Deubiquitylation of Protein Cargo Is Not an Essential Step in Exosome Formation.

Exosomes, derived from multivesicular bodies (MVBs), contain proteins and genetic materials from their cell of origin and are secreted from various cells types, including kidney epithelial cells. In general, it is thought that protein cargo is ubiquitylated but that ubiquitin is cleaved by specific deubiquitylases during the process of cargo incorporation into MVBs. Here, we provide direct evidence that, in vivo, deubiquitylation is not essential. Ubiquitin was detected within human MVBs and urinary exosomes by electron microscopy. Of the >6000 proteins identified in human urinary exosomes was mass spectrometry, 15% were ubiquitylated with various topologies (Lys63>Lys48> Lys11>Lys6>Lys29>Lys33>Lys27). A significant preference for basic amino acids upstream of ubiquitylation sites suggests specific ubiquitylation motifs. The current studies demonstrate that, in vivo, deubiquitylation of proteins is not necessary for their incorporation into MVBs and highlight that urinary exosomes are an enriched source for studying ubiquitin modifications in physiological or disease states.

Ubiquilin2 as a novel marker for detection of urothelial carcinoma cells in urine.

BACKGROUND: Ubiquilin 2 (UBQLN2), an ubiquitin-related protein, is strongly expressed in urothelial carcinoma cells, in contrast to no or less expression in non-neoplastic cells; it protects cancer cells from reactive oxygen species (ROS)-induced cytotoxicity. In this study, we investigated whether UBQLN2 immunostaining, using liquid-based cytology sample could improve the accuracy of cytological urine diagnosis. METHODS: Two-hundred and forty-five urinary samples, including 143 negative controls and 102 urothelial carcinomas, consisting of 42 low-grade and 60 high-grade urothelial carcinomas, were used for immunocytochemical analysis of UBQLN2. RESULTS: Urothelial carcinoma cells were positive for UBQLN2-staining, while non-neoplastic cells, including renal tubular cells and degenerative atypical cells, were negative. Interestingly, percentage of nuclear stain immunopositive for UBQLN2 was significantly higher in carcinoma cells with high grade/invasive phenotype than in those with low grade/noninvasive phenotype. UBQLN2 immunostaining had an overall sensitivity of 87.6%, specificity of 98.6%, positive predictive value of 97.8% and negative predictive value of 92.8% for the detection of urothelial carcinoma. CONCLUSIONS: UBQLN2 immunostaining is a practical test for urine cytology, even in samples with few cells, with slight atypia or severe degenerative changes. In addition, it allows prediction of tumor grade and stage by examining the cellular localization of UBQLN2.

Characterization of diabetic nephropathy by urinary proteomic analysis: identification of a processed ubiquitin form as a differentially excreted protein in diabetic nephropathy patients.

BACKGROUND: Identification of markers for prediction of the clinical course of diabetic nephropathy remains a major challenge in disease management. We established a proteomics approach for identification of diabetic nephropathy-related biomarkers in urine. METHODS: We used SELDI-TOF mass spectrometry and SAX2 protein arrays to compare protein profiles from urine of 4 defined patient groups. Samples from patients with type 2 diabetes (DM; n = 45) without nephropathy and without microalbuminuria (DM-WNP), patients with DM with macro- or microalbuminuria (DM-NP; n = 38), patients with proteinuria due to nondiabetic renal disease (n = 34), and healthy controls (n = 45) were analyzed. Anionic exchange, reversed-phase fractionation, gel electrophoresis, and mass spectrometry were used to isolate and identify proteins with high discriminatory power. RESULTS: A protein with m/z 6188 (P <0.0000004) was strongly released in the urine of healthy controls, patients with proteinuria due to nondiabetic disease, and DM-WNP in contrast to DM-NP patients. An m/z 14 766 protein (P <0.00008) was selectively excreted in the urine of DM-NP patients, whereas the protein with m/z 11 774 (P <0.000004) was significantly excreted by patients with proteinuria and DM-NP. The m/z 11 774 and m/z 14 766 mass peaks were identified as beta(2)-microglobulin and UbA52, a ubiquitin ribosomal fusion protein, respectively. The protein with m/z 6188 was identified as a processed form of ubiquitin. CONCLUSION: The release of high amounts of UbA52 in urine of DM-NP patients could serve as a diagnostic marker, whereas the lack of the short form of ubiquitin raises interesting questions about the pathophysiology.