Injury to nerves and the initiation of amphibian limb regeneration

Neural MMP-28 expression precedes myelination during development and peripheral nerve repair

Mammalian matrix metalloproteinase 28 (MMP-28) is expressed in several normal adult tissues, and during cutaneous wound healing. We show that, in frog and mouse embryos, MMP-28 is expressed predominantly throughout the nervous system. Xenopus expression increases during neurulation and remains elevated through early limb development where it is expressed in nerves. In the mouse, neural expression peaks at embryonic day (E) 14 but remains detectable through E17. During frog hindlimb regeneration XMMP-28 is not initially expressed in the regenerating nerves but is detectable before myelination. Following hindlimb denervation, XMMP-28 expression is detectable along regenerating nerves before myelination. In embryonic rat neuron-glial co-cultures, MMP-28 decreases after the initiation of myelination. Incubation of embryonic brain tissue with purified MMP-28 leads to the degradation of multiple myelin proteins. These results suggest that MMP-28 plays an evolutionarily conserved role in neural development and is likely to modulate the axonal-glial extracellular microenvironment.

Alteration and recovery of appetitive behaviour following nerve section in the starfish Asterias rubens

The starfish Asterias rubens is an invertebrate deuterostome whose nervous system shows remarkable regenerative properties. To understand when full functionality of a damaged part of the nervous system recovers, and to follow nerve regeneration in detail, we carried out behavioural experiments with 29 starfishes that had the nerve in one of the arms sectioned in a mid-arm position. Loss and recovery of normal behaviour was followed by video analysis of animal performance in an appetitive behavioural test. When compared to 13 control (unoperated) animals, the appetitive response of freshly sectioned animals is normal initially, progressively deteriorates up to 40 days after the lesion, and then gradually improves until 60 days, when recovery is complete. This is true only when one of the leading arms in the appetitive test is a sectioned arm; turning the starfish so that both the leading arms facing the prey are unlesioned, results in normal behaviour even at 40 days after the cut. Thus, regeneration is a multi-step process whose time course coincides with anatomical regeneration. At intermediate times the animals have coordination problems in an appetitive behaviour test and these give some insights into how arms may inter-communicate to organize concerted movements.

Skeletal muscle transformation into electric organ in S. macrurus depends on innervation

The cells of the electric organ, called electrocytes, of the weakly electric fish Sternopygus macrurus derive from the fusion of mature fast muscle fibers that subsequently disassemble and downregulate their sarcomeric components. Previously, we showed a reversal of the differentiated state of electrocytes to that of their muscle fiber precursors when neural input is eliminated. The dependence of the mature electrocyte phenotype on neural input led us to test the hypothesis that innervation is also critical during formation of electrocytes. We used immunohistochemical analyses to examine the regeneration of skeletal muscle and electric organ in the presence or absence of innervation. We found that blastema formation is a nerve-dependent process because regeneration was minimal when tail amputation and denervation were performed at the same time. Denervation at the onset of myogenesis resulted in the differentiation of both fast and slow muscle fibers. These were fewer in number, but in a spatial distribution similar to controls. However, in the absence of innervation, fast muscle fibers did not progress beyond the formation of closely apposed clusters, suggesting that innervation is required for their fusion and subsequent transdifferentiation into electrocytes. This study contributes further to our knowledge of the influence of innervation on cell differentiation in the myogenic lineage.

Expression and biological effect of urodele fibroblast growth factor 1: relationship to limb regeneration

Fibroblast growth factors (FGFs) have been previously implicated in urodele limb regeneration. Here, we examined expression of FGF-1 by blastema cells and neurons and investigated its involvement in wound epithelial formation and function and in the trophic effect of nerves. Neurons innervating the limb and blastema cells in vivo and in vitro expressed the FGF-1 gene. The peptide was present in blastemas in vivo. Wound epithelium thickened when recombinant newt FGF-1 was provided on heparin-coated beads, demonstrating that the FGF-1 was biologically active and that the wound epithelium is a possible target tissue of FGF. FGF-1 did not stimulate accessory limb formation. FGF-1 was as effective as 10% fetal bovine serum in maintaining proliferative activity of blastema cells in vitro but was unable to maintain growth of denervated, nerve-dependent stage blastemas when provided on beads or by injection. FGF-1 had a strong stimulating effect on blastema cell accumulation and proliferation of limbs inserted into the body cavity that were devoid of an apical epithelial cap (AEC). These results show that FGF-1 can signal wound epithelium cap formation and/or function and can stimulate mesenchyme accumulation/proliferation in the absence of the AEC but that FGF-1 is not directly involved in the neural effect on blastema growth.

Lack of proliferative response in denervated, reamputated limb regenerates of larval Ambystoma

This paper describes the response of early four-digit regenerates of axolotls to reamputation and denervation. Reamputation of innervated regenerates led to sharp increases in 3H-thymidine labeling index (LI) and mitotic index (MI) on days 2-5 post-reamputation. This resembles the response of innervated limbs following initial amputation. Regenerates that were denervated at the time of reamputation exhibited no proliferative response through day 5. This is in marked contrast to denervated, original amputation limb stumps, in which LI and MI rise for several days (as in innervated stumps) before falling to background levels. Although myelin was scarce near the level of reamputation, the lack of proliferation cannot be explained solely on that basis. The results are consistent with the possibility that the "neurotrophic factor" that causes stump and blastema cell mitosis is not present in unamputated limbs but is made in response to amputation.

Ability of various injuries to promote resorption of denervated, nerve independent regenerates of adult newts

Young blastemas of the newt resorb if the limb is denervated, and are thus called "nerve dependent". Late bud and later stage regenerates are termed "nerve independent" because, while denervation inhibits their growth, they proceed through differentiation to form a normally patterned regenerate. Schotté and Liversage ('59) found that reamputation of a denervated nerve independent regenerate causes it to resorb. The present study asked whether injuries of varying severity are equally effective at promoting resorption. Newt forelimbs were amputated through the mid-radius/ulna. At nerve independent stages, the regenerates were denervated and injured in one of a variety of ways, then monitored for signs of resorption. Reamputation of the tip or incisions which created large gaps in the wound epidermis promoted resorption in 77-90% of the cases. Histology showed that the tissue removed by tip reamputation was a small proportion of the entire regenerate, suggesting that blastema resorption is not determined simply by the number of cells directly affected by the injury. Pin prick injuries, which created small disruptions of the wound epidermis, never caused resorption. Devascularization, caused by severing the brachial artery, promoted resorption in 17% of cases. These results are not consistent with the hypothesis that avascularity is a major causative factor in nerve dependence.

Changes in the extracellular matrix and glycosaminoglycan synthesis during the initiation of regeneration in adult newt forelimbs

The extracellular matrix (ECM) of the distal tissues in a newt limb stump is completely reorganized in the 2-3-week period following amputation. In view of numerous in vitro studies showing that extracellular material influences cellular migration and proliferation, it is likely that the changes in the limb's ECM are important activities in the process leading to regeneration of such limbs. Using biochemical, autoradiographic, and histochemical techniques we studied temporal and spatial differences in the synthesis of glycosaminoglycans (GAGs) during the early, nerve-dependent phase of limb regeneration. Hyaluronic acid synthesis began with the onset of tissue dedifferentiation, became maximal within 1 weeks, and continued throughout the period of active cell proliferation. Chondroitin sulfate synthesis began somewhat later, increased steadily, and reached very high levels during chondrogenesis. During the first 10 days after amputation, distributions of sulfated and nonsulfated GAGs were both uniform throughout dedifferentiating tissues, except for a heavier localization near the bone. Since nerves are necessary to promote the regenerative process, we examined the neural influence on synthesis and accumulation of extracellular GAGs. Denervation decreased GAG production in all parts of the limb stump by approximately 50%. Newt dorsal root ganglia and brain-derived fibroblast growth factor each produced twofold stimulation of GAG synthesis in cultured 7-day regenerates. The latter effect was primarily on synthesis of hyaluronic acid. The results indicate that the trophic action of nerves on amphibian limb regeneration includes a positive influence on synthesis and extracellular accumulation of GAGs. Since the ECM exerts a major influence on cellular proliferation and migration, the effect of nerves on GAG metabolism may have considerable importance for growth and development of the early regenerate.