Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro

Human Muscle Progenitor Cells Overexpressing Neurotrophic Factors Improve Neuronal Regeneration in a Sciatic Nerve Injury Mouse Model

The peripheral nervous system has an intrinsic ability to regenerate after injury. However, this process is slow, incomplete, and often accompanied by disturbing motor and sensory consequences. Sciatic nerve injury (SNI), which is the most common model for studying peripheral nerve injury, is characterized by damage to both motor and sensory fibers. The main goal of this study is to examine the feasibility of administration of human muscle progenitor cells (hMPCs) overexpressing neurotrophic factor (NTF) genes, known to protect peripheral neurons and enhance axon regeneration and functional recovery, to ameliorate motoric and sensory deficits in SNI mouse model. To this end, hMPCs were isolated from a human muscle biopsy, and manipulated to ectopically express brain-derived neurotrophic factor (BDNF), glial-cell-line-derived neurotrophic factor (GDNF), vascular endothelial growth factor (VEGF), and insulin-like growth factor (IGF-1). These hMPC-NTF were transplanted into the gastrocnemius muscle of mice after SNI, and motor and sensory functions of the mice were assessed using the CatWalk XT system and the hot plate test. ELISA analysis showed that genetically manipulated hMPC-NTF express significant amounts of BDNF, GDNF, VEGF, or IGF-1. Transplantation of 3 × 106 hMPC-NTF was shown to improve motor function and gait pattern in mice following SNI surgery, as indicated by the CatWalk XT system 7 days post-surgery. Moreover, using the hot-plate test, performed 6 days after surgery, the treated mice showed less sensory deficits, indicating a palliative effect of the treatment. ELISA analysis following transplantation demonstrated increased NTF expression levels in the gastrocnemius muscle of the treated mice, reinforcing the hypothesis that the observed positive effect was due to the transplantation of the genetically manipulated hMPC-NTF. These results show that genetically modified hMPC can alleviate both motoric and sensory deficits of SNI. The use of hMPC-NTF demonstrates the feasibility of a treatment paradigm, which may lead to rapid, high-quality healing of damaged peripheral nerves due to administration of hMPC. Our approach suggests a possible clinical application for the treatment of peripheral nerve injury.

Macrophage biology in the peripheral nervous system after injury

Neuroinflammation has positive and negative effects. This review focuses on the roles of macrophage in the PNS. Transection of PNS axons leads to degeneration and clearance of the distal nerve and to changes in the region of the axotomized cell bodies. In both locations, resident and infiltrating macrophages are found. Macrophages enter these areas in response to expression of the chemokine CCL2 acting on the macrophage receptor CCR2. In the distal nerve, macrophages and other phagocytes are involved in clearance of axonal debris, which removes molecules that inhibit nerve regeneration. In the cell body region, macrophage trigger the conditioning lesion response, a process in which neurons increase their regeneration after a prior lesion. In mice in which the genes for CCL2 or CCR2 are deleted, neither macrophage infiltration nor the conditioning lesion response occurs in dorsal root ganglia (DRG). Macrophages exist in different phenotypes depending on their environment. These phenotypes have different effects on axonal clearance and neurite outgrowth. The mechanism by which macrophages affect neuronal cell bodies is still under study. Overexpression of CCL2 in DRG in uninjured animals leads to macrophage accumulation in the ganglia and to an increase in the growth potential of DRG neurons. This increased growth requires activation of neuronal STAT3. In contrast, in acute demyelinating neuropathies, macrophages are involved in stripping myelin from peripheral axons. The molecular mechanisms that trigger macrophage action after trauma and in autoimmune disease are receiving increased attention and should lead to avenues to promote regeneration and protect axonal integrity.

Chondroitinase C Selectively Degrades Chondroitin Sulfate Glycosaminoglycans that Inhibit Axonal Growth within the Endoneurium of Peripheral Nerve

The success of peripheral nerve regeneration is highly dependent on the regrowth of axons within the endoneurial basal lamina tubes that promote target-oriented pathfinding and appropriate reinnervation. Restoration of nerve continuity at this structural level after nerve transection injury by direct repair and nerve grafting remains a major surgical challenge. Recently, biological approaches that alter the balance of growth inhibitors and promoters in nerve have shown promise to improve appropriate axonal regeneration and recovery of peripheral nerve function. Chondroitin sulfate proteoglycans (CSPGs) are known inhibitors of axonal growth. This growth inhibition is mainly associated with a CSPG's glycosaminoglycan chains. Enzymatic degradation of these chains with chondroitinase eliminates this inhibitory activity and, when applied in vivo, can improve the outcome of nerve repair. To date, these encouraging findings were obtained with chondroitinase ABC (a pan-specific chondroitinase). The aim of this study was to examine the distribution of CSPG subtypes in rodent, rabbit, and human peripheral nerve and to test more selective biological enzymatic approaches to improve appropriate axonal growth within the endoneurium and minimize aberrant growth. Here we provide evidence that the endoneurium, but not the surrounding epineurium, is rich in CSPGs that have glycosaminoglycan chains readily degraded by chondroitinase C. Biochemical studies indicate that chondroitinase C has degradation specificity for 6-sulfated glycosaminoglycans found in peripheral nerve. We found that chondroitinase C degrades and inactivates inhibitory CSPGs within the endoneurium but not so much in the surrounding nerve compartments. Cryoculture bioassays (neurons grown on tissue sections) show that chondroitinase C selectively and significantly enhanced neuritic growth associated with the endoneurial basal laminae without changing growth-inhibiting properties of the surrounding epineurium. Interestingly, chondroitinase ABC treatment increased greatly the growth-promoting properties of the epineurial tissue whereas chondroitinase C had little effect. Our evidence indicates that chondroitinase C effectively degrades and inactivates inhibitory CSPGs present in the endoneurial Schwann cell basal lamina and does so more specifically than chondroitinase ABC. These findings are discussed in the context of improving nerve repair and regeneration and the growth-promoting properties of processed nerve allografts.

Effects of proinflammatory cytokines on axonal outgrowth from adult rat lumbar dorsal root ganglia using a novel three-dimensional culture system

BACKGROUND CONTEXT

Degeneration of the intervertebral disc is often associated with low back pain and increased infiltration of nerve fibers originating from dorsal root ganglia (DRG). The degenerated disc is also characterized by the presence of proinflammatory cytokines, which may influence axonal outgrowth. Toward an improved understanding of the growth of DRG neurons into compliant extracellular matrices, we developed a novel experimental system to measure axonal outgrowth of adult rat lumbar DRG neurons within three-dimensional (3D) collagen hydrogels and used this system to examine the effects of interleukin 1β (IL-1β) and tumor necrosis factor (TNF)-α treatment.

PURPOSE

The aim was to investigate the effects of proinflammatory cytokines on 3D neuronal growth into collagen matrices.

STUDY DESIGN

This was an in vitro study of neurite outgrowth from adult rat lumbar DRG into collagen gels in response to IL-1β and TNF-α.

METHODS

Lumbar DRG were obtained from adult Sprague Dawley rats, bisected to expose cell bodies and placed onto collagen gel constructs prepared in 24-well Transwell inserts. Dorsal root ganglia were then treated with nerve growth factor (NGF)-free Neurobasal media (negative control) or NGF-supplemented media containing 0, 1, and 10 ng/mL of IL-1β and TNF-α. After 7 days, collagen gel-DRG constructs were immunostained for phosphorylated neurofilament, an axonal marker. Simple Neurite Tracer (Fiji/ImageJ) was used to quantify 3D axonal outgrowth from confocal image stacks. Data were analyzed using one-way analysis of variance, with Tukey HSD post hoc correction at a level of p<.05.

RESULTS

Immunostaining showed robust axonal outgrowth into collagen gels from all NGF-treated DRG. The negative control demonstrated very few and short neurites. Tumor necrosis factor-α (1 and 10 ng/mL) significantly inhibited axonal outgrowth compared with NGF-only media (p<.026 and p<.02, respectively). After IL-1β treatment, average axon length was 10% lower at 1 ng/mL and 7.5% higher at 10 ng/mL, but these differences were not statistically significant. Among cytokine treatments, however, average axon length in the IL-1β (10 ng/mL) group was significantly higher than that in the other groups (p<.05).

CONCLUSIONS

A novel 3D collagen gel culture system was used to investigate factors modulating neuronal ingrowth. Our results showed that NGF was necessary to promote neurite growth into collagen gels. In the presence of proinflammatory cytokines, high concentrations of IL-1β induced significantly higher axonal outgrowth than TNF-α and low levels of IL-1β.

A critical role for macrophages near axotomized neuronal cell bodies in stimulating nerve regeneration

Macrophages have been implicated in peripheral nerve regeneration for some time, supposedly through their involvement in Wallerian degeneration, the process by which the distal nerve degenerates after axotomy and is cleared by phagocytosis. Thus, in several studies in which macrophage accumulation in the distal nerve was reduced and Wallerian degeneration inhibited, regeneration was delayed. However, this interpretation ignores the more recent findings that macrophages also accumulate around axotomized cell bodies. The function of macrophage action at this second site has not been clear. In two mutant strains of mice, the slow Wallerian degeneration (Wld(s)) mouse and the chemokine receptor CCR2 knock-out mouse, we report that macrophage accumulation after axotomy was abolished in both the dorsal root ganglion (DRG) and the distal sciatic nerve. To measure neurite outgrowth, DRG neurons were given a conditioning lesion, and outgrowth was measured in vitro 7 d later in the absence of the distal nerve segment. The increased growth normally seen after a conditioning lesion did not occur or was reduced in Wld(s) or CCR2(-/-) mice. In the superior cervical ganglion (SCG), particularly in Wld(s) mice, macrophage accumulation was reduced but not abolished after axotomy. In SCG neurons from Wld(s) mice, the conditioning lesion response was unchanged; however, in CCR2(-/-) mice in which the effect on macrophage accumulation was greater, SCG neurite outgrowth was significantly reduced. These results indicate that macrophages affect neurite outgrowth by acting at the level of peripheral ganglia in addition to any effects they might produce by facilitation of Wallerian degeneration.

The effects of cold preservation and subimmunosuppressive doses of FK506 on axonal regeneration in murine peripheral nerve isografts

BACKGROUND

FK506 is a frequently used immunosuppressant with neuroregenerative effects. The neuroregenerative and immunosuppressive mechanisms of FK506, however, are distinct, suggesting that FK506 may stimulate nerve regeneration at lower doses than are needed to induce immunosuppression. The effects of cold preservation, a technique known to improve axonal regeneration through nerve allografts, are not well studied in nerve isografts and are also reported here.

OBJECTIVES

To determine the effects of subimmunosuppressive doses of FK506 and cold preservation on nerve regeneration in isografts.

METHODS

The neuroregenerative properties of immunosuppressive and subimmunosuppressive doses of FK506 were compared in a murine model receiving either fresh or cold preserved nerve isografts. Sixty female BALB/cJ mice were randomized into six groups. Animals in groups I, III and V received fresh nerve isografts. Animals in groups II, IV and VI received cold-preserved nerve isografts. Mice in groups I and II received no medical therapy, while those in groups III and IV received subimmunosuppressive doses of FK506, and those in groups V and VI received immunosuppressive doses as confirmed by mixed lymphocyte reactivity assays. Nerve regeneration was evaluated with histomorphometry and functional recovery was evaluated with walking track analysis.

RESULTS

Pretreatment with cold preservation did not significantly affect neural regeneration. The potent neuroregenerative effect of immunosuppressive doses of FK506 was confirmed, and the ability of subimmunosuppressive doses of FK506 to stimulate axonal regeneration in murine nerve isografts is reported.

CONCLUSIONS

Less toxic subimmunosuppressive doses of FK506 retaining some neuroregenerative properties may have a clinical role in treating extensive nerve injuries.

Role of glial cells in axonal regeneration

Axonal regeneration is critical for functional recovery following neural injury. In addition to intrinsic differences between regenerative responses of axons in peripheral versus central nervous systems, environmental factors such as glial cells and related molecules in the extracellular matrix (ECM) play an important role in axonal regeneration. Schwann cells in the peripheral nervous system (PNS) are recognized as favorable factors that promote axonal regeneration, while astrocytes and oligodendrocytes in the central nervous system (CNS) are not. In this review, we evaluate the roles of Schwann cells and astrocytes in axonal regeneration and examine recent evidence that suggests a dual function of astrocytes in regenerative responses. We also discuss the role of Cdc2 pathways in axonal regeneration, which is commonly activated in Schwann cells and astrocytes. Greater insight on the roles of glial cells in axonal regeneration is key to establishing baseline interventions for improving functional recovery following neural injury.

Critical role of p38 MAPK for regeneration of the sciatic nerve following crush injury in vivo

BACKGROUND

The physiological function of p38α, which is an isoform of p38 MAPK, has been investigated previously in several studies using pharmacological inhibitors. However, the results regarding whether p38α promotes or inhibits nerve regeneration in vivo have been controversial.

METHODS

We generated novel p38α mutant mice (sem mice) with a point mutation in the region encoding the p38α substrate-docking-site, which serves as a limited loss-of-function model of p38α. In the present study, we utilized sem mice and wild-type littermates (wt mice) to investigate the physiological role of p38α in nerve regeneration following crush injuries.

RESULTS

At four weeks after crush injury, the average axon diameter and the average axon area in sem mice were significantly smaller than those in wt mice. The average myelin sheath thickness in sem mice was reduced compared to wt mice, but no significant difference was observed in the G-ratio between the two groups. The sciatic functional index value demonstrated that functional nerve recovery in sem mice following crush injury was delayed, which is consistent with the histological findings. To investigate the underlying mechanisms of these findings, we examined inflammatory responses of the sciatic nerve by immunohistochemistry and western blotting. At an early phase following crush injury, sem mice showed remarkably lower expression of inflammatory cytokines, such as TNF-α and IL-1β, than wt mice. The expression of Caspase-3 and Tenascin-C were also lower in sem mice. Conversely, at a late phase of the response, sem mice showed considerably higher expression of TNF-α and of IL-1β with lower expression of S-100 than wt mice.

CONCLUSIONS

This is the first study of the physiological role of p38 MAPK in nerve regeneration that does not rely on the use of pharmacological inhibitors. Our results indicate that p38α insufficiency may cause an inflammatory disorder, resulting in a delay of histological and functional nerve recovery following crush injury. We conclude that p38 MAPK has an important physiological role in nerve regeneration and may be important for controlling both initiation of inflammation and recovery from nerve injury.

Critical role of p38 MAPK for regeneration of the sciatic nerve following crush injury in vivo

Background

The physiological function of p38α, which is an isoform of p38 MAPK, has been investigated previously in several studies using pharmacological inhibitors. However, the results regarding whether p38α promotes or inhibits nerve regeneration in vivo have been controversial.

Methods

We generated novel p38α mutant mice (sem mice) with a point mutation in the region encoding the p38α substrate-docking-site, which serves as a limited loss-of-function model of p38α. In the present study, we utilized sem mice and wild-type littermates (wt mice) to investigate the physiological role of p38α in nerve regeneration following crush injuries.

Results

At four weeks after crush injury, the average axon diameter and the average axon area in sem mice were significantly smaller than those in wt mice. The average myelin sheath thickness in sem mice was reduced compared to wt mice, but no significant difference was observed in the G-ratio between the two groups. The sciatic functional index value demonstrated that functional nerve recovery in sem mice following crush injury was delayed, which is consistent with the histological findings. To investigate the underlying mechanisms of these findings, we examined inflammatory responses of the sciatic nerve by immunohistochemistry and western blotting. At an early phase following crush injury, sem mice showed remarkably lower expression of inflammatory cytokines, such as TNF-α and IL-1β, than wt mice. The expression of Caspase-3 and Tenascin-C were also lower in sem mice. Conversely, at a late phase of the response, sem mice showed considerably higher expression of TNF-α and of IL-1β with lower expression of S-100 than wt mice.

Conclusions

This is the first study of the physiological role of p38 MAPK in nerve regeneration that does not rely on the use of pharmacological inhibitors. Our results indicate that p38α insufficiency may cause an inflammatory disorder, resulting in a delay of histological and functional nerve recovery following crush injury. We conclude that p38 MAPK has an important physiological role in nerve regeneration and may be important for controlling both initiation of inflammation and recovery from nerve injury.

The lipid sulfatide is a novel myelin-associated inhibitor of CNS axon outgrowth

CNS myelin is strongly inhibitory to growing axons and is thought to be a major contributor to CNS axon regenerative failure. Although a number of proteins present in myelin, including Nogo, MAG, and oligodendrocyte-myelin glycoprotein (OMgp), have been identified as myelin-associated inhibitors, studies of mice lacking these genes suggest that additional inhibitors present in CNS myelin remain to be identified. Here we have investigated the hypothesis that myelin lipids contribute to CNS regenerative failure. We identified sulfatide, a major constituent of CNS myelin, as a novel myelin-associated inhibitor of neurite outgrowth. Sulfatide, but not galactocerebroside or ceramide, strongly inhibited the neurite outgrowth of retinal ganglion cells (RGCs) when used as a purified lipid substrate. The mechanism involved in sulfatide-mediated inhibition may share features with other known inhibitors, because the Rho inhibitor C3 transferase lessened these effects. Myelin in which sulfatide was lacking or blocked using specific antibodies was significantly less inhibitory to RGC neurite outgrowth in vitro than was wild-type myelin, indicating that sulfatide is a major component of the inhibitory activity of CNS myelin. Mice unable to make sulfatide did not regenerate RGC axons more robustly after optic nerve crush than wild-type littermates under normal conditions but did exhibit a small but significant enhancement in the extent of zymosan-induced regeneration. These results demonstrate that specific lipids can powerfully inhibit axon growth, identify sulfatide as a novel myelin-associated axon growth inhibitor, and provide evidence that sulfatide inhibition contributes to axon regenerative failure in vivo.

Roles of channels and receptors in the growth cone during PNS axonal regeneration

Neurons in the peripheral nervous system (PNS) are known to maintain a regenerative capacity and will normally regenerate their axons within a permissive growth environment. The success of regeneration in the PNS largely depends on maintenance of the supportive basal lamina membrane, efficient removal of axonal and myelin debris by macrophages and Schwann cells, expression of neurotrophic factors by Schwann cells, and up-regulation of the intrinsic growth program in PNS neurons. The PNS regenerative process is well characterized through initial Wallerian degeneration followed by axonal sprouting, formation of neuronal growth cones, active axonal growth to the target, and finally sensory and motor functional recovery. The initiation and maintenance of active growth cones during peripheral nerve regeneration recapitulate many aspects of early neural development and are achieved through the activation of complex signaling cascades, involving various receptors, channels, cytoplasmic signaling cascades, as well as transcriptional and translational programs. This review focuses on roles of cell surface ion channels and receptors in the growth cone during Wallerian degeneration and axon regeneration in the PNS.

The potentiation of peripheral nerve sheaths in regeneration and repair

Traumatic injury to the nervous system often results in life changing loss of neurological function. Spontaneous neural regeneration occurs rarely and the outcome of therapeutic intervention is most often unacceptable. An intensive effort is underway to improve methods and technologies for nervous system repair. To date, the most success has been attained in the outcomes of peripheral nerve restoration. The importance of the peripheral nerve sheaths in successful nerve regeneration has been long recognized. In particular, Schwann cells and their basal laminae play a central role in axon development, maintenance, physiology, and response to injury. The endoneurial basal lamina is rich in components that promote axonal growth. It is now evident that the bioactivities of these components are counterbalanced by various factors that impede axonal growth. The growth-promoting potential of peripheral nerve is realized in the degenerative processes that occur distal to a lesion. This potentiation involves precise spatiotemporal alterations in the balance of antagonistic regulators of axonal growth. Experimental alteration of nerve sheath composition can also potentiate nerve and improve key features of nerve regeneration. For instance, enzymatic degradation of inhibitory chondroitin sulfate proteoglycan mimics endogenous processes that potentiate degenerated nerve and improves the outcome of direct nerve repair and grafting in animal models. This review provides a perspective of the essential role that peripheral nerve sheaths play in regulating axonal regeneration and focuses on discoveries leading to the inception and development of novel therapies for nerve repair.

Negative regulation of myelination: relevance for development, injury, and demyelinating disease

Dedifferentiation of myelinating Schwann cells is a key feature of nerve injury and demyelinating neuropathies. We review recent evidence that this dedifferentiation depends on activation of specific intracellular signaling molecules that drive the dedifferentiation program. In particular, we discuss the idea that Schwann cells contain negative transcriptional regulators of myelination that functionally complement positive regulators such as Krox-20, and that myelination is therefore determined by a balance between two opposing transcriptional programs. Negative transcriptional regulators should be expressed prior to myelination, downregulated as myelination starts but reactivated as Schwann cells dedifferentiate following injury. The clearest evidence for a factor that works in this way relates to c-Jun, while other factors may include Notch, Sox-2, Pax-3, Id2, Krox-24, and Egr-3. The role of cell-cell signals such as neuregulin-1 and cytoplasmic signaling pathways such as the extracellular-related kinase (ERK)1/2 pathway in promoting dedifferentiation of myelinating cells is also discussed. We also review evidence that neurotrophin 3 (NT3), purinergic signaling, and nitric oxide synthase are involved in suppressing myelination. The realization that myelination is subject to negative as well as positive controls contributes significantly to the understanding of Schwann cell plasticity. Negative regulators are likely to have a major role during injury, because they promote the transformation of damaged nerves to an environment that fosters neuronal survival and axonal regrowth. In neuropathies, however, activation of these pathways is likely to be harmful because they may be key contributors to demyelination, a situation which would open new routes for clinical intervention.

Guidance of glial cell migration and axonal growth on electrospun nanofibers of poly-ε-caprolactone and a collagen/poly-ε-caprolactone blend

Our long-term goal is to develop an artificial implant as a conduit for axonal regeneration after peripheral nerve injury. In this study, biodegradable, aligned poly-epsilon-caprolactone (PCL) and collagen/PCL (C/PCL) nanofibers designed as guidance structures were produced by electrospinning and tested in cell culture assays. We compared fibers of 100% PCL with fibers consisting of a 25:75% C/PCL blend. To test their biocompatibility, assays of cell adhesion, survival, migration, effects on cell morphology, axonal growth and axonal guidance were performed. Both types of eletrospun fibers supported oriented neurite outgrowth and glial migration from dorsal root ganglia (DRG) explants. Schwann cell migration, neurite orientation, and process formation of Schwann cells, fibroblasts and olfactory ensheathing cells were improved on C/PCL fibers, when compared to pure PCL fibers. While the velocity of neurite elongation from DRG explants was higher on PCL fibers, analysis of isolated sensory neurons showed significantly better axonal guidance by the C/PCL material. The data demonstrate that electrospun fibers composed of a collagen and PCL blend represent a suitable substrate for supporting cell proliferation, process outgrowth and migration and as such would be a good material for artificial nerve implants.

Chondroitinase treatment increases the effective length of acellular nerve grafts

Acellular nerve allografts have been explored as an alternative to nerve autografting. It has long been recognized that there is a distinct limit to the effective length of conventional acellular nerve grafts, which must be overcome for many grafting applications. In rodent models nerve regeneration fails in acellular nerve grafts greater than 2 cm in length. In previous studies we found that nerve regeneration is markedly enhanced with acellular nerve grafts in which growth-inhibiting chondroitin sulfate proteoglycan was degraded by pretreatment with chondroitinase ABC (ChABC). Here, we tested if nerve regeneration can be achieved through 4-cm acellular nerve grafts pretreated with ChABC. Adult rats received bilateral sciatic nerve segmental resection and repair with a 4 cm, thermally acellularized, nerve graft treated with ChABC (ChABC graft) or vehicle-treated acellularized graft (Control graft). Nerve regeneration was examined 12 weeks after implantation. Our findings confirm that functional axonal regeneration fails in conventional long acellular grafts. In this condition we found very few axons in the distal host nerve, and there were marginal signs of sciatic nerve reinnervation in few (2/9) rats. This was accompanied by extensive structural disintegration of the distal graft and abundant retrograde axonal regeneration in the proximal nerve. In contrast, most (8/9) animals receiving nerve repair with ChABC grafts showed sciatic nerve reinnervation by direct nerve pinch testing. Histological examination revealed much better structural preservation and axonal growth throughout the ChABC grafts. Numerous axons were found in all but one (8/9) of the host distal nerves and many of these regenerated axons were myelinated. In addition, the amount of aberrant retrograde axonal growth (originating near the proximal suture line) was markedly reduced by repair with ChABC grafts. Based on these results we conclude that ChABC treatment substantially increases the effective length of acellular nerve grafts.

Retinal ganglion cell axons regenerate in the presence of intact sensory fibres

A novel allograft paradigm was used to test whether adult mammalian central axons regenerate within a peripheral nerve environment containing intact sensory axons. Retinal ganglion cell axon regeneration was compared following anastomosis of dorsal root ganglia grafts or conventional peripheral nerve grafts to the adult rat optic nerve. Dorsal root ganglia grafts comprised intact sensory and degenerate motor axons, whereas conventional grafts comprised both degenerating sensory and motor axons. Retinal ganglion cell axons were traced after 2 months. Dorsal root ganglia survived with their axons persisting throughout the graft. Comparable numbers of retinal ganglion cells regenerated axons into both dorsal root ganglia (1053+/-223) and conventional grafts (1323+/-881; P>0.05). The results indicate that an intact sensory environment supports central axon regeneration.

Schwann cells and endoneurial extracellular matrix molecules as potential cues for sorting of regenerated axons: a review

Besides very well elaborated microsurgical management of severed peripheral nerves, the clinical results of functional recovery following surgical repair of mixed nerves are disappointing. An improvement of functional recovery after peripheral nerve lesion requires the accurate regeneration of axons to their original target tissues and structures. Therefore, better clinical results could be obtained by a greater understanding of the cellular and molecular biology of selective nerve regeneration. The studies concerning Schwann cells and their endoneurial extracellular matrix as potent cues for selective promotion and influence of regenerating motor and sensory axons are reviewed. Knowledge of the sorting mechanisms of regenerated motor and sensory axons is needed not only for improvement of functional recovery, but also for the development of biocompatible nerve prostheses.

Factors limiting motor recovery after facial nerve transection in the rat: combined structural and functional analyses

It is believed that a major reason for the poor functional recovery after peripheral nerve lesion is collateral branching and regrowth of axons to incorrect muscles. Using a facial nerve injury protocol in rats, we previously identified a novel and clinically feasible approach to combat axonal misguidance--the application of neutralizing antibodies against neurotrophic factors to the injured nerve. Here, we investigated whether reduced collateral branching at the lesion site leads to better functional recovery. Treatment of rats with antibodies against nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor, insulin-like neurotrophic factor I, ciliary neurotrophic factor or glial cell line-derived neurotrophic factor increased the precision of reinnervation, as evaluated by multiple retrograde labelling of motoneurons, more than two-fold as compared with control animals. However, biometric analysis of vibrissae movements did not show positive effects on functional recovery, suggesting that polyneuronal reinnervation--rather than collateral branching --may be the critical limiting factor. In support of this hypothesis, we found that motor end-plates with morphological signs of multiple innervation were much more frequent in reinnervated muscles of rats that did not recover after injury (51% of all end-plates) than in animals with good functional performance (10%). Because polyneuronal innervation of muscle fibres is activity-dependent and can be manipulated, the present findings raise hopes that clinically feasible and effective therapies could be soon designed and tested.

Stimulation of neurite outgrowth in a human nerve scaffold designed for peripheral nerve reconstruction

The clinical outcome of microsurgical reconstruction of large peripheral nerve lesions depends on the availability of suitable graft material. Allogenic nerve grafts are rejected by the immune system. Extracellular-matrix proteins, in contrast to the resident cells, are of low immunogenicity in allografts. Here, human tibial nerve segments were extracted with lysophosphatidyl choline. The obtained cell-free and myelin-free scaffold consisted of empty endoneural tubes with maintained extracellular matrix architecture. The nerve scaffold had mechanical properties comparable to intact nerve, making it suitable for microsurgical reconstruction. Sections of the nerve scaffold were tested as a substrate for the adhesion and neuronal differentiation of human neuroblastoma-derived LAN-5 cells. Nerve extraction removed laminin-2, an isoform of laminin important for peripheral nerve regeneration. Laminin-2 reloading of the nerve scaffold did not improve cell adhesion and axon growth. Chemical crosslinking of heparan sulfate, on the other hand, increased the percentage of adherent cells with outgrowing neurites from 48 to 85%. Combined laminin-2 reloading and heparan sulfate crosslinking reduced the percentage of neurite-forming cells to 22% of the number of adherent cells. Implantation of the nerve scaffold into the peritoneal cavity of mice was not cytotoxic, and neovascularization of the graft material was observed within weeks. In conclusion, extraction of human nerve with detergents revealed a biocompatible nerve scaffold supporting neuronal cell adhesion. Heparan sulfate crosslinking to the scaffold surface improved neurite outgrowth, presumably mediated by midkine, a member of the neurokine family of growth factors, which is secreted by neuroblastoma-derived cells and binds to heparan sulfate.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

X‐irradiation of adult spinal cord increases its capacity to support neurite regeneration in vitro

Previous in vitro studies have shown that X-irradiation during early postnatal life can change the environment of CNS tissue in later adult life such that it becomes more supportive of neurite regeneration from adult dorsal root ganglion (DRG) neurons than non-irradiated tissue. The question arises whether or not x-irradiation during adult life can alter the CNS environment such that it also becomes more supportive of neurite regeneration. This was investigated by exposing portions of the spinal cord of adult rats to 10, 20 or 40 Gray of X-irradiation and later using this tissue to prepare cryosections suitable for use as a substrate in a cryoculture assay. Fixed cryocultures were immunolabelled using anti-glial fibrillary acidic protein (GFAP) to visualise the tissue sections and anti-growth associated protein (GAP-43) to visualise the regenerating neurites. Tissue sections from sham-irradiated animals and from those irradiated with 10 Gray did not support the regeneration of neurites. However, sections of spinal cords from rats treated with either 20 or 40 Gray of X-irradiation 4 or 32 days prior to sampling were found to support a certain degree of neurite regeneration. It is concluded that X-irradiation of adult CNS tissue can alter its environment such that it becomes more supportive of neurite regeneration and it is speculated that this change may be the result of alterations in the glial cell populations in the post-irradiated tissues.

Macrophages contribute to the maintenance of stable regenerating neurites following peripheral nerve injury

Normal adult uninjured nerve is unable to support axonal regeneration. We have studied the mechanisms underlying the regeneration of peripheral nerve by culturing adult mouse dorsal root ganglia (DRG) explants on unfixed, longitudinal cryosections of either the uninjured sciatic nerve or the distal segment of the transected sciatic nerve. We found that, initially, DRG grew vigorously on cryosections of both uninjured and postinjury sciatic nerves. However, the neurites began to degenerate shortly after contact with the uninjured nerve, whereas those growing on postinjury nerve substrate remained healthy for up to 9 days in culture. This ability to support stable outgrowth peaked at 8 days, gradually decreased by 10 days, and disappeared by 20 days after injury. Macrophages appeared in the distal segment by 4 days postinjury and had infiltrated its entire length by 8 days. Uninjured nerve cryosections could be rendered supportive of stable outgrowth by preincubation with macrophage-conditioned medium or by brief trypsinization. The activity of the macrophage-conditioned medium was augmented upon activation of macrophages. Together these findings suggest that the environment of the sciatic nerve undergoes a transformation during Wallerian degeneration such that it becomes transiently supportive of the stable outgrowth of neurites. This transformation may be mediated by a proteolytic activity, generated by activated macrophages, that removes a putative "degeneration signal" protein normally present in the adult nerve and thus contributes to the maintenance of stable regenerating neurites.

Metalloproteinase-dependent predegeneration in vitro enhances axonal regeneration within acellular peripheral nerve grafts

Injury to peripheral nerve initiates a degenerative process that converts the denervated nerve from a suppressive environment to one that promotes axonal regeneration. We investigated the role of matrix metalloproteinases (MMPs) in this degenerative process and whether effective predegenerated nerve grafts could be produced in vitro. Rat peripheral nerve explants were cultured for 1-7 d in various media, and their neurite-promoting activity was assessed by cryoculture assay, in which neurons are grown directly on nerve sections. The neurite-promoting activity of cultured nerves increased rapidly and, compared with uncultured nerve, a maximum increase of 72% resulted by 2 d of culture in the presence of serum. Remarkably, the neurite-promoting activity of short-term cultured nerves was also significantly better than nerves degenerated in vivo. We examined whether in vitro degeneration is MMP dependent and found that the MMP inhibitor N-[(2R)-2(hydroxamidocarbonylmethyl)-4-methylpantanoyl]-l-tryptophan methylamide primarily blocked the degenerative increase in neurite-promoting activity. In the absence of hematogenic macrophages, MMP-9 was trivial, whereas elevated MMP-2 expression and activation paralleled the increase in neurite-promoting activity. MMP-2 immunoreactivity localized to Schwann cells and the endoneurium and colocalized with gelatinolytic activity as demonstrated by in situ zymography. Finally, in vitro predegenerated nerves were tested as acellular grafts and, compared with normal acellular nerve grafts, axonal ingress in vivo was approximately doubled. We conclude that Schwann cell expression of MMP-2 plays a principal role in the degenerative process that enhances the regeneration-promoting properties of denervated nerve. Combined with their low immunogenicity, acellular nerve grafts activated by in vitro predegeneration may be a significant advancement for clinical nerve allografting.

Regeneration of axons after nerve transection repair is enhanced by degradation of chondroitin sulfate proteoglycan

Our past work indicates that growth-inhibiting chondroitin sulfate proteoglycan (CSPG) is abundant in the peripheral nerve sheaths and interstitium. In this study we tested if degradation of CSPG by chondroitinase enhances axonal regeneration through the site of injury after (a) nerve crush and (b) nerve transection and coaptation. Adult rats received the same injury bilaterally to the sciatic nerves and then chondroitinase ABC was injected near the injury site on one side, and the contralateral nerve was injected with vehicle alone. Nerves were examined 2 days after injury in the nerve crush model and 4 days after injury in the nerve transection model. Chondroitinase-dependent neoepitope immunolabeling showed that CSPG was thoroughly degraded around the injury site in the chondroitinase-treated nerves. Axonal regeneration through the injury site and into the distal nerve was assessed by GAP-43 immunolabeling. Axonal regeneration after crush injury was similar in chondroitinase-treated and control nerves. In contrast, axonal regrowth through the coaptation of transected nerves was markedly accelerated and the ingress of axons into the distal segment was increased severalfold in nerves injected with chondroitinase. On the basis of these results we concluded that growth inhibition by CSPG contributes critically to the poor regenerative growth of axons in nerve transection repair. In addition, degradation of CSPG by injection of chondroitinase ABC at the site of nerve repair increased the ingress of axonal sprouts into basal laminae of the distal nerve segment, presumably by enabling more latitude in growth at the interface of coapted nerve. This suggests that chondroitinase application may be used clinically to improve the outcome of primary peripheral nerve repair.

Focal application of neutralizing antibodies to soluble neurotrophic factors reduces collateral axonal branching after peripheral nerve lesion

A major reason for the insufficient recovery of function after motor nerve injury are the numerous axonal branches which often re-innervate muscles with completely different functions. We hypothesized that a neutralization of diffusable neurotrophic factors at the lesion site in rats could reduce the branching of transected axons. Following analysis of local protein expression by immunocytochemistry and by in situ hybridization, we transected the facial nerve trunk of adult rats and inserted both ends into a silicon tube containing (i) collagen gel with neutralizing concentrations of antibodies to NGF, BDNF, bFGF, IGF-I, CNTF and GDNF; (ii) five-fold higher concentrations of the antibodies and (iii) combination of antibodies. Two months later, retrograde labelling was used to estimate the portion of motoneurons the axons of which had branched and projected into three major branches of the facial trunk. After control entubulation in collagen gel containing non-immune mouse IgG 85% of all motoneurons projecting along the zygomatic branch sprouted and sent at least one twin axon to the buccal and/or marginal-mandibular branches of the facial nerve. Neutralizing concentrations of anti-NGF, anti-BDNF and anti-IGF-I significantly reduced sprouting. The most pronounced effect was achieved after application of anti-BDNF, which reduced the portion of branched neurons to 18%. All effects after a single application of antibodies were concentration-dependent and superior to those observed after combined treatment. This first report on improved quality of reinnervation by antibody-therapy implies that, in rats, the post-transectional collateral axonal branching can be reduced without obvious harmful effects on neuronal survival and axonal elongation.

Immunological approaches to the treatment of spinal cord injury

The innate and adaptive arms of the immune system, represented principally by macrophages and by T and B cells, respectively, provide body tissues with mechanisms of defence, protection and repair. In the central nervous system (CNS), probably because of its status of 'immune privilege', any immune activity has long been viewed as detrimental. Recent studies have provided evidence, however, that immune activity after traumatic CNS injury may have a beneficial effect, manifested by promotion of regeneration and reduction in the secondary degeneration of neurons that escaped direct injury. Rigorous regulation of immune system activity allows the individual to derive the benefit of such neuroprotection without the risk of detrimental side effects. Recently, our research group found a way to boost the T-cell-mediated autoimmune protection while avoiding the risk of autoimmune disease.

The source of reactive cells during central Wallerian degeneration in the goldfish: a differential irradiation protocol

We have used a partial irradiation paradigm to examine the provenance of cells that participate in Wallerian cellular responses in the goldfish visual system. Animals which received 50 Gy whole-body gamma-irradiation showed virtually complete inhibition of the proliferative burst usually seen after optic nerve section. These animals did, however, show a robust hyperplastic response in the optic tract that we believe represents the migration of nearby microglial cells into the affected tract. When only the postcephalic body was irradiated, proliferating cells in the major hematopoietic organs of the fish, the kidney and pronephros, were substantially inhibited. Despite this, the Wallerian cellular response in the visual paths was essentially normal. Thus, there is no obligate requirement for peripheral proliferative cells to participate in central Wallerian degeneration in the fish. However, when only the head was irradiated, and the hematopoietic organs were spared, there was a proliferative response in the visual system. We believe this represents the invasion of the visual pathways by peripheral blood cells through the optic nerve lesion and blood vessels in the nerve itself. This invasion, however, is not sufficient to generate substantial hyperplasia. In summary, although we find evidence for a small contribution by exogenous cells, the major source of reactive cells during central Wallerian degeneration in the fish is the endogenous microglia. Our data underscore the importance of elucidating the mechanisms by which microglial cells are activated and the role that they play in regeneration.

Axonal regeneration into acellular nerve grafts is enhanced by degradation of chondroitin sulfate proteoglycan

Although the peripheral nerve has the potential to regenerate after injury, degenerative processes may be essential to promote axonal growth into the denervated nerve. One hypothesis is that the nerve contains growth inhibitors that must be neutralized after injury for optimal regeneration. In the present study, we tested whether degradation of chondroitin sulfate proteoglycan, a known inhibitor of axon growth, enhances the growth-promoting properties of grafts prepared from normal donor nerves. Excised segments of rat sciatic nerve were made acellular by freeze-killing before treatment with chondroitinase ABC. Chondroitinase-dependent neoepitope immunolabeling showed that chondroitin sulfate proteoglycan was thoroughly degraded throughout the treated nerve segments. In addition, neuronal cryoculture assays revealed that the neurite-promoting activity of acellular nerves was significantly increased by chondroitinase treatment. Control and chondroitinase-treated acellular nerves were then used as interpositional grafts in a rat nerve injury model. Axonal regeneration into the grafts was assessed 4 and 8 d after implantation by growth-associated protein-43 immunolabeling. At both time points, the number of axons regenerating into acellular grafts treated with chondroitinase was severalfold greater than in control grafts. Growth into the chondroitinase-treated grafts was pronounced after only 4 d, suggesting that the delay of axonal growth normally associated with acellular grafts was attenuated as well. These findings indicate that chondroitinase treatment significantly enhanced the growth-promoting properties of freeze-killed donor nerve grafts. Combined with the low immunogenicity of acellular grafts, the ability to improve axonal penetration into interpositional grafts by preoperative treatment with chondroitinase may be a significant advancement for clinical nerve allografting.

Cryosections of pre-irradiated adult rat spinal cord tissue support axonal regeneration in vitro

Neonatal X-irradiation of central nervous system (CNS) tissue markedly reduces the glial population in the irradiated area. Previous in vivo studies have demonstrated regenerative success of adult dorsal root ganglion (DRG) neurons into the neonatally-irradiated spinal cord. The present study was undertaken to determine whether these results could be replicated in an in vitro environment. The lumbosacral spinal cord of anaesthetised Wistar rat pups, aged between 1 and 5 days, was subjected to a single dose (40 Gray) of X-irradiation. A sham-irradiated group acted as controls. Rats were allowed to reach adulthood before being killed. Their lumbosacral spinal cords were dissected out and processed for sectioning in a cryostat. Cryosections (10 microm-thick) of the spinal cord tissue were picked up on sterile glass coverslips and used as substrates for culturing dissociated adult DRG neurons. After an appropriate incubation period, cultures were fixed in 2% paraformaldehyde and immunolabelled to visualise both the spinal cord substrate using anti-glial fibrillary acidic protein (GFAP) and the growing DRG neurons using anti-growth associated protein (GAP-43). Successful growth of DRG neurites was observed on irradiated, but not on non-irradiated, sections of spinal cord. Thus, neonatal X-irradiation of spinal cord tissue appears to alter its environment such that it can later support, rather than inhibit, axonal regeneration. It is suggested that this alteration may be due, at least in part, to depletion in the number of and/or a change in the characteristics of the glial cells.

Protein synthesis in axons and terminals: significance for maintenance, plasticity and regulation of phenotype. With a critique of slow transport theory

This article focuses on local protein synthesis as a basis for maintaining axoplasmic mass, and expression of plasticity in axons and terminals. Recent evidence of discrete ribosomal domains, subjacent to the axolemma, which are distributed at intermittent intervals along axons, are described. Studies of locally synthesized proteins, and proteins encoded by RNA transcripts in axons indicate that the latter comprise constituents of the so-called slow transport rate groups. A comprehensive review and analysis of published data on synaptosomes and identified presynaptic terminals warrants the conclusion that a cytoribosomal machinery is present, and that protein synthesis could play a role in long-term changes of modifiable synapses. The concept that all axonal proteins are supplied by slow transport after synthesis in the perikaryon is challenged because the underlying assumptions of the model are discordant with known metabolic principles. The flawed slow transport model is supplanted by a metabolic model that is supported by evidence of local synthesis and turnover of proteins in axons. A comparison of the relative strengths of the two models shows that, unlike the local synthesis model, the slow transport model fails as a credible theoretical construct to account for axons and terminals as we know them. Evidence for a dynamic anatomy of axons is presented. It is proposed that a distributed "sprouting program," which governs local plasticity of axons, is regulated by environmental cues, and ultimately depends on local synthesis. In this respect, nerve regeneration is treated as a special case of the sprouting program. The term merotrophism is proposed to denote a class of phenomena, in which regional phenotype changes are regulated locally without specific involvement of the neuronal nucleus.

MMP-2 and MMP-9 increase the neurite-promoting potential of schwann cell basal laminae and are upregulated in degenerated nerve

Compared to degenerated nerves, the ability of normal adult peripheral nerve to support axonal regeneration is poor and may be attributed to the inhibition of endoneurial laminin by chondroitin sulfate proteoglycan (CSPG). In cryoculture assays, neuritic growth of neonatal and adult peripheral neurons was increased on sections of normal nerve by pretreatment with CSPG-degrading enzymes, including the matrix metalloproteinases MMP-2 and MMP-9. Axonal regeneration is known to occur within the Schwann cell basal laminae of degenerated nerve. Similarly, deconvolution microscopy revealed that neuritic growth on nerve tissue sections occurred principally on the lumenal surface of enzymatically modified basal laminae. Compared to normal nerve, there was a marked increase in the neurite-promoting activity of the degenerated nerve, and this activity was not increased significantly by subsequent MMP treatment. Additionally, the expression and activation of MMP-2 and MMP-9 were elevated in degenerated nerve, suggesting that degradation of inhibitory CSPG by the MMPs contributes to the growth-promoting properties of degenerated nerve.

Respective roles of inflammation and axonal breakdown in the regulation of peripheral nerve hemopexin: an analysis in rats and in C57BL/Wlds mice

We have previously demonstrated that one of the peripheral nerve responses to injury is the overexpression of hemopexin (HPX). Here, we demonstrate that Wallerian degeneration is required for this response, since HPX does not increase in C57BL/Wlds mice, which display a severely impaired Wallerian degeneration. We also show that HPX synthesis is dramatically increased in macrophages during their activation or after IL-6 stimulation. However, IL-6-driven HPX overexpression occurs in vivo and in vitro in the absence of substantial macrophage invasion. We conclude that, after nerve injury, HPX overexpression occurs first in Schwann cells as a result of axotomy and is subsequently regulated by inflammation. Furthermore, our results and those already described suggest that IL-6, synthesized by the various cell types producing HPX, control nerve HPX expression via paracrine and autocrine mechanisms.

Nerve regeneration in Wld(s) mice is normalized by actinomycin D

Injured nerves of Wld(s) mice neither degenerate nor regenerate for several weeks. We have conjectured that Wld(s) axons have the ability to regenerate but its expression is impaired by the Schwann cells of the undegenerated distal stump. To test this conjecture, transcription was locally arrested with actinomycin D (ActD), nerves were crushed, and regrowth was evaluated. In normal CD1 nerves injected with ActD 3 days before the crush, the rate of elongation was not affected but the delay of regrowth was shortened. In sharp contrast, ActD normalized the elongation of Wld(s) axons. When Wld(s) nerves were crushed past the treated segment, axons did not regenerate. After 7, but not 4, days of treatment, intact CD1 and Wld(s) axons presented a local sprouting response. We conclude that Wld(s) axons can regenerate in a normal way but do not do so because the undegenerated Schwann cells of the distal stump repress the regrowth program. We present a model axon that includes a destruction program and a post-transcriptional trophic regulation of its phenotype.

A dynamic regulation of GDNF-family receptors correlates with a specific trophic dependency of cranial motor neuron subpopulations during development

Glial cell line-derived neurotrophic factor (GDNF) family ligands promote the survival of developing motor neurons in vivo and in vitro. However, not all neurons survive with any single ligand in culture and GDNF null mutant mice display only a partial motor neuron loss. An interesting possibility is that subpopulations of motor neurons based on their function and/or their myotopic organization require distinct members of GDNF family ligands. Because responsiveness to the different ligands depends on the expression of their cognate ligand-binding receptor we have herein addressed this issue by examining the expression of GDNF-family receptors (gfr) during development and in the adult in cranial motor nuclei subpopulations. We have furthermore examined the in vivo role of GDNF for cranial motor neuron subpopulations. The shared ret receptor was expressed in all somatic, branchial and visceral cranial embryonic motor nuclei examined, showing that they are all competent to respond to GDNF family ligands during development. At early stages of development both the GDNF receptor, gfralpha1, and the neurturin (NTN) receptor, gfralpha2, were expressed in the oculomotor, facial and spinal accessory, and only gfralpha1 in the trochlear, superior salivatory, trigeminal, hypoglossal and weakly in the dorsal motor nucleus of the vagus and the ambiguous nucleus. The abducens nucleus was negative for both gfralpha1 and gfralpha2. The artemin (ART) receptor, gfralpha3, was expressed only in the superior salivatory nucleus. A motor neuron subnuclei-specific expression of gfralpha1 and gfralpha2 was seen in the facial and trigeminal nuclei which corresponded to their dependence on GDNF in null mutant mice. We found that the expression was dynamic in these nuclei, which may reflect developmental changes in their trophic factor dependency. Analysis of GDNF null mutant mice revealed that the dynamic receptor expression is regulated by the ligand in vivo, indicating that the attainment of changes in dependency could be ligand induced. Our results indicate that specific GDNF family ligands support selective muscle-motor neuron circuits during development.

White matter of the CNS supports or inhibits neurite outgrowth in vitro depending on geometry

Axonal regeneration is normally limited within myelinated fiber tracts in the CNS of higher vertebrates. Numerous studies suggest that CNS myelin contains inhibitors that may contribute to abortive axonal growth. In contrast to the evidence of myelin-associated neurite inhibitors, embryonic neurons transplanted into the CNS can regenerate extensively within myelinated tracts in vivo. It has been speculated that embryonic neurons do not yet express the appropriate receptors for myelin-associated inhibitors. Recently, however, extensive regeneration from transplanted adult neurons has also been reported within myelinated tracts of the CNS, casting doubt on the role myelin-associated inhibitors play in abortive regeneration. The present study reexamined the potential of white matter to support neurite growth in vitro. By the use of Neurobasal medium, neurons were cultured onto unfixed cryostat sections of mature rat CNS tissue. As documented previously, robust neuronal attachment and neurite outgrowth occurred on gray matter but these neurites were sharply inhibited by white matter. In addition, however, increased rates of neuronal attachment directly to white matter occurred with neurite outgrowth comparable in length with that on gray matter but limited to directions parallel to the fiber tract. Frequently, the same section of white matter was found to inhibit neurite outgrowth from neurons on gray matter while supporting parallel neurite outgrowth from neurons on white matter. These results suggest that whether white matter supports or inhibits axonal growth depends on the geometric relationship between the axon and the fiber tract; more specifically, white matter supports parallel growth but inhibits nonparallel growth.

Proteolysis and cell migration: creating a path?

Both serine and metalloproteinases have been implicated in the complex integrated events underlying cell migration but no definitive single mechanism has emerged. Work over the past two years linking both membrane and soluble proteinases with integrins and other adhesion proteins and with intracellular signalling systems could herald the beginnings of a potential expansion of our understanding of the role and regulation of proteolysis in cell migration.

Modulation of macrophage and microglial responses to axonal injury in the peripheral and central nervous systems

OBJECTIVE

After axonal injury, macrophages rapidly infiltrate and become activated in the mammalian peripheral nervous system (PNS) but not the central nervous system (CNS). We used the dorsal root pathway to study factors that modulate the response of macrophages to degenerating axons in both the PNS and the CNS.

METHODS

Lewis rats underwent transection of dorsal roots (Group 1), stab within the spinal cord (Group II), crush at the dorsal root entry zone (Group III), transection of dorsal roots combined with a CNS lesion (Group IV), or systemic administration of a known activator of macrophages, lipopolysaccharide, alone (Group V) or combined with transection of dorsal roots (Group VI). ED-1 antibody stained for macrophages and activated microglia at 7, 14, and 42 days postinjury.

RESULTS

At early time points, Group I demonstrated ED-1 cells in the PNS but not the CNS portion of the degenerating dorsal roots. Group II revealed ED-1 cells near the stab lesion. Group III demonstrated ED-1 cells adjacent to the dorsal root entry zone crush site. Group IV revealed ED-1 cells along both the PNS and the CNS portions of the degenerating dorsal roots when the CNS lesion was placed near the transected roots. Group V demonstrated few ED-1 cells in the PNS and the CNS, whereas Group VI revealed a marked ED-1 cellular response along both the PNS and the CNS portions of the transected dorsal roots.

CONCLUSION

Local CNS trauma and systemic administration of lipopolysaccharide can "prime" macrophages/microglia, resulting in an enhanced response to degenerating axons in the CNS. Such priming might prove useful in promoting axonal regeneration.

Neuronal age influences the response to neurite outgrowth inhibitory activity in the central and peripheral nervous systems

Axonal regeneration is abortive in the central nervous system (CNS) of adult mammals, but readily occurs in the injured peripheral nervous system (PNS). Recent experiments indicate an important role for both intrinsic neuronal features and extrinsic substrate properties in determining the propensity for axonal regrowth. In particular, certain components of adult mammalian CNS myelin have been shown to exert a strong inhibitory influence on neurite outgrowth. To determine whether the potent neurite outgrowth inhibitory activity found in CNS myelin may also be present in PNS myelin and to study the influence of neuronal age on neurite outgrowth, we used a cryoculture assay in which dissociated rat dorsal root ganglion (DRG) neurons of different ages were challenged to extend neurites on fractionated myelin and cryostat sections from the PNS (sciatic nerve and myelin-free degenerated sciatic nerve) and CNS (optic nerve) of adult rats. The CNS environment of the optic nerve did not support E17 to P8 DRG neurite adhesion or outgrowth. E17 DRG neurons, unlike their older counterparts, however, were able to attach and extend neurites onto normal sciatic nerve and onto purified PNS myelin. In contrast, a vigorous neurite outgrowth response from all the ages tested was observed on the myelin-free degenerated sciatic nerve. These results indicate that PNS myelin is a potent inhibitor of neurite outgrowth and that DRG neuronal age plays an important role in determining the propensity for neurite outgrowth and regenerative response on inhibitory PNS and CNS substrata.

I-type lectins in the nervous system

The number of animal lectins, basically defined upon their interaction with specific carbohydrate structures, is growing considerably during the last few years. Among these proteins the recently identified subfamily of I-type lectins consists of mainly transmembranous glycoproteins belonging to the immunoglobulin superfamily. Most of the I-type lectins participate in cell adhesion events, as are the different sialoadhesins recognizing sialylated glycan structures, which represent the best characterized subgroup. I-type lectins are abundant in the nervous system and have been implicated in a number of morphogenetic processes as fundamental as axon growth, myelin formation and growth factor signaling. In the present review, we summarize the structural and functional properties of I-type lectins expressed in neural tissues with a main focus on the sialoadhesin myelin-associated glycoprotein, the neural cell adhesion molecule and the fibroblast growth factor receptors.

Potential repair of rat spinal cord injuries using stimulated homologous macrophages

The failure of the adult mammalian central nervous system (CNS) to regenerate after injury has long been viewed as a unique phenomenon resulting from the specific nature of this system. The finding that some CNS axons could be induced to regrow if provided with a permissive environment suggested that this failure is a result, at least in part, of the nature of the postinjury neuronal environment. It was further shown that the involvement of inflammatory cells, particularly macrophages, in postinjury processes in the CNS is limited. We have suggested that, to achieve recovery after injury, the adult mammalian CNS may require the assistance of the same postinjury factors as those involved in the recovery of spontaneously regenerating systems but that its accessibility to such assistance is restricted. Accordingly, we proposed that it might be possible to circumvent the restriction, allowing regeneration to occur. We showed that the implantation of autologous macrophages, which had been prestimulated by exposure to a regenerative (sciatic) nerve, into completely transected spinal cords of adult rats led to partial motor recovery. This treatment intervenes in the postinjury process by simulating in the axotomized CNS the events that occur naturally in spontaneously regenerating systems.

Adult rat olfactory nerve ensheathing cells are effective promoters of adult central nervous system neurite outgrowth in coculture

A coculture method is described for ensheathing glial cells from adult rat olfactory nerve, serving as a substrate for the regrowth of neurites from adult rat retinal ganglion cells. Immunocytochemically identified phenotypes present in primary cultures of olfactory nerve cells are described, and their ability to promote neurite outgrowth is compared with neonatal astrocytes and Schwann cells, with other nonglial cells, and with laminin. Ensheathing cell cultures were more effective than any other substrate tested and also directed the orientation of regrowing neurites. In comparison with cultured Schwann cells, which released neurotrophic factors into the culture medium, there was no evidence of a similar activity in ensheathing cell cultures. Combinations of ensheathing cell-conditioned medium and substrates of laminin, merosin, or 3T3 cells also failed to show the release of factors enhancing either survival or neurite outgrowth from retinal ganglion cells. Evidence is presented for a partial inhibition of neurite outgrowth in the presence of calcium channel antagonists or an intracellular calcium-chelating reagent. This provides evidence for a contribution from an intracellular calcium signaling mechanism, possibly implicating ensheathing cell adhesion molecules in promoting neurite outgrowth.

Constitutive expression of GAP-43 correlates with rapid, but not slow regrowth of injured dorsal root axons in the adult rat

It has been postulated that the neuronal growth-associated protein GAP-43 plays an essential role in axon elongation. Although termination of developmental axon growth is generally accompanied by a decline in expression of GAP-43, a subpopulation of dorsal root ganglion (DRG) neurons retains constitutive expression of GAP-43 throughout adulthood. Peripheral nerve regeneration occurring subsequent to injury of the peripheral axon branches of adult DRG neurons is accompanied by renewed elevation of GAP-43 expression. Lesions of DRG central axon branches in the dorsal roots are also followed by some regenerative growth, but little or no increase in GAP-43 expression above the constitutive level is observed. To determine whether dorsal root axon regeneration occurs only from neurons which constitutively express GAP-43, we have used retrograde fluorescent labeling to identify those DRG neurons which extend axons beyond a crush lesion of the dorsal root. Only GAP-43 immunoreactive neurons supported axon regrowth of 7 mm or greater within the first week. At later times, axon regrowth is seen to occur from neurons both with and without GAP-43 immunoreactivity. We conclude that regeneration of injured axons within the dorsal root is not absolutely dependent on the presence of GAP-43, but that expression of GAP-43 is correlated with a capacity for rapid growth.

Regulation of hemopexin synthesis in degenerating and regenerating rat sciatic nerve

In injured peripheral nerves, hemopexin mRNA is expressed by fibroblasts, Schwann cells, and invading blood macrophages, and the protein accumulates in the extracellular matrix. This and its absence of regulation in injured central optic nerve suggest that hemopexin could play a positive role in peripheral nerve repair. Here, we studied the regulation of hemopexin expression in degenerating and regenerating nerves. After a sciatic nerve injury, both the synthesis of hemopexin and the level of its mRNA increase sharply during the first 2 days, leading to an accumulation of hemopexin in the nerve. Afterward, hemopexin expression decreases progressively in regenerating nerves. In permanently degenerated nerves, it is again transiently increased and then strongly decreased, whereas hemopexin from blood origin is accumulating. As part of the elucidation of the complex regulation of hemopexin expression in injured nerves, we demonstrate that interleukin-6 increases hemopexin synthesis in intact nerves, whereas adult rat serum, but not purified hemopexin, inhibits it in degenerated nerves. Hemopexin, known as acute-phase protein, is therefore one of the molecules rapidly and specifically up-regulated in injured peripheral nerves. More generally, our findings suggest that the acute phase could be not only a systemic liver-specific response but also a reaction of injured tissues themselves.

Link between optic nerve regrowth failure and macrophage stimulation in mammals

The adult mammalian central nervous system (CNS) fails to regenerate its axons following injury. A comparison between its postinjury response and that of axons of nervous systems capable of regeneration reveals major differences with respect to inflammation. In regenerative systems, a large number of macrophages rapidly invade the injured site during the first few hours and days after the injury. Following their activation/differentiation through interaction with the host tissue, they play a central role in tissue healing through phagocytosis of cell debris and communication with cellular and molecular elements of the damaged tissue. Relative to the peripheral nervous system (PNS), macrophage recruitment in the adult mammalian CNS is delayed and is restricted in amount and activity. It was recently proposed that in injured mammalian CNS tissue, implantation of macrophages stimulated by prior co-culture with segments of peripheral (sciatic) nerves can compensate, at least in part, of the restricted postinjury inflammatory reaction. In the present study, this experimental paradigm is further explored and shows that there is no conflict between the systemic use of anti-inflammatory compounds and treatment with stimulated macrophages to promote regrowth of neuronal tissue.

Contributions of pathway and neuron to preferential motor reinnervation

Motor axons regenerating after transection of mixed nerve preferentially reinnervate distal muscle branches, a process termed preferential motor reinnervation (PMR). Motor axon collaterals appear to enter both cutaneous and muscle Schwann cell tubes on a random basis. Double-labeling studies suggest that PMR is generated by pruning collaterals from cutaneous pathways while maintaining those in motor pathways (the "pruning hypothesis"). If all collaterals projecting to muscle are saved, then stimulation of regenerative sprouting should increase specificity by increasing the number of motoneurons with at least one collateral in a muscle pathway. In the current experiments, collateral sprouting is stimulated by crushing the nerve proximal to the repair site before suture, a maneuver that also conditions the neuron and predegenerates the distal pathway. Control experiments are performed to separate these effects from those of collateral generation. Experiments were performed on the rat femoral nerve and evaluated by exposing its terminal cutaneous and muscle branches to HRP or Fluoro-Gold. Crush proximal to the repair site increased motor axon collaterals at least fivefold and significantly increased the percentage of correctly projecting motoneurons, consistent with the pruning hypothesis. Conditioning the nerve with distal crushes before repair had no effect on specificity. A graft model was used to separate the effects of collateral generation and distal stump predegeneration. Previous crush of the proximal femoral nerve significantly increased the specificity of fresh graft reinnervation. Stimulation of regenerative collateral sprouting thus increased PMR, confirming the pruning hypothesis. However, this effect was overshadowed by the dramatic specificity with which predegenerated grafts were reinnervated by fresh uncrushed proximal axons. These unexpected effects of predegeneration on specificity could involve a variety of possible mechanisms and warrant further study because of their mechanistic and clinical implications.