Tissue Specific Stem Cells

Question 1

Tissue Homeostasis

Question 2

Blood Homeostasis

Question 3

Identification of Hematopoietic Stem Cells

Answer 3

• HSCs are a type of SC that give rise to all of the different blood cells in the body (red, white, platelets)
• Identified by presence of certain surface markers (CD34 and CD38) and ability to self renew

Transgenic mice:
- Applying X-Ray exposure leads to suppression of blood cell production > mouse dies
- Injection of bone marrow from a healthy mouse into the mouse, then applying X-Ray, leads to mouse surviving

Haematopoietic stem cell in the bone marrow:
- Colonies are formed by COLONY-FORMING UNITS > they contain differentiated blood cells (DIFFERENTIATION) and new “colony-forming units” (SELF-RENEWAL)
- (CFU-S)
- Can be obtained from spleens of “saved” mice
- Insertion of these into irradiated mouse leads to survival

Question 4

The Classical Model of HCS lineage commitment

Answer 4

• HSCs differentiate into common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs).
• SCs > Multipotent progenitors (ST-HSC to MPP) > Oligopotent progenitors (CMPs, CLPs, MEP and GMP)
• CLPs give rise to T cells and B cells
• CMPs give rise to myeloid lineage (RBCs, platelets, myeloid cells like granulocytes and monocytes)

Question 5

Multipotent versus Pluripotent Stem Cells

Question 6

Functional Assays for identifying HSC

Answer 6

Cell surface markers:

Transgenic Mice:

Question 7

TFs and Lineage Determination:

Answer 7

TFs:
- determine gene expression patterns
- recruit coactivators/corepressors of transcription
- often components of multiprotein complexes

GATA-1 regulates MEP to Erythroid fate
PU. 1 regulates GMP to Myleoid fate

Question 8

Master regulators:

Answer 8

DOMINANT ROLES:
- Required for dev into a specific lineage > e.g/ absence of Gata-1 means the HSC can’t lead to erthroid fate through CMP > MEP > Erythroid fate.
- Can change fate of cells they are introduced to > introducing Gata- 1 to GMP cells turns it to MEP and Erythoid fate

ANTAGONISTIC EFFECTS:
- Antagonise the opposite lineage programs > upregulation of Erythroid markers DOWNREGULATE myleoid markers

Question 9

TF cross-antagonism

Answer 9

• GATA-1 mediated antagonism of PU. 1:
  GATA-1 binds to site of PU. 1 where c-Jun binds to > inhibits PU. 1 target gene

“GATA-1 and PU.1 have been shown to have an antagonistic relationship in the differentiation of hematopoietic cells. Studies have shown that GATA-1 can bind to the same cis-elements as PU.1 and repress its activity, thus promoting erythroid and megakaryocytic differentiation and preventing myeloid and lymphoid differentiation.”

• PU.1 mediated antagonism of GATA-1:
  PU. 1 represses GATA-1 from binding to the GATA coding within the genetic coding, repressing further Erythroid fate.

“PU.1 can bind to GATA-1 and repress its activity, by recruiting histone deacetylases (HDACs) that leads to the formation of a repressive chromatin structure at the GATA-1 promoter, inhibiting its expression.

This mechanism of antagonism between GATA-1 and PU.1, helps to ensure that HSCs differentiate into the appropriate cell types, and that the balance of cell production is maintained in the process of lineage commitment.

It should be noted that both GATA-1 and PU.1 have the ability to bind to and modulate the activity of other transcription factors, and thus the regulation of their activity is complex, and also depends on the cellular context.”
- Regulators negatively influence each other
- Autoregulate
- Positively/negatively regulate target genes
- Leads to different cell fates > an undifferentiated/uncommitted cell can have a higher conc of one of the TF to make it favour a certain fate

Question 10

HSC: Heterogeneity

Answer 10

The classic model of HCS lineage is very rigid > based on single cell models, it might not be this way.
- If every HSC is heterogenous, you would expect the same outcome at the end.

• Heterogenous in respect to: self-renewal upon transplantation (not all HSCs are equal, not all propogate in the same way), cell cycle properties (non-cycling, some are), differentiation.

Balanced HSCs = expected the same output
HOWEVER they are biased towards lineage > not always the same amount produced, certain fates favoured.

*Population based studies can mask heterogeneity