In signal transduction, extracellular signals are converted into intracellular signals: A signaling molecule (ligand) reaches its target cell and binds to a specific receptor. This activates a signaling cascade involving intracellular enzymes and molecules (second messengers), which again leads to a specific reaction. Via signal amplification, the number of signaling molecules is increased at every step of the signal cascade.
- Receptor: Proteins that receive signals from outside of the cell by binding specific ligands, thereby initiating an intracellular signaling cascade.
- Ligand (first messenger): A chemical messenger that binds specifically to one receptor (e.g. proteins, steroids, hormones, neurotransmitters, small organic molecules such as nitric oxide (NO)).
- Second messenger: small molecules (e.g. cGMP, IP3, Ca2+ ions) that mediate the intracellular response to an extracellular stimulus
- Signaling cascade: The interconnection of individual steps in a signaling pathway.
- Signal amplification: The increase in the number of signaling molecules at every step of the signal cascade.
Extracellular messengers have to bind to a receptor to exert their effect. Lipophilic messengers can pass through the cell membrane and bind to intracellular receptors, while hydrophilic messengers cannot due to the lipophilic properties of the cell membrane. Therefore, hydrophilic messengers typically act on integral membrane receptors, which translate the signal of the extracellular messenger into an intracellular signal.
Overview of receptor types
|Receptor types||Examples of ligands|
|Cell surface receptors||G protein-coupled receptors||Catecholamines|
|Receptor tyrosine kinases|
Receptors with associated kinases
|Receptor protein serine/threonine kinases||TGF-β (cytokine)|
Other enzyme-linked receptors
|Ligand-gated ion channels||Acetylcholine|
- Definition: receptors that are located inside the cell
- IP3-receptor: in the endoplasmatic reticulum
Nuclear receptors: ligand-dependent transcription factors that act within the nucleus
- Location prior to ligand binding
- Activation principle
- Hydrophilic hormones transmit signals by binding to receptors present in the cell membrane (= cell surface receptors).
- There are three types of cell surface receptors:
- Examples of ligands: catecholamines, anterior pituitary hormones (ACTH, LH, FSH, TSH), glucagon
- Receptor structure
- A heterotrimeric protein composed of three subunits
- Activation principle
- GTPase: a small G protein composed of only an α subunit that functions independently to hydrolyze GTP to GDP and phosphate
- Effector molecules
|Receptor types and their connected G proteins|
|Gq proteins||Gs proteins||Gi proteins|
|Sympathetic β||β1, β2, β3|
|Parasympathetic muscarinergic||M1, M3||M2|
Receptor tyrosine kinases (RTKs)
Receptor tyrosine kinases are transmembrane receptors that are generally activated by ligand-induced dimerization and autophosphorylation of cytoplasmic tyrosine residues, which triggers activation of downstream signaling cascades.
- Examples of ligands: insulin , growth factors (e.g., EGF, IGF)
- Extracellular domain
- Single transmembrane domain
- Intracellular domain with tyrosine kinase activity
- Ligand binding to the extracellular domain results in receptor dimerization.
- The two adjacent tyrosine kinase domains phosphorylate one another to tyrosine residues (autophosphorylation).
- Increased kinase activity through autophosphorylation
- A number of different signal transduction molecules with SH2 domains bind to the phosphorylated tyrosine residues and are activated → activation of various effectors of different signaling pathways
Examples of effectors
- Phospholipase C
- Monomeric, membrane-bound GTPase
- Activated Ras activates further signal transduction pathways such as the mitogen-activated protein (MAP) kinase cascade → transcription of target genes for cell growth and proliferation
- The Ras genes (KRas, HRas, NRas) are proto-oncogenes
- Examples of signaling pathways with Ras involvement: insulin, platelet-derived growth factor (PDGF)
- Examples of ligands: growth hormones, prolactin, erythropoietin, thrombopoietin, interferons
- Receptor structure
Activation principle (similar to receptor tyrosine kinases)
- Ligand binding leads to receptor dimerization.
- Two neighboring tyrosine kinase domains of JAK phosphorylate each other (autophosphorylation) → JAK activation
- Development (at the phosphorylated tyrosine residues) of binding sites for SH2 domains of signal proteins (STAT proteins).
- STAT proteins are phosphorylated and dimerized by JAK.
- STAT dimers exert their effect directly in the nucleus as a transcription factor for JAK-STAT regulated genes.
- Examples of ligands: TGFβ (transforming growth factor β)
- Receptor structure: two subunits (type I and type II receptors) each with serine/threonine kinase activity
- Examples of ligands: acetylcholine, GABA, glutamate, IP3
- Receptor structure: Cell surface receptors that act as ion channels
- Activation principle (example)
Second messengers are small molecules that mediate the intracellular response to an extracellular stimulus.
cAMP (cyclic adenosine monophosphate) and protein kinase A
- A membrane-bound adenylyl cyclase synthesizes cAMP from ATP
- Regulation of adenylyl cyclase: depends on the type of G protein of the G protein-coupled receptor
Effects of cAMP: activation of protein kinase A (PKA)
Protein kinase A
- Enzyme class: serine/threonine kinase
- Activation: cAMP leads to the release of catalytic subunits of PKA
- Mechanism: PKA controls the activity (activation or inactivation) of numerous enzymes via phosphorylation of serine and threonine residues.
- Example: glycogen metabolism to ↑ blood glucose concentration
- Protein kinase A
- cAMP degradation: by phosphodiesterase to adenosine monophosphate (AMP)
- cGMP synthesis: cGMP is synthesized from GTP by the guanylate cyclase. There are two subforms of guanylate cyclase:
Effects of cGMP
- Activates cGMP-dependent protein kinase G in smooth muscle cells → inhibits Ca2+ outflow from the sarcoplasmic reticulum → ↓ intracellular Ca2+ → relaxed smooth vascular muscles → vasodilation
- cGMP-dependent ion channels in the photoreceptor cells of the retina → preserve the unstimulated state → dark signal
- cGMP degradation: by phosphodiesterase to GMP
The second messengers cAMP and cGMP are degraded and inactivated to AMP and GMP by various phosphodiesterases (PDE). A decrease in cAMP or cGMP causes contractions in smooth muscles. PDE inhibitors are used in the treatment of pulmonary hypertension (PDE-4 inhibitor roflumilast) and erectile dysfunction (PDE-5 inhibitor sildenafil).
- Synthesis: produced from L-arginine in two NADPH-dependent reactions, catalyzed by endothelial nitric oxide synthase (eNOS) in the endothelial cells of blood vessels
- Function: causes smooth muscle relaxation and subsequent dilation of blood vessels
Nitric oxide (NO) has a half-life of only a few seconds. It is not stored by the body but is synthesized as a result of activation. Nitrate drugs stimulate the formation and release of NO. Relaxation of smooth muscle cells in vessel walls leads to dilation of coronary arteries and peripheral veins. Peripheral vasodilation leads to a decrease in cardiac preload.
- IP3 (hydrophilic) diffuses into the cytoplasm → activation of IP3 receptors at the membrane of the endoplasmic reticulum (ER) → Ca2+ release from the ER via the IP3 receptor-coupled calcium channel → ↑ intracellular Ca2+ concentration → smooth muscle contraction
DAG (lipophilic) remains in the membrane → activates protein kinase C (PKC)
- PKC is Ca2+-dependent (depends on IP3-mediated Ca2+ release!)
- Mechanism of action: regulates the activity of various enzymes via phosphorylation of serine and threonine residues, e.g., regulatory proteins that influence cell growth (actin cytoskeleton) or differentiation (EGFR)
- Examples of the effect of PKC: cell growth and proliferation
Ca2+ as a second messenger
- Intracellular Ca2+ levels are usually very low.
- Second messengers (such as IP3) or depolarization by action potentials result in opening of the Ca2+ channels in the membrane of cells or ER → Ca2+ spikes (= strong peak concentration)
- Effect of Ca2+, especially as a complex with calmodulin: One molecule of calmodulin binds up to four Ca2+ ions → conformational change → regulation of calmodulin-dependent kinases/phosphatases, e.g., CaM kinase III (protein synthesis)
Examples of Ca2+-mediated effects
- ↓ Glycogen synthesis
- ↓ Cholesterol synthesis
Ca2+ mediates the effect of other second messengers such as IP3 and DAG via PKC activation. It also functions as a second messenger by acting directly, i.e., without activating another signaling molecule!