Background The clinical phenotype of corticobasal syndrome can be caused by aggregation of various proteins in the brain, including 4-repeat tau in a substantial proportion of the patients. Reliable biomarkers for differentiating corticobasal syndrome with aggregation of 4-repeat tau from related parkinsonian disorders remain limited. We therefore aimed to (i) map syndrome-specific CSF proteomic signatures across various parkinsonian disorders, (ii) identify associated molecular pathways and (iii) explore protein biomarkers capable of differentiating β-amyloid negative corticobasal syndrome. Methods We quantified 127 CSF proteins using the Nucleic acid Linked Immuno-Sandwich Assay (NULISA) on a biomarker-enhanced cohort of clinically characterized patients with Parkinson's disease, multiple system atrophy, progressive supranuclear palsy and corticobasal syndrome. Cohort inclusion required fulfillment of established clinical diagnostic criteria supported by complementary biomarkers indicative of the presumed underlying proteinopathy. Differential protein abundance was assessed using age- and sex-adjusted generalized linear models. Pathway-level alterations were examined using panel-aware gene set enrichment analysis. Biomarker candidates for distinguishing β-amyloid-negative corticobasal syndrome from other parkinsonian disorders were identified using elastic-net feature selection and validated by receiver operating characteristic analysis. Results β-amyloid-negative corticobasal syndrome exhibited the most pronounced proteomic alterations, characterized by widespread increases in protein abundance and enrichment of inflammatory and vascular pathways, including IL-10 associated signaling when compared to controls. A core set of 37 proteins was consistently altered relative to other parrkinsonian syndromes, with a subset of proteins converging across multiple analytical approaches, including inflammatory mediators (CXCL8, IL-18, CX3CL1) and vascular/metabolic proteins (VEGFA, PGK1). These proteins also demonstrated high discriminatory potential, with an area under the curve above 0.85. Conclusions High-plex CSF proteomics suggests that β-amyloid-negative corticobasal syndrome is associated with a distinct vascular-immune molecular profile, indicating molecular differences from clinically similar parkinsonian disorders. These findings provide insight into biological processes linked to corticobasal syndrome and highlight candidate protein signatures that to our knowledge have not yet been systematically associated with β-amyloid-negative CBS and may support molecular stratification in parkinsonian disorders.

Lecanemab approval in the European Union (EU) was granted after a delay. This delay resulted in concerns from many stakeholders, but attitudes of patients with early symptomatic Alzheimer's disease receiving specialist memory-clinic care remained insufficiently assessed. Therefore, we evaluated attitudes of specialist memory-clinic patients with early symptomatic Alzheimer's disease towards lecanemab in Europe. In this anonymous, international, multicentre, cross-sectional survey conducted from October 14, 2024 to February 18, 2025, a standardized, expert-developed questionnaire assessed attitudes towards lecanemab treatment and EU approval. Before answering four binary questions, participants received brief explanatory information on expected clinical benefit, amyloid-related imaging abnormalities (ARIA), and the increased ARIA risk associated with APOE ε4 homozygosity. The survey was conducted in specialist memory clinics within the European Alzheimer's Disease Consortium (EADC), the German memory clinic network (DNG), and Austrian memory centers. Available recruitment-flow data were limited to completed questionnaires because the survey was anonymous and distributed locally. 281 patients with early symptomatic Alzheimer's disease completed the survey. Network-level sample sizes were EADC n = 202, DNG n = 60, and Austria n = 19; country-specific sample sizes within the EADC and response rates were not available. Endorsement was high for both individual treatment with lecanemab (81.9%, 95% confidence interval [CI] 76.8-86.2) and general EU approval (91.8%, 95% CI 87.9-94.7). Endorsement remained substantial, but was lower, in the context of APOE ε4 homozygosity (treatment: 61.2%, 95% CI 55.2-66.9; approval: 76.5%, 95% CI 71.1-81.3). Approval-related questions received higher endorsement than treatment-related questions (84% vs. 72%; p < 0.001). Support for approval for APOE ε4 homozygotes declined after regulatory recommendations excluded this group (from 87% to 73%; p = 0.025); this comparison reflects independent respondents completing the anonymous survey before versus after November 14, 2024. Network-level comparisons were descriptive and underpowered for geographic inference. High endorsement within this specialist memory-clinic sample suggests perceived value of access to lecanemab. Greater endorsement for approval than for individual treatment may reflect support for treatment access beyond personal treatment choice, but alternative explanations such as social desirability, acquiescence, misunderstanding, or effects of the survey information cannot be excluded. The findings should not be generalized beyond specialist memory-clinic patients and should be interpreted in view of potential selection and response biases, absent response-rate data, and the brief, non-validated binary questionnaire.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a central regulator of microglial activity and loss-of-function coding variants are major risk factors for late onset Alzheimer’s disease (LOAD). To better understand the molecular and functional changes associated with TREM2 signalling in microglia, we generated a TREM2 reporter mouse. In APP transgenic animals, bulk RNA-sequencing of isolated microglia sorted based on reporter expression highlighted TREM2 level-related changes in major immunometabolic pathways, and enrichment of genes in oxidative phosphorylation and cholesterol metabolism in microglia with increased TREM2 expression. Metabolic and lipidomic profiling of sorted microglia showed that, independent of Aβ pathology, TREM2 expression correlated with signatures consistent with increased cellular redox, energetics, and cholesterol homoeostasis. In accordance, metabolic activity correlated with phagocytic capacity. Finally, we performed chronic treatment with a TREM2 agonist antibody and identified a window of TREM2 expression where microglia are most responsive, thereby informing clinical applications of TREM2 agonists. TREM2 is an important AD risk factor playing essential roles in the microglial response to amyloid pathology. Here, authors show using a TREM2 reporter mouse that TREM2 levels are critical for efficacy of TREM2 agonism informing current clinical efforts.

TDP-43 proteinopathy is a hallmark of neurodegenerative disorders such as amyotrophic lateral sclerosis and frontotemporal dementia where mislocalization of TDP-43 has been observed in neurons and glial cells. However, the role of TDP-43 in microglia and the consequences of its loss of function remain unexplored. Combining magnetic resonance imaging, and confocal, and electron microscopy, we uncovered structural changes and myelin abnormalities in the early postnatal brain of mice lacking microglial TDP-43. Spatial transcriptomics further revealed an enriched interferon-responsive signature associated with oligodendrocyte dysfunction. Early depletion of microglial TDP-43 led to motor deficits in adult mice. Mechanistically, knocking out TDP-43 impaired microglial ability to engulf and degrade myelin. It also led to cryptic exon inclusion in the Tyrobp mRNA, resulting in truncated DAP12 protein, thus causing defective TREM2 signaling. Our findings reveal a role for TDP-43 in regulating the TREM2-DAP12 axis in mice, highlighting a previously unrecognized mechanism through which TDP-43 controls microglial function.

Dementia with Lewy bodies (DLB) is characterized by marked biological heterogeneity, only partly explained by the frequent presence of Alzheimer's disease (AD) copathology. Whether microglial responses contribute to this heterogeneity and how they are modulated by APOE genotype remain poorly understood. Using a specific immunoassay targeting cleaved soluble TREM2 (sTREM2), we investigated TREM2-dependent microglial responses across two independent DLB cohorts (n = 129) with molecular biomarker profiling and longitudinal follow-up. sTREM2 showed stage-dependent associations with AD-related biomarkers, being associated with amyloid-related changes during prodromal DLB and predominantly to tau-related markers at dementia stage. APOEε4 carriers exhibited approximately two-fold lower sTREM2 levels than non-carriers specifically during prodromal DLB, independently of AD copathology, with more pronounced effects observed in women. Longitudinal analyses showed that higher baseline sTREM2 levels during prodromal DLB were associated with slower cognitive decline independently of AD-related biomarkers. These findings suggest that APOEε4 attenuates early TREM2-mediated microglial responses in DLB through mechanisms beyond AD copathology. Together, our results identify the APOE-TREM2 axis as a potential contributor to disease heterogeneity and support stage-specific, biomarker-guided therapeutic strategies targeting TREM2 signaling in Lewy body disorders.

RNA binding proteins have multiple diverse cellular functions and are often mis-regulated in disease. Despite their many cellular functions and implications in disease, very little is known about their physiological functions. Here we describe a novel zebrafish knockout model of the RNA binding proteins Hnrnpa1 and Hnrnpa3. Loss of Hnrnpa3 in zebrafish has no obvious morphological phenotype. Similarly, single mutants of the duplicated zebrafish hnrnpa1 genes, hnrnpa1a and hnrnpa1b, have no discernible phenotype, whereas the hnrnpa1a; hnrnpa1b double mutants are embryonic lethal. They display muscle, vascular and developmental defects with a reduced volume of the yolk extension. Metabolic profiling revealed severe changes in lipid metabolism in the hnrnpa1a; hnrnpa1b double mutants. Our analysis identified the involvement of Hnrnpa1 in many cellular pathways including the regulation of lipid metabolism and opens the door for future therapeutic studies in HNRNPA-associated diseases.

Stem-cell-based in vitro models offer promising potential to elucidate human brain cell functions and interactions, but limitations in reproducibility, maturation and cell-type diversity persist. Especially, prolonged incorporation of mature microglia and studies of neuroinflammation have proven challenging. Here, we developed a human induced pluripotent stem cell-based three-dimensional cortical brain tissue model (3BTM) containing neurons, astrocytes and microglia with high reproducibility, maturity and viability. 3BTMs show morphological, functional and proteomic maturation of all cell types, leading to high similarity to their in vivo counterparts. Incorporated microglia survive for over 6 months and display mature morphology, functions and gene expression. Importantly, when engineered to model Alzheimer's disease pathology, 3BTMs recapitulate key disease hallmarks, including amyloid deposition, increased phospho-tau levels and neuroinflammation, with microglia shifting their transcriptional landscape to disease-relevant signatures. Treatment of Alzheimer's disease 3BTMs with anti-Aβ immunotherapy cleared deposits and largely reversed disease signatures in glia. Together, our microglia-containing model provides a platform for studying physiological and pathological states of human brain tissue.

The molecular basis for accelerated cognitive decline seen in Alzheimer's Disease (AD) cases presenting with cortical alpha-Synuclein (⍺-Syn) co-pathology is not well understood. We show that such co-pathology brains express higher levels of microtubule- associated protein tau and that increasing ⍺-Syn expression is sufficient to drive tau accumulation. Our results reveal a hitherto unknown link between the pathogenesis of AD and Parkinson's Disease whereby tau and ⍺-Syn synergistically drive dementia-related pathology. ### Competing Interest Statement The authors have declared no competing interest.

APOEε4 is a genetic risk factor for both Alzheimer's Disease (AD) and dementia with Lewy bodies (DLB). TREM2-dependent microglial activation is considered protective in AD and has been proposed to interact with APOE in this context. Since DLB often exhibits Alzheimer's copathology, we investigate the interplay between TREM2 response, APOEε4 carriage and Alzheimer's co-pathology, and its influence on disease evolution in DLB. We measured cerebrospinal fluid (CSF) cleaved soluble TREM2 (cTREM2), as a marker of TREM2-dependent microglial response, core AD biomarkers and determine APOEε4 carriage in 76 DLB patients (prodromal DLB [prodDLB], n  = 39; DLB-dementia, n  = 37). Forty one patients additionally underwent [ 18 F]Florbetapir-PET (FBP-PET). We quantified cTREM2 by an in-house MSD-based immunoassay; core AD biomarkers (Aβ42, t-tau, p -tau 181 ), by ELISA; and FBP-PET uptake, by standard uptake value ratio (SUVr). We stratified patients according to the A/T classification. Clinical follow-up (>1 year) was available for 69 patients. APOEε4 carriers had lower cTREM2 levels compared to non-carriers in prodDLB (3.72±1.79vs.6.83±2.25ng/mL, p -value=0.0005, Figure 1). Furthermore, APOEε4 carriage itself was associated with lower cTREM2 levels in prodDLB (β(carriers)=-0.42, p -value=0.025) independently of AD core biomarkers. Conversely, APOEe4 carriage did not impact cTREM2 levels in DLB-dementia. cTREM2 levels across A-/+ and T-/+ DLB groups are represented in Figure 1. In prodDLB, higher cTREM2 levels were associated with higher Aβ42 (β=0.77, p -value=0.0002) and p -tau 181 levels (β=0.612, p -value=0.002). In DLB-dementia, cTREM2 levels were associated only with p -tau 181 (β=0.58, p -value=0.0001). Additionally, higher cTREM2 levels were associated with lower FBP-PET SUVr in prodDLB (β=-1.71, p -value=0.04). Notably, higher sTREM2 levels at baseline in prodDLB were related to a smaller subsequent longitudinal decrease in MMSE scores (β=1.11, p -value=0.01). No significant relationship was observed between baseline cTREM2 at a DLB-dementia stage and subsequent cognitive decline (β=-0.3, p -value=0.5). APOEε4 carriage attenuates the TREM2-dependent microglial response in prodromal DLB, as reflected by lower CSF cTREM2 levels. Elevated cTREM2 levels in the prodromal phase are associated with slower cognitive decline, suggesting a protective role of microglial activation during early disease stages. These findings suggest an early modulation of TREM2-driven microglial response by APOEε4 which influences DLB progression.

The molecular basis for accelerated cognitive decline seen in Alzheimer's Disease (AD) cases presenting with cortical alpha-Synuclein co-pathology is not well understood. Mouse experiments have shown adverse interactions between tau (encoded by MAPT ) and alpha-Synuclein (encoded by SNCA ), but how this finding translates to humans from a genome-centered point of view remains unknown. Whole genome sequencing was performed on 137 neuropathologically defined AD cases, 36 of which presented with neocortical alpha-Synuclein co-pathology (Braak stage 6). Polygenic risk scores were calculated. Single-nucleus RNA sequencing and Western Blot data were collected from post-mortem tissue. Transcriptomic and proteomic results were validated in the MSBB cohort ( n  >300). Cellular, molecular and epigenetic consequences were assessed in isogenic iPSCs-derived neurons carrying a triplication of SNCA (AST) or a normal SNCA copy number (CAS). AD brains with alpha-Synuclein co-pathology had significantly higher polygenic risk scores for Parkinson's Disease, which could be partially explained by variants associated with higher expression of SNCA . Single-nucleus RNA sequencing and immunoblot analysis revealed a higher expression of MAPT and phosphorylated tau in alpha-Synuclein co-pathology cases. Protein and mRNA expression of MAPT and SNCA were positively correlated in the MSBB cohort. The employed iPSCs differentiation protocol accelerated neuronal maturation due to transient inhibition of EZH2 . Day50 AST neurons exhibited significantly increased pathological tau and alpha-Synuclein at both the RNA and protein levels compared to CAS neurons. AST neurons also showed highly activated GSK3β and decreased PSD95 (post-synaptic protein) in the immunofluorescence and immunoblot analyses. ATAC profiles identified dysregulated accessibility in the cAMP signaling pathway, as well as pathways related to axon guidance, postsynaptic density, and calcium signaling, among others. We demonstrate that alpha-Synuclein co-pathology in AD is characterized by higher phosphorylated tau levels in patients and iPSC-derived neurons. Our results provide insights into the complex molecular processes through which alpha-Synuclein and tau synergistically drive dementia-related pathology.

The triggering receptor expressed on myeloid cells 2 (TREM2) plays a pivotal role in the activation of myeloid cells and is currently being investigated as a potential therapeutic target in several diseases. In this study, we established enhanced quantification of PET images of a 64Cu-labeled antibody-based PET radiotracer as a noninvasive tool for the assessment of TREM2 expression in the brain and peripheral organs of mice. We used TREM2 knockout mice that lack target expression to investigate data-driven blood normalization of PET images against percentage of injected dose normalization. Methods: TREM2 knockout and wild-type mice (n = 11 each) were injected with the radiotracer [64Cu]Cu-NODAGA-ATV:4D9 (ATV is antibody transport vehicle). Twenty hours after injection, TREM2 PET was conducted and blood samples were collected. A voxelwise analysis with statistical parametric mapping served to determine voxels that correlate with ex vivo blood radioactivity levels. Furthermore, TREM2 PET signals were compared between mice with and those without TREM2 expression using image-derived blood normalization. Correlation with TREM2 protein expression levels in the lung, liver, spleen, and bone marrow was used to validate organ-specific PET results. Disease models of brain amyloidosis and myocardial infarction were investigated to test for the value of image-derived normalization in mice. Results: Blood radioactivity levels derived from a statistical parametric mapping-derived region of interest demonstrated a robust correlation with radioactivity measurements obtained from ex vivo blood samples. Voxelwise clusters of TREM2 PET signals were more robustly detected after blood normalization of the PET images. Significant voxelwise clusters of TREM2 PET signals in peripheral organs correlated with TREM2 protein expression levels. Furthermore, image-derived normalization enhanced the significance of voxelwise clusters of TREM2 in the brains of App SAA;TfRmu/hu mice, as well as the TREM2 signal in the myocardial infarct region. Both strongly correlated with ex vivo autoradiography. Conclusion: Normalization of PET images to account for blood levels enhanced the detection of TREM2. This improved methodology for TREM2 PET analysis provides a promising basis for future assessments of TREM2 imaging.

Anti-amyloid β-peptide (Aβ) immunotherapy was developed to reduce amyloid plaque pathology and slow cognitive decline during progression of Alzheimer’s disease. Efficient amyloid plaque clearance has been proven in clinical trials testing anti-Aβ antibodies, with the impact on cognitive endpoints correlating with the extent of plaque removal. However, treatment is associated with adverse side-effects, such as oedema and haemorrhages, which are potentially linked to the induced immune response. To improve the safety profile of these molecules, it is imperative to understand the consequences of anti-Aβ antibody treatment on immune cell function. Here, we investigated the effects of long-term chronic anti-Aβ treatment on amyloid plaque pathology and microglial response in the APP-SAA triple knock-in mouse model. Mice were treated weekly with anti-Aβ antibody from 4-8 months of age. Long-term treatment with anti-Aβ results in a robust and dose-dependent removal of amyloid plaque pathology, with a higher efficiency for removing diffuse over dense-core plaques. Analysis of the CSF proteome indicates a reduction of markers for neurodegeneration including Tau and α-Synuclein, as well as immune cell related proteins. Bulk RNA-seq revealed a dose-dependent decrease in brain-wide disease-associated microglial (DAM) and glycolytic gene expression, which is supported by a parallel decrease of glucose uptake and protein levels of Triggering receptor of myeloid cells 2 (Trem2) protein, a major immune receptor involved in DAM activation of microglia. In contrast, DAM activation around remaining plaques remains high regardless of treatment dose. In addition, microglia surrounding remaining plaques display a dose-dependent increase in microglial clustering and a selective increase in antigen presenting and immune signalling proteins. These findings demonstrate that long-term chronic anti-Aβ mediated removal of Aβ leads to a dose dependent decrease in brain-wide microglial DAM activation and neurodegeneration, while microglia at residual plaques display a combined DAM and antigen presenting phenotype that suggests a continued treatment response. ![Figure][1] Graphical abstract: Schematic overview of the effects of chronic long-term anti-Aβ treatment in APP-SAA mice Schematic was created with BioRender.com ### Competing Interest Statement C.H. and K.S. collaborate with Denali Therapeutics Inc. and C.H. is a member of the advisory boards of AviadoBio, Cure Ventures and Curie.Bio. M.B. is a member of the Neuroimaging Committee of the EANM. M.B. has received speaker honoraria from Roche, GE Healthcare, Iba, and Life Molecular Imaging; has advised Life Molecular Imaging and GE healthcare; and is currently on the advisory board of MIAC. T.S., C.H., S.S.D., V.W., D.X., J.W.L. and K.M.M. are full time employees of Denali Therapeutics Inc. * α-Syn : α-Synuclein Aβ : Amyloid β-peptide AD : Alzheimer’s disease APP : Amyloid precursor protein ARIA : Amyloid-related imaging abnormalities ARIA-E : ARIA-related oedema ARIA-H : ARIA-related haemorrhage BCA : Bicinchoninic acid BSA : Bovine serum albumin CAA : Cerebral amyloid-angiopathy CE : Cholesterol ester COA : Cortico-amygdala area CSF : Cerebrospinal fluid CTF : C-terminal fragment DAM : Disease associated microglia DAPI : 40,6-diamidino-2-phenylindole DEA : Diethylamine DEG : Differentially expressed gene diaPASEF : Data Independent Acquisition Parallel Accumulation–Serial Fragmentation ELISA : Enzyme-linked immunosorbent assay EtOH : Ethanol FA : Formic acid FcγR : Fc gamma receptor FBB : Florbetaben FDG : Fluorodeoxyglucose FDR : False discovery rate Gfap : Glial fibrillary acidic protein GM3 : Ganglioside mannose 3 GSEA : Gene set enrichment analysis HBSS : Hanks’ buffered salt solution hTfR : Human transferrin receptor IFN : Intereferon Il1rn : Interleukin-1 receptor anatagonist i.p. : intraperitoneal KI : Knock-in LAMP1 : Lysosomal-associated membrane protein 1 LC-MS : Liquid chromatography - mass spectrometry LC-MS/MS : Liquid chromatography - tandem mass spectrometry LOAD : Late-onset Alzheimer’s disease MACS : Magnetic-activated cell sorting MBq : Megabecquerel MCI : Mild cognitive impairment MHC : Major histocompatibility complex MMF : Medetomidine-midazolam-fentanyl MR : Magnetic resonance MRI : Magnetic resonance imaging MSD : Meso Scale Discovery MX-04 : Methoxy-04 NaCl : Sodium chloride NDS : Normal donkey serum PBS : Phosphate-buffered saline PET : Positron-emission tomography PFA : Paraformaldehyde RIPA : Radioimmunoprecipitation assay RNA-seq : RNA-sequencing ROI : Region of interest ROS : Reactive oxygen species RT : Room temperature SEM : Standard error of the mean SUV : Standard uptake value TBS : Tris-buffered saline TIMS : Trapped Ion Mobility Spectrometry Trem2 : Triggering receptor expressed on myeloid cells 2 VOI : Voxel of interest VT : Total volume of distribution [1]: pending:yes

The cell surface receptor TREM2 is a key genetic risk factor and drug target in Alzheimer’s disease (AD). In the brain, TREM2 is expressed in microglia, where it undergoes proteolytic cleavage, linked to AD risk, but the responsible protease in microglia is still unknown. Another microglial-expressed AD risk factor is catalytically inactive rhomboid 2 (iRhom2, RHBDF2), which binds to and acts as a non-catalytic subunit of the metalloprotease ADAM17. A potential role in TREM2 proteolysis is not yet known. Using microglial-like BV2 cells, bone marrow–derived macrophages, and primary murine microglia, we identify iRhom2 as a modifier of ADAM17-mediated TREM2 shedding. Loss of iRhom2 increased TREM2 in cell lysates and at the cell surface and enhanced TREM2 signaling and microglial phagocytosis of the amyloid β-peptide (Aβ). This study establishes ADAM17 as a physiological TREM2 protease in microglia and suggests iRhom2 as a potential drug target for modulating TREM2 proteolysis in AD.

Phospholipase C gamma 2, proline 522 to arginine (PLCγ2-P522R) is a protective variant that reduces the risk of Alzheimer’s disease (AD). Recently, it was shown to mitigate β-amyloid pathology in a 5XFAD mouse model of AD. Here, we investigated the protective functions of the PLCγ2-P522R variant in a less aggressive APP/PS1 mouse model of AD and assessed the underlying cellular mechanisms using mouse and human microglial models. The effects of the protective PLCγ2-P522R variant on microglial activation, AD-associated β-amyloid and neuronal pathologies, and behavioral changes were investigated in PLCγ2-P522R knock-in variant mice crossbred with APP/PS1 mice. Transcriptomic, proteomic, and functional studies were carried out using microglia isolated from mice carrying the PLCγ2-P522R variant. Finally, microglia-like cell models generated from human blood and skin biopsy samples of PLCγ2-P522R variant carriers were employed. The PLCγ2-P522R variant decreased β-amyloid plaque count and coverage in female APP/PS1 mice. Moreover, the PLCγ2-P522R variant promoted anxiety in these mice. The area of the microglia around β-amyloid plaques was also increased in mice carrying the PLCγ2-P522R variant, while β-amyloid plaque-associated neuronal dystrophy and the levels of certain cytokines, including IL-6 and IL-1β, were reduced. These alterations were revealed through [18F]FEPPA PET imaging and behavioral studies, as well as various cytokine immunoassays, transcriptomic and proteomic analyses, and immunohistochemical analyses using mouse brain tissues. In cultured mouse primary microglia, the PLCγ2-P522R variant reduced the size of lipid droplets. Furthermore, transcriptomic and proteomic analyses revealed that the PLCγ2-P522R variant regulated key targets and pathways involved in lipid metabolism, mitochondrial fatty acid oxidation, and inflammatory/interferon signaling in acutely isolated adult mouse microglia and human monocyte-derived microglia-like cells. Finally, the PLCγ2-P522R variant also increased mitochondrial respiration in human iPSC-derived microglia. These findings suggest that the PLCγ2-P522R variant exerts protective effects against β-amyloid and neuronal pathologies by increasing microglial responsiveness to β-amyloid plaques in APP/PS1 mice. The changes observed in lipid/fatty acid and mitochondrial metabolism revealed by the omics and metabolic assessments of mouse and human microglial models suggest that the protective effects of the PLCγ2-P522R variant are potentially associated with increased metabolic capacity of microglia.

Biallelic loss-of-function variants in TYROBP and TREM2 cause autosomal recessive presenile dementia with bone cysts known as Nasu-Hakola disease (NHD, alternatively polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy, PLOSL). Some other TREM2 variants contribute to the risk of Alzheimer’s disease (AD) and frontotemporal dementia, while deleterious TYROBP variants are globally extremely rare and their role in neurodegenerative diseases remains unclear. The population history of Finns has favored the enrichment of deleterious founder mutations, including a 5.2 kb deletion encompassing exons 1–4 of TYROBP and causing NHD in homozygous carriers. We used here a proxy marker to identify monoallelic TYROBP deletion carriers in the Finnish biobank study FinnGen combining genome and health registry data of 520,210 Finns. We show that monoallelic TYROBP deletion associates with an increased risk and earlier onset age of AD and dementia when compared to noncarriers. In addition, we present the first reported case of a monoallelic TYROBP deletion carrier with NHD-type bone cysts. Mechanistically, monoallelic TYROBP deletion leads to decreased levels of DAP12 protein (encoded by TYROBP) in myeloid cells. Using transcriptomic and proteomic analyses of human monocyte-derived microglia-like cells, we show that upon lipopolysaccharide stimulation monoallelic TYROBP deletion leads to the upregulation of the inflammatory response and downregulation of the unfolded protein response when compared to cells with two functional copies of TYROBP. Collectively, our findings indicate TYROBP deletion as a novel risk factor for AD and suggest specific pathways for therapeutic targeting.

Background: Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) exhibit significant clinical, genetic and neuropathological abnormalities, and are regarded as belonging to a common disease spectrum, referred to as the ALS-FTD spectrum disorders. Our understanding of the underlying mechanisms of these diseases has advanced significantly, including molecular neuropathology, genetics and molecular pathophysiology. The heterogeneity of these diseases poses significant challenges to translational research and drug development, particularly in sporadic cases. Consequently, there is an urgent need to improve patient stratification for the successful execution of future clinical trials. Methods/Results: We here describe the study design of the DESCRIBE-ALS/FTD study which aims to address this research gap by undertaking a systematic sampling of patients from the ALS FTD spectrum, encompassing all possible disease variants. The main objective of the study is to systematically document detailed cross-sectional phenotyping and the temporal progression of motor and neuropsychological abnormalities that occur in both ALS and FTD. Additionally, it seeks to systematically correlate these abnormalities with genetics and potentially predictive biomarkers including longitudinal biomaterial sampling, brain imaging and brain banking. Furthermore, first-degree relatives of patients with disease-causing gene variants undergo the same assessments to also sample presymptomatic risk gene carriers. Conclusion: With this prospective registry study we aim to generate datasets which will help researchers identifying different disease traits in people with sporadic and genetic ALS and FTD and to develop biomarkers to identify preclinical and prodromal disease stages.

Neural circuit responses arise from computations across diverse synapses within a neuron, making synaptic integration a crucial parameter to restore function after injury. Neuron transplantation offers a promising approach for circuit restoration after injury, but our knowledge of transplanted neuron (tN) synaptic connectivity remains limited. Here, we used a stab wound injury model, to examine how synapses of tNs mature and integrate using multimodal read-outs. We analyzed the morphological, ultrastructural and functional aspects of tN synapses and found surprising misalignments, such as many spines without synapses or many shaft synapses that are not inhibitory, pointing towards immature aspects even 3 months post transplantation. Spatial transcriptomics revealed persistent inflammatory signatures at the transplant site including Trem2 upregulation. Indeed, the excessive pruning of the brain-wide input connectome of tNs was much improved in an environment devoid of TREM2, highlighting the importance of tackling the chronic inflammation for adequate tN integration. ### Competing Interest Statement The authors have declared no competing interest.

Progranulin (PGRN) haploinsufficiency is a major risk factor for frontotemporal lobar degeneration with TDP-43 pathology (FTLD- GRN ). Multiple therapeutic strategies are in clinical development to restore PGRN levels in the CNS, including gene therapy. However, a limitation of current gene therapy approaches aimed to alleviate FTLD-associated pathologies may be their inefficient brain exposure and biodistribution. We therefore developed an adeno-associated virus (AAV) targeting the liver (L) to achieve sustained peripheral expression of a transferrin receptor (TfR) binding, brain-penetrant (b) PGRN variant (AAV(L):bPGRN) in two mouse models of FTLD- GRN , namely Grn knockout and GrnxTmem106b double knockout mice. This therapeutic strategy avoids potential safety and biodistribution issues of CNS-administered AAVs while maintaining sustained levels of PGRN in the brain following a single dose. AAV(L):bPGRN treatment reduced several FTLD- GRN associated disease pathologies including severe motor function deficits, aberrant TDP-43 solubility and phosphorylation, dysfunctional protein degradation, lipid metabolism, gliosis and neurodegeneration in the brain. Translatability of our findings was confirmed in a novel human in vitro model using co-cultured human induced pluripotent stem cell (hiPSC)-derived microglia lacking PGRN and TMEM106B and wild-type hiPSC-derived neurons. As in mice, aberrant TDP-43, lysosomal dysfunction and neuronal loss were ameliorated after treatment with exogenous TfR-binding protein transport vehicle fused to PGRN (PTV:PGRN). Together, our studies suggest that peripherally administered brain-penetrant PGRN replacement strategies can ameliorate FTLD- GRN relevant phenotypes including TDP-43 pathology, neurodegeneration and behavioral deficits. Our data provide preclinical proof of concept for the use of this AAV platform for treatment of FTLD- GRN and potentially other CNS disorders. One sentence summary Peripheral AAV-mediated delivery of brain-penetrant PGRN rescues TDP-43 pathology, neurodegeneration and motor phenotypes in FTLD- GRN models.