A Grow-and-Lock Model for the Control of Flagellum Length in Trypanosomes

Abstract

Highlights d The total amount of IFT material increases linearly during flagellum elongation d Reducing IFT rate by knocking down IFT kinesin motors slows down the growth rate d Increasing IFT rate in a locked flagellum does not lead to further elongation d The locking event is initiated prior to cell division SUMMARY Several models have been proposed to explain how eukaryotic cells control the length of their cilia and flagella.Here, we investigated this process in the protist Trypanosoma brucei, an excellent model system for cells with stable cilia like photoreceptors or spermatozoa. We show that the total amount of intraflagellar transport material (IFT, the machinery responsible for flagellum construction) increases during flagellum elongation, consistent with constant delivery of precursors and the previously reported linear growth. Reducing the IFT frequency by RNAi knockdown of the IFT kinesin motors slows down the elongation rate and results in the assembly of shorter flagella. These keep on elongating after cell division but fail to reach the normal length. This failure is neither due to an absence of precursors nor to a morphogenetic control by the cell body. We propose that the flagellum is locked after cell division, preventing further elongation or shortening. This is supported by the fact that subsequent increase in the IFT rate does not lead to further elongation. The distal tip FLAM8 protein was identified as a marker for the locking event. It is initiated prior to cell division, leading to an arrest of elongation in the daughter cell. Here, we propose a new model termed ''grow and lock'' where the flagellum elongates until a locking event takes place in a timely defined manner, hence fixing length. Alteration in the growth rate and/ or in the timing of the locking event would lead to the formation of flagella of different lengths.

A key regulatory protein for flagellum length control in stable flagella

“…Previous work showed that when the cell division inhibitor teniposide was applied for 8, 16 or 24hrs, the new flagellum grew to the same length as the old flagellum but never exceeded it. This illustrates an intrinsic locking event was initiated on the new flagellum to prevent further growth before cytokinesis [5]. We reasoned that if Cep164C was involved in the locking mechanism then Cep164C it could accumulate at the base of the new flagellum in these teniposide-treated cells once it had reached the length of the old.…”

“…During the next cell cycle, this flagellum becomes the old flagellum and does not increase in length or disassemble whilst an entirely new flagellum grows alongside. A locking mechanism is proposed to prevent the old flagellum from growing any longer or shortening in subsequent cell cycles [5]. Previous work showed that when the cell division inhibitor teniposide was applied for 8, 16 or 24hrs, the new flagellum grew to the same length as the old flagellum but never exceeded it.…”

“…It is not currently understood how this is achieved. A grow-and-lock model was proposed for the maintenance of stable flagella where a molecular lock is applied to prevent flagellum length change after assembly [5]. The molecular mechanisms of how this lock operates are unknown, but could be important in cells where an existing flagellum must be maintained whilst a new flagellum assembles.…”

Dealing with several flagella in the same cell

“…We termed this a locked axoneme (Bertiaux, Morga, Blisnick, Rotureau, & Bastin, 2018). We termed this a locked axoneme (Bertiaux, Morga, Blisnick, Rotureau, & Bastin, 2018).…”

“…The first model implies a modification of the axoneme in such a way that it can undergo neither growth nor shrinkage. We termed this a locked axoneme (Bertiaux, Morga, Blisnick, Rotureau, & Bastin, 2018). This could be achieved by the addition of a cap at the distal end that would prevent incorporation or loss of tubulin.…”