Glial growth factor 2, a soluble neuregulin, directly increases Schwann cell motility and indirectly promotes neurite outgrowth

The Role of c-Jun and Autocrine Signaling Loops in the Control of Repair Schwann Cells and Regeneration

After nerve injury, both Schwann cells and neurons switch to pro-regenerative states. For Schwann cells, this involves reprogramming of myelin and Remak cells to repair Schwann cells that provide the signals and mechanisms needed for the survival of injured neurons, myelin clearance, axonal regeneration and target reinnervation. Because functional repair cells are essential for regeneration, it is unfortunate that their phenotype is not robust. Repair cell activation falters as animals get older and the repair phenotype fades during chronic denervation. These malfunctions are important reasons for the poor outcomes after nerve damage in humans. This review will discuss injury-induced Schwann cell reprogramming and the concept of the repair Schwann cell, and consider the molecular control of these cells with emphasis on c-Jun. This transcription factor is required for the generation of functional repair cells, and failure of c-Jun expression is implicated in repair cell failures in older animals and during chronic denervation. Elevating c-Jun expression in repair cells promotes regeneration, showing in principle that targeting repair cells is an effective way of improving nerve repair. In this context, we will outline the emerging evidence that repair cells are sustained by autocrine signaling loops, attractive targets for interventions aimed at promoting regeneration.

Tacrolimus-Induced Neurotrophic Differentiation of Adipose-Derived Stem Cells as Novel Therapeutic Method for Peripheral Nerve Injury

Peripheral nerve injuries (PNIs) are frequent traumatic injuries across the globe. Severe PNIs result in irreversible loss of axons and myelin sheaths and disability of motor and sensory function. Schwann cells can secrete neurotrophic factors and myelinate the injured axons to repair PNIs. However, Schwann cells are hard to harvest and expand in vitro, which limit their clinical use. Adipose-derived stem cells (ADSCs) are easily accessible and have the potential to acquire neurotrophic phenotype under the induction of an established protocol. It has been noticed that Tacrolimus/FK506 promotes peripheral nerve regeneration, despite the mechanism of its pro-neurogenic capacity remains undefined. Herein, we investigated the neurotrophic capacity of ADSCs under the stimulation of tacrolimus. ADSCs were cultured in the induction medium for 18 days to differentiate along the glial lineage and were subjected to FK506 stimulation for the last 3 days. We discovered that FK506 greatly enhanced the neurotrophic phenotype of ADSCs which potentiated the nerve regeneration in a crush injury model. This work explored the novel application of FK506 synergized with ADSCs and thus shed promising light on the treatment of severe PNIs.

Dexpanthenol reduces fibrosis and aids repair following nerve laceration and neurorrhaphy

The aim of the present study was to investigate the effect of dexpanthenol on nerve healing following neurorrhaphy in lacerated peripheral nerves. A total of 30 mature Sprague Dawley rats were used. Surgical sciatic nerve dissection and repair was performed on an experimental group of 20 rats. The remaining 10 rats were designated as the control group. The experimental group was divided into 2 subgroups. The surgery + saline group (SSLE; n=10) was given 1 ml/kg 0.9% sodium chloride saline intraperitoneally. The surgery + dexpanthenol group (SDPL; n=10) rats were given 500 mg/kg/day dexpanthenol intraperitoneally. Histological evaluation of the sciatic nerve tissue revealed that the fibrosis score was significantly lower in the SDPL group than in the SSLE group (P<0.001). Electrophysiological evaluation of compound muscle action potential (CMAP) indicated that the CMAP level in the SDPL group was significantly higher than that of the SSLE group (P<0.001), and the CMAP latency period was lower in the SDPL group compared with the SSLE group (P<0.001). In addition, the SDPL group malondialdehyde level was significantly lower than that of the SSLE group (P<0.001). Functional evaluation with an inclined plane test revealed a significant difference between the SSLE (39.6±5.5^˚) and SDPL (79.1±6.93^˚) groups (P<0.001). Dexpanthenol was observed to have a positive effect on nerve tissue repaired with neurorrhaphy in a rat sciatic model of laceration-type injuries similar to those frequently encountered in the clinic.

Schwann Cell-Like Cells: Origin and Usability for Repair and Regeneration of the Peripheral and Central Nervous System

Functional recovery after neurotmesis, a complete transection of the nerve fiber, is often poor and requires a surgical procedure. Especially for longer gaps (>3 mm), end-to-end suturing of the proximal to the distal part is not possible, thus requiring nerve graft implantation. Artificial nerve grafts, i.e., hollow fibers, hydrogels, chitosan, collagen conduits, and decellularized scaffolds hold promise provided that these structures are populated with Schwann cells (SC) that are widely accepted to promote peripheral and spinal cord regeneration. However, these cells must be collected from the healthy peripheral nerves, resulting in significant time delay for treatment and undesired morbidities for the donors. Therefore, there is a clear need to explore the viable source of cells with a regenerative potential similar to SC. For this, we analyzed the literature for the generation of Schwann cell-like cells (SCLC) from stem cells of different origins (i.e., mesenchymal stem cells, pluripotent stem cells, and genetically programmed somatic cells) and compared their biological performance to promote axonal regeneration. Thus, the present review accounts for current developments in the field of SCLC differentiation, their applications in peripheral and central nervous system injury, and provides insights for future strategies.

Migrating Schwann cells direct axon regeneration within the peripheral nerve bridge

Schwann cells within the peripheral nervous system possess a remarkable regenerative potential. Current research shows that peripheral nerve-associated Schwann cells possess the capacity to promote repair of multiple tissues including peripheral nerve gap bridging, skin wound healing, digit tip repair as well as tooth regeneration. One of the key features of the specialized repair Schwann cells is that they become highly motile. They not only migrate into the area of damaged tissue and become a key component of regenerating tissue but also secrete signaling molecules to attract macrophages, support neuronal survival, promote axonal regrowth, activate local mesenchymal stem cells, and interact with other cell types. Currently, the importance of migratory Schwann cells in tissue regeneration is most evident in the case of a peripheral nerve transection injury. Following nerve transection, Schwann cells from both proximal and distal nerve stumps migrate into the nerve bridge and form Schwann cell cords to guide axon regeneration. The formation of Schwann cell cords in the nerve bridge is key to successful peripheral nerve repair following transection injury. In this review, we first examine nerve bridge formation and the behavior of Schwann cell migration in the nerve bridge, and then discuss how migrating Schwann cells direct regenerating axons into the distal nerve. We also review the current understanding of signals that could activate Schwann cell migration and signals that Schwann cells utilize to direct axon regeneration. Understanding the molecular mechanism of Schwann cell migration could potentially offer new therapeutic strategies for peripheral nerve repair.

Neuregulin-1/ErbB network: An emerging modulator of nervous system injury and repair

Neuregulin-1 (Nrg-1) is a member of the Neuregulin family of growth factors with essential roles in the developing and adult nervous system. Six different types of Nrg-1 (Nrg-1 type I-VI) and over 30 isoforms have been discovered; however, their specific roles are not fully determined. Nrg-1 signals through a complex network of protein-tyrosine kinase receptors, ErbB2, ErbB3, ErbB4 and multiple intracellular pathways. Genetic and pharmacological studies of Nrg-1 and ErbB receptors have identified a critical role for Nrg-1/ErbB network in neurodevelopment including neuronal migration, neural differentiation, myelination as well as formation of synapses and neuromuscular junctions. Nrg-1 signaling is best known for its characterized role in development and repair of the peripheral nervous system (PNS) due to its essential role in Schwann cell development, survival and myelination. However, our knowledge of the impact of Nrg-1/ErbB on the central nervous system (CNS) has emerged in recent years. Ongoing efforts have uncovered a multi-faceted role for Nrg-1 in regulating CNS injury and repair processes. In this review, we provide a timely overview of the most recent updates on Nrg-1 signaling and its role in nervous system injury and diseases. We will specifically highlight the emerging role of Nrg-1 in modulating the glial and immune responses and its capacity to foster neuroprotection and remyelination in CNS injury. Nrg-1/ErbB network is a key regulatory pathway in the developing nervous system; therefore, unraveling its role in neuropathology and repair can aid in development of new therapeutic approaches for nervous system injuries and associated disorders.

The Complex Work of Proteases and Secretases in Wallerian Degeneration: Beyond Neuregulin-1

After damage, axons in the peripheral nervous system (PNS) regenerate and regrow following a process termed Wallerian degeneration, but the regenerative process is often incomplete and usually the system does not reach full recovery. Key steps to the creation of a permissive environment for axonal regrowth are the trans-differentiation of Schwann cells and the remodeling of the extracellular matrix (ECM). In this review article, we will discuss how proteases and secretases promote effective regeneration and remyelination. We will detail how they control neuregulin-1 (NRG-1) activity at the post-translational level, as the concerted action of alpha, beta and gamma secretases cooperates to balance activating and inhibitory signals necessary for physiological myelination and remyelination. In addition, we will discuss the role of other proteases in nerve repair, among which A Disintegrin And Metalloproteinases (ADAMs) and gamma-secretases substrates. Moreover, we will present how matrix metalloproteinases (MMPs) and proteases of the blood coagulation cascade participate in forming newly synthetized myelin and in regulating axonal regeneration. Overall, we will highlight how a deeper comprehension of secretases and proteases mechanism of action in Wallerian degeneration might be useful to develop new therapies with the potential of readily and efficiently improve the regenerative process.

Advances in Controlling Differentiation of Adult Stem Cells for Peripheral Nerve Regeneration

Adult stems cells, possessing the ability to grow, migrate, proliferate, and transdifferentiate into various specific phenotypes, constitute a great asset for peripheral nerve regeneration. Adult stem cells' ability to undergo transdifferentiation is sensitive to various cell-to-cell interactions and external stimuli involving interactions with physical, mechanical, and chemical cues within their microenvironment. Various studies have employed different techniques for transdifferentiating adult stem cells from distinct sources into specific lineages (e.g., glial cells and neurons). These techniques include chemical and/or electrical induction as well as cell-to-cell interactions via co-culture along with the use of various 3D conduit/scaffold designs. Such scaffolds consist of unique materials that possess controllable physical/mechanical properties mimicking cells' natural extracellular matrix. However, current limitations regarding non-scalable transdifferentiation protocols, fate commitment of transdifferentiated stem cells, and conduit/scaffold design have required new strategies for effective stem cells transdifferentiation and implantation. In this progress report, a comprehensive review of recent advances in the transdifferentiation of adult stem cells via different approaches along with multifunctional conduit/scaffolds designs is presented for peripheral nerve regeneration. Potential cellular mechanisms and signaling pathways associated with differentiation are also included. The discussion with current challenges in the field and an outlook toward future research directions is concluded.

Clinical relevance of terminal Schwann cells: An overlooked component of the neuromuscular junction

The terminal Schwann cell (tSC), a type of nonmyelinating Schwann cell, is a significant yet relatively understudied component of the neuromuscular junction. In addition to reviewing the role tSCs play on formation, maintenance, and remodeling of the synapse, we review studies that implicate tSCs in neuromuscular diseases including spinal muscular atrophy, Miller-Fisher syndrome, and amyotrophic lateral sclerosis, among others. We also discuss the importance of these cells on degeneration and regeneration after nerve injury. Knowledge of tSC biology may improve our understanding of disease pathogenesis and help us identify new and innovative therapeutic strategies for the many patients who suffer from neuromuscular disorders and nerve injuries.

Schwann cell lamellipodia regulate cell-cell interactions and phagocytosis

Lamellipodia in Schwann cells (SCs) are crucial for myelination, but their other biological functions remain largely uncharacterised. Two types of lamellipodia exist in SCs: axial lamellipodia at the outermost edge of the cell processes, and radial lamellipodia appearing peripherally along the entire cell. We have previously shown that radial lamellipodia on olfactory glia (olfactory ensheathing cells; OECs) promote cell-cell adhesion, contact-mediated migration and phagocytosis. Here we have investigated whether lamellipodia in SCs have similar roles. Using live-cell imaging, we show that the radial lamellipodia in SCs are highly motile, appear at multiple cellular sites and rapidly move in a wave-like manner. We found that axial and radial lamellipodia had strikingly different roles and are regulated by different intracellular pathways. Axial lamellipodia initiated interactions with other SCs and with neurons by contacting radial lamellipodia on SCs, and budding neurites/axons. Most SC-SC interactions resulted in repulsion, and, lamellipodial activity (unlike in OECs) did not promote contact-mediated migration. We show that lamellipodia are crucial for SC-mediated phagocytosis of both axonal debris and bacteria, and demonstrated that inhibition of lamellipodial activity by blocking the Rho/Rac pathways also inhibits phagocytosis. We also show that heregulin, which induces SC differentiation and maturation, alters lamellipodial behaviour but does not affect phagocytic activity. Overall, the results show that SC lamellipodia are important for cell interactions and phagocytosis.

KIAA1199: A novel regulator of MEK/ERK-induced Schwann cell dedifferentiation

The molecular mechanisms that regulate Schwann cell (SC) plasticity and the role of the Nrg1/ErbB-induced MEK1/ERK1/2 signalling pathway in SC dedifferentiation or in myelination remain unclear. It is currently believed that different levels of MEK1/ERK1/2 activation define the state of SC differentiation. Thus, the identification of new regulators of MEK1/ERK1/2 signalling could help to decipher the context-specific aspects driving the effects of this pathway on SC plasticity. In this perspective, we have investigated the potential role of KIAA1199, a protein that promotes ErbB and MEK1/ERK1/2 signalling in cancer cells, in SC plasticity. We depleted KIAA1199 in the SC-derived MSC80 cell line with RNA-interference-based strategy and also generated Tamoxifen-inducible and conditional mouse models in which KIAA1199 is inactivated through homologous recombination, using the Cre-lox technology. We show that the invalidation of KIAA1199 in SC decreases the expression of cJun and other negative regulators of myelination and elevates Krox20, driving them towards a pro-myelinating phenotype. We further show that in dedifferentiation conditions, SC invalidated for KIAA1199 exhibit lower myelin clearance as well as increased myelination capacity. Finally, the Nrg1-induced activation of the MEK/ERK/1/2 pathway is severely reduced when KIAA1199 is absent, indicating that KIAA1199 promotes Nrg1-dependent MEK1 and ERK1/2 activation in SCs. In conclusion, this work identifies KIAA1199 as a novel regulator of MEK/ERK-induced SC dedifferentiation and contributes to a better understanding of the molecular control of SC dedifferentiation.

Neuregulin 1 functionalization of organic fibers for Schwann cell guidance

The repair of peripheral nerve lesions is a clinical problem where the functional recovery is often far from being satisfactory, although peripheral nerves generally retain good potential for regeneration. Here, we develop a novel scaffold approach based on bioactive fibers of poly(ε-caprolactone) where nanotopographical guidance and neuregulin 1 (NRG1) cues are combined. We interface them with rat primary Schwann cells (SCs), the peripheral glial cells that drive initial regeneration of injured nerves, and found that the combination of NRG1 with parallel nano-fibrous topographies is effective in improving SC growth up to 72 h, alignment to fiber topography, and bipolar differentiation, opening original perspectives for nerve repair applications.

Gelatin-based 3D conduits for transdifferentiation of mesenchymal stem cells into Schwann cell-like phenotypes

In this study, gelatin-based 3D conduits with three different microstructures (nanofibrous, macroporous and ladder-like) were fabricated for the first time via combined molding and thermally induced phase separation (TIPS) technique for peripheral nerve regeneration. The effects of conduit microstructure and mechanical properties on the transdifferentiation of bone marrow-derived mesenchymal stem cells (MSCs) into Schwann cell (SC) like phenotypes were examined to help facilitate neuroregeneration and understand material-cell interfaces. Results indicated that 3D macroporous and ladder-like structures enhanced MSC attachment, proliferation and spreading, creating interconnected cellular networks with large numbers of viable cells compared to nanofibrous and 2D-tissue culture plate counterparts. 3D-ladder-like conduit structure with complex modulus of ∼0.4×10⁶Pa and pore size of ∼150μm provided the most favorable microenvironment for MSC transdifferentiation leading to ∼85% immunolabeling of all SC markers. On the other hand, the macroporous conduits with complex modulus of ∼4×10⁶Pa and pore size of ∼100μm showed slightly lower (∼65% for p75, ∼75% for S100 and ∼85% for S100β markers) immunolabeling. Transdifferentiated MSCs within 3D-ladder-like conduits secreted significant amounts (∼2.5pg/mL NGF and ∼0.7pg/mL GDNF per cell) of neurotrophic factors, while MSCs in macroporous conduits released slightly lower (∼1.5pg/mL NGF and 0.7pg/mL GDNF per cell) levels. PC12 cells displayed enhanced neurite outgrowth in media conditioned by conduits with transdifferentiated MSCs. Overall, conduits with macroporous and ladder-like 3D structures are promising platforms in transdifferentiation of MSCs for neuroregeneration and should be further tested in vivo.

STATEMENT OF SIGNIFICANCE

This manuscript focuses on the effect of microstructure and mechanical properties of gelatin-based 3D conduits on the transdifferentiation of mesenchymal stem cells to Schwann cell-like phenotypes. This work builds on our recently accepted manuscript in Acta Biomaterialia focused on multifunctional 2D films, and focuses on 3D microstructured conduits designed to overcome limitations of current strategies to facilitate peripheral nerve regeneration. The comparison between conduits fabricated with nanofibrous, macroporous and ladder-like microstructures showed that the ladder-like conduits showed the most favorable environment for MSC transdifferentiation to Schwann-cell like phenotypes, as seen by both immunolabeling as well as secretion of neurotrophic factors. This work demonstrates the importance of controlling the 3D microstructure to facilitate tissue engineering strategies involving stem cells that can serve as promising approaches for peripheral nerve regeneration.

TNFa/TNFR2 signaling is required for glial ensheathment at the dorsal root entry zone

Somatosensory information from the periphery is routed to the spinal cord through centrally-projecting sensory axons that cross into the central nervous system (CNS) via the dorsal root entry zone (DREZ). The glial cells that ensheath these axons ensure rapid propagation of this information. Despite the importance of this glial-axon arrangement, how this afferent nerve is assembled during development is unknown. Using in vivo, time-lapse imaging we show that as centrally-projecting pioneer axons from dorsal root ganglia (DRG) enter the spinal cord, they initiate expression of the cytokine TNFalpha. This induction coincides with ensheathment of these axons by associated glia via a TNF receptor 2 (TNFR2)-mediated process. This work identifies a signaling cascade that mediates peripheral glial-axon interactions and it functions to ensure that DRG afferent projections are ensheathed after pioneer axons complete their navigation, which promotes efficient somatosensory neural function.

Molecular Mechanisms Involved in Schwann Cell Plasticity

Schwann cell incredible plasticity is a hallmark of the utmost importance following nerve damage or in demyelinating neuropathies. After injury, Schwann cells undergo dedifferentiation before redifferentiating to promote nerve regeneration and complete functional recovery. This review updates and discusses the molecular mechanisms involved in the negative regulation of myelination as well as in the reprogramming of Schwann cells taking place early following nerve lesion to support repair. Significant advance has been made on signaling pathways and molecular components that regulate SC regenerative properties. These include for instance transcriptional regulators such as c-Jun or Notch, the MAPK and the Nrg1/ErbB2/3 pathways. This comprehensive overview ends with some therapeutical applications targeting factors that control Schwann cell plasticity and highlights the need to carefully modulate and balance this capacity to drive nerve repair.

Generation of pure cultures of autologous Schwann cells by use of biopsy specimens of the dorsal cutaneous branches of the cervical nerves of young adult dogs

OBJECTIVE To identify an optimal technique for isolation, purification, and amplification of Schwann cells (SCs) from biopsy specimens of the dorsal cutaneous branches of the cervical nerves of dogs. SAMPLE Biopsy specimens of dorsal cervical cutaneous nerves from the cadavers of three 1- to 2-year-old dogs. PROCEDURES Nerve specimens were dissected, predegenerated, and dissociated to isolate single cells. After culture to enhance SC growth, cells were immunopurified by use of magnetic beads. Cell purity was evaluated by assessing expression of cell surface antigens p75 (to detect SCs) and CD90 (to detect fibroblasts). Effects of various concentrations of recombinant human glial growth factor 2 (rhGGF2) on SC proliferation were tested. Cell doubling time was assessed in SC cultures with selected concentrations of rhGGF2. RESULTS Mean ± SD wet weight of nerve fascicles obtained from the biopsy specimens was 16.8 ± 2.8 mg. A mean predegeneration period of 8.6 days yielded approximately 6,000 cells/mg of nerve tissue, and primary culture yielded 43,000 cells/mg of nerve tissue in a mean of 11 days, of which 39.9 ± 9.1% expressed p75. Immunopurification with magnetic beads yielded a mean of 85.4 ± 1.9% p75-positive cells. Two passages of subculture with 10μM cytosine arabinoside further enhanced SC purity to a mean of 97.8 ± 1.2% p75-positive cells. Finally, rhGGF2 supplementation at a range of 40 to 100 ng/mL increased the SC proliferation rate up to 3-fold. CONCLUSIONS AND CLINICAL RELEVANCE SCs could be cultured from biopsy specimens of dorsal cervical cutaneous nerves and purified and expanded to generate adequate numbers for autologous transplants to treat dogs with spinal cord and peripheral nerve injuries.

Sulfatase-mediated manipulation of the astrocyte-Schwann cell interface

Schwann cell (SC) transplantation following spinal cord injury (SCI) may have therapeutic potential. Functional recovery is limited however, due to poor SC interactions with host astrocytes and the induction of astrogliosis. Olfactory ensheathing cells (OECs) are closely related to SCs, but intermix more readily with astrocytes in culture and induce less astrogliosis. We previously demonstrated that OECs express higher levels of sulfatases, enzymes that remove 6-O-sulfate groups from heparan sulphate proteoglycans, than SCs and that RNAi knockdown of sulfatase prevented OEC-astrocyte mixing in vitro. As human OECs are difficult to culture in large numbers we have genetically engineered SCs using lentiviral vectors to express sulfatase 1 and 2 (SC-S1S2) and assessed their ability to interact with astrocytes. We demonstrate that SC-S1S2s have increased integrin-dependent motility in the presence of astrocytes via modulation of NRG and FGF receptor-linked PI3K/AKT intracellular signaling and do not form boundaries with astrocytes in culture. SC-astrocyte mixing is dependent on local NRG concentration and we propose that sulfatase enzymes influence the bioavailability of NRG ligand and thus influence SC behavior. We further demonstrate that injection of sulfatase expressing SCs into spinal cord white matter results in less glial reactivity than control SC injections comparable to that of OEC injections. Our data indicate that sulfatase-mediated modification of the extracellular matrix can influence glial interactions with astrocytes, and that SCs engineered to express sulfatase may be more OEC-like in character. This approach may be beneficial for cell transplant-mediated spinal cord repair. GLIA 2016 GLIA 2017;65:19-33.

A novel alternative splicing isoform of NF2 identified in human Schwann cells

Vestibular schwannoma (VS) is a benign, slow-growing cranial tumor that originates from the hypertrophy of Schwann cells. The majority of sporadic VS are unilateral, and the mechanisms underlying VS tumorigenesis are not fully understood. The human neurofibromin 2 (NF2) gene encodes the tumor suppressor protein merlin and the NF2 transcript can be alternatively spliced to form numerous isoforms. The present study investigated human Schwann cells (HSCs) at the mRNA and protein level to understand the function of the alternative splicing (AS) isoform of NF2. The total RNA of HSCs was isolated and the full-length coding sequence of NF2 was amplified. The amplified products were excised from agarose gels, purified and sequenced. NF2 at a protein level was assayed by immunoprecipitation and western blot analysis. The full-length and spliced NF2 forms were amplified by polymerase chain reaction (PCR) from the HSC complementary DNA and ligated into eukaryotic expression vector pcDNA3.1(+). The plasmids were transfected into the HSC HEI-193 cell line and cell proliferation assays were performed using Cell Counting Kit-8. PCR analysis using HSC total RNA as a template revealed the presence of a shortened NF2 transcript, which was due to splicing at the 3'-end of the NF2 mRNA. Sequence analysis confirmed that this AS isoform omitted exons 11, 12, 13, 14, 15 and 16. Immunoprecipitation and western blot analysis demonstrated that the AS isoform was highly expressed in the HSCs at 38 kDa, while the wild-type (WT) isoform, which was expected at 66 kDa, was undetectable. Transfection and cell proliferation assays revealed that the WT isoform exhibited significant growth inhibition, while the AS isoform did not suppress cell growth. In conclusion, the present study detected AS NF2 isoforms in HSC for the first time, and investigated the function of the principle AS isoform. The present study suggests that although HSCs have an undetectable level of WT isoform of the NF2 protein merlin, they are not merlin-null, since they express the AS isoform. Although the AS merlin isoform has no suppressive effect on cell growth, certain mechanisms may exist that underlie this phenomenon, and this may be associated with the genesis and development of VS.

Review : The Role of Schwann cells in Neural Regeneration

The difference in regenerative capacity between the PNS and the CNS is not due to an intrinsic inability of central neurons to extend fibers. Rather, it is probably related to the environment in the CNS that is either repulsive to axonal outgrowth and/or nonsupportive of axonal elongation. In contrast, the PNS both supports and allows for axonal elongation after injury. The Schwann cell, which is the glial cell of the PNS, is strictly required for peripheral regeneration. Here we discuss recent work describing the biology of Schwann cell- dependent regeneration, discuss what is known of the molecular basis of this phenomenon, and how it might apply to the damaged CNS. NEUROSCIENTIST 5:208-216, 1999

Neuregulin/ErbB Signaling in Developmental Myelin Formation and Nerve Repair

Myelin is essential for rapid and accurate conduction of electrical impulses by axons in the central and peripheral nervous system (PNS). Myelin is formed in the early postnatal period, and developmental myelination in the PNS depends on axonal signals provided by Nrg1/ErbB receptors. In addition, Nrg1 is required for effective nerve repair and remyelination in adulthood. We discuss here similarities and differences in Nrg1/ErbB functions in developmental myelination and remyelination after nerve injury.

The scales and tales of myelination: using zebrafish and mouse to study myelinating glia

Myelin, the lipid-rich sheath that insulates axons to facilitate rapid conduction of action potentials, is an evolutionary innovation of the jawed-vertebrate lineage. Research efforts aimed at understanding the molecular mechanisms governing myelination have primarily focused on rodent models; however, with the advent of the zebrafish model system in the late twentieth century, the use of this genetically tractable, yet simpler vertebrate for studying myelination has steadily increased. In this review, we compare myelinating glial cell biology during development and regeneration in zebrafish and mouse and enumerate the advantages and disadvantages of using each model to study myelination. This article is part of a Special Issue entitled SI: Myelin Evolution.

Laser fabricated discontinuous anisotropic microconical substrates as a new model scaffold to control the directionality of neuronal network outgrowth

Patterning of neuronal outgrowth in vitro is important in tissue engineering as well as for the development of neuronal interfaces with desirable characteristics. To date, this has been achieved with the aid of micro- and nanofabrication techniques giving rise to various anisotropic topographies, either in the form of continuous or discontinuous structures. In this study we propose a currently unexplored geometry of a 3D culture substrate for neuronal cell growth comprising discontinuous subcellular microstructures with anisotropic geometrical cross-section. Specifically, using laser precision 3D micro/nano fabrication techniques, silicon substrates comprising arrays of parallel oriented elliptical microcones (MCs) were fabricated to investigate whether a discontinuous geometry comprising anisotropic features at the subcellular level could influence the alignment of peripheral nervous system cell populations. It was shown that both Schwann cells and axons of sympathetic neurons were parallel oriented onto the MCs of elliptical shape, while they exhibited a random orientation onto the MCs of arbitrary shape. Notably, this topography-induced guidance effect was also observed in more complex cell culture systems, such as the organotypic culture whole dorsal root ganglia (DRG) explants. Our results suggest that a discontinuous topographical pattern could promote Schwann cell and axonal alignment, provided that it hosts anisotropic geometrical features, even though the sizes of those range at the subcellular lengthscale. The laser-patterned arrays of MCs presented here could potentially be a useful platform for patterning neurons into artificial networks, allowing the study of neuronal cells interactions under 3D ex-vivo conditions.

Intrinsic mechanisms underlying the neurotrophic activity of adipose derived stem cells

Adipose derived stem cells (ADSC) can be differentiated into Schwann cell-like cells which enhance nerve function and regeneration. However, the signalling mechanisms underlying the neurotrophic potential of ADSC remain largely unknown. In this study, we hypothesised that ADSC, upon stimulation with a combination of growth factors, could rapidly produce brain derived neurotrophic factor (BDNF) with a similar molecular mechanism to that functioning in the nervous system. Within 48 h of stimulation, ADSC demonstrated potent neurotrophic effects on dorsal root ganglion neurons, at a magnitude equivalent to that of the longer term differentiated Schwann cell-like cells. Stimulated ADSC showed rapid up-regulation of the neuronal activity dependent promoter BDNF exon IV along with an augmented expression of full length protein encoding BDNF exon IX. BDNF protein was secreted at a concentration similar to that produced by differentiated Schwann cell-like cells. Stimulation also activated the BDNF expression gating transcription factor, cAMP responsive element binding (CREB) protein. However, blocking phosphorylation of CREB with the protein kinase A small molecule inhibitor H89 did not suppress secretion of BDNF protein. These results suggest rapid BDNF production in ADSC is mediated through multiple compensatory pathways independent of, or in addition to, the CREB neuronal activation cascade.

Schwann cell response on polypyrrole substrates upon electrical stimulation

Current injury models suggest that Schwann cell (SC) migration and guidance are necessary for successful regeneration and synaptic reconnection after peripheral nerve injury. The ability of conducting polymers such as polypyrrole (PPy) to exhibit chemical, contact and electrical stimuli for cells has led to much interest in their use for neural conduits. Despite this interest, there has been very little research on the effect that electrical stimulation (ES) using PPy has on SC behavior. Here we investigate the mechanism by which SCs interact with PPy in the presence of an electric field. Additionally, we explored the effect that the adsorption of different serum proteins on PPy upon the application of an electric field has on SC migration. The results indicate an increase in average displacement of the SC with ES, resulting in a net anodic migration. Moreover, indirect effects of protein adsorption due to the oxidation of the film upon the application of ES were shown to have a larger effect on migration speed than on migration directionality. These results suggest that SC migration speed is governed by an integrin- or receptor-mediated mechanism, whereas SC migration directionality is governed by electrically mediated phenomena. These data will prove invaluable in optimizing conducting polymers for their different biomedical applications such as nerve repair.

Skin derived precursor Schwann cells improve behavioral recovery for acute and delayed nerve repair

Previous work has shown that infusion of skin-derived precursors pre-differentiated into Schwann cells (SKP-SCs) can remyelinate injured and regenerating axons, and improve indices of axonal regeneration and electrophysiological parameters in rodents. We hypothesized that SKP-SC therapy would improve behavioral outcomes following nerve injury repair and tested this in a pre-clinical trial in 90 rats. A model of sciatic nerve injury and acellular graft repair was used to compare injected SKP-SCs to nerve-derived Schwann cells or media, and each was compared to the gold standard nerve isograft repair. In a second experiment, rats underwent right tibial nerve transection and received either acute or delayed direct nerve repair, with injections of either 1) SKP-SCs distal to the repair site, 2) carrier medium alone, or 3) dead SKP-SCs, and were followed for 4, 8 or 17weeks. For delayed repairs, both transected nerve ends were capped and repaired 11weeks later, along with injections of cells or media as above, and followed for 9 additional weeks (total of 20weeks). Rats were serially tested for skilled locomotion and a slip ratio was calculated for the horizontal ladder-rung and tapered beam tasks. Immediately after nerve injury and with chronic denervation, slip ratios were dramatically elevated. In the GRAFT repair study, the SKP-SC treated rats showed statistically significant improvement in ladder rung as compared to all other groups, and exhibited the greatest similarity to the sham controls on the tapered beam by study termination. In the ACUTE repair arm, the SKP-SC group showed marked improvement in ladder rung slip ratio as early as 5weeks after surgery, which was sustained for the duration of the experiment. Groups that received media and dead SKP-SCs improved with significantly slower progression. In the DELAYED repair arm, the SKP-SC group became significantly better than other groups 7weeks after the repair, while the media and the dead SKP-SCs showed no significant improvement in slip ratios. On histomorphometrical analysis, SKP-SC group showed significantly increased mean axon counts while the percent myelin debris was significantly lower at both 4 and 8weeks, suggesting that a less inhibitory micro-environment may have contributed to accelerated axonal regeneration. For delayed repair, mean axon counts were significantly higher in the SKP-SC group. Compound action potential amplitudes and muscle weights were also improved by cell therapy. In conclusion, SKP-SC therapy improves behavioral recovery after acute, chronic and nerve graft repair beyond the current standard of microsurgical nerve repair.

Lysophosphatidic acid (LPA) and its receptor, LPA1 , influence embryonic schwann cell migration, myelination, and cell-to-axon segregation

Schwann cell (SC) migration is an important step preceding myelination and remyelination in the peripheral nervous system, and can be promoted by peptide factors like neuregulins. Here we present evidence that a lipid factor, lysophosphatidic acid (LPA), influences both SC migration and peripheral myelination through its cognate G protein-coupled receptor (GPCR) known as LPA1 . Ultrastructural analyses of peripheral nerves in mouse null-mutants for LPA1 showed delayed SC-to-axon segregation, polyaxonal myelination by single SCs, and thinner myelin sheaths. In primary cultures, LPA promoted SC migration through LPA1 , while analysis of conditioned media from purified dorsal root ganglia neurons using HPLC/MS supported the production of LPA by these neurons. The heterotrimeric G-alpha protein, Gαi , and the small GTPase, Rac1, were identified as important downstream signaling components of LPA1 . These results identify receptor mediated LPA signaling between neurons and SCs that promote SC migration and contribute to the normal development of peripheral nerves through effects on SC-axon segregation and myelination.

Axonal neuregulin 1 is a rate limiting but not essential factor for nerve remyelination

Neuregulin 1 acts as an axonal signal that regulates multiple aspects of Schwann cell development including the survival and migration of Schwann cell precursors, the ensheathment of axons and subsequent elaboration of the myelin sheath. To examine the role of this factor in remyelination and repair following nerve injury, we ablated neuregulin 1 in the adult nervous system using a tamoxifen inducible Cre recombinase transgenic mouse system. The loss of neuregulin 1 impaired remyelination after nerve crush, but did not affect Schwann cell proliferation associated with Wallerian degeneration or axon regeneration or the clearance of myelin debris by macrophages. Myelination changes were most marked at 10 days after injury but still apparent at 2 months post-crush. Transcriptional analysis demonstrated reduced expression of myelin-related genes during nerve repair in animals lacking neuregulin 1. We also studied repair over a prolonged time course in a more severe injury model, sciatic nerve transection and reanastamosis. In the neuregulin 1 mutant mice, remyelination was again impaired 2 months after nerve transection and reanastamosis. However, by 3 months post-injury axons lacking neuregulin 1 were effectively remyelinated and virtually indistinguishable from control. Neuregulin 1 signalling is therefore an important factor in nerve repair regulating the rate of remyelination and functional recovery at early phases following injury. In contrast to development, however, the determination of myelination fate following nerve injury is not dependent on axonal neuregulin 1 expression. In the early phase following injury, axonal neuregulin 1 therefore promotes nerve repair, but at late stages other signalling pathways appear to compensate.

Isolation and culture of Schwann cells

Primarily cultured Schwann cells are essential for the investigation of molecular mechanisms regulating proliferation, survival, differentiation, and myelination of Schwann cell and for the development of efficient transplantation for regeneration of injured spinal cord or peripheral nervous system. Here we describe a basic protocol for isolation and purification of primary Schwann cell from neonatal rat or mouse and discuss some modifications adapted to the culturing from adult nerves and optional methods for Schwann cell enrichment.

Neuregulin 1 role in Schwann cell regulation and potential applications to promote peripheral nerve regeneration

Neuregulin 1 (NRG1) is a multifunctional and versatile protein: its numerous isoforms can signal in a paracrine, autocrine, or juxtacrine manner, playing a fundamental role during the development of the peripheral nervous system and during the process of nerve repair, suggesting that the treatment with NRG1 could improve functional outcome following injury. Accordingly, the use of NRG1 in vivo has already yielded encouraging results. The aim of this review is to focus on the role played by the different NRG1 isoforms during peripheral nerve regeneration and remyelination and to identify good candidates to be used for the development of tissue engineered medical devices delivering NRG1, with the objective of promoting better nerve repair.

Intrinsic migratory properties of cultured Schwann cells based on single-cell migration assay

The migration of Schwann cells is critical for development of peripheral nervous system and is essential for regeneration and remyelination after nerve injury. Although several factors have been identified to regulate Schwann cell migration, intrinsic migratory properties of Schwann cells remain elusive. In this study, based on time-lapse imaging of single isolated Schwann cells, we examined the intrinsic migratory properties of Schwann cells and the molecular cytoskeletal machinery of soma translocation during migration. We found that cultured Schwann cells displayed three motile phenotypes, which could transform into each other spontaneously during their migration. Local disruption of F-actin polymerization at leading front by a Cytochalasin D or Latrunculin A gradient induced collapse of leading front, and then inhibited soma translocation. Moreover, in migrating Schwann cells, myosin II activity displayed a polarized distribution, with the leading process exhibiting higher expression than the soma and trailing process. Decreasing this front-to-rear difference of myosin II activity by frontal application of a ML-7 or BDM (myosin II inhibitors) gradient induced the collapse of leading front and reversed soma translocation, whereas, increasing this front-to-rear difference of myosin II activity by rear application of a ML-7 or BDM gradient or frontal application of a Caly (myosin II activator) gradient accelerated soma translocation. Taken together, these results suggest that during migration, Schwann cells display malleable motile phenotypes and the extension of leading front dependent on F-actin polymerization pulls soma forward translocation mediated by myosin II activity.

Molecular control of Schwann cell migration along peripheral axons: keep moving!

The development of the peripheral nervous system (PNS) is a highly dynamic process, during which motor and sensory axons innervate distal targets, such as skeletal muscles and skin. Axonal function depends critically on support from Schwann cells, the main glial cell type in the PNS. Schwann cells originate from the neural crest, migrate along outgrowing axons and associate with axons along their entire length prior to ensheathment or myelination. How axonal growth and the migration of Schwann cells is coordinated at the level of reciprocal axon-glial signaling is the fascinating subject of ongoing research. Neuregulin-1 (NRG1) type III, an axonal membrane-bound ligand for receptor tyrosine kinases of the ErbB family, acts as a "master regulator" of peripheral myelination. In addition, NRG1-ErbB signaling directs the development of the Schwann cell lineage and regulates the proliferation and survival of Schwann cells. Studies in zebrafish have identified a direct role of NRG1 type III in Schwann cell migration, but to what extend NRG1 serves a similar function in the mammalian PNS is not clear. We have employed a mouse superior cervical ganglion explant culture system, in which the migration of endogenous Schwann cells along outgrowing axons can be visualized by time-lapse imaging. Using this approach, we found that NRG1 type III-ErbB signaling regulates the colonization of distal axonal segments by Schwann cells. However, our data suggest an indirect effect of NRG1 type III-ErbB signaling via the support of Schwann cell survival in proximal axonal regions rather than a direct effect on Schwann cell motility.

Dose-dependent facilitation of peripheral nerve regeneration by bone marrow-derived mononuclear cells: a randomized controlled study: laboratory investigation

OBJECT

Bone marrow-derived stem cells enhance the rate of regeneration of neuronal cells leading to clinical improvement in nerve injury, spinal cord injury, and brain infarction. Recent experiments in the local application of bone marrow-derived mononuclear cells (BM-MNCs) in models of sciatic nerve transection in rats have suggested their beneficial role in nerve regeneration, although the effects of variable doses of stem cells on peripheral nerve regeneration have never been specifically evaluated in the literature. In this paper, the authors evaluated the dose-dependent role of BM-MNCs in peripheral nerve regeneration in a model of sciatic nerve transection in rats.

METHODS

The right sciatic nerve of 60 adult female Wistar rats (randomized into 2 test groups and 1 control group, 20 rats in each group) underwent transection under an operating microscope. The cut ends of the nerve were approximated using 2 epineural microsutures. The gap was filled with low-dose (5 million BM-MNCs/100 μl phosphate-buffered saline [PBS]) rat BM-MNCs in one group, high-dose (10 million BM-MNCs/100 μl PBS) rat BM-MNCs in another group, and only PBS in the control group, and the approximated nerve ends were sealed using fibrin glue. Histological assessment was performed after 30 days by using semiquantitative and morphometric analyses and was done to assess axonal regeneration, percentage of myelinated fibers, axonal diameter, fiber diameter, and myelin thickness at distal-most sites (10 mm from site of repair), intermediate distal sites (5 mm distal to the repair site), and site of repair.

RESULTS

The recovery of nerve cell architecture after nerve anastomosis was far better in the high-dose BM-MNC group than in the low-dose BM-MNC and control groups, and it was most evident (p < 0.02 in the majority of the parameters [3 of 4]) at the distal-most site. Overall, the improvement in myelin thickness was most significant with incremental dosage of BM-MNCs, and was evident at the repair, intermediate distal, and distal-most sites (p = 0.001).

CONCLUSIONS

This study emphasizes the role of BM-MNCs, which can be isolated easily from bone marrow aspirates, in peripheral nerve injury and highlights their dose-dependent facilitation of nerve regeneration.

Schwann cells migrate along axons in the absence of GDNF signaling

Background

During development neural crest derived Schwann Cell (SC) precursors migrate to nerve trunks and populate nascent nerves. Axonal ensheathment by SC is a prerequisite for normal nerve function and the integrity of myelinated as well as nonmyelinated axons. To provide adequate support functions, SC colonize entire nerves. One important prerequisite for this is their migration into distal axonal regions.

Results

Here, we studied the role of Glial cell line derived neurotrophic factor (GDNF), a TGF-beta related growth factor, for SC migration. To this end we used a superior cervical ganglion (SCG) explant-SC migration assay, GDNF null mutant mouse embryos and a chemical inhibitor for GDNF signaling in combination with time-lapse imaging. We found that GDNF signaling is dispensable for SC migration along murine embryonic sympathetic axons. Furthermore, in vivo analyzes revealed that SC migration along the sciatic nerve is also not dependent on GDNF.

Conclusions

In contrast to previous in vitro findings in the sciatic nerve and a SC precursor cell line, our results clearly indicate that GDNF is dispensable for embryonic SC migration. This is demonstrated for the sympathetic nervous system and also for the sciatic nerve in mouse.

Neuregulin1 administration increases axonal elongation in dissociated primary sensory neuron cultures

Neuregulin1 is a family of growth and differentiation factors involved in various functions of both peripheral and central nervous system including the regenerative processes that underlie regeneration of damaged peripheral nerves. In the present study we tested in vitro the effect of Neuregulin1 administration on dissociated rat dorsal root ganglion (DRG). Activity of neuregulin1 was compared to the activity of nerve growth factor in the same in vitro experimental model. Results showed that neurite outgrowth is enhanced by the addition of both neuregulin1 and nerve growth factor to the culture medium. While neuregulin1 was responsible for the growth of longer neurites, DRG neurons incubated with nerve growth factor showed shorter and more branched axons. Using enzyme-linked immunosorbent assay we also showed that the release of nerve growth factor, but not of brain derived neurotrophic factor is improved in DRG neuron treated with neuregulin1. On the other hand, the assay with growth factor blocking antibody, showed that effects exerted by neuregulin1 on neurite outgrowth is only partially due to the release of nerve growth factor. Taken together the results of this study provide a better understanding on the role of neuregulin1 in sensory neurons.

Isolation of adult stem cells and their differentiation to Schwann cells

Peripheral nerve injuries are an economic burden for society in general and despite advanced microsurgical reconstruction of the damaged nerves the functional result is unsatisfactory with poor sensory recovery and reduced motor functions (Wiberg and Terenghi, Surg Technol Int 11:303-310, 2003). In the treatment of nerve injuries transplantation of a nerve graft is often necessary, especially in nerve gap injuries.Schwann cells (SC) are the key facilitators of peripheral nerve regeneration and are responsible for the formation and maintenance of the myelin sheath around axons in peripheral nerve fibers. They are essential for nerve regeneration after nerve injuries as they produce extracellular matrix molecules, integrins, and trophic factors providing guidance and trophic support for regenerating axons (Wiberg and Terenghi, Surg Technol Int 11:303-310, 2003; Bunge, J Neurol 242:S19-21, 1994; Ide, Neurosci Res 25:101-121, 1996; Mahanthappa et al. J Neurosci 16:4673-4683, 1996). However, the use of ex vivo cultured SC within conduits is limited in its clinical application because of the concomitant donor site morbidity and the slow growth of these cells in vitro (Tohill et al. Tissue Eng 10:1359-1367, 2004).Mesenchymal stem cells (MSC or bone marrow stromal cells) and adipose-derived stem cells (ASC) are easily accessible non-hematopoietic stem cells that have proven essential for research purposes due to their plasticity and ability to differentiate into several functional cell types. This alternative source of cells is relatively simple to isolate and expand in culture. We have demonstrated that MSC and ASC can trans-differentiate along a SC lineage with functional properties and growth factor synthesis activities similar to those of native SC and could provide nerve fiber support and guidance during nerve regeneration.

Neuregulin 1 type III/ErbB signaling is crucial for Schwann cell colonization of sympathetic axons

Analysis of Schwann cell (SC) development has been hampered by the lack of growing axons in many commonly used in vitro assays. As a consequence, the molecular signals and cellular dynamics of SC development along peripheral axons are still only poorly understood. Here we use a superior cervical ganglion (SCG) explant assay, in which axons elongate after treatment with nerve growth factor (NGF). Migration as well as proliferation and apoptosis of endogenous SCG-derived SCs along sympathetic axons were studied in these cultures using pharmacological interference and time-lapse imaging. Inhibition of ErbB receptor tyrosine kinases leads to reduced SC proliferation, increased apoptosis and thereby severely interfered with SC migration to distal axonal sections and colonization of axons. Furthermore we demonstrate that SC colonization of axons is also strongly impaired in a specific null mutant of an ErbB receptor ligand, Neuregulin 1 (NRG1) type III. Taken together, using a novel SC development assay, we demonstrate that NRG1 type III serves as a critical axonal signal for glial ErbB receptors that drives SC development along sympathetic axons.

The role of neuregulin-1 in the response to nerve injury

Axons and Schwann cells exist in a highly interdependent relationship: damage to one cell type invariably leads to pathophysiological changes in the other. Greater understanding of communication between these cell types will not only give insight into peripheral nerve development, but also the reaction to and recovery from peripheral nerve injury. The type III isoform of neuregulin-1 (NRG1) has emerged as a key signaling factor that is expressed on axons and, through binding to erbB2/3 receptors on Schwann cells, regulates multiple phases of their development. In adulthood, NRG1 is dispensable for the maintenance of the myelin sheath; however, this factor is required for both axon regeneration and remyelination following nerve injury. The outcome of NRG1 signaling depends on interactions with other pathways within Schwann cells such as Notch, integrin and cAMP signaling. In certain circumstances, this signaling pathway may be maladaptive; for instance, direct binding of Mycobacterium leprae onto erbB2 receptors produces excessive activation and can actually promote demyelination. Attempts to modulate this pathway in order to promote nerve repair will therefore need to give consideration to the exact isoform used, as well as how it is processed and the context in which it is presented to the Schwann cell.

Neuronal Neuregulin 1 type III directs Schwann cell migration

During peripheral nerve development, each segment of a myelinated axon is matched with a single Schwann cell. Tight regulation of Schwann cell movement, proliferation and differentiation is essential to ensure that these glial cells properly associate with axons. ErbB receptors are required for Schwann cell migration, but the operative ligand and its mechanism of action have remained unknown. We demonstrate that zebrafish Neuregulin 1 (Nrg1) type III, which signals through ErbB receptors, controls Schwann cell migration in addition to its previously known roles in proliferation and myelination. Chimera analyses indicate that ErbB receptors are required in all migrating Schwann cells, and that Nrg1 type III is required in neurons for migration. Surprisingly, expression of the ligand in a few axons is sufficient to induce migration along a chimeric nerve constituted largely of nrg1 type III mutant axons. These studies also reveal a mechanism that allows Schwann cells to fasciculate axons regardless of nrg1 type III expression. Time-lapse imaging of transgenic embryos demonstrated that misexpression of human NRG1 type III results in ectopic Schwann cell migration, allowing them to aberrantly enter the central nervous system. These results demonstrate that Nrg1 type III is an essential signal that controls Schwann cell migration to ensure that these glia are present in the correct numbers and positions in developing nerves.

Age affects reciprocal cellular interactions in neuromuscular synapses following peripheral nerve injury

Studies of the influence of age on regeneration and reinnervation in the peripheral nervous system (PNS) and neuromuscular junction (NMJ) are reviewed, with a particular focus on aged and denervated skeletal muscles. The morphological and functional features of incomplete regeneration and reinnervation are compared between adult and aged animals. In addition, some possible mechanisms of the age-related defects will be discussed. Increased fragmentation or damage in individual components of the NMJ (terminal Schwann cells (TSCs), axon terminals and acetylcholine receptor sites occurs during muscle reinnervation following PNS injury in the aged animals. The capacity to produce ultraterminal sprouting or multiple innervation secondary to PNS injury is maintained, but not the capacity to eliminate such anomalous axonal profiles. The frequency and accuracy of reoccupation of the synaptic sites by TSCs and axon terminals are impaired. Thus, despite the capability of extending neural processes, the rate at which regenerating nerve fibers grow, mature and precisely appose the postsynaptic muscle fiber is impaired, resulting in the failure of re-establishment of the normal single motor innervation in the NMJ. A complex set of cellular interactions in the NMJ are known to participate in the neurotrophism and neurotrophism to support growth of the regenerating and sprouting axons and their pathfinding to direct the target muscle fiber. Besides the capability of α-motoneurons, signaling originating from the TSCs and muscle may be impaired during aging.

A dual role for ErbB2 signaling in cardiac trabeculation

Cardiac trabeculation is a crucial morphogenetic process by which clusters of ventricular cardiomyocytes extrude and expand into the cardiac jelly to form sheet-like projections. Although it has been suggested that cardiac trabeculae enhance cardiac contractility and intra-ventricular conduction, their exact function in heart development has not been directly addressed. We found that in zebrafish erbb2 mutants, which we show completely lack cardiac trabeculae, cardiac function is significantly compromised, with mutant hearts exhibiting decreased fractional shortening and an immature conduction pattern. To begin to elucidate the cellular mechanisms of ErbB2 function in cardiac trabeculation, we analyzed erbb2 mutant hearts more closely and found that loss of ErbB2 activity resulted in a complete absence of cardiomyocyte proliferation during trabeculation stages. In addition, based on data obtained from proliferation, lineage tracing and transplantation studies, we propose that cardiac trabeculation is initiated by directional cardiomyocyte migration rather than oriented cell division, and that ErbB2 cell-autonomously regulates this process.

Use of stem cells to augment nerve injury repair

OBJECTIVE

The purpose of this review is to summarize the basic science literature related to chronic nerve injuries, and to then use this as the background to provide emerging insights into the promising role of cellular therapy for nerve injury repair.

METHODS

The literature pertinent to the experimental and clinical aspects of chronic nerve injury was reviewed, as was emerging literature and our own recent experience in using cellular therapy to repair injured nerves.

RESULTS

Peripheral nerves have the potential to regenerate axons and reinnervate end organs. Yet, outcome after peripheral nerve injury, even after nerve repair, remains relatively poor. The single most important quantitative contributor to poor motor recovery is chronic denervation of the distal nerve. Chronic denervation is common because of the often extensive injury zone that prevents any axonal outgrowth or (even if outgrowth occurs) the relatively slow rate of regeneration. As a consequence, the distal nerve remains chronically devoid of regrowing axons. In turn, prolonged denervation of Schwann cells (SCs) seems to be the critical factor that makes them unreceptive for axonal regeneration. Regenerative success was demonstrated when denervated SCs were replaced with healthy SCs cultured from a secondary nerve. This cell-replacement strategy is, however, limited in the clinical setting by the inability to obtain sufficient numbers of cells and the requirement for sacrifice of additional nerve tissue. We, along with several other groups, have therefore begun investigating stem cell therapies to improve the regenerative environment.

CONCLUSION

There are several avenues of stem cell-based approaches to peripheral nerve repair. One of these, skin-derived precursor cells, are easily accessible, autologous adult stem cells that can survive and myelinate in the peripheral nerve environment and become SC-like in their apparent differentiation.

Repair of transected facial nerve with mesenchymal stromal cells: histopathologic evidence of superior outcome

OBJECTIVES/HYPOTHESIS

Despite advanced surgical techniques, clinical results of the transected facial nerve are still far from the desired outcome. Mesenchymal stromal cells (MSCs) were shown to transdifferentiate into Schwann cells and express some growth factors beneficial in peripheral nerve injury. We aimed to document histopathological improvement obtained from application of the homograft bone marrow-derived MSCs immediately after conventional anastomosis of a transected facial nerve branch in rats, and to compare the results with those nerves anastomosed only.

STUDY DESIGN

Animal, prospective, and controlled study.

METHODS

The study was performed in 15 rats. The right buccal branch was completely transected and repaired with epineural sutures. The right-side anastomosis was additionally treated with MSCs thereafter. The right marginal mandibular branch was kept intact, but in contact with MSCs. The left buccal branch was transected and repaired in a similar fashion except for MSC application. The left-side marginal mandibular branch was left intact. Rats were sacrificed at month 1, 3, and 6. Four branches of each rat were sampled, and nerve segments distal to the anastomosis were histopathologically examined.

RESULTS

The examination revealed that intact nerve segments and nerve segments in contact with MSCs had completely normal appearance regardless of the time interval. Samples from the nerves anastomosed and treated with MSCs did better than those nerves anastomosed only in terms of axonal organization and myelin thickness.

CONCLUSIONS

This preliminary report witnessed beneficial effects of MSCs application onto the injured facial nerve as evidenced by the histopathological examination.

Neuregulin-1 beta and neuregulin-1 alpha differentially affect the migration and invasion of malignant peripheral nerve sheath tumor cells

Malignant peripheral nerve sheath tumors (MPNSTs) are the most common malignancy associated with neurofibromatosis Type 1 (NF1). These Schwann cell lineage-derived sarcomas aggressively invade adjacent nerve and soft tissue, frequently precluding surgical resection. Little is known regarding the mechanisms underlying this invasive behavior. We have shown that MPNSTs express neuregulin-1 (NRG-1) beta isoforms, which promote Schwann cell migration during development, and NRG-1 alpha isoforms, whose effects on Schwann cells are poorly understood. Hypothesizing that NRG-1 beta and/or NRG-1 alpha promote MPNST invasion, we found that NRG-1 beta promoted MPNST migration in a substrate-specific manner, markedly enhancing migration on laminin but not on collagen type I or fibronectin. The NRG-1 receptors erbB3 and erbB4 were present in MPNST invadopodia (processes mediating invasion), partially colocalized with focal adhesion kinase and the laminin receptor beta(1)-integrin and coimmunoprecipitated with beta(1)-integrin. NRG-1 beta stimulated human and murine MPNST cell migration and invasion in a concentration-dependent manner in three-dimensional migration assays, acting as a chemotactic factor. Both baseline and NRG-1 beta-induced migration were erbB-dependent and required the action of MEK 1/2, SAPK/JNK, PI-3 kinase, Src family kinases and ROCK-I/II. In contrast, NRG-1 alpha had no effect on the migration and invasion of some MPNST lines and inhibited the migration of others. While NRG-1 beta potently and persistently activated Erk 1/2, SAPK/JNK, Akt and Src family kinases, NRG-1 alpha did not activate Akt and activated these other kinases with kinetics distinct from those evident in NRG-1 beta-stimulated cells. These findings suggest that NRG-1 beta enhances MPNST migration and that NRG-1 beta and NRG-1 alpha differentially modulate this process.

Practical considerations concerning the use of stem cells for peripheral nerve repair

In this review the authors intend to demonstrate the need for supplementing conventional repair of the injured nerve with alternative therapies, namely transplantation of stem or progenitor cells. Although peripheral nerves do exhibit the potential to regenerate axons and reinnervate the end organ, outcome following severe nerve injury, even after repair, remains relatively poor. This is likely because of the extensive injury zone that prevents axon outgrowth. Even if outgrowth does occur, a relatively slow growth rate of regeneration results in prolonged denervation of the distal nerve. Whereas denervated Schwann cells (SCs) are key players in the early regenerative success of peripheral nerves, protracted loss of axonal contact renders Schwann cells unreceptive for axonal regeneration. Given that denervated Schwann cells appear to become effete, one logical approach is to support the distal denervated nerve environment by replacing host cells with those derived exogenously. A number of different sources of stem/precursor cells are being explored for their potential application in the scenario of peripheral nerve injury. The most promising candidate, transplant cells are derived from easily accessible sources such as the skin, bone marrow, or adipose tissue, all of which have demonstrated the capacity to differentiate into Schwann cell-like cells. Although recent studies have shown that stem cells can act as promising and beneficial adjuncts to nerve repair, considerable optimization of these therapies will be required for their potential to be realized in a clinical setting. The authors investigate the relevance of the delivery method (both the number and differentiation state of cells) on experimental outcomes, and seek to clarify whether stem cells must survive and differentiate in the injured nerve to convey a therapeutic effect. As our laboratory uses skin-derived precursor cells (SKPCs) in various nerve injury paradigms, we relate our findings on cell fate to other published studies to demonstrate the need to quantify stem cell survival and differentiation for future studies.

Growth factor and stem cell enhanced conduits in peripheral nerve regeneration and repair

OBJECTIVE

Despite the capacity for spontaneous axonal regeneration, recovery after severe peripheral nerve injury remains variable and often very poor. In addition, autologous nerve grafts, considered to be the 'gold standard' in nerve repair technique, are plagued by restricted donor tissue availability and donor site morbidity. Our primary objective is to highlight new and emerging methods of nerve repair, which have the potential to significantly improve both the functional and behavioral outcome after clinical nerve injury.

METHODS

A critical analysis of nerve injury and regeneration literature concentrating on outcome measures from both immediate and chronically denervated experimental works was conducted.

RESULTS

Results of numerous works employing both growth factor and stem cell enhanced nerve guidance conduits have shown encouraging results. However, further research is needed to optimize guidance conduit dynamics, bioavailability and delivery of both growth factors and stem cells to enhance peripheral nerve regeneration and functional recovery.

DISCUSSION

This review discusses current animal and clinical growth factor and stem cell studies, specifically focusing on future bio-engineering approaches in developing a nerve guidance conduit in the future.

The role of cells, neurotrophins, extracellular matrix and cell surface molecules in peripheral nerve regeneration

Wallerian degeneration is a complicated process whereby axons and myelin sheaths undergo degeneration, and eventually are phagocytosed by macrophages and Schwann cells following nerve damage. Schwann cells proliferate and the endoneural tubes persist. In addition, neurotrophins, neural cell adhesion molecules, cytokines and other soluble factors are upregulated to facilitate regeneration. The important role of cellular components, neurotrophins, and extracellular matrix components, including cell surface molecules involved in this regenerative process, is highlighted and discussed in this review.

Development of the Schwann cell lineage: from the neural crest to the myelinated nerve

The myelinating and nonmyelinating Schwann cells in peripheral nerves are derived from the neural crest, which is a transient and multipotent embryonic structure that also generates the other main glial subtypes of the peripheral nervous system (PNS). Schwann cell development occurs through a series of transitional embryonic and postnatal phases, which are tightly regulated by a number of signals. During the early embryonic phases, neural crest cells are specified to give rise to Schwann cell precursors, which represent the first transitional stage in the Schwann cell lineage, and these then generate the immature Schwann cells. At birth, the immature Schwann cells differentiate into either the myelinating or nonmyelinating Schwann cells that populate the mature nerve trunks. In this review, we will discuss the biology of the transitional stages in embryonic and early postnatal Schwann cell development, including the phenotypic differences between them and the recently identified signaling pathways, which control their differentiation and maintenance. In addition, the role and importance of the microenvironment in which glial differentiation takes place will be discussed.

Nerve fibroblast impact on Schwann cell behavior

In order to reveal non-neuronal cell interactions after peripheral nerve lesions, we began to analyze the impact of sciatic nerve fibroblasts on Schwann cells in vitro. Both cell types are considered to have opposite effects on axonal regeneration. Few data are available on how repulsive nerve fibroblasts affect neuritotrophic Schwann cells and thus might indirectly influence axonal regrowth. Using different culture systems in conjunction with time-lapse video recording, metabolic labeling, pharmacological intervention, RNAi knockdown, Western blotting and RT-PCR analysis, we found that nerve fibroblasts differentially modify the various responses of Schwann cells. In the presence of collagen type IV and heparan sulfate proteoglycan but not of laminin, diffusible fibroblast factors slow down Schwann cell proliferation. In contrast, fibroblast factors increase the migratory activity of Schwann cells without being chemoattractive. One pro-migratory fibroblast factor turned out to be neuregulin. The pro-migratory activity of nerve fibroblasts and of recombinant neuregulin-1beta1 can be counteracted by neuregulin-specific pharmacological intervention and by neuregulin RNA interference. We show for the first time that nerve fibroblasts play antagonistic and agonistic roles for Schwann cells in a context-dependent manner. The data shed light on cellular mechanisms and have implications for some neuro-tissue engineering strategies.