A specific androgen-binding protein (ABP) in Necturus testis and its zonal distribution

Primordial germ cells (spermatogonial stem cells) of bullfrogs express sex hormone-binding globulin and steroid receptors during seasonal spermatogenesis

In vertebrate species, testosterone seems to inhibit spermatogonial differentiation and proliferation. However, this androgen can also be converted, via aromatase, into estrogen which stimulates spermatogonial differentiation and mitotic activity. During seasonal spermatogenesis of adult bullfrogs Lithobates catesbeianus, primordial germ cells (PGCs) show enhanced testosterone cytoplasm immunoexpression in winter; however, in summer, weak or no testosterone immunolabelling was observed. The aim of this study was to confirm if PGCs express stem cell markers - alkaline phosphatase (AP) activity and GFRα1 (glial-cell-line-derived neurotrophic factor) - and verify whether testosterone is maintained in these cells by androgen receptors (ARs) and/or sex hormone-binding globulin (SHBG) in winter. Furthermore, regarding the possibility that testosterone is converted into estrogen by PGCs in summer, the immunoexpression of estrogen receptor (ER)β was investigated. Bullfrog testes were collected in winter and in summer and were embedded in glycol methacrylate for morphological analyses or in paraffin for the histochemical detection of AP activity. GFRα1, AR, SHBG and ERβ expression were detected by Western blot and immunohistochemical analyses. The expression of AP activity and GFRα1 in the PGCs suggest that these cells are spermatogonial stem cells. In winter, the cytoplasmic immunoexpression of ARs and SHBG in the PGCs indicates that testosterone is maintained by these proteins in these cells. The cytoplasmic immunoexpression of ERβ, in summer, also points to an ER-mediated action of estrogen in PGCs. The results indicate a participation of testosterone and estrogen in the control of the primordial spermatogonia during the seasonal spermatogenesis of L. catesbeianus.

The sea lamprey (Petromyzon marinus) has a receptor for androstenedione

The use of nuclear steroid receptors as ligand-activated transcription factors is a critical event in vertebrate evolution. It is believed that nuclear steroid receptors arose at or before the vertebrate radiation, except for an androgen receptor (Ar) that evolved only in the gnathostome line. We report an androgen-Ar complex in the male sea lamprey (Petromyzon marinus), an extant jawless vertebrate. The androgen with the highest affinity is not testosterone, but its direct precursor, androstenedione (Ad). To establish that the binding moiety in lamprey testis is a receptor-and not an "androgen-binding protein"-we have shown that it can be extracted from the nucleus as well as the cytosol, that the Ad-receptor complex binds to DNA, and that the receptor is approximately twice the size of an androgen-binding protein extracted from the Atlantic salmon testis. The capacity (and high affinity) of binding of the lamprey Ar is such that much of the Ad present in male lampreys becomes sequestered within the testis (as opposed to circulating in the plasma). Concentrations of Ad (but not of testosterone) in plasma and testis tissue are upregulated by injection of lamprey GnRH. Implantation of male lampreys with exogenous Ad significantly accelerates the development of the testis and growth of at least one secondary male characteristic. It appears that all classes of steroid hormones have contributed to the evolution of the regulatory complexity of steroid receptors found in modern vertebrates.

Testosterone-immunopositive primordial germ cells in the testis of the bullfrog, Rana catesbeiana

In amphibia, steroidogenesis remains quiescent in distinct seasonal periods, but the mechanism by which spermatogenesis is maintained under low steroidogenic conditions is not clear. In the present study, testosterone location in the testes of Rana catesbeiana was investigated immunohistochemically during breeding (summer) and nonbreeding (winter) periods. In winter, the scarce interstitial tissue exhibited occasional testosterone immunopositivity in the interstitial cells but the cytoplasm of primordial germ cells (PG cells) was clearly immunopositive. By contrast, in summer, PG cells contained little or no immunoreactivity whereas strong immunolabelling was present in the well-developed interstitial tissue. These results suggest that PG cells could retain testosterone during winter. This androgen reservoir could be involved in the control of early spermatogenesis in winter and/or to guarantee spermiogenesis and spermiation in the next spring/summer. The weak or negative immunoreaction in the summer PG cells might reflect consumption of androgen reservoir by the intense spermatogenic activity from spring to summer. Thus, besides acting as stem cells, PG cells of R. catesbeiana could exert an androgen regulatory role during seasonal spermatogenesis.

Glycoconjugates of the urodele amphibian testis shown by lectin cytochemical methods

Lectin histochemistry is a useful method that allows the in situ identification of the terminal sugar moieties of the carbohydrates that form the glycoconjugates. Moreover, when it is combined with chemical or enzymatic deglycosylation pretreatments, lectin histochemistry can be employed to determine if carbohydrates are linked to the protein core by means of an N- or O-glycosidic linkage or, indeed, to partially sequence the sugar chains. One of the most interesting model organs for the study of spermatogenesis is the amphibian urodele testis. However, this organ has not been very widely investigated with lectin histochemical research. In the last few years, we have carried out a research project to identify and locate glycoconjugates in the testis of the urodele Pleurodeles waltl, the Spanish newt, as a first approach to identify possible carbohydrates with key roles in spermatogenesis. Our findings reveal some glycan chains located in a fusome-like structure in early (diploid) germ cells, oligosaccharides with terminal GalNAc in the acrosome, the occurrence of glycan modifications in the acrosomal contents during spermiogenesis, and changes in glycan composition of follicle and interstitial cells during the spermatogenetic cycle. Furthermore, the similar labeling pattern of follicle and duct cells supports the hypothesis for a common origin of both cell types.

Glycan composition of follicle (Sertoli) cells of the amphibian Pleurodeles waltl. A lectin histochemical study

The glycan composition of the N- and O-linked oligosaccharides of the follicle (Sertoli) cells of the urodele amphibian Pleurodeles waltl testis were identified by lectin histochemistry, performed alone or in combination with enzymatic and chemical deglycosylation methods. The follicle cells were shown to contain: (1) Fuc, Galbeta(1,4)GlcNAc, GalNAc and Neu5Acalpha(2,3)Galbeta(1,4)GlcNAc in both N- and O-linked oligosaccharides; (2) Man in N-linked glycans; and (3) Galbeta(1,3)GalNAc in O-linked sugar chains. The follicle cells at the pre- and postmeiotic stages showed some differences in the UEA-1-positive Fuc characterisation, suggesting differences in the glycan composition. In addition, the sequence Neu5Acalpha(2,6)Gal/GalNAc was shown in the follicle cells only after spermiation, in the sperm-empty lobules of the developing glandular tissue. These results suggest that the follicle cells modify their glycoprotein content, probably for the performance of new roles, as the spermatogenetic cells develop. Thus the follicle cells surrounding male germ cells at different spermatogenetic stages would contain different glycoproteins involved in specific roles during male germ cell proliferation and maturation.

Identification of an androgen receptor in the zonal testis of the salamander (Necturus maculosus)

The testis of the salamander, Necturus maculosus, is advantageous for studying biochemical changes during spermatogenesis because germ cells and associated Sertoli and Leydig cells are topographically separated by stage of development. Using extracts of staged tissue samples and [3H]testosterone (T) in a standard binding assay, followed by Sephadex LH-20 or DNA-cellulose chromatography to separate free and bound steroid, we have identified a T-binding protein having physicochemical characteristics of a classical androgen receptor (AR): high affinity (Kd = 10(-9) M), limited capacity (Bmax) = 10(-10) M or 350 fmol/g tissue) and androgen specificity (T = 5 alpha - dihydrotestosterone greater than progesterone = corticosterone greater than estradiol). AR was present in nuclear extracts, where greater than 80% of binding sites were occupied by endogenous ligand, but was not detectable in cytosol. On linear sucrose gradients, nuclear AR sedimented at 3-4 S in both low and high ionic-strength buffers and, by this and other criteria, was distinguishable from the nonreceptor androgen binding protein (ABP) of the same species. The diffuse distribution of AR in germinal and nongerminal (glandular) tissues at all developmental stages is consistent with a dual localization in Sertoli cells and Leydig cells, as previously reported in mammals, and further suggests a regulatory role of androgen throughout spermatogenesis.

Autocrine and paracrine role of steroids during spermatogenesis: studies in Squalus acanthias and Necturus maculosus

Due to a cystic mode of spermatogenesis and zonal testes, the spiny dogfish shark (Squalus acanthias) and salamander (Necturus maculosus) are advantageous for studying the relationship between steroidogenesis and germ cell development. Direct analysis of steroidogenic enzymes and steroid receptors in staged tissue samples, together with light and electron microscopic observation, suggests that androgens and estrogens produced by Sertoli cells or Leydig cells immediately before or after spermiation serve as intratesticular regulators of events specific to premeiotic stages of spermatogenesis. These correlative data are consistent with limited observations in mammalian testis and provide a conceptual framework for direct testing of functional interactions between spermatogenesis and steroidogenesis in future studies.