Effects of receptor-mediated endocytosis and tubular protein composition on volume retention in experimental glomerulonephritis

Abstract

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+-Pi 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 α-epithelial Na+ channel (ENaC) and γ-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.

Lipocalin-2 (24p3/Neutrophil Gelatinase-associated Lipocalin (NGAL)) Receptor Is Expressed in Distal Nephron and Mediates Protein Endocytosis

“…MT with a K D of ϳ100 M (18)) or due to exhaustion of the reabsorptive capacity of the system (e.g. following glomerular or PT damage and ensuing proteinuria) (6,13,14,53). A high-affinity protein reabsorption system in the distal nephron, as suggested by the present study, could contribute to exhaustive protein reabsorption by the nephron under physiological conditions, as deduced from previous micropuncture studies, or limit protein losses associated with renal diseases, including various forms of inherited or acquired Fanconi syndrome (53).…”

“…In fact, several micropuncture studies indicated that distal parts of the nephron are able to reabsorb significant amounts of filtered proteins under physiological conditions (3,(7)(8)(9)(10)(11)(12). However, the contribution of the DT to protein reabsorption varies depending on the nature of the proteins, their concentration, the tubular flow rate, and the experimental animal and ranges between 3 and 25% of filtered proteins (3,(7)(8)(9)(10)(11)(12) but may further increase when the capacity for protein reabsorption in the PT is exceeded, as in the case of glomerular or PT damage (6,13,14).…”

Short-Term Functional Adaptation of Aquaporin-1 Surface Expression in the Proximal Tubule, a Component of Glomerulotubular Balance

“…45 Differences between values obtained for AQP1 and the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase (DCt) were calculated and compared between groups.…”

Constraint-based, Homology Model of the Extracellular Domain of the Epithelial Na+ Channel α Subunit Reveals a Mechanism of Channel Activation by Proteases

“…In cystic fibrosis, enhanced ENaC activity leads to drying of the airways and mucociliary dysfunction (17-19). In the kidney, glomerular injury allows for the filtration of ENaC-activating proteases that are not usually found in the urinary space, contributing to the enhanced renal Na ϩ reabsorption and edema observed in proteinuric states (4,9,15,20,21).Although proteases activate ENaC by cleaving specific subunits and releasing inhibitory tracts, the structural basis for channel activation by proteases has not been elucidated. In an earlier work (22), we identified a number of sites where mutations altered the apparent affinity of an inhibitory peptide * This work was supported, in whole or in part, by National Institutes of Health Grants DK078734 (to O.…”