EGFR signaling is required for re-establishing the proximodistal axis during distal leg regeneration in the cricket Gryllus bimaculatus nymph

Two transcriptional cascades orchestrate cockroach leg regeneration

The mystery of appendage regeneration has fascinated humans for centuries, while the underlying regulatory mechanisms remain unclear. In this study, we establish a transcriptional landscape of regenerating leg in the American cockroach, Periplaneta americana, an ideal model in appendage regeneration studies showing remarkable regeneration capacity. Through a large-scale in vivo screening, we identify multiple signaling pathways and transcription factors controlling leg regeneration. Specifically, zfh-2 and bowl contribute to blastema cell proliferation and morphogenesis in two transcriptional cascades: bone morphogenetic protein (BMP)/JAK-STAT-zfh-2-B-H2 and Notch-drm/bowl-bab1. Notably, we find zfh-2 is working as a direct target of BMP signaling to promote cell proliferation in the blastema. These mechanisms might be conserved in the appendage regeneration of vertebrates from an evolutionary perspective. Overall, our findings reveal that two crucial transcriptional cascades orchestrate distinct cockroach leg regeneration processes, significantly advancing the comprehension of molecular mechanism in appendage regeneration.

ERK-activated CK-2 triggers blastema formation during appendage regeneration

Appendage regeneration relies on the formation of blastema, a heterogeneous cellular structure formed at the injury site. However, little is known about the early injury-activated signaling pathways that trigger blastema formation during appendage regeneration. Here, we provide compelling evidence that the extracellular signal-regulated kinase (ERK)-activated casein kinase 2 (CK-2), which has not been previously implicated in appendage regeneration, triggers blastema formation during leg regeneration in the American cockroach, Periplaneta americana. After amputation, CK-2 undergoes rapid activation through ERK-induced phosphorylation within blastema cells. RNAi knockdown of CK-2 severely impairs blastema formation by repressing cell proliferation through down-regulating mitosis-related genes. Evolutionarily, the regenerative role of CK-2 is conserved in zebrafish caudal fin regeneration via promoting blastema cell proliferation. Together, we find and demonstrate that the ERK-activated CK-2 triggers blastema formation in both cockroach and zebrafish, helping explore initiation factors during appendage regeneration.

Molecular aspects of regeneration in insects

Regeneration is a fascinating phenomenon observed in various organisms across the animal kingdom. Different orders of class Insecta are reported to possess comprehensive regeneration abilities. Several signalling molecules, such as morphogens, growth factors, and others trigger a cascade of events that promote wound healing, blastema formation, growth, and repatterning. Furthermore, epigenetic regulation has emerged as a critical player in regulating the process of regeneration. This report highlights the major breakthrough research on wound healing and tissue regeneration. Exploring and reviewing the molecular basis of regeneration can be helpful in the area of regenerative medicine advancements. The understanding gathered from this framework can potentially contribute to hypothesis designing with implications in the field of synthetic biology and human health.

Physiological and molecular mechanisms of insect appendage regeneration

Regeneration, as a fascinating scientific field, refers to the ability of animals replacing lost tissue or body parts. Many metazoan organisms have been reported with the regeneration phenomena, but showing evolutionarily variable abilities. As the most diverse metazoan taxon, hundreds of insects show strong appendage regeneration ability. The regeneration process and ability are dependent on many factors, including macroscopic physiological conditions and microscopic molecular mechanisms. This article reviews research progress on the physiological conditions and internal underlying mechanisms controlling appendage regeneration in insects.

Toll signalling promotes blastema cell proliferation during cricket leg regeneration via insect macrophages

Hemimetabolous insects, such as the two-spotted cricket Gryllus bimaculatus, can recover lost tissues, in contrast to the limited regenerative abilities of human tissues. Following cricket leg amputation, the wound surface is covered by the wound epidermis, and plasmatocytes, which are insect macrophages, accumulate in the wound region. Here, we studied the function of Toll-related molecules identified by comparative RNA sequencing during leg regeneration. Of the 11 Toll genes in the Gryllus genome, expression of Toll2-1, Toll2-2 and Toll2-5 was upregulated during regeneration. RNA interference (RNAi) of Toll, Toll2-1, Toll2-2, Toll2-3 or Toll2-4 produced regeneration defects in more than 50% of crickets. RNAi of Toll2-2 led to a decrease in the ratio of S- and M-phase cells, reduced expression of JAK/STAT signalling genes, and reduced accumulation of plasmatocytes in the blastema. Depletion of plasmatocytes in crickets using clodronate also produced regeneration defects, as well as fewer proliferating cells in the regenerating legs. Plasmatocyte depletion also downregulated the expression of Toll and JAK/STAT signalling genes in the regenerating legs. These results suggest that Spz-Toll-related signalling in plasmatocytes promotes leg regeneration through blastema cell proliferation by regulating the Upd-JAK/STAT signalling pathway.

The genome of a daddy-long-legs (Opiliones) illuminates the evolution of arachnid appendages

Chelicerate arthropods exhibit dynamic genome evolution, with ancient whole-genome duplication (WGD) events affecting several orders. Yet, genomes remain unavailable for a number of poorly studied orders, such as Opiliones (daddy-long-legs), which has hindered comparative study. We assembled the first harvestman draft genome for the species Phalangium opilio, which bears elongate, prehensile appendages, made possible by numerous distal articles called tarsomeres. Here, we show that the genome of P. opilio exhibits a single Hox cluster and no evidence of WGD. To investigate the developmental genetic basis for the quintessential trait of this group-the elongate legs-we interrogated the function of the Hox genes Deformed (Dfd) and Sex combs reduced (Scr), and a homologue of Epidermal growth factor receptor (Egfr). Knockdown of Dfd incurred homeotic transformation of two pairs of legs into pedipalps, with dramatic shortening of leg segments in the longest leg pair, whereas homeosis in L3 is only achieved upon double Dfd + Scr knockdown. Knockdown of Egfr incurred shortened appendages and the loss of tarsomeres. The similarity of Egfr loss-of-function phenotypic spectra in insects and this arachnid suggest that repeated cooption of EGFR signalling underlies the independent gains of supernumerary tarsomeres across the arthropod tree of life.

Hedgehog signaling regulates regenerative patterning and growth in Harmonia axyridis leg

Appendage regeneration has been widely studied in many species. Compared to other animal models, Harmonia axyridis has the advantage of a short life cycle, is easily reared, has strong regeneration capacity and contains systemic RNAi, making it a model organism for research on appendage regeneration. Here, we performed transcriptome analysis, followed by gene functional assays to reveal the molecular mechanism of H. axyridis leg regenerative growth process. Signaling pathways including Decapentaplegic (Dpp), Wingless (Wg), Ds/Ft/Hippo, Notch, Egfr, and Hedgehog (Hh) were all upregulated during the leg regenerative patterning and growth. Among these, Hh and its auxiliary receptor Lrp2 were required for the proper patterning and growth of the regenerative leg. The targets of canonical Hh signaling were required for the regenerative growth which contributes to the leg length, but were not essential for the pattern formation of the regenerative leg. dpp, wg and leg developmental-related genes including rn, dac and Dll were all regulated by hh and lrp2 and may play an essential role in the regenerative patterning of the leg.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

The Cricket Gryllus bimaculatus: Techniques for Quantitative and Functional Genetic Analyses of Cricket Biology

All extant species are an outcome of nature's "experiments" during evolution, and hence multiple species need to be studied and compared to gain a thorough understanding of evolutionary processes. The field of evolutionary developmental biology (evo-devo) aspires to expand the number of species studied, because most functional genetic studies in animals have been limited to a small number of "traditional" model organisms, many of which belong to the same phylum (Chordata). The phylum Arthropoda, and particularly its component class Insecta, possesses many important characteristics that are considered favorable and attractive for evo-devo research, including an astonishing diversity of extant species and a wide disparity in body plans. The development of the most thoroughly investigated insect genetic model system to date, the fruit fly Drosophila melanogaster (a holometabolous insect), appears highly derived with respect to other insects and indeed with respect to most arthropods. In comparison, crickets (a basally branching hemimetabolous insect lineage compared to the Holometabola) are thought to embody many developmental features that make them more representative of insects. Here we focus on crickets as emerging models to study problems in a wide range of biological areas and summarize the currently available molecular, genomic, forward and reverse genetic, imaging and computational tool kit that has been established or adapted for cricket research. With an emphasis on the cricket species Gryllus bimaculatus, we highlight recent efforts made by the scientific community in establishing this species as a laboratory model for cellular biology and developmental genetics. This broad toolkit has the potential to accelerate many traditional areas of cricket research, including studies of adaptation, evolution, neuroethology, physiology, endocrinology, regeneration, and reproductive behavior. It may also help to establish newer areas, for example, the use of crickets as animal infection model systems and human food sources.

Bone morphogenetic protein signaling in distal patterning and intercalation during leg regeneration of the cricket, Gryllus bimaculatus

The cricket, Gryllus bimaculatus, is a classic model of leg regeneration following amputation. We previously demonstrated that Gryllus decapentaplegic (Gb'dpp) is expressed during leg regeneration, although it remains unclear whether it is essential for this process. In this study, double-stranded RNA targeting the Smad mathers-against-dpp homolog, Gb'mad, was used to examine the role of bone morphogenetic protein (BMP) signaling in the leg regeneration process of Gryllus bimaculatus. RNA interference (RNAi)-mediated knockdown of Gb'mad led to a loss of tarsus regeneration at the most distal region of regenerating leg segments. Moreover, we confirmed that the phenotype obtained by knockdown of Dpp type I receptor, Thick veins (Gb'tkv), closely resembled that observed for Gb'mad RNAi crickets, thereby suggesting that the BMP signaling pathway is indispensable for the initial stages of tarsus formation. Interestingly, knockdown of Gb'mad and Gb'tkv resulted in significant elongation of regenerating tibia along the proximodistal axis compared with normal legs. Moreover, our findings indicate that during the regeneration of tibia, the BMP signaling pathway interacts with Dachsous/Fat (Gb'Ds/Gb'Ft) signaling and dachshund (Gb'dac) to re-establish positional information and regulate determination of leg size. Based on these observations, we discuss possible roles for Gb'mad in the distal patterning and intercalation processes during leg regeneration in Gryllus bimaculatus.

The role of the EGFR signaling pathway in stem cell differentiation during planarian regeneration and homeostasis

Cell signaling is essential for cells to adequately respond to their environment. One of the most evolutionarily conserved signaling pathways is that of the epidermal growth factor receptor (EGFR). Transmembrane receptors with intracellular tyrosine kinase activity are activated by the binding of their corresponding ligands. This in turn activates a wide variety of intracellular cascades and induces the up- or downregulation of target genes, leading to a specific cellular response. Freshwater planarians are an excellent model in which to study the role of cell signaling in the context of stem-cell based regeneration. Owing to the presence of a population of pluripotent stem cells called neoblasts, these animals can regenerate the entire organism from a tiny piece of the body. Here, we review the current state of knowledge of the planarian EGFR pathway. We describe the main components of the pathway and their functions in other animals, and focus in particular on receptors and ligands identified in the planarian Schmidtea mediterranea. Moreover, we summarize current data on the function of some of these components during planarian regeneration and homeostasis. We hypothesize that the EGFR pathway may act as a key regulator of the terminal differentiation of distinct populations of lineage-committed progenitors.

Insect RNAi: Integrating a New Tool in the Crop Protection Toolkit

Protecting crops against insect pests is a major focus area in crop protection. Over the past two decades, biotechnological interventions, especially Bt proteins, have been successfully implemented across the world and have had major impacts on reducing chemical pesticide applications. As insects continue to adapt to insecticides, both chemical and protein-based, new methods, molecules, and modes of action are necessary to provide sustainable solutions. RNA interference (RNAi) has emerged as a significant tool to knock down or alter gene expression profiles in a species-specific manner. In the past decade, there has been intense research on RNAi applications in crop protection. This chapter looks at the current state of knowledge in the field and outlines the methodology, delivery methods, and precautions required in designing targets. Assessing the targeting of specific gene expression is also an important part of a successful RNAi strategy. The current literature on the use of RNAi in major orders of insect pests is reviewed, along with a perspective on the regulatory aspects of the approach. Risk assessment of RNAi would focus on molecular characterization, food/feed risk assessment, and environmental risk assessment. As more RNAi-based products come through regulatory systems, either via direct application or plant expression based, the impact of this approach on crop protection will become clearer.

The Drosophila Duox maturation factor is a key component of a positive feedback loop that sustains regeneration signaling

Regenerating tissue must initiate the signaling that drives regenerative growth, and sustain that signaling long enough for regeneration to complete. How these key signals are sustained is unclear. To gain a comprehensive view of the changes in gene expression that occur during regeneration, we performed whole-genome mRNAseq of actively regenerating tissue from damaged Drosophila wing imaginal discs. We used genetic tools to ablate the wing primordium to induce regeneration, and carried out transcriptional profiling of the regeneration blastema by fluorescently labeling and sorting the blastema cells, thus identifying differentially expressed genes. Importantly, by using genetic mutants of several of these differentially expressed genes we have confirmed that they have roles in regeneration. Using this approach, we show that high expression of the gene moladietz (mol), which encodes the Duox-maturation factor NIP, is required during regeneration to produce reactive oxygen species (ROS), which in turn sustain JNK signaling during regeneration. We also show that JNK signaling upregulates mol expression, thereby activating a positive feedback signal that ensures the prolonged JNK activation required for regenerative growth. Thus, by whole-genome transcriptional profiling of regenerating tissue we have identified a positive feedback loop that regulates the extent of regenerative growth.

Regenerating tissue must initiate the signaling that drives regenerative growth, and then sustain that signaling long enough for regeneration to complete. Drosophila imaginal discs, the epithelial structures in the larva that will form the adult animal during metamorphosis, have been an important model system for tissue repair and regeneration for over 60 years. Here we show that damage-induced JNK signaling leads to the upregulation of a gene called moladietz, which encodes a co-factor for an enzyme, NADPH dual oxidase (Duox), that generates reactive oxygen species (ROS), a key tissue-damage signal. High expression of moladietz induces continuous production of ROS in the regenerating tissue. The sustained production of ROS then continues to activate JNK signaling throughout the course of regeneration, ensuring maximal tissue regrowth.

Gene expression reveals evidence for EGFR-dependent proximal-distal limb patterning in a myriapod

Evolution of segmented limbs is one of the key innovations of Arthropoda, allowing development of functionally specific specialized head and trunk appendages, a major factor behind their unmatched evolutionary success. Proximodistal limb patterning is controlled by two regulatory networks in the vinegar fly Drosophila melanogaster, and other insects. The first is represented by the function of the morphogens Wingless (Wg) and Decapentaplegic (Dpp); the second by the EGFR-signaling cascade. While the role of Wg and Dpp has been studied in a wide range of arthropods representing all main branches, that is, Pancrustacea (= Hexapoda + Crustacea), Myriapoda and Chelicerata, investigation of the potential role of EGFR-signaling is restricted to insects (Hexapoda). Gene expression analysis of Egfr, its potential ligands, and putative downstream factors in the pill millipede Glomeris marginata (Myriapoda: Diplopoda), reveals that-in at least mandibulate arthropods-EGFR-signaling is likely a conserved regulatory mechanism in proximodistal limb patterning.

Comparison of Leg Regeneration Potency Between Holometabolous Helicoverpa armigera (Lepidoptera: Noctuidae) and Hemimetabolous Locusta migratoria manilensis (Orthoptera: Acrididae)

After injury many insects could regenerate lost limb. In this study, Helicoverpa armigera Hubner and Locusta migratoria manilensis (Meyen, 1835) were chosen to compare the regeneration potency of holometabolous and hemimetabolous insects. We employed the classical approach of surgical excision to verify the regeneration ability and to investigate the factors that affect the extent of regeneration. The results found that H. armigera could regenerate intact legs when the larval legs were excised at the first and second instar and that legs of adult H. armigera had a close relationship with their larval counterparts. However, the adult legs became malformed or disappeared when excised at other older instars. For the L. migratoria, we found the legs have weak partial regeneration ability when amputation was conducted at the joint of two segments. The regeneration potency might be stronger the more proximal the operation. Regeneration process had a negative impact on the larval development. This is the first report of complete leg regeneration capacity having a strong correlation with the instar but not with the position where amputation occurred for H. armigera, while for the L. migratoria, partial regenerative ability had a close relationship with the position where amputation occurred but not with instars.

Short germ insects utilize both the ancestral and derived mode of Polycomb group-mediated epigenetic silencing of Hox genes

In insect species that undergo long germ segmentation, such as Drosophila, all segments are specified simultaneously at the early blastoderm stage. As embryogenesis progresses, the expression boundaries of Hox genes are established by repression of gap genes, which is subsequently replaced by Polycomb group (PcG) silencing. At present, however, it is not known whether patterning occurs this way in a more ancestral (short germ) mode of embryogenesis, where segments are added gradually during posterior elongation. In this study, two members of the PcG family, Enhancer of zeste (E(z)) and Suppressor of zeste 12 (Su(z)12), were analyzed in the short germ cricket, Gryllus bimaculatus. Results suggest that although stepwise negative regulation by gap and PcG genes is present in anterior members of the Hox cluster, it does not account for regulation of two posterior Hox genes, abdominal-A (abd-A) and Abdominal-B (Abd-B). Instead, abd-A and Abd-B are predominantly regulated by PcG genes, which is the mode present in vertebrates. These findings suggest that an intriguing transition of the PcG-mediated silencing of Hox genes may have occurred during animal evolution. The ancestral bilaterian state may have resembled the current vertebrate mode of regulation, where PcG-mediated silencing of Hox genes occurs before their expression is initiated and is responsible for the establishment of individual expression domains. Then, during insect evolution, the repression by transcription factors may have been acquired in anterior Hox genes of short germ insects, while PcG silencing was maintained in posterior Hox genes.

RNA interference tools for the western flower thrips, Frankliniella occidentalis

The insect order Thysanoptera is exclusively comprised of small insects commonly known as thrips. The western flower thrips, Frankliniella occidentalis, is an economically important pest amongst thysanopterans due to extensive feeding damage and tospovirus transmission to hundreds of plant species worldwide. Geographically-distinct populations of F. occidentalis have developed resistance against many types of traditional chemical insecticides, and as such, management of thrips and tospoviruses are a persistent challenge in agriculture. Molecular methods for defining the role(s) of specific genes in thrips-tospovirus interactions and for assessing their potential as gene targets in thrips management strategies is currently lacking. The goal of this work was to develop an RNA interference (RNAi) tool that enables functional genomic assays and to evaluate RNAi for its potential as a biologically-based approach for controlling F. occidentalis. Using a microinjection system, we delivered double-stranded RNA (dsRNA) directly to the hemocoel of female thrips to target the vacuolar ATP synthase subunit B (V-ATPase-B) gene of F. occidentalis. Gene expression analysis using real-time quantitative reverse transcriptase-PCR (qRT-PCR) revealed significant reductions of V-ATPase-B transcripts at 2 and 3 days post-injection (dpi) with dsRNA of V-ATPase-B compared to injection with dsRNA of GFP. Furthermore, the effect of knockdown of the V-ATPase-B gene in females at these two time points was mirrored by the decreased abundance of V-ATPase-B protein as determined by quantitative analysis of Western blots. Reduction in V-ATPase-B expression in thrips resulted in increased female mortality and reduced fertility, i.e., number of viable offspring produced. Survivorship decreased significantly by six dpi compared to the dsRNA-GFP control group, which continued decreasing significantly until the end of the bioassay. Surviving female thrips injected with dsRNA-V-ATPase-B produced significantly fewer offspring compared to those in the dsRNA-GFP control group. Our findings indicate that an RNAi-based strategy to study gene function in thrips is feasible, can result in quantifiable phenotypes, and provides a much-needed tool for investigating the molecular mechanisms of thrips-tospovirus interactions. To our knowledge, this represents the first report of RNAi for any member of the insect order Thysanoptera and demonstrates the potential for translational research in the area of thrips pest control.

Embryonic development of the cricket Gryllus bimaculatus

Extensive research into Drosophila melanogaster embryogenesis has improved our understanding of insect developmental mechanisms. However, Drosophila development is thought to be highly divergent from that of the ancestral insect and arthropod in many respects. We therefore need alternative models for arthopod development that are likely to be more representative of basally-branching clades. The cricket Gryllus bimaculatus is such a model, and currently has the most sophisticated functional genetic toolkit of any hemimetabolous insect. The existing cricket embryonic staging system is fragmentary, and it is based on morphological landmarks that are not easily visible on a live, undissected egg. To address this problem, here we present a complementary set of "egg stages" that serve as a guide for identifying the developmental progress of a cricket embryo from fertilization to hatching, based solely on the external appearance of the egg. These stages were characterized using a combination of brightfield timelapse microscopy, timed brightfield micrographs, confocal microscopy, and measurements of egg dimensions. These egg stages are particularly useful in experiments that involve egg injection (including RNA interference, targeted genome modification, and transgenesis), as injection can alter the speed of development, even in control treatments. We also use 3D reconstructions of fixed embryo preparations to provide a comprehensive description of the morphogenesis and anatomy of the cricket embryo during embryonic rudiment assembly, germ band formation, elongation, segmentation, and appendage formation. Finally, we aggregate and schematize a variety of published developmental gene expression patterns. This work will facilitate further studies on G. bimaculatus development, and serve as a useful point of reference for other studies of wild type and experimentally manipulated insect development in fields from evo-devo to disease vector and pest management.

Involvement of dachshund and Distal-less in distal pattern formation of the cricket leg during regeneration

Cricket nymphs have the remarkable ability to regenerate a functional leg following amputation, indicating that the regenerating blastemal cells contain information for leg morphology. However, the molecular mechanisms that underlie regeneration of leg patterns remain poorly understood. Here, we analyzed phenotypes of the tibia and tarsus (three tarsomeres) obtained by knockdown with regeneration-dependent RNA interference (rdRNAi) against Gryllus dachshund (Gb'dac) and Distal-less (Gb'Dll). We found that depletion of Gb'Dll mRNA results in loss of the tarsal segments, while rdRNAi against Gb'dac shortens the tibia at the two most distal tarsomeres. These results indicate that Gb'Dll expression is indispensable for formation of the tarsus, while Gb'dac expression is necessary for elongation of the tibia and formation of the most proximal tarsomere. These findings demonstrate that mutual transcriptional regulation between the two is indispensable for formation of the tarsomeres, whereas Gb'dac is involved in determination of tibial size through interaction with Gb'ds/Gb'ft.

Regenerated leg segment patterns are regulated epigenetically by histone H3K27 methylation in the cricket Gryllus bimaculatus

Hemimetabolous insects such as the cricket Gryllus bimaculatus regenerate lost tissue parts using blastemal cells, which is a population of dedifferentiated-proliferating cells. The gene expression of several epigenetic factors is upregulated in the blastema compared with the expression in differentiated tissue, suggesting that epigenetic changes in gene expression may control the differentiation status of blastema cells during regeneration. To clarify the molecular basis of epigenetic regulation during regeneration, we focused on the function of the Gryllus Enhancer of zeste (Gb’E(z)) and Ubiquitously-transcribed tetratricopeptide repeat gene on the X chromosome (Gb’Utx) homologues that regulate the methylation and demethylation on histone H3 27th lysine residue (H3K27), respectively. Methylated histone H3K27 in the regenerating leg was diminished by Gb’E(z)RNAi and was increased by Gb’UtxRNAi. Regenerated Gb’E(z)RNAi cricket legs exhibited extra leg segment formation between the tibia and tarsus, and regenerated Gb’UtxRNAi cricket legs showed leg joint formation defects in the tarsus. In the Gb’E(z)RNAi-regenerating leg, the Gb’dac expression domain expanded in the tarsus. In contrast, in the Gb’UtxRNAi-regenerating leg, Gb’Egfr expression in the middle of the tarsus was diminished. These results suggest that regulation of the histone H3K27 methylation state is involved in the repatterning process during leg regeneration among cricket species via the epigenetic regulation of leg patterning gene expression.

Cell proliferation dynamics in regeneration of the operculum head appendage in the annelid Pomatoceros lamarckii

Regeneration of lost or damaged appendages is a widespread and ecologically important ability in the animal kingdom, and also of great significance to developing regenerative medicine. The operculum of serpulid polychaetes is one among the many diverse appendages found in the lophotrochozoan superphylum, a clade hitherto understudied with respect to the mechanisms of appendage regeneration. In this study, we establish the normal time course of opercular regeneration in the serpulid Pomatoceros lamarckii and describe cell proliferation patterns in the regenerating opercular filament. The P. lamarckii operculum regenerates through a rapid and consistent series of morphogenetic events. Based on 5-bromo-2'-deoxyuridine (BrdU) labeling and anti-phosphohistone H3 immunohistochemistry, opercular regeneration appears to be a mixture of an early morphallactic stage, and a later phase characterized by widespread proliferative activity within the opercular filament. Tracking residual pigmentation suggests that the distal part of the stump gives rise to the most distal structures of the operculum via morphallactic remodeling, whereas more proximal structures are derived from the proximal stump. Our work underscores the diversity of regenerative strategies employed by animals and introduces P. lamarckii as an emerging model of appendage regeneration.

A bioinformatics expert system linking functional data to anatomical outcomes in limb regeneration

Amphibians and molting arthropods have the remarkable capacity to regenerate amputated limbs, as described by an extensive literature of experimental cuts, amputations, grafts, and molecular techniques. Despite a rich history of experimental efforts, no comprehensive mechanistic model exists that can account for the pattern regulation observed in these experiments. While bioinformatics algorithms have revolutionized the study of signaling pathways, no such tools have heretofore been available to assist scientists in formulating testable models of large-scale morphogenesis that match published data in the limb regeneration field. Major barriers preventing an algorithmic approach are the lack of formal descriptions for experimental regenerative information and a repository to centralize storage and mining of functional data on limb regeneration. Establishing a new bioinformatics of shape would significantly accelerate the discovery of key insights into the mechanisms that implement complex regeneration. Here, we describe a novel mathematical ontology for limb regeneration to unambiguously encode phenotype, manipulation, and experiment data. Based on this formalism, we present the first centralized formal database of published limb regeneration experiments together with a user-friendly expert system tool to facilitate its access and mining. These resources are freely available for the community and will assist both human biologists and artificial intelligence systems to discover testable, mechanistic models of limb regeneration.

Developmental gene discovery in a hemimetabolous insect: de novo assembly and annotation of a transcriptome for the cricket Gryllus bimaculatus

Most genomic resources available for insects represent the Holometabola, which are insects that undergo complete metamorphosis like beetles and flies. In contrast, the Hemimetabola (direct developing insects), representing the basal branches of the insect tree, have very few genomic resources. We have therefore created a large and publicly available transcriptome for the hemimetabolous insect Gryllus bimaculatus (cricket), a well-developed laboratory model organism whose potential for functional genetic experiments is currently limited by the absence of genomic resources. cDNA was prepared using mRNA obtained from adult ovaries containing all stages of oogenesis, and from embryo samples on each day of embryogenesis. Using 454 Titanium pyrosequencing, we sequenced over four million raw reads, and assembled them into 21,512 isotigs (predicted transcripts) and 120,805 singletons with an average coverage per base pair of 51.3. We annotated the transcriptome manually for over 400 conserved genes involved in embryonic patterning, gametogenesis, and signaling pathways. BLAST comparison of the transcriptome against the NCBI non-redundant protein database (nr) identified significant similarity to nr sequences for 55.5% of transcriptome sequences, and suggested that the transcriptome may contain 19,874 unique transcripts. For predicted transcripts without significant similarity to known sequences, we assessed their similarity to other orthopteran sequences, and determined that these transcripts contain recognizable protein domains, largely of unknown function. We created a searchable, web-based database to allow public access to all raw, assembled and annotated data. This database is to our knowledge the largest de novo assembled and annotated transcriptome resource available for any hemimetabolous insect. We therefore anticipate that these data will contribute significantly to more effective and higher-throughput deployment of molecular analysis tools in Gryllus.

Analysis of RNA-Seq data reveals involvement of JAK/STAT signalling during leg regeneration in the cricket Gryllus bimaculatus

In the cricket Gryllus bimaculatus, missing distal parts of the amputated leg are regenerated from the blastema, a population of dedifferentiated proliferating cells that forms at the distal tip of the leg stump. To identify molecules involved in blastema formation, comparative transcriptome analysis was performed between regenerating and normal unamputated legs. Components of JAK/STAT signalling were upregulated more than twofold in regenerating legs. To verify their involvement, Gryllus homologues of the interleukin receptor Domeless (Gb’dome), the Janus kinase Hopscotch (Gb’hop) and the transcription factor STAT (Gb’Stat) were cloned, and RNAi was performed against these genes. Gb’domeRNAi, Gb’hopRNAi and Gb’StatRNAi crickets showed defects in leg regeneration. Blastema expression of Gb’cyclinE was decreased in the Gb’StatRNAi cricket compared with that in the control. Hyperproliferation of blastema cells caused by Gb’fatRNAi or Gb’wartsRNAi was suppressed by RNAi against Gb’Stat. The results suggest that JAK/STAT signalling regulates blastema cell proliferation during leg regeneration.

Delivery of dsRNA for RNAi in insects: an overview and future directions

RNA interference (RNAi) refers to the process of exogenous double-stranded RNA (dsRNA) silencing the complementary endogenous messenger RNA. RNAi has been widely used in entomological research for functional genomics in a variety of insects and its potential for RNAi-based pest control has been increasingly emphasized mainly because of its high specificity. This review focuses on the approaches of introducing dsRNA into insect cells or insect bodies to induce effective RNAi. The three most common delivery methods, namely, microinjection, ingestion, and soaking, are illustrated in details and their advantages and limitations are summarized for purpose of feasible RNAi research. In this review, we also briefly introduce the two possible dsRNA uptake machineries, other dsRNA delivery methods and the history of RNAi in entomology. Factors that influence the specificity and efficiency of RNAi such as transfection reagents, selection of dsRNA region, length, and stability of dsRNA in RNAi research are discussed for further studies.

Functional analysis of the role of eyes absent and sine oculis in the developing eye of the cricket Gryllus bimaculatus

In the cricket Gryllus bimaculatus, a hemimetabolous insect, the compound eyes begin to form in the embryo and increase 5-6 fold in size during the postembryonic development of the nymphal stage. Retinal stem cells in the anteroventral proliferation zone (AVPZ) of the nymphal eye proliferate to increase retinal progenitors, which then differentiate to form new ommatidia in the anterior region of the eye. However, mechanisms underlying this type of eye formation have not been well elucidated yet. Here, we found that the homologues of the retinal determination transcription factor genes of eyes absent (eya) and sine oculis (so) are expressed during the cricket embryonic eye formation. eya is also expressed intensely in the AVPZ of the nymphal eye. To explore their functions, we performed knockdown by RNA interference (RNAi). Knockdown of Gb'eya resulted in loss of the embryonic eye. In the nymphal eye, RNAi against Gb'eya or Gb'so impaired retinal morphology by apparently transforming cornea structures into head cuticle. These results imply that Gb'eya and Gb'so are essential for the differentiation of the retinal progenitor cells and maintaining retinal structures during eye development.

ERK signaling controls blastema cell differentiation during planarian regeneration

The robust regenerative ability of planarians depends on a population of somatic stem cells called neoblasts, which are the only mitotic cells in adults and are responsible for blastema formation after amputation. The molecular mechanism underlying neoblast differentiation associated with blastema formation remains unknown. Here, using the planarian Dugesia japonica we found that DjmkpA, a planarian mitogen-activated protein kinase (MAPK) phosphatase-related gene, was specifically expressed in blastema cells in response to increased extracellular signal-related kinase (ERK) activity. Pharmacological and genetic [RNA interference (RNAi)] approaches provided evidence that ERK activity was required for blastema cells to exit the proliferative state and undergo differentiation. By contrast, DjmkpA RNAi induced an increased level of ERK activity and rescued the differentiation defect of blastema cells caused by pharmacological reduction of ERK activity. These observations suggest that ERK signaling plays an instructive role in the cell fate decisions of blastema cells regarding whether to differentiate or not, by inducing DjmkpA as a negative regulator of ERK signaling during planarian regeneration.

Ancestral functions of Delta/Notch signaling in the formation of body and leg segments in the cricket Gryllus bimaculatus

Delta/Notch signaling controls a wide spectrum of developmental processes, including body and leg segmentation in arthropods. The various functions of Delta/Notch signaling vary among species. For instance, in Cupiennius spiders, Delta/Notch signaling is essential for body and leg segmentation, whereas in Drosophila fruit flies it is involved in leg segmentation but not body segmentation. Therefore, to gain further insight into the functional evolution of Delta/Notch signaling in arthropod body and leg segmentation, we analyzed the function of the Delta (Gb′Delta) and Notch (Gb′Notch) genes in the hemimetabolous, intermediate-germ cricket Gryllus bimaculatus. We found that Gb′Delta and Gb′Notch were expressed in developing legs, and that RNAi silencing of Gb′Notch resulted in a marked reduction in leg length with a loss of joints. Our results suggest that the role of Notch signaling in leg segmentation is conserved in hemimetabolous insects. Furthermore, we found that Gb′Delta was expressed transiently in the posterior growth zone of the germband and in segmental stripes earlier than the appearance of wingless segmental stripes, whereas Gb′Notch was uniformly expressed in early germbands. RNAi knockdown of Gb′Delta or Gb′Notch expression resulted in malformation in body segments and a loss of posterior segments, the latter probably due to a defect in posterior growth. Therefore, in the cricket, Delta/Notch signaling might be required for proper morphogenesis of body segments and posterior elongation, but not for specification of segment boundaries.

Cricket body size is altered by systemic RNAi against insulin signaling components and epidermal growth factor receptor

A long-standing problem of developmental biology is how body size is determined. In Drosophila melanogaster, the insulin/insulin-like growth factor (I/IGF) and target of rapamycin (TOR) signaling pathways play important roles in this process. However, the detailed mechanisms by which insect body growth is regulated are not known. Therefore, we have attempted to utilize systemic nymphal RNA interference (nyRNAi) to knockdown expression of insulin signaling components including Insulin receptor (InR), Insulin receptor substrate (chico), Phosphatase and tensin homologue (Pten), Target of rapamycin (Tor), RPS6-p70-protein kinase (S6k), Forkhead box O (FoxO) and Epidermal growth factor receptor (Egfr) and observed the effects on body size in the Gryllus bimaculatus cricket. We found that crickets treated with double-stranded RNA (dsRNA) against Gryllus InR, chico, Tor, S6k and Egfr displayed smaller body sizes, while Gryllus FoxO nyRNAi-ed crickets exhibited larger than normal body sizes. Furthermore, RNAi against Gryllus chico and Tor displayed slow growth and RNAi against Gryllus chico displayed longer lifespan than control crickets. Since no significant difference in ability of food uptake was observed between the Gryllus chico(nyRNAi) nymphs and controls, we conclude that the adult cricket body size can be altered by knockdown of expressions of Gryllus InR, chico, Tor, S6k, FoxO and Egfr by systemic RNAi. Our results suggest that the cricket is a promising model to study mechanisms underlying controls of body size and life span with RNAi methods.

EGFR signaling regulates cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis

Similarly to development, the process of regeneration requires that cells accurately sense and respond to their external environment. Thus, intrinsic cues must be integrated with signals from the surrounding environment to ensure appropriate temporal and spatial regulation of tissue regeneration. Identifying the signaling pathways that control these events will not only provide insights into a fascinating biological phenomenon but may also yield new molecular targets for use in regenerative medicine. Among classical models to study regeneration, freshwater planarians represent an attractive system in which to investigate the signals that regulate cell proliferation and differentiation, as well as the proper patterning of the structures being regenerated. Recent studies in planarians have begun to define the role of conserved signaling pathways during regeneration. Here, we extend these analyses to the epidermal growth factor (EGF) receptor pathway. We report the characterization of three epidermal growth factor (EGF) receptors in the planarian Schmidtea mediterranea. Silencing of these genes by RNA interference (RNAi) yielded multiple defects in intact and regenerating planarians. Smed-egfr-1(RNAi) resulted in decreased differentiation of eye pigment cells, abnormal pharynx regeneration and maintenance, and the development of dorsal outgrowths. In contrast, Smed-egfr-3(RNAi) animals produced smaller blastemas associated with abnormal differentiation of certain cell types. Our results suggest important roles for the EGFR signaling in controlling cell proliferation, differentiation and morphogenesis during planarian regeneration and homeostasis.

Leg regeneration in Drosophila abridges the normal developmental program

Regeneration of lost body parts has traditionally been seen as a redeployment of embryonic development. However, whether regeneration and embryonic development are controlled by identical, similar or different genetic programmes has not been fully tested. Here, we analyse proximal-distal regeneration in Drosophila leg imaginal discs using the expression of positional markers, and by cell-lineage experiments, and we compare it with the pattern already known in normal development. During regeneration, the first proximal-distal positional markers reappear in overlapping patterns. As the regenerate expands, they segregate and further markers appear until the normal pattern is produced, following a proximal to distal sequence that is in fact the reverse of normal leg imaginal disc development. The results of lineage tracing support this interpretation and show that regenerated structures derive from cells near the wound edge. Although leg development and leg regeneration are served by a set of identical genes, the ways their proximal-distal patterns are achieved are distinct from each other. Such differences can result from similar developmental gene interactions acting under different starting conditions.

Functional convergence of signalling by GPI-anchored and anchorless forms of a salamander protein implicated in limb regeneration

The GPI-anchor is an established determinant of molecular localisation and various functional roles have been attributed to it. The newt GPI-anchored three-finger protein (TFP) Prod1 is an important regulator of cell behaviour during limb regeneration, but it is unclear how it signals to the interior of the cell. Prod1 was expressed by transfection in cultured newt limb cells and activated transcription and expression of matrix metalloproteinase 9 (MMP9) by a pathway involving ligand-independent activation of epidermal growth factor receptor (EGFR) signalling and phosphorylation of extracellular regulated kinase 1 and 2 (ERK1/2). This was dependent on the presence of the GPI-anchor and critical residues in the α-helical region of the protein. Interestingly, Prod1 in the axolotl, a salamander species that also regenerates its limbs, was shown to activate ERK1/2 signalling and MMP9 transcription despite being anchorless, and both newt and axolotl Prod1 co-immunoprecipitated with the newt EGFR after transfection. The substitution of the axolotl helical region activated a secreted, anchorless version of the newt molecule. The activity of the newt molecule cannot therefore depend on a unique property conferred by the anchor. Prod1 is a salamander-specific TFP and its interaction with the phylogenetically conserved EGFR has implications for our view of regeneration as an evolutionary variable.

Loss-of-function analyses of the fragile X-related and dopamine receptor genes by RNA interference in the cricket Gryllus bimaculatus

In order to explore a possibility that the cricket Gryllus bimaculatus would be a useful model to unveil molecular mechanisms of human diseases, we performed loss-of-function analyses of Gryllus genes homologous to human genes that are responsible for human disorders, fragile X mental retardation 1 (fmr1) and Dopamine receptor (DopR). We cloned cDNAs of their Gryllus homologues, Gb'fmr1, Gb'DopRI, and Gb'DopRII, and analyzed their functions with use of nymphal RNA interference (RNAi). For Gb'fmr1, three major phenotypes were observed: (1) abnormal wing postures, (2) abnormal calling song, and (3) loss of the circadian locomotor rhythm, while for Gb'DopRI, defects of wing posture and morphology were found. These results indicate that the cricket has the potential to become a novel model system to explore human neuronal pathogenic mechanisms and to screen therapeutic drugs by RNAi.

Regulation of leg size and shape by the Dachsous/Fat signalling pathway during regeneration

An amputated cricket leg regenerates all missing parts with normal size and shape, indicating that regenerating blastemal cells are aware of both their position and the normal size of the leg. However, the molecular mechanisms regulating this process remain elusive. Here, we use a cricket model to show that the Dachsous/Fat (Ds/Ft) signalling pathway is essential for leg regeneration. We found that knockdown of ft or ds transcripts by regeneration-dependent RNA interference (rdRNAi) suppressed proliferation of the regenerating cells along the proximodistal (PD) axis concomitantly with remodelling of the pre-existing stump, making the regenerated legs shorter than normal. By contrast, knockdown of the expanded (ex) or Merlin (Mer) transcripts induced over-proliferation of the regenerating cells, making the regenerated legs longer. These results are consistent with those obtained using rdRNAi during intercalary regeneration induced by leg transplantation. We present a model to explain our results in which the steepness of the Ds/Ft gradient controls growth along the PD axis of the regenerating leg.

Triggering the regeneration and tissue repair programs

In early October 2008, researchers from diverse backgrounds gathered at an EMBO conference entitled 'The Molecular and Cellular Basis of Regeneration and Tissue Repair' to discuss the basic biology of regeneration. Topics included cell plasticity in regenerative and developmental contexts, and the link between wound healing and regeneration. The meeting also highlighted the progress made in identifying the molecular networks that underlie regeneration in a variety of model systems.

The Two-Spotted Cricket Gryllus bimaculatus: An Emerging Model for Developmental and Regeneration Studies

INTRODUCTIONThe two-spotted cricket Gryllus bimaculatus De Geer (Orthoptera: Gryllidae), which is one of the most abundant cricket species, inhabits the tropical and subtropical regions of Asia, Africa, and Europe. G. bimaculatus can be easily bred in the laboratory and has been widely used to study insect physiology and neurobiology. Recently, this species has become established as a model animal for studies on molecular mechanisms of development and regeneration because its mode of development is more typical of arthropods than that of Drosophila melanogaster, and the cricket is probably ancestral for this phylum. Moreover, the cricket is a hemimetabolous insect, in which nymphs possess functional legs with a remarkable capacity for regeneration after damage. Because RNA interference (RNAi) works effectively in this species, the elucidation of mechanisms of development and regeneration has been expedited through loss-of-function analyses of genes. Furthermore, because RNAi-based techniques for analyzing gene functions can be combined with assay systems in other research areas (such as behavioral analyses), G. bimaculatus is expected to become a model organism in various fields of biology. Thus, it may be possible to establish the cricket as a simple model system for exploring more complex organisms such as humans.