Nerve growth activities in rat peripheral nerve

Experimental Model Systems for Understanding Human Axonal Injury Responses

Neurons are structurally unique and have dendrites and axons that are vulnerable to injury. Some neurons in the peripheral nervous system (PNS) can regenerate their axons after injuries. However, most neurons in the central nervous system (CNS) fail to do so, resulting in irreversible neurological disorders. To understand the mechanisms of axon regeneration, various experimental models have been utilized in vivo and in vitro. Here, we collate the key experimental models that revealed the important mechanisms regulating axon regeneration and degeneration in different systems. We also discuss the advantages of experimenting with the rodent model, considering the application of these findings in understanding human diseases and for developing therapeutic methods.

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Nerve growth factor promoter activity revealed in mice expressing enhanced green fluorescent protein

Nerve growth factor (NGF) and its precursor proNGF are perhaps the best described growth factors of the mammalian nervous system. There remains, however, a paucity of information regarding the precise cellular sites of proNGF/NGF synthesis. Here we report the generation of transgenic mice in which the NGF promoter controls the ectopic synthesis of enhanced green fluorescent protein (EGFP). These transgenic mice provide an unprecedented resolution of both neural cells (e.g., neocortical and hippocampal neurons) and non-neural cells (e.g., renal interstitial cells and thymic reticular cells) that display NGF promoter activity from postnatal development to adulthood. Moreover, the transgene is inducible by injury. At 2 days after sciatic nerve ligation, a robust population of EGFP-positive cells is seen in the proximal nerve stump. These transgenic mice offer novel insights into the cellular sites of NGF promoter activity and can be used as models for investigating the regulation of proNGF/NGF expression after injury.

Effects of Low-dose X-irradiation on Mouse-brain Aggregation Cultures

Biochemical and morphological differentiation in reaggregating mouse-brain cell cultures after low-dose radiation (0.5 Gy) in vitro was studied. Cells were irradiated on culture day 2, corresponding to embryonic day 15-16, and different glial and neuronal markers were followed through development to postnatal day 40. The shape and size of irradiated aggregates were more irregular and smaller compared with controls. Total amounts of DNA and protein were significantly lower in irradiated aggregates than in controls between days 8 and 20. After 30 days in culture activities of the glial markers glutamine synthetase (GS) and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) were lower in X-irradiated aggregates than in controls. However, after 40 days the CNP activity in irradiated aggregates increased to levels above those of the controls. Irradiated and control aggregates did not differ significantly in neuronal marker enzyme activities, i.e. choline acetyltransferase (ChAT), acetylcholine esterase (AChE) and glutamic acid decarboxylase (GAD) measured on a per mg protein basis. On days 20 and 30 the amount of nerve growth factor (NGF) was two-fold higher in irradiated aggregates compared with non-irradiated ones, suggesting that, after irradiation, surviving cells in culture were induced to produce more NGF. After 40 days the amount of NGF in irradiated aggregates had decreased to the level found in the control aggregates.

Effects of electro-acupuncture on CNTF expression in spared dorsal root ganglion and the associated spinal lamina II and nucleus dorsalis following adjacent dorsal root ganglionectomies in cats

It is well known that plasticity occurs in deafferented spinal cord, and that electro-acupuncture (EA) could promote functional restoration. The underlying mechanism is, however, unknown. Ciliary neurotrophic factor (CNTF) plays a crucial role in neurite outgrowth and neuronal survival both in vivo and in vitro, and its expression might explain some of the mechanism. In this study, we investigated the effects of EA on CNTF expression in the spared L(6) dorsal root ganglion (DRG), and spinal lamina II at spinal segments L(3) and L(6) as well as nucleus dorsalis (ND) of L(3) spinal segment following removal of L(1)-L(5) and L(7)-S(2) (DRG) in the cat. After ganglionectomies, the total and small-to-medium-sized numbers of immunoreactive neurons decreased at 3 dpo, and returned to the sham-operated level as early as 7 dpo. After EA, immunoreactive neurons in L(6) DRG noticeably increased at 7 dpo, compared with the non-acupunctured group. Notable increase in the large neurons was seen at 14 dpo, while their numbers in L(3) and L(6) spinal cord segments significantly declined at 3 dpo. Those in L(3) segment did not reach the sham-operated level until 14 dpo, but their numbers in L(6) segment returned to the sham-operated level as early as 7 dpo. CNTF immunopositive neurons in the ND of L(3) segment returned to the sham-operated level at 14 dpo. After EA, their number significantly increased as early as 7 dpo in lamina II of L(6) segment, and as late as 14 dpo in ND of L(3) segment. Western blot analysis showed CNTF changes corresponding to those shown in immunohistochemical staining. It is concluded that CNTF expression was involved in the EA promoted plastic changes in L(6) DRG and the associated deafferented spinal lamina and ND.

Impaired axonal regeneration by isolectin B4-binding dorsal root ganglion neurons in vitro

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, alpha7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding alpha7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both alpha7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

Pleiotrophin is a neurotrophic factor for spinal motor neurons

Regeneration in the peripheral nervous system is poor after chronic denervation. Denervated Schwann cells act as a "transient target" by secreting growth factors to promote regeneration of axons but lose this ability with chronic denervation. We discovered that the mRNA for pleiotrophin (PTN) was highly up-regulated in acutely denervated distal sciatic nerves, but high levels of PTN mRNA were not maintained in chronically denervated nerves. PTN protected spinal motor neurons against chronic excitotoxic injury and caused increased outgrowth of motor axons out of the spinal cord explants and formation of "miniventral rootlets." In neonatal mice, PTN protected the facial motor neurons against cell death induced by deprivation from target-derived growth factors. Similarly, PTN significantly enhanced regeneration of myelinated axons across a graft in the transected sciatic nerve of adult rats. Our findings suggest a neurotrophic role for PTN that may lead to previously unrecognized treatment options for motor neuron disease and motor axonal regeneration.

Improved sciatic nerve regeneration by local thyroid hormone treatment in adult rat is accompanied by increased expression of SCG10

Thyroid hormone plays an important role in regulating the development and regeneration of the nervous system. Our previous work showed that local administration of triiodothyronine (T3) at the level of transected rat sciatic nerve increased the number and diameter of regenerated axons, but the mechanism underlying the improved regeneration is still unclear. Here, we have investigated the effect of T3 on the expression of SCG10, a regulator of microtubule dynamics in growth cones. After transection of adult rat sciatic nerves, silicone tubes were implanted and filled with T3 or phosphate-buffered solution. At various time points following surgery, the expression of SCG10 protein and mRNA was analyzed. Semi-quantitative Western blot analysis revealed that sciatic nerve transection induced a more than 20-fold upregulation of SCG10 protein in proximal nerve segments at 1 day post-lesion, while at this time point, SCG10 mRNA in dorsal root ganglion neurons was not increased yet. The increase in SCG10 protein and mRNA could be observed over 30 days. Local T3 treatment significantly enhanced the increase in SCG10 protein levels about two-fold in the different segments of transected nerve during the regeneration period. Also SCG10 mRNA levels in lumbar ganglia were enhanced. Immunohistochemical analysis showed that T3 treatment not only increased the number of SCG10 positive axons but also the intensity of their staining. These results suggest that SCG10 is involved in the regulation of regeneration. The stimulating effect of T3 on SCG10 expression could provide a mechanism by which T3 enhances peripheral nerve regeneration.

A real-time quantitative PCR comparative study between rat optic and sciatic nerves: determination of neuregulin-1 mRNA levels

Injured axons from peripheral nervous system (PNS) possess the ability to regenerate. In contrast, regeneration of injured axons does not occur in the central nervous system (CNS) or occurs to a limited extent. Previous works have shown that rat sciatic nerve conditioned medium (CM) produced PC12 cells neuronal-like differentiation and neurite outgrowth. In the present work, we compared the expression of neuregulin-1s (NRG-1s) from rat sciatic and optic nerves as members of the PNS and CNS, respectively. Sciatic nerve CM showed a higher neurotrophic activity on PC12 cells than rat optic nerve CM. RT-PCR analysis verified the presence of all three types of NRG-1 mRNAs and their receptors in both types of nerves. Real-time quantitative PCR (QPCR) assays showed that the relative expression levels of all three types of NRG-1 mRNAs were higher in optic nerves than in sciatic nerves. Eleven-day cultured optic nerves showed an increased in NDF and SMDF when compared to freshly isolated optic nerves, whereas GGF decreased. However, 11-day-cultured sciatic nerves only showed an increase in SMDF mRNA. Western blots corroborated the differences in NRG-1 expression profile for both types of nerves and their CMs. Incubation of both CMs with the anti-pan-NRG-1 antibody showed that the neurotrophic activity of the optic nerve CM increased, whereas the sciatic nerve CM remained unchanged. These results indicated that different NRG-1 levels are expressed upon nerve degeneration and the balance between those levels and other neurotrophic factors could have an important role on nerve regeneration.

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.

Role of glypican-1 in the trophic activity on PC12 cells induced by cultured sciatic nerve conditioned medium: identification of a glypican-1-neuregulin complex

Glypican-1 is an extracellular matrix component found by microsequencing in a medium conditioned by cultured rat-sciatic nerves (CM). This CM was concentrated by ultrafiltration and fractionated by quaternary ammonium chromatography, followed by Hi-Trap blue affinity chromatography to obtain the active fraction B1.2. Previously, we have reported a 54 kDa neuregulin (NRG) in the same B1.2 fraction [Villegas et al., Brain Res. 852 (2001) 304]. The effect of Glypican-1 on the neuron-like differentiation of PC12 cells was investigated by immunoprecipitation, Western blot and cellular image analysis. Removal of glypican-1 by immunoprecipitation with increasing concentrations of specific antibodies revealed a gradual decrease of the differentiation activity of fraction B1.2, which paralleled the results obtained by removal of the 54 kDa NRG protein. Colorless native electrophoresis and Western blot analysis was used to identify a glypican-1-NRG protein complex, which could be afterwards separated by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis into its individual components. Additionally, it was demonstrated that glypican-1, in cooperation with the 54 kDa NRG, is involved in the neuronal-like differentiation of PC12 cells and could play an important role on the regeneration responses of peripheral nerves.

Differential Regulation of Nerve Growth Factor (NGF) Synthesis in Neurons and Astrocytes by Glucocorticoid Hormones

Glucocorticoid hormones are important regulators of brain development and ageing. Here we show that dexamethasone, a synthetic glucocorticoid, differentially affects the expression of nerve growth factor (NGF) in cultured neurons and astrocytes. Dexamethasone increased the levels of NGF mRNA in cultured hippocampal neurons in a time- and concentration-dependent manner, whereas it down-regulated the NGF mRNA levels in astrocytes. However, dexamethasone had no effect on the mRNA levels of brain-derived neurotrophic factor in the hippocampal neurons. Aldosterone, a mineralocorticoid, in higher concentrations also up-regulated NGF mRNA levels in the hippocampal neurons. Dexamethasone increased the levels of NGF mRNA in the rat hippocampus in vivo, but not to the same extent as observed with kainic acid, a glutamate receptor agonist. There is no apparent diurnal rhythm in the hippocampal NGF protein levels corresponding to circadian variations in the levels of glucocorticoid hormones in serum. The increase in NGF mRNA in the hippocampus in vivo following dexamethasone treatments may reflect the physiological response of hippocampal neurons to high glucocorticoid levels reached under conditions of stress.

Ionic currents in PC12 cells differentiated into neuron-like cells by a cultured-sciatic nerve conditioned medium

The present work deals with the identification of the ionic currents found in PC12 cells differentiated into neuron-like cells by a 9-11-day cultured-sciatic nerve conditioned medium (CM). PC12 whole-cell currents were measured after chronic exposure to CM. The results obtained in these CM-treated cells reveal that the functional expression of Ca(2+) currents is increased, that Na+ currents are not affected, and that a transient K+ current and a K+ delayed rectifier (K+ dr) current are increased. The combination of nifedipine and omega-conotoxin GVIA (omega-CgTX) does not block completely the increased functional expression of the Ca(2+) current. The remaining current is blocked by omega-agatoxin TK indicating that P/Q-type channels are additionally contributing to the increase in Ca(2+) current. NGF-treated PC12 cells, used as positive controls, confirm that NGF increases the expression of voltage-dependent Na+ currents and of Ca(2+) currents. In addition, we found that NGF also increases a K+ dr-type current in these cells. The results obtained with the CM might be due to a molecule or a mixture of molecules released into the medium by the 9-11-day cultured sciatic nerves.

A two-dimensional gel electrophoretic study of proteins synthesized and released by degenerating adult mouse sciatic nerve

Previous two-dimensional (2-D) gel electrophoretic studies of proteins secreted by degenerating mammalian peripheral nerves (Ignatius et al., 1986, Proc. Natl. Acad. Sci. USA 83: 1125-1129; Muller et al., 1986, J. Cell Biol. 102: 393-402) detected the up-regulation of two proteins of 67-70 and 34-37 kDa, although they failed to resolve proteins smaller than about 15 kDa or with isoelectric points greater than 8. In the present study, we have used 2-D gels that can resolve proteins in the molecular mass range 3.6-200 kDa and isoelectric point range 2.4-10.6. This revealed that the incorporation of radiolabel by three diffusible proteins with apparent molecular mass/isoelectric point values of 38/5-6, 27-31/4-5, and 8/>10 was increased in the distal stumps of sciatic nerves 4 days after lesion, while the radiolabel incorporation by a further two proteins (15/5.3 and 12.5-17.5/6.8-7.5) was increased in the distal nerve stump 15 days after lesion. The possible cellular sources of these proteins were assessed by comparing protein secretion from unoperated nerves with nerve segments maintained in culture for 4 days (in which the contribution from recruited macrophages would be expected to be minimal) and segments of nerve that had been frozen and then replaced in situ for 4 days (in which the contribution from nerve sheath cells would be expected to be minimal). This revealed that three of the proteins up-regulated in lesioned nerves (27-31/4-5, 15/5.3, and 12.5-17.5/6.8-7.5) are probably sheath cell products, while the other two (38/5-6 and 8/>10) may be secreted mainly by macrophages (or other cells) that infiltrate the frozen nerve segments. The identity of these proteins and their possible involvement in axonal regeneration remain to be determined.

Neuregulin found in cultured-sciatic nerve conditioned medium causes neuronal differentiation of PC12 cells

The present work deals with the search and identification of the molecule or combination of molecules, present in a medium conditioned by cultured rat-sciatic nerves (CM), able to cause neuronal differentiation of PC12 cells. The molecular mass range of the active fraction, as well as the thermostability and heparin affinity of the active component found in previous work, all characteristics shared with neuregulin (NRG) family members, led us to search for a NRG protein in the CM. Nerves were previously cultured for 8 days and the CM collected every 24 h, the following 3 days. The CM was concentrated (30,000 NMWL) and fractionated by quaternary ammonium chromatography and Cibacron blue affinity chromatography. The most active fraction B1.2 was further characterized by heparin affinity chromatography, size exclusion HPLC, Western blotting and immunoprecipitation. Results reveal abundance of NRG mRNA in the cultured nerves, presence of a 54 kDa NRG protein in the CM that increases along fractionation, and progressive diminution of fraction B1.2 differentiation activity on PC12 cells by gradual removal of the NRG protein by immunoprecipitation. The abundance of Schwann cells and the lack of axons in the cultured nerves suggest Schwann cells as the main NRG source, to which fibroblasts and perineurial cells might contribute.

Saliva and esophageal protection

There has been much interest in recent years in the potential protective role of saliva in the esophagus. Variables such as salivary volume and neutralizing capacity have been studied both during basal conditions and in response to esophageal acid exposure, in healthy subjects and in patients with esophagitis. In addition to its known neutralizing capacity, saliva also contains growth factors. These polypeptides (of which epidermal growth factor has been studied most) have cytoprotective and healing properties in various segments of the gastrointestinal tract. Therefore, a deficiency in one or more of these growth factors might be a contributing factor in the development of gastroesophageal reflux disease (GERD) or its complication, such as Barrett's metaplasia. However, human studies have produced contradictory results regarding salivary growth factor deficiency in such patients. Current methods of investigation make it difficult to assess the importance of saliva in GERD. This may be due in part to the multifactorial nature of the disease and the difficulty in long-term, selective manipulation of salivary function in humans. Given the present data in the literature, it is therefore unknown if saliva plays an important role in esophageal protection.

Regenerative sprouting of retinal ganglion cells of adult hamsters induced by the epineurium of a peripheral nerve

Although it is known that transplantation of a peripheral nerve (PN) to the damaged central nervous system (CNS) promotes axonal regeneration, the interactions of cellular components of the PN with CNS neurons are still not well defined. Schwann cells in the PN are thought to be the major element involved in supporting CNS regeneration, but very little information exists with regard to whether other PN components also play an active role. Using our previously established model of transplanting a PN segment into the vitreous to stimulate regenerative sprouting of retinal ganglion cells (RGCs), we found that the epineurium isolated from a PN which had been pre-injured by transection was able to induce RGC sprouting when implanted intravitreally. Since the epineurium is composed mainly of connective tissue components and is devoid of Schwann cells, our results suggest that other cellular elements of the PN besides Schwann cells may have the potential to support CNS regeneration.

Nerve growth factor and proprotein convertases furin and PC7 in transected sciatic nerves and in nerve segments cultured in conditioned media: their presence in Schwann cells, macrophages, and smooth muscle cells

Synthesis of proteins such as nerve growth factor (NGF) is induced after nerve lesion. The NGF precursor (pro-NGF) requires a posttranslational processing by proprotein convertases to become active. In this report, we re-examine the localization of NGF protein and mRNA in injured nerve and show that the candidate pro-NGF convertases furin and PC 7 colocalize with NGF in non-neuronal cells in nerve. By Northern blot analysis, 1.5-kb and 1.3-kb NGF mRNAs were shown to be increased in distal and immediately proximal nerve segments on days 1, 4, and 14 after lesion; by Western blot analysis, NGF proteins of high molecular weight were detected after injury. In vivo, two phases of NGF immunopositivity were observed, in macrophages and perivascular cells shortly after lesion and in endoneurial cells on day 1 and 4. To identify the cells containing NGF, nerve segments were incubated in serum-containing medium with or without conditioning by white blood cells isolated from the circulation. Both hybridization and immunoreactivity signals for NGF were elevated after incubation of nerve segments for 4 hours in conditioned media, so that cells with NGF immunoreactivity could be identified by antibodies to specific cell markers. In these nerve fragments, Schwann cells, perivascular smooth muscle cells, and macrophages contained NGF immunoreactivity. The concentration of furin and PC7 mRNA also increased in lesioned nerves. By immunocytochemical investigation of nerve explants, furin and PC7 were detected in endoneurial cells, macrophages and perivascular cells and were colocalized with NGF. These in vitro and in vivo findings suggest that both furin and PC7 are associated with NGF in several cell types of the sciatic nerve and, hence, may be implicated in intracellular processing of pro-NGF.

The pro-protein convertase PC1 is induced in the transected sciatic nerve and is present in cultured Schwann cells: comparison with PC5, furin and PC7, implication in pro-BDNF processing

Injury of peripheral nerves induces expression of several pro-protein convertases (PCs) involved in processing of precursor proteins into their diverse active end-products. In this study, the focus was on convertase PC1 which, although undetectable in control nerves, is strongly induced in injured nerves. High concentrations of PC1 mRNA of 9.0, 5.5, 3.0, 2.5 and 1.6 kb were observed on day 4 post-lesion in proximal and distal segments. By in situ hybridization PC1 mRNA was detected in most of endoneurial cells, which were further identified by immunocytochemistry as myelin 2', 3'-cyclic nucleotide 3'-phosphodiesterase containing Schwann cells. PC1 mRNA and protein were also present in cultured Schwann cells also containing convertases PC5, furin and PC7 as well as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF). Mostly unprocessed pro-NGF of 35 kDa and pro-BDNF of 35 kDa were found on Western blotting of Schwann cells. Expression of exogenous neurotrophins by infection with vaccinia virus vector showed that mouse pro-NGF and rat pro-BDNF are cleaved intracellularly on smaller forms of 13.5 kDa NGF and 14 kDa BDNF. Infection experiments demonstrated that Schwann cells contain active processing enzymes. In conclusion, this work provides in vivo evidence of the presence of several PCs in the injured rat sciatic nerve and ex vivo in cultured Schwann cells.

Peripheral nerve grafts exert trophic and tropic effects on anterior thalamic neurons

Peripheral nerve grafting into the central nervous system (CNS) has been used to study the regenerative capabilities of central neurons given access to a peripheral nervous system (PNS) environment. It is well documented that many CNS neurons regenerate axons along peripheral nerve grafts placed in close proximity to their cell bodies and that these grafts can ameliorate axotomy-induced retrograde degeneration. In the present study, we placed peripheral nerve grafts in proximity to axotomized neurons of the anterior thalamus. Standard histological and retrograde tracing techniques were used to examine these preparations 2 months after grafting. Three effects of these grafts were observed: amelioration of retrograde degeneration of axotomized anterior thalamic neurons, hypertrophy of many thalamic neurons in the local environment of the graft, and ingrowth of axons of axotomized anterior thalamic neurons as well as nonaxotomized neurons from surrounding nuclei. We conclude from these studies that peripheral nerve grafts not only provide a matrix for axonal outgrowth but also exert marked trophic and tropic effects on axotomized anterior thalamic neurons.

A possible role for saliva as a diagnostic fluid in patients with chronic pain

OBJECTIVES

The focus of this review was on proteins and peptides found in saliva. Of greatest interest were those neuropeptides relevant to nociception and to the pathogenesis of chronic pain syndromes. An additional goal was to develop a standardized protocol to collect saliva for laboratory assessment.

METHODS

Data were obtained through discussion with experts at the medical schools in San Antonio and Heidelberg and a Medline literature search involving all relevant studies from 1966 to 1997. The literature search was based on the following key terms: saliva, serotonin, neuropeptide, substance P (SP), calcitonin gene-related peptide (CGRP), and nerve growth factor (NGF).

RESULTS

The mean concentration of SP in the saliva of healthy normal controls ranged from 9.6 to 220 pg/mL. Generally, the concentration of SP was approximately three times higher in saliva than in plasma. In a number of painful conditions, particularly tension headache, substantial elevations of salivary SP were found. Mean values for salivary CGRP in healthy controls were approximately 22 pmol/L and were significantly elevated in patients with migraine attacks or cluster headache. There were no data to indicate prior quantitative determination of NGF in human saliva.

CONCLUSIONS

After sampling and processing techniques have been standardized, measurement of neuropeptides in human saliva could provide a valuable tool for study of patients with chronic painful disorders such as rheumatoid arthritis, osteoarthritis, and even fibromyalgia syndrome.

The expression of the low affinity nerve growth factor receptor in long-term denervated Schwann cells

Schwann cells in the distal stump of injured peripheral nerves synthesize the low affinity nerve growth factor receptor (p75). In this study we used short-term (1 week) and long-term (1-12 months) transected distal sciatic nerves of rats to determine the variations of p75 expression by using immunocytochemistry and in situ hybridization. Semi-quantitative analysis revealed that the synthesis of the protein product of the p75 gene is rapidly enhanced to reach a peak within the 1 month after denervation. After that it gradually decreased and was barely detectable 6 months following denervation. Double immunocytochemistry for p75 and the S100 protein revealed that p75 immunoreactivity is confined to the Schwann cells. Quantitative analysis of our in situ hybridization experiments revealed that the upregulation of the p75 mRNA parallels the enhanced synthesis of the corresponding protein and reaches a peak within 1 month, which is maintained until the second month after the transection and declines thereafter to reach background levels at 4 months. The electron microscopic observations reveal that the increase in the number of nuclei in the distal stump belong to severely atrophied Schwann cells and fibroblasts. Since the presence of p75 in the Schwann cells is necessary for reinnervation, our results indicate that, based on the expression of p75, the Schwann cells will provide a most suitable environment for the regenerating axons up to the first month. At later stages the ability of the Schwann cells to synthesize p75 and cell adhesion proteins such as N-CAM and GAP 43 decreases which may be one of the factors that contribute to poor functional recovery if the regenerating axons reach the distal stump after long periods of time.

Growth cones turn up concentration gradients of diffusible peripheral target-derived factors

Severed peripheral nerve axons grow across a gap in their pathway to regenerate into the distal nerve stump. At the turn of the century this observation led to the proposal that concentration gradients of factors released from the cells of the distal nerve segment orient the growth of the regenerating axons. Recently, several central nervous system target-derived factors have been shown to direct process outgrowth via concentration gradients of the factors during development. The demonstration of target-derived tropic influences in the peripheral nervous system has however remained elusive. We have examined whether concentration gradients of diffusible factors released by the cells of a length of peripheral nerve influence the outgrowth of adult sensory neuron growth cones in vitro. We demonstrate that the growth cones turn and grow up the concentration gradients, providing evidence for the presence of diffusible peripheral target-derived neurotropic factors that can act in the peripheral nervous system.

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

Axonal outgrowth and neuronal apoptosis in cultured adult mouse dorsal root ganglion preparations: effects of neurotrophins, of inhibition of neurotrophin actions and of prior axotomy

Dorsal root ganglia (L4 and L5) with attached spinal roots and nerve stumps were isolated from young adult mice and cultured in a layer of extracellular matrix material (matrigel). Within one day, a large number of axons grew out from the cut ends of the nerve and the dorsal root. The average outgrowth length was more than doubled by nerve growth factor, which also strongly increased the number of fibres, showing extensive branching. There was also a significant outgrowth stimulation by neurotrophin-3, but no observable effect by brain-derived neurotrophic factor. In preparations isolated and cultured six days after peripheral nerve transection in vivo, there was an increase in both the outgrowth length (about 1.5- to 2-fold) and in the number of axons. Stimulation of axonal outgrowth, which concerned outgrowth from both the peripheral nerve and the dorsal root, could be further enhanced by the addition of nerve growth factor to the culture. K-252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity, did not affect either the normal outgrowth or the increased outgrowth in pre-axotomized preparations, at a concentration which abolished the stimulating effects by exogenous nerve growth factor and neurotrophin-3. Under the culturing conditions used, spontaneous apoptosis occurred, but none of the neurotrophins tested, nor K-252a, affected the number of apoptotic neuronal cells analysed by nick-labelling DNA breaks at the end of a 48-h culturing period. Altogether, the present data suggest that for most dorsal root ganglia neurons, signalling through the trk receptors does not influence the apoptosis in vitro and is not required for either the spontaneous axonal outgrowth in matrigel or the increased outgrowth which occurs after prior axotomy in vivo.

Nerve growth factor enhances nerve regeneration through fibronectin grafts

Soluble fibronectin and nerve growth factor (NGF) promote axonal regeneration when placed in silicone tubes. We investigated the ability of orientated fibronectin mats to bind and release bioactive NGF and the possibility of augmenting axonal regeneration following axotomy by using fibronectin conduits impregnated with NGF. The release of NGF was quantified using a fluorometric ELISA and bioactivity confirmed with a neuronal culture bioassay. Immunohistochemical techniques and computerized image analysis were used to assess the rate and volume of axonal and Schwann cell regeneration. The delivery of NGF to the site of injury produced an increase in the rate (P < or = 0.007) and volume (P < or = 0.004) of both axonal and Schwann cell regeneration when compared to conduits of plain fibronectin. We conclude that the local delivery of NGF by impregnated fibronectin conduits enhances axonal regeneration.

Chemoattraction of sensory neuron growth cones by diffusible concentration gradients of acetylcholine

Axon guidance cues are critical for the development and repair of both the central and peripheral nervous systems. These cues serve to help select the pathways taken by axon growth cones, by attracting or repulsing them. During development and following injury to the adult peripheral nervous system, neurons must extend processes, often over long distances, through a variety of cellular environments composed of innervated and uninnervated cells, to find, recognize, and synapse on their appropriate targets. The responsibility for recognizing and responding to the extensive number of cues that are encountered as axons elongate falls on the growth cones at the tip of the elongating axons. Cajal (1928) proposed that denervated target cells release diffusible factors that assist in orienting the direction of out-growth of peripheral axons. However, it is only relatively recently that experiments to identify the molecules responsible for serving this function, and the molecular mechanisms by which they function, have begun to bear fruit. Gradients of both substrate-bound and diffusible factors have now been shown to play critical roles in directing axon outgrowth. The present experiments were aimed at determining whether the neurotransmitter acetylcholine (ACh) can act as a chemoattractant for adult sensory neuron growth cones.

Triiodothyronine exerts a trophic action on rat sensory neuron survival and neurite outgrowth through different pathways

Apart from several growth factors which play a crucial role in the survival and development of the central and peripheral nervous systems, thyroid hormones can affect different processes involved in the differentiation and maturation of neurons. The present study was initiated to determine whether triiodothyronine (T3) affects the survival and neurite outgrowth of primary sensory neurons in vitro. Dorsal root ganglia (DRG) from 19-day-old embryos or newborn rats were plated in explant or dissociated cell cultures. The effect of T3 on neuron survival was tested, either in mixed DRG cell cultures, where neurons grow with non-neuronal cells, or in neuron-enriched cultures where non-neuronal cells were eliminated at the outset. T3, in physiological concentrations, promoted the growth of neurons in mixed DRG cell cultures as well as in neuron-enriched cultures without added nerve growth factor (NGF). Since neuron survival in neuron-enriched cultures cannot be promoted by endogenous neurotrophic factors synthesized by non-neuronal cells, the increased number of surviving neurons was due to a direct trophic action of T3. Another trophic effect was revealed in this study: T3 sustained the neurite outgrowth of sensory neurons in DRG explants. The stimulatory effect of T3 on nerve fibre outgrowth was considerably reduced when non-neuronal cell proliferation was inhibited by the antimitotic agent cytosine arabinoside, and was completely suppressed when the great majority of non-neuronal cells were eliminated in neuron-enriched cultures. These results indicate that the stimulatory effect of T3 on neurite outgrowth is mediated through non-neuronal cells. It is conceivable that T3 up-regulates Schwann cell expression of a neurotrophic factor, which in turn stimulates axon growth of sensory neurons. Together, these results demonstrate that T3 promotes both survival and neurite outgrowth of primary sensory neurons in DRG cell cultures. The trophic actions of T3 on neuron survival and neurite outgrowth operate under two different pathways.

Cellular migration and axonal outgrowth from adult mammalian peripheral nerves in vitro

It is known that following peripheral nerve transections, sheath cells proliferate and migrate to form a bridge between nerve stumps, which may facilitate axonal regeneration. In the present investigations, cellular migration and axonal outgrowth from nerves of adult mice were studied in vitro using collagen gels. During the first 3 days in culture, profuse migration of fibroblasts and macrophages occurred from the ends of sciatic nerve segments, which had been lesioned in situ a few days prior to explantation, but not from segments of normal nerves. The mechanism of cellular activation in the lesioned nerves was not determined, but migration was blocked by suramin, which inhibits the actions of several growth factors. The migrating cells, which form the bridge tissue, may promote axonal regeneration in two ways. Firstly, axonal outgrowth from isolated intercostal nerves was significantly increased in co-cultures with bridges from lesioned sciatic nerves. This stimulatory effect was inhibited by antibodies to 2.5S nerve growth factor. Secondly, the segments of bridge tissue contracted when removed from animals. It is possible that fibroblasts within the bridge exert traction that would tend to pull the lesioned stumps of peripheral nerve together, as in the healing of skin wounds. The traction may also influence deposition of extracellular matrix materials, such as collagen fibrils, which could orient the growth of the regenerating axons toward the distal nerve stump.

Ciliary neurotrophic factor stimulates neurite outgrowth from spinal cord neurons

Ciliary neurotrophic factor (CNTF) has been shown to promote the survival of motoneurons, but its effects on axonal outgrowth have not been examined in detail. Since nerve growth factor (NGF) promotes the outgrowth of neurites within the same populations of neurons that depend on NGF for survival, we investigated whether CNTF would stimulate neurite outgrowth from motoneurons in addition to enhancing their survival. We found that CNTF is a powerful promoter of neurite outgrowth from cultured chick embryo ventral spinal cord neurons. An effect of CNTF on neurite outgrowth was detectable within 7 hours, and at a concentration of 10 ng/ml, CNTF enhanced neurite length by about 3- to 4-fold within 48 hours. The neurite growth-promoting effect of CNTF does not appear to be a consequence of its survival-promoting effect. To determine whether the effect of CNTF on spinal cord neurons was specific for motoneurons, we analyzed cell survival and neurite outgrowth for motoneurons labeled with diI, as well as for neurons taken from the dorsal half of the spinal cord, which lacks motoneurons. We found that the effect of CNTF was about the same for motoneurons as it was for neurons from the dorsal spinal cord. The responsiveness of a variety of spinal cord neurons to CNTF may broaden the appeal of CNTF as a candidate for the treatment of spinal cord injury or disease.

Reinnervation of denervated iris by transplanted sympathetic ganglia: effect of neuronal age

Neuronal outgrowth in vivo is aggressive postnatally, but is diminished with increasing age. This may be attributable to intrinsic features of the neuron or its interaction with other components of the developing organism. The purpose of this study was to determine if there is an age-dependent reduction in the intrinsic ability of sympathetic neurons to initiate fiber outgrowth. Superior cervical ganglia from donor rats aged 3-4, 11-12, 27-28 and 45-46 days were removed and transplanted to the anterior chamber of the sympathectomized eye of host rats 85-89 days of age. Ganglia with host irides were removed at 3, 6 and 10 days post-transplant and whole mounts were analysed using catecholamine histofluorescence for maximum sympathetic fiber density, length and initial rate of outgrowth. Fluorescent fibers were present in host irides of donors of all ages and at all post-transplant times. However, maximum fiber density was less for the 3-4-day-old donor ganglia (e.g. 43-71% of 11-46-day-old donor ganglia at 600 microns, 10 days post-transplant). Maximum fiber length was also less in the youngest group (e.g. 35-49% of 11-46-day-old donor ganglia, 10 days post-transplant). Further, the initial rate of outgrowth was decreased for the 3-4-day-old donor ganglia (128 +/- 46 microns/day for the 3-4-day-old ganglia vs 253 +/- 48 microns/day for the 11-12-day-old ganglia, 307 +/- 35 microns/day for the 27-28-day-old ganglia and 260 +/- 22 microns/day for the 45-46-day-old ganglia).(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal differentiation of PC12 and chick embryo ganglion cells induced by a sciatic nerve conditioned medium: characterization of the neurotrophic activity

The present work deals with the finding and characterization of a neurotrophic factor present in serum-free Dulbecco's modified Eagle's medium in which rat sciatic nerves previously cultured for 9 days were maintained for 24 h. This sciatic nerve conditioned medium (SNCM) produced neuronal differentiation and neurite outgrowth on PC12 cells, as well as survival and differentiation of eight-day old chick embryo dorsal root ganglion (E8-DRG) and ciliary ganglion (E8-CG) neurons. SNCM activity was decreased by dilution, heating and trypsin treatment; it was not inhibited by anti-NGF and anti-bFGF antibodies; and it was not mimicked by CNTF, laminin and fibronectin. By utilizing its neurite-promoting activity on PC12 cells, experiments oriented to purify the factor were carried out. Ultrafiltration, heparin-affinity chromatography and size-exclusion high pressure liquid chromatography (HPLC) were employed. The ability of SNCM to induce PC12 cell, E8-DRG and E8-CG neuronal differentiation, the heparin affinity of the active SNCM protein, and the size-exclusion HPLC elution characteristics of the active protein suggest that the active component of the SNCM is, in all probability, a novel sciatic nerve neurotrophic factor (SNTF).

Reinnervation of peripheral nerve segments implanted into the hemisected spinal cord estimated by transgenic mice

We investigated how far Schwann cells, which are the peripheral nerve elements supporting axonal regrowth, penetrate into the hemisected recipient spinal cord. C57BL/6 mice, which carry carcinoembryonic antigen as transgene, were used for transplantation study. These CEA transgenic mice were syngenic to C57BL/6 mice except for the expression of human CEA DNA. In the syngenic transplantation study, C57BL/6 mice were transplanted with the sciatic nerve of CEA-transgenic mice to the hemisected spinal cord. Schwann cell migration into the recipient spinal cord was detected by the PCR method. Transplanted Schwann cells migrated into the recipient spinal cord both rostrally and caudally at a distance of 2 mm from the graft-host interface until 21 days after transplantation. At 28 days after transplantation, the Schwann cells migrated rostrally at a distance of 2 mm and caudally at a distance of 4 mm. C57BL/6-CEA sciatic nerve was transplanted to BALB/C mice as the allogenic transplant. CEA DNA was detected until 14 days after transplantation, but disappeared at 21 days. In addition, C57BL/6-CEA sciatic nerves were transplanted into Wistar rats to study xenogenic transplantation. The CEA band disappeared at 10 days after transplantation. In conclusion, by using CEA transgenic mice and the PCR method, we could evaluate the mobility of Schwann cells which are thought to play an important role in axonal regeneration.

Oriented growth of regenerating axons in axolotl forelimbs is consistent with guidance by diffusible factors from distal nerve stumps

Previous studies have shown that when peripheral nerves in axolotl limbs are cut and surgically misdirected, regenerating axons grow back to the original pathways and innervate their correct muscles. In the present study however, we demonstrate that when given a choice between their correct nerve stump and an incorrect stump (forearm flexor nerve), regenerating extensor cranialis nerve axons grow towards both pathways. This result suggests that the directed growth of regenerating axons in the peripheral nervous system may be in response to factor(s) released from the distal nerve stumps, but that in this region of the limb, axons were unable to differentiate between correct and incorrect pathways. Growing axons appeared to be accompanied by neural sheath cells, whilst activated macrophages remained near the cut nerve stumps. Possible mechanisms by which regenerating axons may eventually innervate their correct targets are discussed.

Neuronal growth via hybrid system of self-growing and diffusion based grammar rules: I

The formation of neuronal networks requires axonal growth towards target neurons. A simple set of grammar rules is introduced to describe axonal growth towards target cells situated both at short and long distances from the growing neuron. Growth for short distances is described by growth following the highest gradient of a chemical compound (which is spread by diffusion from the targets). This approach fails to describe long-distance growth, which is addressed by adopting a graph grammar theory for growing trees. With these rules a flexible tool to draw network of neurons by computer can be developed.

Nerve growth factor depletion reduces collateral sprouting of cutaneous mechanoreceptive and tooth-pulp axons in ferrets

1. Electrophysiological experiments were carried out to determine whether or not collateral sprouting of cutaneous low-threshold mechanoreceptive fibres could be detected and to investigate the effect of nerve growth factor (NGF) deprivation on the sprouting of these fibres and the fibres innervating tooth pulps. 2. In twenty-one ferrets (eleven of which had been autoimmunized against NGF) the right inferior alveolar nerve (IAN) was sectioned and prevented from regenerating. After 12 weeks, transmedian innervation from the left IAN was determined by stimulating the nerve whilst recording from electrodes implanted in the contralateral anterior teeth and also by single unit recordings from the nerve whilst mechanically and electrically stimulating the skin. The results were compared with those from ten control animals. 3. Transmedian innervation of contralateral teeth was found in none of the control animals; in all ten of the animals which had undergone denervation without immunization (4/10 canines, 17/20 incisors); but in only six of the eleven immunized and denervated animals (0/11 canines, 7/22 incisors). 4. Of 270 cutaneous mechanoreceptive units sampled in the controls, only four units had transmedian receptive fields, extending a maximum of 1 mm across the mid-line. After denervation, significantly more units (42 of 274) crossed the mid-line and extended up to 4 mm. After immunization and denervation only eleven of 305 units crossed the midline by a maximum of 1 mm. 5. These data show that cutaneous low-threshold mechanoreceptive A beta and A delta fibres, as well as A delta tooth pulp fibres, are able to undergo collateral sprouting. This sprouting is partially blocked by NGF depletion, suggesting that NGF plays an essential role in the process.

Ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) was first identified and partially purified from embryonic chick eye tissues. Subsequently, it was shown that CNTF is also present in large amounts in sciatic nerves of adult rats and rabbits, which led to its final purification and cloning. CNTF is not secreted by the classical secretory pathway involving the endoplasmatic reticulum and Golgi complex, but can be detected in high quantities within the cytoplasm of myelinating Schwann cells and astrocytes using immunohistochemistry. CNTF supports survival and/or differentiation of a variety of neuronal cell types including sensory, sympathetic, and motoneurons. Also, nonneuronal cells, such as oligodendrocytes, microglial cells, liver cells, and skeletal muscle cells, respond to exogenously administered CNTF, both in vitro and in vivo. During development, expression of CNTF is very low, if indeed it is expressed at all, and the phenotype of mice lacking endogenous CNTF after inactivation of the CNTF gene by homologous recombination suggests that CNTF does not play a crucial role for responsive cells during embryonic development. However, motoneurons are lost postnatally in mice lacking endogenous CNTF, suggesting that CNTF acts physiologically on the maintenance of these cells. The ability of exogenous CNTF to protect against motoneuron loss following lesion or in other animal models indicates that CNTF might be useful in the treatment of human motoneuron disorders, provided appropriate means of administration can be found.

Motoneuron survival is not affected by the proximo-distal level of axotomy but by the possibility of regenerating axons to gain access to the distal nerve stump

The aim of this study was to examine whether axotomy-induced motoneuron death in adult mammals differ: (1) with the distance between the site of injury and the nerve cell body, and (2) if contact between the transected nerve stumps is established after the injury, compared with cases where contact is prevented. The hypoglossal nerve of adult rats was transected either proximally in the neck (proximal injury) or close to the tongue (distal injury). The nerve stumps were then either deflected from each other in order to prevent axon regeneration into the distal nerve stump, or sutured. Three months later, the extent of nerve cell loss was examined bilaterally in cresyl violet-stained sections of the hypoglossal nucleus. In addition, we examined hypoglossal neuron survival twelve months after a proximal nerve transection with prevented regeneration. Our results show that there was no significant difference in neuronal survival after a proximal nerve transection compared with a distal one, neither if contact between the nerve stumps was established nor if it was prevented. However, contact between the transected nerve stumps increased the likelihood of neuronal survival significantly after both proximally and distally located injury compared to nerve injury with prevented regeneration. There was no significant decrease in nerve cell survival after twelve months with prevented reinnervation compared with survival after three months. These observations indicate that the extent of axotomy-induced motoneuron death in adult mammals does correlate with the proximo-distal level of peripheral injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Choroid coat extract and ciliary neurotrophic factor strongly promote neurite outgrowth in the embryonic chick retina

Previous studies have shown that extracts from the target optic tectum stimulate neurite outgrowth from retinal explants. The present study indicates that the choroid coat is an even richer source of retinotrophic activity. We thus studied the effects of recombinant rat ciliary neurotrophic factor (CNTF) on primary cultures of dissociated chick ciliary ganglion neurons and retinal explants for a comparison with choroid coat extract from the E18 chick. For our assays, E9 ciliary neurons were incubated in collagen gels and retinal explants were cultured on collagen gels with the addition of the trophic factors and maintained for two or four days. Survival of ciliary neurons per area as well as maximal neurite length in retinal cultures were determined. Growth responses occurred in a dose-dependent manner both to CNTF and choroid extract. Immunofluorescence examination of cells and developing processes showed 200 kdal neurofilament positivity demonstrating that the cells studied were neurons with neurites. It is concluded that a trophic activity of the choroid as well as the recombinant CNTF stimulate retinal neuron survival and neurite extension. The results suggest that CNTF may have developmental functions in the establishment of the visual pathways.

Neurotrophic influence of denervated sciatic nerve on adult dorsal root ganglion neurons

Isolated adult frog dorsal root ganglion neurons survive in vitro in a defined medium for more than 4 weeks and extend processes. When co-cultured with a 1-mm piece of peripheral nerve the average total process length per neuron was 10 times longer than that of control neurons by 8 days, and the processes had a significantly different morphology from that of control neurons. This influence on process length increased with increasing time of nerve denervation prior to co-culturing. These results suggest the release of a neurotrophic factor/s from the cells of the peripheral nerve. The neurotrophic influence was completely blocked by antibodies against mouse nerve growth factor (NGF). Although NGF increased the average process length by twofold over control neurons, its influence nerve reached that of the nerve-released factor, and the NGF-induced processes had a distinctly different morphology. The frog nerve-released factor promoted process outgrowth from E11 chick sympathetic ganglia, although the process number, length, and their fasciculation differed greatly from those induced by NGF. These results suggest that the nerve-released factor/s are immunologically and functionally related to NGF but have not established whether a single factor or an aggregate of several secreted molecules are responsible. This article presents a new preparation in which the varied influences of different neurotrophic factors can be studied in great detail on large populations of isolated adult vertebrate neurons and sets the stage for the characterization and isolation of the frog peripheral nerve neurotrophic factor, as well as examining the influence of this factor on neuronal morphology and its ability to direct process outgrowth.

Long-term effects of deprivation of cell support in the distal stump on peripheral nerve regeneration

The distal stump of an injured peripheral nerve supports regenerating axons by offering a favourable growth substratum and several cell-produced growth factors. Deprivation of cellular factors alone has been shown not to prevent fairly rapid axonal elongation after nerve injury if the growth substratum was preserved. The present study examined possible long-term untoward effects of cell support deprivation during an early phase of nerve regeneration. Rat sciatic nerve was crushed and a 25 mm long distal nerve segment was made acellular by freezing-thawing, while the integrity of the growth substratum for the regenerating axons was preserved. Toe-spreading reflex and skin sensitivity to pinch in the foot were monitored to follow recovery of motor and sensory function, respectively. The number of myelinated axons was determined in the sciatic nerve proximally to the lesion site, and distally in the predominantly sensory sural nerve as well as in the mixed motor nerve to the soleus muscle. Except for a short delay in the onset of recovery, explainable by the reduced elongation rate of axons growing through the acellular nerve segment, we found no deleterious effect of cell support deprivation on sensory or motor function recovery after nerve crush. Most of regenerating sensory neurons did not critically depend on the distal stump cell support. However, a 15% and 25% loss of myelinated axons both proximally to the lesion and distally in the sensory sural nerve, respectively, indicated that a corresponding minor loss of injured sensory neurons occurred when they were deprived of such cell support even if provided with a favourable growth substratum for successful regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

An improved procedure for the immunohistochemical localization of nerve growth factor-like immunoreactivity

Nerve growth factor (NGF) is a survival factor required by a number of neuronal populations including most post-ganglionic sympathetic neurones. NGF has been detected and quantified in many tissues but there is little information regarding its cellular localization. Although it has been argued that histological detection has proven difficult due to the low levels of NGF present, other factors may contribute to prevent its identification. In the present study, we report a method for the histological detection of NGF-like immunoreactivity in the rat superior cervical ganglia (SCG). Adult Wistar-Kyoto rats were perfused briefly with either a high or low pH buffer prior to fixation and routine immunohistochemistry. Polyclonal antibodies to native mouse NGF used in the present study recognized mouse NGF but not recombinant human neurotrophin 3 (rhNT3) or brain-derived neurotrophic factor (rhBDNF) by immunoblot analysis. NGF-like immunoreactivity was localized to most sympathetic neurones. Immunoreactivity was detected in the cytoplasm with dense labelling around nuclei. No stain was seen in sections incubated with normal sheep IgG or from animals perfused with phosphate buffer (pH 7.4) prior to fixation. In addition, axotomy resulted in the disappearance of NGF immunoreactivity which was confirmed by biochemical quantification. Finally, no NGF immunoreactivity was found in neurones of rats treated systemically with NGF antiserum 3 days earlier. Possible mechanisms underlying the improvement of NGF immunohistochemistry by pH manipulation before fixation are discussed.

Promoting and directing axon outgrowth

Establishment of appropriate neuronal connections during development and regeneration requires the extension of processes that must then grow in the correct direction, find and recognize their targets, and make synapses with them. During development, embryonic neurons gradually establish central and peripheral connections in an evolving cellular environment in which neurotrophic factors are provided by supporting and target cells that promote neuronal survival, differentiation, and process outgrowth. Some cells also release neurotropic factors that direct the outgrowth of neuronal processes toward their targets. Following development the neurotrophic requirements of some adult neurons change so that, although they respond to neurotrophic factors, they no longer require exogenous neurotrophins to survive or to extend processes. Within the central nervous system (CNS), the ability of neurons to extend processes is eventually lost because of a change in their cellular environment from outgrowth permissive to inhibitory. Thus, neuronal connections that are lost in the adult CNS are rarely reestablished. In contrast, the environment of the adult peripheral nervous system fosters process outgrowth and synapse formation. This article discusses the neurotrophic requirements of embryonic and adult neurons, as well as the importance of neurotropic factors in directing the outgrowth of regenerating adult axons.

Ciliary neurotrophic factor: a review

Ciliary neurotrophic factor (CNTF) is a 22-kDa protein predicted to share with leukemia inhibitory factor (LIF) and interleukin-6 a common amphipathic helical domain. Consistent with this prediction, the CNTF receptor complex is composed of the CNTF alpha receptor, the LIF beta receptor and gp130 a signalling molecule for LIF and interleukin-6. The major sources of synthesis of CNTF are Schwann cells and astrocytes, but it remains unclear how much CNTF is released from these glial cells and by what mechanism. In vitro, CNTF supports the survival of all classes of peripheral nervous system neurons plus many CNS neurons, induces neurite outgrowth, promotes a cholinergic phenotype in sympathetic neurons and arrests division of neuronal precursor cells. Several cell lines also respond to CNTF. In vivo, CNTF rescues several types of neurons from axotomy-induced death. The functions of CNTF in the development and maintenance of the nervous system remain enigmatic.

Median nerve neurotization by peripheral nerve grafts directly implanted into the spinal cord: anatomical, behavioural and electrophysiological evidences of sensorimotor recovery

Over the years, peripheral nerve grafts, a favorable environment to support axonal elongation, have given rise to increasing interest as a possible solution for promoting spinal cord repair. In the experiments described here, following an avulsion injury of the rat brachial plexus, the median nerve was repaired by a peripheral nerve graft (PN) inserted directly into the dorsal side of the spinal cord. Eight months later the animals were submitted to behavioral tests, electrophysiological and histological studies. Regrowth of axons from both motoneurons and ganglionic neurons was demonstrated following a single superficial dorsal implantation of a PN. Sensorimotor peripheral reinnervation allowed most of the studied animals to recover enough flexor activity for grasping. Reinnervation was achieved even without prior root avulsion suggesting that the presence of a PN is sufficient to induce sprouting in the spinal cord from axotomized and non-axotomized neurons.

Repair of digital nerve defect with autogenous vein graft during flexor tendon surgery in zone 2

Autogenous vein graft was used to fill 18 digital nerve defects between 0.5 to 5.8 cm in length during flexor tendon surgery in zone 2. The vein was taken from the forearm and reversed to bridge the digital nerve. For nerves with defects over 2.0 cm, normal nerve slices were inserted inside vein conduits. Recovery of sensibility was evaluated by von Frey test, pin-prick detection, localization of stimulus, moving two-point discrimination and sweating on the finger pulp. Follow-up revealed excellent recovery in two digital nerves, good in nine, fair in five and poor in two. The results suggest that vein graft provides a simple and practical method to reconstruct a digital nerve defect during tendon repair in zone 2.