MOLECULAR ADAPTATIONS TO FLUCTUATIONS IN OXYGEN LEVELS

Abstract

Oxygen is essential for cell metabolism, hence oxygen deprivation can be severely detrimental. A diverse array of organisms can adapt to changes in oxygen levels, and conserved adaptive responses represent therapeutic targets. The organisms that tolerate the widest range of oxygen levels adapt to these changes by suppressing energy-demanding activities until oxygen is restored, a response known as metabolic suppression. One such organism is the zebrafish, which adapts to low oxygen conditions (hypoxia) or no oxygen (anoxia). Many organisms capable of surviving the oxygen deprivation do so by suppressing energy-demanding cellular activities and only using those that promote survival. Here, we have asked to what degree transcriptional changes mediate adaptation to anoxia. Previous studies from the Brewster laboratory identified Ndrg1a as a protein that mediates metabolic suppression under anoxia. We observe that ndrg1a transcript is increased following exposure to anoxia, with expression observed in tissues sensitive to hypoxic damage such as the kidney and inner ear. We next more broadly asked what other genes are transcribed under anoxia by performing a transcriptome-wide analysis. To our surprise, we found that anoxia elicits differential expression of over 2800 genes, nearly 1200 of which are upregulated. While some of these genes may mediate metabolic suppression, the majority appear to be required for adaptation following the return to normal oxygen conditions. These findings suggest that anoxia triggers an active transcriptional response, that may represent an energetic investment for surviving reoxygenation that can be more damaging to cells than hypoxia itself. One of the most significantly increased genes, gadd45ba, was chosen for further study. Analysis of Gadd45b expression revealed that it is expressed in erythrocyte (red blood cell) precursors in the hematopoietic stem cell niche. In response to anoxia, its expression levels increased and Gadd45b-positive cells were now mobilized into circulation. To address the functional relevance of these observations, gadd45ba was depleted using a morpholino that disrupts splicing. Embryos injected with the morpholino were defective in red blood cell differentiation, with fewer and larger cells that failed to enter circulation following exposure to anoxia. Erythropoiesis, or red blood cell production, is an essential process for supplying our tissues with sufficient oxygen under normal conditions, and this process must be further increased to cope with hypoxic conditions. Our work identifies gadd45ba as a novel gene implicated in hypoxia-induced erythropoiesis.