Protein folding stress transcriptionally reprograms muscle metabolism
Cellular stress responses crosstalk with many physiological and metabolic pathways. Muscle cells constantly respond to various endogenous stressors while actively maintaining critical metabolic functions for the tissue and whole animal. The molecular mechanisms of how muscle stress responses transcriptionally reprogram metabolic networks are complex and inadequately understood. Using a multi-omics approach of metabolomics, lipidomics, and single-nuclei RNA-sequencing in Drosophila, we reconstructed the physiological landscape of muscle during chronic activation of endoplasmic reticulum unfolded protein response (UPR), a stress response that ensures the secretion of vital proteins from muscle, known as myokines. By ectopically expressing a constitutively active form of X-box binding protein 1 (Xbp1), a highly conserved transcription factor (TF) and UPR effector, we found that UPR downregulates key metabolic pathways in muscle, including carbohydrate and purine metabolism, while upregulating a robust lipogenic program enriched for phospholipids and several antioxidant metabolic pathways. Using gene regulatory network (GRN) analysis, we linked these metabolic changes to distinct TF regulon activities. The activation of a single TF, Xbp1, increased the activity of other stress response TFs in muscle, including cap-n-collar (cnc/Nrf2), cryptocephal (crc/Atf4), and sterol regulatory element binding protein (SREBP). Simultaneously, we observed decreased activity of TFs, namely Forkhead box O (FoxO), that resulted in downregulated metabolic pathways critical to muscle function, including oxidative phosphorylation and glycolysis. We propose that these GRNs antagonize each other downstream of UPR to reprogram muscle metabolism away from carbohydrates and towards lipogenesis, offering novel insight into how metabolic rewiring can be transcriptionally controlled in response to chronic tissue damage, even to the detriment of organ function.