N-linked glycosylation determines cell surface expression of two-pore-domain K+ channel TRESK

Chloroform is a potent activator of cardiac and neuronal Kir3 channels

Chloroform has been used over decades in anesthesia before it was replaced by other volatile anesthetics like halothane or sevoflurane. Some of the reasons were inadmissible side effects of chloroform like bradycardia or neural illness. In the present study, we identified members of the G protein-activated inwardly rectifying potassium channel family (Kir3) expressed in Xenopus oocytes as potential common molecular targets for both the neural and cardiac effects of chloroform. Millimolar concentration currents representing a 1:10000 dilution of commercially available chloroform were used in laboratories that augment neuronal Kir3.1/3.2 currents as well as cardiac Kir3.1/3.4. This effect was selective and only observed in currents from Kir3 subunits but not in currents from Kir2 subunits. Augmentation of atrial Kir3.1/3.4 currents leads to an effective drop of the heart rate and a reduction in contraction force in isolated mouse atria.

N-Glycosylation of TREK-1/hK2P2.1 Two-Pore-Domain Potassium (K2P) Channels

Mechanosensitive hTREK-1 two-pore-domain potassium (hK_2P2.1) channels give rise to background currents that control cellular excitability. Recently, TREK-1 currents have been linked to the regulation of cardiac rhythm as well as to hypertrophy and fibrosis. Even though the pharmacological and biophysical characteristics of hTREK-1 channels have been widely studied, relatively little is known about their posttranslational modifications. This study aimed to evaluate whether hTREK-1 channels are N-glycosylated and whether glycosylation may affect channel functionality. Following pharmacological inhibition of N-glycosylation, enzymatic digestion or mutagenesis, immunoblots of Xenopus laevis oocytes and HEK-293T cell lysates were used to assess electrophoretic mobility. Two-electrode voltage clamp measurements were employed to study channel function. TREK-1 channel subunits undergo N-glycosylation at asparagine residues 110 and 134. The presence of sugar moieties at these two sites increases channel function. Detection of glycosylation-deficient mutant channels in surface fractions and recordings of macroscopic potassium currents mediated by these subunits demonstrated that nonglycosylated hTREK-1 channel subunits are able to reach the cell surface in general but with seemingly reduced efficiency compared to glycosylated subunits. These findings extend our understanding of the regulation of hTREK-1 currents by posttranslational modifications and provide novel insights into how altered ion channel glycosylation may promote arrhythmogenesis.

N-glycosylation-dependent regulation of hK2P17.1 currents

Two pore-domain potassium (K_2P) channels mediate potassium background currents that stabilize the resting membrane potential and facilitate action potential repolarization. In the human heart, hK_2P17.1 channels are predominantly expressed in the atria and Purkinje cells. Reduced atrial hK_2P17.1 protein levels were described in patients with atrial fibrillation or heart failure. Genetic alterations in hK_2P17.1 were associated with cardiac conduction disorders. Little is known about posttranslational modifications of hK_2P17.1. Here, we characterized glycosylation of hK_2P17.1 and investigated how glycosylation alters its surface expression and activity. Wild-type hK_2P17.1 channels and channels lacking specific glycosylation sites were expressed in Xenopus laevis oocytes, HEK-293T cells, and HeLa cells. N-glycosylation was disrupted using N-glycosidase F and tunicamycin. hK_2P17.1 expression and activity were assessed using immunoblot analysis and a two-electrode voltage clamp technique. Channel subunits of hK_2P17.1 harbor two functional N-glycosylation sites at positions N65 and N94. In hemi-glycosylated hK_2P17.1 channels, functionality and membrane trafficking remain preserved. Disruption of both N-glycosylation sites results in loss of hK_2P17.1 currents, presumably caused by impaired surface expression. This study confirms diglycosylation of hK_2P17.1 channel subunits and its pivotal role in cell-surface targeting. Our findings underline the functional relevance of N-glycosylation in biogenesis and membrane trafficking of ion channels.

Activation of TRESK channels by the inflammatory mediator lysophosphatidic acid balances nociceptive signalling

In dorsal root ganglia (DRG) neurons TRESK channels constitute a major current component of the standing outward current IK_SO. A prominent physiological role of TRESK has been attributed to pain sensation. During inflammation mediators of pain e.g. lysophosphatidic acid (LPA) are released and modulate nociception. We demonstrate co-expression of TRESK and LPA receptors in DRG neurons. Heterologous expression of TRESK and LPA receptors in Xenopus oocytes revealed augmentation of basal K⁺ currents upon LPA application. In DRG neurons nociception can result from TRPV1 activation by capsaicin or LPA. Upon co-expression in Xenopus oocytes LPA simultaneously increased both depolarising TRPV1 and hyperpolarising TRESK currents. Patch-clamp recordings in cultured DRG neurons from TRESK[wt] mice displayed increased IK_SO after application of LPA whereas under these conditions IK_SO in neurons from TRESK[ko] mice remained unaltered. Under current-clamp conditions LPA application differentially modulated excitability in these genotypes upon depolarising pulses. Spike frequency was attenuated in TRESK[wt] neurons and, in contrast, augmented in TRESK[ko] neurons. Accordingly, excitation of nociceptive neurons by LPA is balanced by co-activation of TRESK channels. Hence excitation of sensory neurons is strongly controlled by the activity of TRESK channels, which therefore are good candidates for the treatment of pain disorders.

Amplified cold transduction in native nociceptors by M-channel inhibition

Topically applied camphor elicits a sensation of cool, but nothing is known about how it affects cold temperature sensing. We found that camphor sensitizes a subpopulation of menthol-sensitive native cutaneous nociceptors in the mouse to cold, but desensitizes and partially blocks heterologously expressed TRPM8 (transient receptor potential cation channel subfamily M member 8). In contrast, camphor reduces potassium outward currents in cultured sensory neurons and, in cold nociceptors, the cold-sensitizing effects of camphor and menthol are additive. Using a membrane potential dye-based screening assay and heterologously expressed potassium channels, we found that the effects of camphor are mediated by inhibition of Kv7.2/3 channels subtypes that generate the M-current in neurons. In line with this finding, the specific M-current blocker XE991 reproduced the cold-sensitizing effect of camphor in nociceptors. However, the M-channel blocking effects of XE991 and camphor are not sufficient to initiate cold transduction but require a cold-activated inward current generated by TRPM8. The cold-sensitizing effects of XE991 and camphor are largest in high-threshold cold nociceptors. Low-threshold corneal cold thermoreceptors that express high levels of TRPM8 and lack potassium channels are not affected by camphor. We also found that menthol--like camphor--potently inhibits Kv7.2/3 channels. The apparent functional synergism arising from TRPM8 activation and M-current block can improve the effectiveness of topical coolants and cooling lotions, and may also enhance TRPM8-mediated analgesia.

Functional analysis of a migraine-associated TRESK K+ channel mutation

Recent genetic and functional studies suggest that migraine may result from abnormal activities of ion channels and transporters. A frameshift mutation in the human TWIK-related spinal cord K(+) (TRESK) channel has been identified in migraine with aura patients in a large pedigree. In Xenopus oocytes, mutant TRESK subunits exert a dominant-negative effect on whole-cell TRESK currents. However, questions remain as to whether and how mutant TRESK subunits affect the membrane properties and the excitability of neurons in the migraine circuit. Here, we investigated the functional consequences of the mutant TRESK subunits in HEK293T cells and mouse trigeminal ganglion (TG) neurons. First, we found that mutant TRESK subunits exhibited dominant-negative effects not only on the size of the whole-cell TRESK currents, but also on the level of TRESK channels on the plasma membrane in HEK293T cells. This likely resulted from the heterodimerization of wild-type and mutant TRESK subunits. Next, we expressed mutant TRESK subunits in cultured TG neurons and observed a significant decrease in the lamotrigine-sensitive K(+) current, suggesting that the mutant TRESK subunits have a dominant-negative effect on currents through the endogenous TRESK channels. Current-clamp recordings showed that neurons expressing mutant TRESK subunits had a higher input resistance, a lower current threshold for action potential initiation, and a higher spike frequency in response to suprathreshold stimuli, indicating that the mutation resulted in hyperexcitability of TG neurons. Our results suggest a possible mechanism through which the TRESK mutation increases the susceptibility of migraine headache.

Genes and primary headaches: discovering new potential therapeutic targets

Genetic studies have clearly shown that primary headaches (migraine, tension-type headache and cluster headache) are multifactorial disorders characterized by a complex interaction between different genes and environmental factors. Genetic association studies have highlighted a potential role in the etiopathogenesis of these disorders for several genes related to vascular, neuronal and neuroendocrine functions. A potential role as a therapeutic target is now emerging for some of these genes. The main purpose of this review is to describe new advances in our knowledge regarding the role of MTHFR, KCNK18, TRPV1, TRPV3 and HCRTR genes in primary headache disorders. Involvement of these genes in primary headaches, as well as their potential role in the therapy of these disorders, will be discussed.

TRESK potassium channel in human T lymphoblasts

TRESK (TWIK-related spinal cord K(+)) channel, encoded by KCNK18 gene, belongs to the double-pore domain K(+) channel family and in normal conditions is expressed predominantly in the central nervous system. In our previous patch-clamp study on Jurkat T lymphoblasts we have characterized highly selective K(+) channel with pharmacological profile identical to TRESK. In the present work, the presence of KCNK18 mRNA was confirmed in T lymphoblastic cell lines (Jurkat, JCaM, H9) but not in resting peripheral blood lymphocytes of healthy donors. Positive immunostaining for TRESK was demonstrated in lymphoblastic cell lines, in germinal centers of non-tumoral lymph nodes, and in clinical samples of T acute lymphoblastic leukemias/lymphomas. Besides detection in the plasma membrane, intracellular TRESK localization was also revealed. Possible involvement of TRESK channel in lymphocyte proliferation and tumorigenesis is discussed.

N-glycosylation-dependent control of functional expression of background potassium channels K2P3.1 and K2P9.1

Two-pore domain potassium (K(2P)) channels play fundamental roles in cellular processes by enabling a constitutive leak of potassium from cells in which they are expressed, thus influencing cellular membrane potential and activity. Hence, regulation of these channels is of critical importance to cellular function. A key regulatory mechanism of K(2P) channels is the control of their cell surface expression. Membrane protein delivery to and retrieval from the cell surface is controlled by their passage through the secretory and endocytic pathways, and post-translational modifications regulate their progression through these pathways. All but one of the K(2P) channels possess consensus N-linked glycosylation sites, and here we demonstrate that the conserved putative N-glycosylation site in K(2P)3.1 and K(2P)9.1 is a glycan acceptor site. Patch clamp analysis revealed that disruption of channel glycosylation reduced K(2P)3.1 current, and flow cytometry was instrumental in attributing this to a decreased number of channels on the cell surface. Similar findings were observed when cells were cultured in reduced glucose concentrations. Disruption of N-linked glycosylation has less of an effect on K(2P)9.1, with a small reduction in number of channels on the surface observed, but no functional implications detected. Because nonglycosylated channels appear to pass through the secretory pathway in a manner comparable with glycosylated channels, the evidence presented here suggests that the decreased number of nonglycosylated K(2P)3.1 channels on the cell surface may be due to their decreased stability.

TRESK background K(+) channel is inhibited by phosphorylation via two distinct pathways

The two-pore domain K(+) channel, TRESK (TWIK-related spinal cord K(+) channel, KCNK18) is directly regulated by the calcium/calmodulin-dependent phosphatase calcineurin and 14-3-3 adaptor proteins. The calcium signal robustly activates the channel via calcineurin, whereas the anchoring of 14-3-3 interferes with the return of the current to the resting state after the activation in Xenopus oocytes. In the present study, we report that the phosphorylation of TRESK at two distinct regulatory regions, the 14-3-3 binding site (Ser-264) and the cluster of three adjacent serine residues (Ser-274, Ser-276, and Ser-279), are responsible for channel inhibition. The phosphorylation of Ser-264 by protein kinase A accelerated the return of the current of S276E mutant TRESK to the resting state after the calcineurin-dependent activation. In the presence of 14-3-3, the basal current of the S276E mutant was reduced, and its calcineurin-dependent activation was augmented, suggesting that the direct binding of the adaptor protein to TRESK contributed to the basal inhibition of the channel under resting conditions. Unexpectedly, we found that 14-3-3 impeded the recovery of the current of S264E mutant TRESK to the resting state after the calcineurin-dependent activation, despite of the mutated 14-3-3 binding site. This suggests that 14-3-3 inhibited the kinase phosphorylating the regulatory cluster of Ser-274, Ser-276, and Ser-279, independently of the direct interaction between TRESK and 14-3-3. In conclusion, two distinct inhibitory kinase pathways converge on TRESK, and their effect on the calcineurin-dependent regulation is differentially modulated by the functional availability of 14-3-3.