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Cell-Cell Communication in

Development

Biology 4361 - Developmental Biology

June 23, 2009

Cell-Cell CommunicationConcepts

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.

1) A signal

2) A receptor for that signal

3) A mechanism to translate and/or transport the signal

4) A mechanism to translate the signal to a stimulation

(or repression) of gene expression

The components of this signal/response system must include:

Cell-Cell Communication

Juxtacrine signaling / extracellular matrix signaling

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

Maintenance of the differentiated state

- how to ensure that a tissue remains stable

Cell death pathways

Cross-talk

Induction and Competence

Competence – the ability of a cell or tissue to respond to a

specific inductive signal.

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 =

Induction - Vertebrate Eye Development

MRC Human Embryo Bank / Wellcome Photo Library

Induction - Vertebrate Eye Development

The developing lens then induces brain to

form the optic cup

Lens placode (tissue thickening) induced in head

ectoderm by close contact with neural (brain) tissue

Induction and Competence

Pax6

Pax6

Competence Factors

Competence - ability of a cell or tissue to respond

to a specific inductive signal

- actively acquired (and can also be transient)

During lens induction Pax6 is expressed

in the head ectoderm, but not in other

regions of surface ectoderm

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 InductionInducers

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

A signal from the inducing cell is necessary for initiating new

gene expression in the responding cell

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.

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

Instructive:

Instructive and Permissive Interactions

Permissive:

Permissive interactions tend to regulate the degree of expression

of the remaining developmental potential of the cell.

The responding tissue has already been specified; needs only an

environment that allows the expression of those traits.

A signal from the inducing cell is necessary for initiating new

gene expression in the responding cell

Instructive:

Epithelia and Mesenchyme

Many inductions involve interactions between

epithelia and mesenchyme.

Mesenchymal-epithelial interactions:

Mesenchyme initiates gene activity in epithelial cells

Mesenchyme – loosely packed, unconnected

- derived from mesoderm or neural crest

All organs consist of an epithelium

and an associated mesenchyme.

Epithelia – sheets or tubes of connected cells

- originate from any cell layer

Skin Epithelium & Mesenchyme

Epithelium

Mesenchyme

inductive

signals

epithelial derivatives:

- feathers

- hair - mammary glands- scales - sweat glands

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

Juxtacrine signaling / extracellular matrix signaling

Overview

Induction and competence

- how do cells and tissues “know” how to develop?

Paracrine factors – a type of inducer molecule

- inter- and intracellular signals

Maintenance of the differentiated state

- how to ensure that a tissue remains stable

Signal transduction / signal transduction cascades

- response to signals at the molecular level

- signaling pathways gone bad – proto-oncogenes

Cell death pathways

Cross-talk

Cell-Cell CommunicationConcepts

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.

1) A signal

2) A receptor for that signal

3) A mechanism to translate and/or transport the signal

4) A mechanism to translate the signal to a stimulation

(or repression) of gene expression

The components of this signal/response system must include:

Inducing Signals

Also:

autocrine (self-generated) signals

endocrine signals

Paracrine Factors

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

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

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

e.g. receptor tyrosine kinase (RTK)

(kinase = enzyme that phosphorylates a protein)

- most intercellular and intracellular signals are part of larger

sets of pathways

Signal transduction cascades

- activated products or intermediates trigger other pathways

Signal Transduction; e.g. RTK

= hormone or

paracrine factor

autophosphorylation

intracellular

signal

receptor

spans

membrane

ligand binding =

conformational change

Receptor Tyrosine Kinase (RTK)

RTK Pathway - Generic

2. RTK dimerized

1. Ligand binding

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-STAT PathwayJAK – Janus kinase

- non-receptor

tyrosine kinase

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 PathwayCanonical Wnt pathway

GSK-3 - Glycogen synthase kinase 3

- prevents -catenin dissociation from APC

APC - adenomatosis polyposis coli

(tumor suppressor)

- targets -catenin for degradation

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

Drosophila Wingless

mouse IntegrationWnt

Wnt PathwaysPlanar cell polarity pathway

cell morphology, movement, division Ca2+-dependent gene expression

Ca2+ pathway

phosphatase

phospholipase C

- IP3

- diacylglycerol

b. Rac:

GTPase

Jun kinase

a. Rho:

GTPase

Rho-

associated

kinase

tether

(inactive)

SMAD Pathway

R-Smad

co-Smad

C. elegans Sma

Drosophila MadSmad

TGF-β superfamily ligands

TGF-βs

BMPs

Activins

Dpp

Inhibin

Nodal

Vg1

etc.

Proto-Oncogenes

Proto-oncogenes

(onco = cancer)

e.g. mutated Ras found in

20-30% of all tumors

- many cancers have

mutated proto-oncogenes

Mutations (e.g. constitutive

activation) = oncogene

- active during development

- repressed/silenced in adult

- cause tumor formation

when inappropriately

activated

Cell-Cell Communication

Juxtacrine signaling / extracellular matrix signaling

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

Maintenance of the differentiated state

- how to ensure that a tissue remains stable

Cell death pathways

Cross-talk

Juxtracrine Signaling

Proteins from the inducing cell interact with

receptors from adjacent responding cells

without diffusing from the cell producing them.

Notch Pathway

1. Delta binds Notch

(Serrate)

(Jagged)

3. Proteolytic fragment

moves to nucleus

- displaces repressor

- recruits p300 HAT

- activates

transcription

2. Binding activates

proteolytic cleavage

of Notch inner portion

Extracellular Matrix SignalsECM – 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

Cell death pathways

Juxtacrine signaling / extracellular matrix signaling

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

Maintenance of the differentiated state

- how to ensure that a tissue remains stable

Cross-talk

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

Mechanism – caspases (proteases) – cause autodigestion of the cell.

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)

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 – 33) 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.