Axonal growth and connectivity from neural stem cell grafts in models of spinal cord injury

Promotion of nerve regeneration and motor function recovery in SCI rats using LOCAS-iPSCs-NSCs

BACKGROUND

Spinal cord injury (SCI) is a severe traumatic spinal condition with a poor prognosis. In this study, a scaffold called linearly ordered collagen aggregates (LOCAS) was created and loaded with induced pluripotent stem cells (iPSCs)-derived neural stem cells (NSCs) from human umbilical cord blood derived mesenchymal stem cells (hUCB-MSCs) to treat SCI in a rat model.

METHODS

The rats underwent a complete transection SCI resulting in a 3-mm break at either the T9 or T10 level of the spinal cord.

RESULTS

Scanning electron microscope analysis revealed a uniform pore structure on the coronal plane of the scaffold. The LOCAS had a porosity of 88.52% and a water absorption of 1161.67%. Its compressive modulus and stress were measured at 4.1 MPa and 205 kPa, respectively, with a degradation time of 10 weeks. After 12 weeks, rats in the LOCAS-iPSCs-NSCs group exhibited significantly higher BBB scores (8.6) compared to the LOCAS-iPSCs-NSCs group (5.6) and the Model group (4.2). The CatWalk analysis showed improved motion trajectory, regularity index (RI), and swing speed in the LOCAS-iPSCs-NSCs group compared to the other groups. Motor evoked potentials latency was lower and amplitude was higher in the LOCAS-iPSCs-NSCs group, indicating better neural function recovery. Histological analysis demonstrated enhanced neuronal differentiation of NSCs and nerve fiber regeneration promoted by LOCAS-iPSCs-NSCs, leading to improved motor function recovery in rats. The LOCAS scaffold facilitated ordered neurofilament extension and guided nerve regeneration.

CONCLUSIONS

The combination of LOCAS and iPSCs-NSCs demonstrated a positive therapeutic impact on motor function recovery and tissue repair in rats with SCI. This development offers a more resilient bionic microenvironment and presents novel possibilities for clinical SCI repair.

Regenerative rehabilitation: a novel multidisciplinary field to maximize patient outcomes

Regenerative rehabilitation is a novel and rapidly developing multidisciplinary field that converges regenerative medicine and rehabilitation science, aiming to maximize the functions of disabled patients and their independence. While regenerative medicine provides state-of-the-art technologies that shed light on difficult-to-treated diseases, regenerative rehabilitation offers rehabilitation interventions to improve the positive effects of regenerative medicine. However, regenerative scientists and rehabilitation professionals focus on their aspects without enough exposure to advances in each other's field. This disconnect has impeded the development of this field. Therefore, this review first introduces cutting-edge technologies such as stem cell technology, tissue engineering, biomaterial science, gene editing, and computer sciences that promote the progress pace of regenerative medicine, followed by a summary of preclinical studies and examples of clinical investigations that integrate rehabilitative methodologies into regenerative medicine. Then, challenges in this field are discussed, and possible solutions are provided for future directions. We aim to provide a platform for regenerative and rehabilitative professionals and clinicians in other areas to better understand the progress of regenerative rehabilitation, thus contributing to the clinical translation and management of innovative and reliable therapies.

Mesenchymal stem cells overexpressing XIST induce macrophage M2 polarization and improve neural stem cell homeostatic microenvironment, alleviating spinal cord injury

Spinal cord injury (SCI) is a significant cause of disability worldwide, with limited treatment options. This study investigated the potential of bone marrow-derived mesenchymal stem cells (BMSCs) modified with XIST lentiviral vector to modulate macrophage polarization and affect neural stem cell (NSC) microenvironment reconstruction following SCI. Bioinformatics analysis revealed that MID1 might be crucial for BMSCs' treatment of SCI. XIST overexpression enriched Zmynd8 to the promoter region of MID1 and inhibited MID1 transcription, which promoted macrophage M2 polarization. In vitro experiments showed that BMSCs-XIST promoted NSC proliferation, migration, differentiation, and axonal growth by inducing macrophage M2 polarization, suppressing inflammation, and accelerating the re-establishment of the homeostatic microenvironment of NSCs. In vivo, animal experiments confirmed that BMSCs-XIST significantly alleviated SCI by promoting NSC differentiation and axon formation in the injured area. The study demonstrated the potential of XIST-overexpressing BMSCs for treating SCI by regulating macrophage polarization and homeostasis of the NSC microenvironment. These findings provide new insights into the development of stem cell-based therapies for SCI.

Functional Hydrogel Co-Remolding Migration and Differentiation Microenvironment for Severe Spinal Cord Injury Repair

Spinal cord injury (SCI) activates nestin+ neural stem cells (NSCs), which can be regarded as potential seed cells for neuronal regeneration. However, the lesion microenvironment seriously hinders the migration of the nestin+ cells to the lesion epicenter and their differentiation into neurons to rebuild neural circuits. In this study, a photosensitive hydrogel scaffold is prepared as drug delivery carrier. Genetically engineered SDF1α and NT3 are designed and the scaffold is binary modified to reshape the lesion microenvironment. The binary modified scaffold can effectively induce the migration and neuronal differentiation of nestin+ NSCs in vitro. When implanted into a rat complete SCI model, many of the SCI-activated nestin+ cells migrate into the lesion site and give rise to neurons in short-term. Meanwhile, long-term repair results also show that implantation of the binary modified scaffold can effectively promote the maturation, functionalization and synaptic network reconstruction of neurons in the lesion site. In addition, animals treated with binary scaffold also showed better improvement in motor functions. The therapeutic strategy based on remolding the migration and neuronal differentiation lesion microenvironment provides a new insight into SCI repair by targeting activated nestin+ cells, which exhibits excellent clinical transformation prospects.

Mimicked Spinal Cord Fibers Trigger Axonal Regeneration and Remyelination after Injury

Spinal cord injury (SCI) causes tissue structure damage and composition changes of the neural parenchyma, resulting in severe consequences for spinal cord function. Mimicking the components and microstructure of spinal cord tissues holds promise for restoring the regenerative microenvironment after SCI. Here, we have utilized electrospinning technology to develop aligned decellularized spinal cord fibers (A-DSCF) without requiring synthetic polymers or organic solvents. A-DSCF preserves multiple types of spinal cord extracellular matrix proteins and forms a parallel-oriented structure. Compared to aligned collagen fibers (A-CF), A-DSCF exhibits stronger mechanical properties, improved enzymatic stability, and superior functionality in the adhesion, proliferation, axonal extension, and myelination of differentiated neural progenitor cells (NPCs). Notably, axon extension or myelination has been primarily linked to Agrin (AGRN), Laminin (LN), or Collagen type IV (COL IV) proteins in A-DSCF. When transplanted into rats with complete SCI, A-DSCF loaded with NPCs improves the survival, maturation, axon regeneration, and motor function of the SCI rats. These findings highlight the potential of structurally and compositionally biomimetic scaffolds to promote axonal extension and remyelination after SCI.

3D collagen porous scaffold carrying PLGA-PTX/SDF-1α recruits and promotes neural stem cell differentiation for spinal cord injury repair

Spinal Cord Injury (SCI), one of the major factors of disability, can cause irreversible motor and sensory impairment. There are no effective therapeutic drugs and technologies available in domestic or foreign countries currently. Neural stem cells (NSCs), with the potential for multidirectional differentiation, are a potential treatment for SCI. However, it has been demonstrated that NSCs primarily differentiated into astrocytes rather than neurons due to the inflammatory microenvironment, and the current challenge remains to direct the differentiation of NSCs into neurons in the lesion site. It was reported that the microtubule-stabilizing agent paclitaxel (PTX) was able to promote the differentiation of NSCs into neurons rather than astrocytes after SCI. SDF-1α can recruit NSCs and thus guide the migration of stem cells. In this study, we developed a functional collagen scaffold by loading SDF-1α and nanoparticle-encapsulated PLGA-PTX into a 3D collagen porous scaffold, allowing for slow release of PTX. When the functional scaffolds were implanted into the injury site, it provided a neural regeneration conduit channel for the migration of NSCs and neuronal differentiation. Neural regeneration promoted the recovery of motor function and reduced glial scar formation after SCI. In conclusion, a 3D collagen porous scaffold combined with PLGA-PTX and SDF-1α is a promising therapeutic strategy for SCI repair.

Biomedical applications of supramolecular hydrogels with enhanced mechanical properties

Supramolecular hydrogels bound by hydrogen bonding, host-guest, hydrophobic, and other non-covalent interactions are among the most attractive biomaterials available. Supramolecular hydrogels have attracted extensive attention due to their inherent dynamic reversibility, self-healing, stimuli-response, excellent biocompatibility, and near-physiological environment. However, the inherent contradiction between non-covalent interactions and mechanical strength makes the practical application of supramolecular hydrogels a great challenge. This review describes the mechanical strength of hydrogels mediated by supramolecular interactions, and focuses on the potential strategies for enhancing the mechanical strength of supramolecular hydrogels and illustrates their applications in related fields, such as flexible electronic sensors, wound dressings, and three-dimensional (3D) scaffolds. Finally, the current problems and future research prospects of supramolecular hydrogels are discussed. This review is expected to provide insights that will motivate more advanced research on supramolecular hydrogels.

Engineered human spinal cord-like tissues with dorsal and ventral neuronal progenitors for spinal cord injury repair in rats and monkeys

Transplanting human neural progenitor cells is a promising method of replenishing the lost neurons after spinal cord injury (SCI), but differentiating neural progenitor cells into the diverse types of mature functional spinal cord neurons in vivo is challenging. In this study, engineered human embryonic spinal cord-like tissues with dorsal and ventral neuronal characters (DV-SC) were generated by inducing human neural progenitor cells (hscNPCs) to differentiate into various types of dorsal and ventral neuronal cells on collagen scaffold in vitro. Transplantation of DV-SC into complete SCI models in rats and monkeys showed better therapeutic effects than undifferentiated hscNPCs, including pronounced cell survival and maturation. DV-SC formed a targeted connection with the host's ascending and descending axons, partially restored interrupted neural circuits, and improved motor evoked potentials and the hindlimb function of animals with SCI. This suggests that the transplantation of pre-differentiated hscNPCs with spinal cord dorsal and ventral neuronal characteristics could be a promising strategy for SCI repair.

Moving CNS axon growth and regeneration research into human model systems

Axon regeneration is limited in the adult mammalian central nervous system (CNS) due to both intrinsic and extrinsic factors. Rodent studies have shown that developmental age can drive differences in intrinsic axon growth ability, such that embryonic rodent CNS neurons extend long axons while postnatal and adult CNS neurons do not. In recent decades, scientists have identified several intrinsic developmental regulators in rodents that modulate growth. However, whether this developmentally programmed decline in CNS axon growth is conserved in humans is not yet known. Until recently, there have been limited human neuronal model systems, and even fewer age-specific human models. Human in vitro models range from pluripotent stem cell-derived neurons to directly reprogrammed (transdifferentiated) neurons derived from human somatic cells. In this review, we discuss the advantages and disadvantages of each system, and how studying axon growth in human neurons can provide species-specific knowledge in the field of CNS axon regeneration with the goal of bridging basic science studies to clinical trials. Additionally, with the increased availability and quality of 'omics datasets of human cortical tissue across development and lifespan, scientists can mine these datasets for developmentally regulated pathways and genes. As there has been little research performed in human neurons to study modulators of axon growth, here we provide a summary of approaches to begin to shift the field of CNS axon growth and regeneration into human model systems to uncover novel drivers of axon growth.

Strategies and mechanisms of neuronal reprogramming

Traumatic injury and neurodegenerative diseases of the central nervous system (CNS) are difficult to treat due to the poorly regenerative nature of neurons. Engrafting neural stem cells into the CNS is a classic approach for neuroregeneration. Despite great advances, stem cell therapy still faces the challenges of overcoming immunorejection and achieving functional integration. Neuronal reprogramming, a recent innovation, converts endogenous non-neuronal cells (e.g., glial cells) into mature neurons in the adult mammalian CNS. In this review, we summarize the progress of neuronal reprogramming research, mainly focusing on strategies and mechanisms of reprogramming. Furthermore, we highlight the advantages of neuronal reprogramming and outline related challenges. Although the significant development has been made in this field, several findings are controversial. Even so, neuronal reprogramming, especially in vivo reprogramming, is expected to become an effective treatment for CNS neurodegenerative diseases.

Current Concepts of Biomaterial Scaffolds and Regenerative Therapy for Spinal Cord Injury

Spinal cord injury (SCI) is a catastrophic condition associated with significant neurological deficit and social and financial burdens. It is currently being managed symptomatically, with no real therapeutic strategies available. In recent years, a number of innovative regenerative strategies have emerged and have been continuously investigated in preclinical research and clinical trials. In the near future, several more are expected to come down the translational pipeline. Among ongoing and completed trials are those reporting the use of biomaterial scaffolds. The advancements in biomaterial technology, combined with stem cell therapy or other regenerative therapy, can now accelerate the progress of promising novel therapeutic strategies from bench to bedside. Various types of approaches to regeneration therapy for SCI have been combined with the use of supportive biomaterial scaffolds as a drug and cell delivery system to facilitate favorable cell-material interactions and the supportive effect of neuroprotection. In this review, we summarize some of the most recent insights of preclinical and clinical studies using biomaterial scaffolds in regenerative therapy for SCI and summarized the biomaterial strategies for treatment with simplified results data. One hundred and sixty-eight articles were selected in the present review, in which we focused on biomaterial scaffolds. We conducted our search of articles using PubMed and Medline, a medical database. We used a combination of "Spinal cord injury" and ["Biomaterial", or "Scaffold"] as search terms and searched articles published up until 30 April 2022. Successful future therapies will require these biomaterial scaffolds and other synergistic approaches to address the persistent barriers to regeneration, including glial scarring, the loss of a structural framework, and biocompatibility. This database could serve as a benchmark to progress in future clinical trials for SCI using biomaterial scaffolds.

Research on the function of the Cend1 regulatory mechanism on p75NTR signaling in spinal cord injury

How to use NSC repair mechanisms, minimize the loss of neurons, and recover the damaged spinal cord functions are hotspots and difficulties in spinal cord injury research. Studies have shown that Cend1 signaling is involved in regulating the NSC differentiation, that p75NTR signaling is involved in the regulation of mature neuronal apoptosis and that NSC differentiation decreases mature neuron apoptosis. Our research group found an interaction between Cend1 and p75NTR, and there was a correlation with spinal cord injury. Therefore, we speculate that Cend1 regulates p75NTR signals and promotes the differentiation of NSCs, and inhibits neuronal apoptosis. Therefore, this study first analyzed the expression of p75NTR and Cend1 in spinal cord injury and its relationship with NSCs and neurons and then analyzed the regulatory mechanism and the mechanism of survival on neuronal apoptosis and differentiation of NSCs. Finally, we analyzed the effect of p75NTR and the regulation of Cend1 damage on functional recovery of the spinal cord with overall intervention. The completion of the subject will minimize the loss of neurons, innovative use of NSC repair mechanisms, and open up a new perspective for the treatment of spinal cord injury.

Effects of biological sex mismatch on neural progenitor cell transplantation for spinal cord injury in mice

Despite advancement of neural progenitor cell transplantation to spinal cord injury clinical trials, there remains a lack of understanding of how biological sex of transplanted cells influences outcomes after transplantation. To address this, we transplanted GFP-expressing sex-matched, sex-mismatched, or mixed donor cells into sites of spinal cord injury in adult male and female mice. Biological sex of the donor cells does not influence graft neuron density, glial differentiation, formation of the reactive glial cell border, or graft axon outgrowth. However, male grafts in female hosts feature extensive hypervascularization accompanied by increased vascular diameter and perivascular cell density. We show greater T-cell infiltration within male-to-female grafts than other graft types. Together, these findings indicate a biological sex-specific immune response of female mice to male donor cells. Our work suggests that biological sex should be considered in the design of future clinical trials for cell transplantation in human injury.

In this study, Pitonak et al. report that transplantation of neural progenitor cells derived from male donors trigger an immune rejection response following transplantation into sites of spinal cord injury in female mice.

Release of O-GlcNAc transferase inhibitor promotes neuronal differentiation of neural stem cells in 3D bioprinted supramolecular hydrogel scaffold for spinal cord injury repair

Precise fabrication of biomimetic three-dimensional (3D) structure and effective neuronal differentiation under the pathological environment are the key to neural stem cell (NSC)-based spinal cord injury (SCI) therapy. In this study, we have developed a spinal cord-like bioprinted scaffold loading with OSMI-4, a small molecule O-GlcNAc transferase (OGT) inhibitor, to induce and guide the neuron differentiation of NSCs for efficient SCI repair. To achieve this, we developed a supramolecular bioink (SM bioink) consisting of methacrylated gelatin and acrylated β-cyclodextrins to load NSCs and OSMI-4. This bioink showed fast gelation and stable mechanical properties, facilitating bioprinting of functional neural scaffolds. Moreover, the weak host-guest cross-linking of the SM scaffolds significantly improved the cell-matrix interaction for the infiltration and migration of NSCs. What's more, the sustained delivery of OSMI-4 remarkably enhanced the intrinsic neuronal differentiation of the encapsulated NSCs in vitro by inhibiting Notch signaling pathway. In vivo experiment further revealed that the functional bioprinted scaffolds promoted the neuronal regeneration and axonal growth, leading to significant locomotor recovery of the SCI model rats. Together, the NSC-laden bioprinted SM scaffolds in combination with sustained release of the therapeutic agent OSMI-4 largely induced neuronal differentiation of NSCs and thus leading to efficient SCI repair. STATEMENT OF SIGNIFICANCE: Efficient neuronal differentiation of neural stem cells (NSCs) under the complex pathological microenvironment of spinal cord injury (SCI) is a major challenge of neural regeneration. By the use of a supramolecular bioink, we bioprinted a spinal cord-like scaffold loaded with NSCs and a small molecule drug OSMI-4 to significantly induce neuronal differentiation of NSCs for efficient SCI repair in vivo. The scaffolds with spinal cord-like structure can support the interaction and neuronal differentiation of NSCs by providing a dynamic matrix and a source of molecular release of OSMI-4. The influences of OSMI-4 on NSCs and its molecular mechanism were investigated for the first time in this study. Altogether, three-dimensional bioprinting fabrication of NSC- and small molecule drug-laden biomimetic construct may represent a promising therapeutic strategy for SCI repair.

Regenerative Rehabilitation and Stem Cell Therapy Targeting Chronic Spinal Cord Injury: A Review of Preclinical Studies

Stem cell medicine has led to functional recovery in the acute-to-subacute phase of spinal cord injury (SCI), but not yet in the chronic phase, during which various molecular mechanisms drastically remodel the tissue and render it treatment-resistant. Researchers are attempting to identify effective combinatorial treatments that can overcome the refractory state of the chronically injured spinal cord. Regenerative rehabilitation, combinatorial treatment with regenerative medicine that aims to elicit synergistic effects, is being developed. Rehabilitation upon SCI in preclinical studies has recently attracted more attention because it is safe, induces neuronal plasticity involving transplanted stem cells and sensorimotor circuits, and is routinely implemented in human clinics. However, regenerative rehabilitation has not been extensively reviewed, and only a few reviews have focused on the use of physical medicine modalities for rehabilitative purposes, which might be more important in the chronic phase. Here, we summarize regenerative rehabilitation studies according to the effector, site, and mechanism. Specifically, we describe effects on transplanted cells, microstructures at and distant from the lesion, and molecular changes. To establish a treatment regimen that induces robust functional recovery upon chronic SCI, further investigations are required of combinatorial treatments incorporating stem cell therapy, regenerative rehabilitation, and medication.

Long-Term Cultures of Spinal Cord Interneurons

Spinal cord interneurons (SpINs) are highly diverse population of neurons that play a significant role in circuit reorganization and spontaneous recovery after spinal cord injury. Regeneration of SpIN axons across rodent spinal injuries has been demonstrated after modification of the environment and neurotrophin treatment, but development of methods to enhance the intrinsic regenerative ability of SpINs is needed. There is a lack of described in vitro models of spinal cord neurons in which to develop new regeneration treatments. For this reason, we developed a new model of mouse primary spinal cord neuronal culture in which to analyze maturation, morphology, physiology, connectivity and regeneration of identified interneurons. Isolated from E14 mice, the neurons mature over 15 days in vitro, demonstrated by expression of maturity markers, electrophysiological patch-clamp recordings, and formation of synapses. The neurons express markers of SpINs, including Tlx3, Lmx1b, Lbx1, Chx10, and Pax2. The neurons demonstrate distinct morphologies and some form perineuronal nets in long-term cultivation. Live neurons in various maturation stages were axotomized, using a 900 nm multiphoton laser and their fate was observed overnight. The percentage of axons that regenerated declined with neuronal maturity. This model of SpINs will be a valuable tool in future regenerative, developmental, and functional studies alongside existing models using cortical or hippocampal neurons.

The Role of Biomaterials in Peripheral Nerve and Spinal Cord Injury: A Review

Peripheral nerve and spinal cord injuries are potentially devastating traumatic conditions with major consequences for patients’ lives. Severe cases of these conditions are currently incurable. In both the peripheral nerves and the spinal cord, disruption and degeneration of axons is the main cause of neurological deficits. Biomaterials offer experimental solutions to improve these conditions. They can be engineered as scaffolds that mimic the nerve tissue extracellular matrix and, upon implantation, encourage axonal regeneration. Furthermore, biomaterial scaffolds can be designed to deliver therapeutic agents to the lesion site. This article presents the principles and recent advances in the use of biomaterials for axonal regeneration and nervous system repair.

Transplantation of collagen sponge-based three-dimensional neural stem cells cultured in a RCCS facilitates locomotor functional recovery in spinal cord injury animals

Numerous studies have indicated that microgravity induces various changes in the cellular functions of neural stem cells (NSCs), and the use of microgravity to culture tissue engineered seed cells for the treatment of nervous system diseases has drawn increasing attention. The goal of this study was to verify the efficacy of collagen sponge-based 3-dimensional (3D) NSCs cultured in a rotary cell culture system (RCCS) in treating spinal cord injury (SCI). The Basso-Beattie-Bresnahan score, inclined plane test, and electrophysiology results all indicated that 3D cultured NSCs cultured in a RCCS had better therapeutic effects than those cultured in a traditional cell culture environment, suggesting that the microgravity provided by the RCCS could enhance the therapeutic effect of 3D cultured NSCs. Our study indicates the feasibility of combining the RCCS with collagen sponge-based 3D cell culture for producing tissue engineered seed cells for the treatment of SCI. This novel and effective method shows promise for application in cell-based therapy for SCI in the future.

Reducing host aldose reductase activity promotes neuronal differentiation of transplanted neural stem cells at spinal cord injury sites and facilitates locomotion recovery

Neural stem cell (NSC) transplantation is a promising strategy for replacing lost neurons following spinal cord injury. However, the survival and differentiation of transplanted NSCs is limited, possibly owing to the neurotoxic inflammatory microenvironment. Because of the important role of glucose metabolism in M1/M2 polarization of microglia/macrophages, we hypothesized that altering the phenotype of microglia/macrophages by regulating the activity of aldose reductase (AR), a key enzyme in the polyol pathway of glucose metabolism, would provide a more beneficial microenvironment for NSC survival and differentiation. Here, we reveal that inhibition of host AR promoted the polarization of microglia/macrophages toward the M2 phenotype in lesioned spinal cord injuries. M2 macrophages promoted the differentiation of NSCs into neurons in vitro. Transplantation of NSCs into injured spinal cords either deficient in AR or treated with the AR inhibitor sorbinil promoted the survival and neuronal differentiation of NSCs at the injured spinal cord site and contributed to locomotor functional recovery. Our findings suggest that inhibition of host AR activity is beneficial in enhancing the survival and neuronal differentiation of transplanted NSCs and shows potential as a treatment of spinal cord injury.

Neural Stem Cells: Promoting Axonal Regeneration and Spinal Cord Connectivity

Spinal cord injury (SCI) leads to irreversible functional impairment caused by neuronal loss and the disruption of neuronal connections across the injury site. While several experimental strategies have been used to minimize tissue damage and to enhance axonal growth and regeneration, the corticospinal projection, which is the most important voluntary motor system in humans, remains largely refractory to regenerative therapeutic interventions. To date, one of the most promising pre-clinical therapeutic strategies has been neural stem cell (NSC) therapy for SCI. Over the last decade we have found that host axons regenerate into spinal NSC grafts placed into sites of SCI. These regenerating axons form synapses with the graft, and the graft in turn extends very large numbers of new axons from the injury site over long distances into the distal spinal cord. Here we discuss the pathophysiology of SCI that makes the spinal cord refractory to spontaneous regeneration, the most recent findings of neural stem cell therapy for SCI, how it has impacted motor systems including the corticospinal tract and the implications for sensory feedback.

Stem cell-derived neuronal relay strategies and functional electrical stimulation for treatment of spinal cord injury

The inability of adult mammals to recover function lost after severe spinal cord injury (SCI) has been known for millennia and is mainly attributed to a failure of brain-derived nerve fiber regeneration across the lesion. Potential approaches to re-establishing locomotor function rely on neuronal relays to reconnect the segregated neural networks of the spinal cord. Intense research over the past 30 years has focused on endogenous and exogenous neuronal relays, but progress has been slow and the results often controversial. Treatments with stem cell-derived neuronal relays alone or together with functional electrical stimulation offer the possibility of improved repair of neuronal networks. In this review, we focus on approaches to recovery of motor function in paralyzed patients after severe SCI based on novel therapies such as implantation of stem cell-derived neuronal relays and functional electrical stimulation. Recent research progress offers hope that SCI patients will one day be able to recover motor function and sensory perception.

Nerve growth factor (NGF) with hypoxia response elements loaded by adeno-associated virus (AAV) combined with neural stem cells improve the spinal cord injury recovery

The ischemia and hypoxia microenvironment after spinal cord injury (SCI) makes SCI repair a challenging problem. With various stimulus, chances for neural stem cells (NSCs) to differentiate into neurons, astrocytes, oligodendrocytes are great and is considered as a potential source of the stem cell therapy to SCI. Our research used adeno-associated virus (AAV) to carry the target gene to transfect neural stem cells. Transfected NSCs can express nerve growth factor (NGF) navigated by five hypoxia-responsive elements (5HRE). Therefore, the 5HRE-NGF-NSCs could express NGF specifically in hypoxia sites to promote the tissue repair and function recovery. Based on the regeneration of neurocytes and promotion of the recovery found in SCI models, via locomotor assessment, histochemical staining and molecular examinations, our results demonstrated that 5HRE-NGF-NSCs could improve the motor function, neurons survival and molecules expression of SCI rats. Meanwhile, the downregulated expression of autophagy-related proteins indicated the inhibitive effect of 5HRE-NGF-NSCs on autophagy. Our research showed that 5HRE-NGF-NSCs contribute to SCI repair which might via inhibiting autophagy and improving the survival rate of neuronal cells. The new therapy also hampered the hyperplasia of neural glial scars and induced axon regeneration. These positive functions of 5HRE-NGF-NSCs all indicate a promising SCI treatment.

The Restorative Effect of Human Amniotic Fluid Stem Cells on Spinal Cord Injury

Spinal cord injury (SCI) is a debilitating condition within the neural system which is clinically manifested by sensory-motor dysfunction, leading, in some cases, to neural paralysis for the rest of the patient’s life. In the current study, mesenchymal stem cells (MSCs) were isolated from the human amniotic fluid, in order to study their juxtacrine and paracrine activities. Flow cytometry analysis was performed to identify the MSCs. A conditioned medium (CM) was collected to measure the level of BDNF, IL-1β, and IL-6 proteins using the ELISA assay. Following the SCI induction, MSCs and CM were injected into the lesion site, and also CM was infused intraperitoneally in the different groups. Two weeks after SCI induction, the spinal cord samples were examined to evaluate the expression of the doublecortin (DCX) and glial fibrillary acid protein (GFAP) markers using immunofluorescence staining. The MSCs’ phenotype was confirmed upon the expression and un-expression of the related CD markers. Our results show that MSCs increased the expression level of the DCX and decreased the level of the GFAP relative to the injury group (p < 0.001). Additionally, the CM promoted the DCX expression rate (p < 0.001) and decreased the GFAP expression rate (p < 0.01) as compared with the injury group. Noteworthily, the restorative potential of the MSCs was higher than that of the CM (p < 0.01). Large-scale meta-analysis of transcriptomic data highlighted PAK5, ST8SIA3, and NRXN1 as positively coexpressed genes with DCX. These genes are involved in neuroactive ligand–receptor interaction. Overall, our data revealed that both therapeutic interventions could promote the regeneration and restoration of the damaged neural tissue by increasing the rate of neuroblasts and decreasing the astrocytes.

Corticospinal Motor Circuit Plasticity After Spinal Cord Injury: Harnessing Neuroplasticity to Improve Functional Outcomes

Spinal cord injury (SCI) is a devastating condition that affects approximately 294,000 people in the USA and several millions worldwide. The corticospinal motor circuitry plays a major role in controlling skilled movements and in planning and coordinating movements in mammals and can be damaged by SCI. While axonal regeneration of injured fibers over long distances is scarce in the adult CNS, substantial spontaneous neural reorganization and plasticity in the spared corticospinal motor circuitry has been shown in experimental SCI models, associated with functional recovery. Beneficially harnessing this neuroplasticity of the corticospinal motor circuitry represents a highly promising therapeutic approach for improving locomotor outcomes after SCI. Several different strategies have been used to date for this purpose including neuromodulation (spinal cord/brain stimulation strategies and brain-machine interfaces), rehabilitative training (targeting activity-dependent plasticity), stem cells and biological scaffolds, neuroregenerative/neuroprotective pharmacotherapies, and light-based therapies like photodynamic therapy (PDT) and photobiomodulation (PMBT). This review provides an overview of the spontaneous reorganization and neuroplasticity in the corticospinal motor circuitry after SCI and summarizes the various therapeutic approaches used to beneficially harness this neuroplasticity for functional recovery after SCI in preclinical animal model and clinical human patients' studies.

Elucidating the Pivotal Neuroimmunomodulation of Stem Cells in Spinal Cord Injury Repair

Spinal cord injury (SCI) is a distressing incident with abrupt onset of the motor as well as sensory dysfunction, and most often, the injury occurs as result of high-energy or velocity accidents as well as contact sports and falls in the elderly. The key challenges associated with nerve repair are the lack of self-repair as well as neurotrophic factors and primary and secondary neuronal apoptosis, as well as factors that prevent the regeneration of axons locally. Neurons that survive the initial traumatic damage may be lost due to pathogenic activities like neuroinflammation and apoptosis. Implanted stem cells are capable of differentiating into neural cells that replace injured cells as well as offer local neurotrophic factors that aid neuroprotection, immunomodulation, axonal sprouting, axonal regeneration, and remyelination. At the microenvironment of SCI, stem cells are capable of producing growth factors like brain-derived neurotrophic factor and nerve growth factor which triggers neuronal survival as well as axonal regrowth. Although stem cells have proven to be of therapeutic value in SCI, the major disadvantage of some of the cell types is the risk for tumorigenicity due to the contamination of undifferentiated cells prior to transplantation. Local administration of stem cells via either direct cellular injection into the spinal cord parenchyma or intrathecal administration into the subarachnoid space is currently the best transplantation modality for stem cells during SCI.

Current Concepts of Stem Cell Therapy for Chronic Spinal Cord Injury

Chronic spinal cord injury (SCI) is a catastrophic condition associated with significant neurological deficit and social and financial burdens. It is currently being managed symptomatically with no real therapeutic strategies available. In recent years, a number of innovative regenerative strategies have emerged and have been continuously investigated in clinical trials. In addition, several more are coming down the translational pipeline. Among ongoing and completed trials are those reporting the use of mesenchymal stem cells, neural stem/progenitor cells, induced pluripotent stem cells, olfactory ensheathing cells, and Schwann cells. The advancements in stem cell technology, combined with the powerful neuroimaging modalities, can now accelerate the pathway of promising novel therapeutic strategies from bench to bedside. Various combinations of different molecular therapies have been combined with supportive scaffolds to facilitate favorable cell–material interactions. In this review, we summarized some of the most recent insights into the preclinical and clinical studies using stem cells and other supportive drugs to unlock the microenvironment in chronic SCI to treat patients with this condition. Successful future therapies will require these stem cells and other synergistic approaches to address the persistent barriers to regeneration, including glial scarring, loss of structural framework, and immunorejection.

Upregulation of Apol8 by Epothilone D facilitates the neuronal relay of transplanted NSCs in spinal cord injury

BACKGROUND

Microtubule-stabilizing agents have been demonstrated to modulate axonal sprouting during neuronal disease. One such agent, Epothilone D, has been used to treat spinal cord injury (SCI) by promoting axonal sprouting at the lesion site after SCI. However, the role of Epothilone D in the differentiation of neural stem cells (NSCs) in SCI repair is unknown. In the present study, we mainly explored the effects and mechanisms of Epothilone D on the neuronal differentiation of NSCs and revealed a potential new SCI treatment.

METHODS

In vitro differentiation assays, western blotting, and quantitative real-time polymerase chain reaction were used to detect the effects of Epothilone D on NSC differentiation. Retrograde tracing using a pseudotyped rabies virus was then used to detect neuronal circuit construction. RNA sequencing (RNA-Seq) was valuable for exploring the target gene involved in the neuronal differentiation stimulated by Epothilone D. In addition, lentivirus-induced overexpression and RNA interference technology were applied to demonstrate the function of the target gene. Last, an Apol8-NSC-linear ordered collagen scaffold (LOCS) graft was prepared to treat a mouse model of SCI, and functional and electrophysiological evaluations were performed.

RESULTS

We first revealed that Epothilone D promoted the neuronal differentiation of cultured NSCs and facilitated neuronal relay formation in the injured site after SCI. Furthermore, the RNA-Seq results demonstrated that Apol8 was upregulated during Epothilone D-induced neuronal relay formation. Lentivirus-mediated Apol8 overexpression in NSCs (Apol8-NSCs) promoted NSC differentiation toward neurons, and an Apol8 interference assay showed that Apol8 had a role in promoting neuronal differentiation under the induction of Epothilone D. Last, Apol8-NSC transplantation with LOCS promoted the neuronal differentiation of transplanted NSCs in the lesion site as well as synapse formation, thus improving the motor function of mice with complete spinal cord transection.

CONCLUSIONS

Epothilone D can promote the neuronal differentiation of NSCs by upregulating Apol8, which may provide a promising therapeutic target for SCI repair.

A unique macrophage subpopulation signals directly to progenitor cells to promote regenerative neurogenesis in the zebrafish spinal cord

Central nervous system injury re-initiates neurogenesis in anamniotes (amphibians and fishes), but not in mammals. Activation of the innate immune system promotes regenerative neurogenesis, but it is fundamentally unknown whether this is indirect through the activation of known developmental signaling pathways or whether immune cells directly signal to progenitor cells using mechanisms that are unique to regeneration. Using single-cell RNA-seq of progenitor cells and macrophages, as well as cell-type-specific manipulations, we provide evidence for a direct signaling axis from specific lesion-activated macrophages to spinal progenitor cells to promote regenerative neurogenesis in zebrafish. Mechanistically, TNFa from pro-regenerative macrophages induces Tnfrsf1a-mediated AP-1 activity in progenitors to increase regeneration-promoting expression of hdac1 and neurogenesis. This establishes the principle that macrophages directly communicate to spinal progenitor cells via non-developmental signals after injury, providing potential targets for future interventions in the regeneration-deficient spinal cord of mammals.

Valproic Acid Labeled Chitosan Nanoparticles Promote the Proliferation and Differentiation of Neural Stem Cells After Spinal Cord Injury

Chitosan nanoparticles and valproic acid are demonstrated as the protective agents in the treatment of spinal cord injury (SCI). However, the effects of valproic acid-labeled chitosan nanoparticles (VA-CN) on endogenous spinal cord neural stem cells (NSCs) following SCI and the underlying mechanisms involved remain to be elucidated. In this study, the VA-CN was constructed and the effects of VA-CN on NSCs were assessed in a rat model of SCI. We found VA-CN treatment promoted recovery of the tissue and locomotive function following SCI. Moreover, administration of VA-CN significantly enhanced neural stem cell proliferation and the expression levels of neurotrophic factors following SCI. Furthermore, administration of VA-CN led to a decrease in the number of microglia following SCI. In addition, VA-CN treatment significantly increased the Tuj 1- positive cells in the spinal cord of the SCI rats, suggesting that VA-CN could enhance the differentiation of NSCs following SCI. In conclusion, these results demonstrated that VA-CN could improve the functional and histological recovery through promoting the proliferation and differentiation of NSCs following SCI, which would provide a newly potential therapeutic manner for the treatment of SCI.

Regeneration of Corticospinal Axons into Neural Progenitor Cell Grafts After Spinal Cord Injury

Spinal cord injuries leave patients with lifelong paralysis. To date, there are no therapies that promote the critical step required for the recovery of voluntary motor function: corticospinal axon regeneration. Spinal cord-derived neural progenitor cell (NPC) grafts integrate into the injured host spinal cord, enable robust corticospinal axon regeneration, and restore forelimb function following spinal cord injury in rodents. Consequently, engineered stem cell differentiation and transplantation techniques harbor promising potential for the design and implementation of therapies promoting corticospinal axon regeneration. However, in order to optimize the outcome of clinical trials, it is critical to fully understand the cellular and molecular mechanisms underlying this regeneration. Our recent study highlights the unexpected intrinsic potential of corticospinal neurons to regenerate and allows us to investigate new hypotheses exploiting this newly discovered potential.

Stem-like cells of various origins showed therapeutic effect to improve the recovery of spinal cord injury

We aimed to evaluate the therapeutic effects of exosomes, which were collected from human neuroepithelial stem cells (HNESCs) treated by miR-29b mimics, on the treatment of spinal cord injury (SCI). Computational analysis, real-time PCR, Western blot analysis and TUNEL assay, a BBB score system, the Nissl staining and IHC assay were conducted to explore the molecular signalling pathway underlying the function of exosomes in SCI. Exosomes isolated from cells treated with HNESC exhibited the strongest inhibitory effect on cell apoptosis while exhibiting the highest level of miR-29b expression and the lowest levels of PTEN and caspase-3 expression. Moreover, PTEN and caspase-3 were identified as the direct target genes of miR-29b. The exosomes isolated from the groups of HNESC and HNESC + miR-29b mimics exhibited in vivo therapeutic effects by restoring the BBB score and apoptosis index of post-SCI neuron cells to those of normal neuron cells, with the exosomes collected from the group of HNESC + miR-29b mimics showing the strongest effect. We suggested that the exosomes derived from the group of HNESC + miR-29b mimics exerted therapeutic effects on SCI by down-regulating the expression of PTEN/caspase-3 and subsequently suppressing the apoptosis of neuron cells.

Transsynaptic tracing and its emerging use to assess graft-reconstructed neural circuits

Fetal neural progenitor grafts have been evaluated in preclinical animal models of spinal cord injury and Parkinson's disease for decades, but the initial reliance on primary tissue as a cell source limited the scale of their clinical translatability. With the development of robust methods to differentiate human pluripotent stem cells to specific neural subtypes, cell replacement therapy holds renewed promise to treat a variety of neurodegenerative diseases and injuries at scale. As these cell sources are evaluated in preclinical models, new transsynaptic tracing methods are making it possible to study the connectivity between host and graft neurons with greater speed and detail than was previously possible. To date, these studies have revealed that widespread, long-lasting, and anatomically appropriate synaptic contacts are established between host and graft neurons, as well as new aspects of host-graft connectivity which may be relevant to clinical cell replacement therapy. It is not yet clear, however, whether the synaptic connectivity between graft and host neurons is as cell-type specific as it is in the endogenous nervous system, or whether that connectivity is responsible for the functional efficacy of cell replacement therapy. Here, we review evidence suggesting that the new contacts established between host and graft neurons may indeed be cell-type specific, and how transsynaptic tracing can be used in the future to further elucidate the mechanisms of graft-mediated functional recovery in spinal cord injury and Parkinson's disease.

Combined Method of Neuronal Cell-Inducible Vector and Valproic Acid for Enhanced Gene Expression under Hypoxic Conditions

BACKGROUND

Gene therapy shows the ability to restore neuronal dysfunction via therapeutic gene expression. The efficiency of gene expression and delivery to hypoxic injury sites is important for successful gene therapy. Therefore, we established a gene/stem cell therapy system using neuron-specific enolase promoter and induced neural stem cells in combination with valproic acid to increase therapeutic gene expression in hypoxic spinal cord injury.

METHODS

To examine the effect of combined method on enhancing gene expression, we compared neuronal cell-inducible luciferase levels under normoxia or hypoxia conditions in induced neural stem cells with valproic acid. Therapeutic gene, vascular endothelial growth factor, expression with combined method was investigated in hypoxic spinal cord injury model. We verified gene expression levels and the effect of different methods of valproic acid administration in vivo.

RESULTS

The results showed that neuron-specific enolase promoter enhanced gene expression levels in induced neural stem cells compared to Simian Virus 40 promoter under hypoxic conditions. Valproic acid treatment showed higher gene expression of neuron-specific enolase promoter than without treatment. In addition, gene expression levels and cell viability were different depending on the various concentration of valproic acid. The gene expression levels were increased significantly when valproic acid was directly injected with induced neural stem cells in vivo.

CONCLUSION

In this study, we demonstrated that the combination of neuron-specific enolase promoter and valproic acid induced gene overexpression in induced neural stem cells under hypoxic conditions and also in spinal cord injury depending on valproic acid administration in vivo. Combination of valproic acid and neuron-specific enolase promoter in induced neural stem cells could be an effective gene therapy system for hypoxic spinal cord injury.

Tissue-Engineered Neural Network Graft Relays Excitatory Signal in the Completely Transected Canine Spinal Cord

Tissue engineering produces constructs with defined functions for the targeted treatment of damaged tissue. A complete spinal cord injury (SCI) model is generated in canines to test whether in vitro constructed neural network (NN) tissues can relay the excitatory signal across the lesion gap to the caudal spinal cord. Established protocols are used to construct neural stem cell (NSC)-derived NN tissue characterized by a predominantly neuronal population with robust trans-synaptic activities and myelination. The NN tissue is implanted into the gap immediately following complete transection SCI of canines at the T10 spinal cord segment. The data show significant motor recovery of paralyzed pelvic limbs, as evaluated by Olby scoring and cortical motor evoked potential (CMEP) detection. The NN tissue survives in the lesion area with neuronal phenotype maintenance, improves descending and ascending nerve fiber regeneration, and synaptic integration with host neural circuits that allow it to serve as a neuronal relay to transmit excitatory electrical signal across the injured area to the caudal spinal cord. These results suggest that tissue-engineered NN grafts can relay the excitatory signal in the completely transected canine spinal cord, providing a promising strategy for SCI treatment in large animals, including humans.

Bibliometric and Visualized Analysis of Stem Cells Therapy for Spinal Cord Injury Based on Web of Science and CiteSpace in the Last 20 Years

OBJECTIVE

To provide an analysis of Web of Science (WoS) indexed literature related to stem cells therapy in spinal cord injury published between 1999 and 2018.

METHODS

Data were obtained from the WoS Core Collection on March 30, 2019. Qualitative and quantitative analysis was conducted based on WoS. Co-citation analysis, collaboration analysis, and co-words analysis of keywords was conducted by using CiteSpace.

RESULTS

A total of 4188 references were obtained. The number of publications continually increased over the investigated period. Articles were the most frequently document type. Cell Transplantation (127) was the most productive journal. Experimental Neurology (2180) was the most frequently co-cited journal. H. Okano was the most productive and influential author, with 98 publications and 4860 cited counts. The most productive country and institution were the United States and University of Toronto, respectively. Researchers and institutions from Canada, the United States, Japan, and China were the core research forces. There was a broad and close cooperation worldwide. The Lu et al.'s (2012) article (co-citation counts, 177) was the most representative and symbolic reference. Transplantation, functional recovery, marrow-derived mesenchymal stem cell treatment, and progenitor cells were the hot spots. Inflammation, glial scar, nerve regeneration, neurite outgrowth, and bone marrow stromal cell were research frontiers.

CONCLUSIONS

Research on stem cells for spinal cord injury is a well-developed and promising research field. There is broad global scientific research cooperation. More cooperation among top authors, institutions, and countries is needed. Our results may be helpful for researchers in identifying further potential perspectives on collaborators, research frontiers, and hot topics.

Restoring Motor Neurons in Spinal Cord Injury With Induced Pluripotent Stem Cells

Spinal cord injury (SCI) is a devastating neurological disorder that damages motor, sensory, and autonomic pathways. Recent advances in stem cell therapy have allowed for the in vitro generation of motor neurons (MNs) showing electrophysiological and synaptic activity, expression of canonical MN biomarkers, and the ability to graft into spinal lesions. Clinical translation, especially the transplantation of MN precursors in spinal lesions, has thus far been elusive because of stem cell heterogeneity and protocol variability, as well as a hostile microenvironment such as inflammation and scarring, which yield inconsistent pre-clinical results without a consensus best-practice therapeutic strategy. Induced pluripotent stem cells (iPSCs) in particular have lower ethical and immunogenic concerns than other stem cells, which could make them more clinically applicable. In this review, we focus on the differentiation of iPSCs into neural precursors, MN progenitors, mature MNs, and MN subtype fates. Previous reviews have summarized MN development and differentiation, but an up-to-date summary of technological and experimental advances holding promise for bench-to-bedside translation, especially those targeting individual MN subtypes in SCI, is currently lacking. We discuss biological mechanisms of MN lineage, recent experimental protocols and techniques for MN differentiation from iPSCs, and transplantation of neural precursors and MN lineage cells in spinal cord lesions to restore motor function. We emphasize efficient, clinically safe, and personalized strategies for the application of MN and their subtypes as therapy in spinal lesions.

The Struggle to Make CNS Axons Regenerate: Why Has It Been so Difficult?

Axon regeneration in the CNS is inhibited by many extrinsic and intrinsic factors. Because these act in parallel, no single intervention has been sufficient to enable full regeneration of damaged axons in the adult mammalian CNS. In the external environment, NogoA and CSPGs are strongly inhibitory to the regeneration of adult axons. CNS neurons lose intrinsic regenerative ability as they mature: embryonic but not mature neurons can grow axons for long distances when transplanted into the adult CNS, and regeneration fails with maturity in in vitro axotomy models. The causes of this loss of regeneration include partitioning of neurons into axonal and dendritic fields with many growth-related molecules directed specifically to dendrites and excluded from axons, changes in axonal signalling due to changes in expression and localization of receptors and their ligands, changes in local translation of proteins in axons, and changes in cytoskeletal dynamics after injury. Also with neuronal maturation come epigenetic changes in neurons, with many of the transcription factor binding sites that drive axon growth-related genes becoming inaccessible. The overall aim for successful regeneration is to ensure that the right molecules are expressed after axotomy and to arrange for them to be transported to the right place in the neuron, including the damaged axon tip.

Regeneration of Spinal Cord Connectivity Through Stem Cell Transplantation and Biomaterial Scaffolds

Significant progress has been made in the treatment of spinal cord injury (SCI). Advances in post-trauma management and intensive rehabilitation have significantly improved the prognosis of SCI and converted what was once an “ailment not to be treated” into a survivable injury, but the cold hard fact is that we still do not have a validated method to improve the paralysis of SCI. The irreversible functional impairment of the injured spinal cord is caused by the disruption of neuronal transduction across the injury lesion, which is brought about by demyelination, axonal degeneration, and loss of synapses. Furthermore, refractory substrates generated in the injured spinal cord inhibit spontaneous recovery. The discovery of the regenerative capability of central nervous system neurons in the proper environment and the verification of neural stem cells in the spinal cord once incited hope that a cure for SCI was on the horizon. That hope was gradually replaced with mounting frustration when neuroprotective drugs, cell transplantation, and strategies to enhance remyelination, axonal regeneration, and neuronal plasticity demonstrated significant improvement in animal models of SCI but did not translate into a cure in human patients. However, recent advances in SCI research have greatly increased our understanding of the fundamental processes underlying SCI and fostered increasing optimism that these multiple treatment strategies are finally coming together to bring about a new era in which we will be able to propose encouraging therapies that will lead to appreciable improvements in SCI patients. In this review, we outline the pathophysiology of SCI that makes the spinal cord refractory to regeneration and discuss the research that has been done with cell replacement and biomaterial implantation strategies, both by itself and as a combined treatment. We will focus on the capacity of these strategies to facilitate the regeneration of neural connectivity necessary to achieve meaningful functional recovery after SCI.

Scaffold-facilitated locomotor improvement post complete spinal cord injury: Motor axon regeneration versus endogenous neuronal relay formation

Complete transected spinal cord injury (SCI) severely influences the quality of life and mortality rates of animals and patients. In the past decade, many simple and combinatorial therapeutic treatments have been tested in improving locomotor function in animals with this extraordinarily challenging SCI. The potential mechanism for promotion of locomotor function relies either on direct motor axon regeneration through the lesion gap or indirect neuronal relay bridging to functionally reconnect transected spinal stumps. In this review, we first compare the advantages and problems of complete transection SCI animal models with other prevailing SCI models used in motor axon regeneration research. Next, we enumerate some of the popular bio-scaffolds utilized in complete SCI repair in the last decade. Then, the current state of motor axon regeneration as well as its role on locomotor improvement of animals after complete SCI is discussed. Last, the current approach of directing endogenous neuronal relays formation to achieve motor function recovery by well-designed functional bio-scaffolds implantation in complete transected SCI animals is reviewed. Although facilitating neuronal relays formation by bio-scaffolds implantation appears to be more practical and feasible than directing motor axon regeneration in promoting locomotor outcome in animals after complete SCI, there are still challenges in neuronal relays formation, maintaining and debugging for spinal cord regenerative repair.

Insulin-like Growth Factor-1 Receptor Dictates Beneficial Effects of Treadmill Training by Regulating Survival and Migration of Neural Stem Cell Grafts in the Injured Spinal Cord

Survival and migration of transplanted neural stem cells (NSCs) are prerequisites for therapeutic benefits in spinal cord injury. We have shown that survival of NSC grafts declines after transplantation into the injured spinal cord, and that combining treadmill training (TMT) enhances NSC survival via insulin-like growth factor-1 (IGF-1). Here, we aimed to obtain genetic evidence that IGF-1 signaling in the transplanted NSCs determines the beneficial effects of TMT. We transplanted NSCs heterozygous (+/-) for Igf1r, the gene encoding IGF-1 receptor, into the mouse spinal cord after injury, with or without combining TMT. We analyzed the influence of genotype and TMT on locomotor recovery and survival and migration of NSC grafts. In vitro experiments were performed to examine the potential roles of IGF-1 signaling in the migratory ability of NSCs. Mice receiving +/- NSC grafts showed impaired locomotor recovery compared with those receiving wild-type (+/+) NSCs. Locomotor improvement by TMT was more pronounced with +/+ grafts. Deficiency of one allele of Igf1r significantly reduced survival and migration of the transplanted NSCs. Although TMT did not significantly influence NSC survival, it substantially enhanced the extent of migration for only +/+ NSCs. Cultured neurospheres exhibited dynamic motility with cytoplasmic protrusions, which was regulated by IGF-1 signaling. IGF-1 signaling in transplanted NSCs may be essential in regulating their survival and migration. Furthermore, TMT may promote NSC graft-mediated locomotor recovery via activation of IGF-1 signaling in transplanted NSCs. Dynamic NSC motility via IGF-1 signaling may be the cellular basis for the TMT-induced enhancement of migration.

Peptides and Astroglia Improve the Regenerative Capacity of Alginate Gels in the Injured Spinal Cord

IMPACT STATEMENT

Axonal bridging across a lesion in the injured spinal cord requires a growth substrate and guidance cues. Using alginate hydrogels with capillary channels we show that poly-l-ornithine and laminin can be stably bound and improve cell adhesion and neurite growth in vitro, and axon growth in vivo by enhancing host cell infiltration in the injured spinal cord. Filling of coated hydrogels with postnatal astrocytes further increases short-distance axon growth and results in a continuous astroglial substrate across the host/graft interface. Thus, positively charged bioactive molecules can be stably bound to anisotropic capillary alginate hydrogels and early astrocytes further promote tissue integration.

Human neuroepithelial stem cell regional specificity enables spinal cord repair through a relay circuit

Traumatic spinal cord injury results in persistent disability due to disconnection of surviving neural elements. Neural stem cell transplantation has been proposed as a therapeutic option, but optimal cell type and mechanistic aspects remain poorly defined. Here, we describe robust engraftment into lesioned immunodeficient mice of human neuroepithelial stem cells derived from the developing spinal cord and maintained in self-renewing adherent conditions for long periods. Extensive elongation of both graft and host axons occurs. Improved functional recovery after transplantation depends on neural relay function through the grafted neurons, requires the matching of neural identity to the anatomical site of injury, and is accompanied by expression of specific marker proteins. Thus, human neuroepithelial stem cells may provide an anatomically specific relay function for spinal cord injury recovery.

The optimal type or regional origin of stem cells for regenerative applications in the nervous system has not yet been established. Here the authors show that human neuroepithelial stem cells from the developing spinal cord, but not those from the developing cortex, show good host-graft interaction when transplanted to rodent models of spinal cord injury.

Transplantation of Neural Progenitors and V2a Interneurons after Spinal Cord Injury

There is growing interest in the use of neural precursor cells to treat spinal cord injury (SCI). Despite extensive pre-clinical research, it remains unclear as to which donor neuron phenotypes are available for transplantation, whether the same populations exist across different sources of donor tissue (e.g., developing tissue vs. cultured cells), and whether donor cells retain their phenotype once transplanted into the hostile internal milieu of the injured adult spinal cord. In addition, while functional improvements have been reported after neural precursor transplantation post-SCI, the extent of recovery is limited and variable. The present work begins to address these issues by harnessing ventrally derived excitatory pre-motor V2a spinal interneurons (SpINs) to repair the phrenic motor circuit after cervical SCI. Recent studies have demonstrated that Chx10-positive V2a SpINs contribute to anatomical plasticity within the phrenic circuitry after cervical SCI, thus identifying them as a therapeutic candidate. Building upon this discovery, the present work tests the hypothesis that transplantation of neural progenitor cells (NPCs) enriched with V2a INs can contribute to neural networks that promote repair and enhance respiratory plasticity after cervical SCI. Cultured NPCs (neuronal and glial restricted progenitor cells) isolated from E13.5 Green fluorescent protein rats were aggregated with TdTomato-mouse embryonic stem cell-derived V2a INs in vitro, then transplanted into the injured cervical (C3-4) spinal cord. Donor cells survive, differentiate and integrate with the host spinal cord. Functional diaphragm electromyography indicated recovery 1 month following treatment in transplant recipients. Animals that received donor cells enriched with V2a INs showed significantly greater functional improvement than animals that received NPCs alone. The results from this study offer insight into the neuronal phenotypes that might be effective for (re)establishing neuronal circuits in the injured adult central nervous system.

[Review of the regeneration mechanism of complete spinal cord injury]

Spinal cord injury (SCI), especially the complete SCI, usually results in complete paralysis below the level of the injury and seriously affects the patient's quality of life. SCI repair is still a worldwide medical problem. In the last twenty years, Professor DAI Jianwu and his team pioneered complete SCI model by removing spinal tissue with varied lengths in rodents, canine, and non-human primates to verify therapeutic effect of different repair strategies. Moreover, they also started the first clinical study of functional collagen scaffold on patients with acute complete SCI on January 16th, 2015. This review mainly focusses on the possible mechanisms responsible for complete SCI. In common, recovery of some sensory and motor functions post complete SCI include the following three contributing reasons. ① Regeneration of long ascending and descending axons throughout the lesion site to re-connect the original targets; ② New neural circuits formed in the lesion site by newly generated neurons post injury, which effectively re-connect the transected stumps; ③ The combined effect of ① and ②. The numerous studies have confirmed that neural circuits rebuilt across the injury site by newborn neurons might be the main mechanisms for functional recovery of animals from rodents to dogs. In many SCI model, especially the complete spinal cord transection model, many studies have convincingly demonstrated that the quantity and length of regenerated long descending axons, particularly like CST fibers, are too few to across the lesion site that is millimeters in length to realize motor functional recovery. Hence, it is more feasible in guiding neuronal relays formation by bio-scaffolds implantation than directing long motor axons regeneration in improving motor function of animals with complete spinal cord transection. However, some other issues such as promoting more neuronal relays formation, debugging wrong connections, and maintaining adequate neural circuits for functional recovery are urgent problems to be addressed.

Neural stem cell mediated recovery is enhanced by Chondroitinase ABC pretreatment in chronic cervical spinal cord injury

Traumatic spinal cord injuries (SCIs) affect millions of people worldwide; the majority of whom are in the chronic phase of their injury. Unfortunately, most current treatments target the acute/subacute injury phase as the microenvironment of chronically injured cord consists of a well-established glial scar with inhibitory chondroitin sulfate proteoglycans (CSPGs) which acts as a potent barrier to regeneration. It has been shown that CSPGs can be degraded in vivo by intrathecal Chondroitinase ABC (ChABC) to produce a more permissive environment for regeneration by endogenous cells or transplanted neural stem cells (NSCs) in the subacute phase of injury. Using a translationally-relevant clip-contusion model of cervical spinal cord injury in mice we sought to determine if ChABC pretreatment could modify the harsh chronic microenvironment to enhance subsequent regeneration by induced pluripotent stem cell-derived NSCs (iPS-NSC). Seven weeks after injury—during the chronic phase—we delivered ChABC by intrathecal osmotic pump for one week followed by intraparenchymal iPS-NSC transplant rostral and caudal to the injury epicenter. ChABC administration reduced chronic-injury scar and resulted in significantly improved iPSC-NSC survival with clear differentiation into all three neuroglial lineages. Neurons derived from transplanted cells also formed functional synapses with host circuits on patch clamp analysis. Furthermore, the combined treatment led to recovery in key functional muscle groups including forelimb grip strength and measures of forelimb/hindlimb locomotion assessed by Catwalk. This represents important proof-of-concept data that the chronically injured spinal cord can be ‘unlocked’ by ChABC pretreatment to produce a microenvironment conducive to regenerative iPS-NSC therapy.

In Vivo Reprogramming for CNS Repair: Regenerating Neurons from Endogenous Glial Cells

Neuroregeneration in the CNS has proven to be difficult despite decades of research. The old dogma that CNS neurons cannot be regenerated in the adult mammalian brain has been overturned; however, endogenous adult neurogenesis appears to be insufficient for brain repair. Stem cell therapy once held promise for generating large quantities of neurons in the CNS, but immunorejection and long-term functional integration remain major hurdles. In this Perspective, we discuss the use of in vivo reprogramming as an emerging technology to regenerate functional neurons from endogenous glial cells inside the brain and spinal cord. Besides the CNS, in vivo reprogramming has been demonstrated successfully in the pancreas, heart, and liver and may be adopted in other organs. Although challenges remain for translating this technology into clinical therapies, we anticipate that in vivo reprogramming may revolutionize regenerative medicine by using a patient's own internal cells for tissue repair.

Optogenetic Interrogation of Functional Synapse Formation by Corticospinal Tract Axons in the Injured Spinal Cord

To restore function after injury to the CNS, axons must be stimulated to extend into denervated territory and, critically, must form functional synapses with appropriate targets. We showed previously that forced overexpression of the transcription factor Sox11 increases axon growth by corticospinal tract (CST) neurons after spinal injury. However, behavioral outcomes were not improved, raising the question of whether the newly sprouted axons are able to form functional synapses. Here we developed an optogenetic strategy, paired with single-unit extracellular recordings, to assess the ability of Sox11-stimulated CST axons to functionally integrate in the circuitry of the cervical spinal cord. Initial time course experiments established the expression and function of virally expressed Channelrhodopsin (ChR2) in CST cell bodies and in axon terminals in cervical spinal cord. Pyramidotomies were performed in adult mice to deprive the left side of the spinal cord of CST input, and the right CST was treated with adeno-associated virus (AAV)-Sox11 or AAV-EBFP control, along with AAV-ChR2. As expected, Sox11 treatment caused robust midline crossing of CST axons into previously denervated left spinal cord. Clear postsynaptic responses resulted from optogenetic activation of CST terminals, demonstrating the ability of Sox11-stimulated axons to form functional synapses. Mapping of the distribution of CST-evoked spinal activity revealed overall similarity between intact and newly innervated spinal tissue. These data demonstrate the formation of functional synapses by Sox11-stimulated CST axons without significant behavioral benefit, suggesting that new synapses may be mistargeted or otherwise impaired in the ability to coordinate functional output.

SIGNIFICANCE STATEMENT

As continued progress is made in promoting the regeneration of CNS axons, questions of synaptic integration are increasingly prominent. Demonstrating direct synaptic integration by regenerated axons and distinguishing its function from indirect relay circuits and target field plasticity have presented technical challenges. Here we force the overexpression of Sox11 to stimulate the growth of corticospinal tract axons in the cervical spinal cord and then use specific optogenetic activation to assess their ability to directly drive postsynaptic activity in spinal cord neurons. By confirming successful synaptic integration, these data illustrate a novel optogenetic-based strategy to monitor and optimize functional reconnection by newly sprouted axons in the injured CNS.

Combinatorial Therapies After Spinal Cord Injury: How Can Biomaterials Help?

Traumatic spinal cord injury (SCI) results in an immediate loss of motor and sensory function below the injury site and is associated with a poor prognosis. The inhibitory environment that develops in response to the injury is mainly due to local expression of inhibitory factors, scarring and the formation of cystic cavitations, all of which limit the regenerative capacity of endogenous or transplanted cells. Strategies that demonstrate promising results induce a change in the microenvironment at- and around the lesion site to promote endogenous cell repair, including axonal regeneration or the integration of transplanted cells. To date, many of these strategies target only a single aspect of SCI; however, the multifaceted nature of SCI suggests that combinatorial strategies will likely be more effective. Biomaterials are a key component of combinatorial strategies, as they have the potential to deliver drugs locally over a prolonged period of time and aid in cell survival, integration and differentiation. Here we summarize the advantages and limitations of widely used strategies to promote recovery after injury and highlight recent research where biomaterials aided combinatorial strategies to overcome some of the barriers of spinal cord regeneration.