Microglial activation induces cell death, inhibits neurite outgrowth and causes neurite retraction of differentiated neuroblastoma cells

SRY-box containing gene 11 (Sox11) transcription factor is required for neuron survival and neurite growth

The transcription factor Sox11 is expressed at high levels in developing sensory neurons and injured adult neurons but little is known about its transcriptional targets and function. In this study we examined the role of Sox11 using Neuro2a neuroblastoma cells and cultured mouse dorsal root ganglia (DRG) neurons. Results show Sox11 has an essential role in regulation of neuron survival and neurite outgrowth in Neuro2a cells and primary sensory neurons. Neuro2a cells increase expression of Sox11 as they differentiate in culture. Following addition of 20 microM retinoic acid (RA), a stimulus for differentiation that enhances neurite growth and differentiation, Sox11 level rises. RNAi-mediated knockdown of Sox11 in RA-differentiated Neuro2a cells caused a decrease in neurite growth and an increase in the percent of apoptotic cells. RNA expression analysis showed that Sox11 knockdown modulated the level of mRNAs encoding several genes related to cell survival and death. Further validation in the Neuro2a model showed Sox11 knockdown increased expression of the pro-apoptotic gene BNIP3 (BclII interacting protein 1 NIP3) and decreased expression of the anti-apoptotic gene TANK (TNF receptor-associated factor family member-associated NFkappaB activator). Cultured primary DRG neurons also express Sox11 and treatment with Sox11 small interfering RNA (siRNA) caused a significant decrease in neurite growth and branching and a decrease in mRNA encoding actin-related protein complex 3 (Arpc3), an actin organizing protein that may be involved in axon growth. The percent of apoptotic neurons also increased in cultures of DRG neurons treated with Sox11 siRNA. Similar to Neuro2a cells, a decrease in TANK gene expression occurred, suggesting at least some overlap in Sox11 transcriptional targets in Neuro2a and DRG neurons. These data are consistent with a central role for Sox11 in regulating events that promote neurite growth and neuron survival.

Involvement of nitric oxide synthase and ROS-mediated activation of L-type voltage-gated Ca2+ channels in NMDA-induced DPYSL3 degradation

Dihydropyrimidinase-like 3 (DPYSL3), a member of TUC (TOAD-64/Ulip/CRMP), is believed to play a role in neuronal differentiation, axonal outgrowth and possibly in neuronal regeneration. Recently, we have shown that in primary cortical neurons (PCN) NMDA and oxidative stress (H(2)O(2)) caused a calpain-dependent cleavage of DPYSL3 (62 kDa) resulting in the appearance of a lower molecular weight form (60 kDa) of DPYSL3. Our preliminary results had shown that antioxidants significantly reduced NMDA-induced DPYSL3 degradation, indicating involvement of ROS in calpain activation. The aim of this study was to investigate the possible involvement of NOS in NMDA-induced DPYSL3 degradation. We found that NOS inhibitor (L-NAME) significantly prevented NMDA-induced ROS formation, as well as intracellular Ca(2+) increase [Ca(2+)](i), DPYSL3 degradation and cell death. Further, exposure of PCN to NO donor (SNP) resulted in significant [Ca(2+)](i) increase, ROS generation and probable calpain-mediated DPYSL3 truncation. The NMDA- and oxidative stress (ROS)-induced DPYSL3 truncation was totally dependent on extracellular [Ca(2+)](i). While NMDA-induced DPYSL3 truncation was blocked by both NMDA receptor antagonist (MK801) [Kowara, R., Chen, Q., Milliken, M., Chakravarthy, B., 2005. Calpain-mediated degradation of dihydropyrimidinase-like 3 protein (DPYSL3) in response to NMDA and H(2)O(2) toxicity. J. Neurochem. 95 (2), 466-474] and L-VGCC (nimodipine) inhibitors, H(2)O(2)-induced increase in [Ca(2+)](i), ROS generation and DPYSL3 truncation was blocked only by nimodipine. These results indicate that changes in Ca(2+) homeostasis resulting from ROS-dependent activation of L-VGCC are sufficient to induce probable calpain-mediated DPYSL3 truncation and demonstrate for the first time the role of ROS in the mechanism leading to glutamate-induced calpain activation and DPYSL3 protein degradation. The probable calpain-mediated DPYSL3 truncation may have significant impact on its interaction with actin and its assembly, and in turn on growth cone integrity.

Microglia induce neural cell death via a proximity-dependent mechanism involving nitric oxide

Microglial cells play a major role in the pathogenesis of many neurological diseases by exacerbating neuronal and non-neuronal cell death, but the mechanisms involved are unclear. To investigate the microglial-neuronal interactions, we used the murine BV-2 microglial cell line and the human neuronal-like SK-N-SH neuroblastoma cell line in a co-culture system that enabled proximity-dependent interaction and communication, a trans-well system that allowed proximity-independent communication through diffusible molecules only, and a conditioned media system through which no proximity-dependent interactions or cell-to-cell communication is possible. Activation of BV-2 cells with lipopolysaccharide and interferon-gamma (LPS/IFN-gamma) decreased viability of the BV-2 cells alone and in co-cultures with SK-N-SH cells, but not SK-N-SH cells grown alone. In contrast, activation of BV-2 cells in the trans-well and conditioned media system did not have any effect on the viability of SK-N-SH cells, suggesting that microglia must be in close proximity to the neural cells to elicit cytotoxicity. To determine the molecules involved in proximity-dependent cell death, inhibitors of microglial activation were investigated. Only the specific inducible nitric oxide synthase (iNOS) inhibitor S-methylisothiourea, and hypothermia, which is known to suppress microglial iNOS expression, prevented cell death after LPS/IFN-gamma activation. These results suggest that activated microglia release nitric oxide that is, at least partially, responsible for proximity-dependent microglial-mediated neural toxicity.

Calpain-mediated truncation of dihydropyrimidinase-like 3 protein (DPYSL3) in response to NMDA and H2O2 toxicity

Dihydropyrimidinase-like protein 3 (DPYSL3), a member of TUC (TOAD-64/Ulip/CRMP), is believed to play a role in neuronal differentiation, axonal outgrowth and, possibly, neuronal regeneration. In primary cortical cultures, glutamate (NMDA) excitotoxicity and oxidative stress (H2O2) caused the cleavage of DPYSL3, resulting in the appearance of a doublet of 62 kDa and 60 kDa. Pre-treatment of cell cultures with calpain inhibitors, but not caspase 3 inhibitor, before exposure to NMDA or H2O2 completely blocked the appearance of the doublet, suggesting calpain-mediated truncation. Furthermore, in vitro digestion of DPYSL3 in cell lysate with purified calpain revealed a cleavage product identical to that observed in NMDA- and H2O2-treated cells, and its appearance was blocked by calpain inhibitors. Analysis of the DPYSL3 protein sequence revealed a possible cleavage site for calpain (Val-Arg-Ser) on the C-terminus of DPYSL3. Collectively, these studies demonstrate for the first time that DPYSL3 is a calpain substrate. The physiological relevance of the truncated DPYSL3 protein remains to be determined.

Antibody-bound β-amyloid precursor protein stimulates the production of tumor necrosis factor-α and monocyte chemoattractant protein-1 by cortical neurons

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by the accumulation of extracellular depositions of fibrillar beta-amyloid (A beta), which is derived from the alternative processing of beta-amyloid precursor protein (APP). Although APP is thought to function as a cell surface receptor, its mode of action still remains elusive. In this study, we found that the culture medium derived from cortical neurons treated with an anti-APP antibody triggers the death of naive neurons. Biochemical and immunocytochemical analyses revealed the presence, both in the conditioned medium and in neurons, of increased levels of tumor necrosis factor-alpha and monocyte chemoattractant protein-1. Furthermore, the expression of these proinflammatory mediators occurred through a c-Jun N-terminal protein kinase/c-Jun-dependent mechanism. Taken together, our findings provide evidence for a novel mechanism whereby neuronal APP in its full-length configuration induces neuronal death. Such a mechanism might be relevant to neuroinflammatory processes as those observed in AD.

Inhibition of microglial inflammation by the MLK inhibitor CEP-1347

CEP-1347 is a potent inhibitor of the mixed lineage kinases (MLKs), a distinct family of mitogen-activated protein kinase kinase kinases (MAPKKK). It blocks the activation of the c-Jun/JNK apoptotic pathway in neurons exposed to various stressors and attenuates neurodegeneration in animal models of Parkinson's disease (PD). Microglial activation may involve kinase pathways controlled by MLKs and might contribute to the pathology of neurodegenerative diseases. Therefore, the possibility that CEP-1347 modulates the microglial inflammatory response [tumour necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and monocyte chemotactic protein-1 (MCP-1)] was explored. Indeed, the MLK inhibitor CEP-1347 reduced cytokine production in primary cultures of human and murine microglia, and in monocyte/macrophage-derived cell lines, stimulated with various endotoxins or the plaque forming peptide Abeta1-40. Moreover, CEP-1347 inhibited brain TNF production induced by intracerebroventricular injection of lipopolysaccharide in mice. As expected from a MLK inhibitor, CEP-1347 acted upstream of p38 and c-Jun activation in microglia by dampening the activity of both pathways. These data imply MLKs as important, yet unrecognized, modulators of microglial inflammation, and demonstrate a novel anti-inflammatory potential of CEP-1347.

Sex steroids, APOE genotype and the innate immune system

Microglia are a primary cellular component of the CNS innate immune system. Their response to conserved pathogen motifs is inherent and leads to the release of cytoactive factors that impact surrounding neurons and glia. The microglial response is modified by the local tissue environment and by "global" factors such as gender. Exposure to estrogen and testosterone, in general, down regulate microglia and peripheral macrophage function, promoting an anti-inflammatory phenotype. Other global factors, however, can "override" the gender-based effects demonstrated by estrogen or testosterone. Apolipoprotein E (APOE) genotype and the expression of specific isoforms of apolipoprotein E differentially regulate microglial and peripheral macrophage function. Our studies have shown that the presence of the APOE4 gene, a known risk factor for AD and other neurodegenerative diseases, promotes a pro-inflammatory macrophage phenotype in neonatal microglia. However, in adult mice, the APOE genotype-specific effect depends on gender. Peritoneal macrophages from female adult APOE3 and APOE4 targeted replacement mice do not demonstrate an APOE genotype-specific response, whereas adult male APOE4 targeted replacement mice show enhanced macrophage responsiveness compared to adult male APOE3 mice. At least part of the altered macrophage response in APOE4 male mice may be due to differences in androgen receptor sensitivity to testosterone. These data re-enforce the concept that classical activation in macrophages has multiple levels of regulation, dictated by competing or synergistic factors and genotype.

Nitric oxide synthase inhibition during synaptic maturation decreases synapsin I immunoreactivity in rat brain

During the development of the brain, nitric oxide and synapsins, the latter being phosphoproteins associated to presynaptic membrane vesicles, are abundant in presynaptic terminals and play important and similar roles in neurotransmitter release, morphogenesis, synaptogenesis, and synaptic plasticity. These mechanisms are fundamental for neuronal development and plasticity and constitute important factors for the formation of neuroanatomical structures. Neural nitric oxide synthase (nNOS), synapsin I, and nNOS adapter protein (CAPON) constitute a ternary complex necessary for specific NO and synapsin functions at a presynaptic level. It is not known whether NO absence may affect the presence and/or activity of synapsins during brain development. To understand the role of NO in synaptogenesis, we studied the effects of NOS inhibition on synapsin I expression at a postnatal stage. Rat pups were treated with a competitive NOS antagonist, N-nitro-L-arginine methyl ester, from postnatal days 3 to 23. Control pups received exclusively an equivalent volume of saline solution. Histochemical and immunochemical techniques for NADPH-d and synapsin I, respectively, were carried out. NOS inhibition elicited a significant reduction in synapsin I immunoreactive density and NADPH-d activity in the brain in the analyzed areas-prefrontal cortex, hippocampus, and dorsal thalamus. These data show that the alterations originated by NO and synapsin deficiencies produce a diminution in synaptic density. Thus, functions that depend on the formation of synaptic connections such as learning and memory could be affected by NO deficiency.

Cytokine toxicity to oligodendrocyte precursors is mediated by iron

Inflammatory processes play a key role in the pathogenesis of a number of common neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). Abnormal iron accumulation is frequently noted in these diseases and compelling evidence exists that iron is involved in inflammatory reactions. Histochemical stains for iron repeatedly demonstrate that oligodendrocytes, under normal conditions, stain more prominently than any other cell type in the brain. Therefore, we examined the hypothesis that cytokine toxicity to oligodendrocytes is iron mediated. Oligodendrocytes in culture were exposed to interferon-gamma (IFN-gamma), interleukin-1beta (IL-1beta), and tumor necrosis factor-alpha (TNF-alpha). Toxicity was observed in a dose-dependent manner for IFN-gamma and TNF-alpha. IL-1beta was not toxic in the concentrations used in this study. The toxic concentration of IFN-gamma, and TNF-alpha was lower if the cells were iron loaded, but iron loading had no effect on the toxicity of IL-1beta. These data provide insight into the controversy regarding the toxicity of cytokines to oligodendrocytes by revealing that iron status of these cells will significantly impact the outcome of cytokine treatment. The exposure of oligodendrocytes to cytokines plus iron decreased mitochondrial membrane potential but activation of caspase 3 is limited. The antioxidant, TPPB, which targets mitochondria, protected the oligodendrocytes from the iron-mediated cytotoxicity, providing further support that mitochondrial dysfunction may underlie the iron-mediated cytokine toxicity. Therapeutic strategies involving anti-inflammatory agents have met with limited success in the treatment of demyelinating disorders. A better understanding of these agents and the contribution of cellular iron status to cytokine toxicity may help develop a more consistent intervention strategy.

Effect of anti-inflammatory agents on transforming growth factor beta over-expressing mouse brains: a model revised

BACKGROUND: The over-expression of transforming growth factor beta-1(TGF-beta1) has been reported to cause hydrocephalus, glia activation, and vascular amyloidbeta (Abeta) deposition in mouse brains. Since these phenomena partially mimic the cerebral amyloid angiopathy (CAA) concomitant to Alzheimer's disease, the findings in TGF-beta1 over-expressing mice prompted the hypothesis that CAA could be caused or enhanced by the abnormal production of TGF-beta1. This idea was in accordance with the view that chronic inflammation contributes to Alzheimer's disease, and drew attention to the therapeutic potential of anti-inflammatory drugs for the treatment of Abeta-elicited CAA. We thus studied the effect of anti-inflammatory drug administration in TGF-beta1-induced pathology. METHODS: Two-month-old TGF-beta1 mice and littermate controls were orally administered pioglitazone, a peroxisome proliferator-activated receptor-gamma agonist, or ibuprofen, a non steroidal anti-inflammatory agent, for two months. Glia activation was assessed by immunohistochemistry and western blot analysis; Abeta precursor protein (APP) by western blot analysis; Abeta deposition by immunohistochemistry, thioflavin-S staining and ELISA; and hydrocephalus by measurements of ventricle size on autoradiographies of brain sections. Results are expressed as means +/- SD. Data comparisons were carried with the Student's T test when two groups were compared, or ANOVA analysis when more than three groups were analyzed. RESULTS: Animals displayed glia activation, hydrocephalus and a robust thioflavin-S-positive vascular deposition. Unexpectedly, these deposits contained no Abeta or serum amyloid P component, a common constituent of amyloid deposits. The thioflavin-S-positive material thus remains to be identified. Pioglitazone decreased glia activation and basal levels of Abeta42- with no change in APP contents - while it increased hydrocephalus, and had no effect on the thioflavin-S deposits. Ibuprofen mimicked the reduction of glia activation caused by pioglitazone and the lack of effect on the thioflavin-S-labeled deposits. CONCLUSIONS: i) TGF-beta1 over-expressing mice may not be an appropriate model of Abeta-elicited CAA; and ii) pioglitazone has paradoxical effects on TGF-beta1-induced pathology suggesting that anti-inflammatory therapy may reduce the damage resulting from active glia, but not from vascular alterations or hydrocephalus. Identification of the thioflavin-S-positive material will facilitate the full appraisal of the clinical implication of the effects of anti-inflammatory drugs, and provide a more thorough understanding of TGF-beta1 actions in brain.

(?)-Epigallocatechin gallate inhibits lipopolysaccharide-induced microglial activation and protects against inflammation-mediated dopaminergic neuronal injury

Microglial activation is believed to play a pivotal role in the selective neuronal injury associated with several neurodegenerative disorders, including Parkinson's disease (PD) and Alzheimer's disease. We provide evidence that (-)-epigallocatechin gallate (EGCG), a major monomer of green tea polyphenols, potently inhibits lipopolysaccharide (LPS)-activated microglial secretion of nitric oxide (NO) and tumor necrosis factor-alpha (TNF-alpha) through the down-regulation of inducible NO synthase and TNF-alpha expression. In addition, EGCG exerted significant protection against microglial activation-induced neuronal injury both in the human dopaminergic cell line SH-SY5Y and in primary rat mesencephalic cultures. Our study demonstrates that EGCG is a potent inhibitor of microglial activation and thus is a useful candidate for a therapeutic approach to alleviating microglia-mediated dopaminergic neuronal injury in PD.