Next lecture: Induction/Signaling

1. Next lecture: Induction/Signaling • Requirements of inducer and responder cells • Cascades of inductive events are involved in forming organs • Examples of the kinds of cell communication • Important signaling pathways in development • Notch, TGFb, SHH, Wnt, FGF

2. “Official glossary” from Wolpert • Induction: The process whereby one group of cells signals to another group of cells in the embryo and so affects their development • Competence: The ability of a tissue to respond to an inducing signal. Embryonic tissues remain competent for a limited time (Can depend on prior inductive events)

3. Competence factors • Not all tissues are able to respond to the same signals. Competence is actively acquired • Receptors, signaling molecule • Transcription factors (chromatin state) • Competence factors are specifically required in the responding tissue, not the inducer • Demonstrated in recombination experiments

4. Recombination experiments: First name the players…(fig 6.1 from Gilbert)

5. Then play…Fig 6.3 Gilbert

6. Cascades of inductive events form the organ: Reciprocal induction Of Gilbert

7. Two kinds of induction(plus one) • Instructive interaction: A signal or factor which tells the developing cell what it is • Permissive interaction: A signal or factor that allows the cell to become what it is • Selective interaction. A factor allows the cell to be “selected” into one lineage or another after a stochastic (random) change

8. For example, CD4 vs. CD8 T cells Selection vs. Instruction TCR-I CD8 TCR-II CD4

9. Juxtacrine interactions • Involve cell surface receptors on inducer and responding cells (no soluble factors) • Cell death/apoptosis pathways • Fas (CD95)/FasL • Notch/delta pathway • Involved in many binary cell fate decisions • Examples in flies, worms and mice

10. Cell death pathways Figure 6.27 of Gilbert

11. Notch is involved in a wide array of binary cell fate decisions • C. elegans:Ventral uterine vs. Anchor cell and vulval development • Drosophila: Neural vs. Epidermal cells • Mouse: Embryonic lethal, Demonstrated affects in the immune system and others • Human: Notch deficiencies cause birth defects

12. Notch/delta pathway(fig 6.29 of Gilbert) This model is simplistic little evidence for nuclear localization of Notch in developing organisms

13. Players in the Notch pathway • Receptor family: Notch/lin-12, glp-1 • Ligands: (DSL) Delta, Serrate, Lag2, Jagged • Processing: ADAMs (Kuz, TACE) and Secretases (Sel-12/presenilins) • Downstream effectors: CBF1, Su(Hairless), Lag1 (Collectively called CSL)

14. Proteolytic processing of Notch is complicatedFigure 1 from Weinmaster (2000)Curr. Opin. Genet. Dev. 10:363-369

15. Presenilins • Multipass transmembrane proteins • Mutated in inherited, early onset Alzheimer's disease • Involved in cleavage of amyloid precursor protein (APP) • Evidence that they are proteases or co-factors of a protease

16. Evidence linking presenilins with Notch signaling • Homology between PS1 and Sel-12 of C.elegans (sel-12 facilitates Notch signaling) • PS and Notch deficient animals have similar phenotypes (Drosophila and mice) • PS is required for access of Notch to the nucleus and, thus, Notch signaling • Defects can be rescued by providing exogenous PS

17. Current view of Notch signalingFigure 2 from Weinmaster (2000) Curr Opin Genet. Dev. 10:363-369

18. Notch involvement in cell fate • Examples of Notch involvement in inductive interactions (signaling between non-equivalent cells) • Lateral specification (occurs in a population of equivalent cells) • Involves the amplification of a stochastic small difference between equivalent cells

19. Lateral specification in C. elegansFrom Greenwald (1998) Genes. Dev. 12:1751-62

20. Evidence for feedback mechanism in lateral specification • Constitutive active mutants of lin12 have no AC • Mutants eliminating lin12 activity have 2 AC • Mixing experiments between two types above • Lin12 - cells always became AC • Lin12 WT cells always became VU • Behavior different from WT in non-mosaic situation where these cells have equal chance to become AC or VU

21. One way to generate a bias in Notch activity-C. elegans vulval developmentFrom Greenwald (1998) Genes. Dev. 12:1751-62 fate adoption is 2o-1o-2o

22. Gilbert Chapter 6 errors • Lag2 is not secreted but transmembrane. This is the ligand for lin12/Notch (p.169) • Figure 6.40 on T-lymphocyte signaling: The pathways leading from the receptors are not correct. (Notice, there is no reference) • Figure 6.19: Details appear to be in dispute

23. Paracrine factors and interactions • Transforming Growth Factor (TGF)-b • Sonic Hedgehog • Wnt • Fibroblast Growth Factor (FGF) • Retinoic Acid (RA)

24. TGF-b • Over 30 members of the TGF-b family • Subfamilies, TGFb, Activin, BMP, Vg1 • Processed proteolytically with the C-terminal region conatining the mature peptide • TGFb peptides can homo- or hetero-dimerize • TGFb signal transduction involves multiple receptors which activate cytoplasmic “Smads”

25. Smad pathway (fig 6.20) p.159

26. Division of labor among SmadsCourtesy of J.F. Doody (J. Massague lab @MSKCC) Smad 6 and 7 are inhibitory proteins induced by antagonists of the TGFb signaling pathway, such as g-interferon (Smad7)

27. The Smad familyFrom Piek, et. al. (1999) FASEB J. 13:2105-2124

28. A more complete TGFb pathway From Piek, et. al. (1999) FASEB J. 13:2105-2124

29. Evidence for non-redundant TGFb family involvement in development(due to expression patterns) • TGF-b1 knockout mice have defects in blood and vasculature (many die prenatally) • TGF-b2 knockout mice have multiple organ malformation (perinatal death) • TGF-b3 knockout mice die shortly after birth due to defects in pulmonary development • Information from Piek, et. al. (1999) FASEB J.

30. Next lecture: Induction/Signaling • Requirements of inducer and responder cells • Cascades of inductive events are involved in forming organs • Examples of the kinds of cell communication • Important signaling pathways in development • Notch, TGFb, SHH, Wnt, FGF