Cell Signaling Notes

Chapter 16: Cell Signaling

Intracellular Signaling Molecules: Functions

  • Intracellular signaling molecules perform several crucial functions:
    • Relay: Pass the signal onward.
    • Amplify: Turn one message into many through second messengers.
    • This involves a single signal leading to a cascade of events, greatly increasing the magnitude of the response.
    • Integrate: Detect signals from multiple pathways before relaying a message.
    • This function allows the cell to consider various inputs to coordinate an appropriate response.
    • Distribute: Regulate one or more effector proteins.
      • Signaling molecules can activate different effector proteins, leading to diverse cellular outcomes.

Diagram of Intracellular Signaling Pathways Components:

  • The diagram (Figure 15-55 from Molecular Biology of the Cell 6e) illustrates various components and their interactions within intracellular signaling pathways:
    • Signal Molecule: The initial trigger that binds to a receptor.
    • GPCR (G Protein-Coupled Receptor): A common type of receptor that activates G proteins.
    • RTK (Receptor Tyrosine Kinase): Another type of receptor that phosphorylates tyrosine residues on target proteins.
    • G protein: A protein that relays signals from GPCRs.
    • Phospholipase C: An enzyme activated by G proteins, leading to the production of second messengers.
    • Grb2: An adaptor protein that binds to activated RTKs.
    • PI 3-kinase: Phosphoinositide 3-kinase, which phosphorylates lipids to create docking sites for signaling proteins.
    • Diacylglycerol (DAG): A second messenger produced by phospholipase C that activates protein kinase C (PKC).
    • IP3 (Inositol Trisphosphate): A second messenger that triggers the release of Ca2+Ca^{2+} from the endoplasmic reticulum.
    • Adenylyl Cyclase: An enzyme that produces cyclic AMP (cAMP).
    • Ras-GEF (Sos): A guanine nucleotide exchange factor that activates Ras.
    • PI(3,4,5)P3: A phosphoinositide that acts as a docking site for signaling proteins.
    • Ras: A small GTPase involved in cell growth and differentiation.
    • MAP kinase kinase kinase: A kinase that phosphorylates and activates MAP kinase kinase.
    • Cyclic AMP (cAMP): A second messenger that activates protein kinase A (PKA).
    • Calmodulin: A calcium-binding protein that activates CaM-kinase.
    • PKA: Protein kinase A, which phosphorylates target proteins.
    • CaM-kinase: Calmodulin-dependent protein kinase, which phosphorylates target proteins.
    • PKC: Protein kinase C, which phosphorylates target proteins.
    • MAP kinase kinase: A kinase that phosphorylates and activates MAP kinase.
    • PDK1: Phosphoinositide-dependent kinase 1, which phosphorylates and activates Akt kinase.
    • MAP kinase: Mitogen-activated protein kinase, which phosphorylates transcription regulators and other target proteins.
    • Akt kinase: A kinase involved in cell survival and growth.
    • Transcription Regulators: Proteins that control gene expression.
    • Many Target Proteins: The ultimate effectors of the signaling pathway.

Integration of Multiple Extracellular Signals

  • Cells integrate multiple extracellular signals to produce a coordinated response.
  • Different signals (A, B, C, D) bind to receptors and activate different intracellular kinases.
  • These kinases (kinase 1, kinase 2) phosphorylate a target protein.
  • The combined effect of these phosphorylations determines the cell response.

Modulation of Signaling Pathways by Feedback

  • Signaling pathways are often modulated by feedback loops.
    • Positive Feedback: Pathway 1 stimulates the production of Y, which further activates pathway 1 (+).
      • This can lead to a strong, sustained response.
    • Negative Feedback: Pathway 2 stimulates the production of Y, which inhibits pathway 2 (T).
      • This provides a means for self-regulation and prevents over-activation.

GPCR Negative Feedback: Phosphorylation by Activated Kinases

  • Activated GPCRs can be desensitized through phosphorylation by GPCR kinases (GRKs).
  • Steps:
    • Activated GPCR stimulates GRK to phosphorylate it.
    • Arrestin binds to the phosphorylated GPCR at multiple sites.
    • This binding prevents further signaling by the GPCR.
    • ATPATP is converted to ADPADP during the phosphorylation by GRK.

Transcription Regulator Negative Feedback: NF-κB

  • NF-κB is a transcription regulator inhibited by IκBα.
  • Extracellular signals inhibit IκBα, leading to NF-κB activation.
  • Activated NF-κB induces increased transcription of IκBα, creating a negative feedback loop.
  • This loop helps to fine-tune the inflammatory response regulated by NF-κB.

Testing a Signaling Pathway Initiated by a New Drug

  • Hypothesis: Drug C protects against neurodegeneration by activating Nrf2.
  • Experiment: (The transcript does not provide details of the experiment or results.)
  • Result: (The transcript does not provide results.)
  • Conclusion: (A conclusion would be drawn based on the experimental results to support or refute the hypothesis.)