Purothionin from wheat endosperm reversibly blocks myogenic differentiation of chick embryonic muscle cells in culture

Puroindoline-a and alpha1-purothionin form ion channels in giant liposomes but exert different toxic actions on murine cells

Puroindoline-a (PIN-a) and alpha1-purothionin (alpha1-PTH), isolated from wheat endosperm of Triticum aestivum sp., have been suggested to play a role in plant defence mechanisms against phytopathogenic organisms. We investigated their ability to form pores when incorporated into giant liposomes using the patch-clamp technique. PIN-a formed cationic channels (approximately 15 pS) with the following selectivity K(+) > Na(+) >> Cl(-). Also, alpha1-PTH formed channels of approximately 46 pS and 125 pS at +100 mV, the selectivity of which was Ca(2+) > Na(+) approximately K(+) >> Cl(-) and Cl(-) >> Na(+), respectively. In isolated mouse neuromuscular preparations, alpha1-PTH induced muscle membrane depolarization, leading to blockade of synaptic transmission and directly elicited muscle twitches. Also, alpha1-PTH caused swelling of differentiated neuroblastoma NG108-15 cells, membrane bleb formation, and disorganization of F-actin. In contrast, similar concentrations of PIN-a had no detectable effects. The cytotoxic actions of alpha1-PTH on mammalian cells may be explained by its ability to induce cationic-selective channels.

Inhibitors of the proteasome block the myogenic differentiation of rat L6 myoblasts

Myogenesis is characterized by membrane fusion and accumulation of muscle specific proteins. We have previously shown that nitric oxide acts as a messenger for membrane fusion. Here we show that inhibitors of the proteasome, such as lactacystin, reversibly block both the fusion of L6 myoblasts and the accumulation of muscle specific proteins, such as myosin heavy chain (MHC). The inhibitors also reversibly prevented the induction of the NF-kappaB activity, which is required for the expression of nitric oxide synthase (NOS). Moreover, the inhibition of the NF-kappaB activity occurred in parallel with that of the NOS activity upon treatment with increasing concentrations of lactacystin. While pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB, blocked both membrane fusion and accumulation of MHC, N(G)-monomethyl-L-arginine, a specific inhibitor of NOS, inhibited only the fusion. These results suggest that the proteasome plays an essential role in the regulation of myogenic differentiation through the activation of NF-kappaB and that the target of NF-kappaB for the expression of muscle specific proteins is distinct from that for myoblast fusion.

Increase in the level of m-calpain correlates with the elevated cleavage of filamin during myogenic differentiation of embryonic muscle cells

The activity of Ca(2+)-activated proteinase requiring millimolar Ca2+ (m-calpain) was found to increase dramatically in cultured chick embryonic myoblasts during the early period of myogenic differentiation. Furthermore, the protein level of m-calpain also markedly increased in parallel with the rise in its activity, and both remained elevated thereafter. On the other hand, the activity level of calpastatin, an endogenous inhibitor of the proteinase, remained similar during the entire period of the culture. In addition, the activity of Ca(2+)-activated proteinase requiring micromolar Ca2+ (mu-calpain) was not detected in either proliferating or differentiated myoblasts. Thus, the overall capacity of Ca(2+)-dependent proteolysis is likely to increase in differentiating myoblasts and should be contributed by m-calpain. Filamin (250 kDa), that is known to facilitate actin microfilament assembly and interfere with actin-myosin filament formation, was found to be cleaved in cultured myoblasts to 240 kDa products. This filamin-cleavage occurred in a manner similar to the in vitro cleavage of the cytoskeletal protein by the purified m-calpain. Moreover, the filamin-cleavage was most evident at the period of the cell fusion. Thus, it seems likely that the in vivo cleavage of filamin is mediated by m-calpain. These results suggest that m-calpain may play an important role in cytoskeletal reorganization that is requisite for myoblast fusion.

Ca2+/calmodulin-dependent phosphorylation of the 100-kDa protein in chick embryonic muscle cells in culture

The pattern of protein phosphorylation was found to change in differentiating chick embryonic myoblasts in culture. The extent of phosphorylation of 42-, 50-, and 100-kDa proteins increased while that of a 63-kDa protein declined in extracts of myoblasts that had been cultured for increasing periods. Of these, the increase in phosphorylation of the 100-kDa protein occurred most dramatically in extracts of myoblasts in an early stage of differentiation and was specifically inhibited by trifluoperazine (TFP) and other calmodulin (CaM) antagonists including chlorpromazine and N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7). Treatment of increasing concentrations of TFP to culture medium also decreased the phosphorylation state of the 100-kDa protein and the degree of myoblast fusion in parallel. In addition, levels of both the kinase activity and the 100-kDa protein but not of CaM appeared to rise in the cells cultured for longer periods. These results suggest that (1) a Ca2+/CaM-dependent protein kinase is responsible for phosphorylation of the 100-kDa protein, (2) the TFP-mediated myoblast fusion block may be associated with the inhibitory effect of the drug against the kinase activity, and (3) the increase in phosphorylation state of the 100-kDa protein during myogenic differentiation is due to the rise in levels of the kinase and its substrate.

Okadaic acid blocks membrane fusion of chick embryonic myoblasts in culture

Okadaic acid was found to block membrane fusion of chick embryonic myoblasts in culture. It also induced morphological change of the cells from bipolar to spherical shape. These effects were dose-dependent, and could be reversed upon removal of the drug from the culture medium. It showed, however, no effect on the induction of muscle specific proteins including tropomyosin and creatine kinase. When okadaic acid was treated to the cell lysates, the phosphorylation state of many proteins significantly increased. These results suggest that the inhibition of myoblast fusion by okadaic acid may be mediated by the increase in the phosphorylation of certain, unknown protein(s) that regulate the fusion process.