Biology Reference
In-Depth Information
stimulation on the basis of distinctions in the kinetics of their signal response
networks (
Kubota et al., 2012
). Adjusting the downstream response to insu-
lin also occurs by regulating InR levels via a feedback loop involving the
transcription factor FOXO and the MAPK/Erk pathway (
Puig & Tjian,
2005; Zhang, Thompson, Hietakangas, & Cohen, 2011
), either sensitizing
or desensitizing the cellular response.
5. A DEVELOPMENTAL CHECKPOINT COUPLES ORGAN
GROWTH AND TISSUE REGENERATION TO TIMING
An important physiological aspect of growth is to produce an adult
body with the correct size and dimensions. This is determined by the mech-
anisms that keep individual organ size proportions and time the termination
of their growth at the correct size. Compared to the abundant larval-specific
cells, organs that are destined to form the mature adult respond differently to
nutrient deprivation and are to some degree “spared” under starvation
(
Britton & Edgar, 1998; Cheng et al., 2011
). While most endoreplicative
larval-specific cells become quiescent, adult precursor tissues continue to
grow under nutrient restrictions, at least during post critical weight devel-
opment. The protection of these critical organs against starvation reflects
their important role in the formation of the adult. The brain and discs are
organs that have the ability to sustain growth even during nutrient depriva-
tion by mechanisms that are independent of DILPs secreted from the IPCs.
These organs do not scale up or down with the overall body size in response
to manipulation of DILP secretion from the IPCs. In the brain, activation of
the anaplastic lymphoma kinase (Alk) receptor by its ligand Jelly belly makes
the growth of neuroblasts independent of both the growth promoting
DILP/InR and TOR pathways (
Cheng et al., 2011
). Remarkably, PI3K
is activated in neuroblasts by Alk under nutrient restriction rather than by
InR. Alk signaling also bypasses neuroblast requirements for TORmediated
nutrient sensing by interacting with downstream effectors including 4E-BP
(
Cheng et al., 2011
). Jelly belly is constitutively expressed in glia cells show-
ing that these cells play a central role in sustaining brain development under
limiting nutrient conditions. Growth of the imaginal discs is also more resis-
tant to nutrient deprivation than larval-specific tissues that shut down all
growth under such conditions. In contrast to neuroblasts, disc growth is
TOR dependent but may be decoupled from nutrition due to the expression
of
DILP2
in the discs (
Brogiolo et al., 2001
). Together this demonstrates that
the growth of the central nervous system and the imaginal discs is buffered
against fluctuations in the diet at the expense of other larval-specific organs,
