BIOC2306 apoptosis and senescence

are cell contents liberated in apoptosis

no (but is in necrosis)

so doesnt invoke immune response

how are apoptotic cells removed

engulfed partly or whole by phagocyte

extracellular/extrinsic pathway to activate apop

signalling from death receptors on cell surface

intacellular/intrinsic activation of apop

dsDNA breaks

p53

UV radiation

hypoxia

what are caspases

cyc-catalysed aspartate targeting proteases

induce apoptosis

activation of caspases

exp as pro-caspases

activated by cleavage between large and small domains, and cleave N term prodomains off, allowing large and small subunits to heterodimerise, then form tetramer together

caspase cascade

are caspases highly specific

yes, recognise and degrade v few proteins (a proteases)

initiator vs executioner caspases

8,9 and 10: activate a cascade that results in activation of executioner caspases

3 and 7 are executioner, induce cell killing

what activates initiator caspases 8 and 10

signalling via death receptors (via FADD)

what activates caspase 9

activated by APAF-1, cytochrome c and ATP via CARD domain (intracellular pathways)

what are caspase inhibitors

inhib caspase by binding to catalytic site, so inhib apoptosis

e.g. c-FLIP inhibs casp 8 (therefore inhib executioner capsase activation)

effect of overexpression of c-FLIP in tumours

prev caspase 8 activation, so limit executioner caspase activation

how do executioner caspases induce apop

cleave cyto proteins and nuclear lamina

role of FasL and TRAIL

both bind to Fas and TRAIL death receptors, causing receptor to trimerise, which recruits FADD in cyto (via death domain cluster)

FADD is adaptor protein between receptor and caspase 8 and 10

(form death induced signalling complex, DISC)

TRAIL receptors aka death receptor 4 and 5

what cells contain FasL and TRAIL

killer lymphocytes

mechanisms to prevent signalling via death receptors (external)

soluble and membrane decoy receptors, intracellular activators and inhibitors

intrinsic pathway that activates apoptosis

cytochrome c rel from mitochrondria

cytc binds Apaf1

causes exchange of ADP for GTP which triggers Apaf1 to assemble into apoptosome (many Apaf1 int. via their CARD domain)

apoptosome recruits procaspase9 (also has CARD domain) causing its cleavage

BCL-2 family (proteins)

anti apoptotic

found in B cell lymphomas

are mito memb proteins

what causes cytochrome c release from mito

normally have inactive BH123 proteins (BCL-2 proteins with BH1,2,3 so are proapop) in mito memb that are prev from dimerising by antiapop BLC-2 protein (have BH1,2,3,4) in cyto

apoptotic stimulus activates proapop BH3-only BCL-2 proteins in cyto that cause inhibition of the antipop BCL-2 proteins, allowing BH123 proteins to dimerise in memb, act as pore that lets cyt c exit mito

(anti-IAP rel too)

role of inhibitors of apoptosis

inhib apoptosis e.g. c-FLIP

bind caspases so inactivate them

often upreg in cancers

role of anti-IAPs and where they come from

bind and therefore inhib IAP

get rel from mito with cyt c

what are telomeres

termini of eukaryotic linear chromosomes

repeat 6 nt seq TTAGGG

approx 12 kb at e ach end

most of telomere is dsDNA but has G rich single strand that forms terminal 3’ overhang

noncoding

specific proteins bind it

structure of a telomere

TTAGGG repeat, with terminal 3’ overgang (G-rich), invades dsDNA to form T-loop

bound by many proteins

function of telomeres

protect chromosome ends from shortening

prev chromosome ends from becoming entangled and adhering to eachother (T-loop)

assist in pairing of homologous chromosomes during prophase of meiosis I

how much of telomere is lost each mitosis

100 bp

(16 × 16 repeats)

how is DNA synth

use RNA primer to synth okazaki fragments

remove primer, pol synth between fragments and ligate together

always get 3’ overhang

what is telomerase

a ribonucleoprotein complex

synth telomeres from RNA template (using activity of TERT domain (telomerase reverse transcriptase, so is a specialised reverse transcriptase)

RNA template is the TERC domain (telomerase RNA component) (451 nt in humans)

that provs AAUCCC template

mech of action of telomerase

binds, elongation, translocation (moves along), so can elongate further. does it in 2 syth steps, where binds 3’ overhang and syth some of telomere, it then moves and binds this new DNA that it can then extend from

where is telomerase found

germline cells inc. embryonic stem cells

certain, poss. all adult stem cells and progenitor cells (so can keep proliferating)

cancer cells

unicellular eukaryotes like tetrahymena

what eventually blocks mitotic division

shortening of telomeres, prev genomic instability (lose coding DNA each rep)

but in cancer, have telomerase making them immortal