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Giles E Hardingham

Publications and source records attributed to Giles E Hardingham.

9 recordsLinked to original sources
PubMed2006

Extracellular calcium regulates postsynaptic efficacy through group 1 metabotropic glutamate receptors.

Bursts of synaptic transmission are known to induce transient depletion of Ca2+ within the synaptic cleft. Although Ca2+ depletion has been shown to lower presynaptic release probability, effects on the postsynaptic cell have not been reported. In this study, we show that physiologically relevant reductions in extracellular Ca2+ lead to a decrease in synaptic strength between synaptically coupled layer 2/3 cortical pyramidal neurons. Using quantal analysis and mEPSP analysis, we demonstrate that a lowered extracellular Ca2+ produces a reduction in the postsynaptic quantal size in addition to its known effect on release probability. An elevated Mg2+ level can prevent this reduction in postsynaptic efficacy at subphysiological Ca2+ levels. We show that the calcium-dependent effect on postsynaptic quantal size is mediated by group 1 metabotropic glutamate receptors, acting via CaMKII (Ca2+/calmodulin-dependent protein kinase II) and PKC. Therefore, physiologically relevant changes in extracellular Ca2+ can regulate information transfer at cortical synapses via both presynaptic and postsynaptic mechanisms.

Animals↗
PubMed2006

Preconditioning doses of NMDA promote neuroprotection by enhancing neuronal excitability.

Neuroprotection can be induced by low doses of NMDA, which activate both synaptic and extrasynaptic NMDA receptors. This is in apparent contradiction with our recent findings that extrasynaptic NMDA receptor signaling exerts a dominant inhibitory effect on prosurvival signaling from synaptic NMDA receptors. Here we report that exposure to low preconditioning doses of NMDA results in preferential activation of synaptic NMDA receptors because of a dramatic increase in action potential firing. Both acute and long-lasting phases of neuroprotection in the face of apoptotic or excitotoxic insults are dependent on this firing enhancement. Key mediators of synaptic NMDA receptor-dependent neuroprotection, phosphatidylinositol 3 kinase-Akt (PI3 kinase-Akt) signaling to Forkhead box subgroup O (FOXO) export and glycogen synthase kinase 3beta (GSK3beta) inhibition and cAMP response element-binding protein-dependent (CREB-dependent) activation of brain-derived neurotrophic factor (BDNF), can be induced only by low doses of NMDA via this action potential-dependent route. In contrast, NMDA doses on the other side of the toxicity threshold do not favor synaptic NMDA receptor activation because they strongly suppress firing rates below baseline. The classic bell-shaped curve depicting neuronal fate in response to NMDA dose can be viewed as the net effect of two antagonizing (synaptic vs extrasynaptic) curves: via increased firing the synaptic signaling dominates at low doses, whereas firing becomes suppressed and extrasynaptic signaling dominates as the toxicity threshold is crossed.

Action Potentials↗
PubMed2006

Cellular Notch responsiveness is defined by phosphoinositide 3-kinase-dependent signals.

BACKGROUND: Notch plays a wide-ranging role in controlling cell fate, differentiation and development. The PI3K-Akt pathway is a similarly conserved signalling pathway which regulates processes such as differentiation, proliferation and survival. Mice with disrupted Notch and PI3K signalling show phenotypic similarities during haematopoietic cell development, suggesting functional interaction between these pathways. RESULTS: We show that cellular responsiveness to Notch signals depends on the activity of the PI3K-Akt pathway in cells as diverse as CHO cells, primary T-cells and hippocampal neurons. Induction of the endogenous PI3K-Akt pathway in CHO cells (by the insulin pathway), in T-cells (via TCR activation) or in neurons (via TrKB activation) potentiates Notch-dependent responses. We propose that the PI3K-Akt pathway exerts its influence on Notch primarily via inhibition of GSK3-beta, a kinase known to phosphorylate and regulate Notch signals. CONCLUSION: The PI3K-Akt pathway acts as a "gain control" for Notch signal responses. Since physiological levels of intracellular Notch are often low, coincidence with PI3K-activation may be crucial for induction of Notch-dependent responses.

Animals↗
PubMed2005

Nuclear Ca2+ and the cAMP response element-binding protein family mediate a late phase of activity-dependent neuroprotection.

The mechanism by which physiological synaptic NMDA receptor activity promotes neuronal survival is not well understood. Here, we show that that an episode of synaptic activity can promote neuroprotection for a long time after that activity has ceased. This long-lasting or "late phase" of neuroprotection is dependent on nuclear calcium signaling and cAMP response element (CRE)-mediated gene expression. In contrast, neuroprotection evoked acutely by ongoing synaptic activity relies solely on the activation of the phosphatidylinositol 3-kinase/Akt pathway. This "acute phase" does not require nuclear calcium signaling and is independent of activation of the CRE-binding protein (CREB) family of transcription factors. Thus, activity-dependent neuroprotection comprises two mechanistically distinct phases that differ in their spatial requirements for calcium and in their reliance on the CREB family.

Animals↗
PubMed2005

Nuclear Ca2+ and CaM kinase IV specify hormonal- and Notch-responsiveness.

Many neuronal processes require gene activation by synaptically evoked Ca(2+) transients. Ca(2+)-dependent signal pathways activate some transcription factors outright, but here we report that such signals also potentiate the activation of nuclear receptors by their cognate hormone, and of CBF1 by Notch, transcription factors hitherto not thought to be Ca(2+)-responsive. This potentiation is occluded by histone deacetylase inhibition, indicating a mechanism involving inactivation of co-repressors associated with these transcription factors. Synaptic activity, acting via the nuclear Ca(2+)-dependent activation of CaM kinase IV, triggers the disruption of subnuclear domains containing class II histone deacetylases (HDACs) and silencing mediator of retinoic acid and thyroid hormone receptors (SMRT), a broad-specificity co-repressor which represses nuclear hormone receptors and CBF1. The sequential loss of class II HDACs and SMRT from the subnuclear domains, followed by nuclear export, is associated with disruption of SMRT interaction with its target transcription factors and sensitization of these factors to their activating signal. Counterbalancing these changes, protein phosphatase 1 promotes nuclear localization of SMRT and inactivation of nuclear receptors and CBF1. Thus, the synaptically controlled kinase-phosphatase balance of the neuron determines the efficacy of SMRT-mediated repression and the signal-responsiveness of a variety of transcription factors.

4-Aminopyridine↗
PubMed2003

The Yin and Yang of NMDA receptor signalling.

Ca(2+) entry through the NMDA subtype of glutamate receptors has the power to determine whether neurons survive or die. Too much NMDA receptor activity is harmful to neurons - but so is too little. Is it a case of too much or too little Ca(2+) influx causing cell death or do other factors, such as receptor location or receptor-associated proteins, play a role? Understanding the mechanisms behind this dichotomous signalling is an important area of molecular neuroscience with direct clinical implications.

Animals↗
PubMed2003

Synaptic activity induces signalling to CREB without increasing global levels of cAMP in hippocampal neurons.

Nuclear calcium signals associated with electrical activation of neurons can control the activity of the transcription factor cAMP-response element binding protein (CREB). Yet, cAMP is thought to be the key messenger that links synaptic activity to the regulation of CREB-mediated transcription. It is generally assumed that synaptic activity increases the intracellular levels of cAMP; this causes activation of the cAMP-dependent protein kinase (PKA) that regulates CREB-mediated transcription either directly or through controlling nuclear signalling of the MAP kinases/extracellular signal-regulated kinases (ERK1/2) pathway. Here we show that, in hippocampal neurons, synaptic activity failed to increase global levels of cAMP that would be required for the cAMP-PKA system to induce nuclear events. Even near-continuous bursting of action potentials, giving rise to large nuclear calcium signals and robust CREB-dependent transcription, left global intracellular levels of cAMP unchanged. These results suggest that the cAMP-PKA system does not function as the transducer of synaptic signals to the nucleus. They indicate that the known inhibitory effects of blockers of PKA on gene expression and long-lasting plasticity triggered by calcium entry reflect a gating function of basal activity of PKA that renders neurons permissive for nuclear calcium-regulated, CREB/CBP-dependent gene expression.

Action Potentials↗
PubMed2002

Coupling of extrasynaptic NMDA receptors to a CREB shut-off pathway is developmentally regulated.

Electrical activation of hippocampal neurons can cause calcium influx through different entry sites which may specify nuclear signalling and induction of gene transcription and downstream physiological outputs. Genomic responses initiated by NMDA receptors (NMDARs) are critically dependent on whether synaptically or extrasynaptically located receptors are stimulated; calcium flux through synaptic NMDARs activates CREB whereas flux through extrasynaptic NMDARs triggers a CREB shut-off signal. Here we investigated the possibility that the coupling of extrasynaptic NMDARs to the CREB shut-off pathway is regulated during in vitro development. Cultured hippocampal neurons were analyzed after 7 or 12 days of in vitro culturing. We found that synaptic NMDAR activity induced CREB phosphorylation at day in vitro (DIV) 7 and DIV 12. In contrast, the extrasynaptic NMDAR-dependent CREB shut-off signal is developmentally regulated. At DIV 12 extrasynaptic NMDAR activation shuts down CREB and overrides the CREB-activating signal triggered by synaptic NMDAR activation. In contrast, at DIV 7 this shut off signal is absent; both synaptic and extrasynaptic NMDARs activate CREB function. Developmental changes in NMDAR signaling have been proposed to contribute to the emergence of glutamate excitotoxicity, which causes apoptosis or necrosis depending on the severity of the insult. Since CREB regulates a number of pro-survival genes, the emergence of this shut-off around DIV 7 may contribute to the increase in susceptibility of neurons to glutamate-induced neuropathology in vitro and in vivo during post-natal development.

Animals↗