Profiling of renal tubule Na+ transporter abundances in NHE3 and NCC null mice using targeted proteomics

1. The Na+-H+ exchanger NHE3 and the thiazide-sensitive Na+-C1(-) cotranspo rter NCC are the major apical sodium transporters in the proximal convolute d tubule and the distal convoluted tubule of the kidney, respectively. We i nvestigated the mechanism of compensation that allows maintenance of sodium balance in NHE3 knockout mice and in NCC knockout mice. 2. We used a so-called 'targeted proteomics' approach, which profiles the e ntire renal tubule with regard to changes in Na+ transporter and aquaporin abundance in response to the gene deletions, Specific antibodies to the Na transporters and aquaporins expressed along the nephron were utilized to d etermine the relative abundance of each transporter. Semiquantitative immun oblotting was used which gives an estimate of the percentage change in abun dance of each transporter in knockout compared with wild-type mice. 3. In NHE3 knockout mice three changes were identified which could compensa te for the loss of NHE3-mediated sodium absorption. (a) The proximal sodium -phosphate cotransporter NaPi-2 was markedly upregulated. (b) In the collec ting duct, the 70 kDa form of the gamma -subunit of the epithelial sodium c hannel, ENaC, exhibited an increase in abundance. This is thought to be an aldosterone-stimulated form of gamma -ENaC. (c) Glomerular filtration was s ignificantly reduced. 4. In the NCC knockout mice, amongst all the sodium transporters expressed along the renal tubule, only the 70 kDa form of the gamma -subunit of the e pithelial sodium channel, ENaC, exhibited an increase in abundance. 5. In conclusion, both mouse knockout models, demonstrated successful compe nsation for loss of the deleted transporter. More extensive adaptation occu rred in the case of the NHE3 knockout, presumably because NHE3 is responsib le for much more sodium absorption in normal mice than in NCC knockout mice .