Kidney-specific inactivation of the KIF3A subunit of kinesin-II inhibits renal ciliogenesis and produces polycystic kidney disease

The canonical Wnt signaling pathway is not involved in renal cyst development in the kidneys of inv mutant mice

Recent studies have identified several genes whose defects cause hereditary renal cystic diseases with most of the gene products located in the primary cilia. It has been suggested that primary cilia are involved in signaling pathways, defects of which result in abnormal cell proliferation and randomization of oriented cell division in the kidney leading to cyst formation. Mice with a mutation in the inv gene are a model for human nephronophthisis type 2 and develop multiple renal cysts. Inv protein (also called inversin) is located in the base of primary cilia and acts as a switch from canonical to non-canonical Wnt signaling. Here, we studied the orientation of cell division and proliferation in the kidneys of inv mutant mice, as its loss is thought to maintain activation of the canonical Wnt signaling. To establish if canonical signaling was involved in this process, we mated inv mutant with BATlacZ mice to measure canonical Wnt activity. Based on these reporter mice, nuclear localization and phosphorylation of β-catenin, and responsiveness to Wnt ligands in inv mutant cells, we found that random oriented cell division is an initial event for renal tubule expansion and precedes cell proliferation. Thus, our results do not support the hypothesis that canonical Wnt signaling causes renal cyst development in these mice.

Polycystic kidney disease: pathogenesis and potential therapies

Autosomal dominant polycystic kidney disease (ADPKD) is a prevalent, inherited condition for which there is currently no effective specific clinical therapy. The disease is characterized by the progressive development of fluid-filled cysts derived from renal tubular epithelial cells which gradually compress the parenchyma and compromise renal function. Current interests in the field focus on understanding and exploiting signaling mechanisms underlying disease pathogenesis as well as delineating the role of the primary cilium in cystogenesis. This review highlights the pathogenetic pathways underlying renal cyst formation as well as novel therapeutic targets for the treatment of PKD. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Kif3a deficiency reverses the skeletal abnormalities in Pkd1 deficient mice by restoring the balance between osteogenesis and adipogenesis

Pkd1 localizes to primary cilia in osteoblasts and osteocytes. Targeted deletion of Pkd1 in osteoblasts results in osteopenia and abnormalities in Runx2-mediated osteoblast development. Kif3a, an intraflagellar transport protein required for cilia function, is also expressed in osteoblasts. To assess the relationship between Pkd1 and primary cilia function on bone development, we crossed heterozygous Pkd1- and Kif3a-deficient mice to create compound Pkd1 and Kif3a-deficient mice. Pkd1 haploinsufficiency (Pkd1(+/Δ)) resulted in osteopenia, characterized by decreased bone mineral density, trabecular bone volume, and cortical thickness. In addition, deficiency of Pkd1 resulted in impaired osteoblastic differentiation and enhanced adipogenesis in both primary osteoblasts and/or bone marrow stromal cell cultures. These changes were associated with decreased Runx2 expression, increased PPARγ expression, and impaired hedgehog signaling as evidenced by decreased Gli2 expression in bone and osteoblast cultures. In contrast, heterozygous Kif3a(+/Δ) mice display no abnormalities in skeletal development or osteoblast function, but exhibited decreased adipogenic markers in bone and impaired adipogenesis in vitro in association with decreased PPARγ expression and upregulation of Gli2. Superimposed Kif3a deficiency onto Pkd1(+/Δ) mice paradoxically corrected the effects of Pkd1 deficiency on bone mass, osteoblastic differentiation, and adipogenesis. In addition, Runx2, PPARγ and Gli2 expression in bone and osteoblasts were normalized in compound double Pkd1(+/Δ) and Kif3a(+/Δ) heterozygous mice. The administration of sonic hedgehog, overexpression of Gli2, and the PC1 C-tail construct all increased Gli2 and Runx2-II expression, but decreased PPARγ2 gene expression in C3H10T1/2 cells. Our findings suggest a role for Pkd1 and Kif3a to counterbalance the regulation of osteogenesis and adipogenesis through differential regulation of Runx2 and PPARγ by Gli2.

Molecular genetics and pathogenic mechanisms for the severe ciliopathies: insights into neurodevelopment and pathogenesis of neural tube defects

Meckel-Gruber syndrome (MKS) is a severe autosomal recessively inherited disorder characterized by developmental defects of the central nervous system that comprise neural tube defects that most commonly present as occipital encephalocele. MKS is considered to be the most common syndromic form of neural tube defect. MKS is genetically heterogeneous with six known disease genes: MKS1, MKS2/TMEM216, MKS3/TMEM67, RPGRIP1L, CEP290, and CC2D2A with the encoded proteins all implicated in the correct function of primary cilia. Primary cilia are microtubule-based organelles that project from the apical surface of most epithelial cell types. Recent progress has implicated the involvement of cilia in the Wnt and Shh signaling pathways and has led to an understanding of their role in normal mammalian neurodevelopment. The aim of this review is to provide an overview of the molecular genetics of the human disorder, and to assess recent insights into the etiology and molecular cell biology of severe ciliopathies from mammalian animal models of MKS.

Disease stage characterization of hepatorenal fibrocystic pathology in the PCK rat model of ARPKD

The rat Pck gene is orthologous to the human PKHD1 gene responsible for autosomal recessive polycystic kidney disease (ARPKD). Both renal and hepatic fibrocystic pathology occur in ARPKD. Affected humans have a variable rate of progression, from morbidly affected infants to those surviving into adulthood. This study evaluated the PCK rat, a model of slowly progressive ARPKD. This model originated in Japan and was rederived to be offered commercially by Charles River Laboratories (Wilmington, MA). Previous studies have described the basic aspects of PCK pathology from privately held colonies. This study provides a comprehensive characterization of rats from those commercially available. Rats were bred, maintained on a 12:12 hr light/dark cycle, fed (7002 Teklad), and water provided ad libitum. Male and female rats were evaluated from 4 through 35 weeks of age with histology and serum chemistry. As the hepatorenal fibrocystic disease progressed beyond 18 weeks, the renal pathology (kidney weight, total cyst volume) and renal dysfunction (BUN and serum creatinine) tended to be more severe in males, whereas liver pathology (liver weight as % of body weight and hepatic fibrocystic volume) tended to be more severe in females. Hyperlipidemia was evident in both genders after 18 weeks. Bile secretion was increased in PCK rats compared with age-matched Sprague Dawley rats. The PCK is an increasingly used orthologous rodent model of human ARPKD. This characterization study of hepatorenal fibrocystic pathology in PCK rats should help researchers select stages of pathology to study and/or monitor disease progression during their longitudinal studies.

The zebrafish foxj1a transcription factor regulates cilia function in response to injury and epithelial stretch

Cilia are essential for normal organ function and developmental patterning, but their role in injury and regeneration responses is unknown. To probe the role of cilia in injury, we analyzed the function of foxj1, a transcriptional regulator of cilia genes, in response to tissue damage and renal cyst formation. Zebrafish foxj1a, but not foxj1b, was rapidly induced in response to epithelial distension and stretch, kidney cyst formation, acute kidney injury by gentamicin, and crush injury in spinal cord cells. Obstruction-induced up-regulation of foxj1a was not inhibited by cycloheximide, identifying foxj1a as a primary response gene to epithelial injury. Foxj1 was also dramatically up-regulated in murine cystic kidney disease epithelia [jck/jck (nek8) and Ift88Tg737Rpw(-/-)] as well as in response to kidney ischemia-reperfusion injury. Obstruction of the zebrafish pronephric tubule caused a rapid increase in cilia beat rate that correlated tightly with expanded tubule diameter and epithelial stretch. Zebrafish foxj1a was specifically required for cilia motility. Enhanced foxj1a expression in obstructed tubules induced cilia motility target genes efhc1, tektin-1, and dnahc9. foxj1a-deficient embryos failed to up-regulate efhc1, tektin-1, and dnahc9 and could not maintain enhanced cilia beat rates after obstruction, identifying an essential role for foxj1 in modulating cilia function after injury. These studies reveal that activation of a Foxj1 transcriptional network of ciliogenic genes is an evolutionarily conserved response to multiple forms of tissue damage and highlight enhanced cilia function as a previously uncharacterized component of organ homeostasis.

Planar cell polarity signaling: the developing cell's compass

Cells of many tissues acquire cellular asymmetry to execute their physiologic functions. The planar cell polarity system, first characterized in Drosophila, is important for many of these events. Studies in Drosophila suggest that an upstream system breaks cellular symmetry by converting tissue gradients to subcellular asymmetry, whereas a downstream system amplifies subcellular asymmetry and communicates polarity between cells. In this review, we discuss apparent similarities and differences in the mechanism that controls PCP as it has been adapted to a broad variety of morphological cellular asymmetries in various organisms.

Tankyrase is necessary for canonical Wnt signaling during kidney development

Recent studies using small molecule antagonists have revealed that the poly(ADP-ribose) polymerases (PARPs) Tankyrase 1 and 2 are critical regulators of canonical Wnt signaling in some cellular contexts. However, the absence of any activity during zebrafish embryogenesis suggested that the tankyrases may not be general/core components of the Wnt pathway. Here, we show that Tnks1 and 2 are broadly expressed during mouse development and are essential during kidney and lung development. In the kidney, blockage of tankyrase activity phenocopies the effect of blocking production of all Wnt ligands. Tankyrase inhibition can be rescued by activation of beta-catenin demonstrating its specificity for the Wnt pathway. In addition, treatment with tankyrase inhibitors appears to be completely reversible in some cell types. These studies suggest that the tankyrases are core components of the canonical Wnt pathway and their inhibitors should enjoy broad usage as antagonists of Wnt signaling.

Polycystic kidney disease: inheritance, pathophysiology, prognosis, and treatment

Both autosomal dominant and recessive polycystic kidney disease are conditions with severe associated morbidity and mortality. Recent advances in the understanding of the genetic and molecular pathogenesis of both ADPKD and ARPKD have resulted in new, targeted therapies designed to disrupt cell signaling pathways responsible for the abnormal cell proliferation, dedifferentiation, apoptosis, and fluid secretion characteristic of the disease. Herein we review the current understanding of the pathophysiology of these conditions, as well as the current treatments derived from our understanding of the mechanisms of these diseases.

Intraflagellar transport proteins are essential for cilia formation and for planar cell polarity

The highly conserved intraflagellar transport (IFT) proteins are essential for cilia formation in multiple organisms, but surprisingly, cilia form in multiple zebrafish ift mutants. Here, we detected maternal deposition of ift gene products in zebrafish and found that ciliary assembly occurs only during early developmental stages, supporting the idea that maternal contribution of ift gene products masks the function of IFT proteins during initial development. In addition, the basal bodies in multiciliated cells of the pronephric duct in ift mutants were disorganized, with a pattern suggestive of defective planar cell polarity (PCP). Depletion of pk1, a core PCP component, similarly led to kidney cyst formation and basal body disorganization. Furthermore, we found that multiple ift genes genetically interact with pk1. Taken together, these data suggest that IFT proteins play a conserved role in cilia formation and planar cell polarity in zebrafish.

The small GTPases ARL-13 and ARL-3 coordinate intraflagellar transport and ciliogenesis

Intraflagellar transport (IFT) machinery mediates the bidirectional movement of cargos that are required for the assembly and maintenance of cilia. However, little is known about how IFT is regulated in vivo. In this study, we show that the small guanosine triphosphatase (GTPase) adenosine diphosphate ribosylation factor-like protein 13 (ARL-13) encoded by the Caenorhabditis elegans homologue of the human Joubert syndrome causal gene ARL13B, localizes exclusively to the doublet segment of the cilium. arl-13 mutants have shortened cilia with various ultrastructural deformities and a disrupted association between IFT subcomplexes A and B. Intriguingly, depletion of ARL-3, another ciliary small GTPase, partially suppresses ciliogenesis defects in arl-13 mutants by indirectly restoring binding between IFT subcomplexes A and B. Rescue of arl-13 mutants by ARL-3 depletion is mediated by an HDAC6 deacetylase-dependent pathway. Thus, we propose that two conserved small GTPases, ARL-13 and ARL-3, coordinate to regulate IFT and that perturbing this balance results in cilia deformation.

Kidney-specific inactivation of Ofd1 leads to renal cystic disease associated with upregulation of the mTOR pathway

The oral-facial-digital type I syndrome (OFDI; MIM 311200) is a rare syndromic form of inherited renal cystic disease. It is transmitted as an X-linked dominant, male lethal disorder and is caused by mutations in the OFD1 gene. Previous studies demonstrated that OFDI belongs to the growing number of disorders ascribed to dysfunction of primary cilia. We generated a conditional inactivation of the mouse Ofd1 gene using the Ksp-Cre transgenic line, which resulted in a viable model characterized by renal cystic disease and progressive impairment of renal function. The study of this model allowed us to demonstrate that primary cilia initially form and then disappear after the development of cysts, suggesting that the absence of primary cilia is a consequence rather than the primary cause of renal cystic disease. Immunofluorescence and western blotting analysis revealed upregulation of the mTOR pathway in both dilated and non-dilated renal structures. Treatment with rapamycin, a specific inhibitor of the mTOR pathway, resulted in a significant reduction in the number and size of renal cysts and a decrease in the cystic index compared with untreated mutant animals, suggesting that dysregulation of this pathway in our model is mTOR-dependent. The animal model we have generated could thus represent a valuable tool to understand the molecular link between mTOR and cyst development, and eventually to the identification of novel drug targets for renal cystic disease.

Kif26b, a kinesin family gene, regulates adhesion of the embryonic kidney mesenchyme

The kidney develops through reciprocal interactions between two precursor tissues: the metanephric mesenchyme and the ureteric bud. We previously demonstrated that the zinc finger protein Sall1 is essential for ureteric bud attraction toward the mesenchyme. Here, we show that Kif26b, a kinesin family gene, is a downstream target of Sall1 and that disruption of this gene causes kidney agenesis because of impaired ureteric bud attraction. In the Kif26b-null metanephros, compact adhesion between mesenchymal cells adjacent to the ureteric buds and the polarized distribution of integrin alpha8 were impaired, resulting in failed maintenance of Gdnf, a critical ureteric bud attractant. Overexpression of Kif26b in vitro caused increased cell adhesion through interactions with nonmuscle myosin. Thus, Kif26b is essential for kidney development because it regulates the adhesion of mesenchymal cells in contact with ureteric buds.

Differential role of Rab proteins in ciliary trafficking: Rab23 regulates Smoothened levels

The structure and function of the primary cilium as a sensory organelle depends on a motor-protein-powered intraflagellar transport system (IFT); defective IFT results in retinal degeneration and pleiotropic disorders such as the Bardet Biedl syndrome (BBS) and defective hedgehog (HH) signaling. Protein transport to the cilium involves Rab GTPases. Rab8, together with a multi protein complex of BBS proteins, recruits cargo to the basal body for transport to the cilium. Loss of Rab23 in mice recapitulates the HH phenotype but its function in HH signaling is unknown. Here we established a novel protocol, based on fluorescence recovery after photo-bleaching (FRAP), allowing the quantitative analysis of protein transport into the cilium of MDCK cells. We compared the effect of Rab8, Rab5 and Rab23 on the ciliary transport of the HH-associated transmembrane receptor Smoothened, the microtubular tip protein EB1, and the receptor protein Kim1. Ciliary FRAP confirmed the role of Rab8 in protein entry to the cilium. Dominant negative Rab5 had no impact on the ciliary transport of Smoothened or EB1, but slowed the recovery of the apical protein Kim1 in the cilium. Depletion of Rab23 or expression of dominant-negative Rab23 decreased the ciliary steady state specifically of Smoothened but not EB1 or Kim1, suggesting a role of Rab23 in protein turnover in the cilium.

The primary cilium: a signalling centre during vertebrate development

The primary cilium has recently stepped into the spotlight, as a flood of data show that this organelle has crucial roles in vertebrate development and human genetic diseases. Cilia are required for the response to developmental signals, and evidence is accumulating that the primary cilium is specialized for hedgehog signal transduction. The formation of cilia, in turn, is regulated by other signalling pathways, possibly including the planar cell polarity pathway. The cilium therefore represents a nexus for signalling pathways during development. The connections between cilia and developmental signalling have begun to clarify the basis of human diseases associated with ciliary dysfunction.

Cystic diseases of the kidney: molecular biology and genetics

CONTEXT

Cystic diseases of the kidney are a very heterogeneous group of renal inherited conditions, with more than 33 genes involved and encompassing X-linked, autosomal dominant, and autosomal recessive inheritance. Although mostly monogenic with mendelian inheritance, there are clearly examples of oligogenic inheritance, such as 3 mutations in 2 genes, while the existence of genetic modifiers is perhaps the norm, based on the extent of variable expressivity and the broad spectrum of symptoms.

OBJECTIVES

To present in the form of a mini review the major known cystic diseases of the kidney for which genes have been mapped or cloned and characterized, with some information on their cellular and molecular biology and genetics, and to pay special attention to commenting on the issues of molecular diagnostics, in view of the genetic and allelic heterogeneity. Data Sources.-We used major reviews that make excellent detailed presentation of the various diseases, as well as original publications.

CONCLUSIONS

There is already extensive genetic heterogeneity in the group of cystic diseases of the kidney; however, there are still many more genes awaiting to be discovered that are implicated or mutated in these diseases. In addition, the synergism and interaction among this repertoire of gene products is largely unknown, while a common unifying aspect is the expression of nearly all of them at the primary cilium or the basal body. A major interplay of functions is anticipated, while mutations in all converge in the unifying phenotype of cyst formation.

The ADPKD genes pkd1a/b and pkd2 regulate extracellular matrix formation

Mutations in polycystin1 (PKD1) account for the majority of autosomal dominant polycystic kidney disease (ADPKD). PKD1 mutations are also associated with vascular aneurysm and abdominal wall hernia, suggesting a role for polycystin1 in extracellular matrix (ECM) integrity. In zebrafish, combined knockdown of the PKD1 paralogs pkd1a and pkd1b resulted in dorsal axis curvature, hydrocephalus, cartilage and craniofacial defects, and pronephric cyst formation at low frequency (10-15%). Dorsal axis curvature was identical to the axis defects observed in pkd2 knockdown embryos. Combined pkd1a/b, pkd2 knockdown demonstrated that these genes interact in axial morphogenesis. Dorsal axis curvature was linked to notochord collagen overexpression and could be reversed by knockdown of col2a1 mRNA or chemical inhibition of collagen crosslinking. pkd1a/b- and pkd2-deficient embryos exhibited ectopic, persistent expression of multiple collagen mRNAs, suggesting a loss of negative feedback signaling that normally limits collagen gene expression. Knockdown of pkd1a/b also dramatically sensitized embryos to low doses of collagen-crosslinking inhibitors, implicating polycystins directly in the modulation of collagen expression or assembly. Embryos treated with wortmannin or LY-29400 also exhibited dysregulation of col2a1 expression, implicating phosphoinositide 3-kinase (PI3K) in the negative feedback signaling pathway controlling matrix gene expression. Our results suggest that pkd1a/b and pkd2 interact to regulate ECM secretion or assembly, and that altered matrix integrity may be a primary defect underlying ADPKD tissue pathologies.

Molecular advances in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic disease (ADPKD) is the most common form of inherited kidney disease that results in renal failure. The understanding of the pathogenesis of ADPKD has advanced significantly since the discovery of the 2 causative genes, PKD1 and PKD2. Dominantly inherited gene mutations followed by somatic second-hit mutations inactivating the normal copy of the respective gene result in renal tubular cyst formation that deforms the kidney and eventually impairs its function. The respective gene products, polycystin-1 and polycystin-2, work together in a common cellular pathway. Polycystin-1, a large receptor molecule, forms a receptor-channel complex with polycystin-2, which is a cation channel belonging to the TRP family. Both polycystin proteins have been localized to the primary cilium, a nonmotile microtubule-based structure that extends from the apical membrane of tubular cells into the lumen. Here we discuss recent insights in the pathogenesis of ADPKD including the genetics of ADPKD, the properties of the respective polycystin proteins, the role of cilia, and some cell-signaling pathways that have been implicated in the pathways related to PKD1 and PKD2.

Pkd1 transgenic mice: adult model of polycystic kidney disease with extrarenal and renal phenotypes

While high levels of Pkd1 expression are detected in tissues of patients with autosomal dominant polycystic kidney disease (ADPKD), it is unclear whether enhanced expression could be a pathogenetic mechanism for this systemic disorder. Three transgenic mouse lines were generated from a Pkd1-BAC modified by introducing a silent tag via homologous recombination to target a sustained wild-type genomic Pkd1 expression within the native tissue and temporal regulation. These mice specifically overexpressed the Pkd1 transgene in extrarenal and renal tissues from approximately 2- to 15-fold over Pkd1 endogenous levels in a copy-dependent manner. All transgenic mice reproducibly developed tubular and glomerular cysts leading to renal insufficiency. Interestingly, Pkd1(TAG) mice also exhibited renal fibrosis and calcium deposits in papilla reminiscent of nephrolithiasis as frequently observed in ADPKD. Similar to human ADPKD, these mice consistently displayed hepatic fibrosis and approximately 15% intrahepatic cysts of the bile ducts affecting females preferentially. Moreover, a significant proportion of mice developed cardiac anomalies with severe left-ventricular hypertrophy, marked aortic arch distention and/or valvular stenosis and calcification that had profound functional impact. Of significance, Pkd1(TAG) mice displayed occasional cerebral lesions with evidence of ruptured and unruptured cerebral aneurysms. This Pkd1(TAG) mouse model demonstrates that overexpression of wild-type Pkd1 can trigger the typical adult renal and extrarenal phenotypes resembling human ADPKD.

Polycystic kidney disease, cilia, and planar polarity

Cystic kidney diseases are characterized by dilated or cystic kidney tubular segments. Changes in planar cell polarity, flow sensing, and/or proliferation have been proposed to explain these disorders. Over the last few years, several groups have suggested that ciliary dysfunction is a central component of cyst formation. We review evidence for and against each of these models, stressing some of the inconsistencies that should be resolved if an accurate understanding of cyst formation is to be achieved. We also comment on data supporting a model in which ciliary function could play different roles at different developmental stages and on the relevance of dissecting potential differences between pathways required for tubule formation and/or maintenance.

Constructing and deconstructing roles for the primary cilium in tissue architecture and cancer

Primary cilia are exquisitely designed sensory machines that have evolved at least three distinct sensory modalities to monitor the extracellular environment. The presence and activation of growth factor, morphogen, and hormone receptors within the confines of the ciliary membrane, the intrinsic physical relationship between the ciliary axoneme and the centriole, and the preferential assembly of primary cilia on the apical surfaces of tissue epithelia highlight the importance of this organelle in the establishment and maintenance of tissue architecture and homeostasis. Accordingly, recent studies begin to suggest roles for these organelles in oncogenesis and tumor suppression. Here, we review the sensory properties of primary cilia, assess the "history" of the primary cilium in cancer, and draw upon recent findings in a discussion of how the primary cilium may influence tissue architecture and neoplasia.

KAP, the accessory subunit of kinesin-2, binds the predicted coiled-coil stalk of the motor subunits

Kinesin-2 is an anterograde motor involved in intraflagellar transport and certain other intracellular transport processes. It consists of two different motor subunits and an accessory protein KAP (kinesin accessory protein). The motor subunits were shown to bind each other through the coiled-coil stalk domains, while KAP was proposed to bind the tail domains of the motor subunits. Although several genetic studies established that KAP plays an important role in kinesin-2 functions, its exact role remains unclear. Here, we report the results of a systematic analysis of the KAP binding sites by using recombinant Drosophila kinesin-2 subunits as well as the endogenous proteins. These show that at least one of the coiled-coil stalks is sufficient to bind the N-terminal region of DmKAP. The soluble complex involving the recombinant kinesin-2 fragments is reconstituted in vitro at high salt concentrations, suggesting that the interaction is primarily nonionic. Furthermore, independent distant homology modeling indicated that DmKAP may bind along the coiled-coil stalks through a combination of predominantly hydrophobic interactions and hydrogen bonds. These observations led us to propose that KAP would stabilize the motor subunit heterodimer and help assemble a greater kinesin-2 complex in vivo.

Gli2 trafficking links Hedgehog-dependent activation of Smoothened in the primary cilium to transcriptional activation in the nucleus

Stimulation by the extracellular Hedgehog (Hh) protein signal has been shown to alter ciliary localization of the mammalian Hh receptor components Smoothened (Smo) and Patched (Ptc), and mutations that disrupt the structure and function of the cilium also disrupt Hh-induced changes in gene expression. But how ciliary events affect gene expression in the nucleus is not known, and to address this question we have characterized the cellular trafficking of Gli2, the principal mediator of Hh-dependent transcriptional activation. From a combination of pharmacological and genetic manipulations we find in resting cells that both Gli2 and Smo appear to shuttle in and out of the cilium, with Gli2 but not Smo requiring intact cytoplasmic microtubules for ciliary entry and both requiring the ciliary retrograde motor, cytoplasmic dynein 2, for ciliary exit. We also find that changes in ciliary and nuclear trafficking of Gli2 are triggered by the Hh-dependent accumulation of activated Smo in the cilium, resulting in a shift from primarily cytoplasmic localization to accumulation at the distal tip of the cilium and within the nucleus. Gli2 thus functions as a dynamic monitor of Smo activity in the cilium and thereby links Hh pathway activation in the cilium to transcriptional activation in the nucleus.

Loss of oriented cell division does not initiate cyst formation

Polycystic kidney disease (PKD) can arise from either developmental or postdevelopmental processes. Recessive PKD, caused by mutations in PKHD1, is a developmental defect, whereas dominant PKD, caused by mutations in PKD1 or PKD2, occurs by a cellular recessive mechanism in mature kidneys. Oriented cell division is a feature of planar cell polarity that describes the orientation of the mitotic axes of dividing cells during development with respect to the luminal vector of the elongating nephron. In polycystic mutant mice, the loss of oriented cell division may also contribute to the pathogenesis of PKD. Here, we examined the role of oriented cell division in mouse models based on mutations in Pkd1, Pkd2, and Pkhd1. Precystic tubules after kidney-selective inactivation of either Pkd1 or Pkd2 did not lose oriented division before cystic dilation but lost oriented division after tubular dilation began. In contrast, Pkhd1(del4/del4) mice lost oriented cell division but did not develop kidney cysts. Increased intercalation of cells into the plane of the tubular epithelium maintained the normal tubular morphology in Pkhd1(del4/del4) mice, which had more cells present in transverse tubular profiles. In conclusion, loss of oriented cell division is a feature of Pkhd1 mutation and cyst formation, but it is neither sufficient to produce kidney cysts nor required to initiate cyst formation after mutation in Pkd1 or Pkd2.

Trafficking of membrane proteins to cone but not rod outer segments is dependent on heterotrimeric kinesin-II

Heterotrimeric kinesin-II is a molecular motor localized to the inner segment, connecting cilium and axoneme of mammalian photoreceptors. Our purpose was to identify the role of kinesin-II in anterograde intraflagellar transport by photoreceptor-specific deletions of kinesin family member 3A (KIF3A), its obligatory motor subunit. In cones lacking KIF3A, membrane proteins involved in phototransduction did not traffic to the outer segments resulting in complete absence of a photopic electroretinogram and progressive cone degeneration. Rod photoreceptors lacking KIF3A degenerated rapidly between 2 and 4 weeks postnatally, but the phototransduction components including rhodopsin trafficked to the outer segments during the course of degeneration. Furthermore, KIF3A deletion did not affect synaptic anterograde trafficking. The results indicate that trafficking of membrane proteins to the outer segment is dependent on kinesin-II in cone, but not rod photoreceptors, even though rods and cones share similar structures, and closely related phototransduction polypeptides.

Conditional mutation of Pkd2 causes cystogenesis and upregulates beta-catenin

Loss of polycystin-2 (PC2) in mice (Pkd2(-/-)) results in total body edema, focal hemorrhage, structural cardiac defects, abnormal left-right axis, hepatorenal and pancreatic cysts, and embryonic lethality. The molecular mechanisms by which loss of PC2 leads to these phenotypes remain unknown. We generated a model to allow targeted Pkd2 inactivation using the Cre-loxP system. Global inactivation of Pkd2 produced a phenotype identical to Pkd2(-/-) mice with undetectable PC2 protein and perinatal lethality. Using various Cre mouse lines, we found that kidney, pancreas, or time-specific deletion of Pkd2 led to cyst formation. In addition, we developed an immortalized renal collecting duct cell line with inactive Pkd2; these cells had aberrant cell-cell contact, ciliogenesis, and tubulomorphogenesis. They also significantly upregulated beta-catenin, axin2, and cMyc. Our results suggest that loss of PC2 disrupts normal behavior of renal epithelial cells through dysregulation of beta-catenin-dependent signaling, revealing a potential role for this signaling pathway in PC2-associated ADPKD.

FEZ1 interacts with CLASP2 and NEK1 through coiled-coil regions and their cellular colocalization suggests centrosomal functions and regulation by PKC

FEZ1 was initially described as a neuronal protein that influences axonal development and cell polarization. CLASP2 and NEK1 proteins are present in a centrosomal complex and participate in cell cycle and cell division mechanisms, but their functions were always described individually. Here, we report that NEK1 and CLASP2 colocalize with FEZ1 in a perinuclear region in mammalian cells, and observed that coiled-coil interactions occur between FEZ1/CLASP2 and FEZ1/NEK1 in vitro. These three proteins colocalize and interact with endogenous gamma-tubulin. Furthermore, we found that CLASP2 is phosphorylated and interacts with active PKC isoforms, and that FEZ1/CLASP2 colocalization is inhibited by PMA treatment. Our results provide evidence that these three proteins cooperate in centrosomal functions and open new directions for future studies.

Arrestin development: emerging roles for beta-arrestins in developmental signaling pathways

Arrestins were identified as mediators of G protein-coupled receptor (GPCR) desensitization and endocytosis. However, it is now clear that they scaffold many intracellular signaling networks to modulate the strength and duration of signaling by diverse types of receptors--including those relevant to the Hedgehog, Wnt, Notch, and TGFbeta pathways--and downstream kinases such as the MAPK and Akt/PI3K cascades. The involvement of arrestins in many discrete developmental signaling events suggests an indispensable role for these multifaceted molecular scaffolds.

Planar cell polarity and cilia

In the last few years, evidence has come to light suggesting that planar cell polarity signaling in vertebrates may be controlled and modulated by primary cilia, subcellular organelles that emerge from the plasma membrane of most cell types. This characteristic distinguishes vertebrate planar cell polarity signaling from that in insects. We review here some of the experimental evidence contributing to this finding. These observations have begun to suggest molecular and cellular mechanisms of the so-called ciliopathies, important human diseases characterized by defective ciliary functions.

The primary cilium as a complex signaling center

Respect for the primary cilium has undergone a remarkable renaissance over the past decade, and it is now thought to be an essential regulator of numerous signaling pathways. The primary cilium's functions range from the movement of cells and fluid, to sensory inputs involved with olfaction and photoreception. Disruption of cilia function is involved in multiple human syndromes collectively called 'ciliopathies'. The cilium's activities are mediated by targeting of receptors, channels, and their downstream effector proteins to the ciliary or basal body compartment. These combined properties of the cilium make it a critical organelle facilitating the interactions between the cell and its environment. Here, we review many of the recent advances contributing to the ascendancy of the primary cilium and how the extraordinary complexity of this organelle inevitably assures many more exciting future discoveries.

Kinesin superfamily motor proteins and intracellular transport

Intracellular transport is fundamental for cellular function, survival and morphogenesis. Kinesin superfamily proteins (also known as KIFs) are important molecular motors that directionally transport various cargos, including membranous organelles, protein complexes and mRNAs. The mechanisms by which different kinesins recognize and bind to specific cargos, as well as how kinesins unload cargo and determine the direction of transport, have now been identified. Furthermore, recent molecular genetic experiments have uncovered important and unexpected roles for kinesins in the regulation of such physiological processes as higher brain function, tumour suppression and developmental patterning. These findings open exciting new areas of kinesin research.

Aberrant planar cell polarity induced by urinary tract obstruction

Flow sensing by primary cilia of the epithelial cells is involved in cystogenesis in polycystic kidney disease. We investigate whether a similar mechanism applies to the pathogenesis of cyst-like tubular dilatation induced by ureteral obstruction in mice. Robust proliferation occurs in the obstructed tubules when urine flow is interrupted as well as in the repairing tubules when urine flow is reestablished after relief of the obstruction, suggesting a urine flow-independent mechanism of proliferation. In the urothelium, proliferation is only detected above the obstruction, although urine flow ceased both above and below the obstruction. Our results support mechanical strain- rather than flow-mediated proliferation in obstructive uropathy. To understand the mechanism of cell proliferation leading to increased tubular diameter in cyst-like tubular dilatation, we examine planar cell polarity (PCP), which is necessary for oriented cell division and maintenance of tubular diameter. In dilated tubules, the orientation of cell division is randomized, atypical PKC (aPKC) is mislocalized, and the pattern of the expression of a core PCP protein, Frizzled3 (Fz3), is altered. In addition, the level of Fz3 expression is increased. These results indicate that aberrant PCP may contribute to cyst-like tubular dilatation in obstructive uropathy. Interestingly, the orientation of cell division, localization of aPKC, and Fz3 expression return to normal when obstruction is relieved, which suggest a role of normal PCP signaling in tubular repair.

The kidney and ear: emerging parallel functions

The association between renal dysplasia and minor malformations of the external ear is weak. However, there is a remarkable list of syndromes that link the kidney to the inner ear. To organize these seemingly disparate syndromes, we cluster representative examples into three groups: (a) syndromes that share pathways regulating development; (b) syndromes involving dysfunction of the primary cilium, which normally provides critical information to epithelial cells about the fluid in which they are bathed; (c) syndromes arising from dysfunction of specialized proteins that transport ions and drugs in and out of the extracellular fluid or provide structural support.

INPP5E mutations cause primary cilium signaling defects, ciliary instability and ciliopathies in human and mouse

The primary cilium is an antenna-like structure that protrudes from the cell surface of quiescent/differentiated cells and participates in extracellular signal processing. Here, we report that mice deficient for the lipid 5-phosphatase Inpp5e develop a multiorgan disorder associated with structural defects of the primary cilium. In ciliated mouse embryonic fibroblasts, Inpp5e is concentrated in the axoneme of the primary cilium. Inpp5e inactivation did not impair ciliary assembly but altered the stability of pre-established cilia after serum addition. Blocking phosphoinositide 3-kinase (PI3K) activity or ciliary platelet-derived growth factor receptor alpha (PDGFRalpha) restored ciliary stability. In human INPP5E, we identified a mutation affecting INPP5E ciliary localization and cilium stability in a family with MORM syndrome, a condition related to Bardet-Biedl syndrome. Together, our results show that INPP5E plays an essential role in the primary cilium by controlling ciliary growth factor and PI3K signaling and stability, and highlight the consequences of INPP5E dysfunction.

Ahi1, whose human ortholog is mutated in Joubert syndrome, is required for Rab8a localization, ciliogenesis and vesicle trafficking

The primary non-motile cilium, a membrane-ensheathed, microtubule-bundled organelle, extends from virtually all cells and is important for development. Normal functioning of the cilium requires proper axoneme assembly, membrane biogenesis and ciliary protein localization, in tight coordination with the intraflagellar transport system and vesicular trafficking. Disruptions at any level can induce severe alterations in cell function, giving rise to a myriad of human genetic diseases known as ciliopathies. Here we show that the Abelson helper integration site 1 (Ahi1) gene, whose human ortholog is mutated in Joubert syndrome, regulates cilium formation via its interaction with Rab8a, a small GTPase critical for polarized membrane trafficking. We find that the Ahi1 protein localizes to a single centriole, the mother centriole, which becomes the basal body of the primary cilium. In order to determine whether Ahi1 functions in ciliogenesis, loss of function analysis of Ahi1 was performed in cell culture models of ciliogenesis. Knockdown of Ahi1 expression by shRNAi in cells or targeted deletion of Ahi1 (Ahi1 knockout mouse) leads to impairments in ciliogenesis. In Ahi1-knockdown cells, Rab8a is destabilized and does not properly localize to the basal body. Since Rab8a is implicated in vesicular trafficking, we next examined this process in Ahi1-knockdown cells. Defects in the trafficking of endocytic vesicles from the plasma membrane to the Golgi and back to the plasma membrane were observed in Ahi1-knockdown cells. Overall, our data indicate that the distribution and functioning of Rab8a is regulated by Ahi1, not only affecting cilium formation, but also vesicle transport.

Roles of the primary cilium component Polaris in synchondrosis development

Primary cilia regulate several developmental processes and mediate hedgehog signaling. To study their roles in cranial base development, we created conditional mouse mutants deficient in Polaris, a critical primary cilium component, in cartilage. Mutant post-natal cranial bases were deformed, and their synchondrosis growth plates were disorganized. Expression of Indian hedgehog, Patched-1, collagen X, and MMP-13 was reduced and accompanied by decreases in endochondral bone. Interestingly, there was excessive intramembranous ossification along the perichondrium, accompanied by excessive Patched-1 expression, suggesting that Ihh distribution was wider and responsible for such excessive response. Indeed, expression of heparan sulfate proteoglycans (HS-PGs), normally involved in restricting hedgehog distribution, was barely detectable in mutant synchondroses. Analyses of the data provides further evidence for the essential roles of primary cilia and hedgehog signaling in cranial base development and chondrocyte maturation, and point to a close interdependence between cilia and HS-PGs to delimit targets of hedgehog action in synchondroses.

Polycystic kidney disease in the medaka (Oryzias latipes) pc mutant caused by a mutation in the Gli-Similar3 (glis3) gene

Polycystic kidney disease (PKD) is a common hereditary disease in humans. Recent studies have shown an increasing number of ciliary genes that are involved in the pathogenesis of PKD. In this study, the Gli-similar3 (glis3) gene was identified as the causal gene of the medaka pc mutant, a model of PKD. In the pc mutant, a transposon was found to be inserted into the fourth intron of the pc/glis3 gene, causing aberrant splicing of the pc/glis3 mRNA and thus a putatively truncated protein with a defective zinc finger domain. pc/glis3 mRNA is expressed in the epithelial cells of the renal tubules and ducts of the pronephros and mesonephros, and also in the pancreas. Antisense oligonucleotide-mediated knockdown of pc/glis3 resulted in cyst formation in the pronephric tubules of medaka fry. Although three other glis family members, glis1a, glis1b and glis2, were found in the medaka genome, none were expressed in the embryonic or larval kidney. In the pc mutant, the urine flow rate in the pronephros was significantly reduced, which was considered to be a direct cause of renal cyst formation. The cilia on the surface of the renal tubular epithelium were significantly shorter in the pc mutant than in wild-type, suggesting that shortened cilia resulted in a decrease in driving force and, in turn, a reduction in urine flow rate. Most importantly, EGFP-tagged pc/glis3 protein localized in primary cilia as well as in the nucleus when expressed in mouse renal epithelial cells, indicating a strong connection between pc/glis3 and ciliary function. Unlike human patients with GLIS3 mutations, the medaka pc mutant shows none of the symptoms of a pancreatic phenotype, such as impaired insulin expression and/or diabetes, suggesting that the pc mutant may be suitable for use as a kidney-specific model for human GLIS3 patients.

Advances in the pathogenesis and treatment of polycystic kidney disease

PURPOSE OF REVIEW

Polycystic kidney disease (PKD) is the most common genetic cause of chronic renal failure. Mouse models of PKD, especially those with mutations in genes that are orthologous to human disease genes, have provided insights into the pathogenesis of cyst formation and advanced the preclinical testing of new drugs.

RECENT FINDINGS

PKD is a ciliopathy that arises from abnormalities in the primary cilium, a sensory organelle present on the surface of most cells. The primary cilium is required for the maintenance of planar cell polarity, which regulates tubular diameter. Acute kidney injury stimulates cell proliferation and promotes cyst formation in a mouse model of PKD. Studies of signaling pathways that are perturbed in PKD have identified new potential therapeutic targets. Drugs that have shown beneficial effects in orthologous animal models of PKD include tolvaptan, octreotide, src inhibitors, CFTR inhibitors, pioglitazone, etanercept, and triptolide.

SUMMARY

Abnormalities in the primary cilium perturb signaling pathways that regulate renal epithelial cell growth and differentiation and lead to the formation of kidney cysts. Acute kidney injury promotes cyst formation and may underlie the variability in disease progression that is observed in affected individuals. Several promising new therapeutic agents that have been validated in orthologous animal models have entered clinical trials in humans.

Wnt9b signaling regulates planar cell polarity and kidney tubule morphogenesis

Although many vertebrate organs, such as kidneys, lungs and liver, are composed of epithelial tubules, little is known of the mechanisms that establish the length or diameter of these tubules. In the kidney, defects in the establishment and/or maintenance of tubule diameter are associated with one of the most common inherited human disorders, polycystic kidney disease. Here, we show that attenuation of Wnt9b signaling during kidney morphogenesis affects the planar cell polarity of the epithelium and leads to tubules with significantly increased diameter. Although previous studies showed that polarized cell divisions maintain the diameter of postnatal kidney tubules, we find cell divisions are randomly oriented during embryonic development. Our data suggest that diameter is established during early morphogenetic stages by convergent extension processes and maintained by polarized cell divisions. Wnt9b, signaling through the non-canonical Rho/Jnk branch of the Wnt pathway, is necessary for both of these processes.

Ciliary and centrosomal defects associated with mutation and depletion of the Meckel syndrome genes MKS1 and MKS3

Meckel syndrome (MKS) is a lethal disorder characterized by renal cystic dysplasia, encephalocele, polydactyly and biliary dysgenesis. It is highly genetically heterogeneous with nine different genes implicated in this disorder. MKS is thought to be a ciliopathy because of the range of phenotypes and localization of some of the implicated proteins. However, limited data are available about the phenotypes associated with MKS1 and MKS3, and the published ciliary data are conflicting. Analysis of the wpk rat model of MKS3 revealed functional defects of the connecting cilium in the eye that resulted in lack of formation of the outer segment, whereas infertile wpk males developed spermatids with very short flagella that did not extend beyond the cell body. In wpk renal collecting duct cysts, cilia were generally longer than normal, with additional evidence of cells with multiple primary cilia and centrosome over-duplication. Kidney tissue and cells from MKS1 and MKS3 patients showed defects in centrosome and cilia number, including multi-ciliated respiratory-like epithelia, and longer cilia. Stable shRNA knockdown of Mks1 and Mks3 in IMCD3 cells induced multi-ciliated and multi-centrosomal phenotypes. These studies demonstrate that MKS1 and MKS3 are ciliopathies, with new cilia-related eye and sperm phenotypes defined. MKS1 and MKS3 functions are required for ciliary structure and function, including a role in regulating length and appropriate number through modulating centrosome duplication.

Death by committee: organellar trafficking and communication in apoptosis

Apoptosis proceeds through a set of evolutionarily conserved processes that co-ordinate the elimination of damaged or unneeded cells. This program of cell death is carried out by organelle-directed regulators, including the Bcl-2 proteins, and ultimately executed by proteases of the caspase family. Although the biochemical mechanisms of apoptosis are increasingly understood, the underlying cell biology orchestrating programmed cell death remains enigmatic. In this review, we summarize the current understanding of Bcl-2 protein regulation and caspase activation while examining cell biological mechanisms and consequences of apoptotic induction. Organellar contributions to apoptotic induction include death receptor endocytosis, mitochondrial and lysosomal permeabilization, endoplasmic reticulum calcium release and fragmentation of the Golgi apparatus. These early apoptotic events are accompanied by stabilization of the microtubule cytoskeleton and translocation of organelles to the microtubule organizing center. Together, these phenomena establish a model of apoptotic induction whereby a cytoskeletal-dependent coalescence and 'scrambling' of organelles in the paranuclear region co-ordinates apoptotic communication, caspase activation and cell death.

The primary cilium as a cellular signaling center: lessons from disease

Genetic diseases known as ciliopathies have recently entered the limelight, placing new importance on a previously mysterious organelle: the primary cilium. Mutations affecting the primary cilium in both humans and animal models can lead to a plethora of distinct phenotypes including retinal degeneration, kidney cysts, and brain malformations. New findings are quickly lending insight into the functions of this cellular extension that seems to be especially important in modulation of subcellular signaling cascades at various stages of development and adult homeostasis.

Olfactory cilia: linking sensory cilia function and human disease

The olfactory system gives us an awareness of our immediate environment by allowing us to detect airborne stimuli. The components necessary for detection of these odorants are compartmentalized in the cilia of olfactory sensory neurons. Cilia are microtubule-based organelles, which can be found projecting from the surface of almost any mammalian cell, and are critical for proper olfactory function. Mislocalization of ciliary proteins and/or the loss of cilia cause impaired olfactory function, which is now recognized as a clinical manifestation of a broad class of human diseases, termed ciliopathies. Future work investigating the mechanisms of olfactory cilia function will provide us important new information regarding the pathogenesis of human sensory perception diseases.

The vertebrate primary cilium in development, homeostasis, and disease

Cilia are complex structures that have garnered interest because of their roles in vertebrate development and their involvement in human genetic disorders. In contrast to multicellular invertebrates in which cilia are restricted to specific cell types, these organelles are found almost ubiquitously in vertebrate cells, where they serve a diverse set of signaling functions. Here, we highlight properties of vertebrate cilia, with particular emphasis on their relationship with other subcellular structures, and explore the physiological consequences of ciliary dysfunction.