Effects of receptor-mediated endocytosis and tubular protein composition on volume retention in experimental glomerulonephritis.
Human glomerulonephritis (GN) is characterized by sustained proteinuria, sodium retention, hypertension, and edema formation. Increasing quantities of filtered protein enter the renal tubule, where they may alter epithelial transport functions. Exaggerated endocytosis and consequent protein overload may affect proximal tubules, but intrinsic malfunction of distal epithelia has also been reported. A straightforward assignment to a particular tubule segment causing salt retention in GN is still controversial. We hypothesized that 1) trafficking and surface expression of major transporters and channels involved in volume regulation were altered in GN, and 2) proximal tubular endocytosis may influence locally as well as downstream expressed tubular transporters and channels. Effects of anti-glomerular basement membrane GN were studied in controls and megalin-deficient mice with blunted proximal endocytosis. Mice displayed salt retention and elevated systolic blood pressure when proteinuria had reached 10-15 mg/24 h. Surface expression of proximal Na(+)-coupled transporters and water channels was in part [Na(+)-P(i) cotransporter IIa (NaPi-IIa) and aquaporin-1 (AQP1)] increased by megalin deficiency alone, but unchanged (Na(+)/H(+) exchanger 3) or reduced (NaPi-IIa and AQP1) in GN irrespective of the endocytosis defect. In distal epithelia, significant increases in proteolytic cleavage products of alpha-epithelial Na(+) channel (ENaC) and gamma-ENaC were observed, suggesting enhanced tubular sodium reabsorption. The effects of glomerular proteinuria dominated over those of blunted proximal endocytosis in contributing to ENaC cleavage. Our data indicate that ENaC-mediated sodium entry may be the rate-limiting step in proteinuric sodium retention. Enhanced proteolytic cleavage of ENaC points to a novel mechanism of channel activation which may involve the action of filtered plasma proteases.
Kastner
Christian
C
Charité-Universitätsmedizin Berlin, Institut für Vegetative Anatomie, Philippstr. 12, 10115 Berlin, Germany.
Pohl
Marcus
M
Sendeski
Mauricio
M
Stange
Gerti
G
Wagner
Carsten A
CA
Jensen
Boye
B
Patzak
Andreas
A
Bachmann
Sebastian
S
Theilig
Franziska
F
D000450:Aldosterone / Q000097:blood; D000818:Animals; D051398:Aquaporin 1 / Q000378:metabolism; D051399:Aquaporin 2 / Q000378:metabolism; D001323:Autoantibodies; D001794:Blood Pressure; D051546:Cyclooxygenase 2 / Q000097:blood; D004195:Disease Models, Animal; D004705:Endocytosis; D053503:Epithelial Sodium Channels / Q000378:metabolism; D005260:Female; D005921:Glomerulonephritis / Q000276:immunology / Q000378:metabolism* / Q000503:physiopathology; D007684:Kidney Tubules / Q000201:enzymology / Q000276:immunology / Q000378:metabolism*; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000172:deficiency / Q000235:genetics; D026901:Membrane Transport Proteins / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D021381:Protein Transport; D011507:Proteinuria / Q000276:immunology / Q000378:metabolism* / Q000503:physiopathology; D012083:Renin / Q000097:blood; D012964:Sodium / Q000097:blood / Q000378:metabolism* / Q000652:urine; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000378:metabolism; D014809:Vitamin D-Binding Protein / Q000652:urine
C496021:Aqp1 protein, mouse; C496024:Aqp2 protein, mouse; D051399:Aquaporin 2; D001323:Autoantibodies; D053503:Epithelial Sodium Channels; D026561:Low Density Lipoprotein Receptor-Related Protein-2; C055127:Lrp2 protein, mouse; D026901:Membrane Transport Proteins; C507053:Scnn1g protein, mouse; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa; D014809:Vitamin D-Binding Protein; C051951:antiglomerular basement membrane antibody; D051398:Aquaporin 1; D000450:Aldosterone; D012964:Sodium; C077623:Ptgs2 protein, mouse; D051546:Cyclooxygenase 2; D012083:Renin