Centrosomes, DNA Damage and Aneuploidy - The Centrosome - page 218

13.4 Centrosome Pathways
There are two key pathways by which centrosomes can arise: the normal,
templated pathway in which the pre-existing centrioles serve as the scaffolding for
new centriole formation during S-phase, and a de novo pathway (Loncarek and
Khodjakov 2009 ). However, these are not distinct in terms of the controlling
activities, but differ in the sense that the existing mother centrioles serve as reg-
ulators of the 'templated' process (Rodrigues-Martins et al. 2007b ).
In experiments where centrosomes were removed from monkey kidney cells by
micromanipulation (Hinchcliffe et al. 2001 ; Maniotis and Schliwa 1991 ) or laser
microsurgery (Khodjakov et al. 2000 ; Khodjakov and Rieder 2001 ), the centro-
somes did not regenerate. However, subsequent work that examined what hap-
pened when the centrosomes were removed from S-phase arrested CHO cells by
laser ablation (Khodjakov et al. 2002 ), or from Chlamydomonas cells by a
mutation that causes a fraction of the daughter cells to have no centrioles (Marshall
et al. 2001 ), demonstrated that cells can form centrosomes de novo. These
observations were further supported by the finding of de novo centriole assembly
in transformed (La Terra et al. 2005 ) and normal (Uetake et al. 2007 ) human cells.
A p53-dependent cell cycle arrest in late G1 phase is caused by the loss or damage
of centrosomes (Mikule et al. 2007 ; Srsen et al. 2006 ), which suggests a reason
why the potentiation of de novo centrosome formation was only observed when
cells were treated after this point. The formation of the de novo structures and the
maturation of these centrioles require passage through an entire cycle (Khodjakov
et al. 2002 ; La Terra et al. 2005 ). Once activated, this de novo pathway allows
cells to produce multiple centrosomes, suggesting that numerical control of the
centrosome resides in the existing centrosomes (Khodjakov et al. 2002 ; La Terra
et al. 2005 ). Together, these findings indicate a general pathway of de novo
centrosome formation that is normally inhibited by the presence of existing cen-
trioles (La Terra et al. 2005 ) but which, upon activation or loss of inhibition, can
generate large numbers of centrioles.
An evolutionarily conserved series of proteins govern the process by which
centrioles normally duplicate (Carvalho-Santos et al. 2010 ). A key polo box-
containing kinase, PLK4 in human (Habedanck et al. 2005 ; Kleylein-Sohn et al.
2007 ), SAK in Drosophila melanogaster (Bettencourt-Dias et al. 2005 ) is recruited
to the centrosome by the coiled-coil protein SPD2/CEP192, which also directs the
recruitment of the pericentriolar material (PCM) to the nascent centriole (Kemp et al.
2004 ; Pelletier et al. 2004 ; Zhu et al. 2008 ). PLK4/SAK is required for the recruit-
ment of the coiled-coil proteins, SAS-4 (CPAP/CENP-J in human cells) and SAS-6,
which specify the base of the forming centriole, direct the elongation of its micro-
tubules and are required for centriole duplication (Kirkham et al. 2003 ; Leidel et al.
2005 ; Leidel and Gonczy 2003 ; Pelletier et al. 2006 ; Rodrigues-Martins et al. 2007a ;
Strnad et al. 2007 ). A further coiled-coil component of the Caenorhabditis elegans
centriole regulatory apparatus, SAS-5 (Ana2 in Drosophila), is also required for
centriole duplication (Pelletier et al. 2006 ; Stevens et al. 2010 ). ZYG-1 plays a role