The cell recognition molecule, cognin, mediates choline acetyltransferase activity in embryonic chick retina

Cell signaling and cell-cell interactions play an important role in neuronal differentiation in the embryonic CNS. Previous work (Hausman, R.E., Vivek Sagar, G.D. and Shah, B.H., Dev. Brain Res., 59 (1991) 31-37) had shown that cholinergic differentiation in the embryonic chick retina depends on insulin and neuron-neuron interactions. Here, we pursued the molecular nature of that dependence on cell interactions. The embryonic chick retina is known to contain several cell adhesion or recognition molecules. We asked if retina cognin, a 50 kDa cell surface-associated protein, played a role in controlling cholinergic differentiation in the developing chick retina. As previously, cholinergic differentiation was measured by two markers: choline acetyltransferase (ChAT) activity and high-affinity choline uptake. We used polyclonal antibody to cognin to determine if blocking cognin-mediated cell interactions would affect the normal embryonic increases in these cholinergic markers. We demonstrated a 40% inhibition of the normal developmental appearance of ChAT activity in retina neuronal cultures from early development, but no effect in cultures from more differentiated retina. The inhibition was selective for retina, since it was not seen in neural tissues like cerebrum and cerebellum that also express ChAT. In contrast to the effect of insulin, choline uptake was not affected by treatment with cognin antibody. Antibodies to two other cell recognition molecules present in the retina (Ng-CAM and N-cadherin) did not block the normal developmental appearance of ChAT. These results suggest that cognin-mediated interactions play a unique role in the control of one aspect of cholinergic differentiation in the developing chick retina.