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Fig. 6. EGFR regulates cortical myosin accumulation. (A) Cell
boundaries with high myosin-GFP accumulation often corresponded to the outer
boundary of dp-ERK. b and b' are higher magnification images of a and
a', respectively. Arrows indicate the high myosin-GFP accumulation at
the interface of cells with high and low dp-ERK levels. (B,C)
MHC accumulated in arc-like patterns in Egfr heterozygous embryos,
but these patterns were lost in Egfr homozygotes (see text for
quantitative assessment). In B', 8 out of 12 class T junctions were
judged to be in arcs. Two examples of arcs are highlighted with yellow curved
lines 1 and 2. (D) Two classes of tricellular junctions. In class T
junctions, `horizontal' cell boundaries are colored pink, and `vertical' ones,
blue. Myosin concentration was more than 50% enriched in horizontal cell
junctions compared to the vertical one (mean±s.e.m., arbitrary unit,
number of measurement indicated in parentheses, P<0.002, Student's
t-test). (E) Expression of sSPI (sSpi) by the prd
enhancer in even-numbered parasegments induced the massive activation of ERK
and accumulation of MHC at the cell boundaries between cells with high and low
dp-ERK expression (arrowheads). sSPI also induced the precocious invagination
of the segmental furrow (asterisk) that becomes contiguous with the tracheal
pit (tr2). An arrow indicates unaffected segment boundary. (F)
Enrichment of junctional myosin by sSPI. The intensity of myosin signal was
compared between the border along the stripe of elevated dp-ERK (DV border,
red lines) and those intersecting them (AP border, black lines;
mean±s.e.m., arbitrary unit). sSpi significantly increased the DV
boundary signal compared to AP boundary (*,
P<10-5, Student's t-test: two-sample assuming
equal variances, two-tail). No such enrichment was observed in the control
segments. (G) A model for the EGFR-dependent coordination of cell
movement. Left: Myosin accumulates at the boundary of cells with high and low
EGFR activity. The contractile force of myosin (blue arrows) helps shrinkage
of cell boundary and cell intercalation (juxtaposed cell boundaries shown in
green). In addition, the contractile force smoothens other cell-row
boundaries. Right: once the cell-row boundaries form a continuous arc, the net
force in the convex contractile supracellular actomyosin cable (pink) is
oriented toward the inside of the arc (magenta arrows). Red asterisk indicates
the invagination site. Scale bars: 10 µm.
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