Types of cell death

Question 1

What are the two main types of cell death?

Answer 1

• Necrosis
• Apoptosis

Question 2

What is necrosis?

Answer 2

A passive, accidental process resulting from severe trauma or toxicity - tends to affect clusters of cells.

Necrosis is the oldest known form of death, and was thought to be an “accidental” form

Question 3

What is apoptosis?

Answer 3

• Active & programmed cell death, undertaken by individual cells in response to stimuli.
• Apoptosis requires ATP and enzyme activity.

Question 4

What is the difference in outcome of necrosis compared to that of apoptosis?

Answer 4

• Necrosis releases cell contents and leads to inflammation
• Apoptosis dismantles cells in an organized way and usually does not cause inflammation (or to a lesser extent).

Question 5

For many years these were the only 2 recognized forms of cell death, and were thought to be mutually exclusive:

What additional mechism of cell death was subsequently discovered?

Answer 5

Autophagy was discovered as an additional mechanisms by which specific cells could die.

Now, many variants on these types are known, and it is also clear that there is overlap and the processes are not separate, but can cross over.

Question 6

What is autophagy?

Answer 6

A regulated form of cell degradation (via lysosomes) in response to specific stimuli (e.g. cellular starvation).

• It can be cytoprotective or lead to cell death

Question 7

State the morphologies (physical characteristics) of apoptosis

Answer 7

• Cell shrinking
• Blebbing
• Membrane integrity maintained in early steps
• Chromatin condensation and DNA laddering
• Nuclear fragmentation
• Condensed membrane-bound cellular fragments
• Depolymerization of cytoskeleton

Question 8

State the morphologies (physical characteristics) of necrosis

Answer 8

• Cell swelling
• Loss os membrane integrity
• Organelle swelling
• No DNA laddering

Question 9

State the morphologies (physical characteristics) of autophagy

Answer 9

• No dramatic changes in cell size
• Increased number of double membrane autophagic vesicles
• Degradation of Golgi, polyribosomes and the ER
• Partial chromatin condensation
• No DNA laddering

Question 10

Draw a diagram showing the multiple overlapping pathways of cell death

Answer 10

• Necroptosis
• Ferroptosis
• Anoikis
• Mitophagy

They are multiple overlapping pathways, some triggered by similar signals but dependent on cell environment.

Question 11

What is necroptosis?

Answer 11

A regulated form of necrosis that occurs if apoptosis is blocked

Question 12

What is ferroptosis?

Answer 12

An iron-dependent form of apoptosis

Question 13

What is anoikis?

Answer 13

A specific form of apoptosis triggered by loss of cell-cell adhesion

Question 14

What is mitopathy?

Answer 14

Mitophagy is cell death involving mitochondrial degradation

Question 15

Explain autophagy

Answer 15

Autophagy has a house-keeping function. In contrast to the ubiquitin-proteasome system,which only degrades ubiquitinated proteins, autophagy can engulf and degrade large portion of cytoplasm in a highly regulated manner. There are 3 main types:

• Macroautophagy
• Microautophagy
• Chaperone-mediated autophagy

Autophagy was discovered by Christian de Duve.

Question 16

What is Macroautophagy characterised by?

Answer 16

The formation of a unique double-membrane organelle called the autophagosome.

Question 17

What is Microautophagy characterised by?

Answer 17

lysosomes engulf cytoplasmic materials by inward invagination of the lysosomal membrane.

Question 18

What is Chaperone-mediated autophagy?

Answer 18

This type of autophagy requires the chaperone hsc70, co-chaperones, and a lysosomal-associated membrane protein.

Question 19

Macroautophagy is a complex process consisting of several sequential steps:

Answer 19

1. induction
2. autophagosome formation
3. fusion of lysosome and autophagosome
4. degradation
5. efflux transportation of degradation products
6. autophagic lysosome reformation.

It requires several autophagy-related genes and their proteins (Atg)

Question 20

Under which conditions does autophagy occur?

Answer 20

• Autophagy occurs at a low, basal rate in most cells, but is up-regulated by stresses, especially starvation and lack of amino acids.
• mTOR (mammalian target of rapamycin) is one of the key regulators of cell growth. When nutrients are limited, mTOR is inactivated, which in turn induces autophagy.

Question 21

Dysfunctions of autophagy are involved in which diseases?

Answer 21

Dysfunctions of autophagy are involved in several diseases:

Crohn’s disease, a major type of inflammatorybowel disease.

Mutations in Atg16L1 have also been linked to the disease.

Question 22

Explain how mitophagy occurs

Answer 22

• Mitophagy is a variant of autophagy, involving the selective degradation of mitochondria. It removes damaged or stressed mitochondria, which is essential for cellular health.
• When mitochondria are damaged and lose their membrane potential mitochondrial PTEN-induced putative kinase protein 1 (PINK1) is stabilized and recruits Parkin, which ubiquitinates a number of mitochondrial membrane proteins, triggering selective mitophagy.

Question 23

Dysfunctions of mitophagy are involved in which diseases?

Answer 23

• Dysfunctions of mitophagy are involved in the neurodegenerative disease Parkinson’s disease.
• Mutations in PINK1, Parkin, PARK2, and PARK6 have been implicated in the disease, which involves excessive mitochondrial damage because damaged mitochondrial cannot be removed.

Question 24

Explain how nectroptosis occurs

Answer 24

• Necroptosis is often described as a regulated “necrosis” mediated by death receptors. It is an important defence against pathogen-mediated infections, especially viruses as infected cells cannot always undergo apoptosis.
• Basically, it starts as the extrinsic pathway of apoptosis but Caspase-8 activation fails. Morphologically it is similar to necrosis as cell swelling followed by rupturing of plasma membrane occurs, but it is not necrosis as it is regulated and not simply membrane damage.
• Instead, it involves a regulated pore formation in the plasma membrane to gives the same outcome as necrosis.

Question 25

Explain the decision point between the extrinsic pathway and necroptosis

Answer 25

1. Binding of immune ligands such as Fas, TNF, and LPS to death receptors (extrinsic pathway of apoptosis). This would normally activate caspase-8 via the DISC.
2. In normal circumstances, caspase-8 activates apoptosis and cleaves RIPK1 (inhibiting it), blocking the necroptosis pathway.
3. If caspase-8 isn’t activated, RIPK3 which further activates the MLKL by phosphorylation. This allows MLKL to oligomerize.
4. Phosphorylated MLKL translocates into the inner leaflet of the plasma membrane and disrupts the integrity of the cell, forming ion channels that allow ion to flood in, the cell swells and bursts.

Question 26

Explain ferroptosis

Answer 26

• Ferroptosis is a relatively recent discovery (2012). It is an iron- and oxidative stress-dependent form of cell death, caspase-independent, so clearly not apoptosis.
• The morphological characteristics include decreased mitochondria cristae, a ruptured outer mitochondrial membrane, and a condensed mitochondrial membrane.
• It has been suggested that it is important to control malignant cells.

Question 27

Explain how ferroptosis occurs

Answer 27

• Iron (Fe) with hydrogen peroxide oxidizes polyunsaturated fatty acid in membrane phospholipids.
• Normally these would be repaired by the antioxidant enzyme GPx4, using glutathione (GSH)
• If iron levels are too high or glutathione synthesis too low, the oxidized lipids cause membrane damage and cell death.