Purification of P26h: a hamster sperm protein

Testicular descent, sperm maturation and capacitation. Lessons from our most distant relatives, the monotremes

The present review examines whether monotremes may help to resolve three questions relating to sperm production in mammals: why the testes descend into a scrotum in most mammals, why spermatozoa are infertile when they leave the testes and require a period of maturation in the specific milieu provided by the epididymides, and why ejaculated spermatozoa cannot immediately fertilise an ovum until they undergo capacitation within the female reproductive tract. Comparisons of monotremes with other mammals indicate that there is a need for considerable work on monotremes. It is hypothesised that testicular descent should be related to epididymal differentiation. Spermatozoa and ova from both groups share many of the proteins that are thought to be involved in gamete interaction, and although epididymal sperm maturation is significant it is probably less complex in monotremes than in other mammals. However, the monotreme epididymis is unique in forming spermatozoa into bundles of 100 with greatly enhanced motility compared with individual spermatozoa. Bundle formation involves a highly organised interaction with epididymal proteins, and the bundles persist during incubation in vitro, except in specialised medium, in which spermatozoa separate after 2–3 h incubation. It is suggested that this represents an early form of capacitation.

Identification of a Hamster Sperm 26-Kilodalton Dehydrogenase/Reductase That Is Exclusively Localized to the Mitochondria of the Flagellum1

Sperm mitochondria undergo remodeling during posttesticular maturation that includes extensive disulfide cross-linking of proteins of the outer membrane to form the insoluble mitochondrial capsule. The relationship of these changes to mitochondrial function in mature gametes is unclear. The phospholipid hydroperoxide glutathione peroxidase (GPX4; also termed PHGPx) represents a major disulfide bond-stabilized protein of the mitochondrial capsule, and it is readily released by disulfide-reducing agents. However, in addition to GPX4, we detected a second major protein of 26 kDa (MP26) that was eluted from purified hamster sperm tails by the disulfide-reducing agent dithiothreitol. The objectives of the present study were to identify and characterize MP26 and to explore its potential role in mitochondrial function. Proteomic analysis of MP26 by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) identified 14 peptides with sequence identity to a member of the short-chain dehydrogenase/reductase superfamily termed P26h, which was implicated previously in hamster sperm-zona binding, and with high sequence similarity to mouse lung carbonyl reductase. Indirect immunofluorescence localized MP26 to the midpiece, and two-dimensional PAGE and immunoblot analysis identified a single MP26 isoform of pI 9.0. Immunoblot analyses of cauda epididymal fluid and of purified sperm plasma membranes and mitochondria revealed the exclusive localization of MP26 to the mitochondrial fraction. These data indicate that MP26 does not function in zona binding; instead, like GPX4, it may be associated with the mitochondrial capsule and play an important role in sperm mitochondrial function.

P26h and dicarbonyl/L-xylulose reductase are two distinct proteins present in the hamster epididymis

We have previously identified a 34 kDa protein (P34H) on the human sperm surface covering the acrosome. Using the hamster, we have also described a sperm protein, P26h, which is acquired by spermatozoa during epididymal transit. Both P34H and P26h belong to the carbonyl reductase family. Using molecular tools derived from P34H, we searched in the hamster epididymis for another protein related to the human sperm protein. Cloning and sequencing of P31h cDNA revealed 100% homology with the kidney DCXR (Dicarbonyl/L-Xylulose reductase). Northern Blot experiments revealed a single mRNA that was more expressed in the caput than in the corpus and cauda segment of adult epididymides. In situ hybridization was performed on sexually mature hamsters showing that the mRNA was localized in the principal cells throughout the epididymis. Using an anti-P34H antibody we have identified a P34H related protein named P31h (for 31 kDa). This protein showed 2D-electrophoretic behavior different from P26h and was detectable all along the epididymis (caput, corpus, and cauda) by Western Blot analysis. Immunohistochemistry techniques showed that P31h was localized in the perinuclear region of the principal cells of the epididymal epithelium within the three sections, both in sexually mature and immature animals. Results are discussed with regards to the potential function of DCXR in the epididymis.

Sperm-zona pellucida interaction involves a carbonyl reductase activity in the hamster

For successful fertilization to occur, the spermatozoa must transit through an egg-specific extracellular matrix or zona pellucida (zp) to reach and ultimately fuse with the oocyte plasma membrane. This process involves ligand-receptor recognition between the zp and the acrosomal cap of the sperm. The hamster sperm protein P26h, a receptor which is acquired during epididymal transit, has been suggested to act in sperm-zp binding. The cloning and characterization of the full-length cDNA-encoding hamster P26h revealed 85% identity with a porcine lung carbonyl reductase. To better understand the mechanism by which P26h interacts with zp proteins, we investigated carbonyl reductase activity during gamete interactions. In the present study, we show that specific inhibitors of carbonyl reductase such as diclofenac and phenylbutazone decreases sperm-zp binding without affecting the motility, progressivity or acrosome integrity of sperm. We also detected, and partly purified, carbonyl reductase activities from cauda epididymal sperm protein extract and this activity was associated with an enriched fraction of P26h. Removing P26h from the partly purified protein fractions by immunoaffinity chromatography led to the loss of carbonyl reductase activity. The findings that sperm-zp binding is blocked by carbonyl reductase inhibitors and that P26h is active in mature sperm suggest that P26h could play an important role in the fertilization process.

Differential extraction and enrichment of human sperm surface proteins in a proteome: identification of immunocontraceptive candidates

The objective of this study was to discover previously unknown human sperm surface proteins that may be candidate contraceptive vaccinogens. To this end, methods of concentrating human sperm proteins for microsequencing by mass spectrometry were used, which increased the likelihood of identifying surface proteins. Vectorial labeling, differential extraction and two-dimensional (2-D) gel electrophoresis were employed to identify and isolate proteins accessible at the cell surface. Percoll harvested or swim-up sperm were either solubilized directly or solubilized after surface labeling with sulfo-succinimidyl-6-(biotinamido)hexanoate (sulfo-NHS-LC-biotin). Comparisons were made of proteins extracted with four lysis buffers: (i) Celis buffer containing 9.8 M urea and 2% Igepal CA-630; (ii) 1% Triton X (TX)-100; (iii) 1.7% TX-114 followed by phase partitioning; or (iv) 1 M NaCl. Blots of proteins separated by high-resolution 2-D electrophoresis were probed with avidin and antibodies to known proteins specific for three domains: the sperm surface (SAGA-1), the acrosome (SP-10), and the cytoskeleton (alpha-tubulin). Celis buffer (45 min) extracted proteins from all three major compartments. However, a 20-s extraction in Celis buffer enriched for several proteins and enabled the identification of several novel peptides by mass spectrometry. Mild extraction with TX-100 or 1 M NaCl solubilized mainly membrane and acrosomal proteins, but not cytoskeletal proteins. Comparison of biotinylated proteins extracted by each method showed that the major vectorially labeled proteins solubilized by Celis buffer were also solubilized by TX-100, TX-114, and 1 M NaCl. Extraction with TX-114 followed by phase-partitioning significantly enriched hydrophobic surface proteins and aided resolution and isolation. Eight protein spots microsequenced following all these extraction methods proved to be novel sperm molecules.

Expression of the hamster sperm protein P26h during spermatogenesis

P26h is a hamster sperm protein of 26 kDa that has been previously characterized as a surface protein covering the acrosome acquired during epididymal transit. P26h is involved in sperm-egg interactions. Recently, it has been shown that the P26h transcript is highly expressed in the testis, and the P26h cDNA has been cloned from a hamster testicular cDNA library. Herein we report the production of a fusion protein (maltose binding protein-P26h) with the whole P26h cDNA encoding sequence and the production of a polyclonal antiserum against it. In Western blots, this antiserum recognized both the P26h extracted from cauda epididymal spermatozoa and the MBP-P26h. We also determined the age of appearance of P26h and which germ cell types express P26h mRNA and its translational product. Northern blots and in situ hybridization analysis showed that P26h transcripts appear at 3 wk of age, within the first round of spermatogenesis in the golden hamster. In situ hybridization showed that P26h transcripts are expressed in spermatocytes and round spermatids, whereas immunostaining revealed the presence of P26h in the cytoplasm of round spermatids and elongated spermatids. P26h was undetectable in testicular spermatozoa. Both in situ hybridization and immunostaining showed P26h expression to be dependent of the testicular cell type and the epithelium cycle. The implications for P26h in sperm-egg interaction and the testicular origin of P26h are discussed.

Prostasome-like particles are involved in the transfer of P25b from the bovine epididymal fluid to the sperm surface

In bulls, P25b is a sperm protein associated with the plasma membrane covering the acrosome. The amount of P25b bound to a constant number of spermatozoa varies from one individual to the other, low levels being associated with bull subfertility. In this study, we describe the epididymal origin of P25b using Western blot analysis. Whereas P25b was undetectable on caput spermatozoa, the amount of P25b associated to a constant number of spermatozoa increases from the corpus to the cauda. Prostasome-like particles were prepared by ultracentrifugation of epididymal fluid. P25b appears to be also associated with those membranous vesicles in increasing amounts along the epididymis. P25b is anchored to the plasma membrane of spermatozoa through glycosylphosphatidyl-inositol as shown by the ability of phospholipase C. but not of high salt treatment, to release P25b. Coincubation experiments revealed that prostasome-like particles are able to transfer P25b to spermatozoa, this process being more efficient at slightly acidic pH. P25b thus appears to be a marker of sperm epididymal maturation in bulls.

Hamster sperm protein, p26h: a member of the short-chain dehydrogenase/reductase superfamily

For successful fertilization to occur, mammalian spermatozoa must undergo a series of modifications in order to reach and penetrate the oocyte. Some of these modifications occur during passage through the epididymis, the site where spermatozoa acquire their fertilizing ability. We have previously described hamster sperm protein, P26h, which is acquired by spermatozoa during epididymal transit, and have proposed that this protein is involved in sperm-egg binding. In the present study, we report the cloning and characterization of the full-length cDNA encoding hamster P26h. A database search using the predicted hamster P26h amino acid sequence revealed 85% identity with mouse AP27 protein and porcine carbonyl reductase, members of the short-chain dehydrogenase/reductase (SDR) family of proteins. Northern blot analysis revealed a major P26h 1-kilobase transcript in the testis. No signal was detected in other somatic tissues of the hamster. In situ hybridization experiments revealed that the P26h gene was predominantly transcribed in seminiferous tubules of the testis and at a lower level in the corpus epididymidis. The identity of the cloned P26h was confirmed by immunoprecipitating in vitro-translated P26h using polyclonal antiserum raised against purified hamster sperm P26h. Taken together, these results identify P26h as a new member of the SDR family of proteins involved in the processes of mammalian gamete interactions.