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Adhesion molecules are the protein molecules that are situated on the surface of the cells or the intracellular organelle.
The adhesion molecules attach to adjacent cells and help with binding with these cells or extracellular structures. There are several functions described like transmitting the information (cellular signaling pathways) to communicate between cells and within the cells. The adhesion molecules usually share a common basic structure that they have extracellular and intracellular domains that are connected to transcellular domain. This facilitates their function to act as messenger across the cellular membrane on either side.
Adhesion molecules are classified based on the function, structure, and location. Cadherins, integrins, selectins, and immunoglobulin (Ig)-related cell adhesion molecules (CAMS) are four major types of adhesion molecules. Brief summary of these molecules are described below.
The cadherins are calcium-dependent adhesion molecules, and they are activated when the extracellular domain is attached to calcium. The intracellular domain connects to various intracellular proteins to perform several functions. The details about epithelial (E)-cadherin, neural (N)-cadherin, and placental (P)-cadherin have been described in relation to several tissue function and also embryogenesis.
The integrins are transmembrane proteins that are composed of different subunits (e.g., alpha and beta subunits). These have been described in the signal transfer from within cell to extracellular matrix and also from extracellular to intracellular structures.
The selectins are the type of adhesion molecules, and they are glycoproteins in their composition. When the specific carbohydrate related to cellular signaling pathway attaches to the extracellular domain, these selectins transmit the signal across the cellular membrane. Platelet-selectins, leukocyte-selectins, and endothelial-selectins are subtypes of selectins, and they are involved with various functions related to platelet, leukocyte, and platelet activity at the cellular level.
The immunoglobulin (Ig)-related cellular adhesion molecules (CAM) are independent of calcium for their activity. Vascular-cell adhesion molecules (VCAM-1), neural (N)-cell adhesion molecule (NCAM), intercellular adhesion molecule (ICAM), and platelet-endothelial cell adhesion molecule (PECAM) have been related to dysregulations of the vascular system, nervous system, and platelet-thrombosis.
These adhesion molecules may be involved with each other in the process of various functions of the cellular signaling pathway. For example, during the inflammatory process, white blood cells (leukocytes) are transferred across the endothelial lining to the subendothelial layer of the vasculature which is a multistep process and an important part of the early pathogenesis of atherosclerosis. Initially, leukocytes come in close proximity to endothelial cells which are selectin mediated. The integrins activate the surface adhesion molecules in the presence of several pro-inflammatory factors including various extracellular proteins and cytokines. The integrins also help with the attachment of the leukocytes to the endothelium. With the help of PECAM, the leukocytes migrate across the endothelium (diapedesis) to the subendothelial space. Subsequent complex changes occur, leading to the development and progression of atherosclerosis. Various adhesion molecules have been described in the context of cancer metastasis, growth of tumor cells, and embryogenesis.