Use of large-scale hydrazinolysis in the preparation of N-linked oligosaccharide libraries: application to brain tissue

Human-specific features and developmental dynamics of the brain N-glycome

Comparative "omics" studies have revealed unique aspects of human neurobiology, yet an evolutionary perspective of the brain N-glycome is lacking. We performed multiregional characterization of rat, macaque, chimpanzee, and human brain N-glycomes using chromatography and mass spectrometry and then integrated these data with complementary glycotranscriptomic data. We found that, in primates, the brain N-glycome has diverged more rapidly than the underlying transcriptomic framework, providing a means for rapidly generating additional interspecies diversity. Our data suggest that brain N-glycome evolution in hominids has been characterized by an overall increase in complexity coupled with a shift toward increased usage of α(2-6)-linked N-acetylneuraminic acid. Moreover, interspecies differences in the cell type expression pattern of key glycogenes were identified, including some human-specific differences, which may underpin this evolutionary divergence. Last, by comparing the prenatal and adult human brain N-glycomes, we uncovered region-specific neurodevelopmental pathways that lead to distinct spatial N-glycosylation profiles in the mature brain.

Human-specific features and developmental dynamics of the brain N-glycome

Comparative "omics" studies have revealed unique aspects of human neurobiology, yet an evolutionary perspective of the brain N-glycome is lacking. Here, we performed multi-regional characterization of rat, macaque, chimpanzee, and human brain N-glycomes using chromatography and mass spectrometry, then integrated these data with complementary glycotranscriptomic data. We found that in primates the brain N-glycome has evolved more rapidly than the underlying transcriptomic framework, providing a mechanism for generating additional diversity. We show that brain N-glycome evolution in hominids has been characterized by an increase in complexity and α(2-6)-linked N-acetylneuraminic acid along with human-specific cell-type expression of key glycogenes. Finally, by comparing the prenatal and adult human brain N-glycome, we identify region-specific neurodevelopmental pathways that lead to distinct spatial N-glycosylation profiles in the mature brain.

One-Sentence Summary

Evolution of the human brain N-glycome has been marked by an increase in complexity and a shift in sialic acid linkage.

The dynamic brain N-glycome

The attachment of carbohydrates to other macromolecules, such as proteins or lipids, is an important regulatory mechanism termed glycosylation. One subtype of protein glycosylation is asparagine-linked glycosylation (N-glycosylation) which plays a key role in the development and normal functioning of the vertebrate brain. To better understand the role of N-glycans in neurobiology, it's imperative we analyse not only the functional roles of individual structures, but also the collective impact of large-scale changes in the brain N-glycome. The systematic study of the brain N-glycome is still in its infancy and data are relatively scarce. Nevertheless, the prevailing view has been that the neuroglycome is inherently restricted with limited capacity for variation. The development of improved methods for N-glycomics analysis of brain tissue has facilitated comprehensive characterisation of the complete brain N-glycome under various experimental conditions on a larger scale. Consequently, accumulating data suggest that it's more dynamic than previously recognised and that, within a general framework, it has a given capacity to change in response to both intrinsic and extrinsic stimuli. Here, we provide an overview of the many factors that can alter the brain N-glycome, including neurodevelopment, ageing, diet, stress, neuroinflammation, injury, and disease. Given this emerging evidence, we propose that the neuroglycome has a hitherto underappreciated plasticity and we discuss the therapeutic implications of this regarding the possible reversal of pathological changes via interventions. We also briefly review the merits and limitations of N-glycomics as an analytical method before reflecting on some of the outstanding questions in the field.

Post-natal developmental changes in the composition of the rat neocortical N-glycome

Asparagine-linked glycosylation (N-glycosylation) plays a key role in many neurodevelopmental processes, including neural cell adhesion, neurite outgrowth and axon targeting. However, little is known about the dynamics of N-glycosylation during brain development and, in particular, how the N-glycome of the developing neocortex differs from that of the adult. The aim of this study, therefore, was to perform a thorough characterization of N-glycosylation in both the adult and neonatal rat neocortex in order to gain insights into the types of changes occurring in the N-glycome during neurodevelopment. To this end, we used hydrophilic interaction ultraperformance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry to compare the adult neocortical N-glycome with that of 24- and 48-h neonates. We report that the abundance of complex N-glycans is significantly lower in adults compared with neonates. Furthermore, the proportion of charged complex N-glycans is also greatly reduced. This decrease in the abundance of complex N-glycans is offset by a corresponding increase in the proportion of truncated and, to a lesser extent, hybrid N-glycans. Lastly, we report that although the proportion of oligomannose N-glycans remains constant at around 24%, the distribution of high-mannose subtypes shifts from predominantly large subtypes in neonates to smaller subtypes in the adult. In summary, our findings indicate that N-glycan synthesis in the rat neocortex is fundamentally different in neonates compared with adults with a general shift occurring from large, sialylated N-glycans towards smaller, neutral structures as neonates develop into adults, coupled with a parallel shift towards smaller oligomannose structures.

Region-Specific Characterization of N-Glycans in the Striatum and Substantia Nigra of an Adult Rodent Brain

N-glycan alterations in the nervous system can result in different neuropathological symptoms such as mental retardation, seizures, and epilepsy. Studies have reported the characterization of N-glycans in rodent brains, but there is a lack of spatial resolution as either the tissue samples were homogenized or specific proteins were selected for analysis of glycosylation. We hypothesize that region-specific resolution of N-glycans isolated from the striatum and substantia nigra (SN) can give an insight into the establishment and pathophysiological degeneration of neural circuitry in Parkinson's disease. Specific objectives of the study include isolation of N-glycans from the rat striatum and SN; reproducibility, resolution, and relative quantitation of N-glycome using ultra-performance liquid chromatography (UPLC), weak anion exchange-UPLC, and lectin histochemistry. The total N-glycomes from the striatum and SN were characterized using database mining (GlycoStore), exoglycosidase digestions, and liquid chromatography-mass spectrometry. It revealed significant differences in complex and oligomannose type N-glycans, sialylation (mono-, di-, and tetra-), fucosylation (tri-, core, and outer arm), and galactosylation (di-, tri-, and tetra-) between striatum and SN N-glycans with the detection of phosphorylated N-glycans in SN which were not detected in the striatum. This study presents the most comprehensive comparative analysis of relative abundances of N-glycans in the striatum and SN of rodent brains, serving as a foundation for identifying "brain-type" glycans as biomarkers or therapeutic targets and their modulation in neurodegenerative disorders.

Isomeric Separation and Recognition of Anionic and Zwitterionic N-glycans from Royal Jelly Glycoproteins

Royal jelly has received attention because of its necessity for the development of queen honeybees as well as claims of benefits on human health; this product of the hypopharyngeal glands of worker bees contains a large number of proteins, some of which have been claimed to have various biological effects only in their glycosylated state. However, although there have been glycomic and glycoproteomic analyses in the past, none of the glycan structures previously defined would appear to have potential to trigger specific biological functions. In the current study, whole royal jelly as well as single protein bands were subject to off-line LC-MALDI-TOF MS glycomic analyses, complemented by permethylation, Western blotting and arraying data. Similarly to recent in-depth studies on other insect species, previously overlooked glucuronic acid termini, sulfation of mannose residues and core β-mannosylation of the N-glycans were found; additionally, a relatively rare zwitterionic modification with phosphoethanolamine is present, in contrast to the phosphorylcholine occurring in lepidopteran species. Indicative of tissue-specific remodelling of glycans in the Golgi apparatus of hypopharyngeal gland cells, only a low amount of fucosylated or paucimannosidic glycans were detected as compared with other insect samples or even bee venom. The unusual modifications of hybrid and multiantennary structures defined here may not only have a physiological role in honeybee development, but represent epitopes recognized by pentraxins with roles in animal innate immunity.

Rapid chemical de-N-glycosylation and derivatization for liquid chromatography of immunoglobulin N-linked glycans

Glycan analysis may result in exploitation of glycan biomarkers and evaluation of heterogeneity of glycosylation of biopharmaceuticals. For N-linked glycan analysis, we investigated alkaline hydrolysis of the asparagine glycosyl carboxamide of glycoproteins as a deglycosylation reaction. By adding hydroxylamine into alkaline de-N-glycosylation, we suppressed the degradation of released glycans and obtained a mixture of oximes, free glycans, and glycosylamines. The reaction was completed within 1 h, and the mixture containing oximes was easily tagged with 2-aminobenzamide by reductive amination. Here, we demonstrated N-linked glycan analysis using this method for a monoclonal antibody, and examined whether this method could liberate glycans without degradation from apo-transferrin containing NeuAc and NeuGc and horseradish peroxidase containing Fuc α1-3 GlcNAc at the reducing end. Furthermore, we compared glycan recoveries between conventional enzymatic glycan release and this method. Increasing the reaction temperature and reaction duration led to degradation, whereas decreasing these parameters resulted in lower release. Considering this balance, we proposed to carry out the reaction at 80°C for 1 h for asialo glycoproteins from mammals and at 50°C for 1 h for sialoglycoproteins.

Isomer Information from Ion Mobility Separation of High-Mannose Glycan Fragments

Extracted arrival time distributions of negative ion CID-derived fragments produced prior to traveling-wave ion mobility separation were evaluated for their ability to provide structural information on N-linked glycans. Fragmentation of high-mannose glycans released from several glycoproteins, including those from viral sources, provided over 50 fragments, many of which gave unique collisional cross-sections and provided additional information used to assign structural isomers. For example, cross-ring fragments arising from cleavage of the reducing terminal GlcNAc residue on Man₈GlcNAc₂ isomers have unique collision cross-sections enabling isomers to be differentiated in mixtures. Specific fragment collision cross-sections enabled identification of glycans, the antennae of which terminated in the antigenic α-galactose residue, and ions defining the composition of the 6-antenna of several of the glycans were also found to have different cross-sections from isomeric ions produced in the same spectra. Potential mechanisms for the formation of the various ions are discussed and the estimated collisional cross-sections are tabulated. Graphical Abstract ᅟ.

Travelling-wave ion mobility mass spectrometry and negative ion fragmentation of hybrid and complex N-glycans

Nitrogen collisional cross sections (CCSs) of hybrid and complex glycans released from the glycoproteins IgG, gp120 (from human immunodeficiency virus), ovalbumin, α1-acid glycoprotein and thyroglobulin were measured with a travelling-wave ion mobility mass spectrometer using dextran as the calibrant. The utility of this instrument for isomer separation was also investigated. Some isomers, such as Man₃ GlcNAc₃ from chicken ovalbumin and Man₃ GlcNAc₃ Fuc₁ from thyroglobulin could be partially resolved and identified by their negative ion fragmentation spectra obtained by collision-induced decomposition (CID). Several other larger glycans, however, although existing as isomers, produced only asymmetric rather than separated arrival time distributions (ATDs). Nevertheless, in these cases, isomers could often be detected by plotting extracted fragment ATDs of diagnostic fragment ions from the negative ion CID spectra obtained in the transfer cell of the Waters Synapt mass spectrometer. Coincidence in the drift times of all fragment ions with an asymmetric ATD profile in this work, and in the related earlier paper on high-mannose glycans, usually suggested that separations were because of conformers or anomers, whereas symmetrical ATDs of fragments showing differences in drift times indicated isomer separation. Although some significant differences in CCSs were found for the smaller isomeric glycans, the differences found for the larger compounds were usually too small to be analytically useful. Possible correlations between CCSs and structural types were also investigated, and it was found that complex glycans tended to have slightly smaller CCSs than high-mannose glycans of comparable molecular weight. In addition, biantennary glycans containing a core fucose and/or a bisecting GlcNAc residue fell on different mobility-m/z trend lines to those glycans not so substituted with both of these substituents contributing to larger CCSs. Copyright © 2016 John Wiley & Sons, Ltd.

Fragmentation and ion mobility properties of negative ions from N-linked carbohydrates: Part 7. Reduced glycans

RATIONALE

Negative ion collision-induced dissociation (CID) spectra of released N-glycans provide very informative structural information relating to branching patterns and location of residues such as fucose. For some structural studies, particularly those involving chromatography, glycans are often reduced to avoid production of multiple peaks from α- and β-anomers. We examined the effect of reduction on the production of diagnostic fragment ions and on the ion mobility properties of N-glycans.

METHODS

Released N-glycans from the glycoproteins bovine fetuin, ribonuclease B, chicken ovalbumin, and porcine thyroglobulin were reduced with sodium cyanoborohydride and both negative ion CID spectra and ion mobility properties of their phosphate adducts were examined with a Waters Synapt G2Si travelling-wave ion mobility mass spectrometer with electrospray sample introduction. Estimated collisional cross sections were measured with dextran as the calibrant,

RESULTS

Fragment ions were similar to those from the unreduced glycans with the exception that the prominent (2,4) A cleavage ion from the reducing terminus was replaced by a prominent [M-H3 PO4](-) ion. Other ions arising from the chitobiose core were of lower relative abundance than those from the unreduced glycans. Estimated collisional cross sections were similar to those of the unreduced compounds but with symmetrical arrival time distribution (ATD) profiles, unlike those of the unreduced glycans whose peaks often contained prominent asymmetry. This observation showed that this asymmetry was due to anomer separation.

CONCLUSIONS

Reduction of the reducing terminal GlcNAc residue resulted in fewer diagnostic ions from the chitobiose core but fragmentation of the remainder of the molecules generally paralleled that of the unreduced glycans. Thus, most structural information, with the exception of the linkage position of fucose on the core GlcNAc, was available. ATD peaks were symmetrical with the result that cross sections were more appropriate for data-base searching than those from the non-reduced compounds where asymmetry produced lower precision in the measurement.

Travelling-wave ion mobility and negative ion fragmentation of high-mannose N-glycans

The isomeric structure of high-mannose N-glycans can significantly impact biological recognition events. Here, the utility of travelling-wave ion mobility mass spectrometry for isomer separation of high-mannose N-glycans is investigated. Negative ion fragmentation using collision-induced dissociation gave more informative spectra than positive ion spectra with mass-different fragment ions characterizing many of the isomers. Isomer separation by ion mobility in both ionization modes was generally limited, with the arrival time distributions (ATD) often showing little sign of isomers. However, isomers could be partially resolved by plotting extracted fragment ATDs of the diagnostic fragment ions from the negative ion spectra, and the fragmentation spectra of the isomers could be extracted by using ions from limited areas of the ATD peak. In some cases, asymmetric ATDs were observed, but no isomers could be detected by fragmentation. In these cases, it was assumed that conformers or anomers were being separated. Collision cross sections of the isomers in positive and negative fragmentation mode were estimated from travelling-wave ion mobility mass spectrometry data using dextran glycans as calibrant. More complete collision cross section data were achieved in negative ion mode by utilizing the diagnostic fragment ions. Examples of isomer separations are shown for N-glycans released from the well-characterized glycoproteins chicken ovalbumin, porcine thyroglobulin and gp120 from the human immunodeficiency virus. In addition to the cross-sectional data, details of the negative ion collision-induced dissociation spectra of all resolved isomers are discussed.

Estimating collision cross sections of negatively charged N-glycans using traveling wave ion mobility-mass spectrometry

Glycosylation is one of the most common post-translational modifications occurring in proteins. A detailed structural characterization of the involved carbohydrates, however, is still one of the greatest challenges in modern glycoproteomics, since multiple regio- and stereoisomers with an identical monosaccharide composition may exist. Recently, ion mobility-mass spectrometry (IM-MS), a technique in which ions are separated according to their mass, charge, and shape, has evolved as a promising technique for the separation and structural analysis of complex carbohydrates. This growing interest is based on the fact that the measured drift times can be converted into collision cross sections (CCSs), which can be compared, implemented into databases, and used as additional search criteria for structural identification. However, most of the currently used commercial IM-MS instruments utilize a nonuniform traveling wave field to propel the ions through the IM cell. As a result, CCS measurements cannot be performed directly and require calibration. Here, we present a calibration data set consisting of over 500 reference CCSs for negatively charged N-glycans and their fragments. Moreover, we show that dextran, already widely used as a calibrant in high performance liquid chromatography, is also a suitable calibrant for CCS estimations. Our data also indicate that a considerably increased error has to be taken into account when reference CCSs acquired in a different drift gas are used for calibration.

Mass spectrometry of glycans

Powerful new strategies based on mass spectrometry are revolutionizing the structural analysis and profiling of glycans and glycoconjugates. We survey here the major biosynthetic pathways that underlie the biological diversity in glycobiology, with emphasis on glycoproteins, and the approaches that can be used to address the resulting heterogeneity. Included among these are derivatizations, on- and off-line chromatography, electrospray and matrix-assisted laser desorption/ionization, and a variety of dissociation methods, the recently introduced electron-based techniques being of particular interest.

Ion mobility-mass spectrometry of complex carbohydrates: collision cross sections of sodiated N-linked glycans

Currently, the vast majority of complex carbohydrates are characterized using mass spectrometry (MS)-based techniques. Measuring the molecular mass of a sugar, however, immediately poses a fundamental problem: entire classes of the constituting monosaccharide building blocks exhibit an identical atomic composition and, consequently, also an identical mass. Therefore, carbohydrate MS data can be highly ambiguous and often it is simply not possible to clearly assign a particular molecular structure. A promising approach to overcome the above-mentioned limitation is to implement an additional gas-phase separation dimension using ion mobility spectrometry (IMS), which is a method in which molecules of identical mass and structure but different structure can be separated according to their shape and collision cross section (CCS). With the emergence of commercially available hybrid ion mobility-mass spectrometry (IM-MS) instruments in 2006, IMS technology became readily available. Because of the nonhomogeneous, traveling wave (TW) field utilized in these instruments, however, CCS values currently cannot be determined directly from the drift times measured. Instead, an external calibration using compounds of known CCS and similar molecular identity is required. Here, we report a calibration protocol for TW IMS instruments using a series of sodiated N-glycans that were released from commercially available glycoproteins using an easy-to-follow protocol. The underlying CCS values were determined using a modified Synapt HDMS instrument with a linear drift tube, which was described in detail previously. Our data indicate that, under in-source fragmentation conditions, only a few glycans are required to obtain a TW IMS calibration of sufficient quality. In this context, however, the type of glycan was shown to be of tremendous importance. Furthermore, our data clearly demonstrate that carbohydrate isomers with identical mass but different conformation can be distinguished based on their CCS when all the associated errors are taken into account.

MALDI-MS/MS with traveling wave ion mobility for the structural analysis of N-linked glycans

The preference for singly charged ion formation by MALDI makes it a better choice than electrospray ionization for profiling mixtures of N-glycans. For structural analysis, fragmentation of negative ions often yields more informative spectra than fragmentation of positive ones but such ions are more difficult to produce from neutral glycans under MALDI conditions. This work investigates conditions for the formation of both positive and negative ions by MALDI from N-linked glycans released from glycoproteins and their subsequent MS/MS and ion mobility behaviour. 2,4,6-Trihydroxyacetophenone (THAP) doped with ammonium nitrate was found to give optimal ion yields in negative ion mode. Ammonium chloride or phosphate also yielded prominent adducts but anionic carbohydrates such as sulfated N-glycans tended to ionize preferentially. Carbohydrates adducted with all three adducts (phosphate, chloride, and nitrate) produced good negative ion CID spectra but those adducted with iodide and sulfate did not yield fragment ions although they gave stronger signals. Fragmentation paralleled that seen following electrospray ionization providing superior spectra than could be obtained by PSD on MALDI-TOF instruments or with ion traps. In addition, ion mobility drift times of the adducted glycans and the ability of this technique to separate isomers also mirrored those obtained following ESI sample introduction. Ion mobility also allowed profiles to be obtained from samples whose MALDI spectra showed no evidence of such ions allowing the technique to be used in conditions where sample amounts were limiting. The method was applied to N-glycans released from the recombinant human immunodeficiency virus glycoprotein, gp120.

Production and fragmentation of negative ions from neutral N-linked carbohydrates ionized by matrix-assisted laser desorption/ionization

Although negative ion fragmentation mass spectra of neutral N-linked carbohydrates (those attached to Asn in glycoproteins) provide much more structural information than spectra recorded in positive ion mode, neutral carbohydrates are reluctant to form negative ions by matrix-assisted laser desorption/ionization (MALDI) unless ionized from specific matrices such as nor-harmane or adducted with anions such as chloride. This paper reports the results of experiments to optimize negative ion formation from adducts of N-linked glycans with respect to ion abundance and fragment ion production. The best results were obtained with 2,4,6-trihydroxyacetophenone (THAP) as the matrix with added ammonium nitrate as the salt providing the anion. This approach is demonstrated to be applicable for a wide range of N-linked glycan structures. Phosphate adducts, analogous to those that are usually encountered in electrospray spectra from N-glycans released by protein N-glycosidase F, were produced by addition of ammonium phosphate to the matrix but in relatively low yield allowing competitive ionization of endogenous anionic compounds leading to complex spectra. Fragmentation of the nitrate adducts, which were formed in higher yield, generally paralleled that seen by collision-induced dissociation following ionization by electrospray, with the first stage of the dissociation being the elimination of the nitrate with a proton from one of the hydroxyl groups of the sugar. The spectra of the resulting [M-H](-) species displayed very specific fragment ions, mainly cross-ring and C-type glycosidic cleavage products, that revealed more structural (linkage and branching) information of the compounds than the mainly glycosidic cleavage products that dominated the positive ion spectra.

Fragmentation of negative ions from N-linked carbohydrates, part 4. Fragmentation of complex glycans lacking substitution on the 6-antenna

Negative ion CID spectra of N-linked glycans released from glycoproteins contain many ions that are diagnostic for specific structural features such as the detailed arrangement of antennae and the location of fucose residues. Identification of such ions requires reference glycans that are often difficult to acquire in a pure state. The recent acquisition of a sample of N-glycans from a patient lacking the enzyme N-acetylglucosaminyltransferase-2 provided an opportunity to investigate fragmentation of glycans lacking a 6-antenna. These glycans contained one or two galactose-N-acetylglucosamine-chains attached to the 3-linked mannose residue of the trimannosyl-chitobiose core with and without fucose substitution. The spectra from the patient sample clearly defined the antenna distribution and showed striking differences from the spectra of isomeric compounds obtained from normal subjects. Furthermore, they provided additional information on previously identified antenna-specific fragment ions and indicated the presence of additional ions that were diagnostic of fucose substitution. Glycans obtained from such enzyme-deficient patients can, thus, be a valuable way of obtaining spectra of specific isomers in a relatively pure state for interpretation of mass spectra.

Derivatization of sialic acids for stabilization in matrix-assisted laser desorption/ionization mass spectrometry and concomitant differentiation of alpha(2 --> 3)- and alpha(2 --> 6)-isomers

Sialylated carbohydrates usually decompose by loss of sialic acid when ionized by matrix-assisted laser desorption/ionization (MALDI) as the result of the labile carboxylic proton. Stabilization has previously been achieved by forming methyl esters with methyl iodide, a procedure that eliminates the labile proton. In this paper, we describe an alternative procedure for methyl ester formation that provides information on the sialic acid linkage directly from the MALDI spectrum. The sugars were desalted, dissolved in methanol, and treated with 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMT-MM). After removal of the solvent, the products were transferred directly to the MALDI target and examined from 2,5-dihydroxybenzoic acid. Small amounts of N-glycans derived from biological sources benefited from an additional clean-up stage involving Nafion 117. alpha(2 --> 6)-Linked sialic acid produced only methyl esters whereas alpha(2 --> 3)-linked sialic acids were converted into their lactones providing a 32 Da difference in mass. Negative ion collision-induced decomposition (CID) mass spectra of these neutralized glycans provided information, in many cases, on the antenna of N-linked glycans to which the variously linked sialic acids were attached. The method was applied to N-linked glycans released from bovine fetuin and porcine thyroglobulin.

Differentiation between isomeric triantennary N-linked glycans by negative ion tandem mass spectrometry and confirmation of glycans containing galactose attached to the bisecting (beta1-4-GlcNAc) residue in N-glycans from IgG

Negative ion tandem mass spectrometry (MS/MS) spectra of three isomeric triantennary N-linked glycans provided clear differentiation between the isomers and confirmed the occurrence of an isomer that was substituted with galactose on a bisecting GlcNAc (1 --> 4-substituted on the core mannose) residue recently reported by Takegawa et al. from N-glycans released from human immunoglobulin G (IgG). We extend this analysis of human serum IgG to reveal an analogue of the fucosylated triantennary glycan reported by Takegawa et al. together with a third compound that lacked both the sialic acid and the fucose residues. In addition, we demonstrate the biosynthesis of bisected hybrid-type glycans with the galactose modification, with and without core fucose, on the stem cell marker glycoprotein, 19A, expressed in a partially ricin-resistant human embryonic kidney cell line. It would appear, therefore, that this modification of N-linked glycans containing a galactosylated bisecting GlcNAc residue may be more common than originally thought. Negative ion MS/MS analysis of glycans is likely to prove an invaluable tool in the analysis and monitoring of therapeutic glycoproteins.

Collisionally activated dissociation and electron capture dissociation provide complementary structural information for branched permethylated oligosaccharides

Doubly charged sodiated and permethylated linear malto-oligosaccharides ({Glc}6-{Glc}9), branched N-linked glycans (high-mannose type GlcNAc2Man5-9, and complex asialo- and disialylated-biantennary glycans) were analyzed by tandem mass spectrometry using collisionally-activated dissociation (CAD) and "hot" electron capture dissociation (ECD) available in a custom-built ESI FTICR mass spectrometer. For linear permethylated malto-oligosaccharides, both CAD and "hot" ECD produced glycosidic cleavages (B, Y, C, and Z ions), cross-ring cleavages (A- and X-type), and internal cleavages (B/Y and C/Y type) to provide sequence and linkage information. For the branched N-linked glycans, CAD and "hot" ECD provided complementary structural information. CAD generated abundant B and Y fragment ions by glycosidic cleavages, whereas "hot" ECD produced dominant C and Z ions. A-type cross-ring cleavages were present in CAD spectra. Complementary A- and X-type cross-ring fragmentation pairs were generated by "hot" ECD, and these delineated the branching patterns and linkage positions. For example, 0, 4An and 3, 5An ions defined the linkage position of the major branch as the 6-position of the central core mannose residue. The internal fragments observed in CAD were more numerous and abundant than in "hot" ECD spectra. Since the triply charged (sodiated) molecular ion of the permethylated disialylated-biantennary N-linked glycan has relatively high abundance, it was isolated and fragmented in a "hot" ECD experiment and extensive fragment ions (glycosidic and complementary pairs of cross-ring cleavages) were generated to fully confirm the sequence, branching, and linkage assignments for this glycan.

A practical approach to N-glycan production by hydrazinolysis using hydrazine monohydrate

Hydrazinolysis is a versatile method to liberate N-linked glycans from glycoproteins. However, the method is usually performed with anhydrous hydrazine, a highly toxic and explosive chemical used in rocket fuel. Thus despite the need to produce functionally important glyco-materials, hydrazinolysis is limited to small scale (e.g., 0.2-1 mL) reactions. In the present study, we report an alternative procedure for hydrazinolysis using hydrazine monohydrate in place of anhydrous hydrazine. The developed procedure was applied to both purified glycoproteins (Taka-amylase and transferrin) and hen egg yolk protein fraction with comparable yields to the traditional method using anhydrous hydrazine. The sialyl linkage of alpha2-6disialobiantennary oligosaccharides proved to be fully stable. The developed procedure facilitated the large-scale preparation of N-linked glycans. The new method should make a substantial contribution to both small- and large-scale production of functional glycans, including therapeutically relevant human-type glycans.

Glycan analysis of the chicken synaptic plasma membrane glycoproteins--a major synaptic N-glycan carries the LewisX determinant

The majority of synaptic plasma membrane components are glycosylated. It is now widely accepted that this post-translational modification is crucial during the establishment, maintenance and function of the nervous system. Despite its significance, structural information about the glycosylation of nervous system specific glycoproteins is very limited. In the present study the major glycan structures of the chicken synaptic plasma membrane (SPM) associated glycoprotein glycans were determined. N-glycans were released by hydrazinolysis, labelled with 2-aminobenzamide, treated with neuraminidase and subsequently fractionated by size exclusion chromatography. Individual fractions were characterized by the combination of high-pressure liquid chromatography, exoglycosidase treatment or reagent array analysis method (RAAM). In addition to oligomannose-type glycans, core-fucosylated complex glycans with biantennary bisecting glycans carrying the LewisX epitope were most abundant. The overall chicken glycan profile was strikingly similar to the rat brain glycan profile. The presence of the LewisX determinant in relatively large proportions suggests a tissue-specific function for these glycans.

Collision-induced fragmentation of negative ions from N-linked glycans derivatized with 2-aminobenzoic acid

N-Linked glycans from bovine ribonuclease B, chicken ovalbumin, bovine fetuin, porcine thyroglobulin and human alpha(1)-acid glycoprotein were derivatized with 2-aminobenzoic acid by reductive amination and their tandem mass spectra were recorded by negative ion electrospray ionization with a quadrupole time-of-flight mass spectrometer. Derivatives were also prepared from 2-amino-5-methyl- and 2-amino-4,5-dimethoxybenzoic acid in order to confirm the identity of fragment ions containing the reducing terminus. Major fragments from the [M - H](-) ions from the neutral glycans retained the derivative (Y-type cleavages) and provided information on sequence and branching. Other major fragments were products of A-type cross-ring cleavages giving information on antenna structure. Singly doubly and triply charged ions were formed from sialylated glycans. They produced major fragments by loss of sialic acid and a series of singly charged ions that were similar to those from the neutral analogues. Doubly charge ions were also produced by the neutral glycans and were fragmented to form product ions with one and two charges. Again, the fragment ions with a single charge were similar to those from the singly charged parents, but branching information was less obvious because of the occurrence of more abundant ions produced by multiple cleavages. Detection limits were around 200 fmol (3 : 1 signal-to-noise ratio).

Fragmentation of negative ions from carbohydrates: part 2. Fragmentation of high-mannose N-linked glycans

[M + NO3]- And [M + (NO3)2]2- ions were produced by electrospray from neutral high-mannose ([Man](5-9)[GlcNAc]2, [Glc](1-3)[Man](4-9)[GlcNAc]2) N-linked glycans and their 2-aminobenzamide derivatives sprayed from methanol:water containing ammonium nitrate. Low energy collision-induced decomposition (CID) spectra of both types of ions were almost identical and dominated by cross-ring and C-type fragments, unlike the corresponding spectra of the positive ions that contained mainly B- and Y-type glycosidic fragments. This behavior could be rationalized by an initial proton abstraction from various hydroxy groups by the initially-formed anionic adduct. These negative ion spectra were more informative than the corresponding positive ion spectra and contained prominent ions that were diagnostic of structural features such as the composition of individual antennas that were not easily obtainable by other means. C-ions defined the sequence of the constituent monosaccharide residues. Detailed fragmentation mechanisms are proposed to account for many of the diagnostic ions.

Fragmentation of negative ions from carbohydrates: part 1. Use of nitrate and other anionic adducts for the production of negative ion electrospray spectra from N-linked carbohydrates

Negative ion spectra of N-linked glycans were produced by electrospray from a dilute solution of the glycans and various salts in methanol:water using a Waters-Micromass Q-TOF Ultima Global tandem quadrupole/time-of-flight (Q-TOF) mass spectrometer. Stable anionic adducts were formed with chloride, bromide, iodide, nitrate, sulphate, and phosphate. Unstable adducts that fragmented by a cross-ring cleavage of the reducing N-acetylglucosamine (GlcNAc) residue, were formed with fluoride, nitride, sulphide, carbonate, bicarbonate, hydroxide, and acetate. Nitrate adducts prepared from ammonium nitrate produced the most satisfactory spectra as they were relatively free from in-source fragmentation products and gave signals that were about ten times as strong as those from corresponding [M - H]- ions prepared from solutions containing ammonium hydroxide. Detection limits were in the region of 20 fmol. Neutral glycans gave both singly- and doubly-charged ions with the larger glycans preferring the formation of doubly-charged ions. Acidic glycans with several acidic groups gave ions in higher charge states as the result of ionization of the anionic groups. Low energy collision-induced decomposition (CID) spectra of the singly-charged ions were dominated by cross-ring and C-type fragments, unlike the corresponding spectra of the positive ions that contained mainly B- and Y-type glycosidic fragments. Formation of these ions could be rationalized by proton abstraction from various hydroxy groups by an initially-formed anionic adduct. Prominent glycosidic and cross-ring cleavage ions defined structural features such as the specific composition of each of the two antennae, presence of a bisecting GlcNAc residue and location of fucose residues, details that were difficult to determine by conventional techniques. Acidic glycans fragmented differently on account of charge localization on the acid functions rather than the hydroxy groups.

Structural determination ofN-linked glycans by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry

This paper reviews methods for the analysis of N-linked glycans by mass spectrometry with emphasis on studies conducted at the Oxford Glycobiology Institute. Topics covered are the release of glycans from sodium dodecyl sulphate-polyacrylamide gel electrophoresis gels, their purification for analysis by mass spectrometry, methods based on matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization for producing fragment ions, and details of their fragmentation. MALDI mass spectrometry provided a rapid method for profiling neutral N-linked glycans as their [M + Na](+) ions which could be fragmented by collision-induced decomposition to give spectra containing both glycosidic and cross-ring fragments. Electrospray ionization mass spectrometry was more versatile in that it was relatively easy to change the type of ion that was formed and, furthermore, unlike MALDI, electrospray did not cause extensive loss of sialic acids from sialylated glycans. Negative ions formed by addition of anions such as chloride and, particularly, nitrate, to the electrospray solvent were stable and enabled singly charged ions to be obtained from larger glycans than was possible in positive ion mode. Fragmentation of negative ions followed specific pathways that defined structural details of the glycans that were difficult to obtain by classical methods such as exoglycosidase digestion.

Fragmentation of negative ions from carbohydrates: part 3. Fragmentation of hybrid and complex N-linked glycans

Hybrid and complex N-linked glycans were ionized by electrospray in the presence of ammonium nitrate to give [M + NO3]- and [M + (NO3)2]2- ions. Low energy collision-induced decomposition (CID) spectra of both types of ions were almost identical and were dominated by C-type glycosidic and cross-ring fragments, unlike the corresponding spectra of the positive ions that contained mainly B- and Y-type glycosidic fragments. Also, in contrast to fragments in the positive ion spectra, many of these ions appeared to be produced by single pathways following proton abstraction from specific hydroxy groups. Consequently, many ions were diagnostic for specific structural features. Such features included the composition of each of the two antennas, the presence or absence of a bisecting GlcNAc residue, and the location of fucose residues on the core GlcNAc residues and on the antennas. C-ions defined the sequence of the constituent monosaccharide residues. Detailed fragmentation mechanisms are proposed to account for several of the diagnostic ions.

Ionization and fragmentation of N-linked glycans as silver adducts by electrospray mass spectrometry

[M+Ag]+ ions were produced by electrospray from neutral high-mannose, hybrid and complex N-linked glycans obtained from bovine ribonuclease, chicken egg glycoproteins, bovine fetuin and porcine thyroglobulin by the addition of silver nitrate to the electrospray solvent. Both singly and doubly charged ions were produced but, as the signals were split between the two silver isotopes, sensitivity was not as high as with the sodium adducts reported earlier. Collision-induced dissociation (CID) spectra were dominated by ions produced by glycosidic cleavages, mainly of the B- and Y-type. Internal cleavage ions involving both B and Y cleavages were very prominent but cross-ring fragments were generally of very low abundance or absent. Silver was very efficient at cleaving the glycosidic bonds, so much so that spectra tended to contain glycosidic ions at most possible combinations of the constituent monosaccharides.

Halogeno-substituted 2-aminobenzoic acid derivatives for negative ion fragmentation studies of N-linked carbohydrates

Negative ion electrospray mass spectra of high-mannose N-linked glycans derivatised with 2-aminobenzoic acids and ionised from solutions containing ammonium hydroxide gave prominent [M-H](-) ions accompanied by weaker [M-2H](2-) ions. Fragmentation of both types of ions gave prominent singly charged glycosidic cleavage ions containing the derivatised reducing terminus and ions from the non-reducing terminus that appeared to be products of cross-ring cleavages. Differentiation of these two groups of ions was conveniently achieved in a single spectrum by use of chloro- or bromo-substituted benzoic acids in order to label ions containing the derivative with an atom with a distinctive isotope pattern. Fragmentation of the doubly charged ions gave more abundant fragments, both singly and doubly charged, than did fragmentation of the singly charged ions, but information of chain branching was masked by the appearance of prominent ions produced by internal cleavages.

Fragmentation of N-linked glycans with a matrix-assisted laser desorption/ionization ion trap time-of-flight mass spectrometer

N-Linked glycans were ionized from several matrices with a Shimadzu-Biotech AXIMA-QIT matrix-assisted laser desorption/ionization quadrupole ion trap time-of-flight mass spectrometer. [M+Na]+ ions were produced from all matrices and were accompanied by varying amounts of in-source fragmentation products. The least fragmentation was produced by 2,5-dihydroxybenzoic acid and the most by alpha-cyano-4-hydroxycinnamic acid and 6-aza-2-thiothymine. Sialic acid loss was extensive but could be prevented by formation of methyl esters. Fragmentation produced typical low-energy-type spectra dominated by ions formed by glycosidic cleavages. MS(n) spectra (n = 3 and 4) were used to probe the pathways leading to the major diagnostic ions. Thus, for example, an ion that was formed by loss of the core GlcNAc residues and the 3-antenna was confirmed as being formed by a B/Y rather than a C/Z mechanism. The proposed structures of several cross-ring cleavage ions were confirmed and it was shown that MS3 spectra could be obtained from as little as 10 fmol of glycan.

Reversed-phase ion-pairing liquid chromatography/ion trap mass spectrometry for the analysis of negatively charged, derivatized glycans

The significant complexity, similar polarity and lack of ionizable sites make the analysis of glycans an analytical challenge. These compounds are often derivatized and separated by normal-phase high-performance liquid chromatography (HPLC) or capillary electrophoresis (CE) followed by UV or fluorescence detection. Due to widespread use of reversed-phase chromatography coupled to electrospray mass spectrometry as an analytical tool, our laboratory has developed this methodology for the analysis of glycans derivatized with a negatively charged tag, 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS). It is possible to exploit the ability of this negatively charged tag to interact with a mobile phase ion-pairing reagent, allowing retention on a reversed-phase C(18) column for subsequent on-line UV or MS analysis. ANTS-derivatized samples, including a maltooligosaccharide ladder and glycans released from bovine ribonuclease B, bovine fetuin, and chicken ovalbumin, were analyzed using this method. In addition to reversed-phase retention, ribonuclease B and ovalbumin derivatives displayed highly desirable isomeric separation. With the use of mass spectrometric detection for glycan identity, this allowed relative quantitation of individual components.

Use of proteomic methodology for the characterization of human milk fat globular membrane proteins

Characterization of the major human milk fat globular membrane proteins was carried out using proteomic techniques comprising two-dimensional polyacrylamide gel electrophoresis, followed by in situ PNGase F and trypsin digestion. Matrix-assisted laser desorption/ionization quadrupole time-of-flight and electrospray ionization mass spectrometry identified seven major protein components: alpha-lactalbumin, lysozyme precursor, beta-casein, clusterin, lactotransferrin, polymeric immunoglobulin receptor precursor, and human milk fat globule EGF-factor 8 protein. Sequence information on the protein-associated glycans was determined by matrix-assisted laser desorption-ionization quadrupole time-of-flight hybrid mass spectrometry. This glycan analysis revealed interesting fucosylation branching patterns which may be influential in maternal protection of the newborn against bacterial and viral pathogenic attack.

Capillary electrophoresis/electrospray ion trap mass spectrometry for the analysis of negatively charged derivatized and underivatized glycans

The increasing interest in the development of glycoproteins for therapeutic purposes has created a greater demand for methods to characterize the sugar moieties bound to them. Traditionally, released carbohydrates are derivatized using such methods as permethylation or fluorescent tagging prior to analysis by high performance liquid chromatography (HPLC), capillary electrophoresis (CE), or direct infusion mass spectrometry. However, little research has been performed using CE with on-line mass spectrometry (MS) detection. The CE separation of neutral oligosaccharides requires the covalent attachment of a charged species for electrophoretic migration. Among charged labels which have shown promise in assisting CE and HPLC separation is the fluorophore 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS). This report describes the qualitative profiling of charged ANTS-derivatized and underivatized complex glycans by CE with on-line electrospray ion trap mass spectrometry. Several neutral standard glycans including a maltooligosaccharide ladder were derivatized with ANTS and subjected to CE/UV and CE/MS using low pH buffers consisting of citric and 6-aminocaproic acid salts. The ANTS-derivatized species were detected as negative ions, and multiple stage MS analysis provided valuable structural information. Fragment ions were easily identified, showing promise for the identification of unknowns. N-Linked glycans released from bovine fetuin were used to demonstrate the applicability of ANTS derivatization followed by CE/MS for the analysis of negatively charged glycans. Analyses were performed on both underivatized and ANTS-derivatized species, and sialylated glycans were separated and detected in both forms. The ability of the ion trap mass spectrometer to perform multiple stage analysis was exploited, with MS5 information obtained on selected glycans. This technique presents a complementary method to existing methodologies for the profiling of glycan mixtures.

Enhanced 3-O-sulfation of galactose in Asn-linked glycans and Maackia amurensis lectin binding in a new Chinese hamster ovary cell line

We report the characterization of two Chinese hamster ovary cell lines that produce large amounts of sulfated N-linked oligosaccharides. Clones 26 and 489 were derived by stable transfection of the glycosaminoglycan-deficient cell mutant pgsA-745 with a cDNA library prepared from wild-type cells. Peptide:N-glycanase F released nearly all of the sulfate label, indicating that sulfation had occurred selectively on the Asn-linked glycans. Hydrazinolysis followed by nitrous acid treatment at pH 4 and borohydride reduction yielded reduced sulfated disaccharides that comigrated with standard Gal3SO4beta1-4anhydromannitol. The disaccharides were resistant to periodate oxidation but became sensitive after the sulfate group was removed by methanolysis, indicating that the sulfate was located at C3 of the galactose residues. Maackia amurensis lectin bound to the sulfated glycopeptides on the cell surface and in free form, even after sialidase treatment. This finding indicates that the lectin requires only a charged group at C3 of the galactose unit and not an intact sialic acid. Growth of cells with chlorate restored sialidase sensitivity to lectin binding, indicating that sulfation and sialylation occurred largely at the same sites. The enhanced sulfation was due to elevated sulfotransferase activity that catalyzed transfer of sulfate from phosphoadenosine-5'-phosphosulfate to Galbeta1-4(3)GlcNAcbeta-O-naphthalenemethanol.

Do rodent and human brains have different N-glycosylation patterns?

A large number of studies on the structure of N-glycosidically linked oligosaccharides from glycoproteins of different organs and/or different species have been carried out in the past using various combinations of techniques such as monosaccharide analysis, permethylation, peracteylation, exoglycosidase sequencing, normal and reversed phase HPLC, mass spectrometry and nuclear magnetic resonance spectroscopy. Although it is widely accepted that the processing of N-glycans in the ER and Golgi of mammalian cells follows the same principal metabolic rules, analyses have revealed that the glycosylation pattern of a particular protein may differ depending on the cell type in which it is expressed. N-glycans from brain glycoproteins have been shown to include a variety of hybrid- and complex-type structures with structural features that are not so commonly found on glycoproteins from other organs and which have, therefore, been classified as 'brain-specific'. Comparison of the N-glycans of glycoproteins from homogenates of rat, mouse and human brains confirm that, in general, glycoproteins from human brain show a similar profile of brain-specific N-glycans as glycoproteins from mouse and rat brain.

Exploring the glycan repertoire of genetically modified mice by isolation and profiling of the major glycan classes and nano-NMR analysis of glycan mixtures

The production of mice with genetic alterations in glycosyltransferases has highlighted the need to isolate and study complex mixtures of the major classes of oligosaccharides (glycans) from intact tissues. We have found that nano-NMR spectroscopy of whole mixtures of N- and O-glycans can complement HPLC profiling methods for elucidating structural details. Working toward obtaining such glycan mixtures from mouse tissues, we decided to develop an approach to isolate not only N- and O-glycans, but also to separate out glycosphingolipids, glycosaminoglycans and glycosylphosphatidylinositol anchors. We describe here a comprehensive Glycan Isolation Protocol that is based primarily upon the physicochemical characteristics of the molecules, and requires only commonly available reagents and equipment. Using radiolabeled internal tracers, we show that recovery of each major class of glycans is as good or better than with conventional approaches for isolating individual classes, and that cross-contamination is minimal. The recovered glycans are of sufficient purity to provide a "glycoprofile" of a cell type or tissue. We applied this approach to compare the N- and O-glycans from wild type mouse tissues with those from mice genetically deficient in glycosyltransferases. N- and O-glycan mixtures from organs of mice deficient in ST6Gal-I (CMP-Sia:Galbeta1-4GlcNAc alpha2-6 sialyltransferase) were studied by the nano-NMR spectroscopy approach, showing no detectable alpha2-6-linked sialic acids. Thus, ST6Gal-I is likely responsible for generating most or all of these residues in normal mice. Similar studies indicate that this linkage is very rare in ganglioside glycans, even in wild-type tissues. In mice deficient in GalNAcT-8 (UDP-GalNAc:polypeptide O-Ser/Thr GalNAc transferase 8), HPLC profiling indicates that O-glycans persist in the thymus in large amounts, without a major change in overall profile, suggesting that other enzymes can synthesize the GalNAc-O-Ser/Thr linkage in this tissue. These results demonstrate the applicability of nano-NMR spectroscopy to complex glycan mixtures, as well as the versatility of the Glycan Isolation Protocol, which makes possible the concurrent examination of multiple glycan classes from intact vertebrate tissues.

Characterisation of tissue-specific oligosaccharides from rat brain and kidney membrane preparations enriched in Na+,K+-ATPase

The organ-specific nature of the glycosylation of Na+,K+-ATPase-enriched preparations from kidney and brain tissues has earlier been indicated by the use of lectin-staining techniques. Na+,K+-ATPase is ubiquitous and abundant, and subject to upregulation during cell-division and in certain pathological conditions. Lectins specific for the different carbohydrates displayed by the Na+,K+-ATPases may, therefore, be useful carriers/mediators in tissue-specific targeting. N-linked oligosaccharides purified from Na+,K+-ATPase-enriched preparations from rat brain and kidney were consequently characterised in detail in this study using weak anion exchange and normal phase HPLC (combined with serial glycosidase digestions) and matrix-assisted laser desorption/ionisation mass spectrometry. The oligomannose series of glycans were most abundant in the brain tissue preparation and this contrasted with the renal-associated oligosaccharides that were dominated by families of tetra-antennary glycans (with/without a core fucose) with up to four lactosaminylglycan residues in either branched or linear formation.

Developmental changes in Asn-linked neutral oligosaccharides in murine cerebrum

The changes in Asn-linked oligosaccharide composition in the murine cerebrum during development have been examined by high-performance liquid chromatography (HPLC) and electrospray ionization mass spectrometry (ESI-MS). The oligosaccharides, obtained from murine cerebrum in several developmental stages, were separated by HPLC on anion-exchange and reverse-phase columns. We found that two Asn-linked oligosaccharides, designated oligosaccharide I and oligosaccharide II, had their expression changed during postnatal development. Whereas oligosaccharide I was reduced during brain development, oligosaccharide II was increased. The structures of oligosaccharides I and II were analyzed by ESI-MS and sequential exoglycosidase digestions. Judging from the molecular and fragment ions in each oligosaccharide, the oligosaccharide I was composed of 5Hex+2HexNAc+ABOE (MW 1467.2) and the oligosaccharide II was 3Hex+4HexNAc+DoHex+ABOE (MW 1695.2). The results of sequential exoglycosidase digestion indicated that the oligosaccharide I was an oligomannose type saccharide and the oligosaccharide II was a biantennary complex type saccharide including fucose. The proposed structures are shown below. These results offer an important clue to the role of Asn-linked oligosaccharides associated with development of the central nervous system.

The solution NMR structure of glucosylated N-glycans involved in the early stages of glycoprotein biosynthesis and folding

Glucosylated oligomannose N-linked oligosaccharides (Glc(x)Man9GlcNAc2 where x = 1-3) are not normally found on mature glycoproteins but are involved in the early stages of glycoprotein biosynthesis and folding as (i) recognition elements during protein N-glycosylation and chaperone recognition and (ii) substrates in the initial steps of N-glycan processing. By inhibiting the first steps of glycan processing in CHO cells using the alpha-glucosidase inhibitor N-butyl-deoxynojirimycin, we have produced sufficient Glc3Man7GlcNAc2 for structural analysis by nuclear magnetic resonance (NMR) spectroscopy. Our results show the glucosyl cap to have a single, well-defined conformation independent of the rest of the saccharide. Comparison with the conformation of Man9GlcNAc2, previously determined by NMR and molecular dynamics, shows the mannose residues to be largely unaffected by the presence of the glucosyl cap. Sequential enzymatic cleavage of the glucose residues does not affect the conformation of the remaining saccharide. Modelling of the Glc3Man9GlcNAc2, Glc2Man9GlcNAc2 and Glc1Man9GlcNAc2 conformations shows the glucose residues to be fully accessible for recognition. A more detailed analysis of the conformations allows potential recognition epitopes on the glycans to be identified and can form the basis for understanding the specificity of the glucosidases and chaperones (such as calnexin) that recognize these glycans, with implications for their mechanisms of action.

Brain contains HNK-1 immunoreactive O-glycans of the sulfoglucuronyl lactosamine series that terminate in 2-linked or 2,6-linked hexose (mannose)

The monoclonal antibody HNK-1 originally raised to an antigenic marker of natural killer cells also binds to selected regions in nervous tissue. The antigen is a carbohydrate that has attracted much interest as its expression is developmentally regulated in nervous tissue, and it is found, and proposed to be a ligand, on several of the adhesive glycoproteins of the nervous system. It is also expressed on glycolipids and proteoglycans, and is the target of monoclonal auto-antibodies that give rise to a demyelinating disease. The epitope, as characterized on glycolipids isolated from the nervous system, is expressed on 3-sulfated glucuronic acid joined by beta1-3-linkage to a neolacto backbone. Here we exploit the neoglycolipid technology, in conjunction with immunodetection and in situ liquid secondary ion mass spectrometry, to characterize HNK-1-positive oligosaccharide chains derived by reductive alkaline release from total brain glycopeptides. The immunoreactive oligosaccharides detected are tetra- to octasaccharides that are very minor components among a heterogeneous population, each representing less than 0.1% of the starting material. Their peripheral and backbone sequences resemble those of the HNK-1-positive glycolipids. An unexpected finding is that they terminate not with N-acetylgalactosaminitol but with hexitol (2-substituted and 2,6-disubstituted). In a tetrasaccharide investigated in the greatest detail, the hexitol is identified as 2-substituted mannitol.

Unusual anionic N-linked oligosaccharides from bovine lung

We previously described a diverse family of sulfated anionic N-linked oligosaccharides released by peptide: N-glycosidase F (PNGaseF) from calf pulmonary artery endothelial (CPAE) cells (Roux, L., Holoyda, S., Sundblad, G., Freeze, H.H., and Varki, A. (1988) J. Biol. Chem. 263, 8879-8889). Since a major fraction of the intact lung consists of endothelial cells, we reasoned that bovine lung might be a rich source of similar molecules. Total N-linked oligosaccharides from bovine lung acetone powder were released by PNGaseF, labeled by [3H]NaBH4 reduction, and the anionic fractions were studied with a variety of techniques. The sugar chains with lesser negative charge (designated Class I) share several properties of conventional multiantennary complex-type chains. However, unlike the case with CPAE cells, sialic acids account only for a minority of the anionic properties and only a small proportion carry sulfate esters. A variety of different treatments indicate that most of the unexplained negative charge is due to multiple carboxylic acid groups. Resistance to beta-glucuronidase and alpha-iduronidase suggests that these may be previously undescribed modifications of mammalian oligosaccharides. The most highly charged N-linked chains (designated Class II) are more similar in general structure to the corresponding ones from CPAE cells, although relatively more abundant. Their high charge is primarily due to chondroitin sulfate, heparin/heparan sulfate, or keratan sulfate glycosaminoglycan chains. Sequential digestion studies suggest that a significant proportion of these molecules have more than one type of glycosaminoglycan chain associated with them. Compositional analysis indicates the presence of xylose residues in Class II, but not Class I molecules. However, unlike the case with conventional glycosaminoglycans, these residues are not at the reducing terminus. Most previously reported structures of complex-type N-linked oligosaccharides are derived from the glycoproteins of blood cells, plasma, or the secretions of cultured mammalian cells. This library of N-linked oligosaccharides from an intact mammalian organ (lung) contains a high proportion of novel anionic sugar chains whose structures are different from conventional complex-type sialylated chains and only partially related to those from CPAE cells. Further exploration of the N-linked chains of intact mammalian tissues seems warranted.

Glycosylation of CD4 and Thy-1

The site-specific glycosylation of soluble recombinant variants of human and rat CD4 (sCD4) expressed in Chinese hamster ovary (CHO) cells has been characterized. The presence of identical oligosaccharides at the conserved glycosylation site in domain 3 of rat and human sCD4 and the greater abundance of oligomannose and hybrid type glycans at the non-conserved glycosylation site of rat sCD4 clearly indicate that the protein structure influences oligosaccharide processing. Comparisons of rat sCD4 glycopeptides with mutant molecules with only single glycosylation sites and with a truncated form containing only the two NH2-terminal domains, indicate that independent processing occurs at each glycosylation site and that domain interactions can also affect oligosaccharide processing. These and other analyses of sCD2 expressed in CHO cells and Thy-1 purified from various tissues suggest that the diversity of oligosaccharide structures on a protein is regulated by the location of the glycosylation sites and the nature of the target protein, cell and tissue. The functional significance of this control remains to be determined.