Common Cell Signaling Pathway

Cell Signaling Overview

  • Cells send out chemical signals that act on the receptors of other cells.

Classification of Signals

  • Signals can be classified based on the distance between the signaling cell and the target cell:
    • Autocrine Signals:
    • Definition: Signals produced by a cell that act on its own receptors, effectively signaling to itself.
    • Paracrine Signals:
    • Definition: Signals produced by a cell that act on nearby target cells.
    • Endocrine Signals:
    • Definition: Signals produced by a cell that travel to target cells that are located further away through the bloodstream.
Examples of Signals
  • Hormones:
    • Secreted into the bloodstream, acting on distant target cells.
  • Cytokines:
    • Released at sites of injury, acting on the brain to induce fever.

Properties of Signaling Molecules (Ligands)

  • Hydrophobic Molecules:

    • Characteristics: Tend to repel water, cannot freely float in extracellular spaces, require carrier proteins to be transported into target cells.
    • Function: Bind to receptor proteins that are located inside target cells (cytoplasm or nucleus).

  • Hydrophilic Molecules:

    • Characteristics: Can diffuse across cell membranes, bind to cell surface receptors for signaling.
    • Majority of signal molecules fall within this category, enabling them to bind to receptors on the extracellular surface of target cells.

Cell Signaling Pathway

  • The pathway can be broken down into three key stages:
    1. Reception:
    • Process: Target cell receptors bind to the ligand (analogous to a key fitting into a lock).
    1. Transduction:
    • Process: The receptor protein undergoes a change activating intracellular molecules known as second messengers.
    1. Response:
    • Process: The target cell responds to the signal.
Transmembrane Receptors

  • Receptors can be classified into three major classes:

    • G Protein Coupled Receptors (GPCRs):

    • Structure: Seven-pass transmembrane proteins with an extracellular domain that binds ligands, and an intracellular domain that activates G proteins.

    • Function:

      • G proteins consist of three subunits:
      • Alpha: Anchored to the cell membrane.
      • Beta and Gamma: Work together with alpha subunit.
      • Inactive State: Binds guanosine diphosphate (GDP).
      • Active State: When ligand binds, GDP is released, and guanosine triphosphate (GTP) binds; this causes the alpha subunit to dissociate from beta and gamma.
      • Each alpha subunit turns GTP back to GDP, re-closing the G protein complex.
      • Types of G Proteins:
      • Gq: Activates phospholipase C, cleaving phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3) and diacylglycerol (DAG).
        • IP3: Diffuses to the endoplasmic reticulum opening calcium channels causing calcium influx and cell depolarization.
        • DAG: Remains bound to the cell membrane and activates protein kinase C (PKC) upon rising calcium levels.
      • Gi: Inhibits adenylate cyclase, implementing negative feedback.
      • Gs: Stimulates adenylate cyclase, converting ATP to cyclic adenosine monophosphate (cAMP).
        • cAMP: Activates protein kinase A (PKA) by binding its regulatory subunit, allowing the catalytic subunit to function.

    • Enzyme Coupled Receptors:

    • Structure: Usually single-pass transmembrane proteins with intrinsic enzymatic activity, typically protein kinases.

    • Function:

      • Receptor Tyrosine Kinases (RTKs): Dimerize upon ligand binding, cross-phosphorylate each other, and activate downstream signaling.
      • Cytokine Receptors: Associate with cytoplasmic tyrosine kinases instead of having intrinsic activity.
      • Receptor Serine/Threonine Kinases: Phosphorylate target proteins upon dimerization.

    • Ion Channel Receptors:

    • Status: Generally closed but open upon ligand binding.

    • Function: Allow ions (e.g., Na+, K+, Ca2+) to passively flow across, leading to a shift in membrane potential impacting cellular responses.

Recap of Key Points

  • Signal Mechanisms: Autocrine signals affect the same cell; paracrine signals affect nearby cells; endocrine signals affect distant cells.
  • Ligand Types: Hydrophobic ligands can diffuse; hydrophilic ligands must bind to transmembrane receptors where signaling pathways are initiated.
  • Major Receptor Classes: G Protein Coupled Receptors, Enzyme Coupled Receptors, Ion Channel Receptors.