Cell Signalling: Mechanisms and Pathways

Cell Signalling

Introduction

Cell signalling is a fundamental process by which cells detect, interpret, and respond to signals from their environment or from other cells. This process is essential for coordinating cellular activities, development, and homeostasis in multicellular organisms.

General Model of Signalling Events

Overview of Signal Transduction

• Synthesis of signalling molecules: Cells produce signalling molecules (ligands) such as hormones, neurotransmitters, or growth factors.

• Release of signalling molecules: These molecules are secreted or displayed on the cell surface.

• Transport of signal to target cells: Signals may diffuse locally or travel long distances via the bloodstream.

• Detection of signal / binding to a specific receptor: Target cells possess specific receptors that recognize and bind the signalling molecule.

• Signal transduction: Binding of the ligand to its receptor initiates a cascade of intracellular events.

• Signal amplification: The signal is often amplified, resulting in a large cellular response from a small initial stimulus.

• Cell responses: The cell executes a specific response, such as gene expression, secretion, or metabolic changes.

• Signal removal and response termination: Mechanisms exist to terminate the signal and reset the system.

Types of Cell Signalling

Modes of Intercellular Communication

• Autocrine signalling: A cell targets itself. Example: Some T lymphocytes secrete growth factors that act on themselves.

• Juxtacrine signalling: A cell targets another cell in direct contact; no release of diffusible signal. Example: Notch and Delta signalling in development.

• Paracrine signalling: A cell targets a nearby cell. Example: Transforming growth factor-β (TGF-β), fibroblast growth factors (FGFs), and neurotransmitters at synapses.

• Endocrine signalling: A cell targets a distant cell via the bloodstream. Example: Hormones such as antidiuretic hormone (ADH) and thyroxine.

Types of Signals According to Chemical Nature

Chemical Classes of Signals

• Peptides/proteins: e.g., insulin, growth factors.

• Steroids: e.g., cortisol, estrogen.

• Amino acid derivatives: e.g., epinephrine, thyroxine.

• Gases: e.g., nitric oxide (NO).

Additional info: The chemical nature of the signal determines its receptor type and mechanism of action.

Cell Signalling Receptors

Localization and Mechanisms

• Receptors can be located on the cell surface (membrane-bound) or inside the cell (cytosolic or nuclear).

• Ligand binding triggers conformational changes or clustering, initiating intracellular signalling cascades.

• Receptors differ in their activation mechanisms and the pathways they regulate.

Integration and Crosstalk in Signalling Pathways

Network Properties of Signalling

• Signalling pathways are interconnected; components from one pathway can affect another (signalling crosstalk).

• Signalling is best understood as a network of biochemical pathways rather than a simple linear sequence.

G-Protein Coupled Receptors (GPCRs)

Structure and Function

• G-protein: Guanine-nucleotide binding protein involved in signal transduction.

• GPCR structure:

  • 7 transmembrane alpha helices

  • Extracellular N-terminus

  • Cytosolic C-terminus

  • Ligand binding site

  • Cytosolic loop between segments 5 and 6 binds specific G proteins

• Example: Adrenergic receptors (respond to adrenaline/epinephrine)

Types and Regulation of G-Proteins

• Monomeric G-proteins: e.g., Ras, Rab

• Heterotrimeric G-proteins: Composed of α, β, and γ subunits

• Molecular switch: Active when GTP-bound, inactive when GDP-bound

Regulation of G-Proteins

• GAP (GTPase activating protein): Activates GTPase activity, inactivating the G protein

• GEF (Guanine nucleotide exchange factor): Promotes exchange of GDP for GTP, activating the G protein

• GDI (Guanine nucleotide dissociation inhibitor): Prevents interaction with GEFs and GAPs

Second Messengers

Concept and Examples

• Second messengers are small molecules generated inside the cell in response to receptor activation by hydrophilic signalling molecules.

• They propagate and amplify the signal within the cell.

• Examples: Cyclic AMP (cAMP), Ca2+, inositol trisphosphate (IP3), diacylglycerol (DAG)

Cyclic AMP (cAMP) as a Second Messenger

• Formed from cytosolic ATP by adenylyl cyclase, an enzyme anchored in the plasma membrane.

• Adenylyl cyclase is activated by Gsα (stimulatory G protein) and inhibited by Giα (inhibitory G protein).

Examples of Cell Functions Regulated by cAMP

Regulated Function	Target Tissues	Hormone
Glycogen degradation	Muscle, liver	Epinephrine
Fatty acid production	Adipose	Epinephrine
Heart rate, blood pressure	Cardiovascular	Epinephrine
Water reabsorption	Kidney	Antidiuretic hormone
Bone resorption	Bone	Parathyroid hormone

Ca2+ as a Second Messenger

• Calcium ions act as versatile second messengers in many signalling pathways.

• Release of Ca2+ from intracellular stores (e.g., endoplasmic reticulum) triggers various cellular responses.

Regulation of G-Protein Coupled Receptors

Mechanisms of Regulation

• Phosphorylation of cytosolic domains by G protein-linked receptor kinases (GRKs) reduces receptor activity.

• β-arrestin binds phosphorylated GPCRs, preventing further G protein interaction.

• Protein kinase A (PKA) is activated by cAMP and mediates many downstream effects.

G-Protein Signalling and Disease

Example: Cholera Toxin

• Cholera toxin enters intestinal cells and activates Gsα, locking it in the GTP-bound (active) state.

• This leads to constant production of cAMP, causing efflux of Cl- and Na+ ions and water, resulting in watery diarrhoea.

Receptor Tyrosine Kinases (RTKs)

Structure and Activation

• RTKs are membrane receptors with intrinsic tyrosine kinase activity.

• Ligand binding induces receptor dimerization (clustering) and autophosphorylation of tyrosine residues on the cytosolic domain.

• Non-receptor tyrosine kinases (e.g., Src) can also be activated by receptor binding.

Examples of Growth Factor Families and Their Receptors

Growth Factor	Target Cells	Type of Receptor Complex
Epidermal growth factor (EGF)	Wide variety of epithelial and mesenchymal cells	Tyrosine kinase
Transforming growth factor-β (TGFβ)	Same as EGF	Serine/threonine kinase
Platelet-derived growth factor (PDGF)	Mesenchyme, smooth muscle, trophoblast	Tyrosine kinase
Fibroblast growth factor (FGF)	Mesenchyme, fibroblasts, many other cell types	Tyrosine kinase
Colony-stimulating factor-1 (CSF-1)	Macrophage precursors	Tyrosine kinase

Activated RTKs Bind Specific Cytosolic Proteins

• Phosphorylated tyrosines on RTKs are recognized by cytosolic proteins with SH2 (Src Homology 2) domains.

• Binding of these proteins initiates multiple signalling cascades simultaneously.

MAPK and IP3 Pathways

MAPK Pathway

• Adaptor protein GRB2 (with SH2 domain) binds a GEF called Sos, which activates Ras (a small GTPase).

• Activated Ras triggers a phosphorylation cascade:

  • Raf (MAPKKK) phosphorylates MEK (MAPKK), which phosphorylates MAPK (ERK).

• MAPK activates transcription factors (e.g., Myc), leading to expression of genes such as E2F, G1 cyclin, and cdk.

• Ras and Myc are proto-oncogenes; mutations can lead to cancer.

IP3 Pathway

• Phospholipase C (PLC) is activated by RTKs or GPCRs, generating IP3 and DAG from membrane phospholipids.

• IP3 triggers release of Ca2+ from the endoplasmic reticulum, acting as a second messenger.

Summary of Key Concepts

1. Cells communicate via a variety of signals and means to detect these signals, including autocrine, juxtacrine, paracrine, and endocrine signalling.

2. Receptors differ in their localisation, structure, activation mechanism, signal transduction, and regulation.

3. G-protein coupled receptors change conformation upon ligand binding, interact with heterotrimeric G proteins, and elicit responses in target cells.

4. Receptor tyrosine kinases undergo clustering and autophosphorylation upon ligand binding. Adaptor proteins with SH2 domains interact with phosphorylated RTKs, triggering different cascade events.

5. RTK signalling leads to activation of transcription factors, driving expression of specific proteins and cellular responses.

6. Cyclic AMP and Ca2+ are second messengers that propagate signals inside the cell.

Further Reading: Becker's World of the Cell, Chapter 14: Signal Transduction Mechanisms II: Messengers and Receptors