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

Abstract

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.

Figure 1

Structure of the flagellum in Chlamydomonas reinhardtii and Trypanosoma brucei. Scanning electron microscopy shows the global structure of (A)Chlamydomonas and (A’)Trypanosoma. Sections through the base reveal (B) the emergence of the two flagella from the cell wall (CW) and (B’) the single flagellum through the flagellar pocket (FP). The basal body (BB) and the transition zone (TZ) are also visible. (C,D,E,F,C’,D’,E’) Longitudinal and cross-sections through the flagellum reveal the structure of the axoneme (and of the PFR in T. brucei), and the presence of IFT trains are indicated by brackets or arrows. Scale bars: (A) 5 μm; (B) 1 μm; and (C,D,E,F) 0.1 μm. Image credit: (A,B,C,D,E,F) provided by Elisa Vannuccini and Pietro Lupetti (University of Siena, Italy). (A’,B’) reproduced with permission from Buisson and Bastin [15] and (C’,D’,E’) reproduced with permission from Absalon et al. [16]. The scale bar size is indicated on each image. BB, basal body; CW, cell wall; FP, flagellar pocket; IFT, intraflagellar transport; PFR, paraflagellar rod; TZ, transition zone.

Figure 2

Mode of flagellum formation and relationship with the cell cycle. (A) In Chlamydomonas, the flagella are disassembled prior to mitosis during the asexual life cycle. (B) In the procyclic form of Trypanosoma brucei, a new flagellum is constructed while the old one remains in place. Mature and assembling flagella are shown in green and red, respectively (see text for details). The tip of the elongating flagellum is indicated with arrowheads and the plane of cleavage is shown by dotted lines.

Figure 3

Canonical model for IFT. Step 1: IFT-A and IFT-B complexes, kinesin-2, and inactive cDynein1b gather at the base of the flagellum. Step 2: the active kinesin-2 transport IFT-A and IFT-B complexes, inactive cytoplasmic dynein 2, and axonemal precursors from the base to the tip. Step 3: kinesin-2 reaches the distal end, where axonemal cargo proteins and IFT particles are released into the ciliary tip compartment. IFT-A and IFT-B complexes dissociate from each other. Complex A binds to active cytoplasmic dynein 2. Step 4: active cytoplasmic dynein 2 transports complexes IFT-A and IFT-B and kinesin to the cell body. IFT, intraflagellar transport.

Figure 4

Evolution of the amounts of various IFT mRNA during the cell cycle in Chlamydomonas. The relative concentrations of IFT27 protein (purple line) and IFT27 mRNA (red line), IFT46 mRNA (blue line), IFT140 mRNA (green line), and Fla10 mRNA (yellow line) are plotted along with flagellum length (solid grey line). IFT27 protein concentration decreases continuously during G1 and reaches its lowest level just before division. IFT27, IFT46, IFT140, and Fla10 mRNA and protein are normally synthesized during S/M which resets its levels for the next cell cycle. Figure modified from Wood et al. [64]. IFT, intraflagellar transport.

Figure 5

Expression of the mRNA encoding flagellar proteins during the Trypanosoma brucei cell cycle. (1) Early G1: cells with one flagellum. (2) Late G1: maturation and duplication of the basal body. (3) S phase: construction of the new flagellum. (4) G2/M phase: elongation of the new flagellum. IFT, basal body, and membrane and matrix genes peak first, whereas axoneme and PFR transcripts emerge later when flagellum elongation takes place. Original data are from Archer et al.[66] and transcripts encoding proteins belonging to different structures are listed in Additional file 1: Table S1. IFT, intraflagellar transport; PFR, paraflagellar rod.

Figure 6

An accumulation of electron-dense material precedes flagellum elongation. (A) Cross-sections through the flagellum base of Chlamydomonas cells that undergo regeneration fixed shortly after pH shock-induced deflagellation. IFT particles (arrowheads) are visible in all flagella. In short flagella, numerous particles fill the space distal to the basal body, but by the time microtubules have formed (D), particles have become organized to form linear arrays. IFT particles are linked to the microtubules (small arrowheads) and to the membrane (small arrowheads). Scale bars: 0.1 μm. Reproduced with permission from Dentler [74]. (B) Cross-sections through the flagellar pocket in which the new flagellum is built in procyclic Trypanosoma brucei. The short new flagellum contains a large amount of electron-dense material, while microtubules are not yet assembled. Once microtubules have started to elongate, this material is much more discrete. Scale bars: 500 nm, except where indicated. Reproduced with permission from Pazour et al. [18]. IFT, intraflagellar transport.

Figure 7

Four different models illustrating the possible fate of IFT trains after they are returned to the base of the flagellum. (A,B) Closed model, (C) semi-open model, and (D) open model (see text for details). Large blue boxes, anterograde trains; small blue boxes, retrograde trains; and blue dots, IFT complexes particles. The orange, red, and green colors indicate the cytoplasmic, flagellum base, and flagellum compartment, respectively. IFT, intraflagellar transport.