Zona glomerulosa cells of the mouse adrenal cortex are intrinsic electrical oscillators

T- and L-Type Calcium Channels Maintain Calcium Oscillations in the Murine Zona Glomerulosa

BACKGROUND

The zona glomerulosa of the adrenal gland is responsible for the synthesis and release of the mineralocorticoid aldosterone. This steroid hormone regulates salt reabsorption in the kidney and blood pressure. The most important stimuli of aldosterone synthesis are the serum concentrations of angiotensin II and potassium. In response to these stimuli, voltage and intracellular calcium levels in the zona glomerulosa oscillate, providing the signal for aldosterone synthesis. It was proposed that the voltage-gated T-type calcium channel CaV3.2 is necessary for the generation of these oscillations. However, Cacna1h knock-out mice have normal plasma aldosterone levels, suggesting additional calcium entry pathways.

METHODS

We used a combination of calcium imaging, patch clamp, and RNA sequencing to investigate calcium influx pathways in the murine zona glomerulosa.

RESULTS

Cacna1h-/- glomerulosa cells still showed calcium oscillations with similar concentrations as wild-type mice. No calcium channels or transporters were upregulated to compensate for the loss of CaV3.2. The calcium oscillations observed were instead dependent on L-type voltage-gated calcium channels. Furthermore, we found that L-type channels can also partially compensate for an acute inhibition of CaV3.2 in wild-type mice. Only inhibition of both T- and L-type calcium channels abolished the increase of intracellular calcium caused by angiotensin II in wild-type.

CONCLUSIONS

Our study demonstrates that T-type calcium channels are not strictly required to maintain glomerulosa calcium oscillations and aldosterone production. Pharmacological inhibition of T-type channels alone will likely not significantly impact aldosterone production in the long term.

Adrenal Anion Channels: New Roles in Zona Glomerulosa Physiology and in the Pathophysiology of Primary Aldosteronism

The mineralocorticoid aldosterone is produced in the zona glomerulosa of the adrenal cortex. Its synthesis is regulated by the serum concentrations of the peptide hormone angiotensin II and potassium. The primary role of aldosterone is to control blood volume and electrolytes. The autonomous production of aldosterone (primary aldosteronism, PA) is considered the most frequent cause of secondary hypertension. Aldosterone-producing adenomas and (micro-)nodules are frequent causes of PA and often carry somatic mutations in ion channels and transporters. Rare familial forms of PA are due to germline mutations. Both somatic and germline mutations in the chloride channel gene CLCN2, encoding ClC-2, have been identified in PA. Clinical findings and results from cell culture and animal models have advanced our knowledge about the role of anions in PA. The zona glomerulosa of the adrenal gland has now been firmly established as a tissue in which anions play a significant role for signaling. In this overview, we aim to summarize the current knowledge and highlight novel concepts as well as open questions.

Relevance of KCNJ5 in Pathologies of Heart Disease

Abnormalities in G-protein-gated inwardly rectifying potassium (GIRK) channels have been implicated in diseased states of the cardiovascular system; however, the role of GIRK4 (Kir3.4) in cardiac physiology and pathophysiology has yet to be completely understood. Within the heart, the KACh channel, consisting of two GIRK1 and two GIRK4 subunits, plays a major role in modulating the parasympathetic nervous system's influence on cardiac physiology. Being that GIRK4 is necessary for the functional KACh channel, KCNJ5, which encodes GIRK4, it presents as a therapeutic target for cardiovascular pathology. Human variants in KCNJ5 have been identified in familial hyperaldosteronism type III, long QT syndrome, atrial fibrillation, and sinus node dysfunction. Here, we explore the relevance of KCNJ5 in each of these diseases. Further, we address the limitations and complexities of discerning the role of KCNJ5 in cardiovascular pathophysiology, as identical human variants of KCNJ5 have been identified in several diseases with overlapping pathophysiology.

CaV3.2 (CACNA1H) in Primary Aldosteronism

Aldosterone is a steroid hormone produced in the zona glomerulosa (ZG) of the adrenal cortex. The most prominent function of aldosterone is the control of electrolyte homeostasis and blood pressure via the kidneys. The primary factors regulating aldosterone synthesis are the serum concentrations of angiotensin II and potassium. The T-type voltage-gated calcium channel Ca_V3.2 (encoded by CACNA1H) is an important component of electrical as well as intracellular calcium oscillations, which govern aldosterone production in the ZG. Excessive aldosterone production that is (partially) uncoupled from physiological stimuli leads to primary aldosteronism, the most common cause of secondary hypertension. Germline gain-of-function mutations in CACNA1H were identified in familial hyperaldosteronism, whereas somatic mutations are a rare cause of aldosterone-producing adenomas. In this review, we summarize these findings, put them in perspective, and highlight missing knowledge.

Somatic mutations of CADM1 in aldosterone-producing adenomas and gap junction-dependent regulation of aldosterone production

Aldosterone-producing adenomas (APAs) are the commonest curable cause of hypertension. Most have gain-of-function somatic mutations of ion channels or transporters. Herein we report the discovery, replication and phenotype of mutations in the neuronal cell adhesion gene CADM1. Independent whole exome sequencing of 40 and 81 APAs found intramembranous p.Val380Asp or p.Gly379Asp variants in two patients whose hypertension and periodic primary aldosteronism were cured by adrenalectomy. Replication identified two more APAs with each variant (total, n = 6). The most upregulated gene (10- to 25-fold) in human adrenocortical H295R cells transduced with the mutations (compared to wildtype) was CYP11B2 (aldosterone synthase), and biological rhythms were the most differentially expressed process. CADM1 knockdown or mutation inhibited gap junction (GJ)-permeable dye transfer. GJ blockade by Gap27 increased CYP11B2 similarly to CADM1 mutation. Human adrenal zona glomerulosa (ZG) expression of GJA1 (the main GJ protein) was patchy, and annular GJs (sequelae of GJ communication) were less prominent in CYP11B2-positive micronodules than adjacent ZG. Somatic mutations of CADM1 cause reversible hypertension and reveal a role for GJ communication in suppressing physiological aldosterone production.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

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.

Targeting the T-type calcium channel Cav3.2 in GABAergic arcuate nucleus neurons to treat obesity

OBJECTIVE

Cav3.2, a T-type low voltage-activated calcium channel widely expressed throughout the central nervous system, plays a vital role in neuronal excitability and various physiological functions. However, the effects of Cav3.2 on energy homeostasis remain unclear. Here, we examined the role of Cav3.2 expressed by hypothalamic GABAergic neurons in the regulation of food intake and body weight in mice and explored the underlying mechanisms.

METHODS

Male congenital Cana1h (the gene coding for Cav3.2) global knockout (Cav3.2KO) mice and their wild type (WT) littermates were first used for metabolic phenotyping studies. By using the CRISPR-Cas9 technique, Cav3.2 was selectively deleted from GABAergic neurons in the arcuate nucleus of the hypothalamus (ARH) by specifically overexpressing Cas9 protein and Cav3.2-targeting sgRNAs in ARH Vgat (Vgat^ARH) neurons. These male mutants (Cav3.2KO-Vgat^ARH) were used to determine whether Cav3.2 expressed by Vgat^ARH neurons is required for the proper regulation of energy balance. Subsequently, we used an electrophysiological patch-clamp recording in ex vivo brain slices to explore the impact of Cav3.2KO on the cellular excitability of Vgat^ARH neurons.

RESULTS

Male Cav3.2KO mice had significantly lower food intake than their WT littermate controls when fed with either a normal chow diet (NCD) or a high-fat diet (HFD). This hypophagia phenotype was associated with increased energy expenditure and decreased fat mass, lean mass, and total body weight. Selective deletion of Cav3.2 in Vgat^ARH neurons resulted in similar feeding inhibition and lean phenotype without changing energy expenditure. These data provides an intrinsic mechanism to support the previous finding on ARH non-AgRP GABA neurons in regulating diet-induced obesity. Lastly, we found that naringenin extract, a predominant flavanone found in various fruits and herbs and known to act on Cav3.2, decreased the firing activity of Vgat^ARH neurons and reduced food intake and body weight. These naringenin-induced inhibitions were fully blocked in Cav3.2KO-Vgat^ARH mice.

CONCLUSION

Our results identified Cav3.2 expressed by Vgat^ARH neurons as an essential intrinsic modulator for food intake and energy homeostasis, which is a potential therapeutic target in the treatment of obesity.

Diseases caused by mutations in the Na+/K+ pump α1 gene ATP1A1

Human cell survival requires function of the Na⁺/K⁺ pump; the heteromeric protein that hydrolyzes ATP to extrude Na⁺ and import K⁺ across the plasmalemma, thereby building and maintaining these ions' electrochemical gradients. Numerous dominant diseases caused by mutations in genes encoding for Na⁺/K⁺ pump catalytic (α) subunit isoforms highlight the importance of this protein. Here, we review literature describing disorders caused by missense mutations in ATP1A1, the gene encoding the ubiquitously expressed α1 isoform of the Na⁺/K⁺ pump. These various maladies include primary aldosteronism with secondary hypertension, an endocrine syndrome, Charcot-Marie-Tooth disease, a peripheral neuropathy, complex spastic paraplegia, another neuromuscular disorder, as well as hypomagnesemia accompanied by seizures and cognitive delay, a condition affecting the renal and central nervous systems. This article focuses on observed commonalities among these mutations' functional effects, as well as on the special characteristics that enable each particular mutation to exclusively affect a certain system, without affecting others. In this respect, it is clear how somatic mutations localized to adrenal adenomas increase aldosterone production without compromising other systems. However, it remains largely unknown how and why some but not all de novo germline or familial mutations (where the mutant must be expressed in numerous tissues) produce a specific disease and not the other diseases. We propose hypotheses to explain this observation and the approaches that we think will drive future research on these debilitating disorders to develop novel patient-specific treatments by combining the use of heterologous protein-expression systems, patient-derived pluripotent cells, and gene-edited cell and mouse models.

Rosette morphology in zona glomerulosa formation and function

How morphology informs function is a fundamental biological question. Here, we review the morphological features of the adrenal zona glomerulosa (zG), highlighting recent cellular and molecular discoveries that govern its formation. The zG consists of glomeruli enwrapped in a Laminin-β1-enriched basement membrane (BM). Within each glomerulus, zG cells are organized as rosettes, a multicellular structure widely used throughout development to mediate epithelial remodeling, but not often found in healthy adult tissues. Rosettes arise by constriction at a common cellular contact point mediated/facilitated by adherens junctions (AJs). In mice, small, dispersed AJs first appear postnatally and enrich along the entire cell-cell contact around 10 days after birth. Subsequently, these AJ-rich contacts contract, allowing rosettes to form. Concurrently, flat sheet-like domains in the nascent zG, undergo invagination and folding, gradually giving rise to the compact round glomeruli that comprise the adult zG. How these structures impact adrenal function is discussed.

Enhanced Ca2+ signaling, mild primary aldosteronism, and hypertension in a familial hyperaldosteronism mouse model (Cacna1h M1560V/+ )

Primary aldosteronism (increased production of the adrenal steroid hormone aldosterone) is the most common cause of secondary hypertension. We here generated a mouse model of familial hyperaldosteronism type IV with a heterozygous gain-of-function mutation in a calcium channel gene (Cacna1h^M1560V/+). Cacna1h^M1560V/+ mice have about twofold elevated aldosterone:renin ratios (a screening parameter for primary aldosteronism) and elevated blood pressure, with an overall mild phenotype. Elevated adrenal aldosterone synthase expression in Cacna1h^M1560V/+ mice is associated with increased intracellular calcium concentrations in glomerulosa cells. This model allows for the ex vivo analysis of calcium signaling in aldosterone-producing glomerulosa cells of the adrenal gland. Cacna1h^−/− mice have normal aldosterone synthase expression, with implications for the evaluation of CACNA1H as a therapeutic target.

Gain-of-function mutations in the CACNA1H gene (encoding the T-type calcium channel Ca_V3.2) cause autosomal-dominant familial hyperaldosteronism type IV (FH-IV) and early-onset hypertension in humans. We used CRISPR/Cas9 to generate Cacna1h^M1560V/+ knockin mice as a model of the most common FH-IV mutation, along with corresponding knockout mice (Cacna1h^−/−). Adrenal morphology of both Cacna1h^M1560V/+ and Cacna1h^−/− mice was normal. Cacna1h^M1560V/+ mice had elevated aldosterone:renin ratios (a screening parameter for primary aldosteronism). Their adrenal Cyp11b2 (aldosterone synthase) expression was increased and remained elevated on a high-salt diet (relative autonomy, characteristic of primary aldosteronism), but plasma aldosterone was only elevated in male animals. The systolic blood pressure of Cacna1h^M1560V/+ mice was 8 mmHg higher than in wild-type littermates and remained elevated on a high-salt diet. Cacna1h^−/− mice had elevated renal Ren1 (renin-1) expression but normal adrenal Cyp11b2 levels, suggesting that in the absence of Ca_V3.2, stimulation of the renin-angiotensin system activates alternative calcium entry pathways to maintain normal aldosterone production. On a cellular level, Cacna1h^M1560V/+ adrenal slices showed increased baseline and peak intracellular calcium concentrations in the zona glomerulosa compared to controls, but the frequency of calcium spikes did not rise. We conclude that FH-IV, on a molecular level, is caused by elevated intracellular Ca²⁺ concentrations as a signal for aldosterone production in adrenal glomerulosa cells. We demonstrate that a germline Cacna1h gain-of-function mutation is sufficient to cause mild primary aldosteronism, whereas loss of Ca_V3.2 channel function can be compensated for in a chronic setting.

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.

The Effect of Extracellular Calcium Metabolism on Aldosterone Biosynthesis in Physiological and Pathological Status

Primary aldosteronism (PA) was reported to frequently harbor not only cardiovascular diseases but also some metabolic disorders including secondary calcium metabolic diseases. Recently, the potential association between aldosterone producing cells and systemic calcium metabolism has been proposed. For instance, PA is frequently associated with hypercalciuria or hypocalcemia, which subsequently stimulates parathyroid hormone (PTH) secretion. This altered calcium metabolism in PA patients could frequently result in secondary osteoporosis and fracture in some patients. On the other hand, extracellular calcium itself directly acts on adrenal cortex and has been also proposed as an independent regulator of aldosterone biosynthesis in human adrenals. However, it is also true that both PTH and vitamin D pathways stimulate endocrine functions of adrenal cortical adenomas to co-secret both aldosterone and cortisol. Therefore, it has become pivotal to explore the potential crosstalk between aldosterone and systemic calcium metabolism. We herein reviewed recent advances in these fields.

Inflammasome activation in podocytes: a new mechanism of glomerular diseases

INTRODUCTION

Inflammasome is a multi-protein complex which is an important constituent of innate immunity. It mainly consists of three parts, apoptosis-associated speck-like protein containing caspase recruitment domain (ASC), caspase protease, and a NOD-like receptor (NLR) family protein (such as NLRP1) or an HIN200 family protein (such as AIM2). Inflammasome is widely studied in many autoimmune diseases and chronic inflammatory reactions, such as familial periodic autoinflammatory response, type 2 diabetes, Alzheimer's disease, and atherosclerosis. Activation of inflammasome in the kidney has been widely reported in glomerular and tubular-interstitial diseases. Podocytes play a critical role in maintaining the normal structure and function of glomerular filtration barrier. Recently, it has been demonstrated that podocytes, as a group of renal residential cells, can express all necessary components of NLRP3 inflammasome, which is activated and contribute to inflammatory response in the local kidney.

METHODS

Literature review was conducted to further summarize current evidence of podocyte NLRP3 inflammasome activation and related molecular mechanisms under different disease conditions.

RESULTS

Podocytes are a key component of the glomerular filtration barrier, and the loss of podocyte regeneration is a major limiting factor in the recovery of proteinuria. Through a more comprehensive study of inflammasome in podocytes, it will provide new targets and possibilities for the treatment of kidney diseases.

Angiotensin II induces coordinated calcium bursts in aldosterone-producing adrenal rosettes

Aldosterone-producing zona glomerulosa (zG) cells of the adrenal gland arrange in distinct multi-cellular rosettes that provide a structural framework for adrenal cortex morphogenesis and plasticity. Whether this cyto-architecture also plays functional roles in signaling remains unexplored. To determine if structure informs function, we generated mice with zG-specific expression of GCaMP3 and imaged zG cells within their native rosette structure. Here we demonstrate that within the rosette, angiotensin II evokes periodic Cav3-dependent calcium events that form bursts that are stereotypic in form. Our data reveal a critical role for angiotensin II in regulating burst occurrence, and a multifunctional role for the rosette structure in activity-prolongation and coordination. Combined our data define the calcium burst as the fundamental unit of zG layer activity evoked by angiotensin II and highlight a novel role for the rosette as a facilitator of cell communication.

L- and T-type calcium channels control aldosterone production from human adrenals

Aldosterone, which plays a key role in the regulation of blood pressure, is produced by zona glomerulosa (ZG) cells of the adrenal cortex. Exaggerated overproduction of aldosterone from ZG cells causes primary hyperaldosteronism. In ZG cells, calcium entry through voltage-gated calcium channels plays a central role in the regulation of aldosterone secretion. Previous studies in animal adrenals and human adrenal adrenocortical cell lines suggest that the T-type but not the L-type calcium channel activity drives aldosterone production. However, recent clinical studies show that somatic mutations in L-type calcium channels are the second most prevalent cause of aldosterone-producing adenoma. Our objective was to define the roles of T and L-type calcium channels in regulating aldosterone secretion from human adrenals. We find that human adrenal ZG cells mainly express T-type CaV3.2/3.3 and L-type CaV1.2/1.3 calcium channels. TTA-P2, a specific inhibitor of T-type calcium channel subtypes, reduced basal aldosterone secretion from acutely prepared slices of human adrenals. Surprisingly, nifedipine, the prototypic inhibitor of L-type calcium channels, also decreased basal aldosterone secretion, suggesting that L-type calcium channels are active under basal conditions. In addition, TTA-P2 or nifedipine also inhibited aldosterone secretion stimulated by angiotensin II- or elevations in extracellular K+. Remarkably, blockade of either L- or T-type calcium channels inhibits basal and stimulated aldosterone production to a similar extent. Low concentrations of TTA-P2 and nifedipine showed additive inhibitory effect on aldosterone secretion. We conclude that T- and L-type calcium channels play equally important roles in controlling aldosterone production from human adrenals.

Splice-variant-specific effects of primary aldosteronism point mutations on human CaV3.2 calcium channels

Ca_V3.2 calcium channels play important roles in both neural excitability and aldosterone secretion. Recent clinical studies found four germline mutations (S196 L, M1549I, V1951E and P2083 L) in Ca_V3.2 channels. All four mutations caused primary aldosteronism (PA), while only the M1549I mutation resulted in obvious neural malfunctions besides PA. In human, there are two major Ca_V3.2 channel gene (CACNA1H) splice variants, either with or without exon 26. In this study, we tested the expression of the two CACNA1H splice variants in zona glomerulosa (ZG) cells of human adrenal cortex and the possibility that Ca_V3.2 (-26) and Ca_V3.2 (+26) channels have different functional responses to the four PA mutations. We found that human ZG cells only express long form Ca_V3.2(+26) channels. The M1549I mutation slowed the inactivation of Ca_V3.2(+26) more than 5 fold, and Ca_V3.2(-26) more than 2 fold. The S196 L, V1951E and P2083 L mutations accelerated channel recovery from inactivation for Ca_V3.2(+26), but not Ca_V3.2(-26) channels. All four mutations resulted in gain of function of Ca_V3.2(+26) channels, leading to overproduction of aldosterone. In conclusion, the four PA mutations caused more profound changes on Ca_V3.2 (+26) currents than on Ca_V3.2 (-26) currents, and except the M1549I mutation, the S196 L, V1951E and P2083 L have little effect on the electrophysiological properties of Ca_V3.2(-26) currents, which may partially explain the limitation of the phenotype associated with the V1951E, S196 L and P2083 L germline mutations to PA.

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.

Aldosterone Jeopardizes Myocardial Insulin and β-Adrenergic Receptor Signaling via G Protein-Coupled Receptor Kinase 2

Hyperaldosteronism alters cardiac function, inducing adverse left ventricle (LV) remodeling either via increased fibrosis deposition, mitochondrial dysfunction, or both. These harmful effects are due, at least in part, to the activation of the G protein-coupled receptor kinase 2 (GRK2). In this context, we have previously reported that this kinase dysregulates both β-adrenergic receptor (βAR) and insulin (Ins) signaling. Yet, whether aldosterone modulates cardiac Ins sensitivity and βAR function remains untested. Nor is it clear whether GRK2 has a role in this modulation, downstream of aldosterone. Here, we show in vitro, in 3T3 cells, that aldosterone impaired insulin signaling, increasing the negative phosphorylation of insulin receptor substrate 1 (^ser307pIRS1) and reducing the activity of Akt. Similarly, aldosterone prevented the activation of extracellular signal-regulated kinase (ERK) and the production of cyclic adenosine 3′,5′-monophosphate (cAMP) in response to the β₁/β₂AR agonist, isoproterenol. Of note, all of these effects were sizably reduced in the presence of GRK2-inhibitor CMPD101. Next, in wild-type (WT) mice undergoing chronic infusion of aldosterone, we observed a marked GRK2 upregulation that was paralleled by a substantial β1AR downregulation and augmented ^ser307pIRS1 levels. Importantly, in keeping with the current in vitro data, we found that aldosterone effects were wholly abolished in cardiac-specific GRK2-knockout mice. Finally, in WT mice that underwent 4-week myocardial infarction (MI), we observed a substantial deterioration of cardiac function and increased LV dilation and fibrosis deposition. At the molecular level, these effects were associated with a significant upregulation of cardiac GRK2 protein expression, along with a marked β1AR downregulation and increased ^ser307pIRS1 levels. Treating MI mice with spironolactone prevented adverse aldosterone effects, blocking GRK2 upregulation, and thus leading to a marked reduction in cardiac ^ser307pIRS1 levels while rescuing β1AR expression. Our study reveals that GRK2 activity is a critical player downstream of the aldosterone signaling pathway; therefore, inhibiting this kinase is an attractive strategy to prevent the cardiac structural disarray and dysfunction that accompany any clinical condition accompanied by hyperaldosteronism.

Pathogenesis of Familial Hyperaldosteronism Type II: New Concepts Involving Anion Channels

PURPOSE OF REVIEW

The application of advanced genetic techniques has recently begun to unravel the genetic basis for familial primary aldosteronism type 2 (FH-II).

RECENT FINDINGS

Whole-exome sequencing in a large family with FH-II revealed a shared rare damaging heterozygous variant in CLCN2 (chr.3: g.184075850C>T, p.Arg172Gln) in three severely affected members. The gene encodes a chloride channel, ClC-2. A cohort of 80 unrelated individuals diagnosed with early-onset primary aldosteronism was also examined for CLCN2 mutations finding three further occurrences of p.Arg172Gln mutations and four single cases of other potentially damaging heterozygous mutations for an overall prevalence of 9.9%. A concurrent report also found a different CLCN2 mutation (p.Gly24Asp) in a single severely affected patient from a cohort of 12 with early-onset PA for a prevalence of 8.3%. Cases of primary aldosteronism associated with CLCN2 mutations appear to be bilateral and respond well to medical treatment. In the adrenal, ClC-2 has been demonstrated to localize predominantly to the zona glomerulosa (ZG), and functional analysis suggests that mutations in ClC-2 predispose ZG cells to depolarization, thus leading to calcium influx via activation of voltage-gated calcium channels and increased aldosterone production. Germline CLCN2 mutations appear to account for a substantial proportion of early-onset primary aldosteronism cases, and genetic testing for mutations in this gene should be considered in appropriate cases.

Na/K Pump Mutations Associated with Primary Hyperaldosteronism Cause Loss of Function

Primary hyperaldosteronism (Conn's syndrome), a common cause of secondary hypertension, is frequently produced by unilateral aldosterone-producing adenomas that carry mutations in ion-transporting genes, including ATP1A1, encoding the Na/K pump's α1 subunit. Whether Na/K pump mutant-mediated inward currents are required to depolarize the cell and increase aldosterone production remains unclear, as such currents were observed in four out of five mutants described so far. Here, we use electrophysiology and uptake of the K⁺ congener ⁸⁶Rb⁺, to characterize the effects of eight additional Na/K pump mutations in transmembrane segments TM1 (delM102-L103, delL103-L104, and delM102-I106), TM4 (delI322-I325 and I327S), and TM9 (delF956-E961, delF959-E961, and delE960-L964), expressed in Xenopus oocytes. All deletion mutants induced abnormal inward currents of different amplitudes at physiological voltages, while I327S lacked such currents. A detailed functional characterization revealed that I327S significantly reduces intracellular Na⁺ affinity without altering affinity for external K⁺. ⁸⁶Rb⁺-uptake experiments show that I327S dramatically impairs function under physiological concentrations of Na⁺ and K⁺. Since Na/K pumps in the adrenal cortex may be formed by association of α1 with β3 instead of β1 subunits, we evaluated whether G99R (another mutant without inward currents when associated with β1) would show inward currents when associated with β3. We found that the kinetic characteristics of either mutant or wild-type α1β3 pumps expressed in Xenopus oocytes to be indistinguishable from those of α1β1 pumps. The observed functional consequences of each hyperaldosteronism mutant point to the loss of Na/K pump function as the common feature of all mutants, which is sufficient to induce hyperaldosteronism.

M3-subtype muscarinic receptor activation stimulates intracellular calcium oscillations and aldosterone production in human adrenocortical HAC15 cells

A previous body of work in bovine and rodent models shows that cholinergic agonists modulate the secretion of steroid hormones from the adrenal cortex. In this study we used live-cell Ca²⁺ imaging to investigate cholinergic activity in the HAC15 human adrenocortical carcinoma cell line. The cholinergic agonists carbachol and acetylcholine triggered heterogeneous Ca²⁺ oscillations that were strongly inhibited by antagonists with high affinity for the M₃ muscarinic receptor subtype, while preferential block of M₁ or M₂ receptors was less effective. Acute exposure to carbachol and acetylcholine modestly elevated aldosterone secretion in HAC15 cells, and this effect was also diminished by M₃ inhibition. HAC15 cells expressed relatively high levels of mRNA for M₃ and M₂ receptors, while M₁ and M₅ mRNA were much lower. In conclusion, our data extend previous findings in non-human systems to implicate the M₃ receptor as the dominant muscarinic receptor in the human adrenal cortex.

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.

Aldosterone and Mineralocorticoid Receptor System in Cardiovascular Physiology and Pathophysiology

The mineralocorticoid hormone aldosterone (Aldo) has been intensively studied for its ability to influence both the physiology and pathophysiology of the cardiovascular system. Indeed, although research on Aldo actions for decades has mainly focused on its effects in the kidney, several lines of evidence have now demonstrated that this hormone exerts disparate extrarenal adverse effects, especially in the circulatory system. Accordingly, in the last lusters, a number of studies in preclinical models (in vitro and in vivo) and in humans have established that Aldo, following the interaction with its receptor-the mineralocorticoid receptor (MR)-is able to activate specific intracellular genomic and nongenomic pathways, thus regulating the homeostasis of the cardiovascular system. Importantly, through this mechanism of action, this hormone becomes a crucial regulator of the function and growth of different types of cells, including fibroblasts, cardiomyocytes, and vascular cells. For this main reason, it is plausible that when Aldo is present at high levels in the blood, it profoundly modifies the physiology of these cells, therefore being at the foundation of several cardiovascular disorders, such as heart failure (HF). On these grounds, in this review, we will provide an updated account on the current knowledge concerning Aldo activity in the cardiovascular system and the most recent preclinical studies and clinical trials designed to test better approaches able to counter the hyperactivity of the Aldo/MR signaling pathway in the setting of cardiovascular diseases.

A genetically encoded fluorescent acetylcholine indicator for in vitro and in vivo studies

The neurotransmitter acetylcholine (ACh) regulates a diverse array of physiological processes throughout the body. Despite its importance, cholinergic transmission in the majority of tissues and organs remains poorly understood owing primarily to the limitations of available ACh-monitoring techniques. We developed a family of ACh sensors (GACh) based on G-protein-coupled receptors that has the sensitivity, specificity, signal-to-noise ratio, kinetics and photostability suitable for monitoring ACh signals in vitro and in vivo. GACh sensors were validated with transfection, viral and/or transgenic expression in a dozen types of neuronal and non-neuronal cells prepared from multiple animal species. In all preparations, GACh sensors selectively responded to exogenous and/or endogenous ACh with robust fluorescence signals that were captured by epifluorescence, confocal, and/or two-photon microscopy. Moreover, analysis of endogenous ACh release revealed firing-pattern-dependent release and restricted volume transmission, resolving two long-standing questions about central cholinergic transmission. Thus, GACh sensors provide a user-friendly, broadly applicable tool for monitoring cholinergic transmission underlying diverse biological processes.

CLCN2 chloride channel mutations in familial hyperaldosteronism type II

Primary aldosteronism, a common cause of severe hypertension¹, features constitutive production of the adrenal steroid aldosterone. We analyzed a multiplex family with familial hyperaldosteronism type II (FH-II)² and 80 additional probands with unsolved early-onset primary aldosteronism. Eight probands had novel heterozygous variants in CLCN2, including two de novo mutations and four independent occurrences of the identical p.Arg172Gln mutation; all relatives with early-onset primary aldosteronism carried the CLCN2 variant found in probands. CLCN2 encodes a voltage-gated chloride channel expressed in adrenal glomerulosa that opens at hyperpolarized membrane potentials. Channel opening depolarizes glomerulosa cells and induces expression of aldosterone synthase, the rate-limiting enzyme for aldosterone biosynthesis. Mutant channels cause gain of function, with higher open probabilities at the glomerulosa resting potential. These findings for the first time demonstrate a role of anion channels in glomerulosa membrane potential determination, aldosterone production and hypertension. They establish the cause of a substantial fraction of early-onset primary aldosteronism.

Genetic Causes of Functional Adrenocortical Adenomas

Aldosterone and cortisol, the main mineralocorticoid and glucocorticoid hormones in humans, are produced in the adrenal cortex, which is composed of three concentric zones with specific functional characteristics. Adrenocortical adenomas (ACAs) can lead to the autonomous secretion of aldosterone responsible for primary aldosteronism, the most frequent form of secondary arterial hypertension. In the case of cortisol production, ACAs lead to overt or subclinical Cushing syndrome. Genetic analysis driven by next-generation sequencing technology has enabled the discovery, during the past 7 years, of the genetic causes of a large subset of ACAs. In particular, somatic mutations in genes regulating intracellular ionic homeostasis and membrane potential have been identified in aldosterone-producing adenomas. These mutations all promote increased intracellular calcium concentrations, with activation of calcium signaling, the main trigger for aldosterone production. In cortisol-producing adenomas, recurrent somatic mutations in PRKACA (coding for the cyclic adenosine monophosphate-dependent protein kinase catalytic subunit α) affect cyclic adenosine monophosphate-dependent protein kinase A signaling, leading to activation of cortisol biosynthesis. In addition to these specific pathways, the Wnt/β-catenin pathway appears to play an important role in adrenal tumorigenesis, because β-catenin mutations have been identified in both aldosterone- and cortisol-producing adenomas. This, together with different intermediate states of aldosterone and cortisol cosecretion, raises the possibility that the two conditions share a certain degree of genetic susceptibility. Alternatively, different hits might be responsible for the diseases, with one hit leading to adrenocortical cell proliferation and nodule formation and the second specifying the hormonal secretory pattern.

T-type Ca2+ channels are required for enhanced sympathetic axon growth by TNFα reverse signalling

Tumour necrosis factor receptor 1 (TNFR1)-activated TNFα reverse signalling, in which membrane-integrated TNFα functions as a receptor for TNFR1, enhances axon growth from developing sympathetic neurons and plays a crucial role in establishing sympathetic innervation. Here, we have investigated the link between TNFα reverse signalling and axon growth in cultured sympathetic neurons. TNFR1-activated TNFα reverse signalling promotes Ca²⁺ influx, and highly selective T-type Ca²⁺ channel inhibitors, but not pharmacological inhibitors of L-type, N-type and P/Q-type Ca²⁺ channels, prevented enhanced axon growth. T-type Ca²⁺ channel-specific inhibitors eliminated Ca²⁺ spikes promoted by TNFα reverse signalling in axons and prevented enhanced axon growth when applied locally to axons, but not when applied to cell somata. Blocking action potential generation did not affect the effect of TNFα reverse signalling on axon growth, suggesting that propagated action potentials are not required for enhanced axon growth. TNFα reverse signalling enhanced protein kinase C (PKC) activation, and pharmacological inhibition of PKC prevented the axon growth response. These results suggest that TNFα reverse signalling promotes opening of T-type Ca²⁺ channels along sympathetic axons, which is required for enhanced axon growth.

On the effect of hyperaldosteronism-inducing mutations in Na/K pumps

Mutated Na/K pumps in adrenal adenomas are thought to cause hyperaldosteronism via a gain-of-function effect involving a depolarizing inward current. The findings of Meyer et al. suggest instead that the common mechanism by which Na/K pump mutants lead to hyperaldosteronism is a loss-of-function.

Primary aldosteronism, a condition in which too much aldosterone is produced and that leads to hypertension, is often initiated by an aldosterone-producing adenoma within the zona glomerulosa of the adrenal cortex. Somatic mutations of ATP1A1, encoding the Na/K pump α1 subunit, have been found in these adenomas. It has been proposed that a passive inward current transported by several of these mutant pumps is a "gain-of-function" activity that produces membrane depolarization and concomitant increases in aldosterone production. Here, we investigate whether the inward current through mutant Na/K pumps is large enough to induce depolarization of the cells that harbor them. We first investigate inward currents induced by these mutations in Xenopus Na/K pumps expressed in Xenopus oocytes and find that these inward currents are similar in amplitude to wild-type outward Na/K pump currents. Subsequently, we perform a detailed functional evaluation of the human Na/K pump mutants L104R, delF100-L104, V332G, and EETA963S expressed in Xenopus oocytes. By combining two-electrode voltage clamp with [³H]ouabain binding, we measure the turnover rate of these inward currents and compare it to the turnover rate for outward current through wild-type pumps. We find that the turnover rate of the inward current through two of these mutants (EETA963S and L104R) is too small to induce significant cell depolarization. Electrophysiological characterization of another hyperaldosteronism-inducing mutation, G99R, reveals the absence of inward currents under many different conditions, including in the presence of the regulator FXYD1 as well as with mammalian ionic concentrations and body temperatures. Instead, we observe robust outward currents, but with significantly reduced affinities for intracellular Na⁺ and extracellular K⁺. Collectively, our results point to loss-of-function as the common mechanism for the hyperaldosteronism induced by these Na/K pump mutants.

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.

Dynamic Changes in the Renin-Angiotensin-Aldosterone System and the Beneficial Effects of Renin-Angiotensin-Aldosterone Inhibitors on Spatial Learning and Memory in a Rat Model of Chronic Cerebral Ischemia

Renin-angiotensin-aldosterone system (RAAS) plays an important role in the regulation of blood pressure and brain function. Therefore, we studied the dynamic changes in the RAAS in the blood, cerebral cortex, and hippocampus and the effects of RAAS inhibitors on spatial learning and memory and hippocampal apoptosis in a rat model of chronic cerebral ischemia (CCI) established by bilateral ligation of the common carotid arteries of rats. The levels of renin, angiotensin II (Ang II), and aldosterone (ALD) in the plasma, and the homogenates of the left side of cerebral cortex and whole hippocampus of rats were detected on day 1, 3, 7, 14, 21, and 30 by radioimmunoassay. Spatial learning and memory and hippocampal apoptosis were evaluated on day 30 by Morris water maze test (navigation and space exploration tests) and terminal dexynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) assay, respectively, after rats were orally administered with distilled water (DW), renin inhibitor aliskiren (30 mg/kg), Ang converting enzyme inhibitor enalapril (4 mg/kg), or Ang II receptor antagonist candesartan (2 mg/kg) daily for 30 days. The results showed that the levels of renin and Ang II were significantly higher but ALD fluctuated in the blood, cerebral cortex, and hippocampus in CCI rats compared to normal rats. However, aliskiren and enalapril could significantly decrease (p < 0.05) the levels of renin, Ang II and ALD in the blood, cerebral cortex, and hippocampus compared to DW treatment; while candesartan had similar effect on renin and ALD but no effect on Ang II in CCI rats. Furthermore, spatial learning and memory were significantly decreased but apoptosis in the hippocampus was obviously increased in CCI rats compared to normal rats (p < 0.05). However, aliskiren, enalapril, and candesartan were equally effective to improve spatial learning and memory and decrease apoptosis in the hippocampus. Therefore, RAAS plays an important role in the development of cerebral ischemia and RAAS inhibitors aliskiren, enalapril, and candesartan improve spatial learning and memory and protect brain injury by inhibiting hippocampal apoptosis in CCI rats.

Physiological and Pathological Roles in Human Adrenal of the Glomeruli-Defining Matrix Protein NPNT (Nephronectin)

Primary aldosteronism is a common cause of hypertension, which becomes refractory if undiagnosed, but potentially curable when caused by an aldosterone-producing adenoma (APA). The discovery of somatic mutations and differences in clinical presentations led to recognition of small but common zona glomerulosa (ZG)-like adenomas, distinct from classical large zona fasciculata-like adenomas. The inverse correlation between APA size and aldosterone synthase expression prompted us to undertake a systematic study of genotype-phenotype relationships. After a microarray comparing tumor subtypes, in which NPNT (nephronectin) was the most highly (>12-fold) upregulated gene in ZG-like APAs, we aimed to determine its role in physiological and pathological aldosterone production. NPNT was identified by immunohistochemistry as a secreted matrix protein expressed exclusively around aldosterone-producing glomeruli in normal adrenal ZG and in aldosterone-dense ZG-like APAs; the highest expression was in ZG-like APAs with gain-of-function CTNNB1 mutations, whose removal cured hypertension in our patients. NPNT was absent from normal zona fasciculata, zona fasciculata-like APAs, and ZG adjacent to an APA. NPNT production was regulated by canonical Wnt pathway, and NPNT overexpression or silencing increased or reduced aldosterone, respectively. NPNT was proadhesive in primary adrenal and APA cells but antiadhesive and antiapoptotic in immortalized adrenocortical cells. The discovery of NPNT in the adrenal helped recognition of a common subtype of APAs and a pathway by which Wnt regulates aldosterone production. We propose that this arises through NPNT's binding to cell-surface integrins, stimulating cell-cell contact within glomeruli, which define ZG. Therefore, NPNT or its cognate integrin could present a novel therapeutic target.

NLRP3 inflammasome activation contributes to aldosterone-induced podocyte injury

Aldosterone (Aldo) has been shown as an important contributor of podocyte injury. However, the underlying molecular mechanisms are still elusive. Recently, the pathogenic role of NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome in mediating renal tubular damage was identified while its role in podocyte injury still needs evidence. Thus the present study was undertaken to investigate the role of NLRP3 inflammasome in Aldo-induced podocyte damage. In vitro, exposure of podocytes to Aldo enhanced NLRP3, caspase-1, and IL-18 expressions in dose- and time-dependent manners, indicating an activation of NLRP3 inflammasome, which was significantly blocked by the mineralocorticoid receptor antagonist eplerenone or the antioxidant N-acetylcysteine. Silencing NLRP3 by a siRNA approach strikingly attenuated Aldo-induced podocyte apoptosis and nephrin protein downregulation in line with the blockade of caspase-1 and IL-18. In vivo, since day 5 of Aldo infusion, NLRP3 inflammasome activation and podocyte injury evidenced by nephrin reduction occurred concurrently. More importantly, immunofluorescence analysis showed a significant induction of NLRP3 in podocytes of glomeruli following Aldo infusion. In the mice with NLRP3 gene deletion, Aldo-induced downregulation of nephrin and podocin, podocyte foot processes, and albuminuria was remarkably improved, indicating an amelioration of podocyte injury. Finally, we observed a striking induction of NLRP3 in glomeruli and renal tubules in line with an enhanced urinary IL-18 output in nephrotic syndrome patients with minimal change disease or focal segmental glomerular sclerosis. Together, these results demonstrated an important role of NLRP3 inflammasome in mediating the podocyte injury induced by Aldo.

CACNA1H(M1549V) Mutant Calcium Channel Causes Autonomous Aldosterone Production in HAC15 Cells and Is Inhibited by Mibefradil

We recently demonstrated that a recurrent gain-of-function mutation in a T-type calcium channel, CACNA1H(M1549V), causes a novel Mendelian disorder featuring early-onset primary aldosteronism and hypertension. This variant was found independently in five families. CACNA1H(M1549V) leads to impaired channel inactivation and activation at more hyperpolarized potentials, inferred to cause increased calcium entry. We here aimed to study the effect of this variant on aldosterone production. We heterologously expressed empty vector, CACNA1H(WT) and CACNA1H(M1549V) in the aldosterone-producing adrenocortical cancer cell line H295R and its subclone HAC15. Transfection rates, expression levels, and subcellular distribution of the channel were similar between CACNA1H(WT) and CACNA1H(M1549V). We measured aldosterone production by an ELISA and CYP11B2 (aldosterone synthase) expression by real-time PCR. In unstimulated cells, transfection of CACNA1H(WT) led to a 2-fold increase in aldosterone levels compared with vector-transfected cells. Expression of CACNA1H(M1549V) caused a 7-fold increase in aldosterone levels. Treatment with angiotensin II or increased extracellular potassium levels further stimulated aldosterone production in both CACNA1H(WT)- and CACNA1H(M1549V)-transfected cells. Similar results were obtained for CYP11B2 expression. Inhibition of CACNA1H channels with the T-type calcium channel blocker Mibefradil completely abrogated the effects of CACNA1H(WT) and CACNA1H(M1549V) on CYP11B2 expression. These results directly link CACNA1H(M1549V) to increased aldosterone production. They suggest that calcium channel blockers may be beneficial in the treatment of a subset of patients with primary aldosteronism. Such blockers could target CACNA1H or both CACNA1H and the L-type calcium channel CACNA1D that is also expressed in the adrenal gland and mutated in patients with primary aldosteronism.

Small-Conductance Ca2+-Activated Potassium Channels Negatively Regulate Aldosterone Secretion in Human Adrenocortical Cells

Aldosterone, which plays a key role in maintaining water and electrolyte balance, is produced by zona glomerulosa cells of the adrenal cortex. Autonomous overproduction of aldosterone from zona glomerulosa cells causes primary hyperaldosteronism. Recent clinical studies have highlighted the pathological role of the KCNJ5 potassium channel in primary hyperaldosteronism. Our objective was to determine whether small-conductance Ca(2+)-activated potassium (SK) channels may also regulate aldosterone secretion in human adrenocortical cells. We found that apamin, the prototypic inhibitor of SK channels, decreased membrane voltage, raised intracellular Ca(2+) and dose dependently increased aldosterone secretion from human adrenocortical H295R cells. By contrast, 1-Ethyl-2-benzimidazolinone, an agonist of SK channels, antagonized apamin's action and decreased aldosterone secretion. Commensurate with an increase in aldosterone production, apamin increased mRNA expression of steroidogenic acute regulatory protein and aldosterone synthase that control the early and late rate-limiting steps in aldosterone biosynthesis, respectively. In addition, apamin increased angiotensin II-stimulated aldosterone secretion, whereas 1-Ethyl-2-benzimidazolinone suppressed both angiotensin II- and high K(+)-stimulated production of aldosterone in H295R cells. These findings were supported by apamin-modulation of basal and angiotensin II-stimulated aldosterone secretion from acutely prepared slices of human adrenals. We conclude that SK channel activity negatively regulates aldosterone secretion in human adrenocortical cells. Genetic association studies are necessary to determine whether mutations in SK channel subtype 2 genes may also drive aldosterone excess in primary hyperaldosteronism.

Pharmacology of L-type Calcium Channels: Novel Drugs for Old Targets?

Inhibition of voltage-gated L-type calcium channels by organic calcium channel blockers is a well-established pharmacodynamic concept for the treatment of hypertension and cardiac ischemia. Since decades these antihypertensives (such as the dihydropyridines amlodipine, felodipine or nifedipine) belong to the most widely prescribed drugs 
world-wide. Their tolerability is excellent because at therapeutic doses their pharmacological effects in humans are limited to the cardiovascular system. During the last years substantial progress has been made to reveal the physiological role of different L-type calcium channel isoforms in many other tissues, including the brain, endocrine and sensory cells. 
Moreover, there is accumulating evidence about their involvement in various human diseases, such as Parkinson's disease, neuropsychiatric disorders and hyperaldosteronism. In this review we discuss the pathogenetic role of L-type calcium channels, potential new indications for existing or isoform-selective compounds and strategies to minimize potential side effects.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

Adipocytes, aldosterone and obesity-related hypertension

Understanding the mechanisms linking obesity with hypertension is important in the current obesity epidemic as it may improve therapeutic interventions. Plasma aldosterone levels are positively correlated with body mass index and weight loss in obese patients is reported to be accompanied by decreased aldosterone levels. This suggests a relationship between adipose tissue and the production/secretion of aldosterone. Aldosterone is synthesized principally by the adrenal glands, but its production may be regulated by many factors, including factors secreted by adipocytes. In addition, studies have reported local synthesis of aldosterone in extra-adrenal tissues, including adipose tissue. Experimental studies have highlighted a role for adipocyte-secreted aldosterone in the pathogenesis of obesity-related cardiovascular complications via the mineralocorticoid receptor. This review focuses on how aldosterone secretion may be influenced by adipose tissue and the importance of these mechanisms in the context of obesity-related hypertension.

Transcriptional changes in steroidogenesis by perfluoroalkyl acids (PFOA and PFOS) regulate the synthesis of sex hormones in H295R cells

Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) are two of the most widely used perfluoroalkyl acids (PFAAs). Because of their strong persistence, they have become widely distributed throughout the environment and human bodies. PFOA and PFOS are suspected to disrupt the endocrine system based upon many in vivo studies, but the underlying mechanisms are currently unclear. In this study, we investigated the endocrine-related effects of PFOA and PFOS using in vitro estrogen receptor (ER) and androgen receptor (AR) transactivation assays and steroidogenesis assay. The results showed that PFOA and PFOS exhibited weak antagonistic ER transactivation but did not exhibit agonistic ER or AR transactivation. In the steroidogenesis assay, PFOA and PFOS induced 17β-estradiol (E2) level and reduced testosterone level, which would be caused by the induction of aromatase activity. The qPCR analysis of genes involved in steroidogenesis indicates that PFOA and PFOS associate with sex hormone synthesis by the transcriptional induction of two genes, cyp19 and 3β-hsd2. Moreover, the transcriptional induction of cyp11b2 by PFOS suggests that this chemical may underlie the disruption of several physiological functions related to aldosterone. The results of the current study suggest that PFOA and PFOS are potential endocrine disrupting chemicals (EDCs) and provide information for further studies on the molecular events that initiate the adverse endocrine effects.

Novel genetic determinants of adrenal aldosterone regulation

PURPOSE OF REVIEW

Aldosterone regulation in the adrenal plays an important role in blood pressure. The commonest curable cause of hypertension is primary aldosteronism. Recently, mutations in novel genes have been identified to cause primary aldosteronism. Elucidating the mechanism of action of these genetic abnormalities may help understand the cause of primary aldosteronism and the physiological regulation of aldosterone in the zona glomerulosa.

RECENT FINDINGS

KCNJ5, ATP1A1, ATP2B3, CACNA1D, CTNNB1, and CACNA1H mutations are causal of primary aldosteronism. ARMC5 may cause bilateral lesions resulting in primary aldosteronism.LGR5, DACH1, and neuron-specific proteins are highly expressed in the zona glomerulosa and regulate aldosterone production.

SUMMARY

Most mutations causing primary aldosteronism are in genes encoding cation channels or pumps, leading to increased calcium influx. Genotype-phenotype analyses identified two broad subtypes of aldosterone-producing adenomas (APAs), zona fasciculata-like and zona glomerulosa-like, and the likelihood of under-diagnosed zona glomerulosa-like APAs because of small size. Zona fasciculata-like APAs are only associated with KCNJ5 mutations, whereas zona glomerulosa-like APAs are associated with mutations in ATPase pumps, CACNA1D, and CTNNB1. The frequency of APAs, and the multiplicity of causal mutations, suggests a pre-existing drive for these mutations. We speculate that these mutations are selected for protecting against tonic inhibition of aldosterone in human zona glomerulosa, which express genes inhibiting aldosterone production.

Association between the CYP11 family and six cancer types

Cytochromes P450 (CYPs) are a major source of variability in pharmacokinetics and drug response. CYPs utilize a variety of small and large molecules as substrates in enzymatic reactions. The CYP genes may be divided into two groups: Endogenous CYPs (CYP family 7–51) and xenobiotic CYPs (CYP family 1–4). The aim of the present study was to investigate whether endogenous CYPs exhibit similar gene expression and mutations in various cancer types. The gene expression profiles and somatic mutations exhibited in colon adenocarcinoma, kidney renal clear cell carcinoma, liver hepatocellular carcinoma, lung squamous cell carcinoma, prostate adenocarcinoma and uterine corpus endometrial carcinoma were analyzed using data obtained from The Cancer Genome Atlas. The expression of CYP11A1 was significantly downregulated in all six cancer types. In addition, CYP11B1 and CYP11B2 exhibited the highest number of mutations among endogenous CYPs in all samples. As the CYP11 family is important for steroid biosynthesis, and previous studies have demonstrated that steroid hormones are associated with certain cancers, these results indicate a common role of the CYP11 family in various cancer types.

Role of voltage-gated calcium channels in the regulation of aldosterone production from zona glomerulosa cells of the adrenal cortex

Zona glomerulosa cells (ZG) of the adrenal gland constantly integrate fluctuating ionic, hormonal and paracrine signals to control the synthesis and secretion of aldosterone. These signals modulate Ca²⁺ levels, which provide the critical second messenger to drive steroid hormone production. Angiotensin II is a hormone known to modulate the activity of voltage-dependent L- and T-type Ca²⁺ channels that are expressed on the plasma membrane of ZG cells in many species. Because the ZG cell maintains a resting membrane voltage of approximately -85 mV and has been considered electrically silent, low voltage-activated T-type Ca²⁺ channels are assumed to provide the primary Ca²⁺ signal that drives aldosterone production. However, this view has recently been challenged by human genetic studies identifying somatic gain-of-function mutations in L-type Ca_V 1.3 channels in aldosterone-producing adenomas of patients with primary hyperaldosteronism. We provide a review of these assumptions and challenges, and update our understanding of the state of the ZG cell in a layer in which native cellular associations are preserved. This updated view of Ca²⁺ signalling in ZG cells provides a unifying mechanism that explains how transiently activating Ca_V 3.2 channels can generate a significant and recurring Ca²⁺ signal, and how Ca_V 1.3 channels may contribute to the Ca²⁺ signal that drives aldosterone production.

Cav 1.3 (CACNA1D) L-type Ca2+ channel dysfunction in CNS disorders

Ca_v1.3 belongs to the family of voltage‐gated L‐type Ca²⁺ channels and is encoded by the CACNA1D gene. Ca_v1.3 channels are not only essential for cardiac pacemaking, hearing and hormone secretion but are also expressed postsynaptically in neurons, where they shape neuronal firing and plasticity. Recent findings provide evidence that human mutations in the CACNA1D gene can confer risk for the development of neuropsychiatric disease and perhaps also epilepsy. Loss of Ca_v1.3 function, as shown in knock‐out mouse models and by human mutations, does not result in neuropsychiatric or neurological disease symptoms, whereas their acute selective pharmacological activation results in a depressive‐like behaviour in mice. Therefore it is likely that CACNA1D mutations enhancing activity may be disease relevant also in humans. Indeed, whole exome sequencing studies, originally prompted to identify mutations in primary aldosteronism, revealed de novo CACNA1D missense mutations permitting enhanced Ca²⁺ signalling through Ca_v1.3. Remarkably, apart from primary aldosteronism, heterozygous carriers of these mutations also showed seizures and neurological abnormalities. Different missense mutations with very similar gain‐of‐function properties were recently reported in patients with autism spectrum disorders (ASD). These data strongly suggest that CACNA1D mutations enhancing Ca_v1.3 activity confer a strong risk for – or even cause – CNS disorders, such as ASD.

Context-dependent mechanisms modulating aldosterone signaling in the kidney

The aldosterone-mineralocorticoid receptor (MR) system serves as the major regulator of fluid homeostasis, and is an important drug target for the treatment of hypertension, heart failure, and chronic kidney disease. While the ligand aldosterone plays a central role in facilitating MR activity, recent studies have revealed that MR signaling is modulated through distinct mechanisms at the levels of the receptor and the downstream targets. Notably, phosphorylation of the ligand-binding domain in MR regulates the ability of the receptor to bind to ligand in renal intercalated cells, providing an additional layer of regulation that allows the cell-selective control of MR signaling. These mechanisms are involved in the context-dependent effects of aldosterone in the distal nephron. In this article, the recent progress in the understanding of mechanisms regulating the action of aldosterone is discussed, focusing on the connecting tubules and collecting duct in the kidney.