Conserved extracellular cysteine residues in the inwardly rectifying potassium channel Kir2.3 are required for function but not expression in the membrane

Extracellular Kir2.1C122Y Mutant Upsets Kir2.1-PIP2 Bonds and Is Arrhythmogenic in Andersen-Tawil Syndrome

BACKGROUND

Andersen-Tawil syndrome type 1 is a rare heritable disease caused by mutations in the gene coding the strong inwardly rectifying K+ channel Kir2.1. The extracellular Cys (cysteine)122-to-Cys154 disulfide bond in the channel structure is crucial for proper folding but has not been associated with correct channel function at the membrane. We evaluated 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 its open state.

METHODS

We identified a Kir2.1 loss-of-function mutation (c.366 A>T; p.Cys122Tyr) in an ATS1 family. To investigate its pathophysiological implications, we generated an AAV9-mediated cardiac-specific mouse model expressing the Kir2.1C122Y variant. We employed a multidisciplinary approach, integrating patch clamping and intracardiac stimulation, molecular biology techniques, molecular dynamics, and bioluminescence resonance energy transfer experiments.

RESULTS

Kir2.1C122Y mice recapitulated the ECG features of ATS1 independently of sex, including corrected QT prolongation, conduction defects, and increased arrhythmia susceptibility. Isolated Kir2.1C122Y cardiomyocytes showed significantly reduced inwardly rectifier K+ (IK1) and inward Na+ (INa) current densities independently of normal trafficking. Molecular dynamics predicted that the C122Y mutation provoked a conformational change over the 2000-ns simulation, characterized by a greater loss of hydrogen bonds between Kir2.1 and phosphatidylinositol 4,5-bisphosphate than wild type (WT). Therefore, the phosphatidylinositol 4,5-bisphosphate-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 phosphatidylinositol 4,5-bisphosphate concentrations. In addition, the Kir2.1C122Y mutation resulted in channelosome degradation, demonstrating temporal instability of both Kir2.1 and NaV1.5 proteins.

CONCLUSIONS

The extracellular Cys122-to-Cys154 disulfide bond in the tridimensional Kir2.1 channel structure is essential for the channel function. We demonstrate that breaking disulfide bonds in the extracellular domain disrupts phosphatidylinositol 4,5-bisphosphate-dependent regulation, leading to channel dysfunction and defects in Kir2.1 energetic stability. The mutation also alters functional expression of the NaV1.5 channel and ultimately leads to conduction disturbances and life-threatening arrhythmia characteristic of Andersen-Tawil syndrome type 1.

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.

Intramolecular Disulfide Bonds for Biogenesis of CALHM1 Ion Channel Are Dispensable for Voltage-Dependent Activation

Calcium homeostasis modulator 1 (CALHM1) is a membrane protein with four transmembrane helices that form an octameric ion channel with voltage-dependent activation. There are four conserved cysteine (Cys) residues in the extracellular domain that form two intramolecular disulfide bonds. We investigated the roles of C42-C127 and C44-C161 in human CALHM1 channel biogenesis and the ionic current (ICALHM1). Replacing Cys with Ser or Ala abolished the membrane trafficking as well as ICALHM1. Immunoblotting analysis revealed dithiothreitol-sensitive multimeric CALHM1, which was markedly reduced in C44S and C161S, but preserved in C42S and C127S. The mixed expression of C42S and wild-type did not show a dominant-negative effect. While the heteromeric assembly of CALHM1 and CALHM3 formed active ion channels, the co-expression of C42S and CALHM3 did not produce functional channels. Despite the critical structural role of the extracellular cysteine residues, a treatment with the membrane-impermeable reducing agent tris(2-carboxyethyl) phosphine (TCEP, 2 mM) did not affect ICALHM1 for up to 30 min. Interestingly, incubation with TCEP (2 mM) for 2-6 h reduced both ICALHM1 and the surface expression of CALHM1 in a time-dependent manner. We propose that the intramolecular disulfide bonds are essential for folding, oligomerization, trafficking and maintenance of CALHM1 in the plasma membrane, but dispensable for the voltage-dependent activation once expressed on the plasma membrane.

The ATP-sensitive K(+)-channel (K(ATP)) controls early left-right patterning in Xenopus and chick embryos

Consistent left-right asymmetry requires specific ion currents. We characterize a novel laterality determinant in Xenopus laevis: the ATP-sensitive K(+)-channel (K(ATP)). Expression of specific dominant-negative mutants of the Xenopus Kir6.1 pore subunit of the K(ATP) channel induced randomization of asymmetric organ positioning. Spatio-temporally controlled loss-of-function experiments revealed that the K(ATP) channel functions asymmetrically in LR patterning during very early cleavage stages, and also symmetrically during the early blastula stages, a period when heretofore largely unknown events transmit LR patterning cues. Blocking K(ATP) channel activity randomizes the expression of the left-sided transcription of Nodal. Immunofluorescence analysis revealed that XKir6.1 is localized to basal membranes on the blastocoel roof and cell-cell junctions. A tight junction integrity assay showed that K(ATP) channels are required for proper tight junction function in early Xenopus embryos. We also present evidence that this function may be conserved to the chick, as inhibition of K(ATP) in the primitive streak of chick embryos randomizes the expression of the left-sided gene Sonic hedgehog. We propose a model by which K(ATP) channels control LR patterning via regulation of tight junctions.

Variable loss of Kir4.1 channel function in SeSAME syndrome mutations

SeSAME syndrome is a complex disease characterized by seizures, sensorineural deafness, ataxia, mental retardation and electrolyte imbalance. Mutations in the inwardly rectifying potassium channel Kir4.1 (KCNJ10 gene) have been linked to this condition. Kir4.1 channels are weakly rectifying channels expressed in glia, kidney, cochlea and possibly other tissues. We determined the electrophysiological properties of SeSAME mutant channels after expression in transfected mammalian cells. We found that a majority of mutations (R297C, C140R, R199X, T164I) resulted in complete loss of Kir4.1 channel function while two mutations (R65P and A167V) produced partial loss of function. All mutant channels were rescued upon co-transfection of wild-type Kir4.1 but not Kir5.1 channels. Cell-surface biotinylation assays indicate significant plasma membrane expression of all mutant channels with exception of the non-sense mutant R199X. These results indicate the differential loss of Kir channel function among SeSAME syndrome mutations.

Contribution of cytosolic cysteine residues to the gating properties of the Kir2.1 inward rectifier

The topological model proposed for the Kir2.1 inward rectifier predicts that seven of the channel 13 cysteine residues are distributed along the N- and C-terminus regions, with some of the residues comprised within highly conserved domains involved in channel gating. To determine if cytosolic cysteine residues contribute to the gating properties of Kir2.1, each of the N- and C-terminus cysteines was mutated into either a polar (S, D, N), an aliphatic (A,V, L), or an aromatic (W) residue. Our patch-clamp measurements show that with the exception of C76 and C311, the mutation of individual cytosolic cysteine to serine (S) did not significantly affect the single-channel conductance nor the channel open probability. However, mutating C76 to a charged or polar residue resulted either in an absence of channel activity or a decrease in open probability. In turn, the mutations C311S (polar), C311R (charged), and to a lesser degree C311A (aliphatic) led to an increase of the channel mean closed time due to the appearance of long closed time intervals (T(c) >or= 500 ms) and to a reduction of the reactivation by ATP of rundown Kir2.1 channels. These changes could be correlated with a weakening of the interaction between Kir2.1 and PIP(2), with C311R and C311S being more potent at modulating the Kir2.1-PIP(2) interaction than C311A. The present work supports, therefore, molecular models whereby the gating properties of Kir2.1 depend on the presence of nonpolar or neutral residues at positions 76 and 311, with C311 modulating the interaction between Kir2.1 and PIP(2).

Functional properties of Kch, a prokaryotic homologue of eukaryotic potassium channels

To test the hypothesis that the Kch gene of Escherichia coli encodes a potassium channel, we have transformed E. coli with an expression vector containing the Kch sequence and observed the effect of over-expression of Kch on E. coli. We found that: (i) over-expression of Kch is toxic to E. coli, but the toxicity could be prevented by supplementing the growth medium with K(+), Rb(+), and NH(4)(+), but not Na(+), consistent with the properties of a potassium selective pore; (ii) Cs(+), a blocker of potassium channels, rescues the growth of Kch over-expressing cells; and (iii) when the putative pore-forming region of Kch, containing the signature sequence, was replaced with the corresponding region of the eukaryotic Shaker potassium channel, and the resultant construct expressed in E. coli, the cells became critically dependent on K(+) supply for survival. These data are consistent with the proposed function of Kch, i.e., K(+) conduction.

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.