Uromodulin is expressed in renal primary cilia and UMOD mutations result in decreased ciliary uromodulin expression

Abstract

Uromodulin (UMOD) mutations are responsible for three autosomal dominant tubulo-interstitial nephropathies including medullary cystic kidney disease type 2 (MCKD2), familial juvenile hyperuricemic nephropathy and glomerulocystic kidney disease. Symptoms include renal salt wasting, hyperuricemia, gout, hypertension and end-stage renal disease. MCKD is part of the 'nephronophthisis-MCKD complex', a group of cystic kidney diseases. Both disorders have an indistinguishable histology and renal cysts are observed in either. For most genes mutated in cystic kidney disease, their proteins are expressed in the primary cilia/basal body complex. We identified seven novel UMOD mutations and were interested if UMOD protein was expressed in the primary renal cilia of human renal biopsies and if mutant UMOD would show a different expression pattern compared with that seen in control individuals. We demonstrate that UMOD is expressed in the primary cilia of renal tubules, using immunofluorescent studies in human kidney biopsy samples. The number of UMOD-positive primary cilia in UMOD patients is significantly decreased when compared with control samples. Additional immunofluorescence studies confirm ciliary expression of UMOD in cell culture. Ciliary expression of UMOD is also confirmed by electron microscopy. UMOD localization at the mitotic spindle poles and colocalization with other ciliary proteins such as nephrocystin-1 and kinesin family member 3A is demonstrated. Our data add UMOD to the group of proteins expressed in primary cilia, where mutations of the gene lead to cystic kidney disease.

No Amelioration of Uromodulin Maturation and Trafficking Defect by Sodium 4-Phenylbutyrate in Vivo

“…It is a highly glycosylated protein processed in the endoplasmic reticulum (ER) and Golgi apparatus, translocated to the luminal cell membrane, and released in the urine by proteolytic cleavage. So far, Ͼ70 different UAKDcausing UMOD mutations have been published, of which all but four (in-frame deletions) lead to amino acid substitutions (4,7,11). Mutant uromodulin isoforms exhibit a protein maturation defect and are retained in the ER (12,13).…”

Uromodulin: old friend with new roles in health and disease

“…New insight into the pathophysiology of UAKD has been provided by recent reports linking uromodulin to the central apical cilium of TAL cells and demonstrating decreased ciliary uromodulin expression in biopsy tissue from UAKD patients [32], suggesting that UAKD belongs to the group of diseases termed "ciliopathies." This group includes polycystic kidney diseases (PKD) and nephronophthisis.…”

Molecular and Cellular Effects of Tamm-Horsfall Protein Mutations and Their Rescue by Chemical Chaperones

“…After proper folding, modification with high mannose glycomoieties, and addition of a C-terminal glycophosphatidylinositol, the protein exits the rough ER and transits to the Golgi apparatus where it is further modified with complex-type carbohydrates and then destined for the apical plasma membrane (5,6). Mature THP is believed to anchor via its glycophosphatidylinositol tail onto the luminal leaflet of the lipid bilayer of the apical membrane of the thick ascending limb of the loop of Henle, although a recent study found the protein also to be associated with the primary cilia and the mitotic spindle poles of these tubular cells (7). Apically located THP is eventually released by proteases and/or phospholipases into the urine where it constitutes the most abundant urinary protein in most mammals (8 -10).…”