Sirt5 Plays a Critical Role in Mitochondrial Protein Acylation and Mitochondrial Metabolic Homeostasis in Brown Fat

Brown adipose tissue (BAT) is rich in mitochondria and plays important roles in energy expenditure, thermogenesis and glucose homeostasis. Recent studies have shown that mitochondrial activity can be regulated by protein succinylation and malonylation, processes that are in part controlled by Sirt5, the major mitochondrial desuccinylase and demalonylase. Compared to other tissues, brown fat has high levels of protein malonylation and succinylation, which were further increased in mice lacking Sirt5. BAT specific Sirt5 KO mice (Sirt5-BKO) are intolerant to acute cold. Furthermore, when stressed with high fat diet or aging, Sirt5-BKO mice develop impaired glucose homeostasis. Mass Spec analysis reveals that this correlates with an increase in succinylation of a variety of mitochondria enzymes and proteins, including succinate dehydrogenase (SDH), glutamate dehydrogenase (GDH), and UCP1. Lysine to glutamine mutations of UCP1 at the two target sites, which mimic acylation, resulted in significantly decreased UCP1 activity. Likewise, hyperacylation of SDH and GDH secondary to Sirt5 deficiency also impaired their activities. The reduced function of these and other proteins in Sirt5KO BAT resulted in decreased mitochondrial membrane potential and increased accumulation of PINK1, a mitochondrial serine/threonine kinase that protect cells from stress-induced mitochondrial dysfunction. Depolarized mitochondria are often smaller in size. Consistent with this, mitochondria from BAT of Sirt5KO mice had higher Fis1 expression during fasting, leading to increased mitochondrial fission and smaller mitochondria. Thus, succinylation and malonylation of key metabolic enzymes in mitochondria is regulated by Sirt5 and plays an important role in BAT activity. Over-acylation impairs mitochondrial protein function and results in depolarized mitochondria and disturbed systemic metabolic homeostasis.