Do polycystins function as cation channels?

New mutation associated with autosomal dominant polycystic kidney disease with founder effect located in the alpujarra region of granada

OBJECTIVE

To demonstrate that the variant not described in PKD1 gene c.7292T> A, identified in four families from the Alpujarra in Granada, is the cause of autosomal dominant polycystic kidney disease (ADPKD). This variant consists of a transversion of thymine (T) by adenine (A) that at the level of the Polycystin 1 protein produces a change of leucine (Leu / L) by Glutamine (Gln / Q) in position 2431 (p.Leu2431Gln).

METHOD

Sociodemographic and clinical variables were registered using clinical histories, genealogical trees, ultrasounds and genetic analysis to ADPKD and healthy individuals belonging to these families in the context of segregation study.

RESULTS

All PKD individuals carried the c.7292T>A variant in heterozygosis, whereas healthy ones did not. Among all ADPKD patients, 62.9% were women. ADPKD diagnosis was made at 29.3 ± 15.82 years, after having the first child in 64.8%. The main reasons for diagnosis were family history and hematuria episodes. The onset of renal replacement therapy (RRT) occurred at 55.8 ± 7.62 years (range 44-67), and death at 63 ± 92.2 years (range 48-76), being the cause unknown, cardiovascular and insufficiency kidney the most frequent; the median of renal survival was established at 58.5 ± 0.77 years and the median survival of patients at 67.2 ± 3.54 years. No differences in kidney and patient survivals were observed according to sex. Among deceased patients, 52.2% required RRT and 94.4% suffered from renal failure.

CONCLUSIONS

The variant c.7292T>A in PKD1 gene is responsible for the disease, and its distribution in the Alpujarra region of Granada suggests a founder effect. In ADPKD it is necessary to perform segregation studies that help us to reclassify genetic variants, in this case from indeterminate to pathogenic.

Antihypertensive treatments in adult autosomal dominant polycystic kidney disease: network meta-analysis of the randomized controlled trials

Background

Blood pressure (BP) control is one of the most important treatments of Autosomal dominant polycystic kidney disease (ADPKD). The comparative efficacy of antihypertensive treatments in ADPKD patients is inconclusive.

Methods

Network meta-analysis was used to evaluate randomized controlled trials (RCT) which investigated antihypertensive treatments in ADPKD. PubMed, Embase, Ovid, and Cochrane Collaboration were searched. The primary outcome was estimated glomerular filtration rate (eGFR). Secondary outcomes were serum creatinine (Scr), urinary albumin excretion (UAE), systolic BP (SBP), diastolic BP (DBP), mean artery pressure (MAP) and left ventricular mass index (LVMI).

Results

We included 10 RCTs with 1386 patients and six interventions: angiotensin-converting enzyme inhibitors (ACEI), Angiotensin II receptor blocker (ARB), combination of ACEI and ARB, calcium channel blockers (CCB), β-blockers and dilazep. There was no difference of eGFR in all the treatments in both network and direct comparisons. No significant differences of Scr, SBP, DBP, MAP, and LVMI were found in network comparisons. However, ACEI significantly reduced SBP, DBP, MAP and LVMI when compared to CCB. Significantly increased UAE was observed in CCB compared with ACEI or ARB. Bayesian probability analysis found ARB ranked first in the surrogate measures of eGFR, UAE and SBP.

Conclusions

There is little evidence to detect differences of antihypertensive treatments on kidney disease progression in ADPKD patients. More RCTs will be needed in the future. Use of ARB may be an optimal choice in clinical practice.

Polycystin-1 interacts with inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling with implications for polycystic kidney disease

The PKD1 or PKD2 genes encode polycystins (PC) 1 and 2, which are associated with polycystic kidney disease. Previously we demonstrated that PC2 interacts with the inositol 1,4,5-trisphosphate receptor (IP(3)R) to modulate Ca(2+) signaling. Here, we investigate whether PC1 also regulates IP(3)R. We generated a fragment encoding the last six transmembrane (TM) domains of PC1 and the C-terminal tail (QIF38), a section with the highest homology to PC2. Using a Xenopus oocyte Ca(2+) imaging system, we observed that expression of QIF38 significantly reduced the initial amplitude of IP(3)-induced Ca(2+) transients, whereas a mutation lacking the C-terminal tail did not. Thus, the C terminus is essential to QIF38 function. Co-immunoprecipitation assays demonstrated that through its C terminus, QIF38 associates with the IP(3)-binding domain of IP(3)R. A shorter PC1 fragment spanning only the last TM and the C-terminal tail also reduced IP(3)-induced Ca(2+) release, whereas another C-terminal fragment lacking any TM domain did not. Thus, only endoplasmic reticulum-localized PC1 can modulate IP(3)R. Finally, we show that in the polarized Madin-Darby canine kidney cells, heterologous expression of full-length PC1 resulted in a smaller IP(3)-induced Ca(2+) response. Overexpression of the IP(3)-binding domain of IP(3)R reversed the inhibitory effect of PC1, suggesting interaction of full-length PC1 (or its cleavage forms) with endogenous IP(3)R in Madin-Darby canine kidney cells. These results indicate that the behavior of full-length PC1 in mammalian cells is congruent with that of PC1 C-terminal fragments in the oocyte system. These data demonstrate that PC1 inhibits Ca(2+) release, perhaps opposing the effect of PC2, which facilitates Ca(2+) release through the IP(3)R.

Elevated calcium phosphate product after renal transplantation is a risk factor for graft failure

BACKGROUND

Abnormal mineral metabolism is not uncommon after renal transplant (TXP). In dialysis patients, elevated serum phosphorous (P), calcium (Ca), CaP product, and parathyroid hormone (PTH) are associated with increased morbidity and mortality. The effect of these abnormalities on recipient and graft survival after renal transplantation is unknown.

METHODS

We retrospectively analyzed 422 kidney-only transplants performed between June 1996 and June 2003. Cases with graft or recipient survival less than three months, pre-TXP parathyroidectomy (PTX), cinacalcet therapy and incomplete records were excluded, leaving 303 cases for analysis using Cox models that included post-TXP PTX, levels of albumin-adjusted Ca(Ca(adj)), P, Ca(adj)P product and PTH.

RESULTS

There was an 11-25% prevalence of abnormal serum Ca(adj), P or Ca(adj)P product within the first year post-TXP. At least 24% of recipients not undergoing PTX with an equation estimated GFR of 40-60 mL/min had PTH levels >130 pg/mL at one yr post-TXP. This is above levels recommended by the U.S National Kidney Foundation kidney disease quality initiative for patients with stages I-IV chronic kidney disease. Adjusted Ca > 10.5 mg/dL at three months post-TXP was an independent risk for recipient death (OR 3.0; 95% CI: 1.2-7.4). A Ca(adj)P product >35 mg(2)/dL(2) at six months (OR 4.0; 95% CI: 1.2-13.1), and Ca >10.5 mg/dL at 12 months post-TXP (OR 4.0; 95% CI: 1.2-14) were independent risks for death-censored graft loss. Twenty-two recipients underwent PTX for severe hyperparathyroidism.

CONCLUSION

Abnormalities of mineral metabolism are common early after renal TXP. An elevated serum Ca(adj) at three months post-TXP increases the risk for recipient death, while an elevated Ca(adj)P and Ca(adj) later in the first post-TXP year increases the risk of long-term death-censored graft loss.

TRPP2 and autosomal dominant polycystic kidney disease

Mutations in TRPP2 (polycystin-2) cause autosomal dominant polycystic kidney disease (ADPKD), a common genetic disorder characterized by progressive development of fluid-filled cysts in the kidney and other organs. TRPP2 is a Ca(2+)-permeable nonselective cation channel that displays an amazing functional versatility at the cellular level. It has been implicated in the regulation of diverse physiological functions including mechanosensation, cell proliferation, polarity, and apoptosis. TRPP2 localizes to different subcellular compartments, such as the endoplasmic reticulum (ER), the plasma membrane and the primary cilium. The channel appears to have distinct functions in different subcellular compartments. This functional compartmentalization is thought to contribute to the observed versatility and specificity of TRPP2-mediated Ca(2+) signaling. In the primary cilium, TRPP2 has been suggested to function as a mechanosensitive channel that detects fluid flow in the renal tubule lumen, supporting the proposed role of the primary cilium as the unifying pathogenic concept for cystic kidney disease. This review summarizes the known and emerging functions of TRPP2, focusing on the question of how channel function translates into complex morphogenetic programs regulating tubular structure.

Sperm channel diversity and functional multiplicity

Ion channels are extraordinarily efficient machines that move ions in diversely controlled manners, allowing cells to rapidly exchange information with the outside world and with other cells. Communication is the currency of fertilization, as it is of most fundamental cell signaling events. Ion channels are deeply involved in the dialogue between sperm, its surroundings, and the egg. How sperm swim, find the egg and fertilize it depend on ion permeability changes modulated by environmental cues and components of the egg outer layer. Different ion channels distinctly localized in these tiny, amazing cells perform specific decoding functions that shape the sophisticated behavior of sperm. It is not surprising that certain sperm ion channels are turning out to be unique. New strategies to characterize sperm ion transport have opened exciting possibilities to dissect sperm-egg signaling and unveil novel contraception targets.

Calcium channel blocker versus angiotensin II receptor blocker in autosomal dominant polycystic kidney disease

BACKGROUND

Although hypertension is commonly found in patients with autosomal dominant polycystic kidney disease (ADPKD), there is no consensus about which antihypertensive agents are most appropriate. The effects of calcium channel blockers (CCB) and angiotensin II receptor blockers (ARB) on blood pressure and renoprotection were compared in hypertensive patients with ADPKD.

METHODS

We randomly assigned 49 participants to CCB amlodipine-based (2.5-10 mg/day) or ARB candesartan-based (2-8 mg/day) regimens. Twenty-five patients (13 males and 12 females) received amlodipine, and 24 patients (13 males and 11 females) received candesartan. This was followed up for 36 months.

RESULTS

Baseline characteristics were similar, and blood pressure was well controlled in both groups throughout the study period. Six out of 25 (24.0%) amlodipine and 1 out of 24 (4.2%) candesartan patients were terminated from the protocol due to a twofold increase in serum creatinine and/or decrease in creatinine clearance (Ccr) to half of the baseline. The renal event-free survival rate was significant (p < 0.05, Breslow-Gehan-Wilcoxon test). Serum creatinine was higher in the amlodipine group than in the candesartan group at 24 and 36 months (p < 0.05). The decrease in Ccr at 36 months was larger in the amlodipine group than in the candesartan group (DeltaCcr: -20.9 +/- 13.1 vs. -4.8 +/- 13.8 ml/min, p < 0.01). Urinary protein excretion was significantly lower in the candesartan group than in the amlodipine group at 36 months. Urinary albumin excretion was significantly lower in the candesartan group than in the amlodipine group at 12, 24 and 36 months.

CONCLUSIONS

The renoprotective effect of candesartan is considered more favorable than amlodipine in the treatment of ADPKD. This is independent of the antihypertensive effect per se.

Polycystin 2 interacts with type I inositol 1,4,5-trisphosphate receptor to modulate intracellular Ca2+ signaling

Autosomal dominant polycystic kidney disease, a common cause of renal failure, arises from mutations in either the PKD1 or the PKD2 gene. The precise function of both PKD gene products polycystins (PCs) 1 and 2 remain controversial. PC2 has been localized to numerous cellular compartments, including the endoplasmic reticulum, plasma membrane, and cilia. It is unclear what pools are the most relevant to its physiological function as a putative Ca2+ channel. We employed a Xenopus oocyte Ca2+ imaging system to directly investigate the role of PC2 in inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling. Cytosolic Ca2+ signals were recorded following UV photolysis of caged IP3 in the absence of extracellular Ca2+. We demonstrated that overexpression of PC2, as well as type I IP3 receptor (IP3R), significantly prolonged the half-decay time (t1/2) of IP3-induced Ca2+ transients. However, overexpressing the disease-associated PC2 mutants, the point mutation D511V, and the C-terminally truncated mutation R742X did not alter the t1/2. In addition, we found that D511V overexpression significantly reduced the amplitude of IP3-induced Ca2+ transients. Interestingly, overexpression of the C terminus of PC2 not only significantly reduced the amplitude but also prolonged the t1/2. Co-immunoprecipitation assays indicated that PC2 physically interacts with IP3R through its C terminus. Taken together, our data suggest that PC2 and IP3R functionally interact and modulate intracellular Ca2+ signaling. Therefore, mutations in either PC1 or PC2 could result in the misregulation of intracellular Ca2+ signaling, which in turn could contribute to the pathology of autosomal dominant polycystic kidney disease.

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.

Organic cation permeation through the channel formed by polycystin-2

Polycystin-2 (PC2), a member of the transient receptor potential family of ion channels (TRPP2), forms a calcium-permeable cation channel. Mutations in PC2 lead to polycystic kidney disease. From the primary sequence and by analogy with other channels in this family, PC2 is modeled to have six transmembrane domains. However, most of the structural features of PC2, such as how large the channel is and how many subunits make up the pore of the channel, are unknown. In this study, we estimated the pore size of PC2 from the permeation properties of the channel. Organic cations of increasing size were used as current carriers through the PC2 channel after PC2 was incorporated into lipid bilayers. We found that dimethylamine, triethylamine, tetraethylammonium, tetrabutylammonium, tetrapropylammonium, and tetrapentylammonium were permeable through the PC2 channel. The slope conductance of the PC2 channel decreased as the ionic diameter of the organic cation increased. For each organic cation tested, the currents were inhibited by gadolinium and anti-PC2 antibody. Using the dimensions of the largest permeant cation, the minimum pore diameter of the PC2 channel was estimated to be at least 11 A. The large pore size suggests that the primary state of this channel found in vivo is closed to avoid rundown of cation gradients across the plasma membrane and excessive calcium leak from endoplasmic reticulum stores.

Subcellular localization and trafficking of polycystins

Polycystin-2 is a member of the transient receptor potential (TRP) family of ion channels that is mutated in autosomal dominant polycystic kidney disease. Although its function as a non-selective cation channel has been demonstrated in several model systems, the precise subcellular localization of polycystin-2 (TRPP2) in tubular epithelial cells has remained controversial. Recent evidence suggests that the subcellular localization of TRPP2 is regulated by multiple protein interactions. This review will summarize our current knowledge about polycystin trafficking and highlight the experimental data that supports a compartment-specific function of 'cystogenic' proteins.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Calcium restriction allows cAMP activation of the B-Raf/ERK pathway, switching cells to a cAMP-dependent growth-stimulated phenotype

cAMP can be either mitogenic or anti-mitogenic, depending on the cell type. We demonstrated previously that cAMP inhibited the proliferation of normal renal epithelial cells and stimulated the proliferation of cells derived from the cysts of polycystic kidney disease (PKD) patients. The protein products of the genes causing PKD, polycystin-1 and polycystin-2, are thought to regulate intracellular calcium levels, suggesting that abnormal polycystin function may affect calcium signaling and thus cause a switch to the cAMP growth-stimulated phenotype. To test this hypothesis, we disrupted intracellular calcium mobilization by treating immortalized mouse M-1 collecting duct cells and primary cultures of human kidney epithelial cells with calcium channel blockers and by lowering extracellular calcium with EGTA. Calcium restriction for 3-5 h converted both cell types from a normal cAMP growth-inhibited phenotype to an abnormal cAMP growth-stimulated phenotype, characteristic of PKD. In M-1 cells, we showed that calcium restriction was associated with an elevation in B-Raf protein levels and cAMP-stimulated, Ras-dependent activation of B-Raf and ERK. Moreover, the activity of Akt, a negative regulator of B-Raf, was decreased by calcium restriction. Inhibition of Akt or phosphatidylinositol 3-kinase also allowed cAMP-dependent activation of B-Raf and ERK in normal calcium. These results suggest that calcium restriction causes an inhibition of the phosphatidylinositol 3-kinase/Akt pathway, which relieves the inhibition of B-Raf to allow the cAMP growth-stimulated phenotypic switch. Finally, M-1 cells stably overexpressing an inducible polycystin-1 C-terminal cytosolic tail construct were shown to exhibit a cAMP growth-stimulated phenotype involving B-Raf and ERK activation, which was reversed by the calcium ionophore A23187. We conclude that disruption of calcium mobilization in cells that are normally growth-inhibited by cAMP can derepress the B-Raf/ERK pathway, thus converting these cells to a phenotype that is growth-stimulated by cAMP.

Polycystin-2 associates with the polycystin-1 homolog, suREJ3, and localizes to the acrosomal region of sea urchin spermatozoa

Polycystin-2, the protein mutated in type 2 autosomal dominant polycystic kidney disease, is an integral transmembrane protein with nonselective cation channel activity. Here we report on the sea urchin sperm homolog of polycystin-2 (suPC2). Like other polycystin-2 family members, suPC2 is a six-pass transmembrane protein containing C-terminal cytoplasmic EF hand and coiled-coil domains. The protein localizes exclusively to the plasma membrane over the sperm acrosomal vesicle. This localization coincides with the previously reported localization of the sea urchin PC1 homolog, suREJ3. Co-immunoprecipitation shows that suPC2 and suREJ3 are associated in the membrane. The location of suPC2 suggests that it may function as a cation channel mediating the sperm acrosome reaction. The low cation selectivity of PC2 channels would explain data indicating that Na(+) and Ca(2+) may enter sea urchin sperm through the same channel during the acrosome reaction.

A third sea urchin sperm receptor for egg jelly module protein, suREJ2, concentrates in the plasma membrane over the sperm mitochondrion

Sea urchin spermatozoa are model cells for studying signal transduction events underlying flagellar motility and the acrosome reaction. We previously described the sea urchin sperm receptor for egg jelly 1 (suREJ1) which consists of 1450 amino acids, has one transmembrane segment and binds to the fucose sulfate polymer of egg jelly to induce the sperm acrosome reaction. We also cloned suREJ3 which consists of 2681 amino acids and has 11 putative transmembrane segments. Both these proteins localize to the plasma membrane over the acrosomal vesicle. While cloning suREJ1, we found suREJ2, which consists of 1472 amino acids, has two transmembrane segments and is present in the entire sperm plasma membrane, but is concentrated over the sperm mitochondrion. Experimental evidence suggests that, unlike suREJ1 and suREJ3, suREJ2 does not project extracellularly from the plasma membrane, but is an intracellular plasma membrane protein. All three sea urchin sperm REJ proteins possess a protein module of > 900 amino acids, termed 'the REJ module', that is shared by the human autosomal dominant polycystic kidney disease protein, polycystin-1, and PKDREJ, a testis-specific protein in mammals whose function is unknown. In the present study, we describe the sequence, domain structure and localization of suREJ2 and speculate on its possible function.

Ca2+-dependent potentiation of the nonselective cation channel TRPV4 is mediated by a C-terminal calmodulin binding site

Most Ca2+-permeable ion channels are inhibited by increases in the intracellular Ca2+ concentration ([Ca2+]i), thus preventing potentially deleterious rises in [Ca2+]i. In this study, we demonstrate that currents through the osmo-, heat- and phorbol ester-sensitive, Ca2+-permeable nonselective cation channel TRPV4 are potentiated by intracellular Ca2+. Spontaneous TRPV4 currents and currents stimulated by hypotonic solutions or phorbol esters were reduced strongly at all potentials in the absence of extracellular Ca2+. The other permeant divalent cations Ba2+ and Sr2+ were less effective than Ca2+ in supporting channel activity. An intracellular site of Ca2+ action was supported by the parallel decrease in spontaneous currents and [Ca2+]i on removal of extracellular Ca2+ and the ability of Ca2+ release from intracellular stores to restore TRPV4 activity in the absence of extracellular Ca2+. During TRPV4 activation by hypotonic solutions or phorbol esters, Ca2+ entry through the channel increased the rate and extent of channel activation. Currents were also potentiated by ionomycin in the presence of extracellular Ca2+. Ca2+-dependent potentiation of TRPV4 was often followed by inhibition. By mutagenesis, we localized the structural determinant of Ca2+-dependent potentiation to an intracellular, C-terminal calmodulin binding domain. This domain binds calmodulin in a Ca2+-dependent manner. TRPV4 mutants that did not bind calmodulin lacked Ca2+-dependent potentiation. We conclude that TRPV4 activity is tightly controlled by intracellular Ca2+. Ca2+ entry increases both the rate and extent of channel activation by a calmodulin-dependent mechanism. Excessive increases in [Ca2+]i via TRPV4 are prevented by a Ca2+-dependent negative feedback mechanism.

Identification of two novel polycystic kidney disease-1-like genes in human and mouse genomes

Mutations to the prototypical members of the two general classes of polycystins, polycystin-1 encoded by PKD1 and polycystin-2 encoded by PKD2, underlie autosomal-dominant polycystic kidney disease. Here we report the identification of a pair of genes homologous to PKD1 from both the human and mouse genomes. PKD1L2 and PKD1L3 are located on human chromosome 16q22-q23 and mouse chromosome 8 and are alternatively spliced. The human and mouse forms of PKD1L2 are highly conserved, with each one consisting of 43 exons and approximately 2,460 codons. PKD1L3 shows regional sequence divergence, with the mouse form having two additional exons and a much larger exon 5. The predicted protein products of PKD1L2 and PKD1L3 contain the combination of GPS and PLAT/LH2 domains that uniquely define them as polycystin-1 family members. They are predicted to have 11 membrane-spanning regions with a large extracellular domain consistent with the proposed receptor function of this protein family. PKD1L2 and PKD1L3 contain strong ion channel signature motifs that suggest their possible function as components of cation channel pores. Polycystin-1-related proteins may not only regulate channels, but may actually be part of the pore-forming unit.

Structure-function relationships of the extracellular domain of the autosomal dominant polycystic kidney disease-associated protein, polycystin-1

Polycystin-1 (PC-1) is a member of a novel family of proteins that have a multidomain structure. Although the C-terminal intracellular segments have been extensively studied, mainly with respect to their putative involvement in cell signalling, the potential function of the extracellular domains has received less attention. Mutations in PC-1 result in autosomal dominant polycystic kidney disease (ADPKD) which is characterised by perturbation of transport resulting in fluid accumulation, cell proliferation and modification of the extracellular matrix. The possibility that the interaction of a component of the extracellular matrix or some external factor with PC-1 may be important in the initiation or progression of ADPKD cannot currently be ruled out. The purpose of this review is to assess current evidence for the function of the PC-1 extracellular domains, and their potential implications for ADPKD.

Recent advances in the cell biology of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a significant familial disorder, crossing multiple ethnicities as well as organ systems. The goal of understanding and, ultimately, curing ADPKD has fostered collaborative efforts among many laboratories, mustered on by the opportunity to probe fundamental cellular biology. Here we review what is known about ADPKD including well-accepted data such as the identification of the causative genes and the fact that PKD1 and PKD2 act in the same pathway, fairly well-accepted concepts such as the "two-hit hypothesis," and somewhat confusing information regarding polycystin-1 and -2 localization and protein interactions. Special attention is paid to the recently discovered role of the cilium in polycystic kidney disease and the model it suggests. Studying ADPKD is important, not only as an evaluation of a multisystem disorder that spans a lifetime, but as a testament to the achievements of modern biology and medicine.