Biology Reference
In-Depth Information
downregulated at climax and 48 h were very different from each other.
The basis of this observation is not understood but may relate to different
hormone environments.
Transcriptional repression of most genes encoding the steps leading to gly-
colysis, electron transport chain (including most of the genes that encode the
ATP synthase complex), and tricarboxylic acid cycle genes are downregulated
in tadpole tail muscle, which is consistent with cell death and tail resorption.
These major metabolic pathways are shut off in all of the tail muscle at climax
when the muscle fibers appear to be healthy and before the upregulation of the
proteolytic enzymes and the morphological evidence of dying fibers. This
global downregulation is followed by the more localized upregulation pattern
that coincides with the activation of caspase-3. The upregulation of proteins
involved in transport (e.g., Na
รพ
/dicarboxylate transporter) and proteins of the
ATP Na/K pump are perhaps required to support altered metabolism and
apoptosis, but elucidating their actual role requires functional studies.
Many regulated genes with various biological functions did not have
predicted roles in metamorphosis, because their biological activities did
not match known physiological or morphological or biochemical phenom-
ena. One of the largest groups of upregulated tail genes is signal transduction,
including MAPK phosphatase, TGF-
b
pathway, GTPase-mediated signal-
ing, and Rho-mediated signaling. The signaling genes that were tested by
in situ
hybridization were expressed in fibroblasts (
Das et al., 2006
). A role
for most of these signaling pathways in metamorphosis is not known, but
cyclin C/cdk8 phosphorylation was examined (
Skirrow, Veldhoen,
Domanski, & Helbing, 2008
). One of the most highly upregulated genes
in the tail is corticotropin releasing hormone binding protein, whose func-
tion was elegantly tracked down as contributing to regulation of timing of
tail resorption (
Boorse, Kholdani, Seasholtz, & Denver, 2006
). The roles of
many other genes await similar analysis.
TH action during metamorphosis is modulated by other hormones, prin-
cipally corticosterone (CORT), the stress hormone in frogs. CORT syn-
ergizes with TH via many mechanisms, including at the level of gene
regulation, to accelerate metamorphic processes induced by TH (
Bonett,
Hoopfer, & Denver, 2010
). Two genes have been identified as induced by
TH and CORT and TR
b
and KLF9. Both of these genes are induced in syn-
ergy and/or by either hormone alone, depending on tissue. Amicroarray anal-
ysis was performed to identify more regulated genes and more gene regulation
interaction patterns by these two hormones (
Kulkarni & Buchholz, 2012
).
Premetamorphic
X. tropicalis
tadpole tails were harvested 18 h after treatment
