Destabilization of cortical dendrites and spines by BDNF.
Particle-mediated gene transfer and two-photon microscopy were used to monitor the behavior of dendrites of individual cortical pyramidal neurons coexpressing green fluorescent protein (GFP) and brain-derived neurotrophic factor (BDNF). While the dendrites and spines of neurons expressing GFP alone grew modestly over 24-48 hr, coexpressing BDNF elicited dramatic sprouting of basal dendrites, accompanied by a regression of dendritic spines. Compared to GFP-transfected controls, the newly formed dendrites and spines were highly unstable. Experiments utilizing Trk receptor bodies, K252a, and overexpression of nerve growth factor (NGF) demonstrated that these effects were mediated by secreted BDNF interacting with extracellular TrkB receptors. Thus, BDNF induces structural instability in dendrites and spines, which, when restricted to particular portions of a dendritic arbor, may help translate activity patterns into specific morphological changes. (+info)
Activation of TrkA by nerve growth factor upregulates expression of the cholinergic gene locus but attenuates the response to ciliary neurotrophic growth factor.
Nerve growth factor (NGF) stimulates the expression of the cholinergic gene locus, which encodes choline acetyltransferase (ChAT) and vesicular acetylcholine transporter (VAChT), the proteins necessary for the synthesis and storage of the neurotransmitter acetylcholine (ACh). To determine whether this action of NGF is mediated by the p140TrkA NGF receptor (a member of the Trk tyrosine kinase family) we used a murine basal forebrain cholinergic cell line, SN56, stably transfected with rat trkA cDNA. Treatment of these transfectants with NGF activated mitogen-activated protein kinase and increased cytosolic free calcium concentrations, confirming the reconstitution of TrkA-mediated signalling pathways. The expression of ChAT and VAChT mRNA, as well as ACh content, were coordinately up-regulated by NGF in SN56-trkA transfectants. None of these responses occurred in the parental SN56 cells, which do not express endogenous TrkA, indicating that these actions of NGF required TrkA. We previously reported that ciliary neurotrophic factor (CNTF) upregulates the expression of ChAT and VAChT, as well as ACh production, in SN56 cells. The combined treatment of SN56-trkA cells with CNTF and NGF revealed a complex interaction of these factors in the regulation of cholinergic gene locus expression. At low concentrations of CNTF (<1 ng/ml), the upregulation of ACh synthesis evoked by these factors was additive. However, at higher concentrations of CNTF (>1 ng/ml), NGF attenuated the stimulatory effect of CNTF on ChAT and VAChT mRNA and ACh content. This attenuation was not due to interference with early steps of CNTF receptor signalling, as pre-treatment of SN56-trkA cells with NGF did not affect the nuclear translocation of the transcription factor, Stat3, evoked by CNTF. (+info)
The synergistic action of ethanol and nerve growth factor in the induction of neuronal nitric oxide synthase.
Ethanol alone had no effect on neuronal nitric oxide synthase (nNOS) expression in PC12 cells. However, in the presence of nerve growth factor (NGF), nNOS expression was amplified (threefold, P < 0.05), compared to NGF alone. This increase was eliminated with pretreatment of PC12 cells with staurosporine, suggesting that the effects of ethanol on nNOS expression are mediated by a protein kinase C-dependent pathway. (+info)
Overexpression of atypical PKC in PC12 cells enhances NGF-responsiveness and survival through an NF-kappaB dependent pathway.
Removal of atypical PKC blocks NGF-induced differentiation of PC12 cells.1 We now examine the consequences that overexpression of atypical PKCs had upon NGF responses. PC12 cells were stably transfected with either PKC-iota or PKC-zeta. Overexpression of atypical PKCs markedly enhanced NGF- induced neurite outgrowth as well as enhanced NGF-stimulated JNK kinase. Cotransfection of HA-JNK1 along with increasing concentrations of PKC-iota, resulted in dose-dependent phosphorylation of GST c-Jun (1 - 79). NGF treatment of PC12 cells resulted in activation of NF-kappaB. In comparison, overexpression of atypical PKC-iota was by itself sufficient to activate NF-kappaB and shift the kinetics of NGF-induced kappaB activity. Furthermore, transfection of full-length antisense PKC-iota blocked basal and NGF-stimulated NF-kappaB. Differentiated and undifferentiated PC12 cells overexpressing atypical PKC-iota were protected from serum deprivation-induced cell death. Collectively, these findings demonstrate that atypical PKC-iota lies in a pathway that regulates NF-kappaB and contributes to both neurotrophin-mediated differentiation and survival signaling. (+info)
The ras suppressor, RSU-1, enhances nerve growth factor-induced differentiation of PC12 cells and induces p21CIP expression.
The Rsu-1 Ras suppressor gene was isolated based on its ability to inhibit v-Ras transformation. Using Rsu-1 transfectants of the pheochromocytoma cell line PC12, we demonstrated previously that Rsu-1 expression inhibited Jun kinase activation but enhanced Erk2 activation in response to epidermal growth factor. In the present study, the Rsu-1 PC12 transfectants were used to investigate the role of Rsu-1 in nerve growth factor (NGF)- and v-Ki-ras-mediated neuronal differentiation. NGF-induced neurite extension was enhanced, not inhibited, by the expression of Rsu-1 in PC12 cells. The activation of Erk kinase activity in response to NGF was sustained longer in the Rsu-1 transfectants compared with the vector control cells. During NGF-mediated differentiation, an increase in the expression of specific mRNAs for the early response genes Fos, cJun, and NGF1a was detected in both the vector control and Rsu-1 transfectants. The expression of the differentiation-specific genes VGF8 and SCG10 was similar in Rsu-1 transfectants compared with the vector control cells. The induction of Rsu-1 expression in these cell lines did not inhibit v-Ki-ras-induced differentiation, as measured by neurite extension. These data suggest that although Rsu-1 blocked some Ras-dependent response(s), these responses were not required for differentiation. Moreover, the induction of Rsu-1 expression in the PC12 clones resulted in growth inhibition and p21(WAF/CIP) expression. Hence, Rsu-1 expression enhances NGF-induced differentiation while inhibiting the growth of cells. (+info)
Developmental changes in the response of trigeminal neurons to neurotrophins: influence of birthdate and the ganglion environment.
Previous studies have shown that most neurons in cultures established during the early stages of neurogenesis in the embryonic mouse trigeminal ganglion are supported by BDNF whereas most neurons cultured from older ganglia survive with NGF. To ascertain to what extent these developmental changes in neurotrophin responsiveness result from separate phases of generation of BDNF- and NGF-responsive neurons or from a developmental switch in the response of neurons from BDNF to NGF, we administered BrdU to pregnant mice at different stages of gestation to identify neurons born at different times and studied the survival of labelled neurons in dissociated cultures established shortly after BrdU administration. Most early-generated neurons responded to BDNF, neurons generated at intermediate times responded to both factors and late-generated neurons responded to NGF, indicating that there are overlapping phases in the generation of BDNF- and NGF-responsive neurons and that late-generated neurons do not switch responsiveness from BDNF to NGF. To ascertain if early-generated neurons do switch their response to neurotrophins during development, we used repeated BrdU injection to label all neurons generated after an early stage in neurogenesis and studied the neurotrophin responsiveness of the unlabelled neurons in cultures established after neurogenesis had ceased. The response of these early-generated neurons had decreased to BDNF and increased to NGF, indicating that at least a proportion of early-generated neurons switch responsiveness to neurotrophins in vivo. Because early-generated neurons do not switch responsiveness from BDNF to NGF in long-term dissociated cultures, we cultured early trigeminal ganglion explants with and without their targets for 24 hours before establishing dissociated cultures. This period of explant culture was sufficient to enable many early-generated neurons to switch their response from BDNF to NGF and this switch occurred irrespective of presence of target tissue. Our findings conclusively demonstrate for the first time that individual neurons switch their neurotrophin requirements during development and that this switch depends on cell interactions within the ganglion. In addition, we show that there are overlapping phases in the generation of BDNF- and NGF-responsive neurons in the trigeminal ganglion. (+info)
Differential ability of astroglia and neuronal cells to accumulate lead: dependence on cell type and on degree of differentiation.
The apparent ability of astroglia to serve as a lead (Pb) sink in the mature brain may result from either their strategic location, between the blood-brain barrier and neurons, or from intrinsic differences between the ability of astroglia and neurons to accumulate this metal. This phenomenon may be dependent on the degree of cell differentiation. In order to address the latter possibility, Pb accumulation was compared among the following cell culture models: (1) mature and immature rat astroglia, (2) undifferentiated SY5Y human neuroblastoma cells and SY5Y cells differentiated with nerve growth factor, (3) immature rat astroglia grown in differently conditioned media, some of which induce partial differentiation, and (4) rat astroglia and SY5Y cells in co-culture. Astroglial cultures, prepared from 1-day-old rat cerebral hemispheres, were exposed to 1 microM Pb after either 14 (immature) or 21 (mature) days in culture. Pb content of the cells was measured by atomic absorption spectroscopy. Immature astroglia took up less Pb when glutathione (GSH) was added to the medium, suggesting that GSH may regulate Pb uptake in these cells. Undifferentiated neuroblastoma cells accumulated more Pb than did the differentiated ones. Astroglia accumulated up to 24 times more Pb than did neuronal cells. This ability was enhanced by exposure to conditioned medium from a neuroblastoma cell line, but not by endothelial cell-conditioned medium, although this medium induced the expression of a glutamate-activated Ca2+ response. Our findings are in agreement with in vivo studies, and thus validate the use of these cell-culture models for future studies on differential mechanisms of Pb uptake. (+info)
Axonal transport of activating transcription factor-2 is modulated by nerve growth factor in nociceptive neurons.
The aim of this study was to determine whether axonal transport of activating transcription factor-2 (ATF2) occurs in adult sensory neurons, and whether this process is under neurotrophin control. Antisera to both total ATF2 and to the activated (i.e., phosphorylated) form were used for immunocytochemistry and Western blotting. ATF2 was localized to predominantly nociceptive dorsal root ganglion cells in adult rats and shown to accumulate proximal and distal to a sciatic nerve ligature as a result of axonal transport. Subcutaneous injection of nerve growth factor (NGF) decreased the levels of fast retrograde axonal transport of activated ATF2 by 97% (p < 0.05) and elevated levels of retrograde axonal transport of total ATF2 by twofold (p < 0.02). In contrast, blocking endogenous NGF using an anti-NGF antibody induced an elevation in retrograde axonal transport of activated ATF2 of 4. 5-fold (p < 0.05) and decreased retrograde axonal transport of total ATF2 by 72% (p < 0.05). NGF or anti-NGF treatment had no effect on the anterograde transport levels of total or activated ATF2. This study shows that signaling by target-derived NGF to the cell bodies of sensory neurons consists, in part, of the modulation of levels and activation status of a retrogradely transported transcription factor, ATF2. (+info)