Cell Signaling III - signaling through receptors

Signaling Overview

  • Signaling through receptors: Involves extracellular signal molecules binding to receptor proteins, which then activate intracellular signaling pathways affecting target proteins that alter metabolism, gene expression, and cell shape/movement.

Types of Receptors

  • Cell-Surface Receptors: Located on the plasma membrane, and interact with hydrophilic signaling molecules.

  • Intracellular Receptors: Found within the cell and bind to small hydrophobic signal molecules which often require carrier proteins for cellular transport.

Molecular Switches in Signaling

  • Signaling pathways often involve molecular switches to activate proteins:

    • Phosphorylation: Reversible process involving the addition/removal of phosphate groups from ATP. Proteins in inactive state can be activated in this way.

    • GTP-binding Proteins: Similar to phosphorylation but involves GTP. Activation involves GDP-GTP exchange (binding of GTP and release of GDP), altering protein conformations to initiate signaling.

Second Messengers in Signaling

  • Important in signal amplification.

  • Types of secondary molecules such as:

    • 3',5'-Cyclic AMP (cAMP)

    • First 2nd messenger discovered by Sutherland.

    • Generated from ATP by adenylate cyclase which is activated by Gs (stimulatory G-protein) protein.

    • Targeted by phosphodiesterase, which degrades cAMP to AMP turning off the signal.

    • cAMP activates Protein Kinase A (PKA), leading to phosphorylation of target proteins.

    • 3',5'-Cyclic GMP (cGMP): Generated from GTP; also degraded by phosphodiesterase; activates PKG; involved in nitric oxide signally pathway.

    • Inositol 1,4,5-trisphosphate (IP3) and Diacylglycerol (DAG): Produced from phospholipase C-β (PLC) activity on phosphatidylinositol 4,5-bisphosphate (PIP2).

      • (aq subunit of G-protein activates PLCβ)

    • IP3 receptor is a ligand-gated channel in the membrane of ER. Binding of IP3 opens the channel and releases Ca2+ from ER. The released Ca2+ ions bind to calmodulin (CaM), activating CaM-dependent protein kinase II (CaMK-II).

    • Ca2+ induces a conformational change in CaMK-II and subsequent binding to target proteins.

    • Ca2+ signals are terminated (degraded) by Ca2+ pumps and exchangers.

    • DAG activates PKC in the presence of calcium released by IP3.

G-Protein-Coupled Receptors (GPCRs)

  • Large family of receptors (~800).

  • Activation of GPCRs involves the exchange of GDP for GTP on G-proteins.

  • Structure: Seven transmembrane domains; activated by various ligands including hormones (adrenaline) and neurotransmitters (glutamate).

  • Function: GPCRs are coupled to heterotrimeric G-proteins, facilitating downstream signaling once activated.

GPCR Activation Cycle

  1. Ligand binding causes a conformational change in the receptor.

  2. This activates associated G-proteins by exchanging GDP for GTP.

  3. Activated G-protein subunits relay the signal to effectors like adenylyl cyclase or phospholipase C.

  4. Subunits alpha, beta/gamma each signals for a different sets of responses.

cAMP Signaling Pathway

  • Generated by adenylyl cyclase from ATP.

  • Regulated by:

    • G-protein α subunits:

    • αs increases cAMP levels.

    • αi decreases cAMP levels.

  • cAMP activates protein kinase A (PKA) by freeing (dissociating) the catalytic (C) subunit from regulatory (R) subunit. The catalytic subunit (C) is what phosphorylates target proteins.

  • Degradation of cAMP is performed by phosphodiesterases.

    • Example: Adrenaline activates β-adrenergic receptor (a GPCR) and the cAMP pathway in the heart.

Calcium Signaling

  • Calmodulin Activation: Ca2+ binds to calmodulin, leading to changes in protein conformation that activates downstream targets such as calcium-dependent protein kinases.

  • Termination: Ca2+ levels are regulated by pumps and exchangers; actions cease when levels normalize.

Receptor Tyrosine Kinases (RTKs) - a smart protein

  • Different structure from GPCRs, typically one or two transmembrane regions.

  • Often activated by growth factors like insulin, epidermal growth factor, fibroblast growth factor.

  • RTKs dimerize upon ligand binding, activating their intrinsic kinase activity (autophosphorylation).

  • Autophosphorylation leads to the recruitment of adaptor proteins (like Grb-2) and activation of downstream proteins e.g. Ras, a monomeric G-protein, instigating a MAPK signaling cascade. This is known as a transducer event.

Mitogen Activated Protein Kinase (MAPK) Pathway

  • Initiates following activation of RTKs.

  • Involves a cascade of phosphorylation events starting with MAPK kinase kinase (MAP3K), then MAPK kinase (MAP2K), and finally MAPK itself.

  • MAPKKK e.g. Raf

  • MAPKK e.g. MEK

  • MAPK e.g. ERK p38 and JNK, which are involved in various cellular responses including proliferation, differentiation (particularly mesoderm induction during early development), and stress responses.