Apoptosis/Cell Death

What are the 3 types of cell death?

Type I: programmed apoptotic cell death (apoptosis)

Type II: programmed non-apoptotic cell death (autophagy)

Type III: non-programmed cell death (necrosis)

What happens in the plasma membrane during necrosis?

Some blebbing and the membrane ruptures, releasing cell content

What happens in the plasma membrane during apoptosis?

Blebbing and plasma membrane integrity preserved

What happens in the nucleus during necrosis?

Some chromatin condensation

What happens in the nucleus during apoptosis?

Very distinct chromatin condensation

What happens in the cytoplasmic organelles during necrosis?

Swelling

What happens in the cytoplasmic organelles during apoptosis?

Minor changes

What is unique about necrosis?

There are no apoptotic bodies and the process is heavily inflammatory

What is unique about apoptosis?

There is formation of apoptotic bodies but the process is non-inflammatory

Which type of cell death involves irreparable damage?

Necrosis

What are the possible causes of damage in necrosis?

Poisons and toxins, mechanical injuries, and physical injuries

What poisons and toxins can lead to necrosis?

Chemical (alkali, mercury), infectious agents (bacteria) and toxins (snake venom)

What mechanical injuries can lead to necrosis?

Cutting and crushing

What physical injuries lead to necrosis?

Radiation (such as with cancer therapy)

Why does the plasma membrane rupture in necrosis? What are the consequences?

Swelling of membrane-bound organelles from injury leads to rupture. Leakage of cellular content into the ECM results, and this leads to inflammation (recruit immune cells)

Which cell organelles swell in necrosis?

ER

lysosomes

mitochondria

nucleus

Golgi

What is blebbing?

Bulge of the plasma membrane as a result of cytoskeleton changes. Bulges contain cytoplasm, DNA, organelles and are released as apoptotic bodies

Slight occurrence in necrosis but prominent in apoptosis

What results from freed lysosomal enzymes in necrosis?

This is the final stage of plasma membrane rupture

Degradation of nucleic acids and proteins causes inflammation and increases phagocytotic activity of immune cells to eat the decayed material

How is p53 activated in apoptosis?

Normally, p53 is stabilized by MDM2 and degraded, but DNA damage triggers the phosphorylation of p53 and release from MDM2, allowing p53 to bind to DNA and activate transcription of pro-apoptotic genes

When does p53 first arrest cell growth?

G1/S checkpoint. If DNA can be repaired, it is fixed and S phase begins. If DNA damage is too extensive, p53 induces apoptosis

What are the pro-apoptotic proteins activated by p53?

PUMA, BID, BAX

What do pro-apoptotic proteins inhibit?

BCL-2 survival

What initiates the extrinsic pathway of apoptosis?

Signal comes from outside the cell as a reduction of signaling molecules or infection from a virus

What initiates the intrinsic pathway of apoptosis?

Signal comes from inside the cell as oxidative stress, ROS free radicals, or radiation

Tumor necrosis factor (TNF) family

death receptors triggered by ligands which can transduce apoptotic OR survival signals

Fas ligand

Transmembrane protein belonging to killer cell in TNF family.

Fas as a soluble ligand vs transmembrane

Soluble: released from cell surface but not efficient at inducing apoptosis

Transmembrane: binds to Fas receptor on dying cell to induce cell death

Fas receptor

Transmembrane receptor on dying cell that forms trimer complex upon binding Fas ligand.

aka Fas, CD95

What is the death domain of Fas receptor?

Death effector domain at C terminus allows receptor to bind to other proteins

Fas-associated protein with death domain (FADD)

Adaptor protein that binds pro-caspase 8

Death-inducing signaling complex (DISC)

Fas receptor + FADD + procaspase 8

can also include other proteins

Caspases

Family of cysteine-aspartic acid proteases that regulate apoptosis as initiators and executioners

Initiators

Caspase 8 and 9

Executioners

aka effectors

Caspase 3

Which cells have high levels of DISC formation and increased levels of active caspase 8?

Type I

What does activated caspase-8 do?

After cleavage, procaspase 8 becomes active caspsase 8 and cleaves inactive caspase 3 to activate it

What does Caspase 3 do?

The executioner: cleaves multiple structural and regulatory proteins which are necessary for cell survival, inducing apoptosis

Caspsase-activated DNase (CAD)

ICAD activated by cleavage of Caspase 3, and active CAD degrades chromosomal DNA within nuclei -> condensation

Other functions of Caspase 3

Induces cytoskeletal reorganization and cellular disintegration into apoptotic bodies

How is apoptotic material safely disposed of?

apoptotic bodies are internalized by phagocytes

Which type of cells have lower levels of DISC and lower levels of activation of caspase 8?

Type II cells

How is apoptosis induced in Type II cells?

Apoptotic signal is amplified, achieved when caspase 8 cleaves protein Bid into t-Bid

Anti-apoptosis protein

Bcl2

Pro-apoptotic proteins

Bid, Bax, Bak, Bad

How does Bad work?

Binds Bcl2 and makes Bcl2 unable to prevent Bax/Bak interaction

Which proteins oppose Bcl2 and form an oligomeric pore in the MOM?

Bak/Bax

Which protein promotes insertion of Bax into the MOM?

Bid

Where does the intrinsic apoptotic pathway take place?

Mitochondrial outer membrane (MOM)

t-Bid

translocates into mitochondria and inserts in MOM where it facilitates formation of pore by BAX/BAK complex

What is the purpose of the pore in the MOM?

Allows cytochrome c to leave inner membrane and escape into cytoplasm

Cytochrome C

part of the electron transport chain in the mitochondria that binds Apaf-1 to form apoptosome

Apaf-1

Apoptotic protease activating factor-1

Apoptosome

Apaf-1, cytochrome c, caspase 9 complex

Pro-caspase 9 vs Caspase 9

Pro-caspase 9 joins Apaf-1 and cytochrome C to make the apoptosome, where pro-caspase is activated as part of this complex

How is the caspase cascade initiated in the intrinsic pathway?

Caspase-9 in the apoptosome cleaves pro-caspase 3

BAD as a regulator of the intrinsic pathway

Binds Bcl2 to break Bcl2 contact with Bax and Bak, allowing t-Bid to bind Bax/Bak and start formation of MOM pore

What promotes early phagocytic uptake and disposal of cellular constituents?

Externalization of phosphatidylserine

Phosphatidylserine

normally on the inner leaflet of the plasma membrane; becomes exposed on the outer surface during apoptosis

How is the inflammatory response avoided in apoptosis?

Phosphatidylserine (PS) exposure during apoptosis promotes phagocytosis, degrading cellular components without inflammation

T/F: apoptosis is always bad

False! Apoptosis is what allows for tissue remodeling as the hand forms, removing tissue between digits. It also allows for maintenance of organ size and function.

Autophagy

cellular constituents are moved to the lysosome for degradation and can be recycled

what cellular constituents are degraded in autophagy?

Organelles and macroproteins

Phagophore

multiprotein complex generated in the cell that forms autophagosome

Autophagosome

double-membrane vesicular structure that fuses with lysosomes to become autophagolysosomes

Autophagolysosome

autophagosome + lysosome

degrades cellular constituents and recycles them to be used in later processes

What processes can degraded material from autophagy be recycled for?

Amino acids for protein synthesis

Fatty acids for energy and ATP generation

What happens when there is too much autophagy activity?

Excessive-self destruction and activation of apoptosis