Factors secreted by Schwann cells stimulate the regeneration of neonatal retinal ganglion cells

Melatonin Enhances Neural Differentiation of Adipose-Derived Mesenchymal Stem Cells

Adipose-derived mesenchymal stem cells (ASCs) are adult multipotent stem cells, able to differentiate toward neural elements other than cells of mesodermal lineage. The aim of this research was to test ASC neural differentiation using melatonin combined with conditioned media (CM) from glial cells. Isolated from the lipoaspirate of healthy donors, ASCs were expanded in a basal growth medium before undergoing neural differentiation procedures. For this purpose, CM obtained from olfactory ensheathing cells and from Schwann cells were used. In some samples, 1 µM of melatonin was added. After 1 and 7 days of culture, cells were studied using immunocytochemistry and flow cytometry to evaluate neural marker expression (Nestin, MAP2, Synapsin I, GFAP) under different conditions. The results confirmed that a successful neural differentiation was achieved by glial CM, whereas the addition of melatonin alone did not induce appreciable changes. When melatonin was combined with CM, ASC neural differentiation was enhanced, as demonstrated by a further improvement of neuronal marker expression, whereas glial differentiation was attenuated. A dynamic modulation was also observed, testing the expression of melatonin receptors. In conclusion, our data suggest that melatonin's neurogenic differentiation ability can be usefully exploited to obtain neuronal-like differentiated ASCs for potential therapeutic strategies.

The Role of Sensory Innervation in Homeostatic and Injury-Induced Corneal Epithelial Renewal

The cornea is the window through which we see the world. Corneal clarity is required for vision, and blindness occurs when the cornea becomes opaque. The cornea is covered by unique transparent epithelial cells that serve as an outermost cellular barrier bordering between the cornea and the external environment. Corneal sensory nerves protect the cornea from injury by triggering tearing and blink reflexes, and are also thought to regulate corneal epithelial renewal via unknown mechanism(s). When protective corneal sensory innervation is absent due to infection, trauma, intracranial tumors, surgery, or congenital causes, permanent blindness results from repetitive epithelial microtraumas and failure to heal. The condition is termed neurotrophic keratopathy (NK), with an incidence of 5:10,000 people worldwide. In this report, we review the currently available therapeutic solutions for NK and discuss the progress in our understanding of how the sensory nerves induce corneal epithelial renewal.

Phase 1 Safety Trial of Autologous Human Schwann Cell Transplantation in Chronic Spinal Cord Injury

A phase 1 open-label, non-randomized clinical trial was conducted to determine feasibility and safety of autologous human Schwann cell (ahSC) transplantation accompanied by rehabilitation in participants with chronic spinal cord injury (SCI). Magnetic resonance imaging (MRI) was used to screen eligible participants to estimate an individualized volume of cell suspension to be implanted. The trial incorporated standardized multi-modal rehabilitation before and after cell delivery. Participants underwent sural nerve harvest, and ahSCs were isolated and propagated in culture. The dose of culture-expanded ahSCs injected into the chronic spinal cord lesion of each individual followed a cavity-filling volume approach. Primary outcome measures for safety and trend-toward efficacy were assessed. Two participants with American Spinal Injury Association Impairment Scale (AIS) A and two participants with incomplete chronic SCI (AIS B, C) were each enrolled in cervical and thoracic SCI cohorts (n = 8 total). All participants completed the study per protocol, and no serious adverse events related to sural nerve harvest or ahSC transplantation were reported. Urinary tract infections and skin abrasions were the most common adverse events reported. One participant experienced a 4-point improvement in motor function, a 6-point improvement in sensory function, and a 1-level improvement in neurological level of injury. Follow-up MRI in the cervical (6 months) and thoracic (24 months) cohorts revealed a reduction in cyst volume after transplantation with reduced effect over time. This phase 1 trial demonstrated the feasibility and safety of ahSC transplantation combined with a multi-modal rehabilitation protocol for participants with chronic SCI.

Nerve guidance conduit development for primary treatment of peripheral nerve transection injuries: A commercial perspective

Commercial nerve guidance conduits (NGCs) for repair of peripheral nerve discontinuities are of little use in gaps larger than 30 mm, and for smaller gaps they often fail to compete with the autografts that they are designed to replace. While recent research to develop new technologies for use in NGCs has produced many advanced designs with seemingly positive functional outcomes in animal models, these advances have not been translated into viable clinical products. While there have been many detailed reviews of the technologies available for creating NGCs, none of these have focussed on the requirements of the commercialisation process which are vital to ensure the translation of a technology from bench to clinic. Consideration of the factors essential for commercial viability, including regulatory clearance, reimbursement processes, manufacturability and scale up, and quality management early in the design process is vital in giving new technologies the best chance at achieving real-world impact. Here we have attempted to summarise the major components to consider during the development of emerging NGC technologies as a guide for those looking to develop new technology in this domain. We also examine a selection of the latest academic developments from the viewpoint of clinical translation, and discuss areas where we believe further work would be most likely to bring new NGC technologies to the clinic. STATEMENT OF SIGNIFICANCE: NGCs for peripheral nerve repairs represent an adaptable foundation with potential to incorporate modifications to improve nerve regeneration outcomes. In this review we outline the regulatory processes that functionally distinct NGCs may need to address and explore new modifications and the complications that may need to be addressed during the translation process from bench to clinic.

Schwann-like cell conditioned medium promotes angiogenesis and nerve regeneration

Vascular network reconstruction plays a pivotal role in the axonal regeneration and nerve function recovery after peripheral nerve injury. Increasing evidence indicates that Schwann cells (SCs) can promote nerve function repair, and the beneficial effects attributed to SCs therapy may exert their therapeutic effects through paracrine mechanisms. Recently, the previous research of our group demonstrated the promising neuroregenerative capacity of Schwann-like cells (SCLCs) derived from differentiated human embryonic stem cell-derived neural stem cells (hESC-NSCs) in vitro. Herein, the effects of SC-like cell conditioned medium (SCLC-CM) on angiogenesis and nerve regeneration were further explored. The assays were performed to show the pro-angiogenic effects of SCLC-CM, such as promoted endothelial cell proliferation, migration and tube formation in vitro. In addition, Sprague-Dawley rats were treated with SCLC-CM after sciatic nerve crush injury, SCLC-CM was conducive for the recovery of sciatic nerve function, which was mainly manifested in the SFI increase, the wet weight ratio of gastrocnemius muscle, as well as the number and thickness of myelin. The SCLC-CM treatment reduced the Evans blue leakage and increased the expression of CD34 microvessels. Furthermore, SCLC-CM upregulated the expressions of p-Akt and p-mTOR in endothelial cells. In conclusion, SCLC-CM promotes angiogenesis and nerve regeneration, it is expected to become a new treatment strategy for peripheral nerve injury.

Adult Neural Plasticity in Naked Mole-Rats: Implications of Fossoriality, Longevity and Sociality on the Brain's Capacity for Change

Naked mole-rats (Heterocephalus glaber) are small African rodents that have many unique behavioral and physiological adaptations well-suited for testing hypotheses about mammalian neural plasticity. In this chapter, we focus on three features of naked mole-rat biology and how they impact neural plasticity in this species: (1) their fossorial lifestyle, (2) their extreme longevity with a lack of demonstrable senescence, and (3) their unusual social structure. Critically, each of these features requires some degree of biological flexibility. First, their fossorial habitat situates them in an environment with characteristics to which the central nervous system is particularly sensitive (e.g., oxygen content, photoperiod, spatial complexity). Second, their long lifespan requires adaptations to combat senescence and declines in neural functioning. Finally, their extreme reproductive skew and sustained ability for release from reproductive suppression indicates remarkable neural sensitivity to the sociosexual environment that is distinct from chronological age. These three features of naked mole-rat life are not mutually exclusive, but they do each offer unique considerations for the possibilities, constraints, and mechanisms associated with adult neural plasticity.

Trichostatin A increases BDNF protein expression by improving XBP-1s/ATF6/GRP78 axis in Schwann cells of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is the common complication of diabetes mellitus. Histone deacetylase (HDAC) inhibitor trichostatin A (TSA) is reported to ameliorate the peripheral nerves degeneration of DPN. However, the exact mechanism is still not well elucidated. Here, we first revealed that TSA promoted nerve conduction and brain derived neurotrophic factor (BDNF) expression in the sciatic nerves of diabetic mice. In line, TSA also reversed high glucose-reduced mature BDNF expression in vitro cultured rat Schwann cells (RSC96). Then unexpectedly, the downstream targets of TSA HDAC1 and HDAC5 were not involved in TSA-improved BDNF expression. Furthermore, unfolded protein response (UPR) chaperone GRP78 was revealed to be downregulated with high glucose stimulation in RSC96 cells, which was avoided with TSA treatment. Also, GRP78 upregulation mediated TSA-improved mature BDNF expression in high glucose-cultured RSC96 cells by binding with BDNF. As well, TSA treatment enhanced the binding of GRP78 with BDNF in RSC96 cells. Again, UPR-associated transcription factors XBP-1s and ATF6 were involved in TSA-increased GRP78 expression in high glucose-stimulated RSC96 cells. Finally, conditioned medium from high glucose-cultured RSC96 cells delayed neuron SH-SY5Y differentiation and that from TSA-treated high glucose-cultured RSC96 cells promoted SH-SY5Y cell differentiation. Taken together, our findings suggested that TSA increased BDNF expression to ameliorate DPN by improving XBP-1s/ATF6/GRP78 axis in Schwann cells.

Bioprinting: From Tissue and Organ Development to in Vitro Models

Bioprinting techniques have been flourishing in the field of biofabrication with pronounced and exponential developments in the past years. Novel biomaterial inks used for the formation of bioinks have been developed, allowing the manufacturing of in vitro models and implants tested preclinically with a certain degree of success. Furthermore, incredible advances in cell biology, namely, in pluripotent stem cells, have also contributed to the latest milestones where more relevant tissues or organ-like constructs with a certain degree of functionality can already be obtained. These incredible strides have been possible with a multitude of multidisciplinary teams around the world, working to make bioprinted tissues and organs more relevant and functional. Yet, there is still a long way to go until these biofabricated constructs will be able to reach the clinics. In this review, we summarize the main bioprinting activities linking them to tissue and organ development and physiology. Most bioprinting approaches focus on mimicking fully matured tissues. Future bioprinting strategies might pursue earlier developmental stages of tissues and organs. The continuous convergence of the experts in the fields of material sciences, cell biology, engineering, and many other disciplines will gradually allow us to overcome the barriers identified on the demanding path toward manufacturing and adoption of tissue and organ replacements.

Hierarchical fibrous guiding cues at different scales influence linear neurite extension

Surface topographies at micro- and nanoscales can influence different cellular behavior, such as their growth rate and directionality. While different techniques have been established to fabricate 2-dimensional flat substrates with nano- and microscale topographies, most of them are prone to high costs and long preparation times. The 2.5-dimensional fiber platform presented here provides knowledge on the effect of the combination of fiber alignment, inter-fiber distance (IFD), and fiber surface topography on contact guidance to direct neurite behavior from dorsal root ganglia (DRGs) or dissociated primary neurons. For the first time, the interplay of the micro-/nanoscale topography and IFD is studied to induce linear nerve growth, while controlling branching. The results demonstrate that grooved fibers promote a higher percentage of aligned neurite extension, compensating the adverse effect of increased IFD. Accordingly, maximum neurite extension from primary neurons is achieved on grooved fibers separated by an IFD of 30 μm, with a higher percentage of aligned neurons on grooved fibers at a large IFD compared to porous fibers with the smallest IFD of 10 µm. We further demonstrate that the neurite "decision-making" behavior on whether to cross a fiber or grow along it is not only dependent on the IFD but also on the fiber surface topography. In addition, axons growing in between the fibers seem to have a memory after leaving grooved fibers, resulting in higher linear growth and higher IFDs lead to more branching. Such information is of great importance for new material development for several tissue engineering applications. STATEMENT OF SIGNIFICANCE: One of the key aspects of tissue engineering is controlling cell behavior using hierarchical structures. Compared to 2D surfaces, fibers are an important class of materials, which can emulate the native ECM architecture of tissues. Despite the importance of both fiber surface topography and alignment to direct growing neurons, the current state of the art did not yet study the synergy between both scales of guidance. To achieve this, we established a solvent assisted spinning process to combine these two crucial features and control neuron growth, alignment, and branching. Rational design of new platforms for various tissue engineering and drug discovery applications can benefit from such information as it allows for fabrication of functional materials, which selectively influence neurite behavior.

Recent Advances in Carbon Nanotubes for Nervous Tissue Regeneration

Regenerative medicine has taken advantage of several nanomaterials for reparation of diseased or damaged tissues in the nervous system involved in memory, cognition, and movement. Electrical, thermal, mechanical, and biocompatibility aspects of carbon-based nanomaterials (nanotubes, graphene, fullerenes, and their derivatives) make them suitable candidates to drive nerve tissue repair and stimulation. This review article focuses on key recent advances on the use of carbon nanotube- (CNT-) based technologies on nerve tissue engineering, outlining how neurons interact with CNT interfaces for promoting neuronal differentiation, growth and network reconstruction. CNTs still represent strong candidates for use in therapies of neurodegenerative pathologies and spinal cord injuries.

Peripheral nerve grafts implanted into the substantia nigra in patients with Parkinson's disease during deep brain stimulation surgery: 1-year follow-up study of safety, feasibility, and clinical outcome

OBJECTIVECurrently, there is no treatment that slows or halts the progression of Parkinson's disease. Delivery of various neurotrophic factors to restore dopaminergic function has become a focus of study in an effort to fill this unmet need for patients with Parkinson's disease. Schwann cells provide a readily available source of such factors. This study presents a 12-month evaluation of safety and feasibility, as well as the clinical response, of implanting autologous peripheral nerve grafts into the substantia nigra of patients with Parkinson's disease at the time of deep brain stimulation (DBS) surgery.METHODSStandard DBS surgery targeting the subthalamic nucleus was performed in 8 study participants. After DBS lead implantation, a section of the sural nerve containing Schwann cells was harvested and unilaterally grafted to the substantia nigra. Adverse events were continually monitored. Baseline clinical data were obtained during standard preoperative evaluations. Clinical outcome data were obtained with postoperative clinical evaluations, neuropsychological testing, and MRI at 1 year after surgery.RESULTSAll 8 participants were implanted with DBS systems and grafts. Adverse event profiles were comparable to those of standard DBS surgery with the exception of 1 superficial infection at the sural nerve harvest site. Three participants also reported numbness in the distribution of the sural nerve distal to the harvest site. Motor scores on Unified Parkinson's Disease Rating Scale (UPDRS) part III while the participant was off therapy at 12 months improved from baseline (mean ± SD 25.1 ± 15.9 points at 12 months vs 32.5 ± 9.7 points at baseline). An analysis of the lateralized UPDRS scores also showed a greater overall reduction in scores on the side contralateral to the graft.CONCLUSIONSPeripheral nerve graft delivery to the substantia nigra at the time of DBS surgery is feasible and safe based on the results of this initial pilot study. Clinical outcome data from this phase I trial suggests that grafting may have some clinical benefit and certainly warrants further study to determine if this is an efficacious and neurorestorative therapy.Clinical trial registration no.: NCT01833364 (clinicaltrials.gov).

Biomimetic Architectures for Peripheral Nerve Repair: A Review of Biofabrication Strategies

Biofabrication techniques have endeavored to improve the regeneration of the peripheral nervous system (PNS), but nothing has surpassed the performance of current clinical practices. However, these current approaches have intrinsic limitations that compromise patient care. The "gold standard" autograft provides the best outcomes but requires suitable donor material, while implantable hollow nerve guide conduits (NGCs) can only repair small nerve defects. This review places emphasis on approaches that create structural cues within a hollow NGC lumen in order to match or exceed the regenerative performance of the autograft. An overview of the PNS and nerve regeneration is provided. This is followed by an assessment of reported devices, divided into three major categories: isotropic hydrogel fillers, acting as unstructured interluminal support for regenerating nerves; fibrous interluminal fillers, presenting neurites with topographical guidance within the lumen; and patterned interluminal scaffolds, providing 3D support for nerve growth via structures that mimic native PNS tissue. Also presented is a critical framework to evaluate the impact of reported outcomes. While a universal and versatile nerve repair strategy remains elusive, outlined here is a roadmap of past, present, and emerging fabrication techniques to inform and motivate new developments in the field of peripheral nerve regeneration.

Schwann Cell Transplantation Improves Reticulospinal Axon Growth and Forelimb Strength after Severe Cervical Spinal Cord Contusion

Schwann cell (SC) implantation alone has been shown to promote the growth of propriospinal and sensory axons, but not long-tract descending axons, after thoracic spinal cord injury (SCI). In the current study, we examined if an axotomy close to the cell body of origin (so as to enhance the intrinsic growth response) could permit supraspinal axons to grow onto SC grafts. Adult female Fischer rats received a severe (C5) cervical contusion (1.1 mm displacement, 3 KDyn). At 1 week postinjury, 2 million SCs ex vivo transduced with lentiviral vector encoding enhanced green fluorescent protein (EGFP) were implanted within media into the injury epicenter; injury-only animals served as controls. Animals were tested weekly using the BBB score for 7 weeks postimplantation and received at end point tests for upper body strength: self-supported forelimb hanging, forearm grip force, and the incline plane. Following behavioral assessment, animals were anterogradely traced bilaterally from the reticular formation using BDA-Texas Red. Stereological quantification revealed a twofold increase in the numbers of preserved NeuN+ neurons rostral and caudal to the injury/graft site in SC implanted animals, corroborating previous reports of their neuroprotective efficacy. Examination of labeled reticulospinal axon growth revealed that while rarely an axon was present within the lesion site of injury-only controls, numerous reticulospinal axons had penetrated the SC implant/lesion milieu. This has not been observed following implantation of SCs alone into the injured thoracic spinal cord. Significant behavioral improvements over injury-only controls in upper limb strength, including an enhanced grip strength (a 296% increase) and an increased self-supported forelimb hanging, accompanied SC-mediated neuroprotection and reticulospinal axon growth. The current study further supports the neuroprotective efficacy of SC implants after SCI and demonstrates that SCs alone are capable of supporting modest supraspinal axon growth when the site of axon injury is closer to the cell body of the axotomized neuron.

Safety of Autologous Human Schwann Cell Transplantation in Subacute Thoracic Spinal Cord Injury

The rationale for implantation of autologous human Schwann cells (SCs) in persons with subacute spinal cord injury (SCI) is based on evidence that transplanted SCs are neuroprotective, support local axonal plasticity, and are capable of myelinating axons. A Phase I clinical trial was conducted to evaluate the safety of autologous human SC transplantation into the injury epicenter of six subjects with subacute SCI. The trial was an open-label, unblinded, non-randomized, non-placebo controlled study with a dose escalation design and standard medical rehabilitation. Participants were paraplegics with neurologically complete, trauma-induced spinal lesions. Autologous SCs were cultured in vitro from a sural nerve harvested from each participant and injected into the epicenter of the spinal lesion. Outcome measures for safety were protocol compliance, feasibility, adverse events, stability of neurological level, absence of detectable mass lesion, and the emergence of clinically significant neuropathic pain or muscle spasticity no greater than expected for a natural course cohort. One year post-transplantation, there were no surgical, medical, or neurological complications to indicate that the timing or procedure for the cell transplantation was unsafe. There were no adverse events or serious adverse events related to the cell therapy. There was no evidence of additional spinal cord damage, mass lesion, or syrinx formation. We conclude that it is feasible to identify eligible candidates, appropriately obtain informed consent, perform a peripheral nerve harvest to obtain SCs within 5-30 days of injury, and perform an intra-spinal transplantation of highly purified autologous SCs within 4-7 weeks of injury.

Predegenerated Schwann cells--a novel prospect for cell therapy for glaucoma: neuroprotection, neuroregeneration and neuroplasticity

Glaucoma is an optic neuropathy that leads to irreversible blindness. Because the current therapies are not sufficient to protect against glaucoma-induced visual impairment, new treatment approaches are necessary to prevent disease progression. Cell transplantation techniques are currently considered to be among the most promising opportunities for nervous system damage treatment. The beneficial effects of undifferentiated cells have been investigated in experimental models of glaucoma, however experiments were accompanied by various barriers, which would make putative treatment difficult or even impossible to apply in a clinical setting. The novel therapy proposed in our study creates conditions to eliminate some of the identified barriers described for precursor cells transplantation and allows us to observe direct neuroprotective and pro-regenerative effects in ongoing optic neuropathy without additional modifications to the transplanted cells. We demonstrated that the proposed novel Schwann cell therapy might be promising, effective and easy to apply, and is safer than the alternative cell therapies for the treatment of glaucoma.

Schwann-cell cylinders grown inside hyaluronic-acid tubular scaffolds with gradient porosity

Cell transplantation therapies in the nervous system are frequently hampered by glial scarring and cell drain from the damaged site, among others. To improve this situation, new biomaterials may be of help. Here, novel single-channel tubular conduits based on hyaluronic acid (HA) with and without poly-l-lactide acid fibers in their lumen were fabricated. Rat Schwann cells were seeded within the conduits and cultured for 10days. The conduits possessed a three-layered porous structure that impeded the leakage of the cells seeded in their interior and made them impervious to cell invasion from the exterior, while allowing free transport of nutrients and other molecules needed for cell survival. The channel's surface acted as a template for the formation of a cylindrical sheath-like tapestry of Schwann cells continuously spanning the whole length of the lumen. Schwann-cell tubes having a diameter of around 0.5mm and variable lengths can thus be generated. This structure is not found in nature and represents a truly engineered tissue, the outcome of the specific cell-material interactions. The conduits might be useful to sustain and protect cells for transplantation, and the biohybrids here described, together with neuronal precursors, might be of help in building bridges across significant distances in the central and peripheral nervous system.

STATEMENT OF SIGNIFICANCE

The paper entitled "Schwann-cell cylinders grown inside hyaluronic-acid tubular scaffolds with gradient porosity" reports on the development of a novel tubular scaffold and on how this scaffold acts on Schwann cells seeded in its interior as a template to produce macroscopic hollow continuous cylinders of tightly joined Schwann cells. This cellular structure is not found in nature and represents a truly engineered novel tissue, which obtains as a consequence of the specific cell-material interactions within the scaffold.

Strategies for regeneration of components of nervous system: scaffolds, cells and biomolecules

Nerve diseases including acute injury such as peripheral nerve injury (PNI), spinal cord injury (SCI) and traumatic brain injury (TBI), and chronic disease like neurodegeneration disease can cause various function disorders of nervous system, such as those relating to memory and voluntary movement. These nerve diseases produce great burden for individual families and the society, for which a lot of efforts have been made. Axonal pathways represent a unidirectional and aligned architecture allowing systematic axonal development within the tissue. Following a traumatic injury, the intricate architecture suffers disruption leading to inhibition of growth and loss of guidance. Due to limited capacity of the body to regenerate axonal pathways, it is desirable to have biomimetic approach that has the capacity to graft a bridge across the lesion while providing optimal mechanical and biochemical cues for tissue regeneration. And for central nervous system injury, one more extra precondition is compulsory: creating a less inhibitory surrounding for axonal growth. Electrospinning is a cost-effective and straightforward technique to fabricate extracellular matrix (ECM)-like nanofibrous structures, with various fibrous forms such as random fibers, aligned fibers, 3D fibrous scaffold and core-shell fibers from a variety of polymers. The diversity and versatility of electrospinning technique, together with functionalizing cues such as neurotrophins, ECM-based proteins and conductive polymers, have gained considerable success for the nerve tissue applications. We are convinced that in the future the stem cell therapy with the support of functionalized electrospun nerve scaffolds could be a promising therapy to cure nerve diseases.

Gene therapy strategies for the treatment of spinal cord injury

Spinal cord injury is a complex pathology often resulting in functional impairment and paralysis. Gene therapy has emerged as a possible solution to the problems of limited neural tissue regeneration through the administration of factors promoting axonal growth, while also offering long-term local delivery of therapeutic molecules at the injury site. Of note, gene therapy is our response to the requirements of neural and glial cells following spinal cord injury, providing, in a time-dependent manner, growth substances for axonal regeneration and eliminating axonal growth inhibitors. Herein, we explore different gene therapy strategies, including targeting gene expression to modulate the presence of neurotrophic growth or survival factors and increase neural tissue plasticity. Special attention is given to describing advances in viral and non-viral gene delivery systems, as well as the available routes of gene delivery. Finally, we discuss the future of combinatorial gene therapies and give consideration to the implementation of gene therapy in humans.

Concepts for regulation of axon integrity by enwrapping glia

Long axons and their enwrapping glia (EG; Schwann cells (SCs) and oligodendrocytes (OLGs)) form a unique compound structure that serves as conduit for transport of electric and chemical information in the nervous system. The peculiar cytoarchitecture over an enormous length as well as its substantial energetic requirements make this conduit particularly susceptible to detrimental alterations. Degeneration of long axons independent of neuronal cell bodies is observed comparatively early in a range of neurodegenerative conditions as a consequence of abnormalities in SCs and OLGs . This leads to the most relevant disease symptoms and highlights the critical role that these glia have for axon integrity, but the underlying mechanisms remain elusive. The quest to understand why and how axons degenerate is now a crucial frontier in disease-oriented research. This challenge is most likely to lead to significant progress if the inextricable link between axons and their flanking glia in pathological situations is recognized. In this review I compile recent advances in our understanding of the molecular programs governing axon degeneration, and mechanisms of EG’s non-cell autonomous impact on axon-integrity. A particular focus is placed on emerging evidence suggesting that EG nurture long axons by virtue of their intimate association, release of trophic substances, and neurometabolic coupling. The correction of defects in these functions has the potential to stabilize axons in a variety of neuronal diseases in the peripheral nervous system and central nervous system (PNS and CNS).

Inducible expression of neurotrophic factors by mesenchymal progenitor cells derived from traumatically injured human muscle

Peripheral nerve damage frequently accompanies musculoskeletal trauma and repair of these nerves could be enhanced by the targeted application of neurotrophic factors (NTFs), which are typically expressed by endogenous cells that support nerve regeneration. Injured muscle tissues express NTFs to promote reinnervation as the tissue regenerates, but the source of these factors from within the muscles is not fully understood. We have previously identified a population of mesenchymal progenitor cells (MPCs) in traumatized muscle tissue with properties that support tissue regeneration, and our hypothesis was that MPCs also secrete the NTFs that are associated with muscle tissue reinnervation. We determined that MPCs express genes associated with neurogenic function and measured the protein-level expression of specific NTFs with known functions to support nerve regeneration. We also demonstrated the effectiveness of a neurotrophic induction protocol to enhance the expression of the NTFs, which suggests that the expression of these factors may be modulated by the cellular environment. Finally, neurotrophic induction affected the expression of cell surface markers and proliferation rate of the MPCs. Our findings indicate that traumatized muscle-derived MPCs may be useful as a therapeutic cell type to enhance peripheral nerve regeneration following musculoskeletal injury.

Transplantation of autologous activated Schwann cells in the treatment of spinal cord injury: six cases, more than five years of follow-up

Schwann cells (SCs) are the main glial cells of the peripheral nervous system, which can promote neural regeneration. Grafting of autologous SCs is one of the well-established and commonly performed procedures for peripheral nerve repair. With the aim to improve the clinical condition of patients with spinal cord injury (SCI), a program of grafting autologous activated Schwann cells (AASCs), as well as a series of appropriate neurorehabilitation programs, was employed to achieve the best therapeutic effects. We selected six patients who had a history of SCI before transplantation. At first, AASCs were obtained by prior ligation of sural nerve and subsequently isolated, cultured, and purified in vitro. Then the patients accepted an operation of laminectomy and cell transplantation, and no severe adverse event was observed in any of these patients. Motor and sensitive improvements were evaluated by means of American Spinal Injury Association (ASIA) grading and Functional Independence Measure (FIM); bladder and urethral function were determined by clinical and urodynamic examination; somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) were used to further confirm the functional recovery following transplantation. The patients were followed up for more than 5 years. All of the patients showed some signs of improvement in autonomic, motor, and sensory function. So we concluded that AASC transplantation might be feasible, safe, and effective to promote neurorestoration of SCI patients.

Analysis of Schwann-astrocyte interactions using in vitro assays

Schwann cells are one of the commonly used cells in repair strategies following spinal cord injuries. Schwann cells are capable of supporting axonal regeneration and sprouting by secreting growth factors (1,2) and providing growth promoting adhesion molecules (3) and extracellular matrix molecules (4). In addition they myelinate the demyelinated axons at the site of injury (5). However following transplantation, Schwann cells do not migrate from the site of implant and do not intermingle with the host astrocytes (6,7). This results in formation of a sharp boundary between the Schwann cells and astrocytes, creating an obstacle for growing axons trying to exit the graft back into the host tissue proximally and distally. Astrocytes in contact with Schwann cells also undergo hypertrophy and up-regulate the inhibitory molecules (8-13). In vitro assays have been used to model Schwann cell-astrocyte interactions and have been important in understanding the mechanism underlying the cellular behaviour. These in vitro assays include boundary assay, where a co-culture is made using two different cells with each cell type occupying different territories with only a small gap separating the two cell fronts. As the cells divide and migrate, the two cellular fronts get closer to each other and finally collide. This allows the behaviour of the two cellular populations to be analyzed at the boundary. Another variation of the same technique is to mix the two cellular populations in culture and over time the two cell types segregate with Schwann cells clumped together as islands in between astrocytes together creating multiple Schwann-astrocyte boundaries. The second assay used in studying the interaction of two cell types is the migration assay where cellular movement can be tracked on the surface of the other cell type monolayer (14,15). This assay is commonly known as inverted coverslip assay. Schwann cells are cultured on small glass fragments and they are inverted face down onto the surface of astrocyte monolayers and migration is assessed from the edge of coverslip. Both assays have been instrumental in studying the underlying mechanisms involved in the cellular exclusion and boundary formation. Some of the molecules identified using these techniques include N-Cadherins 15, Chondroitin Sulphate proteoglycans(CSPGs) (16,17), FGF/Heparin (18), Eph/Ephrins(19). This article intends to describe boundary assay and migration assay in stepwise fashion and elucidate the possible technical problems that might occur.

17β-estradiol protects Schwann cells against H2O2-induced cytotoxicity and increases transplanted Schwann cell survival in a cervical hemicontusion spinal cord injury model

Schwann cell (SC) transplantation is a promising repair strategy after spinal cord injury (SCI); however, a large number of SCs do not survive following transplantation. Previous studies have shown that 17β-estradiol (E2) protects several cell types against cytotoxicity. Thus, this study evaluated the protective potential of E2 on SCs in vitro and investigated the effect of E2 on transplanted SC survival in a rat model of SCI. Primary SC cultures were found to robustly express estrogen receptors (ER) and incubation with E2 protected SCs against hydrogen peroxide-induced cell death. This protection was not inhibited by the ER antagonist ICI 182,780, suggesting that genomic signaling is not necessary for protection. In a subsequent experiment, cervical hemicontusion SCI was induced in male rats followed by sustained administration of E2 or placebo. Eight days after SCI, SCs were transplanted into the injury epicenter. E2 treatment significantly increased the number of surviving labeled transplanted SCs evaluated 7 days after transplantation. These data demonstrate that E2 protects SCs against oxidative stress and improves transplanted SC survival, which suggests that E2 administration may be an intervention of choice for enhancing survival of transplanted SCs after SCI.

Trophic responsiveness of purified postnatal and adult rat retinal ganglion cells

Central neurons lose the ability for axonal re-growth during development and typically do not regenerate their axons following axotomy once they become mature unless given a growth-permissive environment i.e. peripheral nerve graft. In the present study, the growth responsiveness of purified retinal ganglion cells (RGCs) at different ages to neurotrophic factors and Schwann cell (SC)-secreted factors were examined directly. The purity of adult RGCs was 97% as assessed by retrograde labelling with 4,6-diamidino-2-phenylindole. The stability of cultures were demonstrated by long-term survival (30 days) in medium contained brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and forskolin (F) (BCF). RGCs from postnatal (P) (P0, P4, P8, P21) and adult (P90) rats showed decreasing levels of survival and neuritogenesis when grown in BCF. In contrast, the opposite was observed in SC-conditioned medium (CM)-treated P0-P8 RGCs which were increasingly responsive. SCCM induced maximal neurite outgrowth in P8 RGCs via the activation of extracellular regulated kinase 1/2 (Erk1/2). Inhibition of mitogen-activated protein kinase-Erk1/2 signaling using an Erk1/2-specific inhibitor (UO126) abolished SCCM-induced Erk1/2 phosphorylation and neuritogenesis completely. Although both SCCM and BCF failed to sustain the same levels of growth in P21 or P90 cultures as observed in P8 cultures, SCCM promoted higher survival and neuritogenesis than BCF-treated adult RGCs. This study is the first report of adult rat RGC purification and demonstrates that mature RGCs need multiple factors for survival and neurite outgrowth.

Synergistic effects of osteonectin and brain-derived neurotrophic factor on axotomized retinal ganglion cells neurite outgrowth via the mitogen-activated protein kinase-extracellular signal-regulated kinase 1/2 pathways

Our previous study identified osteonectin (ON) in a screen of factors made by Schwann cells (SCs) which promoted peripheral and central neurons survival and neuritogenesis, however, the mechanisms of ON promoting effects are largely unknown. In the present study, we investigated the effects of ON-deficient SC-conditioned medium (SCCM) and molecular mechanisms of ON, in regulating retinal ganglion cells (RGCs) survival and neurite outgrowth. Neonatal rat RGCs and SCs were purified by immunopanning technique. RGC survival and neuritogenesis reduced significantly when treated with either ON-null mice SCCM or ON-immunodepleted (IP) SCCM (P<0.05). In contrast to wild type SCCM, in the presence of a tyrosine kinase receptor (Trk) inhibitor (K252a), ON-null mice SCCM-induced neuritogenesis were further reduced by 24%. The Trk-mediated signaling pathways became more sensitive to K252a inhibition in the absence of ON. We also showed the synergistic effects of ON and brain-derived neurotrophic factor (BDNF) in promoting RGCs growth and the involvement of ON in two major neurotrophin-mediated signaling pathways, PI-3K-Akt and MAPK-Erk1/2. ON alone activated Akt phosphorylation and increased survival. Blockage of TrkB signalling pathway by TrkB-Fc chimera (BDNF scavenger) or K252a in ON-treated cultures reduced Akt-P level significantly. This suggests that ON induces BDNF synthesis and secretion from RGCs. The enhancement of neuritogenesis and Erk1/2 phosphorylation by ON in BDNF-treated cultures further demonstrate the signaling pathways responsible for the synergistic effect of ON on BDNF-induced neurite outgrowth. To the best of our knowledge, this is the first report showing the synergistic effects of ON on classical neurotrophins which participate in the same signalling pathways in regulating RGC neurite outgrowth.

Synergistic effects of osteonectin and NGF in promoting survival and neurite outgrowth of superior cervical ganglion neurons

Schwann cells (SCs) play a major role in the successful regeneration of peripheral nerves regeneration. Here we examined the effects of osteonectin (ON), a major factor secreted by SCs, on survival and neuritogenesis of mouse superior cervical ganglion (SCG) neurons. SC conditioned medium (SCCM) not only promoted the survival and neuritogenesis of SCG neurons at a level comparable to nerve growth factor (NGF) but also doubled the neurite length of NGF-treated SCG neurons. SCCM neuritogenic effects were not blocked by the tyrosine kinase receptor (Trk) inhibitor K252a demonstrating that these are not due solely to classical neurotrophic factors. Anti-ON neutralizing antibody diminished the SCCM-induced survival and neuritogenesis significantly. In the presence of K252a, the SCCM neuritogenic effects were blocked completely by anti-ON which suggests synergistic effects of ON with Trk-mediated growth factors. ON alone increased the survival and neurite outgrowth of SCG neurons significantly at high density cultures. ON at low concentration acts synergistically with NGF which induced maximum survival and neurite outgrowth (>50% increase). However, ON at high concentration was detrimental to survival (64% decrease) and neurite outgrowth (87% decrease) even in the presence of NGF. The well documented counter-adhesive effect of ON may account for this observation. Nevertheless, the growth promoting effects of ON became more pronounced as the cell density increased which suggests a possible interaction of ON with growth factors secreted by SCG neurons (autocrine or paracrine effects). Taken together, our study indicates that ON plays important roles in nervous system repair through its synergistic effects with growth factors.

Different factors promote axonal regeneration of adult rat retinal ganglion cells after lens injury and intravitreal peripheral nerve grafting

We have investigated the differential mediators of the neurotrophic effects of intravitreal peripheral nerve grafting and lens injury on adult rat retinal ganglion cells (RGC). Lens injury and intravitreal peripheral nerve grafting both stimulated RGC neurite growth in vitro and axon regeneration past the optic nerve lesion site in vivo concomitant with activation of retinal glia and invasion of macrophages into the eye. These observations, together with the results of coculture studies using a macrophage-free intact peripheral nerve segment, a macrophage-free intact lens, a macrophage-rich peripheral nerve segment, or a macrophage-rich injured lens in retinal cultures suggest that the stimulation of RGC axon regeneration by lens injury and intravitreal peripheral nerve grafting share a common macrophage-derived component overlain by distinct lens-derived and peripheral nerve-derived neurotrophic factors, respectively. RGC axon regeneration following lens injury and intravitreal peripheral nerve grafting was similar in vivo, correlating with similar retinal glia activation whereas, in vitro, the level of RGC neurite outgrowth was significantly higher following intravitreal peripheral nerve grafting compared with lens injury, concomitant with the presence of increased numbers of activated retinal glia. This suggests that in vivo RGC axon regeneration induced by lens injury and peripheral nerve grafting may be limited, in part, by factors derived from activated retinal glia.

Schwann cell: A source of neurotrophic activity on cortical glutamatergic neurons in culture

Glial cells secrete numerous soluble molecules that enhance the development and the survival of different neuronal types cultured in vitro. Schwann cells (SC) play an important role as they are the source of different trophic substances and present a great neurotrophic activity. The aim of this study is to investigate the influence of postnatal SC on embryonic glutamatergic neurons. Co-cultures of SC from sciatic nerve of postnatal rats and neurons from rat embryonic cerebral cortex were successfully established, and cells were immunocytochemically characterized using mono and polyclonal antibodies as different glial and neuronal markers. Furthermore, some neuronal cultures were added with Nerve Growth Factor (NGF) and Insulin-like Growth Factor (IGF) to compare to co-cultures. Our results show that SC promote an increase in the number of glutamatergic cortical neurons; moreover, these neurons present an evidence of dense axonal and dendritic outgrowth even when were fed with conditioned medium obtained from SC cultures. In conclusion, our data suggest that substances produced by SC exert a positive effect on central neuron survival and differentiation as indicated by processes of elongation and that this activity is mediated by soluble factors. Therefore, it is possible to consider the SC as a source of growth factors and might be suitable for the development of a neuroprotective effect in neurodegenerative disorders.

Tissue-engineering approaches for axonal guidance

Owing to the profound impact of nervous system damage, extensive studies have been carried out aimed at facilitating axonal regeneration following injury. Tissue engineering, as an emerging and rapidly growing field, has received extensive attention for nervous system axonal guidance. Numerous engineered substrates containing oriented extracellular matrix molecules, cells or channels have displayed potential of supporting axonal regeneration and functional recovery. Most attempts are focused on seeking new biomaterials, new cell sources, as well as novel designs of tissue-engineered neuronal bridging devices, to generate safer and more efficacious neuronal tissue repairs.