Biology Reference
In-Depth Information
What drives boundary shortening? The first suspect for any contractile activity in cells is
the actin-myosin system. During convergent extension, epithelial cells involved display an
actin-myosin network that surrounds the whole of their apical poles ('apical' refers, as
always, to the apico-basal polarity of the epithelium and has nothing to do with vertices
of hexagons). This network shows a series of contractile pulses, the pulses correlating with
a constriction of the whole of the apical area of the cell and with the shrinkage of selected
boundaries that accompanies this.
7
Significantly, strains of D. melanogaster that carry inacti-
vating mutants in either the myosin II regulatory chain (encoded by the gene Spaghetti squash)
or the myosin II heavy chain (encoded by zipper) show severe disruption to germ band exten-
sion because the cell boundaries that are perpendicular to the antero-posterior axis do not
shorten.
6
Contracting an actin-myosin network would not in itself be able to shorten cell
boundaries unless that network were connected to the adhaerens junctions that are spaced
along the boundary to connect adjacent cells to one another. In the epithelium of the germ
band, actin microfilaments are connected to junctions through the adaptor protein Canoe
(Cno). This protein is not essential for cell adhesion but, in its absence, the contracting apical
actin-myosin network pulls itself away from the junctions and boundary shrinkage fails,
meaning that convergent extension fails as well.
7
In vertebrate systems, p120-catenin, which
physically links adhaerens junctions to the myosin-activating enzyme, ROCK,
8
and therefore
functionally links these junctions to actin-myosin contraction, is similarly necessary for
convergent extension.
9
Current evidence therefore points to the actin-myosin network being attached to cell junc-
tions and pulling opposite sides of the cell together as contractile pulses occur. The presence
of pulses suggests the existence of a ratchet mechanism, in which one cycle of contraction
contracts the network somewhat, and this is stabilized while the system prepares for the
next contraction: there is, however, no direct evidence for such a ratchet.
If themechanismdescribed abovewere applied to all boundaries of a cell at the same time, it
would simply achieve apical constriction of the cell (Chapter 18) rather than convergent exten-
sion. For convergent extension to occur, shortening must be targeted only to those cell bound-
aries that lie perpendicular to the axis of extension. The first edition of this topic, written before
most of the details of the process were discovered, suggested that, because boundaries
between cells are their joint property, it might only be necessary for one of a pair of adjacent
cells to drive the shortening of any particular boundary. The intracellular mechanism of
convergent extension might therefore operate directly on only one side of each cell
(
Figure 16.6
). Data obtained during the last decade indicate that this is indeed the case.
The type of polarization that could specify one cell-cell boundary as being different from
the others that an epithelial cell makes with its neighbours must be distinct from the polar-
ization that makes the apical side of an epithelium distinct from its basal side. Apico-basal
polarity runs across the plane of the epithelium, whereas the polarity required for convergent
extension must be orientated within that plane (
Figure 16.7
). It is therefore usually called
'planar polarity', a name that was very well chosen but is now suffering from two types of
confusing misuse: planar polarity was defined originally as a cell-level phenomenon and
early authors on the subject made no link with any specific biochemical pathways.
10
Inten-
sive investigation of a few specific examples of planar polarity resulted in the discovery of
a set of signalling proteins that are critical for planar polarity in those examples (the proteins
are described in detail below). These discoveries prompted researchers to begin to write
