@article{HornBaumannPereiraetal.2012, author = {Horn, Michael and Baumann, Reto and Pereira, Jorge A. and Sidiropoulos, P{\´a}ris N. M. and Somandin, Christian and Welzl, Hans and Stendel, Claudia and L{\"u}hmann, Tessa and Wessig, Carsten and Toyka, Klaus V. and Relvas, Jo{\~a}o B. and Senderek, Jan and Suter, Ueli}, title = {Myelin is dependent on the Charcot-Marie-Tooth Type 4H disease culprit protein FRABIN/FGD4 in Schwann cells}, series = {Brain}, volume = {135}, journal = {Brain}, doi = {10.1093/brain/aws275}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-125390}, pages = {3567-3583}, year = {2012}, abstract = {Studying the function and malfunction of genes and proteins associated with inherited forms of peripheral neuropathies has provided multiple clues to our understanding of myelinated nerves in health and disease. Here, we have generated a mouse model for the peripheral neuropathy Charcot-Marie-Tooth disease type 4H by constitutively disrupting the mouse orthologue of the suspected culprit gene FGD4 that encodes the small RhoGTPase Cdc42-guanine nucleotide exchange factor Frabin. Lack of Frabin/Fgd4 causes dysmyelination in mice in early peripheral nerve development, followed by profound myelin abnormalities and demyelination at later stages. At the age of 60 weeks, this was accompanied by electrophysiological deficits. By crossing mice carrying alleles of Frabin/Fgd4 flanked by loxP sequences with animals expressing Cre recombinase in a cell type-specific manner, we show that Schwann cell-autonomous Frabin/Fgd4 function is essential for proper myelination without detectable primary contributions from neurons. Deletion of Frabin/Fgd4 in Schwann cells of fully myelinated nerve fibres revealed that this protein is not only required for correct nerve development but also for accurate myelin maintenance. Moreover, we established that correct activation of Cdc42 is dependent on Frabin/Fgd4 function in healthy peripheral nerves. Genetic disruption of Cdc42 in Schwann cells of adult myelinated nerves resulted in myelin alterations similar to those observed in Frabin/Fgd4-deficient mice, indicating that Cdc42 and the Frabin/Fgd4-Cdc42 axis are critical for myelin homeostasis. In line with known regulatory roles of Cdc42, we found that Frabin/Fgd4 regulates Schwann cell endocytosis, a process that is increasingly recognized as a relevant mechanism in peripheral nerve pathophysiology. Taken together, our results indicate that regulation of Cdc42 by Frabin/Fgd4 in Schwann cells is critical for the structure and function of the peripheral nervous system. In particular, this regulatory link is continuously required in adult fully myelinated nerve fibres. Thus, mechanisms regulated by Frabin/Fgd4-Cdc42 are promising targets that can help to identify additional regulators of myelin development and homeostasis, which may crucially contribute also to malfunctions in different types of peripheral neuropathies.}, language = {en} } @article{NiemannHuberWagneretal.2014, author = {Niemann, Axel and Huber, Nina and Wagner, Konstanze M. and Somandin, Christian and Horn, Michael and Lebrun-Julien, Fr{\´e}d{\´e}ric and Angst, Brigitte and Pereira, Jorge A. and Halfter, Hartmut and Welzl, Hans and Feltri, M. Laura and Wrabetz, Lawrence and Young, Peter and Wessig, Carsten and Toyka, Klaus V. and Suter, Ueli}, title = {The Gdap1 knockout mouse mechanistically links redox control to Charcot-Marie-Tooth disease}, series = {Brain}, volume = {137}, journal = {Brain}, number = {3}, doi = {10.1093/brain/awt371}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-120731}, pages = {668-82}, year = {2014}, abstract = {The ganglioside-induced differentiation-associated protein 1 (GDAP1) is a mitochondrial fission factor and mutations in GDAP1 cause Charcot-Marie-Tooth disease. We found that Gdap1 knockout mice (\(Gdap1^{-/-}\)), mimicking genetic alterations of patients suffering from severe forms of Charcot-Marie-Tooth disease, develop an age-related, hypomyelinating peripheral neuropathy. Ablation of Gdap1 expression in Schwann cells recapitulates this phenotype. Additionally, intra-axonal mitochondria of peripheral neurons are larger in \(Gdap1^{-/-}\) mice and mitochondrial transport is impaired in cultured sensory neurons of \(Gdap1^{-/-}\) mice compared with controls. These changes in mitochondrial morphology and dynamics also influence mitochondrial biogenesis. We demonstrate that mitochondrial DNA biogenesis and content is increased in the peripheral nervous system but not in the central nervous system of \(Gdap1^{-/-}\) mice compared with control littermates. In search for a molecular mechanism we turned to the paralogue of GDAP1, GDAP1L1, which is mainly expressed in the unaffected central nervous system. GDAP1L1 responds to elevated levels of oxidized glutathione by translocating from the cytosol to mitochondria, where it inserts into the mitochondrial outer membrane. This translocation is necessary to substitute for loss of GDAP1 expression. Accordingly, more GDAP1L1 was associated with mitochondria in the spinal cord of aged \(Gdap1^{-/-}\) mice compared with controls. Our findings demonstrate that Charcot-Marie-Tooth disease caused by mutations in GDAP1 leads to mild, persistent oxidative stress in the peripheral nervous system, which can be compensated by GDAP1L1 in the unaffected central nervous system. We conclude that members of the GDAP1 family are responsive and protective against stress associated with increased levels of oxidized glutathione.}, language = {en} } @article{NorrmenFigliaLebrunJulienetal.2014, author = {Norrmen, Camilla and Figlia, Gianluca and Lebrun-Julien, Frederic and Pereira, Jorge A. and Tr{\"o}tzm{\"u}ller, Martin and K{\"o}feler, Harald C. and Rantanen, Ville and Wessig, Carsten and van Deijk, Anne-Lieke F. and Smit, August B. and Verheijen, Mark H. G. and R{\"u}egg, Markus A. and Hall, Michael N. and Suter, Ueli}, title = {mTORC1 Controls PNS Myelination along the mTORC1-RXR gamma-SREBP-Lipid Biosynthesis Axis in Schwann Cells}, series = {Cell Reports}, volume = {9}, journal = {Cell Reports}, number = {2}, issn = {2211-1247}, doi = {10.1016/j.celrep.2014.09.001}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-114847}, pages = {646-660}, year = {2014}, abstract = {Myelin formation during peripheral nervous system (PNS) development, and reformation after injury and in disease, requires multiple intrinsic and extrinsic signals. Akt/mTOR signaling has emerged as a major player involved, but the molecular mechanisms and downstream effectors are virtually unknown. Here, we have used Schwann-cell-specific conditional gene ablation of raptor and rictor, which encode essential components of the mTOR complexes 1 (mTORC1) and 2 (mTORC2), respectively, to demonstrate that mTORC1 controls PNS myelination during development. In this process, mTORC1 regulates lipid biosynthesis via sterol regulatory element-binding proteins (SREBPs). This course of action is mediated by the nuclear receptor RXRg, which transcriptionally regulates SREBP1c downstream of mTORC1. Absence of mTORC1 causes delayed myelination initiation as well as hypomyelination, together with abnormal lipid composition and decreased nerve conduction velocity. Thus, we have identified the mTORC1-RXR gamma-SREBP axis controlling lipid biosynthesis as a major contributor to proper peripheral nerve function.}, language = {en} }