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

Stage- and sex-specific transcriptome analyses reveal distinctive sensory gene expression patterns in a butterfly

Background

Animal behavior is largely driven by the information that animals are able to extract and process from their environment. However, the function and organization of sensory systems often change throughout ontogeny, particularly in animals that undergo indirect development. As an initial step toward investigating these ontogenetic changes at the molecular level, we characterized the sensory gene repertoire and examined the expression profiles of genes linked to vision and chemosensation in two life stages of an insect that goes through metamorphosis, the butterfly Bicyclus anynana.

Results

Using RNA-seq, we compared gene expression in the heads of late fifth instar larvae and newly eclosed adults that were reared under identical conditions. Over 50 % of all expressed genes were differentially expressed between the two developmental stages, with 4,036 genes upregulated in larval heads and 4,348 genes upregulated in adult heads. In larvae, upregulated vision-related genes were biased toward those involved with eye development, while phototransduction genes dominated the vision genes that were upregulated in adults. Moreover, the majority of the chemosensory genes we identified in the B. anynana genome were differentially expressed between larvae and adults, several of which share homology with genes linked to pheromone detection, host plant recognition, and foraging in other species of Lepidoptera.

Conclusions

These results revealed promising candidates for furthering our understanding of sensory processing and behavior in the disparate developmental stages of butterflies and other animals that undergo metamorphosis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07819-4.

De novo genome assembly of the tobacco hornworm moth (Manduca sexta)

The tobacco hornworm, Manduca sexta, is a lepidopteran insect that is used extensively as a model system for studying insect biology, development, neuroscience, and immunity. However, current studies rely on the highly fragmented reference genome Msex_1.0, which was created using now-outdated technologies and is hindered by a variety of deficiencies and inaccuracies. We present a new reference genome for M. sexta, JHU_Msex_v1.0, applying a combination of modern technologies in a de novo assembly to increase continuity, accuracy, and completeness. The assembly is 470 Mb and is ∼20× more continuous than the original assembly, with scaffold N50 > 14 Mb. We annotated the assembly by lifting over existing annotations and supplementing with additional supporting RNA-based data for a total of 25,256 genes. The new reference assembly is accessible in annotated form for public use. We demonstrate that improved continuity of the M. sexta genome improves resequencing studies and benefits future research on M. sexta as a model organism.

Morphological changes and patterns of ecdysone receptor B1 immunolocalization in the anterior silk gland undergoing programmed cell death in the silkworm, Bombyx mori

The silk gland is a specific larval tissue of Lepidopteran insects and begins to degenerate shortly before pupation. The steroid hormone ecdysone triggers the stage specific programmed cell death of the anterior silk glands during metamorphosis in the silkworm, Bombyx mori. The anterior silk gland expresses ecdysone receptors, which are involved in regulation processes in response to ecdysone. In this study, the morphological changes, immunohistochemical localization and protein levels of ecdysone receptor B1 (EcR-B1) in the anterior silk gland of B. mori were investigated during programmed cell death. Morphological changes observed during the degeneration process involve the appearance of large vacuoles, probably autophagic vacuoles, which increase in number in pupal anterior silk glands. No macrophages were found in the silk gland during the prepupal and pupal stage unlike in apoptosis, which strongly suggests that programmed cell death of the anterior silk gland is carried out by autophagy. Morphological changes of the silk glands were accompanied by changes in the immunolocalization and protein levels of EcR-B1. The differences in tissue distribution and protein levels of EcR-B1 during the programmed cell death indicate that the receptor plays a major role in the modulation and function of ecdysone activity in Bombyx anterior silk glands. Our results indicate that EcR-B1 expression may be important for the process of programmed cell death in the anterior silk glands.

Detection of autophagy in cell death

Activation of autophagosomes in cell death has been described since the late 1950s as a form of cell death characterized by consumption of the bulk of the cytoplasm by lysosomal derivatives. However, it is not yet established that autophagy is a primary, causative mechanism of death rather than a response to initial problems. Methods to assess autophagic cell death are similar to those used to detect and measure autophagy, with further evidence that the affected cells are indeed dying. These methods include structural analysis using electron microscopy, examination of the activity of lysosomal enzymes, assessment of the number, size, and location of lysosomes by the uptake of fluorescent molecules; measurement of the activity of autophagy-related genes such as the cleavage and activation of LC3 by Western blotting or green fluorescent protein tagging; evaluation of the effects of inhibition of one or more lysosomal enzymes, inhibition of fusion of organelles, or inhibition of intercompartmental transfer of molecules by putatively specific inhibitors; and interference with cells in order to change the expression of components of the lysosomal system to study the effect of this change on autophagy and autophagic cell death.

Programmed cell death in flight muscle histolysis of the house cricket

We have characterized the process of flight muscle histolysis in the female house cricket, Acheta domesticus, through analysis of alterations of tissue wet weight, total protein content, and percent shortening of the dorsal longitudinal flight muscles (DLMs). Our objectives were to (1) define the normal course of histolysis in the cricket, (2) analyze the effects of juvenile hormone (JH) removal and replacement, (3) determine the effects of cycloheximide treatment, and (4) examine patterns of protein expression during histolysis. Our results suggest that flight muscle histolysis in the house cricket is an example of an active, developmentally regulated cell death program induced by an endocrine signal. Initial declines of total protein in DLMs indicated the JH signal that induced histolysis occurred by Day 2 and that histolysis was essentially complete by Day 3. Significant reductions in tissue weight and percent muscle shortening were observed in DLMs from Day 3 crickets. Cervical ligation of Day 1 crickets prevented histolysis but this inhibition could be reversed by continual topical treatments with methoprene (an active JH analog) although ligation of Day 2 crickets did not prevent histolysis. A requirement for active protein expression was demonstrated by analysis of synthesis block by cycloheximide and short-term incorporation of (35)S-methionine. Treatment with cycloheximide prevented histolysis. Autofluorographic imaging of DLM proteins separated by electrophoresis revealed apparent coordinated regulation of protein expression.

Cell death in the heart

The role of apoptosis in cardiac disease remains controversial. Much of the apoptosis detected, by chemical or molecular means, reflects inflammatory reaction and responding blood cells rather than myocytes, though their apoptosis in situ may exacerbate a bad situation, and their direct action against myocytes has not been excluded definitely. Myocyte apoptosis may reflect end-stage cardiac failure rather than causing it. If this is the case, then preventing apoptosis so that the cells can undergo necrosis does not accomplish much. Apoptosis is a consistent and important finding in many forms of cardiovascular disease. As determined by ultra-structure, apoptosis is common in cardiomyocytes, fibroblasts, vascular endothelial cells, and smooth muscle cells in cardiovascular disease of many origins. (62) Even though smooth muscle cells in atheromatous plaques appear to be necrotic,l it is likely that this is an evolved situation of apoptotic cells that were not removed. Given the prevalence of apoptotic processes in diseased heart and the very limited capacity of this organ to repair itself, (56) it is appropriate and justified to continue to explore the significance of apoptosis in cardiac disease and, above all, to explore the use of antiapoptotic agents in acute situations. Researchers must pay explicit attention to how they document cell death and in what tissues or cells it occurs. Otherwise, clinicians risk being deluded by preservation of morphology in nonfunctional cells and by confusion of what happened and where death occurred in the sequence of causality. Cell death in the heart is a matter of substantial theoretical and practical concern. A major problem in analyzing it is that, although apoptosis may be demonstrated easily in myocytes, particularly embryonic myocytes, under conditions of culture, interpretation is much more complex in an intact organ. The first issue is one of timing. In situations of severe, acute loss of cells, such as in an infarct, apoptotic cells may not be cleared rapidly and may progress to a more oncotic or necrotic morphology. Second, in situations of inflammation, biochemical or molecular techniques may confound apoptosis of inflammatory cells with apoptosis of myocytes. Third, priorities in the sequence of apoptosis differ between large, generally nonmitotic cells with massive cytoplasm (as differentiated myocytes) and small mitotic cells in culture, which usually are studied. The appearance and many markers of physiological cell death may differ from the most widely recognized forms of apoptosis, including late collapse of the nucleus and primacy of lysosomal or other proteases as opposed to caspases. Investigators should always strive to establish multiple criteria for apoptosis, with good documentation of timing and cell type. When these factors are taken into consideration, it seems that aggressive action against apoptosis may be of value in acute situations, such as infarct, in which buying short increments of time may reduce damage. In more chronic situations, much of the apoptosis detected derives from invading lymphocytes, mast cells, or other cells relating to inflammation. The apoptosis of these cells may exacerbate an already difficult situation, and intervention may prove of value. Otherwise, apoptosis of myocytes is more typically an end-stage situation, and it is more fruitful to alleviate the problem before this stage is reached.

Protein synthesis, DNA degradation, and morphological changes during programmed cell death in labial glands of Manduca sexta

Labial glands of the tobacco hornworm Manduca sexta (Lepidoptera: Sphingiidae, homologues of Drosophila salivary glands, undergo programmed cell death (PCD) in a 4-day period during larva-to-pupa metamorphosis. The programmed death of the labial gland was examined by electron microscopy and measurement of protein synthesis as well as measurement of DNA synthesis, end-labeling of single strand breaks, and pulsed-field gel electrophoresis. One of the earliest changes observed is a sharp drop in synthesis of most proteins, coupled with synthesis of a glycine-rich protein, reminiscent of silk-like proteins. From a morphological standpoint, during the earliest phases the most prominent changes are the formation of small autophagic vacuoles containing ribosomes and an apparent focal dissolution of the membranes of the endoplasmic reticulum, whereas later changes include differing destruction at the lumenal and basal surfaces of the cell and erosion of the basement membrane. By the fourth day of metamorphosis, individual cells become rapidly vacuolated in a cell-independent manner. In the vacuolated cells on day 3, chromatin begins to coalesce. It is at this period that unequivocal nucleosomal ladders are seen and end-labeling in situ or electrophoretic techniques document single on double-strand breaks, respectively. DNA synthesis ceases shortly after the molt to the fifth instar, as detected by incorporation of tritiated thymidine and weak TUNEL labeling. Large size fragments of DNA are seen shortly after DNA synthesis ceases and thence throughout the instor, raising the possibility of potential limitations built into the cells before their final collapse.

Induction and maintenance of autonomous flock house virus RNA1 replication

The nodavirus flock house virus (FHV) has a bipartite, positive-sense, RNA genome that encodes the catalytic subunit of the RNA replicase and the viral capsid protein precursor on separate genomic segments (RNA1 and RNA2, respectively). RNA1 can replicate autonomously when transfected into permissive cells, allowing study of the kinetics of RNA1 replication in the absence of either RNA2 or capsid proteins. However, RNA1 replication ceases ca. 3 days after transfection despite the presence of replication-competent RNA. We examined this inhibition by inducing the expression of RNA1 in cells from a cDNA copy that was under the control of a hormone-regulated RNA polymerase II promoter. This system reproduced the shutoff of RNA replication when DNA-templated primary transcription was turned off. Continued primary transcription partially alleviated the shutoff and maintained the rate of RNA replication for several days at a steady-state level approximately one-third that of the peak rate. After shutoff, RNA replication could be restored by transferring the resulting intracellular RNA to fresh cells or by reinducing primary transcription, indicating that cessation of replication occurred despite the competence of both the viral RNA and the cytoplasmic environment. These data suggest that there is a mechanism by which replication is shut off at late times after transfection, which may reflect the natural endpoint of the replicative cycle.

Signaling in unicellular eukaryotes

Aspects of intercellular and intracellular signaling systems in cell survival, proliferation, differentiation, chemosensory behavior, and programmed cell death in free-living unicellular eukaryotes have been reviewed. Comparisons have been made with both bacteria and metazoa. The central organisms were flagellates (Trypanosoma, Leishmania, and Crithidia), slime molds (Dictyostelium), yeast cells (Saccharomyces cerevisiae), and ciliates (Paramecium, Euplotes, and Tetrahymena). There are two novel aspects in this review. First, cellular responses are viewed in an evolutionary perspective, rather than from the more prevailing one, in which the unicellular eukaryotes are seen by the mammalian organisms. Second, results obtained with cell cultures in minimal, chemically defined nutrient media at low cell densities where intercellular signaling is strongly reduced are discussed. These results shed light on control mechanisms and their cooperation inside the living cell. Intracellular systems have many common features in unicellular and multicellular organisms.

DAP-5, a novel homolog of eukaryotic translation initiation factor 4G isolated as a putative modulator of gamma interferon-induced programmed cell death

A functional approach to gene cloning was applied to HeLa cells in an attempt to isolate cDNA fragments which convey resistance to gamma interferon (IFN-gamma)-induced programmed cell death. One of the rescued cDNAs, described in this work, was a fragment of a novel gene, named DAP-5. Analysis of a DAP-5 full-length cDNA clone revealed that it codes for a 97-kDa protein that is highly homologous to eukaryotic translation initiation factor 4G (eIF4G, also known as p220). According to its deduced amino acid sequence, this novel protein lacks the N-terminal region of eIF4G responsible for association with the cap binding protein eIF4E. The N-terminal part of DAP-5 has 39% identity and 63% similarity to the central region of mammalian p220. Its C-terminal part is less homologous to the corresponding region of p220, suggesting that it may possess unique functional properties. The rescued DAP-5 cDNA fragment which conveyed resistance to IFN-gamma-induced cell death was expressed from the vector in the sense orientation. Intriguingly, it comprised part of the coding region which corresponds to the less conserved C-terminal part of DAP-5 and directed the synthesis of a 28-kDa miniprotein. The miniprotein exerted a dual effect on HeLa cells. Low levels of expression protected the cells from IFN-gamma-induced programmed cell death, while high levels of expression were not compatible with continuous cell growth. The relevance of DAP-5 protein to possible changes in a cell's translational machinery during programmed cell death and growth arrest is discussed.

Involvement of cellular proteolytic machinery in apoptosis

Programmed cell death (PCD), a genetically controlled cell deletion process, plays an important role in the regulation of cellular and tissue homeostasis. The requisite for proteolysis during PCD-induced apoptosis is well documented. The cellular proteolytic machinery includes numerous proteases localized in membranes, cytoplasm, and nucleus. This machinery may function to remove denatured or misfolded protein from the cytoplasm on a routine basis and may also cleave proteins thereby implementing their activation. The well established role of some proteases is to maintain fundamental cellular processes; however, the precise cellular location and function of other proteases which make a contribution to a unique unidirectional process such as apoptosis remains unclear. The functional overlap between 'scheduled' and 'unscheduled' proteolysis may potentially lead to confusion in this research area. In this review we will discuss certain cellular proteolytic systems and highlight the possible involvement of each in apoptosis.

Programmed cell death of the prothoracic glands of Manduca sexta during pupal-adult metamorphosis

The degeneration of the prothoracic glands of Manduca sexta during pupal-adult metamorphosis was analyzed by both light and transmission electron microscopy, and correlated with a functional analysis of the ability of the prothoracic glands to synthesize ecdysteroids in vitro. Apoptosis of degenerating glandular cells was also examined by in situ indirect immunofluorescein detection of digoxigenin-labeled genomic DNA cleavage (the TUNEL method). The ability of the prothoracic gland to synthesize ecdysteroids reached a peak 4 days after pupation, decreased drastically on day 6 and then decreased further to the basal level 2 days later. Prothoracic gland degeneration was initiated on day 6 and was characterized by nuclear condensation, cytoplasmic budding, giant autophagic vacuoles, the disappearance of the smooth endoplasmic reticulum and intercellular channels, and the fragmentation of the cytoplasm into membrane-bound bodies. The cell debris of these degenerating cells was then engulfed by numerous phagocytic hemocytes. The results of the analysis of apoptosis by immunofluorescence detection are in complete accord with the histological, ultrastructural and radioimmunoassay data, suggesting that apoptosis is the basic mechanism for programmed cell death of the cells comprising these vital glands.

Evidence for transcriptional modulation but not acid phosphatase expression during programmed cell death in the colonial tunicate Botryllus schlosseri

Botryllus schlosseri is a clonally modular ascidian in which asexually derived adults (zooids) exhibit developmental synchrony. At the conclusion of the blastogenic (asexual) cycle every 5 days at 21 degrees C, all zooids within a colony die simultaneously in 24 hours and are replaced by a new asexual generation of zooids. This cyclical process, called takeover, involves the selective destruction of the zooid's visceral tissues which include the pharynx, esophagus, stomach, intestine, endostyle, neural complex and heart, whereas bud tissues and mesenchymal components (muscle and blood cells) remain unaffected. Ultrastructural analysis indicates that the most prevalent form of cell death occurs by apoptosis, although necrotic changes are also observed in several tissues (i.e., stomach and intestine). Blood-derived macrophages and neighboring cells subsequently engulf visceral tissues, reducing the zooid to the size of a small vesicle. Here, we have tested the possibility that acid phosphatase, a hydrolase whose presence is associated with cell death in several invertebrate systems, could account for some of the regressive changes observed during takeover. Our observations indicate that acid phosphatase (AP) activity was selectively localized in the gut of parent zooids during the growth phase of the cycle, with the stomach exhibiting the most intense histochemical staining on tissue sections. As zooid regression progressed during takeover, stomach AP staining gradually disappeared. Other visceral tissues never became AP-positive. Therefore, this hydrolase appears to play a minimal role in zooid death. In order to characterize genes whose expression pattern was selectively altered during takeover, we have carried out differential mRNA display analysis. We report on two genes, 790.3 and 790.4, that are down- and upregulated, respectively, during this process. Collectively, these findings indicate that the takeover phase of blastogenesis in Botryllus involves modulated gene expression.