NOTES: CH 13 - Signaling

Chapter 13: Signal-Transduction Pathways
13.1 Signal Transduction Depends on Molecular Circuits

  • Basic Components of Signal-Transduction Pathways:
    Signal transduction pathways are essential for cellular communication and involve several critical components that facilitate the process.

    1. Release of Primary Messenger:

      • Triggered by various physiological stimuli such as hormones, neurotransmitters, or environmental signals, which initiate the signaling process. These primary messengers are often water-soluble and include molecules such as peptides, amino acids, and gases.

    2. Reception by Receptor:

      • Depending on the type of primary messenger, receptors are typically integral membrane proteins that specifically bind to the messenger. These receptors can be classified into several types, including ligand-gated ion channels, G-protein-coupled receptors (GPCRs), and enzyme-linked receptors.

    3. Relay to Cell Interior:

      • Following ligand-receptor binding, the signal must be relayed into the cell. This often involves the generation of an intracellular second messenger, which amplifies the signal and facilitates rapid cellular responses. Common second messengers include cyclic AMP (cAMP), inositol trisphosphate (IP3), and calcium ions (Ca2+).

    4. Activation of Effector Molecules:

      • The activation of downstream effector molecules can result in a diverse range of physiological responses, such as changes in gene expression, alterations in cellular metabolism, and modifications of the cytoskeleton. Effectors typically include protein kinases, phosphatases, and other enzymes that modulate cellular functions.

    5. Termination of Signal Cascade:

      • To prevent overactivity and ensure precise control, the signaling cascade must be terminated. This can happen through several mechanisms such as dephosphorylation, hydrolysis of second messengers, and internalization or degradation of receptors.

13.2 Receptor Proteins Transmit Information into the Cell
Type 1: Seven-Transmembrane-Helix Receptors

  • Types of Membrane Receptors:

    • These receptors play crucial roles in sensing extracellular signals and are characterized by having seven transmembrane spanning helices.

    1. Seven-transmembrane-helix Receptors (GPCRs):

      • These receptors respond to a multitude of signal molecules, including hormones like epinephrine and glucagon. Their activation results in conformational changes that allow for effective signal transduction.

    2. Dimeric Receptors that Recruit Protein Kinases:

      • An example is the human growth hormone receptor, which upon ligand binding activates associated protein kinases, initiating a cascade that regulates growth and metabolism.

    3. Dimeric Receptors that Are Protein Kinases:

      • The insulin receptor serves as an example, functioning intrinsically as a protein kinase to mediate glucose uptake and metabolic regulation.

  • Functionality:

    • The operation of these receptor types showcases the diversity in cellular signaling. Ligand binding typically induces a structural change in the receptor that is transmitted to the intracellular domain, activating downstream signaling pathways.

Ligand Binding Activates G Proteins

  • Activation Process:

    • For example, the β-adrenergic receptor responds to epinephrine, activating the associated G-protein. This involves the exchange of GDP for GTP on the Gα subunit, leading to dissociation from the βγ dimer. The activated Gα subunit transmits the signal internally.

G Proteins Transmit Signals

  • Activation of Adenylate Cyclase:

    • Activated Gαs stimulates adenylate cyclase to convert ATP into cyclic AMP (cAMP), serving as a crucial second messenger in numerous signaling pathways.

  • cAMP Activation of Protein Kinase A (PKA):

    • cAMP binds to the regulatory subunits of PKA, triggering a conformational change that releases the catalytic subunits, which then phosphorylate target proteins, leading to physiological responses.

Termination of the Signal Cascade

  • Mechanisms:

    1. Hydrolysis of GTP to GDP by Gα leads to G protein re-association and inactivation.

    2. Phosphodiesterase enzymes convert cAMP to AMP, thus decreasing its levels in the cell and altering downstream signal effects.

    3. Epinephrine-receptor interactions can be reversed either by receptor desensitization or by the cellular uptake and degradation of the ligand.

Clinical Insight: Cholera

  • Overview of Cholera:

    • Cholera is caused by the toxin choleragen produced by Vibrio cholerae, leading to the permanent activation of Gαs, resulting in continuous production of cAMP. This disrupts intestinal ion transport, causing massive loss of NaCl and water, leading to severe dehydration and diarrhea.

The Hydrolysis of Phosphatidylinositol Bisphosphate

  • Second Messengers Generated:

    • When phospholipase C is activated, it hydrolyzes phosphatidylinositol bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG), both of which serve as crucial second messengers in various pathways.

Role of IP3 and DAG in Signaling

  • IP3 Functionality:

    • IP3 binds to receptors on the endoplasmic reticulum, prompting the release of stored Ca2+ into the cytosol, which further propagates the signaling cascade.

  • DAG Functionality:

    • DAG, together with Ca2+, activates protein kinase C (PKC), which plays a vital role in various signaling pathways, including those regulating cell growth, differentiation, and gene expression.

13.3 Type 2: Dimeric Receptors That Recruit Protein Kinases

  • Human Growth Hormone Receptor:

    • This receptor is initially a monomer that undergoes dimerization upon hormone binding, which brings the kinase domains into proximity, activating JAK2 (Janus kinase 2) and initiating a downstream signaling cascade that influences growth and metabolism.

Receptor Dimerization and Activation

  • JAK2 Activation:

    • Activated JAK2 phosphorylates partner tyrosine residues, which creates docking sites for signaling molecules such as transcription factors like STAT5, resulting in altered gene expression profiles influenced by growth hormone.

13.4 Type 3: Insulin Receptor

  • Insulin Functionality:

    • The insulin receptor is a receptor tyrosine kinase that can dimerize, even in the absence of insulin. Insulin binding enhances its intrinsic kinase activity, leading to downstream signaling that promotes glucose uptake, lipid synthesis, and cell growth.

Insulin Signaling Pathway

  • Phosphoinositide-3 Kinase (PI3K) Activation:

    • Activated by insulin receptor substrates (IRS), PI3K catalyzes the conversion of PIP2 to PIP3, activating downstream kinases including Akt, which is involved in cellular metabolism, survival, and growth regulation.

Termination of Insulin Signaling

  • Mechanisms:

    • Phosphatases dephosphorylate key signaling intermediates, and lipid phosphatases convert PIP3 back to PIP2, effectively terminating the insulin signaling pathway.

Quick Quiz 2

  • Insight Question:

    • How does increasing the number of glucose transporters in the cell membrane enhance cellular glucose uptake in response to insulin? Discuss the physiological implications of glucose homeostasis.

General Steps of a Signal Transduction Pathway:

  1. Release of Primary Messenger: Triggered by physiological circumstances.

  2. Reception by Receptor: Typically an integral membrane protein.

  3. Relay to Cell Interior: Involves generation of an intracellular second messenger.

  4. Activation of Effector Molecules: Result in physiological responses.

  5. Termination of Signal Cascade: Ends the response initiated by the primary messenger.

Three Major Types of Membrane Receptors:

  1. Seven-transmembrane-helix Receptors (GPCRs): Examples: Epinephrine, glucagon receptors.

  2. Dimeric Receptors that Recruit Protein Kinases: Example: Human growth hormone receptor.

  3. Dimeric Receptors that Are Protein Kinases: Example: Insulin receptor.

Mechanisms of Receptors:

  • Epinephrine Receptors (GPCR): Ligand binding activates G proteins, stimulating adenylate cyclase to produce cAMP, activating PKA, which leads to physiological changes in target cells.

  • Human Growth Hormone Receptor: Dimerizes upon hormone binding, activating JAK2 which phosphorylates downstream signaling factors, thus regulating various biological responses.

  • Insulin Receptor: Dimerizes continually, activating its kinase activity upon insulin binding, leading to downstream effects including the activation of PI3K, which is pivotal for metabolic regulation.

Regulation of Protein Kinase A:

  • The regulation of PKA is crucial for cellular signaling; cAMP binds to its regulatory subunits, thereby releasing and activating the catalytic subunits to modulate target enzyme activities.

Cholera Mechanism of Disease:

  • Cholera’s pathological mechanism is linked to choleragen, which traps Gαs in the active state, leading to uncontrolled adenylate cyclase activation, resulting in massive ion loss and dehydration due to overwhelming diarrhea.