TASK-3 channel deletion in mice recapitulates low-renin essential hypertension

The ex vivo perfused mouse adrenal gland-a new model to study aldosterone secretion

Aldosterone is a steroid hormone that is important for maintaining the volume and ionic composition of extracellular fluids and is produced in the zona glomerulosa of the adrenal cortex. The basic mechanisms controlling aldosterone secretion are known. However, more detailed studies on the regulation of aldosterone secretion often fail due to the lack of suitable models: although secretion can be studied in cultured adrenocortical cells under defined conditions, the differentiation status of the cells is difficult to control and the complex anatomy of the adrenal cortex is lost. In living animals, the physiological context is intact, but the influences are manifold and the examination conditions cannot be sufficiently controlled. One method that closes the gap between cell models and studies in living animals is the isolated perfused adrenal gland. In the past, this method has provided important data on the pathophysiology of adrenal glands from larger animals, but the technique was not used in mice. Here, we developed a method for isolation and perfusion of the mouse adrenal gland to study aldosterone secretion. This technique preserves the complex anatomical and functional context of the mouse adrenal cortex, to ensure defined experimental conditions and to minimize extra-adrenal influences. Initial series of experiments with the ex vivo perfused mouse adrenal gland show that this model offers the possibility for unique insights into pathophysiological regulatory principles and is suitable for the use of genetically modified mouse models.

Primary aldosteronism: molecular medicine meets public health

Primary aldosteronism is the most common single cause of hypertension and is potentially curable when only one adrenal gland is the culprit. The importance of primary aldosteronism to public health derives from its high prevalence but huge under-diagnosis (estimated to be <1% of all affected individuals), despite the consequences of poor blood pressure control by conventional therapy and enhanced cardiovascular risk. This state of affairs is attributable to the fact that the tools used for diagnosis or treatment are still those that originated in the 1970-1990s. Conversely, molecular discoveries have transformed our understanding of adrenal physiology and pathology. Many molecules and processes associated with constant adrenocortical renewal and interzonal metamorphosis also feature in aldosterone-producing adenomas and aldosterone-producing micronodules. The adrenal gland has one of the most significant rates of non-silent somatic mutations, with frequent selection of those driving autonomous aldosterone production, and distinct clinical presentations and outcomes for most genotypes. The disappearance of aldosterone synthesis and cells from most of the adult human zona glomerulosa is the likely driver of the mutational success that causes aldosterone-producing adenomas, but insights into the pathways that lead to constitutive aldosterone production and cell survival may open up opportunities for novel therapies.

Systematic characterization of regulatory variants of blood pressure genes

High blood pressure (BP) is the major risk factor for cardiovascular disease. Genome-wide association studies have identified genetic variants for BP, but functional insights into causality and related molecular mechanisms lag behind. We functionally characterize 4,608 genetic variants in linkage with 135 BP loci in vascular smooth muscle cells and cardiomyocytes by massively parallel reporter assays. High densities of regulatory variants at BP loci (i.e., ULK4, MAP4, CFDP1, PDE5A) indicate that multiple variants drive genetic association. Regulatory variants are enriched in repeats, alter cardiovascular-related transcription factor motifs, and spatially converge with genes controlling specific cardiovascular pathways. Using heuristic scoring, we define likely causal variants, and CRISPR prime editing finally determines causal variants for KCNK9, SFXN2, and PCGF6, which are candidates for developing high BP. Our systems-level approach provides a catalog of functionally relevant variants and their genomic architecture in two trait-relevant cell lines for a better understanding of BP gene regulation.

Low sodium intake ameliorates hypertension and left ventricular hypertrophy in mice with primary aldosteronism

The goal of this paper is to elucidate the effects of sodium restriction on hypertension and left ventricular (LV) hypertrophy in a mouse model with primary aldosteronism (PA). Mice with genetic deletion of TWIK-related acid-sensitive K (TASK)-1 and TASK-3 channels (TASK^-/-) were used as the animal model of PA. Parameters of the LV were assessed using echocardiography and histomorphology analysis. Untargeted metabolomics analysis was conducted to reveal the mechanisms underlying the hypertrophic changes in the TASK^-/- mice. The TASK^-/- adult male mice exhibited the hallmarks of PA, including hypertension, hyperaldosteronism, hypernatremia, hypokalemia, and mild acid-base balance disorders. Two weeks of low sodium intake significantly reduced the 24-h average systolic and diastolic BP in TASK^-/- but not TASK^+/+ mice. In addition, TASK^-/- mice showed increasing LV hypertrophy with age, and 2 weeks of the low-sodium diet significantly reversed the increased BP and LV wall thickness in adult TASK^-/- mice. Furthermore, a low-sodium diet beginning at 4 weeks of age protected TASK^-/- mice from LV hypertrophy at 8-12 weeks of age. Untargeted metabolomics demonstrated that the disturbances in heart metabolism in the TASK^-/- mice (e.g., Glutathione metabolism; biosynthesis of unsaturated fatty acids; amino sugar and nucleotide sugar metabolism; pantothenate and CoA biosynthesis; D-glutamine and D-glutamate metabolism), some of which were reversed after sodium restriction, might be involved in the development of LV hypertrophy. In conclusion, adult male TASK^-/- mice exhibit spontaneous hypertension and LV hypertrophy, which are ameliorated by a low-sodium intake.

Intrinsic Adrenal TWIK-Related Acid-Sensitive TASK Channel Dysfunction Produces Spontaneous Calcium Oscillations Sufficient to Drive AngII (Angiotensin II)-Unresponsive Hyperaldosteronism

Background:

Ion channel mutations in calcium regulating genes strongly associate with AngII (angiotensin II)-independent aldosterone production. Here, we used an established mouse model of in vivo aldosterone autonomy, Cyp11b2 -driven deletion of TWIK-related acid-sensitive potassium channels (TASK-1 and TASK-3, termed zona glomerulosa [zG]-TASK-loss-of-function), and selective pharmacological TASK channel inhibition to determine whether channel dysfunction in native, electrically excitable zG cell rosette-assemblies: (1) produces spontaneous calcium oscillatory activity and (2) is sufficient to drive substantial aldosterone autonomy.

Methods:

We imaged calcium activity in adrenal slices expressing a zG-specific calcium reporter (GCaMP3), an in vitro experimental approach that preserves the native rosette assembly and removes potentially confounding extra-adrenal contributions. In parallel experiments, we measured acute aldosterone production from adrenal slice cultures.

Results:

Absent from untreated WT slices, we find that either adrenal-specific genetic deletion or acute pharmacological TASK channel inhibition produces spontaneous oscillatory bursting behavior and steroidogenic activity (2.4-fold) that are robust, sustained, and equivalent to activities evoked by 3 nM AngII in WT slices. Moreover, spontaneous activity in zG-TASK-loss-of-function slices and inhibitor-evoked activity in WT slices are unresponsive to AngII regulation over a wide range of concentrations (50 pM to 3 µM).

Conclusions:

We provide proof of principle that spontaneous activity of zG cells within classic rosette assemblies evoked solely by a change in an intrinsic, dominant resting-state conductance can be a significant source of AngII-independent aldosterone production from native tissue.

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

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.

Cellular Pathophysiology of Mutant Voltage-Dependent Ca2+ Channel CACNA1H in Primary Aldosteronism

The physiological stimulation of aldosterone production in adrenocortical glomerulosa cells by angiotensin II and high plasma K+ depends on the depolarization of the cell membrane potential and the subsequent Ca2+ influx via voltage-activated Ca2+ channels. Germline mutations of the low-voltage activated T-type Ca2+ channel CACNA1H (Cav3.2) have been found in patients with primary aldosteronism. Here, we investigated the electrophysiology and Ca2+ signaling of adrenal NCI-H295R cells overexpressing CACNA1H wildtype and mutant M1549V in order to understand how mutant CACNA1H alters adrenal cell function. Whole-cell patch-clamp measurements revealed a strong activation of mutant CACNA1H at the resting membrane potential of adrenal cells. Both the expression of wildtype and mutant CACNA1H led to a depolarized membrane potential. In addition, cells expressing mutant CACNA1H developed pronounced action potential-like membrane voltage oscillations. Ca2+ measurements showed an increased basal Ca2+ activity, an altered K+ sensitivity, and abnormal oscillating Ca2+ changes in cells with mutant CACNA1H. In addition, removal of extracellular Na+ reduced CACNA1H current, voltage oscillations, and Ca2+ levels in mutant cells, suggesting a role of the partial Na+ conductance of CACNA1H in cellular pathology. In conclusion, the pathogenesis of stimulus-independent aldosterone production in patients with CACNA1H mutations involves several factors: i) a loss of normal control of the membrane potential, ii) an increased Ca2+ influx at basal conditions, and iii) alterations in sensitivity to extracellular K+ and Na+. Finally, our findings underline the importance of CACNA1H in the control of aldosterone production and support the concept of the glomerulosa cell as an electrical oscillator.

Chemogenetic activation of adrenocortical Gq signaling causes hyperaldosteronism and disrupts functional zonation

The mineralocorticoid aldosterone is produced in the adrenal zona glomerulosa (ZG) under the control of the renin-angiotensin II (AngII) system. Primary aldosteronism (PA) results from renin-independent production of aldosterone and is a common cause of hypertension. PA is caused by dysregulated localization of the enzyme aldosterone synthase (Cyp11b2), which is normally restricted to the ZG. Cyp11b2 transcription and aldosterone production are predominantly regulated by AngII activation of the Gq signaling pathway. Here, we report the generation of transgenic mice with Gq-coupled designer receptors exclusively activated by designer drugs (DREADDs) specifically in the adrenal cortex. We show that adrenal-wide ligand activation of Gq DREADD receptors triggered disorganization of adrenal functional zonation, with induction of Cyp11b2 in glucocorticoid-producing zona fasciculata cells. This result was consistent with increased renin-independent aldosterone production and hypertension. All parameters were reversible following termination of DREADD-mediated Gq signaling. These findings demonstrate that Gq signaling is sufficient for adrenocortical aldosterone production and implicate this pathway in the determination of zone-specific steroid production within the adrenal cortex. This transgenic mouse also provides an inducible and reversible model of hyperaldosteronism to investigate PA therapeutics and the mechanisms leading to the damaging effects of aldosterone on the cardiovascular system.

Genetic and Genomic Mechanisms of Primary Aldosteronism

Aldosterone-producing adenoma (APA) and bilateral adrenal hyperplasia are the main cause of primary aldosteronism (PA), the most frequent form of secondary hypertension. Mutations in ion channels and ATPases have been identified in APA and inherited forms of PA, highlighting the central role of calcium signaling in PA development. Different somatic mutations are also found in aldosterone-producing cell clusters in adrenal glands from healthy individuals and from patients with unilateral and bilateral PA, suggesting additional pathogenic mechanisms. Recent mouse models have also contributed to a better understanding of PA. Application of genetic screening in familial PA, development of surrogate biomarkers for somatic mutations in APA, and use of targeted treatment directed at mutated proteins may allow improved management of patients.

Mineralocorticoid Receptor-Dependent Impairment of Baroreflex Contributes to Hypertension in a Mouse Model of Primary Aldosteronism

Primary aldosteronism (PA) is the most common cause of secondary hypertension. The paucity of good animal models hinders our understanding of the pathophysiology of PA and the hypertensive mechanism of PA remains incompletely known. It was recently reported that genetic deletion of TWIK-related acid-sensitive potassium-1 and potassium-3 channels from mice (TASK^−/−) generates aldosterone excess and mild hypertension. We addressed the hypertensive mechanism by assessing autonomic regulation of cardiovascular activity in this TASK^−/− mouse line that exhibits the hallmarks of PA. Here, we demonstrate that TASK^−/− mice were hypertensive with 24-h ambulatory arterial pressure. Either systemic or central blockade of the mineralocorticoid receptor (MR) markedly reduced elevated arterial pressure to normal level in TASK^−/− mice. The response of heart rate to the muscarinic cholinergic receptor blocker atropine was similar between TASK^−/− and wild-type mice. However, the responses of heart rate to the β-adrenergic receptor blocker propranolol and of arterial pressure to the ganglion blocker hexamethonium were enhanced in TASK^−/− mice relative to the counterparts. Moreover, the bradycardiac rather than tachycardiac gain of the arterial baroreflex was significantly attenuated and blockade of MRs to a large degree rescued the dysautonomia and baroreflex gain in TASK^−/− mice. Overall, the present study suggests that the MR-dependent dysautonomia and reduced baroreflex gain contribute to the development of hyperaldosteronism-related hypertension.

Molecular Mechanisms of Primary Aldosteronism

Primary aldosteronism (PA) results from excess production of mineralocorticoid hormone aldosterone by the adrenal cortex. It is normally caused either by unilateral aldosterone-producing adenoma (APA) or by bilateral aldosterone excess as a result of bilateral adrenal hyperplasia. PA is the most common cause of secondary hypertension and associated morbidity and mortality. While most cases of PA are sporadic, an important insight into this debilitating disease has been derived through investigating the familial forms of the disease that affect only a minor fraction of PA patients. The advent of gene expression profiling has shed light on the genes and intracellular signaling pathways that may play a role in the pathogenesis of these tumors. The genetic basis for several forms of familial PA has been uncovered in recent years although the list is likely to expand. Recently, the work from several laboratories provided evidence for the involvement of mammalian target of rapamycin pathway and inflammatory cytokines in APAs; however, their mechanism of action in tumor development and pathophysiology remains to be understood.

Adrenal Tissue-Specific Deletion of TASK Channels Causes Aldosterone-Driven Angiotensin II-Independent Hypertension

The renin-angiotensin system tightly controls aldosterone synthesis. Dysregulation is evident in hypertension (primary aldosteronism), low renin, and resistant hypertension) but also can exist in normotension. Whether chronic, mild aldosterone autonomy can elicit hypertension remains untested. Previously, we reported that global genetic deletion of 2 pore-domain TWIK-relative acid-sensitive potassium channels, TASK-1 and TASK-3, from mice produces striking aldosterone excess, low renin, and hypertension. Here, we deleted TASK-1 and TASK-3 channels selectively from zona glomerulosa cells and generated a model of mild aldosterone autonomy with attendant hypertension that is aldosterone-driven and Ang II (angiotensin II)-independent. This study shows that a zona glomerulosa-specific channel defect can produce mild autonomous hyperaldosteronism sufficient to cause chronic blood pressure elevation.

Elevated aldosterone and blood pressure in a mouse model of familial hyperaldosteronism with ClC-2 mutation

Gain-of-function mutations in the chloride channel ClC-2 were recently described as a cause of familial hyperaldosteronism type II (FH-II). Here, we report the generation of a mouse model carrying a missense mutation homologous to the most common FH-II-associated CLCN2 mutation. In these Clcn2^R180Q/+ mice, adrenal morphology is normal, but Cyp11b2 expression and plasma aldosterone levels are elevated. Male Clcn2^R180Q/+ mice have increased aldosterone:renin ratios as well as elevated blood pressure levels. The counterpart knockout model (Clcn2^−/−), in contrast, requires elevated renin levels to maintain normal aldosterone levels. Adrenal slices of Clcn2^R180Q/+ mice show increased calcium oscillatory activity. Together, our work provides a knockin mouse model with a mild form of primary aldosteronism, likely due to increased chloride efflux and depolarization. We demonstrate a role of ClC-2 in normal aldosterone production beyond the observed pathophysiology.

Mutations in the chloride channel ClC-2 have been associated with familial forms of hyperaldosteronism. Here, Schewe et al. generated a mouse model carrying the most common mutation found in patients and find it recapitulates key features of the disease, providing a unique tool for future studies on its pathogenesis.

Genetic causes of primary aldosteronism

Primary aldosteronism is characterized by at least partially autonomous production of the adrenal steroid hormone aldosterone and is the most common cause of secondary hypertension. The most frequent subforms are idiopathic hyperaldosteronism and aldosterone-producing adenoma. Rare causes include unilateral hyperplasia, adrenocortical carcinoma and Mendelian forms (familial hyperaldosteronism). Studies conducted in the last eight years have identified somatic driver mutations in a substantial portion of aldosterone-producing adenomas, including the genes KCNJ5 (encoding inwardly rectifying potassium channel GIRK4), CACNA1D (encoding a subunit of L-type voltage-gated calcium channel Ca_V1.3), ATP1A1 (encoding a subunit of Na⁺/K⁺-ATPase), ATP2B3 (encoding a Ca²⁺-ATPase), and CTNNB1 (encoding ß-catenin). In addition, aldosterone-producing cells were recently reported to form small clusters (aldosterone-producing cell clusters) beneath the adrenal capsule. Such clusters accumulate with age and appear to be more frequent in individuals with idiopathic hyperaldosteronism. The fact that they are associated with somatic mutations implicated in aldosterone-producing adenomas also suggests a precursor function for adenomas. Rare germline variants of CYP11B2 (encoding aldosterone synthase), CLCN2 (encoding voltage-gated chloride channel ClC-2), KCNJ5, CACNA1H (encoding a subunit of T-type voltage-gated calcium channel Ca_V3.2), and CACNA1D have been reported in different subtypes of familial hyperaldosteronism. Collectively, these studies suggest that primary aldosteronism is largely due to genetic mutations in single genes, with potential implications for diagnosis and therapy.

ANO4 (Anoctamin 4) Is a Novel Marker of Zona Glomerulosa That Regulates Stimulated Aldosterone Secretion

Microarray comparison of the transcriptomes of human adrenal zona glomerulosa (ZG) and zona fasciculata found several ZG-specific genes that negatively regulate aldosterone secretion. The third and most significantly upregulated ZG-gene (19.9-fold compared with zona fasciculata, P=6.58×10-24) was ANO4, a putative Ca2+-activated chloride channel. We have investigated the role of ANO4 in human adrenal, and whether it functions like the prototype anoctamin, ANO1. We evaluated ANO4 mRNA and protein expression in human adrenal by qPCR and immunohistochemistry, compared the effects of ANO4 and ANO1 overexpression on baseline and stimulated aldosterone secretion and cell proliferation in H295R cells, and analyzed ANO4 activity as a Ca2+-activated chloride channel in comparison with other anoctamins by a fluorescence-based functional assay. The expression of ANO4 in ZG was confirmed by qPCR as 23.21-fold upregulated compared with zona fasciculata (n=18; P=4.93×10-7). Immunohistochemistry found cytoplasmic, ZG-selective expression of ANO4 (anoctamin 4) protein. ANO4 overexpression in H295R cells attenuated calcium-mediated aldosterone secretion and cell proliferation in comparison to controls. The latter effects were in a different direction to those of ANO1. The functional assay showed that, in contrast to ANO1, ANO4 expression results in low levels of calcium-dependent anion transport. In conclusion, ANO4 is one of the most highly expressed genes in ZG. It attenuates stimulated aldosterone secretion and cell proliferation. Although belonging to a family of Ca2+-activated chloride channels, it does not generate significant plasma membrane chloride channel activity.

Pathogenesis of hypertension in a mouse model for human CLCN2 related hyperaldosteronism

Human primary aldosteronism (PA) can be caused by mutations in several ion channel genes but mouse models replicating this condition are lacking. We now show that almost all known PA-associated CLCN2 mutations markedly increase ClC-2 chloride currents and generate knock-in mice expressing a constitutively open ClC-2 Cl⁻ channel as mouse model for PA. The Clcn2^op allele strongly increases the chloride conductance of zona glomerulosa cells, provoking a strong depolarization and increasing cytoplasmic Ca²⁺ concentration. Clcn2^op mice display typical features of human PA, including high serum aldosterone in the presence of low renin activity, marked hypertension and hypokalemia. These symptoms are more pronounced in homozygous Clcn2^op/op than in heterozygous Clcn2^+/op mice. This difference is attributed to the unexpected finding that only ~50 % of Clcn2^+/op zona glomerulosa cells are depolarized. By reproducing essential features of human PA, Clcn2^op mice are a valuable model to study the pathological mechanisms underlying this disease.

Mutations in the chloride channel ClC-2 have been found in primary aldosteronism (PA). Here, Göppner et al. generate transgenic mice expressing a mutant form of ClC-2 that displays increased chloride currents like patient mutations, and find it recapitulates the key pathological features of PA.

Mutations of the Twik-Related Acid-Sensitive K+ Channel 2 Promoter in Human Primary Aldosteronism

Because blunted expression of the twik-related acid-sensitive K+ channel 2 (TASK-2) is a common feature of aldosterone-producing adenoma (APA) causing primary aldosteronism (PA), we sequenced the promoter region of the TASK-2 gene (KCNK5) in APAs (n = 76), primary hypertensive patients (n = 98), and 20-year-old healthy volunteers (n = 71), searching for variants that could affect expression of this channel. We found TASK-2 promoter mutations in 25% of the APAs: C999T in 6.6%, G595A in 5.3%, G36A in 5.3%, and C562T, Gins468, G265C, C1247T, G1140T, and C1399T in 1.3% each. The C999T mutation was found in only one of the 98 primary hypertensive patients, but mutations were detected also in 12% of volunteers: 4 carried the C999T, 3 G1288C, 1 the G1140T mutation, and 1 the 468ins mutation. After a 16-year follow-up, none of these patients developed hypertension or PA. The effect of C999T mutation was investigated in H295R cells using reporter vectors with the mutated or the wild-type (WT) TASK-2 promoters. TASK-2 gene expression was decreased by 31% ± 18% (P = 0.01) in mutated compared with WT APA. Likewise, in transfected H295R cells, the C999T mutation decreased TASK-2 transcriptional activity by 35% (normalized luciferase signal fold change: 0.65 ± 0.25, P < 0.001). Thus, mutations in the promoter region of the TASK-2 gene can account for the low expression in ∼25% of APAs. As they did not result in hypertension or PA during long-term follow-up in healthy participants, these mutations do not seem to be a factor in causing PA by themselves.

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.

Somatic and inherited mutations in primary aldosteronism

Primary aldosteronism (PA), the most common form of secondary hypertension, is caused in the majority of cases by unilateral aldosterone-producing adenoma (APA) or bilateral adrenal hyperplasia. Over the past few years, somatic mutations in KCNJ5, CACNA1D, ATP1A1 and ATP2B3 have been proven to be associated with APA development, representing more than 50% of sporadic APA. The identification of these mutations has allowed the development of a model for APA involving modification on the intracellular ionic equilibrium and regulation of cell membrane potential, leading to autonomous aldosterone overproduction. Furthermore, somatic CTNNB1 mutations have also been identified in APA, but the link between these mutations and APA development remains unknown. The sequence of events responsible for APA formation is not completely understood, in particular, whether a single hit or a double hit is responsible for both aldosterone overproduction and cell proliferation. Germline mutations identified in patients with early-onset PA have expanded the classification of familial forms (FH) of PA. The description of germline KCNJ5 and CACNA1H mutations has identified FH-III and FH-IV based on genetic findings; germline CACNA1D mutations have been identified in patients with very early-onset PA and severe neurological abnormalities. This review summarizes current knowledge on the genetic basis of PA, the association of driver gene mutations and clinical findings and in the contribution to patient care, plus the current understanding on the mechanisms of APA development.

Of channels and pumps: different ways to boost the aldosterone?

The mineralocorticoid aldosterone is a major factor controlling the salt and water balance and thereby also the arterial blood pressure. Accordingly, primary aldosteronism (PA) characterized by an inappropriately high aldosterone secretion is the most common form of secondary hypertension. The physiological stimulation of aldosterone synthesis in adrenocortical glomerulosa cells by angiotensin II and an increased plasma K⁺ concentration depends on a membrane depolarization and an increase in the cytosolic Ca²⁺ activity. Recurrent gain-of-function mutations of ion channels and transporters have been identified in a majority of cases of aldosterone-producing adenomas and in familial forms of PA. In this review, the physiological role of these genes in the regulation of aldosterone synthesis and the altered function of the mutant proteins as well are described. The specific changes of the membrane potential and the cellular ion homoeostasis in adrenal cells expressing the different mutants are compared, and their impact on autonomous aldosterone production and proliferation is discussed.

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

Ca²⁺ drives aldosterone synthesis in the cytosolic and mitochondrial compartments of the adrenal zona glomerulosa cell. Membrane potential across each of these compartments regulates the amplitude of the Ca²⁺ signal; yet, only plasma membrane ion channels and their role in regulating cell membrane potential have garnered investigative attention as pathological causes of human hyperaldosteronism. Previously, we reported that genetic deletion of TASK-3 channels (tandem pore domain acid-sensitive K⁺ channels) from mice produces aldosterone excess in the absence of a change in the cell membrane potential of zona glomerulosa cells. Here, we report using yeast 2-hybrid, immunoprecipitation, and electron microscopic analyses that TASK-3 channels are resident in mitochondria, where they regulate mitochondrial morphology, mitochondrial membrane potential, and aldosterone production. This study provides proof of principle that mitochondrial K⁺ channels, by modulating inner mitochondrial membrane morphology and mitochondrial membrane potential, have the ability to play a pathological role in aldosterone dysregulation in steroidogenic cells.

Primary Aldosteronism: Changing Definitions and New Concepts of Physiology and Pathophysiology Both Inside and Outside the Kidney

In the 60 years that have passed since the discovery of the mineralocorticoid hormone aldosterone, much has been learned about its synthesis (both adrenal and extra-adrenal), regulation (by renin-angiotensin II, potassium, adrenocorticotrophin, and other factors), and effects (on both epithelial and nonepithelial tissues). Once thought to be rare, primary aldosteronism (PA, in which aldosterone secretion by the adrenal is excessive and autonomous of its principal regulator, angiotensin II) is now known to be the most common specifically treatable and potentially curable form of hypertension, with most patients lacking the clinical feature of hypokalemia, the presence of which was previously considered to be necessary to warrant further efforts towards confirming a diagnosis of PA. This, and the appreciation that aldosterone excess leads to adverse cardiovascular, renal, central nervous, and psychological effects, that are at least partly independent of its effects on blood pressure, have had a profound influence on raising clinical and research interest in PA. Such research on patients with PA has, in turn, furthered knowledge regarding aldosterone synthesis, regulation, and effects. This review summarizes current progress in our understanding of the physiology of aldosterone, and towards defining the causes (including genetic bases), epidemiology, outcomes, and clinical approaches to diagnostic workup (including screening, diagnostic confirmation, and subtype differentiation) and treatment of PA.

KCNK3 Variants Are Associated With Hyperaldosteronism and Hypertension

Blood pressure (BP) is a complex trait that is the consequence of an interaction between genetic and environmental determinants. Previous studies have demonstrated increased BP in mice with global deletion of TASK-1 channels contemporaneous with diverse dysregulation of aldosterone production. In humans, genome-wide association studies in ≈100 000 individuals of European, East Asian, and South Asian ancestry identified a single nucleotide polymorphism (SNP) in KCNK3 (the gene encoding TASK-1) associated with mean arterial pressure. The current study was motivated by the hypotheses that (1) association of KCNK3 SNPs with BP and related traits extends to blacks and Hispanics, and (2) KCNK3 SNPs exhibit associations with plasma renin activity and aldosterone levels. We examined baseline BP measurements for 7840 participants from the Multi-Ethnic Study of Atherosclerosis (MESA), and aldosterone levels and plasma renin activity in a subset of 1653 MESA participants. We identified statistically significant association of the previously reported KCNK3 SNP (rs1275988) with mean arterial pressure in MESA blacks (P=0.024) and a nearby SNP (rs13394970) in MESA Hispanics (P=0.031). We discovered additional KCNK3 SNP associations with systolic BP, mean arterial pressure, and hypertension. We also identified statistically significant association of KCNK3 rs2586886 with plasma aldosterone level in MESA and demonstrated that global deletion of TASK-1 channels in mice produces a mild-hyperaldosteronism, not associated with a decrease in renin. Our results suggest that genetic variation in the KCNK3 gene may contribute to BP variation and less severe hypertensive disorders in which aldosterone may be one of several causative factors.

Cellular Pathophysiology of an Adrenal Adenoma-Associated Mutant of the Plasma Membrane Ca(2+)-ATPase ATP2B3

Adrenal aldosterone-producing adenomas (APAs) are a main cause for primary aldosteronism leading to arterial hypertension. Physiologically, aldosterone production in the adrenal gland is stimulated by angiotensin II and high extracellular potassium. These stimuli lead to a depolarization of the plasma membrane and, as a consequence, an increase of intracellular Ca(2+). Mutations of the plasma membrane Ca(2+)-ATPase ATP2B3 have been found in APAs with a prevalence of 0.6%-3.1%. Here, we investigated the effects of the APA-associated ATP2B3(Leu425_Val426del) mutation in adrenocortical NCI-H295R and human embryonic kidney (HEK-293) cells. Ca(2+) measurements revealed a higher basal Ca(2+) level in cells expressing the mutant ATP2B3. This rise in intracellular Ca(2+) was even more pronounced under conditions with high extracellular Ca(2+) pointing to an increased Ca(2+) influx associated with the mutated protein. Furthermore, cells with the mutant ATP2B3 appeared to have a reduced capacity to export Ca(2+) suggesting a loss of the physiological pump function. Surprisingly, expression of the mutant ATP2B3 caused a Na(+)-dependent inward current that strongly depolarized the plasma membrane and compromised the cytosolic cation composition. In parallel to these findings, mRNA expression of the cytochrome P450, family 11, subfamily B, polypeptide 2 (aldosterone synthase) was substantially increased and aldosterone production was enhanced in cells overexpressing mutant ATP2B3. In summary, the APA-associated ATP2B3(Leu425_Val426del) mutant promotes aldosterone production by at least 2 different mechanisms: 1) a reduced Ca(2+) export due to the loss of the physiological pump function; and 2) an increased Ca(2+) influx due to opening of depolarization-activated Ca(2+) channels as well as a possible Ca(2+) leak through the mutated pump.

Local Control of Aldosterone Production and Primary Aldosteronism

Primary aldosteronism (PA) is caused by excessive production of aldosterone by the adrenal cortex and is determined by a benign aldosterone-producing adenoma (APA) in a significant proportion of cases. Local mechanisms, as opposed to circulatory ones, that control aldosterone production in the adrenal cortex are particularly relevant in the physiopathological setting and in the pathogenesis of PA. A breakthrough in our understanding of the pathogenetic mechanisms in APA has been the identification of somatic mutations in genes controlling membrane potential and intracellular calcium concentrations. However, recent data show that the processes of nodule formation and aldosterone hypersecretion can be dissociated in pathological adrenals and suggest a model envisaging different molecular events for the pathogenesis of APA.

Mouse models of adrenocortical tumors

The molecular basis of the organogenesis, homeostasis, and tumorigenesis of the adrenal cortex has been the subject of intense study for many decades. Specifically, characterization of tumor predisposition syndromes with adrenocortical manifestations and molecular profiling of sporadic adrenocortical tumors have led to the discovery of key molecular pathways that promote pathological adrenal growth. However, given the observational nature of such studies, several important questions regarding the molecular pathogenesis of adrenocortical tumors have remained. This review will summarize naturally occurring and genetically engineered mouse models that have provided novel tools to explore the molecular and cellular underpinnings of adrenocortical tumors. New paradigms of cancer initiation, maintenance, and progression that have emerged from this work will be discussed.

Potassium channels related to primary aldosteronism: Expression similarities and differences between human and rat adrenals

Three potassium channels have been associated with primary aldosteronism (PA) in rodents and humans: KCNK3 (TASK-1), KCNK9 (TASK-3), and KCNJ5 (Kir3.4). Mice with deficiency in Kcnk3 and Kcnk9 have elevated aldosterone production and blood pressure. In humans, adrenal tumors with somatic mutations in KCNJ5 cause PA. However, there are very few reports on the expression patterns of these genes in humans versus rodents. Herein, we compared human and rat mRNA expression (by quantitative real-time polymerase chain reaction (qPCR) and protein levels (by immunohistochemistry) across three tissues (adrenal, brain, heart) and two laser-captured adrenal zones (zona glomerulosa, zona fasciculata). Our findings show that expression patterns of KCNK3, KCNK9, and KCNJ5 are inconsistent between rats and humans across both tissues and adrenal zones. Thus, species variation in the expression of PA-related potassium channels indicates an evolutionary divergence in their role in regulating adrenal aldosterone production.

Pathogenesis of Adrenal Aldosterone-Producing Adenomas Carrying Mutations of the Na(+)/K(+)-ATPase

Aldosterone-producing adenoma (APA) is a major cause of primary aldosteronism, leading to secondary hypertension. Somatic mutations in the gene for the α1 subunit of the Na(+)/K(+)-ATPase were found in about 6% of APAs. APA-related α1 subunit of the Na(+)/K(+)-ATPase mutations lead to a loss of the pump function of the Na(+)/K(+)-ATPase, which is believed to result in membrane depolarization and Ca(2+)-dependent stimulation of aldosterone synthesis in adrenal cells. In addition, H(+) and Na(+) leak currents via the mutant Na(+)/K(+)-ATPase were suggested to contribute to the phenotype. The aim of this study was to investigate the cellular pathophysiology of adenoma-associated Na(+)/K(+)-ATPase mutants (L104R, V332G, G99R) in adrenocortical NCI-H295R cells. The expression of these Na(+)/K(+)-ATPase mutants depolarized adrenal cells and stimulated aldosterone secretion. However, an increase of basal cytosolic Ca(2+) levels in Na(+)/K(+)-ATPase mutant cells was not detectable, and stimulation with high extracellular K(+) hardly increased Ca(2+) levels in cells expressing L104R and V332G mutant Na(+)/K(+)-ATPase. Cytosolic pH measurements revealed an acidification of L104R and V332G mutant cells, despite an increased activity of the Na(+)/H(+) exchanger. The possible contribution of cellular acidification to the hypersecretion of aldosterone was supported by the observation that aldosterone secretion of normal adrenocortical cells was stimulated by acetate-induced acidification. Taken together, mutations of the Na(+)/K(+)-ATPase depolarize adrenocortical cells, disturb the K(+) sensitivity, and lower intracellular pH but, surprisingly, do not induce an overt increase of intracellular Ca(2+). Probably, the autonomous aldosterone secretion is caused by the concerted action of several pathological signaling pathways and incomplete cellular compensation.

Bilateral Idiopathic Adrenal Hyperplasia: Genetics and Beyond

Bilateral adrenal hyperplasia currently accounts for up to 2 thirds of cases of primary aldosteronism. As such, it represents a major opportunity for targeted medical management as opposed to unilateral surgically correctable forms of the disease. Although the majority of cases of primary aldosteronism are sporadic, bilateral adrenal hyperplasia may occur in the context of familial hyperaldosteronism where it is associated with specific germline mutations. Over the past 5 years, impressive progress has been made in our understanding of the genetic basis underlying primary aldosteronism, allowing us to identify and characterize new familial forms of the disease and to understand the mechanisms involved in the formation of aldosterone producing adenoma. In contrast, our knowledge of the genetic contribution to the development of bilateral adrenal hyperplasia, and in a larger context, to renin and aldosterone levels in the general population, is still poor. This review summarizes our current knowledge on the genetics of bilateral adrenal hyperplasia and addresses some open questions to be addressed by future research. In particular, genome-wide association studies in large populations may provide clues to understanding the genetic susceptibility underlying the development of primary aldosteronism.

Breathing Stimulant Compounds Inhibit TASK-3 Potassium Channel Function Likely by Binding at a Common Site in the Channel Pore

Compounds PKTHPP (1-{1-[6-(biphenyl-4-ylcarbonyl)-5,6,7,8-tetrahydropyrido[4,3-d]-pyrimidin-4-yl]piperidin-4-yl}propan-1-one), A1899 (2''-[(4-methoxybenzoylamino)methyl]biphenyl-2-carboxylic acid 2,4-difluorobenzylamide), and doxapram inhibit TASK-1 (KCNK3) and TASK-3 (KCNK9) tandem pore (K2P) potassium channel function and stimulate breathing. To better understand the molecular mechanism(s) of action of these drugs, we undertook studies to identify amino acid residues in the TASK-3 protein that mediate this inhibition. Guided by homology modeling and molecular docking, we hypothesized that PKTHPP and A1899 bind in the TASK-3 intracellular pore. To test our hypothesis, we mutated each residue in or near the predicted PKTHPP and A1899 binding site (residues 118-128 and 228-248), individually, to a negatively charged aspartate. We quantified each mutation's effect on TASK-3 potassium channel concentration response to PKTHPP. Studies were conducted on TASK-3 transiently expressed in Fischer rat thyroid epithelial monolayers; channel function was measured in an Ussing chamber. TASK-3 pore mutations at residues 122 (L122D, E, or K) and 236 (G236D) caused the IC50 of PKTHPP to increase more than 1000-fold. TASK-3 mutants L122D, G236D, L239D, and V242D were resistant to block by PKTHPP, A1899, and doxapram. Our data are consistent with a model in which breathing stimulant compounds PKTHPP, A1899, and doxapram inhibit TASK-3 function by binding at a common site within the channel intracellular pore region, although binding outside the channel pore cannot yet be excluded.

Two-pore domain potassium channels in the adrenal cortex

The physiological control of steroid hormone secretion from the adrenal cortex depends on the function of potassium channels. The "two-pore domain K(+) channels" (K2P) TWIK-related acid sensitive K(+) channel 1 (TASK1), TASK3, and TWIK-related K(+) channel 1 (TREK1) are strongly expressed in adrenocortical cells. They confer a background K(+) conductance to these cells which is important for the K(+) sensitivity as well as for angiotensin II and adrenocorticotropic hormone-dependent stimulation of aldosterone and cortisol synthesis. Mice with single deletions of the Task1 or Task3 gene as well as Task1/Task3 double knockout mice display partially autonomous aldosterone synthesis. It appears that TASK1 and TASK3 serve different functions: TASK1 affects cell differentiation and prevents expression of aldosterone synthase in the zona fasciculata, while TASK3 controls aldosterone secretion in glomerulosa cells. TREK1 is involved in the regulation of cortisol secretion in fasciculata cells. These data suggest that a disturbed function of K2P channels could contribute to adrenocortical pathologies in humans.

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

Regulation of aldosterone synthesis and secretion

Aldosterone is a steroid hormone synthesized in and secreted from the outer layer of the adrenal cortex, the zona glomerulosa. Aldosterone is responsible for regulating sodium homeostasis, thereby helping to control blood volume and blood pressure. Insufficient aldosterone secretion can lead to hypotension and circulatory shock, particularly in infancy. On the other hand, excessive aldosterone levels, or those too high for sodium status, can cause hypertension and exacerbate the effects of high blood pressure on multiple organs, contributing to renal disease, stroke, visual loss, and congestive heart failure. Aldosterone is also thought to directly induce end-organ damage, including in the kidneys and heart. Because of the significance of aldosterone to the physiology and pathophysiology of the cardiovascular system, it is important to understand the regulation of its biosynthesis and secretion from the adrenal cortex. Herein, the mechanisms regulating aldosterone production in zona glomerulosa cells are discussed, with a particular emphasis on signaling pathways involved in the secretory response to the main controllers of aldosterone production, the renin-angiotensin II system, serum potassium levels and adrenocorticotrophic hormone. The signaling pathways involved include phospholipase C-mediated phosphoinositide hydrolysis, inositol 1,4,5-trisphosphate, cytosolic calcium levels, calcium influx pathways, calcium/calmodulin-dependent protein kinases, diacylglycerol, protein kinases C and D, 12-hydroxyeicostetraenoic acid, phospholipase D, mitogen-activated protein kinase pathways, tyrosine kinases, adenylate cyclase, and cAMP-dependent protein kinase. A complete understanding of the signaling events regulating aldosterone biosynthesis may allow the identification of novel targets for therapeutic interventions in hypertension, primary aldosteronism, congestive heart failure, renal disease, and other cardiovascular disorders.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

An update on novel mechanisms of primary aldosteronism

Primary aldosteronism (PA) is the most common and curable form of secondary hypertension. It is caused in the majority of cases by either unilateral aldosterone overproduction due to an aldosterone-producing adenoma (APA) or by bilateral adrenal hyperplasia. Recent advances in genome technology have allowed researchers to unravel part of the genetic abnormalities underlying the development of APA and familial hyperaldosteronism. Recurrent somatic mutations in genes coding for ion channels (KCNJ5 and CACNA1D) and ATPases (ATP1A1 and ATP2B3) regulating intracellular ionic homeostasis and cell membrane potential have been identified in APA. Similar germline mutations of KCNJ5 were identified in a severe familial form of PA, familial hyperaldosteronism type 3 (FH3), whereas de novo germline CACNA1D mutations were found in two cases of hyperaldosteronism associated with a complex neurological disorder. These results have allowed a pathophysiological model of APA development to be established. This model involves modifications in intracellular ionic homeostasis and membrane potential, accounting for ∼50% of all tumors, associated with specific gender differences and severity of PA. In this review, we describe the different genetic abnormalities associated with PA and discuss the mechanisms whereby they lead to increased aldosterone production and cell proliferation. We also address some of the foreseeable consequences that genetic knowledge may contribute to improve diagnosis and patient care.

The family of K2P channels: salient structural and functional properties

Potassium channels participate in many biological functions, from ion homeostasis to generation and modulation of the electrical membrane potential. They are involved in a large variety of diseases. In the human genome, 15 genes code for K(+) channels with two pore domains (K2P ). These channels form dimers of pore-forming subunits that produce background conductances finely regulated by a range of natural and chemical effectors, including signalling lipids, temperature, pressure, pH, antidepressants and volatile anaesthetics. Since the cloning of TWIK1, the prototypical member of this family, a lot of work has been carried out on their structure and biology. These studies are still in progress, but data gathered so far show that K2P channels are central players in many processes, including ion homeostasis, hormone secretion, cell development and excitability. A growing number of studies underline their implication in physiopathological mechanisms, such as vascular and pulmonary hypertension, cardiac arrhythmias, nociception, neuroprotection and depression. This review gives a synthetic view of the most noticeable features of these channels.

Molecular and Cellular Mechanisms of Aldosterone Producing Adenoma Development

Primary aldosteronism (PA) is the most common form of secondary hypertension with an estimated prevalence of ~10% in referred patients. PA occurs as a result of a dysregulation of the normal mechanisms controlling adrenal aldosterone production. It is characterized by hypertension with low plasma renin and elevated aldosterone and often associated with hypokalemia. The two major causes of PA are unilateral aldosterone producing adenoma (APA) and bilateral adrenal hyperplasia, accounting together for ~95% of cases. In addition to the well-characterized effect of excess mineralocorticoids on blood pressure, high levels of aldosterone also have cardiovascular, renal, and metabolic consequences. Hence, long-term consequences of PA include increased risk of coronary artery disease, myocardial infarction, heart failure, and atrial fibrillation. Despite recent progress in the management of patients with PA, critical issues related to diagnosis, subtype differentiation, and treatment of non-surgically correctable forms still persist. A better understanding of the pathogenic mechanisms of the disease should lead to the identification of more reliable diagnostic and prognostic biomarkers for a more sensitive and specific screening and new therapeutic options. In this review, we will summarize our current knowledge on the molecular and cellular mechanisms of APA development. On one hand, we will discuss how various animal models have improved our understanding of the pathophysiology of excess aldosterone production. On the other hand, we will summarize the major advances made during the last few years in the genetics of APA due to transcriptomic studies and whole exome sequencing. The identification of recurrent and somatic mutations in genes coding for ion channels (KCNJ5 and CACNA1D) and ATPases (ATP1A1 and ATP2B3) allowed highlighting the central role of calcium signaling in autonomous aldosterone production by the adrenal.

The molecular basis of primary aldosteronism: from chimeric gene to channelopathy

Primary aldosteronism (PA) is the most common endocrine cause of high blood pressure. Only a minority of the PA cases are familial and due to known (CYP11B2/CYP11B1 chimeric gene or mutations in the KCNJ5 gene) or unknown causes. In the most common sporadic cases the mechanisms by which the excess aldosterone production persists in spite of high blood pressure, sodium retention, suppression of the renin angiotensin system and low potassium levels, all factors that by themselves would be expected to shut off aldosterone production, were a puzzle for decades. Only recently the discovery of functional mutations and down-regulation of potassium channels provided some explanations. We herein reviewed these recent findings and their mechanistic implications. We also propose a clinical molecular classification of familial hyperaldosteronism, which can be important from the practical standpoint as it considers besides the molecular features also the responsiveness to treatment and the imaging features.

Regulation of aldosterone biosynthesis by the Kir3.4 (KCNJ5) potassium channel

The G-protein-activated inwardly rectifying potassium channel Kir3.4 is expressed in the zona glomerulosa cell membrane and transports potassium out of the cell.  Angiotensin II stimulation of aldosterone secretion is mediated, in part, by suppression of the transcription of KCNJ5, the gene coding for Kir3.4, and blocking channel activity. This results in membrane depolarization, mobilization of intracellular calcium, activation of the calcium-calmodulin pathway and increasing gene transcription of steroidogenic enzymes required for aldosterone secretion.  In 40-60% of aldosterone-producing adenomas there is a somatic mutation in the region of the KCNJ5 gene that codes for the selectivity filter that decreases potassium selectivity, allowing sodium to leak into the cells, thus depolarizing the membrane and initiating events that result in increased aldosterone synthesis.  The mechanism by which mutated KCNJ5 induces cell proliferation and adenoma formation remains unclear.

Lower expression of the TWIK-related acid-sensitive K+ channel 2 (TASK-2) gene is a hallmark of aldosterone-producing adenoma causing human primary aldosteronism

CONTEXT

The molecular mechanisms of primary aldosteronism, a common cause of human hypertension, are unknown, but alterations of K(+) channels can play a key role.

OBJECTIVE

The objective of the study was to investigate the following: 1) the expression of the Twik-related acid-sensitive K(+) channels (TASK) in aldosterone producing adenomas (APAs); 2) the role of TASK-2 in aldosterone synthesis; and 3) the determinants of TASK-2-blunted expression in APAs.

DESIGN

We analyzed the transcriptome and the microRNA profiles of 32 consecutive APAs and investigated the protein expression and localization of TASK-2 in APA and adrenocortical cell lines (H295R and HAC15) using immunoblotting and confocal microscopy. The functional effect of TASK-2 blunted activity caused by a dominant-negative mutation on steroidogenic enzymes, and aldosterone production was also assessed. TASK-2 regulation by selected microRNA was studied by a luciferase assay.

RESULTS

TASK-2 was consistently less expressed at the transcript and protein levels in APAs than in the normal human adrenal cortex. H295R cell transfection with a TASK-2 dominant-negative mutant construct significantly increased the aldosterone production by 153% and the gene expression of aldosterone synthase (CYP11B2, gene expression fold change 3.1 vs control, P < .05) and the steroidogenic acute regulatory protein (gene expression fold change 1.8 vs control, P < .05). Two microRNAs, hsa-miR-23 and hsa-miR-34, were found to decrease the TASK-2 expression by binding to the 3' untranslated region of the TASK-2 gene.

CONCLUSIONS

The TASK-2 channel lower expression represents a hallmark of APA and is associated with a higher expression of hsa-miR-23 and hsa-miR-34. The ensuing blunted TASK-2 activity increased the production of aldosterone in vitro and the expression of steroidogenic acute regulatory protein and CYP11B2. Hence, the lower expression of TASK-2 channel in APA cells can explain high aldosterone secretion in human primary aldosteronism despite the suppression of angiotensin II, hypertension, and hypokalemia.

Pharmacology and pathophysiology of mutated KCNJ5 found in adrenal aldosterone-producing adenomas

Somatic mutations of the potassium channel KCNJ5 are found in 40% of aldosterone producing adenomas (APAs). APA-related mutations of KCNJ5 lead to a pathological Na(+) permeability and a rise in cytosolic Ca(2+), the latter presumably by depolarizing the membrane and activating voltage-gated Ca(2+) channels. The aim of this study was to further investigate the effects of mutated KCNJ5 channels on intracellular Na(+) and Ca(2+) homeostasis in human adrenocortical NCI-H295R cells. Expression of mutant KCNJ5 led to a 2-fold increase in intracellular Na(+) and, in parallel, to a substantial rise in intracellular Ca(2+). The increase in Ca(2+) appeared to be caused by activation of voltage-gated Ca(2+) channels and by an impairment of Ca(2+) extrusion by Na(+)/Ca(2+) exchangers. The mutated KCNJ5 exhibited a pharmacological profile that differed from the one of wild-type channels. Mutated KCNJ5 was less Ba(2+) and tertiapin-Q sensitive but was inhibited by blockers of Na(+) and Ca(2+)-transporting proteins, such as verapamil and amiloride. The clinical use of these drugs might influence aldosterone levels in APA patients with KCNJ5 mutations. This might implicate diagnostic testing of APAs and could offer new therapeutic strategies.

Association of the variations in the HSD3β gene with primary aldosteronism

OBJECTIVE

In mice, a lack of cryptochrome results in up-regulation of aldosterone production due to high expression of the 3β-hydroxysteroid dehydrogenases (HSD3β) gene. The HSD3β pathway might play a pivotal role in aldosterone synthesis. This study aimed to determine the association of HSD3β and HSD3β2 gene variations with primary aldosteronism in a Taiwanese population.

METHOD

In this case-control cohort, 688 consecutive ethnically matched unrelated individuals including 362 primary aldosteronism and 326 essential hypertension cases were recruited. Nineteen tag single-nucleotide polymorphisms (SNPs) across HSD3β1, HSD3β2, and CYP11β2 were genotyped. Expression of HSD3β mRNA and immunohistochemical stain of HSD3β in the specimens of aldosterone-producing adenoma (APA) was compared with that in nonfunctional incidentaloma.

RESULTS

The SNPs of rs12410453 A allele in HSD3β2 gene [odds ratio (OR) 1.92, 95% confidence interval (CI) 1.13-3.32, P=0.018] and rs6203 C allele in the HSD3β1 gene (OR 2.21, 95% CI 1.28-3.95, P=0.006) showed significant association with primary aldosteronism, with corresponding population attributable risk of 6.7 and 30.7%, respectively. Primary aldosteronism patients of non-CC in rs6203 and non-GA in rs12401453 had lower plasma aldosterone-to-renin ratio. A haplotype in a linkage disequilibrium block containing rs6203 associated significantly with serum potassium level (OR 1.24, 95% CI 1.02-1.24, P=0.026). The expressions of HSD3β1 mRNA, HSD3β2 mRNA and HSD3β protein were increased in APA, as compared to incidentaloma.

CONCLUSION

Risk-conferring genetic variations in the HSD3β gene influenced susceptibility of primary aldosteronism. Concomitant presence of rs6203 CC and rs12410453 GA genotypes synergistically increased aldosterone-to-renin ratio.

Minireview: potassium channels and aldosterone dysregulation: is primary aldosteronism a potassium channelopathy?

Primary aldosteronism is the most common form of secondary hypertension and has significant cardiovascular consequences. Aldosterone-producing adenomas (APAs) are responsible for half the cases of primary aldosteronism, and about half have mutations of the G protein-activated inward rectifying potassium channel Kir3.4. Under basal conditions, the adrenal zona glomerulosa cells are hyperpolarized with negative resting potentials determined by membrane permeability to K(+) mediated through various K(+) channels, including the leak K(+) channels TASK-1, TASK-3, and Twik-Related Potassium Channel 1, and G protein inward rectifying potassium channel Kir3.4. Angiotensin II decreases the activity of the leak K(+) channels and Kir3.4 channel and decreases the expression of the Kir3.4 channel, resulting in membrane depolarization, increased intracellular calcium, calcium-calmodulin pathway activation, and increased expression of cytochrome P450 aldosterone synthase (CYP11B2), the last enzyme for aldosterone production. Somatic mutations of the selectivity filter of the Kir3.4 channel in APA results in loss of selectivity for K(+) and entry of sodium, resulting in membrane depolarization, calcium mobilization, increased CYP11B2 expression, and hyperaldosteronism. Germ cell mutations cause familial hyperaldosteronism type 3, which is associated with adrenal zona glomerulosa hyperplasia, rather than adenoma. Less commonly, somatic mutations of the sodium-potassium ATPase, calcium ATPase, or the calcium channel calcium channel voltage-dependent L type alpha 1D have been found in some APAs. The regulation of aldosterone secretion is exerted to a significant degree by activation of membrane K(+) and calcium channels or pumps, so it is not surprising that the known causes of disorders of aldosterone secretion in APA have been channelopathies, which activate mechanisms that increase aldosterone synthesis.

Recovery of current through mutated TASK3 potassium channels underlying Birk Barel syndrome

TASK3 (TWIK-related acid-sensitive K(+) channel 3) potassium channels are members of the two-pore-domain potassium channel family. They are responsible for background leak potassium currents found in many cell types. TASK3 channels are genetically imprinted, and a mutation in TASK3 (G236R) is responsible for Birk Barel mental retardation dysmorphism syndrome, a maternally transmitted developmental disorder. This syndrome may arise from a neuronal migration defect during development caused by dysfunctional TASK3 channels. Through the use of whole-cell electrophysiologic recordings, we have found that, although G236R mutated TASK3 channels give rise to a functional current, this current is significantly smaller in an outward direction when compared with wild-type (WT) TASK3 channels. In contrast to WT TASK3 channels, the current is inwardly rectifying. Furthermore, the current through mutated channels is differentially sensitive to a number of regulators, such as extracellular acidification, extracellular zinc, and activation of Gαq-coupled muscarinic (M3) receptors, compared with WT TASK3 channels. The reduced outward current through mutated TASK3_G236R channels can be overcome, at least in part, by both a gain-of-function additional mutation of TASK3 channels (A237T) or by application of the nonsteroidal anti-inflammatory drug flufenamic acid (FFA; 2-{[3-(trifluoromethyl)phenyl]amino}benzoic acid). FFA produces a significantly greater enhancement of current through mutated channels than through WT TASK3 channels. We propose that pharmacologic enhancement of mutated TASK3 channel current during development may, therefore, provide a potentially useful therapeutic strategy in the treatment of Birk Barel syndrome.

Severe hyperaldosteronism in neonatal Task3 potassium channel knockout mice is associated with activation of the intraadrenal renin-angiotensin system

Task3 K(+) channels are highly expressed in the adrenal cortex and contribute to the angiotensin II and K(+) sensitivity of aldosterone-producing glomerulosa cells. Adult Task3(-/-) mice display a partially autonomous aldosterone secretion, subclinical hyperaldosteronism, and salt-sensitive hypertension. Here, we investigated the age dependence of the adrenal phenotype of Task3(-/-) mice. Compared with adults, newborn Task3(-/-) mice displayed a severe adrenal phenotype with strongly increased plasma levels of aldosterone, corticosterone, and progesterone. This adrenocortical dysfunction was accompanied by a modified gene expression profile. The most strongly up-regulated gene was the protease renin. Real-time PCR corroborated the strong increase in adrenal renin expression, and immunofluorescence revealed renin-expressing cells in the zona fasciculata. Together with additional factors, activation of the local adrenal renin system is probably causative for the severely disturbed steroid hormone secretion of neonatal Task3(-/-) mice. The changes in gene expression patterns of neonatal Task3(-/-) mice could also be relevant for other forms of hyperaldosteronism.