Detection of brain-derived neurotrophic factor-like activity in fibroblasts and Schwann cells: inhibition by antibodies to NGF

Towards personalized cell-replacement therapies for brain repair

The inability of the CNS to efficiently repair damage caused by trauma and neurodegenerative or demyelinating diseases has underlined the necessity for developing novel therapeutic strategies. Cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. In this review, we will address the prospects of using stem cells or myelinating glial cells of the PNS, as well as olfactory ensheathing cells, in cell-replacement therapies. The recent generation of induced pluripotent stem cells from adult somatic cells by introduction of three or four genes controlling ‘stemness’ and their subsequent differentiation to desired phenotypes, constitutes a significant advancement towards personalized cell-replacement therapies.

Reduced NGF secretion by Schwann cells under the high glucose condition decreases neurite outgrowth of DRG neurons

BACKGROUND

Schwann cells (SCs) have been supposed to play prominent roles in axonal regeneration under various diseases. Here, to evaluate the direct interaction between SCs and dorsal root ganglion (DRG) neurons under a diabetic condition, the effects of Schwann cell-conditioned media on neurite outgrowth of DRG neurons were investigated.

METHODS

Immortalized mouse Schwann cells (IMS) were cultured under 5.5 mM glucose (NG) or 30 mM glucose (HG) conditions for 4 days. IMS-conditioned media (IMS-media) were added to the culture media of neurons isolated from 8-week-old DDY mice. Neurons were cultured for 48 h with or without mouse recombinant NGF (mrNGF) or nerve growth factor (NGF) neutralizing antibody. The concentrations of NGF in IMS-media by ELISA and neurite outgrowth by a computed image analysis system were evaluated.

RESULTS

Neurite outgrowth was significantly enhanced by IMS-media (IMS-media (-): 177+/-177 microm, IMS-media (+): 1648+/-726). The neurite outgrowth cultured with IMS-media obtained under the HG condition was significantly reduced compared with that under the NG condition (NG: 1474+/-652, HG: 734+/-331). The NGF concentrations were significantly lower in IMS-media under the HG condition than in those under the NG condition. The accelerated neurite outgrowth by IMS-media was inhibited by NGF neutralizing antibody.

CONCLUSIONS

These results suggest that SCs play important roles in neurite outgrowth of DRG neurons, and that the decreased NGF secretion by SCs under the diabetic condition would cause a defect of axonal regeneration, resulting in the development of diabetic neuropathy.

Current status of experimental cell replacement approaches to spinal cord injury

Despite advances in medical and surgical care, the current clinical therapies for spinal cord injury (SCI) are largely ineffective. During the last 2 decades, the search for new therapies has been revolutionized by the discovery of stem cells, which has inspired scientists and clinicians to search for a stem cell-based reparative approaches to many diseases, including neurotrauma. In the present study, the authors briefly summarize current knowledge related to the pathophysiology of SCI, including the concepts of primary and secondary injury and the importance of posttraumatic demyelination. Key inhibitory obstacles that impede axonal regeneration include the glial scar and a number of myelin inhibitory molecules including Nogo. Recent advancements in cell replacement therapy as a therapeutic strategy for SCI are summarized. The strategies include the use of pluripotent human stem cells, embryonic stem cells, and a number of adult-derived stem and progenitor cells such as mesenchymal stem cells, Schwann cells, olfactory ensheathing cells, and adult-derived neural precursor cells. Although current strategies to repair the subacutely injured cord appear promising, many obstacles continue to render the treatment of chronic injuries challenging. Nonetheless, the future for stem cell-based reparative strategies for treating SCI appears bright.

Cross talk between vestibular neurons and Schwann cells mediates BDNF release and neuronal regeneration

It is now well-established that an active cross-talk occurs between neurons and glial cells, in the adult as well as in the developing and regenerating nervous systems. These functional interactions not only actively modulate synaptic transmission, but also support neuronal growth and differentiation. We have investigated the possible existence of a reciprocal interaction between inner ear vestibular neurons and Schwann cells maintained in primary cultures. We show that ATP released by the extending vestibular axons elevates intracellular calcium levels within Schwann cells. Purinergic activation of the Schwann P2X(7) receptor induces the release of neurotrophin BDNF, which occurs via a regulated, tetanus-toxin sensitive, vesicular pathway. BDNF, in turn, is required by the vestibular neuron to support its own survival and growth. Given the massive release of ATP during tissue damage, cross-talk between vestibular neurons and Schwann cells could play a primary role during regeneration.

Transplantation of Schwann cells and/or olfactory ensheathing glia into the contused spinal cord: Survival, migration, axon association, and functional recovery

Schwann cells (SCs) and olfactory ensheathing glia (OEG) have shown promise for spinal cord injury repair. We sought their in vivo identification following transplantation into the contused adult rat spinal cord at 1 week post-injury by: (i) DNA in situ hybridization (ISH) with a Y-chromosome specific probe to identify male transplants in female rats and (ii) lentiviral vector-mediated expression of EGFP. Survival, migration, and axon-glia association were quantified from 3 days to 9 weeks post-transplantation. At 3 weeks after transplantation into the lesion, a 60-90% loss of grafted cells was observed. OEG-only grafts survived very poorly within the lesion (<5%); injection outside the lesion led to a 60% survival rate, implying that the injury milieu was hostile to transplanted cells and or prevented their proliferation. At later times post-grafting, p75(+)/EGFP(-) cells in the lesion outnumbered EGFP(+) cells in all paradigms, evidence of significant host SC infiltration. SCs and OEG injected into the injury failed to migrate from the lesion. Injection of OEG outside of the injury resulted in their migration into the SC-injected injury site, not via normal-appearing host tissue but along the pia or via the central canal. In all paradigms, host axons were seen in association with or ensheathed by transplanted glia. Numerous myelinated axons were found within regions of grafted SCs but not OEG. The current study details the temporal survival, migration, axon association of SCs and OEG, and functional recovery after grafting into the contused spinal cord, research previously complicated due to a lack of quality, long-term markers for cell tracking in vivo.

Metabosensitive Afferent Fiber Responses after Peripheral Nerve Injury and Transplantation of an Acellular Muscle Graft in Association with Schwann Cells

Studies dedicated to the repair of peripheral nerve focused almost exclusively on motor or mechanosensitive fiber regeneration. Poor attention has been paid to the metabosensitive fibers from group III and IV (also called ergoreceptor). Previously, we demonstrated that the metabosensitive response from the tibialis anterior muscle was partially restored when the transected nerve was immediately sutured. In the present study, we assessed motor and metabosensitive responses of the regenerated axons in a rat model in which 1 cm segment of the peroneal nerve was removed and immediately replaced by an autologous nerve graft or an acellular muscle graft. Four groups of animals were included: control animals (C, no graft), transected animals grafted with either an autologous nerve graft (Gold Standard-GS) or an acellular muscle filled with Schwann Cells (MSC) or Culture Medium (MCM). We observed that (1) the tibialis anterior muscle was atrophied in GS, M(SC) and M(CM) groups, with no significant difference between grafted groups; (2) the contractile properties of the reinnervated muscles after nerve stimulation were similar in all groups; (3) the metabosensitive afferent responses to electrically induced fatigue was smaller in M(SC) and MCM groups; and (4) the metabosensitive afferent responses to two chemical agents (KCl and lactic acid) was decreased in GS, M(SC) and M(CM) groups. Altogether, these data indicate a motor axonal regeneration and an immature metabosensitive afferent fiber regrowth through acellular muscle grafts. Similarities between the two groups grafted with acellular muscles suggest that, in our conditions, implanted Schwann cells do not improve nerve regeneration. Future studies could include engineered conduits that mimic as closely as possible the internal organization of uninjured nerve.

Immunohistochemical distribution of NGF, BDNF, NT-3, and NT-4 in adult rhesus monkey brains

Immunohistochemical distribution and cellular localization of neurotrophins was investigated in adult monkey brains using antisera against nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Western blot analysis showed that each antibody specifically recognized appropriate bands of approximately 14.7 kDa, 14.2 kDa, 13.6 kDa, and 14.5 kDa, for NGF, BDNF, NT-3, and NT-4, respectively. These positions coincided with the molecular masses of the neurotrophins studied. Furthermore, sections exposed to primary antiserum preadsorbed with full-length NGF, BDNF, NT-3, and NT-4 exhibited no detectable immunoreactivity, demonstrating specificities of the antibodies against the tissues prepared from rhesus monkeys. The study provided a systematic report on the distribution of NGF, BDNF, NT-3, and NT-4 in the monkey brain. Varying intensity of immunostaining was observed in the somata and processes of a wide variety of neurons and glial cells in the cerebrum, cerebellum, hippocampus, and other regions of the brain. Neurons in some regions such as the cerebral cortex and the hippocampus, which stained for neurotrophins, also expressed neurotrophic factor mRNA. In some other brain regions, there was discrepancy of protein distribution and mRNA expression reported previously, indicating a retrograde or anterograde action mode of neurotrophins. Results of this study provide a morphological basis for the elucidation of the roles of NGF, BDNF, NT-3, and NT-4 in adult primate brains.

Cellular repair strategies for spinal cord injury

The implantation of exogenous cells or tissues has been a popular and successful strategy to overcome physical discontinuity and support axon growth in experimental models of spinal cord injury (SCI). Cellular therapies exhibit a multifarious potential for SCI restoration, providing not only a supportive substrate upon which axons can traverse the injury site, but also reducing progressive tissue damage and scarring, facilitating remyelination repair, and acting as a source for replacing and re-establishing lost neural tissue and its circuitry. The past two decades of research into cell therapies for SCI repair have seen the progressive evolution from whole tissue strategies, such as peripheral nerve grafts, to the use of specific, purified cell types from a diverse range of sources and, recently, to the employment of stem or neural precursor cell populations that have the potential to form a full complement of neural cell types. Although the progression of cell therapies from laboratory to clinical implementation has been slow, human SCI safety and efficacy trials involving several cell types within the US appear to be close at hand.

Schwann cell transplantation for repair of the adult spinal cord

The Schwann cell is one of the most widely studied cell types for repair of the spinal cord. These cells play a crucial role in endogenous repair of peripheral nerves due to their ability to dedifferentiate, migrate, proliferate, express growth promoting factors, and myelinate regenerating axons. Following trauma to the spinal cord, Schwann cells migrate from the periphery into the injury site, where they apparently participate in endogenous repair processes. For transplantation into the spinal cord, large numbers of Schwann cells are necessary to fill injury-induced cystic cavities. Several culture systems have been developed that provide large, highly purified populations of Schwann cells. Importantly, the development of in vitro systems to harvest human Schwann cells presents a unique opportunity for autologous transplantation in the clinic. In animal models of spinal cord injury (SCI), grafting Schwann cells or peripheral nerve into the lesion site has been shown to promote axonal regeneration and myelination. However, axons do not regenerate beyond the transplant due to the inhibitory nature of the glial scar surrounding the injury. To overcome the glial scar inhibition, additional approaches such as increasing the intrinsic capacity of axons to regenerate and/or removal of the inhibitory molecules associated with reactive astrocytes and/or oligodendrocyte myelin should be incorporated. Clearly, Schwann cells have great potential for repair of the injured spinal cord, but they need to be combined with other interventions to maximize axonal regeneration and functional recovery.

Grafts of brain-derived neurotrophic factor and neurotrophin 3-transduced primate Schwann cells lead to functional recovery of the demyelinated mouse spinal cord

Experimental studies provided overwhelming proof that transplants of myelin-forming cells achieve efficient remyelination in the CNS. Among cellular candidates, Schwann cells can be used for autologous transplantation to ensure robust remyelination of lesions and to deliver therapeutic factors in the CNS. In the present study, macaque Schwann cells expressing green fluorescent protein (GFP) were infected with human immunodeficiency virus-derived vectors overexpressing brain-derived neurotrophic factor (BDNF) or Neurotrophin 3 (NT-3), two neurotrophins that also modulate glial cell biology. The ability of transgenic Schwann cells to secrete growth factors was assessed by ELISA and showed 35- and 62-fold increases in BDNF and NT-3, respectively, in transduced macaque Schwann cell supernatants. Conditioned media of BDNF- and NT-3-transduced Schwann cells reduced Schwann cell proliferation and favored their differentiation in vitro. Transgenic cells were grafted in demyelinated spinal cords of adult nude mice. Two behavioral assays showed that NT-3- and BDNF-transduced Schwann cells promoted faster and stronger functional recovery than GFP-transduced Schwann cells. Morphological analysis indicated that functional recovery correlated with enhanced proliferation and differentiation of resident oligodendrocyte progenitors and enhanced oligodendrocyte and Schwann cell differentiation. Moreover, NT-3-transduced Schwann cells provided neuroprotection and reduced astrogliosis. These results underline the potential therapeutic benefit of combining neuroprotection and activation of myelin-forming cells to restore altered functions in demyelinating diseases of the CNS.

Neurotrophic factor synergy is required for neuronal survival and disinhibited axon regeneration after CNS injury

The therapeutic effects of individual neurotrophic factors (NTF) have proved disappointing in clinical trials for neuronal repair and axon regeneration. Here, we demonstrate NTF synergistic neuronal responses after a combination of basic fibroblast growth factor, neurotrophin-3 and brain derived growth factor delivered to the somata of retinal ganglion cells promoted greater survival and axon growth than did the sum of the effects of each NTF alone. Triple and not single NTF treatments potentiated regulated intramembraneous proteolysis of p75(NTR), and ectodomain shedding of Nogo receptor, correlated with a 30% decrease in activation of Rho-A, a key signalling molecule in the axon growth inhibitory cascade. Thus, combinatorial NTF administration synergistically enhanced neuronal survival, disinhibited axon growth and promoted axon regeneration through the hostile CNS environment without the intervention of scar tissue at the lesion site.

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Effects of glial transplantation on functional recovery following acute spinal cord injury

Numerous efforts have been made to maximize the efficacy of treatment for spinal cord injury (SCI). Recently, oligodendrocyte-type 2 astrocyte (O-2A) progenitor cells have been reported to remyelinate focal areas of demyelinated spinal cord in adult rats. We conducted a study to investigate the therapeutic potential of transplantation of O-2A cells in a rat model of acute SCI. SCI was induced with an NYU Impactor at T9 of rats. O-2A cells labeled with bromodeoxyuridine (BrdU) were transplanted into sites of SCI at 1 week after the induction of SCI. At 6 weeks after cell transplantation, a behavioral test showed significant functional improvement in animals that had received O-2A-cell transplants as compared to animals given cell-culture medium alone. An electrophysiological study revealed that the transplants did not improve the amplitude or latency of somatosensory evoked potentials, but a recording of motor evoked potentials showed that the latency of these potentials in the O-2A-cell-transplant group was significantly shorter than that in the group treated with cell-culture medium. Following transplantation of BrdU-labeled O-2A cells, cells positive for BrdU were detected at and near sites of SCI. Cells labeled for both BrdU and 2',3' -cyclic nucleotide-3-phosphodiesterase were also detected, showing that the transplanted O-2A cells differentiated into oligodendrocytes. By contrast, cells labeled for BrdU and glial fibrillary acidic protein, or for neuronal nuclei antigen, were not detected. Furthermore, a tract-tracing study showed that numbers of retrogradely labeled neurons increased in areas of the brain stem after O-2A-cell transplantation. The study data showed that after being transplanted into an animal with SCI, O-2A cells migrated to the area adjacent to the site of injury and differentiated into oligodendrocytes. The behavioral test and the electrophysiological and morphological studies showed that transplantation of O-2A cells may play an important role in functional recovery and the regeneration of axons after SCI.

Hyperoxia enhances brain-derived neurotrophic factor and tyrosine kinase B receptor expression in peribronchial smooth muscle of neonatal rats

Airway hyperreactivity is one of the hallmarks of hyperoxic lung injury in early life. As neurotrophins such as brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) are potent mediators of neuronal plasticity, we hypothesized that neurotrophin levels in the pulmonary system may be disturbed by hyperoxic exposure. We therefore evaluated the effects of hyperoxia on the expression of BDNF, NGF, and their corresponding high-affinity receptors, TrkB and TrkA, respectively, in the lung of rat pups. Five-day-old Sprague-Dawley rat pups were randomized to hyperoxic or control groups and then continuously exposed to hyperoxia (>95% oxygen) or normoxia over 7 days. At both mRNA and protein levels, BDNF was detected in lung but not in trachea; its level was substantially enhanced in lungs from the hyperoxia-exposed rat pups. Distribution of BDNF mRNA by in situ hybridization indicates that peribronchial smooth muscle was the major source of increased BDNF production in response to hyperoxic exposure. Interestingly, hyperoxia-induced elevation of BDNF was not accompanied by any changes of NGF levels in lung. Furthermore, hyperoxic exposure increased the expression of TrkB in peribronchial smooth muscle but had no effect on the distribution of the specific NGF receptor TrkA. These findings indicate that hyperoxic stress not only upregulates BDNF at mRNA and protein levels but also enhances TrkB within peribronchial smooth muscle. However, there was no corresponding effect on NGF and TrkA receptors. We speculate that the increased level of BDNF may contribute to hyperoxia-induced airway hyperresponsiveness in early postnatal life.

Repair of avulsed ventral nerve roots by direct ventral intraspinal implantation after brachial plexus injury

Currently, the authors' research confirms that,in humans, communication between the cord and effector muscles can be re-established after multi-ple nerve root avulsion by the implantation of peripheral nerve grafts. Outcomes are still modest,but the possibility of improvement exists. The technique of reimplantation makes it possible to envisage global repair with the possibility of repair of all avulsed regions. The most important factor that could maximize the extent of functional recovery is reducing the time between the injury and corrective surgery: the diagnosis of avulsion within 10 days and reparative surgery within 3 weeks is the objective. This goal will involve a global re-evaluation of how these patients are managed. The problem of the recovery of sensory function (tactile and fine perception and proprioception) warrants further work. It seems likely that methods combining medullary reimplantation with neurotization will be the best way of correcting these lesions of the brachial plexus. In this context, cross-disciplinary collaboration is probably more important than ever. The place that methods based on reimplantation will have in the final picture remains to be seen. The key question is in which patients should medullary reimplantation be attempted and which method should be used. Moreover, medullary reimplantation should be considered as an adjunct to all other surgical options and should not compromise the chance of the latter modalities to be effective.An important point remains: are physicians going to be able to map out all the boundaries of this question in the future?

Effects of Schwann cell secreted factors on PC12 cell neuritogenesis and survival

We have used PC12 cells to examine the effects of factors secreted by Schwann cells that promote cell survival and neurite outgrowth, and hence are likely candidates for promoting neuronal regeneration. RT-PCR showed that primary Schwann cells produced a range of neurotrophins, excluding NT3, but this profile was different from either of two cell lines SCTM41 or PVGSCSV40T, or forskolin-expanded Schwann cells. The effects of Schwann cell conditioned media on neurite outgrowth was tested against a range of factors, and showed clear neuritogenic effects. Of the factors tested, only NGF had a significant response on neuritogenesis. Western blotting for neurofilaments showed that primary Schwann cells induced a strong response close to that of NGF. The Trk tyrosine kinase inhibitor K252a did not block the neuritogenic effects of primary Schwann cells. In contrast, K252a blocked both NGF and the SCTM41 cell effects. Schwann cell conditioned media also enhanced PC12 cell survival. Again, in contrast with NGF or SCTM41 cells, the primary Schwann cell effect was Trk tyrosine kinase independent. The Schwann cell conditioned medium contains a protein factor (greater than 12 kDa and broken down by trypsin treatment) with remarkable thermal stability (unaffected at 95 degrees C for 15 min) and the ability to bind heparin. Our results provide clear evidence that Schwann cells produce factors other than those already known to stimulate a neural phenotype in PC12 cells, and which thus have potential regeneration enhancing effects.

Activity-unrelated neural control of myogenic factors in a slow muscle

The properties of skeletal muscles are modulated by neural and nonneural factors, and the neural factors can be modulated by activity-independent as well as activity-dependent mechanisms. Given that daily activation of fast muscles is considerably less than of slow muscles, we hypothesized that the myogenic properties of the rat soleus (a slow muscle) would be more dependent on activity-dependent than activity-independent factors. Muscle mass, MyoD, and myogenin mRNA and protein levels, and satellite cell proliferation and differentiation rates (bromodeoxyuridine incorporation) were examined at 3, 14, and 28 days after either spinal cord isolation (SI, neuromuscular connectivity intact with minimal activation) or denervation (no neural influence). Soleus atrophy was similar in the SI and denervated groups at each time point, although increases in whole-muscle expression of myogenin and, to a lesser degree, MyoD were lower (P < 0.05) in SI than denervated soleus muscles. Proliferation and differentiation of satellite cells, as well as mitotic activity of connective tissue cells, were lower (P < 0.05) in SI than denervated soleus muscles. In some instances, these changes were not observed until the later time points, i.e., 14 or 28 days. These results demonstrate that the motoneurons that innervate the slow soleus muscle have a significant modulatory influence on some muscle properties via mechanisms that are independent of activation. These activity-independent modulatory influences, however, are less in the slow soleus than previously observed in fast muscles.

Intravitreal transplants of Schwann cells and fibroblasts promote the survival of axotomized retinal ganglion cells in rats

Schwann cells (SCs) are considered one of the major cellular components to maintain the integrity of the peripheral nervous system (PNS) neurons after injury. Intravitreal transplant of peripheral nerves or Schwann cells has been shown to enhance the regenerative ability of retinal ganglion cells (RGCs). In the present study, we compared the effects of intravitreal transplants of Schwann cells and fibroblasts, two major components of peripheral nerves, on the survival of retinal ganglion cells in adult rats after optic nerve (ON) transection. Purified Schwann cells and fibroblasts from neonatal sciatic nerves were injected into the vitreous body of adult rats. Three days after the injection, the optic nerves were transected intraorbitally. After 1 week or 1 month, surviving retinal ganglion cells were retrogradely labelled with Fluoro-Gold (FG) and the number of surviving retinal ganglion cells was counted. The retinas were further processed for 200-kDa neurofilament RT-97 immunohistochemistry. It was found that intravitreally injected- Schwann cells and -fibroblasts delayed the death of axotomized retinal ganglion cells for 1 week. In addition, in the animal group with 1 month survival time after optic nerve transection, those received a larger number of Schwann cells had more surviving retinal ganglion cells and more profusely ramified axonal processes near the optic disc. These findings reveal that both Schwann cells and fibroblasts isolated from the peripheral nerve can promote retinal ganglion cell survival after optic nerve transection, presumably by secreting neurotrophic factors. In addition, the data also demonstrate that Schwann cells could promote intraretinal axonal sprouting. Our findings demonstrate a remarkable glial source of neurotrophic factors with potential clinical applications, as autologous Schwann cells and fibroblasts can be feasibly obtained from peripheral nerves.

Selective expression of L-serine synthetic enzyme 3PGDH in schwann cells, perineuronal glia, and endoneurial fibroblasts along rat sciatic nerves and its upregulation after crush injury

Non-essential amino acid L-serine functions as a highly potent, glia-derived neurotrophic factor, because it is a precursor for syntheses of proteins, other amino acids, membrane lipids, and nucleotides, and also because its biosynthetic enzyme 3-phosphoglycerate dehydrogenase (3PGDH) is preferentially expressed in particular glial cells within the brain. Here we pursued 3PGDH expression in peripheral nerves and its change after crush injury. In the pathway of rat sciatic nerves, 3PGDH was selectively expressed in non-neuronal elements: Schwann sheaths and endoneurial fibroblasts in sciatic nerves, satellite cells in dorsal root ganglia, and astrocytes and oligodendrocytes in the spinal ventral horn. In contrast, 3PGDH was immunonegative in axons, somata of spinal motoneurons and ganglion cells, and endoneurial macrophages. One week after crush injury, 3PGDH was upregulated in the distal segment of injured nerves, where 3PGDH was intensified in activated Schwann cells and fibroblasts. 3PGDH was still negative in activated macrophages, which were instead associated or surrounded by activated Schwann cells with intensified 3PGDH. These results suggest that in the peripheral nervous system, these non-neuronal cells synthesize and may supply L-serine to satisfy metabolic demands for maintenance and regeneration of peripheral nerves and for proliferation and activation of macrophages upon nerve injury.

Combined demyelination plus Schwann cell transplantation therapy increases spread of cells and axonal regeneration following contusion injury

Several cell populations have been shown to provide a permissive environment for axonal extension following transplantation to injury sites. The limited spread of transplanted cells from implantation sites in the mature CNS, and the superior substrate and trophic environment that they provide, likely contribute to the fact that few transplantation-based therapies have elicited axonal extension beyond the transplant. The aim of this study was to determine whether (1) regions of demyelination cranial and caudal to a spinal cord injury site would improve the spread of Schwann cells transplanted into the site of injury, and (2) whether this combination therapy was associated with improved anatomical regeneration. Three days following contusion injury, anti-galactocerebroside antibodies plus complement proteins were injected into the dorsal column cranial and caudal to the injury site, resulting in complete and well defined regions of demyelination that extended 8 mm either side of the injury site. One day later, naïve Schwann cells in suspension were injected into the contusion site. Transplanted Schwann cells homogeneously redistributed throughout the contusion site and the adjacent regions of demyelination cranial and caudal to the contusion site, providing a long-distance prospective path for repair that was free of myelin and contained transplanted cells. Animals that received demyelination plus transplantation therapy, but not untreated or single-treatment groups, exhibited robust axonal regeneration beyond the contusion site within the treated dorsal column. Axonal regeneration in these animals was not associated with an improvement in locomotor ability. These findings suggest that this combination therapy may overcome a central limitation of transplant strategies in which the permissive environment provided remains at the implantation site.

Sensitization of recombinant vanilloid receptor-1 by various neurotrophic factors

The vanilloid receptor (VR1) is a central integrator molecule of nociceptive stimuli. In this study, we have measured the effects of various neurotrophins (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and -4) on recombinant rat VR1-mediated intracellular calcium rise in response to capsaicin in VR1/C6 cells. Our results clearly show that all neurotrophins sensitize the VR1 to capsaicin. Furthermore, using K252a, an inhibitor of tyrosine kinases, we present that actions of neurotrophins are mediated by the trk (A, B, C) receptors expressed in these cells. These data argue for the putative roles of neurotrophins in inducing inflammatory (thermal) hyperalgesia via VR1.

Epithelial growth control by neurotrophins: leads and lessons from the hair follicle

Neurotrophins (NTs) exert many growth-regulatory functions beyond the nervous system. For example, murine hair follicles (HF) show developmentally and spatio-temporally stringently controlled expression of NTs, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, and their cognate receptors, tyrosine kinase A-C (TrkA-C) and p75 neurotrophin receptor (p75NTR). Follicular NT and NT receptor expression exhibit significant, hair cycle-dependent fluctuations on the gene and protein level, which are mirrored by changes in nerve fiber density and neurotransmitter/neuropeptide content in the perifollicular neural networks. NT-3/TrkC and NGF/TrkA signaling stimulate HF development, while NT-3, NT-4 and BDNF inhibit the growth (anagen) of mature HF by the induction of apoptosis-driven HF regression (catagen). p75NTR stimulation inhibits HF development and stimulates catagen. Since the HF is thus both a prominent target and key peripheral source of NT, dissecting the role of NTs in the control of HF morphogenesis and cyclic remodeling provides a uniquely accessible, and easily manipulated, clinically relevant experimental model, which has many lessons to teach. Given that our most recent data also implicate NTs in human hair growth control, selective NT receptor agonists and antagonists may become innovative therapeutic tools for the management of hair growth disorders (alopecia, effluvium, hirsutism). Since, however, the same NT receptor agonists that inhibit hair growth (e.g., BDNF, NT-4) can actually stimulate epidermal keratinocyte proliferation, NT may exert differential effects on defined keratinocyte subpopulations. The studies reviewed here provide new clues to understanding the complex roles of NT in epithelial tissue biology and remodeling in vivo, and invite new applications for synthetic NT receptor ligands for the treatment of epithelial growth disorders, exploiting the HF as a lead model.

Nerve activity-independent regulation of skeletal muscle atrophy: role of MyoD and myogenin in satellite cells and myonuclei

Electrical activity is thought to be the primary neural stimulus regulating muscle mass, expression of myogenic regulatory factor genes, and cellular activity within skeletal muscle. However, the relative contribution of neural influences that are activity-dependent and -independent in modulating these characteristics is unclear. Comparisons of denervation (no neural influence) and spinal cord isolation (SI, neural influence with minimal activity) after 3, 14, and 28 days of treatment were used to demonstrate whether there are neural influences on muscle that are activity independent. Furthermore, the effects of these manipulations were compared for a fast ankle extensor (medial gastrocnemius) and a fast ankle flexor (tibialis anterior). The mass of both muscles plateaued at approximately 60% of control 2 wk after SI, whereas both muscles progressively atrophied to <25% of initial mass at this same time point after denervation. A rapid increase in myogenin and, to a lesser extent, MyoD mRNAs and proteins was observed in denervated and SI muscles: at the later time points, these myogenic regulatory factors remained elevated in denervated, but not in SI, muscles. This widespread neural activity-independent influence on MyoD and myogenin expression was observed in myonuclei and satellite cells and was not specific for fast or slow fiber phenotypes. Mitotic activity of satellite and connective tissue cells also was consistently lower in SI than in denervated muscles. These results demonstrate a neural effect independent of electrical activity that 1) helps preserve muscle mass, 2) regulates muscle-specific genes, and 3) potentially spares the satellite cell pool in inactive muscles.

In vitro development of a tissue-engineered model of peripheral nerve regeneration to study neurite growth

A unique tissue-engineered model of peripheral nerve regeneration was developed in vitro to study neurite outgrowth. Mouse dorsal root ganglia neurons were seeded on a collagen sponge populated with human endothelial cells and/or human fibroblasts. Addition of nerve growth factor (NGF; 10 ng/ml) was not required for sensory neurons survival but was necessary to promote neurite outgrowth, as assessed by immunostaining of the 150 kDa neurofilament. A vigorous neurite elongation was detected inside the reconstructed tissue after 14 and 31 days of neurons culture, reaching up to 770 microm from day 14. Axons were often observed closely associated with the capillary-like tubes reconstructed in the model, in a similar pattern as in the human dermis. The presence of endothelial cells induced a significant increase of the neurite elongation after 14 days of culture. The addition of human keratinocytes totally avoided the twofold decrease in the amount of neurites observed between 14 and 31 days in controls. Besides the addition of NGF, axonal growth did not necessitate B27 supplement or glial cell coculture to be promoted and stabilized for long-term culture. Thus, this model might be a valuable tool to study the effect of various cells and/or attractive or repulsive molecules on neurite outgrowth in vitro.

Thyroid hormone reduces the loss of axotomized sensory neurons in dorsal root ganglia after sciatic nerve transection in adult rat

We have shown that a local administration of thyroid hormones (T3) at the level of transected rat sciatic nerve induced a significant increase in the number of regenerated axons. To address the question of whether local administration of T3 rescues the axotomized sensory neurons from death, in the present study we estimated the total number of surviving neurons per dorsal root ganglion (DRG) in three experimental group animals. Forty-five days following rat sciatic nerve transection, the lumbar (L4 and L5) DRG were removed from PBS-control, T3-treated as well as from unoperated rats, and serial sections (1 microm) were cut. The physical dissector method was used to estimate the total number of sensory neurons in the DRGs. Our results revealed that in PBS-control rats transection of sciatic nerve leads to a significant (P < 0.001) decrease in the mean number of sensory neurons (8743.8 +/- 748.6) compared with the number of neurons in nontransected ganglion (mean 13,293.7 +/- 1368.4). However, administration of T3 immediately after sciatic nerve transection rescues a great number of axotomized neurons so that their mean neuron number (12,045.8 +/- 929.8) is not significantly different from the mean number of neurons in the nontransected ganglion. In addition, the volume of ganglia showed a similar tendency. These results suggest that T3 rescues a high number of axotomized sensory neurons from death and allows these cells to grow new axons. We believe that the relative preservation of neurons is important in considering future therapeutic approaches of human peripheral nerve lesion and sensory neuropathy.

Regenerating crayfish motor axons assimilate glial cells and sprout in cultured explants

Phasic and tonic motor nerves originating from crayfish abdominal ganglia, in 2-3-day-old cultured explants, display at their transected distal ends growth zones from which axonal sprouts arise. The subcellular morphology of this regenerative response was examined with thin serial-section electron microscopy and reveals two major remodeling features. First, the external sprouts that exit the nerve are a very small part of a much more massive sprouting response by individual axons comprising several orders of internal sprouts confined to the nerve. Both internal and external sprouts have a simple construction: a cytoskeleton of microtubules and populations of mitochondria, clear synaptic vesicles, membranous sacs, and extrasynaptic active zone dense bars, features reminiscent of motor nerve terminals. Close intermingling of the sprouts of several axons give rise to a neuropil-like arbor within the nerve. Thus, extensive sprouting is an intrinsic response of crayfish motor axons to transection. Second, an equally dramatic remodeling feature is the appearance of nuclei, which resemble those of adjacent glial cells, within the motor axons. These nuclei often appear where the adjoining membranes of the axon and glial cell are disrupted and where free-standing lengths of the double membrane are present. These images signify a breakdown of the dividing membranes and assimilation of the glial cell by the axon, the nucleus being the most visible sign of such assimilation. Thus, crayfish motor axons respond to transection by assimilating glial cells that may provide regulatory and trophic support for the sprouting response.

Growth factor enhancement of peripheral nerve regeneration through a novel synthetic hydrogel tube

OBJECT

The authors' long-term goal is repair of peripheral nerve injuries by using synthetic nerve guidance devices that improve both regeneration and functional outcome relative to an autograft. They report the in vitro processing and in vivo application of synthetic hydrogel tubes that are filled with collagen gel impregnated with growth factors.

METHODS

Poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (PHEMA-MMA) porous 12-mm-long tubes with an inner diameter of 1.3 mm and an outer diameter of 1.8 mm were used to repair surgically created 10-mm gaps in the rat sciatic nerve. The inner lumen of the tubes was filled with collagen matrix alone or matrix supplemented with either neurotropin-3 at 1 microg/ml, brain-derived neurotrophic factor at 1 microg/ml, or acidic fibroblast growth factor (FGF-1) at 1 or 10 microg/ml. Nerve regeneration through the growth factor-enhanced tubes was assessed at 8 weeks after repair by histomorphometric analysis at the midgraft level and in the nerve distal to the tube repair. The tubes were biostable and biocompatible, and supported nerve regeneration in more than 90% of cases. Nerve regeneration was improved in tubes in which growth factors were added, compared with empty tubes and those containing collagen gel alone (negative controls). Tubes filled with 10 microg/ml of FGF-1 dispersed in collagen demonstrated regeneration comparable to autografts (positive controls) and showed significantly better regeneration than the other groups.

CONCLUSIONS

The PHEMA-MMA tubes augmented with FGF-1 in their lumens appear to be a promising alternative to autografts for repair of nerve injuries. Studies are in progress to assess the long-term biocompatibility of these implants and to enhance regeneration further.

Preferential motor reinnervation in the mouse: comparison of femoral nerve repair using a fibrin sealant or suture

Previous studies in rat femoral nerve demonstrated that regenerating motor axons preferentially reinnervate a nerve branch to muscle as opposed to skin, a process that has been termed preferential motor reinnervation (PMR). This process has not been previously reported in the mouse, where the use of transgenic animals could be a powerful tool to study the basic mechanisms that determine accuracy of regenerating motor axons. In the mouse, we applied the same nerve repair (suture) and retrograde labeling strategies that successfully demonstrated PMR in the rat femoral nerve but surprisingly were unable to demonstrate PMR. However, if the mouse femoral nerve was repaired with a fibrin sealant, PMR was readily apparent, suggesting that PMR in the mouse is dependent on the method of nerve repair.

A hypothetical mathematical construct explaining the mechanism of biological amplification in an experimental model utilizing picoTesla (PT) electromagnetic fields

We seek to answer the conundrum: What is the fundamental mechanism by which very weak, low frequency Electromagnetic fields influence biosystems? In considering the hydrophobicity of intramembranous protein (IMP) H-bonds which cross the phospholipid bilayer of plasma membranes, and the necessity for photonic recycling in cell surface interactions after dissipation of energetic states, present models lack structure and thermodynamic properties to maintain (DeltaE) sufficient energy sources necessary for amplifications by factors of 10(12). Even though one accepts that the ligand-receptor association alters the conformation of extracellular, extruding portions of IMP's at the cell surface, and that this change can be transmitted to the cytoplasm by the transmembranous helical segments by nonlinear vibrations of proteins with generation of soliton waves, one is still unable to account for repair and balanced function. Indeed, responses of critical molecules to certain magnetic field signals may include enhanced vibrational amplitudes, increased quanta of thermal energies and order inducing interactions. We may accept that microtrabecular reticulum-receptor is associated with actin filaments and ATP molecules which contribute to the activation of the cyclase enzyme system through piezoelectricity. Magnetic fields will pass through the membrane which sharply attenuates the electric field component of an EM field, due to its high impedance. Furthermore, EM oscillations are converted to mechanical vibrations; i.e., photon-phonon transduction, to induce molecular vibrations of frequencies specifically responsible for bioamplifications of weak triggers at the membrane surface, as well as GAP junctions. The hydrogen bonds of considerable importance are those in proteins (10(12)Hz) and DNA (10(11)Hz) and may be viewed as centers of EM radiation emission in the range from the mm microwaves to the far IR. However, classical electrodynamical theory does not yield a model for biomolecular resonant responses which are integrated over time and account for the connection between the phonon field and photons. Jacobson Resonance does supply an initial physical mechanism, as equivalencies in energy to that of Zeeman Resonance (i.e., zero-order magnetic resonance) and cyclotron resonance may be derived from the DeBroglie wave particle equation. For the first time, we view the introduction of Relativity Theory to biology in the expression, mc(2)=BvLq, where m is the mass of a particle in the 'box' or 'string' (molecule in a biosystem), c is the velocity of electromagnetic field in space, independent of its inertial frame of reference, B is the magnetic flux density,v is the velocity of the carrier or 'string' (a one or two dimensional 'box') in which the particle exists, L is its dimension (length) and q represents a unit charge q=1C, by defining electromotive force as energy per unit charge. Equivalencies suggest that qvBL is one of the fundamental expressions of energy of a charged wave-particle in magnetic fields, just as Zeeman and cyclotron resonance energy expressions, gbetaB and qhB/2pim, and is applicable to all charged particles (molecules in biological systems). There may exist spontaneous, independent and incessant interactions of magnetic vector B and particles in biosystems which exert Lorentz forces. Lorentz forces may be transmitted from EM field to gravitational field as a gravity wave which return to the phonon field as microgravitational fluctuations to therein produce quantum vibrational states that increase quanta of thermal energies integrated over time. This may account for the differential of 10(12) between photonic energy of ELF waves and the Boltzman energy kT. Recent data from in vivo controlled studies are included as empirical support for the various hypotheses presented.

BDNF and activity-dependent synaptic modulation

It is widely accepted that neuronal activity plays a pivotal role in synaptic plasticity. Neurotrophins have emerged recently as potent factors for synaptic modulation. The relationship between the activity and neurotrophic regulation of synapse development and plasticity, however, remains unclear. A prevailing hypothesis is that activity-dependent synaptic modulation is mediated by neurotrophins. An important but unresolved issue is how diffusible molecules such as neurotrophins achieve local and synapse-specific modulation. In this review, I discuss several potential mechanisms with which neuronal activity could control the synapse-specificity of neurotrophin regulation, with particular emphasis on BDNF. Data accumulated in recent years suggest that neuronal activity regulates the transcription of BDNF gene, the transport of BDNF mRNA and protein into dendrites, and the secretion of BDNF protein. There is also evidence for activity-dependent regulation of the trafficking of the BDNF receptor, TrkB, including its cell surface expression and ligand-induced endocytosis. Further study of these mechanisms will help us better understand how neurotrophins could mediate activity-dependent plasticity in a local and synapse-specific manner.

Parasympathetic nerve fibers invade the upper dermis following sensory denervation of the rat lower lip skin

The sympathetic division of the autonomic nervous system is known to play a role in the genesis of neuropathic pain. In the skin of the rat lower lip (hairy skin), sympathetic and parasympathetic fibers normally innervate the same blood vessels in the lower dermis but do not occur in the upper dermis. However, we have shown that sympathetic fiber migration into the upper dermis occurs following mental nerve lesions (Ruocco et al. [2000] J. Comp. Neurol. 422:287-296). As sensory denervation has a dramatic effect on sympathetic fiber innervation patterns in the rat lower lip skin, we decided to investigate the possible changes in the other autonomic fiber type in the skin-the parasympathetic fiber. Sensory denervation of the rat lower lip was achieved by bilateral transection of the mental nerve, and animals were allowed to recover for 1-8 weeks. Lower lip tissue was processed for double-labeling light microscopic immunocytochemistry (ICC), using antibodies against substance P (SP), which labels a subpopulation of peptidergic sensory fibers, and against the vesicular acetycholine transporter (VAChT), as a marker for parasympathetic fibers. In sham-operated rats, SP-immunoreactive (IR) sensory fibers were found in the epidermis and upper and lower dermal regions, whereas VAChT-IR fibers were confined to the lower dermis. Mental nerve lesions induced the gradual disappearance of SP-IR fibers from all skin layers accompanied by the progressive migration of VAChT-IR fibers into the upper dermis. Cholinergic fiber migration was evident by the second week post surgery, and the ectopic innervation of the upper dermis by these fibers persisted even at the last time point studied (8 weeks) when SP-IR fibers have completely regrown. VAChT-IR fibers were observed in the upper dermis, well above the opening of the sebaceous glands into the hair follicles. These results show that considerable changes occur in the innervation patterns of parasympathetic fibers following mental nerve lesions.

Schwann cell but not olfactory ensheathing glia transplants improve hindlimb locomotor performance in the moderately contused adult rat thoracic spinal cord

Cultured adult rat Schwann cells (SCs) or olfactory ensheathing glia (OEG), or both, were transplanted in the adult Fischer rat thoracic (T9) spinal cord 1 week after a moderate contusion (10 gm, 12.5 mm, NYU impactor). Rats received either a total of 2 x 10(6) cells suspended in culture medium or culture medium only (controls). At 12 weeks after injury, all grafted animals exhibited diminished cavitation. Although in medium-injected rats 33% of spinal tissue within a 5-mm-long segment of cord centered at the injury site was spared, significantly more tissue was spared in SC (51%), OEG (43%), and SC/OEG (44%) grafted animals. All three types of glial grafts were filled with axons, primarily of spinal origin. SC grafts contained more myelinated axons than SC/OEG and OEG grafts. Both types of SC-containing grafts expressed more intense staining for glial fibrillary acidic protein and chondroitin sulfate proteoglycan compared with OEG-only grafts. Retrograde tracing demonstrated that the number of propriospinal and brainstem axons reaching 5-6 mm beyond the grafted area was significantly higher with SC and SC/OEG grafts but not with OEG-only grafts compared with controls. Corticospinal fibers terminated closer to the lesion epicenter in all grafted animals than in controls. With SC-only grafts, a modest but statistically significant improvement in hindlimb locomotor performance was detected at 8-11 weeks after injury. Thus, in addition to this functional improvement, our results show that an SC graft is more effective in promoting axonal sparing/regeneration than an SC/OEG or OEG graft in the moderately contused adult rat thoracic spinal cord.

Myelin phagocytosis by macrophages and nonmacrophages during Wallerian degeneration

The literature concerning Schwann cells (SCs) and macrophages in myelin phagocytosis during Wallerian degeneration is reviewed. SCs carry out the first step in the removal of myelin by segmenting myelin and then incorporating the degraded myelin. The recruited macrophages then join in the myelin-phagocytosis event, appearing to make full use of their original phagocyte abilities until the end of myelin clearance. The molecular mechanisms of the two cells underlying myelin phagocytosis are thought to be different; myelin phagocytosis by SCs being lectin-mediated, i.e., opsonin-independent, whereas that of macrophages is mainly opsonin-dependent. It is important to note that SCs and macrophages cooperatively accomplish myelin phagocytosis.

Neurotrophins are key mediators of the myelination program in the peripheral nervous system

Although knowledge of the functions of neurotrophins has advanced rapidly in recent years, studies concerning the involvement of neurotrophins in glial-neuronal interactions rarely extend further than their roles in supporting the survival and differentiation of neuronal cells. In this study endogenous brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) were identified in Schwann cell/dorsal root ganglia neuronal cocultures and shown to modulate the myelination program of the peripheral nervous system. The differential expression of BDNF and NT3 were examined and compared with the expression profiles of myelin proteins in the cocultures throughout the myelination process. BDNF levels correlated with active myelin formation, whereas NT3 expression was initially high and then down regulated throughout the proliferation and premyelination periods. Addition of exogenous BDNF enhanced myelination, whereas the removal of the endogenous BDNF by using the BDNF receptor TrkB-Fc fusion protein inhibited the formation of mature myelin internodes. Interestingly, exogenous NT3 significantly inhibited myelination, whereas the removal of the endogenous NT3 by using the NT3 receptor TrkC-Fc fusion protein resulted in an enhancement similar to that obtained with the addition of BDNF. In addition, in vivo studies were performed during the development of the mouse sciatic nerve. Subcutaneous injections of BDNF resulted in an enhancement of myelin formation in the sciatic nerve, whereas the removal of the endogenous BDNF dramatically inhibited myelination. Injections of NT3 inhibited myelin formation, and the removal of the endogenous NT3 enhanced myelination. These results demonstrate that BDNF and NT3 possess different modulatory roles in the myelination program of the peripheral nervous system and that their mechanisms of action are specific and highly regulated.

The influence of inhalative corticosteroids on circulating Nerve Growth Factor, Brain-Derived Neurotrophic Factor and Neurotrophin-3 in allergic asthmatics

BACKGROUND

The neurotrophins Nerve Growth Factor (NGF), Brain-Derived Neurotrophic Factor (BDNF) and Neurotrophin (NT)-3 are produced, stored and released by various immunological cells. The influence of NTs upon the function of these cells is described. Elevated plasma levels were found in inflammatory, autoimmune and allergic diseases with the highest levels in allergic asthma. A connection between bronchial hyper-responsiveness and serum levels has been reported.

OBJECTIVE

Little is known about the influence of treatment with inhaled corticosteroids (ICS) on serum NT levels and their influence on the asthmatic state.

METHODS

Eighty-seven volunteers were studied. Thirty-eight were stable allergic asthmatics with constant ICS doses, 29 were asthmatics not receiving anti-asthmatic treatment and 20 were age- and sex-matched healthy controls. Demographic and lung function data were evaluated. NT serum levels were determined by ELISA.

RESULTS

NGF and BDNF levels were significantly increased in untreated asthmatics compared to the control and the treated group, while NT-3 demonstrated significantly higher levels in treated asthmatics compared to healthy controls. After stabilization of untreated subjects with ICS, the NT levels decreased significantly.

CONCLUSIONS

These results suggest that NTs participate in allergic inflammation and asthma. Effective treatment leads to a decrease of circulating neurotrophic factors.

Neurotrophic factors and gene therapy in spinal cord injury

Although it was once thought that the central nervous system (CNS) of mammals was incapable of substantial recovery from injury, it is now clear that the adult CNS remains responsive to various substances that can promote cell survival and stimulate axonal growth. Among these substances are growth factors, including the neurotrophins and cytokines, and growth-supportive cells such as Schwann cells, olfactory ensheathing glia, and stem cells. We review the effects of these substances on promoting axonal growth after spinal cord injury, placing particular emphasis on the genetic delivery of nervous system growth factors to specific sites of injury as a means of promoting axonal growth and, in limited instances, functional recovery.

Conduction of impulses by axons regenerated in a Schwann cell graft in the transected adult rat thoracic spinal cord

Central nervous system axons regenerate into a Schwann cell implant placed in the transected thoracic spinal cord of an adult rat. The present study was designed to test whether these regenerated axons are capable of conducting action potentials. Following the transection and removal of a 4- to 5-mm segment of the thoracic spinal cord (T8-T9), a polymer guidance channel filled with a mixture of adult rat Schwann cells and Matrigel was grafted into a 4- to 5-mm-long gap in the transected thoracic spinal cord. The two cut ends of the spinal cord were eased into the guidance channel openings. Transected control animals received a channel containing Matrigel only. Three months after implantation, electrophysiological studies were performed. Tungsten microelectrodes were used for monopolar stimulation of regenerated axons within the Schwann cell graft. Glass microelectrodes were used to record responses in the spinal cord rostral to the stimulation site. Evoked responses to electrical stimulation of the axon cable were found in two out of nine Schwann cell-grafted animals. These responses had approximate latencies in the range of those of myelinated axons. No responses were seen in any of the Matrigel-grafted animals. Histological analysis revealed that the two cases that showed evoked potentials had the largest number of myelinated axons present in the cable. This study demonstrates that axons regenerating through Schwann cell grafts in the complete transected spinal cord can produce measurable evoked responses following electrical stimulation.

Safe injection of cultured schwann cells into peripheral nerve allografts

The effects of cultured host Schwann cells on axonal regeneration in peripheral nerve allografts were studied. Fischer rats served as recipient animals and Buffalo rats provided nerve allografts. Animals were randomized into 9 groups. Rats receiving tibial nerve isografts were left untreated (group I), or injected with isogeneic Fischer Schwann cells (group II) or placebo suspension (group III). Allografts obtained from Buffalo rats were left untreated (group IV), or received isogeneic Fischer Schwann cells (group V), 2 mg/kg Cyclosporin A and Fischer Schwann cells (group VI), 5 mg/kg Cyclosporin A (group VII), or 5 mg/kg Cyclosporin A with Schwann cells (group VIII). No Schwann cell tumors were identified 4 or 8 weeks postoperatively. Group IX animals, harvested 3 days postoperatively, demonstrated no evidence of injection injury. Schwann cells modestly improved axonal regeneration in both isografts and allografts and may have a clinical role in the treatment of peripheral nerve allografts.

Stimulation of corticospinal tract regeneration in the chronically injured spinal cord

Acute spinal cord injury models have proved popular in studies aimed at identifying factors capable of influencing axonal regeneration within the central nervous system. In these models, the test factors (e.g. graft tissues or cells, antibodies, growth factors, etc.) are typically administered at the time of spinal cord injury. In this study, we use a rat chronic spinal cord injury model to identify possible factors which can stimulate regeneration of the chronically lesioned corticospinal tract axons. We demonstrate that surgical grafting of segments of autologous, preligated sural nerve, into the syrinx, stimulates sprouting and regeneration of the corticospinal tract as evidenced by the presence of anterograde labelled corticospinal tract processes within the cavity walls two or more weeks after treatment. Regrowing corticospinal processes were not observed within control animals. The anterogradely labelled corticospinal tract axons were found exclusively within the central grey tissue comprising the cavity walls with no regrowing corticospinal process observed within the white matter. A similar pattern of regeneration was observed following injection into the cavity of a suspension of minced autologous preligated sural nerve. Evidence of corticospinal tract regeneration was seen when either wheat germ agglutinin--horseradish peroxidase or biotinylated--dextran was used as an anterograde tracer. These data demonstrate that the chronically injured cortical motor neurons retain the capacity to regenerate for extended periods and that regeneration can be stimulated using grafts of minced, preligated autologous peripheral nerve tissue.

Neural transplantation in spinal cord injury

Although medical advancements have significantly increased the survival of spinal cord injury patients, restoration of function has not yet been achieved. Neural transplantation has been studied over the past decade in animal models as a repair strategy for spinal cord injury. Although spinal cord neural transplantation has yet to reach the point of clinical application and much work remains to be done, reconstructive strategies offer the greatest hope for the treatment of spinal cord injury in the future. This article presents the scientific basis of neural transplantation as a repair strategy and reviews the current status of neural transplantation in spinal cord injury.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Effects of short- and long-term Schwann cell denervation on peripheral nerve regeneration, myelination, and size

Poor functional recovery after peripheral nerve injury has been generally attributed to inability of denervated muscles to accept reinnervation and recover from denervation atrophy. However, deterioration of the Schwann cell environment may play a more vital role. This study was undertaken to evaluate the effects of chronic denervation on the capacity of Schwann cells in the distal nerve stump to support axonal regeneration and to remyelinate regenerated axons. We used a delayed cross-suture anastomosis technique in which the common peroneal (CP) nerve in the rat was denervated for 0-24 weeks before cross-suture of the freshly axotomized tibial (TIB) and chronically denervated CP nerve stumps. Motor neurons were backlabeled with either fluoro-ruby or fluorogold 12 months later, to identify and count TIB motor neurons that regenerated axons into chronically denervated CP nerve stumps. Number, size, and myelination of regenerated sensory and motor axons were determined using light and electron microscopy. We found that short-term denervation of < or =4 weeks did not affect axonal regeneration but more prolonged denervation profoundly reduced the numbers of backlabeled motor neurons and axons in the distal nerve stump. Yet, atrophic Schwann cells retained their capacity to remyelinate regenerated axons. In fact, the axons were larger and well myelinated by long-term chronically denervated Schwann cells. These findings demonstrate a progressive inability of chronically denervated Schwann cells to support axonal regeneration and yet a sustained capacity to remyelinate the axons which do regenerate. Thus, axonal interaction can effectively switch the nonmyelinating phenotype of atrophic Schwann cells back into the myelinating phenotype.

A new type of biocompatible bridging structure supports axon regrowth after implantation into the lesioned rat optic tract

We have developed a new type of polymer/cell/matrix implant and tested whether it can promote the regrowth of retinal ganglion cell (RGC) and other axons across surgically induced tissue defects in the CNS. The constructs, which consisted of 2-2.5-mm-long polycarbonate tubes filled with lens capsule-derived extracellular matrix coated with cultured neonatal Schwann cells, were implanted into lesion cavities made in the left optic tract (OT) of 18-21-day-old rats. In one group, to promote Schwann cell proliferation and perhaps also to stimulate axon regrowth, basic fibroblast growth factor (bFGF) was added to the lens capsule matrix prior to implantation. In another group, to determine whether application of growth factors to the somata of cells enhances the regrowth of distally injured axons, the neurotrophin NT-4/5 was injected into the eye contralateral to the OT lesion. NT-4/5 and bFGF treatments were combined in some rats. After medium-term (4-10 weeks) or long-term (15-20 weeks) survivals, axon growth into implants was assessed immunohistochemically using a neurofilament (RT97) antibody. RGC axons were visualized after injection of WGA/HRP into the right eye. Viable Schwann cells were present in implants at all times after transplantation. Large numbers of RT97+ axons were consistently found within the bridging implants, often associated with the peripheral glia. Axons were traced up to 1.7 mm from the nearest CNS neuropil and there was immunohistochemical evidence of myelination by Schwann cells and by host oligodendrocytes. There were fewer RGC axons in the implants, fibers growing up to 1.6 mm from the thalamus. Neither NT-4/5 nor bFGF, alone or in combination, significantly increased the extent of RGC axon growth within the implants. A group of OT-lesioned rats was implanted with polymer tubes filled with 2-2.5-mm-long pieces of predegenerate peripheral nerve. Surprisingly, polymer/cell/matrix constructs contained comparatively greater numbers of RGC and other axons and supported more extensive axon elongation. Thus, implants of this type may potentially be useful in bridging large tissue defects in the CNS.

Heme oxygenase1 (HSP-32) is induced in myelin-phagocytosing Schwann cells of injured sciatic nerves in the rat

Schwann cells participate in myelin phagocytosis in the early stage of Wallerian degeneration, prior to the recruitment of macrophages. This is the first report that Schwann cells induce heme oxygenase-1 (HO-1), a 32-kDa heat shock protein, only when they have transformed into myelin-phagocytosing cells from myelinating cells (days 2-3) immediately after crush injury of rat sciatic nerves. Double immunofluorescent labelling for HO-1 and transferrin receptors revealed that HO-1-immunoreactive Schwann cells also expressed transferrin receptors suggesting activation of iron metabolism. The transient induction of HO-1 in Schwann cells may contribute to the adaptive function in an altered environment when the cells have lost contact with axons, and may play a crucial role in the ensuing regeneration.

Olfactory ensheathing cells: bridging the gap in spinal cord injury

Spinal cord injury (SCI) continues to be an insidious and challenging problem for scientists and clinicians. Recent neuroscientific advances have changed the pessimistic notion that axons are not capable of significant extension after transection. The challenges of recovering from SCI have been broadly divided into four areas: 1) cell survival; 2) axon regeneration (growth); 3) correct targeting by growing axons; and 4) establishment of correct and functional synaptic appositions. After acute SCI, there seems to be a therapeutic window of opportunity within which the devastating consequences of the secondary injury can be ameliorated. This is supported by several observations in which apoptotic glial cells have been identified up to 1 week after acute SCI. Moreover, autopsy studies have identified anatomically preserved but unmyelinated axons that could potentially subserve normal physiological properties. These observations suggest that therapeutic strategies after SCI can be directed into two broad modalities: 1) prevention or amelioration of the secondary injury, and 2) restorative or regenerative interventions. Intraspinal transplants have been used after SCI as a means for restoring the severed neuraxis. Fetal cell transplants and, more recently, progenitor cells have been used to restore intraspinal circuitry or to serve as relay for damaged axons. In an attempt to remyelinate anatomically preserved but physiologically disrupted axons, newer therapeutic interventions have incorporated the transplantation of myelinating cells, such as Schwann cells, oligodendrocytes, and olfactory ensheathing cells. Of these cells, the olfactory ensheathing cells have become a more favorable candidate for extensive remyelination and axonal regeneration. Olfactory ensheathing cells are found along the full length of the olfactory nerve, from the basal lamina of the epithelium to the olfactory bulb, crossing the peripheral nervous system-central nervous system junction. In vitro, these cells promote robust axonal growth, in part through cell adhesion molecules and possibly by secretion of neurotrophic growth factors that support axonal elongation and extension. In animal models of SCI, transplantation of ensheathing cells supports axonal remyelination and extensive migration throughout the length of the spinal cord. Although the specific properties of these cells that govern enhanced axon regeneration remain to be elucidated, it seems certain that they will contribute to the establishment of new horizons in SCI research.