Nanofibrous collagen nerve conduits for spinal cord repair

Cellulose acetate/poly(vinyl alcohol) hybrid fibrous mat containing tetracycline hydrochloride and phenytoin sodium: Morphology, drug release, antibacterial, and cell culture studies

The objective of this study was to introduce an electrospun hybrid fibrous mat (a dual-fiber drug delivery system) based on cellulose acetate and poly(vinyl alcohol) containing tetracycline hydrochloride and phenytoin sodium, respectively. Characterization of samples was carried by morphology, drug release, cell cytotoxicity, adhesion, antibacterial property, and wettability investigations. The results showed a uniform shape and a narrow diameter distribution of fibers (between 160 ± 20 nm) for fabricated cellulose acetate/poly(vinyl alcohol) hybrid fibrous mat. The tetracycline hydrochloride release from cellulose acetate significantly decreased due to gel formation of poly(vinyl alcohol) in aqueous media. The best fit for drug release kinetic of hybrid sample was Higuchi model. Sample with tetracycline hydrochloride and phenytoin sodium drugs showed improved cell growth, viability, and antibacterial activity against Escherichia coli (~89%) and Staphylococcus aureus (~98%) in comparison with sample without drugs. The hydrophilic property of cellulose acetate/poly(vinyl alcohol) fibrous sample containing the drugs was also remarkable (~45°). To consider the obtained results, the presented hybrid fibrous mat shows a high potent for biomedical applications.

Using biomaterials to promote pro-regenerative glial phenotypes after nervous system injuries

Trauma to either the central or peripheral nervous system (PNS) often leads to significant loss of function and disability in patients. This high rate of long-term disability is due to the overall limited regenerative potential of nervous tissue, even though the PNS has more regenerative potential than the central nervous system (CNS). The supporting glial cells in the periphery, Schwann cells, are part of the reason for the improved recovery observed in the PNS. In the CNS, the glial populations, astrocytes and oligodendrocytes (OLs), do not have as much potential to promote regeneration and are at times inhibitory to neuronal growth. In particular, the inhibitory roles astrocytes play following trauma has led to a historical focus on neurons and OLs instead of astrocytes. Recently, this focus has shifted as new, regenerative astrocyte phenotypes have been described. From these observations, glial cells clearly play critical roles in native recovery pathways in both the CNS and PNS. This makes the ability to manipulate both transplanted and native glial cell phenotypes a potentially successful strategy to improve nerve injury outcomes. This review focuses on factors that cause glial cells to adopt repair phenotypes and biomaterials that manipulate and/or harness these glial phenotypes.

Nanopatterned Scaffolds for Neural Tissue Engineering and Regenerative Medicine

Biologically inspired approaches employing nanoengineering techniques have been influential in the progress of neural tissue repair and regeneration. Neural tissues are exposed to complex nanoscale environments such as nanofibrils. In this chapter, we summarize representative nanotechniques, such as electrospinning, lithography, and 3D bioprinting, and their use in the design and fabrication of nanopatterned scaffolds for neural tissue engineering and regenerative medicine. Nanotopographical cues in combination with other cues (e.g., chemical cues) are crucial to neural tissue repair and regeneration using cells, including various types of stem cells. Production of biologically inspired nanopatterned scaffolds may encourage the next revolution for studies aiming to advance neural tissue engineering and regenerative medicine.

Taxol-modified collagen scaffold implantation promotes functional recovery after long-distance spinal cord complete transection in canines

LOCS + Taxol implantation, a promising treatment for acute spinal cord injury, promotes endogenous neurogenesis, axon regeneration and locomotion recovery.

Transplantation of Schwann Cells Inside PVDF-TrFE Conduits to Bridge Transected Rat Spinal Cord Stumps to Promote Axon Regeneration Across the Gap

Among various models for spinal cord injury in rats, the contusion model is the most often used because it is the most common type of human spinal cord injury. The complete transection model, although not as clinically relevant as the contusion model, is the most rigorous method to evaluate axon regeneration. In the contusion model, it is difficult to distinguish regenerated from sprouted or spared axons due to the presence of remaining tissue post injury. In the complete transection model, a bridging method is necessary to fill the gap and create continuity from the rostral to the caudal stumps in order to evaluate the effectiveness of the treatments. A reliable bridging surgery is essential to test outcome measures by reducing the variability due to the surgical method. The protocols described here are used to prepare Schwann cells (SCs) and conduits prior to transplantation, complete transection of the spinal cord at thoracic level 8 (T8), insert the conduit, and transplant SCs into the conduit. This approach also uses in situ gelling of an injectable basement membrane matrix with SC transplantation that allows improved axon growth across the rostral and caudal interfaces with the host tissue.

3D-Printed pHEMA Materials for Topographical and Biochemical Modulation of Dorsal Root Ganglion Cell Response

Understanding and controlling the interactions occurring between cells and engineered materials are central challenges toward progress in the development of biomedical devices. In this work, we describe materials for direct ink writing (DIW), an extrusion-based type of 3D printing, that embed a custom synthetic protein (RGD-PDL) within the microfilaments of 3D-hydrogel scaffolds to modify these interactions and differentially direct tissue-level organization of complex cell populations in vitro. The RGD-PDL is synthesized by modifying poly-d-lysine (PDL) to varying extents with peptides containing the integrin-binding motif Arg-Gly-Asp (RGD). Compositional gradients of the RGD-PDL presented by both patterned and thin-film poly(2-hydroxyethyl) methacrylate (pHEMA) substrates allow the patterning of cell-growth compliance in a grayscale form. The surface chemistry-dependent guidance of cell growth on the RGD-PDL-modified pHEMA materials is demonstrated using a model NIH-3T3 fibroblast cell line. The formation of a more complex cellular system-organotypic primary murine dorsal root ganglion (DRG)-in culture is also achieved on these scaffolds, where distinctive forms of cell growth and migration guidance are seen depending on their RGD-PDL content and topography. This experimental platform for the study of physicochemical factors on the formation and the reorganization of organotypic cultures offers useful capabilities for studies in tissue engineering, regenerative medicine, and diagnostics.

Astrocyte spreading and migration on aggrecan-laminin dot gradients

The surface concentration gradient of two extracellular matrix (ECM) macromolecules was developed to study the migratory and morphological responses of astrocytes to molecular cues typically found in the central nervous system injury environment. The gradient, prepared using microcontact printing, was composed of randomly positioned micrometer-sized dots of aggrecan (AGG) printed on a substrate uniformly coated with laminin (LN). AGG dots were printed in an increasing number along the 1000 μm long and 50 μm wide gradient area which had on each end either a full surface coverage of AGG or LN. Each dot gradient was surrounded by a 100 μm-wide uniform field of AGG printed over laminin. Seeded astrocytes were found to predominantly attach to LN regions on the gradient. Cellular extensions of these cells were longer than the similar processes for cells seeded on uniform substrates of AGG or LN serving as controls. Astrocyte extensions were the largest and spanned a distance of 150 μm when the cells were attached to the mixed AGG+LN patches on the gradient. As evidenced by their increased area and perimeter, the cells extended processes in a stellate fashion upon initial attachment and maintained extensions when seeded in AGG+LN regions but not on uniform laminin controls. The cells migrated short distances, ∼20-35 μm, over 24 h and in doing so preferentially shifted from AGG areas to higher LN surface coverage regions. The results indicated that presenting mixed ECM cues caused astrocytes to sample larger areas of the substrate and made the cells to preferentially relocate to a more permissive ECM region.

Nanofiber Scaffolds as Drug Delivery Systems to Bridge Spinal Cord Injury

The complex pathophysiology of spinal cord injury (SCI) may explain the current lack of an effective therapeutic approach for the regeneration of damaged neuronal cells and the recovery of motor functions. A primary mechanical injury in the spinal cord triggers a cascade of secondary events, which are involved in SCI instauration and progression. The aim of the present review is to provide an overview of the therapeutic neuro-protective and neuro-regenerative approaches, which involve the use of nanofibers as local drug delivery systems. Drugs released by nanofibers aim at preventing the cascade of secondary damage (neuro-protection), whereas nanofibrous structures are intended to re-establish neuronal connectivity through axonal sprouting (neuro-regeneration) promotion, in order to achieve a rapid functional recovery of spinal cord.

Biomaterials for Local, Controlled Drug Delivery to the Injured Spinal Cord

Affecting approximately 17,000 new people each year, spinal cord injury (SCI) is a devastating injury that leads to permanent paraplegia or tetraplegia. Current pharmacological approaches are limited in their ability to ameliorate this injury pathophysiology, as many are not delivered locally, for a sustained duration, or at the correct injury time point. With this review, we aim to communicate the importance of combinatorial biomaterial and pharmacological approaches that target certain aspects of the dynamically changing pathophysiology of SCI. After reviewing the pathophysiology timeline, we present experimental biomaterial approaches to provide local sustained doses of drug. In this review, we present studies using a variety of biomaterials, including hydrogels, particles, and fibers/conduits for drug delivery. Subsequently, we discuss how each may be manipulated to optimize drug release during a specific time frame following SCI. Developing polymer biomaterials that can effectively release drug to target specific aspects of SCI pathophysiology will result in more efficacious approaches leading to better regeneration and recovery following SCI.

Design and criteria of electrospun fibrous scaffolds for the treatment of spinal cord injury

The complex pathophysiology of spinal cord injury may explain the current lack of an effective therapeutic approach for the regeneration of damaged neuronal cells and the recovery of motor functions. Many efforts have been performed to design and develop suitable scaffolds for spinal cord regeneration, keeping in mind that the reconstruction of a pro-regenerative environment is the key challenge for an effective neurogenesis. The aim of this review is to outline the main features of an ideal scaffold, based on biomaterials, produced by the electrospinning technique and intended for the spinal cord regeneration. An overview of the polymers more investigated in the production of neural fibrous scaffolds is also provided.

Electrospun Fibers for Drug Delivery after Spinal Cord Injury and the Effects of Drug Incorporation on Fiber Properties

There is currently no cure for individuals with spinal cord injury (SCI). While many promising approaches are being tested in clinical trials, the complexity of SCI limits several of these approaches from aiding complete functional recovery. Several different categories of biomaterials are investigated for their ability to guide axonal regeneration, to deliver proteins or small molecules locally, or to improve the viability of transplanted stem cells. The purpose of this study is to provide a brief overview of SCI, present the different categories of biomaterial scaffolds that direct and guide axonal regeneration, and then focus specifically on electrospun fiber guidance scaffolds. Much like other polymer guidance approaches, electrospun fibers can retain and deliver therapeutic drugs. The experimental section presents new data showing the incorporation of two therapeutic drugs into electrospun poly-L-lactic acid fibers. Two different concentrations of either riluzole or neurotrophin-3 were loaded into the electrospun fibers to examine the effect of drug concentration on the physical characteristics of the fibers (fiber alignment and fiber diameter). Overall, the drugs were successfully incorporated into the fibers and the release was related to the loading concentration. The fiber diameter decreased with the inclusion of the drug, and the decreased diameter was correlated with a decrease in fiber alignment. Subsequently, the study includes considerations for successful incorporation of a therapeutic drug without changing the physical properties of the fibers.

Extracellular matrix components as therapeutics for spinal cord injury

There is no treatment for people with spinal cord injury that leads to significant functional improvements. The extracellular matrix is an intricate, 3-dimensional, structural framework that defines the environment for cells in the central nervous system. The components of extracellular matrix have signaling and regulatory roles in the fate and function of neuronal and non-neuronal cells in the central nervous system. This review discusses the therapeutic potential of extracellular matrix components for spinal cord repair.

Electrospun Fibers for Spinal Cord Injury Research and Regeneration

Electrospinning is the process by which a scaffold containing micrometer and nanometer diameter fibers are drawn from a polymer solution or melt using a large voltage gradient between a polymer emitting source and a grounded collector. Ramakrishna and colleagues first investigated electrospun fibers for neural applications in 2004. After this initial study, electrospun fibers are increasingly investigated for neural tissue engineering applications. Electrospun fibers robustly support axonal regeneration within in vivo rodent models of spinal cord injury. These findings suggest the possibility of their eventual use within patients. Indeed, both spinal cord and peripheral nervous system regeneration research over the last several years shows that physical guidance cues induce recovery of limb, respiration, or bladder control in rodent models. Electrospun fibers may be an alternative to the peripheral nerve graft (PNG), because PNG autografts injure the patient and are limited in supply, and allografts risk host rejection. In addition, electrospun fibers can be engineered easily to confront new therapeutic challenges. Fibers can be modified to release therapies locally or can be physically modified to direct neural stem cell differentiation. This review summarizes the major findings and trends in the last decade of research, with a particular focus on spinal cord injury. This review also demonstrates how electrospun fibers can be used to study the central nervous system in vitro.

Regeneration strategies after the adult mammalian central nervous system injury-biomaterials

The central nervous system (CNS) has very restricted intrinsic regeneration ability under the injury or disease condition. Innovative repair strategies, therefore, are urgently needed to facilitate tissue regeneration and functional recovery. The published tissue repair/regeneration strategies, such as cell and/or drug delivery, has been demonstrated to have some therapeutic effects on experimental animal models, but can hardly find clinical applications due to such methods as the extremely low survival rate of transplanted cells, difficulty in integrating with the host or restriction of blood–brain barriers to administration patterns. Using biomaterials can not only increase the survival rate of grafts and their integration with the host in the injured CNS area, but also sustainably deliver bioproducts to the local injured area, thus improving the microenvironment in that area. This review mainly introduces the advances of various strategies concerning facilitating CNS regeneration.

Application of Collagen Scaffold in Tissue Engineering: Recent Advances and New Perspectives

Collagen is the main structural protein of most hard and soft tissues in animals and the human body, which plays an important role in maintaining the biological and structural integrity of the extracellular matrix (ECM) and provides physical support to tissues. Collagen can be extracted and purified from a variety of sources and offers low immunogenicity, a porous structure, good permeability, biocompatibility and biodegradability. Collagen scaffolds have been widely used in tissue engineering due to these excellent properties. However, the poor mechanical property of collagen scaffolds limits their applications to some extent. To overcome this shortcoming, collagen scaffolds can be cross-linked by chemical or physical methods or modified with natural/synthetic polymers or inorganic materials. Biochemical factors can also be introduced to the scaffold to further improve its biological activity. This review will summarize the structure and biological characteristics of collagen and introduce the preparation methods and modification strategies of collagen scaffolds. The typical application of a collagen scaffold in tissue engineering (including nerve, bone, cartilage, tendon, ligament, blood vessel and skin) will be further provided. The prospects and challenges about their future research and application will also be pointed out.

Bicomponent electrospun scaffolds to design extracellular matrix tissue analogs

In the last decade, bicomponent fibers have been proposed to fabricate bio-inspired systems for tissue repair, regenerative medicine, medical healthcare and clinical applications. In comparison with monocomponent fibers, key advantage concerns their ability of self-adapting to the physiological conditions through an extended pattern of signals--morphological, chemical and physical ones--confined at the single fiber level. Hydrophobic/hydrophilic phases may be variously organized by tuneable processing modes (i.e., blending, core/shell, interweaving) thus offering different benefits in terms of biological activity, fluid sorption and molecular transport properties (first generation). The possibility to efficiently graft cell-adhesive proteins and peptide sequences onto the fiber surface mediated by spacers or impregnating hydrogels allows to trigger cell late activities by a controlled and sustained release in vitro of specific biomolecules (i.e., morphogens, growth factors). Here, we introduce an overview of current approaches based on bicomponent fiber use as extra cellular matrix analogs with cell-instructive functions and hierarchal organization of living tissues.

Magnetic NGF-releasing PLLA/iron oxide nanoparticles direct extending neurites and preferentially guide neurites along aligned electrospun microfibers

Nerve growth factor releasing composite nanoparticles (NGF-cNPs) were developed to direct the extension of neurite outgrowth from dorsal root ganglia (DRG). Iron oxide magnetic nanoparticles were incorporated into poly-l-lactic acid (PLLA) nanoparticles in order to position the NGF-cNPs in a culture dish. Neurites growing from DRG extended toward the NGF released from the NGF-cNPs. DRG were then cultured on aligned PLLA microfibers in the presence of NGF-cNPs, and these biomaterials combined to align DRG neurite extension along one axis and preferentially toward the NGF-cNPs. This combinatorial biomaterial approach shows promise as a strategy to direct the extension of regenerating neurites.

Intrinsic and extrinsic determinants of central nervous system axon outgrowth into alginate-based anisotropic hydrogels

Appropriate target reinnervation and functional recovery after spinal cord injury depend on longitudinally directed regrowth of injured axons. Anisotropic alginate-based capillary hydrogels (ACH) support peripheral nervous system derived axon growth, which is accompanied by glial supporting cell migration into the ACH. The aim of the present study was to analyze central nervous system (CNS) derived (entorhinal cortex, spinal cord slice cultures) axon regrowth into ACH containing linearly aligned capillaries of defined capillary sizes without and with gelatin constituent. Anisotropic ACH were prepared by ionotropic gel formation using Ba(2+), Cu(2+), Sr(2+), or Zn(2+) ions resulting in gels with average capillary diameters of 11, 13, 29, and 89μm, respectively. Postnatal rat entorhinal cortex or spinal cord slice cultures were placed on top of 500μm thick ACH. Seven days later axon growth and astroglial migration into the ACH were determined. Axon density within capillaries correlated positively with increasing capillary diameters, whereas longitudinally oriented axon outgrowth diminished with increasing capillary diameter. Axons growing into the hydrogels were always accompanied by astrocytes strongly suggesting that respective cells are required to mediate CNS axon elongation into ACH. Overall, midsize capillary diameter ACH appeared to be the best compromise between axon density and orientation. Taken together, ACH promote CNS axon ingrowth, which is determined by the capillary diameter and migration of slice culture derived astroglia into the hydrogel.

STATEMENT OF SIGNIFICANCE

Biomaterials are investigated as therapeutic tools to bridge irreversible lesions following traumatic spinal cord injury. The goal is to develop biomaterials, which promote longitudinally oriented regeneration of as many injured axons as possible as prerequisite for substantial functional recovery. Optimal parameters of the biomaterial have yet to be defined. In the present study we show that increasing capillary diameters within such hydrogels enhanced central nervous system axon regeneration at the expense of longitudinal orientation. Axon ingrowth into the hydrogels was only observed in the presence of glial supporting cells, namely astrocytes. This suggests that alginate-based hydrogels need to be colonized with respective cells in order to facilitate axon ingrowth.

The Effect of Surface Modification of Aligned Poly-L-Lactic Acid Electrospun Fibers on Fiber Degradation and Neurite Extension

The surface of aligned, electrospun poly-L-lactic acid (PLLA) fibers was chemically modified to determine if surface chemistry and hydrophilicity could improve neurite extension from chick dorsal root ganglia. Specifically, diethylenetriamine (DTA, for amine functionalization), 2-(2-aminoethoxy)ethanol (AEO, for alcohol functionalization), or GRGDS (cell adhesion peptide) were covalently attached to the surface of electrospun fibers. Water contact angle measurements revealed that surface modification of electrospun fibers significantly improved fiber hydrophilicity compared to unmodified fibers (p < 0.05). Scanning electron microscopy (SEM) of fibers revealed that surface modification changed fiber topography modestly, with DTA modified fibers displaying the roughest surface structure. Degradation of chemically modified fibers revealed no change in fiber diameter in any group over a period of seven days. Unexpectedly, neurites from chick DRG were longest on fibers without surface modification (1651 ± 488 μm) and fibers containing GRGDS (1560 ± 107 μm). Fibers modified with oxygen plasma (1240 ± 143 μm) or DTA (1118 ± 82 μm) produced shorter neurites than the GRGDS or unmodified fibers, but were not statistically shorter than unmodified and GRGDS modified fibers. Fibers modified with AEO (844 ± 151 μm) were significantly shorter than unmodified and GRGDS modified fibers (p<0.05). Based on these results, we conclude that fiber hydrophilic enhancement alone on electrospun PLLA fibers does not enhance neurite outgrowth. Further work must be conducted to better understand why neurite extension was not improved on more hydrophilic fibers, but the results presented here do not recommend hydrophilic surface modification for the purpose of improving neurite extension unless a bioactive ligand is used.

Bridging the lesion-engineering a permissive substrate for nerve regeneration

Biomaterial-based strategies to restore connectivity after lesion at the spinal cord are focused on bridging the lesion and providing an favourable substrate and a path for axonal re-growth. Following spinal cord injury (SCI) a hostile environment for neuronal cell growth is established by the activation of multiple inhibitory mechanisms that hamper regeneration to occur. Implantable scaffolds can provide mechanical support and physical guidance for axon re-growth and, at the same time, contribute to alleviate the hostile environment by the in situ delivery of therapeutic molecules and/or relevant cells. Basic research on SCI has been contributing with the description of inhibitory mechanisms for regeneration as well as identifying drugs/molecules that can target inhibition. This knowledge is the background for the development of combined strategies with biomaterials. Additionally, scaffold design is significantly evolving. From the early simple hollow conduits, scaffolds with complex architectures that can modulate cell fate are currently being tested. A number of promising pre-clinical studies combining scaffolds, cells, drugs and/or nucleic acids are reported in the open literature. Overall, it is considered that to address the multi-factorial inhibitory environment of a SCI, a multifaceted therapeutic approach is imperative. The progress in the identification of molecules that target inhibition after SCI and its combination with scaffolds and/or cells are described and discussed in this review.

Electrospun Collagen Fibers with Spatial Patterning of SDF1α for the Guidance of Neural Stem Cells

Producing gradients of biological cues into nerve conduits is crucial for nerve guidance and regeneration. Herein, the fabrication of gradients of stromal cell-derived factor-1α (SDF1α) on electrospun collagen mats is reported using an electrohydrodynamic jet printing technique. The fabrication of various SDF1α gradated patterns on collagen fibrous mats is successfully demonstrated including shallow continuous gradient, steep continuous gradient, and step gradient by controlling the processing parameters. The SDF1α graded collagen scaffolds show a long-term stable gradient, as SDF1α is fused with a unique peptide of collagen binding domain (CBD), and CBD-SDF1α can specifically bind to the collagen mat. Such graded scaffolds exhibit sustained release of SDF1α. Further examination of neural stem cell (NSC) response to the CBD-SDF1α gradients with various patterns show that the NSCs can sense the CBD-SDF1α gradients, display a polarized morphology, and tend to migrate toward the region with a higher CBD-SDF1α content. The collagen mats with CBD-SDF1α gradients guide gradual distribution of NSCs, and NSC-differentiated neurons and astrocytes after seeding for 1 and 7 d. This new class of CBD-SDF1α gradient scaffolds can potentially be employed for guided nerve regeneration.

The role of the surface on microglia function: implications for central nervous system tissue engineering

In tissue engineering, it is well accepted that a scaffold surface has a decisive impact on cell behaviour. Here we focused on microglia-the resident immune cells of the central nervous system (CNS)-and on their response to poly(trimethylene carbonate-co-ε-caprolactone) (P(TMC-CL)) fibrous and flat surfaces obtained by electrospinning and solvent cast, respectively. This study aims to provide cues for the design of instructive surfaces that can contribute to the challenging process of CNS regeneration. Cell morphology was evidently affected by the substrate, mirroring the surface main features. Cells cultured on flat substrates presented a round shape, while cells with elongated processes were observed on the electrospun fibres. A higher concentration of the pro-inflammatory cytokine tumour necrosis factor-α was detected in culture media from microglia on fibres. Still, astrogliosis is not exacerbated when astrocytes are cultured in the presence of microglia-conditioned media obtained from cultures in contact with either substrate. Furthermore, a significant percentage of microglia was found to participate in the process of myelin phagocytosis, with the formation of multinucleated giant cells being observed only on films. Altogether, the results presented suggest that microglia in contact with the tested substrates may contribute to the regeneration process, putting forward P(TMC-CL) substrates as supporting matrices for nerve regeneration.

Advances in regenerative therapies for spinal cord injury: a biomaterials approach

Spinal cord injury results in the permanent loss of function, causing enormous personal, social and economic problems. Even though neural regeneration has been proven to be a natural mechanism, central nervous system repair mechanisms are ineffective due to the imbalance of the inhibitory and excitatory factors implicated in neuroregeneration. Therefore, there is growing research interest on discovering a novel therapeutic strategy for effective spinal cord injury repair. To this direction, cell-based delivery strategies, biomolecule delivery strategies as well as scaffold-based therapeutic strategies have been developed with a tendency to seek for the answer to a combinatorial approach of all the above. Here we review the recent advances on regenerative/neural engineering therapies for spinal cord injury, aiming at providing an insight to the most promising repair strategies, in order to facilitate future research conduction.

Nanofiber-mediated microRNA delivery to enhance differentiation and maturation of oligodendroglial precursor cells

Remyelination in the central nervous system (CNS) is critical in the treatment of many neural pathological conditions. Unfortunately, the ability to direct and enhance oligodendrocyte (OL) differentiation and maturation remains limited. It is known that microenvironmental signals, such as substrate topography and biochemical signaling, regulate cell fate commitment. Therefore, in this study, we developed a nanofiber-mediated microRNA (miR) delivery method to control oligodendroglial precursor cell (OPC) differentiation through a combination of fiber topography and gene silencing. Using poly(ε-caprolactone) nanofibers, efficient knockdown of OL differentiation inhibitory regulators were achieved by either nanofiber alone (20-40%, p<0.05) or the synergistic integration with miR-219 and miR-338 (up to 60%, p<0.05). As compared to two-dimensional culture, nanofiber topography enhanced OPC differentiation by inducing 2-fold increase in RIP(+) cells (p<0.01) while the presence of miRs further enhanced the result to 3-fold (p<0.001). In addition, nanofiber-mediated delivery of miR-219 and miR-338 promoted OL maturation by increasing the number of MBP(+) cells significantly (p<0.01). Taken together, the results demonstrate the efficacy of nanofibers in providing topographical cues and microRNA reverse transfection to direct OPC differentiation. Such scaffolds may find useful applications in directing oligodendrocyte differentiation and myelination for treatment of CNS pathological conditions that require remyelination.

New trends in spinal cord tissue engineering

ABSTRACT 

Restoration of lost neuronal function after spinal cord injury still remains a considerable challenge for current medicine. Over the last decade, regenerative medicine has recorded rapid and promising advancements in stem cell research, genetic engineering and the progression of new sophisticated biomaterials as well as nanotechnology. This advancement has also been reflected in neural tissue engineering, where, along with the development of a new generation of well-designed biopolymer scaffolds, multifactorial therapeutic strategies are being validated in order to determine the greatest possible repair efficacy of the complex CNS pathophysiology. Much attention is currently focused on the designing of multifunctional polymer scaffolds as systems for targeted drug or gene delivery, electrical stimulation or as substrates creating a special micro-environment, promoting the growth and desired differentiation of various cell lines. In this review, the latest advances in biomaterial technology together with various combinatorial strategies designed to treat spinal cord injury treatment are summarized and discussed.

Nebulized solvent ablation of aligned PLLA fibers for the study of neurite response to anisotropic-to-isotropic fiber/film transition (AFFT) boundaries in astrocyte-neuron co-cultures

Developing robust in vitro models of in vivo environments has the potential to reduce costs and bring new therapies from the bench top to the clinic more efficiently. This study aimed to develop a biomaterial platform capable of modeling isotropic-to-anisotropic cellular transitions observed in vivo, specifically focusing on changes in cellular organization following spinal cord injury. In order to accomplish this goal, nebulized solvent patterning of aligned, electrospun poly-l-lactic acid (PLLA) fiber substrates was developed. This method produced a clear topographic transitional boundary between aligned PLLA fibers and an isotropic PLLA film region. Astrocytes were then seeded on these scaffolds, and a shift between oriented and non-oriented astrocytes was created at the anisotropic-to-isotropic fiber/film transition (AFFT) boundary. Orientation of chondroitin sulfate proteoglycans (CSPGs) and fibronectin produced by these astrocytes was analyzed, and it was found that astrocytes growing on the aligned fibers produced aligned arrays of CSPGs and fibronectin, while astrocytes growing on the isotropic film region produced randomly-oriented CSPG and fibronectin arrays. Neurite extension from rat dissociated dorsal root ganglia (DRG) was studied on astrocytes cultured on anisotropic, aligned fibers, isotropic films, or from fibers to films. It was found that neurite extension was oriented and longer on PLLA fibers compared to PLLA films. When dissociated DRG were cultured on the astrocytes near the AFFT boundary, neurites showed directed orientation that was lost upon growth into the isotropic film region. The AFFT boundary also restricted neurite extension, limiting the extension of neurites once they grew from the fibers and into the isotropic film region. This study reveals the importance of anisotropic-to-isotropic transitions restricting neurite outgrowth by itself. Furthermore, we present this scaffold as an alternative culture system to analyze neurite response to cellular boundaries created following spinal cord injury and suggest its usefulness to study cellular responses to any aligned-to-unorganized cellular boundaries seen in vivo.

Astrocytes alignment and reactivity on collagen hydrogels patterned with ECM proteins

To modulate the surface properties of collagen and subsequent cell-surface interactions, a method was developed to transfer protein patterns from glass coverslips to collagen type I hydrogel surfaces. Two proteins and one proteoglycan found in central nervous system extracellular matrix as well as fibrinogen were patterned in stripes onto collagen hydrogel and astrocytes were cultured on these surfaces. The addition of the stripe protein patterns to hydrogels created astrocyte layers in which cells were aligned with underlying patterns and had reduced chondroitin sulfate expression compared to the cells grown on collagen alone. Protein patterns were covalently cross-linked to the collagen and stable over four days in culture with no visible cellular modifications. The present method can be adapted to transfer other types of protein patterns from glass coverslips to collagen hydrogels.

Method to form a fiber/growth factor dual-gradient along electrospun silk for nerve regeneration

Concentration gradients of guidance molecules influence cell behavior and growth in biological tissues and are therefore of interest for the design of biomedical scaffolds for regenerative medicine. We developed an electrospining method to generate a dual-gradient of bioactive molecules and fiber density along electrospun nanofibers without any post spinning treatment. Functionalization with fluorescent molecules demonstrated the efficiency of the method to generate a discontinuous concentration gradient along the aligned fibers. As a proof of concept for tissue engineering, the silk nanofibers were functionalized with increasing concentrations of nerve growth factor (NGF) and the biological activity was assessed and quantified with rat dorsal root ganglion (DRG) neurons cultures. Protein assays showed the absence of passive release of NGF from the functionalized fibers. The results demonstrated that the NGF concentration gradient led to an oriented and increased growth of DRG neurons (417.6 ± 55.7 μm) compared to a single uniform NGF concentration (264.5 ± 37.6 μm). The easy-to-use electrospinning technique combined with the multiple molecules that can be used for fiber functionalization makes this technique versatile for a broad range of applications from biosensors to regenerative medicine.

Biologic scaffold for CNS repair

Injury to the CNS typically results in significant morbidity and endogenous repair mechanisms are limited in their ability to restore fully functional CNS tissue. Biologic scaffolds composed of individual purified components have been shown to facilitate functional tissue reconstruction following CNS injury. Extracellular matrix scaffolds derived from mammalian tissues retain a number of bioactive molecules and their ability for CNS repair has recently been recognized. In addition, novel biomaterials for dural mater repairs are of clinical interest as the dura provides barrier function and maintains homeostasis to CNS. The present article describes the application of regenerative medicine principles to the CNS tissues and dural mater repair. While many approaches have been exploring the use of cells and/or therapeutic molecules, the strategies described herein focus upon the use of extracellular matrix scaffolds derived from mammalian tissues that are free of cells and exogenous factors.

Directed neurite growth of rat dorsal root ganglion neurons and increased colocalization with Schwann cells on aligned poly(methyl methacrylate) electrospun nanofibers

Electrospun nanofibers are promising scaffolds for peripheral and central nervous system repair. The aim of this study was to examine the details of neurite growth of rat dorsal root ganglion neurons (DRGn) on randomly oriented and aligned poly(methyl methacrylate) (PMMA) nanofibers and the relationship between neurites and nanofibers on each substrate. Our substrate design involved electrospinning PMMA nanofibers directly onto bare glass coverslips with acceptable biocompatibility. We cocultured DRGn and Schwann cells on PMMA nanofibers and evaluated their response to each substrate. Compared with neurons cultured on PMMA film and randomly oriented nanofibers, DRGn on aligned PMMA nanofibers formed longer, parallel neurites in accordance with the orientation of the substrate nanofibers, although the average neurite number did not differ among the three groups. Regarding the relationship between neurites and nanofibers, the neurites of DRGn were in close contact with the substrate nanofibers, and the neurites seemed to follow aligned nanofibers more than randomly oriented nanofibers. Coculturing DRGn and Schwann cells on PMMA nanofibers revealed that on aligned nanofibers, neurites and Schwann cells had a higher chance of colocalization than on randomly oriented nanofibers or film; this colocalization may be beneficial during the process of myelination that follows. The results of this study enhance our understanding of the ability of aligned electrospun nanofibers to provide contact guidance to neural cells and strengthen the rationale for future in vivo studies.

Tissue engineering is a promising method for the repair of spinal cord injuries (Review)

Spinal cord injury (SCI) may lead to a devastating and permanent loss of neurological function, which may place a great economic burden on the family of the patient and society. Methods for reducing the death of neuronal cells, inhibiting immune and inflammatory reactions, and promoting the growth of axons in order to build up synapses with the target cells are the focus of current research. Target cells are located in the damaged spinal cord which create a connect with the scaffold. As tissue engineering technology is developed for use in a variety of different areas, particularly the biomedical field, a clear understanding of the mechanisms of tissue engineering is important. This review establishes how this technology may be used in basic experiments with regard to SCI and considers its potential future clinical use.

Enhanced GLT-1 mediated glutamate uptake and migration of primary astrocytes directed by fibronectin-coated electrospun poly-L-lactic acid fibers

Bioengineered fiber substrates are increasingly studied as a means to promote regeneration and remodeling in the injured central nervous system (CNS). Previous reports largely focused on the ability of oriented scaffolds to bridge injured regions and direct outgrowth of axonal projections. In the present work, we explored the effects of electrospun microfibers on the migration and physiological properties of brain astroglial cells. Primary rat astrocytes were cultured on either fibronectin-coated poly-L-lactic acid (PLLA) films, fibronectin-coated randomly oriented PLLA electrospun fibers, or fibronectin-coated aligned PLLA electrospun fibers. Aligned PLLA fibers strongly altered astrocytic morphology, orienting cell processes, actin microfilaments, and microtubules along the length of the fibers. On aligned fibers, astrocytes also significantly increased their migration rates in the direction of fiber orientation. We further investigated if fiber topography modifies astrocytic neuroprotective properties, namely glutamate and glutamine transport and metabolism. This was done by quantifying changes in mRNA expression (qRT-PCR) and protein levels (Western blotting) for a battery of relevant biomolecules. Interestingly, we found that cells grown on random and/or aligned fibers increased the expression levels of two glutamate transporters, GLAST and GLT-1, and an important metabolic enzyme, glutamine synthetase, as compared to the fibronectin-coated films. Functional assays revealed increases in glutamate transport rates due to GLT-1 mediated uptake, which was largely determined by the dihydrokainate-sensitive GLT-1. Overall, this study suggests that aligned PLLA fibers can promote directed astrocytic migration, and, of most importance, our in vitro results indicate for the first time that electrospun PLLA fibers can positively modify neuroprotective properties of glial cells by increasing rates of glutamate uptake.

Nanomedicine for treating spinal cord injury

Spinal cord injury results in significant mortality and morbidity, lifestyle changes, and difficult rehabilitation. Treatment of spinal cord injury is challenging because the spinal cord is both complex to treat acutely and difficult to regenerate. Nanomaterials can be used to provide effective treatments; their unique properties can facilitate drug delivery to the injury site, enact as neuroprotective agents, or provide platforms to stimulate regrowth of damaged tissues. We review recent uses of nanomaterials including nanowires, micelles, nanoparticles, liposomes, and carbon-based nanomaterials for neuroprotection in the acute phase. We also review the design and neural regenerative application of electrospun scaffolds, conduits, and self-assembling peptide scaffolds.

Paracrine exchanges of molecular signals between alginate-encapsulated pericytes and freely suspended endothelial cells within a 3D protein gel

Paracrine signals, essential for the proper survival and functioning of tissues, may be mimicked by delivery of therapeutic proteins within engineered tissue constructs. Conventional delivery methods are of limited duration and are unresponsive to the local environment. We developed a system for sustained and regulated delivery of paracrine signals by encapsulating living cells of one type in alginate beads and co-suspending these cell-loaded particles along with unencapsulated cells of a second type within a 3D protein gel. This system was applied to vascular tissue engineering by placing human placental microvascular pericytes (PCs) in the particulate alginate phase and human umbilical vein endothelial cells (HUVECs) in the protein gel phase. Particle characteristics were optimized to keep the encapsulated PCs viable for at least two weeks. Encapsulated PCs were bioactive in vitro, secreting hepatocyte growth factor, an angiogenic protein, and responding to externally applied HUVEC-derived signals. Encapsulated PCs influenced HUVEC behavior in the surrounding gel by enhancing the formation of vessel-like structures when compared to empty alginate bead controls. In vivo, encapsulated PCs modulated the process of vascular self-assembly by HUVECs in 3D gels following implantation into immunodeficient mice. We conclude that alginate encapsulated cells can provide functional paracrine signals within engineered tissues.

Host reaction to poly(2-hydroxyethyl methacrylate) scaffolds in a small spinal cord injury model

Tissue engineered scaffolds and matrices have been investigated over the past decade for their potential in spinal cord repair. They provide a 3-D substrate that can be permissive for nerve regeneration yet have other roles including neuroprotection, altering the inflammatory cascade and mechanically stabilizing spinal cord tissue after injury. In this study we investigated very small lesions (approx. 0.25 μL in volume) of the dorsal column into which a phase-separated poly(2-hydroxyethyl methacrylate) hydrogel scaffold is implanted. Using fluorescent immunohistochemistry to quantify glial scarring, the poly(2-hydroxyethyl methacrylate) scaffold group showed reduced intensity compared to lesion controls for GFAP and the chondroitin sulfate proteoglycan neurocan after 6 days. However, the scaffold and tissue was also pushed dorsally after 6 days while the scaffold was not integrated into the spinal cord after 28 days. Overall, this small-lesion spinal cord injury model provided information on the host tissue reaction of a TE scaffold while reducing animal discomfort and care.

Axon regeneration and exercise-dependent plasticity after spinal cord injury

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

Preparation and characterization of genipin-crosslinked rat acellular spinal cord scaffolds

The feasibility of rat acellular spinal cord scaffolds for tissue engineering applications was investigated. Fresh rat spinal cords were decellularized and crosslinked with genipin (GP) to improve their structural stability and mechanical properties. The GP-crosslinked spinal cord scaffolds possessed a porous structure with an average pore diameter of 31.1 μm and a porosity of 81.5%. The resultant scaffolds exhibited a water uptake ratio of 229%, and moderate in vitro degradation rates of less than 5% in phosphate-buffered saline (PBS) and slightly more than 20% in trypsin-containing buffer, within 14 days. The ultimate tensile strength and elastic modulus of GP-crosslinked spinal cord scaffolds were determined to be 0.193±0.064 MPa and 1.541±0.082 MPa, respectively. Compared with glutaraldehyde (GA)-crosslinked acellular spinal cord scaffolds, GP-crosslinked scaffolds demonstrated similar microstructure and mechanical properties but superior biocompatibility as indicated by cytotoxicity evaluation and rat mesenchymal stem cell (MSC) adhesion behavior. Cells were able to penetrate throughout the crosslinked scaffold due to the presence of an interconnected porous structure. The low cytotoxicity of GP facilitated cell proliferation and extracellular matrix (ECM) secretion in vitro on the crosslinked scaffolds over 7 days. Thus, these GP-crosslinked spinal cord scaffolds show great promise for tissue engineering applications.

Using extracellular matrix for regenerative medicine in the spinal cord

Regeneration within the mammalian central nervous system (CNS) is limited, and traumatic injury often leads to permanent functional motor and sensory loss. The lack of regeneration following spinal cord injury (SCI) is mainly caused by the presence of glial scarring, cystic cavitation and a hostile environment to axonal growth at the lesion site. The more prominent experimental treatment strategies focus mainly on drug and cell therapies, however recent interest in biomaterial-based strategies are increasing in number and breadth. Outside the spinal cord, approaches that utilize the extracellular matrix (ECM) to promote tissue repair show tremendous potential for various application including vascular, skin, bone, cartilage, liver, lung, heart and peripheral nerve tissue engineering (TE). Experimentally, it is unknown if these approaches can be successfully translated to the CNS, either alone or in combination with synthetic biomaterial scaffolds. In this review we outline the first attempts to apply the potential of ECM-based biomaterials and combining cell-derived ECM with synthetic scaffolds.

Tissue engineering and regenerative medicine -where do we stand?

Tissue Engineering (TE) in the context of Regenerative Medicine (RM) has been hailed for many years as one of the most important topics in medicine in the twenty-first century. While the first clinically relevant TE efforts were mainly concerned with the generation of bioengineered skin substitutes, subsequently TE applications have been continuously extended to a wide variety of tissues and organs. The advent of either embryonic or mesenchymal adult stem-cell technology has fostered many of the efforts to combine this promising tool with TE approaches and has merged the field into the term Regenerative Medicine. As a typical example in translational medicine, the discovery of a new type of cells called Telocytes that have been described in many organs and have been detected by electron microscopy opens another gate to RM. Besides cell-therapy strategies, the application of gene therapy combined with TE has been investigated to generate tissues and organs. The vascularization of constructs plays a crucial role besides the matrix and cell substitutes. Therefore, novel in vivo models of vascularization have evolved allowing axial vascularization with subsequent transplantation of constructs. This article is intended to give an overview over some of the most recent developments and possible applications in RM through the perspective of TE achievements and cellular research. The synthesis of TE with innovative methods of molecular biology and stem-cell technology appears to be very promising.

Regenerative medicine for the treatment of spinal cord injury: more than just promises?

Spinal cord injury triggers a complex set of events that lead to tissue healing without the restoration of normal function due to the poor regenerative capacity of the spinal cord. Nevertheless, current knowledge about the intrinsic regenerative ability of central nervous system axons, when in a supportive environment, has made the prospect of treating spinal cord injury a reality. Among the range of strategies under investigation, cell-based therapies offer the most promising results, due to the multifactorial roles that these cells can fulfil. However, the best cell source is still a matter of debate, as are clinical issues that include the optimal cell dose as well as the timing and route of administration. In this context, the role of biomaterials is gaining importance. These can not only act as vehicles for the administered cells but also, in the case of chronic lesions, can be used to fill the permanent cyst, thus creating a more favourable and conducive environment for axonal regeneration in addition to serving as local delivery systems of therapeutic agents to improve the regenerative milieu. Some of the candidate molecules for the future are discussed in view of the knowledge derived from studying the mechanisms that facilitate the intrinsic regenerative capacity of central nervous system neurons. The future challenge for the multidisciplinary teams working in the field is to translate the knowledge acquired in basic research into effective combinatorial therapies to be applied in the clinic.