Evolution of corticosteroid receptor signalling pathways in vertebrates

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Steroid hormone receptors are proteins mediating the actions of steroid hormones. Similar to a lock and its key, any given receptor binds only one, or few similar, hormones. The binding of a steroid to its receptor induces the transport of the receptor-hormone complex into the cellular nucleus. Here, the complex binds to specific sequences of target genes, the expression of which is in turn modulated. Hence, steroid receptors can be seen as cellular switches through which steroid hormones turn gene expression on or off. Specific receptors exist for the sexual steroids, which include oestrogens, androgens and progestins, and for the steroids synthesised by the adrenal cortex, also called corticosteroids, which include cortisol and aldosterone. The present project focuses on the evolution of corticosteroid receptors. In mammals including man, two corticosteroid receptors (CRs) exist: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Cortisol acts through the GR and has important roles in the regulation of general metabolism, stress response and immune function. By contrast, aldosterone has an important role in the maintenance of the hydromineral balance, and these effects are mediated through the MR. Teleost fish (bony fish, the most diverse and evolutionary most recent group of fishes), lack aldosterone. By contrast, cortisol plays an important role in hydromineral regulation of teleosts and is traditionally considered their main and most important corticosteroid hormone. Recently, however, an MR was discovered in teleosts. This finding resurrected the question whether teleost fish could possibly possess a specific mineralocorticoid hormone. In the present project, we chose an evolutionary approach to better our understanding of corticosteroid signalling in vertebrates. Using molecular biology, we would like to determine the gene sequences for CRs in two relic fish that diverged at crucial points in vertebrate evolution, the lamprey and the bichir. This will enable us to study the properties and tissue distribution of these hormone receptors, which will allow first conclusions about the possible roles of the CRs in these animals. The lamprey, a jawless fish that has diverged from the line leading to the jawed vertebrates 500 MYA, has only one CR, the sequence of which is known partially, and can synthesise cortisol, but not aldosterone. An analysis of the sequence of the lamprey CR indicated that the common ancestor of lampreys and jawed vertebrates had only one CR. Studying the lamprey CR will allow us to gain insights into the properties and functions of the ancestral CR. In the lineage leading to the jawed vertebrates, the ancestral CR gene was duplicated to give rise to the ancestral GR and MR. In sarcopterygians (a systematic group that includes mammals) these two genes were retained, but adopted different roles (see above). Moreover, the enzymes needed for the biosynthesis of aldosterone must have evolved. In the actinopterygians (ray-finned fish), the sister group of sarcopterygians, the ancestral GR and MR were also retained, as they are still present in teleosts, a derived subgroup of actinopterygians. Teleost fish, however, are in evolutionary terms peculiar because in their ancestry a whole genome-duplication occurred, meaning that a given mammalian gene potentially can have two teleost counterparts. While teleosts appear later to have lost many of the duplicated genes, they retained two GRs. We will study the CRs in the bichir, because it represents a basal actinopterygian, having diverged before the teleost-specific genome duplication. We therefore expect that the bichir has, as sarcopterygians, one GR and one MR. Our question is in how far the properties and tissue distribution of the bichir CRs resemble those in the few studied teleosts. Because of the particular evolutionary history of bichir, similarities between bichir and the studied teleosts should apply to the majority of teleosts.