Postnatal requirement of the epithelial sodium channel for maintenance of epidermal barrier function

Repurposing an epithelial sodium channel inhibitor as a therapy for murine and human skin inflammation

Inflammatory skin disease is characterized by a pathologic interplay between skin cells and immunocytes and can result in disfiguring cutaneous lesions and systemic inflammation. Immunosuppression is commonly used to target the inflammatory component; however, these drugs are often expensive and associated with side effects. To identify previously unidentified targets, we carried out a nonbiased informatics screen to identify drug compounds with an inverse transcriptional signature to keratinocyte inflammatory signals. Using psoriasis, a prototypic inflammatory skin disease, as a model, we used pharmacologic, transcriptomic, and proteomic characterization to find that benzamil, the benzyl derivative of the US Food and Drug Administration-approved diuretic amiloride, effectively reversed keratinocyte-driven inflammatory signaling. Through three independent mouse models of skin inflammation (Rac1G12V transgenic mice, topical imiquimod, and human skin xenografts from patients with psoriasis), we found that benzamil disrupted pathogenic interactions between the small GTPase Rac1 and its adaptor NCK1. This reduced STAT3 and NF-κB signaling and downstream cytokine production in keratinocytes. Genetic knockdown of sodium channels or pharmacological inhibition by benzamil prevented excess Rac1-NCK1 binding and limited proinflammatory signaling pathway activation in patient-derived keratinocytes without systemic immunosuppression. Both systemic and topical applications of benzamil were efficacious, suggesting that it may be a potential therapeutic avenue for treating skin inflammation.

Upregulation of α-ENaC induces pancreatic β-cell dysfunction, ER stress, and SIRT2 degradation

Islet beta cells (β-cells) produce insulin in response to high blood glucose levels, which is essential for preserving glucose homeostasis. Voltage-gated ion channels in β-cells, including Na +, K +, and Ca 2+ channels, aid in the release of insulin. The epithelial sodium channel alpha subunit (α-ENaC), a voltage-independent sodium ion channel, is also expressed in human pancreatic endocrine cells. However, there is no reported study on the function of ENaC in the β-cells. In the current study, we found that α-ENaC was expressed in human pancreatic glandule and pancreatic islet β-cells. In the pancreas of db/db mice and high-fat diet-induced mice, and in mouse islet β-cells (MIN6 cells) treated with palmitate, α-ENaC expression was increased. When α-ENaC was overexpressed in MIN6 cells, insulin content and glucose-induced insulin secretion were significantly reduced. On the other hand, palmitate injured islet β-cells and suppressed insulin synthesis and secretion, but increased α-ENaC expression in MIN6 cells. However, α-ENaC knockout ( Scnn1a -/-) in MIN6 cells attenuated β-cell disorder induced by palmitate. Furthermore, α-ENaC regulated the ubiquitylation and degradation of sirtuin 2 in β-cells. α-ENaC also modulated β-cell function in correlation with the inositol-requiring enzyme 1 alpha/X-box binding protein 1 (IRE1α/XBP1) and protein kinase RNA-like endoplasmic reticulum kinase/C/EBP homologous protein (PERK/CHOP) endoplasmic reticulum stress pathways. These results suggest that α-ENaC may play a novel role in insulin synthesis and secretion in the β-cells, and the upregulation of α-ENaC promotes islet β-cell dysfunction. In conclusion, α-ENaC may be a key regulator involved in islet β-cell damage and a potential therapeutic target for type 2 diabetes mellitus.

Capsazepine activates amiloride-insensitive ENaC-like channels in human leukemia cells

Sodium influx carried out by ion channels is one of the main regulators of water-salt and volume balance in cells of blood origin. Previously, we described amiloride-insensitive ENaC-like channels in human myeloid leukemia K562 cells; the intracellular regulatory mechanisms of the channels are associated with actin cytoskeleton dynamics. Recently, an extracellular mechanism of ENaC-like channels activation in K562 cells by the action of serine protease trypsin has been revealed. The other extracellular pathways that modulate ENaC (epithelial Na+ channel) activity and sodium permeability in transformed blood cells are not yet fully investigated. Here, we study the action of capsazepine (CPZ), as δ-ENaC activator, on single channel activity in K562 cells in whole-cell patch clamp experiments. Addition of CPZ (2 μM) to the extracellular solution caused an activation of sodium channels with typical features; unitary conductance was 15.1 ± 0.8 pS. Amiloride derivative benzamil (50 μM) did not inhibit their activity. Unitary currents and conductance of CPZ-activated channels were higher in Na+-containing extracellular solution than in Li+, that is one of the main fingerprints of δ-ENaC. The results of RT-PCR analysis and immunofluorescence staining also confirmed the expression of δ-hENaC (as well as α-, β-, γ-ENaC) at the mRNA and protein level. These findings allow us to speculate that CPZ activates amiloride-insensitive ENaC-like channels that contain δ-ENaC in К562 cells. Our data reveal a novel extracellular mechanism for ENaC-like activation in human leukemia cells.

Role of epithelial sodium channel-related inflammation in human diseases

The epithelial sodium channel (ENaC) is a heterotrimer and is widely distributed throughout the kidneys, blood vessels, lungs, colons, and many other organs. The basic role of the ENaC is to mediate the entry of Na+ into cells; the ENaC also has an important regulatory function in blood pressure, airway surface liquid (ASL), and endothelial cell function. Aldosterone, serum/glucocorticoid kinase 1 (SGK1), shear stress, and posttranslational modifications can regulate the activity of the ENaC; some ion channels also interact with the ENaC. In recent years, it has been found that the ENaC can lead to immune cell activation, endothelial cell dysfunction, aggravated inflammation involved in high salt-induced hypertension, cystic fibrosis, pseudohypoaldosteronism (PHA), and tumors; some inflammatory cytokines have been reported to have a regulatory role on the ENaC. The ENaC hyperfunction mediates the increase of intracellular Na+, and the elevated exchange of Na+ with Ca2+ leads to an intracellular calcium overload, which is an important mechanism for ENaC-related inflammation. Some of the research on the ENaC is controversial or unclear; we therefore reviewed the progress of studies on the role of ENaC-related inflammation in human diseases and their mechanisms.

MLL4 mediates differentiation and tumor suppression through ferroptosis

The epigenetic regulator, MLL4 (KMT2D), has been described as an essential gene in both humans and mice. In addition, it is one of the most commonly mutated genes in all of cancer biology. Here, we identify a critical role for Mll4 in the promotion of epidermal differentiation and ferroptosis, a key mechanism of tumor suppression. Mice lacking epidermal Mll4, but not the related enzyme Mll3 (Kmt2c), display features of impaired differentiation and human precancerous neoplasms, all of which progress with age. Mll4 deficiency profoundly alters epidermal gene expression and uniquely rewires the expression of key genes and markers of ferroptosis (Alox12, Alox12b, and Aloxe3). Beyond revealing a new mechanistic basis for Mll4-mediated tumor suppression, our data uncover a potentially much broader and general role for ferroptosis in the process of differentiation and skin homeostasis.

Five Functional Aspects of the Epidermal Barrier

The epidermis is a living, multilayered barrier with five functional levels, including a physical, a chemical, a microbial, a neuronal, and an immune level. Altogether, this complex organ contributes to protect the host from external aggression and to preserve its integrity. In this review, we focused on the different functional aspects.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Intergenerational effects of manipulating DNA methylation in the early life of an iconic invader

In response to novel environments, invasive populations often evolve rapidly. Standing genetic variation is an important predictor of evolutionary response but epigenetic variation may also play a role. Here, we use an iconic invader, the cane toad (Rhinella marina), to investigate how manipulating epigenetic status affects phenotypic traits. We collected wild toads from across Australia, bred them, and experimentally manipulated DNA methylation of the subsequent two generations (G1, G2) through exposure to the DNA methylation inhibitor zebularine and/or conspecific tadpole alarm cues. Direct exposure to alarm cues (an indicator of predation risk) increased the potency of G2 tadpole chemical cues, but this was accompanied by reductions in survival. Exposure to alarm cues during G1 also increased the potency of G2 tadpole cues, indicating intergenerational plasticity in this inducible defence. In addition, the negative effects of alarm cues on tadpole viability (i.e. the costs of producing the inducible defence) were minimized in the second generation. Exposure to zebularine during G1 induced similar intergenerational effects, suggesting a role for alteration in DNA methylation. Accordingly, we identified intergenerational shifts in DNA methylation at some loci in response to alarm cue exposure. Substantial demethylation occurred within the sodium channel epithelial 1 subunit gamma gene (SCNN1G) in alarm cue exposed individuals and their offspring. This gene is a key to the regulation of sodium in epithelial cells and may help to maintain the protective epidermal barrier. These data suggest that early life experiences of tadpoles induce intergenerational effects through epigenetic mechanisms, which enhance larval fitness. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Epithelial Sodium Channel-α Mediates the Protective Effect of the TNF-Derived TIP Peptide in Pneumolysin-Induced Endothelial Barrier Dysfunction

BACKGROUND

Streptococcus pneumoniae is a major etiologic agent of bacterial pneumonia. Autolysis and antibiotic-mediated lysis of pneumococci induce release of the pore-forming toxin, pneumolysin (PLY), their major virulence factor, which is a prominent cause of acute lung injury. PLY inhibits alveolar liquid clearance and severely compromises alveolar-capillary barrier function, leading to permeability edema associated with pneumonia. As a consequence, alveolar flooding occurs, which can precipitate lethal hypoxemia by impairing gas exchange. The α subunit of the epithelial sodium channel (ENaC) is crucial for promoting Na⁺ reabsorption across Na⁺-transporting epithelia. However, it is not known if human lung microvascular endothelial cells (HL-MVEC) also express ENaC-α and whether this subunit is involved in the regulation of their barrier function.

METHODS

The presence of α, β, and γ subunits of ENaC and protein phosphorylation status in HL-MVEC were assessed in western blotting. The role of ENaC-α in monolayer resistance of HL-MVEC was examined by depletion of this subunit by specific siRNA and by employing the TNF-derived TIP peptide, a specific activator that directly binds to ENaC-α.

RESULTS

HL-MVEC express all three subunits of ENaC, as well as acid-sensing ion channel 1a (ASIC1a), which has the capacity to form hybrid non-selective cation channels with ENaC-α. Both TIP peptide, which specifically binds to ENaC-α, and the specific ASIC1a activator MitTx significantly strengthened barrier function in PLY-treated HL-MVEC. ENaC-α depletion significantly increased sensitivity to PLY-induced hyperpermeability and in addition, blunted the protective effect of both the TIP peptide and MitTx, indicating an important role for ENaC-α and for hybrid NSC channels in barrier function of HL-MVEC. TIP peptide blunted PLY-induced phosphorylation of both calmodulin-dependent kinase II (CaMKII) and of its substrate, the actin-binding protein filamin A (FLN-A), requiring the expression of both ENaC-α and ASIC1a. Since non-phosphorylated FLN-A promotes ENaC channel open probability and blunts stress fiber formation, modulation of this activity represents an attractive target for the protective actions of ENaC-α in both barrier function and liquid clearance.

CONCLUSION

Our results in cultured endothelial cells demonstrate a previously unrecognized role for ENaC-α in strengthening capillary barrier function that may apply to the human lung. Strategies aiming to activate endothelial NSC channels that contain ENaC-α should be further investigated as a novel approach to improve barrier function in the capillary endothelium during pneumonia.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Transcriptome adaptation of the bovine mammary gland to diets rich in unsaturated fatty acids shows greater impact of linseed oil over safflower oil on gene expression and metabolic pathways

BACKGROUND

Nutritional strategies can decrease saturated fatty acids (SFAs) and increase health beneficial fatty acids (FAs) in bovine milk. The pathways/genes involved in these processes are not properly defined. Next-generation RNA-sequencing was used to investigate the bovine mammary gland transcriptome following supplemental feeding with 5% linseed oil (LSO) or 5% safflower oil (SFO). Holstein cows in mid-lactation were fed a control diet for 28 days (control period) followed by supplementation with 5% LSO (12 cows) or 5% SFO (12 cows) for 28 days (treatment period). Milk and mammary gland biopsies were sampled on days-14 (control period), +7 and +28 (treatment period). Milk was used to measure fat(FP)/protein(PP) percentages and individual FAs while RNA was subjected to sequencing.

RESULTS

Milk FP was decreased by 30.38% (LSO) or 32.42% (SFO) while PP was unaffected (LSO) or increased (SFO). Several beneficial FAs were increased by LSO (C18:1n11t, CLA:10t12c, CLA:9c11t, C20:3n3, C20:5n3, C22:5n3) and SFO (C18:1n11t, CLA:10t12c, C20:1c11, C20:2, C20:3n3) while several SFAs (C4:0, C6:0, C8:0, C14:0, C16:0, C17:0, C24:0) were decreased by both treatments (P < 0.05). 1006 (460 up- and 546 down-regulated) and 199 (127 up- and 72 down-regulated) genes were significantly differentially regulated (DE) by LSO and SFO, respectively. Top regulated genes (≥ 2 fold change) by both treatments (FBP2, UCP2, TIEG2, ANGPTL4, ALDH1L2) are potential candidate genes for milk fat traits. Involvement of SCP2, PDK4, NQO1, F2RL1, DBI, CPT1A, CNTFR, CALB1, ACADVL, SPTLC3, PIK3CG, PIGZ, ADORA2B, TRIB3, HPGD, IGFBP2 and TXN in FA/lipid metabolism in dairy cows is being reported for the first time. Functional analysis indicated similar and different top enriched functions for DE genes. DE genes were predicted to significantly decrease synthesis of FA/lipid by both treatments and FA metabolism by LSO. Top canonical pathways associated with DE genes of both treatments might be involved in lipid/cholesterol metabolism.

CONCLUSION

This study shows that rich α-linolenic acid LSO has a greater impact on mammary gland transcriptome by affecting more genes, pathways and processes as compared to SFO, rich in linoleic acid. Our study suggest that decrease in milk SFAs was due to down-regulation of genes in the FA/lipid synthesis and lipid metabolism pathways while increase in PUFAs was due to increased availability of ruminal biohydrogenation metabolites that were up taken and incorporated into milk or used as substrate for the synthesis of PUFAs.

Embryonic expression of the common progeroid lamin A splice mutation arrests postnatal skin development

Hutchinson-Gilford progeria syndrome (HGPS) and restrictive dermopathy (RD) are two laminopathies caused by mutations leading to cellular accumulation of prelamin A or one of its truncated forms, progerin. One proposed mechanism for the more severe symptoms in patients with RD compared with HGPS is that higher levels of farnesylated lamin A are produced in RD. Here, we show evidence in support of that hypothesis. Overexpression of the most common progeroid lamin A mutation (LMNA c.1824C>T, p.G608G) during skin development results in a severe phenotype, characterized by dry scaly skin. At postnatal day 5 (PD5), progeroid animals showed a hyperplastic epidermis, disorganized sebaceous glands and an acute inflammatory dermal response, also involving the hypodermal fat layer. PD5 animals also showed an upregulation of multiple inflammatory response genes and an activated NF-kB target pathway. Careful analysis of the interfollicular epidermis showed aberrant expression of the lamin B receptor (LBR) in the suprabasal layer. Prolonged expression of LBR, in 14.06% of the cells, likely contributes to the observed arrest of skin development, clearly evident at PD4 when the skin had developed into single-layer epithelium in the wild-type animals while progeroid animals still had the multilayered appearance typical for skin at PD3. Suprabasal cells expressing LBR showed altered DNA distribution, suggesting the induction of gene expression changes. Despite the formation of a functional epidermal barrier and proven functionality of the gap junctions, progeroid animals displayed a greater rate of water loss as compared with wild-type littermates and died within the first two postnatal weeks.

The role of sphingolipid metabolism in cutaneous permeability barrier formation

The epidermal permeability barrier of mammalian skin is localized in the stratum corneum. Corneocytes are embedded in an extracellular, highly ordered lipid matrix of hydrophobic lipids consisting of about 50% ceramides, 25% cholesterol and 15% long and very long chain fatty acids. The most important lipids for the epidermal barrier are ceramides. The scaffold of the lipid matrix is built of acylceramides, containing ω-hydroxylated very long chain fatty acids, acylated at the ω-position with linoleic acid. After glucosylation of the acylceramides at Golgi membranes and secretion, the linoleic acid residues are replaced by glutamate residues originating from proteins exposed on the surface of corneocytes. Removal of their glucosyl residues generates a hydrophobic surface on the corneocytes used as a template for the formation of extracellular lipid layers of the water permeability barrier. Misregulation or defects in the formation of extracellular ceramide structures disturb barrier function. Important anabolic steps are the synthesis of ultra long chain fatty acids, their ω-hydroxylation, and formation of ultra long chain ceramides and glucosylceramides. The main probarrier precursor lipids, glucosylceramides and sphingomyelins, are packed in lamellar bodies together with hydrolytic enzymes such as glucosylceramide-β-glucosidase and acid sphingomyelinase and secreted into the intercelullar space between the stratum corneum and stratum granulosum. Inherited defects in the extracellular hydrolytic processing of the probarrier acylglucosylceramides impair epidermal barrier formation and cause fatal diseases: such as prosaposin deficiency resulting in lack of lysosomal lipid binding and transfer proteins, or the symptomatic clinical picture of the "collodion baby" in the absence of glucocerebrosidase. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Coculturing human endometrial epithelial cells and stromal fibroblasts alters cell-specific gene expression and cytokine production

OBJECTIVE

To determine the effects of coculturing endometrial epithelial cells (eEC) with paired endometrial stromal fibroblasts (eSF) on cell-specific gene expression and cytokine secretion patterns.

DESIGN

In vitro study.

SETTING

University research laboratory.

PATIENT(S)

Endometrial biopsies were obtained from premenopausal women.

INTERVENTION(S)

Polarized eEC and subject-paired eSF were cultured for 12.5 hours alone (monoculture) or combined in a two-chamber coculture system without cell-cell contact. Cells and conditioned media were analyzed for global gene expression and cytokine secretion, respectively. Purified, endometrial tissue-derived eEC and eSF isolated by fluorescent activated cell sorting (FACS) were used as noncultured controls.

MAIN OUTCOME MEASURE(S)

Cell-specific global gene expression profiling and analysis of secreted cytokines in eEC/eSF cocultures and respective monocultures.

RESULT(S)

Transepithelial resistance, diffusible tracer exclusion, expression of tight junction proteins, and apical/basolateral vectorial secretion confirmed eEC structural and functional polarization. Distinct transcriptomes of eEC and eSF were consistent with their respective lineages and their endometrial origin. Coculture of eEC with eSF resulted in altered cell-specific gene expression and cytokine secretion.

CONCLUSION(S)

This coculture model provides evidence that interactions between endometrial functionally polarized epithelium and stromal fibroblasts affect cell-specific gene expression and cytokine secretion underscoring their relevance when modeling endometrium in vitro.

The epithelial sodium channel mediates the directionality of galvanotaxis in human keratinocytes

Cellular directional migration in an electric field (galvanotaxis) is one of the mechanisms guiding cell movement in embryogenesis and in skin epidermal repair. The epithelial sodium channel (ENaC), in addition to its function of regulating sodium transport in kidney, has recently been found to modulate cell locomotory speed. Here we tested whether ENaC has an additional function of mediating the directional migration of galvanotaxis in keratinocytes. Genetic depletion of ENaC completely blocks only galvanotaxis and does not decrease migration speed. Overexpression of ENaC is sufficient to drive galvanotaxis in otherwise unresponsive cells. Pharmacologic blockade or maintenance of the open state of ENaC also decreases or increases, respectively, galvanotaxis, suggesting that the channel open state is responsible for the response. Stable lamellipodial extensions formed at the cathodal sides of wild-type cells at the start of galvanotaxis; these were absent in the ENaC knockout keratinocytes, suggesting that ENaC mediates galvanotaxis by generating stable lamellipodia that steer cell migration. We provide evidence that ENaC is required for directional migration of keratinocytes in an electric field, supporting a role for ENaC in skin wound healing.

Tight junctions and differentiation--a chicken or the egg question?

Skin barrier function is indispensable to prevent the uncontrolled loss of water and solutes and to protect the body from external assaults. To fulfil this function, keratinocytes undergo a complex pathway of differentiation that terminates in the formation of the stratum corneum. Additionally, tight junctions (TJs), which are cell-cell junctions localized in the stratum granulosum, are involved in the barrier function of the skin. Important biological and clinical roles of TJs are strongly suggested by altered TJ protein levels and distribution in skin diseases like psoriasis, ichthyosis and atopic dermatitis. Because these skin diseases show alterations in differentiation and TJs, it was suggested that changes in TJs might simply be a consequence of altered differentiation. However, in this viewpoint, we like to argue that the situation is not as simple and depends on the specific microenvironment. We discuss three hypotheses regarding the interplay between TJs/TJ proteins and differentiation: (1) TJs/TJ proteins are influenced by differentiation, (2) differentiation is influenced by TJs/TJ proteins, and (3) TJs/TJ proteins and differentiation are independent of each other. In addition, the concept is introduced that both processes are going on at the same time, which means that while one specific TJ protein/barrier component might be influenced by differentiation, the other may influence differentiation.

The structure and function of the stratum corneum

Over the past 150 years the skin's structure and function has been the subject of much investigation by scientists. The stratum corneum (SC), the skin's outermost layer and interface with the outside world is now well recognized as the barrier that prevents unwanted materials from entering, and excessive loss of water from exiting the body. This review summarizes the major advances in our understanding of this formidable membrane. The structure of the SC is outlined as well as techniques to visualize the barrier. The lipid organization and ionic gradients, as well as the metabolic responses and underlying cellular signalling that lead to barrier repair and homeostasis are discussed. Finally, a brief overview of the molecular and genetic factors that determine the development of a competent permeability barrier is provided.

ENaCs and ASICs as therapeutic targets

The epithelial Na(+) channel (ENaC) and acid-sensitive ion channel (ASIC) branches of the ENaC/degenerin superfamily of cation channels have drawn increasing attention as potential therapeutic targets in a variety of diseases and conditions. Originally thought to be solely expressed in fluid absorptive epithelia and in neurons, it has become apparent that members of this family exhibit nearly ubiquitous expression. Therapeutic opportunities range from hypertension, due to the role of ENaC in maintaining whole body salt and water homeostasis, to anxiety disorders and pain associated with ASIC activity. As a physiologist intrigued by the fundamental mechanics of salt and water transport, it was natural that Dale Benos, to whom this series of reviews is dedicated, should have been at the forefront of research into the amiloride-sensitive sodium channel. The cloning of ENaC and subsequently the ASIC channels has revealed a far wider role for this channel family than was previously imagined. In this review, we will discuss the known and potential roles of ENaC and ASIC subunits in the wide variety of pathologies in which these channels have been implicated. Some of these, such as the role of ENaC in Liddle's syndrome are well established, others less so; however, all are related in that the fundamental defect is due to inappropriate channel activity.

Membrane-anchored serine proteases in vertebrate cell and developmental biology

Analysis of vertebrate genome sequences at the turn of the millennium revealed that a vastly larger repertoire of enzymes execute proteolytic cleavage reactions within the pericellular and extracellular environments than was anticipated from biochemical and molecular analysis. Most unexpected was the unveiling of an entire new family of structurally unique multidomain serine proteases that are anchored directly to the plasma membrane. Unlike secreted serine proteases, which function primarily in tissue repair, immunity, and nutrient uptake, these membrane-anchored serine proteases regulate fundamental cellular and developmental processes, including tissue morphogenesis, epithelial barrier function, ion and water transport, cellular iron export, and fertilization. Here the cellular and developmental biology of this fascinating new group of proteases is reviewed. Particularly highlighted is how the study of membrane-anchored serine proteases has expanded our knowledge of the range of physiological processes that require regulated proteolysis at the cell surface.

CFTR-deficiency renders mice highly susceptible to cutaneous symptoms during mite infestation

Pruritus, also known as itch, is a sensation that causes a desire to scratch. Prolonged scratching exacerbates skin lesions in several skin diseases such as atopic dermatitis. Here, we identify the cystic fibrosis transmembrane conductance regulator (CFTR/Cftr), an integral membrane protein that mediates transepithelial chloride transport, as a determinant factor in mice for the susceptibility to several cutaneous symptoms during mite infestation. Mice that endogenously express dysfunctional Cftr (Cftr(ΔF508/ΔF508)) show significant increase of scratching behavior and skin fibrosis after mite exposure. These phenotypes were due to the increased expression of nerve growth factor (NGF) that augments the sensitization of peripheral nerve fibers. Moreover, protein gene product 9.5 (PGP9.5)-positive neurites were abundant in the epidermis of mite-infested Cftr(ΔF508/ΔF508) mice. Furthermore, mite-infested Cftr(+/+) mice orally administered with a chloride channel inhibitor glibenclamide had higher scratching count and increased level of NGF than vehicle-treated mice. Consistently, mite extract-exposed primary and transformed human keratinocytes, treated with CFTR inhibitor, had significantly higher level of NGF mRNA compared with vehicle-treated, mite extract-exposed cells. These results reveal that CFTR in keratinocytes plays a critical role for the regulation of peripheral nerve function and pruritus sensation, and suggest that Cftr(ΔF508/ΔF508) mice may serve as a novel mouse model that represents NGF-dependent generation of pruritus.

PAR2 absence completely rescues inflammation and ichthyosis caused by altered CAP1/Prss8 expression in mouse skin

Altered serine protease activity is associated with skin disorders in humans and in mice. The serine protease channel-activating protease-1 (CAP1; also termed protease serine S1 family member 8 (Prss8)) is important for epidermal homeostasis and is thus indispensable for postnatal survival in mice, but its roles and effectors in skin pathology are poorly defined. In this paper, we report that transgenic expression in mouse skin of either CAP1/Prss8 (K14-CAP1/Prss8) or protease-activated receptor-2 (PAR2; Grhl3^PAR2/+), one candidate downstream target, causes epidermal hyperplasia, ichthyosis and itching. K14-CAP1/Prss8 ectopic expression impairs epidermal barrier function and causes skin inflammation characterized by an increase in thymic stromal lymphopoietin levels and immune cell infiltrations. Strikingly, both gross and functional K14-CAP1/Prss8-induced phenotypes are completely negated when superimposed on a PAR2-null background, establishing PAR2 as a pivotal mediator of pathogenesis. Our data provide genetic evidence for PAR2 as a downstream effector of CAP1/Prss8 in a signalling cascade that may provide novel therapeutic targets for ichthyoses, pruritus and inflammatory skin diseases.

The activity of serine proteases, including CAP1/Prss8, is altered in some human skin disorders; however, the downstream effectors of these proteins are relatively unknown. Here, using animal models, the authors show that protease-activated receptor-2 is a critical component of the CAP1/Prss8 signalling cascade.

Apical serine protease activity is necessary for assembly of a high-resistance renal collecting duct epithelium

AIM

We hypothesized that the serine protease prostasin is necessary for differentiation of a high-resistance renal collecting duct epithelium governed by glucocorticoid.

METHODS

Postnatal rat kidney and adult human kidney was used to study the expression and localization of prostasin. The murine collecting duct cell line (M-1) was cultured in Snapwell inserts to investigate the significance of prostasin for the development of transepithelial electrical resistance (TER).

RESULTS

In the cortex and medulla of rat kidney, prostasin mRNA and protein increased significantly between birth and weaning (day 21) and was detected in collecting ducts. Immunoreactive prostasin was associated with collecting ducts and loops of Henle in human kidney. In rat, adrenalectomy at day 10 had no effect on prostasin mRNA level in kidney at day 20. Cultured M-1 cells exhibited parallel increases in prostasin mRNA, protein and TER 5 days after seeding. Apical addition of the serine protease inhibitor aprotinin to M-1 cell cultures inhibited development of TER and led to aberrant localization of E-cadherin. This effect was mimicked by the protease inhibitor nafamostat. Apical addition of phospholipase C to cleave glycosylphosphatidylinositol (GPI) anchors released prostasin to the medium and attenuated development of TER with time of culture. Disruption of lipid rafts by methyl-β-cyclodextrin attenuated development of TER in M-1 cells. Omission of dexamethasone impaired development of TER in M-1 cells, while prostasin protein abundance and E-cadherin distribution did not change.

CONCLUSION

Apical, GPI-anchored, lipid raft-associated serine protease activity, compatible with prostasin, is necessary for the development of a high-resistance collecting duct epithelium.

Sodium and potassium balance depends on αENaC expression in connecting tubule

Mutations in α, β, or γ subunits of the epithelial sodium channel (ENaC) can downregulate ENaC activity and cause a severe salt-losing syndrome with hyperkalemia and metabolic acidosis, designated pseudohypoaldosteronism type 1 in humans. In contrast, mice with selective inactivation of αENaC in the collecting duct (CD) maintain sodium and potassium balance, suggesting that the late distal convoluted tubule (DCT2) and/or the connecting tubule (CNT) participates in sodium homeostasis. To investigate the relative importance of ENaC-mediated sodium absorption in the CNT, we used Cre-lox technology to generate mice lacking αENaC in the aquaporin 2-expressing CNT and CD. Western blot analysis of microdissected cortical CD (CCD) and CNT revealed absence of αENaC in the CCD and weak αENaC expression in the CNT. These mice exhibited a significantly higher urinary sodium excretion, a lower urine osmolality, and an increased urine volume compared with control mice. Furthermore, serum sodium was lower and potassium levels were higher in the genetically modified mice. With dietary sodium restriction, these mice experienced significant weight loss, increased urinary sodium excretion, and hyperkalemia. Plasma aldosterone levels were significantly elevated under both standard and sodium-restricted diets. In summary, αENaC expression within the CNT/CD is crucial for sodium and potassium homeostasis and causes signs and symptoms of pseudohypoaldosteronism type 1 if missing.

ENaC-mediated alveolar fluid clearance and lung fluid balance depend on the channel-activating protease 1

Sodium transport via epithelial sodium channels (ENaC) expressed in alveolar epithelial cells (AEC) provides the driving force for removal of fluid from the alveolar space. The membrane-bound channel-activating protease 1 (CAP1/Prss8) activates ENaC in vitro in various expression systems. To study the role of CAP1/Prss8 in alveolar sodium transport and lung fluid balance in vivo, we generated mice lacking CAP1/Prss8 in the alveolar epithelium using conditional Cre-loxP-mediated recombination. Deficiency of CAP1/Prss8 in AEC induced in vitro a 40% decrease in ENaC-mediated sodium currents. Sodium-driven alveolar fluid clearance (AFC) was reduced in CAP1/Prss8-deficient mice, due to a 48% decrease in amiloride-sensitive clearance, and was less sensitive to β₂-agonist treatment. Intra-alveolar treatment with neutrophil elastase, a soluble serine protease activating ENaC at the cell surface, fully restored basal AFC and the stimulation by β₂-agonists. Finally, acute volume-overload increased alveolar lining fluid volume in CAP1/Prss8-deficient mice. This study reveals that CAP1 plays a crucial role in the regulation of ENaC-mediated alveolar sodium and water transport and in mouse lung fluid balance.

Activation of the epithelial sodium channel (ENaC) by serine proteases

The study of human monogenic diseases [pseudohypoaldosteronism type 1 (PHA-1) and Liddle's syndrome] as well as mouse models mimicking the salt-losing syndrome (PHA-1) or salt-sensitive hypertension (Liddle's syndrome) have established the epithelial sodium channel ENaC as a limiting factor in vivo in the control of ionic composition of the extracellular fluid, regulation of blood volume and blood pressure, lung alveolar clearance, and airway mucociliary clearance. In this review, we discuss more specifically the activation of ENaC by serine proteases. Recent in vitro and in vivo experiments indicate that membrane-bound serine proteases are of critical importance in the activation of ENaC in different organs, such as the kidney, the lung, or the cochlea. Progress in understanding the basic mechanism of proteolytic activation of ENaC is accelerating, but uncertainty about the most fundamental aspects persists, leaving numerous still-unanswered questions.

The delta-subunit of the epithelial sodium channel (ENaC) enhances channel activity and alters proteolytic ENaC activation

The epithelial sodium channel (ENaC) is probably a heterotrimer with three well characterized subunits (alphabetagamma). In humans an additional delta-subunit (delta-hENaC) exists but little is known about its function. Using the Xenopus laevis oocyte expression system, we compared the functional properties of alphabetagamma- and deltabetagamma-hENaC and investigated whether deltabetagamma-hENaC can be proteolytically activated. The amiloride-sensitive ENaC whole-cell current (DeltaI(ami)) was about 11-fold larger in oocytes expressing deltabetagamma-hENaC than in oocytes expressing alphabetagamma-hENaC. The 2-fold larger single-channel Na(+) conductance of deltabetagamma-hENaC cannot explain this difference. Using a chemiluminescence assay, we demonstrated that an increased channel surface expression is also not the cause. Thus, overall channel activity of deltabetagamma-hENaC must be higher than that of alphabetagamma-hENaC. Experiments exploiting the properties of the known betaS520C mutant ENaC confirmed this conclusion. Moreover, chymotrypsin had a reduced stimulatory effect on deltabetagamma-hENaC whole-cell currents compared with its effect on alphabetagamma-hENaC whole-cell currents (2-fold versus 5-fold). This suggests that the cell surface pool of so-called near-silent channels that can be proteolytically activated is smaller for deltabetagamma-hENaC than for alphabetagamma-hENaC. Proteolytic activation of deltabetagamma-hENaC was associated with the appearance of a delta-hENaC cleavage product at the cell surface. Finally, we demonstrated that a short inhibitory 13-mer peptide corresponding to a region of the extracellular loop of human alpha-ENaC inhibited DeltaI(ami) in oocytes expressing alphabetagamma-hENaC but not in those expressing deltabetagamma-hENaC. We conclude that the delta-subunit of ENaC alters proteolytic channel activation and enhances base-line channel activity.

Conditional gene targeting of the ENaC subunit genes Scnn1b and Scnn1g

Epithelial sodium channels (ENaC) are members of the degenerin/ENaC superfamily of non-voltage-gated, highly amiloride-sensitive cation channels that are composed of three subunits (alpha-, beta-, and gamma-ENaC). Since complete gene inactivation of the beta- and gamma-ENaC subunit genes (Scnn1b and Scnn1g) leads to early postnatal death, we generated conditional alleles and obtained mice harboring floxed and null alleles for both gene loci. Using quantitative RT-PCR analysis, we showed that the introduction of the loxP sites did not interfere with the mRNA transcript expression level of the Scnn1b and Scnn1g gene locus, respectively. Upon a regular and salt-deficient diet, both beta- and gamma-ENaC floxed mice showed no difference in their mRNA transcript expression levels, plasma electrolytes, and aldosterone concentrations as well as weight changes compared with control animals. These mice can now be utilized to dissect the role of ENaC function in classical and nonclassic target organs/tissues.