A search for immunoreactive substance P and other neural peptides in the limb regenerate of the newt Notophthalmus viridescens

Neural dependency in wound healing and regeneration

In response to injury, humans and many other mammals form a fibrous scar that lacks the structure and function of the original tissue, whereas other vertebrate species can spontaneously regenerate damaged tissues and structures. Peripheral nerves have been identified as essential mediators of wound healing and regeneration in both mammalian and nonmammalian systems, interacting with the milieu of cells and biochemical signals present in the post-injury microenvironment. This review examines the diverse functions of peripheral nerves in tissue repair and regeneration, specifically during the processes of wound healing, blastema formation, and organ repair. We compare available evidence in mammalian and nonmammalian models, identifying critical nerve-mediated mechanisms for regeneration and providing future perspectives toward integrating these mechanisms into a therapeutic framework to promote regeneration.

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.

Cellular and molecular processes of regeneration, with special emphasis on fish fins

The phenomenon of 'epimorphic regeneration', a complete reformation of lost tissues and organs from adult differentiated cells, has been fascinating many biologists for many years. While most vertebrate species including humans do not have a remarkable ability for regeneration, the lower vertebrates such as urodeles and fish have exceptionally high regeneration abilities. In particular, the teleost fish has a high ability to regenerate a variety of tissues and organs including scales, muscles, spinal cord and heart among vertebrate species. Hence, an understanding of the regeneration mechanism in teleosts will provide an essential knowledge base for rational approaches to tissue and organ regeneration in mammals. In the last decade, small teleost fish such as the zebrafish and medaka have emerged as powerful animal models in which a variety of developmental, genetic and molecular approaches are applicable. In addition, rapid progress in the development of genome resources such as expressed sequence tags and genome sequences has accelerated the speed of the molecular analysis of regeneration. This review summarizes the current status of our understanding of the cellular and molecular basis of regeneration, particularly that regarding fish fins.

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 neurohormones and growth factors in echinoderms

There has been much recent interest in the presence and biological functions of growth regulators in invertebrates. In spite of the different distribution patterns of these molecules in different phyla (from molluscs, insects, and annelids to echinoderms and tunicates), they seem always to be extensively involved in developmental processes, both embryonic and regenerative. Echinoderms are well known for their striking regenerative potential and many can completely regenerate arms that, for example, are lost following self-induced or traumatic amputation. Thus, they provide a valuable experimental model for the study of regenerative processes from the macroscopic to the molecular level. In crinoids as well as probably all ophiuroids, regeneration is rapid and occurs by means of a mechanism that involves blastema formation, known as epimorphosis, where the new tissues arise from undifferentiated cells. In asteroids, morphallaxis is the mechanism employed, replacement cells being derived from existing tissues following differentiation and (or) transdifferentiation. This paper focuses on the possible contribution of neurohormones and growth factors during both repair and regenerative processes. Three different classes of regulatory molecules are proposed as plausible candidates for growth-promoting factors in regeneration: neurotransmitters (monoamines), neuropeptides (substance P, SALMFamides 1 and 2), and growth-factor-like molecules (TGF-β (transforming growth factor β), NGF (nerve growth factor), RGF-2 (basic fibroblast growth factor)).

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.

Effect of a dipeptide inhibiting ubiquitin-mediated protein degradation nerve-dependent limb regeneration in the newt

The dipeptide Leu-Ala, which inhibits ubiquitin-mediated protein degradation, has been shown to act in vitro as an inhibitor of neurite outgrowth of PC12 cells (Hondermarck et al. [1992] Biochem. Biophys. Res. Commun. 189:280). Using agarose beads as vehicles, we tested, in vivo, the effect of this dipeptide (and the inactive inverse, Ala-Leu, as a control) on limb regeneration in the newt (Triturus cristatus), a nerve-dependent developmental process. Leu-Ala inhibited the growth of mid-bud blastemas without altering blastema differentiation, while Ala-Leu had no effect. Cytological observations of dipeptide-treated blastemas using Bodian staining or neurofilament antibodies showed that all the blastema tissues were unmodified except with regard to innervation. Leu-Ala-treated blastemas were devoid of nerve fibers in the epidermal cap, while the mesenchyme distal to the dipeptide impregnated bead exhibited fewer nerve fibers than did Ala-Leu-treated blastemas, which were similar to the control nontreated blastemas. Thus, Leu-Ala, in reducing blastema innervation, inhibits its growth in the same manner as surgical denervation.

Pattern of substance P- and cholecystokinin-like immunoreactivity during regeneration of the neural complex in the ascidian Ciona intestinalis

The neural ganglion of ascidians exhibits a novel and rapid pattern of regeneration whereby within approximately 28-35 days of total ablation an entirely new neural complex is formed. In normal adults, neuronal cell bodies expressing substance P- (SP-Li), neurokinin A-(NKA-Li), CCK/gastrin- (CCK-Li), and insulin-like immunoreactivity exhibit a clearly defined pattern of localization in the cortical rind of the ganglion with characteristic long processes arising from the perikarya running throughout the neuropile. CCK-Li cell bodies are particularly concentrated close to the points of exit of the main nerve trunks. We have used antisera raised against these peptides to monitor the process of regeneration up to postoperative (pa) day 35. Only SP and CCK antisera produced positive staining in the regenerating tissue. Immunoreactive cell bodies first appear following 14 days pa. At this time CCK-Li neurons are more abundant than SP-Li neurons and in contrast to the pattern found in the normal adult ganglion, immunoreactive cell bodies are located both peripherally and centrally in the core of the ganglion and processes were rarely seen. Later stages exhibited an increasing number of SP-Li neurons and at 35 days pa SP-Li cell bodies clearly predominate. CCK-Li neurons typically become clustered close to the points of emergence of the anterior nerve roots. The early expression of CCK-Li and SP-Li molecules during regeneration is considered in terms of their potential role in development and cell proliferation in the newly forming ganglion.

Localization of beta-endorphin-like immunoreactivity in the nervous system of the adult newt, Notophthalmus viridescens

Using indirect immunofluorescence methods, we have localized for the first time in the newt, Notophthalmus viridescens, beta-endorphin (beta-ep)-like immunoreactivity in the neurons of spinal ganglia (SPG), spinal cord (SPC), as well as in the hypothalamic region of the brain. An examination of serially sectioned SPG showed that the beta-ep-positive neurons, cell bodies, and nerve fibers were distributed at all levels of SPG. Peripheral regions of the perikarya of beta-ep-positive SPG neurons exhibited intense staining for beta-ep, the central nuclear region remaining nonreactive. In SPC, brightly staining fibers were seen entering the afferent nociceptive input areas, namely the Lissauer's tracts, substantia gelatinosa, and the dorsal ascending columns. Dot-fiber immunofluorescence pattern was observed throughout the gray matter of SPC representing beta-ep-positive, secondary sensory neurons as well as interneurons. Also, discrete cluster of neurons located deep in the gray matter of SPC stained positively to beta-ep antisera. This study not only demonstrates for the first time the presence of beta-ep like material in the newt, more specifically in SPG and SPC, but also raises the question of a possible link between beta-ep and newt limb regeneration as previous work has shown that SPG support limb regeneration in a denervated-amputated newt forelimb.