Biochemistry of apoptosis, examples of pro- and anti-apoptotic genes/proteins. Caspases. Role of mitochondria in cell death.

Apoptosis is recognized as a pathway for cell death. The name apoptosis was attributed to the process due its appearance as the cell breaks into fragments. Unlike its counterpart, Apoptosis is well known for its inability to elicit an inflammatory response, which one of the many ways it differs from necrosis.

Causes of apoptosis

often tied to the purpose apoptosis serves in the body, thus when reviewing apoptosis one may consider classifying them into two categories : Physiological apoptosis and Pathological apoptosis

Physiological apoptosis

Seen to occur during natural development of an organism particularly in highly proliferative tissue and hormone response tissue such as the skin, gastrointestinal; lining, endometrium, bone marrow. The pathway of apoptosis removes unwanted cells from the tissues without leading to an inflammatory response.

Pathological apoptosis

In the scenario of a pathological process such as infection, this pathway plays an important role important role in the removal of damaged cells that cannot be repaired. Other examples may include the exposure to radiation or cytotoxic drugs leads to damage of the DNA.

Biochemistry of apoptosis

Apoptosis when simplified may be viewed as process that is elicited when the balance between "death signals" and "survival signals" has been disrupted which consequently lead to the activation of enzymes called Caspases

Caspases: cysteine proteases that cleave proteins after aspartic acid residues

Two pathways lead to the activation of caspases : Mitochondrial pathway and death receptor pathway

Mitochondrial (Intrinsic) pathway

• implicated in both physiological and pathological apoptosis.

At a glance :

• Increase permeability of the mitochondrial membrane-> cytochrome C leaking into cytoplasm (a protein capable of inducing apoptosis) -> caspase activation -> cell death!!!

In depth

• Anti apoptotic proteins : a family of 20 proteins, the prototype of which is Bcl-2. another well known protein, Bcl-xL
  • Bcl-2, Bcl-XL, Bcl-w, Mcl-1
  • These two proteins are often seen in healthy cells in the presence of sufficient GF (growth factor) and other stimuli
  • These proteins maintain the normal permeability of the mitochondrial membrane.
• Pro apoptotic proteins : Bak and Bax
  • the increase the permeability of the mitochondrial membrane to cytochrome C and release of it into the cytosol.
• BH3 proteins "sensors" are activated by changes that favor apoptosis such as; decrease in GF and survival signals, DNA damage as a result of exposure to certain agents, or a significant accumulation of misfolded proteins
• When BH3 are activated -> the balance tips in favor of apoptosis which leads to dimerization and then facilitate the escape to cytochrome C from mitochondria matrix to cytosol
  • In healthy cells, Bax is found as monomer in the cytosol however, they are found in the mitochondria during apoptosis.
• When the cytochrome C enters the cytosol, Cytochrome C along with other ceratin cofactors facilitates the activation of caspase 9.

Death (Extrinsic) pathway

• On the surface of many cells are what some call Death receptors
  • Examples:
    • members of TNF receptor family: type I TNF
    • Fas (CD95)
• Death receptor elicit response from the T cells because they exhibit Fas ligand (FasL) which crosslink with Fas molecule.
  • thanks to the death domains they bind to adaptor proteins
• The former step leads to recruitment and activation of caspase 9
• This followed by the activation of downstream caspases which are responsible for cleaving targets required for the activation of other enzymes that lead to the degradation of the cells
• One example of where the death receptor pathway is involved : the removal of cells that has lost self tolerance.

These two pathways which respectively activates caspase 8 and caspase 9 converge and eventually lead to the activation of the enzymes that cleave the degrade cellular proteins and nucleus.

• Apoptosis is often considered a self cleaning process as the apoptotic fragments/blebs is produce signals that lead to phagocytosis.
  • One example: the presence of phospatidylyserine which is typically present on the inner leaflet of the plasma membrane.
  • Another example soluble factors released by apoptotic cells
• The function of this clearance is important because if they remain, they retain the potential to be able to induce inflammation.

Morphological features of Apoptosis

Microscopic features

• Karyorhexis
• Chromatin condensation
• Fragmented DNA
• Apoptotic bodies : membrane bound pieces of cytosol and organelles

Kumar, V., Abbas, A. K., Aster, J. C., & Perkins, J. A. (2018). Robbins basic pathology (10th ed.). Elsevier. Chapter 2 : Cell injury, Cell death and Adaptations Pg38-40

Wang C, Youle RJ. The role of mitochondria in apoptosis*. Annu Rev Genet. 2009;43:95-118. doi: 10.1146/annurev-genet-102108-134850. PMID: 19659442; PMCID: PMC4762029.