Proheparanase action at excitatory synapses : implication on synaptic plasticity

Abstract

Synaptic plasticity is the activity-dependent modification of the strength of synaptic transmission. It is important for learning and memory. One of the mechanisms mediating synaptic plasticity at glutamatergic synapses is regulation of the postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor (AMPAR), which governs excitatory synaptic transmission. Perineuronal heparan sulfates (HS) have been implicated in controlling the open-state of AMPARs. Earlier findings in our laboratory showed heparanase expression and secretion of proheparanase by hippocampal neurons. Recombinant proheparanase triggered neuronal co-internalization of HS-proteoglycans and AMPAR subunits; this led to decreases in basal synaptic strength and long term potention of synaptic transmission at Schaffer collateral synapses of the hippocampus. The findings suggested proheparanase acting as a negative regulator of synaptic plasticity but the underlying mechanism remained unclear. We hypothesized that neuronal secretion of proheparanase is localized toperi-synaptic regions such that proheparanase mediates clustering of peri-synaptic HS-proteoglycans and AMPARs for internalization of the protein cluster.

To address this, protein kinase C-mediated secretion of proheparanase was enhanced by phorbol ester treatment of hippocampal slices. Synaptosome preparations from the treated slices indicated enrichment in proheparanase, suggesting that proheparanase was directed to synaptic terminals for localized secretion. With use of the hippocampal synaptosomes, pull-down experiments targeting syndecan-3and heparanase found AMPAR subunits, both GluR1 and GluR2/3, indicating that they formed clusters in the peri-synaptic area. Heparitinase pre-treated hippocampal neurons in culture led to lower levels of internalized AMPAR subunits, both GluR1 and GluR2/3,upon recombinant proheparanase treatment. This suggested that HS moieties were critical for proheparanase-mediated AMPAR internalization. Recombinant proheparanase treatment of the neuronal cultures also led to decreases in glutamate-induced calcium influx, in terms of both the number of responsive cells and the change in intracellular calcium level, consistent with proheparanase-mediated neuronal internalization of AMPARs. Taken together, these results support our hypothesis and highlight dependence on the HS moiety for proheparanase-mediated neuronal internalization of AMPARs.

We further investigated if proheparanase action at synapses can be found in other brain regions. The finding of neuronal heparanase expression in vestibular nucleus (VN) microexplant culture led us to study the role of proheparanase in synaptic plasticity in the VN. PKC activation enhanced heparanase expression in VN microexplant cultures. Recombinant proheparanase also triggered the uptake of HS and led to decreases in glutamate-induced calcium influxin VN microexplant cultures. These results support that proheparanase plays a role in synaptic plasticity in the VN but the effect and mechanism of action of proheparanase in VN neurons remain to be elucidated.

This study demonstrated that neuronal secretion of proheparanase at synaptic terminals regulates AMPAR internalization, resets peri-synaptic HS levels and lowers calcium dependent signaling in responsive cells. This work has revealed a novel role of neuronal proheparanase in synaptic plasticity.