The C. elegans homolog of the murine cystic kidney disease gene Tg737 functions in a ciliogenic pathway and is disrupted in osm-5 mutant worms

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

The ion channel polycystin-2 is required for left-right axis determination in mice

Generation of laterality depends on a pathway which involves the asymmetrically expressed genes nodal, Ebaf, Leftb, and Pitx2. In mouse, node monocilia are required upstream of the nodal cascade. In chick and frog, gap junctions are essential prior to node/organizer formation. It was hypothesized that differential activity of ion channels gives rise to unidirectional transfer through gap junctions, resulting in asymmetric gene expression. PKD2, which if mutated causes autosomal dominant polycystic kidney disease (ADPKD) in humans, encodes the calcium release channel polycystin-2. We have generated a knockout allele of Pkd2 in mouse. In addition to malformations described previously, homozygous mutant embryos showed right pulmonary isomerism, randomization of embryonic turning, heart looping, and abdominal situs. Leftb and nodal were not expressed in the left lateral plate mesoderm (LPM), and Ebaf was absent from floorplate. Pitx2 was bilaterally expressed in posterior LPM but absent anteriorly. Pkd2 was ubiquitously expressed at headfold and early somite stages, with higher levels in floorplate and notochord. The embryonic midline, however, was present, and normal levels of Foxa2 and shh were expressed, suggesting that polycystin-2 acts downstream or in parallel to shh and upstream of the nodal cascade.

A proteomic analysis of human cilia: identification of novel components

Cilia play an essential role in protecting the respiratory tract by providing the force necessary for mucociliary clearance. Although the major structural components of human cilia have been described, a complete understanding of cilia function and regulation will require identification and characterization of all ciliary components. Estimates from studies of Chlamydomonas flagella predict that an axoneme contains > or = 250 proteins. To identify all the components of human cilia, we have begun a comprehensive proteomic analysis of isolated ciliary axonemes. Analysis by two-dimensional (2-D) PAGE resulted in a highly reproducible 2-D map consisting of over 240 well resolved components. Individual protein spots were digested with trypsin and sequenced using liquid chromatography/tandem mass spectrometry (LC/MS/MS). Peptide matches were obtained to 38 potential ciliary proteins by this approach. To identify ciliary components not resolved by 2-D PAGE, axonemal proteins were separated on a one-dimensional gel. The gel lane was divided into 45 individual slices, each of which was analyzed by LC/MS/MS. This experiment resulted in peptide matches to an additional 110 proteins. In a third approach, preparations of isolated axonemes were digested with Lys-C, and the resulting peptides were analyzed directly by LC/MS/MS or by multidimensional LC/MS/MS, leading to the identification of a further 66 proteins. Each of the four approaches resulted in the identification of a subset of the proteins present. In total, sequence data were obtained on over 1400 peptides, and over 200 potential axonemal proteins were identified. Peptide matches were also obtained to over 200 human expressed sequence tags. As an approach to validate the mass spectrometry results, additional studies examined the expression of several identified proteins (annexin I, sperm protein Sp17, retinitis pigmentosa protein RP1) in cilia or ciliated cells. These studies represent the first proteomic analysis of the human ciliary axoneme and have identified many potentially novel components of this complex organelle.

A healthy understanding of intraflagellar transport

A microtubule-dependent motility process called intraflagellar transport (IFT) occurs beneath the plasma membrane of cilia and flagella. IFT was first observed in Chlamydomonas, and orthologs of some of the polypeptides involved in IFT have recently been identified in other organisms, including C. elegans and the mouse. In addition to a role in the assembly and maintenance of cilia and flagella, evidence is reviewed here that indicates defects in the process of IFT may be related to problems with human health. Moreover, recent data suggest the possibility of two new roles for IFT in cell function. The first is in transcriptional control of the genes encoding ciliary and flagellar proteins. IFT could provide a mechanism whereby the cell senses the presence or absence of its cilia or flagella and responds by turning on gene transcription resulting in replacement of the missing organelle. The second role is in signal transduction, whereby cilia act as sensors of the external cellular environment and transduce information about the surroundings into intracellular signals that are sent via IFT to the cell body, thus inducing an appropriate cellular response to the environment.

The intraflagellar transport protein, IFT88, is essential for vertebrate photoreceptor assembly and maintenance

Approximately 10% of the photoreceptor outer segment (OS) is turned over each day, requiring large amounts of lipid and protein to be moved from the inner segment to the OS. Defects in intraphotoreceptor transport can lead to retinal degeneration and blindness. The transport mechanisms are unknown, but because the OS is a modified cilium, intraflagellar transport (IFT) is a candidate mechanism. IFT involves movement of large protein complexes along ciliary microtubules and is required for assembly and maintenance of cilia. We show that IFT particle proteins are localized to photoreceptor connecting cilia. We further find that mice with a mutation in the IFT particle protein gene, Tg737/IFT88, have abnormal OS development and retinal degeneration. Thus, IFT is important for assembly and maintenance of the vertebrate OS.

Kinesin-II is required for flagellar sensory transduction during fertilization in Chlamydomonas

The assembly and maintenance of eucaryotic flagella and cilia depend on the microtubule motor, kinesin-II. This plus end-directed motor carries intraflagellar transport particles from the base to the tip of the organelle, where structural components of the axoneme are assembled. Here we test the idea that kinesin-II also is essential for signal transduction. When mating-type plus (mt+) and mating-type minus (mt-) gametes of the unicellular green alga Chlamydomonas are mixed together, binding interactions between mt+ and mt- flagellar adhesion molecules, the agglutinins, initiate a signaling pathway that leads to increases in intracellular cAMP, gamete activation, and zygote formation. A critical question in Chlamydomonas fertilization has been how agglutinin interactions are coupled to increases in intracellular cAMP. Recently, fla10 gametes with a temperature-sensitive defect in FLA10 kinesin-II were found to not form zygotes at the restrictive temperature (32 degrees C). We found that, although the rates and extents of flagellar adhesion in fla10 gametes at 32 degrees C are indistinguishable from wild-type gametes, the cells do not undergo gamete activation. On the other hand, fla10 gametes at 32 degrees C regulated agglutinin location and underwent gamete fusion when the cells were incubated in dibutyryl cAMP, indicating that their capacity to respond to the cAMP signal was intact. We show that the cellular defect in the fla10 gametes at 32 degrees C is a failure to undergo increases in cAMP during flagella adhesion. Thus, in addition to being essential for assembly and maintenance of the structural components of flagella, kinesin-II/intraflagellar transport plays a role in sensory transduction in these organelles.

Cystin, a novel cilia-associated protein, is disrupted in the cpk mouse model of polycystic kidney disease

The congenital polycystic kidney (cpk) mutation is the most extensively characterized mouse model of polycystic kidney disease (PKD). The renal cystic disease is fully expressed in homozygotes and is strikingly similar to human autosomal recessive PKD (ARPKD), whereas genetic background modulates the penetrance of the corresponding defect in the developing biliary tree. We now describe the positional cloning, mutation analysis, and expression of a novel gene that is disrupted in cpk mice. The cpk gene is expressed primarily in the kidney and liver and encodes a hydrophilic, 145-amino acid protein, which we term cystin. When expressed exogenously in polarized renal epithelial cells, cystin is detected in cilia, and its expression overlaps with polaris, another PKD-related protein. We therefore propose that the single epithelial cilium is important in the functional differentiation of polarized epithelia and that ciliary dysfunction underlies the PKD phenotype in cpk mice.

Polaris, a protein disrupted in orpk mutant mice, is required for assembly of renal cilium

Cilia are organelles that play diverse roles, from fluid movement to sensory reception. Polaris, a protein associated with cystic kidney disease in Tg737(o)(rpk) mice, functions in a ciliogenic pathway. Here, we explore the role of polaris in primary cilia on Madin-Darby canine kidney cells. The results indicate that polaris localization and solubility change dramatically during cilia formation. These changes correlate with the formation of basal bodies and large protein rafts at the apical surface of the epithelia. A cortical collecting duct cell line has been derived from mice with a mutation in the Tg737 gene. These cells do not develop normal cilia, which can be corrected by reexpression of the wild-type Tg737 gene. These data suggest that the primary cilia are important for normal renal function and/or development and that the ciliary defect may be a contributing factor to the cystic disease in Tg737(o)(rpk) mice. Further characterization of these cells will be important in elucidating the physiological role of renal cilia and in determining their relationship to cystic disease.

Localization of intraflagellar transport protein IFT52 identifies basal body transitional fibers as the docking site for IFT particles

Intraflagellar transport (IFT) is a motility in which particles composed of at least 17 polypeptides move underneath the flagellar membrane. Anterograde (outward) and retrograde (inward) movements of these IFT particles are mediated by FLA10 kinesin-II and cytoplasmic dynein DHC1b, respectively. Mutations affecting IFT particle polypeptides or motors result in the inability to assemble flagella. IFT particles and the motors moving them are located principally around the basal bodies as well as in the flagella. Here, we clone the cDNA encoding one of the IFT particle proteins, IFT52, and show by immunofluorescence that while some IFT52 is in the flagella, the majority is found in two horseshoe-shaped rings around the basal bodies. Immunoelectron microscopy indicates that IFT52 is associated with the periphery of the transitional fibers, which extend from the distal portion of the basal body to the cell membrane and demarcate the entrance to the flagellar compartment. This localization suggests that the transitional fibers form a docking complex for the IFT particles destined for the flagellum. Finally, the flagellaless mutant bld1 completely lacks IFT52 due to a deletion in the gene encoding IFT52.

The Caenorhabditis elegans autosomal dominant polycystic kidney disease gene homologs lov-1 and pkd-2 act in the same pathway

Autosomal dominant polycystic kidney disease (ADPKD) strikes 1 in 1000 individuals and often results in end-stage renal failure. Mutations in either PKD1 or PKD2 account for 95% of all cases [1-3]. It has recently been demonstrated that polycystin-1 and polycystin-2 (encoded by PKD1 and PKD2, respectively) assemble to form a cation channel in vitro [4]. Here we determine that the Caenorhabditis elegans PKD1 and PKD2 homologs, lov-1 [5] and pkd-2, act in the same pathway in vivo. Mutations in either lov-1 or pkd-2 result in identical male sensory behavioral defects. Also, pkd-2;lov-1 double mutants are no more severe than either of the single mutants, indicating that lov-1 and pkd-2 act together. LOV-1::GFP and PKD-2::GFP are expressed in the same male-specific sensory neurons and are concentrated in cilia and cell bodies. Cytoplasmic, nonnuclear staining in cell bodies is punctate, suggesting that one pool of PKD-2 is localized to intracellular membranes while another is found in sensory cilia. In contrast to defects in the C. elegans autosomal recessive PKD gene osm-5 [6-8], the cilia of lov-1 and pkd-2 single mutants and of lov-1;pkd-2 double mutants are normal as judged by electron microscopy, demonstrating that lov-1 and pkd-2 are not required for ultrastructural development of male-specific sensory cilia.