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Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas 66160
Address all correspondence and requests for reprints to: Dr. Michael J. Soares, Department of Molecular and Integrative Physiology, University of Kansas Medical Center, Kansas City, Kansas 66160-7401. E-mail: msoares{at}kumc.edu
| Abstract |
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| Introduction |
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Members of the placental PRL family can be divided into two groups based on their biological activity (reviewed in Ref.2). Classical PRL family members, including PL-I, PL-Iv, and PL-II, bind the PRL receptor and stimulate PRL-like bioactivity. Nonclassical PRL family members possess distinct biological activities. Progress in identifying the functions of nonclassical PRL family members has been limited to two mouse members. Proliferin (PLF) stimulates uterine cell proliferation (3) and blood vessel development (4). In contrast, PLF-related protein (PLF-RP) opposes blood vessel development (4).
Although the structure and patterns of expression of d/tPRP in rat uteroplacental tissues have been reported (1, 5, 6, 7), the biological role of d/tPRP during pregnancy has yet to be fully resolved. Approaches for investigating the physiology of d/tPRP and other nonclassical PRL family members would be significantly advanced by the availability of a mouse model and the application of gene manipulation strategies.
Evidence for a mouse d/tPRP came originally from Northern blot analysis of mouse decidual RNA using a rat d/tPRP complementary DNA (cDNA) probe (Orwig, K. E., and M. J. Soares, unpublished results). Further evidence for a mouse d/tPRP was derived from inspection of the National Center for Biotechnology Information (Bethesda, MD) database of expressed sequence tags (dbEST) containing information on expressed sequence tags from several mouse conceptus cDNA libraries (8). In this report, we establish the presence of d/tPRP in the mouse, characterize its cDNA, and describe its expression pattern during pregnancy.
| Materials and Methods |
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Animals and tissue preparation
CD-1 mice were obtained from Charles River Laboratories
(Wilmington, MA). Holtzman rats were acquired from Harlan
Sprague-Dawley (Indianapolis, IN). The animals were housed in an
environmentally controlled facility, with lights on from 06002000 h,
and allowed free access to food and water. Timed pregnancies and tissue
dissections were performed as previously described (9, 10). The
presence of a copulatory plug was designated day 1 of pregnancy.
Pseudopregnancy was induced by mating with vasectomized males.
Deciduomal reactions were induced on day 4 of pseudopregnancy by
injection of 5075 µl sesame oil/uterine horn. Protocols for the
care and use of animals were approved by the University of Kansas
animal care and use committee.
Generation of recombinant mouse and rat d/tPRP
The 293 human fetal kidney cell line was used as a host for the
expression of recombinant mouse d/tPRP. 293 cells were routinely
maintained in MEM supplemented with 20 mM HEPES, 100 U/ml
penicillin, 100 µg/ml streptomycin, and 10% FBS with an atmosphere
of 5% CO2-95% air in a 37 C humidified incubator. Mouse
d/tPRP cDNA in the pCMV-SPORT2 expression vector was cotransfected with
pSV2 neo (a plasmid providing neomycin resistance) into 293
cells via electroporation. Cells were selected for 2 weeks in the
presence of geneticin (G418; 500 µg/ml) as previously described (11, 12). Cells were grown to confluence and transferred to serum-free
culture medium. Conditioned medium was collected after 72 h,
clarified by centrifugation, and stored at -20 C until use.
Recombinant rat d/tPRP was generated as previously described (12).
Characterization of d/tPRP cDNA
Examination of the dbEST from day 8.5 mouse embryo revealed the
presence of several cDNA clones exhibiting a high degree of homology
with rat d/tPRP. We obtained one of these clones (EST name: mp06g07.rl;
GenBank accession no. AA108035) from IMAGE Consortium and Research
Genetics (8). The cDNA clone was inserted into the SalI and
NotI sites of the pCMV-SPORT2 vector. DNA sequencing was
performed by the dideoxy chain termination method using Sequenase and
[35S]deoxy-ATP (13). Both strands of the mouse d/tPRP
cDNA were completely sequenced using primers for T7 and SP6 flanking
regions as well as internal oligonucleotides. Reaction products were
resolved on 6% polyacrylamide urea gels, dried, and exposed to Kodak
XAR film (Eastman Kodak, Rochester, NY).
Chromosomal assignment
Chromosomal localization of the mouse d/tPRP gene was determined
using the Jackson Laboratory interspecific backcross panel (14).
Initially, genomic DNA was obtained from C57BL/6JEi and SPRET/Ei mice
for the purpose of identifying polymorphisms between the two strains.
Based on sequence analysis of the mouse d/tPRP cDNA (present study) and
alignment with the rat d/tPRP gene (6), primers were designed to
amplify each intron and the 3'-flanking sequence of mouse d/tPRP by
PCR. A distinct polymorphism was identified within the putative intron
C of mouse d/tPRP. Amplification of intron C from the C57BL/6JEi strain
and subsequent digestion with the NlaIV restriction enzyme
resulted in the generation of two fragments, 840 and 80 bp in size.
Similar analysis of the SPRET/Ei strain resulted in the generation of a
single 920-bp fragment due to the absence of the NlaIV
restriction site. This polymorphism was used to determine the
chromosomal localization of mouse Dtprp using the Jackson
Laboratory 94 animal interspecific backcross panel (C57BL/6JEi x
SPRET/Ei)F1 x SPRET/Ei, known as Jackson BSS (14).
Analysis of d/tPRP expression
Northern blot analysis. Northern blots were performed as
previously described by our laboratory (6, 15). RNA was extracted from
tissues essentially as described by Chomczynski and Sacchi (16), using
TRIzol. Total RNA (15 µg) was separated on a 1% agarose gel and
transferred to a nylon membrane. Blots were probed with
32P-labeled mouse d/tPRP cDNA. The ribosomal protein L7
(rpL7) control probe was generated by PCR (17). Specific
oligonucleotide primers amplified a 246-bp rpL7 fragment that was
random primer labeled using Klenow and
[32P]deoxy-ATP.
In situ hybridization. d/tPRP messenger RNA (mRNA) was detected in frozen tissue sections as previously described (7, 15). The full-length mouse d/tPRP cDNA was linearized and used as a template for the synthesis of 35S-labeled sense and antisense RNA probes.
Western blot analysis. Western blot analysis for d/tPRP was performed as previously described (12). Samples were separated by electrophoresis in 12% polyacrylamide gels under reducing conditions and transferred to nitrocellulose membranes. Immunoreactive bands were visualized using a chemiluminescent detection system. Native and recombinant preparations of rat d/tPRP (12) were used as positive controls in the immunoblotting experiments.
Immunocytochemistry. Tissue and cellular localization of d/tPRP was determined by immunocytochemistry using a streptavidin-biotin immunoperoxidase kit for rabbit IgG (7). The immunostained sections were counterstained with hematoxylin. The specificity of the immunoreactions was demonstrated using preimmune serum.
| Results |
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We have characterized one representative of mouse d/tPRP-like cDNA
clones (mp06g07.rl) in the present study (GenBank accession no.
AF015729). The mouse cDNA was 81% identical to rat d/tPRP at the
nucleotide level and encodes a 239-amino acid protein. Like rat d/tPRP,
the predicted amino acid sequence of the mouse protein contains two
putative N-linked glycosylation sites and six homologously
located cysteine residues (1). Based on homology with rat d/tPRP, it
was determined that the mouse protein contains a 28-amino acid signal
peptide (1). A polyadenylation signal is located 113 nucleotides
downstream of the stop codon (TAA). Amino acid comparisons with members
of the rat PLP-C subfamily (d/tPRP, PLP-D, PLP-C, and PLP-Cv) are shown
in Fig. 1
(sequences were aligned using
the ClustalW multiple sequence alignment program). The predicted amino
acid sequence for mouse d/tPRP shares 66% identity and 84% similarity
with rat d/tPRP (Table 1
). Mouse d/tPRP
showed progressively less amino acid similarity with the other members
of the PLP-C subfamily (Fig. 1
and Table 1
). Boxes and
Roman numerals in Fig. 1
indicate several regions of
interest. Regions I (amino acids 1230), IV (amino acids 149161),
and VI (amino acids 205216) appear to be conserved in all PRL family
members. Regions II (amino acids 6981) and V (amino acids 169178)
are unique to members of the rat PLP-C subfamily (Fig. 1
) and mouse
PLF-RP (18). Recent characterization of the rat d/tPRP and rPLP-Cv
genes demonstrated that region II is encoded by a unique exon (exon 3)
relative to the prototypical PRL gene structure (6, 19). Although the
gene structures for mouse d/tPRP, rPLP-D, rPLP-C, and PLF-RP have not
yet been characterized, it is likely that they also contain an
additional exon. Region II may contribute to the structure and/or
function of PLP-C subfamily members. Finally, regions III (amino acids
109118) and VII (amino acids 220233) seem to be unique to mouse and
rat d/tPRP. The locations of these regions coincide with the locations
of regions of human PRL that are essential for PRL receptor binding and
bioactivity (20). All amino acid numbering is for mouse d/tPRP.
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d/tPRP expression patterns
The tissue distribution of mouse d/tPRP mRNA was determined by
Northern blot analysis (Fig. 2
). d/tPRP
expression was observed in day 8 deciduoma and placentas from days 13,
16, and 19 of pregnancy. d/tPRP transcripts were not detected in mouse
brain, thymus, heart, lung, diaphragm, liver, spleen, kidney, or ovary.
This pattern of expression is identical to that of rat d/tPRP (6, 7).
The integrity of the RNA was verified by hybridization with the rpL7
probe.
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| Discussion |
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Comparisons between mouse and rat d/tPRP and other members of the PLP-C subfamily were useful in identifying regions that are unique to d/tPRP, regions that are shared between all members of the PLP-C subfamily, and regions that are characteristic of all PRL family members. Inclusion of mouse d/tPRP in the PLP-C subfamily is supported by its sequence homology with rat d/tPRP and the presence of region II that is characteristic of PLP-C subfamily members as well as PLF-RP (reviewed in Ref.2). In the genes for rat d/tPRP and PLP-Cv, region II is encoded by an extra exon located between exons 23 of the prototypical PRL gene structure (6, 19). Although the gene structures for mouse d/tPRP and the other PLP-C subfamily members have not been characterized, it is likely that they also contain an additional exon.
Mouse and rat d/tPRP contain two regions of homology (regions III and VII) that are distinct from all other members of the PLP-C subfamily. It is possible that these regions contribute to a distinct mode of action for d/tPRP. Interestingly, the locations of these regions coincide exactly with the locations of regions of human, mouse, and rat PRL and PLs that are important for PRL receptor binding and bioactivity (20). The tertiary structure of the PRL protein brings helix 1 from the N-terminus, and loop 1 (connecting helixes 1 and 2) and helix 4 from the C-terminus into close proximity to form receptor-binding site 1 (20). Key PRL receptor binding determinants in these regions have not been conserved in d/tPRP (see Ref. 20 for a review), thus explaining its inability to stimulate the PRL signaling cascade (12). It is possible that d/tPRP has retained the PRL protein scaffolding to generate a receptor-binding site, but has unique amino acid determinants that direct binding to its cognate receptor. There is limited sequence homology between members of the PLP-C subfamily in these regions, suggesting that each member may have unique binding determinants and possibly a unique receptor.
The identity of a specific receptor for d/tPRP has yet to be determined. It is not known whether d/tPRP coevolved with its own specific receptor, uses a preexisting receptor and signaling cascade, or acts through some novel signaling system. PLF is known to exert its effects on endothelial cells through the mannose 6-phosphate/insulin-like growth factor II receptor (21, 22, 23). We have previously reported that rat d/tPRP associates with heparin-containing molecules and is localized at least in part in the decidual extracellular matrix. Therefore, d/tPRP is ideally situated to influence the behavior of decidual cells and cells traversing the decidua, including trophoblast, endothelial, and immune cells (7). Whether mouse d/tPRP is similarly distributed within the decidual extracellular matrix remains to be determined.
The Dtprp gene maps to mouse chromosome 13 along with other members of the mouse PRL family (24). The accumulation of data regarding the clustering of PRL family members on the same chromosome supports the hypothesis that individual members arose from duplication and divergent evolution from a common ancestral gene (25, 26). Colocalization of PRL family genes may also allow for common or coordinated regulation of expression. The observations that no recombination events occurred between the Dtprp and Pl1 loci on mouse chromosome 13 in the Jackson BSS cross suggests that these genes are closely linked. In contrast, the Plf locus is separated from Dtprp and Pl1 by several intervening genes and several cross-over events. The significance of this physical separation is not known.
The tissue-specific and temporal expression pattern of d/tPRP in the mouse closely parallels its counterpart in the rat. In both species, d/tPRP mRNA and protein are present at high levels from the time of implantation until parturition. The maintenance of d/tPRP expression throughout pregnancy requires a complex interplay between both maternal and fetal tissues. Collectively, these observations suggest potential physiological importance for d/tPRP during pregnancy. The availability of a mouse model creates new opportunities for studying d/tPRP physiology.
| Acknowledgments |
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| Footnotes |
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2 Supported by a fellowship from the Lalor Foundation. ![]()
3 Present address: Laboratory of Cellular Biochemistry, University of
Tokyo, Tokyo, Japan. ![]()
4 Supported by a fellowship from the Deutsche Forschungsgemeinschaft
of Germany (Mu1183/11). ![]()
Received July 25, 1997.
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