Ectopic expression of negative ARNT2 factor disrupts fish development

ARNT factors are a duster of bHLH-PAS factors that heterodimerize with othe r specific bHLH-PAS factors to mediate a wide range of biological responses . Previously, we obtained a truncated form of ARNT2-like factor, ARNT2A, fr om zebrafish, which encompasses the basic-helix-loop-helix and PAS A/B doma ins, but lacks a transactivation domain at its carboxyl end. Herein, we rep ort another truncated ARNT2-like factor, ARNT2X, in zebrafish, which differ s from ARNT2A at its N-terminal region. In cultured ZLE cells, transiently expressed ARNT2X and ARNT2A inhibited 2,3,7,8-TCDD-activated cyp1a1 transcr iption with different efficiencies. In the developing embryo, arnt2X mRNA w as consistently expressed in the retinal and neural tube regions until the hatching stages, but it exhibited a more specific pattern at larval stages, including expression in the brain, eyes, hypothalamus, pharyngeal skeleton , heart, liver, pronephros duct, pectoral fin, and epithelial cells of the swim bladder. In contrast, arnt2A transcription diminished after hatching. Microinjecting a recombinant arnt2X-expression vector into fertilized eggs before cleavage stages caused severe defects in brain, eyes, pectoral fin, heart, and gut development. This suggests that the ARNT-mediated signal tra nsduction pathways play important roles in fish tissue development. (C) 200 1 Academic Press.