ENaC activation by proteases

Abstract

Proteases are fundamental for a plethora of biological processes, including signalling and tissue remodelling, and dysregulated proteolytic activity can result in pathogenesis. In this review, we focus on a subclass of membrane-bound and soluble proteases that are defined as channel-activating proteases (CAPs), since they induce Na⁺ ion transport through an autocrine mechanism when co-expressed with the highly amiloride-sensitive epithelial sodium channel (ENaC) in Xenopus oocytes. These experiments first identified CAP1 (channel-activating protease 1, prostasin) followed by CAP2 (channel-activating protease 2, TMPRSS4) and CAP3 (channel-activating protease 3, matriptase) as in vitro mediators of ENaC current. Since then, more serine-, cysteine- and metalloproteases were confirmed as in vitro CAPs that potentially cleave and regulate ENaC, and thus this nomenclature was not further followed, but is accepted as functional term or alias. The precise mechanism of ENaC modulation by proteases has not been fully elucidated. Studies in organ-specific protease knockout models revealed evidence for their role in increasing ENaC activity, although the proteases responsible for ENaC activation are yet to be identified. We summarize recent findings in animal models of these CAPs with respect to their implication in ENaC activation. We discuss the consequences of dysregulated CAPs underlying epithelial phenotypes in pathophysiological conditions, and the role of selected protease inhibitors. We believe that these proteases may present interesting therapeutic targets for diseases with aberrant sodium homoeostasis.

Keywords: epithelial phenotype; epithelial sodium channel; homoeostasis; kidney disease.

FIGURE 1

Structural schematic of identified ENaC CAPs. CAP1 (Prss8, prostasin),, CAP2 (Tmprss4), CAP3 (St14/Matriptase), Tmprss3, Tmprss2,, uPA (urokinase‐type plasminogen activator),, plasminogen,, , , trypsin, chymotrypsin, tissue, and plasma kallikrein, elastase, furin, factor VII activating protease, cathepsin B,, cathepsin S, meprin β and serralysin. Predicted structural domains of mouse proteases are indicated: CUB, complement C1r/C2s, urchin embryonic growth factor, bone morphogenic protein 1; EGF, epidermal growth factor‐like; apple; kringle; GPI, glycophosphatidylinositol anchor; LDL‐A, low density lipoprotein A; MAM, meprin, A5 protein, receptor protein phosphatase μ; MATH, meprin and TRAF‐C homology; P/Homo B, paired basic amino acid residue‐cleaving enzyme/homo sapiens B; PAN, PAN/apple; SEA, sperm protein, enterokinase and agrin; SRCR, scavenger receptor cysteine‐rich; TM, transmembrane

FIGURE 2

ENaC and CAP transcriptional expression in male C57BL/6 mouse organs and nephron segments shown as transcripts per million. A, Data according to the EMBL‐EBI expression atlas data., B, RNA expression data across 14 mouse renal tubule segments from 6‐ to 8‐week‐old mouse microdissected tubules. ATL, thin ascending limb of the loop of Henle; CCD, cortical collecting duct; CNT, connecting tubule; CTAL, cortical thick ascending limb of the loop of Henle; DCT, distal convoluted tubule; DTL1, short descending limb of the loop of Henle; DTL2, long descending limb of the loop of Henle in the outer medulla; DTL3, long descending limb of the loop of Henle in the inner medulla; IMCD, inner medullary collecting duct; MTAL, medullary thick ascending limb of the loop of Henle; OMCD, outer medullary collecting duct; PST1, initial segment of the proximal tubule; PST2, proximal straight tubule in cortical‐medullary rays; PST3, last segment of the proximal straight tubule in the outer stripe of outer medulla

FIGURE 3

Identified consensus sites for proteolytic ENaC cleavage by CAPs in mouse. Illustration of the mouse α‐, β‐ and γ‐ENaC subunits depicting in vitro identified mouse CAP cleavage sites. α‐ENaC amino acid residues numbered in blue and γ‐ENaC in green. Furin cleaves at two sites in the α subunit and at one site in the γ subunit. CAP1, CAP2, tissue kallikrein, plasma kallikrein and plasmin cleave mouse γ‐ENaC distal to the furin cleavage site

FIGURE 4

In vitro identified consensus sites for proteolytic ENaC cleavage by CAPs. Alignment of human, rat and mouse α‐ and γ‐ENaC subunits showing a high degree of conservation between species at cleavage sites. Amino acid numbering at the end of transcripts indicates the portion of sequence analysed. CAPs shown to cleave human/rat (indicated above) and mouse (indicated below) ENaC consensus sites in vitro are specified., , , CTSS, cathepsin S; ELANE, neutrophil elastase; FSAP, factor VII‐activating protease; PKLK, plasma kallikrein; PLM, plasmin; TKR, tissue kallikrein; Tryp, trypsin; uPA, urokinase‐type plasminogen