Keratan sulfate (KS)‐proteoglycans and neuronal regulation in health and disease: the importance of KS‐glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly‐N‐acetyllactosamine chain of d‐galactose‐GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi‐sulfated high‐charge density, monosulfated and non‐sulfated poly‐N‐acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit‐Robbo and Ephrin‐Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS‐PG abakan defines functional margins of the brain and is up‐regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti‐oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK‐1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS‐PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS‐PGs in development and regenerative neural processes. KS‐proteoglycans (KSPGs) are a diverse collection of intracellular, transmembrane and extracellular matrix PGs in the CNS/PNS with roles in growth of the cytoskeleton and directional dendrite extension, neurotransmitter storage and transport and the regulation of neuronal outgrowth and synaptic plasticity. These PGs are also decorated with CS and HNK‐1 trisaccharide to variable degree in perineuronal nets and extracellular matrix introducing additional ligand interactive capability. Emerging studies show that