Cell­Cell Communication

IBS 8102 – Cell, Molecular, and Developmental Biology

January 29, 2008

Communicate What?

Why do cells communicate? To govern or modify each other for the benefit of the organism

­ differentiate ­ multiply ­ perform specialized physiology; e.g. ­ secrete ­ contract

­ die (i.e. undergo apoptosis)

Mechanism I. Signaling molecules

­ intercellular ­ intracellular A. Long­distance

­ neurotransmitters ­ hormones

B. Short­distance ­ paracrine factors ­ juxtacrine factors ­ autocrine factors ­ ECM components

D. Receptors ­ membrane­bound ­ cytoplasmic ­ cytosolic ­ nuclear

II. Cellular interaction

B. Ion channels

A. Diffusion ­ lipophilic molecules may diffuse through membrane ­ NO ­ steroid hormones

C. Gap junctions

III. Intracellular signaling proteins (Signal Transduction components) ­ distribute external signals ­ often enzymatic ­ phosphatases ­ kinases ­ GTP binding/hydrolysis

IV. Target proteins ­ transcriptional regulators ­ ion channels ­ metabolic pathways ­ cytoskeletal components ­ etc.

Intrercellular Signaling

Fibroblast growth factor (FGF)

Hedgehog family

Wingless family (Wnt)

TGF­β superfamily (TGF = transforming growth factor)

­ TGF­β family

­ Activin family

­ Bone morphogenic proteins (BMPs)

­ Vg1 family

Major Paracrine Factor Families

Signal Transduction Extracellular signals are 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 = protein phosphorylating enzyme)

Signal transduction cascades ­ most intercellular and intracellular signals are part of larger set of pathways ­ activated products or intermediates trigger other pathways

Signal Transduction Model

= hormone or paracrine factor

autophosphorylation

intracellular signal

receptor spans

membrane

ligand binding =

conformational change

e.g. Receptor Tyrosine Kinase (RTK)

RTK Pathway – FGF (Generic) 2. RTK dimerized

1. Ligand binding

3. RTK autophosphorylation

4. Adaptor protein binding

5. GNRP binding (guanine NT releasing protein)

6. GNRP activates Ras (G protein)

7. Ras­GDP → Ras­GTP

8. [GAP recycles Ras]

9. Ras activates Raf (protein kinase C;PKC)

10. Raf phosphorylates MEK (a kinase)

11. MEK phosphorylates ERK (a kinase)

12. ERK phosphorylates transcription factor

13. Transcription modulation

Proto­Oncogenes

Proto­oncogenes

(onco = cancer)

e.g. mutated Ras found in 20­30% of all tumors

Proto­oncogenes are active during development ­ repressed/silenced in adult

­ many cancers have mutated proto­oncogenes

Mutations (e.g. constitutive activation) = oncogene

RTK Pathway – MITF

Stem cell factor (paracrine factor) stimulates genes needed for melanocyte production.

MITF ­ microphthalmia transcription factor

JAK­STAT Pathway

JAK – Janus kinase ­ non­receptor 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 ­ non­receptor tyrosine kinases; e.g. c­src

Hedgehog Pathway ­ Generic

Drosophila

Mammalian Ci homolog ­ Gli ­ zinc finger TF

Wnt Pathways Canonical Wnt pathway

GSK­3 ­ Glycogen synthase kinase 3 ­ prevents β­catenin dissociation from APC

APC ­ adenomatosis polyposis coli (tumor suppressor) ­ targets β­catenin for degradation

Wnt binds to Frizzled receptor family ­ activates Disheveled ­ Disheveled blocks GSK­3 ­ β­catenin released from APC ­ enters nucleus ­ associates with LEF/TCF TFs

NOTE ­ actual picture more complex than this; many other possible participants; e.g. ­ at surface ­ co­receptors, etc. ­ cytoplasmic ­ G­protein, other actors

Drosophila Wingless mouse Integration Wnt

Wnt Pathways Planar cell polarity pathway

cell morphology, movement, division Ca 2+ ­dependent gene expression

Ca 2+ pathway

phosphatase

phospholipase C ­ IP3 ­ diacylglycerol

b. Rac: GTPase

Jun kinase

a. Rho: GTPase

Rho­ associated

kinase

tether (inactive)

SMAD Pathway TGF­β superfamily ligands TGF­βs BMPs Activins Dpp

Inhibin Nodal Vg1 etc.

R­Smad

co­Smad

C. elegans Sma Drosophila Mad Smad

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 Mechanism

homologs

Mechanism: CEDs, Caspases ­ proteases

Apoptosis triggered by developmental and metabolic signals

Often pre­programmed: ­ inhibited until signal received

Signaling pathways:

Adult: ­ TNFR (Tumor necrosis factor receptor) ­ TNF ­ FAS

Developmental: ­ BMP ­ JAK­STAT ­ Hedgehog

Notch Pathway Juxtacrine signaling: Proteins from the inducing cell interact with receptors from adjacent responding cells without diffusing from the cell producing them.

(Serrate) (Jagged)

Delta protein bind Notch ­ protease cleaves both outer and inner Notch portions

­ inner portion moves to nucleus ­ displaces repressor ­ recruits p300 HAT

­ activates transcription

­ outer portion remains with ligand ­ endocytosed into ligand­expressing cell ­ may act as signal

e.g. Notch/Delta, Ephrin/EphR, EGF/EGFR, IL­15/IL­15Rα, etc.

Extracellular Matrix

ECM – macromolecules secreted by cells into their immediate environment

­ form a region of non­cellular material in the intersticies between the cells

­ cell adhesion, migration, formation of epithelial sheets and tubes

­ proteoglycans: e.g. heparan sulfate, chondroitin sulfate, keratan sulfate

­ polysaccharides; e.g. hyaluronic acid

­ proteins: e.g. collagen, fibronectin, elastin, laminin

ECM Function

­ intracellular signaling

ECM­Mediated Cell Adhesion

actin microfilament system = anchoring and movement

Different cells have different cadherins.

Different cadherins have different affinities for each other.

Thus, cell types can segregate themselves based on membrane components.

NOTE – Ca 2+ ­dependent binding:

Ca 2+ can control both strength and reversibility of binding

Extracellular Matrix Signals

ECM Signal Transduction

ECM components; e.g. fibronectin

Trans­membrane receptor; e.g. integrin ­ binds to ECM RGD sequence ­ binds to cytoskeletal components ­ regulation inside and out ­ integrin subunit α ­ Ca 2+

kinase regulation

Signal transduction: ­ cytoskeleton ­ talin, vinculin, paxillin, α­actinin can regulate kinase activity ­ Src family ­ focal adhesion kinase (FAK)

Cross­Talk

Signal transduction is often not a linear event; e.g. ­ cascades ­ multiple signals required ­ multiple products required

also: ­ inhibitory signals ­ promiscuous signals/receptors

Cross­talk provides opportunities for emergent properties; e.g. ­ hypersensitivity ­ stability ­ bistability