Apolysis and the turnover of plasma membrane plaques during cuticle formation in an insect

AOP Report: Inhibition of Chitin Synthase 1 Leading to Increased Mortality in Arthropods

Arthropods (including insects, crustaceans, and arachnids) rely on the synthesis of chitin to complete their life cycles (Merzendorfer 2011). The highly conserved chitin synthetic process and the absence of this process in vertebrates make it an exploitable target for pest management and veterinary medicines (Merzendorfer 2013; Junquera et al. 2019). Susceptible, nontarget organisms, such as insects and aquatic invertebrates, exposed to chitin synthesis inhibitors may suffer population declines, which may have a negative impact on ecosystems and associated services. Hence, it is important to properly identify, prioritize, and regulate relevant chemicals posing potential hazards to nontarget arthropods. The need for a more cost-efficient and mechanistic approach in risk assessment has been clearly evident and triggered the development of the adverse outcome pathway (AOP) framework (Ankley et al. 2010). An AOP links a molecular initiating event (MIE) through key events (KEs) to an adverse outcome. The mechanistic understanding of the underlying toxicological processes leading to a regulation-relevant adverse outcome is necessary for the utilization of new approach methodologies (NAMs) and efficient coverage of wider chemical and taxonomic domains. In the last decade, the AOP framework has gained traction and expanded within the (eco)toxicological research community. However, there exists a lack of mature invertebrate AOPs describing molting defect-associated mortality triggered by direct inhibition of relevant enzymes in the chitin biosynthetic pathway (chitin synthesis inhibitors) or interference with associated endocrine systems by environmental chemicals (endocrine disruptors). Arthropods undergo molting to grow and reproduce (Heming 2018). This process is comprised of the synthesis of a new exoskeleton, followed by the exuviation of the old exoskeleton (Reynolds 1987). The arthropod exoskeleton (cuticle) can be divided into 2 layers, the thin and nonchitinous epicuticle, which is the outermost layer of the cuticle, and the underlying chitinous procuticle. A single layer of epithelial cells is responsible for the synthesis and secretion of both cuticular layers (Neville 1975). The cuticle protects arthropods from predators and desiccation, acts as a physical barrier against pathogens, and allows for locomotion by providing support for muscular function (Vincent and Wegst 2004). Because the procuticle mainly consists of chitin microfibrils embedded in a matrix of cuticular proteins supplemented by lipids and minerals in insects (Muthukrishnan et al. 2012) and crustaceans (Cribb et al. 2009; Nagasawa 2012), chitin is a determinant factor for the appropriate composition of the cuticle and successful molting (Cohen 2001). A detailed overview of the endocrine mechanisms regulating chitin synthesis is given in Supplemental Data, Figure S1. The shedding of the old exoskeleton in insects is mediated by a sequence of distinct muscular contractions, the ecdysis motor program (EMP; Ayali 2009; Song et al. 2017a). Like the expression of chitin synthase isoform 1 (CHS-1), the expression of peptide hormones regulating the EMP is also controlled by ecdysteroids (Antoniewski et al. 1993; Gagou et al. 2002; Ayali 2009). Cuticular chitin is polymerized from uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) by the transmembrane enzyme CHS-1, which is localized in the epithelial plasma membrane in insects (Locke and Huie 1979; Binnington 1985; Merzendorfer and Zimoch 2003; Merzendorfer 2006). Because crustaceans are also dependent on the synthesis of chitin, the underlying mechanisms are believed to be similar, although less is known about different CHS isoforms and their localization (Rocha et al. 2012; Qian et al. 2014; Uddowla et al. 2014; Harðardóttir et al. 2019). Disruption of either chitin synthesis or the upstream endocrine pathways can lead to lethal molting disruption (Arakawa et al. 2008; Merzendorfer et al. 2012; Song et al. 2017a, 2017b). In the case of chitin synthesis inhibition, molting disruption can be referred to as "premature molting." If ecdysis cannot be completed because of decreased chitin synthesis, the organism may not successfully molt. Even if ecdysis can be completed on inhibition of chitin synthesis, the organism may not survive because of the poor integrity of the new cuticle. These effects are observed in arthropods following molting, which fail to survive subsequent molts (Arakawa et al. 2008; Chen et al. 2008) or animals being stuck in their exuviae (Wang et al. 2019) and ultimately dying as a result of insufficient food or oxygen intake (Camp et al. 2014; Song et al. 2017a). The term "premature molting" is used to differentiate from the term "incomplete ecdysis," which describes inhibition of ecdysis on a behavioral level, namely through reduction of the EMP (Song et al. 2017a). The present AOP describes molting-associated mortality through direct inhibition of the enzyme CHS-1. It expands the small but increasing number of invertebrate AOPs that have relevance to arthropods, the largest phylum within the animal kingdom (Bar-On et al. 2018). The development of this AOP will be useful in further research and regulatory initiatives related to assessment of CHS inhibitors and identification of critical knowledge gaps and may suggest new strategies for ecotoxicity testing efforts. Environ Toxicol Chem 2021;40:2112-2120. © 2021 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Insect Cuticular Chitin Contributes to Form and Function

Chitin contributes to the rigidity of the insect cuticle and serves as an attachment matrix for other cuticular proteins. Deficiency of chitin results in abnormal embryos, cuticular structural defects and growth arrest. When chitin is not turned over during molting, the developing insect is trapped inside the old cuticle. Partial deacetylation of cuticular chitin is also required for proper laminar organization of the cuticle and vertical pore canals, molting, and locomotion. Thus, chitin and its modifications strongly influence the structure of the exoskeleton as well as the physiological functions of the insect. Internal tendons and specialized epithelial cells called "tendon cells" that arise from the outer layer of epidermal cells provide attachment sites at both ends of adult limb muscles. Membrane processes emanating from both tendon and muscle cells interdigitate extensively to strengthen the attachment of muscles to the extracellular matrix (ECM). Protein ligands that bind to membrane-bound integrin complexes further enhance the adhesion between muscles and tendons. Tendon cells contain F-actin fiber arrays that contribute to their rigidity. In the cytoplasm of muscle cells, proteins such as talin and other proteins provide attachment sites for cytoskeletal actin, thereby increasing integrin binding and activation to mechanically couple the ECM with actin in muscle cells. Mutations in integrins and their ligands, as well as depletion of chitin deacetylases, result in defective locomotion and muscle detachment from the ECM. Thus, chitin in the cuticle and chitin deacetylases strongly influence the shape and functions of the exoskeleton as well as locomotion of insects.

Nanopore Formation in the Cuticle of an Insect Olfactory Sensillum

Nanometer-level patterned surface structures form the basis of biological functions, including superhydrophobicity, structural coloration, and light absorption [1-3]. In insects, the cuticle overlying the olfactory sensilla has multiple small (50- to 200-nm diameter) pores [4-8], which are supposed to function as a filter that admits odorant molecules, while preventing the entry of larger airborne particles and limiting water loss. However, the cellular processes underlying the patterning of extracellular matrices into functional nano-structures remain unknown. Here, we show that cuticular nanopores in Drosophila olfactory sensilla originate from a curved ultrathin film that is formed in the outermost envelope layer of the cuticle and secreted from specialized protrusions in the plasma membrane of the hair forming (trichogen) cell. The envelope curvature coincides with plasma membrane undulations associated with endocytic structures. The gore-tex/Osiris23 gene encodes an endosomal protein that is essential for envelope curvature, nanopore formation, and odor receptivity and is expressed specifically in developing olfactory trichogen cells. The 24-member Osiris gene family is expressed in cuticle-secreting cells and is found only in insect genomes. These results reveal an essential requirement for nanopores for odor reception and identify Osiris genes as a platform for investigating the evolution of surface nano-fabrication in insects.

Chitin in Arthropods: Biosynthesis, Modification, and Metabolism

Chitin is a structural constituent of extracellular matrices including the cuticle of the exoskeleton and the peritrophic matrix (PM) of the midgut in arthropods. Chitin chains are synthesized through multiple biochemical reactions, organized in several hierarchical levels and associated with various proteins that give their unique physicochemical characteristics of the cuticle and PM. Because, arthropod growth and morphogenesis are dependent on the capability of remodeling chitin-containing structures, chitin biosynthesis and degradation are highly regulated, allowing ecdysis and regeneration of the cuticle and PM. Over the past 20 years, much progress has been made in understanding the physiological functions of chitinous matrices. In this chapter, we mainly discussed the biochemical processes of chitin biosynthesis, modification and degradation, and various enzymes involved in these processes. We also discussed cuticular proteins and PM proteins, which largely determine the physicochemical properties of the cuticle and PM. Although rapid advances in genomics, proteomics, RNA interference, and other technologies have considerably facilitated our research in chitin biosynthesis, modification, and metabolism in recent years, many aspects of these processes are still partially understood. Further research is needed in understanding how the structural organization of chitin synthase in plasma membrane accommodate chitin biosynthesis, transport of chitin chain across the plasma membrane, and release of the chitin chain from the enzyme. Other research is also needed in elucidating the roles of chitin deacetylases in chitin organization and the mechanism controlling the formation of different types of chitin in arthropods.

Group I chitin deacetylases are essential for higher order organization of chitin fibers in beetle cuticle

Roles in the organization of the cuticle (exoskeleton) of two chitin deacetylases (CDAs) belonging to group I, TcCDA1 and TcCDA2, as well as two alternatively spliced forms of the latter, TcCDA2a and TcCDA2b, from the red flour beetle, Tribolium castaneum, were examined in different body parts using transmission EM and RNAi. Even though all TcCDAs are co-expressed in cuticle-forming cells from the hardened forewing (elytron) and ventral abdomen, as well as in the softer hindwing and dorsal abdomen, there are significant differences in the tissue specificity of expression of the alternatively spliced transcripts. Loss of either TcCDA1 or TcCDA2 protein by RNAi causes abnormalities in organization of chitinous horizontal laminae and vertical pore canals in all regions of the procuticle of both the hard and soft cuticles. Simultaneous RNAi for TcCDA1 and TcCDA2 produces the most serious abnormalities. RNAi of either TcCDA2a or TcCDA2b affects cuticle integrity to some extent. Following RNAi, there is accumulation of smaller disorganized fibers in both the horizontal laminae and pore canals, indicating that TcCDAs play a critical role in elongation/organization of smaller nanofibers into longer fibers, which is essential for structural integrity of both hard/thick and soft/thin cuticles. Immunolocalization of TcCDA1 and TcCDA2 proteins and effects of RNAi on their accumulation indicate that these two proteins function in concert exclusively in the assembly zone in a step involving the higher order organization of the procuticle.

Development and ultrastructure of the rigid dorsal and flexible ventral cuticles of the elytron of the red flour beetle, Tribolium castaneum

Insect exoskeletons are composed of the cuticle, a biomaterial primarily formed from the linear and relatively rigid polysaccharide, chitin, and structural proteins. This extracellular material serves both as a skin and skeleton, protecting insects from environmental stresses and mechanical damage. Despite its rather limited compositional palette, cuticles in different anatomical regions or developmental stages exhibit remarkably diverse physicochemical and mechanical properties because of differences in chemical composition, molecular interactions and morphological architecture of the various layers and sublayers throughout the cuticle including the envelope, epicuticle and procuticle (exocuticle and endocuticle). Even though the ultrastructure of the arthropod cuticle has been studied rather extensively, its temporal developmental pattern, in particular, the synchronous development of the functional layers in different cuticles during a molt, is not well understood. The beetle elytron, which is a highly modified and sclerotized forewing, offers excellent advantages for such a study because it can be easily isolated at precise time points during development. In this study, we describe the morphogenesis of the dorsal and ventral cuticles of the elytron of the red flour beetle, Tribolium castaneum, during the period from the 0 d-old pupa to the 9 d-old adult. The deposition of exocuticle and mesocuticle is substantially different in the two cuticles. The dorsal cuticle is four-fold thicker than the ventral. Unlike the ventral cuticle, the dorsal contains a thicker exocuticle consisting of a large number of horizontal laminae and vertical pore canals with pore canal fibers and rib-like veins and bristles as well as a mesocuticle, lying right above the enodcuticle. The degree of sclerotization appears to be much greater in the dorsal cuticle. All of these differences result in a relatively thick and tanned rigid dorsal cuticle and a much thinner and less pigmented membrane-like ventral cuticle.

Post-embryonic changes in the hindgut of honeybee Apis mellifera workers: Morphology, cuticle deposition, apoptosis, and cell proliferation

In insects, the hindgut is a homeostatic region of the digestive tract, divided into pylorus, ileum, and rectum, that reabsorbs water, ions, and small molecules produced during hemolymph filtration. The hindgut anatomy in bee larvae is different from that of adult workers. This study reports the morphological changes and cellular events that occur in the hindgut during the metamorphosis of the honeybee Apis mellifera. We describe the occurrence of autophagosomes and the ultrastructure of the epithelial cells and cuticle, suggesting that cuticular degradation begins in prepupae, with the cuticle being reabsorbed and recycled by autophagosomes in white- and pink-eyed pupae, followed by the deposition of new cuticle in light-brown-eyed pupae. In L5S larvae and prepupae, the hindgut undergoes cell proliferation in the anterior and posterior ends. In the pupae, the pylorus, ileum, and rectum regions are differentiated, and cell proliferation ceases in dark-brown-eyed pupae. Apoptosis occurs in the hindgut from the L5S larval to the pink-eyed pupal stage. In light-brown- and dark-brown-eyed pupae, the ileum epithelium changes from pseudostratified to simple only after the production of the basal lamina, whereas the rectal epithelium is always flattened. In black-eyed pupae, ileum epithelial cells have large vacuoles and subcuticular spaces, while in adult forager workers these cells have long invaginations in the cell apex and many mitochondria, indicating a role in the transport of compounds. Our findings show that hindgut morphogenesis is a dynamic process, with tissue remodeling and cellular events taking place for the formation of different regions of the organ, the reconstruction of a new cuticle, and the remodeling of visceral muscles.

An interommatidial exocrine gland with a "nail-headed" structure in the water strider Aquarius remigis (Hemiptera, Gerridae)

The fine structure of the interommatidial exocrine glands, found in the compound eyes of the water strider Aquarius remigis, is described using light, scanning, and transmission electron microscopy. The glandular pores of the glands are specialized into minute "nail-headed" structures (NS), which are described for the first time in arthropod compound eyes. Each NS is composed of two components: a rod-like stalk and a cup-like depression. The TEM study shows that the glands are class 3 epidermal glands as defined by Noirot and Quennedey (1974, 1991). Each gland consists of 3 cells: a gland cell, an intermediary cell, and a duct (canal) cell. The gland cell contains abundant electron-lucent vesicles, while the intermediary cell contains a large number of osmiophilic secretory granules. These two cells might secrete different substances which mix together in the dilated sac-like portion of the conducting canal before final release. The possible functions of the secretions released from these glands are discussed.

Exoskeletal cuticle differentiation during intramarsupial development of Porcellio scaber (Crustacea: Isopoda)

Exoskeletal crustacean cuticle is a calcified apical extracellular matrix of epidermal cells, illustrating the chitin-based organic scaffold for biomineralization. Studies of cuticle formation during molting reveal significant dynamics and complexity of the assembly processes, while cuticle formation during embryogenesis is poorly investigated. This study reveals in the terrestrial isopod Porcellio scaber, the ultrastructural organization of the differentiating precuticular matrices and exoskeletal cuticles during embryonic and larval intramarsupial development. The composition of the epidermal matrices was obtained by WGA lectin labelling and EDXS analysis. At least two precuticular matrices, consisting of loosely arranged material with overlying electron dense lamina, are secreted by the epidermis in the mid-stage embryo. The prehatching embryo is the earliest developmental stage with a cuticular matrix consisting of an epicuticle and a procuticle, displaying WGA binding and forming cuticular scales. In newly hatched marsupial larva manca, a new cuticle is formed and calcium sequestration in the cuticle is evident. Progression of larval development leads to the cuticle thickening, structural differentiation of cuticular layers and prominent cuticle calcification. Morphological characteristics of exoskeleton renewal in marsupial manca are described. Elaborated cuticle in marsupial larvae indicates the importance of the exoskeleton in protection and support of the larval body in the marsupium and during the release of larvae in the external environment.

Functional specialization among members of Knickkopf family of proteins in insect cuticle organization

Our recent study on the functional analysis of the Knickkopf protein from T. castaneum (TcKnk), indicated a novel role for this protein in protection of chitin from degradation by chitinases. Knk is also required for the laminar organization of chitin in the procuticle. During a bioinformatics search using this protein sequence as the query, we discovered the existence of a small family of three Knk-like genes (including the prototypical TcKnk) in the T. castaneum genome as well as in all insects with completed genome assemblies. The two additional Knk-like genes have been named TcKnk2 and TcKnk3. Further complexity arises as a result of alternative splicing and alternative polyadenylation of transcripts of TcKnk3, leading to the production of three transcripts (and by inference, three proteins) from this gene. These transcripts are named TcKnk3-Full Length (TcKnk3-FL), TcKnk3-5' and TcKnk3-3'. All three Knk-family genes appear to have essential and non-redundant functions. RNAi for TcKnk led to developmental arrest at every molt, while down-regulation of either TcKnk2 or one of the three TcKnk3 transcripts (TcKnk3-3') resulted in specific molting arrest only at the pharate adult stage. All three Knk genes appear to influence the total chitin content at the pharate adult stage, but to variable extents. While TcKnk contributes mostly to the stability and laminar organization of chitin in the elytral and body wall procuticles, proteins encoded by TcKnk2 and TcKnk3-3' transcripts appear to be required for the integrity of the body wall denticles and tracheal taenidia, but not the elytral and body wall procuticles. Thus, the three members of the Knk-family of proteins perform different essential functions in cuticle formation at different developmental stages and in different parts of the insect anatomy.

The pygidial defense gland system of the Steninae (Coleoptera, Staphylinidae): morphology, ultrastructure and evolution

The pygidial defense glands of the Steninae consist of two big (r1) and two smaller (r2) secretion filled sac-like reservoirs with associated secretory tissues and basal eversible membrane structures. The secretion is made up of deterrent and antimicrobial alkaloids stored in r1 as well as terpenes in r2. The gland cells filling r1 form a band shaped secretory tissue (g1) in an invagination of the reservoir membrane. The content of r2 is secreted by a tissue (g2) surrounding the efferent duct of r1 opposite to r2. In both gland tissues the secretion is produced in type IIIt gland cells and accumulates in an extracellular cavity surrounded by numerous microvilli of the gland cell membrane. After exocytosis the secretion enters an epicuticular duct and is transported to the corresponding reservoir via a conducting canal enclosed in at least one canal cell. While the structure of g1 is very similar in all species of the Steninae, g2 is often reduced. This reduction of the system r2/g2 is accompanied by a decreasing amount of terpenes in the total secretion and could be of interest for phylogenetic studies in the subfamily of the Steninae.

Knickkopf protein protects and organizes chitin in the newly synthesized insect exoskeleton

During each molting cycle of insect development, synthesis of new cuticle occurs concurrently with the partial degradation of the overlying old exoskeleton. Protection of the newly synthesized cuticle from molting fluid enzymes has long been attributed to the presence of an impermeable envelope layer that was thought to serve as a physical barrier, preventing molting fluid enzymes from accessing the new cuticle and thereby ensuring selective degradation of only the old one. In this study, using the red flour beetle, Tribolium castaneum, as a model insect species, we show that an entirely different and unexpected mechanism accounts for the selective action of chitinases and possibly other molting enzymes. The molting fluid enzyme chitinase, which degrades the matrix polysaccharide chitin, is not excluded from the newly synthesized cuticle as previously assumed. Instead, the new cuticle is protected from chitinase action by the T. castaneum Knickkopf (TcKnk) protein. TcKnk colocalizes with chitin in the new cuticle and organizes it into laminae. Down-regulation of TcKnk results in chitinase-dependent loss of chitin, severe molting defects, and lethality at all developmental stages. The conservation of Knickkopf across insect, crustacean, and nematode taxa suggests that its critical roles in the laminar ordering and protection of exoskeletal chitin may be common to all chitinous invertebrates.

Dynamic shape changes of ECM-producing cells drive morphogenesis of ball-and-socket joints in the fly leg

Animal body shape is framed by the skeleton, which is composed of extracellular matrix (ECM). Although how the body plan manifests in skeletal morphology has been studied intensively, cellular mechanisms that directly control skeletal ECM morphology remain elusive. In particular, how dynamic behaviors of ECM-secreting cells, such as shape changes and movements, contribute to ECM morphogenesis is unclear. Strict control of ECM morphology is crucial in the joints, where opposing sides of the skeleton must have precisely reciprocal shapes to fit each other. Here we found that, in the development of ball-and-socket joints in the Drosophila leg, the two sides of ECM form sequentially. We show that distinct cell populations produce the 'ball' and the 'socket', and that these cells undergo extensive shape changes while depositing ECM. We propose that shape changes of ECM-producing cells enable the sequential ECM formation to allow the morphological coupling of adjacent components. Our results highlight the importance of dynamic cell behaviors in precise shaping of skeletal ECM architecture.

The biology of the Gaucher cell: the cradle of human chitinases

Gaucher disease (GD) is the most common lysosomal storage disorder and is caused by inherited deficiencies of glucocerebrosidase, the enzyme responsible for the lysosomal breakdown of the lipid glucosylceramide. GD is characterized by the accumulation of pathological, lipid laden macrophages, so-called Gaucher cells. Following the development of enzyme replacement therapy for GD, the search for suitable surrogate disease markers resulted in the identification of a thousand-fold increased chitinase activity in plasma from symptomatic Gaucher patients and that decreases upon successful therapeutic intervention. Biochemical investigations identified a single enzyme, named chitotriosidase, to be responsible for this activity. Chitotriosidase was found to be an excellent marker for lipid laden macrophages in Gaucher patients and is now widely used to assist clinical management of patients. In the wake of the identification of chitotriosidase, the presence of other members of the chitinase family in mammals was discovered. Amongst these is AMCase, an enzyme recently implicated in the pathogenesis of asthma. Chitinases are omnipresent throughout nature and are also produced by vertebrates in which they play important roles in defence against chitin-containing pathogens and in food processing.

Insect chitin synthases: a review

Chitin is the most widespread amino polysaccharide in nature. The annual global amount of chitin is believed to be only one order of magnitude less than that of cellulose. It is a linear polymer composed of N-acetylglucosamines that are joined in a reaction catalyzed by the membrane-integral enzyme chitin synthase, a member of the family 2 of glycosyltransferases. The polymerization requires UDP-N-acetylglucosamines as a substrate and divalent cations as co-factors. Chitin formation can be divided into three distinct steps. In the first step, the enzymes' catalytic domain facing the cytoplasmic site forms the polymer. The second step involves the translocation of the nascent polymer across the membrane and its release into the extracellular space. The third step completes the process as single polymers spontaneously assemble to form crystalline microfibrils. In subsequent reactions the microfibrils combine with other sugars, proteins, glycoproteins and proteoglycans to form fungal septa and cell walls as well as arthropod cuticles and peritrophic matrices, notably in crustaceans and insects. In spite of the good effort by a hardy few, our present knowledge of the structure, topology and catalytic mechanism of chitin synthases is rather limited. Gaps remain in understanding chitin synthase biosynthesis, enzyme trafficking, regulation of enzyme activity, translocation of chitin chains across cell membranes, fibrillogenesis and the interaction of microfibrils with other components of the extracellular matrix. However, cumulating genomic data on chitin synthase genes and new experimental approaches allow increasingly clearer views of chitin synthase function and its regulation, and consequently chitin biosynthesis. In the present review, I will summarize recent advances in elucidating the structure, regulation and function of insect chitin synthases as they relate to what is known about fungal chitin synthases and other glycosyltransferases.

Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases

Chitin is one of the most important biopolymers in nature. It is mainly produced by fungi, arthropods and nematodes. In insects, it functions as scaffold material, supporting the cuticles of the epidermis and trachea as well as the peritrophic matrices lining the gut epithelium. Insect growth and morphogenesis are strictly dependent on the capability to remodel chitin-containing structures. For this purpose, insects repeatedly produce chitin synthases and chitinolytic enzymes in different tissues. Coordination of chitin synthesis and its degradation requires strict control of the participating enzymes during development. In this review, we will summarize recent advances in understanding chitin synthesis and its degradation in insects.

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.

Chitin synthesis and inhibition: a revisit

Chitin is an abundant biologically important aminopolysaccharide composed of N-acetyl-D-glucosamine units. Individual polymers, which are synthesized intracellularly by chitin synthase (CS), a membrane-bound glycosyl transferase, are translocated across the plasma membrane and coalesce to form rigid crystallites. These crystallites, inter alia, are integral parts of septa and cell walls in yeast and filamentous fungi, respectively, and of cuticles in invertebrates, notably crustaceans and insects. Despite decades of intensive research, many events associated with the complexity of chitin formation and deposition are still obscure, or only partially understood. The list includes the hormonal control of CS at the transcriptional and translational levels as well as the post-translational CS packaging; trafficking and guidance of CS clusters to proper sites in the cells and their intricate insertion into the plasma membranes; activation of the catalytic step and its control or modulation; and translocation of chitin chains across cell membranes, their orientation, fibrillogenesis and association with other extracellular structural components such as polysaccharides (fungi) and cuticular proteins (insects). Also the precise biochemical lesions inflicted by CS inhibitors, such as the acylurea insect growth regulators, are largely unclear. The recent isolation and sequencing of insect CS genes should help in elucidating various aspects of chitin biochemistry and inhibition. In particular, the large number of transmembrane segments, characteristic of the insect CS, are speculated to be involved in chitin translocation and are expected to shed light on the mode of action of acylurea insecticides.

A cuticular protein from the moulting stages of an insect

A 22 kDa peptide was purified from prepupal cuticles of 5th instar Calpodes ethlius caterpillars. It was absent earlier in the stadium and from the egg and adult, i.e. it is related to cuticle turnover rather than cuticle structure. It was present at larval and metamorphic moults, showing that it is related to moulting not just metamorphosis. The cDNA corresponding to the 22 kDa peptide was isolated by antibody screening of an epidermal cDNA expression library. Hybridization to Calpodes genomic DNA showed that the gene was present as a single copy. The deduced amino acid sequence is not like any of the sequences of cuticular structural proteins that have been published, but has a 47 amino acid sequence similar to bacteriophage T7 N-acetylmuramoyl-L-alanine amidase (34% identical, 51% similar). The amino acid sequence, the timing of expression in development, and the similarity between the substrate of the bacteriophage amidase and components of insect cuticle, all suggest that the 22 kDa protein may have a role in cleaving chitin-peptide bonds as a prerequisite for digestion of the cuticle by chitinases and proteases.

The cuticular localization of integument peptides from particular routing categories

The distribution of integument peptides in relation to chitin and structural features has been studied in the surface epidermis of the caterpillar of Calpodes ethlius by immunoblotting and immunogold labelling using antibodies prepared to peptides isolated from lamellate endocuticle or from hemolymph. The intermoult cuticle consists of an epicuticle, an endocuticle of many chitin containing lamellae, and a chitin containing assembly zone directly above the apical epidermal microvilli and the perimicrovillar space. During the intermoult, the epidermis secretes peptides constitutively, that is, secretory vesicles containing peptides exocytose without accumulating, traverse the perimicrovillar space and form lamellae in the assembly zone. At moulting, the epidermis deposits ecdysial droplets in addition. These interrupt the last few lamellae which later go on to become the perforated ecdysial membrane. The integument is involved with four routing classes of peptide. Secretion is apical into the cuticle (C), basal into the hemolymph (H), bidirectional (BD), or transported to the cuticle across the epidermis from the hemolymph (T). Some peptides change their routing at moulting. There are several patterns of localization. (1) C and BD cuticular peptides occur mainly in chitin containing lamellate cuticle. (2) Some are also present in epicuticle, and are therefore not obligatorily linked to chitin or matrix between chitin fibers. Cuticular peptides that also occur in the hemolymph are glycosylated, whereas most that are only secreted apically into the cuticle are not. All BD but few C peptides carry alpha-D-glucose/alpha-D-mannose. Some C and BD peptides carry N-acetyl glucosamine. (3) C36 extracted from cuticle has most N-acetyl glucosamine and colocalizes with chitin rather than the protein matrix. It is therefore probably the main link between chitin fibers and the matrix. (4) H235 is barely detectable at the apical cell surface during the intermoult but is abundant at moulting around and below the ecdysial droplets. (5) T66 occurs in intermoult lamellate cuticle. At moulting, alone among the peptides examined, it is in ecdysial droplets. Intermoult C and BD peptides are not in ecdysial droplets but continue to be present in the ecdysial membrane, suggesting that constitutive secretion is independent from the exocytosis of transported moult peptides. T66 differs from most hemolymph peptides in that it does not carry N-acetyl glucosamine or alpha-D-glucose/alpha-D-mannose. (6) Weakly reacting BD peptides (and some H peptides barely detectable in cuticle) localize near the apical surface. Their distribution therefore favours apical secretion and retrieval as a mechanism for basal secretion.

Preservation and contrast without osmication or section staining

Conventional treatment of tissues for sectioning and transmission electron microscopy uses aldehyde fixation and osmium tetroxide postfixation. Although the result is aesthetically pleasing, osmication destroys some cell components and reduces the chemical activity of others, such as reaction with antibodies and lectins. We have found that aldehyde fixation followed by uranyl acetate preserves and contrasts most structures and visualizes some that are not easily seen after osmication. Aldehyde/UA treated tissues have enough contrast to be observed without section staining while retaining some of the chemical activity that is lost through osmication. Sections of tissues with good preservation and contrast can be used for immunogold and lectin-gold labelling of at least some molecules.

Chitin synthesis and degradation as targets for pesticide action

Various pesticides are being used to destabilize, perturb, or inhibit crucial biochemical and physiological targets related to metabolism, growth, development, nervous communication, or behavior in pestiferous organisms. Chitin is an eukaryotic extracellular aminosugar biopolymer, massively produced by most fungal systems and by invertebrates, notably arthropods. Being an integral supportive component in fungal cell wall, insect cuticle, and nematode egg shell, chitin has been considered as a selective target for pesticide action. Throughout the elaborate processes of chitin formation and deposition, only the polymerization events associated with the cell membrane compartment are so far available for chemical interference. Currently, the actinomycetes-derived nucleoside peptide fungicides such as the polyoxins and the insecticidal benzoylaryl ureas have reached commercial pesticide status. The polyoxins and other structurally-related antibiotics like nikkomycins are strong competitive inhibitors of the polymerizing enzyme chitin synthase. The exact biochemical lesion inflicted by the benzoylaryl ureas is still elusive, but a post-polymerization event, such as translocation of chitin chains across the cell membrane, is suggested. Hydrolytic degradation of the chitin polymer is essential for hyphal growth, branching, and septum formation in fungal systems as well as for the normal molting of arthropods. Recently, insect chitinase activity was strongly and specifically suppressed by allosamidin, an actimomycetes-derived metabolite. In part, the defense mechanism in plants against invasion of pathogens is associated with induced chitinases. Chitin, chitosan, and their oligomers are able to act as elicitors which induce enhanced levels of chitinases in various plants. Lectins which bind to N-acetyl-D-glucosamine strongly interfere with fungal and insect chitin synthases. Plant lectins with similar properties may be involved in plant-pathogen interaction inter alia by suppressing fungal invasion.

Fine structure of a developing insect olfactory organ: morphogenesis of the silkmoth antenna

The olfactory organ of the silkmoth Antheraea polyphemus is the feathered antenna which carries about 70,000 olfactory sensilla in the male. It develops within 3 weeks from a leaf-shaped epidermal sac by means of segmental primary and secondary indentations which proceed from the periphery towards the centerline. During the first day post-apolysis, the antennal epidermis differentiates into segmentally arranged, alternating sensillogenic and non-sensillogenic regions. Within the first 2 days post-apolysis, the anlagen of olfactory sensilla arise from electron-dense mother cells in the sensillogenic epidermis. The axons of the developing sensilla begin to form the primary innervation pattern during the second day. The sensilla develop approximately within the first 10 days to their final shape, while the indentations are completed during the same period of time. The indentations are most probably driven by long basal extensions of epidermal cells, the epidermal feet. Primary indentations follow the course of segmentally arranged tracheal bundles and form the segments of the antenna. The secondary indentations follow the course of the primary segmental nerves which are reconstructed by this process. During the remaining time of development, the cuticle of the antenna and the sensory hairs is secreted by the epidermal and the hair-forming cells.

The pupal cuticle of Drosophila: biphasic synthesis of pupal cuticle proteins in vivo and in vitro in response to 20-hydroxyecdysone

We investigated the synthesis and localization of Drosophila pupal cuticle proteins by immunochemical techniques using both a complex antiserum and monoclonal antibodies. A set of low molecular weight (15,000-25,000) pupal cuticle proteins are synthesized by the imaginal disk epithelium before pupation. After pupation, synthesis of the low molecular weight proteins ceases and a set of unrelated high molecular weight proteins (40,000-82,000) are synthesized and incorporated into the pupal cuticle. Ultrastructural changes in the cuticle deposited before and after pupation correlate with the switch in cuticle protein synthesis. A similar biphasic accumulation of low and high molecular weight pupal cuticle proteins is also seen in imaginal discs cultured in vitro. The low molecular weight pupal cuticle proteins accumulate in response to a pulse of the insect steroid hormone 20-hydroxyecdysone and begin to appear 6 h after the withdrawal of the hormone from the culture medium. The high molecular weight pupal cuticle proteins accumulate later in culture; a second pulse of hormone appears to be necessary for the accumulation of two of these proteins.

Ultrastructural changes in the cuticle of the sheep blowfly, Lucilia, induced by certain insecticides and biological inhibitors

The ultrastructural effects on larval cuticle of Lucilia cuprina of two inhibitors of chitin synthesis, diflubenzuron and polyoxin D and an inhibitor of dihydrofolate reductase, aminopterin, are compared with those of the insecticide, cyromazine. Diflubenzuron and polyoxin D both prevent the formation of a normal lamellate appearance in procuticle and interfere with deposition of epicuticle. Aminopterin and cyromazine cause necrotic lesions in the cuticle which, in the case of cyromazine, are contiguous with invasive processes of epidermal cells. There is an accumulation of electron-dense granules in some epidermal cells in larvae poisoned with aminopterin or cyromazine. Aminopterin has a more drastic cytotoxic effect than cyromazine and it also interferes with the formation of epicuticle. The lesions produced by cyromazine treatment are not mimicked precisely by any of the other chemicals. However, there is closer accord between the effects of cyromazine and aminopterin than between cyromazine and the inhibitors of chitin formation.

Design of an insect cuticle associated with osmoregulation: the porous plates of chloride cells in a mayfly nymph

In mayfly nymphs of the genus Coloburiscoides, cell complexes with an osmoregulatory function (so-called chloride cells) are found in the integuments of the oral gills, the abdominal gills and gill filaments, the coxae and the thoracic sternites. The cuticle overlying each cell complex is a rigid circular plate which is known to be porous to colloidal lanthanum suspensions. The present study shows that the plate is composed only of the cuticulin and dense layers of the epicuticle. Both layers have substructures built of subunits on almost perfect hexagonal lattices. The lattice spacings are 53 and 9.5 nm for the dense layer and the cuticulin layer respectively. During moulting the apical plasma membrane of the chloride cell remains adpressed to the old porous plate. The new porous plate is formed from a new chloride cell which intrudes from the base of the integument. Throughout the moult small pores persist in the new and otherwise continuous cuticle to allow continuity of the cytoplasm of the apical and basal portions of the old chloride cell. It is thought that this phenomenon allows osmoregulatory function of the chloride cell complex to be maintained during the moult.

Very tight contact of tormogen cell membrane and sensillum cuticle: ultrastructural basis for high electrical resistance between receptor-lymph and subcuticular spaces in silkmoth olfactory hairs

A very tight contact is present between the apical membrane of the tormogen cell and the cuticle of the hair base in olfactory sensilla trichodea of Antheraea polyphemus. The contact zone is characterized by numerous hemidesmosome-like structures of the cell membrane, which closely attach the latter to the cuticle. If apically opened hairs are incubated in a LaCl3 solution, the tracer ions do not penetrate the contact zone. It is concluded that the tight contacts are the morphological correlates of the electrical isolation of the receptor-lymph space (cf. de Kramer et al., in press).

The dynamics of exoskeletal-epidermal structure during molt in juvenile lobster by electron microscopy and electron spectroscopic imaging

The exoskeletal-epidermal complex of juvenile lobsters at various stages throughout the molt cycle was examined by conventional electron microscopy, freeze-etch replicas, and electron spectroscopic imaging. This latter technique which enables the direct localization of atomic elements superimposed over morphological fine structure has been applied to this tissue complex to determine the spatial distributions and interrelationships of calcium, phosphorus, and sulphur. Chitin microfibril assembly is visualized in thin sections as occurring at the surface of apical membrane plaques which in freeze-etch replicas invariably possess a rich distribution of intramembrane particles on both P and E faces. In early stages of mineralization the exo- and endocuticular zones of the exoskeleton possess a dense Ca-containing lamellar repeat. These bands are unrelated to the helicoidal arrangement of chitin microfibrils. At later stages of development mineral deposits occur within the exocuticle and advance through to the endocuticle. These deposits align with chitin microfibrils and exhibit a helicoidal pattern. Morphological and chemical alterations associated with mineralization and demineralization of the exoskeleton are discussed.

Structure of the egg funiculus and deposition of embryonic envelopes in a crab

The newly laid egg of Carcinus maenas is attached to a maternal ovigerous seta by a funiculus which consists of the two superimposed vitelline envelopes 1a + 1b, highly stretched and concurrently showing important structural alterations. The funiculus is glued to the specialized seta merely owing to the strong adhesiveness of its external face comprising the outermost vitelline envelope 1a, without any added adhesive. The subjacent envelope 2, originated from the cortical reaction, is not involved in such a funiculus elaboration. In the course of the embryonic development, four new coatings are successively secreted from the ectodermal embryonic cells, underneath the (1a + 1b + 2) fertilization envelope or embryonic capsule. They will remain until hatching in this concentric order, thus giving evidence of successive embryonic moulting cycles, with apolysis but without exuviation. In addition, the successive secretory phases, regarding to the embryonic envelope elaborations, happen in presence of high concentrations of the ecdysteroid ponasterone A which might be involved consequently in such secretory processes.

Organization of the cuticle of an aquatic fly larva

The aquatic, apneustic larva of the midge, Chironomus riparius, has a very thin (up to 5 micrometers), readily deformable, post-cephalic cuticle. The ultrastructure of this cuticle from newly moulted and older final instar animals, and exuvia shed at pupation, has been examined using routine methods and also after the extraction of proteins with formamide and acetic acid. From the results described, and using established criteria, it is inferred that an exocuticle is present and represents about 25% of the thickness of the mature procuticle, the remainder being endocuticle. Therefore, it would seem that this exceptionally delicate cuticle conforms to the conventional plan of tanned or sclerotized solid cuticles, unlike those reported in the larvae of cyclorrhaphous dipterans such as blowflies or other soft-bodied insects. This is the first account, using experimental techniques, of the fine structure of sectioned cuticle from nematocerous dipteran larvae. It also indicates the value of the exuvium as a source of information about cuticle structure.

Oenocyte differentiation correlated with the formation of ectodermal coating in the embryo of a cockroach

The first signs of 'embryonic membrane' deposition could be observed at the 11th/12th stage of the embryonic development, while serosal apolysis occurs, and the first signs of oenocyte differentiation could be detected at the 15th stage. When pleuropodial cuticle deposition occurs, at the 16th stage, there is a rapid increase in the number of differentiating oenocytes. At the 19th stage there are some fully differentiated oenocytics, whereas, just before the cuticulin layer of the embryonic cuticle is laid down, another wave of oenocyte differentiation could be observed. The differentiation process of oenocytes and of vertebrate cells with a rapid cell membrane biogenesis (steroid secreting cells and hepatocytes) are compared. The correlation of oenocyte differentiation with ectodermal coating deposition, with molting hormone titer and with prothoracic gland differentiation is discussed.

Autoradiographic and fine structural study of chitin deposition in the cuticle of a barnacle using [3H]-D-glucosamine incorporation

[3H]-D-Glucosamine was injected into the rostral sinus of Balanus eburneus (barnacle) and the distribution of labelled chitin in the cuticle was studied with autoradiography and electron microscopy. When the pattern of labelling was examined in different body regions of the same organism where thickness of fully formed cuticle varied, it was observed that the rate of chitin deposition varied, being greater in thick than in thin regions. The density of Ag grains overlying cuticle was also greater in the thick regions. When the pattern of labelling was examined in regions of cuticle, comparable in thickness, taken from a series of organisms sacrificed at different time points a comparable value for the rate of chitin deposition was obtained. In addition, asynchrony in deposition of cuticle in different body regions of the same organism as well as uptake of the label by substances other than chitin, i.e. glycogen and glycoproteins were described.

The 'embryonic moults' of the milkweed bug as seen by the S.E.M

Deposition, detachment and removal of the three embryonic cuticles are studied. The membrane-like cuticle 1 covers the embryo during katatrepsis and 'disappears' thereafter. Cuticle 2 deposition starts shortly before dorsal closure. Its apolysis is accompanied by contractions of the embryo. Ecdysis of cuticle 2 takes place during hatching. Only cuticle 3 (= first larval cuticle) shows differentiations like sensilla and cornea. Peaks of ecdysteroid (and probably JH) titre are observed during apolysis of cuticle 1 and cuticle 2 (Dorn, 1981). Transition from ectoderm to epidermis proper takes place shortly before and during onset of cuticle 2 synthesis.