Exam 2: Chapter 3-8 Flashcards
What is the goal of the generation phase of LTP?
Inserting GluA1 receptors into the post-synaptic density
Generation of LTP
What is the role of Ionotropic glutamate receptors and where are they found?
Ionotropic receptors are located in the plasma membrane in dendritic spines on the PSD. When Glu binds to the receptor, the channel opens and allows ions (such as Na+ or Ca2+) to enter the cell.
Generation of LTP
What is an NMDA antagonist?
APV
Generation of LTP
What is an AMPA receptor antagonist?
CNQX
Generation of LTP
What receptors are required for generation of an LTP and when?
Induction vs. expression
- AMPA is required for both induction & expression of LTP
- NMDA is only required for induction of LTP
What were Collingridge’s results that established the importance of NMDA receptors to our understanding of LTP?
He found that inhibiting NMDA receptors prevented the induction of LTP but did not influence its expression.
Generation of LTP
What are the 2 events that open the NMDA channel?
- Glu binds to the both AMPA & NMDA receptors
- Na+ entering the cell must depolarize it in order to remove the Mg2+ plug allowing Ca2+ to enter the cell.
Generation of LTP
What were Gary Lynch’s 2 complimentary ideas?
- An LTP-inducing stimulus can rapidly incr. the number of AMPA receptors in the dendritic spine
- This is the fundamental outcome that supports the expression of LTP
Generation of LTP
How are AMPA receptors moved?
AMPA receptors traffic into and out of dendritic spine regulated constitutively by synaptic activity.
Generation of LTP
What is stargazin?
What are its structural features?
Stargazin is a transmembrane AMPAr regulatroy protein (TARP) that co-assembles with AMPA receptors.
- Terminal end binds to membrane
- Has serine site for phosphorylation
Generation of LTP
Describe the constitutive process of AMPA trafficking.
- AMPA receptors are in constant random motion laterally diffusing along the plasma membrane.
- Receptors may be caught in the endocytotic zone and packaged for recycling.
- Endosomes (intracellular vesicles containing AMPA receptors) deliver the receptors to the peri-synaptic region near the PSD.
Generation of LTP
How does protein kinase A (PKA) affect GluA1?
PKA phosphorylates the AMPAr to traffic the endosomed receptor to the peri-synaptic region.
Generation of LTP
How does protein kinase C (PKC) affect GluA1?
What is PKC activated by?
PKC, activated by Ca2+ (synaptic activity), phosphorylates the receptor and helps localize it to the PSD.
Generation of LTP
How does CaMKII affect GluA1?
CaMKII phosphorylates the receptor and changes the GluA1 channel to allow influx of Ca2+ and Na+.
Generation of LTP
Describe the process of lateral diffusion increasing the AMPA receptors in the PSD.
- Influx of calcium into the spine via NMDA receptors
- Activates CaMKII
- CaMKII phosphorylates serine residues on the terminal of Stargazin
- Release of Stargazin terminals from membrane
- Binding of Stargazin terminals to PSD-95 complexes, thereby trapping the receptor in the PSD
- The stability of Stargazin incr as CaMKII phosphorylates more residues
Generation of LTP
How is Ca2+-Calmodulin built?
Why is Calmodulin considered a surrogate second messenger?
Calmodulin undergoes a conformational change when it binds to Calcium. The new shape enables it to bind to other proteins that do no have Ca2+ binding sites.
Generation of LTP
Describe the structure of CaMKII?
Describe each domain. How is it activated?
The CaMKII subunit consists of 2 domains: an autoinhibitory domain and catalytic domain.
The regulatory domain contains a phosphorylation site (Thr286) that is not accessible when the kinase is in an inactive state. Ca2+ + calmodulin serves as a second messenger and activates the kinase to expose the catalytic domain and Thr286
Generation of LTP
How does CaMKII autophosphorylate?
Subunits of the kinase assemble into a ring-like structure called a holoenzyme. A subunit can now phosphorylate its neighbor.
This process of autophosphorylation enables the subunits to remain active even when Ca2+ + calmodulin is no longer present.
Generation of LTP
What is the second, but slower way to deliver AMPA receptors to the PSD?
- Neuronal activation leads to an influx of Ca2+
- Incr Ca2+ induces a conformation change in the motor protein, myosin Vb
- The unfolding of myosin Vb mediates the recruitment of the motor to the membrane-associated Rab11 effector complex on the recycling endosome
- The activated myosin then delivers AMPA receptors to synaptic sites
Generation of LTP
What are the 2 pools of AMPA receptors to the PSD?
What are there roles?
- Preexsisting Surface Pool (Lateral Diffusion Pool): important for rapid generation of LTP
- Intracellular Pool (Endosomed Pool): sustains a slower incr. in LTP
Generation of LTP
What happens if lateral diffusion is inhibited?
If lateral diffusion is inhibited, then the intial delivery of AMPA receptors is prevented and the rapid generation of LTP does not occur.
Generation of LTP
What happens if exocytosis is inhibited?
Inhibiting the delivery of the intracellular pool results in the rapid reduction of LTP, which returns to baseline in about 10 min.
Generation of LTP
What is actin?
What is its role in the generation of LTP?
Actin is a cytoskeleton protein that determines the architecture of neurons, including dendritic spines.
The rapid insertion of AMPA receptors to the PSD requires disassembling the actin networks.
Generation of LTP
What are the 2 states actin is found in?
- Monomer state (G-actin)
- Polymer state (F-actin)
Generation of LTP
How are actin networks disassembled in the calpain-spectrin pathway?
- Filament actin in the spine head is crosslinked with spectrins
- The protease calpain is activated when Ca2+ enters the spine
- Calpain degrades spectrins and facilitates the disassembling of actin
- This activity contributes to the rapid insertion of additional AMPA receptors in the PSD
Generation of LTP
Summarize the Generation Phase.
- LTP is generated within about a minute following the induction stimulus.
- Following the entry of Calcium, the actin cytoskeleton is degraded by a calpain-spectrin pathway
- AMPA receptor lateral diffusion trafficking processes rapidly insert and trap GluA1 receptors into PSD
- AMPA receptors within recyling endosomes are mobilized to the PSD via a Ca2+ conformational change on myosin Vb
Stabalization of LTP
When LTP returns to baseline, it is most likely a result of what?
Constitutive endocytotic processes want to remove AMPA receptors from the PSD
Stabalization of LTP
What produces bigger spine sizes that are more stable?
Theta burst stimulation that produces LTP increases spine size.
Larger spines have increased AMPAr
Stabalization of LTP
Describe the 3 stages of the stabalization phase?
- Long peices of actin cytoskeleton are cut up into smaller peices
- Use the samll peices of actin cytoskeleton to repolymerize into bigger peices
- Defend the bigger spine that was just created
Stabalization of LTP
What regulates actin polymerizatoin?
What are the active and inactive states?
Cofilin regulates actin polymerization.
In its normal unphosphorylated state, cofilin severs and depolymerizes actin filaments.
When phosphorylated, cofilin no longer interfers with actin polymerization.
What are the states of actin in both generation of LTP and stabalization.
Generation = actin depolymerization
Stabalization = actin polymerization
Stabalization of LTP
How do drugs that prevent actin polymerization applied before TBS affect LTP?
They do not prevent the induction of LTP, but it will rapidly decay.
Stabalization of LTP
What are the processes invovled in stabalization of LTP?
The disassembling step
- CAMKII detaches from F-actin strands & unbundles actin
- Unbundled actin is exposed to cofilin that helps depolymerize actin
- Actin binds to actin-interacting proteins that cap the fragments to prevent rejoining
- Myosin IIb shearing of long to short polymerized actin
Stabalization of LTP
What is the role of Myosin IIb?
How do drugs that inhibt myosin IIb affect LTP?
Myosin IIb exerts a shearing action that breaks actin filament into smaller units that can be re-assembled elsewhere in the spine.
Applying drugs that inhibit myosin IIb prior to TBS prevents the stabalization of LTP but does not interfere with its induction.
Stabalization of LTP
In an unpotentiated spine what is the intial state of the actin cytoskeleton?
Actin filaments are connected in a meshlike network in the spine head. Strands of neck actin filaments are bundled by* inactive CaMKII* complexes
Stabalization of LTP
What happens to the CaMKII bundling the actin filaments when an inducing stimulus is applied?
An LTP-inducing stimulus opens NMDA receptors and calcium enters the spine. CaMKII is actviated. CaMKII disengages from the actin filaments and unbundles them.
Stabalization of LTP
How is the depolymerization of actin turned off?
- Cofilin is phosphorylated by LIMK halting the severing process
- Phosphatases dephosphorylate actin
- Allows rapid polymerization of long actin strands that promotes spine extension.
Stabalization of LTP
How is the dendritic spine elongated during stabalization?
- Profilin enters the spine to promote polymerization
- Arp2/3 promotes actin branching & expansion of the spine head
- Alpha-actinin promotes crosslinking fibers
- Inactive CaMKII promotes actin bundling
Stabalization of LTP
Describe uncaging glutamate.
You can tag glutamate with a really heavy moleucle so that it is to big to move and function. A specific directed UV light breaks the bond to free glutamate. Once uncaged the glutamate binds to AMPA and NMDA receptors and leads to massive restructuring of the cell.
Stabalization of LTP
What are the 2 pathways that regulate polymerization and reorganization?
What are they activated by?
Calcium entering the synapse through NMDA receptors activates the GTPases Rho-Rock & Rac-PAK. These enymes phosphorylate LIMK, a kinase that allows cofilin to allow actin polymerization.
Stabalization of LTP
What are cell adhesion molecules?
What are the 2 classes?
Proteins that are located on the cell surface and bind with other cells or with the extracellular matrix. Activated by synaptic activity.
- Neural Cadherins
- Integrin receptors