Structure-function relationships of the mineralocorticoid receptor
The mineralcorticoid receptor (MR) is a nuclear receptor for the steroid hormones aldosterone and cortisol. Pathological activation of the MR promotes cardiac fibrosis and heart failure, making it an important therapeutic target in cardiovascular disease. Often used in the treatment of heart attacks, there are significant side effects linked to current MR therapies. Understanding the underlying interactions that enable blockade of the MR may lead to the development of new safer therapeutic agents.
The adrenal steroid, aldosterone, is a key regulator of blood pressure. High levels of aldosterone are linked to hypertension and cardiac disease. Our research aims to provide insights into aldosterone action and its role in cardiovascular disease. We are also examining the underlying interactions of the mineralocorticoid receptor (MR), which acts as a transcription factor to regulate gene expression.
A number of drugs effective at inhibiting the action of MR are routinely administered to heart attack patients. However, many of these drugs, including eplerenone and spironolactone, also dangerously elevate potassium levels. In collaboration with Dr Morag Young and Dr. Jun Yang in the Cardiovascular Endocrinology Group, we are working to identify proteins that act as potent MR co-regulating proteins. We hope to translate these findings to develop novel approaches to explore the underlying molecular mechanisms of MR inhibition.
Currently, there are four areas of focus in this work:
- We are using a cross-species approach, comparing human and zebra fish to MR to understand the structural basis of the interaction of the antagonist. Somewhat surprisingly, spironolactone like aldosterone acts as an agonist in the zebra fish MR. By creating chimeras between the zebra fish MR ligand-binding domain and the human MR ligand-binding domain, we hope to identify the amino acid differences (in what are otherwise two highly conserved molecules), that differentiate between a ligand being agonist and antagonist. This has implications for the future design of drugs to specifically and unambiguously block the MR.
- We have identified an interaction between two distinct domains in the full-length receptor, an interaction between the N-terminal domain and the ligand-binding domain, which is at the C-terminus of the receptor (N/C-interaction). In a series of published studies, we have started the characterisation of this interaction in the MR, which is unique in distinguishing aldosterone and cortisol. We are identifying the structural basis of this interaction and this unexpected difference between aldosterone and cortisol. We are currently generating a transgenic mouse, which has an MR lacking the N/C interaction.
- The MR is able to signal through three principal mechanisms: the classic transrepression mechanism in which it binds to DNA and directly regulates gene expression; an interaction with other transcription factors, which involves DNA but not the MR binding to the DNA; and a rapid action that occurs at the cell membrane, which does not involve DNA. We have created a transgenic mouse lacking DNA binding in order to identify the relative contribution of these three signalling mechanisms to the actions of the MR.
- After binding to the DNA, nuclear receptors interact with the transcriptional apparatus to regulate gene expression through co-regulatory molecules. We have used yeast 2-hybrid screens to identify a series of co-regulatory molecules of the mineralocorticoid receptor, which we are now in the process of characterising. It is of interest that a subset of these receptors is able to distinguish between the aldosterone-bound MR and the cortisol-bound MR. This is a novel finding and has potential implications for tissue-specific differences and thus for drug design. This work has an important clinical correlate with
the work being done by Dr. Jun Yang and Professor Fuller in the Monash Health Endocrinology Unit to improve the care of patients with primary hyperaldosteronism.
Dr. Jun Yang, MIMR-PHI Institute
Associate Professor Tim Cole, Biochemistry, Monash University
Dr Brian Smith, LaTrobe University
Associate Professor Tom Ratajacz – University of Western Australia
Professor David Pearce, University of California, San Francisco
Pippal, J.P., Cheung, C.M.I., Yao, Y.Z., Brennan, F.E., and Fuller, P.J. Characterization of the zebrafish (Danio rerio) mineralocorticoid receptor. Molecular and Cellular Endocrinology 332:58-66, 2011.
Yang, J. and Fuller, P.J. Interactions of the Mineralocorticoid Receptor – Within and Without. Molecular and Cellular Endocrinology 350: 196-205, 2012.
Cluning, C., Ward, B.K., Arulpragasam, A., Fuller, P.J. and Ratajczak, T. The helix 1-3 loop in the glucocorticoid receptor LBD is a regulatory element for FKBP cochaperones. Molecular Endocrinology 27: 1020-1035, 20133.
Rogerson, F.M., Yao, Y-Z., Young, M.J. and Fuller, P.J. Identification and characterisation of a ligand-selective mineralocorticoid receptor coactivator. FASEB Journal, pii: fj.13-242479, 2014. Epub ahead of print.
Pippal, J. B., Yao, Y., Rogerson, F. M. and Fuller, P.J. Structural and functional characterisation of the interdomain interaction in the mineralocorticoid receptor.
Molecular Endocrinology 23: 1360-1370, 2009.