Mtu1-Mediated Thiouridine Formation of Mitochondrial tRNAs Is Required for Mitochondrial Translation and Is Involved in Reversible Infantile Liver Injury

摘要

Reversible infantile liver failure (RILF) is a unique heritable liver disease characterized by acute liver failure followed by spontaneous recovery at an early stage of life. Genetic mutations in MTU1 have been identified in RILF patients. MTU1 is a mitochondrial enzyme that catalyzes the 2-thiolation of 5-taurinomethyl-2-thiouridine (τm~(5)s~(2)U) found in the anticodon of a subset of mitochondrial tRNAs (mt-tRNAs). Although the genetic basis of RILF is clear, the molecular mechanism that drives the pathogenesis remains elusive. We here generated liver-specific knockout of Mtu1 (Mtu1~(LKO)) mice, which exhibited symptoms of liver injury characterized by hepatic inflammation and elevated levels of plasma lactate and AST. Mechanistically, Mtu1 deficiency resulted in a loss of 2-thiolation in mt-tRNAs, which led to a marked impairment of mitochondrial translation. Consequently, Mtu1~(LKO)mice exhibited severe disruption of mitochondrial membrane integrity and a broad decrease in respiratory complex activities in the hepatocytes. Interestingly, mitochondrial dysfunction induced signaling pathways related to mitochondrial proliferation and the suppression of oxidative stress. The present study demonstrates that Mtu1-dependent 2-thiolation of mt-tRNA is indispensable for mitochondrial translation and that Mtu1 deficiency is a primary cause of RILF. In addition, Mtu1 deficiency is associated with multiple cytoprotective pathways that might prevent catastrophic liver failure and assist in the recovery from liver injury. Author Summary Mitochondrial transfer tRNA (mt-tRNA) contains a variety of chemical modifications that are introduced post-transcriptionally. Three mt-tRNAs for Lys, Gln and Glu contain 5-taurinomethyl-2-thiouridine (τm~(5)s~(2)U) in their anticodons. It is known that the loss of 2-thiolation of τm~(5)s~(2)U is strongly associated with the development of reversible infantile liver failure (RILF) because pathogenic mutations of RILF were found in the MTU1 gene, which encodes an enzyme responsible for the 2-thiolation of τm~(5)s~(2)U. However, the molecular mechanism underlying RILF pathogenesis associated with a lack of MTU1 remains elusive. To understand the physiological function of MTU1 and its association with liver failure, we generated liver-specific Mtu1-deficient (Mtu1~(LKO)) mice. Mtu1 deficiency abolished 2-thiouridine formation in the three mt-tRNAs. Loss of the 2-thiouridine modification resulted in a marked impairment of mitochondrial translation and abnormal mitochondrial structure. Consequently, the Mtu1~(LKO)mice exhibited liver injury, which resembles the symptoms of RILF patients. Furthermore, mitochondrial dysfunction in Mtu1~(LKO)mice induced mitochondrial biogenesis and suppressed oxidative stress. These findings elucidate the cellular and physiological functions of Mtu1 and provide a mouse model for understanding RILF pathogenesis.