RESUMEN
Desmin, a muscle-specific intermediate filament protein, is expressed in all muscle tissues. Its absence leads to a multisystemic disorder involving cardiac, skeletal, and smooth muscles. In skeletal muscle, structural abnormalities include lack of alignment of myofibrils, Z disk streaming, and focal muscle degeneration. In this study, we have examined the consequences of an absence of desmin on the mechanisms of regeneration and the integrity of the neuromuscular junction. The muscles of desmin knock-out and wild-type mice were made to regenerate by injecting cardiotoxin and were examined 7 to 42 days following the injection. The absence of desmin resulted in a delayed and modified regeneration and an accumulation of adipocytes. This was associated with a persistence of small diameter muscle fibers containing both N-CAM and developmental myosin isoforms. The amount of the slow myosin was increased, whereas there was a decrease in the fast isoform in the regenerated muscles of desmin knock-out mice. Both regeneration and aging led to the appearance of elongated neuromuscular junctions with diffuse acetylcholinesterase staining and a decrease in the overall acetylcholinesterase activity in the muscles of these mice. The neuromuscular junctions were markedly disorganised and in some cases postjunctional folds were absent. We conclude that desmin is essential for terminal muscle regeneration, maturation of muscle fibers, and maintaining the complex folded structure of the postsynaptic apparatus of the neuromuscular junctions.
Asunto(s)
Desmina/fisiología , Corazón/fisiología , Músculo Esquelético/fisiología , Músculo Liso/fisiología , Unión Neuromuscular/ultraestructura , Regeneración/fisiología , Animales , Moléculas de Adhesión Celular Neuronal/metabolismo , Desmina/genética , Ratones , Ratones Endogámicos C57BL , Ratones Noqueados , Fibras Musculares Esqueléticas/metabolismo , Músculo Esquelético/metabolismo , Músculo Liso/metabolismo , Miocardio/metabolismo , Cadenas Pesadas de Miosina/biosíntesis , Miosinas/metabolismo , Unión Neuromuscular/anomalías , FenotipoRESUMEN
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.
Asunto(s)
Citoesqueleto/metabolismo , Órgano Eléctrico/citología , Membranas Sinápticas/metabolismo , Torpedo/fisiología , Animales , Proteínas del Citoesqueleto/metabolismo , Citoesqueleto/ultraestructura , Distrofina/metabolismo , Órgano Eléctrico/metabolismo , Órgano Eléctrico/ultraestructura , Proteínas de la Membrana/metabolismo , Modelos Biológicos , Proteínas Musculares/metabolismo , Receptores Nicotínicos/metabolismo , Membranas Sinápticas/ultraestructura , Torpedo/crecimiento & desarrollo , UtrofinaRESUMEN
Tyrosine phosphorylation is thought to play a critical role in the clustering of acetylcholine receptors (AChR) at the developing neuromuscular junction. Yet, in vitro approaches have led to conflicting conclusions regarding the function of tyrosine phosphorylation of AChR beta subunit in AChR clustering. In this work, we followed in situ the time course of tyrosine phosphorylation of AChR in developing Torpedo electrocyte. We observed that tyrosine phosphorylation of the AChR beta and delta subunits occurs at a late stage of embryonic development after the accumulation of AChRs and rapsyn in the membrane and the onset of innervation. Interestingly, in the mature postsynaptic membrane, we observed two populations of AChR differing both in their phosphotyrosine content and distribution. Our data are consistent with the notion that tyrosine phosphorylation of the AChR is related to downstream events in the pathway regulating AChR accumulation rather than to initial clustering events.
Asunto(s)
Envejecimiento/metabolismo , Órgano Eléctrico/embriología , Órgano Eléctrico/metabolismo , Receptores Nicotínicos/metabolismo , Tirosina/metabolismo , Animales , Western Blotting , Órgano Eléctrico/citología , Técnica del Anticuerpo Fluorescente , Proteínas Musculares/metabolismo , Fosforilación , Fosfotirosina/metabolismo , Fracciones Subcelulares/metabolismo , Distribución Tisular , Torpedo/embriología , Torpedo/crecimiento & desarrollo , Torpedo/metabolismoRESUMEN
Spinal muscular atrophy (SMA) is a common autosomal recessive neuromuscular disorder characterized by degeneration of motor neurons of the spinal cord and muscular atrophy. SMA is caused by alterations to the survival of motor neuron (SMN) gene, the function of which has hitherto been unclear. Here, we present immunoblot analyses showing that normal SMN protein expression undergoes a marked decay in the postnatal period compared with fetal development. Morphological and immunohistochemical analyses of the SMN protein in human fetal tissues showed a general distribution in the cytoplasm, except in muscle cells, where SMN protein was immunolocalized to large cytoplasmic dot-like structures and was tightly associated with membrane-free heavy sedimenting complexes. These cytoplasmic structures were similar in size to gem. The SMN protein was markedly deficient in tissues derived from type I SMA fetuses, including skeletal muscles and, as previously shown, spinal cord. While our data do not help decide whether SMA results from impaired SMN expression in spinal cord, skeletal muscle or both, they suggest a requirement for SMN protein during embryo-fetal development.
Asunto(s)
Feto/química , Atrofia Muscular Espinal/metabolismo , Proteínas del Tejido Nervioso/metabolismo , Animales , Western Blotting , Fraccionamiento Celular , Proteína de Unión a Elemento de Respuesta al AMP Cíclico , Femenino , Humanos , Inmunohistoquímica , Ratones , Ratones Endogámicos BALB C , Músculo Esquelético/química , Músculo Esquelético/embriología , Atrofia Muscular Espinal/embriología , Embarazo , Proteínas de Unión al ARN , Proteínas del Complejo SMN , Fracciones Subcelulares/química , Distribución TisularRESUMEN
Ankyrins are a multi-gene family of peripheral proteins that link ion channels and cell adhesion molecules to the spectrin-based skeleton in specialized membrane domains. In the mammalian skeletal myofiber, ankyrins were immunolocalized in several membrane domains, namely the costameres, the postsynaptic membrane and the triads. Ank1 and Ank3 transcripts were previously detected in skeletal muscle by northern blot analysis. However, the ankyrin isoforms associated with these domains were not identified, with the exception of an unconventional Ank1 gene product that was recently localized at discrete sites of the sarcoplasmic reticulum. Here we study the expression and subcellular distribution of the Ank3 gene products, the ankyrinsG, in the rat skeletal muscle fiber. Northern blot analysis of rat skeletal muscle mRNAs using domain-specific Ank3 cDNA probes revealed two transcripts of 8.0 kb and 5.6 kb containing the spectrin-binding and C-terminal, but not the serine-rich, domains. Reverse transcriptase PCR analysis of rat skeletal muscle total RNA confirmed the presence of Ank3 transcripts that lacked the serine-rich and tail domains, a major insert of 7813 bp at the junction of the spectrin-binding and C-terminal domains that was previously identified in brain Ank3 transcripts. Immunoblot analysis of total skeletal muscle homogenates using ankyrinG-specific antibodies revealed one major 100 kDa ankyrinG polypeptide. Immunofluorescence labeling of rat diaphragm cryosections showed that ankyrin(s)G are selectively associated with (1) the depths of the postsynaptic membrane folds, where the voltage-dependent sodium channel and N-CAM accumulate, and (2) the sarcoplasmic reticulum, as confirmed by codistribution with the sarcoplasmic reticulum Ca2+-ATPase (SERCA 1). At variance with ankyrin(s)G, ankyrin(s)R (ank1 gene products) accumulate at the sarcolemma and at sarcoplasmic structures, in register with A-bands. Both ankyrin isoforms codistributed over Z-lines and at the postsynaptic membrane. These data extend the notion that ankyrins are differentially localized within myofibers, and point to a role of the ankyrinG family in the organization of the sarcoplasmic reticulum and the postsynaptic membrane.
Asunto(s)
Ancirinas/análisis , Músculo Esquelético/química , Retículo Sarcoplasmático/química , Membranas Sinápticas/química , Empalme Alternativo , Secuencia de Aminoácidos , Animales , Ancirinas/genética , Diafragma/química , Masculino , Datos de Secuencia Molecular , Fibras Musculares Esqueléticas/química , Unión Neuromuscular/química , ARN Mensajero/análisis , Ratas , Ratas Sprague-DawleyRESUMEN
Agrin, an extracellular matrix protein synthesized by nerves and muscles is known to promote the clustering of acetylcholine receptors and other synaptic proteins in cultured myotubes. This observation suggests that agrin may provide at least part of the signal for synaptic specialization in vivo. The extracellular matrix components agrin, laminin and merosin bind to alpha-dystroglycan, a heavily glycosylated peripheral protein part of the dystrophin-glycoprotein complex, previously characterized in the sarcolemma of skeletal and cardiac muscles and at the neuromuscular junction. In order to understand further the function of agrin and alpha DG in the genesis of the acetylcholine receptor-rich membrane domain, the settling of components of the dystrophin-glycoprotein complex and agrin was followed by immunofluorescence localization in developing Torpedo marmorata electrocytes. In 40-45 mm Torpedo embryos, a stage of development at which the electrocytes exhibit a definite structural polarity, dystrophin, alpha/beta-dystroglycan and agrin accumulated concomitantly with acetylcholine receptors at the ventral pole of the cells. Among these components, agrin appeared as the most intensely concentrated and sharply localized. The scarcity of the nerve-electrocyte synaptic contacts at this stage of development, monitored by antibodies against synaptic vesicles, further indicates that before innervation, the machinery for acetylcholine receptor clustering is provided by electrocyte-derived agrin rather than by neural agrin. These observations suggest a two-step process of acetylcholine receptor clustering involving: (i) an instructive role of electrocyte-derived agrin in the formation of a dystrophin-based membrane scaffold upon which acetylcholine receptor molecules would accumulate according to a diffusion trap model; and (ii) a maturation and/or stabilization step controlled by neural agrin. In the light of these data, the existence of more than one agrin receptor is postulated to account for the action of agrin variants at different stages of the differentiation of the postsynaptic membrane in Torpedo electrocytes.
Asunto(s)
Agrina/análisis , Neuronas/metabolismo , Receptores Colinérgicos/análisis , Membranas Sinápticas/metabolismo , Torpedo , Agrina/metabolismo , Animales , Diferenciación Celular , Técnica del Anticuerpo Fluorescente Indirecta , Neuronas/citología , Receptores Colinérgicos/metabolismoRESUMEN
We investigated the production of Gag particles by Vero, CV-1, or 1D cells infected with different vaccinia virus recombinants expressing HIV gag or gag-pol genes. Immunoblots of (centrifuged) culture media from 1D cells infected with vMM5, a vaccinia virus recombinant expressing the HIV-2 gag-pol genes, revealed the presence of abundant particles that contained (mostly processed) Gag antigens. In contrast, Gag particles were found only in low amounts in the culture medium from Vero cells infected with the same HIV gag-pol vaccinia virus recombinant; the Gag precursor remained associated with the infected Vero cells and was efficiently processed. This low excretion of Gag particles after infection of Vero cells with vMM5 was also demonstrated by assays of reverse transcriptase activity in the pellet of centrifuged culture medium. Cell fractionation showed that Gag proteins were predominantly found in the membrane fraction from both 1D and Vero cells. Electron microscopy observations of 1D or of Vero cells infected with vMM5 vaccinia virus recombinant revealed in both cases the presence of particles budding at the plasma membrane. However, the shape of the budding particles was different in the two cell lines, with immature forms present in the membrane from the infected Vero cells. An inefficient excretion of Gag particles was also observed after infection of Vero cells with different vaccinia virus recombinants expressing either an uncleaved HIV-2 Gag protein or the HIV-1 gag-pol genes, as judged both by immunoblot and reverse transcriptase activity assays.(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Productos del Gen gag/biosíntesis , VIH-1/genética , VIH-2/genética , Virus Vaccinia/genética , Animales , Línea Celular , Expresión Génica , Productos del Gen gag/genética , Genes gag , Genes pol , Ratones , Microscopía Electrónica , Ácidos Mirísticos/metabolismo , Precursores de Proteínas/biosíntesis , Precursores de Proteínas/genética , Procesamiento Proteico-Postraduccional , ADN Polimerasa Dirigida por ARN/metabolismo , Recombinación Genética , Células VeroRESUMEN
The immunological identification of dystrophin isoforms at the neuromuscular junction and Torpedo marmorata electromotor synapse was attempted using various antibodies. A polyclonal antibody raised against electrophoretically purified dystrophin from T. marmorata electrocyte has been thoroughly investigated. This antibody recognized dystrophin in the electric tissue as well as sarcolemmal and synaptic neuromuscular junction dystrophin in all studies species (T. marmorata, rat, mice and human) at serum dilutions as high as 1:10,000. At variance, no staining of either the sarcolemma or neuromuscular junction was observed in Duchenne muscular dystrophy or mdx mice skeletal muscles. In these muscles, other members of the dystrophin superfamily, in particular the dystrophin-related protein(s) encoded by autosomal genes are present. These data thus demonstrate the specificity of our antibodies for dystrophin. Anti-dystrophin-related protein antibodies [Khurana et al. (1991) Neuromusc. Disorders 1, 185-194] which gave a strong immunostaining of the neuromuscular junction in various species, including T. marmorata, cross-reacted weakly with the postsynaptic membrane of the electrocyte. Taken together, these observations are in favor of the existence of a protein very homologous to dystrophin at the electromotor synapse in T. marmorata, whereas both dystrophin and dystrophin-related protein co-localize at the neuromuscular junction as in all species studied. The electrocyte thus offers the unique opportunity to study the interaction of dystrophin with components of the postsynaptic membrane.
Asunto(s)
Proteínas del Citoesqueleto/análisis , Distrofina/análisis , Proteínas de la Membrana , Unión Neuromuscular/ultraestructura , Sinapsis/ultraestructura , Animales , Anticuerpos , Órgano Eléctrico/citología , Técnica del Anticuerpo Fluorescente , Inmunohistoquímica/métodos , Músculos/citología , Receptores Colinérgicos/análisis , Torpedo , UtrofinaRESUMEN
Desmosomes are specialized domains of epithelial cell plasma membranes engaged in the anchoring of intermediate filaments (IF). So far, the desmosomal component(s) responsible for this binding has not been unambiguously identified. In the present work, we have examined bovine muzzle epidermis desmosomes for the presence of protein(s) structurally and functionally related to lamin B, the major receptor for IF in the nuclear envelope (Georgatos, S. D., and G. Blobel. 1987. J. Cell Biol. 105:105-115). By using polyclonal antibodies to lamin B in immunoblotting experiments, we find that a desmosomal protein of 140-kD shares epitope(s) with lamin B. Immunoelectron microscopic and urea extraction experiments show that this protein is a peripheral protein localized at the cytoplasmic side of the desmosomes (desmosomal plaques). Furthermore, this protein binds vimentin in an in vitro assay. Since this binding is inhibited by lamin B antibodies, the epitopes common to the 140-kD protein and to lamin B may be responsible for anchoring of intermediate filaments to desmosomes. These data suggest that lamin B-related proteins (see also Cartaud, A., J. C. Courvalin, M. A. Ludosky, and J. Cartaud. 1989. J. Cell Biol. 109:1745-1752) together with lamin B, provide cells with several nucleation sites, which can account for the multiplicity of IF organization in tissues.
Asunto(s)
Citoesqueleto/ultraestructura , Desmosomas/ultraestructura , Filamentos Intermedios/ultraestructura , Proteínas Nucleares/metabolismo , Piel/ultraestructura , Animales , Autoanticuerpos/inmunología , Bovinos , Electroforesis en Gel de Poliacrilamida , Técnica del Anticuerpo Fluorescente , Humanos , Inmunohistoquímica , Lamina Tipo B , Laminas , Lupus Eritematoso Sistémico/inmunología , Peso Molecular , Proteínas Nucleares/análisis , Proteínas Nucleares/inmunología , Unión Proteica , Vimentina/metabolismoRESUMEN
Dystrophin has been shown to occur in Torpedo electrocyte [Chang, H. W., Bock, E. & Bonilla, E. (1989) J. Biol. Chem. 264, 20831-20834], a highly polarized syncytium that is embryologically derived from skeletal muscle and displays functionally distinct plasma membrane domains on its innervated and noninnervated faces. In the present study, we investigated the subcellular distribution of dystrophin in the adult electrocyte from Torpedo marmorata and the evolution of its distribution during embryogenesis. Immunofluorescence experiments performed on adult electrocytes with a polyclonal antibody directed against chicken dystrophin revealed that dystrophin immunoreactivity codistributed exclusively with the acetylcholine receptor along the innervated membrane. At the ultrastructural level, dystrophin immunoreactivity appears confined to the face of the subsynaptic membrane exposed to the cytoplasm. In developing electrocytes (45-mm embryo), dystrophin is already detectable at the acetylcholine receptor-rich ventral pole of the cells before the entry of the electromotor axons. Furthermore, we show that dystrophin represents a major component of purified membrane fractions rich in acetylcholine receptor. A putative role of dystrophin in the organization and stabilization of the subsynaptic membrane domain of the electrocyte is discussed.
Asunto(s)
Órgano Eléctrico/crecimiento & desarrollo , Proteínas Musculares/análisis , Receptores Colinérgicos/análisis , Envejecimiento , Animales , Anticuerpos , Membrana Celular/ultraestructura , Distrofina , Órgano Eléctrico/citología , Electroforesis en Gel de Poliacrilamida , Técnica del Anticuerpo Fluorescente , Peso Molecular , Proteínas Musculares/aislamiento & purificación , Receptores Colinérgicos/ultraestructura , TorpedoRESUMEN
The Torpedo electrocyte is a flattened syncytium derived from skeletal muscle, characterized by two functionally distinct plasma membrane domains. The electrocyte is filled up with a transversal network of intermediate filaments (IF) of desmin which contact in an end-on fashion both sides of the cell. In this work, we show that polyclonal antibodies specific for lamin B recognizes a component of the plasma membrane of Torpedo electrocyte. This protein which thus shares epitopes with lamin B has a relative molecular mass of 54 kD, an acidic IP of 5.4. It is localized exclusively on the cytoplasmic side of the innervated membrane of the electrocyte at sites of IF-membrane contacts. Since our previous work showed that the noninnervated membrane contains ankyrin (Kordeli, E., J. Cartaud, H. O. Nghiêm, L. A. Pradel, C. Dubreuil, D. Paulin, and J.-P. Changeux. 1986. J. Cell Biol. 102:748-761), the present results suggest that desmin filaments may be anchored via the 54-kD protein to the innervated membrane and via ankyrin to the noninnervated membrane. These findings would represent an extension of the model proposed by Georgatos and Blobel (Georgatos, S. D., and G. Blobel. 1987a. J. Cell Biol. 105:105-115) in which type III intermediate size filaments are vectorially inserted to plasma and nuclear membranes by ankyrin and lamin B, respectively.
Asunto(s)
Órgano Eléctrico/ultraestructura , Proteínas Nucleares/análisis , Membranas Sinápticas/ultraestructura , Animales , Órgano Eléctrico/análisis , Órgano Eléctrico/citología , Electroforesis en Gel Bidimensional , Electroforesis en Gel de Poliacrilamida , Técnica del Anticuerpo Fluorescente , Immunoblotting , Lamina Tipo B , Laminas , Peso Molecular , Músculos/análisis , Proteínas Nucleares/inmunología , Ratas , Membranas Sinápticas/análisis , TorpedoRESUMEN
The structural properties of the cell wall and cell membrane of several mycobacteria and of Leprosy Derived Corynebacteria are investigated by freeze-etching and freeze-fracture. In all cases the freeze-fracture split the cell wall in two asymmetric halves. The cell wall fracture faces of the mycobacteria are characterized by a filamentous network which vary with respect to the amount and complexity among microorganism of the same species and even more of different species. In LDC the structure organization of the cell wall and cell membrane differs from that of mycobacteria. The most stricking difference is the presence on the fracture faces of the LDC cell wall of different classes of particulated entities of yet unknown nature. In the mycobacteria and LDC the periseptal annuli likely provide a potential frame for cell envelope and cell membrane assembly.
Asunto(s)
Corynebacterium/ultraestructura , Lepra/microbiología , Mycobacterium/ultraestructura , Animales , Armadillos/microbiología , Membrana Celular/ultraestructura , Pared Celular/ultraestructura , Grabado por Congelación , Técnica de Fractura por Congelación , Humanos , Ratones , Mycobacterium avium/ultraestructura , Mycobacterium leprae/ultraestructura , Mycobacterium lepraemurium/ultraestructuraRESUMEN
Motor end-plate disease (med) in the mouse is an hereditary defect of the neuromuscular system, with partial functional denervation and muscle inactivity in late stages of the disease. Motor end-plate disease is characterized by an intense ultraterminal sprouting of the motor nerves from swollen nerve terminal branches in the soleus muscle. At the ultrastructural level, the neuromuscular junctions extend to very wide territories, often outside the original motor end-plate, in regions where the nerve sprouts are in simple apposition to the muscle fiber, with no secondary synaptic folds. The nerve terminals are rich in neurofilaments and poor in synaptic vesicles. Freeze fracture analysis of the pre-synaptic and post-synaptic membrane specializations fails to reveal any important structural alteration which could suggest a defect in acetylcholine release or in muscle membrane excitability. However, the non-junctional sarcolemmal specializations (the so-called 'square arrays') are found with a frequency slightly higher than in normal muscle. The nerve abnormalities at the neuromuscular junction may be either a consequence of muscle inactivity or the morphological expression of some primary nerve abnormality. Further studies of the soleus muscle at early stages of the disease may provide evidence in favor of either possibility.
Asunto(s)
Neuronas Motoras/ultraestructura , Unión Neuromuscular/ultraestructura , Enfermedades del Sistema Nervioso Periférico/patología , Sarcolema/ultraestructura , Animales , Técnica de Fractura por Congelación/métodos , Ratones , Placa MotoraRESUMEN
The plasma membrane and its derivative, the phagosome membrane, were studied during and after ingestion of yeast of latex beads in Dictyostelium discoideum. Freeze-fracture electron microscopy, which provides information on the internal architecture of the membranes, and observation of thin sections of cells treated by cytochemical methods were used in parallel. For visualization of membrane sterols in the replicas, the cells were fixed in the presence of digitonin or the antibiotic filipin. No lateral phase separation occurred during yeast engulfment: the intramembranous particles (IMPs), phospholipids and sterols remained distributed at random in the forming phagosome membrane. In contrast architectural modifications of the membrane were observed upon phagosome internalization. Compared to the plasma membrane, the phagosome membrane displayed 2-3 times more IMPs a shift in the IMP size distribution and a higher sterol content. These changes were completed soon after phagosome closure; they were not related either to the nature of the ingested particles (yeast, latex beads) or to the pH in the membrane environment. The membrane changes too place when the phagosomes began to fuse with pre-existing digestive or autophagic vacuoles and lysosomes. Some of the experimental evidence suggests that the restructuring of the membrane may be related to the presence of hydrolases.