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  Glial and vascular contributions to neurodegenerative diseases
    PP33
Astrocytic BACE1 deficiency enhances amyloid beta clearance by increasing clusterin
John Zhou1, Riqiang Yan1
1University of Connecticut Health Center, Farmington, USA
 Background: Abnormal accumulation of amyloid beta peptide (Aβ) in the brain is regarded as the possible causative agent of Alzheimer’s disease (AD). Increased production of Aβ or impaired clearance of Aβ results in the aggregation of Aβ into plaques, which are surrounded by glia cells. BACE1 is the rate-limiting secretase required for production of Aβ. Therefore, inhibiting BACE1 as a means to reduce Aβ production is being tested for treating Alzheimer’s disease. A recent study using conditional deletion of BACE1 in an AD mouse during adulthood showed a reversal of previously formed plaques and rescues Aβ- associated behavioral deficits. One possibility for this phenomenon is that BACE1 inhibition facilitates astrocytic Aβ clearance.
Materials and Methods: Single cell RNAseq (scRNAeq) was used to identify and compare transcriptomes of reactive astrocytes from BACE1- null and wild type mice. We further confirmed differentially expressed genes and interrogated possible pathways using primary astrocytes cultures from BACE1-null and wild type mice. Finally, we crossed mouse model with astrocyte-specific knockout of BACE1 with 5xFAD mouse to investigate the effect of astrocyte specific BACE1 deficiency on amyloid levels.
Results: Using scRNA seq, we found that BACE1 knockout mice reactive astrocytes have transcriptomes distinct from wild-type reactive astrocytes. Clusterin was significantly upregulated in BACE1 deficient astrocytes cultures and siRNA knockdown of clusterin significantly attenuated astrocytic Aβ clearance. Furthermore, in astrocyte specific BACE1 knockout in a 5xFAD background, we found significantly reduced plaque accumulation.
Discussion: Global BACE1 inhibition results in synaptic deficits due to alteration of neuronal BACE1 substrates. By inhibiting BACE1 specifically in astrocytes, we found a pathway that enhances astrocytic Aβ clearance and reduces overall Aβ plaque levels that might spare neuronal BACE1 substrates and subsequent side effects.
Conclusion: Together, our study suggests a novel function of BACE1 in astrocytes and Aβ clearance.
homozygous Plcg2-P522R (Plcg2-P522R-Homo) alleles onto 5xFAD, an amyloidosis mouse model of AD. We analyzed amyloid as well as microglial pathology and performed in vitro microglial phagocytosis assay. Moreover, we performed both bulk cortical tissue and single- nuclei RNA-seq analysis to evaluate the impact of Plcg2-P522R variants on the 5xFAD transcriptomic signatures.
Results & Discussions: We found that 5xFAD/Plcg2-P522R-Homo mice showed significant reduction and remodeling of amyloid plaques and a robust decrease of dystrophic neurites compared to 5xFAD mice. Interestingly, the microglia derived from Plcg2-P522R mice exhibited enhanced phagocytosis of fibrillary Aβ and microbeads. Consistent with this finding, the plaque-associated microglia in 5xFAD/Plcg2- P522R-Homo showed a significant increase in CD68, a lysosomal marker indicative of phagocytic microglia. Our cortical bulk tissue RNA-seq and NeuN- single-nuclei RNA-seq surprisingly did not show an enhanced damaged-associated microglial (DAM) transcriptomic signatures or TREM2-gene-doage-dependent signatures in 5xFAD/Plcg2-P522R- Homo mice compared to wildtype mice.
Conclusions: Our study using a novel Plcg2-P522R knockin mouse model crossed to 5xFAD mice showed an overall beneficial effect of this AD-protective variant. It enhanced the efficiency of microglia to phagocytosing and reducing Aβ amyloids in vitro and in vivo, and revealed that elevated DAM or TREM2-gene-doage dependent transcriptomic responses do not appear to be a component of altered microglial function in the Plcg2-P522R variant in amyloid AD mice. Funding: This work is supported by NIH/NIA (RF1 AG056114).
  PP35
Extracellular matrix protein decorin is increased in CSF of App knock-in mice and early stage of Alzheimer’s disease
Richeng Jiang1, Una Smailovic1, Hazal Haytural1,
Betty M. Tijms2, Hao Li1, Robert Mihai Haret3, Ganna Shevchenko4, Gefei Chen5, Axel Abelein5, Johan Gobom6,7, Susanne Frykman1, Misaki Sekiguchi8, Ryo Fujioka8,
Naoto Watamura8, Hiroki Sasaguri8, Sofie Nyström9,
Per Hammarström9, Takaomi Saido8, Vesna Jelic1, Stina Syvänen10, Henrik Zetterberg6,7,11,12,13, Bengt Winblad1, Jonas Bergquist4, Pieter Jelle Visser1,2,14, Per Nilsson1
1Department of Neurobiology, Care Sciences and Society, Karolinska Institutet, Stockholm, Sweden, 2Alzheimer Center Amsterdam, Department of Neurology, Amsterdam Neuroscience, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands, 3Division of Physiology and Neuroscience, Carol Davila University of Medicine and Pharmacy , Bucharest, Romania, 4Department of Chemistry – BMC, Analytical Chemistry and Neurochemistry, Uppsala University, Uppsala, Sweden, 5Department of Biosciences and Nutrition, Karolinska
Institutet , Huddinge, Sweden, 6Department of Psychiatry and Neurochemistry, Institute of Neuroscience and Physiology, the Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden, 7Clinical Neurochemistry Laboratory, Sahlgrenska University Hospital , Mölndal, Sweden, 8Laboratory for Proteolytic Neuroscience, RIKEN Center for Brain Science, Wako, Japan, 9IFM-Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden, 10Molecular Geriatrics, Department of Public Health and Caring Sciences, Uppsala University, Uppsala, Sweden, 11Department of Neurodegenerative Disease, UCL Institute of Neurology , London, United Kingdom, 12UK Dementia Research Institute at UCL, London, United Kingdom, 13Hong Kong Center for Neurodegenerative Diseases, Hong Kong,
China, 14Alzheimer Center Limburg, School for Mental Health and Neuroscience, Maastricht University , Maastricht, the Netherlands
   PP34
AD-protective Plcg2-P522R Variant Ameliorates Plaque and Neuritic Pathologies, Enhances Microglial Phagocytosis, and Attenuates Disease-Associated Microglial Activation in AD Mice
Xiaofeng Gu1, Daniel Lee1, Amberlene De La Rocha1, Peter Langfelder1, Zoe Pamonag1, Lalini Ramanathan1, Weihong Ge3, Aparna Bhaduri3, William Yang1,2
1Center for Neurobehavioral Genetics, Jane and Terry Semel Institute of Neuroscience and Human Behavior; Dept. Psychiatry and Biobehavioral Science, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, USA, 2Brain Research Institute,
University of California, Los Angeles, Los Angeles, USA, 3Department of Biological Chemistry, University of California, Los Angeles, Los Angeles, USA
 Background: Phospholipase C-γ2 (PLCG2) is highly expressed in microglia in the brain and functions as a signaling enzyme. Mutations in PLCG2 in human lead to autoimmune disorders and immunodeficiency, suggesting a key role for this enzyme in the regulation of myeloid cell functions. A rare P522R hypermorphic variant has been reported to associate with reduced risk against Alzheimer’s disease (AD).
Material and Methods: We used CRISPR/Cas9 genome editing to generate a novel Plcg2-P522R knockin mouse model and bred
Alzheimer’s disease (AD) is caused by amyloid-beta (Aβ) amyloidosis. In addition, autophagy is disturbed in the AD brain. Here we aimed at deepening the understanding of how these brain pathologies translate to the CSF to identify potential biomarkers by combining preclinical and clinical data. We used two state-of-the-art App knock-in AD mouse models, App-NL-F and App-NL-G-F, exhibiting AD-like Aβ pathology to analyze how the brain pathologies translate to CSF proteomes by label-free mass spectrometry (MS). The mouse CSF proteomes were further stratified to a previous CSF proteome dataset obtained from
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