Transcript imaginal discs

Developmental biology:
imaginal discs
Adult structures?
hedgehog/engrailed
time
wingless
* after several hours of interdependence of wingless and hedgehog:
situation changes.
Development is a timed process; once a situation is established. Next
one is set up…
* engrailed expression is permanent in posterior compartment cells
February 06
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Developmental biology:
imaginal discs
What does this mean?
A-P compartment division is now stable (and with it engrailed expression)
Where do adult fly structures come from?
Each part of the adult fly is derived
from an imaginal disc.
Imaginal discs metamorphose into
adult structures in pupal stage
Each part of body grows and
develops independently
February 06
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Developmental biology:
imaginal discs
Where do discs come from?
Each disc is derived from
set of ectodermal cells in
early embryo.
During metamorphosis
The flat sheets of cells, shoot out to from
3-D structure.
February 06
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Developmental biology:
imaginal discs
In pupae, discs turn into structures (by extension mainly),
cells have a “molecular definition”.
Fate map
* Can we use development of fly wings/legs as a model for
“making” an extremity (understand the underlying genes)?
* Study molecular basis of disc patterning….
February 06
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Developmental biology:
imaginal discs
Step 1: what gene activities “control” where discs are going to form?
wingless and hedgehog/engrailed and intersection with:
in dorsal ventral axis: dpp
dpp expresssion
A
wingless expression
PP
decapentaplegic expression
dorsal-ventral restricted stripe
distalless
expression
February 06
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Developmental biology:
imaginal discs
In red: expression of distalless
gene: trancription factor;
distalless mutant misses extremities!
Imaginal discs provide
two (and three-dimensional?)
models for patterning
of complex structures?
February 06
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February 06
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Developmental biology:
imaginal discs
* mathematical models for patterning of complex structures
(often based on regenerating model-systems)
Polar co-ordinate model:
- repair by intercalation
- not-Cartesian (non-flat)
- beginning and end specified
- local cell interactions
Proposed by French and Bryant.
February 06
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Developmental biology:
imaginal discs
Boundaries model (Meinhardt):
- embryonic patterning
- at least three “compartment”
- intersection of these determines
reference point
- morphogenetic activities pattern
Truth?
February 06
Is out there!
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Developmental biology:
imaginal discs
Reality?
Step 2: molecules that set up the patterning within disc
dorsal
Disc derived from intersection of
wingless/hedgehog and dpp
expression (these activate
distalless).
dpp
posterior
ventral
February 06
segment
Expression of these genes is
maintained and drives patterning
“mature”
disc
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Developmental biology:
imaginal discs
Expression of dpp and wingless is maintained by
hedgehog signalling
Stable boundary from embryos (compartment
boundary): forms the source of morphogenetic
molecules.
Patterning in anterior-posterior axis:
Hedgehog, Wingless, decapentaplegic: secreted and act
as morphogen molecules.
“mature”
leg disc
February 06
Their concentration drives cells into particular
differentiation pathways.
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Developmental biology:
imaginal discs
Hedgehog
Expression of Ci
patched
signalling downstream
ptc mutant cells
Loss of Hh pathway components leads to molecular defects;
as in embryos. Now targets are not embryonic epithelial cells
forming cuticle but cells forming bits of leg/wing etc….
February 06
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Developmental biology:
imaginal discs
a
normal wing
b
abnormal increased Hh
signalling
Loss or gain of Hh signalling
leads to external defects;
same for Dpp and Wg.
abnormal decreased Hh
signalling
February 06
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Developmental biology:
imaginal discs
Only anterior-posterior axis determined.
What about dorsal-ventral?
Intersection of dpp/wingless:
“mature”
leg disc
In fact, dorsal ventral boundary was already
there in embryos (cells see either green or
pink or combinations of these).
In a body part sticking out: proximal-distal axis…?
Intersection between dorsal-ventral
and anterior-posterior defines proximal-distal axis!
High point is most distal!
February 06
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Developmental biology:
imaginal discs
Mathematical models and reality?
- both models approach
reality
- non-cartesian model
evokes compartments
- morphogens proposed
by Meinhardt.
- both say distal point
is high point and
circular rings of cell
determination descending
February 06
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Developmental biology:
imaginal discs
Hedgehog and wingless and signalling pathways and medicine?
Cells induced to
express oncogenic
Smoothened
February 06
Assay these cells for
activation of pathway
(increase of Ci transcription factor)
All genes acting in pathway in segment, act here as well;
onogenic smoothened leads to skin cancer
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Developmental biology:
imaginal discs
Dorsal-ventral patterning (molecular):
Compartments like in anterior-posterior
patterning: fringe (with Notch, Delta and
Serrate)
“mature”
wing disc
Activity of these genes
creates signalling
centre at D/V
boundary/.
anterior-posterior
compartment boundary gene
February 06
dorsal-ventral
compartment boundary gene
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Developmental biology:
imaginal discs
More molecular mechanisms:
Intersection of dpp/wingless and hedgehog:
all signalling molecules at highest levels.
How do you keep the “high” point in correct place?
“mature”
leg disc
(Active signalling) And cell autonomous inhibition…
Hedgehog “non” signalling: inhibits expression of hh:
via repressor factor (Gli in vertebrates)
Plus:
Cells do NOT mix….
(mechanism not entirely understood)
February 06
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Developmental biology:
imaginal discs
Is this a model for patterning processes of
fields of cells in other organisms?
“mature”
disc
Development of vertebrate
limb:
naïve group of cells
A patterning activity present
in posterior cells?
Seems to work as morphogen
in anterior-posterior axis?
February 06
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Developmental biology:
imaginal discs
Search for morphogenetic activity in ZPA?
Sonic hedgehog expressed in
ZPA cells.
Ectopic placement of Sonic
hedgehog protein in anterior:
Similar experiment in flies
using human shh gene….
February 06
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Developmental biology:
imaginal discs
* sonic hedgehog gene is expressed in group of posterior cells
* sonic hedgehog activity drives anterior-posterior patterning in
vertebrate limb (and in fly limb)
Effect of mutation in transcription
factor downstream of hh (Ci=Gli)
(gene identified in flies)
Hh/Shh and signalling pathway molecules (segment polarity genes) drive
anterior-posterior patterning in both fly as well as vertebrate limb
* coincidence that it is the same genes?
February 06
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Developmental biology:
imaginal discs
Sonic hedgehog expression in ZPA is responsible for anterior-posterior
patterning in vertebrate limb.
Many gene products (targets as well as interacting) connected to
signalling in fly limb: conserved……...
Hedgehog (sonic hedgehog) /wingless (Wnt) and dpp (BMP) signalling
cooperate to set up 3-D paterning system (many genes originally found
and characterized in flies)
February 06
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Developmental biology:
imaginal discs
Patterning of the simple discs in flies:
- driven by patterning genes: secreted signalling molecules
- 3-dimensional structure building achieved by “simple”
interaction between these molecules: high point of activity (generating
most distal point) is high point of concentrations…..
-wingless, hedgehog, dpp in flies: Wnt,
hedgehog and BMPs in vertebrates
A fly is a human? A limb is a limb??????
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