The regeneration gap

Actin restricts cell proliferation and promotes differentiation during planarian regeneration

Actin is an integral component of the cytoskeleton, which plays an important role in various fundamental cellular processes, such as affecting the polarity of embryonic cells during embryonic development in various model organisms. Meanwhile, previous studies have demonstrated that the polymerization of the actin cytoskeleton can affect cell migration, proliferation, and differentiation. Actin polymerization state regulated osteogenic differentiation and affected cell proliferation. However, the function of actin in regenerative biology has not been thoroughly elucidated. The planarian flatworm, which contains a large number of adult somatic stem cells (neoblasts), is an ideal model organism to study regenerative biology. Here, we identified a homolog of actin in planarian Dugesia japonica and found that RNAi targeting actin during planarian regeneration results in the formation of protrusions on the dorsal side, where the division of phospho-H3 mitotic cells is increased. In addition, a decrease in differentiation is observed in regenerating tissues after Djactin RNAi. These results indicate that Djactin functions in proliferation and differentiation control in planarian regeneration.

Holy grail of tissue regeneration: Size

Cells and tissue within injured organs undergo a complicated healing process that still remains poorly understood. Interestingly, smaller organisms respond to injury with tissue regeneration and restoration of function, while humans and other large organisms respond to injury by forming dysfunctional, fibrotic scar tissue. Over the past few decades, allometric scaling principles have been well established to show that larger organisms experience exponentially higher tissue forces during movement and locomotion and throughout the organism's lifespan. How these evolutionary adaptations may affect tissue injury has not been thoroughly investigated in humans. We discuss how these adapations may affect healing and demonstrate that blocking the most evolutionary conserved biologic force sensor enables large organisms to heal after injury with true tissue regeneration. Future strategies to disrupt tissue force sensors may unlock the key to regenerating after injury in a wide range of organ systems.

Disrupting biological sensors of force promotes tissue regeneration in large organisms

Tissue repair and healing remain among the most complicated processes that occur during postnatal life. Humans and other large organisms heal by forming fibrotic scar tissue with diminished function, while smaller organisms respond with scarless tissue regeneration and functional restoration. Well-established scaling principles reveal that organism size exponentially correlates with peak tissue forces during movement, and evolutionary responses have compensated by strengthening organ-level mechanical properties. How these adaptations may affect tissue injury has not been previously examined in large animals and humans. Here, we show that blocking mechanotransduction signaling through the focal adhesion kinase pathway in large animals significantly accelerates wound healing and enhances regeneration of skin with secondary structures such as hair follicles. In human cells, we demonstrate that mechanical forces shift fibroblasts toward pro-fibrotic phenotypes driven by ERK-YAP activation, leading to myofibroblast differentiation and excessive collagen production. Disruption of mechanical signaling specifically abrogates these responses and instead promotes regenerative fibroblast clusters characterized by AKT-EGR1.

Humans and other large mammals heal wounds by forming fibrotic scar tissue with diminished function. Here, the authors show that disrupting mechanotransduction through the focal adhesion kinase pathway in large animals accelerates healing, prevents fibrosis, and enhances skin regeneration.

Cdc42 regulate the apoptotic cell death required for planarian epidermal regeneration and homeostasis

Rho GTPases have been shown previously to play important roles in several cellular processes by regulating the organization of the actin and microtubule cytoskeletons. However, the mechanisms of Rho GTPases that integrate the cellular responses during regeneration have not been thoroughly elucidated. The planarian flatworm, which contains a large number of adult somatic stem cells (neoblasts), is a unique model to study stem cell lineage development in vivo. Here, we focus on cdc42, which is an extensively characterized member among Rho GTPases. We found that cdc42 is required for the maintenance of epidermal lineage. Cdc42 RNAi induced a sustained increased of cell death and led to a loss of the mature epidermal cells but without affected cell division. Our results indicate that cdc42 function as an inhibitor to block the excessive apoptotic cell death in planarian epidermal regeneration and homeostasis.

Toxic responses of Sox2 gene in the regeneration of the earthworm Eisenia foetida exposed to Retnoic acid

Exogenous retinoic acid delays and disturbs the regeneration of Eisenia foetida. The stem cell pluripotency factor, Sox2, can play a crucial role in cell reprogramming and dedifferentiation. In this study, we compared the regeneration of Eisenia foetida in different segments after amputation and the effects of retinoic acid on the regeneration of different segments. The results showed that the regeneration speed of the head and tail was slightly faster than the middle part, and retinoic acid disrupted and delayed the regeneration of the earthworm. The qRT-PCR and Western blot analysis showed that the expression of the Sox2 gene and Sox2 protein was highest on the seventh day in different segments (p<0.05). After treatment with retinoic acid, the expression level of the Sox2 gene and Sox2 protein was significantly reduced (p<0.05). The results indicated that the regeneration of earthworms and the formation of blastema are related to the expression of the Sox2 gene and protein. Retinoic acid delays and interferes with the regeneration of the earthworm by affecting the expression levels of the Sox2 gene and protein.

Microarray analysis of microRNA expression during axolotl limb regeneration

Among vertebrates, salamanders stand out for their remarkable capacity to quickly regrow a myriad of tissues and organs after injury or amputation. The limb regeneration process in axolotls (Ambystoma mexicanum) has been well studied for decades at the cell-tissue level. While several developmental genes are known to be reactivated during this epimorphic process, less is known about the role of microRNAs in urodele amphibian limb regeneration. Given the compelling evidence that many microRNAs tightly regulate cell fate and morphogenetic processes through development and adulthood by modulating the expression (or re-expression) of developmental genes, we investigated the possibility that microRNA levels change during limb regeneration. Using two different microarray platforms to compare the axolotl microRNA expression between mid-bud limb regenerating blastemas and non-regenerating stump tissues, we found that miR-21 was overexpressed in mid-bud blastemas compared to stump tissue. Mature A. mexicanum (“Amex”) miR-21 was detected in axolotl RNA by Northern blot and differential expression of Amex-miR-21 in blastema versus stump was confirmed by quantitative RT-PCR. We identified the Amex Jagged1 as a putative target gene for miR-21 during salamander limb regeneration. We cloned the full length 3′UTR of Amex-Jag1, and our in vitro assays demonstrated that its single miR-21 target recognition site is functional and essential for the response of the Jagged1 gene to miR-21 levels. Our findings pave the road for advanced in vivo functional assays aimed to clarify how microRNAs such as miR-21, often linked to pathogenic cell growth, might be modulating the redeployment of developmental genes such as Jagged1 during regenerative processes.

A study of low power laser on the regenerative process of Girardia tigrina (Girard,1850) (Turbellaria; Tricladida; Dugesiidae)

The mechanism of regeneration does not start to restore the wound until its corresponding epimorphic phase. A bioestimulation of tissues and cells by laser radiation depends on the wavelength, on the dose, and on the intensity of the light. The goal of this work was to verify the effect of the low power laser at 660 nm on the regenerative process of Girardia tigrina. The specimens were maintained in the laboratory under a temperature ranging from 19° up to 24 °C for 21 days. The planarians were anesthetized by placing them on ice and then cut them with a scalpel. The three treatments were as following: animals individually irradiated with 14 sessions with 1 minute duration (treatment 1), 14 sessions with 3 minutes duration (treatment 2), and without irradiation (control). The planarians were amputated and divided in three study treatments: a control group (without radiation), and two other treatments: irradiated for 1 minute, and irradiated for 3 minutes. The animals were irradiated with diode laser (660 nm) with 3.3 ± 0.3 mW of power, using 0.94 mW.mm-2 power density for each irradiation procedure. During the experiment, 14 irradiation sessions were undertaken. The specimens were fixed in Bouin, and stained with hematoxyline and eosin. From observation and histological analysis, it was possible to assess the effects of interaction between laser and tissue. The head fragment after 1 minute of irradiation presented a better organized tissue scheme, when compared with the other treatments. Aspects of the body fragments submitted to 3 minutes of light treatment were very similar to fragments that had not been injured. It can be concluded that there are changes in the quality of regeneration when treated with low power laser under the conditions mentioned above.

Cellular plasticity in vertebrate regeneration

Within the animal kingdom, there are several examples of organisms with remarkable regenerative abilities. Among vertebrates, newts appear to be the most adept at replacing lost structures and injured organs and can regenerate their limbs, tails, spinal cords, jaws, retinas, lenses, optic nerves, intestines, and heart ventricles. This regenerative ability is dependent on the induction of an unusual degree of cellular plasticity near the site of injury. Mature cells lose their differentiated characteristics and revert to proliferating progenitor cells that will later redifferentiate to replace the lost or injured tissues. This degree of cellular plasticity appears to be restricted to those vertebrates with the most remarkable regenerative abilities and is not observed in mammals. However, in the last several years, there have been a few studies suggesting that certain factors present in newt tissues can induce a dedifferentiation response in mammalian cells. These results suggest that the knowledge gained from studying the molecular basis of cellular plasticity in newts and other regeneration-competent model organisms might one day be used to enhance the regenerative potential in mammals.

Low power laser radiation at 685nm stimulates stem-cell proliferation rate in Dugesia tigrina during regeneration

Today's scientific interest in tissue engineering for organ transplantations and regeneration from stem cells, allied with recent observations on biostimulation of tissues and cells by laser radiation, stands as a strong motivation for the present work, in which we examine the effects of the low power laser radiation onto planarians under regenerative process. To investigate those effects, a number of 60 amputated worms were divided in three study groups: a control group and two other groups submitted to daily 1 and 3 min long laser treatment sections at approximately 910 W/m2 power density. A 685 nm diode laser with 35 mW optical power was used. Samples were sent to histological analysis at the 4th, the 7th and the 15th days after amputation. A remarkable increase in stem cells counts for the fourth day of regeneration was observed when the regenerating worms was stimulated by the laser radiation. Our findings encourage further research works on the influence of optical radiation onto stem cells and tissue regeneration.

Regeneration in the ears of immunodeficient mice: identification and lineage analysis of mesenchymal stem cells

Wound healing in the ears of Athymic Nude-nu mice resembles regeneration. Histological analysis of the ear-punched tissues revealed the initial formation of a blastema-like structure followed by dermal, vascular, cartilage, and muscle regrowth exclusively in Athymic Nude-nu mice but not in wild-type controls (C57BL/6J). A subset of stem cells referred to here as ear mesenchymal stem cells (EMSCs) has been isolated from the external ears of regenerative (Athymic Nude-nu) and nonregenerative strains of mice. Morphological, histochemical, and molecular analysis after the induction of EMSC differentiation revealed multiple mesenchymal cell lineages (adipocytes, chondrocytes, and osteocytes) in all murine strains independent of their ability for regeneration. Thus, the absence of regeneration in wounded ears of C57BL/6J wild-type mice is not related to the absence of mesenchymal stem cell differentiation in tissue culture. Because nude mice lack T lymphocytes, it appears that in this model the absence of T lymphocytes in the wounded ears provides a microenvironment conducive to regeneration of mesenchymal tissues. These findings provide a new model to study the influence of the immune system on tissue regeneration.

DjPum, a homologue of Drosophila Pumilio, is essential to planarian stem cell maintenance

As stem cells are rare and difficult to study in vivo in adults, the use of classical models of regeneration to address fundamental aspects of the stem cell biology is emerging. Planarian regeneration, which is based upon totipotent stem cells present in the adult--the so-called neoblasts--provides a unique opportunity to study in vivo the molecular program that defines a stem cell. The choice of a stem cell to self-renew or differentiate involves regulatory molecules that also operate as translational repressors, such as members of PUF proteins. In this study, we identified a homologue of the Drosophila PUF gene Pumilio (DjPum) in the planarian Dugesia japonica, with an expression pattern preferentially restricted to neoblasts. Through RNA interference (RNAi), we demonstrate that gene silencing of DjPum dramatically reduces the number of neoblasts, thus supporting the intriguing hypothesis that stem cell maintenance may be an ancestral function of PUF proteins.

Regenerative response and endocrine disrupters in crinoid echinoderms: an old experimental model, a new ecotoxicological test

The regenerative phenomena that reproduce developmental processes in adult organisms and are regulated by endocrine and neurohumoral mechanisms can provide new sensitive tests for monitoring the effects of exposure to anthropogenic chemicals such as endocrine disrupter (ED) contaminants. These pollutants in fact can be bioaccumulated by the organisms, causing dysfunctions in steroid hormone production/metabolism and activities and inducing dramatic effects on reproductive competence, development and growth in many animals, man included. Current research is exploring the effects of exposure to different classes of compounds well known for their ED activity, such as polychlorinated biphenyls (PCBs), nonylphenols and organotins, on regenerative potential of echinoderms, a relatively unexplored and promising applied approach which offers the unique chance to study physiological developmental processes in adult animals. The selected test species is the crinoid Antedon mediterranea, which represents a valuable experimental model for investigation into the regenerative process from the macroscopic to the molecular level. The present study employs an integrated approach which combines exposure experiments, chemical analysis and biological analysis utilizing classical methods of light (LM) and electron (TEM and SEM) microscopy and immunocytochemistry. The experiments were carried out on experimentally induced arm regenerations in controlled conditions with exposure concentrations comparable to those of moderately polluted coastal zones in order to reproduce common conditions of exposure to environmental contaminants. The results of the exposure tests were analysed in terms of effects at the whole organism, at the tissue and cellular level, and possible sites of action of EDs. Our results show that prolonged exposure to these compounds significantly affects the regenerative mechanisms by inducing appreciable anomalies in terms of regeneration times, overall growth, general morphology and histological and cellular pattern. A concentration/effect relationship could be found for all substances. Interestingly, contrasting results in terms of inhibition or acceleration of regeneration phenomenon were obtained for different chemicals.

POEtic: an electronic tissue for bio-inspired cellular applications

In this paper, we introduce the general architecture of a new electronic tissue called POEtic. This reconfigurable circuit is designed to ease the implementation of bio-inspired systems that bring cellular applications into play. It contains special features that allow a developer to realize systems that require evolution (Phylogenesis), development (Ontogenesis), and/or learning (Epigenesis). A dynamic routing algorithm has been added to a structure similar to that of common commercial FPGAs, in order to allow the creation of data paths between cells. As the creation of these paths is dynamic, it is possible to add new cells or to repair faulty ones at runtime.

Transplantation of the urinary bladder and other organs in the subcutaneous tissue induces cyst formation and epithelialization: its potential usefulness in regenerative medicine

Certain hollow organs are known to form cysts when heterologously transplanted. In order to examine the usefulness of the phenomenon for regenerative medicine, rat urinary bladders and other organs were allo-transplanted under the subcutaneous tissue of the back. These transplanted tissues very often formed cysts covered with epithelia. The epithelia covered an area about twice the original size. In the case of the urinary bladder, the epithelium started moving from the edge of the transplants around day 3 after the operation, and as time proceeded, the tela submucosa and tunica muscularis also moved to encircle the epithelium, and formed the wall of the cyst. The basal laminae were formed under the newly expanded epithelium slightly behind the leading tip. All of the organs tested had the capability of cyst formation and epithelialization, although their rate differed between organs. The results are discussed with reference to the potential use of cyst formation for regenerating damaged organs.

[Adult stem cells: seing is not being]

Recent unexpected observations in adult rodents that stem/progenitor cells located in the bone marrow, but also in other tissues, could, after their transplantation to an irradiated host contribute to the regeneration of damaged organs such as brain, liver, pancreas or muscle, have raised much hope for future therapeutic applications. These data have also initially been interpreted as a proof of a possible transdifferentiation or plasticity of adult stem cells located in these tissues. Additional experiments rigorously analyzed have tempered initial enthusiasm, by showing that if marrow cells do migrate in damaged muscles and liver, their contribution to organ repair is low, and in some cases, explained by cell fusion. Nevertheless, among bone marrow cells, two categories of stem cells now emerge that have a potentially tremendous interest in cell therapy, if we succeed in understanding how to purify, amplify and differentiate these more efficiently and reproducibly.

CnidBase: The Cnidarian Evolutionary Genomics Database

CnidBase, the Cnidarian Evolutionary Genomics Database, is a tool for investigating the evolutionary, developmental and ecological factors that affect gene expression and gene function in cnidarians. In turn, CnidBase will help to illuminate the role of specific genes in shaping cnidarian biodiversity in the present day and in the distant past. CnidBase highlights evolutionary changes between species within the phylum Cnidaria and structures genomic and expression data to facilitate comparisons to non-cnidarian metazoans. CnidBase aims to further the progress that has already been made in the realm of cnidarian evolutionary genomics by creating a central community resource which will help drive future research and facilitate more accurate classification and comparison of new experimental data with existing data. CnidBase is available at http://cnidbase.bu.edu/.

FGFR-related gene nou-darake restricts brain tissues to the head region of planarians

The study of planarian regeneration may help us to understand how we can rebuild organs and tissues after injury, disease or ageing. The robust regenerative abilities of planarians are based upon a population of totipotent stem cells (neoblasts), and among the organs regenerated by these animals is a well-organized central nervous system. In recent years, methodologies such as whole-mount in situ hybridizations and double-stranded RNA have been extended to planarians with the aim of unravelling the molecular basis of their regenerative capacities. Here we report the identification and characterization of nou-darake (ndk), a gene encoding a fibroblast growth factor receptor (FGFR)-like molecule specifically expressed in the head region of the planarian Dugesia japonica. Loss of function of ndk by RNA interference results in the induction of ectopic brain tissues throughout the body. This ectopic brain formation was suppressed by inhibition of two planarian FGFR homologues (FGFR1 and FGFR2). Additionally, ndk inhibits FGF signalling in Xenopus embryos. The data suggest that ndk may modulate FGF signalling in stem cells to restrict brain tissues to the head region of planarians.

Protein transduction: a novel tool for tissue regeneration

Tissue regeneration in humans is limited and excludes vitals organs like heart and brain. Transformation experiments with oncogenes like T antigen have shown that retrodifferentiation of the respective cells is possible but hard to control. To bypass the risk of cancer formation a protein therapy approach has been developed. The transient delivery of proteins rather than genes could still induce terminally-differentiated cells to reenter the cell cycle. This approach takes advantage of protein-transducing domains that mediate the transfer of cargo proteins into cells. The goal of this brief review is to outline the basics of protein transduction and to discuss potential applications for tissue regeneration.