Neuroglobin expression and function: lessons from single cell transcriptome data and a mouse knock-out model

Neuroglobin (Ngb) is a highly conserved hemoprotein expressed in the vertebrate nervous system. A diverse range of biological functions has been proposed for Ngb, but not yet been proven in vivo. Ngb-expressing cell types are poorly characterized, and controversy remains about the sites of Ngb expression. To address functional hypothesis in vivo and to investigate Ngb's expression pattern, we generated a novel Ngb-knockout (KO) mouse model containing a LacZ reporter gene under control of the Ngb promotor. Using public single-cell (sc) RNA-Seq data, we further analyzed cells from twenty mouse tissues to elucidate the transcriptomic profile of Ngb-expressing cell types. We confirmed a discrete expression pattern of Ngb in mouse brain, wherein peptidergic neurons, particularly of the hypothalamus, form the dominant Ngb expression site. No considerable Ngb signal was found in glia cells. Consistent with previous bulk RNA-Seq results, scRNA-Seq data demonstrated sparse Ngb mRNA expression in the retina, arguing against published functional hypotheses. Surprisingly, Ngb mRNA was found in myofibroblasts of non-neuronal tissues. In single-cell transcriptome data of the central nervous system, indirect evidence was obtained for an association of Ngb expression with genes associated with emotional behavior. Since our unstressed KO mice lacked overt organismic phenotypes, we investigated transcriptomic phenotypes of Ngb-deficiency. Differential expression analysis of hypothalamic bulk RNA-Seq data from Ngb-KO and control mice suggested a potential link between Ngb and developmental processes as well as oxidative stress metabolism in mouse brain.