and giant cells, frequently multinucleated. All these cell types were still present after over 40 passages. A set of clonal derivatives has been obtained from the second in vitro subculture. All the clones showed the same morphological heterogeneity of the parental cells, but differed from one another in the degree of differentiation. Multinucleated myotube-like structures were strongly stained by anti-desmin antibody; most mononuclear cells were weakly stained. About 80% of RMZ and cloned cells were scored as desmin-positive in cytocentrifuged preparations. The expression of embryonic myosin heavy chain, specifically recognized by the monoclonal antibody BF-G6, was found in RMZ cell line and was localised in the myotube-like structures. Only a few giant cells and rare mononucleated polygonal cells were stained. The average proportion of BF-G6 positive cells in cytocentrifuged preparations was of about 6% of the total RMZ cells. In the two RMZ clones studied, the expression of embryonic myosin was correlated to the proportion of myotube-like structures: a BF-G6 positivity of 35% was found in the most differentiated one.

Fetal heavy chain skeletal myosin. An oncofetal antigen expressed by rhabdomyosarcoma

Fetal heavy chain skeletal myosin is normally present in fetal skeletal muscle. The study of 21 cases of rhabdomyosarcoma using specific antisera for fetal myosin, as well as for slow myosin, myoglobin, and desmin, led to positive findings in all cases with at least one antiserum. Desmin was localized in all cases and fetal myosin in 17 cases (81%), while myoglobin and slow myosin were present in 11 and eight cases, respectively. The localization of fetal myosin in rhabdomyosarcoma indicates that it is a type of oncofetal antigen. Because fetal myosin is found in small rhabdomyoblasts, it can be a useful marker in cases that usually constitute diagnostic problems.

[Cardiac myosins and myocardial contraction]

The contractile properties of cardiac muscle cells are determined by the molecular composition of the contractile apparatus and in particular by the structure of myosin. Three isoforms of myosin heavy chains have been recently identified in the mammalian heart: alpha and beta myosin heavy chains, present in atrial and ventricular myocardium, and nodal myosin heavy chain, present in sino-atrial and atrio-ventricular nodes. The alpha and beta isoforms are coded by two distinct genes whose expression is tissue and developmental stage-specific, and can be regulated by hormonal and mechanical factors. The relative concentration of the two isoforms is correlated with the maximal velocity of shortening and with the energy cost of force generation. In hyperthyroid myocardium the predominant isoform is the alpha, high ATPase myosin heavy chain and the contraction is fast but less economical; in hypothyroid and in mechanically overloaded myocardium the beta, low ATPase isoform is predominant and the contraction is slower and more economical.

Myosin types and fiber types in cardiac muscle. III. Nodal conduction tissue

The sinoatrial (SA) and atrioventricular (AV) nodes are specialized centers of the heart conduction system and are composed of muscle cells with distinctive morphological and electrophysiological properties. We report here results of immunofluorescence and immunoperoxidase studies on the bovine heart showing that a large number of SA and AV nodal cells share a distinct type of myosin heavy chain (MHC) which is not found in other myocardial cells and can thus be used as a cell-type-specific marker. The antibody used in this study was raised against fetal skeletal myosin and reacted with fetal skeletal but not with adult skeletal MHCs. Both atrial and ventricular fibers, as well as fibers of the ventricular conduction tissue were unlabeled by this antibody. Specific reactivity was exclusively seen in most cells in the central portions of the SA and AV nodes and rare cells in perinodal areas. However, a number of nodal cells, particularly those located in the peripheral nodal regions, were unreactive with this antibody. The myosin composition of nodal tissues was also explored using two antibodies reacting specifically with alpha-MHC, the predominant atrial isoform, and beta-MHC, the predominant ventricular isoform. Most nodal cells were reactive for alpha-MHC and a number of them also for beta-MHC. Variation in reactivity with the two antibodies was also observed in perinodal areas: at these sites a population of large fibers reacted exclusively for beta-MHC. These findings point to the existence of muscle cell heterogeneity with respect to myosin composition both in nodal and perinodal tissues.

Immunocytochemistry of rhabdomyosarcoma. The use of four different markers

Fast myosin and slow myosin are specific markers of skeletal muscle, in addition to myoglobin. This study of 15 specimens of rhabdomyosarcomas from 13 patients using specific antisera for the three markers as well as for desmin led to positive findings in all cases with at least one antiserum. Desmin was present in all cases; fast myosin and myoglobin were present in 10 cases each. Slow myosin was present in six cases. It appears that the combination of several markers is helpful in differentiating rhabdomyosarcomas from other tumors. The markers considered were generally more abundant in neoplastic elements with large amounts of cytoplasm. This finding suggests that the larger cells of rhabdomyosarcomas are more differentiated than smaller rhabdomyoblasts, which were often negative with some of the antisera used.

Embryonic myosin heavy chain as a differentiation marker of developing human skeletal muscle and rhabdomyosarcoma. A monoclonal antibody study

Hybridoma cell lines were obtained from the fusion of NS-O myeloma cells with spleen cells of mice immunized with bovine fetal skeletal myosin. A stable hybridoma clone, BF-G6, produced immunoglobulin G1 k antibodies reacting specifically with embryonic-type myosin heavy chains present in fetal but not in neonatal or adult human skeletal muscle, as determined by enzyme immunoassay and immunoblot analysis. Fetal but not adult skeletal muscle fibers were stained by this monoclonal antibody in indirect immunofluorescence assays; smooth muscle cells and cardiac muscle cells, as well as non-muscle cells were also unreactive. Solid tumors of infants and children were tested for reactivity with BF-G6 by immunofluorescence and immunoperoxidase staining. Embryonic myosin heavy chain was expressed in rhabdomyosarcomas but not in other types of tumor, except for Wilms' tumor. Rhabdomyosarcoma cells isolated from a bone marrow metastasis and grown in vitro for several months were also labelled by BF-G6. Embryonic myosin heavy chain can thus be used as a specific differentiation marker of normal and neoplastic skeletal muscle tissue.

Heterogeneity of rat neurofilament polypeptides revealed by a monoclonal antibody

A monoclonal antibody obtained from mice immunized with a crude neurofilament preparation from newborn rat brain revealed the existence of heterogeneity of the 200,000- and 150,000-dalton neurofilament polypeptides. On immunoblot the monoclonal antibody iC8 reacted with both the 200,000- and 150,000-dalton components in the CNS, but only with the 150,000-dalton polypeptide in sciatic nerve preparations. In addition, the 150,000-dalton polypeptide appeared as a single band in the sciatic nerve, whereas in the CNS a doublet was labeled by iC8. In contrast a second monoclonal antibody (3H5) reacted with the 200,000-dalton peptide and a single 150,000-dalton component in both the central and peripheral nervous system preparations. The differences revealed by iC8 were probably not due to phosphorylation, as the pattern of antibody binding in immunoblots was not changed by pretreatment with alkaline phosphatase. The findings suggest that different isoforms of neurofilament polypeptides are present in the nervous system.