Glycans That Inhibit or Support Nerve Regeneration: Linking Sulfation Patterns to PTPσ Signaling
Background: The extracellular environment that limits regeneration after nerve injury
The sulfation patterns and structures of glycans surrounding neurons regulate signals that inhibit or promote axon growth. A review published in Frontiers in Molecular Biosciences summarizes prior research on glycosaminoglycan (GAG) structure and receptor interactions, and examines their potential and limitations as therapeutic targets for nerve regeneration.
Axon regeneration in the central nervous system is limited after injury. Regenerative outcomes are influenced by both neurons’ intrinsic growth capacity and the inhibitory extracellular environment at the injury site. Chondroitin sulfate (CS), abundant in glial scars and perineuronal nets, binds cell-surface receptors and inhibits growth-cone motility at axon tips. By contrast, certain heparan sulfate (HS) structures have been reported to support axon extension.
The concept of a GAG sulfation code helps explain this difference. The positions and combinations of sulfate groups on a glycan chain, its length, and the distribution of receptor-binding regions produce different biological responses. The rules connecting structure to function, however, are not yet fully understood.
Key findings: CS and HS act differently at the same receptor
Protein tyrosine phosphatase sigma (PTPσ) is central to a major model described in the review. In this model, CS promotes PTPσ monomerization and enzyme activation, whereas HS can induce receptor clustering and inhibit phosphatase signaling. Depending on glycan structure and the arrangement of binding regions, the same receptor may produce responses in opposite directions.
In prior studies discussed by the authors, receptor binding was examined using chemically defined CS and HS oligosaccharides and surface plasmon resonance. Among the CS-A, CS-C, CS-D, and CS-E structures compared, CS-E showed the highest binding affinity for PTPσ. CS-E has N-acetylgalactosamine sulfated at both the 4 and 6 positions. Among the CS-E oligosaccharides analyzed, a tetrasaccharide comprising four sugars was the smallest unit capable of receptor binding. This was a binding condition identified in that experiment; it does not establish a minimum length applicable to all GAGs.
Inside the cell, cortactin, an actin-binding protein, is linked to autophagic flux. According to the model summarized in the review, CS-activated PTPσ induces cortactin dephosphorylation. This in turn impairs actin stability and autophagosome–lysosome fusion, disrupting intracellular degradation; axon tips may swell and their growth may stall.
Autophagosome accumulation was observed at axon terminals in cultured neurons and in a mouse spinal cord injury model. In culture experiments, fluorescent labeling demonstrated impaired autophagic flux. Reducing proteins required for fusion or pharmacologically inhibiting fusion also caused terminal swelling and inhibited axon extension. These findings illustrate why an increase in autophagosomes alone does not establish increased degradative activity.
Implications and outlook: Glycan degradation and selective signaling control
Therapeutic strategies include degrading inhibitory glycans, blocking specific sulfation structures, regulating biosynthetic enzymes, and targeting receptors. A prominent example, chondroitinase ABC (ChABC), is a bacterium-derived enzyme that degrades CS chains. The review describes animal studies reporting improvements in axon regeneration, neural plasticity, and functional recovery. Enzyme stability, delivery to injury sites, and potential immune responses remain challenges for clinical translation.
The peptide NVG-291, which modulates PTPσ, is also discussed. The clinical observations in patients described in the review are preliminary, and the review itself is not a study demonstrating therapeutic efficacy in humans. These observations do not establish the current state of clinical development or confirmed efficacy.
Because GAGs also contribute to normal neural development and synaptic organization, the effects of therapeutic changes to their structure or signaling on normal function must also be assessed. Future tasks include identifying which structures act on which receptors and developing ways to deliver interventions to the tissues where they are needed. The mechanisms and experimental treatment strategies are detailed in the full review.
Glycosaminoglycans (GAGs) are a structurally and chemically diverse family of sulfated polysaccharides that constitute a major component of the neural extracellular matrix and cell surface proteoglycans, where they exert pivotal regulatory functions in axon growth, guidance, synaptic organization, and regeneration. By forming highly specific and context-dependent interactions with axonal receptors, GAGs orchestrate the spatial patterning and temporal dynamics of signaling events after injury. Accumulating evidence indicates that the biological activities of GAGs are not dictated merely by their presence but are finely tuned by their sulfation codes, chain length, and domain organization. Recent mechanistic studies have revealed that distinct GAG species, particularly chondroitin sulfate (CS) and heparan sulfate (HS), exert opposing effects on axonal behavior through shared receptor systems. In the injured central nervous system (CNS), CS-rich extracellular matrices, prominently associated with reactive astrocytes and perineuronal nets, act as potent inhibitors of axon regeneration. These inhibitory effects are mediated through selective engagement of receptors such as protein tyrosine phosphatase sigma (PTPσ) leading to suppression of cytoskeletal dynamics and growth cone motility. In contrast, specific HS motifs promote axon elongation by inhibiting PTPσ. Based on these insights, therapeutic strategies targeting GAG biology have gained considerable attention. Approaches such as enzymatic digestion of inhibitory CS chains, development of synthetic or biomimetic GAGs, modulation of sulfation patterns, and gene editing of GAG-modifying enzymes have demonstrated encouraging efficacy in preclinical models of spinal cord injury, traumatic brain injury, and neurodegenerative disorders. Together, these findings indicate GAGs not only as passive structural components but as active, druggable regulators of axon growth and regeneration. This review integrates current advances
The review provides a basis for assessing, step by step, whether blocking receptor binding leads to cellular responses and functional recovery when evaluating candidate therapies for nerve regeneration. For example, a study of a candidate that blocks CS-E binding or inhibits PTPσ signaling could separately assess cortactin phosphorylation, autophagic flux, axon extension, and functional recovery in animals. This is a preclinical application scenario derived from the mechanisms described in the review. It has not been established as a validated patient-selection method or clinical assessment standard. Structurally defined glycans and mimetic materials may also inform the design of drugs and biomaterials for nerve regeneration, but their effects on normal neural function and delivery to injury sites must be evaluated together.