Cytoplasmic dynein heavy chain 1b is required for flagellar assembly in Chlamydomonas

Dynamics of the IFT machinery at the ciliary tip

Intraflagellar transport (IFT) is essential for the elongation and maintenance of eukaryotic cilia and flagella. Due to the traffic jam of multiple trains at the ciliary tip, how IFT trains are remodeled in these turnaround zones cannot be determined by conventional imaging. Using PhotoGate, we visualized the full range of movement of single IFT trains and motors in Chlamydomonas flagella. Anterograde trains split apart and IFT complexes mix with each other at the tip to assemble retrograde trains. Dynein-1b is carried to the tip by kinesin-II as inactive cargo on anterograde trains. Unlike dynein-1b, kinesin-II detaches from IFT trains at the tip and diffuses in flagella. As the flagellum grows longer, diffusion delays return of kinesin-II to the basal body, depleting kinesin-II available for anterograde transport. Our results suggest that dissociation of kinesin-II from IFT trains serves as a negative feedback mechanism that facilitates flagellar length control in Chlamydomonas.

Kinesin-2 motors adapt their stepping behavior for processive transport on axonemes and microtubules

Two structurally distinct filamentous tracks, namely singlet microtubules in the cytoplasm and axonemes in the cilium, serve as railroads for long-range transport processes in vivo In all organisms studied so far, the kinesin-2 family is essential for long-range transport on axonemes. Intriguingly, in higher eukaryotes, kinesin-2 has been adapted to work on microtubules in the cytoplasm as well. Here, we show that heterodimeric kinesin-2 motors distinguish between axonemes and microtubules. Unlike canonical kinesin-1, kinesin-2 takes directional, off-axis steps on microtubules, but it resumes a straight path when walking on the axonemes. The inherent ability of kinesin-2 to side-track on the microtubule lattice restricts the motor to one side of the doublet microtubule in axonemes. The mechanistic features revealed here provide a molecular explanation for the previously observed partitioning of oppositely moving intraflagellar transport trains to the A- and B-tubules of the same doublet microtubule. Our results offer first mechanistic insights into why nature may have co-evolved the heterodimeric kinesin-2 with the ciliary machinery to work on the specialized axonemal surface for two-way traffic.

Intraflagellar transport: mechanisms of motor action, cooperation, and cargo delivery

Intraflagellar transport (IFT) is a form of motor-dependent cargo transport that is essential for the assembly, maintenance, and length control of cilia, which play critical roles in motility, sensory reception, and signal transduction in virtually all eukaryotic cells. During IFT, anterograde kinesin-2 and retrograde IFT dynein motors drive the bidirectional transport of IFT trains that deliver cargo, for example, axoneme precursors such as tubulins as well as molecules of the signal transduction machinery, to their site of assembly within the cilium. Following its discovery in Chlamydomonas, IFT has emerged as a powerful model system for studying general principles of motor-dependent cargo transport and we now appreciate the diversity that exists in the mechanism of IFT within cilia of different cell types. The absence of heterotrimeric kinesin-2 function, for example, causes a complete loss of both IFT and cilia in Chlamydomonas, but following its loss in Caenorhabditis elegans, where its primary function is loading the IFT machinery into cilia, homodimeric kinesin-2-driven IFT persists and assembles a full-length cilium. Generally, heterotrimeric kinesin-2 and IFT dynein motors are thought to play widespread roles as core IFT motors, whereas homodimeric kinesin-2 motors are accessory motors that mediate different functions in a broad range of cilia, in some cases contributing to axoneme assembly or the delivery of signaling molecules but in many other cases their ciliary functions, if any, remain unknown. In this review, we focus on mechanisms of motor action, motor cooperation, and motor-dependent cargo delivery during IFT.

Ciliary Motility: Regulation of Axonemal Dynein Motors

Ciliary motility is crucial for the development and health of many organisms. Motility depends on the coordinated activity of multiple dynein motors arranged in a precise pattern on the outer doublet microtubules. Although significant progress has been made in elucidating the composition and organization of the dyneins, a comprehensive understanding of dynein regulation is lacking. Here, we focus on two conserved signaling complexes located at the base of the radial spokes. These include the I1/f inner dynein arm associated with radial spoke 1 and the calmodulin- and spoke-associated complex and the nexin-dynein regulatory complex associated with radial spoke 2. Current research is focused on understanding how these two axonemal hubs coordinate and regulate the dynein motors and ciliary motility.

The N-terminus of IFT46 mediates intraflagellar transport of outer arm dynein and its cargo-adaptor ODA16

A transposon event, resulting in partial suppression of a Chlamydomonas IFT46 null mutant, allowed the function of the N-terminus of IFT46 in flagellar assembly to be explored. The IFT46 N-terminus is not required for IFT complex assembly but is required for transport of outer arm dynein and its adaptor, ODA16, into the flagellum.

Cilia are assembled via intraflagellar transport (IFT). The IFT machinery is composed of motors and multisubunit particles, termed IFT-A and IFT-B, that carry cargo into the cilium. Knowledge of how the IFT subunits interact with their cargo is of critical importance for understanding how the unique ciliary domain is established. We previously reported a Chlamydomonas mutant, ift46-1, that fails to express the IFT-B protein IFT46, has greatly reduced levels of other IFT-B proteins, and assembles only very short flagella. A spontaneous suppression of ift46-1 restored IFT-B levels and enabled growth of longer flagella, but the flagella lacked outer dynein arms. Here we show that the suppression is due to insertion of the transposon MRC1 into the ift46-1 allele, causing the expression of a fusion protein including the IFT46 C-terminal 240 amino acids. The IFT46 C-terminus can assemble into and stabilize IFT-B but does not support transport of outer arm dynein into flagella. ODA16, a cargo adaptor specific for outer arm dynein, also fails to be imported into the flagella in the absence of the IFT46 N-terminus. We conclude that the IFT46 N-terminus, ODA16, and outer arm dynein interact for IFT of the latter.

Dynein-Driven Retrograde Intraflagellar Transport Is Triphasic in C. elegans Sensory Cilia

Cytoplasmic dynein-2 powers retrograde intraflagellar transport that is essential for cilium formation and maintenance. Inactivation of dynein-2 by mutations in DYNC2H1 causes skeletal dysplasias, and it remains unclear how the dynein-2 heavy chain moves in cilia. Here, using the genome-editing technique to produce fluorescent dynein-2 heavy chain in Caenorhabditis elegans, we show by high-resolution live microscopy that dynein-2 moves in a surprising way along distinct ciliary domains. Dynein-2 shows triphasic movement in the retrograde direction: dynein-2 accelerates in the ciliary distal region and then moves at maximum velocity and finally decelerates adjacent to the base, which may represent a physical obstacle due to transition zone barriers. By knocking the conserved ciliopathy-related mutations into the C. elegans dynein-2 heavy chain, we find that these mutations reduce its transport speed and frequency. Disruption of the dynein-2 tail domain, light intermediate chain, or intraflagellar transport (IFT)-B complex abolishes dynein-2's ciliary localization, revealing their important roles in ciliary entry of dynein-2. Furthermore, our affinity purification and genetic analyses show that IFT-A subunits IFT-139 and IFT-43 function redundantly to promote dynein-2 motility. These results reveal the molecular regulation of dynein-2 movement in sensory cilia.

Intraflagellar transport dynein is autoinhibited by trapping of its mechanical and track-binding elements

Cilia are multifunctional organelles that are constructed using intraflagellar transport (IFT) of cargo to and from their tip. It is widely held that the retrograde IFT motor, dynein-2, must be controlled in order to reach the ciliary tip and then unleashed to power the return journey. However, the mechanism is unknown. Here, we systematically define the mechanochemistry of human dynein-2 motors as monomers, dimers, and multimotor assemblies with kinesin-II. Combining these data with insights from single-particle EM, we discover that dynein-2 dimers are intrinsically autoinhibited. Inhibition is mediated by trapping dynein-2's mechanical 'linker' and 'stalk' domains within a novel motor-motor interface. We find that linker-mediated inhibition enables efficient transport of dynein-2 by kinesin-II in vitro. These results suggest a conserved mechanism for autoregulation among dimeric dyneins, which is exploited as a switch for dynein-2's recycling activity during IFT.

Protein arginine methyltransferases interact with intraflagellar transport particles and change location during flagellar growth and resorption

Protein arginine methyl transferases are located at specific sites in the flagellum. They change location during changes in flagellar dynamics (i.e., resorption and regeneration) via interaction with intraflagellar transport trains.

Changes in protein by posttranslational modifications comprise an important mechanism for the control of many cellular processes. Several flagellar proteins are methylated on arginine residues during flagellar resorption; however, the function is not understood. To learn more about the role of protein methylation during flagellar dynamics, we focused on protein arginine methyltransferases (PRMTs) 1, 3, 5, and 10. These PRMTs localize to the tip of flagella and in a punctate pattern along the length, very similar, but not identical, to that of intraflagellar transport (IFT) components. In addition, we found that PRMT 1 and 3 are also highly enriched at the base of the flagella, and the basal localization of these PRMTs changes during flagellar regeneration and resorption. Proteins with methyl arginine residues are also enriched at the tip and base of flagella, and their localization also changes during flagellar assembly and disassembly. PRMTs are lost from the flagella of fla10-1 cells, which carry a temperature-sensitive mutation in the anterograde motor for IFT. The data define the distribution of specific PRMTs and their target proteins in flagella and demonstrate that PRMTs are cargo for translocation within flagella by the process of IFT.

Intraflagellar Transport and Ciliary Dynamics

Cilia and flagella are microtubule-based organelles whose assembly requires a motile process, known as intraflagellar transport (IFT), to bring tubulin and other components to the distal tip of the growing structure. The IFT system uses a multiprotein complex with components that appear to be specialized for the transport of different sets of cargo proteins. The mechanisms by which cargo is selected for ciliary import and transport by IFT remain an area of active research. The complex dynamics of cilia and flagella are under constant regulation to ensure proper length control, and this regulation appears to involve regulation at the stage of IFT injection into the flagellum, as well as regulation of flagellar disassembly and, possibly, of cargo binding. Cilia and flagella thus represent a convenient model system to study how multiple motile and signaling pathways cooperate to control the assembly and dynamics of a complex cellular structure.

Ensemble and single-molecule dynamics of IFT dynein in Caenorhabditis elegans cilia

Cytoplasmic dyneins drive microtubule-based, minus-end directed transport in eukaryotic cells. Whereas cytoplasmic dynein 1 has been widely studied, IFT dynein has received far less attention. Here, we use fluorescence microscopy of labelled motors in living Caenorhabditis elegans to investigate IFT-dynein motility at the ensemble and single-molecule level. We find that while the kinesin composition of motor ensembles varies along the track, the amount of dynein remains relatively constant. Remarkably, this does not result in directionality changes of cargo along the track, as has been reported for other opposite-polarity, tug-of-war motility systems. At the single-molecule level, IFT-dynein trajectories reveal unexpected dynamics, including diffusion at the base, and pausing and directional switches along the cilium. Stochastic simulations show that the ensemble IFT-dynein distribution depends upon the probability of single-motor directional switches. Our results provide quantitative insight into IFT-dynein dynamics in vivo, shedding light on the complex functioning of dynein motors in general.

Functional exploration of the IFT-A complex in intraflagellar transport and ciliogenesis

Intraflagellar transport (IFT) particles or trains are composed of IFT-A and IFT-B complexes. To assess the working mechanism of the IFT-A complex in IFT and ciliogenesis, we have analyzed ift43 mutants of Chlamydomnonas in conjunction with mutants of the other IFT-A subunits. An ift43 null mutant or a mutant with a partial deletion of the IFT43 conserved domain has no or short flagella. The mutants accumulate not only IFT-B but also IFT-Ain the short flagella, which is in contrast to an ift140 null mutant. The IFT43 conserved domain is necessary and sufficient for the function of IFT43. IFT43 directly interacts with IFT121 and loss of IFT43 results in instability of IFT-A. A construct with a partial deletion of the IFT43 conserved domain is sufficient to rescue the instability phenotype of IFT-A, but results in diminishing of IFT-A at the peri-basal body region. We have further provided evidence for the direct interactions within the IFT-A complex and shown that the integrity of IFT-A is important for its stability and cellular localization. Finally, we show that both IFT43 and IFT140 are involved in mobilizing ciliary precursors from the cytoplasmic pool during flagellar regeneration, suggesting a novel role of IFT-A in transporting ciliary components in the cytoplasm to the peri-basal body region.

IFT57 stabilizes assembled intraflagellar transport complex and mediates transport of motility-related flagellar cargo

Intraflagellar Transport (IFT) is essential for flagella/cilia assembly and maintenance. Recent biochemical studies have shown that IFT-B is comprised of two subcomplexes, IFT-B1 and IFT-B2. The IFT-B2 subunit IFT57 lies at the interface between IFT-B1 and IFT-B2. Here, using a Chlamydomonas mutant for IFT57, we tested whether IFT57 is critical for IFT-B complex assembly by bridging IFT-B1 and IFT-B2 together. In the ift57-1 mutant, IFT57 and other IFT-B proteins were greatly reduced at the whole-cell level. Strikingly, in the protease free flagellar compartment, while the level of IFT57 was reduced, other IFT particle proteins were not concomitantly reduced but present at the wild-type level. The IFT movement of the IFT57-deficient-IFT particles was also unchanged. Moreover, IFT57 depletion disrupted the flagellar waveform, leading to cell swimming defects. Analysis of the mutant flagellar protein composition showed that certain axonemal proteins were altered. Taken together, these findings suggest that IFT57 does not play an essential structural role in the IFT particle complex but rather functions to prevent it from degradation. Additionally, IFT57 is involved in transporting specific motility-related proteins.

Primary Cilia in Cystic Kidney Disease

Primary cilia are small, antenna-like structures that detect mechanical and chemical cues and transduce extracellular signals. While mammalian primary cilia were first reported in the late 1800s, scientific interest in these sensory organelles has burgeoned since the beginning of the twenty-first century with recognition that primary cilia are essential to human health. Among the most common clinical manifestations of ciliary dysfunction are renal cysts. The molecular mechanisms underlying renal cystogenesis are complex, involving multiple aberrant cellular processes and signaling pathways, while initiating molecular events remain undefined. Autosomal Dominant Polycystic Kidney Disease is the most common renal cystic disease, caused by disruption of polycystin-1 and polycystin-2 transmembrane proteins, which evidence suggests must localize to primary cilia for proper function. To understand how the absence of these proteins in primary cilia may be remediated, we review intracellular trafficking of polycystins to the primary cilium. We also examine the controversial mechanisms by which primary cilia transduce flow-mediated mechanical stress into intracellular calcium. Further, to better understand ciliary function in the kidney, we highlight the LKB1/AMPK, Wnt, and Hedgehog developmental signaling pathways mediated by primary cilia and misregulated in renal cystic disease.

The Intraflagellar Transport Machinery

Eukaryotic cilia and flagella are evolutionarily conserved organelles that protrude from the cell surface. The unique location and properties of cilia allow them to function in vital processes such as motility and signaling. Ciliary assembly and maintenance rely on intraflagellar transport (IFT), the bidirectional movement of a multicomponent transport system between the ciliary base and tip. Since its initial discovery more than two decades ago, considerable effort has been invested in dissecting the molecular mechanisms of IFT in a variety of model organisms. Importantly, IFT was shown to be essential for mammalian development, and defects in this process cause a number of human pathologies known as ciliopathies. Here, we review current knowledge of IFT with a particular emphasis on the IFT machinery and specific mechanisms of ciliary cargo recognition and transport.

The trafficking of bacterial type rhodopsins into the Chlamydomonas eyespot and flagella is IFT mediated

The bacterial type rhodopsins are present in all the three domains of life. In contrast to the animal type rhodopsin that performs mainly sensory functions in higher eukaryotes, the bacterial type rhodopsin could function as ion channel, pumps and as sensory proteins. The functioning of rhodopsin in higher eukaryotes requires the transport of rhodopsin from its site of synthesis to the ciliated outer segment of the photoreceptive cells. However, the trafficking of bacterial type rhodopsin from its site of synthesis to the position of action is not characterized. Here we present the first report for the existence of an IFT-interactome mediated trafficking of the bacterial type rhodopsins into eyespot and flagella of the Chlamydomonas. We show that there is a light-dependent, dynamic localization of rhodopsins between flagella and eyespot of Chlamydomonas. The involvement of IFT components in the rhodopsin trafficking was elucidated by the use of conditional IFT mutants. We found that rhodopsin can be co-immunoprecipitated with the components of IFT machinery and with other protein components required for the IFT-cargo complex formation. These findings show that light-regulated localization of rhodopsin is not restricted to animals thereby suggesting that rhodopsin trafficking is an IFT dependent ancient process.

Chibby functions to preserve normal ciliary morphology through the regulation of intraflagellar transport in airway ciliated cells

Airway cilia provide the coordinated motive force for mucociliary transport, which prevents the accumulation of mucus, debris, pollutants, and bacteria in our respiratory tracts. As airway cilia are constantly exposed to the environment and, hence, are an integral component of the pathogenesis of several congenital and chronic pulmonary disorders, it is necessary to understand the molecular mechanisms that control ciliated cell differentiation and ciliogenesis. We have previously reported that loss of the basal body protein Chibby (Cby) results in chronic upper airway infection in mice due to a significant reduction in the number of airway cilia. In the present work, we demonstrate that Cby is required for normal ciliary structure and proper distribution of proteins involved in the bidirectional intraflagellar transport (IFT) system, which consists of 2 distinct sub-complexes, IFT-A and IFT-B, and is essential for ciliary biogenesis and maintenance. In fully differentiated ciliated cells, abnormal paddle-like cilia with dilated ciliary tips are observed in Cby-/- airways and primary cultures of mouse tracheal epithelial cells (MTECs). In addition, IFT88, an IFT-B sub-complex protein, robustly accumulates within the dilated tips of both multicilia in Cby-/- MTECs and primary cilia in Cby-/- mouse embryonic fibroblasts (MEFs). Furthermore, we show that only IFT-B components, including IFT20 and IFT57, but not IFT-A and Bardet-Biedl syndrome (BBS) proteins, amass with IFT88 in these distended tips in Cby-/- ciliated cells. Taken together, our findings suggest that Cby plays a role in the proper distribution of IFT particles to preserve normal ciliary morphology in airway ciliated cells.

The role of the dynein light intermediate chain in retrograde IFT and flagellar function in Chlamydomonas

The assembly of cilia and flagella depends on the activity of two microtubule motor complexes, kinesin-2 and dynein-2/1b, but the specific functions of the different subunits are poorly defined. Here we analyze Chlamydomonas strains expressing different amounts of the dynein 1b light intermediate chain (D1bLIC). Disruption of D1bLIC alters the stability of the dynein 1b complex and reduces both the frequency and velocity of retrograde intraflagellar transport (IFT), but it does not eliminate retrograde IFT. Flagellar assembly, motility, gliding, and mating are altered in a dose-dependent manner. iTRAQ-based proteomics identifies a small subset of proteins that are significantly reduced or elevated in d1blic flagella. Transformation with D1bLIC-GFP rescues the mutant phenotypes, and D1bLIC-GFP assembles into the dynein 1b complex at wild-type levels. D1bLIC-GFP is transported with anterograde IFT particles to the flagellar tip, dissociates into smaller particles, and begins processive retrograde IFT in <2 s. These studies demonstrate the role of D1bLIC in facilitating the recycling of IFT subunits and other proteins, identify new components potentially involved in the regulation of IFT, flagellar assembly, and flagellar signaling, and provide insight into the role of D1bLIC and retrograde IFT in other organisms.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Microtubule doublets are double-track railways for intraflagellar transport trains

The cilium is a large macromolecular machine that is vital for motility, signaling, and sensing in most eukaryotic cells. Its conserved core structure, the axoneme, contains nine microtubule doublets, each comprising a full A-microtubule and an incomplete B-microtubule. However, thus far, the function of this doublet geometry has not been understood. We developed a time-resolved correlative fluorescence and three-dimensional electron microscopy approach to investigate the dynamics of intraflagellar transport (IFT) trains, which carry ciliary building blocks along microtubules during the assembly and disassembly of the cilium. Using this method, we showed that each microtubule doublet is used as a bidirectional double-track railway: Anterograde IFT trains move along B-microtubules, and retrograde trains move along A-microtubules. Thus, the microtubule doublet geometry provides direction-specific rails to coordinate bidirectional transport of ciliary components.

Two classes of short intraflagellar transport train with different 3D structures are present in Chlamydomonas flagella

Intraflagellar transport (IFT) is responsible for the bidirectional trafficking of molecular components required for the elongation and maintenance of eukaryotic cilia and flagella. Cargo is transported by IFT 'trains', linear rows of multiprotein particles moved by molecular motors along the axonemal doublets. We have previously described two structurally distinct categories of 'long' and 'short' trains. Here, we analyse the relative number of these trains throughout flagellar regeneration and show that long trains are most abundant at the beginning of flagellar growth whereas short trains gradually increase in number as flagella elongate. These observations are incompatible with the previous hypothesis that short trains are derived solely from the reorganization of long trains at the flagellar tip. We demonstrate with electron tomography the existence of two distinct ultrastructural organizations for the short trains, we name these 'narrow' and 'wide', and provide the first 3D model of the narrow short trains. These trains are characterized by tri-lobed units, which repeat longitudinally every 16 nm and contact protofilament 7 of the B-tubule. Functional implications of the new structural evidence are discussed.

Intraflagellar transport proteins 172, 80, 57, 54, 38, and 20 form a stable tubulin-binding IFT-B2 complex

Intraflagellar transport (IFT) relies on the IFT complex and is required for ciliogenesis. The IFT‐B complex consists of 9–10 stably associated core subunits and six “peripheral” subunits that were shown to dissociate from the core structure at moderate salt concentration. We purified the six “peripheral” IFT‐B subunits of Chlamydomonas reinhardtii as recombinant proteins and show that they form a stable complex independently of the IFT‐B core. We suggest a nomenclature of IFT‐B1 (core) and IFT‐B2 (peripheral) for the two IFT‐B subcomplexes. We demonstrate that IFT88, together with the N‐terminal domain of IFT52, is necessary to bridge the interaction between IFT‐B1 and B2. The crystal structure of IFT52N reveals highly conserved residues critical for IFT‐B1/IFT‐B2 complex formation. Furthermore, we show that of the three IFT‐B2 subunits containing a calponin homology (CH) domain (IFT38, 54, and 57), only IFT54 binds αβ‐tubulin as a potential IFT cargo, whereas the CH domains of IFT38 and IFT57 mediate the interaction with IFT80 and IFT172, respectively. Crystal structures of IFT54 CH domains reveal that tubulin binding is mediated by basic surface‐exposed residues.

Cytoplasmic Dynein Antagonists with Improved Potency and Isoform Selectivity

Cytoplasmic dyneins 1 and 2 are related members of the AAA+ superfamily (ATPases associated with diverse cellular activities) that function as the predominant minus-end-directed microtubule motors in eukaryotic cells. Dynein 1 controls mitotic spindle assembly, organelle movement, axonal transport, and other cytosolic, microtubule-guided processes, whereas dynein 2 mediates retrograde trafficking within motile and primary cilia. Small-molecule inhibitors are important tools for investigating motor protein-dependent mechanisms, and ciliobrevins were recently discovered as the first dynein-specific chemical antagonists. Here, we demonstrate that ciliobrevins directly target the heavy chains of both dynein isoforms and explore the structure–activity landscape of these inhibitors in vitro and in cells. In addition to identifying chemical motifs that are essential for dynein blockade, we have discovered analogs with increased potency and dynein 2 selectivity. These antagonists effectively disrupt Hedgehog signaling, intraflagellar transport, and ciliogenesis, making them useful probes of these and other cytoplasmic dynein 2-dependent cellular processes.

Recombinant Reconstitution and Purification of the IFT-B Core Complex from Chlamydomonas reinhardtii

Eukaryotic cilia and flagella are assembled and maintained by intraflagellar transport (IFT), the bidirectional transport of proteins between the ciliary base and tip. IFT is mediated by the multi-subunit IFT complex, which simultaneously binds cargo proteins and the ciliary motors. So far 22 subunits of the IFT complex have been identified, but insights into the biochemical architecture and especially the three-dimensional structure of this machinery are only starting to emerge because of difficulties in obtaining homogeneous material suitable for structural analysis. Here, we describe a protocol for the purification and reconstitution of a complex containing nine Chlamydomonas reinhardtii IFT proteins, commonly known as the IFT-B core complex. In our hands, this protocol routinely yields several milligrams of pure complex suitable for structural analysis by X-ray crystallography and single-particle cryo-electron microscopy.

Purification and Localization of Intraflagellar Transport Particles and Polypeptides

The growth and maintenance of almost all cilia and flagella are dependent on the proper functioning of the process of intraflagellar transport (IFT). This includes the primary cilia of most human cells that are in the Go phase of the cell cycle. The model system for the study of IFT is the flagella of the biflagellate green alga Chlamydomonas. It is in this organism that IFT was first discovered, and genetic data from a Chlamydomonas mutant first linked the process of IFT to polycystic kidney disease in humans. The information provided in this chapter addresses procedures to purify IFT particles from flagella and localize these particles, and their associated motor proteins, in flagella using light and electron microscopic approaches.

IFT-Cargo Interactions and Protein Transport in Cilia

The motile and sensory functions of cilia and flagella are indispensable for human health. Cilia assembly requires a dedicated protein shuttle, intraflagellar transport (IFT), a bidirectional motility of multi-megadalton protein arrays along ciliary microtubules. IFT functions as a protein carrier delivering hundreds of distinct proteins into growing cilia. IFT-based protein import and export continue in fully grown cilia and are required for ciliary maintenance and sensing. Large ciliary building blocks might depend on IFT to move through the transition zone, which functions as a ciliary gate. Smaller, freely diffusing proteins, such as tubulin, depend on IFT to be concentrated or removed from cilia. As I discuss here, recent work provides insights into how IFT interacts with its cargoes and how the transport is regulated.

Functional differentiation of cooperating kinesin-2 motors orchestrates cargo import and transport in C. elegans cilia

Intracellular transport depends on cooperation between distinct motor proteins. Two anterograde intraflagellar transport (IFT) motors, heterotrimeric kinesin-II and homodimeric OSM-3, cooperate to move cargo along Caenorhabditis elegans cilia. Here, using quantitative fluorescence microscopy, with single-molecule sensitivity, of IFT in living strains containing single-copy transgenes encoding fluorescent IFT proteins, we show that kinesin-II transports IFT trains through the ciliary base and transition zone to a 'handover zone' on the proximal axoneme. There, OSM-3 gradually replaces kinesin-II, yielding velocity profiles inconsistent with in vitro motility assays, and then drives transport to the ciliary tip. Dissociated kinesin-II motors undergo rapid turnaround and recycling to the ciliary base, whereas OSM-3 is recycled mainly to the handover zone. This reveals a functional differentiation in which the slower, less processive kinesin-II imports IFT trains into the cilium and OSM-3 drives their long-range transport, thereby optimizing cargo delivery.

Intraflagellar transport is essential for mammalian spermiogenesis but is absent in mature sperm

Intraflagellar transport (IFT) is necessary for the assembly and maintenance of most cilia, with the exception of gametic flagella in some organisms. IFT is required for assembly of mouse sperm flagella, and defects in IFT lead to male infertility. However, mature sperm lack IFT proteins and thus do not require IFT for maintenance of the axoneme.

Drosophila sperm are unusual in that they do not require the intraflagellar transport (IFT) system for assembly of their flagella. In the mouse, the IFT proteins are very abundant in testis, but we here show that mature sperm are completely devoid of them, making the importance of IFT to mammalian sperm development unclear. To address this question, we characterized spermiogenesis and fertility in the Ift88^Tg737Rpw mouse. This mouse has a hypomorphic mutation in the gene encoding the IFT88 subunit of the IFT particle. This mutation is highly disruptive to ciliary assembly in other organs. Ift88^−/− mice are completely sterile. They produce ∼350-fold fewer sperm than wild-type mice, and the remaining sperm completely lack or have very short flagella. The short flagella rarely have axonemes but assemble ectopic microtubules and outer dense fibers and accumulate improperly assembled fibrous sheath proteins. Thus IFT is essential for the formation but not the maintenance of mammalian sperm flagella.

Reduced tubulin polyglutamylation suppresses flagellar shortness in Chlamydomonas

Tubulin polyglutamylation is a posttranslational modification known to affect ciliary/flagellar motility and assembly. Investigation of Chlamydomonas mutants deficient in axonemal polyglutamylation shows that polyglutamylation functions by increasing tubulin turnover at the flagellar tip and reducing axonemal stability.

Ciliary length control is an incompletely understood process essential for normal ciliary function. The flagella of Chlamydomonas mutants lacking multiple axonemal dyneins are shorter than normal; previously it was shown that this shortness can be suppressed by the mutation suppressor of shortness 1 (ssh1) via an unknown mechanism. To elucidate this mechanism, we carried out genetic analysis of ssh1 and found that it is a new allele of TPG2 (hereafter tpg2-3), which encodes FAP234 functioning in tubulin polyglutamylation in the axoneme. Similar to the polyglutamylation-deficient mutants tpg1 and tpg2-1, tpg2-3 axonemal tubulin has a greatly reduced level of long polyglutamate side chains. We found that tpg1 and tpg2-1 mutations also promote flagellar elongation in short-flagella mutants, consistent with a polyglutamylation-dependent mechanism of suppression. Double mutants of tpg1 or tpg2-1 and fla10-1, a temperature-sensitive mutant of intraflagellar transport, underwent slower flagellar shortening than fla10-1 at restrictive temperatures, indicating that the rate of tubulin disassembly is decreased in the polyglutamylation-deficient flagella. Moreover, α-tubulin incorporation into the flagellar tips in temporary dikaryons was retarded in polyglutamylation-deficient flagella. These results show that polyglutamylation deficiency stabilizes axonemal microtubules, decelerating axonemal disassembly at the flagellar tip and shifting the axonemal assembly/disassembly balance toward assembly.

Mutations in DYNC2LI1 disrupt cilia function and cause short rib polydactyly syndrome

The short rib polydactyly syndromes (SRPSs) are a heterogeneous group of autosomal recessive, perinatal lethal skeletal disorders characterized primarily by short, horizontal ribs, short limbs and polydactyly. Mutations in several genes affecting intraflagellar transport (IFT) cause SRPS but they do not account for all cases. Here we identify an additional SRPS gene and further unravel the functional basis for IFT. We perform whole-exome sequencing and identify mutations in a new disease-producing gene, cytoplasmic dynein-2 light intermediate chain 1, DYNC2LI1, segregating with disease in three families. Using primary fibroblasts, we show that DYNC2LI1 is essential for dynein-2 complex stability and that mutations in DYNC2LI1 result in variable length, including hyperelongated, cilia, Hedgehog pathway impairment and ciliary IFT accumulations. The findings in this study expand our understanding of SRPS locus heterogeneity and demonstrate the importance of DYNC2LI1 in dynein-2 complex stability, cilium function, Hedgehog regulation and skeletogenesis.

TCTEX1D2 mutations underlie Jeune asphyxiating thoracic dystrophy with impaired retrograde intraflagellar transport

The analysis of individuals with ciliary chondrodysplasias can shed light on sensitive mechanisms controlling ciliogenesis and cell signalling that are essential to embryonic development and survival. Here we identify TCTEX1D2 mutations causing Jeune asphyxiating thoracic dystrophy with partially penetrant inheritance. Loss of TCTEX1D2 impairs retrograde intraflagellar transport (IFT) in humans and the protist Chlamydomonas, accompanied by destabilization of the retrograde IFT dynein motor. We thus define TCTEX1D2 as an integral component of the evolutionarily conserved retrograde IFT machinery. In complex with several IFT dynein light chains, it is required for correct vertebrate skeletal formation but may be functionally redundant under certain conditions.

Severe congenital development defects such as Jeune syndrome can result from the malfunction of primary cilia and dynein. Here Schmidts et al. report unique biallelic null mutations in a gene encoding a dynein light chain, helping to explain the nature of ciliopathies in human patients.

The Role of RPGR and Its Interacting Proteins in Ciliopathies

Ciliopathies encompass a group of genetic disorders characterized by defects in the formation, maintenance, or function of cilia. Retinitis pigmentosa (RP) is frequently one of the clinical features presented in diverse ciliopathies. RP is a heterogeneous group of inherited retinal disorders, characterized by the death of photoreceptors and affecting more than one million individuals worldwide. The retinitis pigmentosa GTPase regulator (RPGR) gene is mutated in up to 20% of all RP patients. RPGR protein has different interacting partners to function in ciliary protein trafficking. In this review, we specifically focus on RPGR and its two interacting proteins: RPGRIP1 and RPGRIP1L. We summarize the function of the three proteins and highlight recent studies that provide insight into the cellular function of those proteins.

Somatic CRISPR-Cas9-induced mutations reveal roles of embryonically essential dynein chains in Caenorhabditis elegans cilia

CRISPR–Cas9-induced mutations in intraflagellar transport (IFT) motors reveal that IFT-specific dynein and cytoplasmic dynein have unique compositions but share components and regulatory mechanisms.

Cilium formation and maintenance require intraflagellar transport (IFT). Although much is known about kinesin-2–driven anterograde IFT, the composition and regulation of retrograde IFT-specific dynein remain elusive. Components of cytoplasmic dynein may participate in IFT; however, their essential roles in cell division preclude functional studies in postmitotic cilia. Here, we report that inducible expression of the clustered regularly interspaced short palindromic repeats (CRISPR)–Cas9 system in Caenorhabditis elegans generated conditional mutations in IFT motors and particles, recapitulating ciliary defects in their null mutants. Using this method to bypass the embryonic requirement, we show the following: the dynein intermediate chain, light chain LC8, and lissencephaly-1 regulate retrograde IFT; the dynein light intermediate chain functions in dendrites and indirectly contributes to ciliogenesis; and the Tctex and Roadblock light chains are dispensable for cilium assembly. Furthermore, we demonstrate that these components undergo biphasic IFT with distinct transport frequencies and turnaround behaviors. Together, our results suggest that IFT–dynein and cytoplasmic dynein have unique compositions but also share components and regulatory mechanisms.

Genetic and genomic approaches to identify genes involved in flagellar assembly in Chlamydomonas reinhardtii

Flagellar assembly requires intraflagellar transport of components from the cell body to the flagellar tip for assembly. The understanding of flagellar assembly has been aided by the ease of biochemistry and the availability of mutants in the unicellular green alga, Chlamydomonas reinhardtii. In this chapter, we discuss means to identify genes involved in these processes using forward and reverse genetics. In particular, the ease and low cost of whole genome sequencing (WGS) will help to make gene identification easier and promote the understanding of this important process.

Crystal structures of IFT70/52 and IFT52/46 provide insight into intraflagellar transport B core complex assembly

Cilia are microtubule-based organelles that assemble via intraflagellar transport (IFT) and function as signaling hubs on eukaryotic cells. IFT relies on molecular motors and IFT complexes that mediate the contacts with ciliary cargo. To elucidate the architecture of the IFT-B complex, we reconstituted and purified the nonameric IFT-B core from Chlamydomonas reinhardtii and determined the crystal structures of C. reinhardtii IFT70/52 and Tetrahymena IFT52/46 subcomplexes. The 2.5-Å resolution IFT70/52 structure shows that IFT52330-370 is buried deeply within the IFT70 tetratricopeptide repeat superhelix. Furthermore, the polycystic kidney disease protein IFT88 binds IFT52281-329 in a complex that interacts directly with IFT70/IFT52330-381 in trans. The structure of IFT52C/IFT46C was solved at 2.3 Å resolution, and we show that it is essential for IFT-B core integrity by mediating interaction between IFT88/70/52/46 and IFT81/74/27/25/22 subcomplexes. Consistent with this, overexpression of mammalian IFT52C in MDCK cells is dominant-negative and causes IFT protein mislocalization and disrupted ciliogenesis. These data further rationalize several ciliogenesis phenotypes of IFT mutant strains.

The intraflagellar transport dynein complex of trypanosomes is made of a heterodimer of dynein heavy chains and of light and intermediate chains of distinct functions

Cilia and flagella are assembled by intraflagellar transport (IFT) of protein complexes that bring tubulin and other precursors to the incorporation site at their distal tip. Anterograde transport is driven by kinesin, whereas retrograde transport is ensured by a specific dynein. In the protist Trypanosoma brucei, two distinct genes encode fairly different dynein heavy chains (DHCs; ∼40% identity) termed DHC2.1 and DHC2.2, which form a heterodimer and are both essential for retrograde IFT. The stability of each heavy chain relies on the presence of a dynein light intermediate chain (DLI1; also known as XBX-1/D1bLIC). The presence of both heavy chains and of DLI1 at the base of the flagellum depends on the intermediate dynein chain DIC5 (FAP133/WDR34). In the IFT140(RNAi) mutant, an IFT-A protein essential for retrograde transport, the IFT dynein components are found at high concentration at the flagellar base but fail to penetrate the flagellar compartment. We propose a model by which the IFT dynein particle is assembled in the cytoplasm, reaches the base of the flagellum, and associates with the IFT machinery in a manner dependent on the IFT-A complex.

A differential cargo-loading model of ciliary length regulation by IFT

BACKGROUND

During the assembly and maintenance of cilia, precursor proteins need to be transported from the cell body into the organelle. Intraflagellar transport (IFT) is assumed to be the predominant protein transport pathway in cilia, but it remains largely unknown how ciliary proteins use IFT to reach their destination sites in the cilium and whether the amount of cargo transported by IFT is regulated.

RESULTS

Single-particle imaging showed that DRC4, a structural protein of the axoneme, moves in association with IFT particles inside Chlamydomonas reinhardtii cilia. IFT is required for DRC4 transport both into and within the cilium. DRC4 cargoes dissociate from IFT trains at the tip as well as at various sites along the length of the cilium. Unloaded DRC4 diffuses before docking at its axonemal assembly site. In growing cilia, DRC4 transport by IFT was strongly increased over the steady-state level, and the frequency decreased linearly with the increasing ciliary length. The frequency of DRC4 transport was similarly elevated in short growth-arrested cilia and remained high even when the amount of DRC4 available in the cell body was reduced.

CONCLUSIONS

DRC4 is a bona fide cargo of IFT. Incompletely assembled cilia trigger an increase in the amount of DRC4 cargo transported by IFT particles, and DRC4 transport is downregulated as cilia approach their steady-state length. We propose a model in which ciliary length is controlled by regulating the amount of cargo transported by IFT particles.

Getting to the heart of intraflagellar transport using Trypanosoma and Chlamydomonas models: the strength is in their differences

Cilia and flagella perform diverse roles in motility and sensory perception, and defects in their construction or their function are responsible for human genetic diseases termed ciliopathies. Cilia and flagella construction relies on intraflagellar transport (IFT), the bi-directional movement of ‘trains’ composed of protein complexes found between axoneme microtubules and the flagellum membrane. Although extensive information about IFT components and their mode of action were discovered in the green algae Chlamydomonas reinhardtii, other model organisms have revealed further insights about IFT. This is the case of Trypanosoma brucei, a flagellated protist responsible for sleeping sickness that is turning out to be an emerging model for studying IFT. In this article, we review different aspects of IFT, based on studies of Chlamydomonas and Trypanosoma. Data available from both models are examined to ask challenging questions about IFT such as the initiation of flagellum construction, the setting-up of IFT and the mode of formation of IFT trains, and their remodeling at the tip as well as their recycling at the base. Another outstanding question is the individual role played by the multiple IFT proteins. The use of different models, bringing their specific biological and experimental advantages, will be invaluable in order to obtain a global understanding of IFT.

The ciliary cytoskeleton

Cilia and flagella are surface-exposed, finger-like organelles whose core consists of a microtubule (MT)-based axoneme that grows from a modified centriole, the basal body. Cilia are found on the surface of many eukaryotic cells and play important roles in cell motility and in coordinating a variety of signaling pathways during growth, development, and tissue homeostasis. Defective cilia have been linked to a number of developmental disorders and diseases, collectively called ciliopathies. Cilia are dynamic organelles that assemble and disassemble in tight coordination with the cell cycle. In most cells, cilia are assembled during growth arrest in a multistep process involving interaction of vesicles with appendages present on the distal end of mature centrioles, and addition of tubulin and other building blocks to the distal tip of the basal body and growing axoneme; these building blocks are sorted through a region at the cilium base known as the ciliary necklace, and then transported via intraflagellar transport (IFT) along the axoneme toward the tip for assembly. After assembly, the cilium frequently continues to turn over and incorporate tubulin at its distal end in an IFT-dependent manner. Prior to cell division, the cilia are usually resorbed to liberate centrosomes for mitotic spindle pole formation. Here, we present an overview of the main cytoskeletal structures associated with cilia and centrioles with emphasis on the MT-associated appendages, fibers, and filaments at the cilium base and tip. The composition and possible functions of these structures are discussed in relation to cilia assembly, disassembly, and length regulation.

New mutations in flagellar motors identified by whole genome sequencing in Chlamydomonas

Background

The building of a cilium or flagellum requires molecular motors and associated proteins that allow the relocation of proteins from the cell body to the distal end and the return of proteins to the cell body in a process termed intraflagellar transport (IFT). IFT trains are carried out by kinesin and back to the cell body by dynein.

Methods

We used whole genome sequencing to identify the causative mutations for two temperature-sensitive flagellar assembly mutants in Chlamydomonas and validated the changes using reversion analysis. We examined the effect of these mutations on the localization of IFT81, an IFT complex B protein, the cytoplasmic dynein heavy chain (DHC1b), and the dynein light intermediate chain (D1bLIC).

Results

The strains, fla18 and fla24, have mutations in kinesin-2 and cytoplasmic dynein, respectively. The fla18 mutation alters the same glutamic acid (E₂₄G) mutated in the fla10-14 allele (E₂₄K). The fla18 strain loses flagella at 32?C more rapidly than the E₂₄K allele but less rapidly than the fla10-1 allele. The fla18 mutant loses its flagella by detachment rather than by shortening. The fla24 mutation falls in cytoplasmic dynein and changes a completely conserved amino acid (L₃₂₄₃P) in an alpha helix in the AAA5 domain. The fla24 mutant loses its flagella by shortening within 6 hours at 32?C. DHC1b protein is reduced by 18-fold and D1bLIC is reduced by 16-fold at 21?C compared to wild-type cells. We identified two pseudorevertants (L₃₂₄₃S and L₃₂₄₃R), which remain flagellated at 32?C. Although fla24 cells assemble full-length flagella at 21?C, IFT81 protein localization is dramatically altered. Instead of localizing at the basal body and along the flagella, IFT81 is concentrated at the proximal end of the flagella. The pseudorevertants show wild-type IFT81 localization at 21?C, but proximal end localization of IFT81 at 32?C.

Conclusions

The change in the AAA5 domain of the cytoplasmic dynein in fla24 may block the recycling of IFT trains after retrograde transport. It is clear that different alleles in the flagellar motors reveal different functions and roles. Multiple alleles will be important for understanding structure-function relationships.

Functions and mechanics of dynein motor proteins

Fuelled by ATP hydrolysis, dyneins generate force and movement on microtubules in a wealth of biological processes, including ciliary beating, cell division and intracellular transport. The large mass and complexity of dynein motors have made elucidating their mechanisms a sizable task. Yet, through a combination of approaches, including X-ray crystallography, cryo-electron microscopy, single-molecule assays and biochemical experiments, important progress has been made towards understanding how these giant motor proteins work. From these studies, a model for the mechanochemical cycle of dynein is emerging, in which nucleotide-driven flexing motions within the AAA+ ring of dynein alter the affinity of its microtubule-binding stalk and reshape its mechanical element to generate movement.

A role for the Golgi matrix protein giantin in ciliogenesis through control of the localization of dynein-2

The correct formation of primary cilia is central to the development and function of nearly all cells and tissues. Cilia grow from the mother centriole by extension of a microtubule core, the axoneme, which is then surrounded with a specialized ciliary membrane that is continuous with the plasma membrane. Intraflagellar transport moves particles along the length of the axoneme to direct assembly of the cilium and is also required for proper cilia function. The microtubule motor, cytoplasmic dynein-2 mediates retrograde transport along the axoneme from the tip to the base; dynein-2 is also required for some aspects of cilia formation. In most cells, the Golgi lies adjacent to the centrioles and key components of the cilia machinery localize to this organelle. Golgi-localized proteins have also been implicated in ciliogenesis and in intraflagellar transport. Here, we show that the transmembrane Golgi matrix protein giantin (GOLGB1) is required for ciliogenesis. We show that giantin is not required for the Rab11-Rabin8-Rab8 pathway that has been implicated in the early stages of ciliary membrane formation. Instead we find that suppression of giantin results in mis-localization of WDR34, the intermediate chain of dynein-2. Highly effective depletion of giantin or WDR34 leads to an inability of cells to form primary cilia. Partial depletion of giantin or of WDR34 leads to an increase in cilia length consistent with the concept that giantin acts through dynein-2. Our data implicate giantin in ciliogenesis through control of dynein-2 localization.

Cauli: a mouse strain with an Ift140 mutation that results in a skeletal ciliopathy modelling Jeune syndrome

Cilia are architecturally complex organelles that protrude from the cell membrane and have signalling, sensory and motility functions that are central to normal tissue development and homeostasis. There are two broad categories of cilia; motile and non-motile, or primary, cilia. The central role of primary cilia in health and disease has become prominent in the past decade with the recognition of a number of human syndromes that result from defects in the formation or function of primary cilia. This rapidly growing class of conditions, now known as ciliopathies, impact the development of a diverse range of tissues including the neural axis, craniofacial structures, skeleton, kidneys, eyes and lungs. The broad impact of cilia dysfunction on development reflects the pivotal position of the primary cilia within a signalling nexus involving a growing number of growth factor systems including Hedgehog, Pdgf, Fgf, Hippo, Notch and both canonical Wnt and planar cell polarity. We have identified a novel ENU mutant allele of Ift140, which causes a mid-gestation embryonic lethal phenotype in homozygous mutant mice. Mutant embryos exhibit a range of phenotypes including exencephaly and spina bifida, craniofacial dysmorphism, digit anomalies, cardiac anomalies and somite patterning defects. A number of these phenotypes can be attributed to alterations in Hedgehog signalling, although additional signalling systems are also likely to be involved. We also report the identification of a homozygous recessive mutation in IFT140 in a Jeune syndrome patient. This ENU-induced Jeune syndrome model will be useful in delineating the origins of dysmorphology in human ciliopathies.

Skeletal ciliopathies are an emerging field of human disease in which skeletal birth defects arise due to abnormal communication between cells. This failure in communication arises following mutation in components of the primary cilia, a hair-like structure present on every cell. The skeletal ciliopathies are debilitating and in severe cases lead to death in early infancy. However, the mechanisms by which these malformations come about remains unclear. Mouse models are often used to delineate the causes of human birth defects and we have identified a model that mimics one of these conditions known as Jeune syndrome. It is the first mouse model with a mutation in the Ift140 gene, and these mice exhibit phenotypes that are often seen in this set of human syndromes. We have complimented this study with the discovery of a patient that presents with Jeune Syndrome resulting from mutation of human IFT140. This model will allow us to explore the role of IFT140 and the primary cilia in normal human development and provide insight into the field of human skeletal ciliopathies.

Isolation of intraflagellar transport trains

The intraflagellar transport (IFT) system was first identified in situ by electron microscopy in thin sections of plastic-embedded flagella as linear arrays of electrondense particles, located between the B tubules of the outer doublets and the flagellar membrane. These arrays of particles are referred to as IFT trains. Upon membrane rupture, IFT trains are thought to easily dissociate to yield soluble IFT particles, which are commonly purified through sucrose gradients as ∼16-17S complexes. The latters easily dissociate into two subcomplexes, named A and B. We report here the isolation, visualization, and identification by immunolabeling of flexible strings of IFT particles, which are structurally similar to in situ IFT trains and appear to be formed by both complex A and complex B polypeptides. Moreover, the particles forming isolated IFT trains are structurally similar to the individual particles found in the ∼17S gradient peak. Our results provide the first direct evidence that ∼17S particles do indeed compose the IFT trains. The paper also represents the first isolation of the IFT trains, and opens new possibilities for higher resolution studies on their structure and how particles are attached to each other to form the particle trains.

WD60/FAP163 is a dynein intermediate chain required for retrograde intraflagellar transport in cilia

Retrograde intraflagellar transport (IFT) is required for assembly of cilia. We identify a Chlamydomonas flagellar protein (flagellar-associated protein 163 [FAP163]) as being closely related to the D1bIC(FAP133) intermediate chain (IC) of the dynein that powers this movement. Biochemical analysis revealed that FAP163 is present in the flagellar matrix and is actively trafficked by IFT. Furthermore, FAP163 copurified with D1bIC(FAP133) and the LC8 dynein light chain, indicating that it is an integral component of the retrograde IFT dynein. To assess the functional role of FAP163, we generated an RNA interference knockdown of the orthologous protein (WD60) in planaria. The Smed-wd60(RNAi) animals had a severe ciliary assembly defect that dramatically compromised whole-organism motility. Most cilia were present as short stubs that had accumulated large quantities of IFT particle-like material between the doublet microtubules and the membrane. The few remaining approximately full-length cilia had a chaotic beat with a frequency reduced from 24 to ∼10 Hz. Thus WD60/FAP163 is a dynein IC that is absolutely required for retrograde IFT and ciliary assembly.

Intraflagellar transport drives flagellar surface motility

The assembly and maintenance of all cilia and flagella require intraflagellar transport (IFT) along the axoneme. IFT has been implicated in sensory and motile ciliary functions, but the mechanisms of this relationship remain unclear. Here, we used Chlamydomonas flagellar surface motility (FSM) as a model to test whether IFT provides force for gliding of cells across solid surfaces. We show that IFT trains are coupled to flagellar membrane glycoproteins (FMGs) in a Ca²⁺-dependent manner. IFT trains transiently pause through surface adhesion of their FMG cargos, and dynein-1b motors pull the cell towards the distal tip of the axoneme. Each train is transported by at least four motors, with only one type of motor active at a time. Our results demonstrate the mechanism of Chlamydomonas gliding motility and suggest that IFT plays a major role in adhesion-induced ciliary signaling pathways.

DOI:http://dx.doi.org/10.7554/eLife.00744.001

Cilia and flagella protrude like bristles from the cell surface. They share the same basic ‘9+2’ axoneme structure, being made up of nine microtubule doublets that surround a central pair of singlet microtubules. Flagella are generally involved in cell propulsion, whereas motile cilia help to move fluids over cell surfaces.

Maintaining cilia and flagella is a challenge for cells, which must find a way to send new proteins all the way along the axoneme to the site of assembly at the flagellar tip. Cells achieve this via a process called intraflagellar transport, in which proteins are carried back and forth by kinesin and dynein motors along the axonemal doublet microtubules. Intraflagellar transport has been proposed to influence other functions of cilia and flagella, including the propulsion of cells over surfaces. However, the details of these interactions are unclear.

Through a combination of biophysical and microscopy approaches, Shih et al. describe the mechanism that the green alga Chalmydomonas uses to power flagellar gliding over surfaces. By tracking single fluorescently tagged molecules, Shih et al. observed that flagellar membrane glycoproteins are carried along the axoneme by the intraflagellar transport machinery. During transport, flagellar membrane glycoproteins transiently adhere to the surface, and dynein motors that were previously engaged in carrying these glycoproteins now transmit force that moves the axonemal microtubules. This process, which is dependent on the concentration of calcium ions in the extracellular environment, generates the force that propels the alga's flagella along the surface.

Gliding motility is thought to have been one of the initial driving forces for the evolution of cilia and flagella. How the intricate mechanism of flagellar beat motility could have evolved has been the subject of much discussion, as it would require the flagellum to have evolved first. In demonstrating that gliding motility is powered by the same intraflagellar transport mechanism that is required for flagellar assembly, Shih et al. provide strong evidence for the evolution of primitive flagella before the evolution of flagellar beating. Furthermore, since algal flagella have essentially the same structure as the cilia of human cells, these findings could ultimately aid in the development of treatments for diseases that result from defects in intraflagellar transport, including polycystic kidney disease and retinal degeneration.

DOI:http://dx.doi.org/10.7554/eLife.00744.002

Exome sequencing identifies DYNC2H1 mutations as a common cause of asphyxiating thoracic dystrophy (Jeune syndrome) without major polydactyly, renal or retinal involvement

BACKGROUND

Jeune asphyxiating thoracic dystrophy (JATD) is a rare, often lethal, recessively inherited chondrodysplasia characterised by shortened ribs and long bones, sometimes accompanied by polydactyly, and renal, liver and retinal disease. Mutations in intraflagellar transport (IFT) genes cause JATD, including the IFT dynein-2 motor subunit gene DYNC2H1. Genetic heterogeneity and the large DYNC2H1 gene size have hindered JATD genetic diagnosis.

AIMS AND METHODS

To determine the contribution to JATD we screened DYNC2H1 in 71 JATD patients JATD patients combining SNP mapping, Sanger sequencing and exome sequencing.

RESULTS AND CONCLUSIONS

We detected 34 DYNC2H1 mutations in 29/71 (41%) patients from 19/57 families (33%), showing it as a major cause of JATD especially in Northern European patients. This included 13 early protein termination mutations (nonsense/frameshift, deletion, splice site) but no patients carried these in combination, suggesting the human phenotype is at least partly hypomorphic. In addition, 21 missense mutations were distributed across DYNC2H1 and these showed some clustering to functional domains, especially the ATP motor domain. DYNC2H1 patients largely lacked significant extra-skeletal involvement, demonstrating an important genotype-phenotype correlation in JATD. Significant variability exists in the course and severity of the thoracic phenotype, both between affected siblings with identical DYNC2H1 alleles and among individuals with different alleles, which suggests the DYNC2H1 phenotype might be subject to modifier alleles, non-genetic or epigenetic factors. Assessment of fibroblasts from patients showed accumulation of anterograde IFT proteins in the ciliary tips, confirming defects similar to patients with other retrograde IFT machinery mutations, which may be of undervalued potential for diagnostic purposes.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.