Download Development of the nervous system

Development of the nervous system - I
Raghav Rajan
Bio 334 – Neurobiology I
August 8th 2013
08th August 2013
Bio 334 - Neurobiology I - Development of nervous syste
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Early embryonic development
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Bio 334 - Neurobiology I - Development of nervous syste
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Development of
the nervous system
begins after
gastrulation
Nervous system
and skin develop
from ectoderm
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Stages of neural development
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Neural induction
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Birth and differentiation of neurons
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Polarity, segmentation of nervous system
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Migration of neurons to their final position
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Growth of axons and axon guidance to their post-synaptic
targets
Changes in synapses
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Model systems for studying development
C-elegans
Zebra fish
Drosophila
Chicken
Mouse
Xenopus
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Hydra (diploblast)– neurons develop from a
precursor in the epidermis – interstitial cell
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Molecular mechanisms
poorly understood
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C-elegans – development of nervous system
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~1000 cells
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302 neurons and 56 glial cells
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C-elegans
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Endoderm and
mesoderm
precursors migrate
inward – end of
gastrulation
Proliferation phase
Ventral indentation
– start of
morphogenesis – 4
molts
Neurons form and
migrate inward
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Drosophila – neurons generated from
ventrolateral part of embryo (ectoderm)
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Cellularisation results in
cellular blastoderm
Ventral furrow marks start
of gastrulation
Mesoderm invagination
Ventral midline – future site
of neurogenesis
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Drosophila
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Continuation of neurogenic
region into anterior region
gives rise to brain
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Neuroblasts delaminate
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Divide into GMC and Nb
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GMC gives rise to neurons
or glia
Larval nervous system
Morphogenesis – additional
neurogenesis
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Frog
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Blastopore – small invagination
– point of initiation of
gastrulation
First cells to invaginate occur
at the dorsal side of blastopore
– opposite to sperm entry point
Spemann's organiser
Involuting marginal zone – first
cells form head, last form tail
(mesoderm)
Neural plate
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Frog
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Neural plate folds over to form
neural tube
Gives rise to neurons and glia of
CNS
Neural crest – junction between
neural tube and ectoderm
Unique to vertebrates – gives rise
to PNS
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Zebra fish – small differences in gastrulation due
to differences in amount of yolk
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Epiboly
50% epiboly
marks start of
gastrulation
at future
dorsal margin
– shield
Mesoderm
delaminates,
moves inside
Neural plate
forms
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Chick – “yolky” egg
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Blastodisc
floating on yolk
No epiboly
Mesoderm
invagination in
this disc through
blastopore-like
primitive streak
Hensen's node –
posterior end of
primitive streak
– like dorsal lip
of blastopore
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Human – no yolk
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Inner cell mass of
blastula gives rise to
embryo
Primitive streak – line of
cells migrating into
blastocoel to form
mesoderm
Overlying ectoderm
induced to form neural
tube
Tube rolls up and forms
brain and spinal cord
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Involuting mesoderm “induces” overlying
ectoderm to form neural tissue
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Culture each part of embryo
separately to determine time
of fate determination
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Frog
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Hans Spemann
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Spemann and Mangold identify Spemann
organizer
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Dorsal lip of blastopore can
induce formation of second body
axis including second brain and
spinal cord
Host and donor cells contribute to
new axis
Dorsal lip – Spemann organizer –
not only neural inducer but also
organizes body axis
Who organizes Spemann
organizer?
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Spemann organizer organized by Nieuwkoop
center
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Nieuwkoop found that
mesoderm is induced at the
junction of animal cap
(ectoderm) and vegetal cap
(endoderm)
Dorsal-most part of vegetal
pole can induce Spemann
organizer
Possibly β-catenin
But, again, how is this setup –
sperm entry, cortical rotations,
gray crescent, etc...
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Induction of neural tissue – studying it with the
isolated animal cap assay
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Do candidate factors
induce neural tissue
directly or indirectly?
Assay looked for
increased expression of
neural genes without
increased expression of
mesoderm genes
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Noggin is a secreted molecule that can induce
neural tissue
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Ventralized and
hyperdorsalized embryos
cDNA from dorsal lip (Spemann
organiser) was divided into
smaller and smaller collections
to rescue UV treated embryos
Noggin mRNA expressed in
gastrulating embryos by cells
of dorsal lip of the blastopore
Richard Hartland's group
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Chordin identified as another molecule secreted
by organizer
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DeRobertis' group
Identify genes that were
expressed in the organizer
region
Chordin a secreted molecule
expressed when neural
induction occurs
Overexpressin results in
formation of second axis just
like noggin
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Neural fate is the default for endodermal cells
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Melton's lab
Identified follistatin as a potential
neural inducer
Known to inhibit activin, a TGF-B
family member
Was actually studying the role of
follistatin in mesoderm induction
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Truncated activin
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No mesoderm
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Noggin, chordin – not clear what they
were doing
Also lots of neural tissue despite the
absence of mesoderm!!
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More experiments to show that neural fate is
default for animal cap cells
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Dissociated animal cap cells
form neural tissue
Follistatin also expressed in
the organizer region at the
time of neural induction
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Vertebrate embryos are inverted insect embryos!
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sog (like chordin) inhibits dpp (like
BMP4) in Drosophila to induce
neural tissue – only on the ventral
side
Different neural inducers
antagonize BMP signaling
BMP4 inhibits neural tissue
formation in animal caps
(dissociated animal cells too) – even
with noggin, chordin, follistatin
Antisense BMP4 RNA – neural tissue
forms without addition of inducers
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Wnt and BMP signaling pathways – two that are
important for development
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All three neural inducers block BMP signalling in
a similar fashion
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Noggin and chordin bind to
BMP4 directly
Follistatin binds to related
BMP7 and activin
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Inhibiting BMP signaling – not enough in mammals
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Deletion of noggin and chordin affect head formation and
cerebral hemisphere formation
Some neural tissue does form
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Current understanding of neural induction
: Old problem, new findings, yet more questions
Stern CD Development (2008): Old problem, new findings, yet more questions
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Next step – formation of neuroblasts
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Drosophila, proneural clusters
achaete-scute
complex genes
expressed by clusters
of proneural cells
before delamination
One neuroblast
develops from each
cluster
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Achaete-scute complex genes are required for
formation of the one neuroblast
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Only one of the proneural
cells becomes a neuroblast in
a normal fly
Flies mutant for proneural
genes like achaete scute –
neuroblasts don't form
Flies mutant for notch or
delta – multiple neuroblasts
form
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These genes are basic-helix-loop-helix
transcription factors
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Delaminating neuroblast inhibits neighbouring
cells from becoming a neuroblast
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Ablation of a delaminating
neuroblast makes the
neighbouring ectodermal
cell become a neuroblast
Neuroblast inhibits
neighbouring cells from
becoming neuroblasts
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Notch signaling pathway is involved in lateral
inhibition
Fortini ME Developmental Cell (2009) The
Notch pathway: Core signaling and its
posttranslational regulation
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