Transcriptomics of a cytoglobin knockout mouse: Insights from hepatic stellate cells and brain.

The vertebrate respiratory protein cytoglobin (Cygb) is thought to exert multiple cellular functions. Here we studied the phenotypic effects of a Cygb knockout (KO) in mouse on the transcriptome level. RNA sequencing (RNA-Seq) was performed for the first time on sites of major endogenous Cygb expression, i.e. quiescent and activated hepatic stellate cells (HSCs) and two brain regions, hippocampus and hypothalamus. The data recapitulated the up-regulation of Cygb during HSC activation and its expression in the brain. Differential gene expression analyses suggested a role of Cygb in the response to inflammation in HSCs and its involvement in retinoid metabolism, retinoid X receptor (RXR) activation-induced xenobiotics metabolism, and RXR activation-induced lipid metabolism and signaling in activated cells. Unexpectedly, only minor effects of the Cygb KO were detected in the transcriptional profiles in hippocampus and hypothalamus, precluding any enrichment analyses. Furthermore, the transcriptome data pointed at a previously undescribed potential of the Cygb- knockout allele to produce cis-acting effects, necessitating future verification studies.

Cytoglobin regulates NO-dependent cilia motility and organ laterality during development.

Cytoglobin is a heme protein with unresolved physiological function. Genetic deletion of zebrafish cytoglobin (cygb2) causes developmental defects in left-right cardiac determination, which in humans is associated with defects in ciliary function and low airway epithelial nitric oxide production. Here we show that Cygb2 co-localizes with cilia and with the nitric oxide synthase Nos2b in the zebrafish Kupffer's vesicle, and that cilia structure and function are disrupted in cygb2 mutants. Abnormal ciliary function and organ laterality defects are phenocopied by depletion of nos2b and of gucy1a, the soluble guanylate cyclase homolog in fish. The defects are rescued by exposing cygb2 mutant embryos to a nitric oxide donor or a soluble guanylate cyclase stimulator, or with over-expression of nos2b. Cytoglobin knockout mice also show impaired airway epithelial cilia structure and reduced nitric oxide levels. Altogether, our data suggest that cytoglobin is a positive regulator of a signaling axis composed of nitric oxide synthase-soluble guanylate cyclase-cyclic GMP that is necessary for normal cilia motility and left-right patterning.