Fluid shear in viscous fibronectin gels allows aggregation of fibrous materials for CNS tissue engineering

Advanced approaches to regenerate spinal cord injury: The development of cell and tissue engineering therapy and combinational treatments

Spinal cord injury (SCI) is a central nervous system (CNS) devastate event that is commonly caused by traumatic or non-traumatic events. The reinnervation of spinal cord axons is hampered through a myriad of devices counting on the damaged myelin, inflammation, glial scar, and defective inhibitory molecules. Unfortunately, an effective treatment to completely repair SCI and improve functional recovery has not been found. In this regard, strategies such as using cells, biomaterials, biomolecules, and drugs have been reported to be effective for SCI recovery. Furthermore, recent advances in combinatorial treatments, which address various aspects of SCI pathophysiology, provide optimistic outcomes for spinal cord regeneration. According to the global importance of SCI, the goal of this article review is to provide an overview of the pathophysiology of SCI, with an emphasis on the latest modes of intervention and current advanced approaches for the treatment of SCI, in conjunction with an assessment of combinatorial approaches in preclinical and clinical trials. So, this article can give scientists and clinicians' clues to help them better understand how to construct preclinical and clinical studies that could lead to a breakthrough in spinal cord regeneration.

A high-resolution real-time quantification of astrocyte cytokine secretion under shear stress for investigating hydrocephalus shunt failure

It has been hypothesized that physiological shear forces acting on medical devices implanted in the brain significantly accelerate the rate to device failure in patients with chronically indwelling neuroprosthetics. In hydrocephalus shunt devices, shear forces arise from cerebrospinal fluid flow. The shunt's unacceptably high failure rate is mostly due to obstruction with adherent inflammatory cells. Astrocytes are the dominant cell type bound directly to obstructing shunts, rapidly manipulating their activation via shear stress-dependent cytokine secretion. Here we developed a total internal reflection fluorescence microscopy combined with a microfluidic shear device chip (MSDC) for quantitative analysis and direct spatial-temporal mapping of secreted cytokines at the single-cell level under physiological shear stress to identify the root cause for shunt failure. Real-time secretion imaging at 1-min time intervals enabled successful detection of a significant increase of astrocyte IL-6 cytokine secretion under shear stress greater than 0.5 dyne/cm2, validating our hypothesis and highlighting the importance of reducing shear stress activation of cells.

Sustained delivery of neurotrophic factors to treat spinal cord injury

Acute spinal cord injury (SCI) is a devastating condition that results in tremendous physical and psychological harm and a series of socioeconomic problems. Although neurons in the spinal cord need neurotrophic factors for their survival and development to reestablish their connections with their original targets, endogenous neurotrophic factors are scarce and the sustainable delivery of exogeneous neurotrophic factors is challenging. The widely studied neurotrophic factors such as brain-derived neurotrophic factor, neurotrophin-3, nerve growth factor, ciliary neurotrophic factor, basic fibroblast growth factor, and glial cell-derived neurotrophic factor have a relatively short cycle that is not sufficient enough for functionally significant neural regeneration after SCI. In the past decades, scholars have tried a variety of cellular and viral vehicles as well as tissue engineering scaffolds to safely and sustainably deliver those necessary neurotrophic factors to the injury site, and achieved satisfactory neural repair and functional recovery on many occasions. Here, we review the neurotrophic factors that have been used in trials to treat SCI, and vehicles that were commonly used for their sustained delivery.

Emerging Trends in Information-Driven Engineering of Complex Biological Systems

Synthetic biological systems are used for a myriad of applications, including tissue engineered constructs for in vivo use and microengineered devices for in vitro testing. Recent advances in engineering complex biological systems have been fueled by opportunities arising from the combination of bioinspired materials with biological and computational tools. Driven by the availability of large datasets in the "omics" era of biology, the design of the next generation of tissue equivalents will have to integrate information from single-cell behavior to whole organ architecture. Herein, recent trends in combining multiscale processes to enable the design of the next generation of biomaterials are discussed. Any successful microprocessing pipeline must be able to integrate hierarchical sets of information to capture key aspects of functional tissue equivalents. Micro- and biofabrication techniques that facilitate hierarchical control as well as emerging polymer candidates used in these technologies are also reviewed.

The Molecular Dance of Fibronectin: Conformational Flexibility Leads to Functional Versatility

Fibronectin, a large multimodular protein and one of the major fibrillar components of the extracellular matrix, has been the subject of study for many decades and plays critical roles in embryonic development and tissue homeostasis. Moreover, fibronectin has been implicated in the pathology of many diseases, including cancer, and abnormal depositions of fibronectin have been identified in a number of amyloid and nonamyloid lesions. The ability of fibronectin to carry all these diverse functionalities depends on interactions with a large number of molecules, including adhesive and signaling cell surface receptors, other components of the extracellular matrix, and growth factors and cytokines. The regulation and integration of such large number of interactions depends on the modular architecture of fibronectin, which allows a large number of conformations, exposing or destroying different binding sites. In this Review, we summarize the current knowledge regarding the conformational flexibility of fibronectin, with an emphasis on how it regulates the ability of fibronectin to interact with various signaling molecules and cell-surface receptors and to form supramolecular assemblies and fibrillar structures.

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.

Progress of electrospun fibers as nerve conduits for neural tissue repair

Nerve tissue regeneration approaches have gained much attention in recent years, and nerve conduits (NCs), which facilitate nerve tissue regeneration, have become an attractive alternative to nerve autologous graft. Several methods are proposed to fabricate NCs, including electrospinning, which is a widely used approach for NCs and other tissue scaffolds, and has advantages such as the ability to control the thickness, diameter and porosity of fibers, as well as its simple experimental set up. This article gives an overview of electrospun fibers for nerve conduits utilized in peripheral and central nerve regeneration. Natural and synthetic materials with different mechanical strength, degradation rates and biocompatibility are proposed. Several bioactive proteins that can help the process of nerve regeneration are introduced. Finally, some approaches to control the morphology of electrospun fibers and to deliver bioactive proteins are discussed in detail.

Extracellular matrices, artificial neural scaffolds and the promise of neural regeneration

Over last 20 years, extracellular matrices have been shown to be useful in promoting tissue regeneration. Recently, they have been used and have had success in achieving neurogenesis. Recent developments in extracellular matrix design have allowed their successful in vivo incorporation to engender an environment favorable for neural regeneration in animal models. Promising treatments under investigation include manipulation of the intrinsic extracellular matrix and incorporation of engineered naometer-sized scaffolds through which inhibition of molecules serving as barriers to neuroregeneration and delivery of neurotrophic factors and/or cells for successful tissue regeneration can be achieved. Further understanding of the changes incurred within the extracellular matrix following central nervous system injury will undoubtedly help design a clinically efficacious extracellular matrix scaffold that can mitigate or reverse neural degeneration in the clinical setting.

The ultrastructure of fibronectin fibers pulled from a protein monolayer at the air-liquid interface and the mechanism of the sheet-to-fiber transition

Fibronectin is a globular protein that circulates in the blood and undergoes fibrillogenesis if stretched or under other partially denaturing conditions, even in the absence of cells. Stretch assays made by pulling fibers from droplets of solutions containing high concentrations of fibronectin have previously been introduced in mechanobiology, particularly to ask how bacteria and cells exploit the stretching of fibronectin fibers within extracellular matrix to mechano-regulate its chemical display. Our electron microscopy analysis of their ultrastructure now reveals that the manually pulled fibronectin fibers are composed of densely packed lamellar spirals, whose interlamellar distances are dictated by ion-tunable electrostatic interactions. Our findings suggest that fibrillogenesis proceeds via an irreversible sheet-to-fiber transition as the fibronectin sheet formed at the air-liquid interface of the droplet is pulled off by a sharp tip. This far from equilibrium process is driven by the externally applied force, interfacial surface tension, shear-induced fibronectin self-association, and capillary force-induced buffer drainage. The ultrastructural characterization is then contrasted with previous FRET studies that characterized the molecular strain within these manually pulled fibers. Particularly relevant for stretch-dependent binding studies is the finding that the interior fiber surfaces are accessible to nanoparticles smaller than 10 nm. In summary, our study discovers the underpinning mechanism by which highly hierarchically structured fibers can be generated with unique mechanical and mechano-chemical properties, a concept that might be extended to other bio- or biomimetic polymers.

Comparison of cellular architecture, axonal growth, and blood vessel formation through cell-loaded polymer scaffolds in the transected rat spinal cord

The use of multichannel polymer scaffolds in a complete spinal cord transection injury serves as a deconstructed model that allows for control of individual variables and direct observation of their effects on regeneration. In this study, scaffolds fabricated from positively charged oligo[poly(ethylene glycol)fumarate] (OPF(+)) hydrogel were implanted into rat spinal cords following T9 complete transection. OPF(+) scaffold channels were loaded with either syngeneic Schwann cells or mesenchymal stem cells derived from enhanced green fluorescent protein transgenic rats (eGFP-MSCs). Control scaffolds contained extracellular matrix only. The capacity of each scaffold type to influence the architecture of regenerated tissue after 4 weeks was examined by detailed immunohistochemistry and stereology. Astrocytosis was observed in a circumferential peripheral channel compartment. A structurally separate channel core contained scattered astrocytes, eGFP-MSCs, blood vessels, and regenerating axons. Cells double-staining with glial fibrillary acid protein (GFAP) and S-100 antibodies populated each scaffold type, demonstrating migration of an immature cell phenotype into the scaffold from the animal. eGFP-MSCs were distributed in close association with blood vessels. Axon regeneration was augmented by Schwann cell implantation, while eGFP-MSCs did not support axon growth. Methods of unbiased stereology provided physiologic estimates of blood vessel volume, length and surface area, mean vessel diameter, and cross-sectional area in each scaffold type. Schwann cell scaffolds had high numbers of small, densely packed vessels within the channels. eGFP-MSC scaffolds contained fewer, larger vessels. There was a positive linear correlation between axon counts and vessel length density, surface density, and volume fraction. Increased axon number also correlated with decreasing vessel diameter, implicating the importance of blood flow rate. Radial diffusion distances in vessels significantly correlated to axon number as a hyperbolic function, showing a need to engineer high numbers of small vessels in parallel to improving axonal densities. In conclusion, Schwann cells and eGFP-MSCs influenced the regenerating microenvironment with lasting effect on axonal and blood vessel growth. OPF(+) scaffolds in a complete transection model allowed for a detailed comparative, histologic analysis of the cellular architecture in response to each cell type and provided insight into physiologic characteristics that may support axon regeneration.

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.

Neural ECM mimetics

The consequence of numerous neurological disorders is the significant loss of neural cells, which further results in multilevel dysfunction or severe functional deficits. The extracellular matrix (ECM) is of tremendous importance for neural regeneration mediating ambivalent functions: ECM serves as a growth-promoting substrate for neurons but, on the other hand, is a major constituent of the inhibitory scar, which results from traumatic injuries of the central nervous system. Therefore, cell and tissue replacement strategies on the basis of ECM mimetics are very promising therapeutic interventions. Numerous synthetic and natural materials have proven effective both in vitro and in vivo. The closer a material's physicochemical and molecular properties are to the original extracellular matrix, the more promising its effectiveness may be. Relevant factors that need to be taken into account when designing such materials for neural repair relate to receptor-mediated cell-matrix interactions, which are dependent on chemical and mechanical sensing. This chapter outlines important characteristics of natural and synthetic ECM materials (scaffolds) and provides an overview of recent advances in design and application of ECM materials for neural regeneration, both in therapeutic applications and in basic biological research.

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

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

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.

Scaffolds to promote spinal cord regeneration

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

Engineering an integrated cellular interface in three-dimensional hydrogel cultures permits monitoring of reciprocal astrocyte and neuronal responses

This study reports a new type of three-dimensional (3D) tissue model for studying interactions between cell types in collagen hydrogels. The aim was to create a 3D cell culture model containing separate cell populations in close proximity without the presence of a mechanical barrier, and demonstrate its relevance to modeling the axon growth-inhibitory cellular interfaces that develop in the central nervous system (CNS) in response to damage. This provides a powerful new tool to determine which aspects of the astroglial scar response and subsequent neuronal regeneration inhibition are determined by the presence of the other cell types. Astrocytes (CNS glia) and dissociated dorsal root ganglia (DRG; containing neurons and peripheral nervous system [PNS] glia) were seeded within collagen solution at 4 °C in adjacent chambers of a stainless steel mould, using cells cultured from wild-type or green fluorescent protein expressing rats, to track specific populations. The divider between the chambers was removed using a protocol that allowed the gels to integrate without mixing of the cell populations. Following setting of the gels, they were maintained in culture for up to 15 days. Reciprocal astrocyte and neuronal responses were monitored using confocal microscopy and 3D image analysis. At DRG:astrocyte interfaces, by 5 days there was an increase in the number of astrocytes at the interface followed by hypertrophy and increased glial fibrillary acidic protein expression at 10 and 15 days, indicative of reactive gliosis. Neurons avoided crossing DRG:astrocyte interfaces, and neuronal growth was restricted to the DRG part of the gel. By contrast, neurons were able to grow freely across DRG:DRG interfaces, demonstrating the absence of a mechanical barrier. These results show that in a precisely controlled 3D environment, an interface between DRG and astrocyte cultures is sufficient to trigger reactive gliosis and inhibition of neuronal regeneration across the interface. Different aspects of the astrocyte response could be independently monitored, providing an insight into the formation of a glial scar. This technology has wide potential for researchers wishing to maintain and monitor interactions between adjacent cell populations in 3D culture.

Alignment of astrocytes increases neuronal growth in three-dimensional collagen gels and is maintained following plastic compression to form a spinal cord repair conduit

After injury to the spinal cord, reactive astrocytes form a glial scar consisting of highly ramified cell processes that constitute a major impediment to repair, partly due to their lack of orientation and guidance for regenerating axons. In some nonmammalian vertebrates, successful central nervous system regeneration is attributed to the alignment of reactive glia, which guide axons across the lesion site. Here, a three-dimensional mammalian cell-seeded collagen gel culture system was used to explore the effect of astrocyte alignment on neuronal growth. Astrocyte alignment was mapped within tethered rectangular gels and was significantly greater at the edge and middle of the gels compared to the control unaligned regions. When neurons were seeded on and within astrocyte gels, neurite length was greatest in the areas of astrocyte alignment. There was no difference in expression of astrocyte reactivity markers between aligned and control areas. Having established the potential utility of astrocyte alignment, the aligned gels were plastic compressed, transforming them into mechanically robust implantable devices. After compression, astrocytes remained viable and aligned and supported neurite outgrowth, yielding a novel method for assembling aligned cellular constructs suitable for tissue engineering and highlighting the importance of astrocyte alignment as a possible future therapeutic intervention for spinal cord repair.

Repairing injured peripheral nerves: Bridging the gap

Peripheral nerve injuries that induce gaps larger than 1-2 cm require bridging strategies for repair. Autologous nerve grafts are still the gold standard for such interventions, although alternative treatments, as well as treatments to improve the therapeutic efficacy of autologous nerve grafting are generating increasing interest. Investigations are still mostly experimental, although some clinical studies have been undertaken. In this review, we aim to describe the developments in bridging technology which aim to replace the autograft. A multi-disciplinary approach is of utmost importance to develop and optimise treatments of the most challenging peripheral nerve injuries.

A versatile 3D culture model facilitates monitoring of astrocytes undergoing reactive gliosis

A major impediment to CNS repair is the glial scar, which forms following damage and is composed mainly of ramified, ‘reactive’ astrocytes that inhibit neuronal regrowth. The transition of astrocytes into this reactive phenotype (reactive gliosis) is a potential therapeutic target, but glial scar formation has proved difficult to study in monolayer cultures because they induce constitutive astrocyte activation. Here we demonstrate a 3D collagen gel system in which primary rat astrocytes were maintained in a persistently less reactive state than comparable cells in monolayer, resembling their status in the undamaged CNS. Reactivity, proliferation and viability were monitored and quantified using confocal, fluorescence and time-lapse microscopy, 3D image analysis, RT–PCR and ELISA. To assess the potential of this system as a model of reactive gliosis, astrocytes in 3D were activated with TGFβ1 to a ramified, reactive phenotype (elevated GFAP, Aquaporin 4, CSPG, Vimentin and IL-6 secretion). This provides a versatile system in which astrocytes can be maintained in a resting state, then be triggered to undergo reactive gliosis, enabling real-time monitoring and quantitative analysis throughout and providing a powerful new tool for research into CNS damage and repair. Copyright © 2009 John Wiley & Sons, Ltd.

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

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

A novel biomimetic material for engineering postsurgical adhesion using the injured digital flexor tendon-synovial complex as an in vivo model

BACKGROUND

Many surgical procedures are complicated by adhesions. These restrictive fibrotic bands form between normally separate gliding tissue layers, potentially impairing function. The authors tested the adhesion-modifying effect of a novel fibronectin-derived biomimetic biomaterial in a tendon-synovial complex injury model.

METHODS

The deep flexor tendons of digits 2 and 4 in the right forepaw of 15 New Zealand White rabbits were subjected to 5-mm-long partial tenotomies. Animals were randomized to receive biomaterial tubes enveloping the tendon injuries or left untreated. Digits, amputated at 2 weeks, were randomized to mechanical pullout assessments of adhesion strength or to quantitative histologic cellularity and immunohistochemical proliferation (Ki67) assessments.

RESULTS

The mean peak pullout force required to break the adhesions was reduced from 7.70 N (n = 6) in untreated digits to 0.31 N (n = 7) in biomaterial-treated digits (p = 0.001). The mean structural stiffness of the adhesions was also significantly reduced (p = 0.001). Histologically, treated and untreated digits demonstrated an equal incidence of adhesions. The treated adhesions were 55 percent less cellular at their surface than the untreated injured controls (p = 0.003). Treated tendons were 8 percent more cellular (with equal numbers of proliferating cells) at their surface and significantly more cellular within their bulk than positive controls (p <or= 0.05).

CONCLUSION

This study suggests a significant reduction in the restrictive nature of postsurgical adhesions following treatment with the antiadhesive biomaterial without compromising tendon cellularity.

Approaches to neural tissue engineering using scaffolds for drug delivery

This review seeks to give an overview of the current approaches to drug delivery from scaffolds for neural tissue engineering applications. The challenges presented by attempting to replicate the three types of nervous tissue (brain, spinal cord, and peripheral nerve) are summarized. Potential scaffold materials (both synthetic and natural) and target drugs are discussed with the benefits and drawbacks given. Finally, common methods of drug delivery, including degradable/diffusion-based delivery systems, affinity-based delivery systems, immobilized drug delivery systems, and electrically controlled drug delivery systems, are examined and critiqued. Based on the current body of work, suggestions for future directions of research in the field of neural tissue engineering are presented.

Novel nerve guidance material prepared from bovine aponeurosis

Spinal cord injury (SCI) creates an adverse environment for axon regeneration. As a result, the axons at the injury sites begin to be atrophy, retract and lose their functions. Several strategies to promote axon regeneration at the injury site have been tested, but the progress is very limited. One of the major reasons is that the regenerated axons often extend randomly and do not reach the proper place. Fabricating linearly ordered materials as nerve guidance would be important to solve such problems. In this study, a novel type of nerve guidance material was prepared from the bovine aponeurosis, which mainly consisted of ordered collagen fibers. The processed material showed good cell compatibility and low immunogenisity. Moreover, the processed material guided the neurites outgrowth of in vitro cultured cortical neurons along its fibers. The results suggested that the processed aponeurosis would be a proper nerve guidance biomaterial for SCI repair.

Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

Basic science research in tissue engineering and regenerative medicine aims to investigate and understand the deposition, growth, and remodeling of tissues by drawing together approaches from a range of disciplines. This review discusses approaches that use biomimetic proteins and cellular therapies, both in the development of clinical products and of model platforms for scientific investigation. Current clinical approaches to repairing skin, bone, nerve, heart valves, blood vessels, ligaments, and tendons are described and their limitations identified. Opportunities and key questions for achieving clinical goals are discussed through commonly used examples of biomimetic scaffolds: collagen, fibrin, fibronectin, and silk. The key questions addressed by three-dimensional culture models, biomimetic materials, surface chemistry, topography, and their interaction with cells in terms of durotaxis, mechano-regulation, and complex spatial cueing are reviewed to give context to future strategies for biomimetic technology.

Potential of stem cell based therapy and tissue engineering in the regeneration of the central nervous system

The insufficiency of self-repair and regeneration of the central nervous system (CNS) leads to difficulty of rehabilitation of the injured brain. In the past few decades, the significant progress in cell therapy and tissue engineering has contributed to the functional recovery of the CNS to a great extent. The present review focuses on the potential role of stem cell based therapy and tissue engineering in the regeneration of the CNS.

Phenotypical analysis of adult rat olfactory ensheathing cells on 3-D collagen scaffolds

Olfactory ensheathing cell (OEC) transplantation is a promising or potential therapy for spinal cord injury (SCI). However, the effects of injecting OECs directly into SCI site have been limited and unsatisfied due to the complexity of SCI. To improve the outcome, proper biomaterials are thought to be helpful since these materials would allow the cells to grow three-dimensionally and guide cell migration. In this paper, we have studied the behavior of OECs in two-dimensional (2-D) condition as well as on three-dimensional (3-D) collagen scaffolds by analyzing their phenotypes such as cell proliferation, apoptosis, morphology, and gene activities of some neurotrophic factors and myelin proteins. OECs proliferation rate was increased on 3-D collagen scaffolds compared to the 2-D culture condition. OECs on 3-D collagen scaffolds also showed less apoptosis. In addition, OECs on 3-D collagen scaffolds maintained the original spindle-shape morphology and P75NTR gene activity. NGF, BDNF, and PLP were found to be upregulated in OECs cultured on 3-D collagen scaffolds by the semi-quantitative RT-PCR approach. The results suggested that 3-D collagen scaffolds provide suitable environments for the OECs to maintain their morphology as well as several important functional phenotypes and all these could be helpful for the effective treatment of SCI.

Bioengineered Strategies for Spinal Cord Repair

This article reviews bioengineered strategies for spinal cord repair using tissue engineered scaffolds and drug delivery systems. The pathophysiology of spinal cord injury (SCI) is multifactorial and multiphasic, and therefore, it is likely that effective treatments will require combinations of strategies such as neuroprotection to counteract secondary injury, provision of scaffolds to replace lost tissue, and methods to enhance axonal regrowth, synaptic plasticity, and inhibition of astrocytosis. Biomaterials have major advantages for spinal cord repair because of their structural and chemical versatility. To date, various degradable or non-degradable biomaterial polymers have been tested as guidance channels or delivery systems for cellular and non-cellular neuroprotective or neuroregenerative agents in experimental SCI. There is promise that bioengineering technology utilizing cellular treatment strategies, including Schwann cells, olfactory ensheathing glia, or neural stem cells, can promote repair of the injured spinal cord. This review is divided into three parts: (1) degradable and non-degradable biomaterials; (2) device design; and (3) combination strategies with scaffolds. We will show that bioengineering combinations of cellular and non-cellular strategies have enhanced the potential for experimental SCI repair, although further pre-clinical work is required before this technology can be translated to humans.

Characterization of non-neuronal elements within fibronectin mats implanted into the damaged adult rat spinal cord

Previous studies have shown that mats made from fibronectin (FN) integrate well into spinal cord lesion sites and support extensive axonal growth. Using immunohistochemistry, we have investigated the non-neuronal factors that contribute to these properties. Extensive vascularization was observed in FN mats by 1 week along with heavy macrophage infiltration by 3 days post-implantation. By 1 week post-implantation, laminin tubules had formed and were associated with axons and p75 immunoreactive Schwann cells. By 4 weeks post-implantation, most axons were associated with Schwann cell derived myelin. Few oligodendrocytes were present within the mat, even with an increase in the number of oligodendrocyte precursors around the implant site by 7 days post-implantation. Astrocyte proliferation also occurred in the intact tissue, with a prominent glial scar forming around the implant within 4 weeks. However, by 2 months post-implantation astrocytes were present in the FN implant site and were intermingled with the axons. Axonal ingrowth and integration of the FN mats is probably due to the ability of FN mats to support and organize infiltration of Schwann cells and deposition of laminin. At later time points, myelinated axons remain in the implant site, even after other elements (e.g. macrophages and laminin) have disappeared. Both of these properties are likely to be important in the design of biomaterial bridges for CNS regeneration.

Domain Unfolding Plays a Role in Superfibronectin Formation

Superfibronectin (sFN) is a fibronectin (FN) aggregate that is formed by mixing FN with anastellin, a fragment of the first type III domain of FN. However, the mechanism of this aggregation has not been clear. In this study, we found that anastellin co-precipitated with FN in a ratio of approximately 4:1, anastellin:FN monomer. The primary binding site for anastellin was in the segment (III)1-3, which bound three molecules of anastellin and was able to form a precipitate without the rest of the FN molecule. Anastellin binding to (III)3 caused a conformational change in that domain that exposed a cryptic thermolysin-sensitive site. An additional anastellin binds to (III)11, where it enhances thermolysin digestion of (III)11. An engineered disulfide bond in (III)3 inhibited both aggregation and protease digestion, suggesting that the stability of (III)3 is a key factor in sFN formation. We propose a three-step model for sFN formation: 1) FN-III domains spontaneously unfold and refold; 2) anastellin binds to an unfolded domain, preventing its refolding and leaving it with exposed hydrophobic surfaces and beta-sheet edges; and 3) these exposed elements bind to similar exposed elements on other molecules, leading to aggregation. The model is consistent with our observation that the kinetics of aggregation are first order, with a reaction time of 500-700 s. Similar mechanisms may contribute to the assembly of the native FN matrix.