L6 Adult Stem Cells Flashcards
What was discovered during ionising radiation tests in the 1900s?
In the 1950-60s, when people were researching the effects of ionising radiation, they discovered that mice would stop producing blood cells and die within a few days of being irridated. This was reversed when bone marrow from a healthy mouse was injected.
What was discovered in the spleens of irridated mice?
When the irridated mice had been injected with bone marrow, Till and McCulloh discovered that the spleens of these mice contained colonies or nodules that has arisen from a single cell (the more bone marrow that was injected, the more colonies formed).
These cells were called colony-forming cells and when analysed were shown to contain differentiated blood cells, but also new colony-forming units (CFUs).
When these CFUs were injected into the irridated mice, they survived.
Describe the stages of the hierarchy of haematopoiesis.
1) Stem cells.
2) Multipotent progenitors.
3) Oligopotent progenitors.
4) Lineage-restricted progenitors.
5) Effector cells.
Why are functional assays for identifying HSCs problematic?
Because if you have stem cells and then prove they can be differentiated, this means they are no longer stem cells! This is retroactive classification.
Instead, we use cell surface markers.
What has led to a revised model of haematopoiesis?
Advances in research have shown us the many levels of plasticity within HSCs that were not previously anticipated, leading to a revised model.
Why do different haematopoietic effector cells have very different expression patterns? What does this mean?
The effector cells have their own function in the body and must be able to respond to external cues (such as infection) too.
This means that the differentiation from stem cells must include the upregulation of certain genes and the downregulation of others.
What does differentiation from HSCs involve?
Transcription factors determine the gene expression patterns, recruit co-activators/repressors of transcription and are often components of multi-protein complexes.
What requirements does a master regulator of stem cell fate have to fill? Include examples.
- Must be required for development into a specific lineage.
Gata-1 is considered to be a master regulator of erythmoid lineages. Without Gata-1, the erythmoid lineage would not develop. - Can change the fate of cells they are introduced into.
If Gata-1 is introduced into GMP cells, they will change into MEP cells and follow an erythmoid lineage once more. - Have to have not only a dominant role, but can antagonise the opposite lineage programmes.
Gata-1 upregulates erythmoid markers but down-regulates myeloid markers. PU.1 is the equivalent master regulator that is responsible for the myeloid lineage.
Describe Gata-1 mediated antagonism of PU.1.
Gata-1 outcompetes C-Jun and binds to PU.1 through its zinc-finger domain. With C-Jun no longer present, PU.1 cannot promote transcription of target genes. This happens in erythmoid lineages.
Describe PU.1 mediated antagonism of Gata-1.
PU.1 binds to Gata-1 and displaces it (Gata-1 is usually bound to DNA via the zinc-finger domains). This stops expression of the target genes. This happens in myeloid lineages.
What is multi-lineage priming?
Multipotent stem cells express genes that are involved in different lineages.
How do multipotent stem cells become specified to a particular lineage?
1) Multipotent stem cells have TFs of different lineages present at low levels. These TFs are balanced and cross-antagonised.
2) Something happens in the system (could be an external cue or a random fluctuation of gene transcription) that causes one TF to be expressed more strongly than the others.
3) Cross-antagonism is stopped and reduces the level of other TFs which means the cells now develops as one cell type.
What can extrinsic cues be?
- Instructive = they instruct cells to become a particular fate.
- Selective = the cells differentiate into all cell types but then cytokines eliminate the ones that are not needed.
What did Mossadegh-Keller et al (2013) discover and how?
They wanted to know if M-CSFs were instructive or selective when it comes to the increase in myeloid cells in response to infection and inflammation.
As PU.1 is a marker of myeloid cell activity, they produced a GFP transgenic line with this gene.
A time-lapse video microscopy of PU.1 cell generation from single PU.1 HSCs shows that the cells go from being non-fluorescent to fluorescent. This indicates that the M-CSFs have an instructive role because this occurred without any cell death or division.
Therefore, you can conclude that the extrinsic factor M-CSF can instruct haematopoietic stem cell fates.
