Larval-to-adult conversion of a myogenic system in the frog, Xenopus laevis, by larval-type myoblast-specific control of cell division, cell differentiation, and programmed cell death by triiodo-L-thyronine

Ageing across the great divide: tissue transformation, organismal growth and temperature shape telomere dynamics through the metamorphic transition

Telomere attrition is considered a useful indicator of cellular and whole-organism ageing rate. While approximately 80% of animal species undergo metamorphosis that includes extensive tissue transformations (involving cell division, apoptosis, de-differentiation and de novo formation of stem cells), the effect on telomere dynamics is unknown. We measured telomeres in Xenopus laevis developing from larvae to adults under contrasting environmental temperatures. Telomere dynamics were linked to the degree of tissue transformation during development. Average telomere length in gut tissue increased dramatically during metamorphosis, when the gut shortens by 75% and epithelial cells de-differentiate into stem cells. In the liver (retained from larva) and hindlimb muscle (newly formed before metamorphosis), telomeres gradually shortened until adulthood, likely due to extensive cell division. Tail muscle telomere lengths were constant until tail resorption, and those in heart (retained from larva) showed no change over time. Telomere lengths negatively correlated with larval growth, but for a given growth rate, telomeres were shorter in cooler conditions, suggesting that growing in the cold is more costly. Telomere lengths were not related to post-metamorphic growth rate. Further research is now needed to understand whether telomere dynamics are a good indicator of ageing rate in species undergoing metamorphosis.

Isolation and characterization of bone marrow-derived mesenchymal stem cells in Xenopus laevis

Mesenchymal stem cells (MSCs) are multipotent cells that exist in mesenchymal tissues such as bone marrow and are able to differentiate into osteocytes, chondrocytes, and adipocytes. MSCs are generally collected as adherent cells on a plastic dish, and are positive for markers such as CD44, CD73, CD90, CD105 and CD166, and negative for CD11b, CD14, CD19, CD31, CD34, CD45, CD79a and HLA-DR. MSCs have been established from many kinds of mammals, but MSCs from amphibians have not yet been reported. We cultured adherent cells from the bone marrow of Xenopus laevis by modifying the protocol for culturing mammalian MSCs. The morphology of these cells was similar to that of mammalian MSCs. The amphibian MSCs were positive for cd44, cd73, cd90 and cd166, and negative for cd11b, cd14, cd19, cd31, cd34, cd45, cd79a and hla-dra. Moreover, they could be induced to differentiate into osteocyte-, chondrocyte-, and adipocyte-lineage cells by cytokine induction systems that were similar to those used for mammalian MSC differentiation. Thus, they are considered to be similar to mammalian MSCs. Unlike mammals, amphibians have high regenerative capacity. The findings from the present study will allow for future research to reveal how Xenopus MSCs are involved in the amphibian regenerative capacity and to elucidate the differences in the regenerative capacity between mammals and amphibians.

Isolation and Primary Culture Methods of Adult and Larval Myogenic Cells from Xenopus laevis

During amphibian metamorphosis, larval-to-adult conversion of the myogenic system occurs and there are two distinct types of muscle stem cells; larval myogenic cells have a death-fate by apoptosis in the presence of thyroid hormone T₃, and adult myogenic cells have a life-fate under the same conditions. Here, we describe isolation and culture methods for adult and larval myogenic cells from the frog, Xenopus laevis. Both types of cultured myogenic cells undergo cell division and cell differentiation, i.e., formation of multinucleated myotubes in serum-containing medium. Insulin-like growth factor-1 promotes cell division and differentiation in both cells. Basic properties and some applications of primary culture are also described.

Insulin-like growth factor 1 regulation of proliferation and differentiation of Xenopus laevis myogenic cells in vitro

To understand the mechanism of muscle remodeling during Xenopus laevis metamorphosis, we examined the in vitro effect of insulin-like growth factor 1 (IGF-1) on growth and differentiation of three different-fate myogenic cell populations: tadpole tail, tadpole dorsal, and young adult leg muscle. IGF-1 promoted growth and differentiation of both tail and leg myogenic cells only under conditions where these cells could proliferate. Inhibition of cell proliferation by DNA synthesis inhibitor cytosine arabinoside completely canceled the IGF-1's cell differentiation promotion, suggesting the possibility that IGF-1's differentiation-promotion effect is an indirect effect via IGF-1's cell proliferation promotion. IGF-1 promoted differentiation dose dependently with maximum effect at 100-500 ng/ml. RT-PCR analysis revealed the upregulation (11-fold) of ifg1 mRNA expression in developing limbs, suggesting that IGF-1 plays a role in promoting muscle differentiation during limb development. The combined effect of triiodo-L-thyronine (T₃) and IGF-1 was also examined. In adult leg cells, IGF-1 promoted growth and differentiation irrespective of the presence of T₃. In larval tail cells, cell count was 76% lower in the presence of T₃, and IGF-1 did not promote proliferation and differentiation in T₃-containing medium. In larval dorsal cells, cell count was also lower in the presence of T₃, but IGF-1 enhanced proliferation and differentiation in T₃-containing medium. This result is likely due to the presence among dorsal cells of both adult and larval types (1:1). Thus, IGF-1 affects only adult-type myogenic cells in the presence of T₃ and helps accelerate dorsal muscle remodeling during metamorphosis.

Differential muscle regulatory factor gene expression between larval and adult myogenesis in the frog Xenopus laevis: adult myogenic cell-specific myf5 upregulation and its relation to the notochord suppression of adult muscle differentiation

During Xenopus laevis metamorphosis, larval-to-adult muscle conversion depends on the differential responses of adult and larval myogenic cells to thyroid hormone. Essential differences in cell growth, differentiation, and hormone-dependent life-or-death fate have been reported between cultured larval (tail) and adult (hindlimb) myogenic cells. A previous study revealed that tail notochord cells suppress terminal differentiation in adult (but not larval) myogenic cells. However, little is known about the differences in expression patterns of myogenic regulatory factors (MRF) and the satellite cell marker Pax7 between adult and larval myogenic cells. In the present study, we compared mRNA expression of these factors between the two types. At first, reverse transcription polymerase chain reaction analysis of hindlimb buds showed sequential upregulation of myf5, myogenin, myod, and mrf4 during stages 50-54, when limb buds elongate and muscles begin to form. By contrast, in the tail, there was no such increase during the same period. Secondary, these results were duplicated in vitro: adult myogenic cells upregulated myf5, myod, and pax7 in the early culture period, followed by myogenin upregulation and myotube differentiation, while larval myogenic cells did not upregulate these genes and precociously started myotube differentiation. Thirdly, myf5 upregulation and early-phase proliferation in adult myogenic cells were potently inhibited by the presence of notochord cells, suggesting that notochord cells suppress adult myogenesis through inhibiting the transition from Myf5(-) stem cells to Myf5(+) committed myoblasts. All of the data presented here suggest that myf5 upregulation can be a good criterion for the activation of adult myogenesis during X. laevis metamorphosis.

Non-viral expression of mouse Oct4, Sox2, and Klf4 transcription factors efficiently reprograms tadpole muscle fibers in vivo

Adult mammalian cells can be reprogrammed into induced pluripotent stem cells (iPSCs) by a limited combination of transcription factors. To date, most current iPSC generation protocols rely on viral vector usage in vitro, using cells removed from their physiological context. Such protocols are hindered by low derivation efficiency and risks associated with genome modifications of reprogrammed cells. Here, we reprogrammed cells in an in vivo context using non-viral somatic transgenesis in Xenopus tadpole tail muscle, a setting that provides long term expression of non-integrated transgenes in vivo. Expression of mouse mOct4, mSox2, and mKlf4 (OSK) led rapidly and reliably to formation of proliferating cell clusters. These clusters displayed the principal hallmarks of pluripotency: alkaline phosphatase activity, up-regulation of key epigenetic and chromatin remodeling markers, and reexpression of endogenous pluripotent markers. Furthermore, these clusters were capable of differentiating into derivatives of the three germ layers in vitro and into neurons and muscle fibers in vivo. As in situ reprogramming occurs along with muscle tissue repair, the data provide a link between these two processes and suggest that they act synergistically. Notably, every OSK injection resulted in cluster formation. We conclude that reprogramming is achievable in an anamniote model and propose that in vivo approaches could provide rapid and efficient alternative for non-viral iPSC production. The work opens new perspectives in basic stem cell research and in the longer term prospect of regenerative medicine protocols development.

Adult-type myogenesis of the frog Xenopus laevis specifically suppressed by notochord cells but promoted by spinal cord cells in vitro

Larval-to-adult myogenic conversion occurs in the dorsal muscle but not in the tail muscle during Xenopus laevis metamorphosis. To know the mechanism for tail-specific suppression of adult myogenesis, response character was compared between adult myogenic cells (Ad-cells) and larval tail myogenic cells (La-cells) to a Sonic hedgehog (Shh) inhibitor, notochord (Nc) cells, and spinal cord (SC) cells in vitro. Cyclopamine, an Shh inhibitor, suppressed the differentiation of cultured Ad (but not La) cells, suggesting the significance of Shh signaling in promoting adult myogenesis. To test the possibility that Shh-producing axial elements (notochord and spinal cord) regulate adult myogenesis, Ad-cells or La-cells were co-cultured with Nc or SC cells. The results showed that differentiation of Ad-cells were strongly inhibited by Nc cells but promoted by SC cells. If Ad-cells were "separately" co-cultured with Nc cells without direct cell-cell interactions, adult differentiation was not inhibited but rather promoted, suggesting that Nc cells have two roles, one is a short-range suppression and another is a long-range promotion for adult myogenesis. Immunohistochemical analysis showed both notochord and spinal cord express the N-terminal Shh fragment throughout metamorphosis. The "spinal cord-promotion" and long-range effect by Nc cells on adult myogenesis is thus involved in Shh signaling, while the signaling concerning the short-range "Nc suppression" will be determined by future studies. Interestingly, these effects, "Nc suppression" and "SC promotion" were not observed for La-cells. Situation where the spinal cord/notochord cross-sectional ratio is quite larger in tadpole trunk than in the tail seems to contribute to trunk-specific promotion and tail-specific suppression of adult myogenesis during Xenopus metamorphosis.

Skeletal and cardiac myogenesis accompany adipogenesis in P19 embryonal stem cells

P19 embryonic carcinoma cells resemble normal embryonic stem (ES) cells. They generate cardiac and skeletal myocytes in response to retinoic acid (RA) or oxytocin (OT). RA treatment followed by exposure to triiodothyronine (T3) and insulin induces ES cells differentiation into adipocytes and skeletomyocytes. On the other hand, OT (10(-7) M) was reported to inhibit 3T3 preadipocyte maturation. The present work was undertaken to determine whether P19 cells have an adipogenic potential that could be affected by OT. Cells were treated with RA (10(-6) M)/T3+insulin (adipogenic protocol) or 10(-7) M OT (cardiomyogenic protocol), and analyzed by polymerase chain reaction, immunotechniques, and cytochemistry. Oil-Red-O staining and expression of peroxisome proliferator-activated receptor-gamma (PPARgamma) and aP2 indicated the generation of adipocytes in cultures submitted to the adipogenic protocol. Contracting cells were also generated. Cells positive for sarcomeric actinin and negative for cardiac troponin inhibitor (cTpnI) indicated generation of skeletomyocytes, and cTpnI positive cells revealed generation of cardiomyocytes. Levels of cTpnI and of the skeletal marker MyoD were almost similar in both protocols, whereas no Oil-Red-O staining was associated with the cardiomyogenic protocol. Addition of 10(-7) M OT to the adipogenic protocol did not affect Oil-Red-O staining and PPARgamma expression. Interestingly, Oct3/4 pluripotency marker disappeared in the adipogenic protocol but remained expressed in the cardiomyogenic one. P19 cells thus have an adipogenic potential non affected by 10(-7) M OT. RA/T3+insulin combination generates a larger spectrum of mesodermal cell derivatives and is a more potent morphogenic treatment than OT. P19 cells could help investigating mechanisms of cell fate decision during development.

Muscle development in a biphasic animal: the frog

Knowledge of muscle development in a vertebrate reflects strengths of the particular model system. For example, the origin of mesoderm is very well characterized in Xenopus laevis, where development of somites is less well understood. The major problem in muscle development, presented by frogs, is the complete replacement of larval muscles by adult muscles at thyroid hormone-dependent metamorphosis. All tail muscles die, all leg muscles form de novo, and muscles in the jaw and trunk show both processes. The nature of adult muscle progenitors remains unclear. Comparison of X. laevis development with divergent amphibian patterns, such as direct developers, which lack the larval tadpole, should highlight important steps in adult muscle formation.

Characteristics of initiation and early events for muscle development in the Xenopus limb bud

In Xenopus laevis, limb buds start to develop at a later point of the larval stage, prior to metamorphosis. This onset of limb development in Xenopus is totally different from that in amniotes such as birds and mammals, in which limb buds emerge at an early stage of embryogenesis, in parallel with other organogenesis. We investigated limb myogenesis in Xenopus, focusing on myogenic gene expression, myogenic ability of limb bud cells in the early stage, and the origin of myogenic precursor cells in the limb bud. The Xenopus early limb bud contains myoD/cardiac actin-positive and pax3/pax7-negative cells. Interestingly, results of transplantation experiments have revealed that this early limb bud contains myogenic precursor cells. In order to know the contribution of myogenic cells in somites to myogenic precursor cells in the early limb bud, we used a Cre-LoxP system for tracing over a long period. The results of fate tracing for myogenic cells in somites of the Xenopus embryo suggested that early-specified myogenic cells in somites do not contribute to limb muscle in Xenopus. Taken together, the results suggest that limb muscle development in Xenopus has characteristics of initiation and early events distinct from those of other vertebrate clades.

Muscle formation in regeneratingXenopus froglet limb

A spike, a resultant regenerate made after amputation of a Xenopus froglet limb, has no muscle tissue. This muscle-less phenotype was analyzed by molecular approaches, and the results of analysis revealed that the spike expresses no myosin heavy chain or Pax7, suggesting that neither mature muscle tissue nor satellite cells exist in the spike. The regenerating blastema in the froglet limb lacked some myogenesis-related marker genes, myoD and myf5, but allowed implanted muscle precursor cells to survive and differentiate into myofiber. Implantation of hepatocyte growth factor (HGF) -releasing cell aggregates rescued this muscle-less phenotype and induced muscle regeneration in Xenopus froglet limb regenerates. These results suggest that failure of regeneration of muscle is due to a disturbance of the early steps of myogenesis under a molecular cascade mediated by HGF/c-met. Improvement of muscle regeneration in the Xenopus adult limb that we report here for the first time will give us important insights into epimorphic tissue regeneration in amphibians and other vertebrates.

Regulation of specific developmental fates of larval- and adult-type muscles during metamorphosis of the frog Xenopus

During anuran metamorphosis, larval-type myotubes in both trunk and tail are removed by apoptosis, and only trunk muscles are replaced by newly formed adult-type myotubes. In the present study, we clarified the regulatory mechanisms for specific developmental fates of adult and larval muscles. Two distinct (adult and larval) types of myoblasts were found to exist in the trunk, but no or very few adult myoblasts were found in the tail. Each type of myoblast responded differently to metamorphic trigger, 3,3',5-triiodo-L-thyronine (T(3)) in vitro. T(3)-induced cell death was observed in larval myoblasts but not in adult myoblasts. These results suggest that the fates (life or death) of trunk and tail muscles are determined primarily by the differential distribution of adult myoblasts within the muscles. However, a transplantation study clarified that each larval and adult myoblast was not committed to fuse into particular myotube types, and they could form heterokaryon myotubes in vivo. Cell culture experiments suggested that the following two mechanisms are involved in the specification of myotube fate: (1) Heterokaryon myotubes could escape T(3)-induced death only when the proportion of adult nuclei number was higher than 70% in the myotubes. Apoptosis was not observed in any larval nuclei within the surviving heterokaryon myotubes, suggesting the conversion of larval nuclei fate. (2) Differentiation of adult myoblasts was promoted by the factor(s) released from larval myoblasts in a cell type-specific manner. Taken together, the developmental fate of myotubes is determined by the ratio of nuclei types, and the formation of adult nuclei-rich myotubes was specifically enhanced by larval myoblast factor(s).

Multiple thyroid hormone-induced muscle growth and death programs during metamorphosis in Xenopus laevis

Xenopus laevis tadpole tails contain fast muscle fibers oriented in chevrons and two pairs of slow muscle "cords" along the length of the tail. When tail resorption is inhibited by a number of different treatments, fast muscle but not the slow cord muscle still is lost, demonstrating that the fast tail muscle is a direct target of the thyroid hormone-induced death program. Expression of a dominant negative form of the thyroid hormone receptor (TRDNalpha) was restricted to tadpole muscle by means of a muscle-specific promoter. Even though the transgene protects fast tail muscle from thyroid hormone (TH)-induced death, the tail shortens, and the distal muscle chevrons at the tail tip are degraded. This default pathway for muscle death is probably caused by the action of proteolytic enzymes secreted by neighboring fibroblasts. Non-muscle tissues that are sensitive to TH, such as the fibroblasts, are not protected by the transgene when it is expressed solely in muscle. If allowed to develop to metamorphosis, these transgenic animals die at the climax of metamorphosis before tail resorption has begun. Their limbs have very little muscle even though the rest of limb morphology is normal. Thus, fast tail muscle and limb muscle have their own cell autonomous death and growth programs, respectively, that are independent of the fate of the other neighboring cell types. In contrast, death of the slow muscle is controlled by the other cell types of the tail.

Autonomous regulation of muscle fibre fate during metamorphosis in Xenopus tropicalis

A key event in metamorphosis of anuran amphibians is tail resorption. This composite structure includes epidermal cells, spinal cord, muscle fibres and connective tissue. It is unclear how resorption proceeds and to what extent the signals for the death process are transmitted between cells. We determined the kinetics of metamorphosis, apoptosis, and tail regression in the diploid anuran, Xenopus tropicalis, a species more suited to genetic analysis than the pseudotetraploid, Xenopus laevis. Metamorphosis was found to proceed at a regular and predictable rate in X. tropicalis but not in X. laevis. Caspase 3 activity and mRNA levels were correlated with TdT-mediated dUTP nick end-labeling (TUNEL) signalling and most markedly increased in tail muscle and spinal cord. It has been proposed that muscles die as a result of loss of connectivity with the surrounding matrix. To test this hypothesis, we used direct DNA injection in trunk and tail muscle to overexpress Xenopus Bcl-X(L) (xR11), an anti-apoptotic gene, along with a marker gene (luciferase or GFP). xR11 significantly inhibited the cell death process in both trunk and tail muscle. This protection was functional even up to stage 64 on completion of tail regression. We conclude that (1) somatic gene transfer can be applied to analyse cell fate in X. tropicalis, and (2) that muscle death can be abrogated despite extracellular matrix loss.

Structure and metamorphic remodeling of the larval nervous system and musculature of Phoronis pallida (Phoronida)

The structure of the larval nervous system and the musculature of Phoronis pallida were studied, as well as the remodeling of these systems at metamorphosis. The serotonergic portion of the apical ganglion is a U-shaped field of cell bodies that send projections into a central neuropil. The majority of the serotonergic cells are (at least) bipolar sensory cells, and a few are nonsensory cells. Catecholaminergic cell bodies border the apical ganglion. The second (hood) sense organ develops at competence and is composed of bipolar sensory cells that send projections into a secondary neuropil. Musculature of the competent larva includes circular and longitudinal muscle fibers of the body wall, as well as elevators and depressors of the tentacles and hood. The juvenile nervous system and musculature are developed prior to metamorphosis and are integrated with those of the larva. Components of the juvenile nervous system include a diffuse neural net of serotonergic cell bodies and fibers and longitudinal catecholaminergic fibers. The juvenile body wall musculature consists of longitudinal fibers that overlie circular muscle fibers, except in the cincture regions, where this pattern is reversed. Metamorphosis is initiated by the larval neuromuscular system but is completed by the juvenile neuromuscular system. During metamorphosis, the larval nervous system and the musculature undergo cell death, and the larval tentacles and gut are remodeled into the juvenile arrangement. Although the phoronid nervous system has often been described as deuterostome-like, these data show that several cytological aspects of the larval and juvenile neuromuscular systems also have protostome (lophotrochozoan) characteristics.