Novel mechanism of neural and muscular degeneration

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Ant1 is the muscle/heart/central nervous system (CNS) isoform of adenine nucleotide translocase that is primarily involved in ATP/ADP exchange across the inner mitochondrial membrane (IMM). An increasing number of missense mutations in Ant1 are found to cause dominant diseases that affect skeletal muscle and the central nervous system. These diseases are commonly manifested by fractional mtDNA deletions and mild bioenergetic defects. The mechanism of neuromuscular damage in the diseases is poorly understood. Interestingly, our recent studies in yeast and cultured human cells suggested that the mutant Ant1 is misfolded. This leads to cell death by a novel mechanism that we named mitochondrial Precursor Overaccumulation Stress (mPOS). mPOS is characterized by the toxic accumulation and aggregation of un-imported mitochondrial preproteins in the cytosol. These findings led to the central hypothesis that the mutant Ant1 primarily affects mitochondrial protein import. This results in mPOS in the cytosol, which plays an important role in inducing neural and muscular degeneration. Fractional mtDNA deletions occur independent of nucleotide transport activity, likely as a secondary damage collateral to reduced mitochondrial protein import. In this application, we propose to directly test this hypothesis in mouse models. We successfully generated knock-in (KI) mouse lines expressing misfolded variants of Ant1. Preliminary studies indicated that these mice develop phenotypes consistent with neural and muscular degeneration. In Specific Aim 1, we will use these unique experimental models to test the hypothesis that misfolded Ant1 induces neural and muscular degeneration and mtDNA instability independent of nucleotide transport. In Specific Aim 2, we will use various experimental tools that we developed in yeast, cultured human cells and the Ant1-KI mice to test the hypothesis that the misfolded Ant1 (or Aac2 in yeast) causes structural and functional damage to the mitochondrial protein import machinery and induces mPOS in the cytosol. In Specific Aim 3, we will determine the mechanisms that protect cells against Ant1-induced protein import stress and mPOS. Success of the project will establish a mouse model of protein import stress associated with mPOS. Particularly, validation of the mPOS model would help reconciling the mitochondrial and proteostatic pathways in many neural and muscular degenerative diseases. Finally, the results could have important implications for the understanding and therapy of Ant1-induced diseases, as well as many other clinical conditions that directly or indirectly affect mitochondrial protein import.