Biochemistry

Decellularized extracellular matrix (ECM) biomaterials derived from native tissues and organs are widely used for tissue engineering and wound repair. To boost their regenerative potential, ECM biomaterials can be functionalized via the immobilization of bioactive molecules. To enable ECM functionalization in a chemoselective manner, we have recently reported an effective approach for labeling native organ ECM with the click chemistry-reactive azide ligand via physiologic post-translational glycosylation. Here, using the rat lung as a model, we provide a detailed protocol for in vivo and ex vivo metabolic azide labeling of the native organ ECM using N-Azidoacetylgalactosamine-tetraacylated (Ac₄GalNAz), together with procedures for decellularization and labeling characterization. Our approach enables specific and robust ECM labeling within three days in vivo or within one day during ex vivo organ culture. The resulting ECM labeling remains stable following decellularization. With our approach, ECM biomaterials can be functionalized with desired alkyne-modified biomolecules, such as growth factors and glycosaminoglycans, for tissue engineering and regenerative applications.

We have developed enabling techniques for sulfoglycomics based on MALDI-MS mapping and MS/MS sequencing of permethylated sulfated glycans. We then extended further the analytical workflow to C18 reverse phase (RP)-nanoLC-nanoESI-MS/MS analyses of permethylated sulfated glycans in the negative ion mode. The advantages are that extra sulfates on permethylated di- and multiply sulfated glycans will survive in nanoESI conditions to allow detection of multiply charged intact molecular ions, and more comprehensive MS/MS can be performed in an automated fashion at higher sensitivity, compared with MALDI-MS/MS. Parallel higher energy collision dissociation (HCD) and ion trap collision induced dissociation (CID)-based MS², coupled with product-dependent MS³ in data dependent acquisition mode proved to be highly productive when applied to resolve and identify the isomeric sulfated glycan structures. In-house glycomic data mining software, GlyPick, was developed and used to automate the downstream process of identification and relative quantification of target sulfated glycotopes based on summed intensity of their diagnostic MS² ions extracted from thousands of HCD-MS² and/or CID-MS² data.

Sulfated glycans are barely detectable in routine mass spectrometry (MS)-based glycomic analysis due to ion suppression by the significantly more abundant neutral glycans in the positive ion mode, and sialylated non-sulfated glycans in the negative ion mode, respectively. Nevertheless, the negative charge imparted by sulfate can be advantageous for selective detection in the negative ion mode if the sialic acids can first be neutralized. This is most conveniently achieved by a concerted sample preparation workflow in which permethylation is followed by solid phase fractionation to isolate the sulfated glycans prior to MS analysis. Importantly, we demonstrated that conventional NaOH/DMSO slurry permethylation method can retain the sulfates. Instead of extracting permethylated glycans into chloroform for sample clean-up, reverse phase C18 cartridge coupled with self-packed amine-tip or mixed mode weak anion exchange cartridge can be utilized to obtain in good yield the non-sulfated, mono-sulfated, and multiply sulfated permethylated glycans in separate fractions for sulfoglycomic analysis.

Calnexin is a chaperone protein that plays a critical role in glycoprotein folding in the endoplasmic reticulum (ER). The function of calnexin depends on its binding to monoglucosylated oligosaccharides on nascent glycoproteins, whereas the generation of monoglucosylated oligosaccharides depends on the activity of α-glucosidases I and II, which trim off terminal glucose residues sequentially from triglucosylated N-glycans. This biochemical mechanism can be exploited to study calnexin-assisted folding and subsequent ER exiting of glycoproteins in cells. In our investigation of the intracellular trafficking of N-glycosylated serine proteases, we used an inhibitor of α-glucosidases I and II to block the trimming of triglucosylated oligosaccharides, thereby inhibiting calnexin-assisted glycoprotein folding. The study helped us to discover a key role of calnexin in the folding, ER exiting, and extracellular expression of N-glycosylated serine proteases such as corin, enteropeptidase, and prothrombin. A similar approach of glucosidase inhibition can be used to study the calnexin/calreticulin-dependent folding and intracellular trafficking of other N-glycosylated proteins.

This protocol aims to evaluate folding status of proteins, utilizing peptide:N-glycanase (PNGase) sensitivity. In the cytosol, PNGase works as a deglycosylation-enzyme. N-glycans on unfolded/misfolded proteins are more susceptible to PNGase than N-glycans on folded proteins because of the preference of PNGase to non-native proteins. PNGase is endogenously expressed in various cell types, including HCT116 cells, DT40 cells and mouse embryonic fibroblast cells. Partial deglycosylation by PNGase can be detected by faster migration of band in SDS-PAGE. You can compare tightness of the folding among wild-type and mutant proteins of interest. This method can be used with regular molecular and cell biology equipment, but applied only to glycoproteins.

Arabinogalactan proteins (AGPs) are plant-specific extracellular glycoproteins regulating a variety of processes during growth and development. AGP biosynthesis involves O-galactosylation of hydroxyproline (Hyp) residues followed by a stepwise elongation of the complex sugar chains. The initial Hyp O-galactosylation is mediated by Hyp O-galactosyltransferase (HPGT) that catalyzes the transfer of a D-galactopyranosyl residue to the hydroxyl group of Hyp residues of peptides from the sugar donor UDP-α-D-galactose (Figure 1). Here we describe a LC/MS-based method for the detection of HPGT activity in vitro.


Figure 1. Reaction scheme for Hyp galactosylation by HPGT. HPGT catalyzes the addition of a D-galactopyranose from an UDP-α-D-Gal to the hydroxylgroup of Hyp residues.

The metabolism of the cell surface during bacterial cell division involves synthesis and degradation of peptidoglycan (PGN), the major component of the bacterial cell wall. Bacteria have to ensure that their surface remains capable of withstanding high turgor pressures and, simultaneously, that the PGN at their surface is concealed from receptors produced by the host innate immune system. For cell separation to occur, and for PGN to be kept concealed, “old” PGN is degraded by specific PGN hydrolases, also known as autolysins, that are found at the bacterial cell surface or that are secreted into the growth medium.

Bacterial PGN hydrolases are cell wall lytic enzymes that comprise a broad and diverse group of proteins. It is often difficult to assign a specific function to a PGN hydrolase mainly because an organism can have a large number of hydrolases with redundant activities and one hydrolase can have more than one enzymatic activity and participate in various cell processes (Vollmer et al. 2008). Bacillus subtilis has ca. 35 known or hypothetical PGN hydrolases, whereas Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) have, respectively, ca.16 and 19 PGN hydrolases (Vollmer, 2012; Heidrich et al., 2001; Singh et al., 2012).

PGN hydrolases can be classified in three main classes: glycosidases, amidases and peptidases. Glycosidases cleave the glycan backbone and are divided into N-acetylglucosaminidases and N-acetylmuramidases. Amidases cleave the linkage between the peptide chain and the N-acetylmuramic residue of the glycan chain. Peptidases, such as endopeptidases and carboxypeptidases, are able to cleave peptide bonds between different amino acids of the PGN stem peptide.

Here we describe a method to extract PGN hydrolases, which are non-covalently linked to the S. aureus cell wall (Vollmer, 2008). Analysis of extracts containing denatured PGN hydrolytic enzymes is performed by running a zymogram gel (a SDS-PAGE gel containing crude bacterial cell walls or substrate cells), which is then incubated in a non-denaturing buffer to allow renaturation of the PGN hydrolases. These renatured enzymes can then be identified through the production of clear bands that are observed where cell wall digestion has occurred. The protocol is divided into three steps: A) Preparation of the crude autolytic extracts from S. aureus cells; B) Preparation of substrate cells for gel zymograms; C) Analysis of crude autolytic extracts by gel zymography.

We also show that this method can be used to determine the absence or altered activity of PGN hydrolases produced by different S. aureus mutant strains.

Bacterial glycoproteins are of increasing interest due to their abundance in nature and importance in health and infectious diseases. However, only a very small fraction of bacterial glycoproteins have been characterized and its post-translational modification machinery identified. While analysis of glycoproteins can be achieved through various techniques, this is often limited by the specific characteristics of individual proteins such as type and level of glycosylation. Lectins are sugar-binding proteins that recognize specific glycoconjugates in a manner similar to antigen-antibody interactions. Here, we describe a simple method for the detection of glycoproteins using lectin-based Western blot analysis, which can be applied to different organisms and coupled with various other strategies for complementary analysis.

Glycosaminoglycans (GAGs) are long unbranched polysaccharides consisting of repeating disaccharide units composed of a hexosamine (glucosamine or galactosamine) and a hexuronic acid (glucuronic or iduronic acid). Depending on the disaccharide unit the GAGs can be organized into five groups: chondroitin sulfate, dermatan sulfate, heparan sulfate, keratan sulfate and hyaluronic acid. The GAGs are heterogeneous molecules with great variability in molecular mass and both sulfation density and pattern. Spectrophotometric assays to measure the GAG content in biological fluids and tissue/cell extracts are valuable tools. The dye 1,9-dimethylmethylene is a thiazine chromotrope agent that presents a change in the absorption spectrum due to the induction of metachromasia when bound to sulfated GAGs enabling rapid detection of GAGs in solution (Whitley et al., 1989; Chandrasekhar et al., 1987; Farndale et al., 1982). Moreover, there is a window in which a linear curve may be drawn (approximately between 0.5-5 μg of GAGs) enabling the quantification of GAGs in solution.

Galactofuranose (Galf) is a component of several polysaccharides and glycoconjugates in certain species of filamentous fungi. Galf residues are frequently found in Aspergillus glycoproteins, including N-glycans and O-mannose glycans that modify many cell wall proteins and extracellular enzymes. It is known that furanoses, contained in oligosaccharides, are detected as pyranoses after hydrolysis, and that D-galactopyranose is not contained in the galactomannoproteins of Aspergillus spp. To determine the levels of D-galactofuranose in galactomannoproteins extracted from Aspergillus nidulans (A. nidulans), we measured the amount of D-galactopyranose production after galactomannoproteins hydrolysis. The method described in this manuscript allows determination of the D-galactofuranose content of galactomannoproteins in Aspergillus spp.