Stabilization of cartwheel-less centrioles for duplication requires CEP295-mediated centriole-to-centrosome conversion

Abstract

Vertebrate centrioles lose their geometric scaffold, the cartwheel, during mitosis, concurrently with gaining the ability to recruit the pericentriolar material (PCM) and thereby function as the centrosome. Cartwheel removal has recently been implicated in centriole duplication, but whether "cartwheel-less" centrioles are intrinsically stable or must be maintained through other modifications remains unclear. Here, we identify a newborn centriole-enriched protein, KIAA1731/CEP295, specifically mediating centriole-to-centrosome conversion but dispensable for cartwheel removal. In the absence of CEP295, centrioles form in the S/G2 phase and lose their associated cartwheel in mitosis but cannot be converted to centrosomes, uncoupling the two events. Strikingly, centrioles devoid of both the PCM and the cartwheel progressively lose centriolar components, whereas centrioles associating with either the cartwheel or PCM alone can exist stably. Thus, cartwheel removal can have grave repercussions to centriole stability, and centriole-to-centrosome conversion mediated by CEP295 must occur in parallel to maintain cartwheel-less centrioles for duplication.

Figure 1. CEP295 is a daughter-enriched proximal end protein

(A) U2OS cells at different cell cycle stages were stained with indicated antibodies. DAPI (blue) marks nuclei. (B) CEP295 localizes to the extra daughter centrioles of the rosette centrosome, inducibly formed by Plk4 overexpression in RPE1 cells. (C) G1, or S/G2 centrioles were visualized with indicated antibodies, including the proximal end marker C-Nap1 or SAS-6, and distal end marker centrin or Odf2. (D) CEP295 signals were lost in CEP295-RNAi cells, revealed by both immunostaining and western blot.

Figure 2. Loss of CEP295 does not block centriole assembly but leads to centrosome reduction

(A) U2OS cells transfected with control or CEP295 siRNA for 48hr were stained with indicated antibodies. γ-tubulin (gTub) marks active centrosomes. Only mitotic centrosomes were examined (boxed and magnified), and quantified (n>50, N=3, right). Error bars represent standard deviation. (B) RPE1 cells in which CEP295 gene was mutated by CRISPR/Cas9 gene targeting were analyzed and stained with indicated antibodies. Control cells were nucleofected with Cas9 and empty gRNA vectors. Early prophase cells are shown. ~15% of cells were effectively targeted by CRISPR (not shown). (C) Centriole duplication was analyzed in control or CEP295-RNAi cells in S phase 72hr after transfection, with indicated antibodies. BrdU was added 30 min before fixation. Unlike control cells carrying two centriole doublets (group I), majority of CEP295-RNAi cells contained only one centriole doublet, with or without an additional centrin singlet devoid of SAS-6 (group II & III). (D) De novo centrioles were found in a small percentage of CEP295-depleted cells lacking centrosomes (group V), labeled with centrin and SAS-6, but not with γ-tubulin or C-Nap1. (E) Quantification of centriole duplication (n>160, N=3). Four groups of duplication defects were seen in CEP295-depleted cells as indicated (II–V). Error bars represent standard deviation. See also Figure S1.

Figure 3. Loss of CEP295 blocks centriole-to-centrosome conversion, but has no effect on centriole disengagement and cartwheel removal

(A) Control and CEP295 siRNA-treated cells were transiently labeled with BrdU before fixation. G1 cells, BrdU negative (not shown) and carrying two centrin singlets, were examined with indicated antibodies. Quantifications (n>100, N=3) are shown (right). Error bars represent standard deviation. (B) Cells grown on a gridded coverslip were treated with CEP295 siRNA and filmed for 72 hours before fixation. A cell carrying de novo centrioles was identified (arrow; #1 cell), and its sister cell from the previous cell division was traced through the timelapse movie (arrowhead; #2 cell), and examined with indicated antibodies. (C) Schematic summary of the phenotypes seen in CEP295-depleted cells. (D) Unsynchronized wild-type or CEP295^−/−; p53^−/− RPE1 cells, and CEP295^−/−; p53⁻ cells transiently expressing full-length CEP295 were stained with indicated antibodies. (E) Unsynchronized CEP295^−/−; p53^−/− cells pulsed with BrdU for 30 min and chased for 4 hours were stained with indicated antibodies. (F) CEP295^−/−; p53^−/− cells in late S phase as described in (E), or arrested in S or G1 phase for 24–48 hours as indicated were examined with centrin and centrobin antibodies. Quantifications are shown (n>100, N=3). (G) CEP295^−/−; p53^−/− cells were allowed to enter mitosis in the presence of the Plk1 inhibitor (BI-2536), or Eg5 inhibitor (monastrol) as a control, and release to and arrest at G1 by Cdk inhibition with Roscovitine for 16 hours before fixation and staining with indicated antibodies. Cells displaying donut-shaped, multilobed, or multiple small nuclei (Tsou et al., 2009) were identified for analyses. In Plk1-inhibited cells arrested in G1, centrioles were stably present, and retained the cartwheel and other markers, as shown in the panel on the right. Quantifications are shown (n>50, N=4). See also Figure S2 and Movie S1.

Figure 4. Centriole-to-centrosome conversion stabilizes centrioles devoid of the cartwheel

(A) Mother centrioles and their engaged daughter centrioles in cells treated with control (left) or CEP295 siRNA (right) for 72 hours were examined at early mitosis with antibodies against centrin, γ-tubulin (gTub), and acetylated α-tubulin (AcTu). (B) Cells treated with control or CEP295 siRNA for 72 hours were labeled with BrdU before fixation. G1 cells, BrdU negative (not shown) and inheriting a pair of disengaged centrioles, were examined with indicated antibodies. Quantifications are shown (right) (n>100, N=3). Error bars represent standard deviation. Note that in CEP295 siRNA-treated cells, majority of daughter centrioles failed to convert to centrosomes (group II & III; gTub-), some of which also lost the AcTu staining (group III) (gTub- & AcTu-). (C) Cells were treated as described in (B). BrdU-positive cells inheriting active centrosomes (gTub foci) were examined with indicated antibodies. Cells inheriting no centrosomes were excluded. Quantifications are shown (right) (n>100, N=3). Note that in the majority of CEP295 RNAi cells, the inherited DCP/daughter centrioles either became non-detectable (group IV), or had lost the AcTu staining (group III) (gTub-; AcTu-). (D) DCP centrioles in S-phase cells were examined for the localization of CPAP with indicated antibodies. See also Figure S3.