Development and evaluation of silk fibroin-based nerve grafts used for peripheral nerve regeneration

KLF7 overexpression in bone marrow stromal stem cells graft transplantation promotes sciatic nerve regeneration

OBJECTIVE

Our previous study demonstrated that the transcription factor, Krüppel-like Factor 7 (KLF7), stimulates axon regeneration following peripheral nerve injury. In the present study, we used a gene therapy approach to overexpress KLF7 in bone marrow-derived stem/stromal cells (BMSCs) as support cells, combined with acellular nerve allografts (ANAs) and determined the potential therapeutic efficacy of a KLF7-transfected BMSC nerve graft transplantation in a rodent model for sciatic nerve injury and repair.

APPROACH

We efficiently transfected BMSCs with adeno-associated virus (AAV)-KLF7, which were then seeded in ANAs for bridging sciatic nerve defects.

MAIN RESULTS

KLF7 overexpression promotes proliferation, survival, and Schwann-like cell differentiation of BMSCs in vitro. In vivo, KLF7 overexpression promotes transplanted BMSCs survival and myelinated fiber regeneration in regenerating ANAs; however, KLF7 did not improve Schwann-like cell differentiation of BMSCs within in the nerve grafts. KLF7-BMSCs significantly upregulated expression and secretion of neurotrophic factors by BMSCs, including nerve growth factor, ciliary neurotrophic factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor in regenerating ANA. KLF7-BMSCs also improved motor axon regeneration, and subsequent neuromuscular innervation and prevention of muscle atrophy. These benefits were associated with increased motor functional recovery of regenerating ANAs.

SIGNIFICANCE

Our findings suggest that KLF7-BMSCs promoted peripheral nerve axon regeneration and myelination, and ultimately, motor functional recovery. The mechanism of KLF7 action may be related to its ability to enhance transplanted BMSCs survival and secrete neurotrophic factors rather than Schwann-like cell differentiation. This study provides novel foundational data connecting the benefits of KLF7 in neural injury and repair to BMSC biology and function, and demonstrates a potential combination approach for the treatment of injured peripheral nerve via nerve graft transplant.

Axonotmesis-evoked plantar vasodilatation as a novel assessment of C-fiber afferent function after sciatic nerve injury in rats

Quantitative assessment of the recovery of nerve function, especially sensory and autonomic nerve function, remains a challenge in the field of nerve regeneration research. We previously found that neural control of vasomotor activity could be potentially harnessed to evaluate nerve function. In the present study, five different models of left sciatic nerve injury in rats were established: nerve crush injury, nerve transection/suturing, nerve defect/autografting, nerve defect/conduit repair, and nerve defect/non-regeneration. Laser Doppler perfusion imaging was used to analyze blood perfusion of the hind feet. The toe pinch test and walking track analysis were used to assess sensory and motor functions of the rat hind limb, respectively. Transmission electron microscopy was used to observe the density of unmyelinated axons in the injured sciatic nerve. Our results showed that axonotmesis-evoked vasodilatation in the foot 6 months after nerve injury/repair recovered to normal levels in the nerve crush injury group and partially in the other three repair groups; whereas the nerve defect/non-regeneration group exhibited no recovery in vasodilatation. Furthermore, the recovery index of axonotmesis-evoked vasodilatation was positively correlated with toe pinch reflex scores and the density of unmyelinated nerve fibers in the regenerated nerve. As C-fiber afferents are predominantly responsible for dilatation of the superficial vasculature in the glabrous skin in rats, the present findings indicate that axonotmesis-evoked vasodilatation can be used as a novel way to assess C-afferent function recovery after peripheral nerve injury. This study was approved by the Ethics Committee for Laboratory Animals of Nantong University of China (approval No. 20130410-006) on April 10, 2013.

SilkBridge™: a novel biomimetic and biocompatible silk-based nerve conduit

Silk fibroin (Bombyx mori) was used to manufacture a nerve conduit (SilkBridge™) characterized by a novel 3D architecture. The wall of the conduit consists of two electrospun layers (inner and outer) and one textile layer (middle), perfectly integrated at the structural and functional level. The manufacturing technology conferred high compression strength on the device, thus meeting clinical requirements for physiological and pathological compressive stresses. In vitro cell interaction studies were performed through direct contact assays with SilkBridge™ using the glial RT4-D6P2T cells, a schwannoma cell line, and a mouse motor neuron NSC-34 cell line. The results revealed that the material is capable of sustaining cell proliferation, that the glial RT4-D6P2T cells increased their density and organized themselves in a glial-like morphology, and that NSC-34 motor neurons exhibited a greater neuritic length with respect to the control substrate. In vivo pilot assays were performed on adult female Wistar rats. A 10 mm long gap in the median nerve was repaired with 12 mm SilkBridge™. At two weeks post-operation several cell types colonized the lumen. Cells and blood vessels were also visible between the different layers of the conduit wall. Moreover, the presence of regenerated myelinated fibers with a thin myelin sheath at the proximal level was observed. Taken together, all these results demonstrated that SilkBridge™ has an optimized balance of biomechanical and biological properties, being able to sustain a perfect cellular colonization of the conduit and the progressive growth of the regenerating nerve fibers.

Injection of Fluoro-Gold into the tibial nerve leads to prolonged but reversible functional deficits in rats

Tract tracing with neuronal tracers not only represents a straightforward approach to identify axonal projection connection between regions of the nervous system at distance but also provides compelling evidence for axonal regeneration. An ideal neuronal tracer meets certain criteria including high labeling efficacy, minimal neurotoxicity, rapid labeling, suitable stability in vivo, and compatibility to tissue processing for histological/immunohistochemical staining. Although labeling efficacy of commonly used fluorescent tracers has been studied extensively, neurotoxicity and their effect on neural functions remains poorly understood. In the present study, we comprehensively evaluated motor and sensory nerve function 2-24 weeks after injection of retrograde tracer Fluoro-Gold (FG), True Blue (TB) or Fluoro-Ruby (FR) in the tibial nerve in adult Spague-Dawley rats. We found that motor and sensory nerve functions were completely recovered by 24 weeks after tracer exposure, and that FG lead to a more prolonged delay in functional recovery than TB. These findings shed light on the long-term effect of tracers on nerve function and peripheral axonal regeneration, and therefore have implications in selection of appropriate tracers in relevant studies.

Novel spiral structured nerve guidance conduits with multichannels and inner longitudinally aligned nanofibers for peripheral nerve regeneration

Nerve guidance conduits (NGCs) are artificial substitutes for autografts, which serve as the gold standard in treating peripheral nerve injury. A recurring challenge in tissue engineered NGCs is optimizing the cross-sectional surface area to achieve a balance between allowing nerve infiltration while supporting maximum axonal extension from the proximal to distal stump. In this study, we address this issue by investigating the efficacy of an NGC with a higher cross-sectional surface composed of spiral structures and multi-channels, coupled with inner longitudinally aligned nanofibers and protein on aiding nerve repair in critical sized nerve defect. The NGCs were implanted into 15-mm-long rat sciatic nerve injury gaps for 4 weeks. Nerve regeneration was assessed using an established set of assays, including the walking track analysis, electrophysiological testing, pinch reflex assessment, gastrocnemius muscle measurement, and histological assessment. The results indicated that the novel NGC design yielded promising data in encouraging nerve regeneration within a relatively short recovery time. The performance of the novel NGC for nerve regeneration was superior to that of the control nerve conduits with tubular structures. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1410-1419, 2019.

Review: Bioengineering approach for the repair and regeneration of peripheral nerve

Complex craniofacial surgeries of damaged tissues have several limitations, which present complications and challenges when trying to replicate facial function and structure. Traditional treatment techniques have shown suitable nerve function regeneration with various drawbacks. As technology continues to advance, new methods have been explored in order to regenerate damaged nerves in an effort to more efficiently and effectively regain original function and structure. This article will summarize recent bioengineering strategies involving biodegradable composite scaffolds, bioactive factors, and external stimuli alone or in combination to support peripheral nerve regeneration. Particular emphasis is made on the contributions of growth factors and electrical stimulation on the regenerative process.

Gold nanorods reinforced silk fibroin nanocomposite for peripheral nerve tissue engineering applications

Nowadays, regenerating peripheral nerves injuries (PNIs) remain a major clinical challenge, which has gained a great attention between scientists. Here, we represent a nanocomposite based on silk fibroin reinforced gold nanorods (SF/GNRs) to evaluate the proliferation and attachment of PC12 cells. The morphological characterization of nanocomposites with transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) showed that the fabricated scaffolds have porous structure with interconnected pores that is suitable for cell adhesion and growth. GNRs significantly improved the poor electrical conductivity of bulk silk fibroin scaffold. Evaluating the morphology of PC12 cells on the scaffold also confirmed the normal morphology of cells with good rate of adhesion. SF/GNRs nanocomposites showed better cellular attachment, growth and proliferation without any toxicity compared with bulk SF scaffold. Moreover, immunostaining studies represented the overexpression of neural specific proteins like nestin and neuron specific enolase (NSE) in the cells cultured on SF/GNRs nanocomposites in comparison to neat SF scaffolds.

Piezoelectric Biomaterials for Sensors and Actuators

Recent advances in materials, manufacturing, biotechnology, and microelectromechanical systems (MEMS) have fostered many exciting biosensors and bioactuators that are based on biocompatible piezoelectric materials. These biodevices can be safely integrated with biological systems for applications such as sensing biological forces, stimulating tissue growth and healing, as well as diagnosing medical problems. Herein, the principles, applications, future opportunities, and challenges of piezoelectric biomaterials for medical uses are reviewed thoroughly. Modern piezoelectric biosensors/bioactuators are developed with new materials and advanced methods in microfabrication/encapsulation to avoid the toxicity of conventional lead-based piezoelectric materials. Intriguingly, some piezoelectric materials are biodegradable in nature, which eliminates the need for invasive implant extraction. Together, these advancements in the field of piezoelectric materials and microsystems can spark a new age in the field of medicine.

Bioactive Silk-Based Nerve Guidance Conduits for Augmenting Peripheral Nerve Repair

Repair of peripheral nerve injuries depends upon complex biology stemming from the manifold and challenging injury-healing processes of the peripheral nervous system. While surgical treatment options are available, they tend to be characterized by poor clinical outcomes for the injured patients. This is particularly apparent in the clinical management of a nerve gap whereby nerve autograft remains the best clinical option despite numerous limitations; in addition, effective repair becomes progressively more difficult with larger gaps. Nerve conduit strategies based on tissue engineering approaches and the use of silk as scaffolding material have attracted much attention in recent years to overcome these limitations and meet the clinical demand of large gap nerve repair. This review examines the scientific advances made with silk-based conduits for peripheral nerve repair. The focus is on enhancing bioactivity of the conduits in terms of physical guidance cues, inner wall and lumen modification, and imbuing novel conductive functionalities.

All-Organic Conductive Biomaterial as an Electroactive Cell Interface

Various attractive materials are being used in bioelectronics recently. In this paper, hydroxymethyl-3,4-ethylenedioxythiophene (EDOT-OH) has been in situ integrated and polymerized on the surface of the regenerated silk fibroin (RSF) film to construct a biocompatible electrode. In order to improve the efficiency of in situ polymerization, sodium dodecyl sulfate (SDS) was adopted as surfactant to construct a well-organized and stable poly(hydroxymethyl-3,4-ethylenedioxythiophene) (PEDOT-OH) coating, whereas ammonium persulfate was used as oxidant. The effects of dosages of surfactant and oxidant, initial pH value, and monomer concentration on the polymerization were studied. Under the optimal conditions, the RSF/PEDOT-OH film exhibited a square resistance of 3.28 × 10⁵ Ω corresponding to a conductance of 6.1 × 10^-3 S/cm. Scanning electron microscope images indicated that PEDOT-OH was deposited uniformly on the surface of the RSF film with SDS. Furthermore, Fourier transform infrared spectroscopy confirmed that interactions existed between the peptide linkages of silk fibroin (SF) macromolecules and PEDOT-OH. The RSF/PEDOT-OH film displayed favorable electrochemical stability, biocompatibility, and fastness. This study provides a feasible method to endow conductivity to RSF materials in various forms. In addition, the conductive layer and biocompatible silk substrate make the RSF/PEDOT-OH biomaterial highly suitable for potential applications in bioelectric devices, sensors, and tissue engineering.

Nondestructive, Label-Free Characterization of Mechanical Microheterogeneity in Biomimetic Materials

We propose a novel nondestructive, label-free, mechanical characterization method for composite biomimetic materials. The method combines microscale-force measurement, bright-field microscopy based deformation analysis, and finite-element methods (FEM) to study the heterogeneity in bioengineered composite materials. The method was used to study silk fibroin protein based, donut-shaped scaffolds consisting of a shell (diameter 5 mm) and a core (diameter 2 mm) with a stiff-core or a soft-core configuration. The samples were based on our previously reported bioengineered brain tissue model. Step-wise images of sample deformation were recorded as the automated mechanical stage compressed the sample. The force-compression curves were also recorded with a load cell. A MATLAB program was used to compare and match optically measured strain distribution with that found from the FEM simulations. Iterative processes are used to determine the values that best represent the elastic moduli of the shell and the core regions. The calculated moduli found from the composite models were not significantly different from the values measured separately for each material, demonstrating the efficacy of this new approach. In addition, the method successfully measured multiple distinct regions embedded in a polydimethylsiloxane block. These results demonstrated the feasibility of our method in the microheterogeneity characterization of biomimetic composite structures.

Tunable Enzymatically Cross-Linked Silk Fibroin Tubular Conduits for Guided Tissue Regeneration

Hollow tubular conduits (TCs) with tunable architecture and biological properties are in great need for modulating cell functions and drug delivery in guided tissue regeneration. Here, a new methodology to produce enzymatically cross-linked silk fibroin TCs is described, which takes advantage of the tyrosine groups present in silk structure that are known to allow the formation of a covalently cross-linked hydrogel. Three different processing methods are used as a final step to modulate the properties of the silk-based TCs. This approach allows to virtually adjust any characteristic of the final TCs. The final microstructure ranges from a nonporous to a highly porous network, allowing the TCs to be selectively porous to 4 kDa molecules, but not to human skin fibroblasts. Mechanical properties are dependent both on the processing method and thickness of the TCs. Bioactivity is observed after 30 days of immersion in simulated body fluid only for the TCs submitted to a drying processing method (50 °C). The in vivo study performed in mice demonstrates the good biocompatibility of the TCs. The enzymatically cross-linked silk fibroin TCs are versatile and have adjustable characteristics that can be exploited in a variety of biomedical applications, particularly in guidance of peripheral nerve regeneration.

Tissue-engineered spiral nerve guidance conduit for peripheral nerve regeneration

Recently in peripheral nerve regeneration, preclinical studies have shown that the use of nerve guidance conduits (NGCs) with multiple longitudinally channels and intra-luminal topography enhance the functional outcomes when bridging a nerve gap caused by traumatic injury. These features not only provide guidance cues for regenerating nerve, but also become the essential approaches for developing a novel NGC. In this study, a novel spiral NGC with aligned nanofibers and wrapped with an outer nanofibrous tube was first developed and investigated. Using the common rat sciatic 10-mm nerve defect model, the in vivo study showed that a novel spiral NGC (with and without inner nanofibers) increased the successful rate of nerve regeneration after 6 weeks recovery. Substantial improvements in nerve regeneration were achieved by combining the spiral NGC with inner nanofibers and outer nanofibrous tube, based on the results of walking track analysis, electrophysiology, nerve histological assessment, and gastrocnemius muscle measurement. This demonstrated that the novel spiral NGC with inner aligned nanofibers and wrapped with an outer nanofibrous tube provided a better environment for peripheral nerve regeneration than standard tubular NGCs. Results from this study will benefit for future NGC design to optimize tissue-engineering strategies for peripheral nerve regeneration.

STATEMENT OF SIGNIFICANCE

We developed a novel spiral nerve guidance conduit (NGC) with coated aligned nanofibers. The spiral structure increases surface area by 4.5 fold relative to a tubular NGC. Furthermore, the aligned nanofibers was coated on the spiral walls, providing cues for guiding neurite extension. Finally, the outside of spiral NGC was wrapped with randomly nanofibers to enhance mechanical strength that can stabilize the spiral NGC. Our nerve histological data have shown that the spiral NGC had 50% more myelinated axons than a tubular structure for nerve regeneration across a 10 mm gap in a rat sciatic nerve. Results from this study can help further optimize tissue engineering strategies for peripheral nerve repair.

Nanoscaffolds in promoting regeneration of the peripheral nervous system

The ability to surgically repair peripheral nerve injuries is urgently needed. However, traditional tissue engineering techniques, such as autologous nerve transplantation, have some limitations. Therefore, tissue engineered autologous nerve grafts have become a suitable choice for nerve repair. Novel tissue engineering techniques derived from nanostructured conduits have been shown to be superior to other successful functional neurological structures with different scaffolds in terms of providing the required structures and properties. Additionally, different biomaterials and growth factors have been added to nerve scaffolds to produce unique biological effects that promote nerve regeneration and functional recovery. This review summarizes the application of different nanoscaffolds in peripheral nerve repair and further analyzes how the nanoscaffolds promote peripheral nerve regeneration.

Enhanced Peripheral Nerve Regeneration by a High Surface Area to Volume Ratio of Nerve Conduits Fabricated from Hydroxyethyl Cellulose/Soy Protein Composite Sponges

Multichannel nerve guide conduits (MCNGCs) have been widely studied and exhibited outstanding nerve repair function. However, the effect of the geometric structure of MCNGCs on the nerve repair function was still not clear. Herein, we postulated that MCNGCs with different inner surface area-to-volume ratios (ISA/V) of the channels inside the nerve guide conduits (NGCs) would show different nerve repair functions. Therefore, in current work, we constructed a series of hydroxyethyl cellulose/soy protein sponge-based nerve conduit (HSSN) with low, medium, and high ISA/V from hydroxyethyl cellulose (HEC)/soy protein isolate (SPI) composite sponges, which were abbreviated as HSSN-L, HSSN-M and HSSN-H, respectively. These NGCs were applied to bridge and repair a 10 mm long sciatic nerve defect in a rat model. Finally, the influence of ISA/V on nerve repair function was evaluated by electrophysiological assessment, histological investigation, and in vivo biodegradability testing. The results of electrophysiological assessment and histological investigation showed that the regenerative nerve tissues bridged with HSSN-H and HSSN-M had higher compound muscle action potential amplitude ratio, higher percentage of positive NF200 and S100 staining, larger axon diameter, lower G-ratio, and greater myelination thickness. Furthermore, the regenerative nerve tissues bridged with HSSN-H also showed higher density of regenerated myelinated nerve fibers and more number of myelin sheath layers. On the whole, the repair efficiency of the peripheral nerve in HSSN-H and HSSN-M groups might be better than that in HSSN-L. These results indicated that higher ISA/V based on HEC/SPI composite sponge may result in greater nerve repair functions. The conclusion provided a probable guiding principle for the structural designs of NGCs in the future.

Three-dimensional imaging and analysis of entire peripheral nerves after repair and reconstruction

BACKGROUND

We wanted to achieve a three-dimensional (3D), non-destructive imaging and automatic post-analysis and evaluation of reconstructed peripheral nerves without involving cutting and staining processes.

NEW METHOD

We used a laboratory-based micro computed tomography system for imaging, as well as a custom analysis protocol. The sample preparation was also adapted in order to achieve 3D images with true micrometer resolution and suitable contrast.

RESULTS

Analysis of the acquired tomograms enabled the quantitative assessment of 3D tissue structures, i.e., surface morphology, nerve fascicles, nerve tissue volume, geometry, and vascular regrowth. The resulting data showed significant differences between operated animals and non-operated controls.

COMPARISON WITH EXISTING METHODS

Our approach avoids the sampling error associated with conventional 2D visualization approaches and holds promise for automation of the analysis of large series of datasets.

CONCLUSIONS

We have presented a potential way for 3D imaging and analysis of entire regenerated nerves non-destructively, paving the way for high-throughput analysis of therapeutic conditions of treating adult nerve injuries.

The potential of Antheraea pernyi silk for spinal cord repair

One of the most challenging applications for tissue regeneration is spinal cord damage. There is no cure for this, partly because cavities and scar tissue formed after injury present formidable barriers that must be crossed by axons to restore function. Natural silks are considered increasingly for medical applications because they are biocompatible, biodegradable and in selected cases promote tissue growth. Filaments from wild Antheraea pernyi silkworms can support axon regeneration in peripheral nerve injury. Here we presented evidence that degummed A. pernyi filaments (DAPF) support excellent outgrowth of CNS neurons in vitro by cell attachment to the high density of arginine-glycine-aspartic acid tripeptide present in DAPF. Importantly, DAPF showed stiffness properties that are well suited to spinal cord repair by supporting cell growth mechano-biology. Furthermore, we demonstrated that DAPF induced no activation of microglia, the CNS resident immune cells, either in vitro when exposed to DAPF or in vivo when DAPF were implanted in the cord. In vitro DAPF degraded gradually with a corresponding decrease in tensile properties. We conclude that A. pernyi silk meets the major biochemical and biomaterial criteria for spinal repair, and may have potential as a key component in combinatorial strategies for spinal repair.

Bombyx mori Silk Fibroin Scaffolds with Antheraea pernyi Silk Fibroin Micro/Nano Fibers for Promoting EA. hy926 Cell Proliferation

Achieving a high number of inter-pore channels and a nanofibrous structure similar to that of the extracellular matrix remains a challenge in the preparation of Bombyx mori silk fibroin (BSF) scaffolds for tissue engineering. In this study, Antheraea pernyi silk fibroin (ASF) micro/nano fibers with an average diameter of 324 nm were fabricated by electrospinning from an 8 wt % ASF solution in hexafluoroisopropanol. The electrospun fibers were cut into short fibers (~0.5 mm) and then dispersed in BSF solution. Next, BSF scaffolds with ASF micro/nano fibers were prepared by lyophilization. Scanning electron microscope images clearly showed connected channels between macropores after the addition of ASF micro/nano fibers; meanwhile, micro/nano fibers and micropores could be clearly observed on the pore walls. The results of in vitro cultures of human umbilical vein endothelial cells (EA. hy926) on BSF scaffolds showed that fibrous BSF scaffolds containing 150% ASF fibers significantly promoted cell proliferation during the initial stage.

Electrospun silk fibroin-based neural scaffold for bridging a long sciatic nerve gap in dogs

Silk fibroin (SF)-derived silkworms represent a type of highly biocompatible biomaterial for tissue engineering. We have previously investigated biocompatibility of SF with neural cells isolated from the central nervous system or peripheral nerve system in vitro, and also developed a SF-based nerve graft conduit or tissue-engineered nerve grafts by introducing bone marrow mesenchymal stem cells, as support cells, into SF-based scaffold and evaluated the outcomes of peripheral nerve repair in a rat model. As an extension of the previous study, the electrospun technique was performed here to fabricate SF-based neural scaffold inserted with silk fibres for bridging a 30-mm-long sciatic nerve gap in dogs. Assessments including functional, histological and morphometrical analyses were applied 12 months after surgery. All the results indicated that the SF-based neural scaffold group achieved satisfactory regenerative outcomes, which were close to those achieved by autologous nerve grafts as the golden-standard for peripheral nerve repair. Overall, our results raise a potential possibility for the translation of SF-based electrospun neural scaffolds as an alternative to nerve autografts into the clinic.

A 3D-engineered porous conduit for peripheral nerve repair

End-to-end neurorrhaphy is the most commonly used method for treating peripheral nerve injury. However, only 50% of patients can regain useful function after treating with neurorrhaphy. Here, we constructed a 3D-engineered porous conduit to promote the function recovery of the transected peripheral nerve after neurorrhaphy. The conduit that consisted of a gelatin cryogel was prepared by molding with 3D-printed moulds. Due to its porous structure and excellent mechanical properties, this conduit could be collapsed by the mechanical force and resumed its original shape after absorption of normal saline. This shape-memory property allowed a simply surgery process for installing the conduits. Moreover, the biodegradable conduit could prevent the infiltration of fibroblasts and reduce the risk of scar tissue, which could provide an advantageous environment for nerve regeneration. The efficiency of the conduits in assisting peripheral nerve regeneration after neurorrhaphy was evaluated in a rat sciatic nerve transected model. Results indicated that conduits significantly benefitted the recovery of the transected peripheral nerve after end-to-end neurorrhaphy on the static sciatic index (SSI), electrophysiological results and the re-innervation of the gastrocnemius muscle. This work demonstrates a biodegradable nerve conduit that has potentially clinical application in promoting the neurorrhaphy.

Effect of melatonin supplemented at the light or dark period on recovery of sciatic nerve injury in rats

Peripheral nerve injuries can cause disabilities, social or economic problems. Melatonin, the secretory product of the pineal gland has antioxidant and anti-inflammatory actions. The aim of the present study was to investigate the effect of melatonin on the recovery of sciatic nerve after injury, comparing its effect when given in the light or the dark periods. Forty adult male Albino rats were allocated into four groups: control, nerve injury, nerve injury + melatonin given at light and nerve injury + melatonin given at dark. Nerve injury was initiated by clamping the sciatic nerve. Sciatic functional index (SFI) was measured preoperatively and postoperatively. Melatonin was given daily for six weeks. Recovery of the function was analyzed by functional analysis, electrophysiological analysis and biochemical measurement of Superoxide dismutase (SOD), Interleukin 1-beta (IL-1 β), Nerve growth factor (NGF), and bcl-2. Melatonin improved SFI, nerve conduction velocity (NCV) and the force of gastrocnemius muscle contraction as compared to the untreated rats. SOD activity, NGF, and bcl-2 were significantly increased, while IL-1β was significantly decreased after melatonin treatment as compared to the untreated injury group. SFI reached the control level; muscle contraction and IL-1B were significantly improved in the group treated with melatonin in the dark. Melatonin fastened the neural recovery and may be used in the treatment of nerve injury and it induced better nerve regeneration when the rats were treated during the dark period.

Poly(l-lysine) modified zein nanofibrous membranes as efficient scaffold for adhesion, proliferation, and differentiation of neural stem cells

Cell viability, adhesion, proliferation, and differentiation of neural stem cells (NSCs) on zein nanofibrous membranes could be improved by poly(l-lysine) modification.

Scaffoldless tissue-engineered nerve conduit promotes peripheral nerve regeneration and functional recovery after tibial nerve injury in rats

Damage to peripheral nerve tissue may cause loss of function in both the nerve and the targeted muscles it innervates. This study compared the repair capability of engineered nerve conduit (ENC), engineered fibroblast conduit (EFC), and autograft in a 10-mm tibial nerve gap. ENCs were fabricated utilizing primary fibroblasts and the nerve cells of rats on embryonic day 15 (E15). EFCs were fabricated utilizing primary fibroblasts only. Following a 12-week recovery, nerve repair was assessed by measuring contractile properties in the medial gastrocnemius muscle, distal motor nerve conduction velocity in the lateral gastrocnemius, and histology of muscle and nerve. The autografts, ENCs and EFCs reestablished 96%, 87% and 84% of native distal motor nerve conduction velocity in the lateral gastrocnemius, 100%, 44% and 44% of native specific force of medical gastrocnemius, and 63%, 61% and 67% of native medial gastrocnemius mass, respectively. Histology of the repaired nerve revealed large axons in the autograft, larger but fewer axons in the ENC repair, and many smaller axons in the EFC repair. Muscle histology revealed similar muscle fiber cross-sectional areas among autograft, ENC and EFC repairs. In conclusion, both ENCs and EFCs promoted nerve regeneration in a 10-mm tibial nerve gap repair, suggesting that the E15 rat nerve cells may not be necessary for nerve regeneration, and EFC alone can suffice for peripheral nerve injury repair.

Bone marrow mesenchymal stem cell-derived acellular matrix-coated chitosan/silk scaffolds for neural tissue regeneration

A novel tissue engineered nerve graft (TENG) was used for the first time to bridge a 60 mm long nerve gap in a dog sciatic nerve and achieved satisfactory results.

Silk/agarose scaffolds with tunable properties via SDS assisted rapid gelation

We developed a simple approach to fabricate silk/agarose scaffolds with tunable properties via controlling the gelation degree of silk fibroin.

A multi-walled silk fibroin/silk sericin nerve conduit coated with poly(lactic-co-glycolic acid) sheath for peripheral nerve regeneration

The linearly oriented multi-walled silk fibroin/silk sericin (SF/SS) nerve conduits (NCs) can provide physical cues similar to native peripheral nerve fasciculi, but the mechanical properties of which are not excellent enough. In this study, NCs with a novel and bionic design with dual structures were developed. The important features of our NCs is that the internal skeleton (the multi-walled SF/SS conduits) has a bionic structure similar to the architecture of native peripheral nerve fasciculi, which is beneficial for nerve regeneration, and the outer sheath (the hollow poly(lactic-co-glycolic acid) [PLGA] conduits) could provide strong mechanical protection for the internal skeleton. The linearly oriented multi-walled SF/SS conduit was fabricated and inserted in the hollow PLGA sheath lumen and then used for the bridge across the sciatic nerve defect in rats. The outcome of the peripheral nerve repair post implantation was evaluated. The functional and morphological parameters were examined and showed that the novel PLGA-coated SF/SS NCs could promote peripheral nerve regeneration, approaching those elicited by nerve autografts that are the first candidate for repair of peripheral nerve defects. Thus, these updated NCs have potential usefulness to enhance functional recovery after repair of peripheral nerve defect.

Silk fibroin film from golden-yellow Bombyx mori is a biocomposite that contains lutein and promotes axonal growth of primary neurons

The use of doped silk fibroin (SF) films and substrates from Bombyx mori cocoons for green nanotechnology and biomedical applications has been recently highlighted. Cocoons from coloured strains of B. mori, such as Golden-Yellow, contain high levels of pigments that could have a huge potential for the fabrication of SF based biomaterials targeted to photonics, optoelectronics and neuroregenerative medicine. However, the features of extracted and regenerated SF from cocoons of B. mori Golden-Yellow strain have never been reported. Here we provide a chemophysical characterization of regenerated silk fibroin (RSF) fibers, solution, and films obtained from cocoons of a Golden-Yellow strain of B. mori, by SEM, (1) H-NMR, HPLC, FT-IR, Raman and UV-Vis spectroscopy. We found that the extracted solution and films from B. mori Golden-Yellow fibroin displayed typical Raman spectroscopic and optical features of carotenoids. HPLC-analyses revealed that lutein was the carotenoid contained in the fiber and RSF biopolymer from yellow cocoons. Notably, primary neurons cultured on yellow SF displayed a threefold higher neurite length than those grown of white SF films. The results we report pave the way to expand the potential use of yellow SF in the field of neuroregenerative medicine and provide green chemistry approaches in biomedicine.

Bioactive polymeric scaffolds for tissue engineering

A variety of engineered scaffolds have been created for tissue engineering using polymers, ceramics and their composites. Biomimicry has been adopted for majority of the three-dimensional (3D) scaffold design both in terms of physicochemical properties, as well as bioactivity for superior tissue regeneration. Scaffolds fabricated via salt leaching, particle sintering, hydrogels and lithography have been successful in promoting cell growth in vitro and tissue regeneration in vivo. Scaffold systems derived from decellularization of whole organs or tissues has been popular due to their assured biocompatibility and bioactivity. Traditional scaffold fabrication techniques often failed to create intricate structures with greater resolution, not reproducible and involved multiple steps. The 3D printing technology overcome several limitations of the traditional techniques and made it easier to adopt several thermoplastics and hydrogels to create micro-nanostructured scaffolds and devices for tissue engineering and drug delivery. This review highlights scaffold fabrication methodologies with a focus on optimizing scaffold performance through the matrix pores, bioactivity and degradation rate to enable tissue regeneration. Review highlights few examples of bioactive scaffold mediated nerve, muscle, tendon/ligament and bone regeneration. Regardless of the efforts required for optimization, a shift in 3D scaffold uses from the laboratory into everyday life is expected in the near future as some of the methods discussed in this review become more streamlined.

KLF7-transfected Schwann cell graft transplantation promotes sciatic nerve regeneration

Our former study demonstrated that Krüppel-like Factor 7 (KLF7) is a transcription factor that stimulates axonal regeneration after peripheral nerve injury. Currently, we used a gene therapy approach to overexpress KLF7 in Schwann cells (SCs) and assessed whether KLF7-transfected SCs graft could promote sciatic nerve regeneration. SCs were transfected by adeno-associated virus 2 (AAV2)-KLF7 in vitro. Mice were allografted by an acellular nerve (ANA) with either an injection of DMEM (ANA group), SCs (ANA+SCs group) or AAV2-KLF7-transfected SCs (ANA+KLF7-SCs group) to assess repair of a sciatic nerve gap. The results indicate that KLF7 overexpression promoted the proliferation of both transfected SCs and native SCs. The neurite length of the dorsal root ganglia (DRG) explants was enhanced. Several beneficial effects were detected in the ANA+KLF7-SCs group including an increase in the compound action potential amplitude, sciatic function index score, enhanced expression of PKH26-labeling transplant SCs, peripheral myelin protein 0, neurofilaments, S-100, and myelinated regeneration nerve. Additionally, HRP-labeled motoneurons in the spinal cord, CTB-labeled sensory neurons in the DRG, motor endplate density and the weight ratios of target muscles were increased by the treatment while thermal hyperalgesia was diminished. Finally, expression of KLF7, NGF, GAP43, TrkA and TrkB were enhanced in the grafted SCs, which may indicate that several signal pathways may be involved in conferring the beneficial effects from KLF7 overexpression. We concluded that KLF7-overexpressing SCs promoted axonal regeneration of the peripheral nerve and enhanced myelination, which collectively proved KLF-SCs as a novel therapeutic strategy for injured nerves.

Natural Occurring Silks and Their Analogues as Materials for Nerve Conduits

Spider silk and its synthetic derivatives have a light weight in combination with good strength and elasticity. Their high cytocompatibility and low immunogenicity make them well suited for biomaterial products such as nerve conduits. Silk proteins slowly degrade enzymatically in vivo, thus allowing for an initial therapeutic effect such as in nerve scaffolding to facilitate endogenous repair processes, and then are removed. Silks are biopolymers naturally produced by many species of arthropods including spiders, caterpillars and mites. The silk fibers are secreted by the labial gland of the larvae of some orders of Holometabola (insects with pupa) or the spinnerets of spiders. The majority of studies using silks for biomedical applications use materials from silkworms or spiders, mostly of the genus Nephila clavipes. Silk is one of the most promising biomaterials with effects not only in nerve regeneration, but in a number of regenerative applications. The development of silks for human biomedical applications is of high scientific and clinical interest. Biomaterials in use for biomedical applications have to meet a number of requirements such as biocompatibility and elicitation of no more than a minor inflammatory response, biodegradability in a reasonable time and specific structural properties. Here we present the current status in the field of silk-based conduit development for nerve repair and discuss current advances with regard to potential clinical transfer of an implantable nerve conduit for enhancement of nerve regeneration.

Dry-Spun Silk Produces Native-Like Fibroin Solutions

Silk’s outstanding mechanical properties and energy efficient solidification mechanisms provide inspiration for biomaterial self-assembly as well as offering a diverse platform of materials suitable for many biotechnology applications. Experiments now reveal that the mulberry silkworm Bombyx mori secretes its silk in a practically “unspun” state that retains much of the solvent water and exhibits a surprisingly low degree of molecular order (β-sheet crystallinity) compared to the state found in a fully formed and matured fiber. These new observations challenge the general understanding of silk spinning and in particular the role of the spinning duct for structure development. Building on this discovery we report that silk spun in low humidity appears to arrest a molecular annealing process crucial for β-sheet formation. This, in turn, has significant positive implications, enabling the production of a high fidelity reconstituted silk fibroin with properties akin to the gold standard of unspun native silk.

Multichannel silk protein/laminin grafts for spinal cord injury repair

The physical, chemical, and bioactive cues provided by biomaterials are critical for spinal cord regeneration following injury. In this study, we investigated the bioactivity of a silk-based scaffold for nerve tissue remodeling that featured morphological guidance in the form of ridges as well as bioactive molecules. Multichannel/laminin (LN) silk scaffolds stimulated growth, development, and the extension of primary hippocampal neurons after 7 days of culture in vitro. And then, the multichannel/LN silk scaffolds were implanted into 2-mm-long hemisection defects in Sprague-Dawley rat spinal cords for 70 days to evaluate their bioactivities of spinal cord remolding. Our results demonstrated that animal behavior was significantly improved in the multichannel/LN group, as evaluated by Basso-Beattie-Bresnahan score, whereas the implantation of multichannels and random pores groups resulted in recurring limps. Moreover, histology and immunohistochemical staining revealed an increase in blood vessels and expression of growth associated protein-43 and neurofilament-200 as well as reduced expression of glial fibrillary acidic protein in the multichannel/LN group, which contributed to the rebuilding of spinal cord defects. Thus, multichannel/LN silk scaffolds mediated cell migration, stimulated blood capillary formation, and promoted axonal extension, suggesting the utility of these scaffolds for spinal cord reconstruction. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 104A: 3045-3057, 2016.

Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Peripheral nerve injury is a common clinical entity, which may arise due to traumatic, tumorous, or even iatrogenic injury in craniomaxillofacial surgery. Despite advances in biomaterials and techniques over the past several decades, reconstruction of nerve gaps remains a challenge. Autografts are the gold standard for nerve reconstruction. Using autografts, there is donor site morbidity, subsequent sensory deficit, and potential for neuroma development and infection. Moreover, the need for a second surgical site and limited availability of donor nerves remain a challenge. Thus, increasing efforts have been directed to develop artificial nerve guidance conduits (ANCs) as new methods to replace autografts in the future. Various synthetic conduit materials have been tested in vitro and in vivo, and several first- and second-generation conduits are FDA approved and available for purchase, while third-generation conduits still remain in experimental stages. This paper reviews the current treatment options, summarizes the published literature, and assesses future prospects for the repair of peripheral nerve injury in craniomaxillofacial surgery with a particular focus on facial nerve regeneration.