To characterize the regulatory pathways involved in the inhibition of cell differentiation induced by the impairment of mitochondrial activity, we investigated the relationships occurring between organelle activity and myogenesis using an avian myoblast cell line (QM7). The inhibition of mitochondrial translation by chloramphenicol led to a potent block of myoblast differentiation. Carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone and oligomycin, which affect the organelle at different levels, exerted a similar influence. In addition, we provided evidence that this phenomenon was not the result of an alteration in cell viability. Conversely, overexpression of the mitochondrial T3 receptor (p43) stimulated organelle activity and strongly potentiated myoblast differentiation. The involvement of mitochondrial activity in an actual regulation of myogenesis is further supported by results demonstrating that the muscle regulatory gene myogenin, in contrast to CMD1 (chicken MyoD) and myf5, is a specific transcriptional target of mitochondrial activity. Whereas myogenin mRNA and protein levels were down-regulated by chloramphenicol treatment, they were up-regulated by p43 overexpression, in a positive relationship with the expression level of the transgene. We also found that myogenin or CMD1 overexpression in chloramphenicol-treated myoblasts did not restore differentiation, thus indicating that an alteration in mitochondrial activity interferes with the ability of myogenic factors to induce terminal differentiation.Recent studies emphasize that mitochondria, in addition to their well known involvement in the regulation of energy metabolism, are implicated in the regulation of cell growth and differentiation. In particular, mitochondrial events are involved in the preliminary steps of apoptosis (1), and inhibition of mitochondrial activity, either by deleting mtDNA (rho°cells) or by blocking translation in the organelle, has been shown to stop or decrease the proliferation of different cell lines (2-4). Furthermore, the general activity of the organelle, not restricted to energy production, is implicated in such regulation (5, 6). In addition, mitochondrial protein synthesis inhibition is associated with the impairment of differentiation of different cell lines, such as mouse erythroleukemia (7) and mastocytoma cells (8), neurons (9), and human (10), avian (11) or murine myoblasts (12). In agreement with these data, several pathologies are associated with mitochondrial disorders, even if the links between mitochondrial genome rearrangements or activity and pathological symptoms are not always clearly established. Despite these reports, little is known about the molecular mechanisms involved in these regulations. First, the exclusive use of inhibitors of mitochondrial function in previous reports was not fully adapted to demonstrating the occurrence of an actual regulatory pathway involving mitochondrial activity in the regulation of cell differentiation. Second, the nature of the molecular signals underlying the...
Tripartite multidrug efflux systems of Gram-negative bacteria are composed of an inner membrane transporter, an outer membrane channel and a periplasmic adaptor protein. They are assumed to form ducts inside the periplasm facilitating drug exit across the outer membrane. Here we present the reconstitution of native Pseudomonas aeruginosa MexAB–OprM and Escherichia coli AcrAB–TolC tripartite Resistance Nodulation and cell Division (RND) efflux systems in a lipid nanodisc system. Single-particle analysis by electron microscopy reveals the inner and outer membrane protein components linked together via the periplasmic adaptor protein. This intrinsic ability of the native components to self-assemble also leads to the formation of a stable interspecies AcrA–MexB–TolC complex suggesting a common mechanism of tripartite assembly. Projection structures of all three complexes emphasize the role of the periplasmic adaptor protein as part of the exit duct with no physical interaction between the inner and outer membrane components.
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