Synergistic Activation of ENaC by Three Membrane-bound Channel-activating Serine Proteases (mCAP1, mCAP2, and mCAP3) and Serum- and Glucocorticoid-regulated Kinase (Sgk1) in Xenopus Oocytes

Sodium balance is maintained by the precise regulation of the activity of the epithelial sodium channel (ENaC) in the kidney. We have recently reported an extracellular activation of ENaC-mediated sodium transport (INa) by a GPI-anchored serine protease (mouse channel–activating protein, mCAP1) that was isolated from a cortical collecting duct cell line derived from mouse kidney. In the present study, we have identified two additional membrane-bound serine proteases (mCAP2 and mCAP3) that are expressed in the same cell line. We show that each of these proteases is able to increase INa 6–10-fold in the Xenopus oocyte expression system. INa and the number (N) of channels expressed at the cell surface (measured by binding of a FLAG monoclonal I125-radioiodinated antibody) were measured in the same oocyte. Using this assay, we show that mCAP1 increases INa 10-fold (P < 0.001) but N remained unchanged (P = 0.9), indicating that mCAP1 regulates ENaC activity by increasing its average open probability of the whole cell (wcPo). The serum- and glucocorticoid-regulated kinase (Sgk1) involved in the aldosterone-dependent signaling cascade enhances INa by 2.5-fold (P < 0.001) and N by 1.6-fold (P < 0.001), indicating a dual effect on N and wcPo. Compared with Sgk1 alone, coexpression of Sgk1 with mCAP1 leads to a ninefold increase in INa (P < 0.001) and 1.3-fold in N (P < 0.02). Similar results were observed for mCAP2 and mCAP3. The synergism between CAPs and Sgk1 on INa was always more than additive, indicating a true potentiation. The synergistic effect of the two activation pathways allows a large dynamic range for ENaC-mediated sodium regulation crucial for a tight control of sodium homeostasis

A novel TMPRSS3 missense mutation in a DFNB8/10 family prevents proteolytic activation of the protein

Pathogenic mutations in TMPRSS3, which encodes a transmembrane serine protease, cause non-syndromic deafness DFNB8/10. Missense mutations map in the low density-lipoprotein receptor A (LDLRA), scavenger-receptor cysteine-rich (SRCR), and protease domains of the protein, indicating that all domains are important for its function. TMPRSS3 undergoes proteolytic cleavage and activates the ENaC sodium channel in a Xenopus oocyte model system. To assess the importance of this gene in non-syndromic childhood or congenital deafness in Turkey, we screened for mutations affected members of 25 unrelated Turkish families. The three families with the highest LOD score for linkage to chromosome 21q22.3 were shown to harbor P404L, R216L, or Q398X mutations, suggesting that mutations in TMPRSS3 are a considerable contributor to non-syndromic deafness in the Turkish population. The mutant TMPRSS3 harboring the novel R216L missense mutation within the predicted cleavage site of the protein fails to undergo proteolytic cleavage and is unable to activate ENaC, thus providing evidence that pre-cleavage of TMPRSS3 is mandatory for normal functio

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Collecting duct–specific gene inactivation of αENaC in the mouse kidney does not impair sodium and potassium balance

Aldosterone controls the final sodium reabsorption and potassium secretion in the kidney by regulating the activity of the epithelial sodium channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN). ASDN consists of the last portion of the distal convoluted tubule (late DCT), the connecting tubule (CNT), and the collecting duct (CD) (i.e., the cortical CD [CCD] and the medullary CD [MCD]). It has been proposed that the control of sodium transport in the CCD is essential for achieving sodium and potassium balance. We have tested this hypothesis by inactivating the α subunit of ENaC in the CD but leaving ENaC expression in the late DCT and CNT intact. Under salt restriction or under aldosterone infusion, whole-cell voltage clamp of principal cells of CCD showed no detectable ENaC activity, whereas large amiloride-sensitive currents were observed in control littermates. The animals survive well and are able to maintain sodium and potassium balance, even when challenged by salt restriction, water deprivation, or potassium loading. We conclude that the expression of ENaC in the CD is not a prerequisite for achieving sodium and potassium balance in mice. This stresses the importance of more proximal nephron segments (late DCT/CNT) to achieve sodium and potassium balance

Functional expression of a pseudohypoaldosteronism type I mutated epithelial Na(+) channel lacking the pore-forming region of its α subunit

The autosomal recessive form of type I pseudohypoaldosteronism (PHA-I) is an inherited salt-losing syndrome resulting from diminution-of-function mutations in the 3 subunits of the epithelial Na(+) channel (ENaC). A PHA-I stop mutation (α(R508stop)) of the ENaC α subunit is predicted to lack the second transmembrane domain and the intracellular COOH-terminus, regions of the protein involved in pore function. Nonetheless, we observed a measurable Na(+) current in Xenopus laevis oocytes that coexpress the β and γ subunits with the truncated α subunit. The mutant α was coassembled with β and γ subunits and was present at the cell surface at a lower density, consistent with the lower Na(+) current seen in oocytes with the truncated α subunit. The single-channel Na(+) conductance for the mutant channel was only slightly decreased, and the appearance of the macroscopic currents was delayed by 48 hours with respect to wild-type. Our data suggest novel roles for the α subunit in the assembly and targeting of an active channel to the cell surface, and suggest that channel pores consisting of only the β and γ subunits can provide significant residual activity. This activity may be sufficient to explain the absence of a severe pulmonary phenotype in patients with PHA-I

Renal tubular NEDD4-2 deficiency causes NCC-mediated salt-dependent hypertension

The E3 ubiquitin ligase NEDD4-2 (encoded by the Nedd4L gene) regulates the amiloride-sensitive epithelial Na+ channel (ENaC/SCNN1) to mediate Na+ homeostasis. Mutations in the human β/γENaC subunits that block NEDD4-2 binding or constitutive ablation of exons 6-8 of Nedd4L in mice both result in salt-sensitive hypertension and elevated ENaC activity (Liddle syndrome). To determine the role of renal tubular NEDD4-2 in adult mice, we generated tetracycline-inducible, nephron-specific Nedd4L KO mice. Under standard and high-Na+ diets, conditional KO mice displayed decreased plasma aldosterone but normal Na+/K+ balance. Under a high-Na+ diet, KO mice exhibited hypercalciuria and increased blood pressure, which were reversed by thiazide treatment. Protein expression of βENaC, γENaC, the renal outer medullary K+ channel (ROMK), and total and phosphorylated thiazide-sensitive Na+Cl- cotransporter (NCC) levels were increased in KO kidneys. Unexpectedly, Scnn1a mRNA, which encodes the αENaC subunit, was reduced and proteolytic cleavage of αENaC decreased. Taken together, these results demonstrate that loss of NEDD4-2 in adult renal tubules causes a new form of mild, salt-sensitive hypertension without hyperkalemia that is characterized by upregulation of NCC, elevation of β/γENaC, but not αENaC, and a normal Na+/K+ balance maintained by downregulation of ENaC activity and upregulation of ROMK

β-Liddle mutation of the epithelial sodium channel increases alveolar fluid clearance and reduces the severity of hydrostatic pulmonary oedema in mice

Transepithelial sodium transport via alveolar epithelial Na+ channels and Na+,K+-ATPase constitutes the driving force for removal of alveolar oedema fluid. Decreased activity of the amiloride-sensitive epithelial Na+ channel (ENaC) in the apical membrane of alveolar epithelial cells impairs sodium-driven alveolar fluid clearance (AFC) and predisposes to pulmonary oedema. We hypothesized that hyperactivity of ENaC in the distal lung could improve AFC and facilitate the resolution of pulmonary oedema. AFC and lung fluid balance were studied at baseline and under conditions of hydrostatic pulmonary oedema in the β-Liddle (L) mouse strain harbouring a gain-of-function mutation (R566stop) within the Scnn1b gene. As compared with wild-type (+/+), baseline AFC was increased by 2- and 3-fold in heterozygous (+/L) and homozygous mutated (L/L) mice, respectively, mainly due to increased amiloride-sensitive AFC. The β2-agonist terbutaline stimulated AFC in +/+ and +/L mice, but not in L/L mice. Acute volume overload induced by saline infusion (40% of body weight over 2 h) significantly increased extravascular (i.e. interstitial and alveolar) lung water as assessed by the bloodless wet-to-dry lung weight ratio in +/+ and L/L mice, as compared with baseline. However, the increase was significantly larger in +/+ than in L/L groups (P= 0.01). Volume overload also increased the volume of the alveolar epithelial lining fluid in +/+ mice, indicating the presence of alveolar oedema, but not in L/L mice. Cardiac function as evaluated by echocardiography was comparable in both groups. These data show that constitutive ENaC activation improved sodium-driven AFC in the mouse lung, and attenuated the severity of hydrostatic pulmonary oedema