FOXOs in neural stem cell maintenance Flashcards
Where does adult neurogenesis primarily happen?
Mammalian adult neurogenesis largely happens in the dentate gyrus (memory & learning) and ventricular zone (Migrate to olfactory bulb to become interneurons). This occurs in the OB as there is a lot of plasticity in olfactory circuits in mice.
Describe how radial glial cells develop
Neuroepithelial cells in early development divide symmetrically to generate more neuroepithelial cells in the neuroepithelium. Some neuroepithelial cells likely generate early neurons. As the developing brain epithelium thickens, neuroepithelial cells elongate and convert into radial glial (RG) cells.
How may radial glial cells produce neural cells?
RG divide asymmetrically to generate
neurons directly or indirectly through intermediate progenitor cells (nIPCs; divide symmetrically into neurons). Oligodendrocytes are also derived from RG through intermediate progenitor cells that generate oligodendrocytes (oIPCs). As the progeny from RG and IPCs move into the mantel for differentiation, the brain thickness, further elongating RG cells.
How do radial glial cells have apical basal polarity?
Radial glia have apical-basal polarity: apically (down), RG contact the ventricle, where they project a single primary cilium; basally (up), RG contact the meninges, basal lamina, and blood vessels.
What happens to radial glia at the end of development?
At the end of embryonic development, most RG begin to detach from the apical side and convert into astrocytes while oIPC production continues. Production of astrocytes may also include some IPCs. A subpopulation of RG retain apical contact and continue functioning as NSCs in the neonate. These neonatal RG continue to generate neurons and oligodendrocytes through nIPCs and oIPCS; some convert into ependymal cells, whereas others convert into adult SVZ astrocytes (type B cells) that continue to function as NSCs in the adult. B cells maintain an epithelial organisation with apical contact at the ventricle and basal endings in blood vessels. B cells continue to generate neurons and oligodendrocytes through (n and o) IPCs.
Name the four areas from apical to basal described in this development
Marginal zone
Mantle
sub-ventricular zone
ventricular zone
From this cortical expansion how do the two adult stem cell zones emerge?
During development, the cortical plate progressively expands and gives rise to the adult isocortex, leaving behind the adult stem cell SVZ close to the lateral ventricles. The sub-granular zone (SGZ) of the dentate gyrus within the hippocampus constitutes another adult stem cell compartment.
What three key cells are left in these two respective zones?
Both of these regions are comprised of three main cell types: in turn, the stem cells, the progenitor cells and the neuroblasts.
Describe the progression of maturation depicted in the diagram of the dentate gyrus in the hippocampus
Neural stem cells are located on the subgranular zone of the hippocampus, the develop into neural precursors, then neuroblasts, then immature neurons while they migrate towards the flexure and more towards the granule cell layer as a mature neuron they project out to adult hippocampal neurons
Describe the progression of maturation depicted in the diagram of the subventricular zone
Neural stem cells embedded in glial cells mature outwards into neural precursors, then neuro blasts and begin migrating towards the olfactory bulb as they differentiate into immature neurons and eventually into mature neurons when they reach the olfactory bulb
Describe stem cell types as a global pathway
Totipotent embryonic stem cells are omnipotent and can differentiate into anything (oocytes; fertilised egg cells). They differentiate into endoderm, mesoderm and ectoderm lines which are pluripotent embryonic stem cells and can differentiate into multipotent stem cells. Each step represents a further reduction in potential fates of the cell.
For example endoderm derived multipotent stem cells could differentiate into lung or pancreatic tissue, endoderm derived multipotent stem cells could differentiate into heart muscle or red blood cells and ectoderm derived multipotent stem cells could differentiate into skin or neural cells.
What stem cells do we use as described in this pathway
We take adult bone marrow, skin, cord blood or deciduous teeth as multipotent stem cells and revert them back to human embryonic stem cells as induced pluripotent stem cells. Going from multipotent to pluripotent cells induced are the stem cells we use in the lab, however these are unnatural and retain their former epigenetic profile even if they are reprogrammed. iPSC’s telomeres keep growing even though they should get shorter with age because of telomerase used to maintain them.
Aside from their potential cell fate, what other qualities are reduced as we move down this pathway? (2)
Their ability to self-renew and differentiate
How are brain tumours posited to develop?
Neural stem cells persist in the adult human brain and are the origin of brain tumours. Neural stem cells or precursor cells can develop into cancer cells and develop into glioma cells. The existence of these stem cells and learning about these can give insights into predicting and managing these gliomas.
How can NSC populations develop?
Maintenance: asymmetric division of stem cells occur where a committed cell and a stem cell divide from a stem cell so that the number of stem cells are maintained.
Exhaustion: A stem cell symmetrically divides into two committed cells so that a stem cell is lost.
Expansion: A stem cells symmetrically divides into two stem cells to expand the number of stem cells. Expansion does not occur in the adult that we know of, the number of stem cells decline with age.
In what state are adult neural stem cells?
A quiescent state where they are not actively dividing and have to be first activated which becomes increasingly more difficult with age. Quiescence is stage G0 of the cell cycle; it is a reversible stage and can be activated at any time.
What is the benefit of stem cells being quiescent?
It is best that stem cells stay at this stage for maintenance and so they avoid becoming cancerous stem cells.
How is quiescence different to senescence?
This is opposed to senescence which is essentially going into apoptosis but it doesn’t; it should be dead but it isn’t. It is irreversible; moles are senescent cells.
What might be the mechanism of senescent cells?
Cellular senescence could be a tumour-suppressive mechanism that permanently arrests cells at risk for malignant transformation, however the prevalent view now is that they are cells which should have died. However, accumulating evidence shows that senescent cells can have deleterious effects on the tissue microenvironment. The most significant of these effects is the acquisition of a senescence-associated secretory phenotype (SASP) that turns senescent fibroblasts into proinflammatory cells that have the ability to promote tumour progression.
Do adult stem cells self renew?
Yes, neural stem cell self-renewal is necessary for adult neurogenesis. Stem cells symmetrically divide into other stem cells around 20% of the time and terminally into committed cells 80% of the time, leading to a decrease in the number of available cells over time. This was done in the VZ and hasn’t been done in the hippocampus yet
How has the field of human adult neurogenesis developed?
Carbon 14 can be used as a marker for when cells divide and was found in the adult brain. This was taken as evidence for neurogenesis, however this could be astrocytes or microglia as those cells divide. Recent evidence suggests that adult neurogenesis drops sharply after childhood: Used Ki67 as a marker for dividing cells and thus the cell cycle and Sox2 which is a marker for stem cells. They overlapped and dropped rapidly. These were the black sheep of the field as many people were working on adult neurogenesis (in other animals) and thus these findings undercut their research.
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Briefly describe each stage of the cell cycle
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Interphase (The G1, S, and G2 phases together are known as interphase. The prefix inter- means between, reflecting that interphase takes place between one mitotic (M) phase and the next.):
G1 phase. During G1 phase, also called the first gap phase, the cell grows physically larger, copies organelles, and makes the molecular building blocks it will need in later steps.
S phase. In S phase, the cell synthesises a complete copy of the DNA in its nucleus. It also duplicates a microtubule-organising structure called the centrosome. The centrosomes help separate DNA during M phase.
G2 phase. During the second gap phase, or G2 phase, the cell grows more, makes proteins and organelles, and begins to reorganise its contents in preparation for mitosis. G2 phase ends when mitosis begins.
M phase:
During the mitotic (M) phase, the cell divides its copied DNA and cytoplasm to make two new cells. M phase involves two distinct division-related processes: mitosis and cytokinesis.
In mitosis, the nuclear DNA of the cell condenses into visible chromosomes and is pulled apart by the mitotic spindle, a specialized structure made out of microtubules. Mitosis takes place in four stages: prophase (sometimes divided into early prophase and prometaphase), metaphase, anaphase, and telophase.
In cytokinesis, the cytoplasm of the cell is split in two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. In animals, cell division occurs when a band of cytoskeletal fibers called the contractile ring contracts inward and pinches the cell in two, a process called contractile cytokinesis. The indentation produced as the ring contracts inward is called the cleavage furrow. Animal cells can be pinched in two because they’re relatively soft and squishy.
Some types of cells divide rapidly, and in these cases, the daughter cells may immediately undergo another round of cell division. For instance, many cell types in an early embryo divide rapidly, and so do cells in a tumor.
Other types of cells divide slowly or not at all. These cells may exit the G1 phase and enter a resting state called G0 phase. In G0, a cell is not actively preparing to divide, it’s just doing its job. For instance, it might conduct signals as a neuron (like the one in the drawing below) or store carbohydrates as a liver cell. G0 is a permanent state for some cells, while others may restart division if they get the right signals.
Describe when markers for astrocytes, cell cycle, early neurogenesis and late neurogenesis are present in the transition from quiescent neural stem (qNSC) cell to neuron
Astrocyte markers are present in qNSCs, aNSC early and die out in aNSC mid. Cell cycle markers arise in aNSC mid, through aNSC late and fade during NPC stage. Early markers of neurogenesis begin in aNSC late, through NPC and neuroblast stage. Late markers of neurogenesis begin at NPC stage through neuroblast stage. There is heterogeneity within all aNSCs (early to late).
Describe the stages from quiescent neural stem cell to a neuron and the processes they (can) undergo
Stem cells in deep quiescence can develop into an NSC in shallow quiescence.
From here the NSC can revert back to deep quiescence or activate into an active state and proliferate or self renew.
These can then return to quiescence or undergo neuronal commitment into an intermediate progenitor cell (IPC) from which they cannot return.
IPCs can then differentiate into a neuroblast and undergo proliferation and/or cell survival.
Neuroblasts then undergo maturation requiring cell survival and circuit integration to become a full mature neuron.
In which stage does proliferation happen
Only at neural progenitor cell stage (NPC) and after
Name five things differently regulated in quiescent and activated stem cells
Quiescent stem cells:
- Cell cycle activity is downregulated
- Metabolism is downregulated
- Transcription is downregulated
- Cell cycle inhibitors are upregulated (e.g p21, p57, CDK inhibitors)
- Tumour suppressor genes are upregulated (e.g RB, p53- cancer factor)
Active stem cells:
- Cell cycle is upregulated
- Metabolic activity is upregulated
- RNA content/ translation is upregulated
- Induction of damages is upregulated
- Activation of repair pathways is upregulated
Describe differences in metabolism in qNSCs and aNSCs
Lipid metabolism is high in quiescent NSCs and lowers as protein synthesis increases in activated NSCs. Similarly Glycolysis is high in qNSCs and drops as oxidative phosphorylation increases in aNSCs.
Why might there be this difference in metabolism between iNSCs and aNSCs? (2)
Glycolysis is in the cytoplasm and does not require oxygen, oxidative phosphorylation in the mitochondria requires oxygen. Much more ATP is produced in a short time than can be produced with glycolysis however. Reactive oxygen species are produced during oxidative phosphorylation, this can cause damage and mutations. This is likely why glycolysis is used in qNSCs.
A qNSC has a lot of g-coupled receptors to receive signals for activation. Epidermal growth factor receptors are very important for metabolic signalling as they adjust metabolism for the external environment. These are more important for activated stem cells.
Name 5 quiescence genes
Aqp4: very important for water transport but also a marker for microglia (glial marker).
Id3 (cancer supressor)
Aldoc (cancer supressor)
ApoE3 (lipid metabolism)
FoxO3 (Cancer supressor)
Hes1 (Inhibits differentiation, maybe increases self renewal)
Name 3 genes associated with activation
Fgfr3 (Growth factor)
Dbi (inhibits lipid metabolism)
Egfr (Growth)
Name three genes associated with the cell cycle
CcnD2
Mki67 (marker for dividing cells)
Cdk4
Name five neurogenic genes
Sox11
Dcx
Calb2
Prox1
NeuN
What signals to the NSC what to do?
Surrounding neurons secrete signals. These extrinsic signals regulate NSC homeostasis via transcriptional programs
