Insect oenocytes: a model system for studying cell-fate specification by Hox genes

A Functional Analysis of the Drosophila Gene hindsight: Evidence for Positive Regulation of EGFR Signaling

We have investigated the relationship between the function of the gene hindsight (hnt), which is the Drosophila homolog of Ras Responsive Element Binding protein-1 (RREB-1), and the EGFR signaling pathway. We report that hnt mutant embryos are defective in EGFR signaling dependent processes, namely chordotonal organ recruitment and oenocyte specification. We also show the temperature sensitive hypomorphic allele hnt^pebbled is enhanced by the hypomorphic MAPK allele rolled (rl¹ ). We find that hnt overexpression results in ectopic DPax2 expression within the embryonic peripheral nervous system, and we show that this effect is EGFR-dependent. Finally, we show that the canonical U-shaped embryonic lethal phenotype of hnt, which is associated with premature degeneration of the extraembyonic amnioserosa and a failure in germ band retraction, is rescued by expression of several components of the EGFR signaling pathway (sSpi, Ras85D ^V12 , pnt^P1 ) as well as the caspase inhibitor p35 Based on this collection of corroborating evidence, we suggest that an overarching function of hnt involves the positive regulation of EGFR signaling.

Metabolism in Acute-On-Chronic Liver Failure: The Solution More than the Problem

Chronic inflammatory liver disease with an acute deterioration of liver function is named acute-on-chronic inflammation and could be regulated by the metabolic impairments related to the liver dysfunction. In this way, the experimental cholestasis model is excellent for studying metabolism in both types of inflammatory responses. Along the evolution of this model, the rats develop biliary fibrosis and an acute-on-chronic decompensation. The acute decompensation of the liver disease is associated with encephalopathy, ascites, acute renal failure, an acute phase response and a splanchnic increase of pro- and anti-inflammatory cytokines. This multiorgan inflammatory dysfunction is mainly associated with a splanchnic and systemic metabolic switch with dedifferentiation of the epithelial, endothelial and mesothelial splanchnic barriers. Furthermore, a splanchnic infiltration by mast cells occurs, which suggests that these cells could carry out a compensatory metabolic role, especially through the modulation of hepatic and extrahepatic mitochondrial-peroxisome crosstalk. For this reason, we propose the hypothesis that mastocytosis in the acute-on-chronic hepatic insufficiency could represent the development of a survival metabolic mechanisms that mitigates the noxious effect of the hepatic functional deficit. A better understanding the pathophysiological response of the mast cells in liver insufficiency and portal hypertension would help to find new pathways for decreasing the high morbidity and mortality rate of these patients.

A Hox complex activates and potentiates the Epidermal Growth Factor signaling pathway to specify Drosophila oenocytes

Hox transcription factors specify distinct cell types along the anterior-posterior axis of metazoans by regulating target genes that modulate signaling pathways. A well-established example is the induction of Epidermal Growth Factor (EGF) signaling by an Abdominal-A (Abd-A) Hox complex during the specification of Drosophila hepatocyte-like cells (oenocytes). Previous studies revealed that Abd-A is non-cell autonomously required to promote oenocyte fate by directly activating a gene (rhomboid) that triggers EGF secretion from sensory organ precursor (SOP) cells. Neighboring cells that receive the EGF signal initiate a largely unknown pathway to promote oenocyte fate. Here, we show that Abd-A also plays a cell autonomous role in inducing oenocyte fate by activating the expression of the Pointed-P1 (PntP1) ETS transcription factor downstream of EGF signaling. Genetic studies demonstrate that both PntP1 and PntP2 are required for oenocyte specification. Moreover, we found that PntP1 contains a conserved enhancer (PntP1OE) that is activated in oenocyte precursor cells by EGF signaling via direct regulation by the Pnt transcription factors as well as a transcription factor complex consisting of Abd-A, Extradenticle, and Homothorax. Our findings demonstrate that the same Abd-A Hox complex required for sending the EGF signal from SOP cells, enhances the competency of receiving cells to select oenocyte cell fate by up-regulating PntP1. Since PntP1 is a downstream effector of EGF signaling, these findings provide insight into how a Hox factor can both trigger and potentiate the EGF signal to promote an essential cell fate along the body plan.

Drosophila Spidey/Kar Regulates Oenocyte Growth via PI3-Kinase Signaling

Cell growth and proliferation depend upon many different aspects of lipid metabolism. One key signaling pathway that is utilized in many different anabolic contexts involves Phosphatidylinositide 3-kinase (PI3K) and its membrane lipid products, the Phosphatidylinositol (3,4,5)-trisphosphates. It remains unclear, however, which other branches of lipid metabolism interact with the PI3K signaling pathway. Here, we focus on specialized fat metabolizing cells in Drosophila called larval oenocytes. In the presence of dietary nutrients, oenocytes undergo PI3K-dependent cell growth and contain very few lipid droplets. In contrast, during starvation, oenocytes decrease PI3K signaling, shut down cell growth and accumulate abundant lipid droplets. We now show that PI3K in larval oenocytes, but not in fat body cells, functions to suppress lipid droplet accumulation. Several enzymes of fatty acid, triglyceride and hydrocarbon metabolism are required in oenocytes primarily for lipid droplet induction rather than for cell growth. In contrast, a very long chain fatty-acyl-CoA reductase (FarO) and a putative lipid dehydrogenase/reductase (Spidey, also known as Kar) not only promote lipid droplet induction but also inhibit oenocyte growth. In the case of Spidey/Kar, we show that the growth suppression mechanism involves inhibition of the PI3K signaling pathway upstream of Akt activity. Together, the findings in this study show how Spidey/Kar and FarO regulate the balance between the cell growth and lipid storage of larval oenocytes.

Lipids play diverse roles in health and disease. Some types of lipids function as metabolic fuels for energy homeostasis, whereas others act as components of cell membranes or serve as signals regulating cell behaviors. Much, however, remains to be discovered about the molecular connections between different categories of lipids. Phosphatidylinositide 3-kinase (PI3K) is an enzyme that synthesizes phosphatidylinositide lipids, which act as signals essential for growth during normal development and cancer. Using genetics in the fruit fly, Drosophila, we identify new regulatory links between phosphatidylinositides and lipid oxidoreductases in specialized fat-metabolizing cells called oenocytes. We find that an enzyme metabolizing very long chain fatty acids (VLCFAs) and also a putative lipid dehydrogenase/reductase both act to prevent the inappropriate overgrowth of oenocytes. In the case of the latter enzyme, it suppresses cell growth by inhibiting phosphatidylinositide signaling. Future studies will determine whether similar lipid enzymes regulate PI3K signaling in other cell and tissue types during normal development and tumorigenesis.

Variegated expression of Hsp22 transgenic reporters indicates cell-specific patterns of aging in Drosophila oenocytes

The cytoplasmic chaperone gene Hsp70 and the mitochondrial chaperone gene Hsp22 are upregulated during normal aging in Drosophila in tissue-general patterns. In addition, Hsp22 reporters are dramatically upregulated during aging in a subset of the oenocytes (liver-like cells). Hsp22 reporter expression varied dramatically between individual oenocytes and between groups of oenocytes located in adjacent body segments, and was negatively correlated with accumulation of age pigment, indicating cell-specific and cell-lineage-specific patterns of oenocyte aging. Conditional transgenic systems were used to express 88 transgenes to search for trans-regulators of the Hsp70 and Hsp22 reporters during aging. The wingless gene increased tissue-general upregulation of both Hsp70 and Hsp22 reporters. In contrast, the mitochondrial genes MnSOD and Hsp22 increased expression of Hsp22 reporters in the oenocytes and decreased accumulation of age pigment in these cells. The data suggest that cell-specific and cell lineage-specific patterns of mitochondrial malfunction contribute to oenocyte aging.

The development and functions of oenocytes

Oenocytes have intrigued insect physiologists since the nineteenth century. Many years of careful but mostly descriptive research on these cells highlights their diverse sizes, numbers, and anatomical distributions across Insecta. Contemporary molecular genetic studies in Drosophila melanogaster and Tribolium castaneum support the hypothesis that oenocytes are of ectodermal origin. They also suggest that, in both short and long germ-band species, oenocytes are induced from a Spalt major/Engrailed ectodermal zone by MAPK signaling. Recent glimpses into some of the physiological functions of oenocytes indicate that they involve fatty acid and hydrocarbon metabolism. Genetic studies in D. melanogaster have shown that larval oenocytes synthesize very-long-chain fatty acids required for tracheal waterproofing and that adult oenocytes produce cuticular hydrocarbons required for desiccation resistance and pheromonal communication. Exciting areas of future research include the evolution of oenocytes and their cross talk with other tissues involved in lipid metabolism such as the fat body.

The Spalt transcription factors regulate cell proliferation, survival and epithelial integrity downstream of the Decapentaplegic signalling pathway

The expression of the spalt genes is regulated by the Decapentaplegic signalling pathway in the Drosophila wing. These genes participate in the patterning of the longitudinal wing veins by regulating the expression of vein-specific genes, and in the establishment of cellular affinities in the central region of the wing blade epithelium. The Spalt proteins act as transcription factors, most likely regulating gene expression by repression, but the identity of their target genes in the wing is still unknown. As a preliminary step to unravel the genetic hierarchy controlled by the Spalt proteins, we have analysed their requirements during wing development, and addressed to what extent they mediate all the functions of the Decapentaplegic pathway in this developmental system. We identify additional functions for Spalt in cell division, survival, and maintenance of epithelial integrity. Thus, Spalt activity is required to promote cell proliferation, acting in the G2/M transition of the cell cycle. The contribution of Spalt to cell division is limited to the central region of the wing blade, as they do not mediate the extra growth triggered by Decapentaplegic signalling in the peripheral regions of the wing disc. In addition, Spalt function is required to maintain cell viability in cells exposed to high levels of Decapentaplegic signalling. This aspect of Spalt function is related to the repression of JNK signalling in the spalt domain of expression. Finally, we further characterise the requirements of Spalt to maintain epithelial integrity by regulating cellular affinities between cells located in the central wing region. Our results indicate that Spalt function mediates most of the requirements identified for Decapentaplegic signalling, contributing to establish the cellular qualities that differentiate central versus peripheral territories in the wing blade.

Coupling inflammation with evo-devo

Inflammation integrates diverse mechanisms that are associated not only with pathological conditions, such as cardiovascular diseases, type 2 diabetes, obesity, neurodegenerative diseases and cancer, but also with physiological processes like reproduction i.e. oogenesis and embryogenesis as well as aging. In the current review we firstly propose that the inflammatory response could recapitulate the phylogenia. In this way, highly conserved inflammatory mechanisms that play a main role in the evolutive development of different animal species, both invertebrates as well as vertebrates, are identified. Therefore, we also hypothesize that inflammation could represent a key tool used by nature to modulate organisms according to the environmental conditions in which these develop. Thus, inflammation could be the pathway by which the environmental factors could be related to the evolutionary development. If so, the diverse human chronic inflammatory diseases that nowadays the Western society suffer would represent the way for adapting to the abrupt changes in their lifestyle. Nonetheless, the distribution of the different pathological conditions varies in terms of intensity and magnitude among Western country populations depending on their genetic polymorphism. In this case, it should be considered that this set of diseases, distributed between all the individuals that constitute the Westernized society, would represent a true Social Inflammatory Syndrome whose final result is its remodeling. In this context, the use of inflammation by the Western society could represent the camouflaged expression of efficient mechanisms of evolution and development. In addition, if the different types of the inflammatory response involved in these diverse chronic pathological conditions could trace the biochemical origins of life, perhaps inflammation could represent an archaeological tool of unsuspected usefulness for understanding our own origin.

Oenocyte development in the red flour beetle Tribolium castaneum

Oenocytes are a specialized cell type required for lipid processing, pheromone secretion, and developmental signaling. Their development has been well characterized in Drosophila melanogaster, but it remains unknown whether the developmental program is conserved in other insect species. In this study, we compare and contrast the specification and development of larval oenocytes between Drosophila and the red flour beetle, Tribolium castaneum. First, we identify several useful reagents to label larval oenocytes, including both a Tribolium GFP enhancer trap line and a simple flurophore-conjugated streptavidin staining method that recognizes oenocytes across insect species. Second, we use these tools to describe oenocyte development in Tribolium embryos, and our findings provide evidence for conserved roles of MAP kinase signaling as well as the Spalt, Engrailed, hepatocyte nuclear factor-4, and ventral veins lacking factors in producing abdominal-specific oenocyte cells. However, Tribolium embryos produce four times as many oenocytes per abdominal segment as Drosophila, and unlike in Drosophila, these cells rapidly downregulate the expression of the Spalt transcription factor. Thus, these results provide new insight into the molecular pathways regulating oenocyte specification across insect species.

Sex-biased transcription enhancement by a 5' tethered Gal4-MOF histone acetyltransferase fusion protein in Drosophila

BACKGROUND

In male Drosophila melanogaster, the male specific lethal (MSL) complex is somehow responsible for a two-fold increase in transcription of most X-linked genes, which are enriched for histone H4 acetylated at lysine 16 (H4K16ac). This acetylation requires MOF, a histone acetyltransferase that is a component of the MSL complex. MOF also associates with the non-specific lethal or NSL complex. The MSL complex is bound within active genes on the male X chromosome with a 3' bias. In contrast, the NSL complex is enriched at promoter regions of many autosomal and X-linked genes in both sexes. In this study we have investigated the role of MOF as a transcriptional activator.

RESULTS

MOF was fused to the DNA binding domain of Gal4 and targeted to the promoter region of UAS-reporter genes in Drosophila. We found that expression of a UAS-red fluorescent protein (DsRed) reporter gene was strongly induced by Gal4-MOF. However, DsRed RNA levels were about seven times higher in female than male larvae. Immunostaining of polytene chromosomes showed that Gal4-MOF co-localized with MSL1 to many sites on the X chromosome in male but not female nuclei. However, in female nuclei that express MSL2, Gal4-MOF co-localized with MSL1 to many sites on polytene chromosomes but DsRed expression was reduced. Mutation of conserved active site residues in MOF (Glu714 and Cys680) reduced HAT activity in vitro and UAS-DsRed activation in Drosophila. In the presence of Gal4-MOF, H4K16ac levels were enriched over UAS-lacZ and UAS-arm-lacZ reporter genes. The latter utilizes the constitutive promoter from the arm gene to drive lacZ expression. In contrast to the strong induction of UAS-DsRed expression, UAS-arm-lacZ expression increased by about 2-fold in both sexes.

CONCLUSIONS

Targeting MOF to reporter genes led to transcription enhancement and acetylation of histone H4 at lysine 16. Histone acetyltransferase activity was required for the full transcriptional response. Incorporation of Gal4-MOF into the MSL complex in males led to a lower transcription enhancement of UAS-DsRed but not UAS-arm-lacZ genes. We discuss how association of Gal4-MOF with the MSL or NSL proteins could explain our results.

Dissection of oenocytes from adult Drosophila melanogaster

In Drosophila melanogaster, as in other insects, a waxy layer on the outer surface of the cuticle, composed primarily of hydrocarbon compounds, provides protection against desiccation and other environmental challenges. Several of these cuticular hydrocarbon (CHC) compounds also function as semiochemical signals, and as such mediate pheromonal communications between members of the same species, or in some instances between different species, and influence behavior. Specialized cells referred to as oenocytes are regarded as the primary site for CHC synthesis. However, relatively little is known regarding the involvement of the oenocytes in the regulation of the biosynthetic, transport, and deposition pathways contributing to CHC output. Given the significant role that CHCs play in several aspects of insect biology, including chemical communication, desiccation resistance, and immunity, it is important to gain a greater understanding of the molecular and genetic regulation of CHC production within these specialized cells. The adult oenocytes of D. melanogaster are located within the abdominal integument, and are metamerically arrayed in ribbon-like clusters radiating along the inner cuticular surface of each abdominal segment. In this video article we demonstrate a dissection technique used for the preparation of oenocytes from adult D. melanogaster. Specifically, we provide a detailed step-by-step demonstration of (1) how to fillet prepare an adult Drosophila abdomen, (2) how to identify the oenocytes and discern them from other tissues, and (3) how to remove intact oenocyte clusters from the abdominal integument. A brief experimental illustration of how this preparation can be used to examine the expression of genes involved in hydrocarbon synthesis is included. The dissected preparation demonstrated herein will allow for the detailed molecular and genetic analysis of oenocyte function in the adult fruit fly.

Fly foie gras: modeling fatty liver in Drosophila

Lipids provide an essential source of metabolites and energy in normal development as well as during periods of food deprivation. A recent study in Drosophila (Gutierrez et al., 2007) reveals a novel role in regulating lipid metabolism for specialized cells called oenocytes that present striking functional similarities to mammalian hepatocytes.

Anopheles gambiae P450 reductase is highly expressed in oenocytes and in vivo knockdown increases permethrin susceptibility

We describe an in vivo model for investigation of detoxification mechanisms of the mosquito Anopheles gambiae, important for the development of malaria control programmes. Cytochrome P450s are involved in metabolic insecticide resistance and require NADPH cytochrome P450 reductase (CPR) to function. Here we demonstrate that the major sites of adult mosquito CPR expression are oenocytes, mid-gut epithelia and head appendages. High CPR expression was also evident in Drosophila oenocytes indicating a general functional role in these insect cells. RNAi mediated knockdown drastically reduced CPR expression in oenocytes, and to a lesser extent in mid-gut epithelia; the head was unaffected. These flies showed enhanced sensitivity to permethrin, demonstrating a key role for abdominal/mid-gut P450s in pyrethroid metabolism, aiding the development of insecticides.

EGF Receptor Signaling Regulates Pulses of Cell Delamination from the Drosophila Ectoderm

Many different intercellular signaling pathways are known but, for most, it is unclear whether they can generate oscillating cell behaviors. Here we use time-lapse analysis of Drosophila embryogenesis to show that oenocytes delaminate from the ectoderm in discrete bursts of three. This pulsatile process has a 1 hour period, occurs without cell division, and requires a localized EGF receptor (EGFR) response. High-threshold EGFR targets are sequentially activated in rings of three cells, prefiguring the temporal pattern of delamination. Surprisingly, widespread misexpression of the relevant activating ligand, Spitz, is compatible with robust delamination pulses. Moreover, although Spitz ligand becomes limiting after only two pulses, artificially prolonging its secretion generates up to six additional cycles, revealing a rhythmic underlying mechanism. These findings illustrate how intercellular signaling and cell movements can generate multiple cycles of a cell behavior, despite individual cells experiencing only one cycle of receptor activation.

The conserved glutamine-rich region of chick csal1 and csal3 mediates protein interactions with other spalt family members. Implications for Townes-Brocks syndrome

Members of the spalt family of zinc finger-containing proteins have been implicated in development and disease. However, very little is known about the molecular function of spalt proteins. We have used biochemical approaches to characterize functional domains of two chick spalt homologs, csal1 and csal3. We show that csal1 and csal3 proteins repress transcription and that they can interact with each other. Furthermore, we found that truncated chick spalt proteins, similar to the truncated spalt protein expressed in the human congenital disorder Townes-Brocks syndrome, affect the nuclear localization of full-length spalt. Our findings have implications for the understanding of Townes-Brocks syndrome and the role of spalt genes in normal development. We propose that truncated spalt can exert a dominant negative effect and is able to interfere with the correct function of full-length protein, by causing its displacement from the nucleus. This could affect the transcriptional repressor activity of spalt and DNA binding. Spalt protein truncations could also affect the function of other spalt family members in various tissues.

Hox Genes: it's all a matter of context

Recent studies provide compelling new evidence that Hox gene effects depend on fine-structure spatial and temporal information. Further, in a specific cell type, only one or a few downstream genes may mediate Hox morphogenetic functions. If this is generally true, it will have important implications for how Hox regulatory networks operate and evolve.