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strongly suggest important roles of the epithelial-connective tissue
interactions in adult epithelial development during amphibian intestinal
remodeling just as epithelial-mesenchymal interactions during gut organo-
genesis of higher vertebrates (
Yasugi & Mizuno, 2008
) and during amphibian
skin conversion (
Yoshizato, 2007
).
To demonstrate experimentally that the actions of TH on the connective
tissue are essential for adult stem cell development during intestinal remo-
deling, we used Tg
X. laevis
tadpoles that express a dominant positive thyroid
hormone receptor (dpTR) under the control of a heat shock-inducible pro-
moter (
Hasebe, Buchholz, Shi, & Ishizuya-Oka, 2011
). In these Tg tadpoles,
dpTR specifically binds to TH response elements within promoter regions
of TH target genes and causes metamorphic changes
in vivo
without addition
of TH (
Buchholz, Tomita, Fu, Paul, & Shi, 2004
). By making use of Wt and
dpTR Tg intestines at stage 57 for tissue recombinant culture experiments,
TH target genes can be specifically induced in either the epithelium or non-
E or both in the absence of TH at any time by heat shock. As a result, when-
ever the epithelium was derived from Wt intestine, no stem cells were
detected in any recombinant intestine. In contrast, whenever the epithelium
was derived from Tg intestine, undifferentiated cells expressing Shh, which
is the only stem cell marker known to be directly upregulated by TH
(
Stolow & Shi, 1995
), became detectable after 5 days of cultivation with heat
shock. More importantly, these cells expressed the other stem cell markers
such as Msi1 when Tg epithelium was recombined with Tg non-E (Tg/Tg)
but did not when it was recombined with Wt non-E (Tg/Wt). Then, after
7 days, differentiated absorptive epithelial cells expressing IFABP were
detected in Tg/Tg intestine but not in Tg/Wt intestine. These results pro-
vide experimental evidence that, although some of the larval epithelial cells
begin to express Shh by a direct action of TH, they require TH signaling in
non-E to fully dedifferentiate into adult stem cells that generate the adult
absorptive epithelium (
Fig. 11.4
). Thus, TH response genes expressed in
non-E, other than the epithelium itself, are essential for stem cell develop-
ment as the niche during
X. laevis
intestinal remodeling.
5. SIGNALING PATHWAYS INVOLVED IN
ESTABLISHMENT OF STEM CELL NICHE
Previously, a large number of TH response genes have been identified
in the
X. laevis
intestine by various PCR-based subtractive differential
screens and cDNA microarrays (
Amano & Yoshizato, 1998; Buchholz
