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

Derivation and long-term culture of cells from newt adult limbs and limb blastemas

Notwithstanding the key importance of in vivo models for understanding patterning and cellular interactions in the regenerating tailed amphibian (salamander) limb, dissection of molecular mechanisms, as in other species, can be greatly aided by robust in vitro models. This chapter focuses on derivation and maintenance of cell lines from adult post-metamorphic salamanders and in particular cells derived from normal and regenerating limbs. We also describe a protocol for nucleofecting newt cells that can be used both to investigate the gene function in short-term studies and to establish stable cell lines.

Foamy virus for efficient gene transfer in regeneration studies

BACKGROUND

Molecular studies of appendage regeneration have been hindered by the lack of a stable and efficient means of transferring exogenous genes. We therefore sought an efficient integrating virus system that could be used to study limb and tail regeneration in salamanders.

RESULTS

We show that replication-deficient foamy virus (FV) vectors efficiently transduce cells in two different regeneration models in cell culture and in vivo. Injection of EGFP-expressing FV but not lentivirus vector particles into regenerating limbs and tail resulted in widespread expression that persisted throughout regeneration and reamputation pointing to the utility of FV for analyzing adult phenotypes in non-mammalian models. Furthermore, tissue specific transgene expression is achieved using FV vectors during limb regeneration.

CONCLUSIONS

FV vectors are efficient mean of transferring genes into axolotl limb/tail and infection persists throughout regeneration and reamputation. This is a nontoxic method of delivering genes into axolotls in vivo/ in vitro and can potentially be applied to other salamander species.

Pseudotyped retroviruses for infecting axolotl in vivo and in vitro

Axolotls are poised to become the premiere model system for studying vertebrate appendage regeneration. However, very few molecular tools exist for studying crucial cell lineage relationships over regeneration or for robust and sustained misexpression of genetic elements to test their function. Furthermore, targeting specific cell types will be necessary to understand how regeneration of the diverse tissues within the limb is accomplished. We report that pseudotyped, replication-incompetent retroviruses can be used in axolotls to permanently express markers or genetic elements for functional study. These viruses, when modified by changing their coat protein, can infect axolotl cells only when they have been experimentally manipulated to express the receptor for that coat protein, thus allowing for the possibility of targeting specific cell types. Using viral vectors, we have found that progenitor populations for many different cell types within the blastema are present at all stages of limb regeneration, although their relative proportions change with time.

Functional convergence of signalling by GPI-anchored and anchorless forms of a salamander protein implicated in limb regeneration

The GPI-anchor is an established determinant of molecular localisation and various functional roles have been attributed to it. The newt GPI-anchored three-finger protein (TFP) Prod1 is an important regulator of cell behaviour during limb regeneration, but it is unclear how it signals to the interior of the cell. Prod1 was expressed by transfection in cultured newt limb cells and activated transcription and expression of matrix metalloproteinase 9 (MMP9) by a pathway involving ligand-independent activation of epidermal growth factor receptor (EGFR) signalling and phosphorylation of extracellular regulated kinase 1 and 2 (ERK1/2). This was dependent on the presence of the GPI-anchor and critical residues in the α-helical region of the protein. Interestingly, Prod1 in the axolotl, a salamander species that also regenerates its limbs, was shown to activate ERK1/2 signalling and MMP9 transcription despite being anchorless, and both newt and axolotl Prod1 co-immunoprecipitated with the newt EGFR after transfection. The substitution of the axolotl helical region activated a secreted, anchorless version of the newt molecule. The activity of the newt molecule cannot therefore depend on a unique property conferred by the anchor. Prod1 is a salamander-specific TFP and its interaction with the phylogenetically conserved EGFR has implications for our view of regeneration as an evolutionary variable.

Kidneys of Alb/TGF-beta1 transgenic mice are deficient in retinoic acid and exogenous retinoic acid shows dose-dependent toxicity

BACKGROUND

Alb/TGF-beta(1) transgenic mice overexpress active transforming growth factor-beta(1) (TGF-beta(1)) in the liver, leading to increased circulating levels of the cytokine and progressive renal fibrosis. This study was designed to explore if exogenous all-trans retinoic acid (tRA) prevents renal fibrosis in this animal model.

METHODS

The retinoid profile in kidney and liver of wild-type and Alb/TGF-beta(1) transgenic mice was examined by high-performance liquid chromatography and slow-release pellets containing different amounts of tRA were implanted subcutaneously to treat the Alb/TGF-beta(1) transgenic mice, starting at 1 week of age; mice were sacrificed 2 weeks later.

RESULTS

Kidneys of 3-week-old wild-type mice had abundant tRA, which was completely absent in kidneys of the transgenic mice. Low doses of tRA (6-10.7 mg/kg/day) failed to affect renal fibrosis although it tended to suppress the mRNA expression of some molecular markers of fibrosis and retinal dehydrogenase 2 (RALDH2), a gene encoding a key tRA-synthesising enzyme. These tendencies disappeared, mortality tended to increase and RALDH2 and connective tissue growth factor (CTGF) mRNAs significantly increased in the medium-dose group (12.7-18.8 mg/kg/day). High doses (20.1-27.4 mg/kg/day) showed even higher toxicity with increased renal fibrosis and significant mortality.

CONCLUSIONS

Alb/TGF-beta(1) transgenic mice are characterised by depletion of endogenous renal tRA. Exogenous tRA dose-dependently increases mortality and kidney fibrosis, which is associated with dose-dependent regulation of renal RALDH2 and CTGF mRNA expression.

Transforming growth factor: beta signaling is essential for limb regeneration in axolotls

Axolotls (urodele amphibians) have the unique ability, among vertebrates, to perfectly regenerate many parts of their body including limbs, tail, jaw and spinal cord following injury or amputation. The axolotl limb is the most widely used structure as an experimental model to study tissue regeneration. The process is well characterized, requiring multiple cellular and molecular mechanisms. The preparation phase represents the first part of the regeneration process which includes wound healing, cellular migration, dedifferentiation and proliferation. The redevelopment phase represents the second part when dedifferentiated cells stop proliferating and redifferentiate to give rise to all missing structures. In the axolotl, when a limb is amputated, the missing or wounded part is regenerated perfectly without scar formation between the stump and the regenerated structure. Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele limb regeneration. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-β). In the present study, the full length sequence of the axolotl TGF-β1 cDNA was isolated. The spatio-temporal expression pattern of TGF-β1 in regenerating limbs shows that this gene is up-regulated during the preparation phase of regeneration. Our results also demonstrate the presence of multiple components of the TGF-β signaling machinery in axolotl cells. By using a specific pharmacological inhibitor of TGF-β type I receptor, SB-431542, we show that TGF-β signaling is required for axolotl limb regeneration. Treatment of regenerating limbs with SB-431542 reveals that cellular proliferation during limb regeneration as well as the expression of genes directly dependent on TGF-β signaling are down-regulated. These data directly implicate TGF-β signaling in the initiation and control of the regeneration process in axolotls.

A Single-Cell Analysis of Myogenic Dedifferentiation Induced by Small Molecules

An important direction in chemical biology is the derivation of compounds that affect cellular differentiation or its reversal. The fragmentation of multinucleate myofibers into viable mononucleates (called cellularization) occurs during limb regeneration in urodele amphibians, and the isolation of myoseverin, a trisubstituted purine that could apparently activate this pathway of myogenic dedifferentiation in mammalian cells, generated considerable interest. We have explored the mechanism and outcome of cellularization at a single-cell level, and we report findings that significantly extend the previous work with myoseverin. Using a panel of compounds, including a triazine compound with structural similarity and comparable activity to myoseverin, we have identified microtubule disruption as critical for activation of the response. Time-lapse microscopy has enabled us to analyze the fate of identified mononucleate progeny, and directly assess the extent of dedifferentiation.

Structural and functional properties of CCN proteins

The CCN family currently comprises six members (CCN1-6) that regulate diverse cell functions, including mitogenesis, adhesion, apoptosis, extracellular matrix (ECM) production, growth arrest, and migration. These properties can result in a multiplicity of effects during development, differentiation, wound healing, and disease states, such as tumorigenesis and fibrosis. CCN proteins have emerged as major regulators of chondrogenesis, angiogenesis, and fibrogenesis. CCN proteins are mosaic in nature and consist of up to four structurally conserved modules, at least two of which are involved in binding to cell surfaces via molecules that include integrins, heparan sulfate proteoglycans, and low-density lipoprotein receptor-related protein. CCN proteins use integrins as signal transducing receptors to regulate context-dependent responses in individual cell types. The involvement of integrins in mediating CCN signaling allows for considerable plasticity in response because some effects are specific for certain integrin subtypes and integrin signaling is coordinated with other signaling pathways in the cell. In addition to their own biological properties, CCN proteins regulate the functions of other bioactive molecules (e.g., growth factors) via direct binding interactions. CCN molecules demonstrate complex multifaceted modes of action and regulation and have emerged as important matricellular regulators of cell function.

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

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

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

Role of CTGF/HCS24/ecogenin in skeletal growth control

Connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24 (CTGF/Hcs24) is a multifunctional growth factor for chondrocytes, osteoblasts, and vascular endothelial cells. CTGF/Hcs24 promotes the proliferation and maturation of growth cartilage cells and articular cartilage cells in culture and hypertrophy of growth cartilage cells in culture. The factor also stimulates the proliferation and differentiation of cultured osteoblastic cells. Moreover, CTGF/Hcs24 promotes the adhesion, proliferation, and migration of vascular endothelial cells, as well as induces tube formation by the cells and strong angiogenesis in vivo. Because angiogenesis is critical for the replacement of cartilage with bone at the final stage of endochondral ossification and because gene expression of CTGF/Hcs24 predominates in hypertrophic chondrocytes in the physiological state, a major physiological role for this factor should be the promotion of the entire process of endochondral ossification, with the factor acting on the above three types of cells as a paracrine factor. Thus, CTGF/Hcs24 should be called "ecogenin: endochondral ossification genetic factor." In addition to hypertrophic chondrocytes, osteoblasts activated by various stimuli including wounding also express a significantly high level of CTGF/Hcs24. These findings in conjunction with in vitro findings about osteoblasts mentioned above suggest the involvement of CTGF/Hcs24 in intramembranous ossification and bone modeling/remodeling. Because angiogenesis is also critical for intramembranous ossification and bone remodeling, CTGF/Hcs24 expressed in endothelial cells activated by various stimuli including wounding may also play important roles in direct bone formation. In conclusion, although the most important physiological role of CTGF/Hcs24 is ecogenin action, the factors also play important roles in skeletal growth and modeling/remodeling via its direct action on osteoblasts under both physiological and pathological conditions.

The newt ortholog of CD59 is implicated in proximodistal identity during amphibian limb regeneration

The proximodistal identity of a newt limb regeneration blastema is respecified by exposure to retinoic acid, but its molecular basis is unclear. We identified from a differential screen the cDNA for Prod 1, a gene whose expression in normal and regenerating limbs is regulated by proximodistal location and retinoic acid: Prod 1 is the newt ortholog of CD59. Prod 1/CD59 was found to be located at the cell surface with a GPI anchor which is cleaved by PIPLC. A proximal newt limb blastema engulfs a distal blastema after juxtaposition in culture, and engulfment is specifically blocked by PIPLC, and by affinity-purified antibodies to two distinct Prod 1/CD59 peptides. Prod 1 is therefore a cell surface protein implicated in the local cell-cell interactions mediating positional identity.

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.

Connective tissue growth factor: potential role in glomerulosclerosis and tubulointerstitial fibrosis

Transforming growth factor beta (TGF-beta) is a pivotal driver of glomerulosclerosis and tubulointerstitial fibrosis in renal diseases. Because TGF-beta also plays important anti-inflammatory and antiproliferative roles in mammalian systems, there has been a recent drive to elucidate downstream mediators of TGF-beta's pro-fibrotic effects with the ultimate goal of developing new anti-fibrotic strategies for treatment of chronic diseases. Connective tissue growth factor (CTGF) belongs to the CCN family of immediate early response genes. Several lines of evidence suggest that CTGF is an important pro-fibrotic molecule in renal disease and that CTGF contributes to TGF-beta bioactivity in this setting. CTGF expression is increased in the glomeruli and tubulointerstium in a variety of renal disease in association with scarring and sclerosis of renal parenchyma. In model systems in vitro, mesangial cell CTGF expression is induced by high extracellular glucose, cyclic mechanical strain and TGF-beta. Recombinant human CTGF augments the production of fibronectin and type IV collagen by mesangial cells and the effects of high glucose on mesangial cell CTGF expression and matrix production are attenuated, in part, by anti-TGF-beta antibody. In aggregate, these observations identify CTGF as an attractive therapeutic target in fibrotic renal diseases.

Connective tissue growth factor: what's in a name?

Connective tissue growth factor (CTGF) is a member of the recently described CCN gene family which contains CTGF itself, cyr61, nov, elm1, Cop1, and WISP-3. CTGF is transcriptionally activated by several factors although its stimulation by transforming growth factor beta (TGF-beta) has attracted considerable attention. CTGF acts to promote fibroblast proliferation, migration, adhesion, and extracellular matrix formation, and its overproduction is proposed to play a major role in pathways that lead to fibrosis, especially those that are TGF-beta-dependent. This includes fibrosis of major organs, fibroproliferative diseases, and scarring. CTGF also appears to play a role in the extracellular matrix remodeling that occurs in normal physiological processes such as embryogenesis, implantation, and wound healing. However, recent advances have shown that CTGF is involved in diverse autocrine or paracrine actions in several other cell types such as vascular endothelial cells, epithelial cells, neuronal cells, vascular smooth muscle cells, and cells of supportive skeletal tissues. Moreover, in some circumstances CTGF has negative effects on cell growth in that it can be antimitotic and apoptotic. In light of these discoveries, CTGF has been implicated in a diverse variety of processes that include neovascularization, transdifferentiation, neuronal scarring, atherosclerosis, cartilage differentiation, and endochondral ossification. CTGF has thus emerged as a potential important effector molecule in both physiological and pathological processes and has provided a new target for therapeutic intervention in fibrotic diseases.

Connective tissue growth factor expression in the rat remnant kidney model and association with tubular epithelial cells undergoing transdifferentiation

Connective tissue growth factor (CTGF) has been shown to mediate many actions of transforming growth factor-beta (TGF-beta) in the fibrotic response in several diseases. We compared expression of CTGF, TGF-beta, platelet-derived growth factor (PDGF), TNF-alpha, and interleukin-1 (IL-1) by in situ hybridization in Sprague-Dawley rats euthanized at 0, 2, 4, and 8 weeks after 5/6 nephrectomy using the rat remnant kidney model of renal failure. Collagen was evaluated by trichrome stains, immunohistochemistry, and electron microscopy. We compared expression patterns to cells undergoing metaplasia. Tubular epithelial regeneration and transdifferentiation to myofibroblasts were assessed morphologically and by proliferating cell nuclear antigen, smooth muscle actin, desmin, and vimentin immunohistochemistry. CTGF expression was minimal in controls, mild at 2 weeks and marked by 4 to 8 weeks in interstitial fibroblasts, coinciding with damage, regeneration, and fibrosis. TGF-beta expression was increased in many cell types at 2 weeks, increased further by 4 weeks, then remained constant. PDGF-B messenger RNA was found in many stromal cells at 2-4 weeks, but expression decreased at 8 weeks. No significant IL-1 or TNF-alpha staining was detected. We conclude that CTGF and interacting factors are associated with development or progression of chronic interstitial fibrosis. Proximity of CTGF, TGF-beta, and PDGF mRNA expression to regenerative epithelial cells and those transdifferentiating to myofibroblasts suggests that growth factors may modulate renal tubular epithelial differentiation.

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.

Vaccinia as a tool for functional analysis in regenerating limbs: ectopic expression of Shh

Axolotls, with their extensive abilities to regenerate as adults, provide a useful model in which to study the mechanisms of regeneration in a vertebrate, in hopes of understanding why other vertebrates cannot regenerate. Although the expression of many genes has been described in regeneration, techniques for functional analysis have so far been limited. In this paper we demonstrate a new method for efficient overexpression of foreign genes in axolotls. Using vaccinia virus expressing beta-galactosidase microinjected into regenerating limbs, we show that vaccinia can infect both dividing and nondividing limb cells. The site of infection remains discrete and there is no secondary spread of infection to nearby cells. beta-Gal is expressed at high levels in blastema cells for about a week and in differentiated cells for longer. Blastemas that have been injected with vaccinia at different stages regenerate normally. As a test of the utility of vaccinia for functional analysis in regeneration, we constructed a virus expressing Shh and injected it into the anterior of regenerating limbs. Ectopic Shh expression caused extra digits, carpals, and tarsals in the hands and feet of regenerating limbs, suggesting that despite differences in the timing of expression and the eventual pattern, the function of Shh appears to be similar to that in the developing limbs of other vertebrates. Our results demonstrate that vaccinia virus is an excellent vector for ectopically expressing genes for secreted proteins and is a useful tool to study the function of signaling molecules during the process of regeneration in urodeles.

Apical epithelial cap morphology and fibronectin gene expression in regenerating axolotl limbs

Urodele amphibians (salamanders) are unique among adult vertebrates in their ability to regenerate limbs. The regenerated structure is often indistinguishable from the developmentally produced original. Thus, the two processes by which the limb is produced - development and regeneration - are likely to use many conserved biochemical and developmental pathways. Some of these limb features are also likely to be conserved across vertebrate families. The apical ectodermal ridge (AER) of the developing amniote limb and the larger apical epithelial cap (AEC) of the regenerating urodele limb are both found at the limb's distalmost tip and have been suggested to be functionally similar even though their morphology is quite different. Both structures are necessary for limb outgrowth. However, the AEC is uniformly smooth and thickly covers the entire limb-tip, unlike the AER, which is a protruding ridge covering only the dorsoventral boundary. Previous data from our laboratory suggest the multilayered AEC may be subdivided into separate functional compartments. We used hematoxylin and eosin (H+E) staining as well as in situ hybridization to examine the basal layer of the AEC, the layer that lies immediately over the distal limb mesenchyme. In late-stage regenerates, this basal layer expresses fibronectin (FN) message very strongly in a stripe of cells along the dorso-ventral boundary. H+E staining also reveals the unique shape of basal cells in this area. The stripe of cells in the basal AEC also contains the notch/groove structure previously seen in avian and reptilian AERs. In addition, AEC expression of FN message in the cells around the groove correlates with previous amniote AER localization of FN protein inside the groove. The structural and biochemical analyses presented here suggest that there is a specialized ridge-like compartment in the basal AEC in late-stage regenerates. The data also suggest that this compartment may be homologous to the AER of the developing amniote limb. Thus, the external differences between amniote limb development and urodele limb regeneration may be outweighed by internal similarities, which enable both processes to produce morphologically complete limbs. In addition, we propose that this basal layer of the AEC is uniquely responsible for AEC functions in regeneration, such as secreting molecules to promote mesenchymal cell cycling and dictating the direction of limb outgrowth. Finally, we include here a clarification of existing nomenclature to facilitate further discussion of the AEC and its basal layer.