Pathophysiological role of omega pore current in channelopathies

Abstract

In voltage-gated cation channels, a recurrent pattern for mutations is the neutralization of positively charged residues in the voltage-sensing S4 transmembrane segments. These mutations cause dominant ion channelopathies affecting many tissues such as brain, heart, and skeletal muscle. Recent studies suggest that the pathogenesis of associated phenotypes is not limited to alterations in the gating of the ion-conducting alpha pore. Instead, aberrant so-called omega currents, facilitated by the movement of mutated S4 segments, also appear to contribute to symptoms. Surprisingly, these omega currents conduct cations with varying ion selectivity and are activated in either a hyperpolarized or depolarized voltage range. This review gives an overview of voltage sensor channelopathies in general and focuses on pathogenesis of skeletal muscle S4 disorders for which current knowledge is most advanced.

Keywords: cytotoxic edema; degeneration; epilepsy and neuromyotonia; familial hemiplegic migraine; hyperkalemic and hypokalemic periodic paralysis; long QT syndrome; myotonia and paramyotonia; sodium overload.

Figure 1

Scheme of a voltage-gated ion channel. Bird’s eye view of the channel consisting of four similar repeats (I to IV). The channel has been cut open between repeats I and IV to show the opening and closing of the central pore. The model also shows one of the four voltage sensors S4, which moves outward when the membrane becomes depolarized and remains in this position until repolarization.

Figure 2

Omega pores and currents dependent on the R position. (A) Replacement of the outermost arginine (red) by a neutral amino acid (gray) such as glycine (R1G) opens a conductive pathway through the polarized membrane, resulting in an omega current (red). At depolarized potentials at which the S4 segment moves outward, the conductive pathway is closed by a deeper arginine and the omega current ceases. In contrast the replacement of a deeper arginine (R3G) only opens the omega pore if the membrane is depolarized. (B) Homology model of domain I in hNa_v1.4 based on crystal structure of NaVAb (activated-closed; crystal structure at 0 mV), using Modeller. Positions of arginine and lysine residues of DIS4 are shown, relative to the putative gating pore constriction (arrow). Modified from Groome and Winston (2012). (C) Comparison of current-voltage (I/V) traces for wild type hNa_v1.4 and R222G, with plots of raw I/V, linear leak, and normalized current (linear leak subtracted from IV and normalized to gating current at +40 mV). The mutation R222G causes HypoPP type 2. External solution contained 120 mM K⁺ and 1 μM TTX. Modified from Holzherr et al. (2010).

Figure 3

S4 sequences of channels with an R0 to R6 mutation. Positively charged residues in R0 to R6 positions are delineated with a shaded background (gray). The constriction of the omega pore lies between R2 and R3. Therefore an inward movement of S4 due to hyperpolarization opens the omega pore if R1 or R2 is replaced by a neutral amino acid. An outward movement of S4 due to depolarization opens the omega pore if R3 or R4 is replaced by a neutral amino acid. These gray-background arginines are boxed. The Shaker K⁺ channel serves as reference. Neutral replacements of its arginines R2 and R3 have been described as proton transporters.

Figure 4

Effects of omega currents schematically (A) and in a computer simulation (B). (A) The current-voltage relationship of K_ir potassium channels shows a voltage range characterized by a “negative” resistance. This negativity leads to membrane bi-stability. Whereas small instantaneous changes of P1 will be compensated for (P1 is therefore a stable membrane potential), larger depolarizing artifacts will cause a jump of P1 to the limit point LP2. P1 can be regained by substantial repolarizing influences like the Na/K pump. (B) The curve is downwardly shifted by an omega Na⁺ current. Limit point LP1 is a very instable membrane potential that will be shifted to P2 by even smallest instantaneous changes. In the P2-state the cell membrane will be electrically stable. (C) Membrane potentials P for various [K⁺]_o values and for various omega pore conductances (in μS/cm²). Reducing [K⁺]_o first leads to hyperpolarization until the limit point (LP1) is reached at which the membrane potential becomes instable and jumps to the depolarized state of about −58 mV. From there, increasing [K⁺]_o takes the potential along the curve until LP2 is reached. LP2 is the starting point for the repolarization. The curves of LP1 and LP2 meet in the cusp point, CP. The region inside the cusp (bounded by LP1, LP2, and CP) is bi-stable. In contrast omega pore conductances larger than 18 μS/cm² result in gradual depolarization without bi-stability. The model reveals that an omega pore shifts LP1 and LP2 (the cusp) to the right, i.e., less severe hypokalemia is required to shift cells from the P1- to the P2-state. The membrane potentials yielded by the computer simulation were compared with values measured for muscle fibers by use of microelectrodes: open symbols stand for human controls (−83 ± 5 mV in 95% of fibers at 4 mM K⁺, −99 ± 3 mV for 87% at 1.5 mM K⁺, and −58 mV for 91% at 1 mM K⁺); filled symbols for HypoPP patients with either Ca_v1.1-R1239H (circle: −74 ± 5 mV in 76% of fibers at 4 mM K⁺) or Ca_v1.1-R528H (triangle: −75 ± 5 mV in 91% of fibers at 4 mM K⁺); at 1.5 mM K⁺, 95% of the patients’ fibers −56 mV (square; n = 127). Modified after Jurkat-Rott et al. (2010).

Figure 5

¹H and ²³Na measurements in the calf muscles of HypoPP patients. (A–F) T2-weighted STIR ¹H (left) and ²³Na-MR images (right) from a healthy control (A,B) and the propositus of a HypoPP family, a 37-year-old female harboring the Cav1.1-R1239H mutation (C–F). The images in (C,D) were taken before treatment and the images in (E,F) were taken after treatment with 250 mg/d acetazolamide for 4 weeks. Note the very high proton intensities in STIR (C) and the elevated Na⁺ concentration before treatment (D), arrows pointing at highest Na signal intensities) and their improvement after treatment. The central reference contains 0.3% NaCl solution; occasional side tubes containing 0.3% NaCl in 1% agarose (left) and 0.6% NaCl in H₂O (right) were additional standards. (G–I) Axial T1-weighted MR images from family members patients: the female’s 80-year-old grandmother whose limb muscles were almost completely replaced with fat (G), the female’s 35-year-old sister (I), and the 55-year-old uncle (H,G). Modified after Jurkat-Rott et al. (2009).