Biomedical Engineering Reference
In-Depth Information
in the core- and tissue-specific modules, in which molecular and
morphological asymmetry are typically reduced or abolished. Perhaps, the
most illustrative example is the phenotype seen in large
ft
clones in the wing
(
Ma et al., 2003
). Within these clones, the prehairs form complex swirling
patterns, whereas wild-type hairs form parallel arrays. Furthermore, mole-
cular polarization at the level of the core proteins remains intact, although
misoriented, indicating that the mutant cells polarize but no longer recog-
nize the tissue axes (
Ma et al., 2003
). Additional evidence that the core mod-
ule continues to polarize cells when the global module is disrupted comes
from the observation that small
ft
mutant clones on
Drosophila
wings display
normal polarity (
Ma et al., 2003
); the local alignment property of the core
module aligns the mutant cells with each other (to form swirls) and with the
polarity of the nearby wild-type cells outside the clone (to align with the
tissue axis if the clone is small). Thus, the global module provides direction-
ality to PCP but is not required for cell polarization
per se
, provided the core
module is intact.
It is important to point out that, although the Ft/Ds/Fj global module
as described contributes to PCP in all
Drosophila
tissues so far studied, sev-
eral observations indicate that the current model is missing important
pieces. When the Ds and Fj gradients are experimentally flattened, polarity
of the ommatidia in the fly eye is severely disrupted as predicted (
Simon,
2004
). In contrast, planar polarity of the wing epithelium is more modestly
affected, with much of the wing retaining a predominantly distal polarity.
What accounts for the remaining distal directionality? While Ft is clearly
important for global polarization, as evidenced by the disrupted polarity in
loss-of-function
ft
clones, an extracellularly truncated fragment of Ft that
cannot interact with Ds, and should therefore not be responsive to the Ds
or Fj gradients, can rescue both the viability of
ft
mutant animals and
produce distal polarity in a substantial, predominantly distal, portion of
the wing (
Matakatsu & Blair, 2006
; our unpublished observations). From
these observations, it is reasonable to conclude that there is an additional
directional signal distinct from the gradients of Ds and Fj. Two more recent
studies also provide support for the notion that additional polarizing signals
are present, being organized by unknown cues at the dorsal-ventral
boundary and perhaps also the anterior-posterior boundary (
Brittle,
Thomas, & Strutt, 2012; Sagner et al., 2012
). The nature of such a
signal(s) is unknown. Resolution of these mysteries awaits further
studies, as does determining whether any of these signals contribute to
global PCP signaling in vertebrates, and if so, how their role relates to
that of Wnt proteins.
