RAPHE NEURAL CELLS IMMORTALIZED WITH A TEMPERATURE-SENSITIVE ONCOGENE- DIFFERENTIATION UNDER BASAL CONDITIONS DOWN AN APUD CELL LINEAGE

Dividing cells from the midline of the ventral rhombencephalon and med ulla oblongata have been transduced with a modulatable oncogene, (ts)S V40-T, using retroviral gene transfer. At the permissive temperature o f the oncogene (33 degrees C), cells replicated and were isolated as i ndividual, homogeneous clones. The effects of simply raising the tempe rature to the oncogene's non-permissive value, namely 39 degrees C, we re analyzed by immunohistochemical methods. In one clone in particular (921202-6), cells ceased replication and started to differentiate. Ce rtain neuronal characteristics became apparent: neurone-specific enola se-like immunoreactivity developed, as did the ability to take up exog enously applied 5-hydroxytryptamine (5HT). In addition, the cells took up exogenous 5-hydroxytryptophan (5HTP), and subsequently decarboxyla ted it to 5HT. However, they were unable to synthesize immunohistochem ically detectable amounts of 5HT using L-tryptophan as a precursor. No 5HT uptake was found either in mitotic cells of this clone held at 33 degrees C, or in several other neuronal clones differentiating at 39 degrees C. Neither the neuronal nor the serotoninergic characteristics of clone 921202-6 developed in the presence of retinoic acid. It is c oncluded that 921202-6 cells differentiate under basal conditions down a neuronal pathway typical of an APUD cell, and that the choice of th is pathway is made prior to the end of cell cycling. Furthermore, pred isposition of the precursor cells to the neuronal/APUD phenotype can b e overridden by extraneous epigenetic factors.