Sensing and responding to energetic stress: The role of the AMPK-PGC1α-NRF1 axis in control of mitochondrial biogenesis in fish

Remodeling the muscle metabolic machinery in mammals in response to energetic challenges depends on the energy sensor AMP-activated protein kinase (AMPK) and its ability to phosphorylate PPAR gamma coactivator 1 alpha (PGC1 alpha), which in turn coactivates metabolic genes through direct and indirect association with DNA-binding proteins such as the nuclear respiratory factor 1 (NRF1) (Wu et al., 1999). The integrity of this axis in fish is uncertain because PGC1 alpha i) lacks the critical Thr(177) targeted by AMPK and ii) has mutations that may preclude binding NRF1. In this study we found no evidence that AMPK regulates mitochondrial gene expression through PGC1 alpha in zebrafish and goldfish. AICAR treatment of zebrafish blastula cells increased phosphorylation of AMPK and led to changes in transcript levels of the AMPK targets mTOR and hexolcinase 2. However, we saw no increases in mRNA levels for genes associated with mitochondrial biogenesis, including PGC1 alpha, NRF1, and COX7C, a cytochrome c oxidase subunit. Further, AMPK phosphorylated mammalian peptides of PGC1 alpha but not the corresponding region of zebrafish or goldfish in vitro. In vivo cold acclimation of goldfish caused an increase in mitochondrial enzymes, AMP and ADP levels, however AMPK phosphorylation decreased. In fish, the NRF1-PGC1 alpha axis may be disrupted due to insertions in fish PGC1 alpha orthologs within the region that serves as NRF1 binding domain in mammals. Immunocopurification showed that recombinant NRF1 protein binds mammalian but not fish PGC1 alpha. Collectively, our studies suggest that fish have a disruption in the AMPK-PGC1 alpha-NRF1 pathway due to structural differences between fish and mammalian PGC1 alpha. (C) 2015 Elsevier Inc. All rights reserved.