Contribution of accessory subunits to heteromeric serotonin type 3 receptor function

Abstract

5HT3 receptors are members of the Cys-loop receptor family that mediate fast excitatory transmission in central and pe- ripheral nervous system. Five different subunits (AE) have been identified in humans, and most of them have multiple isoforms. It is known that the A subunit can form functional homomeric (5HT3 A) and heteromeric receptors with the B subunit (5HT3 AB). Here we combined single-channel and macroscopic current recordings to de- termine if the other 5HT3 subunits (C, D or E) can also combine with the A subunit to form heteromeric receptors. After co-expression of the A subunit with each of the tested subunits, single-channel events with different amplitudes to that of 5HT3 A receptors were detected. From the analysis of the single-channel amplitudes, the stoichiom- etry of each heteromeric receptor was inferred. The activation pat- terns of the heteromeric receptors elicited by 1030 μM 5HT showed long-activation episodes composed by openings in quick succes- sion. No statistically significant differences in the open durations were observed among the homomeric and the heteromeric A/C, A/D, A/E receptors. However, the EC50 values for 5HT, which were determined by whole-cell macroscopic recordings, were statistically different between heteromeric and 5HT3 A receptors. In silico studies provided insights into the contribution of the different subunits to the 5HT binding site. Our results demonstrate that all the 5HT3 subunits can combine with the A subunit to form heteromeric receptors, thus leading to a wide variety of receptors showing different functional properties. The functional characterization of different heteromeric 5HT3 receptors, which are expressed in different tissues, contributes to the develop- ment of selective therapies targeting this receptor family