Cellular retinoic acid binding protein: detection and quantitation in regenerating axolotl limbs

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Gene expression patterns specific to the regenerating limb of the Mexican axolotl

Salamander limb regeneration is dependent upon tissue interactions that are local to the amputation site. Communication among limb epidermis, peripheral nerves, and mesenchyme coordinate cell migration, cell proliferation, and tissue patterning to generate a blastema, which will form missing limb structures. An outstanding question is how cross-talk between these tissues gives rise to the regeneration blastema. To identify genes associated with epidermis-nerve-mesenchymal interactions during limb regeneration, we examined histological and transcriptional changes during the first week following injury in the wound epidermis and subjacent cells between three injury types; 1) a flank wound on the side of the animal that will not regenerate a limb, 2) a denervated limb that will not regenerate a limb, and 3) an innervated limb that will regenerate a limb. Early, histological and transcriptional changes were similar between the injury types, presumably because a common wound-healing program is employed across anatomical locations. However, some transcripts were enriched in limbs compared to the flank and are associated with vertebrate limb development. Many of these genes were activated before blastema outgrowth and expressed in specific tissue types including the epidermis, peripheral nerve, and mesenchyme. We also identified a relatively small group of transcripts that were more highly expressed in innervated limbs versus denervated limbs. These transcripts encode for proteins involved in myelination of peripheral nerves, epidermal cell function, and proliferation of mesenchymal cells. Overall, our study identifies limb-specific and nerve-dependent genes that are upstream of regenerative growth, and thus promising candidates for the regulation of blastema formation.

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.

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.

Pattern duplication by retinoic acid treatment in the regenerating limbs of Korean salamander larvae, Hynobius leechii, correlates well with the extent of dedifferentiation

In the regenerating limbs of Korean salamanders, Hynobius leechii, retinoic acid (RA) induces duplication of skeletal structures in the proximodistal (PD) axis and often in the transverse axes. In the present study, the stage-dependent effects of RA for the duplication of limb skeletal structures at two amputation levels, the distal stylopodium and the distal zeugopodium, were studied using larval limbs of Korean salamanders. The results showed that the mean level of proximalization (MLP) by RA treatment increased during the stages of dedifferentiation and early bud formation while the MLP declined thereafter in both amputation levels. The decline of the MLP at the later stages of regeneration was due to the high frequency of hypomorphic regeneration or blocked regeneration. When the effects of RA treatment at two amputation levels were compared, the overall trends were similar but the actual timing was delayed for 2-4 days in the proximal level of amputation. Furthermore, the peak level of proximalization was achieved earlier and the peak level remained longer in the distal stylopodial level of amputation compared to the distal zeugopodial level of amputation. Since the histological observations revealed that the dedifferentiation period was also extended up to 2-4 days in the proximal level of amputation, the acid phosphatase activity during the course of regeneration was measured to look for a quantitative relationship between the enzyme activity and the states of dedifferentiation. The results show that the level and the duration of acid phosphatase activity in the upper arm regenerates are both higher and longer than those in the lower arm regenerates. Furthermore, RA treatment caused an increase in acid phosphatase activity. Thus our results suggest that the state of dedifferentiation might be closely linked to the extent of proximalization of regenerating limbs by RA treatment.

Test of a model for the effects of retinoic acid on urodele limb regeneration

Previous studies have shown that in axolotls (Ambystoma mexicanum), retinoic acid (RA) treatment evokes pattern completion in limb regenerates derived from anterior and dorsal half zeugopodia (lower arms and legs), but causes regenerative failure in posterior and ventral half zeugopodia. Pattern completion in anterior and dorsal half limbs may be explained by postulating that intercalary regeneration occurs in the antero-posterior (AP) and dorsoventral (DV) axes between blastema cells that are posteriorized (anterior half limb) or ventralized (dorsal half limb) by RA, and circumferential anterodorsal cells that remain unaffected by RA and thus maintain their original positional identities. The contrasting regenerative failure of RA-treated posterior and ventral half zeugopodia may likewise be explained by postulating that all the blastema cells in the posterior half are posteriorized, and all the cells in the ventral half are ventralized by RA, thus eliminating differentials in transverse positional identity essential for blastema formation and outgrowth. To test these postulates we grafted blastemas derived from limbs halved in the AP and DV axes of control and RA-treated animals to untreated whole limb stumps and analyzed the patterns of supernumerary (SN) regeneration. The site or location of SN formation will demonstrate (1) whether RA has posteriorized and ventralized the positional identity of the blastema cells and (2) if blastema cells in the periphery of the anterodorsal quadrant of the limb are resistant to these RA-induced changes in positional identity.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Mouse limb bud cells respond to retinoic acid in vitro with reduced growth

Retinoic acid (RA) has dramatic effects on the pattern of developing and regenerating vertebrate limbs. These effects are considered to result from RA-induced changes in the positional identity of limb cells, and involve the formation of extra structures. Whether the growth required to form the supernumerary parts of the pattern is a primary effect of RA treatment or a secondary effect that follows after a change in positional identity is not at present known. In this paper we have investigated the effects of RA treatment on the growth of cells from anterior and posterior halves of mouse limb buds in vitro. We observed that under our culture conditions, limb bud cells treated with 1 nM to 1 microM RA (0.3 ng/ml to 300 ng/ml) continue to grow but do so at a significantly slower rate than control cultures. There is a maximum inhibition of growth (50% of controls) between 10 nM and 100 nM RA, which corresponds to the measured range of concentrations of RA in vivo. Our observation of a significant decrease in growth rate over a wide range of RA concentrations is consistent with comparable reports of growth inhibition for a large number of other cell types in vitro as well as with the observation that exogenous RA inhibits blastemal growth in amphibians during the period of exposure to RA. We propose that the effects of RA on growth, either enhancement in vivo or reduction in vitro, can be seen as consequences of the ability of RA to alter positional identity.(ABSTRACT TRUNCATED AT 250 WORDS)

Retinoic acid enhances monoclonal antibody WE3 reactivity in the regenerate epithelium of the adult newt

Monoclonal antibody (mAb) WE3 recognizes an antigen that is developmentally expressed in the wound epithelium during adult newt limb regeneration. Experiments were designed to determine whether retinoic acid (RA), dissolved in dimethyl sulfoxide (DMSO) and administered by intraperitoneal injection, would enhance the temporal appearance of the WE3 antigen. RA given on days 1 or 4 after amputation, when the WE3 antigen is not yet detectable, resulted in moderate reactivity to mAb 2 days after injection and strong reactivity throughout the wound epithelium 4 days after injection. DMSO alone had no enhancing effect. RA also caused limb skin epidermis to exhibit reactivity to mAb WE3, initially near the amputation level, but then also more proximally. By 4 and 6 days after RA injection, epidermis of the flank, eye lid, and unamputated hind limbs also became strongly reactive to mAb WE3. Outer layers of skin epidermis were shed, resulting in an epidermis only one or two cells thick. Epidermis of newts given DMSO alone remained non-reactive to mAb WE3. When RA was given on days 7 and 10 after amputation, when a low level of mAb WE3 reactivity is already present in the wound epithelium, a considerable enhancement of mAb WE3 reactivity occurred through the next few days. No such enhancement was seen with DMSO alone. RA also greatly increased mAb WE3 reactivity in the wound epithelium of denervated limbs, in which case the wound epithelial reactivity to mAb WE3 is normally low. Retinol palmitate also increased mAb WE3 reactivity. The results raise the possibility that the WE3 antigen is a component of most if not all retinoid target tissues in newts.

Effects of tunicamycin on retinoic acid induced respecification of positional values in regenerating limbs of the larval axolotl, Ambystoma mexicanum

Urodele amphibians possess a remarkable ability to regenerate limbs following experimental or accidental amputation. Since only those parts of the limb distal to the plane of amputation usually regenerate, this suggests the existence of level-specific positional values within the cells of the limb. Vitamin A and other retinoids respecify the positional values of regenerating limbs such that structures proximal to the actual plane of amputation are formed in the regenerating limb producing proximodistal duplications. Regenerating limbs of larval axolotls (Ambystoma mexicanum) treated with sufficient retinoic acid to induce proximodistal duplication were also treated via implantation with tunicamycin, a drug which blocks the synthesis of glycoproteins by blocking N-glycosylation of proteins. Tunicamycin was shown to inhibit the proximalizing effects of retinoic acid. This indicates that asparagine-linked glycoproteins may be essential to the process through which retinoic acid induces these effects in the regenerating limb and that glycoproteins may be responsible for specifying positional values in regeneration blastema cells.

The effect of retinoic acid on positional memory in the dorsoventral axis of regenerating axolotl limbs

We investigated the effect of retinoic acid (RA) on pattern regulation in the dorsoventral (DV) axis of regenerating axolotl limbs. Half and double half dorsal and ventral zeugopodia (lower arms or legs) were amputated through their distal ends, and 4 days later the animals were injected intraperitoneally with 50 (large animals) or 100 (small animals) micrograms RA/g body wt. Half and double half dorsal and ventral zeugopodia of uninjected axolotls, and sham-operated zeugopodia of untreated and RA-treated limbs served as controls. Skeletal patterns and the DV muscle patterns of control and experimental regenerates were then analyzed. Sham-operated zeugopodia of uninjected animals regenerated normally. Sham-operated, RA-treated zeugopodia regenerated normally with proximodistal duplications. Sixty percent of uninjected control dorsal half zeugopodia, 80% of control ventral half zeugopodia, and 100% of control double dorsal and double ventral zeugopodia regenerated distally, but the regenerates did not reconstitute the muscle pattern of the missing half. Thirty-eight percent of RA-treated ventral half zeugopodia and 78% of RA-treated double ventral zeugopodia failed to regenerate distally. Of those cases that did regenerate distally, none regenerated the muscle pattern of the missing half. By contrast, 100% of RA-treated dorsal half zeugopodia regenerated distally and all completed the normal DV muscle pattern. Forty-one percent of RA-treated double dorsal zeugopodia failed to regenerate, but of the remainder that did regenerate, 50% completed the normal DV muscle pattern. These represented eight cases, six of which regenerated single limbs, and two of which regenerated twin limbs, each with a normal DV muscle pattern. We interpret these data to mean that RA ventralizes the positional memory of blastema cells in the DV axis.

Retinoid receptor antisense DNAs inhibit alkaline phosphatase induction and clonogenicity in malignant keratinocytes

Antisense oligodeoxynucleotides [oligo(dN)s] corresponding to human cellular retinol-binding protein I (cRBP) and human nuclear retinoic acid receptor alpha (hnRAR) were synthesized. Exposure of human malignant keratinocytes to these oligo(dN)s significantly attenuated the level of cytoplasmic cRBP and hnRAR in a concentration- and time-dependent manner. Further, the induction of alkaline phosphatase by retinol in these cells was blocked by treatment with 30 microM antisense oligo(dN) to cRBP or hnRAR but not by 30 microM of sense oligo(dN) to cRBP. Antisense oligo(dN) treatments concomitantly induced cell rounding, loss of cell-cell attachment, and cell adhesion to the substratum. By contrast, treatment of cells with an anticytokinetic agent, cytochalasin B, or with a cytostatic concentration of sodium azide failed to reduce cytoplasmic cRBP or hnRAR from nuclear extracts, even though antisense oligo(dN)-like changes in cell morphology were observed. Treatment of the cells for greater than 2.75 hr with 20-40 microM of either antisense oligo(dN) also led to the loss of clonogenic potential. These results show that both cytoplasmic and nuclear receptors for retinoids are important in the transduction of a retinoid signal response critical to cellular growth and differentiation. Our findings also suggest that defined genes, which are specified by retinoids and their receptors, may account for the pleiotropic effect of vitamin A compounds.