Lecture 8: Cell Signaling - Short Term Responses

Organism Responses to Signals

  • Organisms respond to both internal and external signals by:
    • Secretion of digestive enzymes into the pancreatic duct.
    • Hormone secretion into the blood (e.g., insulin).
    • Insulin receptors must be present on the plasma membrane to receive signals.

Classifications of Extracellular Signaling

  • Endocrine Signaling:
    • Signaling molecules are transported via the circulatory system over long distances.
    • Example: Hormones.
  • Paracrine Signaling:
    • Cells are in close proximity; signaling molecules don't travel far.
    • Example: Neuronal cells.
  • Autocrine Signaling:
    • Cells respond to signals they produce themselves.
    • Example: Tumor cells.
  • Plasma Membrane-Attached Signaling:
    • Contact-dependent signaling with membrane-bound ligands targeting receptors on adjacent cells.
    • Example: Immune cells.

Receptor Activation and Ligand Binding

  • Receptors exhibit binding specificity; surfaces of receptors and ligands must be complementary.
  • Ligand binding can cause conformational changes in receptors, triggering activation of downstream signal transducers (allosteric regulation).
  • Receptors can be located on the cell surface or intracellularly (cytosol/nucleus).
  • Receptor-ligand binding is typically reversible; complexes can dissociate.

Signal Transduction Pathways

  • Signal transduction pathways relay the signals detected by receptors, leading from initial detection to final cellular response.
  • These pathways regulate effector proteins that stimulate cellular responses.
  • Second messengers: small molecules or ions synthesized/released in response to receptor signals, activating specific intracellular proteins.

Protein Kinases and Phosphatases

  • Phosphorylation: Common method for activating/deactivating proteins by adding/removing phosphate groups.
    • Phosphorylation alters a protein's ability to bind to others.
    • Protein kinases add phosphate groups while protein phosphatases remove them.
  • Many kinases become activated through phosphorylation themselves.

Signal Amplification

  • One activated receptor can lead to signal amplification.
  • A receptor activates multiple signal transduction and effector proteins, which in turn can activate other downstream proteins.

Feedback Regulation

  • Feedback can suppress or turn off a signaling pathway after adequate response.
  • Activated effector proteins can modify receptors or earlier intermediates, blocking downstream responses.
  • Feedback controls may initiate receptor degradation, decreasing functional receptors and possibly leading to cellular adaptation.

G Protein-Coupled Receptors (GPCRs)

  • GPCRs are a major group of membrane receptors, consisting of 7 transmembrane helical regions and binding heterotrimeric G proteins.
  • GPCR families vary based on ligand-binding sites:
    • Family A: Biological amines, light, odorants.
    • Family B: Peptides.
    • Family C: Amino acids and ions.
Activation of G Proteins
  • G proteins function as molecular switches:
    • Active ("on") when bound to GTP; inactive ("off") when GTP is hydrolyzed to GDP.
  • GTPase Activating Proteins (GAPs) and Guanine Nucleotide Exchange Factors (GEFs) control GTPase activity.

GPCR Mechanism and Effects

  • Ligand-activated GPCRs bind to G proteins, activating them.
  • The Gα subunit dissociates upon GTP binding, activating downstream effectors in signal transduction pathways.
  • Hydrolysis of GTP returns the G protein to its inactive state, reassembling the heterotrimeric complex.

Response to Epinephrine

  • Epinephrine, released from the adrenal gland, mediates the fight or flight response via GPCRs and causes glycogen breakdown (glycogenolysis).
  • Glycogen breakdown:
    • Glucose is secreted into the blood and utilized for energy.
  • Adenylyl Cyclase Activation:
    • Activates cAMP synthesis in response to epinephrine via β2-adrenergic receptor.

Protein Kinase A Regulation by cAMP

  • cAMP activates Protein Kinase A (PKA), which can phosphorylate other proteins, initiating cellular responses.
  • PKA activation leads to gene transcription and regulates pathways related to glycogen breakdown.
  • Glycogen Synthase and Phosphorylase: PKA inactivates glycogen synthase (GS) and activates glycogen phosphorylase (GP).

Phospholipase C and Second Messengers

  • Phosphoinositol bisphosphate (PI(4,5)P2) cleaved by Phospholipase C produces DAG and IP3.
  • IP3 promotes Ca²+ release from the ER, functioning alongside DAG to propagate signaling.

Phototransduction in Rod Cells

  • Rod cells in the eye utilize GPCRs (Rhodopsin) to detect light.
  • Rhodopsin activation leads to signal transduction that results in changes to cGMP concentration, affecting ion channels and neurotransmitter release.

Summary of Cell Signaling Concepts

  • Cell signaling mechanisms include various signaling types (endocrine, paracrine, autocrine) and pathways involving second messengers.
  • GPCRs are crucial signaling molecules that adapt cellular responses to environmental changes.