Insights on the mechanisms of Ca(2+) regulation of connexin26 hemichannels revealed by human pathogenic mutations (D50N/Y)

Because of the large size and modest selectivity of the connexin hemichannel aqueous pore, hemichannel opening must be highly regulated to maintain cell viability. At normal resting potentials, this regulation is achieved predominantly by the physiological extracellular Ca(2+) concentration, which drastically reduces hemichannel activity. Here, we characterize the Ca(2+) regulation of channels formed by wild-type human connexin26 (hCx26) and its human mutations, D50N/Y, that cause aberrant hemichannel opening and result in deafness and skin disorders. We found that in hCx26 wild-type channels, deactivation kinetics are accelerated as a function of Ca(2+) concentration, indicating that Ca(2+) facilitates transition to, and stabilizes, the closed state of the hemichannels. The D50N/Y mutant hemichannels show lower apparent affinities for Ca(2+)-induced closing than wild-type channels and have more rapid deactivation kinetics, which are Ca(2+) insensitive. These results suggest that D50 plays a role in (a) stabilizing the open state in the absence of Ca(2+), and (b) facilitating closing and stabilization of the closed state in the presence of Ca(2+). To explore the role of a negatively charged residue at position 50 in regulation by Ca(2+), this position was substituted with a cysteine residue, which was then modified with a negatively charged methanethiosulfonate reagent, sodium (2-sulfanoethyl) methanethiosulfonate (MTSES)(−). D50C mutant hemichannels display properties similar to those of D50N/Y mutants. Recovery of the negative charge with chemical modification by MTSES(−) restores the wild-type Ca(2+) regulation of the channels. These results confirm the essential role of a negative charge at position 50 for Ca(2+) regulation. Additionally, charge-swapping mutagenesis studies suggest involvement of a salt bridge interaction between D50 and K61 in the adjacent connexin subunit in stabilizing the open state in low extracellular Ca(2+). Mutant cycle analysis supports a Ca(2+)-sensitive interaction between these two residues in the open state of the channel. We propose that disruption of this interaction by extracellular Ca(2+) destabilizes the open state and facilitates hemichannel closing. Our data provide a mechanistic understanding of how mutations at position 50 that cause human diseases are linked to dysfunction of hemichannel gating by external Ca(2+).