The Biochemistry of LTP Induction

From Mechanisms of Memory by J. David Sweatt, Ph.D.

From Sheng and Kim

NeurotransmitterReceptor

NMDA Receptor

AMPA Receptor

K Channels

Ca++ Channels

IP3

Receptor

1

2

23

3

4

4

5

LTP induction machinery

Synaptic Infrastructure

Ca++

6Persisting

Signal

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Glutamate Receptors

NR1 120

NR2A 180

NR2B 180

GluR6 + 7 117

mGluR1a 200

Scaffolding and adaptors

PSD-95 95

ChapSyn110/PSD-93 110

Sap102 115

GKAP/SAPAP 95-140

Shank 200

Homer 28/45

Yotiao 200

AKAP150 150

NSF 83

PKA

PKA catalytic subunit 40

PKA-R2β 53

PKC

PKCβ 80

PKCγ 80

PKCε 90

CaM Kinase

CaM Kinase II β 60

phosph-CaM Kinase 60

Phosphatases

PP1 36

PP2A 36

PP2B(calcineurin) 61

PPs 50

PTPID/SHP2 72

Tyrosine Kinases

Src 60

PYK2 116

MAP Kinase pathway

ERK (pan ERK) 42/44

ERK1 42/44

ERK2 42

MEK1 45

MEK2 46

MKP2 43

Rsk 90

Rsk-2 90

c-Raf1 74

Small G-proteins and modulators

Rac1 21

Rap2 21

SynGAP 10,12,35,60

NF1 60,101

Other signaling molecules

Calmodulin 15

nNOS 155

PI3 Kinase 85

PLCγ 130

cPLA2 110

Citron 183

Arg3.1 55

Cell adhesion and cytoskeletal proteins

N-Cadherin 150

Desmoglein 165

β-Caternin 92

LI 200

pp120cas 120

MAP2B 280

Actin 45

α-actinin 2 110

Spectrin 240/280

Myosin (brain) 205

Tubulin 50

Coractin 80/85

CortBP-1 180/200

Clathryn heavy chain 180

Dynamin 100

Hsp-70 70

Molecule Mr (kD) Molecule Mr (kD) Molecule Mr (kD)

Husi et al. (2001) Nature Neuroscience 3: 661-669.

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Modulator Mechanism Effect

Src family tyrosine kinases (src, fyn) tyrosine phosphorylation enhancement

    loss of Zn inhibition  

Scaffolding proteins    

  RACK1 binding inhibitory

  PSD-95 scaffolding modulatory

PKC   ser/thr phosphoryation (direct) enhancement

    src activation (indirect)  

PKA/PP1/Yotiao phosphorylation enhancement

    dephosphorylation inhibition

Cyclin dependent kinase 5 ser/thr phosphorylation enhancement

Nitric Oxide/redox sulfhydryl nitrosylation inhibition

    or oxidation  

Polyamines (e.g. spermine, spermidine) direct binding to a modulatory augmentation

    site  

Caseine kinase II ser/thr phosphorylation enhancement

    modulation of polyamine effects  

TABLE I – DIRECT MODULATORS OF THE NMDA RECEPTOR

Yotiao

LeptinReceptor

ApoEReceptor

EphBReceptor

NMDA Receptor

NMDA ReceptorNeurotransmitterReceptor Coupled

To PLC

NeurotransmitterReceptor CoupledTo Acetyl Choline

Leptin ApoE Ephrin B

pyk2

ERK

RACK

Src/Fyn

PSD95

Tyr

PO4PO4PI3K/MAPK

??

?

Co

mp

lex

form

atio

nSTEP

PKC

PLC

PIP

X

Ser/Thr

PO4PKA

PP1

CDK5CKII

ATP cAMP

?DAG

Receptor Modulation of the NMDA receptor

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Ionic Current Molecules Involved Role Mechanisms of

Modulation

K Currents      

  Voltage-dependent Kv4.2 (and Kv4.3) limit bpAPs ERK, PKA, CaMKII

  “A” currents   limit EPSP magnitude  

  “H” Currents NCN channels regulate excitabilitycyclic nucleotides (direct)

    (HCN)    

Na Currents      

  AMPA Receptors GluR1, GluR2 depolarize membrane PKA, CaMKII, PKC

    Aka GluR-A,B    

  Voltage-dependent Na(v)1.6, 1.1,1.2 AP propagationPKC (decreased inactivation)

  Na+ currents      

Ca Currents ? – likely many AP propagation PKA

      (hypothetical)  

Cl Currents      

  GABA Receptors all GABA-A AP firing numerous

    receptor subunits excitability  

TABLE II – MECHANISMS UPSTREAM OF THE NMDA RECEPTOR INVOLVED IN MEMBRANE DEPOLARIZATION

Three-way Coincidence Detection

↓Kv4.2

Strong Input

Back propagatingAction Potential

ACh

CA1 Pyramidal Neuron

NMDAR

Glu

1

1

22

3

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Component Targets Role

Cell Adhesion Molecules    

  Integrins src, rho, rac, ras/MAPKs Transmembrane signaling,

      Interactions with extracellular

      matrix, NMDAR regulation

    MLCK, FAK? spine morphology?

  Syndecan-3 fyn, NMDAR signaling from matrix heparan

      sulfates to the NMDA receptor

  N-Cadherin other Cadherins, spine morphology?

    cytoskeleton Pre-post adhesion?

Actin Cytoskeleton/Associated Proteins

  Rho membrane/cytoskeleton regulate synaptic structure

    interactions  

  Cdk5 NMDA receptor increase NMDA receptor function

  Filamin K channels K channel localization

Presynaptic Processes    

  Glutamate release synaptic glutamate NMDA receptor activation

  Glutamate re-uptake synaptic glutamate limiting NMDA receptor

      desensitization

TABLE III – COMPONENTS OF THE SYNAPTIC INFRASTRUCTURE NECESSARY FOR NMDA RECEPTOR FUNCTION

Component Targets Role

Anchoring/Interacting proteins    

  PSD-95 receptors, postsynaptic organization

   signal transduction mechs  

   nNOS, SynGAP, GKAP  

  NMDA receptor multiple proteins effector localization, structural

      organization

  Rack1/fyn NMDA receptordirect regulation of NMDA receptor

  Shank/HOMER metabotropic receptors effector localization, cytoskeleton

  GRIP AMPA receptors, postsynaptic organization

    PICK-1/PKC  

  AKAP PKA, PP2Bkinase and phosphatase localization

  CaMKII signal transductionregulate likelihood of LTP induction

TABLE III – COMPONENTS OF THE SYNAPTIC INFRASTRUCTURE NECESSARY FOR NMDA RECEPTOR FUNCTION ( Continued)

NMDAR NR2

NMDARNR2

AMPARGluR2,GluR3

AMPAR

GAPPSD-95

rap

actin

n-NOS

SynGAP

GKAP PSD95

GKAP

ShankHomer

IP3R

PLC

actinras

IP3 + DAG

SPAR

cortactin-

Spectrin

PICK-1

PKC

GRIP

NSFGRASP1

(GEF for ras)

ras

PKA PKC

AKAP79 PP2B

SAP97

CamKII

β-AR

ReceptorTrafficking liprin

Group I mGluR

PSD-95 as an Anchoring Protein for NMDA Receptors

From Sheng and Kim

Fig. 1. RIM1 and the priming of synaptic vesicle fusion. (a) After docking, synaptic vesicles (SV) are tethered at the active zone by binding of Rab3 to the N-terminal (N) of RIM1 (Rab3-interactive molecule-1). Munc-13 is recruited to the active zone by activity of phospholipase C (PLC) and the second messenger diacylglycerol (DAG). Munc-18 binding to syntaxin (Syntx) keeps syntaxin in a `closed' conformation that cannot bind SNAP-25 (synapstosome-associated protein-25). (b) Activation of second-messenger pathways – such as those involving Ca2+, adenylate cyclase (AC), cAMP and protein kinase A (PKA) – during induction of short-term plasticity leads to a switch in the binding partners of RIM1. Munc-13-1 binds to N-terminal RIM1, competitively inhibiting the binding of Rab3 to RIM1. Thus, a new tethering mechanism holds the SVs at the active zone, as synaptotagmin1/2 (Synat) binds to the C-terminal RIM domains in a Ca2+-dependent manner. Binding of munc-13 to syntaxin removes munc-18 and converts syntaxin's structure to an open conformation. (c) Proximity of synaptotagmin to the plasma membrane, conversion of syntaxin by Munc-13-1 to an open conformation that can interact with SNAP-25, and further increase in cytoplasmic free Ca2+ levels, promote the formation of the synaptobrevin (Syb)–syntaxin–SNAP-25 complex that is required for fusion.

Three Pools of F-Actin in Synaptic Spines

The upper panels are single computed slices through electron tomographic volumes of spines labeled for F-actin using phaloidin-eosin photo conversion, from hippocampus CA1 (A) and cerebellar cortex molecular layer (B) (see Capani et al., 2001  ). Labeling is concentrated between the lamellae of the spine apparatus (SA) and the postsynaptic density (arrowheads). Bundles of actin are seen traversing between these entities (large arrow). In Purkinje cells, which have no spine apparatus, actin filaments fill the head and also can be followed between the smooth ER and the postsynaptic membrane (large arrow). Diffuse staining for actin is also seen (asterisks). The stereo computer graphic reconstruction in the bottom panel is of the CA1 synapse and shows actin bundles (blue) as well as the spine apparatus (yellow) and the postsynaptic density (purple). These figures were kindly provided by Dr. Mark Ellisman.

Figure 1. LIMK Influences Postsynaptic and Presynaptic Function through Modulation of Actin FilamentsDendritic spines are made up of a head, neck, and postsynaptic density (PSD). Within the PSD, scaffold proteins such as Homer, PSD-95, and Shank, as well as others not described here, link the actin cytoskeleton to postsynaptic receptors including AMPA and NMDA glutamate receptors. Results in this issue of Neuron by Meng et al. (2002 ) demonstrate that LIMK-1 is partially responsible for proper dendritic morphology and long-term potentiation (LTP), presumably via its effect on actin filament dynamics, through phosphorylation and inactivation of ADF/cofilin (AC). In LIMK-1−/− mice, the morphology of dendritic spines is altered. The spines have a thicker neck and smaller postsynaptic density length and smaller spine area. Results presented by Meng et al. (2002 ) also reveal that the LIMK-1−/− mice have enhanced basal release of presynaptic vesicles and an enhanced synaptic depression, suggesting a role for LIMK-1 (and most likely actin dynamics) in neurotransmitter release. Figure by Patrick D. Sarmiere and James R. Bamburg

Presynaptic

Postsynaptic

NMDA Receptor

Retrograde Signaling

Integrins

rho

rac

FAK

MLCK

ras

α-actinin

Src/fyn

ERK

β subunit

filamin

?

?

cdk5

talin

vinculin

?

Kv4.2 Channel

actin

actin

actin

DynamicRegulation

Integrins

Extracellular Matrix

Interactions among Integrins and Intracellular Effectors

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Molecule/Organelle Role Modulator/Regulator

VDCCs augment NMDAR-dependent PKA

  Ca influx  

  Ca influx due to bpAPs  

  regulate ERK activation  

Endoplasmic Reticulum Ca efflux from ER, limit LTP? PLC-coupled receptors

(Ca ATPase/IP3R/RyR)  

Presynaptic Mitochondria regulate presynaptic Ca levels unknown

TABLE IV – CALCIUM FEEDBACK AND FEED-FORWARD MECHANISMS

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Regulatory System Molecules Involved Role

The cAMP Gate PKA/PP1/I1/PP2B Phosphatase Inhibition

    Augmented Kinase Signaling

The PKC/Neurogranin PLC/PKC/Neurogranin/CaM Augmenting CaMKII Activation

System   Augmenting Ca-sensitive Cyclase

TABLE V – EXTRINSIC SIGNALS MODULATING THE CALCIUM RESPONSE

Model for the cAMP Gate

Sweatt (2001) Curr. Biol. 11:R391-394.

Phospholipase CNeurogranin

Neurogranin

PO4

+

DAG

PKC

Calmodulin

Calmodulin

MetabotropicReceptor

PKC Phosphorylation of Neurogranin

AugmentedPKC

cAMPGATE

Initial Ca++Signal

IncreasedCa++/CaM

Augmented CaMKIIActivity

AdenylylCyclase

NMDARN

euro

gra

nin

DAG

Cyclase CoupledReceptors

MetabotropicReceptors

The PKC/Neurogranin system and the cAMP Gate

The Biochemistry of LTP Induction

1. Mechanisms upstream of the NMDA receptor that directly regulate NMDA receptor function.

2. Mechanisms upstream of the NMDA receptor that control membrane depolarization.

3. The components of the synaptic infrastructure that are necessary for the NMDA receptor and the synaptic signal transduction machinery to function normally.

4. Feed-forward and feedback mechanisms that regulate the level of calcium attained.

5. Extrinsic signals that regulate the response to the calcium influx.

6. The mechanisms for the generation of the actual persisting biochemical signals.

Four-way Coincidence Detection

↓Kv4.2

Strong Input

Back propagatingAction Potential

ACh

CA1 Pyramidal Neuron

NMDAR

Glu

1

1

22

3

cAMPGATE

Norepinephrine

4

4

The Biochemistry of LTP Induction

From Mechanisms of Memory by J. David Sweatt, Ph.D.