The Physiopathological role of nitric oxide in the brain : from translational regulation of the GluN2B subunit to post-translational modifications of albumin in Alzheimer's disease

Nitric oxide (NO) is a molecule that has pleiotropic effects in brain and vascular system. Physiologically, NO induces the translation of the GluN2B subunit of N-methyl Daspartate receptor (NMDARc) by derepressing it 5’untranslated region (5’UTR) effect. This pathway is due to the activation of the heme regulated eIF2α (HRI) kinase and prevents an excess of GluN2B levels, especially at the extrasynaptic areas, where it can trigger excitotoxicity. Pathologically, NO in a pro-oxidant environment such as in Alzheimer’s Disease (AD) reacts with superoxide anion producing peroxynitrite, which can nitrotyrosinates proteins. There are other concomitant oxidative processes that affect AD patient like protein glycation. Therefore the albumin, the most abundant plasmatic protein, in AD patients is more nitrotyrosinated and glycated, which affects its structure. Modified albumin has a reduced ability as an osmolarity buffer and it is hardly uptaken by hepatoma cells. Moreover, modified albumin binds more Aβ, contributing to maintain higher amount of amyloid in brain and plasma.El óxido nítrico (NO) es una molécula con efectos pleyotrópicos en cerebro y sistema vascular. Fisiológicamente, induce la traducción de la subunidad GluN2B del N-methyl D-aspartate receptor (NMDARc) al revertir la represión de su 5’untranslated region (5’UTR). Este efecto se debe a la activación de la heme regulated eIF2α kinase (HRI) y previene el exceso de GluN2B, especialmente en regiones extrasinápticas, donde desencadena excitotoxicidad. Patológicamente, el NO en un ambiente pro-oxidatvio como el dado en la enfermedad de Alzheimer (AD) reacciona con el anión superóxido produciendo peroxinitrito, y causando entre otros efectos la nitrotirosinación de proteínas. Simultaneamente, las proteínas de pacientes con AD padecen otros procesos oxidativos como la glicación. Por tanto, la albúmina, la proteína plasmática más abundante, en estos pacientes está más nitrotirosinada y glicada, afectando su estructura. La albúmina modificada presenta menos capacidad para tamponar la osmolaridad y apenas es digerida por las células de hepatoma. Además, une más Aβ, contribuyendo a mantener más alta la carga amiloidogénica en cerebro y plasma

The blood-brain barrier: structure, function and therapeutic approaches to cross it

The blood-brain barrier (BBB) is constituted by a specialized vascular endothelium that interacts directly with astrocytes, neurons and pericytes. It protects the brain from the molecules of the systemic circulation but it has to be overcome for the proper treatment of brain cancer, psychiatric disorders or neurodegenerative diseases, which are dramatically increasing as the population ages. In the present work we have revised the current knowledge on the cellular structure of the BBB and the different procedures utilized currently and those proposed to cross it. Chemical modifications of the drugs, such as increasing their lipophilicity, turn them more prone to be internalized in the brain. Other mechanisms are the use of molecular tools to bind the drugs such as small immunoglobulins, liposomes or nanoparticles that will act as Trojan Horses favoring the drug delivery in brain. This fusion of the classical pharmacology with nanotechnology has opened a wide field to many different approaches with promising results to hypothesize that BBB will not be a major problem for the new generation of neuroactive drugs. The present review provides an overview of all state-of-the-art of the BBB structure and function, as well as of the classic strategies and these appeared in recent years to deliver drugs into the brain for the treatment of Central Nervous System (CNS) diseases.This work was supported by the Plan Estatal de I+D+i 2013-2016 and the ISCIII-Subdirección General de Evaluación y Fomento de la Investigación (Grants PI13/00408, PI08/00574 and Red HERACLES RD12/0042/0014 & RD12/0042/0016) and FEDER Funds; Spanish Ministry of Science and Innovation (RD12/0042/0016; BIO2011-25039); Generalitat de Catalunya (SGR09-1369 and SGR-760); and Fundació la Marató de TV3 (100310)

The blood-brain barrier: structure, function and therapeutic approaches to cross it

The blood-brain barrier (BBB) is constituted by a specialized vascular endothelium that interacts directly with astrocytes, neurons and pericytes. It protects the brain from the molecules of the systemic circulation but it has to be overcome for the proper treatment of brain cancer, psychiatric disorders or neurodegenerative diseases, which are dramatically increasing as the population ages. In the present work we have revised the current knowledge on the cellular structure of the BBB and the different procedures utilized currently and those proposed to cross it. Chemical modifications of the drugs, such as increasing their lipophilicity, turn them more prone to be internalized in the brain. Other mechanisms are the use of molecular tools to bind the drugs such as small immunoglobulins, liposomes or nanoparticles that will act as Trojan Horses favoring the drug delivery in brain. This fusion of the classical pharmacology with nanotechnology has opened a wide field to many different approaches with promising results to hypothesize that BBB will not be a major problem for the new generation of neuroactive drugs. The present review provides an overview of all state-of-the-art of the BBB structure and function, as well as of the classic strategies and these appeared in recent years to deliver drugs into the brain for the treatment of Central Nervous System (CNS) diseases.This work was supported by the Plan Estatal de I+D+i 2013-2016 and the ISCIII-Subdirección General de Evaluación y Fomento de la Investigación (Grants PI13/00408, PI08/00574 and Red HERACLES RD12/0042/0014 & RD12/0042/0016) and FEDER Funds; Spanish Ministry of Science and Innovation (RD12/0042/0016; BIO2011-25039); Generalitat de Catalunya (SGR09-1369 and SGR-760); and Fundació la Marató de TV3 (100310)

PKR and PP1C polymorphisms in alzheimer’s disease risk

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by senile plaques and neurofibrillary tangles. Senile plaques are deposits of amyloid ß-peptide (Aß) produced by the cleavage of a transmembrane protein termed Amyloid Precursor Protein (APP). The amyloidogenic cleavage of APP is performed by γ-secretase complex and ß-site APP cleaving enzyme 1 (BACE1), a key enzyme in AD that can be activated by different noxious stimuli. Interestingly, some viruses could activate double-stranded RNA-activated protein kinase (PKR), which phosphorylates Eukaryotic Initiation Factor 2 alpha (eIF2α). This phosphorylation stops global translation to avoid any synthesis of viral infective proteins, but paradoxically up-regulates BACE1 translation. One of the viral mechanisms to circumvent eIF2α phosphorylation is the recruitment of protein phosphatase 1 (PP1), to fully dephosphorylate eIF2α and allow viral protein synthesis. Due to the functional relationship between BACE1, PKR, PP1 and AD we have performed a large (1122 cases and 1191 control individuals) case-control genetic analysis using two biallelic polymorphisms rs2254958 and rs7480390, located within the genes coding for PKR and the catalytic unit A of PP1, respectively. Although a trend to association of the rs2254958 TT genotype with AD risk was found, our results show that neither rs7480390 nor rs2254958 are associated with AD susceptibility.This work was supported by the Spanish Ministerio de Ciencia y Tecnología (FIS: PI07/0593 and PI10/00587; ISCIII-RETIC RED HERACLES RD06/0009/002- FEDER)

Modification of y-secretase by nitrosative stress links neuronal ageing to sporadic Alzheimer's disease

Inherited familial Alzheimer's disease (AD) is characterized by small increases in the ratio of Aβ42 versus Aβ40 peptide which is thought to drive the amyloid plaque formation in the brain of these patients. Little is known however whether ageing, the major risk factor for sporadic AD, affects amyloid beta‐peptide (Aβ) generation as well. Here we demonstrate that the secretion of Aβ is enhanced in an in vitro model of neuronal ageing, correlating with an increase in γ‐secretase complex formation. Moreover we found that peroxynitrite (ONOO−), produced by the reaction of superoxide anion with nitric oxide, promoted the nitrotyrosination of presenilin 1 (PS1), the catalytic subunit of γ‐secretase. This was associated with an increased association of the two PS1 fragments, PS1‐CTF and PS1‐NTF, which constitute the active catalytic centre. Furthermore, we found that peroxynitrite shifted the production of Aβ towards Aβ42 and increased the Aβ42/Aβ40 ratio. Our work identifies nitrosative stress as a potential mechanistic link between ageing and AD.This work was made possible by grants from the Fund for Scientific Research, Flanders; the K.U.Leuven; the VIB, Methusalem (K.U.Leuven and the Flemisch government), the Foundation for Alzheimer Research (SAO/FRMA), the European Research council (BDS), NIH AG15379 (OB), Spanish Ministry of Science and Innovation SAF 2010‐14906, Consolider 2010‐00045 (CGD), Spanish Ministery of Health (Fondo de Investigación Sanitaria‐PI10/00587 and Red HERACLES RD06/0009); The European FEDER Fundings; and Fundació La Marató de TV3 (Catalonia; Spain; no. 100310). We would like to acknowledge the Banc de Teixits Neurologics de l' Hospital Clinic de Barcelona and the Unidad de Neuropatología y Banco de Cerebros of Fundación Hospital Alcorcón for providing the brain samples. BDS is the Arthur Bax and Anna Vanluffelen chair for AD. FG obtained a IEF fellowship of the Marie Curie Actions program in FP7 and a Beatriu de Pinos grant of the Generalitat de Catalunya, Spain. TW was supported by EMBO and DFG long‐term fellowship

PKR and PP1C polymorphisms in alzheimer’s disease risk

Alzheimer’s disease (AD) is a neurodegenerative disease characterized by senile plaques and neurofibrillary tangles. Senile plaques are deposits of amyloid ß-peptide (Aß) produced by the cleavage of a transmembrane protein termed Amyloid Precursor Protein (APP). The amyloidogenic cleavage of APP is performed by γ-secretase complex and ß-site APP cleaving enzyme 1 (BACE1), a key enzyme in AD that can be activated by different noxious stimuli. Interestingly, some viruses could activate double-stranded RNA-activated protein kinase (PKR), which phosphorylates Eukaryotic Initiation Factor 2 alpha (eIF2α). This phosphorylation stops global translation to avoid any synthesis of viral infective proteins, but paradoxically up-regulates BACE1 translation. One of the viral mechanisms to circumvent eIF2α phosphorylation is the recruitment of protein phosphatase 1 (PP1), to fully dephosphorylate eIF2α and allow viral protein synthesis. Due to the functional relationship between BACE1, PKR, PP1 and AD we have performed a large (1122 cases and 1191 control individuals) case-control genetic analysis using two biallelic polymorphisms rs2254958 and rs7480390, located within the genes coding for PKR and the catalytic unit A of PP1, respectively. Although a trend to association of the rs2254958 TT genotype with AD risk was found, our results show that neither rs7480390 nor rs2254958 are associated with AD susceptibility.This work was supported by the Spanish Ministerio de Ciencia y Tecnología (FIS: PI07/0593 and PI10/00587; ISCIII-RETIC RED HERACLES RD06/0009/002- FEDER)

Modification of y-secretase by nitrosative stress links neuronal ageing to sporadic Alzheimer's disease

Inherited familial Alzheimer's disease (AD) is characterized by small increases in the ratio of Aβ42 versus Aβ40 peptide which is thought to drive the amyloid plaque formation in the brain of these patients. Little is known however whether ageing, the major risk factor for sporadic AD, affects amyloid beta‐peptide (Aβ) generation as well. Here we demonstrate that the secretion of Aβ is enhanced in an in vitro model of neuronal ageing, correlating with an increase in γ‐secretase complex formation. Moreover we found that peroxynitrite (ONOO−), produced by the reaction of superoxide anion with nitric oxide, promoted the nitrotyrosination of presenilin 1 (PS1), the catalytic subunit of γ‐secretase. This was associated with an increased association of the two PS1 fragments, PS1‐CTF and PS1‐NTF, which constitute the active catalytic centre. Furthermore, we found that peroxynitrite shifted the production of Aβ towards Aβ42 and increased the Aβ42/Aβ40 ratio. Our work identifies nitrosative stress as a potential mechanistic link between ageing and AD.This work was made possible by grants from the Fund for Scientific Research, Flanders; the K.U.Leuven; the VIB, Methusalem (K.U.Leuven and the Flemisch government), the Foundation for Alzheimer Research (SAO/FRMA), the European Research council (BDS), NIH AG15379 (OB), Spanish Ministry of Science and Innovation SAF 2010‐14906, Consolider 2010‐00045 (CGD), Spanish Ministery of Health (Fondo de Investigación Sanitaria‐PI10/00587 and Red HERACLES RD06/0009); The European FEDER Fundings; and Fundació La Marató de TV3 (Catalonia; Spain; no. 100310). We would like to acknowledge the Banc de Teixits Neurologics de l' Hospital Clinic de Barcelona and the Unidad de Neuropatología y Banco de Cerebros of Fundación Hospital Alcorcón for providing the brain samples. BDS is the Arthur Bax and Anna Vanluffelen chair for AD. FG obtained a IEF fellowship of the Marie Curie Actions program in FP7 and a Beatriu de Pinos grant of the Generalitat de Catalunya, Spain. TW was supported by EMBO and DFG long‐term fellowship

Activation of PKR causes amyloid beta-peptide accumulation via De-Repression of Bace1 expression

BACE1 is a key enzyme involved in the production of amyloid ß-peptide (Aß) in Alzheimer's disease (AD) brains. Normally, its expression is constitutively inhibited due to the presence of the 5′untranslated region (5′UTR) in the BACE1 promoter. BACE1 expression is activated by phosphorylation of the eukaryotic initiation factor (eIF)2-alpha, which reverses the inhibitory effect exerted by BACE1 5′UTR. There are four kinases associated with different types of stress that could phosphorylate eIF2-alpha. Here we focus on the double-stranded (ds) RNA-activated protein kinase (PKR). PKR is activated during viral infection, including that of herpes simplex virus type 1 (HSV1), a virus suggested to be implicated in the development of AD, acting when present in brains of carriers of the type 4 allele of the apolipoprotein E gene. HSV1 is a dsDNA virus but it has genes on both strands of the genome, and from these genes complementary RNA molecules are transcribed. These could activate BACE1 expression by the PKR pathway. Here we demonstrate in HSV1-infected neuroblastoma cells, and in peripheral nervous tissue from HSV1-infected mice, that HSV1 activates PKR. Cloning BACE1 5′UTR upstream of a luciferase (luc) gene confirmed its inhibitory effect, which can be prevented by salubrinal, an inhibitor of the eIF2-alpha phosphatase PP1c. Treatment with the dsRNA analog poly (I:C) mimicked the stimulatory effect exerted by salubrinal over BACE1 translation in the 5′UTR-luc construct and increased Aß production in HEK-APPsw cells. Summarizing, our data suggest that PKR activated in brain by HSV1 could play an important role in the development of ADNeocodex is a private research organization (more than 70 articles in indexes of international research journals in the last 8 years). Neocodex’s work on this project was design, implementation, analysis and interpretation of genetic studies of the PKR gene that is included in this manuscript. Neocodex also coordinates the construction of the DNA bank employed in this draft. The work was carried out with funds from this entity and a project funded by the Alzheimur Foundation. The other funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. This w ork was also supported by the Spanish Ministerio de Ciencia y Tecnología (FIS: PI07/0593; Red HERACLES RD06/0009/002

Posttranslational nitro-glycative modifications of albumin in Alzheimer's disease: implications in cytotoxicity and amyloid-β peptide aggregation

Glycation and nitrotyrosination are pathological posttranslational modifications that make proteins prone to losing their physiological properties. Since both modifications are increased in Alzheimer's disease (AD) due to amyloid-β peptide (Aβ) accumulation, we have studied their effect on albumin, the most abundant protein in cerebrospinal fluid and blood. Brain and plasmatic levels of glycated and nitrated albumin were significantly higher in AD patients than in controls. In vitro turbidometry and electron microscopy analyses demonstrated that glycation and nitrotyrosination promote changes in albumin structure and biochemical properties. Glycated albumin was more resistant to proteolysis and less uptake by hepatoma cells occurred. Glycated albumin also reduced the osmolarity expected for a solution containing native albumin. Both glycation and nitrotyrosination turned albumin cytotoxic in a cell type-dependent manner for cerebral and vascular cells. Finally, of particular relevance to AD, these modified albumins were significantly less effective in avoiding Aβ aggregation than native albumin. In summary, nitrotyrosination and especially glycation alter albumin structural and biochemical properties, and these modifications might contribute for the progression of AD.This work was supported by the Spanish Ministry of Science and Innovation (SAF2012-38140; BIO2011-25039); Fondo de Investigación Sanitaria (PI10/00587; PI11/3035; and Red HERACLES RD06/0009, RD12/0042/0014); FEDER Funds; Generalitat de Catalunya (SGR05-266; SGR09-760); and Fundació la Marató de TV3 (100310). M.A.V. is the recipient of an ICREA Academia Award

Physiological control of nitric oxide in neuronal BACE1 translation by heme-regulated eIF2α kinase HRI induces synaptogenesis

AIMS: Hippocampus is the brain center for memory formation, a process that requires synaptogenesis. However, hippocampus is dramatically compromised in Alzheimer's disease due to the accumulation of amyloid β-peptide, whose production is initiated by β-site APP Cleaving Enzyme 1 (BACE1). It is known that pathological stressors activate BACE1 translation through the phosphorylation of the eukaryotic initiation factor-2α (eIF2α) by GCN2, PERK, or PKR kinases, leading to amyloidogenesis. However, BACE1 physiological regulation is still unclear. Since nitric oxide (NO) participates directly in hippocampal glutamatergic signaling, we investigated the neuronal role of the heme-regulated eukaryotic initiation factor eIF2α kinase (HRI), which can bind NO by a heme group, in BACE1 translation and its physiological consequences. RESULTS: We found that BACE1 is expressed on glutamate activation with NO being the downstream effector by triggering eIF2α phosphorylation, as it was obtained by Western blot and luciferase assay. It is due to the activation of HRI by NO as assayed by Western blot and immunofluorescence with an HRI inhibitor and HRI siRNA. BACE1 expression was early detected at synaptic spines, contributing to spine growth and consolidating the hippocampal memory as assayed with mice treated with HRI or neuronal NO synthase inhibitors. INNOVATION: We provide the first description that HRI and eIF2α are working in physiological conditions in the brain under the control of nitric oxide and glutamate signaling, and also that BACE1 has a physiological role in hippocampal function. CONCLUSION: We conclude that BACE1 translation is controlled by NO through HRI in glutamatergic hippocampal synapses, where it plays physiological functions, allowing the spine growth and memory consolidation.Supported by the Plan Estatal de I+D+i 2013-2016 and the ISCIII-Subdirección General de Evaluación y Fomento de la Investigación (Grants PI13/00408, BFU2012-33500 and Red HERACLES RD12/0042/0014) and FEDER Funds; Generalitat de Catalunya (SGR09-1369); and Fundació la Marató de TV3 (100310)