Biology Reference
In-Depth Information
humans, centrosomes are, like in Xenopus, of biparental origin (Sathananthan
et al.
1996
; Simerly et al.
1999
).
The absence of centrioles in oocytes is not an absolute obstacle for successful
cleavages since the centrioles may form de novo, for example after the fifth
cleavage in normal mouse embryos (Gueth-Hallonet et al.
1993
). De novo for-
mation of centrioles was first observed in sea urchin embryos (Kato and Sugiyama
1971
; Kallenbach
1983
). Later, it was also found in rabbit parthenogenetic embryo
(Szöllosi and Ozile
1991
), and transformed as well as untransformed human cells
(La Terra et al.
2005
; Uetake et al.
2007
). Recently, this pathway of centriole
formation was also identified in Xenopus laevis embryos, and Plk4 kinase seems to
be the key enzyme involved in this process (Eckerdt et al.
2011
). Moreover, this
paper shows that Mos/MAPK pathway suppresses Plk4-dependent centriole for-
mation in oocytes. As Mos/MAPK pathway is activated in all oocytes studied so
far, it seems to prevent maternal inheritance of centrosomes and thus protects
against the parthenogenetic mode of reproduction.
20.4 Molecular Studies of Centrosome Function
in Xenopus laevis
The function and behavior of centrosomes have been extensively studied in
Xenopus egg extracts recapitulating cell cycle events at the molecular level. A role
of CDK/cyclin complexes in MT nucleation activity of centrosome was identified
in Xenopus egg extracts (Buendia et al.
1992
). Importantly, a new role, during
mitosis, of the interphase cytoplasm-nuclear transport machinery was also dis-
covered in these extracts, as importin complexes and the small GTPase Ran
involved in this process are also actively involved in spindle assembly (Nachury
et al.
2001
). The Aurora-A activator TPX2, which is sequestered by importin in G2
phase of the cell cycle, is released during mitosis by the small GTPase Ran to bind
to Aurora-A already present on the centrosomes at the spindle poles. This asso-
ciation results in the activation of the Aurora-A kinase and its relocation to the
MTs of the spindle poles making this kinase a peculiar player in the centrosome-
driven spindle assembly (Kufer et al.
2002
; Tsai et al.
2003
). It is, however, not
known whether TPX2 participates in centrosomal Aurora-A activation. Identifying
the centrosomal activator of Aurora-A is one of the most important challenges in
the field.
Aurora-A kinase gene was first identified in Xenopus laevis embryo as the Eg2
in a screen focused on identification of genes whose maternal mRNA was dif-
ferentially polyadenylated during oocyte maturation and early embryo develop-
ment (Paris and Philippe
1990
). Eg2 was found to be the second gene of a series
of mRNAs deadenylated following fertilization in Xenopus laevis embryo
(Legagneux et al.
1995
; Detivaud et al.
2003
). In the meantime, a gene named
Aurora was identified in Drosophila, and it was shown to code a protein kinase
