Dynamic distribution of an antigen involved in the differentiation of avian myoblasts: II. Possible association of beta1 integrin with myofibril organization

Cell adhesion in zebrafish myogenesis: distribution of intermediate filaments, microfilaments, intracellular adhesion structures and extracellular matrix

To overcome the limitations of in vitro studies, we have been studying myogenesis in situ in zebrafish embryos, at a sub-cellular level. While in previous works we focused on myofibrillogenesis and some aspects of adhesion structures, here we describe in more detail cell adhesion structures and interactions among cytoskeletal components, membrane and extracellular matrix during zebrafish muscle development. We studied the intermediate filaments, and we describe the full range of desmin distribution in zebrafish development, from perinuclear to striated, until its deposition around the intersomite septa of older somites. This adhesion structure, positive for desmin and actin, has not been previously observed in myogenesis in vitro. We also show that actin is initially located in the intersomite septum region whereas it is confined to the myofibrils later on. While actin localization changes during development, the adhesion complex proteins vinculin, paxillin, talin, dystrophin, laminin and fibronectin always appear exclusively at the intersomite septa, and appear to be co-distributed, even though the extracellular proteins accumulates before the intracellular ones. Contrary to the adhesion proteins, that are continuously distributed, desmin and sarcomeric actin form triangular aggregates among the septa and the cytoskeleton. We studied the cytoskeletal linker plectin as well, and we show that it has a distribution similar to desmin and not to actin. We conclude that the in situ adhesion structures differ from their in vitro counterparts, and that the actual zebrafish embryo myogenesis is quite different than that which occurs in in vitro systems.

Integrin α3 subunit participates in myoblast adhesion and fusion in vitro

Satellite cells are myogenic precursor cells, participating in growth, and regeneration of skeletal muscles. The proteins that play a role in myogenesis are integrins. In this report, we show that the integrin alpha3 subunit is expressed in quiescent satellite cells and activated myoblasts. We also find that in myoblasts the integrin alpha3 subunit is localized at cell-cell and cell-extracellular matrix contacts. We notice that increase in protein and mRNA encoding the integrin alpha3 subunit accompanies myoblast differentiation. Using double immunofluorescence and immunoprecipitation experiments, we demonstrate that the integrin alpha3 subunit co-localizes with actin, and binds the integrin beta1 subunit and ADAM12, suggesting that the complex alpha3beta1/ADAM12 is probably involved in myoblast fusion. Importantly, overexpression of the full-length integrin alpha3 subunit increases myoblast fusion whereas an antibody against its extracellular domain inhibits fusion. These data demonstrate that the integrin alpha3 subunit may contribute to satellite cell activation and then myoblast adhesion and fusion.

Formation and characterization of spontaneously formed heterokaryons between quail myoblasts and 3T3-L1 preadipocytes: correlation between differential plasticity and degree of differentiation

Skeletal muscle cells and adipose cells have a close relationship in developmental lineage. Our previous study has shown that the heterokaryons between quail myoblasts and undifferentiated 3T3-L1 cells (preadipocytes) normally differentiated into myotubes, whereas the heterokaryons between myoblasts and differentiated 3T3-L1 cells (adipocytes) failed myogenic differentiation. These results suggest differences between preadipocytes and adipocytes. The purpose of this study was to clarify whether preadipocytes have flexibility in differentiation before terminal adipose differentiation. Presumptive quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) and mouse 3T3-L1 cells (either preadipocytes or adipocytes) were co-cultured for 48 h under conditions allowing myogenic differentiation. On co-culture between myoblasts and undifferentiated 3T3-L1 cells, heterokaryotic myotubes formed spontaneously, but not on co-culture with differentiated 3T3-L1 cells. In addition, the heterokaryotic myotubes expressed mouse myogenin derived from the 3T3-L1 cell gene. Our previous study indicated that the fusion sensitivity of differentiating myoblasts change with decreasing cholesterol of the cell membrane during myoblast fusion. Thus we compared the level of membrane cholesterol between undifferentiated and differentiated 3T3-L1 cells. The result showed that the level of membrane cholesterol in 3T3-L1 cells increases during adipose differentiation. Corresponding to the increase in membrane cholesterol content, differentiated 3T3-L1 cells had lower sensitivity to HVJ (Sendai virus)-mediated cell fusion than undifferentiated 3T3-L1 cells. This study demonstrated that 3T3-L1 cells at an undifferentiated state have a capacity for spontaneous fusion with differentiating myoblasts following myogenic differentiation, and that the capacity is lost after terminal adipose differentiation.

Temperature-sensitive viral infection: inhibition of hemagglutinating virus of Japan (Sendai virus) infection at 41 degrees

While investigating myoblast fusion using enveloped viruses, we unexpectedly found that the production of hemagglutinating virus of Japan (HVJ; Sendai virus) was suppressed temperature dependently in quail myoblasts transformed with a temperature-sensitive Rous sarcoma virus, which proliferate at 35.5 degrees but differentiate at 41 degrees; viral production was normal at 35.5 degrees but suppressed at 41 degrees irrespective of the species of host cells. The production of some viruses, i.e. measles virus, influenza virus, herpes simplex virus type 1 and poliovirus, was also markedly suppressed at 41 degrees, suggesting that a temperature of 41 degrees affects viral infection generally. To clarify the mechanism of the suppression, the infectious pattern of HVJ was examined both at 37 degrees and at 41 degrees in LLC-MK2 cells. The synthesis of HVJ-specific proteins was inhibited at the transcriptional level at 41 degrees, although viral penetration by envelope fusion was not affected. The transcriptional inhibition was also seen in quail fibroblasts, which do not express a 70-kD heat shock protein (HSP70), suggesting that HSP70 is dispensable for the inhibition of viral gene transcription at 41 degrees. Further, when the infected cells were incubated at 41 degrees after the viral proteins had been synthesized at 37 degrees, viral production was also inhibited. Immunofluorescent staining of the cells exposed to 41 degrees showed that HVJ envelope proteins formed large aggregates on the cell surface, into which both M and NP proteins were assembled. Under the electron microscope, HVJ virions appeared normal even at 41 degrees, but were detected in clusters on the cell surface, unlike at 37 degrees. These observations suggested that the release of HVJ virions from the cell surface was inhibited for some reason at 41 degrees. Consequently, it was indicated that two steps, viral gene transcription and the release of virions, were inhibited at 41 degrees.

Ras/MAP kinase pathway is associated with the control of myotube formation but not myofibril assembly in quail myoblasts transformed with Rous sarcoma virus

Tyrosine kinase activity of v-Src from Rous sarcoma virus (RSV) inhibits the differentiation of quail myoblasts. To clarify the inhibitory mechanism, we focused on the signaling pathways from v-Src. When the activation of the Ras/MAP (mitogen-activated protein) kinase pathway was inhibited by a dominant-negative mutant of Ras or PD98059, a specific inhibitor of p42 MAP kinase kinase, differentiation was restored; muscle specific proteins were expressed and myotubes formed even under active conditions of v-Src. Wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (P13-kinase), showed no effects on the inhibition by v-Src. These findings suggest that v-Src activates the Ras/MAP kinase signaling pathway, but not the P13-kinase pathway, and inhibits the differentiation. However, the myotubes derived from the dominant-negative Ras did not form actin fibers, suggesting that myofibril assembly is regulated by other pathway(s) from v-Src.

K252a, an indrocarbazole derivative, causes the membrane of myoblasts to enter a fusion-capable state

K252a, an indrocarbazole derivative and protein kinase inhibitor, is reported to promote myogenic differentiation in C2 mouse myoblasts. We examined the effects of K252a on QM-RSV cells, quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus. K252a promoted myotube formation of QM-RSV cells. Presumptive QM-RSV cells also formed multinucleated cells when exposed to K252a. However, the expression of myogenin, a muscle regulatory factor, was not stimulated in the presence of the drug, suggesting that it promotes membrane fusion but not myogenic differentiation. To confirm the promotion of membrane fusion by K252a, presumptive C2 cells, which are strongly resistant to HVJ-mediated cell fusion, were fused by HVJ (Sendai virus) after K252a treatment. Presumptive C2 cells treated with K252a fused with HVJ, demonstrating that K252a causes the cells to enter a fusion-capable state. The amount of membrane cholesterol, a factor that decreases membrane fluidity, fell in K252a-treated C2 cells. The results suggest that a decrease of membrane cholesterol is a cause of the change that renders myoblast membrane susceptible to fusion in the presence of K252a.

Characterization of heterokaryons between skeletal myoblasts and somatic cells formed by fusion with HVJ (Sendai virus); effects on myogenic differentiation

In skeletal myogenic differentiation, myoblasts fuse with myogenic cells spontaneously, but do not fuse with non-myogenic cells either in vivo or in vitro, suggesting that the fusion of myoblasts with non-myogenic cells is unsuitable for differentiation. To understand the inevitability of the fusion among myoblasts, we prepared heterokaryons in crosses between quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) and rodent non-myogenic cells, such as tumor cells, fibroblasts, or neurogenic cells by HVJ (Sendai virus) and examined how myogenic differentiation was influenced in the prepared heterokaryons, focusing on myogenin expression and myofibril formation as markers of differentiation. When presumptive QM-RSV cells were fused with non-myogenic cells by HVJ and induced to differentiate, both myogenin expression and myofibril formation were suppressed. When myotubes of QM-RSV cells that had already expressed myogenin and formed myofibrils were fused with non-myogenic cells, both myogenin and myofibrils disappeared. Especially, fibrous structures of myofibrils were significantly lost and dots or aggregations of F-actin were formed within 24 hr after formation of heterokaryons. However, the fusion of presumptive or differentiated QM-RSV cells with rodent myoblasts did not disturb myogenin expression or myofibril formation. These results suggest that mutual fusion of myoblasts is indispensable for normal myogenic differentiation irrespective of the species, and that some factors inhibiting myogenic differentiation exist in the cytoplasm of non-myogenic cells, but not in myoblasts.