Biology 4361 - Developmental Biology
Cell-Cell Communication in
Development
June 23, 2009
Cell-Cell Communication
Concepts
Cells develop in the context of their environment, including:
- their immediate cellular neighborhood
- their tissue identity
- their position in the body.
Developing cells receive signals from each of these locations,
and they, in turn, signal the cells around them.
The components of this signal/response system must include:
1)
2)
3)
4)
A signal
A receptor for that signal
A mechanism to translate and/or transport the signal
A mechanism to translate the signal to a stimulation
(or repression) of gene expression
Cell-Cell Communication
Overview
Induction and competence
- how do cells and tissues “know” how to develop?
Paracrine factors – a type of inducer molecule
- inter- and intracellular signals
Signal transduction / signal transduction cascades
- response to signals at the molecular level
Juxtacrine signaling / extracellular matrix signaling
Cross-talk
Cell death pathways
Maintenance of the differentiated state
- how to ensure that a tissue remains stable
Induction and Competence
Development depends on the precise arrangement of tissues and cells.
- organ construction is precisely coordinated in time and space
- arrangements of cells and tissues change over time
Induction – interaction at close range between two or more cells or
tissues with different histories and properties.
Inducer – tissue that produces a signal that changes
cellular behavior
Responder – tissue being induced; the target tissue
Note – the target tissue must be capable of responding =
Competence – the ability of a cell or tissue to respond to a
specific inductive signal.
Induction - Vertebrate Eye Development
MRC Human Embryo Bank / Wellcome Photo Library
Induction - Vertebrate Eye Development
Lens placode (tissue thickening) induced in head
ectoderm by close contact with neural (brain) tissue
The developing lens then induces brain to
form the optic cup
Induction and Competence
Competence Factors
Competence - ability of a cell or tissue to respond
to a specific inductive signal
- actively acquired (and can also be transient)
Pax6
During lens induction Pax6 is expressed
in the head ectoderm, but not in other
regions of surface ectoderm
Pax6
Pax6 is a competence factor for lens induction
Inducers
Inducers are molecular components; e.g. optic vesicle inducers:
- BMP4 (bone morphogenic protein 4)
- induces Sox2 and Sox3 transcription factors
- Fgf8 (fibroblast growth factor 8)
- induces L-Maf transcription factor
Stepwise Induction
Inducers
Often multiple inducer tissues operate on a structure; e.g. for frog lens:
1st inducer - pharyngeal endoderm & heart-forming mesoderm
2nd inducer - anterior neural plate (including signal for
ectoderm Pax6 synthesis)
Reciprocal Induction
A
B
C
D
Mouse Lens – Reciprocal Induction
Inductive Interactions
Interactions between epithelia and mesenchyme:
- mesenchyme plays an instructive role (as the inducing tissue)
- initiates gene activity in epithelial cells
Instructive and Permissive Interactions
Instructive:
A signal from the inducing cell is necessary for initiating new
gene expression in the responding cell
e.g. optic vesicle placed under a new region of head ectoderm
-without the inducing cell, the responding cell is not capable
of differentiating (in that particular way).
Instructive interactions restrict the cell’s developmental options
General principles of instructive interactions:
1. In the presence of tissue A, responding tissue B develops
in a certain way.
2. In the absence of tissue A, responding tissue B does not
develop in that way.
3. In the absence of tissue A, but in the presence of tissue C,
tissue B does not develop in that way.
Instructive and Permissive Interactions
Instructive:
A signal from the inducing cell is necessary for initiating new
gene expression in the responding cell
Permissive:
The responding tissue has already been specified; needs only an
environment that allows the expression of those traits.
Permissive interactions tend to regulate the degree of expression
of the remaining developmental potential of the cell.
Epithelia and Mesenchyme
Epithelia – sheets or tubes of connected cells
- originate from any cell layer
Mesenchyme – loosely packed, unconnected
- derived from mesoderm or neural crest
All organs consist of an epithelium
and an associated mesenchyme.
Mesenchymal-epithelial interactions:
Many inductions involve interactions between
epithelia and mesenchyme.
Mesenchyme initiates gene activity in epithelial cells
Skin Epithelium & Mesenchyme
Epithelium
inductive
signals
Mesenchyme
epithelial derivatives:
- hair
- mammary glands
- scales - sweat glands
- feathers
derivative type depends
on restrictions by region
and genetics
Regional Specificity of Induction
Regional Specificity - source of the mesenchyme (inducing tissue)
determines the structure of the epithelial derivative.
Genetic Specificity of Induction
Mesenchyme induces
epithelial structures…
…but can only induce
what the epithelium is
genetically able to produce
Genetic specificity – epithelial response is limited to genomic capability
Cell-Cell Communication
Overview
Induction and competence
- how do cells and tissues “know” how to develop?
Paracrine factors – a type of inducer molecule
- inter- and intracellular signals
Signal transduction / signal transduction cascades
- response to signals at the molecular level
- signaling pathways gone bad – proto-oncogenes
Juxtacrine signaling / extracellular matrix signaling
Cross-talk
Cell death pathways
Maintenance of the differentiated state
- how to ensure that a tissue remains stable
Cell-Cell Communication
Concepts
Cells develop in the context of their environment, including:
- their immediate cellular neighborhood
- their tissue identity
- their position in the body.
Developing cells receive signals from each of these locations,
and they, in turn, signal the cells around them.
The components of this signal/response system must include:
1)
2)
3)
4)
A signal
A receptor for that signal
A mechanism to translate and/or transport the signal
A mechanism to translate the signal to a stimulation
(or repression) of gene expression
Inducing Signals
Also:
autocrine (self-generated) signals
endocrine signals
Paracrine Factors
Signaling molecules (proteins) produced by one cell (tissue) and
distributed via diffusion to a localized area; often act as inducers.
(Compare “endocrine”, “autocrine”, “juxtracrine” factors)
Paracrine Factor Families
Fibroblast growth factor (FGF)
Hedgehog family
Wingless family (Wnt)
TGF-β superfamily (TGF = transforming growth factor)
- TGF-β family
- Activin family
- Bone morphogenic proteins (BMPs)
- Vg1 family
Signal Transduction
Extracellular signals are received at the membrane and then
transduced to the cytoplasm at the cell membrane
- external signal is transmitted into the interior of the cell
Signal transduction cascades
- most intercellular and intracellular signals are part of larger
sets of pathways
- activated products or intermediates trigger other pathways
e.g. receptor tyrosine kinase (RTK)
(kinase = enzyme that phosphorylates a protein)
Signal Transduction; e.g. RTK
Receptor Tyrosine Kinase (RTK)
= hormone or
paracrine factor
ligand binding =
receptor
spans
membrane
conformational change
autophosphorylation
intracellular
signal
RTK Pathway - Generic
1. Ligand binding
2. RTK dimerized
3. RTK autophosphorylation
4. Adaptor protein binding
5. GNRP binding
6. GNRP activates Ras
(G protein)
7. Ras-GDP → Ras-GTP
(8. GAP recycles Ras)
9. Active Ras activates Raf
(protein kinase C;PKC)
10. Raf phosphorylates
MEK (a kinase)
11. MEK phosphorylates
ERK (a kinase)
12. ERK phosphorylates
transcription factors
13. Transcription modulation
JAK – Janus kinase
- non-receptor
tyrosine kinase
JAK-STAT Pathway
STAT – Signal Transducers
and Activators of
Transcription
- transcription factor
Pathway activators:
prolactin, cytokines,
growth hormones;
- cell proliferation
- differentiation
- apoptosis
NOTE – STATs can be
activated independently
of JAKs
- RTK; e.g. EGF receptor
- non-receptor tyrosine
kinases; e.g. c-src
Hedgehog Pathway - Generic
Drosophila
Mammalian Ci
homolog - Gli
- zinc finger TF
Wnt Pathway
Canonical Wnt pathway
Drosophila Wingless
mouse Integration
Wnt
APC - adenomatosis polyposis coli
(tumor suppressor)
- targets -catenin for degradation
GSK-3 - Glycogen synthase kinase 3
- prevents -catenin dissociation from APC
Wnt binds to Frizzled receptor family
- activates Disheveled
- Disheveled blocks GSK-3
- -catenin released from APC
- enters nucleus
- associates with LEF/TCF TFs
NOTE - actual picture more complex;
many other possible participants; e.g.
- at surface - co-receptors, etc.
- cytoplasmic - G-protein, other actors
Wnt Pathways
Planar cell polarity pathway
tether
(inactive)
a. Rho:
GTPase
b. Rac:
GTPase
Rhoassociated
kinase
Jun kinase
cell morphology, movement, division
Ca2+ pathway
phospholipase C
- IP3
- diacylglycerol
phosphatase
Ca2+-dependent gene expression
SMAD Pathway
TGF-β superfamily ligands
TGF-βs Inhibin etc.
BMPs
Nodal
Activins Vg1
Dpp
C. elegans Sma
Drosophila Mad
Smad
R-Smad
co-Smad
Proto-Oncogenes
Proto-oncogenes
(onco = cancer)
- active during development
- repressed/silenced in adult
- cause tumor formation
when inappropriately
activated
Mutations (e.g. constitutive
activation) = oncogene
- many cancers have
mutated proto-oncogenes
e.g. mutated Ras found in
20-30% of all tumors
Cell-Cell Communication
Overview
Induction and competence
- how do cells and tissues “know” how to develop?
Paracrine factors – a type of inducer molecule
- inter- and intracellular signals
Signal transduction / signal transduction cascades
- response to signals at the molecular level
Juxtacrine signaling / extracellular matrix signaling
Cross-talk
Cell death pathways
Maintenance of the differentiated state
- how to ensure that a tissue remains stable
Juxtracrine Signaling
Proteins from the inducing cell interact with
receptors from adjacent responding cells
without diffusing from the cell producing them.
Notch Pathway
(Serrate)
(Jagged)
1. Delta binds Notch
2. Binding activates
proteolytic cleavage
of Notch inner portion
3. Proteolytic fragment
moves to nucleus
- displaces repressor
- recruits p300 HAT
- activates
transcription
Extracellular Matrix Signals
ECM – macromolecules secreted by cells into their immediate environment
- macromolecules form a region of non-cellular material between the cells
- cell adhesion, migration, formation of epithelial sheets and tubes
- collagen, proteoglycans (fibronectin, laminin)
Cross-Talk
Signal transduction is often not
a linear event; e.g.
- cascades
- multiple signals required
- multiple products required
also:
- inhibitory signals
- promiscuous signals/receptors
Cross-talk provides opportunities
for emergent properties; e.g.
- hypersensitivity
- stability
- bistability
Cell-Cell Communication
Overview
Induction and competence
- how do cells and tissues “know” how to develop?
Paracrine factors – a type of inducer molecule
- inter- and intracellular signals
Signal transduction / signal transduction cascades
- response to signals at the molecular level
Juxtacrine signaling / extracellular matrix signaling
Cross-talk
Cell death pathways
Maintenance of the differentiated state
- how to ensure that a tissue remains stable
Apoptosis
Apoptosis – programmed cell death
Developmental:
- embryonic neural growth
- embryonic brain produces
3X neurons found at birth
- hand and foot
- webbing between digits
- teeth
- middle ear space
- vaginal opening
- male mammary tissue
- frog tails (at metamorphosis)
Adult:
- most cells and tissues
Apoptosis Signals
Paracrine – e.g. BMP4 (connective tissues, frog ectoderm, tooth primordia),
JAK-STAT, Hedgehog
Pre-programming: some cells will die unless “rescued”; e.g. mammalian
RBCs rescued by erythropoietin (hormone; activates JAK-STAT)
Mechanism – caspases (proteases) – cause autodigestion of the cell.
Maintaining Differentiation - 1
1) Activating signal initiates production of a transcription factor
which stimulates transcription of its own gene.
Maintaining Differentiation - 2
2) Synthesized proteins act to stabilize chromatin to keep
gene accessible.
Maintaining Differentiation – 3
3) Autocrine signaling: same cell makes signaling molecule
and receptor.
Community effect - the exchange of signals among equivalent
cells stabilizes the same determined state for all of them.
Maintaining Differentiation – 4
4) Paracrine loop - interaction with neighboring cells such that
each stimulates differentiation of the other.