Alpha and omega in potassium-channel opening
Fredrik Elinder
The self-archived postprint version of this journal article is available at Linköping
University Institutional Repository (DiVA):
http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-154318
N.B.: When citing this work, cite the original publication.
Elinder, F., (2019), Alpha and omega in potassium-channel opening, Acta Physiologica, 225(2), e13240. https://doi.org/10.1111/apha.13240
Original publication available at:
https://doi.org/10.1111/apha.13240
Copyright: Wiley (12 months)
Alpha and omega in potassium-channel opening Fredrik Elinder
Department of Clinical and Experimental Medicine Linköping University
Sweden
fredrik.elinder@liu.se
In this issue of Acta Physiologica, a study from the laboratory of Peter Larsson shows that the distance between the carboxyl group and the first double bond of polyunsaturated fatty acids
(PUFAs) is critical for the affinity of the PUFA molecule to a voltage-gated potassium channel found in the heart.1 A shorter distance increases the affinity, and the bound PUFA alters the channel’s voltage dependence and thereby opens the channel. The efficacy, that is the channel-opening effect of a bound PUFA molecule, partly depends on the distance between the last double bond and the end of the hydrophobic tail. A shorter distance decreases the channel-opening effect.
PUFAs play numerous important roles in the human body, such as being building blocks of the nervous system, and key components in inflammatory and neural signaling.2 They have also been reported to directly affect voltage-gated ion channels and thereby electrical excitability of the cardiac and nervous systems, but the exact mechanism of action is not known.3
PUFAs have a charged carboxyl group in one end of the molecule and a long hydrophobic hydrocarbon tail with at least two double bonds in the other end (Fig. 1A). Docosahexaenoic and arachidonic acids are two physiologically important PUFAs that differ in several aspects from a molecular point of view: (i) the number of carbons (22 vs. 20), (ii) the number of double bonds (6 vs. 4), (iii) the distance between the carboxyl group in the α end and the first double bond (4 vs. 5), and (iv) the distance between the ω end and the last double bond (3 vs. 6). These two distances will be referred to as the α distance and the ω distance respectively. (Note that these distances correspond to the number of carbons, either from the α end or the ω end, despite the fact that the α carbon is the second carbon from the α end; Fig. 1A).
PUFAs affect voltage-gated ion channels by altering the voltage dependence (V0.5) and the maximum conductance (Gmax, Fig. 1B) as well as opening, closing and inactivation kinetics.3 The effects in different types of voltage-gated potassium channels depends on the electric charge of the carboxyl group.4-7 A free PUFA incorporates its hydrophobic tail into the lipid bilayer, and if the fatty acid is close to an ion channel, the negatively charged carboxyl group will electrostatically attract the positively charged voltage sensor of an ion channel to open the ion-conducting pore (Fig. 1C). This mechanism has been referred to as the lipoelectric mechanism.4
Which molecular properties of the PUFAs are important for the effect on ion channel gating? In addition to the negative charge of the carboxyl group, at least two double bonds in cis configuration are required to activate the voltage sensor of voltage-gated ion channels.3 However, the roles of the length of the carbon tail, and the number and location of the double bonds are not known. A general impression is that neither the length of the carbon chain, nor the number or positions of the double bonds have been regarded to be important for the effect, as long as there are at least two double bonds.3
Now Bohannon et al.1 has challenged this view and systematically explored 16 different PUFAs on the slowly activating potassium (IKs) channel, which is important for the repolarization of the cardiac action potential, expressed in Xenopus oocytes. The length of the hydrophobic tail varied from 14 to 22 carbons, the number of double bonds from 2 to 6, the α distance from 4 to 13 carbons, and the ω distance from 3 to 9 carbons. To explore the effect of the PUFAs they focused on three parameters: (1) the maximum conductance, at positive membrane voltages (Gmax), (2) the current (or
conductance) at 0 mV (I0 (or G0)), and (3) the midpoint of the conductance-versus-voltage curve (V0.5) (Fig. 1B). The effect on I0 depends on the effects on both Gmax and V0.5, but it was studied itself because it is a physiologically relevant parameter – the outward current during the long
depolarization phase of the cardiac action potential.
For each compound, Bohannon et al. determined the affinity (the Km-value in their article) and the efficacy (the extrapolated max effect at infinitely high concentrations), both of which are important for the effect, for V0.5, Gmax, and I0. Based on these data they searched for correlations between the effects and the molecular characteristics of the PUFAs. From the data presented in the article, there is a strong correlation between the α distance and Km for V0.5 and I0 but not Gmax. There is also a correlation between the ω distance and the maximum effect (the efficacy) for V0.5 and I0, but not Gmax. There is no correlation between the number of double bonds and any of the studied effects, and only a weak correlation between the length of the carbon chain and Gmax. However, a problem with these correlations is that there is a clear interrelation between them:
C = α + ω + 3∙db – 2,
where C is the number of carbons, α is the α distance, ω is the ω distance, and db is the number of double bonds. Thus if one parameter is changed, at least one other will automatically change. To circumvent this obstacle, Bohannon et al. tested some PUFAs where the inter-double bond distance was changed (the equation above is only valid for PUFAs with hydrocarbon tails with three carbons between each pair of double bonds). Furthermore, they also performed multivariate regression and hierarchical cluster analyses. The data emerging from these analyses suggests that the α distance is critical for the affinity of the PUFAs (Km) to affect the voltage dependence (V0.5; Fig. 1D) and
consequently the current at 0 mV (I0). The number of double bonds, the number of carbons of the tail, and the ω distance (Fig. 1E) are not important for the affinity. However, the ω distance plays a role for the efficacy of PUFAs on V0.5; a longer ω distance increases the shift in the channel’s voltage dependence. This might explain why ω-6 but not ω-3 PUFAs decrease the risk of cardiovascular disease and death.1
The study by Bohannon et al. does not provide a molecular explanation for the increased affinity of compounds with a short α distance and an increased efficacy for compounds with a long ω distance, but one can speculate that while it is problematic for a stiff saturated tail with no double bonds to bind tightly to the channel and allow the carboxyl group to come close to the voltage sensor, it is easier for a more flexible tail with double bonds. This study suggests that not only general flexibility caused by two double bonds is needed, but also that the closer this flexibility reaches to the carboxyl group the tighter is the bonding (Fig. 1F).
The findings described by Bohannon et al. can be important in future design of fatty acids or other lipoelectric compounds8,9 to open specific ion channels to act as pharmaceutical drugs against hyperexcitability diseases such as cardiac arrhythmia.
Conflict of interest
I declare no conflict of interest. References
1. Bohannon BM, Perez ME, Liin SI, Larsson HP. ω-6 and ω-9 polyunsaturated fatty acids with double bonds near the carboxyl head have the highest affinity and largest effects on the cardiac IKs potassium channel. Acta Physiol (Oxf). 2018;e13186.
https://doi.org/10.1111/apha.13186
2. Di Marzo V. New approaches and challenges to targeting the endocannabinoid system. Nat Rev Drug Discov. 2018;17(9):623-639.
3. Elinder F, Liin SI. Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels. Front Physiol. 2017;8:43.
4. Börjesson SI, Hammarstrom S, Elinder F. Lipoelectric modification of ion channel voltage gating by polyunsaturated fatty acids. Biophys J. 2008;95(5):2242-2253.
5. Liin SI, Karlsson U, Bentzen BH, Schmitt N, Elinder F. Polyunsaturated fatty acids are potent openers of human M-channels expressed in Xenopus laevis oocytes. Acta Physiol (Oxf). 2016;218(1):28-37.
6. Liin SI, Silverå Ejneby M, Barro-Soria R, et al. Polyunsaturated fatty acid analogs act
antiarrhythmically on the cardiac IKs channel. Proc Natl Acad Sci U S A. 2015;112(18):5714-5719.
7. Liin SI, Yazdi S, Ramentol R, Barro-Soria R, Larsson HP. Mechanisms Underlying the Dual Effect of Polyunsaturated Fatty Acid Analogs on Kv7.1. Cell Rep. 2018;24(11):2908-2918. 8. Salari S, Silverå Ejneby M, Brask J, Elinder F. Isopimaric acid - a multi-targeting ion channel
modulator reducing excitability and arrhythmicity in a spontaneously beating mouse atrial cell line. Acta Physiol (Oxf). 2018;222(1):e12895.
9. Silverå Ejneby M, Wu X, Ottosson NE, et al. Atom-by-atom tuning of the electrostatic potassium-channel modulator dehydroabietic acid. J Gen Physiol. 2018;150(5):731-750.
Figure 1. Polyunsaturated fatty acids open voltage-gated ion channels. (A) Two examples of
physiologically important PUFAs. The α distance is the number of carbons from the carboxyl group, in the left end, to the first double bond in the carbon chain, 4 and 5 respectively. (Note that the α carbon is the second carbon). The ω distance is the number of carbons from the tail, in the right end, to the last double bond in the carbon chain, 3 and 6 respectively. (B) PUFAs shift the conductance-versus-voltage curve along the voltage axis and increase the amplitude. The shift is measured as the alteration in the midpoint of the curves, V0.5. The increase in maximum conductance, Gmax, is
measured at positive voltages where the curves saturate. The increase in conductance (or current) at 0 mV, G0, is a consequence of the effects on the two other parameters. (C) A cartoon of a voltage-gated ion channel. A PUFA (in red) is incorporated into the lipid bilayer close to an ion channel. The negative charge of the PUFA attracts the positively charged voltage sensor (in blue), which in turn pulls the intracellular gate open and allows positively charged K+ ions to pass through the channel from the intracellular (i.c.) side to the extracellular (e.c.) side. (D) The affinity (Km) of all investigated ω6 PUFAs with 3-4 double bonds closely depends on the α distance; the shorter α distance, the higher affinity (lower binding constant). The explore PUFA stretch is marked by a double-headed arrow. (E) The affinity (Km) of all investigated PUFAs with an α distance of 5 does not depend on the ω distance. The explored PUFA stretch is marked by a double-headed arrow. (F) Tentative binding poses of three different PUFAs. Left: Saturated fatty acid with no double bond has low or no affinity to the voltage sensor. Middle: PUFAs with double bonds far away from the carboxyl group (i.e. long α distance) has an intermediate affinity to the voltage sensor. Right: PUFAs with double bonds close to the carboxyl group (i.e. short α distance) has a high affinity to the voltage sensor.
