Distributions of vimentin and desmin in developing chick myotubes in vivo. II. Immunoelectron microscopic study

The evolution of vimentin and desmin in Pectoralis major muscles of broiler chickens supports their essential role in muscle regeneration

Vimentin (VIM) and desmin (DES) are muscle-specific proteins having crucial roles in maintaining the lateral organization and alignment of the sarcomeric structure during myofibrils’ regeneration. The present experiment was designed to ascertain the evolution of VIM and DES in Pectoralis major muscles (PM) of fast-growing (FG) and medium-growing (MG) meat-type chickens both at the protein and gene levels. MG broilers were considered as a control group whereas the evolution of VIM and DES over the growth period was evaluated in FG by collecting samples at different developmental stages (7, 14, 21, 28, 35, and 42 days). After performing a preliminary classification of the samples based on their histological features, 5 PM/sampling time/genotype were selected for western blot, immunohistochemistry (IHC), and gene expression analyses. Overall, the findings obtained at the protein level mirrored those related to their encoding genes, although a potential time lag required to observe the consequences of gene expression was evident. The two- and 3-fold higher level of the VIM-based heterodimer observed in FG at d 21 and d 28 in comparison with MG of the same age might be ascribed to the beginning and progressive development of the regenerative processes. This hypothesis is supported by IHC highlighting the presence of fibers to co-expressing VIM and DES. In addition, gene expression analyses suggested that, unlike VIM common sequence, VIM long isoform may not be directly implicated in muscle regeneration. As for DES content, the fluctuating trends observed for both the native protein and its heterodimer in FG might be ascribed to its importance for maintaining the structural organization of the regenerating fibers. Furthermore, the higher expression level of the DES gene in FG in comparison with MG further supported its potential application as a marker of muscle fibers’ regeneration. In conclusion, the findings of the present research seem to support the existence of a relationship between the occurrence of muscle regeneration and the growth rate of meat-type chickens and corroborate the potential use of VIM and DES as molecular markers of these cellular processes.

What does desmin do: A bibliometric assessment of the functions of the muscle intermediate filament

Intermediate filaments were first described in muscle in 1968, and desmin was biochemically identified about 10 years afterwards. Its importance grew after the identification of desminopathies and desmin mutations that cause mostly cardiopathies. Since its characterization until recently, different functions have been attributed to desmin. Here, we use bibliometric tools to evaluate the articles published about desmin and to assess its several putative functions. We identified the most productive authors and the relationships between research groups. We studied the more frequent words among 9734 articles (September 2021) containing "desmin" on the title and abstract, to identify the major research focus. We generated an interactive spreadsheet with the 934 papers that contain "desmin" only on the title that can be used to search and quantify terms in the abstract. We further selected the articles that contained the terms "function" or "role" from the spreadsheet, which we then classified according to type of function, organelle, or tissue involved. Based on the bibliographic analysis, we assess comparatively the putative functions, and we propose an alternative explanation for the desmin function.

Knockout of zebrafish desmin genes does not cause skeletal muscle degeneration but alters calcium flux

Desmin is a muscle-specific intermediate filament protein that has fundamental role in muscle structure and force transmission. Whereas human desmin protein is encoded by a single gene, two desmin paralogs (desma and desmb) exist in zebrafish. Desma and desmb show differential spatiotemporal expression during zebrafish embryonic and larval development, being similarly expressed in skeletal muscle until hatching, after which expression of desmb shifts to gut smooth muscle. We generated knockout (KO) mutant lines carrying loss-of-function mutations for each gene by using CRISPR/Cas9. Mutants are viable and fertile, and lack obvious skeletal muscle, heart or intestinal defects. In contrast to morphants, knockout of each gene did not cause any overt muscular phenotype, but did alter calcium flux in myofibres. These results point to a possible compensation mechanism in these mutant lines generated by targeting nonsense mutations to the first coding exon.

Distribution and Expression of Vimentin and Desmin in Broiler Pectoralis major Affected by the Growth-Related Muscular Abnormalities

Desmin (DES) and Vimentin (VIM) exert an essential role in maintaining muscle cytoarchitecture and since are considered reliable markers for muscle regeneration, their expression has been extensively investigated in dystrophic muscles. Thus, exhibiting features similar to those of human dystrophic muscles, the present study aimed at assessing the distribution of VIM and DES proteins and the expression of the corresponding genes in Pectoralis major muscles affected by white striping (WS), wooden breast (WB), and spaghetti meat (SM) abnormalities as well as in those having macroscopically normal appearance (NORM). For this purpose, 20 Pectoralis major muscles (5/group) were collected from the same flock of fast-growing broilers to perform immunohistochemistry, immunoblotting and gene expression. Immunohistochemical analyses showed an increased number of fibers immunoreactive to both VIM and DES in WS and WB, while only a few immunoreactive fibers were observed in NORM. Concerning the protein level, if compared with NORM, a 55% increase in VIM content was found in WB affected cases (P < 0.05) thus suggesting the development of intense regenerative processes in an early-stage within these muscles. The significantly higher amount of DES (+53%) found in WS might be attributed to a progression of the regenerative processes that require its synthesis to preserve the structural organization of the developing fibers. On the other hand, significantly lower VIM and DES contents were found in SM. About gene expression, VIM mRNA levels gradually increased from the NORM to the SM group, with significantly higher gene expressions in WB and SM samples compared to the NORM group (P = 0.009 for WB vs. NORM and P = 0.004 for SM vs. NORM). Similarly, the expression of DES gene showed an increase from the NORM to WB group (P = 0.05). Overall, the findings of the present study suggest that intense regenerative processes take place in both WB and WS muscles although a different progression of regeneration might be hypothesized. On the other hand, the lack of correspondence between VIM gene expression and its protein product observed in SM suggests that VIM may also exert a role in the development of the SM phenotype.

Secreted phospholipases A2 of snake venoms: effects on the peripheral neuromuscular system with comments on the role of phospholipases A2 in disorders of the CNS and their uses in industry

Neuro- and myotoxicological signs and symptoms are significant clinical features of envenoming snakebites in many parts of the world. The toxins primarily responsible for the neuro and myotoxicity fall into one of two categories--those that bind to and block the post-synaptic acetylcholine receptors (AChR) at the neuromuscular junction and neurotoxic phospholipases A2 (PLAs) that bind to and hydrolyse membrane phospholipids of the motor nerve terminal (and, in most cases, the plasma membrane of skeletal muscle) to cause degeneration of the nerve terminal and skeletal muscle. This review provides an introduction to the biochemical properties of secreted sPLA2s in the venoms of many dangerous snakes and a detailed discussion of their role in the initiation of the neurologically important consequences of snakebite. The rationale behind the experimental studies on the pharmacology and toxicology of the venoms and isolated PLAs in the venoms is discussed, with particular reference to the way these studies allow one to understand the biological basis of the clinical syndrome. The review also introduces the involvement of PLAs in inflammatory and degenerative disorders of the central nervous system (CNS) and their commercial use in the food industry. It concludes with an introduction to the problems associated with the use of antivenoms in the treatment of neuro-myotoxic snakebite and the search for alternative treatments.

The desmin coil 1B mutation K190A impairs nebulin Z-disc assembly and destabilizes actin thin filaments

Desmin intermediate filaments intimately surround myofibrils in vertebrate muscle forming a mesh-like filament network. Desmin attaches to sarcomeres through its high-affinity association with nebulin, a giant F-actin binding protein that co-extends along the length of actin thin filaments. Here, we further investigated the functional significance of the association of desmin and nebulin in cultured primary myocytes to address the hypothesis that this association is key in integrating myofibrils to the intermediate filament network. Surprisingly, we identified eight peptides along the length of desmin that are capable of binding to C-terminal modules 160-170 in nebulin. In this study, we identified a targeted mutation (K190A) in the desmin coil 1B region that results in its reduced binding with the nebulin C-terminal modules. Using immunofluorescence microscopy and quantitative analysis, we demonstrate that expression of the mutant desmin K190A in primary myocytes results in a significant reduction in assembled endogenous nebulin and desmin at the Z-disc. Non-uniform actin filaments were markedly prevalent in myocytes expressing GFP-tagged desmin K190A, suggesting that the near-crystalline organization of actin filaments in striated muscle depends on a stable interaction between desmin and nebulin. All together, these data are consistent with a model in which Z-disc-associated nebulin interacts with desmin through multiple sites to provide efficient stability to satisfy the dynamic contractile activity of myocytes.

Analysis of MTMR1 expression and correlation with muscle pathological features in juvenile/adult onset myotonic dystrophy type 1 (DM1) and in myotonic dystrophy type 2 (DM2)

Among genes abnormally expressed in myotonic dystrophy type1 (DM1), the myotubularin-related 1 gene (MTMR1) was related to impaired muscle differentiation. Therefore, we analyzed MTMR1 expression in correlation with CUG-binding protein1 (CUG-BP1) and muscleblind-like1 protein (MBNL1) steady-state levels and with morphological features in muscle tissues from DM1 and myotonic dystrophy type 2 (DM2) patients. Semi-quantitative RT-PCR for MTMR1 was done on muscle biopsies and primary muscle cultures. The presence of impaired muscle fiber maturation was evaluated using immunochemistry for neural cell adhesion molecule (NCAM), Vimentin and neonatal myosin heavy chain. CUG-BP1 and MBNL1 steady-state levels were estimated by Western blot. RNA-fluorescence in situ hybridization combined with immunochemistry for CUG-BP1, MBNL1 and NCAM were performed on serial muscle sections. An aberrant splicing of MTMR1 and a significant amount of NCAM-positive myofibers were detected in DM1 and DM2 muscle biopsies; these alterations correlated with DNA repeat expansion size only in DM1. CUG-BP1 levels were increased only in DM1 muscles, while MBNL1 levels were similar among DM1, DM2 and controls. Normal and NCAM-positive myofibers displayed no differences either in the amount of ribonuclear foci and the intracellular distribution of MBNL1 and CUG-BP1. In conclusion, an aberrant MTMR1 expression and signs of altered myofiber maturation were documented in both DM1 and in DM2 muscle tissues. The more severe dysregulation of MTMR1 expression in DM1 versus DM2, along with increased CUG-BP1 levels only in DM1 tissues, suggests that the mutual antagonism between MBNL1 and CUG-BP1 on alternative splicing is more unbalanced in DM1.

Diagnostic protein expression in human muscle biopsies

Using immunohistochemistry in diagnosing neuromuscular diseases is meant to enhance the diagnostic yield in two ways. The first application aims at visualizing molecules which are developmentally, neurally, and/or immunologically regulated and not expressed by normal muscle. They are upregulated in pathological conditions and may help assign a given muscular biopsy to one of the main diagnostic entities (muscular dystrophies, inflammatory myopathy, neurogenic atrophy). In the past, muscle-specific molecules with a defined expression pattern during fetal myogenesis served as antigens, with the rationale that the developmental program was switched on in new fibers. Recently, myofibers in diseased muscle are thought of as targets of stimuli which are released by macrophages in muscular dystrophy, by lymphocytes in inflammatory myopathies, or by a lesioned peripheral nerve in neurogenic atrophies. This has somewhat blurred the borders between the diagnostic groups, for certain molecules, e.g. cytokines, may be upregulated after experimental necrotization, denervation, and also in inflammatory myopathies. In the second part of this review we summarise the experiences of a Centre in the North of England that specialises in the diagnosis and clinical support of patients with muscular dystrophy. Emphasis is placed on the use of protein expression to guide mutation analysis, particularly in the limb-girdle muscular dystrophies (a group of diseases that are very difficult to differentiate on clinical grounds alone). We confirm that genetic analysis is essential to corroborate the results of protein analysis in certain conditions (particularly in calpainopathy). However, we conclude that analysing biopsies for abnormal protein expression is very useful in aiding the decision between alternative diagnoses.

Expression of late myogenic differentiation markers in sarcoplasmic masses of patients with myotonic dystrophy

Sarcoplasmic masses contain disorganized myofibrillar material and are a striking feature of myotonic dystrophy. However their significance is still unclear. Using immunocytochemistry we studied the expression of cytoskeletal proteins (desmin and vimentin), dystrophin, markers of myogenic differentiation (foetal myosin, neural cell adhesion molecule, bcl-2, insulin-like growth factor-I, fibroblast growth factor, retinoblastoma protein and myoD1), cell cycle regulators (Cdk2, p16, p27 and p57) and muscle proteases (ubiquitin, micro and m calpain and cathepsin D) in muscle biopsies from four patients with myotonic dystrophy. Sarcoplasmic masses were strongly positive for desmin, neural cell adhesion molecule, bcl-2, insulin-like growth factor I, retinoblastoma protein and p57, weakly positive for dystrophin and p16 and negative for vimentin, fibroblast growth factor, myoD1, Cdk2 and p27. Immunoreactivity for foetal myosin was detected only in a few fibres (< 1%). Our data suggest that the late myogenic differentiation programme is activated in sarcoplasmic masses although these areas do not reach complete maturation.

Developmental expression of Pop1/Bves

Initial studies have suggested that Pop1/Bves protein is exclusively expressed in the smooth muscle walls of the coronary vessels, implying its possible importance in coronary diseases. However, the mRNA and activity of this gene are detected in both skeletal and cardiac muscles, not coronary smooth muscle, and Pop1/Bves knockout mice have defects in skeletal muscle regeneration. Here we used specific monoclonal antibodies (MAbs) raised against chicken Pop1/Bves and demonstrated the presence of this protein in cardiomyocytes through development and its apparent absence in coronary vessels. Immunostaining of cardiomyocytes cultured in vitro confirmed the membrane localization of this protein in cells that participate in cell adhesion, with significant intracellular staining seen in isolated cells. In skeletal muscle, Pop1 protein becomes detectable at embryonic day (E) 7, coincident with the differentiation of morphologically distinct muscle masses from the limb muscle blastema, but the protein is not found at high levels in the cell membrane of myotubes until E11, coincident with the formation of secondary myotubes from satellite cells. These data support the hypothesis that Pop1/Bves is a cell adhesion molecule present in skeletal and cardiac muscle.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Muscle fibre breakdown in venom-induced muscle degeneration

We studied the early stages of the degeneration of skeletal muscles using the venom of Notechis scutatus as the myotoxic agent. The venom was used at a dose equivalent to the LD50 in the mouse. There was no mortality amongst the rats. Electron microscopy was used to show the progressive hypercontraction of sarcomeres and the loss of alignment of myofibrils in individual muscle fibres. Between areas of hypercontraction sarcomeres were torn, shedding loosened myofilaments into the cytosol. Western blotting and Coomassie staining were used to compare the respective rates of loss of desmin, titin, actin, myosin and dystrophin. We showed that desmin and titin were the first proteins to be degraded with a time to 50% loss of approximately 1 h and 3 h, respectively. The loss of major contractile proteins, myosin and actin, was rather slower. The loss of dystrophin was also slower than the loss of desmin and titin. Early damage to the plasma membrane of the muscle fibre caused the cells to depolarize, probably promoting the hypercontraction of the sarcomeres, but actual loss of membrane was incomplete even at 24 h. We suggest that the early degradation of desmin and titin was responsible for the disaggregation of the sarcomeres; the liberated contractile proteins myosin and actin were shed into the cytosol, where they were degraded. Phagocytic cells that had invaded the degenerating muscle fibres were primarily involved in the clearance of damaged mitochondria.

Metal ion trafficking in earthworms. Identification of a cadmium-specific metallothionein

Exposure to cadmium poses a considerable risk to human health and environmental safety. Earthworms reside in the most contaminated sites on earth, displaying a phenomenal tolerance to toxic heavy metals. They exhibit a distinct metabolic pathway that allows the bio-accumulation of cadmium to yield body burdens in excess of 1/1000th of total dry body weight, a most impressive figure by any standard. However, the precise molecular mechanism underlying this phenomenon remains to be unraveled. This study meets this challenge by fully characterizing the major metal-binding protein in earthworms, namely the two isoforms of metallothionein. Chemical analysis of recombinant protein showed that although both isoforms bind equimolar amounts of cadmium (6 mol), wMT-2 is more stable during proton competition. Furthermore, isoform-specific transcript analysis demonstrated that only wMT-2 is responsive to cadmium in a dose and temporal manner. The specific sequestration of cadmium to wMT-2 protein was confirmed in situ using polyclonal antisera. The latter also provided the means for mapping the cellular and intracellular distribution of metallothionein, thus yielding a holistic insight into its involvement in cadmium transit during absorption, storage, and excretion. The structure-function relationship of wMT-2 and its role in cadmium detoxification through sequestration and compartmentalization is discussed.

Placental alkaline phosphatase expression at the apical and basal plasma membrane in term villous trophoblasts

Human placental alkaline phosphatase (PLAP) was localized at the apical and basal plasma membrane of syncytiotrophoblasts and at the surface of cytotrophoblasts in term chorionic villi using immunoelectron microscopy. Similarly, apical and basolateral PLAP expression was found in polarized trophoblast-derived BeWo cells. Trophoblasts isolated from term placentas exhibited mainly vesicular PLAP immunofluorescence staining immediately after isolation. After in vitro differentiation into syncytia, PLAP plasma membrane expression was upregulated and exceeded that observed in mononuclear trophoblasts. These data call for caution in using PLAP as a morphological marker to differentiate syncytiotrophoblasts from cytotrophoblasts or as a marker enzyme for placental brush-border membranes. (J Histochem Cytochem 49:1155-1164, 2001)

Paranemin and the organization of desmin filament networks

De novo expression of vimentin, GFAP or peripherin leads to the assembly of an extended intermediate filament network in intermediate filament-free SW13/cl.2 cells. Desmin, in contrast, does not form extended filament networks in either SW13/cl.2 or intermediate filament-free mouse fibroblasts. Rather, desmin formed short thickened filamentous structures and prominent spot-like cytoplasmic aggregates that were composed of densely packed 9–11 nm diameter filaments. Analysis of stably transfected cell lines indicates that the inability of desmin to form extended networks is not due to a difference in the level of transgene expression. Nestin, paranemin and synemin are large intermediate filament proteins that coassemble with desmin in muscle cells. Although each of these large intermediate filament proteins colocalized with desmin when coexpressed in SW-13 cells, expression of paranemin, but not synemin or nestin, led to the formation of an extended desmin network. A similar rescue of desmin network organization was observed when desmin was coexpressed with vimentin, which coassembles with desmin, or with keratins, which formed a distinct filament network. These studies demonstrate that desmin filaments differ in their organizational properties from the other vimentin-like intermediate filament proteins and appear to depend upon coassembly with paranemin, at least when they are expressed in non-muscle cells, in order to form an extended filament network.

FluoroNanogold is a bifunctional immunoprobe for correlative fluorescence and electron microscopy

We applied a fluorescent ultrasmall immunogold probe, FluoroNanogold (FNG), to immunocytochemistry on ultrathin cryosections. FNG has the properties of both a fluorescent dye-conjugated antibody for fluorescence microscopy and a gold particle-conjugated antibody for electron microscopy. Therefore, this bifunctional immunoprobe permits correlative microscopic observation of the same cell profiles labeled in a single labeling procedure by these two imaging methods. We demonstrate the utility of FNG as a secondary antibody for immunocytochemical labeling of myeloperoxidase (a marker protein for azurophilic granules) in ultrathin cryosectioned human neutrophils. Its detection requires high spatial resolution because neutrophils contain many cytoplasmic granules. There was a one-to-one relationship between fluorescent structures labeled with FNG and organelle profiles labeled with the same silver-enhanced FNG in ultrathin cryosections. Use of FNG immunocytochemistry on ultrathin cryosections is an ideal methodology for high-resolution correlative fluorescence and electron microscopy and can provide unique information that may be difficult to obtain with a single imaging regimen.

The torpedo electrocyte: a model system to study membrane-cytoskeleton interactions at the postsynaptic membrane

Many aspects of the organization of the electromotor synapse of electric fish resemble the nerve-muscle junction. In particular, the postsynaptic membrane in both systems share most of their proteins. As a remarquable source of cholinergic synapses, the Torpedo electrocyte model has served to identify the most important components involved in synaptic transmission such as the nicotinic acetylcholine receptor and the enzyme acetylcholinesterase, as well as proteins associated with the subsynaptic cytoskeleton and the extracellular matrix involved in the assembly of the postsynaptic membrane, namely the 43-kDa protein-rapsyn, the dystrophin/utrophin complex, agrin, and others. This review encompasses some representative experiments that helped to clarify essential aspects of the supramolecular organization and assembly of the postsynaptic apparatus of cholinergic synapses.

NCAM, vimentin and neonatal myosin heavy chain expression in human muscle diseases

The intermediate filament protein vimentin, the neonatal isoform of the myosin heavy chain gene (MHCn), and the neural cell adhesion molecule (NCAM) are developmentally and/or neurally regulated molecules that reappear transiently after the induction of necrosis, or denervation. Immunostaining using antibodies against these molecules helps to identify regenerating and/or denervated muscle fibres even if they are not recognized by conventional staining procedures. This study examined the expression of vimentin, MHCn, and NCAM using immunohistochemistry in 82 biopsy specimens from muscular dystrophies, inflammatory myopathies, and neurogenic atrophies. Anti-vimentin labelled significantly more fibres than anti-MHCn staining in the inflammatory myopathies (P<0.03) but not in the muscular dystrophies (P=0.58) and neurogenic atrophies (P=0. 58). The fraction of NCAM+ fibres was always more elevated than vimentin+ or MHCn+ fibres. In the necrotizing myopathies, most NCAM+ fibres were regenerating ones (co-expressing vimentin). In neurogenic atrophies, half the NCAM+ fibres were regenerating and half of them were NCAM+/vimentin- and thus were considered to be denervated. Taken together, anti-vimentin staining detects a broader spectrum of regenerating fibres than anti-MHCn, at least in the inflammatory myopathies. The number of anti-NCAM labelled fibres in the necrotizing myopathies is similar, but not identical, to the number of regenerating fibres. Co-staining with anti-vimentin (or anti-MHCn) and anti-NCAM identifies a subset of fibres that is considered to be denervated.

Enhanced proliferation and migration and altered cytoskeletal proteins in early passage smooth muscle cells from young and old rat aortic explants

Smooth muscle cell (SMC) proliferation, migration, and cytoskeletal protein expression were studied in cultured cells obtained from the aortic explants of young (6-month) and old (30-month) Fischer 344XNB rats. Second-passage SMC were cultured on coverslips, and cytoskeletal fibers were examined by immunofluorescence microscopy using antibodies specific for smooth muscle myosin, alpha-smooth muscle actin, vimentin, desmin, and tubulin. The cytoskeletal fiber density was quantified as fluorescence intensity by confocal microscopy. The proliferation of SMC was analyzed from the growth curve of cells grown in culture from 0 to 14 days, and a Boyden chamber assay was used to quantify the SMC migration rate. The diameter of fresh SMC digested enzymatically from old rat aortae was 52.4% larger than that of the cells from young animals (20.0 +/- 3 microm vs 13.1 +/- 2 microm, P < 0.05). In SMC cultured from old animals, the intensities of smooth muscle myosin, alpha-smooth muscle actin, and vimentin decreased by 59.6, 41.2, and 54.8%, respectively; desmin and tubulin increased by 46.1 and 65.1% (all P < 0.001). Compared to SMC isolated from young rat aortae, the number of SMC cultured (second passage) from the old rat aorta was increased by 48.4, 27.2, and 26.9%, respectively, at Days 3, 7, and 14 in culture (P < 0.05, P < 0.01, and P < 0.001). The migration rate of SMC cultured from old rats was 59.3% higher than that of the cells obtained from young rats. These data show that alterations of the SMC cytoskeleton occur concomitantly with changes in SMC proliferation and migration rate during aging, suggesting that the age-associated changes in cytoskeletal proteins may play a role in remodeling of the aortic wall during aging.

Immunocytochemical colocalization of desmin and vimentin in human fetal skeletal muscle cells

BACKGROUND

Desmin and vimentin are the major intermediate filaments in muscle. They have been extensively studied in animal experiments. This study is the first to identify the distribution and to analyse the correlation of desmin and vimentin in human fetal skeletal muscle. Vimentin might be replaced by or transformed into desmin during myogenesis in chick embryo, although the precise process remains to be elucidated. The aim of this report is to evaluate the ratio of desmin to vimentin in human fetal muscle.

METHODS

By double-labeling immunoelectron microscopy, desmin and vimentin intermediate filaments were localized in developing skeletal muscles of 20-29-week-old human fetuses. The ratio of desmin and vimentin was analyzed statistically.

RESULTS

Two sizes of colloidal gold particles, 5 nm (vimentin) and 10 nm (desmin), were distributed along the intermediate filaments. The commonest distance between gold particles was approximately 40-50 nm. Desmin and vimentin labeled with gold particles were arranged very close together in the same intermediate filament. The ratio of vimentin to desmin varied but the amount of vimentin decreased progressively from the undifferentiated myoblast to the differentiated myocytes. As the fetuses developed, desmin increased and vimentin decreased. Desmin and vimentin intermediate filaments were identified in the intermyofibrils of differentiated myocytes, in subsarcolemmal space, and in myoblast.

CONCLUSIONS

Desmin and vimentin were colocalized in the same intermediate filaments. More vimentin existed in the less differentiated myocytes, although a small number of desmin filaments were already found in undifferentiated myoblasts. These intermediate filaments may not only connect myofibril bundles, cell organelles, and cell membrane but also provide a basis for myofibrillogenesis that is similar to relation between connective fibers and parenchymal cells.

Integration of intermediate filaments into cellular organelles

The intermediate filaments represent core components of the cytoskeleton and are known to interact with several membranous organelles. Classic examples of this are the attachment of keratin filaments to the desmosomes and the association of the lamin filament meshwork with the inner nuclear membrane. At this point, the molecular mechanisms by which the filaments link to membranes are not clearly understood. However, since a substantial body of information has been amassed, the time is now ripe for comparing notes and formulating working hypotheses. With this objective in mind, we review here pioneering studies on this subject, together with work that has appeared more recently in the literature.

Familial fetal akinesia deformation sequence with a skeletal muscle maturation defect

Two female siblings with the fetal akinesia deformation sequence (FADS) are described. Both showed facial anomalies, arthrogrypotic extremities, hypoplastic lungs, and fetal growth retardation. The central nervous system of the second sibling, including the spinal cord, was normal. The skeletal muscle was studied by immunohistochemistry for the expression of several myosin heavy chain isoforms, M-band proteins and intermediate filament proteins. The skeletal muscle was immature and atypical muscle spindles containing up to 31 intrafusal fibers were found. These findings suggest that a lethal FADS phenotype may involve a maturation defect of the skeletal muscle, and, in this family, may be inherited in a recessive fashion.

Immunocytochemistry of the muscle cell cytoskeleton

The muscle cell cytoskeleton is defined for this review as any structure or protein primarily involved in linking or connecting protein filaments to each other or to anchoring sites. In striated muscle, the M line connects thick filaments at their centers to adjacent thick filaments. Titin forms elastic filaments that extend from the M line to the Z line and may contribute to the resting tension properties of striated muscle. Nebulin forms inextensible filaments in skeletal muscle that are closely associated with thin filaments and that may provide a length template for thin filaments. Z lines anchor thin filaments from adjacent sarcomeres via the actin-binding function of alpha-actinin. Other proteins located at the Z line include Cap Z, Z-nin, Z protein, and zeugmatin. Intermediate filaments connect myofibrils to each other at the level of the Z line and to the sarcolemma at the Z- and possibly the M-line levels. Immunolocalization has identified the adhesion plaque proteins spectrin, vinculin, dystrophin, ankyrin, and talin at subsarcolemmal sites where they may be involved with filament attachment. Smooth muscle cell cytoskeletons are believed to include membrane associated dense bodies (MADBs), intermediate filaments, cytoplasmic dense bodies (CDBs), and perhaps a subset of actin filaments. MADBs contain a menu of attachment plaque proteins and anchor both thin filaments and intermediate filaments to the sarcolemma. CDBs are intracellular analogs of striated muscle Z lines and anchor thin filaments and intermediate filaments.

Disruption of intermediate filament organization leads to structural defects at the intersomite junction in Xenopus myotomal muscle

In mature striated muscle, intermediate filaments (IFs) are associated with the periphery of Z-discs and sites of myofibril-membrane attachment. Previously T. Schultheiss, Z. X. Lin, H. Ishikawa, I. Zamir, C. J. Stoeckert and H. Holtzer (1991) J. Cell Biol. 114, 953) reported that the disruption of IF organization in cultured chick myotubes had no detectable effect on muscle cell structure. Cultured muscle is not, however, under the mechanical loads characteristic of muscle in situ. The dorsal myotomal muscle (DMM) of the Xenopus tadpole provides an accessible model system in which to study the effects of mutant IF proteins on an intact, functional muscle. DNAs encoding truncated forms of Xenopus vimentin or desmin were injected into fertilized Xenopus eggs. Embryos were allowed to develop to the tadpole stage and then examined by confocal or electron microscopy. DMM cells containing the truncated IF polypeptides displayed disorganized IF systems. While the alignment of Z-lines appeared unaffected, cells accumulating mutant IF polypeptides displayed abnormal organization at the intersomite junction. Myocyte termini are normally characterized by deep invaginations of the sarcolemma. In myocytes expressing mutated IF polypeptides, these membrane invaginations were reduced or completely absent. Furthermore, the attachment of myofibrils to the junctional membrane was often aberrant or completely disrupted. These results suggest that in active muscle IFs play an important role in the organization and/or stabilization of myofibril-membrane attachment sites.

The expression of vimentin in satellite cells of regenerating skeletal muscle in vivo

The expression of the intermediate filament protein, vimentin, was studied in skeletal muscle during a cycle of degeneration and regeneration. Venom from the Australian tiger snake, Notechis scutatus scutatus, was used to initiate the breakdown of the soleus muscle of young, mature rats in vivo. Cryosections and Western blots of muscle samples were labelled using antibodies to vimentin, and examined at fixed time points after venom injection. Vimentin was absent in control adult muscle fibres, but was identified in activated satellite cells 12 h after venom assault. The amount of this protein rose during the early stages of regeneration, reaching its peak at 2-3 days. At this time, the expression of muscle-specific intermediate filament protein, desmin, began. As the abundance of desmin increased with the maturation of the regenerating myofibres, the abundance of vimentin declined until it was no longer detectable in mature regenerated fibres. It is suggested that vimentin plays an important role during satellite cell activation in the early stages of regeneration, and that the expression of vimentin may act as a stimulus for the expression of desmin at later stages of regeneration.

Differential organization of desmin and vimentin in muscle is due to differences in their head domains

In most myogenic systems, synthesis of the intermediate filament (IF) protein vimentin precedes the synthesis of the muscle-specific IF protein desmin. In the dorsal myotome of the Xenopus embryo, however, there is no preexisting vimentin filament system and desmin's initial organization is quite different from that seen in vimentin-containing myocytes (Cary and Klymkowsky, 1994. Differentiation. In press.). To determine whether the organization of IFs in the Xenopus myotome reflects features unique to Xenopus or is due to specific properties of desmin, we used the injection of plasmid DNA to drive the synthesis of vimentin or desmin in myotomal cells. At low levels of accumulation, exogenous vimentin and desmin both enter into the endogenous desmin system of the myotomal cell. At higher levels exogenous vimentin forms longitudinal IF systems similar to those seen in vimentin-expressing myogenic systems and massive IF bundles. Exogenous desmin, on the other hand, formed a reticular IF meshwork and non-filamentous aggregates. In embryonic epithelial cells, both vimentin and desmin formed extended IF networks. Vimentin and desmin differ most dramatically in their NH2-terminal "head" regions. To determine whether the head region was responsible for the differences in the behavior of these two proteins, we constructed plasmids encoding chimeric proteins in which the head of one was attached to the body of the other. In muscle, the vimentin head-desmin body (VDD) polypeptide formed longitudinal IFs and massive IF bundles like vimentin. The desmin head-vimentin body (DVV) polypeptide, on the other hand, formed IF meshworks and non-filamentous structures like desmin. In embryonic epithelial cells DVV formed a discrete filament network while VDD did not. Based on the behavior of these chimeric proteins, we conclude that the head domains of vimentin and desmin are structurally distinct and not interchangeable, and that the head domain of desmin is largely responsible for desmin's muscle-specific behaviors.

Desmin organization during the differentiation of the dorsal myotome in Xenopus laevis

The reorganization of desmin-type intermediate filaments during muscle differentiation has been studied primarily in cultured cell systems. Here we describe the process of desmin reorganization during the differentiation of the dorsal myotomal muscle of the clawed frog Xenopus laevis. This muscle differs from those described previously primarily in that the desmin system forms de novo, i.e., without the presence of a pre-existing vimentin filament system. The most striking observation is that prior to myotomal segmentation and rotation desmin is concentrated at the medial and lateral tips of the myocytes. It remains concentrated in these regions following somite rotation and is located primarily to the intersomite junctions as late as the stage 33-35 tadpole. As the muscle matures (stage 30 and later) desmin becomes increasingly associated with the sarcolemma and with the Z-discs. The concentration of desmin at the nascent intersomite junction suggests that desmin is involved in coupling somites to one another in the early Xenopus embryo.

Three-dimensional distributions of desmin and vimentin in cultured hamster cardiomyocytes using the immunogold deep-etching replica technique

The distributions of desmin and vimentin intermediate filaments in cultured hamster heart cells were examined by immunofluorescent microscopy and an immunogold deep-etching replica technique in combination with electron microscopy. Fluorescent studies showed the overall staining patterns of the myocytes as well as the fibroblasts. Monoclonal antibodies (Da, D3) to desmin showed punctate staining for the myocytes, while polyclonal desmin (pD) stained in a filamentous pattern. Fibroblasts stained strongly with monoclonal anti-vimentin (Va), but did not stain with the desmin probes. Deep-etched immunogold studies confirmed at the ultrastructural level that monoclonal anti-desmin antibodies stain individual intermediate filaments in an intermittent pattern. Monoclonal (D3) antibody stained the intermediate filaments heavily and continuously at the cell peripheries, while it stained intermittently in the cell body, similar to the Da monoclonal. Monoclonal anti-vimentin stained only intermediate filaments in fibroblasts. Our studies show a heterogeneity of staining within the cultured heart cells when various anti-desmin and anti-vimentin antibodies are used.

Rearrangement of mRNAs for costamere proteins during costamere development in cultured skeletal muscle from chicken

Mature skeletal myofibrils are surrounded by costameres, ribs of metavinculin, vinculin, intermediate filaments, and other proteins that connect the myofibril to the extracellular matrix. Costameres have recently been shown to be the sites at which the forces generated by the myofibril are transduced laterally into the extracellular matrix. We observed costameres developing in cultured skeletal muscles, grown in micromass culture from cells taken from embryonic chicken leg. We detected proteins by immunofluorescence and mRNA by in situ hybridization. Antibody and probe signals were imaged by laser scanning confocal microscopy. Antibody to vimentin protein is first detected in stripes in register with the Z line of the myofibril, at approximately day 12 after fusion; soon thereafter probe to vimentin mRNA is also detected in the same stripes. Optical sections indicate that vimentin mRNA and protein are very close, no more than 0.1 mm apart and possibly in immediate contact. Antibody to vimentin is detected in stripes only in cells that twitch spontaneously. Antibodies and probes to desmin and vinculin protein and mRNA are next detected in stripes of the same periodicity, at approximately day 17 after fusion. Vinculin protein (but not mRNA) is detected at focal contacts much earlier in development. Controls for bleed through of fluorescence, RNase H sensitivity, hybridization without probe, and binding to the myofibril all gave appropriate results. Probes to glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme, stained diffusely and did not associate with the myofibril. These results show that components of the costamere arrive at the structure in a defined sequence, and that mRNA organization is a conspicuous, precise and temporally controlled aspect of costamere development. These results may have wider implications. In these cells, some mRNAs are positioned with submicrometer precision in space and differentially over time. Particular mRNAs differ in the time and place of such positioning. This implies both that cellular structures provide physical cues for such positioning and that mRNA contains information that interacts with such cues in a message-specific manner. If such precision in mRNA location is found in other somatic cells, it could have significant implications for the ways in which cells generate and maintain cellular structures.

Desmin pathology in neuromuscular diseases

Desmin is an intermediate filament protein that in striated muscle is normally located at Z-bands, beneath the sarcolemma, and prominently at neuromuscular junctions. It is abundant during myogenesis and in regenerating fibers, but decreases in amount with maturation; in regenerating and denervated muscle fibers it is co-expressed with vimentin. Aggregates of desmin occur as nonspecific cytoplasmic bodies or cytoplasmic spheroid complexes, similar to the aggregates of keratin filaments in Mallory bodies or the neurofilament aggregates in Lewy bodies. In all three instances, alpha-B crystallin may be associated with desmin. There are now increasing numbers of neuromuscular disorders in which abnormal amounts of desmin, some abnormally phosphorylated, feature prominently in muscle fibres. Several of these diseases, including spheroid body myopathy, granulo-filamentous body myopathy and the dystrophinopathies, are familial. Ultrastructural and immunohistochemical studies of desmin have considerably broadened our understanding of the pathology of the cytoskeleton in muscle fibers and in certain hereditary neuromuscular diseases.

Desmin and vimentin as markers of regeneration in muscle diseases

Localization and distribution of desmin and vimentin have been studied in different neuromuscular disorders using monoclonal antibodies. We have demonstrated that vimentin, although virtually absent in normal human muscle fibers, is expressed in regenerating fibers in different neuromuscular disorders. Moreover, these fibers showed a strong positivity with desmin antibodies. In normal muscle fibers desmin is only localized at Z-line level. These results suggest that desmin and vimentin may be over-expressed during muscle regeneration processes, probably because of their importance in the structural organization of the sarcomere.

The fate of desmin and titin during the degeneration and regeneration of the soleus muscle of the rat

We studied the fate of desmin and titin in rat skeletal muscle during a cycle of degeneration and regeneration induced in vivo by the inoculation of a snake venom. Cryosections of muscle were labelled using antibodies to the two proteins, and examined at fixed time points after venom injection. Early pathological changes in the muscle, such as hypercontraction, preceded the loss of desmin. Immunolabelling using anti-desmin antibodies showed that desmin bridges were still intact when adjacent myofibrils were no longer aligned. The results suggested that although the hydrolysis of desmin is not necessary for the hypercontraction of muscle fibres, it probably contributes to complete fibre breakdown. Titin, or at least the part which lies close to the M-line, remained intact longer than desmin, but was also hydrolysed prior to complete disintegration of the fibres. Both desmin and titin were re-expressed in the regenerating myotubes by 2 days after venom inoculation, and became well organised even before the myofibrils became aligned. We conclude that desmin and titin are involved in both establishing and maintaining the structural integrity of the muscle fibres.

Sarcoplasmic-reticulum biogenesis in contraction-inhibited skeletal-muscle cultures

We have previously shown that inhibition of the spontaneous contractile activity of cultured embryonic-chick skeletal-muscle fibres with tetrodotoxin (TTX) leads to decreased sarcoplasmic-reticulum Ca(2+)-transport rates and steady-state concentrations of the high-energy Ca(2+)-ATPase phosphoenzyme intermediate [Charuk & Holland (1983) Exp. Cell Res. 144, 143-157]. In the present study we used a monoclonal antibody to the Ca(2+)-ATPase to show that there is a decreased amount of enzyme accumulated by contraction-inhibited myotubes. Indirect immunofluorescence microscopy using the monoclonal antibody to the Ca(2+)-ATPase also revealed a disordered subcellular organization of the sarcotubular system in contraction-inhibited myotubes. The biogenesis of sarcoplasmic-reticulum proteins in TTX-paralysed myofibres was studied by labelling cells with [35S]methionine before isolation of the active Ca(2+)-pump membrane fraction. Protein turnover was selectively increased in that fraction from TTX-treated muscle cultures. Electrophoretic analysis and quantitative fluorography confirmed that decreased accumulation of the Ca(2+)-ATPase enzyme in contraction-inhibited myotubes was associated with increased turnover of this protein. The present results demonstrate that biogenesis of the sarcoplasmic-reticulum Ca(2+)-ATPase is regulated by the contractile activity of skeletal-muscle fibres.

Desmin and vimentin in regenerating muscles

Desmin is a normal constituent of skeletal muscle fibers; vimentin is contained in myoblasts and connective tissue cells. The intracellular localization of both intermediate filament proteins in regenerating rat muscles was investigated by immunohisto- and immunocytochemistry. Necrosis was induced by hot Ringer solution. Desmin and vimentin were diffusely distributed in myoblasts and young myotubes. Both proteins became arranged in a sarcomeric fashion between the Z-lines when the sarcomeres got into register. Desmin reactivity persisted, but vimentin disappeared after about 2 weeks. Traces were found for up to 4 weeks. This observation is in contrast to findings in fetal and cultured muscles in which several authors did not find expression of vimentin after myoblast fusion. The presence of vimentin may well be a useful marker for regenerated muscle fibers in muscle biopsies from patients with neuromuscular disorders.

The distribution of desmin and titin in normal and dystrophic human muscle

We have used monoclonal antibodies to desmin and titin, and a combination of immunofluorescence and immunogold labelling to study the disposition of these two proteins in normal human muscle fibres and in fibres at various stages of degeneration in dystrophic muscle. The normal pattern of desmin labelling, in particular the subsarcolemmal labelling, became disrupted at an early stage of fibre breakdown. There was a change from a transverse to a longitudinal orientation of the labelled intermediate filaments as the myofibrils sheared relative to one another. Thus, while it is probable that the desmin filaments are able to play a role in the mechanical integration of the myofibrils in healthy muscle, our results suggest that they cannot withstand the excessive forces generated by the hypercontraction and stretching of dystrophic muscle. However, small accumulations of desmin persisted between the damaged myofibrils until necrosis reached an advanced stage. In general, the degradation of titin appeared to occur before the degradation of desmin, and at the ultrastructural level, labelling with antibodies to epitopes from parts of the titin molecule close to the A-I-band junction was lost before labelling with an antibody to an epitope in the A-band. This suggests that different regions of the titin molecule break down at different stages in the breakdown of the fibre. We propose that lysis of titin in the I-band may underlie 'slippage', an abnormality often seen in dystrophic muscle, in which the A-band slips to one pole of the sarcomere such that it abuts onto the Z-line. Breakdown of the A-band section of titin may facilitate the disassembly of the A-filaments.

Desmin/vimentin intermediate filaments are dispensable for many aspects of myogenesis

An expression vector was prepared containing a cDNA coding for a truncated version of the intermediate filament (IF) protein desmin. The encoded truncated desmin protein lacks a portion of the highly conserved alpha-helical rod region as well as the entire nonhelical carboxy-terminal domain. When transiently expressed in primary fibroblasts, or in differentiating postmitotic myoblasts and multinucleated myotubes, the truncated protein induces the complete dismantling of the preexisting vimentin or desmin/vimentin IF networks, respectively. Instead, in both cell types vimentin and desmin are packaged into hybrid spheroid bodies scattered throughout the cytoplasm. Despite the complete lack of intact IFs, myoblasts and myotubes expressing truncated desmin assemble and laterally align normal striated myofibrils and contract spontaneously in a manner indistinguishable from that of control myogenic cells. In older cultures the spheroid bodies shift from a longitudinal to a predominantly transverse orientation and loosely align along the I-Z-I-regions of striated myofibrils (Bennett, G.S., S. Fellini, Y. Toyama, and H. Holtzer. 1979. J. Cell Biol. 82:577-584), analogous to the translocation of intact desmin/vimentin IFs in control muscle. These results suggest the need for a critical reexamination of currently held concepts regarding the functions of desmin IFs during myogenesis.

Immunocytochemical characterisation of two generations of fibers during the development of the human quadriceps muscle

We have carried out a comprehensive study of the formation of muscle fibers in the human quadriceps in a large series of well dated human foetuses and children. Our results demonstrate that a first generation of muscle fibers forms between 8-10 weeks. These fibers all express slow twitch myosin heavy chain (MHC) in addition to embryonic and foetal MHCs, vimentin and desmin. Between 10-11 weeks, a subpopulation of these fibers express slow tonic MHC, being the first primordia of muscle spindles. Extrafusal fibers of a second generation form progressively and asynchronously around the primary fibers between 10-18 weeks, giving the muscle a very heterogeneous aspect due to different degrees of organization of their proteins. By 20 weeks, these second generation fibers become homogeneous and thereafter undergo a process of maturation and differentiation when they eliminate vimentin, embryonic and foetal MHCs to express either slow twitch or fast MHC. The differentiation of these second generation fibers into slow and fast depends upon different factors, such as motor innervation or level of thyroid hormone. Around the intrafusal first generation fibers, additional subsequent generations of fibers are also progressively formed. Some differ from the extrafusal second generation fibers by expressing slow tonic MHC, others by continuous expression of foetal MHC. The differentiation of intrafusal fibers is probably under the influence of both sensory and motor innervation.

Cytoskeletal filaments in embryonic chick myocardial cells as revealed by the quick-freeze deep-etch method combined with immunocytochemistry

The three-dimensional organization of cytoskeletal filaments associated with the myofibrils and sarcolemma of the myocardial cells of early chick embryos was studied by the rapid-freeze deep-etch method combined with immunocytochemistry. In the endoplasmic region of saponin-treated myocardial cells, 12-14 nm filaments formed a loose network surrounding nascent myofibrils. These 12-14 nm filaments attached to the myofibrils and some of them converged into Z disc regions. In the non-junctional cytocortical region thinner 8-11 nm filaments composed a dense network just beneath the sarcolemma. In myofibril terminating regions at the sarcolemma, i.e., the fascia adherens, 3-5 nm cross-bridges were observed among the thin filaments. In Triton-permeabilized and myosin subfragment 1 (S1)- treated samples, subsarcolemmal 8-11 nm filaments proved to be S1-decorated actin filaments under which there was a loose network of S1-undecorated filaments. Subsarcolemmal S1-decorated actin filaments had mixed polarity and attached to the sarcolemma at one end. A loose network of S1-undecorated filaments among myofibrils in the endoplasmic region was revealed to consist of desmin-containing intermediate filaments after immuno-gold staining for desmin. These networks connecting myofibrils with sarcolemma were assumed to play an important role in integrating and transmitting the contractile force of individual myofibrils within early embryonic myocardial cells.

Deep-etching immunogold replica electron microscopy of cytoskeletal elements in cultured hamster heart cells

A procedure has been developed for the three-dimensional immunoelectron microscopic localization of cytoskeletal filaments by a deep-etching replica method in combination with immunogold labeling and/or myosin subfragment 1 (S1) decoration techniques. Neonatal hamster heart cells grown on glass coverslips were extracted with Triton X-100 or physically permeabilized by breaking open the cell membranes. S1 decoration was performed on some specimens immediately after the permeabilization. After prefixation in formaldehyde, samples were immunostained with poly- or monoclonal antibodies to desmin or vimentin, and indirectly tagged with colloidal gold probes by the biotin-streptavidin method. After postfixation with glutaraldehyde, tannic acid and osmium tetroxide, the cells were freeze-etched and rotary-replicated with platinum and carbon in a freeze-fracture apparatus. Replicas were viewed with a transmission electron microscope using a tilting specimen stage to obtain stereo images. The procedure made it possible to identify the specific filaments within the complex cytoskeletal networks in cultured hamster heart muscle and nonmuscle cells at high resolution and in three dimensions. The method has advantages in its three-dimensionality and feasibility to evaluate the data by comparing them with those obtained by alternative light microscopic methods. Details of the protocol and a description of the results of using three different antibodies are given.

Extra actin filaments at the periphery of skeletal muscle myofibrils

Myofibrils isolated from a variety of vertebrate muscle fibers have a set of peripheral filaments associated with the periphery of the Z line free to move away from the surface of the myofibril. Decoration with myosin subfragment 1 shows that these are actin filaments.

Distribution of myosin heavy chain mRNA in embryonic muscle tissue visualized by ultrastructural in situ hybridization

We have localized myosin heavy chain (MHC) mRNAs in cells of intact embryonic chick muscle using high resolution in situ hybridization. Blocks of muscle were aldehyde-fixed prior to detergent treatment and hybridized with a biotinated cDNA probe, followed by colloidal gold-labeled antibodies, before embedment. Labeling was determined to represent MHC mRNA by extensive quantitative comparisons of electron micrographs from experimental and four different types of control samples. MHC mRNA was localized primarily to peripheral regions of 14-day chick pectoral muscle cells, where the majority of developing myofibrils were found. MHC mRNAs were consistently associated with the nonmyofibrillar cytoskeletal filaments which had diameters ranging from 4 to 10 nm. They were often oriented parallel to the longitudinal axis of the cell. The resolution of the ultrastructural approach allowed us to demonstrate that the mRNA molecules visualized were not directly associated with myofilaments, suggesting that nascent chains read from those messages do not assemble directly into myofilaments simultaneous with translation.

Assembly of amino-terminally deleted desmin in vimentin-free cells

To study the role of the amino-terminal domain of the desmin subunit in intermediate filament (IF) formation, several deletions in the sequence encoding this domain were made. The deleted hamster desmin genes were fused to the RSV promoter. Expression of such constructs in vimentin-free MCF-7 cells as well as in vimentin-containing HeLa cells, resulted in the synthesis of mutant proteins of the expected size. Single- and double-label immunofluorescence assays of transfected cells showed that in the absence of vimentin, desmin subunits missing amino acids 4-13 are still capable of filament formation, although in addition to filaments large numbers of desmin dots are present. Mutant desmin subunits missing larger portions of their amino terminus cannot form filaments on their own. It may be concluded that the amino-terminal region comprising amino acids 7-17 contains residues indispensable for desmin filament formation in vivo. Furthermore it was shown that the endogenous vimentin IF network in HeLa cells masks the effects of mutant desmin on IF assembly. Intact and mutant desmin colocalized completely with endogenous vimentin in HeLa cells. Surprisingly, in these cells endogenous keratin also seemed to colocalize with endogenous vimentin, even if the endogenous vimentin filaments were disturbed after expression of some of the mutant desmin proteins. In MCF-7 cells some overlap between endogenous keratin and intact exogenous desmin filaments was also observed, but mutant desmin proteins did not affect the keratin IF structures. In the absence of vimentin networks (MCF-7 cells), the initiation of desmin filament formation seems to start on the preexisting keratin filaments. However, in the presence of vimentin (HeLa cells) a gradual integration of desmin in the preexisting vimentin filaments apparently takes place.

Immunocytochemistry of intermediate filaments in cultured arterial smooth muscle cells: differences in desmin and vimentin expression related to cell of origin and/or plating time

The objective of this study was to determine whether intermediate filament expression, including desmin and vimentin, in cultured smooth muscle cells (SMCs) is related to cyto-differentiation or proliferation. Using antibodies to desmin and vimentin, we studied by immunoperoxidase technique the distribution of these proteins in subcultured SMCs derived from porcine aorta and coronary artery. In addition, the proliferative potentiality of the cells was estimated by the incorporation of [3H]thymidine into DNA. The frequency of desmin-positive cells in coronary arterial SMCs of 3 and 6 population doubling levels was significantly higher as compared to findings with the aortic SMCs and depended on the plating time. No difference was evident at the 12 population doubling level. In contrast, vimentin was present in the majority of both aortic and coronary arterial SMCs. With regard to the localization of vimentin, two cell types were observed, one had reaction products to vimentin in both perinuclear and cell-peripheral areas (type-I cell), the other only in the cell-peripheral region (type-II cell). The relative proportion of the type-I and -II cells varied with the period of culture. Most of the SMCs showed the type-I cell on the first day and the number of type-II cells was increased on the sixth day. Quiescent SMCs in serum-free media had the same percentage of desmin-positive cells and frequency distribution of type-I and -II cells as did the proliferating SMCs incubated in media containing 5% serum. These results suggest that intermediate filament expression, including desmin and vimentin in cultured SMCs, is related to cell origin and/or plating time, but not to the proliferating activity, per se.