Just as in adulthood, an organism must respond to changes in its external environment during embryogenesis. Oxygen levels can fluctuate within a tissue, and animals have evolved a conserved signalling pathway to orchestrate a cell's response to low oxygen levels (hypoxia). Critical to this pathway is the transcription factor hypoxia-inducible factor α (HIFα), which is degraded in conditions of normoxia. However, in low oxygen, it binds with its partner HIFβ to hypoxia-response elements and activates downstream genes that are important for a cell to cope with oxygen depletion. On p. 2940, Yi-Hsien Su and colleagues demonstrate that the hypoxia signalling pathway is active in the early sea urchin embryo, in a graded manner that mirrors the emerging dorsoventral axis. They report that while hifα mRNA is distributed uniformly throughout the embryo, the protein is stabilised at the dorsal side, and degraded more ventrally. They found that HIFα protein restricts nodal transcripts to the ventral ectoderm only, and that the dorsoventral axis is affected by artificial perturbation of HIFα levels. Interestingly, they also found evidence for an intrinsic hypoxia gradient in embryos, which may be a forerunner to dorsoventral patterning. Together, these results provide a fascinating insight into the question of how environmental signals can impact early development.
