Postsynaptic clustering and activation of Pyk2 by PSD-95

Inhibition of a novel specific neuroglial integrin signaling pathway increases STAT3-mediated CNTF expression

Background

Ciliary neurotrophic factor (CNTF) expression is repressed in astrocytes by neuronal contact in the CNS and is rapidly induced by injury. Here, we defined an inhibitory integrin signaling pathway.

Results

The integrin substrates laminin, fibronectin and vitronectin, but not collagen, thrombospondin or fibrinogen, reduced CNTF expression in C6 astroglioma cells. Antibodies against αv and β5, but not α6 or β1, integrin induced CNTF. Together, the ligand and antibody specificity suggests that CNTF is repressed by αvβ5 integrin. Antibodies against Thy1, an abundant neuronal surface protein whose function is unclear, induced CNTF in neuron-astrocyte co-cultures indicating that it is a neuroglial CNTF repressor. Inhibition of the integrin signaling molecule Focal Adhesion Kinase (FAK) or the downstream c-Jun N-terminal kinase (JNK), but not extracellular regulated kinase (ERK) or p38 MAPK, greatly induced CNTF mRNA and protein expression within 4 hours. This selective inhibitory pathway phosphorylated STAT3 on its inhibitory ser-727 residue interfering with activity of the pro-transcription Tyr-705 residue. STAT3 can activate CNTF transcription because it bound to its promoter and FAK antagonist-induced CNTF was reduced by blocking STAT3. Microinjection of FAK inhibitor directly into the brain or spinal cord in adult mice rapidly induced CNTF mRNA and protein expression. Importantly, systemic treatment with FAK inhibitors over 3 days induced CNTF in the subventricular zone and increased neurogenesis.

Conclusions

Neuron-astroglia contact mediated by integrins serves as a sensor to enable rapid neurotrophic responses and provides a new pharmacological avenue to exploit the neuroprotective properties of endogenous CNTF.

The DAO gene is associated with schizophrenia and interacts with other genes in the Taiwan Han Chinese population

BACKGROUND

Schizophrenia is a highly heritable disease with a polygenic mode of inheritance. Many studies have contributed to our understanding of the genetic underpinnings of schizophrenia, but little is known about how interactions among genes affect the risk of schizophrenia. This study aimed to assess the associations and interactions among genes that confer vulnerability to schizophrenia and to examine the moderating effect of neuropsychological impairment.

METHODS

We analyzed 99 SNPs from 10 candidate genes in 1,512 subject samples. The permutation-based single-locus, multi-locus association tests, and a gene-based multifactorial dimension reduction procedure were used to examine genetic associations and interactions to schizophrenia.

RESULTS

We found that no single SNP was significantly associated with schizophrenia. However, a risk haplotype, namely A-T-C of the SNP triplet rsDAO7-rsDAO8-rsDAO13 of the DAO gene, was strongly associated with schizophrenia. Interaction analyses identified multiple between-gene and within-gene interactions. Between-gene interactions including DAO*DISC1 , DAO*NRG1 and DAO*RASD2 and a within-gene interaction for CACNG2 were found among schizophrenia subjects with severe sustained attention deficits, suggesting a modifying effect of impaired neuropsychological functioning. Other interactions such as the within-gene interaction of DAO and the between-gene interaction of DAO and PTK2B were consistently identified regardless of stratification by neuropsychological dysfunction. Importantly, except for the within-gene interaction of CACNG2, all of the identified risk haplotypes and interactions involved SNPs from DAO.

CONCLUSIONS

These results suggest that DAO, which is involved in the N-methyl-d-aspartate receptor regulation, signaling and glutamate metabolism, is the master gene of the genetic associations and interactions underlying schizophrenia. Besides, the interaction between DAO and RASD2 has provided an insight in integrating the glutamate and dopamine hypotheses of schizophrenia.

Nonselective cation channels and links to hippocampal ischemia, aging, and dementia

Stroke is a very strong risk factor for dementia. Furthermore, ischemic stroke and Alzheimer's disease (AD) share a number of overlapping mechanisms of neuron loss and dysfunction, including those induced by the inappropriate activation of N-methyl-D-aspartate receptors (NMDARs). These receptors form a major subtype of excitatory glutamate receptor. They are nonselective cation channels with appreciable Ca(2+) permeability, and their overactivation leads to neurotoxicity in the cortex and hippocampus. NMDARs have therefore been therapeutic targets in both conditions, but they have failed in the treatment of stroke, and there is limited rationale for using them in treating AD. In this chapter, we discuss current understanding of subtypes of NMDARs and their potential roles in -ischemic stroke and AD. We also discuss the properties of several other nonselective cation channels, transient receptor potential melastatin 2 and 7 channels, and their implications in linking these conditions.

Pyk2 cytonuclear localization: mechanisms and regulation by serine dephosphorylation

Cytonuclear signaling is essential for long-term alterations of cellular properties. Several pathways involving regulated nuclear accumulation of Ser/Thr kinases have been described but little is known about cytonuclear trafficking of tyrosine kinases. Proline-rich tyrosine kinase 2 (Pyk2) is a cytoplasmic non-receptor tyrosine kinase enriched in neurons and involved in functions ranging from synaptic plasticity to bone resorption, as well as in cancer. We previously showed the Ca(2+)-induced, calcineurin-dependent, nuclear localization of Pyk2. Here, we characterize the molecular mechanisms of Pyk2 cytonuclear localization in transfected PC12 cells. The 700-841 linker region of Pyk2 recapitulates its depolarization-induced nuclear accumulation. This region includes a nuclear export motif regulated by phosphorylation at residue S778, a substrate of cAMP-dependent protein kinase and calcineurin. Nuclear import is controlled by a previously identified sequence in the N-terminal domain and by a novel nuclear targeting signal in the linker region. Regulation of cytonuclear trafficking is independent of Pyk2 activity. The region regulating nuclear localization is absent from the non-neuronal shorter splice isoform of Pyk2. Our results elucidate the mechanisms of Ca(2+)-induced nuclear accumulation of Pyk2. They also suggest that Pyk2 nuclear accumulation is a novel type of signaling response that may contribute to specific long-term adaptations in neurons.

Effects of metabotropic glutamate mGlu5 receptor antagonist on tyrosine phosphorylation of NMDA receptor subunits and cell death in the hippocampus after brain ischemia in rats

Tyrosine phosphorylation of the N-methyl-D-aspartate (NMDA) receptor appears to be associated with the regulation of the receptor's ion channel. This study focused on the effect of a metabotropic glutamate mGlu5 receptor antagonist on tyrosine phosphorylation of NMDA receptor subunits and cell death in the hippocampal CA1 region after transient global ischemia and sought to explore their mechanisms. Pretreatment with the mGlu5 receptor antagonist reduced cell death in the hippocampal CA1 region on day 3 after the transient ischemia. Transient ischemia increased the tyrosine phosphorylation of NMDA receptor subunits, which are a major target of Src family tyrosine kinases. Therefore, we investigated the effect of the antagonist on tyrosine phosphorylation of the NMDA receptor subunits after transient ischemia. Tyrosine phosphorylation of the NR2A subunit, but not that of the NR2B one, was inhibited by the mGlu5 receptor antagonist. The administration of the antagonist also attenuated the increase in the amount of active form of Src after the reperfusion. We further demonstrated that the administration of a Src-family kinase inhibitor prevented cell death in the hippocampal CA1 region and attenuated the increase in the tyrosine phosphorylation of the NMDA receptor subunits after the reperfusion. These findings suggest that mGlu5 receptor in the hippocampal CA1 region after transient ischemia is involved in the activation of Src and subsequent tyrosine phosphorylation of NMDA receptor subunits, which actions may contribute to alterations of properties of the NMDA receptor and may be related to pathogenic events leading to neuronal cell death.

Striatal-enriched protein-tyrosine phosphatase (STEP) regulates Pyk2 kinase activity

Proline-rich tyrosine kinase 2 (Pyk2) is a member of the focal adhesion kinase family and is highly expressed in brain and hematopoietic cells. Pyk2 plays diverse functions in cells, including the regulation of cell adhesion, migration, and cytoskeletal reorganization. In the brain, it is involved in the induction of long term potentiation through regulation of N-methyl-d-aspartate receptor trafficking. This occurs through the phosphorylation and activation of Src family tyrosine kinase members, such as Fyn, that phosphorylate GluN2B at Tyr(1472). Phosphorylation at this site leads to exocytosis of GluN1-GluN2B receptors to synaptic membranes. Pyk2 activity is modulated by phosphorylation at several critical tyrosine sites, including Tyr(402). In this study, we report that Pyk2 is a substrate of striatal-enriched protein-tyrosine phosphatase (STEP). STEP binds to and dephosphorylates Pyk2 at Tyr(402). STEP KO mice showed enhanced phosphorylation of Pyk2 at Tyr(402) and of the Pyk2 substrates paxillin and ASAP1. Functional studies indicated that STEP opposes Pyk2 activation after KCl depolarization of cortical slices and blocks Pyk2 translocation to postsynaptic densities, a key step required for Pyk2 activation and function. This is the first study to identify Pyk2 as a substrate for STEP.

Combined β-adrenergic and corticosteroid receptor activation regulates AMPA receptor function in hippocampal neurons

Shortly after stress, limbic neurons are exposed to high levels of noradrenaline and corticosterone. These hormones are necessary for optimal behavioural adaptation. Behavioural effects critically depend on noradrenaline acting via β-adrenergic receptors, but these effects are strongly modulated by corticosterone, indicating putative interactions between the two hormones. Since both noradrenaline and corticosterone are known to quickly affect properties of AMPA-type glutamate receptors (AMPAR), we here examined - in hippocampal neurons - three parameters which give insight in the functionality of AMPARs: phosphorylation, surface expression and spontaneous synaptic transmission. In homogenates of adult hippocampal slices, application of corticosterone (30 nM for 15 min) by itself did not affect phosphorylation of the AMPAR GluA1 subunit at S845 or S831. Co-application of the β-adrenergic receptor agonist isoproterenol (10 µM) largely increased S845 (but not S831) phosphorylation. Corticosterone also did not change GluA1 and GluA2 surface expression in hippocampal primary cultures. However, combined administration of corticosterone and 1 µM isoproterenol - which by itself was ineffective - enhanced surface expression. Interestingly, 10 µM isoproterenol alone enhanced GluA1 surface expression, but this was decreased by corticosterone. Finally, in hippocampal primary cultures, the inter-event interval of miniature excitatory postsynaptic currents (mEPSCs) was decreased by the combination of 1 µM isoproterenol and corticosterone (which were ineffective by themselves) while the same combination did not affect the amplitude. We conclude that AMPAR phosphorylation, surface expression and mEPSC inter-event interval respond most strongly to a combination of corticosterone and β-adrenergic receptors. These combined hormonal effects on glutamate transmission might contribute to their memory-enhancing effects.

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.

MAGUKs, synaptic development, and synaptic plasticity

MAGUKs are proteins that act as key scaffolds in surface complexes containing receptors, adhesion proteins, and various signaling molecules. These complexes evolved prior to the appearance of multicellular animals and play key roles in cell-cell intercommunication. A major example of this is the neuronal synapse, which contains several presynaptic and postsynaptic MAGUKs including PSD-95, SAP102, SAP97, PSD-93, CASK, and MAGIs. Here, they play roles in both synaptic development and in later synaptic plasticity events. During development, MAGUKs help to organize the postsynaptic density via associations with other scaffolding proteins, such as Shank, and the actin cytoskeleton. They affect the clustering of glutamate receptors and other receptors, and these associations change with development. MAGUKs are involved in long-term potentiation and depression (e.g., via their phosphorylation by kinases and phosphorylation of other proteins associated with MAGUKs). Importantly, synapse development and function are dependent on the kind of MAGUK present. For example, SAP102 shows high mobility and is present in early synaptic development. Later, much of SAP102 is replaced by PSD-95, a more stable synaptic MAGUK; this is associated with changes in glutamate receptor types that are characteristic of synaptic maturation.

Proline-rich tyrosine kinase 2 regulates hippocampal long-term depression

Proline-rich tyrosine kinase 2 (PYK2), also known as cell adhesion kinase beta or protein tyrosine kinase 2b, is a calcium-dependent signaling protein involved in cell migration. Phosphorylation of residue Y402 is associated with activation of PYK2 and leads to the recruitment of downstream signaling molecules. PYK2 was previously implicated in long-term potentiation (LTP); however, the role of PYK2 in long-term depression (LTD) is unknown. Here, we report that PYK2 is activated by NMDA receptor stimulation (chemical LTD) in cultured neurons. Small hairpin RNA-mediated knockdown of PYK2 blocks LTD, but not LTP, in hippocampal slice cultures. We find that the Y402 residue and, to a lesser extent, PYK2 kinase activity contribute to PYK2's role in LTD. Knockdown experiments indicate that PYK2 is required to suppress NMDA-induced extracellular signal-regulated kinase (ERK) phosphorylation. Overexpression of PYK2 depresses NMDA-induced ERK phosphorylation and inhibits LTP, but not LTD. Our data indicate that PYK2 is critical for the induction of LTD, possibly in part by antagonizing ERK signaling in hippocampal neurons.