Leukemia inhibitory factor and NGF regulate signal transducers and activators of transcription activation in sympathetic ganglia: convergence of cytokine- and neurotrophin-signaling pathways

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.

Peripheral nerve regeneration and NGF-dependent neurite outgrowth of adult sensory neurons converge on STAT3 phosphorylation downstream of neuropoietic cytokine receptor gp130

After nerve injury, adult sensory neurons can regenerate peripheral axons and reconnect with their target tissue. Initiation of outgrowth, as well as elongation of neurites over long distances, depends on the signaling of receptors for neurotrophic growth factors. Here, we investigated the importance of gp130, the signaling subunit of neuropoietic cytokine receptors in peripheral nerve regeneration. After sciatic nerve crush, functional recovery in vivo was retarded in SNS-gp130(-/-) mice, which specifically lack gp130 in sensory neurons. Correspondingly, a significantly reduced number of free nerve endings was detected in glabrous skin from SNS-gp130(-/-) compared with control mice after nerve crush. Neurite outgrowth and STAT3 activation in vitro were severely reduced in cultures in gp130-deficient cultured neurons. Surprisingly, in neurons obtained from SNS-gp130(-/-) mice the increase in neurite length was reduced not only in response to neuropoietic cytokine ligands of gp130 but also to nerve growth factor (NGF), which does not bind to gp130-containing receptors. Neurite outgrowth in the absence of neurotrophic factors was partially rescued in gp130-deficient neurons by leptin, which activates STAT3 downstream of leptic receptor and independent of gp130. The neurite outgrowth response of gp130-deficient neurons to NGF was fully restored in the presence of leptin. Based on these findings, gp130 signaling via STAT3 activation is suggested not only to be an important regulator of peripheral nerve regeneration in vitro and in vivo, but as determining factor for the growth promoting action of NGF in adult sensory neurons.

Change of fate commitment in adult neural progenitor cells subjected to chronic inflammation

Neural progenitor cells (NPCs) have regenerative capabilities that are activated during inflammation. We aimed at elucidating how NPCs, with special focus on the spinal cord-derived NPCs (SC-NPCs), are affected by chronic inflammation modeled by experimental autoimmune encephalomyelitis (EAE). NPCs derived from the subventricular zone (SVZ-NPCs) were also included in the study as a reference from a distant inflammatory site. We also investigated the transcriptional and functional difference between the SC-NPCs and SVZ-NPCs during homeostatic conditions. NPCs were isolated and propagated from the SVZ and cervical, thoracic, and caudal regions of the SC from naive rats and rats subjected to EAE. Using Affymetrix microarray analyses, the global transcriptome was measured in the different NPC populations. These analyses were paralleled by NPC differentiation studies. Assessment of basal transcriptional and functional differences between NPC populations in naive rat revealed a higher neurogenic potential in SVZ-NPCs compared with SC-NPCs. Conversely, during EAE, the neurogenicity of the SC-NPCs was increased while their gliogenicity was decreased. We detected an overall increase of inflammation and neurodegeneration-related genes while the developmentally related profile was decreased. Among the decreased functions, we isolated a gliogenic signature that was confirmed by differentiation assays where the SC-NPCs from EAE generated fewer oligodendrocytes and astrocytes but more neurons than control cultures. In summary, NPCs displayed differences in fate-regulating genes and differentiation potential depending on their rostrocaudal origin. Inflammatory conditions downregulated gliogenicity in SC-NPCs, promoting neurogenicity. These findings give important insight into neuroinflammatory diseases and the mechanisms influencing NPC plasticity during these conditions.

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.

Regulation of IGF-1 but not TGF-β1 by NGF in the smooth muscle of the inflamed urinary bladder

Intraperitoneal injection of cyclophosphamide (CYP) causes hemorrhagic cystitis with excess growth of muscular layer leading to bladder hypertrophy; this could be attributable to changes in the expression profiles of growth factors in the inflamed urinary bladder. The growth factors characterized in the current study include nerve growth factor (NGF), insulin-like growth factor (IGF)-1, and transforming growth factor (TGF)-β1. We found that following CYP injection for 8 h and 48 h, the mRNA levels of all three factors were increased in the inflamed bladder when compared to control. The level of NGF mRNA was mainly increased in the urothelium layer while the levels of IGF-1 mRNA and TGF-β1 mRNA were increased in the smooth muscle layer. The level of NGF high affinity receptor TrkA mRNA was also increased in both the urothelium and the smooth muscle layers during bladder inflammation. When we blocked NGF action with NGF neutralizing antibody in vivo, we found that the up-regulation of IGF-1 in the inflamed bladder was reversed while the up-regulation of TGF-β1 was not affected by NGF neutralization. The effect of NGF on regulating IGF-1 expression was further confirmed in bladder smooth muscle culture showing that exogenous NGF increased the mRNA level of IGF-1 after 30 min to 1 h stimulation. These results suggested that bladder inflammation induced region-specific changes in the expression profiles of NGF, IGF-1 and TGF-β1. The up-regulation of NGF in the urothelium may have a role in affecting bladder smooth muscle cell physiology by regulating IGF-1 expression.

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.

p38α and p38β mitogen-activated protein kinases determine cholinergic transdifferentiation of sympathetic neurons

Although the p38 mitogen-activated protein kinases are active in many neuronal populations in the peripheral and central nervous systems, little is known about the physiological functions of p38 in postmitotic neurons. We report that p38 activity determines in vitro and in vivo the switch from noradrenergic to cholinergic neurotransmission that occurs in sympathetic neurons on exposure to the neuropoietic cytokines CNTF and LIF. This transdifferentiation serves as a model for the plastic mechanisms that enable mature neurons to change some of their central functions without passing through the cell cycle. We demonstrate that in postmitotic neurons, p38 and STAT pathways are concurrently activated by neuropoietic cytokine treatment for at least 12 h overlapping with changes in neurotransmitter marker gene expression. Inhibition of p38 blocks the upregulation of the nuclear matrix protein Satb2 and of cholinergic markers by CNTF without affecting STAT3 phosphorylation. Conversely, overexpression of p38α or β in the absence of cytokines stimulates cholinergic marker expression. The neurotransmitter switch in vitro is impaired in neurons isolated from p38β(-/-) mice. Consistent with these in vitro results, a substantial loss of cells expressing cholinergic properties is observed in vivo in the stellate ganglion of mature mice deficient in the p38β isoform.

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.

Effects of mood stabilizers on adult dentate gyrus-derived neural precursor cells

Neurogenesis in the adult dentate gyrus (DG) is considered to be partly involved in the action of mood stabilizers. However, it remains unclear how mood stabilizers affect neural precursor cells in adult DG. We have established a culture system of adult rat DG-derived neural precursor cells (ADP) and have shown that lithium, a mood stabilizer, and dexamethasone, an agonist of glucocorticoid receptor, reciprocally regulate ADP proliferation. Neurogenesis constitutes not only proliferation of neural precursor cells but also apoptosis and differentiation. To develop further understanding of mood stabilizer effects on neural precursor cells in adult DG, we investigated and compared the effects of four common mood stabilizers-lithium, valproate, carbamazepine, and lamotrigine-on ADP proliferation, apoptosis, and differentiation. ADP proliferation, decreased by dexamethasone, was examined using Alamar Blue assay. Using TUNEL assay, ADP apoptosis induced by staurosporine was examined. The differentiated ADP induced by retinoic acid was characterized by immunostaining with anti-GFAP or anti-Tuj1 antibody. Lithium and valproate, but not carbamazepine and lamotrigine, recovered ADP proliferation decreased by dexamethasone. All four mood stabilizers decreased ADP apoptosis. Retinoic acid differentiated ADP into both neurons and astrocytes. Lithium and carbamazepine increased the ratio of neurons and decreased that of astrocytes. However, valproate and lamotrigine increased the ratio of astrocytes and decreased that of neurons. Therefore, these four stabilizers exhibited both common and differential effects on ADP proliferation, apoptosis, and differentiation.

Galanin acts as a trophic factor to the central and peripheral nervous systems

The neuropeptide galanin is widely, but not ubiquitously, expressed in the adult nervous system. Its expression is markedly up-regulated in many neuronal tissues after nerve injury or disease. Over the last 10 years, we have demonstrated that the peptide plays a developmental survival role to subsets of neurons in the peripheral and central nervous systems with resulting phenotypic changes in neuropathic pain and cognition. Galanin also appears to play a trophic role to adult sensory neurons following injury, via activation of GalR2, by stimulating neurite outgrowth. Furthermore, galanin also plays a neuroprotective role to the hippocampus following excitotoxic injury, again mediated by activation of GalR2. Most recently, we have shown that galanin expression is markedly up-regulated in multiple sclerosis (MS) lesions and in the experimental autoimmune encephalomyelitis (EAE) model of MS. Over-expression of galanin in transgenic mice abolishes disease in the EAE model, whilst loss-of-function mutations in galanin or GalR2 increase disease severity. In summary, these studies demonstrate that a GalR2 agonist might have clinical utility in a variety of human diseases that affect the nervous system.

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.

Characterization of an enhancer region of the galanin gene that directs expression to the dorsal root ganglion and confers responsiveness to axotomy

Galanin expression markedly increases in the dorsal root ganglion (DRG) after sciatic nerve axotomy and modulates pain behavior and regeneration of sensory neurons. Here, we describe transgenic mice expressing constructs with varying amounts of sequence upstream of the murine galanin gene marked by LacZ. The 20 kb region upstream of the galanin gene recapitulates the endogenous expression pattern of galanin in the embryonic and adult intact DRG and after axotomy. In contrast, 1.9 kb failed to drive LacZ expression in the intact DRG or after axotomy. However, the addition of an additional 2.7 kb of 5' flanking DNA (4.6 kb construct) restored the expression in the embryonic DRG and in the adult after axotomy. Sequence analysis of this 2.7 kb region revealed unique 18 and 23 bp regions containing overlapping putative Ets-, Stat-, and Smad-binding sites, and adjacent putative Stat- and Smad-binding sites, respectively. Deletion of the 18 and 23 bp regions from the 4.6 kb construct abolished the upregulation of LacZ expression in the DRG after axotomy but did not affect expression in the embryonic or intact adult DRG. Also, a bioinformatic analysis of the upstream regions of a number of other axotomy-responsive genes demonstrated that the close proximity of putative Ets-, Stat-, and Smad-binding sites appears to be a common motif in injury-induced upregulation in gene expression.

Genomic and proteomic microglial profiling: pathways for neuroprotective inflammatory responses following nerve fragment clearance and activation

Microglia, a primary immune effector cell of the central nervous system (CNS) affects homeostatic, neuroprotective, regenerative and degenerative outcomes in health and disease. Despite these broad neuroimmune activities linked to specific environmental cues, a precise cellular genetic profile for microglia in the context of disease and repair has not been elucidated. To this end we used nucleic acid microarrays, proteomics, immunochemical and histochemical tests to profile microglia in neuroprotective immune responses. Optic and sciatic nerve (ON and SN) fragments were used to stimulate microglia in order to reflect immune consequences of nervous system injury. Lipopolysaccharide and latex beads-induced microglial activation served as positive controls. Cytosolic and secreted proteins were profiled by surface enhanced laser desorption ionization-time of flight (SELDI-TOF) ProteinChip, 1D and 2D difference gel electrophoresis. Proteins were identified by peptide sequencing with tandem mass spectrometry, ELISA and western blot tests. Temporal expression of pro-inflammatory cytokines, antioxidants, neurotrophins, and lysosomal enzyme expression provided, for the first time, a unique profile of secreted microglia proteins with neuroregulatory functions. Most importantly, this molecular and biochemical signature supports a broad range of microglial functions for debris clearance and promotion of neural repair after injury.

Expression and activation of STAT proteins during mouse retina development

Cytokines and growth factors play important roles in mammalian ocular development and maintenance. Recent studies have indicated that some of these ligands can activate signal transducer and activator of transcription factors (STATs) and modulate gene transcription. The purpose of this study was to investigate the expression and activation of STAT proteins in the developing mouse retina. Anti-STAT and anti-phosphorylated STAT antibodies were used to detect the expression and activation of STATs in embryonic and postnatal neuronal retina, ciliary margin, and retinal pigment epithelium (RPE). In situ hybridization and Western blot were also employed. In embryonic stages, all STAT proteins were expressed in the neuronal retina in distinct cell populations at different embryonic stages. For example, Stat3 expression and activation gradually increased in the inner neuroblast layer and ciliary margin during development. In adult retina, Stat3 was detected in the inner nuclear layer and ganglion cells layers. Stat1 was strongly expressed in both outer and inner plexiform layers. Stat5a was clearly expressed in the outer/inner nuclear layer, the ganglion cell layer, and the inner plexiform layer. Strong expression of Stat3, Stat5a, and Stat6 was observed in the RPE. Activated Stat3 and Stat5a were found in the neural retina and the RPE. Distinct STAT proteins were present in different cell populations in neuronal retina and RPE suggesting multiple functions of STATs in mammalian eye development. Studies of STAT signal pathways in the eye may contribute to the understanding of molecular mechanisms in control of ocular development and pathogenesis.

Role of trophic factors in the development, survival and repair of primary auditory neurons

1. Neurotrophic factors have been identified as crucial for the development of the auditory system and have also been proven to be important for continued survival and maintenance of auditory neural connections. 2. In addition, both in vitro and in vivo studies have demonstrated that these trophic molecules can prevent the secondary wave of auditory neuron degeneration normally seen following the loss of hair cells. 3. Furthermore, neurotrophic factors have been reported to enhance neuronal excitation and to improve the efficacy of synaptic transmission. 4. As such, these molecules are strong candidates to be used as therapeutic agents in conjunction with the cochlear implant, or even to repair and/or regenerate damaged or lost auditory nerve and sensory cells.

Signal transducers and activators of transcription (Stat) are detectable in mouse trigeminal ganglion neurons

We studied signal transducers and activators of transcription (Stat) expression in mouse trigeminal ganglia (TG) to gain an understanding of herpes simplex virus (HSV) infection and reactivation. Mouse TG were harvested and were either frozen for Western blot analysis or preserved in 4% paraformaldehyde for subsequent immunohistochemistry study. The thawed specimens were homogenized, and nuclear/cytoplasmic extractions were performed for Western blots and immunoprecipitation. Immunohistochemistry showed that Stat1, Stat3, Stat4, Stat5b, and phosphotyrosine Stat3 localized to TG neurons, not surrounding satellite cells. Western blot of TG nuclear and cytoplasmic extracts confirmed the presence of these Stat at the appropriate molecular weights. Stat2 was undetectable in TG by these methods. Immunoprecipitation of TG nuclear extracts did not confirm the presence of Stat-Stat dimers in these specimens. These studies show that several Stat, including phosphotyrosine Stat3, are present in TG neurons, the site of HSV latency, where they could act upon latent viral DNA to effect reactivation.

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.

LIF is more potent than BDNF in promoting neurite outgrowth of mammalian auditory neurons in vitro

Neurotrophic factors are known to play a crucial role in the elongation and guidance of auditory nerve fibres to their targets within the organ of Corti. Maintenance of these neural connections following deafness would clearly influence the efficacy of therapies for hearing recovery. The growth factors leukaemia inhibitory factor (LIF), brain-derived neurotrophic factor (BDNF) and transforming growth factor-beta 5 (TGF-beta5) were tested for their efficacy in promoting neurite outgrowth from dissociated cultures of early postnatal rat auditory neurons. Our results indicate that while BDNF enhances neurite outgrowth in a strong fashion, LIF is more potent; moreover, the combined administration of both factors has even greater neuritogenic capacities. TGF-beta5, although neurotrophic, has no neuritogenic activity on cultured auditory neurons. LIF and BDNF may therefore be potential candidates when developing pharmacological therapies for hearing recovery.

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.

The neurotrophins act synergistically with LIF and members of the TGF-beta superfamily to promote the survival of spiral ganglia neurons in vitro

A number of growth factor families have been implicated in normal inner ear development, auditory neuron survival and protection. Several growth factors, including transforming growth factor-beta5 (TGF-beta5) and TGF-beta3, neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF) and leukemia inhibitory factor (LIF) were tested for their ability, individually or in combination, to promote auditory neuron survival in dissociated cell cultures of early rat post-natal spiral ganglion cells (SGCs). The results indicate that at discrete concentrations all growth factors act in an additive fashion and some in synergy when promoting neuronal survival. These findings support the hypothesis that growth factors from different families may be interdependent when sustaining neuronal integrity.

Neurotrophic factors and neuro-muscular disease: II. GDNF, other neurotrophic factors, and future directions

This is the second of two reviews in which we discuss the essential aspects of neurotrophic factor neurobiology, the characteristics of each neurotrophic factor, and their clinical relevance to neuromuscular diseases. The previous paper reviewed the neurotrophin family and neuropoietic cytokines. In the present article, we focus on the GDNF family and other neurotrophic factors and then consider future approaches that may be utilized in neurotrophic factor treatment.