Changes in distribution of phosphomannosyl receptors during maturation of rat spermatozoa

Distribution of the Novel RFRP-3/receptors system in the epididymis of the seasonal desert rodent, Gerbillus tarabuli, during sexual activity

RFamide-related peptide (RFRP-3), the Mammalian ortholog of the Avian gonadotropin-inhibitory hormone (GnIH), is a novel neuropeptide known for its inhibitory regulatory effect on reproduction in various mammalian species. However, a stimulatory action has been reported. This paper aims to: i) study the histology of the epididymis (caput) of Gerbillus tarabuli during the breeding period; and ii) to determine the distribution of the "RFRP-3/receptors system" in the epididymis (caput) of this desert rodent during the active season, and thus, to inspect its potential local interfering in sperm maturation. For that, immunohistochemistry was performed to detect the epididymal immunolocalizations of the three molecules, RFRP-3, GPR147, and GPR74. This is the first report of the epididymis histology in Gerbillus tarabuli, as it is the first evidence of the existence of the RFRP-3/Receptor system in the same organ of the same species. During the breeding season, moderate immunostaining of the RFRP-3/receptors system was present in the caput epididymis' epithelial parts (basal and principal cells) and spermatozoa. In contrast, these three molecules were absent in the peritubular and muscle coat's myoid cells and of the interstitial part of the caput epididymis. The results suggest that the epididymis is a potential source of RFRP-3 in the desert Rodent, Gerbillus tarabuli, which may function as a paracrine and/or autocrine factor affecting the main epididymis' function: sperm maturation.

The human epididymis: its function in sperm maturation

BACKGROUND

Spermatozoa acquire their fertilizing ability and forward motility properties during epididymal transit. Our knowledge of gamete physiology is based on studies conducted in laboratory and domestic species; our knowledge of these processes in humans is limited. Medical indications for assisted reproductive technologies (ART) have progressed to include male infertility. Surgical procedures allow collection of spermatozoa from all along the human excurrent ducts, and the former have been used with some success in reproductive medicine. This has raised questions over the role of the epididymis in human sperm physiology.

OBJECTIVE AND RATIONALE

To reanalyze what we now know about epididymal physiology in humans and to assess the relevance of laboratory animal models for understanding human physiology and the pathophysiology of the epididymis.

SEARCH METHODS

A systematic bibliographic search of PubMed for articles published in English before May 2015 was carried out using the search terms 'epididymis' and 'sperm maturation'. Literature on the consequences of vasectomy on the epididymis was also searched.

OUTCOMES

Whereas the proximal epididymis is almost exclusively occupied by efferent ducts, the sperm reservoir capacity is poorly developed in humans. At the molecular level, the human transcriptome and proteome show some segment specificity; conflicting results persist with regard to secretome variation along the tubule. The number of genes regulated along the excurrent ducts in men is lower when compared to rodent species, but remains significant. It is challenging to reconcile biochemical and physiological studies with clinical data obtained from men undergoing reanastomosis of the vas deferens at different points along the excurrent duct. We propose that vasectomy/vasovasostomy is a model to understand the consequences of obstruction on epididymis function in humans.

WIDER IMPLICATIONS

Despite the scarcity of biological material available, the interspecies variability of the male reproductive tract urges us to use modern molecular and cellular biology tools to better understand human epididymis physiology in order to apply ART in a more responsible manner.

P-Type Lectins: Cation-Dependent Mannose-6-Phosphate Receptor

In eukaryotic cells, post-translational modification of secreted proteins and intracellular protein transport between organelles are ubiquitous features. One of the most studied systems is the N-linked glycosylation pathway in the synthesis of secreted glycoproteins (Schrag et al. 2003). The N-linked glycoproteins are subjected to diverse modifications and are transported through ER and Golgi apparatus to their final destinations in- and outside the cell. Incorporation of cargo glycoproteins into transport vesicles is mediated by transmembrane cargo receptors, which have been identified as intracellular lectins. For example, mannose 6-phosphate receptors (Ghosh et al. 2003) function as a cargo receptor for lysosomal proteins in the trans-Golgi network, whereas ERGIC-53 (Zhang et al. 2003) and its yeast orthologs Emp46/47p (Sato and Nakano 2002) are transport lectins for glycoproteins that are transported out of ER.

Cholesterol content regulates acrosomal exocytosis by enhancing Rab3A plasma membrane association

The acrosome is an exocytic granule that overlies the spermatozoan nucleus. In response to different stimuli, it undergoes calcium-regulated exocytosis. Freshly ejaculated mammalian sperm are not immediately capable of undergoing acrosome reaction. The acquisition of this ability is called capacitation and involves a series of still not well-characterized changes in the sperm physiology. Plasma membrane cholesterol removal is one of the sperm modifications that are associated with capacitation. However, how sterols affect acrosomal exocytosis is unknown. Here, we show that short incubations with cyclodextrin, a cholesterol removal agent, just before stimulation promote acrosomal exocytosis. Moreover, the effect was also observed in permeabilized cells stimulated with calcium, indicating that cholesterol plays a direct role in the calcium-dependent exocytosis associated with acrosome reaction. Using a photo-inhibitable calcium chelator, we show that cholesterol affects an early event of the exocytic cascade rather than the lipid bilayers mixing. Functional data indicate that one target for the cholesterol effect is Rab3A. The sterol content does not affect the Rab3A activation-deactivation cycle but regulates its membrane anchoring. Western blot analysis and immunoelectron microscopy confirmed that cholesterol efflux facilitates Rab3A association to sperm plasma membrane. Our data indicate that the cholesterol efflux occurring during capacitation optimizes the conditions for the productive assembly of the fusion machinery required for acrosome reaction.

Expression and binding properties of the two mannose-6-phosphate receptors differ during perinatal development in rat liver

Mammalian tissues express both cation-dependent (CD-MPR) and cation-independent (CI-MPR) mannose-6-phosphate receptors, which mediate the targeting of acid hydrolases to lysosomes. The coexistence of the two receptors in all cell types and tissues is still poorly understood. To determine whether these receptors might play a role in maturation, we studied their expression and binding properties in rat liver during perinatal development. CI-MPR expression decreases progressively from 18-day fetuses to adults, whereas the CD-MPR showed a transient decrease in newborn and at the 5th day after birth. Immunostaining of the tissues showed that both receptors localize to hepatocytes at all the ages and, additionally, the CD-MPR was reactive in megakaryocytes at early stages. Binding assays showed differences in the B(max) and K(D) values between the ages studied. These results demonstrate that both receptors change differentially during perinatal development, suggesting that they play distinct roles during organ maturation.

Mannose-6-phosphate receptors as a molecular indicator of maturation of epididymal sperm

This review discusses some of the changes in sperm during maturation within the context of current concepts of membrane structure and fertilization. Mammalian sperm are surrounded by a limiting plasma membrane that undergoes remodeling during passage through the epididymis. This process confers on the gamete vigorous motility and the ability to fertilize the egg. The repositioning of some surface proteins may follow redistribution of lipids in the plasmalemma, and thus represent a critical step in the maturation of the gametes. Among the various affected proteins of the sperm plasmalemma, mannose-6-phosphate receptors undergo redistribution as the gametes transit through the epididymal duct. The authors summarize their studies of the redistribution of phosphomannosyl receptors during maturation of sperm and discuss possible roles of these glycoproteins in the fertilizing capability of sperm.