Developmental studies of dystrophin and other cytoskeletal proteins in cultured muscle cells

Ablation of the dystrophin Dp71f alternative C-terminal variant increases sarcoma tumour cell aggressiveness

Alterations in Dp71 expression, the most ubiquitous dystrophin isoform, have been associated with patient survival across tumours. Intriguingly, in certain malignancies, Dp71 acts as a tumour suppressor, while manifesting oncogenic properties in others. This diversity could be explained by the expression of two Dp71 splice variants encoding proteins with distinct C-termini, each with specific properties. Expression of these variants has impeded the exploration of their unique roles. Using CRISPR/Cas9, we ablated the Dp71f variant with the alternative C-terminus in a sarcoma cell line not expressing the canonical C-terminal variant, and conducted molecular (RNAseq) and functional characterisation of the knockout cells. Dp71f ablation induced major transcriptomic alterations, particularly affecting the expression of genes involved in calcium signalling and ECM-receptor interaction pathways. The genome-scale metabolic analysis identified significant downregulation of glucose transport via membrane vesicle reaction (GLCter) and downregulated glycolysis/gluconeogenesis pathway. Functionally, these molecular changes corresponded with, increased calcium responses, cell adhesion, proliferation, survival under serum starvation and chemotherapeutic resistance. Knockout cells showed reduced GLUT1 protein expression, survival without attachment and their migration and invasion in vitro and in vivo were unaltered, despite increased matrix metalloproteinases release. Our findings emphasise the importance of alternative splicing of dystrophin transcripts and underscore the role of the Dp71f variant, which appears to govern distinct cellular processes frequently dysregulated in tumour cells. The loss of this regulatory mechanism promotes sarcoma cell survival and treatment resistance. Thus, Dp71f is a target for future investigations exploring the intricate functions of specific DMD transcripts in physiology and across malignancies.

Archvillin, a muscle-specific isoform of supervillin, is an early expressed component of the costameric membrane skeleton

The membrane skeleton protein supervillin binds tightly to both F-actin and membranes and can potentiate androgen receptor activity in non-muscle cells. We report that muscle, which constitutes the principal tissue source for supervillin sequences, contains a approximately 250 kDa isoform of supervillin that localizes within nuclei and with dystrophin at costameres, regions of F-actin membrane attachment in skeletal muscle. The gene encoding this protein, 'archvillin' (Latin, archi; Greek, árchos; 'principal' or 'chief'), contains an evolutionarily conserved, muscle-specific 5' leader sequence. Archvillin cDNAs also contain four exons that encode approximately 47 kDa of additional muscle-specific protein sequence in the form of two inserts within the function-rich N-terminus of supervillin. The first of these muscle-specific inserts contains two conserved nuclear targeting signals in addition to those found in sequences shared with supervillin. Archvillin, like supervillin, binds directly to radiolabeled F-actin and co-fractionates with plasma membranes. Colocalization of archvillin with membrane-associated actin filaments, non-muscle myosin II, and--to a lesser extent--vinculin was observed in myoblasts. Striking localizations of archvillin protein and mRNA were observed at the tips of differentiating myotubes. Transfected protein chimeras containing archvillin insert sequences inhibited myotube formation, consistent with a dominant-negative effect during early myogenesis. These data suggest that archvillin is among the first costameric proteins to assemble during myogenesis and that it contributes to myogenic membrane structure and differentiation.

Myotonic dystrophy and myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) was designated as a gene responsible for myotonic dystrophy (DM) on chromosome 19, because the gene product has extensive homology to protein kinase catalytic domains. DM is the most common disease with multisystem disorders among muscular dystrophies. The genetic basis of DM is now known to include mutational expansion of a repetitive trinucleotide sequence (CTG)n in the 3'-untranslated region (UTR) of DMPK. Full-length DMPK was detected and various isoforms of DMPK have been reported in skeletal and cardiac muscles, central nervous tissues, etc. DMPK is localized predominantly in type I muscle fibers, muscle spindles, neuromuscular junctions and myotendinous tissues in skeletal muscle. In cardiac muscle it is localized in intercalated dises and Purkinje fibers. Electron microscopically it is detected in the terminal cisternae of SR in skeletal muscle and the junctional and corbular SR in cardia muscle. In central nervous system, it is located in many neurons, especially in the cytoplasm of cerebellar Purkinje cells, hippocampal interneurons and spinal motoneurons. Electron microscopically it is detected in rough endoplasmic reticulum. The functional role of DMPK is not fully understood, however, it may play an important role in Ca2+ homeostasis and signal transduction system. Diseased amount of DMPK may play an important role in the degeneration of skeletal muscle in adult type DM. However, other molecular pathogenetical mechanisms such as dysfunction of surrounding genes by structural change of the chromosome by long trinucleotide repeats, and the trans-gain of function of CUG-binding proteins might be responsible to induce multisystemic disorders of DM such as myotonia, endocrine dysfunction, etc.

Triad proteins and intracellular Ca2+ transients during development of human skeletal muscle cells in aneural and innervated cultures

Dihydropyridine receptors (DHPRs), ryanodine receptors (RyRs), and triadin are major components of triads of mature skeletal muscle and play crucial roles in Ca2+ release in excitation-contraction (E-C) coupling. We investigated the expression and localization of these proteins as well as intracellular Ca2+ transients during development of human muscle cells cultured aneurally and innervated with rat spinal cord. mRNAs encoding skeletal muscle isoforms of the DHPR alpha1 subunit (alpha1S-DHPR), the RyR, and triadin were scarce in myoblasts and increased remarkably after myotube formation. Immunocytochemically, alpha1S-DHPR was expressed after myoblast fusion and localized mainly within the cytoplasmic area of aneural myotubes whereas the cardiac isoform (alpha1C-DHPR) was abundant along the plasma membrane. RyRs and triadin were both detected after myotube formation and colocalized in the cytoplasm of aneural myotubes and innervated muscle fibers. Along the plasma membrane of aneural myotubes, colocalization of alpha1C-DHPR with the RyR was more frequently observed than that of alpha1S-DHPR. In innervated muscle fibers, alpha1S-DHPR and RyR were colocalized first along the plasma membrane and later in the cytoplasmic area and formed regular double rows of cross-striation. The alpha1C-DHPR diminished after innervation. In Ca2+ imaging, spontaneous irregular slow Ca2+ oscillations were observed in aneurally cultured myotubes whereas nerve-driven regular fast oscillations were observed in innervated muscle fibers. Both caffeine and depolarization induced Ca2+ transients in aneurally cultured myotubes and innervated muscle fibers. In aneurally cultured myotubes, depolarization-induced Ca2+ transients were highly dependent on extracellular Ca2+, suggesting immaturity of the Ca2+ release system. This dependence remarkably decreased after innervation. Our present results show that these proteins are expressed differently in aneurally cultured myotubes than in adult skeletal muscle, that Ca2+ release in aneurally cultured myotubes is different from in adult skeletal muscle, and that innervation induces formation of a mature skeletal muscle-like excitation-contraction coupling system in cultured human muscle cells.

Calcium homeostasis and cell death in Sol8 dystrophin-deficient cell line in culture

Abnormalities of calcium homeostasis are involved in the process of cell injuries such as Duchenne muscular dystrophy characterized by the absence of the protein dystrophin. But how the absence of dystrophin leads to cytosolic calcium overload is as yet poorly understood. This question has been addressed with skeletal muscle cells from human DMD muscles or mdx mice. Although easier to obtain than human muscles, mdx muscle cells have provided controversial data concerning the resting intracellular calcium level ([Ca2+](i)). This work describes the culture of Sol8 cell line that expresses neither dystrophin nor adhalin, a dystrophin-associated protein. The [Ca2+](i)and intracellular calcium transients induced by different stimuli (acetylcholine, caffeine and high potassium) are normal during the first days of culture. At later stages, calcium homeostasis exhibits drastic alterations with a breaking down of the calcium responses and a large [Ca2+](i)elevation. Concomitantly, Sol8 cells exhibit morphological signs of cell death like cytoplasmic shrinkage and incorporation of propidium iodide. Cell death could be significantly reduced by blocking the activity of calpains, a type of calcium-regulated proteases. These results suggest that Sol8 cell line provides an alternative model of dystrophin-deficient skeletal muscle cells for which a clear disturbance of the calcium homeostasis is observed in culture in association with calpain-dependent cell death. It is shown that transfection with a plasmid cDNA permits the forced expression of dystrophin in Sol8 myotubes as well as a correct sorting of the protein. This approach could be used to explore possible interactions between dystrophin deficiency, calcium homeostasis alteration, and dystrophic cell death.

Mast cell proliferation and alterations in bFGF amount and localization are involved in the response of muscle to dystrophin deficiency in hypertrophic feline dystrophy

To test the hypothesis that basic fibroblast growth factor and mast cells play a key role in the phenotypic differences between human dystrophinopathies and hypertrophic feline muscular dystrophy, serial sections of dystrophin-deficient, carrier and normal cat muscle biopsy specimens were examined. They were stained immunohistochemically for dystrophin and different markers of differentiation such as desmin, vimentin and utrophin. Basic fibroblast growth factor was increased in the myofibers of dystrophic cats compared to normal controls and carriers. An association of basic fibroblast growth factor with fiber regeneration and necrosis was shown. The amount of mast cells was markedly increased in muscle tissue of dystrophic cats with a clear predominance of tryptase-positive cells present in large amounts in the endomysium. Mast cells, like basic fibroblast growth factor, were concentrated in areas of muscle fiber regeneration and necrosis. Our data concerning basic fibroblast growth factor and mast cells are consistent with a highly abnormal cellular environment in feline dystrophic muscle with very high levels of basic fibroblast growth factor which is likely modulated by mast cells.

The spectrin-based skeleton at the postsynaptic membrane of the neuromuscular junction

Membrane skeletons, in particular the spectrin-based skeleton, are thought to participate in the organization of specialized membrane domains by restricting integral proteins to specific membrane sites. In the neuromuscular junction, discrete isoforms of spectrin and ankyrin, the peripheral protein that links spectrin to the membrane, colocalize with voltage-dependent sodium channels and N-CAM at the troughs of the postsynaptic membrane folds. Moreover, beta-spectrin, N-CAM, and sodium channels become clustered at the endplate during a period of time coincident with postsynaptic fold formation and synapse maturation. These observations suggest a role of the spectrin skeleton in directing and maintaining postsynaptic accumulations of sodium channels and N-CAM. In addition, the coexistence of spectrin and dystrophin at the troughs of the junctional folds raises the question of their respective functions in this membrane domain, where both cytoskeletal proteins have the potential to associate with sodium channels via ankyrin and syntrophin, respectively. Possible scenarios are discussed here with respect to accumulating evidence from studies of assembly of similar membrane domains in neurons.

Dystrophin isoforms DP71 and DP427 have distinct roles in myogenic cells

Duchenne muscular dystrophy is caused by mutations in the dystrophin gene, a complex gene that generates a family of distinct isoforms. In immature muscle cells, two dystrophin isoforms are expressed, Dp427 and Dp71. To characterize the function of Dp71 in myogenesis, we have examined the expression of Dp71 in myogenic cells. The localization of Dp71 in these cells is distinct from the localization of Dp427. Whereas Dp427 localizes to focal adhesions and surface membrane during myogenesis, Dp71 localizes to stress fiberlike structures in myogenic cells. Biochemical fractionation of myogenic cells demonstrates that Dp71 cosediments with the actin bundles thus confirming this interaction. Furthermore, transfection of C2C12 myoblasts with constructs encoding Dp71 fused to green fluorescent protein targeted the protein to the actin microfilament bundles. These results demonstrate involvement of Dp71 with the actin cytoskeleton during myogenesis and suggest a role for Dp71 that is distinct from Dp427.

Developmental regulation of myotonic dystrophy protein kinase in human muscle cells in vitro

From our previous studies, myotonic dystrophy protein kinase: gene product of myotonic dystrophy is localized at the terminal cisternae of sarcoplasmic reticulum of human adult muscle. Now we have studied the developmental expression of myotonic dystrophy protein kinase in aneurally cultured human muscles and contracting cross-striated muscles innervated with fetal rat spinal cord using a semi-quantitative reverse transcription-polymerase chain reaction method for myotonic dystrophy protein kinase messenger RNA expression, Western blot analysis, immunohistochemical examinations by laser scanning confocal microscopy and immunoelectron microscopy. About 65,000 mol. wt myotonic dystrophy protein kinase was detected in aneurally cultured muscles. Myotonic dystrophy protein kinase messenger RNA was expressed in both aneurally and innervated cultured muscles, but in early innervated cultured muscles the message was transiently lower than in aneurally cultured muscles and innervated cultured muscles in long-term co-culture. In aneurally cultured muscles, immature aneurally cultured muscles show a diffuse and irregular distribution of myotonic dystrophy protein kinase in the deeper cytoplasm near the nuclei. Ultrastructurally the immuno-products against myotonic dystrophy protein kinase were observed as dense deposits in parts of the membranes near the mitochondria. In innervated cultured muscles, immunofluorescent microscopy showed myotonic dystrophy protein kinase to be localized regularly in the I bands and A-I junctions. Ultrastructurally myotonic dystrophy protein kinase was localized in branched duct-like membranes in the early stage of innervated cultured muscles and then in small sacs at the I bands and A-I junctions of the sarcolemma in the mature stage. Our present studies strongly suggest that innervation plays an important role in the localization of myotonic dystrophy protein kinase in human skeletal muscle during development. We conclude that the expression of myotonic dystrophy protein kinase during development is under neuronal influence.

Microfilament-membrane interactions in Xenopus myocytes

We used quick-freeze, deep-etch, rotary-replication transmission electron microscopy to determine at molecular resolution the organization of microfilaments at the cytoplasmic surface of the sarcolemma of Xenopus myocytes. We demonstrate that actin microfilaments interact with the sarcolemma in two distinct ways. In one, which resembled focal contacts in Xenopus fibroblasts [Samuelsson et al., 1993: J. Cell Biol. 122:485-496], bundles of microfilaments approached the sarcolemma at sites containing aggregates of membrane-associated particles. Immunogold cytochemistry showed that these particle aggregates contained vinculin, talin and beta 1-integrin. In the second, which covered most of the cytoplasmic surface of the sarcolemma, individual actin microfilaments formed an extensive, lattice-like array. Particle aggregates associated with this array of actin microfilaments also labeled with antibodies to vinculin, talin and beta 1-integrin. The unique, lattice-like association of actin microfilaments with the membrane in Xenopus myocytes suggests that the organization of actin filaments over most of the sarcolemma is distinct from focal contacts, mediating widespread associations of the actin cytoskeleton with the cytoplasmic membrane face.

Cultured rat skeletal muscle cells treated with cytochalasin exhibit normal dystrophin expression and intracellular free calcium control

Many studies performed to elucidate the molecular and cellular processes involved in muscular dystrophies have led to the working hypothesis of a key role for the cytoskeleton elements linking the extracellular matrix to myofibrils. It was recently suggested that cytochalasin B treatment of mouse soleus muscle promoted cell damage mediated by a cytosolic increase in free calcium concentration. Since intracellular calcium overload may be a primary event resulting from the alteration of cytoskeletal structure, this study was intended to evaluate whether or not the integrity of the F-actin microfilament network is necessary for calcium homeostasis. The developmental establishment of the normal cytoarchitecture was altered by treatment of myoblasts with the actin-disrupting agents cytochalasin B and D, and the effects were compared with those in myoblasts treated with colchicine. These drugs modified the morphogenesis in that they prevented the formation of elongated myotubes by myoblast fusion, but did not prevent the maturation of contractile myogenic cells. The subcellular organisation of actin filaments visualised by confocal fluorescence microscopy was modified by colchicine and cytochalasins, but appearance of contractile apparatus and mechanical activity were not precluded. Sarcolemmal addressing of dystrophin, the subsarcolemmal protein lacking in Duchenne muscular dystrophy, was not prevented by cytochalasin. The evaluation of the basal activity of cytosolic calcium measured with indo-1 suggested that the disruption of actin or microtubules did not prevent developing muscle cells to maintain a low basal calcium activity. We propose that the global integrity of the cytoskeleton network is not crucial for the maintenance of calcium homeostasis in muscle cells developing in vitro. These results are discussed with regard to current theories attempting to understand the functional consequences of an abnormal expression of the dystrophin-glycoprotein complex interacting with the extracellular matrix and the cytoskeleton.