CELL-TYPE-SPECIFIC EXPRESSION AND REGULATION OF A C-FOS-NGF FUSION GENE IN NEURONS AND ASTROCYTES OF TRANSGENIC MICE

A mouse line transgenic for nerve growth factor (NGF) was developed us ing the mouse prepro-NGF cDNA inserted within a plasmid containing the proximal region (- 10 to - 550 bp) of the c-fos promoter and the tran scription termination and polyadenylation signals of the rabbit beta-g lobin gene. No significant modification of gross behavior or central n ervous system anatomy was detected in adult animals as assessed by imm unohistochemistry and in situ hybridization for NGF and choline acetyl transferase. The expression of the transgene and the possible regulati on of its expression by agents acting on the promoter were investigate d in vitro. Despite the presence of an additional pool of NGF mRNA spe cific to the transgene, basal levels of NGF in the supernatant of tran sgenic astrocytes were similar to normal ones. On the other hand, tran sgenic neurons spontaneously synthesized and released levels of NGF tw o to three times higher than normal neurons, while mRNA levels were ba rely detectable by conventional Northern blotting. The tissue-specific ity of NGF expression was respected, with higher levels in hippocampal than neocortical neurons. Increases of NGF mRNA by agents acting on t he promoter could be observed in normal and transgenic astrocytes only after inhibition of the protein synthesis by cycloheximide, suggestin g a similar rapid turnover of normal and transgenic transcripts. Cycli c AMP agonists specifically increased the secretion of NGF protein by transgenic astrocytes and neurons, while activators of the protein kin ase C had a similar effect on transgenic and normal cells. Differences between amounts of NGF secreted by neurons and astrocytes with regard s to their respective content in mRNA suggest that transgenic transcri pts are subject to normal cell- and tissue-specific post-transcription al regulations. Agents acting on the c-fos promoter through the protei n kinase C or cyclic AMP routes differentially increased the secretion of NGF by transgenic astrocytes or neurons, supporting this hypothesi s.