Controlled Activation of Heteromeric P2X Receptors by ATP and Magnesium

Abstract

P2X receptor channels are a family of trimeric cation channels that are activated by extracelluar ATP. Seven subtypes of P2X receptors have been identified in mammals, and they are widely distributed throughout the body, serving important roles in sensory signaling and inflammation. In physiological solutions, ATP is ionized and primarily found in complex with Mg2+. We recently found (Li, Silberberg and Swartz, 2013 PNAS 110, E3455-63) that the slowly desensitizing P2X2 and P2X4 receptors can be activated by free ATP, but MgATP2- promotes opening with very low efficacy; Mg2+ thus acts as a competitive antagonist. In contrast, both free ATP and MgATP2− robustly open the rapidly desensitizing P2X3 subtype, although P2X3 receptor channel currents are inhibited by Mg2+ through an allosteric mechanism. Interestingly, both inhibitory effects of Mg2+ are disengaged in heteromeric P2X2/3 channels. In the present study we investigated the properties that P2X6 when forming heteromeric channels with P2X2 and P2X4 subunits. The P2X6 subunit is abundantly expressed in the central nervous system along with P2X2 and P2X4, yet does not form functional homomeric channels on its own. When co-expressed in HEK cells, we find that heteromeric P2X2/6 and P2X4/6 channels are functional, and unlike homomeric P2X2 and P2X4 receptors, can be efficiently activated by MgATP2-. Our results also suggest that Mg2+ allosterically regulates P2X2/6 heteromeric channels and we are currently investigating the underlying mechanism and site of activation. Taken together, our results support the general idea that heteromulterimerization of P2X receptor channels influences the forms of ATP that can activate these channels and the regulatory influences of Mg2+.