INTEGRIN ALPHA(2)BETA(1) MEDIATES INTERACTIONS BETWEEN DEVELOPING EMBRYONIC RETINAL CELLS AND COLLAGEN

In the developing nervous system, the extracellular matrix provides a source of extrinsic cues to guide determination of cell fate, neurobla st migration, axon outgrowth and synapse formation. In the neural reti na, undifferentiated neuroepithelial precursor cells contact extracell ular matrix that contains multiple collagen types. Collagens have been shown to support retinal cell adhesion and neurite outgrowth, but the integrin receptors mediating neuronal responses are not understood, H ere we provide evidence that integrin alpha(2) beta(1) acts as a colla gen receptor in the developing avian retina and examine its expression pattern. Using a recently described monoclonal antibody, MEP-17, alph a(2) protein was detected in the developing retina by immunofluorescen ce in tissue sections and dissociated cells, and by immunoprecipitatio n, At embryonic day 4 (E4), when the majority of retinal cells are und ifferentiated neuroepithelial cells, alpha(2) immunoreactivity in sect ions was widespread and about half of cells dissociated in culture wer e alpha(2) positive. At E6, after the retinal ganglion cell layer had differentiated, immunoreactivity in sections decreased in the central, more developed portion of the retina and 25% of dissociated cells wer e alpha(2) positive, E6 retinal ganglion cells, identified by neurofil ament immunoreactivity, did not express detectable alpha(2) immunoreac tivity. Immunoprecipitation experiments using E6 extracts demonstrated that the alpha(2) subunit was paired with the beta(1) integrin subuni t. By E12, alpha(2) immunoreactivity in sections was confined to the e xtreme peripheral retina, although the antigen may be masked since exp ression levels comparable to or slightly higher than E6 could be detec ted in dissociated cells and extracts. By employing function blocking antibodies, it was shown that alpha(2)beta>(1) integrin is necessary f or cell adhesion and process outgrowth by embryonic retinal cells on c ollagens I and IV. Although alpha(2) expression continued through E12, alpha(2) activity was down regulated with increasing embryonic age, s ince alpha(2)-dependent adhesion and outgrowth declined. These data su ggest a role for alpha(2) beta(1) in neuroepithelial cell interactions with collagen rather than for axon extension by retinal ganglion cell s.