Sungwoo Choi

Peroxisome proliferator-activated receptor alpha ( PPARα ) is a nuclear receptor that orchestrates metabolic adaptation to fasting by regulating hepatic lipid and glucose metabolism. While these functions are well established, its role in protein homeostasis, another energy-intensive process, has remained unclear. Here, we performed multi-omics analyses in wild-type and PPARα knockout mice treated with the PPARα agonist fenofibrate to define its broader role in proteostasis. Interestingly, chronic PPARα activation suppressed hepatic secretory pathways, reducing secretome gene expression and circulating serum proteins, and downregulated genes involved in endoplasmic reticulum (ER) translocation, glycosylation, folding, and trafficking. Concurrently, PPARα activation enhanced proteasome activity, which was associated with selective induction of proteasome 26S subunit non-ATPase (PSMD) family members. In addition, hepatic protein synthesis was strongly attenuated. This was associated with an increase in the inhibitory phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α), that in turn was linked to disrupted iron homeostasis. Together, these findings identify PPARα as a regulator of proteostasis, suppressing protein synthesis and secretion while promoting protein degradation pathways. Beyond its canonical role in lipid and glucose metabolism, we conclude that PPARα exerts an additional energy-conserving function by coordinating proteostasis, expanding our understanding of its systemic metabolic role.

Metabolic alterations have been observed in the brains of patients with bipolar disorder (BD), a neuropsychiatric disorder characterized by alternating episodes of mania and depression. However, the specific contributions of glial cells to these metabolic changes remain largely unknown. Here, we investigate the metabolic characteristics of induced astrocytes (iAstrocytes) derived from induced pluripotent stem cells of BD patients—classified by lithium responsiveness—and healthy controls. Transcriptomic analyses revealed dysregulated expression of genes associated with metabolic diseases in BD iAstrocytes. Compared to control iAstrocytes, BD iAstrocytes showed decreased mitochondrial respiration, increased glycolysis, and elevated lactate secretion, indicating impaired mitochondrial function. These defects were further supported by downregulation of oxidative phosphorylation complex proteins and decreased reactive oxygen species levels. Notably, BD iAstrocytes showed substantial lipid droplet (LD) accumulation, potentially as a consequence of disrupted metabolic homeostasis. Lithium treatment reduced LD levels in lithium-responsive (Li-R) iAstrocytes but failed to restore mitochondrial respiration or normalize lactate secretion. In co-culture with human neurons, lithium-nonresponsive (Li-NR) iAstrocytes exhibited enhanced uptake of neuron-derived lipids and selectively increased neuronal excitability. Metabolomic profiling revealed distinct metabolite signatures between Li-R and Li-NR iAstrocytes, suggesting lithium responsiveness as a key axis of metabolic heterogeneity. Together, our findings identify astrocyte-specific metabolic dysfunction as a hallmark of BD and reveal divergent roles of Li-R and Li-NR iAstrocytes in modulating neuronal function. LD accumulation in Li-NR iAstrocytes may serve as a functional readout for drug screening to identify alternative treatments for patients unresponsive to lithium.

Adipose tissues serve as an energy reservoir and endocrine organ, yet the mechanisms that coordinate these functions remain elusive. Here, we show that the transcriptional coregulators, YAP and TAZ, uncouple fat mass from leptin levels and regulate adipocyte plasticity to maintain metabolic homeostasis. Activating YAP/TAZ signalling in adipocytes by deletion of the upstream regulators Lats1 and Lats2 results in a profound reduction in fat mass by converting mature adipocytes into delipidated progenitor-like cells, but does not cause lipodystrophy-related metabolic dysfunction, due to a paradoxical increase in circulating leptin levels. Mechanistically, we demonstrate that YAP/TAZ–TEAD signalling upregulates leptin expression by directly binding to an upstream enhancer site of the leptin gene. We further show that YAP/TAZ activity is associated with, and functionally required for, leptin regulation during fasting and refeeding. These results suggest that adipocyte Hippo–YAP/TAZ signalling constitutes a nexus for coordinating adipose tissue lipid storage capacity and systemic energy balance through the regulation of adipocyte plasticity and leptin gene transcription.