Microtubule active taxanes inhibit polycystic kidney disease progression in cpk mice

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Cystin genetic variants cause autosomal recessive polycystic kidney disease associated with altered Myc expression

Mutation of the Cys1 gene underlies the renal cystic disease in the Cys1cpk/cpk (cpk) mouse that phenocopies human autosomal recessive polycystic kidney disease (ARPKD). Cystin, the protein product of Cys1, is expressed in the primary apical cilia of renal ductal epithelial cells. In previous studies, we showed that cystin regulates Myc expression via interaction with the tumor suppressor, necdin. Here, we demonstrate rescue of the cpk renal phenotype by kidney-specific expression of a cystin-GFP fusion protein encoded by a transgene integrated into the Rosa26 locus. In addition, we show that expression of the cystin-GFP fusion protein in collecting duct cells down-regulates expression of Myc in cpk kidneys. Finally, we report the first human patient with an ARPKD phenotype due to homozygosity for a deleterious splicing variant in CYS1. These findings suggest that mutations in Cys1/CYS1 cause an ARPKD phenotype in mouse and human, respectively, and that the renal cystic phenotype in the mouse is driven by overexpression of the Myc proto-oncogene.

Nephronophthisis: should we target cysts or fibrosis?

Ciliopathy nephronophthisis (NPHP), a common cause of end-stage renal disease (ESRD) in children and young adults, is characterized by disintegration of the tubular basement membrane accompanied by irregular thickening and attenuation, interstitial fibrosis and tubular atrophy, and occasionally cortico-medullary cyst formation. Pharmacological approaches that delay the development of ESRD could potentially extend the window of therapeutic opportunity for this group of patients, generating time to find an appropriate donor or even for new treatments to mature. In this review we provide an overview of compounds that have been tested to ameliorate kidney cysts and/or fibrosis. We also revisit paclitaxel as a potential strategy to target fibrosis in NPHP. At low dosage this chemotherapy drug shows promising results in rodent models of renal fibrosis. Possible adverse events and safety of paclitaxel treatment in pediatric patients would need to be investigated, as would the efficacy, optimum dose, and administration schedule for the treatment of renal fibrosis in NPHP patients. Paclitaxel is an approved drug for human use with known pharmacokinetics, which could potentially be used in other ciliopathies through targeting the microtubule skeleton.

Accessory proteins for heterotrimeric G-proteins in the kidney

Heterotrimeric G-proteins play a fundamentally important role in regulating signal transduction pathways in the kidney. Accessory proteins are being identified as direct binding partners for heterotrimeric G-protein α or βγ subunits to promote more diverse mechanisms by which G-protein signaling is controlled. In some instances, accessory proteins can modulate the signaling magnitude, localization, and duration following the activation of cell membrane-associated receptors. Alternatively, accessory proteins complexed with their G-protein α or βγ subunits can promote non-canonical models of signaling activity within the cell. In this review, we will highlight the expression profile, localization and functional importance of these newly identified accessory proteins to control the function of select G-protein subunits under normal and various disease conditions observed in the kidney.

Cystogenesis in ARPKD results from increased apoptosis in collecting duct epithelial cells of Pkhd1 mutant kidneys

Mutations in the PKHD1 gene result in autosomal recessive polycystic kidney disease (ARPKD) in humans. To determine the molecular mechanism of the cystogenesis in ARPKD, we recently generated a mouse model for ARPKD that carries a targeted mutation in the mouse orthologue of human PKHD1. The homozygous mutant mice display hepatorenal cysts whose phenotypes are similar to those of human ARPKD patients. By littermates of this mouse, we developed two immortalized renal collecting duct cell lines with Pkhd1 and two without. Under nonpermissive culture conditions, the Pkhd1(-/-) renal cells displayed aberrant cell-cell contacts and tubulomorphogenesis. The Pkhd1(-/-) cells also showed significantly reduced cell proliferation and elevated apoptosis. To validate this finding in vivo, we examined proliferation and apoptosis in the kidneys of Pkhd1(-/-) mice and their wildtype littermates. Using proliferation (PCNA and Histone-3) and apoptosis (TUNEL and caspase-3) markers, similar results were obtained in the Pkhd1(-/-) kidney tissues as in the cells. To identify the molecular basis of these findings, we analyzed the effect of Pkhd1 loss on multiple putative signaling regulators. We demonstrated that the loss of Pkhd1 disrupts multiple major phosphorylations of focal adhesion kinase (FAK), and these disruptions either inhibit the Ras/C-Raf pathways to suppress MEK/ERK activity and ultimately reduce cell proliferation, or suppress PDK1/AKT to upregulate Bax/caspase-9/caspase-3 and promote apoptosis. Our findings indicate that apoptosis may be a major player in the cyst formation in ARPKD, which may lead to new therapeutic strategies for human ARPKD.

Colchicine treatment in autosomal dominant polycystic kidney disease: many points in common

Autosomal dominant polycystic kidney disease (ADPKD) is the most common of the inherited renal cystic diseases and constitutes 10% of the end stage kidney disease population. ADPKD is caused by PKD1 and PKD2 gene mutations in 85% and 15% of the cases respectively. Its high prevalence and negative impact on health outcomes fostered efforts to explain pathophysiologic pathways of cyst formation in kidneys. Among these are increased apoptosis, unopposed proliferation of tubule cells, impaired polarization and planar cell polarity, impaired cAMP pathway, cilier dysfunction, activated mTOR pathway, increased tumor necrosis factor-alpha (TNF-alpha) production. Many drugs have been tried in an attempt to halt cystogenesis in some point. Despite success to some extent in experimental studies, none reached clinical armamentarium yet. Colchicine, originally extracted from Colchicum autunale, is an anti-inflammatory drug that has been in continuous use for more than 3000 years. It has been used successfully to prevent attacks of familial mediterranien fever and amyloidosis, to treat gout and pseudogout attacks for a few decades. Colchicine principally is a microtubule inhibitor, thus prevents cell migration, division, and polarization. It also has anti-apoptotic, anti-proliferative and anti-inflammatory effects and down-regulates (TNF-alpha) receptors. As can easily be seen, many of the effects of colchicine have pathophysiologic counterparts in ADPKD. Thus, we hypothesized that colchicine would be beneficial to prevent or at least delay cyst formation in ADPKD patients. Indirect evidence also support our hypothesis, in which taxol and paclitaxel, other two microtubule inhibitors, were shown to delay cyst formation in experimental models of ADPKD. To our opinion, despite its narrow therapeutic index, widespread experience makes colchicine a suitable candidate for prolonged clinical use, should experimental studies show any benefit in ADPKD.

Molecular and cellular pathogenesis of autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD) is an inherited disease characterized by a malformation complex which includes cystically dilated tubules in the kidneys and ductal plate malformation in the liver. The disorder is observed primarily in infancy and childhood, being responsible for significant pediatric morbidity and mortality. All typical forms of ARPKD are caused by mutations in a single gene, PKHD1 (polycystic kidney and hepatic disease 1). This gene has a minimum of 86 exons, assembled into multiple differentially spliced transcripts and has its highest level of expression in kidney, pancreas and liver. Mutational analyses revealed that all patients with both mutations associated with truncation of the longest open reading frame-encoded protein displayed the severe phenotype. This product, polyductin, is a 4,074-amino acid protein expressed in the cytoplasm, plasma membrane and primary apical cilia, a structure that has been implicated in the pathogenesis of different polycystic kidney diseases. In fact, cholangiocytes isolated from an ARPKD rat model develop shorter and dysmorphic cilia, suggesting polyductin to be important for normal ciliary morphology. Polyductin seems also to participate in tubule morphogenesis and cell mitotic orientation along the tubular axis. The recent advances in the understanding of in vitro and animal models of polycystic kidney diseases have shed light on the molecular and cellular mechanisms of cyst formation and progression, allowing the initiation of therapeutic strategy designing and promising perspectives for ARPKD patients. It is notable that vasopressin V2 receptor antagonists can inhibit/halt the renal cystic disease progression in an orthologous rat model of human ARPKD.

Pulse wave reflection is amplified in normotensive patients with autosomal-dominant polycystic kidney disease and normal renal function

BACKGROUND

In young ADPKD patients, we have previously found an impaired endothelium-dependent relaxation in small resistance vessels but a normal flow-mediated dilatation of the brachial artery. The present study investigated arterial stiffness in early ADPKD by pulse-wave analysis (PWA) and measurement of pulse-wave velocity (PWV).

METHODS

18 young normotensive ADPKD patients with normal renal function and 18 controls were studied by applanation tonometry with the SphygmoCor equipment. Parameters included an estimate of aortic blood pressure, augmentation index (AIx), AIx standardized to heart rate 75 (AIx(HR75)) and PWV. Glomerular filtration rate (GFR) was measured by the 51Cr-EDTA plasma clearance method. Statistical comparisons were made with t tests and multiple linear regression analysis.

RESULTS

GFR was the same in the two groups. Brachial diastolic blood pressure was slightly but significantly higher in ADPKD patients than in controls (81 +/- 9 vs. 73 +/- 9 mm Hg, p < 0.05). No significant difference was present in brachial systolic blood pressure. AIx was significantly higher in ADPKD than in controls: 21.6 +/- 11.3 vs. 11.4 +/- 11.2%, p < 0.02. AIx(HR75) and estimated aortic systolic and diastolic blood pressure was likewise significantly higher in ADPKD than in controls. Multiple linear regression analysis showed AIx and AIx(HR75) to be independently correlated to group (ADPKD/control), p < 0.02 and p < 0.05, respectively. No difference was found between PWV in the two groups.

CONCLUSION

Reflection of the pulse wave was amplified in young normotensive ADPKD patients, demonstrating early pathology in the arterial system.

Polycystin-2 cation channel function in the human syncytiotrophoblast is regulated by microtubular structures

Polycystin-2 (PC2), encoded by PKD2, which is one of the genes whose mutations cause polycystic kidney disease, is abundantly produced in the apical domain of the syncytiotrophoblast (hST) of term human placenta. PC2, a TRP-type (TRPP2) non-selective cation channel, is present in primary cilia of renal epithelial cells, a microtubule-based ancillary structure with sensory function. The hST has abundant cytoskeletal structures, and actin filament dynamics regulate PC2 channel function in this epithelium. However, it is expected that the apical hST excludes microtubular structures. Here, we demonstrated by Western blot and immunocytochemical analyses that hST apical vesicles indeed contain microtubule structural components, including tubulin isoforms, acetylated alpha-tubulin, and the kinesin motor proteins KIF3A and KIF3B. PC2 and tubulin were substantially colocalized in hST vesicles. Treatment of hST vesicles with either the microtubular disrupter colchicine (15 microM) or the microtubular stabilizer paclitaxel (taxol, 15 microM) resulted in distinct patterns of microtubular re-organization and PC2 redistribution. We also observed that changes in microtubular dynamics regulate PC2 channel function. Addition of colchicine rapidly inhibited PC2 channel activity in lipid-bilayer reconstituted hST membranes. Addition of either tubulin and GTP, or taxol, however, stimulated PC2 channel activity in control hST membranes. Interestingly, we found that the kinesin motor protein KIF3A was capable of increasing PC2 channel activity in hST. We believe that the data are the first to provide a direct demonstration of a microtubular interaction with PC2 in the hST. This interaction thus plays an important regulatory role in the control of ion transport in the human placenta.

Galectin-3 associates with the primary cilium and modulates cyst growth in congenital polycystic kidney disease

Several lines of evidence implicate the beta-galactoside-binding lectin galectin-3 in development and pathological processes in renal collecting ducts: galectin-3 is expressed in the ureteric bud/collecting duct lineage during nephrogenesis, modulates collecting duct growth/differentiation in vitro, and is expressed in human autosomal recessive polycystic kidney disease in cyst epithelia, almost all of which arise from collecting ducts. Moreover, exogenous galectin-3 restricts growth of cysts generated by Madin-Darby canine kidney collecting duct-derived cells in three-dimensional culture in collagen. Using the cpk mouse model of recessively inherited polycystic kidney disease, we observed widespread galectin-3 mRNA and protein in cyst epithelia. Exogenous galectin-3 reduced cyst formation in suspension culture, and mice-null mutant for galectin-3 had more extensive renal cysts in vivo. Galectin-3 was also detected for the first time in the centrosome/primary cilium, which has been implicated in diverse polycystic kidney disease. Cilia structure/number appeared normal in galectin-3-null mutants. Finally, paclitaxel, a therapy that retards polycystic kidney disease in cpk mice, increased extracellular galectin-3, in which the lectin could potentially interact with cilia. These data raise the possibility that galectin-3 may act as a natural brake on cystogenesis in cpk mice, perhaps via ciliary roles.

Polycystin-2 cation channel function is under the control of microtubular structures in primary cilia of renal epithelial cells

Mutations in the gene encoding polycystin-2 (PC2) result in autosomal dominant polycystic kidney disease and defects in left-right asymmetry during embryogenesis. PC2 is a TRP-type Ca(2+)-permeable non-selective cation channel, which is expressed in kidney and other organs. PC2 is present and functional in microtubule-containing primary cilia of renal epithelial cells. However, no information is yet available as to whether PC2 interacts with microtubules. Here, we assessed the role of microtubular dynamics in regulating PC2 channel function in primary cilia. Isolated ciliary membranes from LLC-PK1 epithelial cells were reconstituted in a lipid bilayer system. The acute addition of the microtubular disrupter colchicine (15 mum) rapidly abolished, whereas the addition of the microtubular stabilizer paclitaxel (taxol, 15 mum) increased ciliary PC2 channel activity. The further addition of alpha-tubulin plus GTP also stimulated PC2 channel activity in ciliary membranes. However, alpha-tubulin and GTP had no effect on in vitro translated PC2. Using the yeast two-hybrid assay, we found that PC2 interacts with the microtubule-dependent motor kinesin-2 subunit KIF3A, a protein involved in polycystic kidney disease. The interaction occurred through the carboxyl termini domain of both proteins, which was further confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. Co-immunoprecipitation experiments showed that PC2 and KIF3A are in the same complex in native HEK293, Madin-Darby canine kidney cells (MDCK), and LLC-PK1 cells. Immunofluorescent staining also showed substantial PC2 and KIF3A co-localization in primary cilia of renal epithelial cells. The data indicate that microtubular organization regulates PC2 function, which may explain, among others, the regulatory role of PC2 in the sensory function of primary cilia.

Kinesin family member 12 is a candidate polycystic kidney disease modifier in the cpk mouse

The cpk mouse is the most extensively characterized model of autosomal recessive polycystic kidney disease (ARPKD). The major ARPKD-related renal and biliary phenotypes are modulated in F2 mutants by genetic background, suggesting that quantitative trait loci (QTL) modulate disease severity. In 461 F2 cpk mice, kidney length, weight, and volume were scored as quantitative traits (QT), and a semiquantitative method to assess biliary duct number, area (BDA), portal vein area, and total area of each portal field, as well as the severity of cholangitis, was developed. QTL mapping was performed with Pseudomarker v1.02. Candidate genes were identified within the QTL intervals on the basis of expression profiling, reverse transcriptase-PCR, haplotypes, and sequence analysis. The renal QT were normally distributed in the F2 cohort and strongly correlated (P < 0.001). Among the biliary QT, only BDA correlated with the renal QT (P < 0.01). Genome-wide scan identified a major effect QTL on chromosome (Chr) 4 for the renal traits, adjusted BDA, and cholangitis with logarithm of odds scores of 18, 8, and 5, respectively. Regression modeling refined the Chr 4 main effect into an approximately 50-cM region with three distinct QTL peaks at 16, 34, and 54 cM. Kif12, a gene encoding a novel kinesin, mapped beneath the 34 cM QTL peak and has expression level variants and strain-specific sequences that were associated with renal disease severity in affected mice. Therefore, the positional candidate gene, Kif12, fulfills the major criteria for QTL gene discovery established by the Complex Trait Consortium, and, thus, it is proposed that Kif12 is a cpk modifier gene.

The expression and activity of renal nitric oxide synthase and circulating nitric oxide in polycystic kidney disease rats

Nitric oxide (NO) influences tubular fluid and electrolyte transport, and hence possibly also fluid accumulation in renal cysts. The expression and activity of intrarenal constitutive NO synthase (cNOS) [neuronal NOS, nNOS and endothelial NOS, eNOS] and inducible NOS (iNOS) and plasma nitrite/nitrate (PNOx) concentration were assessed in homozygous Han:SPRD polycystic kidney disease (PKD) rats (cy/cy), heterozygous Han:SPRD PKD rats (cy/+), homozygous normal Han:SPRD littermates (+/+) and Sprague Dawley rats (sd). The results showed: 1) nNOS expression was decreased in proximal tubules and thick ascending limbs of the loop of Henle in cy/cy and cy/+ rats compared to +/+ and sd rats (p<0.05). nNOS was weakly expressed in the epithelium of small cysts and unexpressed in epithelium of large cysts. 2) iNOS expression was increased in proximal tubular epithelial cells in cy/+ rats compared to +/+ rats and sd rats (p<0.01). iNOS expression in cyst epithelium was decreased in cy/+ rats (p<0.05) and absent in cy/cy rats. 3) eNOS expression was similar in the endothelium of intrarenal arteries in all groups. 4) The activity of renal cNOS was decreased in cy/cy and cy/+ rats; the activity of iNOS was decreased only in cy/cy rats, with no significant difference among the other three groups. 5) PNOx concentration was higher in cy/cy rats than in the other three groups, and correlated positively with plasma creatinine and urea. In conclusion, NOS expression and activity decreased as cysts developed, suggesting that NO downregulation is involved in the pathogenesis of PKD.

Murine models of polycystic kidney disease: molecular and therapeutic insights

Numerous murine (mouse and rat) models of polycystic kidney disease (PKD) have been described in which the mutant phenotype results from a spontaneous mutation or engineering via chemical mutagenesis, transgenic technologies, or gene-specific targeting in mouse orthologs of human PKD genes. These murine phenotypes closely resemble human PKD, with common abnormalities observed in tubular epithelia, the interstitial compartment, and the extracellular matrix of cystic kidneys. In both human and murine PKD, genetic background appears to modulate the renal cystic phenotype. In murine models, these putative modifying effects have been dissected into discrete factors called quantitative trait loci and genetically mapped. Several lines of experimental evidence support the hypothesis that PKD genes and their modifiers may define pathways involved in cystogenesis and PKD progression. Among the various pathway abnormalities described in murine PKD, recent provocative data indicate that structural and/or functional defects in the primary apical cilia of tubular epithelia may play a key role in PKD pathogenesis. This review describes the most widely studied murine models; highlights the data regarding specific gene defects and genetic modifiers; summarizes the data from these models that have advanced our understanding of PKD pathogenesis; and examines the effect of various therapeutic interventions in murine PKD.

Mutations in INVS encoding inversin cause nephronophthisis type 2, linking renal cystic disease to the function of primary cilia and left-right axis determination

Nephronophthisis (NPHP), an autosomal recessive cystic kidney disease, leads to chronic renal failure in children. The genes mutated in NPHP1 and NPHP4 have been identified, and a gene locus associated with infantile nephronophthisis (NPHP2) was mapped. The kidney phenotype of NPHP2 combines clinical features of NPHP and polycystic kidney disease (PKD). Here, we identify inversin (INVS) as the gene mutated in NPHP2 with and without situs inversus. We show molecular interaction of inversin with nephrocystin, the product of the gene mutated in NPHP1 and interaction of nephrocystin with beta-tubulin, a main component of primary cilia. We show that nephrocystin, inversin and beta-tubulin colocalize to primary cilia of renal tubular cells. Furthermore, we produce a PKD-like renal cystic phenotype and randomization of heart looping by knockdown of invs expression in zebrafish. The interaction and colocalization in cilia of inversin, nephrocystin and beta-tubulin connect pathogenetic aspects of NPHP to PKD, to primary cilia function and to left-right axis determination.

A tale of two tails: ciliary mechanotransduction in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a common lethal genetic disorder, characterized by the progressive development of fluid-filled cysts in the kidney, pancreas and liver, and anomalies of the cardiovascular system. Mutations in PKD1 and PKD2, which encode the transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2) respectively, account for almost all cases of ADPKD. However, the mechanisms by which abnormalities in PKD1 and PKD2 lead to aberrant kidney development remain unknown. Recent progress in the understanding of ADPKD has focused on primary cilia, which act as sensory transducers in renal epithelial cells. New evidence shows that a mechanosensitive signal, cilia bending, activates the PC1-PC2 channel complex. When working properly, this functional complex elicits a transient Ca(2+) influx, which is coupled to the release of Ca(2+) from intracellular stores.

Cystin, a novel cilia-associated protein, is disrupted in the cpk mouse model of polycystic kidney disease

The congenital polycystic kidney (cpk) mutation is the most extensively characterized mouse model of polycystic kidney disease (PKD). The renal cystic disease is fully expressed in homozygotes and is strikingly similar to human autosomal recessive PKD (ARPKD), whereas genetic background modulates the penetrance of the corresponding defect in the developing biliary tree. We now describe the positional cloning, mutation analysis, and expression of a novel gene that is disrupted in cpk mice. The cpk gene is expressed primarily in the kidney and liver and encodes a hydrophilic, 145-amino acid protein, which we term cystin. When expressed exogenously in polarized renal epithelial cells, cystin is detected in cilia, and its expression overlaps with polaris, another PKD-related protein. We therefore propose that the single epithelial cilium is important in the functional differentiation of polarized epithelia and that ciliary dysfunction underlies the PKD phenotype in cpk mice.

Cystin, a novel cilia-associated protein, is disrupted in the cpk mouse model of polycystic kidney disease

The congenital polycystic kidney (cpk) mutation is the most extensively characterized mouse model of polycystic kidney disease (PKD). The renal cystic disease is fully expressed in homozygotes and is strikingly similar to human autosomal recessive PKD (ARPKD), whereas genetic background modulates the penetrance of the corresponding defect in the developing biliary tree. We now describe the positional cloning, mutation analysis, and expression of a novel gene that is disrupted in cpk mice. The cpk gene is expressed primarily in the kidney and liver and encodes a hydrophilic, 145-amino acid protein, which we term cystin. When expressed exogenously in polarized renal epithelial cells, cystin is detected in cilia, and its expression overlaps with polaris, another PKD-related protein. We therefore propose that the single epithelial cilium is important in the functional differentiation of polarized epithelia and that ciliary dysfunction underlies the PKD phenotype in cpk mice.

Treatment prospects for autosomal-dominant polycystic kidney disease

An increased understanding of the molecular genetic and cellular pathophysiologic mechanisms responsible for the development of autosomal-dominant polycystic kidney disease (ADPKD), made possible by the advances in molecular biology and genetics of the last three decades, has laid the foundation for the development of effective therapies. As the concept that a polycystic kidney is a neoplasm in disguise is becoming increasingly accepted, the development of therapies for ADPKD may benefit greatly from the expanding body of information on cancer chemoprevention and chemosuppression. This review summarizes the observations that already have been made and discusses therapies for PKD that deserve investigation.

A possible role for metalloproteinases in renal cyst development

The expansion of cysts in polycystic kidneys bears several similarities to the invasion of the extracellular matrix by benign tumors. We therefore hypothesized that cyst-lining epithelial cells produce extracellular matrix-degrading metalloproteinases and that the inhibition of these enzymes may represent a potential target for therapeutic intervention. Using in situ hybridization, we first analyzed the expression of membrane-type metalloproteinase 1 (MMP-14), an essential matrix metalloproteinase, of its inhibitor TIMP-2, and of the cytokine transforming growth factor (TGF)-beta2 in the (cy/+) rat model of autosomal-dominant polycystic kidney disease. Upregulated MMP-14 mRNA was predominantly located in cyst-lining epithelia and distal tubules, whereas TIMP-2 mRNA was confined almost exclusively to fibroblasts. TGF-beta2, a cytokine known to regulate the expression of matrix metalloproteinases and their inhibitors, was also expressed by cyst wall epithelia. We then treated (cy/+) rats with the metalloproteinase inhibitor batimastat for a period of 8 wk. The treatment with the metalloproteinase inhibitor batimastat resulted in a significant reduction of cyst number and kidney weight. Our study suggests that metalloproteinase inhibitors represent a new therapeutic tool against polycystic kidney disease, which should be applicable independently of the background of the disease.

Endothelium-dependent relaxation of small resistance vessels is impaired in patients with autosomal dominant polycystic kidney disease

Impaired endothelium-dependent relaxation has been demonstrated previously in resistance vessels of Han:SPRD polycystic kidney disease rats. The aim of the present study was to investigate whether endothelium-dependent relaxation is reduced also in patients with autosomal dominant polycystic kidney disease (ADPKD) and whether this is influenced by the nitric oxide (NO) system. Small subcutaneous resistance vessels from normotensive ADPKD patients with normal or near-normal renal function (n = 9) and from healthy control subjects (n = 10) were mounted in a Mulvany-Halpern myograph. The morphology of the vessels and acetylcholine (ACh)-induced endothelium-dependent relaxation, as well as 3-morpholino-sydnonimine (SIN-1, NO donor)-induced endothelium-independent relaxation were investigated. The results showed that: (1) there were no significant differences in morphologic parameters of resistance vessels between the two groups; (2) the maximal ACh-induced relaxation rate was decreased in ADPKD patients compared with control subjects (71.5 +/- 12.1 versus 85.2 +/- 8.7%, P < 0.01); (3) in the presence of L-arginine (a substrate of NO synthase), a left shift of the ACh dose-response curves was found in control subjects, but not in ADPKD patients; (4) in the presence of the N(G)-nitro-L-arginine methyl ester (an inhibitor of NO synthase), a right shift of the ACh dose-response curve was found in control subjects, but not in ADPKD patients; and (5) endothelium-independent relaxation rate induced with SIN-1 was similar in patients and control subjects. In conclusion, endothelium-dependent relaxation was impaired in resistance vessels from patients with ADPKD. The reduced response of the vessels to both the substrate and inhibitor of NO synthase in ADPKD suggests that an impairment of NO synthase may be involved in the mechanism of endothelial dysfunction in ADPKD.

Apoptosis in polycystic kidney disease: involvement of caspases

Polycystic kidney disease (PKD) is characterized by the development of large renal cysts and progressive loss of renal function. Although the cause of the development of renal cysts is unknown, recent evidence suggests that excessive apoptosis occurs in PKD. With the use of terminal deoxynucleotidyl transferase dUTP nick-end labeling staining, we have confirmed the presence of apoptotic bodies in cystic kidneys of congenital polycystic kidney (cpk) disease mice carrying a homozygous mutation at 3 wk of age. Apoptosis was localized primarily to the interstitium with little evidence of cell death in cyst epithelium or noncystic tubules. In addition, we observed that the expression of various caspases, bax and bcl-2, was upregulated in cystic kidneys. With the use of various substrates in enzyme activity assays, we have demonstrated a greater than sevenfold increase in caspase 4 activity and a sixfold increase in caspase 3 activity. These data suggest that there is a caspase-dependent apoptosis pathway associated with PKD and support the hypothesis that apoptotic cell death contributes to cyst formation in PKD.

Microtubule-dependent vesicle transport: modulation of channel and transporter activity in liver and kidney

Microtubule-based vesicle transport driven by kinesin and cytoplasmic dynein motor proteins facilitates several membrane-trafficking steps including elements of endocytosis and exocytosis in many different cell types. Most early studies on the role of microtubule-dependent vesicle transport in membrane trafficking focused either on neurons or on simple cell lines. More recently, other work has considered the role of microtubule-based vesicle transport in other physiological systems, including kidney and liver. Investigation of the role of microtubule-based vesicle transport in membrane trafficking in cells of the kidney and liver suggests a major role for microtubule-based vesicle transport in the rapid and directed movement of ion channels and transporters to and from the apical plasma membranes, events essential for kidney and liver function and homeostasis. This review discusses the evidence supporting a role for microtubule-based vesicle transport and the motor proteins, kinesin and cytoplasmic dynein, in different aspects of membrane trafficking in cells of the kidney and liver, with emphasis on those functions such as maintenance of ion channel and transporter composition in apical membranes that are specialized functions of these organs. Evidence that defects in microtubule-based transport contribute to diseases of the kidney and liver is also discussed.