2010 Lecture 18

From CellBiology

Cell Death 2

Introduction

Neuron necrosis and apoptosis (tem)
Apoptotic nucleus (tem)

This second lecture will focus upon programmed cell death, known as apoptosis. The previous lecture Cell Death 1 looked at mainly stress and pathological processes.

This single term "apoptosis" describes the way in which the majority of cells within our body are removed every day. While the morphological changes associated with this process are the same in all cells, the many different signaling pathways that "trigger" this process can be quite different.

Apoptosis Movie | File:Apoptosis Bcl-2.mov

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Archive

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Nobel Prize 2002

Physiology or Medicine 2002 - genetic regulation of organ development and programmed cell death


C.elegans cell death * Sydney Brenner (b 1927), Berkeley, CA, USA, established C. elegans as a novel experimental model organism. This provided a unique opportunity to link genetic analysis to cell division, differentiation and organ development – and to follow these processes under the microscope. Brenner's discoveries, carried out in Cambridge, UK, laid the foundation for this year's Prize.
  • John Sulston (b 1942), Cambridge, England, mapped a cell lineage where every cell division and differentiation could be followed in the development of a tissue in C. elegans. He showed that specific cells undergo programmed cell death as an integral part of the normal differentiation process, and he identified the first mutation of a gene participating in the cell death process.
  • Robert Horvitz (b 1947), Cambridge, MA, USA, has discovered and characterized key genes controlling cell death in C. elegans. He has shown how these genes interact with each other in the cell death process and that corresponding genes exist in humans.





Regulator → Adaptor → Effector
C. elegans      Ced-9 → Ced-4 → Ced-3 → Death
Vertebrates      Bcl-2 → Apaf-1 → Caspase-9 → Caspase-3 → Death

Regulators can initiate or block apoptosis, the regulators shown block apoptosis.


Apoptosis?

Term used to describe "Programmed Cell Death" originally by Kerr, J. F., Wyllie, A. H. & Currie, A. R. Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br. J. Cancer 26, 239-257 (1972). PMID: 4561027

Greek, ptosis = falling, as in when leaves fall from a tree in autumn.

Apoptosis Signals

Some signal pathways (apoptosis is lower left)

A diverse group of signals can induce apoptosis

Selective process for deletion of cells

Apoptosis - genetic errors

  • Superfluous
  • Infected
  • genetic errors
  • transformed cells


Process required for

  • Embryogenesis
  • Metamorphosis
  • Endocrine dependent tissue atrophy
  • Normal tissue turnover
  • Variety of pathologic conditions

Apoptosis Examples

Development

Apoptosis in digit development

Limb development

Ossification

  • Death of chondrocytes

Nervous System

  • Death of neurons
  • Disruption of Brain Development

Links: limb - FIG5 Development - TUNEL staining

Mammary Gland

Mammary apoptosis

  • balances proliferation
  • occurs each menstrual cycle
  • occurs during "involution" when breast-feeding ceases

Apoptotic Cell Morphology

Apoptosis DNA ladder

The following cellular changes occur in sequence during apoptosis.

  • loss of cell membrane phospholipid asymmetry
  • Condensation of chromatin
  • Reduction in nuclear size JCB - Nucleus changes
  • Internucleosomal DNA cleavage TUNEL staining
    • DNA ladder
  • shrinkage of the cell
  • membrane blebbing
  • breakdown of the cell into membrane-bound apoptotic bodies (apoptosomes)
    • bodies then phagocytosed by other cells

A number of different experimental techniques have been developed to identify these changes. Apoptosis Methods

Two Main Pathways

Apoptosis Signal transduction pathways
Apoptosis cleaved Caspase 3

Apoptosis- Caspase-3 activation.jpg

  • death-receptor pathway (extrinsic)
  • mitochondrial pathway (intrinsic)
    • may also be several alternate pathways

Both pathways

  • Converge on caspase-3 activation
  • branch into many pathways
  • leading to eventual cell death


Links: Apoptosis Signaling Overview

Death Receptor Pathway

TNF receptor model

Receptors

Cell surface receptors contain an intracellular death domain (DD)

  • belong to the tumor necrosis factor (TNF) super family
    • Fas (CD95/Apo1), TNF receptor 1 (p55), TRAMP (WSL-1/Apo3/DR3/LARD), TRAIL-R1 (DR4), TRAIL-R2 (DR5/Apo2/KILLER)
  • trigger apoptosis upon ligand binding

Ligand Binding

  • Fas Ligand (CD95 ligand) binds Fas
  • TNF and lymphotoxin a bind to TNFR1
  • TWEAK (Apo3 ligand) binds to TRAMP
  • TRAIL (Apo2 ligand) binds both TRAIL-R1 (Pan et al., 1997) and TRAIL-R2

Upon ligand binding receptor associates with an adaptor protein

  • Fas-associated death domain (FADD) directly or indirectly
    • through TNFR-associated death domain (TRADD)
  • FADD also interacts with pro-caspase-8
    • form a complex at the receptor called the death inducing signalling complex (DISC)

Death Inducing Signaling Complex (DISC)

  • DISC induces the activation of caspase-8
    • activates downstream effector caspases

Bid is also cleaved by pro-caspase 8 and translocates to the mitochondria to activate the intrinsic mitochondrial pathway, linking the two death pathways.

Decoy Receptors (DcR)

  • tumor necrosis factor (TNF) super family
  • inhibit death signaling by sequestration of ligand
    • DcR1, DcR2 and osteoprotegerin (OPG) bind to TRAIL
    • DcR3 binds Fas ligand

FLICE-like inhibitory protein (c-FLIP)

  • intracellular endogenous inhibitor
  • regulates by interacting with FADD
    • blocks apoptosis

Mitochondrial Pathway

Apoptosis mitochondrial pathway
  • begins with permeabilisation of mitochondrial outer membrane
    • either permeability transition (PT) pore dependent or independent

Permeability Transition (PT) Pore

Structure formed from matrix, inner membrane, outer membrane proteins

  • matrix protein cyclophilin D
  • inner mitochondrial membrane protein adenine nucleotide translocator (ANT)
  • outer mitochondrial membrane protein voltage dependent anion channel (VDAC)

Opening the PT pore

  • triggers the dissipation of the proton gradient created by electron transport
    • uncoupling of oxidative phosphorylation
  • also causes water to enter the mitochondrial matrix
    • results in swelling of the intermembranal space
    • rupturing of the outer membrane
    • releasing apoptogenic proteins - cytochrome c, apoptosis inducing factor (AIF), endonuclease G

Apoptosome

Apoptosome formation
  • Formed by Cytochrome c, apoptosis protease activating factor (APAF-1) and pro-caspase 9 PNAS - Model of apoptosome formation
  • complex promotes the activation of caspase 9
    • activates effector caspases that collectively orchestrate the execution of apoptosis
  • AIF and endonuclease G - DNA fragmentation and chromosomal condensation


Other proteins released upon mitochondrial outer membrane permeabilisation

  • Smac/DIABLO - second mitochondria-derived activator of caspases/direct IAP-associated binding protein with low pI)
  • Omi/HtrA2 (high temperature requirement A2), which antagonize IAPs thereby promoting caspase activation

Bcl-2 Family

Apoptosis Bax regulation

PT pore independent mitochondrial membrane permeabilisation is regulated by Bcl-2 family members anchored to the mitochondria membrane by hydrophobic C-terminal

Anti-apoptotic members

  • Bcl-2 and Bcl-xL
  • Bcl-2 proteins protect integrity of mitochondria
    • prevent cytochrome C release
    • block caspase 9 activation

Pro-apoptotic members

  • two categories based on expression of Bcl-2 homology (BH) domains
  • Multi-domain proteins comprise BH domains 1-3 and include Bax, Bak, and Bok
    • form channels in outer mitochondrial membrane releasing intermembranal space apoptogenic proteins
  • BH3 only proteins consists of Bad, Bik, Bid, Puma, Bim, Bmf and Noxa.
    • activate multi-domain pro-apoptotic species
    • disrupt the function of anti-apoptotic Bcl-2 family members

Possible Bcl Mechanisms

  • Formation of a pore - cytochrome c and other intermembrane proteins escape
  • Heterodimerization - between pro- and anti-apoptotic family members
  • Direct regulation of caspases - by adaptor molecules
  • Interaction with mitochondrial proteins - generate a pore or to modulate mitochondrial homeostasis
  • Oligomerization- form a weakly selective ion channel

p53

Induced by pro-apoptotic stimuli activates apoptosis by intrinsic mitochondrial pathway

Normally p53 low levels

  • by murine double minute-2 (MDM2) or the human homologue (HDM2)
  • inhibit p53 transcription
  • promote p53 degradation (by proteosome)

Activation of p53

  • stabilization of p53 by post-translational modifications
    • disrupt interaction between p53 and MDM2
  • p53 drives expression of APAF-1 and pro-apoptotic Bcl-2 family members and transcriptional independent death pathways

Caspases

Apoptosome formation
  • cysteine proteases
  • central regulators of apoptosis
  • 14 different caspases identified
  • expressed as pro-enzymes
    • three domains NH2 terminal, a large subunit 20 kDa and a small subunit 10 kDa
  • activation by proteolytic processing between domains
    • allows association of large and small subunits
  • active caspases are a tetramer 2 heterodimers of large and small subunits

Initiator caspases

  • caspase 8, 9, 10 and 12
  • closely linked to pro-apoptotic signals
  • once caspases activated cleave and activate effector

Effector caspases

  • caspases 3, 6, and 7
  • cleave cytoskeletal and nuclear proteins
  • induce apoptosis

FasL and Fas-expressing Cells

Caspase Cleavage

  • leads to diverse results
  • depending on substrate and position of cleavage site
  • loss of biological activity eg lamin network
  • limited proteolysis by caspases result in a gain of biological activity eg Bcl-2 or Bcl-x, PAK2, Bid and CAD/ICAD

Caspase Activation Mechanisms

(a) proteolytic cleavage by an upstream caspase activation of downstream effector caspases induction of non-caspase proteases (granzyme B)

(b) Induced proximity aggregation of procaspase-8 molecules somehow results in cross-activation

(c) holoenzyme formation cytochrome c and ATP dependent oligomerization of Apaf-1 recruitment of procaspase-9 into apoptosome complex Activation of caspase-9 mediated by conformational change

Apoptosis Inhibitors

  • directly inhibit caspases or prevent activation

Akt

  • serine/threonine protein kinase
  • important anti-apoptotic factor
  • inhibits pro-apoptotic Bcl-2 family member, Bad
  • directly inhibits caspase 9

TNF-alpha

  • Tumour Necrosis Factor -alpha
  • both anti-apoptotic and pro-apoptotic effect
  • activate the transcription factor NF-kB
  • which then induces expression of IAP
    • an inhibitor of caspases 3, 7,and 9

Neurotrophins

  • regulate apoptosis through protein kinase cascades
  • phosphoinositide 3-kinase/Akt
  • mitogen-activated protein kinase pathways

Final Stages of Apoptosis

  • The membrane enclosed cell fragments are phagocytosed by macrophages and other cells.

Apoptosis - macrophage.jpg

Other Cell Death Terms

Anoikis

  • (Greek, anoikis = "homelessness")
  • probably death by apoptosis
  • A form of programmed cell death that occurs when cells loose contact with the extracellular matrix (ECM)
  • Integrin regulation of cell viability through their interaction with extracellular matrix
  • Protein kinase and apoptosis-related signals can control anoikis both positively and negatively
  • tumorogenesis protection

Oncosis

  • (Greek, onkos = 'swelling')
  • the process occurring in early necrosis

Pyroptosis

  • Greek, pyro = fire or fever; ptosis = falling
  • proinflammatory pathway
  • cell death mediated by the activation of Caspase-1
  • also activates inflammatory cytokines, IL-1ß, and IL-18

References

Textbooks

Molecular Biology of the Cell

Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002

Molecular Cell Biology

Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James E. New York: W. H. Freeman & Co.; c1999

The Cell- A Molecular Approach

Cooper, Geoffrey M. Sunderland (MA): Sinauer Associates, Inc.; c2000

Search Online Textbooks

PubMed

Reviews

  • Morphological and cytochemical determination of cell death by apoptosis. Taatjes DJ, Sobel BE, Budd RC. Histochem Cell Biol. 2008 Jan;129(1):33-43. Epub 2007 Nov 14. Review. PMID: 18000678
  • Frisch SM, Ruoslahti E. Integrins and anoikis. Curr Opin Cell Biol. 1997 Oct;9(5):701-6. Review. PMID: 9330874

Articles

  • Kerr, J. F., Wyllie, A. H. & Currie, A. R. Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br. J. Cancer 26, 239-257 (1972). PMID: 4561027

Search Entrez

Acronyms

  • AIF apoptosis-inducing factor
  • CARD caspase recruitment domain
  • DISC death-inducing signaling complex
  • DED death effector domain
  • FADD Fas-associated death domain protein
  • FADD-DD FADD-death domain
  • ICE interleukin-1β-converting enzyme (-like) protease
  • PARP polyADP ribose polymerase
  • TNF tumor necrosis factor
  • TRADD TNF receptor-associated death domain protein
  • TRAIL TNF-related apoptosis inducing ligand
  • BCL-2

External Links


2010 Course Content

Lectures: Cell Biology Introduction | Cells Eukaryotes and Prokaryotes | Cell Membranes and Compartments | Cell Nucleus | Cell Export - Exocytosis | Cell Import - Endocytosis | Cell Mitochondria | Cell Junctions | Cytoskeleton Introduction | Cytoskeleton 1 Intermediate Filaments | Cytoskeleton 2 Microtubules | Cytoskeleton 3 Microfilaments | Extracellular Matrix 1 | Extracellular Matrix 2 | Cell Cycle | Cell Division | Cell Death 1 | Cell Death 2 | Signal 1 | Signal 2 | Stem Cells 1 | Stem Cells 2 | Development | Revision

Laboratories: Introduction to Lab | Microscopy Methods | Preparation/Fixation | Immunochemistry | Cell Knockout Methods | Cytoskeleton Exercise | Confocal Microscopy | Microarray Visit | Tissue Culture 1 | Tissue Culture 2 | Stem Cells Lab | Stem Cells Analysis

Dr Mark Hill 2015, UNSW Cell Biology - UNSW CRICOS Provider Code No. 00098G