Biology Reference
In-Depth Information
3.8. miR-1
Phenotypic evidence indicates that the muscle-specific
miR-1
miRNA also
plays a role in tissue homeostasis in response to nutritional cues, although a
direct molecular connection to the IIS pathway has not yet been identified.
miR-1
knockout larvae are much smaller than wildtype larvae, have severely
deformed musculature, and become progressively compromised in move-
ment (
Sokol & Ambros, 2005
). This paralysis and death of second instar
miR-1
mutant larvae is triggered by feeding and can be prevented by starving
first instar mutants. Although relevant
miR-1
targets in this context have not
been identified, these data reveal a critical role for
miR-1
in postmitotic
growth of larval muscle (
Sokol & Ambros, 2005
). Owing to the high degree
of evolutionary conservation of
miR-1
, it is likely that this miRNA plays an
important role in maintaining muscle homeostasis in vertebrates.
4. LIPOPHILIC HORMONE SIGNALING PATHWAYS
Lipophilic hormones are endocrine signaling molecules that diffuse
across cell membranes and bind to intracellular proteins, triggering coordi-
nated and system-wide changes in cell behavior.
D. melanogaster
has two pri-
mary classes of lipophilic signaling molecules, ecdysteroids and juvenile
hormones ( JHs), and these hormones activate signaling pathways that tran-
scriptionally control target genes (reviewed in
Jindra, Palli, & Riddiford,
2013
). Recent work has revealed roles for miRNAs in both ecdysteroid
and JH pathways, indicating that these pathways also effect changes via
post-transcriptional control of gene expression. In this section, we review
the ecdysteroid and JH signaling pathways and discuss their relationships
to miRNAs (see
Fig. 4.2
).
4.1. Overview of Ecdysone and JH signaling pathway
Twenty-hydroxy Ecdysone (20E) is the major
D. melanogaster
molting hor-
mone that promotes progression through developmental transitions in
response to environmental cues (
Tennessen & Thummel, 2011
). It is well
known for its developmental role in initiating metamorphosis, but also func-
tions post-developmentally to affect diverse adult process including reproduc-
tion, behavior, stress resistance, and lifespan (
Schwedes & Carney, 2012
). 20E
is derived from a precursor ecdysteroid, termed Ecdysone, which is produced
primarily in the prothoracic gland. Predictable release of Ecdysone from the
prothoracic gland schedules the end of larval development. The exact timing
