Distinct specificities of inwardly rectifying K(+) channels for phosphoinositides

Activation of Smooth Muscle Kir2.1 Channels and Na+/K+-ATPase Mediates Dilation of Porcine Coronary Arterioles at Physiological Levels of Potassium

Metabolic stress on the heart can cause dilation of coronary arterioles for blood flow recruitment. Although potassium ions (K+) released from the myocardium are a major mediator for this response, the underlying signaling pathways for vasodilation are incompletely understood. Herein, the roles of smooth muscle inward-rectifier K+ channel subtype 2.1 (Kir2.1) and Na+/K+-ATPase were examined. Porcine coronary arterioles were isolated, cannulated, and pressurized for vasomotor study. Vessels developed basal tone and dilated concentration-dependently to extraluminal K+ from 7 to 20 mM. Higher K+ concentrations (25-40 mM) caused graded vasoconstriction. Vasodilation to K+ (10 mM) was not altered by endothelial removal, and blockade of ATP-sensitive K+ channels, voltage-sensitive K+ channels, or calcium-activated K+ channels did not affect K+-induced vasodilation. However, sustained but not abrupt transient vasodilation to K+ was reduced by the nonspecific Kir channel inhibitor Ba2+ or Kir2.1 channel blocker chloroethylclonidine. The Na+/K+-ATPase inhibitor ouabain attenuated K+-elicited vasodilation, and ouabain with Ba2+ abolished the response. Transfection of arterioles with Kir2.1 antisense oligonucleotides abolished sustained but not transient dilation. It is concluded that extraluminal K+ elevation within the physiological range induces initial transient dilation of porcine coronary arterioles by activating smooth muscle Na+/K+-ATPase and sustained dilation via smooth muscle Kir2.1 channels.

SUMO Regulation of Ion Channels in Health and Disease

The small ubiquitin-like modifier (SUMO) protein pathway governs a panoply of vital biological processes including cell death, proliferation, differentiation, metabolism, and signal transduction by diversifying the functions, half-lives, and partnerships of target proteins in situ. More recently, SUMOylation has emerged as a key regulator of ion homeostasis and excitability across multiple tissues due to the regulation of a plethora of ion channels expressed in a range of tissue subtypes. Altogether, the balance of SUMOylation states among relevant ion channels can result in graded biophysical effects that tune excitability and contribute to a range of disease states including cardiac arrhythmia, epilepsy, pain transmission, and inflammation. Here, we consolidate these concepts by focusing on the role of ion channel SUMOylation in the central nervous system, peripheral nervous system, and cardiovascular system. In addition, we review what is known about the enigmatic factors that regulate the SUMO pathway and consider the emerging role of small molecule SUMO modulators as potential therapeutics in a range of diseases.

Mechanosensory entities and functionality of endothelial cells

The endothelial cells of the blood circulation are exposed to hemodynamic forces, such as cyclic strain, hydrostatic forces, and shear stress caused by the blood fluid's frictional force. Endothelial cells perceive mechanical forces via mechanosensors and thus elicit physiological reactions such as alterations in vessel width. The mechanosensors considered comprise ion channels, structures linked to the plasma membrane, cytoskeletal spectrin scaffold, mechanoreceptors, and junctional proteins. This review focuses on endothelial mechanosensors and how they alter the vascular functions of endothelial cells. The current state of knowledge on the dysregulation of endothelial mechanosensitivity in disease is briefly presented. The interplay in mechanical perception between endothelial cells and vascular smooth muscle cells is briefly outlined. Finally, future research avenues are highlighted, which are necessary to overcome existing limitations.

Endocannabinoid regulation of inward rectifier potassium (Kir) channels

The inward rectifier potassium channel Kir2.1 (KCNJ2) is an important regulator of resting membrane potential in both excitable and non-excitable cells. The functions of Kir2.1 channels are dependent on their lipid environment, including the availability of PI(4,5)P2, secondary anionic lipids, cholesterol and long-chain fatty acids acyl coenzyme A (LC-CoA). Endocannabinoids are a class of lipids that are naturally expressed in a variety of cells, including cardiac, neuronal, and immune cells. While these lipids are identified as ligands for cannabinoid receptors there is a growing body of evidence that they can directly regulate the function of numerous ion channels independently of CBRs. Here we examine the effects of a panel of endocannabinoids on Kir2.1 function and demonstrate that a subset of endocannabinoids can alter Kir2.1 conductance to varying degrees independently of CBRs. Using computational and Surface plasmon resonance analysis, endocannabinoid regulation of Kir2.1 channels appears to be the result of altered membrane properties, rather than through direct protein-lipid interactions. Furthermore, differences in endocannabinoid effects on Kir4.1 and Kir7.1 channels, indicating that endocannabinoid regulation is not conserved among Kir family members. These findings may have broader implications on the function of cardiac, neuronal and/or immune cells.

Structural basis of the activation of TRPV5 channels by long-chain acyl-Coenzyme-A

Long-chain acyl-coenzyme A (LC-CoA) is a crucial metabolic intermediate that plays important cellular regulatory roles, including activation and inhibition of ion channels. The structural basis of ion channel regulation by LC-CoA is not known. Transient receptor potential vanilloid 5 and 6 (TRPV5 and TRPV6) are epithelial calcium-selective ion channels. Here, we demonstrate that LC-CoA activates TRPV5 and TRPV6 in inside-out patches, and both exogenously supplied and endogenously produced LC-CoA can substitute for the natural ligand phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in maintaining channel activity in intact cells. Utilizing cryo-electron microscopy, we determined the structure of LC-CoA-bound TRPV5, revealing an open configuration with LC-CoA occupying the same binding site as PI(4,5)P2 in previous studies. This is consistent with our finding that PI(4,5)P2 could not further activate the channels in the presence of LC-CoA. Our data provide molecular insights into ion channel regulation by a metabolic signaling molecule.

PI(4,5)P2 regulates the gating of NaV1.4 channels

Voltage-gated sodium (NaV) channels are densely expressed in most excitable cells and activate in response to depolarization, causing a rapid influx of Na+ ions that initiates the action potential. The voltage-dependent activation of NaV channels is followed almost instantaneously by fast inactivation, setting the refractory period of excitable tissues. The gating cycle of NaV channels is subject to tight regulation, with perturbations leading to a range of pathophysiological states. The gating properties of most ion channels are regulated by the membrane phospholipid, phosphatidylinositol (4,5) bisphosphate (PI(4,5)P2). However, it is not known whether PI(4,5)P2 modulates the activity of NaV channels. Here, we utilize optogenetics to activate specific, membrane-associated phosphoinositide (PI)-phosphatases that dephosphorylate PI(4,5)P2 while simultaneously recording NaV1.4 channel currents. We show that dephosphorylating PI(4,5)P2 left-shifts the voltage-dependent gating of NaV1.4 to more hyperpolarized membrane potentials, augments the late current that persists after fast inactivation, and speeds the rate at which channels recover from fast inactivation. These effects are opposed by exogenous diC8PI(4,5)P2. We provide evidence that PI(4,5)P2 is a negative regulator that tunes the gating behavior of NaV1.4 channels.

Molecular stratification of arrhythmogenic mechanisms in the Andersen Tawil syndrome

Andersen-Tawil syndrome (ATS) is a rare inheritable disease associated with loss-of-function mutations in KCNJ2, the gene coding the strong inward rectifier potassium channel Kir2.1, which forms an essential membrane protein controlling cardiac excitability. ATS is usually marked by a triad of periodic paralysis, life-threatening cardiac arrhythmias and dysmorphic features, but its expression is variable and not all patients with a phenotype linked to ATS have a known genetic alteration. The mechanisms underlying this arrhythmogenic syndrome are poorly understood. Knowing such mechanisms would be essential to distinguish ATS from other channelopathies with overlapping phenotypes and to develop individualized therapies. For example, the recently suggested role of Kir2.1 as a countercurrent to sarcoplasmic calcium reuptake might explain the arrhythmogenic mechanisms of ATS and its overlap with catecholaminergic polymorphic ventricular tachycardia. Here we summarize current knowledge on the mechanisms of arrhythmias leading to sudden cardiac death in ATS. We first provide an overview of the syndrome and its pathophysiology, from the patient's bedside to the protein and discuss the role of essential regulators and interactors that could play a role in cases of ATS. The review highlights novel ideas related to some post-translational channel interactions with partner proteins that might help define the molecular bases of the arrhythmia phenotype. We then propose a new all-embracing classification of the currently known ATS loss-of-function mutations according to their position in the Kir2.1 channel structure and their functional implications. We also discuss specific ATS pathogenic variants, their clinical manifestations, and treatment stratification. The goal is to provide a deeper mechanistic understanding of the syndrome toward the development of novel targets and personalized treatment strategies.

PI(4,5)P2-dependent and -independent roles of PI4P in the control of hormone secretion by pituitary cells

Plasma membrane and organelle membranes are home to seven phosphoinositides, an important class of low-abundance anionic signaling lipids that contribute to cellular functions by recruiting cytoplasmic proteins or interacting with the cytoplasmic domains of membrane proteins. Here, we briefly review the functions of three phosphoinositides, PI4P, PI(4,5)P2, and PI(3,4,5)P3, in cellular signaling and exocytosis, focusing on hormone-producing pituitary cells. PI(4,5)P2, acting as a substrate for phospholipase C, plays a key role in the control of pituitary cell functions, including hormone synthesis and secretion. PI(4,5)P2 also acts as a substrate for class I PI3-kinases, leading to the generation of two intracellular messengers, PI(3,4,5)P3 and PI(3,4)P2, which act through their intracellular effectors, including Akt. PI(4,5)P2 can also influence the release of pituitary hormones acting as an intact lipid to regulate ion channel gating and concomitant calcium signaling, as well as the exocytic pathway. Recent findings also show that PI4P is not only a precursor of PI(4,5)P2, but also a key signaling molecule in many cell types, including pituitary cells, where it controls hormone secretion in a PI(4,5)P2-independent manner.

Cholesterol and PIP2 Modulation of BKCa Channels

Ca2+/voltage-gated, large conductance K+ channels (BKCa) are formed by homotetrameric association of α (slo1) subunits. Their activity, however, is suited to tissue-specific physiology largely due to their association with regulatory subunits (β and γ types), chaperone proteins, localized signaling, and the channel's lipid microenvironment. PIP2 and cholesterol can modulate BKCa activity independently of downstream signaling, yet activating Ca2+i levels and regulatory subunits control ligand action. At physiological Ca2+i and voltages, cholesterol and PIP2 reduce and increase slo1 channel activity, respectively. Moreover, slo1 proteins provide sites that seem to recognize cholesterol and PIP2: seven CRAC motifs in the slo1 cytosolic tail and a string of positively charged residues (Arg329, Lys330, Lys331) immediately after S6, respectively. A model that could explain the modulation of BKCa activity by cholesterol and/or PIP2 is hypothesized. The roles of additional sites, whether in slo1 or BKCa regulatory subunits, for PIP2 and/or cholesterol to modulate BKCa function are also discussed.

From Crosstalk to Synergism: The Combined Effect of Cholesterol and PI(4,5)P2 on Inwardly Rectifying Potassium Channels

Inwardly rectifying potassium (Kir) channels are integral membrane proteins that control the flux of potassium ions across cell membranes and regulate membrane permeability. All eukaryotic Kir channels require the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) for activation. In recent years, it has become evident that the function of many members of this family of channels is also mediated by another essential lipid-cholesterol. Here, we focus on members of the Kir2 and Kir3 subfamilies and their modulation by these two key lipids. We discuss how PI(4,5)P₂ and cholesterol bind to Kir2 and Kir3 channels and how they affect channel activity. We also discuss the accumulating evidence indicating that there is interplay between PI(4,5)P₂ and cholesterol in the modulation of Kir2 and Kir3 channels. In particular, we review the crosstalk between PI(4,5)P₂ and cholesterol in the modulation of the ubiquitously expressed Kir2.1 channel and the synergy between these two lipids in the modulation of the Kir3.4 channel, which is primarily expressed in the heart. Additionally, we demonstrate that there is also synergy in the modulation of Kir3.2 channels, which are expressed in the brain. These observations suggest that alterations in the relative levels PI(4,5)P₂ and cholesterol may fine-tune Kir channel activity.

Mechanistic insights on KATP channel regulation from cryo-EM structures

Gated by intracellular ATP and ADP, ATP-sensitive potassium (KATP) channels couple cell energetics with membrane excitability in many cell types, enabling them to control a wide range of physiological processes based on metabolic demands. The KATP channel is a complex of four potassium channel subunits from the Kir channel family, Kir6.1 or Kir6.2, and four sulfonylurea receptor subunits, SUR1, SUR2A, or SUR2B, from the ATP-binding cassette (ABC) transporter family. Dysfunction of KATP channels underlies several human diseases. The importance of these channels in human health and disease has made them attractive drug targets. How the channel subunits interact with one another and how the ligands interact with the channel to regulate channel activity have been long-standing questions in the field. In the past 5 yr, a steady stream of high-resolution KATP channel structures has been published using single-particle cryo-electron microscopy (cryo-EM). Here, we review the advances these structures bring to our understanding of channel regulation by physiological and pharmacological ligands.

Hypoxia inhibits the cardiac I K1 current through SUMO targeting Kir2.1 activation by PIP2

Cardiovascular diseases remain the leading cause of death worldwide. Most deaths are sudden and occur secondary to the occlusion of coronary arteries resulting in a rapid decrease in cellular oxygen levels. Acute hypoxia is proarrhythmic, leading to disordered electrical signals, conduction block, and uncoordinated beating of the myocardium. Although acute hypoxia is recognized to perturb the electrophysiology of heart muscle, the mechanistic basis for the effect has remained elusive, hampering the development of targeted therapeutic interventions. Here, we show that acute hypoxia activates the redox-sensitive SUMO pathway in cardiomyocytes, causing rapid inhibition of the inward-rectifying K+ channel, Kir2.1. We find that SUMOylation decreases the activation of Kir2.1 channels by the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). These data provide a mechanistic basis for the proarrhythmic effects of acute hypoxia and offer a framework for understanding the central role of PIP2 in mediating the sequelae of hypoxia and SUMOylation in cardiovascular disease.

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.

Entropy in the Molecular Recognition of Membrane Protein-Lipid Interactions

Understanding the molecular driving forces that underlie membrane protein-lipid interactions requires the characterization of their binding thermodynamics. Here, we employ variable-temperature native mass spectrometry to determine the thermodynamics of lipid binding events to the human G-protein-gated inward rectifier potassium channel, Kir3.2. The channel displays distinct thermodynamic strategies to engage phosphatidylinositol (PI) and phosphorylated forms thereof. The addition of a 4'-phosphate to PI results in an increase in favorable entropy. PI with two or more phosphates exhibits more complex binding, where lipids appear to bind two nonidentical sites on Kir3.2. Remarkably, the interaction of 4,5-bisphosphate PI with Kir3.2 is solely driven by a large, favorable change in entropy. Installment of a 3'-phosphate to PI(4,5)P2 results in an altered thermodynamic strategy. The acyl chain of the lipid has a marked impact on binding thermodynamics and, in some cases, enthalpy becomes favorable.

Insight into the Phospholipid-Binding Preferences of Kir3.4

The G-protein-gated inwardly rectifying potassium channel 4 (Kir3.4) subunit forms functional tetramers. Previous studies have established that phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) is required for Kir3.4 function. However, the binding preferences of Kir3.4 for the headgroup and acyl chains of phosphorylated phosphatidylinositides (PIPs) and other lipids are not well understood. Here, the interactions between full-length, human Kir3.4 and lipids are characterized using native mass spectrometry (MS) in conjunction with a soluble fluorescent lipid-binding assay. Kir3.4 displays binding preferences for PIPs, and, in some cases, the degree of binding is influenced by the type of acyl chains. The interactions between Kir3.4 and PIPs are weaker in comparison to full-length, human Kir3.2. The binding of PI(4,5)P₂ modified with a fluorophore to Kir3.2 can be enhanced by other lipids, such as phosphatidylcholine. Introduction of S143T, a mutation that enhances Kir3.4 activity, results in an overall reduction in the channel binding PIPs. In contrast, the D223N mutant of Kir3.4 that mimics the sodium-bound state exhibited stronger binding for PI(4,5)P₂, particularly for those with 18:0-20:4 acyl chains. Taken together, these results provide additional insight into the interaction between Kir3.4 and lipids that are important for channel function.

Heritable arrhythmias associated with abnormal function of cardiac potassium channels

Cardiomyocytes express a surprisingly large number of potassium channel types. The primary physiological functions of the currents conducted by these channels are to maintain the resting membrane potential and mediate action potential repolarization under basal conditions and in response to changes in the concentrations of intracellular sodium, calcium, and ATP/ADP. Here, we review the diversity and functional roles of cardiac potassium channels under normal conditions and how heritable mutations in the genes encoding these channels can lead to distinct arrhythmias. We briefly review atrial fibrillation and J-wave syndromes. For long and short QT syndromes, we describe their genetic basis, clinical manifestation, risk stratification, traditional and novel therapeutic approaches, as well as insights into disease mechanisms provided by animal and cellular models.

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.

Methods to study phosphoinositide regulation of ion channels

Ion channel are embedded in the lipid bilayers of biological membranes. Membrane phospholipids constitute a barrier to ion movement, and they have been considered for a long time as a passive environment for channel proteins. Membrane phospholipids, however, do not only serve as a passive amphipathic environment, but they also modulate channel activity by direct specific lipid-protein interactions. Phosphoinositides are quantitatively minor components of biological membranes, and they play roles in many cellular functions, including membrane traffic, cellular signaling and cytoskeletal organization. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P₂] is mainly found in the inner leaflet of the plasma membrane. Its role as a potential ion channel regulator was first appreciated over two decades ago and by now this lipid is a well-established cofactor or regulator of many different ion channels. The past two decades witnessed the steady development of techniques to study ion channel regulation by phosphoinositides with progress culminating in recent cryoEM structures that allowed visualization of how PI(4,5)P₂ opens some ion channels. This chapter will provide an overview of the methods to study regulation by phosphoinositides, focusing on plasma membrane ion channels and PI(4,5)P₂.

KIR channels in the microvasculature: Regulatory properties and the lipid-hemodynamic environment

Basal tone and perfusion control is set in cerebral arteries by the sensing of pressure and flow, key hemodynamic stimuli. These forces establish a contractile foundation within arterial networks upon which local neurovascular stimuli operate. This fundamental process is intimately tied to arterial V_M and the rise in cytosolic [Ca²⁺] by the graded opening of voltage-operated Ca²⁺ channels. Arterial V_M is in turn controlled by a dynamic interaction among several resident ion channels, K_IR being one of particular significance. As the name suggests, K_IR displays strong inward rectification, retains a small outward component, potentiated by extracellular K⁺ and blocked by micromolar Ba²⁺. Cerebrovascular K_IR is unique from other K⁺ currents as it is present in both smooth muscle and endothelium yet lacking in classical regulatory modulation. Such observations have fostered the view that K_IR is nothing more than a background conductance, activated by extracellular K⁺ and which passively facilitates dilation. Recent work in cell model systems has; however, identified two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP₂) and cholesterol, that interact with K_IR2.x, to stabilize the channel in the preferred open or silent state, respectively. Translating this unique form of regulation, recent studies have demonstrated that specific lipid-protein interactions enable unique K_IR populations to sense distinct hemodynamic stimuli and set basal tone. This review summarizes the current knowledge of vascular K_IR channels and how the lipid and hemodynamic impact their activity.

Novel roles of phosphoinositides in signaling, lipid transport, and disease

Phosphoinositides (PPIns) are lipid signaling molecules that act as master regulators of cellular signaling. Recent studies have revealed novel roles of PPIns in myriad cellular processes and multiple human diseases mediated by misregulation of PPIn signaling. This review will present a timely summary of recent discoveries in PPIn biology, specifically their role in regulating unexpected signaling pathways, modification of signaling outcomes downstream of integral membrane proteins, and novel roles in lipid transport. This has revealed new roles of PPIns in regulating membrane trafficking, immunity, cell polarity, and response to extracellular signals. A specific focus will be on novel opportunities to target PPIn metabolism for treatment of human diseases, including cancer, pathogen infection, developmental disorders, and immune disorders.

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.

How is the acyl chain composition of phosphoinositides created and does it matter?

The phosphoinositide (PIPn) family of signalling phospholipids are central regulators in membrane cell biology. Their varied functions are based on the phosphorylation pattern of their inositol ring, which can be recognized by selective binding domains in their effector proteins and be modified by a series of specific PIPn kinases and phosphatases, which control their interconversion in a spatial and temporal manner. Yet, a unique feature of PIPns remains largely unexplored: their unusually uniform acyl chain composition. Indeed, while most phospholipids present a range of molecular species comprising acyl chains of diverse length and saturation, PIPns in several organisms and tissues show the predominance of a single hydrophobic backbone, which in mammals is composed of arachidonoyl and stearoyl chains. Despite evolution having favoured this specific PIPn configuration, little is known regarding the mechanisms and functions behind it. In this review, we explore the metabolic pathways that could control the acyl chain composition of PIPns as well as the potential roles of this selective enrichment. While our understanding of this phenomenon has been constrained largely by the technical limitations in the methods traditionally employed in the PIPn field, we believe that the latest developments in PIPn analysis should shed light onto this old question.

Lipid-Dependent Regulation of Ion Channels and G Protein-Coupled Receptors: Insights from Structures and Simulations

Ion channels and G protein-coupled receptors (GPCRs) are regulated by lipids in their membrane environment. Structural studies combined with biophysical and molecular simulation investigations reveal interaction sites for specific lipids on membrane protein structures. For K channels, PIP₂ plays a key role in regulating Kv and Kir channels. Likewise, several recent cryo-EM structures of TRP channels have revealed bound lipids, including PIP₂ and cholesterol. Among the pentameric ligand-gated ion channel family, structural and biophysical studies suggest the M4 TM helix may act as a lipid sensor, e.g., forming part of the binding sites for neurosteroids on the GABA_A receptor. Structures of GPCRs have revealed multiple cholesterol sites, which may modulate both receptor dynamics and receptor oligomerization. PIP₂ also interacts with GPCRs and may modulate their interactions with G proteins. Overall, it is evident that multiple lipid binding sites exist on channels and receptors that modulate their function allosterically and are potential druggable sites.

Molecular basis of PIP2-dependent regulation of the Ca2+-activated chloride channel TMEM16A

The calcium-activated chloride channel (CaCC) TMEM16A plays crucial roles in regulating neuronal excitability, smooth muscle contraction, fluid secretion and gut motility. While opening of TMEM16A requires binding of intracellular Ca2+, prolonged Ca2+-dependent activation results in channel desensitization or rundown, the mechanism of which is unclear. Here we show that phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates TMEM16A channel activation and desensitization via binding to a putative binding site at the cytosolic interface of transmembrane segments (TMs) 3-5. We further demonstrate that the ion-conducting pore of TMEM16A is constituted of two functionally distinct modules: a Ca2+-binding module formed by TMs 6-8 and a PIP2-binding regulatory module formed by TMs 3-5, which mediate channel activation and desensitization, respectively. PIP2 dissociation from the regulatory module results in ion-conducting pore collapse and subsequent channel desensitization. Our findings thus provide key insights into the mechanistic understanding of TMEM16 channel gating and lipid-dependent regulation.

Phosphoinositides modulate the voltage dependence of two-pore channel 3

Two-pore channels, or two-pore Na⁺ channels (TPCs), contain two homologous domains, each containing a functional unit typical of voltage-dependent cation channels. Each domain is considered to be responsible for either phosphoinositide (PI) binding or voltage sensing. Among the three members of the TPC family, TPC1 and TPC2 are activated by PI(3,5)P₂, while TPC3 has been thought not to be affected by any PIs. Here, we report that TPC3 is sensitive to PI(3,4)P₂ and PI(3,5)P₂, but not to PI(4,5)P₂, and that the extremely slow increase in TPC3 currents induced by depolarization in Xenopus oocytes is due to the production of PI(3,4)P₂ Similarly to TPC1, the cluster of basic amino acid residues in domain I is critical for PI sensitivity, but with a slight variation that may allow TPC3 to be sensitive to both PI(3,4)P₂ and PI(3,5)P₂ We also found that TPC3 has a unique PI-dependent modulation mechanism of voltage dependence, which is achieved by a specific bridging interaction between domain I and domain II. Taken together, these findings show that TPC3 is a unique member of the TPC family that senses PIs and displays a strong coupling between PI binding and voltage-dependent gating.

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.

Selective binding of a toxin and phosphatidylinositides to a mammalian potassium channel

G-protein-gated inward rectifying potassium channels (GIRKs) require Gβγ subunits and phosphorylated phosphatidylinositides (PIPs) for gating. Although studies have provided insight into these interactions, the mechanism of how these events are modulated by Gβγ and the binding affinity between PIPs and GIRKs remains poorly understood. Here, native ion mobility mass spectrometry is employed to directly monitor small molecule binding events to mouse GIRK2. GIRK2 binds the toxin tertiapin Q and PIPs selectively and with significantly higher affinity than other phospholipids. A mutation in GIRK2 that causes a rotation in the cytoplasmic domain, similarly to Gβγ-binding to the wild-type channel, revealed differences in the selectivity towards PIPs. More specifically, PIP isoforms known to weakly activate GIRKs have decreased binding affinity. Taken together, our results reveal selective small molecule binding and uncover a mechanism by which rotation of the cytoplasmic domain can modulate GIRK•PIP interactions.

A nuclear phosphoinositide kinase complex regulates p53

The tumour suppressor p53 (encoded by TP53) protects the genome against cellular stress and is frequently mutated in cancer. Mutant p53 acquires gain-of-function oncogenic activities that are dependent on its enhanced stability. However, the mechanisms by which nuclear p53 is stabilized are poorly understood. Here, we demonstrate that the stability of stress-induced wild-type and mutant p53 is regulated by the type I phosphatidylinositol phosphate kinase (PIPKI-α (also known as PIP5K1A)) and its product phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P₂). Nuclear PIPKI-α binds to p53 upon stress, resulting in the production and association of PtdIns(4,5)P₂ with p53. PtdIns(4,5)P₂ binding promotes the interaction between p53 and the small heat shock proteins HSP27 (also known as HSPB1) and αB-crystallin (also known as HSPB5), which stabilize nuclear p53. Moreover, inhibition of PIPKI-α or PtdIns(4,5)P₂ association results in p53 destabilization. Our results point to a previously unrecognized role of nuclear phosphoinositide signalling in regulating p53 stability and implicate this pathway as a promising therapeutic target in cancer.

Understanding phosphoinositides: rare, dynamic, and essential membrane phospholipids

Polyphosphoinositides (PPIs) are essential phospholipids located in the cytoplasmic leaflet of eukaryotic cell membranes. Despite contributing only a small fraction to the bulk of cellular phospholipids, they make remarkable contributions to practically all aspects of a cell's life and death. They do so by recruiting cytoplasmic proteins/effectors or by interacting with cytoplasmic domains of membrane proteins at the membrane-cytoplasm interface to organize and mold organelle identity. The present study summarizes aspects of our current understanding concerning the metabolism, manipulation, measurement, and intimate roles these lipids play in regulating membrane homeostasis and vital cell signaling reactions in health and disease.

Membrane curvature allosterically regulates the phosphatidylinositol cycle, controlling its rate and acyl-chain composition of its lipid intermediates

Signaling events at membranes are often mediated by membrane lipid composition or membrane physical properties. These membrane properties could act either by favoring the membrane binding of downstream effectors or by modulating their activity. Several proteins can sense/generate membrane physical curvature (i.e. shape). However, the modulation of the activity of enzymes by a membrane's shape has not yet been reported. Here, using a cell-free assay with purified diacylglycerol kinase ϵ (DGKϵ) and liposomes, we studied the activity and acyl-chain specificity of an enzyme of the phosphatidylinositol (PI) cycle, DGKϵ. By systematically varying the model membrane lipid composition and physical properties, we found that DGKϵ has low activity and lacks acyl-chain specificity in locally flat membranes, regardless of the lipid composition. On the other hand, these enzyme properties were greatly enhanced in membrane structures with a negative Gaussian curvature. We also found that this is not a consequence of preferential binding of the enzyme to those structures, but rather is due to a curvature-mediated allosteric regulation of DGKϵ activity and acyl-chain specificity. Moreover, in a fine-tuned interplay between the enzyme and the membrane, DGKϵ favored the formation of structures with greater Gaussian curvature. DGKϵ does not bear a regulatory domain, and these findings reveal the importance of membrane curvature in regulating DGKϵ activity and acyl-chain specificity. Hence, this study highlights that a hierarchic coupling of membrane physical property and lipid composition synergistically regulates membrane signaling events. We propose that this regulatory mechanism of membrane-associated enzyme activity is likely more common than is currently appreciated.

Capturing the Interaction Kinetics of an Ion Channel Protein with Small Molecules by the Bio-layer Interferometry Assay

The bio-layer interferometry (BLI) assay is a valuable tool for measuring protein-protein and protein-small molecule interactions. Here, we first describe the application of this novel label-free technique to study the interaction of human EAG1 (hEAG1) channel proteins with the small molecule PIP2. hEAG1 channel has been recognized as potential therapeutic target because of its aberrant overexpression in cancers and a few gain-of-function mutations involved in some types of neurological diseases. We purified hEAG1 channel proteins from a mammalian stable expression system and measured the interaction with PIP2 by BLI. The successful measurement of the kinetics of binding between hEAG1 protein and PIP2 demonstrates that the BLI assay is a potential high-throughput approach used for novel small-molecule ligand screening in ion channel pharmacology.

Hydrogen sulfide inhibits Kir2 and Kir3 channels by decreasing sensitivity to the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2)

Inwardly rectifying potassium (Kir) channels establish and regulate the resting membrane potential of excitable cells in the heart, brain, and other peripheral tissues. Phosphatidylinositol 4,5-bisphosphate (PIP₂) is a key direct activator of ion channels, including Kir channels. The gasotransmitter carbon monoxide has been shown to regulate Kir channel activity by altering channel-PIP₂ interactions. Here, we tested in two cellular models the effects and mechanism of action of another gasotransmitter, hydrogen sulfide (H₂S), thought to play a key role in cellular responses under ischemic conditions. Direct administration of sodium hydrogen sulfide as an exogenous H₂S source and expression of cystathionine γ-lyase, a key enzyme that produces endogenous H₂S in specific brain tissues, resulted in comparable current inhibition of several Kir2 and Kir3 channels. This effect resulted from changes in channel-gating kinetics rather than in conductance or cell-surface localization. The extent of H₂S regulation depended on the strength of the channel-PIP₂ interactions. H₂S regulation was attenuated when channel-PIP₂ interactions were strengthened and was increased when channel-PIP₂ interactions were weakened by depleting PIP₂ levels. These H₂S effects required specific cytoplasmic cysteine residues in Kir3.2 channels. Mutation of these residues abolished H₂S inhibition, and reintroduction of specific cysteine residues back into the background of the cytoplasmic cysteine-lacking mutant rescued H₂S inhibition. Molecular dynamics simulation experiments provided mechanistic insights into how potential sulfhydration of specific cysteine residues could lead to changes in channel-PIP₂ interactions and channel gating.

Conserved functional consequences of disease-associated mutations in the slide helix of Kir6.1 and Kir6.2 subunits of the ATP-sensitive potassium channel

Cantu syndrome (CS) is a condition characterized by a range of anatomical defects, including cardiomegaly, hyperflexibility of the joints, hypertrichosis, and craniofacial dysmorphology. CS is associated with multiple missense mutations in the genes encoding the regulatory sulfonylurea receptor 2 (SUR2) subunits of the ATP-sensitive K⁺ (K_ATP) channel as well as two mutations (V65M and C176S) in the Kir6.1 (KCNJ8) subunit. Previous analysis of leucine and alanine substitutions at the Val-65-equivalent site (Val-64) in Kir6.2 indicated no major effects on channel function. In this study, we characterized the effects of both valine-to-methionine and valine-to-leucine substitutions at this position in both Kir6.1 and Kir6.2 using ion flux and patch clamp techniques. We report that methionine substitution, but not leucine substitution, results in increased open state stability and hence significantly reduced ATP sensitivity and a marked increase of channel activity in the intact cell irrespective of the identity of the coassembled SUR subunit. Sulfonylurea inhibitors, such as glibenclamide, are potential therapies for CS. However, as a consequence of the increased open state stability, both Kir6.1(V65M) and Kir6.2(V64M) mutations essentially abolish high-affinity sensitivity to the K_ATP blocker glibenclamide in both intact cells and excised patches. This raises the possibility that, at least for some CS mutations, sulfonylurea therapy may not prove to be successful and highlights the need for detailed pharmacogenomic analyses of CS mutations.

Dynamic role of the tether helix in PIP2-dependent gating of a G protein-gated potassium channel

G protein–gated inwardly rectifying potassium (GIRK) channels are activated by the phospholipid phosphatidylinositol 4,5 bisphosphate (PIP₂). Using functional and computational experiments, Lacin et al. reveal that PIP₂ interacts with the tether helix of the neuronal GIRK channel in a dynamic way.

G protein–gated inwardly rectifying potassium (GIRK) channels control neuronal excitability in the brain and are implicated in several different neurological diseases. The anionic phospholipid phosphatidylinositol 4,5 bisphosphate (PIP₂) is an essential cofactor for GIRK channel gating, but the precise mechanism by which PIP₂ opens GIRK channels remains poorly understood. Previous structural studies have revealed several highly conserved, positively charged residues in the “tether helix” (C-linker) that interact with the negatively charged PIP₂. However, these crystal structures of neuronal GIRK channels in complex with PIP₂ provide only snapshots of PIP₂’s interaction with the channel and thus lack details about the gating transitions triggered by PIP₂ binding. Here, our functional studies reveal that one of these conserved basic residues in GIRK2, Lys200 (6′K), supports a complex and dynamic interaction with PIP₂. When Lys200 is mutated to an uncharged amino acid, it activates the channel by enhancing the interaction with PIP₂. Atomistic molecular dynamic simulations of neuronal GIRK2 with the same 6′ substitution reveal an open GIRK2 channel with PIP₂ molecules adopting novel positions. This dynamic interaction with PIP₂ may explain the intrinsic low open probability of GIRK channels and the mechanism underlying activation by G protein Gβγ subunits and ethanol.

Dual activation of neuronal G protein-gated inwardly rectifying potassium (GIRK) channels by cholesterol and alcohol

Activation of G protein-gated inwardly rectifying potassium (GIRK) channels leads to a hyperpolarization of the neuron’s membrane potential, providing an important component of inhibition in the brain. In addition to the canonical G protein-activation pathway, GIRK channels are activated by small molecules but less is known about the underlying gating mechanisms. One drawback to previous studies has been the inability to control intrinsic and extrinsic factors. Here we used a reconstitution strategy with highly purified mammalian GIRK2 channels incorporated into liposomes and demonstrate that cholesterol or intoxicating concentrations of ethanol, i.e., >20 mM, each activate GIRK2 channels directly, in the absence of G proteins. Notably, both activators require the membrane phospholipid PIP₂ but appear to interact independently with different regions of the channel. Elucidating the mechanisms underlying G protein-independent pathways of activating GIRK channels provides a unique strategy for developing new types of neuronal excitability modulators.

Oxytocin (OXT)-stimulated inhibition of Kir7.1 activity is through PIP2-dependent Ca2+ response of the oxytocin receptor in the retinal pigment epithelium in vitro

Oxytocin (OXT) is a neuropeptide that activates the oxytocin receptor (OXTR), a rhodopsin family G-protein coupled receptor. Our localization of OXTR to the retinal pigment epithelium (RPE), in close proximity to OXT in the adjacent photoreceptor neurons, leads us to propose that OXT plays an important role in RPE-retinal communication. An increase of RPE [Ca²⁺]_i in response to OXT stimulation implies that the RPE may utilize oxytocinergic signaling as a mechanism by which it accomplishes some of its many roles. In this study, we used an established human RPE cell line, a HEK293 heterologous OXTR expression system, and pharmacological inhibitors of Ca²⁺ signaling to demonstrate that OXTR utilizes capacitative Ca²⁺ entry (CCE) mechanisms to sustain an increase in cytoplasmic Ca²⁺. These findings demonstrate how multiple functional outcomes of OXT-OXTR signaling could be integrated via a single pathway. In addition, the activated OXTR was able to inhibit the Kir7.1 channel, an important mediator of sub retinal waste transport and K⁺ homeostasis.

Activation of the Ca2+-sensing receptors increases currents through inward rectifier K+ channels via activation of phosphatidylinositol 4-kinase

Inward rectifier K⁺ channels are important for maintaining normal electrical function in many cell types. The proper function of these channels requires the presence of membrane phosphoinositide 4,5-bisphosphate (PIP₂). Stimulation of the Ca²⁺-sensing receptor CaR, a pleiotropic G protein-coupled receptor, activates both G_q/11, which decreases PIP₂, and phosphatidylinositol 4-kinase (PI-4-K), which, conversely, increases PIP₂. How membrane PIP₂ levels are regulated by CaR activation and whether these changes modulate inward rectifier K⁺ are unknown. In this study, we found that activation of CaR by the allosteric agonist, NPSR568, increased inward rectifier K⁺ current (I _K1) in guinea pig ventricular myocytes and currents mediated by Kir2.1 channels exogenously expressed in HEK293T cells with a similar sensitivity. Moreover, using the fluorescent PIP₂ reporter tubby-R332H-cYFP to monitor PIP₂ levels, we found that CaR activation in HEK293T cells increased membrane PIP₂ concentrations. Pharmacological studies showed that both phospholipase C (PLC) and PI-4-K are activated by CaR stimulation with the latter played a dominant role in regulating membrane PIP₂ and, thus, Kir currents. These results provide the first direct evidence that CaR activation upregulates currents through inward rectifier K⁺ channels by accelerating PIP₂ synthesis. The regulation of I _K1 plays a critical role in the stability of the electrical properties of many excitable cells, including cardiac myocytes and neurons. Further, synthetic allosteric modulators that increase CaR activity have been used to treat hyperparathyroidism, and negative CaR modulators are of potential importance in the treatment of osteoporosis. Thus, our results provide further insight into the roles played by CaR in the cardiovascular system and are potentially valuable for heart disease treatment and drug safety.

Human EAG channels are directly modulated by PIP2 as revealed by electrophysiological and optical interference investigations

Voltage-gated ether à go-go (EAG) K⁺ channels are expressed in various types of cancer cells and also in the central nervous system. Aberrant overactivation of human EAG1 (hEAG1) channels is associated with cancer and neuronal disorders such as Zimmermann-Laband and Temple-Baraitser syndromes. Although hEAG1 channels are recognized as potential therapeutic targets, regulation of their functional properties is only poorly understood. Here, we show that the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂) is a potent inhibitory gating modifier of hEAG1 channels. PIP₂ inhibits the channel activity by directly binding to a short N-terminal segment of the channel important for Ca²⁺/calmodulin (CaM) binding as evidenced by bio-layer interferometry measurements. Conversely, depletion of endogenous PIP₂ either by serotonin-induced phospholipase C (PLC) activation or by a rapamycin-induced translocation system enhances the channel activity at physiological membrane potentials, suggesting that PIP₂ exerts a tonic inhibitory influence. Our study, combining electrophysiological and direct binding assays, demonstrates that hEAG1 channels are subject to potent inhibitory modulation by multiple phospholipids and suggests that manipulations of the PIP₂ signaling pathway may represent a strategy to treat hEAG1 channel-associated diseases.

Regulation of the transient receptor potential channel TRPM3 by phosphoinositides

TRPM3 is dynamically regulated by plasma membrane PI(4,5)P₂ and related PIPs.

The transient receptor potential (TRP) channel TRPM3 is a calcium-permeable cation channel activated by heat and by the neurosteroid pregnenolone sulfate (PregS). TRPM3 is highly expressed in sensory neurons, where it plays a key role in heat sensing and inflammatory hyperalgesia, and in pancreatic β cells, where its activation enhances glucose-induced insulin release. However, despite its functional importance, little is known about the cellular mechanisms that regulate TRPM3 activity. Here, we provide evidence for a dynamic regulation of TRPM3 by membrane phosphatidylinositol phosphates (PIPs). Phosphatidylinositol 4,5-bisphosphate (PI[4,5]P₂) and ATP applied to the intracellular side of excised membrane patches promote recovery of TRPM3 from desensitization. The stimulatory effect of cytosolic ATP on TRPM3 reflects activation of phosphatidylinositol kinases (PI-Ks), leading to resynthesis of PIPs in the plasma membrane. Various PIPs directly enhance TRPM3 activity in cell-free inside-out patches, with a potency order PI(3,4,5)P₃ > PI(3,5)P₂ > PI(4,5)P₂ ≈ PI(3,4)P₂ >> PI(4)P_. Conversely, TRPM3 activity is rapidly and reversibly inhibited by activation of phosphatases that remove the 5-phosphate from PIPs. Finally, we show that recombinant TRPM3, as well as the endogenous TRPM3 in insuloma cells, is rapidly and reversibly inhibited by activation of phospholipase C–coupled muscarinic acetylcholine receptors. Our results reveal basic cellular mechanisms whereby membrane receptors can regulate TRPM3 activity.

Mechanism for phosphoinositide selectivity and activation of TRPV1 ion channels

Phosphoinositides bind to a selective site in the proximal C-terminal region to regulate TRPV1.

Although PI(4,5)P₂ is believed to play an essential role in regulating the activity of numerous ion channels and transporters, the mechanisms by which it does so are unknown. Here, we used the ability of the TRPV1 ion channel to discriminate between PI(4,5)P₂ and PI(4)P to localize the region of TRPV1 sequence that interacts directly with the phosphoinositide. We identified a point mutation in the proximal C-terminal region after the TRP box, R721A, that inverted the selectivity of TRPV1. Although the R721A mutation produced only a 30% increase in the EC₅₀ for activation by PI(4,5)P₂, it decreased the EC₅₀ for activation by PI(4)P by more than two orders of magnitude. We used chemically induced and voltage-activated phosphatases to determine that PI(4)P continued to support TRPV1 activity even after depletion of PI(4,5)P₂ from the plasma membrane. Our data cannot be explained by a purely electrostatic mechanism for interaction between the phosphoinositide and the protein, similar to that of the MARCKS (myristoylated alanine-rich C kinase substrate) effector domain or the EGF receptor. Rather, conversion of a PI(4,5)P₂-selective channel to a PI(4)P-selective channel indicates that a structured phosphoinositide-binding site mediates the regulation of TRPV1 activity and that the amino acid at position 721 likely interacts directly with the moiety at the 5′ position of the phosphoinositide.

Requirement of Phosphoinositides Containing Stearic Acid To Control Cell Polarity

Phosphoinositides (PIPs) are present in very small amounts but are essential for cell signaling, morphogenesis, and polarity. By mass spectrometry, we demonstrated that some PIPs with stearic acyl chains were strongly disturbed in a psi1Δ Saccharomyces cerevisiae yeast strain deficient in the specific incorporation of a stearoyl chain at the sn-1 position of phosphatidylinositol. The absence of PIPs containing stearic acid induced disturbances in intracellular trafficking, although the total amount of PIPs was not diminished. Changes in PIPs also induced alterations in the budding pattern and defects in actin cytoskeleton organization (cables and patches). Moreover, when the PSI1 gene was impaired, a high proportion of cells with bipolar cortical actin patches that occurred concomitantly with the bipolar localization of Cdc42p was specifically found among diploid cells. This bipolar cortical actin phenotype, never previously described, was also detected in a bud9Δ/bud9Δ strain. Very interestingly, overexpression of PSI1 reversed this phenotype.

Rise and Fall of Kir2.2 Current by TLR4 Signaling in Human Monocytes: PKC-Dependent Trafficking and PI3K-Mediated PIP2 Decrease

LPSs are widely used to stimulate TLR4, but their effects on ion channels in immune cells are poorly known. In THP-1 cells and human blood monocytes treated with LPS, inwardly rectifying K(+) channel current (IKir,LPS) newly emerged at 1 h, peaked at 4 h (-119 ± 8.6 pA/pF), and decayed afterward (-32 ± 6.7 pA/pF at 24 h). Whereas both the Kir2.1 and Kir2.2 mRNAs and proteins were observed, single-channel conductance (38 pS) of IKir,LPS and small interfering RNA-induced knockdown commonly indicated Kir2.2 than Kir2.1. LPS-induced cytokine release and store-operated Ca(2+) entry were commonly decreased by ML-133, a Kir2 inhibitor. Immunoblot, confocal microscopy, and the effects of vesicular trafficking inhibitors commonly suggested plasma membrane translocation of Kir2.2 by LPS. Both IKir,LPS and membrane translocation of Kir2.2 were inhibited by GF109203X (protein kinase C [PKC] inhibitor) or by transfection with small interfering RNA-specific PKCε. Interestingly, pharmacological activation of PKC by PMA induced both Kir2.1 and Kir2.2 currents. The spontaneously decayed IKir,LPS at 24 h was recovered by PI3K inhibitors but further suppressed by an inhibitor of phosphatidylinositol(3,4,5)-trisphosphate (PIP3) phosphatase (phosphatase and tensin homolog). However, IKir,LPS at 24 h was not affected by Akt inhibitors, suggesting that the decreased phosphatidylinositol(4,5)-bisphosphate availability, that is, conversion into PIP3 by PI3K, per se accounts for the decay of IKir,LPS. Taken together, to our knowledge these data are the first demonstrations that IKir is newly induced by TLR4 stimulation via PKC-dependent membrane trafficking of Kir2.2, and that conversion of phosphatidylinositol(4,5)-bisphosphate to PIP3 modulates Kir2.2. The augmentation of Ca(2+) influx and cytokine release suggests a physiological role for Kir2.2 in TLR4-stimulated monocytes.

Structural Insights into GIRK Channel Function

G protein-gated inwardly rectifying potassium (GIRK; Kir3) channels, which are members of the large family of inwardly rectifying potassium channels (Kir1-Kir7), regulate excitability in the heart and brain. GIRK channels are activated following stimulation of G protein-coupled receptors that couple to the G(i/o) (pertussis toxin-sensitive) G proteins. GIRK channels, like all other Kir channels, possess an extrinsic mechanism of inward rectification involving intracellular Mg(2+) and polyamines that occlude the conduction pathway at membrane potentials positive to E(K). In the past 17 years, more than 20 high-resolution atomic structures containing GIRK channel cytoplasmic domains and transmembrane domains have been solved. These structures have provided valuable insights into the structural determinants of many of the properties common to all inward rectifiers, such as permeation and rectification, as well as revealing the structural bases for GIRK channel gating. In this chapter, we describe advances in our understanding of GIRK channel function based on recent high-resolution atomic structures of inwardly rectifying K(+) channels discussed in the context of classical structure-function experiments.

Identification of the Conformational transition pathway in PIP2 Opening Kir Channels

The gating of Kir channels depends critically on phosphatidylinositol 4,5-bisphosphate (PIP2), but the detailed mechanism by which PIP2 regulates Kir channels remains obscure. Here, we performed a series of Targeted molecular dynamics simulations on the full-length Kir2.1 channel and, for the first time, were able to achieve the transition from the closed to the open state. Our data show that with the upward motion of the cytoplasmic domain (CTD) the structure of the C-Linker changes from a loop to a helix. The twisting of the C-linker triggers the rotation of the CTD, which induces a small downward movement of the CTD and an upward motion of the slide helix toward the membrane that pulls the inner helix gate open. At the same time, the rotation of the CTD breaks the interaction between the CD- and G-loops thus releasing the G-loop. The G-loop then bounces away from the CD-loop, which leads to the opening of the G-loop gate and the full opening of the pore. We identified a series of interaction networks, between the N-terminus, CD loop, C linker and G loop one by one, which exquisitely regulates the global conformational changes during the opening of Kir channels by PIP2.

Phosphoinositides regulate ion channels

Phosphoinositides serve as signature motifs for different cellular membranes and often are required for the function of membrane proteins. Here, we summarize clear evidence supporting the concept that many ion channels are regulated by membrane phosphoinositides. We describe tools used to test their dependence on phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate, and consider mechanisms and biological meanings of phosphoinositide regulation of ion channels. This lipid regulation can underlie changes of channel activity and electrical excitability in response to receptors. Since different intracellular membranes have different lipid compositions, the activity of ion channels still in transit towards their final destination membrane may be suppressed until they reach an optimal lipid environment. This article is part of a Special Issue entitled Phosphoinositides.

Mutations in Nature Conferred a High Affinity Phosphatidylinositol 4,5-Bisphosphate-binding Site in Vertebrate Inwardly Rectifying Potassium Channels

All vertebrate inwardly rectifying potassium (Kir) channels are activated by phosphatidylinositol 4,5-bisphosphate (PIP2) (Logothetis, D. E., Petrou, V. I., Zhang, M., Mahajan, R., Meng, X. Y., Adney, S. K., Cui, M., and Baki, L. (2015) Annu. Rev. Physiol. 77, 81-104; Fürst, O., Mondou, B., and D'Avanzo, N. (2014) Front. Physiol. 4, 404-404). Structural components of a PIP2-binding site are conserved in vertebrate Kir channels but not in distantly related animals such as sponges and sea anemones. To expand our understanding of the structure-function relationships of PIP2 regulation of Kir channels, we studied AqKir, which was cloned from the marine sponge Amphimedon queenslandica, an animal that represents the phylogenetically oldest metazoans. A requirement for PIP2 in the maintenance of AqKir activity was examined in intact oocytes by activation of a co-expressed voltage-sensing phosphatase, application of wortmannin (at micromolar concentrations), and activation of a co-expressed muscarinic acetylcholine receptor. All three mechanisms to reduce the availability of PIP2 resulted in inhibition of AqKir current. However, time-dependent rundown of AqKir currents in inside-out patches could not be re-activated by direct application to the inside membrane surface of water-soluble dioctanoyl PIP2, and the current was incompletely re-activated by the more hydrophobic arachidonyl stearyl PIP2. When we introduced mutations to AqKir to restore two positive charges within the vertebrate PIP2-binding site, both forms of PIP2 strongly re-activated the mutant sponge channels in inside-out patches. Molecular dynamics simulations validate the additional hydrogen bonding potential of the sponge channel mutants. Thus, nature's mutations conferred a high affinity activation of vertebrate Kir channels by PIP2, and this is a more recent evolutionary development than the structures that explain ion channel selectivity and inward rectification.

A Conserved Residue Cluster That Governs Kinetics of ATP-dependent Gating of Kir6.2 Potassium Channels

ATP-sensitive potassium (KATP) channels are heteromultimeric complexes of an inwardly rectifying Kir channel (Kir6.x) and sulfonylurea receptors. Their regulation by intracellular ATP and ADP generates electrical signals in response to changes in cellular metabolism. We investigated channel elements that control the kinetics of ATP-dependent regulation of KATP (Kir6.2 + SUR1) channels using rapid concentration jumps. WT Kir6.2 channels re-open after rapid washout of ATP with a time constant of ∼60 ms. Extending similar kinetic measurements to numerous mutants revealed fairly modest effects on gating kinetics despite significant changes in ATP sensitivity and open probability. However, we identified a pair of highly conserved neighboring amino acids (Trp-68 and Lys-170) that control the rate of channel opening and inhibition in response to ATP. Paradoxically, mutations of Trp-68 or Lys-170 markedly slow the kinetics of channel opening (500 and 700 ms for W68L and K170N, respectively), while increasing channel open probability. Examining the functional effects of these residues using φ value analysis revealed a steep negative slope. This finding implies that these residues play a role in lowering the transition state energy barrier between open and closed channel states. Using unnatural amino acid incorporation, we demonstrate the requirement for a planar amino acid at Kir6.2 position 68 for normal channel gating, which is potentially necessary to localize the ϵ-amine of Lys-170 in the phosphatidylinositol 4,5-bisphosphate-binding site. Overall, our findings identify a discrete pair of highly conserved residues with an essential role for controlling gating kinetics of Kir channels.