JASN Express. Published on December 31, 2008 as doi: 10.1681/ASN.2008101118 EDITORIALS www.jasn.org UP FRONT MATTERS Targeting Cyst Initiation in There are several promising agents that either are under development or are already in clinical trials to reduce the ADPKD increase in cyst volume by regulating proliferation, cell growth, or transepithelial secretion and thereby delaying Stephanie J. Leuenroth* and Craig M. Crews*†‡ time to ESRD. These include the vasopressin receptor an- Departments of *Molecular, Cellular, and Developmental Biology, tagonists OPC-31260 and tolvaptan,1,2 which decrease † ‡ Pharmacology, and Chemistry, Yale University, New Haven, 3 Connecticut cAMP production; the somatostatin analog octreotide ; antagonists of the ClϪ channel cystic fibrosis transmem- J Am Soc Nephrol ●●: 000–000, 2009. brane conductance regulator4,5; angiotensin-converting doi: 10.1681/ASN.2008101118 enzyme inhibitors6,7; and small molecule inhibitors of the ϩ basolateral KCa3.1 K channel.8 Other candidates include With approximately 12.5 million people worldwide af- inhibitors of the inflammatory mediator TNF-␣,9 cell fected by autosomal dominant polycystic kidney disease growth regulators such as the mTOR antagonist rapamy- (ADPKD), one hopes we are now at the beginning of mul- cin10; and the antiproliferative cyclin-dependent kinase tiple therapeutic breakthroughs. ADPKD is a hereditary inhibitor roscovitine.11 disorder caused by a genetic defect in either polycystin-1 These potential therapies could be used for adult pa- (PKD1; PC1) or polycystin-2 (PKD2; PC2) that leads to tients in whom average cystic burden increases at a rate of progressive kidney cyst formation and enlargement. The 5 to 6% per year as reported by the Consortium for Radio- resulting high cystic burden in the kidney destroys the logic Imaging Studies of Polycystic Kidney Disease normal parenchyma and ultimately results in ESRD for the (CRISP).12 The impact of these types of drugs would be majority of patients. Although many of the cellular events greatest in those with numerous large cysts contributing to leading to cystogenesis are not yet identified, the process increasing renal volume. Although this strategy targeting can be roughly divided into the following stages after so- the isolated, expanding kidney cyst is central to current matic inactivation of the remaining normal PKD allele disease intervention, there is the opportunity for a com- through loss of heterozygosity: Loss of calcium-mediated plementary approach as well: One that targets the origins cellular quiescence leading to an initial proliferative phase of cyst formation at the site of the epithelial tubule cell. By of tubule epithelial cells; out-pocketing of the tubule wall focusing on a much earlier stage in disease progression, and eventual cyst separation from the parent nephron; and before a cyst disconnects from its parent nephron, we can continued cell growth, fluid secretion, and progressive ex- begin to question how to prevent the initial proliferative pansion of isolated cysts. phase stemming from cilial defects. Tremendous progress has been made in understanding Although all of the cellular changes and mechanisms the various biologic mechanisms responsible for these patho- involved in initial cyst formation are not yet elucidated, logic stages of cyst formation and growth that range from the interference with primary cilia formation or its resident identification of the primary cilia as a mechanosensor medi- PC1/PC2 complex leads to the formation of cysts. For ex- ating PC1/PC2 calcium influx to various perturbations of ample, murine models with functional or structural de- fluid regulation and cell proliferation. Because many proteins fects in the primary cilium, such as Tg737orpk,13 cpk,14 associated with these processes have been identified, we have inv,15 and kif3a,16 all result in kidney cyst development. multiple cellular targets to examine for pharmacologic inter- The cilia of the tubular epithelium, acting as a “flow sen- vention strategies, some of which are already under investi- sor,” are central to calcium signaling and control of the gation. proliferative phenotype of the cell.17 Because calcium in- When considering the stages of cystogenesis as de- flux as a result of cilial bending maintains cellular quies- scribed already, it is clear that many small molecule ther- cence, the loss of this signal initiates the earliest events of apeutic agents have focused on cysts that are in the con- cystogenesis. While the PC1/PC2 complex would be the tinued growth and expansion phase or the isolated cyst. most upstream candidate to restore calcium signaling di- rectly by pharmacologic manipulation, other associated proteins involved in cilia formation, maintenance, and Published online ahead of print. Publication date available at www.jasn.org. function could be targeted as well. Correspondence: Dr. Craig M. Crews, Yale University, Department of Molec- Recent studies with “adult” inducible animal models of ular, Cellular, and Developmental Biology, P.O. Box 208103, New Haven, CT 06520-8103. Phone: 203-432-9364; Fax: 203-432-6161; E-mail: craig.crews@ ADPKD demonstrated a delay between the loss of calcium yale.edu signaling (through Pkd1 inactivation) and initial cyst for- 18–20 Copyright ᮊ ●●●● by the American Society of Nephrology mation, perhaps indicating that multiple cellular and J Am Soc Nephrol ●●: 000–000, 2009 ISSN : 1046-6673/●●01-000 1 EDITORIALS www.jasn.org extracellular events are required for this process. Examples proliferation. This would decrease the number of cysts formed include changes in adhesion molecule expression, abnor- at an early age that are ultimately the source of the largest malities in basement membrane composition, and increases volume increases that lead to ESRD; isolated cysts that have in inflammatory mediators that accompany cyst develop- separated from the parent nephron and have begun the process ment. It is anticipated, then, that even intermittent PC2- of cyst expansion through fluid secretion can then be treated mediated calcium release induced by pharmacologic agents with agents that target volume regulation in adulthood. could halt or delay events causing slowly progressive cyst Through the use of this dual strategy, a renoprotective effect formation. may be achieved by modulating a range of early and late cellu- Triptolide, for example, activates PC2-mediated calcium lar events associated with the pathogenesis of disease. Even a release, causes cell growth arrest in murine Pkd1-null cells, and shift from a PKD1 phenotype, in which there are a greater reduces cyst burden by inhibition of cyst initiation.21,22 Al- number of initial cysts, to a clinically milder course of disease though these findings result in an exceptional opportunity to that would mimic the PKD2 phenotype would be of benefit to examine the potential therapeutic effect of restoration of cal- many patients with ADPKD. cium signaling in PKD, it leaves the door open for the discov- In summary, the combination of multiple therapeutic ery and investigation of additional PC2 agonists as well as other agents targeting various aspects of experimental cystic disease, modulators of this signaling process in the kidney. Future ther- such as restoration of PC2-mediated calcium signaling, inhibi- apeutic drugs, for example, could act at the site of the primary tion of inflammatory mediators, and reduction of cyst volume cilium to reestablish calcium influx through the PC2 channel and cell growth, may lead to the preservation of kidney func- directly or target a downstream mediator or associated protein tion and attenuate the severity of renal cystic burden. Most of the PC1/PC2 complex that is directly linked to cell-cycle important, one hopes that combination drug strategies might inhibition. Although all of the components of this PC2-stim- some day give treatment options to thousands of patients with ulated calcium release pathway are not yet known, new associ- ADPKD to extend the quality of life for years beyond what is ated members such as the cell-cycle related kinase Nek823 have expected today. recently been discovered and warrant further investigation as therapeutic targets. As the PKD research community continues to fill in the ACKNOWLEDGMENTS gaps separating cilial mechanosensation and calcium release to cell growth arrest, new targets to inhibit cyst initiation C.M.C. gratefully acknowledges support for research into the molec- will be identified. It is also of note that hepatic cysts derived ular mechanisms of ADPKD cystogenesis from the National Institutes from bile ducts are a common extrarenal manifestation of of Health (AI055914) and the PKD Foundation. ADPKD because these cells are also responsive to cilial PC2- mediated calcium influx. Because treatment with such drugs as the vasopressin receptor antagonists are ineffective DISCLOSURES in the liver as a result of a lack of receptor expression, it is None. plausible that a class of PC2-specific calcium agonists could be therapeutically beneficial to both the kidney and the liver. REFERENCES Although cyst formation is a continual process through- out adulthood, cysts initiated early in life are the main con- 1. Torres VE: Vasopressin antagonists in polycystic kidney disease. Semin tributors to adult cystic burden. Once the cyst has discon- Nephrol 28: 306–317, 2008 nected from its originating nephron and begins to increase 2. Torres VE, Wang X, Qian Q, Somlo S, Harris PC, Gattone VH 2nd: in size, further destruction of the renal parenchyma occurs Effective treatment of an orthologous model of autosomal dominant by the involvement of inflammation, fibrosis, and ischemia; polycystic kidney disease. Nat Med 10: 363–364, 2004 3. 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