Biology Reference
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(
Flamant & Samarut, 1998
), the frogs
E. coqui
(
Callery & Elinson, 2000a
),
X. laevis
(
Eliceiri & Brown, 1994; Kawahara, Baker, & Tata, 1991
), and
Silurana tropicalis
(
Duarte-Guterman, Langlois, Pauli, & Trudeau, 2010
).
RNAs for deiodinases are also present in oocytes with D1 detected in
X. laevis
(
Morvan-Dubois et al., 2006
) and D1, D2, and D3 detected in
S. tropicalis
(
Duarte-Guterman et al., 2010; Tindall, Morris, Pownall, &
Isaacs, 2007
). As mentioned earlier, D2 activates thyroid hormone signaling
by converting T
4
to the more active T
3
, and D3 inactivates both T
4
and T
3
.
D1 has both activating and inactivating activities.
Experiments in
X. laevis
indicate that thyroid hormone receptor plays an
inhibitory role in the early development. Unliganded thyroid hormone re-
ceptor represses the expression of genes activated by thyroid hormone, and
this repressive activity functions to prevent precocious metamorphosis of
tadpoles (
Buchholz et al., 2011; Sachs et al., 2000; Sato, Buchholz, Paul,
& Shi, 2007; Tomita, Buchholz, & Shi, 2004
). In embryos, overexpression
of a thyroid hormone receptor without the repression domain led to under-
developed eyes and other head abnormalities (
Havis et al., 2006; Morvan-
Dubois, Demeneix, & Sachs, 2008
), suggesting a role for repression in the
early development. Head development is also abnormal when embryos,
overexpressing thyroid hormone receptor, are given exogenous T
3
(
Old,
Jones, Sweeney, & Smith, 1992
).
These experiments leave open the possibility that maternal thyroid hor-
mone and thyroid hormone receptor have an activity in the early embryo.
The most straightforward test is to interfere with hormone-receptor interac-
tions in the early embryo. In one approach, transgenic
X. laevis
was produced
expressing a dominant negative thyroid receptor (DN-TR) (
Schreiber et al.,
2001
). Embryogenesis was not affected, but it was not determined whether
sufficient DN-TR protein from the transgene was synthesized early enough
to interfere with embryogenesis. A way to circumvent this problem would be
to inject DN-TRRNA into fertilized eggs, or better into full-grown oocytes
(
Mir & Heasman, 2008
). Another approach is to treat early embryos with a
receptor antagonist, such as NH-3 (
Havis et al., 2006; Grover et al., 2007
).
Treated embryos of
X. laevis
had reduced eyes and abnormal tails and intes-
tines, but yolk retention was not examined. Based on the thyroid hormone
dependency of yolk utilization in the
E. coqui
nutritional endoderm
(
Singamsetty & Elinson, 2010
), it would be particularly interesting to know
whether NH-3 inhibited yolk utilization in
X. laevis
.
In
E. coqui
, RNAs for both thyroid hormone receptors
a
and
b
(
thra
,
thrb
)
(
Callery & Elinson, 2000a
) are present in full-grown oocytes. Based on the
