EDITORIALS www.jasn.org

2. Guay-Woodford LM, Desmond RA: Autosomal recessive polycystic kid- Section of Nephrology, Department of Internal Medicine, Yale ney disease: The clinical experience in North America. Pediatrics 111: School of Medicine, New Haven, Connecticut 1072–1080, 2003 3. Onuchic LF, Furu L, Nagasawa Y, Hou X, Eggermann T, Ren Z, Bergmann J Am Soc Nephrol 19: 418–420, 2008. C, Senderek J, Esquivel E, Zeltner R, Rudnik-Schoneborn S, Mrug M, doi: 10.1681/ASN.2008010084 Sweeney W, Avner ED, Zerres K, Guay-Woodford LM, Somlo S, Germino GG: PKHD1, the polycystic and hepatic disease 1 gene, encodes Autosomal dominant polycystic (ADPKD) is a novel large protein containing multiple immunoglobulin-like plexin- characterized by age-dependent occurrence of bilateral, multi- transcription-factor domains and parallel beta-helix 1 repeats. Am J Hum Genet 70: 1305–1317, 2002 ple renal resulting in kidney enlargement in association 4. Ward CJ, Hogan MC, Rossetti S, Walker D, Sneddon T, Wang X, Kubly V, with a variable spectrum of extrarenal manifestations, most Cunningham JM, Bacallao R, Ishibashi M, Milliner DS, Torres VE, Harris commonly simple cysts arising from bile ducts in the liver. PC: The gene mutated in autosomal recessive polycystic kidney disease Nephronophthisis (NPHP) is an autosomal recessive kidney encodes a large, receptor-like protein. Nat Genet 30: 259–269, 2002 disease characterized by tubular basement membrane disrup- 5. Chevalier RL, Garland TA, Buschi AJ: The neonate with adult-type auto- somal dominant polycystic kidney disease. Int J Pediatr Nephrol 2: 73–77, tion, tubular atrophy, and tubulointerstitial nephritis, associ- 1981 ated with corticomedullary cysts typically without kidney en- 6. Martinez JR, Grantham JJ: Polycystic kidney disease: Etiology, pathogen- largement.1 Liver disease, when it occurs in NPHP, is esis, and treatment. Dis Mon 41: 693–765, 1995 characterized by portal tract fibrosis with little or no bile duct 7. Qian F, Watnick TJ, Onuchic LF, Germino GG: The molecular basis of proliferation. ADPKD and NPHP, respectively, are the most focal formation in human autosomal dominant polycystic kidney disease type I. Cell 87: 979–987, 1996 common genetic causes of end-stage kidney disease in adults 8. Yu S, Hackmann K, Gao J, He X, Piontek K, Garcia Gonzalez MA, Men- and children or adolescents. Cysts are considered the initiating ezes LF, Xu H, Germino GG, Zuo J, Qian F: Essential role of cleavage of pathogenetic event in ADPKD to the point where recent stud- polycystin-1 at G protein-coupled receptor proteolytic site for kidney ies suggest that serial measurements of cyst and kidney vol- tubular structure. Proc Natl Acad Sci U S A 104: 18688–18693, 2007 umes can be used as determinants of disease progression.2 9. Watnick T, Germino G: From cilia to cyst. Nat Genet 34: 355–356, 2003 10. Gattone VH 2nd, Wang X, Harris PC, Torres VE: Inhibition of renal cystic Cysts in NPHP do not have a similar central role in pathogen- disease development and progression by a vasopressin V2 receptor esis of the disease. Nonetheless, a growing body of cellular and antagonist. Nat Med 9: 1323–1326, 2003 molecular evidence has come to suggest an interrelationship 11. Torres VE, Wang X, Qian Q, Somlo S, Harris PC, Gattone VH 2nd: among these diseases and a number of others, such as auto- Effective treatment of an orthologous model of autosomal dominant somal recessive polycystic kidney disease (ARPKD), Bardet- polycystic kidney disease. Nat Med 10: 363–364, 2004 12. Wu Y, Dai XQ, Li Q, Chen CX, Mai W, Hussain Z, Long W, Montalbetti N, Biedl syndrome (BBS), and Meckel-Gruber syndrome. Li G, Glynne R, Wang S, Cantiello HF, Wu G, Chen XZ: Kinesin-2 medi- These relationships are based on the association of many of ates physical and functional interactions between polycystin-2 and fibro- the causative disease gene protein products with the cilia/ cystin. Hum Mol Genet 15: 3280–3292, 2006 basal body complex. 13. Wang S, Zhang J, Nauli SM, Li X, Starremans PG, Luo Y, Roberts KA, Zhou There is little doubt that the primary plays a central J: Fibrocystin/polyductin, found in the same protein complex with poly- cystin-2, regulates calcium responses in kidney epithelia. Mol Cell Biol 27: role in both establishing and maintaining the complex yet re- 3241–3252, 2007 producible three-dimensional structure of the kidney. In addi- 14. Garcia-Gonzalez MA, Menezes LF, Piontek KB, Kaimori J, Huso DL, tion to “guilt by association” arising from the finding that Watnick T, Onuchic LF, Guay-Woodford LM, Germino GG: Genetic in- many gene products that are mutated in fibrocystic kidney teraction studies link autosomal dominant and recessive polycystic kidney diseases in humans and mice localize to cilia, there are several disease in a common pathway. Hum Mol Genet 16: 1940–1950, 2007 15. Kim I, Fu Y, Hui K, Moeckel G, Mai W, Li C, Liang D, Zhao P, Ma J, Chen prospective studies confirming this association. For example, XZ, George AL Jr, Coffey RJ, Feng ZP, Wu G: Fibrocystin/polyductin kidney cysts develop when Kif3a, a component of the antero- modulates renal tubular formation by regulating polycystin-2 expression grade transport machinery required for structural integrity of and function. J Am Soc Nephrol 19: 455–468, 2008 cilia but not otherwise known to be associated with cystic dis- eases, is inactivated in the kidney.3 Similarly, a forward genetic screen in zebrafish using pronephric kidney tubule dilation as See related article, “Fibrocystin/Polyductin Modulates Renal Tubular Formation the phenotype identified 10 mutant genes among which there by Regulating Polycystin-2 Expression and Function,” on pages 455–468. was a marked overrepresentation of cilia-associated protein products.4 This convergence on the importance of the cilia/ basal body complex in fibrocystic diseases has fostered a reduc- Too Much of a Good Thing: tionist approach to mechanism based on the premise that at Does Nek8 Link Polycystic Kidney Disease and Published online ahead of print. Publication date available at www.jasn.org. Correspondence: Dr. Stefan Somlo, Section of Nephrology, Yale University Nephronophthisis? School of Medicine, P.O. Box 208029, 333 Cedar Street, New Haven, CT 06520- 8029. Phone: 203-737-2974; Fax: 203-785-4904; E-mail: [email protected]

Yiqiang Cai and Stefan Somlo Copyright © 2008 by the American Society of Nephrology

418 Journal of the American Society of Nephrology J Am Soc Nephrol 19: 413–423, 2008 www.jasn.org EDITORIALS some level all of these proteins should be working together. In co-immunoprecipitates with Nek8 and that this interaction is fact, disease protein interactions and complexes have been not lost with the jck mutation. Expression of polycystin-1 and readily identified within disease groups. For example, the two -2 at the level of both transcript and protein is increased in jck ADPKD proteins, polycystin-1 and polycystin-2, have long kidneys, although only polycystin-1 shows a marked increase. been known to associate with each other.5 Several of the NPHP In keeping with the previously reported studies in cells,15 this gene products interact among themselves,1 and several of the increase in global cellular expression is associated with in- BBS gene products form a functional complex termed the creased expression of polycystins in the primary cilia as well. BBSome.6 Functional association or complex formation Although these results are suggestive, quantitative conclusions among proteins across these disease entities have been more based on native protein immunofluorescence is very difficult difficult to document. Protein and genetic data raise the to interpret conclusively. This is highlighted by the discrepancy possibility that ADPKD and ARPKD gene products affect a in Nek8 expression in cell lines from jck mice, where it is ab- common pathway,7 but direct links between NPHP or BBS sent,15 and jck kidney tissue, where it expressed along the entire with either autosomal polycystic kidney disease has been length of the cilium.9 In addition, in the jck mouse kidney, less apparent. polycystin-2 has an altered electrophoretic migration, which Two articles in this issue of JASN are among the first to the authors propose is an aberrantly phosphorylated form of suggest a direct link between ADPKD and NPHP, a link based the protein on the basis of its disappearance after treatment on the NIMA (never-in-mitosis gene A)-related kinase family with protein phosphatase-1. In aggregate, the study by Sohara member NEK8. Otto et al.8 identified amino acid substitution et al.9 as well as previous studies15,16 raise the possibility that mutations in NEK8 in families with NPHP and propose NEK8 polycystin-1 and -2 may be downstream targets of Nek8. Some as the NPHP9 gene. Sohara et al.9 report that Nek8 and genetic evidence to support this comes from the observation polycystin-2 exist in a complex and describe increased ex- that doubly heterozygous Pkd1 and Nek8 mutant mice have pression of polycystin-1 and polycystin-2 along with abnor- discernible kidney cysts at 16 weeks, a finding not observed in mal phosphorylation of polycystin-2 in the Nek8 mutant jck singly heterozygous mice for either gene.16 Although this find- mouse. The combined conclusions that NEK8 is an NPHP ing is suggestive of a genetic interaction, it does not permit gene and its protein product may modulate ciliary targeting determination of whether either gene functions upstream or of polycystin-1 and -2 poses Nek8 as a linchpin between downstream of the other in a pathway. Studies examining the NPHP and ADPKD. effects of Pkd1ϩ/Ϫ genotype on disease progression in jck/jck Nek8 belongs to the family of NIMA-related serine-threo- homozygous mice and the reciprocal studies with kidney-spe- nine kinases, which has 11 members in mammalian spe- cific Pkd1 knockouts and jck heterozygous mice could address cies.10,11 Among these, Nek2 is involved in G2-M regulation this hypothesis further. and centrosome separation; Nek6 and Nek7 are components In the second article, Otto et al.8 used a candidate gene ap- of a mitotic kinase cascade; Nek9 plays a role in chromosome proach to screen NEK8 in 188 patients with NPHP using direct alignment and segregation during mitosis; and Nek1 and Nek8 sequencing and an additional 400 patients with NPHP using play important roles in cilia, cell cycle, and polycystic kidney polymorphisms near NEK8 followed by sequencing of those disease.10 The first evidence suggesting involvement of Neks in showing homozygosity. They identified three amino acid sub- cystogenesis was the identification of mutations in Nek1 as stitution variants in highly conserved residues, but only one causing polycystic kidney disease and pleiotropic extrarenal occurred in a homozygous state as would be expected for a effects, including facial dysmorphism, dwarfism, male sterility, causative NPHP gene. One of the two heterozygous changes , and cystic choroid plexus in kat and kat2J mice.12 Sub- occurred in an individual with homozygous NPHP5 muta- sequently, a missense mutation in the RCC1 domain of Nek8 tions. The three variants were not observed among a rela- was identified in a mouse model of autosomal recessive juve- tively small number of control subjects. A directed yeast nile , the jck mouse.13 Nek1 localizes to the two-hybrid screen for interaction of NEK8 with NPHP, centrosomes in interphase, whereas Nek8 localizes to the prox- BBS, , and murine polycystic kidney imal region of the primary cilia of mouse renal epithelial cells.14 disease gene products yielded no interacting partners de- Mutant Nek8 is absent from cilia of cultured kidney tubule spite the high sensitivity and limited specificity of this ap- epithelial cells from jck mice, but expression of polycystin-1 proach. The authors go on to show that NEK8 variants ob- and -2 is enhanced and the cilia are longer in these cells.15 In served in patients with NPHP markedly reduce or abolish addition, Nek8 and polycystin-1 have recently been associated centrosomal and ciliary location of NEK8, suggesting that with reciprocal effects on each other’s cilia location. Nek8 ex- these may be pathogenic mutations. In aggregate, the data pression is increased in cilia of Pkd1Ϫ/Ϫ cultured kidney ex- support the conclusion the mutations in NEK8 may be a rare plants, and, conversely, polycystin-1 is increased in cilia of jck cause of NPHP in humans. kidney explants.16 The work of Otto et al.8 and Sohara et al.9 together pose the The study by Sohara et al.9 extends these findings to kidney question of whether the mechanisms of NPHP and ADPKD tissue and more directly addresses the interrelationship of are interrelated, but they do not answer that question. It is not Nek8 with the polycystins. The authors show that polycystin-2 clear that a disease in which nonmutant polycystins are over-

J Am Soc Nephrol 19: 413–423, 2008 Editorials 419 EDITORIALS www.jasn.org expressed in lengthened cilia as a result of Nek8 mutations 10. Quarmby LM, Mahjoub MR: Caught Nek-ing: Cilia and centrioles. (NPHP9) speaks to the same mechanism as a disease in which J Cell Sci 118: 5161–5169, 2005 11. Parker JD, Bradley BA, Mooers AO, Quarmby LM: Phylogenetic anal- polycystins are nonfunctional or absent from cilia (ADPKD). ysis of the neks reveals early diversification of ciliary-cell cycle kinases. It remains possible that the observed alterations in polycystin PLoS ONE 2: e1076, 2007 protein in cilia are the result of a more generalized alteration in 12. Upadhya P, Birkenmeier EH, Birkenmeier CS, Barker JE: Mutations in cilia composition, and if one looked at a spectrum of cilia pro- a NIMA-related kinase gene, Nek1, cause pleiotropic effects including teins, then a generalized abnormality not restricted to poly- a progressive polycystic kidney disease in mice. Proc Natl Acad Sci USA97: 217–221, 2000 cystins may be observed. It is important in studying cilia-re- 13. Liu S, Lu W, Obara T, Kuida S, Lehoczky J, Dewar K, Drummond IA, Beier lated structural diseases of the kidney that we continue to seek DR: A defect in a novel Nek-family kinase causes cystic kidney disease in unbiased approaches toward understanding mechanism and the mouse and in zebrafish. Development 129: 5839–5846, 2002 remain cautious in focusing only on what we know. This was a 14. Mahjoub MR, Trapp ML, Quarmby LM: NIMA-related kinases defec- lesson well learned once before in this field when cilia were tive in murine models of polycystic kidney diseases localize to primary cilia and centrosomes. J Am Soc Nephrol 16: 3485–3489, 2005 unknown in cystic disease and may yet be the lesson for the 15. Smith LA, Bukanov NO, Husson H, Russo RJ, Barry TC, Taylor AL, Beier future as well. DR, Ibraghimov-Beskrovnaya O: Development of polycystic kidney disease in juvenile cystic kidney mice: insights into pathogenesis, ciliary abnormalities, and common features with human disease. JAm ACKNOWLEDGMENTS Soc Nephrol 17: 2821–2831, 2006 16. Natoli TA, Gareski TC, Dackowski WR, Smith L, Bukanov NO, Russo RJ, Husson H, Matthews D, Piepenhagen P, Ibraghimov-Beskrovnaya The author’s are members of the Yale Center for Polycystic Kidney O: Pkd1 and Nek8 mutations affect cell-cell adhesion and cilia in cysts Disease Research (NIH P50 DK57328). formed in kidney organ cultures. Am J Physiol Renal Physiol 294: F73–F83, 2008

DISCLOSURES See related articles, “Nek8 Regulates the Expression and Localization of Poly- None. cystin-1 and Polycystin-2,” on pages 469–476, and “NEK8 Mutations Affect Ciliary and Centrosomal Localization and May Cause Nephronophthisis” on pages 587–592. REFERENCES

1. Hildebrandt F, Zhou W: Nephronophthisis-associated . Naturally Too Sympathetic to a J Am Soc Nephrol 18: 1855–1871, 2007 2. Grantham JJ, Torres VE, Chapman AB, Guay-Woodford LM, Bae KT, Bad Diet? King BF, Jr., Wetzel LH, Baumgarten DA, Kenney PJ, Harris PC, Klahr S, Bennett WM, Hirschman GN, Meyers CM, Zhang X, Zhu F, Miller JP: Volume progression in polycystic kidney disease. N Engl J Med 354: Roger W. Brown 2122–2130, 2006 Centre for Cardiovascular Science, Queen’s Medical Research Insti- 3. Lin F, Hiesberger T, Cordes K, Sinclair AM, Goldstein LS, Somlo S, tute, Edinburgh, United Kingdom Igarashi P: Kidney-specific inactivation of the KIF3A subunit of kine- J Am Soc Nephrol 19: 420–422, 2008. sin-II inhibits renal and produces polycystic kidney dis- doi: 10.1681/ASN.2008010040 ease. Proc Natl Acad Sci U S A 100: 5286–5291, 2003 4. Sun Z, Amsterdam A, Pazour GJ, Cole DG, Miller MS, Hopkins N: A genetic screen in zebrafish identifies cilia genes as a principal cause of An ominous upward trend in obesity, diabetes, and associ- cystic kidney. Development 131: 4085–4093, 2004 5. Hanaoka K, Qian F, Boletta A, Bhunia AK, Piontek K, Tsiokas L, ated and their consequences warns all of us of Sukhatme VP, Guggino WB, Germino GG: Co-assembly of polycys- a burden in cardiovascular disease already well developed in tin-1 and -2 produces unique cation-permeable currents. Nature 408: the United States and sweeping across Europe, many parts 990–994, 2000 of Asia, and beyond. It seems set to get worse. This epidemic 6. Nachury MV, Loktev AV, Zhang Q, Westlake CJ, Peranen J, Merdes A, includes an alarming rise in chronic renal failure reaching Slusarski DC, Scheller RH, Bazan JF, Sheffield VC, Jackson PK: A core complex of BBS proteins cooperates with the GTPase Rab8 to pro- ESRD and requiring long-term or transplantation. mote ciliary membrane biogenesis. Cell 129: 1201–1213, 2007 In the United States, this rose from approximately 42,000 in 7. Garcia-Gonzalez MA, Menezes LF, Piontek KB, Kaimori J, Huso DL, 1978 to Ͼ484,000 by 2005 (11.5-fold).1 Black individuals of Watnick T, Onuchic LF, Guay-Woodford LM, Germino GG: Genetic in- teraction studies link autosomal dominant and recessive polycystic kidney disease in a common pathway. Hum Mol Genet 16: 1940–1950, 2007 Published online ahead of print. Publication date available at www.jasn.org. 8. Otto EA, Trapp ML, Schultheiss UT, Helou J, Quarmby LM, Hildebrandt F: NEK8 mutations affect ciliary and centrosomal localization and may Correspondence: Dr. Roger Brown, Centre for Cardiovascular Science, cause nephronophthisis. J Am Soc Nephrol 19: 587–592, 2008 Queen’s Medical Research Institute, 47 Little France Crescent, Edinburgh, EH16 4TJ, UK. Phone: ϩ44-131-2426739; Fax: ϩ44-131-2426779; E-mail: 9. Sohara E, Luo Y, Zhang J, Manning DK, Beier DR, Zhou J: Nek8 [email protected] regulates the expression and localization of polycystin-1 and polycys- tin-2. J Am Soc Nephrol 19: 469–476, 2008 Copyright © 2008 by the American Society of Nephrology

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