Correction of cilia structure and function alleviates multi-organ pathology in Bardet-Biedl syndrome mice.

Bardet-Biedl syndrome (BBS) is a pleiotropic autosomal recessive ciliopathy affecting multiple organs. The development of potential disease-modifying therapy for BBS will require concurrent targeting of multi-systemic manifestations. Here, we show for the first time that monosialodihexosylganglioside accumulates in Bbs2-/- cilia, indicating impairment of glycosphingolipid (GSL) metabolism in BBS. Consequently, we tested whether BBS pathology in Bbs2-/- mice can be reversed by targeting the underlying ciliary defect via reduction of GSL metabolism. Inhibition of GSL synthesis with the glucosylceramide synthase inhibitor Genz-667161 decreases the obesity, liver disease, retinal degeneration and olfaction defect in Bbs2-/- mice. These effects are secondary to preservation of ciliary structure and signaling, and stimulation of cellular differentiation. In conclusion, reduction of GSL metabolism resolves the multi-organ pathology of Bbs2-/- mice by directly preserving ciliary structure and function towards a normal phenotype. Since this approach does not rely on the correction of the underlying genetic mutation, it might translate successfully as a treatment for other ciliopathies.

Reduction of ciliary length through pharmacologic or genetic inhibition of CDK5 attenuates polycystic kidney disease in a model of nephronophthisis.

Polycystic kidney diseases (PKDs) comprise a subgroup of ciliopathies characterized by the formation of fluid-filled kidney cysts and progression to end-stage renal disease. A mechanistic understanding of cystogenesis is crucial for the development of viable therapeutic options. Here, we identify CDK5, a kinase active in post mitotic cells, as a new and important mediator of PKD progression. We show that long-lasting attenuation of PKD in the juvenile cystic kidneys (jck) mouse model of nephronophthisis by pharmacological inhibition of CDK5 using either R-roscovitine or S-CR8 is accompanied by sustained shortening of cilia and a more normal epithelial phenotype, suggesting this treatment results in a reprogramming of cellular differentiation. Also, a knock down of Cdk5 in jck cells using small interfering RNA results in significant shortening of ciliary length, similar to what we observed with R-roscovitine. Finally, conditional inactivation of Cdk5 in the jck mice significantly attenuates cystic disease progression and is associated with shortening of ciliary length as well as restoration of cellular differentiation. Our results suggest that CDK5 may regulate ciliary length by affecting tubulin dynamics via its substrate collapsin response mediator protein 2. Taken together, our data support therapeutic approaches aimed at restoration of ciliogenesis and cellular differentiation as a promising strategy for the treatment of renal cystic diseases.

CaMKII as a pathological mediator of ER stress, oxidative stress, and mitochondrial dysfunction in a murine model of nephronophthisis.

Polycystic kidney diseases (PKDs) are genetic diseases characterized by renal cyst formation with increased cell proliferation, apoptosis, and transition to a secretory phenotype at the expense of terminal differentiation. Despite recent progress in understanding PKD pathogenesis and the emergence of potential therapies, the key molecular mechanisms promoting cystogenesis are not well understood. Here, we demonstrate that mechanisms including endoplasmic reticulum stress, oxidative damage, and compromised mitochondrial function all contribute to nephronophthisis-associated PKD. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is emerging as a critical mediator of these cellular processes. Therefore, we reasoned that pharmacological targeting of CaMKII may translate into effective inhibition of PKD in jck mice. Our data demonstrate that CaMKII is activated within cystic kidney epithelia in jck mice. Blockade of CaMKII with a selective inhibitor results in effective inhibition of PKD in jck mice. Mechanistic experiments in vitro and in vivo demonstrated that CaMKII inhibition relieves endoplasmic reticulum stress and oxidative damage and improves mitochondrial integrity and membrane potential. Taken together, our data support CaMKII inhibition as a new and effective therapeutic avenue for the treatment of cystic diseases.

Loss of GM3 synthase gene, but not sphingosine kinase 1, is protective against murine nephronophthisis-related polycystic kidney disease.

Genetic forms of polycystic kidney diseases (PKDs), including nephronophthisis, are characterized by formation of fluid-filled cysts in the kidneys and progression to end-stage renal disease. No therapies are currently available to treat cystic diseases, making it imperative to dissect molecular mechanisms in search of therapeutic targets. Accumulating evidence suggests a pathogenic role for glucosylceramide (GlcCer) in multiple forms of PKD. It is not known, however, whether other structural glycosphingolipids (GSLs) or bioactive signaling sphingolipids (SLs) modulate cystogenesis. Therefore, we set out to address the role of a specific GSL (ganglioside GM3) and signaling SL (sphingosine-1-phosphate, S1P) in PKD progression, using the jck mouse model of nephronopthisis. To define the role of GM3 accumulation in cystogenesis, we crossed jck mice with mice carrying a targeted mutation in the GM3 synthase (St3gal5) gene. GM3-deficient jck mice displayed milder PKD, revealing a pivotal role for ganglioside GM3. Mechanistic changes in regulation of the cell-cycle machinery and Akt-mTOR signaling were consistent with reduced cystogenesis. Dramatic overexpression of sphingosine kinase 1 (Sphk1) mRNA in jck kidneys suggested a pathogenic role for S1P. Surprisingly, genetic loss of Sphk1 exacerbated cystogenesis and was associated with increased levels of GlcCer and GM3. On the other hand, increasing S1P accumulation through pharmacologic inhibition of S1P lyase had no effect on the progression of cystogenesis or kidney GSL levels. Together, these data suggest that genes involved in the SL metabolism may be modifiers of cystogenesis, and suggest GM3 synthase as a new anti-cystic therapeutic target.

VEPTP inhibition with an extracellular domain targeting antibody did not restore albuminuria in a mouse model of diabetic kidney disease.

Diabetic kidney disease (DKD) is the leading cause of end-stage kidney disease. DKD is a heterogeneous disease with complex pathophysiology where early endothelial dysfunction is associated with disease progression. The Tie2 receptor and Angiopoietin 1 and 2 ligands are critical for maintaining endothelial cell permeability and integrity. Tie2 signaling is negatively regulated by the endothelial specific transmembrane receptor Vascular Endothelial Protein Tyrosine Phosphatase (VEPTP). Genetic deletion of VEPTP protects from hypertension and diabetes induced renal injury in a mouse model of DKD. Here, we show that VEPTP inhibition with an extracellular domain targeting VEPTP antibody induced Tie2 phosphorylation and improved VEGF-A induced vascular permeability both in vitro and in vivo. Treatment with the VEPTP blocking antibody decreased the renal expression of endothelial activation markers (Angpt2, Edn1, and Icam1) but failed to improve kidney function in db/db uninephrectomized ReninAAV DKD mice.

Long-lasting arrest of murine polycystic kidney disease with CDK inhibitor roscovitine.

Polycystic kidney diseases (PKDs) are primarily characterized by the growth of fluid-filled cysts in renal tubules leading to end-stage renal disease. Mutations in the PKD1 or PKD2 genes lead to autosomal dominant PKD (ADPKD), a slowly developing adult form. Autosomal recessive polycystic kidney disease results from mutations in the PKHD1 gene, affects newborn infants and progresses very rapidly. No effective treatment is currently available for PKD. A previously unrecognized site of subcellular localization was recently discovered for all proteins known to be disrupted in PKD: primary cilia. Because ciliary functions seem to be involved in cell cycle regulation, disruption of proteins associated with cilia or centrioles may directly affect the cell cycle and proliferation, resulting in cystic disease. We therefore reasoned that the dysregulated cell cycle may be the most proximal cause of cystogenesis, and that intervention targeted at this point could provide significant therapeutic benefit for PKD. Here we show that treatment with the cyclin-dependent kinase (CDK) inhibitor (R)-roscovitine does indeed yield effective arrest of cystic disease in jck and cpk mouse models of PKD. Continuous daily administration of the drug is not required to achieve efficacy; pulse treatment provides a robust, long-lasting effect, indicating potential clinical benefits for a lifelong therapy. Molecular studies of the mechanism of action reveal effective cell-cycle arrest, transcriptional inhibition and attenuation of apoptosis. We found that roscovitine is active against cysts originating from different parts of the nephron, a desirable feature for the treatment of ADPKD, in which cysts form in multiple nephron segments. Our results indicate that inhibition of CDK is a new and effective approach to the treatment of PKD.

Anti-microRNA-21 Therapy on Top of ACE Inhibition Delays Renal Failure in Alport Syndrome Mouse Models.

Col4a3-/- Alport mice serve as an animal model for renal fibrosis. MicroRNA-21 (miR-21) expression has been shown to be increased in the kidneys of Alport syndrome patients. Here, we investigated the nephroprotective effects of Lademirsen anti-miR-21 therapy. We used a fast-progressing Col4a3-/- mouse model with a 129/SvJ background and an intermediate-progressing F1 hybrid mouse model with a mixed genetic background, with angiotensin-converting enzyme inhibitor (ACEi) monotherapy in combination with anti-miR-21 therapy. In the fast-progressing model, the anti miR-21 and ACEi therapies showed an additive effect in the reduction in fibrosis, the decline of proteinuria, the preservation of kidney function and increased survival. In the intermediate-progressing F1 model, the anti-miR-21 and ACEi therapies individually improved kidney pathology. Both also improved kidney function and survival; however, the combination showed a significant additive effect, particularly for survival. RNA sequencing (RNA-seq) gene expression profiling revealed that the anti-miR-21 and ACEi therapies modulate several common pathways. However, anti-miR-21 was particularly effective at normalizing the expression profiles of the genes involved in renal tubulointerstitial injury pathways. In conclusion, significant additive effects were detected for the combination of anti-miR-21 and ACEi therapies on kidney function, pathology and survival in Alport mouse models, as well as a strong differential effect of anti-miR-21 on the renal expression of fibrotic factors. These results support the addition of anti-miR-21 to the current standard of care (ACEi) in ongoing clinical trials in patients with Alport syndrome.

A metabolomics approach using juvenile cystic mice to identify urinary biomarkers and altered pathways in polycystic kidney disease.

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease and affects 1 in 1,000 individuals. Ultrasound is most often used to diagnose ADPKD; such a modality is only useful late in the disease after macroscopic cysts are present. There is accumulating evidence suggesting that there are common cellular and molecular mechanisms responsible for cystogenesis in human and murine PKD regardless of the genes mutated, and, in the case of complex metabolomic analysis, the use of a mouse model has distinct advantages for proof of principle over a human study. Therefore, in this study we utilized a urinary metabolomics-based investigation using gas chromatography-time of flight mass spectrometry to demonstrate that the cystic mouse can be discriminated from its wild-type counterpart by urine analysis alone. At day 26 of life, before there is serological evidence of kidney dysfunction, affected mice are distinguishable by urine metabolomic analysis; this finding persists through 45 days until 64 days, at which time body weight differences confound the results. Using functional score analysis and the KEGG pathway database, we identify several biologically relevant metabolic pathways which are altered very early in this disease, the most highly represented being the purine and galactose metabolism pathways. In addition, we identify several specific candidate biomarkers, including allantoic acid and adenosine, which are augmented in the urine of young cystic mice. These markers and pathway components, once extended to human disease, may prove useful as a noninvasive means of diagnosing cystic kidney diseases and to suggest novel therapeutic approaches. Thus, urine metabolomics has great diagnostic potential for cystic renal disorders and deserves further study.

In vitro cystogenesis: the search for drugs antagonizing cyst development.

Autosomal dominant polycystic kidney disease (ADPKD) cystogenesis has been extensively studied and major characteristic abnormalities were identified including increased proliferation, apoptosis, changes in cellular polarity, abnormal matrix composition and fluid secretion. We set out to understand how mutated polycystin-1 leads to these abnormalities and how well mechanisms of cystogenesis in vivo can be understood using in vitro models. Specifically, we addressed the dynamic role of polycystin-1 in the context of cellular rearrangements accompanying cystogenesis and tubulogenesis in vitro. We demonstrated that polycystin-1 plays an important role in cell-cell adhesion through homophilic interactions of its Ig-like domains. To define the role of polycystin-1 in formation of intercellular contacts and cell polarity during epithelial morphogenesis, we have utilized a 3D MDCK in vitro model of tubulogenesis and cystogenesis. We demonstrate that polycystin-1 is a component of desmosomal junctions of epithelial cells. A striking down-regulation of polycystin-1 mRNA was detected in cysts as compared to tubules, leading to altered protein expression and localization. While polycystin-1 is localized to basolateral membranes of MDCK tubules, it is only detected in cytoplasmic pools in cystic cells. To address the impact of mutated PC-1 on intercellular adhesion, we have analyzed the structure/function of desmosomal junctions in primary cells derived from ADPKD cysts. We demonstrated that, in the absence of functional polycystin-1, desmosomal junctions can not be properly assembled and remain sequestered in cytoplasmic compartments. Our data show that the MDCK in vitro model of cystogenesis and tubulogenesis adequately reflects several aspects of cystogenesis in vivo. We have used in vitro cystogenesis assay for a high throughput screen of small molecule drugs and identified drugs specifically inhibiting cystogenesis but not tubulogenesis in vitro.

Differences in the timing and magnitude of Pkd1 gene deletion determine the severity of polycystic kidney disease in an orthologous mouse model of ADPKD.

Development of a disease-modifying therapy to treat autosomal dominant polycystic kidney disease (ADPKD) requires well-characterized preclinical models that accurately reflect the pathology and biochemical changes associated with the disease. Using a Pkd1 conditional knockout mouse, we demonstrate that subtly altering the timing and extent of Pkd1 deletion can have a significant impact on the origin and severity of kidney cyst formation. Pkd1 deletion on postnatal day 1 or 2 results in cysts arising from both the cortical and medullary regions, whereas deletion on postnatal days 3-8 results in primarily medullary cyst formation. Altering the extent of Pkd1 deletion by modulating the tamoxifen dose produces dose-dependent changes in the severity, but not origin, of cystogenesis. Limited Pkd1 deletion produces progressive kidney cystogenesis, accompanied by interstitial fibrosis and loss of kidney function. Cyst growth occurs in two phases: an early, rapid growth phase, followed by a later, slow growth period. Analysis of biochemical pathway changes in cystic kidneys reveals dysregulation of the cell cycle, increased proliferation and apoptosis, activation of Mek-Erk, Akt-mTOR, and Wnt-ß-catenin signaling pathways, and altered glycosphingolipid metabolism that resemble the biochemical changes occurring in human ADPKD kidneys. These pathways are normally active in neonatal mouse kidneys until repressed around 3 weeks of age; however, they remain active following Pkd1 deletion. Together, this work describes the key parameters to accurately model the pathological and biochemical changes associated with ADPKD in a conditional mouse model.

CDK inhibitors R-roscovitine and S-CR8 effectively block renal and hepatic cystogenesis in an orthologous model of ADPKD.

Autosomal dominant polycystic kidney disease (ADPKD) and other forms of PKD are associated with dysregulated cell cycle and proliferation. Although no effective therapy for the treatment of PKD is currently available, possible mechanism-based approaches are beginning to emerge. A therapeutic intervention targeting aberrant cilia-cell cycle connection using CDK-inhibitor R-roscovitine showed effective arrest of PKD in jck and cpk models that are not orthologous to human ADPKD. To evaluate whether CDK inhibition approach will translate into efficacy in an orthologous model of ADPKD, we tested R-roscovitine and its derivative S-CR8 in a model with a conditionally inactivated Pkd1 gene (Pkd1 cKO). Similar to ADPKD, Pkd1 cKO mice developed renal and hepatic cysts. Treatment of Pkd1 cKO mice with R-roscovitine and its more potent and selective analog S-CR8 significantly reduced renal and hepatic cystogenesis and attenuated kidney function decline. Mechanism of action studies demonstrated effective blockade of cell cycle and proliferation and reduction of apoptosis. Together, these data validate CDK inhibition as a novel and effective approach for the treatment of ADPKD.

Inhibition of glucosylceramide accumulation results in effective blockade of polycystic kidney disease in mouse models.

Polycystic kidney disease (PKD) represents a family of genetic disorders characterized by renal cystic growth and progression to kidney failure. No treatment is currently available for people with PKD, although possible therapeutic interventions are emerging. Despite genetic and clinical heterogeneity, PKDs have in common defects of cystic epithelia, including increased proliferation, apoptosis and activation of growth regulatory pathways. Sphingolipids and glycosphingolipids are emerging as major regulators of these cellular processes. We sought to evaluate the therapeutic potential for glycosphingolipid modulation as a new approach to treat PKD. Here we demonstrate that kidney glucosylceramide (GlcCer) and ganglioside GM3 levels are higher in human and mouse PKD tissue as compared to normal tissue, regardless of the causative mutation. Blockade of GlcCer accumulation with the GlcCer synthase inhibitor Genz-123346 effectively inhibits cystogenesis in mouse models orthologous to human autosomal dominant PKD (Pkd1 conditional knockout mice) and nephronophthisis (jck and pcy mice). Molecular analysis in vitro and in vivo indicates that Genz-123346 acts through inhibition of the two key pathways dysregulated in PKD: Akt protein kinase-mammalian target of rapamycin signaling and cell cycle machinery. Taken together, our data suggest that inhibition of GlcCer synthesis represents a new and effective treatment option for PKD.

Serum and urinary biomarker signatures for rapid preclinical in vivo assessment of CDK inhibition as a therapeutic approach for PKD.

Recent advances in understanding the molecular pathogenesis of polycystic kidney diseases (PKD) are being translated into promising treatments. Currently, a response to therapy in preclinical animal models of PKD can only be evaluated after several weeks of treatment. The availability of biomarkers for rapid efficacy assessment would greatly facilitate the drug development process. Here we applied SELDI-TOF technology to establish serum and urinary biomarker signatures associated with a rapid therapeutic response to cyclin dependent kinase (CDK) inhibitor roscovitine in the jck mouse model of PKD. A set of 74 serum and 56 urinary markers was identified in the group receiving chronic treatment over 5 weeks. This set was further screened for early efficacy biomarkers in acutely (3-5 days) treated animals with mild (26 days of age) and advanced disease (50 days of age). A third group with intermediate disease (33 days of age) received a single injection to monitor rapid changes in protein profiles within 4, 24 or 48 hours after drug administration. Multifactorial comparative analysis of the acutely treated groups identified a set of 20 urinary and 21 serum efficacy biomarkers. This biomarker signature provides a necessary tool for further assessment of CDK inhibitors as therapeutic agents for PKD.

Pkd1 and Nek8 mutations affect cell-cell adhesion and cilia in cysts formed in kidney organ cultures.

Development of novel therapies for polycystic kidney disease (PKD) requires assays that adequately reflect disease biology and are adaptable to high-throughput screening. Here we describe an embryonic cystic kidney organ culture model and demonstrate that a new mutant allele of the Pkd1 gene (Pkd1(tm1Bdgz)) modulates cystogenesis in this model. Cyst formation induced by cAMP is influenced by the dosage of the mutant allele: Pkd1(tm1Bdgz) -/- cultures develop a larger cystic area compared with +/+ counterparts, while Pkd1(tm1Bdgz) +/- cultures show an intermediate phenotype. A similar relationship between the degree of cystogenesis and mutant gene dosage is seen in cystic kidney organ cultures derived from mice with a mutated Nek8 gene (Nek8(jck)). Both Pkd1- and Nek8- cultures display altered cell-cell junctions, with reduced E-cadherin expression and altered desmosomal protein expression, similar to ADPKD epithelia. Additionally, characteristic ciliary abnormalities are identified in cystic kidney cultures, with elevated ciliary polycystin 1 expression in Nek8 homozygous cultures and elevated ciliary Nek8 protein expression in Pkd1 homozygotes. These data suggest that the Nek8 and Pkd1 genes function in a common pathway to regulate cystogenesis. Moreover, compound Pkd1 and Nek8 heterozygous adult mice develop a more aggressive cystic disease than animals with a mutation in either gene alone. Finally, we validate the kidney organ culture cystogenesis assay as a therapeutic testing platform using the CDK inhibitor roscovitine. Therefore, embryonic kidney organ culture represents a relevant model for studying molecular cystogenesis and a rapid tool for the screening for therapies that block cystic growth.

Development of polycystic kidney disease in juvenile cystic kidney mice: insights into pathogenesis, ciliary abnormalities, and common features with human disease.

Significant progress in understanding the molecular mechanisms of polycystic kidney disease (PKD) has been made in recent years. Translating this understanding into effective therapeutics will require testing in animal models that closely resemble human PKD by multiple parameters. Similar to autosomal dominant PKD, juvenile cystic kidney (jck) mice develop cysts in multiple nephron segments, including cortical collecting ducts, distal tubules, and loop of Henle. The jck mice display gender dimorphism in kidney disease progression with more aggressive disease in male mice. Gonadectomy experiments show that testosterone aggravates the severity of the disease in jck male mice, while female gonadal hormones have protective effects. EGF receptor is overexpressed and mislocalized in jck cystic epithelia, a hallmark of human disease. Increased cAMP levels in jck kidneys and activation of the B-Raf/extracellular signal-regulated kinase pathway are demonstrated. The effect of jck mutation on the expression of Nek8, a NIMA-related (never in mitosis A) kinase, and polycystins in jck cilia is shown for the first time. Nek8 overexpression and loss of ciliary localization in jck epithelia are accompanied by enhanced expression of polycystins along the cilia. The primary cilia in jck kidneys are significantly more lengthened than the cilia in wild-type mice, suggesting a role for Nek8 in controlling ciliary length. Collectively, these data demonstrate that the jck mice should be useful for testing potential therapies and for studying the molecular mechanisms that link ciliary structure/function and cystogenesis.

Impaired formation of desmosomal junctions in ADPKD epithelia.

Mutations in polycystin-1 (PC-1) are responsible for autosomal dominant polycystic kidney disease (ADPKD), characterized by formation of fluid-filled tubular cysts. The PC-1 is a multifunctional protein essential for tubular differentiation and maturation found in desmosomal junctions of epithelial cells where its primary function is to mediate cell-cell adhesion. To address the impact of mutated PC-1 on intercellular adhesion, we have analyzed the structure/function of desmosomal junctions in primary cells derived from ADPKD cysts. Primary epithelial cells from normal kidney showed co-localization of PC-1 and desmosomal proteins at cell-cell contacts. A striking difference was seen in ADPKD cells, where PC-1 and desmosomal proteins were lost from the intercellular junction membrane, despite unchanged protein expression levels. Instead, punctate intracellular expression for PC-1 and desmosomal proteins was detected. The N-cadherin, but not E-cadherin was expressed in adherens junctions of ADPKD cells. These data together with co-sedimentation analysis demonstrate that, in the absence of functional PC-1, desmosomal junctions cannot be properly assembled and remain sequestered in cytoplasmic compartments. Taken together, our results demonstrate that PC-1 is crucial for formation of intercellular contacts. We propose that abnormal expression of PC-1 causes disregulation of cellular adhesion complexes leading to increased proliferation, loss of polarity and, ultimately, cystogenesis.

Canine PKD1 is a single-copy gene: genomic organization and comparative analysis.

Most cases of autosomal dominant polycystic kidney disease are caused by mutations in the gene PKD1, encoding polycystin-1. To gain insight into the role of polycystin-1 in tubulogenesis and cystogenesis using the well-characterized canine kidney epithelial cell line MDCK, we have now cloned and characterized the exon/intron structure of the canine gene PKD1. FISH analysis showed that the dog genome lacks the multiple PKD1 homologs present in human. Intron 21 of dog PKD1 lacked the polypyrimidine tract characteristic of the human gene, whereas pyrimidine-rich elements were identified in canine intron 30. Canine polycystin-1 showed a higher degree of homology with the human counterpart and lower homology with mouse and rat. A striking degree of conservation (97% identity) was determined for the leucine-rich repeat domain between dog and human. Also, the homology analysis indicated that 4 of 16 Ig-like repeats (IgIII, IgVII, IgX, and IgXV) are likely to be functionally significant. This is particularly important in light of our recent findings demonstrating that Iglike domains form strong homophilic interactions and can mediate cell-cell adhesion. These data enable detailed analysis of the role of polycystin-1 in cystogenesis and tubulogenesis using the canine MDCK cell line.

Functional analysis of PKD1 transgenic lines reveals a direct role for polycystin-1 in mediating cell-cell adhesion.

The PKD1 protein, polycystin-1, is a large transmembrane protein of uncertain function and topology. To study the putative functions of polycystin-1, conditionally immortalized kidney cells transgenic for PKD1 were generated and an interaction between transgenic polycystin-1 and endogenous polycystin-2 has been recently demonstrated in these cells. This study provides the first functional evidence that transgenic polycystin-1 directly mediates cell-cell adhesion. In non-permeabilized cells, polycystin-1 localized to the lateral cell borders with N-terminal antibodies but not with a C-terminal antibody; there was a clear difference in surface intensity between transgenic and non-transgenic cells. Compared with non-transgenic cells, transgenic cells showed a dramatic increase in resistance to the disruptive effect of a polycystin-1 antibody raised to the PKD domains of polycystin-1 (IgPKD) in both cell adhesion and cell aggregation assays. The differential effect on cell adhesion between transgenic and non-transgenic cells could be reproduced using recombinant fusion proteins encoding non-overlapping regions of the IgPKD domains. In contrast, antibodies raised to other extracellular domains of polycystin-1 had no effect on cell adhesion. Finally, the specificity of this finding was confirmed by the lack of effect of IgPKD antibody on cell adhesion in a PKD1 cystic cell line deficient in polycystin-1. These results demonstrate that one of the primary functions of polycystin-1 is to mediate cell-cell adhesion in renal epithelial cells, probably via homophilic or heterophilic interactions of the PKD domains. Disruption of cell-cell adhesion during tubular morphogenesis may be an early initiating event for cyst formation in ADPKD.