Neuronal proheparanase regulates synaptic function of the central vestibular system

Abstract

Perineuronal heparan sulfates (HS) have been implicated in controlling the open-state of AMPA-type glutamate receptors (AMPARs) which govern excitatory synaptic transmission. Our finding of neuronal heparanase-immunoreactivity in the rat vestibular nucleus (VN) led us to test if the secreted enzyme acts on perineuronal HS to regulate synaptic function in the vestibular circuit. Neuron cultures stimulated with phorbol ester were found to secrete the pro-form. Treatment with recombinant proheparanase triggered internalization of membrane associated HS-proteoglycans in VN cultures. Electrophysiological studies on VN-containing brain slices revealed decrease in amplitude of excitatory postsynaptic currents following treatment with proheparanase or phorbol ester. Taken together, these findings suggest that activity-dependent secretion of proheparanase induces internalization of HSPG and associated AMPAR at the VN synapse. We then asked if proheparanase plays a part in the maturation of the vestibular circuitry. Immunohistochemical analysis of the VN revealed colocalization of syndecan and heparanase immunoreactivities in GABAergic interneurons, contrasting the localization of HS- and perlecan immunoreactivities in perineuronal nets (stained positive with Wisteria floribunda agglutinin). Immunoreactivities of both perineuronal HS and heparanase showed progressive increase with postnatal development, time-matched with consolidation of perineuronal nets of GABAergic neurons. Neonatal treatment of the VN with the GABAA receptor antagonist, bicuculline, delayed the onset of negative geotaxis (an innate reflex of the vestibular circuit). If neonatal treatment of the VN with proheparanase results in similar delay, it is expected that proheparanase-triggered down-regulation of AMPAR levels at GABAergic interneurons contributes a crucial link in the postnatal maturation of vestibular function.