Mitochondrial dysfunction has been associated with obesity and metabolic disorders. However, whether mitochondrial perturbation in a single tissue influences mitochondrial function and metabolic status of another distal tissue remains largely unknown. We analyzed the nonautonomous role of muscular mitochondrial dysfunction in Drosophila. Surprisingly, impaired muscle mitochondrial function via complex I perturbation results in simultaneous mitochondrial dysfunction in the fat body (the fly adipose tissue) and subsequent triglyceride accumulation, the major characteristic of obesity. RNA-sequencing (RNA-seq) analysis, in the context of muscle mitochondrial dysfunction, revealed that target genes of the TGF-β signaling pathway were induced in the fat body. Strikingly, expression of the TGF-β family ligand, Activin-β (Actβ), was dramatically increased in the muscles by NF-κB/Relish (Rel) signaling in response to mitochondrial perturbation, and decreasing Actβ expression in mitochondrial-perturbed muscles rescued both the fat body mitochondrial dysfunction and obesity phenotypes. Thus, perturbation of muscle mitochondrial activity regulates mitochondrial function in the fat body nonautonomously via modulation of Activin signaling.mitochondrial synchrony | Activin-β | complex I perturbation | NF-κB/Relish | lipid metabolism I ndividual organs in a multicellular organism, besides performing their respective roles, must communicate with other organs to maintain systemic homeostasis. The central nervous system (CNS) in particular integrates information regarding the status of peripheral metabolic processes via hormonal signaling and directs energy homeostasis and feeding behavior (1). In addition, metabolic changes in a peripheral organ can affect the physiology of other peripheral organs (2, 3). The skeletal muscle system, which is newly recognized as playing endocrine-related roles, produces myokines after exercise to target other metabolic organs (liver, adipose tissue, pancreas, gut, and bone) and modulates systemic energy homeostasis (4).Mitochondria are semiautonomous organelles that integrate multiple physiological signals. Growing evidence indicates that mitochondrial alterations in one organ leads to abnormalities in biological processes in distal organs through hormonal signaling (5, 6). In addition to exercise, which induces mitochondrial activity and improves muscle performance, mitochondrial perturbationassociated muscle injury is also sufficient to modulate functions of other organs and change systemic outcomes via myokine production. For example, in mammals, mitochondrial dysfunction due to disruption of autophagic function in skeletal muscles results in elevated production of muscular FGF21 that triggers browning of white adipose tissue and increases lipid mobilization (7). Further, in Drosophila, mild mitochondrial distress in adult muscles delays aging via an increase of muscular ImpL2 production and remote suppression of insulin signaling in the fat body and brain (8). Despite these examples, molecula...