Developmental biology: imaginal discs

1. 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. Development is a timed process; once a situation is established. Next one is set up… 5

2. Each part of the adult fly is derived from an imaginal disc. 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? Imaginal discs metamorphose into adult structures in pupal stage Each part of body grows and develops independently 6

3. 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. Developmental biology: imaginal discs Where do discs come from? 7

4. Fate map Developmental biology: imaginal discs 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…. 8

5. wingless and hedgehog/engrailed and intersection with: in dorsal ventral axis: dpp dpp expresssion P A wingless expression A P distalless expression decapentaplegic expression dorsal-ventral restricted stripe Developmental biology: imaginal discs Step 1: what gene activities “control” where discs are going to form? 9

6. 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? 10

7. 7

8. Polar co-ordinate model: - repair by intercalation Developmental biology: imaginal discs * mathematical models for patterning of complex structures (often based on regenerating model-systems) - not-Cartesian (non-flat) - beginning and end specified - local cell interactions Proposed by French and Bryant. 11

9. Truth? Is out there! Developmental biology: imaginal discs Boundaries model (Meinhardt): - embryonic patterning - at least three “compartment” - intersection of these determines reference point - morphogenetic activities pattern 12

10. hedgehog engrailed wingless dpp “mature” disc 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). Expression of these genes is maintained and drives patterning posterior ventral segment 13

11. Patterning in anterior-posterior axis: Hedgehog, Wingless, decapentaplegic: secreted and act as morphogen molecules. “mature” leg disc Their concentration drives cells into particular differentiation pathways. Expression of dpp and wingless is maintained by hedgehog signalling Developmental biology: imaginal discs Stable boundary from embryos (compartment boundary): forms the source of morphogenetic molecules. 14

12. Hedgehog patched signalling downstream Expression of Ci ptc mutant cells Developmental biology: imaginal discs 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…. 15

13. a b abnormal decreased Hh signalling Developmental biology: imaginal discs normal wing abnormal increased Hh signalling Loss or gain of Hh signalling leads to external defects; same for Dpp and Wg. 16

14. 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! Developmental biology: imaginal discs Only anterior-posterior axis determined. What about dorsal-ventral? 17

15. 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 18

16. Assay these cells for activation of pathway (increase of Ci transcription factor) Cells induced to express oncogenic Smoothened Developmental biology: imaginal discs Hedgehog and wingless and signalling pathways and medicine? All genes acting in pathway in segment, act here as well; onogenic smoothened leads to skin cancer 19

17. Compartments like in anterior-posterior patterning: fringe (with Notch, Delta and Serrate) anterior-posterior compartment boundary gene dorsal-ventral compartment boundary gene Developmental biology: imaginal discs Dorsal-ventral patterning (molecular): “mature” wing disc Activity of these genes creates signalling centre at D/V boundary/. 20

18. 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 Hedgehog “non” signalling: inhibits expression of hh: via repressor factor (Gli in vertebrates) Plus: Cells do NOT mix…. (mechanism not entirely understood) Developmental biology: imaginal discs More molecular mechanisms: (Active signalling) And cell autonomous inhibition… 21

19. “mature” disc Development of vertebrate limb: naïve group of cells Developmental biology: imaginal discs Is this a model for patterning processes of fields of cells in other organisms? A patterning activity present in posterior cells? Seems to work as morphogen in anterior-posterior axis? 22

20. Ectopic placement of Sonic hedgehog protein in anterior: Developmental biology: imaginal discs Search for morphogenetic activity in ZPA? Sonic hedgehog expressed in ZPA cells. Similar experiment in flies using human shh gene…. 23

21. Effect of mutation in transcription factor downstream of hh (Ci=Gli) (gene identified in flies) 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) 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? 24

22. 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) 25

23. A fly is a human? A limb is a limb?????? 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 26