Cell Communication

Cell Communication

  • Importance of Cell Communication

    • Essential for multicellular organisms

    • Universal mechanisms of cellular regulation identified by biologists

    • Combined effects of multiple signals determine cell response

    • Example: Dilation of blood vessels controlled by multiple signaling molecules

Evolution of Cell Signaling

  • Signal Transduction Pathway

    • Series of steps converting a signal on a cell surface into a cellular response

    • Converts signals on a cell’s surface into specific cellular reactions

Local and Long-Distance Signaling

  • Chemical Messengers for Communication

    • Animal and plant cells communicate via chemical messengers

    • Cell junctions connect adjacent cell cytoplasm

    • Local Signaling

      • Involves direct contact or cell-cell recognition

    • Long-Distance Signaling

      • Involves chemicals called hormones

Types of Signaling

  • Local Signaling

    • Involves local regulators that travel short distances

    • Examples:

      • Paracrine signaling

      • Synaptic signaling

  • Long-Distance Signaling

    • Hormonal signals travel through bloodstream to target cells

The Three Stages of Cell Signaling

  • Processes Involved

    • Discovered by Earl W. Sutherland with epinephrine

    • Stages:

      • Reception

      • Transduction

      • Response

Reception Phase

  • Binding of Signal Molecule

    • Signal molecule (ligand) binds to a receptor protein, causing shape change

    • Highly specific binding between ligand and receptor

    • Initial transduction of the signal occurs through shape change

    • Most receptors are plasma membrane proteins

Receptors in the Plasma Membrane

  • Types of Membrane Receptors

    1. G Protein-Coupled Receptors

    2. Receptor Tyrosine Kinases

    3. Ion Channel Receptors

G Protein-Coupled Receptors

  • Works with G proteins as on/off switches

  • Inactive when GDP is bound

Receptor Tyrosine Kinases

  • Attach phosphates to tyrosines

  • Can trigger multiple signal pathways simultaneously

Ion Channel Receptors

  • Change shape to open gates for specific ions when signal molecule binds

  • Example: Sodium (Na+) or Calcium (Ca2+) ions

Intracellular Receptors

  • Found in cytosol or nucleus, activated by small/hydrophobic messengers

  • Examples: Steroid and thyroid hormones

  • Can act as transcription factors to turn on specific genes

Transduction Phase

  • Molecular Interaction Cascades

    • Signal transduction involves multiple steps

    • Amplifies signal, facilitating greater cellular response

    • Allows coordination and regulation of cellular response

Protein Phosphorylation and Dephosphorylation

  • Role of Kinases and Phosphatases

    • Protein kinases transfer phosphates from ATP to proteins (phosphorylation)

    • Protein phosphatases remove phosphates (dephosphorylation)

    • Acts as a molecular switch regulating activity

Second Messengers

  • Role in Signaling Pathways

    • First messenger binds to receptor; second messengers are small, nonprotein molecules

    • Example second messengers: Cyclic AMP (cAMP), Calcium ions (Ca2+)

Cyclic AMP

  • Widely used second messenger

  • Produced by adenylyl cyclase from ATP in response to signals

Calcium Ions and Inositol Triphosphate (IP3)

  • Calcium acts as second messenger; can be regulated in cells

  • Phospholipase C pathway generates IP3 as a second messenger

Cellular Responses

  • Outcome of Cell Signaling

    • Result in transcription regulation or cytoplasmic activities

    • Responses may occur in nucleus or cytoplasm involving gene synthesis

    • Final activated molecule can function as transcription factor

Fine-Tuning Responses

  • Benefits of Multistep Pathways

    • Amplification of signals

    • Specificity in cell signaling

Termination of Signal

  • Inactivation Mechanisms

    • Essential for stopping signaling when molecules leave receptors

    • Ensures receptors return to inactive state

Apoptosis (Programmed Cell Death)

  • Integrates multiple signaling pathways

  • Important for development and maintenance of organisms

  • Results in cellular cleanup by scavenger cells

Apoptotic Pathways

  • Triggered by death-signaling ligands, DNA damage, or protein misfolding

  • Involve main proteases called caspases that execute apoptosis

Summary

  • Apoptosis' Role in Development and Disease

    • Important in embryonic development (e.g., C. elegans)

    • Involved in diseases like Parkinson’s, Alzheimer’s, and cancer