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Regular Article |
a Division of Veterinary Physiology and Pharmacology, Department of Veterinary Preclinical Studies, University of Glasgow Veterinary School, Glasgow G61 1QH, United Kingdom
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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-reductase 1 and 3-hydroxysteroid dehydrogenase; group 5 contained sulfonylurea receptor 2 and 3ß-HSD I. Group 1 contained genes that were expressed in fetal and adult Leydig cells and which increased in expression around puberty toward a maximum in the adult. Group 2 contained genes expressed only in the adult Leydig cell population. Group 3 contained genes with predominant fetal/neonatal expression in the interstitial tissue. Group 4 contained genes with a peak of expression around puberty, whereas genes in group 5 show little developmental change in expression. Highest mRNA levels in descending order were RLF, P450c17, EH, 17ß-HSD III, PGD-synthetase, GST5-5, and P450scc. Results identify five genes expressed in the mouse adult Leydig cell population, but not in the fetal population, and one gene (TSP2) that may be expressed only in the fetal Leydig cell population. The developmental pattern of gene expression suggests that three distinct phases of adult Leydig cell differentiation occur.
developmental biology, gene regulation, Leydig cells, testis, testosterone
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The 18 mRNA species examined in this study were chosen because they have been reported to be expressed within the Leydig cell, and sufficient sequence data was available to design primers and probes for real-time PCR. The mRNA species examined, and reference to their expression within the Leydig cell appears in Table 1. A second criteria that was applied to selection of mRNA species was that, within the testis, expression should be limited primarily, if not solely, to the Leydig cells. In the adult animal this applies to most mRNA species studied here, the known exception being GST5-5, which is also expressed in spermatids [5]. There have been few developmental studies, however, of changes in mRNA levels, and it is possible that the main sites of expression may change with age, as has been shown previously for 17ß-HSD III and PGD-synthetase [6, 7]. For these reasons, studies reported here have been carried out using isolated testicular interstitial webs to remove possible tubular expression of mRNA species.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Normal mice were bred at the University of Glasgow Veterinary School and maintained as required under United Kingdom Home Office regulations. The mice used were derived from F1 hybrids of C3H/HeH and 101/H strains. The day of birth was designated as Day 1, and animals were killed on Days 1, 5, 10, 15, 20, 25, 30, and 40, whereas adult animals were 90–180 days old. Testes were removed and either stored whole in liquid nitrogen, or interstitial tissue was prepared by mechanical separation from seminiferous tubules as previously described [6]. Interstitial tissue webs were stored frozen in liquid nitrogen until used for RNA extraction. Testis volume was determined as described previously [1].
RNA Extraction and Reverse Transcription
Total RNA was extracted from interstitial webs using Trizol (Life Technologies, Paisley, U.K.) and residual genomic DNA was removed by DNase treatment (DNA-free, Ambion Inc., supplied by AMS Biotechnology, Abbingdon, U.K.). This is because the TaqMan primers were not specifically designed to produce an amplicon spanning an intron/exon boundary. Total RNA was extracted from whole testes as described for interstitial tissue webs with the exception that 5 ng of luciferase mRNA (Promega U.K., Southampton, U.K.) was added to each testis at the start of the RNA extraction to act as an external control. RNA from both types of tissue was reverse transcribed using random hexamers and Moloney murine leukemia virus reverse transcriptase (Superscript II, Life Technologies) as described previously [8, 9].
Real-Time PCR
Background For quantification of specific mRNA content of tissues, a real-time PCR approach was used, which utilized the TaqMan PCR method. A review of real-time PCR methodology has recently been published [10]. Briefly, the TaqMan assay uses the 5'-nuclease activity of the DNA polymerase to cleave a specific probe that hybridizes to the target amplicon during the annealing/extension phase of the PCR. Each probe contains a fluorescent dye reporter at the 5' end and a quencher dye at the 3' end, which will normally inhibit the reporter emission. Cleavage of the probe therefore separates the reporter and quencher dyes, resulting in increased fluorescent emission of the reporter, which is monitored by a suitable detector. The probe also provides an added degree of specificity to the assay. To measure cDNA levels the threshold cycle at which fluorescence is first detected above baseline is used, and a standard curve is drawn between starting RNA/cDNA concentrations and the threshold cycle. In studies reported here, mRNA levels are expressed relative to an internal control (see Results) and arbitrary standards were generated by serial dilutions of cDNA generated from adult testis interstitial tissue.
Primers and probes
Primers and probes for use in the TaqMan method were designed using Primer Express (Applied Biosystems, Warrington, U.K.) and are listed in Table 1. For most mRNA species sequence information came from characterized mouse cDNAs submitted to GenBank. The exceptions were 5-reductase 1 and 3-HSD, for which no characterized mouse cDNAs were available. To obtain a sequence with which to design TaqMan primers/probes for mouse 5-reductase 1 and 3-HSD mouse, expressed sequence tags (ESTs) were identified through a basic local alignment search using rat 5-reductase 1 (J05035) and 3-HSD (D17310), and pile-ups were generated. The mouse ESTs used to generate the pile up were BE533699, BF165069, BE292197, AW114073, AW491466, and BF158810 for 5-reductase 1; and AI225643, AI881993, AI987623, AI663368, and AI048883 for 3-HSD. The LH receptor undergoes extensive alternate splicing [11, 12] and the localization of the TaqMan primers/probes will determine which isoform is detected. In this study the primer/probe combination was localized to exon 11, which contains the transmembrane and intracellular domains. The VCAM1 primary transcript also undergoes alternate splicing [13], and probes were designed that would amplify the full-length transcript only.
Real-time PCRs were carried out in a 25 µl volume using a 96-well plate format. Components for real-time PCR were purchased from Oswel Ltd (Southampton, U.K.) apart from primers and probes, which came from MWG Biotech (Milton Keynes, U.K.). Each PCR well contained 1x reaction buffer (with passive reference), 5 mM MgCl2, 200 µM dNTPs, 300 nM each primer, 200 nM probe, and 0.02 U/µl enzyme (Hot GoldStar). Reactions were carried out and fluorescence was detected on a GeneAmp 5700 system (Applied Biosystems, Warrington, Cheshire, U.K.). For each sample, a replicate was run omitting the reverse transcription step, and a template-negative control was run for each primer/probe combination. For each 96-well plate, standard curves were included for each mRNA species being measured to ensure that amplification efficiencies were not significantly different.
Assay for Testosterone
The testosterone content of testes was measured by radioimmunoassay after extraction with ethanol as described previously [14].
Statistical Analysis
Results were analyzed by ANOVA followed by the Fisher test. Data for intratesticular testosterone levels showed heterogeneity of variance and were normalized by logarithmic transformation before analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In humans, the KIAA0038 gene (GenBank accession number D26068; Unigene cluster Hs.180900) codes for a putative translation initiation factor and is ubiquitously expressed. This gene also appears to show the least variability of expression between tissues compared with other ubiquitous genes [15]. The mouse homologue of KIAA0038 is the Wbscr1 (Williams-Beuren syndrome chromosome region1) gene, and we used real-time PCR to compare expression of Wbscr1, ß-actin, and 18S relative to an external control (luciferase). To characterize amplification of Wbscr1, ß-actin, 18S, and luciferase cDNA, standard curves were prepared using serial dilutions of an arbitrary sample of cDNA, and the threshold cycle at which amplification was first detectable above background was plotted against the relative cDNA concentration (Fig. 1A). The slope of the curve is inversely proportional to the efficiency of the reaction with a slope of -1.4427 equivalent to 100% efficiency. To be able to compare Wbscr1, ß-actin, and 18S directly to luciferase, it is necessary to show that the curves are not significantly nonparallel (i.e., that the amplification efficiencies are not significantly different) and this was tested using two-factor ANOVA. For probes used in this study the efficiencies of the PCR reactions were similar (97.5% to 98.9%) (Fig. 1A), and the interaction factor from ANOVA was not significant. The ratio of Wbscr1, ß-actin, and 18S cDNA to luciferase cDNA was therefore calculated from the difference between the threshold cycle values (
Ct).
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Expression of the three ubiquitous genes was determined relative to the external standard (luciferase) in whole testes from three different ages of mice (5 days, 20 days, and adult). These values were then corrected for testis volume (Fig. 1B). Results show that ß-actin mRNA levels varied significantly during development of the testis. In contrast, there was no significant variation in Wbscr1 or 18S expression at the different ages, although variation between animals was high, particularly for 18S.
Testosterone Levels During Development
Levels of intratesticular testosterone declined after birth to a nadir around Days 10–20. Thereafter, testosterone levels increased more than 16-fold between Days 25 and 30, and then showed a further doubling up to adulthood (Fig. 2).
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Expression of Leydig Cell mRNA Species in Isolated Testicular Interstitial Tissue
Expression levels of 18 different mRNA species were measured in isolated interstitial tissue from mice of different ages using Wbscr1 as an internal control. Results have been organized to group genes showing a similar developmental pattern of expression.
Group 1 – P450scc, P450c17, RLF, GST5-5, StAR, LH Receptor, and EH
In this group, levels of mRNA per testis were relatively low at birth but began to increase around Day 20, reaching a maximum in the adult animal (Fig. 3). Expression levels varied across the mRNA species in this group with levels of RLF, P450c17, and EH mRNA the highest of the 18 different mRNA species examined in this study. In contrast, levels of LH receptor and StAR protein mRNA were significantly lower.
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Group 2 – 3ß-HSD VI, 17ß-HSD III, VCAM1, EST, and PGD-Synthetase
The expression pattern of this group of genes was characterized by the absence or low levels of mRNA in the interstitial tissue up to 10 days, followed in most cases by detectable expression on Day 15, which increased to a maximum in the adult animal (Fig. 4). Both 17ß-HSD III and PGD mRNA species were detectable in the interstitial tissue before Day 10, but in each case we have shown previously that this is due to contamination of the isolated interstitial tissue with cells from the seminiferous tubules [3, 4]. Expression levels of 17ß-HSD III and PGD were high (at least in the adult), whereas levels of EST and VCAM1 were relatively low. Expression of VCAM1 was also highly variable between animals, with undetectable levels found in some animals up to Day 30.
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Group 3 – Ptc and TSP2
The expression levels of both members of this group were highest in neonatal interstitial tissue, and then declined between 10 and 25 days, and remained low throughout puberty and in the adult animal (Fig. 5).
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Group 4 – 5-Reductase 1 and 3-HSD
The expression pattern of these two mRNA species was characterized by a decrease in mRNA levels after birth, followed by an apparent peak of expression around 25 days, with low levels of mRNA in the adult (Fig. 6). This is particularly clear for 5-reductase 1. Expression levels overall were low.
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Group 5 – 3ßHSD I and SUR2
In this group, expression levels of mRNA were low and there was little clear developmental pattern (Fig. 7).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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After birth, intratesticular testosterone levels declined from around Day 5 to reach a nadir between 10 and 20 days, before rising again around Day 25. This decline in testosterone mirrors the steroidogenic activity of the testes as measured using in vitro studies [17, 18], although the maximum capacity in vitro (in response to LH) begins to increase earlier, before Day 25 [18], which indicates a lack of LH support in vivo during early puberty. The underlying cause of the decline in steroidogenesis after birth is not clear. In the mouse the testes are populated by fetal Leydig cells at the time of birth, and these cells become LH-dependent at this time [19]. It is unlikely that there is a significant drop in circulating LH between birth and 10 days, because pituitary levels of LH rise after birth [19] and circulating levels of LH reflect pituitary levels in the normal animal [20]. Whatever the underlying cause, the decline in androgen production after birth in the mouse testis coincides with the initial differentiation of the adult Leydig cell population, which starts around Day 10 [21, 22]. Adult Leydig cell number increases rapidly in the mouse after initial differentiation, and by Day 20 numbers are 16-fold higher than at Day 5, and about half the number in the adult testis [1, 23]. These newly formed Leydig cells appear to be steroidogenically inactive because androgen levels remain low until after Day 25, although this may be partly due to lack of LH support as described above.
The developmental pattern of expression of the 18 mRNA species examined in this study can be divided broadly into 5 groups. The expression pattern of the first group (P450c17, P450scc, RLF, GST5-5, EH, StAR, and LH receptor) remains fairly stable after birth but shows a pubertal rise in expression, generally around Day 20, which precedes the increase in androgen levels. The developmental profile of mRNA levels indicates that these genes are all expressed in the fetal Leydig cells and the increase in expression around 20 days occurs after the initial differentiation of the adult Leydig cell population, which starts around Day 10. The early rise in adult Leydig cell number before puberty is reported by Hardy et al. [24] to be due largely to precursor differentiation rather than proliferation of preexisting Leydig cells. This would suggest that expression of this group of genes by the Leydig cell is regulated to occur some time after initial differentiation of the cells. It is noticeable that most of the genes known to be required for androgen synthesis fall within this group (cyp11a1 [encoding P450scc], cyp17 [encoding P450c17], StAR, and LH Receptor) and that their expression is therefore a prerequisite for Leydig cell function.
The second group of mRNA species (EST, 17ß-HSD III, 3ßHSD VI, VCAM1, and PGD) contain those that are not expressed by the fetal Leydig cells and appear only in the adult Leydig cell population (this study and [6, 7, 21, 25]). Low levels of 17ß-HSD III and PGD mRNA expression before Day 10 can be explained by tubular contamination of the interstitial compartments used in this study because these two genes are expressed in the tubules at this time [6, 7]. The two mRNA species in this group that encode steroidogenic enzymes (17ß-HSD III and 3ß-HSD VI) show changes in expression levels after Day 15, which resemble those seen in the first group described above. This means that the developmental expression pattern of each of the genes required for androgen synthesis is similar after adult Leydig cell differentiation. In contrast, expression of EST and PGD is delayed and starts to show a significant increase in the adult Leydig cell only at the end of puberty around Days 30–40.
The expression pattern of mRNA species in group 3 (Ptc and TSP2) is what may be expected from genes expressed in the fetal Leydig cell population but not in the adult population. The fate of the fetal Leydig cell population after puberty has been a source of debate for a number of years [26], although recent studies on the Dhh-null mouse support the hypothesis that the cells remain in the testis but are eclipsed by development of the much greater adult population [3]. Thus, as the interstitial tissue develops and total cell numbers increase, the expression of fetal Leydig cell genes will decline relative to the internal control as is seen with both Ptc and TSP2. Nevertheless, it is now not clear whether Ptc is expressed in the fetal Leydig cell population. Earlier studies had indicated that Ptc was expressed in the fetal interstitial tissue and probably in the Leydig cells [27], but more recent work has suggested that Ptc is expressed in the peritubular cells with Leydig cell expression uncertain [3]. Measurement of TSP2 was included in this study because of an earlier report showing immunohistochemical expression in the adult Leydig cell [28]. The pattern of mRNA expression is much more clearly consistent, however, with a fetal Leydig cell origin for this matricellular protein.
The mRNA species in group 4 (3-HSD and 5-reductase 1) both show an apparent peak of activity between 20 and 30 days as androgen levels in the testis are increasing to adult levels. The 5-reductase enzyme is responsible for reduction of testosterone to dihydrotestostosterone (DHT), whereas 3-HSD and 3ß-HSD enzymes metabolize DHT further to 5-androstane 3 (or 3ß), 17ß diol, which is a significantly less-potent androgen. It has been shown in several species including mice [29, 30] that there is a peak of 5-reductase enzyme activity around puberty, which would correlate with changes in mRNA levels observed here. Similarly, in the rat a peak of 5-reductase activity occurs around puberty [31, 32], which correlates with levels of type 1 mRNA [33]. It has been shown previously that the peak of 3-HSD enzyme activity in rat Leydig cells is during the pubertal period [34], which would correlate with mRNA levels reported here. Developmental changes in the activity of these enzymes will serve to regulate testicular DHT levels [34].
The two mRNA species in group 5 (3ß-HSD I and SUR 2) show fairly low levels of expression overall with little convincing change in expression during development. The 3ß-HSD I enzyme is the only 3ß-HSD isoform expressed in the testis during fetal development, and is therefore essential for androgen production during this critical period [19]. As the adult Leydig cell population develops after Day 10, levels of 3ß-HSD VI mRNA increase to become the dominant species in the adult testis (this study and [21]). The sulphonylurea receptor SUR2 has been shown previously to be highly expressed within steroidogenic cells of the rat, including the Leydig cells [35]. In this study, SUR2 was clearly expressed in mouse interstitial tissue, although levels of expression were not particularly high and there was little developmental pattern to the expression.
The data on mRNA expression levels described here indicate that there are probably three functional stages during Leydig cell development: 1) an early stage, which must act to induce Leydig cell differentiation and will involve early genes not measured here, 2) a second stage that may begin shortly after initial differentiation but that requires 20–30 days for completion, and 3) a third stage, which is characterized by an increase in gene expression toward the end of puberty. The first stage will include those genes triggered by the initial stimulus to adult Leydig cell differentiation and which lead to morphological changes in the precursor cells. These morphological changes must occur early because one of the first signs of adult Leydig cell differentiation is the marked increase in morphologically identifiable Leydig cells between Days 10 and 20 [1]. This is before marked changes in expression of other genes such as those encoding the steroidogenic enzymes. The second stage of Leydig cell development includes all of the genes in group 1 along with 17ß-HSD III and 3ß-HSD VI from group 2. It is likely that this second stage of development begins shortly after initial Leydig cell differentiation because it has been shown that expression of 3ß-HSD VI can first be detected in the adult Leydig cell population around the time that cell numbers begin to increase [1, 21], while RLF can be detected before a change in cell number is seen [22]. The third stage of development, which is characterized by genes for EST and PGD, probably also begins shortly after Leydig cell differentiation because expression is detectable at low levels on Day 15. Normal expression of these genes is delayed, however, until after puberty, when a marked increase in activity occurs as the animal develops into adulthood. This hypothesis is consistent with, and develops further, a previously described biphasic model of adult Leydig cell development [36, 37]. It is likely that the first stage in both models is the same but that the second stage can be subdivided by data presented here.
Control of each of these developmental processes within the adult Leydig cell population is not known. It is likely that each of the stages is at least partly dependent upon the gonadotropins and, in particular, LH, although Ariyaratne et al. [2] have suggested that the initial stages of adult Leydig cell differentiation may be independent of LH. The identification of adult Leydig cell-specific genes enables us to test this hypothesis directly using appropriate mutant and transgenic animals.
In interstitial tissue from the adult animal, the highest levels of mRNA were from genes encoding (in descending order) RLF, P450c17, EH, 17ß-HSD III, PGD-synthetase, GST5-5, and P450scc. It has been reported previously that levels of RLF mRNA in the adult mouse Leydig cell are very high [38, 39], although the function of this factor in the adult animal remains uncertain. The only known function of RLF in the male is in the fetal period when it is essential for normal testicular descent [40, 41]. High levels of expression of P450c17, 17ß-HSD III, and P450scc in the Leydig cell are to be expected because these are key enzymes in the biosynthesis of testosterone. It is interesting, however, to compare the relative expression of these mRNA species, including both isoforms of 3ß-HSD (which show relatively low levels of expression), with enzyme activity in the adult mouse testis reported previously [42]. Highest levels of enzyme activity in the normal testis are for 3ß-HSD and 17-ketosteroid reductase (coded by the 17ß-HSD III gene), which both show twice the activity of 17-hydroxylase (coded by the cyp17 gene) and 250 times the activity of cholesterol side-chain cleavage (coded by the cyp11a1 gene). It is clear, therefore, that regulation of the activity of these enzymes in the Leydig cell is due to posttranscriptional events. It has been previously reported that testicular steroidogenic capacity correlates strongly with P450scc protein levels in different strains of mice [43], suggesting that control takes place at the level of protein synthesis or degradation. High levels of EH and GST5-5 protein in the testicular interstitial tissue have been reported previously [5, 44] and this study confirms that mRNA expression levels are also high. These enzymes protect cells by detoxifying electrophilic compounds and show high levels in other steroidogenic tissues [45], suggesting a specific role in the steroidogenic process and, perhaps, also in protecting the testis from xenobiotic attack.
This study has examined the developmental expression pattern of 18 genes known to be expressed in the Leydig cell. Results suggest that three phases of adult Leydig cell differentiation occur with development of most known functions associated with the Leydig cells occurring during the second phase. In addition, from results reported here and from previous studies we can now identify five genes that are expressed in the adult Leydig cell population but not in the fetal population. We have also identified at least one candidate gene that may be expressed only in the fetal population. This information can be used to study mechanisms that regulate differentiation and development of the adult Leydig cell population.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 Grant support: BBSRC and Wellcome Trust. L.W. was supported by a scholarship from the University of Glasgow. 
2 Correspondence: P.J. O'Shaughnessy, Division of Veterinary Physiology and Pharmacology, University of Glasgow Veterinary School, Bearsden Rd., Glasgow G61 1QH, UK. FAX: 141 330 5797; p.j.oshaughnessy{at}vet.gla.ac.uk
Accepted: November 1, 2001.
Received: July 10, 2001.
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), 18S (
), Wbscr1 (
), and ß-actin (
) cDNA are shown. Amplification efficiencies can be determined from the slope of the lines (see Results). B) Cumulative data showing 18S, Wbscr1, and ß-actin mRNA levels relative to luciferase in whole testes from mice of different ages. Mean ± SEM from three animals in each group is shown. Using ANOVA, there was no difference between ages in 18S or Wbscr1, but there was a significant difference between ages in ß-actin levels. Note that the scale for 18S is 1000 times the scale of Wbscr1 and ß-actin.
