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  Glial and vascular contributions to neurodegenerative diseases
  POSTER SESSION I
October 10, 2022, 18:00 - 19:30
5xFAD). Six months later, human microglia were isolated from chimeric mice brains and examined via single-cell RNA sequencing and bulk proteomic analysis.
Results: Analysis of RNA sequencing and proteomic datasets reveals significant and novel impacts of TREM2 deletion on the response of human microglia to beta-amyloid pathology. Examination of these complementary datasets identified both immune- and synapse-related co-expression networks that are significantly altered between TREM2 genotypes in AD mice. RNA sequencing further reveals distinct changes in disease associated microglia (DAM) and regulators of G protein signaling (RGS) cell populations with varying TREM2 genotypes. Conclusions: Taken together, these data reveal important new information about the transcriptional and proteomic changes that occur within human microglia in response to amyloid pathology and loss of TREM2 expression.
  PP01
The P522R protective variant of PLCG2 promotes the expression of antigen presentation genes by human microglia in an Alzheimer’s disease mouse model Christel Claes1, Whitney England1, Emma Danhash1, Sepideh Kiani Shabestari1, Jean Paul Chadarevian1, Amit Jairaman1, Jonathan Hasselmann1, Andy Tsai2, Morgan Coburn1, Jessica Sanchez1, Tau Lim1, Jorge Hidalgo1, Christina Tu1, Michael Cahalan1, Bruce Lamb2, Gary Landreth2, Robert Spitale1, Mathew Blurton-Jones1, Hayk Davtyan1
1University Of California Irvine, Irvine, USA, 2Indiana University School of Medicine, Indianapolis , USA
   PP03
PLASMA APOE3 LEVELS IN RELATION TO SEX, BEHAVIOURAL AND COGNITIVE PHENOTYPES IN APOEε3/ε3 HUMANIZED-LIVER AND TARGETED REPLACEMENT MICE
Andreas Giannisis1, Kat Kessler2, Greg Bial3, Lander Foquet3, Henrietta M. Nielsen1, Jacob Raber2,4
1Department of Biochemistry and Biophysics, Stockholm University, Stockholm, Sweden, 2Department of Behavioral Neuroscience, Oregon Health & Science University, Portland,
USA, 3Yecuris Corporation, Tualatin, USA, 4Department of Neurology and Radiation Medicine, Division of Neuroscience, Oregon National Primate Research Center, Oregon Health & Science University, Portland, USA
 The P522R variant of PLCG2, expressed by microglia, is associated with reduced risk of Alzheimer’s disease (AD). Yet, the impact of this protective mutation on microglial responses to AD pathology remains unknown. Chimeric AD and wild-type mice were generated by transplanting PLCG2-P522R or isogenic wild-type human induced pluripotent stem cell microglia. At 7 months of age, single-cell and bulk RNA sequencing, and histological analyses were performed. The PLCG2-P522R variant induced a significant increase in microglial human leukocyte antigen (HLA) expression and the induction of antigen presentation, chemokine signaling, and T cell proliferation pathways. Examination of immune-intact AD mice further demonstrated that the PLCG2-P522R variant promotes the recruitment of CD8+ T cells to the brain. These data provide the first evidence that the PLCG2-P522R variant increases the capacity of microglia to recruit T cells and present antigens, promoting amicroglial transcriptional state that has recently been shown to be reduced in AD patient brains.
Background: Liver-derived plasma apoE does not cross the blood- brain barrier [1] but is associated with AD risk, CSF AD biomarkers, brain imaging measures, and cognition [2-4]. We previously reported disadvantageous effects of an APOEε4/ε4 liver genotype and plasma apoE4 levels on various markers of brain integrity in FRGN APOEε4/ε4 humanized-liver (HL) mice [5]. Here, we assessed whether plasma apoE3 levels are associated with behavior and cognition in FRGN mice with humanized ‘AD-neutral’ APOEε3/ε3 livers (E3-HL) and APOEε3 targeted replacement (E3-TR) mice.
Materials and Methods: Behavioural and cognitive performance (open field, fear conditioning, water maze, Y-Maze, spatial Y-Maze, and novel object recognition) was assessed in 7-month-old female and male E3- HL and E3-TR mice. Plasma apoE3 levels were assessed by ELISA [3, 5, 6], compared between sexes and models, and assessed for correlations with cognitive test measures.
Results: Plasma apoE3 levels in both models were comparable to those in humans. Female E3-HL exhibited higher plasma apoE3 levels compared to male mice (p<0.0001) and compared to the female E3-TR mice (p<0.0001). Plasma apoE3 levels were associated with cortical, hippocampal, and amygdala functions and linked to learning and memory (working and spatial) (p<0.05). Plasma apoE3 levels were positively related to activity levels and reduced anxiety levels but, with the exception of the performance of E3-TR in the spatial Y maze, negatively to measures of cognitive performance (p<0.05).
Discussion: Plasma apoE levels in E3-HL and E3-TR mice vary by sex only in FRGN E3-HL mice. Besides spontaneous alternation in the Y maze, the relationships between plasma apoE3 levels and behavioural and cognitive measures were model-dependent.
Conclusions: This data support the importance of liver-derived apoE in behavioral and cognitive performance in two mouse models expressing human apoE.
  PP02
UNDERSTANDING HUMAN TREM2-DEPENDENT MICROGLIAL RESPONSES TO ALZHEIMER’S DISEASE PATHOLOGY IN VIVO WITH CHIMERIC MICE
Hayk Davtyan1, Jonathan Hasselmann1, Alex M. Tamburino2, Jean Paul Chadarevian1, Yun Jiao2, Carlo Ramil2, Minxue
Jia2, Dan Chang2, Alina Lahian1, Christina Tu1, Sepideh
Kiani Shabestari1, Joia Capocchi1, Nandita Joshi2, Christian Mirescu2, An Chi2, Vanessa Peterson2, Matthew Kennedy2,
Mathew Blurton-Jones1, Rebecca Mathew2
1University of California Irvine, Irvine, USA, 2Merck Sharp & Dohme Corp. (MSD), Boston, USA
 Aims: Genome wide association studies have identified many Alzheimer’s disease (AD) risk genes that are highly expressed by microglia. Loss-of-function mutations in one such gene, TREM2 (Triggering receptor expressed on myeloid cells-2) have been shown to increase Late-onset AD risk by 2-4-fold. Studies of TREM2 knockout mice have provided consistent evidence that TREM2 is critically involved in the ability of microglia to sense and respond to beta-amyloid plaques. Yet many questions remain regarding whether human TREM2 knockout microglia exhibit similar or perhaps additional functional deficits. Methods: To further examine the impact of TREM2 deletion on human microglia, we used CRISPR to generate TREM2-knockout (TREM2-KO) induced pluripotent stem cells (iPSCs). Isogenic wildtype and TREM2- KO iPSCs were differentiated into hematopoietic progenitors (HPCs) and transplanted into postnatal immunodeficient AD mice (hCSF1-
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