A grow-and-lock model for the control of flagellum length in trypanosomes

Abstract

SUMMARYSeveral cell types such as photoreceptors and spermatozoa possess very stable cilia and flagella, a feature also encountered in numerous protists. We tested an original model for the control of flagellum length in such cells, using Trypanosoma brucei as an experimental system. The grow-and-lock model proposes that the flagellum elongates at a linear rate and that a locking event takes place in a timely defined manner preventing further elongation or shortening. The model implies that modulation of (1) the growth rate or of (2) the timing of the locking event should impact on flagellum length, and (3) that a mature flagellum should display unique characteristics. Here, we provide experimental evidence supporting each of these three postulates. First, reducing the frequency of intraflagellar transport, the machinery responsible for construction, slows down the growth rate and results in the construction of much shorter flagella. Second, we show that the locking event is initiated at a defined stage of the cell cycle and subsequently leads to inhibition of elongation. Third, flagellum maturation is associated to a molecular marker termed FLAM8. Altogether, these results provide support for the grow-and-lock model as a new paradigm for the control of organelle length.

Redistribution of FLAgellar Member 8 during the trypanosome life cycle: consequences for cell fate prediction

“…These findings contrast with the opposite profile seen in PCF dividing in the posterior midgut ( Fig. 4A) and with previous observations in cultured trypanosomes (Bertiaux et al, 2018;Subota et al, 2014). In parallel, the amount of FLAM8 in the old flagellum was significantly decreasing during cell division (Fig.…”

“…In this context, FLAM8 is detected in lower amount in the new flagellum than in the old flagellum in 2K2N early PCF, resulting in the production of a daughter cell with a slighthly shorter flagellum that would further mature and elongate after cytokinesis, up to the same length as that of the old flagellum (Fig. 8), as proposed in the grow-and-lock model (Bertiaux et al, 2018 (Fig. 8).…”

“…4B-C). This demonstrates that in procyclic trypanosomes from the posterior midgut, FLAM8 addition to the distal tip occurs during the assembly of the new flagellum and that the protein reaches its maximum amount in the new flagellum after cell division, as observed in PCF in culture (Bertiaux et al, 2018;Subota et al, 2014). Nevertheless, a different situation was observed in late PCF extracted from the anterior midgut ( Fig.…”

“…Second, alternatively or in addition, there could be two distinct types of PCF divisions: one division type leading to two daughter cells intended to further proliferate, and another division type producing one sibling adapted for progressing in the parasite cycle and a second sibling committed for another round of proliferation. In PCF parasites in culture that could virtually proliferate indefinitely, FLAM8 is progressively added to the new flagellum during its construction in G2/M phase and after cell division in G1 phase (Bertiaux et al, 2018;Subota et al, 2014). In order to confirm this in vivo and to explore the two hypotheses, we scrutinized the localization of FLAM8 in dividing PCF isolated from both the posterior and anterior regions of the tsetse midgut ( (Fig.…”

“…Recently, the grow-and-lock model was proposed to explain how flagellum length could be regulated in procyclic trypanosomes in culture (Bertiaux et al, 2018). In line with the proposal that PCF division results in the production of two daughter cells that are not equivalent (Abeywickrema et al, 2019), we reasoned that the discrepancy between the two FLAM8 distribution patterns observed between the two flagella of early dividing PCF from the posterior midgut (PMG) and late dividing PCF from the anterior midgut (AMG) could somehow reflect the distinct fate of their respective daughter cells bearing the new flagellum (Fig.…”