Proteolytic activation of fatty acid synthase signals pan-stress resolution.

Chronic stress and inflammation are both outcomes and major drivers of many human diseases. Sustained responsiveness despite mitigation suggests a failure to sense resolution of the stressor. Here we show that a proteolytic cleavage event of fatty acid synthase (FASN) activates a global cue for stress resolution in Caenorhabditis elegans. FASN is well established for biosynthesis of the fatty acid palmitate. Our results demonstrate FASN promoting an anti-inflammatory profile apart from palmitate synthesis. Redox-dependent proteolysis of limited amounts of FASN by caspase activates a C-terminal fragment sufficient to downregulate multiple aspects of stress responsiveness, including gene expression, metabolic programs and lipid droplets. The FASN C-terminal fragment signals stress resolution in a cell non-autonomous manner. Consistent with these findings, FASN processing is also seen in well-fed but not fasted male mouse liver. As downregulation of stress responses is critical to health, our findings provide a potential pathway to control diverse aspects of stress responses.

Finishing the egg.

Gamete development is a fundamental process that is highly conserved from early eukaryotes to mammals. As germ cells develop, they must coordinate a dynamic series of cellular processes that support growth, cell specification, patterning, the loading of maternal factors (RNAs, proteins, and nutrients), differentiation of structures to enable fertilization and ensure embryonic survival, and other processes that make a functional oocyte. To achieve these goals, germ cells integrate a complex milieu of environmental and developmental signals to produce fertilizable eggs. Over the past 50 years, Drosophila oogenesis has risen to the forefront as a system to interrogate the sophisticated mechanisms that drive oocyte development. Studies in Drosophila have defined mechanisms in germ cells that control meiosis, protect genome integrity, facilitate mRNA trafficking, and support the maternal loading of nutrients. Work in this system has provided key insights into the mechanisms that establish egg chamber polarity and patterning as well as the mechanisms that drive ovulation and egg activation. Using the power of Drosophila genetics, the field has begun to define the molecular mechanisms that coordinate environmental stresses and nutrient availability with oocyte development. Importantly, the majority of these reproductive mechanisms are highly conserved throughout evolution, and many play critical roles in the development of somatic tissues as well. In this chapter, we summarize the recent progress in several key areas that impact egg chamber development and ovulation. First, we discuss the mechanisms that drive nutrient storage and trafficking during oocyte maturation and vitellogenesis. Second, we examine the processes that regulate follicle cell patterning and how that patterning impacts the construction of the egg shell and the establishment of embryonic polarity. Finally, we examine regulatory factors that control ovulation, egg activation, and successful fertilization.

Beyond energy and growth: the role of metabolism in developmental signaling, cell behavior and diapause.

Metabolism is crucial for development through supporting cell growth, energy production, establishing cell identity, developmental signaling and pattern formation. In many model systems, development occurs alongside metabolic transitions as cells differentiate and specialize in metabolism that supports new functions. Some cells exhibit metabolic flexibility to circumvent mutations or aberrant signaling, whereas other cell types require specific nutrients for developmental progress. Metabolic gradients and protein modifications enable pattern formation and cell communication. On an organism level, inadequate nutrients or stress can limit germ cell maturation, implantation and maturity through diapause, which slows metabolic activities until embryonic activation under improved environmental conditions.

The role of metabolism in cellular quiescence.

Cellular quiescence is a dormant, non-dividing cell state characterized by significant shifts in physiology and metabolism. Quiescence plays essential roles in a wide variety of biological processes, ranging from microbial sporulation to human reproduction and wound repair. Moreover, when the regulation of quiescence is disrupted, it can drive cancer growth and compromise tissue regeneration after injury. In this Review, we examine the dynamic changes in metabolism that drive and support dormant and transiently quiescent cells, including spores, oocytes and adult stem cells. We begin by defining quiescent cells and discussing their roles in key biological processes. We then examine metabolic factors that influence cellular quiescence in both healthy and disease contexts, and how these could be leveraged in the treatment of cancer.

Mitochondrial respiratory quiescence: A new model for examining the role of mitochondrial metabolism in development.

Mitochondria are vital organelles with a central role in all aspects of cellular metabolism. As a means to support the ever-changing demands of the cell, mitochondria produce energy, drive biosynthetic processes, maintain redox homeostasis, and function as a hub for cell signaling. While mitochondria have been widely studied for their role in disease and metabolic dysfunction, this organelle has a continually evolving role in the regulation of development, wound repair, and regeneration. Mitochondrial metabolism dynamically changes as tissues transition through distinct phases of development. These organelles support the energetic and biosynthetic demands of developing cells and function as key structures that coordinate the nutrient status of the organism with developmental progression. This review will examine the mechanisms that link mitochondria to developmental processes. We will also examine the process of mitochondrial respiratory quiescence (MRQ), a novel mechanism for regulating cellular metabolism through the biochemical and physiological remodeling of mitochondria. Lastly, we will examine MRQ as a system to discover the mechanisms that drive mitochondrial remodeling during development.

Highly conserved shifts in ubiquitin-proteasome system (UPS) activity drive mitochondrial remodeling during quiescence.

Defects in cellular proteostasis and mitochondrial function drive many aspects of infertility, cancer, and other age-related diseases. All of these conditions rely on quiescent cells, such as oocytes and adult stem cells, that reduce their activity and remain dormant as part of their roles in tissue homeostasis, reproduction, and even cancer recurrence. Using a multi-organism approach, we show that dynamic shifts in the ubiquitin proteasome system drive mitochondrial remodeling during cellular quiescence. In contrast to the commonly held view that the ubiquitin-proteasome system (UPS) is primarily regulated by substrate ubiquitination, we find that increasing proteasome number and their recruitment to mitochondria support mitochondrial respiratory quiescence (MRQ). GSK3 triggers proteasome recruitment to the mitochondria by phosphorylating outer membrane proteins, such as VDAC, and suppressing mitochondrial fatty acid oxidation. This work defines a process that couples dynamic regulation of UPS activity to coordinated shifts in mitochondrial metabolism in fungi, Drosophila, and mammals during quiescence.

Heritable shifts in redox metabolites during mitochondrial quiescence reprogramme progeny metabolism.

Changes in maternal diet and metabolic defects in mothers can profoundly affect health and disease in their progeny. However, the biochemical mechanisms that induce the initial reprogramming events at the cellular level have remained largely unknown owing to limitations in obtaining pure populations of quiescent oocytes. Here, we show that the precocious onset of mitochondrial respiratory quiescence causes a reprogramming of progeny metabolic state. The premature onset of mitochondrial respiratory quiescence drives the lowering of Drosophila oocyte NAD+ levels. NAD+ depletion in the oocyte leads to reduced methionine cycle production of the methyl donor S-adenosylmethionine in embryos and lower levels of histone H3 lysine 27 trimethylation, resulting in enhanced intestinal lipid metabolism in progeny. In addition, we show that triggering cellular quiescence in mammalian cells and chemotherapy-resistant human cancer cell models induces cellular reprogramming events identical to those seen in Drosophila, suggesting a conserved metabolic mechanism in systems reliant on quiescent cells.

Dietary Lipids Modulate Notch Signaling and Influence Adult Intestinal Development and Metabolism in Drosophila.

Tissue homeostasis involves a complex balance of developmental signals and environmental cues that dictate stem cell function. We found that dietary lipids control enteroendocrine cell production from Drosophila posterior midgut stem cells. Dietary cholesterol influences new intestinal cell differentiation in an Hr96-dependent manner by altering the level and duration of Notch signaling. Exogenous lipids modulate Delta ligand and Notch extracellular domain stability and alter their trafficking in endosomal vesicles. Lipid-modulated Notch signaling occurs in other nutrient-dependent tissues, suggesting that Delta trafficking in many cells is sensitive to cellular sterol levels. These diet-mediated alterations in young animals contribute to a metabolic program that persists after the diet changes. A low-sterol diet also slows the proliferation of enteroendocrine tumors initiated by Notch pathway disruption. Thus, a specific dietary nutrient can modify a key intercellular signaling pathway to shift stem cell differentiation and cause lasting changes in tissue structure and physiology.

The role of metabolic states in development and disease.

During development, cells adopt distinct metabolic strategies to support growth, produce energy, and meet the demands of a mature tissue. Some of these metabolic states maintain a constrained program of nutrient utilization, while others providing metabolic flexibility as a means to couple developmental progression with nutrient availability. Here we discuss our understanding of metabolic programs, and how they support specific aspects of animal development. During phases of rapid proliferation a subset of metabolic programs provide the building blocks to support growth. During differentiation, metabolic programs shift to support the unique demands of each tissue. Finally, we discuss how a model system, such as Drosophila egg development, can provide a versatile platform to discover novel mechanisms controlling programmed shift in metabolism.

Electron Transport Chain Remodeling by GSK3 during Oogenesis Connects Nutrient State to Reproduction.

Reproduction is heavily influenced by nutrition and metabolic state. Many common reproductive disorders in humans are associated with diabetes and metabolic syndrome. We characterized the metabolic mechanisms that support oogenesis and found that mitochondria in mature Drosophila oocytes enter a low-activity state of respiratory quiescence by remodeling the electron transport chain (ETC). This shift in mitochondrial function leads to extensive glycogen accumulation late in oogenesis and is required for the developmental competence of the oocyte. Decreased insulin signaling initiates ETC remodeling and mitochondrial respiratory quiescence through glycogen synthase kinase 3 (GSK3). Intriguingly, we observed similar ETC remodeling and glycogen uptake in maturing Xenopus oocytes, suggesting that these processes are evolutionarily conserved aspects of oocyte development. Our studies reveal an important link between metabolism and oocyte maturation.

Steroid Signaling Establishes a Female Metabolic State and Regulates SREBP to Control Oocyte Lipid Accumulation.

Disruptions in energy homeostasis severely affect reproduction in many organisms and are linked to several reproductive disorders in humans. As a result, understanding the mechanisms that control nutrient accumulation in the oocyte will provide valuable insights into the links between metabolic disease and reproductive dysfunction. We show that the steroid hormone ecdysone functions in Drosophila to control lipid metabolism and support oocyte production. First, local EcR-mediated signaling induces a stage-specific accumulation of lipids in stage-10 oocytes. EcR induces lipid accumulation by promoting the activation of the lipogenic transcription factor SREBP and by controlling the expression of the low-density lipoprotein (LDL) receptor homolog, LpR2. Second, global signaling via the ecdysone receptor, EcR, establishes a female metabolic state and promotes whole-body triglyceride and glycogen storage at high levels. EcR acts in the CNS to mediate these effects, in part by promoting higher levels of feeding in females. Thus, ecdysone functions at two levels to support reproduction: first by inducing lipid accumulation in the late stages of oocyte development and second by providing a signal that coordinates lipid metabolism in the germline with whole-animal lipid homeostasis. Ecdysone regulation allows females to assess the demands of oogenesis and alter their behavior and metabolic state to support the biosynthetic requirements of oocyte production.

Coordination of triacylglycerol and cholesterol homeostasis by DHR96 and the Drosophila LipA homolog magro.

Although transintestinal cholesterol efflux has been identified as an important means of clearing excess sterols, the mechanisms that underlie this process remain poorly understood. Here, we show that magro, a direct target of the Drosophila DHR96 nuclear receptor, is required in the intestine to maintain cholesterol homeostasis. magro encodes a LipA homolog that is secreted from the anterior gut into the intestinal lumen to digest dietary triacylglycerol. Expression of magro in intestinal cells is required to hydrolyze cholesterol esters and promote cholesterol clearance. Restoring magro expression in the intestine of DHR96 mutants rescues their defects in triacylglycerol and cholesterol metabolism. These studies show that the central role of the intestine in cholesterol efflux has been conserved through evolution, that the ancestral function of LipA is to coordinate triacylglycerol and cholesterol metabolism, and that the region-specific activities of magro correspond to the metabolic functions of its upstream regulator, DHR96.

The DHR96 nuclear receptor controls triacylglycerol homeostasis in Drosophila.

Triacylglycerol (TAG) homeostasis is an integral part of normal physiology and essential for proper energy metabolism. Here we show that the single Drosophila ortholog of the PXR and CAR nuclear receptors, DHR96, plays an essential role in TAG homeostasis. DHR96 mutants are sensitive to starvation, have reduced levels of TAG in the fat body and midgut, and are resistant to diet-induced obesity, while DHR96 overexpression leads to starvation resistance and increased TAG levels. We show that DHR96 function is required in the midgut for the breakdown of dietary fat and that it exerts this effect through the CG5932 gastric lipase, which is essential for TAG homeostasis. This study provides insights into the regulation of dietary fat metabolism in Drosophila and demonstrates that the regulation of lipid metabolism is an ancestral function of the PXR/CAR/DHR96 nuclear receptor subfamily.