Chapter 12

ApoptosisBy

Douglas R. Green

12.1 Introduction

• Programmed cell death is a developmental process that usually proceeds by apoptosis.

• Apoptosis is also the mode of cell death occurring in a variety of other settings.– It has roles in:

• normal homeostasis• inhibition of cancer• disease processes

• Most animal cells possess the molecules comprising the pathways that can cause death by apoptosis.– These pathways are activated by

appropriate stimuli.

12.1 Introduction

12.2 Caspases orchestrate apoptosis by cleaving specific substrates

• Proteases called “caspases” fall into three types: – Initiator– Executioner – Inflammatory

• The first two types function in apoptosis.

• The morphological and biochemical features of cells undergoing apoptosis are caused by the action of the executioner caspases on their substrates.

• Many substrates for caspases have been identified.– In some cases the effects of their

cleavage on the cell are known.

12.2 Caspases orchestrate apoptosis by cleaving specific substrates

12.3 Executioner caspases are activated by cleavage, whereas

initiator caspases are activated by dimerization

• Cleavage of executioner caspases at specific sites is necessary and sufficient for their activation.

• This cleavage is usually mediated by the initiator caspases.

• Initiator caspases are activated by adaptor molecules that contain protein-protein interaction domains called death folds.

12.3 Executioner caspases are activated by cleavage, whereas initiator caspases are activated by dimerization

12.4 Some inhibitors of apoptosis proteins (IAPs) block caspases

• The inhibitors of apoptosis proteins comprise a family of proteins with different functions.– Some of these proteins:

• bind to and inhibit caspases • induce their degradation by the

proteasome

• Since executioner caspases are activated by cleavage, and since these caspases can cleave and activate each other…– …any proteolytic activity of the caspases will

be rapidly amplified in cells, resulting in their death by apoptosis.

• It is important that there be mechanisms present to limit potential “accidental” activation of caspases in cells that have not been signaled to die.

12.4 Some inhibitors of apoptosis proteins (IAPs) block caspases

12.5 Some caspases have functions in inflammation

• In addition to the initiator and executioner caspases, another set of proteases in this family acts to process cytokines rather than regulate apoptosis.

12.6 The death receptor pathway of apoptosis transmits external signals

• Two well-characterized pathways of apoptosis are:– the death receptor (extrinsic) pathway– the mitochondrial (intrinsic) pathway

• Caspase activation and apoptosis are induced by the binding of specialized ligands in the TNF family to their receptors (death receptors).

12.7 Apoptosis signaling by TNFR1 is complex

• Binding of TNF to one of its receptors, TNFR1, induces both apoptotic and antiapoptotic signals.

12.8 The mitochondrial pathway of apoptosis

• Most apoptosis in mammalian cells proceeds via a pathway in which:– the mitochondrial outer membranes are

disrupted– thus, releasing the contents of the mitochondrial

intermembrane space into the cytosol

• Mitochondrial outer membrane permeabilization (MOMP) is a key feature of this pathway.

12.9 Bcl-2 family proteins mediate and regulate MOMP and apoptosis

• The Bcl-2 family proteins are central to the mitochondrial pathway of apoptosis.

• There are 3 classes of Bcl-2 proteins that induce, directly cause, or inhibit MOMP.

12.10 The multidomain Bcl-2 proteins Bax and Bak are required

for MOMP• Bax and Bak:

– are essential for the permeabilization of the mitochondrial outer membrane

– are required for the mitochondrial pathway of apoptosis

• Bax and Bak probably directly cause the membrane disruption associated with MOMP.

12.11 The activation of Bax and Bak are controlled by other Bcl-2 family

proteins• The antiapoptotic members of the Bcl-2

family block the permeabilization of the mitochondrial outer membrane by Bax and Bak.

• The BH3-only proteins of the Bcl-2 family either:– directly activate Bax and Bak or – interfere with the antiapoptotic Bcl-2 protein

functions

12.12 Cytochrome c, released upon MOMP, induces caspase activation

• Holocytochrome c triggers the activation of cytosolic APAF-1.

• Cytosolic APAF-1 binds and activates caspase-9.

12.13 Some proteins released upon MOMP block IAPs

• The mitochondrial intermembrane space proteins Smac and Omi antagonize the caspase-inhibitory activity of IAPs.

12.14 The death receptor pathway of apoptosis can engage MOMP

through the cleavage of the BH3- only protein Bid

• Caspase-8, activated upon ligation of death receptors, cleaves the BH3-only protein Bid.– This activates Bid.

• Bid then triggers Bax and Bak to cause MOMP, thereby engaging the mitochondrial pathway of apoptosis.

• Bid acts as a link between the two apoptotic pathways.

12.14 The death receptor pathway of apoptosis can engage MOMP through the cleavage of the BH3- only protein Bid

12.15 MOMP can cause caspase-independent cell death

• Once MOMP occurs, cells generally die even if caspase activation is blocked or disrupted.

• The precise mechanisms of this cell death are not fully known.

12.16 The mitochondrial permeability transition can cause

MOMP• In some forms of cell death, the

mitochondria are disrupted by a change in the mitochondrial inner membrane.– This leads to swelling and rupture of

the organelle.

12.17 Many discoveries about apoptosis were made in nematodes

• Apoptosis in nematodes follows a simple pathway with similarities to the mitochondrial pathway of apoptosis in the vertebrates.

12.18 Apoptosis in insects has features distinct from mammals and

nematodes• Apoptosis in insect cells follows a

pathway with some similarities to the mitochondrial pathway of apoptosis in vertebrates.

12.19 The clearance of apoptotic cells requires cellular interaction

• The removal of apoptotic cells from the body occurs by an active process.

12.20 Apoptosis plays a role in diseases such as viral infection and

cancer• Viral infection and cancer are

conditions in which apoptotic pathways may be blocked.

12.21 Apoptotic cells are gone but not forgotten

• The uptake and clearance of apoptotic cells has lasting effects on the immune system.