L23 cont.

Overview of RTK Signaling Pathways

  • RTK Signaling refers to the signaling mechanisms activated by receptor tyrosine kinases (RTKs), which play a critical role in cell proliferation, differentiation, survival, and migration.

General Topics Discussed

  • Stem Cells: Characterization, differentiation, and tissue formation.

  • RTK Signaling Pathway Examples:
      - Ras/MAPK Pathway
      - PI3K/Akt Pathway
      - Roles of Phosphatidylinositol Phosphates (PIPs) in signaling.

Ras/MAPK Signaling Pathway

  • The Ras/MAPK signaling cascade is crucial for regulating a variety of cellular processes, such as proliferation and migration.

Components of Ras/MAPK Pathway

  1. Key Proteins:
       - RAS (Ras protein family):
         - Interacts with GRB2 protein.
         - Activates upon exchanging GDP for GTP.
         - Activates downstream signaling towards MAPK cascade.
       - GRB2: Adaptor protein that links RTKs to the Ras pathway.
       - SOS: Guanine nucleotide exchange factor (GEF) that activates Ras.
       - RAF: First kinase in the MAPK pathway that gets activated by Ras.
       - MAPKK: Dual-specificity kinase that activates MAPK.
       - MAPK: Mitogen-activated protein kinase that translocates to the nucleus to modulate transcription.
       - AP-1: A transcription factor activated downstream of MAPK pathways.

  2. Functions:
       - Promotes cell proliferation, survival, and migration.

  3. Receptors that activate Ras/MAPK Pathway:
       - EGFR (Epidermal Growth Factor Receptor)
       - IR (Insulin Receptor)
       - FGFR (Fibroblast Growth Factor Receptor)
       - PDGFR (Platelet-Derived Growth Factor Receptor)
       - VEGFR (Vascular Endothelial Growth Factor Receptor)

PI3K/Akt Signaling Pathway

  • The PI3K/Akt pathway is essential for cell growth and survival, and it is activated by various growth factors.

Key Components of the PI3K/Akt Pathway

  1. Phosphoinositide 3-Kinase (PI3K):
       - Responsible for phosphorylating phosphatidylinositol (PI) to produce PI(3,4,5)P3.
       - Class IA PI3K is often activated by RTKs.

  2. Protein Kinase B (Akt):
       - Activated by docking to PI(3,4,5)P3.
       - Involved in promoting cell survival and growth signals.
     

  3. Multiple Steps in Activation:
       - 1. Activation of RTK leads to its phosphorylation.
       - 2. Docking of PI3K to the phosphorylated RTK.
       - 3. Generation of PI(3,4,5)P3, creating binding sites for kinases.

  4. Molecular Mechanisms:
       - PDK1 is activated by PI(3,4,5)P3 and phosphorylates Akt on threonine residues.
       - mTORC2 is a critical complex involved in Akt activation.

Functions

  • Promotes cell survival and prevents apoptosis by phosphorylating and inhibiting pro-apoptotic proteins such as Bad, which facilitates the activation of anti-apoptotic proteins like Bcl-2.

Phosphatidylinositol and Phosphoinositides (PIPs)

  • PIPs are crucial in cellular signaling, where their phosphorylation states can determine their downstream effects.

Characteristics of PIPs

  1. Structure:
       - Composed of an inositol sugar that can be phosphorylated at positions 3, 4, and 5.
       - Various phosphorylated forms include:
         - PI(3,4)P2: Phosphoinositide (3,4) bi-phosphate.
         - PI(3)P: Phosphoinositide (3) mono-phosphate.

  2. Modifications:
       - Red arrows indicate modifications by phosphorylation (kinase activity).
       - Green arrows indicate modifications by dephosphorylation (phosphatase activity).

  3. Functional Importance:
       - PIP compositions act as important markers for specific organelles and functional proteins, driving localization and signaling outcomes in the cell.

Summary of RTK Mechanisms

  1. Dimerization and Autophosphorylation:
       - Upon ligand binding, RTKs dimerize and undergo autophosphorylation which activates their kinase activity.

  2. Docking Proteins:
       - Utilize specific recognition domains for interaction with phosphorylated tyrosines (pY):
       - SH2 domain: Binds to phospho-tyrosine motifs.
       - SH3 domain: Binds to proline-rich regions.
       - PH domain: Binds to phosphorylated inositol lipids.

  3. Intracellular Complex Formation:
       - Assembly of signaling complexes is vital for effective transduction of extracellular signals.

Knowledge Terms and Definitions

  • Enzyme-Coupled Receptors: Proteins that facilitate signal transduction through enzymatic activity, typically involving phosphorylation.

  • Trans-autophosphorylation: Process wherein receptor kinases phosphorylate themselves upon activation.

  • Immunoprecipitation (IP): Technique used to isolate specific proteins from a mixture by exploiting antigen-antibody interactions.

Study Questions

  1. Define enzyme-coupled receptors and their biological role.

  2. Detail the activation process of RTKs and their significance.

  3. Discuss ligands and receptors that act as enzyme-coupled receptors.

  4. Analyze the experimental use of radioactive labeled 32P ATP in signaling studies.

  5. Explain ligand competition experiments and their relevance.

  6. Identify and elaborate on specific protein domains integral to cellular signaling.

  7. Describe the concept of trans-autophosphorylation in receptor activation.