Immunological determinants of nerve growth factor involved in p140trk (Trk) receptor binding

Monoclonal anti-NGF antibodies that specifically inhibit the biological activity of mouse beta-NGF were used to study the structural determinants involved in the interaction of NGF with its receptors gp75LNGFR and Trk. None of the three antibodies--N60, M15, and 27/21--showed any reactivity toward denatured NGF. Three experimental methods--radioimmunoassay (RIA), enzyme-linked immunoassay (ELISA), and slot blots--detected no significant cross reactivity between the antibodies and BDNF or NT-3. RIA showed that M15 and N60 recognize the same or an overlapping antigenic site, but 27/21 recognizes a different epitope. Only 27/21, and not N60 or M15, immunoprecipitated beta-NGF crosslinked to LNGFR receptor. Thus, the epitope recognized by 27/21 does not overlap the LNGFR receptor binding site. N60, M15, and 27/21 all block binding of NGF to Trk in a manner consistent with competitive inhibition. Purified Fab fragments of N60 and M15 gave similar results to the intact antibodies. The other subunits present in the 7S complex of NGF, i.e. the alpha and gamma subunits, competitively inhibited binding of antibodies to beta-NGF. Only the gamma subunit inhibited phosphorylation of Trk and biological activity of beta-NGF. These findings suggest that the M15, N60, and 27/21 antibodies bind to a specific site on the surface of NGF where they competitively inhibit binding to the Trk NGF receptor. The region encompassing the N-terminus, the C-terminus, and the loop on the surface of beta-NGF containing residues 60-80 is proposed as important for binding to the Trk receptor.