Letter to the editor: Accurate cell capacitance determination from a single voltage step: a reminder to avoid unnecessary pitfalls

Electrophysiological Characterization of Monoolein-Fatty Acid Bilayers

Understanding the evolution of protocells, primitive compartments that distinguish self from nonself, is crucial for exploring the origin of life. Fatty acids and monoglycerides have been proposed as key components of protocell membranes due to their ability to self-assemble into bilayers and vesicles capable of nutrient exchange. In this study, we investigate the electrophysiological properties of planar bilayers composed of monoglyceride and fatty acid mixtures, using a droplet interface bilayer system. Three fatty acids with varying hydrocarbon chain lengths─oleic acid (C18), palmitoleic acid (C16), and myristoleic acid (C14)─in combination with monoolein (C18) are examined to evaluate the influence of chain length and composition on bilayer stability, thickness, and ion permeability. The results show that pure monoolein bilayers exhibit enhanced ion permeability compared to phospholipid bilayers, which are characteristic of modern cellular membranes. Furthermore, the incorporation of fatty acids into monoolein bilayers destabilizes the membrane structure and further increases ion permeability. We consider that this increased permeability is likely driven by three molecular characteristics. First, the wedge-like shape of monoolein may disrupt bilayer packing and induce transient pore formation. Second, the rapid flip-flop of fatty acids between bilayer leaflets likely facilitates ion transport. Third, the chain-length mismatch between monoolein and myristoleic acid further destabilizes the bilayer, promoting the formation of structural defects. These findings suggest that compositional motifs in monoglyceride-fatty acid bilayers may provide an alternative ion transport mechanism, such as the flip-flop of amphiphilic molecules, in early protocell membranes before the evolution of protein-based transporters.

Injection of IK1 through dynamic clamp can make all the difference in patch-clamp studies on hiPSC-derived cardiomyocytes

Human-induced stem cell-derived cardiomyocytes (hiPSC-CMs) are a valuable tool for studying development, pharmacology, and (inherited) arrhythmias. Unfortunately, hiPSC-CMs are depolarized and spontaneously active, even the working cardiomyocyte subtypes such as atrial- and ventricular-like hiPSC-CMs, in contrast to the situation in the atria and ventricles of adult human hearts. Great efforts have been made, using many different strategies, to generate more mature, quiescent hiPSC-CMs with more close-to-physiological resting membrane potentials, but despite promising results, it is still difficult to obtain hiPSC-CMs with such properties. The dynamic clamp technique allows to inject a current with characteristics of the inward rectifier potassium current (IK1), computed in real time according to the actual membrane potential, into patch-clamped hiPSC-CMs during action potential measurements. This results in quiescent hiPSC-CMs with a close-to-physiological resting membrane potential. As a result, action potential measurements can be performed with normal ion channel availability, which is particularly important for the physiological functioning of the cardiac SCN5A-encoded fast sodium current (INa). We performed in vitro and in silico experiments to assess the beneficial effects of the dynamic clamp technique in dissecting the functional consequences of the SCN5A-1795insD+/- mutation. In two separate sets of patch-clamp experiments on control hiPSC-CMs and on hiPSC-CMs with mutations in ACADVL and GNB5, we assessed the value of dynamic clamp in detecting delayed afterdepolarizations and in investigating factors that modulate the resting membrane potential. We conclude that the dynamic clamp technique has highly beneficial effects in all of the aforementioned settings and should be widely used in patch-clamp studies on hiPSC-CMs while waiting for the ultimate fully mature hiPSC-CMs.

Therapeutically viable generation of neurons with antisense oligonucleotide suppression of PTB

Methods to enhance adult neurogenesis by reprogramming glial cells into neurons enable production of new neurons in the adult nervous system. Development of therapeutically viable approaches to induce new neurons is now required to bring this concept to clinical application. Here, we successfully generate new neurons in the cortex and dentate gyrus of the aged adult mouse brain by transiently suppressing polypyrimidine tract binding protein 1 using an antisense oligonucleotide delivered by a single injection into cerebral spinal fluid. Radial glial-like cells and other GFAP-expressing cells convert into new neurons that, over a 2-month period, acquire mature neuronal character in a process mimicking normal neuronal maturation. The new neurons functionally integrate into endogenous circuits and modify mouse behavior. Thus, generation of new neurons in the dentate gyrus of the aging brain can be achieved with a therapeutically feasible approach, thereby opening prospects for production of neurons to replace those lost to neurodegenerative disease.

Ca2+ currents in cardiomyocytes: How to improve interpretation of patch clamp data?

OBJECTIVES

Variability of ion currents is major issue when used for significance testing. One of the simplest approach to reduce variability is normalization to cell membrane size. However, efficacy of Ca²⁺ currents (I_Ca) normalization is unknown. Beside absolute variability, the type of distribution since non-Gaussian distribution makes application of nonparametric test necessary.

METHODS

We retrospectively analyzed individual I_Ca amplitudes measured in ventricular cardiomyocytes from mice, rats and humans and in atrial cardiomyocytes from humans in sinus rhythm and in atrial fibrillation. I_Ca was normalized to cell membrane size, estimated from capacitance transients. In addition, data were Log transformed to reach Gaussian distribution. Normalized and transformed data were analyzed for variability and applicability of parametric vs. nonparametric tests.

RESULTS

There was strong correlation between I_Ca and cell membrane size. However, correlation coefficient was rather low. Normalizing I_Ca had an inconsistent effect on variability. Variability of I_Ca in cells from the same patient/animal was not different cardiomyocytes from humans, rat and mice. Calculation of mean values based on mean values of cells from individuals (patients or animals) vs. mean values calculated for all cells drastically reduces statistical power to detect differences between the groups. Log transformation of I_Ca allowed application of much higher sensitive parametric testing, compensating loss of power.

CONCLUSION

Impact of cell membrane size to I_Ca is low and may limit efficacy of normalization of I_Ca to reduce variability. In contrast, Log transformation of I_Ca data reduces variability and can increase statistical power to detect difference between I_Ca datasets.

Accuracy considerations for capacitance estimation by voltage steps in cardiomyocytes

Electrophysiologists routinely use simple voltage steps to evaluate cell membrane capacitance derived from corresponding current responses. Frequently, the resting membrane voltage V_rest is employed as holding potential for the subsequent command voltage step and more or less accurate methods are utilised to analyse the transient current. Another choice as holding potential is the peak of the "quasi steady-state" current to voltage relationship, V_peak. The aim of this study is the systematic evaluation of capacitance estimation accuracy from voltage step experiments depending on the choice of holding potential and analysis method. In this paper, a simulation approach is employed to analyse the current response of a model patch-clamp circuit. Four commonly accepted methods are implemented, utilizing different aspects of the transient current (charge, membrane time constant, and influence of the series resistance) in various combinations and with various degrees of refinement. This simulation study indicates an acceptable accuracy of the elaborated methods for capacitance estimation at holding potentials V_rest and V_peak over a broad range of capacitance as well as series resistance values. Simple integration of the current transient provides sufficient accuracy at holding potentials, which effectively minimizes changes in resistive membrane current flow during command voltage steps (particularly around V_peak). However, biphasic command protocols performed at V_peak activate voltage dependent sodium channels, thereby possibly leading to the threshold voltage for an action potential. Compared to V_rest, all methods utilizing monophasic step protocols, gain additional accuracy, when applied at V_peak as holding potential.

A fundamental evaluation of the electrical properties and function of cardiac transverse tubules

This work discusses active and passive electrical properties of transverse (T-)tubules in ventricular cardiomyocytes to understand the physiological roles of T-tubules. T-tubules are invaginations of the lateral membrane that provide a large surface for calcium-handling proteins to facilitate sarcomere shortening. Higher heart rates correlate with higher T-tubular densities in mammalian ventricular cardiomyocytes. We assess ion dynamics in T-tubules and the effects of sodium current in T-tubules on the extracellular potential, which leads to a partial reduction of the sodium current in deep segments of a T-tubule. We moreover reflect on the impact of T-tubules on macroscopic conduction velocity, integrating fundamental principles of action potential propagation and conduction. We also theoretically assess how the conduction velocity is affected by different T-tubular sodium current densities. Lastly, we critically assess literature on ion channel expression to determine whether action potentials can be initiated in T-tubules.