Within the potassium ion channel family, calcium activated potassium (KCa) channels are unique in their ability to couple intracellular Ca2+ signals to membrane potential variations. KCa channels are diversely distributed throughout the central nervous system and play fundamental roles ranging from regulating neuronal excitability to controlling neurotransmitter release. The physiological versatility of KCa channels is enhanced by alternative splicing and co-assembly with auxiliary subunits, leading to fundamental differences in distribution, subunit composition and pharmacological profiles. Thus, understanding specific KCa channels’ mechanisms in neuronal function is challenging. Based on their single channel conductance, KCa channels are divided into three subtypes: small (SK, 4–14 pS), intermediate (IK, 32–39 pS) and big potassium (BK, 200–300 pS) channels. This review describes the biophysical characteristics of these KCa channels, as well as their physiological roles and pathological implications. In addition, we also discuss the current pharmacological strategies and challenges to target KCa channels for the treatment of various neurological and psychiatric disorders.

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BACKGROUND AND PURPOSEGoSlo-SR compounds are efficacious BK (KCa1.1) channel openers, but little is known about their mechanism of action or effect on bladder contractility. We examined the effects of two closely related compounds on BK currents and bladder contractions. EXPERIMENTAL APPROACHA combination of electrophysiology, molecular biology and synthetic chemistry was used to examine the effects of two novel channel agonists on BK channels from bladder smooth muscle cells and in HEK cells expressing BKα alone or in combination with either β1 or β4 subunits. KEY RESULTSGoSlo-SR-5-6 shifted the voltage required for half maximal activation (V1/2) of BK channels approximately −100 mV, irrespective of the presence of regulatory β subunits. The deaminated derivative, GoSlo-SR-5-130, also shifted the activation V1/2 in smooth muscle cells by approximately −100 mV; however, this was reduced by ∼80% in HEK cells expressing only BKα subunits. When β1 or β4 subunits were co-expressed with BKα, efficacy was restored. GoSlo-SR-5-130 caused a concentration-dependent reduction in spontaneous bladder contraction amplitude and this was abolished by iberiotoxin, consistent with an effect on BK channels. CONCLUSIONS AND IMPLICATIONSGoSlo-SR-5-130 required β1 or β4 subunits to mediate its full effects, whereas GoSlo-SR-5-6 worked equally well in the absence or presence of β subunits. GoSlo-SR-5-130 inhibited spontaneous bladder contractions by activating BK channels. The novel BK channel opener, GoSlo-SR-5-130, is approximately fivefold more efficacious on BK channels with regulatory β subunits and may be a useful scaffold in the development of drugs to treat diseases such as overactive bladder.

GoSlo-SR-5-6 is a novel large-conductance Ca 2+ -activated K + (BK) channel agonist that shifts the activation V 1/2 of these channels in excess of −100 mV when applied at a concentration of 10 μM. Although the structure-activity relationship of this family of molecules has been established, little is known about how they open BK channels. To help address this, we used a combination of electrophysiology, mutagenesis, and mathematical modeling to investigate the molecular mechanisms underlying the effect of GoSlo-SR-5-6. Our data demonstrate that the effects of this agonist are practically abolished when three point mutations are made: L227A in the S4/S5 linker in combination with S317R and I326A in the S6C region. Our data suggest that GoSlo-SR-5-6 interacts with the transmembrane domain of the channel to enhance pore opening. The Horrigan-Aldrich model suggests that GoSlo-SR-5-6 works by stabilizing the open conformation of the channel and the activated state of the voltage sensors, yet decouples the voltage sensors from the pore gate.

Fragile X mental retardation protein (FMRP) is an RNA-binding protein prominently expressed in neurons. Missense mutations or complete loss of FMRP can potentially lead to fragile X syndrome, a common form of inherited intellectual disability. In addition to RNA regulation, FMRP was also proposed to modulate neuronal function by direct interaction with the large conductance Ca2+- and voltage-activated potassium channel (BK) β4 regulatory subunits (BKβ4). However, the molecular mechanisms underlying FMRP regulation of BK channels were not studied in detail. We have used electrophysiology and super-resolution stochastic optical reconstruction microscopy (STORM) to characterize the effects of FMRP on pore-forming BKα subunits, as well as the association with regulatory subunits BKβ4. Our data indicate that, in the absence of coexpressed β4, FMRP alters the steady-state properties of BKα channels by decreasing channel activation and deactivation rates. Analysis using the Horrigan-Aldrich model revealed alterations in the parameters associated with channel opening (L0) and voltage sensor activation (J0). Interestingly, FMRP also altered the biophysical properties of BKαβ4 channels favoring channel opening, although not as dramatically as BKα. STORM experiments revealed clustered multi-protein complexes, consistent with FMRP interacting not only to BKαβ4 but also to BKα. Lastly, we found that a partial loss-of-function mutation in FMRP (R138Q) counteracts many of its functional effects on BKα and BKαβ4 channels. In summary, our data show that FMRP modulates the function of both BKα and BKαβ4 channels.

BK channels are dually regulated by voltage and Ca, providing a cellular mechanism to couple electrical and chemical signalling. Intracellular Ca concentration is sensed by a large cytoplasmic region in the channel known as "gating ring", which is formed by four tandems of regulator of conductance for K (RCK1 and RCK2) domains. The recent crystal structure of the full-length BK channel from Aplysia californica has provided new information about the residues involved in Ca coordination at the high-affinity binding sites located in the RCK1 and RCK2 domains, as well as their cooperativity. Some of these residues have not been previously studied in the human BK channel. In this work we have investigated, through site directed mutagenesis and electrophysiology, the effects of these residues on channel activation by voltage and Ca. Our results demonstrate that the side chains of two non-conserved residues proposed to coordinate Ca in the A. californica structure (G523 and E591) have no apparent functional role in the human BK Ca sensing mechanism. Consistent with the crystal structure, our data indicate that in the human channel the conserved residue R514 participates in Ca coordination in the RCK1 binding site. Additionally, this study provides functional evidence indicating that R514 also interacts with residues E902 and Y904 connected to the Ca binding site in RCK2. Interestingly, it has been proposed that this interaction may constitute a structural correlate underlying the cooperative interactions between the two high-affinity Ca binding sites regulating the Ca dependent gating of the BK channel. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

Large conductance, voltage and Ca activated K channels (BK channels) are abundantly expressed throughout the body and are important regulators of smooth muscle tone and neuronal excitability. Their dysfunction is implicated in various diseases including overactive bladder, hypertension and erectile dysfunction. Therefore, BK channel openers bear significant therapeutic potential to treat the above diseases. GoSlo-SR compounds were designed to be potent and efficacious BK channel openers. Although their structural activity relationships, activation in both BKα and BKαβ channels and the hypothetical mode of action of these compounds has been studied in detail in recent years, their effectiveness to open the BKαγ channels still remains unexplored. In this study, we have examined the efficacy of 3 closely related GoSlo-SR openers, GoSlo-SR-5-6 (SR-5-6), GoSlo-SR-5-44 (SR-5-44) and GoSlo-SR-5-130 (SR-5-130) using inside out patches on BKα channels coexpressed with 4 different LRRC (γ) subunits in HEK293 cells. Our data suggests that the activation effects due to SR-5-6 were not significantly affected in the presence of γ subunits. Interestingly, the effects of more efficacious BK channel opener SR-5-44 were altered by different γ subunits. In cells expressing BKα channels, the shift in V (ΔV) induced by SR-5-44 (3 μM) was -76 ± 3 mV, whereas it was significantly reduced by ∼70 % in BKαγ channels (ΔV= -23 ± 3, p < 0.001, ANOVA). In BKαγ channels the ΔV was -36 ± 1 mV, which was less than that observed in BKαγ and BKαγ channels where the ΔV was -47 ± 5 mV, and -82 ± 5 mV, respectively. Additionally, the excitatory effects of a 'β specific' BK channel opener, SR-5-130 were only partially restored in the patches containing BKαγ channels. Together this data highlights that subtle modifications in GoSlo-SR structures alter their effectiveness on BK channels with accessory γ subunits and this study might provide a scaffold for the development of more tissue specific BK channel openers.

are ongoing at two more locations); for an 871 atom (protein, 50 water molecules, and the ion) pore section, quantum calculations optimized (energy minimized) the section, and the energy and ion hydration at each ion position were determined. The protein structures the water column. It appears that hydration may explain the reason for the strong conservation of the threonine at the bottom of the selectivity filter, which interacts with water in a characteristic way, producing a ''basket'' of four water molecules H-bonded to the threonines. One remaining problem is determining how the ion exchanges its own hydration shell to be cosolvated by the four threonine hydroxyl groups, breaking the water ''basket''. Once that is accomplished, the ion can proceed through the selectivity filter. At the entrance to the pore at the bottom of the water column (i.e., the gate), hydration is strongly rearranged when an ion arrives. A small increase in diameter from the closed state, approximately 5 Å , is adequate to admit the ion and hold it by a local increase in the density of the water hydrating it; a wide open gate would often lose the ion back to solution, greatly diminishing the current. The entering ion remains held as the ion above it moves; following this, the gate ion can move up to the center of the pore cavity. Optimizations were done using HF/6-31G*, while energy calculations used B3LYP/6-31G** on the optimized structures.