THE STRUCTURE AND FORMATION OF PROTEIN GRANULES IN THE FAT BODY OF AN INSECT

Fluorescent Reporters, Imaging, and Artificial Intelligence Toolkits to Monitor and Quantify Autophagy, Heterophagy, and Lysosomal Trafficking Fluxes

Lysosomal compartments control the clearance of cell-own material (autophagy) or of material that cells endocytose from the external environment (heterophagy) to warrant supply of nutrients, to eliminate macromolecules or parts of organelles present in excess, aged, or containing toxic material. Inherited or sporadic mutations in lysosomal proteins and enzymes may hamper their folding in the endoplasmic reticulum (ER) and their lysosomal transport via the Golgi compartment, resulting in lysosomal dysfunction and storage disorders. Defective cargo delivery to lysosomal compartments is harmful to cells and organs since it causes accumulation of toxic compounds and defective organellar homeostasis. Assessment of resident proteins and cargo fluxes to the lysosomal compartments is crucial for the mechanistic dissection of intracellular transport and catabolic events. It might be combined with high-throughput screenings to identify cellular, chemical, or pharmacological modulators of these events that may find therapeutic use for autophagy-related and lysosomal storage disorders. Here, discuss qualitative, quantitative and chronologic monitoring of autophagic, heterophagic and lysosomal protein trafficking in fixed and live cells, which relies on fluorescent single and tandem reporters used in combination with biochemical, flow cytometry, light and electron microscopy approaches implemented by artificial intelligence-based technology.

Unraveling the ultrastructure and dynamics of autophagic vesicles: Insights from advanced imaging techniques

Autophagy, an intracellular self-degradation process, is governed by a complex interplay of signaling pathways and interactions between proteins and organelles. Its fundamental purpose is to efficiently clear and recycle cellular components that are damaged or redundant. Central to this process are autophagic vesicles, specialized structures that encapsulate targeted cellular elements, playing a pivotal role in autophagy. Despite growing interest in the molecular components of autophagic machinery and their regulatory mechanisms, capturing the detailed ultrastructural dynamics of autophagosome formation continues to present significant challenges. However, recent advancements in microscopy, particularly in electron microscopy, have begun to illuminate the dynamic regulatory processes underpinning autophagy. This review endeavors to provide an exhaustive overview of contemporary research on the ultrastructure of autophagic processes. By synthesizing observations from diverse technological methodologies, this review seeks to deepen our understanding of the genesis of autophagic vesicles, their membrane origins, and the dynamic alterations that transpire during the autophagy process. The aim is to bridge gaps in current knowledge and foster a more comprehensive comprehension of this crucial cellular mechanism.

Macrophages play a nutritive role in post-metamorphic maturation in Drosophila

In the body of multicellular organisms, macrophages play an indispensable role in maintaining tissue homeostasis by removing old, apoptotic and damaged cells. In addition, macrophages allow significant remodeling of body plans during embryonic morphogenesis, regeneration and metamorphosis. Although the huge amount of organic matter that must be removed during these processes represents a potential source of nutrients, their further use by the organism has not yet been addressed. Here, we document that, during metamorphosis, Drosophila larval adipose tissue is infiltrated by macrophages, which remove dying adipocytes by efferocytosis and engulf leaking RNA-protein granules and lipids. Consequently, the infiltrating macrophages transiently adopt the adipocyte-like metabolic profile to convert remnants of dying adipocytes to lipoproteins and storage peptides that nutritionally support post-metamorphic development. This process is fundamental for the full maturation of ovaries and the achievement of early fecundity of individuals. Whether macrophages play an analogous role in other situations of apoptotic cell removal remains to be elucidated.

Autophagy: Regulator of cell death

Autophagy is the process by which cells degrade and recycle proteins and organelles to maintain intracellular homeostasis. Generally, autophagy plays a protective role in cells, but disruption of autophagy mechanisms or excessive autophagic flux usually leads to cell death. Despite recent progress in the study of the regulation and underlying molecular mechanisms of autophagy, numerous questions remain to be answered. How does autophagy regulate cell death? What are the fine-tuned regulatory mechanisms underlying autophagy-dependent cell death (ADCD) and autophagy-mediated cell death (AMCD)? In this article, we highlight the different roles of autophagy in cell death and discuss six of the main autophagy-related cell death modalities, with a focus on the metabolic changes caused by excessive endoplasmic reticulum-phagy (ER-phagy)-induced cell death and the role of mitophagy in autophagy-mediated ferroptosis. Finally, we discuss autophagy enhancement in the treatment of diseases and offer a new perspective based on the use of autophagy for different functional conversions (including the conversion of autophagy and that of different autophagy-mediated cell death modalities) for the clinical treatment of tumors.

Genome-wide analysis of the chromatin sites targeted by HEX 70a storage protein in the honeybee brain and fat body

Hexamerins, the proteins massively stored in the larval haemolymph of insects, are gradually used throughout metamorphosis as a source of raw material and energy for the development of adult tissues. Such behaviour defined hexamerins as storage proteins. Immunofluorescence experiments coupled with confocal microscopy show a hexamerin, HEX 70a, in the nucleus of the brain and fat body cells from honeybee workers, an unexpected localization for a storage protein. HEX 70a colocalizes with fibrillarin, a nucleolar-specific protein and H3 histone, thus suggesting a potential role as a chromatin-binding protein. This was investigated through chromatin immunoprecipitation and high-throughput DNA sequencing (ChIP-seq). The significant HEX 70a-DNA binding sites were mainly localized at the intergenic, promoter and intronic regions. HEX 70a targeted DNA stretches mapped to the genomic regions encompassing genes with relevant functional attributes. Several HEX 70a targeted genes were associated with H3K27ac or/and H3K27me3, known as active and repressive histone marks. Brain and fat body tissues shared a fraction of the HEX 70 targeted genes, and tissue-specific targets were also detected. The presence of overrepresented DNA motifs in the binding sites is consistent with specific HEX 70a-chromatin association. In addition, a search for HEX 70a targets in RNA-seq public libraries of fat bodies from nurses and foragers revealed differentially expressed targets displaying hex 70a-correlated developmental expression, thus supporting a regulatory activity for HEX 70a. Our results support the premise that HEX 70a is a moonlighting protein that binds chromatin and has roles in the brain and fat body cell nuclei, apart from its canonical role as a storage protein.

ER-phagy: mechanisms, regulation and diseases connected to the lysosomal clearance of the endoplasmic reticulum

ER-phagy (reticulo-phagy) defines the degradation of portions of the endoplasmic reticulum (ER) within lysosomes or vacuoles. It is part of the self-digestion (i.e., auto-phagic) programs recycling cytoplasmic material and organelles, which rapidly mobilize metabolites in cells confronted with nutrient shortage. Moreover, selective clearance of ER subdomains participates to the control of ER size and activity during ER stress, the re-establishment of ER homeostasis after ER stress resolution and the removal of ER parts, in which aberrant and potentially cytotoxic material has been segregated. ER-phagy relies on the individual and/or concerted activation of the ER-phagy receptors, ER peripheral or integral membrane proteins that share the presence of LC3/Atg8-binding motifs in their cytosolic domains. ER-phagy involves the physical separation of portions of the ER from the bulk ER network, and their delivery to the endolysosomal/vacuolar catabolic district. This last step is accomplished by a variety of mechanisms including macro-ER-phagy (in which ER fragments are sequestered by double-membrane autophagosomes that eventually fuse with lysosomes/vacuoles), micro-ER-phagy (in which ER fragments are directly engulfed by endosomes/lysosomes/vacuoles), or direct fusion of ER-derived vesicles with lysosomes/vacuoles. ER-phagy is dysfunctional in specific human diseases and its regulators are subverted by pathogens, highlighting its crucial role for cell and organism life.

ER-phagy responses in yeast, plants, and mammalian cells and their crosstalk with UPR and ERAD

ER-phagy, literally endoplasmic reticulum (ER)-eating, defines the constitutive or regulated clearance of ER portions within metazoan endolysosomes or yeast and plant vacuoles. The advent of electron microscopy led to the first observations of ER-phagy over 60 years ago, but only recently, with the discovery of a set of regulatory proteins named ER-phagy receptors, has it been dissected mechanistically. ER-phagy receptors are activated by a variety of pleiotropic and ER-centric stimuli. They promote ER fragmentation and engage luminal, membrane-bound, and cytosolic factors, eventually driving lysosomal clearance of select ER domains along with their content. After short historical notes, this review introduces the concept of ER-phagy responses (ERPRs). ERPRs ensure lysosomal clearance of ER portions expendable during nutrient shortage, nonfunctional, present in excess, or containing misfolded proteins. They cooperate with unfolded protein responses (UPRs) and with ER-associated degradation (ERAD) in determining ER size, function, and homeostasis.

Endoplasmic Reticulum (ER) and ER-Phagy

The endoplasmic reticulum (ER) is a biosynthetic organelle in eukaryotic cells. Its capacity to produce proteins, lipids and oligosaccharides responds to physiologic and pathologic demand. The transcriptional and translational unfolded protein response (UPR) programs increase ER size and activity. In contrast, ER-phagy programs in all their flavors select ER subdomains for lysosomal clearance. These programs are activated by nutrient deprivation, accumulation of excess ER (recov-ER-phagy), production of misfolded proteins that cannot be degraded by ER-associated degradation and that are removed from cells by the so-called ER-to-lysosome-associated degradation (ERLAD). Selection of ER subdomains to be cleared from cells relies on ER-phagy receptors, a class of membrane-bound proteins displaying cytosolic domains that engage the cytosolic ubiquitin-like protein LC3. Mechanistically, ER clearance proceeds via macro-ER-phagy, micro-ER-phagy and LC3-regulated vesicular delivery.

ER-phagy in human atherosclerosis: an exploratory ultrastructural study

Autophagy is a vacuolar self-digesting mechanism responsible for the removal of damaged organelles, indigestible aggregates, and nonfunctional long-lived proteins by lysosome. Autophagy is dynamically connected to the endoplasmic reticulum (ER) in several ways. It is capable to counteract the possible harmful effects linked to the impairment of protein folding in the ER; the ER has been proposed as the source for autophagosomal membranes. Also, the ER itself can undergo a selective form of autophagy (called ER-phagy) which ensures the maintenance of ER's morphology and function. Autophagy has been widely investigated in the cardiovascular system however there is no evidence to date regarding the occurrence of ER-phagy into the blood vessel wall. This study has been undertaken to explore the existence of this selective control mechanism in the cells of human atherosclerotic plaques. Transmission Electron Microscopy (TEM) analysis revealed that in the plaque cells the smooth ER profiles reorganized into concentric whorls and closely packed membranes arranged in curved and parallel arrays. Circular, often ring-shaped, ER membranes studded with ribosomes and enclosed in a sequestering vesicle have been also frequently observed. This preliminary study demonstrates the existence of a distinct machinery for the specific turnover of ER membranes in human atherosclerosis and provides the first ultrastructural description of ER-phagy in the diseased vascular tissue. These results may open new perspectives for future investigation in the cardiovascular field.

Autophagy in animal development

Macroautophagy (autophagy) delivers intracellular constituents to the lysosome to promote catabolism. During development in multiple organisms, autophagy mediates various cellular processes, including survival during starvation, programmed cell death, phagocytosis, organelle elimination, and miRNA regulation. Our current understanding of autophagy has been enhanced by developmental biology research during the last quarter of a century. Through experiments that focus on animal development, fundamental mechanisms that control autophagy and that contribute to disease were elucidated. Studies in embryos revealed specific autophagy molecules that mediate the removal of paternally derived mitochondria, and identified autophagy components that clear protein aggregates during development. Importantly, defects in mtDNA inheritance, or removal of paternal mtDNA via mitochondrial autophagy, can contribute to mitochondrial-associated disease. In addition, impairment of the clearance of protein aggregates by autophagy underlies neurodegenerative diseases. Experiments in multiple organisms also reveal conserved mechanisms of tissue remodeling that rely on the cooperation between autophagy and apoptosis to clear cell corpses, and defects in autophagy and apoptotic cell clearance can contribute to inflammation and autoimmunity. Here we provide an overview of key developmental processes that are mediated by autophagy in multiple animals.

The various shades of ER-phagy

Endoplasmic reticulum (ER) is a large and dynamic cellular organelle. ER morphology consists of sheets, tubules, matrixes, and contact sites shared with other membranous organelles. The capacity of the ER to fulfill its numerous biological functions depends on its continuous remodeling and the quality control of its proteome. Selective turnover of the ER by autophagy, termed ER-phagy, plays an important role in maintaining ER homeostasis. ER network integrity and turnover rely on specific ER-phagy receptors, which influence and coordinate alterations in ER morphology and the degradation of ER contents and membranes via the lysosome, by interacting with the LC3/GABARAP family. In this commentary, we discuss general principles and identify the major players in this recently characterized form of selective autophagy, while simultaneously highlighting open questions in the field.

ESCRT-III-driven piecemeal micro-ER-phagy remodels the ER during recovery from ER stress

The endoplasmic reticulum (ER) produces about 40% of the nucleated cell's proteome. ER size and content in molecular chaperones increase upon physiologic and pathologic stresses on activation of unfolded protein responses (UPR). On stress resolution, the mammalian ER is remodeled to pre-stress, physiologic size and function on activation of the LC3-binding activity of the translocon component SEC62. This elicits recov-ER-phagy, i.e., the delivery of the excess ER generated during the phase of stress to endolysosomes (EL) for clearance. Here, ultrastructural and genetic analyses reveal that recov-ER-phagy entails the LC3 lipidation machinery and proceeds via piecemeal micro-ER-phagy, where RAB7/LAMP1-positive EL directly engulf excess ER in processes that rely on the Endosomal Sorting Complex Required for Transport (ESCRT)-III component CHMP4B and the accessory AAA+ ATPase VPS4A. Thus, ESCRT-III-driven micro-ER-phagy emerges as a key catabolic pathway activated to remodel the mammalian ER on recovery from ER stress.

ER-phagy: shaping up and destressing the endoplasmic reticulum

The endoplasmic reticulum (ER) network has central roles in metabolism and cellular organization. The ER undergoes dynamic alterations in morphology, molecular composition and functional specification. Remodelling of the network under fluctuating conditions enables the continual performance of ER functions and minimizes stress. Recent data have revealed that selective autophagy‐mediated degradation of ER fragments, or ER‐phagy, fundamentally contributes to this remodelling. This review provides a perspective on established views of selective autophagy, comparing these with emerging mechanisms of ER‐phagy and related processes. The text discusses the impact of ER‐phagy on the function of the ER‐ and the cell, both in normal physiology and when dysregulated within disease settings. Finally, unanswered questions regarding the mechanisms and significance of ER‐phagy are highlighted.

Eat it right: ER-phagy and recovER-phagy

The endoplasmic reticulum (ER) is the site of protein, lipid, phospholipid, steroid and oligosaccharide synthesis and modification, calcium ion storage, and detoxification of endogenous and exogenous products. Its volume (and activity) must be maintained under normal growth conditions, must be expanded in a controlled manner on activation of ER stress programs and must be reduced to pre-stress size during the recovery phase that follows ER stress termination. ER-phagy is the constitutive or regulated fragmentation and delivery of ER fragments to lysosomal compartments for clearance. It gives essential contribution to the maintenance of cellular homeostasis, proteostasis, lipidostasis and oligosaccharidostasis (i.e. the capacity to produce the proteome, lipidome and oligosaccharidome in appropriate quality and quantity). ER turnover is activated on ER stress, nutrient deprivation, accumulation of misfolded polypeptides, pathogen attack and by activators of macroautophagy. The selectivity of these poorly characterized catabolic pathways is ensured by proteins displayed at the limiting membrane of the ER subdomain to be removed from cells. These proteins are defined as ER-phagy receptors and engage the cytosolic macroautophagy machinery via specific modules that associate with ubiquitin-like, cytosolic proteins of the Atg8/LC3/GABARAP family. In this review, we give an overview on selective ER turnover and on the yeast and mammalian ER-phagy receptors identified so far.

Endoplasmic reticulum turnover: ER-phagy and other flavors in selective and non-selective ER clearance

The endoplasmic reticulum (ER) is a highly dynamic organelle in eukaryotic cells. It is deputed to lipid and protein biosynthesis, calcium storage, and the detoxification of various exogenous and endogenous harmful compounds. ER activity and size must be adapted rapidly to environmental and developmental conditions or biosynthetic demand. This is achieved on induction of thoroughly studied transcriptional/translational programs defined as "unfolded protein responses" that increase the ER volume and the expression of ER-resident proteins regulating the numerous ER functions. Less understood are the lysosomal catabolic processes that maintain ER size at steady state, that prevent excessive ER expansion during ER stresses, or that ensure return to physiologic ER size during recovery from ER stresses. These catabolic processes may also be activated to remove ER subdomains where proteasome-resistant misfolded proteins or damaged lipids have been segregated. Insights into these catabolic mechanisms have only recently emerged with the identification of so-called ER-phagy receptors, which label specific ER subdomains for selective lysosomal delivery for clearance. Here, in eight chapters and one addendum, we comment on recent advances in ER turnover pathways induced by ER stress, nutrient deprivation, misfolded proteins, and live bacteria. We highlight the role of yeast (Atg39 and Atg40) and mammalian (FAM134B, SEC62, RTN3, and CCPG1) ER-phagy receptors and of autophagy genes in selective and non-selective catabolic processes that regulate cellular proteostasis by controlling ER size, turnover, and function.

Methods for Monitoring Autophagy in Silkworm Organs

In holometabolous insects, various larval organs are remodeled by autophagy during metamorphosis. Although moths and butterflies are among the first animal models in which this self-eating process was described, only in recent years autophagy has been analyzed in detail in these insects. In particular, the silkworm Bombyx mori, which represents a well-studied model among Lepidoptera, provides a wide repertoire of cellular and molecular tools useful for studying the occurrence of autophagy and for evaluating its role in postembryonic development. Here, we describe some morphological, biochemical, and molecular methods to monitor autophagy in silkworm organs.

Sensing Membrane Curvature in Macroautophagy

In response to intracellular stress events ranging from starvation to pathogen invasion, the cell activates one or more forms of macroautophagy. The key event in these related pathways is the de novo formation of a new organelle called the autophagosome, which either surrounds and sequesters random portions of the cytoplasm or selectively targets individual intracellular challenges. Thus, the autophagosome is a flexible membrane platform with dimensions that ultimately depend upon the target cargo. The intermediate membrane, termed the phagophore or isolation membrane, is a cup-like structure with a clear concave face and a highly curved rim. The phagophore is largely devoid of integral membrane proteins; thus, its shape and size are governed by peripherally associated membrane proteins and possibly by the lipid composition of the membrane itself. Growth along the phagophore rim marks the progress of both organelle expansion and ultimately organelle closure around a particular cargo. These two properties, a reliance on peripheral membrane proteins and a structurally distinct membrane architecture, suggest that the ability to target or manipulate membrane curvature might be an essential activity of proteins functioning in this pathway. In this review, we discuss the extent to which membranes are naturally curved at each of the cellular sites believed to engage in autophagosome formation, review basic mechanisms used to sense this curvature, and then summarize the existing literature concerning which autophagy proteins are capable of curvature recognition.

Microautophagy in nutritive phagocytes of sea urchins

Two types of cells were observed in germinative epithelium of male and female sea urchins: germ cells and somatic accessory cells; the latter referred to as nutritive phagocytes. At the onset of gametogenesis, nutritive phagocytes accumulate nutrients and greatly increase in their size. As gametogenesis progresses, the accumulated nutrients are transferred from nutritive phagocytes into developing gametes, and size of the nutritive phagocytes decreases. An electron microscopic study of nutritive phagocytes in sea urchins, Strongylocentrotus intermedius, at different stages of annual reproductive cycle showed for the first time that both macro- and microautophagy take place in nutritive phagocytes. Both processes occur simultaneously and regulate size and composition of nutritive phagocytes in male and female sea urchins. Nutritive phagocytes consume redundant cytoplasm via macroautophagy. Microautophagy is probably involved in consumption of redundant membranes that appear within nutritive phagocytes due to destruction of nutrient-storing globules, macroautophagy, and phagocytosis of germ cells or their remnants.

Roles and regulation of autophagy and apoptosis in the remodelling of the lepidopteran midgut epithelium during metamorphosis

We previously showed that autophagy and apoptosis occur in the removal of the lepidopteran larval midgut during metamorphosis. However, their roles in this context and the molecular pathways underlying their activation and regulation were only hypothesized. The results of the present study better clarify the timing of the activation of these two processes: autophagic and apoptotic genes are transcribed at the beginning of metamorphosis, but apoptosis intervenes after autophagy. To investigate the mechanisms that promote the activation of autophagy and apoptosis, we designed a set of experiments based on injections of 20-hydroxyecdysone (20E). Our data demonstrate that autophagy is induced at the end of the last larval stage by the 20E commitment peak, while the onset of apoptosis occurs concomitantly with the 20E metamorphic peak. By impairing autophagic flux, the midgut epithelium degenerated faster, and higher caspase activity was observed compared to controls, whereas inhibiting caspase activation caused a severe delay in epithelial degeneration. Our data demonstrate that autophagy plays a pro-survival function in the silkworm midgut during metamorphosis, while apoptosis is the major process that drives the demise of the epithelium. The evidence collected in this study seems to exclude the occurrence of autophagic cell death in this setting.

ER-phagy mediates selective degradation of endoplasmic reticulum independently of the core autophagy machinery

Selective autophagy of damaged or redundant organelles is an important mechanism for maintaining cell homeostasis. We found previously that endoplasmic reticulum (ER) stress in the yeast Saccharomyces cerevisiae causes massive ER expansion and triggers the formation of large ER whorls. Here, we show that stress-induced ER whorls are selectively taken up into the vacuole, the yeast lysosome, by a process termed ER-phagy. Import into the vacuole does not involve autophagosomes but occurs through invagination of the vacuolar membrane, indicating that ER-phagy is topologically equivalent to microautophagy. Even so, ER-phagy requires neither the core autophagy machinery nor several other proteins specifically implicated in microautophagy. Thus, autophagy of ER whorls represents a distinct type of organelle-selective autophagy. Finally, we provide evidence that ER-phagy degrades excess ER membrane, suggesting that it contributes to cell homeostasis by controlling organelle size.

A molecular view of autophagy in Lepidoptera

Metamorphosis represents a critical phase in the development of holometabolous insects, during which the larval body is completely reorganized: in fact, most of the larval organs undergo remodeling or completely degenerate before the final structure of the adult insect is rebuilt. In the past, increasing evidence emerged concerning the intervention of autophagy and apoptosis in the cell death processes that occur in larval organs of Lepidoptera during metamorphosis, but a molecular characterization of these pathways was undertaken only in recent years. In addition to developmentally programmed autophagy, there is growing interest in starvation-induced autophagy. Therefore we are now entering a new era of research on autophagy that foreshadows clarification of the role and regulatory mechanisms underlying this self-digesting process in Lepidoptera. Given that some of the most important lepidopteran species of high economic importance, such as the silkworm, Bombyx mori, belong to this insect order, we expect that this information on autophagy will be fully exploited not only in basic research but also for practical applications.

Autophagy in Drosophila: from historical studies to current knowledge

The discovery of evolutionarily conserved Atg genes required for autophagy in yeast truly revolutionized this research field and made it possible to carry out functional studies on model organisms. Insects including Drosophila are classical and still popular models to study autophagy, starting from the 1960s. This review aims to summarize past achievements and our current knowledge about the role and regulation of autophagy in Drosophila, with an outlook to yeast and mammals. The basic mechanisms of autophagy in fruit fly cells appear to be quite similar to other eukaryotes, and the role that this lysosomal self-degradation process plays in Drosophila models of various diseases already made it possible to recognize certain aspects of human pathologies. Future studies in this complete animal hold great promise for the better understanding of such processes and may also help finding new research avenues for the treatment of disorders with misregulated autophagy.

Drosophila Golgi membrane protein Ema promotes autophagosomal growth and function

Autophagy is a self-degradative process in which cellular material is enclosed within autophagosomes and trafficked to lysosomes for degradation. Autophagosomal biogenesis is well described; however mechanisms controlling the growth and ultimate size of autophagosomes are unclear. Here we demonstrate that the Drosophila membrane protein Ema is required for the growth of autophagosomes. In an ema mutant, autophagosomes form in response to starvation and developmental cues, and these autophagosomes can mature into autolysosomes; however the autophagosomes are very small, and autophagy is impaired. In fat body cells, Ema localizes to the Golgi complex and is recruited to the membrane of autophagosomes in response to starvation. The Drosophila Golgi protein Lva also is recruited to the periphery of autophagosomes in response to starvation, and this recruitment requires ema. Therefore, we propose that Golgi is a membrane source for autophagosomal growth and that Ema facilitates this process. Clec16A, the human ortholog of Ema, is a candidate autoimmune susceptibility locus. Expression of Clec16A can rescue the autophagosome size defect in the ema mutant, suggesting that regulation of autophagosome morphogenesis may be a fundamental function of this gene family.

Variations in mitochondrial DNA and gene transcription in freezing-tolerant larvae of Eurosta solidaginis (Diptera: Tephritidae) and Gynaephora groenlandica (Lepidoptera: Lymantriidae)

Respiration, mitochondrial (mt)DNA content, and mitochondrial-specific RNA expression in fat body cells from active and cold-adapted larvae of the goldenrod gall fly, Eurosta solidaginis, and the Arctic woolly bear caterpillar, Gynaephora groenlandica, were compared. Reduced amounts of mtDNA were observed in cold-adapted larvae of both E. solidaginis and G. groenlandica collected in fall or winter, compared with summer-collected larvae. mtDNA increased to levels similar to those of summer-collected larvae after incubation at 10 degrees C or 15 degrees C for 5 h. Mitochondrial-specific RNAs (COI and 16S) were observed in fat body cells of both active and cold-adapted E. solidaginis larvae. Our results suggest that mitochondrial proteins required for respiration may be restored rapidly from stable RNAs present in overwintering larvae.

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.

Interaction of the anterior fat body protein with the hexamerin receptor in the blowfly Calliphora vicina

In late larvae of the blowfly, Calliphora vicina, arylphorin and LSP-2 proteins, which belong to the class of hexamerins, are selectively taken up by the fat body from the haemolymph. Hexamerin endocytosis is mediated by a specific membrane-bound receptor, the arylphorin-binding protein (ABP). Using the two-hybrid technique, we found that the anterior fat body protein (AFP) interacts with the hexamerin receptor. AFP, a homologue of the mammalian calcium-binding liver protein regucalcin (senescence marker protein-30), exhibits a strong binding affinity for a naturally occurring C-terminal cleavage fragment of the hexamerin receptor precursor (the P30 peptide) and other receptor cleavage products that contain P30. Expression of AFP mRNA and protein is restricted to the anterior part of the fat body tissue and to haemocytes in last-instar larvae. AFP mRNA occurs in all postembryonic developmental stages. Our results suggest that AFP plays a role in the regulation of hexamerin uptake by fat body cells along the anterior-posterior axis.

Conservation of hexamerin endocytosis in Diptera

In cyclorrhaphan Diptera at least two different types of haemolymph proteins exist which belong to the class of hexamerins. In the last larval instar of Calliphora vicina, the highly aromatic hexamerin, arylphorin, and the second hexamerin, PII, make up about 90% of haemolymph proteins. Both of these proteins are selectively taken up by the fat body cells at the end of larval life and share a common membrane-bound receptor. In addition, hexamerins and possible hexamerin receptors of Calliphora vicina, Calliphora vomitoria, Drosophila melanogaster, Ceratitis capitata, Sarcophaga bullata, Musca domestica and Protophormia terraenovae were investigated. Uptake of arylphorin by the larval fat bodies of Calliphora vicina as well asarylphorin-receptor binding can be competed in vitro by haemolymph from other Diptera. Therefore, hexamerin-receptor binding must be conserved among related cyclorrhaphan Diptera and between different types of hexamerins within a species. As the degree of competition is in good agreement with the presumed phylogenetic distances between these species, the method described here provides a simple tool to estimate evolutionary distances.

Epithelial-mesenchymal transformation during palatal fusion: carboxyfluorescein traces cells at light and electron microscopic levels

During the fusion of rodent embryo palatal shelves, the cells of the outer epithelial layer slough off, allowing the cells of the medial edge basal layer to form a midline seam that undergoes epithelial-mesenchymal transformation, as judged by electron microscopy and immunohistochemistry. In this study, we analyze the fate of the transformed cells using a lipid soluble dye to label the medial edge epithelium in situ. Prefusion E14 mouse palates were exposed in vitro or in vivo to a fluoresceinated lipid soluble marker, carboxydichlorofluorescein diacetate succinimidyl ester (CCFSE), which localizes in epithelia as a lipid insoluble compound that does not pass into the connective tissue compartment. The midline seam that formed after 24 hours contained labelled epithelial cells that were replaced by individually labelled mesenchymal cells where the seam transformed. By light microscopy, the labelled cells were seen to contain intensely fluorescent bodies that do not react for acid phosphatase. We were able for the first time to identify these structures by electron microscopy as CCFSE isolation bodies. The cells with isolation bodies are clearly healthy and able to participate in subsequent development of the palate. At 4 days after labelling, individual CCFSE containing cells present in the palate mesenchyme occupy both midline and lateral areas and can clearly be classified as fibroblasts by electron microscopy. CCFSE is a far more useful marker than another lipid soluble marker, DiI, for following cells, because the cells can be fixed and identified both at the light and electron microscope levels. Interestingly, if labelled palatal shelves are not allowed to fuse in vitro, the basal epithelial cells do not form mesenchyme after sloughing, indicating that formation of the epithelial midline seam is necessary to trigger its epithelial-mesenchymal transformation.

Cyclic modulation of Sertoli cell junctional complexes in a seasonal breeder: the mink (Mustela vison)

The development and modulation of Sertoli cell junctions was studied in newborn and adult mink during the active and inactive spermatogenic phases. The techniques used were electron microscopy of freeze-fractured replicas and thin sections of tissues infused with horseradish peroxidase as a junction permeability tracer. In the newborn, freeze-fractured developing junctions had either spherical or fibrillar particles. In addition, junctional domains where particles were associated preferentially with the E-face, and others where particles were associated preferentially with the P-face, were found developing either singly or conjointly within a given membrane segment, thus yielding a heterogeneous junctional segment. Coincidently with the development of a tubular lumen and the establishment of a competent blood-testis barrier, junctional strands were composed primarily of particulate elements associated preferentially with the E-face. In adult mink during active spermatogenesis, cell junctions were found on the entire lateral Sertoli cell plasma membrane from the basal to the luminal pole of the cell. In the basal third of the Sertoli cell, membranous segments that faced a spermatogonium or a migrating spermatocyte displayed forming tight, gap, and adherens junctions. In the middle third, abutting membrane segments localized above germ cells were involved in continuous zonules and in adherens junctions. In the apical or luminal third, the zonules were discontinuous, and the association of junctional particles with the E-face furrow was lost. Gap junctions increased in both size and numbers. Junctional vesicles that appeared as annular gap and tight-junction profiles in thin sections or as hemispheres in freeze-fracture replicas were present. Reflexive tight and gap junctions were formed through the interaction of plasma membrane segments of the same Sertoli cell. Internalized junctional vesicles were also present in mature spermatids. During the inactive spermatogenic phase, cell junctions were localized principally in the basal third of the Sertoli cell; junctional strands resembled those of the newborn mink. During the active spermatogenic phase, continuous zonules were competent in blocking passage of the protein tracer. During the inactive phase the blood-testis barrier was incompetent in blocking entry of the tracer into the seminiferous epithelium. It is proposed that modulation of the Sertoli cell zonules being formed at the base and dismantled at the apex of the seminiferous epithelium follows the direction of germ cell migration and opposes the apicobasal direction of junction formation reported for most epithelia.(ABSTRACT TRUNCATED AT 400 WORDS)

Sequential structural changes in the fat body of the tobacco hornworm, Manduca sexta, during the fifth larval stadium

Light and electron microscopy revealed a series of structural changes that occur in the fat body of the tobacco hornworm, Manduca sexta, during the fifth, i.e. the final, larval stadium. At each developmental stage studied, the cells of the fat body were homogeneous in structure. We found no evidence suggesting the presence of more than one type of fat body cell. Our structural data are consistent with published observations on biochemical activities of M. sexta fat body at particular developmental stages. Specific points of agreement include: (a) acquisition of Golgi complex (GC) and rough endoplasmic reticulum (RER) concomitant with the time of major protein production; (b) loss of many cellular organelles (such as GC and RER) as protein production drastically decreases; (c) accumulation of protein granules and urate granules after the onset of wandering (i.e. during the pre-pupal period); (d) accumulation of lipid and glycogen throughout the feeding period. In addition we found that (a) the plasma membrane reticular system (PMRS) developed during the period when protein secretion was great; (b) the PMRS was lost abruptly at the onset of wandering; and (c) the nucleus changed in shape from being roughly spherical to elliptoid in the pre-pupal stage. We found that the structure of M. sexta fat body is similar to that published for other Lepidoptera. However, it differs from that of Heliothis zea in that regional differences are not obviously apparent.

Ultrastructure of the preparative phase of cell death in the larval fat body of Drosophila melanogaster

Progressive changes in the ultrastructure of the larval fat body of Drosophila melanogaster were studied during the third instar. In addition to electron microscopy, light microscopy and morphometric stereology were employed to evaluate the tissue at five 12-hr intervals: 48, 60, 72, 84, and 96 hr after hatching from the egg. Lipid and glycogen were found stored throughout the instar, whereas protein is stored in the form of cytoplasmic granules mainly during the final 24 hr. The cells increased in cross-sectional area, and there was a concomitant increase in the relative amounts of these substances. Based on morphological characteristics there were three types of protein granules which we called dense granules (D), heterogeneous granules (H), and autophagic vacuoles. The morphology, size range, time of appearance, and changes in frequency of these granules suggested that the H type arose from D granules, and that the autophagic vacuoles were derived from D and H types. Morphological evidence indicated D granules have the unusual characteristic of forming in the intercellular space before entering the cytoplasm.

The induction and distribution of an insect ferritin--a new function for the endoplasmic reticulum

Three insect tissues have particular roles as filters to maintain the fluid composition of the hemolymph. Water and ions enter and leave through the midgut. The pericardial cells filter circulating hemolymph. Malpighian tubules, often with the rectum, allow resorption from a hemolymph filtrate that passes to the hindgut. All three tissues have plasma membrane infolds making a reticulum on their hemolymph surfaces, and all three have RER leading to SER extensions into their reticula. SER is a catch-all description for membranes lacking ribosomes in the pre-Golgi complex set of compartments of the vacuolar system. Some kinds of SER are well known for their role in housing enzymes for steroid metabolism and for detoxification. The SER ramifying within the plasma membrane reticular systems of tissues concerned with hemolymph filtration contains ferritin, suggesting that this SER has another, different function. In contrast to vertebrate cells, where ferritin is confined to the cytosol and lysosomes, we have found that in Calpodes and perhaps in most insects, ferritin occurs in the vacuolar system and not in the cytosol. Ferritin occurs naturally in the RER and SER of cells at the hind end of the midgut, in pericardial cells and in the yellow region of the Malpighian tubules. Additional ferritin is induced by loading the gut or hemolymph with iron. Overloading with iron causes ferritin secretion to the gut lumen. We propose that the SER in these cells functions in iron homeostasis by holding ferritin for loading and unloading as it moves to and from the reticulum at the cell surface where it can be maximally exposed to extracellular fluid flow.

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.

Previtellogenic development and vitellogenin synthesis in the fat body of a mosquito: an ultrastructural and immunocytochemical study

We describe two phases, previtellogenic and vitellogenic, in the activity of the trophocytes in the fat body of the mosquito Aedes aegypti. The previtellogenic phase, leading to trophocyte competence to synthesize vitellogenin (Vg), occurred during the first 3 days after eclosion. This phase was characterized by enlargement and activation of the nucleoli, proliferation of ribosomes and rough endoplasmic reticulum (RER), development of Golgi complexes, and extensive invaginations of the plasma membrane. During the vitellogenic phase, initiated by a blood meal, Vg was first detected, by immunofluorescence, 1 hr after feeding. The intensity of the immunoreaction increased for the next 24 hr, was declining at 30 hr, and had disappeared by 48 hr. Vg synthesis was characterized ultrastructurally by the enlargement of the RER and the formation of dense secretion granules in Golgi complexes. These secretion granules were two to three times larger at the peak of Vg synthesis than at the beginning. The granules discharged their contents by exocytosis. Two electron microscopical immunocytochemical methods, immunoferritin and peroxidase-antiperoxidase, confirmed this pathway of Vg processing. For the first 12 hr after feeding. Vg synthetic organelles proliferated and the active nucleoli were multilobed; thereafter, while Vg synthesis continued, the nucleoli began to regress into compact bodies. Termination of Vg synthesis was marked by autophagical degradation of Vg synthetic and processing organelles.

Ultrastructure of mucocartilage in the larval anadromous sea lamprey, Petromyzon marinus L

The fine structure of mucocartilage, a tissue unique to larval lampreys, was examined in Petromyzon marinus L. This tissue is surrounded by a perichondrium of vascularized, dense connective tissue composed of fibroblasts, collagen fibrils, and elastic-like microfibrils, but it is avascular itself and consists of elastic-like microfibrils, ground substance, and a few diffusely scattered fibroblasts. Fibroblasts possess rough endoplasmic reticulum, may free ribosomes, a well-developed Golgi apparatus, a tubulo-vesicular network, and a number of secondary lysosomes containing crystalline material. The appearance of the organelles suggests the involvement of the cell in the synthesis and secretion of the ground substance and microfibrils. Tubular microfibrils, 11-13 nm in diameter, comprise the major portion of the matrix, and they are similar to those described in developing mammalian elastic tissue (Ross and Bornstein, 1969). The retention of the microfibrils may represent either a primitive form of elastic fiber in this "primitive" vertebrate or reflect the larval condition of the lampreys under examination. Scattered spherical to polyhedral-shaped mitrix granules and intergranular filaments make up the remainder of the matrix. It was concluded that mucocartilage in larval lampreys is not a conventional type of vertebrate connective tissue.