Why signalling?Multicellular communication

Nematodes (C. Elegans)Fruit Flies (Drosophila) to Man

Types of cellular communication

A B

Direct Cell-Cell

Gap Junctions

Ca2+, cAMP

A B

Juxacrine

Cell-Cell interaction via receptors

TCR: MHCNCAM:NCAMKit:Steel (Neural crest formation)

Coordinated behaviour to benefit whole organism

Autocrine

Massive amplification of signal

IL-2 and T-cell activation

Paracrine

Immediate environmentLocal, short-lived mediators (EGF, PDGF, Cytokines)Little, or no diffusion

Neurones and neurotransmitters

GABA, acetycholine, glycine at a high concentration

Endocrine

Secreted Hormone[Low] =10-8M

Binds high-affinity receptor

Insulin / glucagon (Glucose metabolism)Pituitary Hormones (Metabolism / growth)Catecholamines :adrenaline, nor-adrenaline, dopamine (sympathetic nervous system)

β Cell Adipocyte

Anatomy of a signal pathway

Hormone receptors

Classes of membrane receptors

NeurotransmittersAcChGABAGlycineGlutamate

Cell adhesion and mobility

Signal transduction

Signal amplification

Modulation of signal pathways• Saturation, specificity, competition and antagonism

Hormone Signalling

Tyrosine Kinase Receptors Cytokine Receptors Serine Kinase Receptors G-Protein – Coupled Receptors Nuclear Receptor signalling Second Messengers: - Cyclic nucleotides - Phospholipids; Ca2+

Protein Phosphorylation

Dominant posttranslational protein modification Serine / Threonine (99%); Tyrosine (<1%) Protein Kinases vs Phosphatases Protein Tyrosine Kinases(PTKs):

Receptor PTKs - transmembrane prot. - insulin R - EGFR,PDGFR,FGFR,NGFR

Non-receptor PTKs - intracellular proteins

- c-Src, JAKs,Fak etc.

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The receptor protein-tyrosine kinase families

Hunter et al. 1997

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Non-receptor PTK families

Hunter et al 1997

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Role of noncatalytic segments in PDGFR signalling

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Adaptor proteins and signalling

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MAPK – Pathway Regulation & Function

Insulin Signalling Pathways

MitogenicMetabolic

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The protein tyrosine phosphatase families

Receptor-like

Non-receptor likeCytoplasmic proteins eg.PTP1B, PTP-PEST

Growth Factors GCGC

ERK

MEKPTP

+

Ras

Raf

P

P

P

P = Tyrosine phosphorylation to activate proteins

PTP = Protein Tyrosine Phosphatase

Switch activated proteins OFF by dephosphorylation

Mitosis ERKPNucleus

X

Mainly work by regulating gene transcription

Signalling via “Cytokine Receptors”

Class I (e.g.IL2R,4R,7R; GHR, PRLR, EPOR ) Class II ( e.g.IFN; IL10R ) No intrinsic enzyme activity Some cytokines may signal via MAPK / PI3K Class I/II signals through: JAK / STAT-

pathway*

*JAK = “just another kinase”/ Janus kinases

STAT= signal transducers and activators of transcription

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Class I Receptors

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JAK-STAT Signalling

• cytokine binding

• receptors dimerize

• JAKs approximate and

• transphosphorylate

• JAKs receptor-P, which

• effect STAT docking

• JAKs STAT-P

• STATs dimerize

• translocate to nucleus

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Negative regulation of JAK-STAT signalling

PTPs e.g.SHP-1 dephosphorylate activated JAKs or receptors

Other PTPs dephosphorylate STATs

Suppressor of cytokine signalling proteins (SOCS) bind JAKs,compete with STATS for receptor docking, and target bound signalling components for proteasomal degradation

Protein inhibitor of activated STATS (PIAS) inhibit transcriptional activity of STATs

Signalling via Serine Kinase Receptors

Prototype: TGF-β Others: Pituitary (inhibin;activin)

Bone (BMPs) Sexual diff. (MIS)

Type I and II RSKs Signalling via: Smads / Co-Smads

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SMAD signalling

ligand binds to receptor II or II + I

heterotetrameric complex forms and

phosphorylation of receptor I, which

phosphorylates receptor regulated SMADs (R-SMAD )

R-SMAD associates with Co-SMAD, (e.g.Smad4)

translocation to nucleus

co-factors modulate

gene transcription

Type I & II Receptor Serine Kinases (TGF-βR)

Signalling via G Protein-Coupled Receptors

Class I - Rhodopsin

- Adrenoceptors, Ach, Dopamine Serotonin - Angiotensin, GnRH, TSH, Vasopressin,

Opioid - Somatostatin, Cannabinoid, Prostanoid

Class II - Glucagon, Calcitonin, CRF, PTH, VIP

Class III - Calcium sensor

G-protein/AC/cAMP

G Protein Cycle

GDP/GTP exchange

Dissociation

Activation of effector molecules

Inherent GTPase activity

Reassociation

βARK / PKAPhosphorylation of Receptor

Ligand binds receptor

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The cAMP-Dependent Signal Transduction Pathway

PKA

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Pleiotropic Actions of cAMP

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Cross-talk of cAMP with other signalling pathways

GEF=guanine nucleotide exchange factor

Phospholipase CPTH

Gαq

Calcium as second messenger

Overall

Signalling is essential for cellular coordination Important on a cell, tissue and whole body level

Cross-talk between systems to fine-tune cell regulation

Dysfunctional signalling often leads to pathology

Proto-oncogene to oncogene

Degeneracy in the system to compensate for loss of function

Amplification and propagation of signals

Signalling controls all cellular function