Amyloid beta peptide (Abeta42) activates PLC-delta1 promoter through the NF-kappaB binding site

Cell Type-Specific Roles of NF-κB Linking Inflammation and Thrombosis

The transcription factor NF-κB is a central mediator of inflammation with multiple links to thrombotic processes. In this review, we focus on the role of NF-κB signaling in cell types within the vasculature and the circulation that are involved in thrombo-inflammatory processes. All these cells express NF-κB, which mediates important functions in cellular interactions, cell survival and differentiation, as well as expression of cytokines, chemokines, and coagulation factors. Even platelets, as anucleated cells, contain NF-κB family members and their corresponding signaling molecules, which are involved in platelet activation, as well as secondary feedback circuits. The response of endothelial cells to inflammation and NF-κB activation is characterized by the induction of adhesion molecules promoting binding and transmigration of leukocytes, while simultaneously increasing their thrombogenic potential. Paracrine signaling from endothelial cells activates NF-κB in vascular smooth muscle cells and causes a phenotypic switch to a “synthetic” state associated with a decrease in contractile proteins. Monocytes react to inflammatory situations with enforced expression of tissue factor and after differentiation to macrophages with altered polarization. Neutrophils respond with an extension of their life span—and upon full activation they can expel their DNA thereby forming so-called neutrophil extracellular traps (NETs), which exert antibacterial functions, but also induce a strong coagulatory response. This may cause formation of microthrombi that are important for the immobilization of pathogens, a process designated as immunothrombosis. However, deregulation of the complex cellular links between inflammation and thrombosis by unrestrained NET formation or the loss of the endothelial layer due to mechanical rupture or erosion can result in rapid activation and aggregation of platelets and the manifestation of thrombo-inflammatory diseases. Sepsis is an important example of such a disorder caused by a dysregulated host response to infection finally leading to severe coagulopathies. NF-κB is critically involved in these pathophysiological processes as it induces both inflammatory and thrombotic responses.

Phospholipase C δ1 in macrophages negatively regulates TLR4-induced proinflammatory cytokine production and Fcγ receptor-mediated phagocytosis

Macrophages are key players in the innate immune response. Turnover of phosphoinositides (PI), particularly phosphatidylinositol 4,5 bisphosphate (PI(4,5)P2), has been implicated in macrophage functions such as toll-like receptor (TLR)-mediated cytokine production and phagocytosis. However, PI metabolizing enzymes responsible for macrophage functions are not well defined. The phospholipase C (PLC) family of enzymes is critical in PI(4,5)P2 turnover. In this study, we investigated the role of PLCδ1, a prototype PLC, in macrophages on the expression of inflammation-associated genes and phagocytosis. Lipopolysaccharides (LPS) signal through TLR4 to produce proinflammatory cytokines such as interleukin (IL)-1β. LPS stimulation of both RAW264.7 murine macrophages and murine bone marrow-derived macrophages resulted in lower PLCδ1 mRNA and protein expression levels, compared to that in the control. Using chemical inhibitor compounds, we demonstrated that the up-regulation of p38 MAPK activity led to down-regulation of PLCδ1 mRNA expression in macrophages. PLCδ1 reduction by RNAi or gene deletion resulted in greater LPS-induced IL-1β expression than that observed in the control siRNA-treated cells, without increasing TLR4 cell surface expression. PLCδ1 also negatively regulated LPS-induced cell spreading. Analysis of Fcγ receptor-mediated phagocytosis demonstrated an increased phagocytosis index after PLCδ1 knockdown in RAW264.7 cells. Conversely, overexpression of PLCδ1 reduced phagocytosis whereas catalytic inactive PLCδ1 had no effect. Altered levels of PLCδ1 affected the binding of opsonized latex beads with cells, rather than the phagocytic activity. Taken together, the data suggest that PLCδ1 negatively regulates LPS-induced production of IL-1β and Fcγ receptor-mediated phagocytosis in macrophages.

Endothelial calcium dynamics, connexin channels and blood-brain barrier function

Situated between the circulation and the brain, the blood-brain barrier (BBB) protects the brain from circulating toxins while securing a specialized environment for neuro-glial signaling. BBB capillary endothelial cells exhibit low transcytotic activity and a tight, junctional network that, aided by the cytoskeleton, restricts paracellular permeability. The latter is subject of extensive research as it relates to neuropathology, edema and inflammation. A key determinant in regulating paracellular permeability is the endothelial cytoplasmic Ca(2+) concentration ([Ca(2+)]i) that affects junctional and cytoskeletal proteins. Ca(2+) signals are not one-time events restricted to a single cell but often appear as oscillatory [Ca(2+)]i changes that may propagate between cells as intercellular Ca(2+) waves. The effect of Ca(2+) oscillations/waves on BBB function is largely unknown and we here review current evidence on how [Ca(2+)]i dynamics influence BBB permeability.

Oligomeric amyloid-β peptide affects the expression of genes involved in steroid and lipid metabolism in primary neurons

Amyloid-β peptide (Aβ) is the principal component of plaques in the brains of patients with Alzheimer's disease (AD), and the most toxic form of Aβ may be as soluble oligomers. We report here the results of a microarray study of gene expression profiles in primary mouse cortical neurons in response to oligomeric Aβ(1-42). A major and unexpected finding was the down-regulation of genes involved in the biosynthesis of cholesterol and other steroids and lipids (such as Fdft1, Fdps, Idi1, Ldr, Mvd, Mvk, Nsdhl, Sc4mol), the expression of which was verified by quantitative real-time RT-PCR (qPCR). The ATP-binding cassette gene Abca1, which has a major role in cholesterol transport in brain and other tissues and has been genetically linked to AD, was notably up-regulated. The possible involvement of cholesterol and other lipids in Aβ synthesis and action in Alzheimer's disease has been studied and debated extensively but remains unresolved. These new data suggest that Aβ may influence steroid and lipid metabolism in neurons via multiple gene-expression changes.

Calcium fluxes cause nuclear shrinkage and the translocation of phospholipase C-delta1 into the nucleus

Phospholipase C-delta1 (PLCdelta1) is the most fundamental form of the eukaryotic PLC and thought to play important roles in the regulation of cells. We previously reported that PLCdelta1 shuttles between the cytoplasm and nucleus, and an influx of Ca2+ triggers the nuclear import of PLCdelta1 via Ca2+-dependent interaction with importin beta1, although the physiological meaning of this is unclear. Here we have examined the distribution of PLCdelta1 using primary cultures of rat hippocampal neurons. Treatment of 7DIV neurons with ionomycin or thapsigargin caused the nuclear localization of PLCdelta1 as has been observed in other cell lines. Similar results were obtained with neurons treated with glutamate, suggesting that the nuclear localization of PLCdelta1 plays some roles in excitotoxicity associated with ischemic stress. Generally, cells undergoing ischemic or hypoxic cell death show nuclear shrinkage. We confirmed that a massive influx of Ca2+ caused similar results. Furthermore, overexpression of GFP-PLCdelta1 facilitated ionomycin-induced nuclear shrinkage in embryonic fibroblasts derived from PLCdelta1 gene-knockout mice (PLCdelta1KO-MEF). By contrast, an E341A mutant that cannot bind with importin beta1 and be imported into the nucleus by ionomycin and also lacks enzymatic activity did not cause nuclear shrinkage in PLCdelta1KO-MEF. Nuclear translocation and the PLC activity of PLCdelta1, therefore, may regulate the nuclear shape by controlling the nuclear scaffold during stress-induced cell death caused by high levels of Ca2+.

Transient receptor potential channels in Alzheimer's disease

Cognitive impairment and emotional disturbances in Alzheimer's disease (AD) result from the degeneration of synapses and neuronal death in the limbic system and associated regions of the cerebral cortex. An alteration in the proteolytic processing of the amyloid precursor protein (APP) results in increased production and accumulation of amyloid beta-peptide (Abeta) in the brain. Abeta can render neurons vulnerable to excitotoxicity and apoptosis by disruption of cellular Ca(2+) homeostasis and neurotoxic factors including reactive oxygen species (ROS), nitric oxide (NO), and cytokines. Many lines of evidence have suggested that transient receptor potential (TRP) channels consisting of six main subfamilies termed the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and TRPA (ankyrin) are involved in Ca(2+) homeostasis disruption. Thus, emerging evidence of the pathophysiological role of TRP channels has yielded promising candidates for molecular entities mediating Ca(2+) homeostasis disruption in AD. In this review, we focus on the TRP channels in AD and highlight some TRP "suspects" for which a role in AD can be anticipated. An understanding of the involvement of TRP channels in AD may lead to the development of new target therapies.

β-amyloid (25–35) enhances lipid metabolism and protein ubiquitination in cultured neurons

We investigated the effect of beta-amyloid (Abeta) (25-35), a cytotoxic fragment of Abeta peptide, on lipid metabolism and protein ubiquitination in cultured rat hippocampal neurons. After treatment with Abeta under conditions leading to apoptotis, as assessed by caspase activity assay, the total cell mass of lipids changed following a biphasic behavior, with an increase that reached a maximum after 16 hr of treatment, followed by a decrease. The increase at 16 hr was 15.3% in the case of phospholipids and 103.0% in the case of gangliosides and was due to enhanced biosynthesis as confirmed by increase of radioactivity incorporation (phospholipids +52.0%, gangliosides +193.1%) in cells fed with tritiated palmitic acid. No change with respect to cholesterol was observed. Strikingly, under these conditions, the ubiquitination state of cell proteins strongly increased. These effects were not observed with the (35-25) reverse sequence peptide. Similarly to Abeta, lactacystin treatment increased lipid synthesis and protein ubiquitination; only lactacystin, and not Abeta, induced a strong decrease of proteasome chimotrypsin activity. These results suggest that Abeta enhances protein ubiquitination, without inhibiting proteasomal activity, and lipid synthesis. These results may shed new light on the mechanisms of Abeta toxicity.

Possible role of scavenger receptor SRCL in the clearance of amyloid-βin Alzheimer's disease

Accumulation of beta-amyloid protein (Abeta) in the brain is a hallmark of Alzheimer's disease (AD), and Abeta-mediated pathogenesis could result from increased production of Abeta or insufficient Abeta clearance by microglia, astrocytes, or the vascular system. Cell-surface receptors, such as scavenger receptors, might play a critical role in the binding and clearing of Abeta; however, the responsible receptors have yet to be identified. We show that scavenger receptor with C-type lectin (SRCL), a member of the scavenger receptor family containing coiled-coil, collagen-like, and C-type lectin/carbohydrate recognition domains, is expressed in cultured astrocytes and microglia. In contrast to the low expression of SRCL in the wild-type mouse brain, in a double transgenic mouse model of AD (Tg-APP/PS1), immunohistochemistry showed that SRCL was markedly induced in Abeta-positive astrocytes and Abeta-positive vascular/perivascular cells, which are associated closely with cerebral amyloid angiopathy. In patients with AD, the distribution of SRCL was similar to that seen in the Tg-APP/PS1 temporal cortex. The presence of a large number of SRCL/Abeta double-positive particles in the intracellular compartments of reactive astrocytes and vascular/perivascular cells in Tg-APP/PS1 mice and AD patients suggests a role for SRCL in Abeta clearance. Moreover, CHO-K1 cells transfected with SRCL isoforms were found to bind fibrillar Abeta(1-42). These findings suggest that SRCL could be the receptor involved in the binding or clearing of Abeta by glial and vascular/perivascular cells in AD.

Phospholipase C-delta1 is a critical target for tumor necrosis factor receptor-mediated protection against adriamycin-induced cardiac injury

The clinical application of adriamycin, an exceptionally good chemotherapeutic agent, is limited by its dose-related cardiomyopathy. Our recent study showed that tumor necrosis factor-alpha (TNF-alpha) receptors mediated cytoprotective signaling against adriamycin-induced mitochondrial injury and cardiomyocyte apoptosis. In the present study, we investigated the potential targets of TNF receptor-mediated cytoprotective signaling by global genome microarray analysis using wild-type and TNF receptor-deficient mice. Microarray analysis revealed that adriamycin treatment induced the down-regulation of several mitochondrial functions and energy production-related genes in double TNF receptor-deficient mice, notably, phospholipase C-delta1, a protein involved in fatty acid metabolism and calcium regulation. The role of phospholipase C-delta1 in TNF receptor-mediated cardioprotection against adriamycin-induced injury was evaluated by measuring changes in cardiac function using high-frequency ultrasound biomicroscopy. Selective inhibition of phospholipase C activity in wild-type mice by its inhibitor, U73122, exacerbated adriamycin-induced cardiac dysfunction. Inhibition of phospholipase C-delta1 resulted in the significant decrease of left ventricular ejection fraction and fractional shortening, and the decreased levels were similar to those observed in adriamycin-treated double TNF receptor-deficient mice. The data derived from the global genome analysis identified phospholipase C-delta1 as an important target for TNF receptors and revealed the critical role of TNF receptor signaling in the protection against adriamycin-induced cardiotoxicity.