Hormonal regulation of sulfated glycoprotein-1 synthesis by nonciliated cells of the efferent ducts of adult rats

Acquired Male Hypogonadism in the Post-Genomic Era-A Narrative Review

Although precision medicine took its first steps from genomic medicine, it has gone far beyond genomics, considering the full complexity of cellular physiology. Therefore, the present time can be considered as the "post-genomic era". In detail, proteomics captures the overall protein profile of an analyzed sample, whilst metabolomics has the purpose of studying the molecular aspects of a known medical condition through the measurement of metabolites with low molecular weight in biological specimens. In this review, the role of post-genomic platforms, namely proteomics and metabolomics, is evaluated with a specific interest in their application for the identification of novel biomarkers in male hypogonadism and in the identification of new perspectives of knowledge on the pathophysiological function of testosterone. Post-genomic platforms, including MS-based proteomics and metabolomics based on ultra-high-performance liquid chromatography-HRMS, have been applied to find solutions to clinical questions related to the diagnosis and treatment of male hypogonadism. In detail, seminal proteomics helped us in identifying novel non-invasive markers of androgen activity to be translated into clinical practice, sperm proteomics revealed the role of testosterone in spermatogenesis, while serum metabolomics helped identify the different metabolic pathways associated with testosterone deficiency and replacement treatment, both in patients with insulin sensitivity and patients with insulin resistance.

Prosaposin in the rat oviductal epithelial cells

Prosaposin (PSAP) has two forms: a precursor and a secreted form. The secreted form has neurotrophic, myelinotrophic, and myotrophic properties. The precursor form is a precursor protein of saposins A-D. Although the distribution of PSAP in male reproductive organs is well known, its distribution in female reproductive organs, especially in the oviduct, is unclear. Immunoblots and immunohistochemistry of oviducts showed that oviductal tissues contain PSAP proteins, and a significant increase in PSAP was observed in the estrus-metestrus phase compared to the diestrus-proestrus phase in the ampulla. To identify PSAP trafficking in cells, double-immunostaining was performed with antibodies against PSAP in combination with sortilin, mannose 6 phosphate receptor (M6PR), or low-density lipoprotein receptor-related protein 1 (LRP1). PSAP and sortilin double-positive reactions were observed near the nuclei, as well as in the apical portion of microvillous epithelial cells, whereas these reactions were only observed near the nuclei of ciliated epithelial cells. PSAP and M6PR double-positive reactions were observed near the nuclei of microvillous and ciliated epithelial cells. PSAP and M6PR double-positive reactions were also observed in the apical portion of microvillous epithelial cells. PSAP and LRP1 double-positive reactions were observed in the plasma membrane and apical portion of both microvillous and ciliated epithelial cells. Immunoelectron staining revealed PSAP immunoreactive small vesicles with exocytotic features at the apical portion of microvillous epithelial cells. These findings suggest that PSAP is present in the oviductal epithelium and has a pivotal role during pregnancy in providing an optimal environment for gametes and/or sperm in the ampulla.

The inactivation of the sortilin gene leads to a partial disruption of prosaposin trafficking to the lysosomes

Lysosomes are intracellular organelles which contain enzymes and activator proteins involved in the digestion and recycling of a variety of cellular and extracellular substances. We have identified a novel sorting receptor, sortilin, which is involved in the lysosomal trafficking of the sphingolipid activator proteins, prosaposin and GM(2)AP, and the soluble hydrolases cathepsin D, cathepsin H, and acid sphingomyelinase. Sortilin belongs to a growing family of receptors with homology to the yeast Vps10 protein, which acts as a lysosomal sorting receptor for carboxypeptidase Y. In this study we examined the effects of the sortilin gene inactivation in mice. The inactivation of this gene did not yield any noticeable lysosomal pathology. To determine the existence of an alternative receptor complementing the sorting function of sortilin, we quantified the concentration of prosaposin in the lysosomes of the nonciliated epithelial cells lining the efferent ducts. These cells were chosen because they express sortilin and have a large number of lysosomes containing prosaposin. In addition, the nonciliated cells are known to endocytose luminal prosaposin that is synthesized and secreted by Sertoli cells into the seminiferous luminal fluids. Consequently, the nonciliated cells are capable of targeting both exogenous and endogenous prosaposin to the lysosomes. Using electron microscope immunogold labeling and quantitative analysis, our results demonstrate that inactivation of the sortilin gene produces a significant decrease of prosaposin in the lysosomes. When luminal prosaposin was excluded from the efferent ducts, the level of prosaposin in lysosomes was even lower in the mutant mice. Nonetheless, a significant amount of prosaposin continues to reach the lysosomal compartment. These results strongly suggest the existence of an alternative receptor that complements the function of sortilin and explains the lack of lysosomal storage disorders in the sortilin-deficient mice.

Postnatal development and regulation of beta-hexosaminidase in epithelial cells of the rat epididymis

beta-Hexosaminidase (Hex) catalyzes the hydrolysis of terminal sugar residues from a number of substrates such as GM2 gangliosides, glycoproteins, glycolipids, and glycosaminoglycans. As an enzyme present in lysosomes of epithelial cells of the adult rat epididymis, it serves to degrade substances endocytosed from the epididymal lumen. In this way, it modifies and creates a luminal environment where sperm can undergo their maturational modifications. In this study, the postnatal developmental pattern of expression of Hex was examined in animals from days 7-56. In addition, the role of testicular factors on Hex expression in the different cell types and regions of the epididymis of adult rats was examined in orchidectomized and efferent duct-ligated rats. Both parameters were examined on Bouin-fixed epididymides in conjunction with light microscope immunocytochemistry. At postnatal day 7, the epithelium of the entire epididymis was unreactive for anti-Hex antibody. By day 21, narrow and clear cells of their respective regions became reactive, whereas basal cells became reactive only by day 29. Principal cells displayed only an occasional reactive lysosome at day 21, several by day 29, and numerous reactive lysosomes by day 39, comparable to the region-specific distribution noted for 90-day-old animals, and at an age when high androgen levels are attained. Thus, postnatal onset of Hex expression varies according to the different cell types of the epididymis, suggesting different regulatory factors. This finding was confirmed from studies employing adult orchidectomized and efferent duct-ligated adult rats. Indeed, in all experimental animals, Hex immunostaining in narrow, clear, and basal cells was intense and comparable to control animals. In contrast, there was a notable absence of lysosomal staining in principal cells at all time points after orchidectomy, which was restored, however, following testosterone replacement. No effect on Hex expression was observed in efferent duct-ligated animals. Taken together, the data suggest that Hex expression in lysosomes of principal cells is regulated by testosterone or one of its metabolites. However, the expression of Hex being independent of testicular factors in narrow, clear, and basal cells of adult animals, but occurring at different time points during postnatal development, suggests that different regulatory factors are responsible for onset of Hex expression in these cell types during development.

Regulation of sulfated glycoprotein-1 and cathepsin D expression in adult rat epididymis

Endocytosis, whereby proteins are internalized from the epididymal lumen to be eventually degraded in lysosomes, is one of the major functions of the epididymal epithelial cells in maintaining a proper luminal milieu conducive for sperm maturation. In the present study, using light microscope immunocytochemical methods, we examined the regulation of 2 lysosomal enzymes, sulfated glycoprotein-1 (SGP-1) and cathepsin D, in adult rat epididymides fixed in Bouin fixative and embedded in paraffin. After orchidectomy (O) with or without testosterone (T) supplementation, efferent duct ligation (EDL), or hypophysectomy (H), lysosomes of principal cells were intensely reactive with the anti-SGP-1 antibody, as were narrow, clear, and basal cells, with staining patterns similar to that of control animals. These experimental procedures also had no effect on cathepsin D expression in all cell types, except for clear cells of the corpus and cauda epididymidis, which after orchiedectomy and hypophysectomy, became intensely reactive, unlike their completely unreactive state in control animals. In O+T animals, as well as in EDL animals, clear cells remained unreactive. These data taken together suggest that expression of SGP-1 is not under the control of testicular or pituitary factors, as is also the case for cathepsin D expression by principal, narrow, and basal cells. However, specific inhibition of cathepsin D expression by testosterone or one of its metabolites appears to occur in clear cells of the corpus and cauda epididymidis. Furthermore, in addition to small, typical lysosomes, principal cells also revealed large supranuclear and infranuclear spherical structures that were immunoreactive with both anti-SGP-1 and anti-cathepsin D antibodies, suggesting their lysosomal nature. With electron microscopy, these structures appeared electron-lucent and contained membranous profiles embedded in an electron-dense, granular background. Such images suggest that the various experimental procedures adversely affect the expression of several other lysosomal enzymes in principal cells, leading to a lysosomal phenotype similar to that observed in various lysosomal storage diseases.

Identification of a novel sequence involved in lysosomal sorting of the sphingolipid activator protein prosaposin

Prosaposin is synthesized as a 53-kDa protein, post-translationally modified to a 65-kDa form and further glycosylated to a 70-kDa secretory product. The 65-kDa protein is associated to Golgi membranes and is targeted to lysosomes, where four smaller nonenzymatic saposins implicated in the hydrolysis of sphingolipids are generated by its partial proteolysis. The targeting of the 65-kDa protein to lysosomes is not mediated by the mannose 6-phosphate receptor. The Golgi apparatus appears to accomplish the molecular sorting of the 65-kDa prosaposin by decoding a signal from its amino acid backbone. This investigation deals with the characterization of the sequence involved in this process by deleting the saposin functional domains A, B, C, and D and the highly conserved N and C termini of prosaposin. The truncated cDNAs were subcloned into expression vectors and transfected to COS-7 cells. The destination of the mutated proteins was assessed by immunocytochemistry. Deletion of the C terminus did not interfere with the secretion of prosaposin but abolished its transport to lysosomes. Deletion of saposins and the N-terminal domain did not affect the lysosomal or secretory routing of prosaposin. A chimeric construct of albumin and the C terminus of prosaposin was not directed to lysosomes. However, albumin connected to the C terminus and one or more functional domains of prosaposin reached lysosomes, indicating that the C terminus and at least one saposin domain are required for this process. In summary, we are reporting a novel sequence involved in the targeting of prosaposin to lysosomes.

Cathepsin A is expressed in a cell- and region-specific manner in the testis and epididymis and is not regulated by testicular or pituitary factors

The epithelial cells of the testis are involved in the production, differentiation, and sustenance of sperm, and those of the epididymis play a major role in sperm maturation, protection, and storage. These tissues express various proteins that respond differently to androgens. Cathepsin A is a multifunctional lysosomal carboxypeptidase that also functions as a protective and an activator protein for neuraminidase and beta-galactosidase. In this study, cathepsin A was immunolocalized by light and electron microscopy using a polyclonal affinity-purified antibody on the testis and epididymis of normal, orchidectomized with or without testosterone supplementation, efferent duct-ligated, and hypophysectomized adult rats. In normal rats, cathepsin A expression was noted in lysosomes of Sertoli and Leydig cells but not in germ cells of the testis, as well as nonciliated cells of the efferent ducts. In the epididymis, a cell- and region-specific distribution of cathepsin A was noted. In experimentally treated animals, no changes were noted in the expression of cathepsin A. Immunolabeling of tissues examined at the electron microscopic level revealed that lysosomes were reactive. These data indicate cell- and region-specific expression of cathepsin A in cells of the testis and epididymis and also indicate that cathepsin A expression is not regulated by testicular or pituitary factors.

Structural analysis of the mouse prosaposin (SGP-1) gene reveals the presence of an exon that is alternatively spliced in transcribed mRNAs

SGP-1/prosaposin can be secreted or targeted to the lysosomes where it is processed into smaller saposins A, B, C, and D required for the hydrolysis of glycosphingolipids. The deficiency of saposins B and C results in variant forms of metachromatic leukodystrophy and Gaucher's disease, respectively, which are characterized by lysosomal storage of undegraded glycosphingolipids. A required step to correct these genetic defects, or to understand the targeting mechanism of SGP-1 to the lysosomes, or to the extracellular space as well as its interaction with specific glycosphingolipids, is the analysis of the gene encoding this protein. Thus our investigation dealt with the molecular cloning of the mouse SGP-1 gene. Sequence analysis revealed that the mouse SGP-1 gene consists of 15 exons ranging from nine base pairs to 298 base pairs and 14 introns, which ranged from 89 base pairs to >8 kb in length. Our data show that saposin A is encoded by the exons 3, 4, and 5, saposin B by exons 6, 7, 8, and 9, saposin C by exons 10 and 11, and saposin D by exons 12, 13, and 14. The translation start codon is located within exon 1, and the translation stop codon is located within exon 15. The exon/intron boundaries were in accordance to the AG/GT consensus sequences. Our data also revealed that the SGP-1 gene has an exon consisting of the nine base pairs (CAG GAT CAG) encoding the three amino acids of saposin B, which may be alternatively spliced in the SGP-1 mRNA. The presence of the different forms of alternatively spliced mRNAs in various tissues was analyzed by RT-PCR. This approach demonstrated that prosaposin mRNAs of brain, heart, and muscle contain the nine base pairs of exon 8, whereas the transcripts from testis, lung, pancreas, spleen, and kidney do not contain this exon 8. Sequence comparison between the human and mouse prosaposin showed that exon 11 of mouse SGP-1 consists of 279 base pairs, whereas the human prosaposin gene consists of 187 base pairs. The extra 93 base pairs encode 31 amino acids corresponding to a proline-rich region located between saposin C and saposin D in the mouse prosaposin molecule. Finally, the availability of these genomic clones provides a starting point for further studies on the genetic role of specific sequences on the structure and function of SGP-1/prosaposin and its derived saposin proteins. In conclusion, we cloned and sequenced the mouse prosaposin (SGP-1) gene. The structural analysis of this gene revealed the presence of an exon that is alternatively spliced in transcribed mRNAs in a tissue-specific manner.

Targeting of endogenous sulfated glycoprotein-1 and -2 to lysosomes within nonciliated cells of the efferent ducts during postnatal development of the rat

Sulfated glycoprotein (SGP)-1 and -2, secretory products of Sertoli cells, are secreted into the lumen of seminiferous tubules where they bind to late spermatids. Once released, the spermatozoa traverse the efferent ducts where these proteins detach from their surface and are endocytosed by the nonciliated cells. In adult animals, SGP-1 and SGP-2 are also synthesized by nonciliated cells and targeted from the Golgi apparatus to lysosomes. The purpose of the present study was to determine the pattern of expression of SGP-1 and SGP-2 within nonciliated cells during postnatal development. The efferent ducts of animals at different postnatal ages were prepared for an electron microscopic immunocytochemical quantitative analysis as well as for Northern blot analysis. The data expressed as labeling content (no. gold particles/micron 2 and taking into account the volume of the endocytic organelles and the cell) revealed that anti-SGP-1 labeling in endosomes of nonciliated cells was minimal at 15, 21, and 29 days of age. On the other hand, the lysosomal labeling content showed a significant increase by day 29 compared to 15 and 21-day-old animals indicating that an endogenous form of SGP-1 was being synthesized by nonciliated cells and targeted to lysosomes. By day 39 a significant increase in endosomal labeling occurred; this was attributed to the endocytosis of Sertoli-derived SGP-1 which coincided with the entry of spermatozoa into the lumen of these ducts at this age. Lysosomal labeling showed further significant increases at days 39, 49, and then again at day 90. Northern blot analysis detected SGP-1 mRNA transcripts at all postnatal ages examined. While decreases or increases in transcripts could not be determined due to the greater amount of tissue present with increasing age, these data taken together support the idea of an endogenous form of SGP-1 being synthesized by nonciliated cells and targeted to lysosomes during postnatal development. In the case of SGP-2, endosomal labeling was minimal at 15, 21, and 29 days of age but was significantly increased by day 39, with similar values at all subsequent ages. The high value at day 39 was attributed to the endocytosis of SGP-2 which coincided with the entry of spermatozoa into the lumen at this age. Lysosomal labeling, on the other hand, was low at days 15 and 21 but peaked at day 29 at a time when endosomal labeling was minimal. These results suggested the synthesis of an endogenous form of SGP-2 which was being targeted to lysosomes.(ABSTRACT TRUNCATED AT 250 WORDS)