Transcript 2PatternRegion

Neuronal Patterning and
Regionalization
Steps during neural development:
• Neurogenesis
• Compartmentalization
• Neural differentiation
• Neural migration
• Axonal guidance
• Synaptogenesis
Neural development in vertebrate embryo: Gastrulation
Blastula stage through neurulae, highlighting gastrulation and neurulation.
Blastula stage embryo with 3 germ
layers, first signs of invagination of
dorsal blastopore lip
Embryo in midgastrulation,
involution of dorsal mesoderm
(organizer tissue).
Gastrula stage embryo:
Embryo at end of gastrulation. The 3
germ layers have arrived at their
final destination
Organizing Centers:
Restricted specialized areas that are crucial for the
induction of area specification
•
•
•
•
Spemann’s organizer (dorsoblastopore lip)
Hensen’s node (similar to Spemann’s org)
Roofplate and notochord become organizers
secondary organizers:
– Isthmic organizer (IsO)
– Anterior neural ridge (ANR)
– Cortical hem
Organizer Transplant
experiment
A region just above the blastopore lip (mesodermal tissue) is
excised & transplanted to ventral side of host.
The host embryo develops a secondary dorsal
axis, first evident by a secondary neural plate.
A section through a host embryo with two dorsal axes:
Secondary dorsal axis contains the same tissues as the
primary dorsal axis, including a nervous system.
Note: neural tissue was derived from recipient cells,
not donor cells. Thus, the transplant had altered the
fate of the overlying cells
Default model of neural induction.
Balance between agonists and antagonists!
Importance of inhibition as a developmental regulatory
mechanism.
Expression of signaling factors:
Bone morphogenic protein (BMP), a
TGF-β-like PGF expressed in
ectoderm on ventral side, inducing
ectoderm to become epidermis.
Organizer on the dorsal side releases
inhibitors of the BMPs: noggin,
chordin, and follistatin, which
diffuse into the ectoderm on the
dorsal side, block the effects of
BMPs, and allow neural tissue to
form.
Signaling pathway involving BMPs
Signaling pathway involving BMPs
• Large family of polypeptide growth factors (PGF) related to
transforming growth factor-β (TGF-β): BMP, activin, and GDF group
members.
• Heterodimer receptors, with type I & type II subunits, cytoplasmic
domains with serine/theronine kinase activity.
• Dimerization after binding of a TGF-β-like PGF starts signal
transduction pathway: Activation of cytoplasmic proteins (SMADs),
which translocate to nucleus to activate expression of downstream
target genes.
• Inhibitory mechanisms regulate signaling:
– Extracellular proteins such as chordin, tolloid, and twisted gastrulation
interact with the BMP-like ligands, regulating their diffusion through the
extracellular milieu and their ability to bind receptor
– Cell surface proteins such as BAMBI inhibit signaling by binding up BMPs but
failing to transduce a signal.
– Inhibitory SMADs poison the signal transduction pathway.
Neurulation
The neural plate forms after
gastrulation is completed.
The neural tube narrows
along its medial-lateral
Axis. The plate begins to
role into a tube. The
cells at the midline produce
a medial hinge point
(MHP).
As the tube forms and
segregates into the embryo,
neural crest cells emigrate
from the dorsal aspect of the
neural tube.
G.C. Schoenwolf
Steps during neural development:
• Neurogenesis
• Compartmentalization
• Neural differentiation
• Neural migration
• Axonal guidance
• Synaptogenesis
Pattern Formation
In the early stages of pattern formation, two
perpendicular axes are established
-Anterior/posterior (A/P, head-to-tail) axis
-Dorsal/ventral (D/V, back-to-front) axis
Polarity refers to the acquisition of axial
differences in developing structures
Position information leads to changes in gene
activity, and thus cells adopt a fate
appropriate for their location
11
AP polarity of vertebrate CNS
• Head organizer
becomes
precordal
mesoderm
(PME)
underneath
prechordal plate
• Tail organizer
becomes
notochord and
somites,
underneath
epichordal
neural plate
Early Neural Patterning:
Establishment of AP Axis
• Head and tail organizer release factors which
create a gradient.
Neural Patterning
• A/P polarity and other key organizational features are 1st
established by gradients of positional information of a
gradient of a substance or signal.
• How can a gradient confer positional information?
…can signal relative concentrations correlated with
distance.
A. Evidence of positional info in a single 2-dimensional
system; e.g., the striking stripped patterns on insect wings
(butterflies, moths).
• A chemical signal from eyespot center determine the
pigment elaborated by surrounding cells as a function of
concentration.
• Excision of eyespot center  absence of eyespot
pigmentation.
• Transplantation of an eyespot center to an ectosyne locus
stimulates the development of an eyespot in the
surrounding tissue.
B. In the 3-dimensional system of the embryo, the
initial establishment of A/P polarity is signalled
by the organizer (dorsal lip of the blastopore in
amphibians; Hensen’s node in birds).
• During gastrulation, the organizer tissues come
to underlie the neural plate and differentiate
into the notochord.
The chordal mesoderm, which underlies the future
midbrain, hindbrain, and spinal cord, apparently
sends out distance signals from prechordal
mesoderm.
[These have been reversed in transplantation
experiments to demonstrate this].
The candidate neural inducers, which have been
studied (chordin, noggin, and follistatin)
induce primitive neural tissue that appears to
be forebrain-like; chordin particularly potent.
Recall that these 3 proteins antagonize members
of TGF-β signalling family of molecules. This
suggests that induction of anterior neural
plate differentiation involves inhibitors of TGFβ-like signals that repress neural development.
This would be a “ground state”, which would be
induced to be more posterior by a 2nd signal:
a transforming signal.
In this case, a type of gradient, a ratio between
activating (noggin) and transforming signals
would determine the A/P polarity along the
neuraxis.
Possible candidate “posteriorizers”
(transforming signals) include bFGF and
retinoic acid.
Tail organizer:
Head
organizer:
BMP
Inhibitors
Cordin and
Noggin,
Wnt
inhibitors
Cerberus,
Dickkopf and
frzb1 to
"anteriorize"
neural tube
FGF, WNT, RA &BMP
inhibitors
are posteriorizing
signaling molecules
Regionalization of the Nervous System
I. Segmentation (see below)
II. Developmental control genes (e.g., Hox), which encode
positional values along A/P axis.
Positional signaling mechanism, which activates these
genes may be a (more complex) version of a simple
earlier model (gradients?):
At Henson’s Node, a strong candidate for this signal is a
gradient of retinoic acid, which regulates the pattern of
Hox gene expression.
Different Hox genes at specific locations respond more or
less readily to lower or higher [RA]s, through a family of
receptors, which, bound by RA, become transcription
factors.
I. Regarding Segmentation:
Subdivision of the main body axis by segmentation
is a developmental theme found in many animal
phyla.
This provides compartments, which allocate
precursor cells into a repeated set of similar
molecules, so that developmental fields can
remain small, and specialization of cell types
and patterns can be generated as local
variations on the repetitive theme.
Mesoderm = segmented into somites, yielding
muscle groups.
The neuraxis is also segmented ---------------
Rhombomeres – the clearest subdivision
partition the hindbrain neuroepithelium.
See next slides (Fig. 2.6) for these 8 segments.
In the CNS, segmentation is a mechanism for
specifying pattern during development.
The earliest neurons and neural pathways are
laid out in stripes, which match a
morphological repeat pattern ( a “2-segment
repeat” pattern, which has similar patterns of
development in even- or odd-numbered
segments).
Cf., cerebellum
How do the cells become segregated?
a. Mechanical boundaries (certain extracellular
matrix pattern, such as chondroitin SO4
appear at the boundaries during
development (however, only important
during later devel.).
b. Differential adhesion between cells (reaggregation experiments show that this does
indeed occur through a 2-segment repeat
rule (evens evens; odds  odds), so that
adjacent rhombomeres remain separate.
Pattern Generation does not Involve only the
Migration of Cells themselves, but also the
Axons of Cells
• How does a neuronal axon “know” how to
travel to a given area and make specific
connections?
• Appears to involve three steps:
– pathway selection
– target selection
– address selection
Pattern Generation
• What role
does the
substrate
play in
directing
the
pathway of
axons?
II. Developmental Control Genes.
These genes, which encode txn factors, or signaling
molecules, are expressed in a spatially variable
manner.
These classes of genes and their segmented pattern
of expression in the rhomomeres are shown in
Fig. 2.5
The Hox genes (homeobox family) have a clustered
chromosomal organization.
[The relative position of the gene reflects the
expression along the A/P axis].
This expression of the Hox gene confers positional
value and regional identity.
Pattern Generation
• Are there other
molecules known to
direct the migration
of axons?
Pattern Generation
• What is the function of
neurotrophins?
Pattern Generation
• What is involved in the formation of a synapse?
Pattern Generation
• What is the role of
neurotropic factors in
the survival of neurons?
Pattern Generation
• What is known about
the migration of retinal
ganglion axons?
Pattern Generation
• How do axons
distinguish between
different regions of
optic tectum?
What is the signaling mechanism for expression
of these genes?
As noted earlier, this is a gradient of RA.
The RA signal regulates the pattern of Hox
expression.
There is a direct correspondence between the
location of the Hox gene in its cluster and its
responsiveness to RA.
5’
Genes respond
less rapidly; require
higher [RA]s
Posterior CNS
Genes respond
more rapidly at
lower [RA]s
3’
Anterior CNS
Change in Hox gene expression 
change in morphology along the A/P axis
Patterning of the brain and spinal cord through
compartmentalization:
Melton, Iulianella, Trainor, 2004
Regional patterning: Forebrain (FB), Midbrain (MB), Hindbrain (HB) and Spinal cord (SC).
Graded Wnt signaling functions along the entire length of the neuraxis inducing
progressively more posterior neural fates.
Hox genes play important roles in establishing regional cell identity. This is achieved via
opposing gradients of RA and FGF signaling.
Hox gene expression domains in the CNS
Nested domains of homeotic genes along the AP axis of the Drosophila and mouse CNS. Hox
genes specify a positional value along the AP axis, which is interpreted differently in fly and
mouse in terms of downstream gene activation, resulting in neural structure; after Hirth et
al., (1998).
Compartmental organization of hindbrain into
rhombomeres
Stage 8-9: Genes are
expressed in alternate
stripes that correspond
with presumptive
Rhombomeres.
Stage 9-10: Restriction
of movement of mitotic
Precursor cells across
interfaces.
Stage 13: The interfaces
between
Rhombomeres acquire
molecular and
Morphological
specialization marked
by distinct boundaries.
Julie E. Cooke, Cecilia B. Moens, 2002
Example of odd/even
gene expression in
Drosophila
in situ localization of the
achaete transcript
From Skeath et al, 1992.
Stages in the compartmental organization of
rhombomeres.
Genes such as Krox20 and EphA4
(blue) and ephrin-B2 (pink) are
expressed in alternate, fuzzyedged stripes (left). Subsequently,
restriction to the movement of
mitotic precursor cells occurs at
the interfaces between newly
formed rhombomeres, which are
now sharply defined, and marked
by increased intercellular spaces.
(right)
Sharpening of boundaries and cell
lineage restriction occur through
the interaction of Eph and ephrin
molecules. Data from Fraser et al.
(1990).
Regional specification in the developing
brain
Three-vesicle state of a chick embryo
Five-vesicle state
Dorsal Ventral pattern: Notochord as organizer
Left: During development, the floor plate (red) develops above the
mesodermal notochord (n) and motor neurons (yellow) differentiate in
adjacent ventrolateral region of the neural tube.
Center: Grafting a donor notochord (n') alongside the folding neural plate
results in formation of an additional floor plate and a third column of motor
neurons.
Right: Removing the notochord from beneath the neural plate results in the
permanent absence of both floor plate and motor neurons in the region of the
extirpation. Pax6 expression (blue) extends through the ventral region of the
cord.
Sonic-hedge-hog expression by notochord & floor
plate, control of ventral patterns
Shh activity in the ventral neural
tube (blue dots) is distributed in a
ventral-high, dorsal-low profile
within the ventral neural
epithelium.
5 classes of neurons are generated
in response to graded Shh signalling
T.M. Jessell, 2000
Model for ventral neural patterning by SHH.
Left: Graded SHH signaling from the ventral pole induces expression of some homeobox genes (e.g.,
Nkx2.2, Nkx6.1) and represses existing expression of others (e.g. Pax6, Dbx2).
Center: Cross-repressive interactions between pairs of transcription factors sharpen mutually exclusive
expression domains.
Right: Profiles of homeobox gene expression define progenitor zones and control neuronal fate. After
Briscoe and Ericson, (2001).
Regulation of DV pattern in the telencephalon by SHH.
Cross section of mouse
telencephalon at early (left)
and later (right) stage.
SHH produced in the ventral
midline region controls
development of basal ganglia
primordia and medial and
lateral ganglionic eminences
(MGE, LGE).
First, ventral SHH induces
MGE gene expression; SHH
(partly produced by the
MGE) induces LGE gene
expression later.
The neural tube, shown here for a mouse, is subdivided into four longitudinal
domains: the floor plate, basal plate, alar plate, and roof plate. Motor neurons
are derived from the basal plate.