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

Critical Roles of Dual-Specificity Phosphatases in Neuronal Proteostasis and Neurological Diseases

Protein homeostasis or proteostasis is a fundamental cellular property that encompasses the dynamic balancing of processes in the proteostasis network (PN). Such processes include protein synthesis, folding, and degradation in both non-stressed and stressful conditions. The role of the PN in neurodegenerative disease is well-documented, where it is known to respond to changes in protein folding states or toxic gain-of-function protein aggregation. Dual-specificity phosphatases have recently emerged as important participants in maintaining balance within the PN, acting through modulation of cellular signaling pathways that are involved in neurodegeneration. In this review, we will summarize recent findings describing the roles of dual-specificity phosphatases in neurodegeneration and offer perspectives on future therapeutic directions.

SRF modulates seizure occurrence, activity induced gene transcription and hippocampal circuit reorganization in the mouse pilocarpine epilepsy model

A hallmark of temporal lobe epilepsy (TLE) is hippocampal neuronal demise and aberrant mossy fiber sprouting. In addition, unrestrained neuronal activity in TLE patients induces gene expression including immediate early genes (IEGs) such as Fos and Egr1.

We employed the mouse pilocarpine model to analyze the transcription factor (TF) serum response factor (SRF) in epileptogenesis, seizure induced histopathology and IEG induction. SRF is a neuronal activity regulated TF stimulating IEG expression as well as nerve fiber growth and guidance. Adult conditional SRF deficient mice (Srf^CaMKCreERT2) were more refractory to initial status epilepticus (SE) acquisition. Further, SRF deficient mice developed more spontaneous recurrent seizures (SRS). Genome-wide transcriptomic analysis uncovered a requirement of SRF for SE and SRS induced IEG induction (e.g. Fos, Egr1, Arc, Npas4, Btg2, Atf3). SRF was required for epilepsy associated neurodegeneration, mossy fiber sprouting and inflammation. We uncovered MAP kinase signaling as SRF target during epilepsy. Upon SRF ablation, seizure evoked induction of dual specific phosphatases (Dusp5 and Dusp6) was reduced. Lower expression of these negative ERK kinase regulators correlated with altered P-ERK levels in epileptic Srf mutant animals.

Overall, this study uncovered an SRF contribution to several processes of epileptogenesis in the pilocarpine model.

Dual-specificity phosphatases as molecular targets for inhibition in human disease

SIGNIFICANCE

The dual-specificity phosphatases (DUSPs) constitute a heterogeneous group of cysteine-based protein tyrosine phosphatases, whose members exert a pivotal role in cell physiology by dephosphorylation of phosphoserine, phosphothreonine, and phosphotyrosine residues from proteins, as well as other non-proteinaceous substrates.

RECENT ADVANCES

A picture is emerging in which a selected group of DUSP enzymes display overexpression or hyperactivity that is associated with human disease, especially human cancer, making feasible targeted therapy approaches based on their inhibition. A panoply of molecular and functional studies on DUSPs have been performed in the previous years, and drug-discovery efforts are ongoing to develop specific and efficient DUSP enzyme inhibitors. This review summarizes the current status on inhibitory compounds targeting DUSPs that belong to the MAP kinase phosphatases-, small-sized atypical-, and phosphatases of regenerating liver subfamilies, whose inhibition could be beneficial for the prevention or mitigation of human disease.

CRITICAL ISSUES

Achieving specificity, potency, and bioavailability are the major challenges in the discovery of DUSP inhibitors for the clinics. Clinical validation of compounds or alternative inhibitory strategies of DUSP inhibition has yet to come.

FUTURE DIRECTIONS

Further work is required to understand the dual role of many DUSPs in human cancer, their function-structure properties, and to identify their physiologic substrates. This will help in the implementation of therapies based on DUSPs inhibition.

Dual-specificity phosphatases 2: surprising positive effect at the molecular level and a potential biomarker of diseases

Dual-specificity phosphatases (DUSPs) is an emerging subclass of the protein tyrosine phosphatase gene superfamily, a heterogeneous group of protein phosphatases that can dephosphorylate both phosphotyrosine and phosphoserine/phosphothreonine residues within the one substrate. Recently, a series of investigations of DUSPs defined their essential roles in cell proliferation, cancer and the immune response. This review will focus on DUSP2, its involvement in different diseases and its potential as a therapeutic target.

Analysis of potential biomarkers and modifier genes affecting the clinical course of CLN3 disease

Mutations in the CLN3 gene lead to juvenile neuronal ceroid lipofuscinosis, a pediatric neurodegenerative disorder characterized by visual loss, epilepsy and psychomotor deterioration. Although most CLN3 patients carry the same 1-kb deletion in the CLN3 gene, their disease phenotype can be variable. The aims of this study were to (i) study the clinical phenotype in CLN3 patients with identical genotype, (ii) identify genes that are dysregulated in CLN3 disease regardless of the clinical course that could be useful as biomarkers, and (iii) find modifier genes that affect the progression rate of the disease. A total of 25 CLN3 patients homozygous for the 1-kb deletion were classified into groups with rapid, average or slow disease progression using an established clinical scoring system. Genome-wide expression profiling was performed in eight CLN3 patients with different disease progression and matched controls. The study showed high phenotype variability in CLN3 patients. Five genes were dysregulated in all CLN3 patients and present candidate biomarkers of the disease. Of those, dual specificity phosphatase 2 (DUSP2) was also validated in acutely CLN3-depleted cell models and in CbCln3(Δex7/8) cerebellar precursor cells. A total of 13 genes were upregulated in patients with rapid disease progression and downregulated in patients with slow disease progression; one gene showed dysregulation in the opposite way. Among these potential modifier genes, guanine nucleotide exchange factor 1 for small GTPases of the Ras family (RAPGEF1) and transcription factor Spi-B (SPIB) were validated in an acutely CLN3-depleted cell model. These findings indicate that differential perturbations of distinct signaling pathways might alter disease progression and provide insight into the molecular alterations underlying neuronal dysfunction in CLN3 disease and neurodegeneration in general.

Genetic complementation screen identifies a mitogen-activated protein kinase phosphatase, MKP3, as a regulator of dopamine transporter trafficking

The antidepressant and cocaine sensitive plasma membrane monoamine transporters are the primary mechanism for clearance of their respective neurotransmitters and serve a pivotal role in limiting monoamine neurotransmission. To identify molecules in pathways that regulate dopamine transporter (DAT) internalization, we used a genetic complementation screen in Xenopus oocytes to identify a mitogen-activated protein (MAP) kinase phosphatase, MKP3/Pyst1/DUSP6, as a molecule that inhibits protein kinase C-induced (PKC) internalization of transporters, resulting in enhanced DAT activity. The involvement of MKP3 in DAT internalization was verified using both overexpression and shRNA knockdown strategies in mammalian cell models including a dopaminergic cell line. Although the isolation of MKP3 implies a role for MAP kinases in DAT internalization, MAP kinase inhibitors have no effect on internalization. Moreover, PKC-dependent down-regulation of DAT does not correlate with the phosphorylation state of several well-studied MAP kinases (ERK1/2, p38, and SAPK/JNK). We also show that MKP3 does not regulate PKC-induced ubiquitylation of DAT but acts at a more downstream step to stabilize DAT at the cell surface by blocking dynamin-dependent internalization and delaying the targeting of DAT for degradation. These results indicate that MKP3 can act to enhance DAT function and identifies MKP3 as a phosphatase involved in regulating dynamin-dependent endocytosis.

Extracellular ATP counteracts the ERK1/2-mediated death-promoting signaling cascades in astrocytes

Oxidative stress is the main cause of neuronal death in pathological conditions. Hydrogen peroxide (H(2)O(2)), one of the reactive oxygen species, activates many intracellular signaling cascades including src family and mitogen-activated protein kinases (MAPKs), some of which are critically involved in the induction of cellular damage. We previously showed that H(2)O(2)-induced cell death in astrocytes and adenosine 5(')-triphosphate (ATP), acting on P2Y(1) receptors, had a protective effect. Here, we examined the H(2)O(2)-induced changes in intracellular signaling cascades that promote cell death in astrocytes, showing the molecular mechanisms by which the activation of P2Y(1) receptors counteracts such signals. Although H(2)O(2) activated three MAPKs including ERK1/2, p38, and JNK, only the activation of ERK1/2 participated in the H(2)O(2)-evoked cell death. H(2)O(2) induced a sustained activation of ERK1/2 mainly in the nucleus region, which was well in accordance with the H(2)O(2)-induced cell death. H(2)O(2) also activated the src tyrosine kinase family, which was an upstream signal for ERK1/2. Activation of P2Y(1) receptors by 2methylthio-ADP (2MeSADP) inhibited the H(2)O(2)-evoked activation of src tyrosine kinase, resulting in the inhibition of the phosphorylated-ERK1/2 accumulation in the nucleus. 2MeSADP enhanced the gene expression and activity of protein tyrosine phosphatase (PTP), which was responsible for the inhibition of src tyrosine kinase. Thioredoxin reductase, another cytoprotective gene we previously showed to be upregulated by 2MeSADP, also controlled the activity of PTP. Taken together, ATP, acting on P2Y(1) receptors, upregulates the PTP expression and its activity, which counteracts the H(2)O(2)-promoted death signaling cascades including ERK1/2 and its upstream signal src tyrosine kinase in astrocytes.

Alzheimer's disease beta-secretase BACE1 is not a neuron-specific enzyme

The brains of Alzheimer's disease (AD) patients are morphologically characterized by neurofibrillar abnormalities and by parenchymal and cerebrovascular deposits of beta-amyloid peptides. The generation of beta-amyloid peptides by proteolytical processing of the amyloid precursor protein (APP) requires the enzymatic activity of the beta-site APP cleaving enzyme 1 (BACE1). The expression of this enzyme has been localized to the brain, in particular to neurons, indicating that neurons are the major source of beta-amyloid peptides in brain. Astrocytes, on the contrary, are known to be important for beta-amyloid clearance and degradation, for providing trophic support to neurons, and for forming a protective barrier between beta-amyloid deposits and neurons. However, under certain conditions related to chronic stress, the role of astrocytes may not be beneficial. Here we present evidence demonstrating that astrocytes are an alternative source of BACE1 and therefore may contribute to beta-amyloid plaque formation. While resting astroyctes in brain do not express BACE1 at detectable levels, cultured astrocytes display BACE1 promoter activity and express BACE1 mRNA and enzymatically active BACE1 protein. Additionally, in animal models of chronic gliosis and in brains of AD patients, there is BACE1 expression in reactive astrocytes. This would suggest that the mechanism for astrocyte activation plays a role in the development of AD and that therapeutic strategies that target astrocyte activation in brain may be beneficial for the treatment of AD. Also, there are differences in responses to chronic versus acute stress, suggesting that one consequence of chronic stress is an incremental shift to different phenotypic cellular states.

Zac1 is up-regulated in neural cells of the limbic system of mouse brain following seizures that provoke strong cell activation

Zac1, a new zinc-finger protein that regulates both apoptosis and cell cycle arrest, is abundantly expressed in many proliferative/differentiation areas during brain development. In the present work, we studied Zac1 gene expression and protein in experimental seizure models following i.p. injection of pentylenetetrazole (PTZ) or kainic acid (KA). Following KA treatment, an early and intense up-regulation of Zac1 is detected in the limbic areas, such as the hippocampus, cortex and amygdaloid and hypothalamic nuclei. Pre-treatment with MK-801, an antagonist of the NMDA receptors, fully blocks the effect of KA in the hippocampus, whereas it only attenuates KA-induced Zac1 up-regulation in the other areas of the limbic system. A reduced induction is obtained with PTZ-treated animals, specifically in the entorhinal and piriform cortices as well as in amygdaloid and hypothalamic nuclei. Thus, Zac1 is highly induced in the seizure models that generate strong neuronal stimulation and/or extensive cell damage (cell death), reinforcing its putative role in the control of the cell cycle and/or apoptosis. Moreover, strong induction is observed in the granular cells of the dentate gyrus (which are resistant to neurodegeneration) and in some glial cells of the dentate gyrus and subventricular zone, suggesting that Zac1 may be implicated in the mechanisms of neural plasticity following injury.

Characterization of a murine gene encoding a developmentally regulated cytoplasmic dual-specificity mitogen-activated protein kinase phosphatase

Mitogen-activated protein kinases (MAPKs) play a vital role in cellular growth control, but far less is known about these signalling pathways in the context of embryonic development. Duration and magnitude of MAPK activation are crucial factors in cell fate decisions, and reflect a balance between the activities of upstream activators and specific MAPK phosphatases (MKPs). Here, we report the isolation and characterization of the murine Pyst3 gene, which encodes a cytosolic dual-specificity MKP. This enzyme selectively interacts with, and is catalytically activated by, the 'classical' extracellular signal-regulated kinases (ERKs) 1 and 2 and, to a lesser extent, the stress-activated MAPK p38alpha. These properties define the ability of this enzyme to dephosphorylate and inactivate ERK1/2 and p38alpha, but not JNK (c-Jun N-terminal kinase) in vivo. When expressed in mammalian cells, the Pyst3 protein is predominantly cytoplasmic. Furthermore, leptomycin B causes a complete redistribution of the protein to the nucleus, implicating a CRM (chromosomal region maintenance)1/exportin 1-dependent nuclear export signal in determining the subcellular localization of this enzyme. Finally, whole-mount in situ hybridization studies in mouse embryos reveal that the Pyst3 gene is expressed specifically in the placenta, developing liver and in migratory muscle cells. Our results suggest that this enzyme may have a critical role in regulating the activity of MAPK signalling during early development and organogenesis.

Regulation of stress-activated protein kinase signaling pathways by protein phosphatases

Stress-activated protein kinase (SAPK) signaling plays essential roles in eliciting adequate cellular responses to stresses and proinflammatory cytokines. SAPK pathways are composed of three successive protein kinase reactions. The phosphorylation of SAPK signaling components on Ser/Thr or Thr/Tyr residues suggests the involvement of various protein phosphatases in the negative regulation of these systems. Accumulating evidence indicates that three families of protein phosphatases, namely the Ser/Thr phosphatases, the Tyr phosphatases and the dual specificity Ser/Thr/Tyr phosphatases regulate these pathways, each mediating a distinct function. Differences in substrate specificities and regulatory mechanisms for these phosphatases form the molecular basis for the complex regulation of SAPK signaling. Here we describe the properties of the protein phosphatases responsible for the regulation of SAPK signaling pathways.

c-Jun N-terminal kinase-3 (JNK3)/stress-activated protein kinase-beta (SAPKbeta) binds and phosphorylates the neuronal microtubule regulator SCG10

The neuronal growth-associated protein SCG10 is enriched in the growth cones of neurons where it destabilizes microtubules and thus contributes to the dynamic assembly and disassembly of microtubules. Since its microtubule-destabilizing activity is regulated by phosphorylation, SCG10 may link extracellular signals to rearrangements of the neuronal cytoskeleton. To identify signal transduction pathways that may lead to SCG10 phosphorylation, we tested a series of serine-threonine-directed protein kinases that phosphorylate SCG10 in vitro. We demonstrate that purified SCG10 can be phosphorylated by two subclasses of mitogen-activated protein (MAP) kinases, c-Jun N-terminal/stress-activated protein kinase (JNK/SAPK) and p38 MAP kinase. Moreover, SCG10 was found to bind tightly and specifically to JNK3/SAPKbeta. JNK3/SAPKbeta phosphorylation occurs at Ser-62 and Ser-73, residues that result in reduced microtubule-destabilizing activity for SCG10. Endogenous SCG10 also undergoes increased phosphorylation in sympathetic neurons at times of JNK3/SAPKbeta activation following deprivation from nerve growth factor. Together these observations indicate that activation of JNK/SAPKs provides a pathway for phosphorylation of SCG10 and control of growth cone microtubule formation following neuronal exposure to cellular stresses.

Odor regulates the expression of the mitogen-activated protein kinase phosphatase gene hVH-5 in bilateral entorhinal cortex-lesioned rats

Since it is known that several immediate early genes are induced by olfactory stimuli, we determined whether an olfactory stimulus also induces the expression of the mitogen-activated protein kinase (MAPK) phosphatase gene hVH-5 (homologue of vaccinia virus H1 phosphatase gene, clone 5), a member of a novel class of immediate early genes encoding dual-specificity protein phosphatases. The expression was studied by in situ hybridization in different brain structures involved in odor processing, in control and bilateral entorhinal cortex (EC) lesioned rats. EC-lesion did not significantly affect hVH-5 gene expression in the glomerular cell layer of the olfactory bulb (OB), while odor stimulation induced it in both control and EC-lesioned groups. In contrast, odor-induced expression of hVH-5 gene in mitral/granular cell layers was only evident after lesion of the EC. Similar results were obtained in the piriform cortex (PCx), a structure intimately connected to the mitral cell layer. In the CA1 hippocampal subfield, odor stimulation induced hVH-5 gene expression in both control and EC-lesioned animals, the increase being potentiated in lesioned rats. CA3 and dentate gyrus exhibited a similar pattern of gene expression, the odor stimulating gene expression in both control and lesioned groups. The amygdala (Am) displayed no significant change. It appears that through the induction of a MAPK phosphatase, the EC controls MAPK activities differently after odor stimulation in OB, PCx and hippocampus (Hip). The results illustrate the notion that odor representation in the brain requires plastic modifications at both anatomical and functional levels.

Apolipoprotein E expression by neurons surviving excitotoxic stress

In the adult brain, apolipoprotein E (apoE) mRNA is thought to be expressed by nonneuronal cells. Yet, when a brain damage has occurred, the protein is found in neurons. We have studied apoE expression following systemic kainic acid (KA), injected in rats to induce hippocampal neurodegeneration. We describe two effects. First, a moderate increase of apoE levels in astrocytes. Second, and unexpected, a very strong increase of apoE mRNA levels in clusters of CA1 and CA3 pyramidal neurons. Neuronal identity of these cells is supported by a series of observations. First, apoE hybridization signals were found in cells with morphological characteristics of pyramidal neurons. Second, the cells were positive for the neuronal marker MAP2. Third, the cells were negative for the astrocytic marker GFAP and for the microglia marker OX42. Fourth, the same distribution pattern was found with probes hybridizing to c-fos, a transcription factor transiently expressed in neurons under stress. At 48 and 72 h following KA, most of the excitotoxic cell death had already occurred. Since no morphological signs of programmed cell death were observed in apoE-positive pyramidal neurons, we suggest that expression of apoE by neurons may be part of a rescue program to counteract neurodegeneration.

Cocaine and fluoxetine induce the expression of the hVH-5 gene encoding a MAP kinase phosphatase

A novel class of immediate early genes that encode enzymes of the MAP kinase phosphatase family has recently been described. These enzymes are dual-specificity protein phosphatases and some show tissue-specific distribution, like the hVH-5 gene (homologue of vaccinia virus H1 phosphatase gene clone 5), which is expressed predominantly in the adult brain. In this paper, we investigated whether the hVH-5 gene is induced by psychostimulants in rat brain, as has been demonstrated for immediate early genes encoding transcription factors. Using in situ hybridization, we found that i.p. injection of cocaine, amphetamine and caffeine induced hVH-5 mRNA expression within 40 min in the nucleus accumbens (NAc), caudate putamen, frontal cortex and hippocampus, with a maximal effect in the NAc. The cocaine-induced hVH-5 gene induction involves the serotonergic system, since it was abolished in the NAc by lesioning serotonergic raphé projections with 5,7-dihydroxytryptamine. Moreover, the effect of cocaine was fully mimicked by the selective serotonin uptake inhibitor fluoxetine. In contrast to what has been described for c-fos and egr-1 immediate early genes, we found that hVH-5 mRNA expression in the NAc and hippocampus was as significant after repeated cocaine injections for 10 days as after a single injection. The considerable and prolonged induction of the MAP kinase phosphatase hVH-5 gene by psychostimulant drugs in postmitotic brain cells, particularly in the NAc, could indicate that MAP kinase substrates are involved in the reinforcing properties of drugs of abuse.