Expression of trk in MAH cells lacking the p75 low-affinity nerve growth factor receptor is sufficient to permit nerve growth factor-induced differentiation to postmitotic neurons

Short multiwall carbon nanotubes promote neuronal differentiation of PC12 cells via up-regulation of the neurotrophin signaling pathway

Numerous unique properties of carbon nanotubes make them attractive for applications in neurobiology such as drug delivery, tissue regeneration, and as scaffolds for neuronal growth. In this study, the critical roles of the length of multiwall carbon nanotubes (MWCNTs) on a neuronal-like model cell line PC12 cells are investiaged. Incubation of PC12 cells with carboxylated MWCNTs did not significantly affect cellular morphology and viability at lower concentrations. Short MWCNTs show higher cellular uptake and more obvious removal compared to longer ones, which can result in higher ability to promote PC12 cell differentiation. Pre-incubation of short MWCNTs can up-regulate the expression of neurotrophin signaling pathway-associated TrkA/p75 receptors and Pincher/Gap43/TH proteins, which might be the underlying mechanism for the improved differentiation in PC12 cells. The current results provide insight for future applications of MWCNTs in neuron drug delivery and neurodegenerative disease treatment.

Two novel human NUMB isoforms provide a potential link between development and cancer

We previously identified four functionally distinct human NUMB isoforms. Here, we report the identification of two additional isoforms and propose a link between the expression of these isoforms and cancer. These novel isoforms, NUMB5 and NUMB6, lack exon 10 and are expressed in cells known for polarity and migratory behavior, such as human amniotic fluid cells, glioblastoma and metastatic tumor cells. RT-PCR and luciferase assays demonstrate that NUMB5 and NUMB6 are less antagonistic to NOTCH signaling than other NUMB isoforms. Immunocytochemistry analyses show that NUMB5 and NUMB6 interact and complex with CDC42, vimentin and the CDC42 regulator IQGAP1 (IQ (motif) GTPase activating protein 1). Furthermore, the ectopic expression of NUMB5 and NUMB6 induces the formation of lamellipodia (NUMB5) and filopodia (NUMB6) in a CDC42- and RAC1-dependent manner. These results are complemented by in vitro and in vivo studies, demonstrating that NUMB5 and NUMB6 alter the migratory behavior of cells. Together, these novel isoforms may play a role in further understanding the NUMB function in development and cancer.

NRAGE is a negative regulator of nerve growth factor-stimulated neurite outgrowth in PC12 cells mediated through TrkA-ERK signaling

NRAGE, also denominated as MAGE-D1 or Dlxin-1, is firstly identified as a molecule interacting with NGF low affinity receptor p75NTR. It facilitates cell cycle arrest and NGF-dependent neuronal apoptosis. Here we report that NRAGE is downregulated while p75NTR is upregulated during the process of NGF-induced neuronal differentiation of PC12 cells. Knockdown of NRAGE by RNA interference accelerates NGF-mediated neurite outgrowth. In addition, in the NRAGE-suppressed cells, NGF-induced ERK activation is increased and this activation is MEK-dependent. Conversely, NRAGE overexpression significantly represses NGF-induced ERK activation. Further studies revealed that NRAGE downregulates TrkA expression through a post-transcriptional manner and thereby blocks NGF-induced TrkA phosphrylation at tyrosine-490. Altogether, these data indicate for the first time that NRAGE is an endogenous inhibitor for NGF-induced neuronal differentiation of PC12 cells by regulating TrkA-ERK signaling.

Role of PACAP in the physiology and pathology of the sympathoadrenal system

Sympathetic neurons and chromaffin cells derive from common sympathoadrenal precursors which arise from the neural crest. Cells from this lineage migrate to their final destination and differentiate by acquiring a catecholaminergic phenotype in response to different environmental factors. It has been shown that the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) and its PAC1 receptor are expressed at early stages of sympathetic development, and participate to the control of neuroblast proliferation and differentiation. PACAP also acts as a neurotransmitter to stimulate catecholamine and neuropeptide biosynthesis and release from sympathetic neurons and chromaffin cells, during development and in adulthood. In addition, PACAP and its receptors have been described in neuroblastoma and pheochromocytoma, and the neuropeptide regulates the differentiation and activity of sympathoadrenal-derived tumoral cell lines, suggestive of an important role in the pathophysiology of the sympathoadrenal lineage. Transcriptome studies uncovered genes and pathways of known and unknown roles that underlie the effects of PACAP in the sympathoadrenal system.

Soluble Jagged1 Attenuates Lateral Inhibition, Allowing for the Clonal Expansion of Neural Crest Stem Cells

The activation of Notch signaling in neural crest stem cells (NCSCs) results in the rapid loss of neurogenic potential and differentiation into glia. We now show that the attenuation of endogenous Notch signaling within expanding NCSC clones by the Notch ligand soluble Jagged1 (sJ1), maintains NCSCs in a clonal self-renewing state in vitro without affecting their sensitivity to instructive differentiation signals observed previously during NCSC self-renewal. sJ1 functions as a competitive inhibitor of Notch signaling to modulate endogenous cell-cell communication to levels sufficient to inhibit neural differentiation but insufficient to instruct gliogenic differentiation. Attenuated Notch signaling promotes the induction and nonclassic release of fibroblast growth factor 1 (FGF1). The functions of sJ1 and FGF1 signaling are complementary, as abrogation of FGF signaling diminishes the ability of sJ1 to promote NCSC expansion, yet the secondary NCSCs maintain the dosage sensitivity of the founder. These results validate and build upon previous studies on the role of Notch signaling in stem cell self-renewal and suggest that the differentiation bias or self-renewal potential of NCSCs is intrinsically linked to the level of endogenous Notch signaling. This should provide a unique opportunity for the expansion of NCSCs ex vivo without altering their differentiation bias for clinical cell replacement or transplant strategies in tissue repair. Disclosure of potential conflicts of interest is found at the end of this article.

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

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

Characterization of NADE, NRIF and SC-1 gene expression during mouse neurogenesis

The p75 neurotrophin receptor (p75NTR) is a member of the tumor necrosis factor receptor superfamily. p75NTR signaling events have been implicated in both cell cycle arrest and apoptosis depending on which effector molecules are associated with its intracellular domain after ligand binding. Two such effector proteins, p75NTR-associated cell death executor (NADE) and neurotrophin receptor interacting factor (NRIF) promote p75NTR-mediated apoptosis, whereas Schwann cell factor-1 (SC-1) mediates neurotrophin-dependent withdrawal from the cell cycle. An understanding of the expression profiles of these three interacting proteins and p75NTR during embryogenesis is critical for addressing whether these effector proteins might function outside of p75NTR-mediated signaling events. The distribution of NADE, NRIF and SC-1 mRNAs during murine development suggests that the action of these genes is in fact not limited to regions of p75NTR expression. Specifically, a detailed comparison of the spatial and temporal expression domains of NADE, NRIF and SC-1 during brain development revealed regions of co-expression with p75NTR but also illustrates a distinct and discordant spatial and temporal expression. These results yield novel insights into the unique developmental characteristics of the three p75NTR-interacting proteins, thus revealing their diverse signaling potential during embryonic development.

Overexpression of the signaling adapter FRS2 reconstitutes the cell cycle deficit of a nerve growth factor non-responsive TrkA receptor mutant

We have characterized the cell cycle deficit of a novel TrkA receptor mutant (TrkAS3) that fails to support nerve growth factor (NGF)-dependent cell cycle arrest and neurite outgrowth. TrkAS3 receptors fail to support an NGF-dependent increase in the expression of cyclin D1 and the cell cycle inhibitor, p21(Waf1/Cip1), two important regulators of G(1) /S transition, and do not down-regulate expression of the G(2) /M phase marker, cdc2/cdk1, or the S phase marker, proliferating cell nuclear antigen. Moreover, NGF-activated TrkAS3 receptors do not down-regulate cyclin-dependent kinase 4 phosphorylation of the retinoblastoma protein, essential for G(1) arrest, in comparison to NGF-activated wild-type TrkA. Collectively these data indicate that TrkAS3 receptors fail to support NGF-dependent G(1) arrest. Interestingly, ectopic expression of regulators of G(1) /S arrest, such as cyclin D1 or inhibitors of cell cycle (p21(Waf1/Cip1), p16(INK4A) ), or the fibroblast growth factor (FGF) receptor substrate-2 (FRS2) in cells expressing TrkAS3 reconstitutes NGF-dependent neurite outgrowth. Collectively, these data suggest a model in which NGF-stimulated TrkA-dependent activation of FRS2 supports neurite outgrowth through a mechanism that likely involves the induction of p21(Waf1/Cip1) expression and the arrest of cells at G(1) /S.

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.

Subpopulations of rat B2(+) neuroblasts exhibit differential neurotrophin responsiveness during sympathetic development

Sympathetic neurons comprise a population of postmitotic, tyrosine hydroxylase expressing cells whose survival is dependent upon nerve growth factor (NGF) both in vivo and in vitro. However, during development precursors to rat sympathetic neurons in the thoracolumbar region are not responsive to NGF because they lack the signal transducing NGF receptor, trkA. We have previously shown that acquisition of trkA expression is sufficient to confer a functional response to NGF. Here we describe four subpopulations of thoracolumbar sympathetic neuroblasts which are mitotically active and unresponsive to NGF at E13.5 of rat gestation, but differ based upon their neurotrophic responsiveness in vitro. The survival in culture of the largest sympathetic subpopulation is mediated by neurotrophin-3 (NT-3) or glial-derived neurotrophic factor (GDNF), whereas the cell survival of two smaller subpopulations of neuroblasts are mediated by either solely GDNF or solely NT-3. Finally, we identify a subpopulation of sympathetic neuroblasts in the thoracolumbar region whose survival, exit from the cell cycle, induction of trkA expression, and consequent acquisition of NGF responsiveness in culture appear to be neurotrophin independent and cell autonomous. These subpopulations reflect the diversity of neurotrophic actions that occur in the proper development of sympathetic neurons.

Role of neurotrophins and lectins in prevention of ototoxicity

Degeneration of hair cells (HC) and/or spiral ganglion neurons (SGN) is a major cause of hearing loss. Postnatal rat cochlear explant cultures are used to study the toxic actions of different classes of ototoxins and to identify molecules that can protect SGN and HC from ototoxic damage. Various ototoxins induce differential damage to HC and/or SGN. While gentamicin preferentially causes HC death, sodium salicylate selectively induces degeneration of SGN. In contrast, cisplatin results in destruction of both SGN and HC. Specific neurotrophins, including NT-4/5, BDNF, and NT-3, greatly protect SGN from all three types of ototoxins. In contrast, NGF and other growth factors have no effect. Of the 51 compounds examined, only concanavalin A (Con A), a lectin molecule, significantly protects HC from gentamicin. A dose-dependent study of Con A shows that maximal protection occurred at 100 nM. Further experiments indicates that preincubation of Con A with gentamicin does not form a complex, and coaddition of Con A and gentamicin to bacterial cultures, such as E. Coli cultures, does not interfere with the antibiotic activity of gentamicin. When the other 21 lectins are examined, Erythrina cristagalli lectin and Detura stramonium lectin also show activity similar to Con A. These findings may help elucidate the mechanisms of ototoxins and suggest that specific neurotrophins and lectins may be of therapeutic value in the prevention of ototoxin-induced hearing loss.

Restriction of developmental potential during divergence of the enteric and sympathetic neuronal lineages

In the peripheral nervous system, enteric and sympathetic neurons develop from multipotent neural crest cells. While local environmental signals in the gut and in the region of the sympathetic ganglia play a role in the choice of cell fate, little is known about the mechanisms that underlie restriction to specific neuronal phenotypes. We investigated the divergence and restriction of the enteric and sympathetic neuronal lineages using immuno-isolated neural crest-derived cells from the gut and sympathetic ganglia. Analysis of neuronal and lineage-specific mRNAs and proteins indicated that neural crest-derived cells from the gut and sympathetic ganglia had initiated neuronal differentiation and phenotypic divergence by E14.5 in the rat. We investigated the developmental potential of these cells using expression of tyrosine hydroxylase as a marker for a sympathetic phenotype. Tyrosine hydroxylase expression was examined in neurons that developed from sympathetic and enteric neuroblasts under the following culture conditions: culture alone; coculture with gut monolayers to promote enteric differentiation; or coculture with dorsal aorta monolayers to promote noradrenergic differentiation. Both enteric and sympathetic neuroblasts displayed developmental plasticity at E14.5. Sympathetic neuroblasts downregulated tyrosine hydroxylase in response to signals from the gut environment and enteric neuroblasts increased expression of tyrosine hydroxylase when grown on dorsal aorta or in the absence of other cell types. Tracking of individual sympathetic cells displaying a neuronal morphology at the time of plating indicated that neuroblasts retained phenotypic plasticity even after initial neuronal differentiation had occurred. By E19.5 both enteric and sympathetic neuroblasts had undergone a significant loss of their developmental potential, with most neuroblasts retaining their lineage-specific phenotype in all environments tested. Together our data indicate that the developmental potential of enteric and sympathetic neuroblasts becomes restricted over time and that this restriction takes place not as a consequence of initial neuronal differentiation but during the period of neuronal maturation. Further, we have characterized a default pathway of adrenergic differentiation in the enteric nervous system and have defined a transient requirement for gut-derived factors in the maintenance of the enteric neuronal phenotype.

Therapeutic potential of neurotrophins for treatment of hearing loss

Degeneration of spiral ganglion neurons (SGNs) and hair cells in the cochlea induced by aging, injury, ototoxic drugs, acoustic trauma, and various diseases is the major cause of hearing loss. Discovery of growth factors that can either prevent SGN and hair-cell death or stimulate hair-cell regeneration would be of great interest. Studies over the past several years have provided evidence that specific neurotrophins are potent survival factors for SGNs and protect these neurons from ototoxic drugs in vitro and in vivo. Current research focuses more on understanding the mechanism of hair-cell regeneration/differentiation and identification of growth factors that can stimulate hair-cell regeneration. SGNs are required to relay the signal to the central nervous system even when a cochlear implant is used to replace hair-cell function or in the case that cochlear sensory epithelium can be stimulated to regenerate new hair cells successfully. Therefore, neurotrophins may have their therapeutic value in prevention and treatment of hearing impairment.

Nerve growth factor modulates synaptic transmission between sympathetic neurons and cardiac myocytes

Regulation of heart rate by the sympathetic nervous system involves the release of norepinephrine (NE) from nerve terminals onto heart tissue, resulting in an elevation in beat rate. Nerve growth factor (NGF) is a neurotrophin produced by the heart that supports the survival and differentiation of sympathetic neurons. Here we report that NGF also functions as a modulator of sympathetic synaptic transmission. We determined the effect of NGF on the strength of synaptic transmission in co-cultures of neonatal rat cardiac myocytes and sympathetic neurons from the superior cervical ganglion (SCG). Synaptic transmission was assayed functionally, as an increase in the beat rate of a cardiac myocyte during stimulation of a connected neuron. Application of NGF produced a pronounced, reversible enhancement of synaptic strength. We found that TrkA, the receptor tyrosine kinase that mediates many NGF responses, is expressed primarily by neurons in these cultures, suggesting a presynaptic mechanism for the effects of NGF. A presynaptic model is further supported by the finding that NGF did not alter the response of myocytes to application of NE. In addition to the acute modulatory effects of NGF, we found that the concentration of NGF in the growth medium affects the level of synaptic transmission in cultures of sympathetic neurons and cardiac myocytes. These results indicate that in addition to its role as a survival factor, NGF plays both acute and long-term roles in the regulation of developing sympathetic synapses in the cardiac system.

Nerve growth factor modulates synaptic transmission between sympathetic neurons and cardiac myocytes

Regulation of heart rate by the sympathetic nervous system involves the release of norepinephrine (NE) from nerve terminals onto heart tissue, resulting in an elevation in beat rate. Nerve growth factor (NGF) is a neurotrophin produced by the heart that supports the survival and differentiation of sympathetic neurons. Here we report that NGF also functions as a modulator of sympathetic synaptic transmission. We determined the effect of NGF on the strength of synaptic transmission in co-cultures of neonatal rat cardiac myocytes and sympathetic neurons from the superior cervical ganglion (SCG). Synaptic transmission was assayed functionally, as an increase in the beat rate of a cardiac myocyte during stimulation of a connected neuron. Application of NGF produced a pronounced, reversible enhancement of synaptic strength. We found that TrkA, the receptor tyrosine kinase that mediates many NGF responses, is expressed primarily by neurons in these cultures, suggesting a presynaptic mechanism for the effects of NGF. A presynaptic model is further supported by the finding that NGF did not alter the response of myocytes to application of NE. In addition to the acute modulatory effects of NGF, we found that the concentration of NGF in the growth medium affects the level of synaptic transmission in cultures of sympathetic neurons and cardiac myocytes. These results indicate that in addition to its role as a survival factor, NGF plays both acute and long-term roles in the regulation of developing sympathetic synapses in the cardiac system.

Identification of endogenous sympathetic neuron pituitary adenylate cyclase-activating polypeptide (PACAP): depolarization regulates production and secretion through induction of multiple propeptide transcripts

The vasoactive intestinal peptide/pituitary adenylate cyclase-activating polypeptide (PACAP)/secretin/glucagon family of peptides displays numerous physiological roles in autonomic nervous system development and function. The regulated endogenous production and release of PACAP peptides in sympathetic neurons of the superior cervical ganglion (SCG) was investigated. The two posttranslationally processed forms of PACAP, PACAP27 and PACAP38, were identified in rat adult, neonatal, and cultured SCG neurons. PACAP38 levels were approximately 5-10 fmol/adult SCG and approximately 2 fmol/neonatal SCG; PACAP27 levels were comparable. The authenticity of peptide immunoreactivity in these tissues was verified by coelution with synthetic PACAP in reverse-phase HPLC analysis. Reverse transcription-PCR and sequence-specific hybridization revealed PACAP mRNA in adult, neonatal, and cultured SCG neurons; in situ hybridization histochemistry and immunocytochemistry localized the PACAP peptide and proPACAP mRNA to a subset of the SCG neuronal population. Basal and stimulated release of endogenous PACAP38 from cultured sympathetic neurons was established, suggesting that these peptides may function as signaling molecules at target tissues. Chronic depolarization with 40 mM potassium stimulated the PACAP secretory rate 10- to 20-fold, with concomitant increases in cellular PACAP peptide and mRNA levels. When examined using Northern analysis, depolarizing conditions not only stimulated the 2.2 kb form of PACAP mRNA, but also induced the expression of a shortened, 0.9 kb, transcript. Further reverse-transcription PCR analysis demonstrated that this smaller transcript was not identical to the unique testicular message. These studies identify PACAP38 and PACAP27 as regulated endogenous releasable peptides contributing to the functional diversity and phenotypic plasticity of the sympathetic nervous system.

Identification of endogenous sympathetic neuron pituitary adenylate cyclase-activating polypeptide (PACAP): depolarization regulates production and secretion through induction of multiple propeptide transcripts

The vasoactive intestinal peptide/pituitary adenylate cyclase-activating polypeptide (PACAP)/secretin/glucagon family of peptides displays numerous physiological roles in autonomic nervous system development and function. The regulated endogenous production and release of PACAP peptides in sympathetic neurons of the superior cervical ganglion (SCG) was investigated. The two posttranslationally processed forms of PACAP, PACAP27 and PACAP38, were identified in rat adult, neonatal, and cultured SCG neurons. PACAP38 levels were approximately 5-10 fmol/adult SCG and approximately 2 fmol/neonatal SCG; PACAP27 levels were comparable. The authenticity of peptide immunoreactivity in these tissues was verified by coelution with synthetic PACAP in reverse-phase HPLC analysis. Reverse transcription-PCR and sequence-specific hybridization revealed PACAP mRNA in adult, neonatal, and cultured SCG neurons; in situ hybridization histochemistry and immunocytochemistry localized the PACAP peptide and proPACAP mRNA to a subset of the SCG neuronal population. Basal and stimulated release of endogenous PACAP38 from cultured sympathetic neurons was established, suggesting that these peptides may function as signaling molecules at target tissues. Chronic depolarization with 40 mM potassium stimulated the PACAP secretory rate 10- to 20-fold, with concomitant increases in cellular PACAP peptide and mRNA levels. When examined using Northern analysis, depolarizing conditions not only stimulated the 2.2 kb form of PACAP mRNA, but also induced the expression of a shortened, 0.9 kb, transcript. Further reverse-transcription PCR analysis demonstrated that this smaller transcript was not identical to the unique testicular message. These studies identify PACAP38 and PACAP27 as regulated endogenous releasable peptides contributing to the functional diversity and phenotypic plasticity of the sympathetic nervous system.

Hepatocyte growth factor promotes motor neuron survival and synergizes with ciliary neurotrophic factor

Hepatocyte growth factor (HGF) has been shown to function as a potent mitogen for a variety of cells, transducing its signal through the c-met tyrosine kinase receptor. Ciliary neurotrophic factor (CNTF) is a cytokine that has been shown to promote survival of motor neurons. We show here that c-met mRNA is present in the embryonic rat spinal cord. Peak expression of c-met (at E14) coincides with the period of naturally occurring cell death in motor neurons, suggesting a possible role of HGF in the regulation of this process. Utilizing a neuron-enriched culture system, we established that HGF, like CNTF, stimulates choline acetyltransferase (CAT) activity in motor neurons. When co-administered to motor neuron cultures, saturating concentrations of HGF and CNTF produced a synergistic increase in CAT levels. We show that this synergy reflects enhanced motor neuron survival. Exposure of motor neuron cultures to the cytostatic agent vincristine markedly decreased CAT levels; co-treatment with HGF and CNTF (but not either factor alone) restored CAT activity to control levels. Our findings indicate that HGF is a survival factor for motor neurons, that it acts synergistically with CNTF, and that HGF and CNTF can together be neuroprotective in the face of vincristine toxicity.

Analysis of cutaneous sensory neurons in transgenic mice lacking the low affinity neurotrophin receptor p75

Mice with a targeted mutation of the p75 low affinity neurotrophin receptor display smaller peripheral nerves and dorsal root ganglia. Here we show that transgenic mice have a significant elevation of thresholds to noxious mechanical and heat stimuli compared with p75+/+ control mice. Immunocytochemical analysis using antibodies against PGP 9.5 (a panaxonal marker) and calcitonin gene related peptide (CGRP, which labels peptidergic neurons) showed a reduction to 73% and 54%, respectively, of the epidermal innervation density. However, analysis of the cell size distribution of toluidine blue-stained dorsal root ganglia showed no selective loss of neurons of particular diameters. Moreover, the neurochemical profile of dorsal root ganglia cells as defined by trkA, CGRP, IB4 and RT97 immunostaining revealed no significant differences in comparison with p75+/+ animals. Staining of the dorsal horn of the spinal cord for CGRP and IB4 was also normal in p75-/- animals. Taking into account a previously reported loss of approximately 50% dorsal root ganglion neurons, we conclude that all types of sensory neurons are equally depleted in p75-/- mice and that the absence of p75 impedes the development of more than one neuronal subtype.

Structural determinants of Trk receptor specificities using BDNF-based neurotrophin chimeras

Neurotrophins play very important roles in the development and maintenance of the vertebrate nervous system. In mammals, there are four members of the family: NGF, BDNF, NT-3 and NT-4/5. Members of the neurotrophin family activate different receptors that belong to a class of receptor tyrosine kinases known as "Trks." For example, NGF is the specific ligand of TrkA, while BDNF activates TrkB. To elucidate which regions of the two neurotrophins determine the receptor specificities, chimeric neurotrophins were constructed using BDNF as the backbone, with various regions being substituted by the corresponding regions of NGF. The activity of the chimeras on the Trk receptors was assayed in transfected fibroblasts ectopically expressing the Trk receptors. Our findings revealed that, although BDNF is absolutely conserved in mammals, substitution of several small variable regions from NGF into the BDNF backbone did not lead to significant loss in TrkB activity or gain in TrkA activity. Moreover, important determinants of TrkB activation might be located in the carboxy-terminal half of BDNF. On the other hand, critical elements for TrkA activation might be located within the amino-terminal half of the mature NGF molecule.

TrkA activation is sufficient to rescue axotomized cholinergic neurons

To test the molecular nature of the NGF receptor responsible for the ability of NGF to rescue septal cholinergic neurons following axotomy, we infused polyclonal antibodies that act as specific agonists of trkA (RTA) into the lateral ventricle of fimbria-fornix lesioned animals. Rats receiving chronic intraventricular infusions of RTA showed significantly more low affinity NGF receptor immunoreactive (p75NGFR-IR) neurons on the lesioned side than did control animals 2 weeks following unilateral fimbria-fornix lesion. RTA also initiated cholinergic sprouting. Infusions of RTA in combination with an antibody that blocks p75NGFR (REX) did not reduce the cell savings effect observed with RTA alone. However, animals infused with RTA plus REX demonstrated significantly less sprouting. These findings suggest that antibody-induced trkA activation is sufficient to mediate NGF-promoted survival of axotomized cholinergic neurons in vivo.

Zone mapping of the binding domain of the rat low affinity nerve growth factor receptor by the introduction of novel N-glycosylation sites

The binding of NGF (nerve growth factor) to the rat low affinity nerve growth factor receptor (p75NGFR) has been studied by site-directed mutagenesis of the receptor. Introduction of non-native N-glycosylation sites within the binding domain indicates that the second of the characteristic cysteine-rich repeats may be particularly important to NGF binding. Two mutants of the second repeat, S42N and S66N, are glycosylated and bind NGF at a drastically reduced level, while still maintaining a conformation recognized by the monoclonal antibody against p75, MC192. Alanine substitution at these sites does not affect NGF binding. Two other mutations that result in local structural changes in the second repeat also greatly decrease binding. One of these altered residues, Ser50, appears to play an essential structural role, since it cannot be replaced by Asn, Ala, or Thr without loss of both NGF binding and MC192 recognition on a Western. Glycosylation of selected sites in the other repeats has little effect on NGF binding or antibody recognition. The introduction of non-native N-glycosylation sites may provide a generally useful scanning technique for the study of protein-protein interactions.

EGF triggers neuronal differentiation of PC12 cells that overexpress the EGF receptor

BACKGROUND

Mitogen-activated protein (MAP) kinase is the central component of a signal transduction pathway that is activated by growth factors interacting with receptors that have protein tyrosine kinase activity. The stimulation of PC12 phaeochromocytoma cells with nerve growth factor leads to the sustained activation and nuclear translocation of the p42 and p44 isoforms of MAP kinase and induces the differentiation of these chromaffin cells to a sympathetic-neuron-like phenotype. In contrast, stimulation with epidermal growth factor induces a transient activation of p42 and p44 MAP kinases without pronounced nuclear translocation and does not trigger cell differentiation. We have examined whether the differential activation of MAP kinases forms the basis of the differential response of the cells to the two factors.

RESULTS

By overexpressing either wild-type or mutant receptors for epidermal growth factor in PC12 cells, we found that p42 and p44 MAP kinase activity remains elevated for longer in cells that overexpress receptors than in untransfected cells. Epidermal growth factor promotes both a striking nuclear translocation of p42 MAP kinase and the differentiation of the overexpressing cells.

CONCLUSIONS

Our results strongly suggest that the distinct effects of nerve growth factor and epidermal growth factor on PC12 cell differentiation can be explained by differences in the extent and duration of activation of p42 and p44 MAP kinases in response to the two factors, without invoking a signal transduction pathway specific to nerve growth factor.