Immunoglobulin-like Cell Adhesion Molecules

Ig-domains are characteristic immunoglobulin protein folds found in multiple extracellular binding domains of Ig-CAMs. These domains allow Ig-CAMs to function diversely across different tissue types by mediating both homophilic interactions, where Ig-CAMs bind to identical molecules, and heterophilic interactions, where they bind to different ligands like integrins. This structural versatility enables Ig-CAMs to perform varied roles in cell adhesion and signaling.

Neural CAMs, or NCAMs, expressed on neurons bind to each other via their Ig-domains to form homophilic interactions. These interactions activate signaling pathways during neuronal differentiation and direct contact-dependent processes such as axon and dendrite growth. NCAMs continue to maintain neural connections crucial for learning and memory formation throughout life.

During immune responses, vascular endothelium expresses VCAMs and ICAMs, which are Ig-CAMs that bind integrins on leukocyte surfaces through heterophilic interactions. As leukocytes slow via selectin-mediated interactions, their surface integrins become activated and firmly bind these Ig-CAMs, triggering signaling cascades. This process enables leukocytes to pass between endothelial cells and reach target tissues.

While most Ig-CAMs mediate transient cell adhesion, the nectin class of Ig-CAMs forms stable junctions such as adherens junctions in conjunction with other cell adhesion molecules. This distinction reflects the versatility of Ig-CAMs, which can function in both temporary cell-cell contacts and permanent structural connections depending on their molecular class and associated proteins.

Many cancers including myeloid leukemia, pheochromocytoma, and Wilm's tumor stain positive for NCAM2, making it a valuable diagnostic marker in pathology. Cancer cells exploit the leukocyte recruitment mechanisms mediated by Ig-CAMs during metastasis to infiltrate new sites and form secondary tumors. This hijacking of normal Ig-CAM functions demonstrates how cancer cells misuse cell adhesion pathways.

Some Ig-CAMs function as receptors for viral entry into host cells. The rabies virus and human rhinovirus are examples of pathogens that utilize Ig-CAMs as entry points. This viral exploitation of Ig-CAM receptors highlights how cell surface adhesion molecules can be repurposed by infectious agents to establish infection.

Ig-CAMs possess characteristic immunoglobulin protein domains and other domains such as fibronectin type III domains, with varying degrees of glycosylation across different Ig-CAMs. This structural diversity enables them to exhibit either homophilic or heterophilic binding and perform diverse functions including cell adhesion, signaling, development, and infection across neural, vascular, and immune tissues.