Rat sperm galactosyl receptor: purification and identification by polyclonal antibodies raised against multiple antigen peptides

Role of integrins, tetraspanins, and ADAM proteins during the development of apoptotic bodies by spermatogenic cells

We have previously reported that Sertoli cell geometric changes induced by a Fas (CD95) agonist or by restricting Sertoli cell spreading can trigger spermatogenic cell detachment from Sertoli cell surfaces and initiate a programmed cell death sequence. Here, we have focused on ADAM proteins, tetraspanins CD9 and CD81, and the integrin beta1 subunit, which is co-expressed in testis with integrin alpha3 and integrin alpha6 subunits, to understand how these molecules may stabilize spermatogenic cell attachment to Sertoli cell surfaces. Like ADAM proteins, integrin beta1, alpha3, and alpha6 subunits, and CD9 and CD81 transcripts are expressed in the fetal testis and throughout testicular maturation, as well as, in Sertoli-spermatogenic cell co-cultures. Prespermatogonia (gonocytes) display CD9 and CD81 immunoreactive sites. Integrin alpha6 subunit transcripts have unusual developmental characteristics: fetal testis expresses the integrin alpha6B isoform exclusively. In contrast, the integrin alpha6B isoform co-exists with the integrin alpha6A isoform in prepubertal testes and Sertoli-spermatogenic cell co-cultures. A blocking anti body targeting the extracellular domain (N-terminal) of the integrin beta1 subunit causes rapid contraction of Sertoli cells leading to the gradual detachment of associated spermatogenic cells. In contrast, predicted active site peptides targeting the disintegrin domain of ADAM 1, ADAM 2, ADAM 3 (cyritestin), ADAM 4, ADAM 5, ADAM 6, and ADAM 15 (metragidin) do not disturb significantly the attachment of spermatogenic cells to Sertoli cell surfaces. Spermatogenic cells dislodged from their attachment sites by the integrin beta1 subunit blocking antibody display annexin V immunoreactivity, a sign of early apoptosis. Time-lapse videomicroscopy demonstrates that the removal by apoptosis of a single member of a spermatogenic cell cohort inter-connected by cytoplasmic bridges does not affect the remaining members of the cohort. During spermatogenic cell apoptosis, integrin beta1, alpha3, and alpha6 subunits, and tetraspanins CD9 and C81 become displaced away from the developing apoptotic bodies. In contrast, the intermediate filament protein Sak57, a keratin 5 ortholog, concentrates in the developing apoptotic bodies. We propose that the redistribution of integrin-tetraspanin complexes during spermatogenic cell apoptosis may be evidence of a signaling cascade initiated by Sertoli cell geometric changes. As a result, Sertoli cell reduction in surface area may be a limiting factor of spermatogenic cell survival and in the developmental regulation of spermatogenic cell progenies in the intact seminiferous epithelium.

The role of extrahepatic retinol binding protein in the mobilization of retinoid stores

Although the major tissue site of retinol binding protein (RBP) synthesis in the body is the liver, other sites of synthesis have been reported. The physiological role(s) of circulating RBP that is produced and secreted extrahepatically has not been systematically investigated. To address this question, we used as a model a mouse strain (hRBP(-/-)) that expresses human RBP (hRBP) cDNA under the control of the mouse muscle creatine kinase promoter in an rbp-null background (RBP(-/-)). By comparing hRBP(-/-), RBP(-/-), and wild-type mice, we asked whether extrahepatic RBP can perform all of the physiological functions of RBP synthesized in the liver. We demonstrate that extrahepatically synthesized hRBP, unlike RBP expressed in liver, cannot mobilize liver retinoid stores. Consistent with this conclusion, we find that circulating hRBP is not taken up by hepatocytes. RBP has been proposed to play an essential role in distributing hepatic retinoids between hepatocytes and hepatic stellate cells. We find, however, that the distribution of retinoid in the livers of the three mouse strains described above is identical. Thus, RBP is not required for intrahepatic transport and storage of retinoid. These and other observations are discussed.

Zona-binding inhibitory factor-1 from human follicular fluid is an isoform of glycodelin

Zona-binding inhibitory factor-1 (ZIF-1), a glycoprotein in human follicular fluid, reduces the binding of spermatozoa to the zona pellucida. ZIF-1 has a number of properties similar to those of glycodelin-A from human follicular fluid. The objective of this study was to compare the biochemical characteristics of these two glycoproteins. N-terminal sequencing and protease-digested peptide mapping showed that ZIF-1 and glycodelin-A have the same protein core. However, these glycoproteins differ in their oligosaccharide chains, as demonstrated by fluorophore-assisted carbohydrate electrophoresis, lectin-binding ability, and isoelectric focusing. ZIF-1 inhibited spermatozoa-zona pellucida binding slightly more than did glycodelin-A and significantly suppressed progesterone-induced acrosome reaction of human spermatozoa. Indirect immunofluorescence staining revealed specific binding of glycodelin-A and ZIF-1 to the acrosome region of human spermatozoa, where ZIF-1 produced a stronger signal than did glycodelin-A at the same protein concentration. These data suggest that ZIF-1 is a differentially glycosylated isoform of glycodelin that potently inhibits human sperm-egg interaction. Future study on the function role of ZIF-1 would provide a better understanding of the regulation of fertilization in humans.

Keratin 9 is a component of the perinuclear ring of the manchette of rat spermatids

Previous work in our laboratory has shown that a 62- to 64-kDa protein was a major component of the perinuclear ring of manchettes fractionated from rat spermatids. Mass spectrometry analysis of this protein indicated the presence of a glycine-rich domain homologous to human keratin 9 (K9). Several antibodies to K9, raised against synthetic peptides of human K9, recognized the 64- to 62-kDa protein in the perinuclear ring of the manchette as well as in keratinocytes of the suprabasal layer of the rat and human footpad/sole epidermis in both immunoblotting and immunocytochemical experiments. Based on these data, human-derived K9 primers were used to clone rat K9 cDNA from epidermis by RT-PCR. Rat-specific K9 primers were then used to perform a two-step (nested) PCR to amplify the K9-specific rat testicular RNA and to obtain cDNA to demonstrate K9 gene expression in rat testis. The deduced amino acid sequence of rat K9 cDNA contains 618 amino acids with an estimated molecular mass of 63,020 Da, in agreement with that obtained by electrophoretic fractionation of rat manchette and epidermis footpad proteins. The deduced protein structure correlates with the recognizable pattern of keratins: a rod domain of 304 amino acids with well-conserved initiation and termination sequences (MQNLNSRLASY and EIETYRKLLEG, respectively), flanked by glycine/serine-rich head and tail domains of 141 and 173 amino acids, respectively. A high content of phenylalanine was detected in the head domain and a repetitive motif (SGGSYGGGS) in the tail domain. A comparison with human keratin 9 showed an overall nucleotide and amino acid similarity of 75%. An increased level of K9 transcripts was detected in a cDNA library prepared from fractionated round spermatids. Results of this study show that rat testis expresses K9 and that this protein is a component the perinuclear ring of the manchette of rat spermatids.

Molecular cloning of rat sperm galactosyl receptor, a C-type lectin with in vitro egg binding activity

Rat sperm galactosyl receptor is a member of the C-type animal lectin family showing preferential binding to N-acetylgalactosamine compared to galactose. Binding is mediated by a Ca(2+)-dependent carbohydrate-recognition domain (CRD) identical to that of the minor variant of rat hepatic lectin receptor 2/3 (RHL-2/3). The molecular organization of the genomic DNA, cDNA, and derived amino acid sequence of rat testis galactosyl receptor have been determined and in vitro fertilization studies were conducted to ascertain its role. We have determined that the rat testis galactosyl receptor gene generates two mRNA species: one species, designated liver-type, is identical to RHL-2/3; the other, designated testis-type, contains one unspliced intron (86 nt) which alters the reading frame and changes the amino acid sequence of the carboxyl terminus. As a result, the CRD (glutamine-proline-aspartic acid/QPD) and flanked Ca(2+)-binding amino acid sequences were not present in the testis-type protein. Northern and Southern blots demonstrated presence of transcripts with unspliced intron in rat sperm but not liver. Similarly, antibody, raised against a synthetic 12-amino acid peptide (p12) encoded by the unspliced intron, recognized in immunoblots a 54 kDa receptor protein in protein extracts from testis but not from liver. Immunofluorescence and immunogold electron microscopy studies demonstrated that both protein species localized on the plasma membrane surface of the head and tail of rat sperm. Furthermore, capacitated rat sperm preincubated with polyclonal antisera to RHL-2/3 or to the CRD of the liver-type galactosyl receptor showed a statistically significant decrease in the in vitro fertilization rate. We conclude that rat sperm galactosyl receptor may play a role in egg binding and that an undetermined molecular mechanism operates to generate two proteins with identical intracellular amino terminal domain but only one of them displays a CRD and associated Ca(2+)-binding sites at the carboxyl terminal extracellular domain.

Galactosyl receptor, a cell surface C-type lectin of normal and tumoral prostate epithelial cells with binding affinity to endothelial cells

BACKGROUND

The mechanism of bone metastasis of prostate cancer involves the interaction of cell surface receptor(s) on cancer cells with ligand(s) on bone marrow endothelial cell surfaces. The rat galactosyl receptor gene generates two mRNA species by differential splicing: one species encodes a protein identical to the minor form of hepatocyte asialoglycoprotein receptor and displays a galactose/N-acetyl-galactosamine-recognition domain; the other encodes a protein with identical intracellular and transmembrane domains but with a different extracellular domain lacking the carbohydrate-recognition domain (CRD). Both proteins appear to coexist as a heterooligomer on the surface of normal mouse, rat, and human prostate epithelial cells and human prostate cancer cells, including the PC-3 cell line. The CRD of galactosyl receptor mediates adhesion of normal and tumoral prostate cells to the surfaces of a human bone marrow endothelial cell line. The use of inhibitors targeting the CRD would be very valuable in hindering the binding of prostate cancer cells to endothelial cells, thus decreasing the incidence of hematogenous metastasis to bone.

METHODS

Molecular biology, immunohistochemistry, flow cytometry, and a cell aggregation assay were used to determine the expression and role of the galactosyl receptor in cell adhesion.

RESULTS

Immunoblotting experiments demonstrated that each component of the heterooligomer has a mass of 54 kDa, ascribed in part to associated carbohydrates. An oligonucleotide probe showed the presence of both galactosyl receptor forms in rat prostate and testis, but not in liver, kidney, and spleen. Antibodies to the CRD and a segment of the nonhomologous extracellular domain of the galactosyl receptor blocked cell adhesion to endothelial cell monolayers.

CONCLUSIONS

The galactosyl receptor provides a valuable target for the development and use of synthetic ligands capable of disrupting endothelial cell-prostate cancer cell interaction, the first step in prostate cancer bone metastasis.

Spermadhesins: a new protein family. Facts, hypotheses and perspectives

Spermadhesins are a novel family of secretory proteins expressed in the male genital tract of pig, horse and bull. They are major products of the seminal plasma and have been found to be peripherally associated to the sperm surface. The structure and function of spermadhesins have been thoroughly investigated in the pig, which exhibits the greatest diversity of members: AWN, AQN-1, AQN-2, PSP-I and PSP-II and its glycosylated isoforms. They are multifunctional proteins showing a range of ligand-binding abilities, e.g. carbohydrates, sulfated glycosaminoglycans, phospholipids and protease inhibitors, suggesting that they may be involved in different steps of fertilization. Isolated porcine spermadhesins bind the zona pellucida glycoproteins in a cation-dependent manner with a Kd in a low micromolar range, and AWN, AQN-1 and AQN-3 display similar binding affinity for glycoproteins containing Gal beta(1-3)-GalNAc and Gal beta(1-4)-GlcNAc sequences in O-linked and N-linked oligosaccharides, respectively. During sperm passage through the epididymis AQN-3 and AWN have been shown to bind tightly to the sperm surface by interaction with the phospholipids of the membrane bilayer. At ejaculation the spermadhesins form a protective coat around the sensitive acrosomal region of the sperm head, thus possibly preventing premature acrosome reaction. During in vitro capacitation most of these aggregated sperm adhesins are lost, with the exception of phospholipid-bound spermadhesins. AWN and AQN-3 may now serve as a primary receptor for the oocyte zona pellucida, thus contributing to initial binding and recognition between sperm and egg. The amino acid sequence of spermadhesins does not show any discernible similarity with known carbohydrate recognition domains (CRD). However, they belong to the superfamily of proteins with a CUB domain with a predicted all-beta structure. The crystal structure of the heterodimeric complex of the spermadhesins PSP-I/PSP-II has been solved, showing that the overall structure of both spermadhesins consists of a beta-sandwich with five (parallel and antiparallel) beta-strands. It is the first three-dimensional structure of a zona pellucida-binding protein and reveals the architecture of the CUB domain. The spermadhesins represent a novel class of lectins that may be involved in sequential steps of fertilization, at least in the pig.

Isolation and characterization of a protein with homology to angiotensin converting enzyme from the periacrosomal plasma membrane of equine spermatozoa

The periacrosomal plasma membrane of spermatozoa is involved in sperm binding to oviductal epithelial cells and to the zona pellucida. A protein of 68-70 kD molecular mass was purified biochemically from the isolated periacrosomal plasma membrane of equine spermatozoa as a possible receptor for adhesion of spermatozoa to oviductal epithelial cells. A polyclonal antibody raised in rabbits against the purified equine sperm membrane protein recognized the 70 kD and an antigenically related to 32 kD protein in preparations of isolated periacrosomal sperm plasma membrane and in detergent extracted ejaculated and epididymal spermatozoa. A larger protein (approximately 110 kD) was detected in equine testis. Two antigenically related proteins (64 and 45 kD) were recognized on the plasma membrane of cynomolgus macaque spermatozoa. In vitro sperm-binding assays were performed in the presence of antigen-binding fragments or IgG purified from the polyclonal antiserum to investigate a possible function to the isolated protein in binding of equine spermatozoa to homologous oviductal epithelial cells or zona pellucida. Incubation with antigen-binding fragments or IgG purified from the antiserum did not inhibit binding of equine spermatozoa either to oviductal epithelial cells or the zona pellucida. On ultrastructural examination, the antibody bound exclusively to the cytoplasmic side of the periacrosomal plasma membrane of equine and macaque spermatozoa. Microsequence analysis of 13 residues of sequence showed strong homology with a number of angiotensin converting enzymes: An 84% identity was identified with testis specific and somatic forms of human and mouse angiotensin-converting enzyme. Immunocytochemistry and immunoblot analysis established that the protein is specific for the periacrosomal membrane of ejaculated, epididymal, and testicular stallion spermatozoa.

Sak 57, an intermediate filament keratin present in intercellular bridges of rat primary spermatocytes

We have previously reported the purification of Sak 57 (for spermatogenic cell/sperm-associated keratin of molecular mass 57 kDa) from outer dense fibers of rat sperm tails. Internal protein sequence analysis of Sak 57 revealed 70-100% homology to the 1A and 2A regions of the alpha-helical rod domain of human, mouse, and rat keratins. A multiple antigen peptide was synthesized using the KQYEDIAQK sequence corresponding to the 2A region and a polyclonal antibody was produced in rabbit to detect Sak 57. During spermiogenesis, Sak 57 associates with the microtubular manchette before becoming a component of para-axonemal keratin structures of the developing tail. We now report that during late meiotic prophase, intercellular bridges linking late pachytene-diplotene spermatocytes display a distinct ribbon containing a Sak 57/beta-tubulin complex, separated by a nonimmunoreactive midzone. Indirect immunofluorescence demonstrates that the ribbon is the final stage of a three-step developmental sequence: (1) a spindlelike arrangement radiating from equidistant spherical centers in early pachytene spermatocytes, (2) an ectoplasmic shell-like framework in mid-to-late pachytene spermatocytes, and (3) a Sak 57/beta-tubulin-containing ribbon found in intercellular bridges linking adjacent late pachytene-diplotene spermatocytes. Shear forces causing a breakdown of one of the conjoined spermatocytes do not disrupt the cytoskeletal ribbon. Results of this work, together with previous observations during spermiogenesis, show that Sak 57 associates with cytoplasmic microtubules in a timely fashion. Upon completion of late meiotic prophase, the Sak 57/microtubule complex behaves as an intercellular ligament and contributes to both the strength of intercellular bridges and the cohesiveness of members of a spermatocyte lineage.

Purification, partial characterization, and localization of Sak57, an acidic intermediate filament keratin present in rat spermatocytes, spermatids, and sperm

We have purified a 57 kDa protein (designated Sak57, for spermatogenic cell/sperm-associated keratin) from sodium dodecyl sulfate-beta-mercaptoethanol (SDS-beta ME)-dissociated outer dense fibers isolated from rat sperm tails. Internal protein sequence analysis of Sak57 yielded two 15-mer and 10-mer fragments with 70-100% homology to human, rat, and mouse keratins and corresponding to the 1A and 2A regions of the alpha-helical rod domain of keratins. A multiple antigenic peptide (MAP) was constructed using the 10-mer amino acid sequence KAQYEDIAQK (corresponding to the 2A region) and used as antigen for the production of polyclonal antibodies in rabbit. Anti-MAP sera were used for further analysis of the biochemical characteristics of Sak57 in testis and sperm tails using chromatofocusing, immunoblotting, and [32P] orthophosphate-labeling. We have found that rat testis displays two immunoreactive proteins: a soluble 83 kDa protein with pl range 5.9-6.3, regarded as a precursor, and both detergent-insoluble and soluble 57 kDa protein with pl range 5.0-5.9, corresponding to the mature form Sak57. The testicular soluble form was phosphorylated. Rat sperm tail samples displayed only the Sak57 detergent-insoluble form and its pl was more acidic (4.7-4.8). Whole-mount electron microscopy of negatively stained preparations of sperm-derived Sak57 resuspended in SDS-beta ME revealed a rod-shaped pattern. A decrease in the concentration of SDS-beta ME resulted in the side-by-side aggregation of rod-shaped Sak57 forming thick bundles. Indirect immunofluorescence was used to determine the localization of Sak57 in isolated outer dense fibers, epididymal sperm, spermatids, and pachytene spermatocytes. Confocal laser scanning microscopy was used to analyze the three-dimensional arrangement of Sak57 in pachytene spermatocytes. Isolated outer dense fiber and sperm tails displayed an immunoreactive product in the form of linear clusters. In elongating spermatids (steps 10-11), Sak57 immunoreactivity was predominant in the head region whereas pachytene spermatocytes displayed a cortical cytoplasmic distribution. Results of this study demonstrate that Sak57 has the characteristics of a keratin intermediate filament and is present during meiotic and postmeiotic stages of spermatogenesis.

Sak57, an acidic keratin initially present in the spermatid manchette before becoming a component of paraaxonemal structures of the developing tail

We have previously reported that Sak57 (for Spermatogenic cell/Sperm-associated keratin of molecular mass 57 kDa) is an acidic keratin found in rat spermatocytes, spermatids, and sperm. Sak57 displays conserved amino acid sequences found in the 1A and 2A regions of the alpha-helical rod domain of keratins in human, rat, and mouse. We now report indirect immunofluorescence, confocal laser scanning microscopy and immunogold electron microscopy data showing that Sak57 is associated with the microtubular mantie of the manchette, a transient microtubular structure largely regarded as formed by tubulin and microtubule-associated proteins. The immunocytochemical localization of Sak57 was detected with a polyclonal antiserum to a multiple antigenic peptide (MAP) containing an amino acid sequence known to be present in the 2A region of the alpha-helical rod domain. During spermiogenic steps 8-12, Sak57 immunoreactive sites were restricted to microtubular mantie of the manchette which encircles the spermatid nucleus during shaping and chromatin condensation. At later stages (spermiogenic steps 12-14), Sak57 immunoreactive sites in the spermatid head region disappeared gradually as specific immunoreactivity appeared along the already assembled axoneme of the developing spermatid tail. Immunogold electron microscopy confirmed the presence of Sak57 immunoreactivity among microtubules of the manchette and on outer dense fibers and the longitudinal columns linking the ribs of the fibrous sheath. Mature spermatids (spermiogenic step 19) displayed tails with an immunofluorescent banding pattern contrasting with the lack of Sak57 immunoreactivity in the head region. Results from this study suggest that, during early spermiogenesis, a microtubular-Sak57 scaffolding is associated with the spermatid nucleus during shaping and chromatin condensation. During late spermiogenesis, the dispersion of the manchette coincides with the progressive visualization of Sak57 in the paraaxonemal outer dense fibers and longitudinal columns of the fibrous sheath in the developing spermatid tail.