paranodal junction

Definition

Target type: cellularcomponent

A highly specialized cell-cell junction found in vertebrates, which forms between a neuron and a glial cell, and has structural similarity to Drosophila septate junctions. It flanks the node of Ranvier in myelinated nerve and electrically isolates the myelinated from unmyelinated nerve segments and physically separates the voltage-gated sodium channels at the node from the cluster of potassium channels underneath the myelin sheath. [PMID:11395001, PMID:14630217]

The paranodal junction is a specialized region of the myelinated axon where the axonal membrane interacts with the myelinating glial cell, forming a tight seal that prevents ion leakage and ensures rapid and efficient nerve impulse conduction. This junction is characterized by a unique arrangement of molecular components, including transmembrane proteins, cytoskeletal elements, and adhesion molecules, contributing to its structural integrity and functional properties.

**Transmembrane proteins:**

- **Contactin-associated protein (Caspr) 1 and 2:** These proteins are localized to the axonal side of the paranodal junction and bind to the glial protein, Contactin, forming a trans-synaptic interaction essential for paranodal sealing. Caspr proteins are crucial for the proper formation and maintenance of the paranodal junction.
- **Neurofascin 155 (NF155):** This protein, expressed on the axonal membrane, interacts with the glial protein, Neurofascin 186 (NF186), contributing to the structural integrity of the paranodal junction. NF155 also plays a role in the clustering of voltage-gated potassium channels at the juxtaparanodal region, facilitating the rapid repolarization of the action potential.
- **Contactin:** Localized on the glial side of the paranodal junction, this protein interacts with Caspr proteins on the axon, forming a trans-synaptic complex that strengthens the paranodal seal.

**Cytoskeletal elements:**

- **Ankyrin G:** This protein, present at the axonal side of the paranodal junction, anchors voltage-gated sodium channels and other proteins to the spectrin-based cytoskeleton. Ankyrin G plays a critical role in the clustering of sodium channels at the juxtaparanodal region, facilitating the propagation of action potentials along the axon.
- **βII-spectrin:** This protein interacts with ankyrin G and forms a cytoskeletal meshwork that provides structural support to the paranodal junction.
- **Neurofilaments:** These intermediate filaments, present within the axon, contribute to the overall structural integrity of the paranodal junction by interacting with other cytoskeletal elements.

**Adhesion molecules:**

- **N-cadherin:** This protein, located on the axonal membrane, interacts with its counterpart on the glial cell, contributing to the tight adhesion between the axon and the myelinating glial cell. N-cadherin plays a crucial role in the stability and maintenance of the paranodal junction.
- **β-catenin and α-catenin:** These proteins, associated with N-cadherin, mediate the interaction between N-cadherin and the cytoskeleton, strengthening the adhesion between the axon and the glial cell.

**Other components:**

- **Lipid rafts:** These specialized membrane domains, enriched in cholesterol and sphingolipids, are present at the paranodal junction and are thought to play a role in the localization and clustering of transmembrane proteins.
- **Extracellular matrix:** The extracellular matrix, composed of various proteins and glycoproteins, provides structural support and regulates the interactions between the axon and the myelinating glial cell.

**Functional significance:**

The unique arrangement of molecular components within the paranodal junction plays a critical role in the function of the myelinated axon:

- **Rapid and efficient nerve impulse conduction:** The tight seal formed by the paranodal junction prevents ion leakage, ensuring that the action potential propagates rapidly and efficiently along the axon.
- **Preventative role in demyelinating diseases:** The proper formation and maintenance of the paranodal junction are essential for the stability and function of the myelin sheath. Disruption of the paranodal junction can lead to demyelination, a hallmark of neurological diseases such as multiple sclerosis.

In conclusion, the paranodal junction is a highly specialized and complex structure that plays a critical role in the function of the myelinated axon. Its unique arrangement of molecular components contributes to its structural integrity and ensures rapid and efficient nerve impulse conduction. The proper formation and maintenance of the paranodal junction are essential for the healthy function of the nervous system.'"

Proteins (2)

Compounds (39)