Signaling Pathways in Cellular Function

Overview of Signaling Pathways

  • Signaling pathways are crucial for regulating cellular functions, particularly gene transcription, cell division, and differentiation.

Cytokine Receptors and EGF

  • Cytokine Receptors: First discussed type of receptors; vital in cellular signaling.
  • EGF (Epidermal Growth Factor): Ligand that binds to its specific receptors on the plasma membrane to initiate signaling.

Mechanism of Action: MAP-Kinase Cascade

  • Activation of RAS:

    • RAS is a G-protein activated by binding to GTP.
    • Activation of RAS leads to a cascade involving multiple kinases.

  • RAF Activation:

    • RAF is a cytoplasmic protein held in an inactive state by phosphorylation.
    • Upon activation by RAS, RAF reshapes, allowing a phosphatase to remove inhibitory phosphates.

  • Kinase Cascade:

    • Active RAF phosphorylates MEK, which then phosphorylates MAP kinase (MAPK).
    • Each kinase in the cascade activates the next without direct transfer of phosphate groups.

Early Response Genes

  • Function of MAPK:

    • Activates transcription factors to initiate transcription of early response genes.
    • Early response genes prepare the cell for division by increasing nucleotides and proteins necessary for cell division.

  • Transcription Factors:

    • MAPK phosphorylates p90 ribosomal S6 kinase and TCF (ternary complex factor) to enhance gene transcription.
    • Importance: Ensures rapid and effective initiation of the cell cycle.

Delta-Notch Signaling Pathway

  • Overview: Controls differentiation of adjacent cells.
  • Delta Protein: A transmembrane signal molecule that binds to the Notch receptor on neighboring cells.
  • Mechanism:
    • Notch undergoes shape change upon binding, leading to proteolytic cleavage events.
    • Proteases Involved: ADAM 10 cleaves Notch; gamma secretase facilitates internal cleavage to release the Notch intracellular domain (NICD) that acts as a transcription factor in the nucleus.
  • Lateral Inhibition: One cell sends signals to inhibit differentiation of a neighboring cell, establishing different cell fates in development.

Wnt Pathway and Differentiation

  • McWnt Proteins: Secreted signaling molecules that bind to frizzled receptors, leading to transcription regulation.
  • Beta-Catenin: Acts as a transcription factor by entering the nucleus when not phosphorylated, avoiding ubiquitination and degradation.
  • Gradient Mechanism: Wnt signaling operates in gradients, influencing cell fate based on proximity and concentration:
    • High Wnt Concentration: Specify a tail cell fate.
    • Medium Wnt Concentration: Direct towards body cell fate.
    • Low to No Wnt Concentration: Induce head cell fate.
  • Morphogen: Wnt is classified as a morphogen since different concentrations can lead to distinct cellular outcomes.

Conclusion

  • Signaling pathways are crucial for cellular decision-making, including proliferation and differentiation.
  • The interplay between different pathways, such as RAS-RAF-MEK-MAPK and Delta-Notch, illustrates complex regulation involving phosphorylation and proteolysis.
  • Understanding these pathways is essential for insights into developmental biology and potential therapeutic targets in disease.