Cross-Linking Theory

What it proposes

Aging arises from progressive covalent cross-links forming between proteins, lipids, and DNA over time. Cross-links accumulate because:

• Long-lived structural proteins (collagen, elastin, lens crystallins) have little turnover.
• Reactive aldehydes, sugars (glycation), oxidative species, and enzymatic processes (lysyl oxidase) generate cross-links.
• Cellular machinery cannot easily remove them.

Where it fits

• Vascular aging: cross-linked collagen and elastin produce arterial stiffening; this is measurable as pulse-wave velocity.
• Lens opacity (cataract): crystallin cross-linking drives age-related cataracts.
• Skin aging: dermal collagen cross-linking contributes to wrinkles and reduced elasticity.
• Joint stiffening: cartilage matrix cross-linking.
• Diabetic complications: amplified by hyperglycaemia (glycation theory).

What it doesn’t fully explain

• Why some long-lived species (naked mole-rat) develop less cross-linking.
• Why caloric restriction extends lifespan beyond what cross-link reduction predicts.
• The signalling component of aging.

Therapeutic attempts

• AGE-breakers (alagebrium, ALT-711): cleave specific cross-links; small clinical-trial improvements in arterial stiffness, but development halted.
• Pyridoxamine (vitamin B6 derivative): inhibits AGE formation, in trials for diabetic nephropathy.
• Several emerging AGE-related drug programmes.

Related entries

Glycation & AGEs, Loss of proteostasis, Pulse-wave velocity.