Functional identification of an osmotic response element (ORE) in the promoter region of the killifish deiodinase 2 gene (FhDio2)

Abstract

SUMMARY The physiological role played by thyroid hormones (TH) in hydro-osmotic homeostasis in fish remains a controversial issue. Previous studies have shown that in Fundulus heteroclitus (killifish) hypo-osmotic stress increases liver iodothyronine deiodinase type 2 (D2) mRNA and D2 activity. In this study we identified two conserved osmotic response element (ORE) motifs in the promoter region of the killifish D2 gene (FhDio2) and examined their possible role in the transcriptional regulation of FhDio2during hypo-osmotic stress. As assessed by the electrophoretic mobility shift assay, results from in vivo and in vitro experiments demonstrate that exposure to an abrupt hyposmotic challenge triggers in the liver of killifish a strong nuclear recruitment of a putative osmotic response element binding protein (OREBP). This protein–DNA binding is time-dependent, attains a maximum within 2–8 h after the osmotic stress,and is followed by a significant increase in D2 activity. Furthermore,protein–DNA binding and the subsequent elevation in enzyme activity were blocked by the tyrosine kinase inhibitor genistein. Thus, during hypo-osmotic stress, a putative OREBP kinase-activated pathway stimulates FhDio2transcription and enzymatic activity. These data and the fact that D2 is the major enzyme providing local intracellular T3 suggest that TH plays a direct role in osmoregulation in fish, possibly by participating in hepatic ammonia metabolism. This study provides important insight into the physiological role of TH in hydro-osmotic homeostasis in fish.

Functional Divergence of Type 2 Deiodinase Paralogs in the Atlantic Salmon

“…These elements have previously been linked to salinity adaptation in teleosts, mediating the effects of osmotically sensitive ORE binding proteins, and in the killifish (Fundulus heteroclitus), a single dio2 gene has been identified, containing two OREs in the proximal 1.1 kb of promoter region [22]. In S. salar dio2a, we identified four canonical OREs, three lying within 0.5 kb upstream of the TSS and the fourth in the 5 0 UTR.…”

“…Prominent examples include paracrine TSH signaling between the pituitary and the basal hypothalamus to drive photoperiodic induction of dio2 in mammals and birds [34] and noradrenergic signaling in brown fat, which increases dio2 expression as part of the thermogenic response [35]. In teleosts, dio2 may sense osmotic changes through OREs [22], expanding the potential repertoire of routes to environmental regulation of dio2. Our finding that S. salar retains two dio2 paralogs, dio2a and dio2b, specialized for salt-and light-dependent regulation, respectively, suggests that teleosts have taken a distinctive route to refinement of local TH action.…”

Genomic mechanisms of evolved physiological plasticity in killifish distributed along an environmental salinity gradient

“…4H), where expression is initially much higher in the P1 population such that P3 and VC populations require 3 d to achieve transcript levels comparable with those reached by 6 h in P1. Although the role of thyroid hormone in osmoregulatory processes is controversial (40), DIO2 has been shown to harbor osmotic response elements in its promoter region in F. heteroclitus (41) and is up-regulated by hypoosmotic stress, and the thyroid signaling pathway is a target of adaptive divergence in FW populations of threespine sticklebacks (42).…”

Salinity- and population-dependent genome regulatory response during osmotic acclimation in the killifish (Fundulus heteroclitus) gill

“…DIO proteins are important for the activation and deactivation of thyroid hormones (Orozco and Valverde-R, 2005). Members of the DIO family have been cloned from fish gills (Sanders et al, 1999), contain osmotic response elements in promoter regions in killifish (Lopez-Bojorquez et al, 2007), and deiodination activity is detectable in killifish gills (Orozco et al, 2000). Intriguingly, thyroid hormone signaling pathways have adaptively diverged between marine and freshwater populations of stickleback (Kitano et al, 2010).…”