The formation of plasma membrane reticular systems in the oenocytes of an insect

Drosophila PDGF/VEGF signaling from muscles to hepatocyte-like cells protects against obesity

PDGF/VEGF ligands regulate a plethora of biological processes in multicellular organisms via autocrine, paracrine, and endocrine mechanisms. We investigated organ-specific metabolic roles of Drosophila PDGF/VEGF-like factors (Pvfs). We combine genetic approaches and single-nuclei sequencing to demonstrate that muscle-derived Pvf1 signals to the Drosophila hepatocyte-like cells/oenocytes to suppress lipid synthesis by activating the Pi3K/Akt1/TOR signaling cascade in the oenocytes. Functionally, this signaling axis regulates expansion of adipose tissue lipid stores in newly eclosed flies. Flies emerge after pupation with limited adipose tissue lipid stores and lipid level is progressively accumulated via lipid synthesis. We find that adult muscle-specific expression of pvf1 increases rapidly during this stage and that muscle-to-oenocyte Pvf1 signaling inhibits expansion of adipose tissue lipid stores as the process reaches completion. Our findings provide the first evidence in a metazoan of a PDGF/VEGF ligand acting as a myokine that regulates systemic lipid homeostasis by activating TOR in hepatocyte-like cells.

RiboTag translatomic profiling of Drosophila oenocytes under aging and induced oxidative stress

BACKGROUND

Aging is accompanied with loss of tissue homeostasis and accumulation of cellular damages. As one of the important metabolic centers, liver shows age-related dysregulation of lipid metabolism, impaired detoxification pathway, increased inflammation and oxidative stress response. However, the mechanisms for these age-related changes still remain unclear. In the fruit fly, Drosophila melanogaster, liver-like functions are controlled by two distinct tissues, fat body and oenocytes. Compared to fat body, little is known about how oenocytes age and what are their roles in aging regulation. To characterize age- and stress-regulated gene expression in oenocytes, we performed cell-type-specific ribosome profiling (RiboTag) to examine the impacts of aging and oxidative stress on oenocyte translatome in Drosophila.

RESULTS

We show that aging and oxidant paraquat significantly increased the levels of reactive oxygen species (ROS) in adult oenocytes of Drosophila, and aged oenocytes exhibited reduced sensitivity to paraquat treatment. Through RiboTag sequencing, we identified 3324 and 949 differentially expressed genes in oenocytes under aging and paraquat treatment, respectively. Aging and paraquat exhibit both shared and distinct regulations on oenocyte translatome. Among all age-regulated genes, oxidative phosphorylation, ribosome, proteasome, fatty acid metabolism, and cytochrome P450 pathways were down-regulated, whereas DNA replication and immune response pathways were up-regulated. In addition, most of the peroxisomal genes were down-regulated in aged oenocytes, including genes involved in peroxisomal biogenesis factors and fatty acid beta-oxidation. Many age-related mRNA translational changes in oenocytes are similar to aged mammalian liver, such as up-regulation of innate immune response and Ras/MAPK signaling pathway and down-regulation of peroxisome and fatty acid metabolism. Furthermore, oenocytes highly expressed genes involving in liver-like processes (e.g., ketogenesis).

CONCLUSIONS

Our oenocyte-specific translatome analysis identified many genes and pathways that are shared between Drosophila oenocytes and mammalian liver, highlighting the molecular and functional similarities between the two tissues. Many of these genes were altered in both oenocytes and liver during aging. Thus, our translatome analysis provide important genomic resource for future dissection of oenocyte function and its role in lipid metabolism, stress response and aging regulation.

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.

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.

Insights into the transcriptome of oenocytes from Aedes aegypti pupae

Oenocytes are ectodermic cells present in the fat body of several insect species and these cells are considered to be analogous to the mammalian liver, based on their role in lipid storage, metabolism and secretion. Although oenocytes were identified over a century ago, little is known about their messenger RNA expression profiles. In this study, we investigated the transcriptome of Aedes aegypti oenocytes. We constructed a cDNA library from Ae. aegypti MOYO-R strain oenocytes collected from pupae and randomly sequenced 687 clones. After sequences editing and assembly, 326 high-quality contigs were generated. The most abundant transcripts identified corresponded to the cytochrome P450 superfamily, whose members have roles primarily related to detoxification and lipid metabolism. In addition, we identified 18 other transcripts with putative functions associated with lipid metabolism. One such transcript, a fatty acid synthase, is highly represented in the cDNA library of oenocytes. Moreover, oenocytes expressed several immunity-related genes and the majority of these genes were lysozymes. The transcriptional profile suggests that oenocytes play diverse roles, such as detoxification and lipid metabolism, and increase our understanding of the importance of oenocytes in Ae. aegypti homeostasis and immune competence.

Histochemical and ultrastructural studies of the mosquito Aedes aegypti fat body: effects of aging and diet type

Aedes aegypti is the principal vector of dengue world wide and a major vector of urban yellow fever. Despite its epidemiological importance, not much is known regarding cellular and structural changes in the fat body in this mosquito. Here, we applied light and transmission electron microscopies to investigate structural changes in the fat body of three groups of A. aegypti females: newly emerged, 18-day-old sugar-fed, and 18-day-old blood-fed. The fat body consists of a layer of cells attached to the abdomen integument, formed by trophocytes and oenocytes. Trophocytes are strongly positive for carbohydrates, while oenocytes are strongly positive for proteins and lipids. Ultrastructural analyses of trophocytes from newly emerged and 18-day-old blood-fed indicate that these cells are rich in glycogen and free ribosomes. Many lipid droplets and protein granules, which are broken down after the blood meal, are also detected. In 18-day-old sugar-fed, trophocytes display a disorganized cytoplasm filled with lipid droplets, and reduced numbers of free ribosomes, glycogen, rough endoplasmic reticulum (RER) and mitochondria. Following a blood meal, the RER and mitochondria display enlarged sizes, suggestive of increased activity. With regard to oenocytes, these cells display an electron-dense cytoplasm and plasma membrane infoldings facing the hemolymph. As the A. aegypti female ages, trophocyte and oenocyte cell nuclei become larger but decrease in diameter after blood feeding. Our findings suggest that the trophocytes and oenocytes remodeling is likely involved in functional changes of fat body that take place during aging and following a blood meal in A. aegypti females.

Hydrocarbon synthesis by enzymatically dissociated oenocytes of the abdominal integument of the German Cockroach, Blattella germanica

In insects, hydrocarbons waterproof the cuticle, protect the insect from the external environment, and serve as semiochemicals or their metabolic precursors. In the German cockroach, Blattella germanica, hydrocarbons are synthesized by the abdominal integument, but the precise site of biosynthesis is not known. We developed a method for separation of oenocytes from other cells in the abdominal integument using enzymatic dissociation followed by Percoll gradient centrifugation. Radiolabeled propionate was then used to monitor de novo synthesis of hydrocarbons by dissociated cells. Oenocyte-enriched cell suspensions of abdominal sternites synthesized hydrocarbons, whereas suspensions enriched with epidermal cells did not. Our results show conclusively that hydrocarbons are produced by oenocytes not only in insects whose oenocytes are localized within the hemocoel, but also in those insects whose oenocytes are within the abdominal integument. Furthermore, these data support a hemolymph pathway for transport and delivery of hydrocarbons to both external and internal tissues, including the epicuticle, fat body, and ovaries.

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

Lipophorin-facilitated hydrocarbon uptake by oocytes in the German cockroachBlattella germanica(L.)

Large amounts of hydrocarbons accumulate during vitellogenesis in the developing basal oocytes of the German cockroach Blattella germanica (L.), and all ovarian hydrocarbons are deposited into an egg case (ootheca) during oviposition. Hydrocarbons are not synthesized by the ovaries, but are delivered by hemolymph lipoproteins and accumulate within the basal oocytes. A native B. germanica hydrocarbon, [3H]3,11-dimethylnonacosane, injected into adult females of various ages, was taken up by the ovaries in relation to oocyte growth. Ovarian uptake of the hydrocarbon was low in day 0–1 females, increased dramatically between days 3 and 6 and declined sharply through oviposition on day 8–9; ovarian uptake of the hydrocarbon was low during a 21-day pregnancy that followed. [1-14C]Propionate, which becomes incorporated into methyl-branched hydrocarbons, was injected into 5-day-old vitellogenic females to monitor the de novo biosynthesis of hydrocarbons and the time course of hydrocarbon deposition in the ovary. Propionate was rapidly incorporated into hydrocarbons within 4 h. Hydrocarbon uptake by the ovaries, however, was three times higher 24 h after injection than 4 h after injection, showing that hydrocarbons are slowly and continuously deposited in oocytes. This result was confirmed with topical application of [3H]3,11-dimethylnonacosane: ovarian uptake was three times higher after 24 h than after 4 h. In vitro incubations of sternites, which synthesize hydrocarbons, with [14C]propionate and ovaries, showed that both hemolymph and purified high-density lipophorin facilitated ovarian uptake of newly synthesized hydrocarbons; maximum uptake occurred with 10 % hemolymph or 1 mg ml–1 high-density lipophorin. These results were confirmed with sternites treated with [3H]3,11-dimethylnonacosane and co-incubated with ovaries. This is the first report to show that copious amounts of maternal hydrocarbons are provisioned in oocytes and to demonstrate the existence of a hydrocarbon transport pathway involving hemolymph high-density lipophorin.

Evidence for a transcellular cisternal route across the caecal epithelium of an insect

The cells of the mesenteric caeca in the midgut of certain insects possess a labyrinth of transepithelial cisternae. Their existence can be seen in thin sections of lanthanum-incubated tissue, where the tracer enters not only the intercellular clefts but also membranous cisternae which are inpocketings from, and, in continuity with, both the lateral clefts and basal membrane. These infoldings, which are numerous, run from the basal or lateral surfaces into the perinuclear region of the cells, where they are found, laden with lanthanum, as smooth cisternae or vesicles in the peripheral cytoplasm near the plasma membrane. These can be followed in serial sections and are quite distinct from other sub-surface cisternae of the lateral borders which are studded with ribosomes on the cytoplasmic surface. Near the luminal surface, tracer-laden structures in the form of vesicles and granules become increasingly predominant over those in the form of cisternae. Freeze-fracture replicas confirm the above observations, in that the plasma membrane of the intercellular cleft can be characterized as such unequivocally, since it exhibits smooth septate junctional E face grooves and P face ridges. Lateral infoldings, cisternae and vesicles can be seen arising directly from these junction-bearing membranes. The transepithelial cisternae and vesicles may be the morphological basis of an insect transcellular transport system, comparable to the tubulo-cisternal endoplasmic reticulum present in the transporting secretory and absorptive epithelia of vertebrate tissues. However, in insect midgut caecal epithelia, the cisternae appear to be, albeit presumably transiently, in direct continuity with the extracellular space, forming a plasma membrane reticular system which seems not to be the case with the tubulo-cisternal endoplasmic reticulum which terminates in subsurface cisternae.