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

Abstract

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 (NdpKO) 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 13C-glucose to compare WT and NdpKO retinas. In NdpKO 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 NdpKO 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 NdpKO 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.

Serine Catabolism Feeds NADH when Respiration Is Impaired

“…Not only does serine catabolism persist when respiration is impaired, certain regulatory mechanisms actively promote it. Hypoxia induces SHMT2 via MYC and HIF, which can also upregulate the serine synthesis pathway and other mitochondrial 1C enzymes (MTHFD2, MTHFD1L) (Heng et al, 2019;Martinez-Reyes and Chandel, 2014;Papandreou et al, 2006;Samanta et al, 2016;Ye et al, 2014). We find that this induction of serine catabolism is, at least at the cellular level, maladaptive: when respiration is impaired, genetic or pharmacological inhibition of serine catabolism enhances cell growth.…”

Exosomes mediate sensory hair cell protection in the inner ear

“…012586] were used. GLAST-CreER mice express tamoxifeninducible Cre recombinase under control of the GLAST promoter (79). GLAST-CreER ROSA26 mTmG/+ offspring were backcrossed with ROSA-26 mTmG/mTmG mice to obtain GLAST-CreER ROSA26 mTmG/mTmG offspring.…”

Single Cell Analysis in Vascular Biology

“…Specialized endothelial tip cells are the leading cells in an angiogenic sprout and control the rate of angiogenesis (124). These endothelial tip cells are rare, yet one single cell analysis study of the mouse brain identified a subset of neurovascular endothelial cells that activate tip cell genes during chronic tissue hypoxia (125). This finding suggests that, in a specific population of endothelial cells in the brain, hypoxia induces a tip cell phenotype to promote angiogenesis and reperfusion.…”