Apoptosis

APOPTOSISDR.AYESHA FARHEEN

Objectives

Definition of Apoptosis

ProcessDifferentiation from Necrosis

PathwaysApplied Aspect

APOPTOSIS

Greek word : Falling leaves (like in Autumn)

In the human body about 100,000 cells are produced every second by mitosis and a similar number die by apoptosis !!!

History

Apoptotic principle was first described by KARL VOGT in 1842

“APOPTOSIS” term was coined by JAMES CORMACK

2002 NOBEL PRIZE IN MEDICINE was awarded to SYDNEY

BRENNER,HORVITZ and JOHN.E.SULSTON for their work in identifying genes

that control Apoptosis.

John E Sulton won Nobel Prize in 2002

for his pioneering research in Apoptosis

PROGRAMMED CELL DEATH

CELL SUICIDE

What is Apoptosis?

DEFINITION : Apoptosis is a peculiar well controlled individual cell death that is

caspase mediated and leads to fragmentation of the cell and organelles into

numerous small buds, which are then engulfed by macrophages without

surrounding inflammation

An orderly disposal of cells that need to die

DNA has sustained too many injuries

Cell is infected with a virus

Cell needs to be removed for body parts to

be formed

Cell is just too old and ’ its

time has come’

The tail of a tadpole is absorbed via apoptosis.

Most of the embryo development involves programmed cell death

Importance of Apoptosis

1) Crucial for embryonic development

-Errors in Apoptosis can lead to Birth Defects

2) Important for maintaining homeostasis

- Cell death is balanced with mitosis to regulate cell number.

3) Improper regulation contributes to human disease

- Neurodegenerative diseases

Parkinson’s

Alzheimer’s

- Cancer

- Autoimmune diseases e.g. (diabetes type I)

- Viral diseases

What is Apoptosis NOT?

Apoptosis is NOT cell death after injury

Cell death after injury is called NECROSIS

Cells die by one of two mechanisms – necrosis or apoptosis

Two physiologically different processes

– Necrosis – death by injury

– Apoptosis – death by suicide

Apoptosis and necrosis have different characteristics

How Apoptosis Differs from Necrosis?

1. Apoptosis is intrinsically controlled, necrosis is not

2. Apoptosis is more rapid (12-24 hours) than necrosis

3. Apoptosis is induced by endogenous or exogenous stimuli, necrosis is always induced by exogenous harms

4. Apoptosis is limited to single or few cells at a time, and occurs among healthy cell population, necrosis is usually more extensive & occurs in tissue exposed to injuries

5. Cell cytoplasm shrinks in apoptosis and swells in necrosis.

6. Nucleosomes of apoptotic cells are 180 bp fragments, contrary to the irregular ones in necrosis

7. Apoptosis has no inflammation, while necrosis leads to liberation of pro-inflammatory mediators

8. Apoptosis has no systemic manifestations contrary to most inflammations

What happens during Apoptosis?

A programmed series of events occur

Cell shrinkage (condensation of cytoplasm)

Breakdown of mitochondria; release of cytochrome C

Nuclear condensation

Nuclear fragmentation

Cellular changes associated with apoptosis

Biochemical features of Apoptosis

• By activation of caspases

• Caspases activate DNAses

Protein Cleavage

• Cleavage into oligonucleosomes

• By Ca2+-and Mg2+-dependent endonucleases

DNA Breakdown

• Phosphatidylserine

• Thrombospondin

Phagocytic Recognition

Mechanisms of Apoptosis

Mechanisms of Apoptosis

Active cysteine residue in the catalytic site

Specificity in cleavage after an Asp residue

Synthesized as inactive zymogens (PROCASPASES)

Digestion of DNA starts after

2 hrs

3&4 hrs after initiation of apoptosis DNA is almost all degraded

DNA is fragmented with restriction endonucleases

Apoptosis induces 180 bp Laddering of DNA

• DNA cleaved into non-random fragments

• 180-200 bp fragments & multiples of this unit

Mechanism

Four stages of apoptosis have been defined:

i. Commitment to death by extracellular or intracellular triggers/signals

ii. Cell killing (execution) by activation of intracellular proteases (caspases)

iii. Engulfment of cell corpse by other cells

iv. Degradation of the cell corpse within the lysosomes of phagocytic cells

DAMAGE Physiological death signals

DEATH SIGNAL

PROAPOPTOTICPROTEINS

ANTIAPOPTOTICPROTEINS

MITOCHONDRIAL SIGNALS

APOPTOSIS

Stimuli for Apoptotic Cell Death in Mammals i. Growth factor deficiencies

ii. Ionizing radiation/ viral infection

iii. Free radical toxicity

iv. Death receptor activation (such as Fas or CD95 triggering)

v. Metabolic or cell cycle disturbance

Stimuli for Apoptotic Cell Death

Death Factors

Definition: Cytokines that activate an apoptosis program by binding to their specific receptor. Typical examples of death factors are:

1. Fas ligand,

2. TNF (tumor necrosis factor) and

3. TRAIL (TNF-related apoptosis-inducing ligand).

- Apoptosis can also be induced by cytotoxic T-lymphocytes using the enzyme granzyme.

Apoptosis

Extrinsic pathway

Intrinsic pathway

Granzyme pathway

Extrinsic pathway

Fas engagement. FADD= Fas associated death domain.

Extrinsic pathway

Intrinsic Pathway

AIF= Apoptosis inhibitory factor; IAPs= Inhibitors of apoptosis proteins; Apaf-1= apoptosis protease activating factor

Intrinsic Pathway

Bcl2 was the first apoptosis-related gene ,recognized to play a role tumor genesis

BCL-2 is a human proto-oncogene located on chromosome 18.

Its product is an integral membrane protein (called Bcl-2) located in the membranes of the endoplasmic reticulum , nuclear envelope, and in the outer membrane of mitochondria.

The gene was discovered as the translocated locus in a B-cell leukemia (hence the name). This translocation is also found in some B-cell lymphomas

Apoptosis and Cancer

Apoptosis and Cancer

• Apoptosis does not occur in Cancer

• Cancerous cells trick and skip Apoptosis in number of ways

Inactivation of p53 [shooting the guard]

Produce Bcl-2 or a protein which mimics Bcl-2

Inhibits expression of Apaf-1

Apoptosis and Autoimmune Disease

Autoimmune Lymph Proliferative

Syndrome[ALPS]

Apoptosis doesnot occur in self

reactive T & B cells

RBC

Hemolytic

Anemia

Neutrophil

Neutropenia

Platelets

Thrombocyto

penia

Apoptosis and HIV

Infected CD4

cell induces

Apoptosis in

surrounding

cells

• Deactivated

Bcl-2

• Decreases

CD4

Glycoprotein

markers on

innocent T

cells ,getting

them killed

Infected CD4 cell

avoids Apoptosis in

itself

Decreases

Phosphatdylserine

marker for itself

allowing longer

survival.

Overview

Apoptosis is a good thing

Too little of a good thing is

bad [Cancer]

Too much of a good thing is

also bad [HIV]

Sources

Textbook of Medical Physiology –Guyton & Hall [12th edition]

Review of Medical Physiology-William F Ganong [24th edition]

Internet Sources :

• www.ncbi.nlm.nih.gov/books/NBK26873

• http://en.wikipedia.org/wiki/Apoptosis

Articles :

Apoptosis- Molecular mechanisms and Pathogenicity

http://link.springer.com/article/10.1023/A:1009616228304#pg2

http://www.excli.de/vol8/Rastogi_08_2009/Rastogi_030809_proof.pdf

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“Queries?

THANK YOU