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

Tackling the glial scar in spinal cord regeneration: new discoveries and future directions

Axonal regeneration and functional recovery are poor after spinal cord injury (SCI), typified by the formation of an injury scar. While this scar was traditionally believed to be primarily responsible for axonal regeneration failure, current knowledge takes a more holistic approach that considers the intrinsic growth capacity of axons. Targeting the SCI scar has also not reproducibly yielded nearly the same efficacy in animal models compared to these neuron-directed approaches. These results suggest that the major reason behind central nervous system (CNS) regeneration failure is not the injury scar but a failure to stimulate axon growth adequately. These findings raise questions about whether targeting neuroinflammation and glial scarring still constitute viable translational avenues. We provide a comprehensive review of the dual role of neuroinflammation and scarring after SCI and how future research can produce therapeutic strategies targeting the hurdles to axonal regeneration posed by these processes without compromising neuroprotection.

Mitochondrial function in spinal cord injury and regeneration

Many people around the world suffer from some form of paralysis caused by spinal cord injury (SCI), which has an impact on quality and life expectancy. The spinal cord is part of the central nervous system (CNS), which in mammals is unable to regenerate, and to date, there is a lack of full functional recovery therapies for SCI. These injuries start with a rapid and mechanical insult, followed by a secondary phase leading progressively to greater damage. This secondary phase can be potentially modifiable through targeted therapies. The growing literature, derived from mammalian and regenerative model studies, supports a leading role for mitochondria in every cellular response after SCI: mitochondrial dysfunction is the common event of different triggers leading to cell death, cellular metabolism regulates the immune response, mitochondrial number and localization correlate with axon regenerative capacity, while mitochondrial abundance and substrate utilization regulate neural stem progenitor cells self-renewal and differentiation. Herein, we present a comprehensive review of the cellular responses during the secondary phase of SCI, the mitochondrial contribution to each of them, as well as evidence of mitochondrial involvement in spinal cord regeneration, suggesting that a more in-depth study of mitochondrial function and regulation is needed to identify potential targets for SCI therapeutic intervention.

Regulating Endogenous Neural Stem Cell Activation to Promote Spinal Cord Injury Repair

Spinal cord injury (SCI) affects millions of individuals worldwide. Currently, there is no cure, and treatment options to promote neural recovery are limited. An innovative approach to improve outcomes following SCI involves the recruitment of endogenous populations of neural stem cells (NSCs). NSCs can be isolated from the neuroaxis of the central nervous system (CNS), with brain and spinal cord populations sharing common characteristics (as well as regionally distinct phenotypes). Within the spinal cord, a number of NSC sub-populations have been identified which display unique protein expression profiles and proliferation kinetics. Collectively, the potential for NSCs to impact regenerative medicine strategies hinges on their cardinal properties, including self-renewal and multipotency (the ability to generate de novo neurons, astrocytes, and oligodendrocytes). Accordingly, endogenous NSCs could be harnessed to replace lost cells and promote structural repair following SCI. While studies exploring the efficacy of this approach continue to suggest its potential, many questions remain including those related to heterogeneity within the NSC pool, the interaction of NSCs with their environment, and the identification of factors that can enhance their response. We discuss the current state of knowledge regarding populations of endogenous spinal cord NSCs, their niche, and the factors that regulate their behavior. In an attempt to move towards the goal of enhancing neural repair, we highlight approaches that promote NSC activation following injury including the modulation of the microenvironment and parenchymal cells, pharmaceuticals, and applied electrical stimulation.

Cell transplantation and secretome based approaches in spinal cord injury regenerative medicine

The axonal growth-restrictive character of traumatic spinal cord injury (SCI) makes finding a therapeutic strategy a very demanding task, due to the postinjury events impeditive to spontaneous axonal outgrowth and regeneration. Considering SCI pathophysiology complexity, it has been suggested that an effective therapy should tackle all the SCI-related aspects and provide sensory and motor improvement to SCI patients. Thus, the current aim of any therapeutic approach for SCI relies in providing neuroprotection and support neuroregeneration. Acknowledging the current SCI treatment paradigm, cell transplantation is one of the most explored approaches for SCI with mesenchymal stem cells (MSCs) being in the forefront of many of these. Studies showing the beneficial effects of MSC transplantation after SCI have been proposing a paracrine action of these cells on the injured tissues, through the secretion of protective and trophic factors, rather than attributing it to the action of cells itself. This manuscript provides detailed information on the most recent data regarding the neuroregenerative effect of the secretome of MSCs as a cell-free based therapy for SCI. The main challenge of any strategy proposed for SCI treatment relies in obtaining robust preclinical evidence from in vitro and in vivo models, before moving to the clinics, so we have specifically focused on the available vertebrate and mammal models of SCI currently used in research and how can SCI field benefit from them.

Growth associated protein 43 and neurofilament immunolabeling in the transected lumbar spinal cord of lizard indicates limited axonal regeneration

Previous cytological studies on the transected lumbar spinal cord of lizards have shown the presence of differentiating glial cells, few neurons and axons in the bridge region between the proximal and distal stumps of the spinal cord in some cases. A limited number of axons (20–50) can cross the bridge and re-connect the caudal stump of the spinal cord with small neurons located in the rostral stump of the spinal cord. This axonal regeneration appears to be related to the recovery of hind-limb movements after initial paralysis. The present study extends previous studies and shows that after transection of the lumbar spinal cord in lizards, a glial-connective tissue bridge that reconnects the rostral and caudal stumps of the interrupted spinal cord is formed at 11–34 days post-injury. Following an initial paralysis some recovery of hindlimb movements occurs within 1–3 months post-injury. Immunohistochemical and ultrastructural analysis for a growth associated protein 43 (GAP-43) of 48–50 kDa shows that sparse GAP-43 positive axons are present in the proximal stump of the spinal cord but their number decreased in the bridge at 11–34 days post-transection. Few immunolabeled axons with a neurofilament protein of 200–220 kDa were seen in the bridge at 11–22 days post-transection but their number increased at 34 days and 3 months post-amputation in lizards that have recovered some hindlimb movements. Numerous neurons in the rostral and caudal stumps of the spinal cord were also labeled for GAP43, a cytoplasmic protein that is trans-located into their axonal growth cones. This indicates that GAP-43 biosynthesis is related to axonal regeneration and sprouting from neurons that were damaged by the transection. Taken together, previous studies that utilized tract-tracing technique to label the present observations confirm that a limited axonal re-connection of the transected spinal cord occurs 1–3 months post-injury in lizards. The few regenerating-sprouting axons within the bridge reconnect the caudal with the rostral stumps of the spinal cord, and likely contribute to activate the neural circuits that sustain the limited but important recovery of hind-limb movements after initial paralysis. The surgical procedures utilized in the study followed the regulations on animal care and experimental procedures under the Italian Guidelines (art. 5, DL 116/92).

New insights into glial scar formation after spinal cord injury

Severe spinal cord injury causes permanent loss of function and sensation throughout the body. The trauma causes a multifaceted torrent of pathophysiological processes which ultimately act to form a complex structure, permanently remodeling the cellular architecture and extracellular matrix. This structure is traditionally termed the glial/fibrotic scar. Similar cellular formations occur following stroke, infection, and neurodegenerative diseases of the central nervous system (CNS) signifying their fundamental importance to preservation of function. It is increasingly recognized that the scar performs multiple roles affecting recovery following traumatic injury. Innovative research into the properties of this structure is imperative to the development of treatment strategies to recover motor function and sensation following CNS trauma. In this review, we summarize how the regeneration potential of the CNS alters across phyla and age through formation of scar-like structures. We describe how new insights from next-generation sequencing technologies have yielded a more complex portrait of the molecular mechanisms governing the astrocyte, microglial, and neuronal responses to injury and development, especially of the glial component of the scar. Finally, we discuss possible combinatorial therapeutic approaches centering on scar modulation to restore function after severe CNS injury.

Injury-induced Cavl-expressing cells at lesion rostral side play major roles in spinal cord regeneration

The extent of cellular heterogeneity involved in neuronal regeneration after spinal cord injury (SCI) remains unclear. Therefore, we established stress-responsive transgenic zebrafish embryos with SCI. As a result, we found an SCI-induced cell population, termed SCI stress-responsive regenerating cells (SrRCs), essential for neuronal regeneration post-SCI. SrRCs were mostly composed of subtypes of radial glia (RGs-SrRCs) and neuron stem/progenitor cells (NSPCs-SrRCs) that are able to differentiate into neurons, and they formed a bridge across the lesion and connected with neighbouring undamaged motor neurons post-SCI. Compared to SrRCs at the caudal side of the SCI site (caudal-SrRCs), rostral-SrRCs participated more actively in neuronal regeneration. After RNA-seq analysis, we discovered that caveolin 1 (cav1) was significantly upregulated in rostral-SrRCs and that cav1 was responsible for the axonal regrowth and regenerative capability of rostral-SrRCs. Collectively, we define a specific SCI-induced cell population, SrRCs, involved in neuronal regeneration, demonstrate that rostral-SrRCs exhibit higher neuronal differentiation capability and prove that cav1 is predominantly expressed in rostral-SrRCs, playing a major role in neuronal regeneration after SCI.

Glial Metabolic Rewiring Promotes Axon Regeneration and Functional Recovery in the Central Nervous System

Axons in the mature central nervous system (CNS) fail to regenerate after axotomy, partly due to the inhibitory environment constituted by reactive glial cells producing astrocytic scars, chondroitin sulfate proteoglycans, and myelin debris. We investigated this inhibitory milieu, showing that it is reversible and depends on glial metabolic status. We show that glia can be reprogrammed to promote morphological and functional regeneration after CNS injury in Drosophila via increased glycolysis. This enhancement is mediated by the glia derived metabolites: L-lactate and L-2-hydroxyglutarate (L-2HG). Genetically/pharmacologically increasing or reducing their bioactivity promoted or impeded CNS axon regeneration. L-lactate and L-2HG from glia acted on neuronal metabotropic GABA_B receptors to boost cAMP signaling. Local application of L-lactate to injured spinal cord promoted corticospinal tract axon regeneration, leading to behavioral recovery in adult mice. Our findings revealed a metabolic switch to circumvent the inhibition of glia while amplifying their beneficial effects for treating CNS injuries.

NOGO-A immunolabeling is present in glial cells and some neurons of the recovering lumbar spinal cord in lizards

The transected lumbar spinal cord of lizards was studied for its ability to recover after paralysis. At 34 days post-lesion about 50% of lizards were capable of walking with a limited coordination, likely due to the regeneration of few connecting axons crossing the transection site of the spinal cord. This region, indicated as "bridge", contains glial cells among which oligodendrocytes and their elongation that are immunolabeled for NOGO-A. A main reactive protein band occurs at 100-110 kDa but a weaker band is also observed around 240 kDa, suggesting fragmentation of the native protein due to extraction or to physiological processing of the original protein. Most of the cytoplasmic immunolabeling observed in oligodendrocytes is associated with vesicles of the endoplasmic reticulum. Also, the nucleus is labeled in some oligodendrocytes that are myelinating sparse axons observed within the bridge at 22-34 days post-transection. This suggests that axonal regeneration is present within the bridge region. Immunolabeling for NOGO-A shows that the protein is also present in numerous reactive neurons, in particular motor-neurons localized in the proximal stump of the transected spinal cord. Ultrastructural immunolocalization suggests that NOGO is synthesized in the ribosomes of these neurons and becomes associated with the cisternae of the endoplasmic reticulum, probably following a secretory pathway addressed toward the axon. The present observations suggest that, like for the regenerating spinal cord of fish and amphibians, also in lizard NOGO-A is present in reactive neurons and appears associated to axonal regeneration and myelination.

Brief Electrical Stimulation Triggers an Effective Regeneration of Leech CNS

The search for therapeutic strategies to promote neuronal regeneration following injuries toward functional recovery is of great importance. Brief low-frequency electrical stimulation (ES) has been reported as a useful method to improve neuronal regeneration in different animal models; however, the effect of ES on single neuron behavior has not been shown. Here, we study the effect of brief ES on neuronal regeneration of the CNS of adult medicinal leeches. Studying the regeneration of selected sets of identified neurons allow us to quantify the ES effect per cell type at the single-cell level. Chains of the CNS that were subjected to cut injury were observed for 3 d, and the spontaneous regeneration was compared with the electrically stimulated injured chains. We show that the ES improves the efficiency of regeneration of Retzius cells, as larger masses of the total branching tree traverse the injury site with better directed growth with no effect on the average branching tree length. No antero-posterior polarity was found along regeneration within the leech CNS. Moreover, the microglial cell distribution was examined revealing more microglial cells in proximity to the stimulation site compared with non-stimulated. Our results lay a foundation for future ES-based neuroregenerative therapies.

Liver Progenitors and Adult Cell Plasticity in Hepatic Injury and Repair: Knowns and Unknowns

The liver is a complex organ performing numerous vital physiological functions. For that reason, it possesses immense regenerative potential. The capacity for repair is largely attributable to the ability of its differentiated epithelial cells, hepatocytes and biliary epithelial cells, to proliferate after injury. However, in cases of extreme acute injury or prolonged chronic insult, the liver may fail to regenerate or do so suboptimally. This often results in life-threatening end-stage liver disease for which liver transplantation is the only effective treatment. In many forms of liver injury, bipotent liver progenitor cells are theorized to be activated as an additional tier of liver repair. However, the existence, origin, fate, activation, and contribution to regeneration of liver progenitor cells is hotly debated, especially since hepatocytes and biliary epithelial cells themselves may serve as facultative stem cells for one another during severe liver injury. Here, we discuss the evidence both supporting and refuting the existence of liver progenitor cells in a variety of experimental models. We also debate the validity of developing therapies harnessing the capabilities of these cells as potential treatments for patients with severe and chronic liver diseases.

Heterogeneity in the regenerative abilities of central nervous system axons within species: why do some neurons regenerate better than others?

Some neurons, especially in mammalian peripheral nervous system or in lower vertebrate or in vertebrate central nervous system (CNS) regenerate after axotomy, while most mammalian CNS neurons fail to regenerate. There is an emerging consensus that neurons have different intrinsic regenerative capabilities, which theoretically could be manipulated therapeutically to improve regeneration. Population-based comparisons between “good regenerating” and “bad regenerating” neurons in the CNS and peripheral nervous system of most vertebrates yield results that are inconclusive or difficult to interpret. At least in part, this reflects the great diversity of cells in the mammalian CNS. Using mammalian nervous system imposes several methodical limitations. First, the small sizes and large numbers of neurons in the CNS make it very difficult to distinguish regenerating neurons from non-regenerating ones. Second, the lack of identifiable neurons makes it impossible to correlate biochemical changes in a neuron with axonal damage of the same neuron, and therefore, to dissect the molecular mechanisms of regeneration on the level of single neurons. This review will survey the reported responses to axon injury and the determinants of axon regeneration, emphasizing non-mammalian model organisms, which are often under-utilized, but in which the data are especially easy to interpret.

Meningeal Foam Cells and Ependymal Cells in Axolotl Spinal Cord Regeneration

A previously unreported population of foam cells (foamy macrophages) accumulates in the invasive fibrotic meninges during gap regeneration of transected adult Axolotl spinal cord (salamander Ambystoma mexicanum) and may act beneficially. Multinucleated giant cells (MNGCs) also occurred in the fibrotic meninges. Actin-label localization and transmission electron microscopy showed characteristic foam cell and MNGC podosome and ruffled border-containing sealing ring structures involved in substratum attachment, with characteristic intermediate filament accumulations surrounding nuclei. These cells co-localized with regenerating cord ependymal cell (ependymoglial) outgrowth. Phase contrast-bright droplets labeled with Oil Red O, DiI, and DyRect polar lipid live cell label showed accumulated foamy macrophages to be heavily lipid-laden, while reactive ependymoglia contained smaller lipid droplets. Both cell types contained both neutral and polar lipids in lipid droplets. Foamy macrophages and ependymoglia expressed the lipid scavenger receptor CD36 (fatty acid translocase) and the co-transporter toll-like receptor-4 (TLR4). Competitive inhibitor treatment using the modified fatty acid Sulfo-N-succinimidyl Oleate verified the role of the lipid scavenger receptor CD36 in lipid uptake studies in vitro. Fluoromyelin staining showed both cell types took up myelin fragments in situ during the regeneration process. Foam cells took up DiI-Ox-LDL and DiI-myelin fragments in vitro while ependymoglia took up only DiI-myelin in vitro. Both cell types expressed the cysteine proteinase cathepsin K, with foam cells sequestering cathepsin K within the sealing ring adjacent to the culture substratum. The two cell types act as sinks for Ox-LDL and myelin fragments within the lesion site, with foamy macrophages showing more Ox-LDL uptake activity. Cathepsin K activity and cellular localization suggested that foamy macrophages digest ECM within reactive meninges, while ependymal cells act from within the spinal cord tissue during outgrowth into the lesion site, acting in complementary fashion. Small MNGCs also expressed lipid transporters and showed cathepsin K activity. Comparison of ³H-glucosamine uptake in ependymal cells and foam cells showed that only ependymal cells produce glycosaminoglycan and proteoglycan-containing ECM, while the cathepsin studies showed both cell types remove ECM. Interaction of foam cells and ependymoglia in vitro supported the dispersion of ependymal outgrowth associated with tissue reconstruction in Axolotl spinal cord regeneration.

Cerebrospinal fluid-contacting neurons in the regenerating spinal cord of lizards and amphibians are likely mechanoreceptors

During spinal cord (SC) regeneration in the tail of amphibians and lizards, small neurons in contact with the central canal and cerebrospinal fluid (CSF) are formed. The present review summarizes previous and recent studies that have characterized most of these neurons as cerebrospinal fluid-contacting neurons (CSFCNs), especially in the regenerating caudal SC of lizards. CSFCNs form tufts of stereocilia immersed in the CSF, secrete exosomes, and are often in contact with a secreted protein-rod indicated as Reissner fiber. Ultrastructural, autoradiographic, immunohistochemical, and behavioral studies strongly indicate that most of these cells are mechanoreceptors that differentiate from ependymal cells within 20-30 days after SC amputation. Numerous CSFCNs are gamma amino-butyric acid (GABA)-ergic, uptake amino acids, receive few synaptic boutons, and contain neurofilaments, fibroblast growth factor (FGFs), and other signaling proteins, the latter likely secreted into the central canal. Similar neurons are formed in the SC of the tuatara (Sphenodon puctatus), anurans, and urodeles during tail regeneration. In lizard, most of their projection remains in the SC close to the regenerated tail, but they form synapses with neurons that receive descending nerves from the brainstem, including vestibular nuclei. CSFCNs, aside a possible neurosecretory activity, might sense liquor movements for maintenance of balance, a role that is supported from recent studies on other caudate vertebrates. The regeneration of these cells also in the nervous system of other vertebrates remains unknown.

Transplanted enteric neural stem cells integrate within the developing chick spinal cord: implications for spinal cord repair

Spinal cord injury (SCI) causes paralysis, multisystem impairment and reduced life expectancy, as yet with no cure. Stem cell therapy can potentially replace lost neurons, promote axonal regeneration and limit scar formation, but an optimal stem cell source has yet to be found. Enteric neural stem cells (ENSC) isolated from the enteric nervous system (ENS) of the gastrointestinal (GI) tract are an attractive source. Here, we used the chick embryo to assess the potential of ENSC to integrate within the developing spinal cord. In vitro, isolated ENSC formed extensive cell connections when co-cultured with spinal cord (SC)-derived cells. Further, qRT-PCR analysis revealed the presence of TuJ1⁺ neurons, S100⁺ glia and Sox10⁺ stem cells within ENSC neurospheres, as well as expression of key neuronal subtype genes, at levels comparable to SC tissue. Following ENSC transplantation to an ablated region of chick embryo SC, donor neurons were found up to 12 days later. These neurons formed bridging connections within the SC injury zone, aligned along the anterior/posterior axis, and were immunopositive for TuJ1. These data provide early proof of principle support for the use of ENSCs for SCI, and encourage further research into their potential for repair.

The neuromuscular junction of Xenopus tadpoles: Revisiting a classical model of early synaptogenesis and regeneration

The frog neuromuscular junction (NMJ) has been extensively used as a model system to dissect the mechanisms involved in synapse formation, maturation, maintenance, regeneration, and function. Early NMJ synaptogenesis relies on a combination of cell-autonomous and interdependent pre/postsynaptic communication processes. Due to their transparency, comparatively easy manipulation, and remarkable regenerative abilities, frog tadpoles constitute an excellent model to study NMJ formation and regeneration. Here, we aimed to contribute new aspects on the characterization of the ontogeny of NMJ formation in Xenopus embryos and to explore the morphological changes occurring at the NMJ after spinal cord injury. Following analyses of X. tropicalis tadpoles during development we found that the early pathfinding of rostral motor axons is likely helped by previously formed postsynaptic specializations, whereas NMJ formation in recently differentiated ventral muscles in caudal segments seems to rely on presynaptic inputs. After spinal cord injury of X. laevis tadpoles our results suggest that rostral motor axon projections help caudal NMJ re-innervation before spinal cord connectivity is repaired.

Cellular composition and organization of the spinal cord central canal during metamorphosis of the frog Xenopus laevis

Studying the cellular composition and morphological changes of cells lining the central canal during Xenopus laevis metamorphosis could contribute to understand postnatal development and spinal cord regeneration. Here we report the analysis of central canal cells at different stages during metamorphosis using immunofluorescence for protein markers expression, transmission and scanning electron microscopy and cell proliferation assays. The central canal was regionalized according to expression of glial markers, ultrastructure, and proliferation in dorsal, lateral, and ventral domains with differences between larvae and froglets. In regenerative larvae, all cell types were uniciliated, have a radial morphology, and elongated nuclei with lax chromatin, resembling radial glial cells. Important differences in cells of nonregenerative froglets were observed, although uniciliated cells were found, the most abundant cells had multicilia and revealed extensive changes in the maturation and differentiation state. The majority of dividing cells in larvae corresponded to uniciliated cells at dorsal and lateral domains in a cervical-lumbar gradient, correlating with undifferentiated features. Neurons contacting the lumen of the central canal were detected in both stages and revealed extensive changes in the maturation and differentiation state. However, in froglets a very low proportion of cells incorporate 5-ethynyl-2'-deoxyuridine (EdU), associated with the differentiated profile and with the increase of multiciliated cells. Our work showed progressive changes in the cell types lining the central canal of Xenopus laevis spinal cord which are correlated with the regenerative capacities.

Musashi and Plasticity of Xenopus and Axolotl Spinal Cord Ependymal Cells

The differentiated state of spinal cord ependymal cells in regeneration-competent amphibians varies between a constitutively active state in what is essentially a developing organism, the tadpole of the frog Xenopus laevis, and a quiescent, activatable state in a slowly growing adult salamander Ambystoma mexicanum, the Axolotl. Ependymal cells are epithelial in intact spinal cord of all vertebrates. After transection, body region ependymal epithelium in both Xenopus and the Axolotl disorganizes for regenerative outgrowth (gap replacement). Injury-reactive ependymal cells serve as a stem/progenitor cell population in regeneration and reconstruct the central canal. Expression patterns of mRNA and protein for the stem/progenitor cell-maintenance Notch signaling pathway mRNA-binding protein Musashi (msi) change with life stage and regeneration competence. Msi-1 is missing (immunohistochemistry), or at very low levels (polymerase chain reaction, PCR), in both intact regeneration-competent adult Axolotl cord and intact non-regeneration-competent Xenopus tadpole (Nieuwkoop and Faber stage 62+, NF 62+). The critical correlation for successful regeneration is msi-1 expression/upregulation after injury in the ependymal outgrowth and stump-region ependymal cells. msi-1 and msi-2 isoforms were cloned for the Axolotl as well as previously unknown isoforms of Xenopus msi-2. Intact Xenopus spinal cord ependymal cells show a loss of msi-1 expression between regeneration-competent (NF 50-53) and non-regenerating stages (NF 62+) and in post-metamorphosis froglets, while msi-2 displays a lower molecular weight isoform in non-regenerating cord. In the Axolotl, embryos and juveniles maintain Msi-1 expression in the intact cord. In the adult Axolotl, Msi-1 is absent, but upregulates after injury. Msi-2 levels are more variable among Axolotl life stages: rising between late tailbud embryos and juveniles and decreasing in adult cord. Cultures of regeneration-competent Xenopus tadpole cord and injury-responsive adult Axolotl cord ependymal cells showed an identical growth factor response. Epidermal growth factor (EGF) maintains mesenchymal outgrowth in vitro, the cells are proliferative and maintain msi-1 expression. Non-regeneration competent Xenopus ependymal cells, NF 62+, failed to attach or grow well in EGF+ medium. Ependymal Msi-1 expression in vivo and in vitro is a strong indicator of regeneration competence in the amphibian spinal cord.

Spinal cord self-repair during tail regeneration in Polypedates maculatus and putative role of FGF1 as a neurotrophic factor

Spinal cord injury could be fatal in man and often results in irreversible medical conditions affecting mobility. However, anuran amphibians win over such pathological condition by the virtue of regeneration abilities. The tail of anuran tadpoles therefore allures researchers to study spinal cord injury and self- repair process. In the present study, we inflicted injury to the spinal cord by means of surgical transection of the tail and investigated the self-repair activity in the tadpoles of the Indian tree frog Polypedates maculatus. We also demonstrate for the first time by immunofluorescence localization the expression pattern of Fibroblast Growth Factor1 (FGF1) during spinal cord regeneration which has not been documented earlier in anurans. FGF1, bearer of the mitogenic and neurotrophic properties seems to be expressed by progenitor cells that facilitate regeneration. Spinal cord during tail regeneration in P. maculatus attains functional recovery within a span of 2 weeks thus enabling the organism to survive in an aquatic medium till metamorphosis. Moreover, during the course of spinal cord regeneration in the regenerating tail, melanocytes showed an interesting behaviour as these neural crest derivatives were missing near the early regenerates until their reappearance where they were positioned in close proximity with the regenerated spinal cord as in the normal tail.

Potential Involvement of Snail Members in Neuronal Survival and Astrocytic Migration during the Gecko Spinal Cord Regeneration

Certain regenerative vertebrates such as fish, amphibians and reptiles are capable of regenerating spinal cord after injury. Most neurons of spinal cord will survive from the injury and regrow axons to repair circuits with an absence of glial scar formation. However, the underlying mechanisms of neuronal anti-apoptosis and glia-related responses have not been fully clarified during the regenerative process. Gecko has becoming an inspiring model to address spinal cord regeneration in amniotes. In the present study, we investigated the regulatory roles of Snail family members, the important transcriptional factors involved in both triggering of the cell migration and cell survival, during the spontaneous spinal cord regeneration. Both Snail1 and Snail3 have been shown to promote neuronal survival and astrocytic migration via anti-apoptotic and GTPases signaling following gecko tail amputation. Transforming growth factor-beta (TGFβ), together with other cytokines were involved in inducing expression of Snail protein. Our data indicate a conserved function of Snail proteins in embryonic development and tissue regeneration, which may provide clues for CNS repair in the mammals.

Regeneration of Zebrafish CNS: Adult Neurogenesis

Regeneration in the animal kingdom is one of the most fascinating problems that have allowed scientists to address many issues of fundamental importance in basic biology. However, we came to know that the regenerative capability may vary across different species. Among vertebrates, fish and amphibians are capable of regenerating a variety of complex organs through epimorphosis. Zebrafish is an excellent animal model, which can repair several organs like damaged retina, severed spinal cord, injured brain and heart, and amputated fins. The focus of the present paper is on spinal cord regeneration in adult zebrafish. We intend to discuss our current understanding of the cellular and molecular mechanism(s) that allows formation of proliferating progenitors and controls neurogenesis, which involve changes in epigenetic and transcription programs. Unlike mammals, zebrafish retains radial glia, a nonneuronal cell type in their adult central nervous system. Injury induced proliferation involves radial glia which proliferate, transcribe embryonic genes, and can give rise to new neurons. Recent technological development of exquisite molecular tools in zebrafish, such as cell ablation, lineage analysis, and novel and substantial microarray, together with advancement in stem cell biology, allowed us to investigate how progenitor cells contribute to the generation of appropriate structures and various underlying mechanisms like reprogramming.

Human regeneration: An achievable goal or a dream?

The main objective of regenerative medicine is to replenish cells or tissues or even to restore different body parts that are lost or damaged due to disease, injury and aging. Several avenues have been explored over many decades to address the fascinating problem of regeneration at the cell, tissue and organ levels. Here we discuss some of the primary approaches adopted by researchers in the context of enhancing the regenerating ability of mammals. Natural regeneration can occur in different animal species, and the underlying mechanism is highly relevant to regenerative medicine-based intervention. Significant progress has been achieved in understanding the endogenous regeneration in urodeles and fishes with the hope that they could help to reach our goal of designing future strategies for human regeneration.

Characterization of Proliferating Neural Progenitors after Spinal Cord Injury in Adult Zebrafish

Zebrafish can repair their injured brain and spinal cord after injury unlike adult mammalian central nervous system. Any injury to zebrafish spinal cord would lead to increased proliferation and neurogenesis. There are presences of proliferating progenitors from which both neuronal and glial loss can be reversed by appropriately generating new neurons and glia. We have demonstrated the presence of multiple progenitors, which are different types of proliferating populations like Sox2⁺ neural progenitor, A2B5⁺ astrocyte/ glial progenitor, NG2⁺ oligodendrocyte progenitor, radial glia and Schwann cell like progenitor. We analyzed the expression levels of two common markers of dedifferentiation like msx-b and vimentin during regeneration along with some of the pluripotency associated factors to explore the possible role of these two processes. Among the several key factors related to pluripotency, pou5f1 and sox2 are upregulated during regeneration and associated with activation of neural progenitor cells. Uncovering the molecular mechanism for endogenous regeneration of adult zebrafish spinal cord would give us more clues on important targets for future therapeutic approach in mammalian spinal cord repair and regeneration.

Dynamic membrane depolarization is an early regulator of ependymoglial cell response to spinal cord injury in axolotl

Salamanders, such as the Mexican axolotl, are some of the few vertebrates fortunate in their ability to regenerate diverse structures after injury. Unlike mammals they are able to regenerate a fully functional spinal cord after injury. However, the molecular circuitry required to initiate a pro-regenerative response after spinal cord injury is not well understood. To address this question we developed a spinal cord injury model in axolotls and used in vivo imaging of labeled ependymoglial cells to characterize the response of these cells to injury. Using in vivo imaging of ion sensitive dyes we identified that spinal cord injury induces a rapid and dynamic change in the resting membrane potential of ependymoglial cells. Prolonged depolarization of ependymoglial cells after injury inhibits ependymoglial cell proliferation and subsequent axon regeneration. Using transcriptional profiling we identified c-Fos as a key voltage sensitive early response gene that is expressed specifically in the ependymoglial cells after injury. This data establishes that dynamic changes in the membrane potential after injury are essential for regulating the specific spatiotemporal expression of c-Fos that is critical for promoting faithful spinal cord regeneration in axolotl.

Different Astrocytic Activation between Adult Gekko japonicus and Rats during Wound Healing In Vitro

Glial scar formation is a major obstacle to regeneration after spinal cord injury. Moreover, it has been shown that the astrocytic response to injury differs between species. Gekko japonicas is a type of reptile and it shows differential glial activation compared to that of rats. The purpose of the present study was to compare the proliferation and migration of astrocytes in the spinal cords of geckos and rats after injury in vitro. Spinal cord homogenate stimulation and scratch wound models were used to induce astrocytic activation in adult and embryonic rats, as well as in adult geckos. Our results indicated that astrocytes from the adult rat were likely activated by mechanical stimulation, even though they showed lower proliferation abilities than the astrocytes from the gecko under normal conditions. Furthermore, a transcriptome analysis revealed that the differentially expressed genes in astrocytes from adult rats and those from geckos were enriched in pathways involved in proliferation and the response to stimuli. This implies that intrinsic discrepancies in gene expression patterns might contribute to the differential activation of astrocytes between species.

Regeneration of Xenopus laevis spinal cord requires Sox2/3 expressing cells

Spinal cord regeneration is very inefficient in humans, causing paraplegia and quadriplegia. Studying model organisms that can regenerate the spinal cord in response to injury could be useful for understanding the cellular and molecular mechanisms that explain why this process fails in humans. Here, we use Xenopus laevis as a model organism to study spinal cord repair. Histological and functional analyses showed that larvae at pre-metamorphic stages restore anatomical continuity of the spinal cord and recover swimming after complete spinal cord transection. These regenerative capabilities decrease with onset of metamorphosis. The ability to study regenerative and non-regenerative stages in Xenopus laevis makes it a unique model system to study regeneration. We studied the response of Sox2(/)3 expressing cells to spinal cord injury and their function in the regenerative process. We found that cells expressing Sox2 and/or Sox3 are present in the ventricular zone of regenerative animals and decrease in non-regenerative froglets. Bromodeoxyuridine (BrdU) experiments and in vivo time-lapse imaging studies using green fluorescent protein (GFP) expression driven by the Sox3 promoter showed a rapid, transient and massive proliferation of Sox2(/)3(+) cells in response to injury in the regenerative stages. The in vivo imaging also demonstrated that Sox2(/)3(+) neural progenitor cells generate neurons in response to injury. In contrast, these cells showed a delayed and very limited response in non-regenerative froglets. Sox2 knockdown and overexpression of a dominant negative form of Sox2 disrupts locomotor and anatomical-histological recovery. We also found that neurogenesis markers increase in response to injury in regenerative but not in non-regenerative animals. We conclude that Sox2 is necessary for spinal cord regeneration and suggest a model whereby spinal cord injury activates proliferation of Sox2/3 expressing cells and their differentiation into neurons, a mechanism that is lost in non-regenerative froglets.

Does repair of spinal cord injury follow the evolutionary theory?

Lower vertebrates, such as fish and amphibians, and higher vertebrates in embryonic development can acquire complete regeneration of complex body structures, including the spinal cord, an important part of the central nervous system. However, with species evolution and development, this regenerative capacity gradually weakens and even disappears, but the cellular and molecular mechanisms remain poorly understood. We explored the differences in mechanisms of spinal cord regeneration capability between lower and higher vertebrates, investigated differences in their cellular and molecular mechanisms and between the spinal cord structures of lower vertebrates and mammals, such as rat and monkey, to search for theoretical evidence and therapeutic targets for nerve regeneration in human spinal cord.

Spinal Cord Ependymal Responses to Naturally Occurring Traumatic Spinal Cord Injury in Dogs

The spinal cord ependymal layer (SEL) is a recent focus in spinal cord injury (SCI) research because of its potential to serve as a source of endogenous neural stem cells. Dogs are an important spontaneous model of SCI; however, there is a paucity of information available in the literature regarding the canine SEL. Here we describe the histologic appearance and immunohistochemical staining patterns of the SEL in normal dogs (n = 4) and dogs with acute SCI caused by intervertebral disk extrusion (n = 7). Immunohistochemical staining for PCNA, Ki-67, caspase 3, E-cadherin, GFAP, and vimentin was employed in both groups. Staining for Ki-67 was absent in the SEL of normal and SCI-affected dogs, indicating possible restricted proliferative capacity of the canine SEL acutely after SCI. GFAP-positive cells were increased after SCI at both at the lesion epicenter and at proximal spinal cord sites (P = .001 and P = .006, respectively), supporting the possibility of astrocytic differentiation within the SEL after SCI. Total E-cadherin staining did not differ between normal and SCI-affected dogs (P = .42 for lesion epicenter, P = .09 at proximal sites) and was restricted to the apical cell surface in normal dogs. After SCI, E-cadherin staining was membrane-circumferential and cytosolic in nature, indicating possible loss of cellular polarity after injury that could drive cell migration from the SEL to injury sites. Enhanced GFAP expression and changes in E-cadherin expression patterns support additional studies to evaluate the canine SEL as a source of endogenous neural precursors that may be modulated for future clinical interventions after SCI.

Precise control of miR-125b levels is required to create a regeneration-permissive environment after spinal cord injury: a cross-species comparison between salamander and rat

Most spinal cord injuries lead to permanent paralysis in mammals. By contrast, the remarkable regenerative abilities of salamanders enable full functional recovery even from complete spinal cord transections. The molecular differences underlying this evolutionary divergence between mammals and amphibians are poorly understood. We focused on upstream regulators of gene expression as primary entry points into this question. We identified a group of microRNAs (miRNAs) that are conserved between the Mexican axolotl salamander (Ambystoma mexicanum) and mammals but show marked cross-species differences in regulation patterns following spinal cord injury. We found that precise post-injury levels of one of these miRNAs (miR-125b) is essential for functional recovery, and guides correct regeneration of axons through the lesion site in a process involving the direct downstream target Sema4D in axolotls. Translating these results to a mammalian model, we increased miR-125b levels in the rat through mimic treatments following spinal cord transection. These treatments downregulated Sema4D and other glial-scar-related genes, and enhanced the animal’s functional recovery. Our study identifies a key regulatory molecule conserved between salamander and mammal, and shows that the expression of miR-125b and Sema4D must be carefully controlled in the right cells at the correct level to promote regeneration. We also show that these molecular components of the salamander’s regeneration-permissive environment can be experimentally harnessed to improve treatment outcomes for mammalian spinal cord injuries.

SCO-spondin derived peptide NX210 induces neuroprotection in vitro and promotes fiber regrowth and functional recovery after spinal cord injury

In mammals, the limited regenerating potential of the central nervous system (CNS) in adults contrasts with the plasticity of the embryonic and perinatal periods. SCO (subcommissural organ)-spondin is a protein secreted early by the developing central nervous system, potentially involved in the development of commissural fibers. SCO-spondin stimulates neuronal differentiation and neurite growth in vitro. NX210 oligopeptide was designed from SCO-spondin's specific thrombospondin type 1 repeat (TSR) sequences that support the main neurogenic properties of the molecule. The objective of this work was to assess the neuroprotective and neuroregenerative properties of NX210 in vitro and in vivo for the treatment of spinal cord injury (SCI). In vitro studies were carried out on the B104 neuroblastoma cell line demonstrating neuroprotection by the resistance to oxidative damage using hydrogen peroxide and the measure of cell viability by metabolic activity. In vivo studies were performed in two rat models of SCI: (1) a model of aspiration of dorsal funiculi followed by the insertion of a collagen tube in situ to limit collateral sprouting; white matter regeneration was assessed using neurofilament immunostaining; (2) a rat spinal cord contusion model to assess functional recovery using BBB scale and reflex testing. We demonstrate for the first time that NX210 (a) provides neuroprotection to oxidative stress in the B104 neuroblastoma cells, (b) stimulates axonal regrowth in longitudinally oriented neofibers in the aspiration model of SCI and (c) significantly improves functional recovery in the contusive model of SCI.

Central canal ependymal cells proliferate extensively in response to traumatic spinal cord injury but not demyelinating lesions

The adult mammalian spinal cord has limited regenerative capacity in settings such as spinal cord injury (SCI) and multiple sclerosis (MS). Recent studies have revealed that ependymal cells lining the central canal possess latent neural stem cell potential, undergoing proliferation and multi-lineage differentiation following experimental SCI. To determine whether reactive ependymal cells are a realistic endogenous cell population to target in order to promote spinal cord repair, we assessed the spatiotemporal dynamics of ependymal cell proliferation for up to 35 days in three models of spinal pathologies: contusion SCI using the Infinite Horizon impactor, focal demyelination by intraspinal injection of lysophosphatidylcholine (LPC), and autoimmune-mediated multi-focal demyelination using the active experimental autoimmune encephalomyelitis (EAE) model of MS. Contusion SCI at the T9-10 thoracic level stimulated a robust, long-lasting and long-distance wave of ependymal proliferation that peaked at 3 days in the lesion segment, 14 days in the rostral segment, and was still detectable at the cervical level, where it peaked at 21 days. This proliferative wave was suppressed distal to the contusion. Unlike SCI, neither chemical- nor autoimmune-mediated demyelination triggered ependymal cell proliferation at any time point, despite the occurrence of demyelination (LPC and EAE), remyelination (LPC) and significant locomotor defects (EAE). Thus, traumatic SCI induces widespread and enduring activation of reactive ependymal cells, identifying them as a robust cell population to target for therapeutic manipulation after contusion; conversely, neither demyelination, remyelination nor autoimmunity appears sufficient to trigger proliferation of quiescent ependymal cells in models of MS-like demyelinating diseases.

Anatomical and electrophysiological plasticity of locomotor networks following spinal transection in the salamander

Recovery of locomotor behavior following spinal cord injury can occur spontaneously in some vertebrates, such as fish, urodele amphibians, and certain reptiles. This review provides an overview of the current status of our knowledge on the anatomical and electrophysiological changes occurring within the spinal cord that lead to, or are associated with the re-expression of locomotion in spinally-transected salamanders. A better understanding of these processes will help to devise strategies for restoring locomotor function in mammals, including humans.

Spatial distribution of prominin-1 (CD133)-positive cells within germinative zones of the vertebrate brain

BACKGROUND

In mammals, embryonic neural progenitors as well as adult neural stem cells can be prospectively isolated based on the cell surface expression of prominin-1 (CD133), a plasma membrane glycoprotein. In contrast, characterization of neural progenitors in non-mammalian vertebrates endowed with significant constitutive neurogenesis and inherent self-repair ability is hampered by the lack of suitable cell surface markers. Here, we have investigated whether prominin-1-orthologues of the major non-mammalian vertebrate model organisms show any degree of conservation as for their association with neurogenic geminative zones within the central nervous system (CNS) as they do in mammals or associated with activated neural progenitors during provoked neurogenesis in the regenerating CNS.

METHODS

We have recently identified prominin-1 orthologues from zebrafish, axolotl and chicken. The spatial distribution of prominin-1-positive cells--in comparison to those of mice--was mapped in the intact brain in these organisms by non-radioactive in situ hybridization combined with detection of proliferating neural progenitors, marked either by proliferating cell nuclear antigen or 5-bromo-deoxyuridine. Furthermore, distribution of prominin-1 transcripts was investigated in the regenerating spinal cord of injured axolotl.

RESULTS

Remarkably, a conserved association of prominin-1 with germinative zones of the CNS was uncovered as manifested in a significant co-localization with cell proliferation markers during normal constitutive neurogenesis in all species investigated. Moreover, an enhanced expression of prominin-1 became evident associated with provoked, compensatory neurogenesis during the epimorphic regeneration of the axolotl spinal cord. Interestingly, significant prominin-1-expressing cell populations were also detected at distinct extraventricular (parenchymal) locations in the CNS of all vertebrate species being suggestive of further, non-neurogenic neural function(s).

CONCLUSION/INTERPRETATION

Collectively, our work provides the first data set describing a comparative analysis of prominin-1-positive progenitor cells across species establishing a framework for further functional characterization in the context of regeneration.

Macrophages are required for adult salamander limb regeneration

The failure to replace damaged body parts in adult mammals results from a muted growth response and fibrotic scarring. Although infiltrating immune cells play a major role in determining the variable outcome of mammalian wound repair, little is known about the modulation of immune cell signaling in efficiently regenerating species such as the salamander, which can regrow complete body structures as adults. Here we present a comprehensive analysis of immune signaling during limb regeneration in axolotl, an aquatic salamander, and reveal a temporally defined requirement for macrophage infiltration in the regenerative process. Although many features of mammalian cytokine/chemokine signaling are retained in the axolotl, they are more dynamically deployed, with simultaneous induction of inflammatory and anti-inflammatory markers within the first 24 h after limb amputation. Systemic macrophage depletion during this period resulted in wound closure but permanent failure of limb regeneration, associated with extensive fibrosis and disregulation of extracellular matrix component gene expression. Full limb regenerative capacity of failed stumps was restored by reamputation once endogenous macrophage populations had been replenished. Promotion of a regeneration-permissive environment by identification of macrophage-derived therapeutic molecules may therefore aid in the regeneration of damaged body parts in adult mammals.

HMGB1 protein does not mediate the inflammatory response in spontaneous spinal cord regeneration: a hint for CNS regeneration

Uncontrolled, excessive inflammation contributes to the secondary tissue damage of traumatic spinal cord, and HMGB1 is highlighted for initiation of a vicious self-propagating inflammatory circle by release from necrotic cells or immune cells. Several regenerative-competent vertebrates have evolved to circumvent the second damages during the spontaneous spinal cord regeneration with an unknown HMGB1 regulatory mechanism. By genomic surveys, we have revealed that two paralogs of HMGB1 are broadly retained from fish in the phylogeny. However, their spatial-temporal expression and effects, as shown in lowest amniote gecko, were tightly controlled in order that limited inflammation was produced in spontaneous regeneration. Two paralogs from gecko HMGB1 (gHMGB1) yielded distinct injury and infectious responses, with gHMGB1b significantly up-regulated in the injured cord. The intracellular gHMGB1b induced less release of inflammatory cytokines than gHMGB1a in macrophages, and the effects could be shifted by exchanging one amino acid in the inflammatory domain. Both intracellular proteins were able to mediate neuronal programmed apoptosis, which has been indicated to produce negligible inflammatory responses. In vivo studies demonstrated that the extracellular proteins could not trigger a cascade of the inflammatory cytokines in the injured spinal cord. Signal transduction analysis found that gHMGB1 proteins could not bind with cell surface receptors TLR2 and TLR4 to activate inflammatory signaling pathway. However, they were able to interact with the receptor for advanced glycation end products to potentiate oligodendrocyte migration by activation of both NFκB and Rac1/Cdc42 signaling. Our results reveal that HMGB1 does not mediate the inflammatory response in spontaneous spinal cord regeneration, but it promotes CNS regeneration.

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.

Enhanced functional recovery in MRL/MpJ mice after spinal cord dorsal hemisection

Adult MRL/MpJ mice have been shown to possess unique regeneration capabilities. They are able to heal an ear-punched hole or an injured heart with normal tissue architecture and without scar formation. Here we present functional and histological evidence for enhanced recovery following spinal cord injury (SCI) in MRL/MpJ mice. A control group (C57BL/6 mice) and MRL/MpJ mice underwent a dorsal hemisection at T9 (thoracic vertebra 9). Our data show that MRL/MpJ mice recovered motor function significantly faster and more completely. We observed enhanced regeneration of the corticospinal tract (CST). Furthermore, we observed a reduced astrocytic response and fewer micro-cavities at the injury site, which appear to create a more growth-permissive environment for the injured axons. Our data suggest that the reduced astrocytic response is in part due to a lower lesion-induced increase of cell proliferation post-SCI, and a reduced astrocytic differentiation of the proliferating cells. Interestingly, we also found an increased number of proliferating microglia, which could be involved in the MRL/MpJ spinal cord repair mechanisms. Finally, to evaluate the molecular basis of faster spinal cord repair, we examined the difference in gene expression changes in MRL/MpJ and C57BL/6 mice after SCI. Our microarray data support our histological findings and reveal a transcriptional profile associated with a more efficient spinal cord repair in MRL/MpJ mice.

C. elegans as a genetic model to identify novel cellular and molecular mechanisms underlying nervous system regeneration

Research into conditions that improve axon regeneration has the potential to open a new door for treatment of brain injury caused by stroke and neurodegenerative diseases of aging, such as Alzheimer, by harnessing intrinsic neuronal ability to reorganize itself. Elucidating the molecular mechanisms of axon regeneration should shed light on how this process becomes restricted in the postnatal stage and in CNS and therefore could provide therapeutic targets for developing strategy to improve axon regeneration in adult CNS. In this review, we first discuss the general view about nerve regeneration and the advantages of using C. elegans as a model system to study axon regeneration. We then compare the conserved regeneration patterns and molecular mechanisms between C. elegans and vertebrates. Lastly, we discuss the power of femtosecond laser technology and its application in axon regeneration research.

The challenges of long-distance axon regeneration in the injured CNS

Injury to the central nervous system (CNS) that results in long-tract axonal damage typically leads to permanent functional deficits in areas innervated at, and below, the level of the lesion. The initial ischemia, inflammation, and neurodegeneration are followed by a progressive generation of scar tissue, dieback of transected axons, and demyelination, creating an area inhibitory to regrowth and recovery. Two ways to combat this inhibition is to therapeutically target the extrinsic and intrinsic properties of the axon-scar environment. Scar tissue within and surrounding the lesion site can be broken down using an enzyme known as chondroitinase. Negative regulators of adult neuronal growth, such as Nogo, can be neutralized with antibodies. Both therapies greatly improve functional recovery in animal models. Alternatively, modifying the intrinsic growth properties of CNS neurons through gene therapy or pharmacotherapy has also shown promising axonal regeneration after injury. Despite these promising therapies, the main challenge of long-distance axon regeneration still remains; achieving a level of functional and organized connectivity below the level of the lesion that mimics the intact CNS.

Caltubin, a novel molluscan tubulin-interacting protein, promotes axonal growth and attenuates axonal degeneration of rodent neurons

Axotomized central neurons of most invertebrate species demonstrate a strong regenerative capacity, and as such may provide valuable molecular insights and new tools to promote axonal regeneration in injured mammalian neurons. In this study, we identified a novel molluscan protein, caltubin, ubiquitously expressed in central neurons of Lymnaea stagnalis and locally synthesized in regenerating neurites. Reduction of caltubin levels by gene silencing inhibits the outgrowth and regenerative ability of adult Lymnaea neurons and decreases local α- and β-tubulin levels in neurites. Caltubin binds to α- and/or β-tubulin in both Lymnaea and rodent neurons. Expression of caltubin in PC12 cells and mouse cortical neurons promotes NGF-induced axonal outgrowth and attenuates axonal retraction after injury. This is the first study illustrating that a xenoprotein can enhance outgrowth and prevent degeneration of injured mammalian neurons. These results may open up new avenues in molecular repair strategies through the insertion of molecular components of invertebrate regenerative pathways into mammalian neurons.

Crystal structure of the Nogo-receptor-2

The inhibition of axon regeneration upon mechanical injury is dependent on interactions between Nogo receptors (NgRs) and their myelin-derived ligands. NgRs are composed of a leucine-rich repeat (LRR) region, thought to be structurally similar among the different isoforms of the receptor, and a divergent "stalk" region. It has been shown by others that the LRR and stalk regions of NgR1 and NgR2 have distinct roles in conferring binding affinity to the myelin associated glycoprotein (MAG) in vivo. Here, we show that purified recombinant full length NgR1 and NgR2 maintain significantly higher binding affinity for purified MAG as compared to the isolated LRR region of either NgR1 or NgR2. We also present the crystal structure of the LRR and part of the stalk regions of NgR2 and compare it to the previously reported NgR1 structure with respect to the distinct signaling properties of the two receptor isoforms.

Meningeal cells and glia establish a permissive environment for axon regeneration after spinal cord injury in newts

Background

Newts have the remarkable ability to regenerate their spinal cords as adults. Their spinal cords regenerate with the regenerating tail after tail amputation, as well as after a gap-inducing spinal cord injury (SCI), such as a complete transection. While most studies on newt spinal cord regeneration have focused on events occurring after tail amputation, less attention has been given to events occurring after an SCI, a context that is more relevant to human SCI. Our goal was to use modern labeling and imaging techniques to observe axons regenerating across a complete transection injury and determine how cells and the extracellular matrix in the injury site might contribute to the regenerative process.

Results

We identify stages of axon regeneration following a spinal cord transection and find that axon regrowth across the lesion appears to be enabled, in part, because meningeal cells and glia form a permissive environment for axon regeneration. Meningeal and endothelial cells regenerate into the lesion first and are associated with a loose extracellular matrix that allows axon growth cone migration. This matrix, paradoxically, consists of both permissive and inhibitory proteins. Axons grow into the injury site next and are closely associated with meningeal cells and glial processes extending from cell bodies surrounding the central canal. Later, ependymal tubes lined with glia extend into the lesion as well. Finally, the meningeal cells, axons, and glia move as a unit to close the gap in the spinal cord. After crossing the injury site, axons travel through white matter to reach synaptic targets, and though ascending axons regenerate, sensory axons do not appear to be among them. This entire regenerative process occurs even in the presence of an inflammatory response.

Conclusions

These data reveal, in detail, the cellular and extracellular events that occur during newt spinal cord regeneration after a transection injury and uncover an important role for meningeal and glial cells in facilitating axon regeneration. Given that these cell types interact to form inhibitory barriers in mammals, identifying the mechanisms underlying their permissive behaviors in the newt will provide new insights for improving spinal cord regeneration in mammals.