The quality of mitochondria in skeletal muscle is essential for maintaining metabolic homeostasis during adaptive stress responses. However, the precise control mechanism of muscle mitochondrial quality and its physiological impacts remain unclear. Here, we demonstrate that FUNDC1, a mediator of mitophagy, plays a critical role in controlling muscle mitochondrial quality as well as metabolic homeostasis. Skeletal-muscle-specific ablation of FUNDC1 in mice resulted in LC3-mediated mitophagy defect, leading to impaired mitochondrial energetics. This caused decreased muscle fat utilization and endurance capacity during exercise. Interestingly, mice lacking muscle FUNDC1 were protected against high-fat-diet-induced obesity with improved systemic insulin sensitivity and glucose tolerance despite reduced muscle mitochondrial energetics. Mechanistically, FUNDC1 deficiency elicited a retrograde response in muscle that upregulated FGF21 expression, thereby promoting the thermogenic remodeling of adipose tissue. Thus, these findings reveal a pivotal role of FUNDC1-dependent mitochondrial quality control in mediating the muscle-adipose dialog to regulate systemic metabolism.

The cytoplasmic coat protein complex-II (COPII) is evolutionarily conserved machinery that is essential for efficient trafficking of protein and lipid cargos. How the COPII machinery is regulated to meet the metabolic demand in response to alterations of the nutritional state remains largely unexplored, however. Here, we show that dynamic changes of COPII vesicle trafficking parallel the activation of transcription factor X-box binding protein 1 (XBP1s), a critical transcription factor in handling cellular endoplasmic reticulum (ER) stress in both live cells and mouse livers upon physiological fluctuations of nutrient availability. Using live-cell imaging approaches, we demonstrate that XBP1s is sufficient to promote COPII-dependent trafficking, mediating the nutrient stimulatory effects. Chromatin immunoprecipitation (ChIP) coupled with high-throughput DNA sequencing (ChIP-seq) and RNA-sequencing analyses reveal that nutritional signals induce dynamic XBP1s occupancy of promoters of COPII traffic-related genes, thereby driving the COPII-mediated trafficking process. Liver-specific disruption of the inositol-requiring enzyme 1α (IRE1α)-XBP1s signaling branch results in diminished COPII vesicle trafficking. Reactivation of XBP1s in mice lacking hepatic IRE1α restores COPII-mediated lipoprotein secretion and reverses the fatty liver and hypolipidemia phenotypes. Thus, our results demonstrate a previously unappreciated mechanism in the metabolic control of liver protein and lipid trafficking: The IRE1α-XBP1s axis functions as a nutrient-sensing regulatory nexus that integrates nutritional states and the COPII vesicle trafficking. COPII | metabolic sensing | XBP1s | nutrient availability | liver steatosis T he cytoplasmic coat protein complex-II (COPII) is evolutionarily conserved secretory machinery that is essential for cellular protein and lipid trafficking through cargo sorting and vesicle formation at the endoplasmic reticulum (ER) (1-4). The vast majority of proteins and lipids exported from the ER require the COPII secretory machinery. The assembly of COPII-coated vesicles for facilitating the transport of cellular cargos has been demonstrated to be a highly complex process (1-6). Activated small GTPase SAR1 localizes to the specialized ER exit sites and initiates the COPII coat assembly, by first recruiting the inner coat formed by the heterodimer SEC23/SEC24, followed by the outer coat heterotetramer SEC13/SEC31, to deform the ER membrane and eventually produce carrier vesicles (2, 4, 7-9). Mutations in COPII components or accessory factors have been implicated in several human genetic diseases, including chylomicron retention disease, congenital dyserythropoietic anemia type II, and cranio-lenticulosutural dysplasia (10-14). However, it remains largely unexplored how the COPII machinery is regulated to meet the cellular secretory demand in response to various physiological stimuli.As a metabolically active tissue, the liver possesses a remarkable adaptive capacity to secrete lipids and proteins according to...

Mitochondria are essential for maintaining skeletal muscle metabolic homeostasis during adaptive response to a myriad of physiologic or pathophysiological stresses. The mechanisms by which mitochondrial function and contractile fiber type are concordantly regulated to ensure muscle function remain poorly understood. Evidence is emerging that the Folliculin interacting protein 1 (Fnip1) is involved in skeletal muscle fiber type specification, function, and disease. In this study, Fnip1 was specifically expressed in skeletal muscle in Fnip1-transgenic (Fnip1Tg) mice. Fnip1Tg mice were crossed with Fnip1-knockout (Fnip1KO) mice to generate Fnip1TgKO mice expressing Fnip1 only in skeletal muscle but not in other tissues. Our results indicate that, in addition to the known role in type I fiber program, FNIP1 exerts control upon muscle mitochondrial oxidative program through AMPK signaling. Indeed, basal levels of FNIP1 are sufficient to inhibit AMPK but not mTORC1 activity in skeletal muscle cells. Gain-of-function and loss-of-function strategies in mice, together with assessment of primary muscle cells, demonstrated that skeletal muscle mitochondrial program is suppressed via the inhibitory actions of FNIP1 on AMPK. Surprisingly, the FNIP1 actions on type I fiber program is independent of AMPK and its downstream PGC-1α. These studies provide a vital framework for understanding the intrinsic role of FNIP1 as a crucial factor in the concerted regulation of mitochondrial function and muscle fiber type that determine muscle fitness.