Use of a Gain-Of-Function BK Channel to Suppress Neural Excitability

Abstract

BK channels have a uniquely large single channel conductance and are activated in physiological settings by coincident membrane depolarization and rise in intracellular Ca2+ (Ca2+i). Increasing K+ conductances, in many instances, reduces neuronal activity. In this study we used a ‘gain-of-function’ BK channel as a tool to suppress neural excitability. We created a BK channel construct with an Arginine (R) to Glutamine (Q) point mutation in the S4 voltage sensor. Previous studies in Xenopus oocytes with symmetrical K+ solutions showed that R207Q results in a significant negative shift in the channel's voltage-dependence of activation through reduced coupling between Ca2+ binding and channel opening (Diaz et al, 1998). To address how this mutation would affect excitability in mammalian cells under physiological conditions, we recorded from inside-out patches excised from HEK293T cells at various [Ca2+]i. Wild-type (WT) BK and R207Q cDNAs were expressed separately and together to form homo- and putative heterotetramers. Currents from R207Q alone and WT/R207Q co-expression both display strong gain-of-function properties in HEK cells under physiological conditions. The V0.5s for WT, WT/R207Q and R207Q currents with nominal [Ca2+]i were 113.5 ±3.5, 67.0 ±5.2 and 24.3 ±3.3 mV, respectively. In addition, currents recorded from R207Q- and WT/R207Q-expressing HEK cells with nominal [Ca2+]i activated faster than WT currents with 5 µM [Ca2+]i, at +80 mV (Tau activation=1.6 ±0.2, 2.1 ±0.2 and 5.8 ±2.6 ms, respectively). Overall, WT/R207Q currents in the absence of Ca2+i were similar to WT currents with 50 µM Ca2+i. When an R207Q transgene was expressed in mouse hypothalamic neurons with endogenous BK channels present, action potential frequency was suppressed (WT: 33.3 ±3.2 versus R207Q: 65.5 ±7.3 Hz). This work demonstrates that cells expressing R207Q channels have hyperactive BK currents, resulting in suppression of neural activity under physiological conditions.