14-3-3 proteins promote synaptic localization of N-methyl d-aspartate receptors (NMDARs) in mouse hippocampal and cortical neurons

One of the core pathogenic mechanisms for schizophrenia is believed to be dysfunction in glutamatergic synaptic transmissions, particularly hypofunction of N-methyl d-aspartate receptors (NMDARs). Previously we showed that 14-3-3 functional knockout mice exhibit schizophrenia-associated behaviors accompanied by reduced synaptic NMDARs in forebrain excitatory neurons. To investigate how 14-3-3 proteins regulate synaptic localization of NMDARs, here we examined changes in levels of synaptic NMDARs upon 14-3-3 inhibition in primary neurons. Expression of 14-3-3 protein inhibitor (difopein) in primary glutamatergic cortical and hippocampal neurons resulted in lower number of synaptic puncta containing NMDARs, including the GluN1, GluN2A, or GluN2B subunits. In heterologous cells, 14-3-3 proteins enhanced surface expression of these NMDAR subunits. Furthermore, we identified that 14-3-3ζ and ε isoforms interact with NMDARs via binding to GluN2A and GluN2B subunits. Taken together, our results demonstrate that 14-3-3 proteins play a critical role in NMDAR synaptic trafficking by promoting surface delivery of NMDAR subunits GluN1, GluN2A, and GluN2B. As NMDAR hypofunctionality is known to act as a convergence point for progression of symptoms of schizophrenia, further studies on these signaling pathways may help understand how dysfunction of 14-3-3 proteins can cause NMDAR hypofunctionality and lead to schizophrenia-associated behaviors.

NMDA receptor expression during cell transformation process at early stages of liver cancer in rodent models

Hepatocellular carcinoma (HCC) is the most common primary liver cancer, which is not sensitive to radiotherapy and chemotherapy and very often experiences postoperative relapse. In this regard, effective screening of liver cancer is considered as the most important and urgent task. The aim of our study was to determine whether N-methyl-D-aspartate receptor (NMDAR) and, in particular, its subunits, can serve as biomarkers to distinguish the precancerous liver at early stages of liver fibrosis. We assessed the development of HCC after 10, 15 and 22 weeks using a HCC rat model. The expression of NMDAR subunits was monitored at different stages of HCC by means of immunohistochemistry combined with epifluorescence microscopy imaging, Western blotting and direct bisulfite sequencing. NMDAR subunits were not found in healthy liver tissues. In contrast, NMDAR subunits, in particular NR1 and NR2B, appeared at the stage of severe liver fibrosis (precancerous liver disease) in rats and were expressed during the development of HCC in rats and mice. Using the direct bisulfite sequencing, we detected that increased expression of NMDAR directly correlated with the demethylation of CpG islands in the promoter region of genes encoding receptor subunits. The obtained results confirmed that NMDAR subunits can serve as new biomarkers of precancerous liver disease, severe fibrosis, and its progression towards HCC.

Epileptiform GluN2B–driven excitation in hippocampus as a therapeutic target against temporal lobe epilepsy

NMDAR is an ionotropic glutamate receptor critically involved in excitatory synaptic transmission. The receptor properties are strongly determined by its subunit composition. One of the NMDAR subunits is GluN2B, which displays restricted and spatially different from other subunits expression in the mature brain. GluN2B–containing NMDARs are present in the hippocampus – a structure playing a major role in temporal lobe epilepsy (TLE). However, the contribution of GluN2B to pathophysiology of TLE has not been fully explored. Here, we report the functional alterations of GluN2B–containing NMDAR receptors in the hippocampus in distinct mouse models of temporal lobe epilepsy. In particular, we show the impact of GluN2B on excitatory feedback in granule cells. Based on these results, we propose a mechanism–oriented effective antiepileptic strategy that selectively antagonizes GluN2B–containing NMDARs with ifenprodil, a well–known GluN2B antagonist. Collectively, our research identifies GluN2B as one of the pivotal factors in pathogenesis of temporal lobe epilepsy and associated recurrent seizures. Furthermore, our study indicates the prospective antiepileptic properties of ifenprodil in TLE.

Glutamate Attenuates the Survival Property of IGFR through NR2B Containing N-Methyl-D-aspartate Receptors in Cortical Neurons

Glutamate-induced neurotoxicity is involved in various neuronal diseases, such as Alzheimer’s disease. We have previously reported that glutamate attenuated the survival signaling of insulin-like growth factor-1 (IGF-1) by N-methyl-D-aspartate receptors (NMDARs) in cultured cortical neurons, which is viewed as a novel mechanism of glutamate-induced neurotoxicity. However, the phosphorylation sites of IGF-1 receptor (IGF-1R) affected by glutamate remain to be elucidated, and importantly, which subtype of NMDARs plays a major role in attenuating the prosurvival effect of IGF-1 is still unknown. In the present study, glutamate was found to attenuate the tyrosine phosphorylation of the IGF-1R and the prosurvival effect of IGF-1 in primary cultured cortical neurons. NMDAR inhibitors, MK801 and AP-5, blocked the inhibitory effect of glutamate on the phosphorylation of IGF-1R and increased cell survival, while DNQX, LY341495, and CPCCOEt had no effect. Interestingly, we found that glutamate decreased the phosphorylation of tyrosine residues 1131, 1135/1136, 1250/1251, and 1316, while it had no effect on tyrosine 950 in cortical neurons. Moreover, using specific antagonists and siRNA to downregulate individual NMDAR subunits, we found that the activation of NR2B-containing NMDARs was essential for glutamate to inhibit IGF-1 signaling. These findings indicate that the glutamate-induced attenuation of IGF-1 signaling is mediated by NR2B-containing NMDARs. Our study also proposes a novel mechanism of altering neurotrophic factor signaling by the activation of NMDARs.

M9. RATS REARED IN SOCIAL ISOLATION INDUCES EPIGENETIC MODIFICATIONS IN THE NMDA RECEPTOR SUBUNITS

Background Early-life stress is a key risk for psychiatric disorders that may produce changes in the neurodevelopment. N-methyl-d-aspartate receptor (NMDAR) have been associated with the pathophysiology of schizophrenia and evidence supports that epigenetic changes in NMDAR imply deficiencies in excitatory neurotransmission suggest its role in the neurobiology of psychoses (Uno and Coyle, 2019; Fachim et al., 2019; Gulchina et al., 2017). Aims: Although previous studies have shown abnormalities in the glutamatergic system in animal model of schizophrenia, it is not known if there are equivalent mRNA/protein alterations and DNA methylation changes in the brains of rats reared in isolation. Thus, in order to improve the knowledge of glutamatergic system role in psychosis, we investigated the NR1 and NR2 mRNA/protein and the DNA methylation levels of Grin1, Grin2a and Grin2b promoter region in the prefrontal cortex (PFC) and hippocampus (HIPPO) of male Wistar rats after isolation rearing. Furthermore, because the Parvalbumin (PV) deficit is the most consistent finding across animal models and schizophrenia itself, we also evaluated the expression of PV and other related GABAergic genes (REL and GAD1) in the brain of rats undergoing social isolation rearing as a validation of this animal model. We hypothesized that isolation rearing reduces mRNA and protein expressions of NMDAR subunits and cause DNA methylation changes. Methods Wistar rats were kept isolated or grouped (n=10/group) from weaning (21 days after birth) to 10 weeks and then exposed to the Open Field Test to assess locomotion. Afterwards the behavioural tests, the tissues were dissected for RNA/DNA extraction and NMDAR subunits were analysed using qRT-PCR, ELISA and pyrosequencing. Data were analysed by parametric tests. Results Isolated-reared animals presented: (i) decreased mRNA levels of Grin1 (p=0.011), Grin2a (p=0.039) and Grin2b (p=0.037) in the PFC followed by reduction in the GABAergic markers; (ii) increased NR1 protein levels in the HIPPO (p=0.001); (iii) hypermethylation of Grin1 at CpG5 in the PFC (p=0.047) and Grin2b CpG4 in the HIPPO when compared to grouped (p=0.024). Moreover, isolated and grouped animals presented a negative correlation between Grin1 mRNA and Grin1 methylation levels at CpG5 in the PFC (r: -0.577; p=0.010) and isolated rats presented a negative correlation between Grin2b methylation at CpG4 and NR2 protein levels in the HIPPO (r: -0.753; p=0.012). Discussion This study supports the hypothesis that the NMDAR methylation changes found in the brain tissues may underlie the NMDAR mRNA/protein expression alterations caused by the isolation period. These results highlighted the importance of the environmental influence during the development that may lead to cognitive impairments in adulthood. Moreover, we demonstrated that the social isolation rearing during 10 weeks causes long-lasting behavioral changes that may be more associated with late stages of schizophrenia. Our study contributes to the identification of the epigenetic mechanisms involved in the neuropathophysiology of schizophrenia, which can bring new pharmacotherapeutic strategies and to identify biomarkers that can improve the early interventions in schizophrenia patients. Finally, our data thus reinforce the validity of rats reared in social isolation after weaning in modelling aspects of schizophrenia, highlighting the glutamatergic and GABAergic features involved principally in the cognitive impairments related to prefrontal cortex.

Pituitary Adenylate Cyclase Activating Polypeptide Elicits Neuroprotection Against Acute Ischemic Neuronal Cell Death Associated with NMDA Receptors

Background/Aims: The endogenous neurotrophic peptides pituitary adenylate cyclase-activating polypeptides (PACAP-27/38) protect against stroke, but the molecular mechanism remains unknown. Methods: Primary rat neural cells were exposed to PACAP-27 or PACAP-38 before induction of experimental acute ischemic stroke via oxygen-glucose deprivation-reperfusion (OGD/R) injury. To reveal PACAP’s role in neuroprotection, we employed fluorescent live/dead cell viability and caspase 3 assays, optical densitometry of mitochondrial dehydrogenase and cell growth, glutathione disulfide luciferase activity, ELISA for high mobility group box1 extracellular concentration, ATP bioluminescence, Western blot analysis of PACAP, NMDA subunits, apoptosis regulator Bcl-2, social interaction hormone oxytocin, and trophic factor BDNF, and immunocytochemical analysis of PACAP. Results: Both PACAP-27 and PACAP-38 (PACAP-27/38) increased cell viability, decreased oxidative stress-induced cell damage, maintained mitochondrial activity, prevented the release of high mobility group box1, and reduced cytochrome c/caspase 3-induced apoptosis. PACAP-27/38 increased the protein expression levels of BDNF, Bcl-2, oxytocin, and precursor PACAP. N-methyl-D-aspartate receptor (NMDAR)-induced excitotoxicity contributes to the cell death associated with stroke. PACAP-27/38 modulated the protein expression levels of NMDAR subunits. PACAP-27/38 increased the protein expression levels of the GluN1 subunit, and decreased that of the GluN2B and GluN2D subunits. PACAP-27, but not PACAP-38, increased the expression level of the GluN2C subunit. Conclusion: This study provides evidence that PACAP regulated NMDAR subunits, affording neuroprotection after OGD/R injury.

Protective Role of NMDAR for Microwave-Induced Synaptic Plasticity Injuries in Primary Hippocampal Neurons

Background/Aims: The N-methyl-D-aspartic acid receptor (NMDAR) has been extensively studied for its important roles in synaptic plasticity and learning and memory. However, the effects of microwave radiation on the subunit composition and activity of NMDARs and the relationship between NMDARs and microwave-induced synaptic plasticity have not been thoroughly elucidated to date. Materials: In our study, primary hippocampal neurons were used to evaluate the effects of microwave radiation on synaptic plasticity. Structural changes were observed by diolistic (Dil) labeling and scanning electron microscopy (SEM) observation. Functional synaptic plasticity was reflected by the NMDAR currents, which were detected by whole cell patch clamp. We also detected the expression of NMDAR subunits by real-time PCR and Western blot analysis. To clarify the effects of microwave radiation on NMDAR-induced synaptic plasticity, suitable agonists or inhibitors were added to confirm the role of NMDARs on microwave-induced synaptic plasticity. Dil labeling, SEM observation, whole cell patch clamp, real-time PCR and Western blot analysis were used to evaluate changes in synaptic plasticity after treatment with agonists or inhibitors. Results: Our results found that microwave exposure impaired neurite development and decreased mRNA and protein levels and the current density of NMDARs. Due to the decreased expression of NMDAR subunits after microwave exposure, the selective agonist NMDA was added to identify the role of NMDARs on microwave-induced synaptic plasticity injuries. After adding the agonist, the expression of NMDAR subunits recovered to the normal levels. In addition, the microwave-induced structural and functional synaptic plasticity injuries recovered, including the number and length of neurites, the connections between neurons, and the NMDAR current. Conclusion: Microwave radiation caused neuronal synaptic plasticity injuries in primary hippocampal neurons, and NMDARs played protective roles on the damage process.