Biology Reference
In-Depth Information
TH acts to accomplish these metamorphic changes predominantly if not
exclusively via the nuclear receptors TR
a
and TR
b
(
Das et al., 2010
). TRs
are ligand-activated transcription factors that alter expression levels of genes
responsible for metamorphosis (
Buchholz, Paul, Fu, & Shi, 2006
). Using
transgenic animals, overexpression of a dominant negative form of TR that
cannot bind TH inhibits metamorphosis (
Buchholz, Hsia, Fu, & Shi, 2003
).
On the other hand, overexpression of a constitutively active TR (that does
not require TH to induce T3-response genes) initiates metamorphic trans-
formation in the absence of TH (
Buchholz, Tomita, Fu, Paul, & Shi, 2004
).
TRs regulate gene expression by binding thyroid response elements (TREs)
in promoter regions of TH-response genes. TRs have at least two modes to
regulate genes, in that TH-response genes can have positive or negative
TREs. On positive TREs, the presence of TH induces gene expression,
whereas TH represses expression of genes containing negative TREs. Some
TH-response genes with TREs have been identified as transcription factors
that have gene targets of their own. Thus, TH induces TH-direct response
genes, some of which subsequently regulate other genes, all of which com-
prises a gene regulation cascade, ultimately leading to metamorphosis.
Since the identification of TRs as nuclear receptors, more than 75 years
after the discovery that TH induces frog metamorphosis (
Gudernatsch,
1912; Yaoita, Shi, & Brown, 1990
), a major research direction has been
to identify the genes and pathways underlying TH-dependent metamor-
phosis. A unifying question for studies on frog metamorphosis is how a single
molecule, TH, can trigger such varied developmental outcomes among tis-
sues of the tadpole. The modern paradigm for answering this question cen-
ters around the molecular mechanisms underlying how different tissues can
have TH-induced tissue-specific gene regulation cascades. A critical com-
ponent to answering this question is to identify genes comprising these gene
regulation cascades in different tissues.
At first, genes induced by TH were identified one by one based on their
biochemical activities. For example, tail regression involved protease diges-
tion of intra- and extracellular components, such as collagen, and thus col-
lagenases and cathepsins were found (
Yoshizato, 1996
). The biochemistry of
the liver undergoes metamorphic changes in nitrogen metabolism, such that
urea cycle enzymes were found to be TH-regulated (
Atkinson, Helbing, &
Chen, 1996
). Furthermore, distinct hemoglobins were found between tad-
poles and frogs, and the adult hemoglobin genes were induced by TH
(
Weber, 1996
). In a radically new approach, subtractive hybridization was
employed to identify differentially regulated genes where
a priori
knowledge
