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
Name three signalling molecules involved in signalling for quiescence and genes they upregulate
BMP
GABA
Notch
Genes:
Hes5
FoxO3
p57
REST
Name two signalling molecules involved in signalling for activation and four genes they upregulate
Wnt
IGF
Genes:
Tlx
Ascl1
CcnD2
Sox1
Describe cellular properties that change in adult NSCs during aging
Cellular properties of young NSCs compared to aged:
DNA and chromatin modifying enzymes
Assymetric segregation of damaged/ aggregated proteins
Mitochondrial function
Proteostasis
pro-neurogenic niche signalling
Cellular properties of Aged NSCs compared to young:
Pro-inflammatory signalling
Hallmarks of senescence
Accumulation of lysosomes and protein aggregates
Pro-quiescence niche signalling
Describe some of the effects of pro-inflammatory signalling in aged niches
Inflammation increases in the niche, highlighted by the increase in inflammatory cytokines (negative effects on neurogenesis and cognition in young mice), activated microglia, and T cell infiltration. There is also disruption to the blood brain barrier and glymphatics.
Why do quiescent stem cells in older people stop dividing?
Some research groups retrieve quiescent stem cells from older brains of deceased people and place them in cortical organoids and demonstrate that they are activated and begin dividing. The signals in an aged brain are what make qNSC go into quiescence.
Where are the phosphorylation sites on FOXO1,3,4,6? What phosphorylates these sites?
Each have a phosphorylation site prior to the dna binding domain and the nuclear localisation sequence, all except for FOXO6 have an extra site between the nuclear localisation sequence and the nuclear exclusion sequence. These are regulated by phosphorylation via kinases, the kinase doing this is Pkb/Akt pathway.
What is different about the sequences on the FOXO3 and FOXO6 gene compared to the others?
In FOX06 the NES (nuclear exclusion signal; drives out of the nucleus), is missing as it is only localised in the nucleus.
FOX03 has two NES as it does in and out of the nucleus very quickly.
Name 8 cellular processes in which FoxO transcription factors play a role
DNA damage repair
Apoptosis
Neuropeptide release
Cell cycle arrest
Glucose metabolism
Oxidative stress resistance
Autophagy
Lipid metabolism
Describe the regulation of FoxO activity by translocation
In the presence of growth factors a complex in formed with PI3K, PDK1, AKT, SGK which phosphorylates FOXOs to create binding sites for 14-3-3 proteins which translocate FOXOs and retain them in the cytoplasm as well as block DNA binding. This aids in cell proliferation, stress sensitivity and cell survival.
In the absence of growth factors, FOXOs are localised in the nucleus and and bind to target genes (e.g GADD45, p27, MnSOD, BIM, FasL), assisting in cell cycle arrest, stress resistance and apoptosis.
Describe the Insulin/IDF-1 Signalling pathway
Insulin or IGF-1 bind to Insulin or IGF-1 receptors at the cell membrane trigger a kinase cascade: The canonical insulin and growth factor signalling initiates when secreted insulin or insulin-like growth factors (IGFs) bind to their cell surface receptors. Dimerised receptors trigger a series of autophosphorylation and recruit insulin receptor substrate 1–4 (IRS1-4) and PI3K, the latter increases the local concentrations of PIP3. PIP3 acts as a second messenger to activate PDK1 and AKT.
Active AKT translocates to the nucleus and phosphorylates FoxO at three conserved residues, enhancing the binding of FoxO proteins to 14-3-3 and leading to their cytoplasmic localisation. Inactivation of FoxO favours cellular growth under normal conditions.
Describe the JNK signalling pathway
When cells are in a stressed condition, such as increasing levels of reactive oxygen species (ROS), c-Jun N-terminal kinase (JNK) is activated and phosphorylates cytoplasmic FoxO. This stimulatory phosphorylation induces the release of FoxO from 14-3-3, translocates it to the cytoplasm and upregulates its transcriptional activity.
How is Foxo6 different to the other Foxos in regards to these signalling pathways?
FoxO6 is not able to translocate and is mainly nuclear
Is FoxO6 therefore regulated by these pathways?
Yes, FoxO6 does not translocate but its DNA binding activity is regulated by PI3K/Akt. AKT still phophorylates FoxO6 and inhibits binding.
How variable are these pathways across species?
The opposing regulation of FoxO activity by insulin/IGF pathway and JNK pathway is evolutionarily conserved in C. elegans, Drosophila and vertebrates
In regard to stem cells what effect does FOXO3 have? What would you see in a FOXO3 KO and why?
FoxO3 promotes NSC quiescence and stemness; If you knock out FOX03 in a mouse, all the stem cells are consumed in the first two/three months. Ascl1 pushes stem cells into differentiation, FOX03 inhibits Ascl1. Resultantly the brain grows bigger in fox03 mutant mice.
What is meant by stemness?
Stemness is the capability to differentiate and self renew
Does FOXO6 have this same effect on NSC proliferation?
On the contrary, FoxO6 KO results in decreased NSC proliferation
How did researchers analyse the molecular mechanisms of NSC maintenance?
They dissociated stem cells from a mouse brain and added strong mitogens such as epidermal growth factor (EGF) and fibroblast growth factor 2 (FGF-2) allowing the propagation of endogenous proliferative cells (the putative NSCs) into clumps of cells. They then dissociated them again before culturing with mitogens again to establish cell lines and removed growth factors to allow differentiation to occur.
What are neurospheres and why are they used?
A neurosphere is a culture system composed of free-floating clusters of neural stem cells. The forming spheres after culturing with mitogens, referred to as neurospheres, can be propagated over many passages and have the potential to differentiate into all three neural lineages after the withdrawal of mitogens and/or addition of differentiating factors. The neurosphere assay is the most commonly used technique to analyse the stem cell capacity of isolated brain cells.
How can self renewal be tested in neurospheres?
Self-renewal can be tested by the formation of secondary or tertiary spheres; a single cell is grown in a miniwell until the sphere reaches a certain size, after which the sphere is again dissociated into single cells that can give rise to a new, and thus secondary, multipotent neurosphere. Primary neurospheres are indicative of the number of stem cells in a brain, secondary neurospheres are indicative of capability of self renewal.
Describe the results of FOXO6 KO neurospheres and what this means
FoxO6 KO primary neurospheres are decreased in number, indicating a decreased number of neural stem cells. The secondary neurospheres, however, are increased in number, indicating an increased self-renewal/ stemness.
Describe what was found in FOXO6 KO NSC cell lines (2)
In vitro primary FoxO6 KO NSC cell line shows decreased proliferation and more cells were in stage G0 or G1 of the cell cycle.
This FOXO6 KO cell culture research was followed up with RNA sequencing, what did the find was upregulated (2) and donwregulated (2)?
UP
Glycolysis
Fructose & Mannose metabolism
DOWN
Cell cycle
DNA replication
Holistically, what do these results suggest?
The RNA seq data reflects the transition from glycolysis to oxidative phosphorylation seen in NSCs transitioning towards an active state. It also suggests an activation of the cell cycle. This is in support of the cell culture findings, suggesting that FOXO6 increases proliferation and cell cycle and reduced self-renewal/stemness. These suggest that FoxO6 promotes lineage progression in adult NSCs.
What other research findings suggested opposing effects of FOXO3 and FOXO6? (2)
FoxO3 and FoxO6 mutual targets, especially stemness genes, show opposing pattern of gene regulation (80%). This suggests that FoxO3 and FoxO6 are functionally complementary, providing a balance in the early steps of NSC lineage progression.
Additionally, Fox03 and 06 inhibit each other; in other words if one is gone, the other is higher. This can be done through each other binding to the others binding site and thus blocking transcription. This can also happen through the PI3k/Akt pathway in that it binds directly to other transcription factors or modulates their transcription. FOX-03 likely pushes into quiescence and FOX06 pushes them into differentiation.
When does the OSVZ arise and what is its relevance to Neurogenesis?
Around E55 it arises with the ISVZ between, both between the cortical plate and VZ. It then expands to much larger than the ISVZ which remains quite small. It is the predominant neurogenic zone during mid-gestational cortical development
What four important key features do radial glia in the OSVZ display?
- Generation of oRG via asymmetric division of vRG
- Generation of nIPC via asymmetric division of oRG
- Unipolar with a radial glia process reaching the pial zone
- Mitotic Somal Translocation towards the pial surface while dividing
Describe the movement of the RG when dividing and the relevance of their height
With each division they jump up in the cortex (mitotic somal translocation); the higher they are the more differentiated they are
How many times does an oRG divide?
It divides and self renews twice followed by a IP daughter cell division.
Therefore compare mouse vs human cortical development
In mice neurogenesis happens in the SVZ and VZ in humans in happens in the VZ iSVZ and oSVC. In mice neural migration happens in the IZ while in humans it happens in the IZ/subplate and the oSVZ. There is debate whether there is oRGs in mice but in the view presented by Marco there is not.
What is the relevance of notch signalling in the two developing cortices?
In mice notch is:
low in the intermediate zone and
high in the VZ.
In humans Notch is:
very high in the VZ,
moderate in the oSVZ and
low between IZ and cortical plate.
What is the relevance of this difference in notch signalling
Could explain the self renewal of oRGs?
But the situation is not super comparable. Could mean that each progenitor could create its own niche of cells.
What is the overarching result of these differences?
What you see in the human brain is a huge expansion, you begin with a great number of NEs which then explode with all these divisions.
Give another explanation for the neocortical expansion
Neocortical enlargement in gyrencephalic (Gyri containing) animals could be explained by a greater SVZ-located capacity for transit-amplifying divisions, allowing for multiple rounds of cell division before the production of neurons. RGs can self renew in the VZ to increase the founder stem cell population and send a transit-amplifying cell to the SVZ which can divide 10 or 20 times in increased rounds of transit amplification to create a large subventricular zone. Therefore you this expansion in the outer and inner ventricular zone
Describe a study which examined humans with a disease regarding this and what they found
A human genetic study of a family exhibiting congenital microcephaly identified Tbr2 as silenced.
Tbr2 is a specific marker for iPCs and necessary for neurogenesis
Contrast the progression of three types of neural stem cell found in both the developing human and mouse cortex
Both:
Neuroepithelial cells (NE)
* originate from the neural plate
* Divide symmetrical (self-renewal) several times forming NE * Transform into radial glia
Neuronal intermediate progenitor cells (nIPC)
* Originate from RG
* Divide symmetrically forming IPC or neurons
Different:
(Mouse) Ventricular (VZ) radial glia (vRG or aRG)
* originate from NE
** Divide asymmetrical several times forming RG+Neuron or RG+nIPC or RG+oIPC
* Transform into astrocytes or ependymal cells or adult neural stem cells (B-cells)
(Human)
Inner subventricular zone (ISVZ) radial glia (vRG or aRG)
* originate from NE
** Divide asymmetrical several times forming RG+Neuron or RG+nIPC or RG+oIPC or oRG
* Transform into astrocytes or ependymal cells or adult neural stem cells (B-cells)
Name two neural stem cells only found in the human cortex (probably)
Outer subventricular zone (OSVZ) radial glia (oRG or bRG)
* originate from vRG (or aRG)
* Divide symmetrically several times forming oRG (self-renewal)
* Divide asymmetrically several times forming oRG and nIPC
OSVZ Neuronal intermediate progenitor cells (nIPC)
* Originate from oRG
* Divide symmetrically several times forming nIPC (self-renewal)
* Divide symmetrically forming neurons
Describe how the molecular identity of oRGs was explored
Extracted them and performed single cell RNA-seq and clustered them based on their expression level. They then looked at what were specific to humans such as HOPX and did an ISH to find that they are expressed in different parts of the human cortex.
Describe the molecular identity of oRGs (7)
- Local production of growth factors
- Enhanced expression of EMPs (regulate proliferation and stemness)
- Activation of LIF/Stat pathway
- Activation of mTOR pathway (proliferation)
- Increased communication with surrounding
- Enhanced self-renewal capacity
- Shaping of direct environment (EMPs)
What clinical disorder that these oRG genes are involved in?
They now think these genes are involved in developing autism; they are popping up in those studies.
How could oRG maintenance regulated when Notch levels in the OSVZ are low?
Possible answer: the existence of Notch niches
What is the ratio between the two neural progenitors in the oRG and nIPC?
The human OSVZ contains two main types of neural progenitors:
* oRG: 40% of OSVZ progenitors, founders via symmetric and asymmetric divisions
* nIPC cells: 60% of OSVZ progenitors, prone to differentiate
What do regions with high proliferation predict?
gyri-forming areas
Describe the results of a study detailing the remodeling of the radial scaffold in human.
Carried out DiI labeling of pia contacting cells throughout human neurogenesis. Consistent with the proposed model, no back-labeled of ventricle contacting cells are observed after GW16.5.
They validated cell identity of radial glia fibers in the intermediate zone (IZ) and the cortical plate (CP) using immunohistochemistry (immunostaining for pan radial glia marker, VIM, and the outer radial glia marker, HOPX).
This could indicate that these gyri could form during the stage of development in which there is discontinuous scaffolding: Two stages of human cerebral cortex development during neurogenesis.
Describe further how this stage of neurogenesis could form the gyri. What is the name of this hypothesis?
Subgranular cortex expansion hypothesis.
Migrating neurons can follow the vRGs attached to the ventricle and jump to the oRGs attached to apical sites. In this process they also move a bit tangentially, this creates regions of high density of neurons and low densities. This might cause high density areas to fold in on the low density areas causing folds. The development of the OSVZ results in remodeling of the migration scaffold, with fibers no longer spanning the apical and basal surfaces, forcing migrating neurons to switch fibers and disperse tangentially.
Desce lateral dispersion of migration pyramidal neurons in the cortex
- Radial unit hypothesis: columnar organisation originating from neuronal clones via vertical migration of pyramidal neurons
- Lateral dispersion of migrating pyramidal neurons is now thought to be crucial for the generation of a more advanced cortex
- Lateral or tangential migration of pyramidal neurons is modest in rodents, but is considerable in primates, and could account for the neocortical folding in primates
What two key mammalian features does the rodent cortex have?
- Six-layered organization
- Regionalisation into sensory, motor and association areas
How else can the human cortex be modelled?
Human cerebral organoids
Describe the process of ideal organoid development for clinical purposes
Get patient derived hESCs/ iPSCs
Culture them into an embryoid body (EB)
The EBs expand
An early organoid forms in a matrigrel droplet
Late stage organoids are placed on a shaker
An organoid with a disease phenotype develops
Drug screening can be done and they can be placed on a microfluidic chip (engineered with vascular support, astrocytes, microglial cells (?))
And effective drug prescriptions can be develloped
Describe some issues with organoids
- Hypoxia
- no vasculature/ signalling from other areas
- Many cell types present in the human developing brain are not formed: astrocytes, ependymal cells, microglia
- Global organization is lacking, no layering; barely any signs of layering
- no interneurons (take over a year)
- obviously cannot use for aging related questions as even if you culture for over a year you see a very early brain and when you convert to an iPSC you lose the epigenetic profile related to age. There are ways around this but it is very difficult.
- Human cerebral organoids lack normal signalling from the CSF, meninges and intracerebral tissue
- Allthough all neuronal cell types are present, the relative number is abberant, almost no oRG and nIPC
- Gene expression is far less specific, is impoverished
- Cell identity (transcriptional identity) is blended
- Cerebral organoids are under severe stress, leading to vast metabolic changes
