Differential expression of lysosomal associated membrane protein (LAMP-1) during mammalian spermiogenesis

Cascading effects of hypobaric hypoxia on the testis: insights from a single-cell RNA sequencing analysis

Most mammals tolerate exposure to hypobaric hypoxia poorly as it may affect multiple regulatory mechanisms and inhibit cell proliferation, promote apoptosis, limit tissue vascularization, and disrupt the acid-base equilibrium. Here, we quantified the functional state of germ cell development and demonstrated the interaction between the germ and somatic cells via single-cell RNA sequencing (scRNA-seq). The present study elucidated the regulatory effects of hypobaric hypoxia exposure on germ cell formation and sperm differentiation by applying enrichment analysis to genomic regions. Hypobaric hypoxia downregulates the genes controlling granule secretion and organic matter biosynthesis, upregulates tektin 1 (TEKT1) and kinesin family member 2C (KIF2C), and downregulates 60S ribosomal protein 11 (RPL11) and cilia- and flagella-associated protein 206 (CFAP206). Our research indicated that prosaposin-G protein-coupled receptor 37 (PSAP-GPR37) ligands mediate the damage to supporting cells caused by hypobaric hypoxic exposure. The present work revealed that hypoxia injures peritubular myoid (PTM) cells and spermatocytes in the S phase. It also showed that elongating spermatids promote maturation toward the G2 phase and increase their functional reserve for sperm-egg binding. The results of this study provide a theoretical basis for future investigations on prophylactic and therapeutic approaches toward protecting the reproductive system against the harmful effects of hypobaric hypoxic exposure.

Inceptor facilitates acrosomal vesicle formation in spermatids and is required for male fertility

Spermatogenesis is a crucial biological process that enables the production of functional sperm, allowing for successful reproduction. Proper germ cell differentiation and maturation require tight regulation of hormonal signals, cellular signaling pathways, and cell biological processes. The acrosome is a lysosome-related organelle at the anterior of the sperm head that contains enzymes and receptors essential for egg-sperm recognition and fusion. Even though several factors crucial for acrosome biogenesis have been discovered, the precise molecular mechanism of pro-acrosomal vesicle formation and fusion is not yet known. In this study, we investigated the role of the insulin inhibitory receptor (inceptor) in acrosome formation. Inceptor is a single-pass transmembrane protein with similarities to mannose-6-phosphate receptors (M6PR). Inceptor knockout male mice are infertile due to malformations in the acrosome and defects in the nuclear shape of spermatozoa. We show that inceptor is expressed in early spermatids and mainly localizes to vesicles between the Golgi apparatus and acrosome. Here we show that inceptor is an essential factor in the intracellular transport of trans-Golgi network-derived vesicles which deliver acrosomal cargo in maturing spermatids. The absence of inceptor results in vesicle-fusion defects, acrosomal malformation, and male infertility. These findings support our hypothesis of inceptor as a universal lysosomal or lysosome-related organelle sorting receptor expressed in several secretory tissues.

Human globozoospermia-related genes and their role in acrosome biogenesis

The mammalian acrosome is a secretory vesicle attached to the sperm nucleus whose fusion with the overlying plasma membrane is required to achieve fertilization. Acrosome biogenesis starts during meiosis, but it lasts through the entire process of haploid cell differentiation (spermiogenesis). Acrosome biogenesis is a stepwise process that involves membrane traffic from the Golgi apparatus, but it also seems that the lysosome/endosome system participates in this process. Defective sperm head morphology is accompanied by defective acrosome shape and function, and patients with these characteristics are infertile or subfertile. The most extreme case of acrosome biogenesis failure is globozoospermia syndrome, which is primarily characterized by the presence of round-headed spermatozoa without acrosomes with cytoskeleton defects around the nucleus and infertility. Several genes participating in acrosome biogenesis have been uncovered using genetic deletions in mice, but only a few of them have been found to be deleted or modified in patients with globozoospermia. Understanding acrosome biogenesis is crucial to uncovering the molecular basis of male infertility and developing new diagnostic tools and assisted reproductive technologies that may help infertile patients through more effective treatment techniques. This article is categorized under: Reproductive System Diseases > Environmental Factors Infectious Diseases > Stem Cells and Development Reproductive System Diseases > Molecular and Cellular Physiology.

The stallion sperm acrosome: Considerations from a research and clinical perspective

During the fertilization process, the interaction between the sperm and the oocyte is mediated by a process known as acrosomal exocytosis (AE). Although the role of the sperm acrosome on fertilization has been studied extensively over the last 70 years, little is known about the molecular mechanisms that govern acrosomal function, particularly in species other than mice or humans. Even though subfertility due to acrosomal dysfunction is less common in large animals than in humans, the evaluation of sperm acrosomal function should be considered not only as a complementary but a routine test when individuals are selected for breeding potential. This certainly holds true for stallions, which might display lower levels of fertility in the face of "acceptable" sperm quality parameters determined by conventional sperm assays. Nowadays, the use of high throughput technologies such as flow cytometry or mass spectrometry-based proteomic analysis is commonplace in the research arena. Such techniques can also be implemented in clinical scenarios of males with "idiopathic" subfertility. The current review focuses on the sperm acrosome, with particular emphasis on the stallion. We aim to describe the physiological events that lead to the acrosome formation within the testis, the role of very specific acrosomal proteins during AE, the methods to study the occurrence of AE under in vitro conditions, and the potential use of molecular biology techniques to discover new markers of acrosomal function and subfertility associated with acrosomal dysfunction in stallions.

Mechanism of Acrosome Biogenesis in Mammals

During sexual reproduction, two haploid gametes fuse to form the zygote, and the acrosome is essential to this fusion process (fertilization) in animals. The acrosome is a special kind of organelle with a cap-like structure that covers the anterior portion of the head of the spermatozoon. The acrosome is derived from the Golgi apparatus and contains digestive enzymes. With the progress of our understanding of acrosome biogenesis, a number of models have been proposed to address the origin of the acrosome. The acrosome has been regarded as a lysosome-related organelle, and it has been proposed to have originated from the lysosome or the autolysosome. Our review will provide a brief historical overview and highlight recent findings on acrosome biogenesis in mammals.

Involvement of a P2X7 Receptor in the Acrosome Reaction Induced by ATP in Rat Spermatozoa

The acrosome reaction (AR) is the exocytosis of the acrosomal vesicle in response to different physiological and non-physiological stimuli. Particularly in mammals, the AR is needed for sperm to fuse with the oocyte plasma membrane, and it occurs only in capacitated sperm. Previous evidence in the literature indicates that extracellular ATP induces the AR in capacitated human and bovine spermatozoa, but its receptor has not yet been identified. The aim of this work was to define a putative ATP receptor in rat spermatozoa using pharmacological and biochemical approaches. We found that ATP induced the AR only in capacitated rat spermatozoa, which was inhibited in the presence of two general inhibitors of ATP receptors (P2 receptors), Suramin, and oxidized ATP (oATP), and one inhibitor of P2X receptor (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid [PPADS]). In addition, the AR induced by ATP in capacitated rat spermatozoa was inhibited by brilliant blue-G (BB-G) and 17-β-oestradiol, two blockers of P2X7 receptors. Moreover, the ATP analog 2'(3')-O-(4-benzoylbenzoyl) ATP (BzATP) was almost 500 times more potent than ATP to induce the AR, which agrees with the pharmacology of a P2X7 receptor. Here, we show the presence of P2X7 receptor by Western blot and its localization in the tail and acrosome by indirect immunofluorescence. Finally, we quantify the presence of ATP in the rat oviduct during the estrous cycle. We found that the ATP concentration within the lumen of the oviduct is similar to those required to induce acrosome reaction, which agree with its role during in vivo fertilization. Therefore, our results strongly suggest that ATP induces the AR in capacitated rat spermatozoa through a P2X7 receptor, which may be functional during in vivo fertilization.

"Late" macroendosomes and acidic endosomes in vertebrate motor nerve terminals

Activity at the vertebrate nerve-muscle synapse creates large macroendosomes (MEs) via bulk membrane infolding. Visualized with the endocytic probe FM1-43, most (94%) of the ∼25 MEs/terminal created by brief (30-Hz, 18-second) stimulation dissipate rapidly (∼1 minute) into vesicles. Others, however, remain for hours. Here we study these "late" MEs by using 4D live imaging over a period of ∼1 hour after stimulation. We find that some (51/398 or 13%) disappear spontaneously via exocytosis, releasing their contents into the extracellular milieu. Others (at least 15/1,960 or 1%) fuse or closely associate with a second class of endosomes that take up acidophilic dyes (acidic endosomes [AEs]). AEs are plentiful (∼47/terminal) and exist independent of stimulation. Unlike MEs, which exhibit Brownian motion, AEs exhibit directed motion (average, 83 nm/sec) on microtubules within and among terminal boutons. AEs populate the axon as well, where movement is predominantly retrograde. They share biochemical and immunohistochemical markers (e.g., lysosomal-associated membrane protein [LAMP-1]) with lysosomes. Fusion/association of MEs with AEs suggests a sorting/degradation pathway in nerve terminals wherein the role of AEs is similar to that of lysosomes. Based on our data, we propose that MEs serve as sorting endosomes. Thus their contents, which include plasma membrane proteins, vesicle proteins, and extracellular levels of Ca(2+) , can be targeted either toward the reformation and budding of synaptic vesicles, toward secretion via exocytosis, or toward a degradation process that utilizes AEs either for lysis within the terminal or for transport toward the cell body.

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.

The Role of the Transmembrane RING Finger Proteins in Cellular and Organelle Function

A large number of RING finger (RNF) proteins are present in eukaryotic cells and the majority of them are believed to act as E3 ubiquitin ligases. In humans, 49 RNF proteins are predicted to contain transmembrane domains, several of which are specifically localized to membrane compartments in the secretory and endocytic pathways, as well as to mitochondria and peroxisomes. They are thought to be molecular regulators of the organization and integrity of the functions and dynamic architecture of cellular membrane and membranous organelles. Emerging evidence has suggested that transmembrane RNF proteins control the stability, trafficking and activity of proteins that are involved in many aspects of cellular and physiological processes. This review summarizes the current knowledge of mammalian transmembrane RNF proteins, focusing on their roles and significance.

Historical survey on chromatoid body research

The chromatoid body (CB) is a male reproductive cell-specific organelle that appears in spermatocytes and spermatids. The cytoplasmic granule corresponding to the CB was first discovered some 130 years ago by von Brunn in 1876. Thirty years later the German term "chromatoide Körper" (chromatoid body) was introduced to describe this granule and is still used today. In this review, first, the results obtained by light microscopic studies on the CB for the first 60 years are examined. Next, many findings revealed by electron microscopic studies are reviewed. Finally, recent molecular cell biological studies concerning the CB are discussed. The conclusion obtained by exploring the papers on CB published during the past 130 years is that many of the modern molecular cell biological studies are undoubtedly based on information accumulated by vast amounts of early studies.

MARCH-XI, a novel transmembrane ubiquitin ligase implicated in ubiquitin-dependent protein sorting in developing spermatids

A mechanism by which ubiquitinated cargo proteins are sorted into multivesicular bodies (MVBs) from plasma and trans-Golgi network (TGN) membranes is well established in yeast and mammalian somatic cells. However, the ubiquitin-dependent sorting pathway has not been clearly defined in germ cells. In this study we identified a novel member of the transmembrane RING-finger family of proteins, termed membrane-associated RING-CH (MARCH)-XI, that is expressed predominantly in developing spermatids and weakly in brain and pituitary. MARCH-XI possesses an E3 ubiquitin ligase activity that targets CD4 for ubiquitination. Immunoelectron microscopy of rat round spermatids showed that MARCH-XI is localized to TGN-derived vesicles and MVBs. Fluorescence staining of rat round spermatids and immunoprecipitation of rat testis demonstrated that MARCH-XI forms complexes with the adaptor protein complex-1 and with fucose-containing glycoproteins including ubiquitinated forms. Furthermore, the C-terminal region of MARCH-XI mediates its interaction with mu1-adaptin and Veli through a tyrosine-based motif and a PDZ binding motif, respectively. Our data suggest that MARCH-XI acts as a ubiquitin ligase with a role in ubiquitin-mediated protein sorting in the TGN-MVB transport pathway, which may be involved in mammalian spermiogenesis.

RNF151, a testis-specific RING finger protein, interacts with dysbindin

RING finger proteins play important roles in spermatogenesis. Here, we report that a novel RING finger protein RNF151, with a C3HC4-type RING finger domain, a putative nuclear localization signal (NLS), and a TRAF-type zinc finger domain, was exclusively expressed in the mouse testis and developmentally regulated during spermatogenesis. While RNF151 mRNA was present in round spermatids, its protein was expressed in elongating spermatids of the stage VIII-IX seminiferous tubules. The NLS together with the RING domain were necessary and sufficient for the nuclear localization of RNF151-EGFP in transfected cells. Yeast two-hybrid screening identified the physical interaction of mouse RNF151 and dysbindin, which was confirmed by the co-immunoprecipitation of the proteins and by their co-localization in intact cells. As dysbindin has lately been shown to be involved in membrane biogenesis and fusion, a key process for acrosome formation, we propose that RNF151 may play a role in acrosome formation.

Dynamic distribution of Spatial during mouse spermatogenesis and its interaction with the kinesin KIF17b

The Spatial gene is expressed in highly polarized cell types, such as epithelial cells in the thymus, neurons in the brain and germ cells in the testis. In this study, we report the characterization and distribution of Spatial proteins during mouse spermatogenesis. Besides Spatial-epsilon and -delta, we show that the newly described short isoform Spatial-beta is expressed specifically in round spermatids. Using indirect immunofluorescence, we detected Spatial in the cytosol of the early round spermatid. By the end stages of round spermatids, Spatial is concentrated at the opposite face of the acrosome near the nascent flagellum and in the manchette during the elongation process. Finally in mature sperm, Spatial persists in the principal piece of the tail. Moreover, we found that Spatial colocalizes with KIF17b, a testis-specific isoform of the brain kinesin-2 motor KIF17. This colocalization is restricted to the manchette and the principal piece of the sperm tail. Further, coimmunoprecipitation experiments of native proteins from testis lysates confirmed Spatial-KIF17b association through the long Spatial-epsilon isoform. Together, these findings imply a function of Spatial in spermatid differentiation as a new cargo of kinesin KIF17b, in a microtubule-dependent mechanism specific to the manchette and the principal piece of the sperm tail.

The mammalian acrosome as a secretory lysosome: new and old evidence

The morphological and biochemical characteristics of the acrosome depart well from any other vesicles in somatic cells, making it one of a kind amongst secretory vesicles. The components of the acrosome include a mixture of unique enzymes like acrosin and other enzymes that when present in somatic cells are commonly found in lysosomes, peroxisomes, and even in the cytoplasm. Several observations have pointed out that acrosomal biogenesis has unique features not previously described in secretory vesicle biogenesis of somatic cells. In this review we discuss the evidence supporting a molecular link between the machinery involved in lysosome and acrosome biogenesis, link which may help account for the acrosome unique composition.

Afaf, a novel vesicle membrane protein, is related to acrosome formation in murine testis

As a cell-specific organelle, acrosome (Acr) and its formation are an important event for spermiogenesis. However, the Acr formation is far more complicated than has been proposed. In this study, we have cloned a novel membrane protein Afaf (Acr formation associated factor) that was expressed abundantly in the round spermatids, localized in the inner and outer membrane of forming Acrs, and declined in the maturing Acrs. In the transfected Hela cells, Afaf protein was localized in the plasma membrane, EEA1-positive early endosomes (EEs) and occasionally in the nuclei. Therefore, we propose that EEs and plasma membrane may be also directly involved in the Acr biogenesis.

Modulating expression of LAMPs and ABH histo-blood group antigens in normal and neoplastic human skin

Although the precise biological role of lysosomal membrane-associated glycoproteins (LAMPs) and ABH histo-blood group antigens (HBGAs) remains somewhat unclear, they are thought to be related to cell differentiation, cellular adhesion, and tumorigenesis. Here, we present the first comparative immunohistochemical study of both LAMPs and HBGAs in normal and neoplastic skin. Their localization is compared to that of high molecular weight cytokeratin and cytokeratin MNF 116. LAMPs and HBGA were differentially expressed in the normal stratified squamous epithelium, suggesting that they are involved in the initial steps of the differentiation process, whereas HBGAs are characteristic of terminal keratinocyte differentiation. No change in the reactivity for HBGA was detected in the stratified epithelium overlying squamous cell or basal cell carcinomas, whereas a considerable loss of LAMPs was detected. LAMPs were overexpressed in tumor cells, whereas HBGAs were lost in tumor zones of basocellular carcinomas. In spinocellular carcinomas, HBGAs were detected in tumor keratinocytes and in keratin pearls. These results provide new evidence for the differential expression of LAMPs and HBGAs in the normal stratified squamous epithelium, as well as the presence of a modulating reactivity in basocellular and spinocellular carcinomas, suggesting that these glycoproteins are involved in differentiation and tumorigenesis of human skin.

Expression of peroxisomal proteins provides clear evidence for the presence of peroxisomes in the male germ cell line GC1spg

Peroxisomes are cell organelles that perform multiple functions in the metabolism of lipids and of reactive oxygen species. They are present in most eukaryotic cells. However, they are believed to be absent in spermatozoa and they have never been described in male germ cells. We have used the immortalized germ cell line GC1spg to investigate the expression of peroxisomal proteins in germ cells of mice. The GC1spg cells represent the differentiation state of type B spermatogonia or preleptotene spermatocytes. We could show that peroxisomal membrane proteins like Pmp70 and Pex14p as well as peroxisomal matrix proteins like catalase or acyl CoA oxidase are expressed in GC1spg cells. All these proteins were colocalized in the same structures within the cells. Furthermore, by electron microscopy we have identified subcellular particles with an ultrastructural appearance that is characteristic of peroxisomes. This is the first report demonstrating the peroxisomal compartment in male germ cells of mice.