The Slc25a47-Wars1 locus: characterization of its role in metabolic homeostasis and stress pathways

Our objective was to identify the metabolite transported by SLC25A47, a unique mitochondrial carrier specifically expressed in hepatocytes. To achieve this, we utilized a commercially available mouse model with a hepatocyte-specific knockout of Slc25a47. Unexpectedly, the genetic recombination of the Slc25a47 locus in this mouse model resulted in the knockdown of the cytosolic tryptophanyl tRNA synthetase (Wars1), which is located approximately 3 kb downstream from the Slc25a47 gene. To investigate the acute effects associated with a partial Wars1 loss-of-function, we conducted a co-injection protocol where Slc25a47-Wars1lox/lox mice were administered AAV8_Cre to induce recombination of the locus. Simultaneously, mice were injected with either Slc25a47 or Wars1, allowing us to separate the phenotypic outcomes attributable to Wars1. Knockdown of Wars1 in hepatocytes led to a hypermetabolic phenotype, as evidenced by the improved glucose tolerance and insulin sensitivity, demonstrating that genetic modulation of Wars1, and not Slc25a47, is driving this particular metabolic phenotype. When we compared these phenotypes with those observed in tryptophan-deficient feeding, we discovered that Wars1 knockdown in hepatocytes was sufficient to replicate the whole-body phenotypes associated with an amino acid-deprived diet. Both models exhibited activation of the integrated stress response, characterized by the phosphorylation of eIF2α, which aims to restore cellular homeostasis. Interestingly, we also observed the concurrent induction of the mitochondrial unfolded protein response (UPRmt). The occurrence of UPRmt following Wars1 knockdown was unexpected, considering that the stress originated from the cytosol. This highlights the intricate interplay between the mitochondria and the nucleus, which is crucial for maintaining organelle function and cellular integrity. While research has been conducted on the Mitochondrial-to-Cytosolic Stress Response, the reverse phenomenon, known as the anterograde response, remains unexplored in mammals. Our findings suggest that cytosolic stress, caused by defects in the cytosolic incorporation of amino acids into proteins, can activate the UPRmt. This paradigm shift emphasizes the dynamic bidirectional communication between cellular compartments in orchestrating stress responses and preserving cellular homeostasis.