Tissue-specific expression of a bHLH-PAS protein homologous to ARNT during the development of crustacean Daphnia magna

Abstract cDNAs encoding a Daphnia magna homolog of aryl hydrocarbon receptor nuclear translocator (ARNT) were isolated and the structural and functional features as well as the expression pattern of their product, DmagARNT, were analyzed. Among the known bHLH-PAS proteins, the deduced amino acid sequences of DmagARNT showed the highest degree of identity to that of Drosophila ARNT (TGO). Expression of DmagARNT in ARNT-lacking mouse Hepa-c4 cells resulted in the compensation for the loss of hypoxia response, suggesting the formation of a dimer with mouse HIF-1α and that the resulting heterodimer binds to the hypoxia-responsive elements (HRE), leading to transcription of the downstream luciferase gene. Expression of D. magna ARNT was evident at the middle to late stages of embryonic development (about 25 h to 48 h after ovulation) in several tissues, including a pair of the 1st antenna, 2nd antenna, 2nd maxilla, five pairs of the thoracic limbs, the central nerve system, anus, dorsal organ, maxillary gland, and carapace. As observed in other species, the D. magna ARNT is likely to function broadly as an expressed dimerization partner in developmental processes. In contrast, expression of ARNT in adult D. magna was limited to the epipodites of thoracic limbs, suggesting that ARNT plays a role solely in hypoxia response in adult Daphnia .