Transient forebrain ischemia effects interaction of Src, FAK, and PYK2 with the NR2B subunit of N-methyl-d-aspartate receptor in gerbil hippocampus

Selective vulnerability of hippocampal CA1 and CA3 pyramidal cells: What are possible pathomechanisms and should more attention be paid to the CA3 region in future studies?

Transient ischemia and reperfusion selectively damage neurons in brain, with hippocampal pyramidal cells being particularly vulnerable. Even within hippocampus, heterogeneous susceptibility is evident, with higher vulnerability of CA1 versus CA3 neurons described for several decades. Therefore, numerous studies have focused exclusively on CA1. Pediatric cardiac surgery is increasingly focusing on studies of hippocampal structures, and a negative impact of cardiopulmonary bypass on the hippocampus cannot be denied. Recent studies show a shift in selective vulnerability from neurons of CA1 to CA3. This review shows that cell damage is increased in CA3, sometimes stronger than in CA1, depending on several factors (method, species, age, observation period). Despite a highly variable pattern, several markers illustrate greater damage to CA3 neurons than previously assumed. Nevertheless, the underlying cellular mechanisms have not been fully deciphered to date. The complexity is reflected in possible pathomechanisms discussed here, with numerous factors (NMDA, kainate and AMPA receptors, intrinsic oxidative stress potential and various radicals, AKT isoforms, differences in vascular architecture, ratio of pro- and anti-apoptotic Bcl-2 factors, vulnerability of interneurons, mitochondrial dysregulation) contributing to either enhanced CA1 or CA3 vulnerability. Furthermore, differences in expressed genome, proteome, metabolome, and transcriptome in CA1 and CA3 appear to influence differential behavior after damaging stimuli, thus metabolomics-, transcriptomics-, and proteomics-based analyses represent a viable option to identify pathways of selective vulnerability in hippocampal neurons. These results emphasize that future studies should focus on the CA3 field in addition to CA1, especially with regard to improving therapeutic strategies after ischemic/hypoxic brain injury.

Biliverdin reductase bridges focal adhesion kinase to Src to modulate synaptic signaling

Synapses connect discrete neurons into vast networks that send, receive, and encode diverse forms of information. Synaptic function and plasticity, the neuronal process of adapting to diverse and variable inputs, depend on the dynamic nature of synaptic molecular components, which is mediated in part by cell adhesion signaling pathways. Here, we found that the enzyme biliverdin reductase (BVR) physically links together key focal adhesion signaling molecules at the synapse. BVR-null (BVR-/-) mice exhibited substantial deficits in learning and memory on neurocognitive tests, and hippocampal slices in which BVR was postsynaptically depleted showed deficits in electrophysiological responses to stimuli. RNA sequencing, biochemistry, and pathway analyses suggested that these deficits were mediated through the loss of focal adhesion signaling at both the transcriptional and biochemical level in the hippocampus. Independently of its catalytic function, BVR acted as a bridge between the primary focal adhesion signaling kinases FAK and Pyk2 and the effector kinase Src. Without BVR, FAK and Pyk2 did not bind to and stimulate Src, which then did not phosphorylate the N-methyl-d-aspartate (NMDA) receptor, a critical posttranslational modification for synaptic plasticity. Src itself is a molecular hub on which many signaling pathways converge to stimulate NMDAR-mediated neurotransmission, thus positioning BVR at a prominent intersection of synaptic signaling.

Hippocampal Sector-Specific Metabolic Profiles Reflect Endogenous Strategy for Ischemia-Reperfusion Insult Resistance

The gerbil is a well-known model for studying cerebral ischemia. The CA1 of the hippocampus is vulnerable to 5 min of ischemia, while the CA2–4 and dentate gyrus (DG) are resistant to it. Short-lasting ischemia, a model of transient ischemic attacks in men, results in CA1 neuron death within 2–4 days of reperfusion. Untargeted metabolomics, using LC-QTOF-MS, was used to enrich the knowledge about intrinsic vulnerability and resistance of hippocampal regions and their early post-ischemic response (IR). In total, 30 significant metabolites were detected. In controls, taurine was significantly lower and guanosine monophosphate was higher in CA1, as compared to that in CA2–4,DG. LysoPG and LysoPE were more abundant in CA1, while LysoPI 18:0 was detected only in CA2–4,DG. After IR, a substantial decrease in the citric acid level in CA1, an accumulation of pipecolic acid in both regions, and opposite changes in the amount of PE and LysoPE were observed. The following metabolic pathways were identified as being differentially active in control CA1 vs. CA2–4,DG: metabolism of taurine and hypotaurine, glycerophospholipid, and purine. These results may indicate that a regulation of cell volume, altered structure of cell membranes, and energy metabolism differentiate hippocampal regions. Early post-ischemia, spatial differences in the metabolism of aminoacyl-tRNA biosynthesis, and amino acids and their metabolites with a predominance of those which upkeep their well-being in CA2–4,DG are shown. Presented results are consistent with genetic, morphological, and functional data, which may be useful in further study on endogenous mechanisms of neuroprotection and search for new targets for therapeutic interventions.

An Overview on the Differential Interplay Among Neurons-Astrocytes-Microglia in CA1 and CA3 Hippocampus in Hypoxia/Ischemia

Neurons have been long regarded as the basic functional cells of the brain, whereas astrocytes and microglia have been regarded only as elements of support. However, proper intercommunication among neurons-astrocytes-microglia is of fundamental importance for the functional organization of the brain. Perturbation in the regulation of brain energy metabolism not only in neurons but also in astrocytes and microglia may be one of the pathophysiological mechanisms of neurodegeneration, especially in hypoxia/ischemia. Glial activation has long been considered detrimental for survival of neurons, but recently it appears that glial responses to an insult are not equal but vary in different brain areas. In this review, we first take into consideration the modifications of the vascular unit of the glymphatic system and glial metabolism in hypoxic conditions. Using the method of triple-labeling fluorescent immunohistochemistry coupled with confocal microscopy (TIC), we recently studied the interplay among neurons, astrocytes, and microglia in chronic brain hypoperfusion. We evaluated the quantitative and morpho-functional alterations of the neuron-astrocyte-microglia triads comparing the hippocampal CA1 area, more vulnerable to ischemia, to the CA3 area, less vulnerable. In these contiguous and interconnected areas, in the same experimental hypoxic conditions, astrocytes and microglia show differential, finely regulated, region-specific reactivities. In both areas, astrocytes and microglia form triad clusters with apoptotic, degenerating neurons. In the neuron-astrocyte-microglia triads, the cell body of a damaged neuron is infiltrated and bisected by branches of astrocyte that create a microscar around it while a microglial cell phagocytoses the damaged neuron. These coordinated actions are consistent with the scavenging and protective activities of microglia. In hypoxia, the neuron-astrocyte-microglia triads are more numerous in CA3 than in CA1, further indicating their protective effects. These data, taken from contiguous and interconnected hippocampal areas, demonstrate that glial response to the same hypoxic insult is not equal but varies significantly. Understanding the differences of glial reactivity is of great interest to explain the differential susceptibility of hippocampal areas to hypoxia/ischemia. Further studies may evidence the differential reactivity of glia in different brain areas, explaining the higher or lower sensitivity of these areas to different insults and whether glia may represent a target for future therapeutic interventions.

The role of non-receptor protein tyrosine kinases in the excitotoxicity induced by the overactivation of NMDA receptors

Protein tyrosine phosphorylation is one of the primary modes of regulation of N-methyl-d-aspartate (NMDA) receptors. The non-receptor tyrosine kinases are one of the two types of protein tyrosine kinases that are involved in this process. The overactivation of NMDA receptors is a primary reason for neuron death following cerebral ischemia. Many studies have illustrated the important role of non-receptor tyrosine kinases in ischemia insults. This review introduces the roles of Src, Fyn, focal adhesion kinase, and proline-rich tyrosine kinase 2 in the excitotoxicity induced by the overactivation of NMDA receptors following cerebral ischemia.

Protective Effects of Enriched Environment Against Transient Cerebral Ischemia-Induced Impairment of Passive Avoidance Memory and Long-Term Potentiation in Rats

INTRODUCTION

Enriched Environment (EE), a complex novel environment, has been demonstrated to improve synaptic plasticity in both injured and intact animals. The present study investigated the capacity of an early environmental intervention to normalize the impairment of passive avoidance memory and Long-Term Potentiation (LTP) induced by transient bilateral common carotid artery occlusion (2-vessel occlusion, 2VO) in rats.

METHODS

After weaning, young Wistar rats (22 days old) were housed in EE or Standard Environment (SE) for 40 days. Transient (30-min) incomplete forebrain ischemia was induced 4 days before the passive avoidance memory test and LTP induction.

RESULTS

The transient forebrain ischemia led to impairment of passive avoidance memory and LTP induction in the Perforant Path-Dentate Gyrus (PP-DG) synapses. Interestingly, housing and growing in EE prior to 2VO was found to significantly reverse 2VO-induced cognitive and LTP impairments.

CONCLUSION

Our results suggest that early housing and growing in EE exhibits therapeutic potential to normalize cognitive and LTP abnormalities induced by 2VO ischemic model in rats.

Imipramine administration induces changes in the phosphorylation of FAK and PYK2 and modulates signaling pathways related to their activity

BACKGROUND

Antidepressants can modify neuronal functioning by affecting many levels of signal transduction pathways that are involved in neuroplasticity. We investigated whether the phosphorylation status of focal adhesion kinase (FAK/PTK2) and its homolog, PYK2/PTK2B, and their complex with the downstream effectors (Src kinase, p130Cas, and paxillin) are affected by administration of the antidepressant drug, imipramine. The treatment influence on the levels of ERK1/2 kinases and their phosphorylated forms (pERK1/2) or the Gαq, Gα11 and Gα12 proteins were also assessed.

METHODS

Rats were injected with imipramine (10 mg/kg, twice daily) for 21 days. The levels of proteins investigated in their prefrontal cortices were measured by Western blotting.

RESULTS

Imipramine induced contrasting changes in the phosphorylation of FAK and PYK2 at Tyr397 and Tyr402, respectively. The decreased FAK phosphorylation and increased PYK2 phosphorylation were reflected by changes in the levels of their complex with Src and p130Cas, which was observed predominantly after chronic imipramine treatment. Similarly only chronic imipramine decreased the Gαq expression while Gα11 and Gα12 proteins were untouched. Acute and chronic treatment with imipramine elevated ERK1 and ERK2 total protein levels, whereas only the pERK1 was significantly affected by the drug.

CONCLUSION

The enhanced activation of PYK2 observed here could function as compensation for FAK inhibition.

GENERAL SIGNIFICANCE

These data demonstrate that treatment with imipramine, which is a routine in counteracting depressive disorders, enhances the phosphorylation of PYK2, a non-receptor kinase instrumental in promoting synaptic plasticity. This effect documents as yet not considered target in the mechanism of imipramine action.

FAK/Src family of kinases: protective or aggravating factor for ischemia reperfusion injury in nervous system?

INTRODUCTION

The focal adhesion kinase (FAK) and the Src families of kinases are subfamilies of the non-receptor protein tyrosine kinases. FAK activity is regulated by gene amplification, alternative splicing and phosporylation/dephosphorylation. FAK/Src complex has been found to participate through various pathways in neuronal models of ischemia-reperfusion injury (IRI) with conflicting results. The aim of the present review is to summarize the currently available data on this subject.

AREAS COVERED

The MEDLINE/PubMed database was searched for publications with the medical subject heading IRI and FAK and/or Src, nervous system. We restricted our search till 2014. We identified 93 articles that were available in English as abstracts or/and full-text articles that were deemed appropriate for our review.

EXPERT OPINION

FAK has been found to have a beneficial preconditioning effect on IRI through activation via the protein kinase C (PKC) pathway by anesthetic agents. Of great importance are the interactions between FAK/Src and VEGF that has been already detected as a protective mean for IRI. The effect of VEGF administration might depend on dose as well as on time of administration. A Ca(2+)/calmodulin-dependent protein kinase II or PKC inhibitors seem to have protective effects on IRI by inhibiting ion channels activation.

Tau protein kinases: involvement in Alzheimer's disease

Tau phosphorylation is regulated by a balance between tau kinase and phosphatase activities. Disruption of this equilibrium was suggested to be at the origin of abnormal tau phosphorylation and thereby might contribute to tau aggregation. Thus, understanding the regulation modes of tau phosphorylation is of high interest in determining the possible causes at the origin of the formation of tau aggregates in order to elaborate protection strategies to cope with these lesions in Alzheimer's disease. Among the possible and specific interventions that reverse tau phosphorylation is the inhibition of certain tau kinases. Here, we extensively reviewed tau protein kinases, their physiological roles and regulation, their involvement in tau phosphorylation and their relevance to AD. We also reviewed the most common inhibitory compounds acting on each tau kinase.

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.

Increases of antioxidants are related to more delayed neuronal death in the hippocampal CA1 region of the young gerbil induced by transient cerebral ischemia

In age-related studies, young animals are resistant to ischemic damage. In present study, we investigated the neuronal death of pyramidal neurons and compared changes in the immunoreactivities and levels of antioxidants, Cu/Zn-SOD (SOD1), Mn-SOD (SOD2), catalase (CAT) and glutathione peroxidase (Gpx), in the hippocampal CA1 region between adult and young gerbils after 5 min of transient cerebral ischemia. In the adult ischemia-group, only a few (12%) of CA1 pyramidal neurons survived 4 days after ischemia-reperfusion (I-R); however, in the 4 days after I-R the young group, most of CA1 pyramidal neurons survived. Seven days after I-R, many (about 39%) of CA1 pyramidal neurons survived, thereafter, the neuronal death in the CA1 pyramidal neurons was not significantly changed. The immunoreactivities of all the antioxidants were well detected in CA1 pyramidal neurons in the adult sham-groups; in the young sham-groups, they were distinctively low compared to those in the adult sham-group. Four days after I-R in the adult group, all the immunoreactivities in the pyramidal neurons were dramatically deceased. However, at this time after I-R in the young groups, they were dramatically increased in the pyramidal neurons. From 7 days after I-R, all the immunoreactivities in the pyramidal neurons in the young ischemia-groups were distinctively decreased. In addition, the levels of all the antioxidants in the CA1 region of the young sham-groups were lower than those in the adult sham-group. Four days after I-R in the adult groups, the levels of all the antioxidants were dramatically deceased; however, at this time in the young ischemia-groups, they were distinctively increased in the CA1 region. Seven days after I-R, all the antioxidants levels in the CA1 region were distinctively decreased. In brief, we conclude that the increased antioxidants levels were related to a less and much delayed neuronal death in the CA1 pyramidal neurons in the young group following I-R injury.

Mechanisms of HIV-tat-induced phosphorylation of N-methyl-D-aspartate receptor subunit 2A in human primary neurons: implications for neuroAIDS pathogenesis

HIV infection of the central nervous system results in neurological dysfunction in a large number of individuals. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV-infected neurons. Neuronal damage and dropout must therefore be due to indirect effects of HIV infection of other central nervous system cells through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivator, tat. We previously demonstrated that HIV-tat-induced apoptosis in human primary neurons is dependent on N-methyl-D-aspartate receptor (NMDAR) activity. NMDAR activity is regulated by various mechanisms including NMDAR phosphorylation, which may lead to neuronal dysfunction and apoptosis in pathological conditions. We now demonstrate that tat treatment of human neurons results in tyrosine (Y) phosphorylation of the NMDAR subunit 2A (NR2A) in a src kinase-dependent manner. In vitro kinase assays and in vivo data indicated that NR2A Y1184, Y1325, and Y1425 are phosphorylated. Tat treatment of neuronal cultures enhanced phosphorylation of NR2A Y1325, indicating that this site is tat sensitive. Human brain tissue sections from HIV-infected individuals with encephalitis showed an increased phosphorylation of NR2A Y1325 in neurons as compared with uninfected and HIV-infected individuals without encephalitis. These findings suggest new avenues of treatment for HIV-associated cognitive impairment.

Neuroprotective profile of novel SRC kinase inhibitors in rodent models of cerebral ischemia

Src kinase signaling has been implicated in multiple mechanisms of ischemic injury, including vascular endothelial growth factor (VEGF)-mediated vascular permeability that leads to vasogenic edema, a major clinical complication in stroke and brain trauma. Here we report the effects of two novel Src kinase inhibitors, 4-[(2,4-dichloro-5-methoxyphenyl)amino]-6-methoxy-7-[3-(4-methyl-1-piperazinyl)propoxy]-3-quinolinecarbonitrile (SKI-606) and 4-[(2,4-dichloro-5-methoxyphenyl)amino]-6-methoxy-7-[4-(4-methypiperazin-1-yl)but-1-ynyl]-3-quinolinecarbonitrile (SKS-927), on ischemia-induced brain infarction and short- and long-term neurological deficits. Two well established transient [transient middle cerebral artery occlusion (tMCAO)] and permanent [permanent middle cerebral artery occlusion (pMCAO)] focal ischemia models in the rat were used with drug treatments initiated up to 6 h after onset of stroke to mimic the clinical scenario. Brain penetration of Src inhibitors, their effect on blood-brain barrier integrity and VEGF signaling in human endothelial cells were also evaluated. Our results demonstrate that both agents potently block VEGF-mediated signaling in human endothelial cells, penetrate rat brain upon systemic administration, and inhibit postischemic Src activation and vascular leakage. Treatment with SKI-606 or SKS-927 (at the doses of 3-30 mg/kg i.v.) resulted in a dose-dependent reduction in infarct volume and robust protection from neurological impairments even when the therapy was initiated up to 4- to 6-h after tMCAO. Src blockade after pMCAO resulted in accelerated improvement in recovery from motor, sensory, and reflex deficits during a long-term (3 weeks) testing period poststroke. These data demonstrate that the novel Src kinase inhibitors provide effective treatment against ischemic conditions within a clinically relevant therapeutic window and may constitute a viable therapy for acute stroke.

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.

Oxaloacetate restores the long-term potentiation impaired in rat hippocampus CA1 region by 2-vessel occlusion

Various acute brain pathological conditions are characterized by the presence of elevated glutamate concentrations in the brain interstitial fluids. It has been established that a decrease in the blood glutamate level enhances the brain-to-blood efflux of glutamate, removal of which from the brain may prevent glutamate excitotoxicity and its contribution to the long-lasting neurological deficits seen in stroke. A decrease in blood glutamate level can be achieved by exploiting the glutamate-scavenging properties of the blood-resident enzyme glutamate-oxaloacetate transaminase, which transforms glutamate into 2-ketoglutarate in the presence of the glutamate co-substrate oxaloacetate. The present study had the aim of an evaluation of the effects of the blood glutamate scavenger oxaloacetate on the impaired long-term potentiation (LTP) induced in the 2-vessel occlusion ischaemic model in rat. Transient (30-min) incomplete forebrain ischaemia was produced 72 h before LTP induction. Although the short transient brain hypoperfusion did not induce histologically identifiable injuries in the CA1 region (Fluoro-Jade B, S-100 and cresyl violet), it resulted in an impaired LTP function in the hippocampal CA1 region without damaging the basal synaptic transmission between the Schaffer collaterals and the pyramidal neurons. This impairment could be fended off in a dose-dependent manner by the intravenous administration of oxaloacetate in saline (at doses between 1.5 mmol and 0.1 mumol) immediately after the transient hypoperfusion. Our results suggest that oxaloacetate-mediated blood and brain glutamate scavenging contributes to the restoration of the LTP after its impairment by brain ischaemia.

Extracellular signal-regulated kinase activation during renal ischemia/reperfusion mediates focal adhesion dissolution and renal injury

Acute renal failure due to ischemia/reperfusion involves disruption of integrin-mediated cellular adhesion and activation of the extracellular signal-regulated kinase (ERK) pathway. The dynamics of focal adhesion organization and phosphorylation during ischemia/reperfusion in relation to ERK activation are unknown. In control kidneys, protein tyrosine-rich focal adhesions, containing focal adhesion kinase, paxillin, and talin, were present at the basolateral membrane of tubular cells and colocalized with short F-actin stress fibers. Unilateral renal ischemia/reperfusion caused a reversible protein dephosphorylation and loss of focal adhesions. The focal adhesion protein phosphorylation rebounded in a biphasic manner, in association with increased focal adhesion kinase, Src, and paxillin tyrosine phosphorylation. Preceding phosphorylation of these focal adhesion proteins, reperfusion caused increased phosphorylation of ERK. The specific mitogen-activated protein kinase kinase 1/2 inhibitor U0126 prevented ERK activation and attenuated focal adhesion kinase, paxillin, and Src phosphorylation, focal adhesion restructuring, and ischemia/reperfusion-induced renal injury. We propose a model whereby ERK activation enhanced protein tyrosine phosphorylation during ischemia/reperfusion, thereby driving the dynamic dissolution and restructuring of focal adhesions and F-actin cytoskeleton during reperfusion and renal injury.

Neurosteroid estradiol rescues ischemia-induced deficit in the long-term potentiation of rat hippocampal CA1 neurons

Increasing evidence indicates that neurosteroid 17beta-Estradiol (E2), a type of female sex hormone, has a neuroprotective effect against cerebral injury. However, it remains unknown whether E2 can also protect the hippocampal CA1 neurons from functional deficits in synaptic transmission and plasticity caused by ischemia. To address this issue, adult male Wistar rats were subjected to mild global cerebral ischemia created by four-vessel occlusion (4VO) for 10min, and the effects of E2 administration against the ischemic injury were investigated. The electrophysiological properties of Schaffer collateral-CA1 synapses were examined 7days after ischemia by applying a real-time optical recording technique to the hippocampal slices stained with a voltage-sensitive dye (RH482). The ischemic brain showed a decreased basal synaptic transmission and an impairment of LTP induction, but no alteration in paired-pulse facilitation. The administration of E2 (1mg/kg) 3h before ischemia was able to protect CA1 neurons from these ischemia-induced synaptic dysfunctions. The estrogen receptor-alpha (ERalpha) selective agonist, propyl pyrazole triol (PPT, 2mg/kg), exerted a similar protective effect, but the estrogen receptor-beta (ERbeta) agonist, diarylpropiolnitrile (DPN, 8mg/kg), failed to do so. A histological examination revealed that the transient global cerebral ischemia markedly reduced the density of pyramidal neurons in the CA1 region. The cell loss was significantly attenuated by E2 and PPT but not by DPN, as observed in synaptic functions. These findings suggest that E2 can protect neurons not only from cell death but also from functional damages due to a relatively mild degree of transient cerebral ischemia, and this effect is mediated by ERalpha, but not by ERbeta.

Dehydroepiandrosterone sulfate prevents ischemia-induced impairment of long-term potentiation in rat hippocampal CA1 by up-regulating tyrosine phosphorylation of NMDA receptor

We have reported that dehydroepiandrosterone sulfate (DHEAS) reduces the threshold for long-term potentiation (LTP) in Shaffer collateral-CA1 synapses through the amplification of Src-dependent NMDA receptor signaling. The present study is a follow-up of the above reports, aiming at evaluating the effects of DHEAS on the impaired LTP in reversible forebrain ischemic rats. Transient (20 min) incomplete forebrain ischemia led to an impaired LTP in the hippocampal CA1 region without damages to the basal synaptic transmission between the Shaffer collaterals and pyramidal neurons. Repetitive administrations of DHEAS (20 mg/kg for 3 days) from the first 3 h of reperfusion, but not acute DHEAS application (50 microM), prevent the impairment of LTP produced by ischemia. Co-administration of the specific sigma(1) receptor antagonist NE100 with DHEAS completely prevented the protective effect of DHEAS. In contrast, progesterone (PRGO) not only had no protective effect against the ischemic LTP impairment, but also attenuated the protective effect of DHEAS on the impaired LTP. Tyrosine phosphorylation of NMDA receptor subunit 2B (NR2B) significantly decreased after ischemia, whereas that of NR1 had no obvious change. Furthermore, the repetitive administration of DHEAS improved the reduction in tyrosine phosphorylation of NR2B. These findings suggest that the repetitive activation of sigma(1) receptor induced by DHEAS might prevent the ischemic LTP impairment through regulating the tyrosine phosphorylation of NR2B.

NMDA receptors and the differential ischemic vulnerability of hippocampal neurons

Transient cerebral ischemia causes an inhomogeneous pattern of cell death in the brain. We investigated mechanisms, which may underlie the greater susceptibility of hippocampal CA1 vs. CA3 pyramidal cells to ischemic insult. Using an in vitro oxygen-glucose deprivation (OGD) model of ischemia, we found that N-methyl-D-aspartate (NMDA) responses were enhanced in the more susceptible CA1 pyramidal cells and transiently depressed in the resistant CA3 pyramidal cells. The long-lasting potentiation of NMDA responses in CA1 cells was associated with delayed cell death and was prevented by blocking tyrosine kinase-dependent up-regulation of NMDA receptor function. In CA3 cells, the energy deprivation-induced transient depression of NMDA responses was converted to potentiation by blocking protein phosphatase signalling. These results suggest that energy deprivation differentially shifts the intracellular equilibrium between the tyrosine kinase and phosphatase activities that modulate NMDA responses in CA1 and CA3 pyramidal cells. Therapeutic modulation of tyrosine phosphorylation may thus prove beneficial in mitigating ischemia-induced neuronal death in vulnerable brain areas.

FAK and PYK2 interact with SAP90/PSD-95-Associated Protein-3

Focal adhesion kinase (FAK) and proline-rich tyrosine kinase 2 (PYK2) are two related non-receptor tyrosine kinases highly expressed in brain. Although they are both involved in synaptic plasticity, little is known about their specific neuronal partners. Using a yeast two-hybrid screen and GST pull-down assays we show that SAPAP3 (SAP90/PSD-95-Associated Protein-3) interacts with FAK (residues 676-840) and PYK2. The three proteins partly co-distribute in the same sucrose gradient fractions as the post-synaptic density protein PSD-95 and Src. Our results suggest that SAPAP3 is an anchoring protein for FAK and PYK2 in post-synaptic densities and may contribute to the synaptic function of these tyrosine kinases.