5-HT3 receptor subtypes: functional and structural characterisation

5-HT3 receptors are membrane protein which transmit information in the nervous system. They are also the target of many drugs. They are, for example, the 'gold standard' for treatment of postoperative and chemotherapy- and radiotherapy-induced nausea and vomiting, and studies have revealed many potential disease targets that might be amenable to alleviation by 5-HT3 receptor-selective compounds; these include chronic heart pain, bulimia, addiction, pruritis, emesis, fibromyalgia, migraine, rheumatic diseases and neurological phenomena such as anxiety and cognitive function. The receptors are constituted of 5 subunits, some of which have been associated with specific disorders, but the composition and properties of receptors composed of multiple subunits have not yet been studied. To understand how alterations in these subunits could cause disorders, and to provide a template for novel therapeutic agents that could act at complex 5-HT3 receptors, it is essential to determine the properties of these as yet uncharacterised receptors. This is the aim of this project.

The aim of this study is to characterize heteromultimeric 5-HT3 receptors. We will use functional experiments to characterise heteromeric 5-HT3 receptors containing 2, 3, 4 or 5 different subunits using receptors expressed in Xenopus oocytes and HEK cells. We will define the composition of binding sites at heteromeric receptors and identify the molecular interactions of agonists, partial agonists and antagonists at these binding sites using mutagenesis combined with functional data. We will then incorporate this information into a homology model.

To verify this model we will solve the high resolution structures of an agonist, partial agonist and antagonist bound to a 5-HT3 receptor mimic based on AChBP (5-HTBP) which we will modify to contain the novel binding sites. We have already shown that 5-HTBP can bind to 5-HT and 5-HT3 selective ligands, and we have X-ray crystal structures with 5-HT and granisetron bound; thus we anticipate it will be relatively straightforward to obtain structural data for our heteromeric binding pockets.

Currently 5-HT3 receptor antagonists are used clinically to control postoperative and chemotherapy- and radiotherapy-induced nausea and vomiting, but studies have revealed many potential disease targets that might be amenable to alleviation by 5-HT3 receptor-selective compounds; these include chronic heart pain, bulimia, addiction, pruritis, emesis, fibromyalgia, migraine, rheumatic diseases and neurological phenomena such as anxiety and cognitive function. However there are few studies on the properties of multi heteromeric 5-HT3 receptors which are widely expressed and are likely to contribute to some of these disorders. Characterising the properties of complex heteromeric 5-HT3 receptors, and understanding the molecular interactions of drugs at potentially novel binding sites, could have a significant impact on providing potential new therapeutic options for these disorders.