Book excerpt: Autosomal dominant polycystic kidney disease (ADPKD) is characterized by perturbations in extracellular matrix synthesis, cellular polarity, gene expression, and proliferative potential of the epithelial cells lining renal cysts. The mechanisms whereby these cells express a partially dedifferentiated phenotype have remained enigmatic despite the identification of the causative genetic lesions. A comprehensive assessment of epithelial cell architecture, molecular trafficking, cellular plasticity, and extracellular matrix profile was therefore undertaken. These analyses revealed several anomalies as candidate factors that undoubtedly undermine the integrity of ADPKD tissue. Within explanted ADPKD cells the cytoskeletal networks were biochemically or structurally immature. Moreover, both the adherens junction protein E-cadherin and basolateral targeting patch components were depleted from the basolateral ADPKD cell surface. These derangements correlated with impaired vectorial trafficking: while apical transport proceeded effectively, basolateral trafficking in the diseased cells was inefficient. Stalling of traffic along the basolateral exocytic route was evident as an accumulation of exocytic cargo within the dilated ADPKD cell Golgi and the redistribution of several components of the basolateral trafficking machinery. The architectural and behavioral disturbances in the mutant cells prompted an examination of the interplay between cellular phenotype and extracellular matrix. Deprived of native environmental cues by explantation into monolayer culture, diseased cells were unable to fully redifferentiate. Furthermore, while normal kidney cells formed branching tubules in collagen-I matrices, ADPKD cells assembled spherical cysts or failed altogether to organize into multicellular aggregates. Remarkably, ADPKD cells cultured within purified native extracellular matrix successfully completed branching morphogenesis, demonstrating that genetic lesions did not completely abolish the morphogenetic potential of these cells. The dissimilar behavior of ADPKD cells cultured in different matrices suggested that the extracellular matrix in ADPKD tissue is permissive for cystogenesis. The aberrant deposition of a novel matrix microfibril protein within the basement membrane circumscribing ADPKD cysts represents an environmental cue that likely exacerbates atypical cellular behavior. Considered together, the results presented herein imply the existence of a pathological circuit initiated by the acquisition of genetic mutations and perpetuated by cellular dedifferentiation, ultimately leading to tissue dysmorphogenesis and the degeneration of renal epithelia.