Serum UDP-galactose: glycoprotein galactosyltransferase in diabetics with microangiopathy

Plasma protein N-glycan signatures of type 2 diabetes

BACKGROUND

Little is known about enzymatic N-glycosylation in type 2 diabetes, a common posttranslational modification of proteins influencing their function and integrating genetic and environmental influences. We sought to gain insights into N-glycosylation to uncover yet unexplored pathophysiological mechanisms in type 2 diabetes.

METHODS

Using a high-throughput MALDI-TOF mass spectrometry method, we measured N-glycans in plasma samples of the DiaGene case-control study (1583 cases and 728 controls). Associations were investigated with logistic regression and adjusted for age, sex, body mass index, high-density lipoprotein-cholesterol, non-high-density lipoprotein-cholesterol, and smoking. Findings were replicated in a nested replication cohort of 232 cases and 108 controls.

RESULTS

Eighteen glycosylation features were significantly associated with type 2 diabetes. Fucosylation and bisection of diantennary glycans were decreased in diabetes (odds ratio (OR) = 0.81, p = 1.26E-03, and OR = 0.87, p = 2.84E-02, respectively), whereas total and, specifically, alpha2,6-linked sialylation were increased (OR = 1.38, p = 9.92E-07, and OR = 1.40, p = 5.48E-07). Alpha2,3-linked sialylation of triantennary glycans was decreased (OR = 0.60, p = 6.38E-11).

CONCLUSIONS

While some glycosylation changes were reflective of inflammation, such as increased alpha2,6-linked sialylation, our finding of decreased alpha2,3-linked sialylation in type 2 diabetes patients is contradictory to reports on acute and chronic inflammation. Thus, it might have previously unreported immunological implications in type 2 diabetes.

GENERAL SIGNIFICANCE

This study provides new insights into N-glycosylation patterns in type 2 diabetes, which can fuel studies on causal mechanisms and consequences of this complex disease.

Glycomics and glycoproteomics focused on aging and age-related diseases--Glycans as a potential biomarker for physiological alterations

BACKGROUND

Since glycosylation depends on glycosyltransferases, glycosidases, and sugar nucleotide donors, it is susceptible to the changes associated with physiological and pathological conditions. Therefore, alterations in glycan structures may be good targets and biomarkers for monitoring health conditions. Since human aging and longevity are affected by genetic and environmental factors such as diseases, lifestyle, and social factors, a scale that reflects various environmental factors is required in the study of human aging and longevity.

SCOPE OF REVIEW

We herein focus on glycosylation changes elucidated by glycomic and glycoproteomic studies on aging, longevity, and age-related diseases including cognitive impairment, diabetes mellitus, and frailty. We also consider the potential of glycan structures as biomarkers and/or targets for monitoring physiological and pathophysiological changes.

MAJOR CONCLUSIONS

Glycan structures are altered in age-related diseases. These glycans and glycoproteins may be involved in the pathophysiology of these diseases and, thus, be useful diagnostic markers. Age-dependent changes in N-glycans have been reported previously in cohort studies, and characteristic N-glycans in extreme longevity have been proposed. These findings may lead to a deeper understanding of the mechanisms underlying aging as well as the factors influencing longevity.

GENERAL SIGNIFICANCE

Alterations in glycosylation may be good targets and biomarkers for monitoring health conditions, and be applicable to studies on age-related diseases and healthy aging. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

N-glycomic changes in serum proteins in type 2 diabetes mellitus correlate with complications and with metabolic syndrome parameters

Background

Glycosylation, i.e the enzymatic addition of oligosaccharides (or glycans) to proteins and lipids, known as glycosylation, is one of the most common co-/posttranslational modifications of proteins. Many important biological roles of glycoproteins are modulated by N-linked oligosaccharides. As glucose levels can affect the pathways leading to glycosylation of proteins, we investigated whether metabolic syndrome (MS) and type 2 diabetes mellitus (T2DM), pathological conditions characterized by altered glucose levels, are associated with specific modifications in serum N-glycome.

Methods

We enrolled in the study 562 patients with Type 2 Diabetes Mellitus (T2DM) (mean age 65.6±8.2 years) and 599 healthy control subjects (CTRs) (mean age, 58.5±12.4 years). N-glycome was evaluated in serum glycoproteins.

Results

We found significant changes in N-glycan composition in the sera of T2DM patients. In particular, α(1,6)-linked arm monogalactosylated, core-fucosylated diantennary N-glycans (NG1(6)A2F) were significantly reduced in T2DM compared with CTR subjects. Importantly, they were equally reduced in diabetic patients with and without complications (P<0.001) compared with CTRs. Macro vascular-complications were found to be related with decreased levels of NG1(6)A2F. In addition, NG1(6)A2F and NG1(3)A2F, identifying, respectively, monogalactosylated N-glycans with α(1,6)- and α(1,3)-antennary galactosylation, resulted strongly correlated with most MS parameters. The plasmatic levels of these two glycans were lower in T2DM as compared to healthy controls, and even lower in patients with complications and MS, that is the extreme “unhealthy” phenotype (T2DM+ with MS).

Conclusions

Imbalance of glycosyltransferases, glycosidases and sugar nucleotide donor levels is able to cause the structural changes evidenced by our findings. Serum N-glycan profiles are thus sensitive to the presence of diabetes and MS. Serum N-glycan levels could therefore provide a non-invasive alternative marker for T2DM and MS.

Serum enzymes in the BB rat before and after onset of the overt diabetic syndrome

As part of a six-month prospective study of the effects of neonatal thymectomy in the spontaneously diabetic BB Wistar rat, activities of the following enzymes were determined: alkaline phosphatase (AP), lactate dehydrogenase (LDH), creatine phosphokinase (CPK), glutamic-oxaloacetic transaminase (GOT), glutamic-pyruvic transaminase (GPT) and UDP-galactosyltransferase (UDPG). In prediabetics, AP and LDH levels were higher than in sham-operated, non-diabetic controls; however, this increase was seen in nearly all diabetes-prone BB rats, diminishing the usefulness of these changes in discerning potential diabetics from asymptomatic, diabetes-prone rats. After onset of the syndrome, there was a striking elevation of AP values in all diabetics with no similar alteration in asymptomatic, diabetes-prone rats suggesting this was a diabetes-related phenomenon. By contrast, UDPG was the only enzyme to decrease immediately following the onset of the syndrome. Both UDPG and AP levels correlated with blood glucose, the former negatively and the latter positively, suggesting a close relationship with changes occurring after onset of the syndrome. The remaining enzymes increased only in a portion of diabetics alone (GOT, GPT) or in a portion of both diabetics and asymptomatic, diabetes-prone BB rats (LDH, CPK).

The biochemical properties of basement membrane components in health and disease

Basement membranes are complex macromolecular structures which occupy the extracellular space between cells of different histologic types. Biochemically it is composed of Type IV collagen, several noncollagenous glycoproteins including laminin, fibronectin, GP-2 and PYS glycoprotein, and heparan sulfate. Morphologic changes are commonplace in a number of renal diseases. In diabetic glomerular disease, the basement membrane is markedly thickened but the biochemical basis has not been elucidated. In other disease-associated basement membrane changes, altered glycosylation of glycoprotein components has been described. The most important issue is the effect such alterations have on the interaction of basement membrane components and the function of the basement membrane.

Human serum galactosyltransferase: distinction, separation and product identification of two galactosyltransferase activities

Two different galactosyltransferase activities have been found in normal sera from A and O donors. Galactosyltransferase A incorporated galactose from UDP-Gal into sialic-acid-free ovine submaxillary mucin (asialo-mucin), whereas galactosyltransferase B transferred galactose from UDP-Gal to free N-acetylglucosamine or N-acetylglucosamine-glycoproteins. Specificity, kinetic and stability differences permitted the distinction of the activity of galactosyltransferase A from that of galactosyltransferase B; the only substrate found for galactosyltransferase A was asialo-mucin, whereas galactosyltransferase B showed only low activity towards asialo-mucin and free N-acetyl-galactosamine, but had a main specificity for either free N-acetylglucosamine or N-acetylglucosamine-protein. Galactosyltransferase B was more stable on heat inactivation than galactosyltransferase A; galactosyltransferase B could be separated from galactosyltransferase A by affinity chromatography on N-acetylglucosamine-derivatized agarose. The products of both enzyme activities have been analyzed. The galactosyltransferase A product was cleaved from asialo-mucin by alkaline-borohydride treatment. The acceptor used to identify the galactosyltransferase B product was free N-acetylglucosamine. Periodate oxidation studies performed on the reduced disaccharides indicated the linkage type of the products. The anomeric configuration of the respective galactosyltransferase products were determined with specific galactosidases. Using these methods, galactosyltransferase A was found to form a Galbeta (1 leads to 3)GalNAc-protein linkage and galactosyltransferase B was found to form a Galbeta(1 leads to 4)GlcNAc-linkage.