Biology Reference
In-Depth Information
(
Heyland, Price, Bodnarova-Buganova, & Moroz, 2006
). TR function has
been more extensively investigated in the prochordate, Amphioxus. It has a
poor affinity for T
3
and its endogenous ligand is likely to be TRIAC (
Paris
et al., 2008
). Here, again it is not clear whether the liganded receptor plays
any developmental role. In the same study, Ciona TRwas also shown not to
bind TH, even if it was demonstrated that TH inhibitors are able to block
Ciona metamorphosis (
Patricolo, Cammarata, & D'Agati, 2001
). These in-
vertebrate and prochordate models can represent alternatives to the use of
vertebrates in experimental research, but their physiology appears to be quite
distinct from chordate/vertebrate systems.
2.2. Both activating and inactivating deiodinases are present
in embryos
The cloning of the genes corresponding to the activating (
Dio2
and
Dio1
)
and inactivating (
Dio3 and Dio1
) deiodinases (the enzymes are referred to
as D1, D2, and D3) is more recent than that of the TRs (
Dio1
:
Berry &
Larsen, 1993
;
Dio2
:
Becker, Schneider, Davey, & Galton, 1995; Croteau,
Whittemore, Schneider, & St Germain, 1995
;
Dio3
:
Croteau, Davey,
Galton, & St Germain, 1996
). However, deiodinases have recently moved
to the fore as major actors in TH signaling with their roles in tight regulation
of TH availability (reviewed in
Dentice & Salvatore, 2011
).
Measurable levels of mRNA encoding the three deiodinases were found
in
X. laevis
embryos from neurulation stages onward (
Fini, Le M
´
vel, et al.,
2012; Morvan Dubois et al., 2006
). Expression is also found in the egg where
Dio1
mRNA is expressed at high levels, whereas
dio2
and
dio3
mRNAs are
expressed only at much lower levels (
Fini, Le M´vel, et al., 2012; Morvan
Dubois et al., 2006
). Similarly,
dio2
and
dio3
mRNAs were also found at the
two-cell stage in
Xenopus tropicalis
, indicating that these mRNAs are present
in
X. tropicalis
egg (
Duarte-Guterman et al., 2010
). During embryological
development in
X. laevis
,
dio1
expression decreases significantly between
egg and gastrula stages and then increases dramatically from stage NF24
onward, stabilizing between NF45 and 48 (
Fini, Le M
´
vel, et al., 2012
).
Similar to
dio2
, during embryogenesis two major waves of expression are
discerned in
X. laevis
(
Morvan Dubois et al., 2006
): the first around stages
NF10-14 (neurula) and the second at stages NF31-NF37 (late embryogen-
esis), levels then increase significantly from stage NF41 onward, but not in-
creasing to the same extent as the deactivating deiodinases mRNA of D1 and
D3 (
Fini, Le M´vel, et al., 2012
). D2 activity accounts for most of the outer
ring (activating) deiodinase activity in the developing
Xenopus
embryo.
