Programmed Cell Death APOPTOSIS

Cell commits suicide by apoptosis.

Cellular homicide is necrosis

Definition:

Apoptosis is the programmed cell death in which there is carefully coordinated collapse of cell, protein degradation , DNA fragmentation.

Here the cell destined to die activate enzymes that degrade the cells own nuclear DNA and nuclear and cytoplasmic proteins.

Physiologic causes:

1)The programmed destruction of cell during embryogenesis. It is programmed because ---- it is death of specific cell types at defined times during development.

2)Hormone-dependent physiologic involution, such as involution of the endometrium during the menstrual cycle.

3)Cell deletion in proliferating cell population, such as intestinal crypt epithelium

4) Death of cells that have served there useful purpose, such as neutrophils in an acute inflammatory respone.

5) Elimination of potentially harmful self reactive lymphocytes.

Pathological causes :

Cell death produced by a variety of mild injurious stimuli like

- heat,

- radiation,

- cytotoxic cancer drugs, etc.

that cause irreparable DNA damage that in turn triggers cell suicide pathways.

2) Cell injury in certain viral diseases such as viral hepatitis.

3) Cell death in tumors.

Morphological changes in Apoptosis

Apoptosis usually involves single cells or clusters of cells that appear on H & E-stained sections as round or oval masses with intensely eosinophilic cytoplasm.

Following the morphological changes that charcterize apoptosis:

1)Cell Shrinkage : cell is smaller in size, with dense cytoplasm and the organelles are tightly packed.

2)Chromatin condensation: Most characteristic feature of apoptosis. The chromatin aggreagates peripherally, under the nuclear membrane into dense masses of various shape and sizes.

3) Formation of cytoplasmic blebs : As the cell rapidly shrinks some part of cytoplasm with intact cell membrane will bleb out of the cell forming membrane blebs.

4) Apoptotic bodies :Ultimately, karyorrhexis occurs, this is reflected in fragmentation of DNA into nucleosome-sized pieces, presumably through the activation of endonucleases . The cells rapidly shrink, form cytoplasmic buds, and fragment into apoptotic bodies composed of membrane-bound vesicles of cytosol and organelles .

5) Phagocytosis of apototic cell bodies, by macrophages : these fragments or apoptotic bodies are quickly extruded and phagocytosed or degraded by lysosomes.

Ultrastructural

features of apoptosis.

Some nuclear fragments

show peripheral

crescents of compacted

chromatin,

whereas others are

uniformly dense.

Histological diagnosis of apoptosis becomes difficult as:

1) Because the cell shrinkage and formation of apoptotic bodies are rapid and the fragments are quickly phagocytosed.

2) Apoptosis doesnot elicit inflammatory response.

What are Caspases?

Caspases are members of family of cysteine proteases, and are involved in protein hydrolysis which is the specific feature of apoptosis.

Extrinsic Pathway :

Also referred to as death receptor initiation pathway.

What are death receptors ??

Death receptors are the members of the Tumor Necrosis factor receptor family. (TNF)

The best know death receptors are type 1 TNF receptors (TNF 1) and a related protein Fas or CD95.

These receptors contain a cytoplasmic domain (region) which is referred to as death domain.

Fas, which is a death receptor is present on the cell surface. This Fas is then cross linked with its ligand.

3 or more Fas molecules come together

on cytoplasmic side of this receptor ligand complex there is death domain

which forms a binding site for adapter protein which also contains death

domain and is called FADD i.e

Fas associated death domain.

This complex will then bind to inactive form of caspase 8 i.e procaspase 8

Multiple procaspase will be activated by cleaving on another ,to form active caspase 8

Inhibition of Extrinsic pathway of apoptosis:

Inhibited by a protein called FLIP, which bind to procaspases 8 and inhibit there cleavage.

Intrinsic Pathway:

This pathway of apoptosis is the result of increased mitochondrial permeability and release of proapoptotic molecules in the cytoplasm.

There are certain protective molecules in the mitochondria which are referred to as anti apoptotic molecules which prevents apoptosis in normal cells.

Two main antiapoptotic molecules are:

- Bcl-2

- Bcl-x

These anti apoptotic proteins are members of Bcl family, whose production is stimulated by growth factors.

They normally reside in mitochondrial membrane and cytoplasm of the cell. Under the condition of stress they are replaced by proapoptotic proteins.

Due to cell injury, Mitochondrial membrane becomes permeable.

Release of cytochrome C, which activate caspases cascade.

Cytochrome c binds to protein called Apaf-1 i.e apoptosis activating factor -1.

This complex activates caspase 9.

Thus loss of antiapoptotic factors and activation of proapototic proteins leads to activation of caspases.

The Execution Phase:

This the final phase of apoptosis which is mediated by proteolytic cascade.

caspase 3 and caspase 6 acts as executioner caspases.Intiator caspase are cleaved to there active form by two pathways.

they will activate excecutioner caspases, which act on several cellular components.

Removal of dead cells.

Apoptotic cells and their fragments have marker molecules on their surfaces that facilitate uptake and disposal by adjacent cells or phagocyte.

Alterations on the cell surface leads to formation of marker molecules, which allow the early recognition and phagocytosis of apoptotic cells without release of proinflammatory mediators. The process is so efficient that dead cells disappear without leaving a trace, and inflammation is virtually absent.