Assignment of the Tamm-Horsfall protein/uromodulin gene (Umod) to mouse chromosome bands 7F1-F2 and rat chromosome bands 1q36-->q37 by in situ hybridization

GP2/THP gene family of self-binding, GPI-anchored proteins forms a cluster at chromosome 7F1 region in mouse genome

We investigated the genomic organization of pancreatic zymogen granule membrane-associated protein GP2, a GPI-anchored protein exhibiting self-aggregation at acidic pH, in order to construct a gene-knockout mouse. Cloning and analysis of lambda clones encoding GP2 from 129 Svj mouse genomic DNA libraries showed that the GP2 gene spans about 16.8 kb and includes 11 exons. Identifiable functional domains including a signal sequence, an EGF-like motif, a putative condensing ZP domain, a GPI-anchor attachment site, and a transmembrane sequence for GPI anchoring are encoded in separate exons. Using FISH, the GP2 gene was mapped to mouse chromosome 7F1 near the gene for THP, a GP2 homolog expressed in the cells of thick ascending loop of Henle (TALH) in the kidney. Further analysis of the mouse genome revealed that the THP and GP2 genes are adjacent to one another and are separated by only 3.5 kb in the 7F1 locus. Additionally, the overall structure of the THP gene, 16.2kb with 11 exons, was strikingly similar to that of GP2. This finding suggests that the GP2 and THP genes were generated by gene duplication and evolved separately to acquire regulatory elements leading to tissue-specific expression. Comparative analysis revealed that the 5' flanking region of the THP gene is similar to the first intron of NKCC2, a TALH cell-specific ion-transporter gene. The promoter region of the GP2 gene shares cis-elements found in other pancreas-specific genes. Using this genetic information, a GP2 null mutation was successfully introduced into an ES cell line, and an animal model was established without disruption of THP expression.

Analysis of the C-terminal structure of urinary Tamm-Horsfall protein reveals that the release of the glycosyl phosphatidylinositol-anchored counterpart from the kidney occurs by phenylalanine-specific proteolysis

Tamm-Horsfall protein (THP), also known as uromodulin, is a major glycoprotein synthesized in the kidney. THP is expressed on the luminal surface of the membrane with the glycosyl phosphatidylinositol (GPI) anchor and excreted in urine at a rate of 50-100 mg per day. Although THP is the most abundant urinary protein, the function of THP remains unclear. In addition, little is known about the mechanism by which large amounts of THP are actively released into the urinary fluid. In this study, we examined the C-terminal structure of highly purified THP derived from human urine. Carboxypeptidase Y efficiently degraded urinary THP, indicating that the C-terminal structure of the protein contains an amino acid residue with a free carboxyl moiety. These results are consistent with our previous finding that urinary THP does not bind anti-CRD antibody. We obtained peptides from the complete digestion of urinary THP with lysylendopeptidase. We purified the most C-terminal peptide with p-phenylene diisothiocyanate-controlled pore glass (DITC-CPG) beads. N-terminal sequence analysis indicated the peptide begins with Tyr 520 and ends between E539 and E576. Direct C-terminal amino acid sequencing of highly purified urinary THP gave a sequence of -X-(Q)-G-(R)-F, corresponding to amino acids 544-548, -S-Q-G-R-F. We therefore conclude that urinary THP is generated by a proteolytic cleavage between F548 and S549, 66 amino acids upstream of a possible GPI-anchor attachment site. Because the sequence of THP, including the cleavage site, is highly homologous to that of GP2, a GPI-anchored protein within the pancreas, and both THP and GP2 are abundantly found as soluble forms in the excreted fluids, a common mechanism may exist governing the proteolytic release of GPI-anchored membrane proteins.

Conditions for solubilization of Tamm-Horsfall protein/uromodulin in human urine and establishment of a sensitive and accurate enzyme-linked immunosorbent assay (ELISA) method

Tamm-Horsfall protein (THP), also referred to as uromodulin, is a major glycoprotein produced by kidney cells. The level of THP in urine is an indicator of renal function. THP levels are difficult to quantify accurately, because the protein forms aggregates in the pH and ion concentrations found in urine. Here, we describe conditions by which THP can be solubilized and kept soluble during measurement using Triton X-100, EDTA, and alkaline pH. Establishing conditions for precise quantification of THP by enzyme-linked immunosorbent assay (ELISA) will aid in studying the functions of THP.