The role of specific germ cell types in modulation of the secretion of androgen-regulated proteins (ARPs) by stage VI-VIII seminiferous tubules from the adult rat

Transcriptional profiling of androgen receptor (AR) mutants suggests instructive and permissive roles of AR signaling in germ cell development

The androgen receptor (AR) is a transcription factor that plays a critical role in male sexual development, spermatogenesis, and maintenance of hormonal homeostasis. Despite the extensive knowledge of the phenotypic consequences of mutations in Ar, very little is known about the transcriptional targets of AR within the testis. To identify potential targets of androgen signaling in the testis, we have analyzed the transcriptional profile of adult testes from Ar hypomorphs alone or in combination with Sertoli cell-specific Ar ablation. Using Affymetrix MOE430A mouse genome arrays we interrogated more than 22,000 transcripts. We found the expression level of 62 transcripts in the Ar mutants differed by greater than 2-fold compared with wild type. We also found that more transcripts were up-regulated than down-regulated, highlighting AR's role as a transcriptional repressor in the testis. Twelve transcripts were uniquely affected, and 16 transcripts were more severely affected in Sertoli cell-specific Ar ablation compared with hypomorphic Ar mutants. Using a comparative genomic approach, we analyzed the 6 kb around the transcriptional start sites of affected transcripts for conserved AREs (androgen response elements). We identified at least one conserved ARE in 65% of the genes misregulated in our microarray analysis where clear mouse-human orthologs were available. We used a reporter assay in cell culture to functionally verify the AREs for the kallikrein 27 gene. This suggests that the majority of the misregulated transcripts have a high probability of being direct AR targets. The transcripts affected by these Ar mutations encode a diverse array of proteins whose molecular functions support the contention that AR supports spermatogenesis in both a permissive and instructive fashion.

Hormonal regulation of spermatogenesis

Proper functioning of the mammalian testis is dependent upon an array of hormonal messengers acting through endocrine, paracrine, and autocrine pathways. Within the testis, the primary messengers are the gonadotrophins, follicle stimulating hormone and luteinizing hormone, and the androgens. Abundant evidence indicates that the role of the gonadotrophins is to maintain proper functioning of testicular somatic cells. It is the androgens, primarily testosterone, which act through the somatic cells to regulate germ cell differentiation. Despite extensive research in this area, little is known about the cell-specific requirements for androgens and even less is understood about the downstream effectors of androgen signalling. However, recent work using cell-specific ablation of androgen receptor function has demonstrated a clear requirement for androgen signalling at multiple, discrete time points during spermatogenesis. These models also provide useful tools for identifying the targets of androgen receptor activity. The purpose of this review is to provide a brief overview of recent advances in our understanding of hormonal regulation of spermatogenesis, with an emphasis on the role of testosterone within the testis, and to pose important questions for future research in this field.

1,3-Dinitrobenzene metabolism and toxicity in seminiferous tubules isolated from rats of different ages

Previous in vivo studies in rats have shown that susceptibility to 1,3-dinitrobenzene (DNB)-induced testicular damage increases with age. The present study has used an in vitro approach to investigate the possibility that differences in testicular metabolism contribute to the age-related differences in toxicity. Seminiferous tubules were isolated from Sprague-Dawley rats (30, 75 and 120 days old) and incubated with 100 microM DNB for 22 h. Formation of metabolites and tubular levels of ATP and glutathione (GSH) were monitored over time. There was no difference in seminiferous tubule metabolic capacity among the three ages of rats examined. After 22 h of incubation with DNB, ATP levels were 20-30% of control and GSH levels were 70-90% of control, but neither parameter showed an age-related difference in decline. Based on these biochemical indicators of cell health, this study would suggest that the lack of testicular toxicity in young animals in vivo may be due to the previously described shorter half-life with consequent reduced exposure of the testis to DNB and that the age-related increase in severity of lesion between 75 and 120 days of age cannot be explained by differences in tubular metabolism of DNB or whole-animal toxicokinetics.

Differential expression of ornithine decarboxylase, poly(ADP)ribose polymerase, and mitochondrial mRNAs following testosterone administration to hypophysectomized rats

The mRNAs of the nuclear encoded genes, ornithine decarboxylase (ODCase) and poly(ADP)ribose polymerase (PADPRP), and the mitochondrial encoded genes, cytochrome oxidase I and II (COI and COII) and ATPase 6, are differentially expressed during spermatogenesis (Alcivar et al., 1989: Biol Reprod 41:1133; 1989: Dev Biol 135:263; 1991: Biol Reprod 46:201). In this study, we use Northern blotting to examine the steady state levels of ODCase, PADPRP, COI, COII, and ATPase 6 mRNAs in testes of hypophysectomized male rats following testosterone administration. Four weeks after hypophysectomy, rats received 24 cm subcutaneous implants of testosterone-filled polydimethylsiloxane (PDS) and were killed at 3, 7, 14, 28, and 56 days thereafter. After hypophysectomy, the steady state levels for the PADPRP, COI, COII, and ATPase 6 mRNAs were not significantly different from controls, although hypophysectomy caused a 44% loss of preleptotene spermatocytes and an 88% loss of pachytene spermatocytes, the testicular cell types expressing the highest levels of these mRNAs. In contrast, the levels of the two ODCase mRNAs were greatly decreased after hypophysectomy and mirrored the number of germinal cells present in the testis. After testosterone treatment, ODCase mRNA levels remained low 3 days after treatment and gradually increased at days 14, 28, and 56. No major hybridization signal changes in PADPRP, COI, COII, and ATPase mRNA were observed after testosterone treatment. We conclude that the steady state mRNA levels for the housekeeping ODCase gene respond differently after hypophysectomy and testosterone treatment of male rats than the PADPRP and mitochondrial DNA transcripts.

Macro, micro, and molecular research on spermatogenesis: the quest to understand its control

Synchronous maturation of the germ cells in the seminiferous epithelium has long been recognized by microscopy, and is believed to be a consequence of a complex interaction between the germ cells and the Sertoli cells, largely driven by testosterone and its synergistic action with follicle-stimulating hormone. Overall coordination of the cycle of the seminiferous epithelium is reviewed with regard to the known and possible actions of testosterone upon the Sertoli cells and the germ cells. With gradual refinements of optical instrumentation and development of a wide range of histological, morphometric, biochemical, and molecular techniques, coupled with selective alterations of hormonal stimulation and the cellular composition of the testis, new approaches to the question of how sperm production is regulated are becoming available. Germ cell and Sertoli cell functions are intimately related to each other via local, intratesticular or paracrine signals which are suppressed or triggered at certain defined steps in the spermatogenic process. The coordination of germ cell proliferation and maturation is discussed in terms of the contributions made by microscopical techniques.

Comparative analysis of proteins secreted in vitro by isolated seminiferous tubules from man and the rat

The aims of this study were to provide an overall comparison of the in-vitro protein secretory profile of seminiferous tubules (ST) isolated from man and the rat, and to identify specific proteins secreted by both species. Two-dimensional SDS PAGE was used to compare the profile of proteins secreted by cultured ST from 15 men undergoing orchidectomy with those secreted by ST isolated at stages VI-VIII of the spermatogenic cycle from normal adult rats, and by ST at the same stages isolated from rats pretreated with methoxyacetic acid (MAA) in order to deplete both pachytene spermatocytes and round spermatids. Two abundant groups of proteins not present in the medium of rat ST were secreted consistently by human ST, though the profile of proteins secreted by human ST was otherwise more variable than in the rat. Twelve proteins secreted by ST isolated from humans were identified as possible homologues of rat ST-secreted proteins, including the major rat Sertoli cell products SGP-1 and SGP-2, and an androgen-regulated protein which derives in the rat from round spermatids. Otherwise, the majority of the 12 potential homologues corresponded to proteins which in the rat are secreted by ST from both normal and germ cell-depleted testes or by ST from germ cell-depleted testes only, suggesting that they are probably secreted by Sertoli cells and/or peritubular cells. Based on the potential homology of the proteins identified, our results suggest, first, that these proteins may play an important role in spermatogenesis and second, that the profile of proteins secreted by human ST is more akin to that secreted by ST isolated from germ cell-depleted rats.

Effect of seminiferous tubule size on hCG-induced regeneration of peritubular Leydig cells in hypophysectomized, EDS-treated rats

Following their selective destruction 3 weeks previously by administration of ethane dimethanesulphonate (EDS) the regenerative capacity of Leydig cells was assessed in relation to seminiferous tubule morphology in hypophysectomized adult rats administered 7 daily injections of 100 iu hCG. Total Leydig cell volume per testis in hCG-treated rats (30.2 +/- 3.2 microliters, mean +/- SEM) was significantly (p < 0.01) greater than in the testes of rats at 3 and 4 weeks after EDS-treatment (7.6 +/- 0.7 and 22.7 +/- 1.4 microliters, respectively). Regeneration of Leydig cells in hCG-treated rats significantly (p < 0.05) favoured peritubular locations (18.6 +/- 2.8 microliters/testis) compared to central or perivascular sites of origin (11.6 +/- 1.2 microliters/testis). Partial restoration of spermatogenesis occurred in hCG-treated rats (tubule diameters usually > 250 microns) and a significant inverse correlation was found between peritubular Leydig cell percentage, or total volume per testis, and the volumetric proportion of seminiferous tubules (r = -0.94, p < 0.001) or the seminiferous epithelium (r = -0.73 to -0.79, p < 0.05-0.01). No significant (p > 0.4-0.9) correlation existed between centrally-regenerated Leydig cells and these parameters. The results show that in response to hCG stimulation, Leydig cells are more likely to develop around smaller seminiferous tubules, suggesting that hCG alone cannot mimic the expected pattern of Leydig cell regeneration (central and peritubular origins) which occurs during normal sexual maturation or at 3-4 weeks after EDS treatment. It is concluded that other factors, possibly FSH, are required for typical Leydig cell development which in turn may be influenced by local cellular growth factors originating from either the seminiferous tubules or the adjacent intertubular tissue.

Phosphatidylethanolamine binding protein is an abundant secretory product of haploid testicular germ cells in the rat

An abundant cellular and secretory product of isolated seminiferous tubules from adult rats was identified as having an apparent molecular weight of approximately 24,000 and a pI of 5.3 on autoradiographs of two-dimensional polyacrylamide gels. A protein with identical migration characteristics was identified as a major secretory product of isolated round spermatids. Microsequencing revealed that the protein had homology to phosphatidylethanolamine binding protein (PEBP) identified in rat brain. Primers were used in conjunction with RTPCR to amplify a partial cDNA which was used to probe a rat testis library to obtain full length clones. On Northern blots, PEBP mRNA was abundant in adult rat testis and epididymis and fractions enriched in germ cells but was very low/absent from fetal or immature rat testis or adult rat Sertoli cells. In situ hybridisation identified that abundant mRNA was first detectable in pachytene spermatocytes at stage VII and thereafter at particularly high levels in round and elongating spermatids until step 14. Proteins with significant sequence homology to the rat testis PEBP have been identified previously in mouse testis and epididymis, in rat germ cell cultures and coating the surface of mature rat sperm. Differences in the timing of expression of the PEBP mRNA (first expressed in pachytene spermatocytes) and secretion of the PEBP protein (not a major secretory product until round spermatids) is consistent with PEBP mRNA undergoing delayed translation. The role(s) of secreted lipid binding proteins in spermatogenesis are discussed.

Developmental stage- and spermatogenic cycle-specific expression of transcription factor GATA-1 in mouse Sertoli cells

GATA-1 is an essential factor for the transcriptional activation of erythroid-specific genes, and is also abundantly expressed in a discrete subset of cells bordering the seminiferous epithelium in tubules of the murine testis. In examining normal and germ-line defective mutant mice, we show here that GATA-1 is expressed only in the Sertoli cell lineage in mouse testis. GATA-1 expression in Sertoli cells is induced concomitantly with the first wave of spermatogenesis, and GATA-1-positive cells are uniformly distributed among all tubules during prepubertal testis development. However, the number of GATA-1-positive cells declines thereafter and were found only in the peripheral zone of seminiferous tubules in stages VII, VIII and IX of spermatogenesis in the adult mouse testis. In contrast, virtually every Sertoli cell in mutant W/Wv, jsd/jsd or cryptorchid mice (all of which lack significant numbers of germ cells) expresses GATA-1, thus showing that the expression of this transcription factor is negatively controlled by the maturing germ cells. These observations suggest that transcription factor GATA-1 is a developmental stage- and spermatogenic cycle-specific regulator of gene expression in Sertoli cells.

Effect of age on seminiferous tubule protein secretion and the adverse effects of testicular toxicants in the rat

This study has assessed the effect of age on protein secretion by seminiferous tubules (ST) isolated from rats and their response to Sertoli cell toxicants. ST were isolated from immature (aged 28 days), late pubertal (aged 45 days) and young adult (aged 70 days) rats and cultured in vitro for 24 h with 35S-methionine in the presence or absence of FSH (1 mg/ml), m-dinitrobenzene (m-DNB) or nitrobenzene (NB) (both at 10(-4)M). Incorporation of 35S-methionine into newly synthesized proteins in the culture medium (secreted proteins) was assessed and the pattern of protein secretion evaluated using two-dimensional sodium dodecylsulphate polyacrylamide gel electrophoresis (2-D SDS-PAGE). These data were compared with those obtained using cultures of immature rat Sertoli cells+germ cells (SC+GC). Addition of FSH, m-DNB or NB in vitro either had no effect (NB) or had a small stimulatory effect (FSH and m-DNB) on the incorporation of 35S-methionine into overall secreted proteins by ST isolated from immature rats. At the same doses, addition of either FSH, m-DNB or NB to SC+GC co-cultures resulted in increased incorporation of radiolabel into secreted proteins in all instances. In contrast, the same additions to ST isolated from adult rats resulted in a 20-34% decrease in the overall incorporation of 35S-methionine into secreted proteins. ST isolated from late pubertal rats showed a similar response to adult rats except that the decreases in incorporation induced by FSH, m-DNB and NB were smaller. Analysis by 2-D SDS-PAGE revealed considerable age-dependent differences in the proteins secreted by ST from immature and adult rats, of which 13 were identified as being of potential importance. Most of these proteins were prominent secretory products of ST from adult rats, but were minor or non-detectable products of cultures of ST or SC+GC from immature rats. Most of these proteins disappeared or decreased in abundance after culture of ST with m-DNB or NB. Two proteins showed the reverse pattern, being more prominent secretory products in immature than mature rats, and their secretion was unaffected or was increased by toxicant exposure. These results demonstrate that there are major age-dependent differences in the secretion of total and specific proteins by isolated ST and that these are probably related to changes in the germ cell complement with age.(ABSTRACT TRUNCATED AT 400 WORDS)

In-vitro influence of germ cells on Sertoli cell-secreted proteins: a two-dimensional gel electrophoresis analysis

Two-dimensional polyacrylamide gel electrophoresis (2D PAGE) was used to analyse [35S]-methionine-labelled proteins secreted in vitro by Sertoli cells when cultured in the presence or absence of enriched preparations of pachytene spermatocytes (SPC), early spermatids (SPT) or residual bodies/cytoplasts from elongated spermatids (RB/CES). The presence of germ cells modified the pattern of Sertoli cell secreted proteins in co-culture. Out of 21 Sertoli cell secreted polypeptide families visualized by 2D PAGE, one (referred to as number 12) was stimulated, whereas the secretion of polypeptides 1 and 3 was inhibited by all of the germ cell populations tested. Early spermatids and RB/CES both enhanced the secretion of protein number 10 and inhibited the production of protein 11. The RB/CES fraction specifically inhibited secretion of polypeptide 13. Of particular note was the finding that co-culture with early spermatids or RB/CES induced the secretion of a novel polypeptide, termed GIP (germ cell-induced protein), with an apparent molecular weight of 72 kDa and an isoelectric point of 5.9. Under the present experimental conditions, media conditioned by the different germ cell fractions inhibited the secretion of polypeptide 2 but enhanced the secretion of polypeptides 10 and 18; of note also was the finding that media conditioned by early spermatids or RB/CES induced the appearance of GIP. This study confirms and extends the concept that germ cells influence Sertoli cell function and that the effects observed differ according to the stage of development of the germ cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Stage-dependent expression of mRNA for cyclic protein 2 during spermatogenesis is modulated by elongate spermatids

Cyclic protein 2 (CP-2) is a product of the Sertoli cell which is secreted in a cyclical manner according to the stage of the spermatogenic cycle. This study has assessed the influence of the germ cell complement on expression of CP-2 mRNA. Adult rats were treated with 650 mg/kg methoxyacetic acid (MAA) to induce the specific depletion of > 80% of pachytene and later spermatocytes from most tubules, and expression of CP-2 mRNA was then assessed at various times after treatment when particular germ cell types were depleted selectively. CP-2 mRNA was specifically localised to the Sertoli cells of the seminiferous tubules by non-radioactive in situ hybridisation using a digoxigenin-labelled riboprobe. A stage specific variation in CP-2 mRNA levels was observed, with the mRNA being most abundant at stages IV-VII of the spermatogenic cycle. Northern analysis revealed that treatment with MAA led to an apparent increase in the amount of the major 1.7 kb CP-2 transcript when either pachytene spermatocytes or round spermatids were depleted. In contrast, the level of CP-2 mRNA was decreased by more than half at 21 days after MAA treatment. This decrease was confirmed by in situ hybridisation at 21 days after MAA treatment, when CP-2 mRNA expression was found to be decreased or absent from tubules at stages at which CP-2 mRNA is normally expressed (stages IV-VII) when elongate spermatids were depleted selectively from these tubules. These observations lead us to hypothesise that elongate spermatids positively modulate CP-2 mRNA expression in the Sertoli cell.

Relative roles of testosterone and the germ cell complement in determining stage-dependent changes in protein secretion by isolated rat seminiferous tubules

This study has compared the effect of withdrawal of testosterone (+/- replacement) with that of selective depletion of pachytene spermatocytes (PS) or round/elongating spermatids (RS), or both PS and RS, on the level of overall protein secretion by seminiferous tubules (ST) isolated at particular stage-groups of the spermatogenic cycle. Testosterone withdrawal was induced by destroying the Leydig cells with a single injection of ethane-dimethane sulphonate (EDS), with or without concomitant replacement of testosterone by injection; ST at stages II-V, VI-VIII or IX-XII were then isolated from control and treated rats at 4 days after treatment. Methoxyacetic acid (MAA) was administered, in either one or two doses, to selectively destroy 80-100% of pachytene and later spermatocytes; ST at stages I-V, VI-VIII or IX-XIV were then isolated at specific times after treatment such that ST were depleted selectively of either PS, RS or PS+RS. Isolated ST (5 cm) were then cultured for 22 h at 34 degrees C with 35S-methionine and its incorporation into secreted proteins then quantified. Based on the incorporation of 35S-methionine, ST at stages VI-VIII showed a significantly higher level of protein secretion than did ST at earlier or later stages. This difference was abolished following testosterone withdrawal but was maintained by testosterone replacement. The normal increase in protein secretion by ST at stages VI-VIII was also prevented if either PS or RS were depleted, whereas depletion of either PS or RS alone had no significant effect on protein secretion by ST at stages I-V, and only the depletion of RS significantly reduced protein secretion by ST at stages IX-XIV. Depletion of both PS+RS reduced protein secretion significantly by ST at all stages. In contrast to the data for total protein secretion, the levels of sulphated glycoprotein (SGP)-1 and SGP-2 secreted by ST at stages VI-VIII showed that these two Sertoli cell proteins were unaffected by germ cell depletion except after co-depletion of PS+RS when secretion of SGP-1 was halved.(ABSTRACT TRUNCATED AT 400 WORDS)