A number of neural crest derivatives have been shown, by manipulation of developmental signals in cell culture, to display remarkable plasticity in the range of phenotypes they can assume. The interconversion of cell types observed in culture has been further shown to reflect differentiation choices made by certain derivatives during normal development in vivo. It is proposed to identify the precursors and intermediate cell types that give risk to the various members of the sympathoadrenal lineage, to characterize the cellular and hormonal signals that control these differentiation decisions, and to determine the role of these signals in vivo. I. A protein secreted by heart cells can convert postmitotic, noradrenergic neurons into cholinergic neurons. we have purified this molecule to homogeneity and sequenced its 11 N-terminal amino acids. Antisera against a synthetic peptide of this sequence precipitate the cholinergic factor and directly block its activity. The antibodies and the peptide will be used to perturb the development of cholinergic neurons ia vivo, as well as to devise a RIA for localization of the factor in vivo. Oligonucleotide probes corresponding to this sequence are being used to screen heart cell cDNA libraries in an effort to clone the gene for this protein. Successful cloning would enable us (i) to use an expression system to produce large amounts of the protein for in vivo injections and chemical characterization, and (ii) to examine the possibility that there could be a gene family of such differentiation factors. II. Our previous work on the interconversion of adrenal chromaffin cells, small intensely fluorescent (SIF) cells and sympathetic neurons led too the hypothesis that SIF cells might be the central intermediate cell type of the sympathoadrenal lineage. Our new findings suggest that the putative precursor cell is not a mature SIF cell, but a cell that shares certain markers with both SIF and chromaffin cells. It is proposed to isolate these putative precursors in culture in order to determine what derivatives they can give rise to and what the nature of the signals are that influence these phenotypic choices. The relationship of these precursors of neural crest cells will be probed through the production of new lineage markers using our immunosuppression technique.