An overview of nervous system development

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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?
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Patterning, proliferation and neurogenesis
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Specification of cellular identities
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Wiring
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Processes to consider

• Induction of the nervous system
• Neurulation (formation of the neural tube)
• Patterning of major axes
• Proliferation
• Establishment of cell fates
• Cell migration
• Axon guidance
• Synaptogenesis
• Cell death
• Synaptic refinement
• Myelination

References Jessell and Sanes (2000) Kandel,
Jessell and Schwartz, Principles of Neuroscience
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Clinical relevance

• Birth defects
• Psychiatric disorders
• Regeneration
• Stem cell therapeutics

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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?
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Differential rates of proliferation
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• Microcephaly
• Small head size (small brain)
• Moderate to severe mental retardation
• Seizures (rare)
• Genetically heterogeneous (six loci identified)

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Chuas or rat people Many found at shrine to
17th century Sufi saint 1st cousin marriages -
common in British Pakistani community too
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Can the study of microcephaly tell us anything
about control of proliferation and evolutionary
expansion of the neocortex?
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MCPH5 autosomal recessive, linked to chromosome
1q31
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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.
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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
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Expression of ASPM in developing mouse brain
Ventricular zone
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Neurogenesis and migration in the cerebral cortex
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• 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)

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• 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

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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
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Asymmetric distribution of cytoplasmic factors
coordinated with orientation of mitotic spindle
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Aspm protein localises to centrosomes
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Knockdown of Aspm function leads to asymmetric
division
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Knockdown of Aspm results in more asymmetric
divisions at early stages
Effect is more progeny adopt neuronal fate and
fewer retain neuroepithelial progenitor fate
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• Mutation of Aspm (or other genes implicated in
  microcephaly) causes
• Defect in alignment of mitotic spindle with axis
  of cell
• Increase in asymmetric division at early stages
• Failure to expand progenitor pool
• Premature generation of neurons
• Reduction in brain size

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• 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

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Diversity of cell types and functions
Red blood cells
Hair cells in cochlea
Cardiac muscle cells
Nerve cells
Skin cells
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What makes cells different is they make different
proteins
Some proteins made only in specific cell
types e.g., hemoglobin, insulin
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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?

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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.
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Gastrulation and Neural Induction
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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
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Different neuronal types generated from specific
progenitor pools
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Progenitor pools are specified by code of
transcription factors
(Briscoe et al., 2000)
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Sharp borders between domains
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Floor plate of spinal cord can induce ectopic
motorneurons
motoneurons
Floor plate
Floor plate grafted
Wild-type situation
Floor plate ablated
(Embryological experiments in chick)
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Sonic hedgehog is a secreted protein expressed in
floor plate
Shh conc.
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Gradient of Shh induces different fates
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Gradient of Shh induces some genes and represses
others
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How do you get such sharp borders?
Cross-repression between transcription factors
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• Cross-repression
• Nkx2.2 activates its own transcription and
  represses Pax6
• Pax6 activates its own transcription and
  represses Nkx2.2
• Both genes cant be expressed in same cell
• - slight imbalance amplified
• - graded expression becomes sharp
• - individual cells specified as one fate or
  another

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• 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.

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Shh also patterns midline of brain and face
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Mutations in Shh lead to Holoprosencephaly
(OMIM 142945)
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Specification of clinically important cell types
Midbrain dopaminergic neurons degenerate in
Parkinsons disease
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Parkinsons 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

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Midbrain dopamine neurons induced by Shh and Fgf8
Shh
Fgf8
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Induction of midbrain dopaminergic neurons
(side view)
(dorsal view)
d2
d3
v3
v2
Explants of neural tube in vitro
THve neurons arise only in v3 in vivo and in
explants in vitro
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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)
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Inducing dopaminergic neurons from stem cells in
vitro
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• 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

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Transcription factors induced or repressed by
Shh in concentration-dependent fashion
Explants of medial spinal cord plus increasing
concentrations of Shh
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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)
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High affinity and low affinity binding sites
Gli
Nkx2.2
Low affinity sites Gli binds weakly, not
effective at low concentrations gt 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 gt Nkx6.1
expressed further away from floor plate (where
Shh lower)