cAMP-induced decrease in cell-surface laminin receptor and cellular prion protein attenuates amyloid-β uptake and amyloid-β-induced neuronal cell death

Previous studies have shown that amyloid-β oligomers (AβO) bind with high affinity to cellular prion protein (PrP^C ). The AβO-PrP^C complex binds to cell-surface co-receptors, including the laminin receptor (67LR). Our current studies revealed that in Neuroscreen-1 cells, 67LR is the major co-receptor involved in the cellular uptake of AβO and AβΟ-induced cell death. Both pharmacological (dibutyryl-cAMP, forskolin and rolipram) and physiological (pituitary adenylate cyclase-activating polypeptide) cAMP-elevating agents decreased cell-surface PrP^C and 67LR, thereby attenuating the uptake of AβO and the resultant neuronal cell death. These cAMP protective effects are dependent on protein kinase A, but not dependent on the exchange protein directly activated by cAMP. Conceivably, cAMP protects neuronal cells from AβO-induced cytotoxicity by decreasing cell-surface-associated PrP^C and 67LR.

Mild pericyte deficiency is associated with aberrant brain microvascular flow in aged PDGFRβ+/- mice

The receptor tyrosine kinase PDGFRβ is essential for pericyte migration to the endothelium. In mice lacking one allele of PDGFRβ (PDGFRβ^+/-), previous reports have described an age-dependent loss of pericytes in the brain, leading to cerebrovascular dysfunction and subsequent neurodegeneration reminiscent of that seen in Alzheimer's disease and vascular dementia. We examined 12-20-month-old PDGFRβ^+/- mice to better understand how pericyte loss affects brain microvascular structure and perfusion in vivo. We observed a mild reduction of cortical pericyte number in PDGFRβ^+/- mice (27% fewer cell bodies) compared to controls, but no decrease in pericyte coverage of the endothelium. This mild degree of pericyte loss caused no discernable change in cortical microvascular density, length, basal diameter or reactivity to hypercapnia. Yet, it was associated with an increase in basal blood cell velocity, primarily in pre-capillary arterioles. Taken together, our results suggest that mild pericyte loss can lead to aberrant cerebral blood flow despite a lack of apparent effect on microvascular structure and reactivity.

Dynamic Remodeling of Pericytes In Vivo Maintains Capillary Coverage in the Adult Mouse Brain

Direct contact and communication between pericytes and endothelial cells is critical for maintenance of cerebrovascular stability and blood-brain barrier function. Capillary pericytes have thin processes that reach hundreds of micrometers along the capillary bed. The processes of adjacent pericytes come in close proximity but do not overlap, yielding a cellular chain with discrete territories occupied by individual pericytes. Little is known about whether this pericyte chain is structurally dynamic in the adult brain. Using in vivo two-photon imaging in adult mouse cortex, we show that while pericyte somata were immobile, the tips of their processes underwent extensions and/or retractions over days. The selective ablation of single pericytes provoked exuberant extension of processes from neighboring pericytes to contact uncovered regions of the endothelium. Uncovered capillary regions had normal barrier function but were dilated until pericyte contact was regained. Pericyte structural plasticity may be critical for cerebrovascular health and warrants detailed investigation.

Organizational hierarchy and structural diversity of microvascular pericytes in adult mouse cortex

Smooth muscle cells and pericytes, together called mural cells, coordinate many distinct vascular functions. Canonically, smooth muscle cells are ring-shaped and cover arterioles with circumferential processes, whereas pericytes extend thin processes that run longitudinally along capillaries. In between these canonical mural cell types are cells with features of both smooth muscle cells and pericytes. Recent studies suggest that these transitional cells are critical for controlling blood flow to the capillary bed during health and disease, but there remains confusion on how to identify them and where they are located in the brain microvasculature. To address this issue, we measured the morphology, vascular territory, and α-smooth muscle actin content of structurally diverse mural cells in adult mouse cortex. We first imaged intact 3D vascular networks to establish the locations of major gradations in mural cell appearance as arterioles branched into capillaries. We then imaged individual mural cells occupying the regions within these gradations. This revealed two transitional cells that were often similar in appearance, but with sharply contrasting levels of α-smooth muscle actin. Our findings highlight the diversity of mural cell morphologies in brain microvasculature, and provide guidance for identification and categorization of mural cell types.

Pericyte Structural Remodeling in Cerebrovascular Health and Homeostasis

The biology of brain microvascular pericytes is an active area of research and discovery, as their interaction with the endothelium is critical for multiple aspects of cerebrovascular function. There is growing evidence that pericyte loss or dysfunction is involved in the pathogenesis of Alzheimer’s disease, vascular dementia, ischemic stroke and brain injury. However, strategies to mitigate or compensate for this loss remain limited. In this review, we highlight a novel finding that pericytes in the adult brain are structurally dynamic in vivo, and actively compensate for loss of endothelial coverage by extending their far-reaching processes to maintain contact with regions of exposed endothelium. Structural remodeling of pericytes may present an opportunity to foster pericyte-endothelial communication in the adult brain and should be explored as a potential means to counteract pericyte loss in dementia and cerebrovascular disease. We discuss the pathophysiological consequences of pericyte loss on capillary function, and the biochemical pathways that may control pericyte remodeling. We also offer guidance for observing pericytes in vivo, such that pericyte structural remodeling can be more broadly studied in mouse models of cerebrovascular disease.

Cytokine-induced release of ceramide-enriched exosomes as a mediator of cell death signaling in an oligodendroglioma cell line

Th1 pro-inflammatory cytokines, i.e., TNF-α and IFN-γ, in combination are known to induce cell death in several cell types, including oligodendrocytes, but the mechanism of their synergistic cytotoxicity is unclear. Although ceramide (Cer) has been implicated in cytokine- and stress-induced cell death, its intracellular levels alone cannot explain cytokine synergy. We considered the possibility that Cer released as part of extracellular vesicles may contribute to cytokine-induced synergistic cell death. Using a human oligodendroglioma (HOG) cell line as a model, here we show that exosomes derived from TNF-α-treated "donor" cells, while being mildly toxic to fresh cultures (similar to individual cytokines), induce enhanced cell death when added to IFN-γ-primed target cultures in a fashion resembling the effect of cytokine combination. Further, the sphingolipid profiles of secreted exosomes, as determined by HPLC-MS/MS, revealed that the treatment with the cytokines time-dependently induced the formation and exosomal release, in particular of C16-, C24-, and C24:1-Cer species; C16-, C24-, and C24:1-dihydroCer species; and C16-, C24-, and C24:1-SM species. Finally, exogenous C6-Cer or C16-Cer mimicked and enhanced the cytotoxic effects of the cytokines upon HOG cells, thereby supporting the cell death-signaling role of extracellular Cer.

Vasculoprotection as a Convergent, Multi-Targeted Mechanism of Anti-AD Therapeutics and Interventions

Using a variety of animal models of Alzheimer's disease (AD), there have been a number of recent studies reporting varying degrees of success with anti-AD therapeutics. The efficacies are often discussed in terms of the modulatory effects of the compounds tested on identified or assumed targets among the known (or proposed) pathogenic and neuroprotective mechanisms, largely within the context of the dominant amyloid cascade hypothesis. However, it is clear that several of the relatively more efficacious treatments tend to be multifunctional and target multiple pathological processes associated with AD including most commonly, oxidative and metabolic stress and neuroinflammation. Increasing evidence suggests that vascular and neurodegenerative pathologies often co-exist and that neurovascular dysfunction plays a critical role in the development or progression of AD. In this review, we will discuss the significance of vasculoprotection or neurovascular unit integrity as a common, multi-targeted mechanism underlying the reported efficacy of a majority of anti-AD therapeutics--amyloid-targeted or otherwise--while providing a strong support for future neurovascular-based treatment strategies and interventions.

Optogenetic stimulation of astrocytes in the posterior hypothalamus increases sleep at night in C57BL/6J mice

A distributed network of neurons regulates wake, non‐rapid eye movement (NREM) sleep, and REM sleep. However, there are also glia in the brain, and there is growing evidence that neurons and astroglia communicate intimately to regulate behaviour. To identify the effect of optogenetic stimulation of astrocytes on sleep, the promoter for the astrocyte‐specific cytoskeletal protein, glial fibrillary acidic protein (GFAP) was used to direct the expression of channelrhodopsin‐2 (ChR2) and the linked reporter gene, enhanced yellow fluorescent protein (EYFP), in astrocytes. rAAV‐GFAP‐ChR2 (H134R)‐EYFP or rAAV‐GFAP‐EYFP was microinjected (750 nL) into the posterior hypothalamus (bilateral) of mice. Three weeks later baseline sleep was recorded (0 Hz) and 24 h later optogenetic stimulation applied during the first 6 h of the lights‐off period. Mice with ChR2 were given 5, 10 or 30 Hz stimulation for 6 h (10‐ms pulses; 1 mW; 1 min on 4 min off). At least 36 h elapsed between the stimulation periods (5, 10, 30 Hz) and although 0 Hz was always first, the order of the other three stimulation rates was randomised. In mice with ChR2 (n = 7), 10 Hz, but not 5 or 30 Hz stimulation increased both NREM and REM sleep during the 6‐h period of stimulation. Delta power did not increase. In control mice (no ChR2; n = 5), 10 Hz stimulation had no effect. This study demonstrates that direct stimulation of astrocytes powerfully induces sleep during the active phase of the sleep–wake cycle and underlines the inclusion of astrocytes in network models of sleep–wake regulation.

Increased tau phosphorylation and impaired brain insulin/IGF signaling in mice fed a high fat/high cholesterol diet

Previous studies demonstrated that a high fat/high cholesterol (HFC) diet results in a loss of working memory in mice correlated with neuroinflammatory changes and increased AβPP processing (Thirumangalakudi et al. (2008) J Neurochem 106, 475-485). To further explore the nature of the molecular correlates of cognitive impairment, in this study, we examined changes in tau phosphorylation, insulin/IGF-1 signaling (IIS) including GSK3, and levels of specific synaptic proteins. Immunoblot analysis of hippocampal tissue from C57BL/6 mice fed HFC for 2 months with anti-phospho-tau (i.e., PHF1 and phospho-Thr-231 tau) antibodies demonstrated the presence of hyperphosphorylated tau. The tau phosphorylation correlated with activated GSK3, a prominent tau kinase normally kept inactive under the control of IIS. That IIS itself was impaired due to the hyperlipidemic diet was confirmed by a down-regulation of insulin receptor substrate-1 and phospho-Akt levels. Although no significant changes in the levels of the pre-synaptic protein (i.e., synaptophysin) in response to HFC were apparent in immunoblot analysis, there was a clear down-regulation of the post-synaptic protein, PSD95, and drebrin, a dendritic spine-specific protein, indicative of altered synaptic plasticity. The results, in concert with previous findings with the same model, suggest that high dietary fat/cholesterol elicits brain insulin resistance and altered IIS leading to Alzheimer's disease-like cognitive impairment in 'normal' mice.

The role of tumor progression locus 2 protein kinase in glial inflammatory response

Tumor progression locus 2 (Tpl2)/cancer Osaka thyroid kinase is a newer member of MAP3K family that is now known for its essential role in tumor necrosis factor-aplha (TNFα) expression in macrophages, but its pro-inflammatory signaling, if any, in glia is unknown. When cultures of murine microglia and astrocytes were exposed to lipopolysaccharide, there was a rapid activation (i.e., phosphorylation) of Tpl2 in parallel to the activation of down-stream effector MAPKs, that is, extracellular signal regulated kinase (ERK), p38 MAPK and C-Jun N-terminal kinase (JNK). Pre-incubation of the cultures with a Tpl2 inhibitor selectively suppressed the activation of the primary down-stream target, that is, ERK relative to p38 MAPK and JNK. That Tpl2 activation was functionally involved in glial inflammatory response was indicated by a reduced release of the cytokines, i.e. TNFα and the expression of inducible nitric oxide synthase in the presence of the kinase inhibitor. Furthermore, over-expression of a wild-type Tpl2 construct in C-6 glia resulted in an enhanced transcriptional activation of inducible nitric oxide synthase, while transfection with a dominant negative form of Tpl-2 had the opposite effect. The findings assign an important pro-inflammatory signaling function for Tpl2 pathway in glial cells.

Metabolic stress modulates Alzheimer's β-secretase gene transcription via SIRT1-PPARγ-PGC-1 in neurons

Classic cardio-metabolic risk factors such as hypertension, stroke, diabetes, and hypercholesterolemia all increase the risk of Alzheimer's disease. We found increased transcription of β-secretase/BACE1, the rate-limiting enzyme for Aβ generation, in eNOS-deficient mouse brains and after feeding mice a high-fat, high-cholesterol diet. Up- or downregulation of PGC-1α reciprocally regulated BACE1 in vitro and in vivo. Modest fasting in mice reduced BACE1 transcription in the brains, which was accompanied by elevated PGC-1 expression and activity. Moreover, the suppressive effect of PGC-1 was dependent on activated PPARγ, likely via SIRT1-mediated deacetylation in a ligand-independent manner. The BACE1 promoter contains multiple PPAR-RXR sites, and direct interactions among SIRT1-PPARγ-PGC-1 at these sites were enhanced with fasting. The interference on the BACE1 gene identified here represents a unique noncanonical mechanism of PPARγ-PGC-1 in transcriptional repression in neurons in response to metabolic signals that may involve recruitment of corepressor NCoR.

Engineering an in situ crosslinkable hydrogel for enhanced remyelination

Remyelination has to occur to fully regenerate injured spinal cords or brain tissues. A growing body of evidence has suggested that exogenous cell transplantation is one promising strategy to promote remyelination. However, direct injection of neural stem cells or oligodendrocyte progenitor cells (OPCs) to the lesion site may not be an optimal therapeutic strategy due to poor viability and functionality of transplanted cells resulted from the local hostile tissue environment. The overall objective of this study was to engineer an injectable biocompatible hydrogel system as a supportive niche to provide a regeneration permissive microenvironment for transplanted OPCs to survive, functionally differentiate, and remyelinate central nervous system (CNS) lesions. A highly biocompatible hydrogel, based on thiol-functionalized hyaluronic acid and thiol-functionalized gelatin, which can be crosslinked by poly-(ethylene glycol) diacrylate (PEGDA), was used. These hydrogels were optimized first regarding cell adhesive properties and mechanical properties to best support the growth properties of OPCs in culture. Transplanted OPCs with the hydrogels optimized in vitro exhibited enhanced survival and oligodendrogenic differentiation and were able to remyelinate demyelinated axons inside ethidium bromide (EB) demyelination lesion in adult spinal cord. This study provides a new possible therapeutic approach to treat CNS injuries in which cell therapies may be essential.

Vitamin D3-enriched diet correlates with a decrease of amyloid plaques in the brain of AβPP transgenic mice

In addition to its function in calcium and bone metabolism, vitamin D is neuroprotective and important for mitigating inflammation. Alzheimer's disease (AD) is a progressive neurodegenerative disorder of the central nervous system, characterized by neuronal loss in many areas of the brain, and the formation of senile (neuritic) plaques, which increase in number and size over time. The goal of this project was to investigate whether vitamin D3 supplementation would affect amyloid plaque formation in amyloid-β protein precursor (AβPP) transgenic mice that spontaneously develop amyloid plaques within 3-4 months of birth. AβPP mice were fed control, vitamin D3-deficient or vitamin D3-enriched diets for five months, starting immediately after weaning. At the end of the study, the animals were subjected to behavioral studies, sacrificed, and examined for bone changes and brain amyloid load, amyloid-β (Aβ) peptide levels, inflammatory changes, and nerve growth factor (NGF) content. The results obtained indicate that a vitamin D3-enriched diet correlates with a decrease in the number of amyloid plaques, a decrease in Aβ peptides, a decrease in inflammation, and an increase in NGF in the brains of AβPP mice. These observations suggest that a vitamin D3-enriched diet may benefit AD patients.

Targets for AD treatment: conflicting messages from γ-secretase inhibitors

Current evidence suggests that Alzheimer's disease (AD) is a multi-factorial disease that starts with accumulation of multiple proteins. We have previously proposed that inhibition of γ-secretase may impair membrane recycling causing neurodegeneration starting at synapses (Sambamurti K., Suram A., Venugopal C., Prakasam A., Zhou Y., Lahiri D. K. and Greig N. H. A partial failure of membrane protein turnover may cause Alzheimer's disease: a new hypothesis. Curr. Alzheimer Res., 3, 2006, 81). We also proposed familal AD mutations increase Aβ42 by inhibiting γ-secretase. Herein, we discuss the failure of Eli Lilly's γ-secretase inhibitor, semagacestat, in clinical trials in the light of our hypothesis, which extends the problem beyond toxicity of Aβ aggregates. We elaborate that γ-secretase inhibitors lead to accumulation of amyloid precursor protein C-terminal fragments that can later be processed by γ-secretase to yields bursts of Aβ to facilitate aggregation. Although we do not exclude a role for toxic Aβ aggregates, inhibition of γ-secretase can affect numerous substrates other than amyloid precursor protein to affect multiple pathways and the combined accumulation of multiple peptides in the membrane may impair its function and turnover. Taken together, protein processing and turnover pathways play an important role in maintaining cellular homeostasis and unless we clearly see consistent disease-related increase in their levels or activity, we need to focus on preserving their function rather than inhibiting them for treatment of AD and similar diseases.

Prenatal LPS increases inflammation in the substantia nigra of Gdnf heterozygous mice

Prenatal systemic inflammation has been implicated in neurological diseases, but optimal animal models have not been developed. We investigated whether a partial genetic deletion of glial cell line-derived neurotrophic factor (Gdnf(+/-)) increased vulnerability of dopamine (DA) neurons to prenatal lipopolysaccharide (LPS). LPS [0.01 mg/kg intraperitoneal (i.p.)] or saline was administered to wild-type (WT) or Gdnf(+/-) pregnant mice on gestational day 9.5. Male offspring were examined at 3 weeks, 3 and 12 months of age. There was a progressive degeneration of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra (SN) with age in Gdnf(+/-) but not in WT mice, with no observed effects on locus coeruleus (LC) noradrenergic neurons or DA neurons of the ventral tegmental area. Inflammatory markers were elevated in SN of LPS treated offspring, with exacerbation in Gdnf(+/-) mice. Intracellular accumulation of α-synuclein (α-syn) immunoreactivity in DA neurons of SN was observed in all groups of Gdnf(+/-) and in WT mice with prenatal LPS, with altered distribution between pars reticulata (pr) and pars compacta (pc). The findings suggest that prenatal LPS leads to accelerated neuropathology in the SN with age, and that a partial loss of GDNF exacerbates these effects, providing a novel model for age-related neuropathology of the nigrostriatal DA system.

Linking cardiometabolic disorders to sporadic Alzheimer's disease: a perspective on potential mechanisms and mediators

There is increasing evidence that the incidence of Alzheimer's disease (AD) is significantly influenced by cardiovascular risk factors in association with a cluster of metabolic diseases including diabetes and atherosclerosis. The shared risk is also reflected in the dietary and lifestyle links to both metabolic disorders and AD-type cognitive dysfunction. Recent studies with genetic and diet-induced animal models have begun to illuminate convergent mechanisms and mediators between these two categories of disease conditions with distinct tissue-specific pathologies. Although it is clear that peripheral inflammation and insulin resistance are central to the pathogenesis of the disorders of metabolic syndrome, it seems that the same mechanisms are also in play across the blood-brain barrier that lead to AD-like molecular and cognitive changes. This review highlights these convergent mechanisms and discusses the role of cerebrovascular dysfunction as a conduit to brain emergence of these pathogenic processes that might also represent future therapeutic targets in AD in common with metabolic disorders.

High cholesterol-induced neuroinflammation and amyloid precursor protein processing correlate with loss of working memory in mice

Recent findings suggest that hypercholesterolemia may contribute to the onset of Alzheimer's disease-like dementia but the underlying mechanisms remain unknown. In this study, we evaluated the cognitive performance in rodent models of hypercholesterolemia in relation to neuroinflammatory changes and amyloid precursor protein (APP) processing, the two key parameters of Alzheimer's disease pathogenesis. Groups of normal C57BL/6 and low density lipoprotein receptor (LDLR)-deficient mice were fed a high fat/cholesterol diet for an 8-week period and tested for memory in a radial arm maze. It was found that the C57BL/6 mice receiving a high fat diet were deficient in handling an increasing working memory load compared with counterparts receiving a control diet while the hypercholesterolemic LDLR-/- mice showed impaired working memory regardless of diet. Immunohistochemical analysis revealed the presence of activated microglia and astrocytes in the hippocampi from high fat-fed C57BL/6 mice and LDLR-/- mice. Consistent with a neuroinflammatory response, the hyperlipidemic mice showed increased expression of cytokines/mediators including tumor necrosis factor-alpha, interleukin-1beta and -6, nitric oxide synthase 2, and cycloxygenase 2. There was also an induced expression of the key APP processing enzyme i.e. beta-site APP cleaving enzyme 1 in both high fat/cholesterol-fed C57BL/6 and LDLR-/- mice accompanied by an increased generation of C-terminal fragments of APP. Although ELISA for beta-amyloid failed to record significant changes in the non-transgenic mice, a threefold increase in beta-amyloid 40 accumulation was apparent in a strain of transgenic mice expressing wild-type human APP on high fat/cholesterol diet. The findings link hypercholesterolemia with cognitive dysfunction potentially mediated by increased neuroinflammation and APP processing in a non-transgenic mouse model.

Cell-specific expression of neutral glycosphingolipids in vertebrate brain: immunochemical localization of 3-O-acetyl-sphingosine-series glycolipid(s) in myelin and oligodendrocytes

The tissue- and cell-specific expression of three neutral glycosphingolipids, gangliotetraosylceramide (GA1), gangliopentaosylceramide (GalNAc-GA1), and the novel 3-O-acetyl-sphingosine-series glycolipid (FMC-5), were examined with monospecific polyclonal antibodies. Immunohistochemical studies of rodent brain cross-sections indicated that both GA1 and FMC-5 antibodies stained myelin. In contrast, GalNAc-GA1 antibody distinctly stained neurons in cerebral cortex, but only partially delineated Purkinje cells and other neurons in cerebellum. Preliminary studies of mixed glial cultures suggested the following: 1) both FMC-5 and GA1 antibodies stained oligodendrocytes and oligo progenitors, and 2) GalNAc-GA1 antibody did not stain any cells in the culture. Because the GalNAc-GA1 was associated with neurons, we examined the immunoreactivity of GalNAc-GA1 antibody in primary neuronal cultures. Further studies using primary cultures of rat brain oligodendrocytes, and dissociated cerebellar neuronal cultures indicated that both GA1 and FMC-5 are specifically expressed by oligodendrocytes, whereas GalNAc-GA1 is primarily localized in interneurons and to some extent in Purkinje neurons.

p38alpha MAP kinase mediates hypoxia-induced motor neuron cell death: a potential target of minocycline's neuroprotective action

Hypoxia-ischemia (HI) may play a significant role in motor neuron death associated with the pathology of spinal cord injury and, perhaps, amyotrophic lateral sclerosis. The present study employs an in vitro model of HI to investigate the role of a stress kinase pathway, i.e., p38 MAP kinase, in cell death signaling in a motor neuron cell line, i.e., NSC34, subjected to oxygen-glucose deprivation (OGD). Although the neurons were essentially tolerant to either hypoxia (0.2% O(2)) or low glucose (1 mM) alone, more than 60% of them died in response to combined low oxygen and low-glucose exposure. Minocycline, a semi-synthetic tetracycline known for its neuroprotective effects in models of neurodegeneration, afforded substantial (approximately 50%) protection against hypoxic cell death, assessed by lactate dehydrogenase release and flow cytometry, while suppressing OGD-induced p38 MAP kinase activation. An inhibitor of p38 kinase, SB203580, as well as siRNA-mediated down-regulation of p38 kinase elicited an almost complete blockade of OGD-induced cell death. The use of p38 isoform-specific siRNAs further revealed preferential involvement of the alpha over the beta isoform of p38 MAP kinase in hypoxic neuronal cell death in our model.

p38 MAP kinase regulation of oligodendrocyte differentiation with CREB as a potential target

Despite a substantial understanding of the factors regulating oligodendrocyte differentiation, the signaling mechanisms involved in this process are not well-understood. This study elaborates on the findings (Bhat NR, Zhang P (1997) FASEB J 11:A925; Baron W, Metz B, Bansal R, Hoekstra D, de Vries H (2000) Mol Cell Neurosci 15:314-329) of a role for p38 MAP kinase signaling in oligodendrocyte differentiation and myelin gene expression. When proliferating oligodendrocyte progenitors were switched to a growth factor-free differentiation medium, there was a rapid activation of p38 kinase that correlated with an increased phosphorylation of CREB, a down-stream target and a factor involved in oligodendrocyte differentiation. Addition of forskolin, a known inducer of intracellular c-AMP and of oligodendrocyte differentiation, also stimulated CREB phosphorylation in a p38 kinase dependent way. Pharmacological inhibition of p38 interfered with the morphological and antigenic changes associated with differentiating oligodendrocytes as well as with the developmental and forskolin-induced expression of myelin basic protein, thereby supporting an essential role for p38 MAPK pathway in oligodendrocyte differentiation.

FA2H-dependent fatty acid 2-hydroxylation in postnatal mouse brain

2-Hydroxy fatty acids are relatively minor species of membrane lipids found almost exclusively as N-acyl chains of sphingolipids. In mammals, 2-hydroxy sphingolipids are uniquely abundant in myelin galactosylceramide and sulfatide. Despite the well-documented abundance of 2-hydroxy galactolipids in the nervous system, the enzymatic process of the 2-hydroxylation is not fully understood. To fill this gap, we have identified a human fatty acid 2-hydroxylase gene (FA2H) that is highly expressed in brain. In this report, we test the hypothesis that FA2H is the major fatty acid 2-hydroxylase in mouse brain and that free 2-hydroxy fatty acids are formed as precursors of myelin 2-hydroxy galactolipids. The fatty acid compositions of galactolipids in neonatal mouse brain gradually changed during the course of myelination. The relative ratio of 2-hydroxy versus nonhydroxy galactolipids was very low at 2 days of age ( approximately 8% of total galactolipids) and increased 6- to 8-fold by 30 days of age. During this period, free 2-hydroxy fatty acid levels in mouse brain increased 5- to 9-fold, and their composition was reflected in the fatty acids in galactolipids, consistent with a precursor-product relationship. The changes in free 2-hydroxy fatty acid levels coincided with fatty acid 2-hydroxylase activity and with the upregulation of FA2H expression. Furthermore, mouse brain fatty acid 2-hydroxylase activity was inhibited by anti-FA2H antibodies. Together, these data provide evidence that FA2H is the major fatty acid 2-hydroxylase in brain and that 2-hydroxylation of free fatty acids is the first step in the synthesis of 2-hydroxy galactolipids.

C-Jun N-terminal kinase (JNK) regulation of iNOS expression in glial cells: predominant role of JNK1 isoform

The mitogen-activated protein kinases (MAPKs) play a central role in mediating the activation and transcriptional responses of diverse cells, including glia. c-Jun N-terminal kinase (JNK), a member of the MAPK family, is activated by a variety of stress and proinflammatory signals and in turn phosphorylates its downstream substrates including nuclear factors, leading to transcriptional activation of target genes. There are at least three subtypes of JNK (i.e., JNKs 1-3) that may play isoform-specific roles. This study examined the role of JNK isoforms in the induction of inducible nitric oxide synthase (iNOS) in astrocytes in response to lipopolysachharide (LPS) and interferon (IFN)-gamma. While an inhibitor of the JNK pathway (SP600125) inhibited iNOS expression, ectopic expression of a constitutively active form of MEKK1 (MAPK/ERK kinase kinase- 1), an upstream activator of JNK, led to an induction of co-transfected iNOS promoter activity and, in the presence of LPS, to an enhanced expression of iNOS. RNA knockdown studies with JNK subtype-specific short-interfering RNA (siRNA), indicated that JNK1- but not JNK2- nor JNK3-specific siRNA, interfered with LPS/IFNgamma induction of iNOS. It is concluded that, of the three JNK forms, JNK1 is the major mediator of iNOS induction and perhaps, inflammatory signaling in general, in glial cells.

Hypoxia/reoxygenation differentially modulates NF-kappaB activation and iNOS expression in astrocytes and microglia

Hypoxia/ischemic brain injury accompanies an inflammatory response involving an activation of glial cells. This study, using an in vitro model, investigated the signaling mechanisms mediating hypoxic responses of the two glial cell types (astrocytes and microglia) in relation to the expression of inducible nitric oxide synthase (iNOS). In cultures of rat brain microglia and astrocytes, hypoxia (8 h) followed by reoxygenation (24 h) (H/O) had little (microglia) or no (astrocytes) effect on the expression of iNOS. However, H/O elicited opposite effects on lipopolysaccharide (LPS) induction of iNOS in the two cell types: it potentiated LPS induction of iNOS in microglia but inhibited this response in astrocytes. Similar differential effects of hypoxia were observed on the production of tumor necrosis factor-alpha (TNFalpha). In contrast, there was an upregulation of hemoxygenase- 1 (HO-1), a counter-regulatory pathway, with astrocytes showing a bigger induction than microglia. While hypoxic activation of mitogen-activated protein kinases (MAPKs) was similar in the two glial types, the activation pattern of NFkappaB was clearly different: hypoxia stimulated the activation of NFkappaB pathway and NFkappaB-dependent transcription in microglia but not in astrocytes. Lastly, the two cell types displayed differential vulnerabilities to hypoxia-induced cell death, the astrocytes being relatively more resistant than microglia.

MAP kinase regulation of IP10/CXCL10 chemokine gene expression in microglial cells

Interferongamma inducible protein-10 (IP10 or CXCL10), a Th-1 affiliated chemokine, is expressed by activated glial cells and may contribute to the trafficking of immune cells in the inflamed central nervous system. This study examines the regulation of the expression of this chemokine in cultured microglial cells focusing on the roles of mitogen-activated protein (MAP) kinase cascades. Exposure of a mouse microglial cell line, BV-2, to lipopolysaccharide (LPS) and IFNgamma led to an induction of IP10 mRNA and protein as determined by RT-PCR and ELISA, respectively. This induction was suppressed by pharmacological inhibitors of p38 MAPK (i.e., SB203580) and c-Jun N-terminal kinase (JNK, SP600125), suggesting the involvement of the two kinases in IP10 expression. LPS also induced the activity of an IP10 promoter reporter (luciferase) construct transfected into BV-2 cells in a MAP kinase- and NFkappaB-dependent manner. The use of deletion constructs revealed that the kinase-targeted sequences were within the region between -533 bp and -332 bp upstream of the transcriptional start site. Co-transfection of IP10 luciferase with the active forms of the upstream kinases in the MAP kinase cascades, i.e., MAPK kinase-3 (MKK3), MKK6 (the immediately upstream activators of p38 kinase) and a MAP3K, i.e., TGFbeta-activated kinase-1 (TAK1), produced a marked stimulation of the promoter activity. The results of this study indicate that the MAP kinase cascades prominently regulate IP10 gene expression in microglial cells.

A role for p38 mitogen-activated protein kinase in the regulation of the serotonin transporter: evidence for distinct cellular mechanisms involved in transporter surface expression

The serotonin transporter (SERT) is regulated by various signaling mechanisms that may operate to maintain appropriate levels of synaptic serotonin (5-HT). We demonstrate that one of the mitogen-activated protein kinases (MAPKs), p38 MAPK, regulates SERT. Treatment of rat midbrain synaptosomes with p38 MAPK-specific inhibitors, PD169316 [4-(4-fluorophenyl)-2-(4-nitrophenyl)-5-(4-pyridyl)-1H-imidazole] or SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole], reduced 5-HT uptake. An additive SERT inhibition by PD169316 and beta-phorbol 12-myristate 13-acetate (beta-PMA) indicated the involvement of a protein kinase C (PKC)-independent MAPK pathway. Kinetic studies indicated a significant decrease in the transport capacity (V(max)) after PD169316 treatment of synaptosomes. Biotinylation studies showed reduced SERT proteins in the plasma membrane of synaptosomes after p38 MAPK inhibition and PKC activation. Phosphorylation studies using synaptosomes revealed decreased SERT phosphorylation by PD169316 but increased phosphorylation by beta-PMA. d-Amphetamine enhanced SERT basal phosphorylation and PD169316 blocked this effect. SERT interaction with protein phosphatase 2A catalytic subunit and syntaxin 1A decreased after PD169316 or beta-PMA treatment of synaptosomes. In synaptosomes, PKC activation but not p38 MAPK inhibition resulted in SERT redistribution from cholesterolrich lipid raft fractions to nonlipid raft fractions. The presence of phospho-p38 MAPK in synaptosomes and human embryonic kidney 293 (HEK-293) cells suggested the presence of constitutively active p38 MAPK in these preparations. Cotransfection of HEK-293 cells with SERT and a constitutively active form of MAP kinase kinase 3b(E) [MKK3b(E)] increased 5-HT transport, and RNA interference targeted to p38 MAPK inhibited 5-HT uptake, confirming the involvement of active p38 MAPK in SERT expression. Although PD169316 inhibited SERT insertion to the plasma membrane, beta-PMA increased SERT internalization in HEK-293 cells. Together, these results indicate a distinct role of p38 MAPK in SERT regulation.

Cholesterol and Alzheimer's disease: clinical and experimental models suggest interactions of different genetic, dietary and environmental risk factors

Alzheimer's disease (AD) is a progressive senile dementia characterized by deposition of a 4 kDa peptide of 39-42 residues known as amyloid beta-peptide (Abeta) in the form of senile plaques and the microtubule associated protein tau as paired helical filaments. Genetic studies have identified mutations in the Abeta precursor protein (APP) as the key triggers for the pathogenesis of AD. Other genes such as presenilins 1 and 2 (PS1/2) and apolipoprotein E (APOE) also play a critical role in increased Abeta deposition. Several biochemical and molecular studies using transfected cultured cells and transgenic animals point to mechanisms by which Abeta is generated and aggregated to trigger the neurodegeneration that may cause AD. Three important enzymes collectively known as 'secretases' participate in APP processing leading to the generation of either Abeta or non-amyloid proteins. However, the mechanisms of neurotoxicity of Abeta and the role of APP function in AD remain important unanswered questions. Although early studies recognized the loss of cholesterol and other lipids in the brain, these findings have been poorly connected with AD pathogenesis, despite the identification of the epsilon4 allele of APOE as a major risk factor in AD. The recent finding that cholesterol can modulate the yield of potentially toxic Abeta has boosted research on its role in AD. Consequently, several cholesterol-reducing drugs are currently being evaluated for the treatment of AD. The present review summarizes our current understanding of the relationship of AD pathogenesis with cholesterol, lipids and other genetic and environmental risk factors.

Redox regulation of glial inflammatory response to lipopolysaccharide and interferongamma

Astrocytes and microglia, the two immune-regulatory cells of the central nervous system (CNS), are activated by a variety of pathogens and cytokines to elicit rapid transcriptional responses. This program of activation is initiated by a set of intracellular signaling cascades that includes mitogen-activated protein kinase (MAPK), nuclear factor (NF) kappaB, and Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways. This study defines the critical role that NADPH oxidase(Phox)-derived reactive oxygen species (ROS) play in lipopolysaccharide (LPS)- and interferon (IFN)gamma-induced signaling cascades leading to gene expression in glial cells. Treatment of rat microglia and astrocytes with LPS and IFNgamma resulted in a rapid activation of Phox and the release of ROS followed by an induction of inducible nitric oxide synthase (iNOS) expression. iNOS induction was blocked by inhibitors of Phox, i.e., diphenylene iodonium chloride (DPI) and 4-(2-aminoethyl) benzenesulfonylfluoride (AEBSF), suggesting an involvement of ROS signaling in iNOS gene expression. Exogenous catalase but not superoxide dismutase suppressed the basal activity and completely blocked induced levels of NO/iNOS, suggesting that hydrogen peroxide is the ROS involved. Phox inhibitors and catalase also suppressed LPS/IFNgamma-induced expression of cytokines, i.e., interleukin (IL)-1, IL-6, and tumor necrosis factor (TNF)alpha and blocked LPS activation of MAP kinases (i.e., p38 MAPK, c-Jun N-terminal kinase and extracellular signal-regulated kinase), NFkappaB, and IFNgamma-induced STAT1 phosphorylation. A microglial cell line stably transfected with a mutant form of Phox subunit, i.e., p47(phox) W(193)R, and primary astrocytes derived from Phox-deficient mice showed attenuated ROS production and induction of iNOS in response to LPS/IFNgamma, further strengthening the notion that Phox-derived ROS are crucial for proinflammatory gene expression in glial cells.

Minocycline protects basal forebrain cholinergic neurons from mu p75-saporin immunotoxic lesioning

Two prominent characteristics of Alzheimer's disease are basal forebrain cholinergic degeneration and neuroinflammation characterized by glial activation and the release of pro-inflammatory cytokines. Mu p75- saporin (SAP) is a novel immunotoxin that mimics the selective loss of basal forebrain cholinergic neurons and induces cognitive impairment in mice. We report that cholinergic cell loss in the medial septal nucleus and ventral diagonal band after i.c.v. injection of mu p75-SAP is accompanied by simultaneous activation of microglia and astrocytes in the basal forebrain region as well as significant memory loss. Consistent with a role of glial cells in the pathology of Alzheimer's disease, minocycline, a second-generation tetracycline with known anti-inflammatory and neuroprotective properties, attenuated mu p75-SAP-induced cholinergic cell loss, glial activation and transcription of downstream pro-inflammatory mediators. In addition to neuroprotection, minocycline treatment mitigated the cognitive impairment that appears to be a functional consequence of mu p75-SAP lesioning. The current study demonstrates that glial-related inflammation plays a significant role in the selective neurotoxicity of mu p75-SAP, and suggests that minocycline may provide a viable therapeutic option for degenerating cholinergic systems.

TAK1-mediated induction of nitric oxide synthase gene expression in glial cells

Inflammatory cell signaling leading to transcriptional activation is primarily mediated by signal transduction via mitogen-activated protein kinase (MAPK) and NFkappaB pathways. A common upstream kinase that signals the activation of these pathways is TGFbeta-activated kinase 1 (TAK1), which itself becomes activated in response to cytokines and upon engagement of a class of cell surface receptors involved in innate immunity, that is Toll-like receptors (TLRs) by bacterial and viral pathogens. This study directly tests the role of TAK1 in the induction of inducible nitric oxide (NO) synthase (iNOS) in glial cells, which represent immune-regulatory cells of the CNS, by transient transfection assays. Transfection of C-6 glia, primary astrocytes and a rat microglial cell line with TAK1 (but not its inactive form) along with its activator protein, TAK1-binding protein 1 (TAB1) resulted in a marked stimulation of a co-transfected rat iNOS promoter-reporter construct (iNOS-Luc). TAK1-induced iNOS-Luc activity was substantially inhibited by pharmacological inhibitors of the known downstream kinases, p38 MAPK and JNK (SB203580 and SP620125), and was almost completely blocked by co-expression of a phosphorylation mutant of IkappaB. TAK1/TAB1 also induced the production of NO and the expression of iNOS in microglial cells in a p38 MAPK-, JNK- and NFkappaB-dependent manner. The results of these studies provide evidence for an important role for TAK1-mediated intracellular signaling, via p38 MAPK, JNK and NFkappaB, in the transcriptional activation of iNOS in glial cells.

Adenovirus infection induces microglial activation: involvement of mitogen-activated protein kinase pathways

Non-replicating adenovirus vectors (AdV) represent effective tools for long-term gene expression in the central nervous system (CNS), but they also elicit inflammation. The cellular and molecular mechanisms of such a response are not understood. In the present study, we show that infection with AdV causes activation of microglial cells, the key cells involved in inflammatory and immune-regulatory functions in the brain. Exposure of cultured rat brain microglia to AdV resulted in an induced production of nitric oxide (NO) and the expression of inducible nitric oxide synthase (iNOS) and the pro-inflammatory cytokine, TNFalpha. The roles of signal transduction pathways believed to be involved in microglial activation in particular, mitogen-activated protein kinases (MAPKs) and nuclear factor kappaB (NFkappaB) were explored by determining their activation in response to AdV infection and by testing the effects of specific pharmacological inhibitors. It was found that AdV strongly activates extracellular signal-regulated kinase (ERK) and to a lesser extent, p38 MAPK but not NFkappaB. Addition of the kinase inhibitor, i.e. PD98059 (specific for the ERK pathway), inhibits and, in combination with the p38 MAPK inhibitor, SB203580, drastically suppresses AdV-induced expression of iNOS and TNFalpha. The results suggest that AdV uses cellular signal transduction machinery, in particular the MAPK pathways, to elicit microglial activation and that increased production by these cells of inflammatory mediators may primarily contribute to CNS inflammatory responses commonly seen in models of gene therapy using AdV vectors.

p38 MAPK-mediated transcriptional activation of inducible nitric-oxide synthase in glial cells. Roles of nuclear factors, nuclear factor kappa B, cAMP response element-binding protein, CCAAT/enhancer-binding protein-beta, and activating transcription factor-2

Previous studies have shown that mitogen-activated protein kinase (MAPK) cascades signal the induction of inducible nitric-oxide synthase (iNOS) in glial cells (Bhat, N. R., Zhang, P., Lee, J. C., and Hogan E. L. (1998) J. Neurosci. 18, 1633-1641; Bhat, N. R., Zhang, P., and Bhat, A. N. (1999) J. Neurochem. 72, 472-478). This study further investigates the role of p38 MAPK in the transcriptional activation of the iNOS gene by transient transfection with constitutively active upstream kinases in the pathway (i.e. MAPK kinase 3 (MKK3b(E)) and MAPK kinase 6 (MKK6b(E)). Expression in C-6 glial cells of either MKK3b(E) or MKK6b(E) resulted in an induction of the activity of a cotransfected rat iNOS promoter-reporter (iNOS-luciferase (Luc)) gene and an enhancement of cytokine-induced expression of iNOS mRNA, both of which were inhibitable by the p38 MAPK inhibitor SB203580. The MKK constructs also induced cAMP response element-mediated (CRE-Luc) and nuclear factor kappa B-dependent (nuclear factor kappa B-Luc) transcriptional activities. Transfection with dominant negative (dn) forms of CRE-binding protein (CREB) and CCAAT/enhancer-binding protein (C/EBP), the two CRE-binding transcription factors targeted by the p38 MAPK pathway, resulted in opposite effects; dnCREB enhanced and dnC/EBP inhibited iNOS-Luc parallel to their effects on CRE-Luc. In addition, the induction, by MKK3b(E) and MKK6b(E), of iNOS promoter activity was enhanced by a wild-type activating transcription factor (ATF-2), whereas a phosphorylation-defective form of ATF-2 had a suppressive effect. The results of these molecular studies provide evidence for an important role for the p38 MAPK pathway in the transcriptional activation of the iNOS gene in rat glial cells involving the transcription factors nuclear factor kappa B, C/EBP, and ATF-2.

Microglial activation by the small heat shock protein, alpha-crystallin

Activated microglia regulate immune and inflammatory responses in the CNS under a variety of stresses due to infection, injury and disease. In this study, we show that a stress-inducible small heat shock protein, alpha-crystallin, induces in vitro activation of microglia cultured from newborn rat brain. Exposure of microglia to alpha-crystallin resulted in an increased production of nitric oxide (NO) and the expression of the inducible NO synthase (iNOS) as determined by Western blot and reverse transcriptase-polymerase chain reaction (RT-PCR) analyses. Alpha-crystallin also stimulated the synthesis of the pro-inflammatory cytokine, TNF alpha. The results presented showing microglial induction of the two key immune regulatory and inflammatory molecules, i.e., NO and TNF alpha, in response to a stress-inducible protein, suggest a link between environmental stress and the CNS immune response.

Cytokine induction of inducible nitric oxide synthase in an oligodendrocyte cell line: role of p38 mitogen-activated protein kinase activation

The induction of inducible nitric oxide synthase (iNOS) by proinflammatory cytokines was studied in an oligodendrocyte progenitor cell line in relation to mitogen-activated protein kinase (MAPK) activation and cytokine-mediated cytotoxicity. When introduced individually to cultures of CG4 cells, the cytokines, i.e., tumor necrosis factor-alpha (TNF alpha), interleukin-1 (IL-1), and interferon-gamma (IFN gamma), had either minimal (TNF alpha) or no (IL-1 and IFN gamma) detectable stimulatory effect on the production of nitric oxide. However, combinations of these factors, in particular, TNF alpha plus IFN gamma, elicited a strong enhancement of nitric oxide synthesis and, as revealed by western blot and RT-PCR analysis, the expression of iNOS. TNF alpha and IL-1 were able to activate p38 MAPK in a time- and dose-dependent manner and together showed a combinatorial effect. In contrast, IFN gamma neither activated on its own nor enhanced the activation of p38 MAPK in response to TNF alpha and IL-1. However, a specific inhibitor of p38 MAPK, i.e., SB203580, inhibited the induction of iNOS in cytokine combination-treated cells in a dose-dependent manner, thereby suggesting a role for the MAPK cascade in regulating the induction of iNOS gene expression in cytokine-treated cells. Blocking of nitric oxide production by an inhibitor of iNOS, i.e., nitro-L-arginine methyl ester, had a minimal protective effect against cytokine-mediated cytotoxicity that occurred before the elevation of nitric oxide levels, thereby indicating temporal and functional dissociation of nitric oxide production from cell killing.

Hydrogen peroxide activation of multiple mitogen-activated protein kinases in an oligodendrocyte cell line: role of extracellular signal-regulated kinase in hydrogen peroxide-induced cell death

Oxidative stress is known to induce cell death in a wide variety of cell types, apparently by modulating intracellular signaling pathways. In this study, we have examined the activation of mitogen-activated protein kinase (MAPK) cascades in relation to oxidant-induced cell death in an oligodendrocyte cell line, central glia-4 (CG4). Exposure of CG4 cells to hydrogen peroxide (H2O2) resulted in an increased tyrosine phosphorylation of several protein species, including the abundantly expressed platelet-derived growth factor (PDGF) receptor and the activation of the three MAPK subgroups, i.e., extracellular signal-regulated kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK). Dose-response studies showed differential sensitivities of PDGF receptor phosphorylation (>1 mM) and ERK/p38 MAPK (>0.5 mM) and JNK (>0.1 mM) activation by H2O2. The activation of ERK was inhibited by PD98059, a specific inhibitor of the upstream kinase, MAPK or ERK kinase (MEK). H2O2 also activated MAPK-activated protein kinase-2, and this activation was blocked by SB203580, a specific inhibitor of p38 MAPK. The oxidant-induced cell death was indicated by morphological changes, decreased 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, and DNA fragmentation. These effects were suppressed dose-dependently by the MEK inhibitor PD98059. The results demonstrate that H2O2 induces the activation of multiple MAPKs in oligodendrocyte progenitors and that the activation of ERK is associated with oxidant-mediated cytotoxicity.

TNFalpha potentiates IFNgamma-induced cell death in oligodendrocyte progenitors

Oligodendrocytes in multiple sclerosis brain may be under a direct attack by proinflammatory cytokines, particularly tumor necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma). In this study, we have examined the in vitro cytotoxic effects of the two cytokines, individually and in combination, on oligodendrocyte lineage cells using morphological criteria, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide reduction assay (MTT), terminal deoxynucleotide transferase-mediated dUTP nick end-labeling (TUNEL), and agarose-gel electrophoretic analysis of fragmented DNA. IFNgamma exerted a dose-dependent cytotoxic effect on cultured CG4 cells, an oligodendrocyte progenitor cell line, and in primary cultures of purified oligodendrocyte progenitors. TNFalpha, while by itself being only mildly toxic, greatly potentiated the cytotoxicity of IFNgamma. The cytokine effects were developmentally modified in that their cytotoxic and cooperative effects became less evident in more differentiated cells. A cell-permeable peptide inhibitor (i.e., z-VAD.fmk) of caspases partially suppressed apoptotic changes elicited by the cytokine combination in CG4 cells but not in primary oligodendrocytes. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis of mRNA prepared from cytokine-treated cultures revealed an increased expression of the death receptor, Fas. The results suggest particular vulnerability of oligodendrocyte progenitors to a combination of TNFalpha and IFNgamma involving an activation of the cell death program.

Extracellular signal-regulated kinase and p38 subgroups of mitogen-activated protein kinases regulate inducible nitric oxide synthase and tumor necrosis factor-alpha gene expression in endotoxin-stimulated primary glial cultures

Tumor necrosis factor-alpha (TNFalpha) and nitric oxide (NO), the product of inducible NO synthase (iNOS), mediate inflammatory and immune responses in the CNS under a variety of neuropathological situations. They are produced mainly by "activated" astrocytes and microglia, the two immune regulatory cells of the CNS. In this study we have examined the regulation of TNFalpha and iNOS gene expression in endotoxin-stimulated primary glial cultures, focusing on the role of mitogen-activated protein (MAP) kinase cascades. The bacterial lipopolysaccharide (LPS) was able to activate extracellular signal-regulated kinase (ERK) and p38 kinase subgroups of MAP kinases in microglia and astrocytes. ERK activation was sensitive to PD98059, the kinase inhibitor that is specific for ERK kinase. The activity of p38 kinase was inhibited by SB203580, a member of the novel class of cytokine suppressive anti-inflammatory drugs (CSAIDs), as revealed by blocked activation of the downstream kinase, MAP kinase-activated protein kinase-2. The treatment of glial cells with either LPS alone (microglia) or a combination of LPS and interferon-gamma (astrocytes) resulted in an induced production of NO and TNFalpha. The two kinase inhibitors, at micromolar concentrations, individually suppressed and, in combination, almost completely blocked glial production of NO and the expression of iNOS and TNFalpha, as determined by Western blot analysis. Reverse transcriptase-PCR analysis showed changes in iNOS mRNA levels that paralleled iNOS protein and NO while indicating a lack of effect of either of the kinase inhibitors on TNFalpha mRNA expression. The results demonstrate key roles for ERK and p38 MAP kinase cascades in the transcriptional and post-transcriptional regulation of iNOS and TNFalpha gene expression in endotoxin-activated glial cells.

Expression of the CB1 cannabinoid receptor in macrophage-like cells from brain tissue: immunochemical characterization by fusion protein antibodies

Antibodies designed to recognize a 74 amino acid sequence of the N- or C-terminal domain of the rat CB1 cannabinoid receptor detected a 58 kDa protein in immunoblots of brain and various cells known to express the CB1 cannabinoid receptor. A human B-lymphoblastoid cell line and macrophage-like cells from neonatal rat brain were also positive for CB1 receptor-like immunoreactivity. Immunocytochemical analysis performed with isolated Fab fragments showed surface staining in NG108-15 cells and brain macrophage like cells which also express MHC class II antigens. The data suggest a plausible role for CB1 receptors in the immune function of brain.

Activation of JNK/SAPK in primary glial cultures: II. Differential activation of kinase isoforms corresponds to their differential expression

Recently, we reported on the activation of c-Jun N-terminal kinase (JNK) in primary glial cells noting certain differences in the patterns of kinase activation in astrocytes and oligodendrocytes (Zhang et al., J Neurosci Res 46:114-121;1996). In this extended study, we have examined the activation and expression levels of JNK1 and JNK2 isoforms in different glial cell types including the two in vitro-defined astroglial subtypes (type-1 and type-2), oligodendrocytes and microglia. An in-gel kinase assay of cell extracts and JNK-immunoprecipitates revealed the activation of both JNK1 and JNK2 in type-1 astrocytes in response to TNFalpha, and in microglia, in response to TNFalpha and bacterial lipopolysaccharide. The strong activation of the two JNK isoforms in type-1 astrocytes and microglia contrasted with a predominant activation of JNK1 over JNK2 in type-2 astrocytes and oligodendrocytes, the two glial subtypes sharing a common lineage. Immunoblot and immunocytochemical analyses using isoform-specific antibodies showed a differential expression of the two isoforms in different glial cells thereby accounting for their observed differential activation.

Neutral monoglycosylceramides in rat brain: occurrence, molecular expression and developmental variation

Developmental expression of neutral monoglycosylceramides (MGCs) has been examined in rat brain from embryonic day 15 (E15) to postnatal day 30 (P30) and adulthood. To this point, glucosylceramide (GlcCer) is the only MGC that has been characterized in embryonic brain. Galactosylceramide (GalCer) appears at P1, increases with age until P25 and remains constant thereafter. The developmental occurrence of GlcCer and GalCer agrees well with their respective glucosyl- and galactosyltransferase activities. Cerebroside fatty-acid and base compositions, examined by gas chromatography, also change during development. Several alkali-labile fast-migrating cerebrosides (FMCs) with a higher thin-layer chromatography RF than GalCer/ GlcCer are expressed early at P10, increase in concentration with age (P25-P30) and are unchanged until maturity. They are derivatives of GalCer. By employing a newly developed neutral methylation procedure, we have confirmed the structure of one of the FMCs as 6-acylGalCer. A reduction in brain FMC concentrations along with GalCer in murine genetic dysmyelinating disorders (jimpy and quaking) further supports the conclusion that they are myelin constituents.

Activation of C-jun N-terminal kinase/stress-activated protein kinase in primary glial cultures

Glial cells in the mammalian CNS are subject to environmental stress resulting from a variety of neuro-pathological conditions. In this study, we have examined the activation of a stress signal responsive kinase, i.e., stress-activated protein kinase (SAPK) or c-Jun N-terminal kinase (JNK), in primary cultures of rat brain glial cells (i.e., astrocytes and oligodendrocytes) and an oligodendrocyte progenitor cell line, CG4, in response to cytokines and other stress inducers. JNK/SAPK activity was measured by an immune complex kinase assay using polyclonal anti-JNK antibodies along with GST c-Jun (1-79) as the substrate. Among the cytokines tested, TNF-alpha had the strongest effect on JNK activation followed by TNF-beta in both the glial cell types while a substantial level of kinase activation was observed in response to IL-1 in astrocytes. JNK activation by TNF-alpha in astrocytes, but not in oligodendrocytes, showed a biphasic response. An in-gel kinase assay of cell extracts and immunoprecipitated JNK confirmed the activation of JNK1 in cells treated with TNF-alpha. JNK was also activated by several other stress-inducing factors including. UV light, heat shock, inhibitors of protein synthesis, and mechanical injury. Incubation of cells with bacterial sphingomyelinase and a cell-permeable ceramide stimulated JNK activity, suggesting that the ceramide pathway may play a role in JNK activation, although the time course of activation did not correspond to that of TNF-alpha. The results are discussed in terms of possible roles of JNK activation in signaling for gliosis in astrocytes and as a protective/toxic response in oligodendrocytes.

Activation of mitogen-activated protein kinases in oligodendrocytes

The proliferation and differentiation of oligodendrocyte progenitors are stringently controlled by an interacting network of growth and differentiation factors. Not much is known, however, about the intracellular signaling pathways activated in oligodendrocytes. In this study, we have examined the activation of mitogen-activated protein (MAP) kinase [also called extracellular signal-regulated protein kinases (ERKs)] in primary cultures of developing oligodendrocytes and in a primary oligodendrocyte cell line, CG4, in response to platelet-derived growth factor (PDGF) and basic fibroblast growth factor. MAP kinase activation was determined by an ingel protein kinase renaturation assay using myelin basic protein (MBP) as the substrate. The specificity of MAP kinase activation was further confirmed by an immune complex kinase assay using anti-MAP kinase antibodies. Stimulation of oligodendrocyte progenitors with the growth factors PDGF and basic fibroblast growth factor and a protein kinase C-activating tumor promoter, phorbol 12-myristate 13-acetate, resulted in a rapid activation of p42mapk (ERK2) and, to a lesser extent, p44mapk (ERK1). Immunoblot analysis with anti-phosphotyrosine antibodies revealed an increased Tyr phosphorylation of a 42-kDa phosphoprotein band cross-reacting with anti-MAP kinase antibodies. The phosphorylation of p42mapk in PDGF-treated oligodendrocyte progenitors was preceded by a robust autophosphorylation of the growth factor receptor. Immunoblot analysis with anti-pan-ERK antibodies indicated the presence of ERK-immunoreactive species other than p42mapk and p44mapk in oligodendrocytes. The presence of some of the same pan-ERK-immunoreactive species and certain renaturable MBP kinase activities was also demonstrable in myelin preparations from rat brain, suggesting that MAP kinases (and other MBP kinases) may function not only during oligodendrogenesis but also in myelinogenesis.

Thrombin activates mitogen-activated protein kinase in primary astrocyte cultures

Thrombin is known to evoke numerous inflammatory and proliferative responses in a wide variety of its target cells. Recent studies have demonstrated morphoregulatory and mitogenic effects of thrombin on astroglial cells (astrocytes). The present study deals with thrombin-induced activation of mitogen-activated protein (MAP) kinase in primary cultures of rat astrocytes. Treatment of serum-starved astrocytes with thrombin resulted in a rapid activation of tyrosine (Tyr) phosphorylation of a set of proteins including a prominent one with a molecular mass of 42 kDa (p42). The identity of p42 with MAP kinase was confirmed by MAP kinase-immunoreactivity of isolated [i.e., immunoprecipitated with anti-phosphotyrosine (PY) antibodies] p42 and by increased myelin basic protein (MBP) kinase activity present in MAP kinase immunoprecipitates of thrombin-treated cultures. Pertussis toxin (PTX) pretreatment failed to inhibit thrombin stimulation of p42 phosphorylation, indicating the lack of involvement of PTX sensitive G proteins in the mechanism of activation of MAP kinase by thrombin. Chronic exposure of cultures to phorbol 12-myristate 13-acetate to down-regulate PKC resulted in an attenuation of thrombin-induced p42 Tyr phosphorylation, although H-7, a known PKC inhibitor, failed to block thrombin effect. However, staurosporine, a nonspecific protein kinase inhibitor, prevented the activation of p42 phosphorylation. It is concluded that thrombin induces MAP kinase activation in astrocytes by a mechanism involving a staurosporine-sensitive pathway.

Hyaluronectin binds to laminin and blocks laminin-dependent process formation by astrocytes

The interaction of hyaluronectin (HN), a hyaluronic acid-binding extracellular matrix (ECM) glycoprotein with two other ECM-associated molecules, laminin and fibronectin, was studied by ligand blot and solid phase ligand binding assays. Ligand blot analysis with biotin-labeled HN revealed a strong binding of HN to immobilized laminin and a weaker binding to fibronectin. Ligand binding studies indicated a concentration dependent, Ca(2+)-independent binding of HN to laminin. Binding of HN to laminin but not to fibronectin was resistant to increased salt concentrations indicating a non-electrostatic, protein-protein interaction of HN with laminin. The functional relevance of HN-laminin interaction was demonstrated by an inhibition of laminin-supported astroglial process formation by HN.