Evidence for endogenous proteases, mRNA level and insulin as multiple mechanisms of N-cadherin down-regulation during retinal development

Abstract

Our previous studies of the role of cell adhesion in retinal development have focused on the expression and function of N-cadherin, the predominant calcium-dependent intercellular adhesion protein of neural tissues. During the course of retinal development, N-cadherin expression undergoes significant qualitative and quantitative changes in its pattern of expression, most prominently a sharp down-regulation of expression throughout most of the retina. The present studies were directed at investigating the epigenetic mechanisms that could mediate this loss of N-cadherin from the retina. Using an in vitro intact retinal organ culture system, results were obtained which suggest that insulin enhances the down-regulation of N-cadherin expression in a protein-synthesis-dependent fashion. Furthermore, the metalloprotease inhibitor 1,10-phenanthroline inhibits the loss of N-cadherin from the retina. While N-cadherin is down-regulated in organ culture, other cell adhesion molecules, which are not down-regulated in vivo, are also not down-regulated in organ culture. The defined organ culture medium conditioned by the retina accumulates both a soluble 90 × 10(3) M(r) N-terminal fragment of N-cadherin as well as a number of secreted proteases. Both of these components are also shown to be present in vivo in the vitreous humor. Northern blot analysis indicates a single mRNA encoding N-cadherin in the retina and no evidence for a second message that could encode the 90 × 10(3) M(r) fragment. However, the amount of N-cadherin mRNA detectable on northern blots decreases during development. The results reported here suggest that the down-regulation of N-cadherin that occurs during retinal development is possibly mediated by multiple mechanisms, which include turnover at the cell surface mediated by endogenous proteolysis, reduced levels of N-cadherin mRNA and modulation by growth factors.