Cell cycle phase-specific surface expression of nerve growth factor receptors TrkA and p75(NTR)

A Focus on "Bio" in Bio-Nanoscience: The Impact of Biological Factors on Nanomaterial Interactions

Bio-nanoscience research encompasses studies on the interactions of nanomaterials with biological structures or what is commonly referred to as the biointerface. Fundamental studies on the influence of nanomaterial properties, including size, shape, composition, and charge, on the interaction with the biointerface have been central in bio-nanoscience to assess nanomaterial efficacy and safety for a range of biomedical applications. However, the state of the cells, tissues, or biological models can also influence the behavior of nanomaterials at the biointerface and their intracellular processing. Focusing on the "bio" in bio-nano, this review discusses the impact of biological properties at the cellular, tissue, and whole organism level that influences nanomaterial behavior, including cell type, cell cycle, tumor physiology, and disease states. Understanding how the biological factors can be addressed or exploited to enhance nanomaterial accumulation and uptake can guide the design of better and suitable models to improve the outcomes of materials in nanomedicine.

Valproic acid upregulates the expression of the p75NTR/sortilin receptor complex to induce neuronal apoptosis

The antiepileptic and mood stabilizer agent valproic acid (VPA) has been shown to exert anti-tumour effects and to cause neuronal damage in the developing brain through mechanisms not completely understood. In the present study we show that prolonged exposure of SH-SY5Y and LAN-1 human neuroblastoma cells to clinically relevant concentrations of VPA caused a marked induction of the protein and transcript levels of the common neurotrophin receptor p75NTR and its co-receptor sortilin, two promoters of apoptotic cell death in response to proneurotrophins. VPA induction of p75NTR and sortilin was associated with an increase in plasma membrane expression of the receptor proteins and was mimicked by cell treatment with several histone deacetylase (HDAC) inhibitors. VPA and HDAC1 knockdown decreased the level of EZH2, a core component of the polycomb repressive complex 2, and upregulated the transcription factor CASZ1, a positive regulator of p75NTR. CASZ1 knockdown attenuated VPA-induced p75NTR overexpression. Cell treatment with VPA favoured proNGF-induced p75NTR/sortilin interaction and the exposure to proNGF enhanced JNK activation and apoptotic cell death elicited by VPA. Depletion of p75NTR or addition of the sortilin agonist neurotensin to block proNGF/sortilin interaction reduced the apoptotic response to VPA and proNGF. Exposure of mouse cerebellar granule cells to VPA upregulated p75NTR and sortilin and induced apoptosis which was enhanced by proNGF. These results indicate that VPA upregulates p75NTR apoptotic cell signalling through an epigenetic mechanism involving HDAC inhibition and suggest that this effect may contribute to the anti-neuroblastoma and neurotoxic effects of VPA.

Integrative Functional Genomic Analysis of Molecular Signatures and Mechanistic Pathways in the Cell Cycle Underlying Alzheimer's Disease

Objective

Alzheimer's disease (AD) is associated with cell cycle reentry of mature neurons that subsequently undergo degeneration. This study is aimed to identify key regulators of the cell cycle and their underlying pathways for developing optimal treatment of AD.

Methods

RNA sequencing data were profiled to screen for differentially expressed genes in the cell cycle. Correlation of created modules with AD phenotype was computed by weight gene correlation network analysis (WGCNA). Signature genes for trophic factor receptors were determined using Pearson correlation coefficient (PCC) analysis.

Results

Among the 13,679 background genes, 775 cell cycle genes and 77 trophic factor receptors were differentially expressed in AD versus nondementia controls. Four coexpression modules were constructed by WGCNA, among which the turquoise module had the strongest correlation with AD. According to PCC analysis, 10 signature trophic receptors most strongly interacting with cell cycle genes were filtered and subsequently displayed in the global regulatory network. Further cross-talking pathways of signature receptors, such as glutamatergic synapse, long-term potentiation, PI3K-Akt, and MAPK signaling pathways, were identified.

Conclusions

Our findings highlighted the mechanistic pathways of signature trophic receptors in cell cycle perturbation underlying AD pathogenesis, thereby providing new molecular targets for therapeutic intervention in AD.

Obesity-linked circular RNA circTshz2-2 regulates the neuronal cell cycle and spatial memory in the brain

Metabolic syndromes, including obesity, cause neuropathophysiological changes in the brain, resulting in cognitive deficits. Only a few studies explored the contribution of non-coding genes in these pathophysiologies. Recently, we identified obesity-linked circular RNAs (circRNA) by analyzing the brain cortices of high-fat-fed obese mice. In this study, we scrutinized a conserved and neuron-specific circRNA, circTshz2-2, which affects neuronal cell cycle and spatial memory in the brain. Transcriptomic and cellular analysis indicated that circTshz2-2 dysregulation altered the expression of cell division-related genes and induced cell cycle arrest at the G2/M phase of the neuron. We found that circTshz2-2 bound to the YY1 transcriptional complex and suppressed Bdnf transcription. Suppression of circTshz2-2 increased BDNF expression and reduced G2/M checkpoint proteins such as Cyclin B2 and CDK1 through BDNF/TrkB signaling pathway, resulting in cell cycle arrest and neurite elongation. Inversely, overexpression of circTshz2-2 decreased BDNF expression, induced cell cycle proteins, and shortened the neurite length, indicating that circTshz2-2 regulates neuronal cell cycle and structure. Finally, we showed that circTshz2-2 affects spatial memory in wild-type and obese mice. Our data have revealed potential regulatory roles of obesity-related circTshz2-2 on the neuronal cell cycle and memory function providing a novel link between metabolic syndromes and cognitive deficits.

Related expression of TRKA and P75 receptors and the changing copy number of MYC-oncogenes determine the sensitivity of brain tumor cells to the treatment of the nerve growth factor in combination with cisplatin and temozolomide

Objectives Oncological diseases are an urgent medical and social problem. The chemotherapy induces not only the death of the tumor cells but also contributes to the development of their multidrug resistance and death of the healthy cells and tissues. In this regard, the search for the new pharmacological substances with anticancer activity against drug-resistant tumors is of utmost importance. In the present study we primarily investigated the correlation between the expression of TrkA and p75 receptors with the nerve growth factor (NGF) and cisplatin or temozolomide sensitivity of anaplastic astrocytoma (AA), glioblastoma (GB) and medulloblastoma (MB) cell cultures. We then evaluated the changing of copy numbers of MYCC and MYCN and its correlation with cytotoxicity index (CI) in MB cells under NGF exposition. Methods The primary cell cultures were obtained from the tumor biopsy samples of the patients with AA (n=5), GB (n=7) or MB (n=25) prior to radiotherapy and chemotherapy. The cytotoxicity effect of NGF and its combinations with cisplatin or temozolomide, the relative expression of TrkA and p75 receptors, its correlations with CI in AA, GB and MB primary cell cultures were studied by trypan blue cytotoxicity assay and immunofluorescence staining respectively. The effect of NGF on MYCC and MYCN copy numbers in MB cell cultures was studied by fluorescence in situ hybridization. Results We found that the expression of TrkA and p75 receptors (p=0.03) and its ratio (p=0.0004) depends on the sensitivity of AA and GB cells to treatment with NGF and its combinations with cisplatin or temozolomide. NGF reduces (p<0.05) the quantity of MB cells with six or eight copies of MYCN and three or eight copies of MYCC. Besides, NGF increases (p<0.05) the quantity of MB cells containing two copies of both oncogenes. The negative correlation (r=-0.65, p<0.0001) is established between MYCC average copy numbers and CI of NGF in MB cells. Conclusions The relative expression of NGF receptors (TrkA/p75) and its correlation with CI of NGF and its combinations in AA and GB cells point to the mechanism involving a cell death signaling pathway. NGF downregulates (p<0.05) some increased copy numbers of MYCC and MYCN in the human MB cell cultures, and upregulates normal two copies of both oncogenes (p<0.05).

Related expression of TRKA and P75 receptors and the changing copy number of MYC-oncogenes determine the sensitivity of brain tumor cells to the treatment of the nerve growth factor in combination with cisplatin and temozolomide

OBJECTIVES

Oncological diseases are an urgent medical and social problem. The chemotherapy induces not only the death of the tumor cells but also contributes to the development of their multidrug resistance and death of the healthy cells and tissues. In this regard, the search for the new pharmacological substances with anticancer activity against drug-resistant tumors is of utmost importance. In the present study we primarily investigated the correlation between the expression of TrkA and p75 receptors with the nerve growth factor (NGF) and cisplatin or temozolomide sensitivity of anaplastic astrocytoma (AA), glioblastoma (GB) and medulloblastoma (MB) cell cultures. We then evaluated the changing of copy numbers of MYCC and MYCN and its correlation with cytotoxicity index (CI) in MB cells under NGF exposition.

METHODS

The primary cell cultures were obtained from the tumor biopsy samples of the patients with AA (n=5), GB (n=7) or MB (n=25) prior to radiotherapy and chemotherapy. The cytotoxicity effect of NGF and its combinations with cisplatin or temozolomide, the relative expression of TrkA and p75 receptors, its correlations with CI in AA, GB and MB primary cell cultures were studied by trypan blue cytotoxicity assay and immunofluorescence staining respectively. The effect of NGF on MYCC and MYCN copy numbers in MB cell cultures was studied by fluorescence in situ hybridization.

RESULTS

We found that the expression of TrkA and p75 receptors (p=0.03) and its ratio (p=0.0004) depends on the sensitivity of AA and GB cells to treatment with NGF and its combinations with cisplatin or temozolomide. NGF reduces (p<0.05) the quantity of MB cells with six or eight copies of MYCN and three or eight copies of MYCC. Besides, NGF increases (p<0.05) the quantity of MB cells containing two copies of both oncogenes. The negative correlation (r=-0.65, p<0.0001) is established between MYCC average copy numbers and CI of NGF in MB cells.

CONCLUSIONS

The relative expression of NGF receptors (TrkA/p75) and its correlation with CI of NGF and its combinations in AA and GB cells point to the mechanism involving a cell death signaling pathway. NGF downregulates (p<0.05) some increased copy numbers of MYCC and MYCN in the human MB cell cultures, and upregulates normal two copies of both oncogenes (p<0.05).

Nerve Growth Factor Signaling from Membrane Microdomains to the Nucleus: Differential Regulation by Caveolins

Membrane microdomains or "lipid rafts" have emerged as essential functional modules of the cell, critical for the regulation of growth factor receptor-mediated responses. Herein we describe the dichotomy between caveolin-1 and caveolin-2, structural and regulatory components of microdomains, in modulating proliferation and differentiation. Caveolin-2 potentiates while caveolin-1 inhibits nerve growth factor (NGF) signaling and subsequent cell differentiation. Caveolin-2 does not appear to impair NGF receptor trafficking but elicits prolonged and stronger activation of MAPK (mitogen-activated protein kinase), Rsk2 (ribosomal protein S6 kinase 2), and CREB (cAMP response element binding protein). In contrast, caveolin-1 does not alter initiation of the NGF signaling pathway activation; rather, it acts, at least in part, by sequestering the cognate receptors, TrkA and p75NTR, at the plasma membrane, together with the phosphorylated form of the downstream effector Rsk2, which ultimately prevents CREB phosphorylation. The non-phosphorylatable caveolin-1 serine 80 mutant (S80V), no longer inhibits TrkA trafficking or subsequent CREB phosphorylation. MC192, a monoclonal antibody towards p75NTR that does not block NGF binding, prevents exit of both NGF receptors (TrkA and p75NTR) from lipid rafts. The results presented herein underline the role of caveolin and receptor signaling complex interplay in the context of neuronal development and tumorigenesis.

The role of the cell cycle in the cellular uptake of folate-modified poly(l-amino acid) micelles in a cell population

Nanoparticles are widely recognized as a vehicle for tumor-targeted therapies.

The low-affinity nerve growth factor receptor p75NTR identifies a transient stem cell-like state in oral squamous cell carcinoma cells

BACKGROUND

Although several markers have been used for enrichment of cells with stem cell-like properties in oral squamous cell carcinoma (OSCC), isolation of a pure subpopulation is still a challenging task. Normal oral and esophageal keratinocyte stem cells have been previously isolated using the low-affinity nerve growth factor receptor p75NTR.

OBJECTIVE

To investigate the potential of p75NTR as a marker for identification and isolation of oral cancer cells with stem cell-like properties.

METHODS

Subpopulations of cells with high or low expression of p75NTR were sorted from OSCC-derived cells and compared for sphere/colony formation, in vivo tumor formation ability, expression of stem cell-related molecules, cell cycle distribution and drug resistance.

RESULTS

p75NTR(High) cells exhibited statistically significant higher stem cell properties than p75NTR(Low) cells in all assays performed. Nevertheless, p75NTR(Low) subpopulation did also exhibit some stem cell features, but to a lesser extent. Propagation of p75NTR(Low) cells for several passages in culture showed that the expression of p75NTR could rise spontaneously. This finding was also supported by the similar expression of p75NTR by the xenografts generated by both subpopulations in NOD\SCID IL2Rg(null) mice.

CONCLUSION

p75NTR can be used for isolating a subpopulation enriched for cells with stem cell-like properties in OSCC. De novo generation of p75NTR(High) cells from p75NTR(Low) cells suggests either that there is another subpopulation with stem cell features within the p75NTR(Low) cells, or that the p75NTR(Low) cells can dedifferentiate due to a contextually regulated equilibrium between stem cell-like cells and transit-amplifying neoplastic progenitors.

Poly(dimethylsiloxane) (PDMS) affects gene expression in PC12 cells differentiating into neuronal-like cells

Introduction

Microfluidics systems usually consist of materials like PMMA - poly(methyl methacrylate) and PDMS - poly(dimethylsiloxane) and not polystyrene (PS), which is usually used for cell culture. Cellular and molecular responses in cells grown on PS are well characterized due to decades of accumulated research. In contrast, the experience base is limited for materials used in microfludics chip fabrication.

Methods

The effect of different materials (PS, PMMA and perforated PMMA with a piece of PDMS underneath) on the growth and differentiation of PC12 (adrenal phaeochromocytoma) cells into neuronal-like cells was investigated using cell viability, cell cycle distribution, morphology, and gene expression analysis.

Results/Conclusions

After differentiation, the morphology, viability and cell cycle distribution of PC12 cells grown on PS, PMMA with and without PDMS underneath was the same. By contrast, 41 genes showed different expression for PC12 cells differentiating on PMMA as compared to on PS. In contrast, 677 genes showed different expression on PMMA with PDMS underneath as compared with PC12 cells on PS. The differentially expressed genes are involved in neuronal cell development and function. However, there were also many markers for neuronal cell development and functions that were expressed similarly in cells differentiating on PS, PMMA and PMMA with PDMS underneath. In conclusion, it was shown that PMMA has a minor impact and PDMS a major impact on gene expression in PC12 cells.

Nerve growth factor induces cell cycle arrest of astrocytes

Neurotrophins can influence multiple cellular functions depending on the cellular context and the specific receptors they interact with. These neurotrophic factors have been extensively studied for their ability to support neuronal survival via Trk receptors and to induce apoptosis via the p75(NTR). However, the p75(NTR) is also detected on cell populations that do not undergo apoptosis in response to neurotrophins. In particular, the authors have detected p75(NTR) expression on astrocytes during development and after seizure-induced injury. In this study, the authors investigated the role of Nerve growth factor (NGF) in regulating astrocyte proliferation and in influencing specific aspects of the cell cycle. The authors have demonstrated that NGF prevents the induction of cyclins and their association with specific cyclin-dependent kinases, and thereby prevents progression through the G1 phase of the cell cycle. Since the authors have previously shown that p75(NTR) but not TrkA, is expressed in astrocytes, these data suggest that activation of p75(NTR) promotes withdrawal of astrocytes from the cell cycle, which may have important consequences during development and after injury.

γ-Enolase C-terminal peptide promotes cell survival and neurite outgrowth by activation of the PI3K/Akt and MAPK/ERK signalling pathways

γ-Enolase, a glycolytic enzyme, is expressed specifically in neurons. It exerts neurotrophic activity and has been suggested to regulate growth, differentiation, survival and regeneration of neurons. In the present study, we investigated the involvement of γ-enolase in PI3K (phosphoinositide 3-kinase)/Akt and MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) signalling, the two pathways triggered predominantly by neurotrophic factors. Whereas the PI3K/Akt pathway, rather than the MAPK/ERK pathway, is involved in γ-enolase-enhanced cell survival, γ-enolase-stimulated neurite outgrowth requires both pathways, i.e. the activation of both PI3K and ERK1/2, leading to subsequent expression of the growth-cone-specific protein GAP-43 (growth-associated protein of 43 kDa). MEK (MAPK/ERK kinase) and PI3K inhibition blocked or attenuated the neurite outgrowth associated with dynamic remodelling of the actin-based cytoskeleton. We show that γ-enolase-mediated PI3K activation regulates RhoA kinase, a key regulator of actin cytoskeleton organization. Moreover, the inhibition of RhoA downstream effector ROCK (Rho-associated kinase) results in enhanced γ-enolase-induced neurite outgrowth, accompanied by actin polymerization and its redistribution to growth cones. Our results show that γ-enolase controls neuronal survival, differentiation and neurite regeneration by activating the PI3K/Akt and MAPK/ERK signalling pathways, resulting in downstream regulation of the molecular and cellular processes of cytoskeleton reorganization and cell remodelling, activation of transcriptional factors and regulation of the cell cycle.

Proteolytic processing of the p75 neurotrophin receptor: A prerequisite for signalling?: Neuronal life, growth and death signalling are crucially regulated by intra-membrane proteolysis and trafficking of p75(NTR)

The common neurotrophin receptor (p75(NTR) ) regulates various functions in the developing and adult nervous system. Cell survival, cell death, axonal and growth cone retraction, and regulation of the cell cycle can be regulated by p75(NTR) -mediated signals following activation by either mature or pro-neurotrophins and in combination with various co-receptors, including Trk receptors and sortilin. Here, we review the known functions of p75(NTR) by cell type, receptor-ligand combination, and whether regulated intra-membrane proteolysis of p75(NTR) is required for signalling. We highlight that the generation of the intracellular domain fragment of p75(NTR) is associated with many of the receptor functions, regardless of its ligand and co-receptor interactions.

Exendin-4 improves hepatocyte injury by decreasing proliferation through blocking NGF/TrkA in diabetic mice

The hepatocytes express nerve growth factor (NGF) and its high affinity receptor tyrosine kinase A (TrkA). However, the link between NGF/TrkA system and hepatocyte proliferation in diabetic animals and the effects of exendin-4, a glucagon like peptide-1 (GLP-1) receptor agonist, on this system are not known. BALB/c male mice were divided into four groups. The first group was given citrate buffer only, the second group was administered exendin-4 alone, the third group received streptozotocin (STZ), and the fourth group was given both STZ and exendin-4. Exendin-4 (3μg/kg) was administered by subcutaneous injection daily for 30 days after the animals were rendered diabetic by administration of STZ (200mg/kg). With treatment of exendin-4 to the diabetic mice the following results were noted (i) NGF, TrkA and proliferating cell nuclear antigen positive hepatocytes were decreased; (ii) p75 neurotrophin receptor and caspase-3 positive hepatocyte could not be detected; (iii) liver alanine transaminase and aspartate transaminase activities, lipid peroxidation, protein carbonyl and myeloperoxidase levels were decreased; (iv) liver catalase, superoxide dismutase, glutathione peroxidase activities and glutathione levels were increased. These data suggest that exendin-4 might exerts its anti-proliferative action through blocking NGF/TrkA system and stimulating oxidative defense system in liver of diabetic mice.

Epigallocatechin gallate reduces human monocyte mobility and adhesion in vitro

BACKGROUND AND PURPOSE

Monocytes/macrophages are an important population of immune inflammatory cells that have diverse effector functions in which their mobility and adhesion play a very relevant role. Epigallocatechin gallate (EGCG), a major component of green tea, has been reported to have anti-allergic and anti-inflammatory activities, but its effects on monocytes remain to be determined. Here we investigated the effects of EGCG on the migration and adhesion of monocytes.

EXPERIMENTAL APPROACH

We used a human monocyte cell line (THP-1) to analyse the effects of treatment with EGCG under non-cytotoxic conditions on the expression levels of the monocyte chemotactic protein-1 (MCP-1) and of the MCP-1 receptor (CCR2) and on the activation of beta1 integrin. A functional validation was carried out by evaluating the inhibitory effect of EGCG on monocyte adhesiveness and migration in vitro.

KEY RESULTS

Treatment of THP-1 cells with EGCG decreased MCP-1 and CCR2 gene expression, together with MCP-1 secretion and CCR2 expression at the cell surface. EGCG also inhibited beta1 integrin activation. The effects on these molecular targets were in agreement with the EGCG-induced inhibition of THP-1 migration in response to MCP-1 and adhesion to fibronectin.

CONCLUSIONS AND IMPLICATIONS

Under our experimental conditions, EGCG treatment inhibited the migration and adhesion of monocytes. These inhibitory effects of EGCG on monocyte function should be considered as a promising new anti-inflammatory response with a potential therapeutic role in the treatment of inflammation-dependent diseases.

Cell-specific targeting in the mouse inner ear using nanoparticles conjugated with a neurotrophin-derived peptide ligand: potential tool for drug delivery

Cell specific targeting is an emerging field in nanomedicine. Homing of the multifunctional nanoparticles (MFNPs) is achieved by the conjugation of targeting moieties on the nanoparticle surface. The inner ear is an attractive target for new drug delivery strategies as it is hard to access and hearing loss is a significant worldwide problem. In this work we investigated the utility of a Nerve Growth Factor-derived peptide (hNgf_EE) functionalized nanoparticles (NPs) to target cells of the inner ear. These functionalized NPs were introduced to organotypic explant cultures of the mouse inner ear and to PC-12 rat pheochromocytoma cells. The NPs did not show any signs of toxicity. Specific targeting and higher binding affinity to spiral ganglion neurons, Schwann cells and nerve fibers of the explant cultures were achieved through ligand mediated multivalent binding to tyrosine kinase receptors and to p75 neurotrophin receptors. Unspecific uptake of NPs was investigated using NPs conjugated with scrambled hNgf_EE peptide. Our results indicate a selective cochlear cell targeting by MFNPs, which may be a potential tool for cell specific drug and gene delivery to the inner ear.

Autophagic cell death induced by TrkA receptor activation in human glioblastoma cells

The neurotrophin receptor tropomyosin-related kinase A (TrkA) and its ligand nerve growth factor (NGF) are expressed in astrocytomas, and an inverse association of TrkA expression with malignancy grade was described. We hypothesized that TrkA expression might confer a growth disadvantage to glioblastoma cells. To analyze TrkA function and signaling, we transfected human TrkA cDNA into the human glioblastoma cell line G55. We obtained three stable clones, all of which responded with striking cytoplasmic vacuolation and subsequent cell death to NGF. Analyzing the mechanism of cell death, we could exclude apoptosis and cellular senescence. Instead, we identified several indications of autophagy: electron microscopy showed typical autophagic vacuoles; acridine orange staining revealed acidic vesicular organelles; acidification of acidic vesicular organelles was prevented using bafilomycin A1; cells displayed arrest in G2/M; increased processing of LC3 occurred; vacuolation was prevented by the autophagy inhibitor 3-methyladenine; no caspase activation was detected. We further found that both activation of ERK and c-Jun N-terminal kinase but not p38 were involved in autophagic vacuolation. To conclude, we identified autophagy as a novel mechanism of NGF-induced cell death. Our findings suggest that TrkA activation in human glioblastomas might be beneficial therapeutically, especially as several of the currently used chemotherapeutics also induce autophagic cell death.

Nerve growth factor stimulates the concentration of TrkA within lipid rafts and extracellular signal-regulated kinase activation through c-Cbl-associated protein

Nerve growth factor (NGF) acts through its receptor, TrkA, to elicit the neuronal differentiation of PC12 cells through the action of extracellular signal-regulated kinase 1 (ERK1) and ERK2. Upon NGF binding, TrkA translocates and concentrates in cholesterol-rich membrane microdomains or lipid rafts, facilitating formation of receptor-associated signaling complexes, activation of downstream signaling pathways, and internalization into endosomes. We have investigated the mechanisms responsible for the localization of TrkA within lipid rafts and its ability to activate ERK1 and ERK2. We report that NGF treatment results in the translocation of activated forms of TrkA to lipid rafts, and this localization is important for efficient activation of the ERKs. TrkA is recruited and retained within lipid rafts through its association with flotillin, an intrinsic constituent of these membrane microdomains, via the adapter protein, c-Cbl associated protein (CAP). Mutant forms of CAP that lack protein interaction domains block TrkA localization to lipid rafts and attenuate ERK activation. Importantly, suppression of endogenous CAP expression inhibited NGF-stimulated neurite outgrowth from primary dorsal root ganglion neurons. These data provide a mechanism for the lipid raft localization of TrkA and establish the importance of the CAP adaptor protein for NGF activation of the ERKs and neuronal differentiation.

Analysis of alteration of p75NTR processing and signalling by PS2 mutation and gamma-secretase inhibition

The presenilins (PSs) were identified as causative genes in cases of early-onset familial Alzheimer's disease (AD) and current evidence indicates that PSs are part of the gamma-secretase complex responsible for proteolytic processing of type I membrane proteins. p75NTR, a common neurotrophin receptor, was shown to be subject to gamma-secretase processing. However, it is not clear if the p75NTR downstream signal is altered in response to gamma-secretase cleavage, and further there is a possibility that AD-related PS mutations may affect this cleavage, resulting in pathogenic alterations in signal transduction. In this study, we confirmed that p75NTR downstream signalling is altered by PS2 mutation or gamma-secretase inhibition in SHSY-5Y cells. The activity of the small GTPase RhoA is strongly affected by these treatments. This study demonstrates that gamma-secretase and PS2 play an important role in regulating neurotrophin signal transduction and either mutation of PS2 or inhibition of gamma-secretase disturbs this function.

Bex1, a novel interactor of the p75 neurotrophin receptor, links neurotrophin signaling to the cell cycle

A screening for intracellular interactors of the p75 neurotrophin receptor (p75NTR) identified brain-expressed X-linked 1 (Bex1), a small adaptor-like protein of unknown function. Bex1 levels oscillated during the cell cycle, and preventing the normal cycling and downregulation of Bex1 in PC12 cells sustained cell proliferation under conditions of growth arrest, and inhibited neuronal differentiation in response to nerve growth factor (NGF). Neuronal differentiation of precursors isolated from the brain subventricular zone was also reduced by ectopic Bex1. In PC12 cells, Bex1 overexpression inhibited the induction of NF-kappaB activity by NGF without affecting activation of Erk1/2 and AKT, while Bex1 knockdown accelerated neuronal differentiation and potentiated NF-kappaB activity in response to NGF. Bex1 competed with RIP2 for binding to the p75NTR intracellular domain, and elevating RIP2 levels restored the ability of cells overexpressing Bex1 to differentiate in response to NGF. Together, these data establish Bex1 as a novel link between neurotrophin signaling, the cell cycle, and neuronal differentiation, and suggest that Bex1 may function by coordinating internal cellular states with the ability of cells to respond to external signals.

β-nerve growth factor participates in an auto/paracrine pathway of regulation of the meiotic differentiation of rat spermatocytes

NGF appears to be involved in spermatogenesis. However, mice lacking NGF or TrkA genes do not survive more than a few days whereas p75(NTR) knockout mice are viable and fertile. Therefore, we addressed the effect of betaNGF on spermatogenesis by using the systems of rat germ cell culture we established previously. betaNGF did not modify the number of Sertoli cells, pachytene spermatocytes, secondary spermatocytes nor the half-life of round spermatids, but increased the number of secondary meiotic metaphases and decreased the number of round spermatids formed in vitro. These effects of betaNGF were reversible and maximal at about 4 x 10(-11) M. Conversely, K252a, a Trk-specific kinase inhibitor, enhanced the number of round spermatids above that of control cultures. The presence of betaNGF and its receptors TrkA and p75(NTR) was investigated in testis sections, in Sertoli cell and germ cell fractions, and in germ cell and Sertoli cell co-cultures. betaNGF was detected only in germ cells from pachytene spermatocytes of stages VII up to spermatids of stages IX-X. TrkA and p75(NTR) were detected in Sertoli cells and in these germ cells. Taken together, these results indicate that betaNGF should participate in an auto/paracrine pathway of regulation of the second meiotic division of rat spermatocytes in vivo.

Neurotrophin system expression in human pulmonary carcinoid tumors

Neurotrophin (NT) and NT receptor expression was assessed in 12 typical (TC) and 8 atypical (AC) human pulmonary carcinoids by Western blot and immunohistochemistry. TC and AC carcinoid express to different extent NT and NT receptor proteins. Nerve growth factor (NGF) was expressed by 83% of the TC but not by the AC carcinoids. Brain derived neurotrophic factor (BDNF) was expressed by 33 and 100% of TC and AC carcinoids, respectively. NT-3 was expressed by 58% of the TC and 38% of AC carcinoids. TC carcinoids express high affinity NT receptors while 50% of the AC carcinoids express the TrkB receptor. Our results demonstrate that NGF/TrkA and BDNF/TrkB signaling need to be considered as regulatory pathways that may address survival, differentiation and/or aggressiveness of human pulmonary carcinoids. Contrarily to the BDNF/TrkB, expression of the NGF/TrkA signaling may overcome aggressiveness of carcinoid cells. NTs may be useful as markers in the clinic.

Sustained MAPK activation is dependent on continual NGF receptor regeneration

It still remains intriguing how signal specificity is achieved when different signals are relayed by the common intracellular signal transduction pathways. A well documented example for signal specificity determination is found in rat phaeochromocytoma PC12 cells where epidermal growth factor (EGF) stimulation produces a transient mitogen-activated protein kinase (MAPK) activation and leads to cell proliferation while nerve growth factor (NGF) initiates a sustained MAPK activation and induces cell differentiation. In this simulation, we demonstrated that NGF-induced sustained MAPK activation may mainly depend on continual regeneration of NGF receptors and that the presence of a small pool of surface receptors is enough to maintain a sustained MAPK activation. On the other hand, MAPK activation is not significantly sensitive to the half-life of internalized receptors and the levels of NGF-specific MAPK phosphatase MAP kinase phosphatase-3 (MKP-3), though cytoplasmic persistence of internalized NGF-bound receptors and the MKP-3 dependent feedback control also contribute to the sustaining of MAPK activation. These results are consistent with the recent experimental evidence that persistent tyrosine receptor kinase A (TrkA) activity is necessary to maintain transcription in the differentiating PC12 cells (Chang et al. 2003) and a sustained Src kinase activity is detected in response to NGF stimulation (Gatti 2003). It is suggested that sustained or transient MAPK activation induced by different growth factor and neurotrophins, which is crucial to their signaling specificity, could be satisfactorily accounted for by their specific receptor turnover kinetics rather than by the activation of specific downstream signaling cascades.

p75NTR-Immunoreactivity in the subventricular zone of adult male rats: Expression by cycling cells

While the study of in vitro regulation of neural stem cell lineage from both embryonic and adult neurospheres is greatly advanced, much less is known about factors acting in situ for neural stem cell lineage in adult brain. We reported that neurotrophin low affinity receptor p75(NTR) is present in the subventricular zone (SVZ) in adult male rats. We then characterized co-distribution of markers associated with precursor cells (nestin and PSA-NCAM) with growth factor receptors (p75(NTR), trkA, EGFr) and proliferation-associated antigens (Ki67 and BrDU-uptake) in adult male rat by immunocytochemistry and confocal laser scan microscopy. Distribution of p75(NTR)-immunoreactivity (IR) was investigated using different mono- and polyclonal antisera. p75(NTR-) is not co-distributed with glial fibrillary acid protein. It was found to be co-distributed with a small number of nestin-IR cells, whereas no coexistence with PSA-NCAM-IR was observed. Conversely, p75(NTR)-IR was present in numerous dividing cells (Ki-67-positive) and co-distributed with EGFr. In order to verify the possible association between p75(NTR) and cell death, we investigated co-distribution of p75(NTR)-IR with nuclear condensation images as visualized by Hoechst 33258 staining. While few images indicating nuclear condensation were observed in the SVZ, no coexistence with p75(NTR) was found. TrkA- and trkB-IR was not found in the SVZ. We also investigated p75(NTR) immunostaining on post-natal day 1 and day 16, because of the dramatic reduction of proliferating cells in SVZ over this time-interval. p75(NTR)-IR was not increased in the early post-natal phase. Thus, p75(NTR) seems to be associated with cell cycle regulation in SVZ in adult rat brain.

The p75NTR-interacting protein SC1 inhibits cell cycle progression by transcriptional repression of cyclin E

Schwann cell factor 1 (SC1), a p75 neurotrophin receptor–interacting protein, is a member of the positive regulatory/suppressor of variegation, enhancer of zeste, trithorax (PR/SET) domain-containing zinc finger protein family, and it has been shown to be regulated by serum and neurotrophins. SC1 shows a differential cytoplasmic and nuclear distribution, and its presence in the nucleus correlates strongly with the absence of bromodeoxyuridine (BrdU) in these nuclei. Here, we investigated potential transcriptional activities of SC1 and analyzed the function of its various domains. We show that SC1 acts as a transcriptional repressor when it is tethered to Gal4 DNA-binding domain. The repressive activity requires a trichostatin A–sensitive histone deacetylase (HDAC) activity, and SC1 is found in a complex with HDACs 1, 2, and 3. Transcriptional repression exerted by SC1 requires the presence of its zinc finger domains and the PR domain. Additionally, these two domains are involved in the efficient block of BrdU incorporation by SC1. The zinc finger domains are also necessary to direct SC1's nuclear localization. Lastly, SC1 represses the promoter of a promitotic gene, cyclin E, suggesting a mechanism for how growth arrest is regulated by SC1.

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.

Epithelial growth control by neurotrophins: leads and lessons from the hair follicle

Neurotrophins (NTs) exert many growth-regulatory functions beyond the nervous system. For example, murine hair follicles (HF) show developmentally and spatio-temporally stringently controlled expression of NTs, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, and their cognate receptors, tyrosine kinase A-C (TrkA-C) and p75 neurotrophin receptor (p75NTR). Follicular NT and NT receptor expression exhibit significant, hair cycle-dependent fluctuations on the gene and protein level, which are mirrored by changes in nerve fiber density and neurotransmitter/neuropeptide content in the perifollicular neural networks. NT-3/TrkC and NGF/TrkA signaling stimulate HF development, while NT-3, NT-4 and BDNF inhibit the growth (anagen) of mature HF by the induction of apoptosis-driven HF regression (catagen). p75NTR stimulation inhibits HF development and stimulates catagen. Since the HF is thus both a prominent target and key peripheral source of NT, dissecting the role of NTs in the control of HF morphogenesis and cyclic remodeling provides a uniquely accessible, and easily manipulated, clinically relevant experimental model, which has many lessons to teach. Given that our most recent data also implicate NTs in human hair growth control, selective NT receptor agonists and antagonists may become innovative therapeutic tools for the management of hair growth disorders (alopecia, effluvium, hirsutism). Since, however, the same NT receptor agonists that inhibit hair growth (e.g., BDNF, NT-4) can actually stimulate epidermal keratinocyte proliferation, NT may exert differential effects on defined keratinocyte subpopulations. The studies reviewed here provide new clues to understanding the complex roles of NT in epithelial tissue biology and remodeling in vivo, and invite new applications for synthetic NT receptor ligands for the treatment of epithelial growth disorders, exploiting the HF as a lead model.

Hepatocytes express nerve growth factor during liver injury: evidence for paracrine regulation of hepatic stellate cell apoptosis

A key feature of recovery from liver fibrosis is hepatic stellate cell (HSC) apoptosis, which serves the dual function of removing the major source of neomatrix and tissue inhibitors of metalloproteinases thereby facilitating matrix degradation. The mechanisms regulating HSC apoptosis remain undefined but may include the interaction of nerve growth factor (NGF) with its receptor, p75, on HSC. In this study, by TaqMan polymerase chain reaction in situ hybridization and immunohistochemistry, we demonstrate that NGF is expressed by hepatocytes during fibrotic injury. Peak hepatocyte expression of NGF (48 hours after CCl(4) injection) coincides with maximal rate of apoptosis of HSC by terminal dUTP nick-end labeling staining. Addition of recombinant NGF to HSC in tissue culture causes a dose-dependent increase in apoptosis. NGF regulates nuclear factor (NF)-kappaB activity, reducing p50/p65 binding detected by electromobility shift assay and reduced NF-kappaB CAT reporter activities from both basal unstimulated levels and after NF-kappaB induction by tumor necrosis factor. In each case, a relative reduction in NF-kappaB binding was associated with a significant increase in caspase 3 activity. These data provide evidence that NGF is expressed during fibrotic liver injury and may regulate number of activated HSCs via induction of apoptosis.

Osmotic swelling induces p75 neurotrophin receptor (p75NTR) expression via nitric oxide

Brain injuries by physical trauma, epileptic seizures, or microbial infection upset the osmotic homeostasis resulting in cell swelling (cerebral edema), inflammation, and apoptosis. Expression of the neurotrophin receptor p75NTR is increased in the injured tissue and axon regeneration is repressed by the Nogo receptor using p75NTR as the signal transducer. Hence, p75NTR seems central to the injury response and we wished to determine the signals that regulate its expression. Here, we demonstrate that tonicity mediated cell swelling rapidly activates transcription of the endogenous p75NTR gene and of a p75NTR promoter-reporter gene in various cell types. Transcription activation is independent of de novo protein synthesis and requires the activities of phospholipase C, protein kinase C, and nitric-oxide synthase. Hence, p75NTR is a nitric oxide effector gene regulated by osmotic swelling, thereby providing a strategy for therapeutic intervention to modulate p75NTR functions following injury.

Differential cross-regulation of TrkA and TrkC tyrosine kinase receptors with p75

The neurotrophins neurotrophin-3 (NT-3), brain-derived growth factor (BDNF) and nerve growth factor (NGF) bind to the p75 receptor, but each neurotrophin also binds a more selective Trk receptor (e.g. TrkA-NGF and TrkC-NT-3). The biochemical signals following engagement of either Trk or p75 with ligands are well understood, but long-term biological outcomes (trophic, proapoptotic or differentiative) remain unclear because they are cell/tissue specific. For example, Trk receptors are usually trophic but when overexpressed they can be proapoptotic in neuroblastomas and medulloblastomas. We hypothesized that coexpression of Trk and p75 receptors may lead to cross-regulation of signals and different biological outcomes; and used receptor-selective ligands to study cross-regulation by these receptors. We show that in the absence of Trk activation, expression of TrkC is permissive of p75 trophic and differentiation signals induced by p75 ligands, whereas expression of TrkA abolishes trophic and differentiation signals induced by p75 ligands. In contrast, in the presence of Trk activation, p75 ligands can regulate TrkA-mediated survival and TrkC-mediated differentiation. Therefore, a complex homeostasis of p75-selective and Trk-selective signals may determine the fate of cells expressing both receptors.

Ligand-induced internalization of the p75 neurotrophin receptor: a slow route to the signaling endosome

The nerve growth factor (NGF) family of neurotrophins binds two classes of cell-surface receptors, trk receptor tyrosine kinases and the shared p75 receptor. Rapid internalization and retrograde trafficking of neurotrophin-trk complexes have been demonstrated in a number of systems and are thought to transmit trophic signals from terminals to neuronal cell bodies. In contrast, the internalization and trafficking of neurotrophin-p75 complexes are not well understood. In this study, we used biotinylated NGF and a fluorescent-labeled anti-p75 antibody to follow the kinetics and route of ligand-induced internalization of the p75 receptor in cycling and differentiated PC12 cells. Binding of neurotrophins to p75 induced internalization at a rate approximately three times slower than that of transferrin and NGF-TrkA complexes in the same cells. The ligand-p75 complex was internalized via clathrin-coated pits into early endosomes and eventually accumulated in recycling endosomes in the cell body and vesicles colabeled by the cholera toxin B-subunit in the growth cones. Both internalized ligand and p75 were protected from proteolytic degradation and accumulated in vesicles that did not undergo acidification. Finally, NGF induced endosomal association of p75 and its MAGE interactors, necdin and NRAGE. These data suggest that signaling endosomes containing activated p75 are involved in neurotrophin signaling, and that such endosomes may be temporally and spatially distinct from those containing trk receptors.

Trafficking of TrkA-green fluorescent protein chimerae during nerve growth factor-induced differentiation

A chimera of the nerve growth factor (NGF) receptor, TrkA, and green fluorescent protein (GFP) was engineered by expressing GFP in phase with the carboxyl terminus of TrkA. TrkA-GFP becomes phosphorylated on tyrosine residues in response to NGF and is capable of initiating signaling cascades leading to prolonged MAPK activation and differentiation in PC12 nnr5 cells. TrkA constructs, progressively truncated in the carboxyl-terminal domain, were prepared as GFP chimerae in order to identify which part of the receptor intracellular domain is involved in its trafficking. Immunofluorescence observations show that TrkA-GFP is found mainly in cell surface membrane ruffles and in endosomes. Biochemical analysis indicated that the cytoplasmic domain of TrkA is not necessary for correct maturation and cell surface translocation of the receptor. An antibody against the extracellular domain of TrkA (RTA) was used as ligand to stimulate internalization and phosphorylation of TrkA. Co-localization studies with anti-phosphorylated TrkA antibodies support a role for such complexes in the propagation of signaling from the cell surface, resulting in the activation of TrkA in areas of the endosome devoid of receptor-ligand complexes. Confocal time-lapse analysis reveals that the TrkA-GFP chimera shows highly dynamic trafficking between the cell surface and internal locations. TrkA-positive vesicles were estimated to move 0.46 +/- 0.09 microm/s anterograde and 0.48 +/- 0.07 microm/s retrograde. This approach and the fidelity of the biochemical properties of the TrkA-GFP demonstrate that real-time visualization of trafficking of tyrosine kinase receptors in the presence or absence of the ligand is feasible.

Molecular kinetics of nerve growth factor receptor trafficking and activation

A growing body of evidence indicates a close relationship between tyrosine kinase receptor trafficking and signaling. Biochemical and molecular analyses of the expression, fate, and kinetics of membrane trafficking of the nerve growth factor (NGF) receptor TrkA were performed in PC12 cells. Pulse-chase experiments indicate that TrkA is synthesized as a 110-kDa N-glycosylated precursor that leads to the mature 140-kDa form of the receptor with a half-life of conversion of approximately 24 +/- 0.5 min. Neuraminidase digestion shows that modification of the carbohydrate moiety of the receptor by sialylation occurs during maturation. The 140-kDa form is rapidly translocated to the cell surface as assessed by cell surface biotinylation performed on intact PC12 cells. Mature receptor half-life is approximately 138 +/- 4 min and is shortened to 86 +/- 8 min by NGF treatment. Flow cytometric analysis indicates that NGF induces clearing of this receptor from the cell surface within minutes of treatment. The addition of NGF decreases the half-life of cell surface gp140(TrkA) from 100 to 35 min and leads to enhanced lysosomal degradation of the receptor. The process of NGF-induced TrkA internalization is clearly affected by interfering with ligand binding to p75(NTR). An analysis of receptor activation kinetics also shows that receptor signaling primarily takes place from an intracellular location. Together, these data show that the primary effect of NGF treatment is a p75(NTR)-modulated decrease in TrkA transit time at the cell surface.

Co-expression of trkA and p75 neurotrophin receptor in extracranial olfactory neuroblastoma cells

Olfactory neuroblastoma (ON, esthesioneuroblastoma) is a high-grade malignant tumour of neuronal origin. Little is known about the neurobiological behaviour of this tumour. Ten cases of ON and five cases of nasopharyngeal carcinoma were examined for expression of trkA and p75 neurotrophin receptor (p75NTR) using immunohistochemistry and double labelling fluorescence. We found that all ON tissues from 10 cases expressed both trkA and p75NTR at different levels. Double staining revealed that almost all trkA-immunoreactive ON cells also contained p75NTR immunoreactivity. By contrast, no trkA or p75NTR immunoreactivity was detected in nasopharyngeal carcinoma cells from five patients. These results suggest that nerve growth factor may play a role in the generation of ON and staining of trkA and p75NTR may assist in the diagnosis of ON.

Trk receptor tyrosine kinases: a bridge between cancer and neural development

The proto-oncogene Trks encode the high-affinity receptor tyrosine kinases for neurotrophins of a nerve growth factor (NGF) family. The Trk signals spatiotemporally regulate neural development and maintenance of neural network. However, Trk was originally cloned as an oncogene fused with the tropomyosin gene in the extracellular domain. Accumulating evidence has demonstrated that the rearranged Trk oncogene is often observed in non-neuronal neoplasms such as colon and papillary thyroid cancers, while the signals through the receptors encoded by the proto-oncogene Trks regulate growth, differentiation and apoptosis of the tumors with neuronal origin such as neuroblastoma and medulloblastoma. The intracellular Trk signaling pathway is also different depending on the Trk family receptors, cell types and the grade of transformation. Furthermore, developmentally programmed cell death of neuron, which is largely regulated by neurotrophin signaling, is at least in part controlled by tumor suppressors p53 and p73 as well as their antagonist DeltaNp73. Thus, the Trks and their downstream signaling function in both ontogenesis and oncogenesis. In this short review, the dynamic role of the Trk family receptors signaling in neural development, neurogenic tumors and other cancers will be discussed.

Differential regulation of NGF receptors in primary sensory neurons by adjuvant-induced arthritis in the rat

In the adult brain, neurotrophins play a key role in adaptive processes linked to increased neuronal activity. A growing body of evidence suggests that chronic pain results from long-term plasticity of central pathways involved in nociception. We have investigated the involvement of nerve growth factor (NGF) in adaptive responses of primary sensory neurons during the course of a long-lasting inflammatory pain model. The amount and distribution of the NGF receptors p75(NTR) and TrkA were measured in the dorsal horn and dorsal root ganglia (DRG) of animals subjected to Freund's adjuvant-induced arthritis (AIA). We observed an increased immunoreactivity of both receptors in the central terminals of primary sensory neurons in the arthritic state. The increases were seen in the same population of afferent terminals in deep dorsal horn laminae. These changes paralleled the variations of clinical and behavioral parameters that characterize the course of the disease. They occurred in NGF-sensitive, but not GDNF-sensitive, nerve terminals. However, p75(NTR) and TrkA protein levels in the DRG (in the cell body of these neurons) showed different response patterns. An immediate rise of p75(NTR) was seen in parallel with the initial inflammation that developed after administration of Freund's adjuvant in hindpaws. In contrast, increases of the mature (gp140(trk)) form of TrkA occurred later and seemed to be linked to the development of the long-lasting inflammatory response. The changes in receptor expression were observed exclusively at lumbar levels, L3-L5, somatotopically appropriate for the inflammation. Together, these results implicate NGF in long-term mechanisms accompanying chronic inflammatory pain, via the up-regulation of its high affinity receptor, and offer additional evidence for differential processes underlying short- versus long-lasting inflammatory pain.

Distinction between differentiation, cell cycle, and apoptosis signals in PC12 cells by the nerve growth factor mutant delta9/13, which is selective for the p75 neurotrophin receptor

The common neurotrophin receptor p75(NTR) (low affinity nerve growth factor receptor) participates in the high-affinity binding with the TrkA nerve growth factor (NGF) receptor, may mediate apoptosis, and may signal independently in a cell-specific manner. The potential of p75(NTR) to signal independently of TrkA was investigated with an NGF mutant protein (NGFdelta9/13) that binds poorly to TrkA (Woo et al. [1995] J. Biol. Chem. 270:6278-6285). The NGFdelta9/13 mutant does not activate TrkA autophosphorylation and fails to stimulate the normal NGF-induced growth arrest, demonstrating that TrkA activation is required to arrest PC12 cells at the NGF-activated G1/S cell cycle checkpoint. However, apoptosis is successfully blocked and cell survival is promoted by the NGFdelta9/13 mutant in naive PC12 cells after serum withdrawal, suggesting that p75(NTR) can signal for survival autonomously of TrkA. Annexin V binding, an indication of apoptotic plasma membrane disruption, is inhibited by both NGF and the NGFdelta9/13 mutant after serum deprivation. Both NGF and the NGFdelta9/13 mutant inhibit the rapid apoptotic internucleosomal DNA cleavage of PC12 cells upon serum deprivation. Furthermore, the level of caspase3-like activity that is rapidly activated by serum withdrawal from PC12 cells is reduced by both the NGFdelta9/13 protein and NGF. Finally, upon serum withdrawal, both NGF and the NGFdelta9/13 mutant activate nuclear translocation of the transcriptional factor NF-kappaB (nuclear factor kappaB), a process involved in cell survival. These results are consistent with p75(NTR) inhibition of caspase-mediated apoptosis in PC12 cells. The different physiologic responses elicited by NGFdelta9/13 indicate the potential for individual signaling by the two NGF receptors and also demonstrate the utility of NGF mutants for receptor-selective signal transduction.

Mediation of nerve growth factor-driven cell cycle arrest in PC12 cells by p53. Simultaneous differentiation and proliferation subsequent to p53 functional inactivation

Upon stimulation with nerve growth factor (NGF), PC12 cells extend neurites and cease to proliferate by influencing cell cycle proteins. Previous studies have shown that neuritogenesis and a block at the G(1)/S checkpoint correlate with the nuclear translocation of and an increase in the p53 tumor suppressor protein. This study was designed to determine if p53 plays a direct role in mediating NGF-driven G(1) arrest. A retroviral vector that overexpresses a temperature-sensitive p53 mutant protein (p53ts) was used to extinguish the function of endogenous p53 in PC12 cells in a dominant-negative manner at the nonpermissive temperature. NGF treatment led to transactivation of a p53 response element in a luciferase reporter construct in PC12 cells, whereas this response to NGF was absent in PC12(p53ts) cells at the nonpermissive temperature. With p53 functionally inactivated, NGF failed to activate growth arrest, as measured by bromodeoxyuridine incorporation, and also failed to induce p21/WAF1 expression, as measured by Western blotting. Since neurite outgrowth proceeded unharmed, 50% of the cells simultaneously demonstrated neurite morphology and were in S phase. Both PC12 cells expressing SV40 T antigen and PC12 cells treated with p53 antisense oligonucleotides continued through the cell cycle, confirming the dependence of the NGF growth arrest signal on a p53 pathway. Activation of Ras in a dexamethasone-inducible PC12 cell line (GSRas1) also caused p53 nuclear translocation and growth arrest. Therefore, wild-type p53 is indispensable in mediating the NGF antiproliferative signal through the Ras/MAPK pathway that regulates the cell cycle of PC12 cells.

Modulation of cell proliferation in the embryonic retina of zebrafish (Danio rerio)

We describe light-microscopically the development of the embryonic zebrafish eye with particular attention to cell number, cell proliferation, and cell death. The period from 16 to 36 hr post fertilization (hpf) comprises two phases; during the first (16-27 hpf) the optic vesicle becomes the eye cup, and during the second (27-36 hpf) the eye cup begins to differentiate into the neural retina and pigmented epithelium. All cells in the eye primordium are proliferative prior to 28 hpf, and the length of the cell cycle has been estimated to be 10 hr at 24-28 hpf (Nawrocki, 1985). Our cell counts are consistent with that estimate at that age, but not at earlier ages. A 10-hr cell cycle predicts that the cell number should increase by 7% per hr, but during 16-24 hpf the cell number increased by only 1.5% per hr. Despite the low rate of increase, all cells labeled with bromo-deoxyuridine, so all were proliferative. We considered three possible explanations for the nearly-constant cell number in the first phase: proliferation balanced by cell emigration from the eye, proliferation balanced by cell death, and low proliferation caused by a transient prolongation of the cell cycle. We excluded the first two, and found direct support for the third. Previous examinations of the cell cycle length in vertebrate central nervous system have concluded that it increases monotonically, in contrast to the modulation that we have shown. Modulation of the cell cycle length is well-known in flies, but it is generally effected by a prolonged arrest at one phase, in contrast to the general deceleration that we have shown.

Glial growth factor 2 induces proliferation and structural changes in ensheathing cells

Ensheathing cells were isolated from neonatal rat olfactory bulbs and cultured in the presence of glial growth factor 2 (GGF2). Proliferation assay showed that at concentrations of up to 60 ng/ml GGF2, ensheathing cells underwent a modest increase in proliferation rate. This stimulation was not maintained at high doses of GGF2 at 100 ng/ml or more. Chemotaxis chambers and scanning electron microscopy were used to determine whether GGF2 was a chemoattractant for ensheathing cells. Although the results showed no chemotactic response to GGF2, ensheathing cells demonstrated structural changes when cultured in the presence of 20 ng/ml GGF2. Ultrastructural observations revealed that GGF2 promoted increased deposition of extracellular matrix on the cell membrane, more cytoskeletal elements in the processes and as a possible consequence, contributed to a more rigid support. Ensheathing cells cultured in the absence of GGF2 often extended thinner and curved processes. Reverse transcription-polymerase chain reaction confirmed the presence of GGF2 transcripts in ensheathing cells, suggesting that ensheathing cells themselves are a source of GGF2.

A role for p75 neurotrophin receptor in the control of hair follicle morphogenesis

During hair follicle (HF) morphogenesis, p75 neurotrophin receptor (p75NTR) reportedly is the first growth factor receptor found to be expressed by those fibroblasts that later develop into the dermal papilla (DP) of the HF. However, the functional role of p75NTR in HF morphogenesis is still unknown. Studying HF development in fetal and neonatal C57BL/6 murine back skin, we show that p75NTR-immunoreactivity (IR) is prominently expressed by DP fibroblasts as well as by skin nerves during the early steps of HF development. In contrast, p75NTR-IR disappears from the DP in the fully developed HF and it is expressed only in the epithelial outer root sheath of the HF. Compared to age-matched wild-type animals, p75NTR knockout (-/-) mice show significant acceleration of HF morphogenesis, and DP fibroblasts of p75NTR knockout mice show reduced proliferative activity in situ, indicating alterations in their transition from proliferation to differentiation. Although no significant differences in the expression of adhesion molecules (NCAM), selected morphogens (TGFbeta-2, HGF/SF, FGF-2, KGF), or their receptors (TGFbetaR-II, m-met, FGFR-1) were seen between DP of p75NTR knockout and wild-type mice, p75NTR mutants showed a prominent upregulation of FGFR-2, a high-affinity receptor for KGF, in both follicular DP and epithelium. Furthermore, the administration of anti-KGF neutralizing antibody significantly inhibited acceleration of HF morphogenesis in p75NTR knockout mice in vivo. These observations suggest that p75NTR plays an important role during HF morphogenesis, functioning as a receptor that negatively controls HF development, most likely via alterations in DP fibroblast proliferation/differentiation and via downregulation of KGF/FGFR-2 signaling in the HF.

Immunolesioning of nerve growth factor p75 receptor-containing neurons in the rat brain by a novel immunotoxin: anti-p75-anti-mouse IgG-trichosanthin conjugates

In the present study, a comparison of potency between a commercially available immunotoxin, 192-immunoglobulin-SAP (192-IgG), and a novel immunotoxin produced in our laboratory, anti-p75-anti-mouse IgG-trichosanthin conjugates (p75-TCS), was conducted. Both of the immunotoxins were specific for nerve growth factor p75 receptor. Cholinergic neurons in the rat basal forebrain and in the neostriatum were depleted after the injection of either 192-IgG or p75-TCS. These indicate that both types of immunotoxins are potent and useful in performing immunolesioning experiments. In addition, there were variations in potency among the two immunotoxins in different routes of administration. The 192-IgG was more potent than the p75-TCS in the case of ventricular injections. In case of striatal injections, 192-IgG caused serious tissue necrosis and considerable tissue damage in the brain region. In contrast, p75-TCS was potent and caused a selective and specific depletion of cholinergic neurons in the neostriatum. These results indicate that indirect immunotoxins may be more useful for performing immunolesioning experiments in case of brain parenchyma administration.

Differential expression of NGF receptors in human thymic epithelial tumors

NGF receptor (TrkA and p75NGFR) expression was investigated in human thymuses, including normal thymuses, thymic hyperplasias, thymomas and thymic carcinomas. TrkAI but not TrkAII transcripts were demonstrated by RT-PCR. In normal thymuses, immunohistochemistry revealed a restricted TrkA-immunoreactivity to epithelial and interdigitated reticular cells, while only interdigitaded reticular cells were immunoreactive for p75NGFR. Thymocytes were negative for both receptors. A switch from the normal TrkA positive-p75NGFR negative phenotype to a TrkA negative-p75NGFR positive phenotype was found in histologically aggressive epithelial cell tumors, suggesting that NGF and its receptors are potentially involved in thymus stroma organogenesis and proliferation.

Nerve growth factor (NGF) exerts its pro-apoptotic effect via the P75NTR receptor in a cell cycle-dependent manner

Nerve growth factor (NGF), the prototypic member of the neurotrophin family of growth factors, exerts its action via two receptors, P75NTR and TrkA, the expression of which varies at the cell surface of neuroblastoma cells (SH-SY5Y cells) in a cycle phase-specific manner. NGF was pro-apoptotic on growing cells expressing preferentially P75NTR and exhibited a potent anti-apoptotic effect on quiescent cells, when TrkA was prevalent at the cell surface, showing that NGF can have a dual action on SH-SY5Y cells depending on the relative cell surface expression of TrkA and P75NTR. The pro-apoptotic activity of NGF but not its anti-apoptotic activity was abrogated by an antibody against the extracellular domain of P75NTR and in cell isolated from P75NTR knock-out mice indicating that NGF exhibits a proapoptotic activity via P75NTR exclusively. On the other hand, we showed that the anti-apoptotic activity of NGF was specifically mediated by an interaction with TrkA with no contribution of P75NTR, as demonstrated on SK-N-BE cells transfected with TrkA in which NGF was a potent anti-apoptotic compound but did not exhibit any pro-apoptotic activity. These results support the hypothesis that the survival response to NGF depends on its binding to TrkA without any involvement of P75NTR which in turn selectively mediates the pro-apoptotic activity of NGF with no contribution of TrkA and show that, depending on the growth state of the cells, NGF exhibits dual pro- or anti-apoptotic properties via P75NTR and TrkA, respectively.

Expression of neurotrophins and their receptors in human bone marrow

The expression of neurotrophins and their receptors, the low-affinity nerve growth factor receptor (p75LNGFR) and the Trk receptors (TrkA, TrkB, and TrkC), was investigated in human bone marrow from 16 weeks fetal age to adulthood. Using reverse transcription-polymerase chain reaction, all transcripts encoding for catalytic and truncated human TrkB or TrkC receptors were detected together with trkAI transcripts, whereas trkAII transcripts were found only in control nerve tissues. Transcripts for the homologue of the rat truncated TrkC(ic113) receptor were identified for the first time in human tissue. Stromal adventitial reticular cells were found immunoreactive for all neutrophin receptors. In contrast, hematopoietic cell types were not immunoreactive for p75LNGFR but showed immunoreactivity for one or several Trk receptors. TrkA immunoreactivity was found in immature erythroblasts. Catalytic TrkB immunoreactivity was observed in eosinophilic metamyelocytes and polymorphonuclear cells. Truncated TrkB immunoreactivity was found in erythroblasts and megacaryocytes. Immunoreactivity for both catalytic and truncated TrkC receptor was observed in promyelocytes, myelocytes, some polymorphonuclear cells and megacaryocytes. Neutrophin transcript levels appeared higher at fetal than at adult stages, no variation in Trk family transcript levels was observed. The local expression of neurotrophin genes suggests a wide range of paracrine and/or autocrine mode of action through their corresponding receptors within the bone marrow.