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Laboratoire de Génétique Moléculaire, Institut de Recherches Cliniques de Montréal, and the Département de Biochimie, Université de Montréal (C.L., J.D.), Montréal, Québec, Canada H2W 1R7
Address all correspondence and requests for reprints to: Dr. Jacques Drouin, Institut de Recherches Cliniques de Montréal, Laboratory of Molecular Genetics, 110 des Pins ouest, Montréal, Québec, Canada H2W 1R7. E-mail: drouinj{at}ircm.qc.ca
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Abstract |
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-glycoprotein subunit-expressing cells appear.
Coimmunolocalization studies reveal that -glycoprotein
subunit-positive cells express the highest levels of Ptx1 throughout
development and in the adult gland. The quantitative differences in
Ptx1 expression in pituitary cell lineages may relate to a role in cell
proliferation, lineage commitment, and/or the control of organ
development. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionNote Added in ProofReferences |
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The temporal and spatial expression patterns of pituitary hormone genes
have been well characterized and were found to be conserved in
mammalian species (8, 9). In the mouse, the -glycoprotein subunit
(GSU) mRNA is first detected on E11.5 in cells of the forming pars
tuberalis (8). POMC expression then appears in the ventral region of
the pars distalis on E12.5 and in the intermediate lobe on E14.5. The
GH and PRL genes are turned on in the posterior part of the pars
distalis on E15.5. Before birth, the gonadotropins are expressed at low
levels in the medioventral region starting on E16.5. Based on these
expression patterns and on expression of homeobox transcription
factors, it has been suggested that differentiation of the developing
pituitary follows a ventral to dorsal gradient (10).
The homeobox is a 60-amino acid helix-turn-helix motif that acts as a DNA-binding domain (11, 12). Homeobox-containing genes are usually involved in developmental processes, such as embryonic patterning, organogenesis, and/or cell differentiation. Pit-1 is the best characterized homeobox-containing transcription factor specifically expressed in the pituitary. Pit-1 activates transcription of the GH and PRL genes and promotes differentiation and proliferation of the somatolactotroph lineage (13, 14, 15). Other homeobox genes expressed in the pituitary include Rpx and Lhx3 (16, 17). Rpx gene expression is down-regulated as differentiation of pituitary cells occurs; this suggested that Rpx may repress target genes involved in differentiation (18). Inactivation of the Lhx3 gene results in the arrest of Rathke’s pouch development on E11.5, presumably due to a blockade in pituitary cell proliferation (19). Thus, each pituitary homeogene is active in all or a subset of cells and often during a specific period of development. These observations have supported a model in which the early expression of genes such as Rpx and Lhx3 define a homeotic code at the molecular level (16, 17). This code is sequentially acted upon by cell-restricted transcription factors such as Prop-1 and Pit1 in the case of the somatolactotroph lineage (10), SF-1 in the case of the gonadotroph lineage (20, 21), and NeuroD1 in the case of the corticotroph lineage (22). The consequence of this code would be the appearance of hormone-producing cells in a spatially and temporally controlled manner.
We have previously cloned another homeobox-containing gene expressed in the pituitary, pituitary homeobox 1 (Ptx1), through its property to bind a cis-acting element of the POMC gene (23). Originally, we showed Ptx1 expression in corticotroph cells; however, mRNA expression was also detected throughout Rathke’s pouch, well before pituitary cell differentiation (23). In a more detailed developmental study, we characterized the onset of Ptx1 expression in the stomodeal ectoderm from which Rathke’s pouch arises (24). We have most recently shown that all pituitary hormone-coding gene promoters are activated by Ptx1; thus, Ptx1 is a pan-pituitary regulator of transcription (21). The present work was undertaken to define the ontogeny of Ptx1 mRNA and protein expression throughout pituitary organogenesis.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionNote Added in ProofReferences |
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In situ hybridization
The protocol for in situ hybridization has been
described previously (24). Two complementary DNA fragments were
transcribed into complementary RNA probes: a 573-nucleotide fragment
(from nucleotides 604-1177) centered around the homeodomain and a
310-nucleotide fragment derived mainly from the 3'-untranslated region.
Both gave identical results.
Preparation and characterization of affinity-purified
anti-Ptx1 antibodies
A PCR fragment encoding amino acids 24–56 of mouse Ptx1 (mPtx1)
was subcloned in pMal-c to generate mattose-binding protein (MBP)-Ptx1
peptide chimeras. The fusion protein was produced and purified
according to the manufacturer’s protocol (New England Biolabs, Inc., Beverley, MA). Antibodies were raised in rabbits using 100
µg purified protein for the primary injection and three subsequent
boosts. Antiserum was collected and purified by a two-step affinity
chromatography procedure. Antibodies against MBP were first removed
from the antiserum by three passages through an MBP-Sepharose column.
The flow-through from this step was next passed through an MBP-Ptx1
column. After washing in 0.5 M NaCl, antibodies against the
Ptx1 peptide were eluted under acidic conditions (0.1 M
glycine, pH 2.8), neutralized, dialyzed against PBS, and concentrated
to 0.8 mg/ml. Depletion of anti-MBP antibodies was partial (
60% as
judged by Western analysis), but this did not affect subsequent
detection of mPtx1. The specificity of the affinity-purified antibodies
was tested using electrophoretic mobility shift assays and Western
analysis of extracts of cells overexpressing full-length mPtx1
(21).
Immunohistochemistry
Immunohistochemistry was performed using the antigen retrieval
technique (25). Sections were deparaffinized, rehydrated, and submitted
to the following procedure: three microwave treatments of 5 min each in
0.1 M sodium citrate, pH 6.0, at 240 watts followed by 20
min at room temperature and a final 5-min microwave treatment in the
same buffer at 240 watts. Slides were then treated with 3%
H2O2, and reactive amines were quenched by
incubation in 300 mM glycine. All immunohistochemistry
reactions were performed in PBS-0.1% Tween-20 (PBT). Nonspecific
background was blocked with 10% normal goat serum in PBT for 1 h
at room temperature. Slides were rinsed in PBT, and sections were
incubated with mPtx1 affinity-purified antibodies (typical
concentration, 20 µg/ml) for 16 h at 4 C. Further dilution of
the antibody results in loss of the low level signal such that only
intensely labeled nuclei are revealed (data not shown). Immune
complexes were revealed using biotinylated antirabbit antibodies
followed by horseradish peroxidase-conjugated avidin
(Vectastain) and diaminobenzidine as substrate. In
colocalization experiments, pituitary hormones were detected with a
1:100 dilution of antisera raised in guinea pig (provided by A. F.
Parlow, Torrance, CA) and revealed using alkaline
phosphatase-conjugated antiguinea pig antibodies and
bromochloroindoylphosphate/nitrozolium blue as substrate. The following
antisera were used: anti-GSU (lot AFP5351791), anti-TSHß
(lot 4492192), and anti-LHß (lot AFP222238790). Monoclonal antibodies
against ACTH were purchased from Cortex Biochem (San Leandro, CA) and
used at 5 µg/ml.
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Results |
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, A and A'). By E16.5,
however, differential expression of Ptx1 was clearly seen (Fig. 1, B
and B'). Higher levels of Ptx1 mRNA were detected in the pars
tuberalis, which is located rostrally to the anterior lobe. Scattered
cells in the ventral region of the anterior lobe also exhibit higher
levels of Ptx1 (arrows in Fig. 1B'). The position of the
intensely labeled pars tuberalis is better revealed on a transverse
section on E15.5 (Fig. 1, C and C'); it is situated in front of the
anterior lobe and surrounds the infundibular stalk. The junction
between pars tuberalis and pars distalis is the entry point of the
angiogenic mesenchyme that will form the capillary bed of the gland.
This mesenchyme does not express Ptx1 (arrow in Fig. 1C').
Intermediate gestational stages were analyzed to determine the time at
which this differential pattern of expression appears. As shown in Fig. 2A', higher levels of Ptx1 mRNA were
already detectable in cells of the forming pars tuberalis on E13.5. To
determine whether differential expression might arise through
down-regulation in some cells or up-regulation in others, we compared
the levels of Ptx1 mRNA on E13.5 with pituitaries at an earlier
developmental stage. In situ hybridization using sections of
E11.5 pituitaries in the same experiment and conditions similar to
those used in Fig. 2A' suggested that Ptx1 expression is down-regulated
from the Rathke’s pouch stage on E11.5–E13.5 in the anterior lobe but
not in the pars tuberalis (compare signals in Fig. 2B' with those in
Fig. 2A').
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GSU gene expressionGSU expression (7, 8), we determined whether structures intensely
labeled for Ptx1 mRNA correspond to sites of GSU expression.
In situ hybridization with an GSU probe indicated that
the gene is initially turned on in the forming pars tuberalis in the
mouse (Fig. 3, A and A'). As development
proceeds, cells of the anterior lobe begin to express GSU. As shown
in Fig. 3B', these cells are mainly found in the ventral region of the
pituitary on E16.5. Expression of GSU is also maintained in the pars
tuberalis at this stage. These results are consistent with earlier
reports (7, 8), and we used protein colocalization to further define
the relationship between GSU and Ptx1 expression.
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, Ptx1
protein is present in all nuclei of the anterior lobe of the adult
pituitary, with some cells expressing higher levels than others. The
protein is also present at low levels in the intermediate lobe (data
not shown). Ptx1 immunoreactivity is completely competed by the
addition of a 6-fold molar excess of purified fusion protein (MBP-Ptx1;
Fig. 4B), but not by the addition of similarly purified MBP fused to an
unrelated peptide (Fig. 4C). It can be seen that cells heavily stained
by eosin do not express high levels of Ptx1 (Fig. 4A). These so-called
acidophils are the somatotrophs and lactotrophs (26). To determine
which pituitary cells express Ptx1 at higher levels, we performed
colocalization experiments with several hormone antisera. As shown in
Fig. 4D, there is a very good correlation between high level expression
of Ptx1 (brown) and expression of GSU (blue). GSU-expressing
cells include both gonadotrophs and thyrotrophs. About 15% of
intensely labeled cells express TSHß (Fig. 4E), and a large
proportion of the remainder express LHß (Fig. 4F). On the other hand,
we have observed that most corticotrophs (revealed by ACTH
immunolocalization) do not express Ptx1 at high levels. Representative
results are shown in Fig. 4G. However, a fraction of POMC-expressing
cells also appeared to have high levels of Ptx1.
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), a graded signal was detected from rostral to caudal. The
future intermediate lobe was almost devoid of immunoreactivity, whereas
the forming pars tuberalis was intensely labeled, in agreement with the
in situ hybridization results (Fig. 2A'). Ptx1
immunoreactivity was not detected in the mesenchyme surrounding the
pituitary or in neural tissues, consistent with previous in
situ hybridization studies (24). However, Ptx1 protein was readily
detected in other stomodeal derivatives, such as the oral and olfactory
epithelia (not shown). Figure 4I shows an E16.5 pituitary labeled with
anti-Ptx1. The quantitative difference in levels of Ptx1 expression is
more subtle than that in the adult gland (compare with Fig. 4A). At
higher magnification (Fig. 4J), high Ptx1-expressing cells are shown to
be predominantly GSU positive. Most intensely labeled nuclei are
found in the ventral part of the gland. Figure 4K shows Ptx1 (brown)
and ACTH (blue) immunoreactivities. Note the intensely labeled nuclei
of the pars tuberalis that do not express ACTH. At higher magnification
(Fig. 4L), corticotrophs were found in clusters, and unlike GSU
cells, we detected ACTH-positive cells that were both intensely and
weakly labeled for nuclear Ptx1. Interestingly, corticotrophs
expressing high levels of Ptx1 were mostly found in the ventral region
of the gland (Fig. 4, K and L).
In summary, we have detected Ptx1 protein in the nuclei of all
pituitary cells (Fig. 4). Our analysis reveals higher levels of Ptx1
expression in GSU cells relative to other pituitary lineages. A
similar pattern was detected by in situ hybridization in
both the developing pituitary (Figs. 1 and 2) and the adult gland
(23).
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionNote Added in ProofReferences |
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GSU
lineage. The GSU gene is first activated on E11.5 in cells of the
rostro-ventral part of the pituitary primordium. On E12.5, GSU
expression is concentrated in the budding pars tuberalis (Fig. 3A) and
extends to the pars distalis as development proceeds (Fig. 3B). High
expression of Ptx1 is first detected throughout Rathke’s pouch and is
later maintained in the proliferating pars tuberalis. In contrast, the
low level Ptx1 expression in the developing anterior lobe appears to
result from a down-regulation of Ptx1 expression. This interpretation
is based on the comparison of signal intensity in pituitaries sectioned
at different stages (E13.5 vs. E11.5; Fig. 2). We can
formulate three hypotheses to account for the maintenance of the higher
expression levels of Ptx1 in cells of the pars tuberalis during
development and in adult GSU-expressing cells.
First, it is possible that early in pituitary development, higher
levels of Ptx1 maintain pituitary cells in a proliferative state. This
is consistent with reports that higher levels of homeobox-containing
genes are associated with proliferation of undifferentiated cells (27).
Such is the case of HoxB genes during hemopoiesis (28). Furthermore,
expression of another homeobox-containing gene, Rpx, has been inversely
correlated with pituitary differentiation. Expression of this gene is
initially detected throughout Rathke’s pouch, but is progressively
extinguished along a ventral to dorsal gradient as differentiation
occurs (16). In the case of Ptx1, down-regulation of the gene occurs
when the pars tuberalis differentiates from the pars distalis.
Down-regulation of Ptx1 expression may be required for progression of
pars distalis differentiation. Zones of proliferation dependent on high
levels of Ptx1 could thus result in the pars tuberalis and in the
ventral region of the pars distalis. These high-expressing Ptx1 cells
also express GSU, but these cells are clearly different from fully
differentiated gonadotrophs or thyrotrophs that will appear later. This
is exemplified by the transient expression of TSHß in the pars
tuberalis, independently of Pit-1 and the adult TSHß lineage
(29).
During development, quantitative differences in Ptx1 expression may segment the developing pituitary by activation of different target genes in a spatially restricted fashion. It has recently been suggested that development of the pituitary is controlled by a combination of homeoproteins and other transcription factors, a phenomenon analogous to the patterning of the trunk by the so-called Hox code (10). Hence, Ptx1 might be another gene involved in the molecular definition of this hypothetical code.
In the adult gland, maintenance of higher levels of Ptx1 in the
GSU lineage may relate to the important role of Ptx1 in
transcription of the GSU and Lhx-3 genes (21). Indeed, we have shown
previously that Ptx1 lies upstream of a differentiation cascade in a
pregonadotroph cell model; in these cells, knockdown of Ptx1 results in
loss of Lhx-3 and GSU expression. In addition, the levels of Ptx1
may contribute to set the levels of target gene expression in adult
differentiated cells; expression of genes, such as GSU, Lhx-3, and
LHß, appear sensitive to the levels of Ptx1 expression (21). The idea
that gene dosage or the quantitative level of regulatory gene
expression may be relevant to gene function is highlighted by recent
human genetics studies. Indeed, it was recently shown that mutations of
the Ptx2 (RIEG) gene, a close relative of Ptx1, causes Rieger’s
syndrome by haploinsufficiency (30). It is believed that the striking
phenotype observed in the patients results from decreased Ptx2
expression due to deleterious mutations in one allele of the gene.
Hence, quantitative differences in expression of regulatory genes such
as those observed in the present work for Ptx1 in the pituitary might
very well have crucial consequences for cell differentiation and/or
organogenesis.
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Note Added in Proof |
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GSU-expressing cells at birth in agreement with
observations reported in the present work.
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Acknowledgments |
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GSU
probe. |
Footnotes |
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Received April 27, 1998.
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References |
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-subunit in the pars tuberalis of the rat pituitary gland during
ontogenesis. Neuroendocrinology 58:616–624[Medline]
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