Glycation stimulates amyloid formation

Alpha-synuclein expression in GnRH neurons of young and old bovine hypothalami

Context Understanding of central nervous system mechanisms related to age-related infertility remains limited. Fibril α-synuclein, distinct from its monomer form, is implicated in age-related diseases and propagates among neurons akin to prions. Aims We compared α-synuclein expression in gonadotropin-releasing hormone-expressing neurons (GnRH neurons) in the pre-optic area, arcuate nucleus, and median eminence of healthy heifers and aged cows to determine its role in age-related infertility. Methods We analysed mRNA and protein expression, along with fluorescent immunohistochemistry for GnRH and α-synuclein, followed by Congo red staining to detect amyloid deposits, and confocal microscopy. Key results Both mRNA and protein expressions of α-synuclein were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) and western blots in bovine cortex, hippocampus, and anterior and posterior hypothalamus tissues. Significant differences in α-synuclein mRNA expression were observed in the cortex and hippocampus between young and old cows. Western blots showed five bands of α-synuclein, probably reflecting monomer, dimer, and oligomers, in the cortex, hippocampus, hypothalamus tissues, and there were significant differences in some bands between young and old cows. Bright-field and polarised light microscopy did not detect obvious amyloid deposition in aged hypothalami; however, higher-sensitive confocal microscopy unveiled strong positive signal of Congo red and α-synuclein in GnRH neurons in aged hypothalami. Additionally, α-synuclein expression was detected in immortalised GnRH neurons, GT1-7 cells. Conclusion Alpha-synuclein was expressed in GnRH neurons, and some differences were observed between young and old hypothalami. Implications Alpha-synuclein may play an important role in aging-related infertility.

Alpha-synuclein expression in oxytocin neurons of young and old bovine brains

Understanding of central nervous system mechanisms underlying age-related infertility remains limited. Fibril α-synuclein, distinct from its monomeric form, is implicated in age-related diseases. Notably, fibril α-synuclein spreads among neurons, similar to prions, from damaged old neurons in cortex and hippocampus to healthy neurons. However, less is known whether α-synuclein propagates into oxytocin neurons, which play crucial roles in reproduction. We compared α-synuclein expression in the oxytocin neurons in suprachiasmatic nucleus (SCN), supraoptic nucleus (SON), paraventricular hypothalamic nucleus (PVN), and posterior pituitary (PP) gland of healthy heifers and aged cows to determine its role in age-related infertility. We analyzed mRNA and protein expression, along with Congo red histochemistry and fluorescent immunohistochemistry for oxytocin and α-synuclein, followed by confocal microscopy with Congo red staining. Both mRNA and protein expressions of α-synuclein were confirmed in the bovine cortex, hippocampus, SCN, SON, PVN, and PP tissues. Significant differences in α-synuclein mRNA expressions were observed in the cortex and hippocampus between young heifers and old cows. Western blots showed five bands of α-synuclein, probably reflecting monomers, dimers, and oligomers, in the cortex, hippocampus, SCN, SON, PVN, and PP tissues, and there were significant differences in some bands between the young heifers and old cows. Bright-field and polarized light microscopy did not detect obvious amyloid deposition in the aged hypothalami; however, higher-sensitive confocal microscopy unveiled strong positive signals for Congo red and α-synuclein in oxytocin neurons in the aged hypothalami. α-synuclein was expressed in oxytocin neurons, and some differences were observed between young and old hypothalami.

Methylglyoxal-induced modification of myoglobin: An insight into glycation mediated protein aggregation

Post-translational modification of proteins by Maillard reaction, known as glycation, is thought to be the root cause of different complications, particularly in diabetes mellitus and age-related disorders. Methylglyoxal (MG), a reactive α-oxoaldehyde, increases in diabetic condition and reacts with the proteins to form advanced glycation end products (AGEs) following a Maillard-like reaction. In a time-dependent reaction study of MG with the heme protein myoglobin (Mb), MG was found to induce significant structural alterations of the heme protein, such as heme loss, changes in tryptophan fluorescence, and decrease of α-helicity with increased β-sheet content. These changes were found to occur gradually with increasing period of incubation. Incubation of Mb with MG induced the formation of several AGE adducts, including, carboxyethyllysine at Lys-16, carboxymethyllysine at Lys-87, carboxyethyllysine or pyrraline-carboxymethyllysine at Lys-133, carboxyethyllysine at Lys-42 and hydroimidazolone or argpyrimidine at Arg-31 and Arg-139. MG induced amyloid-like aggregation of Mb was detected at a longer period of incubation. MG-derived AGEs, therefore, appear to have an important role as the precursors of protein aggregation, which, in turn, may be associated with pathophysiological complications.

An overview on glycation: molecular mechanisms, impact on proteins, pathogenesis, and inhibition

The formation of a heterogeneous set of advanced glycation end products (AGEs) is the final outcome of a non-enzymatic process that occurs in vivo on long-life biomolecules. This process, known as glycation, starts with the reaction between reducing sugars, or their autoxidation products, with the amino groups of proteins, DNA, or lipids, thus gaining relevance under hyperglycemic conditions. Once AGEs are formed, they might affect the biological function of the biomacromolecule and, therefore, induce the development of pathophysiological events. In fact, the accumulation of AGEs has been pointed as a triggering factor of obesity, diabetes-related diseases, coronary artery disease, neurological disorders, or chronic renal failure, among others. Given the deleterious consequences of glycation, evolution has designed endogenous mechanisms to undo glycation or to prevent it. In addition, many exogenous molecules have also emerged as powerful glycation inhibitors. This review aims to provide an overview on what glycation is. It starts by explaining the similarities and differences between glycation and glycosylation. Then, it describes in detail the molecular mechanism underlying glycation reactions, and the bio-molecular targets with higher propensity to be glycated. Next, it discusses the precise effects of glycation on protein structure, function, and aggregation, and how computational chemistry has provided insights on these aspects. Finally, it reports the most prevalent diseases induced by glycation, and the endogenous mechanisms and the current therapeutic interventions against it.

Clinical Manifestation of AGE-RAGE Axis in Neurodegenerative and Cognitive Impairment Disorders

The receptor of Advanced Glycation Endproducts (RAGE) and Advanced Glycation Endproducts (AGE) have multiple functions in our body and their restraint are being observed in neurodegenerative and memory impairment disorders. The review of different pathways allows an understanding of the probable mechanism of neurodegeneration and memory impairment involving RAGE and AGE. Commonly we observe AGE accumulation in neural cells and tissues but the extent of accumulation increases with the presence of memory impairment disorder. The presence of AGEs can also be seen in morbid accumulation, pathological structures in the form of amyloid clots, and nervous fibrillary tangles in Alzheimer's Disease (AD) and memory impairment disease.Many neuropathological and biochemical aspects of AD are explained by AGEs, including widespread protein crosslinking, glial activation of oxidative stress, and neuronal cell death. Oxidative stress is due to different reasons and glycation end products set in motion and form or define various actions which are normally due to AGE changes in a pathogenic cascade. By regulating the transit of ß-amyloid in and out of the brain or altering inflammatory pathways, AGE and it's ensnare receptor such as soluble RAGE may function as blockage or shield AD development. RAGE activates the transcription-controlling factor Necrosis Factor (NF-κB) and increases the protraction of cytokines, like a higher number of Tumor Necrosis Factor (TNF-α) and Interleukin (IL-I) by inducing several signal transduction cascades. Furthermore, binding to RAGE can pro-activate reactive oxygen species (ROS), which is popularly known to cause neuronal death.

Current Approach to the Pathogenesis of Diabetic Cataracts

Cataracts remain the first or second leading cause of blindness in all world regions. In the diabetic population, cataracts not only have a 3–5 times higher incidence than in the healthy population but also affect people at a younger age. In patients with type 1 diabetes, cataracts occur on average 20 years earlier than in the non-diabetic population. In addition, the risk of developing cataracts increases with the duration of diabetes and poor metabolic control. A better understanding of the mechanisms leading to the formation of diabetic cataracts enables more effective treatment and a holistic approach to the patient.

Natural Product Co-Metabolism and the Microbiota-Gut-Brain Axis in Age-Related Diseases

Complementary alternative medicine approaches are growing treatments of diseases to standard medicine practice. Many of these concepts are being adopted into standard practice and orthomolecular medicine. Age-related diseases, in particular neurodegenerative disorders, are particularly difficult to treat and a cure is likely a distant expectation for many of them. Shifting attention from pharmaceuticals to phytoceuticals and "bugs as drugs" represents a paradigm shift and novel approaches to intervention and management of age-related diseases and downstream effects of aging. Although they have their own unique pathologies, a growing body of evidence suggests Alzheimer's disease (AD) and vascular dementia (VaD) share common pathology and features. Moreover, normal metabolic processes contribute to detrimental aging and age-related diseases such as AD. Recognizing the role that the cerebral and cardiovascular pathways play in AD and age-related diseases represents a common denominator in their pathobiology. Understanding how prosaic foods and medications are co-metabolized with the gut microbiota (GMB) would advance personalized medicine and represents a paradigm shift in our view of human physiology and biochemistry. Extending that advance to include a new physiology for the advanced age-related diseases would provide new treatment targets for mild cognitive impairment, dementia, and neurodegeneration and may speed up medical advancements for these particularly devastating and debilitating diseases. Here, we explore selected foods and their derivatives and suggest new dementia treatment approaches for age-related diseases that focus on reexamining the role of the GMB.

Biophysical and mass spectrometry based characterization of methylglyoxal-modified myoglobin: Role of advanced glycation end products in inducing protein structural alterations

Methylglyoxal (MG) is a highly reactive α-dicarbonyl compound which reacts with proteins to form advanced glycation end products (AGEs). MG-induced AGE (MAGE) formation is particularly significant in diabetic condition. In the current study, we have undertaken a time-dependant characterization of MG-modified myoglobin following incubation of the heme protein with the α-dicarbonyl compound for different time periods. Interestingly, mass spectrometric studies indicated modifications at two specific lysine residues, Lys-87 and Lys-133. The AGE adducts identified at Lys-87 were carboxymethyllysine and carboxyethyllysine, while those detected at Lys-133 included pyrraline-carboxymethyllysine and carboxyethyllysine, respectively. Far-UV CD studies revealed a decrease in the native α-helical content of the heme protein gradually with increasing time of MG incubation. In addition, MG modification was found to induce changes in tertiary structure as well as surface hydrophobicity of the heme protein. MG-derived AGE adducts thus appear to alter the structure of Mb considerably. Considering the increased level of MG in diabetic condition, the current study appears physiologically relevant in terms of understanding AGE-mediated protein modification and subsequent structural changes.

Strategic approaches to target the enzymes using natural compounds for the management of Alzheimer's disease: A review

Alzheimer's disease (AD) is a chronic neurodegenerative disease. It is clinically characterized by memory loss and intellectual decrease, among other neurological deficits. The etiology of AD is not completely understood but includes amyloid plaques and intracellular helical filaments as well as neurofibrillary tangles with hyperphosphorylated tau protein. AD is also associated with alterations in amyloid processing genes, such as PSEN1 or PSEN2 and APP. The modulation immune system, cholesterol metabolism, and synaptic vesicle endocytosis have all been shown to remediate AD. In this review, enzymes such as AChE, BuChE, β-secretase, γ-secretase, MAO, and RAGE are discussed as potential targets for AD treatment. The aim of this review was to addresses the molecular mechanisms as well as various genetic factors in AD etiology. The use of natural compounds against these targets might be beneficial for the management of AD.

Ribosylation induced structural changes in Bovine Serum Albumin: understanding high dietary sugar induced protein aggregation and amyloid formation

Non-enzymatic glycation of proteins is believed to be the root cause of high dietary sugar associated pathophysiological maladies. We investigated the structural changes in protein during progression of glycation using ribosylated Bovine Serum Albumin (BSA). Non enzymatic attachment of about 45 ribose molecules to BSA resulted in gradual reduction of hydrophobicity and aggregation as indicated by red-shifted tryptophan fluorescence, reduced ANS binding and lower anisotropy of FITC-conjugated protein. Parallely, there was a significant decrease of alpha helicity as revealed by Circular Dichroism (CD) and Fourier transformed-Infra Red (FT-IR) spectra. The glycated proteins assumed compact globular structures with enhanced Thioflavin-T binding resembling amyloids. The gross structural transition affected by ribosylation led to enhanced thermostability as indicated by melting temperature and Transmission Electron Microscopy. At a later stage of glycation, the glycated proteins developed non-specific aggregates with increase in size and loss of amyloidogenic behaviour. A parallel non-glycated control incubated under similar conditions indicated that amyloid formation and associated changes were specific for ribosylation and not driven by thermal denaturation due to incubation at 37 °C. Functionality of the glycated protein was significantly altered as probed by Isothermal Titration Calorimetry using polyphenols as substrates. The studies demonstrated that glycation driven globular amyloids form and persist as transient intermediates during formation of misfolded glycated adducts. To the best of our knowledge, the present study is the first systematic attempt to understand glycation associated changes in a protein and provides important insights towards designing therapeutics for arresting dietary sugar induced amyloid formation.

The Microbiota-Gut-Brain Axis-Heart Shunt Part II: Prosaic Foods and the Brain-Heart Connection in Alzheimer Disease

There is a strong cerebrovascular component to brain aging, Alzheimer disease, and vascular dementia. Foods, common drugs, and the polyphenolic compounds contained in wine modulate health both directly and through the gut microbiota. This observation and novel findings centered on nutrition, biochemistry, and metabolism, as well as the newer insights we gain into the microbiota-gut-brain axis, now lead us to propose a shunt to this classic triad, which involves the heart and cerebrovascular systems. The French paradox and prosaic foods, as they relate to the microbiota-gut-brain axis and neurodegenerative diseases, are discussed in this manuscript, which is the second part of a two-part series of concept papers addressing the notion that the microbiota and host liver metabolism all play roles in brain and heart health.

Methylglyoxal modification reduces the sensitivity of hen egg white lysozyme to stress-induced aggregation: Insight into the anti-amyloidogenic property of α-dicarbonyl compound

The reactive α-oxoaldehyde, methylglyoxal reacts with different proteins to form Advanced Glycation End Products (AGEs) through Maillard reaction. Its level increases significantly in diabetic condition. Here, we have investigated the effect of different concentrations of methylglyoxal (200-400 µM) on the monomeric protein, hen egg white lysozyme (HEWL) following incubation for 3 weeks. Reaction of methylglyoxal with HEWL induced considerable changes in tertiary structure of the protein, but no significant alteration in secondary structure, as evident from different spectroscopic and biophysical studies. Interestingly, methylglyoxal modification was found to enhance the thermal stability of the protein and reduce its sensitivity to stress-induced aggregation. Finally, peptide mass fingerprinting revealed modification of arginine (Arg-45, Arg-14, Arg-68 or Arg-72) and lysine (Lys-116) residues of the protein to AGE adducts, namely, hydroimidazolone, tetrahydropyrimidine, and carboxyethyllysine. Methylglyoxal-derived AGE adducts (MAGE) appear to be responsible for the observed changes in protein. As demonstrated in the present study, the findings may highlight a possible therapeutic potential of the α-oxoaldehyde against protein misfolding and conformational disorder.Communicated by Ramaswamy H. Sarma.

Glycation of Lysozyme by Glycolaldehyde Provides New Mechanistic Insights in Diabetes-Related Protein Aggregation

Glycation occurs in vivo as a result of the nonenzymatic reaction of carbohydrates (and/or their autoxidation products) with proteins, DNA, or lipids. Protein glycation causes loss-of-function and, consequently, the development of diabetic-related diseases. Glycation also boosts protein aggregation, which can be directly related with the higher prevalence of aggregating diseases in diabetic people. However, the molecular mechanism connecting glycation with aggregation still remains unclear. Previously we described mechanistically how glycation of hen egg-white lysozyme (HEWL) with ribose induced its aggregation. Here we address the question of whether the ribose-induced aggregation is a general process or it depends on the chemical nature of the glycating agent. Glycation of HEWL with glycolaldehyde occurs through two different scenarios depending on the HEWL concentration regime (both within the micromolar range). At low HEWL concentration, non-cross-linking fluorescent advanced glycation end-products (AGEs) are formed on Lys side chains, which do not change the protein structure but inhibit its enzymatic activity. These AGEs have little impact on HEWL surface hydrophobicity and, therefore, a negligible effect on its aggregation propensity. Upon increasing HEWL concentration, the glycation mechanism shifts toward the formation of intermolecular cross-links, which triggers a polymerization cascade involving the formation of insoluble spherical-like aggregates. These results notably differ with the aggregation-modulation mechanism of ribosylated HEWL directed by hydrophobic interactions. Additionally, their comparison constitutes the first experimental evidence showing that the mechanism underlying the aggregation of a glycated protein depends on the chemical nature of the glycating agent.

Formation of Pentosidine Cross-Linking in Myoglobin by Glyoxal: Detection of Fluorescent Advanced Glycation End Product

Glyoxal, a reactive α-oxoaldehyde, increases in diabetic condition and reacts with proteins to form advanced glycation end products (AGEs) following Maillard-like reaction. Considering the significance of protein modification by glyoxal-derived AGEs, we investigated the in vitro effect of glyoxal (200 μM) on the monomeric heme protein myoglobin (Mb) (100 μM) after incubation for one week at 25 °C. Glyoxal-treated Mb exhibited increased absorbance around the Soret region, decreased α-helicity and thermal stability compared to control Mb. Intrinsic fluorescence spectrum of the treated Mb showed an additional signal in the 400-500 nm region on excitation at 280 nm that was absent in control Mb. When excited at 335 nm, the glyoxal-treated sample gave a strong fluorescence indicating AGE formation. Mass spectrometric studies revealed formation of glyoxal-derived fluorescent AGE adduct pentosidine between Lys-145 and Arg-139 residues of Mb. Other than pentosidine, additional AGE adducts, namely, carboxymethyllysine at Lys-133, hydroimidazolone at Arg-31 and pyrrolidone-carboxymethyllysine at Lys-145 were also detected. Lys-145 was thus found to contain two different types of AGE adducts, indicating the heterogeneous nature of in vitro glycation reaction. AGE-induced protein modifications might be associated with complications in disease conditions.

Advanced glycation end products induce differential structural modifications and fibrillation of albumin

Glycation induced amyloid fibrillation is fundamental to the development of many neurodegenerative and cardiovascular complications. Excessive non-enzymatic glycation in conditions such as hyperglycaemia results in the increased accumulation of advanced glycation end products (AGEs). AGEs are highly reactive pro-oxidants, which can lead to the activation of inflammatory pathways and development of oxidative stress. Recently, the effect of non-enzymatic glycation on protein structure has been the major research area, but the role of specific AGEs in such structural alteration and induction of fibrillation remains undefined. In this study, we determined the specific AGEs mediated structural modifications in albumin mainly considering carboxymethyllysine (CML), carboxyethyllysine (CEL), and argpyrimidine (Arg-P) which are the major AGEs formed in the body. We studied the secondary structural changes based on circular dichroism (CD) and spectroscopic analysis. The AGEs induced fibrillation was determined by Congo red binding and examination of scanning and transmission electron micrographs. The amyloidogenic regions in the sequence of BSA were determined using FoldAmyloid. It was observed that CEL modification of BSA leads to the development of fibrillar structures, which was evident from both secondary structure changes and TEM analysis.

Methylglyoxal-induced modification causes aggregation of myoglobin

Post-translational modification of proteins by Maillard reaction, known as glycation, is thought to be the root cause of different complications, particularly in diabetes mellitus and age-related disorders. Methylglyoxal (MG), a reactive α-oxoaldehyde, increases in diabetic condition and reacts with proteins to form advanced glycation end products (AGEs) following Maillard-like reaction. We have investigated the in vitro effect of MG (200μM) on the monomeric heme protein myoglobin (Mb) (100μM) in a time-dependent manner (7 to 18days incubation at 25°C). MG induces significant structural alterations of the heme protein, including heme loss, changes in tryptophan fluorescence, decrease of α-helicity with increased β-sheet content etc. These changes occur gradually with increased period of incubation. Incubation of Mb with MG for 7days results in formation of the AGE adducts: carboxyethyllysine at Lys-16, carboxymethyllysine at Lys-87 and carboxyethyllysine or pyrraline-carboxymethyllysine at Lys-133. On increasing the period of incubation up to 14days, additional AGEs namely, carboxyethyllysine at Lys-42 and hydroimidazolone or argpyrimidine at Arg-31 and Arg-139 have been detected. MG also induces aggregation of Mb, which is clearly evident with longer period of incubation (18days), and appears to have amyloid nature. MG-derived AGEs may thus have an important role as the precursors of protein aggregation, which, in turn, may be associated with physiological complications.

Insulin glycation coupled with liposomal lipid peroxidation and microglial cell death

Type 2 diabetes is characterized jointly by hyperglycemia and hyperinsulinemia, which make insulin prone to glycation then fibrillation.

The role of advanced glycation end products in various types of neurodegenerative disease: a therapeutic approach

Protein glycation is initiated by a nucleophilic addition reaction between the free amino group from a protein, lipid or nucleic acid and the carbonyl group of a reducing sugar. This reaction forms a reversible Schiff base, which rearranges over a period of days to produce ketoamine or Amadori products. The Amadori products undergo dehydration and rearrangements and develop a cross-link between adjacent proteins, giving rise to protein aggregation or advanced glycation end products (AGEs). A number of studies have shown that glycation induces the formation of the β-sheet structure in β-amyloid protein, α-synuclein, transthyretin (TTR), copper-zinc superoxide dismutase 1 (Cu, Zn-SOD-1), and prion protein. Aggregation of the β-sheet structure in each case creates fibrillar structures, respectively causing Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, familial amyloid polyneuropathy, and prion disease. It has been suggested that oligomeric species of glycated α-synuclein and prion are more toxic than fibrils. This review focuses on the pathway of AGE formation, the synthesis of different types of AGE, and the molecular mechanisms by which glycation causes various types of neurodegenerative disease. It discusses several new therapeutic approaches that have been applied to treat these devastating disorders, including the use of various synthetic and naturally occurring inhibitors. Modulation of the AGE-RAGE axis is now considered promising in the prevention of neurodegenerative diseases. Additionally, the review covers several defense enzymes and proteins in the human body that are important anti-glycating systems acting to prevent the development of neurodegenerative diseases.

Mechanistic insights in glycation-induced protein aggregation

Protein glycation causes loss-of-function through a process that has been associated with several diabetic-related diseases. Additionally, glycation has been hypothesized as a promoter of protein aggregation, which could explain the observed link between hyperglycaemia and the development of several aggregating diseases. Despite its relevance in a range of diseases, the mechanism through which glycation induces aggregation remains unknown. Here we describe the molecular basis of how glycation is linked to aggregation by applying a variety of complementary techniques to study the nonenzymatic glycation of hen lysozyme with ribose (ribosylation) as the reducing carbohydrate. Ribosylation involves a chemical multistep conversion that induces chemical modifications on lysine side chains without altering the protein structure, but changing the protein charge and enlarging its hydrophobic surface. These features trigger lysozyme native-like aggregation by forming small oligomers that evolve into bigger insoluble particles. Moreover, lysozyme incubated with ribose reduces the viability of SH-SY5Y neuroblastoma cells. Our new insights contribute toward a better understanding of the link between glycation and aggregation.

Glycation accelerates fibrillization of the amyloidogenic W7FW14F apomyoglobin

Neurodegenerative diseases are associated with misfolding and deposition of specific proteins, either intra or extracellularly in the nervous system. Advanced glycation end products (AGEs) originate from different molecular species that become glycated after exposure to sugars. Several proteins implicated in neurodegenerative diseases have been found to be glycated in vivo and the extent of glycation is related to the pathologies of the patients. Although it is now accepted that there is a direct correlation between AGEs formation and the development of neurodegenerative diseases, several questions still remain unanswered: whether glycation is the triggering event or just an additional factor acting on the aggregation pathway. To this concern, in the present study we have investigated the effect of glycation on the aggregation pathway of the amyloidogenic W7FW14F apomyoglobin. Although this protein has not been related to any amyloid disease, it represents a good model to resemble proteins that intrinsically evolve toward the formation of amyloid aggregates in physiological conditions. We show that D-ribose, but not D-glucose, rapidly induces the W7FW14F apomyoglobin to generate AGEs in a time-dependent manner and protein ribosylation is likely to involve lysine residues on the polypeptide chain. Ribosylation of the W7FW14F apomyoglobin strongly affects its aggregation kinetics producing amyloid fibrils within few days. Cytotoxicity of the glycated aggregates has also been tested using a cell viability assay. We propose that ribosylation in the W7FW14F apomyoglobin induces the formation of a cross-link that strongly reduces the flexibility of the H helix and/or induce a conformational change that favor fibril formation. These results open new perspectives for AGEs biological role as they can be considered not only a triggering factor in amyloidosis but also a player in later stages of the aggregation process.

Arguing for the motion: yes, RAGE is a receptor for advanced glycation endproducts

Advanced glycation endproducts (AGEs) are an heterogenous class of compounds formed by diverse stimuli, including hyperglycemia, oxidative stress, inflammation, renal failure, and innate aging. Recent evidence suggests that dietary sources of AGE may contribute to pathology. AGEs impart diverse effects in cells; evidence strongly suggests that crosslinking of proteins by AGEs may irrevocably alter basement membrane integrity and function. In addition, the ability of AGEs to bind to cells and activate signal transduction, thereby affecting broad properties in the cellular milieu, indicates that AGEs are not innocent bystanders in the diseases of AGEing. Here, we present evidence that receptor for AGE (RAGE) is a receptor for AGEs.

Receptor for advanced glycation end products (RAGE) in a dash to the rescue: inflammatory signals gone awry in the primal response to stress

The multiligand receptor for advanced glycation end products (RAGE) of the Ig superfamily transduces the biological impact of discrete families of ligands, including advanced glycation end products, certain members of the S100/calgranulin family, high mobility group box-1, Mac-1 (alpha(M)beta(2), CD11b/CD18), and amyloid-beta peptide and beta-sheet fibrils. Although structurally dissimilar, at least at the monomeric level, recent evidence suggests that oligomeric forms of these RAGE ligands may be especially apt to activate the receptor and up-regulate a program of inflammatory and tissue injury-provoking genes. The challenge in probing the biology of RAGE and its impact in acute responses to stress and the potential development of chronic disease is to draw the line between mechanisms that evoke repair versus those that sustain inflammation and tissue damage. In this review, we suggest the concept that the ligands of RAGE comprise a primal program in the acute response to stress. When up-regulated in environments laden with oxidative stress, inflammation, innate aging, or high glucose, as examples, the function of these ligand families may be transformed from ones linked to rapid repair to those that drive chronic disease. Identification of the threshold beyond which ligands of RAGE mediate repair versus injury is a central component in delineating optimal strategies to target RAGE in the clinic.

Compensatory secondary structure alterations in protein glycation

Glycation, a local covalent interaction, leads to alterations in secondary and tertiary structures of hemoglobin, the changes produced by fructose being more pronounced than those caused by glucose. The Stokes diameter of hemoglobin increases upon glycation from 7 to 14 nm and a concurrent inter-chain cross-linking and heme loss are also observed, particularly in the later stage of glycation. An initial increase of tryptophan (trp) fluorescence was observed in both glucation and fructation. In case of frucation however there was a decrease in tryptophan fluorescence that was accompanied by an increase in fluorescence of the advanced glycosylation end products (AGEs). This fluorescence behavior is indicative of energy transfer between tryptophan and the AGEs formed during the late stage of glycation. Emergence of an isosbestic point in the fluorescence spectra (taken at different time intervals) implies existence of two distinct glycation stages. The late glycation stage is also marked by an increase of beta structure and random coil at the expense of alpha helix. It is further observed that this compensatory loss of alpha helix (reported for the first time) and increase in beta sheet and random coil elements depend on the number of solvent-accessible glycation sites (rather than total number of such sites) and the subunit assembly of the protein.

A role for protein misfolding in immunogenicity of biopharmaceuticals

For largely unknown reasons, biopharmaceuticals evoke potentially harmful antibody formation. Such antibodies can inhibit drug efficacy and, when directed against endogenous proteins, cause life-threatening complications. Insight into the mechanisms by which biopharmaceuticals break tolerance and induce an immune response will contribute to finding solutions to prevent this adverse effect. Using a transgenic mouse model, we here demonstrate that protein misfolding, detected with the use of tissue-type plasminogen activator and thioflavin T, markers of amyloid-like properties, results in breaking of tolerance. In wild-type mice, misfolding enhances protein immunogenicity. Several commercially available biopharmaceutical products were found to contain misfolded proteins. In some cases, the level of misfolded protein was found to increase upon storage under conditions prescribed by the manufacturer. Our results indicate that misfolding of therapeutic proteins is an immunogenic signal and a risk factor for immunogenicity. These findings offer novel possibilities to detect immunogenic protein entities with tPA and reduce immunogenicity of biopharmaceuticals.