Generation of mononucleate cells from post-mitotic myotubes proceeds in the absence of cell cycle progression

Insights into intestinal regeneration signaling mechanisms

The cellular mechanisms underlying the amazing ability of sea cucumbers to regenerate their autotomized intestines have been widely described by us and others. However, the signaling pathways that control these mechanisms are unknown. Previous studies have shown that Wnt homologs are upregulated during early intestinal regenerative stages, suggesting that the Wnt/β-catenin pathway is active during this process. Here, we used small molecules, putative disruptors of the Wnt pathway, to determine the potential role of the canonical Wnt pathway on intestine regeneration in the sea cucumber Holothuria glaberrima. We evaluated their effects in vivo by using histological analyses for cell dedifferentiation, cell proliferation and apoptosis. We found that iCRT14, an alleged Wnt pathway inhibitor, decreased the size of the regenerating intestine, while LiCl, a presumed Wnt pathway activator, increased its size. The possible cellular mechanisms by which signaling pathway disruptors affect the gut rudiment size were further studied in vitro, using cultures of tissue explants and additional pharmacological agents. Among the tested signaling activators, those that act through GSK-3 inhibition, LiCl, 1-Azakenpaullone, and CHIR99021 were found to increase muscle cell dedifferentiation, while the inhibitor iCRT14 blocked cell dedifferentiation. Differently, cell proliferation was reduced by all GSK-3 inhibitors, as well as by iCRT14 and C59, which interferes with Wnt ligand secretion. The in vivo temporal and spatial pattern of β-catenin activity was determined using an antibody against phosphorylated β-catenin and shown to correlate with cell proliferative activity. In vitro treatment using C59 decreased the number of cells immunostained for nuclear phosphorylated β-catenin. Our results showed that the cell dedifferentiation observed during intestinal regeneration can be decoupled from the cell proliferation event and that these cellular processes can be modulated by particular signaling pathway inhibitors and activators. These results open the door for future studies where the cellular signaling pathways involved at each regeneration stage can be determined.

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?

Skeletal muscle dedifferentiation during salamander limb regeneration

Salamanders can regenerate entire limbs throughout their life. A critical step during limb regeneration is formation of a blastema, which gives rise to the new extremity. Salamander limb regeneration has historically been tightly linked to the term dedifferentiation, however, with refined research tools it is important to revisit the definition of dedifferentiation in the context. To what extent do differentiated cells revert their differentiated phenotypes? To what extent do progeny from differentiated cells cross lineage boundaries during regeneration? How do cell cycle plasticity and lineage plasticity relate to each other? What is the relationship between dedifferentiation of specialized cells and activation of tissue resident stem cells in terms of their contribution to the new limb? Here we highlight these problems through the case of skeletal muscle.

Dedifferentiation, Redifferentiation, and Transdifferentiation of Striated Muscles During Regeneration and Development

In some rare and striking cases, striated muscle fibers of the skeleton or body wall, which consist of terminally differentiated syncytia with complex ultrastructures, were found to be capable of dedifferentiating and fragmenting into mononucleate cells. Examples of such events will be discussed in which the dedifferentiated cells reenter the cell cycle, proliferate, and rebuilt damaged muscle fibers during limb regeneration or transdifferentiate to generate new types of muscles during normal development.

Sustained ERK activation underlies reprogramming in regeneration-competent salamander cells and distinguishes them from their mammalian counterparts

In regeneration-competent vertebrates, such as salamanders, regeneration depends on the ability of various differentiated adult cell types to undergo natural reprogramming. This ability is rarely observed in regeneration-incompetent species such as mammals, providing an explanation for their poor regenerative potential. To date, little is known about the molecular mechanisms mediating natural reprogramming during regeneration. Here, we have identified the extent of extracellular signal-regulated kinase (ERK) activation as a key component of such mechanisms. We show that sustained ERK activation following serum induction is required for re-entry into the cell cycle of postmitotic salamander muscle cells, partially by promoting the downregulation of p53 activity. Moreover, ERK activation induces epigenetic modifications and downregulation of muscle-specific genes such as Sox6. Remarkably, while long-term ERK activation is found in salamander myotubes, only transient activation is seen in their mammalian counterparts, suggesting that the extent of ERK activation could underlie differences in regenerative competence between species.

•

Sustained ERK activation is required for serum reprogramming of salamander cells

•

Only transient ERK activation is observed in their mammalian counterparts

•

Constant ERK activation promotes expression of S phase genes in mammalian myotubes

•

The extent of ERK activation could underlie differences in regenerative competence

Twist reverses muscle cell differentiation through transcriptional down-regulation of myogenin

Some higher vertebrates can display unique muscle regenerative abilities through dedifferentiation. Research evidence suggests that induced dedifferentiation can be achieved in mammalian cells. TWIST is a bHLH (basic helix-loop-helix) transcription factor that is expressed during embryonic development and plays critical roles in diverse developmental systems including myogenesis. Several experiments demonstrated its role in inhibition of muscle cell differentiation. We have previously shown that overexpression of TWIST can reverse muscle cell differentiation in the presence of growth factors. Here we show that TWIST reverses muscle cell differentiation through binding and down-regulation of myogenin. Moreover, it can reverse cellular morphology in the absence of growth factors.

Cell cycle regulation and regeneration

Regeneration of ear punch holes in the MRL mouse and amputated limbs of the axolotl show a number of similarities. A large proportion of the fibroblasts of the uninjured MRL mouse ear are arrested in G2 of the cell cycle, and enter nerve-dependent mitosis after injury to form a ring-shaped blastema that regenerates the ear tissue. Multiple cell types contribute to the establishment of the regeneration blastema of the urodele limb by dedifferentiation, and there is substantial reason to believe that the cells of this early blastema are also arrested in G2, and enter mitosis under the influence of nerve-dependent factors supplied by the apical epidermal cap. Molecular analysis reveals other parallels, such as; (1) the upregulation of Evi5, a centrosomal protein that prevents mitosis by stabilizing Emi1, a protein that inhibits the degradation of cyclins by the anaphase promoting complex and (2) the expression of sodium channels by the epidermis. A central feature in the entry into the cell cycle by MRL ear fibroblasts is a natural downregulation of p21, and knockout of p21 in wild-type mice confers regenerative capacity on non-regenerating ear tissue. Whether the same is true for entry into the cell cycle in regenerating urodele limbs is presently unknown.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

Transient inactivation of Rb and ARF yields regenerative cells from postmitotic mammalian muscle

An outstanding biological question is why tissue regeneration in mammals is limited, whereas urodele amphibians and teleost fish regenerate major structures, largely by cell cycle reentry. Upon inactivation of Rb, proliferation of postmitotic urodele skeletal muscle is induced, whereas in mammalian muscle this mechanism does not exist. We postulated that a tumor suppressor present in mammals but absent in regenerative vertebrates, the Ink4a product ARF (alternative reading frame), is a regeneration suppressor. Concomitant inactivation of Arf and Rb led to mammalian muscle cell cycle reentry, loss of differentiation properties, and upregulation of cytokinetic machinery. Single postmitotic myocytes were isolated by laser micro-dissection-catapulting, and transient suppression of Arf and Rb yielded myoblast colonies that retained the ability to differentiate and fuse into myofibers upon transplantation in vivo. These results show that differentiation of mammalian cells is reversed by inactivation of Arf and Rb and support the hypothesis that Arf evolved at the expense of regeneration.

Schwann cell dedifferentiation is independent of mitogenic signaling and uncoupled to proliferation: role of cAMP and JNK in the maintenance of the differentiated state

Myelinating Schwann cells (SCs) are highly plastic cells that are able to dedifferentiate and re-enter the cell cycle. However, the molecular signals controlling dedifferentiation are not completely understood. Because a connection between mitogenic signaling and myelin loss has been suggested, we investigated the role of cAMP, a strong inducer of the myelinating phenotype, and mitogenic factors activating receptor tyrosine kinases (RTKs) on SC dedifferentiation. We herein provide evidence indicating that cAMP was required to not only initiate but also maintain a state of differentiation because SCs rapidly dedifferentiated and became competent to resume proliferation upon the removal of cAMP stimulation. Surprisingly, isolated SCs could undergo multiple cycles of differentiation and dedifferentiation upon cAMP addition and removal, respectively, in the absence of mitogenic factors and without entering the cell cycle. Conversely, the activation of RTKs and the ERK cascade by a variety of growth factors, including neuregulin, was not sufficient to initiate dedifferentiation in the presence of cAMP. Importantly, a reduction of cAMP triggered dedifferentiation through a mechanism that required JNK, rather than ERK, activity and an induction of the expression of c-Jun, a transcriptional inhibitor of myelination. In summary, the reversible transition from an undifferentiated to a myelinating state was dependent on cAMP but independent of RTK signaling and cell cycle progression, further indicating that dedifferentiation and proliferation are uncoupled and differentially regulated events in SCs.

Human diploid fibroblast cells in senescence; cycling through polyploidy to mitotic cells

Previously, it was found that senescent cells can undergo a modified cell cycle with mitotic cells as the end results. The major cycling events started with polyploidization, followed by depolyploidization to multinucleated cells (MNCs). These latter cells produced mononuclear offspring cells that could express mitotic cell divisions. In this report the emphasis is on late senescent fibroblasts that exhibited the senescence-associated change in cell morphology to large flat cells. Prior to live cell photography, flat cell cultures were maintained for months in the same culture flasks and therefore judged to be in a late senescent phase. All of the cellular events outlined above were present in these old cell cultures. Time lapse pictures showed movements of mitotic daughter cells away from each other and alignment of the chromosomes on the metaphase plate was visible in other mitotic cells. These data challenge the common view that cell senescence is irreversible and, therefore, an antitumor mechanism. A new finding was that the spike in polyploid cells in the near senescent phase consisted of cells with pairs of sister chromosomes from endoreduplication of DNA (two rounds of DNA synthesis and no mitosis). The lack of cells with 92 single chromosomes (e.g., G2 tetraploid cells) suggested that these polyploid cells also went through a changed cell cycle. The question now is whether these atypical polyploid cells are a subpopulation in senescence that can undergo the cycling from polyploidy to genome-reduced mitotic cells.

Appendage regeneration in adult vertebrates and implications for regenerative medicine

The regeneration of complex structures in adult salamanders depends on mechanisms that offer pointers for regenerative medicine. These include the plasticity of differentiated cells and the retention in regenerative cells of local cues such as positional identity. Limb regeneration proceeds by the local formation of a blastema, a growth zone of mesenchymal stem cells on the stump. The blastema can regenerate autonomously as a self-organizing system over variable linear dimensions. Here we consider the prospects for limb regeneration in mammals from this viewpoint.

The regenerative plasticity of isolated urodele myofibers and its dependence on MSX1

The conversion of multinucleate postmitotic muscle fibers to dividing mononucleate progeny cells (cellularisation) occurs during limb regeneration in salamanders, but the cellular events and molecular regulation underlying this remarkable process are not understood. The homeobox gene Msx1 has been studied as an antagonist of muscle differentiation, and its expression in cultured mouse myotubes induces about 5% of the cells to undergo cellularisation and viable fragmentation, but its relevance for the endogenous programme of salamander regeneration is unknown. We dissociated muscle fibers from the limb of larval salamanders and plated them in culture. Most of the fibers were activated by dissociation to mobilise their nuclei and undergo cellularisation or breakage into viable multinucleate fragments. This was followed by microinjection of a lineage tracer into single fibers and analysis of the labelled progeny cells, as well as by time-lapse microscopy. The fibers showing morphological plasticity selectively expressed Msx1 mRNA and protein. The uptake of morpholino antisense oligonucleotides directed to Msx1 led to a specific decrease in expression of Msx1 protein in myonuclei and marked inhibition of cellularisation and fragmentation. Myofibers of the salamander respond to dissociation by activation of an endogenous programme of cellularisation and fragmentation. Lineage tracing demonstrates that cycling mononucleate progeny cells are derived from a single myofiber. The induction of Msx1 expression is required to activate this programme. Our understanding of the regulation of plasticity in postmitotic salamander cells should inform strategies to promote regeneration in other contexts.

Amphibians such as the salamander can regenerate their limbs. This paper explores how multinucleate muscle cells transform into mononuclear cells and begin to proliferate during regeneration

Amphibian regeneration and stem cells

Larval and adult urodeles and anuran tadpoles readily regenerate their limbs via a process of histolysis and dedifferentiation of mature cells local to the amputation surface that accumulate under the wound epithelium as a blastema of stem cells. These stem cells require growth and trophic factors from the apical epidermal cap (AEC) and the nerves that re-innervate the blastema for their survival and proliferation. Members of the fibroblast growth factor (FGF) family synthesized by both AEC and nerves, and glial growth factor, substance P, and transferrin of nerves are suspected survival and proliferation factors. Stem cells derived from fibroblasts and muscle cells can transdifferentiate into other cell types during regeneration. The regeneration blastema is a self-organizing system based on positional information inherited from parent limb cells. Retinoids, which act through nuclear receptors, have been used in conjunction with assays for cell adhesivity to show that positional identity of blastema cells is encoded in the cell surface. These molecules are involved in the cell-cell signaling network that re-establishes the original structural pattern of the limb. Other systems of interest that regenerate by histolysis and dedifferentiation of pigmented epithelial cells are the neural retina and lens. Members of the FGF family are also important to the regeneration of these structures. The mechanism of amphibian regeneration by dedifferentiation is of importance to the development of a regenerative medicine, since understanding this mechanism may offer insights into how we might chemically induce the regeneration of mammalian tissues.

Heterogeneous proliferative potential in regenerative adult newt cardiomyocytes

Adult newt cardiomyocytes, in contrast to their mammalian counterparts, can proliferate after injury and contribute to the functional regeneration of the heart. In order to understand the mechanisms underlying this plasticity we performed longitudinal studies on single cardiomyocytes in culture. We find that the majority of cardiomyocytes can enter S phase, a process that occurs in response to serum-activated pathways and is dependent on the phosphorylation of the retinoblastoma protein. However, more than half of these cells stably arrest at either entry to mitosis or during cytokinesis, thus resembling the behaviour observed in mammalian cardiomyocytes. Approximately a third of the cells progress through mitosis and may enter successive cell divisions. When cardiomyocytes divided more than once, the proliferative behaviour of sister cells was significantly correlated, in terms of whether they underwent a subsequent cell cycle, and if so, the duration of that cycle. These observations suggest a mechanism whereby newt heart regeneration depends on the retention of proliferative potential in a subset of cardiomyocytes. The regulation of the remaining newt cardiomyocytes is similar to that described for their mammalian counterparts, as they arrest during mitosis or cytokinesis. Understanding the nature of this block and why it arises in some but not other newt cardiomyocytes may lead to an augmentation of the regenerative potential in the mammalian heart.

[Regeneration of vertebrate appendage: an old experimental model to study stem cells in the adult]

The application of stem cell therapy to cure degenerative diseases offers immense possibilities, but the research in this field is the subject of ethical debates raised by the question of destructive research on early human embryos. Stem cells taken in the adult constitute an alternative to human embryonic stem cells, but our knowledge on totipotent or pluripotent cells is currently insufficient. Furthermore, many questions must be solved before selection and differentiation of these cells in a given cellular type can be controlled on a routine basis. What are the molecular characteristics of an adult stem cell? What are the mechanisms involved in cell reprogramming? Which signals control stem cell replication and differentiation? Basic research activities must be carried out in order to clarify all these points. In this context, the regeneration of vertebrate appendages provides a model for this type of research. The regeneration process is defined by both the morphological and functional reconstruction of a part of a living organism, which has previously been destroyed. But why are some vertebrates able to regenerate complex structures and others apparently not? Among most vertebrates, the capacity to regenerate is limited to some tissues. It is however possible to observe the regeneration of appendages (limb, tail, fin, jaw, etc.) among several amphibians and fish. This regeneration leads to re-forming of the amputated part with a complete restoration of its shape, segmentation and function. Why is the amputation of limbs not followed by regeneration in mammals and birds: absence of stem cells, absence of recruitment signals for these cells, or absence of signal receptivity? This review constitutes a report on the current understanding of the basis of on regeneration of legs in tetrapods and of fins in fish with an emphasis in the role of the nervous system in this process.

Muscle regeneration in amphibians and mammals: passing the torch

Skeletal muscle in both amphibians and mammals possesses a high regenerative capacity. In amphibians, a muscle can regenerate in two distinct ways: as a tissue component of an entire regenerating limb (epimorphic regeneration) or as an isolated entity (tissue regeneration). In the absence of epimorphic regenerative ability, mammals can regenerate muscles only by the tissue mode. This review focuses principally on the regeneration of entire muscles and covers what is known and what remains to be elucidated about fundamental mechanisms underlying muscle regeneration at this level.

Plasticity and reprogramming of differentiated cells in amphibian regeneration

Adult urodele amphibians, such as the newt, can regenerate their limbs and various other structures. This is the result of the plasticity and reprogramming of residual differentiated cells, rather than the existence of a 'reserve-cell' mechanism. The recent demonstrations of plasticity in mouse myotubes should facilitate comparative studies of the pathways that underlie the regenerative response, as well as proposing new approaches to promote mammalian regeneration.

Mammalian postmitotic nuclei reenter the cell cycle after serum stimulation in newt/mouse hybrid myotubes

Cell cycle reentry and dedifferentiation of postmitotic cells are important aspects of the ability of an adult newt and other urodele amphibians to regenerate various tissues and appendages [1]. In contrast to their mammalian counterparts, newt A1 myotubes are able to reenter S phase after serum stimulation of a pathway leading to phosphorylation of the retinoblastoma protein, pRb [2]. The activity in serum is not due to mitogenic growth factors but is generated indirectly by the activation of thrombin and subsequent proteolysis [3]. In this paper we describe the formation of interspecies hybrid (heterokaryon) myotubes by the fusion of mouse C2C12 [4] and newt A1 [5, 6] myogenic cells. The C2C12 nuclei reenter the cell cycle upon serum stimulation of the hybrids, while C2C12 homokaryon myotubes remain arrested under these conditions. These findings indicate that the postmitotic arrest of the mouse nuclei is undermined by the pathway activated in the newt cytoplasm. The hybrid myotubes provide a new model for the manipulation of the postmitotic arrest in both mammalian and newt differentiated cells.

Regeneration as an evolutionary variable

Regeneration poses a distinctive set of problems for evolutionary biologists, but there has been little substantive progress since these issues were clearly outlined in the monograph of T. H. Morgan (1901). The champions at regeneration among vertebrates are the urodele amphibians such as the newt, and we tend to regard urodele regeneration as an exceptional attribute. The ability to regenerate large sections of the body plan is widespread in metazoan phylogeny, although it is not universal. It is striking that in phylogenetic contexts where regeneration occurs, closely related species are observed which do not possess this ability. It is a challenge to reconcile such variation between species with a conventional selective interpretation of regeneration. The critical hypothesis from phylogenetic analysis is that regeneration is a basic, primordial attribute of metazoans rather than a mechanism which has evolved independently in a variety of contexts. In order to explain its absence in closely related species, it is postulated to be lost secondarily for reasons which are not understood. Our approach to this question is to compare a differentiated newt cell with its mammalian counterpart in respect of the plasticity of differentiation.