Transforming growth factors-beta 1 and beta 2 are mitogens for rat Schwann cells

Development of myelinating glia: An overview

Myelin is essential to nervous system function, playing roles in saltatory conduction and trophic support. Oligodendrocytes (OLs) and Schwann cells (SCs) form myelin in the central and peripheral nervous systems respectively and follow different developmental paths. OLs are neural stem-cell derived and follow an intrinsic developmental program resulting in a largely irreversible differentiation state. During embryonic development, OL precursor cells (OPCs) are produced in distinct waves originating from different locations in the central nervous system, with a subset developing into myelinating OLs. OPCs remain evenly distributed throughout life, providing a population of responsive, multifunctional cells with the capacity to remyelinate after injury. SCs derive from the neural crest, are highly dependent on extrinsic signals, and have plastic differentiation states. SC precursors (SCPs) are produced in early embryonic nerve structures and differentiate into multipotent immature SCs (iSCs), which initiate radial sorting and differentiate into myelinating and non-myelinating SCs. Differentiated SCs retain the capacity to radically change phenotypes in response to external signals, including becoming repair SCs, which drive peripheral regeneration. While several transcription factors and myelin components are common between OLs and SCs, their differentiation mechanisms are highly distinct, owing to their unique lineages and their respective environments. In addition, both OLs and SCs respond to neuronal activity and regulate nervous system output in reciprocal manners, possibly through different pathways. Here, we outline their basic developmental programs, mechanisms regulating their differentiation, and recent advances in the field.

WNT5A inhibition alters the malignant peripheral nerve sheath tumor microenvironment and enhances tumor growth

Malignant peripheral nerve sheath tumors (MPNST) are aggressive soft-tissue sarcomas that cause significant mortality in adults with neurofibromatosis type 1. We compared gene expression of growth factors in normal human nerves to MPNST and normal human Schwann cells to MPNST cell lines. We identified WNT5A as the most significantly upregulated ligand-coding gene and verified its protein expression in MPNST cell lines and tumors. In many contexts WNT5A acts as an oncogene. However, inhibiting WNT5A expression using shRNA did not alter MPNST cell proliferation, invasion, migration, or survival in vitro. Rather, shWNT5A-treated MPNST cells upregulated mRNAs associated with the remodeling of extracellular matrix and with immune cell communication. In addition, these cells secreted increased amounts of the proinflammatory cytokines CXCL1, CCL2, IL6, CXCL8, and ICAM1. Versus controls, shWNT5A-expressing MPNST cells formed larger tumors in vivo. Grafted tumors contained elevated macrophage/stromal cells, larger and more numerous blood vessels, and increased levels of Mmp9, Cxcl13, Lipocalin-1, and Ccl12. In some MPNST settings, these effects were mimicked by targeting the WNT5A receptor ROR2. These data suggest that the non-canonical Wnt ligand WNT5A inhibits MPNST tumor formation by modulating the MPNST microenvironment, so that blocking WNT5A accelerates tumor growth in vivo.

How scars shape the neural landscape: Key molecular mediators of TGF-β1's anti-neuritogenic effects

Following injury to the peripheral and central nervous systems, tissue levels of transforming growth factor (TGF)-β1 often increase, which is key for wound healing and scarring. However, active wound regions and scars appear to inhibit process outgrowth by regenerating neurons. We recently showed that corneal wound myofibroblasts block corneal nerve regeneration in vivo, and sensory neurite outgrowth in vitro in a manner that relies critically on TGF-β1. In turn, delayed, abnormal re-innervation contributes to long-term sensory dysfunctions of the ocular surface. Here, we exposed morphologically and biochemically-differentiated sensory neurons from the ND7/23 cell line to TGF-β1 to identify the intracellular signals regulating these anti-neuritogenic effects, contrasting them with those of Semaphorin(Sema)3A, a known inhibitor of neurite outgrowth. Neuronal morphology was quantified using phase-contrast imaging. Western blotting and specific inhibitors were then used to identify key molecular mediators. Differentiated ND7/23 cells expressed neuron-specific markers, including those involved in neurite extension and polarization. TGF-β1 increased phosphorylation of collapsin response mediator protein-2 (CRMP2), a molecule that is key for neurite extension. We now show that both glycogen synthase kinase (GSK)-3β and Smad3 modulate phosphorylation of CRMP2 after treatment with TGF-β1. GSK-3β appeared to exert a particularly strong effect, which could be explained by its ability to phosphorylate not only CRMP2, but also Smad3. In conclusion, TGF-β1's inhibition of neurite outgrowth in sensory neurons appears to be regulated through a highly-conserved signaling pathway, which involves the GSK-3β/CRMP-2 loop via both canonical and non-canonical mechanisms. It is hoped that by defining the signaling pathways that control neurite outgrowth in wound environments, it will become possible to identify optimal molecular targets to promote re-innervation following injury.

Peripheral nerve injury induced changes in the spinal cord and strategies to counteract/enhance the changes to promote nerve regeneration

Peripheral nerve injury leads to morphological, molecular and gene expression changes in the spinal cord and dorsal root ganglia, some of which have positive impact on the survival of neurons and nerve regeneration, while the effect of others is the opposite. It is crucial to take prompt measures to capitalize on the positive effects of these reactions and counteract the negative impact after peripheral nerve injury at the level of spinal cord, especially for peripheral nerve injuries that are severe, located close to the cell body, involve long distance for axons to regrow and happen in immature individuals. Early nerve repair, exogenous supply of neurotrophic factors and Schwann cells can sustain the regeneration inductive environment and enhance the positive changes in neurons. Administration of neurotrophic factors, acetyl-L-carnitine, N-acetyl-cysteine, and N-methyl-D-aspartate receptor antagonist MK-801 can help counteract axotomy-induced neuronal loss and promote regeneration, which are all time-dependent. Sustaining and reactivation of Schwann cells after denervation provides another effective strategy. FK506 can be used to accelerate axonal regeneration of neurons, especially after chronic axotomy. Exploring the axotomy-induced changes after peripheral nerve injury and applying protective and promotional measures in the spinal cord which help to retain a positive functional status for neuron cell bodies will inevitably benefit regeneration of the peripheral nerve and improve functional outcomes.

The evolution and multi-molecular properties of NF1 cutaneous neurofibromas originating from C-fiber sensory endings and terminal Schwann cells at normal sites of sensory terminations in the skin

In addition to large plexiform neurofibromas (pNF), NF1 patients are frequently disfigured by cutaneous neurofibromas (cNF) and are often afflicted with chronic pain and itch even from seemingly normal skin areas. Both pNFs and cNF consist primarily of benign hyperproliferating nonmyelinating Schwann cells (nSC). While pNF clearly arise within deep nerves and plexuses, the role of cutaneous innervation in the origin of cNF and in chronic itch and pain is unknown. First, we conducted a comprehensive, multi-molecular, immunofluorescence (IF) analyses on 3mm punch biopsies from three separate locations in normal appearing, cNF-free skin in 19 NF1 patients and skin of 16 normal subjects. At least one biopsy in 17 NF1 patients had previously undescribed micro-lesions consisting of a small, dense cluster of nonpeptidergic C-fiber endings and the affiliated nSC consistently adjoining adnexal structures—dermal papillae, hair follicles, sweat glands, sweat ducts, and arterioles—where C-fiber endings normally terminate. Similar micro-lesions were detected in hind paw skin of mice with conditionally-induced SC Nf1^-/- mutations. Hypothesizing that these microlesions were pre-cNF origins of cNF, we subsequently analyzed numerous overt, small cNF (s-cNF, 3–6 mm) and discovered that each had an adnexal structure at the epicenter of vastly increased nonpeptidergic C-fiber terminals, accompanied by excessive nSC. The IF and functional genomics assays indicated that neurturin (NTRN) and artemin (ARTN) signaling through cRET kinase and GFRα2 and GFRα3 co-receptors on the aberrant C-fiber endings and nSC may mutually promote the onset of pre-cNF and their evolution to s-cNF. Moreover, TrpA1 and TrpV1 receptors may, respectively, mediate symptoms of chronic itch and pain. These newly discovered molecular characteristics might be targeted to suppress the development of cNF and to treat chronic itch and pain symptoms in NF1 patients.

Transforming Growth Factor Beta 1 Regulates Fibroblast Growth Factor 7 Expression in Schwann Cells

Background: Our previous work demonstrated that application of transforming growth factor beta 1 (TGF-β1) and forskolin to the repair site after chronic denervation and axotomy has a mitogenic effect, reactivates Schwann cells (SCs), and supports axonal regeneration. We found decreased expression of fibroblast growth factor 7 (FGF-7), a factor involved in synaptic organization and maintenance. Using an in vitro system, we examined the molecular mechanism of TGF-β1 and forskolin on the regulation of FGF-7 expression in SCs. Methods: SCs were prepared from the sciatic nerve and stimulated with forskolin (0.5 μM), TGF-β1 (1 ng/mL), or TGF-β1 + forskolin for 6 or 24 hours. SCs were also pretreated with LY2109761 (0.5 μM), a TGF-β receptor inhibitor, prior to stimulation with TGF-β1 + forskolin for 6 hours. Real-time TaqMan quantitative polymerase chain reaction analyses for FGF-7, myelin basic protein, and peripheral myelin protein 22 expression were performed. Cycle threshold (Ct) data were normalized to a reference gene, and fold changes relative to untreated SCs were determined using the 2^-ΔΔCt method. Statistical analysis was done using t test (P<0.05). Results: TGF-β1 alone or in combination with forskolin for 24 hours resulted in a 3.3- and 2.8-fold decrease in FGF-7 expression in SCs, respectively. No change in FGF-7 expression was found with forskolin alone. TGF-β1 + forskolin treatment for 6 hours resulted in a 4.0-fold decrease in FGF-7 expression, while the addition of LY2109761 resulted in a 2.7-fold decrease in FGF-7 expression. Conclusion: We showed that SC expression of FGF-7 is regulated by TGF-β1. The positive effect of TGF-β1 and forskolin on SC reactivation and axonal regeneration may involve modulation of FGF-7 expression and activity in SCs.

Single Local Application of TGF-β Promotes a Proregenerative State Throughout a Chronically Injured Nerve

BACKGROUND

The lack of nerve regeneration and functional recovery occurs frequently when injuries involve large nerve trunks because insufficient mature axons reach their targets in the distal stump and because of the loss of neurotrophic support, primarily from Schwann cells (SCs).

OBJECTIVE

To investigate whether a single application of transforming growth factor-beta (TGF-β) plus forskolin or forskolin alone can promote and support axonal regeneration through the distal nerve stump.

METHODS

Using a delayed repair rat model of nerve injury, we transected the tibial nerve. After 8 wk, end-to-end repair was done and the repair site was treated with saline, forskolin, or TGF- β plus forskolin. After 6 wk, nerve sections consisting of the proximal stump, distal to the site of repair, and the most distal part of the nerve stump were removed for nerve histology, axon counts, and immunohistochemistry for activated SCs (S100), macrophages (CD68), cell proliferation (Ki67), p75NGFR, and apoptosis (activated caspase-3).

RESULTS

TGF-β plus forskolin significantly increased the numbers of axons regenerated distal to the repair site and the most distal nerve sections. Both treatments significantly increased the numbers of axons regenerated in the most distal nerve sections compared to saline treated. Both treatments exhibited extended expression of regeneration-associated marker proteins.

CONCLUSION

TGF-β plus forskolin treatment of chronically injured nerve improved axonal regeneration and increased expression of regeneration-associated proteins beyond the repair site. This suggests that a single application at the site of repair has mitogenic effects that extended distally and may potentially overcome the decrease in regenerated axon over long distance.

MicroRNA Mediated Regulation of Schwann Cell Migration and Proliferation in Peripheral Nerve Injury

Schwann cells (SCs) contribute to nerve repair following injury; however, the underlying molecular mechanism is poorly understood. MicroRNAs (miRNAs), which are short noncoding RNAs, have been shown to play a role in neuronal disease. In this work, we show that miRNAs regulate the peripheral nerve system by modulating the migration and proliferation of SCs. Thus, miRNAs expressed in peripheral nerves may provide a potential therapeutic target for peripheral nerve injury or repair.

Phenotypic and Functional Characteristics of Human Schwann Cells as Revealed by Cell-Based Assays and RNA-SEQ

This study comprehensively addresses the phenotype, function, and whole transcriptome of primary human and rodent Schwann cells (SCs) and highlights key species-specific features beyond the expected donor variability that account for the differential ability of human SCs to proliferate, differentiate, and interact with axons in vitro. Contrary to rat SCs, human SCs were insensitive to mitogenic factors other than neuregulin and presented phenotypic variants at various stages of differentiation, along with a mixture of proliferating and senescent cells, under optimal growth-promoting conditions. The responses of human SCs to cAMP-induced differentiation featured morphological changes and cell cycle exit without a concomitant increase in myelin-related proteins and lipids. Human SCs efficiently extended processes along those of other SCs (human or rat) but failed to do so when placed in co-culture with sensory neurons under conditions supportive of myelination. Indeed, axon contact-dependent human SC alignment, proliferation, and differentiation were not observed and could not be overcome by growth factor supplementation. Strikingly, RNA-seq data revealed that ~ 44 of the transcriptome contained differentially expressed genes in human and rat SCs. A bioinformatics approach further highlighted that representative SC-specific transcripts encoding myelin-related and axon growth-promoting proteins were significantly affected and that a deficient expression of key transducers of cAMP and adhesion signaling explained the fairly limited potential of human SCs to differentiate and respond to axonal cues. These results confirmed the significance of combining traditional bioassays and high-resolution genomics methods to characterize human SCs and identify genes predictive of cell function and therapeutic value.

Pre-degenerated peripheral nerves co-cultured with bone marrow-derived cells: a new technique for harvesting high-purity Schwann cells

Schwann cells play an important role in the peripheral nervous system, especially in nerve repair following injury, so artificial nerve regeneration requires an effective technique for obtaining purified Schwann cells. In vivo and in vitro pre-degeneration of peripheral nerves have been shown to obtain high-purity Schwann cells. We believed that in vitro pre-degeneration was simple and controllable, and available for the clinic. Thus, we co-cultured the crushed sciatic nerves with bone marrow-derived cells in vitro. Results demonstrated that, 3 hours after injury, a large number of mononuclear cells moved to the crushed nerves and a large number of bone marrow-derived cells infiltrated the nerve segments. These changes promoted the degradation of the nerve segments, and the dedifferentiation and proliferation of Schwann cells. Neural cell adhesion molecule and glial fibrillary acidic protein expression were detected in the crushed nerves. Schwann cell yield was 9.08 ± 2.01 × 104/mg. The purity of primary cultured Schwann cells was 88.4 ± 5.79%. These indicate a successful new method for obtaining Schwann cells of high purity and yield from adult crushed sciatic nerve using bone marrow-derived cells.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

Schwann Cells: Development and Role in Nerve Repair

Schwann cells develop from the neural crest in a well-defined sequence of events. This involves the formation of the Schwann cell precursor and immature Schwann cells, followed by the generation of the myelin and nonmyelin (Remak) cells of mature nerves. This review describes the signals that control the embryonic phase of this process and the organogenesis of peripheral nerves. We also discuss the phenotypic plasticity retained by mature Schwann cells, and explain why this unusual feature is central to the striking regenerative potential of the peripheral nervous system (PNS).

Combinatorial actions of Tgfβ and Activin ligands promote oligodendrocyte development and CNS myelination

In the embryonic CNS, development of myelin-forming oligodendrocytes is limited by bone morphogenetic proteins, which constitute one arm of the transforming growth factor-β (Tgfβ) family and signal canonically via Smads 1/5/8. Tgfβ ligands and Activins comprise the other arm and signal via Smads 2/3, but their roles in oligodendrocyte development are incompletely characterized. Here, we report that Tgfβ ligands and activin B (ActB) act in concert in the mammalian spinal cord to promote oligodendrocyte generation and myelination. In mouse neural tube, newly specified oligodendrocyte progenitors (OLPs) are first exposed to Tgfβ ligands in isolation, then later in combination with ActB during maturation. In primary OLP cultures, Tgfβ1 and ActB differentially activate canonical Smad3 and non-canonical MAP kinase signaling. Both ligands enhance viability, and Tgfβ1 promotes proliferation while ActB supports maturation. Importantly, co-treatment strongly activates both signaling pathways, producing an additive effect on viability and enhancing both proliferation and differentiation such that mature oligodendrocyte numbers are substantially increased. Co-treatment promotes myelination in OLP-neuron co-cultures, and maturing oligodendrocytes in spinal cord white matter display strong Smad3 and MAP kinase activation. In spinal cords of ActB-deficient Inhbb(-/-) embryos, apoptosis in the oligodendrocyte lineage is increased and OLP numbers transiently reduced, but numbers, maturation and myelination recover during the first postnatal week. Smad3(-/-) mice display a more severe phenotype, including diminished viability and proliferation, persistently reduced mature and immature cell numbers, and delayed myelination. Collectively, these findings suggest that, in mammalian spinal cord, Tgfβ ligands and ActB together support oligodendrocyte development and myelin formation.

Effect of local application of transforming growth factor-β at the nerve repair site following chronic axotomy and denervation on the expression of regeneration-associated genes. Laboratory investigation

OBJECTIVES

Although peripheral nerves can regenerate after traumatic injury, functional recovery is often suboptimal, especially after injuries to large nerve trunks such as the sciatic nerve or brachial plexus. Current research with animal models suggests that the lack of functional recovery resides in the lack of sufficient mature axons reaching their targets due to the loss of neurotrophic support by Schwann cells in the distal stump of injured nerves. This study was designed to investigate the effect of one-time application of transforming growth factor-β (TGF-β) at the repair site of chronically injured nerve.

METHODS

The authors used the rat tibial nerve injury and repair model to investigate the effects of application of physiological concentrations of TGF-β plus forskolin or forskolin alone in vivo at the repair site on gene and protein expression and axon regeneration at 6 weeks after nerve repair. They used gene expression profiling and immunohistochemical analysis of indicative activated proteins in Schwann cells to evaluate the effects of treatments on the delayed repair. They also quantified the regenerated axons distal to the repair site by microscopy of paraffin sections.

RESULTS

Both treatment with forskolin only and treatment with TGF-β plus forskolin resulted in increased numbers of axons regenerated compared with saline-only control. There was robust activation and proliferation of both Schwann cells and macrophages reminiscent of the processes during Wallerian degeneration. The treatment also induced upregulation of genes implicated in cellular activation and growth as detected by gene array.

CONCLUSIONS

Addition of TGF-β plus forskolin to the repair after chronic nerve injury improved axonal regeneration, probably via upregulation of required genes, expression of growth-associated protein, and reactivation of Schwann cells and macrophages. Further studies are required to better understand the mechanism of the positive effect of TGF-β treatment on old nerve injuries.

Epigenetic induction of the Ink4a/Arf locus prevents Schwann cell overproliferation during nerve regeneration and after tumorigenic challenge

The number of Schwann cells is fitted to axonal length in peripheral nerves. This relationship is lost when tumorigenic stimuli induce uncontrolled Schwann cell proliferation, generating tumours such us neurofibromas and schwannomas. Schwann cells also re-enter the cell cycle following nerve injury during the process of Wallerian degeneration. In both cases proliferation is finally arrested. We show that in neurofibroma, the induction of Jmjd3 (jumonji domain containing 3, histone lysine demethylase) removes trimethyl groups on lysine-27 of histone-H3 and epigenetically activates the Ink4a/Arf-locus, forcing Schwann cells towards replicative senescence. Remarkably, blocking this mechanism allows unrestricted proliferation, inducing malignant transformation of neurofibromas. Interestingly, our data suggest that in injured nerves, Schwann cells epigenetically activate the same locus to switch off proliferation and enter the senescence programme. Indeed, when this pathway is genetically blocked, Schwann cells fail to drop out of the cell cycle and continue to proliferate. We postulate that the Ink4a/Arf-locus is expressed as part of a physiological response that prevents uncontrolled proliferation of the de-differentiated Schwann cell generated during nerve regeneration, a response that is also activated to avoid overproliferation after tumorigenic stimuli in the peripheral nervous system.

Spatiotemporal expression of SKIP after rat sciatic nerve crush

Ski-interacting protein (SKIP) is a highly conserved protein from yeast to Human. As an essential spliceosomal component and transcriptional co-regulator it plays an important role in preinitiation, splicing and polyadenylation. SKIP can also combine with Ski to overcome the G1 arrest and the growth-suppressive activities of pRb. Furthermore SKIP has the capacity to augment TGF-β dependent transcription. While the distribution and function of SKIP in peripheral nervous system lesion and regeneration remain unclear. Here, we investigated the spatiotemporal expression of SKIP in an acute sciatic nerve crush model in adult rats. Western Blot analysis revealed that SKIP was expressed in normal sciatic nerves. It gradually increased, reached a peak at 1 week after crush, and then returned to the normal level at 4 weeks. Besides, we observed that up-regulation of SKIP was approximately in parallel with Proliferating cell nuclear antigen (PCNA), and numerous Schwann cells (SCs) expressing SKIP were PCNA and Ki-67 positive. Collectively, we hypothesized peripheral nerve crush induced up-regulation of SKIP in the sciatic nerve, which was associated with SCs proliferation.

Clinical strategies to enhance nerve regeneration in composite tissue allotransplantation

Reinnervation of a hand transplant ultimately dictates functional recovery but provides a significant regenerative challenge. This article highlights interventions to enhance nerve regeneration through acceleration of axonal regeneration or augmentation of Schwann cell support and discuss their relevance to composite tissue allotransplantation. Surgical techniques that may be performed at the time of transplantation to optimize intrinsic muscle recovery--including appropriate alignment of ulnar nerve motor and sensory components, transfer of the distal anterior interosseous nerve to the recurrent motor branch of the median nerve, and prophylactic release of potential nerve entrapment points--are also presented.

Effects of insulin-like growth factor-I and platelet-rich plasma on sciatic nerve crush injury in a rat model

OBJECT

Local administration of insulin-like growth factor-I (IGF-I) has been shown to increase the rate of axon regeneration in crush-injured and freeze-injured rat sciatic nerves. Local administration of platelet-rich plasma (PRP) has been also shown to have a measurable effect on facial nerve regeneration after transection in a rat model. The objective of the study was to compare the effects of locally administered IGF-I and PRP on the parameters of the Sciatic Function Index (SFI), sensory function (SF), axon count, and myelin thickness/axon diameter ratio (G-ratio) in a rat model of crush-injured sciatic nerves.

METHODS

The right sciatic nerve of Wistar albino rats (24 animals) was crushed using a Yasargil-Phynox aneurysm clip for 45 minutes. All animals were randomly divided into 3 groups: Group 1 (control group) was treated with saline, Group 2 was treated with IGF-I, and Group 3 was treated with PRP. Injections were performed using the tissue expander's injection port with a connecting tube directed at the crush-injured site. Functional recovery was assessed with improvement in the SFI. Recovery of sensory function was using the pinch test. Histopathological examination was performed 3 months after the injury.

RESULTS

The SFI showed an improved functional recovery in the IGF-I-treated animals (Group 2) compared with the saline-treated animals (Group 1) 30 days after the injury. In IGF-I-treated rats, sensory function returned to the baseline level significantly faster than in saline-treated and PRP-treated rats as shown in values between SF-2 and SF-7. The G-ratios were found to be significantly higher in both experimental groups than in the control group.

CONCLUSIONS

This study suggests that the application of IGF-I to the crush-injured site may expedite the functional recovery of paralyzed muscle by increasing the rate of axon regeneration.

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.

Role of chronic Schwann cell denervation in poor functional recovery after nerve injuries and experimental strategies to combat it

OBJECTIVE

To present our data about the role of chronic denervation (CD) of the distal nerve stumps as compared with muscle denervation atrophy and experimental strategies to promote better functional recovery.

METHODS

A rat model of nerve injury and repair was used. The common peroneal branch of the sciatic nerve was subjected to 0 to 24 weeks of CD before cross-suture with the tibial motoneurons. Our outcome measures included the numbers of motoneurons that regenerated their axons and the numbers that reinnervated muscle targets (motor units). To overcome the effects of CD, we used subcutaneous injection of FK506 and in vitro reactivation of Schwann cells that had been subjected to 24 weeks of CD with transforming growth factor beta.

RESULTS

Numbers of regenerated motoneurons and reinnervated motor units decreased as a function of duration of CD. However, axons that regenerated through the distal nerve stumps reinnervated the muscle targets and even formed enlarged motor unit size regardless of the duration of CD. FK506 doubled the numbers of tibial motoneurons that regenerated their axons into the common peroneal nerve even after delayed repair. Reactivation of chronically denervated Schwann cells with transforming growth factor beta significantly increased their capacity to support axonal regeneration.

CONCLUSION

CD of the distal nerve stumps is the primary factor that results in poor axonal regeneration and subsequently poor functional recovery. Acceleration of the rate of axonal regeneration and/or reactivation of Schwann cells of the distal nerve stumps are effective experimental strategies to promote axonal regeneration and functional recovery.

ERK1/2-mediated Schwann cell proliferation in the regenerating sciatic nerve by treadmill training

Proliferation of Schwann cells in the injured peripheral nerve supports axonal regeneration, and physical training in experimental animals has been shown to promote nerve regeneration. Extracellular signal-regulated kinase 1/2 (ERK1/2) activity can mediate neuronal responses to lesion signals, but its role in non-neuronal cells in the injured area is largely unknown. Here we report that treadmill training (TMT) facilitates axonal regeneration via the upregulation of phospho-ERK1/2 protein levels in Schwann cells in the injured sciatic nerve. Low-intensity, but not high-intensity, TMT increased neurite outgrowth of dorsal root ganglion (DRG) sensory neurons and potentiated Schwann cell proliferation. TMT elevated levels of GAP-43 mRNA and protein, and phospho-ERK1/2 protein in the injured sciatic nerves. TMT also enhanced phospho-c-Jun protein levels in the injured nerve. In-vivo administration of the ERK1/2 inhibitor PD98059 eliminated phospho-c-Jun, suggesting ERK1/2 phosphorylation of the c-Jun protein. PD98059 treatment decreased levels of BrdU-labeled proliferating Schwann cells in the distal portion of the injured nerve, and delayed the axonal regrowth that was promoted by TMT. The present data suggest that increased ERK1/2 activity in Schwann cells may play an important role in TMT-mediated enhancement of axonal regeneration in the injured peripheral nerve.

BDNF mRNA expression is significantly upregulated in vestibular schwannomas and correlates with proliferative activity

The expression of neurotrophic factors, such as artemin, glial cell line-derived neurotrophic factor (GDNF), neurturin, transforming growth factors (TGF)-beta1/beta2 and brain-derived neurotrophic factor (BDNF), is enhanced in vestibular schwannomas compared to peripheral nerves. Furthermore, this upregulation may correlate with mitotic activity. Vestibular schwannoma arising from Schwann cells of the vestibular nerve are mostly benign and slow-growing. Most of the pathogenic mechanisms regulating the vestibular schwannoma growth process are unknown. An impaired growth regulation and imbalance between mitosis and apoptosis can be assumed. However, molecular mechanisms interfering with regulation of the vestibular schwannoma growth also modulated by mitogenic factors have to be identified. Neurotrophic factors are involved in regulation of developmental processes in neuronal tissues and regeneration after peripheral nerve trauma and also reveal mitogenic effects on glial cell populations. Gene expression profiles of artemin, BDNF, GDNF, TGF-beta1/beta2 and Ret were determined in the vestibular schwannoma in comparison to the peripheral nerve tissues by using semiquantitative RT-PCR. The expression data were correlated to the proliferation-associated Ki-67 labelling index. A significant higher BDNF expression was observed in the vestibular schwannoma, whereas gene expression of artemin and GDNF was upregulated in peripheral nerves. The correlation between LI and BDNF, TGF-beta1 and Ret was found to be significant in the vestibular schwannoma. Our results demonstrate a coherence between BDNF expression and proliferative activity in the vestibular schwannoma. Based on these results, we propose a pivotal role for BDNF in modulating the vestibular schwannoma growth.

Non-antagonistic relationship between mitogenic factors and cAMP in adult Schwann cell re-differentiation

The expression of myelination-associated genes (MGs) can be induced by cyclic adenosine monophosphate (cAMP) elevation in isolated Schwann cells (SCs). To further understand the effect of known SC mitogens in the regulation of SC differentiation, we studied the response of SCs isolated from adult nerves to combined cAMP, growth factors, including neuregulin, and serum. In adult SCs, the induction of MGs by cAMP coincided with the loss of genes expressed in non-myelin-forming SCs and with a change in cell morphology from a bipolar to an expanded epithelial-like shape. Prolonged treatment with high doses of cAMP-stimulating agents, as well as low cell density, was required for the induction of SC differentiation. Stimulation with serum, neuregulin alone, or other growth factors including PDGF, IGF and FGF, increased SC proliferation but did not induce the expression of MGs or the associated morphological change. Most importantly, when these factors were administered in combination with cAMP-stimulating agents, SC proliferation was synergistically increased without reducing the differentiating activity of cAMP. Even though the initiation of DNA synthesis and the induction of differentiation were mostly incompatible events in individual cells, SCs were able to differentiate under conditions that also supported active proliferation. Overall, the results indicate that in the absence of neurons, cAMP can trigger SC re-differentiation concurrently with, but independently of, growth factor signaling.

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.

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.

Expression and localization of Ski determine cell type-specific TGFbeta signaling effects on the cell cycle

Transforming growth factor β (TGFβ) promotes epithelial cell differentiation but induces Schwann cell proliferation. We show that the protooncogene Ski (Sloan-Kettering viral oncogene homologue) is an important regulator of these effects. TGFβ down-regulates Ski in epithelial cells but not in Schwann cells. In Schwann cells but not in epithelial cells, retinoblastoma protein (Rb) is up-regulated by TGFβ. Additionally, both Ski and Rb move to the cytoplasm, where they partially colocalize. In vivo, Ski and phospho-Rb (pRb) appear to interact in the Schwann cell cytoplasm of developing sciatic nerves. Ski overexpression induces Rb hyperphosphorylation, proliferation, and colocalization of both proteins in Schwann cell and epithelial cell cytoplasms independently of TGFβ treatment. Conversely, Ski knockdown in Schwann cells blocks TGFβ-induced proliferation and pRb cytoplasmic relocalization. Our findings reveal a critical function of fine-tuned Ski levels in the control of TGFβ effects on the cell cycle and suggest that at least a part of Ski regulatory effects on TGFβ-induced proliferation of Schwann cells is caused by its concerted action with Rb.

Peripheral regeneration

Whereas the central nervous system (CNS) usually cannot regenerate, peripheral nerves regenerate spontaneously after injury because of a permissive environment and activation of the intrinsic growth capacity of neurons. Functional regeneration requires axon regrowth and remyelination of the regenerated axons by Schwann cells. Multiple factors including neurotrophic factors, extracellular matrix (ECM) proteins, and hormones participate in Schwann cell dedifferentiation, proliferation, and remyelination. We describe the current understanding of peripheral axon regeneration and focus on the molecules and potential mechanisms involved in remyelination.

Imatinib mesylate inhibits cell invasion of malignant peripheral nerve sheath tumor induced by platelet-derived growth factor-BB

Malignant peripheral nerve sheath tumor (MPNST) is rare, highly aggressive, resistant to radiochemotherapy, and associated with poor prognosis. Basic research to develop new treatment regimes is critically needed. This study was designed to identify motogenic factor(s) involved in MPNST cell invasion and inhibitor(s) of such invasive activity. We profiled the invasion-inducing activities of eight motogenic growth factors on two human MPNST cell lines, FU-SFT8611 and 9817, using in vitro Matrigel invasion assays. Platelet-derived growth factor-BB (PDGF-BB) was identified as the most effective MPNST cell invasion-inducing factor. Epidermal growth factor (EGF) and hepatocyte growth factor (HGF) also stimulated invasion in one MPNST cell line. Expressions of PDGF-BB and EGF receptors (PDGFR-beta and EGFR) mRNAs were detected more frequently and their proteins were expressed at higher levels in MPNST tissues than benign peripheral nerve sheath tumors (schwannomas and neurofibromas). In both MPNST cell lines, PDGF-BB induced tyrosine phosphorylation of PDGFR-beta but not of PDGFR-alpha, and specific PDGFR-beta inhibition by small interfering RNA to the receptor inhibited PDGF-BB-stimulated MPNST cell invasion, suggesting the predominant role of PDGFR-beta. Inhibition of PDGFR-beta phosphorylation by pretreatment with herbimycin A and imatinib mesylate effectively suppressed basement membrane invasion and cell growth in vitro. No mutations were present in exons 12 and 18 of PDGFR-beta in both MPNST cell lines and 10 human MPNST tissues examined. Our results indicated that PDGF-BB enhanced the invasive activity of MPNST cells through PDGFR phosphorylation and that imatinib inhibited such activity. The results provide the ground for further assessment of the therapeutic potential of imatinib in suppressing the invasion and growth of MPNST.

Transplantation of preconditioned schwann cells in peripheral nerve grafts after contusion in the adult spinal cord. Improvement of recovery in a rat model

BACKGROUND

Recovery after injury to the peripheral nervous system is based on the pro-regenerative relationship between axons and the extracellular matrix, a relationship established by Schwann cells. As mechanical conditioning of Schwann cells has been shown to stimulate their regenerative behavior, we sought to determine whether transplantation of these cells to the central nervous system (i.e., the spinal cord), with its limited regenerative capacity after injury, would improve axonal regeneration and functional recovery.

METHODS

A moderate contusion injury of the spinal cord was created with a force-directed impactor in forty-eight adult Sprague-Dawley rats, and, at one week postinjury, the spinal cords were reexposed in all animals. In twenty-four of these animals, peripheral nerve grafts with Schwann cells that had been obtained from the sciatic nerves of donor animals, and had been either untreated or subjected to mechanical conditioning, were transplanted to the contused area of the cords following resection of the glial scar. Another group of animals was treated with glial scar excision only, and a fourth group had the contusion injury but neither glial excision nor transplantation. Scores according to the Basso, Beattie, Bresnahan (BBB) Locomotor Rating Scale were assigned preoperatively and weekly thereafter. Tract tracing of descending and ascending spinal cord tracts was performed at six weeks postoperatively for quantitative histological evaluation of axonal regeneration.

RESULTS

While the recovery following glial scar excision without peripheral nerve transplantation was significantly worse than the recovery in the other groups, both transplantation groups had significantly higher BBB scores than the controls (no transplantation) in the early postoperative period (p < 0.05). Moreover, histological analysis showed markedly increased axonal regeneration at the lesional sites in the animals treated with the mechanically conditioned grafts than in the other groups (p < 0.05).

CONCLUSIONS

Functional recovery after spinal cord contusion improved following glial scar excision with transplantation of Schwann cells in peripheral nerve grafts to the contusion areas. Although recovery did not differ significantly between the transplantation groups, only the preconditioned grafts led to axonal regeneration at and past the lesional site. These grafts may further enhance functional recovery as the descending tracts eventually reach their target end-organs.

TGFbeta type II receptor signaling controls Schwann cell death and proliferation in developing nerves

During development, Schwann cell numbers are precisely adjusted to match the number of axons. It is essentially unknown which growth factors or receptors carry out this important control in vivo. Here, we tested whether the type II transforming growth factor (TGF) beta receptor has a role in this process. We generated a conditional knock-out mouse in which the type II TGFbeta receptor is specifically ablated only in Schwann cells. Inactivation of the receptor, evident at least from embryonic day 18, resulted in suppressed Schwann cell death in normally developing and injured nerves. Notably, the mutants also showed a strong reduction in Schwann cell proliferation. Consequently, Schwann cell numbers in wild-type and mutant nerves remained similar. Lack of TGFbeta signaling did not appear to affect other processes in which TGFbeta had been implicated previously, including myelination and response of adult nerves to injury. This is the first in vivo evidence for a growth factor receptor involved in promoting Schwann cell division during development and the first genetic evidence for a receptor that controls normal developmental Schwann cell death.

Secretory products of central nervous system glial cells induce Schwann cell proliferation and protect from cytokine-mediated death

There continues to be interest in Schwann cells (SC) as a possible source of myelinating cells for transplantation into the central nervous system (CNS) of patients with multiple sclerosis (MS) and spinal cord injury. It has been suggested that CNS glial cells interfere with SC migration, survival, maturation, and clinically significant remyelination in the CNS. To investigate the effects of CNS glial cells on SC, we examined the effects of serum-free supernatants obtained from rat mixed CNS glial cultures on rat neonatal SC cultures. Supernatants from 1-, 3-, and 5-day CNS glial cultures induced proliferation of SC assayed at 5 days in vitro but did not induce SC differentiation as measured by induction of surface expression of galactolipids (GalL). High concentrations of cAMP simulate many of the effects of axolemma on SC; CNS glial cell supernatants did not inhibit cAMP induction of SC differentiation. CNS glial cell supernatants had no apparent effect on SC viability at 48 hr as measured by trypan blue exclusion. We have previously demonstrated that incubation of SC with transforming growth factor-beta1 (TGF-beta1) + tumor necrosis factor-alpha (TNF-alpha) induces SC death via apoptosis. We now show that CNS glial supernatants inhibits TGF-beta1/TNF-alpha-induced SC death. Our data show that soluble products of CNS glial cells do not induce or inhibit SC differentiation or increase cell death but have the potential to increase proliferation of SC and their resistance to cytokine-mediated death, and thus may affect the outcome of SC transplantation into the CNS.

Immunohistochemical profile of transforming growth factor-β1 and basic fibroblast growth factor in sciatic nerve anastomosis following pinealectomy and exogenous melatonin administration in rats

Collagen scar formation at the cut end of a peripheral nerve, an important problem in clinical practice for neurosurgeons, obstructs sprouting of axons into appropriate distal fascicles, and thereby limits the regeneration process. Researchers have attempted to control collagen accumulation and neuroma formation with various physical and chemical methods, but with limited functional success. Recently, it has been demonstrated that transforming growth factor (TGF)-beta and basic fibroblast growth factor (bFGF) play an important role in collagen production by fibroblasts and in Schwann cell activity. In our study, rats were divided into a control group, a melatonin-treated group, a surgical pinealectomy group, and a group treated with melatonin following pinealectomy. They then underwent a surgical sciatic nerve transection and primary suture anastomosis. At 2 months after anastomosis, the animals were sacrificed and unilateral sciatic nerve specimens, including the anastomotic region, were removed and processed for immunohistochemical study from two animals in each group. For each antibody, immunoreactivity was assessed using a semiquantitative scoring system. Strong TGF-beta1 and/or bFGF expression was observed in the epineurium of animals that underwent pinealectomy, but no or weak staining was observed in animals in the control and melatonin treatment groups. Based on these data, we suggest that both TGF-beta1 and bFGF have important roles in control of collagen accumulation and neuroma formation at the anastomotic site, and that the pineal neurohormone melatonin has a beneficial effect on nerve regeneration.

ErbB2 signaling in Schwann cells is mostly dispensable for maintenance of myelinated peripheral nerves and proliferation of adult Schwann cells after injury

Neuregulin/erbB signaling is critically required for survival and proliferation of Schwann cells as well as for establishing correct myelin thickness of peripheral nerves during development. In this study, we investigated whether erbB2 signaling in Schwann cells is also essential for the maintenance of myelinated peripheral nerves and for Schwann cell proliferation and survival after nerve injury. To this end, we used inducible Cre-loxP technology using a PLP-CreERT2 allele to ablate erbB2 in adult Schwann cells. ErbB2 expression was markedly reduced after induction of erbB2 gene disruption with no apparent effect on the maintenance of already established myelinated peripheral nerves. In contrast to development, Schwann cell proliferation and survival were not impaired in mutant animals after nerve injury, despite reduced levels of MAPK-P (phosphorylated mitogen-activated protein kinase) and cyclin D1. ErbB1 and erbB4 do not compensate for the loss of erbB2. We conclude that adult Schwann cells do not require major neuregulin signaling through erbB2 for proliferation and survival after nerve injury, in contrast to development and in cell culture.

Cytokine responses during chronic denervation

Background

The aim of the present study was to examine inflammatory responses during Wallerian degeneration in rat peripheral nerve when the regrowth of axons was prevented by suturing.

Methods

Transected rat sciatic nerve was sutured and ligated to prevent reinnervation. The samples were collected from the left sciatic nerve distally and proximally from the point of transection. The endoneurium was separated from the surrounding epi- and perineurium to examine the expression of cytokines in both of these compartments. Macrophage invasion into endoneurium was investigated and Schwann cell proliferation was followed as well as the expression of cytokines IL-1β, IL-10, IFN-γ and TNF-α mRNA. The samples were collected from 1 day up to 5 weeks after the primary operation.

Results

At days 1 to 3 after injury in the epi-/perineurium of the proximal and distal stump, a marked expression of the pro-inflammatory cytokines TNF-α and IL-1β and of the anti-inflammatory cytokine IL-10 was observed. Concurrently, numerous macrophages started to gather into the epineurium of both proximal and distal stumps. At day 7 the number of macrophages decreased in the perineurium and increased markedly in the endoneurium of both stumps. At this time point marked expression of TNF-α and IFN-γ mRNA was observed in the endo- and epi-/perineurium of the proximal stump. At day 14 a marked increase in the expression of IL-1β could be noted in the proximal stump epi-/perineurium and in the distal stump endoneurium. At that time point many macrophages were observed in the longitudinally sectioned epineurium of the proximal 2 area as well as in the cross-section slides from the distal stump. At day 35 TNF-α, IL-1β and IL-10 mRNA appeared abundantly in the proximal epi-/perineurium together with macrophages.

Conclusion

The present studies show that even during chronic denervation there is a cyclic expression pattern for the studied cytokines. Contrary to the previous findings on reinnervating nerves the studied cytokines show increased expression up to 35 days. The high expressions of pro-inflammatory and anti-inflammatory cytokines in the proximal epi-/perineurial area at day 35 may be involved in the formation of fibrosis due to irreversible nerve injury and thus may have relevance to the formation of traumatic neuroma.

The Protooncogene Ski Controls Schwann Cell Proliferation and Myelination

Schwann cell proliferation and subsequent differentiation to nonmyelinating and myelinating cells are closely linked processes. Elucidating the molecular mechanisms that control these events is key to the understanding of nerve development, regeneration, nerve-sheath tumors, and neuropathies. We define the protooncogene Ski, an inhibitor of TGF-beta signaling, as an essential component of the machinery that controls Schwann cell proliferation and myelination. Functional Ski overexpression inhibits TGF-beta-mediated proliferation and prevents growth-arrested Schwann cells from reentering the cell cycle. Consistent with these findings, myelinating Schwann cells upregulate Ski during development and remyelination after injury. Myelination is blocked in myelin-competent cultures derived from Ski-deficient animals, and genes encoding myelin components are downregulated in Ski-deficient nerves. Conversely, overexpression of Ski in Schwann cells causes an upregulation of myelin-related genes. The myelination-regulating transcription factor Oct6 is involved in a complex modulatory relationship with Ski. We conclude that Ski is a crucial signal in Schwann cell development and myelination.

Co-expression of Transforming Growth Factor-β1 and Glial Cell Line–Derived Neurotrophic Factor in Vestibular Schwannoma

HYPOTHESIS

Transforming growth factor-beta1, glial cell line-derived neurotrophic factor, and their receptors are expressed in vestibular schwannoma, and the expression data correlate with the proliferation activity (Ki-67 labeling index) and the clinical growth rate of vestibular schwannoma tissue.

BACKGROUND

Glial cell line-derived neurotrophic factor is a potent growth factor for the central and peripheral nervous system. Recent results demonstrate that glial cell line-derived neurotrophic factor requires transforming growth factor-beta to exert its trophic effect on neural tissue. A functional role, including that in Schwann cell proliferation, is discussed for both transforming growth factor-beta1 and glial cell line-derived neurotrophic factor.

METHODS

Immunohistochemical analysis for transforming growth factor-beta1 and glial cell line-derived neurotrophic factor and their receptors TbetaR II, GFRalpha-1, and Ret was performed on formalin-fixed, paraffin-embedded archival surgical specimens. The Ki-67 labeling index (mean Ki-67 labeling index and highest Ki-67 labeling index for Antoni Type A and Type B regions) and the clinical growth rate of vestibular schwannoma were determined and correlated with the expression patterns of the examined neurotrophic factors and their receptors.

RESULTS

Results demonstrate co-expression of transforming growth factor-beta1 and glial cell line-derived neurotrophic factor with higher levels in Antoni Type A than in Antoni Type B regions. Ninety-five percent of vestibular schwannomas exhibited transforming growth factor-beta1 immunoreactivity, and glial cell line-derived neurotrophic factor expression was found in 100% of vestibular schwannoma specimens. Fifty percent of vestibular schwannoma displayed TbetaR II immunostaining, 100% showed positive reactions for GFRalpha-1, and 86% showed positive reactions for Ret. Statistical analysis revealed no significant correlation in neurotrophin expression related to sex, age, tumor size, clinical growth rate, or Ki-67-labeling indices.

CONCLUSIONS

Expression of transforming growth factor-beta1 and glial cell line-derived neurotrophic factor may suggest a biological role for both growth factors in vestibular schwannomas. Trophic transforming growth factor-beta/glial cell line-derived neurotrophic factor synergism seems possible and is underscored by co-expression of both neurotrophic factors and their receptors.

The effects of treatment with antibodies to transforming growth factor β1 and β2 following spinal cord damage in the adult rat

We recently showed axonal ingrowth into fibronectin (FN) mats implanted into the spinal cord. However, little axonal growth was found from FN mats into intact spinal cord. Previous research has shown that this is due in part to astrocytosis around an area of CNS damage. Antibodies to transforming growth factor beta (TGFbeta) can diminish this astrocytosis. TGFbeta also has effects on macrophages and Schwann cells, both of which infiltrate the spinal cord following damage. We examined the axonal, Schwann cell, and macrophage infiltration into FN mats as well as the level of astrocytosis and chondroitin sulfate proteoglycan NG2 around FN implants incubated in TGFbeta antibodies and implanted into a lesion cavity in the spinal cord. We also examined the effects of applying TGFbeta antibodies to a spinal cord hemisection site. Anti-TGFbeta1 within FN mats resulted in extensive cavitation, with the area of damage being larger than the original lesion. Cavitation was also seen following application of anti-TGFbeta1 to a spinal cord hemisection site. No cavitation was seen following saline, non-immune IgG or anti-TGFbeta2 treatment. However, anti-TGFbeta2 treatment did result in diminished axonal growth and Schwann cell and macrophage infiltration. Around the implant site, anti-TGFbeta2 treatment resulted in a reduction in the level of astrocytosis but had not effect on levels of NG2. Similar effects were seen following anti-TGFbeta2 application to spinal cord hemisection sites. The results suggest that anti-TGFbeta1 exacerbates secondary damage by preventing the anti-inflammatory effect of endogenous TGFbeta1. Anti-TGFbeta2 did not enhance axonal regeneration in this model but did slightly reduce astrocytosis.

Contralateral non-operated nerve to transected rat sciatic nerve shows increased expression of IL-1beta, TGF-beta1, TNF-alpha, and IL-10

Recent reports indicate that after a peripheral nerve injury, the uninjured contralateral nerve is also affected. Because cytokines play an important role in the peripheral nerve injury, we studied the expression of five different mRNAs (interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10), transforming growth factor-beta1 (TGF-beta1) and interleukin-4 (IL-4)) in the contralateral, non-operated, left sciatic nerve when the right rat sciatic nerve was transected. This study extended up to 42 days after the transection. No IL-4 expression was noted. During the first 3 days, high expression of the other studied cytokines was noted in the endoneurium. At day 7, the expression diminished to the control levels. After this, a cyclic expression pattern appeared, which was most pronounced in the endoneurium at 35 days. We also show that the expression pattern in the endoneurium is different from that in the surrounding epi- and perineurium. Also, our present study shows clearly that contralateral nerves are poor controls after injury.

TGFbeta1 modulates the phenotype of Schwann cells at the transcriptional level

We have examined the effects of transforming growth factor beta1 (TGFbeta1) on gene expression in cultured rat Schwann cells (SCs). TGFbeta1 decreased the steady-state mRNA levels of several genes that are expressed by myelinating SCs but had varied effects on the mRNA levels of NCAM, L1, GAP-43, and p75-genes that are expressed by denervated and nonmyelinating SCs. TGFbeta1 antagonized the effects of forskolin on the mRNA levels of the transcription factors Oct-6/tst-1/SCIP and Krox20. Transcriptional run-off analysis demonstrated that the effects of TGFbeta1 on gene expression occur at least in part at the level of transcription. Thus, TGFbeta1 suppresses the expression of genes that characterize the different phenotypes of SCs, and these changes occur at least in part at a transcriptional level.

Transforming growth factor-beta and forskolin attenuate the adverse effects of long-term Schwann cell denervation on peripheral nerve regeneration in vivo

Transforming growth factor-beta (TGF-beta) plays a central role in the regulation of Schwann cell (SC) proliferation and differentiation and is essential for the neurotrophic effects of several neurotrophic factors (reviewed by Unsicker and Krieglstein, 2000; Unsicker and Strelau, 2000). However, its role in peripheral nerve regeneration in vivo is not yet understood. Our studies were carried out to characterize (1) the effects of duration of regeneration, and chronic SC denervation on the number of tibial (TIB) motor neurons that regenerated axons over a fixed distance (25 mm into distal common peroneal [CP] nerve stumps), and (2) the effect of in vitro incubation of 6-month chronically denervated sciatic nerve explants with TGF-beta and forskolin on their capacity to support axonal regeneration in vivo. TIB--CP cross-suture in Silastic tubing was used, and regeneration into 0-24-week chronically denervated CP stumps was allowed for either 1.5 or 3 months. Chronically denervated rat sciatic nerve explants (3 x 3 mm(2)) were incubated in vitro with either DMEM and 15% fetal calf serum (D-15) plus TGF-beta/forskolin or D-15 alone for 48 h and placed into a 10-mm Silastic tube that bridged the proximal and distal nerve stumps of a freshly cut TIB nerve. The number of tibial motor neurons that regenerated axons through the explants and 25 mm into the distal nerve stump after 6 months, and TIB regeneration into the CP nerve stumps, were assessed using retrograde tracers, fluorogold, or fluororuby. We found that all tibial motor neurons regenerate their axons 25 mm into 0-4-week denervated CP nerve stumps after a regeneration period of 3 months. Reducing regeneration time to 1.5 months and chronic denervation, reduced the number of motor neurons that regenerated axons over 25 mm. Exposure of 6-month denervated nerve explants to TGF-beta/forskolin increased the number of motor neurons that regenerated through them from 258 +/-13; mean +/- SE to 442 +/- 22. Hence, acute treatment of atrophic SC with TGF-beta can reactivate the growth-permissive SC phenotype to support axonal regeneration.

Pigment epithelium-derived factor (PEDF) in neuroblastoma: a multifunctional mediator of Schwann cell antitumor activity

Neuroblastoma is notable for its cellular heterogeneity and unpredictable outcome. Tumors are a variable mixture of primitive malignant neuroblasts, more differentiated ganglionic cells, Schwann and endothelial cells. Although often fatal, neuroblastomas can spontaneously regress, possibly due to favorable autocrine and paracrine interactions among these cells. Here, pigment epithelium-derived factor (PEDF), a potent inhibitor of angiogenesis and inducer of neural differentiation, is shown to be produced by ganglionic cells and Schwann cells, but not by more primitive tumor cells. Although undifferentiated neuroblastoma tumor cell secretions were angiogenic primarily due to vascular endothelial growth factor, secretions of Schwann cells were anti-angiogenic due to PEDF. In addition, PEDF was the major factor responsible for Schwann cell’s ability to induce tumor cell differentiation in vitro and recombinant PEDF had the same effect in vitro and in vivo. Both the growth and the survival of Schwann cells were enhanced by PEDF. Thus PEDF may serve as a multifunctional antitumor agent in neuroblastomas, inhibiting angiogenesis while promoting the numbers of Schwann cells and differentiated tumor cells that in turn produce PEDF, suggesting that its clinical administration could stimulate a multifaceted antitumor feedback loop with the potential to limit and possibly regress tumor growth.

Interferon-gamma, tumor necrosis factor-alpha, and transforming growth factor-beta inhibit cyclic AMP-induced Schwann cell differentiation

Schwann cells differentiate in vivo in response to contact with axons, and cAMP simulates some of these aspects of differentiation in vitro, particularly morphologic changes and expression of certain phenotypic molecules. Unfractionated inflammatory cytokines inhibit cAMP-induced Schwann cell expression of galactolipids (Gal). We sought to identify which cytokines were responsible for this inhibition and to determine whether other phenotypic indicators of Schwann cell differentiation were also affected. Neonatal rat Schwann cells were incubated in vitro with 1 mM 8 Bromo cAMP (8 Br cAMP) with or without the addition of interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-2, IL-6, tumor necrosis factor-alpha (TNF-alpha), interferon-gamma (IFN-gamma), or transforming growth factor-beta (TGF-beta). Cells were then examined for morphologic changes and for expression of surface Gal and low-affinity nerve growth factor receptor (NGFRp75), employing indirect immunofluorescence. 8 Br cAMP induced Schwann cell upregulation of Gal, downregulation of NGFRp75, and the cells became enlarged and somewhat amorphous and irregular in appearance. Cells treated with IFN-gamma or TNF-alpha alone were more bipolar and more evenly distributed on coverslips than were control cells, whereas TGF-beta alone induced elongated cells often in a swirling pattern. None of the cytokines alone induced upregulation of Gal or downregulation of NGFRp75. TNF-alpha, IFN-gamma, and TGF-beta inhibited the 8 Br cAMP-induced morphologic changes, as well as the upregulation of Gal and downregulation of NGFRp75. The other cytokines had no effects on Gal or NGFRp75 expression. Thus, these three cytokines, which are present in inflammatory lesions in the peripheral nervous system, are capable of inhibiting Schwann cell differentiation.

Gene expression of transforming growth factor beta isoforms in interposition nerve grafting

Scar production and neuroma formation at nerve graft coaptation sites may limit axonal regeneration and impair functional outcome. Transforming growth factor beta (TGF-beta) is a family of growth factors that is involved in scar formation, wound healing, and nerve regeneration. Fifteen adult Sprague-Dawley rats underwent autogenous nerve grafting. The nerve grafts were analyzed by in situ hybridization to determine the temporal and spatial expression of TGF-beta1 and TGF-beta3 messenger RNA (mRNA). The grafted nerves showed increased expression of TGF-beta1 and TGF-beta3 mRNA in the nerve and the surrounding connective tissue during the first postoperative week. These data suggest that modulation of TGF-beta levels in the first postoperative week may be effective in helping to control scar formation and improve nerve regeneration.

Malignant peripheral nerve sheath tumour arising within neurofibroma. An immunohistochemical analysis in the comparison between benign and malignant components

AIMS

To compare the expression of immunohistochemical variables between benign and malignant components of malignant peripheral nerve sheath tumour (MPNST) arising within neurofibroma.

METHODS

Eight cases of MPNST arising within a neurofibroma, associated with neurofibromatosis type 1 (NF1), were studied. The areas of MPNST and neurofibroma were compared immunohistochemically with regard to the expression of proliferative activity (MIB-1), growth factors, p53, bcl-2, neural cell adhesion molecule (N-CAM), and CD34.

RESULTS

The expression of transforming growth factor beta 1 (TGF-beta 1), TGF-beta receptor type II, hepatocyte growth factor alpha (HGF-alpha), c-met, p53, and N-CAM was higher in the areas of MPNST than in the neurofibromatous areas in four, five, five, eight, five, and three of the eight cases, respectively. CD34 expression was lower in the areas of MPNST than in the neurofibroma areas in three of the eight cases.

CONCLUSIONS

On the basis of these findings, TGF-beta 1, HGF-alpha, and p53 might be involved in the malignant transformation of neurofibroma to MPNST.