CM12 - Signaling Transduction

Biomedical Sciences I

What is cell signaling?

Complex cellular responses triggered by extracellular signals.

What is the role of signaling pathways?

• Cellular differentiation and growth

• Cell division

• Cell motility

• Apoptosis, necrosis, senescence

• Neurotransmission

• Metabolic regulation

• Muscle contraction

What are the key steps of signal-transduction pathways?

1. Release of primary signal (chemical messenger).

2. Reception of signal by membrane receptors or intracellular diffusion.

3. Conversion and amplification (transduction) of the signal by secondary messengers.

4. Activation/inhibition of effectors to induce a physiological response.

5. Termination of signal.

What are the signaling actions?

• Endocrine

• Paracrine

• Synaptic signaling

• Autocrine

• Cell-cell contact

What are the types of chemical messengers in the nervous system?

Neurotransmitters (acetylcholine, epinephrine, GABA) and neuropeptides.

What are the types of chemical messengers in the endocrine system?

Polypeptide hormones (insulin, glucagon), catecholamines (epinephrine), thyroid hormones, steroid hormones (cortisol, aldosterone, sex hormones).

What are the types of chemical messengers in the immune system?

Cytokines (interleukins, tumor necrosis factors, interferons) and chemokines.

What are eicosanoids?

Lipid-based signaling molecules including prostaglandins, thromboxanes, and leukotrienes.

What are growth factors?

Examples include epidermal growth factor (EGF) and platelet-derived growth factor (PDGF).

Which signals use intracellular receptors?

Hydrophobic signals (steroid hormones, thyroid hormones, retinoids). These diffuse across the membrane and interact with cytosolic/nuclear receptors.

How do intracellular receptors function?

Hydrophobic signals diffuse across membrane and interact with cytosolic/nuclear receptors. Ligand-receptor complexes function as transcription factors to induce gene expression.

Which signals use membrane receptors?

Hydrophilic ligands (polypeptide and monoamine hormones, growth factors, cytokines, acetylcholine).

How do membrane receptors signal?

Binding of a ligand induces a signaling cascade by:

• Altering the catalytic or regulatory activity of the receptor.

• Production of secondary messengers.

What are the classes of membrane receptors?

• Ion-channel receptors

• G-protein coupled receptors (heptahelical receptors)

• Kinase or kinase-binding receptors

Ion-channel receptors

• Ligands are neurotransmitters or neuropeptides

• Signal transduction is initiated by conformational changes induced by ligand binding.

G-protein coupled receptors (heptahelical receptors)

Signal transduction occurs through secondary messengers.

Kinase or kinase-binding receptors

Signal transduction is initiated by phosphorylation of receptor or receptor-associated proteins:

• Tyrosine kinase receptors

• JAK-STAT receptors

• Serine-threonine receptors

What is the structure of GPCRs?

Monomeric proteins with 7 transmembrane α-helical domains.

• Extracellular N-terminus binds a specific ligand.

• Intracellular C-terminus interacts with a heterotrimeric G-protein (α, β, γ subunits).

What happens when a ligand binds a GPCR?

Receptors undergo a conformational change → G-protein activation → production of secondary messengers.

What are the states of G-proteins?

• Inactive: α-subunit bound to GDP.

• Activation: Binding of ligand to receptor → exchange of GDP for GTP on Gα → conformational change in Gα → decreased affinity for receptor and βγ → complex dissociation.

How do activated G-proteins function?

Either activated Gα (bound to GTP) or released Gβγ can interact with effectors to induce production of intracellular secondary messengers.

How is G-protein signaling terminated?

Intrinsic GTPase activity of Gα hydrolyzes GTP to GDP → reassociation of heterotrimeric G-protein.

What are the types of trimeric G-proteins in humans?

• 21 Gα subunits encoded by 16 genes (some generated by alternative splicing) with specific functions.

• 6 Gβ subunits.

• 12 Gγ subunits.

• Gβγ subunits have interchangeable activities.

What is the function of secondary messengers?

Amplify the effects of extracellular signals.

How is cAMP produced?

Adenylyl cyclase catalyzes cAMP production from ATP in response to extracellular stimuli (glucagon, epinephrine).

What does cAMP do?

Induces the activation of protein kinase A (PKA).

How is cAMP activity terminated?

By hydrolysis of cAMP to 5’-AMP by cAMP phosphodiesterases.

How does cAMP activate PKA?

4 molecules of cAMP bind to the regulatory subunits of heterotetrameric PKA (R2C2: 2 regulatory, 2 catalytic subunits) → conformational change activates the 2 C subunits.

What does PKA do?

Substrate phosphorylation modulates the activity of transcription factors → induces or represses gene expression.

How is phosphoinositide signaling activated?

Receptors coupled to a Gαq subunit transmit activation signals (oxytocin, vasopressin, angiotensin II) to phospholipase C (PLC).

• PLC cleaves PIP2 (phosphatidylinositol 4,5-bisphosphate) into two secondary messengers:

• DAG (diacylglycerol) → remains in plasma membrane.

• IP3 (inositol triphosphate) → diffuses into cytosol.

What does DAG do?

Anchored in plasma membrane, activates protein kinase C (PKC). High cytosolic Ca2+ promotes interaction between inactive PKC and DAG.

What does IP3 do?

Opens ligand-gated Ca2+ channels in endoplasmic reticulum (ER) → mobilizes Ca2+ into cytoplasm.

What is the role of Ca2+ in signaling?

Ca2+ is a ubiquitous enzyme activator with tightly regulated low intracellular levels.

• 4 cytosolic Ca2+ bind to calmodulin, which activates target protein kinases.

What is the structure of RTKs?

Monomers with a single transmembrane α-helix:

• Extracellular ligand binding domain.

• Intracellular domain with weak tyrosine kinase activity.

How are RTKs activated?

• Ligand binds 2 RTK extracellular domains → homodimerizes the receptors.

• Kinase domains phosphorylate each other → activated.

• Activated kinase phosphorylates additional tyrosine residues in cytosolic domain.

• Creates binding sites for adaptor proteins → initiates signaling process.

What are examples of RTK pathways?

Ras-MAPK (cancer), insulin.

How do Jak-STAT receptors function?

• Ligand = cytokine.

• Receptors are monomers with no intrinsic activity before binding to Janus-like kinases (Jak).

• Ligand binding → homodimerization → activates Jak.

• Cross phosphorylation forms phosphotyrosine-binding sites for STATs.

• STATs phosphorylated by Jak → activated.

• Active STATs dissociate, dimerize, and form an active transcription factor.

What is the ligand for serine/threonine receptors?

Transforming growth factors (TGF).

How do serine/threonine receptors signal?

• TGF receptor complex = type I and type II subunits.

• Activated type II recruits a type I and phosphorylates its serine residue → activated receptor complex.

• Type I phosphorylates serines on Smad.

• Activated Smad dissociates and, with other Smads, enters the nucleus → acts as a transcription factor.

How quickly are different signals terminated?

• Rapid for metabolic signals.

• Slow for proliferative signals.

• Never for differentiation signals.

What are the levels of termination?

• Removal of stimulus.

• GTP hydrolysis for GPCR (by GTPases).

• Secondary messenger degradation (e.g., phosphodiesterases for cAMP).

• Removal of phosphorylation (by phosphatases).

Many chronic diseases result from inappropriate signal termination.

How are signaling pathways integrated in a cell?

Integration of signal pathways means that each cell has multiple signaling pathways happening at once, and these pathways often work together or influence one another to regulate cellular responses.