Slo1

BK channel (also called Slo, Slo1, KCa1.1 or maxi K channel) are ion channels characterized by their large conductance of potassium ions (K+) through cell membranes (260 pS). These channels are activated (opened) by changes in membrane electrical potential and/or by increases in concentration of intracellular calcium ion (Ca2+) [558][556]. Under typical physiological conditions, this results in an efflux of K+ from the cell, which leads to cell membrane hyperpolarization and a decrease in cell excitability. It is expressed by a wide variety of cells, including neurons and myocytes, and has pleiotropic functions. It can respond to transient Ca2+ increases (“sparks”) originated by opening of Ca2+ channels or sarcoplasmic reticulum ryanodine receptors, and it has been observed to form clusters optimally positioned for such local responses, for which it seems to have been particularly designed by evolution since it responds to [Ca2+] in the µM range [1458].

The functional diversity of BK channels are due, in part, to various spliced transcript variants of the α subunit encoding different isoforms that have been identified.
http://www.ncbi.nlm.nih.gov/gene/3778

The primary sequences among different mammalian BK channels are almost identical, and share a high degree of homology with the sequences of the six transmembrane segments Sl-S6 of the superfamily of voltage-gated potassium (K,) channels. The homology among positively charged amino acids in the S4 segment that forms part of the voltage sensor in K [1100]

As with other potassium channels, BK channels have a tetrameric structure of the channel-forming alpha subunit. Modulatory beta subunits (encoded by KCNMB1, KCNMB2, KCNMB3, or KCNMB4) can then associate with the tetrametic channel.

Common to all K+ channel subunits is a structural core consisting of two transmembrane helices, readily identified by hydrophobicity algorithms, separated by a re-entrant pore-loop carrying the signature sequence (Figure 1 in [556]). Atypically, BK channels comprise seven transmembrane domains (S0-S6) placing the short NH2 terminus extracellularly and the COOH terminus (two-thirds of the protein) at the intracelular side of the membrane (see Figure 1 fo Berkefeld et al. 2010 [1457]). This intracellular domain contains four hydrophobic segments (S7- S10), two regulating conductance of potassium (RCK domains) and a stretch of aspartate residues that are known as the “Calcium bowl”. Calcium binding and membrane depolarization, converge allosterically on the gating the channel [1457].

BK channels are a prime example of modular protein evolution. Each BK channel alpha subunit consists of (from N- to C-terminal):

1. A unique transmembrane domain (S0)[560] that precedes the 6 transmembrane domains (S1-S6) conserved in all voltage-dependent K+ channels.
2. A voltage sensing domain (S1-S4).
3. A K+ channel pore domain (S5, selectivity filter, and S6).
4. A cytoplasmic C-terminal domain (CTD) consisting of a pair of RCK domains that assemble into an octameric gating ring on the intracellular side of the tetrameric channel [558], [562], [561], [563], [564]. Sequence homology and experimental evidence suggest that the structure of the voltage sensor domain (VSD) in BK channels may resemble that of other Kv channels, while the cytoplasmic domain of BK channels may adopt a similar structure as that of the MthK channel (Jiang [625], Hou [1131], Shi [1132], Yang [1133], Jiang [625], Fodor [1134]).

Calcium-sensing domains: the calcium bowl

The region between S9 and S1O binds calcium and participate in activation and is termed “the calcium bowl’. This region contains many negatively charged residues, mostly aspartates, and is highly conserved among different species and is highly selective for calcium [564], [558], [561].

Available X-ray structures:

• 3MT5 Crystal Structure of the Human BK Gating Apparatus [558]
• 3NAF Structure of the Intracellular Gating Ring from the Human High-conductance Ca2+ gated K+ Channel (BK Channel) [561]

BK channels mediate fast afterhyperpolarization in neurones, electrical tuning of nonspiking properties of cochlear hair cells, presynaptic regulation of neurotransmitter release, effector of calcium sparks in smooth muscles [539].

BK channels are widely expressed throughout the body: both in brain (cerebellum, habenula, striatum, olfactory bulb, neocortex, granule and pyramidal cells of the hippocampus) and peripheral tissue (skeletal muscle, smooth muscle (vascular, uterine, gastric, bladder), adrenal cortex, cochlear hair cells, odontoblasts, pancreatic islet cells, colonic and kidney epithelium) [1459] [539].
BK is also expressed in human glioma [1465].

In the brain, BK is found in the soma, dendrites, and presynaptic terminals of neurons and is thought to underlie the fast afterhyperpolarization current and to regulate synaptic transmission by limiting the Ca2+ influx through CaV channels [1459]. BK channels are described in mitochondrial membrane of several cells, such human glioma cells and ventricular cells in guinea pigs [1465] [1466]

BK channels differential function are related with their differential implication in physiological regulations [1457]:

• Smooth muscle: BKs are activated by calcium sparks and/or by L-type Cav channels, promoting relaxation of the muscle cell.
  
• In the two types of chromaffin cells in adrenal glands, BK channels show distinct gating properties and show different firing patterns. The slowly deactivating BK-beta2/3 channels give rise to a pronounced afterhyperpolarization that relieves voltage-dependent sodium channels from inactivation and promotes repetitive or tonic firing. In contrast, the rapidly deactivating channels lead to only small afterhyperpolarizations that promote firing at a more phasic pattern.
  
• In CNS neurons, BK channels contribute to repolarization of action potentials (AP) and give rise to a fast afterhyperpolarization (fAHP) impacting on neuronal firing by “spike sharpening”. In hippocampal pyramidal cells, inactivating BK-beta2 channels promote frequency-dependent AP broadening along a spike train. In hippocampal granule cells, the slowed activation kinetics induced by coassembly with beta4 appears to operate as a “low-pass filter” that prevents high-frequency firing and spike sharpening.
  
• In auditory sensory hair cells of amphibians, birds, and fish, BK channels participate in “electrical ringing,” a resonance phenomenon fundamental for hearing. Basically, electrical ringing is depolarization-repolarization cycles that are generated by serial and repetitive activation of L-type Cav channels and BK channels. The frequency of these electrical oscillations is determined by the amplitude and kinetics of the BK currents and varies between hair cells along the axis of the hearing organ as a result of an expression gradient of the beta1.

BK channels are essential for the regulation of several key physiological processes including smooth muscle tone and neuronal excitability.
http://www.ncbi.nlm.nih.gov/gene/3778

BK channels exhibit a very high single-channel conductance, are potassium selective and they are activated by the concerted influences of membrane depolarization and increases in calcium concentration. These characteristics explain their proposed role as feedback modulators of the activity of voltage-dependent calcium channels with whom they coexist in both neurons and smooth muscle cells [1467].

BK channels present different sensitivity to calcium depending on the tissues and therefore, differential inactivation properties and pharmacology. These differences are due to alternative splicing of the alfa-subunit and coassembly with four different auxiliary beta-subunits. While beta-1 is primarily expressed in smooth muscle, hair cells, and some neurons, beta-2 is found in ovary and endocrine tissue, beta-3 in testis, and beta-4 is the most abundant beta-subunit in the brain [1459]. Moreover, the high ratio of β-subunit:α-subunit of BK channels in brain leads them higher calcium sensitivity in cerebral vasculature in comparison with skeletal muscle [1468]. Specifically, the beta1 subunit influence on calcium sensitivity and has a key role on vasoregulation [1169]. Changes in intracellular pH also affect the unitary properties of BK channels [1469].

BK channels are implicated in distinct physiological functions related to the tissue where they are expressed:

• BK channels regulate electrical activity in β-cells of mouse pancreatic islets exposed to elevated glucose [1470].
• These channels activity underlie neuroadaptation to alcohol and neuronal plasticity [1471].BK channels also contribute to the behavioral effects of ethanol in the worm C. elegans under high concentrations (> 100 mM, or approximately 0.50% BAC) [559]
• BK potassium channels regulate neuronal firing and control transmitter release and synaptic efficacy at CA3-CA3 synapses in the rat hippocampus [1472]
• In cardiac myocytes, BKCa channel activators can be used as specific activators for mitochondrial BKCa channels because they are only expressed in the mitochondrial membrane and not in the cell membrane, which may be pivotal in the cardioprotective effect in cardiac myocytes [1217]; Kang et al., 2007 [1218]).

Channels activators may have applications in several physiological alterations such stroke, epilepsy, bladder over-reactivity, asthma, hypertension, gastric hypermotility and psychoses [539] [1214] [1172] [1474].

Mouse models:

• Slo-/- , erectil dysfunction [1475] and overactive bladder and incontinence [1476]
• BKbeta1-/- (hearing loss) [1477]
• BK-/- (cerebellar ataxia) [1478]

Previous studies on Mg2+-dependent activation of BK channels have revealed structural details that are important for BK channel function. Particularly, two acidic amino acids (Glu374 and Glu399) in the cytoplasmic RCK1 domain of BK channel may contribute to Mg2+ coordination (Yang [1134], Xiao [1136]). Removal of the side chain carboxylate groups from these two residues completely abolishes Mg2+ sensing. These residues in the cytoplasmic domain are located close to the C-terminus of the transmembrane segment S4, enabling the bound Mg2+ to engage in an electrostatic interaction with the voltage-sensing residue Arg213 at the C-terminus of S414 (Fig. 1c in Yang [167]).

CO

Carbon monoxide (CO) is a potent activator of slo1 when heterologously expressed in human embryonic kidney 293 (HEK 293) cells (Jaggar [1123], Williams [1124]) or when natively expressed in vascular myocytes (Jaggar [1125], Wang [1126], Wang [1127]) and carotid body glomus cells (Riesco-Fagundo [1128]).
Regulation by CO of BKCa channels is emerging as a widespread and physiologically important phenomenon that is intimately involved in the control of smooth muscle contractility (both systemic and pulmonary) and excitability of neurosecretory and neuronal cell populations. (Williams [166]) A motif in the S9–S10 part of the C-terminal of Slo1 is essential for CO activation. (Williams [166])

Hemoxygenase (hemeoxygenase-2; HO-2) is a protein partner closely associated with the BKCa channel complex (Williams [1124]).

In contrast to the relatively small number of BK inhibitors, a large number of both natural and synthetic BK activators have been reported (see Figure 6 Wulff and Zhorov 2008 [1459]).

Some of the molecules and substances that interact with BK channels:

Calmodulin
Calmodulin (CaM) is constitutively bound to the C terminus of the channel and binds calcium leading to a conformational change enabling opening of the cannel and potassium efflux [1454] [1158]

Kinases
The protein has multiple predicted phosphorylation sites that can modulate channel function by phosphorylation [1454]. The activation of protein kinase C completely suppresses the opening BK channels in intact rat anterior pituitary cells in physiological saline. In fact, in rat cerebellar Purkinje neurons there are multiple endogenous protein kinases and phosphatases that functionally couple to the BK channel modulating their activity. Moreover, each type of neuronal BK channels are differentially sensitive to PKA-dependent phosphorylation. [1455]

Adrenal glucocorticoids
Adrenal glucocorticoids regulate adaptation mechanisms to environmental challenges ([1461]) and can modify the electrical excitability of BK channels in human embryonic kidney cells. [1462]

Aldosterone
Aldosterone-induced K(+) secretion occurs via increased expression of luminal BK channels [1463].

Leptin
Leptin activates BK channels via PI 3-kinase controlling neuronal excitability. As uncontrolled excitability in the hippocampus is one underlying cause of temporal lobe epilepsy, leptin could be considered as therapeutic target [1464]

Toxins
Mammalian BK channels are characterized by their high sensitivity to blockade by iberiotoxin (IbTx) and charybdotoxin [1164]

Drugs
BMS-204352 (BMS) was developed as a neuroprotective drug for disease conditions such as ischemic stroke and it has been shown to have a protective effect, due to activation of big-conductance calcium-activating potassium (BKCa) channels (Gribkoff et al., 2001 [1214]; Cheney et al., 2001 [1215]; Nardi and Olesen, 2008 [1216]).

BK channels are pharmacological targets for the treatment of stroke. Various pharmaceutical companies developed synthetic molecules activating these channels [565] in order to prevent excessive neurotoxic calcium entry in neurons [566]. But BMS-204352 (MaxiPost) a molecule developed by Bristol-Myers Squibb failed to improve clinical outcome in stroke patients compared to placebo [571]. BK channels have also been found to be activated by exogenous pollutants and endogenous gazotransmitters carbon monoxide [568], [567] and hydrogen sulphide [569]. BK channels are blocked by tetraethylammonium (TEA), paxilline (http://www.fermentek.co.il/paxilline.htm) and iberiotoxin [570].

vBK channels are activated by voltage and intracellular Ca2+ and Mg2+. The BK channel contains the transmembrane voltage sensitive domain as well as Ca2+ and Mg2+ binding. The voltage and metal sensors all control the opening of the same ionic pore in response to various physiological signals [1460]. There is evidence of changes in BK channel activity by phosphorylation and/or interaction by G proteins, by mechanical stretch, and by various endothelium-derived vasoactive substances [540].