The assembly of the human centriole

Cells contain various specialised structures - such as the nucleus, mitochondria or peroxisomes - known as “organelles’’. Tracing their genesis and determining their structure is fundamental to understanding cell function and the pathologies linked to their dysfunction.

Scientists have combined high resolution microscopy and kinematic reconstruction techniques to visualise, in motion, the genesis of the human centriole. This organelle, essential to the organisation of the cell skeleton, is associated - in case of dysfunction - with certain cancers, brain disorders or retinal diseases. This work, published in the journal Cell, elucidates the complexities of centriole assembly. It also opens up many new avenues for the study of other cell organelles.

Organelle genesis proceeds according to a precise sequence of successive protein recruitment events. Visualising this assembly in real time provides a better understanding of the role of these proteins in organelle structure or function. However, obtaining a video sequence with sufficient resolution to distinguish such complex microscopic components faces a number of technical limitations.

This is particularly true of the centriole. This organelle, measuring less than 500 nanometers (half a thousandth of a millimeter), is constituted of around 100 different proteins organised into six substructural domains. Until a few years ago, it was impossible to visualise the structure of the centriole in detail.

Obtaining images of the centriole with such high resolution enables the exact location of proteins at a given time but gives no information on the order of appearance of substructural domains or of individual proteins. The first author of the study used expansion microscopy to analyse the location of 24 proteins in the six domains in over a thousand centrioles at different stages of growth.

‘‘This very tedious work was followed by a pseudo-temporal kinematic reconstruction. In other words, we were able to put these thousands of images taken at random during centriole biogenesis back into chronological order, to reconstruct the various stages in the formation of centriole substructures, using a computer analysis we developed,’’ explains a co-leader of the study. 

The authors found that the process initiates with the formation of a naked cartwheel devoid of microtubules. Next, the bloom phase progresses with microtubule blade assembly, concomitantly with radial separation and rapid cartwheel growth.

In the subsequent elongation phase, the tubulin backbone grows linearly with the recruitment of the A-C linker, followed by proteins of the inner scaffold (IS). By following six structural modules, the authors modeled 4D assembly of the human centriole.

 ‘‘Our work will not only deepen our understanding of centriole formation, but also open up incredible prospects in cellular and molecular biology, since this method can be applied to other macromolecules and cellular structures to study their assembly in space and time,’’ concludes the senior author.