Early events in neurotrophin signalling via Trk and p75 receptors

The effects of a mutant p53 protein on the proliferation and differentiation of PC12 rat phaeochromocytoma cells

PC12 rat phaeochromocytoma cells show neuronal differentiation upon NGF treatment. NGF induces prolonged activation of the Ras/Raf/MEK/ERK pathway in which the 42/44 kDa mitogen-activated protein kinases (MAPKs), ERK 1 and 2 are thought to be the key mediators of the differentiation signals. Activation of ERKs leads to the increased transcription of early response genes resulting in cell cycle arrest. Upon NGF treatment the p53 protein, the most commonly mutated tumor suppressor in human cancers, translocates to the nucleus and may play a role in the mediation of NGF-induced cell cycle arrest and neuronal differentiation. Here we demonstrate that in PC12 cells expressing both wild-type and V143A mutant p53 proteins (p143p53PC12 cells), p53-mediated biological responses are critically influenced. p143p53PC12 cells are not able to cease their proliferation and begin their neuronal differentiation program upon NGF treatment. The presence of mutant p53 also reduces the DNA-binding activity of endogenous p53 and disturbs the regulatory machinery of p53 including both the phosphorylation of ERK 1/2, p38 and SAPK/JNK MAP kinases and itself.

Functional importance of PMCA isoforms in growth and development of PC12 cells

Intracellular Ca2+ in neuronal cells is an essential regulatory ion responsible for excitability, synaptic plasticity, and neurite outgrowth. Plasma membrane calcium ATPase (PMCA) is the most sensitive enzyme in decreasing of the Ca2+ concentration. The diverse PMCA isoforms composition in the membranes suggests their specific function in the cell, and whereas PMCA1 and 4 appear to be ubiquitous, PMCA2 and 3 are characteristic isoforms for excitable cells. The aim of our study was to elucidate if and how the elimination of neuron-specific isoforms affects the pattern of cell growth and development. We have obtained stable-transfected PC12 cell lines with a suppressed expression of PMCA2, PMCA3, or both neuron-specific isoforms. The modified profile of PMCA generated considerable changes in morphology of examined PC12 lines, suggesting the activation of a differentiation process to pseudoneuronal phenotype. Experiments with Fura-2/AM-loaded cells revealed an increased cytosolic Ca2+ concentration in the cell lines with blocked PMCA2 isoform. The suppression of PMCA2 concomitantly altered expression of sarco/endoplasmic Ca2+-ATPase 2 isoform (SERCA2) at the protein level. Comparative flow cytometry analysis, using Annexin V/PI conjugate, showed the difference in the mean percentage of apoptotic cells in modified PC12 lines. Our data suggest that specific PMCA isoforms presence can regulate the intact cell development; however, it may involve multiple unidentified yet signaling pathways.

Role of nerve growth factor and its receptors in non-nervous cancer growth: efficacy of a tyrosine kinase inhibitor (AG879) and neutralizing antibodies antityrosine kinase receptor A and antinerve growth factor: an in-vitro and in-vivo study

Neurotrophins, originally identified as neuronal survival and differentiation factors, exert their actions through tyrosine kinase receptors such as TrKA, in the case of the nerve growth factor. Neurotrophins also interact with p75, a common receptor devoid of kinase activity and connected to apoptosis. Here we show that nerve growth factor, TrKA and p75 are expressed in cell lines of human cancers of various non-neuronal lineages, including a panel of muscular sarcomas, and we show that all cell lines investigated actively release nerve growth factor into the medium. Treatment by AG879 (a tyrosine kinase inhibitor that inhibits TrKA phosphorylation, but not TrKB and TrKC) or by neutralizing antibodies anti-nerve growth factor and anti-TrKA dramatically decreases their proliferation with a variable increase in apoptosis. Similarly, p75 transfection induced a significant increase in apoptosis. Furthermore, for the first time we have determined by high-performance liquid chromatography the pharmacokinetic profile of a novel preparation of AG879 and we have established an optimal plasmatic concentration for in-vivo administration. Treatment with AG879 in immunodepressed mice grafted with leiomyosarcoma or promyelocytic leukemia cells resulted in dramatic reductions in tumor sizes. In conclusion, our data have a novel preclinical potential for revealing a possible therapeutical utility in targeting in-vivo nerve growth factor/TrKA by AG879 or neutralizing antibody anti-TrKA in cancer proliferation and in muscle sarcomas, in particular.

Nucleotide sequence variation within the human tyrosine kinase B neurotrophin receptor gene: association with antisocial alcohol dependence

To identify sequence variants in genes that may have roles in neuronal responses to alcohol, we resequenced the 5' region of tyrosine kinase B neurotrophin receptor gene (NTRK2) and determined linkage disequilibrium (LD) values, haplotype structure, and performed association analyses using 43 single nucleotide polymorphisms (SNPs) covering the entire NTRK2 region in a Finnish Caucasian sample of 229 alcohol-dependent subjects with antisocial personality disorder (ASPD) and 287 healthy controls. Individually, three SNPs were associated with alcohol dependence and alcohol abuse (AD) (P-value from 0.0019 to 0.0059, significance level was set at P<or=0.01 corrected for multiple testing), whereas a common 18 locus haplotype within the largest LD block of NTRK2, a 119-kb region containing the 5' flanking region and exons 1-15, was marginally overrepresented in control subjects compared to AD individuals (global P=0.057). Taken together, these results support a role for the NTRK2 gene in addiction in a Caucasian population with AD and a subtype of ASPD.

Label-free detection of neuron–drug interactions using acoustic and Kelvin vibrational fields

Kelvin and acoustic fields of high-frequency have been employed in the non-invasive investigation of immortalized hypothalamic neurons, in order to assess their response to different concentrations of specific drugs, toxins, a stress-reducing hormone and neurotrophic factors. In an analytical systems biology approach, this work constitutes a first study of living neuron cultures by scanning Kelvin nanoprobe (SKN) and thickness shear mode (TSM) acoustic wave techniques. N-38 hypothalamic mouse neurons were immobilized on the gold electrode of 9 MHz TSM acoustic wave devices and gold-coated slides for study by SKN. The neurons were exposed to the neurochemicals betaseron, forskolin, TCAP, and cerebrolysin. Signals were collected with the TSM in real-time mode, and with the SKN in scanning and real-time modes, as the drugs were applied at biologically significant concentrations. With the TSM, for all drugs, some frequency and resistance shifts were in the same direction, contrary to normal functioning for this type of instrument. Possible mechanisms are presented to explain this behaviour. An oscillatory signal with periodicity of approximately 2 min was observed for some neuron-coated surfaces, where the amplitude of these oscillations was altered upon application of certain neurotrophic factors. These two new techniques present novel and non-invasive electrodeless methods for detecting changes at the cellular level caused by a variety of neuroactive compounds, without killing or destroying the neurons.

The role of ubiquitin in neurotrophin receptor signalling and sorting

NGF (nerve growth factor) binding to TrkA (tropomyosin receptor kinase A) induces dimerization, autophosphorylation and internalization of the receptor to signalling vesicles for delivery of differentiation signals. TrkA interacts with p75 receptor through the p62–TRAF-6 (tumour-necrosis-factor-receptor-associated factor 6) complex bridging the two receptors. The atypical protein kinase C is activated and recruited to the receptor complex as well. TrkA is Lys63-polyubiquitinated on Lys485 by the E3 (ubiquitin ligase), TRAF-6, and E2 (ubiquitin-conjugating enzyme), UbcH7. Inhibition of polyubiquitination has been observed to interrupt signalling and internalization. Furthermore, an absence of p62 prevents endosomal localization and signalling. Altogether, these findings reveal Lys63-linked polyubiquitin chains and the shuttling protein p62 co-ordinately regulate TrkA internalization, trafficking and sorting.

Involvement of TNF-alpha in glutamate-induced apoptosis in a differentiated neuronal cell line

We examined the involvement of tumor necrosis factor (TNF)-alpha on glutamate-induced cytotoxicity in a differentiated neuronal cell line. In this study, we used nerve growth factor (NGF)-differentiated PC12h cells. Glutamate cytotoxicity was assessed using the MTS and TUNEL assays. To detect TNF-alpha levels in culture supernatants after glutamate exposure, we used ELISA methods. The involvement of caspase-8, which is downstream from TNF receptor 1 (TNF-R1) in glutamate-induced cytotoxicity, was determined by Western blot analysis. The MTS assay showed that the addition of glutamate resulted in dose-dependent cell death, while the TUNEL assay showed that glutamate induced apoptosis in differentiated PC12h cells in a dose-dependent manner. TNF-alpha levels in the supernatant of glutamate-exposed cells were significantly increased compared with those in unexposed cells. In addition, glutamate caused increases in the levels of caspase-8 protein. The increases in caspase-8 levels were ameliorated by pretreatment with soluble TNF-R1. Moreover, soluble TNF-R1 significantly ameliorated the cell death induced by glutamate. These results suggest that TNF-alpha released from neuronal cells may be associated with glutamate-induced neuronal cell death.

Interaction of survival and death signaling in basal forebrain neurons: roles of neurotrophins and proneurotrophins

Proneurotrophins bind with high affinity to p75 neurotrophin receptor (p75NTR) and lack the capacity to bind Trk receptors, suggesting that proneurotrophins can elicit apoptosis via p75NTR even in cells expressing survival-promoting Trk receptors. In the CNS, basal forebrain (BF) neurons are particularly vulnerable to degeneration in Alzheimer's disease, and are among the few populations of brain neurons that express p75NTR throughout life. These neurons also express Trk receptors and may be concomitantly exposed to both proneurotrophins and mature neurotrophins during development, disease, or after injury. We investigated the interaction of mature and proneurotrophin signaling in these CNS neurons. Kainic acid-induced seizures elicited production of pro-NGF by BF astrocytes before caspase activation in p75NTR-positive BF neurons, demonstrating local production of proneurotrophins under pathological conditions and suggesting apoptotic signaling in vivo. Mechanisms of proneurotrophin-induced death were analyzed in cultured BF neurons, and required both p75NTR and its coreceptor sortilin. Surprisingly, exposure to both mature neurotrophins and proneurotrophins demonstrated that Trk phosphorylation did not prevent pro-NGF-induced apoptosis via p75NTR. However, activation of PI3K (phosphatidylinositol 3-kinase)/Akt and MEK (mitogen-activated protein kinase kinase)/Erk pathways prevented pro-NGF-induced apoptosis, revealing a novel critical checkpoint in survival versus apoptotic signaling downstream of Trk activation, and suggesting that pro-NGF blocks survival signaling by preventing Akt and Erk activation. This study shows that proneurotrophins are produced in the brain under pathological conditions, and can elicit apoptosis of BF neurons even when Trk receptors are activated.

Interleukin-1 beta contributes to the upregulation of kappa opioid receptor mrna in dorsal root ganglia in response to peripheral inflammation

During local painful inflammation, axonal transport of opioid receptors from dorsal root ganglia toward the periphery is increased, associated with a higher receptor density and enhanced efficacy of opioid analgesics at the injured site. To examine whether this increase is related to transcription, mRNA of the kappa opioid receptor in lumbar dorsal root ganglia was quantified by real time light cycler polymerase chain reaction. In dorsal root ganglia of naive rats, kappa opioid receptor mRNA expression was three-fold higher than previously shown for delta opioid receptor and two times lower than mu opioid receptor mRNA, respectively. After induction of unilateral paw inflammation by Freund's complete adjuvant, kappa opioid receptor mRNA was significantly upregulated with a peak at 12 h in ipsilateral dorsal root ganglia. This effect could be mimicked by intraplantar injection of the proinflammatory cytokine interleukin-1 beta. Kappa opioid receptor mRNA upregulation lasted longer in interleukin-1 beta-treated rats compared with Freund's complete adjuvant-treated rats. Furthermore, a significant increase in kappa opioid receptor positive neurons was detected by immunohistochemistry 24 h after local injection of Freund's complete adjuvant or interleukin-1 beta. In Freund's complete adjuvant-induced inflammation, kappa opioid receptor upregulation was blocked by treatment with interleukin-1 receptor antagonist without changing the leukocyte infiltration in the paw. In conclusion, kappa opioid receptor mRNA and protein in dorsal root ganglia are upregulated in response to peripheral inflammation. This effect can be mimicked by a single local injection of interleukin-1 beta, and Freund's complete adjuvant-induced upregulation in kappa opioid receptor mRNA and protein can be prevented by treatment with interleukin-1 receptor antagonist. These data suggest that the peripheral production of the proinflammatory cytokine interleukin-1 beta is a specific inducer of kappa opioid receptor expression in the dorsal root ganglia.

CyclinB2 and BIRC5 genes as surrogate biomarkers for neurite outgrowth in SH-SY5Y subclonal cells

Neurite outgrowth plays a key role in neuronal development and regeneration, and is the hallmark assay for the effects of neurotrophic factors such as nerve growth factor (NGF). However, measuring neurite outgrowth is a slow and resource-intensive process. We therefore wanted to identify surrogate biomarkers for neurite outgrowth activity by gene expression analysis in SH-O10 cells, a subclone of the human SH-SY5Y neuroblastoma cell line but with much higher NGF-induced neurite outgrowth activity. Microarray analysis identified seven genes where mRNA levels were changed. NGF-induced decreases in levels of two genes, CyclinB2 and BIRC5, were confirmed by quantitative real-time RT-PCR. Levels of NGF-induced decreases in CyclinB2 and BIRC5 mRNA in several SH-SY5Y subclones with different neurite outgrowth responses correlated with their neurite outgrowth activities. Decreases in CyclinB2 and BIRC5 mRNA induced by FK506 or retinoic acid, both of which exert potentiation of NGF-induced neurite outgrowth effects but with different mechanisms, also correlated with their neurite outgrowth activities. In conclusion, decreasing levels of CyclinB2 and BIRC5 mRNA strongly correlate with neurite outgrowth activities in terms of NGF-related effect in SH-SY5Y subclonal cells, and have potential to become quantitative surrogate biomarkers for measuring NGF-related neurite outgrowth.

Expression of Nerve Growth Factor and Its Receptors NTRK1 and TNFRSF1B Is Regulated by Estrogen and Progesterone in the Uteri of Golden Hamsters1

Experiments were conducted using female golden hamsters to identify the presence of nerve growth factor (NGF) and its receptors NTRK1 and TNFRSF1B in the uteri of female animals and regulation on their expression by estrogen and progesterone. NGF and its receptor NTRK1 were immunolocalized to luminal epithelial cells, glandular cells, and stromal cells. TNFRSF1B was immunolocalized in luminal epithelial and glandular cells, with no staining found in stromal cells of the uterine horns of normal cyclic golden hamsters. Strong immunostaining of NGF and its receptors NTRK1 and TNFRSF1B was observed in uteri on the day of proestrus as compared to the other stages of the estrous cycle. Results of immunoblot analysis of NGF revealed that there was a positive correlation between uterine NGF expression and plasma concentrations of estradiol-17beta. To clarify the effects of estrogen and progesterone on NGF, NTRK1, and TNFRSF1B expression, adult female golden hamsters were ovariectomized and treated with estradiol-17beta and/or progesterone. Immunoblot analysis and immunohistochemistry indicated that estradiol-17beta stimulated expression of NGF and its two receptors in the uterus. Treatment with progesterone also increased NGF and NTRK1 expression in the uterus. However, no additive effect of these steroids on expression of NGF and its receptors was observed. Changes in uterine weights induced by estradiol-17beta and/or progesterone showed the same profile with that of NGF, suggesting that a proliferative act of NGF may be involved in uterine growth. These results suggest that NGF may play important roles in action of steroids on uterine function.

Null mutations for exon III and exon IV of the p75 neurotrophin receptor gene enhance sympathetic sprouting in response to elevated levels of nerve growth factor in transgenic mice

Under normal conditions, expression of the p75 neurotrophin receptor (p75NTR) by sympathetic neurons can increase the affinity of the signaling receptor, trkA, to target-derived nerve growth factor (NGF) at distal axons. We have previously reported that sprouting of sympathetic axons into NGF-rich target tissues is enhanced when p75NTR expression is perturbed, leading to the postulate that p75NTR may restrain sympathetic sprouting in response to elevated NGF levels. These observations were made using mice having a null mutation of the third p75NTR exon, a line that may express a hypomorphic form of this receptor. Since mice carrying a null mutation of the fourth p75NTR exon may not express a similar splice variant, we sought to determine whether these animals possess the same phenotype of enhanced sympathetic sprouting in response to elevated levels of NGF. Both lines of transgenic mice lacking p75NTR displayed similar degrees of sympathetic axonal sprouting into the cerebellum and trigeminal ganglia, two target tissues having elevated levels of NGF protein. Furthermore, the densities of sympathetic axons in both targets were significantly greater than those observed in age-matched NGF transgenic siblings expressing full-length p75NTR. Our new findings provide a comparative analysis of the phenotype in two independent mutations of the same neurotrophin receptor, revealing that p75NTR plays an important role in restricting sympathetic sprouting in response to higher NGF levels.

The role of cytokines and trophic factors in epiretinal membranes: Involvement of signal transduction in glial cells

Idiopathic epiretinal membranes (ERMs) in the macular region can cause a reduction in vision and sometimes recurs after surgical removal, but its pathogenic mechanisms are still unknown. On the other hand, the presence of secondary ERMs has been associated with various clinical conditions including proliferative diabetic retinopathy (PDR) and proliferative vitreoretinopathy (PVR). Recent studies have shown a significant association between clinical grades of PDR or PVR, and the expression levels of specific cytokines and/or growth factors in the vitreous fluid. Expression of these factors and their receptors are also observed in secondary ERMs. ERMs are composed of many cell types such as retinal pigment epithelial cells and vascular endothelial cells, however the role of glial cells is yet unclear. Interestingly, glial cells in ERMs express some trophic factor receptors and transcription factors, such as NF-kappaB, suggesting an involvement of glial signal transduction in the pathogenesis of ERMs. In this review, we summarize recent progress regarding the clinical and laboratory findings of ERMs.

The biological differences between ovarian serous carcinoma and diffuse peritoneal malignant mesothelioma

Recent improvements in immunohistochemistry panels used for differentiating ovarian serous carcinoma/primary peritoneal carcinoma (OC/PPC) from diffuse malignant peritoneal mesothelioma (DMPM) have resulted in improved diagnostic rates for these tumors in both cytological and histological material. However, little is known about the biological characteristics that differentiate these two cancer types. We performed a comparative analysis of cancer-associated molecule expression data for a cohort consisting of up to 270 serous OC/PPC specimens (only peritoneal lesions) and 32 peritoneal MM. The molecules studied were nerve growth factor receptors (p75, p-TrkA), angiogenic factors (VEGF, IL-8, bFGF, heparanase), laminin receptors (the 67-kDa receptor and the alpha 6 integrin subunit), proteases (MMP-2), immune response mediators (HLA-G), and signaling molecules (the MAPK members ERK, JNK, and p38). The methods used were immunohistochemistry, Western blotting, and RT-PCR. DMPM specimens showed significantly higher expression of p75 (P < 0.001), p-TrkA (P < 0.001), and bFGF (P < 0.001), and significantly lower expression of the 67-kDa receptor (P < 0.001), alpha 6 integrin subunit (P = 0.025), VEGF (P < 0.001), IL-8 (P < 0.001), and HLA-G (P = 0.039) compared with OC/PPC. DMPM specimens showed higher activation ratio (phosphorylated/total enzyme ratio) of all three MAPK members (ERK, P = 0.017; JNK, P < 0.001; p38, P = 0.009) compared with OC/PPC. These data document significant differences in the expression of cancer- and metastasis-associated molecules in MM compared with ovarian carcinoma, and suggest that different biological pathways are involved in tumorigenesis and disease progression in these two tumors.

Creatine and neurotrophin-4/5 promote survival of nitric oxide synthase-expressing interneurons in striatal cultures

Nitric oxide (NO) mediates a variety of physiological functions in the central nervous system and acts as an important developmental regulator. Striatal interneurons expressing neuronal nitric oxide synthase (nNOS) have been described to be relatively spared from the progressive cell loss in Huntington's disease (HD). We have recently shown that creatine, which supports the phosphagen energy system, induces the differentiation of GABAergic cells in cultured striatal tissue. Moreover, neurotrophin-4/5 (NT-4/5) has been found to promote the survival and differentiation of cultured striatal neurons. In the present study, we assessed the effects of creatine and NT-4/5 on nNOS-immunoreactive (-ir) neurons of E14 rat ganglionic eminences grown for 1 week in culture. Chronic administration of creatine [5mM], NT-4/5 [10ng/ml], or a combination of both factors significantly increased numbers of nNOS-ir neurons. NT-4/5 exposure also robustly increased levels of nNOS protein. Interestingly, only NT-4/5 and combined treatment significantly increased general viability but no effects were seen for creatine supplementation alone. In addition, NT-4/5 and combined treatment resulted in a significant larger soma size and number of primary neurites of nNOS-ir neurons while creatine administration alone exerted no effects. Double-immunolabeling studies revealed that all nNOS-ir cells co-localized with GABA. In summary, our findings suggest that creatine and NT-4/5 affect differentiation and/or survival of striatal nNOS-ir GABAergic interneurons. These findings provide novel insights into the biology of developing striatal neurons and highlight the potential of both creatine and NT-4/5 as therapeutics for HD.

Differences in gene expression profile among SH-SY5Y neuroblastoma subclones with different neurite outgrowth responses to nerve growth factor

Nerve growth factor (NGF) plays a key role in the differentiation of neurons. In this study, we established three NGF-induced neurite-positive (NIN+) subclones that showed high responsiveness to NGF-induced neurite outgrowth and three NGF-induced neurite-negative (NIN-) subclones that abolished NGF-induced neurite outgrowth from parental SH-SY5Y cells, and analyzed differences in the NGF signaling cascade. The NIN+ subclones showed enhanced responsiveness to FK506-mediated neurite outgrowth as well. To clarify the mechanism behind the high frequency of NGF-induced neurite outgrowth, we investigated differences in NGF signaling cascade among subclones. Expression levels of the NGF receptor TrkA, and NGF-induced increases in mRNAs for the immediate-early genes (IEGs) c-fos and NGF inducible (NGFI) genes NGFI-A, NGFI-B and NGFI-C, were identical among subclones. Microarray analysis revealed that the NIN+ cell line showed a very different gene expression profile to the NIN- cell line, particularly in terms of axonal vesicle-related genes and growth cone guidance-related genes. Thus, the difference in NGF signaling cascade between the NIN+ and NIN- cell lines was demonstrated by the difference in gene expression profile. These differentially expressed genes might play a key role in neurite outgrowth of SH-SY5Y cells in a region downstream from the site of induction of IEGs, or in a novel NGF signaling cascade.

The excitoprotective effect of N-methyl-D-aspartate receptors is mediated by a brain-derived neurotrophic factor autocrine loop in cultured hippocampal neurons

The neuroprotective effect and molecular mechanisms underlying preconditioning with N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons have not been described. Pre-incubation with subtoxic concentrations of the endogenous neurotransmitter glutamate protects vulnerable neurons against NMDA receptor-mediated excitotoxicity. As a result of physiological preconditioning, NMDA significantly antagonizes the neurotoxicity resulting from subsequent exposure to an excitotoxic concentration of glutamate. The protective effect of glutamate or NMDA is time- and concentration-dependent, suggesting that sufficient agonist and time are required to establish an intracellular neuroprotective state. In these cells, the TrkB ligand, brain-derived neurotrophic factor (BDNF) attenuates glutamate toxicity. Therefore, we tested the hypothesis that NMDA protects neurons via a BDNF-dependent mechanism. Exposure of hippocampal cultures to a neuroprotective concentration of NMDA (50 microM) evoked the release of BDNF within 2 min without attendant changes in BDNF protein or gene expression. The accumulated increase of BDNF in the medium is followed by an increase in the phosphorylation (activation) of TrkB receptors and a later increase in exon 4-specific BDNF mRNA. The neuroprotective effect of NMDA was attenuated by pre-incubation with a BDNF-blocking antibody and TrkB-IgG, a fusion protein known to inhibit the activity of extracellular BDNF, suggesting that BDNF plays a major role in NMDA-mediated survival. These results demonstrate that low level stimulation of NMDA receptors protect neurons against glutamate excitotoxicity via a BDNF autocrine loop in hippocampal neurons and suggest that activation of neurotrophin signaling pathways plays a key role in the neuroprotection of NMDA.

Glutamate receptor antagonists and growth factors modulate dentate granule cell neurogenesis in organotypic, rat hippocampal slice cultures

Generation of dentate granule cells and its modulation by glutamate receptor antagonists, growth factors and pilocarpine-induced seizure-like activity was investigated in rat hippocampal slice cultures derived from 1-week-old rats and grown for 2 weeks. Focussing on the dentate granule cell layer facing CA1 and the immediate subgranular zone, exposure for 3 days to the NMDA receptor blocking agents MK-801 (10 microM) or APV (25 microM) in the culture medium, increased the number of TOAD-64/Ulip/CRMP-4 (TUC-4)-positive cells as counted in the slice cultures at the end of the 3-day treatment period. Exposure to IGF-I (200 ng/ml) and EGF (20 ng/ml) also increased the number of TUC-4-positive cells. Combining APV with IGF-I/EGF had an additive effect. Similar results were obtained by 3 days treatment with the AMPA receptor antagonist CNQX (25 microM). Surprisingly, addition of 5 mM pilocarpine reduced the number of TUC-4-positive cells, just as combining pilocarpine with the neurogenesis-stimulating compounds, prevented or reduced the increase of TUC-4-positive cells. None of the treatments were found to induce dentate granule cell death within the observed period. Labeling of dividing cells by adding 5-bromo-2-deoxyuridine (BrdU) to the culture medium did not result in cells double-labeled with BrdU and TUC-4. The induced increase in TUC-4-positive cells therefore represent neuronal differentiation of existing neural precursor cells when investigated at the 3-day time point. We conclude that 3 days treatment of 2-week-old hippocampal slice cultures with IGF-I and EGF and NMDA and AMPA glutamate receptor antagonists increase granule cell neurogenesis from preexisting neural precursors.

Effects of Thimerosal on NGF Signal Transduction and Cell Death in Neuroblastoma Cells

Signaling through neurotrophic receptors is necessary for differentiation and survival of the developing nervous system. The present study examined the effects of the organic mercury compound thimerosal on nerve growth factor signal transduction and cell death in a human neuroblastoma cell line (SH-SY5Y cells). Following exposure to 100 ng/ml NGF and increasing concentrations of thimerosal (1 nM-10 microM), we measured the activation of TrkA, MAPK, and PKC-delta. In controls, the activation of TrkA MAPK and PKC-delta peaked after 5 min of exposure to NGF and then decreased but was still detectable at 60 min. Concurrent exposure to increasing concentrations of thimerosal and NGF for 5 min resulted in a concentration-dependent decrease in TrkA and MAPK phosphorylation, which was evident at 50 nM for TrkA and 100 nM for MAPK. Cell viability was assessed by the LDH assay. Following 24-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence or absence of NGF was 596 nM and 38.7 nM, respectively. Following 48-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence and absence of NGF was 105 nM and 4.35 nM, respectively. This suggests that NGF provides protection against thimerosal cytotoxicity. To determine if apoptotic versus necrotic cell death was occurring, oligonucleosomal fragmented DNA was quantified by ELISA. Control levels of fragmented DNA were similar in both the presence and absence of NGF. With and without NGF, thimerosal caused elevated levels of fragmented DNA appearing at 0.01 microM (apoptosis) to decrease at concentrations >1 microM (necrosis). These data demonstrate that thimerosal could alter NGF-induced signaling in neurotrophin-treated cells at concentrations lower than those responsible for cell death.

Militarinone A induces differentiation in PC12 cells via MAP and Akt kinase signal transduction pathways

The fungal metabolite militarinone A (MILI A) promotes neurite outgrowth in PC12 cells. This study was conducted to investigate the signaling pathways involved in the cellular differentiation processes induced by the compound, with a focus on cascades implicated with nerve growth factor (NGF)-mediated neuritogenesis. MILI A possessed pronounced amphiphilic properties. The compound rapidly accumulated in the cell membrane and was slowly released into the cytoplasma. In primed PC12 cells, an early activation of protein kinase B (Akt), representing a downstream target of phosphoinositol 3 (PI3) kinase, and a delayed phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2), and of transcription factor cAMP responsive element binding protein (CREB) was found. The NGF-dependent activation of c-Jun amino terminal kinase (SAPK/JNK1) was potentiated. Morphological differentiation of cells and the phosphorylation of specific signal molecules were blocked by the MAP kinase (MEK1) inhibitor PD098059, the PI3-kinase (PI3K) inhibitor wortmannin and the adenylyl cyclase inhibitor 9-cyclopentyladenine.

Rapid upregulation of mu opioid receptor mRNA in dorsal root ganglia in response to peripheral inflammation depends on neuronal conduction

S.c. painful inflammation leads to an increase in axonal transport of opioid receptors from dorsal root ganglia (DRG) toward the periphery, thus causing a higher receptor density and enhanced opioid analgesia at the injured site. To examine whether this increase is related to transcription, the mRNA of Delta- (DOR) and mu-opioid receptor (MOR) in lumbar DRG was quantified by real time Light Cycler polymerase chain reaction (LC-PCR), and correlated to ligand binding in DRG and sciatic nerve. In normal DRG, DOR mRNA was seven times less abundantly expressed than MOR mRNA. After induction of unilateral paw inflammation, mRNA content for DOR remained unchanged, but a bi-phasic upregulation for MOR mRNA with an early peak at 1-2 h and a late increase at 96 h was found in ipsilateral DRG. As no changes were observed in DRG of the non-inflamed side, this effect was apparently not systemically mediated. A significant increase in binding of the MOR ligand DAMGO was detected after 24 h in DRG, and after early and late ligation in the sciatic nerve, indicating an enhanced axonal transport of MOR in response to inflammation. The early increase in MOR mRNA could be completely prevented by local anesthetic blockade of neuronal conduction in sciatic nerve. These data suggest that mRNA of the two opioid receptors DOR and MOR is differentially regulated in DRG during peripheral painful inflammation. The apparently increased axonal transport of MOR in response to this inflammation is preceded by upregulated mRNA-transcription, which is dependent on neuronal electrical activity.

Comparison of target innervation by sympathetic axons in adult wild type and heterozygous mice for nerve growth factor or its receptor trkA

Nerve growth factor (NGF), a neurotrophin required for the survival and maintenance of postganglionic sympathetic neurons, mediates its trophic effects by activation of its high-affinity receptor trkA. Null mutant mice lacking either NGF or trkA have profound sympathetic deficits, thus revealing the vital importance of NGF synthesis in target tissues and trkA expression by sympathetic neurons. In this study, we sought to assess whether sympathetic neurons of the superior cervical ganglion (SCG) display alterations in their neurochemical phenotype in adult mice carrying one mutated allele for either NGF or trkA, and whether such differences result in altered patterns of innervation to the submandibular salivary gland and pineal gland. In comparison with adult siblings, levels of trkA protein in the SCG were reduced in age-matched NGF(+/-) and trkA(+/-) mice. While NGF(+/-) mice also had significantly fewer sympathetic axons innervating both the submandibular salivary gland and pineal gland, densities of sympathetic axons in both tissues reached normal levels in trkA(+/-) mice. These findings reveal that while levels of trkA are reduced in SCG neurons of adult NGF(+/-) and trkA(+/-) mice (compared with their wild type counterparts), sympathetic axons are capable of achieving normal patterns of target innervation in trkA(+/-) mice but not in NGF(+/-) mice. As NGF protein levels are not depleted in the submandibular salivary gland and pineal gland of NGF(+/-) mice, a loss of sympathetic neurons [Nat Neurosci 1999; 2:699-705], in combination with reduced levels of trkA protein, may account for perturbed patterns of sympathetic innervation to peripheral tissues.

TrkA alternative splicing: a regulated tumor-promoting switch in human neuroblastoma

We identify a novel alternative TrkA splice variant, TrkAIII, with deletion of exons 6, 7, and 9 and functional extracellular IG-C1 and N-glycosylation domains, that exhibits expression restricted to undifferentiated early neural progenitors, human neuroblastomas (NBs), and a subset of other neural crest-derived tumors. This NGF-unresponsive isoform is oncogenic in NIH3T3 cells and promotes tumorigenic NB cell behavior in vitro and in vivo (cell survival, xenograft growth, angiogenesis) resulting from spontaneous tyrosine kinase activity and IP3K/Akt/NF-kappaB but not Ras/MAPK signaling. TrkAIII antagonizes NGF/TrkAI signaling, which is responsible for NB growth arrest and differentiation through Ras/MAPK, and its expression is promoted by hypoxia at the expense of NGF-responsive receptors, providing a mechanism for converting NGF/TrkA/Ras/MAPK antioncogenic signals to TrkAIII/IP3K/Akt/NF-kappaB tumor-promoting signals during tumor progression.

The many functions of nerve growth factor: multiple actions on nociceptors

Nerve growth factor (NGF) initially interested neurobiologists because of its effects in the developing nervous system. It is now clear that NGF functions throughout the life of the animal with a wide repertoire of actions. In the sensory nervous system it primarily influences the structure and function of nociceptors. Here, we provide a brief review of these actions and raise the overriding biological questions of why these multiple actions occur and how they are carried out.

Microglia-derived pronerve growth factor promotes photoreceptor cell death via p75 neurotrophin receptor

Reports implicating microglia-derived nerve growth factor (NGF) during programmed cell death in the developing chick retina led us to investigate its possible role in degenerative retinal disease. Freshly isolated activated retinal microglia expressed high molecular weight forms of neurotrophins including that of nerve growth factor (NGF), brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4. Conditioned media from cultured retinal microglia (MGCM) consistently yielded a approximately 32-kDa NGF-reactive band when supplemented with bovine serum albumin (BSA) or protease inhibitors (PI); and promoted cell death that was suppressed by NGF immunodepletion in a mouse photoreceptor cell line (661w). The approximately 32 kDa protein was partially purified (MGCM/p32) and was highly immunoreactive with a polyclonal anti-pro-NGF antibody. Both MGCM/p32 and recombinant pro-NGF protein promoted cell death in 661w cultures. Increased levels of pro-NGF mRNA and protein were observed in the RCS rat model of retinal dystrophy. MGCM-mediated cell death was reversed by p75NTR antiserum in p75NTR(+)/trkA(-) 661w cells. Our study shows that a approximately 32 kDa pro-NGF protein released by activated retinal microglia promoted degeneration of cultured photoreceptor cells. Moreover, our study suggests that defective post-translational processing of NGF might be involved in photoreceptor cell loss in retinal dystrophy.

Adapter protein SH2-B beta undergoes nucleocytoplasmic shuttling: implications for nerve growth factor induction of neuronal differentiation

The adapter protein SH2-B has been shown to bind to activated nerve growth factor (NGF) receptor TrkA and has been implicated in NGF-induced neuronal differentiation and the survival of sympathetic neurons. However, the mechanism by which SH2-B enhances and maintains neurite outgrowth is unclear. We examined the ability of truncation mutants to regulate neuronal differentiation and observed that certain truncation mutants localized in the nucleus rather than in the cytoplasm or at the plasma membrane as reported for wild-type SH2-B beta. Addition of the nuclear export inhibitor leptomycin B caused both overexpressed wild-type and endogenous SH2-B beta to accumulate in the nucleus of both PC12 cells and COS-7 cells as did deletion of a putative nuclear export sequence (amino acids 224 to 233) or mutation of two critical lysines in that sequence. Deleting or mutating the nuclear export signal caused SH2-B beta to lose its ability to enhance NGF-induced differentiation of PC12 cells. Neither the NGF-induced phosphorylation of ERKs 1 and 2 nor their subcellular distribution was altered in PC12 cells stably expressing the nuclear export-defective SH2-B beta(L231A, L233A). These data provide strong evidence that SH2-B beta shuttles constitutively between the nucleus and cytoplasm. However, SH2-B beta needs continuous access to the cytoplasm and/or plasma membrane to participate in NGF-induced neurite outgrowth. These data also suggest that the stimulatory effect of SH2-B beta on NGF-induced neurite outgrowth of PC12 cells is either downstream of ERKs or via some other pathway yet to be identified.

The tyrosine phosphatase inhibitor orthovanadate mimics NGF-induced neuroprotective signaling in rat hippocampal neurons

Activation of the high affinity neurotrophin receptor tropomyosin-related kinase A (TrkA) by nerve growth factor (NGF) leads to phosphorylation of intracellular tyrosine residues of the receptor with subsequent activation of signaling pathways involved in neuronal survival such as the phosphoinositide-3-kinase (PI3-K)/protein kinase B (PKB/Akt) pathway and the mitogen-activated protein kinase (MAPK) cascade. In the present study, we tested whether inhibition of protein-tyrosine phosphatases (PTP) by orthovanadate could enhance tyrosine phosphorylation of TrkA thereby stimulating NGF-like survival signaling in embryonic hippocampal neurons. We found that the PTP inhibitor orthovanadate (1 microM) enhanced TrkA phosphorylation and protected neurons against staurosporine (STS)-induced apoptosis in a time-and concentration-dependent manner. Inhibition of PTP enhanced TrkA phosphorylation also in the presence of NGF antibodies indicating that NGF binding to TrkA was not required for the effects of orthovanadate. Moreover, orthovanadate enhanced phosphorylation of Akt and the MAPK Erk1/2 suggesting that the signaling pathways involved in the protective effect were similar to those activated by NGF. Accordingly, inhibition of PI3-K by wortmannin and MAPK-kinase (MEK) inhibition by UO126 abolished the neuroprotective effects. In conclusion, the results indicate that orthovanadate mimics the effect of NGF on survival signaling pathways in hippocampal neurons. Thus, PTP inhibition appears to be an appropriate strategy to trigger neuroprotective signaling pathways downstream of neurotrophin receptors.

Altered expression and activation of the nerve growth factor receptors TrkA and p75 provide the first evidence of tumor progression to effusion in breast carcinoma

The aim of this study was to characterize phenotypic alterations along the progression of breast carcinoma from primary tumor to pleural effusion through analysis of the expression of nerve growth factor (NGF) and its receptors phospho-TrkA (p-TrkA activated receptor) and p75. Sections from 42 malignant pleural effusions from breast cancer patients and 65 corresponding solid tumors (34 primary, 31 metastatic) were evaluated for protein expression of the activated p-TrkA receptor. The majority of lesions were additionally studied for NGF and p75 expression. Six effusions and four breast carcinoma cell lines were studied for expression of p-TrkA using immunoblotting (IB). Membrane expression of p-TrkA was high in carcinoma cells in effusions (39/42, 93%) and locoregional recurrences (12/13, 92%), with significantly lower expression in both primary tumors (14/34, 41%) and lymph node metastases (8/18, 44%), respectively (p < 0.001 for effusions vs. primary tumors; p = 0.001 for effusions vs. lymph nodes). In contrast, p75 expression was less frequent in effusions compared to both primary tumors and lymph node metastases, significantly so for the latter (p = 0.019). NGF expression was comparable at all sites, but its expression in tumor cells in effusions (7/21 cases) was limited to cases in which time to progression (TTP) to effusion occurred within 5 years or less from primary operation. In univariate analysis of survival, mean and median TTP were 6.3 and 6 years for NGF-negative effusions, compared to 3 and 4 years for NGF-positive cases (p = 0.013). IB confirmed expression of p-TrkA in five of six effusions, while all four breast cancer cell lines were p-TrkA-negative. Our data provide the first documented evidence of molecular events that occur along tumor progression of breast carcinoma from primary tumors to effusion. The almost universal expression of p-TrkA in cancer cells in effusions and late recurrences is in full agreement with our recent report linking this factor with poor prognosis in ovarian cancer. Furthermore, the rapid progression to effusion in cases showing NGF expression in tumor cells underscores the aggressive clinical behavior of tumors that are able to utilize this pathway in an autocrine manner.

Expression of the nerve growth factor receptors TrkA and p75 in malignant mesothelioma

The objective of the present report was to study the expression of the low affinity nerve growth factor (NGF) receptor p75 and of the activated high-affinity NGF receptor TrkA in malignant mesothelioma (MM). In addition, to analyze whether expression of these receptors is site-related (pleural versus peritoneal MM, solid lesions versus effusions). Sections from 81 MM (57 biopsies, 24 effusions) were analyzed. Sixty-one mesotheliomas were of pleural origin, while the remaining 20 were peritoneal. Effusion specimens consisted of 6 peritoneal and 18 pleural effusions, while biopsies consisted of 14 peritoneal and 43 pleural lesions. Specimens were immunohistochemically stained using antibodies against p75 and phospho-TrkA (p-TrkA). Six effusions were additionally analyzed for p-TrkA expression using immunoblotting (IB). p-TrkA membrane expression (66/81 specimens; 81%) was by far more frequent than that of p75 (26/81 specimens; 32%). In addition, p-TrkA expression was significantly higher in peritoneal MM compared to their pleural counterparts (20/20 versus 46/61 positive tumors; P = 0.014). p-TrkA membrane expression was marginally higher in effusions (P = 0.058), while the opposite was true for p75 membrane expression (P = 0.008) and p-TrkA cytoplasmic expression (P = 0.003). In conclusion, our results document for the first time frequent expression of p-TrkA and lower expression of p75 in MM, in agreement with the biological aggressiveness of this tumor. The enhanced expression of p-TrkA in peritoneal MM, tumors that appear in younger patients, and in effusions as compared to solid tumors, suggest that p-TrkA plays a significant role in the biology of this disease and may aid in defining tumor progression in this setting.

Expression of nerve growth factor in the airways and its possible role in asthma

Nerve growth factor (NGF), in addition to its essential role in neuronal growth and survival, may also act as an inflammatory mediator. As several animal studies have shown, NGF appears to play a part in the development of airway hyperresponsiveness and in the increased sympathetic and sensory innervation of the lung. It also has a profound effect on airway inflammation and asthma-related symptoms. Sources of NGF in the airways are numerous: inflammatory cells infiltrated into the bronchial mucosa, and structural cells including lung fibroblasts, airway epithelial and smooth muscle cells. These cells, by releasing more NGF in inflammatory conditions, may contribute to the increased NGF levels observed in bronchoalveolar lavage fluid and serum from patients with asthma. Taken together, these results suggest that NGF is an important mediator in both inflammation and asthma.

Signalling mechanisms for survival of lesioned motoneurons

Mechanisms controlling neuronal survival play an important role both during development and after birth, in particular when the nervous system is lesioned. Isolated embryonic motoneurons and other types of primary neurons have been a useful tool for studying basic mechanisms underlying neuronal cell death during development and under pathophysiological conditions after neurotrauma. These studies have led to the identification of neurotrophic factors which under physiological conditions regulate survival and functional properties, and after neurotrauma promote regeneration and plasticity. Functional analysis of these molecules, in particular by generation of gene knockout mice, has led to a more detailed understanding of complex requirements of individual types of neurons for their survival and also paved the way for a better understanding of the signalling pathways in lesioned neurons which decide on cell death or survival after axotomy and other pathophysiological conditions. These findings could ultimately lead to a rational basis for therapeutic approaches aiming at improving neuronal survival and regeneration after neurotrauma.

Prevention of apoptotic but not necrotic cell death following neuronal injury by neurotrophins signaling through the tyrosine kinase receptor

Object. Neurotrophins prevent the death of neurons during embryonal development and have potential as therapeutic agents. During development, neuronal death occurs only by apoptosis and not by necrosis. Following injury, however, neurons can die by both processes. Data from prior studies have not clearly indicated whether neurotrophins can decrease apoptosis compared with necrosis. The goal of this study was to determine the effect of neurotrophin treatment on each of these processes following injury and to characterize the receptor(s) required.

Methods. The authors used an in vitro model of injury with the aid of primary cortical neurons obtained from rat embryos. After 9 days in culture and the elimination of glia, homogeneous and mature neurons were available for experimentation. Noxious stimuli were applied, including radiation, hypoxia, and ischemia. Subsequent cell death by apoptosis or necrosis was noted based on morphological and enzymatic assessments (such as lactate dehydrogenase [LDH] release) and assays for DNA fragmentation. The effect of treatment with nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 was determined. Finally, Western blot analyses were performed to note the neurotrophin receptor status in the neurons (tyrosine kinase receptors [Trks] and p75).

The authors studied different stimuli-induced cell death by using different processes. With the application of radiation, cells died primarily by apoptosis, as evidenced by cell shrinkage, the presence of apoptotic bodies, and specific DNA fragmentation. This was a delayed process (> 6 hours) that could be reduced by gene transcription or protein synthesis inhibitors. With ischemia, cells died immediately by necrosis, showing cell enlargement and rupture. Ischemic cell death was not affected by the inhibition of macromolecular synthesis. Hypoxia produced a mixture of the two cell death processes.

Both BDNF and neurotrophin-3 demonstrated protection against apoptotic cell death only. Statistically significant decreases of both LDH release and apoptosis-specific DNA fragmentation were noted following radiation and hypoxia, but not for ischemia. Nerve growth factor, unlike the other neurotrophins, did not affect apoptosis because a functional receptor, Trk A, was not expressed by the cortical neurons. There was expression of both Trk B and Trk C, which bind BDNF and neurotrophin-3.

Conclusions. These findings have significant clinical implications. Neurotrophins may only be effective in disorders in which apoptosis, and not necrosis, is the major process. Furthermore, the Trk signaling cascade must be activated for this response to occur. Because the expression of these receptors diminishes in adulthood, neurotrophin application may be most appropriate in the pediatric population.

Leukemia inhibitory factor receptor signaling negatively modulates nerve growth factor-induced neurite outgrowth in PC12 cells and sympathetic neurons

Nerve growth factor (NGF) is required for the development of sympathetic neurons and subsets of sensory neurons. Our current knowledge on the molecular mechanisms underlying the biological functions of NGF is in part based on the studies with PC12 rat pheochromocytoma cells, which differentiate into sympathetic neuron-like cells upon NGF treatment. Here we report that the expression of leukemia inhibitory factor receptor (LIFR), one of the signaling molecules shared by several neuropoietic cytokines of the interleukin-6 family, is specifically up-regulated in PC12 cells following treatment with NGF. Attenuation of LIFR signaling through stable transfection of antisense- or dominant negative-LIFR constructs enhances NGF-induced neurite extension in PC12 cells. On the contrary, overexpression of LIFR retards the growth of neurites. More importantly, whereas NGF-induced Rac1 activity is enhanced in antisense-LIFR and dominant negative-LIFR expressing PC12 cells, it is reduced in LIFR expressing PC12 cells. Following combined treatment with NGF and ciliary neurotrophic factor, sympathetic neurons exhibit attenuated neurite growth and branching. On the other hand, in sympathetic neurons lacking LIFR, neurite growth and branching is enhanced when compared with wild type controls. Taken together, our findings demonstrate that LIFR expression can be specifically induced by NGF and, besides its known function in cell survival and phenotype development, activated LIFR signaling can exert negative regulatory effects on neurite extension and branching of sympathetic neurons.

Antiapoptotic and pronecrotic actions of neurotrophins

Neurotrophins promote the differentiation, growth, and survival of neurons in the nervous system. Specifically, neurotrophins promote neuronal survival by interfering with programmed cell death or apoptosis. In addition to roles of neurotrophins as survival factors, neurotrophins can act as risk factors of neuronal injury under various pathological conditions. Neurotrophins markedly potentiate neuronal cell necrosis induced by activation of N-methyl-D-aspartate receptors, deprivation of oxygen and glucose, and free radicals. Moreover, prolonged exposure to neurotrophins results in widespread neuronal necrosis through free radical-mediated mechanisms. Whereas cellular and molecular mechanisms underlying antiapoptosis action of neurotrophins have been well documented, extensive study will be needed to delineate mechanisms for the neurotrophin-induced neuronal necrosis through activation of Trk tyrosine kinase receptors.

Glutamate stimulates neurotrophin expression in cultured Müller cells

The uptake of excess extracellular glutamate and the secretion of neurotrophins by glial cells have been suggested to protect CNS neurons from glutamate-induced toxicity. In the retina, perturbation of glutamate transport and decreased retrograde transport of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) may contribute to ganglion cell death in experimental glaucoma. Although many studies show a clear relationship between glutamate and neurotrophic factors, such relationship has not been thoroughly investigated in the retinal environment. In the following study, we determined the effects of glutamate on early passaged rat Müller cells, specifically their expression of neurotrophic factors including BDNF, nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), and glial-cell line derived neurotrophic factor (GDNF); and of glutamate receptors and transporters using immunoblots or enzyme-linked immunosorbent assays. Binding of BDNF to its cognate receptor TrkB was also determined using co-immunoprecipitation studies. Cultured Müller cells grown in the presence of glutamate were also assayed for survival using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS). Our study showed that while glutamate treatment did not promote cell death, it upregulated secretion of BDNF, NGF, NT-3, NT-4, and GDNF by Müller cells. While solitary bands at approximately 13-14 kDa were observed for NGF, NT-3, and NT-4; two BDNF-reactive bands were observed in immunoblots: a faster migrating band at the reported size of the BDNF monomer (approximately 13 kDa); and a more intense band at approximately 36 kDa. GDNF-reactive bands were observed at approximately 22, approximately 28, and approximately 55 kDa. Glutamate also induced significant changes in glutamate receptor and transporter proteins, as well maintained the association of BDNF to TrkB in Müller cells. The decreased N-methyl-D-aspartate receptor (NMDAR) levels and sustained activation of TrkB by BDNF could serve as protective mechanisms for Müller cell survival. Moreover, the increased secretion of neurotrophic factors and upregulation of L-glutamate/L-aspartate transporter (GLAST) expression in Müller cells may protect retinal neurons from glutamate toxicity.

Two classes of astrocytes in the adult human and pig retina in terms of their expression of high affinity NGF receptor (TrkA)

Astrocytes have been implicated in axon guidance and synaptic regeneration in the retina and these processes involve activation of the high affinity nerve growth factor receptor, known as the tyrosine kinase A (TrkA) receptor. The purpose of the present study was to characterize the expression of TrkA in astrocytes of the adult pig and human retina. To this end, sections of human and pig retinas were immunolabeled with a combination of antibodies to glial fibrillary acidic protein (GFAP) and TrkA. Our study revealed that most of the GFAP-positive cells express TrkA, whereas a rare, novel subpopulation of astrocytes was found to be devoid of TrkA. Our results support the idea that astrocytes play an important neurotrophic role in the retina.

Cellular and molecular connections between sleep and synaptic plasticity

The hypothesis that sleep promotes learning and memory has long been a subject of active investigation. This hypothesis implies that sleep must facilitate synaptic plasticity in some way, and recent studies have provided evidence for such a function. Our knowledge of both the cellular neurophysiology of sleep states and of the cellular and molecular mechanisms underlying synaptic plasticity has expanded considerably in recent years. In this article, we review findings in these areas and discuss possible mechanisms whereby the neurophysiological processes characteristic of sleep states may serve to facilitate synaptic plasticity. We address this issue first on the cellular level, considering how activation of T-type Ca(2+) channels in nonREM sleep may promote either long-term depression or long-term potentiation, as well as how cellular events of REM sleep may influence these processes. We then consider how synchronization of neuronal activity in thalamocortical and hippocampal-neocortical networks in nonREM sleep and REM sleep could promote differential strengthening of synapses according to the degree to which activity in one neuron is synchronized with activity in other neurons in the network. Rather than advocating one specific cellular hypothesis, we have intentionally taken a broad approach, describing a range of possible mechanisms whereby sleep may facilitate synaptic plasticity on the cellular and/or network levels. We have also provided a general review of evidence for and against the hypothesis that sleep does indeed facilitate learning, memory, and synaptic plasticity.

Upregulation of TrkA neurotrophin receptor expression in the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration

Neuroimmune networks in the thymic microenvironment are thought to be involved in the regulation of T cell development. Here, we report upon an examination of the expression of the TrkA neurotrophin receptor, the high affinity receptor for nerve growth factor, during regeneration following acute involution induced by cyclophosphamide in the rat thymus. Light and electron microscopic immunocytochemistry demonstrated enhanced expression of the TrkA receptor in the subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration. In addition, various morphological alterations, suggestive of a hyperfunctional and dynamic state, of the subcapsular, paraseptal, and perivascular epithelial cells were also observed. The presence of TrkA protein in extracts from the control and regenerating rat thymus was confirmed by western blot. Furthermore, RT-PCR analysis supported these results by demonstrating that thymic extracts contain TrkA mRNA at higher levels during thymus regeneration. Thus, our results suggest that the TrkA receptor located on the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells could play a role in the development of new T cells to replace T cells damaged during thymus regeneration.

Restoration of synapse formation in Musk mutant mice expressing a Musk/Trk chimeric receptor

Mice lacking Musk, a muscle-specific receptor tyrosine kinase that is activated by agrin, fail to form neuromuscular synapses and consequently die at birth because of their failure to move or breathe. We produced mice that express a chimeric receptor, containing the juxtamembrane region of Musk and the kinase domain of TrkA, selectively in muscle, and we crossed this transgene into Musk mutant mice. Expression of this chimeric receptor restores presynaptic and postsynaptic differentiation, including the formation of nerve terminal arbors, synapse-specific transcription, and clustering of postsynaptic proteins, allowing Musk mutant mice to move, breathe and survive as adults. These results show that the juxtamembrane region of Musk, including a single phosphotyrosine docking site, even in the context of a different kinase domain, is sufficient to activate the multiple pathways leading to presynaptic and postsynaptic differentiation in vivo. In addition, we find that Musk protein can be clustered at synaptic sites, even if Musk mRNA is expressed uniformly in muscle. Moreover, acetylcholine receptor clustering and motor terminal branching are restored in parallel, indicating that the extent of presynaptic differentiation is matched to the extent of postsynaptic differentiation.

The basic region and leucine zipper transcription factor MafK is a new nerve growth factor-responsive immediate early gene that regulates neurite outgrowth

We used serial analysis of gene expression to identify new NGF-responsive immediate early genes (IEGs) with potential roles in neuronal differentiation. Among those identified was MafK, a small Maf family basic region and leucine zipper transcriptional repressor and coactivator expressed in immature neurons. NGF treatment elevates the levels of both MafK transcripts and protein. In contrast, there is no effect on expression of the closely related MafG. Unlike many other NGF-responsive IEGs, MafK regulation shows selectivity and is unresponsive to epidermal growth factor, depolarization, or cAMP derivatives. Inhibitor studies indicate that NGF-promoted MafK regulation is mediated by an atypical isoform of PKC but not by mitogen-activated kinase kinase, phospholipase Cgamma, or phosphoinositide 3'-kinase. Interference with MafK expression or activity by small interfering RNA and dominant negative strategies, respectively, suppresses NGF-promoted outgrowth and maintenance of neurites by PC12 cells and neurite outgrowth by immature telencephalic neurons. Our findings support a role for MafK as a novel regulator of neuronal differentiation.

Neuroserpin regulates neurite outgrowth in nerve growth factor-treated PC12 cells

Neuroserpin is a serine protease inhibitor widely expressed in the developing and adult nervous systems and implicated in the regulation of proteases involved in processes such as synaptic plasticity, neuronal migration and axogenesis. We have analysed the effect of neuroserpin on growth factor-induced neurite outgrowth in PC12 cells. We show that small changes in neuroserpin expression result in changes to the number of cells extending neurites and total neurite length following NGF treatment. Increased expression of neuroserpin resulted in a decrease in the number of cells extending neurites and a reduction in total free neurite length whereas reduced levels of neuroserpin led to a small increase in the number of neurite extending cells and a significant increase in total free neurite length compared to the parent cell line. Neuroserpin also altered the response of PC12 cells to bFGF and EGF treatment. Neuroserpin was localised to dense cored secretory vesicles in PC12 cells but was unable to complex with its likely enzyme target, tissue plasminogen activator at the acidic pH found in these vesicles. These data suggest that modulation of neuroserpin levels at the extending neurite growth cone may play an important role in regulating axonal growth.

Role of low-affinity p75 receptor in nerve growth factor-inducible growth arrest of PC12 cells

Mutant PC12 cell clones (PC84 cells) were obtained by transfection with nerve growth factor (NGF) cDNA. These cells secreted active NGF, extended short processes, and proliferated faster than the parental PC12 cells. These features are of great interest because the parental PC12 cells cease proliferation and extend long processes when transfected with NGF cDNA. PC84 cells expressed a high level of acetylcholinesterase activity and neurofilament M, which indicates that PC84 cells were differentiated. The inhibition of TrkA by K252a diminished the short processes of PC84 cells but had no effect on their fast proliferation. The expression level of TrkA in PC84 cells was comparable to that in PC12 cells; whereas that of another NGF receptor, p75, was significantly lower. These data suggest that the decrease of p75 contributed to the continuous growth of PC84 cells, which was confirmed by suppressing p75 activity of PC12 cells with the antisense oligonucleotide of p75 or with anti-p75 neutralizing antibody. The treated cells did not cease proliferation in the presence of NGF and extended short processes. Our results suggest that NGF signaling via TrkA affects the differentiation characteristics of PC12 cells but that an additional signaling via p75 is necessary for the growth arrest of the cells.

Two separate domains in the golli myelin basic proteins are responsible for nuclear targeting and process extension in transfected cells

The golli products of the myelin basic protein (MBP) gene are expressed in neurons and oligodendrocytes (OLs). In certain neuronal populations, golli proteins undergo translocation between the nucleus and cytoplasm/processes during development. The proteins consist of two domains, a golli domain of 133 amino acids and an MBP domain of variable length. One objective of this study was to identify the sequences responsible for nuclear targeting. Site-directed mutagenesis and deletion analyses were used to generate a series of golli-green fluorescent protein (GFP) DNA constructs that were transfected into OL and neuronal cell lines to follow localization by confocal microscopy. The results indicated that a 36-residue stretch in the MBP domain is essential for nuclear targeting, and the sequence appears to be a nontraditional localization signal motif. The studies also revealed that overexpression of golli proteins could induce dramatic changes in cell morphology. In OL lines, overexpression of intact golli proteins, or golli peptide alone, caused an increase in the length and number of processes, and the elaboration of membrane sheets. In the neuronal lines, there was a dramatic increase in number and length of extensions. The results, consistent with the timing of golli expression in cells during neural development, suggest that golli proteins may be involved in process formation/extension in OLs and neurons during development. These studies have defined two functional domains in the golli protein. Sequences in the MBP domain target the protein into the nucleus and sequences within the golli domain induce process sheet extension in OLs and neurons.

Expression of p75(NTR) in photoreceptor cells of dystrophic rat retinas

Although a gene mutation in the Royal College of Surgeons (RCS) dystrophic rat results in defective phagocytosis and in accumulation of debris in the subretinal space, the molecular mechanisms leading to photoreceptor cell death remain unclear. In this study, the expression of p75(NTR), the low-affinity neurotrophin receptor incriminated in the apoptosis of developing neurons, was investigated at various stages of retinal degeneration in dystrophic rats using immunohistochemistry, in situ reverse transcription polymerase chain reaction (RT-PCR), Western blot, and relative RT-PCR. In normal adult retinas, p75(NTR) immunolabeling was observed mainly in the outer limiting membrane, with punctate labeling in the inner nuclear and ganglion cell layers. In 18- to 30-day-old dystrophic retinas, the immunostaining appeared to increase especially in the photoreceptor outer and inner segments. Dense staining was also observed in the retinal pigment epithelium (RPE) and choroid. In 60-day-old dystrophic rat retinas, the density of immunolabeling for p75(NTR) increased dramatically in the remaining inner retina, especially in the inner nuclear, inner plexiform, and ganglion cell layers. Post-embedding immunogold labeling of normal retinas verified the distribution of p75(NTR) in photoreceptor cells within the inner segments, cell bodies, and outer segments. The apparent increased intensity in p75(NTR) immunostaining in dystrophic retinas was verified by Western blots and densitometric analyses. In situ RT-PCR and relative RT-PCR further established increased synthesis of p75(NTR) in dystrophic retinas. The increased levels of p75(NTR) in the RPE and photoreceptor cells, the initial sites of injury, during retinal degeneration in dystrophic rats strongly suggest that altered expression of p75(NTR) may be directly involved in photoreceptor death.

Delayed NGF infusion fails to reverse axotomy-induced degeneration of basal forebrain cholinergic neurons in adult p75(LNTR)-deficient mice

The p75 low-affinity neurotrophin receptor (p75(LNTR)) appears to have various functions that include enhancing nerve growth factor (NGF)-mediated survival by increasing TrkA (high-affinity NGF receptor) efficiency, and mediating apoptosis by acting as a ligand-regulated pro-apoptotic receptor. Here, we investigated the role of p75(LNTR) for adult cholinergic basal forebrain neurons by comparing neuronal responses to injury in control and p75(LNTR)-deficient mice. In both types of mice, approximately 70% of the cholinergic neurons in the ipsilateral medial septum had lost their markers choline acetyltransferase and tyrosine kinase A by 28 days following unilateral transection of the dorsal septohippocampal pathway (fimbria fornix). A 7-day delayed infusion of NGF that started 28 days after the injury resulted in reversal of choline acetyltransferase expression and cell atrophy in control, but not in p75(LNTR)-deficient, mice. This lack of response to delayed NGF treatment in p75(LNTR)-deficient mice was most likely not due to cell death, as all of the septohippocampal neurons, labeled with Fluorogold before the lesion, were present at 28 days post-lesion, similar to control mice. p75(LNTR)-deficient cholinergic neurons can respond to NGF as they were protected by NGF infusions that started immediately after the injury. These observations, the fact that lesioned p75(LNTR)-deficient neurons atrophy faster, and that non-lesioned neurons hypertrophy in response to NGF in control but not in p75(LNTR)-deficient mice, suggest that p75(LNTR) is needed for tyrosine kinase A and NGF signaling efficiency.In conclusion, during adulthood p75(LNTR) appears to play a beneficial role in the response of cholinergic neurons to injury, consistent with the proposed role of p75(LNTR) in the enhancement of TrkA signaling and the transport of neurotrophins by these neurons.

A quantitative bioassay for nerve growth factor, using PC12 clones expressing different levels of trkA receptors

Nerve growth factor (NGF) is a neurotrophin required for differentiation, development, and survival of the sympathetic nervous system, with many of its biological effects being mediated via trkA receptors. There is a need for a standard quantitative bioassay for NGF, to be used in basic research and in pharmaceutical studies. The objective of the present research was to develop a selective, quantitative, and reliable bioassay for NGF, using a morphological criterion: neurite cell outgrowth. In addition, we aimed to apply the aforementioned bioassay to measure NGF administered to mice. Pheochromocytoma PC12 cell variants including wild-type cultures, and a trkA-overexpressing stable transfectant PC12-6.24-I, PC12nnr5, and PC12EN lacking trkA receptors, were used. Dose-response curves were generated with NGF beta-subunit (2.5S) purified from mouse submaxillary glands. Our results demonstrated that the bioassay was sensitive to 0.3-20 ng/mL, and selective, as neurite outgrowth was not seen by any other growth factor other than NGF. In addition, variant clones PC12nnr5 and PC12EN, lacking trkA receptors, did not respond to NGF. The bioassay detected NGF in serum of mice injected with NGF. This novel developed bioassay can serve as a model system for various neuroscience purposes.

Neurotrophic factor receptors in epiretinal membranes after human diabetic retinopathy

OBJECTIVE

Formation of epiretinal membranes (ERMs) in the posterior fundus results in progressive deterioration of vision. ERMs have been associated with numerous clinical conditions, including proliferative diabetic retinopathy (PDR), but its pathogenic mechanisms are still unknown. This study was conducted to determine whether neurotrophic factor receptors (tyrosine kinase receptors trkA, trkB, and trkC; low-affinity neurotrophin [NT] receptor p75 [p75(NTR)]; glial cell line-derived neurotrophic factor receptor-alpha1 [GFR alpha 1] and GFR alpha 2; and Ret) are involved in the formation of ERMs after PDR.

RESEARCH DESIGN AND METHODS

ERM samples were obtained by vitrectomy from 19 subjects with PDR aged 57 +/- 8 years with 17 +/- 8 years of diabetes and 15 subjects with idiopathic ERM. They were processed for RT-PCR analysis. In addition, 11 ERM samples from PDR patients aged 47 +/- 18 years with 13 +/- 4 years of diabetes were processed for immunohistochemical analysis.

RESULTS

Expressions of trkA, trkB, trkC, p75(NTR), and Ret mRNAs were similar in both groups. In contrast, GFR alpha 2 expression levels were significantly higher (17 of 19 vs. 2 of 15 subjects in idiopathic ERM, P < 0.0001) in PDR subjects. Accordingly, immunohistochemical analysis revealed expression of GFR alpha 2 protein in all of the 11 ERMs derived from PDR patients, and that region was double-labeled with glial cell-specific markers. On the other hand, GFR alpha 1 expression was lower (8 of 19 vs. 12 of 15 subjects with idiopathic ERM, P = 0.0258) in PDR subjects.

CONCLUSIONS

These results suggest a possibility that glial cell line-derived neurotrophic factor receptor (GDNF) subtypes are differently involved in the formation of ERMs.