Biology Reference
In-Depth Information
polarity is established by different sets of molecules. The apical cortex of neuro-
blasts is bounded by Par and Pins complexes. The Par complex consists of
Bazooka (PAR3), Par6, and atypical protein kinase C (aPKC), while the Pins
complex consists of Partner of Inscuteable (Pins), Discs large (Dlg), and Gai.
Proteins bound to the basal cortex of NBs include Miranda, Prospero, Brain tumor
(Brat), and Numb. The orientation of the mitotic spindle is directed along the
apical/basal axis, generating an apical daughter cell and a basal daughter cell that
receive different fate determinants and asymmetric fates: the one receiving apical
determinants remains a NB and the other, which receives basal determinants,
becomes the ganglion mother cell (GMC), respectively. It was found that the ratio
of apical to basal determinants also plays an important role in determining the
distinct fates of the daughter cells (Cabernard and Doe
2009
). In experiments
where spindle orientation was disrupted, apical determinants were partitioned
between two daughter cells, while the basal determinants were segregated asym-
metrically into only one daughter cell, resulting in two NBs. Recently, Doe and
colleagues found that the position of the cleavage furrow, which leads to the
segregation of determinants, is not solely dependent upon the mitotic spindle, but
is also correlated with the polarity proteins, particularly the Pins complex at the
apical tip (Cabernard et al.
2010
).
6.3 Asymmetric Behavior of Centrosomes in Stem Cells
In recent years, due to the centrosome's important role in organizing the mitotic
spindle and directing mitotic spindle orientation in dividing cells, it has been
increasingly recognized that it plays critical roles in cell divisions. Recent pro-
gresses in studying centrosome behavior during asymmetric stem cell divisions
make it clear that the structural asymmetry of mother and daughter centrosomes is
an important underlying mechanism by which stem cells coordinate their fate and
cellular asymmetry. A few examples of asymmetric centrosome behaviors during
stem cell divisions will be discussed below.
In Drosophila male GSCs, the mother centrosome stereotypically remains at the
apical side of the stem cell near the hub, while the daughter centrosome migrates to
the basal end of the cell, setting up a centrosome orientation perpendicular to the hub
(Yamashita et al.
2003
; Yamashita et al.
2007
). This stereotypical centrosome ori-
entation ensures asymmetric GSC division; one daughter cell retains stem cell
identity containing the mother centrosome, while the other daughter cell inheriting
the daughter centrosome undergoes differentiation (Fig.
6.3
a). This stereotypical
centrosome orientation and asymmetric centrosome inheritance requires centroso-
min (Cnn). Cnn, an integral centrosome component, is important in PCM recruitment
and normal astral microtubule function (Dobbelaere et al.
2008
; Megraw et al.
2001
).
If cnn is mutated, most PCM components cannot properly localize to the centrosomes
(Conduit et al.
2010
). In cnn mutant flies, male GSCs display abnormal centrosome
orientation and show an almost random mother/daughter centrosome inheritance
