Fucosylation of glycoproteins in rat spermatocytes and spermatids

Assessment of the degree of contamination of rat germ cell preparations using specific cDNA probes

Recent reports showing a decrease in sperm count in men have brought new concerns about male infertility. Animal models have been widely used to provide some relevant information about the human male gamete, and extrapolations are made to men and to the clinical context. The present-study assesses one of the methods used for separation of germ cells of the adult rat testis, namely centrifugal elutriation followed by density gradients (Percoll). This method was chosen since it presents the best results for cell purity in separating germ cells from the rat testis. A comparison between continuous and discontinuous Percoll gradients was performed in order to identify the best type of gradient to separate the cells. Maximal cell purity was obtained for spermatocytes (81 +/- 8.2%, mean +/- SEM) and spermatids (84 +/- 2.6%) using centrifugal elutriation followed by continuous Percoll gradients. A significant difference in purity was observed between elongating spermatids harvested from continuous Percoll gradients and from discontinuous gradients. Molecular analysis was used to assess cell contamination by employing specific probes, namely transition protein 2 (TP2), mitochondrial cytochrome C oxidase II (COX II), and sulfated glycoprotein 1 (SGP1). Molecular analysis of the samples demonstrated that morphological criteria are efficient in characterizing the main composition of the cell suspension, but are not reliable for identifying minimal contamination from other cells. Reliable cell purity data should be established using molecular analysis.

Activin receptor mRNA expression in rat testicular cell types

cDNA encoding the extracellular domain of the rat activin receptor was cloned using the polymerase chain reaction (PCR). This cDNA is highly homologous to cDNA encoding the extracellular domain of the mouse activin receptor, whereas at the protein level the extracellular domains of both receptors are identical. Employing this cDNA as a probe in Northern blot analysis, expression of two activin receptor mRNAs (6 kb and 4 kb) was observed, in testes of immature and mature rats. Between day 21 and 28 of postnatal development, a large increase in testicular expression of the 4 kb mRNA was found, suggesting expression of this activin receptor mRNA in germ cells. The 4 kb mRNA was indeed present in isolated pachytene spermatocytes and round spermatids, but was absent in elongating spermatids. Sertoli cells obtained from immature and mature rats expressed both the 6 kb and 4 kb mRNAs, whereas the expression of these mRNAs in Leydig cell preparations was very low. These results may imply that activin has multiple actions in the control of testicular function.

A new rat gene RT7 is specifically expressed during spermatogenesis

We report the isolation of a new rat male germ cell-specific gene, RT7, by differential cDNA cloning procedures. The RT7 cDNA nucleotide sequence is not homologous to any sequences present in the GenBank library. RT7 RNA is expressed at very high levels in rat early spermatids, while its expression is not detectable in any other organ or tissue examined. Mapping of the RT7 transcription start site by two independent procedures demonstrated that RT7 has two major and a number of upstream minor start sites for transcription. RT7 encodes a putative 90-amino acid protein, of which the N-terminus is predicted to fold as an amphipathic alpha helix with features resembling the leucine zipper structure found in a family of transcription factors. However, unlike the leucine zipper proteins the RT7 alpha helix is not preceded by a basic region. Analysis of the RT7 promoter sequence indicates that it contains a putative testis-specific regulatory sequence found in protamine P1 and P2 promoters, as well as binding sites for several other transcription factors.

Characterization of fucosyltransferase activity during mouse spermatogenesis: evidence for a cell surface fucosyltransferase

Fucosyltransferase activity was quantified in mouse germ cells at different stages of spermatogenesis. Specifically, fucosyltransferase activities of pachytene spermatocytes, round spermatids, and cauda epididymal sperm were compared. Fucosyltransferase activity of mixed germ cells displayed an apparent Vmax of 17 pmol (mg of protein)-1 min-1 and an apparent Km of approximately 13 microM for GDP-L-[14C]fucose in the presence of saturating amounts of asialofetuin at 33 degrees C. Under these conditions, cellular fucosyltransferase activity was found to increase during spermatogenesis. In agreement with assays of intact cells, examination of subcellular fractions indicated that a large fraction of fucosyltransferase activity was associated with the cell surface. The fraction of fucosyltransferase activity that was associated with the cell surface progressively increased throughout spermatogenesis and epididymal maturation so that nearly all of the fucosyltransferase in epididymal sperm was on the cell surface. Specifically, by comparison of activities in the presence and absence of the detergent NP-40, the fraction of fucosyltransferase activity that was associated with the cell surface in pachytene spermatocytes, round spermatids, and epididymal sperm was 0.36, 0.5, and 0.85, respectively. These results suggest that a cell surface fucosyltransferase may be important during differentiation of spermatogenic cells in the testis as well as during epididymal maturation and fertilization.

Fucosylation events during mammalian spermatogenesis

It will be necessary to conduct further studies to establish more precisely the localization of FT on mouse male germ cells. Antibodies to FTs are not yet available, so an immunocytochemical approach is not currently feasible. Additional cell fractionation protocols can be designed to compare plasma membrane fractions with enriched fractions of Golgi apparatus and to compare directly the activities of multiple glycosyltransferase enzymes and Golgi-specific markers in these preparations. Schachter et al. and Nyquist and colleagues have already provided experimental techniques for the isolation of Golgi fractions of good purity from rodent pachytene spermatocytes and spermatids. Ample opportunity exists, then, for a detailed analysis of the number, specificity, and localization of FT enzymes during mammalian spermatogenesis. All available data imply that these enzymes will prove to be vital components in the differentiation of cells within the seminiferous epithelium.

Distribution of glycoconjugates in human testis. A histochemical study using fluorescein- and rhodamine-conjugated lectins

Differentiation in the seminiferous epithelium involves the orderly transformation of germ cells into spermatozoa. We have employed ten fluorescein- and rhodamine-labeled lectins to visualize distinctive changes in the distribution of carbohydrate containing compounds during spermatogenesis and noticed the increase in RCA I, PNA, SBA and HPA binding sites during germ cell differentiation, suggesting the appearance of certain galactose and N-acetylgalactosamine containing glycoconjugates. Besides, in the cytoplasm of all germ cell types the positive reactions with Con A, LCA, WGA, LPA and UEA I indicate the presence of mannose, N-acetylglucosamine, sialic acid and fucose containing glycosubstances. Developing acrosomes demonstrated binding sites for most lectins, and particular HPA binding glycoconjugates were expressed in the equatorial segment region of late spermatids and testicular spermatozoa. In addition, the characteristic staining patterns of other testicular compartments are described. Our results suggest that human germ cells are rich in various carbohydrate containing compounds and there are specific alterations in cellular glycoconjugates during germ cell differentiation.

Ultrastructural localization of labeled acrosomal glycoproteins during in vivo fertilization in the rabbit

Rabbit spermatozoa were labeled predominantly in their acrosomal glycoproteins by 1-3H-glucosamine during spermiogenesis. Ova fertilized in vivo by spermatozoa labeled 22 days earlier were analyzed by fine-structure autoradiography for the localization of the label. The latter was found associated with 1) the fused membranes of the acrosomal cap remaining on the zona pellucida surface, 2) the material released on the zona surface after the acrosome reaction and possibly detectable after tannic acid fixation, 3) the equatorial segment of the sperm head and the preequatorial swellings, and 4) other sperm components, eg, the sperm tail. No labeling, on the other hand, was detected on the denuded leading edge of spermatozoa found either in the penetration slit or in the perivitelline space. Our observations suggest the involvement of acrosomal glycoproteins in different mechanisms of sperm/zona pellucida interaction but are not in favor of a major role of (enzymatic) glycoproteins bound to the inner acrosomal membrane during the penetration of the zona pellucida.

Testicular toxicity in vitro: Sertoli-germ cell co-cultures as a model system

Testicular damage is a comparatively common finding in toxicity studies. Part of the susceptibility of the testis to chemical toxicity stems from the complexity of its normal structure, function and regulatory control. This complexity has stimulated the development and extensive use of testicular cell cultures in the study of normal testicular function. However, the potential of such systems for investigating chemically induced testicular toxicity has only recently been considered. Using primary co-cultures of Sertoli and germ cells many features of the well characterized testicular damage produced by phthalate esters and glycol ethers could be reproduced in vitro. These encouraging findings are discussed in relation to some of the likely limitations to the application of in vitro models for testicular toxicity.

Comparison of lectin-staining pattern in testis and epididymis of gerbil, guinea pig, mouse, and nutria

The testis and epididymis of gerbil, guinea pig, nutria, and mouse were studied after staining with seven rhodamine-conjugated lectins to disclose the distribution of glycoproteins with different sugar residues. In the testis, the lectins showed a variable affinity for Leydig cells, tubular basement membrane, cytoplasm, acrosome, and plasma membrane of maturing spermatids as well as for Sertoli cell extensions. During acrosomal development, the staining pattern showed characteristic changes with different lectins indicating a gradual processing of the glycoprotein components. The staining in the Sertoli cell extensions displayed a cyclic change linked with the release of spermatozoa. A nuclear staining was prominent in zygotene and pachytene spermatocytes in the mouse, weak in the nutria, but absent in gerbil and guinea pig. The principal cells of epididymis showed a lectin-stained Golgi region as well as a similar staining in the apical surface, microvilli, and tubular contents. This staining was most prominent in the caput/corpus regions with some interspecies differences indicating the epididymal areas active in secretion. Narrow cells active in absorption of testis-derived material were lectin-positive in the initial segment of mouse, gerbil, and nutria epididymis. Large light cells with a strong affinity for some lectins were found in the proximal cauda of gerbil and guinea-pig epididymis. In the nutria, corresponding cells were arranged as islands within the low epithelium. The distal cauda of mouse, gerbil, and nutria was the site for lectin-stained light cells interspersed among the low principal cells. It is concluded that the high and low light cells may be active in the absorption and phagocytosis of residual bodies/cytoplasmic droplets and surplus epididymal secretory material, respectively. Thus, labeled lectins formed a useful tool in the analysis of glycoprotein distribution, processing, secretion, absorption, and degradation in the male reproductive tissues.

Protein synthesis by isolated pachytene spermatocytes in the absence of Sertoli cells

Isolated rat pachytene spermatocytes were incubated in chemically defined medium supplemented with pyruvate and lactate, which are known to be essential energy substrates for these germ cells. Protein synthesis by the isolated cells was investigated by means of two-dimensional polyacrylamide gel electrophoresis. The electrophoretic patterns of (35S)-labeled proteins, synthesized by the pachytene spermatocytes during incubation in the presence of (35S)methionine either from 0-2 h or from 24-26 h after isolation, were almost completely identical. The patterns of newly synthesized proteins of freshly isolated spermatocytes and spermatids, however, showed several stage-specific proteins in addition to many proteins common to both spermatogenic cell types. Hence, it was concluded that a stage-specific pattern of protein synthesis can be maintained by pachytene spermatocytes during incubation for a period of 24 h in the absence of Sertoli cells but in the presence of a proper energy source.

The role of glucose, pyruvate and lactate in ATP production by rat spermatocytes and spermatids

The ATP content of pachytene spermatocytes and round spermatids, isolated from rat testes, was not maintained during incubation of the germ cells in the presence of glucose. Glucose was metabolized via glycolysis at a considerable rate, but the rate of oxidation of the resulting endogenous pyruvate in the mitochondria was too low to support fully ATP production. Exogenous pyruvate (0.25 mM) or exogenous L-lactate (3-6 mM), however, were effective energy substrates. The lactate dehydrogenase reaction in isolated germ cells favoured the rapid conversion of pyruvate to lactate, at the expense of reducing equivalents from mitochondrial NADH. Hence, to support ATP production by the germ cells via mitochondrial metabolism of endogenous pyruvate, a relatively high concentration of exogenous lactate may be essential. In the spermatogenic microenvironment in vivo, such high concentrations of lactate could result from the net production of lactate by Sertoli cells. The mitochondria of the isolated germ cells produced ATP probably at a close to maximal rate, and spermatogenesis therefore may be extremely sensitive to compounds which interfere with mitochondrial energy metabolism and respiratory control.

Evidence of protein secretion by cultured pachytene spermatocytes

Pachytene spermatocytes isolated from immature rat testis were cultured for 6 h in the presence of [35S]methionine and the macromolecules present in the culture medium were analyzed by one-dimensional and two-dimensional gel electrophoresis. The electrophoretic profiles obtained showed a limited number of polypeptides, some of them consisting of families of spots with the same molecular weight but a different isoelectric point. The reproducibility of the results and the unaltered metabolic activity of the cells during culture time, indicate that the macromolecules present in the medium do not represent degradative products of the cells. Part of the detected macromolecules are considered secreted proteins since the addition of monensin to the cells induces their disappearance from the culture medium.