Mutant uromodulin expression leads to altered homeostasis of the endoplasmic reticulum and activates the unfolded protein response

<div><p>Uromodulin is the most abundant urinary protein in physiological conditions. It is exclusively produced by renal epithelial cells lining the thick ascending limb of Henle’s loop (TAL) and it plays key roles in kidney function and disease. Mutations in <i>UMOD</i>, the gene encoding uromodulin, cause autosomal dominant tubulointerstitial kidney disease uromodulin-related (ADTKD-<i>UMOD</i>), characterised by hyperuricemia, gout and progressive loss of renal function. While the primary effect of <i>UMOD</i> mutations, retention in the endoplasmic reticulum (ER), is well established, its downstream effects are still largely unknown. To gain insight into ADTKD-<i>UMOD</i> pathogenesis, we performed transcriptional profiling and biochemical characterisation of cellular models (immortalised mouse TAL cells) of robust expression of wild type or mutant GFP-tagged uromodulin. In this model mutant uromodulin accumulation in the ER does not impact on cell viability and proliferation. Transcriptional profiling identified 109 genes that are differentially expressed in mutant cells relative to wild type ones. Up-regulated genes include several ER resident chaperones and protein disulphide isomerases. Consistently, pathway enrichment analysis indicates that mutant uromodulin expression affects ER function and protein homeostasis. Interestingly, mutant uromodulin expression induces the Unfolded Protein Response (UPR), and specifically the IRE1 branch, as shown by an increased splicing of XBP1. Consistent with UPR induction, we show increased interaction of mutant uromodulin with ER chaperones Bip, calnexin and PDI. Using metabolic labelling, we also demonstrate that while autophagy plays no role, mutant protein is partially degraded by the proteasome through ER-associated degradation. Our work demonstrates that ER stress could play a central role in ADTKD-<i>UMOD</i> pathogenesis. This sets the bases for future work to develop novel therapeutic strategies through modulation of ER homeostasis and associated protein degradation pathways.</p></div

Associations between genotyped variants identified from sequencing brought forward to the entire FHS sample.

<p>Associations between genotyped variants identified from sequencing brought forward to the entire FHS sample.</p

Haplotypes at the <i>UMOD</i> locus in the 200 sequenced FHS participants.

<p>Haplotypes are composed of the 6 rare variants detected in this study (bold) and 6 common variants tagging the GWAS signal. Frequencies of each haplotype are provided to the right, and the haplotypes containing the high CKD-risk allele at rs12917707 (C) are shaded in gray. Rare alleles associated in this study with lower THP concentrations are shown in light blue, and rare alleles associated with higher THP concentrations are shown in dark blue.</p