Notes on Cell Communication and Signaling

Cell Communication

Overview of Cell Communication

• Cells detect and respond to signals in their extracellular environment.
• These signals coordinate activities in multicellular organisms.
• Upon binding of a signal to its receptor:
  • Conformation of the receptor changes.
  • This change causes a response inside the cell.
• Some signals can intentionally lead to cell death, known as apoptosis (programmed cell death).

Cells Respond to Environmental Changes

• Example in yeast cells:
  • Presence of glucose leads to the production of more glucose transporters, facilitating glucose import into the cell.

Phototropism in Plants

• Environmental Signal: Blue light.
• Mechanism: Auxin hormone moves to shaded part of the plant stem, causing cell elongation on the shaded side, resulting in the stem bending towards light.

Types of Cell Signaling

• Based on the distance signaling occurs and if cell contact happens:
  • Direct Intercellular Contact
  • Direct Signaling: Immediate transmission via cell junctions.
  • Paracrine Signaling: Short-distance signals affecting nearby cells.
  • Autocrine Signaling: Cells respond to signals they secrete.
  • Endocrine Signaling: Long-distance signals (hormones traveling through the bloodstream).

Signals Relayed Between Cells

• Direct Intercellular Signaling: Molecules pass through gap junctions (e.g., electrical signaling in cardiac muscle).
• Contact-Dependent Signaling: Membrane-bound signals bind to adjacent cells (e.g., neuron growth, immune system response).
• Autocrine Signaling: Cells secrete signals affecting themselves and neighboring cells, regulating growth (ex: cell density limits growth).
• Paracrine Signaling: Signals affect nearby cells, such as neurotransmitters.
• Endocrine Signaling: Hormones act on distant target cells, usually having long-lasting effects.

Signals and Receptors

• Only target cells with correct receptors can respond to signals.
• Signal-receptor interaction is likened to a lock and key mechanism.

Types of Signal Molecules

• Hormones: Secreted by endocrine cells, long-distance signals affecting different tissues.
• Local Mediators: Act on neighboring cells (e.g., autocrine and paracrine signals).
• Neurotransmitters: Released into synaptic cleft, facilitating communication across short distances.

Cell Surface and Intracellular Receptors

• Cell Surface Receptors: Hydrophilic signals do not pass through the PM; examples include most protein hormones.
• Intracellular Receptors: Hydrophobic signals (like steroid hormones) enter the cell where they bind to receptors in the cytoplasm or nucleus, often influencing gene expression.

Specific Receptor Types

• Catalytic Receptors (Tyrosine Kinase): Have dual roles as receptors and enzymes.
• Ion Channel Receptors: Binding opens ion channels; involved in muscle contraction (acetylcholine receptor example).
• G-Protein Linked Receptors: Involve three components and are part of signal transduction pathways leading to cellular responses.

Signal Transduction Pathways

• Signal transduction involves:
  1. Receptor Activation
  2. Signal transduction through a cascade of reactions (often involving second messengers).
  3. Cellular responses which could alter enzyme activity, structural protein functions, or gene expression.

Amplification of Signals

• One hormone can trigger a cascade where one receptor activates multiple G-proteins, resulting in a large cellular response via multiple enzymes.

Variable Responses to the Same Signal

• Depending on cell type, one signal can produce different effects:
  • Epinephrine: Stimulates glycogen breakdown in muscles/liver, fat production in adipose tissue, increases heart rate and blood pressure, and breathing rate.

Apoptosis: Controlled Cell Death

• Essential for development and removing damaged cells.
• Necessary steps in apoptosis:
  1. Chromosomal condensation and cytoplasmic volume decrease.
  2. DNA fragmentation without damaging adjacent cells.
  3. Phagocytic cells then remove apoptotic bodies.

Triggers of Apoptosis

• Apoptosis can be triggered by withdrawal of survival factors or extensive DNA damage via signaling pathways involving specific proteins like caspases.

Cancer and its Mechanisms

• Cancer signifies uncontrolled cell growth due to imbalanced cell division and apoptosis.
• Mutation leads to under-expressed genes for apoptosis and over-expressed genes promoting cell division.

Conclusion

• Understanding cell communication is fundamental in fields like developmental biology, immunology, and cancer therapy. Abnormalities in signaling pathways contribute significantly to disease processes, emphasizing the importance of these cellular communication systems.