Kinesin-II, coming and going

Hedgehog-Related Mutation Causes Bone Malformations with or without Hereditary Gene Mutations

The hedgehog (Hh) family consists of numerous signaling mediators that play important roles at various stages of development. Thus, the Hh pathway is essential for bone tissue development and tumorigenesis. Gorlin syndrome is a skeletal and tumorigenic disorder caused by gain-of-function mutations in Hh signaling. In this review, we first present the phenotype of Gorlin syndrome and the relationship between genotype and phenotype in bone and craniofacial tissues, including the causative gene as well as other Hh-related genes. Next, the importance of new diagnostic methods using next-generation sequencing and multiple gene panels will be discussed. We summarize Hh-related genetic disorders, including cilia disease, and the genetics of Hh-related bone diseases.

Phosphosite T674A mutation in kinesin family member 3A fails to reproduce tissue and ciliary defects characteristic of CILK1 loss of function

BACKGROUND

Kinesin family member 3A (KIF3A) is a molecular motor protein in the heterotrimeric kinesin-2 complex that drives anterograde intraflagellar transport. This process plays a pivotal role in both biogenesis and maintenance of the primary cilium that supports tissue development. Ciliogenesis associated kinase 1 (CILK1) phosphorylates human KIF3A at Thr672. CILK1 loss of function causes ciliopathies that manifest profound and multiplex developmental defects, including hydrocephalus, polydactyly, shortened and hypoplastic bones and alveoli airspace deficiency, leading to perinatal lethality. Prior studies have raised the hypothesis that CILK1 phosphorylation of KIF3A is critical for its regulation of organ development.

RESULTS

We produced a mouse model with phosphorylation site Thr674 in mouse Kif3a mutated to Ala. Kif3a T674A homozygotes are viable and exhibit no skeletal and brain abnormalities, and only mildly reduced airspace in alveoli. Mouse embryonic fibroblasts carrying Kif3a T674A mutation show a normal rate of ciliation and a moderate increase in cilia length.

CONCLUSION

These results indicate that eliminating Kif3a Thr674 phosphorylation by Cilk1 is insufficient to reproduce the severe developmental defects in ciliopathies caused by Cilk1 loss of function. This suggests KIF3A-Thr672 phosphorylation by CILK1 is not essential for tissue development and other substrates are involved in CILK1 ciliopathies.

Primary Cilia and Mammalian Hedgehog Signaling

It has been a decade since it was discovered that primary cilia have an essential role in Hedgehog (Hh) signaling in mammals. This discovery came from screens in the mouse that identified a set of genes that are required for both normal Hh signaling and for the formation of primary cilia. Since then, dozens of mouse mutations have been identified that disrupt cilia in a variety of ways and have complex effects on Hedgehog signaling. Here, we summarize the genetic and developmental studies used to deduce how Hedgehog signal transduction is linked to cilia and the complex effects that perturbation of cilia structure can have on Hh signaling. We conclude by describing the current status of our understanding of the cell-type-specific regulation of ciliogenesis and how that determines the ability of cells to respond to Hedgehog ligands.

Microtubule Motors Drive Hedgehog Signaling in Primary Cilia

The mammalian Hedgehog (Hh) signaling pathway is required for development and for maintenance of adult stem cells, and overactivation of the pathway can cause tumorigenesis. All responses to Hh family ligands in mammals require the primary cilium, an ancient microtubule-based organelle that extends from the cell surface. Genetic studies in mice and humans have defined specific functions for cilium-associated microtubule motor proteins: they act in the construction and disassembly of the primary cilium, they control ciliary length and stability, and some have direct roles in mammalian Hh signal transduction. These studies highlight how integrated genetic and cell biological studies can define the molecular mechanisms that underlie cilium-associated health and disease.

Omics in Chlamydomonas for Biofuel Production

In response to demands for sustainable domestic fuel sources, research into biofuels has become increasingly important. Many challenges face biofuels in their effort to replace petroleum fuels, but rational strain engineering of algae and photosynthetic organisms offers a great deal of promise. For decades, mutations and stress responses in photosynthetic microbiota were seen to result in production of exciting high-energy fuel molecules, giving hope but minor capability for design. However, '-omics' techniques for visualizing entire cell processing has clarified biosynthesis and regulatory networks. Investigation into the promising production behaviors of the model organism C. reinhardtii and its mutants with these powerful techniques has improved predictability and understanding of the diverse, complex interactions within photosynthetic organisms. This new equipment has created an exciting new frontier for high-throughput, predictable engineering of photosynthetically produced carbon-neutral biofuels.

The kinesin-4 protein Kif7 regulates mammalian Hedgehog signalling by organizing the cilium tip compartment

Mammalian Hedgehog (Hh) signal transduction requires a primary cilium, a microtubule-based organelle, and the Gli-Sufu complexes that mediate Hh signalling, which are enriched at cilia tips. Kif7, a kinesin-4 family protein, is a conserved regulator of the Hh signalling pathway and a human ciliopathy protein. Here we show that Kif7 localizes to the cilium tip, the site of microtubule plus ends, where it limits cilium length and controls cilium structure. Purified recombinant Kif7 binds the plus ends of growing microtubules in vitro, where it reduces the rate of microtubule growth and increases the frequency of microtubule catastrophe. Kif7 is not required for normal intraflagellar transport or for trafficking of Hh pathway proteins into cilia. Instead, a central function of Kif7 in the mammalian Hh pathway is to control cilium architecture and to create a single cilium tip compartment, where Gli-Sufu activity can be correctly regulated.

Insights into the Mechanical Properties of the Kinesin Neck Linker Domain from Sequence Analysis and Molecular Dynamics Simulations

The 14-18 amino acid kinesin neck linker domain links the core motor to the coiled-coil dimerization domain. One puzzle is that the neck linker appears too short for the 4 nm distance each linker must stretch to enable an 8 nm step - when modeled as an entropic spring, high inter-head forces are predicted when both heads are bound to the microtubule. We addressed this by analyzing the length of the neck linker across different kinesin families and using molecular dynamics simulations to model the extensibility of Kinesin-1 and Kinesin-2 neck linkers. The force-extension profile from molecular dynamics agrees with the Worm Like Chain (WLC) model for Kinesin-1 and supports the puzzling prediction that extending the neck linker 4 nm requires forces multiple times the motor stall force. Despite being 3 amino acids longer, simulations suggest that extending the Kinesin-2 neck linker by 4 nm requires similarly high forces. A possible resolution to this dilemma is that helix α-6 may unwind to enable the two-head bound state. Finally, simulations suggest that cis/trans isomerization of a conserved proline residue in Kinesin-2 accounts for the differing predictions of molecular dynamics and the WLC model, and may contribute to motor regulation in vivo.

The intraflagellar transport protein IFT57 is required for cilia maintenance and regulates IFT-particle-kinesin-II dissociation in vertebrate photoreceptors

Defects in protein transport within vertebrate photoreceptors can result in photoreceptor degeneration. In developing and mature photoreceptors, proteins targeted to the outer segment are transported through the connecting cilium via the process of intraflagellar transport (IFT). In studies of vertebrate IFT, mutations in any component of the IFT particle typically abolish ciliogenesis, suggesting that IFT proteins are equally required for IFT. To determine whether photoreceptor outer segment formation depends equally on individual IFT proteins, we compared the retinal phenotypes of IFT57 and IFT88 mutant zebrafish. IFT88 mutants failed to form outer segments, whereas IFT57 mutants formed short outer segments with reduced amounts of opsin. Our phenotypic analysis revealed that IFT57 is not essential for IFT, but is required for efficient IFT. In co-immunoprecipitation experiments from whole-animal extracts, we determined that kinesin II remained associated with the IFT particle in the absence of IFT57, but IFT20 did not. Additionally, kinesin II did not exhibit ATP-dependent dissociation from the IFT particle in IFT57 mutants. We conclude that IFT20 requires IFT57 to associate with the IFT particle and that IFT57 and/or IFT20 mediate kinesin II dissociation.

Kinesin-2 is a Motor for Late Endosomes and Lysosomes

The bidirectional nature of late endosome/lysosome movement suggests involvement of at least two distinct motors, one minus-end directed and one plus-end directed. Previous work has identified dynein as the minus-end-directed motor for late endosome/lysosome localization and dynamics. Conventional kinesin (kinesin-1) has been implicated in plus-end-directed late endosome/lysosome movement, but other kinesin family members may also be involved. Kinesin-2 is known to drive the movement of pigment granules, a type of lysosomally derived organelle, and was recently found to be associated with purified late endosomes. To determine whether kinesin-2 might also power endosome movement in non-pigmented cells, we overexpressed dominant negative forms of the KIF3A motor subunit and KAP3 accessory subunit and knocked down KAP3 levels using RNAi. We found kinesin-2 to be required for the normal steady-state localization of late endosomes/lysosomes but not early endosomes or recycling endosomes. Despite the abnormal subcellular distribution of late endosomes/lysosomes, the uptake and trafficking of molecules through the conventional endocytic pathway appeared to be unaffected. The slow time-course of inhibition suggests that both kinesin-2 itself and its attachment to membranes do not turn over quickly.

Kinesin II and regulated intraflagellar transport of Chlamydomonas aurora protein kinase

The assembly and functioning of cilia and flagella depend on a complex system of traffic between the organelles and the cell body. Two types of transport into these organelles have been identified. The best characterized is constitutive: in a process termed intraflagellar transport (IFT), flagellar structural components are continuously carried into cilia and flagella on transport complexes termed IFT particles via the microtubule motor protein kinesin II. Previous studies have shown that the flagella of the unicellular green alga Chlamydomonas exhibit a second type of protein import that is regulated. During fertilization, the Chlamydomonas aurora protein kinase CALK undergoes regulated translocation from the cell body into the flagella. The motor that powers this second, regulated type of movement is unknown. Here, we have examined the cellular properties of the CALK in Chlamydomonas and used a kinesin II mutant to test the idea that the motor protein is essential for regulated translocation of proteins into flagella. We found that the CALK that is transported into flagella of wild-type gametes becomes part of a membrane-associated complex, that kinesin II is essential for the normal localization of this Chlamydomonas aurora protein kinase in unactivated gametes and that the cAMP-induced translocation of the protein kinase into flagella is disrupted in the fla10 mutants. Our results indicate that, in addition to its role in the constitutive transport of IFT particles and their cargo, kinesin II is essential for regulated translocation of proteins into flagella.

A complete inventory of fungal kinesins in representative filamentous ascomycetes

Complete inventories of kinesins from three pathogenic filamentous ascomycetes, Botryotinia fuckeliana, Cochliobolus heterostrophus, and Gibberella moniliformis, are described. These protein sequences were compared with those of the filamentous saprophyte, Neurospora crassa and the two yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. Data mining and phylogenetic analysis of the motor domain yielded a constant set of 10 kinesins in the filamentous fungal species, compared with a smaller set in S. cerevisiae and S. pombe. The filamentous fungal kinesins fell into nine subfamilies when compared with well-characterized kinesins from other eukaryotes. A few putative kinesins (one in B. fuckeliana and two in C. heterostrophus) could not be defined as functional, due to unorthodox organization and lack of experimental data. The broad representation of filamentous fungal kinesins across most of the known subfamilies and the ease of gene manipulation make fungi ideal models for functional and evolutionary investigation of these proteins.

Towards a molecular understanding of Drosophila hearing

The Drosophila auditory system is presented as a powerful new genetic model system for understanding the molecular aspects of development and physiology of hearing organs. The fly's ear resides in the antenna, with Johnston's organ serving as the mechanoreceptor. New approaches using electrophysiology and laser vibrometry have provided useful tools to apply to the study of mutations that disrupt hearing. The fundamental developmental processes that generate the peripheral nervous system are fairly well understood, although specific variations of these processes for chordotonal organs (CHO) and especially for Johnston's organ require more scrutiny. In contrast, even the fundamental physiologic workings of mechanosensitive systems are still poorly understood, but rapid recent progress is beginning to shed light. The identification and analysis of mutations that affect auditory function are summarized here, and prospects for the role of the Drosophila auditory system in understanding both insect and vertebrate hearing are discussed.

Kinesin-II is not essential for mitosis and cell growth in Chlamydomonas

The FLA10 gene product (Fla10p) in Chlamydomonas, a heterotrimeric kinesin-II, plays a crucial role in flagellar assembly as a motor protein driving intraflagellar transport. This protein has also been suggested to play a role in mitosis based on its localization to mitotic spindle. A role for Fla10p in mitosis has been difficult to test because to date only conditional (temperature-sensitive) mutant alleles were available, and it is not known whether these retain residual function for mitosis at the non-permissive temperature. In this report, we describe a null allele of fla10 produced by insertional mutagenesis. This mutant does not assemble flagella, but proliferates at a rate identical to that of wild type cells. Observation of microtubule organization in the cell body revealed that normal mitotic spindles are formed in dividing mutant cells. Thus, we conclude that FLA10 kinesin plays no significant roles in mitosis.

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.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

Expression of KIF3C kinesin during neural development and in vitro neuronal differentiation

KIF3A, KIF3B and KIF3C are kinesin-related motor subunits of the KIF3 family that associate to form the kinesin-II motor complex in which KIF3C and KIF3B are alternative partners of KIF3A. We have analysed the expression of Kif3 mRNAs during prenatal murine development. Kif3c transcripts are detectable from embryonic day 12.5 and persist throughout development both in the CNS and in some peripheral ganglia. Comparison of the expression patterns of the Kif3 genes revealed that Kif3c and Kif3a mRNAs colocalize in the CNS, while only Kif3a is also present outside the CNS. In contrast, Kif3b is detectable in several non-neural tissues. We have also performed immunocytochemical analyses of the developing rat brain and have found the presence of the KIF3C protein in selected brain regions and in several fibre systems. Using neuroblastoma cells as an in vitro model for neuronal differentiation, we found that retinoic acid stimulated the expression of the three Kif3 and the kinesin-associated protein genes, although with different time courses. The selective expression of Kif3c in the nervous system during embryonic development and its up-regulation during neuroblastoma differentiation suggest a role for this motor during maturation of neuronal cells.

Expression and partial characterization of kinesin-related proteins in differentiating and adult skeletal muscle

Using pan-kinesin antibodies to screen a differentiating C2C12 cell library, we identified the kinesin proteins KIF3A, KIF3B, and conventional kinesin heavy chain to be present in differentiating skeletal muscle. We compared the expression and subcellular localization characteristics of these kinesins in myogenic cells to others previously identified in muscle, neuronal, and mitotic systems (KIF1C, KIF3C, and mitotic-centromere-associated kinesin). Because members of the KIF3 subfamily of kinesin-related proteins showed altered subcellular fractionation characteristics in differentiating cells, we focused our study of kinesins in muscle on the function of kinesin-II. Kinesin-II is a motor complex comprised of dimerized KIF3A and KIF3B proteins and a tail-associated protein, KAP. The Xenopus homologue of KIF3B, Xklp3, is predominantly localized to the region of the Golgi apparatus, and overexpression of motorless-Xklp3 in Xenopus A6 cells causes mislocalization of Golgi components (). In C2C12 myoblasts and myotubes, KIF3B is diffuse and punctate, and not primarily associated with the Golgi. Overexpression of motorless-KIF3B does not perturb localization of Golgi components in myogenic cells, and myofibrillogenesis is normal. In adult skeletal muscle, KIF3B colocalizes with the excitation-contraction-coupling membranes. We propose that these membranes, consisting of the transverse-tubules and sarcoplasmic reticulum, are dynamic structures in which kinesin-II may function to actively assemble and maintain in myogenic cells.

Chlamydomonas IFT88 and its mouse homologue, polycystic kidney disease gene tg737, are required for assembly of cilia and flagella

Intraflagellar transport (IFT) is a rapid movement of multi-subunit protein particles along flagellar microtubules and is required for assembly and maintenance of eukaryotic flagella. We cloned and sequenced a Chlamydomonas cDNA encoding the IFT88 subunit of the IFT particle and identified a Chlamydomonas insertional mutant that is missing this gene. The phenotype of this mutant is normal except for the complete absence of flagella. IFT88 is homologous to mouse and human genes called Tg737. Mice with defects in Tg737 die shortly after birth from polycystic kidney disease. We show that the primary cilia in the kidney of Tg737 mutant mice are shorter than normal. This indicates that IFT is important for primary cilia assembly in mammals. It is likely that primary cilia have an important function in the kidney and that defects in their assembly can lead to polycystic kidney disease.

Sorting and transport in C. elegans: aA model system with a sequenced genome

In the past few years, yeast and cultured cells have been the model systems of choice for the study of protein sorting and transport. Recently, there has been a surge in research in these areas in Caenorhabditis elegans, with advances in experimental techniques and genomics. New in vivo assays that monitor endocytosis and neuronal transport have been used to delineate roles for several genes in these processes.