Neural cell adhesion molecule

Neural cell adhesion molecule (NCAM), also called CD56, is a homophilic binding glycoprotein expressed on the surface of is_associated_with::neurons, is_associated_with::glia, is_associated_with::skeletal muscle and is_associated_with::natural killer cells. NCAM has been implicated as having a role in cell–cell adhesion, neurite outgrowth, synaptic plasticity, and learning and memory.

Forms, domains and homophilic binding
NCAM is a glycoprotein of Immunoglobulin (Ig) superfamily. At least 27 alternatively spliced NCAM mRNAs are produced, giving a wide diversity of NCAM isoforms. The three main isoforms of NCAM vary only in their is_associated_with::cytoplasmic domain:
 * NCAM-120kDa (GPI anchored)
 * NCAM-140kDa (short cytoplasmic domain)
 * NCAM-180kDa (long cytoplasmic domain)

The extracellular domain of NCAM consists of five is_associated_with::immunoglobulin-like (Ig) domains followed by two fibronectin type III (FNIII) domains. The different domains of NCAM have been shown to have different roles, with the Ig domains being involved in homophilic binding to NCAM, and the FNIII domains being involved signaling leading to neurite outgrowth.

Homophilic binding occurs between NCAM molecules on opposing surfaces (trans-) and NCAM molecules on the same surface (cis-)1. There is much controversy as to how exactly NCAM homophilic binding is arranged both in trans- and cis-. Current models suggest trans- homophilic binding occurs between two NCAM molecules binding antiparallel between all five Ig domains or just IgI and IgII. cis- homophilic binding is thought to occur by interactions between both IgI and IgII, and IgI and IgIII, forming a higher order NCAM multimer. Both cis- and trans- NCAM homophilic binding have been shown to be important in NCAM “activation” leading to neurite outgrowth.

Minor exons
Another layer of complexity is created by the insertion of other "minor" exons in the NCAM transcript. The two most notable are:
 * the VASE (VAriable domain Spliced Exon) exon which is thought to correlate with an inhibition of the neurite outgrowth promoting properties of NCAM.
 * the MSD (Muscle Specific Domain), which is thought to play a positive role in myoblast fusion. In skeletal muscle it is found in all three NCAM isoforms, increasing their MW, giving NCAM-125, NCAM-145, and NCAM-185 isoforms, but is most commonly found in the NCAM-125 isoform.

Posttranslational modification
NCAM exhibits is_associated_with::glycoforms as it can be posttranslationally modified by the addition of is_associated_with::polysialic acid (PSA) to the fifth Ig domain, which is thought to abrogate its homophilic binding properties and can lead to reduced cell adhesion important in cell migration and invasion. PSA has been shown to be critical in learning and memory. Removal of PSA from NCAM by the enzyme is_associated_with::endoneuraminidase (EndoN) has been shown to abolish is_associated_with::long-term potentiation (LTP) and is_associated_with::long-term depression (LTD).

Function
NCAM is thought to signal to induce neurite outgrowth via the is_associated_with::Fibroblast growth factor receptor (FGFR) and act upon the p59Fyn signaling pathway.

Pathology
In is_associated_with::anatomic pathology, pathologists make use of CD56 is_associated_with::immunohistochemistry to recognize certain tumors.


 * Normal cells that stain positively for CD56 include is_associated_with::NK cells, activated is_associated_with::T cells, the is_associated_with::brain and is_associated_with::cerebellum, and is_associated_with::neuroendocrine tissues.


 * Tumors that are CD56-positive are is_associated_with::myeloma, is_associated_with::myeloid leukemia, is_associated_with::neuroendocrine tumors, is_associated_with::Wilms' tumor, is_associated_with::neuroblastoma, NK/T cell is_associated_with::lymphomas, is_associated_with::pancreatic acinar cell carcinoma, is_associated_with::pheochromocytoma, is_associated_with::paraganglioma, is_associated_with::small cell lung carcinoma, and the is_associated_with::Ewing's sarcoma family of tumors.

Anti-NCAM therapy
NCAM has been used as a target molecule for experimental antibody-based immunotherapy. Successful radioimmunolocalisation of metastases was demonstrated after giving injections of NCAM-binding 123J-UJ13a or 131J-UJ13a radioimmunoconjugates to children with neuroblastoma. Patients with small cell lung cancer were treated with the anti-NCAM immunotoxine huN901-DM1 in two different clinical studies, revealing acceptable toxicity and signs of clinical response.