Project description:Analysis of murine cerebrospinal fluid (CSF) by quantitative mass spectrometry is challenging due to low CSF volume, low total protein concentration and the presence of highly abundant proteins such as albumin. We demonstrate that the CSF proteome of individual mice can be analyzed in a quantitative manner to a depth of several hundred proteins in a robust and simple workflow consisting of single ultra HPLC runs on a benchtop mass spectrometer. The workflow is validated by a comparative analysis of BACE1-/- and wild type mice using label-free quantification. The protease BACE1 cleaves the amyloid precursor protein (APP) as well as several other substrates and is a major drug target in Alzheimer’s disease. We identified a total of 715 proteins with at least 2 unique peptides and quantified 522 of those proteins in CSF from BACE1-/- and wild type mice. Several proteins, including the known BACE1 substrates APP, APLP1, CHL1 and contactin-2 showed lower abundance in the CSF of BACE1-/- mice, demonstrating that BACE1 substrate identification is possible from CSF. Additionally, ectonucleotide pyrophosphatase 5 was identified as a novel BACE1 substrate and validated by immunoblot and in vitro BACE1 protease assay. Taken together, our study shows the deepest characterization of the mouse CSF proteome to date and the first quantitative analysis of the CSF proteome of individual mice. The BACE1 substrates identified in CSF may serve as biomarkers to monitor BACE1 activity in Alzheimer patients treated with BACE inhibitors.
Project description:Lysosomal failure underlies pathogenesis of numerous congenital neurodegenerative disorders and is an early and progressive feature of Alzheimer’s disease (AD) pathogenesis. Here, we report that lysosomal dysfunction in Down Syndrome (Trisomy 21) requires the extra gene copy of amyloid precursor protein (APP) and is mediated by the beta cleaved carboxy terminal fragment of APP (APP-βCTF, C99). In primary fibroblasts from individuals with Down Syndrome (DS), lysosomal degradation of autophagic and endocytic substrates is selectively impaired causing them to accumulate in enlarged autolysosomes/lysosomes. Direct measurements of lysosomal pH uncovered a significant elevation (0.6 units) associated with slowed LC3 turnover and the inactivation of cathepsin D (CTSD) and other lysosomal hydrolases known to be unstable or less active when lysosomal pH is persistently elevated. RNA sequencing analysis excluded a transcriptional contribution to hydrolase declines. Normalizing lysosome pH by delivering acidic nanoparticles to lysosomes ameliorated lysosomal deficits, implicating pH elevation as their primary basis. Cortical neurons cultured from the Ts2 mouse model of DS exhibited lysosomal deficits similar to those in DS cells. Lowering APP expression with siRNA or BACE1 inhibition reversed cathepsin deficits in both fibroblasts and neurons. Deleting one BACE1 allele from adult Ts2 mice had similar rescue effects in vivo. The modest elevation of endogenous APP-βCTF needed to disrupt lysosomal function in DS is relevant to sporadic AD where APP-βCTF, but not APP, is also elevated. Our results extend evidence that impaired lysosomal acidification drives progressive lysosomal failure in multiple forms of AD.
Project description:Mapttm1(EGFP)Klt/J mice (Mapt-EGFP; The Jackson Laboratory, Bar Harbor, ME, USA; stock 004779) carry a knock-in of the EGFP coding sequence in the first exon of the microtubule-associated protein tau (Mapt) gene producing a cytoplasmic EGFP fused to the first 31 amino acids of MAPT. EGFP expression marks neurons including enteric neurons regardless of their lineage, closely patterning the expression of neuron-specific beta-tubulin III (TUBB3). Mapt-EGFP ice were backcrossed to C57BL/6J (Jackson Laboratory strain #:000664) for three to five generations at Mayo Clinic. Six male and six female Mapt-EGFP mice (54-98 days of age) underwent surgical laparotomy in 3 groups (surgery #1: 1 male and 1 female, surgery #2: 3 males and 1 female, surgery #3: 2 males and 4 females) under pentobarbital (50mg/kg) anesthesia. The celiac ganglion of each mouse was injected with 3-5 μL of 25 mg/mL Alexa Fluor 647-labeled cholera toxin subunit B (CTB-AF647; Thermo Fisher Scientific, Waltham, MA, USA) with the intention of labeling the cell soma of intestinofugal neurons in the myenteric plexus of the colon. The animals were killed 3-4 days after surgery. The muscularis externa of the colon from each Mapt-EGFP mouse was pooled together between all mice of the same surgery date (2, 4, and 6 mice) and mechanically and enzymatically dissociated into single cells with a two-step process that first enriches for cells within myenteric ganglia (PMCID: PMC8114175). The pooled cells from each group of mice formed one biological replicate and subjected to FACS immediately after dissociation to generate populations of Mapt-EGFP+ neurons with or without the CTB-AF647 tracer and Mapt-EGFP− non-neuronal cells. The frequency of Mapt-EGFP+CTB-AF647+ neurons was approximately 125-fold lower than that of Mapt-EGFP+CTB-AF647− neurons and RNA from these preparations did not pass quality control. Therefore, only data from Mapt-EGFP+CTB-AF647− neurons were analyzed and referred to as Mapt-EGFP+ cells. Total RNA was isolated from Mapt-EGFP+ colonic neurons and Mapt-EGFP− myenteric cells using RNA-Bee (AMSBIO, Cambridge, MA, USA) and purified with RNeasy Mini Kit (Qiagen, Germantown, MD, USA). RNA quality was tested using Agilent Electropherogram (Agilent Technologies, Santa Clara, CA, USA) and hybridized to Affymetrix Mouse Genome 430.2 gene expression microarrays (Thermo Fisher Scientific, Waltham, MA, USA). This study utilized Affymetrix Mouse Genome 430.2 oligonucleotide microarray analysis to charaterize the transcriptome of Mapt-EGFP+ neurons and Mapt-EGFP- non-neuronal myenteric cells isolated from the colon of Mapttm1(EGFP)Klt/J mice.
Project description:The cell surface proteome is dynamic and has fundamental roles in cell signaling. Many surface membrane proteins are proteolytically released into a cell’s secretome, where they can have additional functions in cell-cell-communication. Yet, it remains challenging to determine the surface proteome and to compare it to the cell secretome, in particular under serum-containing cell culture conditions. Here, we set-up and evaluated the ‘surface-spanning protein enrichment with click sugars’ (SUSPECS) method for cell surface membrane glycoprotein biotinylation, enrichment and label-free quantitative mass spectrometry. SUSPECS is based on click chemistry-mediated labeling of glycoproteins, is fully compatible with labeling of living cells and can be combined with secretome analyses in the same experiment. Immunofluorescence-based confocal microscopy demonstrated that SUSPECS selectively labeled proteins at the cell surface, but not within cells. Nearly 700 transmembrane glycoproteins were quantified at the surface of primary murine neurons. To demonstrate the utility of SUSPECS, we tested how the protease BACE1, which is a key drug target in Alzheimer’s disease, affects the cell surface glycoproteome. Pharmacological inhibition of BACE1 selectively remodeled the neuronal surface proteome, resulting in up to seven-fold increased abundance of the BACE1 substrates APP, SEZ6, SEZ6L, CNTN2, CHL1 and L1, while other substrates were not or only mildly affected. Protein changes at the cell surface only partly correlated with changes in the secretome. Additionally, apparent non-substrates, such as TSPAN6, were also increased. Several altered proteins were validated by immunoblots in neurons and BACE1-deficient murine brains and indicate that BACE1-inhibition may lead to unexpected secondary effects.
Project description:BACE1 role in reactive astrocytes are unknown. We used single cell RNA sequencing (scRNA-seq) to analyze reactive astrocytes in mice with and without germline Bace1. We also examine reactive astrocytes in the case of adult Bace1 conditional knockout on a 5xFAD Alzheimer's disease mouse model.
Project description:The β-amyloid precursor protein APP and the related APLPs, undergo complex proteolytic processing giving rise to several fragments. Whereas it is well established that Aβ accumulation is a central trigger for Alzheimer disease (AD), the physiological role of APP family members and their diverse proteolytic products is still largely unknown. The secreted APPsα ectodomain has been shown to be involved in neuroprotection and synaptic plasticity. The γ-secretase generated APP intracellular domain AICD, functions as a transciptional regulator in heterologous reporter assays, although its role for endogenous gene regulation has remained controversial. To gain further insight into the molecular changes associated with knockout phenotypes and to elucidate the physiological functions of APP family members including their proposed role as transcriptional regulators we performed a DNA microarray transcriptome profiling of the frontal cortex of adult wild type, APP-/-, APLP2-/- and APPsα knockin (KI) mice, APPα/α, expressing solely the secreted APPsα ectodomain. Biological pathways affected by the lack of APP family members included regulation of neurogenesis, regulation of transcription and regulation of neuron projection development. Comparative analysis of transcriptome changes and qPCR validation identified co-regulated gene sets. Interestingly, these included heat shock proteins and plasticity related genes that were down-regulated in knock-out cortices. In contrast, we failed to detect significant differences in expression of previously proposed AICD target genes including Bace1, Kai1, Gsk3b, p53, Tip60 and Vglut2. Only Egfr was slightly up-regulated in APLP2-/- mice. Comparison of APP-/- and APPα/α with wild-type mice revealed a high proportion of co-regulated genes indicating an important role of the C-terminus for cellular signaling. Finally, comparison of APLP2-/- on different genetic backgrounds revealed that background related transcriptome changes may dominate over changes due to the knockout of a single gene. Shared transcriptome profiles corroborated closely related physiological functions of APP family members in the adult central nervous system. As expression of proposed AICD target genes was not altered in adult cortex, this may indicate that these genes are not affected by lack of APP under resting conditions or only in a small subset of cells. Prefrontal cortices of adult male mice (24 - 28 weeks) of the following genotypes were analyzed: WT (n=3), APP-/- (n=3), APPα/α (n=3), APLP2-/- (n=3), APLP2(R1)-/- (n=3). WT, APP-/-, APPα/α, APLP2-/- had been backcrossed for six generations to C57BL/6 mice. APLP2(R1)-/- harbors the identical knockout allele as APLP2-/- but was back-crossed only once.
Project description:The beta-secretase BACE1 (beta-site amyloid precursor (APP)-cleaving enzyme 1) is a major drug target for Alzheimer’s disease (AD), as it catalyzes the first step in amyloid beta (Abeta) generation, but has additional substrates and functions, in particular in the brain. Several advanced clinical trials with BACE1 inhibitors were stopped because of an adverse event, a mild cognitive worsening. The underlying mechanism is not yet known but may result from co-inhibition of the BACE1-homolog BACE2. While a cerebrospinal fluid (CSF) biomarker for measuring BACE2 activity is not yet established, VCAM-1 has been suggested as such a biomarker, but has not yet been tested upon prolonged dosing in vivo. Using CSF pharmacoproteomics and a subchronic dosing paradigm in non-human primates, we demonstrate that compound 89, a BACE inhibitor not yet tested in humans, and the clinically tested drug elenbecestat inhibit BACE1 in vivo, with little or no effect on BACE2, as seen with a reduction of substrates of BACE1, but not of the BACE2 substrate VCAM-1. As a control, verubecestat, which inhibits both BACE2 and BACE1, reduced CSF abundance of BACE1 substrates as well as of VCAM-1. This study demonstrates the suitability of VCAM-1 as a pharmacodynamic biomarker for measuring BACE2 target engagement in CSF.
Project description:The beta-secretase BACE1 (beta-site amyloid precursor (APP)-cleaving enzyme 1) is a major drug target for Alzheimer’s disease (AD), as it catalyzes the first step in amyloid beta (Abeta) generation, but has additional substrates and functions, in particular in the brain. Several advanced clinical trials with BACE1 inhibitors were stopped because of an adverse event, a mild cognitive worsening. The underlying mechanism is not yet known but may result from co-inhibition of the BACE1-homolog BACE2. While a cerebrospinal fluid (CSF) biomarker for measuring BACE2 activity is not yet established, VCAM-1 has been suggested as such a biomarker, but has not yet been tested upon prolonged dosing in vivo. Using CSF pharmacoproteomics and a subchronic dosing paradigm in non-human primates, we demonstrate that compound 89, a BACE inhibitor not yet tested in humans, and the clinically tested drug elenbecestat inhibit BACE1 in vivo, with little or no effect on BACE2, as seen with a reduction of substrates of BACE1, but not of the BACE2 substrate VCAM-1. As a control, verubecestat, which inhibits both BACE2 and BACE1, reduced CSF abundance of BACE1 substrates as well as of VCAM-1. This study demonstrates the suitability of VCAM-1 as a pharmacodynamic biomarker for measuring BACE2 target engagement in CSF.