THYROID-HORMONES PRECOCIOUSLY INCREASE NERVE GROWTH-FACTOR GENE-EXPRESSION IN THE SUBMANDIBULAR-GLAND OF NEONATAL MICE

The developmental regulation of the expression of nerve growth factor (NGF) was studied in the mouse submandibular gland (SMG). Having demon strated that, in the neonatal mouse, maturation of the SMG can be acce lerated by treatment with thyroid hormones, with the resulting inducti on in SMG content of NGF, studies were undertaken to further examine t he locus of thyroid hormone action. Because of the sexual dimorphism o f the SMG, both male and female neonatal mice were used. NGF messenger RNA levels were undetectable in SMGs from untreated immature mice, wh ile hybridization to total RNA from T4-treated mice was easily observa ble for NGF complementary DNA. Treatment for 14 days compared to 7 day s resulted in a 7-fold increase in SMG NGF mRNA levels. A signal was o btained in 21-day-old control mice using S1 nuclease protection analys is; T4 increased NGF mRNA levels by 100-fold in both male and female i mmature mice. Heteronuclear RNA levels were induced 20-fold by T4. No sex differences were readily observable. Determination of the effect o f thyroid hormone treatment on SMG NGF gene expression by nuclear run- on assay demonstrated a significant transcriptional effect of T4. Init ial experiments using the pmngf6 vector, which is a pBR322-derived pro be containing the full length NGF cDNA, showed a 2.5-fold induction of gene transcription. When an internal fragment of pmngf6 was subcloned into pTZ18R, thus removing the dC/dG tails contained in pmngf6, the b ackground hybridization was considerably reduced and a 12.5-fold induc tion in NGF gene transcription was obtained after T4 treatment of neon atal mice. The results show that thyroid hormones increase NGF gene ex pression in the SMG of the immature male and female mouse. This effect is due in part to a significantly enhanced rate of gene transcription .