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A biomarker of synapse loss, an early event in Alzheimer's disease (AD) pathophysiology that precedes neuronal death and symptom onset, would be a much-needed prognostic biomarker. With direct access to the brain interstitial fluid, the cerebrospinal fluid (CSF) is a potential source of synapse-derived proteins. In this study, we aimed to identify and validate novel CSF biomarkers of synapse loss in AD. Discovery: Combining shotgun proteomics of the CSF with an exhaustive search of the literature and public databases, we identified 251 synaptic proteins, from which we selected 22 for further study. Verification: Twelve proteins were discarded because of poor detection by Selected Reaction Monitoring (SRM). We confirmed the specific expression of 9 of the remaining proteins (Calsynytenin-1, GluR2, GluR4, Neurexin-2A, Neurexin-3A, Neuroligin-2, Syntaxin-1B, Thy-1, Vamp-2) at the human synapse using Array Tomography microscopy and biochemical fractionation methods. Exploration: Using SRM, we monitored these 9 synaptic proteins (20 peptides) in a cohort of CSF from cognitively normal controls and subjects in the pre-clinical and clinical AD stages (n = 80). Compared with controls, peptides from 8 proteins were elevated 1.3 to 1.6-fold (p < 0.04) in prodromal AD patients. Validation: Elevated levels of a GluR4 peptide at the prodromal stage were replicated (1.3-fold, p = 0.04) in an independent cohort (n = 60). Moreover, 7 proteins were reduced at preclinical stage 1 (0.6 to 0.8-fold, p < 0.04), a finding that was replicated (0.7 to 0.8-fold, p < 0.05) for 6 proteins in a third cohort (n = 38). In a cross-cohort meta-analysis, 6 synaptic proteins (Calsyntenin-1, GluR4, Neurexin-2A, Neurexin-3A, Syntaxin-1B and Thy-1) were reduced 0.8-fold (p < 0.05) in preclinical AD, changes that precede clinical symptoms and CSF markers of neurodegeneration. Therefore, these proteins could have clinical value for assessing disease progression, especially in preclinical stages of AD. This work was supported by the following research grants: Fondos de Investigaciones Sanitarias (PI15/00058, PI14/01561, PI18/000327) from the “Fondo Europeo de Desarrollo Regional (FEDER)”, Unión Europea, “Una manera de hacer Europa” and the “Instituto de Salud Carlos III, Ministerio de Economia y Competitividad, Gobierno de España” (ISCIII). Additional funding came from the “Departament de Salut, Generalitat de Cataluña - Pla Estratègic de Recerca i Innovació en Salut (PERIS) 2016 –2020, 2017–2019” (SLT002/16/00408, 2017SGR547), “Programa 1 Enfermedad de Alzheimer y otras demencias degenerativas” from the Centro de Investigación Biomédica en Red Enfermedades Neurodegenerativas (CIBERNED), the “Fundació Bancaria La Caixa” (4560/6393) and “La Marató” organized by the television channel, TV3 (20142610). OB is supported by the Miguel Servet program (CP13/00091) from the ISCIII and FEDER. AB is supported by the Ministerio de Economia y Competitividad, Gobierno de España and FEDER (BFU2012-34398 and BFU2015-69717-P), by the Career Integration Grant from the European Union (ref. 304111, Marie Curie Actions), by the Ramón y Cajal Fellowship (ref. RYC-2011-08391) from the Ministerio de Economia y Competitividad, Gobierno de España and by the CERCA Programme from the Generalitat de Catalunya. RN-L is supported by the research grant from the Generalitat de Catalunya (PERIS SLT/2381/2016/00099). The proteomics analyses were performed in the CRG/UPF Proteomics Unit which is part of the Proteored, PRB3 and is supported by grant PT17/0019, of the PE IDi 2013–2016, funded by ISCIII and ERDF

Background The innate immune system is known to be involved early in the pathogenesis of Alzheimer’s disease (AD) and other neurodegenerative disorders. The inflammatory response in the central nervous system can be measured postmortem or through a series of inflammatory mediator surrogates. YKL-40 (also named Chitinase-3-like I) has been frequently investigated in body fluids as a surrogate marker of neuroinflammation in AD and other neurological disorders. However, the expression pattern of YKL-40 in the human brain with neurodegenerative pathology remains poorly investigated. Our aim was to study the cellular expression pattern of YKL-40 in the brain of patients with clinical and neuropathological criteria for AD (n = 11); three non-AD tauopathies: Pick’s disease (PiD; n = 8), corticobasal degeneration (CBD; n = 8) and progressive supranuclear palsy (PSP; n = 9) and a group of neurologically healthy controls (n = 6). Methods Semiquantitative neuropathological evaluation and quantitative confocal triple immunofluorescence studies were performed. An in-house algorithm was used to detect and quantify pathology burden of random regions of interest on a full tissue-section scan. Kruskal-Wallis and Dunn’s multiple comparison tests were performed for colocalization and quantification analyses. Results We found that brain YKL-40 immunoreactivity was observed in a subset of astrocytes in all four diseases and in controls. There was a strong colocalization between YKL-40 and the astroglial marker GFAP but not with neuronal nor microglial markers. Intriguingly, YKL-40-positive astrocytes were tau-negative in PSP, CBD and PiD. The number of YKL-40-positive astrocytes was increased in tauopathies compared with that in controls. A positive correlation was found between YKL-40 and tau immunoreactivities. Conclusions This study confirms that YKL-40 is expressed by a subset of astrocytes in AD and other tauopathies. YKL-40 expression is elevated in several neurodegenerative conditions and correlates with tau pathology. Electronic supplementary material The online version of this article (doi:10.1186/s12974-017-0893-7) contains supplementary material, which is available to authorized users.

The MAPT H1 haplotype has been linked to several disorders, but its relationship with Alzheimer's disease (AD) remains controversial. A rare variant in MAPT (p.A152T) has been linked with frontotemporal dementia (FTD) and AD. We genotyped H1/H2 and p.A152T MAPT in 11,572 subjects from Spain (4,327 AD, 563 FTD, 648 Parkinson's disease (PD), 84 progressive supranuclear palsy (PSP), and 5,950 healthy controls). Additionally, we included 101 individuals from 21 families with genetic FTD. MAPT p.A152T was borderline significantly associated with FTD [odds ratio (OR)=2.03; p=0.063], but not with AD. MAPT H1 haplotype was associated with AD risk (OR=1.12; p=0.0005). Stratification analysis showed that this association was mainly driven by APOE ε4 noncarriers (OR=1.14; p=0.0025). MAPT H1 was also associated with risk for PD (OR=1.30; p=0.0003) and PSP (OR=3.18; p=8.59 × 10-8) but not FTD. Our results suggest that the MAPT H1 haplotype increases the risk of PD, PSP, and non-APOE ε4 AD., Spanish Ministry of Science and Innovation SAF 2006-10126 (2006–2009) and SAF2010-22329-C02-01 (2011–2013) to P.P and by the UTE project FIMA to P.P. Grants from the Ministry of Science (SAF2010-15558) and CIBERNED. Agust´ın Ruiz is supported by grant PI13/02434 (Acción Estratégica en Salud. Instituto de Salud Carlos III. Ministerio de Economía y Competitividad, Spain). Grant: Consolider (CSD2010-00045).