Cell-Cell Communication in Development

Biology 4361 - Developmental Biology Cell-Cell Communication in Development June 23, 2009

Concepts Cell-Cell Communication 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) 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

Overview Cell-Cell Communication 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) During lens induction Pax6 is expressed in the head ectoderm, but not in other regions of surface ectoderm Pax6 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

Inducers Stepwise Induction Often multiple inducer tissues operate on a structure; e.g. for frog lens: 1 st inducer - pharyngeal endoderm & heart-forming mesoderm 2 nd 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: Instructive and Permissive Interactions 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

Overview Cell-Cell Communication 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

Inducing Signals Also: autocrine (self-generated) signals endocrine signals

Fibroblast growth factor (FGF) Hedgehog family Wingless family (Wnt) 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 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) receptor spans membrane ligand binding = conformational change = hormone or paracrine factor 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 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 JAK-STAT Pathway

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

Planar cell polarity pathway Wnt Pathways Ca 2+ pathway tether (inactive) phospholipase C - IP3 - diacylglycerol a. Rho: GTPase Rhoassociated kinase b. Rac: GTPase Jun kinase phosphatase cell morphology, movement, division Ca 2+ -dependent gene expression

SMAD Pathway TGF-β superfamily ligands TGF-βs BMPs Activins Dpp Inhibin Nodal Vg1 etc. 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

Juxtracrine Signaling Proteins from the inducing cell interact with receptors from adjacent responding cells without diffusing from the cell producing them.

1. Delta binds Notch Notch Pathway (Serrate) (Jagged) 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

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.