Centriole duplication: analogue control in a digital age

Abstract

In preparation for mitosis, the centrosome doubles once and only once to provide the two poles of the mitotic spindle. The presence of more than two centrosomes increases the chances that mitosis will be multipolar, and chromosomes will be distributed unequally. Since the number of mother-daughter centriole pairs determines the number of centrosomes, it is important that only one daughter centriole is assembled at, but slightly separated from, the proximal end of each mother centriole. This numerical and spatial specificity has led to the belief that a 'template' on the mother centriole provides a unique site for procentriole assembly. We review observations that are leading to the demise of this intuitively attractive idea. In its place, we are left with the notion that pericentriolar material at the wall of the mother centriole provides a local environment that promotes the assembly of a macromolecular complex that seeds the daughter centriole. Even though the system normally behaves in a digital fashion to go from zero to just one daughter centriole per mother, this behaviour appears to be based in the precise analogue control of multiple proteins, their activities, and the structure provided by the mother centriole.

Figure 1. Centrosome ultrastructure in HeLa cells at times during S-phase arrest

First frame: the mother centriole is cut in cross-section revealing the nine triplet microtubule barrel of the centriole surrounded by a diffuse cloud of pericentriolar material. The procentriole (cut in longitudinal section – arrow) typically assembles at right angles to and at a slight distance from the wall of the mother centriole. In early S-phase, the procentriole is short. Second and third frames: Later times in S-phase-arrested cells showing elongation of the daughter centrioles (arrows). Images taken from Figure S1 of Loncarek J, Hergert P, Magidson V, Khodjakov A. Control of daughter centriole formation by the pericentriolar material. Nat Cell Biol 2008;10:322–8 with permissions of the authors and Nature Press.

Figure 2. Fluorescence microscopic images of centrosomes in mammalian somatic cells

The green contrast shows the localization of GFP-centrin 1, which is concentrated in the distal lumen of centrioles and thus serves as a centriole marker. The red contrast is gamma tubulin located in the pericentriolar material. (a) A single centriole associated with pericentriolar material during G1. (b) A centrosome in G2 containing a mother centriole (larger centrin dot) and a daughter centriole. The pair of centrioles is ofter referred to as a diplosome. (c) Spontaneous occurrence of two daughter centrioles with one mother centriole (a triplosome). This is seen at low frequency in some cell types such as CHO cells. (d) Overduplication of a mother centriole when Plk 4 is overexpressed. The mother centriole (seen weakly as the yellow dot in the middle of the gamma tubulin cloud) is driven to make more than one procentriole at one time giving a figure that resembles a rosette. (e) Multiple centrioles assembled de novo in a cell whose resident centrioles were ablated with a laser microbeam. Note the smaller size of the pericentriolar material and foci of gamma tubulin without centrioles.

Figure 3

Schematic representation of the centriole cycle during the cell cycle in mammalian cells. Centrosomes are shown as a hollow circle of fine lines enclosing the centrioles, which are represented by paired parallel lines as if in longitudinal section. Cell at 1 o’clock is a G1 daughter cell that has inherited a single centrosome with two centrioles. In many cell types, the centrioles remain in close proximity, whereas in others (e.g. HeLa), the original mother and daughter centrioles can be widely separate. The 2 o’clock cell is in late G1, and the centrioles have separated slightly and have lost their orthogonal arrangement. Centriole disorientation was once thought to signal the initiation of centriole duplication, but more recent work has revealed that the centrioles become disengaged from each other starting in late mitosis. The cell at 4 o’clock is in early S-phase, and centriole duplication is underway with the assembly of short procentrioles at the proximal ends of the mother centrioles. The procentrioles elongate throughout the rest of interphase, reaching their mature length in mitosis or the following G1. The 6 o’clock cell is in late S or early G2. The procentrioles have become longer. The 8 o’clock cell is in G2, and the mother–daughter centrioles pairs have started to separate as the centrosome is resolving itself into two sister centrosomes. With time, the sister centrosomes continue to separate around the nucleus as the cell cycle approaches mitosis. At mitosis (10 o’clock), the sister centrosomes organize the two poles of the spindle. Each centrosome contains a mother centriole and its daughter. The cell at 12 o’clock is in late telophase, as it is completing cleavage. Centriole duplication is said to be conservative because the procentriole is assembled from subunits in the cytoplasm, not from components of the mother centriole. Centriole distribution to sister centrosomes is said to be semiconservative because parental centrioles are distributed to both centrosomes. Diagram after Wheatley (1982), by permission of Elsevier and the author.