Different domains of the F3 neuronal adhesion molecule are involved in adhesion and neurite outgrowth promotion

The mouse F3 cell surface protein is preferentially expressed on axons of subpopulations of neurons and is anchored to the membrane by a glycosyl-phosphatidylinositol group. It consists of six immunoglobulin-like domains and four fibronectin type III homologous repeats, and can be found both in membrane-anchored and soluble forms. We have previously established that F3 fulfils the operational criteria of a cell adhesion molecule when anchored to the plasma membrane and that its soluble form stimulates neurite initiation and neurite outgrowth. To further characterize F3-mediated adhesion and to investigate whether adhesion and neurite outgrowth promoting activities are displayed by different parts of the molecule, we (i) selected F3 transfected CHO cells expressing increasing levels of F3 at their surface and (ii) prepared transfectants expressing an F3 molecule with its fibronectin type III repeats deleted. We show that the F3 molecule mediates divalent-cation-independent, temperature-dependent binding. The levels of aggregation of F3 transfectants are proportional to the level of F3 expression. Transfectants expressing F3 deleted of the fibronectin type III repeats lose their adhesive properties; conversely, cells expressing wild-type F3 and treated with collagenase, specifically removing the immunoglobulin-like domains, are still able to aggregate. Therefore, in this model adhesion site(s) mapped to the fibronectin type III repeats. By contrast, transfectants expressing deleted F3, as well as the soluble forms of this F3 deleted molecule, were able to stimulate neurite outgrowth of sensory neurons similarly to wild-type F3. Our data indicate that F3 is a multifunctional molecule and that adhesion and neurite outgrowth promoting properties are expressed by distinct and independent domains.