Mitochondrial Hyperfusion Via Metabolic Sensing of Regulatory Amino Acids

Abstract

Here, we aimed to understand the relationship between nutrient starvation and mitophagy. We found that amino acid starvation of 4T1 breast cancer cells led to clear mitochondrial fusion, entirely consistent with the established model in which elongating mitochondria evade autophagosomes to sustain cellular bioenergetics. As we further dissected amino acid signals, we surprisingly found that supplementation of regulatory amino acids glutamine, leucine and arginine did not reverse, but led to strong mitochondrial hyperfusion. This hyperfusion response to Gln+Leu+Arg appeared distinct from other known remodelling mechanisms, but was dependent upon mitochondrial fusion proteins Mfn1 and Opa1. However, amino-acid dependent hyperfusion did not involve phosphorylation and inhibition of the mitochondrial fission protein Drp1. The ability of Gln+Leu+Arg supplementation to promote mitochondrial hyperfusion was independent of MTORC1, suggesting that the amino acids were being sensed by the mitochondria directly. Accordingly, metabolite profiling confirmed that Gln+Leu+Arg addback led to metabolic reprogramming and replenishment of cell amino acid and nucleotide pools. We found that loss of function in fumarate hydratase blocked Gln+Leu+Arg-dependent mitochondrial hyperfusion, suggesting that TCA-mediated metabolism of supplemented amino acids was driving remodelling. In addition, inhibition of inosine monophosphate dehydrogenase blocked Gln+Leu+Arg-dependent mitochondrial hyperfusion, suggesting a critical role of purine biosynthesis. Amino acid-dependent hyperfusion, like the previously reported stress-induced mitochondrial hyperfusion response, led to a decreased sensitivity towards apoptosis. Thus, we describe here a novel metabolic mechanism for the direct sensing of cellular amino acid levels that is able to control mitochondrial fusion and cell fate.