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.

The expression pattern of tomoregulin-1 in urodele limb regeneration and mouse limb development

Tomoregulin-1 (TMEFF1) was first identified as a gene implicated in pituitary secretion in Xenopus laevis. The predicted structure of TMEFF1 is that of a transmembrane protein with a highly conserved cytoplasmic tail, two follistatin domains and one modified EGF domain in its extracellular region. We report the cloning of the newt orthologue, and show that the expression of TMEFF1 is upregulated in the blastema during limb regeneration, and is also expressed in mouse embryonic limb development.

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.

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

The remarkable regenerative ability of adult urodele amphibians depends in part of the plasticity of differentiated cells at the site of injury. Limb regeneration proceeds by formation of a mesenchymal growth zone or blastema under the wound epidermis at the end of the stump. Previous work has shown that when cultured post-mitotic newt myotubes are introduced into the blastema, they re-enter the cell cycle and undergo conversion to mononucleate cells which divide and contribute to the regenerate [11, 13]. In order to investigate the interdependence of these two aspects of plasticity, we have blocked cell cycle progression of the myotubes either by X-irradiation or by transfection of the CDK4/6 inhibitor p16. In each case, the efficacy of the block was evaluated in culture after activation of S phase re-entry by serum stimulation. The experimental myotubes were implanted into limb blastemas along with a differentially labelled control population of myotubes containing an equivalent number of nuclei. X-irradiated myotubes gave rise to mononucleate cells in the limb blastema, and the progeny were blocked in respect of S phase entry. Comparable results were obtained with the p16-expressing myotubes. We conclude that progression through S or M phase is not required for generation of mononucleate cells and suggest that such cells may arise by budding from the muscle syncytium.

Plasticity of retrovirus-labelled myotubes in the newt limb regeneration blastema

Two important indices of myogenic differentiation are the formation of syncytial myotubes and the postmitotic arrest from the cell cycle, both of which occur after fusion of mononucleate cells. We show here that these indices are reversed in the environment of the urodele limb regeneration blastema. In order to introduce an integrated (genetic) marker into newt myotubes, we infected mononucleate cells in culture with a pseudotyped retrovirus expressing human placental alkaline phosphatase (AP). After fusion the myotubes expressed AP and could be purified by sieving and micromanipulation so as to remove all mononucleate cells. When such purified retrovirus-labelled myotubes were implanted into a limb blastema they gave rise to mononucleate progeny with high efficiency. Purified myotubes labelled with fluorescent lipophilic cell tracker dye also gave rise to mononucleate cells; myotubes which were double labelled with the tracker dye and a nuclear stain gave rise to double-labelled mononucleate progeny. Nuclei within retrovirus-labelled myotubes entered S phase as evidenced by widespread labelling after injection of implanted newts with BrdU. The relation between the two aspects of plasticity is a critical further question.

Topics in prion cell biology

Recent studies of a transmembrane form of the prion protein (PrP) have indicated its importance for neuropathogenesis in certain contexts, and have analysed the transacting factors at the endoplasmic reticulum and the mutations within PrP that regulate its appearance. A significant focus for our understanding of the normal role of PrP has emerged from its interaction with copper ions. Studies on two yeast prions have analysed the structure and phenotype of the aggregated conformers underlying the prion state, as well as the interactions regulating their formation and turnover within a dividing cell.

Thrombin regulates S-phase re-entry by cultured newt myotubes

BACKGROUND

Adult urodele amphibians such as the newt have remarkable regenerative ability, and a critical aspect of this is the ability of differentiated cells to re-enter the cell cycle and lose their differentiated characteristics. Unlike mammalian myotubes, cultured newt myotubes are able to enter and traverse S phase, following serum stimulation, by a pathway leading to phosphorylation of the retinoblastoma protein. The extracellular regulation of this pathway is unknown.

RESULTS

Like their mammalian counterparts, newt myotubes were refractory to mitogenic growth factors such as the platelet-derived growth factor (PDGF), which act on their mononucleate precursor cells. Cultured newt myotubes were activated to enter S phase by purified thrombin in the presence of subthreshold amounts of serum. The activation proceeded by an indirect mechanism in which thrombin cleaved components in serum to generate a ligand that acted directly on the myotubes. The ligand was identified as a second activity present in preparations of crude thrombin and that was active after removal of all thrombin activity. It induced newt myotubes to enter S phase in serum-free medium, and it acted on myotubes but not on the mononucleate precursor cells. Cultured mouse myotubes were refractory to this indirect mechanism of S-phase re-entry.

CONCLUSIONS

These results provide a link between reversal of differentiation and the acute events of wound healing. The urodele myotube responds to a ligand generated downstream of thrombin activation and re-enters the cell cycle. Although this ligand can be generated in mammalian sera, the mammalian myotube is unresponsive. These results provide a model at the cellular level for the difference in regenerative ability between urodeles and mammals.

Identification of newt connective tissue growth factor as a target of retinoid regulation in limb blastemal cells

In order to analyse target genes regulated by retinoic acid in urodele limb regeneration, we have used pseudotyped retroviruses to obtain stably transfected newt limb blastemal (progenitor) cells in culture which express chimeric retinoic acid/thyroid hormone receptors delta1 or delta2. After treatment with thyroid hormone to activate the chimeric receptors, we used a polymerase chain reaction (PCR)-based subtraction method to identify target genes which are retinoid regulated. Newt connective tissue growth factor, a secreted protein recognised in several vertebrates, has been identified in this way and found to be expressed in the limb blastema and regulated by retinoic acid. This approach should permit a systematic analysis of retinoid target genes in limb regeneration.

A target of thrombin activation promotes cell cycle re-entry by urodele muscle cells

A key early event of newt limb regeneration is the local dedifferentiation of cells to form dividing progenitor cells. This involves the plasticity of differentiation and the ability to re-enter the cell cycle. In culture, differentiated newt myotubes are able to re-enter S-phase in response to serum stimulation. Here, we analyzed the intracellular and extracellular requirements for this process. Cell cycle re-entry depends on the phosphorylation of the retinoblastoma protein, which is a key regulator of the G1-S transition. This is in contrast to mammalian myotubes, which are refractory to serum stimulation and cannot phosphorylate retinoblastoma protein in response to serum. The serum factor responsible for this phosphorylation appears to be distinct from common polypeptide growth factors and is enriched in crude preparations of bovine thrombin. Fractionation and analysis of this preparation indicate that the factor is regulated by thrombin and plasmin proteolysis. These results indicate that factors involved in acute responses to wounding such as clotting may be important initiators of the regenerative response.

Hedgehog family member is expressed throughout regenerating and developing limbs

Members of the hedgehog family have been shown to play a key role in many developmental processes, including limb patterning and chondrogenesis. We have therefore investigated whether members of this family are also expressed during regeneration of the adult urodele limb and are regulated by retinoic acid (RA), since this derivative induces proximodistal duplications in regenerating limbs, and has been shown to regulate sonic hedgehog (shh) in the developing limbs of birds and mammals. We report here that a newt homologue of Xenopus banded hedgehog, called N-bhh, is uniformly expressed by mesenchymal blastemal cells from the initial stages of regeneration and is up-regulated by RA. In addition, we show that N-bhh is uniformly expressed in the early limb bud of the newt embryo. Since bhh has not been detected in developing limbs of higher vertebrates, its expression in developing and regenerating newt limbs may be related to the regenerative capability of urodeles.

Immortalization of rat embryo fibroblasts by a 3'-untranslated region

We have exploited a cross-species expression screen to search for cellular immortalizing activities. A newt blastemal cDNA expression library was transfected into rat embryo fibroblasts and immortal cell lines were selected. This identified a 1-kb cDNA fragment which has a low representation in the cDNA library and is derived from the 3'-UTR of an alpha-glucosidase-related mRNA. Expression of this sequence in rat embryo fibroblasts has shown that it is active in promoting colony formation and immortalization. It is also able to cooperate with an immortalization-defective deletion mutant of SV40 T antigen, indicating that it can exert its growth-stimulatory activity in the pathway activated by a viral immortalizing oncogene. This is the first example of an immortalizing activity mediated by an RNA sequence, and further analysis of its mechanism should provide new insights into senescence and immortalization.

Amphibian limb regeneration: rebuilding a complex structure

The ability to regenerate complex structures is widespread in metazoan phylogeny, but among vertebrates the urodele amphibians are exceptional. Adult urodeles can regenerate their limbs by local formation of a mesenchymal growth zone or blastema. The generation of blastemal cells depends not only on the local extracellular environment after amputation or wounding but also on the ability to reenter the cell cycle from the differentiated state. The blastema replaces structures appropriate to its proximodistal position. Axial identity is probably encoded as a graded property that controls cellular growth and movement through local cell interactions. The molecular basis is not understood, but proximodistal identity in newt blastemal cells may be respecified by signaling through a retinoic acid receptor isoform. The possibility of inducing a blastema on a mammalian limb cannot be discounted, although the molecular constraints are becoming clearer as we understand more about the mechanisms of urodele regeneration.

Newt myotubes reenter the cell cycle by phosphorylation of the retinoblastoma protein

Withdrawal from the cell cycle is an essential aspect of vertebrate muscle differentiation and requires the retinoblastoma (Rb) protein that inhibits expression of genes needed for cell cycle entry. It was shown recently that cultured myotubes derived from the Rb-/- mouse reenter the cell cycle after serum stimulation (Schneider, J.W., W. Gu, L. Zhu, V. Mahdavi, and B. Nadal-Ginard. 1994. Science (Wash. DC). 264:1467-1471). In contrast with other vertebrates, adult urodele amphibians such as the newt can regenerate their limbs, a process involving cell cycle reentry and local reversal of differentiation. Here we show that myotubes formed in culture from newt limb cells are refractory to several growth factors, but they undergo S phase after serum stimulation and accumulate 4N nuclei. This response to serum is inhibited by contact with mononucleate cells. Despite the phenotypic parallel with Rb-/- mouse myotubes, Rb is expressed in the newt myotubes, and its phosphorylation via cyclin-dependent kinase 4/6 is required for cell cycle reentry. Thus, the postmitotic arrest of urodele myotubes, although intact in certain respects, can be undermined by a pathway that is inactive in other vertebrates. This may be important for the regenerative ability of these animals.

Is retinoic acid an endogenous ligand during urodele limb regeneration?

The effects of retinoids on a regenerating urodele limb make them interesting candidates for endogenous ligands during regeneration. We review the evidence for considering this possibility. This includes analysis of retinoids and retinoic acid receptors in the regenerate, and studies on activation of retinoid reporter genes. Recent work has provided evidence that the wound epidermis is a source of 9-cis retinoic acid, and may be a favorable model for studying the synthesis and release of this modulator.

Receptor isoform specificity in a cellular response to retinoic acid

The effects of retinoic acid on cell proliferation, differentiation and patterning are thought to be mediated by the various retinoic acid receptors. Different receptor types are encoded by distinct genes (alpha, beta, and gamma), whereas various isoforms within each type are encoded by splicing variants resulting from the use of alternative promoters. The only region that differs between isoforms is the N-terminal A region containing a transcriptional activating domain. It has been proposed that these alternative A regions confer distinct activities on the receptors, thus allowing each to mediate specific effects of retinoic acid, but it has been difficult to demonstrate such isoform specificity as most cells express a number of different retinoic acid receptors. In an attempt to test whether different isoforms can mediate distinct biological effects we are focusing on retinoic-acid-dependent growth inhibition of newt limb cells. We have constructed chimaeric receptors in which the retinoic acid binding domain of each of five newt retinoic acid receptors has been replaced with a thyroid hormone (T3) binding domain. These constructs were introduced individually into cells whose growth rate was then measured in the presence of T3. The chimaeric alpha 1 receptor mediated T3-dependent inhibition of proliferation that was comparable to that given by retinoic acid, whereas the alpha 2 isoform had no activity in this assay, nor did the delta 1A, delta 1B and Delta 2 receptors. When the A region was deleted from the alpha 1 chimaera it remained a potent T3-dependent transcriptional activator, but no longer mediated T3-dependent growth inhibition. In contrast, when the A region of alpha 1 was transferred to a delta chimaeric receptor, the resulting molecule was fully active in T3-dependent growth inhibition. This is the first direct evidence for isoform specificity in a biological response to retinoic acid, and demonstrates that the specificity of this response is confined to the A region.

Semi-automated positional analysis using laser scanning microscopy of cells transfected in a regenerating newt limb

Limb regeneration in urodele amphibians such as the newt is a key system for investigating the positional identity of cells. The regenerate arises locally from blastemal cells, mesenchymal progenitors that normally give rise to structures distal to the amputation plane but which can be respecified (proximalized) by treatment with retinoic acid (RA) such that proximal structures are formed. To establish an assay for positional identity, cells of distal and RA-treated distal blastemas are labeled by transfection with an alkaline phosphatase marker gene using particle bombardment (biolistics). After grafting the distal blastema to a proximal stump, a context known as intercalary regeneration, the proximodistal distribution of labeled cells in the resulting regenerate is an index of positional identity. We use enzyme-labeled fluorescence (ELF) in conjunction with laser scanning microscopy to detect transfected cells within a section of the entire regenerate. A semi-automated analysis of the positional distribution of marked cells along the proximal-distal axis demonstrates that cells from both distal and RA-treated blastemas contribute to the regenerate. This procedure provides an efficient and accurate tool for positional analysis of transfected cells, and should be applicable for studying genes that play a role in specifying cell position during morphogenesis.

Activation of a single retinoic acid receptor isoform mediates proximodistal respecification

BACKGROUND

The regenerating limb of urodele amphibians is an important system for evaluating the effects of retinoic acid (RA) on pattern formation. Regeneration proceeds by local formation of the blastema, a mesenchymal growth zone which normally only gives rise to structures distal to its level of origin. RA can respecify proximodistal identity in amphibian limb regeneration, and this activity of RA on the blastema is observed in two contexts. First, exposure to RA proximalizes a distal blastema resulting in duplication of structures proximal to the level of amputation. Second, after transplantation of a distal blastema to a proximal stump, the transplanted cells normally make only a minor contribution to the intercalary regenerate, but if transplanted cells are exposed to RA they occupy positions proximal to their level of origin and contribute to the regeneration of the intermediate tissue. Multiple isoforms of RA receptors (RARs) are expressed in the newt limb and are thought to mediate the respecification of positional identity.

RESULTS

To identify which receptor(s) mediates proximodistal respecification, we have used the biolistics (particle bombardment) technique to transfect the blastemal mesenchyme with plasmids encoding chimeric proteins containing partial amino-acid sequences of the various newt RAR isoforms fused to a partial sequence of the thyroid hormone (3,5, 3'-triiodothyronine; T3) receptor. We then used T3 treatment to selectively activate individual RAR isoforms in vivo. By analyzing the distributions of transfected cells in regenerates derived from distal-to-proximal transplantation we find that activation of a single RAR isoform, delta 2, specifically mediates proximalization.

CONCLUSIONS

These results demonstrate that the ability of RA to respecify proximodistal identity is mediated by a specific RAR isoform, delta 2. Activation of the RA pathway in individual cells indicates that positional respecification can be cell-autonomous. RA can respecify axial identity in several contexts in vertebrate development, but this is the first case where the RAR mediating respecification has been identified.

Synthesis and release of 9-cis retinoic acid by the urodele wound epidermis

The wound epidermis is a transient secretory epithelium that apposes the mesenchymal blastema of a regenerating urodele limb, and is required for regeneration. Previous studies have shown that the positional identity of the blastema is respecified by retinoic acid (RA; Maden, M. (1982) Nature 295, 672–675), that the blastema contains RA (Scadding, S. R. and Maden, M. (1994) Dev. Biol. 162, 608- 617), and that an RA-reporter gene introduced into the blastema is differentially activated along the proximo-distal axis (Brockes, J. P. (1992) Proc. Natl. Acad. Sci. USA 89, 11386–11390). The newt limb wound epidermis has been explanted with minimal mesenchymal contamination and cultured under conditions where it retains expression and inducibility of marker antigens. We have assayed for the release of retinoids from the wound epidermis by co-culture with cells transfected with an RA- responsive reporter gene. The reporter was activated to a level corresponding to stimulation by 0.1-1 nM RA, and this activation was substantially conferred by medium conditioned by the wound epidermis. No significant activation was observed for cells transfected with mutated reporter plasmids and analysed in parallel co-cultures. Wound epidermis from contralateral proximal and distal blastemas were compared for reporter activation, and gave a P/D activation ratio significantly greater than 1. Wound epidermis explants were cultured in the presence of tritiated retinol, and extracts were analysed by HPLC on three different columns. Radioactivity was detected in peaks corresponding to didehydroretinol, 9-cis RA and other unidentified metabolites. Analysis of conditioned media samples, some after pulse chase experiments, detected significant release of retinol, 9-cis RA and other metabolites. Although all-trans RA was detectable, the predominant acidic metabolite was 9-cis RA. These experiments establish the wound epidermis as a source of RA for local cellular interactions in the blastema.

Expression and activity of the newt Msx-1 gene in relation to limb regeneration

The Msx-1 homeobox gene is expressed in various contexts during vertebrate development, including the progress zone of the avian and mouse limb bud. Expression of mouse Msx-1 in a cultured myogenic cell line conferred a transformed phenotype and inhibited fusion into myotubes. It has been proposed that Msx-1 expression is required to maintain certain cells in a proliferating and undifferentiated state and may be associated with the ability to regenerate limbs. Urodele amphibians such as the newt regenerate their limbs by formation of a growth zone or blastema, and we have isolated and sequenced newt Msx-1 (NvMsx-1) from a limb blastemal cDNA library. NvMsx-1 expression was detectable in RNA preparations from both limb and tail and their regeneration blastemas, although cultured cells established from limb blastemal mesenchyme gave negative results. When either COS cells or cultured newt blastemal cells were cotransfected with an expression vector for NvMsx-1 and reporter plasmids containing multiple homeobox protein binding sites, NvMsx-1 repressed reporter expression. If NvMsx-1 was expressed together with a marker enzyme in cultured newt blastemal cells, no significant difference in DNA synthesis was observed relative to control transfectants. When myogenic mononucleate cells were transfected with NvMsx-1 and subsequently exposed to low serum to promote fusion, the fraction of Msx-1 positive cells in myotubes was comparable to a control transfected population analysed in the same culture. These results indicate that although Msx-1 expression could be important for limb regeneration, it does not exert a cell-autonomous effect on proliferation or myogenic differentiation of cultured blastemal cells.

New approaches to amphibian limb regeneration

Limb regeneration in urodele amphibians proceeds by the formation of a blastema, a growth zone of mesenchymal progenitor cells. Here, I review the origin of the blastema and its positional identity. The ability to culture and reimplant newt blastemal cells, and to transfect them in culture and in situ using a particle delivery system, has yielded new information about the action of retinoic acid on the blastema and the reversal of muscle differentiation during limb regeneration.

Reversibility of the mononucleate-to-multinucleate myogenic transition during amphibian limb regeneration

Muscle differentiation involves the fusion of mononucleate myoblasts to form multinucleate syncytial myotubes. In order to assess reversibility of the mononucleate-to-multinucleate transition in urodele limb regeneration, myotube formation was induced in cultured newt limb blastemal cells. Myotubes were purified, replated at low density and injected with a cytoplasmic lineage tracer. In some cases nuclei of myotubes were labelled by incorporation of tritiated thymidine. Labelled myotubes were stable in culture for 6-8 weeks and no transfer to mononucleate cells was observed. The myotubes were implanted under the wound epidermis of a hindlimb blastema. Labelled mononucleate cells were observed 1 week after implantation and such cells could derive both the cytoplasmic lineage tracer and the nuclear marker. The number of such cells increased by 2-3 weeks after implantation. These results provide strong support for the reversibility of muscle differentiation during urodele limb regeneration, and raise questions about the mechanism of such a reversal.

Isoform-specific induction of a retinoid-responsive antigen after biolistic transfection of chimaeric retinoic acid/thyroid hormone receptors into a regenerating limb

Retinoic acid (RA) induces secretory differentiation in the wound epidermis of a regenerating amphibian limb. We investigated the role of individual RA receptor (RAR) types in the newt wound epidermis by introducing chimaeric RA/thyroid hormone (T3) receptors (chi alpha 1 and chi delta 1) that can be activated by T3. A biolistic particle delivery system was employed to transfect cells in the wound epidermis of a regenerating limb and approximately 10% of the cells in targeted surface areas expressed marker genes. Both chi alpha 1 and chi delta 1 were comparable in their ability to stimulate transcription of a synthetic reporter construct through a RA response element after activation with T3 in situ. This activation was also comparable to that obtained by the endogenous complement of RARs in the RA-treated, transfected wound epidermis. The RA-inducible WE3 antigen, a marker for secretory differentiation, which distinguishes the wound epidermis from normal skin (Tassava, R. A., Johnson-Wint, B. and Gross, J. 1986, J. Exp. Zool. 239, 229–240), was used to assess the functional role of chi alpha 1 and chi delta 1. Chimaeric receptors were transfected with an alkaline phosphatase marker gene, activated with T3, and the expression of both the marker and WE3 was analyzed by double-label immunofluorescence. Newt limbs transfected with chi delta 1 showed many double-labelled cells dependent on the presence of T3, whereas contralateral limbs transfected with an alkaline phosphatase marker lacking chimaeric receptor sequences did not.(ABSTRACT TRUNCATED AT 250 WORDS)

The PML-retinoic acid receptor alpha translocation converts the receptor from an inhibitor to a retinoic acid-dependent activator of transcription factor AP-1

We report here that the fusion of PML, a nuclear protein defined by the t(15;17) chromosomal translocation in acute promyelocytic leukemia, with retinoic acid receptor alpha (RAR alpha) changes the RAR alpha from a retinoic acid (RA)-dependent inhibitor to a RA-dependent activator of AP-1 transcriptional activity. The PML-RAR alpha chimera cooperates with c-Jun and, strikingly, with c-Fos to stimulate the transcription of both synthetic and natural reporter genes containing an AP-1 site. Stimulation is dependent on the concentration of RA and its dose-response curve is comparable to that for activation by RAR alpha of transcription on RA-responsive genes. Further, in the absence of RA, a circumstance in which RAR alpha has no effect on AP-1 activity, PML-RAR alpha is an inhibitor. Deletion of the dimerization, transactivation, or DNA-binding domains of c-Jun and removal of the PML dimerization domain in the PML-RAR alpha hybrid abrogates their transcriptional cooperatively. In view of the association between AP-1 activity and hemopoietic differentiation, we suggest that these properties of PML-RAR alpha could contribute to the leukemic phenotype and its response to RA.

Chimeric retinoic acid/thyroid hormone receptors implicate RAR-alpha 1 as mediating growth inhibition by retinoic acid

Retinoic acid (RA) affects the growth and differentiation of cells in culture, usually to decrease the growth rate. In amphibian limb regeneration RA has the remarkable ability to affect pattern formation by changing positional identity, but its initial action on the limb is to inhibit division of the blastemal progenitor cells. Newt limb blastemal cells also show this inhibition in culture. In order to investigate the role of different RA receptors (RARs) in the RA response, the hormone binding domain of the newt RARs alpha 1 and delta 1 was replaced with the corresponding region from the Xenopus thyroid hormone receptor-alpha (TR-alpha). In COS cells transfected with each of the chimeras, transcription was activated after exposure to thyroid hormone (T3). Their profile of activity on three different response elements was indicative of RAR specificity and not TR specificity. After transfection of cultured newt blastemal cells with a DNA particle gun, the chimeras were equally active in stimulating T3-dependent transcription of two different synthetic reporter genes. Blastemal cells were transfected with chimeras or control plasmids along with a marker plasmid expressing beta-galactosidase, exposed to RA or T3 and labelled with [3H]thymidine followed by autoradiography. The alpha 1 chimera gave T3-dependent inhibition of growth, comparable to the effect exerted by RA itself, whereas the delta 1 chimera and control plasmids were inactive. The results imply that RAR-alpha 1 mediates the effects of RA on blastemal cell growth.

Reversal of muscle differentiation during urodele limb regeneration

Recent studies suggest that maintenance of the differentiated state requires continuous regulation. Limb regeneration in urodele amphibians provides a context in which to address this issue, as limb regeneration may involve the dedifferentiation of multinucleate myotubes to yield mononucleate blastemal cells, which then proliferate and contribute to regenerate tissues. To evaluate this possibility, cultured newt limb myotubes were selectively microinjected with the lineage tracer rhodamine-dextran and introduced into regenerating limbs. In culture, such labeled myotubes were stable after 6-8 weeks, and transfer of the tracer to mononucleate cells was not observed. In contrast, after implantation of labeled myotubes under the wound epidermis of limb blastemas, strongly labeled mononucleate cells were observed after 1 week. These cells could be double-labeled with the cytoplasmic lineage tracer and [3H]thymidine that had been incorporated into the nuclei of implanted myotubes. The number of labeled mononucleate cells increased significantly by 2-3 weeks after implantation, indicating that these cells proliferated. Although the fate of these cells at later times was uncertain, we provide evidence consistent with their subsequent differentiation. These results demonstrate reversal in the mononucleate-to-multinucleate transition of vertebrate myogenesis.

Isoform-specific immunological detection of newt retinoic acid receptor delta 1 in normal and regenerating limbs

Retinoic acid (RA) exerts a variety of effects on the regenerating urodele limb including positional respecification of the blastema. The major RA receptor expressed in the newt limb and blastema is the delta 1 isoform and, in order to detect delta 1 in this context, we have made five affinity-purified antibodies against fusion proteins and peptides from non-overlapping regions of the molecule. These antibodies have been evaluated by reaction with transfected COS-7 cells, newt limb cells in culture and newt limb tissue sections. The most informative antibodies were RP6, directed against N-terminal region A sequence, and RP8, directed against C-terminal sequence. In western blots of blastemal extracts, delta 1 protein was detected as two major bands of immunoreactivity at positions consistent with the employment of two candidate methionine initiators identified by cDNA sequencing. Staining of adult limb sections with RP6 and RP8 showed reactivity in half of the nuclei in epidermal and mesenchymal tissues, a heterogeneity that was observed with adjacent nuclei in muscle fibres. In the regenerating limb, nuclei in the blastemal mesenchyme and wound epidermis were strongly reactive, although no axial variation in expression was detected.

Delta retinoic acid receptor isoform delta 1 is distinguished by its exceptional N-terminal sequence and abundance in the limb regeneration blastema

In amphibian limb regeneration memory for position in the proximal-distal axis can be respecified by retinoic acid. The favoured candidates to mediate this effect are the retinoic acid receptors (RARs) and of the RARs identified in the regeneration blastema, the delta receptor is the most abundant. The presence in blastemal mesenchyme of at least two delta receptor isoforms, delta 1 and delta 2, alternatively spliced at the A-B junction, was demonstrated in expression studies and by PCR cloning. The delta 1 receptor is abundant in regenerative structures such as the limb and tail, whereas the delta 2 and alpha receptors show a more uniform pattern of expression across adult newt tissues. Full-length cloning of the delta 1 receptor established the presence of an unusually long open reading frame and N-terminal sequence that appears unique among vertebrate retinoic acid receptors. Transient transfection of expression constructs into COS cells followed by Western blotting confirmed the existence of at least three potential initiation sites for delta 1 translation. The possibility that delta 1 RAR expression may specify positional memory directly was tested in RNase protection experiments. delta 1 receptor message is increased on amputation, but does not exhibit a pronounced differential distribution along the proximal-distal axis in normal and regenerating limbs, nor does it show a persistent alteration in expression levels following a dose of retinoic acid sufficient to respecify position. The possibility that the morphogenetic effects of RA may be mediated through receptor interactions is raised by the finding that single mesenchymal blastemal cells in culture can express multiple RAR subtypes (delta 1 and alpha) and isoforms (delta 1 and delta 2).

Introduction of a retinoid reporter gene into the urodele limb blastema

After amputation of the limb of an adult urodele amphibian at any point along the proximodistal axis, blastemal cells (the progenitor cells of the regenerate) give rise only to the missing structures. Retinoic acid (RA) is able to respecify the positional identity of the blastema to a more proximal value, thus raising the possibility that the RA response system is activated during limb regeneration. Cultured newt (Notophthalmus viridescens) limb cells were transfected by nuclear microinjection of plasmids which provided RA-sensitive reporter activity that could be normalized for differences in cell recovery and transfection efficiency. Such cells showed a dose-dependent response to RA in culture, and this required a functional RA response element. The cells were implanted under the wound epidermis of newt hindlimb blastemas. After injection of a proximalizing dose of RA there was a significant difference in the level of reporter activity dependent on a functional response element. When cells were implanted into contralateral proximal and distal hindlimb blastemas the proximal-to-distal ratio for activation of the reporter through the response element was approximately 3.5-fold, suggesting that a gene whose expression is regulated by RA could be differentially activated along the proximodistal axis during limb regeneration.

Identification and expression pattern of a second isoform of the newt alpha retinoic acid receptor

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Retinoids and their targets in vertebrate development

The elaboration of the effect of retinoic acid on limb morphogenesis has prompted renewed investigation into the teratology of retinoic acid treatment, with the hope that such analysis might give insight into mechanisms of vertebrate patterning. Retinoids, their nuclear receptors and their cytoplasmic binding proteins are now known to be deployed throughout development, but the extent to which they are natural agents of morphogenesis remains obscure. The study of retinoic acid receptors may offer molecular insight into gene regulation underlying vertebrate pattern formation.

Some current problems in amphibian limb regeneration

Limb regeneration in adult urodele amphibians proceeds by formation of a blastema at the amputation plane. This paper discusses how the blastema forms, and how its positional identity on the proximodistal axis is manifest. Retinoic acid is able to reset axial specification and there is particular interest in determining how it acts. Although limb regeneration is restricted among vertebrates to the urodeles, its mechanism poses fundamental questions in development biology.

Identification and expression of a regeneration-specific homeobox gene in the newt limb blastema

Adult urodele amphibians are able to regenerate their limbs through the formation of a blastema, a growth zone of undifferentiated mesenchymal cells that arises locally at the plane of amputation. In an effort to define genes involved in pattern formation by the blastema, we isolated from a newt forelimb blastema library cDNA clones that identify a homeobox gene termed NvHbox 2. The amino acid sequence of the homeodomain is identical to that of the recently identified human HOX-4f gene (Acampora et al. 1989) and of the mouse Hox-5.5 (Dolle et al. 1989). NvHbox 2 is expressed in the limb blastema as a transcript of 3.4 kb that is not detectable in the normal limb. Analysis by RNAase protection demonstrates expression in limb and tail blastemas, but not in any of the adult tissues or organs tested. In the limb blastema NvHbox 2 was expressed in mesenchymal but not epithelial tissue. When matched and normalised samples of RNA from proximal (mid-humerus) and distal forelimb (mid-radius) blastemas were compared, the level of expression of NvHbox 2 was found to be 3- to 5-fold higher proximally. At two time points after injection of a proximalising dose of retinoic acid, the level of expression in a distal blastema was not increased in comparison to controls.

A newt type II keratin restricted to normal and regenerating limbs and tails is responsive to retinoic acid

In order to understand the molecular mechanisms underlying the regenerative ability of the urodele limb, it is important to identify regeneration-associated proteins and to study their regulation. We have recently shown that the anti-cytokeratin monoclonal antibody LP1K reacts strongly with newt blastemal cells, while its reactivity is restricted in normal limbs. By screening a cDNA expression library from the newt blastema with LP1K, we have identified cDNA clones coding for a type II keratin (NvKII) expressed both in the mesenchyme and the specialized wound epithelium of the blastema. While the rod domain of the protein is highly conserved, the homology between NvKII and mammalian type II keratins drops markedly at the N- and C-terminal regions. The expression of this keratin was analysed by Northern blotting and RNAase protection analysis of various newt tissues, and appears to be organ specific, since it is restricted to normal and regenerating limbs and tails. In particular, we have investigated the expression of this keratin mRNA in normal and regenerating limbs. The transcript is barely detectable in the proximal portion of the normal limb, but its level is high in the distal one. After amputation, NvKII mRNA is expressed both in proximal and distal blastemas, although at higher levels distally, indicating that this keratin is regeneration associated. The NvKII transcript is detectable both in mesenchyme and in the wound epithelium of the regenerate, while no transcript is detectable in normal epidermis. The level of NvKII mRNA is markedly down-regulated both in normal and regenerating limbs following intraperitoneal injection with retinoic acid, a putative endogenous morphogen in limb regeneration.

Cell origin and identity in limb regeneration and development

The occurrence of limb regeneration in adult urodele amphibians raises fundamental questions about the relationship between development and regeneration. The use of monoclonal antibodies as cell markers has provided clear evidence that blastemal cells, the progenitor cells of the regenerate, are not the same as limb bud cells in the embryo. For one of these antibodies the distinction has been traced to the relationship between limb regeneration and the nerve supply. Innervation of the limb bud during development appears to establish nerve-dependent growth control for regeneration. The cell markers have also contributed to the problem of how the blastemal cells arise after amputation, although several important questions remain to be answered.

The monoclonal antibody 22/18 recognizes a conformational change in an intermediate filament of the newt, Notophthalmus viridescens, during limb regeneration

Previous work has shown that the monoclonal antibody 22/18 identifies progenitor cells (blastemal cells) which depend on the nerve for their division in the early stages of limb regeneration in the newt, Notophthalmus viridescens. This antibody also reacts with cultured cells derived from the newt limb, and the intensity of immunoreactivity appears related to cell density and differentiation into myotubes. We report here that the monoclonal antibody 22/18 recognizes a polypeptide (22/18 antigen) which is intracellular and filamentous. Double staining of cells with 22/18 monoclonal antibody and antibodies against various cytoskeletal components indicates that the epitope is expressed on an intermediate filament component. Although this antibody is specific for blastemal cells in cryostat sections of the regenerating limb, its reactivity on immunoblots is not confined to this tissue. The 22/18 antigen is differentially affected by aldehyde fixatives distinguished by the spacing of their reactive groups. While formaldehyde fixation impairs detection of the antigen, ethylene glycol-bis[succinic acid n-hydroxysuccinimide ester] reveals the antigen in sections of normal and regenerating limbs in a distribution that is consistent with the one obtained from immunoblots. We suggest that the 22/18 monoclonal antibody detects a change in protein conformation, probably related to changes in the physiological state of the cell, that occurs transiently during regeneration and possibly during development.

Retinoic acid and limb regeneration

A key problem in the study of vertebrate development is to determine the molecular basis of positional value along a developmental axis. In amphibian regeneration, retinoic acid is able to respecify positional value in a graded fashion that is dependent on its concentration. In view of the fact that retinoic acid is a naturally occurring metabolite of vitamin A, this raises the possibility that it is deployed in vivo as an endogenous morphogen. Furthermore, the recent evidence that its effects are mediated by nuclear receptors of the steroid/thyroid hormone superfamily suggests the possibility of understanding the mechanism of its graded effects on morphogenesis. Such insights would be of crucial importance for our understanding of vertebrate patterning along an axis.

Identification of a novel retinoic acid receptor in regenerative tissues of the newt

In urodele amphibians, the progenitor cells that regenerate amputated limbs (known as the blastema) normally replace only the missing structures. After systemic delivery of retinoic acid (RA), more proximal structures are also formed, indicating that RA can control position specification in the proximal-distal axis of the regenerating limb. According to dose and experimental context, retinoids can also re-specify the anteroposterior axis of the limb, induce deletions of skeletal elements, or block re-growth completely. To study the molecular basis of these morphogenetic effects, we screened complementary DNA libraries of newt regenerative tissues (limbs and tails) for hormone nuclear receptors activated by RA. Two functional retinoic acid receptors (RARs) were identified, one of which is the newt homologue of the human alpha-receptor (RAR alpha). The second receptor, called RAR delta, is novel. Sequence analysis suggests that the composite newt RAR previously reported is chimaeric, consisting of 5'RAR-beta-like and 3' RAR delta clones. We conclude that multiple RARs are expressed during limb regeneration in amphibians and suggest that receptor heterogeneity may underlie the different effects of retinoids on limb morphogenesis.

Position dependent expression of a homeobox gene transcript in relation to amphibian limb regeneration

Adult urodele amphibians such as the newt Notophthalmus viridescens are capable of regenerating their limbs and tail by formation of a blastema, a growth zone of mesenchymal progenitor cells. In an attempt to identify genes implicated in specification of the regenerate, we screened a newt forelimb blastema cDNA library with homeobox probes, and isolated and sequenced clones that identify a 1.8 kb polyadenylated transcript containing a homeobox. The transcript is derived from a single gene called NvHbox 1, the newt homologue of XIHbox 1 (Xenopus), HHO.c8 (human) and Hox-6.1 (mouse). The cDNA for the 1.8 kb transcript has two exons as determined by isolation and partial sequencing of a genomic clone. The expression of the transcript shows several interesting features in relation to limb regeneration: (i) Hybridization of Northern blots of poly(A)+ RNA from limb and tail and their respective blastemas shows that the transcript in limb tissues has exons 1 and 2, whereas a 1.8 kb transcript in tail tissues has only exon 2. (ii) The transcript is expressed in limbs of adult newt but not of adult Xenopus, raising the possibility that this contributes to an explanation of the loss of regenerative ability with maturation in adult anurans. (iii) The transcript is expressed at a higher level in a proximal (mid-humerus) blastema than in a distal one (mid-radius). When distal blastemas were proximalized by treatment with retinoic acid, no change in the level of the transcript was detected by Northern analysis at a single time point after amputation.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for dedifferentiation and metaplasia in amphibian limb regeneration from inheritance of DNA methylation

Amphibian limb regeneration is a process in which it has been suggested that cells of one differentiated type may dedifferentiate and give rise to cells of another type in the regenerate. We have used two tissue-specific hypomethylations in the newt cardioskeletal myosin heavy chain gene as lineage markers to follow the fate of cells during limb regeneration. Analysis of genomic DNA from different muscle cell populations allowed the assignment of one marker to the muscle (Hypo A) lineage and the other, more tentatively, to the ‘connective tissue’ (Hypo B) component of muscle. The contribution to regenerated limb cartilage and limb blastemal tissue by cells carrying these markers was estimated by quantitative analysis of Southern blot hybridizations using DNA from regenerate tissues. The results are consistent with a contribution of cells from both muscle and connective tissue lineages to cartilage in regenerated limbs. In addition, removal of the humerus at the time of amputation (eliminating any contribution from pre-existing cartilage), has provided evidence for an increased representation of cells carrying the connective tissue marker in regenerate cartilage but did not affect the representation of cells carrying the muscle cell marker.

Appearance and regulation of an antigen associated with limb regeneration in Notophthalmus viridescens

Previous reports from this laboratory have documented a cellular antigen, 22/18, associated with regeneration of forelimbs in the newt Notophthalmus viridescens (Kintner and Brockes: Nature 308:67-69, 1984; Journal of Embryology and Experimental Morphology 89:37-55, 1985; Brockes: Science 225:1280-1287, 1984; Kintner et al.: Molecular Basis of Neural Development, 1985). The antigen is expressed at various stages of regeneration in several types of cells including Schwann cells and muscle cells. We report here that, in addition to the previous results, the expression of 22/18 occurs very early in the regeneration process and is initially confined to an apparently nonmuscle, non-neural population of cells. The well-defined context of the antigen's first appearance has allowed us to investigate the regulation of its expression. In particular, injury is sufficient to elicit the appearance of 22/18. It can appear prior to mitosis and despite denervation. In circumstances where regeneration is inhibited, expression of 22/18 can persist.

Structure and expression of a newt cardio-skeletal myosin gene. Implications for the C value paradox

As part of our studies on the fate of the muscle lineage during amphibian limb regeneration, we have isolated genomic and cDNA sequences from a myosin heavy chain in the newt (Notophthalmus viridescens). Notwithstanding the technical problems inherent in analysing the large newt genome, genomic and cDNA sequences have been isolated and subjected to analysis by restriction mapping. Northern hybridization, Southern hybridization and DNA sequencing. We believe these to be the first single copy newt gene sequences to have been subjected to this type of analysis. The newt gene sequences showed a striking difference from mammalian myosins in both the estimated sizes of the gene and its intervening sequences; these being much larger than in the mammalian models, it is speculated that this could contribute to the exceptional size of the newt genome. By contrast, the coding sequences displayed very high levels of sequence homology to mammalian myosins. In particular, the amino acid sequence of the newt myosin was found to have greatest homology with rat and human myosin isotypes having a similar cardio-skeletal muscle expression pattern. Despite a long evolutionary separation, newt and mammalian cardio-skeletal myosins have remained more similar to each other than have the human or rat cardiac forms to skeletal myosins within their own respective species.

Evidence that the nerve controls molecular identity of progenitor cells for limb regeneration

Adult urodele amphibians can regenerate their limbs after amputation by a process that requires the presence of axons at the amputation plane. Paradoxically, if the limb develops in the near absence of nerves (the ‘aneurogenic’ limb) it can subsequently regenerate in a nerve-independent fashion. The growth zone (blastema) of regenerating limbs normally contains progenitor cells whose division is nerve-dependent. A monoclonal antibody that marks these nerve-dependent cells in the normal blastema does not stain the mesenchymal cells of developing limb buds and only stains the amputated limb bud when axons have reached the plane of amputation. This report shows that the blastemal cells of the regenerating aneurogenic limb also fail to react with the antibody in situ. These data suggest that the blastemal cells arising during normal regeneration have been altered by the nerve. This regulation may occur either at the time of amputation (when the antigen is expressed) or during development (when the limb is first innervated).

Culture of newt cells from different tissues and their expression of a regeneration-associated antigen

We have established culture conditions for cells from normal limb, early limb regenerate (blastema), heart, and liver of the newt Notophthalmus viridescens. Whereas heart and liver cells had a relatively short life in culture, limb cells have shown no sign of senescence over more than 1 year in culture. Cultured cells from all these tissues express to differing extents the regeneration-associated antigen 22/18. The antigen is intracellular and filamentous, and its expression appears to be regulated by culture density. Furthermore, 22/18 antigen is turned off in limb and blastemal cultures following differentiation into muscle, as also occurs in vivo.

The nerve dependence of amphibian limb regeneration

The regeneration of limbs in urodele amphibians is a context where the developing and regenerating peripheral nervous system interacts with the mechanisms of epimorphic regeneration. After amputation of a limb, there arise at the amputation plane the blastemal cells which are the progenitor cells of the regenerate. These cells divide rapidly and subsequently differentiate to give rise to the internal tissues (cartilage, muscle and connective tissue) of the regenerate. Division of the blastemal cells requires the presence of nerve axons at the amputation plane, at least during the initial stages of regeneration. This requirement can be circumvented by allowing a limb to develop in the absence of a nerve supply (the ‘aneurogenic limb’), but the underlying mechanisms have been unclear. We have derived a monoclonal antibody called 22/18 that has provided new information about these issues. It is specific for blastemal cells versus normal tissue in the limb, specific for regeneration versus development, and specific for blastemal cells that arise after amputation in the presence of the nervous system versus its absence (in either development or the aneurogenic limb). The antibody reactivity appears to mark a cell transition involved in the imposition of nerve-dependent growth control.

A monoclonal antibody detects a difference in the cellular composition of developing and regenerating limbs of newts

We have previously described a monoclonal antibody (called 22/18) that reacts with the early blastemal cells of the regenerating limb of the newt (Notophthalmus viridescens). In embryos of two newt species the antibody reacts with the epidermis, glial cells in the neural tube, the lens and cells in a restricted region of the aorta. In the developing limb bud less than 1% of the mesenchymal cells were reactive with 22/18, although most cells stained brightly with an antibody to another cytoskeletal component. When limbs were amputated prior to the arrival of nerves (axons and Schwann cells) at the amputation plane there was no extra reactivity with 22/18 as compared to the contralateral unamputated control, even though the amputated buds regenerated satisfactorily. Limbs amputated after nerves are present at the plane of amputation respond by forming a 22/18-positive blastema. The appearance of the 22/18 responses is a function of the stage of limb development as shown by amputation of forelimb and hindlimb buds at a larval stage where development of the forelimb is greatly advanced relative to the hindlimb. The distribution of the 22/18-positive cells in larval blastemas showed them to be closely associated with axons as detected by double staining with an antiserum to a neurofilament subunit. The clear antigenic difference between development and regeneration may be related to the relationship between embryonic regulation and epimorphic regeneration, and also to the acquisition of nerve-dependent proliferation of blastemal cells.