Trophic effects of skeletal muscle extracts on ventral spinal cord neurons in vitro: separation of a protein with morphologic activity from proteins with cholinergic activity.

Protein factors derived from skeletal muscle separately promote neurite elongation and acetylcholine synthesis in cultured rat ventral spinal neurons. Morphologic factor activity (neurite-inducing activity) is specifically found in rat skeletal muscle and cord neuron extracts, decreases with the postnatal age of the rats from which muscle extract is prepared, and increases in rat hindlimb muscle after 5 d of denervation. Cholinergic factor activity (acetylcholine synthesis-stimulating activity) is found in extracts of rat cerebral cortex and cardiac muscle in addition to spinal cord and skeletal muscle, increases with animal age, and decreases following 5 d of denervation. Biochemically, the factors responsible for these activities differ in their lability to denaturing conditions, apparent molecular weights, isoelectric points, and lectin-binding specificities. Under reducing conditions, morphologic activity is isolated in a single acidic glycoprotein with an Mr of 35,000, while acetylcholine synthesis-stimulating activity is found in multiple species of different molecular weights. Thus, acetylcholine synthesis-promoting activities and neurite growth-promoting activity appear to reside in different molecules. Significant purification of several of these factors has been achieved.