Mammalian alpha-mannosidases--multiple forms but a common purpose?

Deconstructing Best-in-Class Neoglycoclusters as a Tool for Dissecting Key Multivalent Processes in Glycosidase Inhibition

Multivalency represents an appealing option to modulate selectivity in enzyme inhibition and transform moderate glycosidase inhibitors into highly potent ones. The rational design of multivalent inhibitors is however challenging because global affinity enhancement relies on several interconnected local mechanistic events, whose relative impact is unknown. So far, the largest multivalent effects ever reported for a non-polymeric glycosidase inhibitor have been obtained with cyclopeptoid-based inhibitors of Jack bean α-mannosidase (JBα-man). Here, we report a structure-activity relationship (SAR) study based on the top-down deconstruction of best-in-class multivalent inhibitors. This approach provides a valuable tool to understand the complex interdependent mechanisms underpinning the inhibitory multivalent effect. Combining SAR experiments, binding stoichiometry assessments, thermodynamic modelling and atomistic simulations allowed us to establish the significant contribution of statistical rebinding mechanisms and the importance of several key parameters, including inhitope accessibility, topological restrictions, and electrostatic interactions. Our findings indicate that strong chelate-binding, resulting from the formation of a cross-linked complex between a multivalent inhibitor and two dimeric JBα-man molecules, is not a sufficient condition to reach high levels of affinity enhancements. The deconstruction approach thus offers unique opportunities to better understand multivalent binding and provides important guidelines for the design of potent and selective multiheaded inhibitors.

Biochemical characteristics of point mutated Capra hircus lysosome α-mannosidase

Locoweeds, a type of poisonous weedare, are widely distributed throughout the world and have a significant impact on the development of herbivore animal husbandry. Swainsonine (SW), the main toxin in locoweeds, can competitively inhibit lysosomes α-mannosidase (LAM) in animal cells, resulting in α-mannosidosis. However, the specifics of the interaction between SW and LAM are still unclear. Here, we used molecular docking to predicte the interaction points between SW and LAM, built mutated lysosomes α-mannosidase (LAM^M), and analyzed its biochemical properties changes in presumption points. The Trp at the 28th position and the Tyr at the 599th position of the LAM were interaction point candidates, and the above two amino acids in Capra hircus LAM (chLAM), were successfully mutated to glycine by constructing recombinant yeast GS115/PIC9K- LAM^M. The results showed that the sensitivity of Capra hircus LAM^M (chLAM^M), to SW decreased significantly compared with wild-type LAM, the enzyme activity of LAM decreased approximately threefold, the optimum temperature of LAM^M decreased from 55°C to 50°C, the optimum pH value increased from 4.5 to 5.0, and the effects of Mn²⁺, Fe³⁺, Al³⁺, Co²⁺, Cr³⁺, and ethylenediaminetetraacetic acid (EDTA) on LAM enzyme activity before and after point mutation changed significantly. These findings help us better understanding the molecular mechanism of the interaction mechanism between SW and chLAM, and provide new reference for solving locoweeds poisoning.

Defining the altered glycoproteomic space of the early secretory pathway by class I mannosidase pharmacological inhibition

N-glycosylation is a key process for various biological functions like protein folding, maturation and sorting for the conventional secretory compartment, cell-cell communication and immune response. This is usually accomplished by a complex system of mannosidases in which those from class I have an outstanding role, commonly involved in the early protein sorting associated to the Endoplasmic Reticulum (ER) in the N-glycan dependent quality control (ERQC) and ER-associated degradation (ERAD). Although these are vital processes in maintaining cellular homeostasis, large-scale analysis studies for this pool of molecules, further denoted as proteins from the early secretory pathway (ESP), were limited addressed. Here, using a custom workflow employing a combination of glycomics and deglycoproteomics analyses, using lectin affinity and selective Endoglycosidase H (Endo H) digestion, we scrutinize the steady-state oligomannosidic glycoprotein load and delineate ESP fraction in melanoma cells. All of these were assessed by applying our workflow for glycosite relative quantification of both the peptide chain and carbohydrate structure in cells with inhibited activity of class I mannosidases after kifunensine treatment. We found that most of the ESP are transient clients involved in cell communication via extracellular matrix, particularly integrin-mediated communication which adopt Man9 N-glycans in kifunensine-treated cells. Moreover, our results reveal that core-fucosylation is decreased subsequent inhibition of class I mannosidases and this could be explained by a general lower protein level of FUT8, the enzyme responsible for fucosylation. By comparing our data with results obtained following downregulation of a key mannosidase in misfolded protein degradation, we mapped both novel and previously suggested endogenous substrate candidates like PCDH2, HLA-B, LAMB2 or members of the integrin family of proteins such as ITGA1 and ITGA4, thus validating the findings obtained using our workflow regarding accumulation and characterization of ESP transitory members following mannosidase class I inhibition. This workflow and the associated dataset not only allowed us to investigate the oligomannosidic glycoprotein fraction but also to delineate differences mediated at glycosite-level upon kifunensine treatment and outline the potential associated cellular responses.

Multivalent Pyrrolidine Iminosugars: Synthesis and Biological Relevance

Recently, the strategy of multivalency has been widely employed to design glycosidase inhibitors, as glycomimetic clusters often induce marked enzyme inhibition relative to monovalent analogs. Polyhydroxylated pyrrolidines, one of the most studied classes of iminosugars, are an attractive moiety due to their potent and specific inhibition of glycosidases and glycosyltransferases, which are associated with many crucial biological processes. The development of multivalent pyrrolidine derivatives as glycosidase inhibitors has resulted in several promising compounds that stand out. Herein, we comprehensively summarized the different synthetic approaches to the preparation of multivalent pyrrolidine clusters, from total synthesis of divalent iminosugars to complex architectures bearing twelve pyrrolidine motifs. Enzyme inhibitory properties and multivalent effects of these synthesized iminosugars were further discussed, especially for some less studied therapeutically relevant enzymes. We envision that this comprehensive review will help extend the applications of multivalent pyrrolidine iminosugars in future studies.

Hyperphosphatemia in chronic kidney disease exacerbates atherosclerosis via a mannosidases-mediated complex-type conversion of SCAP N-glycans

Blood phosphate levels are linked to atherosclerotic cardiovascular disease in patients with chronic kidney disease (CKD), but the molecular mechanisms remain unclear. Emerging studies indicate an involvement of hyperphosphatemia in CKD accelerated atherogenesis through disturbed cholesterol homeostasis. Here, we investigated a potential atherogenic role of high phosphate concentrations acting through aberrant activation of sterol regulatory element-binding protein (SREBP) and cleavage-activating protein (SCAP)-SREBP2 signaling in patients with CKD, hyperphosphatemic apolipoprotein E (ApoE) knockout mice, and cultured vascular smooth muscle cells. Hyperphosphatemia correlated positively with increased atherosclerotic cardiovascular disease risk in Chinese patients with CKD and severe atheromatous lesions in the aortas of ApoE knockout mice. Mice arteries had elevated SCAP levels with aberrantly activated SCAP-SREBP2 signaling. Excess phosphate in vitro raised the activity of α-mannosidase, resulting in delayed SCAP degradation through promoting complex-type conversion of SCAP N-glycans. The retention of SCAP enhanced transactivation of SREBP2 and expression of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, boosting intracellular cholesterol synthesis. Elevated α-mannosidase II activity was also observed in the aortas of ApoE knockout mice and the radial arteries of patients with uremia and hyperphosphatemia. High phosphate concentration in vitro elevated α-mannosidase II activity in the Golgi, enhanced complex-type conversion of SCAP N-glycans, thereby upregulating intracellular cholesterol synthesis. Thus, our studies explain how hyperphosphatemia independently accelerates atherosclerosis in CKD.

Biometals and glycosylation in humans: Congenital disorders of glycosylation shed lights into the crucial role of Golgi manganese homeostasis

About half of the eukaryotic proteins bind biometals that participate in their structure and functions in virtually all physiological processes, including glycosylation. After reviewing the biological roles and transport mechanisms of calcium, magnesium, manganese, zinc and cobalt acting as cofactors of the metalloproteins involved in sugar metabolism and/or glycosylation, the paper will outline the pathologies resulting from a dysregulation of these metals homeostasis and more particularly Congenital Disorders of Glycosylation (CDGs) caused by ion transporter defects. Highlighting of CDGs due to defects in SLC39A8 (ZIP8) and TMEM165, two proteins transporting manganese from the extracellular space to cytosol and from cytosol to the Golgi lumen, respectively, has emphasized the importance of manganese homeostasis for glycosylation. Based on our current knowledge of TMEM165 structure and functions, this review will draw a picture of known and putative mechanisms regulating manganese homeostasis in the secretory pathway.

Multivalent Effect in Glycosidase Inhibition: The End of the Beginning

Glycosidases are ubiquitous enzymes involved in a diversity of key biological processes such as energy uptake or cell wall degradation. The design of specific glycosidase inhibitors has been therefore the subject of intense research efforts in academia and pharmaceutical industry. However, until recently, the study of the impact of multivalency on glycosidase inhibition was almost completely neglected. The following account will review our ten year journey on the design of multivalent glycomimetics within our research group, from the discovery of the first strong multivalent effect in glycosidase inhibition to the high-resolution crystal structures of Jack bean α-mannosidase in complex with the multimeric inhibitor displaying the largest binding enhancements reported so far.

Biochemical Characterization of a Lysosomal α-Mannosidase from the Starfish Asterias rubens

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.

Structural Basis of Outstanding Multivalent Effects in Jack Bean α-Mannosidase Inhibition

Multivalent design of glycosidase inhibitors is a promising strategy for the treatment of diseases involving enzymatic hydrolysis of glycosidic bonds in carbohydrates. An essential prerequisite for successful applications is the atomic-level understanding of how outstanding binding enhancement occurs with multivalent inhibitors. Herein we report the first high-resolution crystal structures of the Jack bean α-mannosidase (JBα-man) in apo and inhibited states. The three-dimensional structure of JBα-man in complex with the multimeric cyclopeptoid-based inhibitor displaying the largest binding enhancements reported so far provides decisive insight into the molecular mechanisms underlying multivalent effects in glycosidase inhibition.

Origin of α-mannosidase activity in CSF

The α-mannosidase activity in human frontal gyrus, cerebrospinal fluid and plasma has been analyzed by DEAE-cellulose chromatography to investigate the origin of the α-mannosidase activity in cerebrospinal fluid (CSF). The profile of α-mannosidase isoenzymes obtained in CSF was similar to that in the frontal gyrus but different from that in human plasma. In particular the two characteristic peaks of lysosomal α-mannosidase, A and B, which have a pH-optimum of 4.5 and are found in human tissues, were present in both the frontal gyrus and CSF. In contrast the majority of α-mannosidase activity in human plasma was due to the so called intermediate form, which has a pH-optimum of 5.5. The results suggest that the intermediate form of α-mannosidase in plasma does not cross the blood-brain barrier and that the α-mannosidase activity present in the cerebrospinal fluid is of lysosomal type and of brain origin. Thus the α-mannosidase activity in cerebrospinal fluid might mirror the brain pathological changes linked to neurodegenerative disorders such as Parkinson's disease.

Developmental and loco-like effects of a swainsonine-induced inhibition of α-mannosidase in the honey bee, Apis mellifera

Background

Deficiencies in lysosomal a-mannosidase (LAM) activity in animals, caused either by mutations or by consuming toxic alkaloids, lead to severe phenotypic and behavioural consequences. Yet, epialleles adversely affecting LAM expression exist in the honey bee population suggesting that they might be beneficial in certain contexts and cannot be eliminated by natural selection.

Methods

We have used a combination of enzymology, molecular biology and metabolomics to characterise the catalytic properties of honey bee LAM (AmLAM) and then used an indolizidine alkaloid swainsonine to inhibit its activity in vitro and in vivo.

Results

We show that AmLAM is inhibited in vitro by swainsonine albeit at slightly higher concentrations than in other animals. Dietary exposure of growing larvae to swainsonine leads to pronounced metabolic changes affecting not only saccharides, but also amino acids, polyols and polyamines. Interestingly, the abundance of two fatty acids implicated in epigenetic regulation is significantly reduced in treated individuals. Additionally, swainsonie causes loco-like symptoms, increased mortality and a subtle decrease in the rate of larval growth resulting in a subsequent developmental delay in pupal metamorphosis.

Discussion

We consider our findings in the context of cellular LAM function, larval development, environmental toxicity and colony-level impacts. The observed developmental heterochrony in swainsonine-treated larvae with lower LAM activity offer a plausible explanation for the existence of epialleles with impaired LAM expression. Individuals carrying such epialleles provide an additional level of epigenetic diversity that could be beneficial for the functioning of a colony whereby more flexibility in timing of adult emergence might be useful for task allocation.

Jack bean α-mannosidase: amino acid sequencing and N-glycosylation analysis of a valuable glycomics tool

Jack bean (Canavalia ensiformis) seeds contain several biologically important proteins among which α-mannosidase (EC 3.2.1.24) has been purified, its biochemical properties studied and widely used in glycan analysis. In the present study, we have used the purified enzyme and derived its amino acid sequence covering both the known subunits (molecular mass of ∼66,000 and ∼44,000 Da) hitherto not known in its entirety. Peptide de novo sequencing and structural elucidation of N-glycopeptides obtained either directly from proteolytic digestion or after zwitterionic hydrophilic interaction liquid chromatography solid phase extraction-based separation were performed by use of nanoelectrospray ionization quadrupole time-of-flight mass spectrometry and low-energy collision-induced dissociation experiments. De novo sequencing provided new insights into the disulfide linkage organization, intersection of subunits and complete N-glycan structures along with site specificities. The primary sequence suggests that the enzyme belongs to glycosyl hydrolase family 38 and the N-glycan sequence analysis revealed high-mannose oligosaccharides, which were found to be heterogeneous with varying number of hexoses viz, Man8-9GlcNAc2 and Glc1Man9GlcNAc2 in an evolutionarily conserved N-glycosylation site. This site with two proximal cysteines is present in all the acidic α-mannosidases reported so far in eukaryotes. Further, a truncated paucimannose type was identified to be lacking terminal two mannose, Man1(Xyl)GlcNAc2 (Fuc).

Traffic of human α-mannosidase in plant cells suggests the presence of a new endoplasmic reticulum-to-vacuole pathway without involving the Golgi complex

The transport of secretory proteins from the endoplasmic reticulum to the vacuole requires sorting signals as well as specific transport mechanisms. This work is focused on the transport in transgenic tobacco (Nicotiana tabacum) plants of a human α-mannosidase, MAN2B1, which is a lysosomal enzyme involved in the turnover of N-linked glycoproteins and can be used in enzyme replacement therapy. Although ubiquitously expressed, α-mannosidases are targeted to lysosomes or vacuoles through different mechanisms according to the organisms in which these proteins are produced. In tobacco cells, MAN2B1 reaches the vacuole even in the absence of mannose-6-phosphate receptors, which are responsible for its transport in animal cells. We report that MAN2B1 is targeted to the vacuole without passing through the Golgi complex. In addition, a vacuolar targeting signal that is recognized in plant cells is located in the MAN2B1 amino-terminal region. Indeed, when this amino-terminal domain is removed, the protein is retained in the endoplasmic reticulum. Moreover, when this domain is added to a plant-secreted protein, the resulting fusion protein is partially redirected to the vacuole. These results strongly suggest the existence in plants of a new type of vacuolar traffic that can be used by leaf cells to transport vacuolar proteins.

Expression, purification and preliminary crystallographic analysis of Drosophila melanogaster lysosomal α-mannosidase

The lysosomal α-mannosidases are class II mannosidases that belong to glycoside hydrolase family 38 and play an important role in the degradation of asparagine-linked carbohydrates of glycoproteins. Based on peptide similarity to human and bovine lysosomal mannosidase (LM), recombinant α-mannosidase from Drosophila melanogaster (dLM408) was cloned and heterologously expressed in Pichia pastoris. The recombinant form of dLM408 designed for structural analysis lacks the transmembrane domain and was crystallized using standard vapour-diffusion and counter-diffusion techniques. The crystals grew as flat plates and as tetragonal bipyramids, respectively. The plate-shaped crystals exhibited the symmetry of space group P2(1)2(1)2(1) and diffracted to a minimum d-spacing of 3.5 Å.

Lysosomal di-N-acetylchitobiase-deficient mouse tissues accumulate Man2GlcNAc2 and Man3GlcNAc2

Most lysosomal storage diseases are caused by defects in genes encoding for acidic hydrolases. Deficiency of an enzyme involved in the catabolic pathway of N-linked glycans leads to the accumulation of the respective substrate and consequently to the onset of a specific storage disorder. Di-N-acetylchitobiase and core specific α1-6mannosidase represent the only exception. In fact, to date no lysosomal disease has been correlated to the deficiency of these enzymes. We generated di-N-acetylchitobiase-deficient mice by gene targeting of the Ctbs gene in murine embryonic stem cells. Accumulation of Man2GlcNAc2 and Man3GlcNAc2 was evaluated in all analyzed tissues and the tetrasaccharide was detected in urines. Multilamellar inclusion bodies reminiscent of polar lipids were present in epithelia of a scattered subset of proximal tubules in the kidney. Less constantly, enlarged Kupffer cells were observed in liver, filled with phagocytic material resembling partly digested red blood cells. These findings confirm an important role for lysosomal di-N-acetylchitobiase in glycans degradation and suggest that its deficiency could be the cause of a not yet described lysosomal storage disease.

Human lysosomal α-D-mannosidase regulation in promyelocytic leukaemia cells

Lysosomal α-D-mannosidase is an exoglycosidase involved in the ordered degradation of N-linked oligosaccharides. It is ubiquitously expressed, although the main transcript is more abundant in peripheral blood leucocytes. Here we report that α-D-mannosidase enzyme activity is very high in the promyelocytic leukaemia cell lines HL60 and NB4, as compared with other leukaemic cell lines or cells from different human sources. The MAN2B1 transcript level correlates with enzyme activity, indicating a transcriptional up-regulation of the α-D-mannosidase gene. The promoter was then characterized in HEK-293 cells (human embryonic kidney 293 cells) and HL60 cells; regulatory sequences crucial for its activity were determined by reporter gene assay in HEK-293 cells and located in the region -101/-71 with respect to the first ATG codon. Supershift assay demonstrated that Sp1 (specificity protein 1) bound to this sequence both in HEK-293 and HL60 cells. However, 5'-RACE (5'-rapid amplification of cDNA ends) indicated the use of multiple upstream TSSs (transcription start sites) in HL60 with respect to HEK-293 cells and gel shift analysis of the sequence -373/-269 demonstrated a specific binding by NF-κB (nuclear factor κB) transcription factor in HL60 but not in HEK-293 cells. We concluded that despite the α-D-mannosidase promoter showing typical features of housekeeping gene promoters, α-D-mannosidase transcription is specifically regulated in HL60 by NF-κB transcription factor.

Human α-mannosidase produced in transgenic tobacco plants is processed in human α-mannosidosis cell lines

Deficiency in human lysosomal α-mannosidase (MAN2B1) results in α-mannosidosis, a lysosomal storage disorder; patients present a wide range of neurological, immunological, and skeletal symptoms caused by a multisystemic accumulation of mannose-containing oligosaccharides. Here, we describe the expression of recombinant MAN2B1 both transiently in Nicotiana benthamiana leaves and in the leaves and seeds of stably transformed N. tabacum plants. After purification from tobacco leaves, the recombinant enzyme was found to be N-glycosylated and localized in vacuolar compartments. In the fresh leaves of tobacco transformants, MAN2B1 was measured at 10,200 units/kg, and the purified enzyme from these leaves had a specific activity of 32-45 U/mg. Furthermore, tobacco-produced MAN2B1 was biochemically similar to the enzyme purified from human tissues, and it was internalized and processed by α-mannosidosis fibroblast cells. These results strongly indicate that plants can be considered a promising expression system for the production of recombinant MAN2B1 for use in enzyme replacement therapy.

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Glycosylation is a conserved posttranslational modification that is found in all eukaryotes, which helps generate proteins with multiple functions. Our knowledge of glycosylation mainly comes from the investigation of the yeast Saccharomyces cerevisiae and mammalian cells. However, during the last decade, glycosylation in the human pathogenic mold Aspergillus fumigatus has drawn significant attention. It has been revealed that glycosylation in A. fumigatus is crucial for its growth, cell wall synthesis, and development and that the process is more complicated than that found in the budding yeast S. cerevisiae. The present paper implies that the investigation of glycosylation in A. fumigatus is not only vital for elucidating the mechanism of fungal cell wall synthesis, which will benefit the design of new antifungal therapies, but also helps to understand the role of protein glycosylation in the development of multicellular eukaryotes. This paper describes the advances in functional analysis of protein glycosylation in A. fumigatus.

Cerebellar alterations and gait defects as therapeutic outcome measures for enzyme replacement therapy in α-mannosidosis

α-Mannosidosis is a rare lysosomal storage disease with accumulation of undegraded mannosyl-linked oligosaccharides in cells throughout the body, most notably in the CNS. This leads to a broad spectrum of neurological manifestations, including progressive intellectual impairment, disturbed motor functions, and cerebellar atrophy. To develop therapeutic outcome measures for enzyme replacement therapy that could be used for human patients, a gene knockout model of α-mannosidosis in mice was analyzed for CNS pathology and motor deficits. In the cerebellar molecular layer, α-mannosidosis mice display clusters of activated Bergman glia, infiltration of phagocytic macrophages, and accumulation of free cholesterol and gangliosides (GM1), notably in regions lacking Purkinje cells. α-Mannosidosis brain lysates also displayed increased expression of Lamp1 and hyperglycosylation of the cholesterol binding protein NPC2. Detailed assessment of motor function revealed age-dependent gait defects in the mice that resemble the disturbed motor function in human patients. Short-term enzyme replacement therapy partially reversed the observed cerebellar pathology with fewer activated macrophages and astrocytes but unchanged levels of hyperglycosylated NPC2, gangliosides, and cholesterol. The present study demonstrates cerebellar alterations in α-mannosidosis mice that relate to the motor deficits and pathological changes seen in human patients and can be used as therapeutic outcome measures.

Calystegine B3 as a specific inhibitor for cytoplasmic alpha-mannosidase, Man2C1

Cytoplasmic α-mannosidase (Man2C1) has been implicated in non-lysosomal catabolism of free oligosaccharides derived from N-linked glycans accumulated in the cytosol. Suppression of Man2C1 expression reportedly induces apoptosis in various cell lines, but its molecular mechanism remains unclear. Development of a specific inhibitor for Man2C1 is critical to understanding its biological significance. In this study, we identified a plant-derived alkaloid, calystegine B(3), as a potent specific inhibitor for Man2C1 activity. Biochemical enzyme assay revealed that calystegine B(3) was a highly specific inhibitor for Man2C1 among various α-mannosidases prepared from rat liver. Consistent with this in vitro result, an in vivo experiment also showed that treatment of mammalian-derived cultured cells with this compound resulted in drastic change in both structure and quantity of free oligosaccharides in the cytosol, whereas no apparent change was seen in cell-surface oligosaccharides. Calystegine B(3) could thus serve as a potent tool for the development of a highly specific in vivo inhibitor for Man2C1.

Overexpression of Man2C1 leads to protein underglycosylation and upregulation of endoplasmic reticulum-associated degradation pathway

Unfolded glycoproteins retained in the endoplasmic reticulum (ER) are degraded via the ER-associated degradation (ERAD) pathway. These proteins are subsequently transported to the cytosol and degraded by the proteasomal complex. Although the sequential events of ERAD are well described, its regulation remains poorly understood. The cytosolic mannosidase, Man2C1, plays an essential role in the catabolism of cytosolic free oligomannosides, which are released from the degraded proteins. We have investigated the impact of Man2C1 overexpression on protein glycosylation and the ERAD process. We demonstrated that overexpression of Man2C1 led to modifications of the cytosolic pool of free oligomannosides and resulted in accumulation of small Man(2-4)GlcNAc(1) glycans in the cytosol. We further correlated this accumulation with incomplete protein glycosylation and truncated lipid-linked glycosylation precursors, which yields an increase in N-glycoprotein en route to the ERAD. We propose a model in which high mannose levels in the cytosol interfere with glucose metabolism and compromise N-glycan synthesis in the ER. Our results show a clear link between the intracellular mannose-6-phosphate level and synthesis of the lipid-linked precursors for protein glycosylation. Disturbance in these pathways interferes with protein glycosylation and upregulated ERAD. Our findings support a new concept that regulation of Man2C1 expression is essential for maintaining efficient protein N-glycosylation.

The molecular characterization of a novel GH38 α-mannosidase from the crenarchaeon Sulfolobus solfataricus revealed its ability in de-mannosylating glycoproteins

α-Mannosidases, important enzymes in the N-glycan processing and degradation in Eukaryotes, are frequently found in the genome of Bacteria and Archaea in which their function is still largely unknown. The α-mannosidase from the hyperthermophilic Crenarchaeon Sulfolobus solfataricus has been identified and purified from cellular extracts and its gene has been cloned and expressed in Escherichia coli. The gene, belonging to retaining GH38 mannosidases of the carbohydrate active enzyme classification, is abundantly expressed in this Archaeon. The purified α-mannosidase activity depends on a single Zn(2+) ion per subunit is inhibited by swainsonine with an IC(50) of 0.2 mM. The molecular characterization of the native and recombinant enzyme, named Ssα-man, showed that it is highly specific for α-mannosides and α(1,2), α(1,3), and α(1,6)-D-mannobioses. In addition, the enzyme is able to demannosylate Man(3)GlcNAc(2) and Man(7)GlcNAc(2) oligosaccharides commonly found in N-glycosylated proteins. More interestingly, Ssα-man removes mannose residues from the glycosidic moiety of the bovine pancreatic ribonuclease B, suggesting that it could process mannosylated proteins also in vivo. This is the first evidence that archaeal glycosidases are involved in the direct modification of glycoproteins.

Purification and biochemical characterisation of endoplasmic reticulum alpha1,2-mannosidase from Sporothrix schenckiil

Alpha 1,2-mannosidases from glycosyl hydrolase family 47 participate in N-glycan biosynthesis. In filamentous fungi and mammalian cells, alpha1,2-mannosidases are present in the endoplasmic reticulum (ER) and Golgi complex and are required to generate complex N-glycans. However, lower eukaryotes such Saccharomyces cerevisiae contain only one alpha1,2-mannosidase in the lumen of the ER and synthesise high-mannose N-glycans. Little is known about the N-glycan structure and the enzyme machinery involved in the synthesis of these oligosaccharides in the dimorphic fungus Sporothrix schenckii. Here, a membrane-bound alpha-mannosidase from S. schenckii was solubilised using a high-temperature procedure and purified by conventional methods of protein isolation. Analytical zymograms revealed a polypeptide of 75 kDa to be responsible for enzyme activity and this purified protein was recognised by anti-alpha1,2-mannosidase antibodies. The enzyme hydrolysed Man(9)GlcNAc(2) into Man(8)GlcNAc(2) isomer B and was inhibited preferentially by 1-deoxymannojirimycin. This alpha1,2-mannosidase was localised in the ER, with the catalytic domain within the lumen of this compartment. These properties are consistent with an ER-localised alpha1,2-mannosidase of glycosyl hydrolase family 47. Our results also suggested that in contrast to other filamentous fungi, S. schenckii lacks Golgi alpha1,2-mannosidases and therefore, the processing of N-glycans by alpha1,2-mannosidases is similar to that present in lower eukaryotes.

Impaired lysosomal trimming of N-linked oligosaccharides leads to hyperglycosylation of native lysosomal proteins in mice with alpha-mannosidosis

Alpha-mannosidosis is caused by the genetic defect of the lysosomal alpha-d-mannosidase (LAMAN), which is involved in the breakdown of free alpha-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 alpha-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 alpha-mannosidosis as indicated by the predominance of extended Man3GlcNAc2 oligosaccharides. Treatment with recombinant human alpha-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.

Tumor suppressor function of BCSC-1 in nasopharyngeal carcinoma

BCSC-1 is dramatically upregulated in CNE-2L2 human nasopharyngeal carcinoma cells with reduced malignancy (AS cells) and is proposed to be a candidate tumor suppressor gene. We therefore examined the effect of BCSC-1 expression on malignant behaviors of CNE-2L2 cells. Growth in vitro and tumorigenesis in nude mice of wild-type CNE-2L2 cells (W cells) were inhibited by ectopic BCSC-1, and those of AS cells were promoted by BCSC-1 suppression. The tumor suppressor function of BCSC-1 was further confirmed by a study showing that intratumor BCSC-1 injection caused growth suppression of the tumor from W cells inoculated in nude mice. Immunohistochemistry exhibited marked reduction of BCSC-1 expression in 11 of 39 human nasopharyngeal carcinoma specimens. Because BCSC-1 expression was as rich as that in normal cells in the rest of the carcinoma specimens and was poor in CNE-2L2 cells, HNE-1 human nasopharyngeal carcinoma cells with rich BCSC-1 expression were used as a control in the study. No effect of BCSC-1 transfection on growth of the cells was observed. The data suggest that BCSC-1 suppression might play roles in tumorigenesis of some nasopharyngeal carcinomas and that BCSC-1 might be a potential gene therapy target in nasopharyngeal carcinomas with poor BCSC-1 expression. Enhanced aggregation of cells together with increased E-cadherin and alpha-catenin expression and reduced Wnt signaling might be involved in the mechanisms of tumor suppressor function of BCSC-1.

hMan2c1 transgene promotes tumor progress in mice

In order to study the biological significance of alpha-mannosidase Man2c1, hMan2c1 transgenic mice were developed. In 113 F0 mice, eight were found to be genomic PCR positive for hMan2c1; 9/20 (45%) F1 mice, 16/21 (76.2%) F2 mice, and 12/14 (85.7%) F3 mice were genomic PCR positive for hMan2c1. RT-PCR demonstrated hMan2c1 mRNA transcription in four of eight transgenic lines. Enzymatic activity on p-nitrophenyl-alpha-D: -mannopyranoside was enhanced in 35# and 54# transgenic mice and real-time RT-PCR showed hMan2c1 mRNA expression in these mice. Reduced Con A binding to splenocytes implied N-glycosylation modification of host proteins by hMan2c1 transgene. hMan2c1 transgene promoted growth, invasion, and metastasis to lung of implanted hepatoma H22 and sarcoma S180. The average weights of H22 and S180 tumors were 3.98 +/- 1.62, 3.29 +/- 0.76, 1.69 +/- 1.09, and 3.19 +/- 0.44, 2.72 +/- 1.38, 0.97 +/- 0.41 g for 35#, 54# transgenic mice and wild type mice (W), respectively, (35# or 54# versus W, paired t-test, P < 0.05). In 35# and 54# mice 5/10 and 3/10 showed lung metastasis of H22 tumor in contrast with 1/10 in W mice. In 35# and 54# mice 1/6 and 2/6 showed lung metastasis of S180 tumor in contrast with 0/6 in W mice. The possible mechanism of the promotion was explored on both humoral and cellular immunity. Reduced antibody response to BSA was observed in transgenic mice, suggesting that specific antibody response to tumor antigens might be suppressed by hMan2c1 transgene. However, NK cytotoxicity in splenocytes was not affected by the transgene.

Class IIC alpha-mannosidase AfAms1 is required for morphogenesis and cellular function in Aspergillus fumigatus

The mammalian ER/cytosolic alpha-mannosidase (Man2C1p), yeast vacuolar alpha-mannosidase (Ams1p) and the Aspergillus nidulans alpha-mannosidase are members of Class IIC subgroup, which is involved in oligosaccharide catabolism and N-glycan processing. Unlike their mammalian counterparts, the yeast Ams1p and A. nidulans Class IIC alpha-mannosidase are not essential for morphogenesis and cellular function. In this study, the Afams1, a gene encoding a member of Class IIC alpha-mannosidases, was identified in the opportunistic pathogen Aspergillus fumigatus. Deletion of the Afams1 led to a severe defect in conidial formation, especially at a higher temperature. In addition, abnormalities of polarity and septation were associated with the DeltaAfams1 mutant. Our results showed that the Afams1 gene, in contrast to its homolog in yeast or A. nidulans, was required for morphogenesis and cellular function in A. fumigatus.

Kex2 protease converts the endoplasmic reticulum alpha1,2-mannosidase of Candida albicans into a soluble cytosolic form

Cytosolic α-mannosidases are glycosyl hydrolases that participate in the catabolism of cytosolic free N-oligosaccharides. Two soluble α-mannosidases (E-I and E-II) belonging to glycosyl hydrolases family 47 have been described in Candida albicans. We demonstrate that addition of pepstatin A during the preparation of cell homogenates enriched α-mannosidase E-I at the expense of E-II, indicating that the latter is generated by proteolysis during cell disruption. E-I corresponded to a polypeptide of 52 kDa that was associated with mannosidase activity and was recognized by an anti-α1,2-mannosidase antibody. The N-mannan core trimming properties of the purified enzyme E-I were consistent with its classification as a family 47 α1,2-mannosidase. Differential density-gradient centrifugation of homogenates revealed that α1,2-mannosidase E-I was localized to the cytosolic fraction and Golgi-derived vesicles, and that a 65 kDa membrane-bound α1,2-mannosidase was present in endoplasmic reticulum and Golgi-derived vesicles. Distribution of α-mannosidase activity in a kex2Δ null mutant or in wild-type protoplasts treated with monensin demonstrated that the membrane-bound α1,2-mannosidase is processed by Kex2 protease into E-I, recognizing an atypical cleavage site of the precursor. Analysis of cytosolic free N-oligosaccharides revealed that cytosolic α1,2-mannosidase E-I trims free Man₈GlcNAc₂ isomer B into Man₇GlcNAc₂ isomer B. This is believed to be the first report demonstrating the presence of soluble α1,2-mannosidase from the glycosyl hydrolases family 47 in a cytosolic compartment of the cell.

Inhibition of metastasis to lung of a human nasopharyngeal carcinoma cell line CNE- 2L2 transfected with pRc/ CMV-antisense 6A8 cDNA in nude mice

The growth of CNE-2L2 cell, a cloned line of human nasopharyngeal carcinoma with a high potentiality of metastasis to lung was inhibited to a certain extent after transfection with a recombinant antisense expression vector of a cDNA encoding a human alpha-mannosidase (pRc/CMV-antisense 6A8 cDNA) (the Genbank accession number of 6A8 cDNA is U37248) in comparison with that of the cell transfected with the Mock and of the wild cell. Two months after a subcutaneous inoculation of CNE-2L2 cell into the axilla of nude mice metastatic lesions in the lung were observed in 9/10 mice (90%) with grade III in 8 mice and grade II in one mouse in the wild cell group, in 6/8 mice (75 %) with grade III in one mouse, grade II in 2 mice and grade I in 3 mice in the Mock-transfection group, in only 3/10 mice (30%) with all grade I in pRc/CMV-antisense 6A8 cDNA-transfection group.

Alpha-mannosidosis

Alpha-mannosidosis is an inherited lysosomal storage disorder characterized by immune deficiency, facial and skeletal abnormalities, hearing impairment, and intellectual disability. It occurs in approximately 1 of 500,000 live births. The children are often born apparently normal, and their condition worsens progressively. Some children are born with ankle equinus or develop hydrocephalus in the first year of life. Main features are immune deficiency (manifested by recurrent infections, especially in the first decade of life), skeletal abnormalities (mild-to-moderate dysostosis multiplex, scoliosis and deformation of the sternum), hearing impairment (moderate-to-severe sensorineural hearing loss), gradual impairment of mental functions and speech, and often, periods of psychosis. Associated motor function disturbances include muscular weakness, joint abnormalities and ataxia. The facial trait include large head with prominent forehead, rounded eyebrows, flattened nasal bridge, macroglossia, widely spaced teeth, and prognathism. Slight strabismus is common. The clinical variability is significant, representing a continuum in severity. The disorder is caused by lysosomal alpha-mannosidase deficiency. Alpha-mannosidosis is inherited in an autosomal recessive fashion and is caused by mutations in the MAN2B1 gene located on chromosome 19 (19 p13.2-q12). Diagnosis is made by measuring acid alpha-mannosidase activity in leukocytes or other nucleated cells and can be confirmed by genetic testing. Elevated urinary secretion of mannose-rich oligosaccharides is suggestive, but not diagnostic. Differential diagnoses are mainly the other lysosomal storage diseases like the mucopolysaccharidoses. Genetic counseling should be given to explain the nature of the disease and to detect carriers. Antenatal diagnosis is possible, based on both biochemical and genetic methods. The management should be pro-active, preventing complications and treating manifestations. Infections must be treated frequently. Otolaryngological treatment of fluid in the middle ear is often required and use of hearing aids is invariably required. Early educational intervention for development of social skills is needed and physiotherapy is important to improve bodily function. Orthopedic surgery may be necessary. The long-term prognosis is poor. There is an insidiously slow progression of neuromuscular and skeletal deterioration over several decades, making most patients wheel-chair dependent. No patients manage to be completely socially independent. Many patients are over 50 years of age.

Glucosylated free oligosaccharides are biomarkers of endoplasmic- reticulum α-glucosidase inhibition

The inhibition of ER (endoplasmic reticulum) α-glucosidases I and II by imino sugars, including NB-DNJ (N-butyl-deoxynojirimycin), causes the retention of glucose residues on N-linked oligosaccharides. Therefore, normal glycoprotein trafficking and processing through the glycosylation pathway is abrogated and glycoproteins are directed to undergo ERAD (ER-associated degradation), a consequence of which is the production of cytosolic FOS (free oligosaccharides). Following treatment with NB-DNJ, FOS were extracted from cells, murine tissues and human plasma and urine. Improved protocols for analysis were developed using ion-exchange chromatography followed by fluorescent labelling with 2-AA (2-aminobenzoic acid) and purification by lectin-affinity chromatography. Separation of 2-AA-labelled FOS by HPLC provided a rapid and sensitive method that enabled the detection of all FOS species resulting from the degradation of glycoproteins exported from the ER. The generation of oligosaccharides derived from glucosylated protein degradation was rapid, reversible, and time- and inhibitor concentration-dependent in cultured cells and in vivo. Long-term inhibition in cultured cells and in vivo indicated a slow rate of clearance of glucosylated FOS. In mouse and human urine, glucosylated FOS were detected as a result of transrenal excretion and provide unique and quantifiable biomarkers of ER-glucosidase inhibition.

Effect of all trans retinoic acid on lysosomal alpha-D-mannosidase activity in HL-60 cell: correlation with HL-60 cells differentiation

Human promyelocytic leukemia HL-60 cells represent an in vitro model of acute promyelocytic leukemia (APL), and are inducible to terminally differentiate into morphologically mature granulocytes by incubation with all trans retinoic acid (ATRA). Lysosomal glycohydrolases are involved in the changes of the membrane surface proteins' glycosylation, linked to the metastatic progression potential of neoplastic cells. In particular, it has been demonstrated that the Asn-linked glucidic residues were directly responsible for the metastatic potential, and it is known that the glycohydrolase alpha-D-mannosidase specifically hydrolyze the Asn-linked oligosaccharides. In this report, we present an in vitro study on the ATRA effects on lysosomal glycohydrolases expression and the eventual relationship with the retinoic acid-induced differentiation of HL-60 cells. We have investigated two highly expressed lysosomal glycohydrolases, namely beta-D-hexosaminidase and alpha-D-mannosidase, and showed that they were differently affected by ATRA differentiating action. In particular, due to the specific action on Asn-linked oligosaccharides, we tested alpha-D: -mannosidase enzymatic activity and observed that it was dramatically decreased after ATRA incubation, indicating a relationship with the differentiation state of the cells. These observations may directly be linked with the loss of metastatic progession of differentiated HL-60.

Characterization and subcellular localization of human neutral class IIα-mannosidase cytosolic enzymes/free oligosaccharides/glycosidehydrolase family 38/M2C1/N-glycosylation

A glycosyl hydrolase family 38 enzyme, neutral alpha-mannosidase, has been proposed to be involved in hydrolysis of cytosolic free oligosaccharides originating either from ER-misfolded glycoproteins or the N-glycosylation process. Although this enzyme has been isolated from the cytosol, it has also been linked to the ER by subcellular fractionations. We have studied the subcellular localization of neutral alpha-mannosidase by immunofluorescence microscopy and characterized the human recombinant enzyme with natural substrates to elucidate the biological function of this enzyme. Immunofluorescence microscopy showed neutral alpha-mannosidase to be absent from the ER, lysosomes, and autophagosomes, and being granularly distributed in the cytosol. In experiments with fluorescent recovery after photo bleaching, neutral alpha-mannosidase had slower than expected two-phased diffusion in the cytosol. This result together with the granular appearance in immunostaining suggests that portion of the neutral alpha-mannosidase pool is somehow complexed. The purified recombinant enzyme is a tetramer and has a neutral pH optimum for activity. It hydrolyzed Man(9)GlcNAc to Man(5)GlcNAc in the presence of Fe(2+), Co(2+), and Mn(2+), and uniquely to neutral alpha-mannosidases from other organisms, the human enzyme was more activated by Fe(2+) than Co(2+). Without activating cations the main reaction product was Man(8)GlcNAc, and Cu(2+) completely inhibited neutral alpha-mannosidase. Our findings from enzyme-substrate characterizations and subcellular localization studies support the suggested role for neutral alpha-mannosidase in hydrolysis of soluble cytosolic oligomannosides.

Multivariate neurocognitive and emotional profile of a mannosidosis murine model for therapy assessment

alpha-Mannosidosis is a lysosomal storage disorder caused by lysosomal alpha-mannosidase (LAMAN) deficiency that leads to neurocognitive dysfunctions, psychotic symptoms and emotional changes in human patients. A murine mannosidosis model, LAMAN-deficient mice, was examined on a behavioral task battery that included test for neuromotor, exploratory and neurocognitive (spatial learning and memory) abilities, and multivariate statistical analyses were used to identify behavioral and neurocognitive domains that are most heavily affected by LAMAN deficiency. In addition, we further investigated synaptic plasticity recordings on hippocampal slices that may relate to these behavioral alterations. Correlation analysis revealed significant intra- and intertask correlations and factor analysis that included all 21 behavioral variables identified three main factors (exploration/emotionality, locomotion and learning/memory abilities). Significant correlations were observed between genotype, and factor 1 (exploration/emotionality) and factor 3 (learning/memory abilities). Discriminant function analysis showed that "path length in the open field test" and "time spent in the target quadrant during the water maze probe trial" were the most decisive variables to distinguish between the genotypes. We therefore suggest that these variables would be especially important in forthcoming therapy assessment experiments using this murine mannosidosis model. LAMAN-deficient mice displayed severe changes in synaptic plasticity, which may have contributed to the neurocognitive impairments observed. The present report further shows that targeted deletion of the LAMAN gene in mice mimics many aspects of human alpha-mannosidosis, and these data provide a basis for future therapeutic experiments.

Man2C1, an alpha-mannosidase, is involved in the trimming of free oligosaccharides in the cytosol

The endoplasmic-reticulum-associated degradation of misfolded (glyco)proteins ensures that only functional, correctly folded proteins exit from the endoplasmic reticulum and that misfolded ones are degraded by the ubiquitin-proteasome system. During the degradation of misfolded glycoproteins, they are deglycosylated by the PNGase (peptide:N-glycanase). The free oligosaccharides released by PNGase are known to be further catabolized by a cytosolic alpha-mannosidase, although the gene encoding this enzyme has not been identified unequivocally. The findings in the present study demonstrate that an alpha-mannosidase, Man2C1, is involved in the processing of free oligosaccharides that are formed in the cytosol. When the human Man2C1 orthologue was expressed in HEK-293 cells, most of the enzyme was localized in the cytosol. Its activity was enhanced by Co2+, typical of other known cytosolic alpha-mannosidases so far characterized from animal cells. The down-regulation of Man2C1 activity by a small interfering RNA drastically changed the amount and structure of oligosaccharides accumulating in the cytosol, demonstrating that Man2C1 indeed is involved in free oligosaccharide processing in the cytosol. The oligosaccharide processing in the cytosol by PNGase, endo-beta-N-acetylglucosaminidase and alpha-mannosidase may represent the common 'non-lysosomal' catabolic pathway for N-glycans in animal cells, although the molecular mechanism as well as the functional importance of such processes remains to be determined.

Molecular and biochemical characterization of alpha-glucosidase and alpha-mannosidase and their clustered genes from the thermoacidophilic archaeon Picrophilus torridus

The genes encoding a putative alpha-glucosidase (aglA) and an alpha-mannosidase (manA) appear to be physically clustered in the genome of the extreme acidophile Picrophilus torridus, a situation not found previously in any other organism possessing aglA or manA homologs. While archaeal alpha-glucosidases have been described, no alpha-mannosidase enzymes from the archaeal kingdom have been reported previously. Transcription start site mapping and Northern blot analysis revealed that despite their colinear orientation and the small intergenic space, the genes are independently transcribed, both producing leaderless mRNA. aglA and manA were cloned and overexpressed in Escherichia coli, and the purified recombinant enzymes were characterized with respect to their physicochemical and biochemical properties. AglA displayed strict substrate specificity and hydrolyzed maltose, as well as longer alpha-1,4-linked maltooligosaccharides. ManA, on the other hand, hydrolyzed all possible linkage types of alpha-glycosidically linked mannose disaccharides and was able to hydrolyze alpha3,alpha6-mannopentaose, which represents the core structure of many triantennary N-linked carbohydrates in glycoproteins. The probable physiological role of the two enzymes in the utilization of exogenous glycoproteins and/or in the turnover of the organism's own glycoproteins is discussed.