Regulation of AMPA-type glutamate receptor trafficking by proheparanase in synaptic plasticity

Abstract

Synaptic plasticity is the activity-dependent modification of the strength of synaptic transmission. Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors (AMPARs) at excitatory synapse mediate fast neurotransmission. Membrane depolarization of postsynaptic neurons then triggers Ca2+ influx that upregulates membrane insertion of AMPARs, resulting in long-lasting potentiation of excitatory post-synaptic potential. Perineuronal heparan sulfates (HS) are implicated in controlling the open-state of AMPARs for the maintenance of synaptic strength. Treatment of hippocampal slice cultures with heparitinase, a bacterial HS-cleaving enzyme, decreases hippocampal LTP on a dose-dependent manner. Our finding of neuronal expression of heparanase, a mammalian HS-cleaving enzyme, led us to test if (1) neuronal heparanase is secreted as the pro-form or the active form and (2) if binding of the secreted form to perineuronal HS plays a role in synaptic plasticity. Hippocampal neurons in culture were treated with phorbol ester to stimulate secretion from vesicular stores. Western blot analysis of secreted material revealed the pro-form. Synaptosome preparations also indicated enrichment in proheparanse. Synaptosome preparations subjected to co-immunoprecipitation studies revealed AMPAR subunits (GluA1 and GluA2/3) binding to both syndecan-3 (a transmembrane HS proteoglycan of neurons) and proheparanase, suggesting their clustering in a functional complex. Treatment of hippocampal neuron cultures with recombinant proheparanase triggered internalization of proheparanase together with perineuronal HS-proteoglycans and AMPARs. Treatment of hippocampal slices with recombinant proheparanase resulted in declines in both basal synaptic strength and LTP. Taken together, the results indicate neuronal secretion of proheparanase as a novel mechanism that contributes to synaptic plasticity by re-setting AMPAR level at the synapse.