A first step towards the understanding of the 5-HT3 receptor subunitheterogeneity from a computational point of view

The functional serotonin type-3 receptor (5-HT3-R), which is the target of many neuroactive drugs, isknown to be a homopentamer made of five identical subunits A (5-HT3A-R) or a binary heteropentamermade of subunits A and B (5-HT3A/B-R) with a still debated arrangement and stoichiometry. Thiscomplex picture has been recently further complicated by the discovery of additional 5-HT3-R subunits,C, D, and E, which, similarly to the B subunit, are apparently able to form functional receptors only ifco-expressed with subunit A. Being the binding site for both serotonin and antagonists (i.e. drugs)located at the extracellular interface between two adjacent subunits, the large variability of the 5-HT3-Rcomposition becomes a crucial issue, since it can originate many different interfaces providing nonequivalentligand binding sites and complicating the pharmacological modulation. Here, the different5-HT3-R interfaces are analysed, on the bases of the structural conformations of previously built 3Dhomology models and of the known subunit sequences, by addressing their physicochemicalcharacterization. The results confirm the presence of an aromatic cluster located in the core of the A–Ainterface as a key determinant for having an interface both stable and functional. This is used as adiscriminant to make hypotheses about the capability of all the other possible interfaces constituted bythe known 5-HT3-R sequences A, B, C, D, and E to build active receptors

Approaching the 5-HT3 receptor heterogeneity by computational studies of the transmembrane and intracellular domains

5-hydroxytryptamine type-3 receptor (5-HT3), an important target of many neuroactive drugs, is a cation selective transmembrane pentamer whose functional stoichiometries and subunit arrangements are still debated, due to the extreme complexity of the system. The three dimensional structure of the 5-HT3R subunits has not been solved so far. Moreover, most of the available structural and functional data is related to the extracellular ligand-binding domain, whereas the transmembrane and the intracellular receptor domains are far less characterised, although they are crucial for receptor function. Here, for the first time, 3D homology models of the transmembrane and the intracellular receptor domains of all the known human 5-HT3 subunits have been built and assembled into homopentameric (5-HT3AR, 5-HT3BR, 5-HT3CR, 5-HT3DR and 5-HT3ER) and heteropentameric receptors (5-HT3AB, 5-HT3AC, 5-HT3AD and 5-HT3AE), on the basis of the known three-dimensional structures of the nicotinic-acetylcholine receptor and of the ligand gated channel from Erwinia chrysanthemi. The comparative analyses of sequences, modelled structures, and computed electrostatic properties of the single subunits and of the assembled pentamers shed new light both on the stoichiometric composition and on the physicochemical requirements of the functional receptors. In particular, it emerges that a favourable environment for the crossing of the pore at the transmembrane and intracellular C terminus domain levels by Ca2+ ions is granted by the maximum presence of two B subunits in the 5-HT3 pentamer