Transcript Overview of NS Development

An overview of nervous system development
How are all the different regions and cell types specified?
How do they arise in the correct areas?
How do all these regions/cell types get connected together?
Patterning, proliferation and neurogenesis
Specification of cellular identities
Wiring
Processes to consider:
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Induction of the nervous system
Neurulation (formation of the neural tube)
Patterning of major axes
Proliferation
Establishment of cell fates
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Cell migration
Axon guidance
Synaptogenesis
Cell death
Synaptic refinement
Myelination
References: Jessell and Sanes (2000);
Kandel, Jessell and Schwartz, Principles of Neuroscience
Clinical relevance
 Birth defects
 Psychiatric disorders
 Regeneration
 Stem cell therapeutics
Proliferation and Neurogenesis
Amount of proliferation controlled by amount of asymmetric
cell division
When a progenitor cell divides does it make:
- Two progenitors?
- One progenitor and one neuron?
- Two neurons?
Differential rates of proliferation
Microcephaly
 Small head size (small brain)
 Moderate to severe mental retardation
 Seizures (rare)
 Genetically heterogeneous (six loci identified)
Chuas or “rat people”
Many found at shrine to 17th century Sufi saint
1st cousin marriages - common in British Pakistani community too
Can the study of microcephaly tell us anything
about control of proliferation and evolutionary
expansion of the neocortex?
MCPH5: autosomal recessive, linked to chromosome 1q31
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Principle of linkage analysis
Recombination
in meiosis:
Variants near each other on the same chromosome
(“linked”) tend to be inherited together.
The co-inheritance of a neutral molecular marker with a
disorder implies the mutant gene is near that marker.
MCPH5 mapped to ASPM gene
Homologous to abnormal spindle (asp) gene in Drosophila
Mutations lead to truncated protein
Bond et al., (2002) Nature Genet. 32: 316
Expression of ASPM
in developing mouse
brain
Ventricular zone
Neurogenesis and migration in the cerebral cortex
A number of other genes that cause Microcephaly
have also been identified:
 MCPH1: Microcephalin
- control of mitosis
(Jackson et al., (2002) Am J Hum Genet. 71, 136-42)
 MCPH3: CDK5RAP2
 MCPH6: CENPJ
- both involved in chromosome segregation
in mitosis
(Bond et al., (2005) Nat Genet. 37, 353-5)
How do mutations in genes controlling mitosis
lead to microcephaly?
Aspm mRNA expressed at early stages:
- Divisions are symmetric
- Progenitor pool expanding
Aspm mRNA downregluated at later stages:
- Divisions are asymmetric
- Neurons being generated
Symmetric divisions at early stages generate two
neuroepithelial progenitors
- expand pool of progenitors
Asymmetric divisions at later stages generate one
postmitotic neuron and one progenitor
- as each progenitor can only generate a limited
number of neurons this eventually depletes pool
of progenitors and leads to fewer neurons
Asymmetric distribution of cytoplasmic factors
coordinated with orientation of mitotic spindle
Aspm protein localises to centrosomes
Knockdown of Aspm function leads to asymmetric division
Knockdown of Aspm results in more asymmetric divisions at
early stages
Effect is more progeny adopt neuronal fate and fewer retain
neuroepithelial progenitor fate
Mutation of Aspm (or other genes implicated in microcephaly)
causes:
1. Defect in alignment of mitotic spindle with axis of cell
2. Increase in asymmetric division at early stages
3. Failure to expand progenitor pool
4. Premature generation of neurons
5. Reduction in brain size
Conclusions:
 Microcephaly caused by mutations in many genes
 All involved in mitosis somehow
 Defects in Aspm affect symmetric division
 Progenitor pool fails to expand - depleted too early
 Small brain results
 ASPM, MCPH1, CDK5RAP2 all show evidence of
positive selection in lineage leading to humans
 Inference: Mutations in these genes that increased
brain size may have been selected for in human
lineage
Diversity of cell types and functions
Red blood cells
Hair cells in cochlea
Cardiac muscle cells
Nerve
cells
Skin cells
What makes cells different is they make different proteins
Some proteins made only in specific cell types:
e.g., hemoglobin, insulin
Each tissue/cell type has a different profile
- Express different genes related to their specific functions
(neurotransmitter receptors, ion channels, etc.)
- Express specific code of transcription
factors that control expression of all the
other genes that make each cell unique
(i.e. that specify its “identity”)
- How do they come to express that
spectrum of transcription factors?
Process of reiterative subdivision of embryo and
progressive restriction of potential.
- specification of intermediate fates of dividing cells
en route to specification of final fates of postmitotic cells
Occurs through series of cellular interactions beginning
at the first cell division and continuing throughout
development as morphogenetic movements shape
embryo.
Gastrulation and Neural Induction
Patterning and establishment of cell fates
1. Gradients of diffusible molecules specify different
fates at different concentrations
2. Interactions between neighbouring cells also influence
cell fates
Different neuronal types generated from
specific progenitor pools
Progenitor pools are specified by code of transcription factors
(Briscoe et al., 2000)
Sharp borders between domains
Floor plate of spinal cord can induce ectopic motorneurons
motoneurons
Floor plate
Wild-type situation
Floor plate ablated
Floor plate grafted
(Embryological experiments in chick)
Sonic hedgehog is a secreted protein expressed in floor plate
Shh conc.
Gradient of Shh induces different fates
Gradient of Shh induces some genes and represses others
How do you get such sharp borders?
Cross-repression between transcription factors:
Cross-repression:
 Nkx2.2 activates its own transcription and represses Pax6
 Pax6 activates its own transcription and represses Nkx2.2
 Both genes can’t be expressed in same cell
- slight imbalance amplified
- graded expression becomes sharp
- individual cells specified as one fate or another
Combinatorial code of transcription factors
 Control expression of other genes
(i.e., turn on whole “profile” of gene expression
for different subtypes of neurons)
 These downstream “effector” genes control various
aspects of cell fate:
- Connectivity
- Neurotransmitter expression
- Expression of ion channels/receptors, etc.
Shh also patterns midline of brain and face
Mutations in Shh lead to Holoprosencephaly
(OMIM: 142945)
Specification of clinically important cell types
Midbrain dopaminergic neurons degenerate in Parkinson’s disease
Parkinson’s disease
Primary symptoms:
 Tremor: an uncontrollable trembling or shaking
 Rigidity: an abnormal stiffness of the muscles
 Bradykinesia: an extreme slowness of movement and reflexes.
Caused by progressive loss of midbrain dopaminergic neurons
- can be familial (often early-onset)
Current therapies (L-dopa) only moderately effective
Midbrain dopamine neurons induced by Shh and Fgf8
Shh
Fgf8
Induction of midbrain dopaminergic neurons
(side view)
(dorsal view)
d2 d3
Explants of
neural tube
in vitro:
v2 v3
TH+ve neurons arise
only in v3 in vivo and
in explants in vitro
Add FP (source of Shh)
to d3: dopaminergic
neurons (TH +ve):
Add isthmus (source of
Fgf8) to v2: dopaminergic
neurons (TH +ve):
Block Shh function in v3
explant with antibody: no
dopaminergic neurons
(TH -ve):
Inducing dopaminergic neurons from stem cells in vitro
Summary
Development of the nervous system involves many
distinct processes in two main phases:
- establishment of cell identities
(patterning, proliferation, neurogenesis)
- wiring
(migration, axonal extension, synaptogenesis)
Defects (due to genetic or environmental causes)
in any of these processes can lead to specific clinical
disorders
Knowledge of developmental mechanisms can inform
efforts to promote regeneration or stem cell replacement
therapies
Transcription factors induced or repressed by Shh
in concentration-dependent fashion
Explants of
medial spinal
cord plus
increasing
concentrations
of Shh
Diffusible Shh bound by transmembrane receptor proteins
that transduce a signal intracellularly, eventually leading to
activation of transcription factors.
- At different concentrations this has different effects (it is a morphogen)
High affinity and low affinity binding sites
Gli
Nkx2.2
Low affinity sites: Gli binds weakly, not effective at
low concentrations => Nkx2.2 only expressed very
near floor plate ([Shh] high)
Gli
Gli
Gli
Nkx6.1
High affinity sites: Gli binds strongly, effective even at
low concentrations => Nkx6.1 expressed further away
from floor plate (where [Shh] lower)