Functional TASK-3-Like Channels in Mitochondria of Aldosterone-Producing Zona Glomerulosa Cells

Role of mitochondrial potassium channels in ageing

Ageing is described as an inevitable decline in body functions over time and an increase in susceptibility to age-related diseases. Therefore, the increase of life expectancy is also viewed as a condition in which many elderly will develop age-related diseases and disabilities, such as cardiovascular, metabolic, neurological and oncological ones. Currently, several recognized cellular hallmarks of senescence are taken in consideration to evaluate the level of biological ageing and are the topic to plan preventive/curative anti-ageing interventions, including genomic instability, epigenetic alterations, and mitochondrial dysfunction. In this scenario, alterations in the function/expression of mitochondrial ion channels have been found in ageing and associated to an impairment of calcium cycling and a reduced mitochondrial membrane potential. Although several ion channels have been described at mitochondrial level, undoubtedly the mitochondrial potassium (mitoK) channels are the most investigated. Therefore, this review summarized the evidence that sheds to light a correlation between age-related diseases and alteration of mitoK channels, focusing the attention of the main age-related diseases, i.e. cardiovascular, neurological and oncological ones.

Redox Regulation of Mitochondrial Potassium Channels Activity

Redox reactions exert a profound influence on numerous cellular functions with mitochondria playing a central role in orchestrating these processes. This pivotal involvement arises from three primary factors: (1) the synthesis of reactive oxygen species (ROS) by mitochondria, (2) the presence of a substantial array of redox enzymes such as respiratory chain, and (3) the responsiveness of mitochondria to the cellular redox state. Within the inner mitochondrial membrane, a group of potassium channels, including ATP-regulated, large conductance calcium-activated, and voltage-regulated channels, is present. These channels play a crucial role in conditions such as cytoprotection, ischemia/reperfusion injury, and inflammation. Notably, the activity of mitochondrial potassium channels is intricately governed by redox reactions. Furthermore, the regulatory influence extends to other proteins, such as kinases, which undergo redox modifications. This review aims to offer a comprehensive exploration of the modulation of mitochondrial potassium channels through diverse redox reactions with a specific focus on the involvement of ROS.

TALK-1-mediated alterations of β-cell mitochondrial function and insulin secretion impair glucose homeostasis on a diabetogenic diet

Mitochondrial Ca2+ ([Ca2+]m) homeostasis is critical for β-cell function and becomes disrupted during the pathogenesis of diabetes. [Ca2+]m uptake is dependent on elevations in cytoplasmic Ca2+ ([Ca2+]c) and endoplasmic reticulum Ca2+ ([Ca2+]ER) release, both of which are regulated by the two-pore domain K+ channel TALK-1. Here, utilizing a novel β-cell TALK-1-knockout (β-TALK-1-KO) mouse model, we found that TALK-1 limited β-cell [Ca2+]m accumulation and ATP production. However, following exposure to a high-fat diet (HFD), ATP-linked respiration, glucose-stimulated oxygen consumption rate, and glucose-stimulated insulin secretion (GSIS) were increased in control but not TALK1-KO mice. Although β-TALK-1-KO animals showed similar GSIS before and after HFD treatment, these mice were protected from HFD-induced glucose intolerance. Collectively, these data identify that TALK-1 channel control of β-cell function reduces [Ca2+]m and suggest that metabolic remodeling in diabetes drives dysglycemia.

Mitochondrial Ion Channels

Mitochondria are involved in multiple cellular tasks, such as ATP synthesis, metabolism, metabolite and ion transport, regulation of apoptosis, inflammation, signaling, and inheritance of mitochondrial DNA. The majority of the correct functioning of mitochondria is based on the large electrochemical proton gradient, whose component, the inner mitochondrial membrane potential, is strictly controlled by ion transport through mitochondrial membranes. Consequently, mitochondrial function is critically dependent on ion homeostasis, the disturbance of which leads to abnormal cell functions. Therefore, the discovery of mitochondrial ion channels influencing ion permeability through the membrane has defined a new dimension of the function of ion channels in different cell types, mainly linked to the important tasks that mitochondrial ion channels perform in cell life and death. This review summarizes studies on animal mitochondrial ion channels with special focus on their biophysical properties, molecular identity, and regulation. Additionally, the potential of mitochondrial ion channels as therapeutic targets for several diseases is briefly discussed.

Advances in the Understanding of Two-Pore Domain TASK Potassium Channels and Their Potential as Therapeutic Targets

TWIK-related acid-sensitive K⁺ (TASK) channels, including TASK-1, TASK-3, and TASK-5, are important members of the two-pore domain potassium (K_2P) channel family. TASK-5 is not functionally expressed in the recombinant system. TASK channels are very sensitive to changes in extracellular pH and are active during all membrane potential periods. They are similar to other K_2P channels in that they can create and use background-leaked potassium currents to stabilize resting membrane conductance and repolarize the action potential of excitable cells. TASK channels are expressed in both the nervous system and peripheral tissues, including excitable and non-excitable cells, and are widely engaged in pathophysiological phenomena, such as respiratory stimulation, pulmonary hypertension, arrhythmia, aldosterone secretion, cancers, anesthesia, neurological disorders, glucose homeostasis, and visual sensitivity. Therefore, they are important targets for innovative drug development. In this review, we emphasized the recent advances in our understanding of the biophysical properties, gating profiles, and biological roles of TASK channels. Given the different localization ranges and biologically relevant functions of TASK-1 and TASK-3 channels, the development of compounds that selectively target TASK-1 and TASK-3 channels is also summarized based on data reported in the literature.

Alternative Targets for Modulators of Mitochondrial Potassium Channels

Mitochondrial potassium channels control potassium influx into the mitochondrial matrix and thus regulate mitochondrial membrane potential, volume, respiration, and synthesis of reactive oxygen species (ROS). It has been found that pharmacological activation of mitochondrial potassium channels during ischemia/reperfusion (I/R) injury activates cytoprotective mechanisms resulting in increased cell survival. In cancer cells, the inhibition of these channels leads to increased cell death. Therefore, mitochondrial potassium channels are intriguing targets for the development of new pharmacological strategies. In most cases, however, the substances that modulate the mitochondrial potassium channels have a few alternative targets in the cell. This may result in unexpected or unwanted effects induced by these compounds. In our review, we briefly present the various classes of mitochondrial potassium (mitoK) channels and describe the chemical compounds that modulate their activity. We also describe examples of the multidirectional activity of the activators and inhibitors of mitochondrial potassium channels.

TASK-1 regulates mitochondrial function under hypoxia

TASK-1, TWIK-related acid-sensitive potassium channel 1, is a member of the two-pore- domain potassium channel family. It is constitutively active at resting potentials and strongly expressed in the heart. However, little is known about the role of TASK-1 channels in hypoxia. A cellular model of hypoxia and reoxygenation from rat heart-derived H9c2 cells or TASK-1 deficient HEK293T cells was employed to explore the role of TASK-1 channels in cytoprotection against hypoxia. The cell viability assay revealed that TASK-1 expression increased the number of viable cells subjected to 2 h of hypoxia followed by 2 h of reoxygenation (H/R). To dissect the protective role of TASK-1 on mitochondrial function, mitochondrial membrane potential (MMP) was assessed by tetramethylrhodamine fluorescence. It was demonstrated that MMP was significantly decreased by H/R, but it was maintained by TASK-1 expression or pretreatment with cyclosporin A, an inhibitor of mitochondrial permeability transition pore (mPTP). The effect of cyclosporin A on MMP was not further altered by TASK-1 expression. Moreover, TASK-1 expression significantly blocked cytochrome c release induced by H/R. While a small fraction of endogenous TASK-1 was found to colocalize with the mitochondrial marker MitoTracker in H9c2 cells, H/R did not alter the extent of colocalization of TASK-1 with MitoTracker. The total TASK-1 protein level was not significantly affected by H/R. In summary, we provided the evidence that TASK-1 channels confer cytoprotection against hypoxia-reoxygenation injury, possibly by their capacity of maintaining the mitochondrial membrane potential via inhibiting MPTP opening.

Mitochondrial ion channels in cardiac function

Mitochondria have been recognized as key organelles in cardiac physiology and are potential targets for clinical interventions to improve cardiac function. Mitochondrial dysfunction has been accepted as a major contributor to the development of heart failure. The main function of mitochondria is to meet the high energy demands of the heart by oxidative metabolism. Ionic homeostasis in mitochondria directly regulates oxidative metabolism, and any disruption in ionic homeostasis causes mitochondrial dysfunction and eventually contractile failure. The mitochondrial ionic homeostasis is closely coupled with inner mitochondrial membrane potential. To regulate and maintain ionic homeostasis, mitochondrial membranes are equipped with ion transporting proteins. Ion transport mechanisms involving several different ion channels and transporters are highly efficient and dynamic, thus helping to maintain the ionic homeostasis of ions as well as their salts present in the mitochondrial matrix. In recent years, several novel proteins have been identified on the mitochondrial membranes and these proteins are actively being pursued in research for roles in the organ as well as organelle physiology. In this article, the role of mitochondrial ion channels in cardiac function is reviewed. In recent times, the major focus of the mitochondrial ion channel field is to establish molecular identities as well as assigning specific functions to them. Given the diversity of mitochondrial ion channels and their unique roles in cardiac function, they present novel and viable therapeutic targets for cardiac diseases.

Routes for Potassium Ions across Mitochondrial Membranes: A Biophysical Point of View with Special Focus on the ATP-Sensitive K+ Channel

Potassium ions can cross both the outer and inner mitochondrial membranes by means of multiple routes. A few potassium-permeable ion channels exist in the outer membrane, while in the inner membrane, a multitude of different potassium-selective and potassium-permeable channels mediate K+ uptake into energized mitochondria. In contrast, potassium is exported from the matrix thanks to an H+/K+ exchanger whose molecular identity is still debated. Among the K+ channels of the inner mitochondrial membrane, the most widely studied is the ATP-dependent potassium channel, whose pharmacological activation protects cells against ischemic damage and neuronal injury. In this review, we briefly summarize and compare the different hypotheses regarding the molecular identity of this patho-physiologically relevant channel, taking into account the electrophysiological characteristics of the proposed components. In addition, we discuss the characteristics of the other channels sharing localization to both the plasma membrane and mitochondria.

Mitochondrial K+ channels and their implications for disease mechanisms

The field of mitochondrial ion channels underwent a rapid development during the last decade, thanks to the molecular identification of some of the nuclear-encoded organelle channels and to advances in strategies allowing specific pharmacological targeting of these proteins. Thereby, genetic tools and specific drugs aided definition of the relevance of several mitochondrial channels both in physiological as well as pathological conditions. Unfortunately, in the case of mitochondrial K⁺ channels, efforts of genetic manipulation provided only limited results, due to their dual localization to mitochondria and to plasma membrane in most cases. Although the impact of mitochondrial K⁺ channels on human diseases is still far from being genuinely understood, pre-clinical data strongly argue for their substantial role in the context of several pathologies, including cardiovascular and neurodegenerative diseases as well as cancer. Importantly, these channels are druggable targets, and their in-depth investigation could thus pave the way to the development of innovative small molecules with huge therapeutic potential. In the present review we summarize the available experimental evidence that mechanistically link mitochondrial potassium channels to the above pathologies and underline the possibility of exploiting them for therapy.

Synthesis and cellular effects of a mitochondria-targeted inhibitor of the two-pore potassium channel TASK-3

The two-pore potassium channel TASK-3 has been shown to localize to both the plasma membrane and the mitochondrial inner membrane. TASK-3 is highly expressed in melanoma and breast cancer cells and has been proposed to promote tumor formation. Here we investigated whether pharmacological modulation of TASK-3, and specifically of mitochondrial TASK-3 (mitoTASK-3), had any effect on cancer cell survival and mitochondrial physiology. A novel, mitochondriotropic version of the specific TASK-3 inhibitor IN-THPP has been synthesized by addition of a positively charged triphenylphosphonium moiety. While IN-THPP was unable to induce apoptosis, mitoIN-THPP decreased survival of breast cancer cells and efficiently killed melanoma lines, which we show to express mitoTASK-3. Cell death was accompanied by mitochondrial membrane depolarization and fragmentation of the mitochondrial network, suggesting a role of the channel in the maintenance of the correct function of this organelle. In accordance, cells treated with mitoIN-THPP became rapidly depleted of mitochondrial ATP which resulted in activation of the AMP-dependent kinase AMPK. Importantly, cell survival was not affected in mouse embryonic fibroblasts and the effect of mitoIN-THPP was less pronounced in human melanoma cells stably knocked down for TASK-3 expression, indicating a certain degree of selectivity of the drug both for pathological cells and for the channel. In addition, mitoIN-THPP inhibited cancer cell migration to a higher extent than IN-THPP in two melanoma cell lines. In summary, our results point to the importance of mitoTASK-3 for melanoma cell survival and migration.

Ion Channel Function and Electrical Excitability in the Zona Glomerulosa: A Network Perspective on Aldosterone Regulation

Aldosterone excess is a pathogenic factor in many hypertensive disorders. The discovery of numerous somatic and germline mutations in ion channels in primary hyperaldosteronism underscores the importance of plasma membrane conductances in determining the activation state of zona glomerulosa (zG) cells. Electrophysiological recordings describe an electrically quiescent behavior for dispersed zG cells. Yet, emerging data indicate that in native rosette structures in situ, zG cells are electrically excitable, generating slow periodic voltage spikes and coordinated bursts of Ca²⁺ oscillations. We revisit data to understand how a multitude of conductances may underlie voltage/Ca²⁺ oscillations, recognizing that zG layer self-renewal and cell heterogeneity may complicate this task. We review recent data to understand rosette architecture and apply maxims derived from computational network modeling to understand rosette function. The challenge going forward is to uncover how the rosette orchestrates the behavior of a functional network of conditional oscillators to control zG layer performance and aldosterone secretion.

TASK-1 and TASK-3 channels modulate pressure overload-induced cardiac remodeling and dysfunction

Tandem pore domain acid-sensitive K⁺ (TASK) channels are present in cardiac tissue; however, their contribution to cardiac pathophysiology is not well understood. Here, we investigate the role of TASK-1 and TASK-3 in the pathogenesis of cardiac dysfunction using both human tissue and mouse models of genetic TASK channel loss of function. Compared with normal human cardiac tissue, TASK-1 gene expression is reduced in association with either cardiac hypertrophy alone or combined cardiac hypertrophy and heart failure. In a pressure overload cardiomyopathy model, TASK-1 global knockout (TASK-1 KO) mice have both reduced cardiac hypertrophy and preserved cardiac function compared with wild-type mice. In contrast to the TASK-1 KO mouse pressure overload response, TASK-3 global knockout (TASK-3 KO) mice develop cardiac hypertrophy and a delayed onset of cardiac dysfunction compared with wild-type mice. The cardioprotective effects observed in TASK-1 KO mice are associated with pressure overload-induced augmentation of AKT phosphorylation and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) expression, with consequent augmentation of cardiac energetics and fatty acid oxidation. The protective effects of TASK-1 loss of function are associated with an enhancement of physiologic hypertrophic signaling and preserved metabolic functions. These findings may provide a rationale for TASK-1 channel inhibition in the treatment of cardiac dysfunction.NEW & NOTEWORTHY The role of tandem pore domain acid-sensitive K⁺ (TASK) channels in cardiac function is not well understood. This study demonstrates that TASK channel gene expression is associated with the onset of human cardiac hypertrophy and heart failure. TASK-1 and TASK-3 strongly affect the development of pressure overload cardiomyopathies in genetic models of TASK-1 and TASK-3 loss of function. The effects of TASK-1 loss of function were associated with enhanced AKT phosphorylation and expression of peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1) transcription factor. These data suggest that TASK channels influence the development of cardiac hypertrophy and dysfunction in response to injury.

Novel Potassium Channels in Kidney Mitochondria: The Hyperpolarization-Activated and Cyclic Nucleotide-Gated HCN Channels

Hyperpolarization-activated cationic HCN channels comprise four members (HCN1–4) that control dendritic integration, synaptic transmission and action potential firing. In the kidney, HCN1, HCN2 and HCN3 are differentially expressed and contribute to the transport of sodium, potassium (K⁺) and ammonium into the nephrons. HCN3 is regulated by K⁺ diets in the kidney. In this work we performed a proteomic analysis of HCN3 expressed in human embryonic kidney cells (HEK293 cells). More than 50% of the interacting proteins belonged to mitochondria. Therefore, we explored the presence of HCN channels in kidney mitochondria. By immunoblotting and immunogold electron microscopy HCN3 protein expression was found in rat kidney mitochondria; it was also confirmed in human kidney. Patch-clamp recordings of renal mitochondria and mitochondria from HEK293 cells overexpressing HCN1, HCN2 and HCN3 channels, stained with MitoTracker Green FM, indicated that only HCN3 could produce inwardly K⁺ currents that were inhibited by ZD7288, a specific blocker of HCN channels. Furthermore, ZD7288 caused inhibition of the oxygen consumption coupled to ATP synthesis and hyperpolarization of the inner mitochondrial membrane. In conclusion, we show for the first time that pacemaker HCN channels contribute to K⁺ transport in mitochondria facilitating the activity of the respiratory chain and ATP synthesis by controlling the inner mitochondrial membrane potential.

The Potassium Channel Odyssey: Mechanisms of Traffic and Membrane Arrangement

Ion channels are transmembrane proteins that conduct specific ions across biological membranes. Ion channels are present at the onset of many cellular processes, and their malfunction triggers severe pathologies. Potassium channels (KChs) share a highly conserved signature that is necessary to conduct K⁺ through the pore region. To be functional, KChs require an exquisite regulation of their subcellular location and abundance. A wide repertoire of signatures facilitates the proper targeting of the channel, fine-tuning the balance that determines traffic and location. These signature motifs can be part of the secondary or tertiary structure of the protein and are spread throughout the entire sequence. Furthermore, the association of the pore-forming subunits with different ancillary proteins forms functional complexes. These partners can modulate traffic and activity by adding their own signatures as well as by exposing or masking the existing ones. Post-translational modifications (PTMs) add a further dimension to traffic regulation. Therefore, the fate of a KCh is not fully dependent on a gene sequence but on the balance of many other factors regulating traffic. In this review, we assemble recent evidence contributing to our understanding of the spatial expression of KChs in mammalian cells. We compile specific signatures, PTMs, and associations that govern the destination of a functional channel.

The Biology of Normal Zona Glomerulosa and Aldosterone-Producing Adenoma: Pathological Implications

The identification of several germline and somatic ion channel mutations in aldosterone-producing adenomas (APAs) and detection of cell clusters that can be responsible for excess aldosterone production, as well as the isolation of autoantibodies activating the angiotensin II type 1 receptor, have rapidly advanced the understanding of the biology of primary aldosteronism (PA), particularly that of APA. Hence, the main purpose of this review is to discuss how discoveries of the last decade could affect histopathology analysis and clinical practice. The structural remodeling through development and aging of the human adrenal cortex, particularly of the zona glomerulosa, and the complex regulation of aldosterone, with emphasis on the concepts of zonation and channelopathies, will be addressed. Finally, the diagnostic workup for PA and its subtyping to optimize treatment are reviewed.

Mouse Models of Primary Aldosteronism: From Physiology to Pathophysiology

Primary aldosteronism (PA) is a common form of endocrine hypertension that is characterized by the excessive production of aldosterone relative to suppressed plasma renin levels. PA is usually caused by either a unilateral aldosterone-producing adenoma or bilateral adrenal hyperplasia. Somatic mutations have been identified in several genes that encode ion pumps and channels that may explain the aldosterone excess in over half of aldosterone-producing adenomas, whereas the pathophysiology of bilateral adrenal hyperplasia is largely unknown. A number of mouse models of hyperaldosteronism have been described that recreate some features of the human disorder, although none replicate the genetic basis of human PA. Animal models that reproduce the genotype-phenotype associations of human PA are required to establish the functional mechanisms that underlie the endocrine autonomy and deregulated cell growth of the affected adrenal and for preclinical studies of novel therapeutics. Herein, we discuss the differences in adrenal physiology across species and describe the genetically modified mouse models of PA that have been developed to date.

TALK-1 channels control β cell endoplasmic reticulum Ca2+ homeostasis

Ca²⁺ handling by the endoplasmic reticulum (ER) serves critical roles in controlling pancreatic β cell function and becomes perturbed during the pathogenesis of diabetes. ER Ca²⁺ homeostasis is determined by ion movements across the ER membrane, including K⁺ flux through K⁺ channels. We demonstrated that K⁺ flux through ER-localized TALK-1 channels facilitated Ca²⁺ release from the ER in mouse and human β cells. We found that β cells from mice lacking TALK-1 exhibited reduced basal cytosolic Ca²⁺ and increased ER Ca²⁺ concentrations, suggesting reduced ER Ca²⁺ leak. These changes in Ca²⁺ homeostasis were presumably due to TALK-1-mediated ER K⁺ flux, because we recorded K⁺ currents mediated by functional TALK-1 channels on the nuclear membrane, which is continuous with the ER. Moreover, overexpression of K⁺-impermeable TALK-1 channels in HEK293 cells did not reduce ER Ca²⁺ stores. Reduced ER Ca²⁺ content in β cells is associated with ER stress and islet dysfunction in diabetes, and islets from TALK-1-deficient mice fed a high-fat diet showed reduced signs of ER stress, suggesting that TALK-1 activity exacerbated ER stress. Our data establish TALK-1 channels as key regulators of β cell ER Ca²⁺ and suggest that TALK-1 may be a therapeutic target to reduce ER Ca²⁺ handling defects in β cells during the pathogenesis of diabetes.