[Show abstract][Hide abstract] ABSTRACT: A spectrum of complex oligogenic disorders called the ciliopathies have been connected to dysfunction of cilia. Among the ciliopathies are Nephronophthisis (NPHP), characterized by cystic kidney disease and retinal degeneration, and Meckel-Gruber syndrome (MKS), a gestational lethal condition with skeletal abnormalities, cystic kidneys and CNS malformation. Mutations in multiple genes have been identified in NPHP and MKS patients, and an unexpected finding has been that mutations within the same gene can cause either disorder. Further, there is minimal genotype-phenotype correlation and despite recessive inheritance, numerous patients were identified as having a single heterozygous mutation. This has made it difficult to determine the significance of these mutations on disease pathogenesis and led to the hypothesis that clinical presentation in an individual will be determined by genetic interactions between mutations in multiple cilia-related genes. Here we utilize Caenorhabditis elegans and cilia-associated behavioral and morphologic assays to evaluate the pathogenic potential of eight previously reported human NPHP4 missense mutations. We assess the impact of these mutations on C. elegans NPHP-4 function, localization and evaluate potential interactions with mutations in MKS complex genes, mksr-2 and mksr-1. Six out of eight nphp-4 mutations analyzed alter ciliary function, and three of these modify the severity of the phenotypes caused by disruption of mksr-2 and mksr-1. Collectively, our studies demonstrate the utility of C. elegans as a tool to assess the pathogenicity of mutations in ciliopathy genes and provide insights into the complex genetic interactions contributing to the diversity of phenotypes associated with cilia disorders.
Full-text · Article · May 2011 · Human Molecular Genetics
[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
We hypothesize that mutations in genes associated with human Meckel-Gruber Syndrome (MKS), Nephronophthisis (NPHP), and Bardet-Biedl Syndrome (BBS) will disrupt normal ciliary entry and/or exit of membrane-bound or membrane-associated proteins. Although disruption of MKS or NPHP genes had no overt effect on cilia formation, mutations in these genes resulted in the ciliary accumulation of proteins that are normally restricted to the cilium base and other regions of the cell. BBS gene mutations, which hinder cilia formation, also caused abnormal ciliary accumulation of non-ciliary proteins.Very little is known about the mechanisms involved in restricting access into the ciliary compartment. Previous reports have indicated that BBS mutations result in the loss of ciliary localization of some G protein-coupled receptors (GPCRs) that directly bind BBS proteins; however, similar analysis with MKS and NPHP mutations has not been performed. This present study indicates that in addition to directly regulating the ciliary entry of some proteins such as GPCRs, the BBS proteins along with the MKS and NPHP proteins will also be required for general gating of protein diffusion at the base of the cilium, which is necessary for maintaining specialization of the ciliary compartment.This work was performed in Caenorhabditis elegans strains carrying mutations in the homologs of human MKS, NPHP and BBS genes. Protein localization was examined in live worms via fluorescent-tagged transgene expression and confocal microscopy.
Factors contributing to the specialization of the cilium membrane were previously unknown. This study demonstrates that without normal function of the MKS, NPHP, or BBS protein complexes, cilium membrane gating is disrupted, and the composition of cilium membrane becomes to a greater extent continuous with the cell membrane. This likely affects the ability of the cilium to function properly as a sensory and signalling center.This study identifies the MKS, NPHP, and BBS complexes as entities involved in cilium membrane gating. Future research should therefore focus on uncovering the molecular mechanisms of how these diverse protein complexes are functioning at the base of the cilium or along the axoneme to keep some proteins outside of the cilium while allowing others entry.
[Show abstract][Hide abstract] ABSTRACT: Meckel-Gruber syndrome (MKS), nephronophthisis (NPHP), and Joubert syndrome (JBTS) are a group of heterogeneous cystic kidney disorders with partially overlapping loci. Many of the proteins associated with these diseases interact and localize to cilia and/or basal bodies. One of these proteins is MKS1, which is disrupted in some MKS patients and contains a B9 motif of unknown function that is found in two other mammalian proteins, B9D2 and B9D1. Caenorhabditis elegans also has three B9 proteins: XBX-7 (MKS1), TZA-1 (B9D2), and TZA-2 (B9D1). Herein, we report that the C. elegans B9 proteins form a complex that localizes to the base of cilia. Mutations in the B9 genes do not overtly affect cilia formation unless they are in combination with a mutation in nph-1 or nph-4, the homologues of human genes (NPHP1 and NPHP4, respectively) that are mutated in some NPHP patients. Our data indicate that the B9 proteins function redundantly with the nephrocystins to regulate the formation and/or maintenance of cilia and dendrites in the amphid and phasmid ciliated sensory neurons. Together, these data suggest that the human homologues of the novel B9 genes B9D2 and B9D1 will be strong candidate loci for pathologies in human MKS, NPHP, and JBTS.
Full-text · Article · Jun 2008 · Molecular biology of the cell
[Show abstract][Hide abstract] ABSTRACT: Defects in cilia are associated with diseases and developmental abnormalities. Proper cilia function is required for sonic hedgehog and PDGFRalpha signaling in mammals and for insulin-like growth factor (IGF) signaling in Caenorhabditis elegans. However, the role of cilia in these pathways remains unknown. To begin addressing this issue, we are characterizing putative cilia proteins in C. elegans that are predicted to have regulatory rather than structural functions. In this report, we characterized the novel cilia protein T28F3.6 (IFTA-2, intraflagellar transport associated protein 2), which is homologous to the mammalian Rab-like 5 protein. We found that, unlike the intraflagellar transport (IFT) genes, disruption of ifta-2 does not result in overt cilia assembly abnormalities, nor did it cause chemotaxis or osmotic avoidance defects typical of cilia mutants. Rather, ifta-2 null mutants have an extended lifespan phenotype and are defective in dauer formation. Our analysis indicates that these phenotypes result from defects in the DAF-2 (insulin-IGF-1-like) receptor signaling pathway in ciliated sensory neurons. We conclude that IFTA-2 is not a ciliogenic protein but rather is a regulator of specific cilia signaling activities. Interestingly, a mammalian IFTA-2 homolog is also found in cilia, raising the possibility that its function has been conserved during evolution.
No preview · Article · Nov 2006 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: 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.
Full-text · Article · Jan 2006 · Journal of Cell Science