Biology subjects

Burtscher, V.

Publications and source records attributed to Burtscher, V..

4 recordsLinked to original sources

bioRxiv2023

Structural Basis for Hyperpolarization-dependent Opening of the Human HCN1 Channel

Hyperpolarization and cyclic-nucleotide (HCN) activated ion channels play a critical role in generating self-propagating action potentials in pacemaking and rhythmic electrical circuits in the human body. Unlike most voltage-gated ion channels, the HCN channels activate upon membrane hyperpolarization, but the structural mechanisms underlying this gating behavior remain unclear. Here, we present cryo-electron microscopy structures of human HCN1 in Closed, Intermediate, and Open states. Our structures reveal that the inward motion of two gating charges past the charge transfer center (CTC) and concomitant tilting of the S5 helix drives the opening of the central pore. In the intermediate state structure, a single gating charge is positioned below the CTC and the pore appears closed, whereas in the open state structure, both charges move past CTC and the pore is fully open. Remarkably, the downward motion of the voltage sensor is accompanied by progressive unwinding of the inner end of S4 and S5 helices disrupting the tight gating interface that stabilizes the Closed state structure. This "melting" transition at the intracellular gating interface leads to a concerted iris-like displacement of S5 and S6 helices, resulting in pore opening. These findings reveal key structural features that are likely to underlie reversed voltage-dependence of HCN channels.

biochemistry↗

bioRxiv2022

Interplay between VSD, pore and membrane lipids in electromechanical coupling in HCN channels

Hyperpolarized-activated and Cyclic Nucleotide-gated (HCN) channels are the only members of the voltage-gated ion channel superfamily in mammals that open upon hyperpolarization, conferring them pacemaker properties that are instrumental for rhythmic firing of cardiac and neuronal cells. Activation of their voltage-sensor domains (VSD) upon hyperpolarization occurs through a downward movement of the S4 helix bearing the gating charges, which triggers a break in the alpha-helical hydrogen bonding pattern at the level of a conserved Serine residue. Previous structural and molecular simulation studies had however failed to capture pore opening that should be triggered by VSD activation, presumably because of a low VSD/pore electromechanical coupling efficiency and the limited timescales accessible to such techniques. Here, we have used advanced modeling strategies, including enhanced sampling molecular dynamics simulations exploiting comparisons between non-domain swapped voltage-gated ion channel structures trapped in closed and open states to trigger pore gating and characterize electromechanical coupling in HCN1. We propose that the coupling mechanism involves the reorganization of the interfaces between the VSD helices, in particular S4, and the pore-forming helices S5 and S6, subtly shifting the balance between hydrophobic and hydrophilic interactions in a "domino effect" during activation and gating in this region. Remarkably, our simulations reveal state-dependent occupancy of lipid molecules at this emergent coupling interface, suggesting a key role of lipids in hyperpolarization-dependent gating. Our model provides a rationale for previous observations and a possible mechanism for regulation of HCN channels by the lipidic components of the membrane.

biophysics↗

bioRxiv2019

An electrophysiological approach to measure changes in the membrane surface potential in real time

Biological membranes carry fixed charges at their surfaces. These arise primarily from phospholipid head groups. In addition, membrane proteins contribute to the surface potential with their charged residues. Membrane lipids are asymmetrically distributed. Because of this asymmetry the net negative charge at the inner leaflet exceeds that at the outer leaflet. Changes in surface potential are predicted to shape the capacitive properties of the membrane (i.e. the ability of the membrane to store electrical charges). Here, we show that it is possible to detect changes in surface potential by an electrophysiological approach: the analysis of cellular currents relies on assuming that the electrical properties of a cell are faithfully described by a three-element circuit - i.e. the minimal equivalent circuit - comprised of two resistors and one capacitor. However, to account for changes in surface potential it is necessary to add a battery to this circuit connected in series with the capacitor. This extended circuit model predicts that the current response to a square-wave voltage pulse harbors information, which allows for separating the changes in surface potential from a true capacitance change. We interrogated our model by investigating changes in capacitance induced by ligand binding to the serotonin transporter (SERT) and to the glycine transporters (GlyT1 and GlyT2). The experimental observations were consistent with the predictions of the extended circuit. We conclude that ligand-induced changes in surface potential (reflecting the binding event) and in true membrane capacitance (reflecting the concomitant conformational change) can be detected in real time even in instances where they occur simultaneously.\n\nStatement of SignificanceThe plasma membrane of a cell possesses fixed charges on both surfaces. Surface charges play an important role in many biological processes. However, the mechanisms, which regulate the surface charge densities at the plasma membrane, are poorly understood. This is in part due to lack of experimental approaches that allow for detecting changes in surface charges in real time. Here, we show that it is possible to track alterations in the electric potential at the membrane surface with high temporal resolution by an electrophysiological approach. Importantly, the described method allows for discriminating between a change in surface potential and a change in true membrane capacitance (e.g. a change in membrane area), even if these occur in parallel.

biophysics↗

bioRxiv2018

A label-free approach to detect ligand binding to cell surface proteins in real time

Electrophysiological recordings allow for monitoring the operation of proteins with high temporal resolution down to the single molecule level. This technique has been exploited to track either ion flow arising from channel opening or the synchronized movement of charged residues and/or ions within the membrane electric field. Here, we describe a novel type of current by using the serotonin transporter (SERT) as a model. We examined transient currents elicited on rapid application of specific SERT inhibitors. Our analysis shows that these currents originate from ligand binding and not from a conformational change. The Gouy-Chapman model predicts that a ligand-induced elimination/neutralization of surface charge must produce a displacement current and related apparent changes in membrane capacitance. Here we verified these predictions with SERT. Our observations demonstrate that ligand binding to a protein can be monitored in real time and in a label-free manner by recording the membrane capacitance.

biophysics↗