The possible role of a disulphide bond in forming functional Kir2.1 potassium channels

Extracellular cysteine disulfide bond break at Cys122 disrupts PIP2-dependent Kir2.1 channel function and leads to arrhythmias in Andersen-Tawil Syndrome

Background

Andersen-Tawil Syndrome Type 1 (ATS1) is a rare heritable disease caused by mutations in the strong inwardly rectifying K+ channel Kir2.1. The extracellular Cys122-to-Cys154 disulfide bond in the Kir2.1 channel structure is crucial for proper folding, but has not been associated with correct channel function at the membrane. We tested whether a human mutation at the Cys122-to-Cys154 disulfide bridge leads to Kir2.1 channel dysfunction and arrhythmias by reorganizing the overall Kir2.1 channel structure and destabilizing the open state of the channel.

Methods and Results

We identified a Kir2.1 loss-of-function mutation in Cys122 (c.366 A>T; p.Cys122Tyr) in a family with ATS1. To study the consequences of this mutation on Kir2.1 function we generated a cardiac specific mouse model expressing the Kir2.1C122Y mutation. Kir2.1C122Y animals recapitulated the abnormal ECG features of ATS1, like QT prolongation, conduction defects, and increased arrhythmia susceptibility. Kir2.1C122Y mouse cardiomyocytes showed significantly reduced inward rectifier K+ (IK1) and inward Na+ (INa) current densities independently of normal trafficking ability and localization at the sarcolemma and the sarcoplasmic reticulum. Kir2.1C122Y formed heterotetramers with wildtype (WT) subunits. However, molecular dynamic modeling predicted that the Cys122-to-Cys154 disulfide-bond break induced by the C122Y mutation provoked a conformational change over the 2000 ns simulation, characterized by larger loss of the hydrogen bonds between Kir2.1 and phosphatidylinositol-4,5-bisphosphate (PIP2) than WT. Therefore, consistent with the inability of Kir2.1C122Y channels to bind directly to PIP2 in bioluminescence resonance energy transfer experiments, the PIP2 binding pocket was destabilized, resulting in a lower conductance state compared with WT. Accordingly, on inside-out patch-clamping the C122Y mutation significantly blunted Kir2.1 sensitivity to increasing PIP2 concentrations.

Conclusion

The extracellular Cys122-to-Cys154 disulfide bond in the tridimensional Kir2.1 channel structure is essential to channel function. We demonstrated that ATS1 mutations that break disulfide bonds in the extracellular domain disrupt PIP2-dependent regulation, leading to channel dysfunction and life-threatening arrhythmias.

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 (CPVT). 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.

A small viral potassium ion channel with an inherent inward rectification

Some algal viruses have coding sequences for proteins with structural and functional characteristics of pore modules of complex K⁺ channels. Here we exploit the structural diversity among these channel orthologs to discover new basic principles of structure/function correlates in K⁺ channels. The analysis of three similar K⁺ channels with ≤ 86 amino acids (AA) shows that one channel (Kmpv₁) generates an ohmic conductance in HEK293 cells while the other two (Kmpv_SP1, Kmpv_PL1) exhibit typical features of canonical Kir channels. Like Kir channels, the rectification of the viral channels is a function of the K⁺ driving force. Reconstitution of Kmpv_SP1 and Kmpv_PL1 in planar lipid bilayers showed rapid channel fluctuations only at voltages negative of the K⁺ reversal voltage. This rectification was maintained in KCl buffer with 1 mM EDTA, which excludes blocking cations as the source of rectification. This means that rectification of the viral channels must be an inherent property of the channel. The structural basis for rectification was investigated by a chimera between rectifying and non-rectifying channels as well as point mutations making the rectifier similar to the ohmic conducting channel. The results of these experiments exclude the pore with pore helix and selectivity filter as playing a role in rectification. The insensitivity of the rectifier to point mutations suggests that tertiary or quaternary structural interactions between the transmembrane domains are responsible for this type of gating.

Inward rectifiers and their regulation by endogenous polyamines

Inwardly-rectifying potassium (Kir) channels contribute to maintenance of the resting membrane potential and regulation of electrical excitation in many cell types. Strongly rectifying Kir channels exhibit a very steep voltage dependence resulting in silencing of their activity at depolarized membrane voltages. The mechanism underlying this steep voltage dependence is blockade by endogenous polyamines. These small multifunctional, polyvalent metabolites enter the long Kir channel pore from the intracellular side, displacing multiple occupant ions as they migrate to a stable binding site in the transmembrane region of the channel. Numerous structure-function studies have revealed structural elements of Kir channels that determine their susceptibility to polyamine block, and enable the steep voltage dependence of this process. In addition, various channelopathies have been described that result from alteration of the polyamine sensitivity or activity of strongly rectifying channels. The primary focus of this article is to summarize current knowledge of the molecular mechanisms of polyamine block, and provide some perspective on lingering uncertainties related to this physiologically important mechanism of ion channel blockade. We also briefly review some of the important and well understood physiological roles of polyamine sensitive, strongly rectifying Kir channels, primarily of the Kir2 family.

Extracellular disulfide bridges stabilize TRPC5 dimerization, trafficking, and activity

Crucial cysteine residues can be involved in the modulation of protein activity via the modification of thiol (-SH) groups. Among these reactions, disulfide bonds (S-S) play a key role in the folding, stability, and activity of membrane proteins. However, the regulation of extracellular cysteines in classical transient receptor potential (TRPC) channels remains controversial. Here, we examine the functional importance of the extracellular disulfide bond in TRPC5 in modulating channel gating and trafficking. Specifically, we investigated TRPC5 activity in transiently transfected HEK293 cells with wild-type (WT) or cysteine (C553 and C558) mutants in the pore loop. Using reducing agents, we determined that a disulfide linkage mediates the tetrameric formation of the TRPC5 channel. By measuring the TRPC5 current, we observed that C553S or C558S mutants completely lose channel activity induced by lanthanides or receptor stimulation. Co-expression of TRPC5 (WT) with mutants demonstrated a dominant-negative function in mutants, which inhibited the activity of TRPC5 (WT). We generated TRPC5-TRPC5 dimers and observed reduced activity of WT-mutant (C553S or C558S) dimers compared to WT-WT dimers. When pretreated with reducing agents for 12 h, the TRPC5 current decreased due to a reduction in membrane TRPC5 distribution. In addition, we identified a reduced expression of C553S mutant in plasma membrane. We analyzed a dimeric interaction of wild-type and mutant TRPC5 using co-immunoprecipitation and FRET method, indicating a weak interaction between dimeric partners. These results indicated that the disulfide bond between conserved extracellular cysteines, especially C553, is essential for functional TRPC5 activity by channel multimerization and trafficking.

Molecular basis and structural insight of vascular K(ATP) channel gating by S-glutathionylation

The vascular ATP-sensitive K(+) (K(ATP)) channel is targeted by a variety of vasoactive substances, playing an important role in vascular tone regulation. Our recent studies indicate that the vascular K(ATP) channel is inhibited in oxidative stress via S-glutathionylation. Here we show evidence for the molecular basis of the S-glutathionylation and its structural impact on channel gating. By comparing the oxidant responses of the Kir6.1/SUR2B channel with the Kir6.2/SUR2B channel, we found that the Kir6.1 subunit was responsible for oxidant sensitivity. Oxidant screening of Kir6.1-Kir6.2 chimeras demonstrated that the N terminus and transmembrane domains of Kir6.1 were crucial. Systematic mutational analysis revealed three cysteine residues in these domains: Cys(43), Cys(120), and Cys(176). Among them, Cys(176) was prominent, contributing to >80% of the oxidant sensitivity. The Kir6.1-C176A/SUR2B mutant channel, however, remained sensitive to both channel opener and inhibitor, which indicated that Cys(176) is not a general gating site in Kir6.1, in contrast to its counterpart (Cys(166)) in Kir6.2. A protein pull-down assay with biotinylated glutathione ethyl ester showed that mutation of Cys(176) impaired oxidant-induced incorporation of glutathione (GSH) into the Kir6.1 subunit. In contrast to Cys(176), Cys(43) had only a modest contribution to S-glutathionylation, and Cys(120) was modulated by extracellular oxidants but not intracellular GSSG. Simulation modeling of Kir6.1 S-glutathionylation suggested that after incorporation to residue 176, the GSH moiety occupied a space between the slide helix and two transmembrane helices. This prevented the inner transmembrane helix from undergoing conformational changes necessary for channel gating, retaining the channel in its closed state.

Kir2.6 regulates the surface expression of Kir2.x inward rectifier potassium channels

Precise trafficking, localization, and activity of inward rectifier potassium Kir2 channels are important for shaping the electrical response of skeletal muscle. However, how coordinated trafficking occurs to target sites remains unclear. Kir2 channels are tetrameric assemblies of Kir2.x subunits. By immunocytochemistry we show that endogenous Kir2.1 and Kir2.2 are localized at the plasma membrane and T-tubules in rodent skeletal muscle. Recently, a new subunit, Kir2.6, present in human skeletal muscle, was identified as a gene in which mutations confer susceptibility to thyrotoxic hypokalemic periodic paralysis. Here we characterize the trafficking and interaction of wild type Kir2.6 with other Kir2.x in COS-1 cells and skeletal muscle in vivo. Immunocytochemical and electrophysiological data demonstrate that Kir2.6 is largely retained in the endoplasmic reticulum, despite high sequence identity with Kir2.2 and conserved endoplasmic reticulum and Golgi trafficking motifs shared with Kir2.1 and Kir2.2. We identify amino acids responsible for the trafficking differences of Kir2.6. Significantly, we show that Kir2.6 subunits can coassemble with Kir2.1 and Kir2.2 in vitro and in vivo. Notably, this interaction limits the surface expression of both Kir2.1 and Kir2.2. We provide evidence that Kir2.6 functions as a dominant negative, in which incorporation of Kir2.6 as a subunit in a Kir2 channel heterotetramer reduces the abundance of Kir2 channels on the plasma membrane.

Molecular mechanisms of EAST/SeSAME syndrome mutations in Kir4.1 (KCNJ10)

Inwardly rectifying potassium channel Kir4.1 is critical for glial function, control of neuronal excitability, and systemic K(+) homeostasis. Novel mutations in Kir4.1 have been associated with EAST/SeSAME syndrome, characterized by mental retardation, ataxia, seizures, hearing loss, and renal salt waste. Patients are homozygous for R65P, G77R, C140R or T164I; or compound heterozygous for A167V/R297C or R65P/R199Stop, a deletion of the C-terminal half of the protein. We investigated the functional significance of these mutations by radiotracer efflux and inside-out membrane patch clamping in COSm6 cells expressing homomeric Kir4.1 or heteromeric Kir4.1/Kir5.1 channels. All of the mutations compromised channel function, but the underlying mechanisms were different. R65P, T164I, and R297C caused an alkaline shift in pH sensitivity, indicating that these positions are crucial for pH sensing and pore gating. In R297C, this was due to disruption of intersubunit salt bridge Glu(288)-Arg(297). C140R breaks the Cys(108)-Cys(140) disulfide bond essential for protein folding and function. A167V did not affect channel properties but may contribute to decreased surface expression in A167V/R297C. In G77R, introduction of a positive charge within the bilayer may affect channel structure or gating. R199Stop led to a dramatic decrease in surface expression, but channel activity was restored by co-expression with intact subunits, suggesting remarkable tolerance for truncation of the cytoplasmic domain. These results provide an explanation for the molecular defects that underlie the EAST/SeSAME syndrome.

Crystal structure of the eukaryotic strong inward-rectifier K+ channel Kir2.2 at 3.1 A resolution

Inward-rectifier potassium (K+) channels conduct K+ ions most efficiently in one direction, into the cell. Kir2 channels control the resting membrane voltage in many electrically excitable cells, and heritable mutations cause periodic paralysis and cardiac arrhythmia. We present the crystal structure of Kir2.2 from chicken, which, excluding the unstructured amino and carboxyl termini, is 90% identical to human Kir2.2. Crystals containing rubidium (Rb+), strontium (Sr2+), and europium (Eu3+) reveal binding sites along the ion conduction pathway that are both conductive and inhibitory. The sites correlate with extensive electrophysiological data and provide a structural basis for understanding rectification. The channel's extracellular surface, with large structured turrets and an unusual selectivity filter entryway, might explain the relative insensitivity of eukaryotic inward rectifiers to toxins. These same surface features also suggest a possible approach to the development of inhibitory agents specific to each member of the inward-rectifier K+ channel family.

Long-pore electrostatics in inward-rectifier potassium channels

Inward-rectifier potassium (Kir) channels differ from the canonical K⁺ channel structure in that they possess a long extended pore (∼85 Å) for ion conduction that reaches deeply into the cytoplasm. This unique structural feature is presumably involved in regulating functional properties specific to Kir channels, such as conductance, rectification block, and ligand-dependent gating. To elucidate the underpinnings of these functional roles, we examine the electrostatics of an ion along this extended pore. Homology models are constructed based on the open-state model of KirBac1.1 for four mammalian Kir channels: Kir1.1/ROMK, Kir2.1/IRK, Kir3.1/GIRK, and Kir6.2/KATP. By solving the Poisson-Boltzmann equation, the electrostatic free energy of a K⁺ ion is determined along each pore, revealing that mammalian Kir channels provide a favorable environment for cations and suggesting the existence of high-density regions in the cytoplasmic domain and cavity. The contribution from the reaction field (the self-energy arising from the dielectric polarization induced by the ion's charge in the complex geometry of the pore) is unfavorable inside the long pore. However, this is well compensated by the electrostatic interaction with the static field arising from the protein charges and shielded by the dielectric surrounding. Decomposition of the static field provides a list of residues that display remarkable correspondence with existing mutagenesis data identifying amino acids that affect conduction and rectification. Many of these residues demonstrate interactions with the ion over long distances, up to 40 Å, suggesting that mutations potentially affect ion or blocker energetics over the entire pore. These results provide a foundation for understanding ion interactions in Kir channels and extend to the study of ion permeation, block, and gating in long, cation-specific pores.

An alternatively spliced isoform of PSD-93/chapsyn 110 binds to the inwardly rectifying potassium channel, Kir2.1

Inwardly rectifying potassium (Kir) channels are prime determinants of resting membrane potential in neurons. Their subcellular distribution and surface density thus help shape neuronal excitability, yet mechanisms governing the membrane targeting and localization of Kir channels are poorly understood. Here we report a direct interaction between the strong inward rectifier, Kir2.1, and a recently identified splice variant of postsynaptic density-93 (PSD-93), a protein involved the subcellular targeting of ion channels and glutamate receptors at excitatory synapses. Yeast two-hybrid screening of a human brain cDNA library using the carboxyl terminus of Kir2.1 as bait yielded cDNA encoding the first two PDZ domains of PSD-93, but with an extended N-terminal region that diverged from other PSD-93 isoforms. This clone represented the human homologue of the mouse PSD-93 splice variant, PSD-93delta. Reverse transcription-polymerase chain reaction analysis showed diffuse low level PSD-93delta expression throughout the brain, with significantly higher levels in spinal cord. In vitro binding studies revealed that a type I PDZ recognition motif at the extreme C terminus of the Kir2.1 mediates interaction with all three PDZ domains of PSD-93delta, and association between Kir2 channels and PSD-93delta was confirmed further by the ability of anti-Kir2.1 antibodies to coimmunoprecipitate PSD-93delta from rat spinal cord lysates. Functionally, coexpression of Kir2.1 and PSD-93delta had no discernible effect upon channel kinetics but resulted in cell surface Kir2.1 clustering and suppression of channel internalization. We conclude that PSD-93delta is potentially an important regulator of the spatial and temporal distribution of Kir2 channels within neuronal membranes of the central nervous system.

Direct interaction between the actin-binding protein filamin-A and the inwardly rectifying potassium channel, Kir2.1

The role of filamins in actin cross-linking and membrane stabilization is well established, but recently their ability to interact with a variety of transmembrane receptors and signaling proteins has led to speculation of additional roles in scaffolding and signal transduction. Here we report a direct interaction between filamin-A and Kir2.1, an isoform of inwardly rectifying potassium channel expressed in vascular smooth muscle and an important regulator of vascular tone. Yeast two-hybrid screening of a porcine coronary artery cDNA library using the carboxyl terminus of Kir2.1 as bait yielded cDNA encoding a fragment of filamin-A (residues 2481-2647). Interaction between filamin-A and Kir2.1 was confirmed by in vitro overlay assay of membrane-bound Kir2.1 with glutathione S-transferase fusion protein of the isolated filamin clone. Additionally, antibodies directed against Kir2.1 coimmunoprecipitated filamin-A from arterial smooth muscle cell lysates, and immunocytochemical analysis of individual arterial smooth muscle cells showed that Kir2.1 and filamin co-localize in "hotspots" at the cell membrane. Interaction with filamin-A was found to have no effect on Kir2.1 channel behavior but, rather, increased the number of functional channels resident within the membrane. We conclude that filamin-A is potentially an important regulator of Kir2.1 surface expression and location within vascular smooth muscle.

Potassium channel ontogeny

Potassium channels are multi-subunit complexes, often composed of several polytopic membrane proteins and cytosolic proteins. The formation of these oligomeric structures, including both biogenesis and trafficking, is the subject of this review. The emphasis is on events in the endoplasmic reticulum (ER), particularly on how, where, and when K(+) channel polypeptides translocate and integrate into the bilayer, oligomerize and fold to form pore-forming units, and associate with auxiliary subunits to create the mature channel complex. Questions are raised with respect to the sequence of these events, when biogenic decisions are made, models for integration of K(+) channel transmembrane segments, crosstalk between the cell surface and ER, and recognition of compatible partner subunits. Also considered are determinants of subunit composition and stoichiometry, their consequence for trafficking, mechanisms for ER retention and export, and sequence motifs that direct channels to the cell surface. It is these mechanistic issues that govern the differential distributions of K(+) conductances at the cell surface, and hence the electrical activity of cells and tissues underlying both the physiology and pathophysiology of an organism.

Redox-dependent gating of G protein-coupled inwardly rectifying K+ channels

G protein-coupled inwardly rectifying K(+) channels (GIRK) play a major role in inhibitory signaling in excitable and endocrine tissues. The gating mechanism of these channels is mediated by a direct interaction of the Gbetagamma subunits of G protein, which are released upon inhibitory neurotransmitter receptor activation. This gating mechanism is further manifested by intracellular factors such as anionic phospholipids and Na(+) and Mg(2+) ions. In addition to the essential role of these components for channel function, phosphorylation events can also modulate channel activity. In this study we explored the involvement of redox modulation on GIRK channel function. Extracellular application of the reducing agent dithiothreitol (DTT), but not reduced glutathione, activated GIRK channels without affecting their permeation or rectification properties. The DTT-dependent activation was found to mimic receptor activation and to act directly on the channel in a membrane delimited fashion. A critical cysteine residue located in the N-terminal cytoplasmic domain was found to be essential for DTT-dependent activation in hetero- and homotetrameric contexts. Interestingly, when mutating this cysteine residue, DTT-dependent activation was abolished, but receptor-mediated channel activation was not affected. These results suggest that intracellular redox potential can play a major role in tuning GIRK channel activity in a receptor-independent manner. This sort of redox modulation can be part of an important cellular protective mechanism against ischemic or hypoxic insults.

Targeting of an A kinase-anchoring protein, AKAP79, to an inwardly rectifying potassium channel, Kir2.1

Protein kinase A (PKA) is targeted to discrete subcellular locations close to its intended substrates through interaction with A kinase-anchoring proteins (AKAPs). Ion channels represent a diverse and important group of kinase substrates, and it has been shown that membrane targeting of PKA through association with AKAPs facilitates PKA-mediated phosphorylation and regulation of several classes of ion channel. Here, we investigate the effect of AKAP79, a membrane-associated multivalent-anchoring protein, upon the function and modulation of the strong inwardly rectifying potassium channel, Kir2.1. Functionally, the presence of AKAP79 enhanced the response of Kir2.1 to elevated intracellular cAMP, suggesting a requirement for a pool of PKA anchored close to the channel. Antibodies directed against a hemagglutinin epitope tag on Kir2.1 coimmunoprecipitated AKAP79, indicating that the two proteins exist together in a complex within intact cells. In support of this, glutathione S-transferase fusion proteins of both the intracellular N and C domains of Kir2.1 isolated AKAP79 from cell lysates, while glutathione S-transferase alone failed to interact with AKAP79. Together, these findings suggest that AKAP79 associates directly with the Kir2.1 ion channel and may serve to anchor kinase enzymes in close proximity to key channel phosphorylation sites.

Inward rectifiers in the heart: an update on I(K1)

The cardiac inward rectifier potassium current (I(K1)), present in all ventricular and atrial myocytes, has been suggested to play a major role in repolarization of the action potential and stabilization of the resting potential. The molecular basis is now ascribed to members of the Kir2 sub-family of inward rectifier K channel genes, and the availability of recombinant expression systems has led to elucidation of the mechanism of inward rectification, as well as additional regulatory mechanisms involving intracellular pH and phosphorylation. In vivo manipulation of the genes encoding I(K1)and regulatory proteins now promise to provide new insights to the role of this conductance in the heart. This review details recent advances and considers the prospects for further elucidation of the role of this conductance in cardiac electrical activity.