Gene transfer to Schwann cells after peripheral nerve injury: a delivery system for therapeutic agents

Intraneural GJB1 gene delivery improves nerve pathology in a model of X-linked Charcot-Marie-Tooth disease

OBJECTIVE

X-linked Charcot-Marie-Tooth disease (CMT1X) is a common inherited neuropathy caused by mutations in the GJB1 gene encoding the gap junction protein connexin32 (Cx32). Clinical studies and disease models indicate that neuropathy mainly results from Schwann cell autonomous, loss-of-function mechanisms; therefore, CMT1X may be treatable by gene replacement.

METHODS

A lentiviral vector LV.Mpz-GJB1 carrying the GJB1 gene under the Schwann cell-specific myelin protein zero (Mpz) promoter was generated and delivered into the mouse sciatic nerve by a single injection immediately distal to the sciatic notch. Enhanced green fluorescent protein (EGFP) reporter gene expression was quantified and Cx32 expression was examined on a Cx32 knockout (KO) background. A gene therapy trial was performed in a Cx32 KO model of CMT1X.

RESULTS

EGFP was expressed throughout the length of the sciatic nerve in up to 50% of Schwann cells starting 2 weeks after injection and remaining stable for up to 16 weeks. Following LV.Mpz-GJB1 injection into Cx32 KO nerves, we detected Cx32 expression and correct localization in non-compact myelin areas where gap junctions are normally formed. Gene therapy trial by intraneural injection in groups of 2-month-old Cx32 KO mice, before demyelination onset, significantly reduced the ratio of abnormally myelinated fibers (p = 0.00148) and secondary inflammation (p = 0.0178) at 6 months of age compared to mock-treated animals.

INTERPRETATION

Gene delivery using a lentiviral vector leads to efficient gene expression specifically in Schwann cells. Restoration of Cx32 expression ameliorates nerve pathology in a disease model and provides a promising approach for future treatments of CMT1X and other inherited neuropathies.

Low-frequency electrical stimulation induces the proliferation and differentiation of peripheral blood stem cells into Schwann cells

BACKGROUND

Functional recovery after peripheral nerve injury remains a tough problem at present. Specifically, a type of glial cell exists in peripheral nerves that promotes axonal growth and myelin formation and secretes various active substances, such as neurotrophic factors, extracellular matrix and adherence factors. These substances have important significance for the survival, growth and regeneration of nerve fibers. Numerous recent studies have shown that electrical stimulation can increase the number of myelinated nerve fibers. However, whether electrical stimulation acts on neurons or Schwann cells has not been verified in vivo. This study investigates low-frequency electrical stimulation-induced proliferation and differentiation of peripheral blood stem cells into Schwann cells and explores possible mechanisms.

METHODS

Peripheral blood stem cells from Sprague-Dawley rats were primarily cultured. Cells in passage 3 were divided into 4 groups: a low-frequency electrical stimulation group (20 Hz, 100 μs, 3 V), a low-frequency electrical stimulation+PD98059 (blocking the extracellular signal-regulated kinase [ERK] signaling pathway) group, a PD98059 group and a control group (no treatment). After induction, the cells were characterized. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazoliumbromide assay was employed to measure the absorbance values at 570 nm in the 4 groups. A Western blot assay was used to detect the expression of cyclin D1 and cyclin-dependent kinase 4 (CDK4) in each group.

RESULTS

No significant difference in cell viability was detected before induction. Peripheral blood stem cells from the 4 groups differentiated into Schwann cells. Phosphorylated ERK 1/2, cyclin D1 and CDK4 protein levels were highest in the low-frequency electrical stimulation group and lowest in the ERK blockage group. Phosphorylated ERK 1/2, cyclin D1 and CDK4 protein levels in the low-frequency electrical stimulation+ERK blockage group were lower than those in the low-frequency electrical stimulation group but higher than those in the ERK blockage group.

CONCLUSIONS

Low-frequency electrical stimulation contributed to the proliferation of peripheral blood stem cells cultured in vitro and induced differentiation into Schwann cells. The ERK signaling pathway underlies cell proliferation and differentiation.

Electric stimulation and decimeter wave therapy improve the recovery of injured sciatic nerves

Drug treatment, electric stimulation and decimeter wave therapy have been shown to promote the repair and regeneration of the peripheral nerves at the injured site. This study prepared a Mackinnon's model of rat sciatic nerve compression. Electric stimulation was given immediately after neurolysis, and decimeter wave radiation was performed at 1 and 12 weeks post-operation. Histological observation revealed that intraoperative electric stimulation and decimeter wave therapy could improve the local blood circulation of repaired sites, alleviate hypoxia of compressed nerves, and lessen adhesion of compressed nerves, thereby decreasing the formation of new entrapments and enhancing compressed nerve regeneration through an improved microenvironment for regeneration. Immunohistochemical staining results revealed that intraoperative electric stimulation and decimeter wave could promote the expression of S-100 protein. Motor nerve conduction velocity and amplitude, the number and diameter of myelinated nerve fibers, and sciatic functional index were significantly increased in the treated rats. These results verified that intraoperative electric stimulation and decimeter wave therapy contributed to the regeneration and the recovery of the functions in the compressed nerves.

Construction of tissue engineered nerve grafts and their application in peripheral nerve regeneration

Surgical repair of severe peripheral nerve injuries represents not only a pressing medical need, but also a great clinical challenge. Autologous nerve grafting remains a golden standard for bridging an extended gap in transected nerves. The formidable limitations related to this approach, however, have evoked the development of tissue engineered nerve grafts as a promising alternative to autologous nerve grafts. A tissue engineered nerve graft is typically constructed through a combination of a neural scaffold and a variety of cellular and molecular components. The initial and basic structure of the neural scaffold that serves to provide mechanical guidance and optimal environment for nerve regeneration was a single hollow nerve guidance conduit. Later there have been several improvements to the basic structure, especially introduction of physical fillers into the lumen of a hollow nerve guidance conduit. Up to now, a diverse array of biomaterials, either of natural or of synthetic origin, together with well-defined fabrication techniques, has been employed to prepare neural scaffolds with different structures and properties. Meanwhile different types of support cells and/or growth factors have been incorporated into the neural scaffold, producing unique biochemical effects on nerve regeneration and function restoration. This review attempts to summarize different nerve grafts used for peripheral nerve repair, to highlight various basic components of tissue engineered nerve grafts in terms of their structures, features, and nerve regeneration-promoting actions, and finally to discuss current clinical applications and future perspectives of tissue engineered nerve grafts.

Nerve conduits and growth factor delivery in peripheral nerve repair

Peripheral nerves possess the capacity of self-regeneration after traumatic injury. Transected peripheral nerves can be bridged by direct surgical coaptation of the two nerve stumps or by interposing autografts or biological (veins) or synthetic nerve conduits (NC). NC are tubular structures that guide the regenerating axons to the distal nerve stump. Early synthetic NC have primarily been made of silicone because of the relative flexibility and biocompatibility of this material and because medical-grade silicone tubes were readily available in various dimensions. Nowadays, NC are preferably made of biodegradable materials such as collagen, aliphatic polyesters, or polyurethanes. Although NC assist in guiding regenerating nerves, satisfactory functional restoration of severed nerves may further require exogenous growth factors. Therefore, authors have proposed NC with integrated delivery systems for growth factors or growth factor-producing cells. This article reviews the most important designs of NC with integrated delivery systems for localized release of growth factors. The various systems discussed comprise NC with growth factors being released from various types of matrices, from transplanted cells (Schwann cells or mesenchymal stem cells), or through genetic modification of cells naturally present at the site of injured tissue. Acellular delivery systems for growth factors include the NC wall itself, biodegradable microspheres seeded onto the internal surface of the NC wall, or matrices that are filled into the lumen of the NC and immobilize the growth factors through physical-chemical interactions or specific ligand-receptor interactions. A very promising and elegant system appears to be longitudinally aligned fibers inserted in the lumen of a NC that deliver the growth factors and provide additional guidance for Schwann cells and axons. This review also attempts to appreciate the most promising approaches and emphasize the importance of growth factor delivery kinetics.

Gene therapy and wound healing

Wound repair involves the sequential interaction of various cell types, extracellular matrix molecules, and soluble mediators. During the past 10 years, much new information on signals controlling wound cell behavior has emerged. This knowledge has led to a number of novel therapeutic strategies. In particular, the local delivery of pluripotent growth factor molecules to the injured tissue has been intensively investigated over the past decade. Limited success of clinical trails indicates that a crucial aspect of the growth factor wound healing strategy is the effective delivery of these polypeptides to the wound site. A molecular approach in which genetically modified cells synthesize and deliver the desired growth factor in regulated fashion has been used to overcome the limitations associated with the (topical) application of recombinant growth factor proteins. We have summarized the molecular and cellular basis of repair mechanisms and their failure, and we give an overview of techniques and studies applied to gene transfer in tissue repair.

Biologically inspired approaches to drug delivery for nerve regeneration

As the biological processes governing nerve regeneration have become elucidated over the past decades, interest has developed in manipulating these processes to improve nerve regeneration. Drug delivery to the regenerating nerve has the potential for major clinical applications in neurodegenerative diseases, spinal cord injury and peripheral nerve injury or sacrifice. This article reviews the evolution of the field of drug delivery to the regenerating nerve, from simple local applications of neurotrophic agents in solution and osmotic pump delivery, to the existing approaches involving novel biomaterials and genetically manipulated cell populations. A discussion of the various known nerve growth-promoting agents, and the chemical considerations involved in their delivery, is included. A perspective on the role of tissue engineering approaches for nerve regeneration in the future is offered.

Therapeutic strategies for the inherited neuropathies

More than 30 genetic causes have been identified for the inherited neuropathies collectively referred to as Charcot-Marie-Tooth (CMT) disease. Previous therapies for CMT were limited to traditional approaches such as rehabilitation medicine, ambulation aids, and pain management. Identification of the genes causing CMT has led to improved genetic counseling and assistance in family planning. Identification of these genes is beginning to delineate common molecular pathways in multiple forms of CMT that can be exploited in future molecular therapies. Scientifically based clinical trials for CMT are currently being implemented. Techniques of gene therapy are advancing to the point that they may become feasible options for patients with CMT and other neurodegenerative diseases.

Schwann-cell injection of cold-preserved nerve allografts

This study investigated the effects of prolonged cold preservation and Schwann-cell injection on nerve regeneration through peripheral nerve allografts. Forty rats were randomized to the following groups: group I, isograft; group II, allograft; group III, isograft + Schwann cells; group IV, 6-week cold-preserved allograft; and group V, 6-week cold-preserved allograft with Schwann cells. Nerves from all animals were harvested at 4 weeks after surgery for histological and histomorphometric analysis. Untreated allograft recipients demonstrated poor nerve regeneration and histological evidence of rejection. The remaining four groups showed robust regeneration without evidence of rejection. In a short nerve allograft model, prolonged cold preservation of allografts supported robust nerve regeneration, but the addition of cultured Schwann cells conferred no additional benefit for nerve regeneration. Further work in large animals is needed to establish the role for exogenous Schwann cells in nerve allotransplantation.

Regeneration into Protected and Chronically Denervated Peripheral Nerve Stumps

OBJECTIVE

Delayed repair of peripheral nerve injuries often results in poor motor functional recovery. This may be a result of the deterioration or loss of endoneurial pathways in the distal nerve stump before motor axons can regenerate into the stump.

METHODS

Using the rat femoral nerve, we protected distal endoneurial pathways of the saphenous nerve with either cross-suture of the quadriceps motor nerve (Group A) or resuture of the saphenous nerve (Group B) to compare later motor regeneration into the "protected" saphenous nerve pathway to chronic denervation and "unprotected" saphenous nerve (Group C). A total of 60 rats, 20 per group, were operated on. After this protection (or lack thereof) for 8 weeks, the motor branch of the femoral nerve was cut and sutured to the distal saphenous nerve to allow motor regeneration into protected and unprotected saphenous nerve stumps. The quantitative assessment of axonal regeneration was performed after 6 weeks by use of nerve sampling for axon counts and retrogradely labeled motor neuron counts.

RESULTS

Significantly more myelinated axons innervated the motor (A) than the sensory (B) and no-protection (C) groups. There were significantly more retrogradely labeled femoral motor neurons in Group A than in the unprotected group (C).

CONCLUSION

We conclude that even 2 months of denervation of the distal nerve pathway is deleterious to regeneration and that protection of the pathway improves subsequent reinnervation and regeneration. Moreover, if the desired regeneration is motor, protection of the distal nerve pathway by a motor nerve conditions is better than a sensory nerve.

Nonviral HVJ (hemagglutinating virus of Japan) liposome-mediated retrograde gene transfer of human hepatocyte growth factor into rat nervous system promotes functional and histological recovery of the crushed nerve

Hepatocyte growth factor (HGF) is well known to be involved in many biological functions, such as organ regeneration and angiogenesis, and to exert neurotrophic effects on motor, sensory, and parasympathetic neurons. In this study, we gave repeated intramuscular injections of the human HGF gene, using nonviral HVJ (hemagglutinating virus of Japan) liposome method, to examine whether transfection of the rat nervous system with this gene is able to exert neurotrophic effects facilitating recovery of a crushed nerve. The expression of HGF protein and HGF mRNA indicated that gene transfer into the nervous system did occur via retrograde axonal transport. At 4 weeks after crush, electrophysiological examination of the crushed nerve showed a significantly shorter mean latency and a significantly greater mean maximum M-wave amplitude with repeated injections of HGF gene. Furthermore, histological findings showed that the mean diameter of the axons, the axon number and the axon population were significantly larger in the group with repeated injections of HGF gene. The above results show that repeated human HGF gene transfer into the rat nervous system is able to promote crushed-nerve recovery, both electrophysiologically and histologically, and suggest that HGF gene transfer has potential for the treatment of crushed nerve.

Nerve Replacement Strategies for Cavernous Nerves

OBJECTIVE

This article reviews novel restorative therapies for cavernous nerves that may be used to replace resected cavernous nerves at the time of pelvic surgery.

METHODS

A literature-based presentation (Medline search) on current nerve replacement strategies was conducted with emphasis on neurobiological factors contributing to the restoration of erectile function after cavernous nerve injuries.

RESULTS

A promising alternative to autologous nerve grafts for extending the length of successful nerve regeneration are artificial nerve guides. The addition of neurotrophic factors, extracellular matrix components and Schwann cells has been shown to promote cavernous nerve regeneration. Neurotrophic factors can be incorporated in the scaffold or can be supplied by cells seeded into the stroma. The regenerative capacity of these cells can be further enhanced by genetic modification with neurotrophic factor encoding genes.

CONCLUSIONS

Artificial nerve guides, especially biodegradable ones containing growth-promoting factors or cells, are a promising option for the repair of cavernous nerve lesions.

[Nerve repair strategies for restoration of erectile function after radical pelvic surgery]

Iatrogenic cavernous nerve lesions occurring during radical pelvic surgery often lead to irreversible erectile dysfunction. The nerve defects after excision of the neurovascular bundles must be reconstructed by interposition grafting to supply a permissive scaffold for oriented axonal regrowth. The use of autologous nerve grafts for the repair of human cavernous nerves during radical prostatectomy has been controversial regarding the limited success achieved with bilateral nerve grafting. Artificial nerve guides consisting of natural or synthetic materials have been successfully used for bridging peripheral nerve defects. The combination with Schwann cells, neurotrophic factors and extracellular matrix components has been shown to promote cavernous nerve regeneration.

PRP-1 Protective Effect against Central and Peripheral Neurodegeneration following n. ischiadicus Transection

We investigated the action of the new hypothalamic proline-rich peptide (PRP-1), normally produced by neurosecretory cells of hypothalamic nuclei (NPV and NSO), 3 and 4 weeks following rat sciatic nerve transection. The impulse activity flow of interneurons (IN) and motoneurons (MN) on stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius--G) and extensor (n. peroneus communis--P) nerves of both injured and symmetric intact sides of spinal cord (SC) was recorded in rats with daily administration of PRP-1 (for a period of 3 weeks) and without it (control). On the injured side of SC in control, there were no responses of IN and MN on ipsilateral G and P stimulation, while responses were elicited on contralateral nerve stimulation. The neuron responses on the intact side of SC were revealed in a reverse ratio. Thus, there were no effects upon stimulation of the injured nerve distal stump in the control because of the absence of fusion between transected nerve stumps. This was also testified by the atrophy of the distal stump and the absence of motor activity of the affected limb. In PRP-1-treated animals, the responses of SC IN and MN in postaxotomy 3 weeks on the injured side of SC at ipsilateral nerve stimulation and on the intact side at contralateral nerve stimulation were recorded because of the obvious fusion of the severed nerve stumps. The histochemical data confirmed the electrophysiological findings. Complete coalescence of transected fibers together with restoration of the motor activity of the affected limb provided evidence for reinnervation on the injured side. Thus, it may be concluded that PRP-1 promotes nerve regeneration and may be used clinically to improve the outcome of peripheral nerve primary repair.

Nonviral gene transfer of human hepatocyte growth factor improves streptozotocin-induced diabetic neuropathy in rats

Peripheral neuropathy is common and ultimately accounts for significant morbidity in diabetes. Recently, several neurotrophic factors have been used to prevent progression of diabetic neuropathy. In this study, we gave repeated intramuscular injections of the human hepatocyte growth factor (HGF) gene percutaneously, using liposomes containing the hemagglutinating virus of Japan (HVJ), to examine therapeutic efficacy of nonviral gene transfer of HGF for experimental diabetic sensorimotor neuropathy in rats. Experimental diabetes induced by intraperitoneal injection of streptozotocin resulted in a marked tactile allodynia (but not in a thermal hyperalgesia), in a reduction of both the conduction velocity and the amplitude, and in a decreased laser Doppler flux of the nerve and the muscle at 6 weeks after the induction. All these changes were significantly reversed by repeated gene transfer of HGF. Furthermore, we analyzed the density of endoneurial capillaries and morphometrical changes of the nerve. The density of endoneurial capillaries, disclosing marked reduction in diabetic rats, was also reversed significantly by repeated gene transfer of HGF; however, no considerable differences were observed morphometrically in either myelinated or unmyelinated axons. These results suggest that nonviral HVJ liposome-mediated gene transfer of human HGF has potential for the safe effective treatment of diabetic sensorimotor neuropathy.

Gene transfer in tissue repair: status, challenges and future directions

Wound repair involves a complex interaction of various cell types, extracellular matrix molecules and soluble mediators. Details on signals controlling wound cell activities are beginning to emerge. In recent years this knowledge has been applied to a number of therapeutic strategies in soft tissue repair. Key challenges include re-adjusting the adult repair process in order to augment diseased healing processes, and providing the basis for a regenerative rather than a reparative wound environment. In particular, the local delivery of pluripotent growth factor molecules to the injured tissue has been intensively investigated over the past decade. Limited success of clinical trials indicates that an important aspect of the growth factor wound-healing paradigm is the effective delivery of these polypeptides to the wound site. A molecular genetic approach in which genetically modified cells synthesise and deliver the desired growth factor in a time-regulated manner is a powerful means to overcome the limitations associated with the (topical) application of recombinant growth factor proteins. This article summarises repair mechanisms and their failure, and gives an overview of techniques and studies applied to gene transfer in tissue repair. It also provides perspectives on potential targets for gene transfer technology.

The brachial plexus: normal anatomy, pathology, and MR imaging

The brachial plexus is the most technically and anatomically challenging area of the peripheral nervous system for diagnostic imaging. Marked improvements in spatial and contrast resolution of plexus images have resulted from the use of phased-array technology and newer MR pulse sequence designs. This article presents case material incorporating these improvements and discusses the primary factors that continue to limit MR image quality, such as inhomogenous fat suppression, motion artifacts, and small vessels that mimic or obscure plexus components, and potential solutions and imaging alternatives. Brachial plexus anatomy and its appearance on multiplanar MR images are reviewed. The morphologic features and MR signal characteristics that have been found useful in distinguishing between normal and abnormal plexus components,and in detecting neuropathic lesions, are addressed in the context of clinical indications for plexus imaging as follows: mass involving the plexus, traumatic injury, entrapment syndrome, posttreatment evaluation, and miscellaneous conditions.

Efficient gene transfer from innervated muscle into rat peripheral and central nervous systems using a non-viral haemagglutinating virus of Japan (HVJ)-liposome method

We evaluated the feasibility of gene delivery into the peripheral and central nervous systems via retrograde axonal transport following injection of a haemagglutinating virus of Japan (HVJ)-liposome-DNA complex vector into an innervated muscle. Transfection efficiency was assessed by measuring luciferase activity, and was compared statistically with that achieved using a liposome-DNA control vector. High luciferase activity was observed in the injected muscle, the ipsilateral sciatic nerve, and the ipsilateral dorsal root ganglia on day 1 after gene transfer. The spinal cord also showed luciferase activity, although this was lower than in the other tissues. However, no activity was observed in the contralateral sciatic nerve or the contralateral dorsal root ganglia. In addition, we performed gene transfer twice, with a 1-week interval, to evaluate the feasibility of repeated therapeutic gene delivery. Again, a high transfection efficiency was observed immediately, even after the second gene transfer, and transfection efficiency was significantly higher at each defined time-point using the HVJ-liposome complex vector than using a control vector. These results indicate that this method could be used for repeated therapeutic gene delivery into muscle, nerve, dorsal root ganglia, and possibly spinal cord, without the need for a surgical approach, making it well suited to clinical applications.

Addition of fibronectin to alginate matrix improves peripheral nerve regeneration in tissue-engineered conduits

Schwann cell (SC) transplantation has been proposed to encourage peripheral nerve regeneration, but an optimal SC-carrying matrix would be needed. The aim of this study was to characterize how the addition of fibronectin to alginate would affect the outcome of nerve regeneration promoted by Schwann cells embedded in this matrix. Genetically labeled rat SCs were obtained by lacZ gene transduction. SCs were suspended in alginate hydrogel matrix with/without addition of liquid fibronectin, and their viability and growth in the different types of matrices were assessed in vitro by AlamarBlue assay. In vivo assessment of SC transplantation in the matrix was carried out with poly-3-hydroxybutyrate (PHB) conduits to bridge a sciatic nerve gap. The grafted conduits were harvested at 2, 3, and 6 weeks and assessed for the presence of labeled SCs in relation to regrowing axons. The amount and rate of axonal regeneration were assessed by quantitative immunohistochemistry. Addition of fibronectin to alginate hydrogel improved SC viability and growth profile in vitro. X-Gal staining confirmed that SCs transplanted in PHB conduits were viable throughout the time course, and that the labeled SCs were clearly associated with regenerating axons. The regeneration rate was enhanced when liquid fibronectin was added to the alginate matrix. Furthermore, the presence of SCs also enhanced regeneration and there was an additive effect when both SCs and fibronectin were combined with alginate. In conclusion, the addition of fibronectin to alginate hydrogel matrix contributed to improve nerve regeneration, supporting SC viability and augmenting their effect on axonal growth when transplanted in a bioengineered nerve conduit.

Engineering an artificial nerve graft for the repair of severe nerve injuries

Nerve repair with tubes has a limit to regeneration depending upon the length of the gap. The characteristics of the guide, in terms of permeability, durability and adhesiveness, also influence regeneration. Considering the importance of the cellular component in regeneration, the development of artificial grafts, composed of a biocompatible nerve guide filled with a neurotropic matrix and seeded with Schwann cells (SCs), is an interesting option to enhance nerve regeneration and provide an alternative to the classical autologous nerve graft. We evaluated the ability of SCs transplanted into a nerve guide to improve regeneration after sciatic nerve resection, leaving a 6-mm gap, in the mouse. Syngeneic, isogeneic and autologous SCs were suspended in Matrigel and seeded in resorbable guides, and compared to acellular guides and to nerve autografts. The immunogenicity of the transplanted SCs clearly influenced the outcome. Transplants of autologous SCs resulted in only slightly lower levels of reinnervation than autografts, but higher recovery and number of regenerated axons than transplants of isologous and syngeneic SCs, and than acellular guides. Thus, by combined developments on nerve guides, extracellular matrix components and cell transplantation, an artificial graft has been designed that allows axonal regeneration across long gaps to levels comparable with an autograft.

A new approach to CNS repair using chimeric peripheral nerve grafts

We have examined whether transplanted freeze-thawed peripheral nerve (PN) sheaths repopulated ex vivo with purified adult Schwann cells (SCs) support the regeneration of adult rat retinal ganglion cell (RGC) axons. Cultured adult SCs were derived from donor rats or from the host animals themselves. We also transplanted PN sheaths filled with neonatal SCs or donor adult olfactory ensheathing glia (OEG). 100,000 cells were injected into 1.5-cm lengths of freeze-thawed PN. After 2 days in culture, repopulated PN segments were grafted onto the transected optic nerve of adult Fischer rats. Three weeks later, 6% fluorogold (FG) was applied to distal PN. Retrogradely labeled RGCs were counted in retinal wholemounts and PN grafts were processed for immunohistochemistry. As expected, there was no RGC axon regeneration in cell-free grafts. Regrowth was also absent in neonatal SC- and adult OEG-filled grafts, which contained only small numbers of surviving donor cells. Many cells were, however, seen in adult SC repopulated PN grafts, intermingled with pan-neurofilament(+) and GAP-43(+) fibers. SCs were aligned along the grafts and were S-100(+), p75(+). Ultrastructurally, SCs were associated with myelinated and unmyelinated axons. Hundreds of FG-labeled RGCs were seen in retinas of rats with congeneic or allogeneic PN sheaths repopulated with either donor or autologous (host-derived) adult SCs. Intraocular CNTF injections significantly increased the number of regenerating RGCs in donor and autologous adult SC groups. The use of chimeric grafts to bridge CNS tissue defects could provide a clinical alternative to using multiple PN autografts, the harvesting of which would exacerbate peripheral dysfunction in already injured patients.

GDNF acts through PEA3 to regulate cell body positioning and muscle innervation of specific motor neuron pools

Target innervation by specific neuronal populations involves still incompletely understood interactions between central and peripheral factors. We show that glial cell line-derived neurotrophic factor (GDNF), initially characterized for its role as a survival factor, is present early in the plexus of the developing forelimb and later in two muscles: the cutaneus maximus and latissimus dorsi. In the absence of GDNF signaling, motor neurons that normally innervate these muscles are mispositioned within the spinal cord and muscle invasion by their axons is dramatically reduced. The ETS transcription factor PEA3 is normally expressed by these motor neurons and fails to be induced in most of them in GDNF signaling mutants. Thus, GDNF acts as a peripheral signal to induce PEA3 expression in specific motor neuron pools thereby regulating both cell body position and muscle innervation.

Critical role for IL-4 in the development of transplant arteriosclerosis in the absence of CD40-CD154 costimulation

Blockade of the CD40-CD154 pathway can inhibit CD4(+) T cell activation but is unable to prevent immune responses mediated by CD8(+) T cells. However, even in the absence of CD8(+) T cells, inhibition of the CD40-CD154 pathway is insufficient to prevent the development of transplant arteriosclerosis. This study investigated the mechanisms of transplant arteriosclerosis in the absence of the CD40 pathway. C57BL/6 CD40(-/-) (H2(b)) recipients were transplanted with MHC-mismatched BALB/c (H2(d)) aortas. Transplant arteriosclerosis was evident in both CD40(-/-) and CD40(+/-) mice (intimal proliferation was 59 +/- 5% for CD40(-/-) mice vs 58 +/- 4% for CD40(+/-) mice) in the presence or absence of CD8(+) T cells (intimal proliferation was 46 +/- 7% for CD40(-/-) anti-CD8-treated mice vs 50 +/- 10% for CD40(+/-) anti-CD8-treated mice), confirming that CD8(+) T cells are not essential effector cells for the development of this disease. In CD40(-/-) recipients depleted of CD8(+) T cells, the number of eosinophils infiltrating the graft was markedly increased (109 +/- 24 eosinophils/grid for CD40(-/-) anti-CD8-treated mice vs 28 +/- 7 for CD40(+/-) anti-CD8-treated mice). The increased presence of eosinophils correlated with augmented intragraft production of IL-4. To test the hypothesis that IL-4 was responsible for the intimal proliferation, CD8 T cell-depleted CD40(-/-) recipients were treated with anti-IL-4 mAb. This resulted in significantly reduced eosinophil infiltration into the graft (12 +/- 5 eosinophils/grid for CD40(-/-) anti-CD8(+), anti-IL-4-treated mice vs 109 +/- 24 for CD40(-/-) anti-CD8-treated mice), intragraft eotaxin, CCR3 mRNA production, and the level of intimal proliferation (18 +/- 5% for CD40(-/-) anti-CD8(+)-, anti-IL-4-treated mice vs 46 +/- 7% for CD40(-/-) anti-CD8-treated mice). In conclusion, elevated intragraft IL-4 production results in an eosinophil infiltrate and is an important mechanism for CD8(+) T cell-independent transplant arteriosclerosis in the absence of CD40-CD154 costimulation.

Retroviral labeling of Schwann cells: in vitro characterization and in vivo transplantation to improve peripheral nerve regeneration

Transplantation of Schwann cells (SCs) is a promising treatment modality to improve neuronal regeneration. Identification of the transplanted cells is an important step when studying the development of this method. Genetic labeling is the most stable and reliable method of cell identification, but it is still unclear whether it has deleterious effect on SC characteristics. Our aim was to achieve a stable population of SCs transduced with the lacZ gene at a high frequency using a retroviral vector in vitro, and to follow the labeled SC in vitro to assess their viability and phenotypic marker expression. Furthermore, we transplanted lacZ-labeled SCs in a conduit to repair peripheral nerve to investigate their effect on nerve regeneration in vivo. Rat and human SCs were cultured and transduced with an MFG lacZ nls marker gene, achieving a transduction rate of 80% and 70%, respectively. Rat SCs were kept in culture for 27 weeks and examined every 4 weeks for expression of lacZ, viability, and phenotypic marker expression of GFAP, p75, MHC I and II. Throughout this period, transduced rat SCs remained viable and continued to proliferate. The proportion of cells expressing lacZ dropped only by 10% and the expression of phenotypic markers remained stable. Transduced human SCs were followed up for 4 weeks in culture. They proliferated and continued to express the lacZ gene and phenotypic marker expression of GFAP and p75 was preserved. Primary culture of transduced rat SCs were transplanted, syngeneically, in a conduit to bridge a 10 mm gap in sciatic nerve and the grafts were examined after 3 weeks for the presence and participation of labeled SCs and for axonal regeneration distance. Transplanted transduced rat SCs were clearly identified, taking part in the regeneration process and enhancing the axonal regeneration rate by 100% (at the optimal concentration) compared to conduits without SCs. Thus, retroviral introduction of lacZ gene has no deleterious effect on SCs in vitro and these SCs take part and enhance nerve regeneration in vivo.

Charcot-Marie-Tooth disease type 1: molecular pathogenesis to gene therapy

Charcot-Marie-Tooth disease type 1 (CMT1) is caused by mutations in the peripheral myelin protein, 22 kDa (PMP22) gene, protein zero (P0) gene, early growth response gene 2 (EGR-2) and connexin-32 gene, which are expressed in Schwann cells, the myelinating cells of the peripheral nervous system. Although the clinical and pathological phenotypes of the various forms of CMT1 are similar, including distal muscle weakness and sensory loss, their molecular pathogenesis is likely to be quite distinct. In addition, while demyelination is the hallmark of CMT1, the clinical signs and symptoms of the disease are probably produced by axonal degeneration, not demyelination itself. In this review we discuss the molecular pathogenesis of CMT1, as well as approaches to an effective gene therapy for this disease.

Adenoviral vectors for in vivo gene delivery to oligodendrocytes: transgene expression and cytopathic consequences

Replication defective viral vectors provide a potentially useful means of gene transfer to oligodendrocytes and thus for studying the pathogenesis of white matter disease. In this study we have examined the expression pattern of E1/E3 deleted adenoviral vectors expressing the reporter gene LacZ (AdlacZ) as a means of establishing the value of these vectors for gene delivery to oligodendrocytes in adult rat white matter. Our results indicate that although such an approach can be used to induce transgene expression in oligodendrocytes, it is complicated by both immunogenic and cytopathic effects. Thus, in normal animals, injection of DeltaE1/E3 adenoviral vectors was associated with a robust immune response that led to a lack of expression by 40 days after injection. In order to overcome this complication, virus was injected into the white matter of immuno-deficient athymic rats. These experiments indi- cated that even in the absence of a T cell response high viral titres of DeltaE1/E3 adenoviral vectors had a profound cytopathic effect leading to death of oligodendrocytes and hence demyelination. A similar cytopathic effect was demonstrated using an adenoviral vector expressing the neurocytokine ciliary neurotrophic factor (AdCNTF). As the titre of injected virus was decreased there was a significant decrease in the number of transgene expressing cells. These experiments therefore indicated that in immunodeficient recipients there is a narrow window of virus titre that results in a high rate of infectivity and expression without significant cytopathic consequences. At higher viral titres the cytopathic effects include oligodendrocyte death and demyelination, while at lower titres there is a significant decrease in the efficiency of the number of cells expressing the transgene.

Modulation of cell-mediated immunity prolongs adenovirus-mediated transgene expression in sciatic nerve

In a previous report, we demonstrated that a first-generation (E1- and E3-deleted) recombinant adenovirus can transduce expression of the E. coli lacZ gene into Schwann cells, both in vitro and in vivo, suggesting that this method might be useful for future therapy of peripheral neuropathy, including CMT1. Adenovirus-mediated gene transfer was limited, however, by demyelination and Wallerian degeneration at the site of virus injection, as well as by attenuation of viral transgene expression over time. In our current work we have optimized adenoviral vector-mediated transgene expression after intraneural injection into sciatic nerve. Using an improved injection protocol, peak expression of lacZ occurs between 10 and 14 days after injection of 2-week-old rats, decreases thereafter, and there is minimal associated tissue injury. In contrast, few lacZ-expressing Schwann cells are found in nerve of adult animals 10 days after injection, probably owing to immune clearance of virus-infected cells. Consistent with this notion, high levels of LacZ are found in sciatic nerve 30 days after injection of adult SCID mice, which have a genetic defect in both cellular and humoral immunity, of adult beta2-microglobulin-deficient mice (beta2M4-/-), which have a genetic defect in cellular immunity, or of adult mice treated with the immunosuppressing agent FK506. In addition, adenovirus-infected Schwann cells cocultured with axons in vitro, in the absence of a host immune response, ensheathe axons and express lacZ for at least 8 weeks. These data thus demonstrate that lacZ transgene expression of first-generation recombinant adenovirus in sciatic nerve in adult mice, as in other tissues, is limited mainly by the host cellular immune response to the virus, which can be overcome by attenuation of host cell-mediated immunity. Adenoviral vectors might thus be used to modulate Schwann cell gene expression in patients with peripheral neuropathy after appropriate immunosuppression.

Adenovirus-mediated transfer of the neurotrophin-3 gene into skeletal muscle of pmn mice: therapeutic effects and mechanisms of action

Several neurotrophic factors (CNTF, BDNF, IGF-1) have been suggested for the treatment of motor neuron diseases. In ALS patients, however, the repeated subcutaneous injection of these factors as recombinant proteins is complicated by their toxicity or poor bioavailability. We have constructed an adenovirus vector coding for neurotrophin-3 (AdNT-3) allowing for stable and/or targeted delivery of NT-3 to motoneurons. The intramuscular administration of this vector was tested in the mouse mutant pmn (progressive motor neuronopathy). AdNT-3-treated pmn mice showed prolonged lifespan, improved neuromuscular function, reduced motor axonal degeneration and efficient reinnervation of muscle fibres. NT-3 protein and also adenovirus vectors, when injected into muscle, can be transported by motoneurons via retrograde axonal transport to their cell bodies in the spinal cord. Using ELISA and RT-PCR analyses in muscle, spinal cord and serum of AdNT-3-treated pmn mice, we have investigated the contribution of these processes to the observed therapeutic effects. Our results suggest that most if not all therapeutic benefit was due to the continuous systemic liberation of adenoviral NT-3. Therefore, viral gene therapy vectors auch as adenoviruses, AAVs, lentiviruses and new types of gene transfer not based on viral vectors that allow for efficient in vivo liberation of neurotrophic factors have potential for the future treatment of human motor neuron diseases.

Nerve injury and regeneration: basic insights and therapeutic interventions

Recent observations have provided new insight into neuronal responses to axotomy, signalling of the Schwann cell switch from 'operating' to 'proliferation' mode and temporal molecular changes in the responsiveness of Schwann cells to neuronal signals, as well as into the role of macrophages in Wallerian degeneration, nerve repair and neuropathic pain. Furthermore, promising therapeutic interventions have been developed to promote axon regeneration and to attenuate axotomy-induced neuronal cell death by means of pharmacological treatment or application of neurotrophic proteins using various strategies and routes of delivery.