Cell Death in CNS Flashcards
Appropriate Cell Death
Developmental Die off:
- more neurons than needed when born
- Fine tuning of connections orchestrated by interactions with the environment (macro and micro).
- developmental cell death occurs without tissue inflammation or disruption of surrounding cells.
- Use it or lose it-growth factors and electrophysiological activity
- Programmed by DNA for establishment of the central nervous system and is therefore considered appropriate.
Cellular Level
Neurons and Glia:
- neurons are vulenrable.
- brain consumes 25% of total O2 and glucose in the blood stream
- neurons use glucose as primary carbon source
- neurons not replaced in signicant numbers
Inappropriate/Pathological Cell Death
Normal cell is in a steady state or homeostasis
Injury is any stimulus bringing changes in cell physiology and/or anatomy-internal/external.
Adaptation (changes in cell)-reversible-irreversible.
Common Routes to Cell Death
- Glutamate Induced neuronal death
- excitotoxicity. Excessive stimulation through receptors for NT glutamate.
- caused by excessive release, failure of glutamate uptake mechanisms, exposure to drugs, poisons that act just like glutamate (agonist)
- too much glutamate activity causes an imbalance of other ions such as calcium and sodium which can result in cell death
- will cause both DNA programmed and necrotic cell death - Reactive O2 Species
- will damage cell membranes and intracellular organelles
- will activate DNA programmed cell death mechanisms - Hypoglycemia
- Loss of glucose leads to rapid depletion of cellular energy reserves
- will activate DNA programmed cell death mechanisms
Cascade of Events
Neurons die in two main modes:
- Necrosis-rapid and somewhat messy death
- Apoptosis-programmed cellular suicide.
Necrosis
Dramatic and very rapid form of cell death in which every comaprtment of the cell disintegrates.
Characterized by marked dysregulation of ion homeostasis causing:
-cell swelling, dilation of mitochondria and ER, formation of vacoules in cytoplasm, activation of enzymes called proteases, which degrade cullular components, normally these proteases are held in packages (lysosomes) but necrotic processes release them.
Chromatinclumps and nuclear membrane is disrupted.
Gene transcription and protein synthesis stops.
ATP is rapidly depleted.
Cell lyse and spill their contents into ECF
contents can damage neighbouring cells.
Spillage of contents cause inflammatory response.
Few cell death triggers that are only capable of inducing either necrosis or apoptosis.
Whether a cell undergoes apoptosis or necrosis is determined primarily by intensity and/ or duration of death-induced stimulus.
Somewhat of a consensus that if stimulus is sever and/or sustained it will induce necrosis
If stimulus is less severe with transient stresses it will induce apoptosis.
eg. glutamate/excitatory, trauma, energy failure/ischemia
Apoptosis-Type 1 PCD
There are multiple forms of programmed cell death but apoptosis is the best characterized form of PCD.
Apoptosis occurs in both appropriate and inappropriate cell death.
Main Pathways:
1. Intrinsic-generated by signals arising within cells.
2. Extrinsic-triggered by death activators-bidning to receptors at cell surface. Death receptor converges on protease called caspases-leads to cell death
3. Direct-to-DNA pathway called caspase-independent pathway
Intrinsic Pathway
Mitochondrial pathway. Conserved. Interplay of family of proteins. Anti-apoptotic. Pro-apoptotic-BCL-2 family. Interplay within the BCL-2 family members-compete and if the pro-apoptotic members won the war then program continues through the activation of the program executors-the caspases (cytosine-dependent, aspartate sopecific proteases) Proapoptosis is bak, bid and bad anti is BCL-2, BCL-xl and Bcl-w.
What Happens in Intrinsic pathway?
Pro-apoptotic winner causes a pore to open in the mitochondria which causes cytochrome C to leak out.
Cytochrome C binds with a molecule called apoptosis activator factor -1 (APAF-1) and induces it to create the first stage of an apoptosome.
Together APAF-1 and cytochrome C capture and bind Caspase-9 which completes the apoptosome.
Nothing stop program then the apoptosome will progress to the next step which is activation of final apoptosis executor-caspase-3.
Extrinsic Pathway
Death receptor pathway.
FasL/FasR, TNFalpha, TNFR1, Apo3L/DR3, Apo2L/DR4, Apo2L/DR5
The death receptors are protiens embedded through the cell membrane with their receptor domains exposed at cell surface.
Death domain activated recruits caspase-8 causes activation of caspase-8
Activated caspase-8 then proceeds with the death plan by activating the final death effector caspase-3.
Extrinsic pathway does have a mechanism to interact with the intrinsic pathway that involves caspase 8-activates a pro-apoptotic molecule typically associated with the intrinsic apoptotic pathway thus the 2 pathways can work towards death simultaneously.
Caspase-Independent Pathway
Through apoptosis-inducing factor-AIF from mitochondria
AIF normally located in intermembrane space of mitochondria.
When cell recieves signal to die:
-AIF released from mitochondria
-migrates to cell nucleus
-Binds to DNA
-Triggers destruction of DNA and cell death
Apoptosis vs. Necrosis
Necrosis can start only and exclusively when the cell dies and is an irreversible process-no return. Non recoverable.
Common Assays for Apoptosis
Cytomorphological changes. DNA fragmentation Detection of caspases, cleaved substrates, regulators and inhibitors Membrane alterations mitochondrial assays nuclear stains Tunnel staining-dUTP nick end labelling.
