Neurotrophic interactions in the development of spinal cord motoneurons

The final number of spinal cord motoneurons is attained by a two-step process involving the proliferation of precursor cells and the loss by cell death of a proportion (approximately 50%) of the post-mitotic neurons. Although the mechanisms responsible for the proliferation of stereotyped numbers of motoneurons are not understood, considerable evidence from in vitro as well as in vivo studies indicates that the second step in attaining population size (cell death) is controlled by the interaction of motoneurons with both their efferent targets and their afferent inputs. Target influences on motoneuron survival are thought to be regulated by muscular activity and by competition for limited amounts of neurotrophic factors derived from striated skeletal muscles. However, evidence that such putative neurotrophic factors actually modulate motoneuron survival in vivo has been lacking. Using crude and partially purified extracts from embryonic hindlimbs (Days 8-9) we have found that the treatment of chick embryos in ovo with these agents during the normal cell death period (Days 5-10) rescues a significant number of motoneurons from degeneration. Kidney or lung extracts and heat-inactivated hindlimb extracts were ineffective. The survival-inducing activity of partially purified extract was dose dependent and developmentally regulated. The survival of sensory, sympathetic and a population of cholinergic sympathetic preganglionic neurons was unaffected by treatment with hindlimb extract. The massive motoneuron death that occurs after early target (hindlimb) removal was partially ameliorated by daily treatment with the hindlimb extract. Survival-inducing activity of the extract is lost after trypsin treatment. Taken collectively these results indicate that a target-derived protein or polypeptide neurotrophic factor is involved in the regulation of motoneuron survival in vivo.