Metabolic alterations caused by the mutation and overexpression of the Tmem135 gene

Mitochondria are dynamic organelles that undergo fission and fusion. While they are essential for cellular metabolism, the effect of dysregulated mitochondrial dynamics on cellular metabolism is not fully understood. We previously found that transmembrane protein 135 (Tmem135) plays a role in the regulation of mitochondrial dynamics in mice. Mice homozygous for a Tmem135 mutation (Tmem135^{FUN025/FUN025}) display accelerated aging and age-related disease pathologies in the retina including the retinal pigment epithelium (RPE). We also generated a transgenic mouse line globally overexpressing the Tmem135 gene (Tmem135 TG). In several tissues and cells that we studied such as the retina, heart, and fibroblast cells, we observed that the Tmem135 mutation causes elongated mitochondria, while overexpression of Tmem135 results in fragmented mitochondria. To investigate how abnormal mitochondrial dynamics affect metabolic signatures of tissues and cells, we identified metabolic changes in primary RPE cell cultures as well as heart, cerebellum, and hippocampus isolated from Tmem135^{FUN025/FUN025} mice (fusion > fission) and Tmem135 TG mice (fusion < fission) using nuclear magnetic resonance spectroscopy. Metabolomics analysis revealed a tissue-dependent response to Tmem135 alterations, whereby significant metabolic changes were observed in the heart of both Tmem135 mutant and TG mice as compared to wild-type, while negligible effects were observed in the cerebellum and hippocampus. We also observed changes in Tmem135^{FUN025/FUN025} and Tmem135 TG RPE cells associated with osmosis and glucose and phospholipid metabolism. We observed depletion of NAD⁺ in both Tmem135^{FUN025/FUN025} and Tmem135 TG RPE cells, indicating that imbalance in mitochondrial dynamics to both directions lowers the cellular NAD⁺ level. Metabolic changes identified in this study might be associated with imbalanced mitochondrial dynamics in heart tissue and RPE cells which can likely lead to functional abnormalities. Impact statement: Mitochondria are dynamic organelles undergoing fission and fusion. Proper regulation of this process is important for healthy aging process, as aberrant mitochondrial dynamics are associated with several age-related diseases/pathologies. However, it is not well understood how imbalanced mitochondrial dynamics may lead to those diseases and pathologies. Here, we aimed to determine metabolic alterations in tissues and cells from mouse models with over-fused (fusion > fission) and over-fragmented (fusion < fission) mitochondria that display age-related disease pathologies. Our results indicated tissue-dependent sensitivity to these mitochondrial changes, and metabolic pathways likely affected by aberrant mitochondrial dynamics. This study provides new insights into how dysregulated mitochondrial dynamics could lead to functional abnormalities of tissues and cells.