Genetic dissection of neurotrophin signaling through the p75 neurotrophin receptor

Structural determinants underlying signaling specificity in the tumor necrosis factor receptor superfamily (TNFRSF) are poorly characterized, and it is unclear whether different signaling outputs can be genetically dissociated. The p75 neurotrophin receptor (p75(NTR)), also known as TNFRSF16, is a key regulator of trophic and injury responses in the nervous system. Here, we describe a genetic approach for dissecting p75(NTR) signaling and deciphering its underlying logic. Structural determinants important for regulation of cell death, NF-κB, and RhoA pathways were identified in the p75(NTR) death domain (DD). Proapoptotic and prosurvival pathways mapped onto nonoverlapping epitopes, demonstrating that different signaling outputs can be genetically separated in p75(NTR). Dissociation of c-Jun kinase (JNK) and caspase-3 activities indicated that JNK is necessary but not sufficient for p75(NTR)-mediated cell death. RIP2 recruitment and RhoGDI release were mechanistically linked, indicating that competition for DD binding underlies crosstalk between NF-κB and RhoA pathways in p75(NTR) signaling. These results provide insights into the logic of p75(NTR) signaling and pave the way for a genetic dissection of p75(NTR) function and physiology.

Assembly and activation of neurotrophic factor receptor complexes

Neurotrophic factors play important roles in the development and function of both neuronal and glial elements of the central and peripheral nervous systems. Their functional diversity is in part based on their ability to interact with alternative complexes of receptor molecules. This review focuses on our current understanding of the mechanisms that govern the assembly and activation of neurotrophic factor receptor complexes. The realization that many, if not the majority, of these complexes exist in a preassembled form at the plasma membrane has forced the revision of classical ligand-mediated oligomerization models, and led to the discovery of novel mechanisms of receptor activation and generation of signaling diversity which are likely to be shared by many different classes of receptors.

Ligand-independent signaling by disulfide-crosslinked dimers of the p75 neurotrophin receptor

Dimerization is recognized as a crucial step in the activation of many plasma membrane receptors. However, a growing number of receptors pre-exist as dimers in the absence of ligand, indicating that, although necessary, dimerization is not always sufficient for signaling. The p75 neurotrophin receptor (p75(NTR)) forms disulfide-linked dimers at the cell surface independently of ligand binding through Cys257 in its transmembrane domain. Here, we show that crosslinking of p75(NTR) dimers by cysteine-scanning mutagenesis results in constitutive, ligand-independent activity in several pathways that are normally engaged upon neurotrophin stimulation of native receptors. The activity profiles of different disulfide-crosslinked p75(NTR) mutants were similar but not identical, suggesting that different configurations of p75(NTR) dimers might be endowed with different functions. Interestingly, crosslinked p75(NTR) mutants did not mimic the effects of the myelin inhibitors Nogo or MAG, suggesting the existence of ligand-specific activation mechanisms. Together, these results support a conformational model of p75(NTR) activation by neurotrophins, and reveal a genetic approach to generate gain-of-function receptor variants with distinct functional profiles.

Activation of the p75 neurotrophin receptor through conformational rearrangement of disulphide-linked receptor dimers

Ligand-mediated dimerization has emerged as a universal mechanism of growth factor receptor activation. Neurotrophins interact with dimers of the p75 neurotrophin receptor (p75(NTR)), but the mechanism of receptor activation has remained elusive. Here, we show that p75(NTR) forms disulphide-linked dimers independently of neurotrophin binding through the highly conserved Cys(257) in its transmembrane domain. Mutation of Cys(257) abolished neurotrophin-dependent receptor activity but did not affect downstream signaling by the p75(NTR)/NgR/Lingo-1 complex in response to MAG, indicating the existence of distinct, ligand-specific activation mechanisms for p75(NTR). FRET experiments revealed a close association of p75(NTR) intracellular domains that was transiently disrupted by conformational changes induced upon NGF binding. Although mutation of Cys(257) did not alter the oligomeric state of p75(NTR), the mutant receptor was no longer able to propagate conformational changes to the cytoplasmic domain upon ligand binding. We propose that neurotrophins activate p75(NTR) by a mechanism involving rearrangement of disulphide-linked receptor subunits.

Cell-specific and concentration-dependent actions of interleukin-1 in acute brain inflammation

Interleukin (IL)-1 is a pivotal pro-inflammatory cytokine and an important mediator of both acute and chronic central nervous system (CNS) injuries. Despite intense research in CNS IL-1 biology over the past two decades, its precise mechanism of action in inflammatory responses to acute brain disorders remains largely unknown. In particular, much effort has been focussed on using in vitro approaches to better understand the cellular and signalling mechanisms of actions of IL-1, yet some discrepancies in the literature regarding the effects produced by IL-1beta in in vitro paradigms of injury still exist, particularly as to whether IL-1 exerts neurotoxic or neuroprotective effects. Here we aim to review the cell-specific and concentration-dependent actions of IL-1 in brain cells, to depict the mechanism by which this cytokine induces neurotoxicity or neuroprotection in acute brain injury.

Mechanisms of regulation for interleukin-1beta in neurodegenerative disease

The interleukin-1 family of cytokines are central to the pathology of acute and chronic diseases of the central nervous system. We describe current evidence on the transcriptional and post-transcriptional regulation of interleukin-1beta production, secretion and activity in the brain. Regarding the induction of protein synthesis, the possible involvement of Toll like receptor-4 is discussed including evidence that ischemic brain damage is reduced in Toll like receptor-4 knockout mice. The post-translational involvement of the P2X7-receptor and caspase-1 in the processing and release of active IL-1beta is also considered, as is evidence suggesting a possible extracellular cleavage of pro-IL-1beta by neutrophil derived proteases. We provide some fresh perspectives on how interleukin-1beta may be regulated and how these mechanisms could be targeted in disease.

Glutamate activatesc-fos in glial cells via a novel mechanism involving the glutamate receptor subtype mGlu5 and the transcriptional repressor DREAM

Activation of c-fos in brain is related to coupling of neuronal activity to gene expression, but also to pathological conditions such as seizures or excitotoxicity-induced cell death. Glutamate activates c-fos in neurons through the calcium-dependent phosphorylation of CREB by ERK and/or CaMKIV kinase pathways downstream NMDA-receptors. In glial cells, however, the activation of c-fos by glutamate is poorly understood. Because glial cells actively modulate neuronal excitability and the brain's response to injury, we studied the mechanisms by which glutamate activates c-fos in rat cortical glial cells. Glutamate potently induced c-fos mRNA in a calcium-dependent manner, as demonstrated by using the calcium chelator BAPTA-AM. Glutamate-induced c-fos mRNA expression was not sensitive to inhibitors of ERK, p38(MAPK), or CaMK pathways, indicating that glial c-fos is activated by a distinct mechanism. Thapsigargin abolished the glutamate effect on c-fos mRNA, indicating ER calcium mobilization. Additionally, glutamate induction of c-fos mRNA was sensitive to the mGluR5 antagonist MPEP but not the NMDA-R antagonist MK-801. In luciferase reporter assays, DRE, which actively represses c-fos by binding the calcium-binding transcriptional repressor DREAM, was activated by glutamate, whereas SRE and CRE were not. Finally, glutamate caused the nuclear export of DREAM in astrocytes, and transfection of astrocytes with a mutant variant of DREAM that constitutively binds DNA inhibited glutamate-induced c-Fos expression. These findings are in sharp contrast to the mechanism described in neurons and suggest a novel pathway activated by glutamate in glial cells that employs mGluR5, ER calcium, and the derepression of c-fos at the DRE.

Activation of c-fos by lipopolysaccharide in glial cells via p38 mitogen-activated protein kinase-dependent activation of serum or cyclic AMP/calcium response element

Pathological conditions such as ischaemic stroke and inflammatory disorders cause c-fos activation in the brain. This activation contributes to the initiation of the brain's inflammatory response, orchestrated by activated glial cells. The inflammatory signalling cascades leading to c-fos activation in glial cells are not well characterized. Thus, we have attempted a detailed analysis of the cis-acting elements, transcription factors and upstream kinase pathways involved in the activation of c-fos by lipopolysaccharide (LPS) in primary rat cortical glial cells. We found that (1) LPS-induced c-fos mRNA levels were sensitive to p38 mitogen-activated protein kinase (MAPK) inhibitors but not to mitogen-activated/extracellular signal-regulated kinase (ERK) or calcium-calmodulin-dependent kinase inhibitors, (2) LPS activated both serum response element (SRE) and cyclic AMP/calcium response element (CRE)-driven luciferase reporters in transient transfection assays, (3) LPS induced the phosphorylation of Elk1 CRE-binding protein (CREB)/activated transcription factor-1 (ATF-1) and the activation of GAL4-Elk1 and GAL4-CREB chimeric proteins, and (4) mutation of both SRE and CRE elements was necessary and sufficient to completely abolish LPS induction of a rat c-fos proximal promoter-luciferase reporter. Thus, c-fos activation by LPS in glial cells occurs via the SRE or CRE in an independent manner, and involves the Elk1 or CREB/ATF-1 transcription factors. Elk1-mediated transactivation was dependent on p38 MAPK, suggesting a crucial role of these factors in mediating inflammatory responses in the CNS.

The neuroprotective agents chlomethiazole and SB203580 inhibit IL-1beta signalling but not its biosynthesis in rat cortical glial cells

Chlomethiazole and pyridinyl imidazole compounds, exemplified by SB203580, are structurally distinct p38 mitogen-activated protein kinase inhibitors with neuroprotective properties in models of cerebral ischaemia. We have examined their effects in interleukin-1beta (IL-1beta) synthesis, release and signalling in rat cortical glial cells, given the important role of IL-1beta in cerebral ischaemia. We analysed (i) IL-1beta mRNA expression by northern blot, (ii) IL-1beta protein precursor levels within the cells by western blot, and (iii) the levels of the mature IL-1beta protein secreted into the medium by enzyme-linked immunosorbent assay (ELISA) after treatment of rat cortical glial cells with lipopolysaccharide. While the induction of IL-1beta expression by lipopolysaccharide or by IL-1beta itself was very sensitive to nuclear factor kappa B (NF-kappaB) inhibitors, chlomethiazole or SB203580 were nearly without effect, indicating a differential regulation as compared to peripheral cells, e.g. monocytes. In contrast, chlomethiazole and SB203580 potently inhibited the IL-1beta-induced expression of c-fos and inducible nitric oxide synthase, as monitored by northern blot and quantitative RT-PCR, respectively. Because IL-1beta-induced expression of c-fos and inducible nitric oxide synthase is believed to directly contribute to the pathology of cerebral ischaemic injury, the results suggest a direct mechanism for the neuroprotective effects of chlomethiazole and SB203580, and further establish the anti-inflammatory properties of chlomethiazole.

Neuroprotective agent chlomethiazole attenuates c-fos, c-jun, and AP-1 activation through inhibition of p38 MAP kinase

Recent evidence suggests that stress-activated protein kinases expressed in glial cells have very important roles during cerebral ischemia. The neuroprotective agent chlomethiazole, which is known to enhance the conductance at the GABA(A) receptor complex, is presently in clinical trials for the treatment of severe stroke. Here the authors suggested that chlormethiazole has anti-inflammatory properties because it potently and selectively inhibited p38 mitogen-activated protein (MAP) kinase in primary cortical glial cultures. The inhibition of p38 MAP kinase resulted in the attenuation of the induction of c-fos and c-jun mRNA and AP-1 DNA binding by lipopolysaccharide (LPS). In addition, chlomethiazole inhibited the activation of an AP-1-dependent luciferase reporter plasmid in SK-N-MC human neuroblastoma cells in response to glutamate. Chlomethiazole inhibited the p38 MAP kinase activity as revealed by the decrease in the LPS-induced phosphorylation of the substrates ATF-2 and hsp27, whereas the phosphorylation status of the p38 MAP kinase itself was unaffected. Interestingly, chlomethiazole exhibited an IC(50) of approximately 2 micromol/L for inhibition of c-fos mRNA expression, indicating 25 to 75 times higher potency than reported EC(50) values for enhancing GABA(A) chloride currents. The results indicated a novel mechanism of action of chlomethiazole, and provided support for a distinctive role of p38 MAP kinase in cerebral ischemia.

Contribution of MAP kinase pathways to the activation of ATF-2 in human neuroblastoma cells

Activated Transcription Factor-2 (ATF-2) is important during development of and during injury to the brain. Both Jun N-terminal Kinases (JNKs) and p38 Mitogen-Activated Protein Kinases (p38MAPKs) may phosphorylate ATF-2, but the contribution of these two pathways in cells has never been investigated. We have assayed endogenous p38MAPK activity in SK-N-MC and SH-SY5Y human neuroblastoma cells for activation of a GAL4/ATF-2 fusionprotein, by means of titrations of transfected expression plasmids and by using the p38MAPK inhibitor SB203580. It was found that basal activation of ATF-2 was independent of p38MAPK and that whereas MAPK kinase-3 (MKK3) was a weak inducer of ATF-2 activation, it was a potent activator of the stress activated transcription factor CHOP. In contrast, ATF-2 was very potently activated by the JNK pathway activator MAPK kinase kinase-1 (MEKK1). Thus, kinases downstream of MEKK1 appear relevant, but it is unlikely that p38MAPKs contribute quantitatively to activation of ATF-2 in these cells.

Post-translational inhibition of cytochrome P-450 2E1 expression by chlomethiazole in Fao hepatoma cells

Chlomethiazole (CMZ) is a sedative and anticonvulsant drug that has been shown to be an efficient transcriptional inhibitor of expression of rat hepatic ethanol-inducible cytochrome P-450 2E1 (CYP2E1). Recent results have shown that human CYP2E1 expression in vivo is almost completely inhibited in control subjects and in alcoholic patients treated with CMZ. In the present investigation, we evaluated the mode of action of CMZ on CYP2E1 expression in Fao rat hepatoma cells. Transcriptional activity of the CYP2E1 gene was monitored using reverse transcription-polymerase chain reaction-based quantification of CYP2E1 heterologous nuclear RNA (hnRNA) against a mimic DNA standard, mRNA was detected by Northern blotting, enzyme protein was detected by Western blotting, and CYP2E1-dependent catalytic activity was detected by assay of chlorzoxazone-6-hydroxylation. Six hours after CMZ treatment, the levels of both CYP2E1 protein and catalytic activity were concomitantly reduced at an IC50 value of about 5 microM. Ethanol treatment of the cells caused a 2-fold induction of CYP2E1 protein levels, which was inhibited by CMZ. Change of medium unexpectedly caused an increase in CYP2E1 gene transcription 4 h later, as monitored by quantitative determination of CYP2E1 hnRNA. However, CMZ failed to influence the expression of CYP2E1 hnRNA or mRNA both constitutively and after medium change, indicating no effect on gene transcription or mRNA synthesis/stability. Cycloheximide treatment of the cells did not abolish the inhibitory action of CMZ, further indicating an action at the post-translational level; in addition, CMZ inhibited CYP2E1 expression in V79 cells with stably expressed CYP2E1 under the control of the SV40 promoter. The data indicate that the CYP2E1 gene is transcriptionally activated in response to medium change and that CMZ, apart from a transcriptional inhibitor of CYP2E1 expression, acts in addition as an efficient high-affinity post-translational inhibitor of CYP2E1, probably due to an allosteric destabilization of the enzyme. This indicates a very rapid and effective CMZ-mediated inhibition of CYP2E1 in vivo.