Tight junction

Tight Junction

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Diagram of a tight junction between epithelial cells.

Tight junctions, also known as zonula occludens, are specialized connections between adjacent epithelial or endothelial cells. They play a crucial role in maintaining the selective permeability barrier of epithelial and endothelial layers, regulating the passage of ions, molecules, and water between cells.

Structure

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Electron micrograph showing tight junctions between cells.

Tight junctions are composed of a complex network of proteins that form a seal between adjacent cell membranes. The primary components of tight junctions include transmembrane proteins such as claudins, occludin, and junctional adhesion molecules (JAMs). These proteins interact with cytoplasmic proteins like ZO-1, ZO-2, and ZO-3, which link the tight junctions to the actin cytoskeleton.

Transmembrane Proteins

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Illustration of tight junction transmembrane proteins.

The transmembrane proteins of tight junctions are integral to their function. Claudins are a large family of proteins that determine the selectivity and permeability of the tight junctions. Occludin is another important protein that contributes to the barrier and signaling functions of tight junctions. Junctional adhesion molecules (JAMs) are involved in cell adhesion and signaling.

Function

Tight junctions serve several critical functions in epithelial and endothelial tissues:

• Barrier Function: They create a paracellular barrier that controls the flow of molecules and ions between cells, maintaining the distinct composition of the apical and basolateral environments.
• Fence Function: Tight junctions prevent the mixing of membrane proteins and lipids between the apical and basolateral surfaces of the cell, preserving cell polarity.
• Signaling: They participate in intracellular signaling pathways that regulate cell proliferation, differentiation, and survival.

Signaling

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Occludin signaling pathways in tight junctions.

Tight junctions are not merely passive barriers; they are dynamic structures involved in signaling pathways. Occludin, for example, is involved in signaling pathways that regulate cell proliferation and apoptosis. The interaction of tight junction proteins with signaling molecules can influence various cellular processes, including gene expression and cytoskeletal organization.

Clinical Significance

Tight junctions are critical for maintaining tissue homeostasis, and their dysfunction is associated with various diseases. Disruption of tight junctions can lead to increased permeability, contributing to conditions such as inflammatory bowel disease, celiac disease, and certain types of cancer. Understanding the molecular mechanisms of tight junction regulation is important for developing therapeutic strategies for these conditions.

Related Pages

• Adherens junction
• Desmosome
• Gap junction
• Epithelial tissue
• Endothelial cell