Skeletal muscle fibre types: detection methods and embryonic determinants

Muscle fibres used to be simply classified as either type I, IIa or IIb. Advances in molecular and histological techniques have, however, lead to the realisation that the phenotypes of muscles are more varied than this. An additional fibre type (IIX/IID) has been discovered, fibres with intermediate fibre types have been described and there is accumulating evidence that the fibres types described from the study of limb muscles are not necessarily applicable to other skeletal muscles, such as the jaw and extra-ocular muscles. Further to this has been the discovery that diversity occurs at all stages of muscle development. There are subpopulations of myoblasts and myotubes as well as various types of muscle fibres. The relationships between the different stages of development is still under study. However, it is clear that each stage of muscle development is influenced to a certain degree by prior events. Consequently, the characteristics of mature fibres reflect both their developmental origins and influences from the adult environment, such as their patterns of muscle activation.

Altered expression levels and impaired steps in the pathway to phosphatidylinositol 3-kinase activation via insulin receptor substrates 1 and 2 in Zucker fatty rats

To elucidate the mechanism of obesity-related insulin resistance, we investigated the impaired steps in the processes of phosphatidylinositol (PI) 3-kinase activation through binding with insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) in liver and muscle of Zucker fatty rats. The expressions of IRS-1 and IRS-2 were shown to be downregulated in both liver and muscle in fatty rats (hepatic IRS-1, 83%; hepatic IRS-2, 45%; muscle IRS-1, 60%; muscle IRS-2, 78%), resulting in decreased tyrosine phosphorylation in response to insulin stimulation. Despite the decrease in the tyrosine phosphorylation levels of hepatic IRS-1 and IRS-2 being mild to moderate, associated PI 3-kinase activities were dramatically decreased in fatty rats (IRS-1, 14%; IRS-2, 10%), which may suggest alteration in the sites of phosphorylated tyrosine residues of hepatic IRS-1 and IRS-2. In addition, we demonstrated that the expressions of p85alpha and p55alpha regulatory subunits of PI 3-kinase were reduced (p85alpha, 67%; p55alpha, 54%), and that the p50alpha regulatory subunit was markedly upregulated (176%) in the livers of fatty rats without apparent alterations in expressions of the catalytic subunits p110alpha and p110beta. These alterations may reflect the obesity-related insulin resistance commonly observed in human NIDDM.

Comparison of the molecular, antigenic and ATPase determinants of fast myosin heavy chains in rat and human: a single-fibre study

Combined methodologies of histochemistry, immunohistochemistry, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), reverse transcriptase polymerase chain reaction (RT-PCR) and a histochemical method specific for myofibrillar ATPase (mATPase) of the type IIX myosin heavy chain (MyHC) isoform were used to study human and rat single fibres to examine the homology between type II MyHC isoform-based fibres of both species. We demonstrate that human type II fibres exhibit antigenic mATPase and 3'-untranslated region (3'-UTR) sequence determinants homologous to the IIA and IIX but not the IIB MyHC isoforms of the rat. Both immunolabelling with anti-MyHC monoclonal antibodies and the mATPase method used with frozen sections confirmed that all human type II fibres express type IIA and/or type IIX MyHC. Quantitative immunohistochemistry failed to recognize human fibres with antigenic characteristics corresponding to hybrid IIXB MyHC-based fibres. Ca2+-stimulated maximum myosin ATPase activity, determined by quantitative histochemistry, revealed that human IIX fibres (with an optical density or OD = 0.707) display enzyme activity which is comparable to that of the rat type IIX (OD = 0.687) but lower than that of the rat type IIB fibres (OD = 0.836). The results do not support the notion that MyHC IIB is expressed in human limb muscles, even in hybrid fibres. We conclude that human type II fibres have been misclassified in numerous previous publications and that this has important implications in attempts to compare the physiological characteristics of fibre types, particularly when animal models are used.

Embryonic gene expression in nonoverloaded ventricles of hereditary hypertrophic cardiomyopathic hamsters

Current information regarding the molecular and biochemical mechanisms of myocardial hypertrophy, as obtained from isolated cardiomyocytes and/or healthy animals with aortic banding, does not permit dissection of the hierarchical relationship among different steps and triggers of the pathogenic process in vivo. The aim of the present study was to depict the temporal relationship among myocardial structural and functional characteristics, the embryonic gene program, and transforming growth factor (TGF) beta 1 expression in euthyroid hereditary hypertrophic cardiomyopathic hamsters (CMPH). This investigation was performed using Western and Northern blot and in situ hybridization techniques. The results show that in CMPH, the severity of the hemodynamic overload is not related to any modification in structural myocardial characteristics (cardiac mass, cardiomyocyte dimensions, total RNA, and protein content), whereas an early activation of the embryonic gene program occurs in not yet overloaded 90-day-old CMPH (left ventricular end diastolic pressure < 15 mm Hg). In these animals, a 30% to 90% decrease in the alpha myosin heavy chain (alpha MHC) relative content was found in ventricles, whereas beta MHC increased 5-fold. In addition, the alpha skeletal actin expression was enhanced 2-fold versus age-matched controls. No modifications were observed in myosin function evaluated by in vitro motility assay, whereas the administration of L-thyroxine (100 micrograms/kg intraperitoneally daily) to CMPH was able to reinduce the ventricular expression of the alpha MHC isoform (5-fold increase). Conversely, no changes were found in alpha cardiac actin and myosin light chain 2 (MLC2) expression. A close temporal relationship occurred in CMPH ventricles between the re-expression of the embryonic gene program and a 3-fold enhancement of the expression of TGF beta 1. These results indicate that the CMPH provides a useful model for investigating the expression of embryonic genes in hypertrophic ventricles in the absence of mechanical and hormonal stimuli, and that TGF beta 1 is involved in regulating in vivo the "embryonic step" of myocardial hypertrophy. Furthermore, the study offers new insights into the pathophysiologic mechanisms leading to heart failure.

Expression of nitric oxide synthase in skeletal muscle: a novel role for nitric oxide as a modulator of insulin action

Previous studies have shown that nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide (NO), is expressed in skeletal muscle. The aim of the present study was to test the hypothesis that NO can modulate glucose metabolism in slow- and fast-twitch skeletal muscles. Calcium-dependent NOS was detected in skeletal muscle, and the enzyme activity was greater in fast-type extensor digitorum longus (EDL) muscles than in slow-type soleus muscles. Both the neuronal-type (nNOS) and endothelial-type (eNOS) enzymes are expressed in resting skeletal muscles. However, nNOS protein was only detected in EDL muscles, whereas eNOS protein contents were comparable in soleus and EDL muscles. NOS expression in muscle cryosections (diaphorase histochemistry) was located in vascular endothelium and in muscle fibers, and the staining was greater in type IIb than in type I and IIa fibers. The macrophage-type inducible NOS (iNOS) was not detected in resting muscle, but endotoxin treatment induced its expression, concomitant with elevated NO production. iNOS induction was associated with impaired insulin-stimulated glucose uptake in isolated rat muscles. In vitro, NOS blockade with specific inhibitors did not affect basal or insulin-stimulated glucose transport in EDL or soleus muscles. In contrast, the NO donors GEA 5024 and sodium nitroprusside induced dose-dependent inhibition (up to 50%) of maximal insulin-stimulated glucose transport in both muscles with minor effects on basal uptake values. GEA 5024 also blunted insulin-stimulated glucose transport and amino acid uptake in cultured L6 muscle cells without affecting insulin binding to its receptor. On the other hand, the permeable cGMP analogue dibutyryl cGMP did not affect muscle glucose transport. These results strongly suggest that NO modulates insulin action in both slow- and fast-type skeletal muscles. This novel autocrine action of NO in muscle appears to be mediated by cGMP-independent pathways.

Postnatal development of the masseter muscles in the Japanese field vole Microtus montebelli, with special attention to differentiation of the fast-twitch oxidative fiber

Postnatal development and differentiation of the masseter muscles consisting only of fast-twitch oxidative (FO) fibers in the adult Japanese field vole Microtus montebelli were studied using histochemical and electron microscopic techniques. The masseter muscles were composed of myotubes and muscle fibers at day 0 (birth day). Most muscle cells showed the strong reaction for myosin ATPase after both alkaline and acid preincubations. For NADH-dehydrogenase (NADH-DH), small granular diformazan deposits were recognized in the sarcoplasm. Afterwards, the masseter muscles consisted of myofibers and satellite cells at day 5. For myosin ATPase, weakly-reactive fibers after acid preincubation (fast-twitch fibers) increased in number. For NADH-DH, granular diformazan deposits in all the myofibers increased in size. Since all the myofibers had numerous sarcoplasmic reticula, and they reacted strongly after alkaline preincubation and weakly after acid preincubation for myosin ATPase at day 10 when the young start to take solid food, it seems that the masseter muscles become contractive fast. At day 15 (before weaning), all the myofibers showed the adult-like strong reaction for NADH-DH and had numerous well-developed mitochondria, thus they acquired the ability of the fast and sustained contraction. It is accordingly considered that the masseter muscles of the vole mature in a short time after birth because of adaptation for herbivorous food habit.

Subcellular localization of myosin-V in the B16 melanoma cells, a wild-type cell line for the dilute gene

The discovery that the dilute gene encodes a class V myosin led to the hypothesis that this molecular motor is involved in melanosome transport and/or dendrite outgrowth in mammalian melanocytes. The present studies were undertaken to gain insight into the subcellular distribution of myosin-V in the melanoma cell line B16-F10, which is wild-type for the dilute gene. Immunofluorescence studies showed some degree of superimposed labeling of myosin-V with melanosomes that predominated at the cell periphery. A subcellular fraction highly enriched in melanosomes was also enriched in myosin-V based on Western blot analysis. Immunoelectron microscopy showed myosin-V labeling associated with melanosomes and other organelles. The stimulation of B16 cells with the alpha-melanocyte-stimulating hormone led to a significant increase in myosin-V expression. This is the first evidence that a cAMP signaling pathway might regulate the dilute gene expression. Immunofluorescence also showed an intense labeling of myosin-V independent of melanosomes that was observed within the dendrites and at the perinuclear region. Although the results presented herein are consistent with the hypothesis that myosin-V might act as a motor for melanosome translocation, they also suggest a broader cytoplasmic function for myosin-V, acting on other types of organelles or in cytoskeletal dynamics.

The A10 cell line: a model for neonatal, neointimal, or differentiated vascular smooth muscle cells?

OBJECTIVES

The A10 cell line was derived from the thoracic aorta of embryonic rat and is a commonly used model of vascular smooth muscle cells (VSMC). Despite its wide use this cell line has not been well characterized. This is especially important in light of recent evidence of phenotypically distinct cell populations isolated from rat vascular tissue. Therefore, the present study was undertaken to confirm the VSMC nature of A10 cells and to investigate whether these cells particularly resemble adult, neonatal, or neointimal rat VSMC.

METHODS

A variety of defining characteristics were used that included immunofluorescent analysis for smooth muscle alpha-actin, smooth and non-muscle myosin heavy chains, desmin and vimentin; Western analysis for smooth muscle and non-muscle myosin heavy chains; mRNA analysis for smooth muscle myosin heavy chain, calponin, SM22 alpha, tropoelastin and PDGF-B peptide; and functional assays of cell migration, proliferation and agonist induced intracellular Ca transients.

RESULTS

A10 cells expressed smooth muscle alpha-actin, SM22 alpha, smooth muscle calponin and vimentin, characteristic of in vivo rat VSMCs; however they also resembled de-differentiated smooth muscle cells in that they expressed non-muscle myosin rather than smooth muscle myosin heavy chain. A10 cells resembled cultured rat neonatal smooth muscle cells ("pup cells") in that they had an epithelioid shape and lacked functional PDGF-alpha receptors: however they did not express PDGF-B mRNA or proliferate in low serum containing medium as do neonatal cells. A10 cells had several characteristics in common with neointimal cells including the expression of alpha-actin, vimentin, and non-muscle myosin and the lack of expression of PDGF-B mRNA as well as the ability to migrate in response to PDGF-BB.

CONCLUSION

In conclusion, A10 cells are nondifferentiated VSMC that differ from neonatal but bear significant resemblance to neointimal cells.

Participation of myosin in gliding motility and host cell invasion by Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that actively invades mammalian cells using a unique form of gliding motility that critically depends on actin filaments in the parasite. To determine if parasite motility is driven by a myosin motor, we examined the distribution of myosin and tested the effects of specific inhibitors on gliding and host cell invasion. A single 90 kDa isoform of myosin was detected in parasite lysates using an antisera that recognizes a highly conserved myosin peptide. Myosin was localized in T. gondii beneath the plasma membrane in a circumferential pattern that overlapped with the distribution of actin. The myosin ATPase inhibitor, butanedione monoxime (BDM), reversibly inhibited gliding motility across serum-coated slides. The myosin light-chain kinase inhibitor, KT5926, also blocked parasite motility and greatly reduced host cell attachment; however, these effects were primarily caused by its ability to block the secretion of microneme proteins, which are involved in cell attachment. In contrast, while BDM partially reduced cell attachment, it prevented invasion even under conditions in which microneme secretion was not affected, indicating a potential role for myosin in cell entry. Collectively, these results indicate that myosin(s) probably participate(s) in powering gliding motility, a process that is essential for cell invasion by T. gondii.

Dexamethasone-induced myocardial hypertrophy in neonatal rats

Clinical trials have shown dexamethasone's beneficial effects on the pulmonary status of infants with bronchopulmonary dysplasia; however, hypertrophic cardiomyopathy has been a reported complication of this therapy with no known mechanism. Our study was designed to test the hypothesis that therapeutic dexamethasone doses would induce myocardial hypertrophy. Newborn Sprague-Dawley rats were randomly assigned to receive dexamethasone 0.125 mg/kg/day, while paired littermate controls received saline placebo. The daily body weights were recorded and pups were sacrificed after 5 or 7 days of treatment. The heart weight to body weight ratio was used as a gross index of myocardial mass. Myocardial protein content, total protein to total DNA ratio, actin content and myosin heavy chain content were used as biochemical indices of hypertrophy. Our results included an increased heart weight to body weight ratio with elevation of the total protein content, actin content and total protein to total DNA ratio after both 5 and 7 days. We conclude that dexamethasone induces myocardial hypertrophy in neonatal rats.

Myosin II is associated with Golgi membranes: identification of p200 as nonmuscle myosin II on Golgi-derived vesicles

A variety of peripheral membrane proteins associate dynamically with Golgi membranes during the budding and trafficking of transport vesicles in eukaryotic cells. A monoclonal antibody (AD7) raised against Golgi membranes recognizes a peripheral membrane protein, p200, which associates with vesicles budding off the trans-Golgi network (TGN). Based on preliminary findings, a potential association between p200 and myosin on Golgi membranes was investigated. Immunofluorescence staining of cultured cells under a variety of fixation conditions was carried out using an antibody raised against chick brush border nonmuscle myosin II. We show that, in addition to being found in the cytoplasm or associated with stress fibres, nonmuscle myosin II is also specifically localized on Golgi membranes. Myosin II was also detected on Golgi membranes by immunoblotting and by immunogold labeling at the electron microscopy level where it was found to be concentrated on Golgi-derived vesicles. The association of myosin II with Golgi membranes is dynamic and was found to be enhanced following activation of G proteins. Myosin II staining of Golgi membranes was also disrupted by brefeldin A (BFA). Colocalization of the AD7 and myosin II antibodies at the light and electron microscopy levels led us to investigate the nature of the 200 kDa protein recognized by both antibodies. The 200 kDa protein immunoprecipiated by the AD7 antibody was isolated from MDCK cells and used for microsequencing. Amino acid sequence data enabled us to identify p200 as the heavy chain of nonmuscle myosin IIA. In addition, an extra protein (240 kDa) recognized by the AD7 antibody specifically in extracts of HeLa cells, was sequenced and identified as another actin-binding protein, filamin. These results show that nonmuscle myosin II is associated with Golgi membranes and that the vesicle-associated protein p200, is itself a heavy chain of myosin II.

Immunoelectron microscopical characterization of the epithelioid type of smooth muscle cells in human glomus organs

The wall structure of arterio-venous anastomoses in human glomus organs was studied by immunohistochemistry and immunoelectron microscopy. Smooth muscle cells of the epithelioid type I and of the dense type II could be found in the media. The immunohistochemical study confirmed the immunopositivity of both smooth muscle cell types for alpha-smooth muscle actin, vimentin, and smooth muscle myosin. All smooth muscle cells also stained positively for caveolin, a recently described structural protein of microvesicles present in selected epithelial and nonepithelial cells. The immunoreactivity for cathepsin D, however, was much higher in the type I cells than in the type II cells. Immunoelectron microscopy revealed that type I cells contain loose arrays of alpha smooth muscle actin positive microfilaments, sometimes arranged in small bundles, whereas the dense medial smooth muscle cells of the type II have tightly packed actin filaments. Only type I cells contained cathepsin D positive lysosomes. The data suggest that two types of phenotypic variants of vascular smooth muscle cells in the human arterio-venous anastomosis exist: a more "synthetic" type I cell and a more contractile type II cell.

Developmental changes in crossbridge properties and myosin isoforms in hamster diaphragm

The aim of this study was to determine the effects of maturation on crossbridge properties and myosin isoform composition in hamster diaphragm muscle. Diaphragm strips were obtained at postnatal Days 1 and 8 and in adults (10 to 12 wk). Peak isometric tension and maximum unloaded shortening velocity (Vmax) increased with age (p < 0.001). The single crossbridge force (pi), the total number of crossbridges normalized per cross-sectional area (m x 10(9)/mm2), the turnover rate of myosin ATPase (kcat), and peak mechanical efficiency (Effmax) were calculated from Huxley's equations. The value of m increased significantly from birth to adulthood (p < 0.001), with no changes in pi or Effmax; kcat increased significantly only after the first week postpartum. There was a strong linear relationship between peak isometric tension and m (p < 0.001). Conversely, changes in Vmax were not related to kcat. Myosin electrophoresis showed that neonatal bands and slow myosin isoforms (S) were present at birth. The number of fast adult myosin isoforms increased progressively from birth to adulthood, whereas S increased during the first week postpartum. In conclusion, development changes in diaphragm muscle force and myosin isoform composition were associated with changes in crossbridge number and kinetics, with no changes in the average force per crossbridge or in mechanical efficiency.

Myosin I is associated with zymogen granule membranes in the rat pancreatic acinar cell

BACKGROUND & AIMS

The mechanisms whereby intracellular messengers mediate zymogen granule transport and exocytosis in the pancreatic acinar cell are not well defined. Electron microscopy has shown a periluminal network of actin in the acinar cell, suggesting a role for actin and myosin in the transport process. The possible involvement of two types of myosin in the secretory process was investigated, and their distribution in acinar cells was determined.

METHODS

Antibodies specific to myosin I or to myosin II were used for immunocytochemistry and Western blot analysis. Ultrastructural studies were also performed.

RESULTS

Western blot analysis showed that myosin I and myosin II were present in total pancreatic homogenate but that only myosin I was present on isolated zymogen granules and their membranes. By immunocytochemistry, myosin I was shown in the apical aspect of acinar cells colocalized with glycoprotein 2, a marker for zymogen granules, and actin. By immunocytochemistry, myosin I was also localized on isolated zymogen granules.

CONCLUSIONS

The immunolocalization of myosin I to zymogen granule membranes and its close association with periluminal actin suggest that myosin I plays a direct role in the process of transport and exocytosis of zymogen granules in the pancreatic acinar cell.

Deficiency of alpha-actinin-3 (ACTN3) occurs in different forms of muscular dystrophy

The alpha-actinins belong to a superfamily of cytoskeletal proteins, and their role in human genetic diseases is still unclear. Therefore, they could be good candidates for muscular dystrophies of unknown etiology. We have analyzed alpha-actinin-3 (ACTN3) in muscle biopsies from a total of 54 patients. A complete deficiency was found in 9 patients: 2/12 with classical merosin-positive congenital MD (CMD), 1/12 with Severe Childhood Autosomal Recessive MD (DLMD), but with a positive IF pattern for the proteins of the sarcoglycan complex: 3/14 with mild limb-girdie MD (1LGMD2A and 2 yet unclassified), 1/10 with sarcoglycanopathies (LGMD2C), and 2/6 with Xp21 Duchenne MD (DMD). Patients within the same family, and with the same disease (DMD, LGMD2A, LGMD2C), were discordant for ACTN3 deficiency. Additionally, no correlation was found with the degree of muscle degeneration, nor with the clinical course. One ACTN3-deficient CMD patient showed no mRNA expression for the muscle ACTN3 gene, but the other ACTN3-deficient patients with different forms of muscular dystrophy showed very low or no mRNA expression as well. These results show that the deficiency of ACTN3 is a secondary effect in these dystrophies.

Relationship of cytoskeletal filaments to annular gap junction expression in human adrenal cortical tumor cells in culture

In addition to the well-characterized surface gap junctions expressed at contact sites between cells, annular gap junction profiles have been localized within the cytoplasm of some cell populations. To study and characterize these annular profiles, gap junction protein type was demonstrated with Western blot and immunocytochemistry. The distribution of annular gap junctions and the relationships to cytoskeletal elements were demonstrated with immunocytochemical, transmission electron microscopic, or image analysis with confocal microscopy techniques. SW-13 adrenal cortical tumor cells expressed alpha1 gap junctions at areas of cell to cell contact. In addition, alpha1 gap junction annular profiles were seen within the cytoplasm. Actin and myosin II were found closely associated with these annular gap junctions, while no physical association between tubulin- or vimentin-containing fibers and gap junction protein could be established. Disruption of microfilaments with cytochalasin B treatment (10 microg/ml, 1 h) resulted in a decrease in the average number and an increase in the average size of annular gap junctions compared to control populations. The results are consistent with a role for cytoskeletal elements containing actin and myosin II in annular gap junction turnover.

Effect of rapid rigor mortis processes on protein functionality in pectoralis major muscle of domestic turkeys

The pale, soft, exudative (PSE) phenomenon in turkey pectoralis major (breast) muscle was studied using a combination of biochemical, meat quality, microscopic, and gel electrophoresis techniques. Breast muscle samples were collected from turkeys characterized by slow vs fast postmortem glycolysis assessed by muscle pH at 20 min after death. The PSE group was characterized by lower muscle ATP (P < .05) and higher lactate levels (P < .05) compared with the normal group. Excess water-holding capacity and cooking yield were significantly lower (P < .05) in the PSE group than in normal turkeys. Breast muscle of the PSE group was also lighter (P < .05) than that in the normal group as determined by Minolta L* values. The SDS-PAGE, Western blotting, and immunofluorescence microscopy revealed that phosphorylase, a soluble enzyme, became tightly associated with the myofibrils in muscle from the PSE group. Also, less myosin could be solubilized from PSE vs normal myofibril samples. The results indicate that irreversible myosin insolubility due to low pH and high-temperature conditions is decisive in the development of PSE turkey breast muscle.

Expression of an alpha-cardiac like myosin heavy chain in diaphragm, chronically stimulated, and denervated fast-twitch muscles of rabbit

An additional slow fibre type, type I alpha, is detected in diaphragm and appears in fast-twitch hindlimb muscles of rabbit under the influence of altered neuromuscular activity. Type I alpha fibres were delineated from fibres expressing myosin heavy chain I beta (type I beta) by immunohistochemistry with a monoclonal antibody raised against the alpha-cardiac MHCI alpha. When stained for mATPase after acid and alkaline preincubations, some type I alpha fibres resembled type I beta and type IIA fibres, respectively. Some type I alpha fibres displayed dissimilar mATPase staining, indicating heterogeneity of this fibre population. The appearance of numerous type I alpha fibres in stimulated muscles, which in addition contain type IIA and type I beta fibres, suggested that they may be interspaced between types IIA and I beta. Electrophoresis under nondenaturing conditions disclosed an additional isomyosin both in normal diaphragm and stimulated muscles. This band displayed the same mobility as the slowest isomyosin in rabbit masseter muscle. It was recognized by the same monoclonal (anti-alpha-cardiac MHC) antibody used for immunohistochemistry. Therefore, this isomyosin appeared to be very similar, but perhaps not identical to the alpha-cardiac MHC-based isomyosin, probably resulting from discrete differences in the MHC complement. This assumption agrees with additional findings suggesting an even greater heterogeneity of the MHCs than generally assumed. In support of this, we show in atrium and masseter muscles the existence of an additional, electrophoretically distinct MHC isoform which migrates in close vicinity to MHCI alpha.

CDK1 inactivation regulates anaphase spindle dynamics and cytokinesis in vivo

Through association with CDK1, cyclin B accumulation and destruction govern the G2/M/G1 transitions in eukaryotic cells. To identify CDK1 inactivation-dependent events during late mitosis, we expressed a nondestructible form of cyclin B (cyclin BDelta90) by microinjecting its mRNA into prometaphase normal rat kidney cells. The injection inhibited chromosome decondensation and nuclear envelope formation. Chromosome disjunction occurred normally, but anaphase-like movement persisted until the chromosomes reached the cell periphery, whereupon they often somersaulted and returned to the cell center. Injection of rhodamine-tubulin showed that this movement occurred in the absence of a central anaphase spindle. In 82% of cells cytokinesis was inhibited; the remainder split themselves into two parts in a process reminiscent of Dictyostelium cytofission. In all cells injected, F-actin and myosin II were diffusely localized with no detectable organization at the equator. Our results suggest that a primary effect of CDK1 inactivation is on spindle dynamics that regulate chromosome movement and cytokinesis. Prolonged CDK1 activity may prevent cytokinesis through inhibiting midzone microtubule formation, the behavior of proteins such as TD60, or through the phosphorylation of myosin II regulatory light chain.

Evidence for the presence of myosin I in the nucleus

We produced and affinity-purified polyclonal antibodies to adrenal myosin I. These antibodies recognize adrenal myosin I by Western blot analysis (116 kDa) and inhibit the actin-activated ATPase activity of purified adrenal myosin I. They also recognize a 120-kDa protein in extracts prepared from many different cell lines. Fluorescence microscopy demonstrated the presence of immunoreactive material in the perinuclear region, the leading edges, and the nuclei of 3T3 cells. Fluorescence microscopy also demonstrated nuclear staining in mouse oocytes at the germinal vesicle stage and in the pronuclei during fertilization. Confocal and immunoelectron microscopy confirmed the intranuclear localization. Electron microscopy also demonstrated staining of structures in nucleoli that are thought to be associated with rDNA transcription. Western blot analyses revealed the presence of the 120-kDa protein in extracts prepared from nuclei that are apparently free of cytosolic contamination. The same nuclear protein binds 125I-calmodulin and is photoaffinity labeled with [alpha-32P]ATP. The 120-kDa protein was partially purified from twice washed nuclei using ammonium sulfate fractionation and gel filtration chromatography. Column fractions containing 120-kDa protein as revealed by Western blot analysis also contain K+-EDTA ATPase activity. The 120-kDa protein was also shown to bind actin in the absence, but not the presence, of ATP. Since K+-EDTA ATPase activity, actin, and ATP binding are defining features of the members of the myosin superfamily of proteins, we propose that the 120-kDa protein is a previously undescribed myosin I isoform that is an intranuclear actin-based molecular motor.

Actomyosin interaction in striated muscle

The mechanics of the actomyosin interaction have been extensively studied using the organized filament array of striated muscle. However, the extrapolation of these data to the events occurring at the level of a single actomyosin interaction has not been simple. Problems arise in part because an active fiber has an ensemble of myosin heads that are spread out through the various steps of the active cycle, and it is likely that only a small fraction of the heads are generating tension at any given time. More recently, two new approaches have greatly extended our knowledge of the actomyosin interaction. First, the three-dimensional crystal structures of both the actin monomer and the myosin head have been determined, and these structures have been fit to lower resolution images to give atomic models of the actin filament and of the actin filament decorated by myosin heads. Second, the technology to measure picoNewton forces and nanometer distances has provided direct determinations of the force and step length generated by a single myosin molecule interacting with a single actin filament. This review synthesizes the existing mechanical data obtained from the more-organized array of the muscle filament with the results obtained by these two technologies.

Unconventional myosins in inner-ear sensory epithelia

To understand how cells differentially use the dozens of myosin isozymes present in each genome, we examined the distribution of four unconventional myosin isozymes in the inner ear, a tissue that is particularly reliant on actin-rich structures and unconventional myosin isozymes. Of the four isozymes, each from a different class, three are expressed in the hair cells of amphibia and mammals. In stereocilia, constructed of cross-linked F-actin filaments, myosin-Ibeta is found mostly near stereociliary tips, myosin-VI is largely absent, and myosin-VIIa colocalizes with crosslinks that connect adjacent stereocilia. In the cuticular plate, a meshwork of actin filaments, myosin-Ibeta is excluded, myosin-VI is concentrated, and modest amounts of myosin-VIIa are present. These three myosin isozymes are excluded from other actin-rich domains, including the circumferential actin belt and the cortical actin network. A member of a fourth class, myosin-V, is not expressed in hair cells but is present at high levels in afferent nerve cells that innervate hair cells. Substantial amounts of myosins-Ibeta, -VI, and -VIIa are located in a pericuticular necklace that is largely free of F-actin, squeezed between (but not associated with) actin of the cuticular plate and the circumferential belt. Our localization results suggest specific functions for three hair-cell myosin isozymes. As suggested previously, myosin-Ibeta probably plays a role in adaptation; concentration of myosin-VI in cuticular plates and association with stereociliary rootlets suggest that this isozyme participates in rigidly anchoring stereocilia; and finally, colocalization with cross-links between adjacent stereocilia indicates that myosin-VIIa is required for the structural integrity of hair bundles.

Single molecule imaging of fluorophores and enzymatic reactions achieved by objective-type total internal reflection fluorescence microscopy

Imaging of single fluorescent molecules has been achieved in a relatively simple manner using objective-type total internal reflection fluorescence microscopy (TIRFM). Switching from epi-fluorescence microscopy to objective-type TIRFM was achieved by translation of a single mirror in the system. Clear images of single molecules of an orange fluorescent dye, Cy3, were obtained with a fluorescence-to-background ratio of 12, using a conventional high aperture objective (PlanApo, 100 x, NA 1.4) with ordinary coverslips and immersion oil. This method allowed visualization of single molecules under scanning probe microscopes. Taking advantage of the technique of single molecule imaging, individual ATP turnovers have been visualized with a fluorescent ATP analogue, Cy3-ATP, using a simple experimental strategy. Clear on/off signals were obtained that correspond to the association and dissociation of single Cy3-ATP/ADP molecules with a single myosin head molecule. This method will allow a variety of single-molecular assays of biomolecular functions to be performed using fluorescently labeled substrates, ligands, messengers, and biologically active molecules. Thus, the present technique provides a simple yet powerful and universal tool for researchers to probe the events of single molecules.

Pure heterologous sarcoma of mixed type of the uterine corpus in a postmenopausal patient: a case report

A case of a pure heterologous sarcoma of mixed type (pleomorphic rhabdomyosarcoma and chondrosarcoma) localized in the uterine corpus of a 75-year-old woman is presented. The tumor was investigated by routine morphologic methods, immunohistochemically, and on ultrastructural level. The histogenesis of the tumor and the problems of differential diagnosis with other tumors of the same localization are under discussion.

Decorin and collagen type I gene expression in avian low score normal pectoral muscle

The avian Low Score Normal (LSN) genetic muscle weakness is phenotypically characterized by a reduction in the ability of the birds to right themselves from a supine position. Compared to normal skeletal muscle, LSN muscle has normal myosin isoform switching and cell-cell recognition, elevated glycosaminoglycan and decorin levels at embryonic Day 20, and a large increase in collagen crosslinking at 6 wk posthatch. To begin to determine the biological mechanism involved in the elevated decorin protein concentration at embryonic Day 20, the steady-state levels of transcripts encoding both decorin and collagen Type I at embryonic Days 14, 19, and 20, and at 1 d and 6 wk posthatch were measured. On embryonic Day 20, collagen Type I transcripts were not different from the control but there was a significant elevation in decorin transcript levels. At 1 d and 6 wk posthatch, transcript levels of decorin and collagen Type I were not different between LSN and controls. The change in decorin transcript steady-state levels is limited to late embryonic development and suggests an alteration in a signal transduction pathway regulating decorin transcription.

Mismatch between myosin heavy chain mRNA and protein distribution in human skeletal muscle fibers

The distribution of myosin heavy chain (MHC) isoforms was analyzed at the protein and mRNA levels in human skeletal muscle biopsies from young normal adult subjects. Using ATPase histochemical reactions, antibodies to fast- and slow-type MHCs, and in situ hybridization with probes specific for MHC-beta/slow, MHC-2A, and MHC-2X, we confirmed our previous results showing that most fibers contain either a single mRNA and isoprotein or a mixed 1/2A or 2A/2X phenotype with coexistence of two mRNAs and isoproteins. However, we also found a minor proportion of fibers showing a mismatch in the relative proportion of mRNA and protein, e.g., fibers containing MHC-2A mRNA but not the corresponding protein or fibers containing MHC-2A protein but not the corresponding transcript. These fibers were more frequent in biopsies obtained after a training or detraining period than before the training period. We propose that these fibers represent transitional fibers and that the relative content of each mRNA and isoprotein gives a clue as to the direction of change in MHC gene expression.

Polymerization of myosin on activation of rat anococcygeus smooth muscle

The in vivo state of assembly of myosin in vertebrate smooth muscle is controversial. In vitro studies on purified smooth muscle myosin show that it is monomeric (10S) under relaxing conditions and filamentous under contraction conditions. Electron microscopic and antibody labelling studies of intact smooth muscles, on the other hand, suggest that myosin is filamentous in the relaxed as well as the contracting state and that 10S myosin occurs only in trace amounts. However, birefringence, conventional EM and X-ray diffraction evidence suggests that in certain smooth muscles in vivo (e.g. rat anococcygeus), while myosin filaments exist in the relaxed state, their number increases on contraction. Here, we have used low temperature electron microscopic techniques (rapid freezing followed by freeze-substitution), which preserve labile components in close to their in vivo state, to detect any change in filament number on contraction. The results from rat anococcygeus have been compared with those from guinea pig taenia coli, in which other techniques have revealed no change in filament number. In the anococcygeus, we find evidence for a 23% increase in filament density in transverse sections of contracting muscle compared with relaxed muscle. In the taenia coli we find no change. These results are in qualitative agreement with earlier findings. They provide evidence for polymerization of myosin in contracting rat anococcygeus, and suggest that this process is subtle and occurs only in some smooth muscles.

Identification and partial characterization of a myosin-like protein from cysticerci and adults of Taenia solium using a monoclonal antibody

The host-parasite relationship in taeniosis due to Taenia solium is practically unknown. Monoclonal antibodies were prepared against whole extracts of adult T. solium parasites and evaluated with tapeworms recovered from experimentally infected hamsters and with cysticerci from naturally infected pigs. With one antibody, mAb 4B3, it was possible to identify, purify and partially characterize a T. solium myosin. Some findings indicate that it corresponds to conventional myosin or myosin type II such as: purification with KCl, high molecular weight, size, structure (dimeric protein with globular and long tail portions), reaction with commercial anti-myosin antibodies, distribution in muscle fibres of parasites and cross-reactivity with antibodies against paramyosin from T. solium cysticerci. The reaction of the mAb was only with taeniids and not with other parasites. Also myosin was detected in faeces of infected animals and in supernatants of parasite cultures. Its presence in biological fluids may be useful for diagnosis of infected hosts.

A case of malignant schwannoma with overexpression of multidrug resistance gene (MDR1) after chemotherapy

A 19-year-old:female patient with malignant schwannoma in the right knee was treated by combination chemotherapy including lipophilic anticancer compounds (cyclophosphamide, doxorubicin, vincristine and dacarbazine). The tumor was radically removed after chemotherapy, but metastatic lesions were noted in the right inguinal nodes. The patient died due to the cachexic state six months after surgery. In human neoplasm, P-glycoprotein (P-Gp) encoded by human multidrug resistance gene MDR1 is known to be related with multidrug resistant phenotype. Northern blot analysis revealed apparent MDR1 expression in the metastatic lesion, while the primary lesion showed faint MDR1 expression detected by only reverse transcriptase-polymerase chain reaction. P-Gp positive tumor cells were immunohistochemically detected both in the metastatic lesion and the primary lesion. The P-Gp-positive tumor cells in the metastatic lesion showed anaplastic features with highly atypical nuclei. These results suggest that MDR1 overexpression is related to the multidrug resistance phenomenon in the malignant schwannoma with morphological differences.

Immunohistochemical and ultrastructural examination of smooth muscle cells in aortocoronary saphenous vein grafts

In this study, phenotypic modulation and remodulation of smooth muscle cells and associated intermediate filament expression were demonstrated by means of immunohistochemistry and ultrastructure to understand the development of intimal hyperplasia in aortocoronary saphenous vein grafts. In nongrafted saphenous veins, all smooth muscle cells expressed vimentin and desmin and were of a contractile form. In saphenous vein grafts showing stenotic intimal hyperplasia (luminal stenosis < 75%), expression of desmin was notably lower, whereas that of vimentin was higher. The cells were shown to be of a synthetic phenotype, suggesting modulation from the original contractile form. In saphenous vein grafts showing occlusive intimal hyperplasia (luminal stenosis > 76%), desmin expression in smooth muscle cells was increased again, and such cells were of a contractile form, suggesting remodulation from the synthetic phenotype. Some of the smooth muscle cells of the synthetic phenotype were positive for an antibody against proliferation cell nuclear antigen. Smooth muscle cells of the contractile form were negative for this antibody. The study suggests that smooth muscle cells of synthetic phenotype are highly responsible for "growing" intimal hyperplasia of aortocoronary saphenous vein grafts.

Skeletal muscle-specific myosin binding protein-H is expressed in Purkinje fibers of the cardiac conduction system

Heart contraction is coordinated by conduction of electrical excitation through specialized tissues of the cardiac conduction system. By retroviral single-cell tagging and lineage analyses in the embryonic chicken heart, we have recently demonstrated that a subset of cardiac muscle cells terminally differentiates as cells of the peripheral conduction system (Purkinje fibers) and that this occurs invariably in perivascular regions of developing coronary arteries. Cis regulatory elements that function in transcriptional regulation of cells in the conducting system have been distinguished from those in contractile cardiac muscle cells; eg, 5' regulatory sequences of the desmin gene act as enhancer elements in skeletal muscle and in the conduction system but not in cardiac muscle. We hypothesize that Purkinje fiber differentiation involves a switch of the gene expression program from that characteristic of cardiac muscle to one typical of skeletal muscle. To test this hypothesis, we examined the expression of myosin binding protein-H (MyBP-H) in Purkinje fibers of chicken hearts. This unique myosin binding protein is present in skeletal but not cardiac myocytes. A site-directed polyclonal antibody (AB105) was generated against MyBP-H. Immunohistological analysis of the myocardium mapped the AB105 antigen predominantly to A bands of myofibrils within Purkinje fibers. Western blot analysis of whole extracts from the ventricular wall of adult chicken hearts revealed that the AB105 epitope was restricted to a single protein of approximately 86 kD, the same size as MyBP-H in skeletal muscle. Biochemical properties of the Purkinje fiber 86-kD protein and RNase protection analyses of its mRNA indicate that Purkinje fiber 86-kD protein is indistinguishable from skeletal muscle MyBP-H. The results provide evidence that skeletal muscle MyBP-H is expressed in a subset of cardiac muscle cells that differentiate into Purkinje fibers of the heart.

Novel isoform of myotonin protein kinase: gene product of myotonic dystrophy is localized in the sarcoplasmic reticulum of skeletal muscle

It is quite important to know the exact localization and function of myotonin protein kinase (MtPK), identified as the gene product of myotonic dystrophy, the most prevalent disease with multisystem disorders among muscular dystrophies. To investigate the localization of MtPK, we raised a polyclonal antibody against a synthetic peptide chosen within the deduced sequence of MtPK. This antibody detected both a membrane-bound 70-kd protein and a soluble 55-kd protein on Western blots of human muscles. By using this antibody for immunohistochemical studies of both biopsied human skeletal muscle fibers and mature innervated cultured muscle fibers, we can now demonstrate by confocal laser scanning microscopy that MtPK is localized mainly in the I-band. By immunoelectron microscopy, it was determined that MtPK is a membrane-bound protein localized mainly in the terminal cisternae of the sarcoplasmic reticulum. To our knowledge, this is the first documentation of the ultrastructural localization of MtPK. This finding is quite important for clarifying the pathophysiological basis of myotonic dystrophy, which might be due to a dysregulation of calcium metabolism.

Changes in the distribution of integrins and their basement membrane ligands during development of human thyroid follicular epithelium

Interactions between cells and basement membrane components are crucial for the regulation of epithelial cell differentiation and polarization. We have studied by immunohistochemical methods the distribution of integrin adhesion proteins and some of their basement membrane ligands in foetal (16-19 weeks) and adult thyroid follicular epithelia. A diffuse immunoreactivity for only alpha 3, alpha v and beta 1 integrins was found in foetal follicular epithelium, whereas in adult follicular epithelium these integrins were expressed basally in a polarized manner. Additionally, beta 3 integrin was seen in a more basolaterally confined manner in adult follicular epithelium. Among basement membrane components, laminin alpha 1, beta 1, gamma 1 and beta 2 chains were found in epithelial basement membranes of the foetal thyroid gland, suggestive of the presence of laminins-1 and -3. In contrast, the basement membranes of adult follicular epithelium presented a much weaker immunoreactivity for the laminin beta 2 chain. Furthermore, immunoreactivity for the laminin alpha 2 chain was occasionally seen in adult thyroid glands, apparently confined to myofibroblasts. Immunoreactivity for type IV collagen alpha 1 and alpha 2 (IV) chains was found in follicular basement membranes of foetal as well as adult thyroid gland. The results suggest that during maturation of foetal thyroid follicular epithelium a distinct polarization of integrins takes place. In mature thyroid follicular epithelium, the presumable adhesion-mediating integrin complexes are alpha 3 beta 1, alpha v beta 1 and/or alpha v beta 3 mediating adhesion to laminin-1 (alpha 1-beta 1-gamma 1) and type IV collagen trimer alpha 1(2) alpha 2 (IV).

Improved electrophoresis and transfer of picogram amounts of protein with hemoglobin

During electrophoresis and electroblotting to transfer membranes, picogram amounts of protein can react irreversibly with the polyacrylamide matrix, preventing complete electrophoresis and efficient electroblotting. Bovine hemoglobin, but not other potential carrier proteins, mitigates this protein loss by migrating with or ahead of other proteins and scavenging reactive groups. Inclusion of 5 micrograms of hemoglobin in sample wells increases by 4-fold the amount of a radiolabeled test protein, myosin I beta, found at its appropriate 120-kDa position in sodium dodecyl sulfate-polyacrylamide gels. For electroblotting, incubating the gel with 0.25 mg/ml hemoglobin prior to transfer improves mobilization of picogram amounts of radiolabeled myosin I beta out of the gel by about 6-fold. For picogram amounts of proteins, therefore, approximately 20-fold more protein transfers to a blotting membrane when hemoglobin is used during both electrophoresis and transfer. This effect is general: transfer of radiolabeled Drosophila embryo proteins is improved dramatically by including hemoglobin in the pretransfer incubation solution. We suggest that electroblot-based detection of small amounts of protein, particularly when in the absence of other potential carrier proteins, can be improved substantially by using hemoglobin.

[Vagal sensory reinnervation and composition of the sterno-cephalic muscle in rabbits]

The crossed nerve anastomosis between the peripheral end of the vagus nerve, cut above the nodose ganglion, and the peripheral end of the accessory nerve has demonstrated the capacity of some vagal afferents to reinnervate, via the accessory nerve stump, certain sternocephalicus muscle fibers in the rabbit. These results add to our understanding of the capacity of these afferents to counter the past-denervational atrophying process that occurs in the reinnervated muscles and to evaluate the changes induced in these muscles during reinnervation. Our work shows that within 3 months, the vagal sensory reinnervation of previously denervated sternocephalicus muscles induces their total weight recovery. This recovery is concomitant on the one hand with the hypertrophy of the four muscle fiber types (I, IIBD, IIC and IIA) identified histochemically in the normal muscles and, on the other, with the appearance of small newly formed myofibers, which are often underlined by characteristic central nuclei. The vagal sensory neurones induce important changes in the percentages and the muscle cross-sectional distribution of the fibers in reinnervated muscles. In these muscles we see also the disappearance of the fast myosin heavy chains MHCIIB and MHCIID, the upholding of the fast MHCIIA percentage and an increase in the slow MHCI isoform.

Asymmetry in the distribution of free versus cytoskeletal myosin II in locomoting microcapillary endothelial cells

Myosin II is required for normal amoeboid locomotion. In order to understand how myosin II elicits its effects on locomotive behavior, we have mapped myosin II-cytoskeleton interactions in locomoting endothelial cells. Bovine microcapillary endothelial cells were microinjected with fluorescently labeled myosin II, and the distribution of myosin II was imaged in the living cells by fluorescence microscopy. The same cells were then permeabilized with Triton X-100 and imaged again. The second set of images showed only myosin II that was associated with detergent-insoluble cytoskeleton. Dividing the image of retained myosin II by that of total myosin II produced a map of the extent to which myosin II was associated with the detergent-resistant cytoskeleton at any point in the cell. In cells migrating at the edge of a scrape wound, myosin II was preferentially retained in a region approximately 10 microm wide located just behind the cells' leading lamellipodia. Relatively little myosin II was retained in perinuclear cytoplasm. A vector representation of the distribution of total versus retained myosin II demonstrated that myosin II retention was sharply polarized with respect to locomotion, favoring the front of migrating cells. Myosin II-enriched cytoskeleton in this region may help polarize protrusive activity and/or move cytoplasmic bulk forward. Patches of myosin II retention were also observed in adherent tails of many cells, consistent with a role in pulling the rear of the cell forward.

Subcellular localization of myosin V in nerve growth cones and outgrowth from dilute-lethal neurons

Myosin V-null mice (dilute-lethal mutants) exhibit apparent neurological defects that worsen from birth until death in the third postnatal week. Although myosin V is enriched in brain, the neuronal function of myosin V is unclear and the underlying cause of the neurological defects in these mice is unknown. To aide in understanding myosin V function, we examined the distribution of myosin V in the rodent superior cervical ganglion (SCG) growth cone, a well characterized neuronal structure in which myosin V is concentrated. Using affinity purified, myosin V-specific antibodies in immunofluorescence and immunoelectron microscopy, we observed that myosin V is concentrated in organelle-rich regions of the growth cone. Myosin V is present on a distinct population of small (50–100 nm) organelles, and on actin filaments and the plasma membrane. Myosin V-associated organelles are present on both microtubules and actin filaments. These results indicate that myosin V may be carried as a passenger on organelles that are transported along microtubules, and that these organelles may also be capable of movement along actin filaments. In addition, we found no abnormalities in outgrowth, morphology, or cytoskeletal organization of SCG growth cones from dilute-lethal mice. These results indicate that myosin V is not necessary for the traction force needed for growth cone locomotion, for organization of the actin cytoskeleton, or for filopodial dynamics.

The complex-type oligosaccharide binding lectin Datura stramonium agglutinin detects type II A muscle fibres in the branchial biceps from man and cat

Complex-type oligosaccharides were detected in the sarcoplasm of muscle fibres from cat and human biceps using lectins and anticarbohydrate antibodies. The lectin Datura stramonium agglutinin strongly stained type II A fibres as identified by myosin ATPase activity after alkaline and acid preincubation. In contrast, all muscle fibres showed a moderate coarse granular staining after incubation with Tetracarpidum conophorum agglutinin and Telfairia occidentalis agglutinin which recognize tri-antennary complex glycans poorly bound by D. stramonium agglutinin. Strong sarcoplasmic staining in all muscle fibres was obtained after incubation with an antibody against branched N-acetyllactosamine structure while an antibody against binary 2 --> 3 sialyllactosamine glycans failed to detect the muscle fibres. Treatment of the muscle sections with sialidase prior to incubation with D. stramonium agglutinin did not influence the lectin staining pattern. Staining of blots from electrophoretically separated muscle proteins obtained by homogenization, solubilization and centrifugation of small muscle pieces showed D. stramonium agglutinin binding to a number of bands ranging from 200 kDa to 30 kDa. No D. stramonium agglutinin positive bands were observed in blots from separated mitochondrial proteins while blots from sarcoplasmic reticulum separated by electrophoresis stained many bands in the range from 200 kDa to 30 kDa. It may be concluded that all muscle fibres in human and cat biceps hold intracellular non-sialylated complex-type oligosaccharides and further, that a specific tri-antennary complex-type glycoform is strongly expressed in type II A fibres as recognized by D. stramonium agglutinin. These results indicate a different glycosylation of certain myofibrillar-associated proteins in muscle fibre types.

Human prostatic smooth muscle cells in culture: estradiol enhances expression of smooth muscle cell-specific markers

Smooth muscle cells (SMCs) constitute a major cellular component of prostatic stroma. SMC tension plays an important role in urethral obstruction secondary to benign prostatic hyperplasia (BPH). We have developed an in vitro procedure for the propagation of human prostatic SMCs. Tissue specimens from patients undergoing radical prostatectomy or cystectomy were enzymatically disaggregated and cultured in MCDB-131 medium supplemented with horse serum, insulin, conditioned medium from the tumor cell line CRL-5813, and steroid hormones. The medium was assembled on the basis of the effects these supplements have on the growth of SMC cultures and on the expression of the two markers desmin and smooth muscle myosin. Addition of 0.1 microM of estradiol to the growth medium dramatically increased expression of these SMC-specific markers. Dihydrotestosterone (DHT) and hydrocortisone had a similar, albeit less pronounced effect. At three to five passages, about two thirds of the cells were immunohistologically positive for smooth muscle myosin or desmin. Almost all cells were positive for the myofibroblast marker smooth muscle alpha-actin throughout 10 passages and more. In SMC cultures, cells staining for smooth muscle myosin and desmin were found to seek direct contact to myofibroblasts. They grew in aggregates on a layer of myofibroblasts which adhered to the surface of the culture vessel. As revealed by transmission electron microscopy the cultured cells exhibited morphological features of myofibroblasts. Characteristics of smooth muscle cells, such as prominent bundles of microfilaments associated with dense bodies, basal laminae investing the cells, and numerous caveolae at the cell surfaces were regularly observed in cultures of low passages. After several passages, these features were markedly decreased and organelles of the biosynthetic system became more prominent. In summary, we present an in vitro model of prostatic SMCs and demonstrate that steroid hormones have characteristic effects on these cells. SMC cultures are expected to facilitate investigation of the functions and properties of human prostatic SMCs.

Actin is a target antigen of anti-neutrophil cytoplasmic antibodies (ANCA) in autoimmune hepatitis type-1

BACKGROUND/AIM

Anti-neutrophil cytoplasmic antibodies (ANCA) are a group of autoantibodies first associated with Wegener's granulomatosis and microscopic polyangiitis. The significance of ANCA in autoimmune hepatitis remains uncertain; the nature of the antigen or antigens has not been defined yet. The purpose of this study was to identify the target antigen of ANCA in patients with autoimmune hepatitis.

METHODS/RESULTS

Sera from 32 type-1 autoimmune hepatitis patients were used in the present study. ANCA were detected in 24 of 32 sera (75%). A diffuse cytoplasmic staining pattern (C-ANCA) was detected in 14 patients; the P-ANCA pattern was observed in 10 patients. An extract of human neutrophils was prepared and subjected to SDS-PAGE and Western Blot analysis. A 43-kD dominant immunoreactive protein was found in 20 (63%) autoimmune hepatitis patients. Aminoacid sequence analysis of the 43 kD protein identified actin. Cytoplasmic or perinuclear staining pattern could be reduced after absorption of sera with actin and after removing anti-actin antibodies by affinity chromatography. This was observed for all C-ANCA and for 8 out of 10 P-ANCA. Moreover in double-staining indirect immunofluorescence, the same type of diffuse cytoplasmic staining was observed with autoimmune hepatitis-sera and anti-actin antibodies. In Western Blot analysis with actin, 17 (53%) patients gave a positive result, while 15 (47%) patients had a positive actin-ELISA.

CONCLUSION

This is the first report to identify the cytoskeletal protein actin as an ANCA antigen.

Expression of HLA class I antigens in skeletal muscle is a diagnostic marker in juvenile dermatomyositis

HLA class I gene products are immunocytochemically detectable in the skeletal muscle of many patients with inflammatory conditions. We report three cases of juvenile dermatomyositis with minimal histologic abnormalities and no inflammation in the muscle biopsies. All three showed strong expression of HLA class I antigens on muscle fibers. Analysis of HLA class I expression is thus a useful marker for the diagnosis of inflammatory muscle disease, even in the absence of histologic changes.

Influence of PDGF-BB on proliferation and transition through the MyoD-myogenin-MEF2A expression program during myogenesis in mouse C2 myoblasts

We have previously demonstrated that PDGF-BB enhances proliferation of C2 myoblasts. This has led us to examine whether the mitogenic influence of PDGF-BB in the C2 model correlates with modulation of specific steps associated with myogenic differentiation. C2 myoblasts transiting through these differentiation specific steps were monitored via immunocytochemistry. We show that the influence of PDGF on enhancing cell proliferation correlates with a delay in the emergence of cells positive for sarcomeric myosin. We further monitored the influence of PDGF-BB on differentiation steps preceding the emergence of myosin+ cells. We demonstrate that mononucleated C2 cells first express MyoD (MyoD+/myogenin- cells) and subsequently, myogenin. Cells negative for both MyoD and myogenin (the phenotype preceding the MyoD+ state) were present at all times in culture and comprised the majority, if not all, of the cells which responded mitogenically to PDGF. Additionally, the frequency of the MyoD+/myogenin+ cell phenotype was reduced in cultures receiving PDGF, suggesting that PDGF can modulate the transition of the cells into the myogenin+ state. We determined that many of the myogenin+ cells subsequently become MEF2A+ and this phenomenon is not influenced by PDGF-BB. FGF-2 also enhanced the proliferation of C2 myoblasts and suppressed the appearance of the myogenin+ cells, but did not influence the subsequent transition into the MEF2A+ state. The study raises the possibility that PDGF-BB and FGF-2 might delay the transition of the C2 cells into the MyoD+/myogenin+ state by depressing a paracrine signal that enhances differentiation.

Old age does not affect shortening velocity or content of contractile and cytoskeletal proteins in the rat detrusor smooth muscle

The influence of old age on mechanical properties of the urinary bladder was investigated using smooth muscle strips from urinary bladders of control (14-16 weeks) and old-age (104 weeks) female Sprague-Dawley rats. Bladder weight of the aged rats had increased by about 30%. The maximal shortening velocity and stiffness in skinned activated urinary bladder fibers from old animals were unchanged compared to controls. The relative content of intermediate filament proteins to actin and the relative content of myosin to actin was unchanged. The concentration of myosin was unchanged (about 6.5 microg/mg wet weight). The results suggest that old age is not associated with pronounced changes in the cellular contractile and cytoskeletal proteins or in the mechanical properties of the contractile machinery. The age-related changes in mechanical properties previously reported for intact smooth muscle from urinary bladder are most likely due to alterations in the activation systems.

Myosin V in the retina: localization in the rod photoreceptor synapse

Brain myosin V is a member of a class of unconventional myosins. Using antibodies against brain myosin V, its distribution in the rat retina has been examined. It was detected in the inner and outer plexiform layers, in the somas of cells forming the inner nuclear layer, and in the ganglion cell layers. Faint labeling was also detected in the inner segments of the photoreceptor cells. In semithin retinal sections and isolated rod photoreceptor cells, a bright punctate labeling pattern was observed in the outer plexiform layer, corresponding to the rod photoreceptor synapses. The results found suggest that a major role of myosin V in the outer retina is in these synapses.

X-ray diffraction studies on thermally induced tension generation in rigor muscle

Muscle fibres in the rigor state and free of nucleotide contract if heated above their physiological working temperature. Kinetic studies on the mechanism of this process, termed rigor contraction, indicate that it has a number of features in common with the contraction of maximally Ca2+ activated fibres. De novo tension generation appears to be associated with a single, tension sensitive, endothermic step in both systems. Rigor contraction differs in that steps associated with crossbridge attachment and detachment are absent. We investigated structural changes associated with rigor contraction using X-ray diffraction. Overall changes in the low angle X-ray diffraction pattern were surveyed using a two-dimensional image plate. Reversible changes in the diffraction pattern included a 28% decrease in intensity of the 14.5 nm meridional reflection, a 12% increase in intensity of 5.9 nm actin layer-line and a somewhat variable 34% increase in intensity of 5.1 nm actin layer-line in laser temperature-jump experiments. When fibres were heated with a temperature ramp, we found that a 70% decrease in intensity of the myosin-related meridional reflection at (14.5 nm)-1 correlated with tension generation. A similar decrease in intensity of the 14.5 nm reflection is seen during tension recovery following a step change in the length of maximally Ca2+ activated fibres. Signals both from actin and actin-bound myosin heads contribute to the 5.1 and 5.9 nm actin layer-lines. Our observed changes in intensity are interpreted as contraction-associated changes in crossbridge shape and/or position on actin.

Myosin isoform transitions in four rabbit muscles during postnatal growth

Four rabbit muscles (i.e. semimembranosus proprius, psoas major, biceps femoris and longissimus lumborum), differing in their fibre type composition in the adult, were investigated during postnatal development. Muscle samples were taken at 1, 7, 14, 21, 28, 35, 49 and 77 days of age. Complementary techniques were used to characterize myosin heavy chain (MHC) isoform transitions, i.e. SDS-PAGE, immunocytochemistry and conventional histochemistry. Good accordance was found between electrophoretic and immunocytochemical techniques. Our results show that rabbit muscles were phenotypically immature at birth. At 1 day of age, perinatal isoform represented 70-90% of the total isoform content of the muscles. Two generations of myofibres could be observed on the basis of their morphology and reaction to specific antibodies. In all muscles, primary fibres expressed slow MHC. In contrast, secondary generation of fibres never expressed slow MHC in future fast muscles, while half of them expressed slow MHC in the future slow-twitch muscle, the semimembranosus proprius. During the postnatal period, all muscles displayed a transition from embryonic to perinatal MHC isoforms, followed by a transition from perinatal to adult MHC isoforms. These transitions occured mainly during the first postnatal month. The embryonic isoform was no longer expressed after 14 days, except in longissimus where it disappeared after 28 days. On the contrary, large differences were found in the timing of disappearance of the perinatal isoform between the four muscles. The perinatal isoform disappeared between 28 and 35 days in semimembranosus proprius and 35 and 49 days in psoas and biceps femoris. Interestingly, the perinatal isoform was still present in 6% of the fibres in longissimus at 77 days, the commercial slaughter age, denoting a great delay in the maturation. Fate of each generation of fibres differed between muscles.

Modulatory role of drebrin on the cytoskeleton within dendritic spines in the rat cerebral cortex

Morphological changes in the dendritic spines have been postulated to participate in the expression of synaptic plasticity. The cytoskeleton is likely to play a key role in regulating spine structure. Here we examine the molecular mechanisms responsible for the changes in spine morphology, focusing on drebrin, an actin-binding protein that is known to change the properties of actin filaments. We found that adult-type drebrin is localized in the dendritic spines of rat forebrain neurons, where it binds to the cytoskeleton. To identify the cytoskeletal proteins that associated with drebrin, we isolated drebrin-containing cytoskeletons using immunoprecipitation with a drebrin antibody. Drebrin, actin, myosin, and gelsolin were co-precipitated. We next examined the effect of drebrin on actomyosin interaction. In vitro, drebrin reduced the sliding velocity of actin filaments on immobilized myosin and inhibited the actin-activated ATPase activity of myosin. These results suggest that drebrin may modulate the actomyosin interaction within spines and may play a role in the structure-based plasticity of synapses.

Healing of incisional wounds in the embryonic chick wing bud: characterization of the actin purse-string and demonstration of a requirement for Rho activation

Small skin wounds in the chick embryo do not heal by lamellipodial crawling of cells at the wound edge as a skin wound does in the adult, but rather by contraction of an actin purse-string that rapidly assembles in the front row of epidermal cells (Martin, P., and J. Lewis. 1992. Nature (Lond.). 360:179-183). To observe the early time course of actin purse-string assembly and to characterize other cytoskeletal components of the contractile machinery, we have followed the healing of incisional or slash wounds on the dorsum of the chick wing; these wounds take only seconds to create and heal within approximately 6 h. Healing of the epithelium depends on a combination of purse-string contraction and zipper-like closure of the gap between the cut edges of the epithelium. Confocal laser scanning microscope studies show that actin initially aligns into a cable at the wound margin in the basal layer of the epidermis within approximately 2 min of wounding. Coincident with actin cable assembly, we see localization of cadherins into clusters at the wound margin, presumably marking the sites where segments of the cable in adjacent cells are linked via adherens junctions. A few minutes later we also see localization of myosin II at the wound margin, as expected if myosin is being recruited into the cable to generate a contractile force for wound healing. At the time of wounding, cells at the wound edge become transiently leaky, allowing us to load them with reagents that block the function of two small GTPases, Rho and Rac, which recently have been shown to play key roles in reorganiztion of the actin cytoskeleton in tissue-culture cells (Hall, A. 1994. Annu. Rev. Cell Biol. 10:31-54). Loading wound edge epidermal cells with C3 transferase, a bacterial exoenzyme that inactivates endogenous Rho, prevents assembly of an actin cable and causes a failure of healing. No such effects are seen with N17rac, a dominant inhibitory mutant Rac protein. These findings support the view that in this system the actin cable is required for healing-both the purse-string contraction and the zipping up-and that Rho is required for formation of the actin cable.

Isoforms of myosin in growing muscles of ky (kyphoscoliotic) mice

Kyphoscoliotic (ky) mice are spontaneous mutants of the BDL strain whose postural muscles atrophy during post-natal growth, resulting in extensive kyphoscoliosis in adult animals. At 21 days of age, the seven muscles examined were already well differentiated into fast, slow and mixed type on the basis of the proportions of their native myosin isoforms or their subunits. During post-natal growth, from 21 to 120 days of age, the normal pattern of myosin maturation was essentially respected by the ky mutation: fast muscles became faster, slow muscles became slower and mixed muscles specialized in both directions. However, the post-natal increases of myosin heavy chain 2B and fast myosin light chain LC3f were depressed in ky muscles, whilst there was novel expression of slow myosin light chains, LC1s and LC2s in muscles which normally did not express them. Intermediate native myosin IM was absent in adult ky soleus, but it increased in adult ky tibialis anterior. We conclude that the ky mutation depresses the normal post-natal transition towards faster muscles and results in adult muscles whose myosin isoforms are generally shifted in a fast-to-slow direction.