Signalling pathways

Signal transduction

Signal transduction is the process by which an extracellular signal is converted into a specific intracellular response

Core Stages of Signalling

• Signal – Extracellular mediator released

• Perception – Ligand binds receptor

• Transduction – Intracellular signalling cascade initiated

• Response – Cellular/metabolic/genetic effect produced

Extracellular Mediators

Chemical messengers released by cells that alter behaviour of target cells.

• hormones

• growth factors

• cytokines

• steroid hormones / hydrophobic signals

Hormones

• Secreted by endocrine glands

• Travel via bloodstream

• Act at distant sites

• Usually cause short-term metabolic effects

Examples:

• Insulin

• Glucagon

• Adrenaline

Growth factors

• Usually act locally

• Stimulate proliferation/differentiation

• Important in development and tissue repair

Examples:

• EGF

• PDGF

• NGF

Cytokines

• Small signalling proteins in immune signalling

• Usually local action

• Regulate immunity, inflammation, hematopoiesis

Examples:

• Interleukins

• Interferons

Steroid hormones / hydrophobic signals

Hydrophobic signals diffuse through membrane directly.

Examples:

• Cortisol

• Oestrogen

• Testosterone

• Retinoids

• Thyroxine

Characteristics:

• No long transduction pathway required

• Usually slower onset

• Long-lasting effects

• Reversible when ligand removed

Steroid hormones / hydrophobic signals mechanism

• Diffuse through plasma membrane

• Bind intracellular receptor

• Receptor acts as transcription factor

• Alters gene expression

Types of Cell-Cell signalling

• endocrine signalling

• paracrine signalling

• autocrine signalling

• Contact-Dependent Signalling (Additional Lodish Concept)

endocrine signalling

Long-range signalling

• Signal enters bloodstream

• Acts on distant target cells

Examples: Insulin, adrenaline

Paracrine signalling

Short-range signalling

• Signal acts on nearby cells

• Does not enter circulation

Examples: Growth factors, cytokines

Autocrine signalling

Cell signals to itself

• Secreted ligand binds receptors on same cell

Examples: Some growth factors in cancer

Contact-Dependent Signalling (Additional Lodish Concept)

• Membrane-bound ligand interacts with adjacent cell receptor

• Requires direct contact

Main families of receptors

• G Protein-Coupled Receptors (GPCRs)

• Enzyme-Coupled Receptors (Intrinsic Enzyme Activity)

• Enzyme-Associated Receptors

• Ligand-Gated Ion Channel Receptors

• Intracellular receptors

G Protein-Coupled Receptors (GPCRs)

Structure

• 7 transmembrane α-helices (7TMR)

• Largest receptor superfamily

Examples:
β-adrenergic receptor, glucagon receptor

G Protein-Coupled Receptors (GPCRs) mechanism

1. Ligand binds GPCR

2. GPCR activates heterotrimeric G protein

3. GDP exchanged for GTP on Gα

4. Gα and Gβγ regulate downstream effectors

Downstream Effects

• Adenylyl cyclase activation/inhibition

• PLC activation

• Ion channel regulation

Functional Outcome

• Short-term metabolic effects

• Rapid responses

• Changes in movement, secretion, metabolism

Enzyme-Coupled Receptors (Intrinsic Enzyme Activity): Receptor Tyrosine Kinases (RTKs)

Examples

• EGFR

• PDGFR

• Insulin receptor

Mechanism

1. Ligand binding

2. Dimerisation

3. Trans-autophosphorylation of tyrosines

4. Recruitment of signalling proteins (SH2/PTB domain proteins)

5. Activation of pathways such as Ras/MAPK

Roles

• Proliferation

• Survival

• Differentiation

Enzyme-Coupled Receptors (Intrinsic Enzyme Activity): Receptor Serine/Threonine Kinases

Example

• TGF-β receptor

Function

• Development

• Tissue remodelling

• Differentiation

Enzyme-Associated Receptors: cytokine receptors

No intrinsic kinase activity.

Associated with cytoplasmic kinases (e.g. JAKs)

Mechanism

1. Ligand binding

2. Receptor dimerisation

3. JAK activation

4. STAT phosphorylation

5. STAT enters nucleus

Outcome

Long-term gene expression changes

Ligand-Gated Ion Channel Receptors

1. Ligand binds receptor

2. Ion channel opens/closes

3. Ion flux alters membrane potential / signalling

Examples

• Nicotinic ACh receptor

• TRP channels

Intracellular Receptors

For hydrophobic ligands only.

Located:

• Cytoplasm

• Nucleus

Act directly as transcription regulators

Second messenger

An intracellular small molecule or ion that couples extracellular receptor activation to intracellular responses

Criteria for second messenger Classification

1. Be Small

• Small metabolite or ion

2. Rapidly Alter Concentration

• Fast synthesis/release

• Fast degradation/removal

3. Be Controlled by Extracellular Stimulus

• Produced only after receptor activation

4. Regulate Enzyme Activity or Protein Function

• Must alter downstream proteins

5. Involve Specific Interactions

• Bind/selectively regulate defined targets

6. Enable Amplification

• One receptor → many messenger molecules

Major second messengers

• cAMP

• cGMP

• DAG

• IP3

• Ca²⁺

cAMP

Produced by adenylyl cyclase from ATP

Activates:

• Protein kinase A (PKA)

Effects:

• ↑ Lipid breakdown

• ↓ Glycogen synthesis

cGMP

Produced by guanylyl cyclase

Activates:

• Protein kinase G (PKG)

Effects:

• Opens cation channels in photoreceptors

• Smooth muscle relaxation

DAG

Generated by PLC cleavage of PIP2

Activates:

• Protein kinase C (PKC)

Effects:

• ↑ Transcription

• ↓ Glycogen synthesis

IP3

Generated with DAG from PIP2 cleavage

Function:

• Opens ER Ca²⁺ channels

Ca²⁺

Universal second messenger

Activates:

• Calmodulin

• CaMKs

• PKC (with DAG)

Protein Kinase Cascades in Signalling

Sequential activation of kinases by phosphorylation.

General Mechanism

1. Receptor activated

2. Kinase 1 activated

3. Kinase 1 phosphorylates kinase 2

4. Kinase 2 phosphorylates kinase 3

5. Final kinase phosphorylates effector protein

Why Kinase Cascades Are Important

1. Signal Amplification

• Each kinase activates multiple downstream molecules.

2. Signal Integration

• Different pathways converge on same kinase.

3. Specificity

• Different scaffold proteins localise cascades.

4. Regulation

• Phosphatases can terminate signalling rapidly.

MAPK Cascade

RTK → Ras → Raf → MEK → ERK

Outcomes

• Gene transcription

• Proliferation

• Differentiation

Amplification in signalling cascades

One signalling event generates many downstream activated molecules

Mechanisms of Amplification

1. Receptor–Effector Amplification

• One receptor activates many G proteins.

2. Enzymatic Amplification

• One enzyme produces many second messenger molecules.

• Example:
  One adenylyl cyclase → thousands of cAMP molecules

3. Kinase Cascade Amplification

• One kinase activates many substrate kinases.

4. Transcriptional Amplification

• One TF induces many mRNA molecules.

Example of Amplification in cAMP Pathway

1 Hormone binds receptor
→ activates multiple G proteins
→ activates adenylyl cyclase
→ generates many cAMP
→ activates many PKA molecules
→ phosphorylates many enzymes

Result:
Massive amplification from tiny initial signal

Reversible Protein Phosphorylation in Signalling

Kinases:

Add phosphate groups to:

• Serine

• Threonine

• Tyrosine

Phosphatases:

• Remove phosphate groups

Importance:

Allows:

• Rapid switching ON/OFF

• Reusability of proteins

• Tight temporal control

Short-Term signalling Responses

Seconds–minutes

Usually affect:

• Metabolism

• Movement

• Ion transport

• Enzyme activity

Often mediated by:

• GPCRs

• Ion channels

Long-Term signalling Responses

Hours–days

Usually affect:

• Gene expression

• Proliferation

• Differentiation

• Development

Often mediated by:

• RTKs

• Cytokine receptors

• Steroid receptors

GPCR summary

• membrane receptor

• no intrinsic enzyme activity

• G proteins main downstream mechanism

• fast

• metabolic typical outcome

RTK summary

• membrane receptor

• intrinsic enzyme activity

• phosphorylation main downstream mechanism

• moderate speed

• growth typical outcome

Cytokine receptor summary

• membrane receptor

• no intrinsic enzyme activity

• JAK/STAT main downstream mechanism

• slower

• Immune/gene expression typical outcome

Ion channel summary

• membrane receptor

• no intrinsic enzyme activity

• Ion flux main downstream mechanism

• very fast

• electrical typical outcome

Intracellular Receptor summary

• no membrane receptor

• gene transcription main downstream mechanism

• slow

• transcriptional typical outcome