Effect of transferrin on amphibian limb regeneration: a blastema cell culture study

Nerves and Proliferation of Progenitor Cells in Limb Regeneration

Nerves, in conjunction with the apical epidermal cap (AEC), play an important role in the proliferation of the mesenchymal progenitor cells comprising the blastema of regenerating urodele amphibian limbs. Reinnervation after amputation requires factors supplied by the forming blastema, and neurotrophic factors must be present at or above a quantitative threshold for mitosis of the blastema cells. The AEC forms independently of nerves, but requires nerves to be maintained. Urodele limb buds are independent of nerves for regeneration, but innervation imposes a regenerative requirement for nerve factors on their cells as they differentiate. There are three main ideas on the functional relationship between nerves, AEC, and blastema cells: (1) nerves and AEC produce factors with different roles in maintaining progenitor status and mitosis; (2) the AEC produces the factors that promote blastema cell mitosis, but requires nerves to express them; (3) blastema cells, nerves, and AEC all produce the same factor(s) that additively attain the required threshold for mitosis.

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?

Do you have the nerves to regenerate? The importance of neural signalling in the regeneration process

The importance of nerve-derived signalling for correct regeneration has been the topic of research for more than a hundred years, but we are just beginning to identify the underlying molecular pathways of this process. Within the current review, we attempt to provide an extensive overview of the neural influences during early and late phases of both vertebrate and invertebrate regeneration. In general, denervation impairs limb regeneration, but the presence of nerves is not essential for the regeneration of aneurogenic extremities. This observation led to the "neurotrophic factor(s) hypothesis", which states that certain trophic factors produced by the nerves are necessary for proper regeneration. Possible neuron-derived factors which regulate regeneration as well as the denervation-affected processes are discussed.

The role of peripheral nerves in urodele limb regeneration

Nerve axons and the apical epidermal cap (AEC) are both essential for the formation of an accumulation blastema by amputated limbs of urodele salamanders. The AEC forms in the absence of axons, but is not maintained, and blastema formation fails. Growth stages of the blastema become nerve-independent for morphogenesis, but remain dependent on the nerve for blastema growth. Denervated growth stage blastemas form smaller than normal skeletal parts, owing to diminished mitosis, but form the full proximodistal array of skeletal elements. This difference in nerve dependency of morphogenesis and proliferation is hypothesized to be the result of a dependence of the AEC on nerves for blastema cell proliferation but not for blastema morphogenesis. Regenerating axons induce the synthesis and secretion of the anterior gradient protein (AGP) by distal Schwann cells during dedifferentiation and by the gland cells of the AEC during blastema growth stages. AGP promotes the regeneration of a denervated limb to digit stages when electroporated into the limb during dedifferentiation. Once a critical mass of blastema cells has been attained, the blastema can undergo morphogenesis in the absence of the nerve, but the regenerate will be a miniature, because the nerve is no longer inducing the AEC to carry out its AGP-mediated proliferative function. AGP expression by both Schwann cells and the AEC is induced by axons, but the nature of the inductive agent is unclear.

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.

Regeneration of the urodele limb: a review

Urodele amphibians have been widely used for studies of limb regeneration. In this article, we review studies on blastema cell proliferation and propose a model of blastemal self-organization and patterning. The model is based on local cell interactions that intercalate positional identities within circumferential and proximodistal boundaries that outline the regenerate. The positional identities created by the intercalation process appear to be reflected in the molecular composition of the cell surface. Transcription factors and signaling molecules involved in patterning are discussed within the context of the boundary/intercalation model.

Nerve-independence of limb regeneration in larval Xenopus laevis is correlated to the level of fgf-2 mRNA expression in limb tissues

In both larval and adult urodele amphibians, limb blastema formation requires the presence of an adequate nerve supply. In previous research, we demonstrated that the hindlimb of early Xenopus laevis larvae formed a regeneration blastema even when denervated, while the denervated limb of late larvae did not. We hypothesized that the nerve-independence was due to the autonomous synthesis of a mitogenic neurotrophic-like factor by undifferentiated limb bud cells. In this paper, we demonstrate that fgf-2 mRNA is present in larval limb tissues and that its level is correlated to the extent of mesenchymal cells populating the limb: in early limbs, fgf-2 mRNA is present at high levels all over the limb, while, in late limbs, the fgf-2 expression is low and detectable only in the distal autopodium. After denervation, fgf-2 mRNA synthesis increases in amputated early limbs but not in amputated late limbs. The implantation of anti-FGF-2 beads into amputated early limbs hardly lowers the mitotic activity of blastema cells. However, FGF-2 beads implanted into the blastema of late limbs prevent the denervation-induced inhibition of mitosis and oppose blastema regression. Our data indicate that FGF-2 is a good candidate for the endogenous mitogenic factor responsible for blastema formation and growth in amputated and denervated early limbs. However, in amputated late limbs, the very limited fgf-2 expression is not sufficient to promote blastema formation in the absence of nerves.

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.

Transferrin is necessary and sufficient for the neural effect on growth in amphibian limb regeneration blastemas

Cell proliferation during the early phase of growth in regenerating amphibian limbs requires a permissive influence of nerves. Based on analyses of proliferative activity in denervated blastemas, it was proposed that nerves provide factors important for cells to complete the proliferative cycle rather than for mitogenesis itself. One such factor, the iron-transport protein transferrin (Tf), is abundant in regenerating peripheral nerves where it is axonally transported and released at growth cones. Using blastemas in organ culture, which have been widely used in previous investigations of the neural effect on growth, it was shown here that the growth-promoting activity of neural extract was completely removed by immuno-absorption with antiserum against Tf and restored by addition of Tf. Purified Tf or a low molecular weight ferric ionophore were as active as the neural extract in this assay, indicating that the trophic effect of Tf involves its capacity for iron delivery. Both Tf and ferric ionophore also maintained DNA synthesis in denervated blastemas in vivo. A dose-response assay indicated that purified axolotl Tf stimulates growth of cultured blastemal cells at concentrations as low as 100 ng/mL. The Tf mRNA in axolotl nervous tissue was shown by northern analysis to be similar in size to that of liver. These results are discussed together with those from previous in vitro studies of blastemal growth and support the hypothesis that cell division in the blastema depends on axonally released Tf during the early, nerve-dependent phase of limb regeneration.

Regenerative responses in cultured hindlimb stumps of larval Xenopus laevis

The regenerative capacity of larval Xenopus laevis hindlimbs amputated through the tarsalia at different stages of development and explanted in vitro was tested. In the first experimental series hindlimb stumps from stage 53, 54, 55, and 57 larvae (according to Nieuwkoop and Faber, '56) were cultured in Leibovitz's L-15 medium supplemented with 10% FCS, and 0.04 U of insulin and 10(-8) mg of L-thyroxine per ml of medium. Results showed that the distal part of the limb stumps from stages 53, 54, and 55 formed a regeneration blastema composed of proliferating mesenchymal cells beneath a typical apical cap. No blastema was formed in the proximal part of the stump. In limb stumps from stage 57, a regeneration blastema did not form either in the proximal or in the distal part of the stump. In a second experimental series, hindlimb stumps from stage 55 larvae, denervated 5 days prior to amputation in order to eliminate any residual neurotrophic factor, were cultured in a simplified L-15 medium containing 2% FCS and lacking insulin and thyroxine. Results showed that also in these experimental conditions the stumps from stage 55 formed a conical regeneration blastema at the distal tip. The blastema cells duplicated their own DNA and divided. At the proximal extremity no regeneration blastema was formed. In the same culture medium, the stumps of larvae at stage 57 did not form a regeneration blastema.(ABSTRACT TRUNCATED AT 250 WORDS)

Lens formation from the cornea following implantation into hindlimbs of larval Xenopus laevis: the influence of limb innervation and extent of differentiation

Corneal fragments of larval Xenopus laevis at stage 48 (according to Nieuwkoop and Faber, '56), were implanted into sham denervated unamputated hindlimbs, denervated unamputated hindlimbs, amputated and sham denervated hindlimbs, and amputated and denervated hindlimbs of larvae at stages 52 and 57. The results show that unamputated limbs at stage 52, either innervated or denervated, manifest a weak capacity to promote the first lens-forming transformations of the outer cornea. This capacity is absent in both limb types at stage 57. After amputation, limbs of both early and late stages form a regenerative blastema and support lens formation from the outer cornea. Denervation of early stage limbs has no appreciable effect on blastema formation and lens-forming transformation of corneal implants. However, denervation of late stage limbs inhibits both processes. These results indicate that the limb tissues of the early stage limbs contain non-neural inductive factors at a low level and that after limb amputation and blastema formation the level of these factors becomes high enough to promote lens formation from implanted cornea, even after denervation. In contrast, the limb tissues of late stage limbs do not contain a suitable level of non-neural inductive factors.

Axonal transport and release of transferrin in nerves of regenerating amphibian limbs

Transferrin, a plasma protein required for proliferation of normal and malignant cells, is abundant in peripheral nerves of birds and mammals and becomes more concentrated in this tissue during nerve regeneration. We are testing the hypothesis that this factor is involved in the growth-promoting effect of nerves during the early, avascular phase of amphibian limb regeneration. A sensitive enzyme-linked immunosorbent assay for axolotl transferrin was developed and used to determine whether this protein meets certain criteria expected of the trophic factor(s) from nerves. During limb regeneration adult sciatic nerves greatly increased their content of transferrin, which immunohistochemistry revealed was distributed in both axons and Schwann cells. Using the double ligature method with sciatic nerves in vivo, it was determined that transferrin is carried by fast anterograde axonal transport at all stages of limb regeneration. An approach based on multicompartment organ culture demonstrated that fast-transported transferrin was secreted in physiologically significant amounts at distal ends of regenerating axons. Finally, the concentration of transferrin in the distal region of larval axolotl limb stumps was found to decrease directly and rapidly in response to axotomy. Since transferrin is important for both axonal regeneration and cell cycling, the present data have significance for various aspects of nerve's trophic activity during limb regeneration.

Production in vitro by spinal cord of growth factor(s) acting on newt limb regeneration: influence of regeneration of the nerve fibers

In order to approach the problem of regulation of growth factor(s) production during limb regeneration in newt, we co-cultivated spinal cord segments and blastemas. First we showed that, like the sensory supply, the spinal cord possesses size-dependent mitogenic capacities for limb blastemas. A 5-mm long spinal segment enhances radiolabelled thymidine incorporation to the same extent as spinal ganglia (1.6-fold). Second, we co-cultivated blastemas with spinal segments, the nerve fibers of which were previously stimulated to regenerate (= stimulated spinal segment) or not (= non-stimulated spinal segment). Only after a 24-h coculture, do stimulated spinal segments enhance thymidine incorporation in blastemas 2-fold more than non-stimulated spinal segments. Our results suggest that during limb regeneration brachial nerves produce more growth factor(s) when regrowing, inducing the proliferation of blastema cells which in return deliver a neuronotrophic factor acting on these nerves.