Novel Treatment of Excitotoxicity:

Targeted Disruption ofIntracellular Signalling Fro

m Glutamate Receptors

GlutamateGlutamate

ExcitotoxicityExcitotoxicity

Glutamate receptor and excitotoxicityGlutamate receptor and excitotoxicity

Types of Glutamate receptorTypes of Glutamate receptor

mGluRsmGluRs

G-protein-coupled membrane receptorsG-protein-coupled membrane receptors Downregulate KDownregulate K++ channel and upregulate non-selection cation channel channel and upregulate non-selection cation channel Inhibit GABA receptor activity and potentiate iGluR functionInhibit GABA receptor activity and potentiate iGluR function Mediate neuronal plasticity, nociception, pain and neurodegeneretionMediate neuronal plasticity, nociception, pain and neurodegeneretion

iGluRsiGluRs

Ligand-gated ion channels, permeable to NaLigand-gated ion channels, permeable to Na++, K, K++ or Ca or Ca2+2+

Mediate synaptic plasticityMediate synaptic plasticity

related to much of the toxicity induced by glutamaterelated to much of the toxicity induced by glutamate

AMPA receptorsAMPA receptors

Permeable to Na+, K+Permeable to Na+, K+

Also permeable to Ca2+ unless it contain GluR2Also permeable to Ca2+ unless it contain GluR2

Loss of GluR2 implicated in delayed death of neurones in ischemiaLoss of GluR2 implicated in delayed death of neurones in ischemia

Structure of GluR2

NMDA receptorsNMDA receptors

Highly permeable to CaHighly permeable to Ca2+2+ and Na and Na++

Calcium transients responsible for the physiologic effects of NMDAR signallingCalcium transients responsible for the physiologic effects of NMDAR signalling

Calcium transients also trigger excitotoxic deathCalcium transients also trigger excitotoxic death

Kainate receptorsKainate receptors

Share many of the same structural characteristicsShare many of the same structural characteristics as as NMDA and AMPA reNMDA and AMPA receptorsceptors

Until recently, little was known about the functional and physiological roleUntil recently, little was known about the functional and physiological roles of kainate receptors in the mammalian CNSs of kainate receptors in the mammalian CNS

Calcium and neurotoxicityCalcium and neurotoxicity

Localised increases in [CaLocalised increases in [Ca2+2+]]ii trigger physiological ev trigger physiological ev

entsents

Excessive CaExcessive Ca2+2+ loading activates processes that lead t loading activates processes that lead to cell deatho cell death

Neurotoxicity mediated by glutamate receptors is largNeurotoxicity mediated by glutamate receptors is largely calcium dependentely calcium dependent

Calcium and neurotoxicityCalcium and neurotoxicity

Calcium load hypothesisCalcium load hypothesis Neurodegeneration is simply a function of the quantitNeurodegeneration is simply a function of the quantit

y of entering the celly of entering the cell

However, some studies show that the calcium channeHowever, some studies show that the calcium channel blockers can prevent Cal blockers can prevent Ca2+2+ accumulation but not neur accumulation but not neurotoxicity during anoxiaotoxicity during anoxia

Calcium and neurotoxicityCalcium and neurotoxicitySource specific hypothesisSource specific hypothesis

CaCa2+2+ toxicity occurs not simply as a function of toxicity occurs not simply as a function of increased Caincreased Ca2+2+ concentration, but is instead linked to concentration, but is instead linked to the route of Cathe route of Ca2+2+ entry and the distinct second entry and the distinct second messenger pathways that are activated as a result.messenger pathways that are activated as a result.

Showing that CaShowing that Ca2+2+ loads produced by voltage-sensitiv loads produced by voltage-sensitive Cae Ca2+2+ channels were not harmful whereas similar [Ca channels were not harmful whereas similar [Ca2+2+]]ii increases via NMDARs were toxic increases via NMDARs were toxic

Postsynaptic organisationPostsynaptic organisation

Postsynaptic density (PSD)Postsynaptic density (PSD) PSD PSD is a multiprotein complex containing membrane

proteins, signaling molecules and core PSD proteins

Postsynaptic organisationPostsynaptic organisation

Membrane receptors and proteinsMembrane receptors and proteins mGluRs, iGluRsmGluRs, iGluRs

Cell junction proteinCell junction protein

Postsynaptic organisationPostsynaptic organisation

Enzymes and modulatorsEnzymes and modulators Src-kinase, CaMKII, PKC , phosphatase calcineurin

nNOS, SPAR, SynGAP

Postsynaptic organisationPostsynaptic organisationCytoskeletal and scaffolding proteinsCytoskeletal and scaffolding proteins

actin, fodrin, tubulin and neurofilamentsactin, fodrin, tubulin and neurofilaments

Spectrin, Spectrin, -actinin-2, AKAP 79 and PDZ-containing -actinin-2, AKAP 79 and PDZ-containing protein protein

Neurotoxic signalling by glutamate Neurotoxic signalling by glutamate receptors within the PSDreceptors within the PSD

Neurotoxic effects of AMPAR signallingNeurotoxic effects of AMPAR signalling

GluR2 hypothesisGluR2 hypothesis

Neurotoxic signalling by glutamate Neurotoxic signalling by glutamate receptors within the PSDreceptors within the PSD

Neurotoxic effects of NMDAR signallingNeurotoxic effects of NMDAR signalling

Role of PSD-95Role of PSD-95

Targeting intracellular signal pathwaysTargeting intracellular signal pathways

Strategies for treating excitotoxic damageStrategies for treating excitotoxic damageo NMDAR and AMPAR blockers?NMDAR and AMPAR blockers?o Particular receptor subunits antagonistsParticular receptor subunits antagonistso Low affinity blockersLow affinity blockerso Targeting the specific intracellular signal pathways Targeting the specific intracellular signal pathways

and uncoupling glutamate receptors from their potand uncoupling glutamate receptors from their potentially neurotoxic downstream effectorsentially neurotoxic downstream effectors

Future directionsFuture directionsLimitations of the use of peptides and small prLimitations of the use of peptides and small proteinsoteins

Protein tranduction domainsProtein tranduction domainso Cross cell membranes independent of specific receCross cell membranes independent of specific rece

ptors or transportersptors or transporterso Ensure efficient delivery of attatched proteins into Ensure efficient delivery of attatched proteins into

cells and across the BBBcells and across the BBBo Particularly suited to the narrow therapeutic windoParticularly suited to the narrow therapeutic windo

w offered during strokew offered during stroke