Molecular basis for a link between complement and the vascular complications of diabetes

Possible mechanism for medial smooth muscle cell injury in diabetic nephropathy: Glycoxidation-mediated local complement activation

BACKGROUND

Although recent studies have emphasized the pathogenic role of intrarenal muscular arteries in patients with diabetic nephropathy, notice has not been taken of their pathological characteristics. We focused on medial smooth muscle cell (SMC) injury and the involvement of glycoxidation and complement activation.

METHODS

Renal samples were obtained at autopsy from patients with diabetes mellitus (DM), patients with hypertension without renal involvement (n = 9), patients with benign nephrosclerosis (n = 7), and age-matched control subjects (n = 12). Patients with DM had glomerulosclerosis classified as severe (n = 9; DM-sev), moderate (n = 11; DM-mod), and minimal (n = 7). Renal samples were immunohistochemically determined. Activation of plasma complement from healthy subjects using advanced glycation end products (AGEs) also was performed.

RESULTS

A marked SMC loss was noted in the media of patients with DM-sev and DM-mod. A membrane attack complex (MAC) observed in the area with SMC loss correlated well with the loss. Considerable carboxymethyllysine (CML), an oxidatively modified AGE, was deposited in the area with SMC loss in patients with DM-mod and DM-sev. Degrees of MAC deposition, SMC loss, and CML deposition were greater in the DM-sev group than the non-DM groups. Another oxidative product, acrolein, colocalized with CML. Plasma complements were not activated by AGE-modified bovine serum albumin in our in vitro assays, which included a complement hemolytic activity assay and determination of complement fragments, including C4d, C3bB, iC3b, and MAC.

CONCLUSION

It is strongly suggested that medial SMC injury in intrarenal arteries is caused by interaction between glycoxidation and complement activation and contributes to the progression of diabetic nephropathy.

Poliovirus binding to its receptor in lipid bilayers results in particle-specific, temperature-sensitive channels

Poliovirus (PV) infection starts with binding to its receptor (PVR), followed by a receptor-aided, temperature-sensitive conformational change of the infectious particle (sedimenting at 160S) to a particle which sediments at 135S. Reported in this communication is the successful incorporation into lipid bilayers of two forms of the receptor: the full-length human receptor and a modified clone in which the extracellular domains of the receptor were fused to a glycosylphosphatidylinositol tail. Addition of virus (160S) to receptor-containing bilayers leads to channel formation, whereas no channels were observed when the receptor-modified viral particle (135S) was added. Increasing the temperature from 21 to 31 degrees C led to a 10-fold increase in the magnitude of the single channel conductance, which can be interpreted as a conformational change in the channel structure. A mutant PV with an amino acid change in VP4 (one of the coat proteins) which is defective in genome uncoating failed to produce channels, suggesting that VP4 might be involved in the channel architecture. These studies provide the first electrophysiological characterization of the interactions between poliovirus and its receptor incorporated into a lipid bilayer membrane. Furthermore, they form the foundation for future studies aiming at defining the molecular architecture of the virus-receptor complex.

Association between mannose-binding lectin and vascular complications in type 1 diabetes

Complement activation and inflammation have been suggested in the pathogenesis of diabetic vascular lesions. We investigated serum mannose-binding lectin (MBL) levels and polymorphisms in the MBL gene in type 1 diabetic patients with and without diabetic nephropathy and associated macrovascular complications. Polymorphisms in the MBL gene and serum MBL levels were determined in 199 type 1 diabetic patients with overt nephropathy and 192 type 1 diabetic patients with persistent normoalbuminuria matched for age, sex, and duration of diabetes, as well as in 100 healthy control subjects. The frequencies of high- and low-expression MBL genotypes were similar in patients with type 1 diabetic and healthy control subjects. High MBL genotypes were significantly more frequent in diabetic patients with nephropathy than in the normoalbuminuric group, and the risk of having nephropathy given a high MBL genotype assessed by odds ratio (OR) was 1.52 (1.02-2.27, P = 0.04). Median serum MBL concentrations were significantly higher in patients with nephropathy than in patients with normoalbuminuria: 2,306 microg/l (interquartile range [IQR] 753-4,867 microg/l) vs. 1,491 microg/l (577-2,944 microg/l), P = 0.0003. In addition, even when comparing patients with identical genotypes, serum MBL levels were higher in the nephropathy group than in the normoalbuminuric group. Patients with a history of cardiovascular disease had significantly elevated MBL levels independent of nephropathy status (3,178 microg/l [IQR 636-5,231 microg/l] vs. 1,741 microg/l [656-3,149 microg/l], P = 0.02). The differences in MBL levels between patients with and without vascular complications were driven primarily by pronounced differences among carriers of high MBL genotypes (P < 0.0001). Our findings suggest that MBL may be involved in the pathogenesis of micro- and macrovascular complications in type 1 diabetes, and that determination of MBL status might be used to identify patients at increased risk of developing these complications.

Effects of diabetes on the vascular system: current research evidence and best practice recommendations

Diabetes mellitus has been referred to as a vascular disease because of its effect on the vascular endothelial wall. In recent years, research has identified specific effects of hyperglycemia and insulin resistance on the vasculature of the diabetic patient. Atherosclerosis is known to develop earlier in the diabetic patient and is more aggressive due to the metabolic effects of hyperglycemia and insulin resistance. The results of many large, randomized, prospective trials have provided practice changes in the management of the patient with diabetes. Trials such as the Framingham Study identified risk factors associated with atherosclerosis. Additional studies, such as the Diabetes Control and Complications Trial and the United Kingdom Prospective Diabetes Study, provided information about risk factors for diabetes and contributed to treatment recommendations for the person with type 1 or type 2 diabetes. Results of these and many other trials continue to change the recommendations for the person with diabetes to reduce mortality and prevent coronary heart disease, blindness, renal failure, and amputation. This paper will identify the effects of diabetes on the vascular system and outline best practice recommendations on the basis of clinical trials.

Advanced Glycation End Products in Clinical Nephrology

As a result of oxidative and carbonyl stress, advanced glycation end products (AGEs) are involved in the pathogenesis of severe and frequent diseases and their fatal vascular/cardiovascular complications, i.e. diabetes mellitus and its complications (nephropathy, angiopathy, neuropathy and retinopathy, renal failure and uremic and dialysis-associated complications), atherosclerosis and dialysis-related amyloidosis, neurodegenerative diseases, and rheumatoid arthritis. They are formed via non-enzymatic glycation which is specifically enhanced through the presence of oxidative and carbonyl stress, and their ability to form glycoxidation products in peptide and protein structures finally modulating or inducing biological reactivity. Food can be another source of AGEs; however, high serum AGEs in hemodialysis patients might reflect nutritional status better. Several methods of renal replacement therapy have been studied in connection with the AGE removal, but unfortunately the possibilities are still unsatisfactory even if high flux dialysis, hemofiltration, or hemodiafiltration give better results than conventional low flux dialysis. AGEs are currently being studied in the patients on peritoneal dialysis as their precursors can be formed in the dialysis fluid. AGEs can cause damage to the peritoneum and so a loss of ultrafiltration capacity. Many compounds give promising results in AGE inhibition (inhibition of formation of AGEs, inhibition of their action or degradation of AGEs), are tested for these properties, and eventually undergo clinical studies (e.g. aminoguanidine, OPB-9195, pyridoxamine, antioxidants, N-phenacylthiazolium bromide, antihypertensive drugs, angiotensin-converting enzyme inhibitors and angiotensin II receptor-1 antagonists).

Modification of proteins in vitro by physiological levels of glucose: pyridoxamine inhibits conversion of Amadori intermediate to advanced glycation end-products through binding of redox metal ions

Hyperglycemic conditions of diabetes accelerate protein modifications by glucose leading to the accumulation of advanced glycation end-products (AGEs). We have investigated the conversion of protein-Amadori intermediate to protein-AGE and the mechanism of its inhibition by pyridoxamine (PM), a potent AGE inhibitor that has been shown to prevent diabetic complications in animal models. During incubation of proteins with physiological diabetic concentrations of glucose, PM prevented the degradation of the protein glycation intermediate identified as fructosyllysine (Amadori) by 13C NMR using [2-13C]-enriched glucose. Subsequent removal of glucose and PM led to conversion of protein-Amadori to AGE Nepsilon-carboxymethyllysine (CML). We utilized this inhibition of post-Amadori reactions by PM to isolate protein-Amadori intermediate and to study the inhibitory effect of PM on its degradation to protein-CML. We first tested the hypothesis that PM blocks Amadori-to-CML conversion by interfering with the catalytic role of redox metal ions that are required for this glycoxidative reaction. Support for this hypothesis was obtained by examining structural analogs of PM in which its known bidentate metal ion binding sites were modified and by determining the effect of endogenous metal ions on PM inhibition. We also tested the alternative hypothesis that the inhibitory mechanism involves formation of covalent adducts between PM and protein-Amadori. However, our 13C NMR studies demonstrated that PM does not react with the Amadori. Because the mechanism of interference with redox metal catalysis is operative under the conditions closely mimicking the diabetic state, it may contribute significantly to PM efficacy in preventing diabetic complications in vivo. Inhibition of protein-Amadori degradation by PM also provides a simple procedure for the isolation of protein-Amadori intermediate, prepared at physiological levels of glucose for relevancy, to study both the biological effects and the chemistry of post-Amadori pathways of AGE formation.

Glycoxidation and inflammation in chronic haemodialysis patients

BACKGROUND

Uraemia and haemodialysis treatment are associated with microinflammation and oxidative as well as carbonyl stress, which result in enhanced formation of glycoxidation products. Although both glycoxidation and inflammation can contribute to severe vascular and cardiovascular complications, the role that these pathogenic mechanisms play in the complex response of the whole organism remains to be elucidated.

METHODS

We performed a cross-sectional study in 34 clinically stable chronic haemodialysis patients and in 14 healthy controls while determining serum concentrations of pentosidine, fluorescent advanced glycation end-products (AGEs), advanced oxidation protein products (AOPPs) and acute phase reactants. We further assessed the relationship between these glycoxidation products and parameters of inflammation.

RESULTS

Glycoxidation products as well as certain acute phase reactants were elevated in haemodialysis patients. There were significant correlations between AOPPs and inflammatory parameters such as orosomucoid (0.39, P < 0.05), fibrinogen (0.49, P < 0.05) and pregnancy-associated protein A (PAPP-A; 0.46, P < 0.05), but no correlations between pentosidine or fluorescent AGEs and any of the inflammatory parameters.

CONCLUSION

Oxidative damage showed a closer relationship to inflammation than advanced glycation (glycoxidation). AOPPs may represent a superior acute biochemical marker, whereas AGEs may better describe chronic long-lasting damage.

Deficiency of the mouse complement regulatory protein mCd59b results in spontaneous hemolytic anemia with platelet activation and progressive male infertility

Basal complement activity presents a potential danger for "self" cells that are tightly protected by complement regulators including CD59. Mice express two Cd59 genes (mCd59a and mCd59b); mCd59b has approximately a 6-fold higher specific activity than mCd59a. Consistently, mCd59b knockout mice present a strong phenotype characterized by hemolytic anemia with increased reticulocytes, anisopoikilocytosis, echinocytosis, schistocytosis, free hemoglobin in plasma, hemoglobinuria with hemosiderinuria, and platelet activation. Remarkably, mCd59b(-/-) males express a progressive loss of fertility associated with immobile dysmorphic and fewer sperm cells after 5 months of age. This work indicates that mCd59b is a key complement regulator in mice and that CD59 is critical in protecting self cells; it also provides a novel model to study complement regulation in human diseases.

Early complement activation and decreased levels of glycosylphosphatidylinositol-anchored complement inhibitors in human and experimental diabetic retinopathy

Diabetic retinal microangiopathy is characterized by increased permeability, leukostasis, microthrombosis, and apoptosis of capillary cells, all of which could be caused or compounded by activation of complement. In this study, we observed deposition of C5b-9, the terminal product of complement activation, in the wall of retinal vessels of human eye donors with 9 +/- 3 years of type 2 diabetes, but not in the vessels of age-matched nondiabetic donors. C5b-9 often colocalized with von Willebrand factor in luminal endothelium. C1q and C4, the complement components unique to the classical pathway, were not detected in the diabetic retinas, suggesting that C5b-9 was generated via the alternative pathway, the spontaneous activation of which is regulated by complement inhibitors. The diabetic donors showed a prominent reduction in the retinal levels of CD55 and CD59, the two complement inhibitors linked to the plasma membrane by glycosylphosphatidylinositol anchors, but not in the levels of transmembrane CD46. Similar complement activation in retinal vessels and selective reduction in the levels of retinal CD55 and CD59 were observed in rats with a 10-week duration of streptozotocin-induced diabetes. Thus, diabetes causes defective regulation of complement inhibitors and complement activation that precede most other manifestations of diabetic retinal microangiopathy. These are novel clues for probing how diabetes affects and damages vascular cells.

Type 1 and type 2 diabetic patients display different patterns of cellular microparticles

The development of vasculopathies in diabetes involves multifactorial processes including pathological activation of vascular cells. Release of microparticles by activated cells has been reported in diseases associated with thrombotic risk, but few data are available in diabetes. The aim of the present work was to explore the number and the procoagulant activity of cell-derived microparticles in type 1 and 2 diabetic patients. Compared with age-matched control subjects, type 1 diabetic patients presented significantly higher numbers of platelet and endothelial microparticles (PMP and EMP), total annexin V-positive blood cell microparticles (TMP), and increased levels of TMP-associated procoagulant activity. In type 2 diabetic patients, only TMP levels were significantly higher without concomitant increase of their procoagulant activity. Interestingly, in type 1 diabetic patients, TMP procoagulant activity was correlated with HbA(1c), suggesting that procoagulant activity is associated with glucose imbalance. These results showed that a wide vesiculation process, resulting from activation or apoptosis of several cell types, occurs in diabetes. However, diabetic patients differ by the procoagulant activity and the cellular origin of microparticles. In type 1 diabetic patients, TMP-procoagulant activity could be involved in vascular complications. Moreover, its correlation with HbA(1c) reinforces the importance of an optimal glycemic control in type 1 diabetes.

A possible role for complement in the pathogenesis of chronic chagasic cardiomyopathy

The membrane attack complex (MAC) of complement participates in several inflammatory and proliferative processes by releasing pro-inflammatory cytokines and growth factors from target cells. Chronic Chagasic cardiomyopathy (CCH) is a parasitic dilated cardiopathy, characterized by severe fibrosis and inflammation, which differs from idiopathic dilated cardiomyopathy (DCM). Trypanosoma cruzi, the pathogenic organism of CCH, is a strong complement activator and can also induce alternative pathway activation by mammalian cells. This study explored whether the myocardium in CCH patients has increased MAC deposition, an expression of complement activation, compared to DCM patients. MAC was semi-quantified in endomyocardial human samples (29 CCH subjects, 18 DCM subjects, and four controls) by immunohistochemistry. MAC was present in the sarcolemma of 38% of CCH, 5.5% of DCM (p<0.02), and 0% of controls, and in interstitial inflammatory cells of CCH. No difference was observed in the expression of the complement regulatory protein CD59, indicating that increased MAC deposition is likely to be the result of complement activation rather than decreased protection. It is proposed that the increased MAC deposition found in CCH, but not in DCM or controls, may help to explain the diffuse myocardial fibrosis and inflammation characteristic of the disease.

How hyperglycemia promotes atherosclerosis: molecular mechanisms

Both type I and type II diabetes are powerful and independent risk factors for coronary artery disease (CAD), stroke, and peripheral arterial disease. Atherosclerosis accounts for virtually 80% of all deaths among diabetic patients. Prolonged exposure to hyperglycemia is now recognized a major factor in the pathogenesis of atherosclerosis in diabetes. Hyperglycemia induces a large number of alterations at the cellular level of vascular tissue that potentially accelerate the atherosclerotic process. Animal and human studies have elucidated three major mechanisms that encompass most of the pathological alterations observed in the diabetic vasculature: 1) Nonenzymatic glycosylation of proteins and lipids which can interfere with their normal function by disrupting molecular conformation, alter enzymatic activity, reduce degradative capacity, and interfere with receptor recognition. In addition, glycosylated proteins interact with a specific receptor present on all cells relevant to the atherosclerotic process, including monocyte-derived macrophages, endothelial cells, and smooth muscle cells. The interaction of glycosylated proteins with their receptor results in the induction of oxidative stress and proinflammatory responses 2) oxidative stress 3) protein kinase C (PKC) activation with subsequent alteration in growth factor expression. Importantly, these mechanisms may be interrelated. For example, hyperglycemia-induced oxidative stress promotes both the formation of advanced glycosylation end products and PKC activation.

Membrane complement regulatory proteins: insight from animal studies and relevance to human diseases

The complement system plays an important role in host defense. However, if not properly regulated, activated complement can also cause significant damage to host tissues. To prevent complement-mediated autologous tissue damage, host cells express a number of membrane-bound complement regulatory proteins. These include decay-accelerating factor (DAF, CD55), membrane cofactor protein (MCP, CD46) and CD59. Recent studies of membrane complement regulatory proteins from various animal species have revealed similarities as well as significant differences from the corresponding human proteins. In this review, we summarize recent advances in this area and contrast the structure, function and tissue distribution of membrane complement regulatory proteins in human and nonprimate mammalian species. We also discuss how the characterization of the animal proteins has provided important clues and might continue to show relevance to the pathogenesis and therapeutics of a number of human diseases.

Complement components as uremic toxins and their potential role as mediators of microinflammation

Cardiovascular disease is the major cause of death in end-stage renal disease (ESRD) patients. There is growing evidence that atherogenesis is an inflammatory rather than a purely degenerative process leading to a state of microinflammation. This raises the issue of whether treatment modalities of ESRD contribute to the microinflammatory state. One potential candidate in this context is the complement system. Here we consider three potential pathways linking complement activation to progression of atherosclerosis: (1) complement activation on artificial surfaces depends on their physicochemical characteristics, the effect of which is amplified because of the accumulation of complement factor D; (2) the exposure of ESRD patients to endotoxin creates a microinflammatory state, and this may amplify complement-induced damage; exposure to endotoxin may result from frequent infections because of the impairment of host-defense mechanisms or from transfer of bacterial contaminants across dialysis membranes into the blood stream; and (3) direct transduction of proinflammatory signals from blood-material interactions to the vascular system. We conclude that the complement system is an important candidate system in the genesis of microinflammation and accelerated atherogenesis in ESRD. We advance the hypothesis that the generation of proinflammatory signals, in which the complement system appears to be involved--both through systemic and local activation--plays a role in the development of late complications of uremia, including coronary heart disease. This hypothesis provides a rationale to maximize the biocompatibility of the dialysis procedure, that is, selection of nonactivating materials, use of ultrapure dialysis fluid, and--still theoretical--high-flux dialysis to remove factor D.