486 lines
13 KiB
Markdown
486 lines
13 KiB
Markdown
# **Detailed Temporal Dynamics of Postsynaptic Response and Plasticity**
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*From glutamate binding to structural consolidation, with concentration changes, receptor trafficking, and calcium signaling across timescales*
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---
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## **Baseline State (Resting Spine)**
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**Time:** Continuous
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**Postsynaptic [Ca²⁺]:** ~50-100 nM
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**Membrane Potential (Vₘ):** -70 mV
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**AMPARs in PSD:** 10-20 receptors (GluA1/GluA2 heteromers)
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**NMDARs in PSD:** 5-10 receptors (GluN1/GluN2B)
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**Mg²⁺ block of NMDARs:** ~80% at -70 mV
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**CaMKII state:** Mostly inactive (α:β ≈ 3:1 ratio)
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**PSD-95 clusters:** ~300 molecules per PSD
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---
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## **PHASE 1: FAST TIMESCALE (0-100 ms) - RECEPTOR ACTIVATION**
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### **0.0-0.2 ms: Glutamate Arrival and Binding**
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```
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Presynaptic glutamate release (~5000 molecules)
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↓
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Diffusion across 20 nm synaptic cleft (t ≈ 0.1 ms)
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↓
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**Glutamate concentration in cleft:**
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- Peak: 1-3 mM at PSD surface
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- Rapid clearance by EAATs (t½ ≈ 1 ms)
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↓
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**Simultaneous binding to:**
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1. **AMPARs (ionotropic, fast):**
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- 2 glutamate molecules bind per channel
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- Binding Kd ≈ 500 µM
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- Channel opens in ~0.2 ms
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2. **NMDARs (ionotropic, slow):**
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- Requires glutamate + glycine/D-serine
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- Binding Kd ≈ 1-5 µM
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- Mg²⁺ block prevents opening at rest
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3. **mGluRs (metabotropic):**
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- Group I mGluRs (mGluR1/5)
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- G-protein coupled, slower signaling
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```
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### **0.2-2.0 ms: AMPAR-Mediated Depolarization**
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```
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**For each open AMPAR:**
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- Conductance: 8-12 pS (single channel)
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- Reversal potential: 0 mV
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- **Na⁺ influx:** ~3000 ions/channel/ms
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- K⁺ efflux: ~1000 ions/channel/ms
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**Net effect at spine head:**
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Without other inputs: EPSP amplitude = 0.5-2 mV
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With 20 AMPARs open: Current = 10-30 pA
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Depolarization to Vₘ ≈ -60 mV
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```
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### **1.0-5.0 ms: NMDAR Activation (if depolarized)**
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```
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**Requirement:** Vₘ > -40 mV to relieve Mg²⁺ block
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**Coincidence detection window:** 5-10 ms
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If depolarized (from AMPARs or bAP):
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↓
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Mg²⁺ expelled from NMDAR channel
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↓
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**NMDAR opens with characteristic:**
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- Slow kinetics (τrise ≈ 10 ms, τdecay ≈ 50-100 ms)
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- High Ca²⁺ permeability (PCa/PNa ≈ 10:1)
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- **Single channel Ca²⁺ influx:** ~5000 Ca²⁺ ions/ms
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↓
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**Local [Ca²⁺] in spine head:**
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- Baseline: 100 nM
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- With NMDAR activation: **→ 1-10 µM**
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- With NMDAR + bAP coincidence: **→ 10-30 µM**
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```
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### **5.0-50 ms: Calcium Dynamics and Clearance**
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```
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**Calcium sources in spine:**
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1. NMDARs (main source for plasticity)
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2. Voltage-gated Ca²⁺ channels (VGCCs) from bAP
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3. Internal stores (IP₃R, RyR)
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**Calcium buffers in spine:**
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- Calbindin-D28K (Kd ≈ 200 nM)
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- Parvalbumin (Kd ≈ 10 nM)
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- Calmodulin (Ca²⁺ sensor, Kd ≈ 1-10 µM)
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**Clearance mechanisms:**
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1. Plasma Membrane Ca²⁺ ATPase (PMCA):
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- High affinity (Kd ≈ 100 nM)
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- Slow: clears ~30 Ca²⁺/sec per pump
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2. Sodium-Calcium Exchanger (NCX):
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- Low affinity (Kd ≈ 1 µM)
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- Fast: 3 Na⁺ in, 1 Ca²⁺ out
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3. SERCA pumps into ER:
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- If spine has smooth ER
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4. Mitochondrial uptake (larger spines):
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- MCU (mitochondrial Ca²⁺ uniporter)
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- Kd ≈ 10-20 µM
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**Result:**
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- 90% Ca²⁺ cleared in 50-100 ms
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- Returns to baseline [Ca²⁺] in 200-500 ms
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```
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---
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## **PHASE 2: MEDIUM TIMESCALE (100 ms - 10 sec) - SIGNALING CASCADES**
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### **Calcium-Decoded Plasticity Decision**
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```
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**The "Calcium Rule":**
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[Ca²⁺] amplitude × duration → plasticity direction
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**Thresholds:**
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- LTD: 1-5 µM sustained (100 ms - 1 sec)
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- LTP: >10 µM brief (10-50 ms)
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- LTP requires **rapid rise** (d[Ca²⁺]/dt > 0.5 µM/ms)
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```
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### **LTD Pathway (Moderate Ca²⁺)**
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```
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[Ca²⁺] = 1-5 µM for >100 ms
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↓
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Calcium binds Calmodulin (CaM)
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↓
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**Activates Calcineurin (CaN, PP2B):**
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- Phosphatase, Kd ≈ 0.5 µM Ca²⁺
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- Activated at lower [Ca²⁺] than CaMKII
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↓
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CaN dephosphorylates Inhibitor-1
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↓
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**Releases inhibition of Protein Phosphatase-1 (PP1)**
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↓
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PP1 dephosphorylates:
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1. GluA1 at S845 → increases endocytosis
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2. Stargazin → reduces AMPAR synaptic retention
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3. Other targets promoting AMPAR removal
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↓
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**Result: AMPAR internalization begins in 30-60 sec**
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```
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### **LTP Pathway (High Ca²⁺)**
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```
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[Ca²⁺] > 10 µM with rapid rise
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↓
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Calcium binds Calmodulin (CaM)
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↓
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**Activates Ca²⁺/Calmodulin Kinase II (CaMKII):**
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- 12-subunit holoenzyme
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- Each subunit has autoinhibitory domain
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- Requires Ca²⁺/CaM binding to activate
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↓
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**Autophosphorylation at T286:**
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- First subunit phosphorylates neighbor
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- Creates Ca²⁺-independent activity
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- **Molecular switch:** stays active after Ca²⁺ clears
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↓
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**Active CaMKII translocates to PSD:**
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- Binds to NR2B subunit of NMDAR
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- Binds to α-actinin (actin linker)
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- Becomes structural component of PSD
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```
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---
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## **PHASE 3: SLOW TIMESCALE (10 sec - 10 min) - RECEPTOR TRAFFICKING**
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### **LTD Execution (1-10 minutes)**
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```
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**Clathrin-mediated endocytosis:**
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PP1 activity → GluA1 S845 dephosphorylated
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↓
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Increased binding to AP2 adaptor complex
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↓
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**Clathrin coats form at spine periphery (t ≈ 1-2 min)**
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↓
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AMPARs internalized via endocytosis
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↓
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**Vesicles transported to early endosomes**
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↓
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Receptors either:
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1. Recycled back to surface (silent synapses)
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2. Degraded in lysosomes (long-term LTD)
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↓
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**By 10 min:**
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- 30-50% reduction in surface AMPARs
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- EPSP amplitude decreases proportionally
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```
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### **LTP Execution (1-10 minutes)**
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```
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**Rapid AMPAR insertion:**
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CaMKII phosphorylates:
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1. **Stargazin (TARP γ-2) at S9:**
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- Increases binding to PSD-95
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- **Traps AMPARs in PSD** (Kd improves 10×)
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2. **SynGAP (RasGAP):**
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- Phosphorylation inhibits Ras inactivation
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- Increases ERK/MAPK signaling
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↓
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**Exocytosis of AMPARs:**
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1. From recycling endosomes (Rab11-dependent)
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2. From intracellular pools
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3. **Insertion at extrasynaptic sites first**
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↓
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**Lateral diffusion into PSD:**
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- AMPARs diffuse in membrane (D ≈ 0.1 µm²/s)
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- Phosphorylated Stargazin binds PSD-95
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- **Trapped in PSD for minutes-hours**
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↓
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**By 10 min:**
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- 50-100% increase in surface AMPARs
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- EPSP amplitude increases 50-200%
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```
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### **Phosphorylation State Changes**
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```
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**AMPAR modifications during LTP:**
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- **GluA1 S831:** Phosphorylated by CaMKII/PKC
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→ Increases single channel conductance (γ from 8→12 pS)
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- **GluA1 S845:** Phosphorylated by PKA
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→ Increases open probability (Po from 0.8→0.95)
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- **GluA2 S880:** Phosphorylated by PKC
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→ Regulates binding to GRIP/ABP vs PICK1
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```
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---
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## **PHASE 4: METABOLIC SUPPORT (10 min - 2 hours) - PROTEIN SYNTHESIS**
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### **Local Translation in Spine**
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```
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**Trigger:**
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1. CaMKII activation
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2. mGluR activation
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3. BDNF-TrkB signaling
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**Pathways:**
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1. **mTOR pathway:**
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- PI3K → Akt → mTORC1
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- Phosphorylates 4E-BP, releases eIF4E
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- **Initiates cap-dependent translation**
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2. **MAPK pathway:**
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- Ras → Raf → MEK → ERK
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- Phosphorylates translation factors
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↓
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**Dendritic mRNA translation begins (t ≈ 20-30 min):**
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Key mRNAs locally translated:
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1. **CaMKIIα** - more kinase molecules
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2. **GluA1** - new AMPAR subunits
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3. **Arc/Arg3.1** - regulates AMPAR trafficking
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4. **PSD-95** - scaffolding protein
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5. **Homer1a** - regulates mGluR signaling
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↓
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**New proteins synthesized locally:**
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- Concentration increases over 1-2 hours
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- Replaces initial plasticity with stable changes
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```
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### **Retrograde Signaling Synthesis**
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```
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**For LTP:**
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Ca²⁺ → activates nNOS (neuronal nitric oxide synthase)
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↓
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**NO synthesis from arginine:**
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- Diffusion constant: ~3300 µm²/s
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- Half-life: ~1-5 seconds
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- Diffuses 10-20 µm to presynaptic terminal
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↓
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**BDNF synthesis and release:**
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- Transcription begins in 30 min
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- Release occurs 1-2 hours post-induction
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```
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---
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## **PHASE 5: STRUCTURAL CONSOLIDATION (2 hours - 24 hours)**
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### **Actin Cytoskeleton Remodeling**
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```
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**Spine enlargement (LTP):**
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Active CaMKII → phosphorylates **Profilin**
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↓
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Profilin binds actin monomers → promotes polymerization
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↓
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**Rho GTPase activation:**
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- Rac1 activated → promotes actin branching (via Arp2/3)
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- Cdc42 activated → promotes filopodia formation
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↓
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**Actin polymerization in spine head:**
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- F-actin increases 2-3×
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- Spine volume increases over 1-3 hours
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↓
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**PSD expansion:**
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- More space for AMPARs
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- More PSD-95 scaffolding
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↓
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**By 6 hours:** Spine volume increased 50-100%
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```
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### **Nuclear Signaling and Gene Expression**
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```
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**Signals reach nucleus (1-3 hours):**
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1. **CaMKIV translocation:**
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- Activated by Ca²⁺ in dendrites
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- Translocates to nucleus when phosphorylated
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2. **MAPK/ERK translocation:**
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- Activated at synapse
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- Travels to nucleus (active transport)
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3. **CREB phosphorylation:**
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- At S133 by CaMKIV/PKA/RSK
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- Recruits CBP/p300 coactivators
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↓
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**Transcriptional activation (3-6 hours):**
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Early genes (IEGs):
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- c-Fos, c-Jun, Egr1/Zif268
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Late genes (plasticity-related):
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- **BDNF** (brain-derived neurotrophic factor)
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- **GluA1** (AMPAR subunit)
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- **CaMKIIα**
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- **Arc**
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- **Homer1a**
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↓
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**New proteins synthesized in soma (6-12 hours)**
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↓
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**Transport to dendrites (12-24 hours)**
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↓
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**Incorporation into spine (24+ hours)**
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```
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---
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## **PHASE 6: VERY SLOW TIMESCALE (Days - Weeks) - STRUCTURAL STABILITY**
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### **Spine Maturation and Stabilization**
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```
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**Day 1-7:**
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- **PSD thickening:** from 30 nm → 50 nm
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- **AMPAR subtype switch:**
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GluA2-lacking (Ca²⁺-permeable) → GluA2-containing
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(Occurs over days via subunit replacement)
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- **Synaptic adhesion molecules:**
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Neuroligin-Neurexin complexes stabilize contact
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**Week 1-4:**
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- **Spine shape changes:**
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Thin → Mushroom (LTP)
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Mushroom → Thin (LTD)
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- **Presynaptic coordination:**
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Active zone aligns with expanded PSD
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- **Perisynaptic astrocyte processes:**
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Enwrap mature synapse for metabolic support
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```
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### **Homeostatic Scaling**
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```
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**Days 2-7:**
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If overall neuron firing rate changes significantly:
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↓
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**Global scaling mechanisms:**
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1. **TNFα signaling:** from astrocytes
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2. **BDNF level changes**
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↓
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All synapses on neuron scaled up or down
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↓
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**AMPAR number adjusted** while relative differences maintained
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↓
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**Preserves signal-to-noise ratio** of individual synapses
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```
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---
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## **COMPLETE LTP TIMELINE EXAMPLE**
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### **Induction (Seconds)**
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```
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T=0 ms: Presynaptic glutamate release
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T=10 ms: bAP arrives at spine (coincidence)
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T=15 ms: [Ca²⁺] peaks at 25 µM
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T=50 ms: Ca²⁺ clears to 1 µM
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T=1 sec: CaMKII autophosphorylated (T286)
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T=10 sec: CaMKII translocates to PSD
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```
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### **Early Expression (Minutes)**
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```
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T=1 min: AMPARs inserted (from recycling endosomes)
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T=2 min: EPSP amplitude increases 100%
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T=5 min: Stargazin phosphorylated, AMPARs trapped
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T=10 min: Early LTP established
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```
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### **Protein Synthesis-Dependent Phase (Hours)**
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```
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T=30 min: Local translation begins (CaMKIIα, GluA1)
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T=1 hour: BDNF transcription initiated
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T=2 hours: Spine volume begins increasing
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T=3 hours: New proteins from local synthesis incorporated
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T=6 hours: Spine volume increased 60%
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```
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### **Late Maintenance (Days)**
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```
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T=12 hours: New proteins from soma arrive
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T=24 hours: Structural changes stabilized
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T=48 hours: GluA2 subunits replace GluA1 homomers
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T=7 days: Mature mushroom spine established
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```
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---
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## **CALCIUM SIGNALING THRESHOLDS SUMMARY**
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| \[Ca²⁺\] Range | Duration | Sensor | Outcome |
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|----------------|----------------|-------------|----------------------------|
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| < 0.5 µM | Any | None | Baseline signaling |
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| 0.5-1 µM | \>1 sec | Calcineurin | Weak LTD |
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| 1-5 µM | 100 ms-1 sec | Calcineurin | Strong LTD |
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| 5-10 µM | Brief (<50 ms) | CaMKII | Weak LTP |
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| \>10 µM | Brief (<50 ms) | CaMKII | Strong LTP |
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| \>20 µM | Any | Calpain | Pathological, spine damage |
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---
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## **KEY BIOLOGICAL PRINCIPLES**
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1. **Spine as Biochemical Compartment:**
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- Neck resistance (50-500 MΩ) restricts diffusion
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- Allows independent [Ca²⁺] signaling in each spine
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- Enables synapse-specific plasticity
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2. **Kinetic Competition:**
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- Calcineurin activates faster at low [Ca²⁺] (Kd ≈ 0.5 µM)
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- CaMKII requires higher [Ca²⁺] but has positive feedback
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- Winner-takes-all decision based on [Ca²⁺] time course
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3. **Energy Requirements:**
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- Each AMPAR insertion: ~1000 ATP
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- CaMKII autophosphorylation: 1 ATP/subunit
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- Protein synthesis: ~4 ATP/amino acid
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- ATP supplied by astrocyte lactate
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4. **Timescale Coupling:**
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- Fast (ms): Receptor activation
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- Medium (min): Trafficking existing proteins
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- Slow (hours): Making new proteins
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- Very slow (days): Structural changes
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This postsynaptic timeline shows how a brief glutamate signal triggers a cascade of events across multiple timescales, converting transient electrical activity into lasting structural and functional changes that underlie learning and memory. |