Excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter and functions to remove glutamate from synapses. A thiopyridazine derivative has been found to increase EAAT2 protein levels in astrocytes. A structure-activity relationship study revealed that several components of the molecule were required for activity, such as the thioether and pyridazine. Modification of the benzylthioether resulted in several derivatives (7–13, 7–15 and 7–17) that enhanced EAAT2 levels by > 6 fold at concentrations < 5 μM after 24 h. In addition, one of the derivatives (7–22) enhanced EAAT2 levels 3.5 – 3.9 fold after 24 h with an EC50 of 0.5 μM.
Overproduction of free radicals can damage cellular components resulting in progressive physiological dysfunction, which has been implicated in many human diseases. Oxidative damage to RNA received little attention until the past decade. Recent studies indicate that RNA, such as messenger RNA and ribosomal RNA, is very vulnerable to oxidative damage. RNA oxidation is not a consequence of dying cells but an early event involved in pathogenesis. Oxidative modification to RNA results in disturbance of the translational process and impairment of protein synthesis, which can cause cell deterioration or even cell death. In this review, we discuss the mechanisms of oxidative damage to RNA and the possible biological consequences of damaged RNA. Furthermore, we review recent evidence suggesting that oxidative damage to RNA may contribute to progression of many human diseases.
RNA oxidation; Oxidative damage; Oxidative stress; Free radical; Neurodegeneration; Alzheimer’s disease; ALS
The glial glutamate transporter EAAT2 is the major mediator of glutamate clearance that terminates glutamate-mediated neurotransmission. Loss of EAAT2 and associated glutamate uptake function has been reported in the brains of patients with Alzheimer’s disease (AD). We previously reported that EAAT2 is associated with lipid raft microdomains of the plasma membrane. In the present study, we demonstrated that association of EAAT2 with lipid rafts is disrupted in AD brains. This abnormality is not a consequence of neuron degeneration, oxidative stress, or amyloid beta toxicity. In AD brains, cholesterol 24S-hydroxylase (CYP46), a key enzyme in maintenance of cholesterol homeostasis in the brain, is markedly increased in astrocytes but decreased in neurons. We demonstrated that increased expression of CYP46 in primary astrocytes results in a reduction of membrane cholesterol levels and leads to the dissociation of EAAT2 from lipid rafts and the loss of EAAT2 and associated glutamate uptake function. These results suggest that a disturbance of cholesterol metabolism may contribute to loss of EAAT2 in AD.
glutamate transporter EAAT2; lipid raft microdomain; Alzheimer’s disease; cholesterol 24S-hydroxylase; excitotoxicity
Excitotoxicity has been implicated as the mechanism of neuronal damage resulting from acute insults such as stroke, epilepsy, and trauma, as well as during the progression of adult-onset neurodegenerative disorders such as Alzheimer’s disease and amyotrophic lateral sclerosis (ALS). Excitotoxicity is defined as excessive exposure to the neurotransmitter glutamate or overstimulation of its membrane receptors, leading to neuronal injury or death. One potential approach to protect against excitotoxic neuronal damage is enhanced glutamate reuptake. The glial glutamate transporter EAAT2 is the quantitatively dominant glutamate transporter and plays a major role in clearance of glutamate. Expression of EAAT2 protein is highly regulated at the translational level. In an effort to identify compounds that can induce translation of EAAT2 transcripts, a cell-based enzyme-linked immunosorbent assay was developed using a primary astrocyte line stably transfected with a vector designed to identify modulators of EAAT2 translation. This assay was optimized for high-throughput screening, and a library of approximately 140,000 compounds was tested. In the initial screen, 293 compounds were identified as hits. These 293 hits were retested at 3 concentrations, and a total of 61 compounds showed a dose-dependent increase in EAAT2 protein levels. Selected compounds were tested in full 12-point dose-response experiments in the screening assay to assess potency as well as confirmed by Western blot, immunohistochemistry, and glutamate uptake assays to evaluate the localization and function of the elevated EAAT2 protein. These hits provide excellent starting points for developing therapeutic agents to prevent excitotoxicity.
excitotoxicity; glutamate transporter; EAAT2; high-throughput screen; neurodegeneration
The heterologous expression and activation of the human P2Y2 nucleotide receptor (P2Y2R) in human 1321N1 astrocytoma cells stimulates α-secretase-dependent cleavage of the amyloid precursor protein (APP), causing extracellular release of the non-amyloidogenic protein sAPPα. To determine whether a similar response occurs in a neuronal cell, we analyzed whether P2Y2R-mediated production of sAPPα occurs in rat primary cortical neurons (rPCNs). In rPCNs, P2Y2R mRNA and receptor activity were virtually absent in quiescent cells, whereas overnight treatment with the pro-inflammatory cytokine interleukin-1β (IL-1β) upregulated both P2Y2R mRNA expression and receptor activity by four-fold. The upregulation of the P2Y2R was abrogated by pre-incubation with Bay 11-7085, an IκB-α phosphorylation inhibitor, which suggests that P2Y2R mRNA transcript levels are regulated through NF-κB signaling. Furthermore, the P2Y2R agonist UTP enhanced the release of sAPPα in rPCNs treated with IL-1β or transfected with P2Y2R cDNA. UTP-induced release of sAPPα from rPCNs was completely inhibited by pretreatment of the cells with the metalloproteinase inhibitor TAPI-2 or the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, and was partially inhibited by the MAPK/ERK kinase (MEK) inhibitor U0126 and the protein kinase C (PKC) inhibitor GF109203. These data suggest that P2Y2R-mediated release of sAPPα from cortical neurons is directly dependent on ADAM10/17 and PI3K activity, whereas ERK1/2 and PI3K activity may indirectly regulate APP processing. These results demonstrate that elevated levels of pro-inflammatory cytokines associated with neurodegenerative diseases, such as IL-1β, can enhance non-amyloidogenic APP processing through upregulation of the P2Y2R in neurons.
Accumulating evidence indicates that RNA oxidation is involved in a wide variety of neurological diseases and may be associated with neuronal deterioration during the process of neurodegeneration. However, previous studies were done in postmortem tissues or cultured neurons. Here, we used transgenic mice to demonstrate the role of RNA oxidation in the process of neurodegeneration.
We demonstrated that messenger RNA (mRNA) oxidation is a common feature in amyotrophic lateral sclerosis (ALS) patients as well as in many different transgenic mice expressing familial ALS-linked mutant copper-zinc superoxide dismutase (SOD1). In mutant SOD1 mice, increased mRNA oxidation primarily occurs in the motor neurons and oligodendrocytes of the spinal cord at an early, pre-symptomatic stage. Identification of oxidized mRNA species revealed that some species are more vulnerable to oxidative damage, and importantly, many oxidized mRNA species have been implicated in the pathogenesis of ALS. Oxidative modification of mRNA causes reduced protein expression. Reduced mRNA oxidation by vitamin E restores protein expression and partially protects motor neurons.
These findings suggest that mRNA oxidation is an early event associated with motor neuron deterioration in ALS, and may be also a common early event preceding neuron degeneration in other neurological diseases.
Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [Ross (Nature 362:801–09, 1993); Fuster et al. (N Engl J Med 326:242–50, 1992); Davies and Woolf (Br Heart J 69:S3–S11, 1993)]. Extracellular nucleotides that are released from a variety of arterial and blood cells [Di Virgilio and Solini (Br J Pharmacol 135:831–42, 2002)] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which are known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [Lafont et al. (Circ Res 76:996–002, 1995)]. In addition, P2 receptors mediate many other functions including platelet aggregation, leukocyte adherence, and arterial vasomotricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that upregulation and activation of P2Y2 receptors in rabbit arteries mediates intimal hyperplasia [Seye et al. (Circulation 106:2720–726, 2002)]. In addition, upregulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [Carpenter et al. (Stroke 32:516–22, 2001)] and in coronary artery of diabetic dyslipidemic pigs [Hill et al. (J Vasc Res 38:432–43, 2001)]. It has been proposed that upregulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [Elmaleh et al. (Proc Natl Acad Sci U S A 95:691–95, 1998)]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.
Atherosclerosis; Inflammation; Migration; Nucleotide receptors; Proliferation; Restenosis; Smooth muscle cell
Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [1–3]. Extracellular nucleotides that are released from a variety of arterial and blood cells  can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which is known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis . In addition, P2 receptors mediate many other functions, including platelet aggregation, leukocyte adherence, and arterial vasomotoricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that up-regulation and activation of P2Y2 receptors in rabbit arteries mediates intimal hyperplasia . In addition, up-regulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model  and in coronary arteries of diabetic dyslipidemic pigs . It has been proposed that up-regulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis . Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.
atherosclerosis; inflammation; migration; nucleotide receptors; proliferation; restenosis; smooth muscle cell
Apoptosis is a major cause of cell death in the nervous system. It plays a role in embryonic and early postnatal brain development and contributes to the pathology of neurodegenerative diseases. Here, we report that activation of the P2X7 nucleotide receptor (P2X7R) in rat primary cortical neurons (rPCNs) causes biochemical (i.e., caspase activation) and morphological (i.e., nuclear condensation and DNA fragmentation) changes characteristic of apoptotic cell death. Caspase-3 activation and DNA fragmentation in rPCNs induced by the P2X7R agonist BzATP were inhibited by the P2X7R antagonist oxidized ATP (oATP) or by pre-treatment of cells with P2X7R antisense oligonucleotide indicating a direct involvement of the P2X7R in nucleotide-induced neuronal cell death. Moreover, Z-DEVD-FMK, a specific and irreversible cell permeable inhibitor of caspase-3, prevented BzATP-induced apoptosis in rPCNs. In addition, a specific caspase-8 inhibitor, Ac-IETD-CHO, significantly attenuated BzATP-induced caspase-9 and caspase-3 activation, suggesting that P2X7R-mediated apoptosis in rPCNs occurs primarily through an intrinsic caspase-8/9/3 activation pathway. BzATP also induced the activation of C-jun N-terminal kinase 1 (JNK1) and extracellular signal-regulated kinases (ERK1/2) in rPCNs, and pharmacological inhibition of either JNK1 or ERK1/2 significantly reduced caspase activation by BzATP. Taken together, these data indicate that extracellular nucleotides mediate neuronal apoptosis through activation of P2X7Rs and their downstream signaling pathways involving JNK1, ERK and caspases 8/9/3.
apoptosis; ATP; BzATP; caspase; P2X7 receptor
The P2Y2 nucleotide receptor (P2Y2R) contains the integrin-binding domain arginine-glycine-aspartic acid (RGD) in its first extracellular loop, raising the possibility that this G protein–coupled receptor interacts directly with an integrin. Binding of a peptide corresponding to the first extracellular loop of the P2Y2R to K562 erythroleukemia cells was inhibited by antibodies against αVβ3/β5 integrins and the integrin-associated thrombospondin receptor, CD47. Immunofluorescence of cells transfected with epitope-tagged P2Y2Rs indicated that αV integrins colocalized 10-fold better with the wild-type P2Y2R than with a mutant P2Y2R in which the RGD sequence was replaced with RGE. Compared with the wild-type P2Y2R, the RGE mutant required 1,000-fold higher agonist concentrations to phosphorylate focal adhesion kinase, activate extracellular signal–regulated kinases, and initiate the PLC-dependent mobilization of intracellular Ca2+. Furthermore, an anti-αV integrin antibody partially inhibited these signaling events mediated by the wild-type P2Y2R. Pertussis toxin, an inhibitor of Gi/o proteins, partially inhibited Ca2+ mobilization mediated by the wild-type P2Y2R, but not by the RGE mutant, suggesting that the RGD sequence is required for P2Y2R-mediated activation of Go, but not Gq. Since CD47 has been shown to associate directly with Gi/o family proteins, these results suggest that interactions between P2Y2Rs, integrins, and CD47 may be important for coupling the P2Y2R to Go.
purinergic receptors; cell surface receptors; integrins; GTP-binding proteins; signal transduction