Quantitative immuno-electron microscopic analysis of depolarization-induced expression of PGC-1α in cultured rat visual cortical neurons

Peroxisome proliferator-activated receptor-γ coactivator 1α (PGC- 1α) is a coactivator of nuclear receptors and other transcription factors that regulate several metabolic processes, including mitochondrial biogenesis, energy homeostasis, respiration, and gluconeogenesis. PGC-1α plays a vital role in stimulating genes that are important to oxidative metabolism and other mitochondrial functions in brown adipose tissue and skeleton muscles, but the significance of PGC-1α in the brain remains elusive. The goal of our present study was to determine by means of quantitative immuno-electron microscopy the expression of PGC-1α in cultured rat visual cortical neurons under normal conditions as well as after depolarizing stimulation for varying periods of time. Our results showed that: (a) PGC-1α was normally located in both the nucleus and the cytoplasm. In the nucleus, PGC-1α was associated mainly with euchromatin rather than heterochromatin, consistent with active involvement in transcription. In the cytoplasm, it was associated mainly with free ribosomes. (b) Neuronal depolarization by KCl for 0.5 h induced a significant increase in PGC-1α labeling density in both the nucleus and the cytoplasm (P < 0.01). The heightened expression continued after 1 and 3 h of depolarizing treatment (P < 0.01), but decreased from 5 h onward and returned to baseline level by 10 h. These results indicate that PGC-1α responds very early to increased neuronal activity by synthesizing more proteins in the cytoplasm and translocating them to the nucleus for gene activation. PGC-1α level in neurons is, therefore, tightly regulated by neuronal activity.