The apical ectodermal ridge (AER) can be re-induced by wounding, wnt-2b, and fgf-10 in the chicken limb bud

Making a new limb out of old cells: exploring endogenous cell reprogramming and its role during limb regeneration

Cellular reprogramming is characterized by the induced dedifferentiation of mature cells into a more plastic and potent state. This process can occur through artificial reprogramming manipulations in the laboratory such as nuclear reprogramming and induced pluripotent stem cell (iPSC) generation, and endogenously in vivo during amphibian limb regeneration. In amphibians such as the Mexican axolotl, a regeneration permissive environment is formed by nerve-dependent signaling in the wounded limb tissue. When exposed to these signals, limb connective tissue cells dedifferentiate into a limb progenitor-like state. This state allows the cells to acquire new pattern information, a property called positional plasticity. Here, we review our current understanding of endogenous reprogramming and why it is important for successful regeneration. We will also explore how naturally induced dedifferentiation and plasticity were leveraged to study how the missing pattern is established in the regenerating limb tissue.

The Accessory Limb Model Regenerative Assay and Its Derivatives

When the Accessory Limb Model (ALM) regenerative assay was first published by Endo, Bryant, and Gardiner in 2004, it provided a robust system for testing the cellular and molecular contributions during each of the basic steps of regeneration: the formation of the wound epithelium, neural induction of the apical epithelial cap, and the formation of a positional disparity between blastema cells. The basic ALM procedure was developed in the axolotl and involves deviating a limb nerve into a lateral wound and grafting skin from the opposing side of the limb axis into the site of injury. In this chapter, we will review the studies that lead to the conception of the ALM, as well as the studies that have followed the development of this assay. We will additionally describe in detail the standard ALM surgery and how to perform this surgery on different limb positions.

To regenerate or not to regenerate: Vertebrate model organisms of regeneration-competency and -incompetency

Why only certain species can regenerate their appendages (e.g. tails and limbs) remains one of the biggest mysteries of nature. Unlike anuran tadpoles and salamanders, humans and other mammals cannot regenerate their limbs, but can only regrow lost digit tips under specific circumstances. Numerous hypotheses have been postulated to explain regeneration-incompetency in mammals. By studying model organisms that show varying regenerative abilities, we now have more opportunities to uncover what contributes to regeneration-incompetency and functionally test which perturbations restore appendage regrowth. Particularly, Xenopus laevis tail and limb, and mouse digit tip model systems exhibit naturally occurring variations in regenerative capacities. Here, we discuss major hypotheses that are suggested to contribute to regeneration-incompetency, and how species with varying regenerative abilities reflect on these hypotheses.

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.

Regeneration and Regrowth Potentials of Digit Tips in Amphibians and Mammals

Tissue regeneration and repair have received much attention in the medical field over the years. The study of amphibians, such as newts and salamanders, has uncovered many of the processes that occur in these animals during full-limb/digit regeneration, a process that is highly limited in mammals. Understanding these processes in amphibians could shed light on how to develop and improve this process in mammals. Amputation injuries in mammals usually result in the formation of scar tissue with limited regrowth of the limb/digit; however, it has been observed that the very tips of digits (fingers and toes) can partially regrow in humans and mice under certain conditions. This review will summarize and compare the processes involved in salamander limb regeneration, mammalian wound healing, and digit regeneration in mice and humans.

Wnt-1 immunodetection in the regenerating tail of lizard suggests it is involved in the proliferation and distal growth of the blastema

Lizard tail regeneration depends from growth of the apical tip and autonomous regeneration of a new spinal cord, cartilaginous tube and muscles. The presence of embryonic signaling pathways is likely involved and we have focused on immunolocalization of Wnt1 protein in regenerating tissues, a protein promoting proliferation and tumorigenesis. Western blot indicates some immunoreactive bands in the expected range at 46 and 33kDa in the regenerating tail. Immunolocalization indicates that Wnt1 is prevalently detected in the apical wound epidermis, blastema, and ependyma ampulla of the regenerating tail while it lowers in other tissues of more proximal regions close to the original tail stump. Although a gradient for Wnt1 was not detected, the higher immunofluorescence present in the apical region of the blastema and around the regenerating spinal cord indicates that the protein could be secreted from the apical wound epidermis and the ependyma and might influence cell proliferation in the blastema, the distal-most growing center of the new tail. The present observations suggest the involvement of the Wnt pathway to direct the process of tail regeneration in lizard. The stimulation of proliferation of epidermal and mesenchymal cells in the apical blastema by Wnt proteins remains to be experimentally validated.

Genome-wide transcriptome analysis of mRNAs and microRNAs in Dorset and Small Tail Han sheep to explore the regulation of fecundity

A variety of sheep species with diverse fecundities are kept as livestock and make up the global agricultural economy. A mutation in the FecB gene has been implicated to be essential and additive for ovulation rate. To uncover potential regulators of fecundity, we performed a genome-wide analysis of mRNAs and miRNAs from Dorset sheep (Dorset), Small Tail Han sheep FecB(B)FecB(B) genotype (Han BB) and Small Tail Han sheep FecB(+)FecB(+) genotype (Han ++). Here we present detailed analyses at both the mRNA and miRNA levels to aid in the identification of candidate genes that might regulate fecundity. We found differentially expressed genes between each of the groups, which are involved in various cellular activities, such as metabolic cascades, catalytic function and signal transduction. Moreover, the miRNA profiling identified specific miRNAs unique to each group of sheep, which may play a role in the controlling fecundity differences. By exploring the miRNA-regulated gene expression network in the different sheep species we can create a stronger profile for regulation of fecundity. Furthermore, quantitative real-time PCR verified the reliability of the RNA-Seq data. To our knowledge, this is the first analysis of intravariety and intervariety in any species in this area. Taken together, this genome-wide analysis of mRNAs and miRNAs in sheep will aid in the ability to identify fecundity regulators between different sheep species.

Transient Inhibition of FGFR2b-ligands signaling leads to irreversible loss of cellular β-catenin organization and signaling in AER during mouse limb development

The vertebrate limbs develop through coordinated series of inductive, growth and patterning events. Fibroblast Growth Factor receptor 2b (FGFR2b) signaling controls the induction of the Apical Ectodermal Ridge (AER) but its putative roles in limb outgrowth and patterning, as well as in AER morphology and cell behavior have remained unclear. We have investigated these roles through graded and reversible expression of soluble dominant-negative FGFR2b molecules at various times during mouse limb development, using a doxycycline/transactivator/tet(O)-responsive system. Transient attenuation (≤24 hours) of FGFR2b-ligands signaling at E8.5, prior to limb bud induction, leads mostly to the loss or truncation of proximal skeletal elements with less severe impact on distal elements. Attenuation from E9.5 onwards, however, has an irreversible effect on the stability of the AER, resulting in a progressive loss of distal limb skeletal elements. The primary consequences of FGFR2b-ligands attenuation is a transient loss of cell adhesion and down-regulation of P63, β1-integrin and E-cadherin, and a permanent loss of cellular β-catenin organization and WNT signaling within the AER. Combined, these effects lead to the progressive transformation of the AER cells from pluristratified to squamous epithelial-like cells within 24 hours of doxycycline administration. These findings show that FGFR2b-ligands signaling has critical stage-specific roles in maintaining the AER during limb development.

Regeneration of the elbow joint in the developing chick embryo recapitulates development

Synovial joints are among the most important structures that give us complex motor abilities as humans. Degenerative joint diseases, such as arthritis, cause loss of normal joint functioning and affect over 40 million people in the USA and approximately 350 million people worldwide. Therapies based on regenerative medicine hold the promise of effectively repairing or replacing damaged joints permanently. Here, for the first time, we introduce a model for synovial joint regeneration utilizing the chick embryo. In this model, a block of tissue that contains the prospective elbow is excised, leaving a window with strips of anterior and posterior tissue intact (window excision, WE). In contrast, we also slice out the same area containing the elbow and the distal piece of the limb is pinned back onto the stump (slice excision, SE). Interestingly, when the elbow is removed via WE, regeneration of the joint takes place, whereas the elbow joint does not regenerate following SE. In order to investigate whether the regeneration response recapitulates the developmental program of forming joints, we used GDF-5 and Autotaxin (Atx) as joint tissue specific markers, and Sox-9 and Col-9 as cartilage markers for in situ hybridization on sections at different time points after WE and SE surgeries. Re-expression of GDF-5 and Atx is observed in the WE samples by 60h after surgery. In contrast, the majority of the samples that underwent SE surgery did not express GDF-5 and Atx. Also, in SE fusion of cartilage elements takes place and the joint interzone does not form. This is indicated by continuous Col-9 expression in SE limbs, whereas Col-9 is downregulated at the joint interzone in the regenerating WE samples. This order and pattern of gene expression observed in regenerates is similar to the development of a joint suggesting that regeneration recapitulates development at the molecular level. This model defines some of the conditions required for inducing joint regeneration in an otherwise nonregenerating environment. This knowledge can be useful for designing new therapeutic approaches for joint loss or for conditions affecting joint integrity in humans.

Sugar-coating wound repair: a review of FGF-10 and dermatan sulfate in wound healing and their potential application in burn wounds

Thousands of patients suffer from burn injuries each year, yet few therapies have been developed to accelerate the wound healing process. Most fibroblast growth factors (FGFs) have been extensively evaluated but only a few have been found to participate in the wound healing process. In particular, FGF-10 is robustly increased in the wound microenvironment after injury and has demonstrated some ability to promote wound healing in vitro and in vivo. Glycosaminoglycans are linear carbohydrates that participate in wound repair by influencing cytokine/growth factor localization and interaction with cognate receptors. Dermatan sulfate (DS) is the most abundant glycosaminoglycan in human wound fluid and has been postulated to be directly involved in the healing process. Recently, the combination of FGF-10 and DS demonstrated the potential to accelerate wound healing via increased keratinocyte proliferation and migration. Based on these preliminary studies, DS may serve as a cofactor for FGF-10, and together they are likely to expedite the healing process by stimulating keratinocyte activity. As a specific subtype of wounds, the overall healing process of burn injuries does not significantly differ from other types of wounds, where optimal repair results in matrix regeneration and complete reepithelialization. At present, standard burn treatment primarily involves topical application of antimicrobial agents, while no routine therapies target acceleration of reepithelialization, the key to wound closure. Thus, this novel therapeutic combination could be used in conjunction with some of the current therapies, but it would have the unique ability to initiate wound healing by stimulating keratinocyte epithelialization.