The roles of perineuronal nets and heparanase in synaptic plasticity at the hippocampus

Abstract

Perineuronal nets (PN), as detectable by Wisteria floribunda agglutinin (WFA) staining, are enriched in chondroitin sulfate (CS) and heparan sulfate (HS) proteoglycans. We hypothesize that the CS and HS moieties of PN regulate synaptic plasticity within the central circuitry. Long-term potentiation (LTP) was assessed in the Schaffer collateral pathway of the rat hippocampus as indicator of activity-dependent synaptic plasticity. Treatment of hippocampal slices with either chondroitinase ABC (Proteus vulgaris) or heparitinase (Flavobacterium heparinum) resulted in dose-dependent attenuation of LTP in correlation with loss of PN, thus supporting roles of perineuronal glycosaminoglycan moieties in maintaining LTP. Our finding of heparanase expression at the CA1 hippocampal neurons further led us to hypothesize that heparanase functions as the mammalian counterpart of heparitinase in the regulation of synaptic plasticity. We approached this with hippocampal neurons in culture and found that only the pro-form of heparanase but not the mature form was secreted and retained in the peri-neuronal environment by neuronal surface HS moieties. The proheparanase was co-internalized with neuronal surface HS proteoglycans into lysosomes and only the internalized form showed HS-cleaving activity, indicating intracellular activation of the internalized proheparanase. Neuronal activity-dependent activation of protein kinase C was mimicked by phorbol 12-myristate 13-acetate in the stimulation of neuronal secretion of proheparanase. We then employed hippocampal slices to show that in the Schaffer collateral pathway, both hippocampal basal synaptic strength and LTP were lowered by treatment with exogenous proheparanase. Taken together, activity-dependent release of proheparanase provides a novel mechanism for the regulation of HS-mediated synaptic plasticity..