In zebrafish, as in most vertebrates, three different isoforms of the hypoxia-inducible transcription factor, Hif-1α, Hif-2α, and Hif-3α, have been identified. The expression data of genes encoding these three proteins, as analyzed so far, show distinct expression patterns for all three isoforms during early development, under hypoxic conditions, and during exercise, suggesting differential roles for all three proteins under these different conditions. While isoform-specific functions for Hif-1α and Hif-2α have been identified in recent years, the role of Hif-3α remains somewhat elusive. Several studies mostly using mammalian cells or tissues discussed Hif-3α as a competitive inhibitor of Hif-1α and Hif-2α. In zebrafish, the expression changes for Hif-1α and Hif-3α observed during development and under environmental stress conditions do not support this hypothesis, and recent studies indicate that Hif-3α is also able to directly control transcriptional activity of certain genes. The Hif signaling pathway is tightly connected to cell circuitries such as glucose and lipid metabolism, and only very recently a further linkage to the circadian clock has been described. In this context a detailed analysis of the mRNA concentrations of hif-1α, hif-2α, and hif-3α also revealed a circadian expression pattern for hif-3α mRNA under normoxic conditions in zebrafish larvae. In addition, accumulation of Hif-1α protein during short-term hypoxia was found to depend on the time within the daily light and dark cycle at which hypoxia was encountered, suggesting that the hypoxia signaling pathway may be regulated by the circadian clock. This is supported by the fact that some of the downstream genes of the Hif signaling pathway, namely, erythropoietin and vascular endothelial growth factor, are known to be clock controlled as well. Furthermore, in developing zebrafish, the disruption of the circadian rhythm was shown to result in a diminished hypoxic response with a modified life cycle of erythrocytes and an altered patterning of the vascular bed, leading to even higher mortality rates of chronodisrupted animals. Hif protein, in turn, is known to affect the circadian clock pathway in zebrafish. Previously, we demonstrated that Hif-1α directly binds to defined E-boxes of the period 1 gene, leading to a sustained dampening of its oscillation amplitude. Here we show that Hif-1α also binds to the promoter of the period 2 gene, indicating that multiple connections between the Hif signaling pathway and the circadian clock exist. The redundancy of the coupling between both pathways might be evidence for the coevolution of both circuits after the great oxygenation event about 2.5 billion years ago. Coupling the circadian clock and the hypoxic signaling pathway may have conferred selective advantages by facilitating a coordinated response of cells and organisms to alternating day-night cycles and concomitant variable food availabilities in the face of varying oxygen supply.
