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
“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)
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
Recombination experiments:
First name the players…(fig 6.1 from Gilbert)
Then play…
Fig 6.3 Gilbert
Cascades of inductive events form
the organ: Reciprocal induction
Of Gilbert
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
For example, CD4 vs. CD8 T cells
Selection
vs.
Instruction
TCR-I
CD8
TCR-II
CD4
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
Cell death pathways
Figure 6.27 of Gilbert
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
Notch/delta pathway(fig 6.29 of Gilbert)
This model is simplistic little evidence for nuclear
localization of Notch in developing organisms
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)
Proteolytic processing of Notch is
complicated Figure 1 from Weinmaster (2000)Curr. Opin. Genet. Dev. 10:363-369
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 cofactors of a protease
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
Current view of Notch signaling
Figure 2 from Weinmaster (2000) Curr Opin Genet. Dev. 10:363-369
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
Lateral specification in C. elegans
From Greenwald (1998) Genes. Dev. 12:1751-62
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
One way to generate a bias in
Notch activity-C. elegans vulval development
From Greenwald (1998) Genes. Dev. 12:1751-62
fate adoption is 2o-1o-2o
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
Paracrine factors and interactions
•
•
•
•
•
Transforming Growth Factor (TGF)-b
Sonic Hedgehog
Wnt
Fibroblast Growth Factor (FGF)
Retinoic Acid (RA)
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”
Smad pathway (fig 6.20) p.159
Division of labor among Smads
Courtesy 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)
The Smad family
From Piek, et. al. (1999) FASE B J. 13:2105-2124
A more complete TGFb pathway
From Piek, et. al. (1999) FASE B J. 13:2105-2124
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.
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