Cell Signaling Notes

Cell Signaling Notes

I. Overview of Multicellularity and Cell Communication

  • Multicellularity: Human body consists of trillions of cells and more than 200 specialized cell types.
    • Differentiation and communication among cells are essential for coordination of:
    • Physiology and metabolism
    • Behavior
    • Growth, proliferation, and differentiation
    • Cancers: Often arise from failures in cell signaling.
    • Signalers: Variety of signaling molecules, from gases to macromolecules.

II. General Principles of Communication

  • Cells communicate through two primary mechanisms:
    1. Nerves
    2. Hormones
    • Hormones are distributed throughout the bloodstream, exposing every cell.
    • Response: Only cells with specific hormone receptors respond to these hormones.
    • Far Signaling: Hormonal signaling over long distances.

III. Types of Signaling Mechanisms

A. Chemical Signaling
  • Chemical Signaling: Impacts cells in close proximity.
    • Characterized by a short half-life due to:
    • Local uptake
    • Destruction
    • Extracellular matrix (ECM) associations limiting diffusion.
    • Autocrine Signaling: Chemical affects the releasing cell itself (e.g., T-cells).
B. Neuronal Signaling
  • Nerve Cell Development: Uses contact-dependent signaling mechanisms to guide development.
    • Involves:
    • Signal Molecules: Membrane-bound inhibitory signals like Delta.
    • Receptors: Notch receptor in target cells responds to Delta.
    • Interaction leads to inhibition of epithelial cells from developing into nerve cells.
C. Contact-dependent Signaling
  • Involves direct interactions between signaling cell and target cell through membrane-bound signals.

IV. Cell Signal Response

  • Cells respond to a finite number of signals based on receptor types.
    • Example: Acetylcholine (ACh) induces cell-specific responses depending on receptor types.
    • Agonist: Binds and activates the receptor.
    • Antagonist: Binds but does not activate the receptor.
    • Cellular response regarding survival, division, differentiation, or apoptosis can be influenced by signal combinations.
    • Signaling often involves a lack of signals leading to apoptosis.

V. Speed of Cell Response

  • The speed of response varies (fast vs slow) based on:
    • Existing proteins already present within the cell.
    • Fast Responses: Occur within seconds to minutes, involving:
    • Altered protein function.
    • Immediate changes in cell behavior.
    • Slow Responses: Take minutes to hours, involving changes in gene expression and cytoplasmic machinery.

VI. Types of Receptors

  • Receptor Classification: Based on solubility of signalers.
    • Lipophilic Hormone Receptors: Can lead to nuclear responses via gene transcription.
    • Non-Lipophilic Hormones: Use second messenger systems to induce transcription.
A. Lipophilic Hormone Mechanism
  • Requires carrier proteins for transport in blood.
  • Binding to intracellular receptors induces dimerization, leading to gene regulation.
    • Results in long-lasting responses, amplifying effects via mRNA production and protein synthesis.
B. Non-lipophilic Hormones
  • Utilize second messenger systems to transduce signals intracellularly.

VII. Role of Gases in Signaling

  • Gases can mediate signaling by acting as soluble signaling molecules.
    • Example: Acetylcholine (ACh) from the PNS generates nitric oxide (NO), facilitating vascular smooth muscle relaxation.
    • cGMP serves as a second messenger leading to vasodilation and decreased blood pressure (BP).

VIII. Plasma Membrane Receptor Transduction

  • Plasma membrane receptors transduce signals from extracellular messengers.
    • Examples include signaling molecules such as calcium ions (Ca2+Ca^{2+}), cyclic AMP (cAMP), and diacylglycerol (DAG).
    • Transducers: Proteins acting to convert signals from one form to another.
    • Amplifying proteins typically involved with ion channels or enzymes to significantly increase the magnitude of signals.

IX. Relaying and Amplifying Signals

  • Reception: Starts upon ligand binding to the receptor.
  • Transduction: Small amounts of first messengers activate a cascade leading to extensive signal amplification through multiple pathways.
    • Graph illustrating the signaling pathway shows how a single molecule can activate a series of proteins involving different magnitudes of activation (e.g., 1 receptor molecule activates 10^2 of G-proteins, leading to 10^6 responses).

X. Differences Between ATP and GTP in Signaling

  • ATP vs. GTP: Differences in signaling functions and mechanisms.
    • ATP: Generally transfers phosphate groups to activate or deactivate signaling molecules.
    • GTP: Exchanged by G-proteins, where active G-protein subunits generate signals.
    • G-proteins possess intrinsic GTPase activity, which helps regulate their signaling states with GEFs and GAPs modulating their intrinsic activity.

XI. Types of Plasma Membrane Receptors

  1. Ion-Channel-Coupled Receptors: Mainly respond to ligand signals; can open and activate ion channels rapidly.
  2. G-Protein-Coupled Receptors: Utilize G-proteins to transduce signals and manifest cellular responses.
  3. Enzyme-Coupled Receptors: Function as enzymes or associate closely with them, primarily requiring dimerization for activation.

XII. Ion Channels

  • Types of Ion Channels:
    1. Ligand-gated: Open upon ligand binding (milliseconds).
    2. Voltage-gated: Open in response to specific changes in membrane potential (milliseconds).
    3. Second messenger-gated: Open via intracellular signals (seconds); e.g., inositol trisphosphate (IP3)-induced calcium channels from the endoplasmic reticulum.

XIII. G-Protein-Coupled Receptors

  • Structure: Seven transmembrane segments, heterotrimeric G-proteins consisting of alpha, beta, and gamma subunits.
    • G-proteins activate target proteins within the plasma membrane using GTP.
    • Final activation of target proteins displaces GDP from the G-protein subunit, leading to signal amplification.

XIV. Activation of Target Proteins by G-Proteins

  • The activated subunit can interact with various target proteins, modulating their activities based on previous interactions and states.
  • Important for understanding regulatory mechanisms and physiological responses.

XV. G-Protein Targets and Second Messengers

  • Key pathways involving G-proteins include adenylyl cyclase activation and phospholipase C activity.
    • These enzymes generate second messengers that disseminate signals across the cell.

XVI. Adrenaline and its Responses

  • Adrenaline (epinephrine) elicits both fast and slow responses within cells, mediated via second messenger systems.
    • Fast Response: Through adenylyl cyclase leading to rapid ATP production and glycogen breakdown.
    • Slow Response: Modulates gene expression through cAMP-dependent transcription.

XVII. cAMP Levels and Fluorescent Protein Use

  • Techniques for monitoring cAMP production include using fluorescent proteins that emit light in relation to intracellular cAMP levels.

XVIII. Activation of Phospholipase C (PLC) Pathway

  • The PLC pathway is activated by the G-protein coupling, regulating other intracellular signals via inositol 1,4,5-trisphosphate (IP3) and calcium ions.

XIX. Intracellular Calcium and Calmodulin Activation

  • Calcium plays a critical role in signaling and mediates effects through proteins like calmodulin.
    • Calmodulin undergoes conformational changes upon binding to calcium, activating specific protein kinases.
    • This interaction leads to broad physiological responses, emphasizing the importance of calcium in cellular signaling.

XX. G-Protein Regulation in Vision

  • Mechanisms of vision regulation are a prime example of G-protein signaling.
    • In darkness, Na+ channels are open, allowing signaling; upon light exposure, channels close leading to the hyperpolarization of rod cells, effectively altering neurotransmitter release.