Effects of analgesics and antidepressants on TREK-2 and TRESK currents

Mini-Review: Two Brothers in Crime - The Interplay of TRESK and TREK in Human Diseases

TWIK-related spinal cord potassium (TRESK) and TWIK-related potassium (TREK) channels are both subfamilies of the two-pore domain potassium (K2P) channel group. Despite major structural, pharmacological, as well as biophysical differences, emerging data suggest that channels of these two subfamilies are functionally more closely related than previously assumed. Recent studies, for instance, indicate an assembling of TRESK and TREK subunits, leading to the formation of heterodimeric channels with different functional properties compared to homodimeric ones. Formation of tandems consisting of TRESK and TREK subunits might thus multiply the functional diversity of both TRESK and TREK activity. Based on the involvement of these channels in the pathophysiology of migraine, we here highlight the role as well as the impact of the interplay of TRESK and TREK subunits in the context of different disease settings. In this regard, we focus on their involvement in migraine and pain syndromes, as well as on their influence on (neuro-)inflammatory processes. Furthermore, we describe the potential implications for innovative therapeutic strategies that take advantage of TRESK and TREK modulation as well as obstacles encountered in the development of therapies related to the aforementioned diseases.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K_2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K_2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K⁺ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K_2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

Block of TREK and TRESK K2P channels by lamotrigine and two derivatives sipatrigine and CEN-092

TREK and TRESK K2P channels are widely expressed in the nervous system, particularly in sensory neurons, where they regulate neuronal excitability. In this study, using whole-cell patch-clamp electrophysiology, we characterise the inhibitory effect of the anticonvulsant lamotrigine and two derivatives, sipatrigine and 3,5-diamino-6-(3,5-bistrifluoromethylphenyl)-1,2,4-triazine (CEN-092) on these channels.

Sipatrigine was found to be a more effective inhibitor than lamotrigine of TREK-1, TREK-2 and TRESK channels. Sipatrigine was slightly more potent on TREK-1 channels (EC₅₀ = 16 μM) than TRESK (EC₅₀ = 34 μM) whereas lamotrigine was equally effective on TREK-1 and TRESK. Sipatrigine was less effective on a short isoform of TREK-2, suggesting the N terminus of the channel is important for both inhibition and subsequent over-recovery. Inhibition of TREK-1 and TREK-2 channels by sipatrigine was reduced by mutation of a leucine residue associated with the norfluoxetine binding site on these channels (L289A and L320A on TREK-1 and TREK-2, respectively) but these did not affect inhibition by lamotrigine. Inhibition of TRESK by sipatrigine and lamotrigine was attenuated by mutation of bulky phenylalanine residues (F145A and F352A) in the inner pore helix. However, phosphorylation mutations did not alter the effect of sipatrigine. CEN-092 was a more effective inhibitor of TRESK channels than TREK-1 channels.

It is concluded that lamotrigine, sipatrigine and CEN-092 are all inhibitors of TREK and TRESK channels but do not greatly discriminate between them. The actions of these compounds may contribute to their current and potential use in the treatment of pain and depression.

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Lamotrigine blocks TREK and TRESK potassium channels at clinical concentrations.

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Sipatrigine is more effective than lamotrigine at blocking TREK and TRESK channels.

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Mutation of norfluoxetine binding site on TREK channels attenuates sipatrigine block.

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Residues in the inner pore region of TRESK channels regulate sipatrigine block.

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The novel lamotrigine derivative, CEN-092, blocks TRESK channels.

Higher expression of KCNK10 (TREK-2) K+ channels and their functional upregulation by lipopolysaccharide treatment in mouse peritoneal B1a cells

Innate-like CD5⁺ B1a cells localized in serous cavities are activated by innate stimuli, such as lipopolysaccharide (LPS), leading to T cell-independent antibody responses. Although ion channels play crucial roles in the homeostasis and activation of immune cells, the electrophysiological properties of B1a cells have not been investigated to date. Previously, in the mouse B cell lymphoma cells, we found that the voltage-independent two-pore-domain potassium (K2P) channels generate a negative membrane potential and drive Ca²⁺ influx. Here, we newly compared the expression and activities of K2P channels in mouse splenic follicular B (FoB), marginal zone B (MZB), and peritoneal B1a cells. Next-generation sequencing analysis showed higher levels of transcripts for TREK-2 and TWIK-2 in B1a cells than those in FoB or MZB cells. Electrophysiological analysis, using patch clamp technique, revealed higher activity of TREK-2 with the characteristic large unitary conductance (~ 250 pS) in B1a than that in FoB or MZB cells. TREK-2 activity was further increased by LPS treatment (>2 h), which was more prominent in B1a than that in MZB or FoB cells. The cytosolic Ca²⁺ concentration of B cells was decreased by high-K⁺-induced depolarization (ΔR_KCl (%)), suggesting the basal Ca²⁺ influx to be driven by negative membrane potential. The LPS treatment significantly increased the ΔR_KCl (%) in B1a, though not in FoB and MZB cells. Our study was the first to compare the K2P channels in mouse primary B cell subsets, elucidating the functional upregulation of TREK-2 and augmentation of Ca²⁺ influx by the stimulation of Toll-like receptor 4 in B1a cells.

Psychopharmacology of chronic pain

Chronic pain is a frequent condition that affects an estimated 20% of people worldwide, accounting for 15%-20% of doctors' appointments (Treede et al., 2015). It lacks the acute warning function of physiologic nociception, and instead involves the activation of multiple neurophysiologic mechanisms in the somatosensory system, a complex neuronal network under the control of powerful autoregulatory loops and able to undergo rapid neuroplastic alteration (Verdu et al., 2008). There is a growing body of research suggesting that some such pathways are shared by major psychologic disorders such as depression and anxiety, opening new avenues in co-treatment strategies. In particular, besides anticonvulsants, which are today used as analgesics, other psychopharmaceuticals, such as the tricyclic antidepressants, are displaying efficacy in the treatment of neuropathic and nociceptive chronic pain. The state of the art regarding the mechanisms of nociception and the pharmacology of both the neurotransmitters involved and the wide range of psychoactive compounds that may be useful in the treatment of chronic pain are discussed.

Tandem pore TWIK-related potassium channels and neuroprotection

TWIK-related potassium channels (TREK) belong to a subfamily of the two-pore domain potassium channels family with three members, TREK1, TREK2 and TWIK-related arachidonic acid-activated potassium channels. The two-pore domain potassium channels is the last big family of channels being discovered, therefore it is not surprising that most of the information we know about TREK channels predominantly comes from the study of heterologously expressed channels. Notwithstanding, in this review we pay special attention to the limited amount of information available on native TREK-like channels and real neurons in relation to neuroprotection. Mainly we focus on the role of free fatty acids, lysophospholipids and other neuroprotective agents like riluzole in the modulation of TREK channels, emphasizing on how important this modulation may be for the development of new therapies against neuropathic pain, depression, schizophrenia, epilepsy, ischemia and cardiac complications.

TREK-2 Mediates the Neuroprotective Effect of Isoflurane Preconditioning Against Acute Cerebral Ischemia in the Rat

It is known that preconditional treatment with volatile anesthetics can induce tolerance of the brain to stroke. A previous study demonstrated that the involvement of TREK-1, a two-pore domain K+ channel, in sevoflurane preconditioning induced neuroprotection against focal cerebral ischemia in rats. The present study testified whether TREK-2, another anesthetic-target K+ channel, is also associated with volatile anesthetic-induced neuroprotection, and further explored its potential mechanism. Rats preconditioned with isoflurane were subjected to 1.4vol% isoflurane plus 98% O₂ (1.5 L/min) inhalation for 1 hour daily and continuing for 5 consecutive days. Then, these rats were subjected to middle cerebral artery occlusion (MCAO) as focal cerebral ischemia model. The expression of TWIK-related K+ channel 2 (TREK-2) was analyzed by western blotting and quantitative real-time RT-PCR, and its downstream signaling molecules, protein kinase C (PKC) alpha, extracellular signal-regulated kinase 1/2 (ERK1/2), and pERK1/2 were detected by western blotting also. Subsequently, the expression of TREK-2 was regulated by siRNA transfection in the brain to clarify its role in the neuroprotection of isoflurane preconditioning. Neurological scores, infarction volume, and TdT-mediated dUTP Nick-End Labeling (TUNEL) staining were examined to evaluate the outcomes. The impact of TREK-2 on the expression of its downstream signaling molecules was also examined for preliminary analysis of the possible mechanisms. Isoflurane preconditioning reduced the infarct volume, inhibited the cell apoptosis, and improved the neurological outcome in rats subjected to MCAO. These effects were parallel with the increase in TREK-2 protein and inhibition of the ERK1/2 phosphorylation. The downregulation of TREK-2 through siRNA could significantly attenuate the isoflurane preconditioning-induced neuroprotective effects. Isoflurane preconditioning-induced neuroprotective effects against ischemia-reperfusion injury are associated with the increase in TREK-2 channel activation. These effects depend on the attenuation of PKC alpha and inhibition of ERK1/2 phosphorylation. Results enrich our understanding on the mechanism of two-pore domain K+ channel in preconditioning-induced tolerance to focal cerebral ischemia.

Identification of critical amino acids in the proximal C-terminal of TREK-2 K+ channel for activation by acidic pHi and ATP-dependent inhibition

TWIK-related two-pore domain K⁺ channels (TREKs) are regulated by intracellular pH (pH_i) and Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). Previously, Glu³⁰⁶ in proximal C-terminal (pCt) of mouse TREK-1 was identified as the pH_i-sensing residue. The direction of PI(4,5)P₂ sensitivity is controversial, and we have recently shown that TREKs are inhibited by intracellular ATP via endogenous PI(4,5)P₂ formation. Here we investigate the anionic and cationic residues of pCt for the pH_i and ATP-sensitivity in human TREK-2 (hTREK-2). In inside-out patch clamp recordings (I_TREK-2,i-o), acidic pH_i-induced activation was absent in E332A and was partly attenuated in E335A. Neutralization of cationic Lys (K330A) also eliminated the acidic pH_i sensitivity of I_TREK-2,i-o. Unlike the inhibition of wild-type (WT) I_TREK-2,i-o by intracellular ATP, neither E332A nor K330A was sensitive to ATP. Nevertheless, exogenous PI(4,5)P₂ (10 μM) abolished I_{TREK-2 i-o} in all the above mutants as well as in WT, indicating unspecific inhibition by exogenous PI(4,5)P₂. In whole-cell recordings of TREK-2 (I_TREK-2,w-c), K330A and E332A showed higher or fully active basal activity, showing attenuated or insignificant activation by 2-APB, arachidonic acid, or acidic pH_e 6.9. I_TREK-1,w-c of WT is largely suppressed by pH_e 6.9, and the inhibition is slightly attenuated in K312A and E315A. The results show concerted roles of the oppositely charged Lys and Glu in pCt for the ATP-dependent low basal activity and pH_i sensitivity.

Selective and state-dependent activation of TRESK (K2P 18.1) background potassium channel by cloxyquin

BACKGROUND AND PURPOSE

Cloxyquin (5-cloroquinolin-8-ol) has been described as an activator of TRESK (K_2P 18.1, TWIK-related spinal cord K⁺ channel) background potassium channel. We have examined the specificity of the drug by testing several K_2P channels. We have investigated the mechanism of cloxyquin-mediated TRESK activation, focusing on the differences between the physiologically relevant regulatory states of the channel.

EXPERIMENTAL APPROACH

Potassium currents were measured by two-electrode voltage clamp in Xenopus oocytes and by whole-cell patch clamp in mouse dorsal root ganglion (DRG) neurons.

KEY RESULTS

Cloxyquin (100 µM) activated mouse and human TRESK 4.4 ± 0.3 (n = 28) and 3.9 ± 0.3-fold (n = 8), respectively. The drug selectively targeted TRESK in the K_2P channel family and exerted state-dependent effects. TRESK was potently activated by cloxyquin in the resting state. However, after robust activation of the current by the calcium signal, evoked by stimulation of Gq-coupled receptors, the compound did not influence mouse TRESK and only slightly affected the human channel. The constitutively active mutant channels, mimicking the dephosphorylated state (S276A) or containing altered channel pore (F156A and F364A), were not further stimulated by cloxyquin. In a subpopulation of isolated DRG neurons, cloxyquin substantially activated the background potassium current.

CONCLUSIONS AND IMPLICATIONS

Cloxyquin activates TRESK by a Ca²⁺ /calcineurin-independent mechanism. The drug is specific for TRESK within the K_2P channel family and useful for studying TRESK currents in native cells. The state-dependent pharmacological profile of this channel should be considered in the development of therapeutics for migraine and other nociceptive disorders.