Proteolytic cleavage and nuclear translocation of fibrocystin is regulated by intracellular Ca2+ and activation of protein kinase C

Trafficking of receptor tyrosine kinases to the nucleus

It has been known for at least 20 years that growth factors induce the internalization of cognate receptor tyrosine kinases (RTKs). The internalized receptors are then sorted to lysosomes or recycled to the cell surface. More recently, data have been published to indicate other intracellular destinations for the internalized RTKs. These include the nucleus, mitochondria, and cytoplasm. Also, it is recognized that trafficking to these novel destinations involves new biochemical mechanisms, such as proteolytic processing or interaction with translocons, and that these trafficking events have a function in signal transduction, implicating the receptor itself as a signaling element between the cell surface and the nucleus.

Polycystic kidney disease and the renal cilium

Polycystic kidney disease (PKD) is a common genetic condition characterized by the formation of fluid-filled cysts in the kidney. Mutations affecting several genes are known to cause PKD and the protein products of most of these genes localize to an organelle called the renal cilium. Renal cilia are non-motile, microtubule-based projections located on the apical surface of the epithelial cells that form the tubules and ducts of the kidney. With the exception of intercalated cells, each epithelial cell bears a single non-motile cilium that projects into the luminal space where it is thought to act as a flow sensor. The detection of fluid flow through the kidney by the renal cilium is hypothesized to regulate a number of pathways responsible for the maintenance of normal epithelial phenotype. Defects of the renal cilium lead to cyst formation, caused primarily by the dedifferentiation and over-proliferation of epithelial cells. Here we discuss the role of renal cilia and the mechanisms by which defects of this organelle are thought to lead to PKD.

Transcriptional regulators in kidney disease: gatekeepers of renal homeostasis

Although we are rapidly gaining a more complete understanding of the genes required for kidney function, the molecular pathways that actively maintain organ homeostasis are only beginning to emerge. The study of the most common genetic cause of renal failure, polycystic kidney disease, has revealed a surprising role for primary cilia in controlling nuclear gene expression and cell division during development as well as maintenance of kidney architecture. Conditions that disturb kidney integrity seem to be associated with reversal of developmental processes that ultimately lead to kidney fibrosis and end-stage renal disease (ESRD). In this review, we discuss transcriptional regulators and networks that are important in kidney disease, focusing on those that mediate cilia function and drive renal fibrosis.

Polycystic kidney diseases: from molecular discoveries to targeted therapeutic strategies

Polycystic kidney diseases (PKDs) represent a large group of progressive renal disorders characterized by the development of renal cysts leading to end-stage renal disease. Enormous strides have been made in understanding the pathogenesis of PKDs and the development of new therapies. Studies of autosomal dominant and recessive polycystic kidney diseases converge on molecular mechanisms of cystogenesis, including ciliary abnormalities and intracellular calcium dysregulation, ultimately leading to increased proliferation, apoptosis and dedifferentiation. Here we review the pathobiology of PKD, highlighting recent progress in elucidating common molecular pathways of cystogenesis. We discuss available models and challenges for therapeutic discovery as well as summarize the results from preclinical experimental treatments targeting key disease-specific pathways.

Ciliary dysfunction in polycystic kidney disease: an emerging model with polarizing potential

The majority of different cell types in the human body have a cilium, a thin rod-like structure of uniquely arranged microtubules that are encapsulated by the surface plasma membrane. The cilium originates from a basal body, a mature centriole that has migrated and docked to the cell surface. The non-motile cilia are microtubule-based organelles that are generally considered sensory structures. The purpose of this review is to discuss the practicality of the ciliary hypothesis as a unifying concept for polycystic kidney disease and to review current literature in the field of cilium biology, as it relates to mechanosensation and planar cell polarity. The polycystins and fibrocystin localization at the cilium and other subcellular localizations are discussed, followed by a hypothetical model for the cilium's role in mechanosensing, planar cell polarity, and cystogenesis.

Substrate specificity of gamma-secretase and other intramembrane proteases

Gamma-Secretase is a promiscuous protease that cleaves bitopic membrane proteins within the lipid bilayer. Elucidating both the mechanistic basis of gamma-secretase proteolysis and the precise factors regulating substrate identification is important because modulation of this biochemical degradative process can have important consequences in a physiological and pathophysiological context. Here, we briefly review such information for all major classes of intramembranously cleaving proteases (I-CLiPs), with an emphasis on gamma-secretase, an I-CLiP closely linked to the etiology of Alzheimer's disease. A large body of emerging data allows us to survey the substrates of gamma-secretase to ascertain the conformational features that predispose a peptide to cleavage by this enigmatic protease. Because substrate specificity in vivo is closely linked to the relative subcellular compartmentalization of gamma-secretase and its substrates, we also survey the voluminous body of literature concerning the traffic of gamma-secretase and its most prominent substrate, the amyloid precursor protein.

Kidney cysts, pancreatic cysts, and biliary disease in a mouse model of autosomal recessive polycystic kidney disease

Mutations in PKHD1 cause autosomal recessive polycystic kidney disease (ARPKD). We produced a mouse model of ARPKD by replacing exons 1-3 of Pkhd1 with a lacZ reporter gene utilizing homologous recombination. This approach yielded heterozygous Pkhd1 (lacZ/+) mice, that expressed beta-galactosidase in tissues where Pkhd1 is normally expressed, and homozygous Pkhd1 (lacZ/lacZ) knockout mice. Heterozygous Pkhd1 (lacZ/+) mice expressed beta-galactosidase in the kidney, liver, and pancreas. Homozygous Pkhd1 (lacZ/lacZ) mice lacked Pkhd1 expression and developed progressive renal cystic disease involving the proximal tubules, collecting ducts, and glomeruli. In the liver, inactivation of Pkhd1 resulted in dilatation of the bile ducts and periportal fibrosis. Dilatation of pancreatic exocrine ducts was uniformly seen in Pkhd1 (lacZ/lacZ ) mice, with pancreatic cysts arising less frequently. The expression of beta-galactosidase, Pkd1, and Pkd2 was reduced in the kidneys of Pkhd1 (lacZ/lacZ ) mice compared with wild-type littermates, but no changes in blood urea nitrogen (BUN) or liver function tests were observed. Collectively, these results indicate that deletion of exons 1-3 leads to loss of Pkhd1 expression and results in kidney cysts, pancreatic cysts, and biliary ductal plate malformations. The Pkhd1 (lacZ/lacZ ) mouse represents a new orthologous animal model for studying the pathogenesis of kidney cysts and biliary dysgenesis that characterize human ARPKD.

Fibrocystin/polyductin modulates renal tubular formation by regulating polycystin-2 expression and function

Autosomal recessive polycystic kidney disease is caused by mutations in PKHD1, which encodes the membrane-associated receptor-like protein fibrocystin/polyductin (FPC). FPC associates with the primary cilia of epithelial cells and co-localizes with the Pkd2 gene product polycystin-2 (PC2), suggesting that these two proteins may function in a common molecular pathway. For investigation of this, a mouse model with a gene-targeted mutation in Pkhd1 that recapitulates phenotypic characteristics of human autosomal recessive polycystic kidney disease was produced. The absence of FPC is associated with aberrant ciliogenesis in the kidneys of Pkhd1-deficient mice. It was found that the COOH-terminus of FPC and the NH2-terminus of PC2 interact and that lack of FPC reduced PC2 expression but not vice versa, suggesting that PC2 may function immediately downstream of FPC in vivo. PC2-channel activities were dysregulated in cultured renal epithelial cells derived from Pkhd1 mutant mice, further supporting that both cystoproteins function in a common pathway. In addition, mice with mutations in both Pkhd1 and Pkd2 had a more severe renal cystic phenotype than mice with single mutations, suggesting that FPC acts as a genetic modifier for disease severity in autosomal dominant polycystic kidney disease that results from Pkd2 mutations. It is concluded that a functional and molecular interaction exists between FPC and PC2 in vivo.

Polyductin undergoes notch-like processing and regulated release from primary cilia

Mutations at a single locus, PKHD1, are responsible for causing human autosomal recessive polycystic kidney disease (ARPKD). Recent studies suggest that the cystic disease might result from defects in planar cell polarity, but how the 4074 amino acid ciliary protein encoded by the longest open reading frame of this transcriptionally complex gene may regulate this process is unknown. Using novel in vitro expression systems, we show that the PKHD1 gene product polyductin/fibrocystin undergoes a complicated pattern of Notch-like proteolytic processing. Cleavage at a probable proprotein convertase site produces a large extracellular domain that is tethered to the C-terminal stalk by disulfide bridges. This fragment is then shed from the primary cilium by activation of a member of the ADAM metalloproteinase disintegrins family, resulting in concomitant release of an intra-cellular C-terminal fragment via a gamma-secretase-dependent process. The ectodomain of endogenous PD1 is similarly shed from the primary cilium upon activation of sheddases. This is the first known example of this process involving a protein of the primary cilium and suggests a novel mechanism whereby proteins that localize to this structure may function as bi-directional signaling molecules. Regulated release from the primary cilium into the lumen may be a mechanism to distribute signal to down-stream targets using flow.