PS-B08-11: SALT DIRECTLY REGULATES UMOD TRAFFICKING IN A NORMOTENSIVE BACKGROUND

Abstract

Objective: Uromodulin (UMOD) is the most abundant protein secreted into urine of healthy individuals and is primarily produced by the renal thick ascending limb cells (TAL) of the loop of Henle. Genetic studies have demonstrated an association between UMOD risk variants, kidney diseases, and hypertension. We have previously demonstrated that salt induces a reduction in urinary UMOD excretion in normotensive rats after three weeks, which was exacerbated in a chronic hypertensive rat model. Our next objective was to determine the influence of salt on UMOD secretion independent of blood pressure effects. Here we investigate the underlying molecular differences in the effects of intermittent and chronic salt-loading on UMOD trafficking dynamics in the TAL. Design and Methods: Wistar-Kyoto (WKY) normotensive rats were maintained on 1% NaCl for three weeks in an intermittent pattern (one week on, one week off, one week on) or continuously (chronic) over three months. Urine was collected over a 24-hour period every experimental week. Total kidneys were harvested for lysate extraction, subcellular fractionation, and immunofluorescence analyses. UMOD levels were measured using ELISAs, qPCR, and Western blotting. Results: No abnormal changes in blood pressure, body weight, or renal parameters including albumin-creatinine-ratio and injury markers KIM-1 and NGAL were observed in either animal study group. During intermittent salt-loading, there was no significant difference in UMOD mRNA expression or total kidney UMOD protein levels. However, UMOD excretion significantly decreased after one week and remained lower than control levels even after salt removal. Similarly, chronic salt-loading did not alter UMOD mRNA levels, however total kidney UMOD protein levels were increased after three months. More specifically, analysis of subcellular fractions of the medulla from these salt-loaded animals showed higher levels of cytosolic UMOD compared to membrane-bound UMOD. Immunofluorescence analyses revealed a trend towards increased localisation of UMOD with endoplasmic reticulum (ER) marker calnexin. Conclusions: Our findings underline the effect of salt in molecular trafficking of UMOD in the kidney independent of blood pressure effects. Long-term salt exposure of TAL cells leads to intracellular accumulation of this abundant renal protein. The relationship between salt, UMOD accumulation, and renal end-organ damage in hypertension requires further research.