Neuronal expression of nuclear transcription factor MafG in the rat medulla oblongata after baroreceptor stimulation.

The medulla oblongata is the site of central baroreceptive neurons in mammals. These neurons express specific basic-leucine zipper transcription factors (bZIP) after baroreceptor stimulation. Previously we showed that activation of baroreceptors induced expression of nuclear transcription factors c-Fos and FosB in central baroreceptive neurons. Here we studied the effects of baroreceptor stimulation on induction of MafG, a member of small Maf protein family that functions as dimeric partners for various bZIP transcription factors by forming transcription-regulating complexes, in the rat medulla oblongata. To determine whether gene expression of MafG is induced by stimulation of arterial baroreceptors, we examined the expression of its mRNA by semi-quantitative reverse transcription-PCR method and its gene product by immunohistochemistry. We found that the number of MafG transcripts increased significantly in the medulla oblongata after baroreceptor stimulation. MafG-immunoreactive neurons were distributed in the nucleus tractus solitarii, the dorsal motor nucleus of the vagus nerve, the ambiguous nucleus and the ventrolateral medulla. The numbers of MafG-immunoreactive neurons in these nuclei were significantly greater in test rats than in saline-injected control rats. We also found approximately 20% of MafG-immunoreactive neurons coexpress FosB after baroreceptor stimulation. Our results suggest that MafG cooperates with FosB to play critical roles as an immediate early gene in the signal transduction of cardiovascular regulation mediated by baroreceptive signals in the medulla oblongata.

Extracellular acidification enhances DNA binding activity of MafG-FosB heterodimer.

Cells are quite sensitive to a change of the extracellular pH and respond to it through detection of the H+/HCO3- level in extracellular fluid. However, little is known about molecular details induced by acidosis, such as intracellular pathways and gene expression. Here we describe properties of gene expression, protein interaction, and DNA binding activity of basic region leucine zipper (bZIP) transcription factor Maf and FosB during extracellular acidification. When cells were incubated with low pH medium, the expressions of small Maf proteins (MafG, MafK, and MafF) and FosB were clearly increased in an extracellular pH-dependent manner and expressed transiently with a peak after 1-2 h after stimulation. Immunofluorescence and protein binding studies indicated that MafG was partially co-localized with FosB in the nucleus and MafG can form heterodimers with FosB at extracellular pH 7.40. Moreover, we found that MafG-FosB complexes are able to bind to AP-1 consensus sequence, TGACTCA. To investigate whether extracellular acidification influences to dimerization and DNA binding activity of MafG and FosB, extracellular pH of cultured cells was decreased from 7.40 to 6.80. The decrease in extracellular pH led to enhanced dimerization of MafG with FosB leading to augmentation of the DNA binding activity of the heterodimer to AP-1 consensus sequence. Moreover, extracellular acidification induces mRNA expression of matrix metalloproteinase-1, one of the genes that are regulated by AP-1. These results suggest that MafG-FosB complexes are involved in transcriptional regulation in response to extracellular acidification.

Neuronal expression of Fos protein in the brain after intravenous injection of gastrin in rats.

The present study examined whether the central neurons are involved in the stimulatory action of gastrin on the secretion of gastric acid. Gastrin (20 microg), which was examined and ascertained to induce a marked increase in gastric acid secretion in gastric-lumen perfused rats, was intravenously injected in Wistar rats under anesthesia with pentobarbital sodium. In the experiments, 1 h after injecting gastrin, rats were perfused and fixed, the brain was removed and sectioned at 40 microm thickness. Every fourth section was treated with anti-c-Fos antiserum, and c-Fos protein was immunohistochemically stained using the avidin-biotin complex method. It was found that c-Fos protein was expressed in neurons of the lateral habenular nucleus, the central nucleus amygdala, the lateral parabrachial nucleus in the pons, and the complex area of the nucleus of the solitary tract and the dorsal motor nucleus of the vagus nerve in the medulla oblongata. The control rats were injected with saline solution, and the brain sections were processed similarly as described above. c-Fos protein was expressed in few neurons in the nuclei above in the control rats. These results suggest that gastrin released into the circulation might stimulate central neurons which, in turn, may relate to the control mechanism for the secretion of gastric acid.