Glycosylation and Glycation in Health and Diseases

Glycobiology aims at structure-function correlational analysis of carbohydrates (sugar or glycan). A monosaccharide is the simplest form of carbohydrate that no longer be hydrolyzed. The other forms of carbohydrates are formed by glycosidic linkages of monosaccharides, such as disaccharides, oligosaccharides, and polysaccharides, comprising two, three to ten, and more than ten monosaccharides, respectively. Carbohydrates act as one of the major energy sources (e.g., ATP) and are also involved in cellular protection, stabilization, organization, and barrier functions. In the cellular system, carbohydrates are present in pure and proteinconjugated forms, which are referred to as glycoproteins. Conjugated carbohydrates are also present in the form of glycolipids and proteoglycans. Notably, N- and O-linked glycosylation as major forms occur in the rough surface endoplasmic reticulum (RER) and Golgi apparatus respectively, adding carbohydrates to proteins and thus making glycoproteins. Relatively fewer common types of glycosylation are the C-linked glycosylation, S-linked glycosylation, glypiation, and phosphoglycosylation. A complex interplay of two enzyme groups such as glycosyl transferases (adding carbohydrates to proteins) and glycosidases/glycosyl hydrolases (removing carbohydrates from proteins) control the glycosylation extent. Prominent cellular factors regulating glycosylation are the availability of carbohydrates, proteins, enzymes, movement of proteins from RER to Golgi, and several other environmental factors regulating post-translational modifications. This chapter describes the various aspects of glycobiology including protein glycosylation, purification, and analysis of glycans, and their role in physiology and pathophysiology.

Glycans and their various conjugates namely glycoproteins, glycolipids, and proteoglycans not only coat all the cells in nature and interact with the extracellular matrix (ECM) molecules but are also located in the intracellular regions of every living organism. Glycans mediate or modulate numerous biological roles that are essential for life. This most abundant cellular molecule is necessary to maintain various general and specialized functions of the cells. Some of the major vital roles of glycans include maintenance of the structural integrity and protection of the cells, cell adhesion, cell-t- -cell communication, crosstalk, and bidirectional cell signaling (both inside out and outside in). Briefly, this chapter predominantly focuses on the role of glycans and their various conjugates in maintaining the structural integrity of biological membranes and the overall cells, the different modulatory functions of glycans, and their implication in nutrient sequestration. Additionally, a brief outline of the role of glycans on intrinsic or intra-species recognition and extrinsic or interspecies recognition is discussed. Overall, the biological importance of glycans and their conjugates is elaborated.

Congenital Disorders of Glycosylation (CDG) encompass a rare and complex group of genetic diseases characterized by abnormalities in the fundamental process of glycosylation. There is an abnormal synthesis or attachment of the glycan moiety of glycoproteins and glycolipids. CDG arises from mutations in genes responsible for various steps in glycosylation within the endoplasmic reticulum and Golgi apparatus. These mutations disrupt the synthesis and transfer of sugar moieties, resulting in the production of defective glycoproteins and glycolipids. Common symptoms of the disease include developmental delays, intellectual disabilities, hypotonia, seizures, and organ dysfunction. The array of CDG subtypes stems from the multitude of underlying genetic mutations and disturbed glycosylation processes making the diagnosis and management challenging. Diagnosis of CDG relies on a multifaceted approach. Clinical evaluation, biochemical analysis, and genetic testing are all essential components. The advent of next-generation sequencing has significantly improved our ability to identify the specific gene mutations responsible for individual CDG subtypes. The management of CDG involves primarily symptom alleviation and enhancing the quality of life. A multidisciplinary approach is fundamental, encompassing supportive care, physical and speech therapies, and medications targeting specific complications.

This chapter is dedicated to the biology of advanced glycation end products (AGEs). In 1912, AGEs were first identified by French chemist Louis-Camille Maillard. Early investigation revealed AGE generation during food preparation (cooking) at high temperatures, wherein carbohydrates (e.g. glucose/glycan) slowly react with various proteins via concomitant generation of Schiff’s base and Amadori products. This non-enzymatic process of AGE generation is termed glycation. Later, subsequent investigations revealed that AGE is exogenously produced during cooking and other processing of foodsand also endogenously generated in the human body including blood, skin, and other tissues. To date, more than 20 AGEs are postulated to prevail within human blood, tissues, and food resources. AGEs are optical sensitive molecules and based on their optical sensitivity AGEs are distinguished into fluorescent and non-fluorescent categories. The most important non-fluorescent components are carboxymethyl-lysine (CML), carboxyethyl-lysine (CEL), and pyrrolidine while pentosidine and methylglyoxal-lysine dimer (MOLD) are prominent compounds having fluorescent sensitivity. AGE binds with several receptor molecules, the prominent among whichare receptors for advanced glycation end products (RAGE). Additional cell surface molecules capable of binding with AGE including macrophage scavenger receptors (MSRs) type A, B1 (CD36), oligosaccharyltransferase-48/OST48, also termed “AGE receptor 1” (AGE-R1), 80K-H phosphoprotein (AGE receptor 2, AGE-R2), and galectin-3 (AGE receptor 3, AGE-R3), the scavenger receptor family (SR-A, SR-B, SR-1, SR-E, LOX-1, FEEL-1, FEEL-2, and CD36). This chapter describes the steps of AGE synthesis, their biochemical characterization, and the implication of the AGE-RAGE interactions at the cellular platform.

The transmembrane protein receptor for advanced glycation end products (mRAGEs) is recognized as an immunoglobulin class of molecule. Mammalian cells produce a carboxy terminus truncated version of RAGE, either as endogenous soluble RAGE (esRAGE) or soluble RAGE (sRAGE), both being generated via proteolytic cleavage or alternative mRAGE-mRNA splicing. Through its extracellular domains (V, C1, and C2), RAGE interacts with seemingly unrelated ligands such as advanced glycation end products (AGEs), high mobility group box protein 1 (HMGB1), S100/calgranulin family, lysophosphatidic acid (LPA), oligomeric forms of amyloid beta peptide (Aβ-peptide), islet amyloid polypeptide (IAPP), attributing to the recognition as multi-ligand receptor. Under physiological conditions, lung tissues exhibit abundant RAGE expression compared to others, being involved in the development, spread, and homeostatic regulation, the prominent of which are lung alveolar type 1 (AT-1) epithelial cells. However, in pathophysiological conditions, supraphysiological expression of RAGE and its ligands and subsequent receptor-ligand interactions result in the aggravation of oxidative stress and inflammation, causing the propagation of various non-communicable disease conditions. The physiological RAGE expression may protect against non-small cell lung cancers (NSCLCs), as suppressed RAGE expression in lung tissues may complicate NSCLCs. The protective role of RAGE in lung tissues is surprisingly contrary to its activities in other cancers, which are unanimously characterized by its enhanced expression-driven propagation of the conditions. Anti-RAGE molecules including esRAGE/sRAGE attenuate RAGEdependent multiple diseased conditions.

The receptor for advanced glycation end products (RAGEs) is a cell surface immunoglobulin class of molecules. RAGE prevails as a multiligand receptor capable of interacting with various ligands, the prominent amongst which is “advanced glycation end products (AGE)”. The ligand-RAGE axis leads to an aggravated extent of inflammation and oxidative stress, activating various pro-inflammatory and prooxidative transcription factors such as nuclear factor kappa B (NF-κB). The binding of NF-κB to the promoter region of the RAGE gene activates its transcription. Once expressed, RAGE interacts further with its multiple ligands including AGE, HMGB1, S100, etc., culminating in aggravated inflammatory and oxidative stress. Thus, RAGE which is a product of an increased level of inflammation and oxidative stress, once produced perpetuates a brutal cycle of self-propagation through sustained interaction with various ligands and subsequent inflammation and oxidation stress. Several levels of crosstalk possibilities prevail between pro-inflammatory and prooxidative reactive molecules. Sustaining a high level of pro-inflammatory and prooxidative reactions is the basic requirement to complicate various non-communicable disease conditions including diabetes-associated vascular complications, cardiovascular disorders (CVDs), pulmonary diseases, cancers, and others. This chapter describes the basic mechanism through which RAGE fuels the inflammatory and oxidative stress on a cellular front.

The receptor for advanced glycation end products (RAGE) is characterized as a multi-ligand pattern recognition receptor molecule exhibiting physiologically profuse expression in the lung alveolar type 1 (AT-1) epithelial cell’s basolateral region. Advanced glycation end products (AGEs) are the most prominent among the multiple ligands of RAGE in lung tissues. Other major RAGE ligands comprise high mobility group box protein 1 (HMGB-1) and S100/calgranulin. In various pathophysiological conditions, lung tissues express the supraphysiological level of RAGE and its multiple ligands. In physiological conditions, the interaction of RAGE with its ligands assists in the maturity, spreading, and maintenance-enabled homeostasis of lung epithelial cells. Thus, physiologically abundant expression of RAGE in the lung AT-1 cells maintains their morphology and specific architecture. In physiological conditions, high basal level expression of RAGE in the lung tissues guards against the development of non-small cell lung cancers (NSCLCs), wherein decreased RAGE extents are correlated with non-small cell lung cancer (NSCLC) complications. However, in the lung tissues under pathophysiological conditions, supraphysiological expression of RAGE and its various ligands stimulates inflammation and oxidative stress-related cell signaling molecules. This aggravated extent of inflammation and oxidative stress in the lung tissues leads to the propagation of manifold lung diseases namely, asthma, chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), and cystic fibrosis (CF), acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), pneumonia, sepsis, bronchopulmonary dysplasia, and pulmonary hypertension. This chapter describes the physiological and pathophysiological role of RAGE in the lungs and the anti-RAGE therapy against various lung diseases.

The Receptor for Advanced Glycation End Products (RAGE) has emerged as a pivotal player in the pathogenesis of cardiovascular and diabetic complications. An in-depth exploration of RAGE involvement in the disease processes, elucidating the molecular mechanisms, signaling pathways, and the associated pathological outcomes, is discussed. In diabetes, chronic hyperglycemia leads to the formation and accumulation of advanced glycation end products (AGEs), which activate RAGE and subsequently initiate a cascade of pro-inflammatory and pro-oxidative events. These processes contribute to the development and progression of diabetic vascular complications, including atherosclerosis, neuropathy, nephropathy, and retinopathy. In the cardiovascular system, RAGE activation promotes vascular inflammation, endothelial dysfunction, and vascular smooth muscle cell proliferation, all of which are critical in the pathogenesis of atherosclerosis and cardiovascular diseases. Furthermore, RAGE-mediated oxidative stress and inflammation have been implicated in the progression of heart failure and post-ischemic injury. Targeting RAGE signaling thereby emerges as a promising therapeutic approach to mitigate the detrimental effects of chronic hyperglycemia and vascular inflammation in diabetic and cardiovascular diseases. A comprehensive understanding of the multifaceted RAGE functions in cardiovascular complications such as atherosclerosis, peripheral arterial disease, atrial fibrillation, thrombotic disorder, myocardial infarction, vascular calcification, and the role of RAGE in diabetes-associated cardiac fibrosis, is discussed with a focus on therapeutic significance.

The receptor for advanced glycation end products (RAGE) was first isolated and characterized in the bovine lungs. Mammalian lungs express a relatively higher level of RAGE than other organs of the mammalian body. Physiologically, RAGE guards from lung cancer development owing to which, a diminished RAGE expression is implicated in the lung cancer complication. Opposed to this, a high-level RAGE expression is associated with the development of various cancers including breast, ovary, prostate, pancreatic, colon and colorectal, hepatocellular, melanoma, and neuronal. Interactions of RAGE and its multiple ligands, namely advanced glycation end products (AGE), high mobility group box protein 1 (HMGB1), S100/calgranulin, Mac 1, amyloid beta (Aβ) peptide, and others are involved in the complications of cancers. Besides their interactions with RAGE, RAGE ligands also independently aggravate the cancer-promoting actions. In cancer cells, the cellular events affected by RAGE include proliferation, survival, angiogenesis, autophagy, invasion, and metastasis. RAGE-ligands interaction aggravates inflammation and oxidative stress, leading to the propagation of various diseases including cancers.

The receptor for advanced glycation end products (RAGE) is a multi-ligand receptor molecule expressed in the cells of the nervous system (neurons and glial cells). Compared to embryonic cells, RAGE expression is significantly decreased within the adult tissues, including the nervous system. Various RAGE ligands such as amyloidbeta peptide (Aβ-peptide), high mobility group box protein 1 (HMGB1), S100/calgranulin, and advanced glycation end products (AGEs) are expressed by the cells of the nervous system. Several studies have predicted the role of RAGE in neurogenesis. Interaction of RAGE with its various ligands has been demonstrated as the responsible factor for complicating multiple diseased conditions such as Neuronal Differentiation and Outgrowth, Alzheimer’s Disease (AD), Parkinson’s Disease (PD), Huntington’s Disease (HD), Amyotrophic Lateral Sclerosis (ALS), Creutzfeldt-Jakob’s Disease (CJD), Peripheral Neuropathies, Familial Amyloid Polyneuropathy (FAP), Spinal Cord Injury (SCI), and epilepsy. The interactions of RAGE with its ligands are critically dependent on the relative extents of inflammation and oxidative stress, controlling the various neurological disease manifestations. Redox sensitivity of such interactions is inferred by their treatment using targeted and sustainable antioxidant delivery at the affected regions. Besides targeting RAGE-ligand interactions via blocking RAGE expression may be useful against various neurological diseases.