Two transcriptional cascades orchestrate cockroach leg regeneration

Protocol for high-resolution 3D visualization of insect regenerating legs through micro-computed tomography

Appendage regeneration occurs within the opaque exoskeleton in arthropods, making it challenging to visualize the regenerative processes dynamically. In this protocol, we present a strategy to scan and capture the high-resolution details of microstructural tissues at certain regeneration points through micro-computed tomography (micro-CT). We describe steps for tissue preparation, fixation, critical point drying, micro-CT scanning, and 3D visualization. This approach promises significant utility in the field of regeneration, particularly in studies involving arthropods such as insects and crustaceans. For complete details on the use and execution of this protocol, please refer to Ren et al.1.

Yorkie negatively regulates the Crustin expression during molting in Chinese mitten crab, Eriocheir sinensis

Molting is a key biological process of crustaceans, which is mainly regulated by 20-hydroxyecdyone (20E). The molting cycle could be divided into three main stages including pre-molt, post-molt and inter-molt stages. The mechanism of immune regulation during molting process still requires further exploration. Yorkie (Yki) is a pivotal transcription factor in the Hippo signaling pathway, and it plays an essential role in regulating cell growth and immune response. In the present study, a Yki gene was identified from Eriocheir sinensis (designed as EsYki), and the regulatory role of EsYki in controlling the expression of antimicrobial peptide genes throughout the molting process was investigated. The mRNA expression level of EsYki was higher at the pre-molt stage compared to the post-molt stage and inter-molt stage. Following the injection of 20E, there was a notable and consistent rise in the EsYki mRNA expression in haemocytes. The increase was observed from 3 h to 48 h with the maximum level at 12 h. And the phosphorylation of Yki in the haemocytes was also significantly up-regulated at 3 h post 20E injection. Moreover, the levels of EsYki mRNA expression at three molting stages were significantly increased post Aeromonas hydrophila stimulation. The maximum level was detected at post-molt stage following A. hydrophila stimulation, while the lowest level was observed at inter-molt stage. The expression pattern of EsCrus was in contrast to EsCrus. After EsYki mRNA transcripts were inhibited by Yki inhibitor (CA3), the mRNA expression levels of EsCrus1 and EsCrus2 following A. hydrophila stimulation were significantly elevated. Furthermore, the phosphorylation level of NF-κB was also increased following the inhibition of Yki. Collectively, our findings indicated that EsYki could be induced by 20E and has a suppressive effect on the expression of EsCrus via inhibiting NF-κB during molting process. This research contributes to the understanding of the immunological regulation mechanism during molting process in crustaceans.

Notch signaling regulates limb regeneration through Hes1 and HeyL in the Chinese mitten crab

Tissue regeneration is an efficient strategy developed by animals to compensate for damaged tissues, involving various types of progenitor cells. Deciphering the signal network that modulates the activity of these progenitors during regeneration is crucial for understanding the differences in regenerative capacities across species. In this study, we evaluated the expression profile and phenotypic function of Notch signaling during limb regeneration in arthropod Chinese mitten crabs. The expression of key components of the Notch signaling pathway was upregulated at 7-day post-autotomy (7 DPA), and declined later at 18-day post-autotomy (18 DPA). To assess the role of Notch, we injected dsRNA targeting the Notch gene into the automized area and evaluated the regeneration efficiency. Our results indicated that blocking Notch signaling led to regenerative defects, manifested by delays in the wound closure and blastema emergence processes. Furthermore, the expression of Notch target genes, Hes1 and HeyL, was significantly reduced following Notch knockdown by dsRNA. Knockdown of Hes1 specifically impaired the proliferation and expression of neural progenitor cell markers, without affecting myogenic cells. In contrast, blockage of HeyL inhibited the proliferation and expression of markers in both activated neurogenic and myogenic progenitor cells, while up-regulating markers of quiescent neural progenitor cells. These findings suggest that Notch signaling plays an important role in limb regeneration of E. sinensis by activating downstream effectors Hes1 and HeyL, regulating neurogenesis and myogenesis through distinct mechanisms.

Gill regeneration in the mayfly Cloeon uncovers new molecular pathways in insect regeneration

The capacity to regenerate lost organs is widespread among animals, and yet the number of species in which regeneration has been experimentally probed using molecular and functional assays is very small. This is also the case for insects, for which we still lack a complete picture of their regeneration mechanisms and the extent of their conservation. Here, we contribute to filling this gap by investigating regeneration in the mayfly Cloeon dipterum. We focus on the abdominal gills of Cloeon nymphs, which are critical for osmoregulation and gas exchange. After amputation, gills re-grow faster than they do during normal development. Direct cell count and EdU assays indicate that growth acceleration involves an uniform increase in cell proliferation throughout the gill, rather than a localized growth zone. Accordingly, transcriptomic analysis reveals an early enrichment in cell cycle-related genes. Other gene classes are also enriched in regenerating gills, including protein neddylation and other proteostatic processes. We then showed the conservation of these mechanisms by functionally testing protein neddylation, the activin signalling pathway or the mRNA-binding protein Lin28, among other genes, in Drosophila larval/pupal wing regeneration. Globally, our results contribute to elucidating regeneration mechanisms in mayflies and the conservation of mechanisms involved in regeneration across insects.

Cephalic ganglia transcriptomics of the American cockroach Periplaneta americana (Blattodea: Blattidae)

The American cockroach Periplaneta americana (L.) (Blattodea, Blattidae) has been a model organism for biochemical and physiological study for almost a century, however, its use does not benefit from the genetic tools found in key model species such as Drosophila melanogaster. To facilitate the use of the cockroach as a model system in neuroscience and to serve as a foundation for functional and translational experimentation, a transcriptome of the cephalic ganglia was assembled and annotated, and differential expression profiles between these ganglia were assessed. The transcriptome assembly yielded >400 k transcripts, with >40 k putative coding sequences. Gene ontology and protein domain searches indicate the cerebral and gnathal ganglia (GNG) have distinct genetic expression profiles. The developmental Toll signaling pathway appears to be active in the adult central nervous system (CNS), which may suggest a separate role for this pathway besides innate immune activation or embryonic development. The catabolic glycolytic and citric acid cycle enzymes are well represented in both ganglia, but key enzymes are more highly expressed in the GNG. Both ganglia express gluconeogenic and trehaloneogenic enzymes, suggesting a larger role of the CNS in regulating hemolymph sugar homeostasis than previously appreciated. The annotation and quantification of the cephalic ganglia transcriptome reveal both canonical and novel pathways in signaling and metabolism in an adult insect and lay a foundation for future functional and genetic analysis.

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.