Aggrecan glycoforms contribute to the molecular heterogeneity of perineuronal nets

The perineuronal net forms the extracellular matrix of many neurons in the CNS, surrounding neuron cell bodies and proximal dendrites in a mesh-like structure with open "holes" at the sites of synaptic contacts. The perineuronal net is first detected late in development, approximately coincident with the transformation of the CNS from an environment conducive to neuronal growth and motility to one that is restrictive, suggesting a role for the perineuronal net in this developmental transition. Perineuronal nets show a great degree of molecular heterogeneity. Using monoclonal antibodies Cat-301, Cat-315, and Cat-316, we have shown previously that although all antibodies recognize chondroitin sulfate proteoglycans of similar sizes, each antibody recognizes perineuronal nets on distinct but overlapping sets of neurons in the adult cat CNS. An understanding of the heterogeneity demonstrated by these antibodies is critical to understanding the organization and function of perineuronal nets. Using aggrecan knock-out mice (cmd), we have now determined that all three antibodies recognize aggrecan. Chemical and enzymatic deglycosylation show that the differences revealed by the three antibodies arise from differential glycosylation of aggrecan. We further demonstrate that aggrecan mRNA is expressed relatively late in development and that neurons themselves are likely the predominant cellular sites of aggrecan expression. This work indicates that neurons can directly regulate the composition of their extracellular matrix by regulated synthesis and differential glycosylation of aggrecan in a cell type-specific manner. These results have important implications for the role of regulated microheterogeneity of glycosylation in the CNS.