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.