Potential role of the membrane in hERG channel functioning and drug-induced long QT syndrome

Comparative study of the structure and interaction of the pore helices of the hERG and Kv1.5 potassium channels in model membranes

The hERG channel is a voltage-gated potassium channel found in cardiomyocytes that contributes to the repolarization of the cell membrane following the cardiac action potential, an important step in the regulation of the cardiac cycle. The lipids surrounding K⁺ channels have been shown to play a key role in their regulation, with anionic lipids shown to alter gating properties. In this study, we investigate how anionic lipids interact with the pore helix of hERG and compare the results with those from Kv1.5, which possesses a pore helix more typical of K⁺ channels. Circular dichroism studies of the pore helix secondary structure reveal that the presence of the anionic lipid DMPS within the bilayer results in a slight unfolding of the pore helices from both hERG and Kv1.5, albeit to a lesser extent for Kv1.5. In the presence of anionic lipids, the two pore helices exhibit significantly different interactions with the lipid bilayer. We demonstrate that the pore helix from hERG causes significant perturbation to the order in lipid bicelles, which contrasts with only small changes observed for Kv1.5. These observations suggest that the atypical sequence of the pore helix of hERG may play a key role in determining how anionic lipids influence its gating.

Phase behavior of a binary lipid system containing long- and short-chain phosphatidylcholines

A new phase state, named the U phase, was observed in DPPC–diC8PC mixtures at low DPPC contents.

Mechanism of Action of Thymosinα1: Does It Interact with Membrane by Recognition of Exposed Phosphatidylserine on Cell Surface? A Structural Approach

Thymosinα1 is a peptidic hormone with pleiotropic activity, which is used in the therapy of several diseases. It is unstructured in water solution and interacts with negative regions of micelles and vesicles assuming two tracts of helical conformation with a structural flexible break in between. The studies of the interaction of Thymosinα1 with micelles of mixed dipalmitoylphosphatidylcholine and sodium dodecylsulfate and vesicles with mixed dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine, the latter the negative component of the membranes, by (1)H and natural abundance (15)N NMR are herewith reported, reviewed, and discussed. The results indicate that the preferred interactions are those where the surface is negatively charged due to sodium dodecylsulfate or due to the presence of dipalmitoylphosphatidylserine exposed on the surface. In fact the unbalance of dipalmitoylphosphatidylserine on the cellular surface is an important phenomenon present in pathological conditions of cells. Moreover, the direct interaction of Thymosinα1 with K562 cells presenting an overexposure of phosphatidylserine as a consequence of resveratrol-induced apoptosis was carried out.

Insight into the molecular interaction between the cyclic nucleotide-binding homology domain and the eag domain of the hERG channel

The gating of the hERG channel is regulated by its eag domain through molecular interaction with either the cyclic nucleotide-binding homology domain (CNBHD) or the linker between transmembrane segments 4 and 5. Our NMR study on the purified CNBHD demonstrated that it contains nine β-strands and does not bind cAMP. We show that the eag domain binds to the CBND through an interface containing several disease-associated mutations. The N-terminal cap domain and R56 in the eag domain are important for the interaction with the CNBHD. Residues from the CNBHD that were affected by the interaction with the eag domain were also identified. A R56Q mutation does not cause major structural changes in the eag domain and showed reduced interaction with the CNBHD.

Lipid concentration and molar ratio boundaries for the use of isotropic bicelles

Bicelles are model membranes generally made of long-chain dimyristoylphosphatidylcholine (DMPC) and short-chain dihexanoyl-PC (DHPC). They are extensively used in the study of membrane interactions and structure determination of membrane-associated peptides, since their composition and morphology mimic the widespread PC-rich natural eukaryotic membranes. At low DMPC/DHPC (q) molar ratios, fast-tumbling bicelles are formed in which the DMPC bilayer is stabilized by DHPC molecules in the high-curvature rim region. Experimental constraints imposed by techniques such as circular dichroism, dynamic light scattering, or microscopy may require the use of bicelles at high dilutions. Studies have shown that such conditions induce the formation of small aggregates and alter the lipid-to-detergent ratio of the bicelle assemblies. The objectives of this work were to determine the exact composition of those DMPC/DHPC isotropic bicelles and study the lipid miscibility. This was done using (31)P nuclear magnetic resonance (NMR) and exploring a wide range of lipid concentrations (2-400 mM) and q ratios (0.15-2). Our data demonstrate how dilution modifies the actual DMPC/DHPC molar ratio in the bicelles. Care must be taken for samples with a total lipid concentration ≤250 mM and especially at q ∼ 1.5-2, since moderate dilutions could lead to the formation of large and slow-tumbling lipid structures that could hinder the use of solution NMR methods, circular dichroism or dynamic light scattering studies. Our results, supported by infrared spectroscopy and molecular dynamics simulations, also show that phospholipids in bicelles are largely segregated only when q > 1. Boundaries are presented within which control of the bicelles' q ratio is possible. This work, thus, intends to guide the choice of q ratio and total phospholipid concentration when using isotropic bicelles.

Cardiac slices as a predictive tool for arrhythmogenic potential of drugs and chemicals

IMPORTANCE OF THE FIELD

cardiac arrhythmia represents one of the primary safety pharmacological concerns in drug development. The most prominent example is drug induced ventricular tachycardia of the Torsade des Pointes type. The mechanism how this type of arrhythmia develops is a complex multi-cellular phenomenon. It can only be insufficiently reflected by cellular or molecular assays. However, organ models - such as Langendorff hearts - or in vivo experiments are expensive and time consuming and not suitable for assays requiring an increased throughput.

AREAS COVERED IN THIS REVIEW

here, we describe and review an assay bridging the gap between cardiomyocyte based assays and organ based systems - cardiac slices. This assay is reviewed in direct comparison with established safety pharmacological assays.

WHAT THE READER WILL GAIN

while slices have played an important role in brain research for > 2 decades, cardiac slices are experiencing a renaissance due to the novel challenges in safety pharmacology just in the last few years. Cardiac slices can be cultured and recorded over several days. It is possible to access electrophysiological data with a high number of electrodes - up to 256 electrodes - embedded in the surface of a microelectrode array.

TAKE HOME MESSAGE

cardiac slices close the gap between cellular and organ based assays in cardiac safety pharmacology. The tissue properties of a functional cardiac syncytium are more accurately reflected by a slice rather than a single cell.