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Proteoglycans are one of the major constituents of the extracellular matrix and cell membranes. In the brain, there are two major proteoglycans, chondroitin sulfate proteoglycans and heparan sulfate proteoglycans. In the adult mammalian central nervous system, some species of chondroitin sulfate proteoglycan are localized in the ‘perineuronal net’ around a restricted number of neurons. The ‘perineuronal net’ is a reticular structure covering the cell bodies and proximal dendrites of certain neurons. Various glycoproteins, proteoglycans and hyaluronic acid are constituents of the perineuronal nets. Recently, an N-terminal proteolytic fragment of neurocan named neurocan-130 was found to be a member of the perineuronal net-constituting molecules. Neurocan is a nervous tissue-unique chondroitin sulfate proteoglycan whose expression and proteolytic cleavage are developmentally regulated. Most of the perineuronal netconstituting molecules are considered to extracellularly exist and some are considered to be localized at the surface of glial cell processes enwrapping the neural cell body. However, neurocan-130 was detected in the cytoplasm of the glial cell processes. Perineuronal nets could be involved in synapse stabilization or neuronal maturation because they appear in the vicinity of the synapses relatively late in neuronal development. The presence of perineuronal nets around a limited number of cells may reflect some functional heterogeneity of the neuron and/or glia.
