A single c-Jun N-terminal kinase isoform (JNK3-p54) is an effector in both neuronal differentiation and cell death

c-Jun N-terminal kinases (JNKs) mediate pro-inflammatory actions of microglia

The activation and function of c-Jun N-terminal kinases (JNKs) were investigated in primary microglia cultures from neonatal rat brain, which express all three JNK isoforms. Lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-alpha), and thrombin preparations induced a rapid and lasting activation of JNKs in the cytoplasm. In the nucleus, the activation patterns were rather complex. In untreated microglia, the small pool of nuclear JNKs was strongly activated, while the high-affinity JNK substrate c-Jun was only weakly phosphorylated. Stimulation with LPS increased the total amount of nuclear JNKs and the phosphorylation of the transcription factor c-Jun. Levels of activated JNKs in the nucleus, however, rapidly decreased. Analysis of the nuclear JNK isoforms revealed that the amount of JNK1 declined, while JNK2 increased, and the weakly expressed JNK3 did not vary. This observation suggests that JNK2 is mainly responsible for the activation of c-Jun in this context. Upstream of JNKs, LPS induced a lasting activation of the constitutively present JNK kinase MKK4. The function of JNKs in LPS-triggered cellular reactions was investigated using SP600125 (0.5-5 microM), a direct inhibitor of JNKs. Inhibition of JNKs reduced the LPS-induced metabolic activity and induction of the AP-1 target genes cyclooxygenase-2 (Cox-2), TNF-alpha, monocyte chemoattractant protein-1 (MCP-1), and interleukin-6 (IL-6) in response to LPS, while ERK1/2 and p38 alpha had a more pronounced effect on LPS-induced cellular enlargement than JNKs. In summary, JNKs are essential mediators of relevant pro-inflammatory functions in microglia with different contributions of the JNK isoforms.

HSV-induced apoptosis in herpes encephalitis

HSV triggers and blocks apoptosis in cell type-specific fashion. This review discusses present understanding of the role of apoptosis and signaling cascades in neuronal pathogenesis and survival and summarizes present findings relating to the modulation of these strictly balanced processes by HSV infection. Underscored are the findings that HSV-1, but not HSV-2, triggers apoptosis in CNS neurons and causes encephalitis in adult subjects. Mechanisms responsible for the different outcomes of infection with the two HSV serotypes are described, including the contribution of viral antiapoptotic genes, notably the HSV-2 gene ICP10PK. Implications for the potential use of HSV vectors in future therapeutic developments are discussed.

Specific pathophysiological functions of JNK isoforms in the brain

We have investigated the effect of JNK1 ko, JNK2 ko, JNK3 ko, JNK2+3 ko and c-JunAA mutation on neuronal survival in adult transgenic mice following ischemia, 6-hydroxydopamine induced neurotoxicity, axon transection and kainic acid induced excitotoxicity. Deletion of JNK isoforms indicated the compartment-specific expression of JNK isoforms with 46-kDa JNK1 as the main phosphorylated JNK isoform. Permanent occlusion of the MCA significantly enlarged the infarct area in JNK1 ko, which showed an increased expression of JNK3 in the penumbra. Survival of dopaminergic neurons in the substantia nigra compacta (SNC) following intrastriatal injection of 6-hydroxydopamine was transiently improved in JNK3 ko and c-JunAA mice after 7 days, but not 60 days. Following transection of the medial forebrain bundle, however, JNK3 ko conferred persisting neuroprotection of axotomised SNC neurons. None of the JNK ko and c-JunAA mutation affected the survival of facial motoneurons following peripheral axotomy when investigated after 90 days. Finally, we determined the impact of JNK ko on the survival of animals and the degeneration of hippocampal neurons following kainic acid. JNK3 ko mice were substantially resistant against and survived kainic acid-induced seizures. JNK3 ko and JNK1 ko showed a nonsignificant tendency for decreased or increased death of hippocampal neurons, respectively. Surprisingly, the deletion of a single JNK isoform did not attenuate the immunocytochemical signal of phosphorylated c-Jun irrespective on the experimental set-up. This comprehensive study provides novel insights into the context-dependent physiological and pathological functions of JNK isoforms.

JNK2 Translocates to the Mitochondria and Mediates Cytochrome c Release in PC12 Cells in Response to 6-Hydroxydopamine

6-Hydroxydopamine (6-OHDA) causes death of dopaminergic neurons by mitochondrial dysfunction with JNKs as central mediators. Here we provide novel insights into specific actions of JNK isoforms in 6-OHDA-induced death of PC12 cells. Twenty five mum 6-OHDA enhanced total JNK activity in the cytoplasm, nucleus, and at the mitochondria. Inhibition of JNKs by 2 mum SP600125 or transfection with dominant-negative JNK2 (dnJNK2) rescued more than 60% of the otherwise dying PC12 cells after 24 h, whereas transfection with dnJNK1 had no protective effects. In contrast to constitutively present JNK1, JNK2 amounts increased in the nucleus and at the mitochondria after 6-OHDA stimulation. JNK inhibition by SP600125 or transfection of dnJNK2 reduced the pool of active JNKs in the nucleus, the release of cytochrome c, as well as the cleavage of caspase-3 and its substrate poly(ADP-ribose) polymerase-1. Transfection with dnJNK1, however, had no effects on the translocation of JNKs to the mitochondria or the release of cytochrome c. Our data provide novel functional insights into the pathological role of individual JNK isoforms, the signalosome at the mitochondria, and the mode of JNK-induced release of cytochrome c.

c‐Jun N‐terminal kinases (JNKs) and the cytoskeleton—functions beyond neurodegeneration

The c-Jun N-terminal kinases (JNKs) are important mediators of neurodegeneration and their actions include the activation of genetic programs by phosphorylation of the nuclear transcription factor c-Jun/AP-1, the release of cytochrome c or the pro-inflammatory actions of microglia. Recent data, however, provide evidence for physiological functions of JNKs in particular JNK1, and this involves a role of JNKs in the development of the brain and the (functional and/or structural) integrity of the cytoskeleton. Here we summarize our findings on the cytoskeleton-associated actions of JNKs. Thus, JNKs the relevant MAP kinases for the NGF-induced formation and elongation of PC12 cells, and this process is also supported by JNK2 and JNK3 which are commonly considered as pro-apoptotic signal transducers. Importantly, JNK3 is also mandatory for the intact differentiation of neurons since the functional deletion of JNK3 caused apoptotic features such as activation of caspase 3 in untreated P0 primary hippocampal neurons and following glutamate excitotoxicity. Finally, we can visualize the presence of JNKs at the cytoskeleton, axon and growth cones of primary hippocampal neurons and PC12 cells, and this pattern changes following excitatory stimulation with glutamate. Thus, the functional role of JNKs during development and differentiation substantially differs from their degenerative actions in the adult brain.

Neurodegenerative and physiological actions of c-Jun N-terminal kinases in the mammalian brain

The research in the field of AP-1 transcription factor expression, such as Jun or Fos proteins, in the brain was a milestone in neurosciences. The last years have provided growing insights into the upstream signal transduction which controls the expression and activation of these transcriptional regulators. In particular, the c-Jun N-terminal kinases (JNKs) were considered to confer degeneration by activation of c-Jun. Recent findings, however, demonstrate an essential physiological role of JNKs in the nervous system. Here we review the specific control and dual functions of JNK isoforms which are relevant for the development of the intact brain on the one hand, and which can confer dramatic neurodegenerative effects and microglial activation on the other hand.

Bcl-2 enhances neurite extension via activation of c-Jun N-terminal kinase

Recent studies suggest that Bcl-2 may play an active role in neuronal differentiation. Here, we showed a marked neurite extension in MN9D dopaminergic neuronal cells overexpressing Bcl-2 (MN9D/Bcl-2) or Bcl-X(L) (MN9D/Bcl-X(L)). We found a specific increase in phosphorylation of c-Jun N-terminal kinase (JNK) accompanied by neurite extension in MN9D/Bcl-2 but not in MN9D/Bcl-X(L) cells. Consequently, neurite extension in MN9D/Bcl-2 but not in MN9D/Bcl-X(L) cells was suppressed by treatment with SP600125, a specific inhibitor of JNK. Inhibition of other mitogen-activated protein kinases-including p38 and extracellular signal-regulated kinase-did not affect Bcl-2-mediated neurite extension in MN9D cells. While the expression levels of such protein markers of maturation as SNAP-25, phosphorylated NF-H, and neuron-specific enolase were increased in MN9D/Bcl-2 cells, only upregulation of SNAP-25 was inhibited after treatment with SP600125. Thus, the JNK signal activated by Bcl-2 seems to play an important role during morphological and certain biochemical differentiation in cultured dopaminergic neurons.

DCX, a new mediator of the JNK pathway

Mutations in the X-linked gene DCX result in lissencephaly in males, and abnormal neuronal positioning in females, suggesting a role for this gene product during neuronal migration. In spite of several known protein interactions, the involvement of DCX in a signaling pathway is still elusive. Here we demonstrate that DCX is a substrate of JNK and interacts with both c-Jun N-terminal kinase (JNK) and JNK interacting protein (JIP). The localization of this signaling module in the developing brain suggests its functionality in migrating neurons. The localization of DCX at neurite tips is determined by its interaction with JIP and by the interaction of the latter with kinesin. DCX is phosphorylated by JNK in growth cones. DCX mutated in sites phosphorylated by JNK affected neurite outgrowth, and the velocity and relative pause time of migrating neurons. We hypothesize that during neuronal migration, there is a need to regulate molecular motors that are working in the cell in opposite directions: kinesin (a plus-end directed molecular motor) versus dynein (a minus-end directed molecular motor).

The concerted signaling of ERK1/2 and JNKs is essential for PC12 cell neuritogenesis and converges at the level of target proteins

Mitogen-activated protein kinase (MAPK) pathways are central signaling elements, which translate and integrate stimuli from cell surface receptors into cytoplasmic and transcriptional responses. Here, we systematically compare the role of MAPKs in the nerve growth factor-induced long-term differentiation of PC12 cells and show the persistent nuclear and dose-dependent cytoplasmic activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and the increasing nuclear and cytoplasmic activation of c-Jun N-terminal kinases (JNKs). Inhibition of ERK1/2 and JNKs significantly reduced neurite outgrowth. Both synergistically controlled the expression of c-Jun, the induction and/or phosphorylation of neurofilament, and the phosphorylation of Elk-1. JNKs alone were responsible for the phosphorylation of c-Jun and activating transcription factor 2 as well as for the expression of MAPK phosphatase 1. In contrast, p38alpha was only transiently activated and marginally involved in these processes. Thus, JNKs and ERK1/2 accomplish differentiation by signaling in parallel cascades that converge only at the target level.

Tumor Necrosis Factor–Mediated E2F1 Suppression in Endothelial Cells

After balloon angioplasty, locally expressed tumor necrosis factor (TNF)-α disrupts endothelial cell (EC) proliferation and reendothelialization of the injured vessel. We have previously reported that TNF inhibits the EC cycle and downregulates the transcription factor E2F1. Ectopic expression of E2F1 at the site of injury improves reendothelialization of the injured vessel. In this study, we report that c-Jun N-terminal kinase (JNK) 1 and p38 mitogen-activated protein kinases (MAPKs) are differentially required for E2F1 expression and activity in ECs. Overexpression of constitutively active JNK1 mimicked TNF-mediated inhibitory events, whereas dominant-negative JNK1 prevented these effects. E2F cis elements in the promoter of E2F1 gene mediate suppressive actions of TNF, because removal of these sites rendered E2F1 promoter activity insensitive to TNF. JNK1 physically interacted with E2F1 and inactivated it via direct phosphorylation. Additionally, TNF inhibited Rb phosphorylation and dissociation from E2F1. Overexpression of constitutively active p38 MAPK facilitated Rb-E2F1 dissociation, whereas that of dominant-negative p38 MAPK did not. Taken together, these data suggest a differential requirement of JNK1 and p38 MAPK in TNF regulation of E2F1. Targeted inactivation of JNK1 at arterial injury sites may represent a potential therapeutic intervention for ameliorating TNF-mediated EC dysfunction.

Sphingosine-1-phosphate-induced ERK activation protects human melanocytes from UVB-induced apoptosis

Ultraviolet B (UVB) is known to induce apoptosis in human melanocytes. Here we show the cytoprotective effect of sphingosine-1-phosphate (S1P) against UVB-induced apoptosis. We also show that UVB-induced apoptosis of melanocytes is mediated by caspase-3 activation and poly(ADP-ribose) polymerase (PARP) cleavage, and that S1P prevents apoptosis by inhibiting this apoptotic pathway. We further investigated three major mitogen-activated protein (MAP) kinases after UVB irradiation. UVB gradually activated c-Jun N-terminal kinase (JNK) and p38 MAP kinase, while extracellular signal-regulated protein kinase (ERK) was inactivated transiently. Blocking of the p38 MAP kinase pathway using SB203580 promoted cell survival and inhibited the activation of caspase-3 and PARP cleavage. These results suggest that p38 MAP kinase activation may play an important role in the UVB-induced apoptosis of human melanocytes. To explain this cytoprotective effect, we next examined whether S1P could inhibit UVB-induced JNK and p38 MAP kinase activation. However, S1P was not found to have any influence on UVB-induced JNK or p38 MAP kinase activation. In contrast, S1P clearly stimulated the phosphorylation of ERK, and the specific inhibition of the ERK pathway using PD98059 abolished the cytoprotective effect of S1P. Based on these results, we conclude that the activation of p38 MAP kinase plays an important role in UVB-induced apoptosis, and that S1P may show its cytoprotective effect through ERK activation in human melanocytes.

JNK1 physically interacts with WW domain-containing oxidoreductase (WOX1) and inhibits WOX1-mediated apoptosis

Transient activation of c-Jun N-terminal kinase (JNK) promotes cell survival, whereas persistent JNK activation induces apoptosis. Bovine testicular hyaluronidase PH-20 activates JNK1 and protects L929 fibroblasts from staurosporine-mediated cell death. PH-20 also induces the expression of a p53-interacting WW domain-containing oxidoreductase (WOX1, also known as WWOX or FOR) in these cells. WOX1 enhances the cytotoxic function of tumor necrosis factor and mediates apoptosis synergistically with p53. Thus, the activated JNK1 is likely to counteract WOX1 in mediating apoptosis. Here it is demonstrated that ectopic JNK1 inhibited WOX1-mediated apoptosis of L929 fibroblasts, monocytic U937 cells, and other cell types. Also, JNK1 blocked WOX1 prevention of cell cycle progression. By stimulating cells with anisomycin or UV light, JNK1 became activated, and WOX1 was phosphorylated at Tyr(33). The activated JNK1 physically interacted with the phosphorylated WOX1, as determined by co-immunoprecipitation. Alteration of Tyr(33) to Arg(33) in WOX1 abrogated its binding interaction with JNK1 and its activity in mediating cell death, indicating that Tyr(33) phosphorylation is needed to activate WOX1. A dominant negative WOX1 was developed and shown to block p53-mediated apoptosis and anisomycin-mediated WOX1 phosphorylation but could not inhibit JNK1 activation. This mutant protein bound p53 but could not interact with JNK1, as determined in yeast two-hybrid analysis. Taken together, phosphorylation of JNK1 and WOX1 is necessary for their physical interaction and functional antagonism.

Chronic morphine treatment and withdrawal induce up-regulation of c-jun n-terminal kinase 3 gene expression in rat brain

Chronic opiate applications produce long-term impacts on many functions of the brain and induce tolerance, dependence, and addiction. It has been demonstrated that opioid drugs are capable to induce apoptosis of neuronal cells, but the mechanism is not clear. c-Jun N-terminal kinase 3 (JNK3), specifically expressed in brain, has been proved to mediate neuronal apoptosis and is involved in opiate-induced cell apoptosis in vitro. The present study investigated the effect of opioid administration on expression of JNK3, an important mediator involved in apoptosis of neurons, in rat brain. Our results showed that single or chronic injection of morphine resulted in a 45-50% increase in the level of JNK3 mRNA in frontal cortex, while no significant change was detected in other brain regions such as thalamus, hippocampus and locus coeruleus. Similar to what was observed after the acute or chronic morphine administration, no significant change in JNK3 expression was detected in locus coeruleus following cessation of the chronic morphine administration. However, interestingly, sustained elevation of JNK3 expression peaked on day 14 after cessation of morphine treatment was observed in the brain regions such as hippocampus and thalamus, where acute or chronic morphine treatment did not cause any significant change in JNK3 gene expression. The increased JNK3 mRNA in these brain areas returned to the control levels in 28 days following cessation of chronic morphine treatment. Taken together, these results demonstrated for the first time that the expression of JNK3 gene is regulated by opioids and that chronic opioid administration and withdrawal could induce sustained elevation of JNK3 mRNA in many important brain areas. The changes in JNK3 gene expression in brain induced by chronic opioid treatment may play a role in opioid-induced apoptosis and neurotoxicity.