Voltage-dependent K+ (Kv) channels can limit Ca2+ influx through voltage-dependent Ca2+ channels by means of their effects on membrane potential and thereby indirectly inhibit the secretion of neurotransmitters, neuropeptides, and hormones. Noting that Kv2.1 channels interact directly with syntaxin 1A and SNAP25, two components of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) exocytotic apparatus, Singer-Lahat et al. investigated the possibility that Kv2.1-syntaxin interactions affect the release of dense-core vesicles (DCVs). Kv2.1 overexpression in undifferentiated PC12 cells (so that current density and voltage dependence of activation were similar to those characteristic of hippocampal neurons) enhanced the release of green fluorescent protein-labeled pro-ANF (atrial natriuretic factor) elicited by increasing the extracellular K+ concentration, thereby depolarizing the cells (conditions under which the effects of K+ flux through Kv channels on membrane potential should be minimal). Kv2.1-dependent facilitation of release did not involve effects on membrane potential or on bulk intracellular Ca2+ concentration; moreover, a Kv2.1 mutant with a nonconducting pore enhanced release as well. Exposing permeabilized cells to calcium after priming with ATP (a treatment known to trigger DCV release) enhanced coimmunoprecipitation of syntaxin with endogenous Kv2.1. Syntaxin-binding peptides derived from cytoplasmic portions of Kv2.1 decreased syntaxin coimmunoprecipitation with Kv2.1 and inhibited Kv2.1-dependent facilitation of release. Indeed, a mutant form of Kv2.1 that lacked the syntaxin-binding C1a domain not only failed to enhance depolarization-induced release but also slightly inhibited it. The authors propose that the apparently antagonistic effects of Kv2.1 on Ca2+-evoked neuropeptide release (inhibiting release through effects on membrane potential and enhancing it through direct interaction with syntaxin) could contribute to the known activity dependence of DCV release.
D. Singer-Lahat, A. Sheinin, D. Chikvashvili, S. Tsuk, D. Greitzer, R. Friedrich, L. Feinshreiber, U. Ashery, M. Benveniste, E. S. Levitan, I. Lotan, K+ channel facilitation of exocytosis by dynamic interaction with syntaxin. J. Neurosci. 27, 1651-1658 (2007). [Abstract] [Full Text]