Alternative processing of sterol regulatory element binding protein during larval development in Drosophila melanogaster

Transcriptional Control of Lipid Metabolism

Transcriptional control of lipid metabolism uses a framework that parallels the control of lipid metabolism at the protein or enzyme level, via feedback and feed-forward mechanisms. Increasing the substrates for an enzyme often increases enzyme gene expression, for example. A paucity of product can likewise potentiate transcription or stability of the mRNA encoding the enzyme or enzymes needed to produce it. In addition, changes in second messengers or cellular energy charge can act as on/off switches for transcriptional regulators to control transcript (and protein) abundance. Insects use a wide range of DNA-binding transcription factors (TFs) that sense changes in the cell and its environment to produce the appropriate change in transcription at gene promoters. These TFs work together with histones, spliceosomes, and additional RNA processing factors to ultimately regulate lipid metabolism. In this chapter, we will first focus on the important TFs that control lipid metabolism in insects. Next, we will describe non-TF regulators of insect lipid metabolism such as enzymes that modify acetylation and methylation status, transcriptional coactivators, splicing factors, and microRNAs. To conclude, we consider future goals for studying the mechanisms underlying the control of lipid metabolism in insects.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

Individual co-variation between viral RNA load and gene expression reveals novel host factors during early dengue virus infection of the Aedes aegypti midgut

Dengue virus (DENV) causes more human infections than any other mosquito-borne virus. The current lack of antiviral strategies has prompted genome-wide screens for host genes that are required for DENV infectivity. Earlier transcriptomic studies that identified DENV host factors in the primary vector Aedes aegypti used inbred laboratory colonies and/or pools of mosquitoes that erase individual variation. Here, we performed transcriptome sequencing on individual midguts in a field-derived Ae. aegypti population to identify new candidate host factors modulating DENV replication. We analyzed the transcriptomic data using an approach that accounts for individual co-variation between viral RNA load and gene expression. This approach generates a prediction about the agonist or antagonist effect of candidate genes on DENV replication based on the sign of the correlation between gene expression and viral RNA load. Using this method, we identified 39 candidate genes that went undetected by conventional pairwise comparison of gene expression levels between DENV-infected midguts and uninfected controls. Only four candidate genes were detected by both methods, emphasizing their complementarity. We demonstrated the value of our approach by functional validation of a candidate agonist gene encoding a sterol regulatory element-binding protein (SREBP), which was identified by correlation analysis but not by pairwise comparison. We confirmed that SREBP promotes DENV infection in the midgut by RNAi-mediated gene knockdown in vivo. We suggest that our approach for transcriptomic analysis can empower genome-wide screens for potential agonist or antagonist factors by leveraging inter-individual variation in gene expression. More generally, this method is applicable to a wide range of phenotypic traits displaying inter-individual variation.

Physiological and stem cell compartmentalization within the Drosophila midgut

The Drosophila midgut is maintained throughout its length by superficially similar, multipotent intestinal stem cells that generate new enterocytes and enteroendocrine cells in response to tissue requirements. We found that the midgut shows striking regional differentiation along its anterior-posterior axis. At least ten distinct subregions differ in cell morphology, physiology and the expression of hundreds of genes with likely tissue functions. Stem cells also vary regionally in behavior and gene expression, suggesting that they contribute to midgut sub-specialization. Clonal analyses showed that stem cells generate progeny located outside their own subregion at only one of six borders tested, suggesting that midgut subregions resemble cellular compartments involved in tissue development. Tumors generated by disrupting Notch signaling arose preferentially in three subregions and tumor cells also appeared to respect regional borders. Thus, apparently similar intestinal stem cells differ regionally in cell production, gene expression and in the ability to spawn tumors. DOI:http://dx.doi.org/10.7554/eLife.00886.001.

The site-2 protease

The site-2 protease (S2P) is an unusually-hydrophobic integral membrane protease. It cleaves its substrates, which are membrane-bound transcription factors, within membrane-spanning helices. Although structural information for S2P from animals is lacking, the available data suggest that cleavage may occur at or within the lipid bilayer. In mammalian cells, S2P is essential owing to its activation of the sterol regulatory element binding proteins (SREBPs); in the absence of exogenous lipid, cells lacking S2P cannot survive. S2P is also important in the endoplasmic reticulum (ER) stress response, activating several different membrane-bound transcription factors. Human patients harboring reduction-of-function mutations in S2P exhibit an array of pathologies ranging from skin defects to neurological abnormalities. Surprisingly, Drosophila melanogaster lacking S2P are viable and fertile. This article is part of a Special Issue entitled: Intramembrane Proteases.

Hypopigmentation and maternal-zygotic embryonic lethality caused by a hypomorphic mbtps1 mutation in mice

The site 1 protease, encoded by Mbtps1, mediates the initial cleavage of site 2 protease substrates, including sterol regulatory element binding proteins and CREB/ATF transcription factors. We demonstrate that a hypomorphic mutation of Mbtps1 called woodrat (wrt) caused hypocholesterolemia, as well as progressive hypopigmentation of the coat, that appears to be mechanistically unrelated. Hypopigmentation was rescued by transgenic expression of wild-type Mbtps1, and reciprocal grafting studies showed that normal pigmentation depended upon both cell-intrinsic or paracrine factors, as well as factors that act systemically, both of which are lacking in wrt homozygotes. Mbtps1 exhibited a maternal-zygotic effect characterized by fully penetrant embryonic lethality of maternal-zygotic wrt mutant offspring and partial embryonic lethality (~40%) of zygotic wrt mutant offspring. Mbtps1 is one of two maternal-zygotic effect genes identified in mammals to date. It functions nonredundantly in pigmentation and embryogenesis.

Expression and characterization of Drosophila signal peptide peptidase-like (sppL), a gene that encodes an intramembrane protease

Intramembrane proteases of the Signal Peptide Peptidase (SPP) family play important roles in developmental, metabolic and signaling pathways. Although vertebrates have one SPP and four SPP-like (SPPL) genes, we found that insect genomes encode one Spp and one SppL. Characterization of the Drosophila sppL gene revealed that the predicted SppL protein is a highly conserved structural homolog of the vertebrate SPPL3 proteases, with a predicted nine-transmembrane topology, an active site containing aspartyl residues within a transmembrane region, and a carboxy-terminal PAL domain. SppL protein localized to both the Golgi and ER. Whereas spp is an essential gene that is required during early larval stages and whereas spp loss-of-function reduced the unfolded protein response (UPR), sppL loss of function had no apparent phenotype. This was unexpected given that genetic knockdown phenotypes in other organisms suggested significant roles for Spp-related proteases.

Peptidylgycine α-amidating monooxygenase and copper: a gene-nutrient interaction critical to nervous system function

Peptidylgycine alpha-amidating monooxygenase (PAM), a highly conserved copper-dependent enzyme, is essential for the synthesis of all amidated neuropeptides. Biophysical studies revealed that the binding of copper to PAM affects its structure, and cell biological studies demonstrated that the endocytic trafficking of PAM was sensitive to copper. We review data indicating that genetic reduction of PAM expression and mild copper deficiency in mice cause similar alterations in several physiological functions known to be regulated by neuropeptides: thermal regulation, seizure sensitivity, and anxiety-like behavior.

New insights into S2P signaling cascades: regulation, variation, and conservation

Regulated intramembrane proteolysis (RIP) is a conserved mechanism that regulates signal transduction across the membrane by recruiting membrane-bound proteases to cleave membrane-spanning regulatory proteins. As the first identified protease that performs RIP, the metalloprotease site-2 protease (S2P) has received extensive study during the past decade, and an increasing number of S2P-like proteases have been identified and studied in different organisms; however, some of their substrates and the related S1Ps remain elusive. Here, we review recent research on S2P cascades, including human S2P, E. coli RseP, B. subtilis SpoIVFB and the newly identified S2P homologs. We also discuss the variation and conservation of characterized S2P cascades. The conserved catalytic motif of S2P and prevalence of amino acids of low helical propensity in the transmembrane segments of the substrates suggest a conserved catalytic conformation and mechanism within the S2P family. The review also sheds light on future research on S2P cascades.

Other ways to skin a cat: activating SREBP without Scap

The sterol regulatory element binding protein (SREBP) pathway plays a central role in the global regulation of lipid homeostasis. SREBPs are membrane-bound transcription factors whose proteolytic activation is regulated by cellular lipid levels; when demand for lipid rises, SREBP travels from the endoplasmic reticulum to the Golgi apparatus where it is cleaved by two distinct proteases. Cleavage releases the transcription factor domain of SREBP from the membrane-bound precursor and transcription of its target genes consequently rises. Previously, we isolated Drosophila mutants null for dsrebp and others lacking site-2 protease (ds2p), the second of two Golgi-resident proteases that cleave dSREBP. dScap is a protein needed to escort dSREBP from the ER to the Golgi apparatus. We recently characterized the phenotypes of dscap mutants as well. Here, we describe additional details of phenotypes arising from the inability to activate SREBP appropriately.

Activation of sterol regulatory element binding proteins in the absence of Scap in Drosophila melanogaster

The escort factor Scap is essential in mammalian cells for regulated activation of sterol regulatory element binding proteins (SREBPs). SREBPs are membrane-bound transcription factors. Cells lacking Scap cannot activate SREBP. They are therefore deficient in the transcription of numerous genes involved in lipid synthesis and uptake; they cannot survive in the absence of exogenous lipid. Here we report that, in contrast to mammalian cells, Drosophila completely lacking dscap are viable. Flies lacking dscap emerge at approximately 70% of the expected rate and readily survive as homozygous stocks. These animals continue to cleave dSREBP in some tissues. Transcription of dSREBP target genes in dscap mutant larvae is reduced compared to wild type. It is greater than in mutants lacking dSREBP and remains responsive to dietary lipids in dscap mutants. Flies lacking dscap do not require the caspase Drice to activate dSREBP. This contrasts with ds2p mutants. ds2p encodes a protease that releases the transcription factor domain of dSREBP from the membrane. Larvae doubly mutant for dscap and ds2p exhibit phenotypes similar to those of ds2p single mutants. Thus, dScap and dS2P, essential components of the SREBP activation machinery in mammalian cells, are dispensable in Drosophila owing to different compensatory mechanisms.

Evolutionary conservation and adaptation in the mechanism that regulates SREBP action: what a long, strange tRIP it's been

Sterol regulatory element-binding proteins (SREBPs) are a subfamily of basic helix-loop-helix leucine zipper (bHLH-LZ) transcription factors that are conserved from fungi to humans and are defined by two key features: a signature tyrosine residue in the DNA-binding domain, and a membrane-tethering domain that is a target for regulated proteolysis. Recent studies including genome-wide and model organism approaches indicate SREBPs coordinate cellular lipid metabolism with other cellular physiologic processes. These functions are broadly related as cellular adaptation to environmental changes ranging from nutrient fluctuations to toxin exposure. This review integrates classic features of the SREBP pathway with newer information regarding the regulation and sensing mechanisms that serve to assimilate different cellular physiologic processes for optimal function and growth.

Activation of sterol regulatory element-binding protein by the caspase Drice in Drosophila larvae

During larval development in Drosophila melanogaster, transcriptional activation of target genes by sterol regulatory element-binding protein (dSREBP) is essential for survival. In all cases studied to date, activation of SREBPs requires sequential proteolysis of the membrane-bound precursor by site-1 protease (S1P) and site-2 protease (S2P). Cleavage by S2P, within the first membrane-spanning helix of SREBP, releases the transcription factor. In contrast to flies lacking dSREBP, flies lacking dS2P are viable. The Drosophila effector caspase Drice cleaves dSREBP, and cleavage requires an Asp residue at position 386, in the cytoplasmic juxtamembrane stalk. The initiator caspase Dronc does not cleave dSREBP, but animals lacking dS2P require both drice and dronc to complete development. They do not require Dcp1, although this effector caspase also can cleave dSREBP in vitro. Cleavage of dSREBP by Drice releases the amino-terminal transcription factor domain of dSREBP to travel to the nucleus where it mediates the increased transcription of target genes needed for lipid synthesis and uptake. Drice-dependent activation of dSREBP explains why flies lacking dS2P are viable, and flies lacking dSREBP itself are not.