Developmentally regulated changes in c-Jun N-terminal kinase signalling determine the apoptotic response of oligodendrocyte lineage cells

c-Jun N-terminal kinase 1 (JNK1) modulates oligodendrocyte progenitor cell architecture, proliferation and myelination

During Central Nervous System ontogenesis, myelinating oligodendrocytes (OLs) arise from highly ramified and proliferative precursors called oligodendrocyte progenitor cells (OPCs). OPC architecture, proliferation and oligodendro-/myelino-genesis are finely regulated by the interplay of cell-intrinsic and extrinsic factors. A variety of extrinsic cues converge on the extracellular signal-regulated kinase/mitogen activated protein kinase (ERK/MAPK) pathway. Here we found that the germinal ablation of the MAPK c-Jun N-Terminal Kinase isoform 1 (JNK1) results in a significant reduction of myelin in the cerebral cortex and corpus callosum at both postnatal and adult stages. Myelin alterations are accompanied by higher OPC density and proliferation during the first weeks of life, consistent with a transient alteration of mechanisms regulating OPC self-renewal and differentiation. JNK1 KO OPCs also show smaller occupancy territories and a less complex branching architecture in vivo. Notably, these latter phenotypes are recapitulated in pure cultures of JNK1 KO OPCs and of WT OPCs treated with the JNK inhibitor D-JNKI-1. Moreover, JNK1 KO and WT D-JNKI-1 treated OLs, while not showing overt alterations of differentiation in vitro, display a reduced surface compared to controls. Our results unveil a novel player in the complex regulation of OPC biology, on the one hand showing that JNK1 ablation cell-autonomously determines alterations of OPC proliferation and branching architecture and, on the other hand, suggesting that JNK1 signaling in OLs participates in myelination in vivo.

The effects of short-term JNK inhibition on the survival and growth of aged sympathetic neurons

During the course of normal aging, certain populations of nerve growth factor (NGF)-responsive neurons become selectively vulnerable to cell death. Studies using dissociated neurons isolated from neonates have shown that c-Jun N-terminal kinases (JNKs) are important in regulating the survival and neurite outgrowth of NGF-responsive sympathetic neurons. Unlike neonatal neurons, adult sympathetic neurons are not dependent on NGF for their survival. Moreover, the NGF precursor, proNGF, is neurotoxic for aging but not young adult NGF-responsive neurons. Because of these age-related differences, the effects of JNK inhibition on the survival and growth of sympathetic neurons isolated from aged mice were studied. Aged neurons, as well as glia, were found to be dependent on JNK for their growth but not their survival. Conversely, proNGF neurotoxicity was JNK-dependent and mediated by the p75-interacting protein NRAGE, whereas neurite outgrowth was independent of NRAGE. These results have implications for the potential use of JNK inhibitors as therapies for ameliorating age-related neurodegenerative disease.

Specific inhibition of c-Jun N-terminal kinase delays preterm labour and reduces mortality

Preterm labour (PTL) is commonly associated with infection and/or inflammation. Lipopolysaccharide (LPS) from different bacteria can be used to independently or mutually activate Jun N-terminal kinase (JNK)/AP1- or NF-κB-driven inflammatory pathways that lead to PTL. Previous studies using Salmonella abortus LPS, which activates both JNK/AP-1 and NF-κB, showed that selective inhibition of NF-κB delays labour and improves pup outcome. Where labour is induced using Escherichia coli LPS (O111), which upregulates JNK/AP-1 but not NF-κB, inhibition of JNK/AP-1 activation also delays labour. In this study, to determine the potential role of JNK as a therapeutic target in PTL, we investigated the specific contribution of JNK signalling to S. Abortus LPS-induced PTL in mice. Intrauterine administration of S. Abortus LPS to pregnant mice resulted in the activation of JNK in the maternal uterus and fetal brain, upregulation of pro-inflammatory proteins COX-2, CXCL1, and CCL2, phosphorylation of cPLA2 in myometrium, and induction of PTL. Specific inhibition of JNK by co-administration of specific D-JNK inhibitory peptide (D-JNKI) delayed LPS-induced preterm delivery and reduced fetal mortality. This is associated with inhibition of myometrial cPLA2 phosphorylation and proinflammatory proteins synthesis. In addition, we report that D-JNKI inhibits the activation of JNK/JNK3 and caspase-3, which are important mediators of neural cell death in the neonatal brain. Our data demonstrate that specific inhibition of TLR4-activated JNK signalling pathways has potential as a therapeutic approach in the management of infection/inflammation-associated PTL and prevention of the associated detrimental effects to the neonatal brain.

TNFR1-JNK signaling is the shared pathway of neuroinflammation and neurovascular damage after LPS-sensitized hypoxic-ischemic injury in the immature brain

Background

Hypoxic-ischemia (HI) and inflammation are the two major pathogenic mechanisms of brain injury in very preterm infants. The neurovascular unit is the major target of HI injury in the immature brain. Systemic inflammation may worsen HI by up-regulating neuroinflammation and disrupting the blood–brain barrier (BBB). Since neurons and oligodendrocytes, microvascular endothelial cells, and microglia may closely interact with each other, there may be a common signaling pathway leading to neuroinflammation and neurovascular damage after injury in the immature brain. TNF-α is a key pro-inflammatory cytokine that acts through the TNF receptor (TNFR), and c-Jun N-terminal kinases (JNK) are important stress-responsive kinases.

Objective

To determine if TNFR1-JNK signaling is a shared pathway underlying neuroinflammation and neurovascular injury after lipopolysaccharide (LPS)-sensitized HI in the immature brain.

Methods

Postpartum (P) day-5 mice received LPS or normal saline (NS) injection before HI. Immunohistochemistry, immunoblotting and TNFR1- and TNFR2-knockout mouse pups were used to determine neuroinflammation, BBB damage, TNF-α expression, JNK activation, and cell apoptosis. The cellular distribution of p-JNK, TNFR1/TNFR2 and cleaved caspase-3 were examined using immunofluorescent staining.

Results

The LPS + HI group had significantly greater up-regulation of activated microglia, TNF-α and TNFR1 expression, and increases of BBB disruption and cleaved caspase-3 levels at 24 hours post-insult, and showed more cortical and white matter injury on P17 than the control and NS + HI groups. Cleaved caspase-3 was highly expressed in microvascular endothelial cells, neurons, and oligodendroglial precursor cells. LPS-sensitized HI also induced JNK activation and up-regulation of TNFR1 but not TNFR2 expression in the microglia, endothelial cells, neurons, and oligodendrocyte progenitors, and most of the TNFR1-positive cells co-expressed p-JNK. Etanercept (a TNF-α inhibitor) and AS601245 (a JNK inhibitor) protected against LPS-sensitized HI brain injury. The TNFR1-knockout but not TNFR2-knockout pups had significant reduction in JNK activation, attenuation of microglial activation, BBB breakdown and cleaved caspase-3 expression, and showed markedly less cortical and white matter injury than the wild-type pups after LPS-sensitized HI.

Conclusion

TNFR1-JNK signaling is the shared pathway leading to neuroinflammation and neurovascular damage after LPS-sensitized HI in the immature brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0215-2) contains supplementary material, which is available to authorized users.

Improving myelin/oligodendrocyte-related dysfunction: a new mechanism of antipsychotics in the treatment of schizophrenia?

Schizophrenia is a severe psychiatric disorder with complex clinical manifestations and its aetiological factors remain unclear. During the past decade, the oligodendrocyte-related myelin dysfunction was proposed as a hypothesis for schizophrenia, supported initially by a series of neuroimaging studies and genetic evidence. Recently, the effects of antipsychotics on myelination and oligodendroglial lineage development and their underlying molecular mechanisms were evaluated. Data from those studies suggest that the antipsychotics-resulting improvement in myelin/oligodendrocyte-related dysfunction may contribute, at least in part, to their therapeutic effect on schizophrenia. Importantly, these findings may provide the basis for a new insight into the therapeutic strategy by targeting the oligodendroglia lineage cells against schizophrenia.

Rosiglitazone negatively regulates c-Jun N-terminal kinase and toll-like receptor 4 proinflammatory signalling during initiation of experimental aortic aneurysms

OBJECTIVE

Development and rupture of aortic aneurysms (AA) is a complex process involving inflammation, cell death, tissue and matrix remodelling. The thiazolidinediones (TZDs) including Rosiglitazone (RGZ) are a family of drugs which act as agonists of the nuclear peroxisome proliferator-activated receptors and have a broad spectrum of effects on a number of biological processes in the cardiovascular system. In our previous study we have demonstrated that RGZ has a marked effect on both aneurysm rupture and development, however, the precise mechanism of this is unknown.

METHODS AND RESULTS

In the present study, we examined possible targets of RGZ action in the early stages of Angiotensin II-induced AA in apolipoprotein E-deficient mice. For this purpose we employed immunoblotting, ELISA and antibody array approaches. We found that RGZ significantly inhibited c-Jun N-terminal kinase (JNK) phosphorylation and down-regulated toll-like receptor 4 (TLR4) expression at the site of lesion formation in response to Angiotensin II infusion in the initiation stage (6-72 h) of experimental AA development. Importantly, this effect was also associated with a decrease of CD4 antigen and reduction in production of TLR4/JNK-dependant proinflammatory chemokines MCP-1 and MIP-1α.

CONCLUSION

These data suggest that RGZ can modulate inflammatory processes by blocking TLR4/JNK signalling in initiation stages of AA development.

JNK signaling is the shared pathway linking neuroinflammation, blood-brain barrier disruption, and oligodendroglial apoptosis in the white matter injury of the immature brain

BACKGROUND

White matter injury is the major form of brain damage in very preterm infants. Selective white matter injury in the immature brain can be induced by lipopolysaccharide (LPS)-sensitized hypoxic-ischemia (HI) in the postpartum (P) day 2 rat pups whose brain maturation status is equivalent to that in preterm infants less than 30 weeks of gestation. Neuroinflammation, blood-brain barrier (BBB) damage and oligodendrocyte progenitor apoptosis may affect the susceptibility of LPS-sensitized HI in white matter injury. c-Jun N-terminal kinases (JNK) are important stress-responsive kinases in various forms of insults. We hypothesized that LPS-sensitized HI causes white matter injury through JNK activation-mediated neuroinflammation, BBB leakage and oligodendroglial apoptosis in the white matter of P2 rat pups.

METHODS

P2 pups received LPS (0.05 mg/kg) or normal saline injection followed by 90-min HI. Immunohistochemistry and immunoblotting were used to determine microglia activation, TNF-α, BBB damage, cleaved caspase-3, JNK and phospho-JNK (p-JNK), myelin basic protein (MBP), and glial fibrillary acidic protein (GFAP) expression. Immunofluorescence was performed to determine the cellular distribution of p-JNK. Pharmacological and genetic approaches were used to inhibit JNK activity.

RESULTS

P2 pups had selective white matter injury associated with upregulation of activated microglia, TNF-α, IgG extravasation and oligodendroglial progenitor apoptosis after LPS-sensitized HI. Immunohistochemical analyses showed early and sustained JNK activation in the white matter at 6 and 24 h post-insult. Immunofluorescence demonstrated upregulation of p-JNK in activated microglia, vascular endothelial cells and oligodendrocyte progenitors, and also showed perivascular aggregation of p-JNK-positive cells around the vessels 24 h post-insult. JNK inhibition by AS601245 or by antisense oligodeoxynucleotides (ODN) significantly reduced microglial activation, TNF-α immunoreactivity, IgG extravasation, and cleaved caspase-3 in the endothelial cells and oligodendrocyte progenitors, and also attenuated perivascular aggregation of p-JNK-positive cells 24 h post-insult. The AS601245 or JNK antisense ODN group had significantly increased MBP and decreased GFAP expression in the white matter on P11 than the vehicle or scrambled ODN group.

CONCLUSIONS

LPS-sensitized HI causes white matter injury through JNK activation-mediated upregulation of neuroinflammation, BBB leakage and oligodendrocyte progenitor apoptosis in the immature brain.

Injury and differentiation following inhibition of mitochondrial respiratory chain complex IV in rat oligodendrocytes

Oligodendrocyte lineage cells are susceptible to a variety of insults including hypoxia, excitotoxicity, and reactive oxygen species. Demyelination is a well-recognized feature of several CNS disorders including multiple sclerosis, white matter strokes, progressive multifocal leukoencephalopathy, and disorders due to mitochondrial DNA mutations. Although mitochondria have been implicated in the demise of oligodendrocyte lineage cells, the consequences of mitochondrial respiratory chain defects have not been examined. We determine the in vitro impact of established inhibitors of mitochondrial respiratory chain complex IV or cytochrome c oxidase on oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes as well as on differentiation capacity of OPCs from P0 rat. Injury to mature oligodendrocytes following complex IV inhibition was significantly greater than to OPCs, judged by cell detachment and mitochondrial membrane potential (MMP) changes, although viability of cells that remained attached was not compromised. Active mitochondria were abundant in processes of differentiated oligodendrocytes and MMP was significantly greater in differentiated oligodendrocytes than OPCs. MMP dissipated following complex IV inhibition in oligodendrocytes. Furthermore, complex IV inhibition impaired process formation within oligodendrocyte lineage cells. Injury to and impaired process formation of oligodendrocytes following complex IV inhibition has potentially important implications for the pathogenesis and repair of CNS myelin disorders. © 2010 Wiley-Liss, Inc.

Attenuation of proliferation in oligodendrocyte precursor cells by activated microglia

Activated microglia can influence the survival of neural cells through the release of cytotoxic factors. Here, we investigated the interaction between Toll-like receptor 4 (TLR4)-activated microglia and oligodendrocytes or their precursor cells (OPC). Primary rat or N9 microglial cells were activated by exposure to TLR4-specifc lipopolysaccharide (LPS), resulting in mitogen-activated protein kinase activation, increased CD68 and inducible nitric oxide synthase expression, and release of the proinflammatory cytokines tumor necrosis factor (TNF) and interleukin-6 (IL-6). Microglial conditioned medium (MGCM) from LPS-activated microglia attenuated primary OPC proliferation without inducing cell death. The microglial-induced inhibition of OPC proliferation was reversed by stimulating group III metabotropic glutamate receptors in microglia with the agonist L-AP4. In contrast to OPC, LPS-activated MGCM enhanced the survival of mature oligodendrocytes. Further investigation suggested that TNF and IL-6 released from TLR4-activated microglia might contribute to the effect of MGCM on OPC proliferation, insofar as TNF depletion of LPS-activated MGCM reduced the inhibition of OPC proliferation, and direct addition of TNF or IL-6 attenuated or increased proliferation, respectively. OPC themselves were also found to express proteins involved in TLR4 signalling, including TLR4, MyD88, and MAL. Although LPS stimulation of OPC did not induce proinflammatory cytokine release or affect their survival, it did trigger JNK phosphorylation, suggesting that TLR4 signalling in these cells is active. These findings suggest that OPC survival may be influenced not only by factors released from endotoxin-activated microglia but also through a direct response to endotoxins. This may have consequences for myelination under conditions in which microglial activation and cerebral infection are both implicated. , Inc.

Paradoxical dysregulation of the neural stem cell pathway sonic hedgehog-Gli1 in autoimmune encephalomyelitis and multiple sclerosis

OBJECTIVE

Neurovascular niches have been proposed as critical components of the neural stem cell (NSC) response to acute central nervous system injury; however, it is unclear whether these potential reparative niches remain functional during chronic injury. Here, we asked how central nervous system inflammatory injury regulates the intrinsic properties of NSCs and their niches.

METHODS

We investigated the sonic hedgehog (Shh)-Gli1 pathway, an important signaling pathway for NSCs, in experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis (MS), and its regulation by inflammatory cytokines.

RESULTS

We show that Shh is markedly upregulated by reactive and perivascular astroglia in areas of injury in MS lesions and during EAE. Astroglia outside the subventricular zone niche can support NSC differentiation toward neurons and oligodendrocytes, and Shh is a critical mediator of this effect. Shh induces differential upregulation of the transcription factor Gli1, which mediates Shh-induced NSC differentiation. However, despite the increase in Shh and the fact that Gli1 was initially increased during early inflammation of EAE and active lesions of MS, Gli1 was significantly decreased in spinal cord oligodendrocyte precursor cells after onset of EAE, and in chronic active and inactive lesions from MS brain. The Th1 cytokine interferon-gamma was unique in inducing Shh expression in astroglia and NSCs, while paradoxically suppressing Gli1 expression in NSCs and inhibiting Shh-mediated NSC differentiation.

INTERPRETATION

Our data suggest that endogenous repair potential during chronic injury appears to be limited by inflammation-induced alterations in intrinsic NSC molecular pathways such as Gli1.

c-Jun expression, activation and function in neural cell death, inflammation and repair

Up-regulation of c-Jun is a common event in the developing, adult as well as in injured nervous system that serves as a model of transcriptional control of brain function. Functional studies employing in vivo strategies using gene deletion, targeted expression of dominant negative isoforms and pharmacological inhibitors all suggest a three pronged role of c-Jun action, exercising control over neural cell death and degeneration, in gliosis and inflammation as well as in plasticity and repair. In vitro, structural and molecular studies reveal several non-overlapping activation cascades via N-terminal c-Jun phosphorylation at serine 63 and 73 (Ser63, Ser73), and threonine 91 and 93 (Thr91, Thr93) residues, the dephosphorylation at Thr239, the p300-mediated lysine acetylation of the near C-terminal region (Lys268, Lys271, Lys 273), as well as the Jun-independent activities of the Jun N-terminal family of serine/threonine kinases, that regulate the different and disparate cellular responses. A better understanding of these non-overlapping roles in vivo could considerably increase the potential of pharmacological agents to improve neurological outcome following trauma, neonatal encephalopathy and stroke, as well as in neurodegenerative disease.

Maturation-dependent sensitivity of oligodendrocyte lineage cells to apoptosis: implications for normal development and disease

Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stage-specific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease.

Deletion of the c-Jun N-terminal kinase 3 gene protects neonatal mice against cerebral hypoxic-ischaemic injury

c-Jun N-terminal kinase 3 (JNK3) is a member of the stress-activated group of mitogen-activated protein kinases. c-Jun N-terminal kinase 3 is a potent mediator of apoptosis and the use of JNK inhibitors or jnk3 gene deletion each protect against brain injury in adults. However, little is known about the role of JNK3 or its mechanism of action in neonatal brain injury. The aim of the present study was to compare the vulnerability of neonatal JNK3 knockout (JNK3 KO) mice and wild-type (WT) mice to cerebral hypoxic-ischaemic injury (HII) using unilateral-carotid occlusion combined with transient hypoxia. The degree of neural tissue loss in JNK3 KO mice was substantially reduced compared with WT mice (JNK3 KO 27.8%+/-2.8% versus WT 48.3%+/-2.0%, P<or=0.0001) after HII. Significant attenuation of injury was observed in the cerebral cortex, hippocampus, striatum, and thalamus of JNK3 KO compared with WT mice. Hypoxic-ischaemic injury increased JNK phosphorylation and activity, with JNK3 as the major isoform. Significantly, in JNK3 KO animals there was no difference in the activation of the upstream kinases mitogen-activated protein kinase kinase (MKK4) or MKK7. Downstream of JNK3, HII lead to increased phosphorylation of the transcription factors c-Jun and adenovirus transcription factor-2 (ATF-2), which was attenuated in JNK3 KO mice. c-Jun N-terminal kinase 3 deletion also decrease caspase-3 cleavage and Bim/PUMA expression, coupled with a upregulation of AKT/FOXO3a levels, linking JNK3 to apoptosis. These findings implicate JNK3 involvement in neural cell loss resulting from cerebral HII in the developing brain.