Crk-associated substrate p130(Cas) interacts with nephrocystin and both proteins localize to cell-cell contacts of polarized epithelial cells

Transport and barrier mechanisms that regulate ciliary compartmentalization and ciliopathies

Primary cilia act as cell surface antennae, coordinating cellular responses to sensory inputs and signalling molecules that regulate developmental and homeostatic pathways. Cilia are therefore critical to physiological processes, and defects in ciliary components are associated with a large group of inherited pleiotropic disorders - known collectively as ciliopathies - that have a broad spectrum of phenotypes and affect many or most tissues, including the kidney. A central feature of the cilium is its compartmentalized structure, which imparts its unique molecular composition and signalling environment despite its membrane and cytosol being contiguous with those of the cell. Such compartmentalization is achieved via active transport pathways that bring protein cargoes to and from the cilium, as well as gating pathways at the ciliary base that establish diffusion barriers to protein exchange into and out of the organelle. Many ciliopathy-linked proteins, including those involved in kidney development and homeostasis, are components of the compartmentalizing machinery. New insights into the major compartmentalizing pathways at the cilium, namely, ciliary gating, intraflagellar transport, lipidated protein flagellar transport and ciliary extracellular vesicle release pathways, have improved our understanding of the mechanisms that underpin ciliary disease and associated renal disorders.

Fluid shear stress triggers cholesterol biosynthesis and uptake in inner medullary collecting duct cells, independently of nephrocystin-1 and nephrocystin-4

Renal epithelial cells are subjected to fluid shear stress of urine flow. Several cellular structures act as mechanosensors-the primary cilium, microvilli and cell adhesion complexes-that directly relay signals to the cytoskeleton to regulate various processes including cell differentiation and renal cell functions. Nephronophthisis (NPH) is an autosomal recessive tubulointerstitial nephropathy leading to end-stage kidney failure before adulthood. NPHP1 and NPHP4 are the major genes which code for proteins that form a complex at the transition zone of the primary cilium, a crucial region required for the maintenance of the ciliary composition integrity. These two proteins also interact with signaling components and proteins associated with the actin cytoskeleton at cell junctions. Due to their specific subcellular localization, we wondered whether NPHP1 and NPHP4 could ensure mechanosensory functions. Using a microfluidic set up, we showed that murine inner medullary collecting ductal cells invalidated for Nphp1 or Nphp4 are more responsive to immediate shear exposure with a fast calcium influx, and upon a prolonged shear condition, an inability to properly regulate cilium length and actin cytoskeleton remodeling. Following a transcriptomic study highlighting shear stress-induced gene expression changes, we showed that prolonged shear triggers both cholesterol biosynthesis pathway and uptake, processes that do not seem to involve neither NPHP1 nor NPHP4. To conclude, our study allowed us to determine a moderate role of NPHP1 and NPHP4 in flow sensation, and to highlight a new signaling pathway induced by shear stress, the cholesterol biosynthesis and uptake pathways, which would allow cells to cope with mechanical stress by strengthening their plasma membrane through the supply of cholesterol.

Nephronophthisis-Pathobiology and Molecular Pathogenesis of a Rare Kidney Genetic Disease

The exponential rise in our understanding of the aetiology and pathophysiology of genetic cystic kidney diseases can be attributed to the identification of cystogenic genes over the last three decades. The foundation of this was laid by positional cloning strategies which gradually shifted towards next-generation sequencing (NGS) based screenings. This shift has enabled the discovery of novel cystogenic genes at an accelerated pace unlike ever before and, most notably, the past decade has seen the largest increase in identification of the genes which cause nephronophthisis (NPHP). NPHP is a monogenic autosomal recessive cystic kidney disease caused by mutations in a diverse clade of over 26 identified genes and is the most common genetic cause of renal failure in children. NPHP gene types present with some common pathophysiological features alongside a diverse range of extra-renal phenotypes associated with specific syndromic presentations. This review provides a timely update on our knowledge of this disease, including epidemiology, pathophysiology, anatomical and molecular features. We delve into the diversity of the NPHP causing genes and discuss known molecular mechanisms and biochemical pathways that may have possible points of intersection with polycystic kidney disease (the most studied renal cystic pathology). We delineate the pathologies arising from extra-renal complications and co-morbidities and their impact on quality of life. Finally, we discuss the current diagnostic and therapeutic modalities available for disease management, outlining possible avenues of research to improve the prognosis for NPHP patients.

The unique function of p130Cas in regulating the bone metabolism

p130 Crk-associated substrate (Cas) functions as an adapter protein and plays important roles in certain cell functions, such as cell proliferation, spreading, migration, and invasion. Furthermore, it has recently been reported to have a new function as a mechanosensor. Since bone is a tissue that is constantly under gravity, it is exposed to mechanical stress. Mechanical unloading, such as in a microgravity environment in space or during bed rest, leads to a decrease in bone mass because of the suppression of bone formation and the stimulation of bone resorption. Osteoclasts are multinucleated bone-resorbing giant cells that recognize bone and then form a ruffled border in the resorption lacuna. p130Cas is a molecule located downstream of c-Src that is important for the formation of a ruffled border in osteoclasts. Indeed, osteoclast-specific p130Cas-deficient mice exhibit osteopetrosis due to osteoclast dysfunction, similar to c-Src-deficient mice. Osteoblasts subjected to mechanical stress induce both the phosphorylation of p130Cas and osteoblast differentiation. In osteocytes, mechanical stress regulates bone mass by shuttling p130Cas between the cytoplasm and nucleus. Oral squamous cell carcinoma (OSCC) cells express p130Cas more strongly than epithelial cells in normal tissues. The knockdown of p130Cas in OSCC cells suppressed the cell growth, the expression of receptor activator of NF-κB ligand, which induces osteoclast formation, and bone invasion by OSCC. Taken together, these findings suggest that p130Cas might be a novel therapeutic target molecule in bone diseases, such as osteoporosis, rheumatoid arthritis, bone loss due to bed rest, and bone invasion and metastasis of cancer.

The N-Terminal Domain of NPHP1 Folds into a Monomeric Left-Handed Antiparallel Three-Stranded Coiled Coil with Anti-apoptotic Function

Mutations in the NPHP1 gene, coding for human nephrocystin-1 (NPHP1), cause the autosomal recessive disease nephronophthisis, the most common cause of end-stage renal disease in children and adolescents. The function and structure of NPHP1 are still poorly characterized. NPHP1 presents a modular structure well in keeping with its role as an adaptor protein: it harbors an SH3 domain flanked by two glutamic acid-rich regions and a conserved C-terminal nephrocystin homology domain (NHD). Similar to other NPHP protein family members, its N-terminus contains a putative coiled-coil domain (NPHP1_CC) that is supposed to play an important role in NPHP1 self-association and/or protein-protein interactions. Structural studies proving its structure and its oligomerization state are still lacking. Here we demonstrate that NPHP1_CC is monomeric in solution and unexpectedly folds into an autonomous domain forming a three-stranded antiparallel coiled coil suitable for protein-protein interactions. Notably, we found that the NPHP1_CC shares remarkable structural similarities with the three-stranded coiled coil of the BAG domain protein family, which is known to mediate the anti-apoptotic function of these proteins, suggesting a possible similar role for NPHP1_CC. In agreement with this hypothesis, we show that in the context of the full-length protein the NPHP1_CC is fundamental to regulate resistance to apoptotic stimuli.

Loss of Tctn3 causes neuronal apoptosis and neural tube defects in mice

Tctn3 belongs to the Tectonic (Tctn) family and is a single-pass membrane protein localized at the transition zone of primary cilia as an important component of ciliopathy-related protein complexes. Previous studies showed that mutations in Tctn1 and Tctn2, two members of the tectonic family, have been reported to disrupt neural tube development in humans and mice, but the functions of Tctn3 in brain development remain elusive. In this study, Tctn3 knockout (KO) mice were generated by utilizing the piggyBac (PB) transposon system. We found that Tctn3 KO mice exhibited abnormal global development, including prenatal lethality, microphthalmia, polysyndactyly, and abnormal head, sternum, and neural tube, whereas Tctn3 heterozygous KO mice did not show abnormal development or behaviors. Further, we found that the mRNA levels of Gli1 and Ptch1, downstream signaling components of the Shh pathway, were significantly reduced. Likewise, neural tube patterning-related proteins, such as Shh, Foxa2, and Nkx2.2, were altered in their distribution. Interestingly, Tctn3 KO led to significant changes in apoptosis-related proteins, including Bcl-2, Bax, and cleaved PARP1, resulting in reduced numbers of neuronal cells in embryonic brains. Tctn3 KO inhibited the PI3K/Akt signaling pathway but not the mTOR-dependent pathway. The small molecule SC79, a specific Akt activator, blocked apoptotic cell death in primary mouse embryonic fibroblasts from Tctn3 KO mice. Finally, NPHP1, a protein with anti-apoptotic ability, was found to form a complex with Tctn3, and its levels were decreased in Tctn3 KO mice. In conclusion, our results show that Tctn3 KO disrupts the Shh signaling pathway and neural tube patterning, resulting in abnormal embryonic development, cellular apoptosis, and prenatal death in mice.

Structural characterization of CAS SH3 domain selectivity and regulation reveals new CAS interaction partners

CAS is a docking protein downstream of the proto-oncogene Src with a role in invasion and metastasis of cancer cells. The CAS SH3 domain is indispensable for CAS-mediated signaling, but structural aspects of CAS SH3 ligand binding and regulation are not well understood. Here, we identified the consensus CAS SH3 binding motif and structurally characterized the CAS SH3 domain in complex with ligand. We revealed the requirement for an uncommon centrally localized lysine residue at position +2 of CAS SH3 ligands and two rather dissimilar optional anchoring residues, leucine and arginine, at position +5. We further expanded the knowledge of CAS SH3 ligand binding regulation by manipulating tyrosine 12 phosphorylation and confirmed the negative role of this phosphorylation on CAS SH3 ligand binding. Finally, by exploiting the newly identified binding requirements of the CAS SH3 domain, we predicted and experimentally verified two novel CAS SH3 binding partners, DOK7 and GLIS2.

Many Genes-One Disease? Genetics of Nephronophthisis (NPHP) and NPHP-Associated Disorders

Nephronophthisis (NPHP) is a renal ciliopathy and an autosomal recessive cause of cystic kidney disease, renal fibrosis, and end-stage renal failure, affecting children and young adults. Molecular genetic studies have identified more than 20 genes underlying this disorder, whose protein products are all related to cilia, centrosome, or mitotic spindle function. In around 15% of cases, there are additional features of a ciliopathy syndrome, including retinal defects, liver fibrosis, skeletal abnormalities, and brain developmental disorders. Alongside, gene identification has arisen molecular mechanistic insights into the disease pathogenesis. The genetic causes of NPHP are discussed in terms of how they help us to define treatable disease pathways including the cyclic adenosine monophosphate pathway, the mTOR pathway, Hedgehog signaling pathways, and DNA damage response pathways. While the underlying pathology of the many types of NPHP remains similar, the defined disease mechanisms are diverse, and a personalized medicine approach for therapy in NPHP patients is likely to be required.

Systematic Analysis of Protein Interaction Network Associated with Azoospermia

Non-obstructive azoospermia is a severe infertility factor. Currently, the etiology of this condition remains elusive with several possible molecular pathway disruptions identified in the post-meiotic spermatozoa. In the presented study, in order to identify all possible candidate genes associated with azoospermia and to map their relationship, we present the first protein-protein interaction network related to azoospermia and analyze the complex effects of the related genes systematically. Using Online Mendelian Inheritance in Man, the Human Protein Reference Database and Cytoscape, we created a novel network consisting of 209 protein nodes and 737 interactions. Mathematical analysis identified three proteins, ar, dazap2, and esr1, as hub nodes and a bottleneck protein within the network. We also identified new candidate genes, CREBBP and BCAR1, which may play a role in azoospermia. The gene ontology analysis suggests a genetic link between azoospermia and liver disease. The KEGG analysis also showed 45 statistically important pathways with 31 proteins associated with colorectal, pancreatic, chronic myeloid leukemia and prostate cancer. Two new genes and associated diseases are promising for further experimental validation.

CSPP-L Associates with the Desmosome of Polarized Epithelial Cells and Is Required for Normal Spheroid Formation

Deleterious mutations of the Centrosome/Spindle Pole associated Protein 1 gene, CSPP1, are causative for Joubert-syndrome and Joubert-related developmental disorders. These disorders are defined by a characteristic mal-development of the brain, but frequently involve renal and hepatic cyst formation. CSPP-L, the large protein isoform of CSPP1 localizes to microtubule ends of the mitotic mid-spindle and the ciliary axoneme, and is required for ciliogenesis. We here report the microtubule independent but Desmoplakin dependent localization of CSPP-L to Desmosomes in apical-basal polarized epithelial cells. Importantly, siRNA conferred depletion of CSPP-L or Desmoplakin promoted multi-lumen spheroid formation in 3D-cultures of non-ciliated human colon carcinoma Caco-2 cells. Multi-lumen spheroids of CSPP1 siRNA transfectants showed disrupted apical cell junction localization of the cytoskeleton organizing RhoGEF ECT2. Our results hence identify a novel, non-ciliary role for CSPP-L in epithelial morphogenesis.

Ciliopathy proteins establish a bipartite signaling compartment in a C. elegans thermosensory neuron

How signaling domains form is an important, yet largely unexplored question. Here, we show that ciliary proteins help establish two contiguous, yet distinct cyclic GMP (cGMP) signaling compartments in Caenorhabditis elegans thermosensory AFD neurons. One compartment, a bona fide cilium, is delineated by proteins associated with Bardet-Biedl syndrome (BBS), Meckel syndrome and nephronophthisis at its base, and requires NPHP-2 (known as inversin in mammals) to anchor a cGMP-gated ion channel within the proximal ciliary region. The other, a subcompartment with profuse microvilli and a different lipid environment, is separated from the dendrite by a cellular junction and requires BBS-8 and DAF-25 (known as Ankmy2 in mammals) for correct localization of guanylyl cyclases needed for thermosensation. Consistent with a requirement for a membrane diffusion barrier at the subcompartment base, we reveal the unexpected presence of ciliary transition zone proteins where no canonical transition zone ultrastructure exists. We propose that differential compartmentalization of signal transduction components by ciliary proteins is important for the functions of ciliated sensory neurons.

Src-mediated caveolin-1 phosphorylation affects the targeting of active Src to specific membrane sites

Src interactions with the plasma membrane are an important determinant of its activity. In turn, Src activity modulates its association with the membrane through binding of activated Src to phosphotyrosylated proteins. Caveolin-1 (Cav-1), a major component of caveolae, is a known Src phosphorylation target, and both were reported to regulate cell transformation. However, the nature of Src-Cav-1 interactions, a potential mechanism of their coregulation, remained unclear. Here we used fluorescence recovery after photobleaching beam-size analysis, coimmunoprecipitation, quantitative imaging, and far-Western studies with cells expressing wild type, as well as structural and activity mutants of Src-green fluorescent protein and Cav-1-monomeric red fluorescent protein, to measure their interactions with the membrane and with each other. We show dynamic Src-plasma membrane interactions, which are augmented and stabilized by Cav-1. The mechanism involves phosphorylation of Cav-1 at Tyr-14 by Src and subsequent binding of the Src SH2 domain to phospho-Cav-1, leading to accumulation of activated Src in focal adhesions. This novel Cav-1 function potentially modulates focal adhesion dynamics.

Cell polarity and cystic kidney disease

Epithelial cell polarity is essential for organ development; aberrations in this process have been implicated in various diseases, including polycystic kidney disease. Establishment and maintenance of cell polarity is governed by a number of molecular processes and how these processes operate remains an interesting question. Conserved protein complexes guide both apical-basolateral polarity and planar cell polarity. In this review we discuss the recent findings that provide insights into polarity mechanisms and the intriguing crosstalk between apical-basolateral polarity and planar cell polarity, and their relationship to cystic kidney disease.

Pooling/bootstrap-based GWAS (pbGWAS) identifies new loci modifying the age of onset in PSEN1 p.Glu280Ala Alzheimer's disease

The literature on GWAS (genome-wide association studies) data suggests that very large sample sizes (for example, 50,000 cases and 50,000 controls) may be required to detect significant associations of genomic regions for complex disorders such as Alzheimer's disease (AD). Because of the challenges of obtaining such large cohorts, we describe here a novel sequential strategy that combines pooling of DNA and bootstrapping (pbGWAS) in order to significantly increase the statistical power and exponentially reduce expenses. We applied this method to a very homogeneous sample of patients belonging to a unique and clinically well-characterized multigenerational pedigree with one of the most severe forms of early onset AD, carrying the PSEN1 p.Glu280Ala mutation (often referred to as E280A mutation), which originated as a consequence of a founder effect. In this cohort, we identified novel loci genome-wide significantly associated as modifiers of the age of onset of AD (CD44, rs187116, P=1.29 × 10⁻¹²; NPHP1, rs10173717, P=1.74 × 10⁻¹²; CADPS2, rs3757536, P=1.54 × 10⁻¹⁰; GREM2, rs12129547, P=1.69 × 10⁻¹³, among others) as well as other loci known to be associated with AD. Regions identified by pbGWAS were confirmed by subsequent individual genotyping. The pbGWAS methodology and the genes it targeted could provide important insights in determining the genetic causes of AD and other complex conditions.

p130Cas: a key signalling node in health and disease

p130Cas/breast cancer anti-oestrogen resistance 1 (BCAR1) is a member of the Cas (Crk-associated substrate) family of adaptor proteins, which have emerged as key signalling nodes capable of interactions with multiple proteins, with important regulatory roles in normal and pathological cell function. The Cas family of proteins is characterised by the presence of multiple conserved motifs for protein-protein interactions, and by extensive tyrosine and serine phosphorylations. Recent studies show that p130Cas contributes to migration, cell cycle control and apoptosis. p130Cas is essential during early embryogenesis, with a critical role in cardiovascular development. Furthermore, p130Cas has been reported to be involved in the development and progression of several human cancers. p130Cas is able to perform roles in multiple processes due to its capacity to regulate a diverse array of signalling pathways, transducing signals from growth factor receptor tyrosine kinases, non-receptor tyrosine kinases, and integrins. In this review we summarise the current understanding of the structure, function, and regulation of p130Cas, and discuss the importance of p130Cas in both physiological and pathophysiological settings, with a focus on the cardiovascular system and cancer.

The extracellular matrix and ciliary signaling

The primary cilium protrudes like an antenna from the cell surface, sensing mechanical and chemical cues provided in the cellular environment. In some tissue types, ciliary orientation to lumens allows response to fluid flow; in others, such as bone, ciliary protrusion into the extracellular matrix allows response to compression forces. The ciliary membrane contains receptors for Hedgehog, Wnt, Notch, and other potent growth factors, and in some instances also harbors integrin and cadherin family members, allowing receipt of a robust range of signals. A growing list of ciliopathies, arising from deficient formation or function of cilia, includes both developmental defects and chronic, progressive disorders such as polycystic kidney disease (PKD); changes in ciliary function have been proposed to support cancer progression. Recent findings have revealed extensive signaling dialog between cilia and extracellular matrix (ECM), with defects in cilia associated with fibrosis in multiple contexts. Further, a growing number of proteins have been determined to possess multiple roles in control of cilia and focal adhesion interactions with the ECM, further coordinating functionality. We summarize and discuss these recent findings.

Crk1/2-dependent signaling is necessary for podocyte foot process spreading in mouse models of glomerular disease

The morphology of healthy podocyte foot processes is necessary for maintaining the characteristics of the kidney filtration barrier. In most forms of glomerular disease, abnormal filter barrier function results when podocytes undergo foot process spreading and retraction by remodeling their cytoskeletal architecture and intercellular junctions during a process known as effacement. The cell adhesion protein nephrin is necessary for establishing the morphology of the kidney podocyte in development by transducing from the specialized podocyte intercellular junction phosphorylation-mediated signals that regulate cytoskeletal dynamics. The present studies extend our understanding of nephrin function by showing that nephrin activation in cultured podocytes induced actin dynamics necessary for lamellipodial protrusion. This process required a PI3K-, Cas-, and Crk1/2-dependent signaling mechanism distinct from the previously described nephrin-Nck1/2 pathway necessary for assembly and polymerization of actin filaments. Our present findings also support the hypothesis that mechanisms governing lamellipodial protrusion in culture are similar to those used in vivo during foot process effacement in a subset of glomerular diseases. In mice, podocyte-specific deletion of Crk1/2 prevented foot process effacement in one model of podocyte injury and attenuated foot process effacement and associated proteinuria in a delayed fashion in a second model. In humans, focal adhesion kinase and Cas phosphorylation - markers of focal adhesion complex-mediated Crk-dependent signaling - was induced in minimal change disease and membranous nephropathy, but not focal segmental glomerulosclerosis. Together, these observations suggest that activation of a Cas-Crk1/2-dependent complex is necessary for foot process effacement observed in distinct subsets of human glomerular diseases.

Mapping the NPHP-JBTS-MKS protein network reveals ciliopathy disease genes and pathways

Nephronophthisis (NPHP), Joubert (JBTS), and Meckel-Gruber (MKS) syndromes are autosomal-recessive ciliopathies presenting with cystic kidneys, retinal degeneration, and cerebellar/neural tube malformation. Whether defects in kidney, retinal, or neural disease primarily involve ciliary, Hedgehog, or cell polarity pathways remains unclear. Using high-confidence proteomics, we identified 850 interactors copurifying with nine NPHP/JBTS/MKS proteins and discovered three connected modules: "NPHP1-4-8" functioning at the apical surface, "NPHP5-6" at centrosomes, and "MKS" linked to Hedgehog signaling. Assays for ciliogenesis and epithelial morphogenesis in 3D renal cultures link renal cystic disease to apical organization defects, whereas ciliary and Hedgehog pathway defects lead to retinal or neural deficits. Using 38 interactors as candidates, linkage and sequencing analysis of 250 patients identified ATXN10 and TCTN2 as new NPHP-JBTS genes, and our Tctn2 mouse knockout shows neural tube and Hedgehog signaling defects. Our study further illustrates the power of linking proteomic networks and human genetics to uncover critical disease pathways.

Nephrocystin-4 is required for pronephric duct-dependent cloaca formation in zebrafish

NPHP4 mutations cause nephronophthisis, an autosomal recessive cystic kidney disease associated with renal fibrosis and kidney failure. The NPHP4 gene product nephrocystin-4 interacts with other nephrocystins, cytoskeletal and ciliary proteins; however, the molecular and cellular functions of nephrocystin-4 have remained elusive. Here we demonstrate that nephrocystin-4 is required for normal cloaca formation during zebrafish embryogenesis. Time-lapse imaging of the developing zebrafish pronephros revealed that tubular epithelial cells at the distal pronephros actively migrate between the yolk sac extension and the blood island towards the ventral fin fold to join the proctodeum and to form the cloaca. Nphp4-deficient pronephric duct cells failed to connect with their ectodermal counterparts, and instead formed a vesicle at the obstructed end of the pronephric duct. Nephrocystin-4 interacts with nephrocystin-1 and Par6. Depletion of zebrafish NPHP1 (nphp1) increased the incidence of cyst formation and randomization of the normal body axis, but did not augment cloaca malformation in nphp4-deficient zebrafish embryos. However, simultaneous depletion of zebrafish Par6 (pard6) aggravated cloaca formation defects in nphp4-depleted embryos, suggesting that nphp4 orchestrates directed cell migration and cloaca formation through interaction with the Par protein complex.

Nephronophthisis: a genetically diverse ciliopathy

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease and a leading genetic cause of established renal failure (ERF) in children and young adults. Early presenting symptoms in children with NPHP include polyuria, nocturia, or secondary enuresis, pointing to a urinary concentrating defect. Renal ultrasound typically shows normal kidney size with increased echogenicity and corticomedullary cysts. Importantly, NPHP is associated with extra renal manifestations in 10-15% of patients. The most frequent extrarenal association is retinal degeneration, leading to blindness. Increasingly, molecular genetic testing is being utilised to diagnose NPHP and avoid the need for a renal biopsy. In this paper, we discuss the latest understanding in the molecular and cellular pathogenesis of NPHP. We suggest an appropriate clinical management plan and screening programme for individuals with NPHP and their families.

Nephronophthisis

Nephronophthisis (NPHP) is an autosomal recessive cystic kidney disease and the most frequent genetic cause of end-stage renal disease up to the third decade of life. It is caused by mutations in 11 different genes, denoted nephrocystins (NPHP1-11, NPHP1L). As an increasing number of these genes are identified, our knowledge of nephronophthisis is changing, thereby improving our understanding of the pathomechanisms in NPHP. Recent publications have described ciliary expression of nephrocystins together with other cystoproteins, such as polycystins 1 and 2 and fibrocystin. These findings have shifted our focus to a pathomechanism involving defects in ciliary function (ciliopathy) and planar cell polarity (PCP). In addition, discoveries of new nephrocystin genes have shown that the disease spectrum of NPHP is much broader than previously anticipated. Different forms of mutations within the same NPHP gene can cause different disease severity. In this review, we highlight the different hypotheses on the pathomechanisms for NPHP and underline the clinical variability of this disease. The clinical spectrum has become even more complex with the possibility of oligogenicity in NPHP.

Mechanisms of nephronophthisis and related ciliopathies

An emerging group of human genetic diseases termed 'ciliopathies' are caused by dysfunction of two functionally and physically associated organelles, the centrosome and cilium. These organelles are central to perception of the physical environment through detection of a diverse variety of extracellular signals such as growth factors, chemicals, light and fluid flow. Many of the described ciliopathies display multi-organ involvement, with renal and retina being the most commonly affected. Nephronophthisis is a recessive disorder of the kidney that is the leading cause of end-stage renal failure in children. Through positional cloning, many of the causative mutations have been mapped to genes involved in centrosome and cilia function. In this review, we discuss the identified causative mutations that give rise to nephronophthisis and how these are related to the disease etiology in both the kidney and other organs.

P130Cas Src-binding and substrate domains have distinct roles in sustaining focal adhesion disassembly and promoting cell migration

The docking protein p130Cas is a prominent Src substrate found in focal adhesions (FAs) and is implicated in regulating critical aspects of cell motility including FA disassembly and protrusion of the leading edge plasma membrane. To better understand how p130Cas acts to promote these events we examined requirements for established p130Cas signaling motifs including the SH3-binding site of the Src binding domain (SBD) and the tyrosine phosphorylation sites within the substrate domain (SD). Expression of wild type p130Cas in Cas −/− mouse embryo fibroblasts resulted in enhanced cell migration associated with increased leading-edge actin flux, increased rates of FA assembly/disassembly, and uninterrupted FA turnover. Variants lacking either the SD phosphorylation sites or the SBD SH3-binding motif were able to partially restore the migration response, while only a variant lacking both signaling functions was fully defective. Notably, the migration defects associated with p130Cas signaling-deficient variants correlated with longer FA lifetimes resulting from aborted FA disassembly attempts. However the SD mutational variant was fully defective in increasing actin assembly at the protruding leading edge and FA assembly/disassembly rates, indicating that SD phosphorylation is the sole p130Cas signaling function in regulating these processes. Our results provide the first quantitative evidence supporting roles for p130Cas SD tyrosine phosphorylation in promoting both leading edge actin flux and FA turnover during cell migration, while further revealing that the p130Cas SBD has a function in cell migration and sustained FA disassembly that is distinct from its known role of promoting SD tyrosine phosphorylation.

Nephrocystin-1 forms a complex with polycystin-1 via a polyproline motif/SH3 domain interaction and regulates the apoptotic response in mammals

Mutations in PKD1, the gene encoding for the receptor Polycystin-1 (PC-1), cause autosomal dominant polycystic kidney disease (ADPKD). The cytoplasmic C-terminus of PC-1 contains a coiled-coil domain that mediates an interaction with the PKD2 gene product, Polycystin-2 (PC-2). Here we identify a novel domain in the PC-1 C-terminal tail, a polyproline motif mediating an interaction with Src homology domain 3 (SH3). A screen for interactions using the PC-1 C-terminal tail identified the SH3 domain of nephrocystin-1 (NPHP1) as a potential binding partner of PC-1. NPHP1 is the product of a gene that is mutated in a different form of renal cystic disease, nephronophthisis (NPHP). We show that in vitro pull-down assays and NMR structural studies confirmed the interaction between the PC-1 polyproline motif and the NPHP1 SH3 domain. Furthermore, the two full-length proteins interact through these domains; using a recently generated model system allowing us to track endogenous PC-1, we confirm the interaction between the endogenous proteins. Finally, we show that NPHP1 trafficking to cilia does not require PC-1 and that PC-1 may require NPHP1 to regulate resistance to apoptosis, but not to regulate cell cycle progression. In line with this, we find high levels of apoptosis in renal specimens of NPHP patients. Our data uncover a link between two different ciliopathies, ADPKD and NPHP, supporting the notion that common pathogenetic defects, possibly involving de-regulated apoptosis, underlie renal cyst formation.

Cilia in cell signaling and human disorders

One of the most widespread cellular organelles in nature is cilium, which is found in many unicellular and multicellular organisms. Formerly thought to be a mostly vestigial organelle, the cilium has been discovered in the past several decades to play critical motile and sensory roles involved in normal organogenesis during development. The role of cilia has also been implicated in an ever increasing array of seemingly unrelated human diseases, including blindness, kidney cysts, neural tube defects and obesity. In this article we review some of the recent developments in research on cilia, and how defects in ciliogenesis and function can give rise to developmental disorders and disease.

Dcas supports cell polarization and cell-cell adhesion complexes in development

Mammalian Cas proteins regulate cell migration, division and survival, and are often deregulated in cancer. However, the presence of four paralogous Cas family members in mammals (BCAR1/p130Cas, EFS/Sin1, NEDD9/HEF1/Cas-L, and CASS4/HEPL) has limited their analysis in development. We deleted the single Drosophila Cas gene, Dcas, to probe the developmental function of Dcas. Loss of Dcas had limited effect on embryonal development. However, we found that Dcas is an important modulator of the severity of the developmental phenotypes of mutations affecting integrins (If and mew) and their downstream effectors Fak56D or Src42A. Strikingly, embryonic lethal Fak56D-Dcas double mutant embryos had extensive cell polarity defects, including mislocalization and reduced expression of E-cadherin. Further genetic analysis established that loss of Dcas modified the embryonal lethal phenotypes of embryos with mutations in E-cadherin (Shg) or its signaling partners p120- and beta-catenin (Arm). These results support an important role for Cas proteins in cell-cell adhesion signaling in development.

CSPP is a ciliary protein interacting with Nephrocystin 8 and required for cilia formation

We described previously the cell cycle- and microtubule-related functions of two splice isoforms of the centrosome spindle pole-associated protein (CSPP and CSPP-L). Here, we show that endogenous CSPP isoforms not only localize to centrosomes and the midbody in cycling cells but also extend to the cilia axoneme in postmitotic resting cells. They are required for ciliogenesis in hTERT-RPE1 cells in vitro and are expressed in ciliated renal, retinal, and respiratory cells in vivo. We report that CSPP isoforms require their common C-terminal domain to interact with Nephrocystin 8 (NPHP8/RPGRIP1L) and to form a ternary complex with NPHP8 and NPHP4. We find CSPP-L to be required for the efficient localization of NPHP8 but not NPHP4 to the basal body. The ciliogenesis defect in hTERT-RPE1 cells is, however, not mediated through loss of NPHP8. Similar to the effects of ectopical expression of CSPP-L, cilia length increased in NPHP8-depleted cells. Our results thus suggest that CSPP proteins may be involved in further cytoskeletal organization of the basal body and its primary cilium. To conclude, we have identified a novel, nonmitotic function of CSPP proteins placing them into a ciliary protein network crucial for normal renal and retinal tissue architecture and physiology.

CAS proteins in normal and pathological cell growth control

Proteins of the CAS (Crk-associated substrate) family (BCAR1/p130Cas, NEDD9/HEF1/Cas-L, EFS/SIN and CASS4/HEPL) are integral players in normal and pathological cell biology. CAS proteins act as scaffolds to regulate protein complexes controlling migration and chemotaxis, apoptosis, cell cycle, and differentiation, and have more recently been linked to a role in progenitor cell function. Reflecting these complex functions, over-expression of CAS proteins has now been strongly linked to poor prognosis and increased metastasis in cancer, as well as resistance to first-line chemotherapeutics in multiple tumor types including breast and lung cancers, glioblastoma, and melanoma. Further, CAS proteins have also been linked to additional pathological conditions including inflammatory disorders, Alzheimer's and Parkinson's disease, as well as developmental defects. This review will explore the roles of the CAS proteins in normal and pathological states in the context of the many mechanistic insights into CAS protein function that have emerged in the past decade.

Enriching the viral-host interactomes with interactions mediated by SH3 domains

Protein-protein interactions play an essential role in the regulation of most cellular processes. The process of viral infection is no exception and many viral pathogenic strategies involve targeting and perturbing host-protein interactions. The characterization of the host protein subnetworks disturbed by invading viruses is a major goal of viral research and may contribute to reveal fundamental biological mechanisms and to identify new therapeutic strategies. To assist in this approach, we have developed a database, VirusMINT, which stores in a structured format most of the published interactions between viral and host proteome. Although SH3 are the most ubiquitous and abundant class of protein binding modules, VirusMINT contains only a few interactions mediated by this domain class. To overcome this limitation, we have applied the whole interactome scanning experiment approach to identify interactions between 15 human SH3 domains and viral proline-rich peptides of two oncogenic viruses, human papillomavirus type 16 and human adenovirus A type 12. This approach identifies 114 new potential interactions between the human SH3 domains and proline-rich regions of the two viral proteomes.

Nephrocystin-1 and nephrocystin-4 are required for epithelial morphogenesis and associate with PALS1/PATJ and Par6

Nephronophthisis (NPH) is an autosomal recessive disorder characterized by renal fibrosis, tubular basement membrane disruption and corticomedullary cyst formation leading to end-stage renal failure. The disease is caused by mutations in NPHP1-9 genes, which encode the nephrocystins, proteins localized to cell–cell junctions and centrosome/primary cilia. Here, we show that nephrocystin mRNA expression is dramatically increased during cell polarization, and shRNA-mediated knockdown of either NPHP1 or NPHP4 in MDCK cells resulted in delayed tight junction (TJ) formation, abnormal cilia formation and disorganized multi-lumen structures when grown in a three-dimensional collagen matrix. Some of these phenotypes are similar to those reported for cells depleted of the TJ proteins PALS1 or Par3, and interestingly, we demonstrate a physical interaction between these nephrocystins and PALS1 as well as their partners PATJ and Par6 and show their partial co-localization in human renal tubules. Taken together, these results demonstrate that the nephrocystins play an essential role in epithelial cell organization, suggesting a plausible mechanism by which the in vivo histopathologic features of NPH might develop.

Nephronophthisis

Nephronophthisis (NPHP) is an autosomal recessive kidney disorder characterized by chronic tubulointerstitial nephritis and leading to end-stage renal failure. NPHP as a renal entity is often part of a multisystem disorder and has been associated with many syndromes including Joubert syndrome (and related disorders) and Senior-Loken syndrome. Recent molecular genetic advances have allowed identification of several genes underlying NPHP. Most of these genes express their protein products, named nephrocystins, in primary cilial/basal body structures. Some nephrocystins are part of adherens junction and focal adhesion kinase protein complexes. This shared localization suggests that common pathogenic mechanisms within the kidney underlie this disease. Functional studies implicate nephrocystins in planar cell polarity pathways, which may be crucial for renal development and maintenance of tubular architecture.

Nephronophthisis: disease mechanisms of a ciliopathy

Nephronophthisis (NPHP), a recessive cystic kidney disease, is the most frequent genetic cause of end-stage kidney disease in children and young adults. Positional cloning of nine genes (NPHP1 through 9) and functional characterization of their encoded proteins (nephrocystins) have contributed to a unifying theory that defines cystic kidney diseases as "ciliopathies." The theory is based on the finding that all proteins mutated in cystic kidney diseases of humans or animal models are expressed in primary cilia or centrosomes of renal epithelial cells. Primary cilia are sensory organelles that connect mechanosensory, visual, and other stimuli to mechanisms of epithelial cell polarity and cell-cycle control. Mutations in NPHP genes cause defects in signaling mechanisms that involve the noncanonical Wnt signaling pathway and the sonic hedgehog signaling pathway, resulting in defects of planar cell polarity and tissue maintenance. The ciliary theory explains the multiple organ involvement in NPHP, which includes retinal degeneration, cerebellar hypoplasia, liver fibrosis, situs inversus, and mental retardation. Positional cloning of dozens of unknown genes that cause NPHP will elucidate further signaling mechanisms involved. Nephrocystins are highly conserved in evolution, thereby allowing the use of animal models to develop future therapeutic approaches.

Targeted disruption of Nphp1 causes male infertility due to defects in the later steps of sperm morphogenesis in mice

Juvenile nephronophthisis type I is the most common genetic disorder causing end-stage renal failure in children and young adults. The defective gene responsible has been identified as NPHP1. Its gene product, nephrocystin-1, is a novel protein of uncertain function that is widely expressed in many tissues and not just confined to the kidney. To gain insight into the physiological function of nephrocystin, Nphp1-targeted mutant mice were generated by homologous recombination. Interestingly, homozygous Nphp1 mutant mice were viable without renal manifestations of nephronophthisis. They appeared normal, but males were infertile with oligoteratozoospermia. Histological analysis of the seminiferous tubules showed that spermatogenesis was blocked at the early stages of spermatid elongation, with degenerating spermatids sloughing off into the lumen. Electron microscopic analysis revealed detachment of early elongating spermatids from Sertoli cells, and a failure of sperm head and tail morphogenesis. However, a few mature spermatozoa were still deposited in the epididymis, though they were frequently dead, immotile, or malformed. These novel findings indicate that nephrocystin is critically required for the differentiation of early elongating spermatids into spermatozoa in mice. The possible roles of nephrocystin in the formation and maintenance of Sertoli-spermatid junctions are still under investigation.

Jouberin localizes to collecting ducts and interacts with nephrocystin-1

Joubert syndrome and related disorders are autosomal recessive multisystem diseases characterized by cerebellar vermis aplasia/hypoplasia, retinal degeneration and cystic kidney disease. There are five known genes; mutations of which give rise to a spectrum of renal cystic diseases the most common of which is nephronophthisis, a disorder characterized by early loss of urinary concentrating ability, renal fibrosis, corticomedullary cyst formation and renal failure. Many of the proteins encoded by these genes interact with one another and are located at adherens junctions or the primary cilia and or basal bodies. Here we characterize Jouberin, a multi-domain protein encoded by the AHI1 gene. Immunohistochemistry with a novel antibody showed that endogenous Jouberin is expressed in brain, kidney and HEK293 cells. In the kidney, Jouberin co-localized with aquaporin-2 in the collecting ducts. We show that Jouberin interacts with nephrocystin-1 as determined by yeast-2-hybrid system and this was confirmed by exogenous and endogenous co-immunoprecipitation in HEK293 cells. Jouberin is expressed at cell-cell junctions, primary cilia and basal body of mIMCD3 cells while a Jouberin-GFP construct localized to centrosomes in subconfluent and dividing MDCK cells. Our results suggest that Jouberin is a protein whose expression pattern supports both the adherens junction and the ciliary hypotheses for abnormalities leading to nephronophthisis.

Nephrocystin-1 interacts directly with Ack1 and is expressed in human collecting duct

Nephronophthisis is characterised by renal fibrosis, tubular basement membrane disruption and corticomedullary cyst formation leading to end stage renal failure. Mutations in NPHP1 account for the underlying genetic defect in 25% of patients with nephronophthisis. Loss of urine concentration ability may be an early feature of nephronophthisis. Using yeast-2-library screening with the SH3 domain of nephrocystin-1 as bait, we identify Ack1 as a novel interaction partner. This interaction is confirmed using exogenous over-expression followed by co-immunoprecipitation. Ack1 is an activated Cdc42-associated kinase, and like nephrocystin-1, is a known interactor of p130Cas. Nephrocystin-1 partially colocalises with Ack1 at cell-cell contacts in IMCD3 cells. In human kidney, nephrocystin-1 expression is limited to cell-cell junctions in renal collecting duct cells. These data define Ack1 as a novel interaction partner of nephrocystin-1 and implicate cell-cell junctions and the renal collecting duct in the pathology of nephronophthisis.

Urinary concentration defects and mechanisms underlying nephronophthisis

The cystic kidney disease nephronophthisis (NPHP) is the commonest genetic cause of end-stage renal failure in young people and children. Histologically the disease is characterized by interstitial fibrosis, tubular atrophy with corticomedullary cyst development and disruption of the tubular basement membrane. Affected children present with polydipsia and polyuria, secondary to a urinary concentration defect, before these structural changes develop. Recently, molecular genetic advances have identified several genes mutated in NPHP, providing novel insights into its pathophysiology for the first time in decades. Here we review the normal physiological mechanisms of urinary concentration and explain, in the context of recent discoveries, the possible mechanisms underlying urinary concentration defects in patients with NPHP. The pattern of a ciliary and adherens junction subcellular localization of nephrocystin proteins is discussed. Recent animal models of cystic kidney disease and treatment with vasopressin V2 receptor antagonists are reviewed and a hypothesis regarding urinary concentration defects in NPHP is proposed. Understanding the cellular mechanisms underlying NPHP and other cystic kidney diseases will provide the rationale for therapeutic interventions in this disease. Early urinary concentration defects provide both a clue to clinical diagnosis of NPHP and potential therapeutic targets for pharmacological treatment of this condition.

The ciliopathies: an emerging class of human genetic disorders

Cilia and flagella are ancient, evolutionarily conserved organelles that project from cell surfaces to perform diverse biological roles, including whole-cell locomotion; movement of fluid; chemo-, mechano-, and photosensation; and sexual reproduction. Consistent with their stringent evolutionary conservation, defects in cilia are associated with a range of human diseases, such as primary ciliary dyskinesia, hydrocephalus, polycystic liver and kidney disease, and some forms of retinal degeneration. Recent evidence indicates that ciliary defects can lead to a broader set of developmental and adult phenotypes, with mutations in ciliary proteins now associated with nephronophthisis, Bardet-Biedl syndrome, Alstrom syndrome, and Meckel-Gruber syndrome. The molecular data linking seemingly unrelated clinical entities are beginning to highlight a common theme, where defects in ciliary structure and function can lead to a predictable phenotypic pattern that has potentially predictive and therapeutic value.

Mouse models of polycystic kidney disease

Polycystic kidney disease (PKD) is a diverse group of human monogenic lethal conditions inherited as autosomal dominant (AD) or recessive (AR) traits. Recent development of genetically engineered mouse models of ADPKD, ARPKD, and nephronophthisis/medullary cystic disease (NPHP) are providing additional insights into the molecular mechanisms governing of these disease processes as well as the developmental differentiation of the normal kidney. Genotypic and phenotypic mouse models are discussed and provide evidence for the fundamental involvement of cell-matrix, cell-cell, and primary cilia-lumen interactions, as well as epithelial proliferation, apoptosis, and polarization. Structure/function relationships between the PKD1, PKD2, PKHD1, and NPHP genes and proteins support the notion of a regulatory multiprotein cystic complex with a mechanosensory function that integrates signals from the extracellular environment. The plethora of intracellular signaling cascades that can impact renal cystic development suggest an exquisitely sensitive requirement for integrated downstream transduction and provide potential targets for therapeutic intervention. Appropriate genocopy models that faithfully recapitulate the phenotypic characteristics of the disease will be invaluable tools to analyze the effects of modifier genes and small molecule inhibitor therapies.

Src kinase activity and SH2 domain regulate the dynamics of Src association with lipid and protein targets

Src functions depend on its association with the plasma membrane and with specific membrane-associated assemblies. Many aspects of these interactions are unclear. We investigated the functions of kinase, SH2, and SH3 domains in Src membrane interactions. We used FRAP beam-size analysis in live cells expressing a series of c-Src–GFP proteins with targeted mutations in specific domains together with biochemical experiments to determine whether the mutants can generate and bind to phosphotyrosyl proteins. Wild-type Src displays lipid-like membrane association, whereas constitutively active Src-Y527F interacts transiently with slower-diffusing membrane-associated proteins. These interactions require Src kinase activity and SH2 binding, but not SH3 binding. Furthermore, overexpression of paxillin, an Src substrate with a high cytoplasmic population, competes with membrane phosphotyrosyl protein targets for binding to activated Src. Our observations indicate that the interactions of Src with lipid and protein targets are dynamic and that the kinase and SH2 domain cooperate in the membrane targeting of Src.

Nephronophthisis-associated ciliopathies

Nephronophthisis (NPHP), an autosomal recessive cystic kidney disease, represents the most frequent genetic cause of end-stage kidney disease in the first three decades of life. Contrary to polycystic kidney disease, NPHP shows normal or diminished kidney size, cysts are concentrated at the corticomedullary junction, and tubulointerstitial fibrosis is dominant. NPHP can be associated with retinitis pigmentosa (Senior-Løken syndrome), liver fibrosis, and cerebellar vermis aplasia (Joubert syndrome) in approximately 10% of patients. Positional cloning of six novel genes (NPHP1 through 6) as mutated in NPHP and functional characterization of their encoded proteins have contributed to the concept of "ciliopathies." It has helped advance a new unifying theory of cystic kidney diseases. This theory states that the products of all genes that are mutated in cystic kidney diseases in humans, mice, or zebrafish are expressed in primary cilia or centrosomes of renal epithelial cells. Primary cilia are sensory organelles that connect mechanosensory, visual, osmotic, and other stimuli to mechanisms of cell-cycle control and epithelial cell polarity. The ciliary theory explains the multiple organ involvement in NPHP regarding retinitis pigmentosa, liver fibrosis, ataxia, situs inversus, and mental retardation. Mutations in NPHP genes cause defects in signaling mechanisms, including the noncanonical Wnt signaling pathway. The "ciliopathy" NPHP thereby is caused by defects in tissue differentiation and maintenance as a result of impaired processing of extracellular cues. Nephrocystins, the proteins that are encoded by NPHP genes, are highly conserved in evolution. Positional cloning of additional causative genes of NPHP will elucidate further signaling mechanisms that are involved, thereby establishing therapeutic approaches using animal models in mouse, zebrafish, and Caenorhabditis elegans.

High NPHP1 and NPHP6 mutation rate in patients with Joubert syndrome and nephronophthisis: potential epistatic effect of NPHP6 and AHI1 mutations in patients with NPHP1 mutations

Joubert syndrome (JS) is an autosomal recessive disorder that is described in patients with cerebellar ataxia, mental retardation, hypotonia, and neonatal respiratory dysregulation. Kidney involvement (nephronophthisis or cystic renal dysplasia) is associated with JS in one fourth of known cases. Mutations in three genes--AHI1, NPHP1, and NPHP6--have been identified in patients with JS. However, because NPHP1 mutations usually cause isolated nephronophthisis, the factors that predispose to the development of neurologic involvement are poorly understood. In an attempt to identify such genetic determinants, a cohort of 28 families with nephronophthisis and at least one JS-related neurologic symptom were screened for mutations in AHI1, NPHP1, and NPHP6 genes. NPHP1 and NPHP6 homozygous or compound heterozygous mutations were found in 13 (46%) and six (21%) unrelated patients, respectively. Two of the 13 patients with NPHP1 mutations carried either a heterozygous truncating mutation in NPHP6 or a heterozygous missense mutation in AHI1. Furthermore, five patients with NPHP1 mutations carried the AHI1 variant R830W, which was predicted to be "possibly damaging" and was found with significantly higher frequency than in healthy control subjects and in patients with NPHP1 mutations without neurologic symptoms (five of 26 versus four of 276 and three of 152 alleles; P < 0.001 and P < 0.002, respectively). In contrast to the variable neurologic and milder retinal phenotype of patients with NPHP1 mutations, patients with NPHP6 mutations presented with a more severe neurologic and retinal phenotype. In conclusion, NPHP1 and NPHP6 are major genes of nephronophthisis associated with JS. Epistatic effects that are provided by heterozygous NPHP6 and AHI1 mutations and variants may contribute to the appearance of extrarenal symptoms in patients with NPHP1 mutations.

Cilium-generated signaling: a cellular GPS?

PURPOSE OF REVIEW

The discovery of the importance of proteins localized to cilia and basal bodies has provided novel insights into the pathogenesis of various human disorders including cystic kidney disease. The physiological role of cilium-generated signaling in most tissues including the kidney has remained elusive, however. This review focuses on the most recent advance in understanding a role for signaling through cilia for kidney biology.

RECENT FINDINGS

Recent work has tied the function of several developmental master regulators to cilium-generated signaling in vertebrates. Hedgehog signaling requires ciliary proteins and loss of targeting of its receptors and effectors to cilia abrogates Hedgehog function. Moreover, the ciliary protein inversin has been shown to act as a molecular switch for the regulation of Wnt signaling cascades.

SUMMARY

Together with recent data on a function of cilia and basal body proteins in planar cell polarity in mice, a novel concept and testable hypothesis for the function of cilia in vertebrates is emerging: cilia may act as a cellular positioning device to allow subcellular asymmetry and polarization of the cells within the plane of the epithelium to develop and maintain regular arrays of cells such as kidney tubes.

Nephrocystin Specifically Localizes to the Transition Zone of Renal and Respiratory Cilia and Photoreceptor Connecting Cilia

Nephronophthisis (NPHP) is a hereditary cystic kidney disorder that causes renal failure in children and young adults and can be associated with various extrarenal disorders, including retinitis pigmentosa. Six NPHP genes, whose functions are disrupted by autosomal recessive mutations in patients with NPHP, have been identified. The majority of patients with NPHP carry homozygous deletions of NPHP1 encoding nephrocystin. Previous data indicate that nephrocystin forms a complex at cell junctions and focal adhesions. Here, it is shown that nephrocystin specifically localizes at the ciliary base to the transition zone of renal and respiratory cilia and to photoreceptor connecting cilia. During in vitro ciliogenesis of primary human respiratory epithelial cells, nephrocystin can be detected first with a diffuse cytoplasmic localization as soon as cell polarization starts, and translocates to the transition zone when cilia are formed. In columnar respiratory cells, nephrocystin is attached tightly to the axonemal structure of the transition zone at a region that contains the calcium-sensitive cilia autotomy site. In patients with homozygous NPHP1 deletions, nephrocystin is absent from the entire respiratory cell, including the transition zone, which might be of interest for future diagnostic approaches. Cilia formation is not altered in primary nephrocystin-deficient respiratory cells, which is consistent with previous findings obtained for the Caenorhabditis elegans ortholog. In addition, it is shown that the localization pattern of intraflagellar transport proteins and nephrocystin differs, suggesting distinct functional roles. In conclusion, nephrocystin deficiency or dysfunction at the transition zone of renal monocilia and the photoreceptor connecting cilium might explain renal failure and retinal degeneration that are observed in patients with NPHP1.

Cilia and centrosomes: a unifying pathogenic concept for cystic kidney disease?

Cystic kidney diseases are among the most frequent lethal genetic diseases. Positional cloning of novel cystic kidney disease genes revealed that their products (cystoproteins) are expressed in sensory organelles called primary cilia, in basal bodies or in centrosomes. Primary cilia link mechanosensory, visual, osmotic, gustatory and other stimuli to mechanisms of cell-cycle control and epithelial cell polarity. The ciliary expression of cystoproteins explains why many other organs might be also affected in patients with cystic kidney disease. Protein-protein interactions among cystoproteins, and their strong evolutionary conservation, provide a basis for a multidisciplinary approach to unravelling the novel signalling mechanisms that are involved in this disease group.

HEF1-aurora A interactions: points of dialog between the cell cycle and cell attachment signaling networks

Regulated timing of cell division cycles, and geometrical precision in the planar orientation of cell division, are critical during organismal development and remain important for the maintenance of polarized structures in adults. Mounting evidence suggests that these processes are coordinated at the centrosome through the action of proteins that mediate both cell cycle and cell attachment. Our recent work identifying HEF1 as an activator of the Aurora A kinase suggests a novel hub for such integrated signaling. We suggest that defects in components of the machinery specifying the temporal and spatial integration of cell division may induce cancer and other diseases through pleiotropic effects on cell migration, proliferation, apoptosis, and genomic stability.

The C. elegans homologs of nephrocystin-1 and nephrocystin-4 are cilia transition zone proteins involved in chemosensory perception

Nephronophthisis (NPH) is a cystic kidney disorder that causes end-stage renal failure in children. Five nephrocystin (nephrocystin-1 to nephrocystin-5) genes, whose function is disrupted in NPH patients, have been identified and data indicate they form a complex at cell junctions and focal adhesions. More recently, the nephrocystin proteins have also been identified in cilia, as have multiple other cystic kidney disease related proteins. Significant insights into this cilia and cystic kidney disease connection have come from analyses in simpler eukaryotic organisms such as Caenorhabditis elegans. In this regard, we became interested in the C. elegans homologs of nephrocystin-1 (nph-1) and nephrocystin-4 (nph-4) from a database screen to identify genes coordinately regulated by the ciliogenic transcription factor DAF-19. Here we show that expression of nph-1 and nph-4 is DAF-19 dependent, that their expression is restricted to ciliated sensory neurons, and that both NPH-1 and NPH-4 concentrate at the transition zones at the base of the cilia, but are not found in the cilium axoneme. In addition, NPH-4 is required for the localization of NPH-1 to this domain. Interestingly, nph-1 or nph-4 mutants have no obvious cilia assembly defects; however, they do have abnormalities in cilia-mediated sensory functions as evidenced by abnormal chemotaxis and lifespan regulation. Our data suggest that rather than having a ciliogenic role, the NPH proteins play an important function as part of the sensory or signaling machinery of this organelle. These findings suggest that the defects in human NPH patients may not be the result of aberrant ciliogenesis but abnormal cilia-sensory functions.

Nephronophthisis

There has been tremendous progress in the past few years in understanding the molecular basis of nephronophthisis, and it is now evident that the disease is characterized by both clinical and genetic heterogeneity. Within the three different clinical forms there is a large spectrum of phenotypes, which have been associated, to date, with five gene defects. These genes encode proteins that localize in different cell compartments - in particular, to the primary apical cilia - as is the case for virtually all gene products involved in cystic kidney diseases. Two animal models with mutations in the mouse orthologs of the genes involved in the adolescent and infantile forms also exist. These models have been of considerable help in deciphering disease pathogenesis.

Functional characterization of the C. elegans nephrocystins NPHP-1 and NPHP-4 and their role in cilia and male sensory behaviors

Autosomal dominant polycystic kidney disease (ADPKD) and nephronophthisis (NPH) share two common features: cystic kidneys and ciliary localized gene products. Mutation in either the PKD1 or PKD2 gene accounts for 95% of all ADPKD cases. Mutation in one of four genes (NPHP1-4) results in nephronophthisis. The NPHP1, NPHP2, PKD1, and PKD2 protein products (nephrocystin-1, nephrocystin-2 or inversin, polycystin-1, and polycystin-2, respectively) localize to primary cilia of renal epithelia. However, the relationship between the nephrocystins and polycystins, if any, is unknown. In the nematode Caenorhabditis elegans, the LOV-1 and PKD-2 polycystins localize to male-specific sensory cilia and are required for male mating behaviors. To test the hypothesis that ADPKD and NPH cysts arise from a common defect in cilia, we characterized the C. elegans homologs of NPHP1 and NPHP4. C. elegans nphp-1 and nphp-4 are expressed in a subset of sensory neurons. GFP-tagged NPHP-1 and NPHP-4 proteins localize to ciliated sensory endings of dendrites and colocalize with PKD-2 in male-specific sensory cilia. The cilia of nphp-1(ok500) and nphp-4(tm925) mutants are intact. nphp-1; nphp-4 double, but not single, mutant males are response defective. We propose that NPHP-1 and NPHP-4 proteins play important and redundant roles in facilitating ciliary sensory signal transduction.

Cilium-generated signaling and cilia-related disorders

Biologists have long known that humans experience their environment through cilia. Light, odorant, and sound perception depend on these microtubule-filled, complex organelles present on cells in primary sensory tissues. Recently, discoveries on the mechanism of assembly of cilia (flagella) in the lowly, biflagellated, eucaryotic green alga Chlamydomonas have triggered a renaissance of interest in the organelles along with a recognition of their key sensory roles in nonsensory tissues. Chlamydomonas researchers uncovered an entirely new set of cellular machinery essential for transporting the protein components of cilia and flagella in all ciliated/flagellated eukaryotic cells between their site of synthesis in the cell body and their site of assembly at the tip of the flagellum (intraflagellar transport: IFT). Prompted by the surprising observations that disruption of IFT genes in mice led to polycystic kidney disease (PKD) and that PKD proteins are present on the sensory cilia of Caenorhabditis elegans, researchers have made a direct connection between PKD and cilia. At least five (and possibly all) of the seven identified human genes disrupted in PKD and a related disorder nephronophthisis encode proteins expressed in the primary cilia that project into the lumen from the epithelial cells that line renal tubules. Moreover, the renal cilia are flow sensors and at least two of the PKD genes encode ciliary transmembrane proteins essential for mechanosensation. Although their roles have not yet been as clearly identified, cilia also are at the center of a rare human disorder, Bardet-Biedl syndrome (BBS), in which patients exhibit phenotypes of common human diseases, including obesity and increased incidence of hypertension and diabetes. Five of the eight known BBS genes encode basal body or cilia proteins in mice or humans, and homologues of two of the remaining genes are present in basal bodies/cilia of model organisms. Here we briefly describe the biology of cilia and flagella, we outline how studies on model organisms have led to our current understanding of the roles of these organelles and their proteins in health and disease, and we highlight the notion that the primary cilia present on cells throughout the body, even those on brain neurons, may be essential for as yet undiscovered cilium-generated signaling functions.