Kidney-Specific Membrane-Bound Serine Proteases CAP1/Prss8 and CAP3/St14 Affect ENaC Subunit Abundances but Not Its Activity

The serine proteases CAP1/Prss8 and CAP3/St14 are identified as ENaC channel-activating proteases in vitro, highly suggesting that they are required for proteolytic activation of ENaC in vivo. The present study tested whether CAP3/St14 is relevant for renal proteolytic ENaC activation and affects ENaC-mediated Na+ absorption following Na+ deprivation conditions. CAP3/St14 knockout mice exhibit a significant decrease in CAP1/Prss8 protein expression with altered ENaC subunit and decreased pNCC protein abundances but overall maintain sodium balance. RNAscope-based analyses reveal co-expression of CAP3/St14 and CAP1/Prss8 with alpha ENaC in distal tubules of the cortex from wild-type mice. Double CAP1/Prss8; CAP3/St14-deficiency maintained Na+ and K+ balance on a Na+-deprived diet, restored ENaC subunit protein abundances but showed reduced NCC activity under Na+ deprivation. Overall, our data clearly show that CAP3/St14 is not required for direct proteolytic activation of ENaC but for its protein abundance. Our study reveals a complex regulation of ENaC by these serine proteases on the expression level rather than on its proteolytic activation.

Lack of Renal Tubular Glucocorticoid Receptor Decreases the Thiazide-Sensitive Na+/Cl- Cotransporter NCC and Transiently Affects Sodium Handling

Chronic glucocorticoid infusion impairs NCC activity and induces a non-dipping profile in mice, suggesting that glucocorticoids are essential for daily blood pressure variations. In this paper, we studied mice lacking the renal tubular glucocorticoid receptor (GR) in adulthood (GR knockouts, Nr3c1^Pax8/LC1). Upon standard salt diet, Nr3c1^Pax8/LC1 mice grow normally, but show reduced NCC activity despite normal plasma aldosterone levels. Following diet switch to low sodium, Nr3c1^Pax8/LC1 mice exhibit a transient but significant reduction in the activity of NCC and expression of NHE3 and NKCC2 accompanied by significant increased Spak activity. This is followed by transiently increased urinary sodium excretion and higher plasma aldosterone concentrations. Plasma corticosterone levels and 11βHSD2 mRNA expression and activity in the whole kidney remain unchanged. High salt diet does not affect whole body Na⁺ and/or K⁺ balance and NCC activity is not reduced, but leads to a significant increase in diastolic blood pressure dipping in Nr3c1^Pax8/LC1 mice. When high sodium treatment is followed by 48 h of darkness, NCC abundance is reduced in knockout mice although activity is not different. Our data show that upon Na⁺ restriction renal tubular GR-deficiency transiently affects Na⁺ handling and transport pathways. Overall, upon standard, low Na⁺ and high Na⁺ diet exposure Na⁺ and K⁺ balance is maintained as evidenced by normal plasma and urinary Na⁺ and K⁺ and aldosterone concentrations.

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.

Severe Salt-Losing Syndrome and Hyperkalemia Induced by Adult Nephron-Specific Knockout of the Epithelial Sodium Channel α-Subunit

Systemic pseudohypoaldosteronism type 1 (PHA-1) is a severe salt-losing syndrome caused by loss-of-function mutations of the amiloride-sensitive epithelial sodium channel (ENaC) and characterized by neonatal life-threatening hypovolemia and hyperkalemia. The very high plasma aldosterone levels detected under hypovolemic or hyperkalemic challenge can lead to increased or decreased sodium reabsorption, respectively, through the Na(+)/Cl(-) cotransporter (NCC). However, the role of ENaC deficiency remains incompletely defined, because constitutive inactivation of individual ENaC subunits is neonatally lethal in mice. We generated adult inducible nephron-specific αENaC-knockout mice (Scnn1a(Pax8/LC1)) that exhibit hyperkalemia and body weight loss when kept on a regular-salt diet, thus mimicking PHA-1. Compared with control mice fed a regular-salt diet, knockout mice fed a regular-salt diet exhibited downregulated expression and phosphorylation of NCC protein, despite high plasma aldosterone levels. In knockout mice fed a high-sodium and reduced-potassium diet (rescue diet), although plasma aldosterone levels remained significantly increased, NCC expression returned to control levels, and body weight, plasma and urinary electrolyte concentrations, and excretion normalized. Finally, shift to a regular diet after the rescue diet reinstated the symptoms of severe PHA-1 syndrome and significantly reduced NCC phosphorylation. In conclusion, lack of ENaC-mediated sodium transport along the nephron cannot be compensated for by other sodium channels and/or transporters, only by a high-sodium and reduced-potassium diet. We further conclude that hyperkalemia becomes the determining factor in regulating NCC activity, regardless of sodium loss, in the ENaC-mediated salt-losing PHA-1 phenotype.

Regulation of blood pressure and renal function by NCC and ENaC: lessons from genetically engineered mice

The activity of the thiazide-sensitive Na(+)/Cl(-) cotransporter (NCC) and of the amiloride-sensitive epithelial Na(+) channel (ENaC) is pivotal for blood pressure regulation. NCC is responsible for Na(+) reabsorption in the distal convoluted tubule (DCT) of the nephron, while ENaC reabsorbs the filtered Na(+) in the late DCT and in the cortical collecting ducts (CCD) providing the final renal adjustment to Na(+) balance. Here, we aim to highlight the recent advances made using transgenic mouse models towards the understanding of the regulation of NCC and ENaC function relevant to the control of sodium balance and blood pressure. We thus like to pave the way for common mechanisms regulating these two sodium-transporting proteins and their potential implication in structural remodeling of the nephron segments and Na(+) and Cl(-) reabsorption.

The CAP1/Prss8 catalytic triad is not involved in PAR2 activation and protease nexin-1 (PN-1) inhibition

Serine proteases, serine protease inhibitors, and protease-activated receptors (PARs) are responsible for several human skin disorders characterized by impaired epidermal permeability barrier function, desquamation, and inflammation. In this study, we addressed the consequences of a catalytically dead serine protease on epidermal homeostasis, the activation of PAR2 and the inhibition by the serine protease inhibitor nexin-1. The catalytically inactive serine protease CAP1/Prss8, when ectopically expressed in the mouse, retained the ability to induce skin disorders as well as its catalytically active counterpart (75%, n=81). Moreover, this phenotype was completely normalized in a PAR2-null background, indicating that the effects mediated by the catalytically inactive CAP1/Prss8 depend on PAR2 (95%, n=131). Finally, nexin-1 displayed analogous inhibitory capacity on both wild-type and inactive mutant CAP1/Prss8 in vitro and in vivo (64% n=151 vs. 89% n=109, respectively), indicating that the catalytic site of CAP1/Prss8 is dispensable for nexin-1 inhibition. Our results demonstrate a novel inhibitory interaction between CAP1/Prss8 and nexin-1, opening the search for specific CAP1/Prss8 antagonists that are independent of its catalytic activity.

The channel-activating protease CAP1/Prss8 is required for placental labyrinth maturation

The serine protease CAP1/Prss8 is crucial for skin barrier function, lung alveolar fluid clearance and has been unveiled as diagnostic marker for specific cancer types. Here, we show that a constitutive knockout of CAP1/Prss8 leads to embryonic lethality. These embryos presented no specific defects, but it is during this period, and in particular at E13.5, that wildtype placentas show an increased expression of CAP1/Prss8, thus suggesting a placental defect in the knockout situation. The placentas of knockout embryos exhibited significantly reduced vascular development and incomplete cellular maturation. In contrary, epiblast-specific deletion of CAP1/Prss8 allowed development until birth. These CAP1/Prss8-deficient newborns presented abnormal epidermis, and died soon after birth due to impaired skin function. We thus conclude that a late placental insufficiency might be the primary cause of embryonic lethality in CAP1/Prss8 knockouts. This study highlights a novel and crucial role for CAP1/Prss8 in placental development and function.

Mutations of the serine protease CAP1/Prss8 lead to reduced embryonic viability, skin defects, and decreased ENaC activity

CAP1/Prss8 is a membrane-bound serine protease involved in the regulation of several different effectors, such as the epithelial sodium channel ENaC, the protease-activated receptor PAR2, the tight junction proteins, and the profilaggrin polypeptide. Recently, the V170D and the G54-P57 deletion mutations within the CAP1/Prss8 gene, identified in mouse frizzy (fr) and rat hairless (fr(CR)) animals, respectively, have been proposed to be responsible for their skin phenotypes. In the present study, we analyzed those mutations, revealing a change in the protein structure, a modification of the glycosylation state, and an overall reduction in the activation of ENaC of the two mutant proteins. In vivo analyses demonstrated that both fr and fr(CR) mutant animals present analogous reduction of embryonic viability, similar histologic aberrations at the level of the skin, and a significant decrease in the activity of ENaC in the distal colon compared with their control littermates. Hairless rats additionally had dehydration defects in skin and intestine and significant reduction in the body weight. In conclusion, we provided molecular and functional evidence that CAP1/Prss8 mutations are accountable for the defects in fr and fr(CR) animals, and we furthermore demonstrate a decreased function of the CAP1/Prss8 mutant proteins. Therefore, fr and fr(CR) animals are suitable models to investigate the consequences of CAP1/Prss8 action on its target proteins in the whole organism.

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.

Comparison of two molecular methods used for subtyping of Legionella pneumophila 1 strains isolated from a hospital water supply

The results of the pulsed-field gel electrophoresis and the sequence-based typing (using the loci flaA, pilE, asd, mip, mompS and proA) were compared for subtyping of Legionella pneumophila 1 strains isolated from a hospital water supply. Molecular typing was carried out on 61 isolates (38% of the positive samples) selected on space and temporal criteria in order to follow the evolution of the water-system colonization. For all the 61 isolates, the sequence of the amplified mip gene fragment identified Legionella pneumophila strain Wadsworth. Genotype testing by PFGE analysis showed three different patterns, correspondent to three SBT types according to the allelic profiles. Both PFGE and SBT indicated the circulation and the persistence in the hospital potable water-system of three types randomly distributed in space and time. The two molecular methods adopted showed a 100% concordance, although a low degree of genetic heterogeneity characterized the isolates. The electrophoretic patterns were sufficiently unambiguous to consider PFGE a highly discriminatory typing method, but the SBT technique besides accurately characterizing isolates, was able to identify Legionella strains through analysis of the mip gene. A typing method with this level of discriminatory power has great potential for assisting in epidemiological studies.