Zebrafish twist2/dermo1 regulates scale shape and scale organization during skin development and regeneration

The Regulatory Role of miRNAs in Zebrafish Fin Regeneration

Since Teleostei fins have a strong regenerative capacity, further research was conducted on the regulation of gene expression during fin regeneration. This research focuses on miRNA, which is a key post-transcriptional regulatory molecule. In this study, a miRNA library for the fin regeneration of zebrafish was constructed to reveal the differential expression of miRNA during fin regeneration and to explore the regulatory pathway for fin regeneration. Following the injection of miRNA agomir into zebrafish, the proliferation of blastema cells and the overall fin regeneration area were significantly reduced. It was observed that the miRNAs impaired blastocyte formation by affecting fin regeneration through the inhibition of the expressions of genes and proteins associated with blastocyte formation (including yap1 and Smad1/5/9), which is an effect associated with the Hippo pathway. Furthermore, it has been demonstrated that miRNAs can impair the patterns and mineralization of newly formed fin rays. The miRNAs influenced fin regeneration by inhibiting the expression of a range of bone-related genes and proteins in osteoblast lineages, including sp7, runx2a, and runx2b. This study provides a valuable reference for the further exploration of morphological bone reconstruction in aquatic vertebrates.

Homeostasis and evolution in relation to regeneration and repair

Homeostasis constitutes a key concept in physiology and refers to self-regulating processes that maintain internal stability when adjusting to changing external conditions. It diminishes internal entropy constituting a driving force behind evolution. Natural selection might act on homeostatic regulatory mechanisms and control mechanisms including homeodynamics, allostasis, hormesis and homeorhesis, where different stable stationary states are reached. Regeneration is under homeostatic control through hormesis. Damage to tissues initiates a response to restore the impaired equilibrium caused by mild stress using cell proliferation, cell differentiation and cell death to recover structure and function. Repair is a homeorhetic change leading to a new stable stationary state with decreased functionality and fibrotic scarring without reconstruction of the 3-D pattern. Mechanisms determining entrance of the tissue or organ to regeneration or repair include the balance between innate and adaptive immune cells in relation to cell plasticity and stromal stem cell responses, and redox balance. The regenerative and reparative capacities vary in different species, distinct tissues and organs, and at different stages of development including ageing. Many cell signals and pathways play crucial roles determining regeneration or repair by regulating protein synthesis, cellular growth, inflammation, proliferation, autophagy, lysosomal function, metabolism and metalloproteinase cell signalling. Attempts to favour the entrance of damaged tissues to regeneration in those with low proliferative rates have been made; however, there are evolutionary constraint mechanisms leading to poor proliferation of stem cells in unfavourable environments or tumour development. More research is required to better understand the regulatory processes of these mechanisms.

Twist2 contributes to skin regeneration and hair follicle formation in mouse fetuses

Unlike adult mammalian wounds, early embryonic mouse skin wounds completely regenerate and heal without scars. Analysis of the underlying molecular mechanism will provide insights into scarless wound healing. Twist2 is an important regulator of hair follicle formation and biological patterning; however, it is unclear whether it plays a role in skin or skin appendage regeneration. Here, we aimed to elucidate Twist2 expression and its role in fetal wound healing. ICR mouse fetuses were surgically wounded on embryonic day 13 (E13), E15, and E17, and Twist2 expression in tissue samples from these fetuses was evaluated via in situ hybridization, immunohistochemistry, and reverse transcription-quantitative polymerase chain reaction. Twist2 expression was upregulated in the dermis of E13 wound margins but downregulated in E15 and E17 wounds. Twist2 knockdown on E13 left visible marks at the wound site, inhibited regeneration, and resulted in defective follicle formation. Twist2-knockdown dermal fibroblasts lacked the ability to undifferentiate. Furthermore, Twist2 hetero knockout mice (Twist + /-) formed visible scars, even on E13, when all skin structures should regenerate. Thus, Twist2 expression correlated with skin texture formation and hair follicle defects in late mouse embryos. These findings may help develop a therapeutic strategy to reduce scarring and promote hair follicle regeneration.

The avian ectodermal default competence to make feathers

Feathers originate as protofeathers before birds, in pterosaurs and basal dinosaurs. What characterizes a feather is not only its outgrowth, but its barb cells differentiation and a set of beta-corneous proteins. Reticula appear concomitantly with feathers, as small bumps on plantar skin, made only of keratins. Avian scales, with their own set of beta-corneous proteins, appear more recently than feathers on the shank, and only in some species. In the chick embryo, when feather placodes form, all the non-feather areas of the integument are already specified. Among them, midventral apterium, cornea, reticula, and scale morphogenesis appear to be driven by negative regulatory mechanisms, which modulate the inherited capacity of the avian ectoderm to form feathers. Successive dermal/epidermal interactions, initiated by the Wnt/β-catenin pathway, and involving principally Eda/Edar, BMP, FGF20 and Shh signaling, are responsible for the formation not only of feather, but also of scale placodes and reticula, with notable differences in the level of Shh, and probably FGF20 expressions. This sequence is a dynamic and labile process, the turning point being the FGF20 expression by the placode. This epidermal signal endows its associated dermis with the memory to aggregate and to stimulate the morphogenesis that follows, involving even a re-initiation of the placode.

An active traveling wave of Eda/NF-κB signaling controls the timing and hexagonal pattern of skin appendages in zebrafish

Periodic patterns drive the formation of a variety of tissues, including skin appendages such as feathers and scales. Skin appendages serve important and diverse functions across vertebrates, yet the mechanisms that regulate their patterning are not fully understood. Here, we have used live imaging to investigate dynamic signals regulating the ontogeny of zebrafish scales. Scales are bony skin appendages that develop sequentially along the anterior-posterior and dorsal-ventral axes to cover the fish in a hexagonal array. We have found that scale development requires cell-cell communication and is coordinated through an active wave mechanism. Using a live transcriptional reporter, we show that a wave of Eda/NF-κB activity precedes scale initiation and is required for scale formation. Experiments decoupling the propagation of the wave from dermal placode formation and osteoblast differentiation demonstrate that the Eda/NF-κB activity wavefront controls the timing of the sequential patterning of scales. Moreover, this decoupling resulted in defects in scale size and significant deviations in the hexagonal patterning of scales. Thus, our results demonstrate that a biochemical traveling wave coordinates scale initiation and proper hexagonal patterning across the fish body.

Evo Devo of the Vertebrates Integument

All living jawed vertebrates possess teeth or did so ancestrally. Integumental surface also includes the cornea. Conversely, no other anatomical feature differentiates the clades so readily as skin appendages do, multicellular glands in amphibians, hair follicle/gland complexes in mammals, feathers in birds, and the different types of scales. Tooth-like scales are characteristic of chondrichthyans, while mineralized dermal scales are characteristic of bony fishes. Corneous epidermal scales might have appeared twice, in squamates, and on feet in avian lineages, but posteriorly to feathers. In contrast to the other skin appendages, the origin of multicellular glands of amphibians has never been addressed. In the seventies, pioneering dermal-epidermal recombination between chick, mouse and lizard embryos showed that: (1) the clade type of the appendage is determined by the epidermis; (2) their morphogenesis requires two groups of dermal messages, first for primordia formation, second for appendage final architecture; (3) the early messages were conserved during amniotes evolution. Molecular biology studies that have identified the involved pathways, extending those data to teeth and dermal scales, suggest that the different vertebrate skin appendages evolved in parallel from a shared placode/dermal cells unit, present in a common toothed ancestor, c.a. 420 mya.

Systemic Prenatal Stress Exposure through Corticosterone Application Adversely Affects Avian Embryonic Skin Development

Prenatal stress exposure is considered a risk factor for developmental deficits and postnatal behavioral disorders. While the effect of glucocorticoid-associated prenatal stress exposure has been comprehensively studied in many organ systems, there is a lack of in-depth embryological investigations regarding the effects of stress on the integumentary system. To approach this, we employed the avian embryo as a model organism and investigated the effects of systemic pathologically-elevated glucocorticoid exposure on the development of the integumentary system. After standardized corticosterone injections on embryonic day 6, we compared the stress-exposed embryos with a control cohort, using histological and immunohistochemical analyses as well as in situ hybridization. The overarching developmental deficits observed in the stress-exposed embryos were reflected through downregulation of both vimentin as well as fibronectin. In addition, a deficient composition in the different skin layers became apparent, which could be linked to a reduced expression of Dermo-1 along with significantly reduced proliferation rates. An impairment of skin appendage formation could be demonstrated by diminished expression of Sonic hedgehog. These results contribute to a more profound understanding of prenatal stress causing severe deficits in the integumentary system of developing organisms.

An active traveling wave of Eda/NF-kB signaling controls the timing and hexagonal pattern of skin appendages in zebrafish

Periodic patterns make up a variety of tissues, including skin appendages such as feathers and scales. Skin appendages serve important and diverse functions across vertebrates, yet the mechanisms that regulate their patterning are not fully understood. Here, we have used live imaging to investigate dynamic signals regulating the ontogeny of zebrafish scales. Scales are bony skin appendages which develop sequentially along the anterior-posterior and dorsal-ventral axes to cover the fish in a hexagonal array. We have found that scale development requires cell-cell communication and is coordinated through an active wave mechanism. Using a live transcriptional reporter, we show that a wave of Eda/NF-κB activity precedes scale initiation and is required for scale formation. Experiments decoupling the propagation of the wave from dermal placode formation and osteoblast differentiation demonstrate that the Eda/NF-kB activity wavefront times the sequential patterning of scales. Moreover, this decoupling resulted in defects in scale size and significant deviations in the hexagonal patterning of scales. Thus, our results demonstrate that a biochemical traveling wave coordinates scale initiation and proper hexagonal patterning across the fish body.

Twist2-driven chromatin remodeling governs the postnatal maturation of dermal fibroblasts

Dermal fibroblasts lose stem cell potency after birth, which prevents regenerative healing. However, the underlying intracellular mechanisms are largely unknown. We uncover the postnatal maturation of papillary fibroblasts (PFs) driven by the extensive Twist2-mediated remodeling of chromatin accessibility. A loss of the regenerative ability of postnatal PFs occurs with decreased H3K27ac levels. Single-cell transcriptomics, assay for transposase-accessible chromatin sequencing (ATAC-seq), and chromatin immunoprecipitation sequencing (ChIP-seq) reveal the postnatal maturation trajectory associated with the loss of the regenerative trajectory in PFs, which is characterized by a marked decrease in chromatin accessibility and H3K27ac modifications. Histone deacetylase inhibition delays spontaneous chromatin remodeling, thus maintaining the regenerative ability of postnatal PFs. Genomic analysis identifies Twist2 as a major regulator within chromatin regions with decreased accessibility during the postnatal period. When Twist2 is genetically deleted in dermal fibroblasts, the intracellular cascade of postnatal maturation is significantly delayed. Our findings reveal the comprehensive intracellular mechanisms underlying intrinsic postnatal changes in dermal fibroblasts.

Scale Development-Related Genes Identified by Transcriptome Analysis

Scales, as key structures of fish skin, play an important role in physiological function. The study of fish scale development mechanisms provides a basis for exploring the molecular-level developmental differences between scaled and non-scaled fishes. In this study, alizarin red staining was used to divide the different stages of zebrafish (Danio rerio) scale development. Four developmental stages, namely stage I (~17 dpf, scales have not started to grow), stage II (~33 dpf, the point at which scales start to grow), stage III (~41 dpf, the period in which the scales almost cover the whole body), and stage IV (~3 mpf, scales cover the whole body), were determined and used for subsequent transcriptome analysis. WGCNA (weighted correlation network analysis) and DEG (differentially expressed gene) analysis were used for screening the key genes. Based on the comparison between stage II and stage I, 54 hub-genes were identified by WGCNA analysis. Key genes including the Scpp family (Scpp7, Scpp6, Scpp5, and Scpp8), the Fgf family (Fgfr1b and Fgfr3), Tcf7, Wnt10b, Runx2b, and Il2rb were identified by DEG analysis, which indicated that these genes played important roles in the key nodes of scale development signal pathways. Combined with this analysis, the TGF-β, Wnt/β-catenin, and FGF signaling pathways were suggested to be the most important signal pathways for scales starting to grow. This study laid a foundation for exploring the scale development mechanism of other fishes. The scale development candidate genes identified in the current study will facilitate functional gene identifications in the future.