THE FAT BODY AS A PROTEIN FACTORY

Chapter 97 - Fat Body

Simple SummaryEfficient and proper functioning of processes within living organisms play key roles in times of climate change and strong human pressure. In insects, the most abundant group of organisms, many important changes occur within their tissues, including the fat body, which plays a key role in the development of insects. Fat body cells undergo numerous metabolic changes in basic energy compounds (i.e., lipids, carbohydrates, and proteins), enabling them to move and nourish themselves. In addition to metabolism, the fat body is involved in the development of insects by determining the time an individual becomes an adult, and creates humoral immunity via the synthesis of bactericidal proteins and polypeptides. As an important tissue that integrates all signals from the body, the processes taking place in the fat body have an impact on the functioning of the entire body.The biodiversity of useful organisms, e.g., insects, decreases due to many environmental factors and increasing anthropopressure. Multifunctional tissues, such as the fat body, are key elements in the proper functioning of invertebrate organisms and resistance factors. The fat body is the center of metabolism, integrating signals, controlling molting and metamorphosis, and synthesizing hormones that control the functioning of the whole body and the synthesis of immune system proteins. In fat body cells, lipids, carbohydrates and proteins are the substrates and products of many pathways that can be used for energy production, accumulate as reserves, and mobilize at the appropriate stage of life (diapause, metamorphosis, flight), determining the survival of an individual. The fat body is the main tissue responsible for innate and acquired humoral immunity. The tissue produces bactericidal proteins and polypeptides, i.e., lysozyme. The fat body is also important in the early stages of an insect’s life due to the production of vitellogenin, the yolk protein needed for the development of oocytes. Although a lot of information is available on its structure and biochemistry, the fat body is an interesting research topic on which much is still to be discovered.

Fat body cells of Amblyomma cajennense partially engorged females (Acari: Ixodidae) and their role on vitellogenesis process

Metamorphic changes in fat body proteins of the southwestern corn borer, Diatraea grandiosella

Insect fat body is the organ for intermediary metabolism, comparable to vertebrate liver and adipose tissue. Larval fat body is disintegrated to individual fat body cells and then adult fat body is remodeled at the pupal stage. However, little is known about the dissociation mechanism. We find that the moth Helicoverpa armigera cathepsin L (Har-CL) is expressed heavily in the fat body and is released from fat body cells into the extracellular matrix. The inhibitor and RNAi experiments demonstrate that Har-CL functions in the fat body dissociation in H. armigera. Further, a nuclear protein is identified to be transcription factor Har-Relish, which was found in insect immune response and specifically binds to the promoter of Har-CL gene to regulate its activity. Har-Relish also responds to the steroid hormone ecdysone. Thus, the dissociation of the larval fat body is involved in the hormone (ecdysone)-transcription factor (Relish)-target gene (cathepsin L) regulatory pathway.

The insect fat body is the major biosynthetic and storage organ involved in lipid, carbohydrate, amino acid and nitrogen metabolism and protein synthesis. It also actively participates in vitellogenesis and ovary development, synthesizing the soluble precursor for yolk, i.e., vitellogenin in stage specific manner. The continual substance exchange among fat body, hemolymph and ovary is controlled by hormones, ecdysteroids and juvenile hormone, during metamorphosis of Bombyx mori (L.). The aim of the present study was to clarify the interactions between major proteins of fat body and hemolymph and their effect on ovarian development under hormonal factors during pupal-adult transformation. Detected fat body, hemolymph and ovary proteins are grouped as follows by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE): storage proteins (72 kDa and 76 kDa), 30 kDa proteins, ApoLp-I (230-250 kDa), egg specific protein (72 kDa and 64 kDa) and vitellogenins (178 kDa). Our results suggest that changes in the well-defined and characterized protein fractions quality and quantity of the fat body had a direct effect under hormonal factors on the ovary and egg development during metamorphosis.

The binding and internalization of a circulating insect lipoprotein, high density lipophorin (HDLp), by insect fat body cells was studied at the electron-microscopic level using ultrasmall gold-labeled HDLp and DiI-labeled HDLp, which were visualized by silver enhancement and diaminobenzidine photoconversion, respectively. Internalization of HDLp seems to conflict with the selective process by which the lipids are transported between HDLp and fat body cells. The pathway followed by the internalized lipoproteins was investigated. In addition, the localizations of HDLp in fat body cells of young and older adult locusts were compared because of the previously reported agerelated differences in distribution of cell-associated and internalized HDLp. In the present study, internalized labeled HDLp was observed in early endosomes, late endosomes, and putative lysosomes. In older adults, these labeled structures were much less abundant than in young adults. Moreover, in these animals, the labeled endosomal/lysosomal vesicles were located close to the plasma membranes. A more intense labeling was observed in the extracellular matrix in older adults compared to young adults. In both developmental stages, an apparent accumulation of labeled HDLp was found in extracellular spaces. We propose that this entrapment of HDLp may be essential for selective lipid transport between HDLp and fat body cells.—Dantuma, N. P., M. A. P. Pijnenburg, J. H. B. Diederen, and D. J. Van der Horst. Multiple interactions between insect lipoproteins and fat body cells: extracellular trapping and endocytic trafficking.

Schi8tocerca gregaria, the haemolymph of which may contain up to 2 % of trehalose (Howden & Kilby, 1956). Treherne (1958a, b) demonstrated that the introduction of radioactive glucose into the alimentary canal or directly into the haemolymph of S. gregaria in vsvo resulted in the appearance of radioactive trehalose in the haemolymph within a short time, but the site and mode of conversion of glucose into trehalose were not investigated. We have been able to show that the fat body is the most active tissue of the locust in this respect. The insect fat body is a conspicuous organ which extends throughout the abdominal and thoracic cavities, and consists of a loose meshwork of anastomosing lobes formed of sheets of single or double layers of yellow cells. It occupies the space between the gut and the body wall and is everywhere in contact with the blood, so a ready interchange of metabolites between fat-body cells and the blood would be expected. One function of the fat body which has long been recognized is that of a storage organ, since the cells become loaded with globules of fat, protein and glycogen during the development of the insect. Recently it has been shown that fat-body tissue is active in carrying out a number of different metabolic reactions (Kilby & Neville, 1957; Bellamy, 1958; Zebe & McShan, 1959), and the organ may possibly be considered as an equivalent in some respects of the mammalian liver as a site of intermediary metabolism. For these reasons, it was thought that it might be involved in trehalose biosynthesis. Fat body forms a very convenient tissue for biochemical investigation as it is readily dissected from the insect and can be obtained almost free from other tissues. It was found that the fat body ofS. gregaria would convert radioactive glucose into trehalose in vitro, and the subsequent preparation of active cell-free extracts from it facilitated the study of the pathway of trehalose biosynthesis. Their yeast preparation also contained a specific phosphatase for trehalose phosphate. In the present paper we describe the identification of similar enzymes in the fat body of S. gregaria, and also of other enzymes for the regeneration of uridine diphosphate glucose and the formation of glucose 6-phosphate. Parts of this work have been briefly reported elsewhere (Candy & Kilby, 1959, 1960).

Secretion and Uptake of 14C Proteins by Fat Body of Calpodes ethlius Stoll. (Lepidoptera, Hesperiidae)

Vitellogenin (Vg) synthesis in cultured tissues was analysed biochemically in a soft tick,Ornithodoros moubata. Nine tissue fractions dissected from reproductive females were incubated in vitro in a specially designed Ringer containing35S-methionine. The synthesis of total protein and Vg was assayed by the radioactivity incorporated into precipitates with trichloroacetic acid and antivitellin (Vn)-serum, respectively. Fat body was the most active tissue in Vg synthesis, which comprised 46% of the Vg synthesis by all tissues and 42% of total protein synthesis by fat body. Protein synthesized by the fat body and precipitated with anti-Vn-serum was shown by electrophoresis and fluorography, to consist of six radioactive polypeptides corresponding to the components of Vg. Vg synthesized in cultured fat body was first accumulated in the tissue and secreted into the medium during incubation. Some tissues other than fat body showed low Vg synthesis (in each, less than 12% of total protein synthesis) which, however, may be due to contamination by fat body cells as seen with the scanning electron microscope (SEM). SEM also showed that fat body cells in the active stage of Vg synthesis expanded about 10-fold in length. Immunohistochemical analysis showed a very strong reaction with anti-Vn-IgG in the cytoplasm of fat body from reproductive females. Fat body from unfed females and other tissues including midgut, did not show any specific fluorescence. A positive reaction was obtained with developing oocytes. These results indicate that the fat body is the only site of Vg synthesis in this tick.

Exogenous substances regulate silkworm fat body protein synthesis through MAPK and PI3K/Akt signaling pathways

Antimicrobial peptides accumulated in the hemolymph in response to infection are a key element of insect innate immunity. The involvement of the fat body and hemocytes in the antimicrobial peptide synthesis is widely acknowledged, although release of the peptides present in the hemolymph from the immune cells was not directly verified so far. Here, we studied the presence of antimicrobial peptides in the culture medium of fat body cells and hemocytes isolated from the blue blowfly Calliphora vicina using complex of liquid chromatography, mass spectrometry, and antimicrobial activity assays. Both fat body and hemocytes are shown to synthesize and release to culture medium defensin, cecropin, diptericins, and proline-rich peptides. The spectra of peptide antibiotics released by the fat body and hemocytes partially overlap. Thus, the results suggest that insect fat body and blood cells are capable of releasing mature antimicrobial peptides to the hemolymph. It is notable that the data obtained demonstrate dramatic difference in the functioning of insect antimicrobial peptides and their mammalian counterparts localized into blood cells' phagosomes where they exert their antibacterial activity.

Diglyceride-carrying lipoproteins in insect hemolymph isolation, purification and properties

BackgroundInsect fat body, a central tissue for nutrient storage, energy metabolism, and protein synthesis, degrades by apoptosis and autophagy during larval metamorphosis. After adult emergence, the fat body grows rapidly with cell proliferation and polyploidization during the previtellogenic period but ceases cell proliferation in the vitellogenic phase. So far, the regulatory mechanisms underlying fat body cell fate decisions in adulthood remain unknown.ResultsTranscriptomic analysis of locust fat body revealed the enrichment of pathways associated with cell cycle, nuclear division, and DNA replication. Decapentaplegic (Dpp) was among the top of differentially expressed genes in the signaling cascades involved in regulating cell proliferation. Abundance of Dpp, phosphorylated Mad (p-Mad), and Medea increased during the previtellogenic stage and subsequently declined in the vitellogenic phase. Knockdown of Dpp, Mad, and Medea resulted in suppressed fat body cell proliferation, along with remarkably reduced cell number and blocked vitellogenin (Vg) expression in the fat body as well as consequent arrest of egg development. Mad/Medea complex bound to the promoters of cyclin B (CycB) and polo-like kinase 1 (Plk1) and stimulated their expression. Depletion of CycB and Plk1 caused the defective phenotypes resembling Dpp, Mad, and Medea knockdown. In the vitellogenic phase, the high levels of juvenile hormone (JH) promoted the degradation of Medea via fizzy-related protein (Fzr)-mediated ubiquitination, leading to inhibited cell proliferation. The results suggest that fat body cell proliferation in the previtellogenic development is promoted by the bone morphogenetic protein (BMP) signaling pathway, whereas high levels of JH in the vitellogenic stage antagonize BMP signaling for ceasing cell proliferation.ConclusionsThe findings provide novel insights into the regulation of fat body cell fate during the transition of previtellogenic growth to vitellogenic Vg synthesis for reproductive requirements.

Protein homeostasis (proteostasis) is crucial for the maintenance of cellular homeostasis. Impairment of proteostasis activates proteotoxic and unfolded protein response pathways to resolve cellular stress or induce apoptosis in damaged cells. However, the responses of individual tissues to proteotoxic stress and evoking cell death program have not been extensively explored in vivo. Here, we show that a reduction in Nascent polypeptide-associated complex protein alpha subunit (Nacα) specifically and progressively induces cell death in Drosophila fat body cells. Nacα mutants disrupt both ER integrity and the proteasomal degradation system, resulting in caspase activation through JNK and p53. Although forced activation of the JNK and p53 pathways was insufficient to induce cell death in the fat body, the reduction of Nacα sensitized fat body cells to intrinsic and environmental stresses. Reducing overall protein synthesis by mTor inhibition or Minute mutants alleviated the cell death phenotype in Nacα mutant fat body cells. Our work revealed that Nacα is crucial for protecting the fat body from cell death by maintaining cellular proteostasis, thus demonstrating the coexistence of a unique vulnerability and cell death resistance in the fat body.