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。     

The expression of the let-7 small regulatory RNA is controlled by ecdysone during metamorphosis in Drosophila melanogaster

In Caenorhabditis elegans, the heterochronic pathway controls the timing of developmental events during the larval stages. A component of this pathway, the let-7 small regulatory RNA, is expressed at the late stages of development and promotes the transition from larval to adult (L/A) stages. The stage-specificity of let-7 expression, which is crucial for the proper timing of the worm L/A transition, is conserved in Drosophila melanogaster and other invertebrates. In Drosophila, pulses of the steroid hormone 20-hydroxyecdysone (ecdysone) control the timing of the transition from larval to pupal to adult stages. To test whether let-7 expression is regulated by ecdysone in Drosophila, we used Northern blot analysis to examine the effect of altered ecdysone levels on let-7 expression in mutant animals, organ cultures, and S2 cultured cells. Experiments were conducted to test the role of Broad-Complex (BR-C), an essential component in the ecdysone pathway, in let-7 expression. We show that ecdysone and BR-C are required for let-7 expression, indicating that the ecdysone pathway regulates the temporal expression of let-7 in Drosophila. These results demonstrate an interaction between steroid hormone signaling and the heterochronic pathway in insects.     

br regulates the expression of the ecdysone biosynthesis gene npc1

The growth and metamorphosis of insects are regulated by ecdysteroid hormones produced in the ring gland. Ecdysone biosynthesis-related genes are both highly and specifically expressed in the ring gland. However, the intrinsic regulation of ecdysone biosynthesis has received little attention. Here we used the Drosophila npc1 gene to study the mechanism of ring gland-specific gene expression. npc1 is important for sterol trafficking in the ring gland during ecdysone biosynthesis. We have identified a conserved ring gland-specific cis-regulatory element (RSE) in the npc1 promoter using promoter fusion reporter analysis. Furthermore, genetic loss-of-function analysis and in vitro electrophoretic mobility shift assays revealed that the ecdysone early response gene broad complex (br) is a vital factor in the positive regulation of npc1 ring gland expression. Moreover, br also affects the ring gland expression of many other ecdysone biosynthetic genes as well as torso and InR, two key factors in the regulation of ecdysone biosynthesis. These results imply that ecdysone could potentially act through its early response gene br to achieve positive feedback regulation of ecdysone biosynthesis during development.     

Spatiotemporally modulated Vestigial gradient by Wingless signaling adaptively regulates cell division for precise wing size control

In animal development, the growth of a tissue or organ is timely arrested when it reaches the stereotyped correct size. How this is robustly controlled remains poorly understood. The prevalent viewpoint, which is that morphogen gradients, due to their organizing roles in development, are directly responsible for growth arrest, cannot explain a number of observations. Recent findings from studies of the Drosophila wing have revealed that the interpretation of the Wingless gradient requires signaling-induced self-inhibition and that cell proliferation is controlled by graded vestigial expression. These findings highlight a growth control mechanism that involves Wingless regulated vestigial expression, but a question is whether they can quantitatively explain the observed preciseness and robustness of wing size control. Quantitative and systematic investigation into Wingless signaling using a mathematical model has elucidated two points. First, negative regulation of the Vestigial gradient by Wingless signaling makes vestigial expression precise and robust. Second, weak Wingless signaling in a primarily small wing pouch causes a short and steep Vestigial gradient, which stimulates more cell divisions and leads to a significant expansion of the wing pouch; however, strong Wingless signaling in a primarily large wing pouch causes a long and smooth Vestigial gradient, which stimulates fewer cell divisions and results in a slight expansion of the wing pouch. These results substantially decipher an inherent mechanism of tissue and organ size control. Our model explains, and is supported by, a number of experimental observations.     

Mob as tumor suppressor is activated at the cell membrane to control tissue growth and organ size in Drosophila

Growth inhibition mediated by Hippo (Hpo) signaling is essential for tissue growth and organ size control in Drosophila. However, the cellular mechanism by which the core components like Mob as tumor suppressor (Mats) and Warts (Wts) protein kinase are activated is poorly understood. In this work, we found that the endogenous Mats is located at the plasma membrane in developing tissues. Membrane targeting constitutively activates Mats to promote apoptosis and reduce cell proliferation, which leads to reduced tissue growth and organ size. Moreover, the ability of membrane-targeted Mats to inhibit tissue growth required the wts gene activity and Wts kinase activity was increased by the activated Mats in developing tissues. Consistent with the idea that Mats is a key component of the Hpo pathway, Mats is required and sufficient to regulate Yki nuclear localization. These results support a model in which the plasma membrane is an important site of action for Mats tumor suppressor to control tissue growth and organ size.     

A dual function for Deep orange in programmed autophagy in the Drosophila melanogaster fat body

Lysosomal degradation of cytoplasm by way of autophagy is essential for cellular amino acid homeostasis and for tissue remodeling. In insects such as Drosophila, autophagy is developmentally upregulated in the larval fat body prior to metamorphosis. Here, autophagy is induced by the hormone ecdysone through down-regulation of the autophagy-suppressive phosphoinositide 3-kinase (PI3K) signaling pathway. In yeast, Vps18 and other members of the HOPS complex have been found essential for autophagic degradation. In Drosophila, the Vps18 homologue Deep orange (Dor) has previously been shown to mediate fusion of multivesicular endosomes with lysosomes. A requirement of Dor for ecdysone-mediated chromosome puffing has also been reported. In the present report, we have tested the hypothesis that Dor may control programmed autophagy at the level of ecdysone signaling as well as by mediating autophagosome-to-lysosome fusion. We show that dor mutants are defective in programmed autophagy and provide evidence that autophagy is blocked at two levels. First, PI3K activity was not down-regulated correctly in dor larvae, which correlated with a decrease in ecdysone reporter activity. The down-regulation of PI3K activity was restored by feeding ecdysone to the mutant larvae. Second, neither exogenous ecdysone nor overexpression of PTEN, a silencer of PI3K signaling, restored fusion of autophagosomes with lysosomes in the fat body of dor mutants. These results indicate that Dor controls autophagy indirectly, via ecdysone signaling, as well as directly, via autolysosomal fusion.     

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.     

昆虫成虫蜕皮激素研究进展

绝大多数成体昆虫羽化后,幼虫期间负责蜕皮激素合成的前胸腺即发生退化,但在一些内部生理及外部环境因子的调控下,某些成体组织(如生殖腺)可扮演类似前胸腺的角色合成与分泌蜕皮激素。蜕皮激素的功能发挥是经受体介导的,包括核受体(如EcR/USP)和膜受体(如DopEcR),它们广泛表达于成体许多组织,参与成虫行为、生殖、寿命、滞育及免疫应答等众多方面的调节,对维持基本的生理功能具有重要作用。就成虫蜕皮激素的产生组织及影响其滴度的因素、成虫蜕皮激素受体概述与组织分布、成虫蜕皮激素信号通路的功能发挥等研究进展方面加以综述。     

A matter of size: developmental control of organ size in plants

The intrinsic size of plant organs is determined by developmental signals, yet the molecular and genetic mechanisms that control organ size are largely unknown. Ongoing functional analysis of Arabidopsis genes is defining important regulators involved in these mechanisms. Key features of this control are the coordinated activation of growth and cell division by growth regulators and the maintenance of meristematic competence by the ANT gene, which acts as an organ-size checkpoint. Alterations of genome size by polyploidization and endoreduplication can reset this checkpoint by ploidy-dependent, epigenetically regulated differential gene expression. In addition, the regulation of polarized growth and phytohormone signaling also affect final organ size. These findings reveal unique aspects of plant organ-size control that are distinct from animal organ-size control.     

The Hox gene Ultrabithorax modulates the shape and size of the third leg of Drosophila by influencing diverse mechanisms

The developmental mechanisms that regulate the relative size and shape of organs have remained obscure despite almost a century of interest in the problem and the fact that changes in relative size represent the dominant mode of evolutionary change. Here, I investigate how the Hox gene Ultrabithorax (Ubx) instructs the legs on the third thoracic segment of Drosophila melanogaster to develop with a different size and shape from the legs on the second thoracic segment. Through loss-of-function and gain-of-function experiments, I demonstrate that different segments of the leg, the femur and the first tarsal segment, and even different regions of the femur, regulate their size in response to Ubx expression through qualitatively different mechanisms. In some regions, Ubx acts autonomously to specify shape and size, whereas in other regions, Ubx influences size through nonautonomous mechanisms. Loss of Ubx autonomously reduces cell size in the T3 femur, but this reduction seems to be partially compensated by an increase in cell numbers, so that it is unclear what effect cell size and number directly have on femur size. Loss of Ubx has both autonomous and nonautonomous effects on cell number in different regions of the basitarsus, but again there is not a strong correlation between cell size or number and organ size. Total organ size appears to be regulated through mechanisms that operate at the level of the entire leg segment (femur or basitarsus) relatively independently of the behavior of individual subpopulations of cells within the segment.     

Mechanisms of size control

The study of organ size control is a discipline of developmental biology that is largely unexplored. Although the size of an organ or organism depends largely on cell numbers and cell size, studies have found that the simple deregulation of cell proliferation or cell growth does not necessarily lead to changes in organ size. Recent genetic screens in Drosophila suggest that mutations that do affect organ size can be classified into three broad categories on the basis of their underlying effects: patterning, proliferation, and growth. Overall, experimental data suggest that organ size might be regulated by a 'total mass checkpoint' mechanism which functions to link the regulation of cell size and cell proliferation. The mechanisms of organ size control could also be critical targets for evolutionary events or disease processes such as tumorigenesis.     

The Arabidopsis EIN2 restricts organ growth by retarding cell expansion

The growth of plant organ to its characteristic size is a fundamental developmental process, but the mechanism is still poorly understood. Plant hormones play a great role in organ size control by modulating cell division and/or cell expansion. ETHYLENE INSENSITVE 2 (EIN2) was first identified by a genetic screen for ethylene insensitivity and is regarded as a central component of ethylene signaling, but its role in cell growth has not been reported. Here we demonstrate that changed expression of EIN2 led to abnormity of cell expansion by morphological and cytological analyses of EIN2 loss-of-function mutants and the overexpressing transgenic plant. Our findings suggest that EIN2 controls final organ size by restricting cell expansion.     

Hippo circuitry and the redox modulation of hippo components in cancer cell fate decisions

Meticulous and precise control of organ size is undoubtedly one of the most pivotal processes in mammalian development and regeneration along with cell differentiation, morphogenesis and programmed cell death. These processes are strictly regulated by complex and highly coordinated mechanisms to maintain a steady growth state. There are a number of extrinsic and intrinsic factors that dictate the total number and/or size of cells by influencing growth, proliferation, differentiation and cell death. Multiple pathways, such as those involved in promoting organ size and others that restrict disproportionate tissue growth act simultaneously to maintain cellular and tissue homeostasis. Aberrations at any level in these organ size-regulating processes can lead to various pathological states with cancers being the most formidable one (Yin and Zhang, 2011). Extensive research in the realm of growth control has led to the identification of the Hippo-signaling pathway as a critical network in modulating tissue growth via its effect on multiple signaling pathways and through intricate crosstalk with proteins that regulate cell polarity, adhesion and cell-cell interactions (Zhao et al., 2011b). The Hippo pathway controls cell number and organ size by transducing signals from the plasma membrane to the nucleus to regulate the expression of genes involved in cell fate determination (Shi et al., 2015). In this review, we summarize the recent discoveries concerning Hippo pathway, its diversiform regulation in mammals as well as its implications in cancers, and highlight the possible role of oxidative stress in Hippo pathway regulation.     

Coordination of cell proliferation and cell expansion in the control of leaf size in Arabidopsis thaliana

Size is an important parameter in the characterization of organ morphology and function. To understand the mechanisms that control leaf size, we previously isolated a number of Arabidopsis thaliana mutants with altered leaf size. Because leaf morphogenesis depends on determinate cell proliferation, the size of a mature leaf is controlled by variation in cell size and number. Therefore, leaf-size mutants should be classified according to the effects of the mutations on the cell number and/or size. A group of mutants represented by angustifolia3/grf-interacting factor1 and aintegumenta exhibits an intriguing cellular phenotype termed compensation: when the leaf cell number is decreased due to the mutation, the leaf cell size increases, leading to compensation in leaf area. Several lines of genetic evidence suggest that compensation is probably not a result of the uncoupling of cell division from cell growth. Rather, the evidence suggests an organ-wide mechanism that coordinates cell proliferation with cell expansion during leaf development. Our results provide a key, novel concept that explains how leaf size is controlled at the organ level.     

Cytochrome P450 CYP307A1/Spook: a regulator for ecdysone synthesis in insects

The prothoracic gland (PG) has essential roles in synthesizing and secreting a steroid hormone called ecdysone that is critical for molting and metamorphosis of insects. However, little is known about the genes controlling ecdysteroidogenesis in the PG. To identify genes functioning in the PG of the silkworm, Bombyx mori, we used differential display PCR and focused on a cytochrome P450 gene designated Cyp307a1. Its expression level positively correlates with a change in the hemolymph ecdysteroid titer. In addition, Drosophila Cyp307a1 is encoded in the spook locus, one of the Halloween mutant family members showing a low ecdysone titer in vivo, suggesting that Cyp307a1 is involved in ecdysone synthesis. While Drosophila Cyp307a1 is expressed in the early embryos and adult ovaries, the expression is not observed in the PGs of embryos or third instar larvae. These results suggest a difference in the ecdysone synthesis pathways during larval development in these insects.