Hypoxia tolerance in the Norrin-deficient retina and the chronically hypoxic brain studied at single-cell resolution

<jats:p> The mammalian CNS is capable of tolerating chronic hypoxia, but cell type-specific responses to this stress have not been systematically characterized. In the Norrin KO ( <jats:italic>Ndp</jats:italic> <jats:sup> <jats:italic>KO</jats:italic> </jats:sup> ) mouse, a model of familial exudative vitreoretinopathy (FEVR), developmental hypovascularization of the retina produces chronic hypoxia of inner nuclear-layer (INL) neurons and Muller glia. We used single-cell RNA sequencing, untargeted metabolomics, and metabolite labeling from <jats:sup>13</jats:sup> C-glucose to compare WT and <jats:italic>Ndp</jats:italic> <jats:sup> <jats:italic>KO</jats:italic> </jats:sup> retinas. In <jats:italic>Ndp</jats:italic> <jats:sup> <jats:italic>KO</jats:italic> </jats:sup> retinas, we observe gene expression responses consistent with hypoxia in Muller glia and retinal neurons, and we find a metabolic shift that combines reduced flux through the TCA cycle with increased synthesis of serine, glycine, and glutathione. We also used single-cell RNA sequencing to compare the responses of individual cell types in <jats:italic>Ndp</jats:italic> <jats:sup> <jats:italic>KO</jats:italic> </jats:sup> retinas with those in the hypoxic cerebral cortex of mice that were housed for 1 week in a reduced oxygen environment (7.5% oxygen). In the hypoxic cerebral cortex, glial transcriptome responses most closely resemble the response of Muller glia in the <jats:italic>Ndp</jats:italic> <jats:sup> <jats:italic>KO</jats:italic> </jats:sup> retina. In both retina and brain, vascular endothelial cells activate a previously dormant tip cell gene expression program, which likely underlies the adaptive neoangiogenic response to chronic hypoxia. These analyses of retina and brain transcriptomes at single-cell resolution reveal both shared and cell type-specific changes in gene expression in response to chronic hypoxia, implying both shared and distinct cell type-specific physiologic responses. </jats:p>