Biomedical Engineering Reference
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anchor the nucleus to the cortex in the posterior E daughter cell (
Sugioka &
Sawa, 2010
). This process is also regulated by Wnt and MES-1/SRC-1 sig-
naling. Although this nuclear anchoring is not essential inWnt signaling, it has
significant effects on POP-1/TCF nuclear localization in the
mom-2
/Wnt
mutant background.
The effects of Wnt signaling in spindle orientation have recently been
reported also in seam cells that are considered to be epithelial stem cells because
of their self-renewing asymmetric divisions (
Wildwater, Sander, de Vreede, &
van den Heuvel, 2011
). The anterior-posterior orientations of seam cell divi-
sions are not affected even in quintupleWnt mutants that have mutations in all
the fiveWnt genes (
Yamamoto, Takeshita, & Sawa, 2011
). However, the ori-
entations become abnormal in situations which perturb both Wnt signaling
and shape of seam cells that is normally elongated along the anterior-posterior
axis, for example, in double mutants between
mig-14
/Wntless required for
Wnt secretion and
dpy-10
whose mutations cause shorter body and rounded
shape of seam cells. The results indicate that orientations of seam cell divisions
are redundantly regulated by Wnt signaling and their elongated shape.
5. Wnts INSTRUCTIVELY ORIENT POLARITY OF THE
EMS AND T CELLS
Important feature of the PCP regulation is the coordination of cell
polarity in a group of cells or tissues (
Gray, Roszko, & Solnica-Krezel,
2011
). During
C. elegans
development, most cell divisions occur along
the anterior-posterior axis with asymmetric localization of Wnt signaling
components described above. In particular, POP-1 asymmetry that is higher
in the anterior daughter nuclei has been observed in most embryonic and
postembryonic cells. Therefore, most
C. elegans
cells are polarized in the
same orientation along the anterior-posterior axis at least when they divide.
One exciting possibility for polarity coordination is that Wnt signals
function as a global cue for orienting cell polarity. Wnts were first shown
to orient polarity in the embryonic EMS and postembryonic T cells
(
Goldstein et al., 2006
). Blastomere isolation and recombination experi-
ments have shown that EMS polarity is determined by the position of the
P2 blastomere, as recombining EMS and P2 results in one of the EMS
daughters attaching to P2 to produce endoderm (
Goldstein, 1993
).
Although P2-EMS signaling involves two proteins MES-1 and MOM-
2/Wnt, it was shown that MOM-2 is the orienting signal by recombining
two P2 (one lacks the
mom-2
function and the other lacks the
mes-1
function)
to the opposite sides of EMS (
Fig. 3.3
A). This results in the endoderm
