Phosphotyrosine-modified proteins are localized in Müller cells of the chick neural retina

Phosphorylated protein substrates of tyrosine-specific protein kinases accumulate within areas of high density of neuronal and glial processes in the avian retina. To identify the cell type(s) containing elevated tyrosine kinase activity, phosphotyrosine-modified proteins were localized in the outer neural retina of the hatchling chick by immunoelectron microscopy using phosphotyrosine antibodies. In the outer retina, phosphotyrosine immunoreactivity was confined to the plasmalemma and cytoplasm of Müller cells. Prominent membrane labeling was associated with sites adjoining photoreceptor inner segments, cell bodies, and synaptic terminals. Immunoreactivity was not associated with pre- or post-synaptic membranes, synaptic ribbons, or synaptic vesicles. Immunoreactivity was also observed at sites of apposition between plasma membranes of adjacent Müller glial processes. Phosphotyrosine antibodies recognized nine principal proteins (106, 83, 68, 60, 52, 46, 44, 39, 38 kDa) in retinal extracts revealed by immunoblotting. The 60, 52, and 38 kDa phosphotyrosine-modified proteins were also prominent in extracts prepared from Müller glia-enriched cultures, but were absent or low in neuronal cultures; consistent with the higher immunolabeling of glia in retinal sections. Protein tyrosine kinases encoded by the fyn and yes proto-oncogenes were expressed in Müller glia-enriched cultures, and could contribute to the observed protein tyrosine phosphorylation. The localization of phosphotyrosine-modified proteins in the Müller glia leads us to suggest a possible role for protein tyrosine kinases in communication between Müller glia and neuronal cells.