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Fig. 5. Uniform PVR activation abolishes cell polarity and generates tumorous wing
discs. (A-C) A constitutively activated 
PVR construct was expressed in
the wing imaginal disc, to follow the consequences of uniform PVR activation.
Five days after egg lay (AEL), the size of these discs was normal, but
aberrant overall tissue organization was observed (compare a wild-type disc in
A with B). These larvae failed to pupariate and continued to grow. Fifteen
days AEL, these larvae contained wing imaginal discs that were approximately
five times larger than normal discs (C). (D,E) Cross-sections show that discs
expressing PVR (E) were multilayered, and the cells appeared
non-polarized, in contrast to the layered epithelium of a wild-type disc (D).
The arrow in E shows a region at the periphery of the disc where
MS1096-Gal4 is not expressed, and the simple epithelial structure was
retained. (F) Apical section through a wild-type disc shows the ordered
organization of F-actin. (G) In MS1096-Gal4/UAS-PVR
discs a higher level of F-actin with a highly unorganized distribution is
observed. (H) Optical cross-section of this disc shows that within the
multilayered structure that is generated, F-actin appears to be distributed in
a uniform, non polarized manner within each cell. (I,J) The above observation
is corroborated by EM studies. While wild-type disc cells show only a single
adherens junction per cell (marked by electron-dense material on both sides of
the membrane bilayer, arrows), multiple adherens junctions per cell were
identified in the cells expressing PVR. (K) The presence of DLG
associated with the membranes of PVR cells suggests that the septate
junctions are retained. However, their distribution is no longer polarized.
(L) In wild-type epithelial cells, LGL is associated with the plasma membrane.
(M,N) In PVR discs, LGL continues to be associated with the membrane
(possibly even at higher levels), indicating that the septate junctions that
mediate its membrane association are functional. Taken together, these results
imply that normal activation of PVR in the wing imaginal disc is polarized, as
uniform PVR activation leads to a complete loss of cell polarity. In addition
to the disrupted distribution of polarity markers, PVR specifically
elevates the levels of F-actin.
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