Reaction of spinal cord central canal cells to cord transection and their contribution to cord regeneration

Involvement of vimentin- and BLBP-positive glial cells and their MMP expression in axonal regeneration after spinal cord transection in goldfish

In goldfish, spinal cord injury triggers the formation of a fibrous scar at the injury site. Regenerating axons are able to penetrate the scar tissue, resulting in the recovery of motor function. Previous findings suggested that regenerating axons enter the scar through tubular structures surrounded by glial elements with laminin-positive basement membranes and that glial processes expressing glial fibrillary acidic protein (GFAP) are associated with axonal regeneration. How glia contribute to promoting axonal regeneration, however, is unknown. Here, we revealed that glial processes expressing vimentin or brain lipid-binding protein (BLBP) also enter the fibrous scar after spinal cord injury in goldfish. Vimentin-positive glial processes were more numerous than GFAP- or BLBP-positive glial processes in the scar tissue. Regenerating axons in the scar tissue were more closely associated with vimentin-positive glial processes than GFAP-positive glial processes. Vimentin-positive glial processes co-expressed matrix metalloproteinase (MMP)-14. Our findings suggest that vimentin-positive glial processes closely associate with regenerating axons through tubular structures entering the scar after spinal cord injury in goldfish. In intact spinal cord, ependymo-radial glial cell bodies express BLBP and their radial processes express vimentin, suggesting that vimentin-positive glial processes derive from migrating ependymo-radial glial cells. MMP-14 expressed in vimentin-positive glial cells and their processes might provide a beneficial environment for axonal regeneration.

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

Vof16-miR-185-5p-GAP43 network improves the outcomes following spinal cord injury via enhancing self-repair and promoting axonal growth

INTRODUCTION

Self-repair of spinal cord injury (SCI) has been found in humans and experimental animals with partial recovery of neurological functions. However, the regulatory mechanisms underlying the spontaneous locomotion recovery after SCI are elusive.

AIMS

This study was aimed at evaluating the pathological changes in injured spinal cord and exploring the possible mechanism related to the spontaneous recovery.

RESULTS

Immunofluorescence staining was performed to detect GAP43 expression in lesion site after spinal cord transection (SCT) in rats. Then RNA sequencing and gene ontology (GO) analysis were employed to predict lncRNA that correlates with GAP43. LncRNA smart-silencing was applied to verify the function of lncRNA vof16 in vitro, and knockout rats were used to evaluate its role in neurobehavioral functions after SCT. MicroRNA sequencing, target scan, and RNA22 prediction were performed to further explore the underlying regulatory mechanisms, and miR-185-5p stands out. A miR-185-5p site-regulated relationship with GAP43 and vof16 was determined by luciferase activity analysis. GAP43-silencing, miR-185-5p-mimic/inhibitor, and miR-185-5p knockout rats were also applied to elucidate their effects on spinal cord neurite growth and neurobehavioral function after SCT. We found that a time-dependent increase of GAP43 corresponded with the limited neurological recovery in rats with SCT. CRNA chip and GO analysis revealed lncRNA vof16 was the most functional in targeting GAP43 in SCT rats. Additionally, silencing vof16 suppressed neurite growth and attenuated the motor dysfunction in SCT rats. Luciferase reporter assay showed that miR-185-5p competitively bound the same regulatory region of vof16 and GAP43.

CONCLUSIONS

Our data indicated miR-185-5p could be a detrimental factor in SCT, and vof16 may function as a ceRNA by competitively binding miR-185-5p to modulate GAP43 in the process of self-recovery after SCT. Our study revealed a novel vof16-miR-185-5p-GAP43 regulatory network in neurological self-repair after SCT and may underlie the potential treatment target for SCI.

Astrocyte-Neuron Interactions in Spinal Cord Injury

Spinal cord injuries cause irreversible loss of sensory and motor functions. In mammals, intrinsic and extrinsic inhibitions of neuronal regeneration obstruct neural repair after spinal cord injury. Although astrocytes have been involved in a growing list of vital homeostatic functions in the nervous system, their roles after injury have fascinated and puzzled scientists for decades. Astrocytes undergo long-lasting morphological and functional changes after injury, referred to as reactive astrogliosis. Although reactive astrogliosis is required to contain spinal cord lesions and restore the blood-spinal cord barrier, reactive astrocytes have detrimental effects that inhibit neuronal repair and remyelination. Intriguingly, elevated regenerative capacity is preserved in some non-mammalian vertebrates, where astrocyte-like glial cells display exclusively pro-regenerative effects after injury. A detailed molecular and phenotypic catalog of the continuum of astrocyte reactivity states is an essential first step toward the development of glial cell manipulations for spinal cord repair.

Small leucine-rich proteoglycans inhibit CNS regeneration by modifying the structural and mechanical properties of the lesion environment

Extracellular matrix (ECM) deposition after central nervous system (CNS) injury leads to inhibitory scarring in humans and other mammals, whereas it facilitates axon regeneration in the zebrafish. However, the molecular basis of these different fates is not understood. Here, we identify small leucine-rich proteoglycans (SLRPs) as a contributing factor to regeneration failure in mammals. We demonstrate that the SLRPs chondroadherin, fibromodulin, lumican, and prolargin are enriched in rodent and human but not zebrafish CNS lesions. Targeting SLRPs to the zebrafish injury ECM inhibits axon regeneration and functional recovery. Mechanistically, we find that SLRPs confer mechano-structural properties to the lesion environment that are adverse to axon growth. Our study reveals SLRPs as inhibitory ECM factors that impair axon regeneration by modifying tissue mechanics and structure, and identifies their enrichment as a feature of human brain and spinal cord lesions. These findings imply that SLRPs may be targets for therapeutic strategies to promote CNS regeneration.

Progenitor-derived glia are required for spinal cord regeneration in zebrafish

Unlike mammals, adult zebrafish undergo spontaneous recovery after major spinal cord injury. Whereas reactive gliosis presents a roadblock for mammalian spinal cord repair, glial cells in zebrafish elicit pro-regenerative bridging functions after injury. Here, we perform genetic lineage tracing, assessment of regulatory sequences and inducible cell ablation to define mechanisms that direct the molecular and cellular responses of glial cells after spinal cord injury in adult zebrafish. Using a newly generated CreERT2 transgenic line, we show that the cells directing expression of the bridging glial marker ctgfa give rise to regenerating glia after injury, with negligible contribution to either neuronal or oligodendrocyte lineages. A 1 kb sequence upstream of the ctgfa gene was sufficient to direct expression in early bridging glia after injury. Finally, ablation of ctgfa-expressing cells using a transgenic nitroreductase strategy impaired glial bridging and recovery of swim behavior after injury. This study identifies key regulatory features, cellular progeny, and requirements of glial cells during innate spinal cord regeneration.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

The neural stem cell properties of Pkd2l1+ cerebrospinal fluid-contacting neurons in vivo

The neural stem cells (NSCs) in the ventricular-subventricular zone of the adult mammalian spinal cord may be of great benefit for repairing spinal cord injuries. However, the sources of NSCs remain unclear. Previously, we have confirmed that cerebrospinal fluid-contacting neurons (CSF-cNs) have NSC potential in vitro. In this study, we verified the NSC properties of CSF-cNs in vivo. In mouse spinal cords, Pkd2l1+ CSF-cNs localized around the central canal express NSC markers. In vitro, Pkd2l1+ CSF-cNs form a neurosphere and express NSC markers. Activation and proliferation of CSF-cNs can be induced by injection of the neurotrophic factors basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) into the lateral ventricle. Spinal cord injury (SCI) also induces NSC activation and proliferation of CSF-cNs. Collectively, our results demonstrate that Pkd2l1+ CSF-cNs have NSC properties in vivo and may be involved in SCI recovery.

Radial Glia and Neuronal-like Ependymal Cells Are Present within the Spinal Cord of the Trunk (Body) in the Leopard Gecko (Eublepharis macularius)

As is the case for many lizards, leopard geckos (Eublepharis macularius) can self-detach a portion of their tail to escape predation, and then regenerate a replacement complete with a spinal cord. Previous research has shown that endogenous populations of neural stem/progenitor cells (NSPCs) reside within the spinal cord of the original tail. In response to tail loss, these NSPCs are activated and contribute to regeneration. Here, we investigate whether similar populations of NSPCs are found within the spinal cord of the trunk (body). Using a long-duration 5-bromo-2′-deoxyuridine pulse-chase experiment, we determined that a population of cells within the ependymal layer are label-retaining following a 20-week chase. Tail loss does not significantly alter rates of ependymal cell proliferation within the trunk spinal cord. Ependymal cells of the trunk spinal cord express SOX2 and represent at least two distinct cell populations: radial glial-like (glial fibrillary acidic protein- and Vimentin-expressing) cells; and neuronal-like (HuCD-expressing) cells. Taken together, these data demonstrate that NSPCs of the trunk spinal cord closely resemble those of the tail and support the use of the tail spinal cord as a less invasive proxy for body spinal cord injury investigations.

Know How to Regrow-Axon Regeneration in the Zebrafish Spinal Cord

The capacity for long-distance axon regeneration and functional recovery after spinal cord injury is poor in mammals but remarkable in some vertebrates, including fish and salamanders. The cellular and molecular basis of this interspecies difference is beginning to emerge. This includes the identification of target cells that react to the injury and the cues directing their pro-regenerative responses. Among existing models of successful spinal cord regeneration, the zebrafish is arguably the most understood at a mechanistic level to date. Here, we review the spinal cord injury paradigms used in zebrafish, and summarize the breadth of neuron-intrinsic and -extrinsic factors that have been identified to play pivotal roles in the ability of zebrafish to regenerate central nervous system axons and recover function.

Adult neurogenesis in the central nervous system of teleost fish: from stem cells to function and evolution

Adult neurogenesis, the generation of functional neurons from adult neural stem cells in the central nervous system (CNS), is widespread, and perhaps universal, among vertebrates. This phenomenon is more pronounced in teleost fish than in any other vertebrate taxon. There are up to 100 neurogenic sites in the adult teleost brain. New cells, including neurons and glia, arise from neural stem cells harbored both in neurogenic niches and outside these niches (such as the ependymal layer and parenchyma in the spinal cord, respectively). At least some, but not all, of the stem cells are of astrocytic identity. Aging appears to lead to stem cell attrition in fish that exhibit determinate body growth but not in those with indeterminate growth. At least in some areas of the CNS, the activity of the neural stem cells results in additive neurogenesis or gliogenesis - tissue growth by net addition of cells. Mathematical and computational modeling has identified three factors to be crucial for sustained tissue growth and correct formation of CNS structures: symmetric stem cell division, cell death and cell drift due to population pressure. It is hypothesized that neurogenesis in the CNS is driven by continued growth of corresponding muscle fibers and sensory receptor cells in the periphery to ensure a constant ratio of peripheral versus central elements. This 'numerical matching hypothesis' can explain why neurogenesis has ceased in most parts of the adult CNS during the evolution of mammals, which show determinate growth.

Building bridges, not walls: spinal cord regeneration in zebrafish

Spinal cord injury is a devastating condition in which massive cell death and disruption of neural circuitry lead to long-term chronic functional impairment and paralysis. In mammals, spinal cord tissue has minimal capacity to regenerate after injury. In stark contrast, the regeneration of a completely transected spinal cord and accompanying reversal of paralysis in adult zebrafish is arguably one of the most spectacular biological phenomena in nature. Here, we review reports from the last decade that dissect the mechanisms of spinal cord regeneration in zebrafish. We highlight recent progress as well as areas requiring emphasis in a line of study that has great potential to uncover strategies for human spinal cord repair.

Summary: Unlike mammals, teleost fish are capable of efficient, spontaneous recovery after a paralyzing spinal cord injury. Here, we highlight the major events through which laboratory model zebrafish regenerate spinal cord tissue.

Unlocking the paradoxical endogenous stem cell response after spinal cord injury

Nearly a century ago, the concept of the secondary injury in spinal cord trauma was first proposed to explain the complex cascade of molecular and cellular events leading to widespread neuronal and glial cell death after trauma. In recent years, it has been established that the ependymal region of the adult mammalian spinal cord contains a population of multipotent neural stem/progenitor cells (NSPCs) that are activated after spinal cord injury (SCI) and likely play a key role in endogenous repair and regeneration. How these cells respond to the various components of the secondary injury remains poorly understood. Emerging evidence suggests that many of the biochemical components of the secondary injury cascade which have classically been viewed as deleterious to host neuronal and glial cells may paradoxically trigger NSPC activation, proliferation, and differentiation thus challenging our current understanding of secondary injury mechanisms in SCI. Herein, we highlight new findings describing the response of endogenous NSPCs to spinal cord trauma, redefining the secondary mechanisms of SCI through the lens of the endogenous population of stem/progenitor cells. Moreover, we outline how these insights can fuel novel stem cell-based therapeutic strategies to repair the injured spinal cord.

Cellular Dynamics during Spinal Cord Regeneration in Larval Zebrafish

The study of spinal cord regeneration using diverse animal models, which range from null to robust regenerative capabilities, is imperative for understanding how regeneration evolved and, eventually, to treat spinal cord injury and paralysis in humans. In this study, we used electroablation to fully transect the spinal cord of zebrafish larvae (3 days postfertilization) and examined regeneration of the tissue over time. We used transgenic lines to follow immune cells, oligodendrocytes, and neurons in vivo during the entire regenerative process. We observed that immune cells are recruited to the injury site, oligodendrocytes progenitor cells (olig2-expressing cells) invade, and axons cross the gap generated upon damage from anterior to reinnervate caudal structures. Together with the recovery of cell types and structures, a complete reversal of paralysis was observed in the lesioned larvae indicating functional regeneration. Finally, using transplantation to obtain mosaic larvae with single-labeled neurons, we show that severed spinal axons exhibited varying regenerative capabilities and plasticity depending on their original dorsoventral position in the spinal cord.

Wnt/β-catenin signaling regulates ependymal cell development and adult homeostasis

In the adult mouse spinal cord, the ependymal cell population that surrounds the central canal is thought to be a promising source of quiescent stem cells to treat spinal cord injury. Relatively little is known about the cellular origin of ependymal cells during spinal cord development, or the molecular mechanisms that regulate ependymal cells during adult homeostasis. Using genetic lineage tracing based on the Wnt target gene Axin2, we have characterized Wnt-responsive cells during spinal cord development. Our results revealed that Wnt-responsive progenitor cells are restricted to the dorsal midline throughout spinal cord development, which gives rise to dorsal ependymal cells in a spatially restricted pattern. This is contrary to previous reports that suggested an exclusively ventral origin of ependymal cells, suggesting that ependymal cells may retain positional identities in relation to their neural progenitors. Our results further demonstrated that in the postnatal and adult spinal cord, all ependymal cells express the Wnt/β-catenin signaling target gene Axin2, as well as Wnt ligands. Genetic elimination of β-catenin or inhibition of Wnt secretion in Axin2-expressing ependymal cells in vivo both resulted in impaired proliferation, indicating that Wnt/β-catenin signaling promotes ependymal cell proliferation. These results demonstrate the continued importance of Wnt/β-catenin signaling for both ependymal cell formation and regulation. By uncovering the molecular signals underlying the formation and regulation of spinal cord ependymal cells, our findings thus enable further targeting and manipulation of this promising source of quiescent stem cells for therapeutic interventions.

Spinal Cord Stem Cells In Their Microenvironment: The Ependyma as a Stem Cell Niche

The ependyma of the spinal cord is currently proposed as a latent neural stem cell niche. This chapter discusses recent knowledge on the developmental origin and nature of the heterogeneous population of cells that compose this stem cell microenviroment, their diverse physiological properties and regulation. The chapter also reviews relevant data on the ependymal cells as a source of plasticity for spinal cord repair.

Neural stem/progenitor cells are activated during tail regeneration in the leopard gecko (Eublepharis macularius)

As for many lizards, the leopard gecko (Eublepharis macularius) can self-detach its tail to avoid predation and then regenerate a replacement. The replacement tail includes a regenerated spinal cord with a simple morphology: an ependymal layer surrounded by nerve tracts. We hypothesized that cells within the ependymal layer of the original spinal cord include populations of neural stem/progenitor cells (NSPCs) that contribute to the regenerated spinal cord. Prior to tail loss, we performed a bromodeoxyuridine pulse-chase experiment and found that a subset of ependymal layer cells (ELCs) were label-retaining after a 140-day chase period. Next, we conducted a detailed spatiotemporal characterization of these cells before, during, and after tail regeneration. Our findings show that SOX2, a hallmark protein of NSPCs, is constitutively expressed by virtually all ELCs before, during, and after regeneration. We also found that during regeneration, ELCs express an expanded panel of NSPC and lineage-restricted progenitor cell markers, including MSI-1, SOX9, and TUJ1. Using electron microscopy, we determined that multiciliated, uniciliated, and biciliated cells are present, although the latter was only observed in regenerated spinal cords. Our results demonstrate that cells within the ependymal layer of the original, regenerating and fully regenerate spinal cord represent a heterogeneous population. These include radial glia comparable to Type E and Type B cells, and a neuronal-like population of cerebrospinal fluid-contacting cells. We propose that spinal cord regeneration in geckos represents a truncation of the restorative trajectory observed in some urodeles and teleosts, resulting in the formation of a structurally distinct replacement.

Light on a sensory interface linking the cerebrospinal fluid to motor circuits in vertebrates

The cerebrospinal fluid (CSF) is circulating around the entire central nervous system (CNS). The main function of the CSF has been thought to insure the global homeostasis of the CNS. Recent evidence indicates that the CSF also dynamically conveys signals modulating the development and the activity of the nervous system. The later observation implies that cues from the CSF could act on neurons in the brain and the spinal cord via bordering receptor cells. Candidate neurons to enable such modulation are the cerebrospinal fluid-contacting neurons (CSF-cNs) that are located precisely at the interface between the CSF and neuronal circuits. The atypical apical extension of CSF-cNs bears a cluster of microvilli bathing in the CSF indicating putative sensory or secretory roles in relation with the CSF. In the brainstem and spinal cord, CSF-cNs have been described in over two hundred species by Kolmer and Agduhr, suggesting an important function within the spinal cord. However, the lack of specific markers and the difficulty to access CSF-cNs hampered their physiological investigation. The transient receptor potential channel PKD2L1 is a specific marker of spinal CSF-cNs in vertebrate species. The transparency of zebrafish at early stages eases the functional characterization of pkd2l1⁺ CSF-cNs. Recent studies demonstrate that spinal CSF-cNs detect spinal curvature via the channel PKD2L1 and modulate locomotion and posture by projecting onto spinal interneurons and motor neurons in vivo. In vitro recordings demonstrated that spinal CSF-cNs are sensing pH variations mainly through ASIC channels, in combination with PKD2L1. Altogether, neurons contacting the CSF appear as a novel sensory modality enabling the detection of mechanical and chemical stimuli from the CSF and modulating the excitability of spinal circuits underlying locomotion and posture.

Wnt signaling controls pro-regenerative Collagen XII in functional spinal cord regeneration in zebrafish

The inhibitory extracellular matrix in a spinal lesion site is a major impediment to axonal regeneration in mammals. In contrast, the extracellular matrix in zebrafish allows substantial axon re-growth, leading to recovery of movement. However, little is known about regulation and composition of the growth-promoting extracellular matrix. Here we demonstrate that activity of the Wnt/β-catenin pathway in fibroblast-like cells in the lesion site is pivotal for axon re-growth and functional recovery. Wnt/β-catenin signaling induces expression of col12a1a/b and deposition of Collagen XII, which is necessary for axons to actively navigate the non-neural lesion site environment. Overexpression of col12a1a rescues the effects of Wnt/β-catenin pathway inhibition and is sufficient to accelerate regeneration. We demonstrate that in a vertebrate of high regenerative capacity, Wnt/β-catenin signaling controls the composition of the lesion site extracellular matrix and we identify Collagen XII as a promoter of axonal regeneration. These findings imply that the Wnt/β-catenin pathway and Collagen XII may be targets for extracellular matrix manipulations in non-regenerating species.

Following spinal injury in zebrafish, non-neural cells establish an extracellular matrix to promote axon re-growth but how this is regulated is unclear. Here, the authors show that Wnt/β-catenin signaling in fibroblast-like cells at a lesion activates axon re-growth via deposition of Collagen XII.

Additive neurogenesis supported by multiple stem cell populations mediates adult spinal cord development: A spatiotemporal statistical mapping analysis in a teleost model of indeterminate growth

The knifefish Apteronotus leptorhynchus exhibits indeterminate growth throughout adulthood. This phenomenon extends to the spinal cord, presumably through the continuous addition of new neurons and glial cells. However, little is known about the developmental dynamics of cells added during adult growth. The present work characterizes the structural and functional development of the adult spinal cord in this model organism through a comprehensive quantitative analysis of the spatial and temporal dynamics of new cells at various developmental stages. This analysis, based on a novel statistical mapping approach, revealed within the adult spinal cord a wide distribution of both mitotically active and quiescent Sox2-expressing stem/progenitor cells (SPCs). While such cells are particularly concentrated within the ependymal layer near the central canal, the majority of them reside in the parenchyma, resembling the distribution of SPCs observed in the mammalian spinal cord. The active SPCs in the adult knifefish spinal cord give rise to transit amplifying progenitor cells that undergo a few additional mitotic divisions before developing into Hu C/D+ neurons and S100+ glial cells. There is no evidence of long-distance migration of the newborn cells. The persistence of cell proliferation and differentiation, combined with low levels of apoptosis, leads to a continuous addition of cells to the existing tissue. Newly generated neurons have functional and behavioral relevance, as indicated by the integration of axons of new electromotor neurons into the electric organ of these weakly electric fish. This results in a gradual increase in the amplitude of the electric organ discharge during adult development. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1269-1307, 2017.

Neurogenesis in the lamprey central nervous system following spinal cord transection

After spinal cord transection, lampreys recover functionally and axons regenerate. It is not known whether this is accompanied by neurogenesis. Previous studies suggested a baseline level of nonneuronal cell proliferation in the spinal cord and rhombencephalon (where most supraspinal projecting neurons are located). To determine whether cell proliferation increases after injury and whether this includes neurogenesis, larval lampreys were spinally transected and injected with 5-bromo-2&prime-deoxyuridine (BrdU) at 0-3 weeks posttransection. Labeled cells were counted in the lesion site, within 0.5 mm rostral and caudal to the lesion, and in the rhombencephalon. One group of animals was processed in the winter and a second group was processed in the summer. The number of labeled cells was greater in winter than in summer. The lesion site had the most BrdU labeling at all times, correlating with an increase in the number of cells. In the adjacent spinal cord, the percentage of BrdU labeling was higher in the ependymal than in nonependymal regions. This was also true in the rhombencephalon but only in summer. In winter, BrdU labeling was seen primarily in the subventricular and peripheral zones. Some BrdU-labeled cells were also double labeled by antibodies to glial-specific (antikeratin) as well as neuron-specific (anti-Hu) antigens, indicating that both gliogenesis and neurogenesis occurred after spinal cord transection. However, the new neurons were restricted to the ependymal zone, were never labeled by antineurofilament antibodies, and never migrated away from the ependyma even at 5 weeks after BrdU injection. They would appear to be cerebrospinal fluid-contacting neurons.

Decreased GFAP expression and improved functional recovery in contused spinal cord of rats following valproic acid therapy

Many studies have illustrated that much of the post-traumatic degeneration of the spinal cord cells is caused by the secondary mechanism. The aim of this study is to evaluate the effect of the anti-inflammatory property of valproic acid (VPA) on injured spinal cords (SC). The rats with the contused SC received intraperitoneal single injection of VPA (150, 200, 300, 400 or 500 mg/kg) at 2, 6, 12 and 24 h post-injury. Basso-Beattie-Bresnahan (BBB) test and H-reflex evaluated the functional outcome for 12 weeks. The SC were investigated 3 months post-injury using morphometry and glial fibrillary acid protein (GFAP) expression. Reduction in cavitation, H/M ratio, BBB scores and GFAP expression in the treatment groups were significantly more than that of the untreated one (P < 0.05). The optimal improvement in the condition of the contused rats was in the ones treated at the acute phase of injury with 300 mg/kg of VPA at 12 h post-injury, they had the highest increase in BBB score and decrease in astrogliosis and axonal loss. We conclude that treating the contused rats with 300 mg/kg of VPA at 12 h post-injury improves the functional outcome and reduces the traumatized SC gliosis.

Neuronal release and successful astrocyte uptake of aminoacidergic neurotransmitters after spinal cord injury in lampreys

In contrast to mammals, the spinal cord of lampreys spontaneously recovers from a complete spinal cord injury (SCI). Understanding the differences between lampreys and mammals in their response to SCI could provide valuable information to propose new therapies. Unique properties of the astrocytes of lampreys probably contribute to the success of spinal cord regeneration. The main aim of our study was to investigate, in the sea lamprey, the release of aminoacidergic neurotransmitters and the subsequent astrocyte uptake of these neurotransmitters during the first week following a complete SCI by detecting glutamate, GABA, glycine, Hu and cytokeratin immunoreactivities. This is the first time that aminoacidergic neurotransmitter release from neurons and the subsequent astrocytic response after SCI are analysed by immunocytochemistry in any vertebrate. Spinal injury caused the immediate loss of glutamate, GABA and glycine immunoreactivities in neurons close to the lesion site (except for the cerebrospinal fluid-contacting GABA cells). Only after SCI, astrocytes showed glutamate, GABA and glycine immunoreactivity. Treatment with an inhibitor of glutamate transporters (DL-TBOA) showed that neuronal glutamate was actively transported into astrocytes after SCI. Moreover, after SCI, a massive accumulation of inhibitory neurotransmitters around some reticulospinal axons was observed. Presence of GABA accumulation significantly correlated with a higher survival ability of these neurons. Our data show that, in contrast to mammals, astrocytes of lampreys have a high capacity to actively uptake glutamate after SCI. GABA may play a protective role that could explain the higher regenerative and survival ability of specific descending neurons of lampreys.

Endogenous proliferation after spinal cord injury in animal models

Spinal cord injury (SCI) results in motor and sensory deficits, the severity of which depends on the level and extent of the injury. Animal models for SCI research include transection, contusion, and compression mouse models. In this paper we will discuss the endogenous stem cell response to SCI in animal models. All SCI animal models experience a similar peak of cell proliferation three days after injury; however, each specific type of injury promotes a specific and distinct stem cell response. For example, the transection model results in a strong and localized initial increase of proliferation, while in contusion and compression models, the initial level of proliferation is lower but encompasses the entire rostrocaudal extent of the spinal cord. All injury types result in an increased ependymal proliferation, but only in contusion and compression models is there a significant level of proliferation in the lateral regions of the spinal cord. Finally, the fate of newly generated cells varies from a mainly oligodendrocyte fate in contusion and compression to a mostly astrocyte fate in the transection model. Here we will discuss the potential of endogenous stem/progenitor cell manipulation as a therapeutic tool to treat SCI.

Spinal transection induces widespread proliferation of cells along the length of the spinal cord in a weakly electric fish

The ability to regenerate spinal cord tissue after tail amputation has been well studied in several species of teleost fish. The present study examined the proliferation and survival of cells following complete spinal cord transection rather than tail amputation in the weakly electric fish Apteronotus leptorhynchus. To quantify cell proliferation along the length of the spinal cord, fish were given a single bromodeoxyuridine (BrdU) injection immediately after spinal transection or sham surgery. Spinal transection significantly increased the density of BrdU⁺ cells along the entire length of the spinal cord at 1 day posttransection (dpt), and most newly generated cells survived up to 14 dpt. To examine longer-term survival of the newly proliferated cells, BrdU was injected for 5 days after the surgery, and fish were sacrificed at 14 or 30 dpt. Spinal transection significantly increased cell proliferation and/or survival, as indicated by an elevated density of BrdU⁺ cells in the spinal cords of spinally transected compared to sham-operated and intact fish. At 14 dpt, BrdU⁺ cells were abundant at all levels of the spinal cord. By 30 dpt, the density of BrdU⁺ cells had decreased at all levels of the spinal cord except at the tip of the tail. Thus, newly generated cells in the caudal-most segment of the spinal cord survived longer than those in more rostral segments. Our findings indicate that spinal cord transection stimulates widespread cellular proliferation; however, there were regional differences in the survival of the newly generated cells.

Spinal cord regeneration in Xenopus tadpoles proceeds through activation of Sox2-positive cells

BACKGROUND

In contrast to mammals, amphibians, such as adult urodeles (for example, newts) and anuran larvae (for example, Xenopus) can regenerate their spinal cord after injury. However, the cellular and molecular mechanisms involved in this process are still poorly understood.

RESULTS

Here, we report that tail amputation results in a global increase of Sox2 levels and proliferation of Sox2(+) cells. Overexpression of a dominant negative form of Sox2 diminished proliferation of spinal cord resident cells affecting tail regeneration after amputation, suggesting that spinal cord regeneration is crucial for the whole process. After spinal cord transection, Sox2(+) cells are found in the ablation gap forming aggregates. Furthermore, Sox2 levels correlated with regenerative capabilities during metamorphosis, observing a decrease in Sox2 levels at non-regenerative stages.

CONCLUSIONS

Sox2(+) cells contribute to the regeneration of spinal cord after tail amputation and transection. Sox2 levels decreases during metamorphosis concomitantly with the lost of regenerative capabilities. Our results lead to a working hypothesis in which spinal cord damage activates proliferation and/or migration of Sox2(+) cells, thus allowing regeneration of the spinal cord after tail amputation or reconstitution of the ependymal epithelium after spinal cord transection.

Teleost fish as a model system to study successful regeneration of the central nervous system

Traumatic brain injury and spinal cord injury are devastating conditions that may result in death or long-term disability. A promising strategy for the development of effective cell replacement therapies involves the study of regeneration-competent organisms. Among this group, teleost fish are distinguished by their excellent potential to regenerate nervous tissue and to regain function after injury to the central nervous system. In this chapter, we summarize our current understanding of the cellular processes that mediate this regenerative potential, and we show that several of these processes are shared with the normal development of the intact central nervous system; we describe how the spontaneous self-repair of the teleostean central nervous system leads to functional recovery, at physiological and behavioral levels; we discuss the possible function of molecular factors associated with the degenerative and regenerative processes after injury; and, finally, we speculate on evolutionary aspects of adult neurogenesis and neuronal regeneration, and on how a better understanding of these aspects could catalyze the development of therapeutic strategies to overcome the regenerative limits of the mammalian CNS.

Neural stem/progenitor cells from the adult human spinal cord are multipotent and self-renewing and differentiate after transplantation

Neural stem/progenitor cell (NSPC) transplantation is a promising therapy for spinal cord injury (SCI). However, little is known about NSPC from the adult human spinal cord as a donor source. We demonstrate for the first time that multipotent and self-renewing NSPC can be cultured, passaged and transplanted from the adult human spinal cord of organ transplant donors. Adult human spinal cord NSPC require an adherent substrate for selection and expansion in EGF (epidermal growth factor) and FGF2 (fibroblast growth factor) enriched medium. NSPC as an adherent monolayer can be passaged for at least 9 months and form neurospheres when plated in suspension culture. In EGF/FGF2 culture, NSPC proliferate and primarily express nestin and Sox2, and low levels of markers for differentiating cells. Leukemia inhibitory factor (LIF) promotes NSPC proliferation and significantly enhances GFAP expression in hypoxia. In differentiating conditions in the presence of serum, these NSPC show multipotentiality, expressing markers of neurons, astrocytes, and oligodendrocytes. Dibutyryl cyclic AMP (dbcAMP) significantly enhances neuronal differentiation. We transplanted the multipotent NSPC into SCI rats and show that the xenografts survive, are post-mitotic, and retain the capacity to differentiate into neurons and glia.

Together, these findings reveal that multipotent self-renewing NSPC cultured and passaged from adult human spinal cords of organ transplant donors, respond to exogenous factors that promote selective differentiation, and survive and differentiate after transplantation into the injured spinal cord.

Spinal cord injury: From inflammation to glial scar

BACKGROUND

Glial scar (GS) is the most important inhibitor factor to neuroregeneration after spinal cord injury (SCI) and behaves as a tertiary lesion. The present review of the literature searched for representative studies concerning GS and therapeutic strategies to neuroregeneration.

METHODS

The author used the PubMed database and Google scholar to search articles published in the last 20 years. Key words used were SCI, spinal cord (SC) inflammation, GS, and SCI treatment.

RESULTS

Both inflammation and GS are considered important events after SCI. Despite the fact that firstly they seem to cause benefit, in the end they cause more harm than good to neuroregeneration. Each stage has its own aspects under the influence of the immune system causing inflammation, from the primary to secondary lesion and from those to GS (tertiary lesion).

CONCLUSION

Future studies should stress the key points where and when GS presents itself as an inhibitory factor to neuroregeneration. Considering GS as an important event after SCI, the author defends GS as being a tertiary lesion. Current strategies are presented with emphasis on stem cells and drug therapy. A better understanding will permit the development of a therapeutic basis in the treatment of the SCI patients considering each stage of the lesion, with emphasis on GS and neuroregeneration.

Cell proliferation and cytoarchitectural remodeling during spinal cord reconnection in the fresh-water turtle Trachemys dorbignyi

In fresh-water turtles, the bridge connecting the proximal and caudal stumps of transected spinal cords consists of regenerating axons running through a glial cellular matrix. To understand the process leading to the generation of the scaffold bridging the lesion, we analyzed the mitotic activity triggered by spinal injury in animals maintained alive for 20-30 days after spinal cord transection. Flow cytometry and bromodeoxyuridine (BrdU)-labeling experiments revealed a significant increment of cycling cells around the lesion epicenter. BrdU-tagged cells maintained a close association with regenerating axons. Most dividing cells expressed the brain lipid-binding protein (BLBP). Cells with BrdU-positive nuclei expressed glial fibrillary acidic protein. As spinal cord regeneration involves dynamic cell rearrangements, we explored the ultra-structure of the bridge and found cells with the aspect of immature oligodendrocytes forming an embryonic-like microenvironment. These cells supported and ensheathed regenerating axons that were recognized by immunocytological and electron-microscopical procedures. Since functional recovery depends on proper impulse transmission, we examined the anatomical axon-glia relationships near the lesion epicenter. Computer-assisted three-dimensional models revealed helical axon-glial junctions in which the intercellular space appeared to be reduced (5-7 nm). Serial-sectioning analysis revealed that fibril-containing processes provided myelinating axon sheaths. Thus, disruption of the ependymal layer elicits mitotic activity predominantly in radial glia expressing BLBP on the lateral aspects of the ependyma. These cycling cells seem to migrate and contribute to the bridge providing the main support and sheaths for regenerating axons.

Reaction of endogenous progenitor cells in a rat model of posttraumatic syringomyelia

OBJECT

Endogenous stem cells theoretically could replace lost tissue and repair deficits caused by syringes. In this study the authors quantitatively examined 1) whether neural progenitor cells exist in an adult rat model of posttraumatic syringomyelia (PTS); 2) and if so, how long an active population of progenitor cells can persist; 3) whether the cell population's location is associated with the syrinx; 4) the degree of differentiation of the progenitor cells; and 5) the phenotypic fate of the progenitor cells.

METHODS

Wistar rats were divided into intact, sham-operated, and experimental syrinx groups. Animals in each group were equally subdivided according to 4 time points: 7, 14, 28, and 56 days post-syrinx induction. Rats in the experimental syrinx group underwent a C-7 and T-1 laminectomy and then received 0.5 μl of a 24-mg/ml quisqualic acid spinal cord injection at the C-8 level to mimic an excitotoxic injury with an initial cyst, and 10 μl of a 250-mg/ml kaolin injection into the subarachnoid space at the C-8 level to create arachnoiditis. The proliferation, distribution, and differentiation of endogenous progenitor cells were identified immunocytochemically.

RESULTS

The authors observed a 20-fold increase in progenitor cells excluding inflammatory cells in the 1st 2 weeks post-syrinx induction. The cells persisted for at least 56 days, and 80% of them were located in the gray matter along the border of cysts. They included neural multipotential progenitor cells, oligodendroglial progenitor cells, and astrocytes.

CONCLUSIONS

Data in this study provide evidence for proliferation, distribution, and differentiation of endogenous progenitor cells in a model of PTS in adult rats. These progenitor cells proliferate rapidly, extend for long periods, and are mainly located in the gray matter along the border of syringes. Neural multipotential progenitor cells are expected to be associated with reparative and regenerative mechanisms of PTS. Glial cells are involved in the formation of a glial scar barrier that surrounds the syrinx and may prevent cyst enlargement. The authors' findings suggest that neural progenitor cells play a protective role in PTS.

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.

Sonic hedgehog is a polarized signal for motor neuron regeneration in adult zebrafish

In contrast to mammals, the spinal cord of adult zebrafish has the capacity to reinitiate generation of motor neurons after a lesion. Here we show that genes involved in motor neuron development, i.e., the ventral morphogen sonic hedgehog a (shha), as well as the transcription factors nkx6.1 and pax6, together with a Tg(olig2:egfp) transgene, are expressed in the unlesioned spinal cord of adult zebrafish. Expression is found in ependymoradial glial cells lining the central canal in ventrodorsal positions that match expression domains of these genes in the developing neural tube. Specifically, Tg(olig2:egfp)(+) ependymoradial glial cells, the adult motor neuron progenitors (pMNs), coexpress Nkx6.1 and Pax6, thus defining an adult pMN-like zone. shha is expressed in distinct ventral ependymoradial glial cells. After a lesion, expression of all these genes is strongly increased, while relative spatial expression domains are maintained. In addition, expression of the hedgehog (hh) receptors patched1 and smoothened becomes detectable in ependymoradial glial cells including those of the pMN-like zone. Cyclopamine-induced knock down of hh signaling significantly reduces ventricular proliferation and motor neuron regeneration. Expression of indicator genes for the FGF and retinoic acid signaling pathways was also increased in the lesioned spinal cord. This suggests that a subclass of ependymoradial glial cells retain their identity as motor neuron progenitors into adulthood and are capable of reacting to a sonic hedgehog signal and potentially other developmental signals with motor neuron regeneration after a spinal lesion.

Enigmatic central canal contacting cells: immature neurons in "standby mode"?

The region that surrounds the central canal of the spinal cord derives from the neural tube and retains a substantial degree of plasticity. In turtles, this region is a neurogenic niche where newborn neurons coexist with precursors, a fact that may be related with the endogenous repair capabilities of low vertebrates. Immunohistochemical evidence suggests that the ependyma of the mammalian spinal cord may contain cells with similar properties, but their actual nature remains unsolved. Here, we combined immunohistochemistry for cell-specific markers with patch-clamp recordings to test the hypothesis that the ependyma of neonatal rats contains immature neurons similar to those in low vertebrates. We found that a subclass of cells expressed HuC/D neuronal proteins, doublecortin, and PSA-NCAM (polysialylated neural cell adhesion molecule) but did not express NeuN (anti-neuronal nuclei). These immature neurons displayed electrophysiological properties ranging from slow Ca(2+)-mediated responses to fast repetitive Na(+) spikes, suggesting different stages of maturation. These cells originated in the embryo, because we found colocalization of neuronal markers with 5-bromo-2'-deoxyuridine when injected during embryonic day 7-17 but not in postnatal day 0-5. Our findings represent the first evidence that the ependyma of the rat spinal cord contains cells with molecular and functional features similar to immature neurons in adult neurogenic niches. The fact that these cells retain the expression of molecules that participate in migration and neuronal differentiation raises the possibility that the ependyma of the rat spinal cord is a reservoir of immature neurons in "standby mode," which under some circumstances (e.g., injury) may complete their maturation to integrate spinal circuits.

Neural reconnection in the transected spinal cord of the freshwater turtle Trachemys dorbignyi

This paper provides the first evidence that freshwater turtles are able to reconnect their completely transected spinal cords, leading to some degree of recovery of the motor functions lost after injury. Videographic analysis showed that some turtles (5 of 11) surviving more than 20 days after injury were able to initiate stepping locomotion. However, the stepping movements were slower than those of normal animals, and swimming patterns were not restored. Even though just 45% of the injured turtles recovered their stepping patterns, all showed axonal sprouting beyond the lesion site. Immunocytochemical and electron microscope images revealed the occurrence of regrowing axons crossing the severed region. A major contingent of the axons reconnecting the cord originated from sensory neurons lying in dorsal ganglia adjacent to the lesion site. The axons bridging the damaged region traveled on a cellular scaffold consisting of brain lipid-binding protein (BLBP)- and glial fibrillary acidic protein (GFAP)-positive cells and processes. Serotonergic varicose nerve fibers and endings were found at early stages of the healing process at the epicenter of the lesion. Interestingly, the glial scar commonly found in the damaged central nervous system of mammals was absent. In contrast, GFAP- and BLBP-positive processes were found running parallel to the main axis of the cord accompanying the crossing axons.

Colocalization of dopamine and GABA in spinal cord neurones in the sea lamprey

In this study, double immunofluorescence methods were used to investigate possible colocalization of the neurotransmitters dopamine [DA] and GABA in rostral spinal cord neurones in the upstream migrating adult sea lamprey (Petromyzon marinus). Double immunofluorescence revealed that all the DA-immunoreactive (ir) cerebrospinal fluid-contacting (CSF-c) cells, approximately 30% of the medioventral DA-ir cells, and most of the DA-ir cells located in the grey lateral to the central canal were also GABA-ir. The results also revealed some DA-ir cells located dorsally to the central canal, which increases the number of dopaminergic cell types known in lamprey. Double-labelled fibres were mainly distributed in the ventral column, and double-labelled boutons contacted some dorsal GABA-ir CSF-c cells, as well as some non-CSF-c GABA-ir cells and ventromedial dendrites of motoneurones. The findings reveal colocalization of dopamine and GABA in some cells and fibres, which suggests co-release of these substances in some synaptic terminals. Although dopaminergic/GABAergic CSF-c cells have been reported in some other vertebrates, the other double-labelled spinal populations appear exclusive to lampreys.

Transplantation of Adult Rat Spinal Cord Stem/Progenitor Cells for Spinal Cord Injury

Stem/progenitor cells derived from the ependymal region of the spinal cord have the ability to self-renew and are multipotential for neurons and glia. These cells may have the ability to regenerate the injured mammalian spinal cord as they do in some lower vertebrates. However, the optimal conditions for transplantation and the fate of transplanted cells are not fully known. In the current study, spinal cord stem/progenitor cells were cultured from adult male rats expressing enhanced green fluorescent protein (eGFP). Neurospheres were transplanted at the time of clip compression injury (35-g force) into the injury site, or 1 mm rostral and caudal to the injury site. Neurospheres were also transplanted into a subacute model (day 9 after injury) and a chronic model (day 28 after injury). Functional recovery was also studied in an acute injury model with weekly locomotor testing over a 16-week period. A significant increase in cell survival at 7 days was seen in rats receiving rostral and caudal injections as compared to injection directly into the site of injury. A significant increase in cell survival was also seen in rats receiving subacute transplants at 9 days after injury. Transplanted cells differentiated primarily into astrocytes (31.2%) and oligodendrocytes (50.3%), and a small number of neurons (1%). No improvement was seen in the Basso, Beattie and Bresnahan (BBB) locomotor rating scale after acute transplantation as compared with injury only, although surviving transplanted cells were identified that had migrated across the injury site from the rostral and caudal injection sites.

Cytological organization of the central gelatinosa in the turtle spinal cord

This paper deals with the cytological organization of the central gelatinosa (CG) in the spinal cord of juvenile (2-12 months) turtles. We found two main cell classes in the CG: one with characteristics of immature neurons, the other identified as radial glia (RG). The cells surrounding the central canal formed radial conglomerates in such a way that the RG lamellae covered the immature neurons. We found three major subpopulations of RG that expressed S-100, glial fibrillary acidic protein, or both proteins. Electron microscopic images showed gap junctions interconnecting RG. As with the mammalian neuroepithelial cells, most CG cells displayed intrinsic polarity expressed by structural and molecular differences between the most apical and basal cell compartments. The apical zone was characterized by the occurrence of a single cilium associated with a conspicuous centrosomal complex. We found a prominent expression of the PCM-1 centrosomal protein concentrated close to the central canal lumen. In the particular case of RG, the peripheral end feet contacted the subpial basement membrane. We also found "transitional cell forms" difficult to classify by the usual imaging approaches. Functional clues obtained by patch-clamp recordings of CG cells defined some of them as already committed to follow the neuronal lineage, whereas others had properties of less mature or migrating cells. The CG appeared as a richly innervated region receiving terminal branches from nerve plexuses expressing gamma-aminobutyric acid, serotonin, and glutamate. The results presented here support our previous studies indicating that the CG is an extended neurogenic niche along the spinal cord of turtles.

Exercise enhances axonal growth and functional recovery in the regenerating spinal cord

We investigated whether enhancing locomotory activity could accelerate the axonal growth underlying the significant recovery of function after a complete spinal transection in the eel, Anguilla. Eels with low spinal transections (at about 60% body length) were kept in holding tanks, where they were inactive, or made to swim continually against a water current at about one body length/s. Their locomotion was periodically assessed by measuring tail beat frequencies at different swimming speeds. Axonal growth was determined from anterograde labeling with 1,1'-diotadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate, inserted postmortem into the spinal cord, just rostral to the transection. Twenty days after surgery, there were significantly more labeled growth cones more than 2 mm caudal from the transection in the exercised fish (74.6+/-2.3%; cf. 34.5+/-1.1%). This difference was still observed at 40 days (57.9+/-1.6% cf. 42.1+/-2% >2 mm), but the regenerated axons were of similar maximum lengths by 120 days (9.8+/-0.3 cf. 7.7+/-2.8 mm). After surgery, each eel undulated its whole body faster at any given swimming speed, thus changing the linear relationship between tail beat frequency and forward speed established before transection. The slope increased by up to 112.5+/-27.4% over the first 8 days post-surgery in inactive animals, while a smaller rise (45.6+/-10.5%) was observed in exercised fish during this period. Thereafter, the slope progressively declined to pre-surgery levels in both groups of animals, but the recovery occurred within 20+/-4 days in exercised eels, as opposed to 40+/-5 days in inactive fish. The locomotory performance of sham-operated fish was unaffected by 10 days of continual locomotion and remained similar to that of naïve eels, pre-transection. These data show that elevated locomotory activity enhances axonal growth and accelerates recovery of locomotory function.

Functional and molecular clues reveal precursor-like cells and immature neurones in the turtle spinal cord

In lower vertebrates, some cells contacting the central canal (CC) retain the ability to proliferate, leading the reconstruction of the spinal cord after injury. A better understanding about the nature of these cells could contribute to the development of novel strategies for spinal cord repair. Here, by combining light and electron microscopy, immunocytochemistry and patch-clamp recordings, we provide evidence supporting the presence of precursor-like cells and immature neurones contacting the CC of juvenile turtles. A class of cells expressed the ependymal and glial cell marker S100 and displayed morphological and electrophysiological features of radial glia: relatively low input resistance, high resting potential, lack of active membrane properties and extensive dye-coupling. A second class of S100 reactive cells were characterized by a higher input resistance and outward rectification. Finally, some CC-contacting cells expressed HuC/D - a marker of immature neurones - and fired action potentials. The coexistence of cells with functional properties of precursor-like cells and immature neurones suggests that the region surrounding the CC is a site of active neurogenesis. It remains to be demonstrated by lineage analysis whether, as in the embryonic cerebral cortex, radial glia are the progenitor cells in the turtle spinal cord.