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

Cells as factories for humanized encapsulation

Biocompatibility is of paramount importance for drug delivery, tumor labeling, and in vivo application of nanoscale bioprobes. Until now, biocompatible surface processing has typically relied on PEGylation and other surface coatings, which, however, cannot minimize clearance by macrophages or the renal system but may also increase the risk of chemical side effects. Cell membranes provide a generic and far more natural approach to the challenges of encapsulation and delivery in vivo. Here we harness for the first time living cells as "factories" to manufacture cell membrane capsules for encapsulation and delivery of drugs, nanoparticles, and other biolabels. Furthermore, we demonstrate that the built-in protein channels of the new capsules can be utilized for controlled release of encapsulated reagents.

Myoblast fusion: when it takes more to make one

Cell-cell fusion is a crucial and highly regulated event in the genesis of both form and function of many tissues. One particular type of cell fusion, myoblast fusion, is a key cellular process that shapes the formation and repair of muscle. Despite its importance for human health, the mechanisms underlying this process are still not well understood. The purpose of this review is to highlight the recent literature pertaining to myoblast fusion and to focus on a comparison of these studies across several model systems, particularly the fly, zebrafish and mouse. Advances in technical analysis and imaging have allowed identification of new fusion genes and propelled further characterization of previously identified genes in each of these systems. Among the cellular steps identified as critical for myoblast fusion are migration, recognition, adhesion, membrane alignment and membrane pore formation and resolution. Importantly, striking new evidence indicates that orthologous genes govern several of these steps across these species. Taken together, comparisons across three model systems are illuminating a once elusive process, providing exciting new insights and a useful framework of genes and mechanisms.

Involvment of c‐abl in the radiation‐induced inhibition of myoblast differentiation

PURPOSE

While radiation-induced differentiation is common in many cell types it has been shown (Ikeda et al. 2000) that irradiation inhibits differentiation of murine myoblast cell line C2C12. To investigate the mechanisms responsible for this phenomenon we looked into the role of the non-receptor tyrosine kinase c-Abl.

MATERIAL AND METHODS

C-Abl activity was determined using an immune-complex kinase assay and the functional role of c-Abl activity was assessed by using the kinase inhibitor STI 571 (Gleevec) and by transfecting a dominant negative c-Abl mutant (K290R).

RESULTS

We found an up-regulation of c-Abl activity 6h after differentiation induction when compared to uninduced control cells. Exposure of C2C12 cells to ionizing radiation, which inhibits myotube formation, inhibited this up-regulation. Transfected dominant negative c-Abl as well as STI 571 treatment and ionizing radiation inhibited myotube formation but did not significantly suppress the expression of the muscle-specific transcription factor Myogenin or of the differentiation marker Myosin heavy chain.

CONCLUSIONS

These data suggest that c-Abl is involved in the radiation-induced inhibition of myoblast differentiation and lead to the hypothesis that c-Abl plays a role in late events during myogenic differentiation probably in the process of myoblast fusion.

Drosophila D-titin is required for myoblast fusion and skeletal muscle striation

An ethylmethane sulfonate (EMS) mutagenesis of Drosophila melanogaster aimed at discovering novel genes essential for neuromuscular development identified six embryonic lethal alleles of one genetic locus on the third chromosome at 62C. Two additional lethal P element insertion lines, l(3)S02001 and l(3)j1D7, failed to complement each other and each of the six EMS alleles. Analysis of genomic sequence bracketing the two insertion sites predicted a protein of 16,215 amino acid residues, encoded by a 70 kb genomic region. This sequence includes the recently characterized kettin, and includes all known partial D-Titin sequences. We call the genetic locus, which encodes both D-Titin and kettin, D-Titin. D-Titin has 53 repeats of the immunoglobulin C2 domain, 6 repeats of the fibronectin type III domain and two large PEVK domains. Kettin appears to be the NH2-terminal one third of D-Titin, presumably expressed via alternative splicing. Phenotype assays on the allelic series of D-Titin mutants demonstrated that D-Titin plays an essential role in muscle development. First, D-Titin has an unsuspected function in myoblast fusion during myogenesis and, second, D-Titin later serves to organize myofilaments into the highly ordered arrays underlying skeletal muscle striation. We propose that D-Titin is instrumental in the development of the two defining features of striated muscle: the formation of multi-nucleate syncitia and the organization of actin-myosin filaments into striated arrays.

Mechanisms of resistance to pathogenesis in muscular dystrophies

A mechanistic definition of the dystrophic process is proposed, and the effects of growth factors vs. down-regulation of growth are critically analyzed. A conceptual scheme is presented to illustrate the steps leading to pathology, and various compensatory systems which ameliorate the pathology are examined, particularly in regards to the mdv mouse which is resistant to the deficiency of dystrophin, the main protein product of the Duchenne and Becker muscular dystrophy (DMD/BMD) gene. These compensatory systems are analyzed in terms of the differential resistance of fiber types to pathogenesis. The generation of a stable population of maturationally arrested centronucleated fibers which express the mature adult myosin isoforms is proposed to be the main strategy of mdx muscle to minimize apoptosis. Physiological properties of these fibers, such as utrophin expression, and high mitochondrial and endoplasmic reticulum content, together with probable increased glycerophosphorylcholine concentrations and facile access to the vascular system, are hypothesized to be instrumental in their resistance to pathogenesis. It is proposed that the major element that determines the susceptibility of most human muscles to the dystrophic process is their inability to arrest the maturation of regenerated fibers at the centronucleated stage with a concomitant expression of the adult myosins.

Drosophila myoblast city encodes a conserved protein that is essential for myoblast fusion, dorsal closure, and cytoskeletal organization

The Drosophila myoblast city (mbc) locus was previously identified on the basis of a defect in myoblast fusion (Rushton et al., 1995. Development [Camb.]. 121:1979-1988). We describe herein the isolation and characterization of the mbc gene. The mbc transcript and its encoded protein are expressed in a broad range of tissues, including somatic myoblasts, cardial cells, and visceral mesoderm. It is also expressed in the pole cells and in ectodermally derived tissues, including the epidermis. Consistent with this latter expression, mbc mutant embryos exhibit defects in dorsal closure and cytoskeletal organization in the migrating epidermis. Both the mesodermal and ectodermal defects are reminiscent of those induced by altered forms of Drac1 and suggest that mbc may function in the same pathway. MBC bears striking homology to human DOCK180, which interacts with the SH2-SH3 adapter protein Crk and may play a role in signal transduction from focal adhesions. Taken together, these results suggest the possibility that MBC is an intermediate in a signal transduction pathway from the rho/rac family of GTPases to events in the cytoskeleton and that this pathway may be used during myoblast fusion and dorsal closure.