Guiding Axons in the Central Nervous System: A Tissue Engineering Approach

Regenerated Silk Fibers Obtained by Straining Flow Spinning for Guiding Axonal Elongation in Primary Cortical Neurons

The recovery of injured nervous tissue, one of the main goals for regenerative therapeutic approaches, is often hindered by the limited axonal regeneration ability of the central nervous system (CNS). In this regard, the identification of scaffolds that support the reconstruction of functional neuronal tissues and guide the alignment of regenerating neurons is a major challenge in tissue engineering. Ideally, the usage of such scaffolds would promote and guide the axonal growth, a crucial phase for the restoration of neuronal connections and, consequently, the nerve function. Among the materials proposed as scaffolds for CNS regeneration, silk has been used to exploit its outstanding features as a biomaterial to promote axonal regeneration. In this study, we explore, for the first time, the possibility of using high-performance regenerated silk fibers obtained by straining flow spinning (SFS) to serve as scaffolds for inducing and guiding the axonal growth. It is shown that SFS fibers promote the spontaneous organization of dissociated cortical primary cells into highly interconnected cellular spheroid-like tissue formations. Neuronal projections (i.e., axons) from these cellular spheroids span hundreds of microns along the SFS fibers that act as guides and allow the connection of distant spheroids. In addition, it is also shown that SFS fibers serve as scaffolds for neuronal migration covering short and long distances. As a consequence, the usage of high-performance SFS fibers appears as a promising basis for the development of novel therapies, leading to directed axonal regeneration.

Human Neural Tissue Construct Fabrication Based on Scaffold-Free Tissue Engineering

Current neural tissue engineering strategies involve the development and application of neural tissue constructs produced by using an anisotropic polymeric scaffold. This study reports a scaffold-free method of tissue engineering to create a tubular neural tissue construct containing unidirectional neuron bundles. The surface patterning of a thermoresponsive culture substrate and a coculture system of neurons with patterned astrocytes can provide an anisotropic structure and easy handling of the neural tissue construct without the use of a scaffold. Furthermore, using a gelatin gel-coated plunger, the neuron bundles can be laid out in the same direction at regulated intervals within multilayered astrocyte sheets. Since the 3D tissue construct is composed only by neurons and astrocytes, they can communicate physiologically without obstruction of a scaffold. The medical benefits of scaffold-free tissue generation provide new opportunities for the development of human cell-based tissue models required to better understand the mechanisms of neurodegenerative diseases. Therefore, this new tissue engineering approach may be useful to establish a technology for regenerative medicine and drug discovery using the patient's own neurons.

The effects of confinement on neuronal growth cone morphology and velocity

Optimizing growth cone guidance through the use of patterned substrates is important for designing regenerative substrates to aid in recovery from neuronal injury. Using laser ablation, we designed micron-scale patterns capable of confining dissociated mouse cerebellar granule neuron growth cones to channels of different widths ranging from 1.5 to 12 μm. Growth cone dynamics in these channels were observed using time-lapse microscopy. Growth cone area was decreased in channels between 1.5 and 6 μm as compared to that in 12 μm and unpatterned substrates. Growth cone aspect ratio was also affected as narrower channels forced growth cones into a narrow, elongated shape. There was no difference in the overall rate of growth cone advance in uniform channels between 1.5 and 12 μm as compared to growth on unpatterned substrates. The percentage of time growth cones advanced, paused, and retracted was also similar. However, growth cones did respond to changes in confinement: growth cones in narrow lanes rapidly sped up when encountering a wide region and then slowed down as they entered another narrow region. Our results suggest that the rate of neurite extension is not affected by the degree of confinement, but does respond to changes in confinement.

Mechanical properties of the lamprey spinal cord: uniaxial loading and physiological strain

During spinal cord injury, nerves suffer a strain beyond their physiological limits which damages and disrupts their structure. Research has been done to measure the modulus of the spinal cord and surrounding tissue; however the relationship between strain and spinal cord fibers is still unclear. In this work, our objective is to measure the stress-strain response of the spinal cord in vivo and in vitro and model this response as a function of the number of fibers. We used the larvae lamprey (Petromyzon Marinus), a model for spinal cord regeneration and animal locomotion. We found that physiologically the spinal cord is pre-stressed to a longitudinal strain of 10% and this strain increases to 15% during swimming. Tensile measurements show that uniaxial, longitudinal loading is independent of the meninges. Stress values for uniaxial strains below 18%, are homogeneous through the length of the body. However, for higher uniaxial strains the Head section shows more resistance to longitudinal loading than the Tail. These data, together with the number of fibers obtained from histological sections were used in a composite-material model to obtain the properties of the spinal cord fibers (2.4 MPa) and matrix (0.017 MPa) to uniaxial longitudinal loading. This model allowed us to approximate the percentage of fibers in the spinal cord, establishing a relationship between uniaxial longitudinal strains and spinal cord composition. We showed that there is a proportional relationship between the number of fibers and the properties of the spinal cord at large uniaxial strains.

Spinal cord regeneration: where fish, frogs and salamanders lead the way, can we follow?

Major trauma to the mammalian spinal cord often results in irreversible loss of function, i.e. paralysis, and current therapies ranging from drugs, implantations of stem cells and/or biomaterials, and electrically stimulated nerve regrowth, have so far offered very limited success in improving quality-of-life. However, in marked contrast with this basic shortcoming of ours, certain vertebrate species, including fish and salamanders, display the amazing ability to faithfully regenerate various complex body structures after injury or ablation, restoring full functionality, even in the case of the spinal cord. Despite the inherently strong and obvious translational potential for improving treatment strategies for human patients, our in-depth molecular-level understanding of these decidedly more advanced repair systems remains in its infancy. In the present review, we will discuss the current state of this field, focusing on recent progress in such molecular analyses using various regenerative species, and how these so far relate to the mammalian situation.

Directional neurite outgrowth on superaligned carbon nanotube yarn patterned substrate

Superaligned carbon nanotube (CNT) yarn patterned substrates were developed as the topographic scaffold for guiding the neurite outgrowth. As-prepared patterned substrates were used for culturing rat hippocampal neurons, without purifying and functionalizing processes on the CNTs. The neurite outgrowth on the patterned substrate exhibited a strong tendency to being aligned along the CNT yarns long axes. The neurite grown along the CNT yarns had much less branching than the one on a uniform planar substrate typically used for neuron culture. These results indicate that the pure CNT yarns possess the main characteristics of a guidance scaffold for neurite outgrowth. Furthermore, the CNT yarns can be mass produced and be easily weaved into desired structures, which may make them attractive for neuronal regeneration and tissue engineering.

Modeling neural differentiation on micropatterned substrates coated with neural matrix components

Topographical and biochemical characteristics of the substrate are critical for neuronal differentiation including axonal outgrowth and regeneration of neural circuits in vivo. Contact stimuli and signaling molecules allow neurons to develop and stabilize synaptic contacts. Here we present the development, characterization and functional validation of a new polymeric support able to induce neuronal differentiation in both PC12 cell line and adult primary skin-derived precursor cells (SKPs) in vitro. By combining a photolithographic technique with use of neural extracellular matrix (ECM) as a substrate, a biocompatible and efficient microenvironment for neuronal differentiation was developed.

Lubricated biodegradable polymer networks for regulating nerve cell behavior and fabricating nerve conduits with a compositional gradient

We present a method of tuning surface chemistry and nerve cell behavior by photo-cross-linking methoxy poly(ethylene glycol) monoacrylate (mPEGA) with hydrophobic, semicrystalline poly(ε-caprolactone) diacrylate (PCLDA) at various weight compositions of mPEGA (ø(m)) from 2 to 30%. Improved surface wettability is achieved with corresponding decreases in friction, water contact angle, and capability of adsorbing proteins from cell culture media because of repulsive PEG chains tethered in the network. The responses of rat Schwann cell precursor line (SpL201), rat pheochromocytoma (PC12), and E14 mouse neural progenitor cells (NPCs) to the modified surfaces are evaluated. Nonmonotonic or parabolic dependence of cell attachment, spreading, proliferation, and differentiation on ø(m) is identified for these cell types with maximal values at ø(m) of 5-7%. In addition, NPCs demonstrate enhanced neuronal differentiated lineages on the mPEGA/PCLDA network at ø(m) of 5% with intermediate wettability and surface energy. This approach lays the foundation for fabricating heterogeneous nerve conduits with a compositional gradient along the wall thickness, which are able to promote nerve cell functions within the conduit while inhibiting cell attachment on the outer wall to prevent potential fibrous tissue formation following implantation.

Injectable hydrogel materials for spinal cord regeneration: a review

Spinal cord injury (SCI) presents a complex regenerative problem due to the multiple facets of growth inhibition that occur following trauma to the cord parenchyma and stroma. Clinically, SCI is further complicated by the heterogeneity in the size, shape and extent of human injuries. Many of these injuries do not breach the dura mater and have continuous viable axons through the injury site that can later lead to some degree of functional recovery. In these cases, surgical manipulation of the spinal cord by implanting a preformed scaffold or drug delivery device may lead to further damage. Given these circumstances, in situ-forming scaffolds are an attractive approach for SCI regeneration. These synthetic and natural polymers undergo a rapid transformation from liquid to gel upon injection into the cord tissue, conforming to the individual lesion site and directly integrating with the host tissue. Injectable materials can be formulated to have mechanical properties that closely match the native spinal cord extracellular matrix, and this may enhance axonal ingrowth. Such materials can also be loaded with cellular and molecular therapeutics to modulate the wound environment and enhance regeneration. This review will focus on the current status of in situ-forming materials for spinal cord repair. The advantages of, and requirements for, such polymers will be presented, and examples of the behavior of such systems in vitro and in vivo will be presented. There are helpful lessons to be learned from the investigations of injectable hydrogels for the treatment of SCI that apply to the use of these biomaterials for the treatment of lesions in other central nervous system tissues and in organs comprising other tissue types.

Synergistic NGF/B27 gradients position synapses heterogeneously in 3D micropatterned neural cultures

Native functional brain circuits show different numbers of synapses (synaptic densities) in the cerebral cortex. Until now, different synaptic densities could not be studied in vitro using current cell culture methods for primary neurons. Herein, we present a novel microfluidic based cell culture method that combines 3D micropatterning of hydrogel layers with linear chemical gradient formation. Micropatterned hydrogels were used to encapsulate dissociated cortical neurons in laminar cell layers and neurotrophic factors NGF and B27 were added to influence the formation of synapses. Neurotrophic gradients allowed for the positioning of distinguishable synaptic densities throughout a 3D micropatterned neural culture. NGF and B27 gradients were maintained in the microfluidic device for over two weeks without perfusion pumps by utilizing a refilling procedure. Spatial distribution of synapses was examined with a pre-synaptic marker to determine synaptic densities. From our experiments, we observed that (1) cortical neurons responded only to synergistic NGF/B27 gradients, (2) synaptic density increased proportionally to synergistic NGF/B27 gradients; (3) homogeneous distribution of B27 disturbed cortical neurons in sensing NGF gradients and (4) the cell layer position significantly impacted spatial distribution of synapses.

Creating permissive microenvironments for stem cell transplantation into the central nervous system

Traumatic injury to the central nervous system (CNS) is highly debilitating, with the clinical need for regenerative therapies apparent. Neural stem/progenitor cells (NSPCs) are promising because they can repopulate lost or damaged cells and tissues. However, the adult CNS does not provide an optimal milieu for exogenous NSPCs to survive, engraft, differentiate, and integrate with host tissues. This review provides an overview of tissue engineering strategies to improve stem cell therapies by providing a defined microenvironment during transplantation. The use of biomaterials for physical support, growth factor delivery, and cellular co-transplantation are discussed. Providing the proper environment for stem cell survival and host tissue integration is crucial in realizing the full potential of these cells in CNS repair strategies.

Electric field-guided neuron migration: a novel approach in neurogenesis

Effective directional neuron migration is crucial in development of the central nervous system and for neurogenesis. Endogenous electrical signals are present in many developing systems and crucial cellular behaviors such as neuronal cell division, cell migration, and cell differentiation are all under the influence of such endogenous electrical cues. Preclinical in vivo studies have used electric fields (EFs) to attempt to enhance regrowth of damaged spinal cord axons with some success. Recent evidence shows that small EFs not only guide axonal growth, but also direct the earlier events of neuronal migration and neuronal cell division. This raises the possibility that applied or endogenous EFs, perhaps in combination, may direct transplanted neural stem cells, or regenerating neurons, to the desired site after brain injury or neuron degeneration. The high complexity of both structure and function of the nervous system, however, poses significant challenges to techniques for applying EFs to promote neurogenesis. The evolution of functional biomaterials and nanotechnology may provide promising solutions for the application of EFs in guiding neuron migration and neurogenesis within the central nervous system.

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.

Stereolithographic bone scaffold design parameters: osteogenic differentiation and signal expression

Scaffold design parameters including porosity, pore size, interconnectivity, and mechanical properties have a significant influence on osteogenic signal expression and differentiation. This review evaluates the influence of each of these parameters and then discusses the ability of stereolithography (SLA) to be used to tailor scaffold design to optimize these parameters. Scaffold porosity and pore size affect osteogenic cell signaling and ultimately in vivo bone tissue growth. Alternatively, scaffold interconnectivity has a great influence on in vivo bone growth but little work has been done to determine if interconnectivity causes changes in signaling levels. Osteogenic cell signaling could be also influenced by scaffold mechanical properties such as scaffold rigidity and dynamic relationships between the cells and their extracellular matrix. With knowledge of the effects of these parameters on cellular functions, an optimal tissue engineering scaffold can be designed, but a proper technology must exist to produce this design to specification in a repeatable manner. SLA has been shown to be capable of fabricating scaffolds with controlled architecture and micrometer-level resolution. Surgical implantation of these scaffolds is a promising clinical treatment for successful bone regeneration. By applying knowledge of how scaffold parameters influence osteogenic cell signaling to scaffold manufacturing using SLA, tissue engineers may move closer to creating the optimal tissue engineering scaffold.

Polymers for Fabricating Nerve Conduits

Peripheral nerve regeneration is a complicated and long-term medical challenge that requires suitable guides for bridging nerve injury gaps and restoring nerve functions. Many natural and synthetic polymers have been used to fabricate nerve conduits as well as luminal fillers for achieving desired nerve regenerative functions. It is important to understand the intrinsic properties of these polymers and techniques that have been used for fabricating nerve conduits. Previously extensive reviews have been focused on the biological functions and in vivo performance of polymeric nerve conduits. In this paper, we emphasize on the structures, thermal and mechanical properties of these naturally derived synthetic polymers, and their fabrication methods. These aspects are critical for the performance of fabricated nerve conduits. By learning from the existing candidates, we can advance the strategies for designing novel polymeric systems with better properties for nerve regeneration.