Schwann Cell Cultures: Biology, Technology and Therapeutics

The phenotypic changes of Schwann cells promote the functional repair of nerve injury

Functional recovery after nerve injury is a significant challenge due to the complex nature of nerve injury repair and the non-regeneration of neurons. Schwann cells (SCs), play a crucial role in the nerve injury repair process because of their high plasticity, secretion, and migration abilities. Upon nerve injury, SCs undergo a phenotypic change and redifferentiate into a repair phenotype, which helps in healing by recruiting phagocytes, removing myelin fragments, promoting axon regeneration, and facilitating myelin formation. However, the repair phenotype can be unstable, limiting the effectiveness of the repair. Recent research has found that transplantation of SCs can be an effective treatment option, therefore, it is essential to comprehend the phenotypic changes of SCs and clarify the related mechanisms to develop the transplantation therapy further.

Identification of cellular and noncellular components of mature intact human peripheral nerve

BACKGROUND AND AIMS

The goal of this study was to define basic constituents of the adult peripheral nervous system (PNS) using intact human nerve tissues.

METHODS

We combined fluorescent and chromogenic immunostaining methods, myelin-selective fluorophores, and routine histological stains to identify common cellular and noncellular elements in aldehyde-fixed nerve tissue sections. We employed Schwann cell (SC)-specific markers, such as S100β, NGFR, Sox10, and myelin protein zero (MPZ), together with axonal, extracellular matrix (collagen IV, laminin, fibronectin), and fibroblast markers to assess the SC's relationship to myelin sheaths, axons, other cell types, and the acellular environment.

RESULTS

Whereas S100β and Sox10 revealed mature SCs in the absence of other stains, discrimination between myelinating and non-myelinating (Remak) SCs required immunodetection of NGFR along with axonal and/or myelin markers. Surprisingly, our analysis of NGFR+ profiles uncovered the existence of at least 3 different novel populations of NGFR+/S100β- cells, herein referred to as nonglial cells, residing in the stroma and perivascular areas of all nerve compartments. An important proportion of the nerve's cellular content, including circa 30% of endoneurial cells, consisted of heterogenous S100β negative cells that were not associated with axons. Useful markers to identify the localization and diversity of nonglial cell types across different compartments were Thy1, CD34, SMA, and Glut1, a perineurial cell marker.

INTERPRETATION

Our optimized methods revealed additional detailed information to update our understanding of the complexity and spatial orientation of PNS-resident cell types in humans.

GRT-X Stimulates Dorsal Root Ganglia Axonal Growth in Culture via TSPO and Kv7.2/3 Potassium Channel Activation

GRT-X, which targets both the mitochondrial translocator protein (TSPO) and the Kv7.2/3 (KCNQ2/3) potassium channels, has been shown to efficiently promote recovery from cervical spine injury. In the present work, we investigate the role of GRT-X and its two targets in the axonal growth of dorsal root ganglion (DRG) neurons. Neurite outgrowth was quantified in DRG explant cultures prepared from wild-type C57BL6/J and TSPO-KO mice. TSPO was pharmacologically targeted with the agonist XBD173 and the Kv7 channels with the activator ICA-27243 and the inhibitor XE991. GRT-X efficiently stimulated DRG axonal growth at 4 and 8 days after its single administration. XBD173 also promoted axonal elongation, but only after 8 days and its repeated administration. In contrast, both ICA27243 and XE991 tended to decrease axonal elongation. In dissociated DRG neuron/Schwann cell co-cultures, GRT-X upregulated the expression of genes associated with axonal growth and myelination. In the TSPO-KO DRG cultures, the stimulatory effect of GRT-X on axonal growth was completely lost. However, GRT-X and XBD173 activated neuronal and Schwann cell gene expression after TSPO knockout, indicating the presence of additional targets warranting further investigation. These findings uncover a key role of the dual mode of action of GRT-X in the axonal elongation of DRG neurons.

Cultivation of Schwann cells from fresh and non-fresh adult equine peripheral nerves

BACKGROUND

Over the past 25 years, acquired equine polyneuropathy (AEP) has emerged as a neurological disease in Scandinavian horses. This condition is characterized by histopathological features including the presence of Schwann cell (SC) inclusions. Cultivated equine SCs would serve as a valuable resource for investigations of factors triggering this Schwannopathy. Ideally, cells should be sampled for cultivation from fresh nerves immediately after death of the animal, however the availability of fresh material is limited, due to the inconsistent case load and the inherent technical and practical challenges to collection of samples in the field. This study aimed to cultivate SCs from adult equine peripheral nerves and assess their ability to survive in sampled nerve material over time to simulate harvesting of SCs in field situations.

NEW METHODS

Peripheral nerves from five non-neurological horses were used. After euthanasia, both fresh and non-fresh nerve samples were harvested from each horse. Flow cytometry was employed to confirm the cellular identity and to determine the SC purity.

RESULTS

The results revealed successful establishment of SC cultures from adult equine peripheral nerves, with the potential to achieve high SC purity from both fresh and non-fresh nerve samples.

COMPARISON WITH EXISTING METHOD

While most SC isolation methods focus on harvest of cells from fresh nerve materials from laboratory animals, our approach highlights the possibility of utilizing SC cultures from field-harvested and transported nerve samples from horses.

CONCLUSIONS

We describe a method for isolating SCs with high purity from both fresh and non-fresh peripheral nerves of adult horses.

Role of transforming growth factor-β in peripheral nerve regeneration

Injuries caused by trauma and neurodegenerative diseases can damage the peripheral nervous system and cause functional deficits. Unlike in the central nervous system, damaged axons in peripheral nerves can be induced to regenerate in response to intrinsic cues after reprogramming or in a growth-promoting microenvironment created by Schwann cells. However, axon regeneration and repair do not automatically result in the restoration of function, which is the ultimate therapeutic goal but also a major clinical challenge. Transforming growth factor (TGF) is a multifunctional cytokine that regulates various biological processes including tissue repair, embryo development, and cell growth and differentiation. There is accumulating evidence that TGF-β family proteins participate in peripheral nerve repair through various factors and signaling pathways by regulating the growth and transformation of Schwann cells; recruiting specific immune cells; controlling the permeability of the blood-nerve barrier, thereby stimulating axon growth; and inhibiting remyelination of regenerated axons. TGF-β has been applied to the treatment of peripheral nerve injury in animal models. In this context, we review the functions of TGF-β in peripheral nerve regeneration and potential clinical applications.

Senescent Schwann cells induced by aging and chronic denervation impair axonal regeneration following peripheral nerve injury

Following peripheral nerve injury, successful axonal growth and functional recovery require Schwann cell (SC) reprogramming into a reparative phenotype, a process dependent upon c-Jun transcription factor activation. Unfortunately, axonal regeneration is greatly impaired in aged organisms and following chronic denervation, which can lead to poor clinical outcomes. While diminished c-Jun expression in SCs has been associated with regenerative failure, it is unclear whether the inability to maintain a repair state is associated with the transition into an axonal growth inhibition phenotype. We here find that reparative SCs transition into a senescent phenotype, characterized by diminished c-Jun expression and secretion of inhibitory factors for axonal regeneration in aging and chronic denervation. In both conditions, the elimination of senescent SCs by systemic senolytic drug treatment or genetic targeting improved nerve regeneration and functional recovery, increased c-Jun expression and decreased nerve inflammation. This work provides the first characterization of senescent SCs and their influence on axonal regeneration in aging and chronic denervation, opening new avenues for enhancing regeneration and functional recovery after peripheral nerve injuries.

Human Schwann Cells in vitro II. Passaging, Purification, Banking, and Labeling of Established Cultures

This manuscript describes step-by-step procedures to establish and manage fresh and cryopreserved cultures of nerve-derived human Schwann cells (hSCs) at the desired scale. Adaptable protocols are provided to propagate hSC cultures through serial passaging and perform routine manipulations such as enzymatic dissociation, purification, cryogenic preservation, live-cell labeling, and gene delivery. Expanded hSCs cultures are metabolically active, proliferative, and phenotypically stable for at least three consecutive passages. Cell yields are expected to be variable as determined by the rate of growth of individual batches and the rounds of subculture. The purity, however, can be maintained high at >95% hSC regardless of passage. The cells obtained in this manner are suitable for various applications, including small drug screens, in vitro modeling of neurodevelopmental processes, and cell transplantation. One caveat of this protocol is that continued expansion of same-batch hSC populations is eventually restricted due to senescence-linked growth arrest.

Schwann cells are axo-protective after injury irrespective of myelination status in mouse Schwann cell-neuron cocultures

Myelinating Schwann cell (SC)-dorsal root ganglion (DRG) neuron cocultures are an important technique for understanding cell-cell signalling and interactions during peripheral nervous system (PNS) myelination, injury, and regeneration. Although methods using rat SCs and neurons or mouse DRG explants are commonplace, there are no established protocols for compartmentalised myelinating cocultures with dissociated mouse cells. There consequently is a need for a coculture protocol that allows separate genetic manipulation of mouse SCs or neurons, or use of cells from different transgenic animals to complement in vivo mouse experiments. However, inducing myelination of dissociated mouse SCs in culture is challenging. Here, we describe a new method to coculture dissociated mouse SCs and DRG neurons in microfluidic chambers and induce robust myelination. Cocultures can be axotomised to study injury and used for drug treatments, and cells can be lentivirally transduced for live imaging. We used this model to investigate axon degeneration after traumatic axotomy and find that SCs, irrespective of myelination status, are axo-protective. At later timepoints after injury, live imaging of cocultures shows that SCs break up, ingest and clear axonal debris.

High-efficient serum-free differentiation of trabecular meshwork mesenchymal stem cells into Schwann-like cells on polylactide electrospun nanofibrous scaffolds

Cell-based therapies of the peripheral nerve injury (PNI) have provided satisfactory outcomes among which Schwann cells (SCs) are the most reliable candidate to improve repair of the damaged nerve, however, it is difficult to obtain sufficient amount of SCs for clinical applications. Trabecular meshwork-derived mesenchymal stem cells (TM-MSCs) are newly introduced neural crest originated MSCs, which may have a desirable potential for Schwann-like differentiation due to their common lineage. On the other hand, one of the challenges of cell-based therapies is usage of serum containing media which is inappropriate for clinical applications. In the present study, we investigated the differentiation potential of TM-MSCs into Schwann-like cells on polylactide (PLA) nanofibrous scaffolds in the presence or absence of serum. Our results revealed that PLA nanofibers had no negative effects on the cell growth and proliferation of TM-MSCs, and improved Schwann-like differentiation compared with tissue culture plates (TCPs). More importantly, when the cells cultured on the scaffold in the presence of serum-free media (SFM), expression mRNA levels of SC markers (S100B, GAP43, GFAP and SOX10) were significantly increased compared with those of serum-rich groups. Immunostaining of TM-MSCs cultured on serum-free PLA nanofibrous scaffolds also showed significant expression of GAP43, GFAP and SOX10 compared to those of control, indicating the efficient role of SFM in the differentiation of TM-MSCs into SCs lineage. Overall, the findings of this study revealed the differentiation potential of TM-MSCs to SC fate for the first time, and also showed the beneficial effects of SFM and PLA nanofibrous scaffolds as a promising approach for peripheral nerve regeneration.

Research progress on the reduced neural repair ability of aging Schwann cells

Peripheral nerve injury (PNI) is associated with delayed repair of the injured nerves in elderly patients, resulting in loss of nerve function, chronic pain, muscle atrophy, and permanent disability. Therefore, the mechanism underlying the delayed repair of peripheral nerves in aging patients should be investigated. Schwann cells (SCs) play a crucial role in repairing PNI and regulating various nerve-repair genes after injury. SCs also promote peripheral nerve repair through various modalities, including mediating nerve demyelination, secreting neurotrophic factors, establishing Büngner bands, clearing axon and myelin debris, and promoting axon remyelination. However, aged SCs undergo structural and functional changes, leading to demyelination and dedifferentiation disorders, decreased secretion of neurotrophic factors, impaired clearance of axonal and myelin debris, and reduced capacity for axon remyelination. As a result, aged SCs may result in delayed repair of nerves after injury. This review article aimed to examine the mechanism underlying the diminished neural repair ability of aging SCs.

In vitro myelination using explant culture of dorsal root ganglia: An efficient tool for analyzing peripheral nerve differentiation and disease modeling

Peripheral nerves conducting motor and somatosensory signals in vertebrate consist of myelinated and unmyelinated axons. In vitro myelination culture, generated by co-culturing Schwann cells (SCs) and dorsal root ganglion (DRG) neurons, is an indispensable tool for modeling physiological and pathological conditions of the peripheral nervous system (PNS). This technique allows researchers to overexpress or downregulate molecules investigated in neurons or SCs to evaluate the effect of such molecules on myelination. In vitro myelination experiments are usually time-consuming and labor-intensive to perform. Here we report an optimized protocol for in vitro myelination using DRG explant culture. We found that our in vitro myelination using DRG explant (IVMDE) culture not only achieves myelination with higher efficiency than conventional in vitro myelination methods, but also can be used to observe Remak bundle and non-myelinating SCs, which were unrecognizable in conventional methods. Because of these characteristics, IVMDE may be useful in modeling PNS diseases, including Charcot Marie Tooth disease (CMT), in vitro. These results suggest that IVMDE may achieve a condition more similar to peripheral nerve myelination observed during physiological development.

Culture Conditions for Human Induced Pluripotent Stem Cell-Derived Schwann Cells: A Two-Centre Study

Adult human Schwann cells represent a relevant tool for studying peripheral neuropathies and developing regenerative therapies to treat nerve damage. Primary adult human Schwann cells are, however, difficult to obtain and challenging to propagate in culture. One potential solution is to generate Schwann cells from human induced pluripotent stem cells (hiPSCs). Previously published protocols, however, in our hands did not deliver sufficient viable cell numbers of hiPSC-derived Schwann cells (hiPSC-SCs). We present here, two modified protocols from two collaborating laboratories that overcome these challenges. With this, we also identified the relevant parameters to be specifically considered in any proposed differentiation protocol. Furthermore, we are, to our knowledge, the first to directly compare hiPSC-SCs to primary adult human Schwann cells using immunocytochemistry and RT-qPCR. We conclude the type of coating to be important during the differentiation process from Schwann cell precursor cells or immature Schwann cells to definitive Schwann cells, as well as the amounts of glucose in the specific differentiation medium to be crucial for increasing its efficiency and the final yield of viable hiPSC-SCs. Our hiPSC-SCs further displayed high similarity to primary adult human Schwann cells.

Behavioral and axon histomorphometric analyses after intraneural transplantation of human skin- and nerve-derived Schwann cells in nude rats

INTRODUCTION/AIMS

We have recently isolated and expanded skin-derived Schwann cells (Sk-SCs) from human skin and showed that they are largely similar to nerve-derived Schwann cells (N-SCs). Here, we extend our investigation into functional assessments of the nude rats that received human Sk-SCs and N-SCs after intraneural delivery into crushed and decellularized tibial nerve in adult nude rats.

METHODS

Sk-SCs, N-SCs, dermal fibroblasts, or control culture medium was injected into the crushed and decellularized tibial nerve using in situ repeated freeze-thaw cycles. Animals were then subjected to a ladder rung walking test, nociceptive von Frey testing, and walking gait analysis weekly. Animals were euthanized 6 weeks after surgery, gastrocnemius and soleus muscles were weighed, distal nerves were harvested, and whole semithin cross-sections were analyzed using segmentation software.

RESULTS

N-SC-injected and dermal fibroblast-injected animals improved significantly at 4 to 6 weeks postinjury in nociceptive assessment compared with medium-injected controls. Sk-SCs recovered more rapidly in tibial functional index at 2 weeks postinjury compared with medium-injected controls. No significant difference was observed for the ladder rung walking test or muscle weight ratio. Histologically, the number of myelinated axons was significantly higher in all cell injection groups compared with medium-injected controls. No significant difference was observed in g ratio, axon diameter, or myelin thickness.

DISCUSSION

Cell injection significantly improved axon regeneration across an in situ decellularized nerve segment. However, a more human cell-permissive animal model is required to delineate functional differences between cell types for preclinical transplantation studies.

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.

Development and In Vitro Differentiation of Schwann Cells

Schwann cells are glial cells of the peripheral nervous system. They exist in several subtypes and perform a variety of functions in nerves. Their derivation and culture in vitro are interesting for applications ranging from disease modeling to tissue engineering. Since primary human Schwann cells are challenging to obtain in large quantities, in vitro differentiation from other cell types presents an alternative. Here, we first review the current knowledge on the developmental signaling mechanisms that determine neural crest and Schwann cell differentiation in vivo. Next, an overview of studies on the in vitro differentiation of Schwann cells from multipotent stem cell sources is provided. The molecules frequently used in those protocols and their involvement in the relevant signaling pathways are put into context and discussed. Focusing on hiPSC- and hESC-based studies, different protocols are described and compared, regarding cell sources, differentiation methods, characterization of cells, and protocol efficiency. A brief insight into developments regarding the culture and differentiation of Schwann cells in 3D is given. In summary, this contribution provides an overview of the current resources and methods for the differentiation of Schwann cells, it supports the comparison and refinement of protocols and aids the choice of suitable methods for specific applications.

Transgenic Schwann cells overexpressing POU6F1 promote sciatic nerve regeneration within acellular nerve allografts

Objective.Acellular nerve allograft (ANA) is an effective surgical approach used to bridge the sciatic nerve gap. The molecular regulators of post-surgical recovery are not well-known. Here, we explored the effect of transgenic Schwann cells (SCs) overexpressing POU domain class 6, transcription factor 1 (POU6F1) on sciatic nerve regeneration within ANAs. We explored the functions of POU6F1 in nerve regeneration by using a cell model of H₂O₂-induced SCs injury and transplanting SCs overexpressing POU6F1 into ANA to repair sciatic nerve gaps.Approach.Using RNA-seq, Protein-Protein Interaction network analysis, gene ontology enrichment, and Kyoto Encyclopedia of Genes and Genomes pathway analysis, we identified a highly and differentially expressed transcription factor, POU6F1, following ANA treatment of sciatic nerve gap. Expressing a high degree of connectivity, POU6F1 was predicted to play a role in peripheral nervous system myelination.Main results.To test the role of POU6F1 in nerve regeneration after ANA, we infected SCs with adeno-associated virus-POU6F1, demonstrating that POU6F1 overexpression promotes proliferation, anti-apoptosis, and migration of SCsin vitro. We also found that POU6F1 significantly upregulated JNK1/2 and c-Jun phosphorylation and that selective JNK1/2 inhibition attenuated the effects of POU6F1 on proliferation, survival, migration, and JNK1/2 and c-Jun phosphorylation. The direct interaction of POU6F1 and activated JNK1/2 was subsequently confirmed by co-immunoprecipitation. In rat sciatic nerve injury model with a 10 mm gap, we confirmed the pattern of POU6F1 upregulation and co-localization with transplanted SCs. ANAs loaded with POU6F1-overexpressing SCs demonstrated the enhanced survival of transplanted SCs, axonal regeneration, myelination, and functional motor recovery compared to the ANA group loaded by SCs-only in line within vitrofindings.Significance.This study identifies POU6F1 as a novel regulator of post-injury sciatic nerve repair, acting through JNK/c-Jun signaling in SCs to optimize therapeutic outcomes in the ANA surgical approach.

Implantable Electroceutical Approach Improves Myelination by Restoring Membrane Integrity in a Mouse Model of Peripheral Demyelinating Neuropathy

Although many efforts are undertaken to treat peripheral demyelinating neuropathies based on biochemical interventions, unfortunately, there is no approved treatment yet. Furthermore, previous studies have not shown improvement of the myelin membrane at the biomolecular level. Here, an electroceutical treatment is introduced as a biophysical intervention to treat Charcot-Marie-Tooth (CMT) disease-the most prevalent peripheral demyelinating neuropathy worldwide-using a mouse model. The specific electrical stimulation (ES) condition (50 mV mm^-1 , 20 Hz, 1 h) for optimal myelination is found via an in vitro ES screening system, and its promyelinating effect is validated with ex vivo dorsal root ganglion model. Biomolecular investigation via time-of-flight secondary ion mass spectrometry shows that ES ameliorates distribution abnormalities of peripheral myelin protein 22 and cholesterol in the myelin membrane, revealing the restoration of myelin membrane integrity. ES intervention in vivo via flexible implantable electrodes shows not only gradual rehabilitation of mouse behavioral phenotypes (balance and endurance), but also restored myelin thickness, compactness, and membrane integrity. This study demonstrates, for the first time, that an electroceutical approach with the optimal ES condition has the potential to treat CMT disease and restore impaired myelin membrane integrity, shifting the paradigm toward practical interventions for peripheral demyelinating neuropathies.

The Effect of Schwann Cells/Schwann Cell-Like Cells on Cell Therapy for Peripheral Neuropathy

Peripheral neuropathy is a common neurological issue that leads to sensory and motor disorders. Over time, the treatment for peripheral neuropathy has primarily focused on medications for specific symptoms and surgical techniques. Despite the different advantages of these treatments, functional recovery remains less than ideal. Schwann cells, as the primary glial cells in the peripheral nervous system, play crucial roles in physiological and pathological conditions by maintaining nerve structure and functions and secreting various signaling molecules and neurotrophic factors to support both axonal growth and myelination. In addition, stem cells, including mesenchymal stromal cells, skin precursor cells and neural stem cells, have the potential to differentiate into Schwann-like cells to perform similar functions as Schwann cells. Therefore, accumulating evidence indicates that Schwann cell transplantation plays a crucial role in the resolution of peripheral neuropathy. In this review, we summarize the literature regarding the use of Schwann cell/Schwann cell-like cell transplantation for different peripheral neuropathies and the potential role of promoting nerve repair and functional recovery. Finally, we discuss the limitations and challenges of Schwann cell/Schwann cell-like cell transplantation in future clinical applications. Together, these studies provide insights into the effect of Schwann cells/Schwann cell-like cells on cell therapy and uncover prospective therapeutic strategies for peripheral neuropathy.

Direct Conversion to Achieve Glial Cell Fates: Oligodendrocytes and Schwann Cells

Glia have been known for its pivotal roles in physiological and pathological conditions in the nervous system. To study glial biology, multiple approaches have been applied to utilize glial cells for research, including stem cell-based technologies. Human glial cells differentiated from pluripotent stem cells are now available, allowing us to study the structural and functional roles of glia in the nervous system, although the efficiency is still low. Direct conversion is an advanced strategy governing fate conversion of diverse cell types directly into the desired lineage. This novel strategy stands as a promising approach for preliminary research and regenerative medicine. Direct conversion employs genetic and environmental cues to change cell fate to that with the required functional cell properties while retaining maturity-related molecular features. As an alternative method, it is now possible to obtain a variety of mature cell populations that could not be obtained using conventional differentiation methods. This review summarizes current achievements in obtaining glia, particularly oligodendrocytes and Schwann cells.

Schwann Cells in Digestive System Disorders

Proper functioning of the digestive system is ensured by coordinated action of the central and peripheral nervous systems (PNS). Peripheral innervation of the digestive system can be viewed as intrinsic and extrinsic. The intrinsic portion is mainly composed of the neurons and glia of the enteric nervous system (ENS), while the extrinsic part is formed by sympathetic, parasympathetic, and sensory branches of the PNS. Glial cells are a crucial component of digestive tract innervation, and a great deal of research evidence highlights the important status of ENS glia in health and disease. In this review, we shift the focus a bit and discuss the functions of Schwann cells (SCs), the glial cells of the extrinsic innervation of the digestive system. For more context, we also provide information on the basic findings regarding the function of innervation in disorders of the digestive organs. We find diverse SC roles described particularly in the mouth, the pancreas, and the intestine. We note that most of the scientific evidence concerns the involvement of SCs in cancer progression and pain, but some research identifies stem cell functions and potential for regenerative medicine.

Models and methods to study Schwann cells

Schwann cells (SCs) are fundamental components of the peripheral nervous system (PNS) of all vertebrates and play essential roles in development, maintenance, function, and regeneration of peripheral nerves. There are distinct populations of SCs including: (1) myelinating SCs that ensheath axons by a specialized plasma membrane, called myelin, which enhances the conduction of electric impulses; (2) non‐myelinating SCs, including Remak SCs, which wrap bundles of multiple axons of small caliber, and perysinaptic SCs (PSCs), associated with motor axon terminals at the neuromuscular junction (NMJ). All types of SCs contribute to PNS regeneration through striking morphological and functional changes in response to nerve injury, are affected in peripheral neuropathies and show abnormalities and a diminished plasticity during aging. Therefore, methodological approaches to study and manipulate SCs in physiological and pathophysiological conditions are crucial to expand the present knowledge on SC biology and to devise new therapeutic strategies to counteract neurodegenerative conditions and age‐derived denervation. We present here an updated overview of traditional and emerging methodologies for the study of SCs for scientists approaching this research field.

A Review on the Role of Endogenous Neurotrophins and Schwann Cells in Axonal Regeneration

Injury to the peripheral nerve is traditionally referred to acquired nerve injury as they are the result of physical trauma due to laceration, stretch, crush and compression of nerves. However, peripheral nerve injury may not be completely limited to acquired physical trauma. Peripheral nerve injury equally implies clinical conditions like Guillain-Barré syndrome (GBS), Carpal tunnel syndrome, rheumatoid arthritis and diabetes. Physical trauma is commonly mono-neuropathic as it engages a single nerve and produces focal damage, while in the context of pathological conditions the damage is divergent involving a group of the nerve causing polyneuropathy. Damage to the peripheral nerve can cause a diverse range of manifestations from sensory impairment to loss of function with unpredictable recovery patterns. Presently no treatment option provides complete or functional recovery in nerve injury, as nerve cells are highly differentiated and inert to regeneration. However, the regenerative phenotypes in Schwann cells get expressed when a signalling cascade is triggered by neurotrophins. Neurotrophins are one of the promising biomolecules that are released naturally post-injury with the potential to exhibit better functional recovery. Pharmacological intervention modulating the expression of these neurotrophins such as brain-derived neurotrophic factor (BDNF) and pituitary adenylyl cyclase-activating peptide (PACAP) can prove to be a significant treatment option as endogenous compounds which may have remarkable innate advantage showing maximum 'biological relevance'.

Deep proteome profiling reveals signatures of age and sex differences in paw skin and sciatic nerve of naïve mice

The age and sex of studied animals profoundly impact experimental outcomes in biomedical research. However, most preclinical studies in mice use a wide-spanning age range from 4 to 20 weeks and do not assess male and female mice in parallel. This raises concerns regarding reproducibility and neglects potentially relevant age and sex differences, which are largely unknown at the molecular level in naïve mice. Here, we employed an optimized quantitative proteomics workflow in order to deeply profile mouse paw skin and sciatic nerves (SCN) - two tissues implicated in nociception and pain as well as diseases linked to inflammation, injury, and demyelination. Remarkably, we uncovered significant differences when comparing male and female mice at adolescent (4 weeks) and adult (14 weeks) age. Our analysis deciphered protein subsets and networks that were correlated with the age and/or sex of mice. Notably, among these were proteins/biological pathways with known (patho)physiological relevance, e.g., homeostasis and epidermal signaling in skin, and, in SCN, multiple myelin proteins and regulators of neuronal development. Extensive comparisons with available databases revealed that various proteins associated with distinct skin diseases and pain exhibited significant abundance changes in dependence on age and/or sex. Taken together, our study uncovers hitherto unknown sex and age differences at the level of proteins and protein networks. Overall, we provide a unique proteome resource that facilitates mechanistic insights into somatosensory and skin biology, and integrates age and sex as biological variables - a prerequisite for successful preclinical studies in mouse disease models.

Matrix stiffness mechanosensing modulates the expression and distribution of transcription factors in Schwann cells

After peripheral nerve injury, mature Schwann cells (SCs) de-differentiate and undergo cell reprogramming to convert into a specialized cell repair phenotype that promotes nerve regeneration. Reprogramming of SCs into the repair phenotype is tightly controlled at the genome level and includes downregulation of pro-myelinating genes and activation of nerve repair-associated genes. Nerve injuries induce not only biochemical but also mechanical changes in the tissue architecture which impact SCs. Recently, we showed that SCs mechanically sense the stiffness of the extracellular matrix and that SC mechanosensitivity modulates their morphology and migratory behavior. Here, we explore the expression levels of key transcription factors and myelin-associated genes in SCs, and the outgrowth of primary dorsal root ganglion (DRG) neurites, in response to changes in the stiffness of generated matrices. The selected stiffness range matches the physiological conditions of both utilized cell types as determined in our previous investigations. We find that stiffer matrices induce upregulation of the expression of transcription factors Sox2, Oct6, and Krox20, and concomitantly reduce the expression of the repair-associated transcription factor c-Jun, suggesting a link between SC substrate mechanosensing and gene expression regulation. Likewise, DRG neurite outgrowth correlates with substrate stiffness. The remarkable intrinsic physiological plasticity of SCs, and the mechanosensitivity of SCs and neurites, may be exploited in the design of bioengineered scaffolds that promote nerve regeneration upon injury.

An electroceutical approach enhances myelination via upregulation of lipid biosynthesis in the dorsal root ganglion

As the myelin sheath is crucial for neuronal saltatory conduction, loss of myelin in the peripheral nervous system (PNS) leads to demyelinating neuropathies causing muscular atrophy, numbness, foot deformities and paralysis. Unfortunately, few interventions are available for such neuropathies, because previous pharmaceuticals have shown severe side effects and failed in clinical trials. Therefore, exploring new strategies to enhance PNS myelination is critical to provide solution for such intractable diseases. This study aimed to investigate the effectiveness of electrical stimulation (ES) to enhance myelination in the mouse dorsal root ganglion (DRG) - an ex vivo model of the PNS. Mouse embryonic DRGs were extracted at E13 and seeded onto Matrigel-coated surfaces. After sufficient growth and differentiation, screening was carried out by applying ES in the 1-100 Hz range at the beginning of the myelination process. DRG myelination was evaluated via immunostaining at the intermediate (19 DIV) and mature (30 DIV) stages. Further biochemical analyses were carried out by utilizing RNA sequencing, qPCR and biochemical assays at both intermediate and mature myelination stages. Imaging of DRG myelin lipids was carried out via time-of-flight secondary ion mass spectrometry (ToF-SIMS). With screening ES conditions, optimal condition was identified at 20 Hz, which enhanced the percentage of myelinated neurons and average myelin length not only at intermediate (129% and 61%) but also at mature (72% and 17%) myelination stages. Further biochemical analyses elucidated that ES promoted lipid biosynthesis in the DRG. ToF-SIMS imaging showed higher abundance of the structural lipids, cholesterol and sphingomyelin, in the myelin membrane. Therefore, promotion of lipid biosynthesis and higher abundance of myelin lipids led to ES-mediated myelination enhancement. Given that myelin lipid deficiency is culpable for most demyelinating PNS neuropathies, the results might pave a new way to treat such diseases via electroceuticals.

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.

Retracing Schwann Cell Developmental Transitions in Embryonic Dissociated DRG/Schwann Cell Cocultures in Mice

Embryonic Dissociated Dorsal Root Ganglia (DRG) cultures are often used to investigate the role of novel molecular pathways or drugs in Schwann cell development and myelination. These cultures largely recapitulate the order of cellular and molecular events that occur in Schwann cells of embryonic nerves. However, the timing of Schwann cell developmental transitions, notably the transition from Schwann Cell Precursors (SCP) to immature Schwann cells (iSC) and then to myelinating Schwann cells, has not been estimated so far in this culture system. In this study, we determined the expression profiles of Schwann cell developmental genes during the first week of culture and then compared our data to the expression profiles of these genes in developing spinal nerves. This helped in identifying that SCP transition into iSC between the 5th and 7th day in vitro. Furthermore, we also investigated the transition of immature cells into pro-myelinating and myelinating Schwann cells upon the induction of myelination in vitro. Our results suggest that Schwann cell differentiation beyond the immature stage can be observed as early as 4 days post the induction of myelination in cocultures. Finally, we compared the myelinating potential of coculture-derived Schwann cell monocultures to cultures established from neonatal sciatic nerves and found that both these culture systems exhibit similar myelinating phenotypes. In effect, our results allow for a better understanding and interpretation of coculture experiments especially in studies that aim to elucidate the role of a novel actor in Schwann cell development and myelination.

Advances in 3D neuronal microphysiological systems: towards a functional nervous system on a chip

Microphysiological systems (MPS) designed to study the complexities of the peripheral and central nervous systems have made marked improvements over the years and have allowed researchers to assess in two and three dimensions the functional interconnectivity of neuronal tissues. The recent generation of brain organoids has further propelled the field into the nascent recapitulation of structural, functional, and effective connectivities which are found within the native human nervous system. Herein, we will review advances in culture methodologies, focused especially on those of human tissues, which seek to bridge the gap from 2D cultures to hierarchical and defined 3D MPS with the end goal of developing a robust nervous system-on-a-chip platform. These advances have far-reaching implications within basic science, pharmaceutical development, and translational medicine disciplines.

A Subpopulation of Schwann Cell-Like Cells With Nerve Regeneration Signatures Is Identified Through Single-Cell RNA Sequencing

Schwann cell-like cells (SCLCs) derived from human amniotic mesenchymal stem cells (hAMSCs) have been shown to promote peripheral nerve regeneration, but the underlying molecular mechanism was still poorly understood. In order to investigate the heterogeneity and potential molecular mechanism of SCLCs in the treatment of peripheral nerve regeneration at a single cell level, single-cell RNA sequencing was applied to profile single cell populations of hAMSCs and SCLCs. We profiled 6,008 and 5,140 single cells from hAMSCs and SCLCs, respectively. Based on bioinformatics analysis, pathways associated with proliferation, ECM organization, and tissue repair were enriched within both populations. Cell cycle analysis indicated that single cells within these two populations remained mostly in the G0/G1 phase. The transformation of single cells from hAMSCs to SCLCs was characterized by pseudotime analysis. Furthermore, we identified a subpopulation of SCLCs that highly expressed genes associated with Schwann cell proliferation, migration, and survival, such as JUN, JUND, and NRG1., Genes such as PTGS2, PITX1, VEGFA, and FGF2 that promote nerve regeneration were also highly expressed in single cells within this subpopulation, and terms associated with inflammatory and tissue repair were enriched in this subpopulation by pathway enrichment analysis. Our results indicate that a subpopulation of SCLCs with nerve regeneration signatures may be the key populations that promote nerve regeneration.

Perspective on Schwann Cells Derived from Induced Pluripotent Stem Cells in Peripheral Nerve Tissue Engineering

Schwann cells play a crucial role in successful peripheral nerve repair and regeneration by supporting both axonal growth and myelination. Schwann cells are therefore a feasible option for cell therapy treatment of peripheral nerve injury. However, sourcing human Schwann cells at quantities required for development beyond research is challenging. Due to their availability, rapid in vitro expansion, survival, and integration within the host tissue, stem cells have attracted considerable attention as candidate cell therapies. Among them, induced pluripotent stem cells (iPSCs) with the associated prospects for personalized treatment are a promising therapy to take the leap from bench to bedside. In this critical review, we firstly focus on the current knowledge of the Schwann cell phenotype in regard to peripheral nerve injury, including crosstalk with the immune system during peripheral nerve regeneration. Then, we review iPSC to Schwann cell derivation protocols and the results from recent in vitro and in vivo studies. We finally conclude with some prospects for the use of iPSCs in clinical settings.

Magnetic separation of peripheral nerve-resident cells underscores key molecular features of human Schwann cells and fibroblasts: an immunochemical and transcriptomics approach

Nerve-derived human Schwann cell (SC) cultures are irreplaceable models for basic and translational research but their use can be limited due to the risk of fibroblast overgrowth. Fibroblasts are an ill-defined population consisting of highly proliferative cells that, contrary to human SCs, do not undergo senescence in culture. We initiated this study by performing an exhaustive immunological and functional characterization of adult nerve-derived human SCs and fibroblasts to reveal their properties and optimize a protocol of magnetic-activated cell sorting (MACS) to separate them effectively both as viable and biologically competent cells. We next used immunofluorescence microscopy imaging, flow cytometry analysis and next generation RNA sequencing (RNA-seq) to unambiguously characterize the post-MACS cell products. High resolution transcriptome profiling revealed the identity of key lineage-specific transcripts and the clearly distinct neural crest and mesenchymal origin of human SCs and fibroblasts, respectively. Our analysis underscored a progenitor- or stem cell-like molecular phenotype in SCs and fibroblasts and the heterogeneity of the fibroblast populations. In addition, pathway analysis of RNA-seq data highlighted putative bidirectional networks of fibroblast-to-SC signaling that predict a complementary, yet seemingly independent contribution of SCs and fibroblasts to nerve regeneration. In sum, combining MACS with immunochemical and transcriptomics approaches provides an ideal workflow to exhaustively assess the identity, the stage of differentiation and functional features of highly purified cells from human peripheral nerve tissues.