JNK3 contributes to c-Jun activation and apoptosis but not oxidative stress in nerve growth factor-deprived sympathetic neurons

The stress activated protein kinase pathway culminates in c-Jun phosphorylation mediated by the Jun Kinases (JNKs). The role of the JNK pathway in sympathetic neuronal death is unclear in that apoptosis is not inhibited by a dominant negative protein of one JNK kinase, SEK1, but is inhibited by CEP-1347, a compound known to inhibit this overall pathway but not JNKs per se. To evaluate directly the apoptotic role of the JNK isoform that is selectively expressed in neurons, JNK3, we isolated sympathetic neurons from JNK3-deficient mice and quantified nerve growth factor (NGF) deprivation-induced neuronal death, oxidative stress, c-Jun phosphorylation, and c-jun induction. Here, we report that oxidative stress in neurons from JNK3-deficient mice is normal after NGF deprivation. In contrast, NGF-deprivation-induced increases in the levels of phosphorylated c-Jun, c-jun, and apoptosis are each inhibited in JNK3-deficient mice. Overall, these results indicate that JNK3 plays a critical role in activation of c-Jun and apoptosis in a classic model of cell-autonomous programmed neuron death.     

beta-Amyloid-induced neuronal apoptosis requires c-Jun N-terminal kinase activation

beta-Amyloid (A beta) has been strongly implicated in the pathophysiology of Alzheimer's disease (AD), but the means by which the aggregated form of this molecule induces neuronal death have not been fully defined. Here, we examine the role of the c-Jun N-terminal kinases (JNKs) and of their substrate, c-Jun, in the death of cultured neuronal PC12 cells and sympathetic neurons evoked by exposure to aggregated A beta. The activities of JNK family members increased in neuronal PC12 cells within 2 h of A beta treatment and reached 3--4-fold elevation by 6 h. To test the role of these changes in death caused by A beta, we examined the effects of CEP-1347 (KT7515), an indolocarbazole that selectively blocks JNK activation. Inclusion of CEP-1347 (100--300 nM) in the culture medium effectively blocked the increases in cellular JNK activity caused by A beta and, at similar concentrations, protected both PC12 cells and sympathetic neurons from A beta-evoked-death. Effective protection required addition of CEP-1347 within 2 h of A beta treatment, indicating that the JNK pathway acts relatively proximally and as a trigger in the death mechanism. A dominant-negative c-Jun construct also conferred protection from A beta-evoked death, supporting a model in which JNK activation contributes to death via activation of c-Jun. Finally, CEP-1347 blocked A beta-stimulated activation of caspase-2 and -3, placing these downstream of JNK activation. These observations implicate the JNK pathway as a required element in death evoked by A beta and hence identify it as a potential therapeutic target in AD.     

Interaction of the c-Jun/JNK pathway and cyclin-dependent kinases in death of embryonic cortical neurons evoked by DNA damage

DNA damage, an important initiator of neuronal death, has been implicated in numerous neurodegenerative conditions. We previously delineated several pathways that control embryonic cortical neuronal death evoked by the DNA-damaging agent, camptothecin. In this model, the tumor suppressor p53 and cyclin-dependent kinases (CDKs) are activated independently and cooperate to mediate the conserved death pathway. To further our understanding, we presently examined whether the c-Jun/JNK pathway modulates death and whether this pathway is regulated by CDKs, p53, and Bax. We show that c-Jun/JNK is activated following DNA damage. Moreover, the c-Jun pathway is one mediator of death, because expression of dominant negative c-Jun and cdc42, and JNK pathway inhibitors are neuroprotective. Although previous evidences indicate that JNK3 is required for neuronal death under certain conditions, we show that JNK3 deficiency only partially mediates c-Jun phosphorylation and its deficiency does not protect neurons from death. Interestingly, we provide evidence that CDK activity regulates c-Jun but does not affect upstream pathways that lead to JNK phosphorylation. Finally, c-Jun activation is independent of p53 and Bax. Accordingly, we propose that c-Jun is regulated by the JNK and CDK pathways and that both must be activated for efficient c-Jun activation to occur.     

Neuronal c-Jun is required for successful axonal regeneration, but the effects of phosphorylation of its N-terminus are moderate

Although neural c-Jun is essential for successful peripheral nerve regeneration, the cellular basis of this effect and the impact of c-Jun activation are incompletely understood. In the current study, we explored the effects of neuron-selective c-Jun deletion, substitution of serine 63 and 73 phosphoacceptor sites with non-phosphorylatable alanine, and deletion of Jun N-terminal kinases 1, 2 and 3 in mouse facial nerve regeneration. Removal of the floxed c-jun gene in facial motoneurons using cre recombinase under control of a neuron-specific synapsin promoter (junΔS) abolished basal and injury-induced neuronal c-Jun immunoreactivity, as well as most of the molecular responses following facial axotomy. Absence of neuronal Jun reduced the speed of axonal regeneration following crush, and prevented most cut axons from reconnecting to their target, significantly reducing functional recovery. Despite blocking cell death, this was associated with a large number of shrunken neurons. Finally, junΔS mutants also had diminished astrocyte and microglial activation and T-cell influx, suggesting that these non-neuronal responses depend on the release of Jun-dependent signals from neighboring injured motoneurons. The effects of substituting serine 63 and 73 phosphoacceptor sites (junAA), or of global deletion of individual kinases responsible for N-terminal c-Jun phosphorylation were mild. junAA mutants showed decrease in neuronal cell size, a moderate reduction in post-axotomy CD44 levels and slightly increased astrogliosis. Deletion of Jun N-terminal kinase (JNK)1 or JNK3 showed delayed functional recovery; deletion of JNK3 also interfered with T-cell influx, and reduced CD44 levels. Deletion of JNK2 had no effect. Thus, neuronal c-Jun is needed in regeneration, but JNK phosphorylation of the N-terminus mostly appears to not be required for its function.     

All JNKs can kill, but nuclear localization is critical for neuronal death

JNKs are implicated in a range of brain pathologies and receive considerable attention as potential therapeutic targets. However, JNKs also regulate physiological and homeostatic processes. An attractive hypothesis from the drug development perspective is that distinct JNK isoforms mediate "physiological" and "pathological" responses. However, this lacks experimental evaluation. Here we investigate the isoforms, subcellular pools, and c-Jun/ATF2 targets of JNK in death of central nervous system neurons following withdrawal of trophic support. We use gene knockouts, gene silencing, subcellularly targeted dominant negative constructs, and pharmacological inhibitors. Combined small interfering RNA knockdown of all JNKs 1, 2, and 3, provides substantial neuroprotection. In contrast, knockdown or knock-out of individual JNKs or two JNKs together does not protect. This explains why the evidence for JNK in neuronal death has to date been largely pharmacological. Complete knockdown of c-Jun and ATF2 using small interfering RNA also fails to protect, casting doubt on c-Jun as a critical effector of JNK in neuronal death. Nonetheless, the death requires nuclear but not cytosolic JNK activity as nuclear dominant negative inhibitors of JNK protect, whereas cytosolic inhibitors only block physiological JNK function. Thus any one of the three JNKs is capable of mediating apoptosis and inhibition of nuclear JNK is protective.     

Regulation of axotomy-induced dopaminergic neuron death and c-Jun phosphorylation by targeted inhibition of cdc42 or mixed lineage kinase

Mechanical transection of the nigrostriatal dopamine pathway at the medial forebrain bundle (MFB) results in the delayed degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc). We have previously demonstrated that c-Jun activation is an obligate component of neuronal death in this model. Here we identified the small GTPase, cdc42, and mixed lineage kinases (MLKs) as upstream factors regulating neuronal loss and activation of c-Jun following MFB axotomy. Adenovirus-mediated expression of a dominant-negative form of cdc42 in nigral neurons blocked MFB axotomy-induced activation (phosphorylation) of MAP kinase kinase 4 (MKK4) and c-Jun, resulting in attenuation of SNpc neuronal death. Pharmacological inhibition of MLKs, MKK4-activating kinases, significantly reduced the phosphorylation of c-Jun and abrogated dopaminergic neuronal degeneration following MFB axotomy. Taken together, these findings suggest that death of nigral dopaminergic neurons following axotomy can be attenuated by targeting cell signaling events upstream of c-Jun N-terminal mitogen-activated protein kinase/c-Jun.     

Pin1 promotes cell death in NGF-dependent neurons through a mechanism requiring c-Jun activity

Developing neurons deprived of trophic support undergo apoptosis mediated by activation of c-Jun N-terminal kinases (JNK) and c-Jun, induction of the Bcl-2 homology 3-only protein Bim_EL, Bax-dependent loss of mitochondrial cytochrome c , and caspase activation. However, the mechanisms that regulate each of these events are only partially understood. Here we show that the prolyl isomerase Pin1 functions as a positive regulator of neuronal death through a c-Jun-dependent mechanism. Ectopic Pin1 promoted caspase-dependent death of NGF-maintained neurons that was associated with an accumulation of Ser⁶³-phosphorylated c-Jun in neuronal nuclei and was partially dependent on Bax. Downregulating Pin1 prior to NGF withdrawal suppressed the accumulation of phosphorylated c-Jun, inhibited the release of cytochrome c , and significantly delayed cell death. Pin1 knockdown inhibited NGF deprivation-induced death to a similar extent in Bim (+/+) and Bim (−/−) neurons. The protective effect of Pin1 knockdown was significantly greater than that caused by loss of Bim and nearly identical to that caused by a dominant negative form of c-Jun. Finally, cell death induced by ectopic Pin1 was largely blocked by expression of dominant negative c-Jun. These results suggest a novel mechanism by which Pin1 promotes cell death involving activation of c-Jun.     

The limited role of NH2-terminal c-Jun phosphorylation in neuronal apoptosis: identification of the nuclear pore complex as a potential target of the JNK pathway

c-Jun is induced in many neuronal death paradigms. A critical step in c-Jun regulation involves phosphorylation of Ser63/Ser73 located in the NH2-terminal transactivation domain. To determine the importance of this phosphorylation for neuronal apoptosis, we analyzed the sympathetic neurons of mice carrying a mutant c-Jun gene that lacks Ser63/Ser73 phosphorylation sites (jun aa). Trophic factor-deprivation or DNA damage-induced death was significantly delayed in jun aa/aa neurons. Neuronal c-Jun induction was only partially inhibited, demonstrating that phosphorylation of Ser63/73 is not required for c-Jun activation. The inductions of proapoptotic BH3-only proteins, Bim and PUMA/Bbc3, were delayed during neuronal apoptosis in mutant neurons. These results demonstrate that NH2-terminal c-Jun phosphorylation is important, but not necessary, for the induction of proapoptotic genes and neuronal apoptosis. Thus, additional JNK substrates may be critical for neuronal death. As potential mediators, we identified additional nuclear MLK/JNK substrates, including Nup214 subunit of the nuclear pore complex.     

Required roles of Bax and JNKs in central and peripheral nervous system death of retinoblastoma-deficient mice

Retinoblastoma-deficient mice show massive neuronal damage and deficits in both CNS and PNS tissue. Previous work in the field has shown that death is regulated through distinct processes where CNS tissue undergoes death regulated by the tumor suppressor p53 and the apoptosome component, APAF1. Death in the PNS, however, is independent of p53 and reliant on the death protease, caspase 3. In the present study, we more carefully delineated the common and distinct mechanisms of death regulation by examining the stress-activated kinases, JNK2 and 3, the conserved Bcl-2 member Bax, and the relationship among these elements including p53. By use of genetic modeling, we show that death in various regions of the CNS and DRGs of the PNS is reliant on Bax. In the CNS, Bax acts downstream of p53. The relevance of the JNKs is more complex, however. Surprisingly, JNK3 deficiency by itself does not inhibit c-Jun phosphorylation and instead, aggravates death in both CNS and PNS tissue. However, JNK2/3 double deficiency blocks death due to Rb loss in both the PNS and CNS. Importantly, the relationships between JNKs, p53, and Bax exhibit regional differences. In the medulla region of the hindbrain in the CNS, JNK2/3 deficiency blocks p53 activation. Moreover, Bax deficiency does not affect c-Jun phosphorylation. This indicates that a JNK-p53-Bax pathway is central in the hindbrain. However, in the diencephalon regions of the forebrain (thalamus), Bax deficiency blocks c-Jun activation, indicating that a Bax-JNK pathway of death is more relevant. In the DRGs of the PNS, a third pathway is present. In this case, a JNK-Bax pathway, independent of p53, regulates damage. Accordingly, our results show that a death regulator Bax is common to death in both PNS and CNS tissue. However, it is regulated by or itself regulates different effectors including the JNKs and p53 depending upon the specific region of the nervous system.     

Sh3rf2/POSHER protein promotes cell survival by ring-mediated proteasomal degradation of the c-Jun N-terminal kinase scaffold POSH (Plenty of SH3s) protein

We report that Sh3rf2, a homologue of the pro-apoptotic scaffold POSH (Plenty of SH3s), acts as an anti-apoptotic regulator for the c-Jun N-terminal kinase (JNK) pathway. siRNA-mediated knockdown of Sh3rf2 promotes apoptosis of neuronal PC12 cells, cultured cortical neurons, and C6 glioma cells. This death appears to result from activation of JNK signaling. Loss of Sh3rf2 triggers activation of JNK and its target c-Jun. Also, apoptosis promoted by Sh3rf2 knockdown is inhibited by dominant-negative c-Jun as well as by a JNK inhibitor. Investigation of the mechanism by which Sh3rf2 regulates cell survival implicates POSH, a scaffold required for activation of pro-apoptotic JNK/c-Jun signaling. In cells lacking POSH, Sh3rf2 knockdown is unable to activate JNK. We further find that Sh3rf2 binds POSH to reduce its levels by a mechanism that requires the RING domains of both proteins and that appears to involve proteasomal POSH degradation. Conversely, knockdown of Sh3rf2 promotes the stabilization of POSH protein and activation of JNK signaling. Finally, we show that endogenous Sh3rf2 protein rapidly decreases following several different apoptotic stimuli and that knockdown of Sh3rf2 activates the pro-apoptotic JNK pathway in neuronal cells. These findings support a model in which Sh3rf2 promotes proteasomal degradation of pro-apoptotic POSH in healthy cells and in which apoptotic stimuli lead to rapid loss of Sh3rf2 expression, and consequently to stabilization of POSH and JNK activation and cell death. On the basis of these observations, we propose the alternative name POSHER (POSH-eliminating RING protein) for the Sh3rf2 protein.     

Role of apoptosis signal-regulating kinase in regulation of the c-Jun N-terminal kinase pathway and apoptosis in sympathetic neurons

We have previously shown that nerve growth factor (NGF) withdrawal-induced death requires the activity of the small GTP-binding protein Cdc42 and that overexpression of an active form of Cdc42 is sufficient to mediate neuronal apoptosis via activation of the c-Jun pathway. Recently, a new mitogen-activated protein (MAP) kinase kinase kinase, apoptosis signal-regulating kinase 1 (ASK1) which activates both the c-Jun N-terminal kinase (JNK) and p38 MAP kinase pathways and plays pivotal roles in tumor necrosis factor- and Fas-induced apoptosis, has been identified. Therefore, we investigated the role of ASK1 in neuronal apoptosis by using rat pheochromocytoma (PC12) neuronal cells and primary rat sympathetic neurons (SCGs). Overexpression of ASK1-DeltaN, a constitutively active mutant of ASK1, activated JNK and induced apoptosis in differentiated PC12 cells and SCG neurons. Moreover, in differentiated PC12 cells, NGF withdrawal induced a four- to fivefold increase in the activity of endogenous ASK1. Finally, expression of a kinase-inactive ASK1 significantly blocked both NGF withdrawal- and Cdc42-induced death and activation of c-jun. Taken together, these results demonstrate that ASK1 is a crucial element of NGF withdrawal-induced activation of the Cdc42-c-Jun pathway and neuronal apoptosis.     

Regulation of apoptotic c-Jun N-terminal kinase signaling by a stabilization-based feed-forward loop

A sequential kinase cascade culminating in activation of c-Jun N-terminal kinases (JNKs) plays a fundamental role in promoting apoptotic death in many cellular contexts. The mechanisms by which this pathway is engaged in response to apoptotic stimuli and suppressed in viable cells are largely unknown. Here, we show that apoptotic stimuli increase endogenous cellular levels of pathway components, including POSH, mixed lineage kinases (MLKs), and JNK interacting protein 1, and that this effect occurs through protein stabilization and requires the presence of POSH as well as activation of MLKs and JNKs. Our findings suggest a self-amplifying, feed-forward loop mechanism by which apoptotic stimuli promote the stabilization of JNK pathway components, thereby contributing to cell death.     

Activation of c-Jun N-terminal kinase 1 by UV irradiation is inhibited by wortmannin without affecting c-iun expression

Activation of c-Jun N-terminal kinases (JNKs)/stress-activated protein kinases is an early response of cells upon exposure to DNA-damaging agents. JNK-mediated phosphorylation of c-Jun is currently understood to stimulate the transactivating potency of AP-1 (e.g., c-Jun/c-Fos; c-Jun/ATF-2), thereby increasing the expression of AP-1 target genes. Here we show that stimulation of JNK1 activity is not a general early response of cells exposed to genotoxic agents. Treatment of NIH 3T3 cells with UV light (UV-C) as well as with methyl methanesulfonate (MMS) caused activation of JNK1 and an increase in c-Jun protein and AP-1 binding activity, whereas antineoplastic drugs such as mafosfamide, mitomycin C, N-hydroxyethyl-N-chloroethylnitrosourea, and treosulfan did not elicit this response. The phosphatidylinositol 3-kinase inhibitor wortmannin specifically blocked the UV-stimulated activation of JNK1 but did not affect UV-driven activation of extracellular regulated kinase 2 (ERK2). To investigate the significance of JNK1 for transactivation of c-jun, we analyzed the effect of UV irradiation on c-jun expression under conditions of wortmannin-mediated inhibition of UV-induced stimulation of JNK1. Neither the UV-induced increase in c-jun mRNA, c-Jun protein, and AP-1 binding nor the activation of the collagenase and c-jun promoters was affected by wortmannin. In contrast, the mitogen-activated protein kinase/ERK kinase inhibitor PD98056, which blocked ERK2 but not JNK1 activation by UV irradiation, impaired UV-driven c-Jun protein induction and AP-1 binding. Based on the data, we suggest that JNK1 stimulation is not essential for transactivation of c-jun after UV exposure, whereas activation of ERK2 is required for UV-induced signaling leading to elevated c-jun expression.