Biology Reference
In-Depth Information
pioneered 9 years ago with the bantam miRNA (
Brennecke
et al
., 2003
).
The bantam “sensor” transgene consisted of a ubiquitously expressed GFP
transcript with several perfectly complementary bantam sites in its 3
0
UTR
(
Fig. 8.4
B); thus, the expression of GFP is lowest where the activity of
bantam is highest (
Fig. 8.4
C). The bantam sensor exhibits spatially modu-
lated activity in imaginal discs, reflecting its function in growth regulation.
The bantam sensor has been widely exploited as a proxy readout of its
response to various signaling pathways, presumably at the transcriptional
level. For example, a complement to the aforementioned epistastic tests
between upstream Hippo pathway members and bantam was to examine
the bantam sensor in Hippo pathway mutant clones. These tests revealed
lower bantam sensor levels in these clones in wing imaginal discs, indicating
increased bantam activity (
Nolo
et al
., 2006
;
Thompson and Cohen, 2006
).
The bantam sensor was later shown to be upregulated by activation of
Notch signaling in a particular region of the wing disc (
Herranz
et al
.,
2008
), suggesting a repressive input of Notch pathway activity onto bantam.
Finally, the bantam sensor helped reveal a direct input of the Homothorax
transcription factor, in complex with Yorkie, that drives proliferation in the
eye imaginal disc (
Peng
et al
., 2009
).
Most recently, exploitation of the bantam sensor uncovered a novel and
direct intersection of the Hippo and TGF-
b
signaling pathways that drives
bantam expression in both wing and eye discs (
Oh and Irvine, 2011
). Indeed,
protein-protein interactions between the mammalian Yki ortholog Yap and
TGF-
b
pathway Smad transcription factors have been documented (
Alarcon
et al
., 2009
;
Ferrigno
et al
., 2002
), although the
in vivo
consequences of this for
Hippo signaling or TGF-
b
signaling were not known. Taking a cue from the
essential role for cell signaling via the
Drosophila
TGF-
b
ligand Dpp for tissue
growth and patterning (
Affolter and Basler, 2007
), it was found that coex-
pression of Yorkie and with an activated Dpp receptor (Tkv
QD
) synergisti-
cally promoted tissue growth and repression of the bantam sensor, indicating
increased bantam activity. Reciprocally, blocking Dpp pathway activity
increased bantam sensor expression, indicating repression of bantam function
(
Martin
et al
., 2004
;
Oh and Irvine, 2011
). (Note that these experiments
involved keeping mutant cells alive using the antiapoptotic factor p35, since
cells are otherwise succumb without Dpp signaling.)
These observations laid a foundation for multifaceted studies involving
many other principles we have discussed in this review. First, epistatic tests
(
Section 4.2
) showed that forced expression of bantam could partially rescue
the failure of cells lacking Dpp signaling to survive and proliferate. Second,
enhancer bashing (
Section 5.2
) identified a bantam CRM that was syner-
gistically responsive to Yorkie and the Dpp pathway function in imaginal
discs. Finally, it was shown that a novel transcriptional complex containing
Yorkie and the Dpp pathway transcription factor Mad directly binds and
activates this bantam CRM.
