Robust regeneration of adult sensory axons in degenerating white matter of the adult rat spinal cord

Inhibitors of myelination: ECM changes, CSPGs and PTPs

After inflammation-induced demyelination, such as in the disease multiple sclerosis, endogenous remyelination often fails. However, in animal models of demyelination induced with toxins, remyelination can be quite robust. A significant difference between inflammation-induced and toxin-induced demyelination is the response of local cells within the lesion, including astrocytes, oligodendrocytes, microglia/macrophages, and NG2+ cells, which respond to inflammatory stimuli with increased extracellular matrix (ECM) protein and chondroitin sulfate proteoglycan (CSPG) production and deposition. Here, we summarize current knowledge of ECM changes in demyelinating lesions, as well as oligodendrocyte responses to aberrant ECM proteins and CSPGs after various types of demyelinating insults. The discovery that CSPGs act through the receptor protein tyrosine phosphatase sigma (PTPσ) and the Rho-ROCK pathway to inhibit oligodendrocyte process extension and myelination, but not oligodendrocyte differentiation (Pendleton et al., Experimental Neurology (2013) vol. 247, pp. 113-121), highlights the need to better understand the ECM changes that accompany demyelination and their influence on oligodendrocytes and effective remyelination.

Targeting the neural extracellular matrix in neurological disorders

The extracellular matrix (ECM) is known to regulate important processes in neuronal cell development, activity and growth. It is associated with the structural stabilization of neuronal processes and synaptic contacts during the maturation of the central nervous system. The remodeling of the ECM during both development and after central nervous system injury has been shown to affect neuronal guidance, synaptic plasticity and their regenerative responses. Particular interest has focused on the inhibitory role of chondroitin sulfate proteoglycans (CSPGs) and their formation into dense lattice-like structures, termed perineuronal nets (PNNs), which enwrap sub-populations of neurons and restrict plasticity. Recent studies in mammalian systems have implicated CSPGs and PNNs in regulating and restricting structural plasticity. The enzymatic degradation of CSPGs or destabilization of PNNs has been shown to enhance neuronal activity and plasticity after central nervous system injury. This review focuses on the role of the ECM, CSPGs and PNNs; and how developmental and pharmacological manipulation of these structures have enhanced neuronal plasticity and aided functional recovery in regeneration, stroke, and amblyopia. In addition to CSPGs, this review also points to the functions and potential therapeutic value of these and several other key ECM molecules in epileptogenesis and dementia.

A comparison of the behavioral and anatomical outcomes in sub-acute and chronic spinal cord injury models following treatment with human mesenchymal precursor cell transplantation and recombinant decorin

This study assessed the potential of highly purified (Stro-1(+)) human mesenchymal precursor cells (hMPCs) in combination with the anti-scarring protein decorin to repair the injured spinal cord (SC). Donor hMPCs isolated from spinal cord injury (SCI) patients were transplanted into athymic rats as a suspension graft, alone or after previous treatment with, core (decorin(core)) and proteoglycan (decorin(pro)) isoforms of purified human recombinant decorin. Decorin was delivered via mini-osmotic pumps for 14 days following sub-acute (7 day) or chronic (1 month) SCI. hMPCs were delivered to the spinal cord at 3 weeks or 6 weeks after the initial injury at T9 level. Behavioral and anatomical analysis in this study showed statistically significant improvement in functional recovery, tissue sparing and cyst volume reduction following hMPC therapy. The combination of decorin infusion followed by hMPC therapy did not improve these measured outcomes over the use of cell therapy alone, in either sub-acute or chronic SCI regimes. However, decorin infusion did improve tissue sparing, reduce spinal tissue cavitation and increase transplanted cell survivability as compared to controls. Immunohistochemical analysis of spinal cord sections revealed differences in glial, neuronal and extracellular matrix molecule expression within each experimental group. hMPC transplanted spinal cords showed the increased presence of serotonergic (5-HT) and sensory (CGRP) axonal growth within and surrounding transplanted hMPCs for up to 2 months; however, no evidence of hMPC transdifferentiation into neuronal or glial phenotypes. The number of hMPCs was dramatically reduced overall, and no transplanted cells were detected at 8 weeks post-injection using lentiviral GFP labeling and human nuclear antigen antibody labeling. The presence of recombinant decorin in the cell transplantation regimes delayed in part the loss of donor cells, with small numbers remaining at 2 months after transplantation. In vitro co-culture experiments with embryonic dorsal root ganglion explants revealed the growth promoting properties of hMPCs. Decorin did not increase axonal outgrowth from that achieved by hMPCs. We provide evidence for the first time that (Stro-1(+)) hMPCs provide: i) an advantageous source of allografts for stem cell transplantation for sub-acute and chronic spinal cord therapy, and (ii) a positive host microenvironment that promotes tissue sparing/repair that subsequently improves behavioral outcomes after SCI. This was not measurably improved by recombinant decorin treatment, but does provide important information for the future development and potential use of decorin in contusive SCI therapy.

Astrocytes specifically remove surface-adsorbed fibrinogen and locally express chondroitin sulfate proteoglycans

Surface-adsorbed fibrinogen (FBG) was recognized by adhering astrocytes, and was removed from the substrates in vitro by a two-phase removal process. The cells removed adsorbed FBG from binary proteins' surface patterns (FBG+laminin, or FBG+albumin) while leaving the other protein behind. Astrocytes preferentially expressed chondroitin sulfate proteoglycan (CSPG) at the loci of fibrinogen stimuli; however, no differences in overall CSPG production as a function of FBG surface coverage were identified. Removal of FBG by astrocytes was also found to be independent of transforming growth factor type β (TGF-β) receptor based signaling as cells maintained CSPG production in the presence of TGF-β receptor kinase inhibitor, SB 431542. The inhibitor decreased CSPG expression, but did not abolish it entirely. Because blood contact and subsequent FBG adsorption are unavoidable in neural implantations, the results indicate that implant-adsorbed FBG may contribute to reactive astrogliosis around the implant as astrocytes specifically recognize adsorbed FBG.

Role of glial cells in axonal regeneration

Axonal regeneration is critical for functional recovery following neural injury. In addition to intrinsic differences between regenerative responses of axons in peripheral versus central nervous systems, environmental factors such as glial cells and related molecules in the extracellular matrix (ECM) play an important role in axonal regeneration. Schwann cells in the peripheral nervous system (PNS) are recognized as favorable factors that promote axonal regeneration, while astrocytes and oligodendrocytes in the central nervous system (CNS) are not. In this review, we evaluate the roles of Schwann cells and astrocytes in axonal regeneration and examine recent evidence that suggests a dual function of astrocytes in regenerative responses. We also discuss the role of Cdc2 pathways in axonal regeneration, which is commonly activated in Schwann cells and astrocytes. Greater insight on the roles of glial cells in axonal regeneration is key to establishing baseline interventions for improving functional recovery following neural injury.

Paxillin phosphorylation counteracts proteoglycan-mediated inhibition of axon regeneration

In the adult central nervous system, the tips of axons severed by injury are commonly transformed into dystrophic endballs and cease migration upon encountering a rising concentration gradient of inhibitory proteoglycans. However, intracellular signaling networks mediating endball migration failure remain largely unknown. Here we show that manipulation of protein kinase A (PKA) or its downstream adhesion component paxillin can reactivate the locomotive machinery of endballs in vitro and facilitate axon growth after injury in vivo. In dissociated cultures of adult rat dorsal root ganglion neurons, PKA is activated in endballs formed on gradients of the inhibitory proteoglycan aggrecan, and pharmacological inhibition of PKA promotes axon growth on aggrecan gradients most likely through phosphorylation of paxillin at serine 301. Remarkably, pre-formed endballs on aggrecan gradients resume forward migration in response to PKA inhibition. This resumption of endball migration is associated with increased turnover of adhesive point contacts dependent upon paxillin phosphorylation. Furthermore, expression of phosphomimetic paxillin overcomes aggrecan-mediated growth arrest of endballs, and facilitates axon growth after optic nerve crush in vivo. These results point to the importance of adhesion dynamics in restoring endball migration and suggest a potential therapeutic target for axon tract repair.

Controlled arrays of self-assembled peptide nanostructures in solution and at interface

Controlling the formation of large and homogeneous arrays of bionanostructures through the self-assembly approach is still a great challenge. Here, we report the spontaneous formation of highly ordered arrays based on aligned peptide nanostructures in a solution as well as at an interface by self-assembly. By controlling the time and temperature of self-assembly in the solution, parallel fibrous alignments and more sophisticated two-dimensional "knitted" fibrous arrays could be formed from aligned rod-like fibers. During the formation of such arrays, the "disorder-to-order" transitions are controlled by the temperature-responsible motile short hydrophobic tails of the gemini-like amphiphilic peptides (GAPs) with asymmetric molecular conformation. In addition, the resulting long-range-ordered "knitted" fibrous arrays are able to direct mineralization of calcium phosphate to form organic-inorganic composite materials. In this study, the self-assembly behavior of these peptide building blocks at an interface was also studied. Highly ordered spatial arrays with vertically or horizontally aligned nanostructures such as nanofibers, microfibers, and microtubes could be formed through interfacial assembly. The regular structures and their alignments on the interface are controlled by the alkyl chain length of building blocks and the hydrophilicity/hydrophobicity property of the interface.

Alterations in sulfated chondroitin glycosaminoglycans following controlled cortical impact injury in mice

Chondroitin sulfate proteoglycans (CSPGs) play a pivotal role in many neuronal growth mechanisms including axon guidance and the modulation of repair processes following injury to the spinal cord or brain. Many actions of CSPGs in the central nervous system (CNS) are governed by the specific sulfation pattern on the glycosaminoglycan (GAG) chains attached to CSPG core proteins. To elucidate the role of CSPGs and sulfated GAG chains following traumatic brain injury (TBI), controlled cortical impact injury of mild to moderate severity was performed over the left sensory motor cortex in mice. Using immunoblotting and immunostaining, we found that TBI resulted in an increase in the CSPGs neurocan and NG2 expression in a tight band surrounding the injury core, which overlapped with the presence of 4-sulfated CS GAGs but not with 6-sulfated GAGs. This increase was observed as early as 7 days post injury (dpi), and persisted for up to 28 dpi. Labeling with markers against microglia/macrophages, NG2+ cells, fibroblasts, and astrocytes showed that these cells were all localized in the area, suggesting multiple origins of chondroitin-4-sulfate increase. TBI also caused a decrease in the expression of aggrecan and phosphacan in the pericontusional cortex with a concomitant reduction in the number of perineuronal nets. In summary, we describe a dual response in CSPGs whereby they may be actively involved in complex repair processes following TBI.

Systematic Review of Induced Pluripotent Stem Cell Technology as a Potential Clinical Therapy for Spinal Cord Injury

Transplantation therapies aimed at repairing neurodegenerative and neuropathological conditions of the central nervous system (CNS) have utilized and tested a variety of cell candidates, each with its own unique set of advantages and disadvantages. The use and popularity of each cell type is guided by a number of factors including the nature of the experimental model, neuroprotection capacity, the ability to promote plasticity and guided axonal growth, and the cells' myelination capability. The promise of stem cells, with their reported ability to give rise to neuronal lineages to replace lost endogenous cells and myelin, integrate into host tissue, restore functional connectivity, and provide trophic support to enhance and direct intrinsic regenerative ability, has been seen as a most encouraging step forward. The advent of the induced pluripotent stem cell (iPSC), which represents the ability to “reprogram” somatic cells into a pluripotent state, hails the arrival of a new cell transplantation candidate for potential clinical application in therapies designed to promote repair and/or regeneration of the CNS. Since the initial development of iPSC technology, these cells have been extensively characterized in vitro and in a number of pathological conditions and were originally reported to be equivalent to embryonic stem cells (ESCs). This review highlights emerging evidence that suggests iPSCs are not necessarily indistinguishable from ESCs and may occupy a different “state” of pluripotency with differences in gene expression, methylation patterns, and genomic aberrations, which may reflect incomplete reprogramming and may therefore impact on the regenerative potential of these donor cells in therapies. It also highlights the limitations of current technologies used to generate these cells. Moreover, we provide a systematic review of the state of play with regard to the use of iPSCs in the treatment of neurodegenerative and neuropathological conditions. The importance of balancing the promise of this transplantation candidate in the light of these emerging properties is crucial as the potential application in the clinical setting approaches. The first of three sections in this review discusses (A) the pathophysiology of spinal cord injury (SCI) and how stem cell therapies can positively alter the pathology in experimental SCI. Part B summarizes (i) the available technologies to deliver transgenes to generate iPSCs and (ii) recent data comparing iPSCs to ESCs in terms of characteristics and molecular composition. Lastly, in (C) we evaluate iPSC-based therapies as a candidate to treat SCI on the basis of their neurite induction capability compared to embryonic stem cells and provide a summary of available in vivo data of iPSCs used in SCI and other disease models.

Overcoming neurite-inhibitory chondroitin sulfate proteoglycans in the astrocyte matrix

Acute trauma to the central nervous system (CNS) can result in permanent damage and loss of function related to the poor regeneration of injured axons. Injured axons encounter several barriers to regeneration, such as the glial scar at the injury site. The glial scar contains extracellular matrix (ECM) macromolecules deposited by reactive astrocytes in response to injury. The scar ECM is rich in chondroitin sulfate proteoglycans (CSPGs), macromolecules that inhibit axonal growth. CSPGs consist of a core protein with attachment sites for glycosaminoglycan (GAG) chains. An extensive literature demonstrates that enzymatic removal of the GAG chains by chondroitinase ABC permits some axonal regrowth; however, the remaining intact core proteins also possess inhibitory domains. Because metalloproteinases can degrade core proteins of CSPGs, we have evaluated five matrix metalloproteinases (MMPs) and a related protease-a disintegrin and metalloproteinase with thrombospondin motifs-4 (ADAMTS-4)-for their capacity to overcome CSPG inhibition of neuritic growth in culture. The metalloproteinases were selected for their known expression after CNS injuries. Of the MMPs, MMP-3, -7 and -8 reduced or abolished inhibition of neurite outgrowth on a purified CSPG substrate and on an astrocyte-derived ECM. ADAMTS-4 also attenuated CSPG inhibition of neurites and had the additional benefits of neither degrading laminin nor causing neurotoxicity. The efficacy of ADAMTS-4 matched that of blocking the EGFR signaling previously reported to mediate CSPG inhibition. These findings highlight ADAMTS-4 as a superior protease for overcoming CSPG inhibition of axonal regeneration in the CNS.

New hypothesis of chronic back pain: low pH promotes nerve ingrowth into damaged intervertebral disks

The pathogenesis of low back pain is still elusive. Here, we proposed a new hypothesis that low pH is a possible cause of the development and progression of low back pain. We propose that low pH promotes the production of the inflammatory mediators and the depletion of proteoglycan in the damaged intervertebral disk. The inflammation response, evoked by the dorsal root ganglia, changes the delicate nutrient balance in the nucleus, resulting in a vicious cycle and leading to choronic back pain. Our hypothesis may explain many of the available clinical and experimental data on low back pain, thus it may help elucidate the pathogenesis of low back pain and improve clinical management.

JAK2 inhibition is neuroprotective and reduces astrogliosis after quinolinic acid striatal lesion in adult mice

Quinolinic acid (QA) striatal lesion in rodents induces neuronal death, astrogliosis and migration of neuroblasts from subventricular zone to damaged striatum. These phenomena occur in some human neurodegenerative illnesses, but the underlying mechanisms are unknown. We investigated the effect of AG490, a Janus-kinase 2 (JAK2) inhibitor, on astrogliosis, neuronal loss and neurogenesis in the striatum of adult mice after unilateral infusion of QA (30 nmol). Animals were given subcutaneous injections of AG490 (10 mg/kg) or vehicle immediately after lesion and then once daily for six days. Brain sections were used for neuronal stereological quantification, immunohistochemical and Western Blotting analyses for GFAP and doublecortin, markers of astrocytes and neuroblasts, respectively. The total area of doublecortin-positive cells (ADPC) and the number of neurons (NN) in the lesioned (L) and contralateral (CL) sides were evaluated. Neurogenesis index (NI=ADPC(L)/ADPC(CL)) and neuronal ratio (NR=NN(L)/NN(CL)) were calculated. After QA administration, blotting for GFAP showed an ipsilateral decrease of 19% in AG490- vs vehicle-treated animals. NR was 25% higher in mice given AG490 vs controls given vehicle. NI showed a decrease of 21% in AG490- vs vehicle-treated mice. Our results indicate that JAK2 inhibition reduces QA lesion and suggest that astrogliosis may impair neuronal survival in this model.

Proteoglycans in the central nervous system: role in development, neural repair, and Alzheimer's disease

Proteoglycans (PGs) are major components of the cell surface and extracellular matrix and play critical roles in development and maintenance of the central nervous system (CNS). PGs are a family of proteins, all of which contain a core protein to which glycosaminoglycan side chains are covalently attached. PGs possess diverse physiological roles, particularly in neural development, and are also implicated in the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). The main functions of PGs in the CNS are reviewed as are the roles of PGs in brain injury and in the development or treatment of AD.

Electroacupuncture Promotes the Differentiation of Transplanted Bone Marrow Mesenchymal Stem Cells Overexpressing TrkC into Neuron-Like Cells in Transected Spinal Cord of Rats

Our previous study indicated that electroacupuncture (EA) could increase neurotrophin-3 (NT-3) levels in the injured spinal cord, stimulate the differentiation of transplanted bone marrow mesenchymal stem cells (MSCs), and improve functional recovery in the injured spinal cord of rats. However, the number of neuron-like cells derived from the MSCs is limited. It is known that NT-3 promotes the survival and differentiation of neurons by preferentially binding to its receptor TrkC. In this study, we attempted to transplant TrkC gene-modified MSCs (TrkC-MSCs) into the spinal cord with transection to investigate whether EA treatment could promote NT-3 secretion in the injured spinal cord and to determine whether increased NT-3 could further enhance transplanted MSCs overexpressing TrkC to differentiate into neuron-like cells, resulting in increased axonal regeneration and functional improvement in the injured spinal cord. Our results showed that EA increased NT-3 levels; furthermore, it promoted neuron-phenotype differentiation, synaptogenesis, and myelin formation of transplanted TrkC-MSCs. In addition, TrkC-MSC transplantation combined with EA (the TrkC-MSCs + EA group) treatment promoted the growth of the descending BDA-labeled corticospinal tracts (CSTs) and 5-HT-positive axonal regeneration across the lesion site into the caudal cord. In addition, the conduction of cortical motor-evoked potentials (MEPs) and hindlimb locomotor function increased as compared to controls (treated with the LacZ-MSCs, TrkC-MSCs, and LacZ-MSCs + EA groups). In the TrkC-MSCs + EA group, the injured spinal cord also showed upregulated expression of the proneurogenic factors laminin and GAP-43 and downregulated GFAP and chondroitin sulfate proteoglycans (CSPGs), major inhibitors of axonal growth. Together, our data suggest that TrkC-MSC transplantation combined with EA treatment spinal cord injury not only increased MSC survival and differentiation into neuron-like cells but also promoted CST regeneration across injured sites to the caudal cord and functional improvement, perhaps due to increase of NT-3 levels, upregulation of laminin and GAP-43, and downregulation of GFAP and CSPG proteins.

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

Neural stem cells (NSCs) expressing GFP were embedded into fibrin matrices containing growth factor cocktails and grafted to sites of severe spinal cord injury. Grafted cells differentiated into multiple cellular phenotypes, including neurons, which extended large numbers of axons over remarkable distances. Extending axons formed abundant synapses with host cells. Axonal growth was partially dependent on mammalian target of rapamycin (mTOR), but not Nogo signaling. Grafted neurons supported formation of electrophysiological relays across sites of complete spinal transection, resulting in functional recovery. Two human stem cell lines (566RSC and HUES7) embedded in growth-factor-containing fibrin exhibited similar growth, and 566RSC cells supported functional recovery. Thus, properties intrinsic to early-stage neurons can overcome the inhibitory milieu of the injured adult spinal cord to mount remarkable axonal growth, resulting in formation of new relay circuits that significantly improve function. These therapeutic properties extend across stem cell sources and species.

Gene delivery to overcome astrocyte inhibition of axonal growth: an in vitro model of the glial scar

After injury to the central nervous system, a glial scar develops that physically and biochemically inhibits axon growth. In the scar, activated astrocytes secrete inhibitory extracellular matrix, of which chondroitin sulfate proteoglycans (CSPGs) are considered the major inhibitory component. An inhibitory interface of CSPGs forms around the lesion and prevents axons from traversing the injury, and decreasing CSPGs can enhance axon growth. In this report, we established an in vitro interface model of activated astrocytes and subsequently investigated gene delivery as a means to reduce CSPG levels and enhance axon growth. In the model, a continuous interface of CSPG producing astrocytes was created with neurons seeded opposite the astrocytes, and neurite crossing, stopping, and turning were evaluated as they approached the interface. We investigated the efficacy of lentiviral delivery to degrade or prevent the synthesis of CSPGs, thereby removing CSPG inhibition of neurite growth. Lentiviral delivery of RNAi targeting two key CSPG synthesis enzymes, chondroitin polymerizing factor and chondroitin synthase-1, decreased CSPGs, and reduced inhibition by the interface. Degradation of CSPGs by lentiviral delivery of chondroitinase also resulted in less inhibition and more neurites crossing the interface. These results indicate that the interface model provides a tool to investigate interventions that reduce inhibition by CSPGs, and that gene delivery can be effective in promoting neurite growth across an interface of CSPG producing astrocytes.

Scar-mediated inhibition and CSPG receptors in the CNS

Severed axons in adult mammals do not regenerate appreciably after central nervous system (CNS) injury due to developmentally determined reductions in neuron-intrinsic growth capacity and extracellular environment for axon elongation. Chondroitin sulfate proteoglycans (CSPGs), which are generated by reactive scar tissues, are particularly potent contributors to the growth-limiting environment in mature CNS. Thus, surmounting the strong inhibition by CSPG-rich scar is an important therapeutic goal for achieving functional recovery after CNS injuries. As of now, the main in vivo approach to overcoming inhibition by CSPGs is enzymatic digestion with locally applied chondroitinase ABC (ChABC), but several disadvantages may prevent using this bacterial enzyme as a therapeutic option for patients. A better understanding of the molecular mechanisms underlying CSPG action is needed in order to develop more effective therapies to overcome CSPG-mediated inhibition of axon regeneration and/or sprouting. Because of their large size and dense negative charges, CSPGs were thought to act by non-specifically hindering the binding of matrix molecules to their cell surface receptors through steric interactions. Although this may be true, recent studies indicate that two members of the leukocyte common antigen related (LAR) phosphatase subfamily, protein tyrosine phosphatase σ (PTPσ) and LAR, are functional receptors that bind CSPGs with high affinity and mediate CSPG inhibitory effects. CSPGs also may act by binding to two receptors for myelin-associated growth inhibitors, Nogo receptors 1 and 3 (NgR1 and NgR3). If confirmed, it would suggest that CSPGs have multiple mechanisms by which they inhibit axon growth, making them especially potent and difficult therapeutic targets. Identification of CSPG receptors is not only important for understanding the scar-mediated growth suppression, but also for developing novel and selective therapies to promote axon sprouting and/or regeneration after CNS injuries, including spinal cord injury (SCI).

Conditional Sox9 ablation reduces chondroitin sulfate proteoglycan levels and improves motor function following spinal cord injury

Chondroitin sulfate proteoglycans (CSPGs) found in perineuronal nets and in the glial scar after spinal cord injury have been shown to inhibit axonal growth and plasticity. Since we have previously identified SOX9 as a transcription factor that upregulates the expression of a battery of genes associated with glial scar formation in primary astrocyte cultures, we predicted that conditional Sox9 ablation would result in reduced CSPG expression after spinal cord injury and that this would lead to increased neuroplasticity and improved locomotor recovery. Control and Sox9 conditional knock-out mice were subject to a 70 kdyne contusion spinal cord injury at thoracic level 9. One week after injury, Sox9 conditional knock-out mice expressed reduced levels of CSPG biosynthetic enzymes (Xt-1 and C4st), CSPG core proteins (brevican, neurocan, and aggrecan), collagens 2a1 and 4a1, and Gfap, a marker of astrocyte activation, in the injured spinal cord compared with controls. These changes in gene expression were accompanied by improved hind limb function and locomotor recovery as evaluated by the Basso Mouse Scale (BMS) and rodent activity boxes. Histological assessments confirmed reduced CSPG deposition and collagenous scarring at the lesion of Sox9 conditional knock-out mice, and demonstrated increased neurofilament-positive fibers in the lesion penumbra and increased serotonin immunoreactivity caudal to the site of injury. These results suggest that SOX9 inhibition is a potential strategy for the treatment of SCI.

Sensory Axon Regeneration: A Review from an in vivo Imaging Perspective

Injured primary sensory axons fail to regenerate into the spinal cord, leading to chronic pain and permanent sensory loss. Re-entry is prevented at the dorsal root entry zone (DREZ), the CNS-PNS interface. Why axons stop or turn around at the DREZ has generally been attributed to growth-repellent molecules associated with astrocytes and oligodendrocytes/myelin. The available evidence challenges the contention that these inhibitory molecules are the critical determinant of regeneration failure. Recent imaging studies that directly monitored axons arriving at the DREZ in living animals raise the intriguing possibility that axons stop primarily because they are stabilized by forming presynaptic terminals on non-neuronal cells that are neither astrocytes nor oligodendrocytes. These observations revitalized the idea raised many years ago but virtually forgotten, that axons stop by forming synapses at the DREZ.

The assessment of adeno-associated vectors as potential intrinsic treatments for brainstem axon regeneration

BACKGROUND

Adeno-associated virus (AAV) vector-mediated transgene expression is a promising therapeutic to change the intrinsic state of neurons and promote repair after central nervous system injury. Given that numerous transgenes have been identified as potential candidates, the present study demonstrates how to determine whether their expression by AAV has a direct intrinsic effect on axon regeneration.

METHODS

Serotype 2 AAV-enhanced green fluorescent protein (EGFP) was stereotaxically injected into the brainstem of adult rats, followed by a complete transection of the thoracic spinal cord and Schwann cell (SC) bridge implantation.

RESULTS

The expression of EGFP in brainstem neurons labeled numerous axons in the thoracic spinal cord and that regenerated into the SC bridge. The number of EGFP-labeled axons rostral to the bridge directly correlated with the number of EGFP-labeled axons that regenerated into the bridge. Animals with a greater number of EGFP-labeled axons rostral to the bridge exhibited an increased percentage of those axons found near the distal end of the bridge compared to animals with a lesser number. This suggested that EGFP may accumulate distally in the axon with time, enabling easier visualization. By labeling brainstem axons with EGFP before injury, numerous axon remnants undergoing Wallerian degeneration may be identified distal to the complete transection up to 6 weeks after injury.

CONCLUSIONS

Serotype 2 AAV-EGFP enabled easy visualization of brainstem axon regeneration. Rigorous models of axonal injury (i.e. complete transection and cell implantation) should be used in combination with AAV-EGFP to directly assess AAV-mediated expression of therapeutic transgenes as intrinsic treatments to improve axonal regeneration.

Angioneural crosstalk in scaffolds with oriented microchannels for regenerative spinal cord injury repair

The aim of our work is to utilize the crosstalk between the vascular and the neuronal system to enhance directed neuritogenesis in uniaxial guidance scaffolds for the repair of spinal cord injury. In this study, we describe a method for angioneural regenerative engineering, i.e., for generating biodegradable scaffolds, produced by a combination of controlled freezing (freeze-casting) and lyophilization, which contain longitudinally oriented channels, and provide uniaxial directionality to support and guide neuritogenesis from neuronal cells in the presence of endothelial cells. The optimized scaffolds, composed of 2.5 % gelatin and 1 % genipin crosslinked, were characterized by an elastic modulus of ~51 kPa and longitudinal channels of ~50 μm diameter. The scaffolds support the growth of endothelial cells, undifferentiated or NGF-differentiated PC12 cells, and primary cultures of fetal chick forebrain neurons. The angioneural crosstalk, as generated by first forming endothelial cell monolayers in the scaffolds followed by injection of neuronal cells, leads to the outgrowth of long aligned neurites in the PC12/endothelial cell co-cultures also in the absence of exogenously added nerve growth factor. Neuritogenesis was not observed in the scaffolds in the absence of the endothelial cells. This methodology is a promising approach for neural tissue engineering and may be applicable for regenerative spinal cord injury repair.

Need for a paradigm shift in therapeutic approaches to CNS injury

Irreversible damage to the nervous system can result from many causes including trauma, disruption of blood supply, pathogen infection or neurodegenerative disease. Common features following CNS injury include a disruption of axons, neuron death and injury, local B-cell and microglial activation, and the synthesis of pathogenic autoantibodies. CNS injury results in a pervasive inhibitory microenvironment that hinders regeneration. Current approaches to eliminate the inhibitory environment have met with limited success. These results argue for a paradigm shift in therapeutic approaches to CNS injury. Targeting CNS cells (neurons, oligodendrocytes and astrocytes) themselves may drive CNS repair. For example, our group and others have demonstrated that autoreactive antibodies can participate in aspects of CNS regeneration, including remyelination. We have developed recombinant autoreactive natural human IgM antibodies with the therapeutic potential for CNS repair in several neurologic diseases.

Neuronal cadherin (NCAD) increases sensory neurite formation and outgrowth on astrocytes

We examined the neurite outgrowth of sensory neurons on astrocytes following the genetic deletion of N-cadherin (NCAD). Deletion abolished immunostaining for NCAD and the other classical cadherins, indicating that NCAD is likely the only classical cadherin expressed by astrocytes. Only 38% of neurons grown on NCAD-deficient astrocytes for 24 h produced neurites, as compared to 74% of neurons grown on NCAD-expressing astrocytes. Of the neurons that produced neurites, those grown on NCAD-deficient astrocytes had a mean total length of 378 μm, as compared to 1093 μm for neurons grown on NCAD-expressing astrocytes. Thus, the loss of NCAD greatly impairs the formation and extension neurites on astrocytes.

Hyaluronic acid-based scaffold for central neural tissue engineering

Central nervous system (CNS) regeneration with central neuronal connections and restoration of synaptic connections has been a long-standing worldwide problem and, to date, no effective clinical therapies are widely accepted for CNS injuries. The limited regenerative capacity of the CNS results from the growth-inhibitory environment that impedes the regrowth of axons. Central neural tissue engineering has attracted extensive attention from multi-disciplinary scientists in recent years, and many studies have been carried out to develop cell- and regeneration-activating biomaterial scaffolds that create an artificial micro-environment suitable for axonal regeneration. Among all the biomaterials, hyaluronic acid (HA) is a promising candidate for central neural tissue engineering because of its unique physico-chemical and biological properties. This review attempts to outline current biomaterials-based strategies for CNS regeneration from a tissue engineering point of view and discusses the main progresses in research of HA-based scaffolds for central neural tissue engineering in detail.

Microglial inhibitory factor (MIF/TKP) mitigates secondary damage following spinal cord injury

Spinal cord injury (SCI) induces an immune response during which microglia, the resident immunocompetent cells of the central nervous system, become activated and migrate to the site of damage. Depending on their state of activation, microglia secrete neurotoxic or neurotrophic factors that influence the surrounding environment and have a detrimental or restorative effect following SCI, including causing or protecting bystander damage to nearby undamaged tissue. Subsequent infiltration of macrophages contributes to the SCI outcome. We show here that suppressing microglia/macrophage activation using the tripeptide macrophage/microglia inhibitory factor (MIF/TKP) reduced secondary injury around the lesion epicenter in the murine dorsal hemisection model of SCI; it decreased the hypertrophic change of astrocytes and caused an increase in the number of axons present within the lesion epicenter. Moreover, timely inhibition of microglial/macrophage activation prevented demyelination and axonal dieback by modulating oligodendrocyte survival and oligodendrocyte precursor maturation. Microglia/macrophages located within or proximal to the lesion produced neurotoxic factors, such as tumor necrosis factor alpha (TNF-α). These results suggest that microglia/macrophages within the epicenter at early time points post injury are neurotoxic, contributing to demyelination and axonal degeneration and that MIF/TKP could be used in combination with other therapies to promote functional recovery.

A chemical genetic approach identifies piperazine antipsychotics as promoters of CNS neurite growth on inhibitory substrates

Injury to the central nervous system (CNS) can result in lifelong loss of function due in part to the regenerative failure of CNS neurons. Inhibitory proteins derived from myelin and the astroglial scar are major barriers for the successful regeneration of injured CNS neurons. Previously, we described the identification of a novel compound, F05, which promotes neurite growth from neurons challenged with inhibitory substrates in vitro, and promotes axonal regeneration in vivo (Usher et al., 2010). To identify additional regeneration-promoting compounds, we used F05-induced gene expression profiles to query the Broad Institute Connectivity Map, a gene expression database of cells treated with >1300 compounds. Despite no shared chemical similarity, F05-induced changes in gene expression were remarkably similar to those seen with a group of piperazine phenothiazine antipsychotics (PhAPs). In contrast to antipsychotics of other structural classes, PhAPs promoted neurite growth of CNS neurons challenged with two different glial derived inhibitory substrates. Our pharmacological studies suggest a mechanism whereby PhAPs promote growth through antagonism of calmodulin signaling, independent of dopamine receptor antagonism. These findings shed light on mechanisms underlying neurite-inhibitory signaling, and suggest that clinically approved antipsychotic compounds may be repurposed for use in CNS injured patients.

The NAMPT inhibitor FK866 reverts the damage in spinal cord injury

Background

Emerging data implicate nicotinamide phosphoribosyl transferase (NAMPT) in the pathogenesis of cancer and inflammation. NAMPT inhibitors have proven beneficial in inflammatory animal models of arthritis and endotoxic shock as well as in autoimmune encephalitis. Given the role of inflammatory responses in spinal cord injury (SCI), the effect of NAMPT inhibitors was examined in this setting.

Methods

We investigated the effects of the NAMPT inhibitor FK866 in an experimental compression model of SCI.

Results

Twenty-four hr following induction of SCI, a significant functional deficit accompanied widespread edema, demyelination, neuron loss and a substantial increase in TNF-α, IL-1β, PAR, NAMPT, Bax, MPO activity, NF-κB activation, astrogliosis and microglial activation was observed. Meanwhile, the expression of neurotrophins BDNF, GDNF, NT3 and anti-apoptotic Bcl-2 decreased significantly. Treatment with FK866 (10 mg/kg), the best known and characterized NAMPT inhibitor, at 1 h and 6 h after SCI rescued motor function, preserved perilesional gray and white matter, restored anti-apoptotic and neurotrophic factors, prevented the activation of neutrophils, microglia and astrocytes and inhibited the elevation of NAMPT, PAR, TNF-α, IL-1β, Bax expression and NF-κB activity.

We show for the first time that FK866, a specific inhibitor of NAMPT, administered after SCI, is capable of reducing the secondary inflammatory injury and partly reduce permanent damage. We also show that NAMPT protein levels are increased upon SCI in the perilesional area which can be corrected by administration of FK866.

Conclusions

Our findings suggest that the inflammatory component associated to SCI is the primary target of these inhibitors.

The role of well-defined patterned substrata on the regeneration of DRG neuron pathfinding and integrin expression dynamics using chondroitin sulfate proteoglycans

Injured neurons intrinsically adapt to and partially overcome inhibitory proteoglycan expression in the central nervous system by upregulating integrin expression. It remains unclear however, to what extent varying proteoglycan concentrations influence the strength of this response, how rapidly neurons adapt to proteoglycans, and how pathfinding dynamics are altered over time as integrin expression is modulated in response to proteoglycan signals. To investigate these quandaries, we created well-defined substrata in which postnatal DRG neuron pathfinding dynamics and growth cone integrin expression were interrogated as a function of proteoglycan substrata density. DRGs responded by upregulating integrin expression in a proteoglycan dose dependent fashion and exhibited robust outgrowth over all proteoglycan densities at initial time frames. However, after prolonged proteoglycan exposure, neurons exhibited decreasing velocities associated with increasing proteoglycan densities, while neurons growing on low proteoglycan levels exhibited robust outgrowth at all time points. Additionally, DRG outgrowth over proteoglycan density step boundaries, and a brief β1 integrin functional block proved that regeneration was integrin dependent and that DRGs exhibit delayed slowing and loss in persistence after even transient encounters with dense proteoglycan boundaries. These findings demonstrate the complexity of proteoglycan regulation on integrin expression and regenerative pathfinding.

Fibronectin supports neurite outgrowth and axonal regeneration of adult brain neurons in vitro

The molecular basis of axonal regeneration of central nervous system (CNS) neurons remains to be fully elucidated. In part, this is due to the difficulty in maintaining CNS neurons in vitro. Here, we show that dissociated neurons from the cerebral cortex and hippocampus of adult mice may be maintained in culture for up to 9 days in defined medium without added growth factors. Outgrowth of neurites including axons was observed from both CNS sources and was significantly greater on plasma fibronectin than on other substrata such as laminin and merosin. Neurite outgrowth on fibronectin appears to be mediated by α5β1 integrin since a recombinant fibronectin fragment containing binding sites for this receptor was as effective as intact fibronectin in supporting neurite outgrowth. Conversely, function-blocking antibodies to α5 and β1 integrin sub-units inhibited neurite outgrowth on intact fibronectin. These results suggest that the axonal regeneration seen in in vivo studies using fibronectin-based matrices is due to the molecule itself and not a consequence of secondary events such as cellular infiltration. They also indicate the domains of fibronectin that may be responsible for eliciting this response.

Isoform diversity and regulation in peripheral and central neurons revealed through RNA-Seq

To fully understand cell type identity and function in the nervous system there is a need to understand neuronal gene expression at the level of isoform diversity. Here we applied Next Generation Sequencing of the transcriptome (RNA-Seq) to purified sensory neurons and cerebellar granular neurons (CGNs) grown on an axonal growth permissive substrate. The goal of the analysis was to uncover neuronal type specific isoforms as a prelude to understanding patterns of gene expression underlying their intrinsic growth abilities. Global gene expression patterns were comparable to those found for other cell types, in that a vast majority of genes were expressed at low abundance. Nearly 18% of gene loci produced more than one transcript. More than 8000 isoforms were differentially expressed, either to different degrees in different neuronal types or uniquely expressed in one or the other. Sensory neurons expressed a larger number of genes and gene isoforms than did CGNs. To begin to understand the mechanisms responsible for the differential gene/isoform expression we identified transcription factor binding sites present specifically in the upstream genomic sequences of differentially expressed isoforms, and analyzed the 3′ untranslated regions (3′ UTRs) for microRNA (miRNA) target sites. Our analysis defines isoform diversity for two neuronal types with diverse axon growth capabilities and begins to elucidate the complex transcriptional landscape in two neuronal populations.

Injectable hydrogel materials for spinal cord regeneration: a review

Spinal cord injury (SCI) presents a complex regenerative problem due to the multiple facets of growth inhibition that occur following trauma to the cord parenchyma and stroma. Clinically, SCI is further complicated by the heterogeneity in the size, shape and extent of human injuries. Many of these injuries do not breach the dura mater and have continuous viable axons through the injury site that can later lead to some degree of functional recovery. In these cases, surgical manipulation of the spinal cord by implanting a preformed scaffold or drug delivery device may lead to further damage. Given these circumstances, in situ-forming scaffolds are an attractive approach for SCI regeneration. These synthetic and natural polymers undergo a rapid transformation from liquid to gel upon injection into the cord tissue, conforming to the individual lesion site and directly integrating with the host tissue. Injectable materials can be formulated to have mechanical properties that closely match the native spinal cord extracellular matrix, and this may enhance axonal ingrowth. Such materials can also be loaded with cellular and molecular therapeutics to modulate the wound environment and enhance regeneration. This review will focus on the current status of in situ-forming materials for spinal cord repair. The advantages of, and requirements for, such polymers will be presented, and examples of the behavior of such systems in vitro and in vivo will be presented. There are helpful lessons to be learned from the investigations of injectable hydrogels for the treatment of SCI that apply to the use of these biomaterials for the treatment of lesions in other central nervous system tissues and in organs comprising other tissue types.

Axons with highly branched terminal regions successfully regenerate across spinal midline transections in the adult cat

We recently reported that some, but not all, axotomized propriospinal commissural interneurons (PCI) of the adult mammal can regenerate through spinal midsagittal transection injury sites (Fenrich and Rose [2009] J Neurosci 29:12145-12158). In this model, regenerating axons grow through a lesion site surrounded by a dense deposition of chondroitin sulfate proteoglycans (CSPG), which are typically inhibitory to regenerating axons. However, the mechanisms that lead some regenerating axons to grow through spinal cord injury (SCI) sites while others remain trapped in the CSPG zones or retract to their soma remain unknown. As a first step toward elucidating these mechanisms, here we show that the ability of PCI axons to regenerate across a SCI site depends on the branching patterns of their distal terminals. Using 3D reconstruction techniques through multiple serial sections and immunohistochemical analyses, we found that at 7 days postinjury a majority of PCI axons terminated in CSPG zones ipsilateral of the spinal midline. Conversely, at 9 days postinjury some PCI axons had regenerated across the midline, but others terminated outside the CSPG zones near their soma. Furthermore, we show that the most successful regenerators were those with the most extensive branching patterns, whereas those that terminated outside the CSPG zones had terminal regions indistinguishable from dystrophic terminals. Our results demonstrate that the morphological characteristics of regenerating axons play an important role in their ability to regenerate across SCI sites, and that the branching patterns of some regenerating axons are more extensive and have a far greater complexity than previously reported.

Combination therapies

Spinal cord injury (SCI) has multiple consequences, ranging from molecular imbalances to glial scar formation to functional impairments. It is logical to think that a combination of single treatments implemented in the right order and at the right time will be required to repair the spinal cord. However, the single treatments that compose the combination therapy will need to be chosen with caution as many have multiple outcomes that may or may not be synergistic. Single treatments may also elicit unwanted side-effects and/or effects that would decrease the repair potential of other components and/or the entire combination therapy. In this chapter a number of single treatments are discussed with respect to their multiplicity of action. These include strategies to boost growth and survival (such as neurotrophins and cyclic AMP) and strategies to reduce inhibitory factors (such as antimyelin-associated growth inhibitors and digestion of glial scar-associated inhibitors). We also present an overview of combination therapies that have successfully or unsuccessfully been tested in the laboratory using animal models. To effectively design a combination therapy a number of considerations need to be made such as the nature and timing of the treatments and the method for delivery. This chapter discusses these issues as well as considerations related to chronic SCI and the logistics of bringing combination therapies to the clinic.

Neural regeneration

Regeneration of the nervous system requires either the repair or replacement of nerve cells that have been damaged by injury or disease. While lower organisms possess extensive capacity for neural regeneration, evolutionarily higher organisms including humans are limited in their ability to regenerate nerve cells, posing significant issues for the treatment of injury and disease of the nervous system. This chapter focuses on current approaches for neural regeneration, with a discussion of traditional methods to enhance neural regeneration as well as emerging concepts within the field such as stem cells and cellular reprogramming. Stem cells are defined by their ability to self-renew as well as their ability to differentiate into multiple cell types, and hence can serve as a source for cell replacement of damaged neurons. Traditionally, adult stem cells isolated from the hippocampus and subventricular zone have served as a source of neural stem cells for replacement purposes. With the advancement of pluripotent stem cells, including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs), new and exciting approaches for neural cell replacement are being developed. Furthermore, with increased understanding of the human genome and epigenetics, scientists have been successful in the direct genetic reprogramming of somatic cells to a neuronal fate, bypassing the intermediary pluripotent stage. Such breakthroughs have accelerated the timing of production of mature neuronal cell types from a patient-specific somatic cell source such as skin fibroblasts or mononuclear blood cells. While extensive hurdles remain to the translational application of such stem cell and reprogramming strategies, these approaches have revolutionized the field of regenerative biology and have provided innovative approaches for the potential regeneration of the nervous system.

Defeating inhibition of regeneration by scar and myelin components

Axon regeneration and the sprouting processes that underlie plasticity are blocked by inhibitory factors in the central nervous system (CNS) environment, several of which are upregulated after injury. The major inhibitory molecules are those associated with myelin and those associated with the glial scar. In myelin, NogoA, MAG, and OMgp are present on normal oligodendrocytes and on myelin debris. They act partly via the Nogo receptor, partly via an unidentified amino-Nogo receptor. In the glial scar, chondroitin sulphate proteoglycans, semaphorins, and the formation of a collagen-based membrane are all inhibitory. Methods to counteract these forms of inhibition have been identified, and these treatments promote axon regeneration in the damaged spinal cord, and in some cases recovery of function through enhanced plasticity.

Repair involves all three surfaces of the glial cell

We propose that severed adult CNS axons are intrinsically capable of regeneration and reestablishing lost functions and that the key to repair lies in reconfiguring the scarring response of the astrocytic network. Astrocytes are multifunctional cells with three distinct surfaces: a glia to glial surface, providing the junctions needed to incorporate the astrocytes into the network; a glia to mesodermal surface, at which astrocytes collaborate with the meningeal fibroblasts to maintain the protective covering of the CNS; and a glia to neuronal surface, which provides the routes along which axons travel. After injury, the astrocytes collaborate with the meningeal fibroblasts to form a scar, which provides the necessary defensive sealing of the opened surface of the CNS, but which also has the detrimental effect of closing off the pathways along which axons could regenerate. Incorporation of glial cells transplanted from the olfactory system into a CNS injury causes a re-arrangement of the scarred astrocyte/fibroblast complex so as to produce the alignment of the glia to neuronal surfaces needed to provide a pathway for the regeneration of severed axons. Olfactory ensheathing cells certainly have a direct stimulatory effect on axons, but without concomitant reorganization of the glial scar, this could not in itself lead to regeneration of severed axons to their targets.

Determination of the therapeutic time window for human umbilical cord blood mononuclear cell transplantation following experimental stroke in rats

Experimental treatment strategies using human umbilical cord blood mononuclear cells (hUCB MNCs) represent a promising option for alternative stroke therapies. An important point for clinical translation of such treatment approaches is knowledge on the therapeutic time window. Although expected to be wider than for thrombolysis, the exact time window for hUCB MNC therapy is not known. Our study aimed to determine the time window of intravenous hUCB MNC administration after middle cerebral artery occlusion (MCAO). Male spontaneously hypertensive rats underwent MCAO and were randomly assigned to hUCB MNC administration at 4, 24, 72, and 120 or 14 days. Influence of cell treatment was observed by magnetic resonance imaging on days 1, 8, and 29 following MCAO and by assessment of functional neurological recovery. On day 30, brains were screened for glial scar development and presence of hUCB MNCs. Further, influence of hUCB MNCs on necrosis and apoptosis in postischemic neural tissue was investigated in hippocampal slices cultures. Transplantation within a 72-h time window resulted in an early improvement of functional recovery, paralleled by a reduction of brain atrophy and diminished glial scarring. Cell transplantation 120 h post-MCAO only induced minor functional recovery without changes in the brain atrophy rate and glial reactivity. Later transplantation (14 days) did not show any benefit. No evidence for intracerebrally localized hUCB MNCs was found in any treatment group. In vitro hUCB MNCs were able to significantly reduce postischemic neural necrosis and apoptosis. Our results for the first time indicate a time window of therapeutic hUCB MNC application of at least 72 h. The time window is limited, but wider than compared to conventional pharmacological approaches. The data furthermore confirms that differentiation and integration of administered cells is not a prerequisite for poststroke functional improvement and lesion size reduction.

Transcriptional regulation of neuronal polarity and morphogenesis in the mammalian brain

The highly specialized morphology of a neuron, typically consisting of a long axon and multiple branching dendrites, lies at the core of the principle of dynamic polarization, whereby information flows from dendrites toward the soma and to the axon. For more than a century, neuroscientists have been fascinated by how shape is important for neuronal function and how neurons acquire their characteristic morphology. During the past decade, substantial progress has been made in our understanding of the molecular underpinnings of neuronal polarity and morphogenesis. In these studies, transcription factors have emerged as key players governing multiple aspects of neuronal morphogenesis from neuronal polarization and migration to axon growth and pathfinding to dendrite growth and branching to synaptogenesis. In this review, we will highlight the role of transcription factors in shaping neuronal morphology with emphasis on recent literature in mammalian systems.

Precursor cell biology and the development of astrocyte transplantation therapies: lessons from spinal cord injury

This review summarizes current progress on development of astrocyte transplantation therapies for repair of the damaged central nervous system. Replacement of neurons in the injured or diseased central nervous system is currently one of the most popular therapeutic goals, but if neuronal replacement is attempted in the absence of appropriate supporting cells (astrocytes and oligodendrocytes), then the chances of restoring neurological functional are greatly reduced. Although the past 20 years have offered great progress on oligodendrocyte replacement therapies, astrocyte transplantation therapies have been both less explored and comparatively less successful. We have now developed successful astrocyte transplantation therapies by pre-differentiating glial restricted precursor (GRP) cells into a specific population of GRP cell-derived astrocytes (GDAs) by exposing the GRP cells to bone morphogenetic protein-4 (BMP) prior to transplantation. When transplanted into transected rat spinal cord, rat and human GDAs(BMP) promote extensive axonal regeneration, rescue neuronal cell survival, realign tissue structure, and restore behavior to pre-injury levels on a grid-walk analysis of volitional foot placement. Such benefits are not provided by GRP cells themselves, demonstrating that the lesion environment does not direct differentiation in a manner optimally beneficial for the restoration of function. Such benefits also are not provided by transplantation of a different population of astrocytes generated from GRP cells exposed to ciliary neurotrophic factor (GDAs(CNTF)), thus providing the first transplantation-based evidence of functional heterogeneity in astrocyte populations. Moreover, lessons learned from the study of rat cells are strongly predictive of outcomes using human cells. Thus, these studies provide successful strategies for the use of astrocyte transplantation therapies for restoration of function following spinal cord injury.

Training and anti-CSPG combination therapy for spinal cord injury

Combining different therapies is a promising strategy to promote spinal cord repair, by targeting axon plasticity and functional circuit reconnectivity. In particular, digestion of chondroitin sulphate proteoglycans at the site of the injury by the activity of the bacterial enzyme chondrotinase ABC, together with the development of intensive task specific motor rehabilitation has shown synergistic effects to promote behavioural recovery. This review describes the mechanisms by which chondroitinase ABC and motor rehabilitation promote neural plasticity and we discuss their additive and independent effects on promoting behavioural recovery.

Alterations in chondroitin sulfate proteoglycan expression occur both at and far from the site of spinal contusion injury

Chondroitin sulfate proteoglycans (CSPGs) present an inhibitory barrier to axonal growth and plasticity after trauma to the central nervous system. These extracellular and membrane bound molecules are altered after spinal cord injuries, but the magnitude, time course, and patterns of expression following contusion injury have not been fully described. Western blots and immunohistochemistry were combined to assess the expression of four classically inhibitory CSPGs, aggrecan, neurocan, brevican and NG2, at the lesion site and in distal segments of cervical and thoracic spinal cord at 3, 7, 14 and 28 days following a severe mid-thoracic spinal contusion. Total neurocan and the full-length (250 kDa) isoform were strongly upregulated both at the lesion epicenter and in cervical and lumbar segments. In contrast, aggrecan and brevican were sharply reduced at the injury site and were unchanged in distal segments. Total NG2 protein was unchanged across the injury site, while NG2+ profiles were distributed throughout the lesion site by 14 days post-injury (dpi). Far from the lesion, NG2 expression was increased at lumbar, but not cervical spinal cord levels. To determine if the robust increase in neurocan at the distal spinal cord levels corresponded to regions of increased astrogliosis, neurocan and GFAP immunoreactivity were measured in gray and white matter regions of the spinal enlargements. GFAP antibodies revealed a transient increase in reactive astrocyte staining in cervical and lumbar cord, peaking at 14 dpi. In contrast, neurocan immunoreactivity was specifically elevated in the cervical dorsal columns and in the lumbar ventral horn and remained high through 28 dpi. The long lasting increase of neurocan in gray matter regions at distal levels of the spinal cord may contribute to the restriction of plasticity in the chronic phase after SCI. Thus, therapies targeted at altering this CSPG both at and far from the lesion site may represent a reasonable addition to combined strategies to improve recovery after SCI.

Extrinsic and intrinsic determinants of nerve regeneration

After central nervous system (CNS) injury axons fail to regenerate often leading to persistent neurologic deficit although injured peripheral nervous system (PNS) axons mount a robust regenerative response that may lead to functional recovery. Some of the failures of CNS regeneration arise from the many glial-based inhibitory molecules found in the injured CNS, whereas the intrinsic regenerative potential of some CNS neurons is actively curtailed during CNS maturation and limited after injury. In this review, the molecular basis for extrinsic and intrinsic modulation of axon regeneration within the nervous system is evaluated. A more complete understanding of the factors limiting axonal regeneration will provide a rational basis, which is used to develop improved treatments for nervous system injury.

Increase of sensitivity to mechanical stimulus after transplantation of murine induced pluripotent stem cell-derived astrocytes in a rat spinal cord injury model

OBJECT

Clinical use of autologous induced pluripotent stem cells (iPSCs) could circumvent immune rejection and bioethical issues associated with embryonic stem cells. Spinal cord injury (SCI) is a devastating trauma with long-lasting disability, and current therapeutic approaches are not satisfactory. In the present study, the authors used the neural stem sphere (NSS) method to differentiate iPSCs into astrocytes, which were evaluated after their transplantation into injured rat spinal cords.

METHODS

Induced pluripotent stem cell-derived astrocytes were differentiated using the NSS method and injected 3 and 7 days after spinal contusion-based SCI. Control rats were injected with DMEM in the same manner. Locomotor recovery was assessed for 8 weeks, and sensory and locomotion tests were evaluated at 8 weeks. Immunohistological parameters were then assessed.

RESULTS

Transplant recipients lived for 8 weeks without tumor formation. Transplanted cells stretched their processes along the longitudinal axis, but they did not merge with the processes of host GFAP-positive astrocytes. Locomotion was assessed in 3 ways, but none of the tests detected statistically significant improvements compared with DMEM-treated control rats after 8 weeks. Rather, iPSC transplantation caused even greater sensitivity to mechanical stimulus than DMEM treatment.

CONCLUSIONS

Astrocytes can be generated by serum treatment of NSS-generated cells derived from iPSCs. However, transplantation of such cells is poorly suited for repairing SCI.