Identification of organ-specific glycosylation of a membrane protein in two tissues using lectins

Capitalizing glycomic changes for improved biomarker-based cancer diagnostics

Cancer serum biomarkers are valuable or even indispensable for cancer diagnostics and/or monitoring and, currently, many cancer serum markers are routinely used in the clinic. Most of those markers are glycoproteins, carrying cancer-specific glycan structures that can provide extra-information for cancer monitoring. Nonetheless, in the majority of cases, this differential feature is not exploited and the corresponding analytical assays detect only the protein amount, disregarding the analysis of the aberrant glycoform. Two exceptions to this trend are the biomarkers α-fetoprotein (AFP) and cancer antigen 19-9 (CA19-9), which are clinically monitored for their cancer-related glycan changes, and only the AFP assay includes quantification of both the protein amount and the altered glycoform. This narrative review demonstrates, through several examples, the advantages of the combined quantification of protein cancer biomarkers and the respective glycoform analysis, which enable to yield the maximum information and overcome the weaknesses of each individual analysis. This strategy allows to achieve higher sensitivity and specificity in the detection of cancer, enhancing the diagnostic power of biomarker-based cancer detection tests.

Uromodulin Isolation and Its N-Glycosylation Analysis by NanoLC-MS/MS

The glycoprotein uromodulin (UMOD) is the most abundant protein in urine, and N-glycans are critical for many biological functions of UMOD. Comprehensive glycan profiling of UMOD provides valuable information to understand the exact mechanisms of glycan-regulated functions. To perform comprehensive glycosylation analysis of UMOD from urine samples with limited volumes, we developed a streamlined workflow that included UMOD isolation from 5 mL of urine from 6 healthy adult donors (3 males and 3 females) and a glycosylation analysis using a highly sensitive and reproducible nanoLC-MS/MS based glycomics approach. In total, 212 N-glycan compositions were identified from the purified UMOD, and 17% were high-mannose glycans, 2% were afucosylated/asialylated, 3% were neutral fucosylated, 28% were sialylated (with no fucose), 46% were fucosylated and sialylated, and 4% were sulfated. We found that isolation of UMOD resulted in a significant decrease in the relative quantity of high-mannose and sulfated glycans with a significant increase of neutral fucosylated glycans in the UMOD-depleted urine relative to the undepleted urine, but depletion had little impact on the sialylated glycans. To our knowledge, this is the first study to perform comprehensive N-glycan profiling of UMOD using nanoLC-MS/MS. This analytical workflow would be very beneficial for studies with limited sample size, such as pediatric studies, and can be applied to larger patient cohorts not only for UMOD interrogation but also for global glycan analysis.

Surface Display Technology for Biosensor Applications: A Review

Surface display is a recombinant technology that expresses target proteins on cell membranes and can be applied to almost all types of biological entities from viruses to mammalian cells. This technique has been used for various biotechnical and biomedical applications such as drug screening, biocatalysts, library screening, quantitative assays, and biosensors. In this review, the use of surface display technology in biosensor applications is discussed. In detail, phage display, bacterial surface display of Gram-negative and Gram-positive bacteria, and eukaryotic yeast cell surface display systems are presented. The review describes the advantages of surface display systems for biosensor applications and summarizes the applications of surface displays to biosensors.

Effect of N-glycosylation on the transport activity of the peptide transporter PEPT1

The intestinal peptide transporter PEPT1 provides bulk quantities of amino acids to epithelial cells. PEPT1 is a high-capacity and low-affinity solute carrier of the SLC15 family found in apical membranes of enterocytes in small intestine and distal colon. Surprisingly, murine PEPT1 (mPEPT1) has an apparent molecular mass of ∼95 kDa in the small intestine but ∼105 kDa in the large intestine. Here we describe studies on mPEPT1 protein glycosylation and how glycans affect transport function. Putative N-glycosylation sites of mPEPT1 were altered by site-directed mutagenesis followed by expression in Xenopus laevis oocytes. Replacement of six asparagine residues (N) at positions N50, N406, N439, N510, N515, and N532 by glutamine (Q) resulted in a decrease of the mPEPT1 mass by around 35 kDa. Electrophysiology revealed all glycosylation-deficient transporters to be functional with comparable expression levels in oocyte membranes. Strikingly, the mutant protein with N50Q exhibited a twofold decreased affinity for Gly-Sar but a 2.5-fold rise in the maximal inward currents compared with the wild-type protein. Elevated maximal transport currents were also recorded for cefadroxil and tri-l-alanine. Tracer flux studies performed with [(14)C]-Gly-Sar confirmed the reduction in substrate affinity and showed twofold increased maximal transport rates for the N50Q transporter. Elimination of individual N-glycosylation sites did not alter membrane expression in oocytes or overall transport characteristics except for the mutant protein N50Q. Because transporter surface density was not altered in N50Q, removal of the glycan at this location appears to accelerate the substrate turnover rate.

Differential adeno-associated virus serotype-specific interaction patterns with synthetic heparins and other glycans

All currently identified primary receptors of adeno-associated virus (AAV) are glycans. Depending on the AAV serotype, these carbohydrates range from heparan sulfate proteoglycans (HSPG), through glycans with terminal α2-3 or α2-6 sialic acids, to terminal galactose moieties. Receptor identification has largely relied on binding to natural compounds, defined glycan-presenting cell lines, or enzyme-mediated glycan modifications. Here, we describe a comparative binding analysis of highly purified, fluorescent-dye-labeled AAV vectors of various serotypes on arrays displaying over 600 different glycans and on a specialized array with natural and synthetic heparins. Few glycans bind AAV specifically in a serotype-dependent manner. Differential glycan binding was detected for the described sialic acid-binding AAV serotypes 1, 6, 5, and 4. The natural heparin binding serotypes AAV2, -3, -6, and -13 displayed differential binding to selected synthetic heparins. AAV7, -8, -rh.10, and -12 did not bind to any of the glycans present on the arrays. For discrimination of AAV serotypes 1 to 6 and 13, minimal binding moieties are identified. This is the first study to differentiate the natural mixed heparin binding AAV serotypes 2, 3, 6, and 13 by differential binding to specific synthetic heparins. Also, sialic acid binding AAVs display differential glycan binding specificities. The findings are relevant for further dissection of AAV host cell interaction. Moreover, the definition of single AAV-discriminating glycan binders opens the possibility for glycan microarray-based discrimination of AAV serotypes in gene therapy.

Na+/K+ATPase as a Signaling Molecule During Bovine Sperm Capacitation1

A heteromeric integral membrane protein, Na+/K+ATPase is composed of two polypeptides, alpha and beta, and is active in many cell types, including testis and spermatozoa. It is a well-known ion transporter, but binding of ouabain, a specific inhibitor of Na+/K+ATPase, to Na+/K+ATPase in somatic cells initiates responses that are similar to signaling events associated with bovine sperm capacitation. The objectives of the present study were to demonstrate the presence of Na+/K+ATPase in bovine sperm and to investigate its role in the regulation of bovine sperm capacitation. The presence of Na+/K+ATPase in sperm from mature Holstein bulls was demonstrated by immunoblotting and immunocytochemistry using a monoclonal antibody developed in mouse against the beta 1 polypeptide of Na+/K+ATPase. Binding of ouabain to Na+/K+ATPase inhibited motility (decreased progressive motility, average path velocity, and curvilinear velocity) and induced tyrosine phosphorylation and capacitation but did not increase intracellular calcium levels in spermatozoa. Furthermore, binding of ouabain to Na+/K+ATPase induced depolarization of sperm plasma membrane. Therefore, binding of ouabain to Na+/K+ATPase induced sperm capacitation through depolarization of sperm plasma membrane and signaling via the tyrosine phosphorylation pathway without an appreciable increase in intracellular calcium. To our knowledge, this is the first report concerning the signaling role of Na+/K+ATPase in mammalian sperm capacitation.

Localization of beta1-adrenergic receptors in the cochlea and the vestibular labyrinth

Sympathetic activation in a "fight or flight reaction" may put the sensory systems for hearing and balance into a state of heightened alert via beta1-adrenergic receptors (beta1-AR). The aim of the present study was to localize beta1-AR in the gerbil inner ear by confocal immunocytochemistry, to characterize beta1-AR by Western immunoblots, and to identify beta1-AR pharmacologically by measurements of cAMP production. Staining for beta1-AR was found in strial marginal cells, inner and outer hair cells, outer sulcus, and spiral ganglia cells of the cochlea, as well as in dark, transitional and supporting cells of the vestibular labyrinth. Receptors were characterized in microdissected inner ear tissue fractions as 55 kDa non-glycosylated species and as 160 kDa high-mannose-glycosylated complexes. Pharmacological studies using isoproterenol, ICI-118551 and CGP-20712A demonstrated beta1-AR as the predominant adrenergic receptor in stria vascularis and organ of Corti. In conclusion, beta1-AR are present and functional in inner ear epithelial cells that are involved in K+ cycling and auditory transduction, as well as in neuronal cells that are involved in auditory transmission.

Changes in glycosylation of rat liver arylsulfatase B in relation to age

The aim of this study was to determine how glycosylation of the rat liver arylsulfatase B was influenced by the age of the animal. The enzyme was purified from a liver lysosomal fraction obtained from male Wistar rats aged 18 days of gestation, 1 week, and 1, 1.5, 3 and 18 months by an affinity chromatography. Examination of the carbohydrate structures was performed after electrophoresis and blotting, followed by a very sensitive detection system with a set of six highly specific digoxygenin-labelled lectins. After densitometric measurement of the intensity of a digoxigenin-labelled lectin binding to arylsulfatase B, it could be stated that, at least, changes in sialylation are related to the growth and development of rats. Sialylation increases while fucosylation slightly decreases with age of the animal.

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.

Glycans of higher plant peroxidases: recent observations and future speculations

Plant peroxidases are composed of a peptide and associated heme, calcium and glycans. The 3D structure of the major cationic peanut peroxidase has revealed the sites of the heme and calcium. But the diffraction of the glycans was not sufficient to show their structure. This review presents research that has been executed to obtain putative glycans and their binding sites, and to gain an indirect insight into these glycans. It also offers approaches that will be used to determine the function of the glycans on the peanut peroxidase. Some comparisons are made with other plant glycoproteins including peroxidases from plants other than peanut.

Isoform-specific monoclonal antibodies to Na,K-ATPase alpha subunits. Evidence for a tissue-specific post-translational modification of the alpha subunit

Monoclonal antibodies to isoforms of the Na,K-ATPase have become important tools in the study of the enzyme's distribution, physiological roles, and gene regulation, and when their epitopes are defined, they are useful in the study of enzyme structure as well. Evidence is presented that the alpha3-specific antibody McBX3 recognizes an unusual epitope that is not present on alpha3 in the heart. The epitope, which is also found in kidney alpha1 from some species, was mapped to a site on the large intracellular loop near the ATP binding site. DNA sequencing of reverse transcribed-PCR products encompassing the corresponding regions from alpha3 from brain (where McBX3 recognizes alpha3) and heart demonstrated that the tissue difference in epitope is not due to alternative splicing of the mRNA. Instead, hydroxylamine sensitivity indicated that the antibody recognizes a post-translational modification. The epitope for a new antibody for alpha3, XVIF9-G10, was mapped to a site near the N terminus, a location analogous to the sites for the well-characterized antibodies McK1 (alpha1) and McB2 (alpha2). The antibody XVIF9-G10 reacted with the alpha3 of the heart as well as that of the brain; however, McBX3 and XVIF9-G10 both stained the same cellular structures in sections of the rat retina. A new alpha1-specific antibody, 6F, was characterized and mapped to another site near the N terminus; this antibody has broader species specificity than the other well-characterized alpha1 antibody, McK1.

Protein-tyrosine phosphatase epsilon. An isoform specifically expressed in mouse mammary tumors initiated by v-Ha-ras OR neu

Transgenic mice that overexpress v-Ha-ras, c-myc, c-neu or int-2 proto-oncogenes in the mammary epithelium develop breast tumors with morphologies that are characteristic of each initiating oncogene. Since these morphological differences reflect distinctive patterns of tumor-specific gene expression, the identification of the products of these genes might shed light on the mechanisms of transformation and/or the identity of target cells that are transformed by specific classes of oncogenes. By focusing on the tyrosine phosphorylation pathway, we have found that the transmembranal protein-tyrosine phosphatase epsilon (PTP epsilon) is highly expressed in murine mammary tumors initiated by c-neu and v-Haras, but not in mammary tumors initiated by c-myc or int-2. This difference is striking and occurs both in primary tumors and in epithelial cells cultured from them. Moreover, PTP epsilon overexpression appears to be mammary tumor-specific in that it is not found in other ras-based tumors and cell lines. These observations suggest that PTP epsilon either plays a role in ras- and neu-mediated transformation of mammary epithelium or marks mammary epithelial cells particularly susceptible to transformation by these oncogenes. Because of its distinctive expression in these mammary tumors, we have further characterized murine PTP epsilon, cloning and determining the complete structures of its cDNAs and showing that it is a glycoprotein that is N-glycosylated in a tissue-specific manner.

Major organ-specific glycoproteins in isolated brain and kidney membranes identified as Na,K-ATPase subunits by combined glycan-, lectin-, and immunoblotting

In the present work combined glycan-, lectin-, and immunoblotting of isolated brain and kidney membranes shows that the alpha and beta subunits of Na,K-ATPase are the most abundant glycoproteins. Further, Datura stramonium and Galanthus nivalis agglutinins recognize the Na,K-ATPase subunits in a mutually exclusive manner in membranes from human, rabbit and rat brain or human, rabbit, rat, pig and dog kidney indicating the presence of species-independent organ-typical glycoforms. The glycosylation status is not related to the ouabain-sensitivity. Taken together, the data reveals organ-specific glycoforms of Na,K-ATPase which might have roles for organ identification and recognition.