TASK channels: channelopathies, trafficking, and receptor-mediated inhibition

TWIK-related acid-sensitive potassium channels TASK-1 and TASK-3 may participate in the process of the coexistence of asthma and OSA

OBJECTIVE

To investigate the role of TWIK-related acid-sensitive potassium channels TASK-1 and TASK-3 in the mechanism of asthma combined with obstructive sleep apnea (OSA) in mice.

METHOD

C57BL/6 mice were randomly divided into four groups: control group (NS-RA), asthma group (OVA-RA), OSA group (NS-IH), and asthma combined with OSA group (OVA-IH). After monitoring lung function in each group, the expression levels of TASK-1 and TASK-3 mRNA and protein in lung tissues were measured, and the correlation between the changes of both and lung function was analyzed.

RESULTS

A total of 64 male mice were studied. Penh, serum IgE concentrations, and the percentage of eosinophils in bronchoalveolar lavage fluid (BALF) were higher in OVA-RA and OVA-IH mice compared with NS-RA (P < 0.05),while the above indexes were slightly elevated in NS-IH mice compared with NS-RA (P > 0.05), where the Penh and the percentage of eosinophils in BALF was higher in OVA-IH mice than NS-IH (P < 0.05).Increased TASK-3 mRNA expression (P < 0.05) as well as TASK-1 and TASK-3 protein expression (P > 0.05) in lung tissues of OVA-RA and NS-IH mice compared with NS-RA, and TASK-3 mRNA expression was slightly more in the OVA-IH group compared with NS-RA (P > 0.05), but less compared with OVA-RA (P < 0.05) or NS-IH (P > 0.05), while TASK-1 and TASK-3 protein expression was increased in the OVA-IH group compared with the remaining three groups, and TASK-3 protein expression was associated with lung function impairment was positively correlated with the degree of lung function impairment (P < 0.05).

CONCLUSION

Task-1 and Task-3 may be involved in the pathogenesis of asthma with OSA by affecting lung function.

Pathophysiology and pathogenic mechanisms of pulmonary hypertension: role of membrane receptors, ion channels, and Ca2+ signaling

The pulmonary circulation is a low-resistance, low-pressure, and high-compliance system that allows the lungs to receive the entire cardiac output. Pulmonary arterial pressure is a function of cardiac output and pulmonary vascular resistance, and pulmonary vascular resistance is inversely proportional to the fourth power of the intraluminal radius of the pulmonary artery. Therefore, a very small decrease of the pulmonary vascular lumen diameter results in a significant increase in pulmonary vascular resistance and pulmonary arterial pressure. Pulmonary arterial hypertension is a fatal and progressive disease with poor prognosis. Regardless of the initial pathogenic triggers, sustained pulmonary vasoconstriction, concentric vascular remodeling, occlusive intimal lesions, in situ thrombosis, and vascular wall stiffening are the major and direct causes for elevated pulmonary vascular resistance in patients with pulmonary arterial hypertension and other forms of precapillary pulmonary hypertension. In this review, we aim to discuss the basic principles and physiological mechanisms involved in the regulation of lung vascular hemodynamics and pulmonary vascular function, the changes in the pulmonary vasculature that contribute to the increased vascular resistance and arterial pressure, and the pathogenic mechanisms involved in the development and progression of pulmonary hypertension. We focus on reviewing the pathogenic roles of membrane receptors, ion channels, and intracellular Ca2+ signaling in pulmonary vascular smooth muscle cells in the development and progression of pulmonary hypertension.

The cellular pathways that maintain the quality control and transport of diverse potassium channels

Potassium channels are multi-subunit transmembrane proteins that permit the selective passage of potassium and play fundamental roles in physiological processes, such as action potentials in the nervous system and organismal salt and water homeostasis, which is mediated by the kidney. Like all ion channels, newly translated potassium channels enter the endoplasmic reticulum (ER) and undergo the error-prone process of acquiring post-translational modifications, folding into their native conformations, assembling with other subunits, and trafficking through the secretory pathway to reach their final destinations, most commonly the plasma membrane. Disruptions in these processes can result in detrimental consequences, including various human diseases. Thus, multiple quality control checkpoints evolved to guide potassium channels through the secretory pathway and clear potentially toxic, aggregation-prone misfolded species. We will summarize current knowledge on the mechanisms underlying potassium channel quality control in the secretory pathway, highlight diseases associated with channel misfolding, and suggest potential therapeutic routes.

Expression and function of mitochondrial inhibitor factor-1 and TASK channels in adrenal cells

Adrenal medullary chromaffin (AMC) cells in the perinatal period and carotid body glomus cells after birth respond to hypoxia with catecholamine secretion. The hypoxia detection mechanism in such O2-sensitive cells is still not well defined. One hypothesis is that a decrease in cellular ATP may be involved in the hypoxia detection. This idea is based on ATP dependence of TASK channel activity that regulates the resting membrane potential and is suppressed by hypoxia in glomus cells. Mitochondrial ATPase inhibitor factor-1 (IF1), a physiological regulator of ATP synthase, helps prevent ATP hydrolysis under hypoxic conditions. In cells where IF1 expression is high, exposure to hypoxia is expected to have no effect on TASK channel activity. This possibility was electrophysiologically and immunocytochemically explored. Single channel recordings revealed that 36-pS TASK3-like channels contribute to the resting membrane potential in young rat adrenal cortical (AC) cells. TASK3-like channel activity in a cell-attached patch was not affected by bath application of mitochondrial inhibitors. Consistent with this finding, IF1-like immunoreactive material was well expressed in rat AC cells. In further support of our hypothesis, IF1-like immunoreactive material was well expressed in adult rat AMC cells that are known to be hypoxia-insensitive and minimally expressed in newborn AMC cells that are hypoxia-sensitive. These results provide evidence for the functional relevance of IF1 expression in excitability in O2-sensitive cells in response to mitochondrial inhibition.

Genetic screening of hypertensive patients with aldosterone hypersecretion under conditions of stress

PURPOSE

Although ACTH is considered a secondary regulator of aldosterone production, patients with apparent essential hypertension have been treated with mineralocorticoid receptor antagonists (MRAs). In this study, we aimed to identify potentially damaging variants that might be implicated in the phenotype of a well-characterized cohort of 21 hypertensive patients without PA but with stress-induced aldosterone hypersecretion. The patients' blood pressure was normalized though MRA administration.

METHODS

Genetic screening was performed through whole-exome sequencing (WES), and variants in PA-associated or in ion-channels of aldosterone-regulating genes were prioritized. Variants with population frequency < 0.01, predicted to alter protein structure and classified as likely pathogenic by in silico tools, were retained.

RESULTS

Qualifying variants were identified in nine of the 21 patients screened. Seven patients were carriers of six potentially damaging variants in six genes associated with PA (KCNK9, KCNK5, ATP13A3, SLC26A2, CACNA1H, and CACNA1D). A novel variant in the KCNK9 gene (p.V221M) is reported. Our analysis revealed two variants in two novel susceptibility genes for aldosterone hypersecretion, namely, KCNK16 (p.P255H) and CACNA2D3 (p.V557I).

CONCLUSION

WES revealed potentially damaging germline variants in genes participating in aldosterone synthesis/regulating pathways in 9/21 patients of our cohort. The variants identified might play a role in aldosterone hypersecretion under conditions of stress. The potential pathogenicity of these variants should be examined in future functional studies.

Gain-of-function mutations in KCNK3 cause a developmental disorder with sleep apnea

Sleep apnea is a common disorder that represents a global public health burden. KCNK3 encodes TASK-1, a K+ channel implicated in the control of breathing, but its link with sleep apnea remains poorly understood. Here we describe a new developmental disorder with associated sleep apnea (developmental delay with sleep apnea, or DDSA) caused by rare de novo gain-of-function mutations in KCNK3. The mutations cluster around the 'X-gate', a gating motif that controls channel opening, and produce overactive channels that no longer respond to inhibition by G-protein-coupled receptor pathways. However, despite their defective X-gating, these mutant channels can still be inhibited by a range of known TASK channel inhibitors. These results not only highlight an important new role for TASK-1 K+ channels and their link with sleep apnea but also identify possible therapeutic strategies.

Expression of p11 and heteromeric TASK channels in mouse adrenal cortical cells and H295R cells

TWIK-related acid-sensitive K⁺ (TASK) channels are thought to contribute to the resting membrane potential in adrenal cortical (AC) cells. However, the molecular identity of TASK channels in AC cells have not yet been elucidated. Thus, immunocytochemical and molecular biological approaches were employed to investigate the expression and intracellular distribution of TASK1 and TASK3 in mouse AC cells and H295R cells derived from human adrenocortical carcinoma. Immunocytochemical study revealed that immunoreactive materials were mainly located in the cytoplasm for TASK1 and at the cell periphery for TASK3 in mouse AC cells. A similar pattern of localization was observed when GFP-TASK1 and GFP-TASK3 were exogenously expressed in H295R cells. In addition, p11 that is known to suppress the endoplasmic reticulum exit of TASK1 was localized in the cytoplasm in mouse AC and H295R cells, but not in adrenal medullary cells. Proximity ligation assay (PLA) suggested formation of heteromeric TASK1-3 channels that were found predominantly in the cytoplasm and weakly at the cell periphery. A similar distribution was observed following exogenous expression of tandem TASK1-3 channels in H295R cells. When stimulated by angiotensin II, however, tandem TASK1-3 channels were present mainly in the cytoplasm in all H295R cells. In contrast to that in H295R cells, tandem channels were exclusively located at the cell periphery in all non-stimulated and exclusively in the cytoplasm in stimulated PC12 cells, respectively. From these results, we conclude that TASK1 proteins are present mainly in the cytoplasm and minimally at the cell periphery as a heteromeric channel with TASK3, whereas the majority of TASK3 is at the cell periphery as homomeric and heteromeric channels.

Expression of p11 and TASK1 Channels in Rat Carotid Body Glomus Cells Subjected to Chronic Intermittent Hypoxia

Chronic intermittent hypoxia (CIH) has been used as a model to mimic nocturnal apnea, which is associated with hypertension. One of the mechanisms for hypertension in patients with nocturnal apnea is an enhancement of the plasma membrane response to acute hypoxia in carotid body glomus cells. Hypoxia is known to induce depolarization via inhibiting TWIK-related acid-sensitive K⁺ (TASK) channels, one type of leak K⁺ channels, in glomus cells. The present experiment was undertaken to immunocytochemically investigate the effects of CIH on the expression and intracellular localization of TASK1 channels and p11 that critically affect the trafficking of TASK1 to the cell surface. The expression levels of TASK1 proteins and p11 and their intracellular localization in rat carotid body glomus cells were not noticeably affected by CIH, suggesting that the enhanced membrane response to acute hypoxia is not due to an increase in surface TASK channels.

Ion channels as convergence points in the pathology of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a fatal disease of the cardiopulmonary system that lacks curative treatments. The main pathological event in PAH is elevated vascular resistance in the pulmonary circulation, caused by abnormal vasoconstriction and vascular remodelling. Ion channels are key determinants of vascular smooth muscle tone and homeostasis, and four PAH channelopathies (KCNK3, ABCC8, KCNA5, TRPC6) have been identified so far. However, the contribution of ion channels in other forms of PAH, which account for the majority of PAH patients, has been less well characterised. Here we reason that a variety of triggers of PAH (e.g. BMPR2 mutations, hypoxia, anorectic drugs) that impact channel function may contribute to the onset of the disease. We review the molecular mechanisms by which these ‘extrinsic’ factors converge on ion channels and provoke their dysregulation to promote the development of PAH. Ion channels of the pulmonary vasculature are therefore promising therapeutic targets because of the modulation they provide to both vasomotor tone and proliferation of arterial smooth muscle cells.

Aging-related modifications to G protein-coupled receptor signaling diversity

Aging is a highly complex molecular process, affecting nearly all tissue systems in humans and is the highest risk factor in developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease, cardiovascular disease and Type 2 diabetes mellitus. The intense complexity of the aging process creates an incentive to develop more specific drugs that attenuate or even reverse some of the features of premature aging. As our current pharmacopeia is dominated by therapeutics that target members of the G protein-coupled receptor (GPCR) superfamily it may be prudent to search for effective anti-aging therapeutics in this fertile domain. Since the first demonstration of GPCR-based β-arrestin signaling, it has become clear that an enhanced appreciation of GPCR signaling diversity may facilitate the creation of therapeutics with selective signaling activities. Such 'biased' ligand signaling profiles can be effectively investigated using both standard molecular biological techniques as well as high-dimensionality data analyses. Through a more nuanced appreciation of the quantitative nature across the multiple dimensions of signaling bias that drugs possess, researchers may be able to further refine the efficacy of GPCR modulators to impact the complex aberrations that constitute the aging process. Identifying novel effector profiles could expand the effective pharmacopeia and assist in the design of precision medicines. This review discusses potential non-G protein effectors, and specifically their potential therapeutic suitability in aging and age-related disorders.