PROJECT SUMMARY Oligodendrocytes are a highly specialized type of glial cell that myelinates axons of the vertebrate CNS, both promoting rapid transmission of nerve impulses via saltatory conduction and providing metabolic and trophic support for the axons. A widely held consensus view in the field holds that the loss of this oligodendrocyte support (as seen in demyelinating conditions such as multiple sclerosis) directly contributes to axonal loss and cumulative disability. Nevertheless, direct experimental evidence for oligodendrocyte support being required for axonal survival is largely lacking. In addition, there is remarkably little information available on how neurons respond to a primary demyelinating insult. To better understand whether loss of oligodendrocyte support of axons directly leads to axonal loss or neuronal changes, we will capitalize on two tamoxifen-inducible conditional knockout mouse strains for the Myrf gene, which encodes a transcription factor required for myelination and myelin maintenance. The Myrf∆iPLP mouse line leads to oligodendrocyte loss and severe CNS demyelination, but largely remyelinates through recruitment of non-recombined OPCs. In contrast, the Myrf∆iSox10 mouse line shows near complete CNS demyelination with remyelination failure, representing a unique mouse model of severe and chronic demyelination. Based on our preliminary data we hypothesize that neurons are initially resilient to even severe demyelinating insults, but that they become vulnerable to loss in the face of subsequent remyelination failure, inflammation or metabolic challenge. In addition, we have found activation of the DLK/JNK/c-Jun axonal stress pathway in chronically demyelinated animals, providing a potential molecular signal linking chronic demyelination to neurodegeneration. We will use retinal ganglion cells (RGCs) in these mice as the ideal myelinated neuronal population to: 1) determine the relative roles of remyelination failure, neuroinflammation and metabolic challenge in inducing neurodegeneration; 2) establish the role of the DLK/JNK/c-Jun in mediating neuronal changes following loss of oligodendrocyte support and 3) Use bulk and single cell RNA-Seq to determine the transcriptional changes in RGCs in response to loss of glial support and to determine whether specific subgroups of RGCs show preferential vulnerability to demyelination.