Cellular Communication and Cell Junctions: Structure and Function

Cellular Communication

Overview of Cellular Communication

Cellular communication refers to the transmission of signals between cells, enabling them to coordinate and perform specific functions. This process is essential for maintaining homeostasis, development, and response to environmental changes.

• Transmission of signals: Cells communicate via chemical signals, physical contact, and receptor-mediated interactions.

• Receptors: Specialized proteins present in the plasma membrane (PM) detect and respond to signaling molecules.

• Cellular communication: Involves both direct cell-cell contact and indirect signaling through extracellular matrix (ECM) components.

Cell-Cell Interaction

Mechanisms of Cell-Cell and Cell-Matrix Adhesion

Cells interact with each other and with the ECM through adhesion molecules, which are critical for tissue structure and function.

• Direct adhesion: Mediated by Cell Adhesion Molecules (CAMs) such as cadherins, integrins, selectins, and members of the Ig superfamily.

• Indirect adhesion: Involves cell-matrix adhesion via receptors in the PM and ECM components like proteoglycan, collagen, laminin, and fibronectin.

Major Types of CAMs

• Cadherins: Calcium-dependent adhesion proteins, crucial for maintaining tissue integrity.

• Ig Superfamily: Mediate adhesion in both immune and non-immune cells; includes vascular and neural CAMs.

• Integrins: Bind to ECM components, facilitating indirect adhesion and signal transduction.

• Selectins: Lectin proteins that interact with sugar molecules on neighboring cells, important in immune cell trafficking.

Types and Domains of CAMs

Classification and Functional Domains

• Cadherins:

  • Classical (adhering junctions)

  • Desmosomal (desmosomes)

  • Protocadherins

  • Unconventional

• Ig Superfamily: Interact with both immune and non-immune cells.

• Integrins:

  • Composed of alpha and beta subunits

  • Transfer signals from ECM to cell interior

  • Contain cytosolic and extracellular domains

• Selectins: Mediate cell-cell interactions via carbohydrate recognition.

Types of Adhesion

• Homotypic adhesion: Interaction between same cell types (e.g., two epithelial cells).

• Heterotypic adhesion: Interaction between different cell types (e.g., epithelial and connective tissue cells).

• Homophilic interaction: Similar CAMs on both cells.

• Heterophilic interaction: Different CAMs on each cell.

Cell-Cell Adhesion by CAMs

Strength and Dynamics of Adhesion

• Transient adhesion: Weak, short-lived (e.g., immune cell interactions).

• Stable adhesion: Strong, long-lasting (e.g., nerve and epithelial cells).

• CAMs bind adapter proteins: These connect to the cytoskeleton, stabilizing cell junctions.

• Cis interaction: CAMs interact laterally on the same cell.

• Trans interaction: CAMs interact with CAMs on adjacent cells.

Epithelial Cells

Structure and Function

Epithelial tissues cover surfaces exposed to the external environment and line internal cavities. They serve protective, absorptive, and secretory roles.

• Apical, lateral, and basal surfaces: Epithelial cells are polarized, with distinct regions for specialized functions.

• Basal surface: Connected to the basal lamina, part of the ECM.

• Examples: Skin, gastrointestinal tract, ducts, glands.

Cell Junctions

Types of Cell Junctions

Cell junctions are specialized structures that connect cells to each other or to the ECM, maintaining tissue integrity and facilitating communication.

• Tight junctions: Located just below the apical region; impermeable to solutes and ions.

• Anchoring junctions: Include adherens junctions, desmosomes, and hemidesmosomes.

• Gap junctions: Allow movement of solutes and ions across cells; enable intercellular communication.

• Hemidesmosomes: Found in the basal region; anchor cells to the ECM.

Functions of Cell Junctions

• Strength and rigidity: Provide mechanical stability to tissues.

• Information transmission: Facilitate signaling between extracellular and intracellular spaces.

• Regulation of passage: Control movement of ions and molecules across cell layers.

Anchoring Junctions

Adherens Junctions

• Main CAMs: Cadherins (especially E-cadherin in epithelial cells).

• Adaptor proteins: α-catenin, β-catenin, p120-catenin link cadherins to the actin cytoskeleton and initiate signaling pathways.

• Function: Maintain tissue integrity and enable cell signaling.

Desmosomes

• Cadherin proteins: Desmoglein and desmocollin.

• Adaptor proteins: Plakoglobin and plakophilin form cytoplasmic plaques, connecting to intermediate filaments.

• Function: Provide strong adhesion and mechanical strength, especially in tissues subject to stress.

Hemidesmosomes

• Location: Basal surface of epithelial cells.

• Function: Anchor cells to the ECM, imparting shape and rigidity.

• Structure: Bundles of intermediate filaments connect spot desmosomes and hemidesmosomes.

Tight Junctions

Structure and Function

• Location: Just below the apical surface of epithelial cells.

• Function: Maintain cell polarity and form a barrier to seal off body cavities (e.g., intestine, blood-brain barrier).

• Permeability: Not all tight junctions have the same properties; some are permeable to specific ions or solutes.

Proteins of Tight Junctions

• Occludin and claudin: Principal integral membrane proteins forming the tight seal.

• Junction adhesion molecules (JAMs): Contribute to homophilic adhesion and other functions.

• Extracellular domains: Rows of occludin, claudin, and JAM proteins in adjacent cells create the tight junction.

Gap Junctions

Structure and Function

• Intercellular communication: Specialized sites between animal cells for direct exchange of ions and small molecules.

• Connexin: Integral membrane protein forming connexons, which span the membrane and create channels.

• Channel properties:

  • Allow diffusion of molecules with molecular mass below ~1000 daltons.

  • Channels are relatively non-selective and gated.

  • Channel closure is triggered by phosphorylation of connexin subunits.

Formation of Gap Junctions

• Connexons in opposing plasma membranes align and link tightly, forming complete intercellular channels.

• These channels connect the cytoplasm of adjacent cells, allowing direct communication.

Plasmodesmata

Structure and Function in Plant Cells

• Plasmodesmata: Cytoplasmic channels connecting plant cells through cell walls.

• Desmotubule: Dense central structure within plasmodesmata, surrounded by cytoplasmic sleeve.

• Function: Enable cell-to-cell communication and transport of substances in plants.

Summary Table: Major Cell Junctions

Junction Type	Main Proteins	Function	Location
Tight Junction	Occludin, Claudin, JAMs	Barrier, maintain polarity	Apical region of epithelial cells
Adherens Junction	E-cadherin, catenins	Cell-cell adhesion, signaling	Lateral surface of epithelial cells
Desmosome	Desmoglein, Desmocollin, Plakoglobin, Plakophilin	Strong adhesion, mechanical strength	Lateral surface of epithelial cells
Hemidesmosome	Integrins, intermediate filaments	Anchor to ECM, shape, rigidity	Basal surface of epithelial cells
Gap Junction	Connexin	Intercellular communication	Lateral surface of animal cells
Plasmodesmata	Desmotubule	Cell-cell communication in plants	Plant cell walls

Additional info:

• Cell junctions are critical for tissue development, maintenance, and function in multicellular organisms.

• Disruption of cell junctions can lead to diseases such as cancer and inflammatory disorders.