Differential expression of myogenic regulatory genes and Msx-1 during dedifferentiation and redifferentiation of regenerating amphibian limbs

Cellular and molecular regulation of muscle regeneration

Under normal circumstances, mammalian adult skeletal muscle is a stable tissue with very little turnover of nuclei. However, upon injury, skeletal muscle has the remarkable ability to initiate a rapid and extensive repair process preventing the loss of muscle mass. Skeletal muscle repair is a highly synchronized process involving the activation of various cellular responses. The initial phase of muscle repair is characterized by necrosis of the damaged tissue and activation of an inflammatory response. This phase is rapidly followed by activation of myogenic cells to proliferate, differentiate, and fuse leading to new myofiber formation and reconstitution of a functional contractile apparatus. Activation of adult muscle satellite cells is a key element in this process. Muscle satellite cell activation resembles embryonic myogenesis in several ways including the de novo induction of the myogenic regulatory factors. Signaling factors released during the regenerating process have been identified, but their functions remain to be fully defined. In addition, recent evidence supports the possible contribution of adult stem cells in the muscle regeneration process. In particular, bone marrow-derived and muscle-derived stem cells contribute to new myofiber formation and to the satellite cell pool after injury.

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.

Dual-regulated myoD- and msx1-based interventions in C2C12-derived cells enable precise myogenic/osteogenic/adipogenic lineage control

BACKGROUND

Advanced gene therapy, tissue engineering and biopharmaceutical manufacturing require sophisticated and well-balanced multiregulated multigene interventions to reprogram desired mammalian cell phenotypes.

METHODS

We have combined the streptogramin (PIP)- and tetracycline (TET)-responsive gene regulation systems for independent expression control of the differentiation determinants myoD and msx1 in C2C12-derived cells.

RESULTS

Different dual-regulated expression scenarios which induce either both, only one or none of the lineage control genes triggered differential differentiation and precise control of myogenic, osteogenic or adipogenic cell phenotypes.

CONCLUSIONS

Our findings substantiate the use of multiregulated multigene interventions in reprogramming cellular differentiation pathways in a desired manner.

Digit regeneration is regulated by Msx1 and BMP4 in fetal mice

The regeneration of digit tips in mammals, including humans and rodents, represents a model for organ regeneration in higher vertebrates. We had previously characterized digit tip regeneration during fetal and neonatal stages of digit formation in the mouse and found that regenerative capability correlated with the expression domain of the Msx1 gene. Using the stage 11 (E14.5) digit, we now show that digit tip regeneration occurs in organ culture and that Msx1, but not Msx2, mutant mice display a regeneration defect. Associated with this phenotype, we find that Bmp4 expression is downregulated in the Msx1 mutant digit and that mutant digit regeneration can be rescued in a dose-dependent manner by treatment with exogenous BMP4. Studies with the BMP-binding protein noggin show that wild-type digit regeneration is inhibited without inhibiting the expression of Msx1, Msx2 or Bmp4. These data identify a signaling pathway essential for digit regeneration, in which Msx1 functions to regulate BMP4 production. We also provide evidence that endogenous Bmp4 expression is regulated by the combined activity of Msx1 and Msx2 in the forming digit tip; however, we discovered a compensatory Msx2 response that involves an expansion into the wild-type Msx1 domain. Thus, although both Msx1 and Msx2 function to regulate Bmp4 expression in the digit tip, the data are not consistent with a model in which Msx1 and Msx2 serve completely redundant functions in the regeneration response. These studies provide the first functional analysis of mammalian fetal digit regeneration and identify a new function for Msx1 and BMP4 as regulators of the regenerative response.

Cranial neural crest-derived mesenchymal proliferation is regulated by Msx1-mediated p19(INK4d) expression during odontogenesis

Neural crest cells are multipotential progenitors that contribute to various cell and tissue types during embryogenesis. Here, we have investigated the molecular and cellular mechanism by which the fate of neural crest cell is regulated during tooth development. Using a two- component genetic system for indelibly marking the progeny of neural crest cells, we provide in vivo evidence of a deficiency of CNC-derived dental mesenchyme in Msx1 null mutant mouse embryos. The deficiency of the CNC results from an elevated CDK inhibitor p19(INK4d) activity and the disruption of cell proliferation. Interestingly, in the absence of Msx1, the CNC-derived dental mesenchyme misdifferentiates and possesses properties consistent with a neuronal fate, possibly through a default mechanism. Attenuation of p19(INK4d) in Msx1 null mutant mandibular explants restores mitotic activity in the dental mesenchyme, demonstrating the functional significance of Msx1-mediated p19(INK4d) expression in regulating CNC cell proliferation during odontogenesis. Collectively, our results demonstrate that homeobox gene Msx1 regulates the fate of CNC cells by controlling the progression of the cell cycle. Genetic mutation of Msx1 may alternatively instruct the fate of these progenitor cells during craniofacial development.

Intercalary and supernumerary regeneration in the limbs of the frog, Xenopus laevis

Anuran amphibians, such as Xenopus laevis, can regenerate their limbs only when they are young tadpoles, whereas urodele amphibians have a regenerative ability throughout their lives. It is still unclear whether anuran and urodele use the same mechanism during regeneration. In the present study, we analyzed intercalary and supernumerary regeneration in Xenopus. In contrast to urodele blastema that induces intercalary regeneration along the proximodistal (PD) axis, intercalation did not occur in the Xenopus limb bud when the presumptive zeugopodium (fibula and tibia) was removed. However, when the limb bud tip (presumptive autopodium) was transplanted to the presumptive stylopodium (femur) with a 180-degree rotation at stage 52, the complete zeugopodium was regenerated. These results were similar to the results of urodele mature limbs, suggesting that Xenopus limb buds are equivalent to the urodele mature limbs but not to the urodele blastemas. We hypothesized that the ability for intercalation depends on the expression pattern of fibroblast growth factor (fgf)-8, because the expression of fgf-8 in the urodele spreads over the whole blastema and is close enough to activate the growth of the stump. To test this hypothesis, an FGF-8-soaked bead was implanted at the boundary between the stump and tip of a Xenopus limb bud. Intercalary regeneration was induced at stages 52 and 53. These results suggest that the Xenopus limb bud possesses the potential for intercalation, but endogenous FGF-8 in the apical ectodermal ridge (AER) does not induce intercalation to the stump because of the long distance between the AER and stump.

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.

The molecular basis of amphibian limb regeneration: integrating the old with the new

Is regeneration close to revealing its secrets? Rapid advances in technology and genomic information, coupled with several useful models to dissect regeneration, suggest that we soon may be in a position to encourage regeneration and enhanced repair processes in humans.

Mechanisms of muscle dedifferentiation during regeneration

For many years people have known that amphibians have an amazing ability to regenerate lost body parts. In contrast humans have limited regeneration capacity and even simple wound healing results in scarring. Despite more than a century of scientific inquiry, this remarkable phenomenon remains poorly understood. Recent research has begun to provide insight into how this unique process that is now fully accepted to occur via the reversal of cell differentiation is executed at the molecular level. As more and more is known about regeneration and dedifferentiation we can begin to address the question: if given the right signals could mammals also regenerate body structures?

The origin of skeletal muscle stem cells in the embryo and the adult

Skeletal muscle progenitors are specified during embryogenesis and in addition have recently been found to be generated from either mesenchymal or neural stem cells in the adult. We review recent progress in identifying the signals and transcription factors that control skeletal muscle formation during embryogenesis and in the adult.

Leaping lopsided: a review of the current hypotheses regarding etiologies of limb malformations in frogs

Recent progress in the investigation of limb malformations in free-living frogs has underlined the wide range in the types of limb malformations and the apparent spatiotemporal clustering of their occurrence. Here, we review the current understanding of normal and abnormal vertebrate limb development and regeneration and discuss some of the molecular events that may bring about limb malformation. Consideration of the differences between limb development and regeneration in amphibians has led us to the hypothesis that some of the observed limb malformations come about through misdirected regeneration. We report the results of a pilot study that supports this hypothesis. In this study, the distal aspect of the right hindlimb buds of X. laevis tadpoles was amputated at the pre-foot paddle stage. The tadpoles were raised in water from a pond in Minnesota at which 7% of surveyed newly metamorphosed feral frogs had malformations. Six percent (6 of 100) of the right limbs of the tadpoles raised in pond water developed abnormally. One truncated right limb was the only malformation in the control group, which was raised in dechlorinated municipal water. All unamputated limbs developed normally in both groups. Three major factors under consideration for effecting the limb malformations are discussed. These factors include environmental chemicals (primarily agrichemicals), encysted larvae (metacercariae) of trematode parasites, and increased levels of ultraviolet light. Emphasis is placed on the necessary intersection of environmental stressors and developmental events to bring about the specific malformations that are observed in free-living frog populations.

NF-kappaB mediates FGF signal regulation of msx-1 expression

The nuclear factor-kappaB (NF-kappaB) family of transcription factors is involved in proliferation, differentiation, and apoptosis in a stage- and cell-dependent manner. Recent evidence has shown that NF-kappaB activity is necessary for both chicken and mouse limb development. We report here that the NF-kappaB family member c-rel and the homeodomain gene msx-1 have partially overlapping expression patterns in the developing chick limb. In addition, inhibition of NF-kappaB activity resulted in a decrease in msx-1 mRNA expression. Sequence analysis of the msx-1 promoter revealed three potential kappaB-binding sites similar to the interferon-gamma (IFN-gamma) kappaB-binding site. These sites bound to c-Rel, as shown by electrophoretic mobility shift assay (EMSA). Furthermore, inhibition of NF-kappaB activity significantly reduced transactivation of the msx-1 promoter in response to FGF-2/-4, known stimulators of msx-1 expression. These results suggest that NF-kappaB mediates the FGF-2/-4 signal regulation of msx-1 gene expression.

In vivo imaging indicates muscle fiber dedifferentiation is a major contributor to the regenerating tail blastema

During tail regeneration in urodele amphibians such as axolotls, all of the tissue types, including muscle, dermis, spinal cord, and cartilage, are regenerated. It is not known how this diversity of cell types is reformed with such precision. In particular, the number and variety of mature cell types in the remaining stump that contribute to the blastema is unclear. Using Nomarski imaging, we followed the process of regeneration in the larval axolotl tail. Combining this with in vivo fluorescent labeling of single muscle fibers, we show that mature muscle dedifferentiates. Muscle dedifferentiation occurs by the synchronous fragmentation of the multinucleate muscle fiber into mononucleate cells followed by rapid cell proliferation and the extension of cell processes. We further show that direct clipping of the muscle fiber and severe tissue damage around the fiber are both required to initiate dedifferentiation. Our observations also make it possible to estimate for the first time how many of the blastema cells arise specifically from muscle dedifferentiation. Calculations based on our data suggest that up to 29% of nondermal-derived cells in the blastema come from dedifferentiation of mature muscle fibers. Overall, these results show that endogenous multinucleate muscle fibers can dedifferentiate into mononucleate cells and contribute significantly to the blastema.

Identification and characterization of newt rad (ras associated with diabetes), a gene specifically expressed in regenerating limb muscle

Formation of the blastema is a key event for limb regeneration in urodele amphibians, and skeletal muscle has been thought to be a major origin of the multipotent blastemal mesenchyme. In the present study, we used differential display to identify the genes expressed differentially in the muscle at the amputation site. We have isolated a cDNA clone that was upregulated during limb regeneration of the Japanese newt, Cynops pyrrhogaster. Deduced amino acid sequence revealed that the cloned cDNA was a newt homolog of rad (ras associated with diabetes), a gene overexpressed in skeletal muscle of Type II diabetic patients. Expression of newt rad (nrad) was not observed in unamputated normal limb muscle, increased within 4 hr after amputation, and then decreased to the level of normal muscle between 11 and 21 days after amputation. In situ hybridization showed that the transcripts of nrad were localized around most of the nuclei of skeletal muscle near the amputation site, indicating the expression of nrad in the multinucleate myotubes. This expression gradually decreased along the distal to proximal axis. No signals were observed in apical epidermal cap or blastemal mesenchyme. However, reverse transcription-PCR analysis detected a very low level of nrad expression in blastema, suggesting the carry-over of nrad expression in blastema from muscle. Administration of retinoic acid, which has been shown to cause an enhanced dedifferentiation in the regenerating limbs, increased nrad expression in more proximally located limb muscle tissues and prolonged the expression period. Thus, it was strongly suggested that the nrad expression is correlated with the dedifferentiation of myotubes of regenerating limbs. We also analyzed the expression of nrad during development. Transcripts were observed in immature oocytes, seen faintly or not seen thereafter until stage 57 when its expression increased again. These results indicated that nrad may play a role(s) in the developmental process as well as limb regeneration.

Dedifferentiation of mammalian myotubes induced by msx1

The process of cellular differentiation culminating in terminally differentiated mammalian cells is thought to be irreversible. Here, we present evidence that terminally differentiated murine myotubes can be induced to dedifferentiate. Ectopic expression of msx1 in C2C12 myotubes reduced the nuclear muscle proteins MyoD, myogenin, MRF4, and p21 to undetectable levels in 20%-50% of the myotubes. Approximately 9% of the myotubes cleave to produce either smaller multinucleated myotubes or proliferating, mononucleated cells. Finally, clonal populations of the myotube-derived mononucleated cells can be induced to redifferentiate into cells expressing chondrogenic, adipogenic, myogenic, and osteogenic markers. These results suggest that terminally differentiated mammalian myotubes can dedifferentiate when stimulated with the appropriate signals and that msx1 can contribute to the dedifferentiation process.

Regeneration in the metazoans: why does it happen?

Why does regeneration occur? And why, when it manifests itself, does it do so in some but not all metazoan species? Hence, what are the permissive or inhibitory factors operating behind this phenomenon? When it comes to regeneration, many questions, such as these, remain unanswered. In fact, the problem of animal regeneration has withstood the probing of scientific inquiry for over 250 years and still awaits a satisfactory mechanistic explanation. In this essay, I will review the distribution and the modes of regeneration that are found in the different metazoan phyla. Also, I will re-examine ideas on its evolutionary origins, and discuss its possible relationship to both asexual reproduction and embryogenesis. This endeavor has two objectives. First, to bring forward an interpretation of regeneration which integrates evolutionary and developmental considerations into its discussion. And second, to suggest a comparative experimental approach to this problem that may bring us closer to understanding the molecular basis of this long-standing biological problem. BioEssays 22:578-590, 2000.

Roles for Msx and Dlx homeoproteins in vertebrate development

This review provides a comparative analysis of the expression patterns, functions, and biochemical properties of Msx and Dlx homeobox genes. These comprise multi-gene families that are closely related with respect to sequence features as well as expression patterns during vertebrate development. Thus, members of the Msx and Dlx families are expressed in overlapping, but distinct, patterns and display complementary or antagonistic functions, depending upon the context. A common theme shared among Msx and Dlx genes is that they are required during early, middle, and late phases of development where their differential expression mediates patterning, morphogenesis, and histogenesis of tissues in which they are expressed. With respect to their biochemical properties, Msx proteins function as transcriptional repressors, while Dlx proteins are transcriptional activators. Moreover, their ability to oppose each other's transcriptional actions implies a mechanism underlying their complementary or antagonistic functions during development.

Expression of helix-loop-helix type negative regulators of differentiation during limb regeneration in urodeles and anurans

The urodele is capable of regenerating its limb by forming a blastema even in the adult. By contrast, the anuran, which is phylogenetically close to the urodele, loses this ability during metamorphosis and forms blastema-like tissues that develop only into a spike-like structure in the adult. In order to compare the molecular mechanism of the formation and maintenance of the blastema between the urodele and anuran, the genes encoding helix-loop-helix (HLH) type negative regulators of differentiation were characterized for both the Japanese newt, Cynops pyrrhogaster, and African clawed frog, Xenopus laevis. Cynops homologs of Id2, Id3, and HES1 and Xenopus Id2 were identified. To learn the roles of these genes in regeneration, their expression was examined. The expression of Id2 and Id3 was low in unamputated limbs, but was up-regulated in blastemas of both adult newt and Xenopus. Interestingly, transcripts of the two Id genes showed specific localizations in the blastema and the expression patterns were very similar in both species through the early to medium bud stage. Id2 was expressed predominantly in the blastemal epidermis, and Id3 was expressed equally in the blastemal epidermis and mesenchyme including cells in precartilage condensations. HES1 expression was up-regulated in the newt blastemal epidermis. It was thought that the up-regulation of these genes in the epidermis was related to the proliferation of the cells and that increased expression of these genes in the mesenchyme was related to the undifferentiated state of the blastemal cells. These results and considerations strongly suggested that the state of differentiation is similar in the early to medium bud blastema of both urodeles and anurans. The expression of Id3 remained high through to the digits stage in newts. In contrast, its expression in Xenopus decreased in spike-like regenerates, which correspond to palette-digits stage of newt regenerates. From these results, it was suggested that the blastema redifferentiates earlier in the frog than in the newt, and therefore the timing of redifferentiation of the cartilage is crucial for complete regeneration.

Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors

The migration of myogenic precursors to the vertebrate limb exemplifies a common problem in development - namely, how migratory cells that are committed to a specific lineage postpone terminal differentiation until they reach their destination. Here we show that in chicken embryos, expression of the Msx1 homeobox gene overlaps with Pax3 in migrating limb muscle precursors, which are committed myoblasts that do not express myogenic differentiation genes such as MyoD. We find that ectopic expression of Msx1 in the forelimb and somites of chicken embryos inhibits MyoD expression as well as muscle differentiation. Conversely, ectopic expression of Pax3 activates MyoD expression, while co-ectopic expression of Msx1 and Pax3 neutralizes their effects on MyoD. Moreover, we find that Msx1 represses and Pax3 activates MyoD regulatory elements in cell culture, while in combination, Msx1 and Pax3 oppose each other's trancriptional actions on MyoD. Finally, we show that the Msx1 protein interacts with Pax3 in vitro, thereby inhibiting DNA binding by Pax3. Thus, we propose that Msx1 antagonizes the myogenic activity of Pax3 in migrating limb muscle precursors via direct protein-protein interaction. Our results implicate functional antagonism through competitive protein-protein interactions as a mechanism for regulating the differentiation state of migrating cells.

Expression of the Third Component of Complement, C3, in Regenerating Limb Blastema Cells of Urodeles

In this study we have shown that complement component C3 is expressed in the regenerating tissue during urodele limb regeneration. C3 was expressed in the dedifferentiated regeneration blastema and in the redifferentiated limb tissues in the axolotl, Amblystoma mexicanum, and in Notophthalmus viridescens. This expression was verified by immunofluorescent staining using an Ab against axolotl C3 and by in situ hybridization with an axolotl C3 cDNA probe. In the early stages of regeneration C3 appeared to be equally present in all mesenchymal cells and in the wound epithelium, whereas in the later stages it was mainly expressed in the differentiating muscle cells. Since no expression was seen in the developing limb, it appears that the C3 expression was specific to the regeneration process. We then demonstrated by hybridization experiments that a blastema cell line of myogenic origin expresses C3. All these findings implicate C3 in the dedifferentiation process and may indicate a new role for this molecule in muscle differentiation.

Expression of Msx-2 during development, regeneration, and wound healing in axolotl limbs

Msx genes are transcription factors that are expressed during embryogenesis of developing appendages in regions of epithelial-mesenchymal interactions. Various lines of evidence indicate that these genes function to maintain embryonic tissues in an undifferentiated, proliferative state. We have identified the axolotl homolog of Msx-2, and investigated its expression during limb development, limb regeneration, and wound healing. As in limb buds of higher vertebrates, axolotl Msx-2 is expressed in the apical epidermis and mesenchyme; however, its expression domain is more extensive, reflecting the broader region of the apical epidermal cap in amphibians. Msx-2 expression is downregulated at late stages of limb development, but is reexpressed within one hour after limb amputation. Msx-2 is also reexpressed during wound healing, and may be essential in the early stages of initiation of the limb regeneration cascade.

Expression of Msx genes in regenerating and developing limbs of axolotl

Msx genes, homeobox-containing genes, have been isolated as homologues of the Drosophila msh gene and are thought to play important roles in the development of chick or mouse limb buds. We isolated two Msx genes, Msx1 and Msx2, from regenerating blastemas of axolotl limbs and examined their expression patterns using Northern blot and whole mount in situ hybridization during regeneration and development. Northern blot analysis revealed that the expression level of both Msx genes increased during limb regeneration. The Msx2 expression level increased in the blastema at the early bud stage, and Msx1 expression level increased at the late bud stage. Whole mount in situ hybridization revealed that Msx2 was expressed in the distal mesenchyme and Msx1 in the entire mesenchyme of the blastema at the late bud stage. In the developing limb bud, Msx1 was expressed in the entire mesenchyme, while Msx2 was expressed in the distal and peripheral mesenchyme. The expression patterns of Msx genes in the blastemas and limb buds of the axolotl were different from those reported for chick or mouse limb buds. These expression patterns of axolotl Msx genes are discussed in relation to the blastema or limb bud morphology and their possible roles in limb patterning.

Protein complex formation between Msx1 and Lhx2 homeoproteins is incompatible with DNA binding activity

Msx genes encode a family of homeoproteins that function as transcription repressors through protein-protein interactions. Here we show that Lhx2, a LIM-type homeoprotein, is a protein partner for Msx1 in vitro and in cellular extracts. The interaction between Msx1 and Lhx2 is mediated through the homeodomain-containing regions of both proteins. Interestingly, the LIM domains, which serve as protein interaction domains for other partners of Lhx2, are not required for the Msx1-Lhx2 association. We show that Msx1 and Lhx2 form a protein complex in the absence of DNA, and that DNA binding by either protein alone can occur at the expense of protein complex formation. The significance of this protein-protein interaction is underscored by the expression patterns of Msx1 and Lhx2, which are partially overlapping during murine embryogenesis. The description of Lhx2 as a protein partner for Msx1 suggests that the functional specificity of homeoproteins in vivo is determined by a balance between their association with DNA and their protein partners.

Structural and functional analysis of mouse Msx1 gene promoter: sequence conservation with human MSX1 promoter points at potential regulatory elements

Vertebrate Msx genes are related to one of the most divergent homeobox genes of Drosophila, the muscle segment homeobox (msh) gene, and are expressed in a well-defined pattern at sites of tissue interactions. This pattern of expression is conserved in vertebrates as diverse as quail, zebrafish, and mouse in a range of sites including neural crest, appendages, and craniofacial structures. In the present work, we performed structural and functional analyses in order to identify potential cis-acting elements that may be regulating Msx1 gene expression. To this end, a 4.9-kb segment of the 5'-flanking region was sequenced and analyzed for transcription-factor binding sites. Four regions showing a high concentration of these sites were identified. Transfection assays with fragments of regulatory sequences driving the expression of the bacterial lacZ reporter gene showed that a region of 4 kb upstream of the transcription start site contains positive and negative elements responsible for controlling gene expression. Interestingly, a fragment of 130 bp seems to contain the minimal elements necessary for gene expression, as its removal completely abolishes gene expression in cultured cells. These results are reinforced by comparison of this region with the human Msx1 gene promoter, which shows extensive conservation, including many consensus binding sites, suggesting a regulatory role for them.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

A minimal murine Msx-1 gene promoter. Organization of its cis-regulatory motifs and their role in transcriptional activation in cells in culture and in transgenic mice

To dissect the cis-regulatory elements of the murine Msx-1 promoter, which lacks a conventional TATA element, a putative Msx-1 promoter DNA fragment (from -1282 to +106 base pairs (bp)) or its congeners containing site-specific alterations were fused to luciferase reporter and introduced into NIH3T3 and C2C12 cells, and the expression of luciferase was assessed in transient expression assays. The functional consequences of the sequential 5' deletions of the promotor revealed that multiple positive and negative regulatory elements participate in regulating transcription of the Msx-1 gene. Surprisingly, however, the optimal expression of Msx-1 promoter in either NIH3T3 or C2C12 cells required only 165 bp of the upstream sequence to warrant detailed examination of its structure. Therefore, the functional consequences of site-specific deletions and point mutations of the cis-acting elements of the minimal Msx-1 promoter were systematically examined. Concomitantly, potential transcriptional factor(s) interacting with the cis-acting elements of the minimal promoter were also studied by gel electrophoretic mobility shift assays and DNase I footprinting. Combined analyses of the minimal promoter by DNase I footprinting, electrophoretic mobility shift assays, and super shift assays with specific antibodies revealed that 5'-flanking regions from -161 to -154 and from -26 to -13 of the Msx-1 promoter contains an authentic E box (proximal E box), capable of binding a protein immunologically related to the upstream stimulating factor 1 (USF-1) and a GC-rich sequence motif which can bind to Sp1 (proximal Sp1), respectively. Additionally, we observed that the promoter activation was seriously hampered if the proximal E box was removed or mutated, and the promoter activity was eliminated completely if the proximal Sp1 site was similarly altered. Absolute dependence of the Msx-1 minimal promoter on Sp1 could be demonstrated by transient expression assays in the Sp1-deficient Drosophila cell line cotransfected with Msx-1-luciferase and an Sp1 expression vector pPacSp1. The transgenic mice embryos containing -165/106-bp Msx-1 promoter-LacZ DNA in their genomes abundantly expressed beta-galactosidase in maxillae and mandibles and in the cellular primordia involved in the formation of the meninges and the bones of the skull. Thus, the truncated murine Msx-1 promoter can target expression of a heterologous gene in the craniofacial tissues of transgenic embryos known for high level of expression of the endogenous Msx-1 gene and found to be severely defective in the Msx-1 knock-out mice.

Heterodimerization of Msx and Dlx homeoproteins results in functional antagonism

Protein-protein interactions are known to be essential for specifying the transcriptional activities of homeoproteins. Here we show that representative members of the Msx and Dlx homeoprotein families form homo- and heterodimeric complexes. We demonstrate that dimerization by Msx and Dlx proteins is mediated through their homeodomains and that the residues required for this interaction correspond to those necessary for DNA binding. Unlike most other known examples of homeoprotein interactions, association of Msx and Dlx proteins does not promote cooperative DNA binding; instead, dimerization and DNA binding are mutually exclusive activities. In particular, we show that Msx and Dlx proteins interact independently and noncooperatively with homeodomain DNA binding sites and that dimerization is specifically blocked by the presence of such DNA sites. We further demonstrate that the transcriptional properties of Msx and Dlx proteins display reciprocal inhibition. Specifically, Msx proteins act as transcriptional repressors and Dlx proteins act as activators, while in combination, Msx and Dlx proteins counteract each other's transcriptional activities. Finally, we show that the expression patterns of representative Msx and Dlx genes (Msx1, Msx2, Dlx2, and Dlx5) overlap in mouse embryogenesis during limb bud and craniofacial development, consistent with the potential for their protein products to interact in vivo. Based on these observations, we propose that functional antagonism through heterodimer formation provides a mechanism for regulating the transcriptional actions of Msx and Dlx homeoproteins in vivo.

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.

A novel family of T-box genes in urodele amphibian limb development and regeneration: candidate genes involved in vertebrate forelimb/hindlimb patterning

In certain urodeles, a lost appendage, including hand and foot, can be completely replaced through epimorphic regeneration. The regeneration process involves cellular activities similar to those described for embryogenesis. Working on the assumption that the morphological pattern specific for a forelimb or a hindlimb is controlled by different gene activities in the two limbs, we employed a mRNA differential display screen for the detection of candidate limb identity genes. Using this approach, we have isolated a newt gene which in regenerating and developing limbs reveals properties expected of a gene having a role in controlling limb morphology: (1) it is exclusively expressed in the forelimbs, but not hindlimbs, (2) during embryonic development its expression is co-incident with forelimb bud formation, (3) it has an elevated message level throughout the undifferentiated limb bud and the blastema, respectively, and (4) it is expressed only in mesenchymal, but not in epidermal tissues. This novel newt gene shares a conserved DNA-binding domain, the T-box, with putative transcription factors including the Brachyury (T) gene product. In a following PCR-based screen, we used the evolutionarily conserved T-box motif and amplified a family of related genes in the newt; their different expression patterns in normal and regenerating forelimbs, hindlimbs and tail suggest, in general, an important role of T-domain proteins in vertebrate pattern formation.

Advanced mRNA differential display: isolation of a new differentially regulated myosin heavy chain-encoding gene in amphibian limb regeneration

In an effort to make mRNA differential display more amenable as a molecular screen, we have optimized the technology for the isotopic and non-isotopic detection of differentially regulated mRNAs. The number of amplification rounds in the displays was significantly reduced, resulting in the semi-quantitative detection of expression patterns of both low- and high-abundance transcripts. Moreover, we extended the method beyond the display of mRNAs by introducing a direct sequencing approach for the fast molecular analysis of isolated cDNAs. Applying this improved technique to the regenerating amphibian limb system, we have identified cDNA PCR products with a temporal difference in expression. This differential regulation was confirmed by Northern analysis, and DNA sequencing uncovered a novel newt differentiation-specific transcript encoding a skeletal myosin heavy chain (MHC).

Stabilizing role of the basement membrane and dermal fibers during newt limb regeneration

BACKGROUND

Following amputation of a newt limb, tissues at the amputation site undergo histolysis to give rise to a growth bud, or blastema, but they also provide a base on which the regenerate is constructed. Studies suggest that dermal tissues may differentially resist histolysis.

METHODS AND RESULTS

To examine stability of tissues at the amputation site, more than 80 preblastemal staged regenerating limbs were examined histologically. Initially, all soft tissues not attached to bone retracted and were covered by migrating epithelium. The dermis was seen to be stable during the first week postamputation. Muscle dedifferentiated and was heavily stained with anti-tenascin antibodies, but the intact overlying dermis was unstained. Fiber bundles, revealed by staining with phosphotungstic acid hematoxylin, isolated the dermis from dedifferentiating deeper tissues during the first week postamputation, but partially broke down during the second week. However, the basement membrane (BM) remained as the distalmost intact structure at the amputation site in all limbs examined. The BM was the foundation for new BM synthesis which preceded dermis synthesis in the base of the blastema during the second week, even while undifferentiated cells were accumulating centrally.

CONCLUSIONS

We suggest that the dermis resists histolysis long enough for new BM to form in continuity with that of the stump. Dermis formation (dermogenesis) distal to the amputation plane begins early as in mammalian healing but is not completed until after blastema formation. Thus, factors that inhibit dermal closure appear to distinguish regenerating from non-regenerating appendages.

'Regeneration' of wing bud stumps of chick embryos and reactivation of Msx-1 and Shh expression in response to FGF-4 and ridge signals

We examined systematically the ability of chick limb bud stumps to regenerate distal structures when fibroblast growth factor (FGF)-4 is applied. When amputations were made within 600 mu m of the tip and FGF-4 applied either posteriorly or both apically and posteriorly, outgrowth of stump tissues occurred and a virtually complete skeleton developed. 'Regeneration' of distal structures was correlated with reactivation of Msx-1 and Shh expression. At proximal amputation levels where FGF-4 did not lead to 'regeneration', neither Msx-1 nor Shh expression was induced. We also grafted cells from progressively more proximal levels of mouse limb buds to chick wing bud tips beneath the apical ridge and the pattern of reactivation of Msx-1 expression along the proximal distal axis of the limb buds was similar to that found in chick limb buds.