Targeted mutation of the murine arylhydrocarbon receptor nuclear translocator 2 (Arnt2) gene reveals partial redundancy with Arnt

The ubiquitously expressed basic helix-loop-helix (bHLH)-PAS protein ARNT ( arylhydrocarbon receptor nuclear transporter) forms transcriptionally activ e heterodimers with a variety of other bHLH-PAS proteins, including HIF-1 a lpha (hypoxia-inducible factor-1 alpha) and AHR (arylhydrocarbon receptor). These complexes regulate gene expression in response to hypoxia and xenobi otics, respectively, and mutation of the murine Arnt locus results in embry onic death by day 10.5 associated with placental, vascular, and hematopoiet ic defects. The closely related protein ARNT2 is highly expressed in the ce ntral nervous system and kidney and also forms complexes with HIF-1 alpha a nd AHR, To assess unique roles for ARNT2 in development, and reveal potenti al functional overlap with ARNT, we generated a targeted null mutation of t he murine Arnt2 locus. Arnt2(-/-) embryos die perinatally and exhibit impai red hypothalamic development, phenotypes previously observed for a targeted mutation in the murine bHLH-PAS gene Sim1(Single-minded 1), and consistent with the recent proposal that ARNT2 and SIM1 form an essential heterodimer in vivo [Michaud. J. L., DeRossi, C,, May, N. R., Holdener. B. C. & Fan, C , (2000) Mech. Dev. 90, 253-261]. In addition, cultured Arnt2(-/-) neurons display decreased hypoxic induction of HIF-1 target genes, demonstrating fo rmally that ARNT2/HIF-1 alpha complexes regulate oxygen-responsive genes. F inally, a strong genetic interaction between Arnt and Arnt2 mutations was o bserved, indicating that either gene can fulfill essential functions in a d ose-dependent manner before embryonic day 8.5. These results demonstrate th at Arnt and Arnt2 have both unique and overlapping essential functions in e mbryonic development.