Cell Fate
Quiescence
Proliferation
Apoptosis
Differentiation
Senescence
Necrosis
Cell states are mutually exclusive
Quiescence
Resting cells
Post- or pre-mitotic
Growth arrested
Metabolically active
Naïve (Potential to differentiate)
Differentiated
cells
Not differentiating!
Potential to
proliferate
Not proliferating!
In G0 of the cell cycle
Stem cells
Resident pool of
progenitor cells
Proliferation
Maintain cell number
Differentiation
Accomplish function
Proliferation (Cell Division /Mitosis)
Cellular division of non-terminally differentiated cells
Mitotic stimulation by hormones,
growth factors and cytokines
Stimulation of Immediate Early Genes
(transcription factors c-fos, c-myc, c-jun)
Needed to maintain
cell population
Protein products of IEGs regulate
transcription of late genes
Change expression of proteins in the cell cycle
Functional tissue
Regulation of cell cycle / mitosis
The cell cycle (a one-way street)
Stages cells pass through during cell division
G0
(Quiescence)
G0
Senescence
Terminal differentiation
G2-M Restriction
Checkpoint 2
Gap
Gap
DNA synthesis
RNA transcription low
G1-S Restriction
Checkpoint 1
Cell cycle is regulated by cyclins and CDKs
Degradation of structural
components and cyclins
Production of
protein
complexes
for M-phase
Cyclin are the regulatory
subunit of the CyclinDependent Kinases
(CDKs)
Activity of CDKs regulated
by the availability of the
corresponding cyclins
Production of protein
complexes for S-phase
Phosphorylation of
complexes for
chromatin synthesis
Degradation of cell
cycle inhibitors
(ubiquitination)
G1-S restriction
Primary regulation point for the control of the cell cycle
Growth factors
Immediate early
Genes
Delayed response genes
Cyclin E, D-cyclins, E2F-1, CDK2/4
Needed to pass G1-S restriction
E2F
Group of
transcription factors
Required for transcription of
genes for G1-S transition (DNA
replication)
Normally (Growth Factor deprived)
E2F inhibited by
retinoblastoma (Rb)
Rb
X
G1 to S
Binds activation
domain of E2F T.F.
Disrupts transcription
Binds histone
deacetylase (HDAC)
Folds DNA into nucleosome
(DNA/histone complex)
Hinders TF
access
Growth factors
[Cyclin] ↑
G1 CDK/cyclin
complex formed
Active kinase
Rb-P
Dissociates from E2F
Transcription of
genes for G1 to S
The cell cycle is regulated by the fine balance
of cyclin expression
CDK2/cyclin E complex is essential for G1 to S transition
The cell cycle is regulated by the fine balance
of cyclin expression
CDK2/cyclin E complex is essential for G1 to S transition
GFs
[D-cyclins] ↑
Complex with CDK 4/6
More cyclin-D
D-cyclin -CDK4 ↑↑↑
P-Rb
E2F ↑
Cyclin E ↑
Sequests p27KIP
Complexes with CDK2 but
inhibited by p27KIP
CDK2-cyclin E Active
G1 to S
DNA damage inhibits G1 to S transition until the DNA is repaired
Normally p53 tumour suppressor is degraded when DNA is intact
DNA damage
S-phase
p53 stability ↑
[p53] ↓
[p53] ↑
Cells stuck in G1
until DNA
damage repaired
[p21CIP] ↑
(CDK inhibitor)
Binds G1 CDKcyclin complex
Alternatively cells may undergo apoptosis
Programmed cell death
Knockout mice
Indicates the in vivo function of the protein
p21CIP-/-
Apoptosis
No inhibition of G1
CDK/cyclin complex
Into S-phase with
DNA damage
No proliferation
Tumouragenesis?
p21CIP is anti-apoptotic!!
NO!
Ratio p21 : p53 important
P21 : p53 ↑
P21 : p53 ↓
Growth arrest
Apoptosis
Fine tuning of expression of cyclins, cell cycle inhibitors
and tumour suppressors to allow mitogenesis
Knockout mice
Rb -/-
No inhibition
of E2F
X
G1 to S
Uncontrolled
proliferation?
NO!
E2F ↑↑↑
Apoptosis
Activates p19ARF
P21 : p53 ↓
Binds to and inhibits MDM2
(Degrades p53)
p53 ↑
There is a fine balance between proliferation and apoptosis
G2 to M restriction
G2 to M transition also
finely controlled
Cyclin B synthesis and
phosphorylation
Controls breakdown
of nuclear lamina
Multiple checks that prevent uncontrolled proliferation (tumourigenesis)
Apoptosis (programmed cell death)
Apoptosis = Falling off
Ordered set of events
Blebbing
Nuclear
fragmentation
Chromatin
condensation and
fragmentation
Cellular fragmentation
into apoptotic bodies
Phagocytosis
Important during embryogenesis
Syndactyly
Development of free and
independent digits
Massive death of interdigital
mesenchymal cells
Brain development
~ 50% neurones undergo apoptosis
before adulthood (selection)
Immune system
Majority of lymphocytes undergo
apoptosis during negative selection of
antigen receptor
Man
Turnover of several billion cells / day!
External activation
Stimulated by cell stress
Nutrient deprivation
Radiation
Heat
Viral infection
Cytokines
Hypoxia
GCs
Common
pathway
Apoptosis vs Necrosis
Injury
Apoptotic pathways
Extrinsic
pathway
Intrinsic pathway
Death-inducing
signal complex
Loss of
membrane
integrity
ATP synthesis stops
Change in redox
potential
Caspase =
Cysteine
proteases
Components of the apoptotic pathway
are always present in the cell
Waiting for
stimulus
Anti-apoptotic pathway
keeps cells viable
Insufficient apoptosis
Cancer
Activating mutations in BCL-2
Many oncogenes sensitise
the cells to apoptosis
Also requires
apoptosis to be
turned off
Autoimmunity
(Failure to eliminate autoimmune antibodies)
Persistent infections
(Failure to eliminate infected cells)
Excessive apoptosis
Neurodegeneration
Alzheimer’s disease, Parkinson’s disease,
Huntington’s disease
Autoimmunity
(Uncontrolled apoptosis in specific organs)
AIDS (depletion of T lymphocytes)
Ischemia (stroke, myocardial infarction)
Differentiation
Differential expression or repression
of genes to confer phenotype and
function
All diploid cells contain the full
complement of genetic material
Whole organism from a single cell
Inhibition of proliferation
Specialised function not conferred
until terminal differentiation
Pre-Phenotype A
Progenitor cell
Pre-Phenotype B
Pre-Phenotype C
Pre-Phenotype D
?
Phenotype A
Phenotype B
Phenotype C
Phenotype D
Differentiation
Differential expression or repression
of genes to confer phenotype and
function
All diploid cells contain the full
complement of genetic material
Whole organism from a single cell
Inhibition of proliferation
Specialised function not conferred
until terminal differentiation
Pre- adipocyte
Mesenchymal
stromal cell
Dedifferentiation
Pre- osteoblast
Pre- myoblast
Pre- chondrocyte
Transdifferentiation
?
Adipocyte
Osteoblast
Myoblast
Chondrocyte
Terminal differentiation
Expression of specific markers indicates state of differentiation
Adipose-derived
stromal cells
Cell-cell contact
is essential!
Coordinated and Synchronized
expression of genes
Adipocyte
Differentiation
medium
Lipid
droplets
Terminal differentiation
Senescence (aging)
Characterised by inability
of cells to proliferate
Cells are viable but
function is compromised
Larger, fatter, express β galactosidase
Limits the number of cell
divisions and possibly
somatic mutations
An alternative to apoptosis
to prevent spread of cancer
Progressive telomere
shortening
Alters RNA splicing in proteins such as
progerin (lamin A) which compromise
functionality
(protective, non-coding DNA at
end of chromosome)
Many cancers
are immortal
Stem cells
Up-regulation
of telomerase
(90% tumours)
Also upregulate
telomerase
Adds non-coding
DNA onto the
telomeres
Proliferation needed
to supply tissue
No telomere
shortening
Aging
Gompetz-Makeham
law of mortality
Mortality rate increases with age
Genetically determined
Man at 80 years
Mouse at 3 years
Old enough for reproductive success!
Longevity
Differences in
Antioxidant enzymes
DNA repair
Free radical production
Hormone signalling over age
Stress
Chronic activation
of the HPA axis
Damage
and repair
Aging ↑
Cell Fate
Quiescence

Proliferation

Apoptosis

Differentiation

Senescence
