Paracrine control of immature Sertoli cells by adult germ cells, in the rat (an in vitro study). Cell-cell interactions within the testis

Pleiotropic Activities of HGF/c-Met System in Testicular Physiology: Paracrine and Endocrine Implications

In the last decades, a growing body of evidence has been reported concerning the expression and functional role of hepatocyte growth factor (HGF) on different aspects of testicular physiology. This review has the aim to summarize what is currently known regarding this topic. From early embryonic development to adult age, HGF and its receptor c-Met appeared to be clearly detectable in the testis. These molecules acquire different distribution patterns and roles depending on the developmental stage or the post-natal age considered. HGF acts as a paracrine modulator of testicular functions promoting the epithelium-mesenchyme cross-talk as described even in other organs. Interestingly, it has been reported that testicular HGF acts even as an autocrine factor and that its receptor might be modulated by endocrine signals that change at puberty: HGF receptor expressed by Sertoli cells, in fact, is up-regulated by FSH administration. HGF is in turn able to modify endocrine state of the organism being able to increase testosterone secretion of both fetal and adult Leydig cells. Moreover, c-Met is expressed in mitotic and meiotic male germ cells as well as in spermatozoa. The distribution pattern of c-Met on sperm cell membrane changes in the caput and cauda epididymal sperms and HGF is able to maintain epididymal sperm motility in vitro suggesting a physiological role of this growth factor in the acquisition of sperm motility. Noteworthy changes in HGF concentration in seminal plasma have been reported in different andrological diseases. All together these data indicate that HGF has a role in the control of spermatogenesis and sperm quality either directly, acting on male germ cells, or indirectly acting on tubular and interstitial somatic cells of the testis.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 5: intercellular junctions and contacts between germs cells and Sertoli cells and their regulatory interactions, testicular cholesterol, and genes/proteins associated with more than one germ cell generation

In the testis, cell adhesion and junctional molecules permit specific interactions and intracellular communication between germ and Sertoli cells and apposed Sertoli cells. Among the many adhesion family of proteins, NCAM, nectin and nectin-like, catenins, and cadherens will be discussed, along with gap junctions between germ and Sertoli cells and the many members of the connexin family. The blood-testis barrier separates the haploid spermatids from blood borne elements. In the barrier, the intercellular junctions consist of many proteins such as occludin, tricellulin, and claudins. Changes in the expression of cell adhesion molecules are also an essential part of the mechanism that allows germ cells to move from the basal compartment of the seminiferous tubule to the adluminal compartment thus crossing the blood-testis barrier and well-defined proteins have been shown to assist in this process. Several structural components show interactions between germ cells to Sertoli cells such as the ectoplasmic specialization which are more closely related to Sertoli cells and tubulobulbar complexes that are processes of elongating spermatids embedded into Sertoli cells. Germ cells also modify several Sertoli functions and this also appears to be the case for residual bodies. Cholesterol plays a significant role during spermatogenesis and is essential for germ cell development. Lastly, we list genes/proteins that are expressed not only in any one specific generation of germ cells but across more than one generation.

Serine proteases and serine protease inhibitors in testicular physiology: the plasminogen activation system

The testis is an organ in which a series of radical remodeling events occurs during development and in adult life. These events likely rely on a sophisticated network of proteases and complementary inhibitors, including the plasminogen activation system. This review summarizes our current knowledge on the testicular occurrence and expression pattern of members of the plasminogen activation system. The various predicted functions for these molecules in the establishment and maintenance of the testicular architecture and in the process of spermatogenesis are presented.

Regulation of testicular function by cell-to-cell interaction

Cell-to-cell interactions in the testis play important roles in regulating testicular function. Investigating the mechanism of such interactions is essential to elucidate the cause of spermatogenic dysfunction and to create a new treatment. (Reprod Med Biol 2006; 5: 9-17).

Effect of compound CDRI 84/35 and synthetic estrogen on the seminiferous epithelium of immature rat

Compound CDRI 84/35 (a piperazine derivative--a potent antispermatogenic agent) has been shown to cause significant inhibition in testicular spermatogenesis without affecting Leydig cell and accessory sex organ function in adult rats. The present study was conducted to determine its effect on the germ cell population and Leydig cell morphology in immature rats (40-50 gm) administered CDRI 84/35 (100 mg/kg/day p.o.), synthetic estradiol benzoate (EB; 5 microg/rat/day) and vehicle at the age of 21 days. Animals were killed 24 h later following 7 and 14 days' treatments. Bouin's fixed testes were sectioned (at 5 microm) and stained with PAS-hematoxylin. Quantitative determination of Sertoli Cell-Germ Cell ratio was carried out in 150 round seminiferous tubules in each group of 5 rats. Results revealed a significant decrease in number of the spermatocytes (non-pachytene and pachytene) and early (round) spermatids in step 1-8 of spermiogenesis without affecting Leydig cell morphology in rats administered CDRI 84/35 for 7 and 14 days as compared to corresponding controls. In contrast, the testes of rats injected with synthetic EB, caused a marked inhibition in these meiotic and post-meiotic germ cell types, as well as in the diameters of round seminiferous tubules, and Leydig cells nuclei (only in 14 days treatment), and testicular weight on autopsy days 8 and 15 as compared to CDRI 84/35-treated rats. While the number of pre-meiotic spermatogoniae was observed to be slightly decreased after only 14 days treatment in both CDRI 84/35 and EB treatment groups, the Sertoli cell number did not show any significant change as compared to controls. The present investigation confirms the antispermatogenic effect of compound CDRI 84/35 in immature rats similar to that reported in adult rats. Marked inhibition in pachytene spermatocytes and other testicular parameters following synthetic estrogen treatment might be due to its antiandrogenic action, contrasting with the non-hormonal profile of CDRI compound.

mRNAs encoding a von Ebner's-like protein and the Huntington disease protein are induced in rat male germ cells by Sertoli cells

The success of spermatogenesis is dependent upon closely coordinated interactions between Sertoli cells and germ cells. To identify specific molecules that mediate interactions between somatic cells and germ cells in the rat testis, Sertoli cell-germ cell co-cultures and mRNA differential display were used. Two cDNAs, clone 1 (660 nucleotides) and clone 2 (390 nucleotides) were up-regulated when Sertoli cells were co-cultured with pachytene spermatocytes or round spermatids. Northern blot analyses confirmed the differential display expression patterns. Sequence analyses indicated that clone 1 was similar to a von Ebner's gland protein (87% at the nucleotide level and 80% at the amino acid level) and clone 2 was identical to a region of the Huntington disease protein. The von Ebner's-like protein mRNA was induced after 4 h of co-culture, while the Huntington disease protein required 18 h of co-culture for expression. The von Ebner's-like protein was induced in germ cells by a secreted Sertoli cell factor(s) smaller than 10 kDa that is sensitive to freezing and thawing or boiling. The Huntington disease protein was induced in germ cells by a Sertoli cell secreted factor(s) larger than 10 kDa which survives freezing and thawing, but is inactivated by boiling.

Up-regulation and down-regulation of genes expressed in cocultures of rat Sertoli cells and germ cells

To better understand the molecular interactions between somatic and germ cells in the mammalian testis, we have begun to analyze with mRNA differential display changes in gene expression induced by coculturing rat Sertoli cells and germ cells. We have identified 10 cDNAs that are either down-regulated or up-regulated in cocultures of germ cells and Sertoli cells. Three genes expressed in Sertoli cells and three genes expressed in germ cells were down-regulated in Sertoli cell-germ cell cocultures, whereas four genes were up-regulated in the cocultures. Northern blot analysis was used to establish the expression pattern of the mRNAs encoded by the cDNAs and to define the sizes of the differentially expressed mRNAs. Sequence analysis of the cDNAs and computer searches against the GenBank and EMBL DNA databases were used to relate the ten cDNAs to known genes. Of the three Sertoli cell cDNAs, one appeared identical to transferin, while the other two shared regions of similarity to an endoplasmic reticulum stress protein and to a pro-alpha 2 XI collagen, respectively. The three germ cell cDNAs shared sequences with fibronectin, with a basic fibroblast growth factor receptor and with an IgG gamma 2b, respectively. The four cDNAs that were up-regulated in the Sertoli-germ cell cocultures showed similarity to an isoform of casein kinase 1 delta, to an epidermal growth factor, to a statin-related protein, and to an integral membrane glycoprotein. These data demonstrate that a number of specific genes are up- and down-regulated when germ cells and Sertoli cells are cocultured, and suggest these genes are important in cell to cell communication during spermatogenesis.

Sertoli cells in testes containing or lacking germ cells: a comparative study of paracrine effects using the W (c-kit) gene mutant mouse model

BACKGROUND

Paracrine effects of germ cells on Sertoli cell structure were examined in a mouse model with the W locus (dominant white spotting) mutation in which animals with the W/Wv genotype (referred to as mutants) lack virtually all germ cells.

RESULTS

Morphometric determination of Sertoli cell parameters in mutant and control (+/+) animals showed that although the testes of mutant animals were about eight times smaller than controls, the numbers of Sertoli cells in the two groups did not differ. Sertoli cell volume, Sertoli cell cytoplasmic and nuclear volumes, and Sertoli cell surface area in mutant animals were significantly smaller than in control animals. Organelle volumes and surface areas, expressed per cell, did not differ significantly in the two groups with one exception: the volume and surface area of smooth endoplasmic reticulum was significantly reduced in mutant animals. Plasma testosterone levels and tissue testosterone levels/testis were normal, indicating that the effects observed in the mutant animal were not a consequence of androgen insufficiency. Plasma FSH was elevated, probably as a consequence of germ cell depletion, and was thought not to affect Sertoli cell parameters observed.

CONCLUSIONS

The data suggest that paracrine interactions with germ cells do affect Sertoli cells by modifying the amount of smooth endoplasmic reticulum. These data focus attention on the function of this abundant Sertoli cell organelle in promoting spermatogenesis.

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.

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.

Seasonal changes of the blood-testis barrier in viscacha (Lagostomus maximus maximus): a freeze-fracture and lanthanum tracer study

Adult male viscachas (Lagostomus maximus maximus) were gathered from their natural habitat during the period of complete spermatogenesis (June) and during the month of maximum testicular regression (August). The testes were processed by conventional electron microscopic technique using lanthanum nitrate (electron-dense intercellular tracer) to define the intercellular spaces below the inter-Sertoli tight junctions and by freeze-fracture techniques. During complete spermatogenesis the tracer surrounds spermatogonia, preleptotene, and leptotene spermatocytes and stops at the level of the inter-Sertoli tight junctions below all germ cells displaying synaptonemal complexes (zygotene-pachytene spermatocytes) and germ cells in more advanced stages of differentiation. Conversely, during testicular regression the tracer percolates all intercellular spaces between Sertoli cells and the remaining germ cells (spermatogonia and few preleptotene and leptotene spermatocytes). During complete spermatogenesis, freeze-fracture replicas exhibit numerous inter-Sertoli tight junction strands parallel to each other and to the basal lamina. During spermatogenesis decay, the inter-Sertoli tight junctions are found to be short, tortuous, frequently interrupted, and often associated with extented membranous areas of gap junctions.

Rat Sertoli cell clusterin, alpha 2-macroglobulin, and testins: biosynthesis and differential regulation by germ cells

Clusterin, alpha 2-macroglobulin and testins are three novel Sertoli cell proteins whose physiological functions may be related to cell-cell interactions in the seminiferous epithelium of the testis. We have demonstrated the biosynthesis of clusterin, alpha 2-macroglobulin, and testins by Sertoli cells in vitro using pulse-chase labeling analysis. For clusterin, two precursors with an apparent molecular weight (M(r)) of 72,000 (PH) and 66,000 (PL) were detected in the Sertoli cell cytosol in addition to the alpha (M(r) 43,000) and beta (M(r) 35,000) subunits of the mature protein. However, the precursors were not secreted into the medium since only the alpha and beta subunits of clusterin were detected. For alpha 2-macroglobulin and testins, no precursor molecules were detected either in the Sertoli cell cytosol or culture medium. The polarized secretory pattern of these proteins and their regulation by follicle stimulating hormone (FSH) and testosterone (T) were examined using a bicameral culture chamber that mimics the in vivo physiological conditions. Clusterin was secreted almost exclusively into the apical chamber of the bicameral culture unit with an apical:basal ratio of 30:1. In contrast, alpha 2-macroglobulin and testins had an apical:basal ratio of 1:1 and 1.5:1, respectively. Thus, the polarized secretory pattern for clusterin is different from alpha 2-macroglobulin and testins. It was noted that FSH and T, the known Sertoli cell regulators, did not affect the secretion of either clusterin or alpha 2-macroglobulin. Due to the morphological intimacy between Sertoli cells and germ cells in the adluminal compartment of the testis, the effects of germ cell-conditioned medium were investigated. Addition of germ cell-conditioned medium (1-30 micrograms protein) to the apical chamber of the bicameral culture unit caused a dose-dependent inhibition of clusterin and testins apical secretion and a slight but statistically significant stimulation of their basal secretion. In contrast, the secretion of alpha 2-macroglobulin by Sertoli cells was stimulated both apically and basally. These observations suggest that germ cell-conditioned medium contains a biological factor(s) that differentially regulates the bidirectional secretion of Sertoli cell proteins. These studies therefore reveal the complicated regulatory processes involved in cell-cell interactions in the seminiferous epithelium.

Immortalization of germ cells and somatic testicular cells using the SV40 large T antigen

We report the immortalization, using the SV40 large T antigen, of all the cell types contributing to a developing seminiferous tubule in the mouse testis. Sixteen peritubular, 22 Leydig, 8 Sertoli, and 1 germ cell line have been established and cultured successfully for 90 generations in a period of 2.5 years. Immortalized peritubular cells were identified by their spindle-like appearance, their high expression of alkaline phosphatase, and their expression of the intermediary filament desmin. They also produce high amounts of collagen. Immortalized Leydig cells are easily identifiable by the accumulation of lipid droplets in their cytoplasm and the production of the enzyme 3-beta-hydroxysteroid dehydrogenase. Some Leydig cell lines also express LH receptors. The immortalized Sertoli cells are able to adopt their typical in vivo columnar appearance when cultured at high density. They exhibit a typical indented nucleus and cytoplasmic phagosomes. Some Sertoli cell lines also express FSH receptors. A germ cell line (GC-1spg) was established that corresponds to a stage between spermatogonia type B and primary spermatocyte, based on its characteristics in phase contrast and electron microscopy. This cell line expresses the testicular cytochrome ct and lactate dehydrogenase-C4 isozyme. These four immortalized cell types, when plated together, are able to reaggregate and form structures resembling two-dimensional spermatogenic tubules in vitro. When only the immortalized somatic cells are cocultured, the peritubular and Sertoli cells form cord-like structures in the presence of Leydig cells. Fresh pachytene spermatocytes cocultured with the immortalized somatic cells integrate within the cords and are able to survive for at least 7 days. The ability to perform coculture experiments with immortalized testicular cell lines represents an important advancement in our ability to study the nature of cell-cell and cell-matrix interactions during spermatogenesis and testis morphogenesis.

Lipopolysaccharide, latex beads and residual bodies are potent activators of Sertoli cell interleukin-1 alpha production

Several recent studies indicate that interleukin-1 (IL-1) may be one of the major regulators of spermatogenesis. In the present work, the effects of various agents on rat Sertoli cell IL-1 production were investigated in vitro at different ages. In control cultures the IL-1 production by Sertoli cells from immature rats (20 days) was barely detectable; it markedly increased with the advancing age of the donors (35 and 45 days). Lipopolysaccharide and latex beads, two inducers of monocyte-macrophage IL-1 production, were able to stimulate the release of IL-1 by Sertoli cells at all ages studied; a decrease in the relative response to these inducers was noted as the age of the Sertoli cell donors increased. Under all the experimental conditions tested it appeared that the IL-1 produced was IL-1 alpha, not IL-1 beta. Whereas pachytene spermatocytes and early spermatids had no effect on IL-1 alpha production, residual bodies/cytoplasts from elongated spermatids dramatically stimulated Sertoli cell IL-1 production. In addition, FSH had no effect on IL-1 alpha levels. It is concluded that Sertoli cell IL-1 secretion can be stimulated in vitro by well known monocyte-macrophage activators. Furthermore, Sertoli cell IL-1 production is most probably crucially dependent on the phagocytosis of residual bodies in vivo.

The second messenger pathway for germ cell-mediated stimulation of Sertoli cells

Treatment of cultured rat Sertoli cells with FSH or dibutyryl cAMP for 30 min resulted in phosphorylation of the same Sertoli cell proteins. Different Sertoli cell proteins were phosphorylated after calcium ionophore A23187 and 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. A23187 stimulated the phosphorylation of hsp27, while TPA alone had no effect. TPA plus A23187 resulted in phosphorylation of a 14 kDa protein, in addition to hsp27. The effect of TPA plus A23187 was identical to that of germ cells on Sertoli cell protein phosphorylation. FSH-stimulated cAMP production by Sertoli cells was reduced by prior exposure of Sertoli cells to germ cells. The results indicate that germ cells stimulate Sertoli cells by the inositol trisphosphate/diacylglycerol mediated second messenger pathway. The results also suggest that the germ cell-activated pathway interacts within Sertoli cells to modulate Sertoli cell response to FSH.

A germ cell factor(s) modulates preproenkephalin gene expression in rat Sertoli cells

Within the seminiferous tubule, both Sertoli and specific germ cells express opioid genes. Little is known about the paracrine regulation or role of opioid gene expression in the tubule. The present study shows that interactions among cells within the tubule may play a role in regulating preproenkephalin (PPenk) gene expression. Rat pachytene spermatocytes (PS) and round spermatids (RSd) were purified by centrifugal elutriation and established as primary cultures or co-cultured with Sertoli cells. The effects of germ cells or germ cell-conditioned media were studied to determine the expression of one of the opioid precursor genes in rat Sertoli cells, the PPenk gene. Following a 24 h co-culture with either PS or RSd, the expression of PPenk gene in Sertoli cells was increased 6.4- and 1.9-fold, respectively. Conditioned media obtained from either PS or RSd cultured for 20 h stimulated PPenk mRNA levels in Sertoli cells from as early as 2 h after exposure; maximum increases of 3.5- and 7.6-fold were observed at 12 h, respectively. The molecular weight of the germ cell factor(s) is greater than 30 kDa. 2 h after the addition of either PS- or RSd-conditioned media to Sertoli cells, small (2- to 2.6-fold, respectively) but significant (p less than 0.02) increases in extracellular cAMP levels were observed. Although both FSH and forskolin activated c-fos and PPenk gene expression in Sertoli cells, the germ cell factor(s) that stimulated PPenk mRNA levels did not affect the expression of this oncogene. These results indicate that germ cells interact with Sertoli cells, possibly by a protein(s) that acts as a short-loop paracrine factor, which regulates the expression of PPenk gene in Sertoli cells. These data suggest that stage-specific regulation of PPenk levels in Sertoli cells may occur in vivo.

Germ cell-Sertoli cell interactions and production of testosterone by purified Leydig cells from mature rat

The addition of seminiferous tubule (ST) culture medium (STM) prepared from testes of either busulfan-treated (Bus) or cryptorchid (Cryp) or genetically sterile (hd) rats, to Percoll purified Leydig cells leads to a further increase of LH-stimulated testosterone (T) output (26, 43 and 14%, respectively). Taking into account that the Sertoli cell number per cm of ST is 2.6, 1.8 and 1.4-fold greater in Bus, Cryp and hd rats than in controls, the above STM effects on T output, expressed per 10(6) Sertoli cells are in fact lower (63, 44 and 43%, respectively) that those of control STM. Similar results have been obtained for the STM transferrin levels which are decreased, 74, 67 and 45%, respectively in Bus, Cryp and hd animals. So, it is likely that the Sertoli cell secretion of both the paracrine factor involved on Leydig cell T production and the transferrin is influenced mainly by spermatids and to a lesser extent by spermatocytes of mature rat testis.

Mannose 6-phosphate receptors: potential mediators of germ cell-Sertoli cell interactions

These studies have demonstrated that mouse pachytene spermatocytes, round spermatids, and Sertoli cells synthesize mannose 6-phosphate receptors and that the proportions of the CI- and CD-MPRs vary markedly between cell types. Isolated spermatogenic cells synthesize predominantly the CD-MPR and lower levels of the CI-MPR. In contrast, cultured Sertoli cells selectively synthesize the CI-MPR, even though transcripts for the CD-MPR have been detected in these cells. These striking differences in the expression of MPRs suggest that these receptors may serve multiple roles during germ cell differentiation. We have hypothesized that MPRs in the seminiferous epithelium mediate interactions between germ cells and Sertoli cells, and participate in the targeting of hydrolytic enzymes to the acrosome. In support of the first hypothesis, we have shown that functional MPRs are localized on the surface of spermatogenic cells and Sertoli cells where they mediate the endocytosis of M6P-containing ligands. As in other somatic cells, the CI-MPR is likely to be responsible for M6P receptor-mediated endocytosis in the seminiferous epithelium. Recent studies have shown that Sertoli cells in culture synthesize and secrete at least ten M6P-containing glycoproteins. Furthermore, pachytene spermatocytes and round spermatids endocytose these Sertoli M6P-glycoproteins and process them to lower molecular weight forms that persist during 17 h culture periods. The identification of relevant ligands for mannose 6-phosphate receptors in the seminiferous epithelium may help define new regulatory mechanisms in cell differentiation. Current efforts to determine if Sertoli M6P-glycoproteins modulate germ cell function should confirm the significance of surface MPRs and clarify their roles in signal transduction and/or the endocytosis of Sertoli cell products.

Spermatids are regulators of Sertoli cell function

Spermatids are major regulators of Sertoli cell function. Specific anatomical structures exist between spermatids and Sertoli cells. Their nature evolves during spermiogenesis and they are essential mediators in the interaction between these two cell types. Spermatids play a crucial role in Sertoli cell gene expression and secretory function at different ages. Early spermatid effect is partly mediated through the secretion of soluble factor(s). The influence of late spermatids on Sertoli cell secretion in the adult testis is conserved throughout evolution and may be mediated via their implication in the conformational changes on Sertoli cells that occur during spermatogenesis and through phagocytosis of the residual bodies.

Effect of epidermal and insulin-like growth factors on vectorial secretion of transferrin by rat Sertoli cells in vitro

Within the seminiferous tubules, the Sertoli cells create an impermeable blood-testis barrier and an unique intratubular microenvironment that fosters the development of spermatozoa. The functional differentiation of spermatozoa therefore requires vectorial secretion by Sertoli cells of substances that cannot cross the blood-testis barrier. We investigated the role of epidermal (EGF) and insulin-like growth factors I and II (IGF-I and IGF-II) in the regulation of vectorial secretion of transferrin by Sertoli cells. In order to study the regulation of vectorial transferrin secretion, we modified culture conditions in the twin chamber culture system to maximise gradients of transferrin secretion. Sertoli cells were plated at high density (3-4 x 10(6) cells/well) into chambers of near equal volume, cultured at 37 degrees C and maintained in simple, fully defined media omitting standard supplements (insulin, EGF, FSH) which affect vectorial transferrin secretion. Using this optimised culture system, maximum gradients of transferrin secretion occurred between days 2 and 3 of culture with preferential secretion (mean ratio 3.7 +/- 0.2) directed towards the apical compartment. The transferrin ratio (ratio of transferrin secreted into the upper over the lower chamber) was decreased by insulin and FSH but not by retinoic acid or testosterone, yet all four stimuli increased total transferrin secretion. IGF-I and IGF-II were effective at physiological concentrations (ED50 = 1 ng/ml) in lowering transferrin ratio and were 100-fold more potent than insulin suggesting that insulin effects on vectorial transferrin secretion by Sertoli cells is mediated through type 1 IGF receptors. EGF also reduced the transferrin ratio (ED50 = 50 ng/ml) as well as stimulating total transferrin secretion. The hormonally mediated reduction in transferrin ratio was consistently due to enhanced secretion of transferrin into the lower chamber. In the first demonstration of a highly polarised response of Sertoli cells to hormonal stimuli, the effects of insulin, FSH and EGF on vectorial transferrin secretion were effected primarily via the basal membrane of the Sertoli cell and operated independent of mechanisms controlling total transferrin secretion. These results establish a potential role for epidermal and insulin-like growth factors in the paracrine regulation of vectorial secretion by the Sertoli cell, in particular the developmental regulation of vectorial transferrin secretion by Sertoli cells. These findings also indicate that previous studies which included insulin and EGF routinely in culture media have systematically underestimated apically directed transferrin secretion.

Study in vitro of the phagocytic function of Sertoli cells in the rat

Aspects of the interaction between residual bodies/cytoplasts from elongated spermatids (RB/CES) and Sertoli cells were studied in vitro. Highly enriched Sertoli cells (91%: experiment A), very highly enriched Sertoli cells (greater than 96%: experiment B), as well as peritubular cells were isolated from testes of 20-day-old rats by means of hypotonic treatment. Isolated Sertoli cells and peritubular cells were also prepared from 45-day-old rats (experiment C). RB/CES were isolated by centrifugal elutriation from testes of rats aged 90-120 days. The kinetics of adhesion of RB/CES to Sertoli cells were similar in all experiments. FSH accelerated binding of RB/CES but markedly reduced the number of RB/CES phagocytosed. Co-culture of the highly enriched Sertoli cells from experiments A and C with isolated peritubular cells did not change the kinetics of adhesion of RB/CES. However, when the contamination of Sertoli cells by peritubular cells was at a minimum (experiment B), addition of peritubular cells induced a slight but significant stimulation of the binding of RB/CES. Transmission electron microscopy revealed the following events within 24 h of co-culture: adhesion of the RB/CES to microvilli of Sertoli cells; internalization of RB/CES; lysis of the membrane of RB/CES; total digestion. Therefore, FSH and peritubular cells modulate the interaction in vitro between Sertoli cells and RB/CES, and the different steps of residual body disposal can be reproduced in co-culture. The co-culture model described in this study provides a useful system for the study of phagocytic activity by Sertoli cells.

Pachytene spermatocyte protein(s) stimulate Sertoli cells grown in bicameral chambers: dose-dependent secretion of ceruloplasmin, sulfated glycoprotein-1, sulfated glycoprotein-2, and transferrin

Interactions between pachytene spermatocytes and Sertoli cells were investigated using the bicameral culture chamber system. Pachytene spermatocytes were isolated from adult rats with a purity in excess of 90% by centrifugal elutriation. The pachytene spermatocytes were cultured in a defined media and pachytene spermatocyte protein prepared from the conditioned media by dialysis and lyophilization. This pachytene spermatocyte protein was reconstituted at various concentrations and incubated with confluent epithelial sheets of immature Sertoli cells cultured in bicameral chambers. Pachytene spermatocyte protein stimulated secretion of total [35S]methionine-labeled protein from Sertoli cells in a dose-dependent manner predominantly in an apical direction. This stimulatory effect of pachytene spermatocyte protein was domain specific from the apical surface of Sertoli cells, and seemed specific for secretion because total intracellular protein did not increase under the influence of pachytene spermatocyte protein. Pachytene spermatocyte protein and follicle-stimulating hormone additively stimulated Sertoli cell secretion. The physicochemical characteristics of the stimulatory pachytene spermatocyte protein are indicative of heat stability, whereas the stimulatory pachytene spermatocyte protein exhibit acid, dithiothreitol and trypsin sensitivity, and partial urea sensitivity. Furthermore, Sertoli cell secretion of ceruloplasmin, sulfated glycoprotein-1, sulfated glycoprotein-2, and transferrin in response to various concentrations of pachytene spermatocyte protein were determined by immunoprecipitate of these [35S]methionine-labeled proteins with polyclonal antibodies. Maximal stimulation of ceruloplasmin and sulfated glycoprotein-1 secretion from Sertoli cells was observed at a dose of 50 micrograms/ml pachytene spermatocyte protein, whereas maximal stimulation of sulfated glycoprotein-2 and transferrin secretion from Sertoli cells was observed at a dose of 100 micrograms/ml of pachytene spermatocyte protein. These results suggest that pachytene spermatocytes modulate Sertoli cell secretory function of at least four proteins in the regulation of spermatogenesis.

Modulation of Sertoli cell secretory function by rat round spermatid protein(s)

The influence of rat round spermatid protein(s) (RSP) on protein synthesis and secretory function of Sertoli cells was used in the bicameral chamber system. Round spermatids (RS) were purified from 90-day-old rats by centrifugal elutriation. RS were incubated in a supplement-enriched culture medium that lacked exogenous proteins. The RS-conditioned media were dialysed and lyophilized to obtain RSP. Most de novo protein synthesized under basal conditions by Sertoli cells (18-day-old) was secreted into the apical chamber (apical/basal ratio: 3.42). Follicle-stimulating hormone (FSH, 100 ng/ml) stimulated total protein secretion from Sertoli cells by a factor of 1.54. The RSP (100 micrograms/ml) stimulated total protein secretion from Sertoli cells by a factor of 2.33. The enhancement of total Sertoli cell protein secretion by FSH and RSP additively increased by a factor of 2.82. The combined effect of FSH and RSP on total protein secretion from Sertoli cells was dose dependent and saturated at approximately 200 micrograms/ml of RSP. Polarity of total protein secretion from Sertoli cells (apical/basal ratio: 3.42) was stimulated by RSP predominantly in the apical direction (apical/basal ratio: 8.48). The modulation of radiolabeled Sertoli cell secretory proteins (ceruloplasmin, CP; sulfated glycoprotein-2, SGP-2; testins and transferrin, Tf) by cold (non-labeled) RSP was investigated by immunoprecipitation followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The secretion of CP, SGP-2 and Tf was stimulated in a dose-dependent manner by the addition of RSP up to a saturating concentration of between 200 and 300 micrograms/ml, whereas the secretion of Sertoli cell testins did not reach saturation at 300 micrograms/ml RSP. These results indicate that FSH and RSP independently modulate Sertoli cell protein secretion, and that Sertoli cell secretory proteins may differentially respond to RSP stimulation.

Regulation of Sertoli cell inhibin production and of inhibin alpha-subunit mRNA levels by specific germ cell types

To elucidate the endocrine and paracrine regulation of testicular inhibin production, the effects of follicle-stimulating hormone (FSH), (Bu)2cAMP, germ cells (either crude or enriched preparations) and germ cell-conditioned media on inhibin production (immuno- and bio-activities) and the levels of alpha- and beta B-subunit mRNAs were assessed in cultured Sertoli cells isolated from 20-day-old rats. FSH and (Bu)2-cAMP stimulated both secreted and intracellular inhibin levels in a dose-dependent manner. Using cDNA probes corresponding to the alpha-subunit and the beta B-subunit of rat inhibin it was also shown that both FSH and (Bu)2cAMP markedly increased the level of alpha-subunit mRNA but had no effect on the beta B-subunit mRNA. Addition of a crude mixture of germ cells to Sertoli cell monolayers was found to enhance inhibin secretion. Of the different germ cell fractions tested in co-culture, early spermatids reproducibly stimulated both basal and (Bu)2cAMP-induced production of inhibin whereas pachytene spermatocytes only increased the latter; cytoplasts from elongated spermatids (CES) had no effect. Co-culture of Sertoli cells with liver epithelial cells (LEC) significantly enhanced (Bu)2cAMP-induced inhibin levels. Media conditioned by early spermatids consistently and dramatically stimulated the secretion of both bioactive and immunoactive inhibin by Sertoli cells while spent media from pachytene spermatocytes displayed less activity. CES-conditioned media had only minor stimulatory effects, which may have resulted from the contamination of this fraction by spermatids. Media conditioned by LEC had no effect on inhibin production, confirming that the activity of this cell line is not mediated via a diffusible factor. Early spermatids were found to increase levels of the alpha-subunit mRNA. The current study provides evidence for the involvement of germ cells, in particular of early spermatids, in the local testicular regulation of inhibin gene expression and production in the rat. This may be of crucial importance for the ontogeny of this parameter of Sertoli cell function, and has important implications with regard to the postulated endocrine and paracrine roles of inhibin.

Influence of germ cells on Sertoli cell secretory activity in direct and indirect co-culture with Sertoli cells from rats of different ages

We examined the effect of direct and indirect Sertoli-germ cell co-culture on androgen binding protein (ABP) and transferrin (TRF) secretion by Sertoli cells (Sc) from 10-, 18-, and 26-day-old rats. Addition of germ cells (Gc), mainly (greater than 80%) pachytene spermatocytes, directly to Sc monolayers enhanced basal and follicle-stimulating hormone (FSH) + testosterone-stimulated ABP and TRF secretion at all three ages. When the Gc were co-cultured indirectly with Sc (separated by a Nucleopore filter), only 50% of the direct stimulatory effect was found at 18- and 26-day-old groups, whereas no difference between direct and indirect co-culture was noted with Sc from 10-day-old rats. With 18- and 26-day-old rat Sc, the Gc effect on ABP and TRF secretion declined after 6 days of Sc culture, reaching the level of Sc-only cultures after 10 days, whereas the direct effect was maintained throughout the entire culture period. With Sc from 10-day-old animals, both direct and indirect effect of Gc decreased after 6 days but the levels of ABP and TRF secretion remained above those of Sc-only cultures. The viability and number of Gc in indirect co-cultures were maintained significantly higher than in Gc-only control cultures. The direct and indirect Gc effect was completely reversed 48 h after the Gc were removed from Sc cultures of 18- and 26-day-old rats, whereas in Sc cultures from 10-day-old rats 40% of the stimulatory effect remained after 48 h of Gc removal. We conclude that Gc can influence Sc secretory activity through both direct contact and some released factor(s). These two pathways may have different relevance at different ages during sexual maturation.

Possible role of elongated spermatids in control of stage-dependent changes in the diameter of the lumen of the rat seminiferous tubule

Adult male rats were treated with a single dose of 650 mg/kg methoxy acetic acid to deplete the seminiferous tubules specifically of pachytene and later spermatocytes. The effect of this treatment and the subsequent maturation-depletion of later germ cell types on the diameter of the seminiferous tubule and its lumen and the area of the seminiferous epithelium were studied in relation to the stages of the spermatogenic cycle. At 21 days after methoxy acetic acid treatment, the diameter of the tubule and the area of the epithelium were reduced below control values at all stages, consistent with the reduced number of early (stage VIII) or late (all other stages) spermatids. Unexpectedly, diameter of the lumen was also reduced at all stages other than VIII, and especially at stage VII. In controls, lumen diameter at stages VII and VIII was increased by approximately 50% compared with earlier and later stages. In rats treated 21 days previously with methoxy acetic acid no change occurred at stage VII (lacking elongated spermatids) while a normal increase did occur at stage VIII (lacking round but not elongated spermatids). At earlier times after methoxy acetic acid treatment when stage VII tubules were depleted of pachytene spermatocytes alone (3 days) or together with early spermatids (7 days), the diameter of the lumen was not significantly different from the control value. These data suggest that lumen diameter may be regulated by elongated spermatids, especially at stages VII and VIII.