Plasma amyloid beta predicts conversion to dementia in subjects with mild cognitive impairment: The BALTAZAR study.

INTRODUCTION: Blood-based biomarkers are the next challenge for Alzheimer's disease (AD) diagnosis and prognosis. METHODS: Mild cognitive impairment (MCI) participants (N = 485) of the BALTAZAR study, a large-scale longitudinal multicenter cohort, were followed-up for 3 years. A total of 165 of them converted to dementia (95% AD). Associations of conversion and plasma amyloid beta (Aß)1-42 , Aß1-40 , Aß1-42 /Aß1-40 ratio were analyzed with logistic and Cox models. RESULTS: Converters to dementia had lower level of plasma Aß1-42 (37.1 pg/mL [12.5] vs. 39.2 [11.1] , P value = .03) and lower Aß1-42 /Aß1-40 ratio than non-converters (0.148 [0.125] vs. 0.154 [0.076], P value = .02). MCI participants in the highest quartile of Aß1-42 /Aß1-40 ratio (>0.169) had a significant lower risk of conversion (hazard ratio adjusted for age, sex, education, apolipoprotein E ε4, hippocampus atrophy = 0.52 (95% confidence interval [0.31-0.86], P value = .01). DISCUSSION: In this large cohort of MCI subjects we identified a threshold for plasma Aß1-42 /Aß1-40 ratio that may detect patients with a low risk of conversion to dementia within 3 years.

Transient increase in sAPPα secretion in response to Aß1-42 oligomers: an attempt of neuronal self-defense?

Amyloid precursor protein (APP), a key molecule of Alzheimer disease, is metabolized in 2 antagonist pathways generating the soluble APP alpha (sAPPα) having neuroprotective properties and the beta amyloid (Aß) peptide at the origin of neurotoxic oligomers, particularly Aß1-42. Whether extracellular Aß1-42 oligomers modulate the formation and secretion of sAPPα is not known. We report here that the addition of Aß1-42 oligomers to primary cortical neurons induced a transient increase in α-secretase activity and secreted sAPPα 6-9 hours later. Preventing the generation of sAPPα by using small interfering RNAs (siRNAs) for the α-secretases ADAM10 and ADAM17 or for APP led to increased Aß1-42 oligomer-induced cell death after 24 hours. Neuronal injuries due to oxidative stress or growth factor deprivation also generated sAPPα 7 hours later. Finally, acute injection of Aß1-42 oligomers into wild-type mouse hippocampi induced transient secretion of sAPPα 48-72 hours later. Altogether, these data suggest that neurons respond to stress by generating sAPPα for their survival. These data must be taken into account when interpreting sAPPα levels as a biomarker in neurological disorders.

Targeting demyelination via α-secretases promoting sAPPα release to enhance remyelination in central nervous system.

Remyelination is an endogenous regenerative process of myelin repair in the central nervous system (CNS) with limited efficacy in demyelinating disorders. As strategies enhancing endogenous remyelination become a therapeutic challenge, we have focused our study on α-secretase-induced sAPPα release, a soluble endogenous protein with neuroprotective and neurotrophic properties. However, the role of sAPPα in remyelination is not known. Therefore, we investigated the remyelination potential of α-secretase-induced sAPPα release following CNS demyelination in mice. Acute demyelination was induced by feeding mice with cuprizone (CPZ) for 5weeks. To test the protective effect and the remyelination potential of etazolate, an α-secretase activator, we designed two treatment protocols. Etazolate was administrated either during the last two weeks or at the end of the CPZ intoxication. In both protocols, etazolate restored the number of myelinated axons in corpus callosum with a corresponding increase in the amount of MBP, one of the major myelin proteins in the brain. We also performed ex vivo studies to decipher etazolate's mechanism of action in a lysolecithin-induced demyelination model using organotypic culture of cerebellar slices. Etazolate treatment was able to i) enhance the release of sAPPα in the culture media of demyelinated slices, ii) protect myelinated axons from demyelination, iii) increase the number of mature oligodendrocytes, iv) promote the reappearance of the paired Caspr+ adjacent to the nodes of Ranvier and v) increase the percentage of myelinated axons with short internodes, an indicator of remyelination. Etazolate failed to promote all the aforementioned effects in the presence of GI254023X, an α-secretase inhibitor. Moreover, the protective effects of etazolate in demyelinated slices were mimicked by sAPPα treatment in a dose-dependent manner. In conclusion, etazolate-induced sAPPα release protects myelinated axons from demyelination while also promoting remyelination. This work, thus, highlights the therapeutic potential of strategies that enhance sAPPα release in demyelinating disorders.

Fluorescent nanodiamond tracking reveals intraneuronal transport abnormalities induced by brain-disease-related genetic risk factors.

Brain diseases such as autism and Alzheimer's disease (each inflicting >1% of the world population) involve a large network of genes displaying subtle changes in their expression. Abnormalities in intraneuronal transport have been linked to genetic risk factors found in patients, suggesting the relevance of measuring this key biological process. However, current techniques are not sensitive enough to detect minor abnormalities. Here we report a sensitive method to measure the changes in intraneuronal transport induced by brain-disease-related genetic risk factors using fluorescent nanodiamonds (FNDs). We show that the high brightness, photostability and absence of cytotoxicity allow FNDs to be tracked inside the branches of dissociated neurons with a spatial resolution of 12 nm and a temporal resolution of 50 ms. As proof of principle, we applied the FND tracking assay on two transgenic mouse lines that mimic the slight changes in protein concentration (∼30%) found in the brains of patients. In both cases, we show that the FND assay is sufficiently sensitive to detect these changes.

sAßPPα Improves Hippocampal NMDA-Dependent Functional Alterations Linked to Healthy Aging.

This study shows a decrease in soluble amyloid-ß protein precursor-α (sAßPPα) levels, but no change in sAßPPß, in the rat hippocampus during healthy aging, associated with the weaker expression of N-methyl-D-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP) in the CA1 area of hippocampal slices. Exogenous application of recombinant sAßPPα increases NMDAR activation in aged animals and could rescue the age-related LTP deficits described. In contrast, it does not affect basal synaptic transmission or glutamate release. These results indicate that improving synaptic sAßPPα availability at synapses helps in reducing the functional NMDAR-related deregulation of hippocampal networks linked to aging.

Cytoplasmic SET induces tau hyperphosphorylation through a decrease of methylated phosphatase 2A.

BACKGROUND: The neuronal cytoplasmic localization of SET, an inhibitor of the phosphatase 2A (PP2A), results in tau hyperphosphorylation in the brains of Alzheimer patients through mechanisms that are still not well defined. RESULTS: We used primary neurons and mouse brain slices to show that SET is translocated to the cytoplasm in a manner independent of both its cleavage and over-expression. The localization of SET in the cytoplasm, either by the translocation of endogenous SET or by internalization of the recombinant full-length SET protein, induced tau hyperphosphorylation. Cytoplasmic recombinant full-length SET in mouse brain slices induced a decrease of PP2A activity through a decrease of methylated PP2A levels. The levels of methylated PP2A were negatively correlated with tau hyperphosphorylation at Ser-202 but not with the abnormal phosphorylation of tau at Ser-422. CONCLUSIONS: The presence of full-length SET in the neuronal cytoplasm is sufficient to impair PP2A methylation and activity, leading to tau hyperphosphorylation. In addition, our data suggest that tau hyperphosphorylation is regulated by different mechanisms at distinct sites. The translocation of SET to the neuronal cytoplasm, the low activity of PP2A, and tau hyperphosphorylation are associated in the brains of Alzheimer patients. Our data show a link between the translocation of SET in the cytoplasm and the decrease of methylated PP2A levels leading to a decrease of PP2A activity and tau hyperphosphorylation. This chain of events may contribute to the pathogenesis of Alzheimer disease.

Mass spectrometric detection and characterization of atypical membrane-bound zinc-sensitive phosphatases modulating GABAA receptors.

BACKGROUND: GABAA receptor (GABAAR) function is maintained by an endogenous phosphorylation mechanism for which the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is the kinase. This phosphorylation is specific to the long intracellular loop I2 of the α1 subunit at two identified serine and threonine residues. The phosphorylation state is opposed by an unknown membrane-bound phosphatase, which inhibition favors the phosphorylated state of the receptor and contributes to the maintenance of its function. In cortical nervous tissue from epileptogenic areas in patients with drug-resistant epilepsies, both the endogenous phosphorylation and the functional state of the GABAAR are deficient. METHODOLOGY/PRINCIPAL FINDINGS: The aim of this study is to characterize the membrane-bound phosphatases counteracting the endogenous phosphorylation of GABAAR. We have developed a new analytical tool for in vitro detection of the phosphatase activities in cortical washed membranes by liquid chromatography coupled to mass spectrometry. The substrates are two synthetic phosphopeptides, each including one of the identified endogenous phosphorylation sites of the I2 loop of GABAAR α1 subunit. We have shown the presence of multiple and atypical phosphatases sensitive to zinc ions. Patch-clamp studies of the rundown of the GABAAR currents on acutely isolated rat pyramidal cells using the phosphatase inhibitor okadaic acid revealed a clear heterogeneity of the phosphatases counteracting the function of the GABAAR. CONCLUSION/SIGNIFICANCE: Our results provide new insights on the regulation of GABAAR endogenous phosphorylation and function by several and atypical membrane-bound phosphatases specific to the α1 subunit of the receptor. By identifying specific inhibitors of these enzymes, novel development of antiepileptic drugs in patients with drug-resistant epilepsies may be proposed.

Citrulline diet supplementation improves specific age-related raft changes in wild-type rodent hippocampus.

The levels of molecules crucial for signal transduction processing change in the brain with aging. Lipid rafts are membrane microdomains involved in cell signaling. We describe here substantial biophysical and biochemical changes occurring within the rafts in hippocampus neurons from aging wild-type rats and mice. Using continuous sucrose density gradients, we observed light-, medium-, and heavy raft subpopulations in young adult rodent hippocampus neurons containing very low levels of amyloid precursor protein (APP) and almost no caveolin-1 (CAV-1). By contrast, old rodents had a homogeneous age-specific high-density caveolar raft subpopulation containing significantly more cholesterol (CHOL), CAV-1, and APP. C99-APP-Cter fragment detection demonstrates that the first step of amyloidogenic APP processing takes place in this caveolar structure during physiological aging of the rat brain. In this age-specific caveolar raft subpopulation, levels of the C99-APP-Cter fragment are exponentially correlated with those of APP, suggesting that high APP concentrations may be associated with a risk of large increases in beta-amyloid peptide levels. Citrulline (an intermediate amino acid of the urea cycle) supplementation in the diet of aged rats for 3 months reduced these age-related hippocampus raft changes, resulting in raft patterns tightly close to those in young animals: CHOL, CAV-1, and APP concentrations were significantly lower and the C99-APP-Cter fragment was less abundant in the heavy raft subpopulation than in controls. Thus, we report substantial changes in raft structures during the aging of rodent hippocampus and describe new and promising areas of investigation concerning the possible protective effect of citrulline on brain function during aging.

New highly sensitive rodent and human tests for soluble amyloid precursor protein alpha quantification: preclinical and clinical applications in Alzheimer's disease.

BACKGROUND: Amyloid precursor protein (APP), a key molecule in Alzheimer's disease (AD), is metabolized in two alternative cleavages, generating either the amyloidogenic peptides involved in AD pathology or the soluble form of APP (sAPPα). The level of amyloidogenic peptides in human cerebrospinal fluid (CSF) is considered to be a biomarker of AD, whereas the level of sAPPα in CSF as a biomarker has not been clearly established. sAPPα has neurotrophic and neuroprotective properties. Stimulating its formation and secretion is a promising therapeutic target in AD research. To this end, very sensitive tests for preclinical and clinical research are required. METHODS: The tests are based on homogenous time-resolved fluorescence and require no washing steps. RESULTS: We describe two new rapid and sensitive tests for quantifying mouse and human sAPPα. These 20 µl-volume tests quantify the levels of: i) endogenous mouse sAPPα in the conditioned medium of mouse neuron primary cultures, as well as in the CSF of wild-type mice, ii) human sAPPα in the CSF of AD mouse models, and iii) human sAPPα in the CSF of AD and non-AD patients. These tests require only 5 µl of conditioned medium from 5 × 10(4) mouse primary neurons, 1 µl of CSF from wild-type and transgenic mice, and 0.5 µl of human CSF. CONCLUSIONS: The high sensitivity of the mouse sAPPα test will allow high-throughput investigations of molecules capable of increasing the secretion of endogenous sAPPα in primary neurons, as well as the in vivo validation of molecules of interest through the quantification of sAPPα in the CSF of treated wild-type mice. Active molecules could then be tested in the AD mouse models by quantifying human sAPPα in the CSF through the progression of the disease. Finally, the human sAPPα test could strengthen the biological diagnosis of AD in large clinical investigations. Taken together, these new tests have a wide field of applications in preclinical and clinical studies.

Secreted amyloid precursor protein ß and secreted amyloid precursor protein α induce axon outgrowth in vitro through Egr1 signaling pathway.

BACKGROUND: sAPPα released after α secretase cleavage of Amyloid Precursor Protein (APP) has several functions including the stimulation of neurite outgrowth although detailed morphometric analysis has not been done. Two domains involved in this function have been described and are present in sAPPß released at the first step of amyloid peptide cleavage, raising the possibility that sAPPß could also stimulate neurite outgrowth. We investigated the morphological effects of sAPPα and sAPPß on primary neurons and identified a key signaling event required for the changes observed. METHODOLOGY/PRINCIPAL FINDINGS: Final concentrations of 50 to 150 nM bacterial recombinant sAPPα or sAPPß added to primary neuronal cultures after 1 day in vitro decreased cell adhesion 24 hours later and primary dendrite length 96 hours later. 150 nM sAPPα and sAPPß induced a similar increase of axon outgrowth, although this increase was already significant at 100 nM sAPPα. These morphological changes induced by sAPPs were also observed when added to differentiated neurons at 5 days in vitro. Real time PCR and immunocytochemistry showed that sAPPα and sAPPß stimulated Egr1 expression downstream of MAPK/ERK activation. Furthermore, in primary neurons from Egr1 -/- mice, sAPPs affected dendritic length but did not induce any increase of axon length. CONCLUSION/SIGNIFICANCE: sAPPα and sAPPß decrease cell adhesion and increase axon elongation. These morphological changes are similar to what has been observed in response to heparan sulfate. The sAPPα/sAPPß stimulated increase in axon growth requires Egr1 signaling. These data suggest that sAPPß is not deleterious per se. Since sAPPß and sAPPα are present in the embryonic brain, these two APP metabolites might play a role in axon outgrowth during development and in response to brain damage.

Modulation of prepulse inhibition and stereotypies in rodents: no evidence for antipsychotic-like properties of histamine H3-receptor inverse agonists.

RATIONALE: H(3)-receptor inverse agonists raise a great interest as innovative therapeutics in several central disorders. Whereas their procognitive properties are well established, their antipsychotic-like properties are still debated. OBJECTIVES: We further explored the effect of maximal doses (3-10 mg/kg) of ciproxifan, BF2.649, and ABT-239, three selective H(3)-receptor inverse agonists, on deficits of prepulse inhibition (PPI) induced by apomorphine, MK-801, and phencyclidine (PCP). Their effect was also investigated on stereotypies induced by apomorphine and methamphetamine. RESULTS: Ciproxifan, BF2.649, and ABT-239 did not reverse the PPI impairment produced by apomorphine (0.5 mg/kg, subcutaneous) in rats. Ciproxifan and BF2.649 did not reverse the impairment induced in mice by MK-801 (0.3 mg/kg). Ciproxifan and BF2.649 also failed to reverse the disruption induced in mice by PCP (5-10 mg/kg). Low to moderate doses of haloperidol (0.1-0.4 mg/kg, intraperitoneal), alone or co-administered with BF2.649, did not reverse MK-801-induced PPI disruption. A high dose (1 mg/kg) of haloperidol partially reversed the MK-801-induced deficit and BF2.649 tended to increase this effect, although nonsignificantly. Whereas stereotypies induced in mice by apomorphine and methamphetamine were totally suppressed by haloperidol, the decrease induced by ciproxifan was partial against apomorphine and very low, if any, against methamphetamine. CONCLUSIONS: Their total absence of effect in several validated animal models of the disease does not support antipsychotic properties of H(3)-receptor inverse agonists. However, their positive effects previously reported in behavioral tasks addressing learning, attention, and memory maintain the interest of H(3)-receptor inverse agonists for the treatment of cognitive symptoms of schizophrenia as adjunctive medications.

Cyclotraxin-B, the first highly potent and selective TrkB inhibitor, has anxiolytic properties in mice.

In the last decades, few mechanistically novel therapeutic agents have been developed to treat mental and neurodegenerative disorders. Numerous studies suggest that targeting BDNF and its TrkB receptor could be a promising therapeutic strategy for the treatment of brain disorders. However, the development of potent small ligands for the TrkB receptor has proven to be difficult. By using a peptidomimetic approach, we developed a highly potent and selective TrkB inhibitor, cyclotraxin-B, capable of altering TrkB-dependent molecular and physiological processes such as synaptic plasticity, neuronal differentiation and BDNF-induced neurotoxicity. Cyclotraxin-B allosterically alters the conformation of TrkB, which leads to the inhibition of both BDNF-dependent and -independent (basal) activities. Finally, systemic administration of cyclotraxin-B to mice results in TrkB inhibition in the brain with specific anxiolytic-like behavioral effects and no antidepressant-like activity. This study demonstrates that cyclotraxin-B might not only be a powerful tool to investigate the role of BDNF and TrkB in physiology and pathology, but also represents a lead compound for the development of new therapeutic strategies to treat brain disorders.

Inhibitors of tripeptidyl peptidase II. 3. Derivation of butabindide by successive structure optimizations leading to a potential general approach to designing exopeptidase inhibitors.

The cholecystokinin-8 (CCK-8)-inactivating peptidase is a serine peptidase that has been shown to be a membrane-bound isoform of tripeptidyl peptidase II (EC 3.4.14.10). It cleaves the neurotransmitter CCK-8 sulfate at the Met-Gly bond to give Asp-Tyr(SO3H)-Met-OH + Gly-Trp-Met-Asp-Phe-NH2. Starting from Val-Pro-NHBu, a dipeptide of submicromolar affinity that had previously been generated to serve as a lead, successive optimization at P3, P1, and then P2 gave Abu-Pro-NHBu (18, Ki = 80 nM). Further transformation (by making a benzologue) gave the indoline analogue, butabindide (33) as a reversible inhibitor having nanomolar affinity (Ki = 7 nM). Retrospective analysis suggested the possibility of a general approach to designing exopeptidase inhibitors starting from the structure of the first hydrolysis product. Application of this approach to CCK-8 led to Abu-Phe-NHBu (37), but this only had Ki = 9.4 microM. Molecular modeling, to determine the minimum energy conformations and explain the 1000-fold better affinity of butabindide, indicated that 37 cannot access the likely active conformation of butabindide.

The Kell protein of the common K2 phenotype is a catalytically active metalloprotease, whereas the rare Kell K1 antigen is inactive. Identification of novel substrates for the Kell protein.

The Kell blood group is a highly polymorphic system containing over 20 different antigens borne by the protein Kell, a 93-kDa type II glycoprotein that displays high sequence homology with members of the M13 family of zinc-dependent metalloproteases whose prototypical member is neprilysin. Kell K1 is an antigen expressed in 9% of the Caucasian population, characterized by a point mutation (T193M) of the Kell K2 antigen, and located within a putative N-glycosylation consensus sequence. Recently, a recombinant, non-physiological, soluble form of Kell was shown to cleave Big ET-3 to produce the mature vasoconstrictive peptide. To better characterize the enzymatic activity of the Kell protein and the possible differences introduced by antigenic point mutations affecting post-translational processing, the membrane-bound forms of the Kell K1 and Kell K2 antigens were expressed either in K562 cells, an erythroid cell line, or in HEK293 cells, a non-erythroid system, and their pharmacological profiles and enzymatic specificities toward synthetic and natural peptides were evaluated. Results presented herein reveal that the two antigens possess considerable differences in their enzymatic activities, although not in their trafficking pattern. Indeed, although both antigens are expressed at the cell surface, Kell K1 protein is shown to be inactive, whereas the Kell K2 antigen binds neprilysin inhibitory compounds such as phosphoramidon and thiorphan with high affinity, cleaves the precursors of the endothelin peptides, and inactivates members of the tachykinin family with enzymatic properties resembling those of other members of the M13 family of metalloproteases to which it belongs.

Cell-specific activity of neprilysin 2 isoforms and enzymic specificity compared with neprilysin.

Neprilysin (NEP) 2 is a recently cloned glycoprotein displaying a high degree of sequence identity with neprilysin (EC 3.4.24.11), the prototypical member of the M13 subfamily of metalloproteases. Whereas NEP is involved in the metabolism of several bioactive peptides by plasma membranes of various cells, the enzymic properties and physiological functions of NEP2 are unknown. Here we characterize the cell-expression modalities and enzymic specificity of two alternatively spliced isoforms of NEP2 in Chinese hamster ovary and AtT20 cells. In the two cell lines, both isoforms are type II glycoproteins inserted in the endoplasmic reticulum as inactive precursors. Maturation detected by Western-blot analysis of glycosidase digests was cell-specific and more efficient in the endocrine cell line. The enzymic activity of both isoforms semi-purified from AtT20 cells reveals comparable specificities in terms of model substrates, pH optima and inhibitory patterns. NEP2 activity was compared with that of NEP regarding potencies of transition-state inhibitors, modes of hydrolysis, maximal hydrolysis rates and apparent affinities of bioactive peptides. Although all transition-state inhibitors of NEP inhibited NEP2 activity, albeit with different potencies, and many peptides were cleaved at the same amide bond by both peptidases, differences could be observed, i.e. in the hydrolysis of gonadotropin-releasing hormone and cholecystokinin, which occurred at different sites and more efficiently in the case of NEP2. Differences in cleavage of bioactive peptides, in cell-trafficking patterns and in tissue distribution indicate that NEP and NEP2 play distinct physiological roles in spite of their high degree of sequence identity.