Theoretical and simulation studies on voltage-gated sodium channels

Impact of a Terahertz electromagnetic field on the ion permeation of potassium and sodium channels

Ion channels are essential for various physiological processes, and their defects are associated with many diseases. Previous research has revealed that a Terahertz electromagnetic field can alter the channel conductance by affecting the motion of chemical groups of ion channels, and hence regulate the electric signals of neurons. In this study, we conducted molecular dynamics simulations to systematically investigate the effects of terahertz electromagnetic fields on the ion permeation of voltage-gated potassium and sodium channels, particularly focusing on the bound ions in the selectivity filters that have not been extensively studied previously. Our results identified multiple new characteristic frequencies and showed that 1.4, 2.2, or 2.9 THz field increases the ion permeability of Kv1.2, and 2.5 or 48.6 THz field increases the ion permeability of Nav1.5. Such effects are specific to the frequencies and directions of the electric field, which are determined by the intrinsic oscillation motions of the permeating ions in the selectivity filter or certain chemical groups of the ion channels. The amplitude of the THz field positively correlates with the change in ion permeation. This study demonstrates that THz fields can specifically regulate ion channel conductances by multiple mechanisms, which may carry great potential in biomedical applications.

Role of Hydration and Amino Acid Interactions on the Ion Permeation Mechanism in the hNaV1.5 Channel

This study explores the ion selectivity and conduction mechanisms of the hNaV1.5 sodium channel using classical molecular dynamics simulations under an externally applied electric field. Our findings reveal distinct conduction mechanisms for Na+ and K+, primarily driven by differences in their hydration states when they diffuse close to the channel's selective filter (DEKA) and extracellular ring (EEDD). The Na+ ions undergo partial dehydration in the EEDD region, followed by a rehydration step in the DEKA ring, resulting in longer retention times and a deeper free energy minimum compared to K+. Conversely, the K+ ions exhibit a continuous dehydration process, facilitating a smoother passage through these key regions. These results indicate that ion selectivity and conductance are primarily governed by solvation dynamics, which, in turn, depend on the interactions with key charged residues within the channel. Additionally, we show that the delicate energetic balance between the interactions of the ions with the protein residues and with their solvation shells during the dehydration and rehydration processes is not properly captured by the force field. As a consequence, the selectivity of the channel is not well described, indicating that more accurate computational models must be applied to simulate ion conduction through eukaryotic NaV channels.

Role of the Interaction Motif in Maintaining the Open Gate of an Open Sodium Channel

Voltage-gated sodium channels undergo transitions between open, closed, and inactivated states, enabling regulation of the translocation of sodium ions across membranes. A recently published crystal structure of the full-length prokaryotic NavMs crystal structure in the activated open conformation has revealed the presence of a novel motif consisting of an extensive network of salt bridges involving residues in the voltage sensor, S4-S5 linker, pore, and C-terminal domains. This motif has been proposed to be responsible for maintaining an open conformation that enables ion translocation through the channel. In this study, we have used long-time molecular dynamics calculations without artificial restraints to demonstrate that the interaction network of full-length NavMs indeed prevents a rapid collapse and closure of the gate, in marked difference to earlier studies of the pore-only construct in which the gate had to be restrained to remain open. Interestingly, a frequently discussed "hydrophobic gating" mechanism at nanoscopic level is also observed in our simulations, in which the discontinuous water wire close to the gate region leads to an energetic barrier for ion conduction. In addition, we demonstrate the effects of in silico mutations of several of the key residues in the motif on the open channel's stability and functioning, correlating them with existing functional studies on this channel and homologous disease-associated mutations in human sodium channels; we also examine the effects of truncating/removing the voltage sensor and C-terminal domains in maintaining an open gate.

Single-molecule study of full-length NaChBac by planar lipid bilayer recording

Planar lipid bilayer device, alternatively known as BLM, is a powerful tool to study functional properties of conducting membrane proteins such as ion channels and porins. In this work, we used BLM to study the prokaryotic voltage-gated sodium channel (Nav) NaChBac in a well-defined membrane environment. Navs are an essential component for the generation and propagation of electric signals in excitable cells. The successes in the biochemical, biophysical and crystallographic studies on prokaryotic Navs in recent years has greatly promoted the understanding of the molecular mechanism that underlies these proteins and their eukaryotic counterparts. In this work, we investigated the single-molecule conductance and ionic selectivity behavior of NaChBac. Purified NaChBac protein was first reconstituted into lipid vesicles, which is subsequently incorporated into planar lipid bilayer by fusion. At single-molecule level, we were able to observe three distinct long-lived conductance sub-states of NaChBac. Change in the membrane potential switches on the channel mainly by increasing its opening probability. In addition, we found that individual NaChBac has similar permeability for Na+, K+, and Ca2+. The single-molecule behavior of the full-length protein is essentially highly stochastic. Our results show that planar lipid bilayer device can be used to study purified ion channels at single-molecule level in an artificial environment, and such studies can reveal new protein properties that are otherwise not observable in in vivo ensemble studies.

Three-dimensional Modelling of the Voltage-gated Sodium Ion Channel from Anopheles gambiae Reveals Spatial Clustering of Evolutionarily Conserved Acidic Residues at the Extracellular Sites

BACKGROUND

The eukaryotic voltage-gated sodium channel(e-Nav) is a large asymmetric transmembrane protein with important functions concerning neurological function. No structure has been resolved at high resolution for this protein.

METHODS

A homology model of the transmembrane and extracellular regions of an Anopheles gambiae para-like channel with emphasis on the pore entrance has been constructed, based upon the templates provided by a prokaryotic sodium channel and a potassium two-pore channel. The latter provides a template for the extracellular regions, which are located above the entrance to the pore, which is likely to open at a side of a dome formed by these loops.

RESULTS

A model created with this arrangement shows a structure similar to low-resolution cryoelectron microscope images of a related structure. The pore entrance also shows favorable electrostatic interface.

CONCLUSION

Residues responsible for the negative charge around the pore have been traced in phylogeny to highlight their importance. This model is intended for the study of pore-blocking toxins.

Lysine and the Na+/K+ Selectivity in Mammalian Voltage-Gated Sodium Channels

Voltage-gated sodium (Na_v) channels are critical in the generation and transmission of neuronal signals in mammals. The crystal structures of several prokaryotic Na_v channels determined in recent years inspire the mechanistic studies on their selection upon the permeable cations (especially between Na⁺ and K⁺ ions), a property that is proposed to be mainly determined by residues in the selectivity filter. However, the mechanism of cation selection in mammalian Na_v channels lacks direct explanation at atomic level due to the difference in amino acid sequences between mammalian and prokaryotic Na_v homologues, especially at the constriction site where the DEKA motif has been identified to determine the Na⁺/K⁺ selectivity in mammalian Na_v channels but is completely absent in the prokaryotic counterparts. Among the DEKA residues, Lys is of the most importance since its mutation to Arg abolishes the Na⁺/K⁺ selectivity. In this work, we modeled the pore domain of mammalian Na_v channels by mutating the four residues at the constriction site of a prokaryotic Na_v channel (Na_vRh) to DEKA, and then mechanistically investigated the contribution of Lys in cation selection using molecular dynamics simulations. The DERA mutant was generated as a comparison to understand the loss of ion selectivity caused by the K-to-R mutation. Simulations and free energy calculations on the mutants indicate that Lys facilitates Na⁺/K⁺ selection by electrostatically repelling the cation to a highly Na⁺-selective location sandwiched by the carboxylate groups of Asp and Glu at the constriction site. In contrast, the electrostatic repulsion is substantially weakened when Lys is mutated to Arg, because of two intrinsic properties of the Arg side chain: the planar geometric design and the sparse charge distribution of the guanidine group.