Differential age-dependent trophic responses of nodose, sensory, and sympathetic neurons to neurotrophins and GDNF: potencies for neurite extension in explant culture

Targeting Vasohibins to Promote Axon Regeneration

Treatments accelerating axon regeneration in the nervous system are still clinically unavailable. However, parthenolide promotes adult sensory neurons' axon growth in culture by inhibiting microtubule detyrosination. Here, we show that overexpression of vasohibins increases microtubule detyrosination in growth cones and compromises growth in culture and in vivo. Moreover, overexpression of these proteins increases the required parthenolide concentrations to promote axon regeneration. At the same time, the partial knockdown of endogenous vasohibins or their enhancer SVBP in neurons facilitates axon growth, verifying them as pharmacological targets for promoting axon growth. In vivo, repeated intravenous application of parthenolide or its prodrug di-methyl-amino-parthenolide (DMAPT) markedly facilitates the regeneration of sensory, motor, and sympathetic axons in injured murine and rat nerves, leading to acceleration of functional recovery. Moreover, orally applied DMAPT was similarly effective in promoting nerve regeneration. Thus, pharmacological inhibition of vasohibins facilitates axon regeneration in different species and nerves, making parthenolide and DMAPT the first promising drugs for curing nerve injury.

Ontogeny and Trophic Factor Sensitivity of Gastrointestinal Projecting Vagal Sensory Cell Types

Vagal sensory neurons (VSNs) located in the nodose ganglion provide information, such as stomach stretch or the presence of ingested nutrients, to the caudal medulla via specialized cell types expressing unique marker genes. Here, we leverage VSN marker genes identified in adult mice to determine when specialized vagal subtypes arise developmentally and the trophic factors that shape their growth. Experiments to screen for trophic factor sensitivity revealed that brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) robustly stimulate neurite outgrowth from VSNs in vitro Perinatally, BDNF was expressed by neurons of the nodose ganglion itself, while GDNF was expressed by intestinal smooth muscle cells. Thus, BDNF may support VSNs locally, whereas GDNF may act as a target-derived trophic factor supporting the growth of processes at distal innervation sites in the gut. Consistent with this, expression of the GDNF receptor was enriched in VSN cell types that project to the gastrointestinal tract. Last, the mapping of genetic markers in the nodose ganglion demonstrates that defined vagal cell types begin to emerge as early as embryonic day 13, even as VSNs continue to grow to reach gastrointestinal targets. Despite the early onset of expression for some marker genes, the expression patterns of many cell type markers appear immature in prenatal life and mature considerably by the end of the first postnatal week. Together, the data support location-specific roles for BDNF and GDNF in stimulating VSN growth, and a prolonged perinatal timeline for VSN maturation in male and female mice.

Nerve growth factor alters microtubule targeting agent-induced neurotransmitter release but not MTA-induced neurite retraction in sensory neurons

Peripheral neuropathy is a dose-limiting side effect of anticancer treatment with the microtubule-targeted agents (MTAs), paclitaxel and epothilone B (EpoB); however, the mechanisms by which the MTAs alter neuronal function and morphology are unknown. We previously demonstrated that paclitaxel alters neuronal sensitivity, in vitro, in the presence of nerve growth factor (NGF). Evidence in the literature suggests that NGF may modulate the neurotoxic effects of paclitaxel. Here, we examine whether NGF modulates changes in neuronal sensitivity and morphology induced by paclitaxel and EpoB. Neuronal sensitivity was assessed using the stimulated release of calcitonin gene-related peptide (CGRP), whereas morphology of established neurites was evaluated using a high content screening system. Dorsal root ganglion cultures, maintained in the absence or presence of NGF, were treated from day 7 to day 12 in culture with paclitaxel (300nM) or EpoB (30nM). Following treatment, the release of CGRP was stimulated using capsaicin or high extracellular potassium. In the presence of NGF, EpoB mimicked the effects of paclitaxel: capsaicin-stimulated release was attenuated, potassium-stimulated release was slightly enhanced and the total peptide content was unchanged. In the absence of NGF, both paclitaxel and EpoB decreased capsaicin- and potassium-stimulated release and the total peptide content, suggesting that NGF may reverse MTA-induced hyposensitivity. Paclitaxel and EpoB both decreased neurite length and branching, and this attenuation was unaffected by NGF in the growth media. These differential effects of NGF on neuronal sensitivity and morphology suggest that neurite retraction is not a causative factor to alter neuronal sensitivity.

The Effects of Nerve Growth Factor on Nicotinic Synaptic Transmission in Mouse Airway Parasympathetic Neurons

In autonomic ganglia, acetylcholine (ACh) is released from preganglionic nerve terminals and binds to nicotinic ACh receptors (nAChRs) on postganglionic neurons, resulting in a brief, short-lived synaptic potential (fast excitatory postsynaptic potential [fEPSP]). Although nerve growth factor (NGF) is known to affect sensory and sympathetic nerves, especially during development, little is known regarding its effect on parasympathetic nerves, especially on adult neurons. Elevated levels of NGF and NGF-mediated neural plasticity may have a role in airway diseases, such as asthma and chronic obstructive pulmonary disease. In this study, we characterize the composition and response of nAChRs in parasympathetic neurons located in lower airways of mice, and note the effects of NGF on fEPSPs and on nicotinic currents. Based on immunohistochemical staining, nAChRs are made up of α-3 and β-4 subunits; in addition, tropomyosin-related kinase A, the receptor for NGF, is also expressed by the neurons. Vagus nerve evoked fEPSPs and inward currents evoked by a nicotinic receptor agonist (1,1-dimethyl-4-phenylpiperazinium) were increased by NGF. NGF also affected the action potential after hyperpolarization. These studies were done in mice, which are routinely used to study airway diseases, such as asthma, where the allergen-induced contraction of airway smooth muscle has a well-defined parasympathetic cholinergic component.

A sympathetic neuron autonomous role for Egr3-mediated gene regulation in dendrite morphogenesis and target tissue innervation

Egr3 is a nerve growth factor (NGF)-induced transcriptional regulator that is essential for normal sympathetic nervous system development. Mice lacking Egr3 in the germline have sympathetic target tissue innervation abnormalities and physiologic sympathetic dysfunction similar to humans with dysautonomia. However, since Egr3 is widely expressed and has pleiotropic function, it has not been clear whether it has a role within sympathetic neurons and if so, what target genes it regulates to facilitate target tissue innervation. Here, we show that Egr3 expression within sympathetic neurons is required for their normal innervation since isolated sympathetic neurons lacking Egr3 have neurite outgrowth abnormalities when treated with NGF and mice with sympathetic neuron-restricted Egr3 ablation have target tissue innervation abnormalities similar to mice lacking Egr3 in all tissues. Microarray analysis performed on sympathetic neurons identified many target genes deregulated in the absence of Egr3, with some of the most significantly deregulated genes having roles in axonogenesis, dendritogenesis, and axon guidance. Using a novel genetic technique to visualize axons and dendrites in a subpopulation of randomly labeled sympathetic neurons, we found that Egr3 has an essential role in regulating sympathetic neuron dendrite morphology and terminal axon branching, but not in regulating sympathetic axon guidance to their targets. Together, these results indicate that Egr3 has a sympathetic neuron autonomous role in sympathetic nervous system development that involves modulating downstream target genes affecting the outgrowth and branching of sympathetic neuron dendrites and axons.

Sciatic nerve regeneration is not inhibited by anti-NGF antibody treatment in the adult rat

Elevated nerve growth factor (NGF) is believed to play a role in many types of pain. An NGF-blocking antibody (muMab 911) has been shown to reduce pain and hyperalgesia in pain models, suggesting a novel therapeutic approach for pain management. Since NGF also plays important roles in peripheral nervous system development and sensory nerve outgrowth, we asked whether anti-NGF antibodies would adversely impact peripheral nerve regeneration. Adult rats underwent a unilateral sciatic nerve crush to transect axons and were subcutaneously dosed weekly for 8weeks with muMab 911 or vehicle beginning 1day prior to injury. Plasma levels of muMab 911 were assessed from blood samples and foot print analysis was used to assess functional recovery. At 8-weeks post-nerve injury, sciatic nerves were prepared for light and electron microscopy. In a separate group, Fluro-Gold was injected subcutaneously at the ankle prior to perfusion, and counts and sizes of retrogradely labeled and unlabeled dorsal root ganglion neurons were obtained. There was no difference in the time course of gait recovery in antibody-treated and vehicle-treated animals. The number of myelinated and nonmyelinated axons was the same in the muMab 911-treated crushed nerves and intact nerves, consistent with observed complete recovery. Treatment with muMab 911 did however result in a small decrease in average cell body size on both the intact and injured sides. These results indicate that muMab 911 did not impair functional recovery or nerve regeneration after nerve injury in adult rats.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

Omega-3 polyunsaturated fatty acids increase the neurite outgrowth of rat sensory neurones throughout development and in aged animals

Polyunsaturated fatty acids (PUFA) of the omega-3 series and omega-6 series modulate neurite outgrowth in immature neurones. However, it has not been determined if their neurotrophic effects persist in adult and aged tissue. We prepared cultures of primary sensory neurones from male and female rat dorsal root ganglia (DRG), isolated at different ages: post-natal day 3 (P3) and day 9 (P9), adult (2-4 months) and aged (18-20 months). Cultures were incubated with the omega-6 PUFA arachidonic acid (AA) and the omega-3 PUFA eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), at 0.8, 4, 8 and 40muM. PUFA increased neurite outgrowth throughout the developmental stages studied. The effects of omega-3 PUFA, in particular DHA, were still prominent in aged tissue. The amplitude of the effects was comparable to that of nerve growth factor (NGF; 50ng/ml) and all-trans-retinoic acid (ATRA; 0.1muM). The effects of PUFA were similar in cells positive or negative for the N52 neurofilament marker. Our results show that omega-3 PUFA have a marked neurite-promoting potential in neurones from adult and aged animals.

Aging impairs collateral sprouting of nociceptive axons in the rat

Sprouting of uninjured nociceptive axons was examined in young adult, middle aged and aged rats. Axon sprouting from the spared sural nerve, both into adjacent denervated skin and into end-to-side coapted nerve graft, was significantly higher in young rats than in aged rats. Cross-transplantations of the end-to-side coapted nerve grafts between young and aged rats demonstrated that axon sprouting from young recipient nerves into aged donor nerve grafts was significantly deteriorated, whereas the axon sprouting from aged recipient nerves into young donor nerve grafts was not statistically significantly affected. The levels of laminin polypeptides in peripheral nerves were 50-100% higher in young adult than in aged rats. However, the levels of peripherin, NGF isoforms and TrkA in skin, peripheral nerves and DRG, respectively, were not significantly reduced in aged rats. Therefore, impaired sprouting of nociceptive axons in aged rats is due rather to the alterations in peripheral neural pathways, than to the limited sprouting capacity of aged sensory neurons. Decreased levels of extracellular matrix components might be important in this respect.

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.

Chapter 26: Age-related differences in the reinnervation after peripheral nerve injury

Numerous and extensive functional, structural, and biochemical changes characterize intact aged peripheral nervous system. Functional recovery after peripheral nerve injury depends on survival of injured neurons and functional reinnervation of target tissue by regeneration of injured axons and collateral sprouting of uninjured (intact) adjacent axons. The rate of axonal regeneration becomes slower and its extent (density and number of regenerating axons) decreases in aged animals. Aging also impairs terminal sprouting of regenerated axons and collateral sprouting of intact adjacent axons, thus further limiting target reinnervation and its functional recovery. Decreased survival of aged noninjured and injured neurons, limited intrinsic growth potential of neuron, alteration in its responsiveness to stimulatory or inhibitory environmental factors, and changes in the peripheral neural pathways and target tissues are possible reasons for impaired reinnervation after peripheral nerve injury in old age. The review of present data suggests that this impairment is mostly due to the age-related changes in the peripheral neural pathways and target tissues, and not due to the limited intrinsic growth capacity of neurons or their reduced responsiveness to trophic factors. Age-related alterations in the soluble target derived neurotrophic factors, like nerve growth factor, and nonsoluble extracellular matrix components of neural pathways, like laminin, might be important in this respect.

Cellular dynamics underlying regeneration of damaged axons differs from initial axon development

While long-distance regeneration may be limited in mammalian species, it is becoming apparent that damaged mature neurons retain some capacity for attempted regeneration and that the adult CNS is not entirely inhibitory to axon growth. Our investigations show that there are critical intrinsic features of postinjury axonal regeneration that differ from initial axon development, and that these distinct differences may account for the limited and inappropriate regenerative response that currently characterizes the mature CNS. We compared the neurochemical and dynamic characteristics of developing axons to relatively mature regenerating axons, utilizing an in vitro model of axonal transection to long-term cultured rat cortical neurons. Immunolabelling studies revealed that regenerating and developing axons have a similar localization of cytoskeletal proteins, but the tips of regenerating axons, although morphologically similar, were smaller with reduced fillopodial extension, relative to developmental growth cones. Live imaging demonstrated that regenerating axons exhibited significantly less outgrowth than developmental neurites. Furthermore, growth cones of regenerating axons had a significant reduction in pausing, considered vital for interstitial branching and pathfinding, than did developmental growth cones. In addition, unlike developing axons, the regenerating axons were unresponsive to the growth factors BDNF and GDNF. Thus, although similar in their cytoskeletal composition, the growth cones of regenerative sprouts differed from their developmental counterparts in their size, their dynamic behaviour and their ability to respond to critical growth factors. These intrinsic differences may account for the inability of post-traumatic locally sprouting axons to make accurate pathway decisions and successfully respond to trauma.

Reduction in nerve growth factor availability leads to a conditioning lesion-like effect in sympathetic neurons

Axotomized peripheral neurons are capable of regeneration, and the rate of regeneration can be enhanced by a conditioning lesion (i.e., a lesion prior to the lesion after which neurite outgrowth is measured). A possible signal that could trigger the conditioning lesion effect is the reduction in availability of a target-derived factor resulting from the disconnection of a neuron from its target tissue. We tested this hypothesis with respect to nerve growth factor (NGF) and sympathetic neurons by administering an antiserum to NGF to adult mice for 7 days prior to explantation or dissociation of the superior cervical ganglion (SCG) and subsequently measuring neurite outgrowth. The antiserum treatment dramatically lowered the concentration of NGF in the SCG and increased the rate of neurite outgrowth in both explants and cell cultures. The increase in neurite outgrowth was similar in magnitude to that seen after a conditioning lesion. To determine if exogenous NGF could block the effect of a conditioning lesion, mice were injected with NGF or cytochrome C immediately prior to unilateral axotomy of the SCG, and for 7 days thereafter. A conditioning lesion effect of similar magnitude was seen in NGF-treated and control animals. While NGF treatment increased NGF levels in the contralateral control ganglion, it did not significantly elevate levels in the axotomized ganglion. The results suggest that the decreased availability of NGF after axotomy is a sufficient stimulus to induce the conditioning lesion effect in sympathetic neurons. While NGF administration did not prevent the conditioning lesion effect, this may be due to the markedly decreased ability of sympathetic neurons to accumulate the growth factor after axotomy.

Glaucoma: thinking in new ways-a rôle for autonomous axonal self-destruction and other compartmentalised processes?

Glaucoma is a common neurodegenerative disease that affects retinal ganglion cells (RGCs). Substantial effort is being expended to determine how RGCs die in glaucoma. As in other neurodegenerative diseases, the majority of effort focuses on characterising apoptotic self-destruct pathways. However, apoptosis is not the only self-destruct mechanism that may be activated in neurons. It is now known that neurons have distinct classes of self-destruct programme that are spatially compartmentalised. In addition to the well-described intracellular suicide machinery in the neuronal soma, responsible for apoptosis, there is another, molecularly distinct, self-destruct programme localised in the axon. Evidence also supports the existence of compartmentalised degeneration programmes in synapses and dendrites. RGCs are no exception to this. Recent data, from in vitro studies and from an inherited mouse model of glaucoma, suggest that molecularly distinct degenerative pathways underlie the destruction of RGC somata and RGC axons. In various neurodegenerative diseases, axons, dendrites and synapses often degenerate well before the cells die, and there is increasing evidence that this is important for the production of clinical symptoms and signs. We hypothesise that such compartmentalised and autonomous programmes are of critical importance in the pathophysiology of glaucoma, and we suggest that studies of these processes are essential for a complete understanding of this complex disease.

Glial-mediated neuroprotection: evidence for the protective role of the NO-cGMP pathway via neuron-glial communication in the peripheral nervous system

The NO-cGMP pathway has emerged as a neuroprotective signaling system involved in communication between neurons and glia. We have previously shown that axotomy or nerve growth factor (NGF)-deprivation of dorsal root ganglion (DRG) neurons leads to increased production of NO and at the same time an increase in cGMP production in their satellite glia cells. Blockade of NO or its receptor, the cGMP synthesizing enzyme soluble guanylate cyclase (sGC), results in apoptosis of neurons and glia. We now show that co-culture of neonatal DRG neurons with either Schwann cells pre-treated with an NO donor or a membrane-permeant cGMP analogue; or neurons maintained in the medium from Schwann cell cultures treated in the same way, prevents neuronal apoptosis. Both NO donor and cGMP treatment of Schwann cells results in synthesis of NGF and NT3. Furthermore, if the Schwann cells are previously infected with adenoviral vectors expressing a dominant negative sGC mutant transgene, treatment of these Schwann cells with an NO donor now fails to prevent neuronal apoptosis. Schwann cells treated in this way also fail to express neither cGMP nor neurotrophins. These findings suggest NO-sGC-cGMP-mediated NGF and NT3 synthesis by Schwann cells protect neurons.

Recombinant Sendai virus vectors deleted in both the matrix and the fusion genes: efficient gene transfer with preferable properties

BACKGROUND

Sendai virus (SeV) is a new type of cytoplasmic RNA vector, which infects and replicates in most mammalian cells, directs high-level expression of the genes on its genome and is free from genotoxicity. In order to improve this vector, both the matrix (M) and fusion (F) genes were deleted from its genome.

METHODS

For the recovery of the M and F genes-deleted SeV (SeV/DeltaMDeltaF), the packaging cell line was established by using a Cre/loxP induction system. SeV/DeltaMDeltaF was characterized and compared with wild-type and F or M gene-deleted SeV vectors in terms of transduction ability, particle formation, transmissible property and cytotoxicity.

RESULTS

SeV/DeltaMDeltaF was propagated in high titers from the packaging cell line. When this vector was administered into the lateral ventricle and the respiratory tissue, many of the ependymal and epithelial cells were transduced, respectively, as in the case of wild-type SeV. F gene-deletion made the SeV vector non-transmissible, and M gene-deletion worked well to inhibit formation of the particles from infected cells. Simultaneous deletions of these two genes in the same genome resulted in combining both advantages. That is, both virus maturation into particles and transmissible property were almost completely abolished in cells infected with SeV/DeltaMDeltaF. Further, the cytopathic effect of SeV/DeltaMDeltaF was significantly attenuated rather than that of wild type in vitro and in vivo.

CONCLUSIONS

SeV/DeltaMDeltaF is an advanced type of cytoplasmic RNA vector, which retains efficient gene transfer, gains non-transmissible properties and loses particle formation with less cytopathic effect.

Oestrogen regulates sympathetic neurite outgrowth by modulating brain derived neurotrophic factor synthesis and release by the rodent uterus

Sympathetic innervation of the adult rodent uterus undergoes cyclic remodelling. Terminal sympathetic axons degenerate when oestrogen levels rise and regenerate when oestrogen levels decline. This study examined the role of neurotrophins in oestrogen-mediated uterine sympathetic nerve remodelling. Oestrogen injection of ovariectomized female rats did not affect uterine NT-3 levels 24 h postinjection, and increased endometrial NGF protein, indicating that reduced NGF or NT-3 is not responsible for the oestrogen-induced denervation. Oestrogen also raised BDNF protein and mRNA in myometrium and endometrium. To assess whether increased BDNF affects uterine receptivity to sympathetic outgrowth, sympathetic ganglion explants were co-cultured with myometrium. Myometrium from ovariectomized rats induced neuritogenesis in oestrogen-free conditions, and this was abolished when BDNF was added to the medium. Neuritogenesis induced by ovariectomized myometrium was suppressed by oestrogen, and restored by a BDNF function-blocking antibody. To determine if target BDNF synthesis is required for oestrogen to suppress sympathetic neurite outgrowth, uteri from wild-type mice and mice homozygous or heterozygous for recombinant mutations of the BDNF gene were cultured with rat sympathetic ganglia. Neuritogenesis induced by wild-type uteri was diminished by oestrogen. Neurite formation in the presence of homozygous BDNF mutant uteri was not affected by oestrogen, but was lower than that of wild-type mice. Uteri from mice heterozygous for the BDNF mutation, who have reduced BDNF synthesis, showed normal neuritogenic properties, but were not affected by oestrogen. These findings suggest that oestrogen alters neuritogenic properties of the rodent uterus by regulating BDNF synthesis, which inhibits sympathetic neurite outgrowth.