How x-rays inhibit amphibian limb regeneration

Positional Memory in Vertebrate Regeneration: A Century's Insights from the Salamander Limb

Salamanders, such as axolotls and newts, can regenerate complex tissues including entire limbs. But what mechanisms ensure that an amputated limb regenerates a limb, and not a tail or unpatterned tissue? An important concept in regeneration is positional memory-the notion that adult cells "remember" spatial identities assigned to them during embryogenesis (e.g., "head" or "hand") and use this information to restore the correct body parts after injury. Although positional memory is well documented at a phenomenological level, the underlying cellular and molecular bases are just beginning to be decoded. Herein, we review how major principles in positional memory were established in the salamander limb model, enabling the discovery of positional memory-encoding molecules, and advancing insights into their pattern-forming logic during regeneration. We explore findings in other amphibians, fish, reptiles, and mammals and speculate on conserved aspects of positional memory. We consider the possibility that manipulating positional memory in human cells could represent one route toward improved tissue repair or engineering of patterned tissues for therapeutic purposes.

Beryllium nitrate inhibits fibroblast migration to disrupt epimorphic regeneration

Epimorphic regeneration proceeds with or without formation of a blastema, as observed for the limb and skin, respectively. Inhibition of epimorphic regeneration provides a means to interrogate the cellular and molecular mechanisms that regulate it. In this study, we show that exposing amputated limbs to beryllium nitrate disrupts blastema formation and causes severe patterning defects in limb regeneration. In contrast, exposing full-thickness skin wounds to beryllium only causes a delay in skin regeneration. By transplanting full-thickness skin from ubiquitous GFP-expressing axolotls to wild-type hosts, we demonstrate that beryllium inhibits fibroblast migration during limb and skin regeneration in vivo Moreover, we show that beryllium also inhibits cell migration in vitro using axolotl and human fibroblasts. Interestingly, beryllium did not act as an immunostimulatory agent as it does in Anurans and mammals, nor did it affect keratinocyte migration, proliferation or re-epithelialization, suggesting that the effect of beryllium is cell type-specific. While we did not detect an increase in cell death during regeneration in response to beryllium, it did disrupt cell proliferation in mesenchymal cells. Taken together, our data show that normal blastema organogenesis cannot occur without timely infiltration of local fibroblasts and highlights the importance of positional information to instruct pattern formation during regeneration. In contrast, non-blastemal-based skin regeneration can occur despite early inhibition of fibroblast migration and cell proliferation.

Angiogenesis is inhibitory for mammalian digit regeneration

The regenerating mouse digit tip is a unique model for investigating blastema formation and epimorphic regeneration in mammals. The blastema is characteristically avascular and we previously reported that blastema expression of a known anti‐angiogenic factor gene, Pedf, correlated with a successful regenerative response (Yu, L., Han, M., Yan, M., Lee, E. C., Lee, J. & Muneoka, K. (2010). BMP signaling induces digit regeneration in neonatal mice. Development, 137, 551–559). Here we show that during regeneration Vegfa transcripts are not detected in the blastema but are expressed at the onset of differentiation. Treating the amputation wound with vascular endothelial growth factor enhances angiogenesis but inhibits regeneration. We next tested bone morphogenetic protein 9 (BMP9), another known mediator of angiogenesis, and found that BMP9 is also a potent inhibitor of digit tip regeneration. BMP9 induces Vegfa expression in the digit stump suggesting that regenerative failure is mediated by enhanced angiogenesis. Finally, we show that BMP9 inhibition of regeneration is completely rescued by treatment with pigment epithelium‐derived factor. These studies show that precocious angiogenesis is inhibitory for regeneration, and provide compelling evidence that the regulation of angiogenesis is a critical factor in designing therapies aimed at stimulating mammalian regeneration.

Nerve independent limb induction in axolotls

Urodele amphibians can regenerate their limbs. During limb regeneration, dermal fibroblasts are transformed into undifferentiated cells called blastema cells. These dermis-blastema cells show multipotency. Such so-called endogenous reprogramming of cell differentiation is one of the main targets of amphibian limb regeneration studies. It is well recognized that nerve presence controls the initiation of limb regeneration. Accordingly, nerve factors have been sought in amphibian limb regeneration. To investigate it, a relatively new study system called the accessory limb model (ALM) was developed. Using ALM, two signaling cascades (Fgf and Gdf5 signaling) came under focus. In the present study, Growth and differentiation factor-5 (Gdf5) application to wounded skin initiated limb regeneration responses and resulted in induction of a blastema-like structure in the absence of a nerve. However, the Gdf5-induced structure showed defects as a regeneration blastema, such as absence of detectable Prrx1 expression by in situ hybridization. The defects could be remedied by additional Fibroblasts growth factor (Fgf) inputs. These two inputs (Gdf5 and Fgfs) were sufficient to substitute for the nerve functions in the induction of limb regeneration. Indeed, Fgf2, Fgf8, and Gdf5 applications with the contralateral skin graft resulted in limb formation without nerve supply. Furthermore, acquisition of cartilage differentiation potential of dermal fibroblasts was tested in an in vivo and in vitro combination assay. Dermal fibroblasts cultured with Gdf5 were difficult to participate in cartilage formation when the cultured cells were grafted into cartilage forming region. In contrast, dermal fibroblasts cultured with Fgf2 and Fgf8 became easier to participate into cartilage formation in the same procedure. These results contribute to our understanding of molecular mechanisms of the early phase of amphibian limb regeneration.

Anteroposterior axis formation in Xenopus limb bud recombinants: a model of pattern formation during limb regeneration

We previously showed that recombinant limb buds with dissociated and reaggregated mesenchyme develop more than 30 digits in Xenopus laevis, which exhibits different capacities for limb regeneration at different developmental stages (Yokoyama et al. [1998] Dev Biol 196:1-10). Cell-cell contact among anterior- and posterior-derived mesenchymal cells is required for anteroposterior (AP) axis formation of recombinant limbs in an intercalary manner. However, whether one-way induction from posterior cells to anterior cells as proposed by the polarizing zone model or interactions between anterior and posterior cells evoke the AP axis formation in recombinant limbs remains unclear. In this study, we found, by a combination of X-ray irradiation and a recombinant limb technique, that not one-way induction but interactions between anterior and posterior cells accompanied by cell contribution are indispensable for AP axis formation in recombinant limbs. Shh was expressed in posterior-derived not anterior-derived cells. We propose that the recombinant limb is an excellent model for examining the axis formation mechanism in regenerating limbs because, as in recombinant limbs, cell-cell contact among cells derived from different positions of an amputation plane occurs in the blastema of regenerating limbs.

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.

Influence of longitudinal whole animal clinorotation on lens, tail, and limb regeneration in urodeles

Two species of newts (Urodela) and two types of clinostats for fast clinorotation (60 rpm) were used to investigate the influence of simulated weightlessness on regeneration and to compare results obtained with data from spaceflight experiments. Seven or fourteen days of weightlessness in Russian biosatellites caused acceleration of lens and limb regeneration by an increase in cell proliferation, differentiation, and rate of morphogenesis in comparison with ground controls. After a comparable time of clinorotation the results obtained with Triturus vulgaris using a horizontal clinostat were similar to those found in spaceflight. In contrast, in Pleurodeles waltl using both horizontal and radial clinostats the results were contradictory compared to Triturus. We speculate that different levels of gravity or/and species specific thresholds for gravitational sensitivity could be responsible for these contradictory results.

The effect of X-irradiation on skeletal muscle regeneration in the adult rat

Muscle regeneration was induced by transplanting the extensor digitorum longus (EDL) muscle in adult rats to examine the effect of X-irradiation on muscle regeneration. The EDL muscles were removed, irradiated with X-rays to administer 650 R, 2,000 R or 10,000 R, and transplanted into the original animal and location. Muscles from non-irradiated control group and each irradiated group were analyzed morphologically at 4, 7, 14 and 30 days post-transplantation. The regeneration pattern in the non-irradiated and 650-R irradiated muscles was similar. A majority of myofibers underwent degeneration followed by regeneration from the precursor myosatellite cells. The myosatellite cells proliferated, differentiated into myoblasts and then fused to form myotubes and myofibers. Muscles exposed to 2,000 R underwent initial degeneration and myosatellite cell activation, however, considerably fewer myotubes regenerated in these muscles. In muscles exposed to 10,000 R, again myofiber degeneration and myosatellite cell activation was evident, but these cells remained undifferentiated and did not fuse to form myotubes. These results show a dose-dependent inhibition in muscle regeneration due to irradiation.

Cellular proliferation in the skin of X-rayed newt limbs (with a note on x-ray-induced limb regression)

Left hind limbs, including the pelvis, of adult newts (Notophthalmus viridescens) were locally irradiated with a dose of x-rays that inhibited regeneration (2,000 R). This x-ray dose and other doses (700-2,000 R) capable of inhibiting limb regeneration also cause limb regression prior to amputation. Before limb regression occurred, there was a latent period of 3 to 6 weeks. Limb regression was characterized by necrotic wasting and resorption of distal elements. The degree of loss was variable and dependent upon dosage. After this further degenerative changes were not noted. Proliferation of epidermal cells was examined 4 days after irradiation prior to limb regression or after x-ray-induced degeneration of the limbs had ended. Proliferative activity in x-rayed limbs was also compared at various stages of contralateral control limb regeneration. Limbs examined after x-ray-induced limb regression had ended showed levels of [3H]-thymidine incorporation into DNA comparable to normal epidermis. In contrast, limbs examined 4 days after irradiation had lower levels of DNA synthesis (P much less than 0.01). Amputation of limbs in both groups caused an increase in DNA synthesis (P much less than 0.01). Histological examination showed that cellular proliferation was associated primarily with the epidermis. These results indicate that epidermal cell proliferation was not resistant to x-rays. However, levels of normal cell division were observed after amputation of after cessation of x-ray-induced limb regression.

The ability of localized implants of whole or minced dermis to disrupt pattern formation in the regenerating forelimb of the axolotl

The ability of localized grafts of dermis to alter pattern formation in the regenerating limb of the axolotl was studied. Longitudinal pieces of skin (1/4 of circumference of the limb) were removed from either the anterior or the posterior surface of the upper forelimb. Epidermis was removed by immersion in versene followed by mechanical stripping. The resulting dermis was cross transplanted directly beneath the skin on the opposite side of the limb from which it originated. After 5 days of healing each limb was amputated through the graft at the midpoint of the humerus. High percentages of multiple regenerates resulted. Similar results were obtained when dermis was minced into 1 mm3 fragments prior to cross-transplantation. Freezing or x-raying (2000 rads) the grafts prior to cross-transplantation abolished the effect. Dermis obtained form head skin rarely caused multiple regeneration when implanted into the upper forelimb followed by amputation 5 days later. These results demonstrate that addition of dermis to an intact limb stump profoundly alters pattern formation during regeneration. The effect is dependent upon viable cells that are capable of cell division.

Distal regeneration and symmetry

A revision of the "polar coordinate model" shows how pattern formation in diverse regenerating systems can be understood in terms of strictly local cell interactions.

Interactions between irradiated and unirradiated tissues during supernumerary limb formation in the newt

The interactions between irradiated and unirradiated blastemas and stumps in the newt forelimb were studied. Irradiated right blastemas at the stage of early digits were grafted to unirradiated left stumps and unirradiated left blastemas were grafted to irradiated right stumps. Grafts were oriented with their anterior-posterior axes opposed to that of the stumps. Supernumerary limbs ranging in completeness from one to four digits were found to arise predominantly on the anterior or posterior sides of the host limb. The graft developed well when the blastema was unirradiated and had reversed handedness with respect to the stump. Irradiated grafts developed poorly. On occasions, limbs with two supernumerary structures were found. The results are discussed in terms of the origin of the cells which comprise the supernumerary limbs and their bearing on a recently presented model concerned with pattern specification and regulation in epimorphic fields.