Date of Award
Doctor of Philosophy (PhD)
The neuron-glia cell adhesion molecule, Ng-CAM, is a cell-surface glycoprotein expressed on neurons and Schwann cells in the chicken nervous system. It is expressed as a group of related components of -200 kD, 135 kD, and 80 kD, which are detected at particular sites during development. Ng-CAM mediates a variety of functions that are known to be important in specific cell-cell interactions and the histogenesis of the nervous system, including neuron-neuron adhesion, neuron-glia adhesion, neurite fasciculation, and neuronal migration on glial cells. This thesis describes the characterization of cDNA that encode the entire Ng-CAM protein and the determination of the origin of the polypeptide components. An analysis of the deduced amino acid sequence reveals the presence of immunoglobulin-like domains and fibronectin-type III repeats, and establishes the relationship of Ng-CAM to other neural CAMs containing similar structures. The thesis also describes the identification of a similar protein in the embryonic chicken nervous system, the Ng-CAM-related cell adhesion molecule (Nr-CAM). Its close structural and sequence similarities to Ng-CAM indicate its probable involvement in cell adhesion, and defines a closely related subfamily of neural CAMs. cDNA libraries prepared from embryonic nervous tissue were used to isolate clones encoding Ng-CAM. These clones encode a polypeptide that includes all three known components of Ng-CAM, and hybridize to a single mRNA species (6 kb) that is restricted to nervous tissue. Single bands were detected in genomic DNA blots with Ng-CAM cDNA probes, indicating that there is a single Ng-CAM gene in chicken. Together these data indicate that the polypeptide components of Ng-CAM all arise from a single translation product, and that the two smaller poljrpeptide components (135 kD and 80 kD) are products of posttranslational cleavage. The deduced amino acid sequence of Ng-CAM predicts the presence of six immunoglobulin-like domains, five fibronectin-type III repeats, a single transmembrane domain, and a short cytoplasmic region. Ng-CAM also contains a single RGD sequence in its third fibronectin-type III repeat, which in an equivalent position in several other proteins is known to mediate a cell-binding activity. Ng-CAM is similar to other neural CAMs containing immunoglobulin-like domains and is most similar to its previously presumed equivalent in mouse, Ll. However, overall the amino acid sequence of Ng-CAM is only 40% identical to Ll, and within the individual domains ranges from only 15% to 66% identity. Known equivalent CAMs (e.g., N-CAM) in the two species are much more closely related (80% identity), indicating that in contrast to previous assumptions Ng-CAM and Ll may be functionally different molecules. cDNA clones encoding another protein were isolated from embryonic chicken nervous system libraries using antibodies against Ng-CAM. The new protein shares at least one polypeptide epitope with Ng-CAM, and was designated the Ng-CAM-related cell adhesion molecule, or Nr-CAM. Like Ng-CAM, it is predicted to contain six immunoglobulin-like domains, five fibronectin-type III repeats, one transmembrane segment and a small cytoplasmic domain, each of which is most similar to corresponding segments in Ng-CAM and Ll. However, overall Nr-CAM shares only 40% identity to Ll, and is thus not the chicken equivalent of Ll. In addition, comparison of several similar Nr-CAM cDNA clones indicated that the fifth fibronectin-type III repeat (93 amino acids) and a segment between the second and third immunoglobulin-like domains (20 amino acids) may be differentially spliced in Nr-CAM mRNA; such alternative forms have not been seen for Ng-CAM and Ll. The combined results indicate that Ng-CAM, Nr-CAM and Ll define a structurally related subfamily of neural CAMs. Their similarities suggest that they may perform similar binding functions in the nervous system, possibly at different times and locations during neural development.
Burgoon, Mark P., "Structure of the Chicken Neuron-Glia Cell Adhesion Molecule, Ng-CAM: Origin of the Polypeptides and Relation to the Ig Superfamily" (1991). Student Theses and Dissertations. 362.