Lecture 11 - Apoptosis summing up

Question 1

What is an example of apoptosis in work in vertebrate embryos?

Answer 1

Ensuring homeostasis within adult tissues - elimination of material to create new shape

Question 2

What are the consequences of mis-folded proteins in the ER?

Answer 2

There is growing evidence that [3-cell failure and death are caused by unresolvable ER stress, leading to chronic and/or strong activation of the unfolded protein response (UPR)
Involves a few pathways
+ IRE1cx
+ ATF6
+ PERK

ER stress due to increased insulin synthesis has been proposed to contribute towards [3-cell failure and cell death observed in type 2 diabetes (T2D).

Question 3

How does the RER respond to unfolded protein?

Answer 3

Properly folded proteins exit the ER and progress through the secretory pathway.

Mis-folded proteins are detected by the unfolded protein response (UPR) system.
* Triggering events to fold proteins properly.

Irreparably unfolded or mis-folded proteins are retro-translocated from the ER and degraded by cytoplasmic proteasomes.
* Through a process known as ER-associated degradation (ERAD).

Question 4

UPR pathways
What is the IRE1 pathway (highly-conserved across species)?

Answer 4

Mis-folded proteins are recognised by a few systems
e.g., is the IRE1 (inositol-requiring protein 1) pathway
+ IRE1 is an ER-resident transmembrane protein kinase, an upstream component of the UPR and a central regulator of UPR-specific downstream gene expression.
IRE1 “senses” the presence of unfolded and misfolded proteins in the ER, which causes dimerization, trans-autophosphorylation and consequent activation of IRE1.

Question 5

What happens in the Ire1 pathway?

Answer 5

Ire1 is a transmembrane protein kinase with an
endo-ribonuclease activity that is able to splice
RNA.
i.e., trans-phosphorylation leads to IRE1
activation of the endo-ribonuclease activity.
Ire splices the X-box binding protein-1 (XBP-1)
pre-mRNA to mature Xbp1 mRNA.
The spliced Xbp1 mRNA is translated to the Xbp1 protein in the cytoplasm.
Xbp1 protein is able to affect gene expression of a chaperone.

Question 6

Example - Ire 1 pathway
What is the role of Xbp1?

Answer 6

Xbp1 is a transcriptional activator that plays a crucial role in the development of professional secretory cells.
Xbp1 is a key component Of the UPR activated by disturbances in RER protein- folding homeostasis.
Xbp1 activates the expression of ER chaperone genes that are required for the folding and trafficking of secretory cargo proteins.
The chaperone when translated at the RER will enter the lumen and help with the folding of the mis-folded proteins.

Question 7

What happens when misfolded proteins cannot be resolved with chaperones?

Answer 7

However, sometimes additional chaperones cannot resolve the mis-folding problem - e.g., mutated proteins
This will trigger a few events
- Endoplasmic reticulum (ER)-associated protein degradation (ERAD) eliminates misfolded or unassembled proteins from the ER. ERAD targets are selected by a quality control system within the ER lumen and are ultimately destroyed by the cytoplasmic ubiquitin—proteasome system (UPS).
- Cell death

Question 8

What is the unfolded protein response (PERK pathway)?

Answer 8

PERK is a kinase that appears up-regulates a specific gene that encodes for a translation initiation factor, elF2a.
Example of PERK response
- elF2a - increases translation of specific genes such as CHOP that encodes for a proapoptotic factor Chop.
- Chop activates the transcription of BIM and PUMA (proapoptic) and represses the expression of MCL-1 (prosurvival)