Organization of uroplakin subunits: transmembrane topology, pair formation and plaque composition

Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain

This review summarizes key aspects of tetraspanin proteins, with a focus on the functional relevance and structural features of these proteins and how they are organized into a novel type of membrane microdomain. Despite the size of the tetraspanin family and their abundance and wide distribution over many cell types, most have not been studied. However, from studies of prototype tetraspanins, information regarding functions, cell biology, and structural organization has begun to emerge. Genetic evidence points to critical roles for tetraspanins on oocytes during fertilization, in fungi during leaf invasion, in Drosophila embryos during neuromuscular synapse formation, during T and B lymphocyte activation, in brain function, and in retinal degeneration. From structure and mutagenesis studies, we are beginning to understand functional subregions within tetraspanins, as well as the levels of connections among tetraspanins and their many associated proteins. Tetraspanin-enriched microdomains (TEMs) are emerging as entities physically and functionally distinct from lipid rafts. These microdomains now provide a context in which to evaluate tetraspanins in the regulation of growth factor signaling and in the modulation of integrin-mediated post-cell adhesion events. Finally, the enrichment of tetraspanins within secreted vesicles called exosomes, coupled with hints that tetraspanins may regulate vesicle fusion and/or fission, suggests exciting new directions for future research.

Rab27b is associated with fusiform vesicles and may be involved in targeting uroplakins to urothelial apical membranes

The terminally differentiated umbrella cells of bladder epithelium contain unique cytoplasmic organelles, the fusiform vesicles, which deliver preassembled crystalline arrays of uroplakin proteins to the apical cell surface of urothelial umbrella cells. We have investigated the possible role of Rab proteins in this delivery process, and found Rab27b to be expressed at an extraordinary high level (0.1% of total protein) in urothelium, whereas Rab27b levels were greatly reduced (to <5% of normal urothelium) in cultured urothelial cells, which synthesized only small amounts of uroplakins and failed to form fusiform vesicles. Immuno-electron microscopy showed that Rab27b was associated with the cytoplasmic face of the fusiform vesicles, but not with that of the apical plasma membrane. The association of Rab27b with fusiform vesicles and its differentiation-dependent expression suggest that this Rab protein plays a role in regulating the delivery of fusiform vesicles to the apical plasma membrane of umbrella cells.

Primary ureteral reflux: emerging insights from molecular and genetic studies

The incidence of primary vesicoureteral reflux is about 1% to 2% of the general population and is as high as 50% in siblings as well as offspring of affected patients, suggesting autosomal dominant inheritance. The current diagnosis of vesicoureteral reflux involves voiding cystourethrograms, which are invasive and costly. Consequently, vesicoureteral reflux screening in siblings and offspring is not routinely practiced, because of the known high risk. Early detection of vesicoureteral reflux will be valuable for prevention of reflux nephropathy, because the incidence of reflux nephropathy can be reduced effectively by antibiotic prophylaxis. Furthermore, the presence of reflux nephropathy can only be accurately assessed currently by dimercapto-succinic acid nuclear scans, which are costly, time and labor intensive, and often require conscious sedation by a pediatric anesthesiology team. As a result, the clinical assessment of reflux nephropathy is also not routinely practiced. There is a pressing need to develop less invasive and less costly tests for the early diagnosis of primary vesicoureteric reflux and reflux nephropathy. Recent molecular and genetic studies have greatly increased our understanding of vesicoureteral reflux and provide a promise of novel non-invasive tests. Targeted disruption of angiotensin type II receptor and uroplakin III genes result in the phenotype of primary vesicoureteral reflux. There are characteristic patterns of message and protein changes in the knockout animals, providing the basis for detection of genetic mutations leading to vesicoureteral reflux in humans by studying differential gene expression by functional genomics methodology. The urothelium is also known to secrete proteins into the urine. Preliminary studies showed unique fingerprints in urinary protein patterns in children with primary VUR, providing the basis for developing novel noninvasive molecular diagnostic tests of vesicoureteral reflux by proteomics methodology.

Potential utility of uroplakin III, thrombomodulin, high molecular weight cytokeratin, and cytokeratin 20 in noninvasive, invasive, and metastatic urothelial (transitional cell) carcinomas

The morphology of urothelial carcinomas, particularly when poorly differentiated or in metastatic sites, is not distinctive and overlaps significantly with other poorly differentiated nonurothelial carcinomas. Currently, there is no widely used single marker or panel of markers to confirm urothelial origin. We evaluated a panel consisting of antibodies to uroplakin III (UROIII), thrombomodulin (THR), high molecular weight cytokeratin (HMWCK), and cytokeratin 20 (CK20) in a wide range of urothelial tumors. Immunohistochemistry was performed on 112 paraffin-embedded urothelial neoplasms: 14 low malignant potential, 16 low-grade noninvasive, 16 high-grade noninvasive, 36 invasive, and 25 metastatic and 5 small cell carcinomas of the urinary bladder. Tissue microarray analysis was used to examine 498 tissue cores of nonurothelial tumors and normal tissue using antibodies to UROIII, THR, and HMWCK. Overall positive staining results in all urothelial tumors are as follows: UROIII, 64 of 112 (57.1%); THR, 77 of 112 (68.8%); HMWCK, 88 of 110 (80%); and CK20, 53 of 110 (48.2%). The expression of the four markers varied with tumor grade and stage. All small cell carcinomas were negative for all markers. Variant morphologic subtypes showed similar staining as conventional urothelial carcinomas. Tissue microarray analysis showed no UROIII immunoreactivity in tissue cores of nonurothelial tumors. THR was expressed by a limited number of nonurothelial cores (10 of 37 [27%] non-small cell lung carcinomas, 2 of 36 [5.6%] lymphomas). HMWCK was expressed by 43.8% of non-small cell lung carcinomas and essentially absent in other nonurothelial tumor cores. Based on the results of the study, the expression of UROIII in a tumor is essentially diagnostic of urothelial origin; however, it is expressed in only slightly more than half of urothelial tumors. The coexpression of THR, HMWCK, and CK20 strongly suggests urothelial origin. The coexpression of two of three non-UROIII markers (THR, HMWCK, CK20) suggests urothelial origin but requires clinicopathologic correlation. The results of the study indicate a role for an antibody panel that includes UROIII, THR, HMWCK, and CK20 in the diagnosis of urothelial tumors.

Specific heterodimer formation is a prerequisite for uroplakins to exit from the endoplasmic reticulum

Much of the lower urinary tract, including the bladder, is lined by a stratified urothelium forming a highly differentiated, superficial umbrella cell layer. The apical plasma membrane as well as abundant cytoplasmic fusiform vesicles of the umbrella cells is covered by two-dimensional crystals that are formed by four membrane proteins named uroplakins (UPs) Ia, Ib, II, and III. UPs are synthesized on membrane-bound polysomes, and after several co- and posttranslational modifications they assemble into planar crystals in a post-Golgi vesicular compartment. Distension of the bladder may cause fusiform vesicles to fuse with the apical plasma membrane. We have investigated the early stages of uroplakin assembly by expressing the four uroplakins in 293T cells. Transfection experiments showed that, when expressed individually, only UPIb can exit from the endoplasmic reticulum (ER) and move to the plasma membrane, whereas UPII and UPIII reach the plasma membrane only when they form heterodimeric complexes with UPIa and UPIb, respectively. Heterodimer formation in the ER was confirmed by pulse-chase experiment followed by coimmunoprecipitation. Our results indicate that the initial building blocks for the assembly of crystalline uroplakin plaques are heterodimeric uroplakin complexes that form in the ER.

Uroplakin IIIb, a urothelial differentiation marker, dimerizes with uroplakin Ib as an early step of urothelial plaque assembly

Urothelial plaques consist of four major uroplakins (Ia, Ib, II, and III) that form two-dimensional crystals covering the apical surface of urothelium, and provide unique opportunities for studying membrane protein assembly. Here, we describe a novel 35-kD urothelial plaque-associated glycoprotein that is closely related to uroplakin III: they have a similar overall type 1 transmembrane topology; their amino acid sequences are 34% identical; they share an extracellular juxtamembrane stretch of 19 amino acids; their exit from the ER requires their forming a heterodimer with uroplakin Ib, but not with any other uroplakins; and UPIII-knockout leads to p35 up-regulation, possibly as a compensatory mechanism. Interestingly, p35 contains a stretch of 80 amino acid residues homologous to a hypothetical human DNA mismatch repair enzyme-related protein. Human p35 gene is mapped to chromosome 7q11.23 near the telomeric duplicated region of Williams-Beuren syndrome, a developmental disorder affecting multiple organs including the urinary tract. These results indicate that p35 (uroplakin IIIb) is a urothelial differentiation product structurally and functionally related to uroplakin III, and that p35-UPIb interaction in the ER is an important early step in urothelial plaque assembly.

Stretch-regulated exocytosis/endocytosis in bladder umbrella cells

The epithelium of the urinary bladder must maintain a highly impermeable barrier despite large variations in urine volume during bladder filling and voiding. To study how the epithelium accommodates these volume changes, we mounted bladder tissue in modified Ussing chambers and subjected the tissue to mechanical stretch. Stretching the tissue for 5 h resulted in a 50% increase in lumenal surface area (from approximately 2900 to 4300 microm(2)), exocytosis of a population of discoidal vesicles located in the apical cytoplasm of the superficial umbrella cells, and release of secretory proteins. Surprisingly, stretch also induced endocytosis of apical membrane and 100% of biotin-labeled membrane was internalized within 5 min after stretch. The endocytosed membrane was delivered to lysosomes and degraded by a leupeptin-sensitive pathway. Last, we show that the exocytic events were mediated, in part, by a cyclic adenosine monophosphate, protein kinase A-dependent process. Our results indicate that stretch modulates mucosal surface area by coordinating both exocytosis and endocytosis at the apical membrane of umbrella cells and provide insight into the mechanism of how mechanical forces regulate membrane traffic in non-excitable cells.

Specific tetraspanin functions

Relatively little attention has been given to the large family of abundantly expressed transmembrane proteins known as tetraspanins. Now, the importance of tetraspanins is strongly supported by emerging genetic evidence, coupled with new insights into the biochemistry and functions of tetraspanin protein complexes.