Inhibition of PAI-1 expression in breast cancer carcinoma cells by siRNA at nanomolar range

Plasminogen activator inhibitor type I (PAI-1) plays a central role in metastatic behavior by increasing cells' migratory capacities as shown in several tumoral cell lines. Moreover, in vivo high expression of this factor helps tumoral growth, both by its role in extracellular matrix remodeling and by favoring angiogenesis. High levels of PAI-1 are correlated with bad prognosis in several cancers, particularly in breast cancer. The effect of PAI-1 upon angiogenesis is also involved in atherosclerosis, in which high levels of PAI-1 expression are observed. Breast carcinoma MDA MB 231 cells are known for both having important metastatic capacities and expressing high levels of PAI-1. We have demonstrated in these cells that the transfection of PAI-1 specific small interfering RNAs (siRNA) specifically inhibited the expression of this factor by 91%. We evaluated siRNA activity by determining PAI-1 mRNA level, as well as intracellular and extracellular PAI-1 protein by using RT Q-PCR, Western blot and ELISA analyses, respectively. Data confirmed inhibition at mRNA levels (primary aim of interference), intracellular protein, and secreted PAI-1, the latter being operative successfully in the cell microenvironment. The lipidic vector Delivery Liposomes System (DLS) used was adapted to siRNA delivery as observed by particle size distribution analysis, confocal microscopy and transfection into MDA MB 231, in the presence of serum. SiRNA activity was clearly detected at concentrations as low as 10 nM. Moreover, the low cytotoxicity of this vector makes it a good candidate for future in vivo siRNA delivery.

Involvement of the [uPAR:uPA:PAI-1:LRP] complex in human myogenic cell motility

The urokinase-type plasminogen activator system is a proteolytic system involved in tissue remodeling and cell migration. At the cell surface, receptor (uPAR)-bound urokinase (uPA) binds its inhibitor PAI-1, localized in the matrix, and the complex is internalized by endocytic receptors, such as the low-density lipoprotein receptor-related protein (LRP). We previously proposed a nonproteolytic role for the uPA system in human myogenic cell differentiation in vitro, i.e., cell fusion, and showed that myogenic cells can use PAI-1 as an adhesion matrix molecule. The aim of this study was to define the role of the uPA system in myogenic cell migration that is necessary for fusion. Using a two-dimensional motility assay and microcinematography, we showed that any interference with the [uPAR:uPA:PAI-1] complex formation, and interference with LRP binding to this complex, markedly decreased myogenic cell motility. This phenomenon was reversible and independent of plasmin activity. Inhibition of cell motility was associated with suppression of both filopodia and membrane ruffling activity. [uPAR:uPA:PAI-1:LRP] complex formation involves high-affinity molecular interactions and results in quick internalization of the complex. It is likely that this complex supports the membrane ruffling activity involved in the guidance of the migrating cell toward appropriate sites for attachment.

Differential expression of the IL-1 system components during in vitro myogenesis: implication of IL-1beta in induction of myogenic cell apoptosis

We evaluated the expression of IL-1 system by normal human myogenic cells during in vitro myogenesis and the effect of exogenous IL-1beta. Expression of IL-1alpha and beta, IL-1 receptor antagonist (IL-1Ra), IL-1RI and II, IL-1R accessory protein (IL-1RAcP) and IL-1beta converting enzyme (ICE) was studied by immunocytochemistry, immunoblotting, ELISA and RT - PCR. Cell proliferation was evaluated by [3H]thymidine incorporation, cell fusion by flow cytometry and cell death by in situ end-labelling. Human normal myogenic cells constitutively produced IL-1beta and ICE, with a maximum expression at time of cell fusion. IL-1Rs and IL-1RAcP expression reached a peak at time of commitment to fusion. Myogenic cells produced small amounts of IL-1Ra at latest stages of culture, and only the intracellular isoform. Exposure of cultures to exogenous IL-1beta (1-5 ng/ml) induced myogenic cell apoptosis, without effect on cell proliferation or fusion. IL-1beta-induced cell death was associated with morphological changes including spreading appearance of cells and alteration of cell alignment. We conclude that (1) human myogenic cells constitutively produce IL-1beta; (2) IL-1 system components are differentially expressed during in vitro myogenesis; (3) IL-1 system participates to the coordinated regulation of cell density during normal myogenesis, which could serve to control the muscle mass in vivo.

In vitro evaluation of human muscle satellite cell migration prior to fusion into myotubes

We developed a short-time assay to evaluate muscle satellite cell migration, based on the fact that during myogenic differentiation, myoblasts migrate preferentially towards high cellular density areas where myotubes would form. This assay consists of a computer-assisted count of cells within a randomly chosen field, performed every hour for eight hours, and compared with the cell number at the start time of the experiment. Nine primary myoblast cultures were tested in triplicate. The method relies on several requisites. (1) Negligible cell proliferation: cell division was nearly absent in 8 h experiments. (2) Directional cell movement: a major flow of cells, either entering or exiting the fields, was constantly observed. 'Counter-flows', detected by visual counting, involved minor percentages of cells. (3) Constant migration rate: a linear increase in cell count variations over 8 h and a very high degree of intra-assay homogeneity were observed. Individual primary cell culture characteristics (depending on characteristics of the different donors) were the sole factor with a significant impact on migration rate. Automatic cell counting conveniently assessed the inhibitory effect of GRGDTP, an inhibitor of integrin-mediated cell adhesion. The method described here is rapid, does not require heavy equipment, and allows studies under serum-free conditions required to test molecules interfering with cell migration, in the course of the in vitro myogenic process.

Differential myogenicity of satellite cells isolated from extensor digitorum longus (EDL) and soleus rat muscles revealed in vitro

Following muscle damage, fast- and slow-contracting fibers regenerate, owing to the activation of their satellite cells. In rats, crush-induced regeneration of extensor digitorum longus (EDL, a fast muscle) and soleus (a slow muscle) present different characteristics, suggesting that intrinsic differences exist among their satellite cells. An in vitro comparative study of the proliferation and differentiation capacities of satellite cells isolated from these muscles is presented there. We observed several differences between soleus and EDL satellite cell cultures plated at high density on gelatin-coated dishes. Soleus satellite cells proliferated more actively and fused into myotubes less efficiently than EDL cells. The rate of muscular creatine kinase enzyme appeared slightly lower in soleus than in EDL cultures at day 11 after plating, when many myotubes were formed, although the levels of muscular creatine kinase mRNA were similar in both cultures. In addition, soleus cultures expressed higher levels of MyoD and myogenin mRNA and of MyoD protein than EDL satellite cell cultures at day 12. A clonal analysis was also carried out on both cell populations in order to determine if distinct lineage features could be detected among satellite cells derived from EDL and soleus muscles. When plated on gelatin at clonal density, cells from both muscles yielded clones within 2 weeks, which stemmed from 3-15 mitotic cycles and were classified into three classes according to their sizes. Myotubes resulting from spontaneous fusion of cells from the progeny of one single cell were seen regardless of the clone size in the standard culture medium we used. The proportion of clones showing myotubes in each class depended on the muscle origin of the cells and was greater in EDL- than in soleus-cell cultures. In addition, soleus cells were shown to improve their differentiation capacity upon changes in the culture condition. Indeed, the proportions of clones showing myotubes, or of cells fusing into myotubes in clones, were increased by treatments with a myotube-conditioned medium, with phorbol ester, and by growth on extra-cellular matrix components (Matrigel). These results, showing differences among satellite cells from fast and slow muscles, might be of importance to muscle repair after trauma and in pathological situations.

Interleukin-1 expression in normal motor endplates and muscle fibers showing neurogenic changes

Histological features of neurogenic muscle involvement include type grouping, muscle fiber atrophy and target fibers. In zidovudine-induced myopathy and dermatomyositis, immunoreactivity for interleukin (IL)-1 has been reported in diseased muscle fibers involving myofibrillar breakdown and atrophy. Since IL-1 is a signal for muscle proteolysis, we studied myofiber expression of IL-1 in neurogenic muscle involvement, specially in atrophic myofibers and target fibers which are associated with myofilament breakdown. Muscle biopsy samples from patients with normal (5 cases) or neurogenic muscle involvement (25 cases) were studied by enzyme histochemistry and immunohistochemistry. In normal muscles, immunoreactivity for IL-1beta was restricted to the postsynaptic domain of motor endplates and that for IL-1alpha had a similar localization but was faint. Immunoreactivity for IL-1alpha and -beta was observed, respectively, in 42.5% and 75.5% of target fibers, in 8.5% and 10.4% of dark angulated fibers, in 0% and 0.3% of non-atrophic type-grouped fibers, in 14.2% and 16.5% of moderately atrophic fibers, and in 65% and 20.9% of severely atrophic fibers. Immunoblot study showed the presence of both proIL-1 (31 kDa) and mature IL-1 (17.5 kDa). From this study, we conclude that IL-1 is normally expressed in the muscular domain of neuromuscular junctions; that IL-1 is mainly expressed in neurogenic target fibers; and that IL-1 expression by muscle fibers in pathological conditions seems to be associated with myofibrillar protein breakdown and regeneration.

Cellular and mitochondrial toxicity of zidovudine (AZT), didanosine (ddI) and zalcitabine (ddC) on cultured human muscle cells

Zidovudine (AZT), didanosine (ddI) and zalcitabine (ddC) are the reference antiretroviral therapy in patients with AIDS. A toxic mitochondrial myopathy can be observed in patients treated with AZT, but not with ddI and ddC. All 3 compounds can inhibit mitochondrial (mt)DNA polymerase and cause termination of synthesis of growing mtDNA strands and mtDNA depletion. The propensity to injure particular target tissues is unexplained. In our work, cultured muscle cells prepared from human muscle biopsies, were exposed to various concentrations of AZT (4-5000 micromol/l), ddI (5-1000 micromol/l) and ddC (1-1000 micromol/l) for 10 days. We evaluated cell proliferation and differentiation and measured lipid droplet accumulation, lactate production and respiratory chain enzyme activities. All 3 compounds induced a dose-related decrease of cell proliferation and differentiation. AZT seemed to be the most potent inhibitor of cell proliferation. AZT, ddI and ddC induced cytoplasmic lipid droplet accumulations, increased lactate production and decreased activities of COX (complex IV) and SDH (part of complex II). NADHR (complex I) and citrate sinthase activities were unchanged. Zalcitabine (ddC) and, to a lesser extent, ddI, were the most potent inhibitors of mitochondrial function. In conclusion, AZT, ddI and ddC all exert cytotoxic effects on human muscle cells and induce functional alterations of mitochondria possibly due to mechanisms other than the sole mtDNA depletion. Our results provide only a partial explanation of the fact that AZT, but not ddI and ddC, can induce a myopathy in HIV-infected patients. AZT myopathy might not simply result from a direct mitochondrial toxic effect of crude AZT.

Cytokines and peripheral nerve disorders

Peripheral nerve production of cytokines originates from resident and recruited macrophages, lymphocytes, mastocytes, Schwann cells, and probably neurons. Cytokines are involved in nerve lesions and repair. Tumor necrosis factor-alpha (TNF-alpha) injected into nerve induces Wallerian degeneration, whereas, interleukin-1 (IL-1) production promotes detersion by scavenger macrophages, and synthesis of neurotrophic factors (nerve growth factor-NGF- and leukemia inhibitory factor-LIF). After experimental axotomy, other neurotrophic factors, including IL-6, LIF and transforming growth factor-beta 1 (TGF-beta 1), are overexpressed in nerve and promote axonal growth until axon/Schwann cell contact. Proinflammatory cytokines are instrumental in the course of inflammatory demyelinating neuropathies. They increase vascular permeability and blood nerve barrier breakdown (TNF-alpha, vascular endothelial growth factor/ vascular permeability factor-VEGF/VPF), favor transmigration of leukocytes into nerve, induce activation and proliferation of lymphocytes (IL-1, IL-2) and macrophages (gamma-interferon-IFN-gamma), and have a direct myelinotoxic activity (TNF-alpha and TNF-beta). In addition, downregulation of the immunosuppressive cytokine TGF-beta 1 may favor the nerve inflammatory reactions.

Primary human muscle satellite cell culture: variations of cell yield, proliferation and differentiation rates according to age and sex of donors, site of muscle biopsy, and delay before processing

The present study was performed to determine the influence on human satellite cell yield, proliferation, and differentiation rates of: 1) sex and age of donors; 2) site of the muscle biopsy; and 3) delay before processing of the muscle biopsy sample. We used a standardized primary muscle cell culture procedure on 206 normal muscle samples obtained from different muscle groups of patients aged from 20 to 88 years, at time of orthopedic surgery. Sex of donors did not influence muscle culture parameters. In contrast, aging tended to affect muscle cell yield (age group 50-59 years vs 70-79 years, P < 0.08), but not myogenic cell abilities to proliferate and to fuse into myotubes. The anatomic origin of muscle samples used for culture appeared to influence culture parameters. In contrast with other tested muscles, the tensor fasciae muscle gave both a good cell yield (174 +/- 25 10(3) cells per gram) and homogeneous proliferation and differentiation rates. Storage of the muscle sample at 4 degrees C in transport medium was associated with a very high cell yield when processing was done in early hours after biopsy (277 +/- 50 10(3) cells/g), a high and stable cell yield when processing was done from day 1 to day 3 after biopsy (185 +/- 15 10(3) cells/g), and a poor cell yield when processing was done after day 4 (111 +/- 13 10(3) cells/g). Storage of muscle biopsy samples at 4 degrees C for 1 to 4 days was associated with good proliferation and fusion rates. In conclusion, these data validate a convenient procedure of primary human muscle cell culture, using tensor fasciae muscle biopsy, which is easily done at time of orthopedic surgery, obtained from men and women of all ages (if possible less than 70 years to obtain good cell yield), and allowing of 1-3 days of storage before processing that may compensate uncertainty of the exact time of availability of muscle samples for the scientist.

Interleukin (IL)-1 beta and IL-1 beta mRNA expression in normal and diseased skeletal muscle assessed by immunocytochemistry, immunoblotting and reverse transcriptase-nested polymerase chain reaction

To confirm the production of IL-1 beta and to optimize detection and semiquantitation of IL-1 beta mRNA by polymerase chain reaction (PCR) techniques in skeletal muscle tissue, immunocytochemistry, immunoblotting and several procedures of RNA extraction and reverse transcription (RT)-PCR amplification were used on muscle samples from 12 patients with conditions associated with local production of IL-1 beta (AZT myopathy: 6 patients; sarcoid myopathy: 6 patients) and from 9 patients with normal muscle used as controls. Abundant IL-1 beta immunoreactivities, corresponding to both pro IL-1 beta and mature IL-1 beta as assessed by immunoblotting, were observed in all diseased muscles, either in inflammatory cells (sarcoid myopathy) or in atrophic muscle fibers (AZT myopathy). Acid guanidinium isothiocyanate phenol-chloroform extraction of RNA appeared less efficient for IL-1 beta mRNA detection by RT-PCR than proteinase K digestion followed by phenol-chloroform extraction. Even using the latter procedure, RT-single PCR for IL-1 beta mRNA was puzzlingly negative in all cases but one; in contrast, RT-nested PCR specified by DNA enzyme immunoassay yielded detection of IL-1 beta mRNA in all diseased muscles and in occasional controls, including the expected PCR product of 391 bp, but also another product of 935 bp, corresponding to IL-1 beta mRNA with unsplicing of the fourth intron. Semi-quantitative PCR showed that production of IL-1 beta mRNA was higher in sarcoid myopathy than in AZT myopathy, and in AZT myopathy than in controls. In conclusion, IL-1 beta expression can be reliably studied using immunocytochemistry, but assessment of IL-1 beta mRNA production in muscle tissue requires optimized extraction and RT-PCR procedures.

Evidence of a non-conventional role for the urokinase tripartite complex (uPAR/uPA/PAI-1) in myogenic cell fusion

Urokinase can form a tripartite complex binding urokinase receptor (uPAR) and plasminogen activator inhibitor type-1 (PAI-1), a component of the extracellular matrix (ECM). The components of the tripartite complex are modulated throughout the in vitro myogenic differentiation process. A series of experiments aimed at elucidating the role of the urokinase tripartite complex in the fusion of human myogenic cells were performed in vitro. Myogenic cell fusion was associated with increased cell-associated urokinase-type plasminogen activator (uPA) activity, cell-associated uPAR, and uPAR occupancy. Incubation of cultures with either uPA anticatalytic antibodies, or the amino-terminal fragment of uPA (ATF), which inhibits competitively uPA binding to its receptor, or anti-PAI-1 antibodies, which inhibit uPA binding to PAI-1, resulted in a 30 to 47% decrease in fusion. Incubation of cultures with the plasmin inhibitor aprotinin did not affect fusion. Decreased fusion rates induced by interfering with uPAR/uPA/PAI-1 interactions were not associated with significant changes in mRNA levels of both the myogenic regulatory factor myogenin and its inhibitor of DNA binding, Id. Incubation of cultures with purified uPA resulted in a decrease in fusion, likely due to a competitive inhibition of PAI-1 binding of endogenous uPA. We conclude that muscle cell fusion largely depends on interactions between the members of the urokinase complex (uPAR/uPA/PAI-1), but does not require proteolytic activation of plasmin. Since the intrinsic muscle cell differentiation program appears poorly affected by the state of integrity of the urokinase complex, and since cell migration is a prerequisite for muscle cell fusion in vitro, it is likely that the urokinase system is instrumental in fusion through its connection with the cell migration process. Our results suggest that the urokinase tripartite complex may be involved in cell migration in a non conventional way, playing the role of an adhesion system bridging cell membrane to ECM.

Binding of urokinase to plasminogen activator inhibitor type-1 mediates cell adhesion and spreading

Urokinase plasminogen activator and its receptor are both found at the surface of the cell membrane in many cell types. The plasminogen activator inhibitor type-1 (PAI-1) is often associated with the extracellular matrix. The spatial localization of these three molecules could account for their involvement in cell adhesion and/or migration. We have shown previously that the urokinase receptor mediates mechanical force transmission across the cell surface to the cytoskeleton. Here we investigated whether immobilized plasminogen activator inhibitor type 1 (PAI-1) could regulate cell spreading and cytoskeleton reorganization. Serum deprived human myogenic cells were plated in serum free medium onto bacteriologic dishes precoated with different extracellular matrix ligands (fibronectin, vitronectin, or type 1 collagen) or PAI-1 at increasing concentrations. The number of adherent cells and their projected area were quantitated after 3 hours of plating. PAI-1 promoted cell adhesion and spreading in a dose dependent manner. Addition of antibodies to PAI-1 inhibited the adhesion on PAI-1 coated dishes in a dose dependent way. The PAI-1 mediated cell adhesion required the presence of urokinase at the cell surface. Removal of the glycosylphosphatidylinositol (GPI)-linked proteins abolished cell adhesion on PAI-1 dish, suggesting its dependence on the presence of the urokinase receptor, a GPI-linked receptor. Furthermore, addition of antibodies against alpha v beta3 integrin completely inhibited cell adhesion on PAI-1, suggesting that alpha v beta3 might be the transmembrane molecule that physically connects the complex of PAI-1, urokinase, and urokinase receptor to the cytoskeleton. Visualization of spread cells stained for filamentous actin with confocal microscopy showed a dose-dependent increase of filopodia on PAI-1 coated dishes and cytoskeletal reorganization, suggesting a migratory profile. These data indicate that PAI-1 plays a direct role in dynamic cell adhesion particularly at the leading edge, where increased levels of urokinase plasminogen activator (uPA) and its receptor (uPAR) are localized in migrating cells. Immobilized PAI-1 could therefore serve to bridge the cell surface with the extracellular matrix via the formation of a multimolecular complex that includes alpha v beta3 integrins in myogenic cells.

Interleukin-1 expression in inflammatory myopathies: evidence of marked immunoreactivity in sarcoid granulomas and muscle fibres showing ischaemic and regenerative changes

The most frequent autoimmune adult inflammatory myopathies are dermatomyositis, polymyositis, inclusion body myositis, and sarcoid myopathy. Interleukin-1 (IL-1) is a pleiotropic molecule, implicated in the inflammatory process, but also in tissue protection and remodelling. We evaluated the immunocytochemical expression of [L,-1alpha and beta in frozen muscle biopsy specimens from patients with dermatomyositis (15 cases), polymyositis (five cases), inclusion body myositis (five cases) and sarcoid myopathy (five cases). Positive immunoreactivities, were observed in both inflammatory cells and muscle fibres. Specificity of the immunostaining was assessed by Western blot experiments. IL-1 positive inflammatory cells were rare in polymyositis and inclusion body myositis, moderately abundant in dermatomyositis, and prominent in sarcoid myopathy granulomas. In sarcoid myopathy, 24.6 +/- 4.1% inflammatory cells were IL-1alpha-positive and 45.2 +/- 2.6% were IL-1beta-positive. IL-1 positive muscle fibres were mainly observed in dermatomyositis, usually remote from inflammatory infiltrates. Positive immunostaining for IL-1 was observed in fibres showing ischaemic punched-out vacuoles, that correspond to areas of myosinolysis, in atrophic perifascicular fibres, and in fibres located within healing microinfarcts. All NCAM-positive regenerating fibres were IL-1 positive. We conclude that: (i) IL-1 is expressed in granulomas of sarcoid myopathy, which is in keeping with the role ascribed to IL-1 in the formation of granulomas: (ii) IL-1 is expressed by muscle fibres undergoing ischaemic damage: and (iii) IL-1 expression by muscle fibres is associated with myofibrillar protein breakdown and regeneration.

Differential intracellular distribution and activities of mu- and m-calpains during the differentiation of human myogenic cells in culture

Calpains are intracellular calcium-dependent cystein proteases active at neutral pH. There have been found in human adult myogenic cells (i.e. satellite cells) 2 forms of calpains requiring either micromolar Ca2+: mu-calpain, or millimolar Ca2+: m-calpain. Calpains could be involved in both intracellular proteolysis and cytoskeleton reorganization required for myogenic cell fusion. We showed significant differences in calpains distribution during differentiation of myogenic cells. Using mono- and polyclonal antibodies against both types of calpains, we localized mu-calpain and m-calpain in cultured human satellite cells. mu-calpain was detected in the nuclei of myoblasts and in the cytoplasm of myotubes. m-calpain was only present in the cytoplasm, and was concentrated near the nuclear envelope. Biochemical assays for calpain activities showed that the amounts of these proteinases were modulated during cell growth and differentiation. m-calpain activity was high at the proliferation phase (day 4 of culture) and reached a maximum with the beginning of fusion (day 8) and decreased slightly when the number of myotubes increased (day 12). This activity profile suggests that m-calpain could play a role in the initiation of fusion of satellite cells. The activity of mu-calpain increased regularly with cell growth, the maximum being reached when the cells differentiate, i.e. when its intracellular localization shifted from the nucleus to the cytoplasm. We conclude that the activity and the intracellular localization of the 2 forms of calpains differ with the state of differentiation of myogenic cells.

Myoblast fusion promotes the appearance of active protease nexin I on human muscle cell surfaces

Protease nexin I (PNI) is a 43- to 50-kDa glycoprotein capable of inhibiting a number of serine proteases and belongs to the serpin superfamily. PNI is identical to glia-derived nexin, a neurite outgrowth promoter by virtue of its thrombin-inhibiting activity. Of particular relevance to neuromuscular biology and pathology, PNI was the first serpin shown to be highly localized to the neuromuscular junction and it maps to precisely the same locus as autosomal recessive amyotrophic lateral sclerosis (ALSJ) at chromosome 2q33-35. In the present report, we now show that in cultures of human skeletal muscle, PNI protein is expressed only after myoblast fusion into multinuclear myotubes and is localized in patches on their surfaces. We performed complex formation experiments with labeled thrombin, another target protease for PNI, with intact human muscle cells in culture. We detected specific SDS-stable PNI/thrombin complexes in myotube extracts only, indicating that active PNI was bound to their surfaces. We studied the gene expression of PNI mRNA using a 300-bp cDNA synthesized from the published sequence of human PNI. Confirming the protein data, upregulation of PNI appears in myotubes using Northern blot analysis. The current results reinforce the hypothesis that the regulation of the balance of serine proteases and serpins, such as PNI, is involved in muscle differentiation. They also prompt us to explore PNI abnormalities in several neuromuscular diseases, including ALSJ.

Urokinase receptor mediates mechanical force transfer across the cell surface

The tripartite complex formed by the urokinase receptor, urokinase, and its inhibitor is an enzymatic system that controls plasmin formation involved in degradation of extracellular matrix proteins. With the use of magnetic twisting cytometry with urokinase-coated ferromagnetic beads, we applied mechanical stress directly to the urokinase receptor on the surface of human myogenic cells in culture. The stiffness and the stiffening response measured through the urokinase receptor resembled those of integrins, which are linked mechanically to the cytoskeleton. Furthermore, stiffness decreased with disruption of actin microfilaments. These results demonstrate that the urokinase receptor is coupled mechanically to the cytoskeleton. Inhibition of the tripartite complex formation with antibodies led to a twofold increase in cytoskeletal stiffness. A stiffened cytoskeleton might impede cytoskeletal remodeling and reorganization and thus impede cell motility. Our results demonstrate that the urokinase receptor mediates mechanical force transfer across the cell surface. As such, it is a novel pathway to regulate cytoskeletal stiffness and, thereby, possibly to modulate motility of normal and abnormal adherent cells.

Plasminogen activators in the neuromuscular system of the wobbler mutant mouse

Wobbler, the neurological mutant mouse, carries an autosomal recessive gene (wr) and has been characterized as a model of lower motoneuron disorders with associated muscle atrophy, denervation and reinnervation. During normal murine neuromuscular development a decrease in muscle plasminogen activator (PA) activity accompanies synapse maturation. In contrast, experimental denervation in adult mice leads to an increase in muscle PA activity. The purpose of the present study was to determine the possible involvement of PAs in the denervation/reinnervation phenomena and motoneuron degeneration that characterize the wobbler mutant mouse. We determined the degree of innervation and its characteristics in wobbler mice by measuring choline acetyltransferase (ChAT) activity. We measured ChAT in the spinal cord as well as in two different muscles known to be differentially affected, biceps brachii and gastrocnemius. We found a sharp decrease of ChAT activity in both muscles but not in spinal cord extracts. We estimated the extent of sprouting by the silver/cholinesterase stain. Motoneuron terminal sprouting, not detected in normal animals, was present in 40% of the neuromuscular junctions in wobbler mice. We estimated specific PA activities in biceps brachii and gastrocnemius muscle extracts, as well as spinal cord extracts, using both an amidolytic assay and fibrin zymography. Increased PA, predominantly urokinase-PA (uPA), was observed in wobbler mouse muscle. A greater uPA was detected in biceps brachii muscle than in gastrocnemius muscle, which is less impaired by the mutation. There was no change in spinal cord PA, although tissue type PA (tPA) is the predominant PA type there.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of activities and RNA level of the components of the plasminogen activation system during fusion of human myogenic satellite cells in vitro

Primary cultures of human myogenic stem cells (satellite cells) mimic myogenic differentiation. During this process, the expression of the components of the plasminogen activation system underwent modulation. Activities and mRNA levels of tissue-type and urokinase-type plasminogen activator were increased in a reproducible pattern during differentiation. A modulation of the mRNA level of PAI-2 was also observed. Human satellite cells expressed a urokinase receptor and also the mRNA level of this component underwent modulation. With the exception of PAI-1 mRNA, the level of all mRNAs increased from Day 4 to Day 8, i.e., just before myoblasts fusion, and then remained high at later stages. The modulation of the plasminogen activating activity indicates that this system is directly involved in the fusion process of myogenic differentiation.

Slow and fast rat skeletal muscles differ in their plasminogen activator activities

Slow and fast contracting muscles differ in their innervation and electrophysiological properties as well as in their regenerating potentialities. The purpose of the present work was to investigate the expression of plasminogen activators and its possible relation to each type of muscle. Slow (Soleus) and fast (Extensor Digitorum Longus) muscles were obtained from white Wistar rats. Before sectioning the muscles, the euthanized rats were perfused with cold phosphate buffer saline to avoid interference by circulating proteases and inhibitors. Muscle extracts were pounded in an ice-cold Potter tube. Plasminogen activators (PAs) were assayed by fibrin zymography and by both liquid and solid-phase fibrin spectrophotometric assays for the detection of PAs activity. Both urokinase (uPA) and tissue-type plasminogen activator (tPA) activities corresponding to proteins of 38 kDa and 65 kDa molecular masses, were detected in the extracts. Slow muscles contained higher amounts of both activators than fast muscles, but the relative amount of uPA was higher in both types of muscles. In addition, the characteristics of each type of extracts differed somewhat: the fast muscle activity curve was typical of an accelerating process, while the slow muscle curve showed an activity probably related to already formed plasmin or to some other trypsin-like enzyme. These results suggest that the amount of plasminogen activators could be a new criterion of discrimination between slow and fast skeletal muscles.

Characterization of myogenesis from adult satellite cells cultured in vitro

We describe several characteristics of in vitro myogenesis from adult skeletal muscle satellite cells from the rat and several amphibian species. The timing of cell proliferation and fusion into myotubes was determined, and in urodeles, myogenesis from satellite cells was clearly demonstrated for the first time. Growth factors are known to stimulate satellite cell proliferation. Acidic FGF mRNA was present in rat satellite cells during proliferation but it was not detected in myotubes. Fibronectin was synthesized in satellite cells during proliferation and expelled into the extracellular medium when the myotubes differentiated. We suggest that fibronectin plays a part in the formation of myotubes, as this process was inhibited by anti-fibronectin IgG. Adult satellite cells might differ from fetal myoblasts since they were observed to exhibit the opposite response to a phorbol ester (TPA) to that of the myoblasts. We therefore examined the possibility that the different levels of protein kinase C activity and different phorbol ester binding characteristics in the two cell types account for these opposite responses. Our results suggest that the difference is not connected with the phorbol ester receptor but might be caused by events subsequent to protein kinase C activation. Localized extracellular proteolytic activity might have a role in cell mobilization and/or fusion when satellite cells are activated. We showed that the content of plasminogen activators, chiefly urokinase, was larger in tissues from slow twitch muscles which regenerate more rapidly than fast muscles. The urokinase level rose sharply in cultures when cells fused into myotubes, and was twice as high in slow muscle cells as in fast ones. We also found that, in vitro, slow muscle satellite cells displayed greater myogenicity, but that phorbol ester inhibited their mitosis and myogenicity. We conclude that satellite cells acquire characteristics which differentiate them from myoblasts and correspond to the fast and slow muscles from which they originate.