Glycoprotein glycans may not be necessary for the differentiation of skeletal myoblasts

Developmental potential of rat L6 myoblasts in vivo following injection into regenerating muscles

To examine the relative importance of myoblast lineage and environmental influences on the development of muscle fiber types in vivo, the phenotype of muscle fibers formed from rat L6 myoblasts was examined following their injection into different regenerating adult muscles. Myoblasts were infected with a retroviral vector carrying a LacZ reporter gene and their fate in vivo was examined using a panel of antibodies against various myosin heavy chain (MyHC) isoforms. Since L6 myoblasts express IIX MyHC following differentiation in vitro, we wanted to determine if they would form IIX muscle fibers in vivo and whether innervation would alter this fate. Following injection, L6 cells either fused with each other to form homotypic fibers or fused with host muscle cells to form heterotypic fibers. Initially, homotypic fibers expressed embryonic MyHC-similar to L6 myotubes in vitro. However, by 4 weeks postinjection IIX MyHC had replaced embryonic MyHC as the predominant isoform. Single fiber analysis using an antibody specific for NCAM indicated that this transition was independent of innervation. Analysis of heterotypic fibers resulting from the incorporation of donor L6 myoblasts into host fast IIA and IIB fibers revealed that L6-derived nuclei express embryonic and IIX MyHCs for up to 8 weeks postinjection, often as nuclear domains surrounding L6 nuclei. These results suggest that MyHC expression in muscle fibers derived from L6 myoblasts is regulated, in part, by intrinsic factors that limit the fiber type potential of these cells in vivo.

Involvement of a cell surface protein and an ecto-protein kinase in myogenesis

Myogenic differentiation is composed of a sequential cascade of multiple steps leading to the formation of multinucleated myotubes. The interference with any one step would abolish myogenesis. The present investigation examined the cell surface components which might be involved in myogenesis. Studies with subconfluent day 2 cultures of rat L6 myoblasts revealed that a cell surface 112 kDa protein was phosphorylated by a Ca(2+)-, F(-)- and Mg(2+)-dependent ecto-protein kinase [Chen & Lo (1991) Biochem. J. 279, 467-474]. We have shown in the present investigation that adequate ATP was present on the cell surface for efficient functioning of this ecto-protein kinase. The phosphorylation of the 112 kDa protein by this ecto-protein kinase was decrease dramatically in confluent cells and in multinucleated myotubes. The following evidence suggests that both the 112 kDa protein and the ecto-protein kinase may play important roles in myogenesis. (i) The highest phosphorylation activity was observed in subconfluent cultures, i.e. before the onset of morphological differentiation. (ii) Treatment of cells with chemical reagents resulted in a corresponding decrease in the ecto-protein kinase, the 112 kDa protein, the phosphorylated 112 kDa protein (p112) and the ability to form myotubes. (iii) The level of p112 in a conditional myogenesis-defective mutant corresponded with the cells' eventual ability to differentiate. (iv) A mutant defective in the ecto-protein kinase was impaired in the phosphorylation of the 112 kDa protein and in myogenesis. (v) A mutant containing only residual levels of the 112 kDa protein was deficient in both p112 and myogenesis. (vi) Since the level of p112 was normal in another myogenesis-defective mutant, the phosphorylation of this protein was not likely to be a consequence of myogenic differentiation. The above findings suggest that the ecto-protein kinase and the 112 kDa protein may directly or indirectly be associated with the myogenic pathway. Since the levels of the ecto-protein kinase, the 112 kDa protein and p112 decreased dramatically upon the formation of myotubes, these proteins were probably not required once morphological differentiation had been initiated.

Towards understanding skeletal muscle regeneration

Factors which effect proliferation and fusion of muscle precursor cells have been studied extensively in tissue culture, although little is known about these events in vivo. This review assesses the tissue culture derived data with a view to understanding factors which may control the regeneration of mature skeletal muscle in vivo. The following topics are discussed in the light of recent developments in cell and molecular biology: 1) Injury and necrosis of mature skeletal muscle fibres 2) Phagocytosis of myofibre debris 3) Revascularisation of injured muscle 4) Activation and proliferation of muscle precursor cells (mpc) in vivo Identification of mpcs; Satellite cell relationships; Extracellular matrix; Growth factors; Hormones; Replication. 5) Differentiation and fusion of muscle precursor cells in vivo Differentiation; Fusion; Extracellular matrix; Cell surface molecules: Growth factors and prostaglandins 6) Myotubes and innervation.

Involvement of hexose transport in myogenic differentiation

A high (HAHT) and a low (LAHT) affinity hexose transport system are present in undifferentiated rat L6 myoblasts; however, only the latter can be detected in multinucleated myotubes. This suggests that HAHT is either down-regulated or modified as a result of myogenesis. The present investigation examined the relationship between HAHT and myogenic differentiation. While myogenesis could be inhibited by the potent hexose transport inhibitor phloretin, it was not affected by phlorizin which had no effect on hexose transport. This relationship was further explored using six different HAHT-defective mutants. All six mutants, altered in either the HAHT transport affinity (Type I mutants) or capacity (Type II mutants), were impaired in myogenesis. Since these mutants were selected from both mutagenized and non-mutagenized cells with different reagents, or with different concentrations of the same reagent, the deficiency in myogenesis was likely due to changes in HAHT properties. This notion was confirmed by the observation that growth of Type I mutants in high D-glucose concentrations could rectify the defect in myogenesis. D-glucose was unlikely to rectify the defect in myogenesis, if this defect was due to a second unrelated mutation that may have arisen during isolation of the mutants. Since both types of mutants were not altered in LAHT, D-glucose should still be taken up into the cells. The fact that the glucose-mediated increase in fusion could not be observed in Type II mutants (deficient in the HAHT transporter) suggested that myogenesis was dependent on the presence of D-glucose or its metabolites in specific HAHT-accessible compartments. It is tempting to speculate that trans-acting regulators involved in myogenesis may be synthesized from the glucose metabolites in these specialized HAHT-accessible compartments.