Glycosaminoglycans (GAGs)

• Glycosaminoglycans (GAGs) are long, linear complex carbohydrates, also known as polysaccharides
• GAGs are composed of repeating disaccharide units containing amino sugars and uronic acid

GAG Composition

• Amino sugars are either N-acetylglucosamine (GlcNAc) or N-acetylgalactosamine (GalNAc)
• Uronic acids are either glucuronic acid (GlcA) or iduronic acid (IdoA)
• Repeating disaccharide units form the long, linear GAG chain

Biophysical Properties of GAGs

• GAGs have multiple negatively charged functional groups like carboxyl (-COO-), sulfate (-SO3-), and N-acetyl groups
• The charged functional groups lead to electrostatic repulsion between adjacent chains
• GAGs attract and retain large volumes of water due to their polyanionic nature
• This contributes to the viscoelasticity of mucous secretions and synovial fluid
• Under compression, water is displaced, reducing volume, but electrostatic repulsion and hydration forces restore the hydrated structure upon release
• GAGs act as critical components of the extracellular matrix (ECM), forming hydrated gel-like networks for cell adhesion, migration, and mechanotransduction

Disaccharide Repeat Units

• Glycosaminoglycans contain amino sugars like glucosamine and galactosamine
• These sugars have an amine (-NH2) or an N-acetyl (-NHCOCH3) group instead of a hydroxyl (-OH) group
• Modifications are crucial for protein interactions and structural diversity

Acidic Sugars in GAGs

• GAGs contain acidic sugars like D-glucuronic acid and L-iduronic acid, the latter being an epimer of glucuronic acid
• These sugars carry carboxyl (-COO-) groups, making GAGs negatively charged and hydrophilic
• They bind water and maintain tissue hydration and elasticity

Biological Relevant GAGs and Locations

• Hyaluronic Acid (Hyaluronan): Skin, synovial fluid, bone, cartilage, vitreous humor, loose connective tissue, umbilical cord
• Chondroitin sulfate: Cartilage, bone, CNS
• Keratan sulfate I and II: Cornea, cartilage, Loose connective tissue
• Heparin: Mast cells, Liver, lung, skin
• Heparan sulfate (HS): Skin, Kidney basement membrane
• Dermatan sulfate (DS): Skin, Wide distribution

Hyaluronic Acid

• HA is a naturally occurring glycosaminoglycan in connective tissues, skin, and synovial fluid
• HA maintains tissue hydration, lubrication, and shock absorption
• HA has clinical applications in dermatology, orthopaedics, ophthalmology, and plastic surgery

Hyaluronic Acid cont.

• In dermatology, HA hydrates and plumps the skin, reducing fine lines and wrinkles, and is used in dermal fillers
• In orthopaedics, intra-articular injections of HA treat osteoarthritis, lubricate joints, reduce inflammation, and improve mobility
• In ophthalmology, HA is a component of artificial tears to treat dry eye syndrome by retaining moisture on the eye surface
• In plastic surgery, HA enhances facial appearance and treats wrinkles as a filler

Chondroitin Sulfate

• Chondroitin sulfate (CS) is a sulfated glycosaminoglycan found in cartilage, connective tissue, and synovial fluid
• CS is used in dietary supplements for joint health and to treat osteoarthritis and musculoskeletal disorders
• CS can join a protein core to form a proteoglycan

CS Linker

• The linker consists of the tetrasaccharide: Xylose (Xyl), Galactose (Gal), Galactose (Gal), Glucuronic Acid (GlcA)
• It is joined with a protein by the Xylulose-Serine O Glycosidic bond

Proteoglycans

• Proteoglycans are formed when GAGs attach to a core protein
• Aggrecan in cartilage is made chondroitin sulfate and a core protein
• Syndecan on cell membranes is made of heparan sulfate and a core protein

Osteoarthritis and Chondroitin Sulfate

• Chondroitin sulfate (CS) manages osteoarthritis (OA), often with glucosamine
• CS reduces inflammation (inhibits IL-1β, TNF-α), enhances cartilage repair (stimulates proteoglycan and collagen synthesis), and prevents joint degeneration
• CS is usually taken as an oral supplement
• Hyaluronic acid (HA) is administered intra-articularly as a viscosupplement for joint lubrication
• HA improves synovial fluid viscosity, while CS protects cartilage and modulates inflammation

Keratan Sulfate Types, Functions, and Significance

• Keratan sulfate (KS) is a sulfated glycosaminoglycan in cartilage, cornea, bone, and the central nervous system
• KS plays a key role in hydration, structural integrity, and cellular signalling
• There are two major types: Keratan Sulfate I (KS-I) and Keratan Sulfate II (KS-II)

Keratan Sulfate I

• Keratan Sulfate I (KS-I) is found in the cornea, lying between collagen fibrils in the stromal layer
• KS-I maintains corneal transparency by regulating hydration and preventing collagen fibril aggregation
• KS-I protects against oxidative stress and mechanical damage in the eye
• KS-I deficiency leads to corneal clouding and opacification, impairing vision
• KS-I deficiency results in Macular Corneal Dystrophy (MCD), a genetic disorder causing progressive vision loss due to defective KS-I sulfation

Keratan Sulfate II

• KS-II is found in cartilage and connective tissues and playing a role in joint biomechanics
• KS-II contributes to cartilage hydration, load-bearing capacity, and resistance to mechanical stress
• KS-II supports extracellular matrix organization, maintaining tissue elasticity
• Impaired KS-II metabolism is linked to Morquio Syndrome characterized by skeletal dysplasia and cartilage and bone deformities

Heparin

• Heparin is a highly sulfated glycosaminoglycan (GAG) found in mast cells and endothelial cells
• Heparin is intracellular, stored in granules of mast cells, and released into circulation
• Heparin plays a crucial role in anticoagulation, inflammation modulation, and vascular homeostasis

Heparin's Anticoagulant Activity

• Heparin potentiates antithrombin III leading to inhibition of thrombin and Factor Xa, preventing blood clot formation
• It is used in thromboprophylaxis, deep vein thrombosis management, and prevention of embolic events

Further Heparin Functions

• Reduces leukocyte adhesion to endothelium, minimizing vascular inflammation
• Inhibits cytokines like IL-6 and TNF-a, reducing inflammatory responses
• Modulates growth factor activity (e.g., FGF), promoting angiogenesis and wound healing
• Contributes to endothelial cell stabilization, preventing excessive capillary leakage

Heparin Deficiency and Dysfunction

• Deficiency or inadequate heparin activity leads to uncontrolled blood clotting, increasing the risk of DVT, PE, and stroke
• Heparin-Induced Thrombocytopenia (HIT) is a severe immune-mediated complication with antibodies forming against heparin-platelet factor 4 complexes, leading to paradoxical thrombosis
• Overdosing results in excessive anticoagulation and haemorrhage and is reversed using protamine sulfate, which binds and neutralizes heparin

Key Takeaways on GAGs

• GAGs are highly anionic and hydrophilic due to negative sulfate and carboxyl groups, allowing them to retain water and providing tissue hydration, shock absorption, and mechanical resilience
• Different GAGs have tissue-specific functions
• Hyaluronic acid lubricates joints
• Keratan sulfate maintains corneal transparency
• Chondroitin sulfate supports cartilage integrity
• Heparin acts as an anticoagulant
• The degree and pattern of sulfation regulate GAG interactions with proteins, enzymes, and signaling molecules, affecting tissue repair, inflammation, and coagulation
• Keratan sulfate deficiency causes macular corneal dystrophy
• Chondroitin sulfate abnormalities contribute to osteoarthritis
• Heparin dysfunction can lead to thrombosis or excessive bleeding