Nerve growth factor inhibits sympathetic neurons' response to an injury cytokine

Ischemia-reperfusion myocardial infarction induces remodeling of left cardiac-projecting stellate ganglia neurons

Neurons in the stellate ganglion (SG) provide sympathetic innervation to the heart, brown adipose tissue (BAT), and other organs. Sympathetic innervation to the heart becomes hyperactive following myocardial infarction (MI). The impact of MI on the morphology of cardiac sympathetic neurons is not known, but we hypothesized that MI would stimulate increased cell and dendritic tree size in cardiac neurons. In this study, we examined the effects of ischemia-reperfusion MI on sympathetic neurons using dual retrograde tracing methods to allow detailed characterization of cardiac- and BAT-projecting neurons. Different fluorescently conjugated cholera toxin subunit B (CTb) tracers were injected into the pericardium and the interscapular BAT pads, respectively. Experimental animals received a 45-min occlusion of the left anterior descending coronary artery and controls received sham surgery. One week later, hearts were collected for assessment of MI infarct and SGs were collected for morphological or electrophysiological analysis. Cardiac-projecting SG neurons from MI mice had smaller cell bodies and shorter dendritic trees compared with sham animals, specifically on the left side ipsilateral to the MI. BAT-projecting neurons were not altered by MI, demonstrating the subpopulation specificity of the response. The normal size and distribution differences between BAT- and cardiac-projecting stellate ganglion neurons were not altered by MI. Patch-clamp recordings from cardiac-projecting left SG neurons revealed increased spontaneous excitatory postsynaptic currents despite the decrease in cell and dendritic tree size. Thus, increased dendritic tree size does not contribute to the enhanced sympathetic neural activity seen after MI.NEW & NOTEWORTHY Myocardial infarction (MI) causes structural and functional changes specifically in stellate ganglion neurons that project to the heart, but not in cells that project to brown adipose fat tissue.

Macrophage biology in the peripheral nervous system after injury

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

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

STAT3 integrates cytokine and neurotrophin signals to promote sympathetic axon regeneration

The transcription factor STAT3 has been implicated in axon regeneration. Here we investigate a role for STAT3 in sympathetic nerve sprouting after myocardial infarction (MI) - a common injury in humans. We show that NGF stimulates serine phosphorylation (S727) of STAT3 in sympathetic neurons via ERK1/2, in contrast to cytokine phosphorylation of Y705. Maximal sympathetic axon regeneration in vitro requires phosphorylation of both S727 and Y705. Furthermore, cytokine signaling is necessary for NGF-induced sympathetic nerve sprouting in the heart after MI. Transfection studies in neurons lacking STAT3 suggest two independent pools of STAT3, phosphorylated on either S727 or Y705, that regulate sympathetic regeneration via both transcriptional and non-transcriptional means. Additional data identify STAT3-microtubule interactions that may complement the well-characterized role of STAT3 stimulating regeneration associated genes. These data show that STAT3 is critical for sympathetic axon regeneration in vitro and in vivo, and identify a novel non-transcriptional mode of action.

gp130 cytokines are positive signals triggering changes in gene expression and axon outgrowth in peripheral neurons following injury

Adult peripheral neurons, in contrast to adult central neurons, are capable of regeneration after axonal damage. Much attention has focused on the changes that accompany this regeneration in two places, the distal nerve segment (where phagocytosis of axonal debris, changes in the surface properties of Schwann cells, and induction of growth factors and cytokines occur) and the neuronal cell body (where dramatic changes in cell morphology and gene expression occur). The changes in the axotomized cell body are often referred to as the "cell body response." The focus of the current review is a family of cytokines, the glycoprotein 130 (gp130) cytokines, which produce their actions through a common gp130 signaling receptor and which function as injury signals for axotomized peripheral neurons, triggering changes in gene expression and in neurite outgrowth. These cytokines play important roles in the responses of sympathetic, sensory, and motor neurons to injury. The best studied of these cytokines in this context are leukemia inhibitory factor (LIF) and interleukin (IL)-6, but experiments with conditional gp130 knockout animals suggest that other members of this family, not yet determined, are also involved. The primary gp130 signaling pathway shown to be involved is the activation of Janus kinase (JAK) and the transcription factors Signal Transducers and Activators of Transcription (STAT), though other downstream pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) may also play a role. gp130 signaling may involve paracrine, retrograde, and autocrine actions of these cytokines. Recent studies suggest that manipulation of this cytokine system can also stimulate regeneration by injured central neurons.

STATus and Context within the Mammalian Nervous System

Effective manipulation of human disease processes may be achieved by understanding transcriptional, posttranscriptional and epigenetic events that orchestrate cellular events. The levels of activation of specific molecules, spatial distribution and concentrations of relevant networks of signaling molecules along with the receptiveness of the chromatin to these signals are some of the parameters which dictate context. Effects elicited by the transcription factor signal transducers and activator of transcription 3 (Stat3) are discussed with respect to the context within which Stat3-mediated effects are elicited within the developing and adult mammalian nervous system. Stat3 signals are pivotal to the proliferation and differentiation of neural stem cells. They also participate in neuronal regeneration and cancers of the nervous system. An analysis of the context in which Stat3 activation occurs in these processes provides a potential predictive paradigm with which novel methods for intervention may be designed.

Glial NF-κB inhibition alters neuropeptide expression after sciatic nerve injury in mice

We utilized a transgenic mouse model where nuclear factor kappa B (NF-κB) is selectively inhibited in glial fibrillary acidic protein (GFAP) expressing cells. The transgene, GFAP-IκBα-dn, overexpresses a dominant negative form of the inhibitor of NF-κB (IκBα) under the control of the GFAP promoter. In the present work, we sought to understand the impact of glial NF-κB inhibition on the expression of pain mediating sensory neuropeptides galanin and calcitonin gene related peptide (CGRP) in a model of neuropathic pain in mice. Chronic constriction injury (CCI) of the left sciatic nerve was performed on wild type (WT) and GFAP-IκBα-dn transgenic mice. RT-PCR and immunohistological staining were performed in sciatic nerve and/or L4-L5 DRG tissue for galanin, CGRP and macrophage marker CD11b. GFAP-IκBα-dn mice had less mechanical and thermal hyperalgesia compared to WT mice post-CCI. After CCI, we observed galanin upregulation in DRG and sciatic nerve, which was less in GFAP-IκBα-dn mice. CGRP gene expression in the DRG increased transiently on day 1 post-CCI in WT but not in GFAP-IκBα-dn mice, and no evidence of CGRP upregulation in sciatic nerve post-CCI was found. After CCI, upregulation of CD11b in sciatic nerve was less in GFAP-IκBα-dn mice compared to WT mice, indicative of less macrophage infiltration. Our results showed that glial NF-κB inhibition reduces galanin and CGRP expression, which are neuropeptides that correlate with pain behavior and inflammation after peripheral nerve injury.

The dependence on gp130 cytokines of axotomy induced neuropeptide expression in adult sympathetic neurons

Adult peripheral neurons exhibit dramatic changes in gene expression after axonal injury, including changes in neuropeptide phenotype. For example, sympathetic neurons in the superior cervical ganglion (SCG) begin to express vasoactive intestinal peptide (VIP), galanin, pituitary adenylate cyclase activating polypeptide (PACAP), and cholecystokinin after axotomy. Before these changes, nonneuronal cells in the SCG begin to express leukemia inhibitory factor (LIF). When the effects of axotomy were compared in LIF-/- and wild-type mice, the increases in VIP and galanin expression were less in the former, though significant increases still occurred. LIF belongs to a family of cytokines with overlapping physiological effects and multimeric receptors containing the subunit gp130. Real-time PCR revealed large increases in the SCG after axotomy in mRNA for three members of this cytokine family, interleukin (IL)-6, IL-11, and LIF, with modest increases in oncostatin M, no changes in ciliary neurotrophic factor, and decreases in cardiotrophin-1. To explore the role of these cytokines, animals with selective elimination of the gp130 receptor in noradrenergic neurons were studied. No significant changes in mRNA levels for VIP, galanin, and PACAP were seen in axotomized ganglia from these mutant mice, while the increase in cholecystokinin was as large as that seen in wild-type mice. The data indicate that the inductions of VIP, galanin, and PACAP after axotomy are completely dependent on gp130 cytokines and that a second cytokine, in addition to LIF, is involved. The increase in cholecystokinin after axotomy, however, does not require the action of these cytokines.

Transregulation of leukemia inhibitory [corrected] factor receptor expression and function by growth factors in neuroblastoma cells

The cytokines that signal through the leukemia inhibitory factor (LIF) receptor are members of the neuropoietic cytokine family and have varied and numerous roles in the nervous system. In this report, we have determined the effects of growth factor stimulation on LIF receptor (LIFR) expression and signal transduction in the human neuroblastoma cell line NBFL. We show here that stimulation of NBFL cells with either epidermal growth factor or fibroblast growth factor decreases the level of LIFR in an extracellular signal-regulated kinase (Erk)1/2-dependent manner and that this down-regulation is due to an increase in the apparent rate of lysosomal LIFR degradation. Growth factor-induced decreases in LIFR level inhibit both LIF-stimulated phosphorylation of signal transducers and activators of transcription 3 and LIFR-mediated gene induction. We also show that Ser1044 of LIFR, which we have previously shown to be phosphorylated by Erk1/2, is required for the inhibitory effects of growth factors. Neurons are exposed to varying combinations and concentrations of growth factors and cytokines that influence their growth, development, differentiation, and repair in vivo. These findings demonstrate that LIFR expression and signaling in neuroblastoma cells can be regulated by growth factors that are potent activators of the mitogen-activated protein kinase pathway, and thus illustrate a fundamental mechanism that underlies crosstalk between receptor tyrosine kinase and neuropoietic cytokine signaling pathways.

Post-infarct cardiac sympathetic hyperactivity regulates galanin expression

The neuropeptide galanin is elevated in the cardiac sympathetic innervation after myocardial infarction (MI). Galanin inhibits vagal transmission and may support the regeneration of sympathetic nerves, thereby contributing to the development of arrhythmia and sudden cardiac death after MI. The reason for increased galanin production in sympathetic neurons after myocardial infarction is not known. Cardiac sympathetic neurons are activated chronically after cardiac ischemia-reperfusion, and activation of sympathetic neurons in culture stimulates galanin expression. Therefore, we tested the hypothesis that increased sympathetic nerve activity stimulates galanin expression in cardiac sympathetic neurons after myocardial infarction. To test this hypothesis we used TGR(ASrAOGEN) transgenic rats, which lack brain angiotensinogen and do not exhibit post-infarct sympathetic hyperactivity. Hearts and stellate ganglia were collected 1 week after ischemia-reperfusion. Galanin mRNA was quantified by real-time PCR and peptide content was assayed by enzyme-linked immunosorbent assay. Galanin mRNA increased approximately 3-fold after MI in cardiac sympathetic neurons of both genotypes compared to unoperated and sham controls. Left ventricular galanin content, however, increased after MI only in Sprague-Dawley rats and not in AOGEN rats. These data suggest that post-infarct cardiac sympathetic hyperactivity stimulates galanin peptide production but is not required for increased galanin mRNA expression.

Neuronal injury-inducible gene is synergistically regulated by ATF3, c-Jun, and STAT3 through the interaction with Sp1 in damaged neurons

Nerve injury requires the expression of large ensembles of genes. The key molecular mechanism for this gene transcription regulation in injured neurons is poorly understood. Among many nerve injury-inducible genes, the gene encoding damage-induced neuronal endopeptidase (DINE) showed most marked expression response to various kinds of nerve injuries in central and peripheral nervous system neurons. This unique feature led us to examine the promoter region of the DINE gene and clarify both the injury-responsive element within the promoter and its related transcriptional machinery. This study showed that DINE promoter was activated by leukemia inhibitory factor and nerve growth factor withdrawal, which were pivotal for the up-regulation of DINE mRNA after nerve injury. The injury-inducible transcription factors such as activating transcription factor 3 (ATF3), c-Jun, and STAT3, which were located at the downstream of leukemia inhibitory factor and nerve growth factor withdrawal, seemed to be involved in the activation of the DINE promoter. Surprisingly, these transcription factors did not bind to the DINE promoter directly. Instead, the general transcription factor, Sp1, bound to a GC box within the promoter. ATF3, c-Jun, and STAT3 interacted with Sp1 and are associated with the GC box region of the DINE gene in injured neurons. These findings suggested that Sp1 recruit ATF3, c-Jun, and STAT3 to obtain the requisite synergistic effect. Of these transcription factors, ATF3 may be the most critical, because ATF3 is specifically expressed after nerve injury.

Activating transcription factor 3 induction in sympathetic neurons after axotomy: response to decreased neurotrophin availability

Activating transcription factor 3 (ATF3) is induced in a high proportion of axotomized sensory and motor neurons after sciatic nerve transection. In the present study, we looked at the expression of this factor in the superior cervical ganglion (SCG) after axotomy and after other manipulations that induce certain aspects of the cell body response to axotomy. Sympathetic ganglia from intact rats and mice exhibit only a very occasional neuronal nucleus with activating transcription factor 3-like immunoreactivity (ATF3-IR); however, as early as 6 h and as late as 3 weeks postaxotomy, many of the neurons showed intense ATF3-IR. A second population of cells had smaller and generally less intensely stained nuclei, and at least some of these cells were satellite cells. Lesions distal to the SCG induced by administration of 6-hydroxydopamine or unilateral removal of the salivary glands produced increases in ATF3-IR similar to those seen after proximal axotomy, indicating that this response is not strictly dependent on the distance of the lesion from the cell body. Two proposed signals for triggering ATF3 expression were examined: reduction in nerve growth factor (NGF) availability and induction of the cytokine leukemia inhibitory factor (LIF). While administration of an antiserum raised against NGF to intact animals induced ATF3-IR, induction of ATF3-IR after axotomy was not reduced in LIF null mutant mice. Since axotomy, 6-hydroxydopamine, and sialectomy are known to decrease the concentration of NGF in the SCG, our data suggest that these decreases in NGF lead to increases in ATF3-IR. Furthermore, since the number of neurons in the SCG expressing ATF3-IR was greater after axotomy than after antiserum against NGF treatment, this raises the possibility that decreased NGF is not the only process regulating ATF3 expression after axotomy.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

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.

Increased galanin expression in the celiac ganglion of BB diabetic rats

BB rats lose >50% of their islet sympathetic nerve terminals soon after diabetes onset, markedly impairing the glucagon response to activation of these nerves. In this study, we sought evidence that this degree of disease-induced nerve terminal damage affected their neuronal cell bodies. Increased galanin expression was used as a marker of the change of phenotype that occurs in neuronal cell bodies when their axons are severely damaged. The celiac ganglion (CG) was analyzed because it is a major source of the sympathetic nerves that project to the pancreatic islets. But we first needed to determine if damaging nerve terminals could increase galanin expression in this ganglion and, if so, when that expression was maximal. Severe, global nerve terminal damage produced a dramatic increase of CG galanin expression which was maximal 5 days later. We next determined if a global, but partial, nerve terminal loss would also increase galanin expression and found a significant increase of galanin mRNA and its peptide in the CG. Finally, we determined if the disease-induced, partial and islet-selective loss of nerve terminals seen in BB diabetic rats was sufficient to increase galanin: we, again, found a significant increase of galanin mRNA and its peptide in their CG. These increases did not occur in their superior cervical ganglia. We conclude that the selective damage to islet sympathetic nerve terminals seen in BB diabetic rats, rather than the systemic factors of diabetic hyperglycemia or insulin deficiency, causes the increased galanin expression observed in the CG of this animal model of type 1 diabetes.

Age-related impairments of synaptic plasticity in the lateral perforant path input to the dentate gyrus of galanin overexpressing mice

In the present study, electrophysiological recordings were made from hippocampal slices obtained from mice overexpressing galanin under the promoter for the platelet-derived growth factor-B (GalOE mice). In these mice, a particularly strong galanin expression is seen in the granule cell layer/mossy fibers. Paired-pulse facilitation (PPF) of excitatory postsynaptic field potentials (fEPSPs) at the lateral perforant path (LPP)-dentate gyrus synapses was elicited in the dentate gyrus after stimulation with different interpulse intervals. Slices from young adult wild-type (WT) animals showed significant PPF of the 2nd EPSP evoked with paired-pulse stimuli, while PPF was reduced in slices from young adult GalOE mice, as well as aged WT mice, but were not observed at all in slices from aged GalOE animals. Application of the putative galanin antagonist M35 increased PPF in slices from aged WT mice as well as from adult and aged GalOE mice, but had no effect in slices taken from young adult WT mice. These data indicate that galanin is involved in hippocampal synaptic plasticity, in particular in age-related reduction of synaptic plasticity in the LPP input to the dentate gyrus. Galaninergic mechanisms may therefore represent therapeutic targets for treatment of age-related memory deficits and Alzheimer's disease.

Myocardial infarction stimulates galanin expression in cardiac sympathetic neurons

Cardiac ischemia-reperfusion alters sympathetic neurotransmission in the heart, but little is known about its effect on neuropeptide expression in sympathetic neurons. Ischemia followed by reperfusion induces the production of inflammatory cytokines in the heart, including interleukin-6 and cardiotrophin-1. These cytokines and related molecules inhibit the expression of neuropeptide Y (NPY), and stimulate the expression of vasoactive intestinal peptide (VIP), substance P (SubP), and galanin (GAL) in cultured sympathetic neurons. Therefore, we quantified NPY, VIP, SubP, and GAL mRNA in neurons of the stellate ganglia 1 week after ischemia-reperfusion to determine if neuropeptide expression was altered in cardiac sympathetic neurons. NPY, VIP, and SubP mRNAs were unchanged compared to unoperated control animals, but GAL mRNA was increased significantly. The increased GAL mRNA was not accompanied by elevated GAL peptide content in the stellate ganglia. Galanin content was increased significantly in the heart, however, indicating that elevated GAL mRNA led to increased peptide production. GAL content was increased in the left ventricle below the coronary artery ligation, but was not increased significantly in the atria or the base of the heart above the ligation. The buildup of GAL specifically in the damaged left ventricle is consistent with previous reports that GAL is transported to regenerating nerve endings after axon damage.

Lymphocyte regulation of neuropeptide gene expression after neuronal injury

The neuropeptides vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase-activating peptide (PACAP) are induced strongly in neurons after several types of injury, and exhibit neuroprotective actions in vitro and in vivo. It is thought that changes in expression of neuropeptides and other molecules in injured neurons are mediated by new factors produced in Schwann and immune cells at the injury site, a loss of target-derived factors, or a combination of mediators. To begin to determine the role of the inflammatory mediators, we investigated axotomy-induced changes in VIP and PACAP gene expression in the facial motor nucleus in severe combined immunodeficient (SCID) mice, and in mice with targeted mutations in specific cytokine genes. In normal mice, VIP and PACAP mRNA was induced strongly in facial motor neurons 4 days after axotomy. The increase in PACAP mRNA was blocked selectively in SCID mice, indicating that mechanisms responsible for VIP and PACAP gene induction are not identical. The loss of PACAP gene expression in SCID mice after axotomy was fully reversed by an infusion of normal splenocytes, suggesting that PACAP mRNA induction requires inflammatory mediators. PACAP and VIP mRNA inductions, however, were maintained in mice lacking leukemia inhibitory factor (LIF) and interleukin-6 (IL-6), and in mice lacking both receptors for tumor necrosis factor alpha (TNFalpha). The data suggest that an inflammatory response, most likely involving T lymphocytes, is necessary for the axotomy-induced increase in PACAP but not in VIP. LIF, IL-6, and TNFalpha, however, are not required for this response to injury.

Local sympathetic denervation in painful diabetic neuropathy

This study assessed whether painful diabetic neuropathy is associated with abnormal sympathetic nervous function in the affected limbs. Nine patients with diabetes (four men, five women; age 61 +/- 7 years) and painful peripheral neuropathy of the feet, but without evidence of generalized autonomic neuropathy, underwent intravenous infusion of tritiated norepinephrine (NE) and sampling of arterial and venous blood in both feet and in one arm to quantify the rate of entry of NE into the local venous plasma (NE spillover). In the same patients, positron emission tomography (PET) scanning after intravenous injection of the sympathoneural imaging agent 6-[(18)F]fluorodopamine was used to visualize sympathetic innervation and after intravenous [(13)N]ammonia to visualize local perfusion. The results were compared with those in the feet of normal volunteers and in an unaffected foot of patients with unilateral complex regional pain syndrome (CRPS). In addition, neurochemical results obtained in painful diabetic neuropathy were compared with those obtained in diabetic control patients with painless neuropathy and diabetic control patients without neuropathy. Local arteriovenous difference in plasma NE levels (DeltaNE(AV)) and NE spillover in the arms did not differ across the groups. However, DeltaNE(AV) in the feet was significantly less in the group with painful diabetic neuropathy than in the control groups. Also NE spillover in the feet tended to be lower in painful neuropathy. DeltaNE(AV) of diabetic control patients without neuropathy (n = 6) resembled values in the control groups without diabetes, whereas patients with painless diabetic neuropathy (n = 6) had evidence suggesting partial loss of sympathetic innervation. PET scanning revealed decreased flow-corrected 6-[(18)F]fluorodopamine-derived radioactivity in patients with painful diabetic neuropathy, compared with values in normal volunteers and patients with CRPS. The results provide neurochemical and neuroimaging evidence for regionally selective sympathetic denervation in the painful feet of patients with diabetic neuropathy.

Long-term injured purkinje cells are competent for terminal arbor growth, but remain unable to sustain stem axon regeneration

Long-distance axon regeneration requires the activation of a specific set of neuronal growth-associated genes. Adult Purkinje cells fail to upregulate these molecules in response to axotomy and show extremely weak regenerative properties. Nevertheless, starting from several months after injury, transected Purkinje axons undergo spontaneous sprouting. Here, we asked whether long-term injured Purkinje cells acquire novel intrinsic growth properties that enable them to upregulate growth-associated genes and sustain axon regeneration. To test this hypothesis, we examined axon growth and cell body changes in adult rat Purkinje neurons following axotomy and implantation of embryonic neocortical tissue or Schwann cells into the injury track. Purkinje cells that survived over 6 months after injury/transplantation displayed profuse sprouting in the injured cerebellum and developed extensive networks of terminal branches into embryonic neocortical grafts. In addition, severed Purkinje axons exposed to these transplants 6 months after injury grew faster than their counterparts confronted with the same environment immediately after axotomy. Nevertheless, long-term injured Purkinje cells failed to regenerate stem neurites into Schwann cell grafts, and, under all experimental conditions, they did not upregulate growth-associated molecules, including c-Jun, GAP-43, SNAP-25, and NADPH-diaphorase. These results indicate that the long-term injured Purkinje cells remain unable to activate the gene program required to sustain axon regeneration and their plasticity is restricted to terminal arbor remodeling. We propose that the delayed growth of injured Purkinje cells reflects an adaptive phenomenon by which the severed axon stump develops a new terminal arbor searching for alternative connections with local partners.

Leukemia inhibitory factor inhibits neuronal terminal differentiation through STAT3 activation

The discovery of stem cells in the adult central nervous system raises questions concerning the neurotrophic factors that regulate postnatal neuronal development. Olfactory receptor neurons (ORNs) are a useful model, because they are capable of robust neurogenesis throughout adulthood. We have investigated the role of leukemia inhibitory factor (LIF) in postnatal neuronal development by using ORNs as a model. LIF is a multifunctional cytokine implicated in various aspects of neuronal development, including phenotype determination, survival, and in response to nerve injury. LIF-deficient mice display significant increases, both in the absolute amount and in the number of cells expressing olfactory marker protein, a marker of mature ORNs. The maturation of ORNs was significantly inhibited by LIF in vitro. LIF activated the STAT3 pathway in ORNs, and transfection of ORNs with a dominant negative form of STAT3 abolished the effect of LIF. These findings demonstrate that LIF negatively regulates ORN maturation via the STAT3 pathway. Thus, LIF plays a critical role in controlling the transition of ORNs to maturity. Consequently, a population of ORNs is maintained in an immature state to facilitate the rapid repopulation of the olfactory epithelium with mature neurons during normal cell turnover or after injury.

Effects of Leukemia Inhibitory Factor on Galanin Expression and on Axonal Growth in Adult Dorsal Root Ganglion Neurons in Vitro

Synthesis of leukemia inhibitory factor (LIF) is increased in lesioned peripheral nerves and it is thought that this may cause increased expression of galanin (GAL) in axotomized dorsal root ganglia (DRG) neurons and also to promote axonal regeneration. We therefore compared effects of LIF and nerve growth factor (NGF) on galanin expression and axonal growth using cultured intact DRGs of adult mice. In control lumbar DRGs cultured for 3 days, only 16% of neurons were immunoreactive for GAL, but this was increased to 38% in preparations cultured with LIF. NGF by itself had no effect on GAL expression, but the proportion of GAL-positive neurons in cultures incubated with LIF and NGF together (22%) was less than that observed in DRGs cultured with LIF alone. Similar results were obtained using thoracic DRGs. In collagen gels, NGF caused marked increases in the numbers and lengths of outgrowing axons as observed in previous studies. In contrast, LIF did not stimulate axonal outgrowth but increased the proportions of axons which were immunoreactive for GAL. The results indicate that expression of LIF in lesioned nerves may affect expression of neuropeptides such GAL rather than stimulating axonal regeneration.

Nerve growth factor antiserum induces axotomy-like changes in neuropeptide expression in intact sympathetic and sensory neurons

Axonal transection of adult sympathetic and sensory neurons leads to a decrease in their content of target-derived nerve growth factor (NGF) and to dramatic changes in the expression of several neuropeptides and enzymes involved in transmitter biosynthesis. For example, axotomy of sympathetic neurons in the superior cervical ganglion (SCG) dramatically increases levels of galanin, vasoactive intestinal peptide (VIP), and substance P and their respective mRNAs and decreases mRNA levels for neuropeptide Y (NPY) and tyrosine hydroxylase (TH). Axotomy of sensory neurons in lumbar dorsal root ganglia (DRG) increases protein and mRNA levels for galanin and VIP and decreases levels for substance P and calcitonin gene-related peptide (CGRP). To assess whether reduction in the availability of endogenous NGF might play an important role in triggering these changes, we injected nonoperated animals with an antiserum against NGF (alphaNGF). alphaNGF increased levels of peptide and mRNA for galanin and VIP in neurons in both the SCG and DRG. NPY protein and mRNA were decreased in the SCG, but levels of TH protein and mRNA remained unchanged. In sensory neurons the levels of SP and CGRP protein decreased after alphaNGF treatment. These data suggest that the reduction in levels of NGF in sympathetic and sensory neurons after axotomy is partly responsible for the subsequent changes in neuropeptide expression. Thus, the peptide phenotype of these axotomized neurons is regulated both by the induction of an "injury factor," leukemia inhibitory factor, as shown previously, and by the reduction in a target-derived growth factor.

Effect of peripheral nerve lesion and lumbar sympathectomy on peptide regulation in dorsal root ganglia in the NGF-overexpressing mouse

Galanin is a peptide normally expressed at low levels both in sensory and in sympathetic neurons. It is strongly upregulated after peripheral nerve lesions, and it has been proposed that nerve growth factor (NGF) plays a role in this regulation. In the present study the effect of both sciatic nerve transection and lumbar sympathectomy on galanin in lumbar dorsal root ganglia (DRGs) was examined in mice overexpressing NGF (NGFOE) in the skin under the keratin promoter. The superior cervical ganglia (SCG) were also studied. In the DRG pericellular baskets containing tyrosine hydroxylase- (TH) and galanin-like immunoreactivity (LI) were found, mostly in the same fibers. Galanin-positive baskets were also found in the trigeminal ganglia. However, only single neuropeptide Y (NPY)-positive baskets were observed within the DRGs. No marked difference in number of galanin-positive neurons was seen between wild-type and NGFOE mice. After sciatic nerve transection galanin was upregulated in DRG neurons to about the same extent in NGFOE mice as in wild-type mice. Galanin-, but not TH-LIs decreased in the pericellular baskets. After lumbar sympathectomy both galanin- and TH-immunoreactive baskets disappeared, suggesting a sympathetic origin. In the SCG the very low galanin mRNA levels were strongly increased after lesion of the carotid nerves, both in wild-type and in NGFOE mice. However, whereas NPY mRNA levels decreased in the SCG after axotomy in the wild-type mice, there was a distinct increase in the NGFOE mice. Our results show that high NGF levels in skin induce formation of pericellular baskets in DRGs expressing galanin- and TH-LI and that galanin in these baskets is strongly influenced by peripheral axotomy. However, overexpression of NGF did not markedly influence galanin expression in DRG neurons, neither normally nor after nerve lesions. Finally, expression of NPY in sympathetic ganglia is differently regulated in NGFOE compared to wild-type mice.

Nerve growth factor antiserum induces axotomy-like changes in neuropeptide expression in intact sympathetic and sensory neurons

Axonal transection of adult sympathetic and sensory neurons leads to a decrease in their content of target-derived nerve growth factor (NGF) and to dramatic changes in the expression of several neuropeptides and enzymes involved in transmitter biosynthesis. For example, axotomy of sympathetic neurons in the superior cervical ganglion (SCG) dramatically increases levels of galanin, vasoactive intestinal peptide (VIP), and substance P and their respective mRNAs and decreases mRNA levels for neuropeptide Y (NPY) and tyrosine hydroxylase (TH). Axotomy of sensory neurons in lumbar dorsal root ganglia (DRG) increases protein and mRNA levels for galanin and VIP and decreases levels for substance P and calcitonin gene-related peptide (CGRP). To assess whether reduction in the availability of endogenous NGF might play an important role in triggering these changes, we injected nonoperated animals with an antiserum against NGF (alphaNGF). alphaNGF increased levels of peptide and mRNA for galanin and VIP in neurons in both the SCG and DRG. NPY protein and mRNA were decreased in the SCG, but levels of TH protein and mRNA remained unchanged. In sensory neurons the levels of SP and CGRP protein decreased after alphaNGF treatment. These data suggest that the reduction in levels of NGF in sympathetic and sensory neurons after axotomy is partly responsible for the subsequent changes in neuropeptide expression. Thus, the peptide phenotype of these axotomized neurons is regulated both by the induction of an "injury factor," leukemia inhibitory factor, as shown previously, and by the reduction in a target-derived growth factor.

AM424: history of a novel drug candidate

1. Leukaemia inhibitory factor (LIF) is a 180 amino acid single-chain protein, named after its effect on haematopoietic cells. Leukaemia inhibitory factor belongs to a group of cytokines that includes ciliary neurotrophic factor, interleukin (IL)-6, IL-11, cardiotrophin-1 and oncostatin M. All group members use the gp130 signal transducing subunit for intracellular signalling, but show differences in biological effect. 2. Research over the past 6-8 years has shown LIF to have potent neuromuscular activity. In vitro and in vivo studies on axotomy and nerve crush models demonstrate a powerful effect of LIF in enhancing the survival of both motor and sensory neurons, while reducing denervation-induced muscle atrophy. In models of both axotomy induced neuronal death and in the wobbler mouse, LIF is active at doses as low as 1 microgram/kg delivered systemically. 3. In muscle, LIF will increase the rate of muscle regeneration in vivo when applied exogenously after injury and will stimulate intrinsic muscle repair following its targeted release to dystrophic muscle in the mdx mouse. Leukaemia inhibitory factor may also have a role as an adjunct to myoblast transfer therapy, with studies showing that the transplantation of genetically competent myoblasts into mdx mouse muscle is enhanced when cells are injected with LIF. 4. Distribution and pharmacokinetic studies have been conducted in primates with doses of 20 micrograms/kg recombinant human LIF given subcutaneously over 2 weeks tolerated without major side effects. 5. A pharmaceutical form of recombinant human LIF (AM424; AMRAD Operations, Richmond, Victoria, Australia) entered human clinical trials during 1997 and a phase I clinical trial in healthy volunteers has been completed. A phase I repeat dose study has also been completed in cancer patients undergoing chemotherapy. The primary indication for a phase II study is the treatment of chemotherapy induced peripheral neuropathy. Other potential indications include muscle wasting diseases, acute nerve trauma and motor neuron disease. 6. The role of LIF in modulating nerve loss should make it an ideal candidate for the treatment of a number of neurological conditions. The phase I study represents the first trial in a programme for the clinical development of AM424.

Learning abilities, NGF and BDNF brain levels in two lines of TNF-alpha transgenic mice, one characterized by neurological disorders, the other phenotypically normal

In this study we used two lines of transgenic mice overexpressing tumor necrosis factor alpha (TNF-alpha) in the central nervous system (CNS), one characterized by reactive gliosis, inflammatory demyelination and neurological deficits (Tg6074) the other showing no neurological or phenotypical alterations (TgK3) to investigate the effect of TNF-alpha on brain nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) levels and learning abilities. The results showed that the amount of NGF in the brain of Tg6074 and TgK3 transgenic mice is low in the hippocampus and in the spinal cord, increases in the hypothalamus of Tg6074 and showed no significant changes in the cortex. BDNF levels were low in the hippocampus and spinal cord of TgK3. BDNF increased in the hypothalamus of TgK3 and Tg6074 while in the cortex, BDNF increased only in Tg6074 mice. Transgenic mice also had memory impairments as revealed by the Morris Water Maze test. These findings indicate that TNF-alpha significantly influences BDNF and NGF synthesis, most probably in a dose-dependent manner. Learning abilities were also differently affected by overexpression of TNF-alpha, but were not associated with inflammatory activity. The possible functional implications of our findings are discussed.

Leukemia inhibitory factor regulates galanin/galanin message-associated peptide expression in cultured mouse dorsal root ganglia; with a note on in situ hybridization methodology

After transection of the sciatic nerve there is a dramatic increase in both galanin/galanin message-associated peptide-like immunoreactivities and preprogalanin messenger RNA levels in rat and mouse lumbar 4 and 5 dorsal root ganglion neurons. There is strong evidence that after nerve injury leukemia inhibitor factor is a key molecule in the control of peptide expression both in sympathetic neurons and in dorsal root ganglion neurons, although the cells of origin of endogenous leukemia inhibitory factor remain to be established. We have therefore studied the effect of leukemia inhibitory factor on galanin expression in 72 h cultured dorsal root ganglion neurons from normal mice, leukemia inhibitory factor-deficient and heterozygous mice with immunohistochemistry and in situ hybridization. In cultures of leukemia inhibitory factor-deficient (-/-) mice only 13% of the dorsal root ganglion neurons expressed galanin message-associated peptide and in cultures from heterozygous (+/-) and wild-type (+/+) mice the corresponding figures were, respectively, 24 and 40%. After addition of leukemia inhibitory factor (10 or 50 ng/ml) to the culture medium, the number of neurons expressing galanin message-associated peptide was increased (up to 41%) in cultures from (-/-) animals after the high concentration and reached similar values in cultures from heterozygous animals incubated with the low concentration. These findings were supported by parallel analysis of prepro-galanin messenger RNA levels, where similar transcript levels and effects in the various cultures were observed in the non-radioactive in situ hybridization experiments. These results support the hypothesis that leukemia inhibitory factor is an important regulator of galanin/galanin message-associated peptide expression following axotomy, and may therefore be involved in the defence mechanisms against neuropathic pain at the level of dorsal root ganglion neurons.

Re-expression of p75NTR by adult motor neurons after axotomy is triggered by retrograde transport of a positive signal from axons regrowing through damaged or denervated peripheral nerve tissue

To investigate different types of potential signalling mechanisms that regulate neuronal reactions to axotomizing injury, we compared the re-expression of the low-affinity neurotrophin receptor, p75NTR, and the down-regulation of choline acetyltransferase expression, after various combinations of axotomy, crush injury and blockade of axonal transport in adult hypoglossal motor neurons in the rat. We found that pure axotomy in the absence of crush injury down-regulated choline acetyltransferase, but did not induce p75NTR re-expression. Blockade of axonal transport with colchicine had an identical effect. In contrast, both a crush injury on its own, or a crush injury proximal to a complete axotomy, induced p75NTR re-expression and down-regulated expression of choline acetyltransferase. Blockade of axonal transport with colchicine or tight ligation proximal to a crush prevented the crush injury-induced re-expression of p75NTR. Infusion of vehicle, nerve growth factor or ciliary neurotrophic factor induced low levels of p75NTR re-expression that were not significantly different from each other and were substantially lower than crush-induced levels. These findings confirm previous suggestions that the loss of choline acetyltransferase expression is due to the interruption of a constitutive retrograde signal, and show that the re-expression of p75NTR by adult motor neurons after axotomy is triggered by the retrograde transport of a positive signal derived from axons that are regrowing through damaged or denervated peripheral nerve tissue. The precise source and nature of this signal are not yet clear.

Analysis of selected regulatory pathways for rat galanin gene transcription and their suitability as putative models for negative regulation by NGF

Nerve growth factor (NGF) is known to negatively regulate the transcription of the rat galanin gene both in vivo and in vitro in dorsal root ganglion neurons, yet it is unclear how this regulation actually occurs. We propose here several possible pathways whereby NGF could interact to exert negative control on galanin regulation. These include: (1) repression of AP1-mediated transcription, (2) repression of nuclear binding protein-mediated transcription, and (3) repression of cytokine-mediated transcription. Although not enough data are available for speculation on which, if any, of these pathways is most relevant for NGF repression of galanin transcription, the mechanisms we describe can provide putative models for regulatory pathways. From here we can carry out further experiments that may help to elucidate the possible mechanisms of NGF repression in vivo.