The origin and fate of microbodies in the fat body of an insect

Fat Body-Multifunctional Insect Tissue

Simple Summary

Efficient 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.

Abstract

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.

A eukaryote without catalase-containing microbodies: Neurospora crassa exhibits a unique cellular distribution of its four catalases

Microbodies usually house catalase to decompose hydrogen peroxide generated within the organelle by the action of various oxidases. Here we have analyzed whether peroxisomes (i.e., catalase-containing microbodies) exist in Neurospora crassa. Three distinct catalase isoforms were identified by native catalase activity gels under various peroxisome-inducing conditions. Subcellular fractionation by density gradient centrifugation revealed that most of the spectrophotometrically measured activity was present in the light upper fractions, with an additional small peak coinciding with the peak fractions of HEX-1, the marker protein for Woronin bodies, a compartment related to the microbody family. However, neither in-gel assays nor monospecific antibodies generated against the three purified catalases detected the enzymes in any dense organellar fraction. Furthermore, staining of an N. crassa wild-type strain with 3,3'-diaminobenzidine and H(2)O(2) did not lead to catalase-dependent reaction products within microbodies. Nonetheless, N. crassa does possess a gene (cat-4) whose product is most similar to the peroxisomal type of monofunctional catalases. This novel protein indeed exhibited catalase activity, but was not localized to microbodies either. We conclude that N. crassa lacks catalase-containing peroxisomes, a characteristic that is probably restricted to a few filamentous fungi that produce little hydrogen peroxide within microbodies.

Autophagy in organelle homeostasis: peroxisome turnover

When cells are confronted with an insufficient supply of nutrients in their extracellular fluid, they may begin to cannibalize some of their internal proteins as well as whole organelles for reuse in the synthesis of new components. This process is termed autophagy and it involves the formation of a double-membrane structure within the cell, which encloses the material to be degraded into a vesicle called an autophagosome. The autophagosome subsequently fuses with a lysosome/vacuole whose hydrolytic enzymes degrade the sequestered organelle. Degradation of peroxisomes is a specific type of autophagy, which occurs in a selective manner and has been mostly studied in yeast. Recently, it was reported that a similar selective process of autophagy occurs in mammalian cells with proliferated peroxisomes. Here we discuss characteristics of the autophagy of peroxisomes in mammalian cells and present a comprehensive model of their likely mechanism of degradation on the basis of known and common elements from other systems.

Autophagic degeneration of motor neurons in a model of slow glutamate excitotoxicity in vitro

There is increasing evidence that so-called "autophagic cell death" participates in cell degeneration in certain pathological conditions. Autophagy might be involved in some neurodegenerative processes, including lateral amyotrophic sclerosis (SLA). The exact mechanism leading to progressive motor neuron (MN) loss remains unclear, but glutamate-mediated mechanism is thought to be responsible. Previous ultrastructural studies by the authors performed on a model of SLA in vitro, based on chronic glutamate excitotoxicity, revealed a subset of morphological features characteristic to different modes of neuronal death, including autophagic degeneration. The contribution of this pathway of MNs death is evaluated in organotypic cultures of rat lumbar spinal cord chronically exposed to specific glutamate uptake blockers: DL-threo-beta-hydroxyaspartate (THA) and L-transpyrrolidine-2,4-dicarboxylate (PDC). The study documents the various steps of authophagy in slowly evolving process of MN neurodegeneration. The cells undergoing autophagy usually exhibited sequestration of some parts of cytoplasm with normal and/or degenerated organelles, whereas other parts of cytoplasm as well as neuronal nucleus remained unchanged. The advanced autophagic changes were often associated with other modes of MN death, especially with apoptosis. Numerous MNs revealed apoptotic nuclear features with typical peripheral margination of nuclear chromatin, accompanied by severe autophagic or autophagic-necrotic degeneration of the cytoplasm. These results support the opinion of unclear distinction between different modes of cell death and indicate the involvement of autophagey in MNs neurodegeneration in vitro.

Catalase from the silkworm, Bombyx mori: gene sequence, distribution, and overexpression

Living organisms require mechanisms regulating reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion. Catalase is one of the regulatory enzymes and facilitates the degradation of hydrogen peroxide to oxygen and water. Biochemical information on an insect catalase is, however, insufficient. Using mRNA from fat body of the silkworm, Bombyx mori, a cDNA encoding a putative catalase was amplified by reverse transcriptase-polymerase chain reaction and sequenced. The deduced amino acid sequence comprised 507 residues with more than seventy residues forming a scaffold for a heme cofactor conserved. The sequence showed 71% and 66% identities to those of the Drosophila melanogaster and Apis mellifera catalases, respectively; the catalase from B. mori was estimated to be phylogenetically close to that from A. mellifera. The transcripts of the gene and the catalase activity were distributed in diverse tissues of B. mori, suggesting its ubiquitous nature. Using the gene, a recombinant catalase (rCAT) was functionally overexpressed in a soluble form using Escherichia coli, purified to homogeneity, and characterized. The pH-optimum of rCAT was broad around pH 8.0. More than 80% of the original rCAT activity was retained after incubation in the following conditions: at pH 8-11 and 4 degrees C for 24 h; at pH 7 and temperatures below 50 degrees C for 30 min. The Michaelis constant for hydrogen peroxide was evaluated to be 28 mM at pH 6.5 and 30 degrees C. rCAT was suggested to be a member of the typical catalase family.

Peroxisome turnover by micropexophagy: an autophagy-related process

Many organisms stringently regulate the number, volume and enzymatic content of peroxisomes (and other organelles). Understanding this regulation requires knowledge of how organelles are assembled and selectively destroyed in response to metabolic cues. In the past decade, considerable progress has been achieved in the elucidation of the roles of genes involved in peroxisome biogenesis, half of which are affected in human peroxisomal disorders. The recent discovery of intermediates and genes in peroxisome turnover by selective autophagy-related processes (pexophagy) opens the door to understanding peroxisome turnover and homeostasis. In this article, we summarize advances in the characterization of genes that are necessary for the transport and delivery of selective and nonselective cargoes to the lysosome or vacuole by autophagy-related processes, with emphasis on peroxisome turnover by micropexophagy.

Apoptosis, autophagy, and more

Cell death has been subdivided into the categories apoptosis (Type I), autophagic cell death (Type II), and necrosis (Type III). The boundary between Type I and II has never been completely clear and perhaps does not exist due to intrinsic factors among different cell types and the crosstalk among organelles within each type. Apoptosis can begin with autophagy, autophagy can end with apoptosis, and blockage of caspase activity can cause a cell to default to Type II cell death from Type I. Furthermore, autophagy is a normal physiological process active in both homeostasis (organelle turnover) and atrophy. "Autophagic cell death" may be interpreted as the process of autophagy that, unlike other situations, does not terminate before the cell collapses. Since switching among the alternative pathways to death is relatively common, interpretations based on knockouts or inhibitors, and therapies directed at controlling apoptosis must include these considerations.

Existence of catalase-less peroxisomes in Sf21 insect cells

Catalase activity, a peroxisomal marker enzyme, was not detectable in any of the subcellular fractions of Spodoptera frugiperda (Sf) 21 insect cells, although marker enzymes in other organelles were distributed in the fractions in a manner similar to that seen in mammalian cells. When a green fluorescent protein fused with peroxisome targeting signal 1 at the C-terminal (GFP-SKL) was expressed in Sf21 cells, punctate fluorescent dots were observed in the cytoplasm. The fraction where GFP-SKL was concentrated exhibited long-chain and very-long-chain fatty acid beta-oxidation activities in the presence of KCN and the density of this fraction was slightly higher than that of mitochondria. Immunoelectron microscopy studies with anti-SKL antibody demonstrated that Sf21 cells have immunoreactive peroxisome-like organelles which are structurally distinct from mitochondria, endoplasmic reticulum, and lysosomes. In contrast to peroxisomal matrix proteins, adrenoleukodystrophy protein, a peroxisomal membrane protein, was not located to peroxisomes. This suggests that the targeting signal for PMP in insect cells is distinct from that in mammalian cells. These results demonstrate that Sf21 insect cells have unique catalase-less peroxisomes capable of beta-oxidation of fatty acids.

Degradation of normal and proliferated peroxisomes in rat hepatocytes: regulation of peroxisomes quantity in cells

Degradation and turnover of peroxisomes is reviewed. First, we describe the historical aspects of peroxisome degradation research and the two major concepts for breakdown of peroxisomes, i.e., autophagy and autolysis. Next, the comprehensive knowledge on autophagy of peroxisomes in mammalian and yeast cells is reviewed. It has been shown that proliferated peroxisomes are degraded by selective autophagy, and studies using yeast cells have been especially helpful in shedding light on the molecular mechanisms of this process. The degradation of extraperoxisomal urate oxidase crystalloid is noted. Overexpressed wild-type urate oxidase in cultured cells has been shown to be degraded through an unknown proteolytic pathway distinct from the lysosomal system including autophagy or the ubiquitin-proteasome system. Finally, peroxisome autolysis mediated by 15-lipoxygenase (15-LOX) is described. 15-LOX is integrated into the peroxisome membrane causing focal membrane disruptions. The content of the peroxisomes is then exposed to cytosol proteases and seems to be digested quickly. In conclusion, the number of peroxisomes appears to be regulated by two selective pathways, autophagy, including macro- and microautophagy, and 15-LOX-mediated autolysis.

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.

Ergastoplasmic paracrystalline inclusion bodies in the adipose gonadal envelope and fat body of the glow worm, Lampyris noctiluca (Insecta, Coleoptera)

The gonads of glow worm larvae are enveloped by adipose tissue which represents a specialized fat body. The adipose gonadal envelope, and also to a lesser extent the fat body cells, contain tubular paracrystalline inclusion bodies (PIBs). Cells of other tissues are devoid of such inclusions. The PIBs form in the cisternae of rough ER. In young larvae PIB formation is sparse, but at advanced larval stages PIBs often occur as bundles in stacks of ergastoplasm. Typically, a PIB within a cisterna consists of four to seven parallel tubules. The outer diameter of a tubule is ca 28.8 nm and the width of the tubule lumen ca 12.2 nm. The "wall" of a tubule contains globular protein subunits of ca 8.3 nm diameter; the subunits are arranged helically. Since the adipose gonadal envelope progresses through a cytological differentiation process during differentiation and maturation of the gonads, the increased number of PIBs may indicate enhanced metabolic activity of the tissue related to nutrition of the growing gonads.

Aspects of the relation between Cyanophora paradoxa (Korschikoff) and its endosymbiotic cyanelles Cyanocyta korschikoffiana (Hall & Claus) - I. Growth, ultrastructure, photosynthesis and the obligate nature of the association

Ultrastructural studies of the symbiotic relation between the apochlorotic cryptomonad Cyanophora paradoxa and the cyanobacterium Cyanocyta korschikoffiana have demonstrated that the endosymbiotic cyano­bacteria are within host cell vacuoles and possess a highly reduced prokaryotic cell wall which becomes particularly evident during binary fission. Division by the endosymbiotic cyanobacteria was found not to be in synchrony with division by the host cryptomonad. The cyano­bacteria divided most rapidly when the host population had reached its asymptote, such that, in cultures in extended stationary phase, it is possible to observe as many as eight cyanobacteria in one host cell. Nutritionally, C. paradoxa was found to be an obligate phototroph; its auxotropic capabilities are confined to acetate and cyanocobalamin. Analysis of the rate of photosynthesis by the intact association demon­strated that saturation occurred at the 140 μE cm -2 s -1 (where E is einstein), and at saturating light intensities, the rate of CO 2 fixation was 30 μmol CO 2 (mg chl a ) -1 h -1 . The best rate of CO 2 fixation obtained by the isolated cyanobacteria represented 12% of the rate found in the intact association. Two independent methods of analysis showed that at best, only 15% of the carbon fixed by the cyanobacteria was translocated to the host, mostly as glucose and probably sucrose. Studies with metabolic inhibitors directed against the prokaryotic cyanobacteria demonstrated the close interdependence of the host with the endosymbionts, in that disruption of the endosymbionts always resulted in death of the host. Our inability to grow the cyanobacteria away from the host, combined with the apparent inability of the host to survive in the absence of the photosynthetic endosymbiotic cyanobacteria provide strong support for the concept that the association represents a well integrated mutually obligate relation, but the concept that the cyanobacteria are chloroplasts cannot be supported.

Programmed cell death in the tobacco hornworm, Manduca sexta: alteration in protein synthesis

The metamorphic death of the labial glands of the tobacco hornworm, Manduca sexta, occurs during a 4 day period during larva-to-pupa metamorphosis. The earliest changes marking the death of the cell, all occurring on the first day, are a sharp drop in protein synthesis, coupled with the selective survival or upregulation of a few messages. An early rearrangement of the rough endoplasmic reticulum is presumably related to the generalized decrease in protein synthesis. Lysosomal acid phosphatase also begins to increase very early, and ultimately the bulk of the cytoplasm is destroyed in autophagic vacuoles, but activation of lysosomes does not account for the decreased rate of synthesis. The mechanism by which most protein synthesis is depressed remains under investigation.

Autophagy and other vacuolar protein degradation mechanisms

Autophagic degradation of cytoplasm (including protein, RNA etc.) is a non-selective bulk process, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lives. The initial autophagic sequestration step, performed by a poorly-characterized organelle called a phagophore, is subject to feedback inhibition by purines and amino acids, the effect of the latter being potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive alpha 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid. Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, on intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy to a variable extent, and that the amphisome may play a central role as a collecting station for material destined for lysosomal degradation. Protein degradation can also take place in a 'salvage compartment' closely associated with the endoplasmic reticulum (ER). In this compartment unassembled protein chains are degraded by uncharacterized proteinases, while resident proteins return to the ER and assembled secretory and membrane proteins proceed through the Golgi apparatus. In the trans-Golgi network some proteins are proteolytically processed by Ca(2+)-dependent proteinases; furthermore, this compartment sorts proteins to lysosomes, various membrane domains, endosomes or secretory vesicles/granules. Processing of both endogenous and exogenous proteins can occur in endosomes, which may play a particularly important role in antigen processing and presentation. Proteins in endosomes or secretory compartments can either be exocytosed, or channeled to lysosomes for degradation. The switch mechanisms which decide between these options are subject to bioregulation by external agents (hormones and growth factors), and may play an important role in the control of protein uptake and secretion.

Peroxisomes in the head of Drosophila melanogaster

Peroxisomes were localized in the head of wild-type and mutant strains of Drosophila melanogaster by use of a cytochemical method for the demonstration of D-amino acid oxidase activity. With similar techniques we had found previously that vertebrate photoreceptors have few, if any, bodies with cytochemically demonstrable oxidase activity, but that the pigment epithelial cells adjacent to the photoreceptors have a substantial population of such bodies. Peroxisomes in Drosophila were very abundant in the fat body. Probable peroxisomes were also present in the peripheral retina of the eye, including in retinular (retinula) and pigment cells, but there were very few of them. Thus, our results suggest that the fat body, which lies adjacent to the eye, is the principal site of peroxisomal function in the head. Peroxisome functions in the Drosophila head may include participation in the genesis of eye pigments.

Cytochemical localization of a D-amino acid oxidizing enzyme in peroxisomes of Drosophila melanogaster

A peroxide generating oxidase is demonstrated cytochemically in the peroxisomes of adult and larval Drosophila melanogaster, Oregon R and Rosy-506 strains. This enzyme activity is demonstrable using D-pipecolate or D-proline, but not L-proline, as substrate and is inhibited by kojic acid. Thus this enzyme shares cytochemical characteristics with vertebrate D-amino acid oxidase.

Peroxisomes in wild-type and rosy mutant Drosophila melanogaster

This study shows that peroxisomes are abundant in the Malpighian tubule and gut of wild-type Oregon R Drosophila melanogaster and that the peroxisomal population of the rosy-506 eye-color mutant differs from that of the wild type. Catalase activity in wild-type flies is demonstrable in bodies of appearance and centrifugal behavior comparable to the peroxisomes of vertebrate tissues. Xanthine oxidase (xanthine:oxygen oxidoreductase, EC 1.1.3.22) activity of the Malpighian tubule of wild-type flies is demonstrable cytochemically in bodies like those containing catalase. The rosy-506 mutant flies, with a deletion in the structural gene for xanthine dehydrogenase (xanthine:NAD+ oxidoreductase, EC 1.1.1.204), lack cytochemically demonstrable peroxisomal xanthine oxidase activity. In addition, peroxisomes in the rosy-506 mutants show less intense cytochemical staining for catalase than those in wild-type flies, and biochemical assays indicate that catalase in the rosy mutant is much more accessible to substrate in the absence of detergent than in the wild type. Thus, the rosy-506 mutation appears to affect peroxisomes and may mimic aspects of the defects of peroxisomes in some human metabolic disorders.

Lenticles: innervated secretory structures that are expressed at every other larval moult

Lenticles are dome-shaped circles or ovals of cuticle with a dark rim. They occur with a precise segmental arrangement in the larvae and pupae of lycaenid and hesperiid butterflies. In Calpodes ethlius (Lepidoptera, Hesperiidae) each lenticle is secreted by a pair of large polyploid epidermal cells. The dark rim or annulus is formed from a ring-shaped cell. The dome, which consists of an epicuticle with a perforate intermediate layer like a pepper-pot, is formed by a central goblet cell. Between the perforate intermediate layer and the cell surfaces there is a cavity that contains material presumed to be secretion. Both cells have elaborate basal plasma membrane reticular systems and the apical microvilli associated with an extensive smooth endoplasmic reticulum that is typical of lipid secreting cells. In addition, there is a plasma membrane reticular system in the ring cell and between it and the goblet cell that contains the endings of nerves having neurosecretory vesicles. Lenticles thus have a structure appropriate for an innervated organ of lipid secretion. However, in their development, lenticles arise from bristles that are presumed to be sensory. Lenticles or their precursors are segmentally arranged in the five larval instars and the pupa, but the pattern changes at each moult. The cells that form a lenticle at one moult have a rest period at the next one when they only secrete surface cuticle. Many lenticles are paired in their cycle of development, with only one of the pair making a lenticle at a particular moult. For example, the dorsal and lateral lenticles alternate in position between anterior and posterior. The second and fourth instar segments have anterior and the third and fifth instars have posterior lenticles. In the first instar the cells that will make lenticles for the second and third instars both make bristles. Lenticles are thus formed by cells that not only change their response to ecdysone qualitatively by switching from bristle to lenticle but also alternate in their later responses, switching back and forth at alternate moults between the formation of a lenticle and the secretion of surface cuticle.

Degradation and turnover of peroxisomes in the yeast Hansenula polymorpha induced by selective inactivation of peroxisomal enzymes

Inactivation of peroxisomal enzymes in the yeast Hansenula polymorpha was studied following transfer of cells into cultivation media in which their activity was no longer required for growth. After transfer of methanol-grown cells into media containing glucose - a substrate that fully represses alcohol oxidase synthesis - the rapid inactivation of alcohol oxidase and catalase was paralleled by a disappearance of alcohol oxidase and catalase protein. The rate and extent of this inactivation was dependent upon conditions of cultivation of cells prior to their transfer. This carbon catabolite inactivation of alcohol oxidase was paralleled by degradation of peroxisomes which occurred by means of an autophagic process that was initiated by the formation of a number of electron-dense membranes around the organelles to be degraded. Sequestration was confined to peroxisomes; other cell-components such as ribosomes were absent in the sequestered cell compartment. Also, cytochemically, hydrolytic enzymes could not be demonstrated in these autophagosomes. The vacuole played a major role in the subsequent peroxisomal breakdown since it provided the enzymes required for proteolysis. Two basically similar mechanisms were observed with respect to the administration of vacuolar enzymes into the sequestered cell compartment. The first mechanism involved incorporation of a small vacuolar vesicle into the sequestered cell compartment. The delimiting membrane of this vacuolar vesicle subsequently disrupted, thereby exposing the contents of the sequestered cell compartment to vacuolar hydrolases which then degraded the peroxisomal proteins. The second mechanism, observed in cells which already contained one or more autophagic vacuoles, included fusion of the delimiting membranes of an autophagosome with the membrane surrounding an autophagic vacuole which led to migration of the peroxisome inside the latter organelle. Peroxisomes of methanol-grown H. polymorpha were degraded individually. In one cell 2 or 3 peroxisomes might be subject to degradation at the same time, but they were never observed together in one autophagosome. However, fusions of autophagic vacuoles in one cell were frequently observed. After inhibition of the cell's energy-metabolism by cyanide ions or during anaerobic incubations the formation of autophagosomes was prevented and degradation was not observed.

Cyclic AMP in the fat body of Mamestra brassicae during the last instar and its possible involvement in the cellular autophagocytosis induced by 20-Hydroxyecdysone

The amount of cAMP was assayed by a competitive protein binding method in fat body cells of Mamestra brassicae, during the last larval stage and after administration of 20-hydroxyecdysone. When expressed as picomoles of cAMP per milligram fresh weight of tissue, two increases in its concentration were observed on the 3rd and on the 6th days. However, only the first peak appeared on the curve when cAMP concentration was expressed as picomoles cAMP per milligram of protein of tissue homogenate. Electron microscopical examination of the tissue showed that the first increase of cAMP level coincided with the beginning of the formation of autophagic vacuoles and revealed a heavy accumulation of protein storage granules in the cells, starting on the 4th day. This process might mask the second rise of cAMP level when tissue protein content is taken as the basis for calculation. 20-Hydroxyecdysone (5 micrograms/g body wt) administered to 48-hr-old larvae induced premature autophagocytosis in the fat body cells and a sharp rise in their cAMP content, reaching within 3 hr a level as high as observed in the 3-day-old untreated larvae. Autophagy was also enhanced in the cells exposed to dibutyryl cAMP or theophylline either in vivo or in vitro. Based on these data we think that cAMP content of the fat body is controlled by ecdysone and that cAMP plays a significant role in the regulation of autophagocytosis in this tissue during metamorphosis.

Tyrosine storage vacuoles in insect fat body

The watery vacuoles first described from larval insect fat body (Chironomus, Voinov, 1927; Aedes, Wigglesworth, 1942; Rhodnius, Wigglesworth, 1967) have been studied in 4th and 5th stage Calpodes larvae. The vacuoles arise at the beginning (E + 6-24 hr) of the 4th stadium from plasma membrane infolds that separate from the cell surface as provacuoles less than 1 micron in diameter. These provacuoles grow and fuse with one another through the intermolt until about half the volume of each fat body cell is occupied by a single, large vacuole. The vacuoles begin to disappear at molting. Their membrane is either incorporated into the plasma membrane by exocytosis or fragmented into vesicles that fuse to become lamellar bodies where the membranes are presumably digested. All the vacuoles have gone by a few hours after ecdysis. The tyrosine content of the fat body increases and decreases in proportion to the size of the vacuoles. As the vacuoles decrease at molting the titre of tyrosine in the hemolymph is transiently elevated at the time when there is most demand for phenolics for cuticle stabilization. Crystals having the form of tyrosine crystallize out from vacuoles separated from the fat body. In fat body extracts separated by thin layer chromatography, similar crystals occur only in the eluates from spots corresponding to tyrosine. The vacuoles are therefore presumed to be tyrosine stores used in cuticle stabilization at molting. They correspond to a type of aqueous storage compartment that is well known in plants but hitherto little recognized in animal cells.

Lead staining in the Golgi complex

Lead ions at similar concentrations to those used for Gomori type phosphatase localization stain some parts of the vacuolar system, particularly compartments of the Golgi complex (GC) and isolation envelopes (im) in a characteristic way in both vertebrates and invertebrates. After fixation in 2.5% glutaraldehyde, lead citrate in acetate or aspartate buffer (pH 5.5-7.2) leaves the contents of GC cisternal compartments with a fine particulate stippling. In the fat body of Calpodes ethlius and in mouse pancreas the staining is faint but definite without further enhancement of contrast, although it is easily overlooked after section staining. The distribution of lead stain differs from that of the lead phosphate precipitated after Gomori type acid phosphatase reactions. Whereas lead stain may be in all GC and im compartments, acid phosphatase is restricted to the innermost saccules and nearby vacuoles. The compartment specific staining by led also differs from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC cisternae are devoid of uranyl stainable material. However, lead staining and acid phosphatase activity in the GC continue. We presume that they mark the environment within these cisternae rather than the proteins passing through them. This environment is itself not static. Several observations suggest that the function of cisternae that is detectable by lead staining is temporally discontinuous and related to a stage of maturation or development. Only early stage ims stain: the staining ceases by the beginning of autophagy after hydrolytic enzymes are presumed to have been added. Condensing vacuoles cease to stain as the central core crystallizes out. Stain may be absent from one or two GC saccules at any position in the stack as though the phase of lead staining (or lack or it) can move progressively through the system. We conclude that in studies characterizing components of the vacuolar system it is necessary to separate those that mark transient occupants of a compartment from those that mark the compartment itself. Both may vary temporally independently from one another.

The evaluation of large test fields for morphometric studies in electron microscopy

The surface area of test fields required for morphometric evaluation of sectioned particles increases with the square of the mean particle diameter, but is inversely proportional to the volume fraction of the particles. A method is presented for an economical evaluation of large test fields. Factors giving rise to variation in the sampling procedure are analyzed. The volume fraction of autophagic vacuoles, for instance, shows a rather broad variation between animals, probably because of the short half-life of these particles. Other applications of the method include bile capillaries under different experimental conditions, and human biopsy specimens, e.g. liver tissue containing very few profiles resembling small peroxisomes in a case of Zellweger syndrome.

Development and cellular differentiation of an insect telotrophic ovary (Rhodnius prolixus)

Differentiation events accompanying the larval-adult ovarian transformation in Rhodnius prolixus can be divided into three phases: proliferative phase (unfed to 3 days post-feed or DPF), early differentiation phase (9-15 DPF) and late differentiation phase (16 DPF to moult at 21 DPF). Ovarioles remain morphologically larval until feeding initiates development. The unfed ovariole contains germ cells surrounding a central trophic core region with the 'germarial lumen' occupying the basal region of the tropharium immediately above the pre-follicular tissue. Mitosis of germ cells during the proliferative phase results in a progressive increase in tropharial size with no differentiation of tissues. Regional specialization within the ovariole marks the beginning of the early differentiation phase. A zone of oocytes is established at the base of the tropharium with nuclei containing synaptonemal complexes and condensing chromosomes. Nurse cell differentiation is characterized by nucleolar elaboration and nucleo-cytoplasmic transport, the cytoplasm becoming rich in ribosomes. Autoradiographic results suggest that functional nurse cell-oocyte divergence occurs concurrently with morphological divergence. Pre-follicular tissue is divided into apical and basal zones with apical zone differentiation occurring during early and late differentiation phases.

Cytochemical analysis of organelle degradation in phagosomes and apoptotic cells of the mucoid epithelium of mice

The activity of mitochondrial cytochrome oxidase and peroxisomal catalase in the phagolysosomes and apoptotic bodies of mucoid epithelial cells was analysed. Tissue from 2-6 day old mice was used. The activity of acid phosphatase in lysosomes was also estimated. Cytochrome oxidase was demonstrated in well-preserved mitochondria inside phagosomes. Mitochondria in cells exhibiting apoptotic death also show activity of cytochrome oxidase. The enzyme activity in swollen mitochondria ceases before the membranes of the cristae disappear completely. Apoptotic bodies are phagocytosed by sister mucoid cells and, later on, they are digested inside the cell. Phagosomes which contain already degraded mitochondria show still active catalase in sequestered peroxisomes. The acid phosphatase involved in degradation of phagocytosed material originates from endocytosed lysosomes and primary and secondary lysosomes which fuse with the membranes of phagosomes.

Developmental changes in Malpighian tubule cell structure

Structural changes which occur in the Malpighian tubule yellow region primary cells during larval-pupal-adult development of the skipper butterfly Calpodes ethlius are described. The developmental changes in cell structure are correlated with functional changes in fluid transport (Ryerse, 1978a) in a way which supports osmotic gradient models of fluid secretion. Larval tubules are specialized for fluid secretion with deep basal infolds and elongate mitochondria-containing apical microvilli which provide channels in which osmotic gradients could be set up. The Malpighian tubule cells are extensively remodelled at pupation when fluid transport is switched off, but they persist intact through metamorphosis. At this time, the basement membrane doubles in thickness, the mitochondria are retracted from the microvilli and are isolated for degradation in autophagic vacuoles, and both apical and basal plasma membranes are internalized via coated vesicles for degradation in multivesicular bodies, which results in the shortening of the microville and the disappearance of the basal infolds. Mitochondria are re-inserted into the microvilli, and the basal infolds re-form in pharate adult stage Malpighian tubules when fluid secretion resumes. Adult tubules are similar in general structure to larval tubules and contain mitochondria in the microvilli and basal infolds. However, they differ from larval tubules in that they are capable of very rapid fluid transport, have a reduced tubule diameter and tubule wall thickness, a much thicker basement membrane and peripherally associated tracheoles. Mineral concretions of calcium phosphate accumulate in larval tubules, persist through metamorphosis and decline in number in adults, suggesting they serve some anabolic role.

Inhibition by insulin of the formation of autophagic vacuoles in rat liver. A morphometric approach to the kinetics of intracellular degradation by autophagy

Electron microscopic morphometry has demonstrated a rapid decrease in the fractional volume of autophagic vacuoles (AV) in hepatocytes of adult male rats after the intraperitoneal administration of insulin (5 U/kg of body weight). Except for a significant decrease in glycogen to about one-half its initial value, no major changes in the composition of the remaining cytoplasm, or in the average volume of the single hepatocyte, were seen. The decrease found in the AVs is attributed to an inhibition of the formation of new AVs-probably the morphologic counterpart of the well-known anticatabolic effects of insulin. The decay of the fractional volume of the AVs appeared to follow first-order kinetics. Thus, the termination of the "life" of an AV by destruction of its contents may not depend directly on the "age" of the AV. The average half-life of the AVs amounted to approximately 9 min. Similar values were found for the different types of AVs, except for those containing glycogen. The half-life of these AVs was approximately 18 min. From the half-life values and from the "segregated fractions" at time zero, which were different for the different cytoplasmic components, rates of removal from the cytoplasm by autophagy were calculated. Expressed as "percent per day", the following rates were found: whole cytoplasm, 2.3; mitochondria, 3.9; microbodies, 8.9; and glycogen, 1.1. The results indicate that autophagy, to some extent, is selective and plays an important, but not an exclusive, role in intracellular turnover.

Cell remodeling in the fat body of an insect

The fat body of the Lepidopteran, Calpodes ethlius, undergoes major functional changes during larval-adult metamorphosis. These changes occur in conjunction with extensive cell remodeling - a process whereby one population of cellular organelles is destroyed and replaced by another during development. Fat body organelles including mitochondria, microbodies, and RER are destroyed on a massive scale shortly before pupation (Locke and Collins, 1965; Locke and McMahon, 1971) a new populations of each are regenerated shortly after emergence of the adult. In addition, protein, lipid and RNA reserves formed shortly before pupation and multivesicular bodies formed shortly before emergence are secreted into the haemocoel during the first few days of adult life. Electron microscopic studies using tracer techniques, cytochemical and enzyme localization procedures, and sterological analyses have been undertaken to determine the time course and mechanism of organelle regeneration and the fate of reserves stored in the fat body.

Cytochemical discrimination between catalases and peroxidases using diaminobenzidine

The influence on diaminobenzidine staining of four variables: prefixation in aldehyde, temperature and pH of incubation, and H2O2 concentration, was investigated in catalase-, as well as in peroxydase-containing material. Catalase from five different sources and five types of peroxidase were examined. It is concluded: (a) when cells are incubated without prior fixation, in a DAB medium at room temperature and pH 7.3 with 0.003% H2O2, peroxidases produce a visible cytochemical stain, while catalases do not; (b) the cytochemical reaction elicited by catalases is stimulated by prior aldehyde fixation in specified conditions, and incubation at 45 degrees C and pH 9.7 with 0.06% H2O2; (c) under the latter circumstances several peroxidases also stain. Ultrastructural preservation is satisfactory in tissues incubated prior to fixation.

Structure, composition, physical properties, and turnover of proliferated peroxisomes. A study of the trophic effects of Su-13437 on rat liver

Peroxisome proliferation has been induced with 2-methyl-2-(p-[1,2,3,4-tetrahydro-1-naphthyl]-phenoxy)-propionic acid (Su-13437). DNA, protein, cytochrome oxidase, glucose-6-phosphatase, and acid phosphatase concentrations remain almost constant. Peroxisomal enzyme activities change to approximately 165%, 50%, 30%, and 0% of the controls for catalase, urate oxidase, L-alpha-hydroxy acid oxidase, and D-amino acid oxidase, respectively. For catalase the change results from a decrease in particle-bound activity and a fivefold increase in soluble activity. The average diameter of peroxisome sections is 0.58 +/- 0.15 mum in controls and 0.73 +/- 0.25 mum after treatment. Therefore, the measured peroxisomal enzymes are highly diluted in proliferated particles. After tissue fractionation, approximately one-half of the normal peroxisomes and all proliferated peroxisomes show matric extraction with ghost formation, but no change in size. In homogenates submitted to mechanical stress, proliferated peroxisomes do not reveal increased fragility; unexpectedly, Su-13437 stabilizes lysosomes. Our results suggest that matrix extraction and increased soluble enzyme activities result from transmembrane passage of peroxisomal proteins. The changes in concentration of peroxisomal oxidases and soluble catalase after Su-13437 allow the calculation of their half-lives. These are the same as those found for total catalase, in normal and treated rats, after allyl isopropyl acetamide: about 1.3 days, a result compatible with peroxisome degradation by autophagy. A sequential increase in liver RNA concentration, [14C]leucine incorporation into DOC-soluble proteins and into immunoprecipitable catalase, and an increase in liver size and peroxisomal volume per gram liver, characterize the trophic effect of the drug used. In males, Su-13437 is more active than CPIB, another peroxisome proliferation-inducing drug; in females, only Su-13437 is active.

The mandibular organ of the lobster, Homarus americanus

The lobster mandibular organ is well vascularized and its polygonal cells are arranged loosely around blood vessels and blood sinuses. Numerous mitochondria and microbodies (peroxisomes) give the acidophilic cytoplasm a finely granular appearance, but there is no evidence of secretory granules. The abundant endoplasmic reticulum is almost entirely agranular and occurs in two morphologically distinct forms: tubular and cisternal. The tubular reticulum is randomly distributed and may represent the site of synthesis and transport of the mandibular organ product. The cisternal reticulum is frequently associated with microbodies. Both forms of endoplasmic reticulum proliferate during mid to late premolt. Mandibular organ ultrastructure closely resembles that of cells known to synthesize steroids or lipids, which suggests that this organ may have a similar function. There is no functional evidence of involvement in molt control in Homarus, but ultrastructural and other evidence suggests an analogy with insect corpus allatum.

Ultrastructure of pea aphid mycetocytes: evidence for symbiote secretion

A detailed investigation into the ultrastructure of the pea aphid mycetocytes and their contained symbiotes and organelles was carried out with the transmission electron microscope. The most striking observation was the presence of small vesicles in the space between the primary symbiote cell wall and membrane envelope (outer membrane space). The vesicles appear to form by a budding process at the outer cell wall layer. Subsequently, the vesicles, we suggest, may move out into the mycetocyte cytoplasm via a similar budding of the membrane envelope; The Golgi apparatus was found to be an important structural component of the primary mycetocyte; it is continuous with the rough endoplasmic reticulum and the latter, in turn, appears to be closely connected to the primary symbiote membrane envelope. This may be of functional significance. A number of other organelles not previously described in mycetocytes were found, including transparent vacuoles, granular bodies, multi-vesicular bodies and microfilaments. The chemical composition of the various vesicles and organelles is unknown at present.