DEGRADATION FRAGMENTS OF L1-ANTIGEN ENHANCE TYROSINE HYDROXYLASE-IMMUNOREACTIVE NEURITE OUTGROWTH IN MESENCEPHALIC CELL-CULTURE

The L1 antigen has been implicated in adhesion events controlling axon al elongation during formation of major fiber tracts, and promotes neu rite outgrowth in culture. It is possible that injury of brain tissue causes neuronal surface molecules such as L1 antigen to be shed, and d egradation fragments may therefore be present adjacent to the damage. These Ll fragments might then influence regeneration or injury-induced growth. We have evaluated neurite outgrowth from tyrosine hydroxylase -positive (TH+) E13 mesencephalic neurons grown in vitro on a substrat e of mouse Ll antigen and on L1 degradation fragments separated by mol ecular weight. Mouse myelin-associated glycoprotein (MAG), laminin, po ly-D-lysine, and fetal calf serum served as control substrates. L1 ant ibodies were added to one set of cultures (experimental), and compared to control cultures containing normal rabbit serum. After 3 days in v itro, the cultures were stained using an antibody against TH, and the length of the TH+ neurites was measured by computer-assisted image ana lysis in a double-blind fashion. TH+ neurites were significantly longe r when grown on Ll antigen, as well as on Ll degradation fragments, as compared to the control substrates. As compared to control normal rab bit serum, L1 antibodies eliminated the neurite-promoting effect of bo th the Ll substrate and the Ll degradation products. In addition, Ll s ubstrates promoted clustering of TH+ cells and the formation of loose bundles of TH+ neurites. It is suggested that the L1 substrates influe nce TH-immunoreactive neurite outgrowth, at least partially, through i ndirect effects on glial cells. OUT data indicate that the presence of L1 fragments in vivo might influence regeneration or synaptic restruc turing. Moreover, we speculate that the studies of large proteins as s ubstrates in cell culture may be influenced by protein instability, an d that some studies may have examined the functions of protein fragmen ts rather then intact proteins.