Differentiation Flashcards
Changes gene expression underlie diversity cell type in body
All cells have same gene content but different sets proteins
Cell diversity primarily results from differences in gene expression
Terminally differentiated state - final cell fate
Cells change expression profile in response signals and other cues in environment
Cells expression fraction of genes
Often express different gene in disease state
How does genome direct differences between cell types?
Stepwise restriction in potency (potency in formate being able to make different types of cells)
Cells gradually become committed to fate:
- Totipotent ~ able create any type cell
- Pluripotent ~ almost all
- Multipotent ~ many
- Bipotent ~ only 2 cell types
- Unipotent ~ committed (only 1 cell type)
Restriction in potency:
- Differential gene activity results in terminal differentiation
- Is controlled by regulatory transcription factors
- Researchers use expression different genes (markers) track stages differentiation
Markers often not transcription factors
Level transcription affects level expression
Transcription factors can control what proteins and amount get made in cell
If cells express different RNA then say they have different transcriptome
Transcriptome different for different cell types, reflects number genes expressed in cell at given time
Proteome reflects number proteins in cell at any given time
DNA binding proteins bonds
DNA binding proteins bind DNA backbone, reach into major groove and form H-bonds
Binding site is stretch DNA with sequence that’s recognised and bound by transcription factor
Form H-bond which aren’t stable
Other names for binding site:
- Cis acting element
- Regulatory element
- Enhancer
- Silencer
Regulatory elements usually found in and around gene
Transcription factors bind in clusters
Enhancer - binding site for transcriptional activities
Silencers - binding site transcriptional repressors
Some TFs act as repressors or activators depending on cellular environment
Enhancers are promiscuous (work on any gene), can be moved and will still be enhancer for gene its then put in front of
Usually many inputs that alter gene expression
Inputs: (look at the diagram for this)
- Weakly activating protein assembly
- Strongly inhibiting protein
- Strongly activating assembly
Each input can be referred to as genetic switch
Each switch responding to extrinsic or intrinsic regulation
Regulation transcription factors and gene activity (go back over this bit)
Regulatory transcription factors modulate the level expression of many genes
One transcription factor will regulate many different genes
Gene activity can be regulated by feedback loops
Activation transcription factor 1 can start transcription in gene 2 which … upregulates gene 1
But gene 4 can repress transcription gene 3
Look at diagram
Transcription factor activity dependent on
Extrinsic signals
Intrinsic factors
Regulatory binding sites
Accessibility of chromatin
Muscle differentiation
Muscle-specific proteins:
- Muscle-specific actin
- Myosin II
- Tropomyosin
- Muscle-specific enzymes (e.g. creatine phosphate kinase)
Muscle differentiation multi-step process involves many external imputs
MyoD
Transcription factor
Expressed only in muscle precursors and muscle cells
Controls expression genes for muscle differentiation
Maintains its own expression
Is MyoD sufficient for muscle cell differentiation?
Fibroblasts don’t normally express MyoD or other muscle-specific proteins
Experiment - transfect fibroblast with activated MyoD gene
Results - Fibroblasts now differentiate into muscle
Conclusion - MyoD sufficient to direct muscle cell fate
Transfect - introduce new DNA/modified gene into cell or animal
Fibroblasts related to mesenchymal cells
Is MyoD required for muscle cell differentiation?
Mice lacking function of MyoD develop with normal skeletal muscle
Therefore MyoD not required for muscle cell differentiation
Redundancy of Myf5
Myf5 (a gene) acts redundantly with MyoD
Mice lacking both MyoD and Myf5 lack all skeletal muscle, both genes have important role in muscle differentiation
Functionally Myf5 and Myod very similar which is why can make each other redundant
Embryonic stem cells Vs adult stem cells
In embryo most muscle come from somites
In adult muscle cells renewed by satellite stem cells that line muscle fibres
Blood cell another classic model differentiation
In embryo blood cells first made in yolk sac then liver
In adults blood made in bone marrow
Self-renewing blood stem cells can differentiate into many different blood cell types
As cells travel toward terminal differentiation, they express different sets transcription factors
Some terminally differentiated states of blood cells are:
- Erythrocyte
- Platelets
- Basophil
- B and T lymphocytes
