Restoration of function after spinal cord transection using a collagen bridge

Transplantation of fibrin-thrombin encapsulated human induced neural stem cells promotes functional recovery of spinal cord injury rats through modulation of the microenvironment

Recent studies have mostly focused on engraftment of cells at the lesioned spinal cord, with the expectation that differentiated neurons facilitate recovery. Only a few studies have attempted to use transplanted cells and/or biomaterials as major modulators of the spinal cord injury microenvironment. Here, we aimed to investigate the role of microenvironment modulation by cell graft on functional recovery after spinal cord injury. Induced neural stem cells reprogrammed from human peripheral blood mononuclear cells, and/or thrombin plus fibrinogen, were transplanted into the lesion site of an immunosuppressed rat spinal cord injury model. Basso, Beattie and Bresnahan score, electrophysiological function, and immunofluorescence/histological analyses showed that transplantation facilitates motor and electrophysiological function, reduces lesion volume, and promotes axonal neurofilament expression at the lesion core. Examination of the graft and niche components revealed that although the graft only survived for a relatively short period (up to 15 days), it still had a crucial impact on the microenvironment. Altogether, induced neural stem cells and human fibrin reduced the number of infiltrated immune cells, biased microglia towards a regenerative M2 phenotype, and changed the cytokine expression profile at the lesion site. Graft-induced changes of the microenvironment during the acute and subacute stages might have disrupted the inflammatory cascade chain reactions, which may have exerted a long-term impact on the functional recovery of spinal cord injury rats.

Hydrogels in Spinal Cord Injury Repair: A Review

Traffic accidents and falling objects are responsible for most spinal cord injuries (SCIs). SCI is characterized by high disability and tends to occur among the young, seriously affecting patients' lives and quality of life. The key aims of repairing SCI include preventing secondary nerve injury, inhibiting glial scarring and inflammatory response, and promoting nerve regeneration. Hydrogels have good biocompatibility and degradability, low immunogenicity, and easy-to-adjust mechanical properties. While providing structural scaffolds for tissues, hydrogels can also be used as slow-release carriers in neural tissue engineering to promote cell proliferation, migration, and differentiation, as well as accelerate the repair of damaged tissue. This review discusses the characteristics of hydrogels and their advantages as delivery vehicles, as well as expounds on the progress made in hydrogel therapy (alone or combined with cells and molecules) to repair SCI. In addition, we discuss the prospects of hydrogels in clinical research and provide new ideas for the treatment of SCI.

Distinct Glycosylation Responses to Spinal Cord Injury in Regenerative and Nonregenerative Models

Traumatic spinal cord injury (SCI) results in disruption of tissue integrity and loss of function. We hypothesize that glycosylation has a role in determining the occurrence of regeneration and that biomaterial treatment can influence this glycosylation response. We investigated the glycosylation response to spinal cord transection in Xenopus laevis and rat. Transected rats received an aligned collagen hydrogel. The response compared regenerative success, regenerative failure, and treatment in an established nonregenerative mammalian system. In a healthy rat spinal cord, ultraperformance liquid chromatography (UPLC) N-glycoprofiling identified complex, hybrid, and oligomannose N-glycans. Following rat SCI, complex and outer-arm fucosylated glycans decreased while oligomannose and hybrid structures increased. Sialic acid was associated with microglia/macrophages following SCI. Treatment with aligned collagen hydrogel had a minimal effect on the glycosylation response. In Xenopus, lectin histochemistry revealed increased levels of N-acetyl-glucosamine (GlcNAc) in premetamorphic animals. The addition of GlcNAc is required for processing complex-type glycans and is a necessary foundation for additional branching. A large increase in sialic acid was observed in nonregenerative animals. This work suggests that glycosylation may influence regenerative success. In particular, loss of complex glycans in rat spinal cord may contribute to regeneration failure. Targeting the glycosylation response may be a promising strategy for future therapies.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

A multi-channel collagen scaffold loaded with neural stem cells for the repair of spinal cord injury

Collagen scaffolds possess a three-dimensional porous structure that provides sufficient space for cell growth and proliferation, the passage of nutrients and oxygen, and the discharge of metabolites. In this study, a porous collagen scaffold with axially-aligned luminal conduits was prepared. In vitro biocompatibility analysis of the collagen scaffold revealed that it enhances the activity of neural stem cells and promotes cell extension, without affecting cell differentiation. The collagen scaffold loaded with neural stem cells improved the hindlimb motor function in the rat model of T8 complete transection and promoted nerve regeneration. The collagen scaffold was completely degraded in vivo within 5 weeks of implantation, exhibiting good biodegradability. Rectal temperature, C-reactive protein expression and CD68 staining demonstrated that rats with spinal cord injury that underwent implantation of the collagen scaffold had no notable inflammatory reaction. These findings suggest that this novel collagen scaffold is a good carrier for neural stem cell transplantation, thereby enhancing spinal cord repair following injury. This study was approved by the Animal Ethics Committee of Nanjing Drum Tower Hospital (the Affiliated Hospital of Nanjing University Medical School), China (approval No. 2019AE02005) on June 15, 2019.

Enhancing the regenerative potential of stem cell-laden, clinical-grade implants through laminin engineering

Protected delivery of neural stem cells (NSCs; a major transplant population) within bioscaffolds has the potential to improve regenerative outcomes in sites of spinal cord injury. Emergent research has indicated clinical grade bioscaffolds (e.g. those used as surgical sealants) may be repurposed for this strategy, bypassing the long approval processes and difficulties in scale-up faced by laboratory grade materials. While promising, clinical scaffolds are often not inherently regenerative. Extracellular molecule biofunctionalisation of scaffolds can enhance regenerative features such as encapsulated cell survival/distribution, cell differentiation into desired cell types and nerve fibre growth. However, this strategy is yet to be tested for clinical grade scaffolds. Here, we show for the first time that Hemopatch™, a widely used, clinically approved surgical matrix, supports NSC growth. Further, functionalisation of Hemopatch™ with laminin promoted homogenous distribution of NSCs and their daughter cells within the matrix, a key regenerative criterion for transplant cells.

Neural stem cell delivery via porous collagen scaffolds promotes neuronal differentiation and locomotion recovery in spinal cord injury

Neural stem cell (NSC) grafts have demonstrated significant effects in animal models of spinal cord injury (SCI), yet their clinical translation remains challenging. Significant evidence suggests that the supporting matrix of NSC grafts has a crucial role in regulating NSC effects. Here we demonstrate that grafts based on porous collagen-based scaffolds (PCSs), similar to biomaterials utilized clinically in induced regeneration, can deliver and protect embryonic NSCs at SCI sites, leading to significant improvement in locomotion recovery in an experimental mouse SCI model, so that 12 weeks post-injury locomotion performance of implanted animals does not statistically differ from that of uninjured control animals. NSC-seeded PCS grafts can modulate key processes required to induce regeneration in SCI lesions including enhancing NSC neuronal differentiation and functional integration in vivo, enabling robust axonal elongation, and reducing astrogliosis. Our findings suggest that the efficacy and translational potential of emerging NSC-based SCI therapies could be enhanced by delivering NSC via scaffolds derived from well-characterized clinically proven PCS.

Electrophysiological properties of neurons grown on soft polymer scaffolds reveal the potential to develop neuromimetic culture environments

Tissue engineering methodologies for various physiological systems are seeing a significant trend towards 3D cell culture in or on 'soft' polymeric hydrogel materials, widely considered to provide a more biomimetic environment for cell growth versus 'hard' materials such as glass or plastic. Progress has been slower with 3D neural cell culture with current studies overwhelmingly reliant on hard substrates. Accordingly, our knowledge of the alterations in electrochemical properties of neurons propagated in soft materials is relatively limited. In this study, primary cortical neurons and glial cells were seeded onto the surface of collagen hydrogels and grown in vitro for 7-8 days. At this time, neurons had formed a complex neurite web interspersed with astrocytes. Neuronal patch clamp recordings revealed voltage-gated Na+ and K+ currents in voltage clamp and action potentials in current clamp. When measured at voltages close to maximum activation, both currents were >1 nA in mean amplitude. When compared to primary cortical neurons cultured on glass coverslips, but otherwise under similar conditions (Evans et al., 2017), the Na+ current from hydrogel neurons was found to be significantly larger although there were no differences in the K+ current amplitude, membrane potential, input resistance or cell capacitance. We speculate that the larger size of the neuronal voltage-dependent Na+ current in the hydrogels is related to the better biomimetic properties of the soft material, being close to values reported for neurons recorded in brain slices. The results highlight the potential benefits offered by neuronal culture on soft and biomimetic polymeric materials for neural tissue engineering studies.

Role and prospects of regenerative biomaterials in the repair of spinal cord injury

Axonal junction defects and an inhibitory environment after spinal cord injury seriously hinder the regeneration of damaged tissues and neuronal functions. At the site of spinal cord injury, regenerative biomaterials can fill cavities, deliver curative drugs, and provide adsorption sites for transplanted or host cells. Some regenerative biomaterials can also inhibit apoptosis, inflammation and glial scar formation, or further promote neurogenesis, axonal growth and angiogenesis. This review summarized a variety of biomaterial scaffolds made of natural, synthetic, and combined materials applied to spinal cord injury repair. Although these biomaterial scaffolds have shown a certain therapeutic effect in spinal cord injury repair, there are still many problems to be resolved, such as product standards and material safety and effectiveness.

Cetuximab and Taxol co-modified collagen scaffolds show combination effects for the repair of acute spinal cord injury

Injury-activated endogenous neural stem cells (NSCs) in the spinal cord have promising therapeutic applications for rebuilding the neuronal relays after spinal cord injury (SCI) because of their lack of immune-rejection following exogenous cell transplantation. However, these NSCs rarely differentiate into neurons and the damaged axonal regenerative ability is drastically reduced due to the adverse SCI microenvironment. Cetuximab, an EGFR signaling antagonist, has demonstrated the ability of promoting NSC differentiation into neurons. Taxol, in addition to stabilizing microtubules, has shown potential for enhancing axonal regeneration and reducing scar formation after SCI. In this study, we further verified the combined therapeutic effects of Cetuximab and Taxol on inhibition of scar deposition and promotion of neuronal differentiation, axonal outgrowth and functional recovery in a rat severe SCI model. A linear orderly collagen scaffold modified with Cetuximab and Taxol was grafted into the SCI site after the complete removal of 4 mm of spinal tissue. The results showed that the combined functional scaffold implantation significantly increased neural regeneration to reconnect the neural network. Moreover, scaffold transplantation decreases the deposition of varied scar-related inhibitors within the lesion center, further reflecting the need for a combination dedicated to increasing motor function following SCI. Collagen scaffold based-combined therapy provides a potential strategy for improving functional restoration of the injured spinal cord.

Label-free multiphoton microscopy reveals relevant tissue changes induced by alginate hydrogel implantation in rat spinal cord injury

The development of therapies promoting recovery after spinal cord injury is a challenge. Alginate hydrogels offer the possibility to develop biocompatible implants with mechanical properties tailored to the nervous tissue, which could provide a permissive environment for tissue repair. Here, the effects of non-functionalized soft calcium alginate hydrogel were investigated in a rat model of thoracic spinal cord hemisection and compared to lesioned untreated controls. Open field locomotion tests were employed to evaluate functional recovery. Tissue analysis was performed with label-free multiphoton microscopy using a multimodal approach that combines coherent anti-Stokes Raman scattering to visualize axonal structures, two-photon fluorescence to visualize inflammation, second harmonic generation to visualize collagenous scarring. Treated animals recovered hindlimb function significantly better than controls. Multiphoton microscopy revealed that the implant influenced the injury-induced tissue response, leading to decreased inflammation, reduced scarring with different morphology and increased presence of axons. Demyelination of contralateral white matter near the lesion was prevented. Reduced chronic inflammation and increased amount of axons in the lesion correlated with improved hindlimb functions, being thus relevant for locomotion recovery. In conclusion, non-functionalized hydrogel improved functional outcome after spinal cord injury in rats. Furthermore, label-free multiphoton microscopy qualified as suitable technique for regeneration studies.

A Dual Functional Scaffold Tethered with EGFR Antibody Promotes Neural Stem Cell Retention and Neuronal Differentiation for Spinal Cord Injury Repair

Neural stem cells (NSCs) transplantation is a promising strategy to restore neuronal relays and neurological function of injured spinal cord because of the differentiation potential into functional neurons, but the transplanted NSCs often largely diffuse from the transplanted site and mainly differentiate into glial cells rather than neurons due to the adverse microenviornment after spinal cord injury (SCI). This paper fabricates a dual functional collagen scaffold tethered with a collagen-binding epidermal growth factor receptor (EGFR) antibody to simultaneously promote NSCs retention and neuronal differentiation by specifically binding to EGFR molecule expressed on NSCs and attenuating EGFR signaling, which is responsible for the inhibition of differentiation of NSCs toward neurons. Compared to unmodified control scaffold, the dual functional scaffold promotes the adhesion and neuronal differentiation of NSCs in vitro. Moreover, the implantation of the dual functional scaffold with exogenous NSCs in rat SCI model can capture and retain NSCs at the injury sites, and promote the neuronal differentiation of the retained NSCs into functional neurons, and finally dedicate to improving motor function of SCI rats, which provides a potential strategy for synchronously promoting stem cell retention and differentiation with biomaterials for SCI repair.

Mechanically Oriented 3D Collagen Hydrogel for Directing Neurite Growth

Recent studies in the field of neuro-tissue engineering have demonstrated the promising effects of aligned contact guidance cue to scaffolds of enhancement and direction of neuronal growth. In vivo, neurons grow and develop neurites in a complex three-dimensional (3D) extracellular matrix (ECM) surrounding. Studies have utilized hydrogel scaffolds derived from ECM molecules to better simulate natural growth. While many efforts have been made to control neuronal growth on 2D surfaces, the development of 3D scaffolds with an elaborate oriented topography to direct neuronal growth still remains a challenge. In this study, we designed a method for growing neurons in an aligned and oriented 3D collagen hydrogel. We aligned collagen fibers by inducing controlled uniaxial strain on gels. To examine the collagen hydrogel as a suitable scaffold for neuronal growth, we evaluated the physical properties of the hydrogel and measured collagen fiber properties. By combining the neuronal culture in 3D collagen hydrogels with strain-induced alignment, we were able to direct neuronal growth in the direction of the aligned collagen matrix. Quantitative evaluation of neurite extension and directionality within aligned gels was performed. The analysis showed neurite growth aligned with collagen matrix orientation, while maintaining the advantageous 3D growth.

Stem cells for spinal cord injury: Strategies to inform differentiation and transplantation

The complex pathology of spinal cord injury (SCI), involving a cascade of secondary events and the formation of inhibitory barriers, hampers regeneration across the lesion site and often results in irreversible loss of motor function. The limited regenerative capacity of endogenous cells after SCI has led to a focus on the development of cell therapies that can confer both neuroprotective and neuroregenerative benefits. Stem cells have emerged as a candidate cell source because of their ability to self-renew and differentiate into a multitude of specialized cell types. While ethical and safety concerns impeded the use of stem cells in the past, advances in isolation and differentiation methods have largely mitigated these issues. A confluence of work in stem cell biology, genetics, and developmental neurobiology has informed the directed differentiation of specific spinal cell types. After transplantation, these stem cell-derived populations can replace lost cells, provide trophic support, remyelinate surviving axons, and form relay circuits that contribute to functional recovery. Further refinement of stem cell differentiation and transplantation methods, including combinatorial strategies that involve biomaterial scaffolds and drug delivery, is critical as stem cell-based treatments enter clinical trials. Biotechnol. Bioeng. 2017;114: 245-259. © 2016 Wiley Periodicals, Inc.

One-year clinical study of NeuroRegen scaffold implantation following scar resection in complete chronic spinal cord injury patients

The objective of this clinical study was to assess the safety and feasibility of the collagen scaffold, NeuroRegen scaffold, one year after scar tissue resection and implantation. Scar tissue is a physical and chemical barrier that prevents neural regeneration. However, identification of scar tissue is still a major challenge. In this study, the nerve electrophysiology method was used to distinguish scar tissue from normal neural tissue, and then different lengths of scars ranging from 0.5-4.5 cm were surgically resected in five complete chronic spinal cord injury (SCI) patients. The NeuroRegen scaffold along with autologous bone marrow mononuclear cells (BMMCs), which have been proven to promote neural regeneration and SCI recovery in animal models, were transplanted into the gap in the spinal cord following scar tissue resection. No obvious adverse effects related to scar resection or NeuroRegen scaffold transplantation were observed immediately after surgery or at the 12-month follow-up. In addition, patients showed partially autonomic nervous function improvement, and the recovery of somatosensory evoked potentials (SSEP) from the lower limbs was also detected. The results indicate that scar resection and NeuroRegen scaffold transplantation could be a promising clinical approach to treating SCI.

Semaphorin 3C Released from a Biocompatible Hydrogel Guides and Promotes Axonal Growth of Rodent and Human Dopaminergic Neurons

Cell therapy in experimental models of Parkinson's disease replaces the lost dopamine neurons (DAN), but we still need improved methods to guide dopaminergic axons (DAx) of grafted neurons to make proper connections. The protein Semaphorin 3C (Sema3C) attracts DAN axons and enhances their growth. In this work, we show that the hydrogel PuraMatrix, a self-assembling peptide-based matrix, incorporates Sema3C and releases it steadily during 4 weeks. We also tested if hydrogel-delivered Sema3C attracts DAx using a system of rat midbrain explants embedded in collagen gels. We show that Sema3C released by this hydrogel attracts DAx, in a similar way to pretectum, which is known to attract growing DAN axons. We assessed the effect of Sema3C on the growth of DAx using microfluidic devices. DAN from rat midbrain or those differentiated from human embryonic stem cells showed enhanced axonal extension when exposed to hydrogel-released Sema3C, similar to soluble Sema3C. Notably, DAN of human origin express the cognate Sema3C receptors, Neuropilin1 and Neuropilin2. These results show that PuraMatrix is able to incorporate and release Sema3C, and such delivery guides and promotes the axonal growth of DAN. This biocompatible hydrogel might be useful as a Sema3C carrier for in vivo studies in parkinsonian animal models.

A Hydrogel Bridge Incorporating Immobilized Growth Factors and Neural Stem/Progenitor Cells to Treat Spinal Cord Injury

Spinal cord injury (SCI) causes permanent, often complete disruption of central nervous system (CNS) function below the damaged region, leaving patients without the ability to regenerate lost tissue. To engineer new CNS tissue, a unique spinal cord bridge is created to deliver stem cells and guide their organization and development with site-specifically immobilized growth factors. In this study, this bridge is tested, consisting of adult neural stem/progenitor cells contained within a methacrylamide chitosan (MAC) hydrogel and protected by a chitosan conduit. Interferon-γ (IFN-γ) and platelet-derived growth factor-AA (PDGF-AA) are recombinantly produced and tagged with an N-terminal biotin. They are immobilized to streptavidin-functionalized MAC to induce either neuronal or oligodendrocytic lineages, respectively. These bridges are tested in a rat hemisection model of SCI between T8 and T9. After eight weeks treatments including chitosan conduits result in a significant reduction in lesion area and macrophage infiltration around the lesion site (p < 0.0001). Importantly, neither immobilized IFN-γ nor PDGF-AA increased macrophage infiltration. Retrograde tracing demonstrates improved neuronal regeneration through the use of immobilized growth factors. Immunohistochemistry staining demonstrates that immobilized growth factors are effective in differentiating encapsulated cells into their anticipated lineages within the hydrogel, while qualitatively reducing glial fibrillary acid protein expression.

A novel composite type I collagen scaffold with micropatterned porosity regulates the entrance of phagocytes in a severe model of spinal cord injury

Traumatic spinal cord injury (SCI) is a damage to the spinal cord that results in loss or impaired motor and/or sensory function. SCI is a sudden and unexpected event characterized by high morbidity and mortality rate during both acute and chronic stages, and it can be devastating in human, social and economical terms. Despite significant progresses in the clinical management of SCI, there remain no effective treatments to improve neurological outcomes. Among experimental strategies, bioengineered scaffolds have the potential to support and guide injured axons contributing to neural repair. The major aim of this study was to investigate a novel composite type I collagen scaffold with micropatterned porosity in a rodent model of severe spinal cord injury. After segment resection of the thoracic spinal cord we implanted the scaffold in female Sprague-Dawley rats. Controls were injured without receiving implantation. Behavioral analysis of the locomotor performance was monitored up to 55 days postinjury. Two months after injury histopathological analysis were performed to evaluate the extent of scar and demyelination, the presence of connective tissue and axonal regrowth through the scaffold and to evaluate inflammatory cell infiltration at the injured site. We provided evidence that the new collagen scaffold was well integrated with the host tissue, slightly ameliorated locomotor function, and limited the robust recruitment of the inflammatory cells at the injury site during both the acute and chronic stage in spinal cord injured rats. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1040-1053, 2017.

Hydrogels and Cell Based Therapies in Spinal Cord Injury Regeneration

Spinal cord injury (SCI) is a central nervous system- (CNS-) related disorder for which there is yet no successful treatment. Within the past several years, cell-based therapies have been explored for SCI repair, including the use of pluripotent human stem cells, and a number of adult-derived stem and mature cells such as mesenchymal stem cells, olfactory ensheathing cells, and Schwann cells. Although promising, cell transplantation is often overturned by the poor cell survival in the treatment of spinal cord injuries. Alternatively, the therapeutic role of different cells has been used in tissue engineering approaches by engrafting cells with biomaterials. The latter have the advantages of physically mimicking the CNS tissue, while promoting a more permissive environment for cell survival, growth, and differentiation. The roles of both cell- and biomaterial-based therapies as single therapeutic approaches for SCI repair will be discussed in this review. Moreover, as the multifactorial inhibitory environment of a SCI suggests that combinatorial approaches would be more effective, the importance of using biomaterials as cell carriers will be herein highlighted, as well as the recent advances and achievements of these promising tools for neural tissue regeneration.

Acellular spinal cord scaffold seeded with bone marrow stromal cells protects tissue and promotes functional recovery in spinal cord-injured rats

Therapy using scaffolds seeded with stem cells plays an important role in repair of spinal cord injury (SCI), with the transplanted cells differentiating into nerve cells to replace the lost tissue while releasing neurotrophic factors that contribute to repair following SCI and enhance the function of the damaged nervous system. The present study investigated the ability to extend the survival time of bone marrow stromal cells (BMSCs) to restore the damaged spinal cord and improve functional recovery by grafting acellular spinal cord (ASC) scaffold seeded or not with BMSCs in a rat model of acute hemisected SCI. BBB scores revealed that treatment with BMSCs seeded into ASC scaffold led to an obvious improvement in motor function recovery compared with treatment with ASC scaffold alone or untreated controls. This improvement was evident at 2 and 8 weeks after surgery (P < 0.05). When BMSCs labeled with 5-bromodeoxyuridine were implanted together with ASC scaffold into the injured sites, they differentiated into glial cells, and some BMSCs could be observed within the graft by immunofluorescent staining at 8 weeks after implantation. Evaluation of caspase-3 activation suggested that the graft group was able to reduce apoptosis compared with SCI alone at 8 weeks after operation (P < 0.05). This study suggests that ASC scaffolds have the ability to enhance BMSC survival and improve differentiation and could also reduce native damaged nerve tissue apoptosis, thus protecting host tissue as well as improving functional recovery after implantation.

Long-term characterization of axon regeneration and matrix changes using multiple channel bridges for spinal cord regeneration

Spinal cord injury (SCI) results in loss of sensory and motor function below the level of injury and has limited available therapies. The host response to SCI is typified by limited endogenous repair, and biomaterial bridges offer the potential to alter the microenvironment to promote regeneration. Porous multiple channel bridges implanted into the injury provide stability to limit secondary damage and support cell infiltration that limits cavity formation. At the same time, the channels provide a path that physically directs axon growth across the injury. Using a rat spinal cord hemisection injury model, we investigated the dynamics of axon growth, myelination, and scar formation within and around the bridge in vivo for 6 months, at which time the bridge has fully degraded. Axons grew into and through the channels, and the density increased overtime, resulting in the greatest axon density at 6 months postimplantation, despite complete degradation of the bridge by that time point. Furthermore, the persistence of these axons contrasts with reports of axonal dieback in other models and is consistent with axon stability resulting from some degree of connectivity. Immunostaining of axons revealed both motor and sensory origins of the axons found in the channels of the bridge. Extensive myelination was observed throughout the bridge at 6 months, with centrally located and peripheral channels seemingly myelinated by oligodendrocytes and Schwann cells, respectively. Chondroitin sulfate proteoglycan deposition was restricted to the edges of the bridge, was greatest at 1 week, and significantly decreased by 6 weeks. The dynamics of collagen I and IV, laminin, and fibronectin deposition varied with time. These studies demonstrate that the bridge structure can support substantial long-term axon growth and myelination with limited scar formation.

Scaffolds to promote spinal cord regeneration

Substantial research effort in the spinal cord injury (SCI) field is directed towards reduction of secondary injury changes and enhancement of tissue sparing. However, pathway repair after complete transections, large lesions, or after chronic injury may require the implantation of some form of oriented bridging structure to restore tissue continuity across a trauma zone. These matrices or scaffolds should be biocompatible and create an environment that facilitates tissue growth and vascularization, and allow axons to regenerate through and beyond the implant in order to reconnect with "normal" tissue distal to the injury. The myelination of regrown axons is another important requirement. In this chapter, we describe recent advances in biomaterial technology designed to provide a terrain for regenerating axons to grow across the site of injury and/or create an environment for endogenous repair. Many different types of scaffold are under investigation; they can be biodegradable or nondegradable, natural or synthetic. Scaffolds can be designed to incorporate immobilized signaling molecules and/or used as devices for controlled release of therapeutic agents, including growth factors. These bridging structures can also be infiltrated with specific cell types deemed suitable for spinal cord repair.

Transection method for shortening the rat spine and spinal cord

Previous studies have presented evidence which indicates that the regeneration of axons in the spinal cord occurs following spinal cord transection in young rats. However, in a transection-regeneration model, the completeness of the transection is often a matter of dispute. We established a method for shortening the rat spine and spinal cord to provide a spinal cord injury (SCI) model in which there was no doubt about whether the axonal transection was complete. In the future, this model may be applied to the chronic period of complete paralysis following SCI. Adult, female Wistar rats (220–250g) were used in the study. The spinal cord was exposed and a 4-mm-long segment of the spinal cord was removed at Th8. Subsequently, the Th7/8 and Th8/9 discs were cut between the stumps of the spinal cord to remove the Th8 vertebra. The stitches which had been passed through the 7th and 9th ribs bilaterally were tied gradually to bring together the stumps of the spinal cord. Almost all the rats survived until the end of the experiment. Uncoordinated movements of the hind limbs in locomotion were observed at 4 weeks after surgery. However coordinated movements of the hind limbs in locomotion were not observed until the end of the experiment. After 12 weeks, an intracardiac perfusion was performed to remove the thoracic spine and the spinal cord. There were no signs of infection. The bone fusion of the Th7 and Th9 vertebrae was observed to be complete in all specimens and the alignment of the thoracic spine was maintained. The spinal canal was also correctly reconstituted. The stumps of the spinal cord were connected. Light microscopy of the cord showed that scar tissue intervened at the connection site. Cavitation inhibiting the axonal regeneration was also observed. This model was also made on the assumption that glial scar tissue inhibits axonal regeneration in chronic SCI. Axonal regeneration was not observed across the transected spinal cord in this model. Attempts should be made to minimize the damage to the spinal cord and the surgery time for successful axonal regeneration to occur. The model developed in this study may be useful in the study of axonal regeneration in SCI.

Biodegradable biomatrices and bridging the injured spinal cord: the corticospinal tract as a proof of principle

Important advances in the development of smart biodegradable implants for axonal regeneration after spinal cord injury have recently been reported. These advances are evaluated in this review with special emphasis on the regeneration of the corticospinal tract. The corticospinal tract is often considered the ultimate challenge in demonstrating whether a repair strategy has been successful in the regeneration of the injured mammalian spinal cord. The extensive know-how of factors and cells involved in the development of the corticospinal tract, and the advances made in material science and tissue engineering technology, have provided the foundations for the optimization of the biomatrices needed for repair. Based on the findings summarized in this review, the future development of smart biodegradable bridges for CST regrowth and regeneration in the injured spinal cord is discussed.

The reparative response to cross-linked collagen-based scaffolds in a rat spinal cord gap model

Prior work demonstrated the improvement of peripheral nerve regeneration in gaps implanted with collagen scaffold-filled collagen tubes, compared with nerve autografts, and the promise of such implants for treating gaps in spinal cord injury (SCI) in rats. The objective of this study was to investigate collagen implants alone and incorporating select therapeutic agents in a 5-mm full-resection gap model in the rat spinal cord. Two studies were performed, one with a 6-week time point and one with a 2-week time point. For the 6-week study the groups included: (1) untreated control, (2) dehydrothermally (DHT)-cross-linked collagen scaffold, (3) DHT-cross-linked collagen scaffold seeded with adult rat neural stem cells (NSCs), and (4) DHT-cross-linked collagen scaffold incorporating plasmid encoding glial cell line-derived neurotropic factor (pGDNF). The 2-week study groups were: (1) nontreated control, (2) DHT-cross-linked collagen scaffold; (3) DHT-cross-linked collagen scaffold containing laminin; and (4) carbodiimide-cross-linked collagen scaffold containing laminin. The tissue filling the defect of all groups at 6 weeks was largely composed of fibrous scar; however, the tissue was generally more favorably aligned with the long axis of the spinal cord in all of the treatment groups, but not in the control group. Quantification of the percentage of animals per group containing cystic cavities in the defect showed a trend toward fewer rats with cysts in the groups in which the scaffolds were implanted compared to control. All of the collagen implants were clearly visible and mostly intact after 2 weeks. A band of fibrous tissue filling the control gaps was not seen in the collagen implant groups. In all of the groups there was a narrowing of the spinal canal within the gap as a result of surrounding soft tissue collapse into the defect. The narrowing of the spinal canal occurred to a greater extent in the control and DHT scaffold alone groups compared to the DHT scaffold/laminin and EDAC scaffold/laminin groups. Collagen biomaterials can be useful in the treatment of SCI to: favorably align the reparative tissue with the long axis of the spinal cord; potentially reduce the formation of fluid-filled cysts; serve as a delivery vehicle for NSCs and the gene for GDNF; and impede the collapse of musculature and connective tissue into the defect.

Neurite extension of primary neurons on electrospun piezoelectric scaffolds

Neural tissue engineering may be a promising option for neural repair treatment, for which a well-designed scaffold is essential. Smart materials that can stimulate neurite extension and outgrowth have been investigated as potential scaffolding materials. A piezoelectric polymer polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) was used to fabricate electrospun aligned and random scaffolds having nano- or micron-sized fiber dimensions. The advantage of using a piezoelectric polymer is its intrinsic electrical properties. The piezoelectric characteristics of PVDF-TrFE scaffolds were shown to be enhanced by annealing. Dorsal root ganglion (DRG) neurons attached to all fibrous scaffolds. Neurites extended radially on random scaffolds, whereas aligned scaffolds directed neurite outgrowth for all fiber dimensions. Neurite extension was greatest on aligned, annealed PVDF-TrFE having micron-sized fiber dimensions in comparison with annealed and as-spun random PVDF-TrFE scaffolds. DRG on micron-sized aligned, as-spun and annealed PVDF-TrFE also had the lowest aspect ratio amongst all scaffolds, including non-piezoelectric PVDF and collagen-coated substrates. Findings from this study demonstrate the potential use of a piezoelectric fibrous scaffold for neural repair applications.

Linear ordered collagen scaffolds loaded with collagen-binding neurotrophin-3 promote axonal regeneration and partial functional recovery after complete spinal cord transection

Neurotrophin-3 (NT3) is an important neurotrophic factor for spinal cord injury (SCI) repair. However, constant exchange of cerebrospinal fluid often decreases the effective dosage of NT3 at the targeted injury site. In the present study, a recombinant collagen-binding NT3 (CBD-NT3), consisting of a collagen-binding domain (CBD) and native NT3, was constructed. Linear rat-tail collagen (LRTC) was used as a physical carrier for CBD-NT3 to construct a LRTC/C3 system. The collagen-binding ability of CBD-NT3 was verified, and the bioactivity of CBD-NT3 was assayed with neurite outgrowth of dorsal root ganglia (DRG) explants and DRG cells in vitro. After complete spinal cord transection in rats, LRTC/CBD-NT3 or the LRTC/NT3 system was transplanted into the injury site. Hindlimb locomotion recovery was closely observed using the Basso-Beattie-Bresnahan (BBB) locomotor rating scale and the grid walk test. Significant improvement was observed in the LRTC/CBD-NT3 group. The results of regenerating nerve fiber and anterograde tracing of biotinylated dextran amine (BDA)-labeled corticospinal tract (CST) fibers demonstrated axonal regeneration of LRTC/CBD-NT3 in the injured spinal cord. Serotonin fiber regrowth also illustrated the effectiveness of LRTC/CBD-NT3. Thus, collagen-binding NT3 with LRTC may provide an effective method for treating SCI.

Bioengineered scaffolds for spinal cord repair

Spinal cord injury can lead to devastating and permanent loss of neurological function, affecting all levels below the site of trauma. Unfortunately, the injured adult mammalian spinal cord displays little regenerative capacity and little functional recovery in large part due to a tissue environment that is nonpermissive for regenerative axon growth. Artificial tissue repair scaffolds may provide a physical guide to allow regenerative axon growth that bridges the lesion cavity and restores functional neural connectivity. By integrating different strategies, including the use of various biomaterials and microstructures as well as incorporation of bioactive molecules and living cells, combined or synergistic effects for spinal cord repair through regenerative axon growth may be achieved. This article briefly reviews the development of bioengineered scaffolds for spinal cord repair, focusing on spinal cord injury and the subsequent cellular response, scaffold materials, fabrication techniques, and current therapeutic strategies. Key issues and challenges are also identified and discussed along with recommendations for future research.

Topography, cell response, and nerve regeneration

In the body, cells encounter a complex milieu of signals, including topographical cues, in the form of the physical features of their surrounding environment. Imposed topography can affect cells on surfaces by promoting adhesion, spreading, alignment, morphological changes, and changes in gene expression. Neural response to topography is complex, and it depends on the dimensions and shapes of physical features. Looking toward repair of nerve injuries, strategies are being explored to engineer guidance conduits with precise surface topographies. How neurons and other cell types sense and interpret topography remains to be fully elucidated. Studies reviewed here include those of topography on cellular organization and function as well as potential cellular mechanisms of response.

Biomaterial design strategies for the treatment of spinal cord injuries

The highly debilitating nature of spinal cord injuries has provided much inspiration for the design of novel biomaterials that can stimulate cellular regeneration and functional recovery. Many experts agree that the greatest hope for treatment of spinal cord injuries will involve a combinatorial approach that integrates biomaterial scaffolds, cell transplantation, and molecule delivery. This manuscript presents a comprehensive review of biomaterial-scaffold design strategies currently being applied to the development of nerve guidance channels and hydrogels that more effectively stimulate spinal cord tissue regeneration. To enhance the regenerative capacity of these two scaffold types, researchers are focusing on optimizing the mechanical properties, cell-adhesivity, biodegradability, electrical activity, and topography of synthetic and natural materials, and are developing mechanisms to use these scaffolds to deliver cells and biomolecules. Developing scaffolds that address several of these key design parameters will lead to more successful therapies for the regeneration of spinal cord tissue.

Current tissue engineering and novel therapeutic approaches to axonal regeneration following spinal cord injury using polymer scaffolds

This review highlights current tissue engineering and novel therapeutic approaches to axonal regeneration following spinal cord injury. The concept of developing 3-dimensional polymer scaffolds for placement into a spinal cord transection model has recently been more extensively explored as a solution for restoring neurologic function after injury. Given the patient morbidity associated with respiratory compromise, the discrete tracts in the spinal cord conveying innervation for breathing represent an important and achievable therapeutic target. The aim is to derive new neuronal tissue from the surrounding, healthy cord that will be guided by the polymer implant through the injured area to make functional reconnections. A variety of naturally derived and synthetic biomaterial polymers have been developed for placement in the injured spinal cord. Axonal growth is supported by inherent properties of the selected polymer, the architecture of the scaffold, permissive microstructures such as pores, grooves or polymer fibres, and surface modifications to provide improved adherence and growth directionality. Structural support of axonal regeneration is combined with integrated polymeric and cellular delivery systems for therapeutic drugs and for neurotrophic molecules to regionalize growth of specific nerve populations.

The identification of proteoglycan, collagen and neuron in precursor cells from human fetal spinal cord

After spinal cord injury (SCI), a loss of myelinating oligodendrocytes and neurons occurs. The functional recovery of injured spinal cords is the principal objective of SCI repair. Cell transplantation may prove beneficial to help replace lost myelin and spinal cord circuitry. In this study, we demonstrated that neural precursor cells (hNPCs) from human fetal spinal cord express three types of proteoglycan proteins-chondroitin sulfate, keratan sulfate, and cartilage proteoglycan (an extracellular matrix detected in normal spinal cord), and non-proteoglycan matrix collagen. Both proteoglycan and collagen evidenced profound immunoreactivity in double-stained cell clusters. In addition, whether or not hNPCs were capable of differentiating into a variety of cells, including GABAergic and cholinergic neurons, were assessed. The differentiated cells of eight passages grown on a monolayer expressed the human nuclear protein (HNu), the progenitor marker nestin, GAD, ChAT, TJU, and MAP-2. These results indicate that hNPCs may prove to be candidate cells for therapeutic SCI strategies.

Delayed implantation of intramedullary chitosan channels containing nerve grafts promotes extensive axonal regeneration after spinal cord injury

OBJECTIVE

We describe a new strategy to promote axonal regeneration after subacute or chronic spinal cord injury consisting of intramedullary implantation of chitosan guidance channels containing peripheral nerve (PN) grafts.

METHODS

Chitosan channels filled with PN grafts harvested from green fluorescent protein rats were implanted in the cavity 1 week (subacute) or 4 weeks (chronic) after 50-g clip injury at T8 and were compared with similarly injured animals implanted with either unfilled channels or no channels. Functional recovery was measured weekly for 12 weeks by open-field locomotion, after which histological examination was performed.

RESULTS

The implanted channels with PN grafts contained a thick tissue bridge containing as many as 35,000 myelinated axons in both the subacute and chronic spinal cord injury groups, with the greatest number of axons in the channels containing PN grafts implanted subacutely. There were numerous green fluorescent protein-positive donor Schwann cells in the tissue bridges in all animals with PN grafts. Moreover, these Schwann cells had high functional capacity in terms of myelination of the axons in the channels. In addition, PN-filled chitosan channels showed excellent biocompatibility with the adjacent neural tissue and no obvious signs of degradation and minimal tissue reaction at 14 weeks after implantation. In control animals that had unfilled chitosan channels implanted, there was minimal axonal regeneration in the channels; in control animals without channels, there were large cavities in the spinal cords, and the bridges contained only a small number of axons and Schwann cells. Despite the large numbers of axons in the chitosan channel-PN graft group, there was no significant difference in functional recovery between treatment and control groups.

CONCLUSION

Intramedullary implantation of chitosan guidance channels containing PN grafts in the cavity after subacute spinal cord injury resulted in a thicker bridge containing a larger number of myelinated axons compared with chitosan channels alone. A chitosan channel containing PN grafts is a promising strategy for spinal cord repair.

Plasmid releasing multiple channel bridges for transgene expression after spinal cord injury

The regeneration of tissues with complex architectures requires strategies that promote the appropriate cellular processes, and can direct their organization. Plasmid-loaded multiple channel bridges were engineered for spinal cord regeneration with the ability to support and direct cellular processes and promote gene transfer at the injury site. The bridges were manufactured with a gas foaming technique, and had multiple channels with controllable diameter and encapsulated plasmid. Initial studies investigating bridge implantation subcutaneously (SC) indicated transgene expression in vivo for 44 days, with gene expression dependent upon the pore size of the bridge. In the rat spinal cord, bridges implanted into a lateral hemisection supported substantial cell infiltration, aligned cells within the channels, axon growth across the channels, and high levels of transgene expression at the implant site with decreasing levels rostral and caudal. Immunohistochemistry revealed that the transfected cells at the implant site were present in both the pores and channels of the bridge and were mainly identified as Schwann cells, fibroblasts, and macrophages, in descending order of transfection. This synergy between gene delivery and the scaffold architecture may enable the engineering of tissues with complex architectures.

Repair of thoracic spinal cord injury by chitosan tube implantation in adult rats

Spinal cord injury (SCI) is a common outcome of traffic accidents and trauma with severe consequences. There has been no cure for such a condition. We performed experiments to evaluate the feasibility of implanting a chitosan tube filled with semifluid type I collagen into the site of surgically induced SCI to facilitate functional recovery. After a segment of the spinal cord, 4mm in length and 2/3 of the spinal cord across its width, at the ninth thoracic level of an adult rat was dissected and removed, the biodegradable chitosan tube was implanted into the lesioned site. One year later, we found that axons from the proximal spinal cord regenerated, traversed the dissected area inside the tube and reentered the distal spinal cord, leading to functional restoration of the essentially paralyzed hind limbs. The nerve regeneration and functional recovery were confirmed by immunohistochemistry, electron microscopy, nerve tracing and Basso-Beattie-Bresnahan behavioral evaluation. Such beneficial outcomes were not observed in the control groups, in which either no tube was implanted or the implanted tube had no collagen filling. We conclude that the newly designed tube implant promotes both axon regeneration and functional recovery following SCI. A similar approach may have clinical implications in humans.

Macro-architectures in spinal cord scaffold implants influence regeneration

Biomaterial scaffold architecture has not been investigated as a tunable source of influence on spinal cord regeneration. This study compared regeneration in a transected spinal cord within various designed-macro-architecture scaffolds to determine if these architectures alone could enhance regeneration. Three-dimensional (3-D) designs were created and molds were built on a 3-D printer. Salt-leached porous poly(epsilon-caprolactone) was cast in five different macro-architectures: cylinder, tube, channel, open-path with core, and open-path without core. The two open-path designs were created in this experiment to compare different supportive aspects of architecture provided by scaffolds and their influence on regeneration. Rats received T8 transections and implanted scaffolds for 1 and 3 months. Overall morphology and orientation of sections were characterized by H&E, luxol fast blue, and cresyl violet staining. Borders between intact gray matter and non-regenerated defect were observed from GFAP immunolabeling. Nerve fibers and regenerating axons were identified with Tuj-1 immunolabeling. The open-path designs allowed extension of myelinated fibers along the length of the defect both exterior to and inside the scaffolds and maintained their original defect length up to 3 months. In contrast, the cylinder, tube, and channel implants had a doubling of defect length from secondary damage and large scar and cyst formation with no neural tissue bridging. The open-path scaffold architectures enhanced spinal cord regeneration compared to the three other designs without the use of biological factors.

Templated agarose scaffolds support linear axonal regeneration

While several strategies can stimulate axonal regeneration within a site of spinal cord injury, the growth of axons is generally disorganized and random. Biocompatible scaffolds that guide and maintain the native organization of axons regenerating through an injury site could be of importance in enhancing recovery of the nervous system after injury. Here we report a novel fabrication process for templated agarose nerve guidance scaffolds composed of uniaxial channels of precise diameter and wall thickness extending through their full length. When tested in an in vivo model of spinal cord injury, scaffolds exhibit excellent integration with host tissue and support linear axonal growth through their channels. Further, when loaded with bone marrow stromal cells genetically engineered to secrete brain-derived neurotrophic factor (BDNF), the number of linear penetrating axons is significantly enhanced. The templating process can be useful in fabricating nerve guidance scaffolds for both central and peripheral nerve injuries, or any materials application requiring a precise array of linearly oriented channels.