Mutations in INPP5E, encoding inositol polyphosphate-5-phosphatase E, link phosphatidyl inositol signaling to the ciliopathies

Trafficking to the ciliary membrane: how to get across the periciliary diffusion barrier?

The primary cilium organizes numerous signal transduction cascades, and an understanding of signaling receptor trafficking to cilia is now emerging. A defining feature of cilia is the periciliary diffusion barrier that separates the ciliary and plasma membranes. Although lateral transport through this barrier may take place, polarized exocytosis to the base of the cilium has been the prevailing model for delivering membrane proteins to cilia. Key players for this polarized exocytosis model include the GTPases Rab8 and Rab11, the exocyst, and possibly the intraflagellar tranport machinery. In turn, the sorting of membrane proteins to cilia critically relies on the recognition of ciliary targeting signals by sorting machines such as the BBSome coat complex or the GTPase Arf4. Finally, some proteins need to exit from cilia, and ubiquitination may regulate this step. The stage is now set to dissect the interplay between signaling and regulated trafficking to and from cilia.

Is hearing loss a feature of Joubert syndrome, a ciliopathy?

OBJECTIVE

To assess if hearing loss is a feature of Joubert syndrome (JBS), one of the ciliopathies and therefore possibly associated with hearing loss.

DESIGN

Retrospective case series.

SETTING

University Children's Hospital.

PATIENTS

Dutch patients with JBS.

MAIN OUTCOME MEASURES

Audiological data.

RESULTS

Data from 22 Dutch Joubert syndrome (JBS) cases (17 males, 5 females) aged 3-40 years were available. Audiological tests were successfully performed in 14 cases. Three cases (aged 17-26 years) showed very mild sensorineural hearing loss (SNHL) at different frequencies. Conductive hearing loss due to middle ear infections occurred frequently in young JBS children (6 out of 22 cases). In three cases (aged 3-13 years) the parents reported the child was hypersensitive to sound.

CONCLUSION

We found no evidence for significant hearing loss in Joubert syndrome patients. However, given the compromised speech development in JBS, conductive hearing loss due to middle ear infections should be treated vigorously. SNHL at a later age cannot be excluded on the basis of our data, given the sample size. Three of the older cases showed discretely increased hearing thresholds. Analogous to the ciliopathy Bardet-Biedl syndrome, where hearing thresholds were reported to be subclinically increased in a group of adolescents patients, we recommend follow-up of JBS patients in view of the possibility of progressive, late-onset SNHL.

A key role for the phosphorylation of Ser440 by the cyclic AMP-dependent protein kinase in regulating the activity of the Src homology 2 domain-containing Inositol 5'-phosphatase (SHIP1)

The Src homology 2 domain-containing inositol 5'-phosphatase 1 (SHIP1) dephosphorylates phosphatidylinositol 3,4,5-trisphosphate to phophatidylinositol 3,4-bisphosphate in hematopoietic cells to regulate multiple cell signaling pathways. SHIP1 can be phosphorylated by the cyclic AMP-dependent protein kinase (PKA), resulting in an increase in SHIP1 activity (Zhang, J., Walk, S. F., Ravichandran, K. S., and Garrison, J. C. (2009) J. Biol. Chem. 284, 20070-20078). Using a combination of approaches, we identified the serine residue regulating SHIP1 activity. After mass spectrometric identification of 17 serine and threonine residues on SHIP1 as being phosphorylated by PKA in vitro, studies with truncation mutants of SHIP1 narrowed the phosphorylation site to the catalytic region between residues 400 and 866. Of the two candidate phosphorylation sites located in this region (Ser(440) and Ser(774)), only mutation of Ser(440) to Ala abolished the ability of PKA to phosphorylate the purified, catalytic domain of SHIP1 (residues 401-866). Mutation of Ser(440) to Ala in full-length SHIP1 abrogated the ability of PKA to increase the activity of SHIP1 in mammalian cells. Using flow cytometry, we found that the PKA activator, Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt hydrate (Sp-cAMPS) blunted the phosphorylation of Akt downstream of B cell antigen receptor engagement in SHIP1-null DT40 B lymphocytes expressing native mouse SHIP1. The inhibitory effect of Sp-cAMPS was absent in cells expressing the S440A mutant of SHIP1. These results suggest that activation of SHIP1 by PKA via phosphorylation on Ser(440) is an important regulatory event in hematopoietic cells.

Co-occurrence of Joubert syndrome and Jeune asphyxiating thoracic dystrophy

Ciliary disorders share typical features, such as polydactyly, renal and biliary cystic dysplasia, and retinitis pigmentosa, which often overlap across diagnostic entities. We report on two siblings of consanguineous parents and two unrelated children, both of unrelated parents, with co-occurrence of Joubert syndrome and Jeune asphyxiating thoracic dystrophy, an association that adds to the observation of common final patterns of malformations in ciliary disorders. Using homozygosity mapping in the siblings, we were able to exclude all known genes/loci for both syndromes except for INVS, AHI1, and three genes from the previously described Jeune locus at 15q13. No pathogenic variants were found in these genes by direct sequencing. In the third child reported, sequencing of RPGRIP1L, ARL13B, AHI1, TMEM67, OFD1, CC2D2A, and deletion analysis of NPHP1 showed no mutations. Although this study failed to identify a mutation in the patients tested, the co-occurrence of Joubert and Jeune syndromes is likely to represent a distinct entity caused by mutations in a yet to be discovered gene. The mechanisms by which certain organ systems are affected more than others in the spectrum of ciliary diseases remain largely unknown.

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.

Mutations of a country: a mutation review of single gene disorders in the United Arab Emirates (UAE)

The United Arab Emirates inhabitants are ethnically diverse, with ancestries from Arabia, Persia, Baluchistan, and Africa. However, the majority of the current five million inhabitants are expatriates from the Asian subcontinent, Middle Eastern, African, and European countries. Consanguineous marriages within most UAE subpopulations are still the norm, leading to the formation of isolates and higher frequencies of recessive conditions. The UAE is ranked sixth in terms of prevalence of birth defects, with more than 270 genetic disorders reported in the national population. The UAE has high frequencies of blood disorders including thalassemias, sickle cell disease, and G6PD. In addition, certain genetic conditions are relatively common including cystic fibrosis, Joubert, and Meckel syndromes. Furthermore, numerous rare congenital malformations and metabolic disorders have been reported. We review the single gene disorders that have been studied at the molecular level in the UAE (which currently stand at 76) and compile the mutations found. Several novel (p.S2439fs) mutations have been reported including c.7317delA in NF1, c.5C>T (p.A2V) in DKC1, c.1766T>A (p.I589N) in TP63, and c.2117G>T (p.R706L) in VLDLR. We hope that this review will form the basis to establish a UAE mutations database and serve as a model for the collection of mutations of a country.

The conserved Bardet-Biedl syndrome proteins assemble a coat that traffics membrane proteins to cilia

The BBSome is a complex of Bardet-Biedl Syndrome (BBS) proteins that shares common structural elements with COPI, COPII, and clathrin coats. Here, we show that the BBSome constitutes a coat complex that sorts membrane proteins to primary cilia. The BBSome is the major effector of the Arf-like GTPase Arl6/BBS3, and the BBSome and GTP-bound Arl6 colocalize at ciliary punctae in an interdependent manner. Strikingly, Arl6(GTP)-mediated recruitment of the BBSome to synthetic liposomes produces distinct patches of polymerized coat apposed onto the lipid bilayer. Finally, the ciliary targeting signal of somatostatin receptor 3 needs to be directly recognized by the BBSome in order to mediate targeting of membrane proteins to cilia. Thus, we propose that trafficking of BBSome cargoes to cilia entails the coupling of BBSome coat polymerization to the recognition of sorting signals by the BBSome.

Phosphatidylinositol-4-phosphate 5-kinases and phosphatidylinositol 4,5-bisphosphate synthesis in the brain

The predominant pathway for phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P(2)) synthesis is thought to be phosphorylation of phosphatidylinositol 4-phosphate at the 5 position of the inositol ring by type I phosphatidylinositol phosphate kinases (PIPK): PIPKIalpha, PIPKIbeta, and PIPKIgamma. PIPKIgamma has been shown to play a role in PI(4,5)P(2) synthesis in brain, and the absence of PIPKIgamma is incompatible with postnatal life. Conversely, mice lacking PIPKIalpha or PIPKIbeta (isoforms are referred to according to the nomenclature of human PIPKIs) live to adulthood, although functional effects in specific cell types are observed. To determine the contribution of PIPKIalpha and PIPKIbeta to PI(4,5)P(2) synthesis in brain, we investigated the impact of disrupting multiple PIPKI genes. Our results show that a single allele of PIPKIgamma, in the absence of both PIPKIalpha and PIPKIbeta, can support life to adulthood. In addition, PIPKIalpha alone, but not PIPKIbeta alone, can support prenatal development, indicating an essential and partially overlapping function of PIPKIalpha and PIPKIgamma during embryogenesis. This is consistent with early embryonic expression of PIPKIalpha and PIPKIgamma but not of PIPKIbeta. PIPKIbeta expression in brain correlates with neuronal differentiation. The absence of PIPKIbeta does not impact embryonic development in the PIPKIgamma knock-out (KO) background but worsens the early postnatal phenotype of the PIPKIgamma KO (death occurs within minutes rather than hours). Analysis of PIP(2) in brain reveals that only the absence of PIPKIgamma significantly impacts its levels. Collectively, our results provide new evidence for the dominant importance of PIPKIgamma in mammals and imply that PIPKIalpha and PIPKIbeta function in the generation of specific PI(4,5)P(2) pools that, at least in brain, do not have a major impact on overall PI(4,5)P(2) levels.

Joubert Syndrome and related disorders

Joubert syndrome (JS) and related disorders (JSRD) are a group of developmental delay/multiple congenital anomalies syndromes in which the obligatory hallmark is the molar tooth sign (MTS), a complex midbrain-hindbrain malformation visible on brain imaging, first recognized in JS. Estimates of the incidence of JSRD range between 1/80,000 and 1/100,000 live births, although these figures may represent an underestimate. The neurological features of JSRD include hypotonia, ataxia, developmental delay, intellectual disability, abnormal eye movements, and neonatal breathing dysregulation. These may be associated with multiorgan involvement, mainly retinal dystrophy, nephronophthisis, hepatic fibrosis and polydactyly, with both inter- and intra-familial variability. JSRD are classified in six phenotypic subgroups: Pure JS; JS with ocular defect; JS with renal defect; JS with oculorenal defects; JS with hepatic defect; JS with orofaciodigital defects. With the exception of rare X-linked recessive cases, JSRD follow autosomal recessive inheritance and are genetically heterogeneous. Ten causative genes have been identified to date, all encoding for proteins of the primary cilium or the centrosome, making JSRD part of an expanding group of diseases called "ciliopathies". Mutational analysis of causative genes is available in few laboratories worldwide on a diagnostic or research basis. Differential diagnosis must consider in particular the other ciliopathies (such as nephronophthisis and Senior-Loken syndrome), distinct cerebellar and brainstem congenital defects and disorders with cerebro-oculo-renal manifestations. Recurrence risk is 25% in most families, although X-linked inheritance should also be considered. The identification of the molecular defect in couples at risk allows early prenatal genetic testing, whereas fetal brain neuroimaging may remain uninformative until the end of the second trimester of pregnancy. Detection of the MTS should be followed by a diagnostic protocol to assess multiorgan involvement. Optimal management requires a multidisciplinary approach, with particular attention to respiratory and feeding problems in neonates and infants. Cognitive and behavioral assessments are also recommended to provide young patients with adequate neuropsychological support and rehabilitation. After the first months of life, global prognosis varies considerably among JSRD subgroups, depending on the extent and severity of organ involvement.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

cAMP and cGMP signaling: sensory systems with prokaryotic roots adopted by eukaryotic cilia

An exciting discovery of the new millennium is that primary cilia, organelles found on most eukaryotic cells, play crucial roles in vertebrate development by modulating Hedgehog, Wnt and PDGF signaling. Analysis of the literature and sequence databases reveals that the ancient signal transduction pathway, which uses cGMP in eukaryotes or related cyclic di-GMP in bacteria, exists in virtually all eukaryotes. However, many eukaryotes that secondarily lost cilia during evolution, including flowering plants, slime molds and most fungi, lack otherwise evolutionarily conserved cGMP signaling components. Based on this intriguing phylogenetic distribution, the presence of cGMP signaling proteins within cilia, and the indispensable roles that cGMP plays in transducing environmental signals in divergent ciliated cells (e.g. vertebrate photoreceptors and Caenorhabditis elegans sensory neurons), we propose that cGMP signaling has a strong ciliary basis. cAMP signaling, also inherent to bacteria and crucial for cilium-dependent olfaction, similarly appears to have widespread usage in diverse cilia. Thus, we argue here that both cyclic nucleotides play essential and potentially ubiquitous roles in modulating ciliary functions.

Functional genomic screen for modulators of ciliogenesis and cilium length

Primary cilia are evolutionarily conserved cellular organelles that organize diverse signaling pathways1,2. Defects in the formation or function of primary cilia are associated with a spectrum of human diseases and developmental abnormalities3. Genetic screens in model organisms have discovered core machineries of cilium assembly and maintenance4. However, regulatory molecules that coordinate the biogenesis of primary cilia with other cellular processes, including cytoskeletal organization, vesicle trafficking and cell-cell adhesion, remain to be identified. Here we report the results of a functional genomic screen using RNA interference (RNAi) to identify human genes involved in ciliogenesis control. The screen identified 36 positive and 13 negative ciliogenesis modulators, which include molecules involved in actin dynamics and vesicle trafficking. Further investigation demonstrated that blocking actin assembly facilitates ciliogenesis by stabilizing the pericentrosomal preciliary compartment (PPC), a previously uncharacterized compact vesiculotubular structure storing transmembrane proteins destined for cilia during the early phase of ciliogenesis. PPC was labeled by recycling endosome markers. Moreover, knockdown of modulators that are involved in the endocytic recycling pathway affected the formation of PPC as well as ciliogenesis. Our results uncover a critical regulatory step that couples actin dynamics and endocytic recycling with ciliogenesis, and also provide potential target molecules for future study.

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.

The emerging mechanisms of isoform-specific PI3K signalling

Phosphoinositide 3-kinases (PI3Ks) function early in intracellular signal transduction pathways and affect many biological functions. A further level of complexity derives from the existence of eight PI3K isoforms, which are divided into class I, class II and class III PI3Ks. PI3K signalling has been implicated in metabolic control, immunity, angiogenesis and cardiovascular homeostasis, and is one of the most frequently deregulated pathways in cancer. PI3K inhibitors have recently entered clinical trials in oncology. A better understanding of how the different PI3K isoforms are regulated and control signalling could uncover their roles in pathology and reveal in which disease contexts their blockade could be most beneficial.

An unnatural PIP simulates growth factor signaling

In this issue of Chemistry & Biology, Laketa et al. describe the synthesis of a membrane permeant phosphoinositide lipid that acts to stimulate PI(3,4,5)P(3)-dependent signaling without the need of growth factor stimulation.

Two closely related endocytic proteins that share a common OCRL-binding motif with APPL1

Mutations of the inositol 5' phosphatase oculocerebrorenal syndrome of Lowe (OCRL) give rise to the congenital X-linked disorders oculocerebrorenal syndrome of Lowe and Dent disease, two conditions giving rise to abnormal kidney proximal tubule reabsorption, and additional nervous system and ocular defects in the case of Lowe syndrome. Here, we identify two closely related endocytic proteins, Ses1 and Ses2, which interact with the ASH-RhoGAP-like (ASPM-SPD-2-Hydin homology and Rho-GTPase Activating Domain-like) domain of OCRL. The interaction is mediated by a short amino acid motif similar to that used by the rab-5 effector APPL1 (Adaptor Protein containing pleckstrin homology [PH] domain, PTB domain and Leucine zipper motif 1) APPL1 for OCRL binding. Ses binding is mutually exclusive with APPL1 binding, and is disrupted by the same missense mutations in the ASH-RhoGAP-like domain that also disrupt APPL1 binding. Like APPL1, Ses1 and -2 are localized on endosomes but reside on different endosomal subpopulations. These findings define a consensus motif (which we have called a phenylalanine and histidine [F&H] motif) for OCRL binding and are consistent with a scenario in which Lowe syndrome and Dent disease result from perturbations at multiple sites within the endocytic pathway.

The role of primary cilia in neuronal function

The "ciliopathies" are a newly defined group of disorders characterized by defects in the structure or function of the cellular primary cilium. Patients with these disorders display variably expressive fibrocystic renal disease, retinal blindness, polydactyly, obesity, and brain dysgenesis as well as neurocognitive impairments. Joubert syndrome is a ciliopathy defined by cerebellar vermis hypoplasia, oculomotor apraxia, intermittent hyperventilation, and mental retardation. Recent evidence suggests important roles for the primary cilium in mediating a host of extracellular signaling events such as morphogen, mitogen, homeostatic and polarity signals. Based upon the clinical features of ciliopathies and cilia mediated signaling pathways, the data support a role for the primary cilium in modulating neurogenesis, cell polarity, axonal guidance and possibly adult neuronal function.

Role of primary cilia in brain development and cancer

The primary cilium, a hair-like extension from a cell's surface, acts as a sensory organelle to receive signals that regulate cellular behavior and physiology. Like most mammalian cells, neural progenitors and neurons have primary cilia. Recent studies show that this tiny projection plays important roles in brain development and diseases. Ciliary mutant mice show defects in brain patterning, progenitor proliferation, and specification of adult neural stem cells. Primary cilia also have dual opposing functions in the development of brain tumors. Ciliary defects are associated with genetic syndromes that frequently have neurological symptoms. Understanding the multifaceted roles that primary cilia have in brain development will provide important insights into the mechanism of brain development and diseases.

Clinical and molecular features of Joubert syndrome and related disorders

Joubert syndrome (JBTS; OMIM 213300) is a rare, autosomal recessive disorder characterized by a specific congenital malformation of the hindbrain and a broad spectrum of other phenotypic findings that is now known to be caused by defects in the structure and/or function of the primary cilium. The complex hindbrain malformation that is characteristic of JBTS can be identified on axial magnetic resonance imaging and is known as the molar tooth sign (MTS); other diagnostic criteria include intellectual disability, hypotonia, and often, abnormal respiratory pattern and/or abnormal eye movements. In addition, a broad spectrum of other anomalies characterize Joubert syndrome and related disorders (JSRD), and may include retinal dystrophy, ocular coloboma, oral frenulae and tongue tumors, polydactyly, cystic renal disease (including cystic dysplasia or juvenile nephronophthisis), and congenital hepatic fibrosis. The clinical course can be variable, but most children with this condition survive infancy to reach adulthood. At least eight genes cause JSRD, with some genotype-phenotype correlations emerging, including the association between mutations in the MKS3 gene and hepatic fibrosis characteristic of the JSRD subtype known as COACH syndrome. Several of the causative genes for JSRD are implicated in other ciliary disorders, such as juvenile nephronophthisis and Meckel syndrome, illustrating the close association between these conditions and their overlapping clinical features that reflect a shared etiology involving the primary cilium.

PI3K: from the bench to the clinic and back

From humble beginnings over 25 years ago as a lipid kinase activity associated with certain oncoproteins, PI3K (phosphoinositide 3-kinase) has been catapulted to the forefront of drug development in cancer, immunity and thrombosis, with the first clinical trials of PI3K pathway inhibitors now in progress. Here, we give a brief overview of some key discoveries in the PI3K area and their impact, and include thoughts on the current state of the field, and where it could go from here.PI3K has become a very intense area of research, with over 2,000 publications on PI3K in PubMed for 2009 alone. The expectations for a therapeutic impact of intervention with PI3K activity are high, and progress in the clinical arena is being monitored by many. However, targeted therapies almost invariably encounter roadblocks, often exposing unresolved questions in the basic understanding of the target. PI3K will most likely be no exception. Below, we describe some of these early "surprises" and how these inform and shape basic science investigations.

Disease gene characterization through large-scale co-expression analysis

BACKGROUND

In the post genome era, a major goal of biology is the identification of specific roles for individual genes. We report a new genomic tool for gene characterization, the UCLA Gene Expression Tool (UGET).

RESULTS

Celsius, the largest co-normalized microarray dataset of Affymetrix based gene expression, was used to calculate the correlation between all possible gene pairs on all platforms, and generate stored indexes in a web searchable format. The size of Celsius makes UGET a powerful gene characterization tool. Using a small seed list of known cartilage-selective genes, UGET extended the list of known genes by identifying 32 new highly cartilage-selective genes. Of these, 7 of 10 tested were validated by qPCR including the novel cartilage-specific genes SDK2 and FLJ41170. In addition, we retrospectively tested UGET and other gene expression based prioritization tools to identify disease-causing genes within known linkage intervals. We first demonstrated this utility with UGET using genetically heterogeneous disorders such as Joubert syndrome, microcephaly, neuropsychiatric disorders and type 2 limb girdle muscular dystrophy (LGMD2) and then compared UGET to other gene expression based prioritization programs which use small but discrete and well annotated datasets. Finally, we observed a significantly higher gene correlation shared between genes in disease networks associated with similar complex or Mendelian disorders.

DISCUSSION

UGET is an invaluable resource for a geneticist that permits the rapid inclusion of expression criteria from one to hundreds of genes in genomic intervals linked to disease. By using thousands of arrays UGET annotates and prioritizes genes better than other tools especially with rare tissue disorders or complex multi-tissue biological processes. This information can be critical in prioritization of candidate genes for sequence analysis.

Cilia localization is essential for in vivo functions of the Joubert syndrome protein Arl13b/Scorpion

arl13b was initially cloned as the novel cystic kidney gene scorpion (sco) in zebrafish and was shown to be required for cilia formation in the kidney duct. In mouse, a null mutant of Arl13b shows abnormal ultrastructure of the cilium and defective sonic hedgehog (Shh) signaling. Importantly, a recent study linked mutations in ARL13B to a classical form of Joubert syndrome (JS), an autosomal recessive disorder characterized by a distinctive cerebellar malformation. In this study, we analyzed the zebrafish arl13b (sco) mutant and gene products in detail. We first demonstrate that Arl13b is a protein that is highly enriched in the cilium and is required for cilia formation in multiple organs in zebrafish, and that knockdown of arl13b leads to multiple cilia-associated phenotypes. We additionally show that multiple regions of Arl13b are required for its localization to the cilium. By means of rescuing experiments with a series of deletion and point mutants, we further demonstrate that the ciliary localization is crucial for the in vivo function of Arl13b. Together, these results strongly support the hypothesis that JS-related disease (JSRD) is a ciliopathy, or a disease caused by ciliary defects, and that Arl13b functions mainly through the cilium.

The CIL-1 PI 5-phosphatase localizes TRP Polycystins to cilia and activates sperm in C. elegans

BACKGROUND

C. elegans male sexual behaviors include chemotaxis and response to hermaphrodites, backing, turning, vulva location, spicule insertion, and sperm transfer, culminating in cross-fertilization of hermaphrodite oocytes with male sperm. The LOV-1 and PKD-2 transient receptor potential polycystin (TRPP) complex localizes to ciliated endings of C. elegans male-specific sensory neurons and mediates several aspects of male mating behavior. TRPP complex ciliary localization and sensory function are evolutionarily conserved. A genetic screen for C. elegans mutants with PKD-2 ciliary localization (Cil) defects led to the isolation of a mutation in the cil-1 gene.

RESULTS

Here, we report that a phosphoinositide (PI) 5-phosphatase, CIL-1, regulates TRPP complex ciliary receptor localization and sperm activation. cil-1 does not regulate the localization of other ciliary proteins, including intraflagellar transport (IFT) components, sensory receptors, or other TRP channels in different cell types. Rather, cil-1 specifically controls TRPP complex trafficking in male-specific sensory neurons and does so in a cell-autonomous fashion. In these cells, cil-1 is required for normal PI(3)P distribution, indicating that a balance between PI(3,5)P2 and PI(3)P is important for TRPP localization. cil-1 mutants are infertile because of sperm activation and motility defects. In sperm, the CIL-1 5-phosphatase and a wortmannin-sensitive PI 3-kinase act antagonistically to regulate the conversion of sessile spermatids into motile spermatozoa, implicating PI(3,4,5)P3 signaling in nematode sperm activation.

CONCLUSION

Our studies identify the CIL-1 5-phosphatase as a key regulator of PI metabolism in cell types that are important in several aspects of male reproductive biology.

OFD1 is mutated in X-linked Joubert syndrome and interacts with LCA5-encoded lebercilin

We ascertained a multi-generation Malaysian family with Joubert syndrome (JS). The presence of asymptomatic obligate carrier females suggested an X-linked recessive inheritance pattern. Affected males presented with mental retardation accompanied by postaxial polydactyly and retinitis pigmentosa. Brain MRIs showed the presence of a "molar tooth sign," which classifies this syndrome as classic JS with retinal involvement. Linkage analysis showed linkage to Xpter-Xp22.2 and a maximum LOD score of 2.06 for marker DXS8022. Mutation analysis revealed a frameshift mutation, p.K948NfsX8, in exon 21 of OFD1. In an isolated male with JS, a second frameshift mutation, p.E923KfsX3, in the same exon was identified. OFD1 has previously been associated with oral-facial-digital type 1 (OFD1) syndrome, a male-lethal X-linked dominant condition, and with X-linked recessive Simpson-Golabi-Behmel syndrome type 2 (SGBS2). In a yeast two-hybrid screen of a retinal cDNA library, we identified OFD1 as an interacting partner of the LCA5-encoded ciliary protein lebercilin. We show that X-linked recessive mutations in OFD1 reduce, but do not eliminate, the interaction with lebercilin, whereas X-linked dominant OFD1 mutations completely abolish binding to lebercilin. In addition, recessive mutations in OFD1 did not affect the pericentriolar localization of the recombinant protein in hTERT-RPE1 cells, whereas this localization was lost for dominant mutations. These findings offer a molecular explanation for the phenotypic spectrum observed for OFD1 mutations; this spectrum now includes OFD1 syndrome, SGBS2, and JS.