Downregulation of postsynaptic density-95-interacting regulator of spine morphogenesis reduces glutamate-induced excitotoxicity by differentially regulating glutamate receptors in rat cortical neurons

The mGluR5-mediated Arc activation protects against experimental traumatic brain injury in rats

INTRODUCTION

Traumatic brain injury (TBI) is a complex pathophysiological process, and increasing attention has been paid to the important role of post-synaptic density (PSD) proteins, such as glutamate receptors. Our previous study showed that a PSD protein Arc/Arg3.1 (Arc) regulates endoplasmic reticulum (ER) stress and neuronal necroptosis in traumatic injury in vitro.

AIM

In this study, we investigated the expression, regulation and biological function of Arc in both in vivo and in vitro experimental TBI models.

RESULTS

Traumatic neuronal injury (TNI) induced a temporal upregulation of Arc in cortical neurons, while TBI resulted in sustained increase in Arc expression up to 24 h in rats. The increased expression of Arc was mediated by the activity of metabotropic glutamate receptor 5 (mGluR5), but not dependent on the intracellular calcium (Ca2+) release. By using inhibitors and antagonists, we found that TNI regulates Arc expression via Gq protein and protein turnover. In addition, overexpression of Arc protects against TBI-induced neuronal injury and motor dysfunction both in vivo and in vitro, whereas the long-term cognitive function was not altered. To determine the role of Arc in mGluR5-induced protection, lentivirus-mediated short hairpin RNA (shRNA) transfection was performed to knockdown Arc expression. The mGluR5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG)-induced protection against TBI was partially prevented by Arc knockdown. Furthermore, the CHPG-induced attenuation of Ca2+ influx after TNI was dependent on Arc activation and followed regulation of AMPAR subunits. The results of Co-IP and Ca2+ imaging showed that the Arc-Homer1 interaction contributes to the CHPG-induced regulation of intracellular Ca2+ release.

CONCLUSION

In summary, the present data indicate that the mGluR5-mediated Arc activation is a protective mechanism that attenuates neurotoxicity following TBI through the regulation of intracellular Ca2+ hemostasis. The AMPAR-associated Ca2+ influx and ER Ca2+ release induced by Homer1-IP3R pathway might be involved in this protection.

Preso enhances mGluR1-mediated excitotoxicity by modulating the phosphorylation of mGluR1-Homer1 complex and facilitating an ER stress after traumatic brain injury

Glutamate receptor (GluR)-mediated excitotoxicity is an important mechanism causing delayed neuronal injury after traumatic brain injury (TBI). Preso, as a core scaffolding protein of postsynaptic density (PSD), is considered an important regulator during excitotoxicity and TBI and combines with glutamate receptors to form functional units for excitatory glutamatergic neurotransmission, and elucidating the mechanisms of these functional units will provide new targets for the treatment of TBI. As a multidomain scaffolding protein, Preso directly interacts with metabotropic GluR (mGluR) and another scaffold protein, Homer. Because the mGluR-Homer complex plays a crucial role in TBI, modulation of this complex by Preso may be an important mechanism affecting the excitotoxic damage to neurons after TBI. Here, we demonstrate that Preso facilitates the interaction between metabotropic mGluR1 and Homer1 to activate mGluR1 signaling and cause excitotoxic neuronal injury and endoplasmic reticulum (ER) stress after TBI. The regulatory effect of Preso on the mGluR1-Homer1 complex is dependent on the direct association between Preso and this complex and also involves the phosphorylation of the interactive binding sites of mGluR1 and Homer1 by Preso. Further studies confirmed that Preso, as an adaptor of cyclin-dependent kinase 5 (CDK5), promotes the phosphorylation of the Homer1-binding site on mGluR1 by CDK5 and thereby enhances the interaction between mGluR1 and Homer1. Preso can also promote the formation of the mGluR1-Homer1 complex by inhibiting the phosphorylation of the Homer1 hinge region by Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα). Based on these molecular mechanisms, we designed several blocking peptides targeting the interaction between Preso and the mGluR1-Homer1 complex and found that directly disrupting the association between mGluR1 and scaffolding proteins significantly promotes the recovery of motor function after TBI.

Downregulation of Preso protects against ischemic/reperfusion-mediated neuronal injury through regulating PSD95-nNOS/YAP pathways

Cerebral ischemic/reperfusion (I/R) injury has become a great challenge harming patients' life. This study aims to explore the regulatory role of Preso during cerebral I/R injury and to elucidate the potential mechanism. Here, we established a middle cerebral artery occlusion/reperfusion (MCAO/IR) rat model and an oxygen-glucose deprivation/reoxygenation (OGD/R)-mediated PC12 cell model to evaluate the expression and role of Preso following cerebral I/R injury. Histopathological injury and infarct size were assessed by hematoxylin and eosin (HE) and 2,3,5-Triphenyltertrazolium chloride (TTC) staining. Double immunofluorescence staining was performed to assess neuronal apoptosis in brain tissues. Cell counting kit-8 (CCK-8) and flow cytometry were performed to evaluate cell viability and apoptosis, respectively. The reactive oxygen species (ROS) and nitric oxide (NO) levels were detected using their respective detection kits, and the expression of corresponding proteins was examined adopting Western blot. The results showed that Preso was upregulated in OGD/R-induced PC12 cells and MCAO rats. Preso knockdown significantly reduced OGD/R-caused viability loss, apoptosis and oxidative stress in PC12 cells, and reduced infarct size, attenuated histological injury, and inhibited apoptosis and oxidative stress in the brain tissues from MCAO rats, as well as inhibiting the expression of postsynaptic density protein-95 (PSD95) and nitric oxide synthase (nNOS) and repressing YAP phosphorylation in vitro. In addition, the protective role of Preso knockdown against cerebral I/R injury was partly strengthened by IC87201, the nNOS/PSD95 interaction inhibitor, or weakened by Verteporfin (Vert), an inhibitor of YAP. In conclusion, Perso knockdown might exert a protective role against cerebral I/R injury via regulating PSD95-nNOS and YAP pathways, providing a potential therapeutic target for the treatment of ischemic stroke.

Arc silence aggravates traumatic neuronal injury via mGluR1-mediated ER stress and necroptosis

Delayed neuronal death is associated with neurological deficits and mortality after traumatic brain injury (TBI), where post-synaptic density (PSD) proteins are thought to play key roles. The immediate-early gene (IEG) coded protein Arc is a brain-specific PSD protein that controls synaptic plasticity and learning behaviors. In this study, we investigated the expression and biological function of Arc in neuronal death after TBI in an in vitro model mimicked by traumatic neuronal injury (TNI) in cortical neurons. TNI caused a temporal increase of Arc expression at 3 and 6 h. Knockdown of Arc expression using small interfering RNA (Si-Arc-3) promoted TNI-induced cytotoxicity and apoptosis. The results of western blot showed that Si-Arc-3 transfection further enhanced the activation of endoplasmic reticulum (ER) stress-associated factors, including glucose-regulated protein 78 (GRP78), C/EBP homologous protein (CHOP) and caspase-12 after TNI. In addition, knockdown of Arc significantly increased expression of (receptor-interacting protein kinase 1) RIP1 and the number of necroptotic cells, which were apparently prevented by necrostatin-1 (Nec-1). The results of immunostaining and western blot showed that knockdown of Arc activated the metabotropic glutamate receptor 1 (mGluR1) and intracellular Ca²⁺ release in neurons. Mechanistically, the Si-Arc-3-induced activation of ER stress-associated factors, RIP1 expression, apoptosis, and necroptosis were partially reversed by the mGluR1 antagonist AIDA. In summary, our data suggest that silence of Arc expression aggravates neuronal death after TNI by promoting apoptosis and necroptosis. These data support for the first time that Arc may represent a novel candidate for therapies against TBI.

Preso regulates NMDA receptor-mediated excitotoxicity via modulating nitric oxide and calcium responses after traumatic brain injury

Traumatic brain injury (TBI) has become a major health concern worldwide, and the poor outcome of TBI increases the need for therapeutic improvement. Secondary injuries following TBI, including excitotoxicity, lead to synaptic dysfunction and provide potential targets for intervention. Postsynaptic scaffold proteins, which are involved in the regulation of excitotoxicity after neuronal injury, play a crucial role in modulating synaptic function. Therefore, exploring the role of postsynaptic scaffold proteins in TBI might uncover new treatments. In this study, we demonstrated that downregulated expression of the postsynaptic scaffold protein Preso protects against neuronal injury after TBI in vitro and in vivo, and these effects are related to the inhibition of N-methyl-D-aspartate receptor (NMDAR) function. Further study showed that Preso facilitates signaling from NMDAR to nitric oxide (NO) and calcium (Ca²⁺) responses. First, the complex constituting NMDAR, postsynaptic density-95 (PSD-95), and neuronal nitric oxide synthase (nNOS) was shown to be involved in the Preso regulation of the NO response. Uncoupling the linkage between Preso and PSD-95 attenuated the stability of this complex and suppressed the regulatory effect of Preso on the NO response. In addition, phosphorylation of NMDAR by cyclin-dependent kinase 5 (CDK5) was shown to be responsible for the Preso-mediated Ca²⁺ response, which was dependent on the interaction between Preso and CDK5. These results suggested that the association of Preso with NMDAR signaling can serve as a target for neuroprotection against TBI.

Metabotropic glutamate receptor trafficking

The metabotropic glutamate receptors (mGlu receptors) are G protein-coupled receptors that bind to the excitatory neurotransmitter glutamate and are important in the modulation of neuronal excitability, synaptic transmission, and plasticity in the central nervous system. Trafficking of mGlu receptors in and out of the synaptic plasma membrane is a fundamental mechanism modulating excitatory synaptic function through regulation of receptor abundance, desensitization, and signaling profiles. In this review, we cover the regulatory mechanisms determining surface expression and endocytosis of mGlu receptors, with particular focus on post-translational modifications and receptor-protein interactions. The literature we review broadens our insight into the precise events defining the expression of functional mGlu receptors at synapses, and will likely contribute to the successful development of novel therapeutic targets for a variety of developmental, neurological, and psychiatric disorders.

Activation of mGluR5 attenuates NMDA-induced neurotoxicity through disruption of the NMDAR-PSD-95 complex and preservation of mitochondrial function in differentiated PC12 cells

Glutamate-mediated toxicity is implicated in various neuropathologic conditions, and activation of ionotropic and metabotropic glutamate receptors is considered to be the most important mechanism. It has been reported that pharmacological saturation of metabotropic glutamate receptors (mGluRs) can facilitate N-methyl-d-aspartate receptor (NMDAR) related signaling cascades, but the mechanism leading to mGluR-NMDAR interactions in excitotoxic neuronal injury has remained unidentified. In the present study, we investigated the role of mGluR5 in the regulation of N-methyl-d-aspartate (NMDA)-induced excitotoxicity in differentiated PC12 cells. We found that activation of mGluR5 with the specific agonist R,S-2-chloro-5-hydroxyphenylglycine (CHPG) increased cell viability and inhibited lactate dehydrogenase (LDH) release in a dose-dependent manner. CHPG also inhibited an increase in the Bax/Bcl-2 ratio, attenuated cleavage of caspase-9 and caspase-3, and reduced apoptotic cell death after NMDA treatment. The NMDA-induced mitochondrial dysfunction, as indicated by mitochondrial reactive oxygen species (ROS) generation, collapse of mitochondrial membrane potential (MMP), and cytochrome c release, was also partly prevented by CHPG treatment. Furthermore, CHPG blocked the NMDA-induced interaction of NMDAR with postsynaptic density protein-95 (PSD-95), but had no effects on intracellular calcium concentrations. All these results indicated that activation of mGluR5 protects differentiated PC12 cells from NMDA-induced neuronal excitotoxicity by disrupting NMDAR-PSD-95 interaction, which might be an ideal target for investigating therapeutic strategies in various neurological diseases where excitotoxicity may contribute to their pathology.