Optimization of Schwann cell adhesion in response to shear stress in an in vitro model for peripheral nerve tissue engineering

The potential of graphene coatings as neural interfaces

Recent advances in nanotechnology design and fabrication have shaped the landscape for the development of ideal cell interfaces based on biomaterials. A holistic evaluation of the requirements for a cell interface is a highly complex task. Biocompatibility is a crucial requirement which is affected by the interface's properties, including elemental composition, morphology, and surface chemistry. This review explores the current state-of-the-art on graphene coatings produced by chemical vapor deposition (CVD) and applied as neural interfaces, detailing the key properties required to design an interface capable of physiologically interacting with neural cells. The interfaces are classified into substrates and scaffolds to differentiate the planar and three-dimensional environments where the cells can adhere and proliferate. The role of specific features such as mechanical properties, porosity and wettability are investigated. We further report on the specific brain-interface applications where CVD graphene paved the way to revolutionary advances in biomedicine. Future studies on the long-term effects of graphene-based materials in vivo will unlock even more potentially disruptive neuro-applications.

Microfluidic Systems for Neural Cell Studies

Whereas the axons of the peripheral nervous system (PNS) spontaneously regenerate after an injury, the occurring regeneration is rarely successful because axons are usually directed by inappropriate cues. Therefore, finding successful ways to guide neurite outgrowth, in vitro, is essential for neurogenesis. Microfluidic systems reflect more appropriately the in vivo environment of cells in tissues such as the normal fluid flow within the body, consistent nutrient delivery, effective waste removal, and mechanical stimulation due to fluid shear forces. At the same time, it has been well reported that topography affects neuronal outgrowth, orientation, and differentiation. In this review, we demonstrate how topography and microfluidic flow affect neuronal behavior, either separately or in synergy, and highlight the efficacy of microfluidic systems in promoting neuronal outgrowth.

The role of mechanobiology on the Schwann cell response: A tissue engineering perspective

Schwann cells (SCs), the glial cells of the peripheral nervous system (PNS), do not only form myelin sheaths thereby insulating the electrical signal propagated by the axons, but also play an essential role in the regeneration of injured axons. SCs are inextricably connected with their extracellular environment and the mechanical stimuli that are received determine their response during development, myelination and injuries. To this end, the mechanobiological response of SCs is being actively researched, as it can determine the suitability of fabricated scaffolds for tissue engineering and regenerative medicine applications. There is growing evidence that SCs are sensitive to changes in the mechanical properties of the surrounding environment (such as the type of material, its elasticity and stiffness), different topographical features provided by the environment, as well as shear stress. In this review, we explore how different mechanical stimuli affect SC behaviour and highlight the importance of exploring many different avenues when designing scaffolds for the repair of PNS injuries.

A tuned gelatin methacryloyl (GelMA) hydrogel facilitates myelination of dorsal root ganglia neurons in vitro

Investigating axonal myelination by Schwann cells (SCs) is crucial for understanding mechanisms underlying demyelination and remyelination, which may help gain insights into incurable disorders like neurodegenerative diseases. In this study, a gelatin-based hydrogel, gelatin methacryloyl (GelMA), was optimized to achieve the biocompatibility, porosity, mechanical stability, and degradability needed to provide high cell viability for dorsal root ganglia (DRG) neurons and SCs, and to enable their long-term coculture needed for myelination studies. The results of cell viability, neurite elongation, SC function and maturation, SC-axon interaction, and myelination were compared with two other commonly used substrates, namely collagen and Poly-d Lysine (PDL). The tuned GelMA constructs (Young's modulus of 32.6 ± 1.9 kPa and the median value of pore size of 10.3 μm) enhanced single axon generation (unlike collagen) and promoted the interaction of DRG neurons and SCs (unlike PDL). While DRG cells exhibited relatively higher viability on PDL after 48 h, i.e., 83.8%, the cells had similar survival rate on GelMA and collagen substrates, 66.7% and 61.5%, respectively. Further adjusting the hydrogel properties to achieve two distinct ranges of relatively small and large pores supported SCs to extend their processes freely and enabled physical contact with and wrapping around their corresponding axons. Staining the cells with myelin basic protein (MBA) and myelin-associated glycoprotein (MAG) revealed enhanced myelination on GelMA hydrogel compared to PDL and collagen. Moreover, the engineered porosity enhanced DRGs and SCs attachments and flexibility of movement across the substrate. This engineered hydrogel structure can now be further explored to model demyelination in neurodegenerative diseases, as well as to study the effects of various compounds on myelin regeneration.

Combined effect of shear stress and laser-patterned topography on Schwann cell outgrowth: synergistic or antagonistic?

Although the peripheral nervous system exhibits a higher rate of regeneration than that of the central nervous system through a spontaneous regeneration after injury, the functional recovery is fairly infrequent and misdirected. Thus, the development of successful methods to guide neuronal outgrowth, in vitro, is of great importance. In this study, a precise flow controlled microfluidic system with specific custom-designed chambers, incorporating laser-microstructured polyethylene terephthalate (PET) substrates comprising microgrooves, was fabricated to assess the combined effect of shear stress and topography on Schwann cells' behavior. The microgrooves were positioned either parallel or perpendicular to the direction of the flow inside the chambers. Additionally, the cell culture results were combined with computational flow simulations to calculate accurately the shear stress values. Our results demonstrated that wall shear stress gradients may be acting either synergistically or antagonistically depending on the substrate groove orientation relative to the flow direction. The ability to control cell alignment in vitro could potentially be used in the fields of neural tissue engineering and regenerative medicine.

Advances in ex vivo models and lab-on-a-chip devices for neural tissue engineering

The technologies related to ex vivo models and lab-on-a-chip devices for studying the regeneration of brain, spinal cord, and peripheral nerve tissues are essential tools for neural tissue engineering and regenerative medicine research. The need for ex vivo systems, lab-on-a-chip technologies and disease models for neural tissue engineering applications are emerging to overcome the shortages and drawbacks of traditional in vitro systems and animal models. Ex vivo models have evolved from traditional 2D cell culture models to 3D tissue-engineered scaffold systems, bioreactors, and recently organoid test beds. In addition to ex vivo model systems, we discuss lab-on-a-chip devices and technologies specifically for neural tissue engineering applications. Finally, we review current commercial products that mimic diseased and normal neural tissues, and discuss the future directions in this field.

Sensitivity to Strain and Shear Stress of Isolated Mechanosensitive Enteric Neurons

Within the enteric nervous system, the neurons in charge to control motility of the gastrointestinal tract reside in a particular location nestled between two perpendicular muscle layers which contract and relax. We used primary cultured myenteric neurons of male guinea pigs to study mechanosensitivity of enteric neurons in isolation. Ultrafast Neuroimaging with a voltage-sensitive dye technique was used to record neuronal activity in response to shear stress and strain. Strain was induced by locally deforming the elastic cell culture substrate next to a neuron. Measurements showed that substrate strain was mostly elongating cells. Shear stress was exerted by hydrodynamic forces in a microchannel. Both stimuli induced excitatory responses. Strain activated 14% of the stimulated myenteric neurons that responded with a spike frequency of 1.9 (0.7/3.2) Hz, whereas shear stress excited only a few neurons (5.6%) with a very low spike frequency of 0 (0/0.6) Hz. Thus, shear stress does not seem to be an adequate stimulus for mechanosensitive enteric neurons (MEN) while strain activates enteric neurons in a relevant manner. Analyzing the adaptation behavior of MEN showed that shear stress activated rapidly/slowly/ultraslowly adapting MEN (2/62/36%) whereas strain only slowly (46%) and ultraslowly (54%) MEN. Paired experiments with strain and normal stress revealed three mechanosensitive enteric neuronal populations: one strain-sensitive (37%), one normal stress-sensitive (17%) and one strain- and stress-sensitive (46%). These results indicate that shear stress does not play a role in the neuronal control of motility but normal stress and strain.

On The Origin of Shear Stress Induced Myogenesis Using PMMA Based Lab-on-Chip

One of the central themes in cell and tissue engineering is to develop an understanding as to how biophysical cues can influence cell functionality changes. The flow induced shear stress is regarded as one such biophysical cue to influence physiological changes in shear-sensitive tissues, in vivo. The origin of such phenomena is, however, poorly understood. While addressing such an issue, the present work demonstrates the intriguing synergistic effect of shear stress and spatial constraints in inducing aligned growth and differentiation of myoblast cells to myotubes. In a planned set of in vitro experiments, the regulation of laminar flow regime within a narrow window was obtained in a PMMA-based Lab-on-Chip (LOC) device, wherein the murine muscle cells (C2C12), chosen for their phenotypical differentiation stages, were cultured under graded shear conditions. The two factors of shear stress and spatial allowance were decoupled by another two sets of experiments. This aspect has been conclusively established using a PMMA device having a fixed width microchannel with varying shear and an identical amount of shear with different width of channels. On the basis of the extensive analysis of biochemical assays (WST-1, picogreen) together with gene expression using qRT-PCR and cell morphological changes (fluorescence/confocal microscopy), extensive differentiation of the myoblasts into myotubes is found to be dependent on both shear stress and spatial allocation with a maximum at an optimal shear of ca. 16 mPa. Quantitatively, the mRNA expression of myogenic biomarkers, i.e., myogenin, MyoD, and neogenin, exhibited 10- to 50-fold changes at ca. 16 mPa shear flow, compared to that under static conditions. Also, myotube aspect ratio and myotube density are modulated with shear stress and are in commensurate with gene expression changes. The flow cytometry analysis further confirmed that the cell cycle arrest at the G1/G0 phase triggers the onset of myogenesis. Taken together, the present study unambiguously establishes qualitative and quantitative biophysical basis for the origin of myogenesis toward the critical shear stress of murine myoblasts in a microfludic device, in vitro.

Galanin protects against nerve injury after shear stress in primary cultured rat cortical neurons

The neuropeptide galanin and its receptors (GalR) are found to be up-regulated in brains suffering from nerve injury, but the specific role played by galanin remains unclear. This study aimed to explore the neuroprotective role of galanin after shear stress induced nerve injury in the primary cultured cortical neurons of rats. Our results demonstrated that no significant changes in cell death and viability were found after galanin treatment when subjected to a shear stress of 5 dyn/cm(2) for 12 h, after increasing magnitude of shear stress to 10 dyn/cm(2) for 12 h, cell death was significantly increased, while galanin can inhibit the nerve injury induced by shear stress with 10 dyn/cm(2) for 12 h. Moreover, Gal2-11 (an agonist of GalR2/3) could also effectively inhibit shear stress-induced nerve injury of primary cultured cortical neurons in rats. Although GalR2 is involved in the galanin protection mechanism, there was no GalR3 expression in this system. Moreover, galanin increased the excitatory postsynaptic currents (EPSCs), which can effectively inhibit the physiological effects of shear stress. Galanin was also found to inhibit the activation of p53 and Bax, and further reversed the down regulation of Bcl-2 induced by shear stress. Our results strongly demonstrated that galanin plays a neuroprotective role in injured cortical neurons of rats.

Chronic nerve compression alters Schwann cell myelin architecture in a murine model

INTRODUCTION

Myelinating Schwann cells compartmentalize their outermost layer to form actin-rich channels known as Cajal bands. Herein we investigate changes in Schwann cell architecture and cytoplasmic morphology in a novel mouse model of carpal tunnel syndrome.

METHODS

Chronic nerve compression (CNC) injury was created in wild-type and slow-Wallerian degeneration (Wld(S) ) mice. Over 12 weeks, nerves were electrodiagnostically assessed, and Schwann cell morphology was thoroughly evaluated.

RESULTS

A decline in nerve conduction velocity and increase in g-ratio is observed without early axonal damage. Schwann cells display shortened internodal lengths and severely disrupted Cajal bands. Quite surprisingly, the latter is reconstituted without improvements to nerve conduction velocity.

CONCLUSIONS

Chronic entrapment injuries like carpal tunnel syndrome are primarily mediated by the Schwann cell response, where decreases in internodal length and myelin thickness disrupt the efficiency of impulse propagation. Restitution of Cajal bands is not sufficient for remyelination after CNC injury.

Sciatic nerve repair by acellular nerve xenografts implanted with BMSCs in rats xenograft combined with BMSCs

Acellular nerves possess the structural and biochemical features similar to those of naive endoneurial tubes, and have been proved bioactive for allogeneil graft in nerve tissue engineering. However, the source of allogenic donators is restricted in clinical treatment. To explore sufficient substitutes for acellular nerve allografts (ANA), we investigated the effectiveness of acellular nerve xenografts (ANX) combined with bone marrow stromal cells (BMSCs) on repairing peripheral nerve injuries. The acellular nerves derived from Sprague-Dawley rats and New Zealand rabbits were prepared, respectively, and BMSCs were implanted into the nerve scaffolds and cultured in vitro. All the grafts were employed to bridge 1 cm rat sciatic nerve gaps. Fifty Wistar rats were randomly divided into five groups (n = 10 per group): ANA group, ANX group, BMSCs-laden ANA group, BMSCs-laden ANX group, and autologous nerve graft group. At 8 weeks post-transplantation, electrophysiological study was performed and the regenerated nerves were assayed morphologically. Besides, growth-promoting factors in the regenerated tissues following the BMSCs integration were detected. The results indicated that compared with the acellular nerve control groups, nerve regeneration and functional rehabilitation for the xenogenic nerve transplantation integrated with BMSCs were advanced significantly, and the rehabilitation efficacy was comparable with that of the autografting. The expression of neurotrophic factors in the regenerated nerves, together with that of brain-derived neurotrophic factor (BDNF) in the spinal cord and muscles were elevated largely. In conclusion, ANX implanted with BMSCs could replace allografts to promote nerve regeneration effectively, which offers a reliable approach for repairing peripheral nerve defects.

Rapid repair of rat sciatic nerve injury using a nanosilver-embedded collagen scaffold coated with laminin and fibronectin

AIM

Scaffold with micro-channels has shown great promise in facilitating axonal regeneration after peripheral nerve injury. Significant research has focused on mimicking, in terms of composition and function, the ability of the basement membrane of Schwann cells to both promote and guide axonal regeneration. We aim to investigate the ability of a tissue-engineered scaffold with nanosilver and collagen to adsorb laminin and fibronectin, and the usefulness of this scaffold for repairing and regenerating a 10-mm peripheral nerve gap in rats.

METHODS

In this study, nanosilver-embedded collagen scaffolds were prepared and coated with laminin (LN) or LN plus fibronectin (FN). Scanning electron microscopy of the transverse and longitudinal sections of the scaffold revealed axially oriented microtubules ranging from 20 to 80 µm in diameter, and the internal surface of microtubules was found to be evenly coated with LN and FN. Energy dispersive spectrometry also confirmed an even distribution of nanosilver particles within the scaffold. To test its effectiveness in restoring neuronal connection, the scaffold was used in order to bridge 10 mm gaps in the severed sciatic nerve of rats. The rats were divided into an experimental group (receiving scaffold coated with LN and FN), a control group (receiving scaffold coated with LN only) and an autologous graft group. The functional recovery 40 days after surgery was examined by electrophysiology and sciatic nerve functional index (SFI) evaluation. FluoroGold™ (FG) retrograde tracing, toluidine blue staining and transmission electron microscopy were also used to examine the regenerated nerve fibers and to establish their myelination status.

RESULTS

The experimental group displayed partially restored nerve function. The recovery was comparable to the effect of autologous nerve graft and was better than that observed in the control group. A better functional recovery correlated with more FG-labeled neurons, higher density of toluidine blue stained nerve fibers and thicker myelin sheath.

CONCLUSION

Our results demonstrated that nanosilver-embedded collagen scaffolds with LN and FN coating is effective in aiding axonal regeneration, and recovery is comparable to the effect of an autologous nerve graft.

Biocompatibility of acellular nerves of different mammalian species for nerve tissue engineering

To explore the biocompatibility of acellular nerves of different mammalian species, for the acellular nerves derived from rats and rabbits, the morphology, immunocompatibility, and cytocompatibility with bone marrow stromal cells (BMSCs) were evaluated. The results indicated that the tridimensional architecture and main proteins of endoneurial tubes in both biomaterials were well retained. The nerve scaffolds did not show immunogenicity or cytotoxicity, but facilitated growth of BMSCs and secretion of neurotrophic factors in vitro. In conclusion, acellular nerves of different species possess favorable biocompatibility, and xenogenic acellular nerves combined with BMSCs have potential to replace allografts for peripheral nerve reconstruction.

Inhibition of N-cadherin and beta-catenin function reduces axon-induced Schwann cell proliferation

N-cadherin and beta-catenin are involved in cell adhesion and cell cycle in tumor cells and neural crest. Both are expressed at key stages of Schwann cell (SC) development, but little is known about their function in the SC lineage. We studied the role of these molecules in adult rat derived SC-embryonic dorsal root ganglion cocultures by using low-Ca(2+) conditions and specific blocking antibodies to interfere with N-cadherin function and by using small interfering RNA (siRNA) to decrease beta-catenin expression in both SC-neuron cocultures and adult rat-derived SC monocultures. N-cadherin blocking conditions decreased SC-axon association and reduced axon-induced SC proliferation. In SC monocultures, beta-catenin reduction diminished the proliferative response of SCs to the mitogen beta1-heregulin, and, in SC-DRG cocultures, beta-catenin reduction inhibited axon-contact-dependent SC proliferation. Stimulation of SC cultures with beta1-heregulin increased total beta-catenin protein amount, phosphorylation of GSK-3beta and beta-catenin presence in nuclear extracts. In conclusion, our findings suggest a previously unrecognized contribution of beta-catenin and N-cadherin to axon-induced SC proliferation.

Demyelination secondary to chronic nerve compression injury alters Schmidt-Lanterman incisures

The role of Schmidt-Lanterman incisures (SLIs) within the myelin sheath remains the subject of much debate. Previous studies have shown a positive correlation between the number of SLIs per internode and internodal width for both normal and pathological myelin internodes. As chronic nerve compression (CNC) injury induces demyelination, we sought to evaluate if CNC injury altered the occurrence of SLIs using nerve-teasing techniques and light microscopy. Rigorous examination of the teased axons from nerves subjected to CNC injury for 1 month, 2 months or 8 months revealed that there is indeed a positive correlation between the number of SLIs per internode and the internodal width. However, unlike previous studies, the degree of positive correlation between these two parameters was greater in the internodes that had undergone remyelination in response to CNC injury as compared with the internodes from control nerves. These findings support the theory that SLIs are likely to assist in the metabolic processes of the myelin sheath, including growth and maintenance of the myelin sheath.

Effects of mechanical stimuli and microfiber-based substrate on neurite outgrowth and guidance

We introduced mechanical stimuli and micropatterned substrate with microfibers to investigate their effects on neurite outgrowth along with nerve growth factor in vitro. Two types of surface morphology were used: a surface that was coated by laminin alone and a surface where in microfibers was added on the laminin surface. PC-12 cells were seeded on both surface types and cultured for 2 d. The magnitudes of shear stresses ranged from 0.10 to 1.50 Pa. Two days after stimulation by shear stress, neurite outgrowth and its direction were measured by F-actin staining and digital image processing. When a shear stress of 0.50 Pa was applied, neurons were most highly aligned with microfibers. The average length of neurite outgrowth with microfibers was largest at a shear stress of 0.25 Pa. The results suggest that micropatterned fibers and fluid-induced shear stress are promising for stimulating neurite outgrowth in a desired direction.

Shear stress alters the expression of myelin-associated glycoprotein (MAG) and myelin basic protein (MBP) in Schwann cells

Schwann cells within a peripheral nerve respond robustly after an axonal injury. Recent results have revealed that Schwann cells undergo concurrent proliferation and apoptosis after a chronic nerve injury that is independent of axonal pathology. Although the exact nature of the stimulus that produces this Schwann cell response remains unknown, we postulated that this response may be triggered directly by mechanical stimuli. Thus, we sought to determine how pure Schwann cells responded to a sustained shear stress in the form of laminar fluid flow by evaluating for proliferation, expression of S-100, myelin-associated glycoprotein (MAG), and myelin basic protein (MBP). Immunohistochemistry demonstrated that the Schwann cells were positive for S-100, MAG, and MBP in greater than 99% of the experimental cells. Stimulated cells also revealed an increased rate of proliferation by as much as 100% (p<.001). The mRNA expression of MAG and MBP was down-regulated by 21% (p<.035) and 18% (p<.015), respectively, in experimental cells from RT-PCR assays. Furthermore, Western blot showed a down-regulation in MAG and MBP protein expression by 29% (p<.035) and 35% (p<.02), respectively. This study provides novel information regarding Schwann cell direct response to this physical stimulus that is not secondary to an axonal injury.

Understanding the biology of compressive neuropathies

Compressive neuropathies are highly prevalent, debilitating conditions with variable functional recovery after surgical decompression. Chronic nerve compression injury induces concurrent Schwann cell proliferation and apoptosis in the early stages of the disorder, independent of axonal injury. These proliferating Schwann cells locally demyelinate and remyelinate in the region of injury. Furthermore, Schwann cells upregulate vascular endothelial growth factor secondary to chronic nerve compression injury and induce neovascularization to facilitate the recruitment of macrophages. In contrast to Wallerian degeneration, macrophage recruitment occurs gradually with chronic nerve compression injury and continues for a longer time. Schwann cells change their gene and protein expression in response to mechanical stimuli as shear stress decreases the expression of myelin associated glycoprotein and myelin basic protein mRNA and protein for in vitro promyelinating Schwann cells. The local down-regulation of myelin associated glycoprotein in the region of compression injury creates an environment allowing axonal sprouting that may be reversed with intraneural injections of purified myelin associated glycoprotein. These studies suggest that while the reciprocal relationship between neurons and glial cells is maintained, chronic nerve compression injury is a Schwann cell-mediated disease.