Stem cells and regenerative medicine Flashcards
What are stem cells?
- Cells that can differentiate into many different cell types depending on signals they recieve
- Self-renewing by cell division
- Gives new cells and replaces cells that are damaged
There are three main sources of stem cells.
Describe adult stem cells
- Rare, supplies new cells and replaces damaged cells
- Varies with different organs, skin has greater ability to replace damaged tissue than heart
- Tissue specific and multipotent
- Some ASC’s show plasticity e.g those in bm, can differentiate into variety of cells
Describe embryonic stem cells
- Supply all cells of developing embryo
- Pluripotent can differentiate into every type of cell
- derived from embryos at blastocyst stage - stem cells are in inner cell mass
- give rise to cells in all 3 embryonic germ layers
- and each germ layer gives rise to specific tissue
- ectoderm - produces nervous, epithelial and sensory tissue
- mesoderm - skeletal, cardiac muscle and blood and connective tissues
- endoderm - lungs, pancreas, stomach, liver, germ cells
How are induced pluripotent stem cells produced?
- Made in lab
- Take normal differentiated tissues such as skin, reprogramme cells by exposure to pluripotency factors e.g OCT4,SOX2,KILF4,CMICK to produce pluripotent stem cells
- cells used for cell therapy, repairing mutations using gene editing like CRISPR, differentiated back to healthy cells then transplanted back into patients
- cells specific to patient so reduces graft rejection by host
- used in disease modelling, drug screening, 3D organoid models, developmental biology, cell replacement therapy, model for basic and translational studies
What are stem cell niches? What are their specific features?
Tissue specific stem cells are maintained in special supportive microenvironments called stem cell niches.
- Supporting ECM (extracellular matrix molecules such as collagen)
- Neighbouring niche cells
- Secreted soluble signalling factors e.g growth factors and cytokines
- Physical parameters; shear stress, tissue stiffness and topography
- Environmental signals (metabolites, hypoxia, inflammation etc)
What considerations do you need to take when assessing the suitability of a particular stem cell type for study or therapy?
Unregulated stem cell proliferation can produce tumours
- Likely with embryonic stem cells due to unlimited growth potential
- IPSC have less growth potential
- Adult stem cells even less growth potential so have lower risk of tumour production
Growth potential also affects number of cells available to work with, embryonic most available.
Risk of rejection
- low with ASCs and IPSCs as derived from and for speciifc patient
- embryonic likely to be genetically different - higher risk
Risk of disease causing mutation being in stem cell
- Embryonic stem cell has lower mutation rate and high genetic stability
- IPSC and ASM more at risk of spontaneous disease causing mutations
Ethical considerations
- human embryonic stem cells derived from surplus in vitro fertilised embryos
- some consider destruction of embryos to make stem cells unethical
What are the four Yamanaka pluripotency factors?
Four transcription factors
- Oct4
- Sox2
- Kilf4
- C-Myc
NANOG can also play a role in reprogramming.
How do the four Yamanaka pluripotency factors work?
- C-Myc promotes DNA replication and relaxes chromatin structure
- Allows Oct4 to access its target genes
- Sox2 and Klf4 also co-operate with Oct4 to activate target genes
- These encode transcription factors which establish the pluripotent transcription factor network
- Results in the activation of the epigenetic processes (more open chromatin) that establish the pluripotent epigenome
- The iPSCs cells have a similar global gene expression profile to that of ES cells
How can stem cells be tracked in animal modes?
Manipulated in vitro, inserting a reporter gene e.g one that makes cells fluorescent.
How can stem cells be used to promote neovascularisation?
- Stem cells that differentiate into new coronary vessels or by cell-three therapies
- improves circulation at injury site, which in turn promotes paracrine stimulation of endogenous cardiomyocytes
What model organisms can regenerated cardiac tissue?
- Zebrafish, apex of heart is removed surgically or by cryoinjury, by two months entire injury site has been regenerated with new cardiomyocytes
- see free expression of developmental gene programmes early after the injury - Raldh2 and WT1
Other organisms like neonatal mice and amphibans can regenerate hearts. Larger mammals like pigs, primates cannot, instead forms fibrotic scar.
How are lymphatics and immune response important in cardiac regeneration?
- In normal/control injury response lymphatic system doesn’t clear excess tissue fluid and inflammatory immune cells efficiently - leads to oedema and inflammation, produces poor cardiac repair and function
- If lymphatic system is stimulated with modified VEGFC there is increased lymphatic response, improves clearance of tissue fluid and inflammatory cells, reduces oedema and inflammation. - improves cardiac repair and function
How are cardiac lineages made from iPSC cells?
- iPSCs specified towards precardiac mesoderm by inhibition of glycogen synthase kinase 3B /GSK-3B (beta)
- acts as downstream switch for signaling pathways
- Wnt signalling inhibited
- Leads to differentiation of cardiac progenitor cells
- Specific signalling molecules differentiates cells towards specialised cardiac lineages e.g epicardium, cardiac fibroblasts, cardiomyocytes, smooth muscle, endothelial cells
What is myocardial thymosin Beta-4 necessary for?
What can its addition in adult hearts do?
It is a small molecule produced by cardiomyocytes.
Necessary for epicardial migration into myocardium where it forms coronary vasculature required for cardiomyocyte survival.
TB4 in adult hearts stimulates epicardial outgrowth and neovascularisation by reactivating epicardial gene programmes.
And by Tb4 priming there is re-expression of embryonic epicardial gene Wt1.
How are stem cell based therapies used for cancer?
- Chemo/radiotherapy kills cancerous cells. Stem cell transplantation rescontitutes healthy cells e.g HSC transplant for blood cells and leukocytes, replaces cancerous blood cells with healthy blood cells and leukocytes (ASC, iPSC)
- Effector immune cells from iPSC/ESCs e.g NK and T cells, targeted immunotherapy against specific cancer cells.
- engineered by treating with specific factors SCF, IL-7, FLT3L
- in anti-cancer vaccines, targets cancer - stem cells attracted to tumours
- Mesencymal/neural stem cells deliver genes, nanoparticles and oncolytic viruses to tumour niches
