Fibroblasts Colonizing Nerve Conduits Express High Levels of Soluble Neuregulin1, a Factor Promoting Schwann Cell Dedifferentiation

Interaction of graphene and WS2 with neutrophils and mesenchymal stem cells: implications for peripheral nerve regeneration

Graphene and bidimensional (2D) materials have been widely used in nerve conduits to boost peripheral nerve regeneration. Nevertheless, the experimental and commercial variability in graphene-based materials generates graphene forms with different structures and properties that can trigger entirely diverse biological responses from all the players involved in nerve repair. Herein, we focus on the graphene and tungsten disulfide (WS2) interaction with non-neuronal cell types involved in nerve tissue regeneration. We synthesize highly crystalline graphene and WS2 with scalable techniques such as thermal decomposition and chemical vapor deposition. The materials were able to trigger the activation of a neutrophil human model promoting Neutrophil Extracellular Traps (NETs) production, particularly under basal conditions, although neutrophils were not able to degrade graphene. Of note is that pristine graphene acts as a repellent for the NET adhesion, a beneficial property for nerve conduit long-term applications. Mesenchymal stem cells (MSCs) have been proposed as a promising strategy for nerve regeneration in combination with a conduit. Thus, the interaction of graphene with MSCs was also investigated, and reduced viability was observed only on specific graphene substrates. Overall, the results confirm the possibility of regulating the cell response by varying graphene properties and selecting the most suitable graphene forms.

Review on electrically conductive smart nerve guide conduit for peripheral nerve regeneration

At present, peripheral nerve injuries (PNIs) are one of the leading causes of substantial impairment around the globe. Complete recovery of nerve function after an injury is challenging. Currently, autologous nerve grafts are being used as a treatment; however, this has several downsides, for example, donor site morbidity, shortage of donor sites, loss of sensation, inflammation, and neuroma development. The most promising alternative is the development of a nerve guide conduit (NGC) to direct the restoration and renewal of neuronal axons from the proximal to the distal end to facilitate nerve regeneration and maximize sensory and functional recovery. Alternatively, the response of nerve cells to electrical stimulation (ES) has a substantial regenerative effect. The incorporation of electrically conductive biomaterials in the fabrication of smart NGCs facilitates the function of ES throughout the active proliferation state. This article overviews the potency of the various categories of electroactive smart biomaterials, including conductive and piezoelectric nanomaterials, piezoelectric polymers, and organic conductive polymers that researchers have employed latterly to fabricate smart NGCs and their potentiality in future clinical application. It also summarizes a comprehensive analysis of the recent research and advancements in the application of ES in the field of NGC.

Peripheral nerve-derived fibroblasts promote neurite outgrowth in adult dorsal root ganglion neurons more effectively than skin-derived fibroblasts

NEW FINDINGS

What is the central question of this study? Although fibroblasts are involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. What is the main finding and its importance? Nerve-derived fibroblasts have a greater neurite-promoting effect than skin-derived fibroblasts, and epineurium-derived fibroblasts can promote neurite outgrowth more effectively than parenchyma-derived fibroblasts. The epineurium-derived fibroblasts and parenchyma-derived fibroblasts have distinctly different molecular profiles, including genes of soluble factors to promote axonal growth. Fibroblasts are molecularly and functionally different depending on their localization in nerve tissue, and epineurium-derived fibroblasts might be involved in axon regeneration after peripheral nerve injury more than previously thought.

ABSTRACT

Although fibroblasts (Fb) are components of a peripheral nerve involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. The objective of the present study was to investigate the capacity of Fb derived from peripheral nerves to stimulate the outgrowth of neurites from adult dorsal root ganglion neurons and to clarify their molecular characteristics. Fibroblasts were prepared from the epineurium and parenchyma of rat sciatic nerves and skin. The Fb derived from epineurium showed the greatest effect on neurite outgrowth, followed by the Fb derived from parenchyma, indicating that Fb derived from nerves promote neurite outgrowth more effectively than skin-derived Fb. Although both soluble and cell-surface factors contributed evenly to the neurite-promoting effect of nerve-derived Fb, in crush and transection injury models, Fb were not closely associated with regenerating axons, indicating that only soluble factors from Fb are available to regenerating axons. A transcriptome analysis revealed that the molecular profiles of these Fb were distinctly different and that the gene expression profiles of soluble factors that promote axonal growth are unique to each Fb. These findings indicate that Fb are molecularly and functionally different depending on their localization in nerve tissue and that Fb derived from epineurium might be involved more than was previously thought in axon regeneration after peripheral nerve injury.

Evaluation and Comparison of the Effects of Mature Silkworm (Bombyx mori) and Silkworm Pupae Extracts on Schwann Cell Proliferation and Axon Growth: An In Vitro Study

Background

Silkworm products were first used by physicians more than 8500 years ago, in the early Neolithic period. In Persian medicine, silkworm extract has several uses for treating and preventing neurological, cardiac, and liver diseases. Mature silkworms (Bombyx mori) and their pupae contain a variety of growth factors and proteins that can be used in many repair processes, including nerve regeneration.

Objectives

The study aimed to evaluate the effects of mature silkworm (Bombyx mori), and silkworm pupae extract on Schwann cell proliferation and axon growth.

Methods

Silkworm (Bombyx mori) and silkworm pupae extracts were prepared. Then, the concentration and type of amino acids and proteins in the extracts were evaluated by Bradford assay, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and liquid chromatograph-mass spectrometer (LC-MS/MS). Also, the regenerative potential of extracts for improving Schwann cell proliferation and axon growth was examined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) assay, electron microscopy, and NeuroFilament-200 (NF-200) immunostaining.

Results

According to the results of the Bradford test, the total protein content of pupae extract was almost twice that of mature worm extract. Also, SDS-PAGE analysis revealed numerous proteins and growth factors, such as bombyrin and laminin, in extracts that are involved in the repair of the nervous system. In accordance with Bradford's results, the evaluation of extracts using LC-MS/MS revealed that the number of amino acids in pupae extract was higher than in mature silkworm extract. It was found that the proliferation of Schwann cells at a concentration of 0.25 mg/mL in both extracts was higher than the concentrations of 0.01 and 0.05 mg/mL. When using both extracts on dorsal root ganglion (DRGs), an increase in length and number was observed in axons.

Conclusions

The findings of this study demonstrated that extracts obtained from silkworms, especially pupae, can play an effective role in Schwann cell proliferation and axonal growth, which can be strong evidence for nerve regeneration, and, consequently, repairing peripheral nerve damage.

Chitosan conduits enriched with fibrin-collagen hydrogel with or without adipose-derived mesenchymal stem cells for the repair of 15-mm-long sciatic nerve defect

Hollow conduits of natural or synthetic origins have shown acceptable regeneration results in short nerve gap repair; however, results are still not comparable with the current gold standard technique "autografts". Hollow conduits do not provide a successful regeneration outcome when it comes to critical nerve gap repair. Enriching the lumen of conduits with different extracellular materials and cells could provide a better biomimicry of the natural nerve regenerating environment and is expected to ameliorate the conduit performance. In this study, we evaluated nerve regeneration in vivo using hollow chitosan conduits or conduits enriched with fibrin-collagen hydrogels alone or with the further addition of adipose-derived mesenchymal stem cells in a 15 mm rat sciatic nerve transection model. Unexpected changes in the hydrogel consistency and structural stability in vivo led to a failure of nerve regeneration after 15 weeks. Nevertheless, the molecular assessment in the early regeneration phase (7, 14, and 28 days) has shown an upregulation of useful regenerative genes in hydrogel enriched conduits compared with the hollow ones. Hydrogels composed of fibrin-collagen were able to upregulate the expression of soluble NRG1, a growth factor that plays an important role in Schwann cell transdifferentiation. The further enrichment with adipose-derived mesenchymal stem cells has led to the upregulation of other important genes such as ErbB2, VEGF-A, BDNF, c-Jun, and ATF3.

Typical and atypical properties of peripheral nerve allografts enable novel strategies to repair segmental-loss injuries

We review data showing that peripheral nerve injuries (PNIs) that involve the loss of a nerve segment are the most common type of traumatic injury to nervous systems. Segmental-loss PNIs have a poor prognosis compared to other injuries, especially when one or more mixed motor/sensory nerves are involved and are typically the major source of disability associated with extremities that have sustained other injuries. Relatively little progress has been made, since the treatment of segmental loss PNIs with cable autografts that are currently the gold standard for repair has slow and incomplete (often non-existent) functional recovery. Viable peripheral nerve allografts (PNAs) to repair segmental-loss PNIs have not been experimentally or clinically useful due to their immunological rejection, Wallerian degeneration (WD) of anucleate donor graft and distal host axons, and slow regeneration of host axons, leading to delayed re-innervation and producing atrophy or degeneration of distal target tissues. However, two significant advances have recently been made using viable PNAs to repair segmental-loss PNIs: (1) hydrogel release of Treg cells that reduce the immunological response and (2) PEG-fusion of donor PNAs that reduce the immune response, reduce and/or suppress much WD, immediately restore axonal conduction across the donor graft and re-innervate many target tissues, and restore much voluntary behavioral functions within weeks, sometimes to levels approaching that of uninjured nerves. We review the rather sparse cellular/biochemical data for rejection of conventional PNAs and their acceptance following Treg hydrogel and PEG-fusion of PNAs, as well as cellular and systemic data for their acceptance and remarkable behavioral recovery in the absence of tissue matching or immune suppression. We also review typical and atypical characteristics of PNAs compared with other types of tissue or organ allografts, problems and potential solutions for PNA use and storage, clinical implications and commercial availability of PNAs, and future possibilities for PNAs to repair segmental-loss PNIs.

Blood Vessels: The Pathway Used by Schwann Cells to Colonize Nerve Conduits

The repair of severe nerve injuries requires an autograft or conduit to bridge the gap and avoid axon dispersion. Several conduits are used routinely, but their effectiveness is comparable to that of an autograft only for short gaps. Understanding nerve regeneration within short conduits could help improve their efficacy for longer gaps. Since Schwann cells are known to migrate on endothelial cells to colonize the “nerve bridge”, the new tissue spontaneously forming to connect the injured nerve stumps, here we aimed to investigate whether this migratory mechanism drives Schwann cells to also proceed within the nerve conduits used to repair large nerve gaps. Injured median nerves of adult female rats were repaired with 10 mm chitosan conduits and the regenerated nerves within conduits were analyzed at different time points using confocal imaging of sequential thick sections. Our data showed that the endothelial cells formed a dense capillary network used by Schwann cells to migrate from the two nerve stumps into the conduit. We concluded that angiogenesis played a key role in the nerve conduits, not only by supporting cell survival but also by providing a pathway for the migration of newly formed Schwann cells.

Gli1 Regulates the Postnatal Acquisition of Peripheral Nerve Architecture

Peripheral nerves are organized into discrete compartments. Axons, Schwann cells (SCs), and endoneurial fibroblasts (EFs) reside within the endoneurium and are surrounded by the perineurium, a cellular sheath comprised of layers of perineurial glia (PNG). SC secretion of Desert Hedgehog (Dhh) regulates this organization. In Dhh nulls, the perineurium is deficient and the endoneurium is subdivided into small compartments termed minifascicles. Human Dhh mutations cause a neuropathy with similar defects. Here we examine the role of Gli1, a canonical transcriptional effector of hedgehog signaling, in regulating peripheral nerve organization in mice of both genders. We identify PNG, EFs, and pericytes as Gli1-expressing cells by genetic fate mapping. Although expression of Dhh by SCs and Gli1 in target cells is coordinately regulated with myelination, Gli1 expression unexpectedly persists in Dhh null EFs. Thus, Gli1 is expressed in EFs noncanonically (i.e., independent of hedgehog signaling). Gli1 and Dhh also have nonredundant activities. Unlike Dhh nulls, Gli1 nulls have a normal perineurium. Like Dhh nulls, Gli1 nulls form minifascicles, which we show likely arise from EFs. Thus, Dhh and Gli1 are independent signals: Gli1 is dispensable for perineurial development but functions cooperatively with Dhh to drive normal endoneurial development. During development, Gli1 also regulates endoneurial extracellular matrix production, nerve vascular organization, and has modest, nonautonomous effects on SC sorting and myelination of axons. Finally, in adult nerves, induced deletion of Gli1 is sufficient to drive minifascicle formation. Thus, Gli1 regulates the development and is required to maintain the endoneurial architecture of peripheral nerves.SIGNIFICANCE STATEMENT Peripheral nerves are organized into distinct cellular/ECM compartments: the epineurium, perineurium, and endoneurium. This organization, with its associated cellular constituents, is critical for the structural and metabolic support of nerves and their response to injury. Here, we show that Gli1, a transcription factor normally expressed downstream of hedgehog signaling, is required for the proper organization of the endoneurium but not the perineurium. Unexpectedly, Gli1 expression by endoneurial cells is independent of, and functions nonredundantly with, Schwann Cell-derived Desert Hedgehog in regulating peripheral nerve architecture. These results further delineate how peripheral nerves acquire their distinctive organization during normal development, and highlight mechanisms that may regulate their reorganization in pathologic settings, including peripheral neuropathies and nerve injury.

Heregulin Activity Assays for Residual Testing of Cell Therapy Products

Background

Heregulin is a ligand for the protooncogene product ErbB/HER that acts as  a key mitogenic factor for human Schwann cells (hSCs). Heregulin is required for sustained hSC growth in vitro but must be thoroughly removed before cell collection for transplantation due to potential safety concerns. The goal of this study was to develop simple cell-based assays to assess the effectiveness of heregulin addition to and removal from aliquots of hSC culture medium. These bioassays were based on the capacity of a β1-heregulin peptide to elicit ErbB/HER receptor signaling in adherent ErbB2+/ErbB3+ cells.

Results

Western blotting was used to measure the activity of three different β1-heregulin/ErbB-activated kinases (ErbB3/HER3, ERK/MAPK and Akt/PKB) using phospho-specific antibodies against key activating residues. The duration, dose-dependency and specificity of β1-heregulin-initiated kinase phosphorylation were investigated, and controls were implemented for assay optimization and reproducibility to detect β1-heregulin activity in the nanomolar range. Results from these assays showed that the culture medium from transplantable hSCs elicited no detectable activation of the aforementioned kinases in independent rounds of testing, indicating that the implemented measures can ensure that the final hSC product is devoid of bioactive β1-heregulin molecules prior to transplantation.

Conclusions

These assays may be valuable to detect impurities such as undefined soluble factors or factors for which other biochemical or biological assays are not yet available. Our workflow can be modified as necessary to determine the presence of ErbB/HER, ERK, and Akt activators other than β1-heregulin using native samples, such as fresh isolates from cell- or tissue extracts in addition to culture medium.

Modified Hyaluronic Acid-Laminin-Hydrogel as Luminal Filler for Clinically Approved Hollow Nerve Guides in a Rat Critical Defect Size Model

Hollow nerve guidance conduits are approved for clinical use for defect lengths of up to 3 cm. This is because also in pre-clinical evaluation they are less effective in the support of nerve regeneration over critical defect lengths. Hydrogel luminal fillers are thought to improve the regeneration outcome by providing an optimized matrix inside bioartificial nerve grafts. We evaluated here a modified hyaluronic acid-laminin-hydrogel (M-HAL) as luminal filler for two clinically approved hollow nerve guides. Collagen-based and chitosan-based nerve guides were filled with M-HAL in two different concentrations and the regeneration outcome comprehensively studied in the acute repair rat sciatic nerve 15 mm critical defect size model. Autologous nerve graft (ANG) repair served as gold-standard control. At 120 days post-surgery, all ANG rats demonstrated electrodiagnostically detectable motor recovery. Both concentrations of the hydrogel luminal filler induced improved regeneration outcome over empty nerve guides. However, neither combination with collagen- nor chitosan-based nerve guides resulted in functional recovery comparable to the ANG repair. In contrast to our previous studies, we demonstrate here that M-HAL slightly improved the overall performance of either empty nerve guide type in the critical defect size model.

Natural-Based Biomaterials for Peripheral Nerve Injury Repair

Peripheral nerve injury treatment is a relevant problem because of nerve lesion high incidence and because of unsatisfactory regeneration after severe injuries, thus resulting in a reduced patient's life quality. To repair severe nerve injuries characterized by substance loss and to improve the regeneration outcome at both motor and sensory level, different strategies have been investigated. Although autograft remains the gold standard technique, a growing number of research articles concerning nerve conduit use has been reported in the last years. Nerve conduits aim to overcome autograft disadvantages, but they must satisfy some requirements to be suitable for nerve repair. A universal ideal conduit does not exist, since conduit properties have to be evaluated case by case; nevertheless, because of their high biocompatibility and biodegradability, natural-based biomaterials have great potentiality to be used to produce nerve guides. Although they share many characteristics with synthetic biomaterials, natural-based biomaterials should also be preferable because of their extraction sources; indeed, these biomaterials are obtained from different renewable sources or food waste, thus reducing environmental impact and enhancing sustainability in comparison to synthetic ones. This review reports the strengths and weaknesses of natural-based biomaterials used for manufacturing peripheral nerve conduits, analyzing the interactions between natural-based biomaterials and biological environment. Particular attention was paid to the description of the preclinical outcome of nerve regeneration in injury repaired with the different natural-based conduits.