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, -ß, and -
during Rat Embryonic Development1
Institut de Biologie Animale, Université de Lausanne, 1015 Lausanne, Switzerland
Address all correspondence and requests for reprints to: Dr. Walter Wahli, Institut de Biologie Animale, Bâtiment de Biologie, Université de Lausanne, CH-1015 Lausanne, Switzerland. E-mail: walter.wahli{at}iba.unil.ch
| Abstract |
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starts late in development,
with increasing levels in organs such as liver, kidney, intestine, and
pancreas, in which it will also be present later in adulthood to
regulate its specific target genes. PPAR
is also transiently
expressed in the embryonic epidermis and central nervous system.
PPAR
presents a very restricted pattern of expression, being
strongly expressed in brown adipose tissue, in which differentiation it
has been shown to participate. Like PPAR
, it is also expressed
transiently in the central nervous system. Interestingly, PPAR
, -ß
and -
are coexpressed at high levels in brown adipose tissue.
Finally, the high and ubiquitous expression of PPARß suggests some
fundamental role(s) that this receptor might play throughout
development. | Introduction |
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, PPARß (also called
, NUC-1, or
FAAR), and PPAR
(2, 3, 4, 5, 6, 7, 8, 9, 10). PPARs were shown to be activated by
substances that induce peroxisome proliferation (2, 3) as well as by
natural fatty acids (4, 11). It is only recently that fatty acids, some
eicosanoids, and some hypolipidemic and antidiabetic drugs have been
shown to directly bind to PPARs (1218; see Ref. 19 for a review).
PPAR target genes encode enzymes involved in peroxisomal and
mitochondrial ß-oxidation, ketone body synthesis, and microsomal
-hydroxylation, as well as the production of fatty acid binding
proteins, apolipoproteins, lipoprotein lipase, malic enzyme,
phosphoenolpyruvate carboxykinase, and the brown adipose tissue (BAT)
uncoupling protein (see Refs. 1, 20, and 21 for reviews; 22). Thus,
PPARs play key roles in different aspects of lipid metabolism and
homeostasis. Consistent with the different pathways they regulate,
PPARs were shown to be expressed in a wide range of tissues of the
adult organism (3, 7, 23, 24, 25). We have shown recently, using in
situ hybridization, that in the adult rat, PPAR
is expressed in
cells with high catabolic rates of fatty acids and high
peroxisome-dependent activities (hepatocytes, cardiomyocytes, proximal
tubules of kidney, intestinal mucosa). PPARß is expressed
ubiquitously and is abundant in most tissues, whereas PPAR
is
restricted mainly to adipose tissue, with some expression also in parts
of the immune system, the retina, and other organs in trace amounts
(25). The localization of PPAR
and PPAR
gene transcripts and
proteins and the nature of their target genes indicates that they play
roles in fatty acid catabolism for PPAR
, on the one hand, and
adipogenesis and lipid storage for PPAR
, on the other hand (see Ref.
1 for a review). In contrast, however, the role of the ubiquitously
expressed PPARß remains elusive.
As the ubiquitous expression of PPARß in the adult rat did not
provide clear information as to its roles, developmental expression of
this isotype was analyzed, with the goal of unraveling differential
patterns of expression that would implicate PPARß in specific events.
Developmental patterns of expression of PPAR
and PPAR
were also
investigated to further understand their roles in the embryo. Previous
reports of Northern blot analyses in Xenopus and mice showed
that PPARß is expressed early and throughout development, whereas
PPAR
and PPAR
appear only later in relatively high amounts (3, 7). However, these studies did provide only limited information, as the
distribution within the embryo could not be resolved by the approach
used. It is precisely to answer this question that analysis of the
differential tissular expression of PPARs during embryonic development
of the rat was undertaken by in situ hybridization. Rat
embryos were studied using specific probes for rat PPAR
, PPARß,
and PPAR
(25). Embryonic days (E) E8.5, E11.5, E13.5, E15.5, and
E18.5 were chosen to cover the most important periods of development:
E8.5 for the beginning of organization of the embryo in three layers;
E11.5E15.5 because of the intense differentiation processes that
occur in most tissues, particularly in the central nervous system
(CNS); and E18.5 because it represents a stage at which many tissues
are already expressing specific functions that can be compared with
those found in adulthood. Our results show that PPAR
and PPAR
start to be expressed only late in development (E13.5), mainly in the
tissues where they will be found postnatally and in the adult. In
addition, both PPAR
and PPAR
present transient expression in the
CNS around E13.5. In contrast to the specific distribution of PPAR
and PPAR
, PPARß is expressed ubiquitously and very early during
embryogenesis, with a peak of expression in the developing nervous
system on E13.5. During late development (E18.5), PPARß decreases to
levels that will be found later ubiquitously in adult tissues. This
strikingly high expression of PPARß during development points to
potential new directions of investigation to analyze the to date
elusive role of this PPAR isotype.
| Materials and Methods |
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(nucleotides 13771766) (4) was obtained (25, 26). cDNAs comprising
part of the A/B domain of the rat PPARß (nucleotides 264398) (8)
and part of the A/B and C domains of the rat PPAR
(403 nucleotides
long, 96% homologous to the mouse PPAR
) (23) were obtained (25).
The three rat PPAR (
, ß, and
) cDNAs were subcloned in
pBluescript KS+ and SK+ vectors (Stratagene,
Heidelberg, Germany) and used to synthesize in vitro
transcribed antisense and sense riboprobes (25, 27). Antisense and
sense riboprobes were labeled with digoxigenin (Boehringer Mannheim,
Mannheim, Germany).
Embryo preparation and in situ hybridization analysis
The age of Sprague-Dawley rat embryos (BRL, Basel, Switzerland)
was determined from the appearance of a vaginal plug in pregnant
females, and uteruses were dissected out. E8.5 embryos were let
in utero, whereas E11.5, E13.5, E15.5, and E18.5 embryos
were removed from the uterus and separated from the placenta. Uteruses
(E8.5) and embryos (E11.5, E13.5, E15.5, and E18.5) were then quickly
washed in diethylpyrocarbonate (Fluka, Buchs, Switzerland)-treated PBS
and fixed 15 h in 4% paraformaldehyde-PBS at room temperature.
Embryos were cryoprotected at 4 C in 12% sucrose-PBS and 18%
sucrose-PBS for 6 and 12 h, respectively, and then embedded in
tissue-freezing medium (Jung, Nussloch, Germany) and frozen in
isopentane and dry ice. Embryos were kept at -80 C until used, and
then cut and analyzed by in situ hybridization as previously
described (25, 27). Briefly, cryosections 12 µm thick were prepared,
and hybridization with digoxigenin-labeled antisense and sense
riboprobes for rat PPAR
, -ß, and -
was carried out at 60 C in
5 x SSC (standard saline citrate) and 50% formamide for 40
h. Washes (30 min in 2 x SSC at room temperature, 1 h in
2 x SSC at 65 C, 1 h in 0.1 x SSC at 65 C) and
alkaline phosphatase staining were performed as previously described
(25, 27). After staining, sections were dehydrated and mounted (Eukitt,
O. Kindler Co., Freiburg, Germany). The specificity of hybridization
was ascertained by the use of sense probes for the PPAR genes with the
same length, GC content, and specificity as the corresponding antisense
probes.
Histological analysis
In situ hybridization slides were observed and
photographed on an Axiophot microscope (Carl Zeiss, Zurich,
Switzerland).
| Results |
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PPAR
Transcripts of the PPAR
gene were first detected at stage
E13.5, where they were found at relatively high levels in the CNS
(brain and spinal cord), tongue, and digestive tract (Fig. 1C
). Lower levels were also detected in
vertebrae, liver, and heart. On E15.5, PPAR
expression decreased in
the CNS, heart, digestive tract, and vertebrae (Fig. 2A
), whereas it remained unchanged in the
liver and was first detected in the epidermis, the cortex of the
kidney, striated muscles, and the lung (Fig. 2C
and Table 1
). On E18.5, the PPAR
gene was highly
expressed in the liver (Fig. 3A
and Table 1
), the mucosa of the digestive tract (Fig. 3B
and Table 1
), BAT (Fig. 3C
and Table 1
), and epidermis (Fig. 3A
and Table 1
). PPAR
transcripts were also well detected in the kidney cortex and pancreas
(Fig. 3B
and Table 1
), but were barely detectable in muscle (Fig. 3
, A
and C) and CNS (not shown and Table 1
). The staining of sections with a
sense control probe for PPAR
was negative at the different stages
tested (Figs. 1D
and 3D
).
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PPAR
The expression of the PPAR
gene was first detected on E13.5,
when relatively high transient levels of transcripts were observed in
the CNS only, particularly in the hindbrain (Fig. 1K
). On E15.5, the
expression of PPAR
in the CNS was already reduced, and no other
tissue was found expressing it (Fig. 2G
and Table 1
). On E18.5, PPAR
mRNAs were only detected in BAT, at very high levels (Fig. 3K
and Table 1
). The staining of sections with a sense control probe for PPAR
was
negative at each stage tested (Figs. 1L
, 2H
, and 3L
).
| Discussion |
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and PPAR
appear late in
development in the tissues where they will continue to be expressed in
adulthood (25, 28), with the exception of a transient expression in the
CNS (
and
) and the epidermis (
). In contrast, PPARß is
already expressed ubiquitously at the earliest stage tested, with a
marked peak on E13.5E15.5, then declining to levels characteristic of
adulthood (25).
Late expression of PPAR
in embryogenesis
The late onset of PPAR
expression (E13.5) in liver, cortex of
kidney, intestinal mucosa, pancreas, and heart (this work and Ref.29)
correlates with the progressive differentiation of these organs (30),
including the maturation of specific metabolic pathways in which
PPAR
has been found to be involved (see Ref. 1 for a review).
Particularly, the expression of PPAR
in the developing liver,
kidney, and intestinal mucosa at the starting point of their
differentiation makes it possible to regulate, early in histogenesis,
the peroxisomal ß-oxidation-encoding genes that are expressed in the
primordials of these tissues before they differentiate (31). The
expression of PPAR
in rat fetal pancreas (E18.5) may indicate its
involvement in insulin-secreting ß-cells just before they change
their metabolism from high fatty acid oxidation toward high glucose
oxidation on E19.5 (32). PPAR
knock-out mice develop normally and
are fertile, without observable gross defects (33). Thus, either the
role(s) of PPAR
in metabolic pathways must be very subtle during
embryogenesis, or compensatory events, e.g. by the other
PPAR isotypes, are responsible for the apparently normal development of
PPAR
knock-out mice. PPARs could not be detected in the adult
epidermal keratinocytes by the technique used herein (25). In contrast,
we show in this study that PPAR
and PPARß are expressed in the rat
embryonic epidermis. In the mouse embryo, PPAR
and PPARß mRNAs
appear even earlier, together with PPAR
, which we did not observe in
the rat (Michalik, L., and W. Wahli, personal communication). The
earlier onset of PPAR
and PPARß in the mouse probably reflects the
2-day advance shift of mouse development compared with that in the rat
(30), whereas the difference in PPAR
expression in embryonic
epidermis as well as liver is illustrative either of differences
between species or of the sensitivity of the methods used. However, it
indicates that PPARs might be involved in the establishment of the skin
lipid barrier, as suggested previously (34), based on a study using a
mouse expressing a dominant negative retinoic acid receptor.
Transient expression of PPAR
and PPAR
in the developing
CNS
Interestingly, a transient peak of expression of PPAR
is
observed in the whole developing CNS, around the E13.5 stage. The same
phenomenon is true for PPAR
, but is restricted to hindbrain. E13.5
corresponds to the onset of differentiation and apoptosis events in
CNS, which would suggest a role for PPAR
and PPAR
during these
processes, but remains to be proven.
Coexpression of PPAR
, -ß, and -
in BAT
White adipose tissue is specialized to store triglycerides
and releases fatty acids upon demand, whereas BAT has the additional
unique faculty to produce heat. This heat dissipation is made possible
by the expression of a specific BAT protein, UCP, which uncouples the
fatty acid oxidation from the synthesis of ATP (35, 36). Transgenic
mice lacking BAT develop obesity (37), providing evidence for an
important role of BAT in fatty acid catabolism. Furthermore, the
specific expression of UCP in BAT is regulated by PPAR
(22), which
also promotes BAT differentiation at the end of fetal life (28). Our
results on PPAR
expression in embryonic rat BAT underscore and add
to these data. Furthermore, we show that the two other isotypes,
PPAR
and PPARß, are concomitantly expressed at high levels with
PPAR
(this work and Ref.29). This raises the question of the
specific role(s) for each of the three receptors for BAT
differentiation and functions. Can they all achieve the PPAR
function in this developing tissue as previously reported (28), or do
PPAR
and PPARß have different specific roles? One would predict
that as in adulthood, PPAR
is involved primarily in BAT
differentiation and fatty acid uptake and storage, whereas PPAR
would regulate BAT fatty acid catabolism. Finally, PPARß would have a
role in basal cellular metabolism, in line with its ubiquitous
expression.
Potential roles of PPARß
PPARß is expressed ubiquitously, at higher levels during
embryogenesis than in adulthood (this study and Ref.25). It has been
proposed as a repressor of other PPAR isotype activities (38), but its
expression levels and pattern during development, compared with those
of the
and
isotypes, argue for a different function. Its
ubiquitous expression suggests a specific role in each cell type, from
the early embryo to adulthood, possibly under the control of specific
ligands. One possibility could be a function in membrane lipid
synthesis and turnover. Such a role is compatible with PPARß
expression in giant neurons of the adult brain (25), as these cells are
characterized by huge dendritic trees and axons that need high amounts
of membrane lipids and an efficient rate of membrane turnover both to
maintain these structures and to remodel synaptic connections. Another
possibility would be a function of PPARß at the onset of
differentiation processes. Indeed, we show that the peak of ubiquitous
expression of PPARß during embryogenesis correlates with that of the
period of greatest cell differentiation activity (30), particularly in
the CNS (39, 40). One hypothesis would view PPARß as a sensor for a
specific ligand that would induce cell differentiation, whereas the
absence of ligand would direct the cell toward apoptosis by default, as
previously suggested (36). This would be consistent with the PPARß
expression pattern in the adult, where it is found at high levels in
tissues with a high rate of cell renewal and differentiation (25).
Conclusion
We have described the expression patterns of the three PPAR
isotypes during embryonic development of the rat. PPAR
and PPAR
appear late in development, in cells where they will be expressed later
in adulthood, e.g. liver, intestine, kidney, and pancreas
for
, and adipose tissue for
. Furthermore, an
interesting transient peak of expression of
and
has been
noticed in the CNS. The intriguing high coexpression of the three
isotypes in the developing BAT and CNS provides opportunities to define
their respective roles in these differentiating structures.
Furthermore, the very strong ubiquitous PPARß expression throughout
development argues for a role of this isotype in basic cellular
metabolic pathways, such as those involved in membrane synthesis and
turnover, and possibly in cell cycle control.
| Acknowledgments |
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| Footnotes |
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2 Present address: Laboratoire de Chimie Clinique, Centre Hospitalier
Universitaire Vaudois, 1011-Lausanne, Suisse. ![]()
Received December 2, 1997.
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