Cell Signaling Summary

  • Cell Signaling Overview

    • Cells detect and respond to environmental changes.
    • In multicellular organisms, there's intercellular communication for growth and division coordination.

  • Types of Cell Signaling

    1. Endocrine Signaling: Hormones released into the bloodstream, affecting distant target cells with specific receptors.
    2. Paracrine Signaling: Local chemical signals acting on nearby cells.
    3. Neuronal Signaling: Electrical signals along axons releasing neurotransmitters to adjacent cells.
    4. Contact-Dependent Signaling: Signal molecules on one cell bind receptors on another adjacent cell.

  • Signaling Molecule Actions

    • Must bind to receptors; hydrophobic molecules can cross the membrane to bind to internal receptors.
    • Same molecule can induce different responses in distinct cell types (e.g., acetylcholine).

  • Response Speed

    • Fast responses: neuronal signaling (ion channels).
    • Slow responses: gene transcription (more steps involved).
    • Signal complexity includes amplification, integration, divergence, and feedback mechanisms.

  • Feedback Mechanisms

    • Positive Feedback: Amplifies the signal.
    • Negative Feedback: Reduces the signal.

  • Molecular Switches

    • Proteins can toggle between "on" and "off" states (e.g., via phosphorylation).
    • GTP-binding proteins (GTPases) switch states with GAP (turns off) and GEF (turns on).

  • Cell Surface Receptor Classes

    1. Ion Channel Coupled Receptors: Open in response to ligand binding.
    2. G-Protein Coupled Receptors (GPCRs): Seven transmembrane helices that activate G-proteins upon ligand binding, leading to signal pathways and second messenger production (e.g., cAMP).
    3. Enzyme Coupled Receptors: Activate enzymatic functions upon ligand binding; e.g., Receptor Tyrosine Kinases (RTKs).

  • Receptor Tyrosine Kinases (RTKs)

    • Dimerize and phosphorylate each other, creating docking sites for signaling proteins.
    • Often engage downstream signaling cascades (e.g., activating Ras).

  • Signal Amplification and Regulation

    • Kinase cascades allow multiple regulation opportunities and signal amplification at each step.

  • Intracellular Receptors

    • Hydrophobic molecules (steroid hormones) cross the membrane, bind internal receptors, and regulate transcription in the nucleus.

  • Integration of Signaling Pathways

    • Multiple pathways may converge on a target protein, with final outcomes depending on various signals’ strengths and amounts.

  • Cell Survival Signals

    • Many cells require continuous signaling to survive; lack of signals can trigger apoptosis (programmed cell death).