Cell Communication and Signaling Pathways

Types of Signals

• Autocrine Signals

  • Produced by a cell to act on its own receptors.
  • Example: A cell signaling itself.

• Paracrine Signals

  • Produced by a cell to act on nearby target cells.

• Endocrine Signals

  • Long-distance signals targeting cells further away.
  • Example: Hormones secreted into the bloodstream or cytokines at injury sites.

Signaling Molecules

• Hydrophobic Molecules

  • Tend to repel water and can’t float freely in extracellular spaces.
  • Transported to target cells by carrier proteins.
  • Can diffuse across the cell membrane, binding to receptors inside the target cell (cytoplasm or nucleus).

• Hydrophilic Molecules

  • Stay in water; freely float in extracellular spaces.
  • Unable to cross cell membranes.
  • Bind to receptors on the cell surface.

Signaling Pathway Stages

1. Reception

   • Target cell’s receptor binds to a ligand (key-lock fit).

2. Transduction

   • Receptor protein changes, activating intracellular molecules (second messengers).

3. Response

   • Cell’s response to the signal.

Major Classes of Transmembrane Receptors

G Protein-Coupled Receptors (GPCR)

• Structure: Seven-pass transmembrane receptors.
• Function: Ligand binds, triggering shape change and G protein activation.
• G Proteins
  • Made of alpha, beta, and gamma subunits; inactive with GDP, active with GTP.
  • Types: Gq, Gi, Gs.
  • Gq: Activates phospholipase C, leading to calcium release and protein kinase C activation.
  • Gs: Stimulates adenylate cyclase, forming cAMP, activating protein kinase A.
  • Gi: Inhibits adenylate cyclase, creates negative feedback.

Enzyme-Coupled Receptors

• Structure: Single-pass transmembrane proteins with enzymatic activity.
• Types
  • Receptor Tyrosine Kinases (RTK)
    • Dimerize and cross-phosphorylate tyrosine residues.
  • Tyrosine Kinase-Associated Receptors
    • Similar to RTKs but rely on cytoplasmic tyrosine kinases.
  • Receptor Serine/Threonine Kinases
    • Type I and Type II; ligand binding activates phosphorylation.

Ion Channel Receptors

• Generally closed, open upon ligand binding.
• Allow passive ion flow, triggering cellular response.

Summary

• Autocrine: Signals self.
• Paracrine: Signals nearby cells.
• Endocrine: Signals distant cells.
• Hydrophobic Ligands: Cross membranes, bind intracellularly.
• Hydrophilic Ligands: Bind to transmembrane receptors triggering internal pathways.
• Receptor Classes: GPCRs, enzyme-coupled receptors, ion channel receptors.