BIOLOGY 2D03 Lecture Notes - Phagocytosis, Tumor Suppressor Gene, Deoxyribonuclease

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28 Jan 2013
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Programmed Cell Death
Cell death critical to embryonic development, tissue homeostasis, establishment of
immune self-tolerance, and for regulation of cell viability by hormones and growth
factors
Apoptosis programmed cell death characterized by distinct morphological changes in
the dying cell
Shrink
Membrane blebs form at plasma membrane
Intracellular organelles disassemble
Chromatin condensed and cleaved
Finally, packaged into membrane bound apoptotic bodies
Apoptosis neat form of cell death (necrosis usually initiates inflammatory response)
2 phases of apoptosis:
Cells are committed to undergo apoptosis during the latent phase
oExtremely variable in length
oCells may look normal
oOnce past point of no return, no rescue possible
Execution phase
oUndergo dramatic morphological changes
oExtremely rapid compared to latent phase
Apoptosis is mediated by an intracellular proteolytic cascade
Caspases
oCystein proteases that cleave at specific aspartic acid residues
oResponsible for dissembling the apoptotic cell
oSynthesized as procaspases (inactive precursors), and are activated by
cleavage
oActive caspase is heterotetramer of 2 large and 2 small subunits
oClassifications
Initiator caspases can be autoactivated when complexed with
appropriate scaffolding (have long prodomains that interact with
other proteins)
Effector caspases activated by cleavage by initiator caspases
Activation of initiator complexes 2 general pathways
Extrinsic signal comes from outside of the cell
oExample – ligation of a death receptor on the plasma membrane
Intrinsic signal comes from within the cell
oDNA damage that cannot be prepared
oUsually involves mitochondria
Adaptor proteins have multiple protein-protein interaction motifs and assemble after
pro-apoptotic stimuli to activate these initiator caspases
Example – ligand outside the cell binds to a death receptor, such as Fas or TNFα
oCaspase 8 is ciritcal initiator caspase in this pathway
Another well studied pathway is activated by the release of critical mitochondrial
factors from the intermembrane space, including cytochrome C
oCytochrome c and dATP associate with the adaptor protein Apaf-1, which
then recruits procaspase-9 and induces autoactivation
Proteolytic cascade
Once an initiator caspase activated, it can cleave and activate downstream effector
caspases (i.e. casp 3)
oThis sets off an irreversible proteolytic cascade
oEffector casp cleave a variety of cellular structural proteins and regulatory
proteins
oDiff casp recognize diff asp residues within diff consensus sites, but only a
specific subset of cellular proteins are cleaved (probably only interferes
with key protein-protein interactions that maintain cell integrity)
oCleavage of DNA into nucelosomal-sized fragments is induced by CAD
(caspase activated DNase)
ICAD inhibitor bound to CAD until cleaved by capsases
Phagocytosis of apoptotic bodies
Critical component of apoptosis removal of cell corpses
Exposure of phosphotidylserine (PS, a phospholipids found normally only in the
inner leaflet) on the extracellular leaflet of the plasma membrane
oRecognition signal for phagocytic cells
Regulation of apoptosis
Highly regulated
Bcl-2 protein family can be pro- or anti-apoptotic
oHave one or more BH (Bcl-2 homology) domains
oSome proteins are integral membrane proteins (with C-terminal membrane
anchor), and some are soluble
oBH3 domains may participate in protein-protein interactions that regulate
cell death
oBcl-2 and Bcl-XL are anti-apoptotic because they can block cytochrome c
release from mitochondria
oBax may directly activate the permeability transition pore in the
mitochondrial outer membrane, or may form a pore itself after assembly
with other pro-apoptotic Bcl-2 proteins like Bak and Bid.
oCleavage of Bcl-XL by caspases renders it pro-apoptotic
Inhibitor of apoptosis proteins (IAPs)
Can block apoptosis in 2 ways
oBind to some procaspases and prevent their activation
oSmac-DIABLO inhibits IAP
Signalling in growth control and apoptosis
Organ and body size determined by 3 fundamental processes:
oCell growth
oCell division
oCell death
Animal cells need signals from other cells, not only to grow and proliferate, but
also to survive
oIf deprived of such survival factors, cells die by apoptosis
oSurvival factors usually bind to cell surface receptors and activate
signaling pathways that keep the death program suppressed
oOften involves the Bcl-2 family, either by stimulating production of anti-
apoptotic Bcl-2 proteins, and/or by inhibiting pro-apoptotic ones
p53
Inactivation of p53 tumor suppressor pathway most common anti-apoptotic
lesion
Excess mitotic stimulation often leads to production of a cell cycle inhibitor
protein (p19ARF), which binds and inhibits Mdm2
Mdm2 normally promotes p53 degradation (so inhibition of mdm2 leads to
stabilization of p53)
Depending on cell type and extracellular conditions, p53 will cause cell cycle
arrest and DNA repair
It also induces expression of genes that induce apoptosis