Differential Effects of Hearing Loss Mutations in Homomeric P2X2 and Heteromeric P2X2/3 Receptors.

P2X receptors are unspecific cation channels activated by ATP. They are expressed in various tissues and found in neuronal and immune cells. In mammals, seven subunits are described, which can assemble into homomeric and heteromeric trimers. P2X2 receptors play important roles in cochlear adaptation to elevated sound levels. Three mutations causing inherited progressive hearing loss have been identified. These mutations localize to the transmembrane domain 1 (V60L), the transmembrane domain 2 (G353R) and a β-sheet linking the ATP binding site to the pore (D273Y). Herein, mutations were studied in human homomeric P2X2 as well as in heteromeric P2X2/3 receptors. We measured their binding of a fluorescently labeled ATP derivative (fATP) and characterized the constructs using the patch-clamp technique. The conclusions from our results are as follows: 1. The mutations V60L and G353R show robust localization on the plasma membrane and binding of fATP, whereas the mutant D273Y has no binding to fATP. 2. The mutation V60L has an increased affinity to fATP compared with the wildtype. 3. The expression of hP2X2 V60L channels reduces cell viability, which may support its role in the pathogenesis of hearing loss. 4. All mutant P2X2 subunits can assemble into P2X2/3 heteromeric channels with distinct phenotypes.