ABSTRACT
Costello syndrome (CS) is a gain of function Rasopathy caused by heterozygous activating mutations in the HRAS gene. Patients show brain dysfunction that can include abnormal brain white matter. Transgenic activation of HRas in the entire mouse oligodendrocyte lineage resulted in myelin defects and behavioral abnormalities, suggesting roles for disrupted myelin in CS brain dysfunction. Here, we studied a mouse model in which the endogenous HRas gene is conditionally replaced by mutant HRasG12V in mature oligodendrocytes, to separate effects in mature myelinating cells from developmental events. Increased myelin thickness due to decompaction was detectable within one month of HRasG12V expression in the corpus callosum of adult mice. Increases in active ERK and Nitric Oxide (NO) were present in HRas mutants and inhibition of NO synthase (NOS) or MEK each partially rescued myelin decompaction. In addition, genetic or pharmacologic inhibition of Notch signaling improved myelin compaction. Complete rescue of myelin structure required dual drug treatments combining MAPK, NO, or Notch inhibition; with MEK + NOS blockade producing the most robust effect. We suggest that individual or concomitant blockade of these pathways in CS patients may improve aspects of brain function.
Subject(s)
MAP Kinase Signaling System/physiology , Mitogen-Activated Protein Kinase Kinases/metabolism , Myelin Sheath/metabolism , Nitric Oxide/metabolism , Oligodendroglia/metabolism , Proto-Oncogene Proteins p21(ras)/metabolism , Receptors, Notch/metabolism , Animals , Corpus Callosum/pathology , Corpus Callosum/ultrastructure , Enzyme Inhibitors/pharmacology , Immunoglobulin J Recombination Signal Sequence-Binding Protein/genetics , Immunoglobulin J Recombination Signal Sequence-Binding Protein/metabolism , MAP Kinase Signaling System/genetics , Mice , Mice, Inbred C57BL , Mice, Transgenic , Microscopy, Electron , Mutation/genetics , Myelin Proteolipid Protein/genetics , Myelin Proteolipid Protein/metabolism , Myelin Sheath/ultrastructure , NG-Nitroarginine Methyl Ester/pharmacology , Oligodendroglia/ultrastructure , Proto-Oncogene Proteins p21(ras)/genetics , Tamoxifen/pharmacologyABSTRACT
The RASopathy neurofibromatosis type 1 (NF1) is one of the most common autosomal dominant genetic disorders. In NF1 patients, neurological issues may result from damaged myelin, and mice with a neurofibromin gene (Nf1) mutation show white matter (WM) defects including myelin decompaction. Using mouse genetics, we find that altered Nf1 gene-dose in mature oligodendrocytes results in progressive myelin defects and behavioral abnormalities mediated by aberrant Notch activation. Blocking Notch, upstream mitogen-activated protein kinase (MAPK), or nitric oxide signaling rescues myelin defects in hemizygous Nf1 mutants, and pharmacological gamma secretase inhibition rescues aberrant behavior with no effects in wild-type (WT) mice. Concomitant pathway inhibition rescues myelin abnormalities in homozygous mutants. Notch activation is also observed in Nf1+/- mouse brains, and cells containing active Notch are increased in NF1 patient WM. We thus identify Notch as an Nf1 effector regulating myelin structure and behavior in a RASopathy and suggest that inhibition of Notch signaling may be a therapeutic strategy for NF1.
