Retention of membrane-localized beta-catenin in cells lacking functional polycystin-1 and tuberin.

The tuberous sclerosis (TSC) 2 tumor suppressor gene encodes the protein tuberin, which has recently been shown to play a crucial role in the intracellular trafficking of polycystin-1, the product of the polycystic kidney disease (PDK) 1 gene. PKD1 is responsible for most cases of autosomal dominant polycystic kidney disease, which has been described as "neoplasia in disguise." Polycystin-1 is a membrane protein localized to adherens junctions in a complex containing E-cadherin and alpha-, beta-, and gamma-catenins. To determine whether loss of membrane localization of polycystin-1 and E-cadherin affects the function of beta-catenin, beta-catenin localization and signaling were characterized in tuberin-null EKT2 and ERC15 cells and in tuberin-positive TRKE2 cells derived from polycystic, neoplastic, and normal rat kidney epithelial cells, respectively. EKT2 cells lacking tuberin because of inactivation of the Tsc2 gene fail to localize polycystin-1 and E-cadherin appropriately to these junctions. However, beta-catenin was retained at lateral cell membranes in both tuberin-null and tuberin-positive cells. Moreover, gene transcription mediated by beta-catenin T-cell--specific transcription factor complexes showed no differences among EKT2, ERC15, and TRKE2 cells. Thus, beta-catenin was stably retained at the lateral cell membrane in tuberin-null renal cells lacking membrane-localized polycystin-1 and E-cadherin. These data suggest that, although loss of Tsc2 tumor suppressor gene function disrupts normal polycystin-1 function and membrane localization of E-cadherin, normal beta-catenin signaling is retained in tuberin-null cells.

Analysis of the role of membrane polarity in polycystic kidney disease of transgenic SBM mice.

Altered membrane polarity has been proposed as an important pathogenetic factor in the development of renal cysts in polycystic kidney disease. To determine whether this alteration in epithelial phenotype is a primary or secondary phenomenon, we examined the epithelial membrane polarity of SBM transgenic mice, in which epithelial proliferation mediated by the c-myc oncogene is an established primary event. Kidneys from 32 transgenic mice and 10 age-matched controls from fetal to adult age were immunostained with antibodies to Na,K-ATPase, fodrin, ankyrin, E-cadherin, and tubule segment-specific lectins. In normal control mice, Na,K-ATPase localization was apical in fetal kidneys but became translocated to the basolateral membrane at maturity. Early microcysts in fetal transgenic kidneys displayed similar (95 to 100%) apical Na,K-ATPase. In young and newborn transgenic mice (1 to 8 days of age), Na,K-ATPase localization was extremely heterogeneous. Noncystic tubules demonstrated either apical (mean 23 to 28%), basolateral (mean 48 to 58%), mixed (mean 4 to 15%), or absent (mean 10 to 13%) staining for Na,K-ATPase. Apical Na,K-ATPase was more frequently observed in early cysts (mean 55%) in young transgenic mice but became less prevalent in adult mice (mean 22%), where 30% of cysts had basolateral staining, 39% mixed patterns, and 9% absent staining. Macrocysts typically lost all Na,K-ATPase reactivity. At all ages, Na,K-ATPase colocalized well with cytoskeletal proteins ankyrin and fodrin. These heterogeneous patterns of Na,K-ATPase staining indicate that although altered cell polarity is frequent in early cystic epithelium of SBM mice, it is not a prerequisite to cystogenesis or progressive cyst enlargement. In conclusion, our results support the view that altered cystic membrane polarity is not a primary process, but represents the persistence of an immature epithelial phenotype characteristic of proliferative polycystic kidney disease epithelia.