Résumé
In previous studies, we have characterized a new hormonal pathway involving a mitochondrial T3 receptor (p43) acting as a mitochondrial transcription factor and consequently stimulating mitochondrial activity and mitochondriogenesis. Numerous studies bring evidence that besides their major role in the regulation of energetic metabolism, mitochondria also regulate important processes involved in development such as apoptosis or cell proliferation and differentiation. We have established the involvement of this T3 pathway in the regulation of myoblast differentiation, through the control of the expression of a set of nuclear genes, regulated by mitochondrial activity (myogenin, calcineurin and the c-myc proto-oncogene). In order to assess the physiological importance of this direct mitochondrial T3 pathway onmuscle development, with a particular attention to the acquisition of metabolic and contractile features of muscle fibres, we have generated mice overexpressing p43 under control of the human a-skeletal actin promoter. In agreement with the previous characterization of this promoter, northern-blot and western-blot experiments confirmed that p43 was specifically overexpressed in skeletal muscle. Studies performedon gastrocnemius permeabilized fibres using a highly sensitive oxygraph (Oroboros) indicated that p43 overexpression stimulates mitochondrial respiration and cytochrome c oxydase activity. Moreover, electron microscopy observations demonstrated the occurrence of a strong increase in muscle mitochondria number. Histoimmunochemistry experiments using antibodies raised against specific Myosin Heavy Chain (I, IIA, IIA+IIB) indicated that p43 overexpression induces significant changes in contractile features of muscle fibres. In gastrocnemius muscle, we observed an increase in the number of type I and type IIa fibres, associated to a decrease in the number of type IIB fibres. These data indicate that, as expected from our previous in vitro studies, p43 regulates mitochondrial activity in vivo, and is an important determinant of muscle fibres metabolic and contractile phenotype.