FGF8 and SHH substitute for anterior–posterior tissue interactions to induce limb regeneration

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.

Optogenetic manipulation of cellular communication using engineered myosin motors

Cells achieve highly efficient and accurate communication through cellular projections such as neurites and filopodia, yet there is a lack of genetically encoded tools that can selectively manipulate their composition and dynamics. Here, we present a versatile optogenetic toolbox of artificial multi-headed myosin motors that can move bidirectionally within long cellular extensions and allow for the selective transport of GFP-tagged cargo with light. Utilizing these engineered motors, we could transport bulky transmembrane receptors and organelles as well as actin remodellers to control the dynamics of both filopodia and neurites. Using an optimized in vivo imaging scheme, we further demonstrate that, upon limb amputation in axolotls, a complex array of filopodial extensions is formed. We selectively modulated these filopodial extensions and showed that they re-establish a Sonic Hedgehog signalling gradient during regeneration. Considering the ubiquitous existence of actin-based extensions, this toolbox shows the potential to manipulate cellular communication with unprecedented accuracy.

Appendage Regeneration in Vertebrates: What Makes This Possible?

The ability to regenerate amputated or injured tissues and organs is a fascinating property shared by several invertebrates and, interestingly, some vertebrates. The mechanism of evolutionary loss of regeneration in mammals is not understood, yet from the biomedical and clinical point of view, it would be very beneficial to be able, at least partially, to restore that capability. The current availability of new experimental tools, facilitating the comparative study of models with high regenerative ability, provides a powerful instrument to unveil what is needed for a successful regeneration. The present review provides an updated overview of multiple aspects of appendage regeneration in three vertebrates: lizard, salamander, and zebrafish. The deep investigation of this process points to common mechanisms, including the relevance of Wnt/β-catenin and FGF signaling for the restoration of a functional appendage. We discuss the formation and cellular origin of the blastema and the identification of epigenetic and cellular changes and molecular pathways shared by vertebrates capable of regeneration. Understanding the similarities, being aware of the differences of the processes, during lizard, salamander, and zebrafish regeneration can provide a useful guide for supporting effective regenerative strategies in mammals.

FGF10 Attenuates Experimental Traumatic Brain Injury through TLR4/MyD88/NF-κB Pathway

Traumatic brain injury (TBI) can induce neuronal apoptosis and neuroinflammation, resulting in substantial neuronal damage and behavioral disorders. Fibroblast growth factors (FGFs) have been shown to be critical mediators in tissue repair. However, the role of FGF10 in experimental TBI remains unknown. In this study, mice with TBI were established via weight-loss model and validated by increase of modified neurological severity scores (mNSS) and brain water content. Secondly, FGF10 levels were elevated in mice after TBI, whereas intraventricular injection of Ad-FGF10 decreased mNSS score and brain water content, indicating the remittance of neurological deficit and cerebral edema in TBI mice. In addition, neuronal damage could also be ameliorated by stereotactic injection of Ad-FGF10. Overexpression of FGF10 increased protein expression of Bcl-2, while it decreased Bax and cleaved caspase-3/PARP, and improved neuronal apoptosis in TBI mice. In addition, Ad-FGF10 relieved neuroinflammation induced by TBI and significantly reduced the level of interleukin 1β/6, tumor necrosis factor α, and monocyte chemoattractant protein-1. Moreover, Ad-FGF10 injection decreased the protein expression level of Toll-like receptor 4 (TLR4), MyD88, and phosphorylation of NF-κB (p-NF-κB), suggesting the inactivation of the TLR4/MyD88/NF-κB pathway. In conclusion, overexpression of FGF10 could ameliorate neurological deficit, neuronal apoptosis, and neuroinflammation through inhibition of the TLR4/MyD88/NF-κB pathway, providing a potential therapeutic strategy for brain injury in the future.

Characterizing the regenerative capacity and growth patterns of the Texas blind salamander (Eurycea rathbuni)

BACKGROUND

Regeneration of complex patterned structures is well described among, although limited to a small sampling of, amphibians. This limitation impedes our understanding of the full range of regenerative competencies within this class of vertebrates, according to phylogeny, developmental life stage, and age. To broaden the phylogenetic breath of this research, we characterized the regenerative capacity of the Texas blind salamander (Eurycea rathbuni), a protected salamander native to the Edwards Aquifer of San Marcos, Texas and colonized by the San Marcos Aquatic Resource Center. As field observations suggested regenerative abilities in this population, the forelimb stump of a live captured female was amputated in the hopes of restoring the structure, and thus locomotion in the animal. Tails were clipped from two males to additionally document tail regeneration.

RESULTS

We show that the Texas blind salamander exhibits robust limb and tail regeneration, like all other studied Plethodontidae. Regeneration in this species is associated with wound epithelium formation, blastema formation, and subsequent patterning and differentiation of the regenerate.

CONCLUSIONS

The study has shown that the Texas blind salamander is a valuable model to study regenerative processes, and that therapeutic surgeries offer a valuable means to help maintain and conserve this vulnerable species.

Extracellular Matrix and Cellular Plasticity in Musculoskeletal Development

Cellular plasticity refers to the ability of cell fates to be reprogrammed given the proper signals, allowing for dedifferentiation or transdifferentiation into different cell fates. In vitro, this can be induced through direct activation of gene expression, however this process does not naturally occur in vivo. Instead, the microenvironment consisting of the extracellular matrix (ECM) and signaling factors, directs the signals presented to cells. Often the ECM is involved in regulating both biochemical and mechanical signals. In stem cell populations, this niche is necessary for maintenance and proper function of the stem cell pool. However, recent studies have demonstrated that differentiated or lineage restricted cells can exit their current state and transform into another state under different situations during development and regeneration. This may be achieved through (1) cells responding to a changing niche; (2) cells migrating and encountering a new niche; and (3) formation of a transitional niche followed by restoration of the homeostatic niche to sequentially guide cells along the regenerative process. This review focuses on examples in musculoskeletal biology, with the concept of ECM regulating cells and stem cells in development and regeneration, extending beyond the conventional concept of small population of progenitor cells, but under the right circumstances even “lineage-restricted” or differentiated cells can be reprogrammed to enter into a different fate.

Role of VIP and Sonic Hedgehog Signaling Pathways in Mediating Epithelial Wound Healing, Sensory Nerve Regeneration, and Their Defects in Diabetic Corneas

Diabetic keratopathy, a sight-threatening corneal disease, comprises several symptomatic conditions including delayed epithelial wound healing, recurrent erosions, and sensory nerve (SN) neuropathy. We investigated the role of neuropeptides in mediating corneal wound healing, including epithelial wound closure and SN regeneration. Denervation by resiniferatoxin severely impaired corneal wound healing and markedly upregulated proinflammatory gene expression. Exogenous neuropeptides calcitonin gene-related peptide (CGRP), substance P (SP), and vasoactive intestinal peptide (VIP) partially reversed resiniferatoxin's effects, with VIP specifically inducing interleukin-10 expression. Hence, we focused on VIP and observed that wounding induced VIP and VIP type 1 receptor (VIPR1) expression in normal (NL) corneas, but not corneas from mice with diabetes mellitus (DM). Targeting VIPR1 in NL corneas attenuated corneal wound healing, dampened wound-induced expression of neurotrophic factors, and exacerbated inflammatory responses, while exogenous VIP had the opposite effects in DM corneas. Remarkably, wounding and diabetes also affected the expression of Sonic Hedgehog (Shh) in a VIP-dependent manner. Downregulating Shh expression in NL corneas decreased while exogenous Shh in DM corneas increased the rates of corneal wound healing. Furthermore, inhibition of Shh signaling dampened VIP-promoted corneal wound healing. We conclude that VIP regulates epithelial wound healing, inflammatory response, and nerve regeneration in the corneas in an Shh-dependent manner, suggesting a therapeutic potential for these molecules in treating diabetic keratopathy.

Introducing www.axolotl-omics.org - an integrated -omics data portal for the axolotl research community

Genomic resources are indispensable for biological investigations in model organisms. In recent years, a number of genomic resources including a full genome assembly, extensive transcriptomic data, as well as genome editing has been developed for the axolotl, a classical model organism for developmental, neurobiological and regeneration studies, making the axolotl a highly versatile system. Here we describe the Axolotl-omics website that allows rapid ortholog searches, and access to genome and transcriptomic resources.

A versatile depigmentation, clearing, and labeling method for exploring nervous system diversity

Tissue clearing combined with deep imaging has emerged as a powerful alternative to classical histological techniques. Whereas current techniques have been optimized for imaging selected nonpigmented organs such as the mammalian brain, natural pigmentation remains challenging for most other biological specimens of larger volume. We have developed a fast DEpigmEntation-Plus-Clearing method (DEEP-Clear) that is easily incorporated in existing workflows and combines whole system labeling with a spectrum of detection techniques, ranging from immunohistochemistry to RNA in situ hybridization, labeling of proliferative cells (EdU labeling) and visualization of transgenic markers. With light-sheet imaging of whole animals and detailed confocal studies on pigmented organs, we provide unprecedented insight into eyes, whole nervous systems, and subcellular structures in animal models ranging from worms and squids to axolotls and zebrafish. DEEP-Clear thus paves the way for the exploration of species-rich clades and developmental stages that are largely inaccessible by regular imaging approaches.

von Willebrand factor D and EGF domains is an evolutionarily conserved and required feature of blastemas capable of multitissue appendage regeneration

Regenerative ability varies tremendously across species. A common feature of regeneration of appendages such as limbs, fins, antlers, and tails is the formation of a blastema—a transient structure that houses a pool of progenitor cells that can regenerate the missing tissue. We have identified the expression of von Willebrand factor D and EGF domains (vwde) as a common feature of blastemas capable of regenerating limbs and fins in a variety of highly regenerative species, including axolotl (Ambystoma mexicanum), lungfish (Lepidosiren paradoxa), and Polpyterus (Polypterus senegalus). Further, vwde expression is tightly linked to the ability to regenerate appendages in Xenopus laevis. Functional experiments demonstrate a requirement for vwde in regeneration and indicate that Vwde is a potent growth factor in the blastema. These data identify a key role for vwde in regenerating blastemas and underscore the power of an evolutionarily informed approach for identifying conserved genetic components of regeneration.

vwde expression is a common feature of blastemas capable of fin and limb regeneration.

vwde expression is tightly tied to regeneration‐competency.

vwde is required for axolotl limb regeneration, with transient knockdown resulting in severe endpoint phenotypes.

Remembering where we are: Positional information in salamander limb regeneration

Fifty years ago, Lewis Wolpert defined an important question in developmental biology: how are cell fates determined by the positions of cells within a system? He proposed that cells retain positional values as if they lie within a coordinate system and that the interpretation of these values produces patterns in development. He referred to this concept as positional information. Though initially controversial, this concept of positional information has proven to be profoundly influential in developmental biology. One area in which the influence of Wolpert's theoretical work can be clearly demonstrated is the study of limb regeneration in salamanders. Here, we review the work in limb regeneration leading up to Wolpert defining the concept of positional information and how his theory has guided regeneration research over the subsequent 50 years.

Identification of Heparan-Sulfate Rich Cells in the Loose Connective Tissues of the Axolotl (Ambystoma mexicanum) with the Potential to Mediate Growth Factor Signaling during Regeneration

Limb regeneration is the outcome of a complex sequence of events that are mediated by interactions between cells derived from the tissues of the amputated stump. Early in regeneration, these interactions are mediated by growth factor/morphogen signaling associated with nerves and the wound epithelium. One shared property of these proregenerative signaling molecules is that their activity is dependent on interactions with sulfated glycosaminoglycans (GAGs), heparan sulfate proteoglycan (HSPG) in particular, in the extracellular matrix (ECM). We hypothesized that there are cells in the axolotl that synthesize specific HSPGs that control growth factor signaling in time and space. In this study we have identified a subpopulation of cells within the ECM of axolotl skin that express high levels of sulfated GAGs on their cell surface. These cells are dispersed in a grid-like pattern throughout the dermis as well as the loose connective tissues that surround the tissues of the limb. These cells alter their morphology during regeneration, and are candidates for being a subpopulation of connective tissue cells that function as the cells required for pattern-formation during regeneration. Given their high level of HSPG expression, their stellate morphology, and their distribution throughout the loose connective tissues, we refer to these as the positional information GRID (Groups that are Regenerative, Interspersed and Dendritic) cells. In addition, we have identified cells that stain for high levels of expression of sulfated GAGs in mouse limb connective tissue that could have an equivalent function to GRID cells in the axolotl. The identification of GRID cells may have important implications for work in the area of Regenerative Engineering.

Positional information and tissue scaling during development and regeneration

In order to contribute to the appropriate tissues during development, cells need to know their position within the embryo. This positional information is conveyed by gradients of signaling molecules, termed morphogens, that are produced in specific regions of the embryo and induce concentration-dependent responses in target tissues. Positional information is remarkably robust, and embryos often develop with the correct proportions even if large parts of the embryo are removed. In this Review, we discuss classical embryological experiments and modern quantitative analyses that have led to mechanistic insights into how morphogen gradients adapt, scale and properly pattern differently sized domains. We analyze these experimental findings in the context of mathematical models and synthesize general principles that apply to multiple systems across species and developmental stages.

Cell type-specific transcriptome analysis unveils secreted signaling molecule genes expressed in apical epithelial cap during appendage regeneration

Wound epidermis (WE) and the apical epithelial cap (AEC) are believed to trigger regeneration of amputated appendages such as limb and tail in amphibians by producing certain secreted signaling molecules. To date, however, only limited information about the molecular signatures of these epidermal structures is available. Here we used a transgenic Xenopus laevis line harboring the enhanced green fluorescent protein (egfp) gene under control of an es1 gene regulatory sequence to isolate WE/AEC cells by performing fluorescence-activated cell sorting during the time course of tail regeneration (day 1, day 2, day 3 and day 4 after amputation). Time-course transcriptome analysis of these isolated WE/AEC cells revealed that more than 8,000 genes, including genes involved in signaling pathways such as those of reactive oxygen species, fibroblast growth factor (FGF), canonical and non-canonical Wnt, transforming growth factor β (TGF β) and Notch, displayed dynamic changes of their expression during tail regeneration. Notably, this approach enabled us to newly identify seven secreted signaling molecule genes (mdk, fstl, slit1, tgfβ1, bmp7.1, angptl2 and egfl6) that are highly expressed in tail AEC cells. Among these genes, five (mdk, fstl, slit1, tgfβ1 and bmp7.1) were also highly expressed in limb AEC cells but the other two (angptl2 and egfl6) are specifically expressed in tail AEC cells. Interestingly, there was no expression of fgf8 in tail WE/AEC cells, whose expression and pivotal role in limb AEC cells have been reported previously. Thus, we identified common and different properties between tail and limb AEC cells.

Can laboratory model systems instruct human limb regeneration?

Regeneration has fascinated scientists since well before the 20th century revolutions in genetics and molecular biology. The field of regenerative biology has grown steadily over the past decade, incorporating advances in imaging, genomics and genome editing to identify key cell types and molecules involved across many model organisms. Yet for many or most tissues, it can be difficult to predict when and how findings from these studies will advance regenerative medicine. Establishing technologies to stimulate regrowth of a lost or amputated limb with a patterned replicate, as salamanders do routinely, is one of the most challenging directives of tissue regeneration research. Here, we speculate upon what research avenues the field must explore to move closer to this capstone achievement.

The multifaceted role of nerves in animal regeneration

The discovery that the nervous system plays a critical role in salamander limb regeneration, in 1823, provided the first mechanistic insights into regenerative phenomena and stimulated a long quest for molecular regulators. A role for nerves in the context of regeneration has been suggested for most vertebrate and invertebrate groups, thus offering a possible shared mechanism for the regulation of regenerative processes among animals. Methodological differences and technical limitations, especially in invertebrate groups, have so far hampered broad comparisons and the search for common principles on the role of nerves. This review considers both old and recent work in this topic and provides a broad perspective on the roles of nerves during regeneration. Nerves are found consistently to have important roles in regeneration, but their mode of action varies across species. The ongoing technological developments in a broad range of invertebrate models are now paving the way for the discovery of the shared and unique roles of nerves in animal regeneration.

Fgf-signaling is compartmentalized within the mesenchyme and controls proliferation during salamander limb development

Although decades of studies have produced a generalized model for tetrapod limb development, urodeles deviate from anurans and amniotes in at least two key respects: their limbs exhibit preaxial skeletal differentiation and do not develop an apical ectodermal ridge (AER). Here, we investigated how Sonic hedgehog (Shh) and Fibroblast growth factor (Fgf) signaling regulate limb development in the axolotl. We found that Shh-expressing cells contributed to the most posterior digit, and that inhibiting Shh-signaling inhibited Fgf8 expression, anteroposterior patterning, and distal cell proliferation. In addition to lack of a morphological AER, we found that salamander limbs also lack a molecular AER. We found that amniote and anuran AER-specific Fgfs and their cognate receptors were expressed entirely in the mesenchyme. Broad inhibition of Fgf-signaling demonstrated that this pathway regulates cell proliferation across all three limb axes, in contrast to anurans and amniotes where Fgf-signaling regulates cell survival and proximodistal patterning.

Salamanders are a group of amphibians that are well-known for their ability to regenerate lost limbs and other body parts. At the turn of the twentieth century, researchers used salamander embryos as models to understand the basic concepts of how limbs develop in other four-limbed animals, including amphibians, mammals and birds, which are collectively known as “tetrapods”. However, the salamander’s amazing powers of regeneration made it difficult to carry out certain experiments, so researchers switched to using the embryos of other tetrapods – namely chickens and mice – instead.

Studies in chickens, later confirmed in mice and frogs, established that there are two major signaling centers that control how the limbs of tetrapod embryos form and grow: a small group of cells known as the “zone of polarizing activity” within a structure called the “limb bud mesenchyme”; and an overlying, thin ridge of cells called the “apical ectodermal ridge”. Both of these centers release potent signaling molecules that act on cells in the limbs. The cells in the zone of polarizing activity produce a molecule often called Sonic hedgehog, or Shh for short. The apical ectodermal ridge produces another group of signals commonly known as fibroblast growth factors, or simply Fgfs.

Several older studies reported that salamander embryos do not have an apical ectodermal ridge suggesting that these amphibian’s limbs may form differently to other tetrapods. Yet, contemporary models in developmental biology treated salamander limbs like those of chicks and mice. To address this apparent discrepancy, Purushothaman et al. studied how the forelimbs develop in a salamander known as the axolotl.

The experiments showed that, along with lacking an apical ectodermal ridge, axolotls did not produce fibroblast growth factors normally found in this tissue. Instead, these factors were only found in the limb bud mesenchyme. Purushothaman et al. also found that fibroblast growth factors played a different role in axolotls than previously reported in chick, frog and mouse embryos. On the other hand, the pattern and function of Shh activity in the axolotl limb bud was similar to that previously observed in chicks and mice.

These findings show that not all limbs develop in the same way and open up questions for evolutionary biologists regarding the evolution of limbs. Future studies that examine limb development in other animals that regenerate tissues, such as other amphibians and lungfish, will help answer these questions.

Stability and plasticity of positional memory during limb regeneration in Ambystoma mexicanum

BACKGROUND

Urodele amphibians are capable of regenerating their organs after severe damage. During such regeneration, participating cells are given differentiation instructions by the surrounding cells. Limb regeneration has been investigated as a representative phenomenon of organ regeneration. Cells known as blastema cells are induced after limb amputation. In this process, dermal fibroblasts are dedifferentiated and become undifferentiated similar to limb bud cells. Just like limb bud cells, the induced blastema cells are positioned along the three limb developmental axes: the dorsoventral, the anteroposterior, and the proximodistal. The accurate developmental axes are essential for reforming the structures correctly. Despite the importance of the developmental axes, the relationship between the newly establishing developmental axes and existing limb axes was not well described with molecular markers.

RESULTS

In this study, we grafted skin from GFP-transgenic axolotls and traced the cell lineage with position-specific gene expressions in order to investigate the correlation of the newly established axes and cellular origin. Shh- and Lmx1b-expressing cells emerged from the posterior skin and dorsal skin, respectively, even though the skin was transplanted to an inconsistent position. Shox2, a posterior marker gene, could be activated in cells derived from distal skin.

CONCLUSIONS

Our results suggest that the location memories on anteroposterior and dorsoventral axes are relatively stable in a regenerating blastema though cellular differentiation is reprogrammed.

Deep evolutionary origin of limb and fin regeneration

Salamanders and lungfishes are the only sarcopterygians (lobe-finned vertebrates) capable of paired appendage regeneration, regardless of the amputation level. Among actinopterygians (ray-finned fishes), regeneration after amputation at the fin endoskeleton has only been demonstrated in polypterid fishes (Cladistia). Whether this ability evolved independently in sarcopterygians and actinopterygians or has a common origin remains unknown. Here we combine fin regeneration assays and comparative RNA-sequencing (RNA-seq) analysis of Polypterus and axolotl blastemas to provide support for a common origin of paired appendage regeneration in Osteichthyes (bony vertebrates). We show that, in addition to polypterids, regeneration after fin endoskeleton amputation occurs in extant representatives of 2 other nonteleost actinopterygians: the American paddlefish (Chondrostei) and the spotted gar (Holostei). Furthermore, we assessed regeneration in 4 teleost species and show that, with the exception of the blue gourami (Anabantidae), 3 species were capable of regenerating fins after endoskeleton amputation: the white convict and the oscar (Cichlidae), and the goldfish (Cyprinidae). Our comparative RNA-seq analysis of regenerating blastemas of axolotl and Polypterus reveals the activation of common genetic pathways and expression profiles, consistent with a shared genetic program of appendage regeneration. Comparison of RNA-seq data from early Polypterus blastema to single-cell RNA-seq data from axolotl limb bud and limb regeneration stages shows that Polypterus and axolotl share a regeneration-specific genetic program. Collectively, our findings support a deep evolutionary origin of paired appendage regeneration in Osteichthyes and provide an evolutionary framework for studies on the genetic basis of appendage regeneration.

Model systems for regeneration: salamanders

Salamanders have been hailed as champions of regeneration, exhibiting a remarkable ability to regrow tissues, organs and even whole body parts, e.g. their limbs. As such, salamanders have provided key insights into the mechanisms by which cells, tissues and organs sense and regenerate missing or damaged parts. In this Primer, we cover the evolutionary context in which salamanders emerged. We outline the varieties of mechanisms deployed during salamander regeneration, and discuss how these mechanisms are currently being explored and how they have advanced our understanding of animal regeneration. We also present arguments about why it is important to study closely related species in regeneration research.

Hierarchical pattern formation during amphibian limb regeneration

In 1901 T.H. Morgan proposed in "Regeneration" that pattern formation in amphibian limb regeneration is a stepwise process. Since, biologist have continued to piece together the molecular components of this process to better understand the "patterning code" responsible for regenerate formation. Within this context, several different models have been proposed; however, all are based on one of two underlying hypotheses. The first is the "morphogen hypothesis" that dictates that pattern emerges from localized expression of signaling molecules, which produce differing position-specific cellular responses in receptive cells depending on the intensity of the signal. The second hypothesis is that cells in the remaining tissues retain memory of their patterning information, and use this information to generate new cells with the missing positional identities. A growing body of evidence supports the possibility that these two mechanisms are not mutually exclusive. Here, we propose our theory of hierarchical pattern formation, which consists of 4 basic steps. The first is the existence of cells with positional memory. The second is the communication of positional information through cell-cell interactions in a regeneration-permissive environment. The third step is the induction of molecular signaling centers. And the last step is the interpretation of these signals by specialized cell types to ultimately restore the limb in its entirety. Biological codes are intertwined throughout this model, and we will discuss their multiple roles and mechanisms.

Parenchymal and stromal tissue regeneration of tooth organ by pivotal signals reinstated in decellularized matrix

Cells are transplanted to regenerate an organs' parenchyma, but how transplanted parenchymal cells induce stromal regeneration is elusive. Despite the common use of a decellularized matrix, little is known as to the pivotal signals that must be restored for tissue or organ regeneration. We report that Alx3, a developmentally important gene, orchestrated adult parenchymal and stromal regeneration by directly transactivating Wnt3a and vascular endothelial growth factor. In contrast to the modest parenchyma formed by native adult progenitors, Alx3-restored cells in decellularized scaffolds not only produced vascularized stroma that involved vascular endothelial growth factor signalling, but also parenchymal dentin via the Wnt/β-catenin pathway. In an orthotopic large-animal model following parenchyma and stroma ablation, Wnt3a-recruited endogenous cells regenerated neurovascular stroma and differentiated into parenchymal odontoblast-like cells that extended the processes into newly formed dentin with a structure-mechanical equivalency to native dentin. Thus, the Alx3-Wnt3a axis enables postnatal progenitors with a modest innate regenerative capacity to regenerate adult tissues. Depleted signals in the decellularized matrix may be reinstated by a developmentally pivotal gene or corresponding protein.

FGF, BMP, and RA signaling are sufficient for the induction of complete limb regeneration from non-regenerating wounds on Ambystoma mexicanum limbs

Some organisms, such as the Mexican axolotl, have the capacity to regenerate complicated biological structures throughout their lives. Which molecular pathways are sufficient to induce a complete endogenous regenerative response in injured tissue is an important question that remains unanswered. Using a gain-of-function regeneration assay, known as the Accessory Limb Model (ALM), we and others have begun to identify the molecular underpinnings of the three essential requirements for limb regeneration; wounding, neurotrophic signaling, and the induction of pattern from cells that retain positional memory. We have previously shown that treatment of Mexican axolotls with exogenous retinoic acid (RA) is sufficient to induce the formation of complete limb structures from blastemas that were generated by deviating a nerve bundle into an anterior-located wound site on the limb. Here we show that these ectopic structures are capable of regenerating and inducing new pattern to form when grafted into new anterior-located wounds. We additionally found that the expression of Alx4 decreases, and Shh expression increases in these anterior located blastemas, but not in the mature anterior tissues, supporting the hypothesis that RA treatment posteriorizes blastema tissue. Based on these and previous observations, we used the ALM assay to test the hypothesis that a complete regenerative response can be generated by treating anterior-located superficial limb wounds with a specific combination of growth factors at defined developmental stages. Our data shows that limb wounds that are first treated with a combination of FGF-2, FGF-8, and BMP-2, followed by RA treatment of the resultant mid-bud stage blastema, will result in the generation of limbs with complete proximal/distal and anterior/posterior limb axes. Thus, the minimal signaling requirements from the nerve and a positional disparity are achieved with the application of this specific combination of signaling molecules.

Structural and biochemical analysis of DNA lesion-induced RNA polymerase II arrest

Transcription, catalyzed by RNA polymerase II (Pol II) in eukaryotes, is the first step in gene expression. RNA Pol II is a 12-subunit enzyme complex regulated by many different transcription factors during transcription initiation, elongation, and termination. During elongation, Pol II encounters various types of obstacles that can cause transcriptional pausing and arrest. Through decades of research on transcriptional pausing, it is widely known that Pol II can distinguish between different types of obstacles by its active site. A major class of obstacles is DNA lesions. While some DNA lesions can cause transient transcriptional pausing, which can be bypassed by Pol II itself or with the help from other elongation factors, bulky DNA damage can cause prolonged transcriptional pausing and arrest, which signals for transcription coupled repair. Using biochemical and structural biology approaches, the outcomes of many different types of DNA lesions, DNA modifications, and DNA binding molecules to transcription were studied. In this mini review, we will describe the in vitro transcription assays with Pol II to investigate the impacts of various DNA lesions on transcriptional outcomes and the crystallization method of lesion-arrested Pol II complex. These methods can provide a general platform for the structural and biochemical analysis of Pol II transcriptional pausing and bypass mechanisms.

Digit Tip Injuries: Current Treatment and Future Regenerative Paradigms

Over the past several decades there has been a profound increase in the understanding of tissue regeneration, driven largely by the observance of the tremendous regenerative capacity in lower order life forms, such as hydra and urodeles. However, it is known that humans and other mammals retain the ability to regenerate the distal phalanges of the digits after amputation. Despite the increased knowledge base on model organisms regarding regenerative paradigms, there is a lack of application of regenerative medicine techniques in clinical practice in regard to digit tip injury. Here, we review the current understanding of digit tip regeneration and discuss gaps that remain in translating regenerative medicine into clinical treatment of digit amputation.

Blastemal progenitors modulate immune signaling during early limb regeneration

Blastema formation, a hallmark of limb regeneration, requires proliferation and migration of progenitors to the amputation plane. Although blastema formation has been well described, the transcriptional programs that drive blastemal progenitors remain unknown. We transcriptionally profiled dividing and non-dividing cells in regenerating stump tissues, as well as the wound epidermis, during early axolotl limb regeneration. Our analysis revealed unique transcriptional signatures of early dividing cells and, unexpectedly, repression of several core developmental signaling pathways in early regenerating stump tissues. We further identify an immunomodulatory role for blastemal progenitors through interleukin 8 (IL-8), a highly expressed cytokine in subpopulations of early blastemal progenitors. Ectopic il-8 expression in non-regenerating limbs induced myeloid cell recruitment, while IL-8 knockdown resulted in defective myeloid cell retention during late wound healing, delaying regeneration. Furthermore, the il-8 receptor cxcr-1/2 was expressed in myeloid cells, and inhibition of CXCR-1/2 signaling during early stages of limb regeneration prevented regeneration. Altogether, our findings suggest that blastemal progenitors are active early mediators of immune support, and identify CXCR-1/2 signaling as an important immunomodulatory pathway during the initiation of regeneration.

Tissue Regeneration Enhancer Elements: A Way to Unlock Endogenous Healing Power

Regenerative capacity is widespread throughout almost all animal phyla. However, the distribution pattern remains incompletely understood. Various examples show that very closely related species display different regenerative capacities. Why and how have diverse regenerative capacities evolved across species? One prevailing thought in the field of regeneration is that most regeneration-associated factors are evolutionarily conserved, suggesting the existence of an innate tissue regeneration ability in all species. However, its regulation is differentially controlled in distinct species, resulting in heterogeneous regenerative capabilities. In this review, we discuss regeneration-associated enhancers, the key cis-regulatory elements controlling gene expression, their underlying molecular mechanisms, and their influence on regenerative capacity. Understanding the regulatory mechanisms of regeneration enhancers can provide fundamental insights into tissue regeneration and further help us develop therapeutic strategies to unlock latent healing powers in humans. Developmental Dynamics 248:34-42, 2019. © 2018 Wiley Periodicals, Inc.

Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration

Amputation of the axolotl forelimb results in the formation of a blastema, a transient tissue where progenitor cells accumulate prior to limb regeneration. However, the molecular understanding of blastema formation had previously been hampered by the inability to identify and isolate blastema precursor cells in the adult tissue. We have used a combination of Cre-loxP reporter lineage tracking and single-cell messenger RNA sequencing (scRNA-seq) to molecularly track mature connective tissue (CT) cell heterogeneity and its transition to a limb blastema state. We have uncovered a multiphasic molecular program where CT cell types found in the uninjured adult limb revert to a relatively homogenous progenitor state that recapitulates an embryonic limb bud-like phenotype including multipotency within the CT lineage. Together, our data illuminate molecular and cellular reprogramming during complex organ regeneration in a vertebrate.

Mechanoresponsive stem cells acquire neural crest fate in jaw regeneration

During both embryonic development and adult tissue regeneration, changes in chromatin structure driven by master transcription factors lead to stimulus-responsive transcriptional programs. A thorough understanding of how stem cells in the skeleton interpret mechanical stimuli and enact regeneration would shed light on how forces are transduced to the nucleus in regenerative processes. Here we develop a genetically dissectible mouse model of mandibular distraction osteogenesis-which is a process that is used in humans to correct an undersized lower jaw that involves surgically separating the jaw bone, which elicits new bone growth in the gap. We use this model to show that regions of newly formed bone are clonally derived from stem cells that reside in the skeleton. Using chromatin and transcriptional profiling, we show that these stem-cell populations gain activity within the focal adhesion kinase (FAK) signalling pathway, and that inhibiting FAK abolishes new bone formation. Mechanotransduction via FAK in skeletal stem cells during distraction activates a gene-regulatory program and retrotransposons that are normally active in primitive neural crest cells, from which skeletal stem cells arise during development. This reversion to a developmental state underlies the robust tissue growth that facilitates stem-cell-based regeneration of adult skeletal tissue.

Differences in neural stem cell identity and differentiation capacity drive divergent regenerative outcomes in lizards and salamanders

While lizards and salamanders both exhibit the ability to regenerate amputated tails, the outcomes achieved by each are markedly different. Salamanders, such as Ambystoma mexicanum, regenerate nearly identical copies of original tails. Regenerated lizard tails, however, exhibit important morphological differences compared with originals. Some of these differences concern dorsoventral patterning of regenerated skeletal and spinal cord tissues; regenerated salamander tail tissues exhibit dorsoventral patterning, while regrown lizard tissues do not. Additionally, regenerated lizard tails lack characteristically roof plate-associated structures, such as dorsal root ganglia. We hypothesized that differences in neural stem cells (NSCs) found in the ependyma of regenerated spinal cords account for these divergent regenerative outcomes. Through a combination of immunofluorescent staining, RT-PCR, hedgehog regulation, and transcriptome analysis, we analyzed NSC-dependent tail regeneration. Both salamander and lizard Sox2⁺ NSCs form neurospheres in culture. While salamander neurospheres exhibit default roof plate identity, lizard neurospheres exhibit default floor plate. Hedgehog signaling regulates dorsalization/ventralization of salamander, but not lizard, NSCs. Examination of NSC differentiation potential in vitro showed that salamander NSCs are capable of neural differentiation into multiple lineages, whereas lizard NSCs are not, which was confirmed by in vivo spinal cord transplantations. Finally, salamander NSCs xenogeneically transplanted into regenerating lizard tail spinal cords were influenced by native lizard NSC hedgehog signals, which favored salamander NSC floor plate differentiation. These findings suggest that NSCs in regenerated lizard and salamander spinal cords are distinct cell populations, and these differences contribute to the vastly different outcomes observed in tail regeneration.

New insight into functional limb regeneration: A to Z approaches

Limb/digit amputation is a common event in humans caused by trauma, medical illness, or surgery. Although the loss of a digit is not lethal, it affects quality of life and imposes high costs on amputees. In recent years, the increasing interest in limb regeneration has led to enhanced scientific knowledge. However, the limited ability to develop functional limb regeneration in the clinical setting suggests that a challenging issue remains in limb regeneration. Recently, the emergence of regenerative engineering is a promising field to address this challenge and close the gap between science and clinical applications. Cell signalling and molecular mechanisms involved in the limb regeneration process have been extensively studied; however, there is still insufficient data on cell therapy and tissue engineering for limb regeneration. In this review, we intend to focus on therapeutic approaches for limb regeneration that are closely related to gene, immune, and stem cell therapies, as well as tissue engineering approaches that take into consideration the peculiar developmental properties of the limbs. In addition, we attempt to identify the challenges of these strategies for limb regeneration studies in terms of clinical settings and as a road map to accomplish the goal of functional human limb regeneration.

The journey of cells through regeneration

The process of building an organ, appendage, or organism requires the precise coordination of cells in space and time. Regeneration of those same tissues adds an additional element of complexity, emerging from the chaos of disease or injury to build a mass of progenitors from mature tissue. Translating insights from natural examples of tissue regeneration into engineered regenerative therapies requires a deep understanding of the journey of a cell directly following injury to its contribution to functional, scaled replacement tissue. Here we step through the chronological phases of regeneration and highlight emerging work that brings us closer to elucidating the unique intrinsic and extrinsic properties of cells during epimorphic regeneration.

Efficient induction of functional ameloblasts from human keratinocyte stem cells

Background

Although adult human tissue-derived epidermal stem cells are capable of differentiating into enamel-secreting ameloblasts and forming teeth with regenerated enamel when recombined with mouse dental mesenchyme that possesses odontogenic potential, the induction rate is relatively low. In addition, whether the regenerated enamel retains a running pattern of prism identical to and acquires mechanical properties comparable with human enamel indeed warrants further study.

Methods

Cultured human keratinocyte stem cells (hKSCs) were treated with fibroblast growth factor 8 (FGF8) and Sonic hedgehog (SHH) for 18 h or 36 h prior to being recombined with E13.5 mouse dental mesenchyme with implantation of FGF8 and SHH-soaked agarose beads into reconstructed chimeric tooth germs. Recombinant tooth germs were subjected to kidney capsule culture in nude mice. Harvested samples at various time points were processed for histological, immunohistochemical, TUNEL, and western blot analysis. Scanning electronic microscopy and a nanoindentation test were further employed to analyze the prism running pattern and mechanical properties of the regenerated enamel.

Results

Treatment of hKSCs with both FGF8 and SHH prior to tissue recombination greatly enhanced the rate of tooth-like structure formation to about 70%. FGF8 and SHH dramatically enhanced stemness of cultured hKSCs. Scanning electron microscopic analysis revealed the running pattern of intact prisms of regenerated enamel is similar to that of human enamel. The nanoindentation test indicated that, although much softer than human child and adult mouse enamel, mechanical properties of the regenerated enamel improved as the culture time was extended.

Conclusions

Application of FGF8 and SHH proteins in cultured hKSCs improves stemness but does not facilitate odontogenic fate of hKSCs, resulting in an enhanced efficiency of ameloblastic differentiation of hKSCs and tooth formation in human–mouse chimeric tooth germs.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-0822-4) contains supplementary material, which is available to authorized users.

Ectopic Fgf signaling induces the intercalary response in developing chicken limb buds

BACKGROUND

Intercalary pattern formation is an important regulatory step in amphibian limb regeneration. Amphibian limb regeneration is composed of multiple steps, including wounding, blastema formation, and intercalary pattern formation. Attempts have been made to transfer insights from regeneration-competent animals to regeneration-incompetent animalsat each step in the regeneration process. In the present study, we focused on the intercalary mechanism in chick limb buds. In amphibian limb regeneration, a proximodistal axis is organized as soon as a regenerating blastema is induced. Intermediate structures are subsequently induced (intercalated) between the established proximal and distal identities. Intercalary tissues are derived from proximal tissues. Fgf signaling mediates the intercalary response in amphibian limb regeneration.

RESULTS

We attempted to transfer insights into intercalary regeneration from amphibian models to the chick limb bud. The zeugopodial part was dissected out, and the distal and proximal parts were conjunct at st. 24. Delivering ectopic Fgf2 + Fgf8 between the distal and proximal parts resulted in induction of zeugopodial elements. Examination of HoxA11 expression, apoptosis, and cell proliferation provides insights to compare with those in the intercalary mechanism of amphibian limb regeneration. Furthermore, the cellular contribution was investigated in both the chicken intercalary response and that of axolotl limb regeneration.

CONCLUSIONS

We developed new insights into cellular contribution in amphibian intercalary regeneration, and found consistency between axolotl and chicken intercalary responses. Our findings demonstrate that the same principal of limb regeneration functions between regeneration-competent and -incompetent animals. In this context, we propose the feasibility of the induction of the regeneration response in amniotes.

Cross-limb communication during Xenopus hind-limb regenerative response: non-local bioelectric injury signals

Regeneration of damaged body-parts requires coordination of size, shape, location, and orientation of tissue with the rest of the body. It is not currently known how far injury sites communicate with the remaining soma during repair, or what information may emanate from the injury site to other regions. We examined the bioelectric properties (resting potential gradients in the epidermis) of Xenopus froglets undergoing hind-limb amputation and observed that the contralateral (un-damaged) limb exhibits apparent depolarization signals immediately after the opposite hind-limb is amputated. The pattern of depolarization matches that of the amputated limb and is correlated to the position and type of injury, revealing that information about damage is available to remote body tissues and is detectable non-invasively in vivo by monitoring of the bioelectric state. These data extend knowledge about the electrophysiology of regenerative response, identify a novel communication process via long-range spread of injury signaling, a phenomenon which we call bioelectric injury mirroring (BIM), and suggests revisions to regenerative medicine and diagnostic strategies focused entirely on the wound site and to the use of contralateral limbs as controls.

Evolution of antero-posterior patterning of the limb: Insights from the chick

The developing limbs of chicken embryos have served as pioneering models for understanding pattern formation for over a century. The ease with which chick wing and leg buds can be experimentally manipulated, while the embryo is still in the egg, has resulted in the discovery of important developmental organisers, and subsequently, the signals that they produce. Sonic hedgehog (Shh) is produced by mesenchyme cells of the polarizing region at the posterior margin of the limb bud and specifies positional values across the antero-posterior axis (the axis running from the thumb to the little finger). Detailed experimental embryology has revealed the fundamental parameters required to specify antero-posterior positional values in response to Shh signaling in chick wing and leg buds. In this review, the evolution of the avian wing and leg will be discussed in the broad context of tetrapod paleontology, and more specifically, ancestral theropod dinosaur paleontology. How the parameters that dictate antero-posterior patterning could have been modulated to produce the avian wing and leg digit patterns will be considered. Finally, broader speculations will be made regarding what the antero-posterior patterning of chick limbs can tell us about the evolution of other digit patterns, including those that were found in the limbs of the earliest tetrapods.

Generic wound signals initiate regeneration in missing-tissue contexts

Despite the identification of numerous regulators of regeneration in different animal models, a fundamental question remains: why do some wounds trigger the full regeneration of lost body parts, whereas others resolve by mere healing? By selectively inhibiting regeneration initiation, but not the formation of a wound epidermis, here we create headless planarians and finless zebrafish. Strikingly, in both missing-tissue contexts, injuries that normally do not trigger regeneration activate complete restoration of heads and fin rays. Our results demonstrate that generic wound signals have regeneration-inducing power. However, they are interpreted as regeneration triggers only in a permissive tissue context: when body parts are missing, or when tissue-resident polarity signals, such as Wnt activity in planarians, are modified. Hence, the ability to decode generic wound-induced signals as regeneration-initiating cues may be the crucial difference that distinguishes animals that regenerate from those that cannot.

Some wounds trigger regeneration, while others simply heal but how this is regulated is unclear. Here, by manipulating ERK and Wnt signalling pathways, the authors create headless planarians and finless zebrafish and show that wounds that normally only trigger wound healing can activate regeneration of heads and bones.

Pseudotyped baculovirus is an effective gene expression tool for studying molecular function during axolotl limb regeneration

Axolotls can regenerate complex structures through recruitment and remodeling of cells within mature tissues. Accessing the underlying mechanisms at a molecular resolution is crucial to understand how injury triggers regeneration and how it proceeds. However, gene transformation in adult tissues can be challenging. Here we characterize the use of pseudotyped baculovirus (BV) as an effective gene transfer method both for cells within mature limb tissue and within the blastema. These cells remain competent to participate in regeneration after transduction. We further characterize the effectiveness of BV for gene overexpression studies by overexpressing Shh in the blastema, which yields a high penetrance of classic polydactyly phenotypes. Overall, our work establishes BV as a powerful tool to access gene function in axolotl limb regeneration.

Fibroblast growth factors: key players in regeneration and tissue repair

Tissue injury initiates a complex repair process, which in some organisms can lead to the complete regeneration of a tissue. In mammals, however, the repair of most organs is imperfect and results in scar formation. Both regeneration and repair are orchestrated by a highly coordinated interplay of different growth factors and cytokines. Among the key players are the fibroblast growth factors (FGFs), which control the migration, proliferation, differentiation and survival of different cell types. In addition, FGFs influence the expression of other factors involved in the regenerative response. Here, we summarize current knowledge on the roles of endogenous FGFs in regeneration and repair in different organisms and in different tissues and organs. Gaining a better understanding of these FGF activities is important for appropriate modulation of FGF signaling after injury to prevent impaired healing and to promote organ regeneration in humans.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Non-model model organisms

Model organisms are widely used in research as accessible and convenient systems to study a particular area or question in biology. Traditionally only a handful of organisms have been widely studied, but modern research tools are enabling researchers to extend the set of model organisms to include less-studied and more unusual systems. This Forum highlights a range of 'non-model model organisms' as emerging systems for tackling questions across the whole spectrum of biology (and beyond), the opportunities and challenges, and the outlook for the future.

Advances in Decoding Axolotl Limb Regeneration

Humans and other mammals are limited in their natural abilities to regenerate lost body parts. By contrast, many salamanders are highly regenerative and can spontaneously replace lost limbs even as adults. Because salamander limbs are anatomically similar to human limbs, knowing how they regenerate should provide important clues for regenerative medicine. Although interest in understanding the mechanics of this process has never wavered, until recently researchers have been vexed by seemingly impenetrable logistics of working with these creatures at a molecular level. Chief among the problems has been the very large size of salamander genomes, and not a single salamander genome has been fully sequenced to date. Recently the enormous gap in sequence information has been bridged by approaches that leverage mRNA as the starting point. Together with functional experimentation, these data are rapidly enabling researchers to finally uncover the molecular mechanisms underpinning the astonishing biological process of limb regeneration.

Serum Proteases Potentiate BMP-Induced Cell Cycle Re-entry of Dedifferentiating Muscle Cells during Newt Limb Regeneration

Limb amputation in the newt induces myofibers to dedifferentiate and re-enter the cell cycle to generate proliferative myogenic precursors in the regeneration blastema. Here we show that bone morphogenetic proteins (BMPs) and mature BMPs that have been further cleaved by serum proteases induce cell cycle entry by dedifferentiating newt muscle cells. Protease-activated BMP4/7 heterodimers that are present in serum strongly induced myotube cell cycle re-entry with protease cleavage yielding a 30-fold potency increase of BMP4/7 compared with canonical BMP4/7. Inhibition of BMP signaling via muscle-specific dominant-negative receptor expression reduced cell cycle entry in vitro and in vivo. In vivo inhibition of serine protease activity depressed cell cycle re-entry, which in turn was rescued by cleaved-mimic BMP. This work identifies a mechanism of BMP activation that generates blastema cells from differentiated muscle.

Msh homeobox 1 (Msx1)- and Msx2-overexpressing bone marrow-derived mesenchymal stem cells resemble blastema cells and enhance regeneration in mice

Amputation of the proximal region in mammals is not followed by regeneration because blastema cells (BCs) and expression of regenerative genes, such as Msh homeobox (Msx) genes, are absent in this animal group. The lack of BCs and positional information in other cells is therefore the main obstacle to therapeutic approaches for limb regeneration. Hence, this study aimed to create blastema-like cells (BlCs) by overexpressing Msx1 and Msx2 genes in mouse bone marrow-derived mesenchymal stem cells (mBMSCs) to regenerate a proximally amputated digit tip. We transduced mBMSCs with Msx1 and Msx2 genes and compared osteogenic activity and expression levels of several Msx-regulated genes (Bmp4, Fgf8, and keratin 14 (K14)) in BlC groups, including MSX1, MSX2, and MSX1/2 (in a 1:1 ratio) with those in mBMSCs and BCs in vitro and in vivo following injection into the amputation site. We found that Msx gene overexpression increased expression of specific blastemal markers and enhanced the proliferation rate and osteogenesis of BlCs compared with mBMSCs and BCs via activation of Fgf8 and Bmp4 Histological analyses indicated full regrowth of digit tips in the Msx-overexpressing groups, particularly in MSX1/2, through endochondral ossification 6 weeks post-injection. In contrast, mBMSCs and BCs formed abnormal bone and nail. Full digit tip was regenerated only in the MSX1/2 group and was related to boosted Bmp4, Fgf8, and K14 gene expression and to limb-patterning properties resulting from Msx1 and Msx2 overexpression. We propose that Msx-transduced cells that can regenerate epithelial and mesenchymal tissues may potentially be utilized in limb regeneration.

Lessons from the swamp: developing small molecules that confer salamander muscle cellularization in mammals

The ability of salamanders, such as newts, to regenerate damaged tissues has been studied for centuries. A prominent example of this regenerative power is the ability to re-grow entire amputated limbs. One important step in this regeneration process is skeletal muscle cellularization, in which the muscle fibers break down into dedifferentiated, mononuclear cells that proliferate and form new muscle in the replacement limb. In contrast, mammalian skeletal muscle does not undergo cellularization after injury. A significant proportion of research about tissue regeneration in salamanders aims to characterize regulatory genes that may have mammalian homologs. A less mainstream approach is to develop small molecule compounds that induce regeneration-related mechanisms in mammals. In this commentary, we discuss progress in discovering small molecules that induce cellularization in mammalian muscle. New research findings using these compounds has also shed light on cellular processes that regulate cellularization, such as apoptotic signaling. Although formidable technical hurdles remain, this progress increases our understanding of tissue regeneration and provide opportunities for developing small molecules that may enhance tissue repair in humans.

The two domain hypothesis of limb prepattern and its relevance to congenital limb anomalies

Functional annotation of mutations that cause human limb anomalies is enabled by basic developmental studies. In this study, we focus on the prepatterning stage of limb development and discuss a recent model that proposes anterior and posterior domains of the early limb bud generate two halves of the future skeleton. By comparing phenotypes in humans with those in model organisms, we evaluate whether this prepatterning concept helps to annotate human disease alleles. WIREs Dev Biol 2017, 6:e270. doi: 10.1002/wdev.270 For further resources related to this article, please visit the WIREs website.