Neurite outgrowth-promoting factors in extracts of denervated chick skeletal muscle

1. An extract of denervated skeletal muscle contained activity for promotion of neurite outgrowth from telencephalic neurons, as well as that from neurons in the spinal cord. A factor responsible for the activity was characterized in cultures of dissociated neurons. 2. The factor acted on neurons only when they were attached to the surface of culture dishes. Since treatments with proteases and lectins reduced the outgrowth-promoting activity, the factor was thought to be a glycoprotein. 3. Among the monoclonal antibodies raised against the partially purified extract, five antibodies were found to inhibit the activity for spinal and telencephalic neurons. The most potent antibody, 4D2a, recognized mainly a 63-kD protein and other minor proteins in the extract. Although the 63-kD protein was confirmed to be chick serum albumin by analysis of amino acid sequence, the purified albumin exhibited no activity. 4. From these observations, the factor was found to be a glycoprotein recognized by the neutralizing antibody as one of the minor components of the extract. This factor exhibits its activity in a substrate-bound form but not in a diffusible one.