Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Maturational differences between vascular and bladder smooth muscle during ovine development

Maturation rates of vascular and visceral smooth muscle (SM) during ovine development were compared by quantifying contractile protein, myosin heavy chain (MHC) isoform contents, and contractile properties of aortas and bladders from female fetal (n = 19) and postnatal (n = 21) sheep. Actin, myosin, and protein contents rose progressively throughout development in both tissues (P </= 0.003); however, expression patterns differed. During the last trimester, i. e., 101-130 days (term approximately 145 days), bladder actin and MHC contents were approximately twofold greater (P < 0.04) than those in the aorta. Although the fractional content of 204-kDa SM1 MHC in the bladder decreased from 74 +/- 3% at midgestation to 48 +/- 2% 3 mo postnatal, the aorta exhibited an increase from 30 +/- 2% to 65 +/- 2%. Bladder MHC (MHC-B) migrating at 200 kDa contained only SM2 throughout development. In contrast, 200-kDa MHC in the aorta was predominantly nonmuscle MHC-B at midgestation, which was gradually replaced by SM2 as development progressed. Along with its early expression of SM2, bladder muscle obtained maximal stress generating capacity (1.7 x 10(5) N/m(2)) by term gestation, whereas the aorta exhibited no contractions until after birth. We conclude that whereas aortic SM maturation is delayed until after birth, bladder SM matures biochemically and functionally during prenatal development, thus supporting early requirements for micturition.

Actomyosin interactions in a novel single muscle fiber in vitro motility assay

A novel in vitro motility assay has been developed to study the actomyosin interaction, in which the molecular motor protein myosin has been extracted and immobilized directly from 2-4 mm single rat skeletal muscle fiber segments. This method study was carried out to investigate: (1) the amount of myofibrillar proteins extracted from the fiber segment; (2) the effects of temperature on the speed at which actin is propelled by fast and slow myosin; and (3) the effects of myosin isoform expression on motility speed. Approximately 80% of the myosin and myosin-associated proteins were extracted from the fiber segments. while no significant extraction was shown of the thin filament proteins. Fluorescently labeled actin filaments moved with constant speed in a bi-directional motion over the high-density myosin region in the experimental chamber, and motility speed was highly dependent on the myosin heavy chain (MyHC) isoform extracted. At 25 degrees C, significant (P < 0.001) differences in motility speed were obtained between type I (1.31 +/- 0.23 microm/s, n = 11) and IIxb (5.81 +/- 0.35 microm/s, n = 6), or llb (6.07 +/- 0.33 microm/s, n = 8) MyHC isoforms. The motility speed and maximum velocity of unloaded shortening (V0) in single fibers were well correlated, indicating that filament speed is a good molecular analogue to contractile speed at the fiber level. The effects of temperature on filament motility speed were analyzed from 10 to 35 degrees C. The Q10 values, calculated in the 10-25 degrees C temperature range, differed between slow (4.20) and fast (2.38) myosin. In conclusion, this in vitro motility assay offers a unique possibility to compare the regulatory and modulatory influence of myosin isoforms and thin filament proteins on shortening velocity, at the cellular and molecular level in the same muscle fiber.

Morphological variability and specializations in bovine extraocular muscle spindles

Bovine extraocular muscles were examined to determine whether the structure of their muscle spindles was notably different from those commonly encountered in mammalian limb muscles. Extraocular muscle spindles on the whole were shorter, and intrafusal fiber counts/spindle were more variable than in somatic muscles. No pronounced nuclear bags were seen in intrafusal fibers. Based on cross-sectional areas, intrafusal fibers in extraocular muscles could be loosely categorized as small or large types. Small fibers expressed more neonatal/fast myosin heavy chain and less embryonic myosin heavy chain than large fibers. When incubated for myosin ATPase, about 70% of the large fibers and 15% of the small fibers in spindles presented profiles that were characteristic of type I extrafusal fibers, and not of nuclear bag or nuclear chain fibers. The ratio of number of small intrafusal fibers to number of large intrafusal fibers in extraocular spindles was on average greater than the ratio of nuclear chain fibers to nuclear bag fibers that is typical for limb spindles of rodents and cats. Structural modifications at muscle spindle sensory regions, extrafusal-like fibers and intrafusal-like fibers with few equatorial nuclei and many myofibrils, may produce distinct afferent signals that are appropriate for sensorimotor integration in the specialized extraocular muscles.

Nonmuscle myosin II localizes to the Z-lines and intercalated discs of cardiac muscle and to the Z-lines of skeletal muscle

To understand the role of nonmuscle myosin II in cardiac and skeletal muscle, we used a number of polyclonal antibodies, three detecting nonmuscle myosin heavy chain II-B (NMHC II-B) and two detecting NMHC II-A, to examine the localization of these two proteins in fresh-frozen, acetone-fixed sections of normal human and mouse hearts and human skeletal muscles. Results were similar in both species and were confirmed by examination of fresh-frozen sections of human hearts subjected to no fixation or to treatment with either 4% p-formaldehyde or 50% glycerol. NMHC II-B was diffusely distributed in the cytoplasm of cardiac myocytes during development, but after birth it was localized to the Z-lines and intercalated discs. Dual labeling showed almost complete colocalization of NMHC II-B with alpha-actinin. Whereas endothelial cells, smooth muscle cells and fibroblasts showed strong immunoreactivity for NMHC II-A and NMHC II-B, cardiac myocytes only showed reactivity for the latter. The Z-lines of human skeletal muscle cells, in contrast to those of cardiac myocytes, gave positive reactions for both NMHC II-A and NMHC II-B. The presence of a motor protein in the Z-lines and intercalated discs raises the possibility that these structures may play a more dynamic role in the contraction/relaxation mechanism of cardiac and skeletal muscle than has been previously suspected.

Lateral musculature in the whitemouth croaker (Micropogonias furnieri): its characterization with respect to different gonadal conditions

Histochemical and ultrastructural studies were performed on the lateral musculature from individual female whitemouth croaker, Micropogonias furnieri, at the anterior, medium and posterior regions. Based upon histochemical myosin-ATPase (m-ATPase) determination, diverse types of red, pink and white fibres were discerned. Red muscle had abundant mitochondria and stained intensely for aerobic enzymes, white muscle scarcely stained for the same enzymes and pink muscle responded in an intermediate manner. Both white and pink muscle had few mitochondria. The relative proportion of red muscle increased towards the caudal region; pink muscle diminished towards this region and white muscle modified its proportion only in the anterior region. m-ATPase activity showed differences in relation to the gonadal condition along the body, particularly in the white fibres at the anterior and medium regions.

A case of myasthenia gravis proven by ultrastructural study

Although light microscopic features of muscle are not pathognomonic in most cases of myasthenia gravis (MG), careful examination of neuromuscular junction by electron microscopy (EM) can reveal important clues for this disease. We report here a case of MG confirmed by EM study to emphasize that tissue diagnosis is still the best adjuvant to confirm the diagnosis. An 18-year-old female visited our hospital complaining of progressive muscle weakness for 3 years. She had difficulty in running, going upstairs and doing routine activities. Symptoms were aggravated with continuous work and resolved after rest. She had weakness of bilateral masseter and facial muscles and proximal portions of extremities without definite diurnal variation. Electromyography showed myopathic changes in proximal muscles of extremities. MG was considered but tensilon test was equivocal. Repetitive nerve stimulation tests revealed 20-30 percent decrease in responses to low and high rate stimulation. Muscle biopsy revealed selective type 2 atrophy. Ultrastructurally, abnormalities of neuromuscular junctions, i.e., wide primary synaptic cleft, and wide and shallow secondary synaptic clefts with mild myopathic features were present. These findings were pathognomonic for MG. Later, her symptoms were improved completely 3 months after thymectomy. The histologic finding of thymus was follicular hyperplasia.

Myofibrillar disruption in hypocontractile myocardium showing perfusion-contraction matches and mismatches

Chronically instrumented dogs underwent 2- or 5-h regional reductions in coronary flow that were followed, respectively, by balanced reductions in myocardial contraction and O(2) consumption ("hibernation") and persistently reduced contraction despite normal myocardial O(2) consumption ("stunning"). Previously unidentified myofibrillar disruption developed during flow reduction in both experimental models and persisted throughout the duration of reperfusion (2-24 h). Aberrant perinuclear aggregates that resembled thick filaments and stained positively with a monoclonal myosin antibody were present in 34 +/- 3.8% (SE) and 68 +/- 5.9% of "hibernating" and "stunned" subendocardial myocytes in areas subjected to flow reduction and in 16 +/- 2.5% and 44 +/- 7.4% of subendocardial myocytes in remote areas of the same ventricles. Areas of myofibrillar disruption also showed glycogen accretion and unusual heterochromatin clumping adjacent to the inner nuclear envelope. The degrees of flow reduction employed were sufficient to reduce regional myofibrillar creatine kinase activity by 25-35%, but troponin I degradation was not evident. The observed changes may reflect an early, possibly reversible, phase of the myofibrillar loss characteristic of hypocontractile myocardium in patients undergoing revascularization.

Presence and morphological variability of Golgi tendon organs in the distal portion of sheep extraocular muscle

This study was undertaken to demonstrate the presence of Golgi tendon organs (GTOs) in the distal portion of sheep extraocular muscle (EOM) and to describe the morphological variability of these receptors. Extraocular muscles of a young and an old sheep were perfusion fixed and/or immersion fixed. Tissue was prepared for light microscopy and transmission electron microscopy. Immunohistochemistry was done to demonstrate the myosin pattern of the intracapsular muscle fibers of the GTOs. All GTOs in the distal portions of the sheep EOMs were located in a distinct muscle layer which was designated in a former investigation as the so-called peripheral patch layer. Each EOM of the young sheep contained GTOs; between four and 15 GTOs were counted in the rectus EOMs. Eight GTOs were found in the superior rectus of the old sheep. Golgi tendon organs in EOMs of the young and the old sheep did not differ in their morphology. In the young sheep the mean length of the GTOs was 447 +/- 132 microm (n = 60) and their mean width 101 +/- 26 microm (n = 60). In the old sheep values were 576 +/- 188 microm (mean length, n = 8) and 103 +/- 18 microm (mean width, n = 8). The GTOs were encapsulated by perineurial cells. In 12 GTOs, only collagen bundles were inside. In the remaining GTOs (56), intracapsular muscle fibers were present. Muscle fibers entered the proximal poles of the GTOs and either terminated inside the receptors or muscle fibers left the GTOs at their distal poles. These intracapsular muscle fibers were of the multiply-innervated type. In the GTOs variably shaped nerve terminals were found which contained a high number of mitochondria. In two GTOs, additionally, nerve terminals with aggregates of densely packed vesicles were present.

Regulated von Willebrand factor secretion is associated with agonist-specific patterns of cytoskeletal remodeling in cultured endothelial cells

von Willebrand factor (vWF), an adhesive glycoprotein involved in primary hemostasis, is stored and released from endothelial secretory granules called Weibel-Palade bodies. Regulated secretion occurs in reaction either to [Ca(2+)](i)-raising agents (histamine or thrombin) or to cAMP-raising agents (epinephrine, adenosine, or forskolin). We investigated the pattern of release and the cytoskeletal requirements for secretion in response to these 2 classes of agonists. Secretion induced by [Ca(2+)](i)-raising agents involves peripheral and central granules and is inhibited by colchicine-induced microtubule disruption. It is accompanied by Rho-dependent stress fiber formation and cell retraction. Secretion and remodeling occur in the same individual cells. However, secretion is potentiated by cytochalasin E and C3 toxin, indicating that stress fiber formation antagonizes vWF secretion. In contrast, vWF secretion induced by cAMP-raising agents involves the release of only peripheral granules (implying less vWF release on a per cell basis) and is not inhibited by microtubule disruption. cAMP-mediated secretion is accompanied by disruption of stress fibers, strengthening of the cortical actin rim, and preservation of cell-cell contacts. It is unaffected by cytochalasins or C3 toxin. In contrast to [Ca(2+)](i)-raising agents, cAMP-raising agents induce secretion without cell retraction/intercellular gap formation. Thus, they are likely to play a physiological role in the regulation of endothelial vWF secretion and, therefore, of plasma vWF levels.

Co-localization of FAM and AF-6, the mammalian homologues of Drosophila faf and canoe, in mouse eye development

The Drosophila fat facets and canoe genes regulate non-neural cell fate decisions during ommatidium formation. We have shown previously that the FAM (fat facets in mouse) de-ubiquitinating enzyme regulates the function of AF-6, (mammalian canoe homologue), in the MDCK epithelial cell line (Taya et al., 1998. The Ras target AF-6 is a substrate of the fam de-ubiquitinating enzyme. J. Cell Biol. 142, 1053-1062). We report here that the expression of the FAM and AF-6 proteins overlaps extensively in the mouse eye from embryogenesis to maturity, especially in the non-neural epithelia including the retinal pigment epithelium, subcapsular epithelium of the lens and corneal epithelium. Expression is not limited to the epithelia however, as FAM and AF-6 also co-localize during lens fibre development as well as in sub-populations of the neural retina.

Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin distinguished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface loop suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins, and indeed a GST-loop construct binds actin. Probably as a consequence, binding of MyoK to actin was not only ATP- but also salt-dependent. Moreover, as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dynamic regions of the cortex. Absence of MyoK resulted in a drop of cortical tension whereas overexpression led to significantly increased tension. Absence and overexpression of MyoK dramatically affected the cortical actin cytoskeleton and resulted in reduced initial rates of phagocytosis. Cells lacking MyoK showed excessive ruffling, mostly in the form of large lamellipodia, accompanied by a thicker basal actin cortex. At early stages of development, aggregation of myoK null cells was slowed due to reduced motility. Altogether, the data indicate a distinctive role for MyoK in the maintenance and dynamics of the cell cortex.

Activity-induced fiber regeneration in rat soleus muscle

In an attempt to understand why muscle recovery is limited following atrophy due to limb immobilization, satellite cell activity and muscle fiber regeneration were analyzed in rat soleus muscles. Adult rat hindlimbs were immobilized in plaster casts for a period of two to ten weeks. Soleus muscles were examined by electron microscopy for evidence of fiber degeneration or regeneration, and to quantify satellite cell nuclei. Immunocytochemical localization of embryonic myosin was used to identify regenerating myofibers. Soleus muscle wet weight to body weight ratios for the casted muscles significantly decreased over the 10-week immobilization period. The casted muscles displayed ultrastructural evidence of minor fiber damage, including myofibrillar atrophy, Z-disc disruption, and abnormal triadic junctions. No ultrastructural evidence of regeneration was seen in the casted animals. The number of satellite cells in the casted muscles significantly decreased from 6.4% to 3. 3% by eight to 10 weeks of immobilization. Approximately 1.0% of extrafusal fibers in the control soleus muscles appeared to be regenerating since they expressed embryonic myosin and were of a small diameter, while in casted muscles, only 0.1% of the fibers were embryonic myosin-positive. Following release from immobilization, a reappearance of embryonic myosin-positive fibers was noted within four days of renewed activity. In contrast to control muscles, embryonic myosin-positive fibers in the recovery muscles included both small and large diameter fibers. Subtle changes in functional activity influence muscle damage and subsequent myofiber regeneration. Reduced activity reduces muscle fiber regeneration, while increased activity, as seen by increased hindlimb weight bearing and return to normal activity following immobilization, increase regenerating fibers and also the expression of embryonic myosin in adult fibers.

Novel myosin VI isoform is abundantly expressed in retina

Several forms of sensory deficit have been associated with unconventional myosin defects in humans and other animals. Normal hearing in mammals has been shown to require functional myosin VI (Avraham et al., 1995) and myosin VIIA (Gibson et al., 1995; Liu et al., 1997), and the combined blindness and deafness of Usher syndrome type IB has been shown to be produced by specific defects in myosin VIIA (Weil et al., 1997). Here we report the cloning and characterization of two distinct myosin VI isoforms (FMVIA and FMVIB) initially identified in a degenerate PCR screen of retinal cDNA from the striped bass, Morone saxatilis. Open reading frames for FMVIA and FMVIB encode predicted proteins of 1304 and 1270 amino acids respectively, which are 83% identical at the amino acid level. Both fish isoforms are likewise approximately 83-86% identical to mammalian class VI myosins (Hasson and Mooseker, 1994). Northern blot analysis revealed that FMVIA mRNA is broadly expressed and most abundant in kidney, a pattern similar to that previously reported for mammalian myosin VI. FMVIB expression is dramatically more abundant in retina than in any other tissue examined. Antibodies directed against pig myosin VI (Hasson and Mooseker, 1994) detect a doublet at approximately 150 kDa in bass retina and RPE. Since both fish VIA and VIB isoforms share high sequence identity with pig myosin VI within the domain used for antibody production, it seems likely that this antibody crossreacts with both FMVIA and FMVIB. Immunocytochemistry with this same affinity-purified rabbit anti-myosin VI antibody shows that myosin VI isoforms are primarily localized in photoreceptors, horizontal cells and Müller cells in both fish and primate retinas. This report is the first demonstration that two myosin VI genes are expressed in the same organism and the same cell type (RPE). The relatively high abundance of FMVIB expression in retina suggests that it may play an important role in retinal motility events.

Up-regulation of type XIX collagen in rhabdomyosarcoma cells accompanies myogenic differentiation

Rhabdomyosarcomas are known to recapitulate some of the early events in skeletal muscle embryogenesis, and cultures derived from these tumors have been extensively used to elucidate processes associated with the differentiation of primitive mesenchymal cells. These neoplasms have also provided important systems for studying different collagen types. This aspect is particularly relevant to type XIX collagen, which was originally identified from rhabdomyosarcoma cDNA clones. Although this collagen has been localized in vivo to basement membrane zones in a wide variety of tissues, including skeletal muscle, the tumor cells appear to be a unique source of its expression in vitro. We have found that one particular cell line-derived from a peritesticular embryonal rhabdomyosarcoma-produced relatively large amounts of type XIX collagen, especially in those rare instances in which these cells appear to spontaneously differentiate. To characterize this phenomenon, tumor cells were grown under conditions known to induce differentiation in normal myoblast cultures. In response to this treatment, the typical tumor cell morphology consistently and reproducibly switched from polygonal to round/spindle-shaped with the subsequent appearance of some structures resembling myotubes. Concurrently, the cultures commenced a dramatic up-regulation of type XIX collagen and skeletal muscle myosin heavy chain and alpha-actinin in a time-dependent fashion, whereas protein and mRNA levels of other matrix proteins were either decreased or unchanged. Moreover, immunocytochemical analysis revealed that only a subpopulation of the cells was responsible for the increased synthesis of type XIX collagen, alpha-actinin, and myosin, and that the same cells which stained positive for the collagen also stained positive for the muscle proteins. Taken together, the results suggested that type XIX collagen may be involved in the initial stages of skeletal muscle cell differentiation.

RhoA inactivation enhances endothelial barrier function

The modulation of endothelial barrier function is thought to be a function of contractile tension mediated by the cell cytoskeleton, which consists of actomyosin stress fibers (SF) linked to focal adhesions (FA). We tested this hypothesis by dissociating SF/FA with Clostridium botulinum exoenzyme C3 transferase (C3), an inhibitor of the small GTP-binding protein RhoA. Bovine pulmonary artery endothelial cell (EC) monolayers given C3, C3 + thrombin, thrombin, or no treatment were examined using a size-selective permeability assay and quantitative digital imaging measurements of SF/FA. C3 treatment disassembled SF/FA, stimulated diffuse myosin II immunostaining, and reduced the phosphotyrosine (PY) content of paxillin and 130- to 140-kDa proteins that included p125(FAK). C3-treated monolayers displayed a 60-85% decline in F-actin content and a 170-300% increase in EC surface area with enhanced endothelial barrier function. This activity correlated with reorganization of F-actin and PY protein(s) to beta-catenin-containing cell-cell junctions. Because C3 prevented the thrombin-induced formation of myosin ribbons, SF/FA, and the increased PY content of proteins, these characteristics were Rho dependent. Our data show that C3 inhibition of Rho proteins leads to cAMP-like characteristics of reduced SF/FA and enhanced endothelial barrier function.

A single-fiber in vitro motility assay. In vitro sliding velocity of F-actin vs. unloaded shortening velocity in skinned muscle fibers

We describe an approach that allows us to form a micro in vitro motility assay with as little myosin as can be retrieved from a short (approximately 10 mm) segment of a single skinned skeletal muscle fiber (diameter some 100 microm). Myosin is directly extracted from the single fiber segment by a high ionic strength solution in the presence of MgATP, and the extracted myosin is immediately applied to a miniaturized flow cell that has been pretreated with BSA. The observed sliding velocities of fluorescently labeled F-actin are essentially identical with those reported in the literature. Since at the single fiber level most muscle fibers contain only a single myosin heavy chain isoform this approach allows us to determine without additional purification steps, the sliding velocity driven by myosins with different heavy chain isoforms. In addition, this approach can be used to directly correlate under identical experimental conditions unloaded shortening velocity measured in segments of skinned muscle fibers with the in vitro sliding velocity of fluorescently labeled F-actin by extraction of myosin from the same skinned fibers. Such direct correlation was performed with different myosin heavy chain isoforms as well as at different temperatures and ionic strengths. Under all conditions studied, unloaded shortening velocity was 4- to 8-fold faster than sliding velocity in the motility assay even at high temperature (22 degrees C) and ionic strengths >50 mM. This suggests that sliding velocity in the motility assay is limited by additional factors beyond those thought to limit velocity of unloaded shortening in muscle fibers. One such factor might be unspecific ionic interactions between F-actin and the substrate in the motility assay resulting in somewhat higher sensitivity for ionic strength of sliding velocity in the motility assay. This might become of special relevance when using in vitro sliding velocity in assessing functional consequences of mutations involving charged residues of actin or myosin.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

[Development of new anti-atherosclerotic drugs based on the phenotypic modulation of vascular smooth muscle cells]

We have established a novel smooth muscle cell line (SVS) which retains the expression of specific markers for smooth muscle cells, such as smooth muscle myosin-1, calponin and SM22 alpha, from temperature-sensitive SV40 large T-antigen transgenic mice. SVS cells showed temperature-dependent growth and the expression of smooth muscle markers, showing that the differentiation stage can be controlled in this cell line by culture temperature. We have constructed luciferase-reporter cell lines for calponin and SM22 alpha genes by using SVS cells. We detected some compounds including ascorbic acid, which stimulated the expression of smooth muscle markers by the reporter assay. Ascorbic acid and its long lasting derivatives stimulated the expression of markers continuously. Moreover, ascorbic acid activated the expression of markers in synthetic SMCs seen in the intima of a rat balloon injury model. Thus, this reporter system should be useful for discovery of new antiatherosclerotic drugs.

Contraction parameters, myosin composition and metabolic enzymes of the skeletal muscles of the etruscan shrew Suncus etruscus and of the common European white-toothed shrew Crocidura russula (Insectivora: soricidae)

In the Etruscan shrew, the isometric twitch contraction times of extensor digitorum longus (EDL) and soleus muscles are shorter than in any other mammal, allowing these muscles to contract at outstandingly high contraction frequencies. This species has the highest mass-specific metabolic rate of all mammals and requires fast skeletal muscles not only for locomotion but also for effective heat production and for an extremely high ventilation rate. No differences could be detected in the fibre type pattern, the myosin heavy and light chain composition, or in the activity of the metabolic enzymes lactate dehydrogenase and citrate synthase of the two limb muscles, the EDL and the soleus, which in larger mammalian species exhibit distinct differences in contractile proteins and metabolic enzymes. All properties determined in EDL and soleus muscles of Suncus etruscus, as well as in the larger Crocidura russula, are typical for fast-oxidative fibres, and the same holds for several other skeletal muscles including the diaphragm muscle of S. etruscus. Nevertheless, the EDL and soleus muscles showed different mechanical properties in the two shrew species. Relaxation times and, in C. russula, time to peak force are shorter in the EDL than in the soleus muscle. This is in accordance with the time course of the Ca(2+) transients in these muscles. Such a result could be due to different parvalbumin concentrations, to a different volume fraction of the sarcoplasmic reticulum in the two muscles or to different Ca(2+)-ATPase activities. Alternatively, the lower content of cytosolic creatine kinase (CK) in the soleus compared with the EDL muscle could indicate that the observed difference in contraction times between these shrew muscles is due to the CK-controlled activity of their sarcoplasmic reticulum Ca(2+)-ATPase.

Characterization of myosin-A and myosin-C: two class XIV unconventional myosins from Toxoplasma gondii

Two class XIV unconventional myosins from Toxoplasma gondii, Myosin-A (TgM-A) and Myosin-C (TgM-C), were characterized in terms of their biochemical properties and their expression in quiescent and motile stages of the parasite life cycle. In cell fractionation studies, both myosins partitioned with the major organelle/cell membrane fraction, and extraction studies indicated that both were tightly associated with membrane domains as detergent was necessary for their solubilization. In addition, both TgM-A and TgM-C demonstrated a hallmark feature of myosins in their ability to bind actin in the absence but not the presence of ATP. In parasites residing within the host cell parasitophorous vacuole, TgM-A was detected by immunofluorescence microscopy as a bright spot near the apical pole of the parasite. This pattern underwent a subtle change as the parasites became motile, with TgM-A then localizing more intimately with the parasite cell membrane domain in apically disposed spots or patches, consistent with the role of this myosin in gliding motility. TgM-C showed a distinct localization to the juxtanuclear region towards the apical pole of the parasite, consistent with an association with the Golgi apparatus.

Undecagold cluster labeling of proteins at reactive cysteine residues

Undecagold cluster labeling of reactive cysteine residues in numerous proteins has allowed the labeled sites to be identified by electron microscopy, providing high-resolution information on the location and orientation of subunits in oligomeric enzymes, virus capsids, crystalline sheets of membrane proteins, and muscle thin filaments. The range of applications of undecagold cluster labeling has been greatly extended by the availability of site-directed mutagenesis to introduce cysteine residues at sites of interest. In this paper I discuss factors that can influence the extent and specificity of labeling, methods for biochemical analysis of undecagold-labeled proteins, and the effects of undecagold cluster labeling on biological activity.

Correlation between histochemically assessed fiber type distribution and isomyosin and myosin heavy chain content in porcine skeletal muscles

Highly sensitive enzyme assays developed to differentiate skeletal muscle fibers allow the recognition of three main fiber types: slow-twitch oxidative (SO), fast-twitch oxidative glycolytic (FOG), and fast-twitch glycolytic (FG). Myosin, the predominant contractile protein in mammalian skeletal muscle, can be separated based on the electrophoretic mobility under nondissociating conditions into SM2, SM1, IM, FM3, and FM2 isoforms, or under dissociating conditions into myosin heavy chain (MHC) I, IIb, IIx/d, and IIa. The purpose of the present study was to determine whether the histochemical method of differentiation of fiber types is consistent with the electrophoretically identified isomyosin and MHC isoforms. These comparisons were made using serratus ventralis (SV), gluteus medius (GM), and longissimus muscles (LM) from 13 pigs. Two calculation methods for the histochemical assessed fiber type distribution were adopted. The first method incorporated the number of fibers counted for each fiber type and calculated a percentage of the total fiber number (fiber number percentage: FNP). The second method expressed the cross-sectional area of each fiber type as a percentage of the total fiber area measured per muscle (fiber area percentage: FAP). Independent of the calculation methods, correlation analyses revealed in all muscles a strong relation between SO fibers, the slow isomyosin (SM1 and SM2), and MHCI, as well as between the FG fibers, the fast isomyosin (FM3 and FM2), and MHCIIx/b content (P<.05). There were no correlations between FOG fiber population assessed by histochemical analysis and intermediate isoform (IM) or MHCIIa content. The present results did not provide conclusive evidence as to which of the calculation methods (FNP or FAP) was more closely related to myosin composition of skeletal muscles. Despite some incompatibility between the methods, the present study shows that histochemical as well as electrophoretic analyses yielded important information about the composition of porcine skeletal muscle. The combination of the two methods may be essential to accurately characterize porcine skeletal muscles.

Thick filament assembly occurs after the formation of a cytoskeletal scaffold

The development of myofibrils involves the formation of contractile filaments and their assembly into the strikingly regular structure of the sarcomere. We analysed this assembly process in cultured human skeletal muscle cells and in rat neonatal cardiomyocytes by immunofluorescence microscopy using antibodies directed against cytoskeletal and contractile proteins. In particular, the question in which temporal order the respective proteins are integrated into developing sarcomeres was addressed. Although sarcomeric myosin heavy chain is expressed as one of the first myofibrillar proteins, its characteristic A band arrangement is reached at a very late stage. In contrast, titin, then myomesin and finally C-protein (MyBP-C) gradually form a regularly arranged scaffold on stress fiber-like structures (SFLS), on non-striated myofibrils (NSMF) and on nascent striated myofibrils (naSMF). Immediately subsequent to the completion of sarcomere cytoskeleton formation, the labeling pattern of myosin changes from the continuous staining of SFLS to the periodic staining characteristic for mature myofibrils. This series of events can be seen most clearly in the skeletal muscle cell cultures and--probably due to a faster developmental progression less well in cardiomyocytes. We therefore conclude that the correct assembly of a cytoskeletal scaffold is a prerequisite for correct thick filament assembly and for the integration of the contractile apparatus into the myofibril.