Glutamate, but not dopamine, stimulates stress-activated protein kinase and AP-1-mediated transcription in striatal neurons

Triggering of neuronal cell death by accumulation of activated SEK1 on nuclear polyglutamine aggregations in PML bodies

BACKGROUND

A novel class of inherited human neurodegenerations is now known to be caused by expanded CAG repeats encoding polyglutamines. Polyglutamine-containing protein fragments have been shown to accumulate as aggregates in the nucleus and in the cytoplasm, and to induce cell death when expressed in cultured cells, leading to the proposal that polyglutamine aggregation is an important step in the pathogenesis. Supporting this, nuclear inclusions containing expanded polyglutamines have been identified in neurones from the brains of patients and in neurones from transgenic mouse models of this class of neural disorders.

RESULTS

We analysed the consequences of polyglutamine expression in PC12 neuronal cells. Activated SEK1 accumulated with nuclear but not cytoplasmic polyglutamine aggregations, which consequently triggers cell death. Cell death induced by polyglutamine expression was inhibited by a dominant-negative SEK1 (DN-SEK1), but not by DN-SEK1 tagged with a nuclear export signal. Steady state SEK1 expression itself was enhanced two to three-fold. Nuclearly aggregated polyglutamines, which were identified in PML bodies, co-localized with not only activated SEK1 but also activated c-Jun. We also observed that nuclear inclusion-positive neurones from brains with Huntington's disease expressed SEK1.

CONCLUSIONS

This study provides molecular links between the neurodegeneration observed in polyglutamine diseases, cell death signalling kinase cascades and nuclear subdomains related to cell death. We propose that the nuclear PML bodies containing polyglutamine aggregates activate the SEK1-JNK kinase cascade, resulting in the transduction of a death signal.

Changes in the regional and compartmental distribution of FosB- and JunB-like immunoreactivity induced in the dopamine-denervated rat striatum by acute or chronic L-dopa treatment

This study was carried out in order to examine the effects of acute or chronic L-DOPA treatment on striatally expressed FosB- and JunB-like proteins in a rat model of Parkinson's disease. Rats with a unilateral, near-total 6-hydroxydopamine lesion of the ascending mesostriatal projection received either an acute challenge or a one-week treatment with 10 mg/kg/day methyl L-DOPA (combined with 15 mg/mg benserazide), and were killed at either 3 h or two days post-injection. Both acute and chronic L-DOPA treatment caused a pronounced, persistent increase in the number of FosB-like immunoreactive cells in the dopamine-denervated striata (five- and seven-fold increase, respectively, above the levels found in lesioned but non-drug-treated controls), but the two treatment groups differed markedly with respect to both the average amount of staining per cell, which was two-fold larger in the chronic L-DOPA cases, and the anatomical distribution of the labeled cells. After an acute injection of L-DOPA, FosB-positive cells were distributed rather uniformly across all striatal subregions, whereas chronic L-DOPA treatment induced discrete clusters of strongly FosB-like immunoreactive cells within medial and central striatal subregions, as well as in a large, yet sharply defined portion of the lateral caudate-putamen. Strongly labeled cell clusters that appeared in the medial and central caudate-putamen were preferentially located within calbindin-poor, mu-opioid receptor-rich striosomes, whereas the lateral area displaying FosB activation encompassed both striosomal and matrix domains. In both the medial and the lateral striatum a near-total overlap was found between strongly FosB-like immunoreactive cell groups and areas showing pronounced dynorphin expression. NADPH-diaphorase-positive striatal interneurons did not express FosB-like immunoreactivity after a 6-hydroxydopamine lesion alone, a negligible proportion of them did after an acute L-DOPA challenge, but about 8% of these interneurons were FosB positive following chronic L-DOPA treatment. Like FosB, JunB was induced in the DA-denervated striatum by both acute and chronic L-DOPA treatment, and exhibited similar distribution patterns. However, JunB did not exhibit prolonged expression kinetics, and was somewhat down-regulated in the chronically compared with the acutely L-DOPA-treated rats. The present results show that L-DOPA administration produces a long-lasting increase in the levels of FosB-, but not JunB-like immunoreactivity in the dopamine-denervated striatum. More importantly, these data show that striatal induction of FosB- and JunB-like proteins by chronic L-DOPA treatment exhibits both regional and compartmental specificity.

Expression of cell death-associated phospho-c-Jun and p53-activated gene 608 in hippocampal CA1 neurons following global ischemia

Persistent activation of c-Jun N-terminal kinases (JNKs) and phosphorylation of c-Jun has been shown in various cell death paradigms. Inhibition of the JNK signal transduction pathway prevented neuronal cell death both in vitro and in vivo. In the present study, nuclear phospho-c-Jun immunoreactivity became apparent selectively in vulnerable hippocampal CA1 neurons at 24 h after transient global cerebral ischemia. A high constitutive expression of phospho-JNK1 was detected by immunoblot analysis of hippocampal extracts. Expression of JNK interacting protein-1 (JIP-1), which facilitates JNK signaling, remained unchanged in post-ischemic hippocampal neurons. By contrast, p53-activated gene 608 (PAG608), which promotes cell death in vitro, was strongly induced in post-ischemic CA1 neurons. Our data suggest that transcription factors p53 and phospho-c-Jun may contribute to programmed CA1 cell death following ischemia.

Endogenous adenosine protects CA1 neurons from kainic acid-induced neuronal cell loss in the rat hippocampus

CA3 pyramidal neurons in the rat hippocampus show selective vulnerability to the intracerebroventricular injection of kainic acid (KA). However, the mechanism of this selective neuronal vulnerability remains unclear. In this study, we examined the contribution of endogenous adenosine, a potent inhibitory neuromodulator, to the differences in the neuronal vulnerability of the hippocampus, using microtubule-associated protein (MAP)-2, phosphorylated c-Jun, and major histocompatibility complex (MHC) class II immunoreactivities as markers for neuronal cell loss, neuronal apoptosis and glial activation, respectively. Pretreatment with 8-cyclopenthyltheophylline (CPT), an A1 adenosine receptor antagonist, significantly exacerbated KA-induced neuronal cell loss in both the CA1 and CA3. Although c-Jun phosphorylation, a critical step in neuronal apoptosis, was not detected in the vehicle-injected rat hippocampus, c-Jun phosphorylation was induced in the CA3 by the injection of KA alone. Pretreatment with CPT induced c-Jun phosphorylation in both the CA1 and CA3. MHC class II antigen was also detected in the regions of c-Jun phosphorylation. Coadministration of N6-cyclopenthyladenosine (CHA), an A1 adenosine receptor agonist, attenuated the neuronal cell loss in the CA1 and CA3 with or without pretreatment with CPT. These results strongly suggest that endogenous adenosine has neuroprotective effects against excitotoxin-induced neurodegeneration in the CA1 through its A1 receptors.

B-myb promoter retargeting of herpes simplex virus gamma34.5 gene-mediated virulence toward tumor and cycling cells

Deletion of the gamma34.5 gene coding for virulence markedly reduces cytotoxicity mediated by herpes simplex virus type 1 (HSV-1) (J. M. Markert et al., Neurosurgery 32:597-603, 1993; N. S. Markovitz et al. , J. Virol. 71:5560-5569, 1997). To target lytic virulence to tumors, we have created a novel HSV-1 mutant, designated Myb34.5. This viral mutant is characterized by a deletion of the gene for infected cell polypeptide 6 (ICP6; also known as UL39 or ribonucleotide reductase) and of the two endogenous copies of the gamma34.5 gene (RL1) and by reintroduction of one copy of gamma34.5 under control of the E2F-responsive, cellular B-myb promoter. On direct intracerebral inoculation in BALB/c mice, the 50% lethal dose (LD(50)) for Myb34.5 was 2.7 x 10(7) PFU while that for HSVs with mutations in the gamma34.5 gene could not be technically achieved with available viral stocks and it was estimated as >1 x 10(7) PFU. The LD(50) for an HSV with a single defect in ICP6 function was 1.3 x 10(6) PFU. Conversely, Myb34.5's oncolytic efficacy against a variety of human glioma cells in culture and in vivo was enhanced compared to that of HSVs with gamma34.5 mutations, and in fact, it was comparable to that of the wild-type F strain and of viral mutants that possess a wild-type gamma34.5 gene. The characteristic shutoff of host protein synthesis, occurring after infection of human SK-N-SH neuroblastoma cells by gamma34.5 mutant viruses (J. Chou and B. Roizman, Proc. Natl. Acad. Sci. USA 89:3266-3270, 1992), was not present after infection with Myb34.5. There was an increase of almost 3 logarithmic units in the production of progeny virus in arrested fibroblasts compared to that in cycling fibroblasts infected with Myb34.5. These results suggest that transcriptional regulation of gamma34.5 by cell cycle-regulated promoters can be used to target HSV-1 virulence toward tumors while maintaining the desirable neuroattenuated phenotype of a gamma34.5 mutant.

Hypoxia induces selective SAPK/JNK-2-AP-1 pathway activation in the nucleus tractus solitarii of the conscious rat

In the nucleus tractus solitarii, NMDA glutamate receptors are critical to the hypoxic ventilatory response. However, the signal transduction pathways underlying the hypoxic ventilatory response remain undefined. To assess the effect of a moderate hypoxic stimulus (10% O2) on tyrosine phosphorylation of proteins in the nucleus tractus solitarii, tissue lysates were harvested by repeated punch sampling at 0, 1, 10, and 60 min of hypoxia and examined for the presence of phosphorylated tyrosine residues by immunoblotting. Time-dependent phosphotyrosine increases occurred in proteins migrating at regions corresponding to molecular masses of 38-42, 50, 55, and 60 kDa, which were attenuated by pretreatment with the NMDA receptor channel blocker, MK-801. As extracellular signal-regulated kinase (Erk) and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) phosphorylation may induce Fos and Jun gene transcription and activator protein-1 (AP-1) DNA binding, the activation of Erk1, Erk2, p38, and SAPK/JNK was examined in the nucleus tractus solitarii and neocortex during hypoxia and following administration of MK-801. Hypoxia enhanced Erk1, Erk2, and p38 activity in the cortex, but not in the nucleus tractus solitarii. Increased phosphorylation of SEK1 and SAPK/JNK-2 occurred in the nucleus tractus solitarii during hypoxia, whereas both SAPK/JNK-1 and SAPK/JNK-2 were recruited in cortex. MK-801 attenuated hypoxia-induced SEK1, SAPK/JNK-2, and AP-1 binding in the nucleus tractus solitarii, and the widespread activation of all MAP kinases in the cortex was also attenuated. We conclude that in conscious rats, a moderate hypoxic stimulus elicits NMDA-dependent widespread mitogen-activated protein kinase activation in cortex, but selective SAPK/JNK-2 and AP-1 activation in the nucleus tractus solitarii, thereby suggesting a functional role for the SAPK/JNK-2-AP-1 pathway.

Intrastriatal dopamine injection induces apoptosis through oxidation-involved activation of transcription factors AP-1 and NF-kappaB in rats

ABSTRACT More and more evidence suggests that increases in dopamine (DA) in striata may participate in neurodegenerative processes during acute ischemia, hypoxia, and excitotoxicity. With a rat model of intrastriatal DA injection, we studied the molecular events involved in DA toxicity. Intrastriatal injections of DA in amounts from 1 to 2 micromol result in apoptotic cell death, as indicated by terminal deoxynucleotidyl transferase labeling of DNA strand breaks and Klenow polymerase-catalyzed [(32)P]deoxycytidine triphosphate-labeled DNA laddering. Injections of DA produce a strong and prolonged activated protein 1 (AP-1) activity that contains c-fos, c-jun, and phosphorylated c-jun protein. DA injections also stimulate the activity of nuclear factor-kappaB (NF-kappaB), an oxidative stress-responsive transcription factor. Injection of curcumin at a dose that selectively inhibits AP-1 activation without affecting NF-kappaB activity attenuates DNA laddering induced by DA. Preinjection with SN50, a specific permeable recombinant NF-kappaB translocation inhibitor peptide, reduces DA-induced NF-kappaB activation and apoptosis. Moreover, preinjection of the antioxidant GSH significantly inhibits both DA-induced activation of transcription factors AP-1 and NF-kappaB and subsequent apoptosis. Thus, our data suggest that DA-oxidative stress-induced apoptosis in vivo is mediated by activation of transcription factors AP-1 and NF-kappaB.

Expression of I2PP2A, an inhibitor of protein phosphatase 2A, induces c-Jun and AP-1 activity

Transient expression of I2PP2A, a potent inhibitor of protein phosphatase 2A (PP2A), in HEK-293 cells increased the concentration and DNA binding of the proto-oncogene c-Jun. In contrast, expression of the catalytic subunit of PP2A (PP2AC) markedly decreased the concentration and DNA binding of c-Jun. Expression of I2PP2A also increased the transcriptional activity of activator protein-1, and this effect was diminished in a dose-dependent manner by expression of PP2AC. Densitometric analysis following Western blotting of extracts with antibodies specific for phospho-Ser63 and Ser73 suggests that the effects of I2PP2A and PP2AC expression might be mediated, in part, by changes in the phosphorylation of c-Jun at Ser63. The results indicate that I2PP2A elicits effects that are consistent with it acting as an inhibitor of PP2A in intact cells, and suggest that PP2A might exhibit site selectivity with respect to c-Jun phosphorylation.

5-Hydroxytryptamine2A receptor stimulation induces activator protein-1 and cyclic AMP-responsive element binding with cyclic AMP-responsive element-binding protein and Jun D as common components in cerebellar neurons

Previous studies from our laboratory have demonstrated that stimulation of 5-hydroxytryptamine2A receptors in rat cerebellar granule cells produces an increase in the levels of 5-hydroxytryptamine2A receptor messenger RNA and binding sites, and that this up-regulation requires de novo RNA and protein synthesis. Here we showed that up-regulation of 5-hydroxytryptamine2A receptor binding sites induced by stimulation with the 5-hydroxytryptamine2A/2C receptor agonist, (+/-)-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), is associated with an increase in the 5-hydroxytryptamine2A receptor transcription rate. To examine the possible role of transcriptional activation in DOI-induced 5-hydroxytryptamine2A receptor up-regulation, we studied the effects of DOI on transcription factor binding to activator protein-1 and cyclic AMP-responsive element (CRE) DNA consensus sequences. We found that DOI induces a time-dependent increase in activator protein-1 and CRE transcription factor binding activity, which is blocked by 5-hydroxytryptamine2A receptor antagonists. Similar to 5-hydroxytryptamine2A receptor up-regulation, DOI-induced activator protein-1 binding is suppressed by inhibitors of calmodulin and Ca2+/calmodulin-dependent kinases. The increased activator protein-1 binding is effectively competed by excessive activator protein-1 and CRE sequences as well as endogenous activator protein-1-like sequences present in the rat 5-hydroxytryptamine2A receptor gene. Supershift assays revealed that cAMP-responsive element-binding protein (CREB) and Jun D are common components of both activator protein-1 and CRE binding complexes. DOI also increased the level of phospho-CREB in a time-dependent manner. The binding of phospho-CREB transcription factor to the activator protein-1 site suggests that CREB may modulate the transcription of genes that contain activator protein-1 but lack CRE site in their promoters, through interaction with the activator protein-1 site. The rat 5-hydroxytryptamine2A receptor up-regulation may involve such a mechanism.

Contrasting calcium dependencies of SAPK and ERK activations by glutamate in cultured striatal neurons

Stress-activated protein kinase (SAPK) and extracellular signal-regulated kinase (ERK), both members of the mitogen-activated protein kinase (MAPK) family, may in some circumstances serve opposing functions with respect to cell survival. However, SAPK and ERK can also be coordinately activated in neurons in response to glutamate stimulation of NMDA receptors. To explore the mechanisms of these MAPK activations, we compared the ionic mechanisms mediating SAPK and ERK activations by glutamate. In primary cultures of striatal neurons, glutamatergic activation of ERK and one of its transcription factor targets, CREB, showed a calcium dependence typical of NMDA receptor-mediated responses. In contrast, extracellular calcium was not required for glutamatergic, NMDA receptor-mediated activation of SAPK and phosphorylation of its substrate, c-Jun. Increasing extracellular calcium enhanced ERK activation but reversed SAPK activation, further distinguishing the calcium dependencies of these two NMDA receptor-mediated effects. Finally, reducing extracellular sodium prevented the glutamatergic activation of SAPK but only partially blocked that of ERK. These contrasting ionic dependencies suggest a mechanism by which NMDA receptor activation may, under distinct conditions, differentially regulate neuronal MAPKs and their divergent functions.

Control of recruitment and transcription-activating function of CBP determines gene regulation by NMDA receptors and L-type calcium channels

Recruitment of the coactivator CBP by signal-regulated transcription factors and stimulation of CBP activity are key regulatory events in the induction of gene transcription following Ca2+ flux through ligand- and/or voltage-gated ion channels in hippocampal neurons. The mode of Ca2+ entry (L-type Ca2+ channels versus NMDA receptors) differentially controls the CBP recruitment step to CREB, providing a molecular basis for the observed Ca2+ channel type-dependent differences in gene expression. In contrast, activation of CBP is triggered irrespective of the route of Ca2+ entry, as is activation of c-Jun, that recruits CBP independently of phosphorylation at major regulatory c-Jun phosphorylation sites, serines 63 and 73. This control of CBP recruitment and activation is likely relevant to other CBP-interacting transcription factors and represents a general mechanism through which Ca2+ signals associated with electrical activity may regulate the expression of many genes.

Dehydroepiandrosterone antagonizes the neurotoxic effects of corticosterone and translocation of stress-activated protein kinase 3 in hippocampal primary cultures

Glucocorticoids are toxic to hippocampal neurons. We report here that the steroid dehydroepiandrosterone protects neurons of primary hippocampal cultures against the toxic effects of corticosterone. Corticosterone (20-500 nM) added for 24h to primary cultures of embryonic day 18 rat hippocampus resulted in significant neurotoxicity. Dissociated cells were grown for at least 10 days, initially in serum-containing medium, but serum was removed before adding steroids for 24 h. Neurotoxicity was measured by counting the number of cells stained either for beta-tubulin III or glial fibrillary acidic protein. Corticosterone-induced toxicity was prevented by co-treatment of the cultures with dehydroepiandrosterone (20-500 nM). Dehydroepiandrosterone on its own had little effect, though the highest concentration used (500 nM) was mildly toxic. Immunohistochemical studies on the nuclear translocation of a range of stress-activated protein kinases showed that stress-activated protein kinases 1, 2, 3 and 4 were all translocated by 10 min exposure to corticosterone (100 nM). Dehydroepiandrosterone (100 nM) attenuated the translocation of stress-activated protein kinase 3, but not the others. These experiments show that dehydroepiandrosterone has potent anti-glucocorticoid actions on the brain, and can protect hippocampal neurons from glucocorticoid-induced neurotoxicity. This protective action may involve stress-activated protein kinase 3-related intracellular pathways, though direct evidence for this has still to be obtained.

Amino-terminal phosphorylation of c-Jun regulates stress-induced apoptosis and cellular proliferation

c-Jun is a major component of the heterodimeric transcription factor AP-1 and is essential for embryonic development, as fetuses lacking Jun die at mid-gestation with impaired hepatogenesis and primary Jun-/- fibroblasts have a severe proliferation defect and undergo premature senescence in vitro. c-Jun and AP-1 activities are regulated by c-Jun N-terminal phosphorylation (JNP) at serines 63 and 73 through Jun N-terminal kinases(JNKs). JNP is thought to be required for the anti-apoptotic function of c-Jun during hepatogenesis, as mice lacking the JNK kinase SEK1 exhibit liver defects similar to those seen in Jun-/- fetuses. To investigate the physiological relevance of JNP, we replaced endogenous Jun by a mutant Jun allele with serines 63 and 73 mutated to alanines (Jun(tm1wag); hereafter referred to as JunAA). Here we show that primary JunAA fibroblasts have proliferation- and stress-induced apoptotic defects, accompanied by reduced AP-1 activity. JunAA mice are viable and fertile, smaller than controls and resistant to epileptic seizures and neuronal apoptosis induced by the excitatory amino acid kainate. Primary mutant neurons are also protected from apoptosis and exhibit unaltered JNK activity. Our results provide evidence that JNP is dispensable for mouse development, and identify c-Jun as the essential substrate of JNK signalling during kainate-induced neuronal apoptosis.

Glutamate induces phosphorylation of Elk-1 and CREB, along with c-fos activation, via an extracellular signal-regulated kinase-dependent pathway in brain slices

In cell culture systems, the TCF Elk-1 represents a convergence point for extracellular signal-related kinase (ERK) and c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) subclasses of mitogen-activated protein kinase (MAPK) cascades. Its phosphorylation strongly potentiates its ability to activate transcription of the c-fos promoter through a ternary complex assembled on the c-fos serum response element. In rat brain postmitotic neurons, Elk-1 is strongly expressed (V. Sgambato, P. Vanhoutte, C. Pagès, M. Rogard, R. A. Hipskind, M. J. Besson, and J. Caboche, J. Neurosci. 18:214-226, 1998). However, its physiological role in these postmitotic neurons remains to be established. To investigate biochemically the signaling pathways targeting Elk-1 and c-fos in mature neurons, we used a semi-in vivo system composed of brain slices stimulated with the excitatory neurotransmitter glutamate. Glutamate treatment leads to a robust, progressive activation of the ERK and JNK/SAPK MAPK cascades. This corresponds kinetically to a significant increase in Ser383-phosphorylated Elk-1 and the appearance of c-fos mRNA. Glutamate also causes increased levels of Ser133-phosphorylated cyclic AMP-responsive element-binding protein (CREB) but only transiently relative to Elk-1 and c-fos. ERK and Elk-1 phosphorylation are blocked by the MAPK kinase inhibitor PD98059, indicating the primary role of the ERK cascade in mediating glutamate signaling to Elk-1 in the rat striatum in vivo. Glutamate-mediated CREB phosphorylation is also inhibited by PD98059 treatment. Interestingly, KN62, which interferes with calcium-calmodulin kinase (CaM-K) activity, leads to a reduction of glutamate-induced ERK activation and of CREB phosphorylation. These data indicate that ERK functions as a common component in two signaling pathways (ERK/Elk-1 and ERK/?/CREB) converging on the c-fos promoter in postmitotic neuronal cells and that CaM-Ks act as positive regulators of these pathways.

Neurosteroids, brain damage, and mental illness

The steroidal environment of the brain has marked consequences for both its structure and function. Social or physical stress has deleterious results on hippocampal function. This can be replicated by raising corticoids, which are also highly responsive to stress. Corticosterone, the major glucocorticoid in the rat, induces neuronal death in primary hippocampal cultures. Elevated corticoids also induce mood changes, and these are well known to be associated with stress, particularly chronic stress such as social adversity accentuated by intercurrent aversive life events. DHEA, a second adrenal steroid, has a very different developmental history, increasing rapidly during childhood, reaching a peak in youth, and declining thereafter in both blood and CSF. DHEA, in contrast to corticoids, has brain protective actions. It reduces the neurotoxic actions of glutamate analogues (such as NMDA) as well as those of corticoids. Evidence from several sources suggests that DHEA can act as an antiglucocorticoid. DHEA levels are reduced in major depressive disorders in both adolescents and adults, and a raised cortisol/DHEA ratio (together with intercurrent life events) predicts delayed recovery. DHEA may have a role in the treatment of depression. Together, these findings suggest that altered steroidal environment, whether induced by stress or aging, can have appreciable results on the cellular structure of the brain as well as on its function, although links between the two sets of findings are still tentative.

Activation of c-jun N-terminal kinase/stress-activated protein kinase and the decreased ratio of Bcl-2 to Bax are associated with the auto-oxidized dopamine-induced apoptosis in PC12 cells

Current concepts of the pathogenesis of Parkinson's disease center on the formation of reactive oxygen species (ROS). Dopamine is one of the major sources of ROS. In this study, the molecular events during the dopamine-induced apoptosis in PC-12 cells were studied using auto-oxidized dopamine. Auto-oxidized-dopamine induced DNA fragmentation and activation of c-jun N-terminal kinase (JNK)/stress-activated protein kinase (SAPK) faster and stronger than dopamine. Furthermore, N-acetylcysteine, an antioxidant, prevented the auto-oxidized dopamine-induced JNK/SAPK activation and DNA fragmentation. Meanwhile, Bcl-2 started to decrease after onset of apoptosis, and Bax was increased up to beginning of apoptosis, and thereafter decreased. Therefore, these results suggested that activation of JNK/SAPK and the decreased ratio of antiapoptotic Bcl-2 to proapoptotic Bax appear to be associated with the dopamine-induced apoptosis.

Expression of polyglutamine-expanded Huntingtin activates the SEK1-JNK pathway and induces apoptosis in a hippocampal neuronal cell line

Huntington's disease is one of a growing number of hereditary neurodegenerative disorders caused by expansion of a polyglutamine stretch at the NH2 terminus of huntingtin. To explore whether polyglutamine-expanded huntingtin induces neuronal toxicity, I examined the expression of the full-length of huntingtin with 16, 48, or 89 polyglutamine repeats in a rat hippocampal neuronal cell (HN33). Expression of mutated huntingtin with 48 or 89 polyglutamine repeats stimulated c-Jun amino-terminal kinases (JNKs) activity and induced apoptotic cell death in HN33 cells while expression of normal huntingtin with 16 polyglutamine repeats had no toxic effect. The JNK activation precedes apoptotic cell death and co-expression of a dominant negative mutant form of stress-signaling kinase (SEK1) nearly completely blocked activation of JNKs and neuronal apoptosis mediated by mutated huntingtin. Taken together, my studies demonstrate that expression of polyglutamine-expanded huntingtin induces neuronal apoptosis via activation of the SEK1-JNK pathway.

Differential expression of SAPK isoforms in the rat brain. An in situ hybridisation study in the adult rat brain and during post-natal development

MAPK pathways transduce a broad variety of extracellular signals into cellular responses. Despite their pleiotropic effects and their ubiquitous distribution, surprisingly little is known about their involvement in the communication network of nerve cells. As a first step to elucidate the role of MAPK pathways in neuronal signalling, we studied the distribution of SAPK alpha/JNK2, SAPK beta/JNK3, and SAPK gamma/JNK1, three isoforms of SAPK/JNK, a stress-activated MAPK subfamily. We compared the mRNA localisation of the three main isoforms in the adult and developing rat brain using in situ hybridisation. In the adult brain, SAPK alpha and beta were widely but heterogeneously distributed, reproducing the pattern of a probe that does not discriminate the isoforms. Differently, high labelling for the SAPK gamma probe was exclusively localised in the endopiriform nucleus and medial habenula. Intermediate staining was detected in the hippocampus. During post-natal development, SAPK beta showed the same localisation as in the adult. Nevertheless, the semi-quantitative analysis of optical densities showed significantly different mRNA levels. In the adult, SAPK gamma signal was weak, whereas in newborn rats the labelling was intense and widely distributed. SAPK gamma mRNA levels decreased during development, to reach the low signals detected in the adult. These results suggest that in the central nervous system SAPK-type MAP kinases perform significant physiological functions which are particularly relevant during post-natal development. The distinct distribution patterns of SAPK isoforms in the adult rat brain support the hypothesis that separate functions are performed by the products of the three SAPK genes.

Quantitative neuronal c-fos and c-jun expression in Alzheimer's disease

Apoptosis, or programmed cell death, has been proposed as a mechanism of neuropathology in Alzheimer's disease (AD). Activation of immediate early genes (IEG) c-jun and c-fos appears to be required for the initiation of apoptosis. Furthermore, the expression of c-jun is induced in cultured neurons that undergo beta-amyloid-mediated apoptosis suggesting a direct role for c-jun in the apoptosis of AD neurons. Using immunohistochemical methods, we calculated the average number of neuronal profiles per unit area expressing c-Jun and c-Fos within hippocampal regions CA1, CA2/3, and CA4 in postmortem brain samples from AD patients and age-matched non-AD patients. There was an increase in c-Jun-positive and c-Fos-positive neuronal profile density in nearly all AD hippocampal regions examined. In cerebellum there was no evidence of apoptosis as determined by using TUNEL technique, and negligible c-Jun labeling.

Glucocorticoid-induced apoptosis and regulation of NF-kappaB activity in human leukemic T cells

Glucocorticoid-induced apoptosis was investigated in glucocorticoid-sensitive 6TG1.1 and resistant ICR27TK.3 human leukemic T cells. Following glucocorticoid treatment of 6TG1.1 cells, chromatin fragmentation was observed after a delay of 24 h. Fragmentation was not observed in ICR27TK.3 cells containing mutant glucocorticoid receptors (L753F) that are activation-deficient but retain the ability to repress AP-1 activity. Nor was fragmentation observed after treatment with RU38486, indicating that repression of AP-1 activity is not involved. As described in other systems, fragmentation required ongoing protein synthesis. However, inhibition of protein synthesis with cycloheximide anytime during the first 18 h of steroid treatment was as effective in blocking chromatin fragmentation as inhibition for the entire period, suggesting that synthesis of a component with a rapid turnover rate is required. Dexamethasone treatment completely blocked 12-O-tetradecanoylphorbol 13-acetate induction of nuclear factor-kappaB (NF-kappaB) activity and elicited an increase in the amount of immunoreactive IkappaB alpha in sensitive 6TG1.1 cells but not in resistant ICR27TK.3 cells. In addition, mild detergent treatment of cell extracts indicated that a substantial amount of cytoplasmic NF-kappaB is complexed with IkappaB alpha or some other inhibitory factor. These results suggest that induction of a labile inhibitory factor such as IkappaB alpha may contribute to glucocorticoid-induced apoptosis.

Drugs of abuse and immediate-early genes in the forebrain

A diverse array of chemical agents have been self administered by humans to alter the psychological state. Such drugs of abuse include both stimulants and depressants of the central nervous system. However, some commonalties must underlie the neurobiological actions of these drugs, since the desire to take the drugs often crosses from one drug to another. Studies have emphasized a role of the ventral striatum, especially the nucleus accumbens, in the actions of all drugs of abuse, although more recent studies have implicated larger regions of the forebrain. Induction of immediate-early genes has been studied extensively as a marker for activation of neurons in the central nervous system. In this review, we survey the literature reporting activation of immediate-early gene expression in the forebrain, in response to administration of drugs of abuse. All drugs of abuse activate immediate-early gene expression in the striatum, although each drug induces a particular neuroanatomical signature of activation. Most drugs of abuse activate immediate-early gene expression in several additional forebrain regions, including portions of the extended amygdala, cerebral cortex, lateral septum, and midline/intralaminar thalamic nuclei, although regional variations are found depending on the particular drug administered. Common neuropharmacological mechanisms responsible for activation of immediate-early gene expression in the forebrain involve dopaminergic and glutamatergic systems. Speculations on the biological significance and clinical relevance of immediate-early gene expression in response to drugs of abuse are presented.

Signal transduction by the c-Jun N-terminal kinase (JNK)--from inflammation to development

The c-Jun amino-terminal kinase (JNK) group of MAP kinases has been identified in mammals and insects. JNK is activated by exposure of cells to cytokines or environmental stress, indicating that this signaling pathway may contribute to inflammatory responses. Genetic and biochemical studies demonstrate that this signaling pathway also regulates cellular proliferation, apoptosis, and tissue morphogenesis. A functional role for JNK is therefore established in both the cellular response to stress and in many normal physiological processes.

A splicing variant of a death domain protein that is regulated by a mitogen-activated kinase is a substrate for c-Jun N-terminal kinase in the human central nervous system

The mitogen-activated kinase activating death domain protein (MADD) that is differentially expressed in neoplastic vs. normal cells (DENN) was identified as a substrate for c-Jun N-terminal kinase 3, the first demonstration of such an activity for this stress-activated kinase that is predominantly expressed in the brain. A splice isoform was identified that is a variant of MADD. A protein identical to MADD has been reported to be expressed differentially in neoplastic vs. normal cells and is termed "DENN." We demonstrated differential effects on DENN/MADD in a stressed vs. basal environment. Using in situ hybridization, we localized both the substrate and the kinase to large pyramidal neurons in the human hippocampus. It was interesting that, in four of four patients with neuropathologically confirmed acute hypoxic changes, we detected a unique translocation of DENN/MADD to the nucleolus. These changes were apparent only in neurons sensitive to hypoxia. Moreover, in those cells, translocation of the substrate was accompanied by nuclear translocation of JNK3. These findings place DENN/MADD and JNK in important hypoxia insult-induced intracellular signaling pathways. Our conclusions are important for future studies for understanding these stress-activated mechanisms.

Peripheral axotomy induces long-term c-Jun amino-terminal kinase-1 activation and activator protein-1 binding activity by c-Jun and junD in adult rat dorsal root ganglia In vivo

One of the earliest documented molecular events after sciatic nerve injury in adult rats is the rapid, long-term upregulation of the immediate early gene transcription factor c-Jun mRNA and protein in lumbar dorsal root ganglion (DRG) neurons, suggesting that c-Jun may regulate genes that are important both in the early post-injury period and during later peripheral axonal regeneration. However, neither the mechanism through which c-Jun protein is increased nor the level of its post-injury transcriptional activity in axotomized DRGs has been characterized. To determine whether transcriptional activation of c-Jun occurs in response to nerve injury in vivo and is associated with axonal regeneration, we have assayed axotomized adult rat DRGs for evidence of jun kinase activation, c-Jun phosphorylation, and activator protein-1 (AP-1) binding. We report that sciatic nerve transection resulted in chronic activation of c-Jun amino-terminal kinase-1 (JNK) in L4/L5 DRGs concomitant with c-Jun amino-terminal phosphorylation in neurons, and lasting AP-1 binding activity, with both c-Jun and JunD participating in DNA binding complexes. The timing of JNK activation was dependent on the distance of the axotomy site from the DRGs, suggesting the requirement for a retrograde transport-mediated signal. AP-1 binding and c-Jun protein returned to basal levels in DRGs as peripheral regeneration was completed but remained elevated in the case of chronic sprouting, indicating that c-Jun may regulate target genes that are involved in axonal outgrowth.

Absence of excitotoxicity-induced apoptosis in the hippocampus of mice lacking the Jnk3 gene

Excitatory amino acids induce both acute membrane depolarization and latent cellular toxicity, which often leads to apoptosis in many neurological disorders. Recent studies indicate that glutamate toxicity may involve the c-Jun amino-terminal kinase (JNK) group of mitogen-activated protein (MAP) kinases. One member of the JNK family, Jnk3, may be required for stress-induced neuronal apoptosis, as it is selectively expressed in the nervous system. Here we report that disruption of the gene encoding Jnk3 in mice caused the mice to be resistant to the excitotoxic glutamate-receptor agonist kainic acid: they showed a reduction in seizure activity and hippocampal neuron apoptosis was prevented. Although application of kainic acid imposed the same level of noxious stress, the phosphorylation of c-Jun and the transcriptional activity of the AP-1 transcription factor complex were markedly reduced in the mutant mice. These data indicate that the observed neuroprotection is due to the extinction of a Jnk3-mediated signalling pathway, which is an important component in the pathogenesis of glutamate neurotoxicity.

Induction of the dual specificity phosphatase PAC1 in rat brain following seizure activity

Recurrent seizure activity leads to delayed neuronal death as well as to inflammatory responses involving microglia in hippocampal subfields CA1, CA3 and CA4. Since mitogen activated protein (MAP) kinases control neuronal apoptosis and trigger generation of inflammatory cytokines, their activation state could determine seizure-related brain damage. PAC1 is a dual specificity protein phosphatase inactivating MAP kinases which we have found to be undetectable in normal brain. Despite this, kainic acid-induced seizure activity lead to rapid (approximately 3 h) but transient appearance of PAC1 mRNA in granule cells of the dentate gyrus as well as in pyramidal CA1 neurons. This pattern changed with time and after 2-3 days PAC1 was induced in dying CA1 and CA3 neurons. At this time PAC1 mRNA was also expressed in white matter microglia as well as in microglia invading the damaged hippocampus. PAC1 may play an important role controlling MAP kinase involvement in both neuronal death and neuro-inflammation following excitotoxic damage.