Investigation of Receptors for the Modulation of Neuronal Growth by Chondroitin Sulfate

Abstract

A major obstacle to neural regeneration after injury in the central nervous system (CNS) is the environment encountered by injured axons. This environment is inhibitory due to proteins expressed by the CNS myelin as well as molecules present in the glial scar. Experimental results have implicated chondroitin sulfate proteoglycans (CSPGs) as major inhibitors of axonal regeneration after CNS injury, but until recently, the mechanisms of this inhibition were not well understood. Furthermore, the complex nature of the chondroitin sulfate (CS) chains made it difficult to study their contribution to CSPG function. This thesis describes a specific carbohydrate epitope, CS-E, that is primarily responsible for the inhibition of CNS axonal regrowth in the presence of CSPGs. We show that removal or blocking of the CS-E motif via genetic elimination of the enzyme responsible for generating CS-E or a monoclonal antibody that binds specifically to the CS-E motif significantly reduces the inhibitory activity of CSPGs on axon growth. Furthermore, we show that CS-E functions as a protein recognition element to engage receptors, including the transmembrane protein tyrosine phosphatase PTPσ, which had been previously established to be a receptor for CSPGs. Finally, we show that the protein tyrosine kinase receptor EphA4 is a novel receptor for the CS-E motif, and as with PTPσ, neurons deficient in EphA4 exhibit reduced inhibition by CS-E. Our results demonstrate that a specific sugar epitope within chondroitin sulfate polysaccharides directs important physiological processes, and establish the importance of the chemical structure of CS chains in modulating the activity of CSPGs in vivo. The identification of receptors that mediate the inhibitory effect of CS-E advances our understanding of the mechanisms of axon regeneration following injury to the CNS when CS-E expression is upregulated. These findings provide us with the opportunity to develop therapies for the recovery of axonal outgrowth after damage to the nervous system, which in conjunction with blocking approaches targeting the CS motif, can provide a powerful strategy for allowing recovery after injury to the CNS.