Direct neuronal and microglia-mediated effects of group I metabotropic glutamate receptors in neurodegenerative and neuroinflammatory processes

The group I of metabotropic glutamate (mGlu) receptor, mGlu1 and mGlu5 receptors are involved in neurodegenerative and in neuro-inflammatory processes. We have examined the role of mGlu5 receptor in a neuroinflammatory context. This receptor is expressed in glial cells and has been reported to play an anti-inflammatory role but also a pro-inflammatory effect. We were particularly involved in establishing whether mGlu5 receptor could modulate the inflammatory response in microglia, more specifically through the release of microvesicles (MVs). These have been shown to represent a more recent mode of intercellular communication and are extracellular vesicles, with a diameter of 100-1000 nm, that bud from the plasma membrane of cell of origin. MVs have been described in microglia where their formation occurs following activation of P2X7 purinergic receptors. We observed MVs development in pure microglial cultures, but, in the present study, they were more easily examined in the murine BV2 cell line. Activation of mGlu5 receptor attenuated the levels of pro-inflammatory cytokines, such as tumor necrosis factor (TNF) alpha, an effect that occurred only under conditions of activated microglia. Stimulation of mGlu5 increased also P2X7 receptor activation as shown by increased purinergic currents and by enhanced formation of MVs in response to the stable ATP analog, benzoyl-ATP. When MVs from mGlu5 receptor-stimulated microglia were transferred onto neurons, they increased neuronal toxicity induced by rotenone. Among possible factors released by MVs, in preliminary studies, we have identified miRNA146a as a potential mediator of this effect. At the same time, the other member of group I mGlu receptor, mGlu1 receptor, was mainly analyzed considering its involvement in neurodegeneration. Specifically, we examined its role in an experimental setting of retinal ganglion cells (RGC) degeneration, a well-established model of several eye disorders including prematurity-induced retinal degeneration, glaucoma, and age-related macular degeneration. All these conditions are associated with excitotoxicity and it has been observed that activation of mGlu1 receptor significantly contributes to excitotoxic degeneration of RGCs. Thus, using postnatal day 9 wild type mice and knockout mice with a spontaneous recessive mutation causing the lack of mGlu1 receptor protein (crv4 mice), we observed that mGlu1 receptor is primarily involved in RGC excitotoxic death induced by monosodium glutamate (MSG). Treatment with the mGlu1 receptor negative allosteric modulator, JNJ16258695, was in fact protective against retinal degeneration induced by MSG. In addition, MSG treatment did not produce any toxic effect in crv4 mice, lacking mGlu1 receptor. Altogether these data confirm a role for mGlu1 and mGlu5 receptor in neurodegeneration and neuroinflammation, respectively and may serve as a starting point for the development of future possible pharmacological intervention targeting these receptors.