Novel role of proheparanase in regulating synaptic function in the central vestibular circuitry

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 form acts on perineuronal HS as means to regulate synaptic function in the vestibular circuit. We however found enhanced secretion of enzymatically inactive proheparanase into the medium of VN explants treated with phorbol ester to induce exocytosis. Treatment of VN cultures with recombinant proheparanase triggered (i) neuronal internalization of HS-associated AMPAR and (ii) decrease in glutamate-induced calcium influx into neurons. VNcontaining brainstem slices subjected to whole-cell patch-clamp studies revealed decrease in amplitudes of excitatory postsynaptic currents following phorbol ester treatment. These findings suggest that activity-dependent secretion of proheparanase induces internalization of HS-associated AMPAR at glutamatergic synapses of VN interneurons. We then asked if proheparanase plays a part in the maturation of the central vestibular circuitry. Immunohistochemical analysis of the VN revealed colocalization of HSsyndecan and heparanase on GABAergic interneurons, contrasting that of HS-perlecan in perineuronal nets. Intensification of both perineuronal HS and cellular heparanase immunoreactivities with postnatal development was time-matched with consolidation of perineuronal nets of GABAergic neurons. In vivo, neonatal perturbation of either glutamatergic input to the neurons with CNQX or GABAergic output of the neuron with bicuculline delayed the postnatal onset of negative geotaxis (readout of vestibular reflex). Taken together, our results suggest that activitydependent production of proheparanase triggers internalization of AMPAR at GABAergic interneurons in the VN within a period critical for the postnatal maturation of vestibular function. [Supported by HKRGC-GRF Grant 774608]