Changes in spinal cord regenerative ability through phylogenesis and development: lessons to be learnt

Origin of Nogo-A by domain shuffling in an early jawed vertebrate

Unlike mammals, fish are able to regenerate axons in their central nervous system. This difference has been partly attributed to the loss/acquisition of inhibitory proteins during evolution. Nogo-A--the longest isoform of the reticulon4 (rtn4) gene product--is commonly found in mammalian myelin where it acts as a potent inhibitor of axonal regeneration. Interestingly, fish RTN4 isoforms were previously reported to lack the most inhibitory Nogo-A-specific region (NSR). Nevertheless, fish axons collapse on contact with mammalian NSR, suggesting that fish possess a functional Nogo-A receptor but not its ligand. To reconcile these findings, we revisited the early evolution of rtn4. Mining of current genome databases established the unequivocal presence of NSR-coding sequences in fish rtn4 paralogues. Further comparative analyses indicate that the common ancestor of fish and tetrapods had an NSR-coding rtn4 gene, which underwent duplication and divergent evolution in bony fish. Our genomic survey also revealed that the cephalochordate Branchiostoma floridae contains a single rtn gene lacking the NSR. Hence, Nogo-A most probably arose independently in the rtn4 gene of a gnathostome ancestor before the split of the fish and tetrapod lineages. Close examination of the NSR uncovered clusters of structural and sequential similarities with neurocan (NCAN), an inhibitory proteoglycan of the glial scar. Notably, the shared presence of transposable elements in ncan and rtn4 genes suggests that Nogo-A originated via insertion of an ncan-like sequence into the rtn4 gene of an early jawed vertebrate with myelinated axons.

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.

Functions of Nogo proteins and their receptors in the nervous system

The membrane protein Nogo-A was initially characterized as a CNS-specific inhibitor of axonal regeneration. Recent studies have uncovered regulatory roles of Nogo proteins and their receptors--in precursor migration, neurite growth and branching in the developing nervous system--as well as a growth-restricting function during CNS maturation. The function of Nogo in the adult CNS is now understood to be that of a negative regulator of neuronal growth, leading to stabilization of the CNS wiring at the expense of extensive plastic rearrangements and regeneration after injury. In addition, Nogo proteins interact with various intracellular components and may have roles in the regulation of endoplasmic reticulum (ER) structure, processing of amyloid precursor protein and cell survival.

Establishment and characterization of two cell lines derived from primary cultures of Gekko japonicus cerebral cortex

Adult Gekko japonicus is one of those vertebrates that are able to regenerate their missing or amputated tail. The most interesting feature of this animal lies in the ability of its spinal cord to regrow a functional tail. A fundamental question is whether the neuroglial cells play a different role compared with high vertebrates. Since in vitro studies using primary neuroglial cells are hampered by the limited lifespan and miscellaneous genetic background of these cells, we generated neuroglial cell lines from primary cell cultures of cerebral cortex of G. japonicus. The SV40 (simian-virus-40) T antigen gene was introduced into primary cell cultures. Cell cycle analysis, cell growth and proliferation, cell colony formation and contact inhibition, as well as karyotype assays were investigated. Two cell colonies, Gsn-1 and Gsn-3, were immunochemically characterized as glial fibrillary acidic protein and galactocerebroside-positive respectively. Compared with parental primary cells, the Gsn cells displayed shorter population doubling time, decreased percentage of cells in the G0/G1 phase, higher cell proliferation index, and increased cell activity. In assays of colony characteristics, Gsn cells showed increased cell activity at the lower cell densities or FBS (fetal bovine serum) supplement. The karyotype of immortalized Gsn cells exhibited transformational characteristics with hyperdiploid and polyploid chromosomes. The cell lines will provide a useful in vitro model for gecko neuroglial cells and facilitate systematic studies investigating the biological functions of specific gene products related to regeneration of the central nervous system.

Soluble factor effects on glial cell reactivity at the surface of gel-coated microwires

A basal lamina gel preparation was incorporated into a modified neuroinflammation cell culture model to test the system as a characterization tool for surface-modified microwires. The extent of gliosis at the surface of gel-coated microwires was quantified in response to titrating the cell culture with a number of soluble factors reported to be involved in reactive gliosis. Positive control conditions (1% FBS, 10 ng/ml bFGF, Neural Basal medium with B27 supplement) induced a layer of GFAP-expressing astrocytes to accumulate on all gel-coated microwires. Serum, inflammatory cytokines IL-1alpha and IL-1beta and the neural progenitor cell (NPC) proliferating growth factors bFGF and PDGF all increased the number of reactive cells on the gel, in agreement with their reported roles in cell activation, migration and proliferation at the site of injury. Technically, this study shows that a fetal neuron-glia culture system is well suited for characterizing the impact of electrode coatings designed to mitigate implant-associated gliosis, and for screening the effect of multiple soluble factors that promote and/or inhibit implant-associated gliosis. Scientifically, this study points to essential roles of serum and inflammatory factors to induce NPC activation and migration to the site of injury, where growth factors like bFGF and PDGF induce proliferation of cells that will eventually form the glial scar.

Considering the evolution of regeneration in the central nervous system

For many years the mammalian CNS has been seen as an organ that is unable to regenerate. However, it was also long known that lower vertebrate species are capable of impressive regeneration of CNS structures. How did this situation arise through evolution? Increasing cellular and molecular understanding of regeneration in different animal species coupled with studies of adult neurogenesis in mammals is providing a basis for addressing this question. Here we compare CNS regeneration among vertebrates and speculate on how this ability may have emerged or been restricted.

Structural and functional regeneration after spinal cord injury in the weakly electric teleost fish, Apteronotus leptorhynchus

In contrast to mammals, teleost fish exhibit an enormous potential to regenerate adult spinal cord tissue after injury. However, the mechanisms mediating this ability are largely unknown. Here, we analyzed the major processes underlying structural and functional regeneration after amputation of the caudal portion of the spinal cord in Apteronotus leptorhynchus, a weakly electric teleost. After a transient wave of apoptotic cell death, cell proliferation started to increase 5 days after the lesion and persisted at high levels for at least 50 days. New cells differentiated into neurons, glia, and ependymal cells. Retrograde tract tracing revealed axonal re-growth and innervation of the regenerate. Functional regeneration was demonstrated by recovery of the amplitude of the electric organ discharge, a behavior generated by spinal motoneurons. Computer simulations indicated that the observed rates of apoptotic cell death and cell proliferation can adequately explain the re-growth of the spinal cord.

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.

The opossum genome: insights and opportunities from an alternative mammal

The strategic importance of the genome sequence of the gray, short-tailed opossum, Monodelphis domestica, accrues from both the unique phylogenetic position of metatherian (marsupial) mammals and the fundamental biologic characteristics of metatherians that distinguish them from other mammalian species. Metatherian and eutherian (placental) mammals are more closely related to one another than to other vertebrate groups, and owing to this close relationship they share fundamentally similar genetic structures and molecular processes. However, during their long evolutionary separation these alternative mammals have developed distinctive anatomical, physiologic, and genetic features that hold tremendous potential for examining relationships between the molecular structures of mammalian genomes and the functional attributes of their components. Comparative analyses using the opossum genome have already provided a wealth of new evidence regarding the importance of noncoding elements in the evolution of mammalian genomes, the role of transposable elements in driving genomic innovation, and the relationships between recombination rate, nucleotide composition, and the genomic distributions of repetitive elements. The genome sequence is also beginning to enlarge our understanding of the evolution and function of the vertebrate immune system, and it provides an alternative model for investigating mechanisms of genomic imprinting. Equally important, availability of the genome sequence is fostering the development of new research tools for physical and functional genomic analyses of M. domestica that are expanding its versatility as an experimental system for a broad range of research applications in basic biology and biomedically oriented research.

Robust axonal growth and a blunted macrophage response are associated with impaired functional recovery after spinal cord injury in the MRL/MpJ mouse

Spinal cord injury (SCI) in mammals leads to a robust inflammatory response followed by the formation of a glial and connective tissue scar that comprises a barrier to axonal regeneration. The inbred MRL/MpJ mouse strain exhibits reduced inflammation after peripheral injury and shows true regeneration without tissue scar formation following an ear punch wound. We hypothesized that following SCI, the unique genetic wound healing traits of this strain would result in reduced glial and connective tissue scar formation, increased axonal growth, and improved functional recovery. Adult MRL/MpJ and C57BL/6J mice were subjected to a mid-thoracic spinal contusion and the distribution of axon profiles and selected cellular and extracellular matrix components was compared at 1, 2, 4 and 6 weeks post-injury. Recovery of hind-limb locomotor function was assessed over the same time period. The MRL/MpJ mice exhibited robust axon growth within the lesion, beginning at 4 weeks post-injury. This growth was accompanied by reduced macrophage staining at 1, 2, 4 and 6 weeks post-injury, decreased chondroitin sulfate proteoglycan staining at 1-2 weeks and increased laminin staining throughout the lesion at 2-6 weeks post-injury. Paradoxically, the extent of locomotor recovery was impaired in the MRL/MpJ mice. Close examination of the chronic lesion site revealed evidence of ongoing degeneration both within and surrounding the lesion site. Thus, the regenerative genetic wound healing traits of the MRL/MpJ mice contribute to the evolution of a lesion environment that supports enhanced axon growth after SCI. However, this response occurs at the expense of meaningful functional recovery.

Regeneration of the radial nerve cord in a holothurian: a promising new model system for studying post-traumatic recovery in the adult nervous system

After a complete transection, the radial nerve cord (RNC) of the adult sea cucumber Eupentacta fraudatrix quickly regrows and reconnects. The description of the major cellular events that accompany this regeneration is derived from light and transmission electron microscopy. Shortly after lesioning, the extensive nerve fiber degeneration and neuronal apoptosis occur. The gap in the cord created by the transection is rapidly bridged, at first by connective tissue and subsequently by regenerating nerve tissue. On either side of the wound, the ectoneural and hyponeural components of the injured RNC form separate tubular rudiments, whose epithelial walls are composed mostly of dedifferentiated glial cells, capable of mitotic division, but also contain some nerve fibers and occasional neuronal perikarya. It is suggested that the glial cells play a crucial role in regeneration not only by providing the supporting guiding scaffold for regrowing nerve fibers, but also by producing new neurons. Other mechanisms of post-traumatic neurogenesis may involve proliferation and/or migration of existing perikarya. The anterior and posterior regenerates grow towards one another and eventually fuse to restore the anatomical continuity of the RNC. Re-differentiation of gliocytes and accumulation of nerve cells in the newly formed regions of the nervous tissue make histological organization of the fully regenerated RNC indistinguishable from that of the intact cord. The authors suggest that the holothurian RNC provides a valuable experimental model, which opens new possibilities for exploring the fundamental mechanisms underlying regeneration of the nervous system in deuterostomes.

The molecular characterization of the brain protein 44-like (Brp44l) gene of Gekko japonicus and its expression changes in spinal cord after tail amputation

The cDNA encoding brain protein 44-like (Brp44l) was identified from the brain and spinal cord cDNA library of Gekko japonicus. It contains a 324 bp open reading frame corresponding to a deduced protein of 107 amino acids. The molecular weight of the BRP44L protein is 12.2664 kDa and its theoretical pI is 9.67. At amino acid level, the BRP44L from gecko shares 85-89% similarity with those from other species. Northern blotting showed that Brp44l was ubiquitously expressed in the major organs of Gekko japonicus including brain, spinal cord, heart, lung, liver, and kidney, suggesting a fundamental biological role of this gene. In situ hybridization (ISH) revealed that positive hybridization signals were present in gray matter of spinal cord. The change of Brp44l expression in spinal cord after tail amputation was examined by reverse transcription polymerase chain reaction (RT-PCR). The expression level of this gene increased in the spinal cord after tail amputation, and reached the highest value at 1 week after tail amputation, which indicated that the expression level of Brp44l might be associated with the process of spinal cord injury and regeneration.

Adult spinal cord progenitor cells are repelled by netrin-1 in the embryonic and injured adult spinal cord

Adult neural progenitor cells (aNPCs) exhibit limited migration in vivo with the exception of the rostral migratory stream and injury-induced movement. Surprisingly little is known regarding those signals regulating attraction or inhibition of the aNPC. These studies demonstrate that aNPCs respond principally to a repulsive cue expressed at the embryonic floor plate (FP) and also the injured adult CNS. Adult spinal cord progenitor cells (aSCPs) were seeded onto organotypic slice preparations of the intact embryonic or injured adult spinal cord. Cell migration assays combined with genetic and molecular perturbation of FP-derived migration cues or aSCP receptors establish netrin-1 (Ntn-1) but not Slit-2, Shh, or Ephrin-B3 as the primary FP-derived repellant. When slices were prepared from injured spinal cord, aSCP migration away from the injury core was Ntn-1-dependent. These studies establish Ntn-1 as a critical regulator of aSCP migration in the intact and injured CNS.

Local protein synthesis in axonal growth cones: what is next?

While initially thought to be essentially a developmental characteristic, observed in artificial in vitro models, local protein synthesis in growth cones has been described in the adult, and more interestingly, during nerve regeneration. This emerging field is under intense investigation, revealing new functions of localized protein synthesis that include axon guidance, growth cone adaptation and sensitivity modulation at intermediate targets or axon regeneration. Here, we will review these functions and provide a short survey of the current knowledge on mechanisms of mRNA transport and regulation of localized protein synthesis. In addition, we will consider what lessons can be learned from localized protein synthesis in dendrites, and what developments can be expected next in the field. This latter question relates to the crucial point of which technical strategy to adopt for an ideal and pertinent analysis of the phenomenon.

Expression change of stem cell-derived neural stem/progenitor cell supporting factor gene in injured spinal cord of rats

OBJECTIVE

To explore the expression change of stem cell-derived neural stem/progenitor cell supporting factor (SDNSF) gene in the injuried spinal cord tissues of rats, and the relation between the expressions of SDNSF and nestin.

METHODS

The spinal cord contusion model of rat was established according to Allen's falling strike method. The expression of SDNSF was studied by RT-PCR and in situ hybridization (ISH), and the expression of nestin was detected by immunochemistry.

RESULTS

RT-PCR revealed that SDNSF mRNA was upregulated on day 4 after injury, peaked on day 8-12, and decreased to the sham operation level on day 16. ISH revealed that SDNSF mRNA was mainly expressed in the gray matter cells, probably neurons, of spinal cord. The immunohistochemistry showed that accompanied with SDNSF mRNA upregulation, the nestin-positive cells showed erupted roots, migrated peripherad and proliferation on the 8-day slice. However, the distribution pattern of these new cells was different from that of SDNSF-positive cells.

CONCLUSION

(1) SDNSF is expressed in the gray matter of spinal cord. The expression of SDNSF mRNA in the spinal cord varies with injured time. (2) The nestin-positive cells proliferate accompanied with spinal cord injury repair, but do not secrete SDNSF.

Antibodies neutralizing Nogo-A increase pan-cadherin expression and motor recovery following spinal cord injury in rats

STUDY DESIGN

A rat model of spinal cord injury was used to test the hypothesis that Nogo-A monoclonal antibody (NEP1-40) promotes morphologic and functional recoveries of injured spinal cord.

OBJECTIVE

Nogo-A is a myelin-associated neurite outgrowth inhibitory protein, which blocks elongation nerve fibers and limits neuronal regeneration after central nervous system injury.

METHODS

Forty-four rats were utilized and allocated into control (vehicle) and NEP1-40-treated groups. In all animals, the spinal cord was hemi-transected at Th-10 and phosphate-buffered saline solution was immediately applied on the injured area in the control group. NEP1-40 solution was immediately applied on the hemi-transected area in the treatment group. Each group was subdivided into three subgroups according to the postsurgical day of killing (3, 8 and 21 days). The spinal cords were removed for analysis.

RESULTS

Motor scores in the NEP1-40-treated groups were significantly higher than those in the vehicle groups both at 8 and 21 days post injury. Immunohistochemical staining for pan-cadherin, a marker of neuronal cell adhesion and axonal sprouting, revealed a significant increase in staining in the NEP1-40 treatment group at 8 and 21 days post injury. Transmission electron microscopical evaluation revealed degeneration of the myelin and loss of cytoarchitectural organization in the axons of controls. Better preservation and normal histologic features were observed in the NEP1-40-treated groups.

CONCLUSION

We have demonstrated improved preservation of injured axons and significant pan-cadherin expression after NEP1-40 treatment after the spinal cord injury. Inhibition of Nogo-A may improve the capacity for neuronal regeneration after spinal cord injury.

Regeneration of neural crest derivatives in the Xenopus tadpole tail

Background

After amputation of the Xenopus tadpole tail, a functionally competent new tail is regenerated. It contains spinal cord, notochord and muscle, each of which has previously been shown to derive from the corresponding tissue in the stump. The regeneration of the neural crest derivatives has not previously been examined and is described in this paper.

Results

Labelling of the spinal cord by electroporation, or by orthotopic grafting of transgenic tissue expressing GFP, shows that no cells emigrate from the spinal cord in the course of regeneration.

There is very limited regeneration of the spinal ganglia, but new neurons as well as fibre tracts do appear in the regenerated spinal cord and the regenerated tail also contains abundant peripheral innervation.

The regenerated tail contains a normal density of melanophores. Cell labelling experiments show that melanophores do not arise from the spinal cord during regeneration, nor from the mesenchymal tissues of the skin, but they do arise by activation and proliferation of pre-existing melanophore precursors. If tails are prepared lacking melanophores, then the regenerates also lack them.

Conclusion

On regeneration there is no induction of a new neural crest similar to that seen in embryonic development. However there is some regeneration of neural crest derivatives. Abundant melanophores are regenerated from unpigmented precursors, and, although spinal ganglia are not regenerated, sufficient sensory systems are produced to enable essential functions to continue.

Recapitulate development to promote axonal regeneration: good or bad approach?

In the past decade there has been an explosion in our understanding, at the molecular level, of why axons in the adult, mammalian central nervous system (CNS) do not spontaneously regenerate while their younger counterparts do. Now a number of inhibitors of axonal regeneration have been described, some of the receptors they interact with to transduce the inhibitory signal are known, as are some of the steps in the signal transduction pathway that is responsible for inhibition. In addition, developmental changes in the environment and in the neurons themselves are also now better understood. This knowledge in turn reveals novel, putative sites for drug development and therapeutic intervention after injury to the brain and spinal cord. The challenge now is to determine which of these putative treatments are the most effective and if they would be better applied in combination rather than alone. In this review I will summarize what we have learnt about these molecules and how they signal. Importantly, I will also describe approaches that have been shown to block inhibitors and encourage regeneration in vivo. I will also speculate on what the differences are between the neonatal and adult CNS that allow the former to regenerate and the latter not to.

Study of the spinal cords of the sturgeonAcipenser schrenckii, garLepisosteus oculatus, and goldfishCarassius auratus by morphological, immunohistochemical, and biochemical approaches

Little is known about the spinal cords of phylogenetically ancient actinopterygeans. The spinal cords of the chondrostean Acipenser schrenckii (Amur sturgeon), holostean Lepisosteus oculatus (spotted gar), and teleost Carassius auratus (goldfish) were, therefore, analyzed by immunohistochemistry, electron microscopy and two-dimensional gel electrophoresis. Morphology showed numerous similarities between sturgeons and gars. In both, a dorsal column between the two dorsal horns was lacking, giving the grey matter an inverted Y-shape. In goldfish, a small dorsal column was seen, the grey matter occupied a larger area, neuronal density was much higher, and a ventral commissure was apparent, which was absent in sturgeons and gars. In the white matter of sturgeons and gars, small caliber axons predominated, whereas larger axons were frequent in goldfish. Choline acetyltransferase immunoreactive neurons were prevalent in the ventral horns of all three fish, mainly in motoneurons, but stained fibers were only found in sturgeons and gars. gamma-aminobutyric acid positive cells were seen in both the ventral and the dorsal horns of all three fish. Distribution of serotonin (5-HT) and tyrosine hydroxylase (TH) immunoreaction was similar in sturgeons and gars, being located in both the ventral and the dorsal horns. In goldfish, 5-HT label was confined to the ventral horn and TH label was mainly observed in a cell group located ventromedially. Two-dimensional gel electrophoresis showed a gradual increase in protein number from sturgeons to gars to goldfish. In conclusion, the spinal cords of sturgeons and gars share many morphological and chemical features, distinguishing them from the goldfish spinal cord.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Can regenerating axons recapitulate developmental guidance during recovery from spinal cord injury?

The precise wiring of the adult mammalian CNS originates during a period of stunning growth, guidance and plasticity that occurs during and shortly after development. When injured in adults, this intricate system fails to regenerate. Even when the obstacles to regeneration are cleared, growing adult CNS fibres usually remain misdirected and fail to reform functional connections. Here, we attempt to fill an important niche related to the topics of nervous system development and regeneration. We specifically contrast the difficulties faced by growing fibres within the adult context to the precise circuit-forming capabilities of developing fibres. In addition to focusing on methods to stimulate growth in the adult, we also expand on approaches to recapitulate development itself.

Glial inhibition of CNS axon regeneration

Damage to the adult CNS often leads to persistent deficits due to the inability of mature axons to regenerate after injury. Mounting evidence suggests that the glial environment of the adult CNS, which includes inhibitory molecules in CNS myelin as well as proteoglycans associated with astroglial scarring, might present a major hurdle for successful axon regeneration. Here, we evaluate the molecular basis of these inhibitory influences and their contributions to the limitation of long-distance axon repair and other types of structural plasticity. Greater insight into glial inhibition is crucial for developing therapies to promote functional recovery after neural injury.

Tail regeneration and ependymal outgrowth in the adult newt, Notophthalmus viridescens, are adversely affected by experimentally produced ischemia

Spinal axons of the adult newt will regenerate when the spinal cord is severed or when the tail is amputated. Ischemia and associated hypoxia have been correlated with poor central nervous system regeneration in mammals. To test the effects of ischemia on newt spinal cord regeneration, the spinal cord and major blood vessels of the newt tail were severed 2 cm caudal to the cloaca as a primary injury. This primary injury severely reduced circulation in the caudal direction for 7 days; by day 8, circulation was largely restored. After various periods of time after primary injury, tails were amputated 1 cm caudal to the primary injury (in the area of ischemia) and tested for regeneration. If the tail was amputated within 5 days of the primary injury, regeneration did not occur. If amputation was 7 days or longer after the primary injury, a regenerative response occurred. Histology showed that in the non-regenerating tails the spinal cord and associated ependyma, known to be important to tail regeneration, had degenerated in the rostral direction. Such degeneration was prevented when tails were first amputated and allowed to form blastemas before the primary injury. The data indicate that the first 5-7 days of blastema formation are particularly sensitive to compromised blood flow (ischemia/hypoxia). It follows that mechanisms must be present in the adult newt to reduce ischemia to a minimum and thus allow ependymal outgrowth and tail regeneration.

The Developing Human Spinal Cord Contains Distinct Populations of Neural Precursors

It is becoming apparent that neural stem cells display some differences in their behaviour depending on the region of the CNS they originate from and on whether they are derived from embryonic or adult tissue. Whereas much work has focused on brain neural stem cells, less attention has been paid to spinal cord neural precursors, particularly in the developing human embryo. We briefly review here some of our work which points at some similarities between neural precursors in developing human spinal cords and in animals which can regenerate their spinal cord (e.g. tailed amphibians), and at differences in the properties of human neural precursors with spinal cord development. Altogether these studies suggest the existence of dynamic neural stem cell populations within the developing spinal cord. They also support the notion that thorough characterization of neural stem cells under different culture conditions and analysis of how these may affect their differentiation in vivo after grafting into different injury models is imperative if we are to develop effective cell therapy strategies for spinal cord injury and diseases.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.

Inhibition of collagen synthesis overrides the age-related failure of regeneration of nigrostriatal dopaminergic axons

To investigate the mechanism of the age-related failure of regeneration of transected axons, nigrostriatal dopaminergic axons were unilaterally transected in the lateral hypothalamus in adult mice and in immature mice aged postnatal days 7, 14, and 21. Ten days after the transection, tyrosine hydroxylase-immunoreactive axons had regenerated from caudally to rostrally across the lesion site in mice transected at postnatal day 7, whereas they stopped and did not extend across the lesion site in mice transected at postnatal day 14 or older. Reactive astrocytes bearing chondroitin sulfate proteoglycans were observed around the lesion in mice transected at all ages. However, a fibrotic scar containing type IV collagen-immunoreactive deposits, which was consistently formed at the lesion site in mice transected at postnatal day 14 or older, was not formed in mice lesioned at postnatal day 7. When 2,2'-dipyridyl, an inhibitor of collagen synthesis, was injected into the lesion site at the time of transection in both postnatal day 14 and adult mice, the deposition of type IV collagen and the formation of a fibrotic scar were completely prevented, and large numbers of tyrosine hydroxylase-immunoreactive axons extended across the lesion and reinnervated the striatum. These results imply that the fibrotic scar formed in the lesion site is a crucial impediment to the regeneration of ascending dopaminergic axons in adult mice and suggest that type IV collagen is required for the development of the fibrotic response to adult brain injury.

Anatomical organization of motoneurons and interneurons in the mudpuppy (Necturus maculosus) brachial spinal cord: the neural substrate for central pattern generation

The isolated brachial spinal cord of the mudpuppy is useful for studies of neural networks underlying forelimb locomotion, but information about its anatomy is scarce. We addressed this issue by combining retrograde labeling with fluorescent tracers and confocal microscopy. Remarkably, the central region of gray matter was aneural and contained only a tenuous meshwork of glial fibers and large extracellular spaces. Somata of motoneurons (MNs) and interneurons (INs), labeled retrogradely from ventral roots or axons in the ventro-lateral funiculus, respectively, were confined within a gray neuropil layer abutting the white matter borders, while their dendrites projected widely throughout the white matter. A considerable fraction of labeled INs was found contralaterally with axons crossing beneath a thick layer of ependyma surrounding the central canal. Dorsal roots (DRs) produced dense presynaptic arbors within a restricted dorsal region containing afferent terminations, within which dorsally directed MN and IN dendrites mingled with dense collections of synaptic boutons. Our data suggest that a major fraction of synaptic interactions takes place within the white matter. This study provides a detailed foundation for electrophysiological experiments aimed at elucidating the neural circuits involved in locomotor pattern generation.

Nogo and Nogo-66 receptor in human and chick: implications for development and regeneration

Antibodies to the myelin protein Nogo increase axonal regrowth after central nervous system injury. We have investigated whether Nogo expression contributes to loss of regenerative potential during development by using chick embryos, which regenerate their spinal cord until embryonic day (E) 13, when myelination begins. We show that Nogo-A and the Nogo receptor (NgR) are developmentally regulated both in chick and human embryos, are first detected at developmental stages when the chick spinal cord regenerates, and are not down-regulated after injury at permissive stages for regeneration. Therefore, expression of Nogo-A and NgR in pre-E13 chick spinal cords is not sufficient to inhibit regeneration. Nogo-A expression in the chick early embryo is primarily observed in axons, whereas NgR is mainly located on neuronal cell bodies, both in spinal cord and eye, and in striated muscle including the heart. With the onset of myelination, there is down-regulation of Nogo-A expression in neurons. Therefore, loss of regenerative potential might be linked to changes in its cellular localization. The possibility that only Nogo expressed in mature oligodendrocytes can exercise inhibitory effects would reconcile the lack of inhibition we observe in developing chick spinal cords before the onset of myelination with evidence from other laboratories on the inhibitory effects of Nogo in mature central nervous system. The distinctive and complementary patterns of Nogo-A and NgR expression and their conservation throughout evolution support the view that Nogo signaling represents a key pathway in nervous system and striated muscle development. Its putative role in target innervation and establishment of neural circuitry is discussed.

CNS regeneration: A morphogen's tale

Tissue regeneration will soon become an avenue for repair of damaged or diseased tissues as stem cell niches have been found in almost every organ of the vertebrate body including the CNS. In addition, different animals display an array of regenerative capabilities that are currently being researched to dissect the molecular mechanisms involved. This review concentrates on the different ways in which CNS tissues such as brain, spinal cord and retina can regenerate or display neurogenic potential and how these abilities are modulated by morphogens.

Cloning and analysis of axolotl ISL2 and LHX2 LIM-homeodomain transcription factors

We cloned and characterized the ISL2 and LHX2 LIM-homeodomain transcription factors of the Mexican salamander, or axolotl, Ambystoma mexicanum. Using a degenerate PCR approach, partial cDNAs representing five LIM-homeodomain genes were cloned, indicating conservation of this class of transcription factors in urodeles. Full-length cDNAs for Isl2 and Lhx2 were identified and sequenced. The predicted ISL2 and LHX2 proteins are well conserved, especially in the LIM and DNA-binding domains. The Isl2 and Lhx2 genes are expressed at all examined stages of embryogenesis and display tissue-restricted expression patterns in adults. In functional tests, axolotl LHX2 was inactive compared to homologous mammalian factors and adopted unusual DNA/protein complexes. However, axolotl ISL2 bound and induced transcription from the rat insulin gene. These experiments demonstrate conservation of key developmental regulatory proteins in salamanders and will allow future studies of their potential roles in the molecular regulation of tissue regeneration in such species.

Functional recovery and axonal growth following spinal cord transection is accelerated by sustained l-DOPA administration

The eel, Anguilla anguilla, as with other fish species, recovers well from spinal cord injury. We assessed the quality of locomotion of spinally transected eels from measurements made from video recordings of individuals swimming at different speeds in a water tunnel. Following transection of the spinal cord just caudal to the anus, the animals displayed higher tail beat frequencies and lower tail beat amplitudes than before surgery, owing to the loss of power in this region. Swimming performance then progressively recovered, appearing normal within 1 month of surgery. Eels with similar transections, but given regular, repeated intraperitoneal injections (50 mg/kg) of l-3,4-dihydroxyphenylalanine (L-DOPA) showed an equivalent pattern of decline and recovery that was 10-20 days shorter than that seen in non-treated fish. Axonal growth into the denervated cord, as determined from anterograde labelling experiments, was also more rapid in the drug-treated fish. L-DOPA treatment increased the activity of all fish for up to 18 h, and accelerated the spontaneous movements ('spinal swimming') made by the denervated, caudal portion of the animal that appeared following transection. We suggest that this enhancement of locomotion underlies the accelerated axonal growth and, hence, functional recovery.