Deletion of specific glycan chains affects differentially the stability, local structures, and activity of lecithin-cholesterol acyltransferase

Discovery of age-related early-stage glycated proteins based on deep quantitative serum glycated proteome analysis

Aging is a pressing global health issue that is linked to various diseases, such as diabetes and Alzheimer's disease. It is well known that glycation plays a pathological role in the aging process and age-related diseases. Thus, it is of great significance to discover protein glycation at an early stage for monitoring and intervention in the aging process. However, the endogenous age-related early-stage glycated proteome remains insufficiently profiled. To address this research gap, our study focuses on assessing glycated proteomics profiles in the serum of mice. We employ a robust and quantitative strategy previously developed by our team, to analyze endogenous glycated proteome in serum samples of 4 age groups of mice (10 weeks, 16 weeks, 48 weeks and 80 weeks). In total, 2959 endogenous glycated peptides corresponding to 296 serum proteins are identified from 48 runs of serum samples from 16 mice across the four age groups. By comparing these glycated peptides between adjacent age groups, we discover 49 glycated peptides from 35 proteins that show significant upregulation between the 48-week and 80-week age groups. Furthermore, we identify 10 glycated proteins (or protein groups) that are significantly upregulated only between the 48-week and 80-week age groups, including lecithin-cholesterol acyltransferase (LCAT) and apolipoprotein A-II (Apo A-II). These novel findings provide unique signatures for understanding the aging process and age-related diseases. By shedding light on the early-stage glycated proteome, our study contributes valuable insights that may have implications for future interventions and therapeutic approaches.

Glycosylation of HDL-Associated Proteins and Its Implications in Cardiovascular Disease Diagnosis, Metabolism and Function

High-density lipoprotein (HDL) particles, long known for their critical role in the prevention of cardiovascular disease (CVD), were recently identified to carry a wide array of glycosylated proteins, and the importance of this glycosylation in the structure, function and metabolism of HDL are starting to emerge. Early studies have demonstrated differential glycosylation of HDL-associated proteins in various pathological states, which may be key to understanding their etiological role in these diseases and may be important for diagnostic development. Given the vast array and specificity of glycosylation pathways, the study of HDL-associated glycosylation has the potential to uncover novel mechanisms and biomarkers of CVD. To date, no large studies examining the relationships between HDL glycosylation profiles and cardiovascular outcomes have been performed. However, small pilot studies provide promising preliminary evidence that such a relationship may exist. In this review article we discuss the current state of the evidence on the glycosylation of HDL-associated proteins, the potential for HDL glycosylation profiling in CVD diagnostics, how glycosylation affects HDL function, and the potential for modifying the glycosylation of HDL-associated proteins to confer therapeutic value.

BacMam production of active recombinant lecithin-cholesterol acyltransferase: Expression, purification and characterization

Lecithin-cholesterol acyltransferase (LCAT) is a key enzyme in the esterification of cholesterol and its subsequent incorporation into the core of high density lipoprotein (HDL) particles. It is also involved in reverse cholesterol transport (RCT), the mechanism by which cholesterol is removed from peripheral cells and transported to the liver for excretion. These processes are involved in the development of atherosclerosis and coronary heart disease (CHD) and may have therapeutic implications. This work describes the use of baculovirus as a transducing vector to express LCAT in mammalian cells, expression of the recombinant protein as a high-mannose glycoform suitable for deglycosylation by Endo H and its purification to homogeneity and characterization. The importance of producing underglycosylated forms of secreted glycoproteins to obtain high-resolution crystal structures is discussed.

Recombinant human lecithin-cholesterol acyltransferase Fc fusion: analysis of N- and O-linked glycans and identification and elimination of a xylose-based O-linked tetrasaccharide core in the linker region

Recombinant human lecithin-cholesterol acyltransferase Fc fusion (huLCAT-Fc) is a chimeric protein produced by fusing human Fc to the C-terminus of the human enzyme via a linker sequence. The huLCAT-Fc homodimer contains five N-linked glycosylation sites per monomer. The heterogeneity and site-specific distribution of the various glycans were examined using enzymatic digestion and LC-MS/MS, followed by automatic processing. Almost all of the N-linked glycans in human LCAT are fucosylated and sialylated. The predominant LCAT N-linked glycoforms are biantennary glycans, followed by triantennary sugars, whereas the level of tetraantennary glycans is much lower. Glycans at the Fc N-linked site exclusively contain typical asialobiantennary structures. HuLCAT-Fc was also confirmed to have mucin-type glycans attached at T407 and S409 . When LCAT-Fc fusions were constructed using a G-S-G-G-G-G linker, an unexpected +632 Da xylose-based glycosaminoglycan (GAG) tetrasaccharide core of Xyl-Gal-Gal-GlcA was attached to S418 . Several minor intermediate species including Xyl, Xyl-Gal, Xyl-Gal-Gal, and a phosphorylated GAG core were also present. The mucin-type O-linked glycans can be effectively released by sialidase and O-glycanase; however, the GAG could only be removed and localized using chemical alkaline β-elimination and targeted LC-MS/MS. E416 (the C-terminus of LCAT) combined with the linker sequence is likely serving as a substrate for peptide O-xylosyltransferase. HuLCAT-Fc shares some homology with the proposed consensus site near the linker sequence, in particular, the residues underlined PPPE416 GS418 GGGGDK. GAG incorporation can be eliminated through engineering by shifting the linker Ser residue downstream in the linker sequence.

Core glycan in the yeast multicopper ferroxidase, Fet3p: a case study of N-linked glycosylation, protein maturation, and stability

Glycosylation is essential to the maintenance of protein quality in the vesicular protein trafficking pathway in eukaryotic cells. Using the yeast multicopper oxidase, Fet3p, the hypothesis is tested that core glycosylation suppresses Fet3p nascent chain aggregation during synthesis into the endoplasmic reticulum (ER). Fet3p has 11 crystallographically mapped N-linked core glycan units. Assembly of four of these units is specifically required for localization of Fet3p to the plasma membrane (PM). Fet3 protein lacking any one of these glycan units is found in an intracellular high-molecular mass species resolvable by blue native gel electrophoresis. Individually, the remaining glycan moieties are not required for ER exit; however, serial deletion of these by N → A substitution correlates with these desglycan species failure to exit the ER. Desglycan Fet3 proteins that localize to the PM are wild type in function indicating that the missing carbohydrate is not required for native structure and biologic activity. This native function includes the interaction with the iron permease, Ftr1p, and wild type high-affinity iron uptake activity. The four essential sequons are found within relatively nonpolar regions located in surface recesses and are strongly conserved among fungal Fet3 proteins. The remaining N-linked sites are found in more surface exposed, less nonpolar environments, and their conservation is weak or absent. The data indicate that in Fet3p the N-linked glycan has little effect on the enzyme's molecular activity but is critical to its cellular activity by maximizing the protein's exit from the ER and assembly into a functional iron uptake complex.

Influence of carbohydrates on the stability and structure of a hyperglycosylated human interferon alpha mutein

Protein physical and chemical instability is one of the major challenges in the development of biopharmaceuticals during every step of the process, ranging from production to final delivery. This is particularly applicable to human recombinant interferon alpha-2b (rhIFN-alpha2b), a pleiotropic cytokine currently used worldwide for the treatment of various cancer and chronic viral diseases, which presents a poor stability in solution. In previous studies, we have demonstrated that the introduction of four N-glycosylation sites in order to construct a heavily glycosylated IFN variant (4N-IFN) resulted in a markedly prolonged plasma half-life which was reflected in an enhanced therapeutic activity in mice in comparison with the commercial non-glycosylated rhIFN-alpha2b (NG-IFN). Herein, we evaluated the influence of glycosylation on the in vitro stability of 4N-IFN towards different environmental conditions. Interestingly, the hyperglycosylated cytokine showed enhanced stability against thermal stress, acid pH and repetitive freeze-thawing cycles in comparison with NG-IFN. Besides, microcalorimetric analysis indicated a much higher melting temperature of 4N-IFN, also demonstrating a higher solubility of this variant as denoted by the absence of precipitation at the end of the experiment, in contrast with the NG-IFN behaviour. Furthermore, far-UV circular dichroism (CD) spectrum of 4N-IFN was virtually superimposed with that of NG-IFN, indicating that the IFN structure was not altered by the addition of carbohydrate moieties. The same conclusion could be inferred from limited proteolysis studies. Our results suggest that glycoengineering could be a useful strategy for protecting rhIFN-alpha2b from inactivation by various external factors and for overcoming aggregation problems during the production process and storage.

Lecithin: cholesterol acyltransferase--from biochemistry to role in cardiovascular disease

PURPOSE OF REVIEW

We discuss the latest findings on the biochemistry of lecithin : cholesterol acyltransferase (LCAT), the effect of LCAT on atherosclerosis, clinical features of LCAT deficiency, and the impact of LCAT on cardiovascular disease from human studies.

RECENT FINDINGS

Although there has been much recent progress in the biochemistry of LCAT and its effect on high-density lipoprotein metabolism, its role in the pathogenesis of atherosclerosis is still not fully understood. Studies from various animal models have revealed a complex interaction between LCAT and atherosclerosis that may be modified by diet and by other proteins that modify lipoproteins. Furthermore, the ability of LCAT to lower apoB appears to be the best way to predict its effect on atherosclerosis in animal models. Recent studies on patients with LCAT deficiency have shown a modest but significant increase in incidence of cardiovascular disease consistent with a beneficial effect of LCAT on atherosclerosis. The role of LCAT in the general population, however, has not revealed a consistent association with cardiovascular disease.

SUMMARY

Recent research findings from animal and human studies have revealed a potential beneficial role of LCAT in reducing atherosclerosis but additional studies are necessary to better establish the linkage between LCAT and cardiovascular disease.

Effects of glycosylation on the stability of protein pharmaceuticals

In recent decades, protein-based therapeutics have substantially expanded the field of molecular pharmacology due to their outstanding potential for the treatment of disease. Unfortunately, protein pharmaceuticals display a series of intrinsic physical and chemical instability problems during their production, purification, storage, and delivery that can adversely impact their final therapeutic efficacies. This has prompted an intense search for generalized strategies to engineer the long-term stability of proteins during their pharmaceutical employment. Due to the well known effect that glycans have in increasing the overall stability of glycoproteins, rational manipulation of the glycosylation parameters through glycoengineering could become a promising approach to improve both the in vitro and in vivo stability of protein pharmaceuticals. The intent of this review is therefore to further the field of protein glycoengineering by increasing the general understanding of the mechanisms by which glycosylation improves the molecular stability of protein pharmaceuticals. This is achieved by presenting a survey of the different instabilities displayed by protein pharmaceuticals, by addressing which of these instabilities can be improved by glycosylation, and by discussing the possible mechanisms by which glycans induce these stabilization effects.

Activation of lecithin:cholesterol acyltransferase by HDL ApoA-I central helices

Lecithin:cholesterol acyltransferase (LCAT) is an enzyme that first hydrolyzes the sn-2 position of phospholipids, preferentially a diacylphosphocholine, and then transfers the fatty acid to cholesterol to yield a cholesteryl ester. HDL ApoA-I is the principal catalytic activator for LCAT. Activity of LCAT on nascent or lipid-poor HDL particles composed of phospholipid, cholesterol and ApoA-I allows the maturation of HDL particles into lipid-rich spherical particles that contain a core of cholesteryl ester surrounded by phospholipid and ApoA-I on the surface. This article reviews the recent progress in elucidating structural aspects of the interaction between LCAT and ApoA-I. In the last decade, there has been considerable progress in understanding the structure of ApoA-I and the central helices 5, 6, and 7 that are known to activate LCAT. However, much less information has been forthcoming describing the 3D structure and conformation of LCAT required to catalyze two separate reactions within a single monomeric peptide.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.