Biology Reference
In-Depth Information
rudimentary understanding of how the timing, amplitude, and duration of a given pulse
are controlled. A key component involved in the timing of ecdysone pulses is PTTH, a
brain-derived neuropeptide. PTTH stimulates ecdysone production through a Ras/Raf/
ERK signaling cascade; however, comparatively little is known about the downstream tar-
gets of this pathway. In recent years, it has become apparent that transcriptional
regulation plays a critical role in regulating the synthesis of ecdysone, but only one
transcription factor has a well-defined link to PTTH. Interestingly, many of the
ecdysteroidogenic transcription factorswere originally characterized as primary response
genes in the ecdysone signaling cascade that elicits the biological responses to the hor-
mone in target tissues. To review these developments, wewill first provide an overviewof
the transcription factors that act in the Drosophila ecdysone regulatory hierarchy. We will
then discuss the roles of these transcriptional regulators in controlling ecdysone synthe-
sis. In the last section, we will briefly outline transcription factors that likely have roles in
regulating ecdysone synthesis but have not been formally identified as downstream ef-
fectors of ecdysone.
1. INTRODUCTION
Metamorphosis translates into “change in form.” While most organ-
isms display some changes in form during development, one typically uses
the term “metamorphosis” in the context of the dramatic remodeling that
occurs during the transition from a larval to an adult stage, as seen in holo-
metabolous insects or amphibians. Insect metamorphosis is arguably one of
the most striking developmental processes in the animal kingdom, allowing
us to witness the transformation of an entire body plan into another. The
change in body plans is not just a developmental necessity. In insects, for
example, the use of different body plans serves as a remarkable adaptation
to different habitats and food sources while undergoing development
(
Truman & Riddiford, 1999
).
Typically, an insect's life cycle comprises embryonic, larval, pupal, and
adult stages, where growth occurs exclusively during larval development.
The succession of different stages and body plans begs the question as to
how these transitions are regulated. Key components responsible for the dra-
matic reprogramming of body plans are small lipophilic hormones, such as
ecdysteroids in insects and thyroid hormones in amphibians (
Galton, 1992;
Riddiford, 1993
). In either case, these hormones act as ligands for members
of the nuclear receptor superfamily that, upon ligand binding, initiate a
cascade of gene programs that drive forward the remodeling process. In
Drosophila
, the ligand-bound ecdysteroid receptor activates a small group
