Cholesterol and Kir channels

Electrocontractile remodeling of isolated cardiomyocytes induced during early-stage hypercholesterolemia

Hypercholesterolemia is one of the most important risk factors for cardiovascular diseases. However, it is mostly associated with vascular dysfunction and atherosclerotic lesions, while evidence of direct effects of hypercholesterolemia on cardiomyocytes and heart function is still incomplete and controversial. In this study, we assessed the direct effects of hypercholesterolemia on heart function and the electro-contractile properties of isolated cardiomyocytes. After 5 weeks, male Swiss mice fed with AIN-93 diet added with 1.25% cholesterol (CHO), developed an increase in total serum cholesterol levels and cardiomyocytes cholesterol content. These changes led to altered electrocardiographic records, with a shortening of the QT interval. Isolated cardiomyocytes displayed a shortening of the action potential duration with increased rate of depolarization, which was explained by increased IK, reduced ICa.L and altered INa voltage-dependent inactivation. Also, reduced diastolic [Ca2+]i was found with preserved adrenergic response and cellular contraction function. However, contraction of isolated hearts is impaired in isolated CHO hearts, before and after ischemia/reperfusion, although CHO heart was less susceptible to arrhythmic contractions. Overall, our results demonstrate that early hypercholesterolemia-driven increase in cellular cholesterol content is associated with direct modulation of the heart and cardiomyocytes' excitability, Ca2+ handling, and contraction.

Super-resolution imaging of potassium channels with genetically encoded EGFP

The plasma membrane is a well-organized structure of lipids and proteins, segmented into lipid compartments under 200 nm in size. This specific spatial patterning is crucial for the function of proteins and necessitates super-resolution imaging for its elucidation. Here, we establish that the genetically encoded enhanced green fluorescent protein (EGFP), when combined with direct optical reconstruction microscopy (dSTORM), tracks shear- and cholesterol-induced nanoscopic patterning of potassium channels overexpressed in HEK293T cells. Leveraging EGFP in dSTORM (EGFP-STORM), our findings indicate that cholesterol directs the C-terminus of TWIK-related potassium channel (TREK-1) to ceramide-enriched lipid ganglioside (GM1) clusters. In the absence of the C-terminus, the channel associates with phosphatidylinositol 4,5-bisphosphate (PIP2) cluster. Similarly, cholesterol derived from astrocytes repositions EGFP-tagged inward-rectifying potassium (Kir) channels into GM1 clusters. Without cholesterol, the channel aligns with PIP2 lipids. We deduce that cholesterol's interaction with Kir sequesters the channel, separating it from its activating lipid PIP2. Fundamentally, a genetically encoded EGFP tag should make any protein amenable to dSTORM analysis.

Nutrigenomics of inward rectifier potassium channels

Inwardly rectifying potassium (Kir) channels play a key role in maintaining the resting membrane potential and supporting potassium homeostasis. There are many variants of Kir channels, which are usually tetramers in which the main subunit has two trans-membrane helices attached to two N- and C-terminal cytoplasmic tails with a pore-forming loop in between that contains the selectivity filter. These channels have domains that are strongly modulated by molecules present in nutrients found in different diets, such as phosphoinositols, polyamines and Mg²⁺. These molecules can impact these channels directly or indirectly, either allosterically by modulation of enzymes or via the regulation of channel expression. A particular type of these channels is coupled to cell metabolism and inhibited by ATP (KATP channels, essential for insulin release and for the pathogenesis of metabolic diseases like diabetes mellitus). Genomic changes in Kir channels have a significant impact on metabolism, such as conditioning the nutrients and electrolytes that an individual can take. Thus, the nutrigenomics of ion channels is an important emerging field in which we are attempting to understand how nutrients and diets can affect the activity and expression of ion channels and how genomic changes in such channels may be the basis for pathological conditions that limit nutrition and electrolyte intake. In this contribution we briefly review Kir channels, discuss their nutrigenomics, characterize how different components in the diet affect their function and expression, and suggest how their genomic changes lead to pathological phenotypes that affect diet and electrolyte intake.

Cholesterol Regulation of Membrane Proteins Revealed by Two-Color Super-Resolution Imaging

Cholesterol and phosphatidyl inositol 4,5-bisphosphate (PIP₂) are hydrophobic molecules that regulate protein function in the plasma membrane of all cells. In this review, we discuss how changes in cholesterol concentration cause nanoscopic (<200 nm) movements of membrane proteins to regulate their function. Cholesterol is known to cluster many membrane proteins (often palmitoylated proteins) with long-chain saturated lipids. Although PIP₂ is better known for gating ion channels, in this review, we will discuss a second independent function as a regulator of nanoscopic protein movement that opposes cholesterol clustering. The understanding of the movement of proteins between nanoscopic lipid domains emerged largely through the recent advent of super-resolution imaging and the establishment of two-color techniques to label lipids separate from proteins. We discuss the labeling techniques for imaging, their strengths and weakness, and how they are used to reveal novel mechanisms for an ion channel, transporter, and enzyme function. Among the mechanisms, we describe substrate and ligand presentation and their ability to activate enzymes, gate channels, and transporters rapidly and potently. Finally, we define cholesterol-regulated proteins (CRP) and discuss the role of PIP₂ in opposing the regulation of cholesterol, as seen through super-resolution imaging.

OxLDL induces membrane structure rearrangement leading to biomechanics alteration and migration deficiency in macrophage

Cholesterol sequestration from plasma membrane has been shown to induce lipid packing disruption, causing actin cytoskeleton reorganization and polymerization, increasing cell stiffness and inducing lysosomal exocytosis in non-professional phagocytes. Similarly, oxidized form of low-density lipoprotein (oxLDL) has also been shown to disrupt lipid organization and packing in endothelial cells, leading to biomechanics alterations that interfere with membrane injury and repair. For macrophages, much is known about oxLDL effects in cell activation, cytokine production and foam cell formation. However, little is known about its impact in the organization of macrophage membrane structured domains and cellular mechanics, the focus of the present study. Treatment of bone marrow-derived macrophages (BMDM) with oxLDL not only altered membrane structure, and potentially the distribution of raft domains, but also induced actin rearrangement, diffuse integrin distribution and cell shrinkage, similarly to observed upon treatment of these cells with MβCD. Those alterations led to decreased migration efficiency. For both treatments, higher co-localization of actin cytoskeleton and GM1 was observed, indicating a similar mechanism of action involving raft-like domain dynamics. Lastly, like MβCD treatment, oxLDL also induced lysosomal spreading in BMDM. We propose that OxLDL induced re-organization of membrane/cytoskeleton complex in macrophages can be attributed to the insertion of oxysterols into the membrane, which lead to changes in lipid organization and disruption of membrane structure, similar to the effect of cholesterol depletion by MβCD treatment. These results indicate that oxLDL can induce physical alterations in the complex membrane/cytoskeleton of macrophages, leading to significant biomechanical changes that compromise cell behavior.

Kir7.1 disease mutant T153I within the inner pore affects K+ conduction

Inward-rectifier potassium channel 7.1 (Kir7.1) is present in polarized epithelium, including the RPE. A single amino acid change at position 153 in the KCNJ13 gene, a substitution of threonine to isoleucine in Kir7.1 protein, causes blindness. We hypothesized that the disease caused by this single amino acid substitution within the transmembrane protein domain could alter the translation, localization, or ion transport properties. We assessed the effects of amino acid side-chain length, arrangement, and polarity on channel structure and function. We showed that the T153I mutation yielded a full-length protein localized to the cell membrane. Whole-cell patch-clamp recordings and chord conductance analyses revealed that the T153I mutant channel had negligible K+ conductance and failed to hyperpolarize the membrane potential. However, the mutant channel exhibited enhanced inward current when Rb+ was used as a charge carrier, suggesting that an inner pore had formed, and the channel was dysfunctional. Substituting with a polar, non-polar, or short side-chain amino acid did not affect the localization of the protein. Still, it had an altered channel function due to differences in pore radius. Polar side chains (cysteine and serine) with inner pore radii comparable to wildtype exhibited normal inward K+ conductance. Short side-chains (glycine and alanine) produced a channel with wider than expected inner pore size and lacked the biophysical characteristics of the wildtype channel. Leucine substitution produced results similar to the T153I mutant channel. This study provides direct electrophysiological evidence for the structure and function of the Kir7.1 channel's narrow inner pore in regulating conductance.

Inward Rectifier Potassium Channels: Membrane Lipid-Dependent Mechanosensitive Gates in Brain Vascular Cells

Cerebral arteries contain two primary and interacting cell types, smooth muscle (SMCs) and endothelial cells (ECs), which are each capable of sensing particular hemodynamic forces to set basal tone and brain perfusion. These biomechanical stimuli help confer tone within arterial networks upon which local neurovascular stimuli function. Tone development is intimately tied to arterial membrane potential (VM) and changes in intracellular [Ca2+] driven by voltage-gated Ca2+ channels (VGCCs). Arterial VM is in turn set by the dynamic interplay among ion channel species, the strongly inward rectifying K+ (Kir) channel being of special interest. Kir2 channels possess a unique biophysical signature in that they strongly rectify, display negative slope conductance, respond to elevated extracellular K+ and are blocked by micromolar Ba2+. While functional Kir2 channels are expressed in both smooth muscle and endothelium, they lack classic regulatory control, thus are often viewed as a simple background conductance. Recent literature has provided new insight, with two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol, noted to (1) stabilize Kir2 channels in a preferred open or closed state, respectively, and (2) confer, in association with the cytoskeleton, caveolin-1 (Cav1) and syntrophin, hemodynamic sensitivity. It is these aspects of vascular Kir2 channels that will be the primary focus of this review.

A Cholesterol Dimer Stabilizes the Inactivated State of an Inward-Rectifier Potassium Channel

Cholesterol oligomers reside in multiple membrane protein X-ray crystal structures. Yet, there is no direct link between these oligomers and a biological function. Here we present the structural and functional details of a cholesterol dimer that stabilizes the inactivated state of an inward-rectifier potassium channel KirBac1.1. K⁺ efflux assays confirm that high cholesterol concentration reduces K⁺ conductance. We then determine the structure of the cholesterol-KirBac1.1 complex using Xplor-NIH simulated annealing calculations driven by solid-state NMR distance measurements. These calculations identified an α-α cholesterol dimer docked to a cleft formed by adjacent subunits of the homotetrameric protein. We compare these results to coarse grain molecular dynamics simulations. This is one of the first examples of a cholesterol oligomer performing a distinct biological function and structural characterization of a conserved promiscuous lipid binding region.

Methyl-Beta-Cyclodextrin Restores KIR Channel Function in Brain Endothelium of Female Alzheimer's Disease Mice

Background:

As the sixth-leading cause of death in the United States, Alzheimer’s disease (AD) entails deteriorating endothelial control of blood flow throughout the brain. In particular, reduced inward-rectifying K⁺ (K_IR) channel function in animal models of aging and AD compromises endothelial function and optimal perfusion of brain parenchyma. Deficient endothelial K_IR channels may result from aberrant interaction with plasma membrane cholesterol as a primary regulator of membrane fluidity and ion channels.

Objective:

We tested the hypothesis that mild methyl-β-cyclodextrin (MβCD) treatment to reduce membrane cholesterol may restore endothelial K_IR channel function in brain endothelium of old AD mice.

Methods:

Membrane potential was continuously measured in isolated endothelial tubes from posterior cerebral arteries of young (1 to 3 months) and old (16 to 19 months) female 3xTg-AD mice before and after mild treatment with the cholesterol-removing agent MβCD (1 mmol/L). Elevated extracellular potassium ([K⁺]_E; 15 mmol/L) and NS309 (1μmol/L) activated K_IR and Ca²⁺-activated K⁺ (SK_Ca/IK_Ca) channels respectively before and after MβCD treatment.

Results:

SK_Ca/IK_Ca channel function for producing hyperpolarization remained stable regardless of age group and MβCD treatment (ΔV_m: ∼–33 mV). However, as deficient during AD, K_IR channel function was restored (ΔV_m: –9±1 mV) versus pre-MβCD conditions (–5±1 mV); a progressive effect that reached –14±1 mV hyperpolarization at 60 min following MβCD washout.

Conclusion:

In female animals, MβCD treatment of brain endothelium selectively restores K_IR versus SK_Ca/IK_Ca channel function during AD. Thus, the endothelial cholesterol-K_IR channel interface is a novel target for ameliorating perfusion of the AD brain.

Cardiovascular mechanosensitive ion channels-Translating physical forces into physiological responses

Cells and tissues are constantly exposed to mechanical stress. In order to respond to alterations in mechanical stimuli, specific cellular machinery must be in place to rapidly convert physical force into chemical signaling to achieve the desired physiological responses. Mechanosensitive ion channels respond to such physical stimuli in the order of microseconds and are therefore essential components to mechanotransduction. Our understanding of how these ion channels contribute to cellular and physiological responses to mechanical force has vastly expanded in the last few decades due to engineering ingenuities accompanying patch clamp electrophysiology, as well as sophisticated molecular and genetic approaches. Such investigations have unveiled major implications for mechanosensitive ion channels in cardiovascular health and disease. Therefore, in this chapter I focus on our present understanding of how biophysical activation of various mechanosensitive ion channels promotes distinct cell signaling events with tissue-specific physiological responses in the cardiovascular system. Specifically, I discuss the roles of mechanosensitive ion channels in mediating (i) endothelial and smooth muscle cell control of vascular tone, (ii) mechano-electric feedback and cell signaling pathways in cardiomyocytes and cardiac fibroblasts, and (iii) the baroreflex.

From Bench to Biomolecular Simulation: Phospholipid Modulation of Potassium Channels

Potassium (K+) ion channels are crucial in numerous cellular processes as they hyperpolarise a cell through K+ conductance, returning a cell to its resting potential. K+ channel mutations result in multiple clinical complications such as arrhythmia, neonatal diabetes and migraines. Since 1995, the regulation of K+ channels by phospholipids has been heavily studied using a range of interdisciplinary methods such as cellular electrophysiology, structural biology and computational modelling. As a result, K+ channels are model proteins for the analysis of protein-lipid interactions. In this review, we will focus on the roles of lipids in the regulation of K+ channels, and how atomic-level structures, along with experimental techniques and molecular simulations, have helped guide our understanding of the importance of phospholipid interactions.

Multiomic analysis of the Arabian camel (Camelus dromedarius) kidney reveals a role for cholesterol in water conservation

The Arabian camel (Camelus dromedarius) is the most important livestock animal in arid and semi-arid regions and provides basic necessities to millions of people. In the current context of climate change, there is renewed interest in the mechanisms that enable camelids to survive in arid conditions. Recent investigations described genomic signatures revealing evolutionary adaptations to desert environments. We now present a comprehensive catalogue of the transcriptomes and proteomes of the dromedary kidney and describe how gene expression is modulated as a consequence of chronic dehydration and acute rehydration. Our analyses suggested an enrichment of the cholesterol biosynthetic process and an overrepresentation of categories related to ion transport. Thus, we further validated differentially expressed genes with known roles in water conservation which are affected by changes in cholesterol levels. Our datasets suggest that suppression of cholesterol biosynthesis may facilitate water retention in the kidney by indirectly facilitating the AQP2-mediated water reabsorption.

Interfacial Binding Sites for Cholesterol on Kir, Kv, K2P, and Related Potassium Channels

Inwardly rectifying, voltage-gated, two-pore domain, and related K+ channels are located in eukaryotic membranes rich in cholesterol. Here, molecular docking is used to detect specific binding sites ("hot spots") for cholesterol on K+ channels with characteristics that match those of known cholesterol binding sites. The transmembrane surfaces of all available high-resolution structures for K+ channels were swept for potential binding sites. Cholesterol poses were found to be located largely in hollows between protein ridges. A comparison between cholesterol poses and resolved phospholipids suggests that not all cholesterol molecules binding to the transmembrane surface of a K+ channel will result in displacement of a phospholipid molecule from the surface. Competition between cholesterol binding and binding of anionic phospholipids essential for activity could explain some of the effects of cholesterol on channel function.

Endothelial inwardly-rectifying K+ channels as a key component of shear stress-induced mechanotransduction

It has been recognized for decades that fluid shear stress plays a major role in vascular function. Acting on the endothelium shear stress induces vasorelaxation of resistance arteries and plays a major role in the propensity of the major arteries to atherosclerosis. Many elements of shear-induced signaling have been identified yet we are just beginning to decipher the roles that mechanosensitive ion channels may play in the signaling pathways initiated by shear stress. Endothelial inwardly-rectifying K⁺ channels were identified as potential primary mechanosensors in the late 1980s yet until our recent works, highlighted in the forthcoming chapter, the functional effect of a shear-activated K⁺ current was completely unknown. In this chapter, we present the physiological effects of shear stress in arteries in health and disease and highlight the most prevalent of today's investigated mechanosensitive ion channels. Ultimately, we focus on Kir2.1 channels and discuss in detail our findings regarding the downstream signaling events that are induced by shear-activated endothelial Kir2.1 channels. Most importantly, we examine our findings regarding hypercholesterolemia-induced inhibition of Kir channel shear-sensitivity and the impact on endothelial function in the context of flow (shear)-mediated vasodilation and atherosclerosis.

Membrane Lipid-K IR 2.x Channel Interactions Enable Hemodynamic Sensing in Cerebral Arteries

Objective—

Inward rectifying K + (K IR ) channels are present in cerebral arterial smooth muscle and endothelial cells, a tandem arrangement suggestive of a dynamic yet undiscovered role for this channel. This study defined whether distinct pools of cerebral arterial K IR channels were uniquely modulated by membrane lipids and hemodynamic stimuli.

Approach and Results—

A Ba 2+ -sensitive K IR current was isolated in smooth muscle and endothelial cells of rat cerebral arteries; molecular analyses subsequently confirmed K IR 2.1/K IR 2.2 mRNA and protein expression in both cells. Patch-clamp electrophysiology next demonstrated that each population of K IR channels was sensitive to key membrane lipids and hemodynamic stimuli. In this regard, endothelial K IR was sensitive to phosphatidylinositol 4,5-bisphosphate content, with depletion impairing the ability of laminar shear stress to activate this channel pool. In contrast, smooth muscle K IR was sensitive to membrane cholesterol content, with sequestration blocking the ability of pressure to inhibit channel activity. The idea that membrane lipids help confer shear stress and pressure sensitivity of K IR channels was confirmed in intact arteries using myography. Virtual models integrating structural/electrical observations reconceptualized K IR as a dynamic regulator of membrane potential working in concert with other currents to set basal tone across a range of shear stresses and intravascular pressures.

Conclusions—

The data show for the first time that specific membrane lipid-K IR interactions enable unique channel populations to sense hemodynamic stimuli and drive vasomotor responses to set basal perfusion in the cerebral circulation.

An Inward-Rectifier Potassium Channel Coordinates the Properties of Biologically Derived Membranes

KirBac1.1 is a prokaryotic inward-rectifier K⁺ channel from Burkholderia pseudomallei. It shares the common inward-rectifier K⁺ channel fold with eukaryotic channels, including conserved lipid-binding pockets. Here, we show that KirBac1.1 changes the phase properties and dynamics of the surrounding bilayer. KirBac1.1 was reconstituted into vesicles composed of ¹³C-enriched biological lipids. Two-dimensional liquid-state and solid-state NMR experiments were used to assign lipid ¹H and ¹³C chemical shifts as a function of lipid identity and conformational degrees of freedom. A solid-state NMR temperature series reveals that KirBac1.1 lowers the primary thermotropic phase transition of Escherichia coli lipid membranes while introducing both fluidity and internal lipid order into the fluid phases. In B. thailandensis liposomes, the bacteriohopanetetrol hopanoid, and potentially ornithine lipids, introduce a similar primary lipid-phase transition and liquid-ordered properties. Adding KirBac1.1 to B. thailandensis lipids increases B. thailandensis lipid fluidity while preserving internal lipid order. This synergistic effect of KirBac1.1 in bacteriohopanetetrol-rich membranes has implications for bilayer dynamic structure. If membrane proteins can anneal lipid translational degrees of freedom while preserving internal order, it could offer an explanation to the nature of liquid-ordered protein-lipid organization in vivo.

Synergistic activation of G protein-gated inwardly rectifying potassium channels by cholesterol and PI(4,5)P2

G-protein gated inwardly rectifying potassium (GIRK or Kir3) channels play a major role in the control of the heart rate, and require the membrane phospholipid phosphatidylinositol-bis-phosphate (PI(4,5)P₂) for activation. Recently, we have shown that the activity of the heterotetrameric Kir3.1/Kir3.4 channel that underlies atrial K_ACh currents was enhanced by cholesterol. Similarly, the activities of both the Kir3.4 homomer and its active pore mutant Kir3.4* (Kir3.4_S143T) were also enhanced by cholesterol. Here we employ planar lipid bilayers to investigate the crosstalk between PI(4,5)P₂ and cholesterol, and demonstrate that these two lipids act synergistically to activate Kir3.4* currents. Further studies using the Xenopus oocytes heterologous expression system suggest that PI(4,5)P₂ and cholesterol act via distinct binding sites. Whereas PI(4,5)P₂ binds to the cytosolic domain of the channel, the putative binding region of cholesterol is located at the center of the transmembrane domain overlapping the central glycine hinge region of the channel. Together, our data suggest that changes in the levels of two key membrane lipids - cholesterol and PI(4,5)P₂ - could act in concert to provide fine-tuning of Kir3 channel function.

Isoform dependent regulation of human HCN channels by cholesterol

Cholesterol has been shown to regulate numerous ion channels. HCN channels represent the molecular correlate of I_f or I_h in sinoatrial node (SAN) and neuronal cells. Previous studies have implicated a role for cholesterol in the regulation of rabbit HCN4 channels with effects on pacing in the rabbit SAN. Using electrophysiological and biochemical approaches, we examined the effect of cholesterol modulation on human HCN1, HCN2 and HCN4 isoforms. Patch-clamp experiments uncovered isoform specific differences in the effect of cholesterol on gating kinetics upon depletion by MβCD or mevastatin or enrichment using MβCD/cholesterol. Most dramatically cholesterol had isoform specific effects on mode-shifting, which has been suggested to play a key role in stabilizing firing rate and preventing arrhythmic firing in SAN cells and neurons. Mode-shifting in HCN1 channels was insensitive to cholesterol manipulation, while HCN2 and HCN4 were strongly affected. Trafficking of each isoform to the plasma membrane was also affected by cholesterol modulation differentially between isoforms, however, each isoform remained localized in lipid raft domains after cholesterol depletion. These effects may contribute to the side effects of cholesterol reducing therapies including disrupted heart rhythm and neuropathic pain, as well as the susceptibility of sinus dysfunction in patients with elevated cholesterol.

Cholesterol binding to ion channels

Numerous studies demonstrated that membrane cholesterol is a major regulator of ion channel function. The goal of this review is to discuss significant advances that have been recently achieved in elucidating the mechanisms responsible for cholesterol regulation of ion channels. The first major insight that comes from growing number of studies that based on the sterol specificity of cholesterol effects, show that several types of ion channels (nAChR, Kir, BK, TRPV) are regulated by specific sterol-protein interactions. This conclusion is supported by demonstrating direct saturable binding of cholesterol to a bacterial Kir channel. The second major advance in the field is the identification of putative cholesterol binding sites in several types of ion channels. These include sites at locations associated with the well-known cholesterol binding motif CRAC and its reversed form CARC in nAChR, BK, and TRPV, as well as novel cholesterol binding regions in Kir channels. Notably, in the majority of these channels, cholesterol is suggested to interact mainly with hydrophobic residues in non-annular regions of the channels being embedded in between transmembrane protein helices. We also discuss how identification of putative cholesterol binding sites is an essential step to understand the mechanistic basis of cholesterol-induced channel regulation. Clearly, however, these are only the first few steps in obtaining a general understanding of cholesterol-ion channels interactions and their roles in cellular and organ functions.

Membrane damaging activity of a maslinic acid analog

Close inspection of human ovarian cancer cells A2780 in the course of an antitumor screening using maslinic acid analogs revealed for one of the compounds, 4-oxa-4-phenyl-butyl 2,3-dihydroxy-olean-12-en-28-oate (1), an unusual behavior. During the incubation of the cells with 1, at the perimeter of the cells or close by crystals were formed consisting of cholesterol and excess 1. Compound 1 was incorporated into the cell's membrane followed by an extrusion of cholesterol from the lipid rafts. As a consequence of the alterations of the cell membrane, a volume decrease was initiated that triggered apoptosis; this extends previous models on apoptosis initiating mechanisms.

Identification of novel cholesterol-binding regions in Kir2 channels

Inwardly rectifying potassium (Kir) channels play an important role in setting the resting membrane potential and modulating membrane excitability. We have recently shown that cholesterol regulates representative members of the Kir family and that in the majority of the cases, cholesterol suppresses channel function. Furthermore, recent data indicate that cholesterol regulates Kir channels by specific sterol-protein interactions, yet the location of the cholesterol binding site in Kir channels is unknown. Using a combined computational-experimental approach, we show that cholesterol may bind to two nonanular hydrophobic regions in the transmembrane domain of Kir2.1 located between adjacent subunits of the channel. The location of the binding regions suggests that cholesterol modulates channel function by affecting the hinging motion at the center of the pore-lining transmembrane helix that underlies channel gating either directly or through the interface between the N and C termini of the channel.

Cholesterol sensitivity of KIR2.1 depends on functional inter-links between the N and C termini

In recent years, cholesterol has been emerging as a major regulator of ion channel function. We have previously shown that cholesterol suppresses Kir2 channels, a subfamily of constitutively active strongly rectifying K (+) channels. Furthermore, our earlier studies have shown that cholesterol sensitivity of Kir2 channels depends on a group of residues that form a belt-like structure around the cytosolic pore of the channel in proximity to the transmembrane domain. In this study, we focus on the contributions of different structural domains of Kir2 channels in the regulation of their cholesterol sensitivity. Focusing on the mildest mutation in the sensitivity belt, L222I, we show that the sensitivity of the channel to cholesterol can be restored by crosstalk between three distinct cytosolic regions: the C-terminal CD loop, the EF and GA loops of the C-terminus, and the βA sheet of the N-terminus. Thus, in addition to the importance of residues that affect the cytosolic G-loop gate in the sensitivity of Kir2 channels to cholesterol, our data suggest an important role to the interactions at the interface between the channel's N- and C- termini that couple the intracellular domains of its four subunits during gating.

Functionally important amino acid residues in the transient receptor potential vanilloid 1 (TRPV1) ion channel--an overview of the current mutational data

This review aims to create an overview of the currently available results of site-directed mutagenesis studies on transient receptor potential vanilloid type 1 (TRPV1) receptor. Systematization of the vast number of data on the functionally important amino acid mutations of TRPV1 may provide a clearer picture of this field, and may promote a better understanding of the relationship between the structure and function of TRPV1. The review summarizes information on 112 unique mutated sites along the TRPV1, exchanged to multiple different residues in many cases. These mutations influence the effect or binding of different agonists, antagonists, and channel blockers, alter the responsiveness to heat, acid, and voltage dependence, affect the channel pore characteristics, and influence the regulation of the receptor function by phosphorylation, glycosylation, calmodulin, PIP2, ATP, and lipid binding. The main goal of this paper is to publish the above mentioned data in a form that facilitates in silico molecular modelling of the receptor by promoting easier establishment of boundary conditions. The better understanding of the structure-function relationship of TRPV1 may promote discovery of new, promising, more effective and safe drugs for treatment of neurogenic inflammation and pain-related diseases and may offer new opportunities for therapeutic interventions.

Distant cytosolic residues mediate a two-way molecular switch that controls the modulation of inwardly rectifying potassium (Kir) channels by cholesterol and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2))

BACKGROUND

Cholesterol modulates inwardly rectifying potassium (Kir) channels.

RESULTS

A two-way molecular cytosolic switch controls channel modulation by cholesterol and PI(4,5)P(2).

CONCLUSION

Cholesterol and PI(4,5)P(2) induce a common gating pathway of Kir2.1 despite their opposite impact on channel function.

SIGNIFICANCE

These findings provide insights into structure-function relationship of ion channels and contribute to understanding of the mechanisms underlying their regulation by lipids. Inwardly rectifying potassium (Kir) channels play an important role in setting the resting membrane potential and modulating membrane excitability. An emerging feature of several Kir channels is that they are regulated by cholesterol. However, the mechanism by which cholesterol affects channel function is unclear. Here we show that mutations of two distant Kir2.1 cytosolic residues, Leu-222 and Asn-251, form a two-way molecular switch that controls channel modulation by cholesterol and affects critical hydrogen bonding. Notably, these two residues are linked by a residue chain that continues from Asn-251 to connect adjacent subunits. Furthermore, our data indicate that the same switch also regulates the sensitivity of the channels to phosphatidylinositol 4,5-bisphosphate, a phosphoinositide that is required for activation of Kir channels. Thus, although cholesterol and phosphatidylinositol 4,5-bisphosphate do not interact with the same region of Kir2.1, these different modulators induce a common gating pathway of the channel.

Influence of cholesterol on cellular signaling and fusion pore kinetics

Cholesterol is an important lipid component of cellular membranes. Recent studies have shown that changes in cellular cholesterol level can affect cellular functions. Here, we summarize our recent findings on the impact of cholesterol on the glucose-stimulated Ca(2+) signaling in rat pancreatic β cells and the fusion pore kinetics of large dense core granules in rat chromaffin cells. In mouse pancreatic β cells, pharmacological elevation of cellular cholesterol (but not cholesterol extraction) reduced the current density of the delayed rectifier K(+) channels, the ATP-dependent K(+) channels, and voltage-gated Ca(2+) channels. Importantly, cholesterol enrichment impaired glucose-stimulated Ca(2+) signaling in mouse pancreatic β cells via a suppression of voltage-gated Ca(2+) channels and a decrease in mitochondrial ATP production, which in turn led to a reduction in the glucose-evoked depolarization. In rat chromaffin cells, we found that the persistence of the semi-stable fusion pore was increased by cholesterol enrichment, and acute cholesterol extraction from the cytosolic side of the cell destabilized the semi-stable fusion pore. Overall, our findings highlight the importance of cholesterol in the regulation of cellular signaling and exocytosis.

Cholesterol depletion facilitates recovery from hypotonic cell swelling in CHO cells

The maintenance of cell volume homeostasis is critical for preventing pathological cell swelling that may lead to severe cellular dysfunction or cell death. Our earlier studies have shown that volume-regulated anion channels that play a major role in the regulation of cell volume are facilitated by a decrease in cellular cholesterol suggesting that cholesterol depletion should also facilitate regulatory volume decrease (RVD), the ability of cells to recover from hypotonic swelling. In this study, we test this hypothesis using a novel methodology developed to measure changes in cell volume using a microfluidics chamber. Our data show that cholesterol depletion of Chinese Hamster Ovary (CHO) significantly facilitates the recovery process, as is apparent from a faster onset of the RVD (162±10 s. vs. 114±5 s. in control and cholesterol depleted cells respectively) and a higher degree of volume recovery after 10 min of the hypotonic challenge (41%±6% vs. 65%±6% in control and cholesterol depleted cells respectively). In contrast, enriching cells with cholesterol had no effect on the RVD process. We also show here that similarly to our previous observations in endothelial cells, cholesterol depletion significantly increases the stiffness of CHO cells suggesting that facilitation of RVD may be associated with cell stiffening. Furthermore, we also show that increasing cell stiffness by stabilizing F-actin with jasplakinolide also facilitates RVD development. We propose that cell stiffening enhances cell mechano-sensitivity, which in turn facilitates the RVD process.

Cholesterol regulates prokaryotic Kir channel by direct binding to channel protein

Cholesterol is a major regulator of a variety of ion channels but the mechanisms underlying cholesterol sensitivity of ion channels are still poorly understood. The key question is whether cholesterol regulates ion channels by direct binding to the channel protein or by altering the physical environment of lipid bilayer. In this study, we provide the first direct evidence that cholesterol binds to prokaryotic Kir channels, KirBac1.1, and that cholesterol binding is essential for its regulatory effect. Specifically, we show that cholesterol is eluted together with the KirBac1.1 protein when separated on an affinity column and that the amount of bound cholesterol is proportional to the amount of the protein. We also show that cholesterol binding to KirBac1.1 is saturable with a K(D) of 390μM. Moreover, there is clear competition between radioactive and non-radioactive cholesterol for the binding site. There is no competition, however, between cholesterol and 5-Androsten 3β-17 β-diol, a sterol that we showed previously to have no effect on KirBac1.1 function. Finally, we show that cholesterol-KirBac1.1 binding is significantly inhibited by trifluoperazine, known to inhibit cholesterol binding to other proteins, and that inhibition of cholesterol-KirBac1.1 binding results in full recovery of the channel activity. Collectively, results from this study indicate that cholesterol-induced suppression of KirBac1.1 activity is mediated by direct interaction between cholesterol and the channel protein.

Identification of a binding motif in the S5 helix that confers cholesterol sensitivity to the TRPV1 ion channel

The TRPV1 ion channel serves as an integrator of noxious stimuli with its activation linked to pain and neurogenic inflammation. Cholesterol, a major component of cell membranes, modifies the function of several types of ion channels. Here, using measurements of capsaicin-activated currents in excised patches from TRPV1-expressing HEK cells, we show that enrichment with cholesterol, but not its diastereoisomer epicholesterol, markedly decreased wild-type rat TRPV1 currents. Substitutions in the S5 helix, rTRPV1-R579D, and rTRPV1-F582Q, decreased this cholesterol response and rTRPV1-L585I was insensitive to cholesterol addition. Two human TRPV1 variants, with different amino acids at position 585, had different responses to cholesterol with hTRPV1-Ile(585) being insensitive to this molecule. However, hTRPV1-I585L was inhibited by cholesterol addition similar to rTRPV1 with the same S5 sequence. In the absence of capsaicin, cholesterol enrichment also inhibited TRPV1 currents induced by elevated temperature and voltage. These data suggest that there is a cholesterol-binding site in TRPV1 and that the functions of TRPV1 depend on the genetic variant and membrane cholesterol content.

Ion channels, phosphorylation and mammalian sperm capacitation

Sexually reproducing animals require an orchestrated communication between spermatozoa and the egg to generate a new individual. Capacitation, a maturational complex phenomenon that occurs in the female reproductive tract, renders spermatozoa capable of binding and fusing with the oocyte, and it is a requirement for mammalian fertilization. Capacitation encompasses plasma membrane reorganization, ion permeability regulation, cholesterol loss and changes in the phosphorylation state of many proteins. Novel tools to study sperm ion channels, image intracellular ionic changes and proteins with better spatial and temporal resolution, are unraveling how modifications in sperm ion transport and phosphorylation states lead to capacitation. Recent evidence indicates that two parallel pathways regulate phosphorylation events leading to capacitation, one of them requiring activation of protein kinase A and the second one involving inactivation of ser/thr phosphatases. This review examines the involvement of ion transporters and phosphorylation signaling processes needed for spermatozoa to achieve capacitation. Understanding the molecular mechanisms leading to fertilization is central for societies to deal with rising male infertility rates, to develop safe male gamete-based contraceptives and to preserve biodiversity through better assisted fertilization strategies.

Cholesterol sensitivity of KIR2.1 is controlled by a belt of residues around the cytosolic pore

Kir channels play an important role in setting the resting membrane potential and modulating membrane excitability. A common feature of several Kir channels is that they are regulated by cholesterol. Yet, the mechanism by which cholesterol affects channel function is unclear. We recently showed that the cholesterol sensitivity of Kir2 channels depends on several CD-loop residues. Here we show that this cytosolic loop is part of a regulatory site that also includes residues in the G-loop, the N-terminus, and the connecting segment between the C-terminus and the inner transmembrane helix. Together, these residues form a cytosolic belt that surrounds the pore of the channel close to its interface with the transmembrane domain, and modulate the cholesterol sensitivity of the channel. Furthermore, we show that residues in this cluster are correlated with residues located in the most flexible region of the G-loop, the major cytosolic gate of Kir2.1, implying that the importance of these residues extends beyond their effect on the channel's cholesterol sensitivity. We suggest that the residues of the cholesterol sensitivity belt are critical for channel gating.

Review: Placental syncytiotrophoblast membranes--domains, subdomains and microdomains

Human placental syncytiotrophoblast (STB) is an epithelium responsible for materno-fetal exchange. Ions play multiple roles in STB, as in other transport epithelia. We have been interested in the character and functional expression of ion channels in STB membrane fractions. Characterization of ion channels and their relationship with different domains, subdomains and microdomains of STB membranes is important to explain the intracellular mechanisms operating in the placental barrier. The aim of this paper is to summarize our work on this subject. We isolated and purified basal membrane (BM) and two fractions from the apical membrane, a classical fraction (MVM) and a light fraction (LMVM). They were used either for reconstitution into giant liposomes or for transplantation into Xenopus oocyte membranes followed by electrophysiological recordings to characterize chloride and cationic channels in STB from term human placenta. In addition, Western blot analysis, using ion channel antibodies, was performed on purified apical and basal membrane fractions. We also reported the presence of two functional microdomains (lipid rafts) in LMVM and MVM, using detergent resistant membranes (DRMs) and cholesterol-sensitive depletion. Moreover we found evidence of cytoskeletal participation in lipid rafts of different composition. Our results contribute to knowledge of the ion channels present in STB membranes and their participation in the physiology of this epithelium in normal and pathological pregnancies.

Acute cholesterol-induced anti-natriuretic effects: role of epithelial Na+ channel activity, protein levels, and processing

The epithelial Na(+) channel (ENaC) is modulated by membrane lipid composition. However, the effect of an in vivo change of membrane composition is unknown. We examined the effect of a 70-day enhanced cholesterol diet (ECD) on ENaC and renal Na(+) handling. Rats were fed a standard chow or one supplemented with 1% cholesterol and 0.5% cholic acid (ECD). ECD animals exhibited marked anti-diuresis and anti-natriuresis (40 and 47%), which peaked at 1-3 weeks. Secondary compensation returned urine output and urinary Na(+) excretion to control levels by week 10. During these initial changes, there were no accompanying effects on systolic blood pressure, serum creatinine, or urinary creatinine excretion, indicating that the these effects of ECD preceded those which modify renal filtration and blood pressure. The effects of ECD on ENaC were evaluated by measuring the relative protein content of α, β, and γ subunits. α and γ blots were further examined for subunit cleavage (a process that activates ENaC). No significant changes were observed in α and β levels throughout the study. However, levels of cleaved γ were elevated, suggesting that ENaC was activated. The changes of γ persisted at week 10 and were accompanied by additional subunit fragments, indicating potential changes of γ-cleaving proteases. Enhanced protease activity, and specifically that which could act on the second identified cleavage site in γ, was verified in a newly developed urinary protease assay. These results predict enhanced ENaC activity, an effect that was confirmed in patch clamp experiments of principal cells of split open collecting ducts, where ENaC open probability was increased by 40% in the ECD group. These data demonstrate a complex series of events and a new regulatory paradigm that is initiated by ECD prior to the onset of elevated blood pressure. These events lead to changes of renal Na(+) handling, which occur in part by effects on extracellular γ-ENaC cleavage.

Comparative analysis of cholesterol sensitivity of Kir channels: role of the CD loop

Kir channels are important in setting the resting membrane potential and modulating membrane excitability. A common feature of Kir2 channels and several other ion channels that has emerged in recent years is that they are regulated by cholesterol, a major lipid component of the plasma membrane whose excess is associated with multiple pathological conditions. Yet, the mechanism by which cholesterol affects channel function is not clear. We have recently shown that the sensitivity of Kir2 channels to cholesterol depends on residues in the CD loop of the cytosolic domain of the channels with one of the mutations, L222I, abrogating cholesterol sensitivity of the channels completely. Here we show that in addition to Kir2 channels, members of other Kir subfamilies are also regulated by cholesterol. Interestingly, while similarly to Kir2 channels, several Kir channels, Kir1.1, Kir4.1 and Kir6.2Delta36 were suppressed by an increase in membrane cholesterol, the function of Kir3.4* and Kir7.1 was enhanced following cholesterol enrichment. Furthermore, we show that independent of the impact of cholesterol on channel function, mutating residues in the corresponding positions of the CD loop in Kir2.1 and Kir3.4*, inhibits cholesterol sensitivity of Kir channels, thus extending the critical role of the CD loop beyond Kir2 channels.

Activation of membrane cholesterol by 63 amphipaths

A few membrane-intercalating amphipaths have been observed to stimulate the interaction of cholesterol with cholesterol oxidase, saponin and cyclodextrin, presumably by displacing cholesterol laterally from its phospholipid complexes. We now report that this effect, referred to as cholesterol activation, occurs with dozens of other amphipaths, including alkanols, saturated and cis- and trans-unsaturated fatty acids, fatty acid methyl esters, sphingosine derivatives, terpenes, alkyl ethers, ketones, aromatics and cyclic alkyl derivatives. The apparent potency of the agents tested ranged from 3 microM to 7 mM and generally paralleled their octanol/water partition coefficients, except that relative potency declined for compounds with >10 carbons. Some small amphipaths activated cholesterol at a membrane concentration of approximately 3 mol per 100 mol of bilayer lipids, about equimolar with the cholesterol they displaced. Lysophosphatidylserine countered the effects of all these agents, consistent with its ability to reduce the pool of active membrane cholesterol. Various amphipaths stabilized red cells against the hemolysis elicited by cholesterol depletion, presumably by substituting for the extracted sterol. The number and location of cis and trans fatty acid unsaturations and the absolute stereochemistry of enantiomer pairs had only small effects on amphipath potency. Nevertheless, potency varied approximately 7-fold within a group of diverse agents with similar partition coefficients. We infer that a wide variety of amphipaths can displace membrane cholesterol by competing stoichiometrically but with only limited specificity for weak association with phospholipids. Any number of other drugs and experimental agents might do the same.

Direct regulation of prokaryotic Kir channel by cholesterol

Our earlier studies have shown that channel activity of Kir2 subfamily of inward rectifiers is strongly suppressed by the elevation of cellular cholesterol. The goal of this study is to determine whether cholesterol suppresses Kir channels directly. To achieve this goal, purified prokaryotic Kir (KirBac1.1) channels were incorporated into liposomes of defined lipid composition, and channel activity was assayed by (86)Rb(+) uptake. Our results show that (86)Rb(+) flux through KirBac1.1 is strongly inhibited by cholesterol. Incorporation of 5% (mass cholesterol/phospholipid) cholesterol into the liposome suppresses (86)Rb(+) flux by >50%, and activity is completely inhibited at 12-15%. However, epicholesterol, a stereoisomer of cholesterol with similar physical properties, has significantly less effect on KirBac-mediated (86)Rb(+) uptake than cholesterol. Furthermore, analysis of multiple sterols suggests that cholesterol-induced inhibition of KirBac1.1 channels is mediated by specific interactions rather than by changes in the physical properties of the lipid bilayer. In contrast to the inhibition of KirBac1.1 activity, cholesterol had no effect on the activity of reconstituted KscA channels (at up to 250 microg/mg of phospholipid). Taken together, these observations demonstrate that cholesterol suppresses Kir channels in a pure protein-lipid environment and suggest that the interaction is direct and specific.