Antagonism of Group I Metabotropic Glutamate Receptors and PLC Attenuates Increases in Inositol Trisphosphate and Reduces Reactive Gliosis in Strain-Injured Astrocytes

Abstract

We have previously found that in vitro traumatic injury uncouples IP3-mediated intracellular free calcium ([Ca2+]i) signaling in astrocytes (Rzigalinski et al., 1998; Floyd et al., 2001). Since Group I metabotropic glutamate receptors (mGluRs) are coupled to IP3-mediated Ca2+ signaling, we investigated their role in the in vitro strain injury of cultured astrocytes. Astrocytes grown on Silastic membranes were labeled with 3H-myo-inositol and strain (stretch)-injured. Cells injured in the presence of LiCl to prevent inositol phosphate metabolism were acid extracted and inositol phosphates (IPx) isolated using anion exchange columns. Reactive gliosis was assessed as increased glial fibrillary acidic protein immunoreactivity (GFAP-IR). Pre- but not post-injury administration of (RS)-1-aminoindan-15-decarboxylic acid (AIDA) or (S)-4-carboxy-3-hydroxyphenylglycine (S4CH3HPG), both group I mGluR antagonists, attenuated injury-induced increases in IPx. Injury increased GFAP-IR in astrocytes at 24 and 48 h post injury, which was reduced or blocked by AIDA or inhibition of phospholipase C (PLC) with U73122. These findings suggest that strain injury activates Group I mGluRs, causing aberrant IPx production and uncoupling of the PLC signaling pathway. Changes in this signaling pathway may be related to induction of reactive gliosis. Additionally, our results suggest a complex physical coupling between G protein receptor, PLC, and IP3 receptor, in support of the conformational coupling model.

Strain rate-dependent induction of reactive astrogliosis and cell death in three-dimensional neuronal–astrocytic co-cultures

“…Previous studies of 2-D stretch injury in astrocytes have demonstrated transient increases in intracellular calcium from extracellular and intracellular sources (Rzigalinski et al, 1998;Floyd et al, 2001;Neary et al, 2003). Activation of Group I metabotropic glutamate receptors, altered inositol phosphate production and uncoupling of the phospholipase C signaling pathway have been reported as acute events in astrocytic dysfunction following trauma (Floyd et al, 2001;Floyd et al, 2004). Also, increases in intracellular calcium have been linked to mitochondrial dysfunction and ATP release (Ahmed et al, 2000).…”

“…For instance, necrotic cell death or disruptions in the plasma membrane may result in the release of glutamate and ATP, which have been shown to induce astrogliosis through receptor binding and subsequent initiation of signaling pathways (Franke et al, 1999;Ahmed et al, 2000;Floyd et al, 2001;Neary et al, 2003;Floyd et al, 2004;Neary et al, 2006). Therefore, the strain rate-dependent cell death may partially explain the variable astrocytotic response at the strain rates evaluated.…”

Novel Neuroinflammatory Targets in the Chronically Injured Spinal Cord

“…Blockade of group I mGluR with the antagonist AIDA in a neuronal/glial co-culture stretch-injury model resulted in reduced axonal signaling abnormalities and GFAP immunoreactivity [88]. Furthermore, N-methyl-D-aspartic acid-mediated currents were reduced in this culture model with mGluR5 agonist (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) administration, but only when glia were present [89].…”

Phosphoinositide pathway and the signal transduction network in neural development

“…During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Glutamate neurotransmission triggers a critical signalling mechanism involving the activation of PI-PLC β1 by mGluRs [94] . Activation of PI-PLC β1 via mGluR5 is critical for the coordinated development of the neocortex [45,95] .…”

“…N-methyl-D-aspartate (NMDA) toxicity might be associated with NO-mediated activation of the PI-PLC δ subfamily [24,97] . Activation of group I mGluRs by (S)-3,5-dihydroxyphenylglycine, prior to NMDA exposure, reduces hippocampal susceptibility to NMDA-induced injury [93,94] . The reduced susceptibility is probably associated with PI-PLC-dependent reduction of synaptic excitation.…”