Body-fat sensor triggers ribosome maturation in the steroidogenic gland to initiate sexual maturation in Drosophila

Leptin- and cytokine-like unpaired signaling in Drosophila

Animals have evolved a multitude of signaling pathways that enable them to orchestrate diverse physiological processes to tightly regulate systemic homeostasis. This signaling is mediated by various families of peptide hormones and cytokines that are conserved across the animal kingdom. In this review, we primarily focus on the unpaired (Upd) family of proteins in Drosophila which are evolutionarily related to mammalian leptin and the cytokine interleukin 6. We summarize expression patterns of Upd in Drosophila and discuss the parallels in structure, signaling pathway, and functions between Upd and their mammalian counterparts. In particular, we focus on the roles of Upd in governing metabolic homeostasis, growth and development, and immune responses. We aim to stimulate future studies on leptin-like signaling in other phyla which can help bridge the evolutionary gap between insect Upd and vertebrate leptin and cytokines like interleukin 6.

Macrophages play a nutritive role in post-metamorphic maturation in Drosophila

In the body of multicellular organisms, macrophages play an indispensable role in maintaining tissue homeostasis by removing old, apoptotic and damaged cells. In addition, macrophages allow significant remodeling of body plans during embryonic morphogenesis, regeneration and metamorphosis. Although the huge amount of organic matter that must be removed during these processes represents a potential source of nutrients, their further use by the organism has not yet been addressed. Here, we document that, during metamorphosis, Drosophila larval adipose tissue is infiltrated by macrophages, which remove dying adipocytes by efferocytosis and engulf leaking RNA-protein granules and lipids. Consequently, the infiltrating macrophages transiently adopt the adipocyte-like metabolic profile to convert remnants of dying adipocytes to lipoproteins and storage peptides that nutritionally support post-metamorphic development. This process is fundamental for the full maturation of ovaries and the achievement of early fecundity of individuals. Whether macrophages play an analogous role in other situations of apoptotic cell removal remains to be elucidated.

Inhibition of Trehalose Synthesis in Lepidoptera Reduces Larval Fitness

Trehalose is synthesized in insects through the trehalose 6-phosphate synthase and phosphatase (TPS/TPP) pathway. TPP dephosphorylates trehalose 6-phosphate to release trehalose. Trehalose is involved in metamorphosis, but its relation with body weight, size, and developmental timing is unexplored. The expression and activity of TPS/TPP fluctuate depending on trehalose demand. Thus, TPS/TPP inhibition can highlight the significance of trehalose in insect physiology. TPS/TPP transcript levels are elevated in the pre-pupal and pupal stages in Helicoverpa armigera. The inhibition of recombinantly expressed TPP by N-(phenylthio)phthalimide (NPP), is validated by in vitro assays. In vivo inhibition of trehalose synthesis reduces larval weight and size, hampers metamorphosis, and reduces its overall fitness. Insufficient trehalose leads to a shift in glucose flux, reduced energy, and dysregulated fatty acid oxidation. Metabolomics reaffirms the depletion of trehalose, glucose, glucose 6-phosphate, and suppressed tricarboxylic acid cycle. Reduced trehalose hampers the energy level affecting larval vitality. Through trehalose synthesis inhibition, the importance of trehalose in insect physiology and development is investigated. Also, in two other lepidopterans, TPP inhibition impedes physiology and survival. NPP is also found to be effective as an insecticidal formulation. Overall, trehalose levels affect the larval size, weight, and metabolic homeostasis for larval-pupal transition in lepidoptera.

The macrophage genetic cassette inr/dtor/pvf2 is a nutritional status checkpoint for developmental timing

A small number of signaling molecules, used reiteratively, control differentiation programs, but the mechanisms that adapt developmental timing to environmental cues are less understood. We report here that a macrophage inr/dtor/pvf2 genetic cassette is a developmental timing checkpoint in Drosophila, which either licenses or delays biosynthesis of the steroid hormone in the endocrine gland and metamorphosis according to the larval nutritional status. Insulin receptor/dTor signaling in macrophages is required and sufficient for production of the PDGF/VEGF family growth factor Pvf2, which turns on transcription of the sterol biosynthesis Halloween genes in the prothoracic gland via its receptor Pvr. In response to a starvation event or genetic manipulation, low Pvf2 signal delays steroid biosynthesis until it becomes Pvr-independent, thereby prolonging larval growth before pupariation. The significance of this developmental timing checkpoint for host fitness is illustrated by the observation that it regulates the size of the pupae and adult flies.

Signaling in cAMP-stimulated ecdysteroidogenesis in prothoracic glands of the silkworm, Bombyx mori

In the present study, we investigated downstream pathways of cyclic adenosine monophosphate (cAMP) signaling (which is related to prothoracicotropic hormone (PTTH)-stimulated ecdysteroidogenesis) in Bombyx mori prothoracic glands (PGs). Results showed that treatment with either dibutyryl cAMP (dbcAMP) or 1-methyl-3-isobutylxanthine (MIX) inhibited phosphorylation of adenosine 5'-monophosphate-activated protein kinase (AMPK) and activated phosphorylation of the translational repressor, 4E-binding protein (4E-BP), a marker of target of rapamycin (TOR) signaling. A chemical activator of AMPK (5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside, AICAR) increased dbcAMP-inhibited AMPK phosphorylation and blocked dbcAMP-stimulated phosphorylation of 4E-BP, indicating that inhibition of AMPK phosphorylation lies upstream of dbcAMP-stimulated TOR signaling. Treatment of PGs with dbcAMP and MIX also stimulated phosphorylation of a 37-kDa protein, as recognized by a protein kinase C (PKC) substrate antibody, indicating that cAMP activates PKC signaling. Treatment with either LY294002 or AICAR did not affect dbcAMP-stimulated phosphorylation of the PKC-dependent 37-kDa protein, indicating that cAMP-stimulated PKC signaling is not related to phosphoinositide 3-kinase (PI3K) or AMPK. In addition, dbcAMP-stimulated ecdysteroidogenesis in PGs was partially inhibited by pretreatment with either LY294002, AICAR, or calphostin C. From these results, we concluded that AMPK/TOR/4E-BP and PKC pathways are involved in ecdysteroidogenesis of PGs stimulated by cAMP signaling in B. mori.

The fat body cortical actin network regulates Drosophila inter-organ nutrient trafficking, signaling, and adipose cell size

Defective nutrient storage and adipocyte enlargement (hypertrophy) are emerging features of metabolic syndrome and type 2 diabetes. Within adipose tissues, how the cytoskeletal network contributes to adipose cell size, nutrient uptake, fat storage, and signaling remain poorly understood. Utilizing the Drosophila larval fat body (FB) as a model adipose tissue, we show that a specific actin isoform-Act5C-forms the cortical actin network necessary to expand adipocyte cell size for biomass storage in development. Additionally, we uncover a non-canonical role for the cortical actin cytoskeleton in inter-organ lipid trafficking. We find Act5C localizes to the FB cell surface and cell-cell boundaries, where it intimately contacts peripheral LDs (pLDs), forming a cortical actin network for cell architectural support. FB-specific loss of Act5C perturbs FB triglyceride (TG) storage and LD morphology, resulting in developmentally delayed larvae that fail to develop into flies. Utilizing temporal RNAi-depletion approaches, we reveal that Act5C is indispensable post-embryogenesis during larval feeding as FB cells expand and store fat. Act5C-deficient FBs fail to grow, leading to lipodystrophic larvae unable to accrue sufficient biomass for complete metamorphosis. In line with this, Act5C-deficient larvae display blunted insulin signaling and reduced feeding. Mechanistically, we also show this diminished signaling correlates with decreased lipophorin (Lpp) lipoprotein-mediated lipid trafficking, and find Act5C is required for Lpp secretion from the FB for lipid transport. Collectively, we propose that the Act5C-dependent cortical actin network of Drosophila adipose tissue is required for adipose tissue size-expansion and organismal energy homeostasis in development, and plays an essential role in inter-organ nutrient transport and signaling.

Wds-Mediated H3K4me3 Modification Regulates Lipid Synthesis and Transport in Drosophila

Lipid homeostasis is essential for insect growth and development. The complex of proteins associated with Set 1 (COMPASS)-catalyzed Histone 3 lysine 4 trimethylation (H3K4me3) epigenetically activates gene transcription and is involved in various biological processes, but the role and molecular mechanism of H3K4me3 modification in lipid homeostasis remains largely unknown. In the present study, we showed in Drosophila that fat body-specific knockdown of will die slowly (Wds) as one of the COMPASS complex components caused a decrease in lipid droplet (LD) size and triglyceride (TG) levels. Mechanistically, Wds-mediated H3K4me3 modification in the fat body targeted several lipogenic genes involved in lipid synthesis and the Lpp gene associated with lipid transport to promote their expressions; the transcription factor heat shock factor (Hsf) could interact with Wds to modulate H3K4me3 modification within the promoters of these targets; and fat body-specific knockdown of Hsf phenocopied the effects of Wds knockdown on lipid homeostasis in the fat body. Moreover, fat body-specific knockdown of Wds or Hsf reduced high-fat diet (HFD)-induced oversized LDs and high TG levels. Altogether, our study reveals that Wds-mediated H3K4me3 modification is required for lipid homeostasis during Drosophila development and provides novel insights into the epigenetic regulation of insect lipid metabolism.

Endoreplication in the Drosophila melanogaster prothoracic gland is dispensable for the critical weight checkpoint

Critical weight (CW) attainment is a key life event in the development of holometabolous insects including Drosophila melanogaster. It indicates that sufficient growth has occurred to initiate the juvenile-to-adult transition. The prothoracic gland (PG), the major insect larval endocrine organ, is a polyploid tissue that plays a key role in the determination of CW via release of the steroid hormone ecdysone. Here we show that when the cells of the PG fail to make the mitotic-to-endocycle switch, but instead remain mitotic, the result is more but smaller cells. Nevertheless, they reach the same CW and produce healthy adults after only a moderate developmental delay. We propose that the CW checkpoint can be reached by either an endocycling or mitotic PG and may simply reflect the attainment of sufficient ecdysone biosynthetic capacity to initiate metamorphosis.

Mammalian puberty: a fly perspective

Mammalian puberty and Drosophila metamorphosis, despite their evolutionary distance, exhibit similar design principles and conservation of molecular components. In this Viewpoint, we review recent advances in this area and the similarities between both processes in terms of the signaling pathways and neuroendocrine circuits involved. We argue that the detection and uptake of peripheral fat by Drosophila prothoracic endocrine cells induces endomembrane remodeling and ribosomal maturation, leading to the acquisition of high biosynthetic and secretory capacity. The absence of this fat-neuroendocrine interorgan communication leads to giant, obese, non-pupating larvae. Importantly, human leptin is capable of signaling the pupariation process in Drosophila, and its expression prevents obesity and triggers maturation in mutants that do not pupate. This implies that insect metamorphosis can be used to address issues related to the biology of leptin and puberty.

Pupal size as a proxy for fat content in laboratory-reared and field-collected Drosophila species

In arthropods, larger individuals tend to have more fat reserves, but data for many taxa are still missing. For the vinegar fly Drosophila melanogaster, only few studies have provided experimental data linking body size to fat content. This is rather surprising considering the widespread use of D. melanogaster as a model system in biology. Here, we hypothesized that fat content in D. melanogaster is positively correlated with body size. To test this, we manipulated the developmental environment of D. melanogaster by decreasing food availability. We then measured pupal size and quantified fat content of laboratory-reared D. melanogaster. We subsequently measured pupal size and fat content of several field-caught Drosophila species. Starvation, crowding, and reduced nutrient content led to smaller laboratory-reared pupae that contained less fat. Pupal size was indeed found to be positively correlated with fat content. The same correlation was found for field-caught Drosophila pupae belonging to different species. As fat reserves are often strongly linked to fitness in insects, further knowledge on the relationship between body size and fat content can provide important information for studies on insect ecology and physiology.

A toolbox to study metabolic status of Drosophila melanogaster larvae

Somatic energy reserves are essential for reproductive success and can govern the onset of sexual maturation. Here, we present a toolkit to analyze the metabolic status of Drosophila larvae using an optimized NMR profiling assay in dissected tissues or whole animals, as well as a complementary protocol for the dissection and staining of key organs in nutrient sensing. This toolkit will aid investigations into critical body weight signaling and how it is sensed for maturation commitment in Drosophila.

For complete details on the use and execution of this profile, please refer to Juarez-Carreño et al. (2021).

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Single-assay quantification of metabolites in dissected tissues or whole animals

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NMR-based profiling of polar and non-polar metabolites

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Protocol for dissection and staining of brains and fat body cells

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Metabolic status of control and obese Drosophila larvae