Hypoxia Regulation of ndrgs

Many organisms rely on oxygen to generate energy in the form of adenosine triphosphate (ATP). During severe hypoxia, the production of ATP decreases due to diminished activity of the electron transport chain, leading to cell damage or death. Conversely, excessive oxygen causes oxidative stress that is equally damaging to cells. To mitigate pathological outcomes, organisms have evolved mechanisms to adapt to fluctuations in oxygen levels. Zebrafish embryos are remarkably hypoxia-tolerant, surviving anoxia (zero oxygen) for hours in a hypometabolic, energy-conserving state. To begin to unravel underlying mechanisms, we analyze here the distribution and hypoxia-dependent regulation of members of the N-myc Downstream Regulated Gene (Ndrg) family, Ndrg 1-4. These genes have primarily been studied in cancer cells, and hence little is understood about their normal function. We show here using in situ hybridization that, under normoxic conditions, ndrgs are expressed in metabolically-demanding organs of the zebrafish embryo, such as the brain, kidney, and heart. Following exposure of embryos to different severity and durations of hypoxia, we observed that ndrgs are differentially regulated and that ndrg1a is the most responsive member of this family, with nine-fold upregulation following prolonged anoxia. We further show that this treatment resulted in de novo expression of ndrg1a in tissues where it is not observed under normoxia, such as head vasculature, the inner ear, and somites. These findings provide an entry point into understanding the role of this conserved gene family in hypoxia adaptation of normal cells.