1. Cellular responses to stress (Adaptations, injury and death) (4 of 5)
Ali Al Khader, MD
Faculty of Medicine
Al-Balqa’ Applied University

2. **Apoptosis**
The cell here decides to die by activating enzymes that degrade its own DNA and proteins
The name means: “falling off” because the fragments of the cell appear falling off
Cell membrane is intact but it is altered that the cell and its fragments appear good targets for phagocytosis
The dead cells and its fragments are cleared rapidly before contents leaked out…inflammation doesn’t occur

3. Apoptosis
Necrosis
Membranes are intact but they are altered and are avid for phagocytosis
Membrane integrity is lost
The cell is shrunken
The cell is swollen
Dead cells and fragments are cleared rapidly before inflammation can occur
Contents are leaked out inducing surrounding inflammation
Can be physiologic…mention 2 examples
Always pathological (always preceded by irreversible injury)

4. Apoptosis in physiologic situations
Embryogenesis
Involution of hormone-dependent tissues upon hormone deprivation
…mention 2 examples
Cell loss in proliferating cell populations, such as intestinal crypt epithelia…tissue renewal
Elimination of cells that have served their useful purpose…like neutrophils after end of inflammation (because there are no signals now)
Elimination of self-reactive lymphocytes

5. Apoptosis in pathologic conditions
DNA damage: -radiation
-cytotoxic anticancer drugs
-extremes of temperatures
-…etc
…even causes of necrosis can also cause apoptosis especially when the
main damage is to the DNA or proteins
Accumulation of misfolded proteins: due to
-gene mutations (DNA damage) or
-direct misfolding of proteins such as by free radicals
…these misfolded proteins accumulate in the ER causing ER
stress which culminates in apoptosis

6. Apoptosis in pathologic conditions, cont’d
Certain infections, esp., viruses:
-apoptosis is induced by the virus…examples: adenovirus and HIV or
-apoptosis is induced by the immune response…cytotoxic T
cells kill virus-infected cells…example: hepatitis viruses
….cytotoxic T cells also kill neoplastic cells
Death of parenchymal cells due to obstruction of organ ducts…e.g., pancreas, parotid, and kidney

7. Morphology of apoptosis
Nucleus: chromatin condensation fragmentation (karyorrhexis) into nucleosome-sized pieces
Cell shrinkage
Formation of cytoplasmic buds these buds fragment into apoptotic bodies
…these fragments are rapidly phagocytosed so: no inflammation and
the whole process may be invisible on H&E

8. Apoptosis in normal colonic mucosa

9. Mechanisms of apoptosis
2 pathways:
1- The Mitochondrial (Intrinsic) Pathway of Apoptosis…most situations
2- The Death Receptor (Extrinsic) Pathway of Apoptosis
Both pathways activate caspases that are the main players here
they are cysteine proteases that cleave proteins after
aspartic residues

10. = BH3 proteins = Caspase 9 in the mitochondrial pathway = CD95
Synthesized when there are survival signals (like growth factors)
= CD95
Also inhibit antiapoptotic proteins (Bcl-2 and Bcl-XL)
These receptors have death domain in their cytoplasmic part
= Caspase 9 in the mitochondrial pathway
Some also inhibit caspase antagonists
= BH3 proteins
= Pro-apoptotc proteins..they dimerize and form channel in mit. membrane..so cytochrome c and others escape to cytosol

11. Its ligand is called: Fas ligand…present on activated T lymphocytes: for killing self-reactive lymphocytes and killing target cells by cytotoxic T cells
Caspase 8 in this pathway
These activate caspase 8 after they bound to the death domain of the receptor that bound to the ligand
May activate pro-apoptotic member of BcL-2 family called Bid which activates mitochondrial pathway
*Caspase antagonist called FLIP block activation of caspases in this pathway…some viruses produce homologues to FLIP to keep the infected cell alive
May also secrete granzyme which directly activates caspases (not mitochondrial or death-receptor pathway)

12. Caspases
The executioner caspases when activated activate nucleases degrade nuclear matrix and cytoskeleton fragmentation
Degrade DNA
and
nucleoproteins

13. Clearance of apoptotic cells
Apoptotic cells produce “eat me” signals
Phosphatidyl serine is normally present on the inner leaflet of plasma membrane…it becomes on the outer leaflet in apoptotic cell…recognized by macrophages
Also secreted factors by dying cells induce phagocytosis
Some apoptotic bodies express adhesive glycoproteins that are recognized by macrophages

14. Necrosis and apoptosis may coexist
DNA damage (seen in apoptosis) activates an enzyme called poly- ADP(ribose) polymerase, which depletes cellular supplies of nicotinamide adenine dinucleotide, leading to a fall in ATP levels and ultimately necrosis
Even in common situations such as ischemia, it has been suggested that early cell death can be partly attributed to apoptosis, with necrosis supervening later as ischemia worsens
Apoptosis induced by some pathologic stimuli may progress to necrosis…like some stimuli of DNA damage

15. Examples of Apoptosis Growth Factor Deprivation:
-hormone-sensitive cells deprived of the hormone,
-lymphocytes that are not stimulated by antigens and cytokines
-neurons deprived of nerve growth factor
*In all these situations, apoptosis is triggered by the mitochondrial pathway:
-activation of pro-apoptotic members of the Bcl-2 family
-decreased synthesis of Bcl-2 and Bcl-xL

16. Examples of Apoptosis, cont’d
DNA Damage:
When DNA is damaged, the p53 protein arrests the cell cycle (at the G1 phase). Why?
…if the damage is too great to be repaired successfully, p53 triggers apoptosis
(mitochondrial pathway)
…When p53 is mutated or absent, cells with damaged DNA survive, and in this case What may happen?

17. Examples of Apoptosis, cont’d
Accumulation of Misfolded Proteins:
-First: unfolded protein response increased synthesis of chaperones in ER (which control proper folding)
-Then: if the accumulation is more severe ER stress
activates mitochondrial pathway
**Cell death by this mechanism is a feature of many neurodegenerative diseases:
-Alzheimer
-Huntington
-Parkinson diseases
-..etc