Arrest of Myelination and Reduced Axon Growth When Schwann Cells Lack mTOR

International audienc

Muscle inactivation of mTOR causes metabolic and dystrophin defects leading to severe myopathy

Mammalian target of rapamycin (mTOR) is a key regulator of cell growth that associates with raptor and rictor to form the mTOR complex 1 (mTORC1) and mTORC2, respectively. Raptor is required for oxidative muscle integrity, whereas rictor is dispensable. In this study, we show that muscle-specific inactivation of mTOR leads to severe myopathy, resulting in premature death. mTOR-deficient muscles display metabolic changes similar to those observed in muscles lacking raptor, including impaired oxidative metabolism, altered mitochondrial regulation, and glycogen accumulation associated with protein kinase B/Akt hyperactivation. In addition, mTOR-deficient muscles exhibit increased basal glucose uptake, whereas whole body glucose homeostasis is essentially maintained. Importantly, loss of mTOR exacerbates the myopathic features in both slow oxidative and fast glycolytic muscles. Moreover, mTOR but not raptor and rictor deficiency leads to reduced muscle dystrophin content. We provide evidence that mTOR controls dystrophin transcription in a cell-autonomous, rapamycin-resistant, and kinase-independent manner. Collectively, our results demonstrate that mTOR acts mainly via mTORC1, whereas regulation of dystrophin is raptor and rictor independent

Muscle inactivation of mTOR causes metabolic and dystrophin defects leading to severe myopathy

mTor, acting mainly via mTORC1, controls dystrophin transcription in a raptor- and rictor-independent mechanism

The ESCRT-0 subcomplex component Hrs/Hgs is a master regulator of myogenesis via modulation of signaling and degradation pathways

International audienceAbstract Background Myogenesis is a highly regulated process ending with the formation of myotubes, the precursors of skeletal muscle fibers. Differentiation of myoblasts into myotubes is controlled by myogenic regulatory factors (MRFs) that act as terminal effectors of signaling cascades involved in the temporal and spatial regulation of muscle development. Such signaling cascades converge and are controlled at the level of intracellular trafficking, but the mechanisms by which myogenesis is regulated by the endosomal machinery and trafficking is largely unexplored. The Endosomal Sorting Complex Required for Transport (ESCRT) machinery composed of four complexes ESCRT-0 to ESCRT-III regulates the biogenesis and trafficking of endosomes as well as the associated signaling and degradation pathways. Here, we investigate its role in regulating myogenesis. Results We uncovered a new function of the ESCRT-0 hepatocyte growth factor-regulated tyrosine kinase substrate Hrs/Hgs component in the regulation of myogenesis. Hrs depletion strongly impairs the differentiation of murine and human myoblasts. In the C2C12 murine myogenic cell line, inhibition of differentiation was attributed to impaired MRF in the early steps of differentiation. This alteration is associated with an upregulation of the MEK/ERK signaling pathway and a downregulation of the Akt2 signaling both leading to the inhibition of differentiation. The myogenic repressors FOXO1 as well as GSK3β were also found to be both activated when Hrs was absent. Inhibition of the MEK/ERK pathway or of GSK3β by the U0126 or azakenpaullone compounds respectively significantly restores the impaired differentiation observed in Hrs-depleted cells. In addition, functional autophagy that is required for myogenesis was also found to be strongly inhibited. Conclusions We show for the first time that Hrs/Hgs is a master regulator that modulates myogenesis at different levels through the control of trafficking, signaling, and degradation pathways

The histone fold is a key structural motif of transcription factor TFIID

International audienc

The Human Transcription Factor IID Subunit Human TATA-binding Protein-associated Factor 28 Interacts in a Ligand-reversible Manner with the Vitamin D 3 and Thyroid Hormone Receptors

International audienceUsing coexpression in COS cells, we have identified novel interactions between the human TATA-binding protein-associated factor 28 (hTAF(II)28) component of transcription factor IID and the ligand binding domains (LBDs) of the nuclear receptors for vitamin D3 (VDR) and thyroid hormone (TRalpha). Interaction between hTAF(II)28 and the VDR and TR LBDs was ligand-reversible, whereas no interactions between hTAF(II)28 and the retinoid X receptors (RXRs) or other receptors were observed. TAF(II)28 interacted with two regions of the VDR, a 40-amino acid region spanning alpha-helices H3-H5 and alpha-helix H8. Interactions were also observed with the H3-H5 region of the TRalpha but not with the equivalent highly related region of the RXRgamma. Fine mapping using RXR derivatives in which single amino acids of the RXRgamma LBD have been replaced with their VDR counterparts shows that the determinants for interaction with hTAF(II)28 are located in alpha-helix H3 and are not identical to those previously identified for interactions with hTAF(II)55. We also describe a mutation in the H3-H5 region of the VDR LBD, which abolishes transactivation, and we show that interaction of hTAF(II)28 with this mutant is no longer ligand-reversible

Dissecting the interaction network of multiprotein complexes by pairwise coexpression of subunits in E. coli11Edited by K. Nagai

International audienc

Human TAF(II)55 Interacts with the Vitamin D(3) and Thyroid Hormone Receptors and with Derivatives of the Retinoid X Receptor That Have Altered Transactivation Properties

We have identified novel interactions between the human (h)TATA-binding protein-associated factor TAF(II)55 and the ligand-binding domains (LBDs) of the nuclear receptors for vitamin D(3) (VDR) and thyroid hormone (TRα). Following expression in Cos cells, hTAF(II)55 interacts with the VDR and TRα LBDs in a ligand-independent manner whereas no interactions with the retinoid X receptors (RXRs) or with other receptors were observed. Deletion mapping indicates that hTAF(II)55 interacts with a 40-amino-acid region spanning α-helices H3 to H5 of the VDR and TRα LBDs but not with the equivalent highly related region of RXRγ. TAF(II)55 also interacts with chimeric receptors in which the H3-to-H5 region of RXRγ has been replaced with that of the VDR or TRα. Furthermore, replacement of two single amino acids of the RXRγ LBD with their VDR counterparts allows the RXRγ LBD to interact with hTAF(II)55 while the corresponding double substitution allows a much stronger interaction. In transfection experiments, the single mutated RXRγ LBDs activate transcription to fivefold higher levels than wild-type RXRγ while the double mutation activates transcription to a level comparable to that observed with the VDR. There is therefore a correlation between the ability of the modified RXRs to interact with hTAF(II)55 and transactivation. These results strongly suggest that the TAF(II)55 interactions with the modified RXR LBDs modulate transcriptional activation

The Human TFIID Components TAF(II)135 and TAF(II)20 and the Yeast SAGA Components ADA1 and TAF(II)68 Heterodimerize to Form Histone-Like Pairs

It has been previously proposed that the transcription complexes TFIID and SAGA comprise a histone octamer-like substructure formed from a heterotetramer of H4-like human hTAF(II)80 (or its Drosophila melanogaster dTAF(II)60 and yeast [Saccharomyces cerevisiae] yTAF(II)60 homologues) and H3-like hTAF(II)31 (dTAF(II)40 and yTAF(II)17) along with two homodimers of H2B-like hTAF(II)20 (dTAF(II)30α and yTAF(II)61/68). However, it has not been formally shown that hTAF(II)20 heterodimerizes via its histone fold. By two-hybrid analysis with yeast and biochemical characterization of complexes formed by coexpression in Escherichia coli, we showed that hTAF(II)20 does not homodimerize but heterodimerizes with hTAF(II)135. Heterodimerization requires the α2 and α3 helices of the hTAF(II)20 histone fold and is abolished by mutations in the hydrophobic face of the hTAF(II)20 α2 helix. Interaction with hTAF(II)20 requires a domain of hTAF(II)135 which shows sequence homology to H2A. This domain also shows homology to the yeast SAGA component ADA1, and we show that yADA1 heterodimerizes with the histone fold region of yTAF(II)61/68, the yeast hTAF(II)20 homologue. These results are indicative of a histone fold type of interaction between hTAF(II)20-hTAF(II)135 and yTAF(II)68-yADA1, which therefore constitute novel histone-like pairs in the TFIID and SAGA complexes

Crystal Structure of a Subcomplex of Human Transcription Factor TFIID Formed by TATA Binding Protein-associated Factors hTAF4 (hTAF II 135) and hTAF12 (hTAF II 20)

International audienc