Stem Cells and Society Flashcards
Stem cells are capable of what 3 things?
- Self renewal to make a copy of itself
- Potency to make a range of cell types
- Differentiate into a range of cell types
What are the 3 fates of stem cells?
- Self renewal
- Differentiation
- Death
Self renewal requires…
Growth factors to promote synthesis of macromolecules
Differentiation requires…
Loss of signals to drive cell down differentiation pathway
Cell death is mediated by…
- Death factors - promote apoptosis
- Survival factors - suppress apoptosis
Components for cell signalling process?
- Signal binds to membrane receptor
- Activation of intracellular effectors
- Activation of transcription factors in nucleus
- Change in gene transcription and state of cell
Chromatin structure
- Chromatin is wrapped around histones to form nucleosomes
- Allows condense packing of chromatin into chromosomes
What effect does acetylation of chromatin have?
How does it do this?
- Turns genes on
- Binds to histone tails and opens up chromatin to increase accessibility for transcription factors
Histone acetyl transferase…
Allows factors to load onto chromatin to increase transcription
Histone deacetylase…
Allows repressor proteins to load onto chromatin to decrease transcription
What effect does methylation of chromatin have?
How does it do this?
- Turns genes off
2 ways: - Direct methylation of CpG islands blocks binding of TFIID to TATA box of promotor to suppress transcription
- Recruits Histone deacetylase to condense chromatin and decrease transcription
Proximal control elements examples
- Promotor regions
- Transcriptional start sites
Distal control elements examples
Enhancer sequences
Introns in primary mRNA…
Are spliced out to form mature mRNA which is then 5’ capped and 3’ tailed
TFIID is transcription factor that binds to…
TATA box
miRNA and siRNA
- miRNA - blocks translation
- siRNA - degradation of mRNA
Ubiquitin tags proteins for…
Destruction by the proteosome
Cell cycle phases
- G1 phase - Cells increase in size and ribosome/RNA production
- S phase - Duplication of DNA content
- G2 Phase - DNA is checked for fidelity
- M phase - Mitosis
Cell cycle of Pluripotent stem cells is different because…
They cycle fast and don’t spend much time in G1
G1 checkpoint
- Is cell big enough
- Is environment favourable
- Is there DNA damage
G2 checkpoint
- Is all DNA replicated
- Is cell big enough
- Is environment favourable
Metaphase checkpoint
- Are all chromosomes aligned on spindle and attached
G0
State of dormancy not going through cell cycle
Cells enter G0 from…
G1
3 components of cell cycle activation
- Cyclin-dependent kinases (CDK)
- Cyclins
- CDK inhibitor proteins
Cyclin and CDK at start of G1
Cyclin D and CDK4/6
Cyclin and CDK at end of G1
Cyclin E and CDK 2
Cyclin and CDK in S phase
Cyclin A and CDK2
Cyclin and CDK in G2 phase
Cyclin A and CDK1
Cyclin and CDK in M phase
Cyclin B and CDK1
Most growth factors act by directly upregulating…
Cyclin D expression to drive cell through G1 for self renewal
G1 restriction point
Cell will either:
- Commit to division and proceed to S phase
- Exit cell cycle into G0
Molecular G1 restriction point
- Rb is bound to E2F
- Rb protein is phosphorylated by Cyclin D-CDK4 complex
- Rb protein is then hyperphosphorylated by Cyclin E-CDK2 complex
- Rb releases E2F
- E2F targets genes to activate S phase
2 families of CDK inhibitor proteins
- INK family - prevents binding of cyclin D to CDK4
- KIP family - binds to Cyclin E-CDK2 complex and inhibits activity
Apoptosis characteristics
- No loss of integrity
- Aggregation of chromatin at nuclear membrane
- Shrinking of cytoplasm and nuclear condensation
- Mitochondria release death signals
- No inflammation
Necrosis characteristics
- Loss of membrane integrity
- Swelling of cytoplasm and mitochondria
- Total cell lysis
- Disintegration of organelle
- Inflammation
Apoptosis activation signals
- Withdrawal of positive signals - adhesion to other cells
- Presence of negative signals - UV light, free radicals
Stages of apoptosis
- Death signal received and commitment to die - reversible
- Execution of cell - irreversible
- Dead cell phagocytosed
- Degradation of apoptotic bodies in phagocyte
Intrinsic pathway of Apoptosis
- Cytochrome C release from mitochondria
- Activates initiator caspase 9
- Activates effector caspase 3
- Triggers apoptosis
Extrinsic pathway of Apoptosis
- Death ligand bind to death receptors
- Activates initiator caspase 8
- Activates effector caspase 3
- Triggers apoptosis
Model of cell signalling
- Reception of signal
- Transduction of signal
- Cellular response
Signal transduction
Receiving information into a cell and acting to make choices on cell fate
5 crucial functions of signalling cascades
- Transduce signal into molecular form to stimulate response
- Relay signal from point of reception to point of action
- Amplify received signal
- Distribute signal to influence responses in parallel
- Each step open to modulation by other signals
2 mechanisms of signalling
Phosphorylation
- Signal taken in and protein is phosphorylated by a kinase
GTP-binding
- Signal taken in and G-portein activated by binding of GTP
Slow and Rapid acting signals
- Fast - directly alters cellular process
- Slow - indirectly alters cellular process by changing gene expression
Direct communication between cells
- Gap junctions
- Cell-cell recognition by surface molecules
Local communication between cells
- Paracrine signalling
- Synaptic signalling
- Hormonal signalling
2 clases of extracellular molecules in signalling
- Small hydrophobic molecules - pass through membrane and bind to intracellular receptors
- Large hydrophilic molecules - cannot pass through membrane and bind to membrane receptor
3 types of cell surface receptors
- Ion channel linked receptor
- G-protein linked receptor
- Enzyme linked receptor
3 types of enzyme linked receptors
- Receptor tyrosine kinase
- Cytokine receptors
- TGF-Beta receptors
Receptor Tyrosine Kinases
- Binding of ligand (soluble/membrane bound peptide hormones) allows tyrosine kinases to autophosphorylate
- This allows binding/activation of other proteins to form signalling complex
- Common component is G-protein Ras
Cytokine receptors
- Cytokines are small secreted proteins
- Control growth and differentiation of many tissues
- Signal to nucleus in direct pathway
TGF-Beta Receptors
- Used to keep pluripotent stem cells in an undifferentiated state
Binding of TGF-Beta to receptors causes phosphorylation of…
SMAD 2 and 3
Binding of BMP to receptors causes phosphorylation of…
SMAD 1, 5 and 8
Signals from TGF-Beta and BMP compete for…
SMAD 4 (coSMAD) to enter the nucleus