An insulin-like peptide regulates size and adult stem cells in planarians

Animal growth depends on nutritional intake during development. In many animals, nutritional status is uncoupled from moderation of adult stature after adult size is achieved. However, some long-lived animals continue to regulate adult size and fertility in a nutrition-dependent manner. For example, the regenerating flatworm Schmidtea mediterranea becomes smaller, or degrows, during periods of starvation. These animals provide an opportunity to readily observe adult stem cell population dynamics in response to nutritional cues. We explored the role of insulin signaling in S. mediterranea. We disrupted insulin signaling via RNA interference and showed that animals, despite eating, degrew similarly to starved animals. Utilizing in situ hybridization and immunofluorescence, we assessed cellular changes in proliferative populations including the planarian adult stem cell population (neoblasts) and the germline. Both impaired insulin signaling and nutritional deprivation correlated with decreased neoblast proliferation. Additionally, insulin signaling played a role in supporting spermatogenesis that was distinct from the effects of starvation. In sum, we have demonstrated that insulin signaling is responsible for regulation of adult animal size and tissue homeostasis in an organism with plastic adult size. Importantly, insulin signaling continued to affect stem cell and germline populations in a mature organism. Furthermore, we have shown that adult organisms can differentially regulate specific cell populations as a result of environmental challenges.     

Size assessment and growth control: how adult size is determined in insects

Size control depends on both the regulation of growth rate and the control over when to stop growing. Studies of Drosophila melanogaster have shown that insulin and Target of Rapamycin (TOR) pathways play principal roles in controlling nutrition-dependent growth rates. A TOR-mediated nutrient sensor in the fat body detects nutrient availability, and regulates insulin signaling in peripheral tissues, which in turn controls larval growth rates. After larvae initiate metamorphosis, growth stops. For growth to stop at the correct time, larvae need to surpass a critical weight. Recently, it was found that the insulin-dependent growth of the prothoracic gland is involved in assessing when critical weight has been reached. Furthermore, mutations in DHR4, a repressor of ecdysone signaling, reduce critical weight and adult size. Thus, the mechanisms that control growth rates converge on those assessing size to ensure that the larvae attain the appropriate size at metamorphosis.     

Big-brained birds survive better in nature

Big brains are hypothesized to enhance survival of animals by facilitating flexible cognitive responses that buffer individuals against environmental stresses. Although this theory receives partial support from the finding that brain size limits the capacity of animals to behaviourally respond to environmental challenges, the hypothesis that large brains are associated with reduced mortality has never been empirically tested. Using extensive information on avian adult mortality from natural populations, we show here that species with larger brains, relative to their body size, experience lower mortality than species with smaller brains, supporting the general importance of the cognitive buffer hypothesis in the evolution of large brains.     

Local requirement of the Drosophila insulin binding protein imp-L2 in coordinating developmental progression with nutritional conditions

In Drosophila, growth takes place during the larval stages until the formation of the pupa. Starvation delays pupariation to allow prolonged feeding, ensuring that the animal reaches an appropriate size to form a fertile adult. Pupariation is induced by a peak of the steroid hormone ecdysone produced by the prothoracic gland (PG) after larvae have reached a certain body mass. Local downregulation of the insulin/insulin-like growth factor signaling (IIS) activity in the PG interferes with ecdysone production, indicating that IIS activity in the PG couples the nutritional state to development. However, the underlying mechanism is not well understood. In this study we show that the secreted Imaginal morphogenesis protein-Late 2 (Imp-L2), a growth inhibitor in Drosophila, is involved in this process. Imp-L2 inhibits the activity of the Drosophila insulin-like peptides by direct binding and is expressed by specific cells in the brain, the ring gland, the gut and the fat body. We demonstrate that Imp-L2 is required to regulate and adapt developmental timing to nutritional conditions by regulating IIS activity in the PG. Increasing Imp-L2 expression at its endogenous sites using an Imp-L2-Gal4 driver delays pupariation, while Imp-L2 mutants exhibit a slight acceleration of development. These effects are strongly enhanced by starvation and are accompanied by massive alterations of ecdysone production resulting most likely from increased Imp-L2 production by neurons directly contacting the PG and not from elevated Imp-L2 levels in the hemolymph. Taken together our results suggest that Imp-L2-expressing neurons sense the nutritional state of Drosophila larvae and coordinate dietary information and ecdysone production to adjust developmental timing under starvation conditions.     

蜕皮激素和IIS-TORC1信号的分子互作

蜕皮激素信号主导调控昆虫的蜕皮和变态,决定昆虫的发育时间;IIS-TORC1信号整合生长因子、激素、营养和能量信号,决定昆虫的生长速率。蜕皮激素和IIS-TORC1信号之间发生3种分子互作:(1)IIS-TORC1信号促进前胸腺和卵巢合成蜕皮激素前体。(2)在蜕皮和变态期间,蜕皮激素抑制脂肪体细胞内IIS-TORC1信号、Myc的转录、细胞生长及其内分泌功能,导致脑神经分泌细胞分泌胰岛素样肽的功能减弱,从而降低昆虫全身性的IIS-TORC1信号。(3)在幼虫摄食期间,胰岛素信号抑制FOXO的转录活性,降低了蜕皮激素受体EcR的转录共激活因子DOR编码基因的转录水平,从而阻碍了蜕皮激素信号传导。蜕皮激素信号和IIS-TORC1信号协同调控发育时间和生长速率共同决定昆虫的个体大小。     

Flies on steroids: the interplay between ecdysone and insulin signaling

In the fruit fly Drosophila melanogaster, the insulin and ecdysone signaling pathways have long been known to regulate growth and developmental timing, respectively. Recent findings reveal that crosstalk between these pathways allows coordination of growth and developmental timing and thus determines final body size.     

Prothoracicotropic hormone regulates developmental timing and body size in Drosophila

In insects, control of body size is intimately linked to nutritional quality as well as environmental and genetic cues that regulate the timing of developmental transitions. Prothoracicotropic hormone (PTTH) has been proposed to play an essential role in regulating the production and/or release of ecdysone, a steroid hormone that stimulates molting and metamorphosis. In this report, we examine the consequences on Drosophila development of ablating the PTTH-producing neurons. Surprisingly, PTTH production is not essential for molting or metamorphosis. Instead, loss of PTTH results in delayed larval development and eclosion of larger flies with more cells. Prolonged feeding, without changing the rate of growth, causes the overgrowth and is a consequence of low ecdysteroid titers. These results indicate that final body size in insects is determined by a balance between growth-rate regulators such as insulin and developmental timing cues such as PTTH that set the duration of the feeding interval.     

Effect of hormones and growth factors on the proliferation of adult cricket neural progenitor cells in vitro

In the adult cricket brain, a cluster of neuroblasts produces new interneurons that integrate into the mushroom body (MB), the main associative structure for multisensory information of the insect brain. In previous study we showed the antagonist role of the two morphogenetic hormones, juvenile hormone (JH) and ecdysone, on the regulation of adult MB neurogenesis in vivo. In order to examine whether these hormones act directly on neural progenitor cells, we developed an organotypic culture of MB cortices. Cell proliferation was assessed by 5-bromo, 2'-deoxyuridine (BrdU) incorporation. We showed that JH increased mushroom body neuroblast (MBNb) proliferation, confirming the mitogenic effect of JH observed in vivo. By contrast, ecdysone did not affect the amount of BrdU-labeled nuclei, suggesting that the inhibitory effect observed in vivo probably proceeded from an indirect pathway. We then examined the role of growth factors known to stimulate neural stem cell/progenitor cell proliferation in vertebrates. As shown by calcium imaging, MBNb only expressed functional receptors for insulin whereas mature interneurons responded to IGF-I and bFGF. Both insulin (10 microg/ml) and IGF-I (10 ng/ml) enhanced MB progenitor cell proliferation in culture, although the insulin effect was more pronounced. This effect was abolished when an inhibitor of polyamine biosynthesis was present in the medium, suggesting a link between polyamines and the insulin signaling pathway. By contrast, bFGF (20-200 ng/ml) failed to stimulate MBNb proliferation. Our results point to conserved and divergent mechanisms between vertebrates and invertebrates in the regulation of adult neural progenitor cell proliferation.     

Genetic modifier screens in Drosophila demonstrate a role for Rho1 signaling in ecdysone-triggered imaginal disc morphogenesis

Drosophila adult leg development provides an ideal model system for characterizing the molecular mechanisms of hormone-triggered morphogenesis. A pulse of the steroid hormone ecdysone at the onset of metamorphosis triggers the rapid transformation of a flat leg imaginal disc into an immature adult leg, largely through coordinated changes in cell shape. In an effort to identify links between the ecdysone signal and the cytoskeletal changes required for leg morphogenesis, we performed two large-scale genetic screens for dominant enhancers of the malformed leg phenotype associated with a mutation in the ecdysone-inducible broad early gene (br1). From a screen of >750 independent deficiency and candidate mutation stocks, we identified 17 loci on the autosomes that interact strongly with br1. In a complementary screen of approximately 112,000 F1 progeny of EMS-treated br1 animals, we recovered 26 mutations that enhance the br1 leg phenotype [E(br) mutations]. Rho1, stubbloid, blistered (DSRF), and cytoplasmic Tropomyosin were identified from these screens as br1-interacting genes. Our findings suggest that ecdysone exerts its effects on leg morphogenesis through a Rho1 signaling cascade, a proposal that is supported by genetic interaction studies between the E(br) mutations and mutations in the Rho1 signaling pathway. In addition, several E(br) mutations produce unexpected defects in midembryonic morphogenetic movements. Coupled with recent evidence implicating ecdysone signaling in these embryonic morphogenetic events, our results suggest that a common ecdysone-dependent, Rho1-mediated regulatory pathway controls morphogenesis during the two major transitions in the life cycle, embryogenesis and metamorphosis.     

Ecdysone differentially regulates metamorphic timing relative to 20-hydroxyecdysone by antagonizing juvenile hormone in Drosophila melanogaster

In insects, a steroid hormone, 20-hydroxyecdysone (20E), plays important roles in the regulation of developmental transitions by initiating signaling cascades via the ecdysone receptor (EcR). Although 20E has been well characterized as the molting hormone, its precursor ecdysone (E) has been considered to be a relatively inactive compound because it has little or no effect on classic EcR mediated responses. I found that feeding E to wild-type third instar larvae of Drosophila melanogaster accelerates the metamorphic timing, which results in elevation of lethality during metamorphosis and reduced body size, while 20E has only a minor effect. The addition of a juvenile hormone analog (JHA) to E impeded their precocious pupariation and thereby rescued the reduced body size. The ability of JHA impeding the effect of E was not observed in the Methoprene-tolerant (Met) and germ-cell expressed (gce) double mutant animals lacking JH signaling, indicating that antagonistic action of JH against E is transduced via a primary JH receptor, Met, or a product of its homolog, Gce. I also found that L3 larvae are susceptible to E around the time when they reach their minimum viable weight. These results indicate that E, and not just 20E, is also essential for proper regulation of developmental timing and body size. Furthermore, the precocious pupariation triggered by E is impeded by the action of JH to ensure that animals attain body size to survive metamorphosis.     

Coordinated regulation of niche and stem cell precursors by hormonal signaling

Stem cells and their niches constitute units that act cooperatively to achieve adult body homeostasis. How such units form and whether stem cell and niche precursors might be coordinated already during organogenesis are unknown. In fruit flies, primordial germ cells (PGCs), the precursors of germ line stem cells (GSCs), and somatic niche precursors develop within the larval ovary. Together they form the 16-20 GSC units of the adult ovary. We show that ecdysone receptors are required to coordinate the development of niche and GSC precursors. At early third instar, ecdysone receptors repress precocious differentiation of both niches and PGCs. Early repression is required for correct morphogenesis of the ovary and for protecting future GSCs from differentiation. At mid-third instar, ecdysone signaling is required for niche formation. Finally, and concurrent with the initiation of wandering behavior, ecdysone signaling initiates PGC differentiation by allowing the expression of the differentiation gene bag of marbles in PGCs that are not protected by the newly formed niches. All the ovarian functions of ecdysone receptors are mediated through early repression, and late activation, of the ecdysone target gene broad. These results show that, similar to mammals, a brain-gland-gonad axis controls the initiation of oogenesis in insects. They further exemplify how a physiological cue coordinates the formation of a stem cell unit within an organ: it is required for niche establishment and to ensure that precursor cells to adult stem cells remain undifferentiated until the niches can accommodate them. Similar principles might govern the formation of additional stem cell units during organogenesis.     

Severe developmental timing defects in the prothoracicotropic hormone (PTTH)-deficient silkworm,Bombyx mori

The insect neuropeptide prothoracicotropic hormone (PTTH) triggers the biosynthesis and release of the molting hormone ecdysone in the prothoracic gland (PG), thereby controlling the timing of molting and metamorphosis. Despite the well-documented physiological role of PTTH and its signaling pathway in the PG, it is not clear whether PTTH is an essential hormone for ecdysone biosynthesis and development. To address this question, we established and characterized a PTTH knockout line in the silkworm, Bombyx mori. We found that PTTH knockouts showed a severe developmental delay in both the larval and pupal stages. Larval phenotypes of PTTH knockouts can be classified into three major classes: (i) developmental arrest during the second larval instar, (ii) precocious metamorphosis after the fourth larval instar (one instar earlier in comparison to the control strain), and (iii) metamorphosis to normal-sized pupae after completing the five larval instar stages. In PTTH knockout larvae, peak levels of ecdysone titers in the hemolymph were dramatically reduced and the timing of peaks was delayed, suggesting that protracted larval development is a result of the reduced and delayed synthesis of ecdysone in the PG. Despite these defects, low basal levels of ecdysone were maintained in PTTH knockout larvae, suggesting that the primary role of PTTH is to upregulate ecdysone biosynthesis in the PG during molting stages, and low basal levels of ecdysone can be maintained in the absence of PTTH. We also found that mRNA levels of genes involved in ecdysone biosynthesis and ecdysteroid signaling pathways were significantly reduced in PTTH knockouts. Our results provide genetic evidence that PTTH is not essential for development, but is required to coordinate growth and developmental timing.     

Coordinating growth and maturation - insights from Drosophila

Adult body size in higher animals is dependent on the amount of growth that occurs during the juvenile stage. The duration of juvenile development, therefore, must be flexible and responsive to environmental conditions. When immature animals experience environmental stresses such as malnutrition or disease, maturation can be delayed until conditions improve and normal growth can resume. In contrast, when animals are raised under ideal conditions that promote rapid growth, internal checkpoints ensure that maturation does not occur until juvenile development is complete. Although the mechanisms that regulate growth and gate the onset of maturation have been investigated for decades, the emerging links between childhood obesity, early onset puberty, and adult metabolic disease have placed a new emphasis on this field. Remarkably, genetic studies in the fruit fly Drosophila melanogaster have shown that the central regulatory pathways that control growth and the timing of sexual maturation are conserved through evolution, and suggest that this aspect of animal life history is regulated by a common genetic architecture. This review focuses on these conserved mechanisms and highlights recent studies that explore how Drosophila coordinates developmental growth with environmental conditions.     

Acquired obesity is associated with changes in the serum lipidomic profile independent of genetic effects--a monozygotic twin study

Both genetic and environmental factors are involved in the etiology of obesity and the associated lipid disturbances. We determined whether acquired obesity is associated with changes in global serum lipid profiles independent of genetic factors in young adult monozygotic (MZ) twins. 14 healthy MZ pairs discordant for obesity (10 to 25 kg weight difference) and ten weight concordant control pairs aged 24-27 years were identified from a large population-based study. Insulin sensitivity was assessed by the euglycemic clamp technique, and body composition by DEXA (% body fat) and by MRI (subcutaneous and intra-abdominal fat). Global characterization of lipid molecular species in serum was performed by a lipidomics strategy using liquid chromatography coupled to mass spectrometry. Obesity, independent of genetic influences, was primarily related to increases in lysophosphatidylcholines, lipids found in proinflammatory and proatherogenic conditions and to decreases in ether phospholipids, which are known to have antioxidant properties. These lipid changes were associated with insulin resistance, a pathogonomic characteristic of acquired obesity in these young adult twins. Our results show that obesity, already in its early stages and independent of genetic influences, is associated with deleterious alterations in the lipid metabolism known to facilitate atherogenesis, inflammation and insulin resistance.     

果蝇胰岛素样肽8(Dilp8)的功能及作用机制

果蝇Drosophila作为一种模式生物,具有生长周期短、繁殖能力强和研究成本低等优点。而且果蝇有65%的基因与人类同源,特别是其遗传背景简单的特点,使其在生物生长发育研究、病理机制研究和基因表达调控等研究中发挥重要作用。目前在果蝇中已发现8种胰岛素样肽,即果蝇胰岛素样肽1-8(Drosophila insulin-like peptide 1-8, Dilp1-8),而对于果蝇胰岛素信号通路的研究主要集中在其调控机体生长发育和能量代谢的方面,这些功能主要通过Dilp1-7来发挥作用。对于Dilp8的功能及其发挥作用的分子机制知之甚少。本文总结了自Dilp8被发现以来,人们对于其功能的研究结果。Dilp8主要在幼虫成虫盘和成年雌果蝇的卵巢中表达,其主要作用是调节果蝇的组织生长和发育时间,使果蝇生长为具有相对固定体型和一定对称性的个体。当果蝇幼虫在生长过程中受到损伤时,Dilp8会通过延缓发育时间来缓解异常生长。Dilp8被激活后,在中枢神经系统与其受体富含亮氨酸重复序列的G蛋白偶联受体3(leucine-rich repeat-containing G protein-coupled ...     

Obesity and colorectal cancer: epidemiology, mechanisms and candidate genes

There is increasing evidence that dysregulation of energy homeostasis is associated with colorectal carcinogenesis. Epidemiological data have consistently demonstrated a positive relation between increased body size and colorectal malignancy, whereas mechanistic studies have sought to uncover obesity-related carcinogenic pathways. The phenomenon of "insulin resistance" or the impaired ability to normalize plasma glucose levels has formed the core of these pathways, but other mechanisms have also been advanced. Obesity-induced insulin resistance leads to elevated levels of plasma insulin, glucose and fatty acids. Exposure of the colonocyte to heightened concentrations of insulin may induce a mitogenic effect within these cells, whereas exposure to glucose and fatty acids may induce metabolic perturbations, alterations in cell signaling pathways and oxidative stress. The importance of chronic inflammation in the pathogenesis of obesity has recently been highlighted and may represent an additional mechanism linking increased adiposity to colorectal carcinogenesis. This review provides an overview of the epidemiology of body size and colorectal neoplasia and outlines current knowledge of putative mechanisms advanced to explain this relation. Family based studies have shown that the propensity to become obese is heritable, but this is only manifest in conditions of excess energy intake over expenditure. Inheritance of a genetic profile that predisposes to increased body size may also be predictive of colorectal cancer. Genomewide scans, linkage studies and candidate gene investigations have highlighted more than 400 chromosomal regions that may harbor variants that predispose to increased body size. The genetics underlying the pathogenesis of obesity are likely to be complex, but variants in a range of different genes have already been associated with increased body size and insulin resistance. These include genes encoding elements of insulin signaling, adipocyte metabolism and differentiation, and regulation of energy expenditure. A number of investigators have begun to study genetic variants within these pathways in relation to colorectal neoplasia, but at present data remain limited to a handful of studies. These pathways will be discussed with particular reference to genetic polymorphisms that have been associated with obesity and insulin resistance.     

Programmed autophagy in the Drosophila fat body is induced by ecdysone through regulation of the PI3K pathway

Eukaryotic cells catabolize their own cytoplasm by autophagy in response to amino acid starvation and inductive signals during programmed tissue remodeling and cell death. The Tor and PI3K signaling pathways have been shown to negatively control autophagy in eukaryotes, but the mechanisms that link these effectors to overall animal development and nutritional status in multicellular organisms remain poorly understood. Here, we reveal a complex regulation of programmed and starvation-induced autophagy in the Drosophila fat body. Gain-of-function genetic analysis indicated that ecdysone receptor signaling induces programmed autophagy whereas PI3K signaling represses programmed autophagy. Genetic interaction studies showed that ecdysone signaling downregulates PI3K signaling and that this represents the effector mechanism for induction of programmed autophagy. Hence, these studies link hormonal induction of autophagy to the regulatory function of the PI3K signaling pathway in vivo.     

Vertebrate insulin induces diapause termination in Pieris brassicae pupae

Summary Due to its close structural homology with the 4K prothoracicotropic hormone isolated from Bombyx mori, we tested the ability of vertebrate insulin to break pupal diapause in a Lepidopteran, Pieris brassicae. Injection of 5g of bovine insulin in diapausing pupae led to diapause termination and synchronous adult eclosion; the effect of insulin was dose-dependent. Bovine insulin-A chain and B chain injected separately failed to show any biological activity suggesting that the intact structure of the molecule is required. Bovine insulin also promoted adult development of decapitated diapausing animals. We show that insulin triggers a reactivation of the neuroendocrine system leading to a neosynthesis of ecdysone beginning 6 days after treatment. This neosynthesis also occurred in beheaded animals suggesting that insulin stimulates the prothoracic glands without acting via the brain.