Signaling 2

Steps of Signaling

  1. Ligand/Receptor Interaction:

    • A ligand binds to its specific receptor, which can be on the cell surface (e.g., GPCRs, RTKs) or intracellular (e.g., steroid hormone receptors).

    • Binding induces conformational changes in the receptor, initiating the signaling process.

  2. Signal Transduction:

    • Conversion of the extracellular signal into an intracellular message.

    • Typically involves secondary messengers like cAMP or Ca²⁺ and proteins like kinases and phosphatases.

  3. Signal Amplification:

    • One ligand-receptor interaction can activate multiple downstream molecules, leading to a significant cellular response.

    • Example: Activation of adenylate cyclase by GPCRs increases cAMP levels, amplifying the signal.

  4. Effectors:

    • Proteins or enzymes that bring about the cellular response. For example:

      • Ion channels

      • Transcription factors

      • Metabolic enzymes

  5. Attenuation and Termination:

    • Mechanisms to stop signaling include:

      • Dephosphorylation of RTKs: Phosphatases remove phosphate groups, inactivating the receptor.

      • Endocytosis: Internalization and degradation of receptors.

      • Re-association of heterotrimeric G proteins: Gα reassociates with Gβγ, inactivating the signal.

      • GPCR desensitization: GPCR kinases (GRKs) phosphorylate the receptor, followed by arrestin binding and internalization.

Types of Signaling

  1. Endocrine:

    • Features: Hormones travel long distances through the bloodstream.

    • Examples: Insulin, epinephrine.

  2. Paracrine:

    • Features: Signals affect nearby cells.

    • Examples: Growth factors, neurotransmitters in a synaptic cleft.

  3. Neuronal:

    • Features: Rapid, specific signals sent via axons and synapses.

    • Examples: Neurotransmitter release.

  4. Contact-Dependent:

    • Features: Requires direct physical contact between cells.

    • Examples: Notch-Delta signaling.

Steroid Hormone Signaling

  • Mechanism:

    • Steroid hormones (e.g., cortisol, estrogen) diffuse across the plasma membrane and bind to intracellular receptors.

    • Hormone-receptor complexes act as transcription factors in the nucleus, regulating gene expression.

  • Distinct Features:

    • Direct gene regulation without reliance on second messengers.

    • Long-lasting effects compared to rapid GPCR signaling.

Processes Controlled by TOR Kinase and MAPK Kinases

  1. TOR Kinase:

    • Controls cell growth and metabolism.

    • Regulates:

      • Protein synthesis.

      • Autophagy inhibition.

      • Nutrient sensing.

  2. MAPK Kinases:

    • Part of a kinase cascade that regulates cell proliferation, differentiation, and stress responses.

    • Activated by Ras and leads to phosphorylation of transcription factors.

Integrins

  • Structure:

    • Heterodimers composed of α and β subunits.

    • Bind to extracellular matrix components (e.g., fibronectin, collagen).

  • Function:

    • Facilitate cell migration, adhesion, and signaling to the cytoskeleton.

    • Engage focal adhesion complexes to signal intracellular responses.

GPCR and Heterotrimeric G Proteins

  1. GTP Binding:

    • Gα subunit binds GTP when active and GDP when inactive.

  2. Regulation:

    • On: GTP-bound state.

    • Off: GDP-bound state.

  3. Protein Interactions:

    • GPCRs act as GEFs (guanine nucleotide exchange factors), activating G proteins.

    • GAPs (GTPase-activating proteins) hydrolyze GTP to inactivate G proteins.

GPCR Signaling

  1. cAMP Pathway:

    • GPCR activates adenylate cyclase, converting ATP to cAMP.

    • cAMP activates PKA, which phosphorylates target proteins.

    • Termination: Phosphodiesterase converts cAMP to AMP.

  2. IP3 and DAG Pathway:

    • GPCR activates PLC, which cleaves PIP2 into IP3 and DAG.

    • IP3: Releases Ca²⁺ from the ER.

    • DAG: Activates PKC, which phosphorylates target proteins.

  3. Ion Channel Activation:

    • GPCRs can indirectly regulate ion channels via second messengers.

RTK Signaling

  1. Signal to Ras and MAPK Cascade:

    • Ligand binding causes dimerization and autophosphorylation of RTKs.

    • RTKs recruit and activate Ras via adaptor proteins and GEFs.

    • Active Ras triggers the MAPK cascade, leading to transcriptional changes.

GEFs and GAPs

  • GEFs (Guanine Exchange Factors):

    • Activate GTPases by promoting the exchange of GDP for GTP.

  • GAPs (GTPase-Activating Proteins):

    • Inactivate GTPases by increasing the hydrolysis of GTP to GDP.

Calmodulin

  • Activation:

    • Activated by binding to Ca²⁺ ions.

  • Role:

    • Mediates calcium signaling by activating various enzymes and kinases (e.g., CaMK).

Notch-Delta Signaling

  • Biological Purpose:

    • Regulates cell differentiation during development.

  • Type:

    • Contact-dependent signaling.

Signaling Pathways: Mechanisms

  1. GDP/GTP Binding:

    • Regulates G proteins and Ras activity.

  2. Phosphorylation/Kinase Cascades:

    • Sequential activation of kinases (e.g., MAPK cascade).

  3. Phosphoproteins:

    • Modify protein activity via addition/removal of phosphate groups.