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Departments of Physiology and Medicine, Dartmouth Medical School, Lebanon, New Hampshire 03756-0001
Address all correspondence and requests for reprints to: Dr. Valerie Anne Galton, Departments of Physiology and Medicine, Dartmouth Medical School, 1 Medical Center Drive, Borwell Building, Lebanon, New Hampshire 03756-0001. E-mail: valerie.a.galton{at}dartmouth.edu
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The mechanisms by which TH regulates developmental processes are poorly
understood. However, TH action is dependent on the presence of TH
receptors (TRs) and their major ligand, T3, and thus the
extent of TH action will be influenced by the concentrations of both.
TH and TRs have been demonstrated in the fetal rat. The TR
gene is
expressed in the neural tube as early as embryonic day 11.5 (E11.5),
and it is widely expressed at low levels in brain by E14 (6).
T4 and T3 have been detected in rat embryos by
E10, only 4 days after their implantation into the uterine wall
(7).
In extrathyroidal tissues, the intracellular concentration of T3 is dependent on the circulating levels of TH, the rates of entry of T4 and T3 into the cell, and the intracellular rates of T4 to T3 conversion and T3 degradation. In adult rats the ratio of T3/T4 in many tissues is much higher than that in plasma due to intracellular conversion of T4 to T3 by 5'-deiodinase (5'D) systems (8). The same is true in the fetus where the T3/T4 ratio approaches 1:1 by midgestation, very much higher than that in placenta or maternal plasma (7). These findings suggest that fetal and/or placental deiodinases play a critical role in development.
Three iodothyronine deiodinases, type 1 (D1), type 2 (D2), and type 3 (D3), have been identified. D1 and D2 catalyze primarily 5'D and thus are responsible for the generation of T3. 5'D activity has been demonstrated in several fetal tissues, including liver (9, 10, 11), intestine (10), lung (11), brown adipose tissue (BAT) (12), and pituitary (13). With the exception of the two latter tissues, levels of activity were very low compared with those in the adult, and generally, it was not determined whether the 5'D activity was characteristic of D1 or D2. D3 catalyzes inner ring or 5-deiodination (5D), a process that results in the degradation of both T4 and T3 to inactive derivatives. This enzyme is found primarily in cerebral cortex and skin of adult rats (14, 15), but is expressed in placenta and several fetal tissues in the rat (16) and may serve to protect fetal brain and other tissues from excessively high levels of T3 (17, 18).
Intracellular T3 levels are likely to be critical in development. Furthermore, as different tissues and organs have specific temporal patterns of development, their T3 requirements may vary widely, suggesting a need for differential regulation of T3 generation at the cellular level. The source of TH in developing mammals is the thyroid gland, initially that of the mother and later that of the fetus, and these glands secrete primarily T4. Thus, differential regulation of intracellular T3 levels in peripheral tissues as a result of different levels of D1, D2, and D3 activity could offer an important means of achieving coordination of T3-dependent developmental processes among tissues. We have strong evidence supporting this concept in Rana catesbeiana. The different temporal patterns of metamorphic events among tissues are very evident in this species. Thus, the hind limbs develop fully during prometamorphosis, whereas tail does not begin to resorb until the animals have entered climax. Yet both tissues express TRs and are exposed to the same plasma levels of TH during these two phases of development. We have shown that D2 is expressed in hind limb from the earliest time that it can be measured, but it is not expressed significantly in tail until the animals have entered metamorphic climax (19). These findings are consistent with the view that tail cannot be resorbed during prometamorphosis, even though plasma TH levels are adequate for development in leg, because it does not express the D2 needed to convert the plasma-derived T4 to T3. Once climax begins, adequate generation of T3 is made possible by the dramatic increase in D2 activity.
The aim of the present study was to demonstrate that this concept obtains also in mammals. We have determined the ontogenic profiles of D1, D2, and D3 expression in a wide spectrum of tissues in rats at different stages of development. The expression patterns were found to be both tissue and developmental stage specific. The findings are consistent with the view that the deiodinases play a major role in achieving the intracellular T3 levels that are optimal for the development of each tissue.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Tissue preparation
Rats were killed by decapitation and exsanguination. Placenta
was obtained from the pregnant rats. Tissues obtained from adults,
neonates, and fetuses included cerebral cortex, cerebellum, liver,
kidney, intestine, BAT, and skin. Pituitaries, thyroids, ovaries, and
testes were also collected from neonates and adults.
For determination of 5'D and 5D activities, tissues were homogenized in a deiodinase buffer (0.25 mM sucrose and 20 mM Tris-HCl, pH 7.6) using a Tissumizer (Tekmar Co., Cincinnati, OH). For some experiments tissues were also homogenized in 1) deiodinase buffer containing 1.2 mM EDTA and 2) deiodinase buffer containing 5 mM dithiothreitol (DTT). The homogenates were centrifuged at 1000 x g for 15 min, and the supernatants were generally stored at -20 C for subsequent assay for 5D or 5'D activity. For comparison, some tissue samples were assayed both before and after freezing. Total RNA was prepared either by the method of Chirgwin et al. (20) as modified by Schneider and Galton (21) or using a commercial RNA isolation reagent (Tri-Reagent, Molecular Research Center, Inc. Cincinnati, OH), according to the manufacturer’s instructions.
5D and 5'D assays in rat tissue homogenates
Tissue samples were assayed for 5D and 5'D according to
published methods (22, 23). The reaction mixture (total volume, 50
µl) contained between 25–250 µg tissue protein for the 5D assay,
and 1 µg (adult liver) to 500 µg (most fetal and neonatal tissues)
for the 5'D assay. Except for some of the preliminary studies, protein
concentrations for both assays were adjusted to ensure that
deiodination was less than 20%. The incubation time for both assays
was 1 h. For the 5D assays, 1 nM
[125I]T3 was used as substrate, and 50
mM DTT was used as cofactor; activity is expressed as
femtomoles of 3,3'-diiodothyronine (T2) generated per h/mg
protein. For the 5'D assays, EDTA (1.2 mM) was included in
the incubation mixture, the substrate was 1.0 nM
[125I]rT3, and the cofactor was 20
mM DTT; activity is expressed as femtomoles or picomoles of
iodide generated per h/mg protein. In determining 5'D activity, the
percentage of iodide generated was multiplied by 2 because the specific
activities of the labeled products were only half that of the
substrate. D1 and D2 5'D activities were distinguished, respectively,
by the inclusion of 1 mM PTU and/or 100 nM
nonradioactive T4 in the incubation medium. Some
incubations were carried out at 0 C, and values obtained at 37 C were
corrected for any nonspecific deiodination observed at 0 C.
[125I]Iodothyronines ([125I]rT3
SA, 
959 µCi/µg; [125I]T3 SA,
2200 Ci/mmol) were obtained from DuPont de Nemours (Wilmington,
DE) and were purified by chromatography using Sephadex LH-20
(Sigma Chemical Co., St. Louis, MO) before use. Protein
concentrations of all samples were determined according to the method
of Comings and Tack (24).
Analysis of RNA
Samples of total RNA were examined for the presence of D1, D2,
and D3 transcripts by slot blot analysis, using the rat D1
complementary DNA (cDNA), G21 (provided by Drs. Reed Larsen and Marla
Berry, Brigham and Womens Hospital, Boston, MA), and our recently
cloned rat D2 (25) and D3 (26) cDNAs as probes. Slot blots and
radioactive probes were prepared as previously described (21), except
that each slot contained 20 µg total RNA, the blots were hybridized
for 16 h at 42 C, and the final wash was carried out at 50 C. All
blots were stripped and reprobed with rat ß-actin. Hybridization
signals were quantified by densitometric measurements of scanned
computer images of the autoradiographs using the IPLab Gel program
(Signal Analytics, Vienna, VA) on a Macintosh computer.
In most instances the D2 hybridization signal was very low, and thus the presence of D2 transcripts in the samples was determined by RT-PCR generally using the Access RT-PCR kit (Promega Corp., Madison, WI). This method, a two-enzyme, single reaction tube system, uses avian myeloblastosis virus reverse transcriptase for first strand cDNA synthesis and Tfl DNA polymerase derived from Thermus flavus for second strand cDNA synthesis and subsequent PCR amplification. The reaction conditions were as follows: RT: 48 C for 45 min; 94 C for 2 min (to inactivate the RT); and PCR: 94 C for 30 sec; 58 C for 1 min; 68 C for 2 min with a 1-sec extension (30–37 cycles). The final concentration of MgSO4 was 0.5 mM. Two micrograms of total RNA were used in each reaction. The PCR products were separated by agarose gel electrophoresis, transferred to nylon filter (Nytran, Schleicher & Schuell, Inc., Keene, NH), and probed with a nested oligonucleotide as previously described (27). The following primers based on the sequences of the rat D2 cDNA were used for the RT-PCR: D2 sense (bp 666–685), 5'-ACTCGGTCATTCTGCTCAAG-3'; and D2 antisense (bp 877–895), 5'-ATCCGCCGTCTTCTCTGA-3'. This latter primer is used in both the RT and PCR reactions. A nested D2 oligo (bp 700–717) 5'CCACTCGCGGAGAGTGGA-3' was used as the probe. In some RT-PCR analyses for D2, the RT step was performed separately, and an aliquot of the reaction mixture was used in the PCR step. In this case the D2 antisense primer (bp 1256–1254) 5'-TTCAAAGGCTACCCCATAAG-3' was used in the RT step. The following PCR conditions were used: 94 C for 30 sec, 55 C for 1 min, and 72 C for 1 min with 1 sec extension (30 cycles).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In fetal tissues and in some adult tissues, the level of 5'D activity
is relatively low and distinguishing D1 from D2 by kinetic analysis has
proved to be impractical. Thus, the possibility of distinguishing
between the two types of activity by including 1 mM PTU
and/or 0.1 µM nonradioactive T4 in the
incubation medium was investigated. PTU inhibits D1 activity, whereas
D2 is insensitive to this compound. As the Km of
the D2 enzyme is in the nanomolar range, the fractional deiodination of
the substrate by D2 should be greatly reduced in the presence of the
T4. However, fractional deiodination of rT3 by
D1 should be little affected by 0.1 µM T4, as
the inhibitory constant for rT3 deiodination by
T4 is 2.7 µM (28). To demonstrate that this
is, in fact, the case under the conditions used in the present
experiments, 5'D activity was determined in the absence and presence of
PTU and/or T4 in liver, which expresses only D1, and BAT,
which expresses only D2. The substrate was 1.0 nM
[125I]rT3. As shown in Fig. 1
, the fraction of rT3
deiodinated in liver was greatly reduced in the presence of PTU, but
was unaffected by the presence of 0.1 µM T4.
In addition, it was evident that the inhibitory effect of PTU was
equally as effective when using 1 or 200 nM rT3
as the substrate concentration. In contrast, BAT 5'D activity was
completely inhibited by the T4, but was unchanged in the
presence of PTU. The results of a similar experiment carried out to
test the effects of PTU and/or T4 in several developing rat
tissues are shown in Table 1. Data for
liver and BAT are included for comparison. The percent deiodination of
[125I]rT3 in fetal skin, fetal cerebrum,
placenta, neonatal pituitary, and neonatal thyroid was only partially
inhibited by PTU. In contrast to liver, the percent deiodination in
these tissues was also inhibited by T4, and both compounds
were required for near-complete inhibition. These findings strongly
suggest that these tissues express both D1 and D2 activities and that
the majority of the PTU-resistant fraction of deiodination is
attributable to the D2. Thus, for the experiments described below, PTU
was used routinely to distinguish between D1 and D2 activities.
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. 5'D
activity was readily detectable in all three tissues on E19/20, and it
increased substantially during the first 10 days of life. In liver and
kidney the highest values were obtained in the adult, whereas in
intestine the highest 5'D activity was observed on P10 and activity was
very low in the adult. These findings in liver and intestine confirm
those in a previous report from this laboratory (10). In contrast to
5'D activity, 5D activity in all three tissues was highest in the
fetus. It decreased rapidly after birth and was minimal or absent in
the adult. In these tissues, the 5D activity was not inhibited by PTU.
This together with the fact that 5'D activity increased in the neonatal
period while 5D activity decreased strongly suggest that this 5D
activity is effected by the D3 rather than the D1 enzyme.
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), efforts
to demonstrate the presence of D2 transcripts in RNA from fetal liver,
even using the more sensitive method of RT-PCR, were not convincing. As
PTU is known to bind to protein (29), it is possible that its relative
effectiveness might have been impaired by the very high protein
concentrations required to determine 5'D activity in fetal liver; up to
1000 times more fetal than adult protein was required in the assays. We
thus conclude that the 5'D activity in these tissues is essentially all
due to the D1.
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). The profiles are similar qualitatively to those of the
corresponding D1 activities, suggesting that D1 expression during
development is regulated at least in part at a pretranslational level.
No signals were obtained when either these blots, after stripping, or
fresh blots were probed with the cDNAs for the rat D3 and D2.
All three types of deiodinase activity were detected in cerebral
cortex, cerebellum, and skin, and their developmental profiles are
shown in Fig. 4. In cerebral cortex and
cerebellum, D1 and D2 activities were detectable, but very low in the
fetus and early neonate, but by P22, levels had increased and were
comparable to those seen in the adult tissues. D3 levels were highest
in the fetus and lowest in the adult. These profiles of 5'D and 5D
activities were qualitatively similar to those in liver and kidney, but
quantitatively very different; D3 activities in fetal brain tissues
were much higher than in those in liver and kidney, and this is true
also between E17 and E20 (Galton, V. A., unpublished
observations). In contrast, D1 activity in liver and kidney after birth
are very much higher than the activity in brain tissue (>500-fold).
Skin exhibited a unique pattern of deiodinase activity. Thus, the
activities of both D1 and D2 were highest in the fetus and decreased
rapidly after birth, whereas D3 activity was relatively low in the
fetus but increased dramatically after birth, and by P12 had reached
levels approaching those found in the near-term placenta. Low levels of
both D1 and D2, along with high levels of D3 activity as previously
noted (30), were also found in placenta (Fig. 5).
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and 6), and levels were highest in the fetus
(Fig. 6).
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). It is
notable that at all three stages of development, D1 activity was much
higher than that of D2 under the in vitro assay conditions
employed, and D1, but not D2 activity, increased with age. Thus,
although D2 represented almost 50% of the total 5'D activity on P12,
it was less than one fourth of the total in the adult.
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). In the ovary, the predominant
activity at all ages was D3, but low levels of both D1 and D2 were
detected by P34, and these increased further in the adult. Compared
with the ovary, the testis exhibited considerably higher levels of D1
and D2 in the younger animals, but, like the ovary, D3 activity in the
developing testis was substantial. In the P12 and P22 testis, but not
in the adult, the predominant 5'D activity was D1. To substantiate this
novel finding that D2 is expressed in testis, 5'D activities were
determined in testes obtained from weanling rats placed on water
containing 0.1% PTU for 8 days to reduced plasma T4
levels. D2, but not D1, activity is known to increase as plasma
T4 levels decrease. D2 activity (femtomoles per h/mg
protein) was significantly increased in the PTU-treated rats: 5.7
± 0.922 (SE) compared with 2.4 ± 0.124
(P < 0.01) in control rats. In the same
experiment, a significant increase in pituitary D2 activity in
hypothyroid rats was also observed: 532 ± 59 vs.
129 ± 13.9 (P < 0.001). This finding that the
PTU-resistant 5'D activity in testis, like that in pituitary, is
enhanced in PTU-treated weanling rats strongly supports the view that
this organ expresses the D2 gene.
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). The activity was
substantial by P12 and P22 and was greatly increased in the adult. In
the adult, the majority of this activity was PTU sensitive and
therefore characteristic of D1. However, in the P12 and P22 rats at
least 50% of the activity was insensitive to PTU, suggesting the
presence of significant D2 activity. Analysis of RNA from neonatal
thyroids, both by slot blot analysis and RT-PCR revealed the presence
of D2 messenger RNA (mRNA) transcripts in this tissue. Figure 10 shows a Southern blot of the
products obtained when RNAs from several neonatal tissues are subjected
to RT-PCR using D2 gene-specific primers. The blot was probed with a
nested D2-specific oligo. D2 products were clearly evident in both
neonatal and maternal thyroid as well as in BAT, pituitary, and skin.
It is notable that no signal was seen in the lane containing neonatal
liver.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The skin presents a striking exception to this pattern. Here D2 is highly expressed in the late embryonic period and then declines markedly after birth in association with a marked increase in D3 activity to a level approaching that observed in the term placenta, and severalfold higher than that noted in the skin of adult animals (15). That the PTU-resistant 5'D activity in fetal skin is, in fact, D2 was substantiated by the finding that the fractional deiodination is reduced in the presence of 0.1 µM T4. It is notable that this high level of D2 expression in skin in late gestation occurs at a critical period of differentiation when the skin permeability barrier is forming in preparation for birth (35). It has also been found that T3 along with glucocorticoids accelerates the development of this barrier by stimulating the expression of two key enzymes, cholesterol sulfotransferase and steroid sulfatase (36, 37). Thus, the situation in skin is comparable to our findings in tadpoles, in which D2 expression is consistently most prominent in tissues at the time of maximum T3-dependent differentiation (19).
It is evident from the present results that not only is D3 expressed in all the fetal tissues studied, except BAT, thyroid, and pituitary, but the levels of activity are seemingly very much higher than those of D2 and even D1 when present, with the possible exception of liver near term. If these relative activities actually reflect those that obtain in vivo, it is highly unlikely that any T3 could exist in the cell, and certainly the intracellular concentration of T3 would be much lower than that in plasma. However, this does not appear to be the case. It has been clearly shown that fetal brain and liver selectively accumulate both T4 and T3 (38) during gestation, and that the T3/T4 ratio is much higher in fetal tissues, especially brain, than in fetal or maternal plasma (7, 39). How this could be achieved in the presence of the seemingly very high levels of D3 activity is not clear.
This apparent paradox may result from the fact that the in vitro assay conditions, which have been optimized for maximum activity with respect to pH, EDTA, and DTT concentrations, do not mimic those in the cell. For example, all three enzymes require a thiol-containing compound as a cofactor. We have shown previously that the level of activity obtained in vitro is highly dependent on the type of cofactor used; in the case of D1, activity is very much lower when glutathione-S-transferase or thioredoxin are employed in place of DTT (10, 22). Thus, the activities determined in vitro using DTT may not reflect accurately relative in vivo activities exhibited under physiological conditions. Determination of relative enzyme protein levels would probably provide some insight into this problem. However, the individual assays presumably do allow for comparison of differences in levels of activity of a given enzyme among tissues. It is also possible that the enzymes are differentially distributed either among cells or within a cell such that they do not each have the same access to intracellular T4 and T3. Studies to determine the distribution of the three deiodinases among the different cell types in a tissue using in situ hybridization techniques are currently in progress.
One of the goals of this study was to determine the relative contributions of D1 and D2 to total 5'D activity. In neonatal thyroid, more than 50% of the 5'D activity was insensitive to inhibition by PTU; the fraction was much less in the adult thyroid. D2 expression has recently been described in the human thyroid gland (40), but to date no evidence of D2 expression in the rat thyroid has been cited. That the PTU-resistant 5'D activity probably represented D2 activity was substantiated by the finding that D2 was expressed in this tissue at the mRNA level; D2 transcripts were detected in neonatal thyroid by both slot blot and RT-PCR analysis. With the latter analysis, D2 transcripts were also detected in the adult thyroid. It is notable that whereas D2 transcripts were readily detected in tissues known to express the D2 gene (neonatal BAT, pituitary, and skin), none was found in neonatal liver RNA subjected to RT-PCR in the same experiment. These findings strongly support the view that at least some of the PTU-insensitive 5'D activity in the thyroid can be attributed to the D2.
The developmental profiles of deiodinase activity in the ovary and testis have not been previously reported, and an important finding was the presence of D3 activity in both tissues at levels comparable to those found in fetal brain just before birth. The levels were highest after weaning and decreased in the adult. 5'D activity was also present in the ovary and testis at levels roughly comparable to those observed in the cerebrum and cerebellum. In both tissues, a significant fraction of the 5'D activity was PTU resistant, even in the adult. The finding that the PTU-resistant fraction in testis, as in pituitary, was up-regulated in PTU-treated rats strongly supports the contention that this fraction represented D2 activity. The coexpression of the 5'- and 5D enzymes suggests a need for tight control of intracellular T3 levels in these tissues.
In the frog, where TH is essential for metamorphosis, the importance of D2 rather than D1 for the intracellular generation of T3 during development is indisputable, as D2 is the only 5'D present at all stages of the life cycle of the frog (41, 42). In mammals, with the exception of liver, kidney, and intestine, D2 also appears to play the major role in the conversion of T4 to T3 in fetal rat tissues. Additional support for this view has been provided by the studies of Rodriguez-Garcia et al. (13), who have shown that D2 activity is the primary 5'D in fetal rat pituitary, and on E19 the level of activity is comparable to that found in adult rat. In contrast, D1 activity is very low on E19, although it increases rapidly thereafter and soon after birth reaches 50% of adult values. As noted in both these studies and the present one, the D2/D1 ratio in pituitary decreased greatly during development to the point that in the adult rat D1 accounted for the major fraction of 5'D activity, a finding consistent with that in a recent report by Kohrle et al. (43). Obregon et al. (12) have shown that fetal rat BAT 5'D activity, which is exclusively D2, reaches a maximum on E20, when levels are comparable to those found in adult rats exposed to acute cold. At this point, levels of T3 in BAT are also very high. Between E20 and birth at E22, D2 activity in BAT decreases markedly and within a few hours of birth, levels of activity are similar to those seen in normal adult rats housed at room temperature.
An important issue raised by our studies relates to quantitating the
relative amounts of D1 and D2 activity in tissues where these enzymes
are coexpressed. Our preliminary studies generally validate the use of
PTU and unlabeled T4 to assign 5'D activity to either D1 or
D2. However, it is important to consider potential artifacts that could
influence the quantitative nature of our results. For example, the
binding of substrates, inhibitors, or thiol cofactors to proteins
present in tissue homogenates can alter artificially levels of
deiodinase activity (44). In addition, PTU appears to inhibit D1 by
reacting with a substrate-induced intermediate (8). Thus, the
concentration of substrate may influence the efficiency of PTU
inhibition and reaction products may be generated during the initial
phase of the first catalytic cycle. These considerations may explain
the failure of PTU, even at high concentrations, to achieve 100%
inhibition of the 5'D activity in the liver, a tissue that expresses D1
but not D2. Such an effect could result in an overestimation of the
amount of PTU insensitive, i.e. D2, activity. However, as
shown in Fig. 1, inhibition by PTU at a variety of substrate
concentrations is nearly complete under the conditions of our assays.
Thus, any degree of overestimation of D2 activity is likely to be small
(<10% of the amount of D1 activity present) and cannot account for
the majority of PTU-insensitive 5'D activity found in the tissues
studied. However, because of such potential ambiguities, definitive
information concerning relative levels of D1 vs. D2
expression will require the development and application of quantitative
purification schemes or sensitive immunochemical methods.
In summary, D1 is the major deiodinase expressed in liver, kidney, and intestine at all stages studied. D2 is not expressed in these tissues, and D3 is expressed only during fetal life, and then at very low levels compared with those in other fetal tissues. Thyroid, pituitary, and BAT express either D2, or D2 plus D1, but do not express D3 at any of the stages studied herein. Other tissues, including cerebrum, cerebellum, ovary, testis, skin, and placenta, express all three deiodinases, generally throughout the life cycle. However, it is notable that the maximum 5'D activity attained, and thus presumably the amount of T3 generated, in these tissues is very much lower than that in liver, kidney, thyroid, and pituitary. This may relate to the fact that these latter tissues are thought to generate T3 for export to plasma (17). Although such a role is not postulated for pituitary, it is known that this tissue participates in major TH-dependent regulatory systems, which may require significant amounts of locally generated T3. Furthermore, the coexpression of D3 with D1 and D2 in a number of tissues suggests the need for tight regulation of intracellular T3 concentrations within these organs at critical stages of the life cycle. Such a pattern mimics that found in several tissues of the metamorphosing tadpole at critical T3-dependent stages of development (19). Additional studies will be required to demonstrate the functional consequences of these deiodinase expression patterns.
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Acknowledgments |
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Footnotes |
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Received March 11, 1998.
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References |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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- and ß-thyroid hormone receptor mRNAs,
including the ß2-subtype, in the developing mammalian nervous system.
J Neurosci 12:2286–2302
mRNA species in tadpole erythrocytes by thyroid
hormone. Mol Endocrinol 5:201–208[Abstract]
but not a ß
c-erbA gene. Endocrinology 133:2488–2495[Abstract]
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S. Yasuo, M. Watanabe, N. Nakao, T. Takagi, B. K. Follett, S. Ebihara, and T. Yoshimura The Reciprocal Switching of Two Thyroid Hormone-Activating and -Inactivating Enzyme Genes Is Involved in the Photoperiodic Gonadal Response of Japanese Quail Endocrinology, June 1, 2005; 146(6): 2551 - 2554. [Abstract] [Full Text] [PDF]
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J. Bernal The Significance of Thyroid Hormone Transporters in the Brain Endocrinology, April 1, 2005; 146(4): 1698 - 1700. [Full Text] [PDF]
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 K. J. Barber, J. A. Franklyn, C. J. McCabe, F. L. Khanim, J. N. Bulmer, G. S. J. Whitley, and M. D. Kilby The in Vitro Effects of Triiodothyronine on Epidermal Growth Factor-Induced Trophoblast Function J. Clin. Endocrinol. Metab., March 1, 2005; 90(3): 1655 - 1661. [Abstract] [Full Text] [PDF]
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B. Gereben, J. Pachucki, A. Kollar, Z. Liposits, and C. Fekete Ontogenic Redistribution of Type 2 Deiodinase Messenger Ribonucleic Acid in the Brain of Chicken Endocrinology, August 1, 2004; 145(8): 3619 - 3625. [Abstract] [Full Text] [PDF]
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M. H. A. Kester, R. Martinez de Mena, M. Jesus Obregon, D. Marinkovic, A. Howatson, T. J. Visser, R. Hume, and G. Morreale de Escobar Iodothyronine Levels in the Human Developing Brain: Major Regulatory Roles of Iodothyronine Deiodinases in Different Areas J. Clin. Endocrinol. Metab., July 1, 2004; 89(7): 3117 - 3128. [Abstract] [Full Text] [PDF]
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