Selective effects of skeletal muscle extract fractions on motoneuron development in vitro.

In vivo labeling of motoneurons with retrogradely transported wheat germ agglutinin, and in situ staining of cholinergic neurons with a monoclonal antibody against CAT, have been used as methods to identify motoneurons in cultures of dissociated 14 d embryonic rat ventral spinal cord. With these two different labeling techniques, we demonstrate that skeletal muscle extracts maintain motoneuron survival, and induce motoneuron morphologic and cholinergic differentiation in vitro. Further, we show that several partially purified muscle extract-derived proteins separately augment each of these specific parameters of motoneuron development. A 55,000 Da neutral glycoprotein specifically induces both ACh synthesis and motoneuron-selective process outgrowth and elongation, while a 1200-1500 Da peptide selectively augments both apparent motoneuron survival and ACh synthesis. A third trophic agent, a 33,000-35,000 Da acidic glycoprotein, induces process outgrowth in both motoneurons and nonmotoneurons of the ventral spinal cord, but has no effect on ACh synthesis.