Perineuronal Net - Composition


PNNs are composed of a condensed matrix of chondroitin sulfate proteoglycans, molecules that consist of a core protein and a glycosaminoglycan (GAG) chain. The CS-GAG chains associated with PNNs differs from those found floating in the extra-cellular matrix in a noncondensed form. PNNs are composed of brevican, neurocan, versican, aggrecan, phosphacan, hyaluronan, tenascin-R and various link proteins. The CSPGs aggrecan, versican, neurocan, breican, and phosphacan are bound to hyaluronan. PNNs found in both the brain and the spinal cord have the same composition. Chondroitinase ABC (ChABC), a bacterial enzyme routinely used to digest CSPGs, works by catalyzing the removal of the CS-GAG chains of CSPGs.

In the cortex and other subcortical areas, PNNs preferentially surround GABAergic interneurons containing the calcium-binding protein parvalbumin. The onset of the critical period corresponds closely to the emergence of parvalbumin-positive cells. Parvalbumin-positive cells synapse onto α1-subunit-containing GABAA receptors. The α1-subunit-containing GABAA receptors have been shown to be the only GABAA receptors that drive cortical plasticity. For this reason, PNNs were first thought to have a strong role in the closure of the critical period.

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