Endocrinology Vol. 141, No. 1 340-345
Copyright © 2000 by The Endocrine Society
Developmental Regulation of Thyrotropin Receptor Gene Expression in the Fetal and Neonatal Rat Thyroid: Relation to Thyroid Morphology and to Thyroid-Specific Gene Expression1
Rosalind S. Brown,
Victoria Shalhoub2,
Sarah Coulter,
Sharon Alex,
Isabelle Joris,
William De Vito,
Jane Lian and
Gary S. Stein
Departments of Pediatrics (R.S.B., S.C.), Cell Biology (V.S., J.L.,
G.S.S.), Pathology (I.J.), and Medicine (W.D.V., S.A.),
University of Massachusetts Medical School, Worcester Massachusetts
01655
Address all correspondence and requests for reprints to: Rosalind S. Brown, M.D., Department of Pediatrics, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, Massachusetts 01655. E-mail: rosalind.brown{at}banyan.ummed.edu
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Abstract
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The TSH receptor plays a pivotal role in thyroid gland function,
growth, and differentiation, but little is known about its role or
regulation in the fetus and neonate. To explore these questions, we
systematically evaluated TSH receptor gene expression at the level of
messenger RNA (mRNA) in thyroid glands obtained from rat fetuses and
neonates, from 14 days gestation to day 5 of postnatal life. Results
were compared with histological evidence of differentiation and to
thyroid-specific gene expression. Northern blot and RT-PCR analysis
revealed that TSH mRNA was first detected at low levels on fetal day
15, but it increased 3- to 15-fold on fetal days 1718. Up-regulation
of TSH receptor mRNA on fetal day 1718 was accompanied by the first
appearance of colloid formation and of follicular development on
morphological examination. It was also paralleled by increased
expression of the thyroid-specific genes thyroglobulin (Tg) and thyroid
peroxidase. Unexpectedly, TSH mRNA abundance was 2- to 3-fold higher in
pregnant dams than in nonpregnant adult females or adult males.
In view of the 8-day lapse between the first appearance of the thyroid
diverticulum and up-regulation of TSH receptor gene expression, we
conclude that pituitary TSH, acting through its receptor, plays an
important role in terminal thyroid maturation, but it is not involved
earlier in gestation. Similarly, these data support previous evidence
that the weak thyrotropic activity of human CG could not be of
significance in early fetal thyroid gland development. The increased
TSH receptor mRNA on fetal day 1718 may be attributable to
up-regulation by TSH, which is first secreted into the fetal
circulation at this time. The significance of the increased TSH
receptor expression during pregnancy remains to be explored.
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Introduction
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THE TSH RECEPTOR plays a pivotal role in
thyroid gland growth, function, and differentiation (1, 2). Through
effects mediated primarily by the cAMP signal-transduction pathway, the
TSH receptor exhibits transcriptional control of the genes for the
major thyroid-specific proteins thyroglobulin (Tg) and thyroid
peroxidase (TPO), and it stimulates an array of cellular events,
including iodine uptake and organification, as well as thyroid hormone
synthesis and secretion (1, 2). A member of the subgroup 2, G
protein-coupled receptor superfamily, it is composed of a large,
extracellular domain, 7 hydrophobic transmembrane-spanning regions, and
a short intracytoplasmic tail. The N-terminal extracellular domain
seems to be sufficient for binding of hormone, whereas the cytoplasmic
loops and C-terminal tail are important in signal transduction (1, 2).
Despite major advances in knowledge of the structure, function, and
molecular biology of the TSH receptor in the adult, very little is
known about its expression and regulation in fetal life or its role in
fetal thyroid gland development. In thyroid follicular cells obtained
from 15-day-old rat fetuses, TSH can stimulate folliculogenesis and
iodine organification in vitro (3), an effect that is
mimicked by forskolin (4). Recently, evidence of TSH receptor message
also has been reported, at 15.5 days gestation, by in situ
hybridization (5). Despite the presence of TSH receptor messenger RNA
(mRNA) and protein at fetal days 1515.5, evidence of TSH-dependent
function, such as a follicular structure (6), TPO activity (7), and
thyroid hormonogenesis (6), does not appear until 2 days later (fetal
day 17). The present studies were initiated to provide insight into
whether this delay might be related to quantitative changes in TSH
receptor expression.
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Materials and Methods
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Animals
Female Sprague Dawley rats (Taconic Farms, Inc.,
Germantown, PA) were housed in an environmentally controlled room (22
C; lights on from 0700 to 1700 h) and mated with males overnight.
The day a vaginal plug was seen was designated day 0 of pregnancy; the
day of birth was designated as day 0 of life. After ketamine/rompun
anesthesia was administered, the thyroid gland was removed, immediately
frozen in liquid N2, and stored at -70 C until
analyzed. Control tissues were removed, frozen, and stored in a similar
manner. Thyroidectomies in fetal rats were performed under a dissecting
microscope. On fetal days 14 and 15, the entire cervical portion of the
fetus was removed; and, on day 16, the thyroid was removed (attached to
the trachea). Thereafter, the thyroid glands were obtained free of
surrounding tissue. Pools of up to 812 fetal glands were used to
obtain sufficient tissue for study. The study was approved by the
Animal Care Committee of the University of Massachusetts Medical
Center.
RNA preparation
Thyroid tissue was homogenized on ice using a Dounce
glass-Teflon homogenizer; occasionally, a power (polytron)
homogenizer was used to homogenize thyroid tissue from older animals.
Total cellular RNA was extracted by a monophasic solution of phenol and
guanidine isothiocyanate (TRIzol Reagent, Life Technologies, Inc., Grand Island, NY), as described by
Chomczynski et al. (8). In brief, addition of
chloroform-isoamyl alcohol (24:1) was followed by centrifugation. RNA
in the supernatant was precipitated with isopropranol, washed with 75%
ethanol, air dried, and dissolved in water treated with diethyl
pyrocarbonate. RNA was quantitated by absorbance at 260nm and stored at
-70 C. Intactness of the RNA and equality of loading were monitored by
electrophoretic fractionation on a 6.6% formaldehyde, 1.2% agarose
gel, and ethidium bromide staining.
Northern hybridization analysis
Northern hybridization was performed as described previously,
with minor modifications (9). In brief, 5 µg of total cellular RNA
was electrophoresed in a 1.2% denaturing agarose gel, transferred to a
Zeta-probe membrane (Bio-Rad Laboratories, Inc., Hercules,
CA), cross-linked to filter by exposure to UV light for 1 min, and
stored in a plastic bag at 4 C until use. The rat TSH receptor gene
probe employed (kindly provided by Dr. Leonard Kohn, NIH) was the
purified insert from clone T8AFB and represents residues -54 to 2780
of the nucleotide sequence reported for the rat FRTL-5 TSH receptor
(10). It was labeled with
-32P-deoxycytidine 5'-triphosphate by the
random primer method to a specific activity of at least 1 x
109 dpm/µg DNA. Prehybridizations and
hybridizations were performed in 50% formamide, 5 x SSC (20 x SSC is
0.3 M sodium chloride, 0.3 M sodium citrate),
10 x Denhardts solution (100 x Denhardts solution is 2% Ficoll,
2% polycinylpyrolidone), 50 mM sodium phosphate (pH 6.5),
1% SDS, 250 µg/ml Escherichia coli DNA at 42 C for 4 and
18 h, respectively. Blots were washed with 2 x SSC, 0.1% SDS at
room temperature, and twice with 0.1 x SSC, 0.1% SDS at 65 C and then
exposed to either preflashed XAR-5 x-ray film (Eastman Kodak Co., Rochester, NY) using a Cronex Lightning Plus screen
at -70 C or, in later experiments, a Storm 840 phosphor screen. The
amount of radioactivity corresponding to each area was quantitated
within the linear range of signals using, respectively, scanning laser
densitometry (LKB 2400 GelScan XL, Amersham Pharmacia Biotech, Inc., Piscataway, NJ) or by the Image Quant
statistical program (Molecular Dynamics, Inc., Sunnyvale,
CA). To control for variability in loading, the relative abundance of
TSH receptor message was normalized by comparison with expression of
the housekeeping gene, rat glyceraldehyde phosphate dehydrogenase
(GAPDH). For this purpose, the oligonucleotide probe for GAPDH
(Oncogene Science, Inc., Uniondale, NY) was used.
Thyroid histology
After thyroidectomy, thyroid glands obtained from the same
litters as those used for Northern analysis were immediately placed in
Bouins solution for overnight fixation, washed in running water, and
stored in 10% formalin until processing. Hematoxylin-eosin and
periodic acid Schiff (PAS) stains were performed using standard
methods.
Slot blot RNA analysis
Slot blot analysis was performed as described previously (9). In
brief, 1 and 2 µg of each RNA sample were immobilized on a Zetaprobe
membrane (Bio-Rad Laboratories, Inc.) using a Minifold ll
slot blot system (Schleicher & Schuell, Inc., Keene,
NH), cross-linked to filters by exposure to UV light for 1 min,
and stored in plastic bags at 4 C. The labeling of probes,
prehybridzation and hybridizations, and washing steps were performed
exactly as described for Northern analysis. Homologous rat probes
included the TSH receptor (described above) and the thyroid-specific
proteins Tg and TPO. The Tg gene probe is a 0.64-kb complementary DNA
(cDNA) kindly provided by G. Vassart (11). The TPO gene probe, a
2.8-kb fragment of FRTL-5 TPO cDNA inserted in pUC9, was kindly
provided by S. Kimura, NIH (12). Results were quantitated using a Storm
840 phosphoimager.
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Results
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Ontogeny of TSH receptor gene expression in the fetal and neonatal
rat thyroid
To address the question as to when the TSH receptor is first
expressed during gestation, TSH receptor mRNA was initially evaluated
by Northern hybridization analysis and quantitated by densitometry
after monitoring the mRNA with respect to representation of ribosomal
RNA (28S and 18 S) as an internal standard. Results were also
normalized by comparison with the housekeeping gene, rat GAPDH (Fig. 1
). The representation of TSH receptor mRNA
transcript levels in total cellular RNA was very low on fetal days 15
and 16 but increased 3- to 15-fold by fetal day 17. There was a
subsequent slight increase in relative mRNA abundance on neonatal day
5, but the extent of this increase was variable in repeated
experiments. Two major transcripts of 5.6-kb and 3.3-kb were observed
at all ages, similar to those observed previously in rat FRTL-5 cells.
An unexpected finding was the 2- to 3-fold increase in TSH receptor
gene expression in mothers, as compared with 5-day neonates.

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Figure 1. Cellular representation of rat TSH (rTSH) receptor
mRNA levels in thyroid glands obtained from fetal and neonatal rats,
pregnant dams, and fetal liver (negative control). A
(top), Total cellular RNA (5 µg) was prepared from
thyroid glands and subjected to Northern blot hybridization analysis,
as described in Materials and Methods. The rTSH receptor
gene probe employed represents residues -54 to 2780 of the nucleotide
sequence reported for the rat FRTL-5 TSH receptor. The exposure time
was 5 days at 70 C, with two intensifying screens.
Middle, The above blot was stripped and probed with the
housekeeping gene GAPDH. Bottom, Ethidium bromide (EtBr)
staining of total RNA resolved on a 1.2% agarose gel. B, Relative TSH
receptor abundance in arbitrary densitometric units, normalized by
comparison with GAPDH and expressed as mean +
SEM of three separate experiments. Note the unexpected 2-
to 3-fold increase in TSH mRNA levels in mothers used as a positive
control. f, Day of fetal life; d, day of postnatal life.
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The first appearance of TSH receptor mRNA on fetal day 15 was verified
by RT-PCR analysis using primers that amplified a 302-bp fragment of
the C-terminal portion of the extracellular domain specific for the rat
TSH receptor. The aforementioned amplified fragment also hybridized
with the TSH receptor cDNA probe on Southern hybridization analysis
(data not shown).
Induction of TSH receptor mRNA correlates with formation of colloid
and follicles
To compare changes in TSH receptor gene expression with
morphological evidence of thyroid maturation thyroid glands obtained
from the same litters as those used for Northern analysis and RT-PCR
were studied under light microscopy (Fig. 2
). Before fetal day 17, the thyroid gland
was difficult to distinguish from surrounding tissue. On fetal day 17,
a well-defined thyroid gland, containing clusters of mitotic epithelial
cells separated by a scanty stromal network (1A) and with minute
amounts of PAS-positive material (1B), indicative of early colloid
formation, was first observed. At 18 days gestation, the follicular
structure was much better developed (2A), and increased amounts of PAS
reactive colloid material were seen (2B). Further increase in thyroid
size and follicular development was observed at 10 days postnatally (3A
and 3B).

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Figure 2. Morphology of the fetal and neonatal thyroid gland
(350x magnification). Thyroid glands obtained from the same litters as
those used for Northern analysis were processed for hematoxylin-eosin
[H & E (A)] and PAS (B) stains. Before fetal day 17,
the thyroid gland was difficult to recognize on inspection. 1A (fetal
day 17), Clusters of epithelial cells containing numerous mitoses and
separated by a scanty stromal network are seen; 1B (fetal day 17),
minute amounts of PAS-positive material, indicated by the
arrows, first appear; 2A (fetal day 18), clear evidence
of follicular development is observed; 2B (fetal day 18), increased
colloid (indicated by the arrows) is observed; 3A and B
(postnatal day 10), further follicular development is seen. Follicles
are filled with colloid.
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Induction of TSH receptor mRNA correlates with increased expression
of Tg and TPO
Further studies were performed to determine whether TSH receptor
gene expression was related to other aspects of thyroid maturation,
such as expression of the thyroid-specific genes Tg and TPO. As seen in
Fig. 3
, up-regulation of TSH receptor mRNA
was accompanied by a concomitant increase in expression of both Tg and
TPO. In contrast, no signal was observed in the lane containing the
negative control, liver.

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Figure 3. Temporal association between gene expression of
the TSH receptor (TSHr) and gene expression of the major
thyroid-specific proteins Tg and TPO. Total cellular RNA was isolated
from fetal thyroid glands of various ages, slotted in 1 and 2 µg
samples, and hybridized with specific probe, as described under
Materials and Methods.
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Up-regulation of TSH receptor mRNA in pregnancy
Because of the observation that TSH receptor gene expression in
mothers was 2- to 3-fold that found in 5 day neonates (Fig. 1
),
experiments were performed to determine whether this was simply
attributable to a further increase in TSH receptor gene expression with
maturation into adulthood or whether the TSH receptor was up-regulated
in pregnancy. As noted in Fig. 4
, TSH
receptor mRNA abundance was again 2-fold higher in pregnant females, as
compared with adult nonpregnant females and adult males.

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Figure 4. Relative abundance of TSH receptor mRNA in thyroid
glands obtained from pregnant (preg) and nonpregnant female rats, adult
male rats, and liver (negative control). A (top),
Northern hybridization analysis of rTSH receptor mRNA was performed as
described in Materials and Methods;
middle, the above blot was stripped and probed with the
housekeeping gene GAPDH; bottom, ethidium bromide
staining of total RNA resolved on a 1% gel. B, relative TSH receptor
abundance, normalized by comparison with rat GAPDH and expressed in
arbitrary densitometric units as mean + SEM
of two separate experiments.
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Discussion
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In the present studies, we have begun to explore the role of the
pivotal thyroid regulator, the TSH receptor, in thyroid gland
development by systematically examining the ontogeny of gene expression
in the thyroid gland of the rat fetus and newborn, a frequently used
model of human thyroid development. Using a cDNA probe specific for the
full-length rat TSH receptor and a short portion of the
51- and 31-untranslated
regions, we detected two major transcripts, of 5.6 kb and 3.3 kb,
similar to those described previously in rat FRTL-5 cells
(10). No apparent difference in the relative proportions
of these transcripts, at the fetal and neonatal ages examined (as
compared with adult animals) was observed.
Although TSH receptor message was detected as early as fetal day 15,
morphological evidence of thyroid differentiation was not present until
fetal day 17. This is consistent with previous studies that have shown
that follicular development, iodine organification, and thyroid hormone
synthesis do not appear until fetal day 17 (6, 7). Immunoreactive Tg,
although demonstrable at 15 days of fetal life, undergoes a similar
increase in abundance on fetal day 17 (6). Because the fetal rat
thyroid can respond to both TSH (3) and forskolin (4) by fetal day 15,
the reason for this 2-day delay presumably is not merely a lag in
translation into functional protein. One important reason would seem to
be that the physiological ligand, pituitary TSH, is not found in the
fetal circulation until fetal day 17 (13). Our findings would suggest
that coincident up-regulation of TSH receptor gene (and, presumably
protein) expression on fetal day 17 may be an additional reason for
this 2-day lag period.
The regulation of steady-state TSH receptor mRNA levels is complex and
involves not only the homologous hormone TSH acting through its cAMP
signal (14, 15) but the coordinate action of multiple hormones
[insulin (15, 16), thyroid hormone (17, 18)], growth factors
[insulin-like growth factor, IGF-1 (15, 16)], and DNA binding
proteins [thyroid-transcription factor, TTF-1 (19), an
as-yet-unidentified single-strand DNA-binding protein (20), a Y box
protein (21), and a cAMP-response element modulator (CREM) (22)].
Multiple studies have shown that stimulation with TSH leads to
desensitization of the TSH receptor (homologous desensitization), a
phenomenon that involves not only a reduction in available receptors
but a decreased coupling of the TSH receptor and the
Gs subunit of the signal transduction machinery
(23, 24, 25, 26). In the best-studied model, the rat FRTL-5 cell line, TSH
inhibits TSH receptor expression at a transcriptional level (14, 15);
although, at lower concentrations of TSH, a stimulatory effect has been
described (15, 25). In human thyroid monolayer cell cultures, TSH
up-regulates TSH receptor mRNA at concentrations of TSH in the
physiological range (27). Thus, the dosage of TSH, study conditions,
and species may all play a role in determining the results obtained.
In vivo, the TSH receptor is down-regulated by TSH in the
adult rat (17). Our finding that TSH receptor mRNA increases on fetal
day 1718, a time when pituitary TSH is first secreted into the
circulation, suggests that (contrary to the adult situation) the TSH
receptor may be up-regulated by TSH in the fetus, though the mechanism
remains unclear. This pattern is reminiscent of the LH receptor, which
is up-regulated by LH in fetal life but down-regulated in the adult
(28).
The association between TSH receptor mRNA expression and both
morphological and molecular evidence of thyroid differentiation
observed in this study provide strong evidence that TSH, acting through
its receptor, plays an important role in terminal thyroid maturation
but is not involved in early thyroid maturation. It is possible that
other factors play a role in the thyroid maturation observed on fetal
day 17. For example, both insulin and IGF-1 have important effects on
both thyroid growth and function (14, 15, 16, 29), but their developmental
expression within the thyroid gland is unknown. Other candidate genes
are TTF-1, TTF-2, and Pax 8, thyroid-specific transcription factors
that are important not only for commitment of progenitor cells to a
thyroid-specific phenotype but for the expression of thyroid-specific
gene expression (30). Evidence to date would suggest that TTF-1 and Pax
8, which are expressed 5 days before the expression of the genes for
Tg, TPO, and the TSH receptor, are important in early thyroid
maturation and are necessary, but not sufficient, for the development
of the fully differentiated thyroid phenotype (30). Clearly, the
pivotal importance of the TSH receptor in thyroid maturation is
underscored by the severe hypothyroidism and hypoplastic thyroid glands
found in hyt/hyt mice (31), an inborn strain of mice with a loss of
function mutation of the TSH receptor (32). Similar findings have been
reported in infants and children with inactivating mutations of the TSH
receptor (33) and in babies born to mothers with potent TSH receptor
blocking antibodies (34, 35).
In summary, these data suggest strongly that pituitary TSH, acting
through its receptor, plays an important role in terminal thyroid
maturation but is not involved earlier in gestation. The increased TSH
receptor mRNA on fetal day 1718 may be attributable to up-regulation
by TSH, which is first secreted into the fetal circulation at this
time. The demonstration that the TSH receptor gene is not expressed
until relatively late in gestation provides further evidence that
placental factors with thyrotropic activity, such as human CG, do not
play a significant role in early fetal thyroid gland development. The
significance of the increased TSH receptor gene expression in pregnancy
remains to be explored.
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Acknowledgments
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We thank Jack Green for assistance with the figures.
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Footnotes
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1 Presented, in part, at the 69th Annual Meeting of The American
Thyroid Association, November 1317, 1996, San Diego, California. This
work was supported, in part, by NIH Grant DK-32520 (to the University
of Massachusetts Medical School). 
2 Current address: Amgen, Inc., Newbury Park,
California 91320-1789 
Received May 27, 1999.
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