Developmental biology segment polarity genes and cell signalling

Developmental biology: segment polarity genes and cell signalling b i c o i d g a p g e n e s time * transcription factors expressed in large blocks * induce expression of downstream transcription factors in smaller blocks * generates segmental expression (14) of segment polarity genes. February 06 1

Developmental biology: segment polarity genes and cell signalling Non-cellular: syncitium Transcription factors are able to determine more than just the nuclei that produce them February 06 2

Developmental biology: segment polarity genes and cell signalling How do you relate the molecular expression of segment polarity genes in stripes in cells in every segment, to the final larval structure…. ? February 06 3

Developmental biology: segment polarity genes and cell signalling Each segment is affected in a wingless mutant: cells making “naked” cuticle are mis-patterned and are lost (death) or redirected to form denticles February 06 normal wingless mutant 4

Developmental biology: segment polarity genes and cell signalling * mutants: each segment polarity mutant shows identical defect repeated in every segment * conclusion: segment polarity proteins are required for pattern formation in each and every segment February 06 * different classes: role? 5

Developmental biology: segment polarity genes and cell signalling * detailed analysis of expression. * wingless expressed in one cell wide stripe February 06 6

Developmental biology: segment polarity genes and cell signalling expression of wingless gene * wingless gene encodes for a secreted signalling protein; Int-1 (now Wnt-1) gene cloned in mice as being involved in breast cancer February 06 * Secreted molecules being involved in patterning consistent with fact that embryo is now cellularised! 7

Developmental biology: segment polarity genes and cell signalling * how do segment polarity genes generate pattern? Targets of signalling? engrailed expression in normal embryo wingless engrailed February 06 engrailed expression in wingless mutant embryo * wingless acts on neighbouring cells to maintain engrailed expression. 8

Developmental biology: segment polarity genes and cell signalling * wingless acts on neighbouring cells to maintain engrailed expression. * hedgehog gene encodes for a secreted signalling protein. * wingless also acts to maintain expression of hedgehog gene in neighbouring cells * target of hedgehog signalling? wingless February 06 hedgehog 9

Developmental biology: segment polarity genes and cell signalling wingless expression in normal embryo wingless expression in hedgehog mutant embryo hedgehog wingless February 06 * two signalling molecules maintain each others expression: maintain a stable reference point in segment 10

Developmental biology: segment polarity genes and cell signalling hedgehog wingless ? How does the “boundary” get there…. Pair rule gene activity Anterior precise’sh (posterior not); Segment polarity interplay refines expression domains and sets up segment boundaries (later, see textbooks). February 06 11

Developmental biology: segment polarity genes and cell signalling Stability? Field of cells: virgin area where no/little prepatterning has taken place. What is the most efficient way of turning this into something useful for the animal? * simple (as little genes/protein as possible) * reliable * accident resistant February 06 12

Developmental biology: segment polarity genes and cell signalling growth Damage: ie loss of blue cell is devastating…. secreted signal: efficient!? Blue cell has to be fixed to stay blue February 06 Not all daughters can stay blue 13

Developmental biology: segment polarity genes and cell signalling * blue and green cell identity is actively kept growth * loss of one blue cell is corrected * regeneration…. February 06 14

Developmental biology: segment polarity genes and cell signalling * regeneration…. flatworms amphibians Molecules that drive regeneration important for stem cell technology February 06 15

Developmental biology: segment polarity genes and cell signalling * genetic studies to unravel segment patterning: hedgehog wingless identical phenotype: molecules work to common end Hedgehog protein X Smo wingless expression in smo-/February 06 16

Developmental biology: segment polarity genes and cell signalling * patched mutant phenotype is very different from hedgehog or wingless mutant phenotype hedgehog normal patched wingless * what does this mean “molecularly”? February 06 17

Developmental biology: segment polarity genes and cell signalling * overexpression of hedgehog using a transgene, leads to same phenotype as patched mutant patched normal patched mutant = hedgehog everywhere overexpression of hedgehog gene patched mutant February 06 18

Developmental biology: segment polarity genes and cell signalling patched-hedgehog double mutant patched mutant = hedgehog over-expression * patched hedgehog double mutant: looks like patched single mutant. hedgehog February 06 patched normal signalling downstream patched hedgehog mutant 19

Developmental biology: segment polarity genes and cell signalling patched-hedgehog double mutant * patched gene expression in all cells that do not express hedgehog gene. * patched required downstream of hedgehog: inhibits signalling hedgehog patched signalling downstream * patched gene encodes for twelve transmembrane protein. * patched protein can bind hedgehog protein February 06 20

Developmental biology: segment polarity genes and cell signalling hedgehog Smo Ptc Fu Su(fu) Cos 2 Ci Ci February 06 21

Developmental biology: segment polarity genes and cell signalling hedgehog protein processing: acyltransferase proteolytic activity cholesterol addition * patched protein contains sterol sensing domain February 06 26

Developmental biology: segment polarity genes and cell signalling February 06 http: //www. stanford. edu/~rnusse/wntwindow. html 22

Developmental biology: segment polarity genes and cell signalling * two secreted signals (hedgehog and wingless) that are required to pattern each segment. * other segment polarity genes encode for proteins that act either in hedgehog or wingless pathway. hedgehog wingless * fine patterning of segment: late roles for wingless and hedgehog. February 06 23

Developmental biology: segment polarity genes and cell signalling hedgehog wingless * patterning via signalling proteins is most common way of development (cellular mode of development). * signalling processes conserved? YES! * wingless is fly homologue of a vertebrate oncogene: Wnt-1 * patterning molecules are heading a differentiation/growth decision pathway! February 06 24

Developmental biology: segment polarity genes and cell signalling Roles for segment polarity proteins in other animals? -in development: midline (ventral Hedgehog; dorsal wnt) -In patterning of arms/legs -In cell differentiation (neurons, germline cells, “organs”) -In control of cell growth (skin, mammary gland) -Ageing? February 06 29