Retinoid Hormone Signaling in Tropical Reef Corals

Ann Tarrant and Adam Reitzel, Biology Department, WHOI

RXRs (retinoic acid X receptors) are evolutionarily conserved hormone receptors that are important for animal development and reproduction. RXRs naturally bind hormones such as 9-cisretinoic acid (9-cis-RA). RXR signaling can be disrupted through binding of pervasive environmental contaminants, such as tributyltin and some pesticides. Chemical disruption of RXR signaling can have important consequences, such as abnormal reproduction or obesity.

RXR-like genes have been identified in cnidarians including corals. The biological functions of coral RXRs have not been elucidated, but it is likely that coral RXRs are activated by 9-cis-RA and regulate expression of coral genes. It is currently unknown whether environmental contaminants such as tributyltin can disrupt coral RXR signaling. Tributyltin (used in antifouling paints) is toxic to corals and impairs metamorphosis of coral larvae through unknown mechanisms. Many tropical coral reefs are near human population centers and increasingly subjected to physical and chemical stresses (including tributylin). Understanding mechanisms of coral signaling and signal disruption (e.g., disruption of RXR by tributyltin) are important in predicting responses of corals to combined stresses.

We propose to study RXR signaling in the tropical reef-building coral Montipora capitata and the related estuarine sea anemone Nematostella vectensis. We will test three hypotheses:
    (1) Coral and anemone RXRs will bind 9-cis-RA and environmental contaminants, such as tributyltin. To test this hypothesis, we will express RXRs from each species in vitro, measure the specific binding of tritiated 9-cis-RA by the RXRs, and determine whether chemicals (e.g., tributlytin and methoprene) can competitively inhibit binding by 9-cis-RA.
    (2) RXR expression varies with developmental stage and/or reproductive condition. We will use quantitative PCR to measure RXR expression (in coral embryos, larvae, and adults in varying degrees of gametogenesis) and identify conditions under which corals might be most vulnerable to disruption of RXR signaling.
    (3) Cnidarian RXRs activate gene expression by binding to conserved DNA sequences (retinoic acid response elements, RAREs). To test this hypothesis, we will take advantage of the sequenced sea anemone genome. We will expose anemones to 9-cis-RA and identify differentially expressed genes using two complementary techniques (subtractive hybridization and quantitative PCR). We will then examine the sea anemone genome for RAREs in DNA regions flanking the differentially expressed genes.

This project will enhance understanding of the role of RXR in coral reproduction and development and will provide insight into the evolution of retinoid hormone signaling in marine animals (Ocean Life Institute theme of Biodiversity in the Ocean). Binding of cnidarian RXRs by tributyltin will provide the first demonstration of chemical disruption of nuclear receptor signaling in a cnidarian. Understanding coral sensitivity to tributyltin would provide new insight into the health and vulnerability of tropical reef ecosystems. This work complements our current and planned efforts to characterize stress responses and hormonal signaling in cnidarians.