A New Splice Variant of Large Conductance Ca2+-activated K+ (BK) Channel Subunit Alters Human Chondrocyte Function

Large conductance Ca2+-activated K+ (BK) channels play essential roles in both excitable and non-excitable cells. For example, in chondrocytes, agonist-induced Ca2+ release from intracellular store activates BK channels, and this hyperpolarizes these cells, augments Ca2+ entry, and forms a positive feed-back mechanism for Ca2+ signaling and stimulation-secretion coupling. In the present study, functional roles of a newly identified splice variant in the BK channel subunit (BKe2) were examined in a human chondrocyte cell line, OUMS-27, and in a HEK293 expression system. Although BKe2 lacks exon2, which codes the intracellular S0-S1 linker (Glu-127-Leu-180), significant expression was detected in several tissues from humans and mice. Molecular image analyses revealed that BKe2 channels are not expressed on plasma membrane but can traffic to the plasma membrane after forming hetero-tetramer units with wild-type BK (BKWT). Single-channel current analyses demonstrated that BK hetero-tetramers containing one, two, or three BKe2 subunits are functional. These hetero-tetramers have a smaller single channel conductance and exhibit lower trafficking efficiency than BKWT homo-tetramers in a stoichiometry-dependent manner. Site-directed mutagenesis of residues in exon2 identified Helix2 and the linker to S1 (Trp-158-Leu-180, particularly Arg-178) as an essential segment for channel function including voltage dependence and trafficking. BKe2 knockdown in OUMS-27 chondrocytes increased BK current density and augmented the responsiveness to histamine assayed as cyclooxygenase-2 gene expression. These findings provide significant new evidence that BKe2 can modulate cellular responses to physiological stimuli in human chondrocyte and contribute under pathophysiological conditions, such as osteoarthritis.