Download Developmental biology 2008 Fates of the ectoderm: The neural tube

Developmental biology 2008
Fates of the ectoderm:
The neural tube and brain development
Lecture 1:
Chapter 12 p. 373-397, 404-405
Chapter 13 p. 424-436
Stine Falsig Pedersen
[email protected]/Room 527
Department of Biology
University of Copenhagen
Fates of the ectoderm
Third lecture
Second lecture
First lecture
Fig. 12.1
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Fates of the ectoderm
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Stages of neurogenesis
1. Induction of a neuron-forming region (requires competence)
2. Birth and start of migration of neurons and glial cells
3. Specification of, and commitment to, neuronal fate
4. Guidance of axon growth cones to specific targets
5. Formation of synaptic connections
6. Binding of trophic factors for survival and differentiation
7. Competitive rearrangement of functional synapses
8. Continued synaptic plasticity
(Gilbert kap 13, p. 424 - after Goodman and Doe, 1993)
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Stages of neurogenesis
1. Induction of a neuron-forming region (requires competence)
2. Birth and start of migration of neurons and glial cells
3. Specification of, and commitment to, neuronal fate
4. Guidance of axon growth cones to specific targets
5. Formation of synaptic connections
6. Binding of trophic factors for survival and differentiation
7. Competitive rearrangement of functional synapses
8. Continued synaptic plasticity
(Gilbert kap 13, p. 424 - after Goodman and Doe, 1993)
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Four stages of neural tube formation
Gastrulation and
neurulation in a
Xenopus embryo
Fig. 12.4: Neurulation in an amphibian embryo
I. Neural plate formation: The dorsal mesoderm (mostly notochord) and pharyngeal endoderm
induce the overlying ectodermal cells to elongate to columnar neural plate cells (competence:
FGF8, specification: BMP antagonists noggin, chordin, follistatin)
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II. Neural plate shaping: The neural tube elongates and narrows by convergent extension
Four stages of neural tube formation
III. Neural plate bending
Medial hinge point (MHP) and
dorsolateral hinge point (DLHP)
cells anchor to notochord and
surface ectoderm, respectively,
and are induced to become
wedge-shaped
The actin cytoskeleton apically in
the neural plate cells contracts (Factin, shroom, myosinII, MARCKs)
BMPs inhibit, and the BMP
antagonist noggin favors, DLHP
formation; sonic hedgehog inhibits
it by reducing noggin expression
The presumptive ectoderm pushes
inward on the neural plate cells
Neural tube folding in the chick embryo
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Four stages of neural tube formation
IV. Neural plate closure
Neural folds merge dorsally, and the neural crest cells eventually migrate away.
Apical lamellipodial cell protrusions interdigitate → adhesion and fusion of the neural folds
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Neural tube closure in a human embryo
Neural plate closure in mammals is initiated at several points along the body axis
Fig. 12.5A-C
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Abnormalities of neural tube closure (dysraphisms)
Failure of neural tube closure in different regions gives rise to different symptoms
Most common is spina bifida, the mildest form (spina bifida occulta) affecting ~10% of the population
Fig. 12.5D
Copp et al Nature Reviews Genetics (2003) 4:784-793
Craniorachischisis
Exencephaly and
open spina bifida
Anencephaly
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Contraction of the apical F-actin cytoskeleton contributes to neural tube closure
Zolessi & Arruti (2001) BMC Developmental Biology 1:7 11
Changes in the expression of specific cell adhesion
molecules are central to proper neural tube closure
Fig. 12.6
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Folate plays a major role in neural tube closure
B
C
Fig. 12.7
Expression of folate binding protein in the closing neural tube of a mouse embryo
Other important factors in neural tube closure are the Pax3, sonic hedgehog, and
openbrain genes, as well as poorly understood dietary and environmental factors
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Primary and secondary neurulation
Primary neurulation, the folding and closure of the neural tube as described above, creates the
brain and most of the spinal cord.
In mammals, neurulation caudal to the future upper sacral level occurs by secondary neurulation.
In the tail bud, a stem-cell population that is the last bit of the retreating primitive streak,
mesenchymal cells undergo condensation and epithelialization to form a tube, the lumen of which is
continuous with that of the primary neural tube.
Fig. 12.8
Secondary neurulation
in a 25 somite chick
embryo
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Fig. 12.2: Neural tube formation in the chick
Neurulation occurs in the direction from rostral to caudal (Hox gene gradient)
Neural tube closure is initiated at midbrain level, from where it closes in both directions
=about 33 h!
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Fig. 12.2: Neural tube formation in the chick – overview at 24 h
Neurulation occurs in the direction from rostral to caudal
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Fig. 12.2: Neural tube formation in the chick - overview
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Differentiation of the neural tube
I. Anterior-posterior: 3 primary vesicles → 5 secondary vesicles
Boron & Boulpaep 2003 Fig. 10-6
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Formation of the brain ventricles requires inflation of the
neural tube lumen due to the activity of the Na+,K+ ATPase
2 K+
3 Na+
H2O
2 K+
3 Na+
H2O
2 K+
3 Na+
H2O
2 K+
Lowery & Sive, 2005; and Fig. 12.11
3 Na+
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Formation of the brain ventricles requires inflation of the
neural tube lumen due to the activity of the Na+,K+ ATPase
How do we know?
The Snakehead mutant lacks functional Na+, K+
ATPases, and cannot inflate the brain ventricles!
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Differentiation of the neural tube
I. Anterior-posterior
side view of developing and adult brain and spinal cord
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Differentiation of the neural tube
II. Dorsal-ventral
Sonic hedgehog (shh) from the notochord induces the medial hinge point cells to become
the floor plate, and TNF-β family proteins from the dorsal ectoderm induces the
dorsalmost part of the neural tube to become the roof plate.
The combination of ventral-to-dorsal shh gradient (ventralizing signal) and dorsal-toventral TNF-β family gradient (dorsalizing signal) induces the development of different
types of neurons along the dorso-ventral axis (i.e. specification of neuronal fate depends
on cell position relative to the floor plate).
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Differentiation of the neural tube
III. At the cell level – the germinal neuroepithelium
The region closest to the lumen of the neural tube is the germinal neuroepithelium, a 1 cell
layer thick neuronal stem cell region. Nuclei move up and down during cell cycle, with
those of dividing cells closest to the lumen. At division (neuronal birthday, after which it is
in Go), one daughter cell looses adhesion and migrates away.
Mouse
embryo, e9
~human
28 days
Lumen
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Differentiation of the neural tube
III. At the cell level – neuronal differentiation
The migrated neuronal progenitor cells and glial cells derived from them create the
mantle- or intermediate zone (gray matter), and myelinated neuronal axons create the
marginal zone (white matter). The germinal neuroepithelium becomes the ventricular
zone.
The cells in the ventricular zone are largely pluripotent – their fate is specified close to or
after the last mitotic division.
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Neuronal growth and differentiation along the neural tube
Spinal cord
In the spinal cord, the three-zone pattern is maintained throughout development
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Interneurons
(alar plate)
Dorsal root ganglion
sensory neurons
(from neural crest cells)
Development of the spinal cord
Motor neurons
(from basal plate)
Mouse embryo e11
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Neuronal growth and differentiation along the neural tube
Cerebellum
A second germinal zone, the external granule cell layer, is formed at the outside border
of the neural tube by migrating neuronal precursors. These form the granule neurons,
that migrate back and form an internal granule layer. The ventricular neuronal stem cells
form the Purkinje cells.
Bergman glia
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Cerebellar organization as visualized by confocal microscopy
of fluorescently labeled slices of rat cerebellum
Fig. 12.18
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Neuronal growth and differentiation along the neural tube
Cerebrum
Neocortex is created by neuronal precursors migrating out from the mantle zone
The neocortex stratifies into 6 functionally distinct layers
The main type of neurons in neocortex is pyramidal neurons, and each layer contains
different proportions of these and various non-pyramidal neurons
Cortical layer 6
5 4
3 21
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Neocortical neurons migrate both radially and tangentially
Nadarajah & Parnavelas 2002 Nat Rev Neurosci
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Pathologies of neuronal migration
”Inside-out” gradient of development: in all of the brain, neurons with the earliest
birthdays populate the inner, and those with later birthdays the outer, layers.
In Reeler and Scrambler mice, the gradient is reversed due to neuronal migration
defects, and the mice exhibit tremors, dystonia, and ataxia
In Lissencephaly, the normal gradient is maintained, but neuronal migration is slowed
and cortical development incomplete, due to a chromosome 17 deletion
Normal brain, MRI
Lissencephaly brain, MRI
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Neuronal migration,
axonal outgrowth,
and guidance cues
”The axon growth cone can in many ways be
thought of as a neural crest cell on a leash”
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General features of some important multipolar neurons
myelination
Growth cone
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Neuronal migration and glial guidance
In many regions of the CNS, neurons migrate on radial glial cell processes.
Ex. Bergman glia in the cerebellum. Weaver mouse: inability of granule neurons to migrate
on Bergman glia → defective neuronal migration in cerebellum.
Not all neurons use radial glia to travel on– but they all need guidance cues to reach the
right location.
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Axonal outgrowth: the axon growth cone
Microspikes/filopodia
Fig. 12.24
Red: microtubuli
Green: F-actin
Huot, J. Prog. Neuro-Psychopharm. & Biol.
Psych. 28 (2004) 813–8
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Neuronal outgrowth: the axon growth cone
The actin microspikes mediate neuronal pathfinding
The microtubules mediate axon elongation
Guidance cues direct the growth cone by modulation of
actin polymerization and organization
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
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Forces working on the axon growth cone
Laminin/integrin
Netrins/DCCs + UNCs
EphA7/ephrinA5
semaphorin 1/plexin
Modified from
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
NB: this is a simplified summary of some important players – whether a
given signal is in fact repulsive or attractive is often cell-type specific!
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Forces working on the axon growth cone
Dorsal spinal cord explant
Floor plate explant
Netrins are secreted proteins, which
interact with UNC or DCC receptors,
and which are most frequently
chemoattractants for axon growth cones
Ex.: netrin-1 and netrin-2 direct
commisural neurons to the ventral
midline
Control COS cell
Dorsal spinal cord explant
Netrin-1 COS cell
Netrin-2 COS cell
Fig. 13.22 & 13.23
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Forces working on the axon growth cone
Netrins can also act as chemorepellants
The outcome depends on the netrin
receptors present on the axon
Ex.: trochlear nerve outgrowth is inhibited
by netrin-1
Dorsal spinal
cord explant
Control
COS cells
Netrin-1
COS cells
Floor plate cells
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Forces working on the axon growth cone
Laminin/integrin
Netrins/DCCs + UNCs
EphA7/ephrinA5
semaphorin 1/plexin
Modified from
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
Eph/ephrin
semaphorin 3,5/plexin
Slit/Robo
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Forces working on the axon growth cone
Ephrins are membrane proteins which interact with Eph receptors on the growth cones,
and generally act as repulsive guidance cues
Ephrins are expressed in the
posterior part of the schlerotome
Eph receptors are expressed in
the motorneurons
Eph/ephrin interactions inhibit
motor neuron migration in the
developing neural tube
Rostral/
anterior
Caudal/
posterior
Ephrin expression
Motor neurons
Wang & Anderson Neuron (1997) 18(3):383-96
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Where are we? The somites and their derivatives
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Forces working on the axon growth cone
Ephrins are membrane proteins which interact with Eph receptors on the growth cones,
and generally act as repulsive guidance cues
Ephrins are expressed in the
posterior part of the schlerotome
Eph receptors are expressed in
the motorneurons
Eph/ephrin interactions inhibit
motor neuron migration in the
developing neural tube
Rostral/
anterior
Caudal/
posterior
Ephrin expression
Motor neurons
Wang & Anderson Neuron (1997) 18(3):383-96
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Forces working on the axon growth cone
Laminin/integrin
Netrins/DCCs + UNCs
EphA7/ephrinA5
semaphorin 1/plexin
Modified from
Hubert et al (2003) Ann
Rev Neurosci 26:509-63
Eph/ephrin
Neurotrophins:
BDNF, NGF,
NT-3, NT-4
semaphorin 3,5/plexin
Slit/Robo
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Forces working on the axon growth cone
Neurotrophins promote survival of specific neuronal and glial populations by
locally counteracting the apoptotic cell death that would occur in their absence.
Survival depends on competition for a limited supply of neurotrophins.
Neurotrophins also act as chemoattractants
Ex.: chemoattraction:
Dorsal root ganglion
neuron turning in
response to NT-3
Fig. 13.27
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Forces working on the axon growth cone
The specific effect of a given neurotrophic factor on axonal
outgrowth differs between different types of neurons
Fig. 13.29
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Neurotrophins and neuronal survival
Parkinsons disease (PD) is a
neurodegenerative disease
associated with loss of
midbrain (substantia nigra)
dopaminergic neurons. GDNF
enhances survival of these
neurons.
Astrocytes transfected to
produce GDNF and injected
into the substantia nigra
rescue dopaminergic neurons
(stained in black in the fig.) in a
mouse model (OHDA lesion)
of PD.
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Cunningham & Su (2002) Exp. Neurol.174, 230–242
Take-home points:
Stages of neurogenesis, and mechanisms involved in each step
Induction of a neuron-forming region (requires competence)
Birth and start of migration of neurons and glial cells
Specification of, and commitment to, neuronal fate
Guidance of axon growth cones to specific targets
Binding of trophic factors for survival and differentiation
Formation of synaptic connections
Competitive rearrangement of functional synapses
Continued synaptic plasticity
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A few extra slides that you might find useful
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Eph/ephrin signaling in development
Two major classes of ephrins:
ephrin A, which are GPI-anchored,
and ephrin B, which are
transmembrane.
Ephrins work via receptor tyrosine
kinases, the eph receptors, to
modulate growth cone F-actin
organisation and polymerization.
Ephrin signaling can be both
forward, in the direction of the eph
receptor, and reverse, in the
direction of the ephrin ligand.
Huot, J. Prog. Neuro-Psychopharm.
& Biol. Psych. 28 (2004) 813– 818
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Neurotrophins and
neuronal survival
Neurotrophins promote
survival of specific
neuronal and glial
populations by locally
counteracting apoptotic
cell death.
Survival depends on
competition for a limited
supply of neurotrophins.
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Vecino et al (2004) Int. J. Dev. Biol. 48: 965-974
Neuronal growth and differentiation along the neural tube
Time course of development of mouse cerebrum
Mouse
Human (approx.)
3½-4 weeks
5 weeks
8 - weeks
Adult
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