Tissue localization of nerve growth factor and nerve growth factor receptors

The nerve growth factor signaling and its potential as therapeutic target for glaucoma

Neuroprotective therapies which focus on factors leading to retinal ganglion cells (RGCs) degeneration have been drawing more and more attention. The beneficial effects of nerve growth factor (NGF) on the glaucoma have been recently suggested, but its effects on eye tissue are complex and controversial in various studies. Recent clinical trials of systemically and topically administrated NGF demonstrate that NGF is effective in treating several ocular diseases, including glaucoma. NGF has two receptors named high affinity NGF tyrosine kinase receptor TrkA and low affinity receptor p75NTR. Both receptors exist in cells in retina like RGC (expressing TrkA) and glia cells (expressing p75NTR). NGF functions by binding to TrkA or p75NTR alone or both together. The binding of NGF to TrkA alone in RGC promotes RGC's survival and proliferation through activation of TrkA and several prosurvival pathways. In contrast, the binding of NGF to p75NTR leads to apoptosis although it also promotes survival in some cases. Binding of NGF to both TrkA and p75NTR at the same time leads to survival in which p75NTR functions as a TrkA helping receptor. This review discusses the current understanding of the NGF signaling in retina and the therapeutic implications in the treatment of glaucoma.

Deposition of the NG2 proteoglycan at nodes of Ranvier in the peripheral nervous system

The node of Ranvier is a complex macromolecular assembly of ion channels and other proteins that is specialized for the rapid propagation of the action potential. A full understanding of the processes responsible for the assembly and maintenance of the node requires first the identification and characterization of the proteins found there. Here we show that NG2, a structurally unique chondroitin sulfate proteoglycan, is a molecular component of the node of Ranvier in the peripheral nervous system. In adult sciatic nerve, NG2 is (1) associated with thin, elongated fibroblast-like cells, (2) on some but not all basal laminae, and (3) at nodes of Ranvier. At the nodes, NG2 is restricted to the nodal gap and is absent from the paranodal or juxtaparanodal region. In dissociated cell cultures of adult sciatic nerve, perineurial fibroblasts but not Schwann cells express NG2 on their surfaces. Approximately 45% of the total NG2 in peripheral nerves is in a soluble, rather than particulate, subcellular compartment. NG2 is also present in membrane fractions that also contain high levels of voltage-dependent sodium channels, caspr, and neuron-glia related cell adhesion molecule. These medium-density membranes likely correspond to the nodal and paranodal region of the axon-Schwann cell unit. These results suggest a model in which perineurial fibroblasts secrete or shed NG2, which subsequently associates with nodes of Ranvier. The growth-inhibitory and anti-adhesive properties of NG2 may limit the lateral extension of myelinating Schwann cells as nodes mature. NG2 may also participate in the barrier functions of the perineurial linings of the nerve.

Primary olfactory axons form ectopic glomeruli in mice lacking p75NTR

The restricted expression of the low affinity nerve growth factor receptor p75NTR by olfactory ensheathing cells suggests that this molecule is involved in the development of the olfactory nerve pathway. To begin to understand the role of p75NTR, we examined the development of the primary olfactory system in p75NTR(-/-) and wild-type mice. Our results demonstrate that, although p75NTR is not essential for the initial assembly of the olfactory nerve, it plays an important role in the postnatal maturation of the olfactory bulb. In the absence of p75NTR, there is exuberant growth of some primary olfactory axons into the olfactory bulb. These axons either aberrantly bypass the glomerular layer and project into deeper lamina or grow into an abnormal bleb of tissue protruding from the medial surface of the dorsocaudal olfactory bulb. These blebs become apparent in neonatal mice and contain axons expressing olfactory marker protein that form ectopic glomerular-like tufts. Histochemical staining with the plant lectin Dolichos biflorus agglutinin revealed that axons sorted out and selectively converged on glomeruli within these blebs. Our results suggest that p75NTR indirectly influences axon growth but not glomerular targeting and plays a role in the postnatal maturation of laminar cytoarchitecture in the olfactory bulb.

Palmitoylation of the p75 neurotrophin receptor has no effect on its interaction with TrkA or on TrkA-mediated down-regulation of cell adhesion molecules

The short- and long-term effects of nerve growth factor (NGF) were studied on fibroblast cell lines stably expressing both TrkA and either wild-type p75 or a mutant that lacks the palmitoylation site of p75. The lack of palmitoylation had no effect on the ability of p75 to enhance the short-term NGF-induced tyrosine phosphorylation of TrkA over a wide range of NGF concentrations. Long-term treatment of the cell lines with NGF led to loss of cell adhesion to the culture dishes that increased with increasing concentrations of NGF and increased expression of TrkA. Treatment of the cell lines with mutant NGFs that bound selectively to TrkA or p75 alone revealed that cell detachment was mediated solely through TrkA. Increased cell detachment correlated with a decrease in the expression levels of fibronectin and cadherin, cell surface molecules involved in cell adhesion. The loss of cell adhesion with the cell line expressing the palmitoylation-deficient p75 were identical to those expressing wild type, as was anticipated from the lack of involvement of p75 in this process.

Neural cell surface differentiation antigen gp130(RB13-6) induces fibroblasts and glioma cells to express astroglial proteins and invasive properties

Transient expression of the differentiation and tumor cell surface antigen gp130(RB13-6) characterizes a subset of rat glial progenitor cells susceptible to ethylnitrosourea-induced neurooncogenesis. gp130(RB13-6) is as a member of an emerging protein family of ecto-phosphodiesterases/nucleotide pyrophosphatases that includes PC-1 and the tumor cell motility factor autotaxin. We have investigated the potential role of gp130(RB13-6) in glial differentiation by transfection of three cell lines of different origin that do not express endogenous gp130(RB13-6) (NIH-3T3 mouse fibroblasts; C6 and BT7Ca rat glioma cells) with the cDNA encoding gp130(RB13-6). The effect of gp130(RB13-6) expression was analyzed in terms of overall cell morphology, the expression of glial cell-specific marker proteins, and invasiveness. Transfectant sublines, consisting of 100% gp130(RB13-6)-positive cells, exhibited an altered, bipolar morphology. Fascicular aggregates of fibroblastoid cells subsequently developed into mesh-like patterns. Contrary to the parental NIH-3T3 and BT7Ca cells, the transfectant cells invaded into collagen type I. As shown by immunofluorescence staining of the transfectant sublines as well as of primary cultures composed of gp130(RB13-6)-positive and -negative cells, expression of gp130(RB13-6) induced coexpression of proteins typical for glial cells and their precursors, i.e., glial fibrillary acidic protein, the low affinity nerve growth factor receptor, and the neural proteins Thy-1, Ran-2, and S-100. In accordance with its expression in the immature rat nervous system, gp130(RB13-6) may thus have a significant role in the glial differentiation program and its subversion in neurooncogenesis.

Neurotrophin stimulation of human melanoma cell invasion: selected enhancement of heparanase activity and heparanase degradation of specific heparan sulfate subpopulations

Heparanase is an endo-beta-D-glucuronidase whose enzymatic targets are the glycosaminoglycan chains of heparan sulfate proteoglycans (50). Elevated levels of heparanase are associated with the metastatic potential of melanoma cells, and treatment of murine and human melanoma cells with the prototypic neurotrophin nerve growth factor (NGF) increases the production of heparanase by melanoma cells. We previously reported that physiological concentrations of NGF increased invasion of early passage human brain-metastatic 70W melanoma cells but not melanoma cells metastatic to other sites or nonmetastatic melanoma cells as measured in Matrigel invasion assays. Here we found that treatment of 70W melanoma cells with neurotrophin-3 (NT-3) increased Matrigel invasion, whereas treatment with neurotrophins other than NGF or NT-3 did not influence invasion. Mutants of NGF that do not bind to the neurotrophin receptor p75NTR or other nonneuronal growth factors were not able to enhance the invasion of 70W melanoma cells. When 70W cells were exposed to antisense oligonucleotides directed against p75NTR mRNA, there was a reduction in NGF and NT-3 binding, and the neurotrophins failed to enhance Matrigel invasion. To study the properties of heparanase in neurotrophin-regulated malignant melanoma invasive processes, we developed a sensitive heparanase assay consisting of purified [35S]HS subpopulations separated by agarose gel electrophoresis. Incubation of 70W cells with NGF or NT-3 but not brain-derived neurotrophic factor, neurotrophin-4/5 or mutant NGF resulted in increased release of heparanase activity that was capable of degrading a subpopulation of heparan sulfate molecules.

Functional roles of neurotrophin 3 in the developing and mature sympathetic nervous system

Nerve growth factor (NGF) is a potent regulator of sympathetic neuronal function in both developing and adult animals. This article reviews the evidence published in recent years indicating that another member of the NGF family, neurotrophin 3 (NT3), plays both a complementary and overlapping role in the development and maturation of sympathetic neurons. In migratory neural crest cells, expression of the high-affinity receptor, trkC, and promotion of mitosis by NT3 suggest an involvement in gangliogenesis, since sympathetic neuroblasts express both NT3 and trkC and require NT3 for their proliferation, differentiation, and survival, it has been proposed that the factor acts at this developmental stage as an autocrine or paracrine factor. However, NT3 also acts in parallel with NGF to promote the survival of postmitotic neurons during late development. Both trkC and trkA are expressed in sympathetic neurons and function as high-affinity receptors for NT3. NT3 is synthesized in sympathetic effector tissues and the endogenous factor is retrogradely transported to accumulate within the cell soma. Thus, in addition to its role in the differentiation of sympathetic neurons, NT3, like NGF, is also an effector tissue-derived neurotrophic factor for these neurons in maturity.

The low-affinity nerve growth factor receptor p75NGFR mediates death of PC12 cells after nerve growth factor withdrawal

We have investigated the role of the low-affinity nerve growth factor (NGF) receptor p75NGFR in determining the death of neuronally differentiated PC12 cells after withdrawal of NGF. A range of high and low p75NGFR-expressing cells were obtained by a combination of fluorescence activated cell sorting (FACS) and stable transfection with a p75NGFR expression vector. Cells were readily differentiated to a neuronal phenotype irrespective of the level of p75NGFR expression. However, the rate and extent of neuronal death following NGF deprivation were extremely sensitive to the level of p75NGFR expression. The highest expressing cells died most rapidly. Cells selected for very low levels of p75NGFR expression exhibited resistance to NGF withdrawal, and remained as viable, differentiated neurons, with minimal cell death, for at least 5 days in the absence of NGF. Antisense oligonucleotides against p75NGFR were shown to down-regulate p75NGFR in PC12 cells and, further, to significantly enhance survival in the absence of NGF. These results consolidate and generalize our previous findings that p75NGFR induces cell death in postnatal sensory neurons in the absence of NGF. The ability to induce cell death in the absence of NGF appears to be a more general role of p75NGFR in differentiated neurons, and an important new paradigm for the mechanism of NGF-dependent survival.

Regulation of glucocorticosteroid receptor expression in rat hippocampal cell cultures by nerve growth factor

Dispersed hippocampal cells cultured in serum-free conditions were used to study the effects of nerve growth factor (NGF) on the expression of type I (mineralocorticosteroid or MR) and type II (glucocorticosteroid or GR) corticosteroid receptors. Cells, plated at a density of 1.2 x 10(6) cells/ml in 60 mm Petri dishes, were mainly identified as neurons (90-95%) and maintained for at least 2 weeks. A 7-day treatment with 10-50 ng NGF/ml induced a concentration-dependent decrease of GR binding (40% decrease) compared to untreated cells. In contrast, MR density was unaffected by a 7-day treatment with 50 ng NGF/ml. Data are discussed as possible direct and/or indirect effects of NGF at the level of both neuronal and glial cells.

The p75 neurotrophin receptor

The low-affinity p75 molecule and trk tyrosine kinases serve as receptors for target-derived neurotrophins. While the mechanism by which receptor tyrosine kinases impart intracellular signaling has become well understood, the precise roles of the p75 receptor are not fully defined. The p75 neurotrophin receptor belongs to a family of transmembrane molecules which also serve as receptors for the tumor necrosis factor family of cytokines. Each receptor shares a common extracellular structure highlighted by conserved cysteine-rich repeats. Because NGF, BDNF, NT-3, and NT-4/5 bind to p75 with similar affinity, p75 may either act as a common subunit in a neurotrophin receptor complex with trk family members, or act by independent mechanisms to mediate biological actions of each neurotrophin.

Expression of trkA cDNA in neuroblastomas mediates differentiation in vitro and in vivo

The human trkA cDNA was transfected into a malignant human neuroblastoma (NB) cell line (HTLA230) to investigate its role in NB growth and differentiation. This cell line lacks expression of both endogenous trkA and gp75NGFR genes. Transfectants expressing the trkA mRNA and surface-bound receptors transcriptionally activate immediate-early genes (c-fos, c-jun, and jun-B) following nerve growth factor (NGF) stimulation. NGF treatment induces growth arrest as well as down-regulation of the amplified N-myc oncogene. Genes selectively expressed in mature neurons (SCG-10, ret proto-oncogene, GAP-43, etc.) are transcriptionally activated, and neurite outgrowth further demonstrates differentiation of transfectants following NGF stimulation. trkA-expressing NB cells remain tumorigenic in nude mice; however, subcutaneous treatment of tumor-bearing mice with NGF induces Schwannian and neuronal cell differentiation similar to the induction seen in human ganglioneuroblastomas. Thus, trkA expression in HTLA230 cells is sufficient to generate a functional NGF receptor complex that leads to growth-arrested and differentiated NB cells in vitro and in vivo in the presence of NGF. Hence, NGF may play a crucial role in NB cell differentiation and regression in vivo.

Expression of trkA cDNA in neuroblastomas mediates differentiation in vitro and in vivo

The human trkA cDNA was transfected into a malignant human neuroblastoma (NB) cell line (HTLA230) to investigate its role in NB growth and differentiation. This cell line lacks expression of both endogenous trkA and gp75NGFR genes. Transfectants expressing the trkA mRNA and surface-bound receptors transcriptionally activate immediate-early genes (c-fos, c-jun, and jun-B) following nerve growth factor (NGF) stimulation. NGF treatment induces growth arrest as well as down-regulation of the amplified N-myc oncogene. Genes selectively expressed in mature neurons (SCG-10, ret proto-oncogene, GAP-43, etc.) are transcriptionally activated, and neurite outgrowth further demonstrates differentiation of transfectants following NGF stimulation. trkA-expressing NB cells remain tumorigenic in nude mice; however, subcutaneous treatment of tumor-bearing mice with NGF induces Schwannian and neuronal cell differentiation similar to the induction seen in human ganglioneuroblastomas. Thus, trkA expression in HTLA230 cells is sufficient to generate a functional NGF receptor complex that leads to growth-arrested and differentiated NB cells in vitro and in vivo in the presence of NGF. Hence, NGF may play a crucial role in NB cell differentiation and regression in vivo.

Mediation of NGF-stimulated extracellular matrix invasion by the human melanoma low-affinity p75 neurotrophin receptor: melanoma p75 functions independently of trkA

Although overexpression of the low-affinity p75 neurotrophin receptor (p75NTR) is frequently associated with advanced stages of human melanoma progression, the functional significance of this finding is unknown. We examined whether the degree of cell surface expression of p75NTR in human melanoma cell variants determines their extent of invasion stimulated by nerve growth factor (NGF). Treatment of MeWo melanoma cells or a metastatic spontaneous wheat germ agglutinin-resistant variant subline (70W) of MeWo cells with 2.5S NGF resulted in a dose-dependent enhancement of invasion through a reconstituted basement membrane. This effect was most pronounced with the 70W subline that exhibits brain-metastasizing potential in nude mice but was not found with a poorly metastatic MeWo variant subline (3S5). The expression of p75NTR as determined by Northern blotting and immunoprecipitation analysis of 125I-labeled cell surface proteins correlated with NGF-stimulated invasion. The MeWo melanoma sublines used in this study did not express p140proto-trkA mRNA or any p140proto-trkA variant transcripts including p70trkA as determined by Northern analysis and RT-PCR analysis. Thus, these melanoma cells would not be expected to form functional p75-p140 heterodimers or p140-p140 homodimers capable of transducing an NGF-generated signal to p140proto-trkA cytoplasmic substrates. These cells did express authentic p145trkC transcripts. However, NGF did not catalytically activate p145trkC receptors via increased tyrosine phosphorylation as would be expected if p145trkC participated in the signaling established by NGF. Furthermore, a NGF-stimulated purine-analogue-sensitive kinase activity was found to coimmunoprecipitate with p75NTR. This p75NTR-associated kinase may coordinate initial signaling events evoked by p75NTR ligand interaction. Addition of 2.5S NGF, at concentrations that should saturate cell surface p75NTR, to matrix-adherent cultures of human MeWo and 70W but not 3S5 melanoma cells suppressed the expression of 92-kDa type IV collagenase and stimulated the production of 72-kDa type IV collagenase in its fully active 68-kDa form. In the absence of p140proto-trkA, the matrix-dependent effects of NGF on metalloproteinase expression of brain-metastatic 70W melanoma cells suggest a signaling role for the low-affinity melanoma p75NTR receptor and its associated purine-analogue-sensitive kinase in signaling enhanced matrix penetration of NGF-rich stromal microenvironments such as the brain.

Nerve growth factor receptor (p75)-immunoreactivity in the normal adult feline trigeminal system and following retrogasserian rhizotomy

The 75 kDa protein nerve growth factor receptor [NGFr(p75)] is a neurotrophin receptor that is able to bind different members of the neurotrophin family of molecules implicated in affecting neuronal survival. Here we describe the light microscopic distribution of NGFr(p75)-immunoreactivity (IR) within the feline trigeminal brainstem sensory nuclear complex and trigeminal ganglion of normal adult subjects and in subjects 10 and 30 days following retrogasserian rhizotomy. Within the trigeminal ganglion of normal subjects, numerous fibers and most of the neuronal cell bodies showed NGFr(p75)-IR that varied in intensity, while cells and fibers with NGFr(p75)-IR were less numerous within the mesencephalic trigeminal nucleus. Within the main sensory and spinal trigeminal nuclei, NGFr(p75)-IR formed a reproducible pattern that varied between the different subnuclei. The NGFr(p75)-IR consisted both of dense pockets and a low level NGFr(p75)-IR that was selective to the trigeminal neuropil. Following rhizotomy, most of the NGFr(p75)-IR was lost from the main sensory and spinal trigeminal nuclei, except in regions where the upper cervical roots and cranial nerves VII, IX, and X project. In contrast, examination of the central root that was still attached to the trigeminal ganglion showed increased NGFr(p75)-IR in fibers and supporting cells, as did the motor root within the peripheral mandibular division. These results indicate that the majority of the NGFr(p75)-IR within the main sensory and spinal trigeminal nuclei originates from primary trigeminal afferents and that retrogasserian rhizotomy leads to an up-regulation of NGFr(p75)-IR in the part of the central root that is contiguous with the ganglion.

Synthesis and localization of ciliary neurotrophic factor in the sciatic nerve of the adult rat after lesion and during regeneration

Ciliary neurotrophic factor (CNTF) is expressed in high quantities in Schwann cells of peripheral nerves during postnatal development of the rat. The absence of a hydrophobic leader sequence and the immunohistochemical localization of CNTF within the cytoplasm of these cells indicate that the factor might not be available to responsive neurons under physiological conditions. However, CNTF supports the survival of a variety of embryonic neurons, including spinal motoneurons in culture. Moreover we have recently demonstrated that the exogenous application of CNTF protein to the lesioned facial nerve of the newborn rat rescued these motoneurons from cell death. These results indicate that CNTF might indeed play a major role in assisting the survival of lesioned neurons in the adult peripheral nervous system. Here we demonstrate that the CNTF mRNA and protein levels and the manner in which they are regulated are compatible with such a function in lesioned peripheral neurons. In particular, immunohistochemical analysis showed significant quantities of CNTF at extracellular sites after sciatic nerve lesion. Western blots and determination of CNTF biological activity of the same nerve segments indicate that extracellular CNTF seems to be biologically active. After nerve lesion CNTF mRNA levels were reduced to less than 5% in distal regions of the sciatic nerve whereas CNTF bioactivity decreased to only one third of the original before-lesion levels. A gradual reincrease in Schwann cells occurred concomitant with regeneration.

The activity of cAMP-dependent protein kinase is required at a posttranslational level for induction of voltage-dependent sodium channels by peptide growth factors in PC12 cells

The synthesis and expression of voltage-dependent sodium (Na) channels is a crucial aspect of neuronal differentiation because of the central role these ion channels play in the generation of action potentials and the transfer of information in the nervous system. We have used rat pheochromocytoma (PC12) cell lines deficient in cAMP-dependent protein kinase (PKA) activity to examine the role of PKA in the induction of Na channel expression by nerve growth factor (NGF) and basic FGF (bFGF). In the parental PC12 cell line both NGF and bFGF elicit an increase in the density of functional Na channels, as determined from whole-cell patch clamp recordings. This increase does not occur in two PC12 cell lines deficient in both isozymes of PKA (PKAI and PKAII), and is strongly reduced in a third line deficient in PKAII, but not PKAI. Despite the inability of the neurotrophic factors to induce functional Na channel expression in the PKA-deficient cells, Northern blot hybridization studies and saxitoxin binding assays of intact cells indicate that NGF and bFGF are still capable of eliciting increases in both Na channel mRNA and Na channel protein in the membrane. Thus, PKA activity appears to be necessary at a posttranslational step in the synthesis and expression of functional Na channels, and thereby plays an important role in determining neuronal excitability.