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>