Signal transduction in neuronal systems lecture 4

Cell death

Regulated apoptosis and unregulated necrosis

Cell death in neurodegenerative diseases

In PD cell loss of dopamine neurons occurs leading to dopamine deficiencies, necrosis is difficult to stop but apoptosis can hypothetically be stopped 

In which field is cell death studied

Cancer field

Extrensic apoptosis pathway

Extracellular factors like Killer T cells, lack of oxygen or nutrients can lead to activation of death receptors which leads to activation of caspase-8 and caspase-3

Intrinsic apoptosis pathway

Intracellular pathway where mitochondrial processes induce apoptosis

Apoptosis pathway

Apoptotic stimulus activates BH3-only proteins which inactivates anti-apoptotic Bcl2 protein allowing aggregated BH123 proteins to form pores in the mitochondrial membrane leading to release of cytochrome-c

Induction of mitochondrial toxicity

Caspase-8, Bax

Effect of mitochondrial toxicity

Cytochrome-C leaks out due to mitochrondial membrane permeabilization, which stimulates APAF-1 to produce the apoptosome, leading to activation of caspase-9 and downstream caspase-3

Overlapping component between extrinsic and intrinsic apoptosis pathways

Caspase-8

How can intracellular stress induce intrinsic apoptosis

Increased calcium or ROS can induce toxicity 

Why is there no immunological response when a cell undergoes apoptosis?

The apoptosome cleans up the cell

Apoptosis during embryogenesis

Morphogens can regulate apoptosis to remove developed parts which are not needed anymore, developmental defects can affect apoptosis leading to parts of the body being present at birth which should’ve been removed

Apoptosis regulation

Mitochondria produces SMAC, which inhibits XIAP proteins needed to inhibits caspase-3 and caspase-9

(X)IAP

Inhibiting apoptosis protein that is always present in healthy cells and bind to caspases to inactivate them

How does the apoptosis counter the effect of apoptosis inhibiting proteins

Release anti-IAPs (SMAC) from the mitochondria during activation of the apoptosis pathway 

What do almost all proteins in the apoptosis pathway do?

Stop apoptosis, because the cell normally doesn’t want to go into apoptosis

BCL2 protein family

Apoptosis regulating proteins

Examples of BCL2 proteins

Bax, BH3-only, Bak, Bim

Pore-forming proteins of the BCL2 family

Bax is bound to the mitochondrial membrane where it can dimerize with BH3-only domain proteins to form proteolytic enzymes that make holes

Function of BCL2 in apoptosis

Inhibiting dimerization of Bax and therefore pore-formation

Pro-apoptotic BCL2-family proteins

Bim, BH3-only, PUMA, Bad, NOXA, Bax

Anti-apoptotic BCL2-family proteins

BCL2, Mcl1

How are BCL2 proteins regulated?

Post-translational modifications, other proteins

BCL2 regulating proteins

PUMA can regulate BCL2 leading to sensitization which is anti-apoptotic or can regulate by derepressing BCL2 and pushing away Bim which is pro-apoptotic, Bim can also help neutralizing by pushin away from Bax and therefore being anti-apoptotic

Apoptosis involvement in synaptic plasticity?

Caspases are involved in pruning, reducing synapses that are less used to filter out unwanted signals

Effect of drugs and schizophrenia on synaptic plasticity

Drugs lead to pruning, too much reduction of synapses leads to depression, schizophrenia associated with too many synapses

Structure of Caspases

Have cleavage sites which are cleaved when activated, prodomains fall off while small dubunit domains migrate to other positions along the large subunit

Caspase cascade

A positive feedforward leading to exponential increase of caspases where one caspase (caspase-9) can activate many executioner caspases (caspase-3/7)

Why does the caspase cascade make regulation difficult?

Once caspases increase exponentially, the IAPs are not able to stop them anymore

What do executioner caspases do?

Cut apoptotic cells and their contents into equal size parts

How does Mcl1 function as an anti-apoptotic protein?

Binds to Bax but inhibits pore-formation

How was found that Mcl1 is an anti-apoptotic protein?

Was found to be upregulated in dopamine neurons, blocking Mcl1 with UMI-77 allowed more pro-apoptotic proteins to bind to Bax, dopamine neurons showed more apoptosis than other neurons

Which period in neurogenesis is good for studying apoptosis?

P3

NOXA

NOXA binds directly to anti-apoptotic Mcl1 and therefore activating pro-apoptotic Bax

How can NOXA be blocked

Bax inhibiting peptide also works on NOXA

What is important for dopamine neurons survival

Balance between anti-apoptosis factor Mcl1 and pro-apoptosis factor NOXA

Mcl1 in PD

Mutation in USP24 which is needed for de-ubiquitination of Mcl1 and does not happen anymore in dopamine neurons leading to NOXA winning the compitition and neuronal cell death

USP24

Protein produced in dopamine neurons which de-ubiquitinates Mcl1 to keep it from degradation

Bax 6a7

Binds to Bax if its actively dimerized