Cell death of asynaptic neurons in regenerating spinal cord

Electric fish: new insights into conserved processes of adult tissue regeneration

Biology is replete with examples of regeneration, the process that allows animals to replace or repair cells, tissues and organs. As on land, vertebrates in aquatic environments experience the occurrence of injury with varying frequency and to different degrees. Studies demonstrate that ray-finned fishes possess a very high capacity to regenerate different tissues and organs when they are adults. Among fishes that exhibit robust regenerative capacities are the neotropical electric fishes of South America (Teleostei: Gymnotiformes). Specifically, adult gymnotiform electric fishes can regenerate injured brain and spinal cord tissues and restore amputated body parts repeatedly. We have begun to identify some aspects of the cellular and molecular mechanisms of tail regeneration in the weakly electric fish Sternopygus macrurus (long-tailed knifefish) with a focus on regeneration of skeletal muscle and the muscle-derived electric organ. Application of in vivo microinjection techniques and generation of myogenic stem cell markers are beginning to overcome some of the challenges owing to the limitations of working with non-genetic animal models with extensive regenerative capacity. This review highlights some aspects of tail regeneration in S. macrurus and discusses the advantages of using gymnotiform electric fishes to investigate the cellular and molecular mechanisms that produce new cells during regeneration in adult vertebrates.

Spinal cord repair in regeneration-competent vertebrates: adult teleost fish as a model system

Spinal cord injuries in mammals, including humans, have devastating long-term consequences. Despite substantial research, therapeutic approaches developed in mammalian model systems have had limited success to date. An alternative strategy in the search for treatment of spinal cord lesions is provided by regeneration-competent vertebrates. These organisms, which include fish, urodele amphibians, and certain reptiles, have a spinal cord very similar in structure to that of mammals, but are capable of spontaneous structural and functional recovery after spinal cord injury. The present review aims to provide an overview of the current status of our knowledge of spinal cord regeneration in one of these groups, teleost fish. The findings are discussed from a comparative perspective, with reference to other taxa of regeneration-competent vertebrates, as well as to mammals.

Regeneration of the radial nerve cord in the sea cucumber Holothuria glaberrima

Background

Regeneration of neurons and fibers in the mammalian spinal cord has not been plausible, even though extensive studies have been made to understand the restrictive factors involved. New experimental models and strategies are necessary to determine how new nerve cells are generated and how fibers regrow and connect with their targets in adult animals. Non-vertebrate deuterostomes might provide some answers to these questions. Echinoderms, with their amazing regenerative capacities could serve as model systems; however, very few studies have been done to study the regeneration of their nervous system.

Results

We have studied nerve cord regeneration in the echinoderm Holothuria glaberrima. These are sea cucumbers or holothurians members of the class Holothuroidea. One radial nerve cord, part of the echinoderm CNS, was completely transected using a scalpel blade. Animals were allowed to heal for up to four weeks (2, 6, 12, 20, and 28 days post-injury) before sacrificed. Tissues were sectioned in a cryostat and changes in the radial nerve cord were analyzed using classical dyes and immmuohistochemistry. In addition, the temporal and spatial distribution of cell proliferation and apoptosis was assayed using BrdU incorporation and the TUNEL assay, respectively.

We found that H. glaberrima can regenerate its radial nerve cord within a month following transection. The regenerated cord looks amazingly similar in overall morphology and cellular composition to the uninjured cord. The cellular events associated to radial cord regeneration include: (1) outgrowth of nerve fibers from the injured radial cord stumps, (2) intense cellular division in the cord stumps and in the regenerating radial nerve cords, (3) high levels of apoptosis in the RNC adjacent to the injury and within the regenerating cord and (4) an increase in the number of spherule-containing cells. These events are similar to those that occur in other body wall tissues during wound healing and during regeneration of the intestine.

Conclusion

Our data indicate that holothurians are capable of rapid and complete regeneration of the main component of their CNS. Regeneration involves both the outgrowth of nerve fibers and the formation of neurons. Moreover, the cellular events employed during regeneration are similar to those involved in other regenerative processes, namely wound healing and intestinal regeneration. Thus, holothurians should be viewed as an alternative model where many of the questions regarding nervous system regeneration in deuterostomes could be answered.

Muscle activity and motor neuron death in the spinal cord of the chick embryo

During embryonic development in vertebrates about half the spinal motor neurons degenerate naturally after an initial period of normal differentiation. Motor neuron survival during this period is regulated by influences associated with both afferent and target contacts. Target-associated influences are regulated, at least in part, by activity (i.e. neuromuscular transmission or muscle contraction). Pharmacological blockade of neuromuscular activity reduces or prevents normal cell death whereas induced hyperactivity of targets enhances the death of motor neurons. Information supporting these assertions is reviewed and evidence is presented from studies which attempt to elucidate the major site at which neuromuscular activity affects motor neuron survival and degeneration in the chick embryo. Finally, a model and some supporting evidence are described in which activity is thought to regulate the production or availability of a target-derived trophic factor required by motor neurons for their survival during certain critical phases of early development.

Norman K. Wessells: a life in science

"In its triple role as locomotory organelle, as a site of deposition of new surface material for the elongating axon, and a source of microspikes (sensory probes), the growth cone becomes the key to axon elongation" Yamada et al. (1971).

Motoneurons dorsolateral to the central canal innervate perineal muscles in the Mongolian gerbil

The Mongolian gerbil provides a model in which sexually dimorphic areas in the hypothalamus are correlated with sociosexual behaviors such as scent marking and male copulatory behavior. To extend this model, investigations were conducted to determine whether sexually dimorphic areas existed in the spinal cord that could be relevant to male sexual behavior. The focus of these investigations was the perineal muscles associated with the penis. Therefore, this research identified the spinal motoneurons that innervate the bulbocavernosus, levator ani, anal sphincter, and ischiocavernosus muscles of Mongolian gerbils. The motoneuron pool that innervates the bulbocavernosus, levator ani, and anal sphincter was designated the spinal nucleus of the bulbocavernosus (SNB), as for other species of rodents. The motoneuron pool innervating the ischiocavernosus was identified as the dorsolateral nucleus, again, to be consistent with the designation for other rodents. The motoneurons of the gerbil SNB were distributed dorsolateral to the central canal in the lumbosacral transition zone of the spinal column. These motoneurons are located in the region classically defined as area X of the spinal cord. The number of SNB motoneurons was sexually dimorphic, with male gerbils having about five times as many SNB motoneurons as do female gerbils. The size of SNB motoneurons was also sexually dimorphic. The SNB motoneurons of males were 1.5 times larger than the SNB motoneurons of females. The effects of adult castration on the male SNB were also studied. After castration, the size, but not the number, of SNB motoneurons in males was significantly decreased. This decrease was prevented by testosterone treatment. The percentage of calcitonin gene-related peptide (CGRP)-immunoreactive SNB motoneurons was also affected by adult castration. The percentage of CGRP-immunoreactive motoneurons was significantly decreased after adult castration. Again, this decrease was reversed by testosterone treatment. These findings suggest that the SNB of gerbils is sexually dimorphic and is sensitive to circulating levels of gonadal steroids. The unique placement of the SNB motoneurons suggests that an alternative laminar organizational scheme may be necessary for Mongolian gerbil.

Neuronal differentiation in vitro from precursor cells of regenerating spinal cord of the adult teleost Apteronotus albifrons

This study documents neuronal differentiation in vitro from undifferentiated precursor cells of caudalmost regenerating spinal cord of the teleost Apteronotus albifrons. At 11 days in vitro, cells from the caudalmost tip of the regenerating cord are flat and polygonal in shape, lack neuronal processes and do not stain with antibody against neuron-specific filaments. At 15 days in vitro, some of the caudalmost cells have developed short, neurite-like processes; at 18 days in vitro, some cells react positively with antibody against neuron-specific filaments. At 26 days in vitro, many of the caudalmost cells have long branching neurites and react positively with anti-neurofilament antibody. Addition of insulin-like growth factor-I to the medium accelerates the process of neuronal differentiation from the caudalmost precursor cells in vitro. The source of these precursor cells is ultimately cells of the ependymal layer of adult spinal cord. Further investigation of the factors that control production and differentiation of these cells will be important in defining the developmental potential possible for vertebrate spinal cord cells and may aid in creating an optimal environment for regeneration of axons within mammalian spinal cord.

The conservation of neurotrophic factors during vertebrate evolution

Neurotrophic factors are a family of extracellular ligands that affect the differentiation, survival (by inhibition of apoptosis) and maintenance of function of neuronal cells in vertebrates. The family includes nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). The survival specificities of NGF and BDNF for different classes of chick neurons are maintained from the fish to the mammalian proteins, implying a conserved interaction with neuronal cell surface receptors (of the Trk family). However, the quantitative effect of a fish neurotrophin can differ significantly from that of the mammalian orthologue.

Differences in growth of neurons from normal and regenerated teleost spinal cord in vitro

Explants and dissociated cells from normal adult spinal cord and regenerating cord of the teleost Apteronotus albifrons were grown in vitro for periods of 8 to 12 wk. During this time the neurons showed extensive neurite outgrowth. Neurite outgrowth from tissue explants and dissociated cells of regenerated spinal cord starts sooner and is more profuse than that from normal (unregenerated) cord. Neurite outgrowth is maximized by using adhesive substrata and a high density of explants or dissociated cells. Inasmuch as Apteronotus does regenerate its spinal cord naturally after injury, whereas mammals do not, this culture system will be useful to study factors that control (permit) regeneration of spinal neurons in this adult vertebrate.

Cell death in the inner and outer nuclear layers of the developing cat retina

The occurrence and distribution of pyknotic profiles (dying cells) in the inner (INL) and outer nuclear layers (ONL) were mapped from radial sections through 13 kitten retinae aged from the 30th postconceptional day (30PCD) to the 81PCD (17th postnatal day). Cell death in the INL occurs in two consecutive waves, each beginning at the area centralis and proceeding toward the retinal edge. The first wave involves cells located in the inner part of the INL, adjacent to the inner plexiform layer. It begins on the 50PCD and lasts until the 66PCD, with a peak of 72,000 profiles on the 58PCD. This wave of cell death seems to be associated with the formation of the inner plexiform layer and may be due to the elimination of amacrine cells which have failed to establish sustaining connections with their appropriate target cells. The second wave involves cells located in the outer part of the INL, adjacent to the outer plexiform layer. It begins on the 60PCD and lasts until after the 81PCD, with a peak of 65,000 on the 71PCD. This wave appears to be due to the elimination of ectopic photoreceptor cells displaced by the formation of the OPL and presumably prevented from establishing sustaining connections with their appropriate target cells. A phase of cell death in the ONL begins on about the 42PCD and lasts until after the 81PCD, with a peak of 10,300 pyknotic profiles on the 60PCD.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurogenesis in adult vertebrate spinal cord in situ and in vitro: a new model system

Phylogenetically "lower" species in some cases use different biological strategies for recovery after injury to the CNS than do "higher" species. One approach that we have taken in our laboratory has been to study the mechanisms of functional recovery of the CNS after injury in those vertebrate species where recovery does occur. The present report reviews recent studies on a model system, the spinal electromotor system of the gymnotiform teleost Sternarchus albifrons, which exhibits regeneration and neurogenesis after injury. Regeneration in this system leads to a recapitulation of relatively normal morphologic structure by the damaged or extirpated spinal cord. In Sternarchus, new spinal cord is generated from ependymal cells; some ependymal cells in the adult remain pluripotent and retain the capability to generate new neurons. The Sternarchus spinal cord thus represents an especially useful model for the study of neurogenesis after injury to the CNS. Recent studies in our laboratory indicate that neurogenesis in adult Sternarchus spinal cord tissue occurs both in vivo and in vitro. Neurogenesis has been demonstrated by incorporation of tritiated thymidine into explant cultures from the spinal cord of adult Sternarchus. Autoradiography reveals the presence of thymidine-labeled neurons. Neuronal identity of 3H-labeled cells has been confirmed by positive staining with neuron-specific monoclonal antibodies. Thymidine labeling occurs in cultured neurons derived from both normal (histologically and functionally mature) and regenerating spinal cord of adult Sternarchus albifrons. These results provide evidence that some cells in spinal cord of adult Sternarchus retain the ability to incorporate thymidine and undergo neuronal differentiation in vitro. This system provides a new model in which neurogenesis from adult tissue can be studied in vivo and in vitro.

Generation of electromotor neurons in Sternarchus albifrons: differences between normally growing and regenerating spinal cord

This study examines the regulation of the number of electromotor neurons during postnatal growth of the spinal cord in the gymnotiform teleost Sternarchus albifrons. It specifically asks whether a large overproduction of electromotor neurons and a wave of cell death, similar to those occurring during spinal cord regeneration in this species, play a role in the on-going growth at the caudal tip of the normal spinal cord. Neurons are produced from ependymal precursors at the caudal end of the spinal cord during both normal growth in the adult and regeneration of the spinal cord in this species. Previous studies have demonstrated that during spinal cord regeneration after amputation of the tail in Sternarchus, there is an initial massive (up to fivefold) overproduction of electromotor neurons, followed by a wave of cell death which reduces the number of these neurons to the normal level. In the present study, transverse sections through the caudalmost spinal segment of normal adult Sternarchus were examined. Proceeding rostrally from the caudal tip of the cord, the number of electromotor neurons increases monotonically to reach the normal number at a site 4-5 mm rostral to the caudal tip. Neither a massive overproduction of electromotor neurons nor a wave of neuronal death are observed during on-going growth of the normal spinal cord. The mechanisms by which the neuronal number is modulated are therefore different in the on-going normal growth of spinal cord versus regeneration of spinal cord in this species.

Retrograde labeling of regenerated electromotor neurons with HRP in a teleost fish, Sternarchus albifrons: relation to cell death

Back-labeling of regenerated electromotor neurons in the teleost Sternarchus albifrons was performed to test the hypothesis that, in regenerated spinal cord, incorrectly located electromotor neurons are eliminated because their axons do not reach the correct target area (electric organ). In each cross section examined, all of the regenerated electromotor neurons ipsilateral to the implantation site were labeled with horseradish peroxidase, including those ectopic cells located at the edge of the cord, which are later eliminated by selective cell death. Retrograde labeling of these ectopic neurons demonstrates that their axons do extend into the correct target area (the regenerated electric organ). Thus total misdirection of the axons cannot be the cause of their subsequent cell death. We conclude that selective neuronal death in this system does not reflect the absence of axonal projection to the correct target area.

Neurogenesis in tissue cultures of adult teleost spinal cord

[3H]Thymidine autoradiography of explant cultures from the spinal cord of an adult teleost, Sternarchus albifrons, reveals the presence of thymidine-labeled cells with neuronal morphology. These cells have also been identified as neurons by positive staining with two monoclonal antibodies against neurofilaments. Thymidine labeling occurs in cultured neurons derived from both normal (histologically and functionally mature) and regenerating spinal cord of adult Sternarchus albifrons. These results provide evidence that some cells in spinal cord of adult Sternarchus retain the ability to incorporate thymidine and undergo neuronal differentiation in vitro.

Responses of host motor and sensory neurons to a neural tube implant in amputated chick limbs

The right wingbuds of stage 23-25 chick embryos were amputated at the future elbow region and a segment of 2-day neural tube was implanted longitudinally into the limb stump of experimental embryos to induce limb regeneration. Control embryos had no implant in the amputated limb stump. To analyze effects of the neural tube implant (NTI) upon the host nervous system, quantitative determinations were made of the peripheral limb field (PLF), dorsal root ganglia (DRG) and spinal cord lateral motor column (LMC) of the amputated side for comparison with similar determinations of the unamputated side in all embryos. The PLF was estimated by determining the area of the skeletal elements of the amputated and unamputated limb of each embryo. The size of the DRG was estimated by determining the sectional-profile area of a pair of ganglia; the LMC was determined by counting the neurons on both sides of a single spinal cord segment. The PLF was less on the amputated than on the unamputated side but was significantly greater in amputated limbs that received a NTI. The size of the DRG was positively correlated with the size of the PLF in all groups, indicating the DRG was not directly affected by the NTI but did respond to target structures. The number of neurons in the LMC was not positively correlated with the PLF and was not reduced by limb amputation in embryos with a NTI. The data suggest that the NTI may have protected host spinal cord cells from the induced-cell death expected to follow limb amputation; perhaps the implant produces a neuronal survival factor.