An unexpected role for the transcriptional coactivator isoform NT-PGC-1α in the regulation of mitochondrial respiration in brown adipocytes

Brown adipose tissue dissipates energy as heat, a process that relies on a high abundance of mitochondria and high levels of electron transport chain (ETC) complexes within these mitochondria. Two regulators of mitochondrial respiration and heat production in brown adipocytes are the transcriptional coactivator PGC-1 alpha and its splicing isoform NT-PGC-1 alpha, which control mitochondrial gene expression in the nucleus. Surprisingly, we found that, in brown adipocytes, some NT-PGC-1 alpha localizes to mitochondria, whereas PGC-1 alpha resides in the nucleus. Here we sought to investigate the role of NT-PGC-1 alpha in brown adipocyte mitochondria. Immunocytochemistry, immunotransmission electron microscopy, and biochemical analyses indicated that NT-PGC-1 alpha was located in the mitochondrial matrix in brown adipocytes. NT-PGC-1 alpha was specifically enriched at the D-loop region of the mtDNA, which contains the promoters for several essential ETC complex genes, and was associated with LRP130, an activator of mitochondrial transcription. Selective expression of NT-PGC-1 alpha and PGC-1 alpha in PGC-1 alpha(-/-) brown adipocytes similarly induced expression of nuclear DNA-encoded mitochondrial ETC genes, including the key mitochondrial transcription factor A(TFAM). Despite having comparable levels of TFAM expression, PGC-1 alpha(-/-) brown adipocytes expressing NT-PGC-1 alpha had higher expression of mtDNA-encoded ETC genes than PGC-1 alpha(-/-) brown adipocytes expressing PGC-1 alpha, suggesting a direct effect of NT-PGC-1 alpha on mtDNA transcription. Moreover, this increase in mtDNA-encoded ETC gene expression was associated with enhanced respiration in NT-PGC-1 alpha-expressing PGC-1 alpha(-/-) brown adipocytes. Our findings reveal a previously unappreciated and isoform-specific role for NT-PGC-1 alpha in the regulation of mitochondrial transcription in brown adipocytes and provide new insight into the transcriptional control of mitochondrial respiration.