Module 1 Lecture 3: Neurogenesis Flashcards
which cells give rise to all neurons and glia in the CNS
neural progenitor cells (NPCs)
interkinetic migration in NPCs
soma moves up and down the thickness of the neural tube in sync with cell cycle
where is NPC soma during S phase (DNA synthesis)
toward pial/basal side
where is NPC soma during G2 (2nd growth) phase
back at ventricular/apical side
where is NPC soma during M-phase (mitosis)
at the ventricular/apical side
where is NPC soma at G1 (1st growth) phase
starts moving back to basal side
first step of lineage tracing for NPCs
make a progenitor with a lineage tracer that can only be passed down to progeny
second characteristic of lineage tracing for NPCs
tracer can produce marker to label the whole cell outline
third step of lineage tracing for NPCs
wait
fourth step of lineage tracing for NPCs
identify progeny by shape and/or co-staining w/ genetic markers
application of tracer for NPCs
replicating incompetent retrovirus
- reverse transcribed viral genome incorporates into genome of dividing host cells
- cannot spread horizontally
application of markers for NPCs
LacZ (can paint the cell) gene engineered into retrovirus
- experiment done pre-GFP
how is the CNS built by NPCs
inside out
first step of birthdating
at chosen time, mark progenitor DNA w/ labeled nucleotide
- radioactive: 3H-thymidine
- chemical: Bromodeoxyuridine (BrdU, thymidine analog)
second characteristic of birthdating
at each round of DNA synthesis, label gets diluted
- contrast with lineage tracing
third characteristic of birthdating
post-mitotic progeny born right after label injection is most heavily marked
what are asymmetric divisions controlled by
Notch signaling
what kind of divisions in the expansion phase
symmetric divisions
what kind of divisions in neurogenic phase
asymmetric divisions
what is a mitogen
factor that promotes cell division
examples of mitogens
EGF, FGF, and Shh
- drive expansion phase
two hypotheses for why Shh is both morphogen and mitogen
- signaling is context dependent (concentration, duration, other molecules, history of the responding cell, etc)
- biology doesn’t care about our labels: possible that these two effects are not separable and/or two sides of the same coin
first step of Notch signaling cascade in signaling cell
proneural genes Ascl1 and Neurog2 lead to expression of Dll1 ligand
second step of Notch signaling cascade in signaling cell
Dll1 is presented at the cell surface
first step of Notch signaling in the receiving cell
Notch binds Dll1
second step of Notch signaling in the receiving cell
Notch intracellular domain (NICD) gets cleaved and translocates to the nucleus
third step of Notch signaling in the receiving cell
NICD binds to Rbpj and Maml
fourth step of Notch signaling in the receiving cell
complex turns on transcription of Hes, Hey
fifth step of Notch signaling in the receiving cell
proneural genes Ascl1 and Neurog2 are downregulated
three key features of Notch signaling
- requires cell-cell contact
- involves feedback loop(s)
- biology uses it to build patterns
what does Ascl1 stand for
Achaete-Scute like 1
what does Maml stand for
Mastermind-like
what does Hes stand for
hairy/suppressor of split
characteristics of neurogenic region
- induced at the ventral midline using similar molecules and mechanisms as vertebrate neural induction
- segmented, arrayed along the ventral midline
function of delaminating neuroblasts
they internalize the future CNS instead of neural tube closure
- they are the NPCs of the fly
role of asymmetric divisions in fly neurogenesis
regenerate the neuroblast and make a ganglion mother cell (GMC)
role of ganglion mother cells (GMCs) in fly neurogenesis
divide once to produce post-mitotic neurons
- similar to vertebrate intermediate precursor cells
characteristics of neurogenic islands
8 per hemisegment
- 5-7 ectodermal cells in each hemisegment
how is the neuroblast determined out of the clusters?
younger embryos express Ascl1 gene in each neurogenic cluster; soon after, only one cell continues to express Ascl1 –> this will be the neuroblast
what happens if you kill a neuroblast?
another one takes its place
- all cells in the neurogenic cluster are competent to become the NB; cell-cell interactions decide which gets picked
what happens if there are no proneural genes in the neurogenic cluster
no neuroblast develops
what happens if there is no Notch or Delta
too many NB
role of proneural genes in neuroblast development
necessary for neuroblasts
role of Notch and Delta in neuroblast development
control the number of neuroblasts
lateral inhibition meaning
small quantitative differences between neighboring equivalent cells are amplified through Notch-Delta positive feedback until two qualitatively different cells emerge
how does lateral inhibition determine which cell becomes the neuroblast
lateral inhibition happens between the 5-7 ectodermal cells in each neurogenic cluster, and each cell either becomes a neuroblast or an epidermal cell (bifurcation)
- the signaling cell with Asc expression becomes the neuroblast
- the receiving cell without Asc expression becomes the epidermal cell
what is the mammalian Notch component
hex/her
what happens if hes/her is autoinhibited?
lack of bifurcation between the signaling and receiving cells; results in fluctuation between the two fates
how do NPCs control asymmetric division
oscillatory circuits
effect of inhibiting Notch in NPC asymmetric divisions
premature differentiation
effect of overactivating Notch in NPC asymmetric divisions
remains progenitor or become glia
what proneural genes are involved in NPC asymmetric division
Ascl1, Neurog2, Olig2
what are the Notch-responsive transcription factors in NPC asymmetric divisions
Hes, Hey (auto-inhibitory)
- relative levels determine which daughter cells differentiation
function of Hes1 oscillations
maintain NPC renewal
what happens if Ascl1 dominates in NPC asymmetric divisions
differentiates as neuron
what happens if Olig2 dominates in NPC asymmetric divisions
differentiate as oligodendrocyte
what happens if Hes1 always dominates in NPC asymmetric divisions
differentiates as astrocyte
what kind of molecule drives symmetric divisions
mitogens (EGF, FGF, Shh)
what kind of signaling drives asymmetric divisions
Notch signaling
what is the neocortex
6 layered derivative of telencephalon
- present in mammals, but not in other vertebrates
main characteristic of mammalian neocortex assembly
depends on interplay between developmentally transient structures that are built through ongoing self-proliferation, migration, & differentiation
lissencephalic vs gyrencephalic cell division
cortical progenitors undergo 11 rounds of cell division in lissencephalic brains, vs >28 in gyrencephalic
why is radial glia a misnomer
they are not actually glia, but are NPCs of corticogenesis; they undergo asymmetric division to form 1 radial glia cell and 1 neuron
- appeared to act as a scaffold for migrating neurons, but play a more complex role
which phase of corticogenesis are intermediate precursor cells (IPCs) involved in
indirect neurogenesis
intermediate precursor cells (IPC) characteristics
- daughters of RGCs
- divides once to make two neurons
- similar to ganglion mother cells (GMC) in the fly
what phase of corticogenesis are outer Radial Glia (oRG) involved in
neurogenesis through oRG cells
characteristics of outer Radial Glia (oRG)
- daughter of RGCs
- unipolar w/ one process attached to the basal membrane
- true progenitors; can re-new, make neurons directly, or through IPCs
ventricular zone characteristics
ventricle/apical side of neuroepithelium
- home to NPCs; most cortex neurons derived from here
- VZ
preplate characteristics
first neurons born from VZ
- gives rise to marginal zone (MZ) and subplate (SP)
- PP
marginal zone characteristics
home to Cajal-Retzius cells; will become L1
- most superficial layer
- MZ
characteristics of sub-ventricular zone
- “above” VZ
- home to IPCs and bRGCs
- route for incoming migratory neurons (GABAergic)
- SVZ
cortical plate characteristics
will become layers 2-6, starting with 6 (inside out)
- CP
characteristics of intermediate zone
home to incoming neurons from other parts of the brain
what do apical radial glial cell (RGC), intermediate precursor cells (IPC), and basal radial ganglion cells (bRGC) have in common
all neural progenitor cells
characteristics of outer subventricular zone (oSVZ)
generated by massive expansion of basal radial glia
- one of the reasons we have gyrencephalic brains
what cells are in the vertebrate marginal zone?
horizontal cells
- not in fish
where are horizontal cells born
cortical hem (transient structure, arises next to the former roof plate)
where do horizontal cells go after being born
migrate out and cover the whole cortex surface
how long do Cajal-Retzius cells stay
in mice, gone by 1 week after birth; in humans, most disappear but a few persist
when were Cajal-Retzius cells discovered
turn of 19th century
- still do not know how they do what they do
- express Reelin
what are reeler mice
RELN-/- mice
- have ataxia (impaired balance and motor coordination)
structural phenotype of RELN+/+ in brain
normal
structural phenotype of RELN+/- in brain
- no growth pathology
- structural alterations
- fine histological changes
- GABAergic neurons
- pyramidal neurons
- dendritic spines
structural phenotype of RELN-/- in brain
- lissencephaly
- cerebellar hypoplasia
- structural alterations
- loss of layered structures
- severe histological changes
what are mutations in Reelin linked to
Lissencephaly 2
effect of Lissencephaly 2
- abnormalities of the skull and face
- severe or profound intellectual disability
- seizures
- abnormally increased muscle tone
- exaggerated reflexes
- severe growth failure
how is cortex assembly affected by Reln-/-
- new neurons cannot migrate past older ones; outside-in (and scrambled) assembly
- disrupted organization
what is Reelin
secreted glyco-protein
what is the only source of Reelin during cortical development
Cajal-Retzius cells
what receptors do migrating cortical neurons express for Reelin
VLDLR and ApoER2 (very low density lipoprotein receptor)
relationship between Dab1 (adaptor protein) and Reelin
gets phosphorylated when Reelin binds its receptors
- Dab1 mutant mice have a reeler-like phenotype
what is the ultimate target of Reelin signalin
regulation of cell migration
characteristics of most neurons born at the ventricular zone of the developing cortex
pyramidal, glutamatergic, projection neurons
where the GABAergic interneurons of the cortex migrate in from
the Medial Ganglionic Eminence (MGE)
what is the medial ganglionic eminence
a temporary subcortical structure which disappears in humans by age 1
- contributes neurons to the basal ganglia
role of subventricular zone in building the cortex
incoming neurons migrate along the sub-ventricular zone and follow the radial glia processes to find their layers
where does neurogenesis occur in the adult mammalian brain
- subventricular zone (SVZ) of the lateral ventricle
- dentate gyrus of the hippocampus
