The contrasting roles of N-CAM and N-cadherin as neurite outgrowthpromoting molecules

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Abstract
The neural cell adhesion molecule (N-CAM) is a prominent member of the immunoglobulin gene superfamily of recognition molecules. It operates in a calcium-independent manner to promote cell–cell adhesion. Alternative splicing of a single gene generates more than twenty N-CAM isoforms and these can be further modified by the differential addition of complex N- and O-linked carbohydrates. In contrast, N-cadherin is a major calcium-dependent adhesion molecule in the brain; it is not a member of the immunoglobulin gene superfamily and, as far as we know, exists as a single gene product with no evidence of differential post-translational modification. Both molecules are believed to operate through a homophilic binding mechanism and both are expressed at key developmental times in a number of tissues including the brain. Antibody perturbation experiments suggest that both of the above cell adhesion molecules (CAMs) can support neurite outgrowth over complex cellular substrata such as astrocytes and Schwann cells. In the present review we discuss the use of a molecular genetic approach to study the neurite outgrowth-promoting activity of these molecules. Using this approach we have found that both CAMs are potent-inducers of neurite outgrowth from a variety of neurons. However, whereas a critical value of N-CAM expression is required for increased neurite outgrowth, with small increases above this value having substantial effects, N-cadherin promotes neurite outgrowth in a highly linear manner. In addition, whereas N-CAM promotes chick retinal ganglion cell (RGC) neurite outgrowth at E6 but not E11, N-cadherin does so throughout this developmental period. These studies show fundamental differences in neuronal responsiveness to CAMs, and suggest a more dynamic regulation for N-CAM-dependent neurite outgrowth than for N-cadherin-dependent neurite outgrowth.