Activation of mGluR5 and Inhibition of NADPH Oxidase Improves Functional Recovery after Traumatic Brain Injury

Abstract

Traumatic brain injury (TBI) induces microglial activation, which can contribute to secondary tissue loss. Activation of mGluR5 reduces microglial activation and inhibits microglial-mediated neurodegeneration in vitro, and is neuroprotective in experimental models of CNS injury. In vitro studies also suggest that the beneficial effects of mGluR5 activation involve nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibition in activated microglia. We hypothesized that activation of mGluR5 by the selective agonist CHPG after TBI in mice is neuroprotective and that its therapeutic actions are mediated by NADPH oxidase inhibition. Vehicle, CHPG, or CHPG plus the mGluR5 antagonist (MPEP), were administered centrally, 30 minutes post-TBI, and functional recovery and lesion volume was assessed. CHPG significantly attenuated post-traumatic sensorimotor and cognitive deficits, and reduced lesion volumes; these effects were blocked by MPEP, thereby indicating neuroprotection involved selective activation of mGluR5. CHPG treatment also reduced NFjB activity and nitrite production in lipopolysaccharide-stimulated microglia and the protective effects of CHPG treatment were abrogated in NADPH oxidase deficient microglial cultures (gp91 phox-/-). To address whether the neuroprotective effects of CHPG are mediated via the inhibition of NADPH oxidase, we administered the NADPH oxidase inhibitor apocynin with or without CHPG treatment after TBI. Both apocynin or CHPG treatment alone improved sensorimotor deficits and reduced lesion volumes when compared with vehicle-treated mice; however, the combined CHPG + apocynin treatment was not superior to CHPG alone. These data suggest that the neuroprotective effects of activating mGluR5 receptors after TBI are mediated, in part, via the inhibition of NADPH oxidase.

Microglial/Macrophage Polarization Dynamics following Traumatic Brain Injury

“…37,38 NOX2 activation contributes to oxidative stress and neuroinflammation after ischemic brain injury 39,40 and TBI ,9,23,41,42 and inhibition of this enzyme, either directly or indirectly, is highly neuroprotective in TBI models. 23,[41][42][43][44][45] Redox signaling is an essential component of M1-like polarization of macrophages, 16 and reports also have confirmed ROS regulation of microglial M2-like activation. 46 Notably, attenuation of microglial ROS through genetic or pharmacological inhibition of NOX2 increases microglial M2-like phenotype markers in response to LPS.…”

Novel mGluR5 Positive Allosteric Modulator Improves Functional Recovery, Attenuates Neurodegeneration, and Alters Microglial Polarization after Experimental Traumatic Brain Injury

“…Following moderate-to-severe spinal cord contusion injury in rats we demonstrated that intrathecal CHPG administration improved motor recovery, reduced lesion volume, and spared white matter loss [14]. Recently we showed that central administration of CHPG at 30 min post-injury resulted in significantly improved sensorimotor and cognitive recovery and reduced lesion volumes after CCI [16]. Furthermore, we also demonstrated that when CHPG was administered to CCI mice at 1 month post-injury it arrested the expansion of the lesion over time, reduced white matter loss and hippocampal neurodegeneration, and improved functional recovery at 4 months [13].…”

“…Previously, we showed that knockout of NOX2 abolished the anti-inflammatory effects of mGluR5 stimulation by CHPG in activated microglia [16], and that CHPG treatment following CCI resulted in significantly reduced microglial activation and NOX2 expression at 4 months post-injury [13]. Here, we evaluated NOX2 expression in microglia in vehicle-and VU0360172-treated CCI groups by double immunofluorescence staining for gp91 phox (NOX2) and Iba1 (microglia).…”

“…Following experimental traumatic brain injury (TBI) we found that acute central administration of CHPG significantly limited post-traumatic sensorimotor and cognitive deficits, reduced lesion volumes, and attenuated post-traumatic microglial activation [16]. Further, we demonstrated persistent microglial activation in the TBI cortex through 1 year postinjury that was associated with progressive lesion expansion and hippocampal neurodegeneration [21].…”

“…Previously, we demonstrated that selective activation of microglial mGluR5 receptors by CHPG had strong antiinflammatory effects and attenuated microglial activation in vitro and in vivo [13,19,16,20]. To test whether VU0360172 has similar anti-inflammatory properties, we pre-treated cultured BV2 microglia for 1 h with increasing concentrations of VU0360172 (1-50 μM) and then stimulated the cells with/without LPS (50 ng/ml) or IFNγ (0.2 ng/ml) for a further 24 h. We included a CHPG treatment group (2,000 μM) as a positive control, and measured TNFα and NO release in the medium to assess microglial activation, as well as LDH release to assess cell cytotoxicity.…”