TENASCIN-CONTACTIN F11 INTERACTIONS - A CLUE FOR A DEVELOPMENTAL ROLE/

To understand how the extracellular matrix glycoprotein tenascin modif ies cell adhesion and neurite outgrowth, we sought to isolate cellular receptors for tenascin. So far, two completely different cell surface ligands for tenascin have been detected. This we achieved by affinity chromatography of tissue extracts and of isolated proteins over tenas cin-Sepharose and by solid-phase assays using the individual proteins. The first receptor, the neuronal cell adhesion molecule contactin/F11 , a member of the immunoglobulin superfamily, binds to tenascin via a site in the N-terminal immunoglobulin-like domains. The binding site i s within the fibronectin type III homology region at the boundary of t he alternatively spliced region of tenascin, requiring that fibronecti n type III homology domains 5 and 9 be adjacent, as they are in the 19 0 kD tenascin isoform. The close similarity in tertiary structure betw een type III domains and immunoglobulin-like repeats raises the possib ility that we are observing a side-by-side interaction between the two molecules in a manner closely analogous to that between paired immuno globulin domains. The second receptor is the heparan sulfate proteogly can, glypican, which, similarly to contactin/F11, is anchored to the m embrane via glycosylphosphatidylinositol. Glypican bound to a column o f tenas-cin-Sepharose cannot be dissociated by chondroitin sulfate or dermatan sulfate, but elutes in a broad peak with a gradient of hepara n sulfate and in a sharper peak with heparin. By means of fusion prote ins, we have identified a potential binding site on the fifth fibronec tin type III homology domain of tenascin. We are trying to define thes e sites more closely by means of site-directed mutagenesis. It will be interesting to see whether the interaction between tenascin and cell surface contactin/F11, and possibly cellular heparan sulfate proteogly cans, contributes to the prominent role played by tenascin in pattern formation during development of the nervous system. In a first step, w e have examined the distribution of tenascin isoforms and contactin/F1 1 during retinal development by means of immunohistochemistry and in s itu hybridization with tenascin isoform-specific probes. Tenascin isof orms 190/200 along with contactin/F11 are particularly prominent in th e inner and outer plexiform layers of embryonic day 8 retina in the ch ick. This coordinate up-regulation was confirmed both by immunoblots a nd Northern blots of retinal extracts. A speculative model is presente d to suggest how the unique hexabrachion may signal the cell via conta ctin/F11.