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From the Department of Clinical Molecular
Biology,*
Faculty of Medicine, Kyoto University, Kyoto and
the Department of Urology,
Faculty of
Medicine, Kansai Medical University, Osaka, Japan
| Abstract |
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| Introduction |
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Most mammals have testes within a scrotum, the temperature of which is maintained 2 to 8°C lower than the body cavity temperature.5,6 The low temperature is believed to be important for normal testicular function.7,8 In experimental animals, surgical induction of cryptorchidism or exposure to heat stress causes disruption of spermatogenesis, leading to infertility.7,9 Various exogenous thermal factors, including those observed in welders and paraplegic patients in wheelchairs, are proposed to be risk factors for human male infertility.10-12 Cryptorchidism and varicocele of the spermatic veins are associated with male infertility, and their pathogenesis is attributed to thermal factors.13,14 Although all cell types in the testis including germ cells, Sertoli cells, and Leydig cells may be affected by the elevated temperature, effects on the germ cells have been most extensively investigated. The earliest cellular changes noticed in experimentally cryptorchid testes are in early pachytene spermatocytes and early spermatids.15-17 Especially in pachytene spermatocytes, apoptotic cell death is induced within 2 to 4 days.17 The initial phase of germ-cell apoptosis has been reported to be p53-dependent and the subsequent apoptosis to be p53-independent.18 The molecular mechanisms of the thermal effect on spermatogenesis are, however, just beginning to be clarified.
Recently we have isolated the cold-inducible RNA-binding protein (Cirp) from a mouse testis cDNA library.19 In the testis of mice and humans, Cirp is constitutively expressed in the germ cells, which is decreased at an elevated temperature.20 In somatic cells, Cirp is induced by mild cold stress (32°C), and mediates the cold-induced suppression of cell growth.19 Human RNA-binding motif protein 3 (RBM3), originally isolated from the Xp11.2 region of chromosome during positional cloning of disease-causing genes, encodes a protein structurally related to Cirp,21 and both Cirp and RBM3 transcripts are induced by cold stress in human cells.22 In the present study, we report an isolation of mouse Rbm3 cDNA. We identified the cells expressing Rbm3 in mouse testis and assessed the effect of temperature stress on its expression.
| Materials and Methods |
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Sterile WBB6F1-W/Wv mutant mice were obtained through the mating between congenic C57BL/6-Wv/+ and WB-W/+ mice. Two-month-old and 4-month-old C57BL/6 wild-type (+/+) mice and 4-month-old WBB6F1-W/Wv mutant mice were purchased from Japan SLC Company (Hamamatsu, Japan).
BALB/3T3 fibroblasts, BMA1 bone marrow stromal cells, and TAMA26 Sertoli cells were cultured in Dulbeccos modified Eagles medium (Life Technologies, Inc., Grand Island, NY) supplemented with 10% calf serum at 37°C in a humidified atmosphere of 5% CO2 in air as described.23 In experiments where effects of growth temperature were examined, cells were grown at 37°C for 96 hours and then shifted to 32°C, 37°C, or 39°C. After 12 hours, cells were scraped, frozen in liquid nitrogen, and stored at -80°C until analysis.
Antisense and sense oligodeoxynucleotides (ODNs) containing phosphorothioates were purchased from Cruachem (Kyoto, Japan). The sequence of the sense ODN was (5'-GCCTTCTGCCATGTCGTCTG-3') spanning the translation initiation site of Rbm3 mRNA, and that of antisense ODN was complementary to this sequence. Cell numbers were determined after 2 days of culture at 32°C or 37°C in the presence of 0.5, 1.0, or 1.5 µmol/L of ODN as described previously.19
Experimental Cryptorchidism
To induce unilateral cryptorchidism, 4-month-old +/+ male mice were anesthetized by intraperitoneal injection of 25 mg/kg pentobarbital sodium, and a small incision was made in the abdomen. On one side, the testis was fixed in the abdominal cavity by suturing its capsule to peritoneum. On the other side of the same animal, the testis was not fixed and used as a control. The testes of sham-operated mice were used as another control. The animals were sacrificed at different intervals after surgery. The testes were removed, frozen in liquid nitrogen, and stored at -80°C until analysis.
Cloning of Mouse Rbm3 cDNA
Mouse EST clone (AA218202) was obtained from Research Genetics, Inc. (Huntsville, AL). The insert was excised from the clone and ligated into the NotI and EcoRI sites of pBluescript SK(-) vectors. The nucleotide sequence was determined by the dideoxy method as described.19
RNA Extraction and Northern Blot Hybridization
Mice tissues and cells were dissolved in TRIzol reagent (Life
Technologies, Tokyo, Japan) and RNA was extracted by following the
manufacturers instructions. Twenty µg of total RNA of each sample
was separated in 1.0% agarose/formaldehyde gels by electrophoresis and
was blotted onto nylon filters (Hybond-N+; Amersham, Buckinghamshire,
UK). The filters were hybridized with [
-32P]
dCTP-labeled random-primed cDNA fragments, then washed under stringent
conditions (65°C for 30 minutes in a washing buffer composed of 0.1x
standard saline citrate and 0.1% sodium dodecyl sulfate), and detected
by autoradiography. A fragment of Rbm3 cDNA (nucleotide
positions 86 to 547) was used as a probe. The filters were stripped and
rehybridized with a cDNA probe for the S26 ribosomal protein as an
internal control.
Western Blot Analysis
The anti-Rbm3 polyclonal antibody was produced by immunizing rabbits with a carboxyl-terminal oligopeptide (SGGNYRDNYDN) common to mice and humans. For Western blot analysis, 20 µg of proteins were separated by 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis, blotted onto polyvinyl difluoride membranes (Millipore, Tokyo, Japan), and treated with the anti-Rbm3 antiserum (1: 20,000 diluted). Protein concentrations were determined using the Bradford reagent (BioRad Labs, Hercules, CA). Bound antibody was detected using a goat anti-rabbit immunoglobulin G (IgG) conjugated with horseradish peroxidase (BioRad) and the enhanced chemiluminescence system (ECL, Amersham Life Science).
Immunohistochemistry
Immunohistochemistry was performed using sections from the mouse testes fixed in 4% paraformaldehyde in phosphate-buffered saline (PBS) and embedded in wax. Five-µm sections were mounted on siran-coated glass slides. Rbm3 was immunolocalized using the indirect immunoperoxidase or immunofluorescence method. Briefly, tissue sections were incubated with 5% bovine serum albumin in PBS, pH 7.5, for 1 hour. They were subsequently reacted with the anti-Rbm3 antibody at a dilution of 1: 20,000 overnight at 4°C. After washing in PBS containing 0.2% Tween-20 and blocking with 5% bovine serum albumin, the sections were incubated with peroxidase-conjugated anti-rabbit IgG for 1 hour at room temperature. Diaminobenzidine was used as substrate for the peroxidase reaction. Sections were counterstained with hematoxylin. As a control, the preimmune antiserum was used at the same dilution. In some experiments, nuclei of testicular germ cell were specifically stained by the rat monoclonal antibody TRA 98, kindly provided by Dr. Y. Nishimune, Osaka University.24 Detection was with fluorescein isothiocyanate-conjugated goat anti- rat IgG (DAKO, Kyoto, Japan) diluted 1:1,000 and a fluorescence microscope (Olympus, Tokyo, Japan) equipped with the fluorescein-isothiocyanate filter set.
| Results |
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To isolate the mouse homolog of human RBM3 cDNA, we
searched the GenBank database for similar cDNA sequences and identified
a mouse EST clone (AA218202). Sequencing of the EST clone revealed that
it contained a 1158-bp insert potentially encoding 153 amino acids
(Figure 1)
. The sequence around the first
ATG codon provided a favorable context for translation
initiation.25
The predicted amino acid sequence displayed
two main features: the presence of an amino-terminal consensus sequence
RNA-binding domain (CS-RBD) and a carboxyl-terminal part rich in
glycine. The predicted amino acid sequence of this clone was compared
to known sequences in GenBank using the Wisconsin Package (Genetics
Computer Group, Madison, WI). The overall amino acid sequence
was 94% identical to that of human RBM3.21
The CS-RBD of
Rbm3 contained consensus sequences of RNP1 and RNP2, and a number of
highly conserved segments that could be perfectly aligned with those of
other CS-RBDs (Figure 2)
. Together with
Cirp and some others, Rbm3 constitutes a glycine-rich RNA-binding
protein subgroup in the family of proteins with CS-RBD.26
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When we analyzed the expression of Rbm3 mRNA in various
mouse tissues by Northern blot analysis, a single band of 1.1 kb was
detected in all tissues examined including the testis and spleen
(Figure 3a)
.
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Expression of Rbm3 in Sertoli Cells
To identify the cells expressing Rbm3 in the testis, adult mouse
testis was analyzed by immunohistochemistry using the anti-Rbm3
antibody. As shown in Figure 4
, a and c,
strong signals were detected in the nuclei of Sertoli cells.
Stage-dependency of the signal intensity was not detected under the
present conditions. Expression of Rbm3 in Sertoli cells was further
confirmed by analysis of the testis of the adult
W/Wv mutant mice which are deficient in
germ cells.20
As shown in Figure 4d
, Sertoli cells were
positively stained with the anti-Rbm3 antibody in all of the tubules
examined. To determine whether the signals were present in germ cells
as well as Sertoli cells, serial sections of testis were stained with
the germ cell-specific TRA 98 antibody24
or the anti-Rbm3
antibody. The cells stained with the anti-Rbm3 antibody (Figure 4e)
were not stained with the TRA 98 antibody (Figure 4f)
, indicating that
the germ cells did not express Rbm3.
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Testes at several developmental stages were immunohistochemically
analyzed for expression of Rbm3. Rbm3 was not detected in the testis of
17-day-old fetus (Figure 5a)
. A low-level
expression was detected in the Sertoli cells of newborn mice (Figure 5b)
. At this age, signals were also observed in the interstitial cells
of the testis. In the testis of 8-day-old mice, strong signals were
present only in the Sertoli cells (Figure 5c)
. Sertoli cells were
located in the periphery of the seminiferous tubules in the testis of
17-day-old mice, and they were strongly positive for the Rbm3
expression (Figure 5e)
. The strong signals were also observed in the
Sertoli cells of 21-day-old mice (Figure 5f)
. Thus, during testicular
development, distinct Rbm3 expression was first observed at early
neonatal period and continued thereafter in Sertoli cells.
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We exposed the mouse cell lines BMA-1 bone marrow stromal cells,
BALB/3T3 fibroblasts, and TAMA26 Sertoli cells to cold (32°C) or heat
(39°C) stress. As shown in Figure 6a
,
Northern blot analysis demonstrated that the levels of Rbm3
mRNA were increased in response to the cold stress in all of the cell
lines examined. By contrast, the heat stress decreased the levels of
Rbm3 expression. The expression was also affected at the
protein level (Figure 6b)
. The anti-Rbm3 antibody specifically
recognized the 18-kd protein in the lysates of TAMA26 cells. In
agreement with the results of Northern blot analysis, the protein level
was increased after the temperature shift from 37°C to 32°C.
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Effect of Cryptorchidism on Rbm3 Expression
To examine the effect of heat stress on Rbm3 expression in the
testis in vivo, we made use of experimental cryptorchidism.
As shown in Figure 7a
, Northern blot
analysis revealed that Rbm3 transcripts were decreased in
the cryptorchid testes within 12 hours after the surgery as compared
with the contralateral testis of the same animal as well as the
sham-operated testis. By immunohistochemistry using the anti-Rbm3
antibody, the decrease in Rbm3 protein expression was detected in
Sertoli cells of the cryptorchid testis (Figure 7c)
.
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| Discussion |
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It is known that some proteins with CS-RBD play important roles in spermatogenesis. For example, chromosomal deletion of the Rb97D gene results in azoospermia in Drosophila.27 The Y-chromosome genes YRRM/RBM1 and DAZ (deleted in azoospermia) are candidate genes for azoospermia factor that controls human spermatogenesis.28,29 The X chromosome gene RBM3 has been found to encode a protein with high amino acid identity to YRRM/RBM1 (51%), and suggested to have a related function.21 Subsequent isolation of Cirp, however, has demonstrated that RBM3 is most similar to Cirp in amino acid sequence, and together they belong to a glycine-rich RNA-binding protein family.26 Cirp is an RNA-binding protein with a growth suppressing activity and its expression is inducible at 32°C in mouse somatic cells.19 It is constitutively expressed in various tissues and most strongly in the testis. In the mouse testes, Cirp is expressed in the germ cells, and the level varies depending on the stage of differentiation.20 Similar to Cirp, the expression of Rbm3 was induced at 32°C in mouse somatic cells. However, Rbm3 did not seem to mediate the cold-induced growth suppression. The kinetics of induction by cold stress is slightly different between Cirp and RBM3.22 The tissue distribution patterns of constitutive expression were different. Although expressions of both Cirp and Rbm3 were decreased in the cryptorchid testis, Cirp is expressed in the germ cells20 whereas Rbm3 was in Sertoli cells. These findings suggest that Cirp and Rbm3 play distinct roles under physiological and stress conditions.
Sertoli cells are known to proliferate most actively in the fetal testis shortly before birth.30 The rate of the Sertoli cell growth declines linearly thereafter and this proliferation ceases entirely by approximately 3 weeks after birth. In contrast, development of mature Sertoli cell function begins in the testis at approximately 2 weeks of age.31 The testis of 2-month-old mice contains fully differentiated, mature Sertoli cells. We observed a high level of Rbm3 expression in Sertoli cells of 8-day-old mice and thereafter. Thus, the timing of the emergence of Rbm3 expression during mouse testis development shows correlation with the Sertoli cell acquisition of developmental function.
Sertoli cells provide essential physical and trophic support for developing spermatogonia, spermatocytes, and spermatids.1-4 This trophic support in turn depends on a network of reciprocal signaling interactions between Leydig cells, Sertoli cells, peritubular cells and germ cells within the testis, and on upstream hormonal regulation from the pituitary. Recently the importance of Sertoli cells in spermatogenesis has definitely been demonstrated by analysis of null mutations in the mouse genes encoding three structurally related tyrosine kinase receptors: Tyro 3, Axl, and Mer.32 They are normally expressed by Sertoli cells during postnatal development, and male animals lacking all three receptors show progressive death of differentiating germ cells. The temperature of the testis is maintained at a specific temperature between 30 to 33°C depending on the species.5,6 The difference of a few degrees between body and scrotum is critical for spermatogenesis. The present study demonstrated that the level of Rbm3 expression was regulated by temperature in Sertoli cells. The Rbm3 mRNA and protein expressions were decreased in the cryptorchid testis. The histological analysis confirmed that the decrease was not due to a decrease in the number of cells. At the body cavity temperature, male germ cells are easily damaged and our findings suggest a possibility that effects of heat on Sertoli cell functions related to those of Rbm3 contribute to the observed damages. Because proteins with CS-RBD are involved in posttranscriptional regulation of gene expression, identification of targets for Rbm3 will help elucidate the molecular mechanisms of spermatogenesis and its sensitivity to heat .
| Acknowledgements |
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| Footnotes |
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Supported in part by Grants-in-Aid from the Ministry of Education, Science, Sports and Culture and the Ministry of Agriculture, Forestry, and Fishery of Japan, and Kansai Medical University Research Grants.
Accepted for publication February 2, 2000.
| References |
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