Scientists make important discovery about sodium channel function

An international study with Brazilian participants identifies the domain responsible for the gating mechanism of voltage-sensisitve sodium channels.

By Fábio de Castro

Agência FAPESP – An international study has established a new protocol to measure the current that passes through the voltage sensors of ion channels and has identified the parts of the channels responsible for the gating mechanism – the opening and closing – of voltage-sensing sodium channels.

The study was conducted by a group of scientists in Brazil, the United States and Canada. Formed by proteins in cell membranes, ion channels are “tunnels” that allow only certain ions to pass through to the interior of a cell. Some of these channels are sensitive to voltage – that is, they are activated by differences in ocal electrical potentials.

Voltage-sensing ion channels are responsible for propagation of the electrical impulses that facilitate communication in the neural system. These channels participate in a series of important processes, including controlling the intracellular concentrations of calcium and hydrogen.

According to the authors of the study, which was published in the Proceedings of the National Academy of Sciences (PNAS), this research paves the way for understanding the mechanisms of action of several drugs and toxins that bind to these channels, by advancing the understanding of how voltage-sensing ion channels work.

Participants in the study included Manoel Arcisio-Miranda of the Department of Biophysics at Universidade Federal de São Paulo (Unifesp) and researchers from the Department of Neuroscience at University of Wisconsin-Madison and the Physiology and Pharmacology Department at the University of Calgary (Canada).

Arcisio-Miranda initiated the study during his post-doctoral studies in Madison – which concluded in 2010 – and continued the work under the auspices of the project “Molecular aspects of the voltage sensor of ionic channels: structure, kinetics and evolution.” This project was coordinated by Arcisio-Miranda and was funded by FAPESP under its Regular Research Award program.

“In this study, we managed to identify the domain in the gating mechanism that is responsible for the transitions that occur in the selectivity filter for voltage-sensing sodium channels. Understanding this mechanism is fundamental for developing new drugs and for understanding how certain toxins block or paralyze the channels and how specific alterations change their functioning,” explained Arcisio-Miranda in an interview with Agência FAPESP.

Voltage-sensing ion channels are made up of four subunits, and researchers have discovered that the domain 4 voltage sensor is responsible for the gating mechanism. In addition to identifying the domain responsible for the gating mechanism, the scientists also developed a new approach for measuring ion concentrations.

“The main advance was ion measurement through a voltage sensor. We did this by blocking the pores of the membrane and measuring a current through the voltage sensor. This procedure allows scientists to determine the spatial position of the voltage and to identify whether the channel is activated and how it is linked to the pore region – that is, the region where the ion passes through the protein,” explained Arcisio-Miranda.

Voltage sensor

In Unifesp’s Biophysical Membranes and Ionic Channels Laboratory, Arcisio-Miranda has initiated a line of research related to studies on the relationship between the structure and function of ion channels.

The studies involve, for example, linking the voltage sensor and the pore channels for potassium and functionally characterizing the negatively charged residues of the domain 4voltage sensor to understand the inactivation of sodium channels.

“As of 1998, a series of crystalline structures of ion channels has been revealed. One of the most important studies indicated that with voltage-sensing potassium channels, in particular, the region that identifies changes in voltage is physically distant from the pore region, as if it were an appendix,” explained Arcisio-Miranda.

“One of the questions that emerged from this study was, if the voltage sensing region is near the protein and if ion transport occurs in other places, how are they coupled?” he asked.

In studies that sought to answer these questions, the international groups demonstrated that a region known as link S4-S5 is responsible for the transmission of information from the voltage sensor to the pores and is thus responsible for the gating mechanism. However, in the other four, scientists observed that the pore regions in potassium channels could also participate in the gating mechanism.

“The potassium channel has four identical subunits, while in the sodium channel these subunits are different. For this reason, we imagine that the sodium channels could have a localized gating mechanism, controlled in the region of the selectivity filter. With this work, we managed to identify the domain that is responsible for the transitions that occur in the selectivity filter for voltage-sensing sodium channels,” he explained.

In addition to identifying the domain responsible for the gating mechanism and developing an improved procedure for measuring ion concentrations, the scientists measured the currents that resulted from the movement of the voltage sensor.

“Before the study, due to technical limitations, it was only possible to obtain this type of measurement with the use of toxins or drugs that blocked the central pore. We managed to develop a method that allowed us to measure the gating current by making the pore non-conductive,” the scientist explains. According to Arcisio-Miranda, previous studies had shown the importance of the voltage sensor in domain IV in the mechanism of inactivation of sodium channels.

“We verified that the coupling between the selectivity filter and the voltage sensor, which we observed, has a much higher probability of occurring when the channel is in an inactive state,” he said.

The article “Gating transitions in the selectivity filter region of a sodium channel are coupled to the domain IV voltage sensor” (doi: 10.1073/pnas.1115575109) by Manoel Arcísio-Miranda and others can be read by PNAS subscribers at www.pnas.org/content/early/2012/01/27/1115575109.abstract.