Long-distance growth and connectivity of neural stem cells after severe spinal cord injury

Signaling regulations of neuronal regenerative ability

Different from physiological axon growth during development, a major limiting factor for successful axon regeneration is the poor intrinsic regenerative capacity in mature neurons in the adult mammalian central nervous system (CNS). Recent studies identified several molecular pathways, including PTEN/mTOR, Jak/STAT, DLK/JNK, providing important probes in investigating the mechanisms by which the regenerative ability is regulated. This review will summarize these recent findings and speculate their implications.

Synthetic control of mammalian-cell motility by engineering chemotaxis to an orthogonal bioinert chemical signal

Directed migration of diverse cell types plays a critical role in biological processes ranging from development and morphogenesis to immune response, wound healing, and regeneration. However, techniques to direct, manipulate, and study cell migration in vitro and in vivo in a specific and facile manner are currently limited. We conceived of a strategy to achieve direct control over cell migration to arbitrary user-defined locations, independent of native chemotaxis receptors. Here, we show that genetic modification of cells with an engineered G protein-coupled receptor allows us to redirect their migration to a bioinert drug-like small molecule, clozapine-N-oxide (CNO). The engineered receptor and small-molecule ligand form an orthogonal pair: The receptor does not respond to native ligands, and the inert drug does not bind to native cells. CNO-responsive migration can be engineered into a variety of cell types, including neutrophils, T lymphocytes, keratinocytes, and endothelial cells. The engineered cells migrate up a gradient of the drug CNO and transmigrate through endothelial monolayers. Finally, we demonstrate that T lymphocytes modified with the engineered receptor can specifically migrate in vivo to CNO-releasing beads implanted in a live mouse. This technology provides a generalizable genetic tool to systematically perturb and control cell migration both in vitro and in vivo. In the future, this type of migration control could be a valuable module for engineering therapeutic cellular devices.

Long-lasting significant functional improvement in chronic severe spinal cord injury following scar resection and polyethylene glycol implantation

We identified a suitable biomatrix that improved axon regeneration and functional outcome after partial (moderate) and complete (severe) chronic spinal cord injury (SCI) in rat. Five weeks after dorsal thoracic hemisection injury the lesion scar was resected via aspiration and the resulting cavity was filled with different biopolymers such as Matrigel™, alginate-hydrogel and polyethylene glycol 600 (PEG) all of which have not previously been used as sole graft-materials in chronic SCI. Immunohistological staining revealed marked differences between these compounds regarding axon regeneration, invasion/elongation of astrocytes, fibroblasts, endothelial and Schwann cells, revascularization, and collagen deposition. According to axon regeneration-supporting effects, the biopolymers could be ranked in the order PEG>>alginate-hydrogel>Matrigel™. Even after complete chronic transection, the PEG-bridge allowed long-distance axon regeneration through the grafted area and for, at least, 1cm beyond the lesion/graft border. As revealed by electron microscopy, bundles of regenerating axons within the matrix area received myelin ensheathment from Schwann cells. The beneficial effects of PEG-implantation into the resection-cavity were accompanied by long-lasting significant locomotor improvement over a period of 8months. Following complete spinal re-transection at the rostral border of the PEG-graft the locomotor recovery was aborted, suggesting a functional role of regenerated axons in the initial locomotor improvement. In conclusion, scar resection and subsequent implantation of PEG into the generated cavity leads to tissue recovery, axon regeneration, myelination and functional improvement that have not been achieved before in severe chronic SCI.

Existence of a potential neurogenic system in the adult human brain

Background

Prevailingly, adult mammalian neurogenesis is thought to occur in discrete, separate locations known as neurogenic niches that are best characterized in the subgranular zone (SGZ) of the dentate gyrus and in the subventricular zone (SVZ). The existence of adult human neurogenic niches is controversial.

Methods

The existence of neurogenic niches was investigated with neurogenesis marker immunostaining in histologically normal human brains obtained from autopsies. Twenty-eight adult temporal lobes, specimens from limbic structures and the hypothalamus of one newborn and one adult were examined.

Results

The neural stem cell marker nestin stained circumventricular organ cells and the immature neuronal marker doublecortin (DCX) stained hypothalamic and limbic structures adjacent to circumventricular organs; both markers stained a continuous structure running from the hypothalamus to the hippocampus. The cell proliferation marker Ki-67 was detected predominately in structures that form the septo-hypothalamic continuum. Nestin-expressing cells were located in the fimbria-fornix at the insertion of the choroid plexus; ependymal cells in this structure expressed the putative neural stem cell marker CD133. From the choroidal fissure in the temporal lobe, a nestin-positive cell layer spread throughout the SVZ and subpial zone. In the subpial zone, a branch of this layer reached the hippocampal sulcus and ended in the SGZ (principally in the newborn) and in the subiculum (principally in the adults). Another branch of the nestin-positive cell layer in the subpial zone returned to the optic chiasm. DCX staining was detected in the periventricular and middle hypothalamus and more densely from the mammillary body to the subiculum through the fimbria-fornix, thus running through the principal neuronal pathway from the hippocampus to the hypothalamus. The column of the fornix forms part of this pathway and appears to coincide with the zone previously identified as the human rostral migratory stream. Partial co-labeling with DCX and the neuronal marker βIII-tubulin was also observed.

Conclusions

Collectively, these findings suggest the existence of an adult human neurogenic system that rises from the circumventricular organs and follows, at minimum, the circuitry of the hypothalamus and limbic system.

Reactive gliosis and the multicellular response to CNS damage and disease

The CNS is prone to heterogeneous insults of diverse etiologies that elicit multifaceted responses. Acute and focal injuries trigger wound repair with tissue replacement. Diffuse and chronic diseases provoke gradually escalating tissue changes. The responses to CNS insults involve complex interactions among cells of numerous lineages and functions, including CNS intrinsic neural cells, CNS intrinsic nonneural cells, and CNS extrinsic cells that enter from the circulation. The contributions of diverse nonneuronal cell types to outcome after acute injury, or to the progression of chronic disease, are of increasing interest as the push toward understanding and ameliorating CNS afflictions accelerates. In some cases, considerable information is available, in others, comparatively little, as examined and reviewed here.

S6 kinase inhibits intrinsic axon regeneration capacity via AMP kinase in Caenorhabditis elegans

The ability of axons to regrow after injury is determined by the complex interplay of intrinsic growth programs and external cues. In Caenorhabditis elegans mechanosensory neuron, axons exhibit robust regenerative regrowth following laser axotomy. By surveying conserved metabolic signaling pathways, we have identified the ribosomal S6 kinase RSKS-1 as a new cell-autonomous inhibitor of axon regeneration. RSKS-1 is not required for axonal development but inhibits axon regrowth after injury in multiple neuron types. Loss of function in rsks-1 results in more rapid growth cone formation after injury and accelerates subsequent axon extension. The enhanced regrowth of rsks-1 mutants is partly dependent on the DLK-1 MAPK cascade. An essential output of RSKS-1 in axon regrowth is the metabolic sensor AMP kinase, AAK-2. We further show that the antidiabetic drug phenformin, which activates AMP kinase, can promote axon regrowth. Our data reveal a new function for an S6 kinase acting through an AMP kinase in regenerative growth of injured axons.

Fgf2 improves functional recovery-decreasing gliosis and increasing radial glia and neural progenitor cells after spinal cord injury

OBJECTIVES

A major impediment for recovery after mammalian spinal cord injury (SCI) is the glial scar formed by proliferating reactive astrocytes. Finding factors that may reduce glial scarring, increase neuronal survival, and promote neurite outgrowth are of major importance for improving the outcome after SCI. Exogenous fibroblast growth factor (Fgf) has been shown to decrease injury volume and improve functional outcome; however, the mechanisms by which this is mediated are still largely unknown.

METHODS

In this study, Fgf2 was administered for 2 weeks in mice subcutaneously, starting 30 min after spinal cord hemisection.

RESULTS

Fgf2 treatment decreased the expression of TNF-a at the lesion site, decreased monocyte/macrophage infiltration, and decreased gliosis. Fgf2 induced astrocytes to adopt a polarized morphology and increased expression of radial markers such as Pax6 and nestin. In addition, the levels of chondroitin sulfate proteoglycans (CSPGs), expressed by glia, were markedly decreased. Furthermore, Fgf2 treatment promotes the formation of parallel glial processes, "bridges," at the lesion site that enable regenerating axons through the injury site. Additionally, Fgf2 treatment increased Sox2-expressing cells in the gray matter and neurogenesis around and at the lesion site. Importantly, these effects were correlated with enhanced functional recovery of the left paretic hind limb.

CONCLUSIONS

Thus, early pharmacological intervention with Fgf2 following SCI is neuroprotective and creates a proregenerative environment by the modulation of the glia response.

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.

Transplanting neural progenitors into a complete transection model of spinal cord injury

Neural progenitor cell (NPC) transplantation is a promising therapeutic strategy for spinal cord injury (SCI) because of the potential for cell replacement and restoration of connectivity. Our previous studies have shown that transplants of NPC, composed of neuron- and glia-restricted progenitors derived from the embryonic spinal cord, survived well in partial lesion models and generated graft-derived neurons, which could be used to form a functional relay. We have now examined the properties of a similar NPC transplant using a complete transection model in juvenile and adult rats. We found poor survival of grafted cells despite using a variety of lesion methods, matrices, and delays of transplantation. If, instead of cultured progenitor cells, the transplants were composed of segmental or dissociated segments of fetal spinal cord (FSC) derived from similar-staged embryos, grafted cells survived and integrated well with host tissue in juvenile and adult rats. FSC transplants differentiated into neurons and glial cells, including astrocytes and oligodendrocytes. Graft-derived neurons expressed glutaminergic and GABAergic markers. Grafted cells also migrated and extended processes into host tissue. Analysis of axon growth from the host spinal cord showed serotonin-positive fibers and biotinylated dextran amine-traced propriospinal axons growing into the transplants. These results suggest that in treating severe SCI, such as complete transection, NPC grafting faces major challenges related to cell survival and formation of a functional relay. Lessons learned from the efficacy of FSC transplants could be used to develop a therapeutic strategy based on neural progenitor cells for severe SCI.

Functional regeneration beyond the glial scar

Astrocytes react to CNS injury by building a dense wall of filamentous processes around the lesion. Stromal cells quickly take up residence in the lesion core and synthesize connective tissue elements that contribute to fibrosis. Oligodendrocyte precursor cells proliferate within the lesion and entrap dystrophic axon tips. Here we review evidence that this aggregate scar acts as the major barrier to regeneration of axons after injury. We also consider several exciting new interventions that allow axons to regenerate beyond the glial scar, and discuss the implications of this work for the future of regeneration biology.

Cell therapy for spinal cord injuries: what is really going on?

During the last two decades, many experiments have examined the ability of cell transplants to ameliorate the loss of function after spinal cord injuries, with the hope of developing interventions to benefit patients. Although many reports suggest positive effects, there is growing concern over the quality of the available preclinical data. It is therefore important to ask whether this worldwide investigative process is close to defining a cell transplant protocol that could be translated into human patients with a realistic chance of success. This review systematically examines the strength of the preclinical evidence and outlines mechanisms by which transplanted cells may mediate their effects in spinal cord injuries. First, we examined changes in voluntary movements in the forelimb associated with cell transplants after partial cervical lesions. Second, we examined the efficacy of transplanted cells to restore electrophysiological conduction across a complete thoracic lesion. We postulated that cell therapies found to be successful in both models could reasonably have potential to treat human patients. We conclude that although there are data to support a beneficial effect of cell transplantation, most reports provide only weak evidence because of deficits in experimental design. The mechanisms by which transplanted cells mediate their functional effects remain unclear.

Survival, Differentiation, and Migration of High-Purity Mouse Embryonic Stem Cell-derived Progenitor Motor Neurons in Fibrin Scaffolds after Sub-Acute Spinal Cord Injury

Embryonic stem (ES) cells can be differentiated into many neural cell types that hold great potential as cell replacement therapies following spinal cord injury (SCI). Coupling stem cell transplantation with biomaterial scaffolds can produce a unified combination therapy with several potential advantages including enhanced cell survival, greater transplant retention, reduced scarring, and improved integration at the transplant/host interface. Undesired cell types, however, are commonly present in ES-cell derived cultures due to the limited efficiency of most ES cell induction protocols. Heterogeneous cell populations can confound the interaction between the biomaterial and specific neural populations leading to undesired outcomes. In particular, biomaterials scaffolds may enhance tumor formation by promoting survival and proliferation of undifferentiated ES cells that can persist after induction. Methods for purification of specific ES cell-derived neural populations are necessary to recognize the full potential of combination therapies involving biomaterials and ES cell-derived neural populations. We previously developed a method for enriching ES cell-derived progenitor motor neurons (pMNs) induced from mouse ES cells via antibiotic selection and showed that the enriched cell populations are depleted of pluripotent stem cells. In this study, we demonstrate the survival and differentiation of enriched pMNs within three dimensional (3D) fibrin scaffolds in vitro and when transplanted into a sub-acute dorsal hemisection model of SCI into neurons, oligodendrocytes and astrocytes.

Neurotrophic factors in spinal cord injury

A major challenge in repairing the injured spinal cord is to assure survival of damaged cells and to encourage regrowth of severed axons. Because neurotrophins are known to affect these processes during development, many experimental approaches to improving function of the injured spinal cord have made use of these agents, particularly Brain derived neurotrophic factor (BDNF) and Neurotrophin-3 (NT-3). More recently, neurotrophins have also been shown to affect the physiology of cells and synapses in the spinal cord. The effect of neurotrophins on circuit performance adds an important dimension to their consideration as agents for repairing the injured spinal cord. In this chapter we discuss the role of neurotrophins in promoting recovery after spinal cord injury from both a structural and functional perspective.

Decorin blocks scarring and cystic cavitation in acute and induces scar dissolution in chronic spinal cord wounds

In the injured central nervous system (CNS), transforming growth factor (TGF)-β1/2-induced scarring and wound cavitation impede axon regeneration implying that a combination of both scar suppression and axogenic treatments is required to achieve functional recovery. After treating acute and chronic dorsal funicular spinal cord lesions (DFL) in adult rats with the pan-TGF-β1/2 antagonist Decorin, we report that in: (1), acute DFL, the development of all injury parameters was significantly retarded e.g., wound cavity area by 68%, encapsulation of the wound by a glia limitans accessoria (GLA) by 65%, GLA basal lamina thickness by 94%, fibronectin, NG2 and Sema-3A deposition by 87%, 48% and 48%, respectively, and both macrophage and reactive microglia accumulations by 60%; and (2), chronic DFL, all the above parameters were attenuated to a lesser extent e.g., wound cavity area by 11%, GLA encapsulation by 25%, GLA basal lamina thickness by 31%, extracellular fibronectin, NG2 and Sema-3A deposition by 58%, 22% and 29%, respectively, and macrophage and reactive microglia accumulations by 44%. Moreover, in acute and chronic DFL, levels of tissue plasminogen activator (tPA) were raised (by 236% and 482%, respectively), as were active-MMP-2 (by 64% and 91%, respectively) and active-MMP-9 (by 122% and 18%, respectively), while plasminogen activator inhibitor-1 (PAI-1) was suppressed (by 56% and 23%, respectively) and active-TIMP-1 and active TIMP-2 were both lower but only significantly suppressed in acute DFL (by 56 and 21%, respectively). These findings demonstrate that both scar tissue mass and cavitation are attenuated in acute and chronic spinal cord wounds by Decorin treatment and suggest that the dominant effect of Decorin during acute scarring is anti-fibrogenic through suppression of inflammatory fibrosis by neutralisation of TGF-β1/2 whereas, in chronic lesions, Decorin-induction of tPA and MMP (concomitant with reduced complimentary levels of TIMP and PAI-1) leads to dissolution of the mature established scar by fibrolysis. Decorin also promoted the regeneration of similar numbers of axons through acute and chronic wounds. Accordingly, intrathecal delivery of Decorin offers a potential translatable treatment for scar tissue attenuation in patients with spinal cord injury.

Partial restoration of cardiovascular function by embryonic neural stem cell grafts after complete spinal cord transection

High-level spinal cord injury can lead to cardiovascular dysfunction, including disordered hemodynamics at rest and autonomic dysreflexia during noxious stimulation. To restore supraspinal control of sympathetic preganglionic neurons (SPNs), we grafted embryonic brainstem-derived neural stem cells (BS-NSCs) or spinal cord-derived neural stem cells (SC-NSCs) expressing green fluorescent protein into the T4 complete transection site of adult rats. Animals with injury alone served as controls. Implanting of BS-NSCs but not SC-NSCs resulted in recovery of basal cardiovascular parameters, whereas both cell grafts alleviated autonomic dysreflexia. Subsequent spinal cord retransection above the graft abolished the recovery of basal hemodynamics and reflexic response. BS-NSC graft-derived catecholaminergic and serotonergic neurons showed remarkable long-distance axon growth and topographical innervation of caudal SPNs. Anterograde tracing indicated growth of medullar axons into stem cell grafts and formation of synapses. Thus, grafted embryonic brainstem-derived neurons can act as functional relays to restore supraspinal regulation of denervated SPNs, thereby contributing to cardiovascular functional improvement.

Inosine enhances axon sprouting and motor recovery after spinal cord injury

Although corticospinal tract axons cannot regenerate long distances after spinal cord injury, they are able to sprout collateral branches rostral to an injury site that can help form compensatory circuits in cases of incomplete lesions. We show here that inosine enhances the formation of compensatory circuits after a dorsal hemisection of the thoracic spinal cord in mature rats and improves coordinated limb use. Inosine is a naturally occurring metabolite of adenosine that crosses the cell membrane and, in neurons, activates Mst3b, a protein kinase that is part of a signal transduction pathway that regulates axon outgrowth. Compared to saline-treated controls, rats with dorsal hemisections that were treated with inosine showed three times as many synaptic contacts between corticospinal tract collaterals and long propriospinal interneurons that project from the cervical cord to the lumbar level. Inosine-treated rats also showed stronger serotonergic reinnervation of the lumbar cord than saline-treated controls, and performed well above controls in both open-field testing and a horizontal ladder rung-walking test. Inosine was equally effective whether delivered intracranially or intravenously, and has been shown to be safe for other indications in humans. Thus, inosine might be a useful therapeutic for improving outcome after spinal cord injury.

Advances in peripheral nerve regeneration

Rodent models of nerve injury have increased our understanding of peripheral nerve regeneration, but clinical applications have been scarce, partly because such models do not adequately recapitulate the situation in humans. In human injuries, axons are often required to extend over much longer distances than in mice, and injury leaves distal nerve fibres and target tissues without axonal contact for extended amounts of time. Distal Schwann cells undergo atrophy owing to the lack of contact with proximal neurons, which results in reduced expression of neurotrophic growth factors, changes in the extracellular matrix and loss of Schwann cell basal lamina, all of which hamper axonal extension. Furthermore, atrophy and denervation-related changes in target tissues make good functional recovery difficult to achieve even when axons regenerate all the way to the target tissue. To improve functional outcomes in humans, strategies to increase the speed of axonal growth, maintain Schwann cells in a healthy, repair-capable state and keep target tissues receptive to reinnervation are needed. Use of rodent models of chronic denervation will facilitate our understanding of the molecular mechanisms of peripheral nerve regeneration and create the potential to test therapeutic advances.

Neurogenesis is required for behavioral recovery after injury in the visual system of Xenopus laevis

Nonmammalian vertebrates have a remarkable capacity to regenerate brain tissue in response to central nervous system (CNS) injury. Nevertheless, it is not clear whether animals recover lost function after injury or whether injury-induced cell proliferation mediates recovery. We address these questions using the visual system and visually-guided behavior in Xenopus laevis tadpoles. We established a reproducible means to produce a unilateral focal injury to optic tectal neurons without damaging retinotectal axons. We then assayed a tectally-mediated visual avoidance behavior to evaluate behavioral impairment and recovery. Focal ablation of part of the optic tectum prevents the visual avoidance response to moving stimuli. Animals recover the behavior over the week following injury. Injury induces a burst of proliferation of tectal progenitor cells based on phospho-histone H3 immunolabeling and experiments showing that Musashi-immunoreactive tectal progenitors incorporate the thymidine analog chlorodeoxyuridine after injury. Pulse chase experiments indicate that the newly-generated cells differentiate into N-β-tubulin-immunoreactive neurons. Furthermore, in vivo time-lapse imaging shows that Sox2-expressing neural progenitors divide in response to injury and generate neurons with elaborate dendritic arbors. These experiments indicate that new neurons are generated in response to injury. To test if neurogenesis is necessary for recovery from injury, we blocked cell proliferation in vivo and found that recovery of the visual avoidance behavior is inhibited by drugs that block cell proliferation. Moreover, behavioral recovery is facilitated by changes in visual experience that increase tectal progenitor cell proliferation. Our data indicate that neurogenesis in the optic tectum is critical for recovery of visually-guided behavior after injury.

Restoration of sensorimotor functions after spinal cord injury

The purpose of this review is to discuss the achievements and perspectives regarding rehabilitation of sensorimotor functions after spinal cord injury. In the first part we discuss clinical approaches based on neuroplasticity, a term referring to all adaptive and maladaptive changes within the sensorimotor systems triggered by a spinal cord injury. Neuroplasticity can be facilitated through the training of movements with assistance as needed, and/or by electrical stimulation techniques. The success of such training in individuals with incomplete spinal cord injury critically depends on the presence of physiological proprioceptive input to the spinal cord leading to meaningful muscle activations during movement performances. The addition of rehabilitation technology, such as robotic devices allows for longer training times and provision of feedback information regarding changes in movement performance. Nevertheless, the improvement of function by such approaches for rehabilitation is limited. In the second part, we discuss preclinical approaches to restore function by compensating for the loss of descending input to spinal networks following complete spinal cord injury. This can be achieved with stimulation of spinal networks or approaches to restore their descending input. Electrical and pharmacological stimulation of spinal neural networks is still in an experimental stage; and despite promising repair studies in animal models, translations to humans up to now have not been convincing. It is likely that combinations of techniques targeting the promotion of axonal regeneration and meaningful plasticity are necessary to advance the restoration of function. In the future, refinement of animal studies may contribute to greater translational success.

Tension-induced neurite growth in microfluidic channels

The generation of an effective method for stimulating neuronal growth in specific directions, along well-defined geometries, and in numerous cells could impact areas ranging from fundamental studies of neuronal evolution and morphogenesis, to applications in biomedical diagnostics and nerve regeneration. Applied mechanical stress can regulate neurite growth. Indeed, previous studies have shown that neuronal cells can develop and extend neurites with rapid growth rates under applied "towing" tensions imparted by micropipettes. Yet, such methods are complex and exhibit low throughputs, as the tension is applied serially to individual cells. Here we present a novel approach to inducing neurite growth in multiple cells in parallel, by using a miniaturized platform with numerous microchannels. Upon connection of a vacuum to these microchannels, tension can be applied on multiple cells simultaneously to induce the growth of neurites. A theoretical model was also developed to understand the effect of tension on the dynamics of neurite development.

Nerve regeneration restores supraspinal control of bladder function after complete spinal cord injury

A life-threatening disability after complete spinal cord injury is urinary dysfunction, which is attributable to lack of regeneration of supraspinal pathways that control the bladder. Although numerous strategies have been proposed that can promote the regrowth of severed axons in the adult CNS, at present, the approaches by which this can be accomplished after complete cord transection are quite limited. In the present study, we modified a classic peripheral nerve grafting technique with the use of chondroitinase to facilitate the regeneration of axons across and beyond an extensive thoracic spinal cord transection lesion in adult rats. The novel combination treatment allows for remarkably lengthy regeneration of certain subtypes of brainstem and propriospinal axons across the injury site and is followed by markedly improved urinary function. Our studies provide evidence that an enhanced nerve grafting strategy represents a potential regenerative treatment after severe spinal cord injury.

SnoN facilitates axonal regeneration after spinal cord injury

Adult CNS neurons exhibit a reduced capacity for growth compared to developing neurons, due in part to downregulation of growth-associated genes as development is completed. We tested the hypothesis that SnoN, an embryonically regulated transcription factor that specifies growth of the axonal compartment, can enhance growth in injured adult neurons. In vitro, SnoN overexpression in dissociated adult DRG neuronal cultures significantly enhanced neurite outgrowth. Moreover, TGF-β1, a negative regulator of SnoN, inhibited neurite outgrowth, and SnoN over-expression overcame this inhibition. We then examined whether SnoN influenced axonal regeneration in vivo: indeed, expression of a mutant form of SnoN resistant to degradation significantly enhanced axonal regeneration following cervical spinal cord injury, despite peri-lesional upregulation of TGF-β1. Thus, a developmental mechanism that specifies extension of the axonal compartment also promotes axonal regeneration after adult CNS injury.

Caudalized human iPSC-derived neural progenitor cells produce neurons and glia but fail to restore function in an early chronic spinal cord injury model

Neural progenitor cells (NPCs) have shown modest potential and some side effects (e.g. allodynia) for treatment of spinal cord injury (SCI). In only a few cases, however, have NPCs shown promise at the chronic stage. Given the 1.275 million people living with chronic paralysis, there is a significant need to rigorously evaluate the cell types and methods for safe and efficacious treatment of this devastating condition. For the first time, we examined the pre-clinical potential of NPCs derived from human induced pluripotent stem cells (hiPSCs) to repair chronic SCI. hiPSCs were differentiated into region-specific (i.e. caudal) NPCs, then transplanted into a new, clinically relevant model of early chronic cervical SCI. We established the conditions for successful transplantation of caudalized hiPSC-NPCs and demonstrate their remarkable ability to integrate and produce multiple neural lineages in the early chronic injury environment. In contrast to prior reports in acute and sub-acute injury models, survival and integration of hiPSC-derived neural cells in the early chronic cervical model did not lead to significant improvement in forelimb function or induce allodynia. These data indicate that while hiPSCs show promise, future work needs to focus on the specific hiPSC-derivatives or co-therapies that will restore function in the early chronic injury setting.

Transplantation of human fetal-derived neural stem cells improves cognitive function following cranial irradiation

Treatment of central nervous system (CNS) malignancies typically involves radiotherapy to forestall tumor growth and recurrence following surgical resection. Despite the many benefits of cranial radiotherapy, survivors often suffer from a wide range of debilitating and progressive cognitive deficits. Thus, while patients afflicted with primary and secondary malignancies of the CNS now experience longer local regional control and progression-free survival, there remains no clinical recourse for the unintended neurocognitive sequelae associated with their cancer treatments. Multiple mechanisms contribute to disrupted cognition following irradiation, including the depletion of radiosensitive populations of stem and progenitor cells in the hippocampus. We have explored the potential of using intrahippocampal transplantation of human stem cells to ameliorate radiation-induced cognitive dysfunction. Past studies demonstrated the capability of cranially transplanted human embryonic (hESCs) and neural (hNSCs) stem cells to functionally restore cognition in rats 1 and 4 months after cranial irradiation. The present study employed an FDA-approved fetal-derived hNSC line capable of large scale-up under good manufacturing practice (GMP). Animals receiving cranial transplantation of these cells 1 month following irradiation showed improved hippocampal spatial memory and contextual fear conditioning performance compared to irradiated, sham surgery controls. Significant newly born (doublecortin positive) neurons and a smaller fraction of glial subtypes were observed within and nearby the transplantation core. Engrafted cells migrated and differentiated into neuronal and glial subtypes throughout the CA1 and CA3 subfields of the host hippocampus. These studies expand our prior findings to demonstrate that transplantation of fetal-derived hNSCs improves cognitive deficits in irradiated animals, as assessed by two separate cognitive tasks.

Acute stimulation of transplanted neurons improves motoneuron survival, axon growth, and muscle reinnervation

Few options exist for treatment of pervasive motoneuron death after spinal cord injury or in neurodegenerative diseases such as amyotrophic lateral sclerosis. Local transplantation of embryonic motoneurons into an axotomized peripheral nerve is a promising approach to arrest the atrophy of denervated muscles; however, muscle reinnervation is limited by poor motoneuron survival. The aim of the present study was to test whether acute electrical stimulation of transplanted embryonic neurons promotes motoneuron survival, axon growth, and muscle reinnervation. The sciatic nerve of adult Fischer rats was transected to mimic the widespread denervation seen after disease or injury. Acutely dissociated rat embryonic ventral spinal cord cells were transplanted into the distal tibial nerve stump as a neuron source for muscle reinnervation. Immediately post-transplantation, the cells were stimulated at 20 Hz for 1 h. Other groups were used to control for the cell transplantation and stimulation. When neurons were stimulated acutely, there were significantly more neurons, including cholinergic neurons, 10 weeks after transplantation. This led to enhanced numbers of myelinated axons, reinnervation of more muscle fibers, and more medial and lateral gastrocnemius muscles were functionally connected to the transplant. Reinnervation reduced muscle atrophy significantly. These data support the concept that electrical stimulation rescues transplanted motoneurons and facilitates muscle reinnervation.

Fibrin scaffolds containing ectomesenchymal stem cells enhance behavioral and histological improvement in a rat model of spinal cord injury

Fibrin has been widely used in wound healing. However, its benefit for spinal cord injury (SCI) is limited. In this study, we investigated the impact of fibrin scaffolds containing ectomesenchymal stem cells (EMSCs) on histological and behavioral recovery after SCI and compared it with fibrin alone. To achieve this, EMSCs derived from adult rat nasal respiratory mucosa were cultured, characterized and transfected with green fluorescent protein adenovirus before transplantation. Then, Sprague-Dawley host rats were randomly assigned into four groups: the control group (laminectomy); the SCI group (laminectomy and transection of spinal cords); the fibrin group (fibrin was transplanted immediately after SCI), and the fibrin cell (FC) group (fibrin scaffolds containing EMSCs were transplanted after SCI). Three days after the operation, a TUNEL assay indicated less apoptotic cells in the FC group than in the fibrin group. Two weeks after SCI, fluorescence staining demonstrated not only the survival and migration of EMSCs into the lesion sites, but also a higher number of nerve fibers in the FC group than in the fibrin group. Histological examination including immunohistochemistry and transmission electron microscopy 12 weeks after the operation showed more nerve fibers and a thicker myelin sheath in the FC group compared to the fibrin group. Western blotting confirmed these morphological results. Consistent with the histological results, Basso, Beattie and Bresnahan locomotor scores of the FC group were higher than those of the fibrin group. These results suggest that fibrin scaffolds containing EMSCs can improve the behavioral and histological recovery after SCI better than fibrin alone.

Amelioration of motor/sensory dysfunction and spasticity in a rat model of acute lumbar spinal cord injury by human neural stem cell transplantation

Introduction

Intraspinal grafting of human neural stem cells represents a promising approach to promote recovery of function after spinal trauma. Such a treatment may serve to: I) provide trophic support to improve survival of host neurons; II) improve the structural integrity of the spinal parenchyma by reducing syringomyelia and scarring in trauma-injured regions; and III) provide neuronal populations to potentially form relays with host axons, segmental interneurons, and/or α-motoneurons. Here we characterized the effect of intraspinal grafting of clinical grade human fetal spinal cord-derived neural stem cells (HSSC) on the recovery of neurological function in a rat model of acute lumbar (L3) compression injury.

Methods

Three-month-old female Sprague–Dawley rats received L3 spinal compression injury. Three days post-injury, animals were randomized and received intraspinal injections of either HSSC, media-only, or no injections. All animals were immunosuppressed with tacrolimus, mycophenolate mofetil, and methylprednisolone acetate from the day of cell grafting and survived for eight weeks. Motor and sensory dysfunction were periodically assessed using open field locomotion scoring, thermal/tactile pain/escape thresholds and myogenic motor evoked potentials. The presence of spasticity was measured by gastrocnemius muscle resistance and electromyography response during computer-controlled ankle rotation. At the end-point, gait (CatWalk), ladder climbing, and single frame analyses were also assessed. Syrinx size, spinal cord dimensions, and extent of scarring were measured by magnetic resonance imaging. Differentiation and integration of grafted cells in the host tissue were validated with immunofluorescence staining using human-specific antibodies.

Results

Intraspinal grafting of HSSC led to a progressive and significant improvement in lower extremity paw placement, amelioration of spasticity, and normalization in thermal and tactile pain/escape thresholds at eight weeks post-grafting. No significant differences were detected in other CatWalk parameters, motor evoked potentials, open field locomotor (Basso, Beattie, and Bresnahan locomotion score (BBB)) score or ladder climbing test. Magnetic resonance imaging volume reconstruction and immunofluorescence analysis of grafted cell survival showed near complete injury-cavity-filling by grafted cells and development of putative GABA-ergic synapses between grafted and host neurons.

Conclusions

Peri-acute intraspinal grafting of HSSC can represent an effective therapy which ameliorates motor and sensory deficits after traumatic spinal cord injury.

Axon regeneration and exercise-dependent plasticity after spinal cord injury

Current dogma states that meaningful recovery of function after spinal cord injury (SCI) will likely require a combination of therapeutic interventions comprised of regenerative/neuroprotective transplants, addition of neurotrophic factors, elimination of inhibitory molecules, functional sensorimotor training, and/or stimulation of paralyzed muscles or spinal circuits. We routinely use (1) peripheral nerve grafts to support and direct axonal regeneration across an incomplete cervical or complete thoracic transection injury, (2) matrix modulation with chondroitinase (ChABC) to facilitate axonal extension beyond the distal graft-spinal cord interface, and (3) exercise, such as forced wheel walking, bicycling, or step training on a treadmill. We and others have demonstrated an increase in spinal cord levels of endogenous neurotrophic factors with exercise, which may be useful in facilitating elongation and/or synaptic activity of regenerating axons and plasticity of spinal neurons below the level of injury.

Potential of olfactory ensheathing cells from different sources for spinal cord repair

Spinal cord injury (SCI) induces a permanent disability in patients. To this day no curative treatment can be proposed to restore lost functions. Therefore, extensive experimental studies have been conducted to induce recovery after SCI. One of the most promising therapies is based on the use of olfactory ensheathing cells (OECs). OECs can be obtained from either the olfactory bulbs (OB-OECs) or from olfactory mucosa (OM-OECs), involving a less invasive approach for autotransplantation. However the vast majority of experimental transplantations have been focusing on OB-OECs although the OM represents a more accessible source of OECs. Importantly, the ability of OM-OECs in comparison to OB-OECs to induce spinal cord recovery in the same lesion paradigm has never been described. We here present data using a multiparametric approach, based on electrophysiological, behavioral, histological and magnetic resonance imaging experiments on the repair potential of OB-OECs and OM-OECs from either primary or purified cultures after a severe model of SCI. Our data demonstrate that transplantation of OECs obtained from OB or OM induces electrophysiological and functional recovery, reduces astrocyte reactivity and glial scar formation and improves axonal regrowth. We also show that the purification step is essential for OM-OECs while not required for OB-OECs. Altogether, our study strongly indicates that transplantation of OECs from OM represents the best benefit/risk ratio according to the safety of access of OM and the results induced by transplantations of OM-OECs. Indeed, purified OM-OECs in addition to induce recovery can integrate and survive up to 60 days into the spinal cord. Therefore, our results provide strong support for these cells as a viable therapy for SCI.

Promotion of neuronal differentiation of neural progenitor cells by using EGFR antibody functionalized collagen scaffolds for spinal cord injury repair

The main challenge for neural progenitor cell (NPC)-mediated repair of spinal cord injury (SCI) is lack of favorable environment to direct its differentiation towards neurons rather than glial cells. The myelin associated inhibitors have been demonstrated to promote NPC differentiation into glial lineage. Herein, to inhibit the downstream signaling activated by myelin associated inhibitors, cetuximab, an epidermal growth factor receptor (EGFR) neutralizing antibody, functionalized collagen scaffold has been developed as a vehicle for NPC implantation. It was found that collagen-cetuximab 1 μg scaffolds enhanced neuronal differentiation and inhibited astrocytic differentiation of NPCs exposed to myelin proteins significantly in vitro. To test the therapeutic effect in vivo, NPCs expressing green fluorescent protein (GFP)-embedded scaffolds have been implanted into the 4 mm-long hemisection lesion of rats. We found that the collagen-cetuximab 5 μg scaffolds induced neuronal differentiation and decreased astrocytic differentiation of NPCs, enhanced axon regeneration, and promoted functional recovery markedly. A well-functionalized scaffold was constructed to improve the recovery of SCI, which could promote the neuronal differentiation of neural progenitor cells in vivo.

Spinal cord regeneration: where fish, frogs and salamanders lead the way, can we follow?

Major trauma to the mammalian spinal cord often results in irreversible loss of function, i.e. paralysis, and current therapies ranging from drugs, implantations of stem cells and/or biomaterials, and electrically stimulated nerve regrowth, have so far offered very limited success in improving quality-of-life. However, in marked contrast with this basic shortcoming of ours, certain vertebrate species, including fish and salamanders, display the amazing ability to faithfully regenerate various complex body structures after injury or ablation, restoring full functionality, even in the case of the spinal cord. Despite the inherently strong and obvious translational potential for improving treatment strategies for human patients, our in-depth molecular-level understanding of these decidedly more advanced repair systems remains in its infancy. In the present review, we will discuss the current state of this field, focusing on recent progress in such molecular analyses using various regenerative species, and how these so far relate to the mammalian situation.

Rodent Models and Behavioral Outcomes of Cervical Spinal Cord Injury

Rodent spinal cord injury (SCI) models have been developed to examine functional and physiological deficits after spinal cord injury with the hope that these models will elucidate information about human SCI. Models are needed to examine possible treatments and to understand histopathology after SCI; however, they should be considered carefully and chosen based on the goals of the study being performed. Contusion, compression, transection, and other models exist and have the potential to reveal important information about SCI that may be related to human SCI and the outcomes of treatment and timing of intervention.

Integration and long distance axonal regeneration in the central nervous system from transplanted primitive neural stem cells

Spinal cord injury (SCI) results in devastating motor and sensory deficits secondary to disrupted neuronal circuits and poor regenerative potential. Efforts to promote regeneration through cell extrinsic and intrinsic manipulations have met with limited success. Stem cells represent an as yet unrealized therapy in SCI. Recently, we identified novel culture methods to induce and maintain primitive neural stem cells (pNSCs) from human embryonic stem cells. We tested whether transplanted human pNSCs can integrate into the CNS of the developing chick neural tube and injured adult rat spinal cord. Following injection of pNSCs into the developing chick CNS, pNSCs integrated into the dorsal aspects of the neural tube, forming cell clusters that spontaneously differentiated into neurons. Furthermore, following transplantation of pNSCs into the lesioned rat spinal cord, grafted pNSCs survived, differentiated into neurons, and extended long distance axons through the scar tissue at the graft-host interface and into the host spinal cord to form terminal-like structures near host spinal neurons. Together, these findings suggest that pNSCs derived from human embryonic stem cells differentiate into neuronal cell types with the potential to extend axons that associate with circuits of the CNS and, more importantly, provide new insights into CNS integration and axonal regeneration, offering hope for repair in SCI.