Biology Reference
In-Depth Information
FIGURE 18.3
Invagination in the early Xenopus laevis embryo. (a) Depicts the surface ectoderm before invag-
ination begins (only the ectoderm cells are shown, for clarity). (b) Depicts the situation after invagination has begun,
when the ectodermal cells have undergone pronounced apical constriction to become bottle cells.
The importance of myosin-driven contraction of apical actin cables in driving apical
constriction has been illustrated by a series of experiments on the events that immediately
precede gastrulation in the frog Xenopus laevis. The first morphological sign that gastrulation
is about to begin is the formation of an invagination in a part of the embryo called the 'dorsal
marginal zone'. The pit formed by the invagination becomes the crevice into which gastrulat-
ing cells will stream (the details of amphibian gastrulation are beyond the scope of this topic,
but a good review can be found in Gilbert
3
). Before invagination begins, ectodermal epithelial
cells are approximately cuboidal (
Figure 18.3
a). At the future site of invagination, cells show
an intense concentration of both actin microfilaments and active (light chain-phosphorylated)
type II non-muscle myosin.
4
They then elongate and undergo a strong apical constriction,
acquiring the thin apical necks that give them their name: 'bottle cells' (
Figure 18.3
b). This
apical constriction depends on functional myosin and actin: it fails if actin filaments are
disrupted by Latrunculin B or cytochalsin D, and it fails if myosin function is blocked with
blebbistatin or ML-7. Failure of apical constriction leads to failure of invagination, and defec-
tive gastrulation.
The demonstration that apical actin-myosin contraction is necessary for apical constriction
raises two questions: what leads to the apical localization of the actin-myosin complexes, and
what activates their contraction? Though incomplete, the answers to the questions seem to be
closely linked.
Myosin activity in non-muscle cells is controlled mainly by phosphorylation of myosin
light chain, either by myosin light chain kinase or by ROCK: ROCK also inactivates myosin
light chain phosphatase, which would otherwise dephosphorylate and thereby deactivate
myosin light chain again. ROCK is itself activated by the small GTPase, RhoA (
Figure 18.4
).
Expression of constitutively active RhoA mutants in cultured epithelial cells, such as MDCK
dog kidney cells, is not itself enough to initiate apical constriction. Expression of a form of
constitutively active RhoA that is targeted to the apical domain of cells is, however, sufficient
to drive a pronounced ROCK- and myosin-dependent apical constriction.
5
Wild-type RhoA
has no such activity even when apically targeted, unless the cell also expresses natural acti-
vators of RhoA such as Trio: this shows that both apical location and activation of RhoA are
