Cell cycle related signaling in neuro2a cells proceeds via the receptor for advanced glycation end products

Role and Therapeutic Potential of RAGE Signaling in Neurodegeneration

Activation of the receptor for advanced glycation end products (RAGE) has been shown to play an active role in the development of multiple neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Amyotrophic Lateral Sclerosis. Although originally identified as a receptor for advanced glycation end products, RAGE is a pattern recognition receptor able to bind multiple ligands. The final outcome of RAGE signaling is defined in a context and cell type specific manner and can exert both neurotoxic and neuroprotective functions. Contributing to the complexity of the RAGE signaling network, different RAGE isoforms with distinctive signaling capabilities have been described. Moreover, multiple RAGE ligands bind other receptors and RAGE antagonism can significantly affect their signaling. Here, we discuss the outcome of celltype specific RAGE signaling in neurodegenerative pathologies. In addition, we will review the different approaches that have been developed to target RAGE signaling and their therapeutic potential. A clear understanding of the outcome of RAGE signaling in a cell type- and disease-specific manner would contribute to advancing the development of new therapies targeting RAGE. The ability to counteract RAGE neurotoxic signaling while preserving its neuroprotective effects would be critical for the success of novel therapies targeting RAGE signaling.

Hyperglycemia-Induced Aberrant Cell Proliferation; A Metabolic Challenge Mediated by Protein O-GlcNAc Modification

Chronic hyperglycemia has been associated with an increased prevalence of pathological conditions including cardiovascular disease, cancer, or various disorders of the immune system. In some cases, these associations may be traced back to a common underlying cause, but more often, hyperglycemia and the disturbance in metabolic balance directly facilitate pathological changes in the regular cellular functions. One such cellular function crucial for every living organism is cell cycle regulation/mitotic activity. Although metabolic challenges have long been recognized to influence cell proliferation, the direct impact of diabetes on cell cycle regulatory elements is a relatively uncharted territory. Among other "nutrient sensing" mechanisms, protein O-linked β-N-acetylglucosamine (O-GlcNAc) modification emerged in recent years as a major contributor to the deleterious effects of hyperglycemia. An increasing amount of evidence suggest that O-GlcNAc may significantly influence the cell cycle and cellular proliferation. In our present review, we summarize the current data available on the direct impact of metabolic changes caused by hyperglycemia in pathological conditions associated with cell cycle disorders. We also review published experimental evidence supporting the hypothesis that O-GlcNAc modification may be one of the missing links between metabolic regulation and cellular proliferation.

RAGE Expression and ROS Generation in Neurons: Differentiation versus Damage

RAGE is a multiligand receptor able to bind advanced glycation end-products (AGEs), amphoterin, calgranulins, and amyloid-beta peptides, identified in many tissues and cells, including neurons. RAGE stimulation induces the generation of reactive oxygen species (ROS) mainly through the activity of NADPH oxidases. In neuronal cells, RAGE-induced ROS generation is able to favor cell survival and differentiation or to induce death through the imbalance of redox state. The dual nature of RAGE signaling in neurons depends not only on the intensity of RAGE activation but also on the ability of RAGE-bearing cells to adapt to ROS generation. In this review we highlight these aspects of RAGE signaling regulation in neuronal cells.

S100B inhibition reduces behavioral and pathologic changes in experimental traumatic brain injury

Neuroinflammation following traumatic brain injury (TBI) is increasingly recognized to contribute to chronic tissue loss and neurologic dysfunction. Circulating levels of S100B increase after TBI and have been used as a biomarker. S100B is produced by activated astrocytes and can promote microglial activation; signaling by S100B through interaction with the multiligand advanced glycation end product-specific receptor (AGER) has been implicated in brain injury and microglial activation during chronic neurodegeneration. We examined the effects of S100B inhibition in a controlled cortical impact model, using S100B knockout mice or administration of neutralizing S100B antibody. Both interventions significantly reduced TBI-induced lesion volume, improved retention memory function, and attenuated microglial activation. The neutralizing antibody also significantly reduced sensorimotor deficits and improved neuronal survival in the cortex. However, S100B did not alter microglial activation in BV2 cells or primary microglial cultures stimulated by lipopolysaccharide or interferon gamma. Further, proximity ligation assays did not support direct interaction in the brain between S100B and AGER following TBI. Future studies are needed to elucidate specific pathways underlying S100B-mediated neuroinflammatory actions after TBI. Our results strongly implicate S100B in TBI-induced neuroinflammation, cell loss, and neurologic dysfunction, thereby indicating that it is a potential therapeutic target for TBI.

Advanced glycation end products are mitogenic signals and trigger cell cycle reentry of neurons in Alzheimer's disease brain

Neurons that reenter the cell cycle die rather than divide, a phenomenon that is associated with neurodegeneration in Alzheimer's disease (AD). Reexpression of cell-cycle related genes in differentiated neurons in AD might be rooted in aberrant mitogenic signaling. Because microglia and astroglia proliferate in the vicinity of amyloid plaques, it is likely that plaque components or factors secreted from plaque-activated glia induce neuronal mitogenic signaling. Advanced glycation end products (AGEs), protein-bound oxidation products of sugar, might be one of those mitogenic compounds. Cyclin D1 positive neurons are colocalized with AGEs or directly surrounded by extracellular AGE deposits in AD brain. However, a direct proof of DNA replication in these cells has been missing. Here, we report by using fluorescent in situ hybridization that consistent with the expression of cell cycle proteins, hyperploid neuronal cells are in colocalization with AGE staining in AD brains but not in nondemented controls. To complement human data, we used apolipoprotein E-deficient mice as model of neurodegeneration and showed that increased oxidative stress caused an intensified neuronal deposition of AGEs, being accompanied by an activation of the MAPK cascade via RAGE. This cascade, in turn, induced the expression of cyclin D1 and DNA replication. In addition, reduction of oxidative stress by application of α-lipoic acid decreased AGE accumulations, and this decrease was accompanied by a reduction in cell cycle reentry and a more euploid neuronal genome.

ERK activation and expression of neuronal cell cycle markers in the hippocampus after entorhinal cortex lesion

Current findings suggest that neuronal cell death is frequently associated with the aberrant expression of cell cycle-regulatory proteins in postmitotic neurons. Aberrant cell cycle reentry has been implicated in diverse neurodegenerative conditions, including Alzheimer's disease (AD). Previously we reported that the appearance of cell cycle markers in postmitotic neurons of the entorhinal cortex (EC) after excitotoxic hippocampal damage is associated with the expression of phospho-tau and amyloid precursor protein (APP). However, the question of the signaling pathway involved in this cell cycle reentry remains unresolved. Differentiated neurons use the molecular mechanisms initially acquired to direct cell proliferation, such as the Ras-extracellular signal-regulated kinase (ERK1/2) pathway, to regulate synaptic plasticity. In this work we explored whether ERK1/2-related signaling might contribute to the cell cycle reentry in hippocampal neurons after a unilateral EC lesion. We showed that, within the first 24 hr after hippocampal deafferentation, numerous neurons expressed phospho-ERK1/2, concomitantly with the gradual increases in cyclin D1 and cyclin B immunoreactivity in the dentate gyrus and hilus. Several of these immunopositive cells to phospho-ERK1/2 and cyclin B in hippocampus are postmitotic neurons, insofar as they are positive to NeuN. The intracisternal administration of U0126 (an MEK inhibitor), previous to the excitotoxic lesion, decreased the activation of ERK1/2 and the expression of cyclin D1 and cyclin B in the hippocampus. The present findings support the notion that ERK1/2 plays a role in cell cycle reactivation in mature neurons efferently connected to the lesion site.

Cell signaling and receptors in toxicity of advanced glycation end products (AGEs): α-dicarbonyls, radicals, oxidative stress and antioxidants

Considerable attention has been paid to the toxicity of advanced glycation end products (AGEs), including relation to various illnesses. AGEs, generated nonenzymatically from carbohydrates and proteins, comprises large numbers of simple and more complicated compounds. Many reports deal with a role for receptors (RAGE) and cell signaling, including illnesses and aging. Reactive oxygen species appear to participate in signaling. RAGE include angiotensin II type 1 receptors. Many signaling pathways are involved, such as kinases, p38, p21, TGF-β, NF-κβ, TNF-α, JNK and STAT. A recent review puts focus on α-dicarbonyl metabolites, formed by carbohydrate oxidation, and imine derivatives from protein condensation, as a source via electron transfer (ET) of ROS and oxidative stress (OS). The toxic species have been related to illnesses and aging. Antioxidants alleviate the adverse effects.

Role of age and uncoupling protein-2 in oxidative stress, RAGE/AGE interaction and inflammatory liver injury

The objective of this study is to clarify whether age-related oxidative stress enhances hepatic vulnerability via increased interaction of advanced glycation endproducts (AGE) with their receptor RAGE. To further address the role of uncoupling of mitochondrial respiration, mitochondrial uncoupling protein-2 wild-type (UCP2+/+) and knock out (UCP2-/-) mice were used and studied at an age of 8 (young), 38 (adult) and 76 weeks (senescent). First, we could show that UCP2 protein expression increased with age in UCP2+/+ mice. Second, in both mouse strains oxidative stress, as measured by malondialdehyde concentrations and the ratio of glutathione to glutathione disulfide, as well as hepatic RAGE expression and highly modified AGE accumulation significantly increased with age. This, however, was far more pronounced in UCP2-/- mice, in particular at the young age of 8 wk. In addition, the hepatic activity of the AGE precursor detoxifying enzyme glyoxalase-I was significantly decreased in 8 wk old UCP2-/- animals and concomitantly caused 2-fold higher levels of methylglyoxal-modified AGE in these animals. We further showed that the numbers of hepatic cells expressing sRAGE which acts as a decoy for RAGE ligands decreased with age and were markedly lower in the UCP2-/- than the UCP2+/+ mice. As a consequence, young 8 wk old UCP2-/- mice benefited from treatment with recombinant mouse RAGE to block the RAGE/AGE interaction, when challenged with galactosamine/lipopolysaccharide for the induction of acute liver injury. They showed less pronounced tissue damage and slightly lower mortality rate, while older UCP2+/+ and UCP2-/- mice revealed comparably high mortality rates and extent of liver injury, irrespective of their treatment with rRAGE. Taken together, the present study underlines the role of UCP2 in the age-related increase of oxidative stress and the oxidative stress-related RAGE/AGE interaction. In young animals, blockade of the RAGE/AGE interaction is of benefit, while in older animals, this protective effect is lost, supposedly due to the fact that with age other factors than enhanced hepatic glycation products predominantly determine liver injury and injury-related mortality rate.

Oxidative stress-associated rise of hepatic protein glycation increases inflammatory liver injury in uncoupling protein-2 deficient mice

Mitochondrial dysfunction seems to be intrinsically involved in the pathogenesis of multiple organ failure because of enhanced production of reactive oxygen species and induction of oxidative damage. Chronic oxidative stress in turn causes an accumulation of advanced glycation end products (AGEs). To investigate whether mitochondrial dysfunction-associated oxidative stress leads to increased formation and accumulation of AGE, we studied hepatic glycation in uncoupling protein-2 (UCP2-/-) knockout mice. Using the galactosamine/lipopolysaccharide (G/L)-induced liver injury model, we further tested the hypothesis that a mitochondrial dysfunction-associated increase of hepatic glycation is causative for increased liver injury. Under baseline conditions, UCP2-/- mice showed higher malondialdehyde levels and reduced glutathione/glutathione disulfide ratios as well as significantly higher hepatic levels of AGE and hepatic expression of receptor for AGE (RAGE) when compared with UCP2+/+ mice, indicative for increased oxidative stress and hepatic glycation. Further, livers of G/L-challenged UCP2-/- mice revealed significantly more pronounced tissue injury and were found to express higher levels of AGE and RAGE compared with wild-type mice. Functional blockade of RAGE by application of recombinant RAGE significantly diminished liver damage particularly in UCP2-/- mice. This in turn increased survival from 30% in UCP2+/+ mice to 50% in UCP2-/- mice. In summary, we show for the first time that mitochondrial dysfunction-associated oxidative stress enhances hepatic protein glycation, which aggravates inflammation-induced liver injury. Targeting the AGE/RAGE interaction by the blockade of RAGE might be of therapeutic value for the oxidative stress-exposed liver.

A novel role for the Receptor for Advanced Glycation End-products in neural progenitor cells derived from adult SubVentricular Zone

The Receptor for Advanced Glycation End-products (RAGE) is a member of the immunoglobulin superfamily of cell surface receptors which interacts with a wide range of ligands, such as High-Mobility Group Box-1 (HMGB-1), S100B, advanced glycation end-products (AGEs). Here we provided evidence for the restricted expression of RAGE in the undifferentiated neural stem/progenitor cells of mouse adult SubVentricular Zone (SVZ) neurogenic region and adult SVZ-derived neurospheres. Additionally, RAGE ligands stimulated both proliferation and neuronal differentiation of SVZ-derived neural progenitor cells (NPC) in vitro. NF-kappaB nuclear translocation occurred upon RAGE activation in SVZ-derived neurospheres and its blockade (by SN-50) or its absence (in p50(-/-) derived NPC) resulted in the inhibition of the ligand-mediated effects on neuronal differentiation. These novel findings delineate an interesting scenario where the RAGE-NF-kappaB axis may contribute to regulate adult neural stem/progenitor cell function in physiological and possibly pathological conditions where this axis is upregulated.

Regulation of the beta1,4-Galactosyltransferase I promoter by E2F1

Cell surface carbohydrate chains are widely known to contribute to cell migration, recognition and proliferation. beta1,4-Galactosyltransferase I (beta1,4GalT I) transfers galactose to the terminal N-acetylglucosamine of complex-type N-glycan, and contributes to cell proliferation, differentiation and migration. Here, we identified beta1,4GalT I as a novel target gene of cell cycle regulator E2F1. E2F1 proteins interact with the promoter of the beta1,4GalT I gene in vivo, and E2F1 over-expression stimulates the activity of beta1,4GalT I promoter and the mRNA and protein expression of beta1,4GalT I, and augments the level of beta1, 4-galactosyltion. Site-specific mutagenesis revealed that this region which contains two E2F1 binding site (nt -215 to -207 and +1 to +6) is necessary for beta1,4GalT I activation by E2F1. Furthermore, down-regulation of beta1,4GalT I expression attenuates E2F1-induced DNA synthesis and cell cycle progression as well as the expression of cell-cycle regulator Cyclin D1. Thus, beta1,4GalT I is an important E2F1 target gene that is required for cell cycle progression in mammalian cells, which elicits a new mechanism of cell growth and a new mechanism of beta1,4GalT I transcription.

Binding of S100 proteins to RAGE: an update

The Receptor for Advanced Glycation Endproducts (RAGE) is a multi-ligand receptor of the immunoglobulin family. RAGE interacts with structurally different ligands probably through the oligomerization of the receptor on the cell surface. However, the exact mechanism is unknown. Among RAGE ligands are members of the S100 protein family. S100 proteins are small calcium binding proteins with high structural homology. Several members of the family have been shown to interact with RAGE in vitro or in cell-based assays. Interestingly, many RAGE ligands appear to interact with distinct domains of the extracellular portion of RAGE and to trigger various cellular effects. In this review, we summarize the modes of S100 protein-RAGE interaction with regard to their cellular functions.