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Drug Discovery for Polycystic Kidney Disease

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Drug Discovery for Polycystic Kidney Disease Acta Pharmacologica Sinica (2011) 32: 805–816 npg © 2011 CPS and SIMM All rights reserved 1671-4083/11 $32.00 www.nature.com/aps Review Drug discovery for polycystic kidney disease Ying SUN, Hong ZHOU, Bao-xue YANG* Department of Pharmacology, School of Basic Medical Sciences, Peking University, and Key Laboratory of Molecular Cardiovascular Sciences, Ministry of Education, Beijing 100191, China In polycystic kidney disease (PKD), a most common human genetic diseases, fluid-filled cysts displace normal renal tubules and cause end-stage renal failure. PKD is a serious and costly disorder. There is no available therapy that prevents or slows down the cystogen- esis and cyst expansion in PKD. Numerous efforts have been made to find drug targets and the candidate drugs to treat PKD. Recent studies have defined the mechanisms underlying PKD and new therapies directed toward them. In this review article, we summarize the pathogenesis of PKD, possible drug targets, available PKD models for screening and evaluating new drugs as well as candidate drugs that are being developed. Keywords: polycystic kidney disease; drug discovery; kidney; candidate drugs; animal model Acta Pharmacologica Sinica (2011) 32: 805–816; doi: 10.1038/aps.2011.29 Introduction the segments of the nephron. Autosomal recessive polycystic Polycystic kidney disease (PKD), an inherited human renal kidney disease (ARPKD) results primarily from the mutations disease, is characterized by massive enlargement of fluid- in a single gene, Pkhd1[14]. Its frequency is estimated to be filled renal tubular and/or collecting duct cysts[1]. Progres- one per 20000 individuals. The PKHD1 protein, fibrocystin, sively enlarging cysts compromise normal renal parenchyma, has been found to be localized to primary cilia and the basal often leading to renal failure. The occurrence of autosomal bodies. The exact function of fibrocystin has not been demon- dominant polycystic kidney disease (ADPKD) is estimated to strated. In ARPKD, smaller, elongated cysts arise as ecstatic be between 1 in 1000 and 1 in 400 individuals by a study in expansions of collecting ducts. Patients with PKD often Olmsted Country, MN[2]. ADPKD is caused by mutations in require dialysis and kidney transplantation, which are exceed- one of two genes (Pkd1 and Pkd2) expressing the interacting ingly costly. There are currently no approved drug or preven- polycystic proteins polycystin-1 (PC1) and polycystin-2 (PC2) tive strategies for PKD. in renal tubular epithelia[3, 4]. Mutation of Pkd1 accounts for approximately 85% cases in clinically identified patients[5]. Mechanisms of renal cyst formation and enlargement in PC1 is a membrane receptor capable of binding and interact- PKD ing with many proteins, including carbohydrates and lipids, The development and growth of PKD cysts involve the abnor- and eliciting intracellular responses through phosphorylation mal proliferation and apoptosis of immature epithelial cells, pathways[6, 7]. PC2 is thought to act as a calcium permeable accumulation of fluid within the cyst cavity, abnormal cell- channel[8, 9]. PC1 and PC2 form a complex that localizes to pri- cell/cell-matrix interactions and abnormal cilia function. mary cilia[10, 11]. The polycystin complex has a role in the regu- lation of the proliferation, differentiation and morphogenesis Role of epithelial cell proliferation and apoptosis in cyst develop­ of renal tubular cells through interactions with protein com- ment in PKD plexes linked to the actin cytoskeleton, intracellular signaling Increased apoptosis and proliferative capacity in renal cascades, and the regulation of gene transcrip tion[12, 13] (Figure epithelial cells are essential processes in PKD. While the pro- 1). In ADPKD, the thousands of large, spherical cysts of vari- liferation of renal tubular epithelial cells halts before birth in ous sizes throughout the cortex and medulla are derived from normal individuals, cystic epithelia proliferate throughout life in patients with ADPKD[15]. Several genetic manipulations * To whom correspondence should be addressed. that increase the proliferation of tubular epithelial cells in mice [16–19] E-mail [email protected] result in PKD . Received 2011-02-06 Accepted 2011-03-17 Epidermal growth factor (EGF), transforming growth fac- npg www.nature.com/aps Sun Y et al 806 Figure 1. A diagram depicting the effects that PC1 and PC2 exert on signaling pathways. Multiple direct and indirect interactions allow the polycystin proteins to be inhibited or stimulated. Pathways was invoved in cell proliferation and liquid secretion. tor alpha (TGF-α) and EGF receptor (EGFR) promote cystic Role of fluid secretion in cyst development in PKD epithelial proliferation and expand renal cysts. EGFR is over- Fluid secretion is a critical pathogenic mechanism associated expressed and mislocalized to the apical membranes of cystic with cyst formation and growth in PKD. Fluid secretion, epithelial cells, which leads to a sustained stimulation of cell coupled with epithelial hyperplasia, is necessary and sufficient proliferation in the cysts[20]. Increased intracellular cAMP to account for the dynamics of cyst growth. In PKD, a large level also plays a crucial role in cystogenesis. The reduced cal- number of cystic lesions lack afferent and efferent tubule con- cium caused by mutation of Pkd1 or Pkd2 can inhibit adenylyl nections, suggesting that cysts, which arise from tubular seg- cyclase 6 leading to increased cAMP. Studies have demon- ments, become disconnected from the glomerular filtrate. The strated that cAMP inhibits the proliferation of normal renal development and expansion of cystic lesions therefore requires epithelial cells. In contrast, cAMP promotes the proliferation net transepithelial fluid secretion. An extensive body of in of cells derived from PKD patients[21]. The switch is caused by vitro data implicates epithelial chloride secretion in the genera- decreased intracellular calcium levels in a polycystic kidney tion and maintenance of fluid-filled cysts[27]. The fluid secre- leading to cAMP-mediated stimulation of the B-Raf/MEK/ tion is driven by mechanisms that are similar to those found in ERK pathway instead of inhibiting the Ras/Raf/MEK/ERK other secretory epithelia. Chloride movement drives fluid into pathway like in the normal kidney[22]. B-Raf is inhibited by the cyst lumen. Fluid accumulation causes cyst enlargement Akt in normal cells, while it is activated because of decreased directly by swelling cysts and indirectly by stretching cells to activation of Akt in calcium-restricted cells. Inhibitors of promote cell division[28]. Akt and PI3K can reproduce the effects of calcium reduc- Cystic fibrosis transmembrane conductance regulator tion. However, activation of Akt has been found in animal (CFTR), a cAMP-regulated chloride channel, is present on the models of PKD, such as Pkd–/– mice, Han:SPRD rats and jck apical membranes of many secretory epithelia. Chloride secre- mice. Additional growth factors, cytokines, lipid factors, and tion through the CFTR has been implicated in the pathway of adenosine triphosphate (ATP) also participate in regulating fluid secretion in PKD. In vitro experiments have suggested the proliferation of renal epithelial cells[23–25]. Cell apoptosis that increased cAMP-mediated chloride secretion provides is also a key factor in the development of PKD. Knocking out the electrochemical driving force for fluid secretion in cystic the anti-apoptotic Bcl-2 and AP-2 genes or overexpression of epithelia[29]. CFTR is expressed in the apical membrane of the pro-apoptotic gene c-myc in mice results in renal cystogen- intact cysts dissected from PKD kidneys[30]. An important esis[26]. role of CFTR in PKD fluid secretion is also supported by the observation that interference with CFTR protein production Acta Pharmacologica Sinica www.chinaphar.com npg Sun Y et al 807 (by treatment of ADPKD monolayers with antisense oligo- its Ig-like domains[39]. A significant downregulation of Pkd1 nucleotide against human CFTR) dramatically reduced fluid mRNA is detected in MDCK cysts compared to tubules, which secretion by these cells[27]. Additional evidence supporting a leads to a striking reduction of membrane PC1 and mislocal- role of CFTR in chloride secretion was obtained from immor- ization to the cytoplasmic pools[40]. It has been demonstrated talized cystic murine collecting duct cell lines isolated from that a controlled level of PC1 expressed at cell-cell junction CFTR mutant and CFTR wild-type mice. The wild-type cell is critical for normal tubular differentiation. In normal renal lines formed numerous fluid-filled cysts in response to EGF cells, PC1 forms a complex with the protein E-cadherin and its and forskolin when cultured in three-dimensional collagen catenins. However, in primary cells from ADPKD patients, gels, whereas the CFTR mutant cell lines failed to form cysts the PC1/E-cadherin/β-catenin complex was disrupted under identical conditions[31]. These results demonstrate that and was accompanied by increased PC1 phosphorylation, CFTR is required for in vitro cyst formation. In a single family reduced E-cadherin and upregulated normal mesenchymal affected with both ADPKD and cystic fibrosis (CF), individual N-cadherin[41]. members with both ADPKD and CF had less severe renal Renal epithelial cells in ADPKD show increased PC1 disease than those family members with only ADPKD[32, 33]. adhesion to type I collagen compared with normal human These studies suggest that in vivo, defective
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