Cell-Cell Communication and Chemical Messengers

Cell-Cell Communication

• Communication between cells is essential for multicellular organisms.
• Cells communicate by using signals, typically chemical messengers.
• Signals can be transmitted directly from one cell to another or indirectly through the extracellular environment.

Types of Chemical Messengers

• Steroids are hydrophobic molecules that can diffuse across cell membranes.
  • Examples include testosterone and cortisol.
  • Steroids bind to intracellular or transmembrane receptors and can trigger a slow response by altering gene expression.
• Biogenic amines possess an amine group (–NH2)
  • Examples include acetylcholine, dopamine, norepinephrine, epinephrine, serotonin, melatonin, histamine, and thyroid hormones.
  • They have diverse effects and can act as hormones, neurotransmitters, or both.
• Eicosanoids are synthesized from fatty acids in cell membranes and act as paracrines.
  • They can be hydrophobic.
  • Examples include prostaglandins and leukotrienes.
• Gases are paracrines that can diffuse across cell membranes.
  • Examples include nitric oxide (NO) and carbon monoxide.
• Purines act as neuromodulators and paracrines.
  • Examples include adenosine, AMP, ATP, and GTP.

Receptor-Ligand Interactions

• Ligand-Gated Ion Channels open a channel when a ligand binds to a transmembrane receptor.
  • The movement of ions across the membrane changes the membrane potential.
  • An example is acetylcholine.
• Receptor Enzymes have a catalytic domain that starts a phosphorylation cascade upon ligand binding to a transmembrane receptor.
  • This cascade initiates a change in the cell.
  • An example is insulin binding to its receptor.
• G-Protein-Coupled Receptors interact with intracellular GTP binding proteins upon ligand binding to a transmembrane receptor.
  • Subunits of the G-protein dissociate and activate other membrane-associated proteins, producing second messengers (e.g. cAMP).

Indirect Signaling

• Paracrine signals diffuse to nearby cells.
• Autocrine signals diffuse back to the signaling cell.
• Endocrine signals are hormones transported by the circulatory system.
• Nervous signals travel along neurons and release neurotransmitters.

Feedback Loops

• Positive feedback amplifies the signal and can cause a response to continue long after the stimulus is removed.
• Negative feedback dampens the signal, reducing the intensity and duration of the response.

Cell Adaptation / Desensitization

• Prolonged exposure to a stimulus can cause a decrease in the cell's response to it.
• This occurs due to receptor downregulation, changes in signal transduction pathways, or other mechanisms.

Cell Communication Pathways

• Water-soluble hormones bind to receptors on the plasma membrane.
  • This triggers a signaling cascade that can involve second messengers, enzymes, and changes in gene expression.
  • An example is epinephrine binding to liver cells, causing the release of glucose into the bloodstream.
• Lipid-soluble hormones diffuse across cell membranes and bind to receptors in the cytoplasm or nucleus.
  • They typically regulate gene expression.
  • An example is a steroid hormone binding to its receptor and moving into the nucleus to act as a transcription factor.

Endocrine System

• The endocrine system is responsible for secreting hormones into the bloodstream.
• These hormones regulate various physiological processes, development, and behavior.

Pheromones

• Pheromones are chemical signals released into the environment that can be detected by other individuals of the same species.
  • They often play a role in communication, especially during reproduction.