Use of explant cultures of peripheral nerves of adult vertebrates to study axonal regeneration in vitro

Fabrication of a Co-Culture System with Human Sweat Gland-Derived Cells and Peripheral Nerve Cells

The interaction of peripheral nerves with different cells of the skin is a relevant aspect of many physiological processes including nociception, temperature control, and wound healing. Here we describe a protocol for the setup of an indirect co-culture system of peripheral nerve cells and sweat gland-derived stem cells, which can be used to quantify neurite outgrowth.

Establishing an Organotypic System for Investigating Multimodal Neural Repair Effects of Human Mesenchymal Stromal Stem Cells

Human mesenchymal stromal stem cells (hMSCs) hold regenerative medicine potential due to their availability, in vitro expansion readiness, and autologous feasibility. For neural repair, hMSCs show translational value in research on stroke, spinal cord injury (SCI), and traumatic brain injury. It is pivotal to establish multimodal in vitro systems to investigate molecular mechanisms underlying neural actions of hMSCs. Here, we describe a platform protocol on how to set up organotypic co-cultures of hMSCs (alone or polymer-scaffolded) with explanted adult rat dorsal root ganglia (DRGs) to determine neural injury and recovery events for designing implants to counteract neurotrauma sequelae. We emphasize in vitro hMSC propagation, polymer scaffolding, hMSC stemness maintenance, hMSC-DRG interaction profiling, and analytical formulas of neuroinflammation, trophic factor expression, DRG neurite outgrowth and tropic tracking, and in vivo verification of tailored implants in rodent models of SCI. © 2018 by John Wiley & Sons, Inc.

Effect of Exosomes from Rat Adipose-Derived Mesenchymal Stem Cells on Neurite Outgrowth and Sciatic Nerve Regeneration After Crush Injury

Peripheral nerve injury requires optimal conditions in both macro-environment and microenvironment for promotion of axonal regeneration. However, most repair strategies of traumatic peripheral nerve injury often lead to dissatisfying results in clinical outcome. Though various strategies have been carried out to improve the macro-environment, the underlying molecular mechanism of axon regeneration in the microenvironment provided by nerve conduit remains unclear. In this study, we evaluate the effects of from adipose-derived mesenchymal stem cells (adMSCs) originating exosomes with respect to sciatic nerve regeneration and neurite growth. Molecular and immunohistochemical techniques were used to investigate the presence of characteristic exosome markers. A co-culture system was established to determine the effect of exosomes on neurite elongation in vitro. The in vivo walking behaviour of rats was evaluated by footprint analysis, and the nerve regeneration was assessed by immunocytochemistry. adMSCs secrete nano-vesicles known as exosomes, which increase neurite outgrowth in vitro and enhance regeneration after sciatic nerve injury in vivo. Furthermore, we showed the presence of neural growth factors transcripts in adMSC exosomes for the first time. Our results demonstrate that exosomes, constitutively produced by adMSCs, are involved in peripheral nerve regeneration and have the potential to be utilised as a therapeutic tool for effective tissue-engineered nerves.

Aquaporin-1 water permeability as a novel determinant of axonal regeneration in dorsal root ganglion neurons

Dorsal root ganglion (DRG) neurons transduce peripheral pain signals through small-diameter, non-myelinated C-fibers, which, when injured, can regenerate to restore pain sensation. Water channel aquaporin-1 (AQP1) is expressed at the plasma membrane of cell bodies and axons of DRG neurons, where it modulates the sensing of certain types of pain. Here, we found that AQP1 is also involved in DRG axonal growth and regeneration by a mechanism that may involve water transport-facilitated extension of axonal outgrowths. Spontaneous and nerve growth factor-stimulated axonal extension was reduced in cultures of AQP1-deficient DRG neurons and DRG explants compared to the wildtype. Axonal growth in AQP1-deficient DRG cultures was rescued by transfection with AQP1 or a different water-transporting AQP (AQP4), but not by a non-water-transporting AQP1 mutant. Following sciatic nerve compression injury AQP1 expression was increased in DRG neurons in wildtype mice, and DRG axonal growth was impaired in AQP1-deficient mice. Our results indicate AQP1 as a novel determinant of DRG axonal regeneration and hence a potential therapeutic target to accelerate neuronal regeneration.

Studying axonal degeneration and regeneration using in vitro and in vivo models: the translational potential

ABSTRACT: 

Since the initial studies by Cajal, multiple models of peripheral nerve degeneration and regeneration have been developed to address the ever-increasing complexity of the mechanisms involved in regeneration. In vitro models offer the principal benefit of a system that can be readily manipulated to address specific mechanistic questions in a deconstructed system. However, in vitro models can be overly simplified and intricacies of the interactions between neurons and glia can be lost. In vivo animal models seek to remedy some of these shortcomings, but most in vivo animal systems fail to precisely model human nerve regeneration. Rodent models of chronic nerve regeneration have been developed to better recapitulate human nerve regeneration, but are not widely used. An important development in the field has been the establishment of experimental nerve regeneration in humans, involving the reinnervation of the epidermis after cutaneous axotomy or topical capsaicin application. Use of such human models will likely accelerate the development and evaluation of new drugs that enhance peripheral nerve regeneration.

Axonal growth towards Xenopus skin in vitro is mediated by matrix metalloproteinase activity

We have previously demonstrated that the growth of peripheral nervous system axons is strongly attracted towards limb buds and skin explants in vitro. Here, we show that directed axonal growth towards skin explants of Xenopus laevis in matrigel is associated with expression of matrix metalloproteinase (MMP)-18 and also other MMPs, and that this long-range neurotropic activity is inhibited by the broad-spectrum MMP inhibitors BB-94 and GM6001. We also show that forced expression of MMP-18 in COS-7 cell aggregates enhances axonal growth from Xenopus dorsal root ganglia explants. Nidogen is the target of MMPs released by cultured skin in matrigel, whereas other components remain intact. Our results suggest a novel link between MMP activity and extracellular matrix breakdown in the control of axonal growth.

Fabrication of growth factor- and extracellular matrix-loaded, gelatin-based scaffolds and their biocompatibility with Schwann cells and dorsal root ganglia

One of the most exciting new avenues of research to repair the injured spinal cord is to combine cells for implantation with scaffolds that protect the cells and release growth factors to improve their survival and promote host axonal regeneration. To realize this goal, we fabricated biodegradable, photocurable gelatin tubes and membranes for exploratory in vitro studies. Detailed methods are described for their fabrication with a high gelatin concentration. Gelatin membranes fabricated in the same way as tubes and photo-co-immobilized with rhBDNF or rhNT-3, with or without Schwann cells (SCs), showed an initial burst of neurotrophin release within 24 h, with release diminishing progressively for 21 days thereafter. SCs attained their typical bipolar conformation on membranes without neurotrophins but adhesion, alignment and proliferation were improved with neurotrophins, particularly rhBDNF. When dorsal root ganglion explants were cultured on membranes containing laminin and fibronectin plus both neurotrophins, neurite outgrowth was lengthier compared to combining one neurotrophin with laminin and fibronectin. Thus, these gelatin membranes allow SC survival and effectively release growth factors and harbor extracellular matrix components to improve cell survival and neurite growth. These scaffolds, based on the combination of cross-linked gelatin technology and incorporation of neurotrophins and extracellular matrix components, are promising candidates for spinal cord repair.

Major histocompatibility complex class I-mediated inhibition of neurite outgrowth from peripheral nerves

Studies of mice deficient in classical major histocompatability complex class I (MHCI) revealed that MHCI plays an important role in neurodevelopment in the central nervous system. We previously studied the effects of recombinant MHCI molecules on wildtype retina explants and observed that MHCI can inhibit retina neurite outgrowth, with self-MHCI molecules having greater inhibitory effect than non-self MHCI molecules. Here, we examined classical MHCI's effects on axon outgrowth from neurons of the peripheral nervous system (PNS). We used the embryonic dorsal root ganglia (DRG) explant model since their neurons express MHCI and because DRG explants have been widely used to assess the effects of molecules on axonal outgrowth from PNS neurons. We observed that picomolar levels of a recombinant self-MHCI molecule, but not non-self MHCI molecules, inhibited axon outgrowth from DRG explants. This differential sensitivity to self- vs. non-self MHCI suggests that early in development, self-MHCI may "educate" PNS neurons to express appropriate MHCI receptors, as occurs during natural killer cell development. Furthermore, we observed that a MHCI tetramer stained embryonic DRG neurons, indicating the expression of classical MHCI receptors. These results suggest that MHCI and MHCI receptors play roles during early stages of PNS development and may provide new targets of therapeutic strategies to promote neuronal outgrowth after PNS injury.

Erythropoietin promotes functional recovery and enhances nerve regeneration after peripheral nerve injury in rats

BACKGROUND AND PURPOSE

EPO has been shown to have beneficial effects in a variety of CNS injury models. The purpose of this study was to evaluate the effects of EPO on nerve regeneration and functional recovery in a rat model of peripheral nerve surgery.

MATERIALS AND METHODS

The sciatic nerve of the rat with a 10-mm defect was bridged with a silicone rubber tube. Forty adult male Sprague-Dawley rats were assigned to the control or experimental groups to receive an intraperitoneal injection of NGF (2000 U/kg daily for 2 weeks) or EPO (5000 U/kg daily for 2 weeks), respectively. Macroscopic, functional, electrophysiologic, ultraminiature, and histologic assessments of nerves were performed 4-8 weeks after surgery.

RESULTS

The results showed that in EPO-treated rats, there was a significant increase in the axon diameter, myelin thickness, and total number of nerve fibers as well as the degree of maturity of regenerated myelinated nerve fibers in comparison with those rats not treated with EPO. In addition, as measured by the SFI and MNCV, the motor function of the re-innervated hind limbs of rats with EPO treatment significantly improved at week 8, whereas there was no significant difference in the motor function between the 2 groups at 4 weeks.

CONCLUSIONS

Our results demonstrated that EPO is able to enhance nerve regeneration and promote functional recovery after peripheral nerve injury in the rat, suggesting the potential clinical application of EPO for the treatment of peripheral nerve injury in humans.

An in vitro model of adult mammalian nerve repair

The role of pathway-derived growth factors in the support of peripheral axon regeneration remains elusive. Few appropriate knock-out mice are available, and gene silencing techniques are rarely 100% effective. To overcome these difficulties, we have developed an in vitro organotypic co-culture system that accurately models peripheral nerve repair in the adult mammal. Spinal cord sections from P4 mice that express YFP in their neurons are used to innervate segments of P4 peripheral nerve. This reconstructed ventral root is then transected and joined to a nerve graft. Growth of axons across the nerve repair and into the graft can be imaged repeatedly with fluorescence microscopy to define regeneration speed, and parent neurons can be labeled in retrograde fashion to identify contributing neurons. Nerve graft harvested from adult mice remains viable in culture by both morphologic and functional criteria. Motoneurons are supported with GDNF for the first week in culture, after which they survive axotomy, and are thus functionally adult. This platform can be modified by using motoneurons from any genetically modified mouse that can be bred to express XFP, by harvesting nerve graft from any source, or by treating the culture systemically with antibodies, growth factors, or pathway inhibitors. The regeneration environment is controlled to a degree not possible in vivo, and the use of experimental animals is reduced substantially. The flexibility and control offered by this technique should thus make it a useful tool for the study of regeneration biology.

Peripheral sensory axon growth: from receptor binding to cellular signaling

Regeneration following axonal injury of the adult peripheral sensory nervous system is heavily influenced by factors located in a neuron's extracellular environment. These factors include neurotrophins, such as Nerve Growth Factor (NGF) and the extracellular matrix, such as laminin. The presence of these molecules in the peripheral nervous system (PNS) is a major contributing factor for the dichotomy between regenerative capacities of central vs. peripheral neurons. Although PNS neurons are capable of spontaneous regeneration, this response is critically dependent on many different factors including the type, location and severity of the injury. In this article, we will focus on the plasticity of adult dorsal root ganglion (DRG) sensory neurons and how trophic factors and the extracellular environment stimulate the activation of intracellular signaling cascades that promote axonal growth in adult dorsal root ganglion neurons.

Enhancement of axonal regeneration by in vitro conditioning and its inhibition by cyclopentenone prostaglandins

Axonal regeneration is enhanced by the prior ;conditioning' of peripheral nerve lesions. Here we show that Xenopus dorsal root ganglia (DRG) with attached peripheral nerves (PN-DRG) can be conditioned in vitro, thereafter showing enhanced neurotrophin-induced axonal growth similar to preparations conditioned by axotomy in vivo. Actinomycin D inhibits axonal outgrowth from freshly dissected PN-DRG, but not from conditioned preparations. Synthesis of mRNAs that encode proteins necessary for axonal elongation might therefore occur during the conditioning period, a suggestion that was confirmed by oligonucleotide microarray analysis. Culturing PN-DRG in a compartmentalized system showed that inhibition of protein synthesis (but not RNA synthesis) in the distal nerve impaired the conditioning response, suggesting that changes in gene expression in cultured DRG depend on the synthesis and retrograde transport of protein(s) in peripheral nerves. The culture system was also used to demonstrate retrograde axonal transport of several proteins, including thioredoxin (Trx). Cyclopentenone prostaglandins, which react with Trx, blocked the in vitro conditioning effect, whereas inhibition of other signalling pathways thought to be involved in axonal regeneration did not. This suggests that Trx and/or other targets of these electrophilic prostaglandins regulate axonal regeneration. Consistent with this hypothesis, morpholino-induced suppression of Trx expression in dissociated DRG neurons was associated with reduced neurite outgrowth.

Erythropoietin accelerates functional recovery after peripheral nerve injury

BACKGROUND

Erythropoietin is a naturally occurring hormone with multiple effects on a number of different cell types. Recent data have suggested neuroprotective and perhaps even neurotrophic roles for erythropoietin. We hypothesized that these functional effects could be demonstrable in standard models of peripheral nerve injury.

METHODS

Experiments were undertaken to evaluate the effect of erythropoietin on the previously reported standard course of healing of sciatic injuries in mice. The injury groups included mice that were subjected to (1) sham surgery, (2) a calibrated sciatic crush injury, (3) transection of the sciatic nerve followed by epineural repair, or (4) a transection followed by burial of the proximal stump in the adjacent muscle tissue (neurectomy). Either erythropoietin or saline solution was administered to the mice in each of these experimental groups twenty-four hours preinjury, immediately after surgical creation of the injury, twenty-four hours postinjury, or one week postinjury. All mice were evaluated on the basis of the published model for recovery of sciatic nerve motor function by measuring footprint parameters at specific times after the injury. Immunohistochemistry was also performed to assess the erythropoietin-receptor expression profile at the site of injury.

RESULTS

In general, the mice treated with erythropoietin recovered sciatic nerve motor function significantly faster than did the untreated controls. This conclusion was based on a sciatic function index that was 60% better in the erythropoietin-treated mice at seven days postinjury (p < 0.05). Although the group that had been given the erythropoietin immediately postinjury showed the best enhancement of recovery, the timing of the administration of the drug was not critical. Histological analysis demonstrated enhanced erythropoietin-receptor positivity in the nerves that recovered fastest, suggesting that accelerated healing correlates with expression of the receptor in nerve tissue.

CONCLUSIONS

Erythropoietin treatment of an acute sciatic nerve crush injury leads to an effect consistent with functional neuroprotection. This protective effect may have clinical relevance, especially since it was detectable even when erythropoietin had been administered up to one week after injury.

Effects of collagen 1, fibronectin, laminin and hyaluronic acid concentration in multi-component gels on neurite extension

Recovery after peripheral nerve injury remains a significant challenge. Extracellular matrix proteins and hydrogels of extracellular matrix components have been shown to improve regeneration in peripheral nerve entubulation models, especially over long distances. The chemical properties, ligand identity and density, and mechanical properties of the hydrogel can affect neurite extension. However, the importance of combinatorial effects between different components in co-gels of several extracellular matrix components is unclear. In this study, we investigated neurite extension from explanted dorsal root ganglia cultured within co-gels made from laminin, fibronectin, collagen 1 and hyaluronic acid. Laminin had a strong, dose-dependent effect on both neurite length and outgrowth. Fibronectin was slightly, but generally not significantly, inhibitory to neurite extension. The concentration of collagen 1 and hyaluronic acid did not have significant effects on neurite extension. The combinatorial effects among the four components were additive rather than synergistic. A co-gel made with 1.5 mg/ml collagen 1 and 1.5 mg/ml laminin was optimum in this study, resulting in an average neurite length of 1532 +/- 91 microm versus 976 +/- 32 microm for controls, and an increase in overall volume outgrowth (reflecting neurite length and branching) of 85.9+/-9.3% over controls. This co-gel provides a mechanically stable scaffold with high ligand density and biochemical affinity. The results of this study support the use of co-gels of laminin and collagen 1 for promoting regeneration in peripheral nerve injuries and suggest that interactions among hydrogel components are not significant.

Three-dimensional culture models of normal and malignant breast epithelial cells

Extracellular matrix is a key regulator of normal homeostasis and tissue phenotype. Important signals are lost when cells are cultured ex vivo on two-dimensional plastic substrata. Many of these crucial microenvironmental cues may be restored using three-dimensional (3D) cultures of laminin-rich extracellular matrix (lrECM). These 3D culture assays allow phenotypic discrimination between nonmalignant and malignant mammary cells, as the former grown in a 3D context form polarized, growth-arrested acinus-like colonies whereas the latter form disorganized, proliferative and nonpolar colonies. Signaling pathways that function in parallel in cells cultured on plastic become reciprocally integrated when the cells are exposed to basement membrane-like gels. Appropriate 3D culture thus provides a more physiologically relevant approach to the analysis of gene function and cell phenotype ex vivo. We describe here a robust and generalized method for the culturing of various human breast cell lines in three dimensions and describe the preparation of cellular extracts from these cultures for molecular analyses. The procedure below describes the 3D 'embedded' assay, in which cells are cultured embedded in an lrECM gel (Fig. 1). By lrECM, we refer to the solubilized extract derived from the Engelbreth-Holm-Swarm mouse sarcoma cells. For a discussion of user options regarding 3D matrices, see Box 1. Alternatively, the 3D 'on-top' assay, in which cells are cultured on top of a thin lrECM gel overlaid with a dilute solution of lrECM, may be used as described in Box 2 (Fig. 1 and Fig. 2).

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.

Optimizing neurotrophic factor combinations for neurite outgrowth

Most neurotrophic factors are members of one of three families: the neurotrophins, the glial cell-line derived neurotrophic factor family ligands (GFLs) and the neuropoietic cytokines. Each family activates distinct but overlapping cellular pathways. Several studies have shown additive or synergistic interactions between neurotrophic factors from different families, though generally only a single combination has been studied. Because of possible interactions between the neurotrophic factors, the optimum concentration of a factor in a mixture may differ from the optimum when applied individually. Additionally, the effect of combinations of neurotrophic factors from each of the three families on neurite extension is unclear. This study examines the effects of several combinations of the neurotrophin nerve growth factor (NGF), the GFL glial cell-line derived neurotrophic factor (GDNF) and the neuropoietic cytokine ciliary neurotrophic factor (CNTF) on neurite outgrowth from young rat dorsal root ganglion (DRG) explants. The combination of 50 ng ml(-1) NGF and 10 ng ml(-1) of each GDNF and CNTF induced the highest level of neurite outgrowth at a 752 +/- 53% increase over untreated DRGs and increased the longest neurite length to 2031 +/- 97 microm compared to 916 +/- 64 microm for untreated DRGs. The optimum concentrations of the three factors applied in combination corresponded to the optimum concentration of each factor when applied individually. These results indicate that the efficacy of future therapies for nerve repair would be enhanced by the controlled release of a combination of neurotrophins, GFLs and neuropoietic cytokines at higher concentrations than used in previous conduit designs.

A conditioning lesion enhances sympathetic neurite outgrowth

Axonal regeneration can be influenced by a conditioning lesion (an axonal injury made prior to a second test lesion). Previously, sympathetic neurons in vivo were shown to respond to a conditioning lesion with decreased neurite outgrowth, in contrast to the enhanced outgrowth observed in all other peripheral neurons examined. The present experiments tested the effects of a conditioning lesion on neurite outgrowth in vitro from the superior cervical ganglion (SCG) and the impact of several factors on that response. Ganglia axotomized 1 week earlier and then explanted in Matrigel or collagen gel responded with a significant increase in neurite extension compared to sham-operated ganglia. A distal axotomy produced by unilateral removal of the salivary glands (sialectomy) caused an increase in neurite outgrowth similar to that of a proximal axotomy. These conditioning lesions induced both an increase in the rate of elongation, and, in the case of the proximally axotomized SCG, a shorter initial delay of outgrowth. The enhanced outgrowth following sialectomy was specific to the nerve containing the majority of axons projecting to the salivary glands, suggesting that the conditioning lesion effect is restricted to previously injured neurons. Deletion of the gene for leukemia inhibitory factor (LIF), a gene induced by axotomy, did not abolish the conditioning lesion effect in SCG explants or dissociated cell cultures. In conclusion, sympathetic neurons are capable of responding to a conditioning lesion with increased neurite outgrowth. The hypothesis that the neuronal cell body response to axotomy plays an important role in the conditioning lesion response is discussed.

Amphiregulin acts as an autocrine survival factor for adult sensory neurons

We studied the effect of amphiregulin on axonal outgrowth and survival in sensory neurons in organ cultured and dissociated mouse dorsal root ganglia (DRG). Amphiregulin at 20 ng/ml stimulated axonal outgrowth in both preparations. The EGF receptor inhibitor AG1478 inhibited outgrowth at 10 microM but not at 1 microM, where it abolished the stimulatory effects of amphiregulin. Fluoro-Jade staining and neuronal counting showed that more neurons survived in culture in the presence of amphiregulin while AG1478 at 10 microM but not 1 microM increased cell death. Small and medium sized neurons were immunopositive for both amphiregulin and the EGF receptor. Taken together these results suggest that amphiregulin can act as an autocrine survival factor for sensory neurons and stimulate axonal outgrowth through the EGF receptor.

Multidimensional long-term time-lapse microscopy of in vitro peripheral nerve regeneration

In order to test the effectiveness of a new advanced time-lapse microscopy imaging and image processing and analysis system, and to do quantitative and qualitative temporal analyses of in vitro peripheral nerve regeneration, long-term time-lapse imaging of cultures of mouse dorsal root ganglia (DRGs) was performed. DRGs were placed in a Petri dish, covered with collagen gel, their attached peripheral nerves were cut in the middle, creating a gap, and the dish was filled with culture medium. Six preparations were kept on the time-lapse imaging system, which provides a suitable incubation environment and enables to capture images from multiple coordinates at x,y,z axes at desired time intervals for 13 days. In general, the time-lapse imaging system proved quite stable and efficient, although some improvements are certainly required. Two main components of peripheral nerve regeneration, outgrowth of axons and activities of resident cells, were examined. Axons started to grow during the first hour of incubation with a 16.5 microm/h rate and showed the slowest rates (0.7 microm/h) on days 8 and 9, after which they resumed higher speeds again. The first cell came out of the proximal end of the cut nerve on the second day and it was a Schwann cell (SC), which was the prominent cell type in the preparations throughout the experiment. SCs were higher in number (83.15% of all cells) but slower in migration (3.4 vs. 7.3 microm/h, P < 0.001) than other cells. Other observed characteristics of axonal outgrowth and cellular activity and interactions between axons and the cells are discussed.

Neurotrophin-independent attraction of growing sensory and motor axons towards developing Xenopus limb buds in vitro

The mechanisms for directing axons to their targets in developing limbs remain largely unknown though recent studies in mice have demonstrated the importance of neurotrophins in this process. We now report that in co-cultures of larval Xenopus laevis limb buds with spinal cords and dorsal root ganglia of Xenopus and axolotl (Ambystoma mexicanum) axons grow directly to the limb buds over distances of up to 800 microm and in particular to sheets of epidermal cells which migrate away from the limb buds and also tail segments in culture. This directed axonal growth persists in the presence of trk-IgG chimeras, which sequester neurotrophins, and k252a, which blocks their actions mediated via trk receptors. These findings indicate that developing limb buds in Xenopus release diffusible factors other than neurotrophins, able to attract growth of sensory and motor axons over long distances.

Role of phosphatidylinositol 3-kinase in neuronal survival and axonal outgrowth of adult mouse dorsal root ganglia explants

Adult ganglionic peripheral neurons have lost dependence on target-derived neurotrophin signaling for survival and regeneration after injury. To understand the mechanisms required to sustain such processes at maturity, we are studying neuronal survival and axonal outgrowth of adult mouse dorsal root ganglia (DRG) explants. We have here examined the role of phosphatidylinositol 3-kinase (PI3-K) activity. Both neuronal survival and axonal outgrowth of spontaneously growing preparations were decreased significantly by the PI3-K inhibitor LY294002 as was the increased outgrowth caused by nerve growth factor or glial cell line-derived factor. Inhibition of PI3-K activity promoted neuronal cell death to the same extent in the presence as in the absence of a growth factor, whereas inhibition of mitogen-activated protein kinase, MAPK, lacked effect. Using a compartmentalized system, it could be shown that only axonal outgrowth was decreased when the outgrowth region only was exposed to LY294002. Already-formed growth cones showed morphological changes within 5-10 min after exposure to LY294002. Akt (PKB) is one downstream effector of PI3-K. Immunofluorescence revealed the presence of activated Akt in DRG cell bodies and in axonal growth cones. Immunoreactivity was decreased by PI3-K inhibition. The results suggest that Akt is constitutively active in adult DRG neurons, and that PI3-K mediated processes are involved in neuronal survival of one or more DRG neuronal subpopulations and also in axonal elongation. The possible significance of Akt signaling for these effects is discussed.

Involvement of alpha7beta1 integrin in the conditioning-lesion effect on sensory axon regeneration

Conditioning lesions of peripheral nerves improve axonal regeneration after injury and involve changes in expression of proteins required for axonal growth. Integrin alpha7beta1 expression in motor and sensory neurons increases following nerve lesions and motor axon regeneration is impaired in alpha7 integrin KO mice (J. Neurosci. 20, 1822-1830). To investigate the role of alpha7beta1 integrin in sensory axon regeneration, dorsal root ganglia of adult mice were cultured in gels of laminin-rich extracellular matrix (Matrigel) or collagen. Normal dorsal root ganglia in Matrigel or collagen supplemented with laminin showed spontaneous axonal outgrowth, which was greatly increased in conditioned preparations, but only in the presence of laminin. Conditioned dorsal root ganglia from normal mice cultured with a blocking antibody to beta1 integrin and from alpha7 integrin KO mice showed reduced axonal growth in both Matrigel- and laminin-supplemented collagen gels. Enhanced axonal regeneration after conditioning lesions therefore involves increased responsiveness to laminin and integrin alpha7beta1 expression.

Hyperglycemia triggers abnormal signaling and proliferative responses in Schwann cells

Peripheral neuropathy is a serious diabetic complication. Delayed nerve regeneration in diabetic animal models suggests abnormalities in proliferation/differentiation of Schwann cells (SC). We recently reported that endothelins (ETs) regulate proliferation and phenotype in primary and immortalized SC (iSC). We now investigated changes in the effects of ETs on SC proliferation and signaling in nerve segments from streptozotocin-induced diabetic rats and in iSC exposed to high glucose. Cultured explants from diabetic rats displayed a delay in the time-course of [3H]-thymidine incorporation as well as enhanced sensitivity to endothelin-1 (ET-1) or insulin. iSC cultured in high (25 mM) glucose-containing media also exhibited higher [3H]-thymidine incorporation, along with an enhanced activation of p38 mitogen-activated protein kinase and phospholipase C in response to ET-1 or platelet-derived growth factor as compared to controls (5.5 mM glucose). These studies support an extra-vascular role of ETs in peripheral nerves and SC. The increased sensitivity to ET-1 in nerves and iSC exposed to high glucose may contribute to abnormal SC proliferation characterizing diabetic neuropathy.

Mitogen-activated protein kinase inhibition reveals differences in signalling pathways activated by neurotrophin-3 and other growth-stimulating conditions of adult mouse dorsal root ganglia neurons

PD98059 blocks mitogen-activated protein kinase (MAPK) by inhibiting its activator, MAP kinase kinase (MEK). We have previously found that PD98059 only transiently inhibits spontaneous axonal outgrowth from adult mouse dorsal root ganglia (DRG) explants, whereas it causes sustained inhibition of nerve growth factor (NGF)-stimulated growth. Surprisingly, the present results showed that outgrowth stimulation by neurotrophin-3 (NT-3), interacting with another neuronal subgroup, was markedly enhanced by PD98059 and also by U0126, another inhibitor of MAPK activation. In contrast, the effects of glial cell line-derived neurotrophic factor (GDNF), which stimulates still another subgroup of DRG neurons, was opposed by PD98059. Axonal outgrowth in vitro can also be strongly increased by a prior axotomy in vivo. The increased outgrowth in preaxotomized explants was effectively inhibited by the presence of PD98059. Immunocytochemistry based on whole-mount labelling revealed the presence of neuronal MAPK, which was found to be activated by NGF, NT-3, and GDNF in separate axonal populations and by a prior axotomy in a majority of growing axons. The results suggest that there are important differences in the NGF and NT-3 signalling pathways, which may involve positive and negative control mechanisms by MAPK activation, respectively. Other findings indicate that GDNF exerts its growth effects by activation of MAPK and that expression of the conditioning effect in vitro in preaxotomized preparations also requires activation of MAPK.

Axonal outgrowth from adult mouse nodose ganglia in vitro is stimulated by neurotrophin-4 in a Trk receptor and mitogen-activated protein kinase-dependent way

The actions of neurotrophic factors on sensory neurons of the adult nodose ganglion were studied in vitro. The ganglia were explanted in an extracellular matrix-based gel that permitted observation of the growing axons. Neurotrophin-4 (NT-4) was a very efficient stimulator of outgrowth of axons from the nodose ganglion and had almost doubled the outgrowth length when this was analyzed after 2 days in culture. Brain-derived neurotrophic factor also stimulated outgrowth, but to a lesser degree, whereas NT-3 gave only weak stimulatory tendencies. Nerve growth factor and glial cell line-derived neurotrophic factor both lacked stimulatory effects. NT-4 is known to act via TrkB receptors, and the presence of these on growing nodose neurons was demonstrated immunohistochemically. In line with a Trk-mediated growth effect, the NT-4 stimulation was abolished by K252a, a selective inhibitor of neurotrophin receptor-associated tyrosine kinase activity. K252a had no effect on the unstimulated preparation. NT-4 treatment led to activation of the mitogen-activated protein kinase and inhibition of the latter pathway by PD98059 significantly reduced the NT-4 stimulated outgrowth, whereas the drug had no effect on the unstimulated growth. In conclusion, the data suggest that NT-4 can serve as a powerful growth factor for neurons of adult nodose ganglia and that the growth stimulation involves TrkB- and mitogen-activated protein kinase.

Sensitivity of NGF-responsive dorsal root ganglion neurons to semaphorin D is maintained in both neonatal and adult mice

Using a coculture assay of DRG neurons and aggregates of cells transfected with individual semaphorins, we have investigated the ability of semaphorins A, D, and E to inhibit axonal growth from DRG neurons. We show that axons of these neurons that grow in response to NGF remain responsive to semaphorin D in neonatal and in adult mice, although sensitivity may decline in the latter. Consistent with these findings, expression of the semaphorin receptor, neuropilin-1, is maintained in the DRGs of adult mice.

Functional recovery of respiratory behavior during axonal regeneration in snails (Lymnaea stagnalis) is experience dependent

This study investigated the role of experience in recovery of pulmonary respiration during axonal regeneration in Lymnaea stagnalis. Pulmonary respiration occurs when snails break the water surface and open the lung orifice, the pneumostome. It was shown that axotomy of all the axons innervating the pneumostome and surrounding area prevents the occurrence of lung respiration in 69% of snails. In the remaining 31%, lung respiration persisted, indicating that peripheral components alone are capable of initiating pneumostome openings and closures. Five weeks postsurgery, all snails with previous nerve crushes showed opening of the pneumostome with normal latency after breaking the water surface. However, prevention of pulmonary respiration during the recovery period dramatically changed the recovered behavior. Thus, experience in pulmonary respiration during axonal regeneration plays a role in the recovery of this behavior.

Vascular endothelial growth factor has neurotrophic activity and stimulates axonal outgrowth, enhancing cell survival and Schwann cell proliferation in the peripheral nervous system

Vascular endothelial growth factor (VEGF) is a mitogen for endothelial cells, and it promotes angiogenesis in vivo. Here we report that VEGF(165) has neurotrophic actions on cultured adult mouse superior cervical ganglia (SCG) and dorsal root ganglia (DRG), measured as axonal outgrowth. Maximal effect was observed at 10-50 ng/ml for SCG and 100 ng/ml for DRG. VEGF-induced axonal outgrowth was inhibited by the mitogen-activated protein kinase kinase inhibitor PD 98059 but not by the protein kinase inhibitor K252a. VEGF also increased survival of both neurons and satellite cells and the number of proliferating Schwann cells. Immunocytochemistry and immunoblotting revealed that VEGF was expressed in virtually all nerve cells in the SCG but only in a population of small-diameter (<35 micrometers) neurons representing approximately 30% of the neurons in DRG. Immunostaining showed that the VEGF receptor fetal liver kinase receptor (flk-1) was found on nerve cell bodies in DRG and to a lesser extent on neurons in SCG. Growth cones of regenerating axons from both types of ganglia exhibited flk-1 immunoreactivity, as did Schwann cells. We conclude that VEGF has both neurotrophic and mitogenic activity on cells in the peripheral nervous system.