Developmental biology: imaginal discs

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February 06 Developmental biology: imaginal discs 5 wingless hedgehog/engrailed after several hours of interdependence of wingless and hedgeho tuation changes. Development is a timed process; once a situation is established. Next one is set up… engrailed expression is permanent in posterior compartment cel Adult structures? time

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hedgehog / engrailed. time. wingless. * engrailed expression is permanent in posterior compartment cells. Developmental biology: imaginal discs. Adult structures?. * after several hours of interdependence of wingless and hedgehog : situation changes. - PowerPoint PPT Presentation

Transcript of Developmental biology: imaginal discs

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wingless

hedgehog/engrailed

* after several hours of interdependence of wingless and hedgehog:situation changes.Development is a timed process; once a situation is established. Nextone is set up…

* engrailed expression is permanent in posterior compartment cells

Adult structures?

time

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What does this mean?

A-P compartment division is now stable (and with it engrailed expression)

Where do adult fly structures come from?

Imaginal discs metamorphose intoadult structures in pupal stage

Each part of body grows and develops independently

Each part of the adult fly is derivedfrom an imaginal disc.

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Where do discs come from?

Each disc is derived from set of ectodermal cells inearly embryo.

During metamorphosis The flat sheets of cells, shoot out to from3-D structure.

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In pupae, discs turn into structures (by extension mainly), cells have a “molecular definition”.

* 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….

Fate map

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wingless and hedgehog/engrailed and intersection with:

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Step 1: what gene activities “control” where discs are going to form?

wingless expression

A PA P

decapentaplegic expressiondorsal-ventral restricted stripe

dpp expresssion

in dorsal ventral axis: dpp

distallessexpression

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In red: expression of distallessgene: trancription factor;distalless mutant misses extremities!

Imaginal discs provide two (and three-dimensional?) models for patterning of complex structures?

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* 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.

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Boundaries model (Meinhardt):- embryonic patterning

Truth? Is out there!

- at least three “compartment”

- intersection of these determinesreference point

- morphogenetic activities pattern

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Reality?Step 2: molecules that set up the patterning within disc

segment

posterior

dorsal

ventral

hedge

hog

en

grail

ed

Disc derived from intersection ofwingless/hedgehog and dppexpression (these activatedistalless).

“mature”disc

Expression of these genes ismaintained and drives patterning

wingl

ess

dpp

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Expression of dpp and wingless is maintained byhedgehog signalling

Stable boundary from embryos (compartmentboundary): forms the source of morphogeneticmolecules.

“mature”leg disc

Patterning in anterior-posterior axis:

Hedgehog, Wingless, decapentaplegic: secreted and actas morphogen molecules.

Their concentration drives cells into particular differentiation pathways.

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ptc mutant cellsExpression of Ci

Loss of Hh pathway components leads to molecular defects;as in embryos. Now targets are not embryonic epithelial cellsforming cuticle but cells forming bits of leg/wing etc….

Hedgehog patched signalling downstream

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normal wing abnormal increased Hhsignalling

a b

abnormal decreased Hhsignalling

Loss or gain of Hh signallingleads to external defects;same for Dpp and Wg.

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“mature”leg disc

Only anterior-posterior axis determined.What about dorsal-ventral?

Intersection of dpp/wingless:

In a body part sticking out: proximal-distal axis…?Intersection between dorsal-ventraland anterior-posterior defines proximal-distal axis!

High point is most distal!

In fact, dorsal ventral boundary was alreadythere in embryos (cells see either green orpink or combinations of these).

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Mathematical models and reality?

- both models approachreality

- non-cartesian modelevokes compartments

- morphogens proposedby Meinhardt.

- both say distal pointis high point andcircular rings of celldetermination descending

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Hedgehog and wingless and signalling pathways and medicine?

Cells induced toexpress oncogenicSmoothened

Assay these cells foractivation 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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Dorsal-ventral patterning (molecular):

“mature”wing disc

Compartments like in anterior-posteriorpatterning: fringe (with Notch, Delta andSerrate)

anterior-posteriorcompartment boundary gene

dorsal-ventralcompartment boundary gene

Activity of these genescreates signallingcentre at D/V boundary/.

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“mature”leg disc

Intersection of dpp/wingless and hedgehog:all signalling molecules at highest levels.

How do you keep the “high” point in correct place?

Hedgehog “non” signalling: inhibits expression of hh:via repressor factor (Gli in vertebrates)

(Active signalling) And cell autonomous inhibition…

More molecular mechanisms:

Cells do NOT mix….(mechanism not entirely understood)

Plus:

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“mature”disc

Is this a model for patterning processes offields of cells in other organisms?

A patterning activity presentin posterior cells?

Seems to work as morphogenin anterior-posterior axis?

Development of vertebratelimb:naïve group of cells

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Search for morphogenetic activity in ZPA?

Sonic hedgehog expressed inZPA cells.

Similar experiment in fliesusing human shh gene….

Ectopic placement of Sonic hedgehog protein in anterior:

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Hh/Shh and signalling pathway molecules (segment polarity genes) drive anterior-posterior patterning in both fly as well as vertebrate limb

* sonic hedgehog gene is expressed in group of posterior cells

* sonic hedgehog activity drives anterior-posterior patterning invertebrate limb (and in fly limb)

* coincidence that it is the same genes?

Effect of mutation in transcriptionfactor downstream of hh (Ci=Gli)(gene identified in flies)

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Sonic hedgehog expression in ZPA is responsible for anterior-posteriorpatterning in vertebrate limb.

Hedgehog (sonic hedgehog) /wingless (Wnt) and dpp (BMP) signallingcooperate to set up 3-D paterning system (many genes originally foundand characterized in flies)

Many gene products (targets as well as interacting) connected to signalling in fly limb: conserved……...

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Patterning of the simple discs in flies:

- driven by patterning genes: secreted signalling molecules

-wingless, hedgehog, dpp in flies: Wnt,hedgehog and BMPs in vertebrates

- 3-dimensional structure building achieved by “simple” interaction between these molecules: high point of activity (generatingmost distal point) is high point of concentrations…..

A fly is a human? A limb is a limb??????