Rapid sequential clustering of NMDARs, CaMKII, and AMPARs upon activation of NMDARs at developing synapses

Rapid, synapse-specific neurotransmission requires the precise alignment of presynaptic neurotransmitter release and postsynaptic receptors. How postsynaptic glutamate receptor accumulation is induced during maturation is not well understood. We find that in cultures of dissociated hippocampal neurons at 11 days in vitro (DIV) numerous synaptic contacts already exhibit pronounced accumulations of the pre- and postsynaptic markers synaptotagmin, synaptophysin, synapsin, bassoon, VGluT1, PSD-95, and Shank. The presence of an initial set of AMPARs and NMDARs is indicated by miniature excitatory postsynaptic currents (mEPSCs). However, AMPAR and NMDAR immunostainings reveal rather smooth distributions throughout dendrites and synaptic enrichment is not obvious. We found that brief periods of Ca2+ influx through NMDARs induced a surprisingly rapid accumulation of NMDARs within 1 min, followed by accumulation of CaMKII and then AMPARs within 2-5 min. Postsynaptic clustering of NMDARs and AMPARs was paralleled by an increase in their mEPSC amplitudes. A peptide that blocked the interaction of NMDAR subunits with PSD-95 prevented the NMDAR clustering. NMDAR clustering persisted for 3 days indicating that brief periods of elevated glutamate fosters permanent accumulation of NMDARs at postsynaptic sites in maturing synapses. These data support the model that strong glutamatergic stimulation of immature glutamatergic synapses results in a fast and substantial increase in postsynaptic NMDAR content that required NMDAR binding to PSD-95 or its homologues and is followed by recruitment of CaMKII and subsequently AMPARs.

Mitochondrial Calcium Uniporter (MCU) is Involved in an Ischemic Postconditioning Effect Against Ischemic Reperfusion Brain Injury in Mice

The phenomenon of ischemic postconditioning (PostC) is known to be neuroprotective against ischemic reperfusion (I/R) injury. One of the key processes in PostC is the opening of the mitochondrial ATP-dependent potassium (mito-KATP) channel and depolarization of the mitochondrial membrane, triggering the release of calcium ions from mitochondria through low-conductance opening of the mitochondrial permeability transition pore. Mitochondrial calcium uniporter (MCU) is known as a highly sensitive transporter for the uptake of Ca2+ present on the inner mitochondrial membrane. The MCU has attracted attention as a new target for treatment in diseases, such as neurodegenerative diseases, cancer, and ischemic stroke. We considered that the MCU may be involved in PostC and trigger its mechanisms. This research used the whole-cell patch-clamp technique on hippocampal CA1 pyramidal cells from C57BL mice and measured changes in spontaneous excitatory post-synaptic currents (sEPSCs), intracellular Ca2+ concentration, mitochondrial membrane potential, and N-methyl-D-aspartate receptor (NMDAR) currents under inhibition of MCU by ruthenium red 265 (Ru265) in PostC. Inhibition of MCU increased the occurrence of sEPSCs (p = 0.014), NMDAR currents (p < 0.001), intracellular Ca2+ concentration (p < 0.001), and dead cells (p < 0.001) significantly after reperfusion, reflecting removal of the neuroprotective effects in PostC. Moreover, mitochondrial depolarization in PostC with Ru265 was weakened, compared to PostC (p = 0.004). These results suggest that MCU affects mitochondrial depolarization in PostC to suppress NMDAR over-activation and prevent elevation of intracellular Ca2+ concentrations against I/R injury.

NMDA Receptor Antagonists Increase the Release of GLP-1 From Gut Endocrine Cells

Type 2 diabetes mellitus (T2DM) remains one of the most pressing health issues facing modern society. Several antidiabetic drugs are currently in clinical use to treat hyperglycaemia, but there is a need for new treatments that effectively restore pancreatic islet function in patients. Recent studies reported that both murine and human pancreatic islets exhibit enhanced insulin release and β-cell viability in response to N-methyl-D-aspartate (NMDA) receptor antagonists. Furthermore, oral administration of dextromethorphan, an over-the-counter NMDA receptor antagonist, to diabetic patients in a small clinical trial showed improved glucose tolerance and increased insulin release. However, the effects of NMDA receptor antagonists on the secretion of the incretin hormone GLP-1 was not tested, and nothing is known regarding how NMDA receptor antagonists may alter the secretion of gut hormones. This study demonstrates for the first time that, similar to β-cells, the NMDA receptor antagonist MK-801 increases the release of GLP-1 from a murine L-cell enteroendocrine model cell line, GLUTag cells. Furthermore, we report the 3′ mRNA expression profiling of GLUTag cells, with a specific focus on glutamate-activated receptors. We conclude that if NMDA receptor antagonists are to be pursued as an alternative, orally administered treatment for T2DM, it is essential that the effects of these drugs on the release of gut hormones, and specifically the incretin hormones, are fully investigated.

Differential regulation of the Rac1 GTPase-activating protein (GAP) BCR during oxygen/glucose deprivation in hippocampal and cortical neurons

Brain ischemia causes oxygen and glucose deprivation (OGD) in neurons, triggering a cascade of events leading to synaptic accumulation of glutamate. Excessive activation of glutamate receptors causes excitotoxicity and delayed cell death in vulnerable neurons. Following global cerebral ischemia, hippocampal CA1 pyramidal neurons are more vulnerable to injury than their cortical counterparts, but the mechanisms that underlie this difference are unclear. Signaling via Rho-family small GTPases, their upstream guanine nucleotide exchange factors, and GTPase-activating proteins (GAPs) is differentially dysregulated in response to OGD/ischemia in hippocampal and cortical neurons. Increased Rac1 activity caused by OGD/ischemia contributes to neuronal death in hippocampal neurons via diverse effects on NADPH oxidase activity and dendritic spine morphology. The Rac1 guanine nucleotide exchange factor Tiam1 mediates an OGD-induced increase in Rac1 activity in hippocampal neurons; however, the identity of an antagonistic GAP remains elusive. Here we show that the Rac1 GAP breakpoint cluster region (BCR) associates with NMDA receptors (NMDARs) along with Tiam1 and that this protein complex is more abundant in hippocampal compared with cortical neurons. Although total BCR is similar in the two neuronal types, BCR is more active in hippocampal compared with cortical neurons. OGD causes an NMDAR- and Ca²⁺-permeable AMPAR-dependent deactivation of BCR in hippocampal but not cortical neurons. BCR knockdown occludes OGD-induced Rac1 activation in hippocampal neurons. Furthermore, disrupting the Tiam1–NMDAR interaction with a fragment of Tiam1 blocks OGD-induced Tiam1 activation but has no effect on the deactivation of BCR. This work identifies BCR as a critical player in Rac1 regulation during OGD in hippocampal neurons.

Inhibition on the S-nitrosylation of MKK4 can protect hippocampal CA1 neurons in rat cerebral ischemia/reperfusion

S-nitrosylation, the nitric oxide-derived post-translational modification of proteins, plays critical roles in various physiological and pathological functions. In this present study, a rat model of cerebral ischemia and reperfusion by four-vessel occlusion was generated to assess MKK4 S-nitrosylation. Immunoprecipitation and immunoblotting were performed to evaluate MKK4 S-nitrosylation and phosphorylation. Neuronal loss was observed using histological detection. These results indicated that endogenous NO promoted the S-nitrosylation of MKK4. However, application of the exogenous NO donor S-nitrosoglutathione (GNSO), an inhibitor of the neuronal nitric oxide synthase 7-nitroindazole (7-NI), and the N-methyl-d-aspartate receptor (NMDAR) antagonist MK801 diminished I/R-induced S-nitrosylation and phosphorylation. These compounds also markedly decreased cerebral I/R-induced degeneration and death of neurons in hippocampal CA1 region in rats. Taken together, we demonstrated for the first time, that cerebral ischemia/reperfusion can induce S-nitrosylation of MKK4. We also found that inhibiting S-nitrosylation and activation of MKK4 resulted in marked decreases in neuronal degeneration and apoptosis, potentially via NMDAR-mediated mechanisms. These findings may lead to a new field of inquiry to investigate the underlying pathogenesis of stoke and the development of novel treatment strategies.

Regulation of ERK Kinase by MEK1 Kinase Inhibition in the Brain

Metabotropic (slow) and ionotropic (fast) neurotransmission are integrated by intracellular signal transduction mechanisms involving protein phosphorylation/dephosphorylation to achieve experience-dependent alterations in brain circuitry. ERK is an important effector of both slow and fast forms of neurotransmission and has been implicated in normal brain function and CNS diseases. Here we characterize phosphorylation of the ERK-activating protein kinase MEK1 by Cdk5, ERK, and Cdk1 in vitro in intact mouse brain tissue and in the context of an animal behavioral paradigm of stress. Cdk5 only phosphorylates Thr-292, whereas ERK and Cdk1 phosphorylate both Thr-292 and Thr-286 MEK1. These sites interact in a kinase-specific manner and inhibit the ability of MEK1 to activate ERK. Thr-292 and Thr-286 MEK1 are phosphorylated in most mouse brain regions to stoichiometries of ~5% or less. Phosphorylation of Thr-292 MEK1 is regulated by cAMP-dependent signaling in mouse striatum in a manner consistent with negative feedback inhibition in response to ERK activation. Protein phosphatase 1 and 2A contribute to the maintenance of the basal phosphorylation state of both Thr-292 and Thr-286 MEK1 and that of ERK. Activation of the NMDA class of ionotropic glutamate receptors reduces inhibitory MEK1 phosphorylation, whereas forced swim, a paradigm of acute stress, attenuates Thr-292 MEK1 phosphorylation. Together, the data indicate that these inhibitory MEK1 sites phosphorylated by Cdk5 and ERK1 serve as mechanistic points of convergence for the regulation of ERK signaling by both slow and fast neurotransmission.