Studies on α-D-mannosidase and ConA during jackbean development and germination

Summary Changes in the level and activity of -D-mannosidase and ConA have been followed during a time-course of development spanning seed formation, desiccation and germination. In parallel studies, immunogold labelling has enabled these changes to be placed within a structural context of the cotyledon parenchyma cells during protein body formation, dehydration and subsequent autolysis during germination.The results indicate that the exo-glycosidase and lectin accumulate in parallel during seed formation and are packaged within the same protein bodies. Several lines of evidence suggest that the function of both proteins is related to events that occur during seed development rather than germination.Abbreviations BSA bovine serum albumin - ConA concanavalin A - DPA days post-anthesis - DPI days post-imbibition - Man D-mannose - PBS phosphate buffered saline - RIA radioimmunoassay - ER endoplasmic reticulum - GA Golgi apparatus - SDS-PAGE polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate     

Purification and characterisation of a novel alkalophilic α-D-mannosidase from Pseudomonas fluorescens

An extracellular alkaline α-D-mannosidase in the cell culture of a marine bacterium Pseudomonas fluorescens JK-02 was purified to homogeneity with a 30.7-fold by ammonium sulphate fractionation, anion-exchange chromatography and gel-filtration chromatography. The molecular weight of the purified enzyme was estimated to be 50.5 kDa based on the sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The optimal pH and temperature of the purified enzyme were 8.5 and 30°C. The K_m and V_max values of the purified enzyme towards p-nitrophenyl-α-D-mannopyranoside were determined to be 77 µM and 0.23 µM min^?1mg^?1 of protein, respectively. The α-D-mannosidase showed higher substrate specificity to α-1,3-mannobiose than other isomeric substrates such as α-1,2- and α-1,6-mannobiose. In addition, molecular characterisation of this enzyme reveals that it belongs to a class II α-mannosidase from the glycosyl hydrolase family 38. To the best of our knowledge, this is the first report on the alkalophilic α-1,3 D-mannosidase of Pseudomonas species, which has selective algal-lytic activity against Alexandrium tamarense, Akashiwo sanguine, Gymnodinium catenatum, Gymnodinium mikimotoi and Prorocentrum dentatum.     

Impaired Lysosomal Trimming of N-Linked Oligosaccharides Leads to Hyperglycosylation of Native Lysosomal Proteins in Mice with α-Mannosidosis

α-Mannosidosis is caused by the genetic defect of the lysosomal α-d-mannosidase (LAMAN), which is involved in the breakdown of free α-linked mannose-containing oligosaccharides originating from glycoproteins with N-linked glycans, and thus manifests itself in an extensive storage of mannose-containing oligosaccharides. Here we demonstrate in a model of mice with α-mannosidosis that native lysosomal proteins exhibit elongated N-linked oligosaccharides as shown by two-dimensional difference gel electrophoresis, deglycosylation assays, and mass spectrometry. The analysis of cathepsin B-derived oligosaccharides revealed a hypermannosylation of glycoproteins in mice with α-mannosidosis as indicated by the predominance of extended Man3GlcNAc2 oligosaccharides. Treatment with recombinant human α-mannosidase partially corrected the hyperglycosylation of lysosomal proteins in vivo and in vitro. These data clearly demonstrate that LAMAN is involved not only in the lysosomal catabolism of free oligosaccharides but also in the trimming of asparagine-linked oligosaccharides on native lysosomal proteins.The lysosomal α-d-mannosidase (LAMAN; EC 3.2.1.24) belongs to the group of at least seven lysosomal exoglycosidases which sequentially degrade oligosaccharides derived from glycoproteins (2, 31). These glycoproteins enter the lysosomal compartment by either endocytic pathways (extracellular and plasma membrane proteins) or autophagic processes (intracellular proteins). In addition, free oligosaccharides originating from lipid-linked oligosaccharides in the endoplasmic reticulum and from glycoproteins by the endoplasmic reticulum-associated protein degradation (ERAD) pathway are transported into the lysosome, where these oligosaccharides are subsequently degraded (9, 45). Inside the lysosome, the degradation of the glycoproteins is described as a bidirectional process in which on the one hand the polypeptide is hydrolyzed by a cohort of lysosomal endo- and exoproteases with partially overlapping specificities like cathepsins and other peptidases (like DPP II and TPP-I [19, 40, 52). On the other hand, the sugar moiety is stepwise hydrolyzed into its monosaccharides by exoglycosidases. The precise order of the bidirectional breakdown of glycoproteins is unclear, although assumptions can be made based on the analysis of the storage products of the different glycoproteinoses (31). Therefore, it is assumed that an efficient degradation of the oligosaccharide chain is highly dependent on the cleavage of the protein-oligosaccharide linkage by the glycosylasparaginase (2, 31). In contrast, the proteolysis of the polypeptide backbone is mainly unaffected by intact oligosaccharide structures on the glycoproteins (1).LAMAN has a broad substrate specificity, cleaving nonreducing terminal α1,2-, α1,3-, and α1,6-mannosyl linkages found in complex-type, hybrid-type, and high-mannose-type asparagine-linked glycans (30, 60). Additionally, a second lysosomal mannosidase (MAN2B2) specific for the core α1,6 branch was characterized and found to be dependent on the prior enzymatic activity of lysosomal glycosylasparaginase or chitobiase, releasing Man3GlcNAc2 and Man3GlcNAc oligosaccharides, respectively (21, 36). The cooperation of this novel core-specific α1,6-mannosidase with chitobiase is also reflected by their similar tissue-specific expression patterns in humans and rodents and their simultaneous absence in cattle and cats (2, 14).LAMAN deficiency results in the rare lysosomal storage disorder (LSD) α-mannosidosis, which is clinically characterized by progressive mental retardation, dysostosis multiplex, impaired hearing, immune defects, and mild hepatosplenomegaly. However, the onset of symptoms varies greatly and the clinical severity of α-mannosidosis patients ranges from mildly affected to severely affected, lacking a genotype-phenotype correlation (29). Patients also show elevated serum and urine oligosaccharide levels and an enlargement of the lysosomal compartment which is considered to be caused by the accumulation of undegraded oligosaccharides. The major lysosomal storage product is the trisaccharide Man2GlcNAc, although oligosaccharides with up to eight mannosyl residues were detected in the urine and serum of patients, indicating their lysosomal accumulation as well (4, 33). From these findings, one can draw the conclusion that beside metabolic intermediates of the glycoprotein degradation, a considerable number of oligosaccharides originate from dolichol-linked oligosaccharides or from glycoproteins that failed quality control in the endoplasmic reticulum and thus are degraded by the proteasome, leaving behind highly mannosylated glycans (23, 32, 41). It is assumed that 70% of the stored oligosaccharides derive from complex- and hybrid-type glycans, 10% derive from high-mannose-type glycans, and 20% derive from biosynthetic intermediates, e.g., lipid-linked oligosaccharides (61).Naturally occurring animal models for α-mannosidosis have been described for cats (8, 55), cattle (6, 24), and guinea pigs (12). The animal models have been the subjects of various studies dealing with neuropathological, behavioral, and therapeutic aspects of α-mannosidosis (3, 13, 38). It was shown with guinea pigs and cats that enzyme replacement therapy (ERT) and bone marrow transplantation, respectively, provided a benefit concerning clinical manifestations and remarkable success in the central nervous system of cats after bone marrow transplantation (13, 56).Aside from the naturally occurring models, a mouse model for α-mannosidosis was generated in which the LAMAN gene was disrupted by gene targeting. This mouse model phenotypically resembled a mild variant of the human disease (46). We exploited this mouse model to develop an ERT approach as already clinically established for other LSDs like Gaucher disease, Hunter disease, or Pompe disease. For this purpose, LAMAN preparations from different species were proven to be efficacious for visceral organs, and most remarkably, we demonstrated that high-dose administration of recombinant human LAMAN (rhLAMAN) affected the central neural storage (39). Very recently, Blanz et al. confirmed the influence of high-dosage ERT on the peripheral as well as the central nervous system in the same mouse model and showed clearance of storage material in hippocampal neurons in particular (5). Here, we report on structural alterations of lysosomal proteins in mice with α-mannosidosis due to hyperglycosylation and the reversibility by ERT.     

Glycosylation-independent Lysosomal Targeting of Acid α-Glucosidase Enhances Muscle Glycogen Clearance in Pompe Mice

We have used a peptide-based targeting system to improve lysosomal delivery of acid α-glucosidase (GAA), the enzyme deficient in patients with Pompe disease. Human GAA was fused to the glycosylation-independent lysosomal targeting (GILT) tag, which contains a portion of insulin-like growth factor II, to create an active, chimeric enzyme with high affinity for the cation-independent mannose 6-phosphate receptor. GILT-tagged GAA was taken up by L6 myoblasts about 25-fold more efficiently than was recombinant human GAA (rhGAA). Once delivered to the lysosome, the mature form of GILT-tagged GAA was indistinguishable from rhGAA and persisted with a half-life indistinguishable from rhGAA. GILT-tagged GAA was significantly more effective than rhGAA in clearing glycogen from numerous skeletal muscle tissues in the Pompe mouse model. The GILT-tagged GAA enzyme may provide an improved enzyme replacement therapy for Pompe disease patients.     

Lysosomal α-galactosidase in endothelial cell cultures established from a fabry hemizygous and normal umbilical veins

Summary An endothelial cell line has been established from the umbilical vein obtained after abortion of a male fetus suffering from Fabry disease. This X-linked inborn error of glycosphingolipid catabolism results from deficiency of the lysosomal hydrolase -galactosidase. A the clinical manifestations of the disease are mainly caused by glycosphingolipid depositions in the endothelium of all vessels. The hemizygous cell line and eight endothelial cell lines originating from the umblical cords of normal newborns were grown for more than 10 passages. They had a short generation time that allowed us to get sufficient cells for qualitative and quantive investigations of -galactosidase. The enzyme in normal endothelial cells had a similar thermostability and isoelectric focusing pattern as that in fibroblasts, but the activity was essentially higher in endothelial cells. The hemizygous endothelial cells were deficient in -galactosidase A. It is concluded that endothelial cell lines are an important alternative to fibroblasts for in vitro studies of the lysosomal storage diseases.     

Lysosomal–mitochondrial cross-talk during cell death

Lysosomes are membrane-bound organelles, which contain an arsenal of different hydrolases, enabling them to act as the terminal degradative compartment of the endocytotic, phagocytic and autophagic pathways. During the last decade, it was convincingly shown that destabilization of lysosomal membrane and release of lysosomal content into the cytosol can initiate the lysosomal apoptotic pathway, which is dependent on mitochondria destabilization. The cleavage of BID to t-BID and degradation of anti-apoptotic BCL-2 proteins by lysosomal cysteine cathepsins were identified as links to the mitochondrial cytochrome c release, which eventually leads to caspase activation. There have also been reports about the involvement of lysosome destabilization and lysosomal proteases in the extrinsic apoptotic pathway, although the molecular mechanism is still under debate. In the present article, we discuss the cross-talk between lysosomes and mitochondria during apoptosis and its consequences for the fate of the cell.     

Characterization of a β-D-mannosidase from a marine gastropod, Aplysia kurodai

A β-D-mannosidase (EC 3.2.1.25) with a molecular mass of approximately 100 kDa was purified from the digestive fluid of a marine gastropod Aplysia kurodai by ammonium sulfate fractionation followed by column chromatographies on TOYOPEARL Butyl-650 M, TOYOPEARL DEAE-650 M, and Superdex 200 10/300 GL. This enzyme, named AkMnsd in the present study, showed optimal activities at pH 4.5 and 40 °C and was stable at the acidic pH range from 2.0 to 6.7 and the temperature below 38 °C. The Km and Vmax values for AkMnsd determined at pH 6.0 and 30 °C with p-nitrophenyl β-d-mannopyranoside were 0.10 mM and 3.75 μmol/min/mg, respectively. AkMnsd degraded various polymer mannans as well as mannooligosaccharides liberating mannose as a major degradation product. Linear mannan from green alga Codium fragile was completely depolymerized by AkMnsd in the presence of AkMan, an endolytic β-mannanase, which we previously isolated from the same animal (Zahura et al., Comp. Biochem. Physiol. B 157, 137-148 (2010)). A cDNA encoding AkMnsd was amplified from the Aplysia hepatopancreas cDNA by the PCR using degenerated primers designed on the basis of N-terminal and internal amino-acid sequences of AkMnsd. The cloned AkMnsd cDNA consisted of 2985 bp and encoded an amino-acid sequence of 931 residues with the calculated molecular mass of 101,970 Da. The deduced sequence of AkMnsd showed 20-43% amino-acid identity to those of glycoside-hydrolase-family 2 (GHF2) β-mannosidases. The catalytically important amino-acid residues determined in GHF2 enzymes were completely conserved in AkMnsd. Thus, AkMnsd is regarded as a new member of GHF2 mannosidase from marine gastropod.     

Biochemical Characterization of a Lysosomal α-Mannosidase from the Starfish <Emphasis Type="Italic">Asterias rubens</Emphasis>

Acidic α-mannosidase is an important enzyme and is reported from many different plants and animals. Lysosomal α-mannosidase helps in the catabolism of glycoproteins in the lysosomes thereby playing a major role in cellular homeostasis. In the present study lysosomal α-mannosidase from the gonads of echinoderm Asterias rubens was isolated and purified. The crude protein sample from ammonium sulfate precipitate contained two isoforms of mannosidase as tested by the MAN2B1 antibody, which were separated by anion exchange chromatography. Enzyme with 75 kDa molecular weight was purified and biochemically characterized. Optimum pH of the enzyme was found to be in the range of 4.5–5 and optimum temperature was 37 °C. The activity of the enzyme was inhibited completely by swainsonine but not by 1-deoxymannojirimycin. Ligand blot assays showed that the enzyme can interact with both the lysosomal enzyme sorting receptors indicating the presence of mannose 6-phosphate in the glycan surface of the enzyme. This is the first report of lysosomal α-mannosidase in an active monomeric form. Its interaction with the receptors suggest that the lysosomal enzyme targeting in echinoderms might follow a mannose 6-phosphate mediated pathway similar to that in the vertebrates.     

Lysosomal Storage Diseases: Is Impaired Apoptosis a Pathogenic Mechanism?

Lysosomal storage disorders are inborn diseases resulting from the lack or activity of lysosomal hydrolases, transporters, or integral membrane proteins. Although most of the genes encoding these proteins have been characterized and many gene defects identified, the molecular bases underlying the pathophysiology of these genetic diseases still remain obscure. In this mini-review, the potential role of apoptotic cell death in the development of the cellular and tissue lesions seen in lysosomal storage disorders, and particularly in neurological diseases, is discussed. A list of observations documenting either a decrease or an exacerbation in apoptosis induction are presented. The putative, yet controversial contribution of certain sphingolipids and cathepsins in the regulation of these phenomena is emphasized.     

Lysosomal acid deoxyribonuclease from vervet monkey livers—II: Enzymic properties

• 1.1. Primate liver lysosomal acid DNase is an endonucleolytic enzyme.
• 2.2. The enzyme has both 3'- and 5'-nucleotidohydrolase activities.
• 3.3. The oligonucleotides produced by DNase are polymers mainly about 30 mononucleotides long.
• 4.4. The Arrhenius plot shows a discontinuity with a transition temperature at 47°C, with an activation energy of 107 kJ/mol below and 67 kJ/mol above this temperature.
• 5.5. The activation enthalpy is 104kJ/mol and the entropy −0.498 kJ/mol/K.
• 6.6. The enzyme is subject to substrate inhibition and the K_m value is 159 × 10⁻³mM DNA-P.

     

Synthesis of new α-heterocyclic α-aminoesters

alpha-Heterocyclic alpha-aminoesters were obtained in good yields by reaction of a glycine cation equivalent and different heterocyclic nucleophiles; diastereoselectivity using a carbohydrate (galactopyranose) as N-protecting group was modest.     

Lysosomal acid deoxyribonuclease from vervet monkey livers—I: Purification and physico-chemical characterization

• 1.1. Acid DNase from monkey liver lysosomes was purified to homogeneity by salt extraction of lysosomal membranes at pH 3.8; (NH₄)₂SO₄ fractionation; low salt precipitation; SP-C₅₀ and G-150 Sephadex chromatography; and polyacrylamide gel electrophoresis.
• 2.2. The pH for optimum activity was dual in character with a labile optimum at pH 3.8 and a less active but stable one at pH 4.2
• 3.3. The estimated molecular weight was 40 K and the pl was 4.4.
• 4.4. Inorganic ions such as Ca²⁺, Mg²⁺, Mn²⁺ and SO₄²⁻ were more than 80% inhibitory at 10-mM levels. Fe³⁺ ions were 80% inhibitory at 0.1-mM levels. 15. Nad at 100 mM is essential for activity but becomes 100% inhibitory above 200 mM.

     

Differential Regulation of Amyloid-β Endocytic Trafficking and Lysosomal Degradation by Apolipoprotein E Isoforms

Aggregation of amyloid-β (Aβ) peptides leads to synaptic disruption and neurodegeneration in Alzheimer disease (AD). A major Aβ clearance pathway in the brain is cellular uptake and degradation. However, how Aβ traffics through the endocytic pathway and how AD risk factors regulate this event is unclear. Here we show that the majority of endocytosed Aβ in neurons traffics through early and late endosomes to the lysosomes for degradation. Overexpression of Rab5 or Rab7, small GTPases that function in vesicle fusion for early and late endosomes, respectively, significantly accelerates Aβ endocytic trafficking to the lysosomes. We also found that a portion of endocytosed Aβ traffics through Rab11-positive recycling vesicles. A blockage of this Aβ recycling pathway with a constitutively active Rab11 mutant significantly accelerates cellular Aβ accumulation. Inhibition of lysosomal enzymes results in Aβ accumulation and aggregation. Importantly, apolipoprotein E (apoE) accelerates neuronal Aβ uptake, lysosomal trafficking, and degradation in an isoform-dependent manner with apoE3 more efficiently facilitating Aβ trafficking and degradation than apoE4, a risk factor for AD. Taken together, our results demonstrate that Aβ endocytic trafficking to lysosomes for degradation is a major Aβ clearance pathway that is differentially regulated by apoE isoforms. A disturbance of this pathway can lead to accumulation and aggregation of cellular Aβ capable of causing neurotoxicity and seeding amyloid.     

Thermostable extracellular α-amylase and α-glucosidase ofLipomyces starkeyi

Summary Thermostable, extracellular -amylase and -glucosidase were produced byLipomyces starkeyi CBS 1809 in a medium containing maize starch and soya bean meal. Contrary to published findings which suggested a single cell-bound amylolytic system for another strain ofL. starkeyi, this study revealed the presence of two enzymes — an -amylase and an -glucosidase inL. starkeyi CBS 1809. The enzymes were separated by solvent and salt precipitation and ion-exchange chromatography on DEAE-Biogel-A. The -amylase and -glucosidase had pH optima at 4.0 and 4.5 and temperature optima at 70°C and 60°C, respectively. While the low pH optima are not unique the enzymes are very distinctive in yeasts in having very high temperature optima. The -glucosidase had highest activities on maltose and isomaltose (100) with relative rates of activity on maltotriose, isomaltotriose and p-nitrophenyl--d-glucoside of 59, 48 and 22, respectively. It was inactive towards sucrose. Both the -amylase and -glucosidase ofL. starkeyi were located extracellularly and had molecular weights of 76,000 and 35,000, respectively.     

Imidazolethyl-Phosphoramidate α-Oligonucleotides

Abstract

α-ODNs conjugated to imidazole groups via phosphoramidate internucleosidic linkages were synthesized. The presence of the imidazolethyl-phosphoramidate linkage improved the affinity of α-ODNs for their nucleic acid targets.