Activity-dependent transcriptional regulation of nuclear respiratory factor-1 in cultured rat visual cortical neurons

Abstract

Nuclear respiratory factor 1 is a transcription factor involved in the regulation of mitochondrial biogenesis by activating the transcription of subunit genes of cytochrome oxidase and other respiratory enzymes. Very little is known of its role in neurons. To determine if neuronal activity regulates nuclear respiratory factor 1 expression, cultured primary neurons from postnatal rat visual cortex were subjected to 20 mM KCl depolarizing treatment for 1, 3, 5, and 7 h, or exposed to 7 h of KCl followed by withdrawal for 1, 3, 5, and 7 h. Nuclear respiratory factor 1 expression was analyzed by immunoblots, immunocytochemistry, quantitative electron microscopy, real-time quantitative PCR, and in situ hybridization. Nuclear respiratory factor 1 protein was expressed at relatively low basal levels in both the nucleus, where it was associated primarily with euchromatin, and in the cytoplasm, where it was localized to free ribosomes and occasionally to the Golgi apparatus and the outer nuclear membrane. Depolarizing treatment progressively up-regulated both nuclear respiratory factor 1 protein and mRNA in a time-dependent manner, increasing above controls after 1 h and remaining high at 3, 5, and 7 h. Both nuclear and cytoplasmic mRNA levels increased with stimulation, and there was an apparent cytoplasmic-to-nuclear translocation of protein. Following the withdrawal of KCl, both nuclear respiratory factor 1 message and protein were significantly reduced after 1 h. The message returned to basal levels by 5 h and the protein by 7 h. These results strongly indicate that the expression and compartmental redistribution of nuclear respiratory factor 1 protein and mRNA in visual cortical neurons are dynamic processes tightly controlled by neuronal activity.