2.6 Cell to cell communication over short and long distances

Cells and Tissues

• Cellular Communication: Involves methods through which cells communicate over short and long distances.

How Cells Talk

• Critical for maintaining bodily functions and responding to environmental changes.

• The distance the signal travels influences the type of communication method:

  • Autocrine Signaling: Signaling cell affects itself.

  • Contact-Dependent Signaling: Cells must be in direct contact.

  • Paracrine Signaling: Signal affects nearby (local) cells.

  • Endocrine Signaling: Signals travel long distances through the circulatory system.

• Signaling Molecules: Responsible for communication; they travel through receptors.

Types of Cellular Signaling

1. Signaling by plasma-membrane-attached proteins:

   • Involves membrane-bound proteins acting as signals.

2. Autocrine Signaling:

   • Cells communicate with themselves, using receptors on the same cell.

3. Paracrine Signaling:

   • Communication between adjacent cells using secreted signaling molecules.

4. Endocrine Signaling:

   • Hormones released into blood affecting distant cells.

Direct Communication

• Cell-to-Cell Recognition:

  • Cells need a system to recognize self from non-self for proper communication.

  • Contact-Dependent Interactions are essential for cell signaling.

Juxtracrine Signaling

• Mechanism:

  • Cells directly interact via membrane-bound ligands and receptors.

  • Involves signaling molecules like Notch and Eph-ephrin that affect:

    • Cell fate and migration

    • Cell adhesion

    • Axon guidance

    • Immune cell function

Direct Communication: Cell Junctions & Adhesion Molecules

• Types of Junctions:

  • Tight Junctions:

    • Seal cells to prevent leakage of molecules

    • consist of transmembrane proteins

    • Fuse cells together through the plasma membranes

    • Provide a barrier between cell compartments

  • Adherens Junctions:

    • Connect adjacent cells through cadherins, providing structural support.

    • Consisting of transmembrane proteins, they play a crucial role in cell adhesion and signaling, facilitating communication between neighboring cells.

  • Desmosomes:

    • Provide mechanical strength to tissues by anchoring cells.

    • Increases cell and tissue stability

    • Consist of protein complexes that connect the intermediate filaments of adjacent cells, facilitating communication and cohesion in epithelial and cardiac tissues.

  • Hemidesmosomes:

    • Anchor cells to the basement membrane (not to other cells).

    • Look like half a desmosome

    • Consist of transmembrane glycoproteins (integrins) that attach to:

      • Intermediate filaments (keratin)

      • Basement membrane protein (laminin)

    • Anchor cells to basement membrane (NOT ANOTHER CELL)

  • Gap Junctions:

    • Allow for cell communication through connexons, connecting adjacent cell cytoplasm.

    • Porous protein tunnels (connexon) directly link the plasma membrane of neighbouring cells

Examples of Direct Communication

• Cardiac Tissue: Shows extensive gap junctions for synchronized contraction.

  • Involves signaling through various cellular structures like desmosomes and T-tubules.

Autocrine Signaling

• Definition: Signaling where the cell releases a ligand (Soluble molecule) that binds to its receptors on the same cell.

• Functions:

  • Regulates cell proliferation, differentiation, and death.

• Examples:

  • Plays a role in embryonic development and inflammation responses.

    • Ensures cells differentiate into the correct cell type

  • Involved in tumor growth, pain regulation, inflammation, destruction f virus infected cells and tumour growth.

Paracrine Signaling

• Definition: Short distance communication where a ligand is released by a secretory cell.

• Mechanism:

  • Ligands diffuse across the extracellular space to bind to receptors on nearby target cells, facilitating rapid and localized responses.

• Characteristics:

  • Quick response, lasts a short time.

  • Can involve hormones acting locally.

• Example: Communication between endothelial cells and vascular smooth muscle cells.

Neural Signalling

• Electrical Synapses:

  • Involves direction communication via gap junctions for rapid communication.

  • Example: Reticnal interneurons

• Chemical Synapses:

  • Indirect communication across synaptic clefts using neurotransmitters.

• When a nerve impulse reaches the presynaptic neuron, it triggers the release of neurotransmitters, which are chemical signals. These neurotransmitters cross the synaptic cleft and attach to specific ion channels on the post-synaptic neuron. This causes the ion channels to open, allowing particular ions to enter the post-synaptic neuron, generating an electrical signal. This process also happens between nerves and target cells, such as muscles.

Endocrine Signalling

• Long distance signals

• Mechanism: Involves hormones released by endocrine glands that travel through the bloodstream, Binds to receptors on target cells.

• Response: Slower but long-lasting effects.

• Example:

  • Thyroid-stimulating hormone, produceed by the thyroid gland

  • Adrenocorticotropic hormone, produced by the Adrenal gland

  • Follicle-stimulating hormone, produced by the Ovary

  • Luteinizing hormone, produce by the Testis

  • Growth hormone, produced by the Musculoskeletal system

  • prolactin, produced by Mammary gland

Neuroendocrine Signaling

• long distance Signals

• Mechanism: Neurohormones released by neuroendocrine or neurosecretory cells, which travels through the bloodstream to bind to a receptor on target cells.

• Response: Slower but long lasting

Lecture Summary

• Contrast between main local and long-distance modes of cell-to-cell communication: Gap junctions, contact dependent, autocrine, paracrine, neural and endocrine

• Compare and contrast neural versus hormonal communication