A novel voltage clamp/dye uptake assay reveals saturable transport of molecules through CALHM1 and connexin channels.

Abstract

Channels that are permeable to small molecules such as ATP, in addition to atomic ions, are emerging as important regulators in health and disease. Nonetheless, mechanisms of molecular permeation and selectivity of these channels remain mostly unexplored due to the lack of quantitative methodologies. To address this need, we developed a novel two-electrode voltage clamp (TEVC)/dye uptake assay to examine the kinetics of molecular permeation of channels formed by human connexins (hCx), and the calcium homeostasis modulator (hCALHM1). We expressed hCx26, hCx30, and hCALHM1 individually in Xenopus laevis oocytes. To quantify the uptake of small molecular dyes through these channels, we developed a protocol that renders oocytes translucent - thereby amenable to optical detection techniques - without affecting the functional properties of the expressed channels. To control membrane potential and to determine functional channel expression accurately, dye uptake was evaluated in conjunction with TEVC. Using this methodology, we found that: (1) CALHM1 and Cx30 hemichannels display saturable transport of molecules that could be described by Michaelis-Menten kinetics, with apparent KM and Vmax; (2) Kinetic parameters for molecular transport through CALHM1 are sensitive to voltage and extracellular calcium; (3) Significant transport of molecules occurs through CALHM1 when there are little or no ionic currents through the channels; (4) Cx mutations in the N-terminal region significantly affect kinetics of transport and permselectivity. Our results reveal that molecular flux through these channels has a rate-limiting step, that the kinetic parameters of molecular transport are sensitive to modulators of channel gating and that molecular transport and ionic currents can be differentially affected. Our methodology allows the analysis of how human mutations causing diseases affect kinetic properties and permselectivity of molecular signaling and enables the study of molecular mechanisms, including selectivity and saturability, associated with molecular transport in large-pore channels.