Methods for Studying Movement of Molecules Within Cilia

Distribution and bulk flow analyses of the intraflagellar transport (IFT) motor kinesin-2 support an "on-demand" model for Chlamydomonas ciliary length control

Most cells tightly control the length of their cilia. The regulation likely involves intraflagellar transport (IFT), a bidirectional motility of multi-subunit particles organized into trains that deliver building blocks into the organelle. In Chlamydomonas, the anterograde IFT motor kinesin-2 consists of the motor subunits FLA8 and FLA10 and the nonmotor subunit KAP. KAP dissociates from IFT at the ciliary tip and diffuses back to the cell body. This observation led to the diffusion-as-a-ruler model of ciliary length control, which postulates that KAP is progressively sequestered into elongating cilia because its return to the cell body will require increasingly more time, limiting motor availability at the ciliary base, train assembly, building block supply, and ciliary growth. Here, we show that Chlamydomonas FLA8 also returns to the cell body by diffusion. However, more than 95% of KAP and FLA8 are present in the cell body and, at a given time, just ~1% of the motor participates in IFT. After repeated photobleaching of both cilia, IFT of fluorescent kinesin subunits continued indicating that kinesin-2 cycles from the large cell-body pool through the cilia and back. Furthermore, growing and full-length cilia contained similar amounts of kinesin-2 subunits and the size of the motor pool at the base changed only slightly with ciliary length. These observations are incompatible with the diffusion-as-a-ruler model, but rather support an "on-demand model," in which the cargo load of the trains is regulated to assemble cilia of the desired length.

Global and local functions of the Fused kinase ortholog CdaH in intracellular patterning in Tetrahymena

Ciliates assemble numerous microtubular structures into complex cortical patterns. During ciliate division, the pattern is duplicated by intracellular segmentation that produces a tandem of daughter cells. In Tetrahymena thermophila, the induction and positioning of the division boundary involves two mutually antagonistic factors: posterior CdaA (cyclin E) and anterior CdaI (Hippo kinase). Here, we characterized the related cdaH-1 allele, which confers a pleiotropic patterning phenotype including an absence of the division boundary and an anterior-posterior mispositioning of the new oral apparatus. CdaH is a Fused or Stk36 kinase ortholog that localizes to multiple sites that correlate with the effects of its loss, including the division boundary and the new oral apparatus. CdaH acts downstream of CdaA to induce the division boundary and drives asymmetric cytokinesis at the tip of the posterior daughter. CdaH both maintains the anterior-posterior position of the new oral apparatus and interacts with CdaI to pattern ciliary rows within the oral apparatus. Thus, CdaH acts at multiple scales, from induction and positioning of structures on the cell-wide polarity axis to local organelle-level patterning.

The Small Interactor of PKD2 protein promotes the assembly and ciliary entry of the Chlamydomonas PKD2-mastigoneme complexes

In Chlamydomonas, the channel polycystin 2 (PKD2) is primarily present in the distal region of cilia, where it is attached to the axoneme and mastigonemes, extracellular polymers of MST1. In a smaller proximal ciliary region that lacks mastigonemes, PKD2 is more mobile. We show that the PKD2 regions are established early during ciliogenesis and increase proportionally in length as cilia elongate. In chimeric zygotes, tagged PKD2 rapidly entered the proximal region of PKD2-deficient cilia, whereas the assembly of the distal region was hindered, suggesting that axonemal binding of PKD2 requires de novo assembly of cilia. We identified the protein Small Interactor of PKD2 (SIP), a PKD2-related, single-pass transmembrane protein, as part of the PKD2-mastigoneme complex. In sip mutants, stability and proteolytic processing of PKD2 in the cell body were reduced and PKD2-mastigoneme complexes were absent from the cilia. Like the pkd2 and mst1 mutants, sip mutant cells swam with reduced velocity. Cilia of the pkd2 mutant beat with an increased frequency but were less efficient in moving the cells, suggesting a structural role for the PKD2-SIP-mastigoneme complex in increasing the effective surface of Chlamydomonas cilia.

Small Interactor of PKD2 (SIP), a novel PKD2-related single-pass transmembrane protein, is required for proteolytic processing and ciliary import of Chlamydomonas PKD2

In Chlamydomonas cilia, the ciliopathy-relevant TRP channel PKD2 is spatially compartmentalized into a distal region, in which PKD2 binds the axoneme and extracellular mastigonemes, and a smaller proximal region, in which PKD2 is more mobile and lacks mastigonemes. Here, we show that the two PKD2 regions are established early during cilia regeneration and increase in length as cilia elongate. In abnormally long cilia, only the distal region elongated whereas both regions adjusted in length during cilia shortening. In dikaryon rescue experiments, tagged PKD2 rapidly entered the proximal region of PKD2-deficient cilia whereas assembly of the distal region was hindered, suggesting that axonemal docking of PKD2 requires de novo ciliary assembly. We identified Small Interactor of PKD2 (SIP), a small PKD2-related protein, as a novel component of the PKD2-mastigoneme complex. In sip mutants, stability and proteolytic processing of PKD2 in the cell body were reduced and PKD2-mastigoneme complexes were absent from mutant cilia. Like the pkd2 and mst1 mutants, sip swims with reduced velocity. Cilia of the pkd2 mutant beat with normal frequency and bending pattern but were less efficient in moving cells supporting a passive role of the PKD2-SIP-mastigoneme complexes in increasing the effective surface of Chlamydomonas cilia.

In vivo imaging reveals independent intraflagellar transport of the nexin-dynein regulatory complex subunits DRC2 and DRC4

Many axonemal proteins enter cilia and flagella on intraflagellar transport (IFT) trains, which move bidirectionally along the axonemal microtubules. Certain axonemal substructures including the radial spokes and outer dynein arms are preassembled in the cell body and transported as multisubunit complexes into flagella by IFT. Here, we used in vivo imaging to analyze the transport and assembly of DRC2 and DRC4, two core subunits of the nexin-dynein regulatory complex (N-DRC). Tagged DRC2 moved by IFT in mutants lacking DRC4 and vice versa, showing that they do not depend on each other for IFT. Simultaneous imaging of tagged DRC2 and DRC4, expressed from transgenes that rescue a corresponding double mutant, mostly showed transport on separate IFT trains, but occasional cotransports were also observed. The results demonstrate that DRC2 and DRC4 are transported largely independently of each other into flagella. These studies suggest that the N-DRC assembles onto the axoneme by the stepwise addition of subunits.

Wbm0076, a candidate effector protein of the Wolbachia endosymbiont of Brugia malayi, disrupts eukaryotic actin dynamics

Brugia malayi, a parasitic roundworm of humans, is colonized by the obligate intracellular bacterium, Wolbachia pipientis. The symbiosis between this nematode and bacterium is essential for nematode reproduction and long-term survival in a human host. Therefore, identifying molecular mechanisms required by Wolbachia to persist in and colonize B. malayi tissues will provide new essential information regarding the basic biology of this endosymbiosis. Wolbachia utilize a Type IV secretion system to translocate so-called "effector" proteins into the cytosol of B. malayi cells to promote colonization of the eukaryotic host. However, the characterization of these Wolbachia secreted proteins has remained elusive due to the genetic intractability of both organisms. Strikingly, expression of the candidate Wolbachia Type IV-secreted effector protein, Wbm0076, in the surrogate eukaryotic cell model, Saccharomyces cerevisiae, resulted in the disruption of the yeast actin cytoskeleton and inhibition of endocytosis. Genetic analyses show that Wbm0076 is a member of the family of Wiskott-Aldrich syndrome proteins (WAS [p]), a well-conserved eukaryotic protein family required for the organization of actin skeletal structures. Thus, Wbm0076 likely plays a central role in the active cell-to-cell movement of Wolbachia throughout B. malayi tissues during nematode development. As most Wolbachia isolates sequenced to date encode at least partial orthologs of wBm0076, we find it likely that the ability of Wolbachia to directly manipulate host actin dynamics is an essential requirement of all Wolbachia endosymbioses, independent of host cell species.

Loss of ARL13 impedes BBSome-dependent cargo export from Chlamydomonas cilia

The GTPase Arl13b participates in ciliary protein transport, but its contribution to intraflagellar transport (IFT), the main motor-based protein shuttle of cilia, remains largely unknown. Chlamydomonas arl13 mutant cilia were characterized by both abnormal reduction and accumulation of select membrane-associated proteins. With respect to the latter, a similar set of proteins including phospholipase D (PLD) also accumulated in BBSome-deficient cilia. IFT and BBSome traffic were apparently normal in arl13. However, transport of PLD, which in control cells moves by BBSome-dependent IFT, was impaired in arl13, causing PLD to accumulate in cilia. ARL13 only rarely and transiently traveled by IFT, indicating that it is not a co-migrating adapter securing PLD to IFT trains. In conclusion, the loss of Chlamydomonas ARL13 impedes BBSome-dependent protein transport, resulting in overlapping biochemical defects in arl13 and bbs mutant cilia.

Chlamydomonas ARMC2/PF27 is an obligate cargo adapter for intraflagellar transport of radial spokes

Intraflagellar transport (IFT) carries proteins into flagella but how IFT trains interact with the large number of diverse proteins required to assemble flagella remains largely unknown. Here, we show that IFT of radial spokes in Chlamydomonas requires ARMC2/PF27, a conserved armadillo repeat protein associated with male infertility and reduced lung function. Chlamydomonas ARMC2 was highly enriched in growing flagella and tagged ARMC2 and the spoke protein RSP3 co-migrated on anterograde trains. In contrast, a cargo and an adapter of inner and outer dynein arms moved independently of ARMC2, indicating that unrelated cargoes distribute stochastically onto the IFT trains. After concomitant unloading at the flagellar tip, RSP3 attached to the axoneme whereas ARMC2 diffused back to the cell body. In armc2/pf27 mutants, IFT of radial spokes was abolished and the presence of radial spokes was limited to the proximal region of flagella. We conclude that ARMC2 is a cargo adapter required for IFT of radial spokes to ensure their assembly along flagella. ARMC2 belongs to a growing class of cargo-specific adapters that enable flagellar transport of preassembled axonemal substructures by IFT.

In vivo imaging shows continued association of several IFT-A, IFT-B and dynein complexes while IFT trains U-turn at the tip

Flagellar assembly depends on intraflagellar transport (IFT), a bidirectional motility of protein carriers, the IFT trains. The trains are periodic assemblies of IFT-A and IFT-B subcomplexes and the motors kinesin-2 and IFT dynein. At the tip, anterograde trains are remodeled for retrograde IFT, a process that in Chlamydomonas involves kinesin-2 release and train fragmentation. However, the degree of train disassembly at the tip remains unknown. Here, we performed two-color imaging of fluorescent protein-tagged IFT components, which indicates that IFT-A and IFT-B proteins from a given anterograde train usually return in the same set of retrograde trains. Similarly, concurrent turnaround was typical for IFT-B proteins and the IFT dynein subunit D1bLIC-GFP but severance was observed as well. Our data support a simple model of IFT turnaround, in which IFT-A, IFT-B and IFT dynein typically remain associated at the tip and segments of the anterograde trains convert directly into retrograde trains. Continuous association of IFT-A, IFT-B and IFT dynein during tip remodeling could balance protein entry and exit, preventing the build-up of IFT material in flagella.

The BBSome restricts entry of tagged carbonic anhydrase 6 into the cis-flagellum of Chlamydomonas reinhardtii

The two flagella of Chlamydomonas reinhardtii are of the same size and structure but display functional differences, which are critical for flagellar steering movements. However, biochemical differences between the two flagella have not been identified. Here, we show that fluorescence protein-tagged carbonic anhydrase 6 (CAH6-mNG) preferentially localizes to the trans-flagellum, which is organized by the older of the two flagella-bearing basal bodies. The uneven distribution of CAH6-mNG is established early during flagellar assembly and restored after photobleaching, suggesting that it is based on preferred entry or retention of CAH6-mNG in the trans-flagellum. Since CAH6-mNG moves mostly by diffusion, a role of intraflagellar transport (IFT) in establishing its asymmetric distribution is unlikely. Interestingly, CAH6-mNG is present in both flagella of the non-phototactic bardet-biedl syndrome 1 (bbs1) mutant revealing that the BBSome is involved in establishing CAH6-mNG flagellar asymmetry. Using dikaryon rescue experiments, we show that the de novo assembly of CAH6-mNG in flagella is considerably faster than the removal of ectopic CAH6-mNG from bbs flagella. Thus, different rates of flagellar entry of CAH6-mNG rather than its export from flagella is the likely basis for its asymmetric distribution. The data identify a novel role for the C. reinhardtii BBSome in preventing the entry of CAH6-mNG specifically into the cis-flagellum.

Diffusion rather than intraflagellar transport likely provides most of the tubulin required for axonemal assembly in Chlamydomonas

Tubulin enters the cilium by diffusion and motor-based intraflagellar transport (IFT). However, the respective contribution of each route in providing tubulin for axonemal assembly remains unknown. Using Chlamydomonas, we attenuated IFT-based tubulin transport of GFP-β-tubulin by altering the IFT74N-IFT81N tubulin-binding module and the C-terminal E-hook of tubulin. E-hook-deficient GFP-β-tubulin was incorporated into the axonemal microtubules, but its transport frequency by IFT was reduced by ∼90% in control cells and essentially abolished when the tubulin-binding site of IFT81 was incapacitated. Despite the strong reduction in IFT, the proportion of E-hook-deficient GFP-β-tubulin in the axoneme was only moderately reduced. In vivo imaging showed more GFP-β-tubulin particles entering cilia by diffusion than by IFT. Extrapolated to endogenous tubulin, the data indicate that diffusion provides most of the tubulin required for axonemal assembly. We propose that IFT of tubulin is nevertheless needed for ciliogenesis, because it augments the tubulin pool supplied to the ciliary tip by diffusion, thus ensuring that free tubulin there is maintained at the critical concentration for plus-end microtubule assembly during rapid ciliary growth.

In vivo analyses of radial spoke transport, assembly, repair and maintenance

Radial spokes (RSs) are multiprotein complexes that regulate dynein activity. In the cell body, RS proteins (RSPs) are present in a 12S precursor, which enters the flagella and converts into the axoneme-bound 20S spokes consisting of a head and stalk. To study RS dynamics in vivo, we expressed fluorescent protein (FP)-tagged versions of the head protein RSP4 and the stalk protein RSP3 to rescue the corresponding Chlamydomonas mutants pf1, lacking spoke heads, and pf14, lacking RSs entirely. RSP3 and RSP4 mostly co-migrated by intraflagellar transport (IFT). The transport was elevated during flagellar assembly and IFT of RSP4-FP depended on RSP3. To study RS assembly independently of ciliogenesis, strains expressing FP-tagged RSPs were mated to untagged cells with, without, or with partial RSs. Tagged RSPs were incorporated in a spotted fashion along wild-type-derived flagella indicating an exchange of RSs. During the repair of pf1-derived axonemes, RSP4-FP is added onto the preexisting spoke stalks with little exchange of RSP3. Thus, RSP3 and RSP4 are transported together but appear to separate inside flagella during the repair of RSs. The 12S RS precursor encompassing both proteins could represent a transport form to ensure stoichiometric delivery of RSPs into flagella by IFT.

In vivo analysis of outer arm dynein transport reveals cargo-specific intraflagellar transport properties

Outer dynein arms (ODAs) are multiprotein complexes that drive flagellar beating. Based on genetic and biochemical analyses, ODAs preassemble in the cell body and then move into the flagellum by intraflagellar transport (IFT). To study ODA transport in vivo, we expressed the essential intermediate chain 2 tagged with mNeonGreen (IC2-NG) to rescue the corresponding Chlamydomonas reinhardtii mutant oda6. IC2-NG moved by IFT; the transport was of low processivity and increased in frequency during flagellar growth. As expected, IFT of IC2-NG was diminished in oda16, lacking an ODA-specific IFT adapter, and in ift46 IFT46ΔN lacking the ODA16-interacting portion of IFT46. IFT loading appears to involve ODA16-dependent recruitment of ODAs to basal bodies followed by handover to IFT. Upon unloading from IFT, ODAs rapidly docked to the axoneme. Transient docking still occurred in the docking complex mutant oda3 indicating that the docking complex stabilizes rather than initiates ODA–microtubule interactions. In full-length flagella, ODAs continued to enter and move inside cilia by short-term bidirectional IFT and diffusion and the newly imported complexes frequently replaced axoneme-bound ODAs. We propose that the low processivity of ODA-IFT contributes to flagellar maintenance by ensuring the availability of replacement ODAs along the length of flagella.

Primary cilia proteins: ciliary and extraciliary sites and functions

Primary cilia are immotile organelles known for their roles in development and cell signaling. Defects in primary cilia result in a range of disorders named ciliopathies. Because this organelle can be found singularly on almost all cell types, its importance extends to most organ systems. As such, elucidating the importance of the primary cilium has attracted researchers from all biological disciplines. As the primary cilia field expands, caution is warranted in attributing biological defects solely to the function of this organelle, since many of these "ciliary" proteins are found at other sites in cells and likely have non-ciliary functions. Indeed, many, if not all, cilia proteins have locations and functions outside the primary cilium. Extraciliary functions are known to include cell cycle regulation, cytoskeletal regulation, and trafficking. Cilia proteins have been observed in the nucleus, at the Golgi apparatus, and even in immune synapses of T cells (interestingly, a non-ciliated cell). Given the abundance of extraciliary sites and functions, it can be difficult to definitively attribute an observed phenotype solely to defective cilia rather than to some defective extraciliary function or a combination of both. Thus, extraciliary sites and functions of cilia proteins need to be considered, as well as experimentally determined. Through such consideration, we will understand the true role of the primary cilium in disease as compared to other cellular processes' influences in mediating disease (or through a combination of both). Here, we review a compilation of known extraciliary sites and functions of "cilia" proteins as a means to demonstrate the potential non-ciliary roles for these proteins.

The IDA3 adapter, required for intraflagellar transport of I1 dynein, is regulated by ciliary length

We determined how the ciliary motor I1 dynein is transported. A specialized adapter, IDA3, facilitates I1 dynein attachment to the ciliary transporter called intraflagellar transport (IFT). Loading of IDA3 and I1 dynein on IFT is regulated by ciliary length.

The Bardet-Biedl syndrome protein complex is an adapter expanding the cargo range of intraflagellar transport trains for ciliary export

Bardet-Biedl syndrome (BBS) is a ciliopathy resulting from defects in the BBSome, a conserved protein complex. BBSome mutations affect ciliary membrane composition, impairing cilia-based signaling. The mechanism by which the BBSome regulates ciliary membrane content remains unknown. Chlamydomonas bbs mutants lack phototaxis and accumulate phospholipase D (PLD) in the ciliary membrane. Single particle imaging revealed that PLD comigrates with BBS4 by intraflagellar transport (IFT) while IFT of PLD is abolished in bbs mutants. BBSome deficiency did not alter the rate of PLD entry into cilia. Membrane association and the N-terminal 58 residues of PLD are sufficient and necessary for BBSome-dependent transport and ciliary export. The replacement of PLD's ciliary export sequence (CES) caused PLD to accumulate in cilia of cells with intact BBSomes and IFT. The buildup of PLD inside cilia impaired phototaxis, revealing that PLD is a negative regulator of phototactic behavior. We conclude that the BBSome is a cargo adapter ensuring ciliary export of PLD on IFT trains to regulate phototaxis.

IFT trains in different stages of assembly queue at the ciliary base for consecutive release into the cilium

Intraflagellar transport (IFT) trains, multimegadalton assemblies of IFT proteins and motors, traffic proteins in cilia. To study how trains assemble, we employed fluorescence protein-tagged IFT proteins in Chlamydomonas reinhardtii. IFT-A and motor proteins are recruited from the cell body to the basal body pool, assembled into trains, move through the cilium, and disperse back into the cell body. In contrast to this ‘open’ system, IFT-B proteins from retrograde trains reenter the pool and a portion is reused directly in anterograde trains indicating a ‘semi-open’ system. Similar IFT systems were also observed in Tetrahymena thermophila and IMCD3 cells. FRAP analysis indicated that IFT proteins and motors of a given train are sequentially recruited to the basal bodies. IFT dynein and tubulin cargoes are loaded briefly before the trains depart. We conclude that the pool contains IFT trains in multiple stages of assembly queuing for successive release into the cilium upon completion.

DOI:http://dx.doi.org/10.7554/eLife.26609.001

Intraflagellar transport protein IFT52 recruits IFT46 to the basal body and flagella

Cilia are microtubule-based organelles and perform motile, sensing and signaling functions. The assembly and maintenance of cilia depend on intraflagellar transport (IFT). Besides ciliary localization, most IFT proteins accumulate at basal bodies. However, little is known about the molecular mechanism of basal body targeting of IFT proteins. We first identified the possible basal body-targeting sequence in IFT46 by expressing IFT46 truncation constructs in an ift46-1 mutant. The C-terminal sequence between residues 246-321, termed BBTS3, was sufficient to target YFP to basal bodies in the ift46-1 strain. Interestingly, BBTS3 is also responsible for the ciliary targeting of IFT46. BBTS3::YFP moves bidirectionally in flagella and interacts with other IFT complex B (IFT-B) proteins. Using IFT and motor mutants, we show that the basal body localization of IFT46 depends on IFT52, but not on IFT81, IFT88, IFT122, FLA10 or DHC1b. IFT52 interacts with IFT46 through residues L285 and L286 of IFT46 and recruits it to basal bodies. Ectopic expression of the C-terminal domain of IFT52 in the nucleus resulted in accumulation of IFT46 in nuclei. These data suggest that IFT52 and IFT46 can preassemble as a complex in the cytoplasm, which is then targeted to basal bodies.