Interactions of postsynaptic density-95 and the NMDA receptor 2 subunit control calpain-mediated cleavage of the NMDA receptor

Sex and age differences in social and cognitive function in offspring exposed to late gestational hypoxia

BACKGROUND

Gestational sleep apnea is a hypoxic sleep disorder that affects 8-26% of pregnancies and increases the risk for central nervous system dysfunction in offspring. Specifically, there are sex differences in the sensitivity of the fetal hippocampus to hypoxic insults, and hippocampal impairments are associated with social dysfunction, repetitive behaviors, anxiety, and cognitive impairment. Yet, it is unclear whether gestational sleep apnea impacts these hippocampal-associated functions and if sex and age modify these effects. To examine the relationship between gestational sleep apnea and hippocampal-associated behaviors, we used chronic intermittent hypoxia (CIH) to model late gestational sleep apnea in pregnant rats. We hypothesized that late gestational CIH would produce sex- and age-specific social, anxiety-like, repetitive, and cognitive impairments in offspring.

METHODS

Timed pregnant Long-Evans rats were exposed to CIH or room air normoxia from GD 15-19. Behavioral testing of offspring occurred during either puberty or young adulthood. To examine gestational hypoxia-induced behavioral phenotypes, we quantified hippocampal-associated behaviors (social function, repetitive behaviors, anxiety-like behaviors, and spatial memory and learning), hippocampal neuronal activity (glutamatergic NMDA receptors, dopamine transporter, monoamine oxidase-A, early growth response protein 1, and doublecortin), and circulating hormones in offspring.

RESULTS

Late gestational CIH induced sex- and age-specific differences in social, repetitive, and memory functions in offspring. In female pubertal offspring, CIH impaired social function, increased repetitive behaviors, and elevated circulating corticosterone levels but did not impact memory. In contrast, CIH transiently induced spatial memory dysfunction in pubertal male offspring but did not impact social or repetitive functions. Long-term effects of gestational CIH on social behaviors were only observed in female offspring, wherein CIH induced social disengagement and suppression of circulating corticosterone levels in young adulthood. No effects of gestational CIH were observed in anxiety-like behaviors, hippocampal neuronal activity, or circulating testosterone and estradiol levels, regardless of sex or age of offspring.

CONCLUSIONS

Our results indicate that hypoxia-associated pregnancy complications during late gestation can increase the risk for behavioral and physiological outcomes in offspring, such as social dysfunction, repetitive behaviors, and cognitive impairment, that are dependent on sex and age.

Prenatal Exposure to Δ9-Tetrahydrocannabinol Affects Hippocampus-Related Cognitive Functions in the Adolescent Rat Offspring: Focus on Specific Markers of Neuroplasticity

Previous evidence suggests that prenatal exposure to THC (pTHC) derails the neurodevelopmental trajectories towards a vulnerable phenotype for impaired emotional regulation and limbic memory. Here we aimed to investigate pTHC effect on hippocampus-related cognitive functions and markers of neuroplasticity in adolescent male offspring. Wistar rats were exposed to THC (2 mg/kg) from gestational day 5 to 20 and tested for spatial memory, object recognition memory and reversal learning in the reinforce-motivated Can test and in the aversion-driven Barnes maze test; locomotor activity and exploration, anxiety-like behaviour, and response to natural reward were assessed in the open field, elevated plus maze, and sucrose preference tests, respectively. The gene expression levels of NMDA NR1-2A subunits, mGluR5, and their respective scaffold proteins PSD95 and Homer1, as well as CB1R and the neuromodulatory protein HINT1, were measured in the hippocampus. pTHC offspring exhibited deficits in spatial and object recognition memory and reversal learning, increased locomotor activity, increased NR1-, decreased NR2A- and PSD95-, increased mGluR5- and Homer1-, and augmented CB1R- and HINT1-hippocampal mRNA levels. Our data shows that pTHC is associated with specific impairment in spatial cognitive processing and effectors of hippocampal neuroplasticity and suggests novel targets for future pharmacological challenges.

The effect of vitamin B12 on synaptic plasticity of hippocampus in Alzheimer's disease model rats

BACKGROUND

Hippocampus cells, responsible for learning and memory, are disturbed in Alzheimer's disease (AD), resulting in production of several inflammatory markers, such as neurexin 1 -neuroligin, cyclooxygenase-2 (COX-2), and caspase-3 proteins, used in measurement of AD's severity and development. Vitamin B₁₂, which plays a role in brain functioning, has anti-inflammatory properties and its impairment is associated with apoptosis in Alzheimer's disease. This study aimed to investigate the effect of vitamin B₁₂ on restoration of Synaptic Plasticity on scopolamine-induced AD in rats.

METHODS

To simulate AD, Rats, except the control group were i.p. injected with 3 mg/kg scopolamine. Before scopolamine the pretreatment group vitamin B₁₂ (0.5, 2, and 4 mg/kg) was injected every day for the next 14 days. After 24 h, sectioning the rats' brains, the concentration of postsynaptic density protein 95 (PSD-95), neurexin 1-neurolgin, COX-2, and caspase-3 proteins in hippocampus were measured using immunoblotting.

RESULTS

B₁₂ significantly enhanced molecular balance. PSD-95 and neurexin 1 and neuroligin concentrations were significantly reduced, whereas COX-2 and activated caspase-3 were enhanced in the hippocampus of scopolamine-injected subjects. Their alterations were decreased after B12 administration.

CONCLUSIONS

Vitamin B₁₂ protected scopolamine-injected rats and inhibited hippocampal inflammation and apoptosis and preserved pre- and post-synaptic proteins and possibly synaptic integrity in hippocampus route.

Functional NMDA receptors are expressed by human pulmonary artery smooth muscle cells

N-methyl-d-aspartate (NMDA) receptors are widely expressed in the central nervous system. However, their presence and function at extraneuronal sites is less well characterized. In the present study, we examined the expression of NMDA receptor subunit mRNA and protein in human pulmonary artery (HPA) by quantitative polymerase chain reaction (PCR), immunohistochemistry and immunoblotting. We demonstrate that both GluN1 and GluN2 subunit mRNAs are expressed in HPA. In addition, GluN1 and GluN2 (A–D) subunit proteins are expressed by human pulmonary artery smooth muscle cells (HPASMCs) in vitro and in vivo. These subunits localize on the surface of HPASMCs and form functional ion channels as evidenced by whole-cell patch-clamp electrophysiology and reduced phenylephrine-induced contractile responsiveness of human pulmonary artery by the NMDA receptor antagonist MK801 under hypoxic condition. HPASMCs also express high levels of serine racemase and vesicular glutamate transporter 1, suggesting a potential source of endogenous agonists for NMDA receptor activation. Our findings show HPASMCs express functional NMDA receptors in line with their effect on pulmonary vasoconstriction, and thereby suggest a novel therapeutic target for pharmacological modulations in settings associated with pulmonary vascular dysfunction.

Sp4 controls constitutive expression of neuronal serine racemase and NF-E2-related factor-2 mediates its induction by valproic acid

Serine racemase (SR) synthesizes l-type serine to its enantimor, d-serine which participates in physiological processes and in pathological conditions. In the central nervous system, SR is highly expressed in neurons and astrocytes but expressed at relatively lower amount in microglia. However, the mechanism by which SR is highly expessed in neurons is hitherto unknown. We report that the SR mRNA and protein levels in Neuro-2a were increased by valproic acid (VPA), a neuron differentiation stimulator as well as a histone deacetylase inhibitor. SR proximal promoter contained nine putative Sp-binding elements and in the exon 1, three putative anti-oxidant elements (AREs) were conservative among human, rat, and mouse genome. The promoter constructs including 5'-, 3'-fragment, and full length fragment from mouse were individually cloned into a luciferase reporter. Using dual-luciferase assay, the promoter harboring 3'-fragment contained much lower activity than the construct containing 5'-fragment which was though resistant to VPA induction, relative to 3'-fragment. Overexpression of Sp4 or Nrf2 increased whereas knockdown of either decreased Srr mRNA and SR protein. Using site-directed mutagenesis, mutation of Sp-binding elements or AREs in the constructs significantly decreased luciferase activity of the corresponding promoter construct. With chromatin immunoprecipitation, Sp4 was demonstrated to interact directly with the Sp-binding elements whereas Nrf2 bound AREs in Srr mRNA promoter. Altogether, our study highlights that Sp4 controls constitutive expression of SR in neuron and VPA mediates SR expression in N2A cells which is associated with its effect on neuron differentiation, that is, the effect is mediated via Nrf2.

Modulation of the MAPKs pathways affects Aβ-induced cognitive deficits in Alzheimer's disease via activation of α7nAChR

Cognitive impairment in Alzheimer's disease (AD) is characterized by being deficient at learning and memory. Aβ_1-42 oligomers have been shown to impair rodent cognitive function. We previously demonstrated that activation of α7nAChR, inhibition of p38 or JNK could alleviate Aβ-induced memory deficits in Y maze test. In this study, we investigated whether the effects of α7nAChR and MAPKs on Y maze test is reproducible with a hippocampus-dependent spatial memory test such as Morris water maze. We also assessed the possible co-existence of hippocampus-independent recognition memory dysfunction using a novel object recognition test and an alternative and stress free hippocampus-dependent recognition memory test such as the novel place recognition. Besides, previous research from our lab has shown that MAPKs pathways regulate Aβ internalization through mediating α7nAChR. In our study, whether MAPKs pathways exert their functions in cognition by modulating α7nAChR through regulating glutamate receptors and synaptic protein, remain little known. Our results showed that activation of α7nAChR restored spatial memory, novel place recognition memory, and short-term and long-term memory in novel object recognition. Inhibition of p38 restored spatial memory and short-term and long-term memory in novel object recognition. Inhibition of ERK restored short-term memory in novel object recognition and novel place recognition memory. Inhibition of JNK restored spatial memory, short-term memory in novel object recognition and novel place recognition memory. Beside this, the activation of α7nAChR, inhibition of p38 or JNK restored Aβ-induced levels of NMDAR1, NMDAR2A, NMDAR2B, GluR1, GluR2 and PSD95 in Aβ-injected mice without influencing synapsin 1. In addition, these treatments also recovered the expression of acetylcholinesterase (AChE). Finally, we found that the inhibition of p38 or JNK resulted in the upregulation of α7nAChR mRNA levels in the hippocampus. Our results indicated that inhibition of p38 or JNK MAPKs could alleviate Aβ-induced spatial memory deficits through regulating activation of α7nAChR via recovering memory-related proteins. Moreover, p38, ERK and JNK MAPKs exert different functions in spatial and recognition memory.

NR2B receptor- and calpain-mediated KCC2 cleavage resulted in cognitive deficiency exposure to isoflurane

BACKGROUND

During brain development, volatile anesthetic can rapidly interfere with physiologic patterns of dendritic development and synaptogenesis and impair the formation of precise neuronal circuits. KCC2 plays vital roles in spine development and synaptogenesis through its Cl- transport function and structural interactions with the spine cytoskeleton protein 4.1 N. The aim of this study was to dissect the mechanism of volatile anesthetics, which impair dendritic development and synaptogenesis via mediation of KCC2 cleavage.

METHODS

Westernblotting was employed to assess the expression change of NR2B, NR2A, calpain-1, calpain-2, KCC2, and 4.1 N protein of rat (PND 5). Co-immunoprecipitation was applied to demonstrate the interaction between KCC2 and 4.1 N protein. Long-term cognitive deficiency was assessed by MWM. Lentivirus-calpain-2 was administered by hippocampus stereotaxic injection.

RESULTS

There was a significant increase in the level of NR2B instead of NR2A exposure to isoflurane. Calpain-2 was excessively activated via NR2B after 6 h of isoflurane exposure. The expression of plasmalemmal KCC2 and 4.1 N protein was significantly decreased treated with isoflurane. The isoflurane group showed longer traveled distance, prolonged escape latency, less time spent in the target quadrant, and decreased platform crossings. Pretreatment with ifenprodil and downregulated calpain-2 expression significantly alleviated these neurotoxicity responses and cognitive deficiency after isoflurane exposure.

CONCLUSIONS

A significant increase in NR2B, excessive activation of calpain-2 and increased cleavage of plasmalemmal KCC2, are involved in isoflurane-induced neurotoxicity and long-term cognitive deficiency. Blocking NR2B and calpain-2 activity significantly attenuated these responses. The KCC2 cleavage mediated by NR2B and calpain-2 is a major determinant of isoflurane-induced long-term cognitive deficiency.

A unique tau conformation generated by an acetylation-mimic substitution modulates P301S-dependent tau pathology and hyperphosphorylation

Abnormal intracellular accumulation of aggregated tau is a hallmark feature of Alzheimer's disease and other tauopathies. Pathological tau can undergo a range of post-translational modifications (PTMs) that are implicated as triggers of disease pathology. Recent studies now indicate that tau acetylation, in particular, controls both microtubule binding and tau aggregation, thereby acting as a central regulator of tau's biochemical properties and providing avenues to exploit for potential therapies. Here, using cell-based assays and tau transgenic mice harboring an acetylation-mimic mutation at residue Lys-280 (K280Q), we evaluated whether this substitution modifies the neurodegenerative disease pathology associated with the aggregate-prone tau P301S variant. Strikingly, the addition of a K280Q-substituted variant altered P301S-mediated tau conformation and reduced tau hyperphosphorylation. We further evaluated neurodegeneration markers in K280Q acetylation-mimic mice and observed reduced neuroinflammation as well as restored levels of N-methyl-d-aspartate receptors and post-synaptic markers compared with the parental mice. Thus, substituting a single lysine residue in the context of a P301S disease-linked mutation produces a unique tau species that abrogates some of the cardinal features of tauopathy. The findings of our study indicate that a complex tau PTM code likely regulates tau pathogenesis, highlighting the potential utility of manipulating and detoxifying tau strains through site-specific tau-targeting approaches.

Developmental NMDA receptor dysregulation in the infantile neuronal ceroid lipofuscinosis mouse model

Protein palmitoylation and depalmitoylation alter protein function. This post-translational modification is critical for synaptic transmission and plasticity. Mutation of the depalmitoylating enzyme palmitoyl-protein thioesterase 1 (PPT1) causes infantile neuronal ceroid lipofuscinosis (CLN1), a pediatric neurodegenerative disease. However, the role of protein depalmitoylation in synaptic maturation is unknown. Therefore, we studied synapse development in Ppt1^-/- mouse visual cortex. We demonstrate that the developmental N-methyl-D-aspartate receptor (NMDAR) subunit switch from GluN2B to GluN2A is stagnated in Ppt1^-/- mice. Correspondingly, Ppt1^-/- neurons exhibit immature evoked NMDAR currents and dendritic spine morphology in vivo. Further, dissociated Ppt1^-/- cultured neurons show extrasynaptic, diffuse calcium influxes and enhanced vulnerability to NMDA-induced excitotoxicity, reflecting the predominance of GluN2B-containing receptors. Remarkably, Ppt1^-/- neurons demonstrate hyperpalmitoylation of GluN2B as well as Fyn kinase, which regulates surface retention of GluN2B. Thus, PPT1 plays a critical role in postsynapse maturation by facilitating the GluN2 subunit switch and proteostasis of palmitoylated proteins.

Regulatory role of calpain in neuronal death

Calpains are a group of calcium-dependent proteases that are over activated by increased intracellular calcium levels under pathological conditions. A wide range of substrates that regulate necrotic, apoptotic and autophagic pathways are affected by calpain. Calpain plays a very important role in neuronal death and various neurological disorders. This review introduces recent research progress related to the regulatory mechanisms of calpain in neuronal death. Various neuronal programmed death pathways including apoptosis, autophagy and regulated necrosis can be divided into receptor interacting protein-dependent necroptosis, mitochondrial permeability transition-dependent necrosis, pyroptosis and poly (ADP-ribose) polymerase 1-mediated parthanatos. Calpains cleave series of key substrates that may lead to cell death or participate in cell death. Regarding the investigation of calpain-mediated programed cell death, it is necessary to identify specific inhibitors that inhibit calpain mediated neuronal death and nervous system diseases.

Hippocampal calpain is required for the consolidation and reconsolidation but not extinction of contextual fear memory

Memory consolidation, reconsolidation, and extinction have been shown to share similar molecular signatures, including new gene expression. Calpain is a Ca2+-dependent protease that exerts its effects through the proteolytic cleavage of target proteins. Neuron-specific conditional deletions of calpain 1 and 2 impair long-term potentiation in the hippocampus and spatial learning. Moreover, recent studies have suggested distinct roles of calpain 1 and 2 in synaptic plasticity. However, the role of hippocampal calpain in memory processes, especially memory consolidation, reconsolidation, and extinction, is still unclear. In the current study, we demonstrated the critical roles of hippocampal calpain in the consolidation, reconsolidation, and extinction of contextual fear memory in mice. We examined the effects of pharmacological inhibition of calpain in the hippocampus on these memory processes, using the N-Acetyl-Leu-Leu-norleucinal (ALLN; calpain 1 and 2 inhibitor). Microinfusion of ALLN into the dorsal hippocampus impaired long-term memory (24 h memory) without affecting short-term memory (2 h memory). Similarly, this pharmacological blockade of calpain in the dorsal hippocampus also disrupted reactivated memory but did not affect memory extinction. Importantly, the systemic administration of ALLN inhibited the induction of c-fos in the hippocampus, which is observed when memory is consolidated. Our observations showed that hippocampal calpain is required for the consolidation and reconsolidation of contextual fear memory. Further, the results suggested that calpain contributes to the regulation of new gene expression that is necessary for these memory processes as a regulator of Ca2+-signal transduction pathway.

The Insula and Taste Learning

The sense of taste is a key component of the sensory machinery, enabling the evaluation of both the safety as well as forming associations regarding the nutritional value of ingestible substances. Indicative of the salience of the modality, taste conditioning can be achieved in rodents upon a single pairing of a tastant with a chemical stimulus inducing malaise. This robust associative learning paradigm has been heavily linked with activity within the insular cortex (IC), among other regions, such as the amygdala and medial prefrontal cortex. A number of studies have demonstrated taste memory formation to be dependent on protein synthesis at the IC and to correlate with the induction of signaling cascades involved in synaptic plasticity. Taste learning has been shown to require the differential involvement of dopaminergic GABAergic, glutamatergic, muscarinic neurotransmission across an extended taste learning circuit. The subsequent activation of downstream protein kinases (ERK, CaMKII), transcription factors (CREB, Elk-1) and immediate early genes (c-fos, Arc), has been implicated in the regulation of the different phases of taste learning. This review discusses the relevant neurotransmission, molecular signaling pathways and genetic markers involved in novel and aversive taste learning, with a particular focus on the IC. Imaging and other studies in humans have implicated the IC in the pathophysiology of a number of cognitive disorders. We conclude that the IC participates in circuit-wide computations that modulate the interception and encoding of sensory information, as well as the formation of subjective internal representations that control the expression of motivated behaviors.

Peripheral Elevation of a Klotho Fragment Enhances Brain Function and Resilience in Young, Aging, and α-Synuclein Transgenic Mice

Cognitive dysfunction and decreased mobility from aging and neurodegenerative conditions, such as Parkinson and Alzheimer diseases, are major biomedical challenges in need of more effective therapies. Increasing brain resilience may represent a new treatment strategy. Klotho, a longevity factor, enhances cognition when genetically and broadly overexpressed in its full, wild-type form over the mouse lifespan. Whether acute klotho treatment can rapidly enhance cognitive and motor functions or induce resilience is a gap in our knowledge of its therapeutic potential. Here, we show that an α-klotho protein fragment (αKL-F), administered peripherally, surprisingly induced cognitive enhancement and neural resilience despite impermeability to the blood-brain barrier in young, aging, and transgenic α-synuclein mice. αKL-F treatment induced cleavage of the NMDAR subunit GluN2B and also enhanced NMDAR-dependent synaptic plasticity. GluN2B blockade abolished αKL-F-mediated effects. Peripheral αKL-F treatment is sufficient to induce neural enhancement and resilience in mice and may prove therapeutic in humans.

Palmitoylation regulates glutamate receptor distributions in postsynaptic densities through control of PSD95 conformation and orientation

Postsynaptic density protein 95 (PSD95) and synapse-associated protein 97 (SAP97) are homologous scaffold proteins with different N-terminal domains, possessing either a palmitoylation site (PSD95) or an L27 domain (SAP97). Here, we measured PSD95 and SAP97 conformation in vitro and in postsynaptic densities (PSDs) using FRET and EM, and examined how conformation regulated interactions with AMPA-type and NMDA-type glutamate receptors (AMPARs/NMDARs). Palmitoylation of PSD95 changed its conformation from a compact to an extended configuration. PSD95 associated with AMPARs (via transmembrane AMPAR regulatory protein subunits) or NMDARs [via glutamate ionotropic receptor NMDA-type subunit 2B (GluN2B) subunits] only in its palmitoylated and extended conformation. In contrast, in its extended conformation, SAP97 associates with NMDARs, but not with AMPARs. Within PSDs, PSD95 and SAP97 were largely in the extended conformation, but had different orientations. PSD95 oriented perpendicular to the PSD membrane, with its palmitoylated, N-terminal domain at the membrane. SAP97 oriented parallel to the PSD membrane, likely as a dimer through interactions of its N-terminal L27 domain. Changing PSD95 palmitoylation in PSDs altered PSD95 and AMPAR levels but did not affect NMDAR levels. These results indicate that in PSDs, PSD95 palmitoylation, conformation, and its interactions are dynamic when associated with AMPARs and more stable when associated with NMDARs. Altogether, our results are consistent with differential regulation of PSD95 palmitoylation in PSDs resulting from the clustering of palmitoylating and depalmitoylating enzymes into AMPAR nanodomains segregated away from NMDAR nanodomains.

Differential role of calpain-dependent protein cleavage in intermediate and long-term operant memory in Aplysia

In addition to protein synthesis, protein degradation or protein cleavage may be necessary for intermediate (ITM) and long-term memory (LTM) to remove molecular constraints, facilitate persistent kinase activity and modulate synaptic plasticity. Calpains, a family of conserved calcium dependent cysteine proteases, modulate synaptic function through protein cleavage. We used the marine mollusk Aplysia californica to investigate the in vivo role of calpains during intermediate and long-term operant memory formation using the learning that food is inedible (LFI) paradigm. A single LFI training session, in which the animal associates a specific netted seaweed with the failure to swallow, generates short (30min), intermediate (4-6h) and long-term (24h) memory. Using the calpain inhibitors calpeptin and MDL-28170, we found that ITM requires calpain activity for induction and consolidation similar to the previously reported requirements for persistent protein kinase C activity in intermediate-term LFI memory. The induction of LTM also required calpain activity. In contrast to ITM, calpain activity was not necessary for the molecular consolidation of LTM. Surprisingly, six hours after LFI training we found that calpain activity was necessary for LTM, although this is a time at which neither persistent PKC activity nor protein synthesis is required for the maintenance of long-term LFI memory. These results demonstrate that calpains function in multiple roles in vivo during associative memory formation.

Selenoprotein K regulation of palmitoylation and calpain cleavage of ASAP2 is required for efficient FcγR-mediated phagocytosis

Effective activation of macrophages through phagocytic Fcγ receptors (FcγR) has been shown to require selenoprotein K (Selk). We set out to determine whether the FcγR-mediated uptake process itself also requires Selk and potential underlying mechanisms. Macrophages from Selk knockout (KO) mice were less efficient compared with wild-type (WT) controls in engulfing IgG-coated fluorescent beads. Using LC-MS/MS to screen for Selk-binding partners involved in FcγR-mediated phagocytosis, we identified Arf-GAP with SH3 domain, ANK repeat, and PH domain-containing protein 2 (ASAP2). Coimmunoprecipitation assays confirmed interactions between Selk and ASAP2. Selk was required for ASAP2 to be cleaved by calpain-2 within the Bin/Amphiphysin/Rvs (BAR) domain of ASAP2. BAR domains promote membrane association, which was consistent with our data showing that Selk deficiency led to retention of ASAP2 within the phagocytic cup. Because Selk was recently identified as a cofactor for the palmitoylation of certain proteins, we investigated whether ASAP2 was palmitoylated and whether this was related to its cleavage by calpain-2. Acyl/biotin exchange assays and MALDI-TOF analysis showed that cysteine-86 in ASAP2 was palmitoylated in WT, but to a much lesser extent in KO, mouse macrophages. Inhibitors of either palmitoylation or calpain-2 cleavage and rescue experiments with different versions of Selk demonstrated that Selk-dependent palmitoylation of ASAP2 leads to cleavage by calpain-2 within the BAR domain, which releases this protein from the maturing phagocytic cup. Overall, these findings identify ASAP2 as a new target of Selk-dependent palmitoylation and reveal a new mechanism regulating the efficiency of FcγR-mediated phagocytosis.

Calpains and neuronal damage in the ischemic brain: The swiss knife in synaptic injury

The excessive extracellular accumulation of glutamate in the ischemic brain leads to an overactivation of glutamate receptors with consequent excitotoxic neuronal death. Neuronal demise is largely due to a sustained activation of NMDA receptors for glutamate, with a consequent increase in the intracellular Ca(2+) concentration and activation of calcium- dependent mechanisms. Calpains are a group of Ca(2+)-dependent proteases that truncate specific proteins, and some of the cleavage products remain in the cell, although with a distinct function. Numerous studies have shown pre- and post-synaptic effects of calpains on glutamatergic and GABAergic synapses, targeting membrane- associated proteins as well as intracellular proteins. The resulting changes in the presynaptic proteome alter neurotransmitter release, while the cleavage of postsynaptic proteins affects directly or indirectly the activity of neurotransmitter receptors and downstream mechanisms. These alterations also disturb the balance between excitatory and inhibitory neurotransmission in the brain, with an impact in neuronal demise. In this review we discuss the evidence pointing to a role for calpains in the dysregulation of excitatory and inhibitory synapses in brain ischemia, at the pre- and post-synaptic levels, as well as the functional consequences. Although targeting calpain-dependent mechanisms may constitute a good therapeutic approach for stroke, specific strategies should be developed to avoid non-specific effects given the important regulatory role played by these proteases under normal physiological conditions.

The Regulation of GluN2A by Endogenous and Exogenous Regulators in the Central Nervous System

The NMDA receptor is the most widely studied ionotropic glutamate receptor, and it is central to many physiological and pathophysiological processes in the central nervous system. GluN2A is one of the two main types of GluN2 NMDA receptor subunits in the forebrain. The proper activity of GluN2A is important to brain function, as the abnormal regulation of GluN2A may induce some neuropsychiatric disorders. This review will examine the regulation of GluN2A by endogenous and exogenous regulators in the central nervous system.

Inhibiting BACE1 to reverse synaptic dysfunctions in Alzheimer's disease

Over the past two decades, many studies have identified significant contributions of toxic β-amyloid peptides (Aβ) to the etiology of Alzheimer's disease (AD), which is the most common age-dependent neurodegenerative disease. AD is also recognized as a disease of synaptic failure. Aβ, generated by sequential proteolytic cleavages of amyloid precursor protein (APP) by BACE1 and γ-secretase, is one of major culprits that cause this failure. In this review, we summarize current findings on how BACE1-cleaved APP products impact learning and memory through proteins localized on glutamatergic, GABAergic, and dopaminergic synapses. Considering the broad effects of Aβ on all three types of synapses, BACE1 inhibition emerges as a practical approach for ameliorating Aβ-mediated synaptic dysfunctions. Since BACE1 inhibitory drugs are currently in clinical trials, this review also discusses potential complications arising from BACE1 inhibition. We emphasize that the benefits of BACE1 inhibitory drugs will outweigh the concerns.

Higher levels of phosphorylated Y1472 on GluN2B subunits in the frontal cortex of aged mice are associated with good spatial reference memory, but not cognitive flexibility

The N-methyl-D-aspartate receptor (NMDAr) is particularly vulnerable to aging. The GluN2B subunit of the NMDAr, compared to other NMDAr subunits, suffers the greatest losses of expression in the aging brain, especially in the frontal cortex. While expression levels of GluN2B mRNA and protein in the aged brain are well documented, there has been little investigation into age-related posttranslational modifications of the subunit. In this study, we explored some of the mechanisms that may promote differences in the NMDAr complex in the frontal cortex of aged animals. Two ages of mice, 3 and 24 months, were behaviorally tested in the Morris water maze. The frontal cortex and hippocampus from each mouse were subjected to differential centrifugation followed by solubilization in Triton X-100. Proteins from Triton-insoluble membranes, Triton-soluble membranes, and intracellular membranes/cytosol were examined by Western blot. Higher levels of GluN2B tyrosine 1472 phosphorylation in frontal cortex synaptic fractions of old mice were associated with better reference learning but poorer cognitive flexibility. Levels of GluN2B phosphotyrosine 1336 remained steady, but there were greater levels of the calpain-induced 115 kDa GluN2B cleavage product on extrasynaptic membranes in these old good learners. There was an age-related increase in calpain activity, but it was not associated with better learning. These data highlight a unique aging change for aged mice with good spatial learning that might be detrimental to cognitive flexibility. This study also suggests that higher levels of truncated GluN2B on extrasynaptic membranes are not deleterious to spatial memory in aged mice.

Biomarkers for acute diagnosis and management of stroke in neurointensive care units

The effectiveness of current management of critically ill stroke patients depends on rapid assessment of the type of stroke, ischemic or hemorrhagic, and on a patient's general clinical status. Thrombolytic therapy with recombinant tissue plasminogen activator (r-tPA) is the only effective treatment for ischemic stroke approved by the Food and Drug Administration (FDA), whereas no treatment has been shown to be effective for hemorrhagic stroke. Furthermore, a narrow therapeutic window and fear of precipitating intracranial hemorrhage by administering r-tPA cause many clinicians to avoid using this treatment. Thus, rapid and objective assessments of stroke type at admission would increase the number of patients with ischemic stroke receiving r-tPA treatment and thereby, improve outcome for many additional stroke patients. Considerable literature suggests that brain-specific protein biomarkers of glial [i.e. S100 calcium-binding protein B (S100B), glial fibrillary acidic protein (GFAP)] and neuronal cells [e.g., ubiquitin C-terminal hydrolase-L1 (UCH-L1), neuron-specific enolase (NSE), αII-spectrin breakdown products SBDP120, SBDP145, and SBDP150, myelin basic protein (MBP), neurofilament light chain (NF-L), tau protein, visinin-like protein-1 (VLP 1), NR2 peptide] injury that could be detected in the cerebrospinal fluid (CSF) and peripheral blood might provide valuable and timely diagnostic information for stroke necessary to make prompt management and decisions, especially when the time of stroke onset cannot be determined. This information could include injury severity, prognosis of short-term and long-term outcomes, and discrimination of ischemic or hemorrhagic stroke. This chapter reviews the current status of the development of biomarker-based diagnosis of stroke and its potential application to improve stroke care.

Physiological Roles of Calpain 1 Associated to Multiprotein NMDA Receptor Complex

We have recently demonstrated that in resting conditions calpain 1, but not calpain 2, is specifically associated to the N-Methyl-D-Aspartate receptor (NMDAR) multiprotein complex. We are here reporting that in SKNBE neuroblastoma cells or in freshly isolated nerve terminals from adult rat hippocampus, the proteolytic activity of calpain 1 resident at the NMDAR is very low under basal conditions and greatly increases following NMDAR stimulation. Since the protease resides at the NMDAR in saturating amounts, variations in Ca2+ influx promote an increase in calpain 1 activity without affecting the amount of the protease originally associated to NMDAR. In all the conditions examined, resident calpain 1 specifically cleaves NR2B at the C-terminal region, leading to its internalization together with NR1 subunit. While in basal conditions intracellular membranes include small amounts of NMDAR containing the calpain-digested NR2B, upon NMDAR stimulation nearly all the receptor molecules are internalized. We here propose that resident calpain 1 is involved in NMDAR turnover, and following an increase in Ca2+ influx, the activated protease, by promoting the removal of NMDAR from the plasma membranes, can decrease Ca2+ entrance through this channel. Due to the absence of calpastatin in such cluster, the activity of resident calpain 1 may be under the control of HSP90, whose levels are directly related to the activation of this protease. Observations of different HSP90/calpain 1 ratios in different ultrasynaptic compartments support this conclusion.

FRET-FLIM investigation of PSD95-NMDA receptor interaction in dendritic spines; control by calpain, CaMKII and Src family kinase

Little is known about the changes in protein interactions inside synapses during synaptic remodeling, as their live monitoring in spines has been limited. We used a FRET-FLIM approach in developing cultured rat hippocampal neurons expressing fluorescently tagged NMDA receptor (NMDAR) and PSD95, two essential proteins in synaptic plasticity, to examine the regulation of their interaction. NMDAR stimulation caused a transient decrease in FRET between the NMDAR and PSD95 in spines of young and mature neurons. The activity of both CaMKII and calpain were essential for this effect in both developmental stages. Meanwhile, inhibition of Src family kinase (SFK) had opposing impacts on this decrease in FRET in young versus mature neurons. Our data suggest concerted roles for CaMKII, SFK and calpain activity in regulating activity-dependent separation of PSD95 from GluN2A or GluN2B. Finally, we found that calpain inhibition reduced spine growth that was caused by NMDAR activity, supporting the hypothesis that PSD95-NMDAR separation is implicated in synaptic remodeling.

Cortical parvalbumin GABAergic deficits with α7 nicotinic acetylcholine receptor deletion: implications for schizophrenia

Dysfunction of cortical parvalbumin (PV)-containing GABAergic interneurons has been implicated in cognitive deficits of schizophrenia. In humans microdeletion of the CHRNA7 (α7 nicotinic acetylcholine receptor, nAChR) gene is associated with cortical dysfunction in a broad spectrum of neurodevelopmental and neuropsychiatric disorders including schizophrenia while in mice similar deletion causes analogous abnormalities including impaired attention, working-memory and learning. However, the pathophysiological roles of α7 nAChRs in cortical PV GABAergic development remain largely uncharacterized. In both in vivo and in vitro models, we identify here that deletion of the α7 nAChR gene in mice impairs cortical PV GABAergic development and recapitulates many of the characteristic neurochemical deficits in PV-positive GABAergic interneurons found in schizophrenia. α7 nAChR null mice had decreased cortical levels of GABAergic markers including PV, glutamic acid decarboxylase 65/67 (GAD65/67) and the α1 subunit of GABAA receptors, particularly reductions of PV and GAD67 levels in cortical PV-positive interneurons during late postnatal life and adulthood. Cortical GABAergic synaptic deficits were identified in the prefrontal cortex of α7 nAChR null mice and α7 nAChR null cortical cultures. Similar disruptions in development of PV-positive GABAergic interneurons and perisomatic synapses were found in cortical cultures lacking α7 nAChRs. Moreover, NMDA receptor expression was reduced in GABAergic interneurons, implicating NMDA receptor hypofunction in GABAergic deficits in α7 nAChR null mice. Our findings thus demonstrate impaired cortical PV GABAergic development and multiple characteristic neurochemical deficits reminiscent of schizophrenia in cortical PV-positive interneurons in α7 nAChR gene deletion models. This implicates crucial roles of α7 nAChRs in cortical PV GABAergic development and dysfunction in schizophrenia and other neuropsychiatric disorders.

Deletion of CPEB3 enhances hippocampus-dependent memory via increasing expressions of PSD95 and NMDA receptors

Long-term memory requires activity-dependent synthesis of plasticity-related proteins (PRPs) to strengthen synaptic efficacy and consequently consolidate memory. Cytoplasmic polyadenylation element binding protein (CPEB)3 is a sequence-specific RNA-binding protein that regulates translation of several PRP RNAs in neurons. To understand whether CPEB3 plays a part in learning and memory, we generated CPEB3 knock-out (KO) mice and found that the null mice exhibited enhanced hippocampus-dependent, short-term fear memory in the contextual fear conditioning test and long-term spatial memory in the Morris water maze. The basal synaptic transmission of Schaffer collateral-CA1 neurons was normal but long-term depression evoked by paired-pulse low-frequency stimulation was modestly facilitated in the juvenile KO mice. Molecular and cellular characterizations revealed several molecules in regulating plasticity of glutamatergic synapses are translationally elevated in the CPEB3 KO neurons, including the scaffolding protein PSD95 and the NMDA receptors along with the known CPEB3 target, GluA1. Together, CPEB3 functions as a negative regulator to confine the strength of glutamatergic synapses by downregulating the expression of multiple PRPs and plays a role underlying certain forms of hippocampus-dependent memories.

Excitotoxicity and stroke: identifying novel targets for neuroprotection

Excitotoxicity, the specific type of neurotoxicity mediated by glutamate, may be the missing link between ischemia and neuronal death, and intervening the mechanistic steps that lead to excitotoxicity can prevent stroke damage. Interest in excitotoxicity began fifty years ago when monosodium glutamate was found to be neurotoxic. Evidence soon demonstrated that glutamate is not only the primary excitatory neurotransmitter in the adult brain, but also a critical transmitter for signaling neurons to degenerate following stroke. The finding led to a number of clinical trials that tested inhibitors of excitotoxicity in stroke patients. Glutamate exerts its function in large by activating the calcium-permeable ionotropic NMDA receptor (NMDAR), and different subpopulations of the NMDAR may generate different functional outputs, depending on the signaling proteins directly bound or indirectly coupled to its large cytoplasmic tail. Synaptic activity activates the GluN2A subunit-containing NMDAR, leading to activation of the pro-survival signaling proteins Akt, ERK, and CREB. During a brief episode of ischemia, the extracellular glutamate concentration rises abruptly, and stimulation of the GluN2B-containing NMDAR in the extrasynaptic sites triggers excitotoxic neuronal death via PTEN, cdk5, and DAPK1, which are directly bound to the NMDAR, nNOS, which is indirectly coupled to the NMDAR via PSD95, and calpain, p25, STEP, p38, JNK, and SREBP1, which are further downstream. This review aims to provide a comprehensive summary of the literature on excitotoxicity and our perspectives on how the new generation of excitotoxicity inhibitors may succeed despite the failure of the previous generation of drugs.

Cortical synaptic NMDA receptor deficits in α7 nicotinic acetylcholine receptor gene deletion models: implications for neuropsychiatric diseases

Microdeletion of the human CHRNA7 gene (α7 nicotinic acetylcholine receptor, nAChR) as well as dysfunction in N-methyl-d-aspartate receptors (NMDARs) have been associated with cortical dysfunction in a broad spectrum of neurodevelopmental and neuropsychiatric disorders including schizophrenia. However, the pathophysiological roles of synaptic vs. extrasynaptic NMDARs and their interactions with α7 nAChRs in cortical dysfunction remain largely uncharacterized. Using a combination of in vivo and in vitro models, we demonstrate that α7 nAChR gene deletion leads to specific loss of synaptic NMDARs and their coagonist, d-serine, as well as glutamatergic synaptic deficits in mouse cortex. α7 nAChR null mice had decreased cortical NMDAR expression and glutamatergic synapse formation during postnatal development. Similar reductions in NMDAR expression and glutamatergic synapse formation were revealed in cortical cultures lacking α7 nAChRs. Interestingly, synaptic, but not extrasynaptic, NMDAR currents were specifically diminished in cultured cortical pyramidal neurons as well as in acute prefrontal cortical slices of α7 nAChR null mice. Moreover, d-serine responsive synaptic NMDAR-mediated currents and levels of the d-serine synthetic enzyme serine racemase were both reduced in α7 nAChR null cortical pyramidal neurons. Our findings thus identify specific loss of synaptic NMDARs and their coagonist, d-serine, as well as glutamatergic synaptic deficits in α7 nAChR gene deletion models of cortical dysfunction, thereby implicating α7 nAChR-mediated control of synaptic NMDARs and serine racemase/d-serine pathways in cortical dysfunction underlying many neuropsychiatric and neurodevelopmental disorders, particularly those associated with deletion of human CHRNA7.

Targeting calpain in synaptic plasticity

INTRODUCTION

Calpains represent a family of neutral, calcium-dependent proteases, which modify the function of their target proteins by partial truncation. These proteases have been implicated in numerous cell functions, including cell division, proliferation, migration, and death. In the CNS, where µ-calpain and m-calpain are the main calpain isoforms, their activation has been linked to synaptic plasticity as well as to neurodegeneration. This review will focus on the role of calpains in synaptic plasticity and discuss the possibility of developing methods to manipulate calpain activity for therapeutic purposes.

AREAS COVERED

This review covers the literature showing how calpains are implicated in synaptic plasticity and in a number of conditions associated with learning impairment. The possibility of developing new drugs targeting these enzymes for treating these conditions is discussed.

EXPERT OPINION

As evidence accumulates that calpain activation participates in neurodegeneration and cancer, there is interest in developing therapeutic approaches using direct or indirect calpain inhibition. In particular, a peptide derived from the calpain truncation site of mGluR1α was shown to decrease neurodegeneration following neonatal hypoxia/ischemia. More selective approaches need to be developed to target calpain or some of its substrates for therapeutic indications associated with deregulation of synaptic plasticity.

N-methyl-D-aspartate receptor- and calpain-mediated proteolytic cleavage of K+-Cl- cotransporter-2 impairs spinal chloride homeostasis in neuropathic pain

Loss of synaptic inhibition by γ-aminobutyric acid and glycine due to potassium chloride cotransporter-2 (KCC2) down-regulation in the spinal cord is a critical mechanism of synaptic plasticity in neuropathic pain. Here we present novel evidence that peripheral nerve injury diminishes glycine-mediated inhibition and induces a depolarizing shift in the reversal potential of glycine-mediated currents (E(glycine)) in spinal dorsal horn neurons. Blocking glutamate N-methyl-D-aspartate (NMDA) receptors normalizes synaptic inhibition, E(glycine), and KCC2 by nerve injury. Strikingly, nerve injury increases calcium-dependent calpain activity in the spinal cord that in turn causes KCC2 cleavage at the C terminus. Inhibiting calpain blocks KCC2 cleavage induced by nerve injury and NMDA, thereby normalizing E(glycine). Furthermore, calpain inhibition or silencing of μ-calpain at the spinal level reduces neuropathic pain. Thus, nerve injury promotes proteolytic cleavage of KCC2 through NMDA receptor-calpain activation, resulting in disruption of chloride homeostasis and diminished synaptic inhibition in the spinal cord. Targeting calpain may represent a new strategy for restoring KCC2 levels and tonic synaptic inhibition and for treating chronic neuropathic pain.

Activity-dependent cleavage of the K-Cl cotransporter KCC2 mediated by calcium-activated protease calpain

The K-Cl cotransporter KCC2 plays a crucial role in neuronal chloride regulation. In mature central neurons, KCC2 is responsible for the low intracellular Cl(-) concentration ([Cl(-)](i)) that forms the basis for hyperpolarizing GABA(A) receptor-mediated responses. Fast changes in KCC2 function and expression have been observed under various physiological and pathophysiological conditions. Here, we show that the application of protein synthesis inhibitors cycloheximide and emetine to acute rat hippocampal slices have no effect on total KCC2 protein level and K-Cl cotransporter function. Furthermore, blocking constitutive lysosomal degradation with leupeptin did not induce significant changes in KCC2 protein levels. These findings indicate a low basal turnover rate of the total KCC2 protein pool. In the presence of the glutamate receptor agonist NMDA, the total KCC2 protein level decreased to about 30% within 4 h, and this effect was blocked by calpeptin and MDL-28170, inhibitors of the calcium-activated protease calpain. Interictal-like activity induced by incubation of hippocampal slices in an Mg(2+)-free solution led to a fast reduction in KCC2-mediated Cl(-) transport efficacy in CA1 pyramidal neurons, which was paralleled by a decrease in both total and plasmalemmal KCC2 protein. These effects were blocked by the calpain inhibitor MDL-28170. Taken together, these findings show that calpain activation leads to cleavage of KCC2, thereby modulating GABAergic signaling.

Perinatal phencyclidine treatment alters neuregulin 1/erbB4 expression and activation in later life

Schizophrenia is a complex and devastating mental disorder of unknown etiology. Hypofunction of N-methyl-D-aspartate (NMDA) receptors are implicated in the disorder, since phencyclidine (PCP) and other NMDA receptor antagonists mimic schizophrenia-like symptoms in humans and animals so well. Moreover, genetic linkage and post mortem studies strongly suggest a role for altered neuregulin 1 (Nrg1)/erbB4 signaling in schizophrenia pathology. This study investigated the relationship between the NMDA receptor and Nrg1 signaling pathways using the perinatal PCP animal model. Rats (n=5/group) were treated with PCP (10 mg/kg) or saline on postnatal days (PN) 7, 9 and 11 and were sacrificed on PN12, 5 weeks and 20 weeks for biochemical analyses. Western blotting was used to determine total and phosphorylated levels of proteins involved in NMDA receptor/Nrg1 signaling in the prefrontal cortex and hippocampus. In the cortex, PCP treatment altered Nrg1/erbB4 expression levels throughout development, including decreased Nrg1 and erbB4 at PN12 (-25-30%; p<0.05); increased erbB4 and p-erbB4 (+18-27%; p<0.01) at 5 weeks; and decreased erbB4 and p-erbB4 (-16-18%; p<0.05) along with increased Nrg1 (+33%; p<0.01) at 20 weeks. In the hippocampus, levels of Nrg1/erbB4 were largely unaffected apart from a significant decrease in p-erbB4 at 20 weeks (-13%; p<0.001); however NMDA receptor subunits and PSD-95 showed increases at PN12 and 5 weeks (+20-32%; p<0.05), and decreases at 20 weeks (-22-29%; p<0.05). This study shows that NMDA receptor antagonism early in development can have long term effects on Nrg1/erbB4 expression which could be important in understanding pathological processes which might be involved in schizophrenia.

Allopregnanolone prevents dieldrin-induced NMDA receptor internalization and neurotoxicity by preserving GABA(A) receptor function

Dieldrin is an endocrine disruptor that accumulates in mammalian adipose tissue and brain. It induces convulsions due to its antagonism of the γ-aminobutyric acid A receptor (GABA(A)R). We have previously reported that long-term exposure to dieldrin causes the internalization of the N-methyl-D-aspartate receptor (NMDAR) as a result of persistent GABA(A)R inhibition. Because the neurosteroids 17β-estradiol (E2) and allopregnanolone are known to modulate the function and trafficking of GABA(A)R and NMDAR, we examined the effects of E2 and allopregnanolone on dieldrin-induced GABA(A)R inhibition, NMDAR internalization, and neuronal death in cortical neurons. We found that 1 nM E2 increased the membrane expression of NR1/NR2B receptors and postsynaptic density 95 but did not induce their physical association. In contrast, 10 nM E2 had no effect on these proteins but reduced NR2A membrane expression. We also found that exposure to 60 nM dieldrin for 6 d in vitro caused the internalization of NR1 and NR2B but not NR2A. Treatment with either 1 nM E2 or 10 μM allopregnanolone prevented the dieldrin-induced reduction in membrane levels of the NR1/NR2B receptors. Furthermore, prolonged exposure to 200 nM dieldrin down-regulated the expression of NR2A; this was inhibited only by allopregnanolone. Although both hormones restored NMDAR function, as measured by the NMDA-induced rise in intracellular calcium, allopregnanolone (but not E2) reversed the inhibition of GABA(A)R and neuronal death caused by prolonged exposure to dieldrin. Our results indicate that allopregnanolone protects cortical neurons against the neurotoxicity caused by long-term exposure to dieldrin by maintaining GABA(A)R and NMDAR functionality.

Molecular mechanisms underlying memory consolidation of taste information in the cortex

The senses of taste and odor are both chemical senses. However, whereas an organism can detect an odor at a relatively long distance from its source, taste serves as the ultimate proximate gatekeeper of food intake: it helps in avoiding poisons and consuming beneficial substances. The automatic reaction to a given taste has been developed during evolution and is well adapted to conditions that may occur with high probability during the lifetime of an organism. However, in addition to this automatic reaction, animals can learn and remember tastes, together with their positive or negative values, with high precision and in light of minimal experience. This ability of mammalians to learn and remember tastes has been studied extensively in rodents through application of reasonably simple and well defined behavioral paradigms. The learning process follows a temporal continuum similar to those of other memories: acquisition, consolidation, retrieval, relearning, and reconsolidation. Moreover, inhibiting protein synthesis in the gustatory cortex (GC) specifically affects the consolidation phase of taste memory, i.e., the transformation of short- to long-term memory, in keeping with the general biochemical definition of memory consolidation. This review aims to present a general background of taste learning, and to focus on recent findings regarding the molecular mechanisms underlying taste–memory consolidation in the GC. Specifically, the roles of neurotransmitters, neuromodulators, immediate early genes, and translation regulation are addressed.

Neuroprotection against traumatic brain injury by a peptide derived from the collapsin response mediator protein 2 (CRMP2)

Neurological disabilities following traumatic brain injury (TBI) may be due to excitotoxic neuronal loss. The excitotoxic loss of neurons following TBI occurs largely due to hyperactivation of N-methyl-d-aspartate receptors (NMDARs), leading to toxic levels of intracellular Ca(2+). The axon guidance and outgrowth protein collapsin response mediator protein 2 (CRMP2) has been linked to NMDAR trafficking and may be involved in neuronal survival following excitotoxicity. Lentivirus-mediated CRMP2 knockdown or treatment with a CRMP2 peptide fused to HIV TAT protein (TAT-CBD3) blocked neuronal death following glutamate exposure probably via blunting toxicity from delayed calcium deregulation. Application of TAT-CBD3 attenuated postsynaptic NMDAR-mediated currents in cortical slices. In exploring modulation of NMDARs by TAT-CBD3, we found that TAT-CBD3 induced NR2B internalization in dendritic spines without altering somal NR2B surface expression. Furthermore, TAT-CBD3 reduced NMDA-mediated Ca(2+) influx and currents in cultured neurons. Systemic administration of TAT-CBD3 following a controlled cortical impact model of TBI decreased hippocampal neuronal death. These findings support TAT-CBD3 as a novel neuroprotective agent that may increase neuronal survival following injury by reducing surface expression of dendritic NR2B receptors.

Induced tolerance to glutamate neurotoxicity through down-regulation of NR2 subunits of N-methyl-D-aspartate receptors in cultured rat striatal neurons

We have previously shown differential vulnerabilities to glutamate (Glu) excitotoxicity mediated by the N-methyl-D-aspartate (NMDA) receptor (NMDAR) between rat cortical and rat hippocampal neurons in culture. In this study, we evaluated the possible induced tolerance to NMDA neurotoxicity in cultured rat striatal neurons with prior sustained activation of NMDAR. Brief exposure to Glu or NMDA for 1 hr led to a significant decrease in cellular vitality determined 24 hr later in cultured rat striatal neurons, whereas no marked loss was seen in cellular survival after exposure to Glu or NMDA in striatal neurons previously cultured with Glu or NMDA. Sustained culture with Glu or NMDA invariably led to a significant decrease in protein levels of NR2, but not NR1, subunits without affecting their mRNA levels. Similar induced tolerance was seen to the excitotoxicity of NMDA in hippocampal neurons in a manner sensitive to an NMDAR antagonist. Prior culture with NMDA induced less effective alterations in both intracellular free Ca(2+) levels and mitochondrial membrane potentials after the addition of NMDA in striatal neurons. However, calpain inhibitor-I significantly prevented the decreased NR2B and NR2C protein levels in striatal neurons cultured with NMDA. These results suggest that prior tonic activation of NMDAR would induce tolerance to the excitotoxicity mediated by NMDAR through a mechanism related to calpain-induced down-regulation of particular NR2 subunits in rat striatal neurons.

Role of the NR2A/2B subunits of the N-methyl-D-aspartate receptor in glutamate-induced glutamic acid decarboxylase alteration in cortical GABAergic neurons in vitro

The vulnerability of brain neuronal cell subpopulations to neurologic insults varies greatly. Among cells that survive a pathological insult, for example ischemia or brain trauma, some may undergo morphological and/or biochemical changes that may compromise brain function. The present study is a follow-up of our previous studies that investigated the effect of glutamate-induced excitotoxicity on the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67)'s expression in surviving DIV 11 cortical GABAergic neurons in vitro [Monnerie and Le Roux, (2007) Exp Neurol 205:367-382, (2008) Exp Neurol 213:145-153]. An N-methyl-D-aspartate receptor (NMDAR)-mediated decrease in GAD expression was found following glutamate exposure. Here we examined which NMDAR subtype(s) mediated the glutamate-induced change in GAD protein levels. Western blotting techniques on cortical neuron cultures showed that glutamate's effect on GAD proteins was not altered by NR2B-containing diheteromeric (NR1/NR2B) receptor blockade. By contrast, blockade of triheteromeric (NR1/NR2A/NR2B) receptors fully protected against a decrease in GAD protein levels following glutamate exposure. When receptor location on the postsynaptic membrane was examined, extrasynaptic NMDAR stimulation was observed to be sufficient to decrease GAD protein levels similar to that observed after glutamate bath application. Blocking diheteromeric receptors prevented glutamate's effect on GAD proteins after extrasynaptic NMDAR stimulation. Finally, NR2B subunit examination with site-specific antibodies demonstrated a glutamate-induced, calpain-mediated alteration in NR2B expression. These results suggest that glutamate-induced excitotoxic NMDAR stimulation in cultured GABAergic cortical neurons depends upon subunit composition and receptor location (synaptic vs. extrasynaptic) on the neuronal membrane. Biochemical alterations in surviving cortical GABAergic neurons in various disease states may contribute to the altered balance between excitation and inhibition that is often observed after injury.

Wnt-5a occludes Abeta oligomer-induced depression of glutamatergic transmission in hippocampal neurons

Background

Soluble amyloid-β (Aβ;) oligomers have been recognized to be early and key intermediates in Alzheimer's disease (AD)-related synaptic dysfunction. Aβ oligomers block hippocampal long-term potentiation (LTP) and impair rodent spatial memory. Wnt signaling plays an important role in neural development, including synaptic differentiation.

Results

We report here that the Wnt signaling activation prevents the synaptic damage triggered by Aβ oligomers. Electrophysiological analysis of Schaffer collaterals-CA1 glutamatergic synaptic transmission in hippocampal slices indicates that Wnt-5a increases the amplitude of field excitatory postsynaptic potentials (fEPSP) and both AMPA and NMDA components of the excitatory postsynaptic currents (EPSCs), without modifying the paired pulse facilitation (PPF). Conversely, in the presence of Aβ oligomers the fEPSP and EPSCs amplitude decreased without modification of the PPF, while the postsynaptic scaffold protein (PSD-95) decreased as well. Co-perfusion of hippocampal slices with Wnt-5a and Aβ oligomers occludes against the synaptic depression of EPSCs as well as the reduction of PSD-95 clusters induced by Aβ oligomers in neuronal cultures. Taken together these results indicate that Wnt-5a and Aβ oligomers inversely modulate postsynaptic components.

Conclusion

These results indicate that post-synaptic damage induced by Aβ oligomers in hippocampal neurons is prevented by non-canonical Wnt pathway activation.

Tyrosine phosphorylation of the 2B subunit of the NMDA receptor is necessary for taste memory formation

We aimed to test whether tyrosine phosphorylation of the NMDA receptor (NMDAR) in the insular cortex is necessary for novel taste learning. We found that in rats, novel taste learning leads to elevated phosphorylation of tyrosine 1472 of the NR2B subunit of the NMDAR and increases the interaction of phosphorylated NR2B with the major postsynaptic scaffold protein PSD-95. Injection of the tyrosine kinase inhibitor genistein directly into the insular cortex of rats before novel taste exposure prevented the increase in NR2B tyrosine phosphorylation and behaviorally attenuated taste-memory formation. Functionally, tyrosine phosphorylation of NR2B after learning was found to determine the synaptic distribution of the NMDAR, since microinjection of genistein to the insular cortex altered the distribution pattern of NMDAR caused by novel taste learning.

Differential role of N-methyl-D-aspartate receptor subunits 2A and 2B in mediating phencyclidine-induced perinatal neuronal apoptosis and behavioral deficits

The mechanism underlying phencyclidine (PCP)-induced apoptosis in perinatal rats and the development of schizophrenia-like behaviors is incompletely understood. We used antagonists for N-methyl-D-aspartate (NMDA) receptor subunit NR2A- and NR2B-containing NMDA receptor to test the hypothesis that the behavioral and apoptotic effects of PCP are mediated by blockade of NR1/NR2A-containing receptors, rather than NR1/NR2B-containing receptors. Sprague-Dawley rats were treated on PN7, PN9, and PN11 with PCP (10 mg/kg), PEAQX (NR2A-preferring antagonist; 10, 20, or 40 mg/kg), or ifenprodil (selective NR2B antagonist; 1, 5, or 10 mg/kg) and sacrificed for measurement of caspase-3 activity (an index of apoptosis) or allowed to age and tested for locomotor sensitization to PCP challenge on PN28-PN35. PCP or PEAQX on PN7, PN9, and PN11 markedly elevated caspase-3 activity in the cortex; ifenprodil showed no effect. Striatal apoptosis was evident only after subchronic treatment with a high dose of PEAQX (20 mg/kg). Animals treated with PCP or PEAQX on PN7, PN9, and PN11 showed a sensitized locomotor response to PCP challenge on PN28-PN35. Ifenprodil treatment had no effect on either measure. Therefore, PCP blockade of cortical NR1/NR2A, rather than NR1/NR2B, appears to be responsible for PCP-induced apoptosis and the development of long-lasting behavioral deficits.

Calpain and the glutamatergic synapse

Calpain is a ubiquitous protease found in different tissue types and in many organisms including mammals. It generally does not destroy its large variety of substrates, but more commonly disrupts their function. In neurons, many of its substrates become dysregulated as a result of cleavage of their regulatory domain by this protease, leading to altered signaling between cells. In glutamatergic synaptic transmission, direct targets of calpain include all of the major glutamate receptors: NMDA receptors, AMPA receptors and mGluR. By cleaving these receptors and associated intracellular proteins, calpain may regulate the physiology at glutamatergic synapses. As a result, calpain-mediated cleavage in neurons might not only be involved in pathological events like excitotoxicity, but may also have neuroprotective effects and roles in physiological synaptic transmission.

A novel protein complex in membrane rafts linking the NR2B glutamate receptor and autophagy is disrupted following traumatic brain injury

Hyperactivation of N-methyl-D-aspartate receptors (NRs) is associated with neuronal cell death induced by traumatic brain injury (TBI) and many neurodegenerative conditions. NR signaling efficiency is dependent on receptor localization in membrane raft microdomains. Recently, excitotoxicity has been linked to autophagy, but mechanisms governing signal transduction remain unclear. Here we have identified protein interactions between NR2B signaling intermediates and the autophagic protein Beclin-1 in membrane rafts of the normal rat cerebral cortex. Moderate TBI induced rapid recruitment and association of NR2B and pCaMKII to membrane rafts, and translocation of Beclin-1 out of membrane microdomains. Furthermore, TBI caused significant increases in expression of key autophagic proteins and morphological hallmarks of autophagy that were significantly attenuated by treatment with the NR2B antagonist Ro 25-6981. Thus, stimulation of autophagy by NR2B signaling may be regulated by redistribution of Beclin-1 in membrane rafts after TBI.

Polyglutamine-modulated striatal calpain activity in YAC transgenic huntington disease mouse model: impact on NMDA receptor function and toxicity

Huntington disease (HD), caused by CAG expansion in the ubiquitously expressed huntingtin gene, is characterized by early dysfunction and death of striatal medium-sized spiny neurons (MSNs). Previous work has shown MSN-specific alterations in NMDA receptor (NMDAR) expression and cell death signaling. Furthermore, studies in HD human brain tissue and a knock-in mouse model demonstrate increases in calpain activity, which can be stimulated by NMDARs and contribute to excitotoxicity. Here, we report increased calpain activity in MSNs from the yeast artificial chromosome (YAC) transgenic mouse model of HD, expressing human full-length huntingtin with 128 polyglutamine repeats (YAC128), compared with wild type. Moreover, the calpain-cleaved product of NMDAR subunit NR2B is increased early, and NR2B expression levels are reduced, in YAC128 striatum. Although steady-state NMDAR surface expression is similar in wild-type and YAC128 MSNs, the rate of loss of NR2B-containing surface receptors is enhanced in YAC128 MSNs, suggesting that NMDAR forward trafficking to the surface is also faster, as previously reported for YAC72 MSNs. Calpain inhibitor-1 treatment normalized the loss rate of surface NMDARs in YAC128 MSNs to that of wild type, and significantly increased surface NMDAR expression in YAC128, but not in wild type or YAC72. With acute NMDAR overstimulation, the increase in calpain activity correlated with polyglutamine length, and calpain inhibitor treatment reduced NMDA-induced apoptosis in YAC72 and YAC128 MSNs to wild-type levels. Thus, the cumulative effect of increasing huntingtin polyglutamine length is to enhance MSN sensitivity to excitotoxicity at least in part by calpain-mediated cell death signaling.

Calpain-mediated signaling mechanisms in neuronal injury and neurodegeneration

Calpain is a ubiquitous calcium-sensitive protease that is essential for normal physiologic neuronal function. However, alterations in calcium homeostasis lead to persistent, pathologic activation of calpain in a number of neurodegenerative diseases. Pathologic activation of calpain results in the cleavage of a number of neuronal substrates that negatively affect neuronal structure and function, leading to inhibition of essential neuronal survival mechanisms. In this review, we examine the mechanistic underpinnings of calcium dysregulation resulting in calpain activation in the acute neurodegenerative diseases such as cerebral ischemia and in the chronic neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, prion-related encephalopathy, and amylotrophic lateral sclerosis. The premise of this paper is that analysis of the signaling and transcriptional consequences of calpain-mediated cleavage of its various substrates for any neurodegenerative disease can be extrapolated to all of the neurodegenerative diseases vulnerable to calcium dysregulation.

Postsynaptic density-95 (PSD-95) and calcineurin control the sensitivity of N-methyl-D-aspartate receptors to calpain cleavage in cortical neurons

The N-methyl-D-aspartate receptor (NMDAR) is a Ca(2+)-permeable glutamate receptor mediating many neuronal functions under normal and pathological conditions. Ca(2+) influx via NMDARs activates diverse intracellular targets, including Ca(2+)-dependent protease calpain. Biochemical studies suggest that NR2A and NR2B subunits of NMDARs are substrates of calpain. Our physiological data showed that calpain, activated by prolonged NMDA treatment (100 microM, 5 min) of cultured cortical neurons, irreversibly decreased the whole-cell currents mediated by extrasynaptic NMDARs. Animals exposed to transient forebrain ischemia, a condition that activates calpain, exhibited the reduced NMDAR current density and the lower full-length NR2A/B level in a calpain-dependent manner. Disruption of the association between NMDARs and the scaffolding protein postsynaptic density (PSD)-95 facilitated the calpain regulation of synaptic NMDAR responses and NR2 cleavage in cortical slices, whereas inhibition of calcineurin activity blocked the calpain effect on NMDAR currents and NR2 cleavage. Calpain-cleaved NR2B subunits were removed from the cell surface. Moreover, cell viability assays showed that calpain, by targeting NMDARs, provided a negative feedback to dampen neuronal excitability in excitotoxic conditions. These data suggest that calpain activation suppresses NMDAR function via proteolytic cleavage of NR2 subunits in vitro and in vivo, and the susceptibility of NMDARs to calpain cleavage is controlled by PSD-95 and calcineurin.

Mechanistic role of calpains in postischemic neurodegeneration

The calpain family of proteases is causally linked to postischemic neurodegeneration. However, the precise mechanisms by which calpains contribute to postischemic neuronal death have not been fully elucidated. This review outlines the key features of the calpain system, and the evidence for its causal role in postischemic neuronal pathology. Furthermore, the consequences of specific calpain substrate cleavage at various subcellular locations are explored. Calpain substrates within synapses, plasma membrane, endoplasmic reticulum, lysosomes, mitochondria, and the nucleus, as well as the overall effect of postischemic calpain activity on calcium regulation and cell death signaling are considered. Finally, potential pathways for calpain-mediated neurodegeneration are outlined in an effort to guide future studies aimed at understanding the downstream pathology of postischemic calpain activity and identifying optimal therapeutic strategies.