Rat epididymal alpha-D-mannosidase: purification, carbohydrate composition, substrate specificity, and antibody production

Sperm surface antigens and the prospects for contraceptive vaccine development

It has been estimated that 500 million couples world-wide have no access to contraception and that approximately 20% of births between 1995 and 2000 will be unwanted. Such statistics have important implications for the rate of world population growth and the possibility of maintaining a sustainable population. Although political change and the empowerment of women across the world will help address these issues, it will also be important to increase the availability of contraceptives; not only the modalities that are in current use, but also novel methods that will satisfy needs that are presently unfulfilled. One such alternative could be a vaccine targeting the human spermatozoon.

Glycan-modifying enzymes in luminal fluid of the mammalian epididymis: an overview of their potential role in sperm maturation

Testicular spermatozoa and those present within the proximal regions of the epididymis are unable to bind to the zona pellucida, the extracellular coat that surrounds the oocyte, and fertilize the egg. They acquire progressive motility and fertilizing ability during passage through the epididymis. Mammalian spermatozoa undergo biochemical and physiological changes during epididymal transit that are collectively termed epididymal maturation. The process involves several intracellular and extracellular changes in the spermatozoon, including remodeling of the sperm plasma membrane and modifications of glycan moieties of the sperm surface glycoconjugates. Two sets of glycan-modifying enzymes, namely glycohydrolases that cleave sugar residues and glycosyltransferases that add sugar residues to the existing glycoconjugates, are present in the epididymal luminal fluid that surrounds spermatozoa. Thus, it is reasonable to expect that glycan chains present on the sperm surface will interact with these glycan-modifying enzymes in the epididymal fluid. In this article, I have attempted to summarize and present an overview on the potential role of these glycan-modifying enzymes in sperm maturation.

In vivo role of alpha-mannosidase IIx: ineffective spermatogenesis resulting from targeted disruption of the Man2a2 in the mouse

Alpha-mannosidase IIx (MX) is an enzyme closely related to the Golgi N-glycan processing enzyme alpha-mannosidase II (MII). The enzymatic activity of MX in vitro is minimal. Therefore, the in vivo role of MX in N-glycan processing is as yet unclear. The targeted disruption of the gene encoding MX in the mouse resulted in an obvious phenotype, i.e., MX-deficient males were found to be infertile. Testes from homozygous mutant male mice are smaller than those from wild-type or heterozygous littermates. Histology of the MX null mouse testis showed significant reduction of spermatogenic cells in the seminiferous tubules. Electron microscopy showed that prominent intercellular spaces surround MX-deficient spermatogenic cells, suggesting a failure of germ cell adhesion to Sertoli cells. Quantitative structural analyses of N-glycans from wild-type and MX-deficient mouse testis showed that wild-type testes contain GlcNAc-terminated complex type N-glycans, while they are significantly reduced in MX-deficient mutant testis. An in vitro assay for adhesion of spermatogenic cells to Sertoli cells was carried out. By testing the effect of each purified N-glycan oligosaccharide, it was demonstrated that a GlcNAc-terminated tri-antennary, fucosylated N-glycan has an activity on the adhesion between germ cells and Sertoli cells. Thus, the targeted disruption of the gene encoding MX uncovered a novel carbohydrate recognition system in a biologically important process, spermatogenesis.

Purification and characterization of the plasma membrane glycosidases of Drosophila melanogaster spermatozoa

Previous studies from our laboratory have demonstrated the presence of two integral proteins with glycosidase activity in the plasma membrane of Drosophila melanogaster spermatozoa and we have suggested that these enzymes might have a role in sperm-egg binding. In this study the glycosidases have been purified and characterized. We have evidenced the presence of three distinct enzymes, two beta-N-acetylhexosaminidase isoforms, named HEX 1 and HEX 2, and an alpha-mannosidase. The molecular size of the native enzymes estimated by gel filtration was 158 kDa for beta-hexosaminidases and 317 kDa for alpha-mannosidase. SDS-PAGE showed that HEX 1 and HEX 2 are dimers formed by subunits with different molecular sizes, whereas alpha-mannosidase consists of three subunits with different molecular weights. All the enzymes are terminally glycosylated. Characterization of the purified enzymes included their 4-methylumbelliferyl-substrate preferences, kinetic properties, inhibitor constants and thermal stability. On the basis of substrate specificity, kinetics and the results of inhibition studies, beta-hexosaminidases appear to differ from each other. HEX 1 and HEX 2 are similar to mammalian isoenzyme A and isoenzyme B, respectively. These findings represent the first report on the characterization of sperm proteins that are potentially involved in interactions with the egg in Insects.

Purification and properties of major alpha-D-mannosidase in the luminal fluid of porcine epididymis

A lysosomal type alpha-D-mannosidase was successfully purified by DEAE-Sephacel, Red-Amicon and Superdex 200 column chromatographies from porcine cauda epididymal fluid. The purified enzyme consisted of 63 and 51 kDa subunits at equimolar amounts. It cleaved alpha1-2 linked mannosyl residues and less but significantly cleaved alpha1-3 and alpha1-6 linked mannosyl residues in the high-mannose oligosaccharides. The optimal pH to hydrolyze oligosaccharide was in the acidic pH range (pH 3.5 approximately 4.0). Total alpha-D-mannosidase activities in the porcine epididymal fluid increased from proximal to distal caput epididymis, which maintained to cauda epididymis. At least two kinds of alpha-D-mannosidase (lysosomal type enzyme and 135 kDa alpha-D-mannosidase (MAN2B2)) were contained in the porcine epididymal fluid. The activity of the lysosomal type enzyme is much higher than MAN2B2 at the physiological pH. These results suggest that the lysosomal type alpha-D-mannosidase is the predominantly active enzyme in the luminal fluid of porcine epididymis and that it participates in the glycoprotein modification on the sperm surface during epididymal transit.

alpha-Mannosidase from rat epididymal fluid is a ligand for phosphomannosyl receptors on the sperm surface

This study demonstrates that alpha-mannosidase from rat epididymal fluid is a ligand for phosphomannosyl receptors on the sperm surface. This enzyme was bound to intact epididymal spermatozoa with high affinity and in saturable form, and the binding was inhibited by mannose-6-phosphate but not by phosphorylated derivatives of fructose. Treatment of the enzyme with sodium periodate inhibited the binding of alpha-mannosidase, confirming that a carbohydrate residue is involved in the interaction with spermatozoa. Evidence is also presented that the cation-independent phosphomannosyl receptors are responsible for the interaction with alpha-mannosidase. These findings suggest a new role for extracellular transport mediated by the mannose-6-phosphate receptor.

Cloning, expression, purification, and characterization of the murine lysosomal acid alpha-mannosidase

Catabolism of alpha-linked mannose residues on eukaryotic glycoproteins is accomplished by a broad specificity lysosomal alpha-mannosidase (EC 3.2.1.24). Based on regions of protein sequence conservation between the lysosomal alpha-mannosidase from Dictyostelium discoideum and the murine Golgi glycoprotein processing alpha 1,3/1,6-mannosidase, alpha-mannosidase II, we have cloned a cDNA encoding the murine lysosomal alpha-mannosidase. The longest of the clones was 3.1 kb in length and encoded a polypeptide of 992 amino acids containing a putative NH2-terminal signal sequence and 11 potential N-glycosylation sites. The deduced amino acid sequence was 76.5% identical to the human lysosomal alpha-mannosidase and 38.1% identical to the lysosomal alpha-mannosidase from D. discoideum. Expression of the cDNA in Pichia pastoris resulted in the secretion of an alpha-mannosidase activity into the culture medium. This recombinant expression product was purified and was shown to have enzymatic characteristics highly similar to the enzyme purified from mammalian sources and to the human lysosomal alpha-mannosidase cDNA expressed in Pichia. These characteristics include a similar pH optimum, Km, Vmax, inhibition by swainsonine, and activity toward natural substrates. Northern blots identified a major 3.5 kb RNA transcript in all murine tissues tested. A minor transcript of 5.4 kb was also detected in some murine tissues similar to the alternatively spliced transcripts that have been previously identified in human tissues.

Genomic structure of the human lysosomal alpha-mannosidase gene (MANB)

Lysosomal alpha-mannosidase (LAMAN) (EC 3.2.1.24) is an exoglycosidase involved in the ordered degradation of N-linked oligosaccharides. Lack of LAMAN activity leads to the lysosomal storage disorder alpha-mannosidosis (MIM No. 248500). We determined the genomic organization of the human lysosomal alpha-mannosidase gene (laman; HGMW-approved symbol MANB) by using oligonucleotide primers designed from the human laman cDNA sequence as part of a PCR-based strategy. The gene spanned 21.5 kb and contained 24 exons. By primer extension analysis, the major transcription initiation sites were mapped to positions -309, -196, and -191 relative to the first in-frame ATG. No CAAT or TATA sequences could be identified within 134 bp upstream of the transcription initiation sites, but the 5' flanking region contained several GC-rich regions with putative binding sites for the transcription factors SP-1, AP-2, and ETF.

Purification of bovine lysosomal alpha-mannosidase, characterization of its gene and determination of two mutations that cause alpha-mannosidosis

Bovine kidney lysosomal alpha-mannosidase was purified to homogeneity and the gene was cloned. The gene was organized in 24 exons that spanned 16 kb and its corresponding cDNA contained an open reading frame of 2997 bp beginning from a putative ATG start codon. The deduced amino acid sequence contained a signal peptide of 50 amino acids adjacent to a protein sequence of 949 amino acids that was cleaved into five peptides in the mature enzyme; starting with the peptide derived from the N-terminal part of this precursor, their molecular masses were 35/38 (peptide a), 11/13 (peptide b), 22 (peptide c), 38 (peptide d) and 13/15 kDa (peptide e). Variation in the degree of N-glycosylation accounts for molecular mass heterogeneities of peptides a, b and e. Peptides a, b and c were disulphide-linked. A T961-->C transition, resulting in Phe321-->Leu substitution, was identified in the cDNA of alpha-mannosidosis-affected Angus cattle. In affected Galloway cattle, a G662-->A transition that causes Arg221-->His substitution was identified. Phe321 and Arg221 are conserved among the alpha-mannosidase class-2 family, indicating that the substitutions resulted from disease-causing mutations in these breeds.

Rat epididymal luminal fluid acid beta-D-galactosidase optimally hydrolyses glycoprotein substrate at neutral pH

Several glycosidases, purified and characterized from mammalian tissues, have been shown to be optimally active under acidic conditions when p-nitrophenyl (PNP) or 4-methylumbelliferyl glycosides are used as substrates. Although high levels of the glycosidases are present in the epididymal lumen, their physiological role remains uncertain. To be functional, the glycosidases are expected to be enzymatically active at or near the physiological pH of luminal fluid. In this report, we demonstrate that the rat epididymal luminal fluid beta-D-galactosidase, optimally active toward PNP beta-D-galactoside at pH 3.5, shows maximum activity towards a glycoprotein substrate ([Gal-3H]fetuin) at neutral pH. Several lines of evidence, including immunoprecipitation studies using antibody to the acid beta-D-galactosidase, and substrate competition studies, indicate that PNP galactosidase and [3H]Gal galactosidase activities are caused by a single enzyme, and that the two substrates are probably cleaved by the same catalytic site(s). Competition studies with various disaccharides indicate that this enzyme is capable of cleaving a variety of galactose linkages found in both O- and N-linked oligosaccharides. Molecular-sieve column chromatography of the beta-D-galactosidase of luminal fluid under several conditions of buffer and pH show that, whereas the enzyme eluted as a tetramer (apparent M(r) 320,000) under acidic conditions (pH 3.5-4.3), only dimers and monomers (apparent M(r) 180,000 and 92,000 respectively) were observed in neutral conditions (pH 6.8). This aggregation/dissociation phenomenon is reversible. These studies indicate that beta-D-galactosidase is present in the luminal fluid in dissociated forms, and is therefore optimally active towards glycoprotein substrates at physiological pH. The potential role of the enzyme in modification of sperm surface glycoproteins is discussed.

In vitro hydrolysis of oligomannosyl oligosaccharides by the lysosomal alpha-D-mannosidases

In vitro incubation of the oligomannosyl oligosaccharides Man9GlcNAc and Man5GlcNAc with isolated disrupted lysosomes yields different oligosaccharide isomers resulting from mannosidase hydrolysis. These isomers were isolated by HPLC and characterized by 1H-NMR spectroscopy. The first steps of the degradation involve an (alpha 1-2)mannosidase activity and lead to the formation of one Man8GlcNAc, one Man7GlcNAc, two Man6GlcNAc and two Man5GlcNAc isomers. These reactions do not require Zn2+ as activator. On the other hand, the following steps, which lead to the formation of Man3GlcNAc and Man2GlcNAc, are Zn2(+)-dependent. This process is characterized by the preferential action of an (alpha 1-3)mannosidase activity, and the formation of Man(alpha 1-6)Man(alpha 1-6)Man(beta 1-4)GlcNAc and Man(alpha 1-6)Man(beta 1-4)GlcNAc. Therefore, the digestion of Man9GlcNAc inside the lysosome appears to follow a very specific pathway, since only nine intermediate compounds can be identified instead of the 38 possible isomers. Our results are consistent both with the existence of several specific enzymes for alpha 1-2, alpha 1-3 and alpha 1-6 linkages, and with the presence of a unique enzyme whose specificity would be dependent either on Zn2+ or on the spatial conformation of the glycan. Nevertheless, previous work on the structural analysis of oligosaccharides excreted in the urine of patients suffering from mannosidosis, demonstrates the absence of the core alpha 1-6-linked mannosyl residue in the major storage product derived from oligomannosyl oligosaccharides. This observation indicates the presence of a specific (alpha 1-6)mannosidase form, unaffected in mannosidosis.

Novel alpha-D-mannosidase of rat sperm plasma membranes: characterization and potential role in sperm-egg interactions

During the course of a study of glycoprotein processing mannosidases in the rat epididymis, we have made an intriguing discovery regarding the presence of a novel alpha-D-mannosidase on the rat sperm plasma membranes. Unlike the sperm acrosomal "acid" mannosidase which has a pH optimum of 4.4, the newly discovered alpha-D-mannosidase has a pH optimum of 6.2, and 6.5 when assayed in sperm plasma membranes and intact spermatozoa, respectively. In addition, the two enzymes show different substrate specificity. The acrosomal alpha-D-mannosidase is active mainly towards synthetic substrate, p-nitrophenyl alpha-D-mannopyranoside, whereas the sperm plasma membrane alpha-D-mannosidase shows activity mainly towards mannose-containing oligosaccharides. Evidence is presented which suggest that the sperm plasma membrane alpha-D-mannosidase is different from several processing mannosidases previously characterized from the rat liver. The newly discovered alpha-D-mannosidase appears to be an intrinsic plasma membrane component, since washing of the purified membranes with buffered 0.4 M NaCl did not release the enzyme in soluble form. The enzyme requires nonionic detergent (Triton X-100) for complete solubilization. The enzyme is activated by Co2+ and Mn2+. However, Cu2+ and Zn2+ are potent inhibitors of the sperm plasma membrane alpha-D-mannosidase. At a concentration of 0.1 mM, these divalent cations caused nearly complete inactivation of the sperm enzyme. In addition methyl-alpha-D-mannoside, methyl-alpha-D-glucoside, mannose, 2-deoxy-D-glucose, and D-mannosamine are inhibitors of the sperm surface alpha-D-mannosidase. The physiological role of the newly discovered enzyme is not yet known. Several published reports in three species, including the rat, suggest that the sperm surface alpha-D-mannosidase may have a role in binding to mannose-containing saccharides presumably present on the zona pellucida.