Response of purified chick motoneurons to myotube conditioned medium: laminin is essential for the substratum-binding, neurite outgrowth-promoting activity.

Purified motoneurons from chick, cultured on polycationic substrata treated with myotube-conditioned (MCM), respond by rapidly extending neurites. When MCM, partially purified by salt precipitation and ion-exchange chromatography, was fractionated on Sepharose CL-4B, the peak of neurite outgrowth-promoting activity (NOPA) corresponded to a peak of laminin (LA) immunoreactivity. Fractions from this peak contained a protein band that comigrated with an LA standard on sodium dodecyl sulfate gels. Antibodies to LA immunoprecipitated all motoneuron NOPA from MCM, and the specifically immunoprecipitated material comigrated with LA in both reducing and non-reducing gels. It thus appears that LA in MCM is essential for the ability of this conditioned medium to promote motoneuron neurite outgrowth.