Activation by cleavage of the epithelial Na+ channel α and γ subunits independently coevolved with the vertebrate terrestrial migration

Comparative Genomics Uncovers Molecular Adaptations for Cetacean Deep-Sea Diving

Cetaceans show remarkable diversity in diving capability, implying a range of adaptive strategies to hazards such as hydrostatic pressure and oxidative stress, but few studies have considered the evolution of extreme diving. Here, we first examined the relationship between morphological and physiological factors and diving capability and then considered the molecular evolution of candidate deep-sea diving traits in a genomic dataset of cetaceans. Our dataset included six super-divers, sperm whales (families Physeteridae and Kogiidae) and beaked whales (Ziphiidae), species that can dive deeper than 1000 m for about an hour or longer. We found a positive association between diving capability and oxygen-linked globins, and super-diver myoglobin (MB) is under positive selection and harbours a reported functional amino acid change. Blubber thickness was positively associated, likely to provide thermal insulation and hydrostatic pressure resistance. Super-divers have gene changes that may contribute to differences in the composition of outer blubber neutral lipids (triacylglycerols and wax esters), fatty acids and cholesterol. Total lung capacity relative to body mass showed a negative association, ostensibly to limit gas bubbles that can cause decompression sickness. A functional assay suggests that an ATP8B1 amino acid substitution may reduce lung injury in super-divers. Super-diver XDH has two unique amino acids and a decreased ability to produce uric acid under hypoxia when its ROS-generating XO isoform is prevalent, suggesting that it reduces cell damage from oxidative stress and uric acid accumulation in species with prolonged dives. Our study deepens the understanding of how deep-sea diving emerged in the cetacean lineage.

The evolutionary path of the epithelial sodium channel δ-subunit in Cetartiodactyla points to a role in sodium sensing

The epithelial sodium channel (ENaC) plays a key role in osmoregulation in tetrapod vertebrates and is a candidate receptor for salt taste sensation. There are four ENaC subunits (α, β, γ, δ) which form αβγ- or δβγ ENaCs. While αβγ-ENaC is a 'maintenance protein' controlling sodium and potassium homeostasis, δβγ-ENaC might represent a 'stress protein' monitoring high sodium concentrations. The δ-subunit emerged with water-to-land transition of tetrapod vertebrate ancestors. We investigated the evolutionary path of ENaC-coding genes in Cetartiodactyla, a group comprising even-toed ungulates and the cetaceans (whales/dolphins) which transitioned from terrestrial to marine environments in the Eocene. The genes SCNN1A (α-ENaC), SCNN1B (β-ENaC) and SCNN1G (γ-ENaC) are intact in all 22 investigated cetartiodactylan families. While SCNN1D (δ-ENaC) is intact in terrestrial Artiodactyla, it is a pseudogene in 12 cetacean families. A fusion of SCNN1D exons 11 and 12 under preservation of the open reading frame was observed in the Antilopinae, representing a new feature of this clade. Transcripts of SCNN1A, SCNN1B and SCNN1G were present in kidney and lung tissues of Bottlenose dolphins, highlighting αβγ-ENaC's role as a maintenance protein. Consistent with SCNN1D loss, Bottlenose dolphins and Beluga whales did not show behavioural differences to stimuli with or without sodium in seawater-equivalent concentrations. These data suggest a function of δ-ENaC as a sodium sensing protein which might have become obsolete in cetaceans after the migration to high-salinity marine environments. Consistently, there is reduced selection pressure or pseudogenisation of SCNN1D in other marine mammals, including sirenians, pinnipeds and sea otter.

Loss of the alpha subunit distal furin cleavage site blunts ENaC activation following Na+ restriction

Epithelial Na+ channels (ENaCs) are activated by proteolysis of the α and γ subunits at specific sites flanking embedded inhibitory tracts. To examine the role of α subunit proteolysis in channel activation in vivo, we generated mice lacking the distal furin cleavage site in the α subunit (αF2M mice). On a normal Na+ control diet, no differences in ENaC protein abundance in kidney or distal colon were noted between wild-type (WT) and αF2M mice. Patch-clamp analyses revealed similar levels of ENaC activity in kidney tubules, while no physiologically relevant differences in blood chemistry or aldosterone levels were detected. Male αF2M mice did exhibit diminished ENaC activity in the distal colon, as measured by amiloride-sensitive short-circuit current (ISC). Following dietary Na+ restriction, WT and αF2M mice had similar natriuretic and colonic ISC responses to amiloride. However, single-channel activity was significantly lower in kidney tubules from Na+-restricted αF2M mice compared with WT littermates. ENaC α and γ subunit expression in kidney and distal colon were also enhanced in Na+-restricted αF2M vs. WT mice, in association with higher aldosterone levels. These data provide evidence that disrupting α subunit proteolysis impairs ENaC activity in vivo, requiring compensation in response to Na+ restriction. KEY POINTS: The epithelial Na+ channel (ENaC) is activated by proteolytic cleavage in vitro, but key questions regarding the role of ENaC proteolysis in terms of whole-animal physiology remain to be addressed. We studied the in vivo importance of this mechanism by generating a mouse model with a genetic disruption to a key cleavage site in the ENaC's α subunit (αF2M mice). We found that αF2M mice did not exhibit a physiologically relevant phenotype under normal dietary conditions, but have impaired ENaC activation (channel open probability) in the kidney during salt restriction. ENaC function at the organ level was preserved in salt-restricted αF2M mice, but this was associated with higher aldosterone levels and increased expression of ENaC subunits, suggesting compensation was required to maintain homeostasis. These results provide the first evidence that ENaC α subunit proteolysis is a key regulator of channel activity in vivo.

Varying Selection Pressure for a Na+ Sensing Site in Epithelial Na+ Channel Subunits Reflect Divergent Roles in Na+ Homeostasis

The epithelial Na+ channel (ENaC) emerged early in vertebrates and has played a role in Na+ and fluid homeostasis throughout vertebrate evolution. We previously showed that proteolytic activation of the channel evolved at the water-to-land transition of vertebrates. Sensitivity to extracellular Na+, known as Na+ self-inhibition, reduces ENaC function when Na+ concentrations are high and is a distinctive feature of the channel. A fourth ENaC subunit, δ, emerged in jawed fishes from an α subunit gene duplication. Here, we analyzed 849 α and δ subunit sequences and found that a key Asp in a postulated Na+ binding site was nearly always present in the α subunit, but frequently lost in the δ subunit (e.g. human). Analysis of site evolution and codon substitution rates provide evidence that the ancestral α subunit had the site and that purifying selection for the site relaxed in the δ subunit after its divergence from the α subunit, coinciding with a loss of δ subunit expression in renal tissues. We also show that the proposed Na+ binding site in the α subunit is a bona fide site by conferring novel function to channels comprising human δ subunits. Together, our findings provide evidence that ENaC Na+ self-inhibition improves fitness through its role in Na+ homeostasis in vertebrates.

Reverse Phenotypes of Patients with Genetically Confirmed Liddle Syndrome

BACKGROUND

Liddle syndrome was initially characterized by hypertension, hypokalemia, metabolic alkalosis, and suppressed plasma renin and aldosterone, resulting from gain-of-function variants in the epithelial Na + channel (ENaC). Efficient treatment with ENaC inhibitors is available, but the phenotypic spectrum of genetically confirmed Liddle syndrome is unknown, and some patients may remain undiagnosed and at risk of inefficient treatment. In this study, we used a reverse phenotyping approach to investigate the Liddle syndrome phenotypic spectrum and genotype-phenotype correlations.

METHODS

Pubmed, Embase, Scopus, and the Human Gene Mutation Database were searched for articles reporting Liddle syndrome variants. The genetic variants were systematically classified to identify patients with genetically confirmed Liddle syndrome. We identified 62 articles describing 45 unique variants within 86 Liddle syndrome families, and phenotypic data were pooled for 268 patients with confirmed Liddle syndrome.

RESULTS

The Liddle syndrome variants localized to exon 13 of SCNN1B and SCNN1G , disrupting the PPPxY motif critical for downregulating ENaC activity. Hypertension sensitive to ENaC inhibition was present in 97% of adults carrying Liddle syndrome variants while hypokalemia, metabolic alkalosis, and plasma renin and aldosterone suppression showed incomplete penetrance. In addition, 95% and 55% of patients had a family history of hypertension or cerebrovascular events, respectively. The genotype had minor phenotypic effects; however, probands compared with relatives showed significant phenotypic discrepancies consistent with selection bias for initial genetic screening.

CONCLUSIONS

Patients with genetically confirmed Liddle syndrome displayed a phenotypic spectrum, with ENaC-sensitive hypertension and family history of hypertension being the most common features. The phenotype seemed independent of the specific gene or variant type involved.

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

MicroRNA-19 is regulated by aldosterone in a sex-specific manner to alter kidney sodium transport

A key regulator of blood pressure homeostasis is the steroid hormone aldosterone, which is released as the final signaling hormone of the renin-angiotensin-aldosterone-signaling (RAAS) system. Aldosterone increases sodium (Na+) reabsorption in the kidney distal nephron to regulate blood volume. Unregulated RAAS signaling can lead to hypertension and cardiovascular disease. The serum and glucocorticoid kinase (SGK1) coordinates much of the Na+ reabsorption in the cortical collecting duct (CCD) tubular epithelial cells. We previously demonstrated that aldosterone alters the expression of microRNAs (miRs) in CCD principal cells. The aldosterone-regulated miRs can modulate Na+ transport and the cellular response to aldosterone signaling. However, the sex-specific regulation of miRs by aldosterone in the kidney distal nephron has not been explored. In this study, we report that miR-19, part of the miR-17-92 cluster, is upregulated in female mouse CCD cells in response to aldosterone activation. Mir-19 binding to the 3'-untranslated region of SGK1 was confirmed using a dual-luciferase reporter assay. Increasing miR-19 expression in CCD cells decreased SGK1 message and protein expression. Removal of this cluster using a nephron-specific, inducible knockout mouse model increased SGK1 expression in female mouse CCD cells. The miR-19-induced decrease in SGK1 protein expression reduced the response to aldosterone stimulation and may account for sex-specific differences in aldosterone signaling. By examining evolution of the miR-17-92 cluster, phylogenetic sequence analysis indicated that this cluster arose at the same time that other Na+-sparing and salt regulatory proteins, specifically SGK1, first emerged, indicating a conserved role for these miRs in kidney function of salt and water homeostasis.NEW & NOTEWORTHY Expression of the microRNA-17-92 cluster is upregulated by aldosterone in mouse cortical collecting duct principal cells, exclusively in female mice. MiR-19 in this cluster targets the serum and glucocorticoid kinase (SGK1) to downregulate both mRNA and protein expression, resulting in a decrease in sodium transport across epithelial cells of the collecting duct. The miR-17-92 cluster is evolutionarily conserved and may act as a novel feedback regulator for aldosterone signaling in females.

Lessons learned about epithelial sodium channels from transgenic mouse models

PURPOSE OF REVIEW

This review provides an up-to-date understanding about the regulation of epithelial sodium channel (ENaC) expression and function. In particular, we will focus on its implication in renal Na+ and K+ handling and control of blood pressure using transgenic animal models.

RECENT FINDINGS

In kidney, the highly amiloride-sensitive ENaC maintains whole body Na+ homeostasis by modulating Na+ transport via epithelia. This classical role is mostly confirmed using genetically engineered animal models. Recently identified key signaling pathways that regulate ENaC expression and function unveiled some nonclassical and unexpected channel regulatory processes. If aberrant, these dysregulated mechanisms may also result in the development of salt-dependent hypertension.The purpose of this review is to highlight the most recent findings in renal ENaC regulation and function, in considering data obtained from animal models.

SUMMARY

Increased ENaC-mediated Na+ transport is a prerequisite for salt-dependent forms of hypertension. To treat salt-sensitive hypertension it is crucial to fully understand the function and regulation of ENaC.