The kidney releases a non-polymerizing form of Uromodulin in the urine and circulation that retains the external hydrophobic patch domain

Abstract

Uromodulin (Tamm-Horsfall protein, THP) is a glycoprotein uniquely produced in the kidney. It is released by cells of the thick ascending limbs (TAL) apically in the urine, and basolaterally in the renal interstitium and systemic circulation. Processing of mature urinary THP, which polymerizes into supra-molecular filaments, requires cleavage of an external hydrophobic patch (EHP) at the C terminus. However, THP in the circulation is not polymerized, and it remains unclear if non-aggregated forms of THP exist natively in the urine. We propose that an alternative processing path, which retains the EHP domain, can lead to a non-polymerizing form of THP. We have generated an antibody that specifically recognizes THP with retained EHP (THP+EHP) and established its presence in the urine in a non-polymerized native state. In the human kidney, THP+EHP predominantly co-localized with urinary THP in TAL cells, but it was also detected, at lower levels, in other tubular segments. We then developed a customized ELISA to measure THP+EHP and used it on urine and plasma samples from a small cohort of patients hospitalized with liver cirrhosis. On admission, urinary THP+EHP, but not total THP, was significantly lower in patients who subsequently developed acute kidney injury during hospitalization. THP+EHP was also detected in the plasma, albeit at very low concentrations. Our findings uncover novel insights into uromodulin biology by establishing the presence of an alternative path for cellular processing. Our proof-of-principle findings in patients warrant further investigations to establish the utility of THP+EHP as a sensitive biomarker of kidney health and susceptibility to injury.