Biology subjects

Brinkmalm, G.

Publications and source records attributed to Brinkmalm, G..

8 recordsLinked to original sources

bioRxiv2026

Differential endothelial cell remodeling in Down syndrome and APP duplication drives distinct blood brain barrier dysfunction

An extra copy of the amyloid precursor protein gene (APP) is present in Down syndrome (DS) and APP duplication (APPdup), increasing the risk of Alzheimer disease (AD) and cerebral amyloid angiopathy (CAA). However, we reported greater vascular A{beta} deposition in APPdup than in DS with AD, suggesting that other triplicated chromosome 21 genes may mitigate CAA severity. We investigated endothelial cell (EC) function and blood brain barrier (BBB) integrity in DS and APPdup using human induced pluripotent stem cell-derived ECs (iECs), assessing their morphology, junctional proteins, A{beta} peptide secretion, barrier function, and transcriptomic profiles. Both DS and APPdup iECs secreted more A{beta}40 A{beta}42 compared with their respective isogenic controls but exhibited distinct A{beta} peptide profiles. DS iECs showed increased junctional protein expression and reduced permeability, whereas APPdup iECs displayed increased permeability. Transcriptomic analysis revealed extensive dysregulation in DS iECs, with enrichment of junctional, adhesion, angiogenic, immune, and apoptosis pathways, and reduced cell-cycle, proliferative, and membrane-targeting signatures. APPdup iECs showed milder changes, including impaired protein targeting and reduced adhesion-related pathways. Despite comparable APP dosage, DS iECs maintained greater junctional integrity and lower permeability than APPdup iECs, potentially contributing to differential CAA vulnerability. However, impaired proliferation and altered angiogenesis may reduce BBB resilience during ageing.

neuroscience↗

bioRxiv2026

Distinct mechanistic pathways of early tauopathy revealed by MAPT mutations

Tau pathology underlies a broad spectrum of neurodegenerative disorders, collectively termed tauopathies, yet these diseases exhibit striking heterogeneity in their biological mechanisms and clinical outcomes. The basis for this heterogeneity remains poorly understood. Here, we address this question using knock-in mouse models expressing two distinct frontotemporal dementia-associated tau mutations to define how different tau variants drive divergent pathogenic programs in vivo. We find that the two mutations give rise to fundamentally different trajectories of tau pathogenesis. One trajectory is marked by progressive tau hyperphosphorylation and cytoskeletal destabilization occurring in the absence of detectable tau seed formation. In contrast, an alternative trajectory is characterized by tau hypophosphorylation, early seed formation, and alterations in nucleotide metabolism and chromatin organization, without overt cytoskeletal disruption. With aging, tau in this latter pathway transitions to a hyperphosphorylated state and forms mature fibrillar aggregates. Genetic enhancement of {beta}-amyloid selectively accelerates fibril formation, particularly in the model exhibiting early seeding. Together, these findings demonstrate that distinct tau mutations can engage separable pathogenic mechanisms, providing a biological framework for the heterogeneity observed across tauopathies, and highlighting the need for mechanism-informed therapeutic strategies and patient stratification. O_FIG O_LINKSMALLFIG WIDTH=199 HEIGHT=200 SRC="FIGDIR/small/705716v1_ufig1.gif" ALT="Figure 1"> View larger version (46K): org.highwire.dtl.DTLVardef@11eb652org.highwire.dtl.DTLVardef@153cc53org.highwire.dtl.DTLVardef@8b670eorg.highwire.dtl.DTLVardef@1afaad_HPS_FORMAT_FIGEXP M_FIG C_FIG Graphical abstractSchematic representation comparing two trajectories of tau pathology. The S305N mutation promotes a 4R isoform shift, cytoskeletal damage and synapse loss, and accumulation of soluble hyperphosphorylated tau. Tau remains soluble even at old ages. In contrast, the P301S mutation generates hypophosphorylated, seed-competent tau that forms fibrils. The effect of amyloid is slow in the S305N, but results in accelerated acceleration of pathology in the P301S. Figure made with Biorender.com.

neuroscience↗

bioRxiv2025

Independent Generation of Amyloid-β via Novel APP Transcripts

The amyloid precursor protein (APP) is processed by multiple enzymes to generate biologically active peptides, including amyloid-{beta} (A{beta}), which aggregates to form the hallmark pathology of Alzheimers disease (AD). A{beta} is produced through an initial {beta}-secretase cleavage of APP, generating a 99-amino acid C-terminal fragment (APP-C99). Subsequent cleavage of APP-C99 by {gamma}-secretase produces A{beta} peptides of varying lengths. To better understand the transcriptional regulation of A{beta} production, we employed long-read RNA sequencing and identified previously unannotated transcripts encoding APP-C99 with an additional methionine residue (APP-C100), generated independently of {beta}-secretase cleavage. These transcripts are expressed separately from full-length APP, and we observed that cells lacking full-length APP can still produce A{beta} through these shorter isoforms. Importantly, mass spectrometry analysis of cerebrospinal fluid (CSF) revealed peptides consistent with the methionine-extended A{beta} species, supporting the in vivo translation of these transcripts. Our findings reveal an alternative pathway for A{beta} generation and aggregation, highlighting a potential new target for modulating A{beta} accumulation in AD.

neuroscience↗

bioRxiv2025

Alterations in retinal tau phosphorylation in Alzheimer's disease patients identified by mass spectrometry

Most neurodegenerative diseases, including Alzheimers disease (AD) and multiple sclerosis (MS), are associated with abnormal phosphorylation of tau (p-tau) in the brain. Immunostaining studies have revealed an accumulation of p-tau in the AD retina and suggested it may reflect tau pathology in the brain. To validate these findings and further investigate the relationship between retinal and brain tau pathology, we used mass spectrometry to measure p-tau peptides in matched retinal and hippocampal samples from non-demented controls (NC, n=8), AD (n=12), and MS (n=4). We analysed the differences in p-tau levels between diagnoses and explored how retinal p-tau variants correlate with hippocampal p-tau and neuropathological changes. Our results revealed peaks corresponding to tau peptides phosphorylated at T181, S199/S202, T231, S396 + T403/S404, and T403/S404. These p-tau peptides were also detected in the hippocampal samples, along with additional p-tau peptides such as T217 and S262. Total tau phosphorylation and phosphorylation at S199/S202 and T231 were significantly higher in the retina of AD cases compared to NC. These two peptides, along with peptides phosphorylated at S396+T403/S404 and T403/S404, were also higher in cases with high amyloid-beta (A{beta}) Braak stages compared to those with low A{beta} Braak stages. Higher A{beta} Braak stages were also associated with higher mass spectrometry peak intensities of peptides phosphorylated at S199/S202 and S396+T403/S404. Additionally, retinal p-tau peptides at S396+T403/S404 and T403/S404 correlated with neurofibrillary tangle (NFT) Braak stages, and p-tau peptides S396+T403/S404 in the retina were linked to corresponding phosphorylation in the CA1 region. These findings underscore the connection between retinal and brain tau pathology and highlight the potential of retinal tau as a biomarker for AD diagnosis and monitoring while also deepening our understanding of tauopathies in both the retina and brain.

neuroscience↗

bioRxiv2024

Age and amyloidβ-dependent initiation of neurofibrillary tau tangles: an improved mouse model of Alzheimers disease without mutations in MAPT.

The lack of models in which ageing and amyloid-{beta} (A{beta}) initiate neurofibrillary tau tangles, independent of MAPT mutations, limits mechanistic insights into Alzheimers disease. Here, we characterise NLFTaum/h, a novel mouse model that recapitulates the gradual transition from A{beta} accumulation to tau pathology by 24 months. We show that combined effects of ageing, A{beta}, and human tau expression promote tau pathology. However, either A{beta} plaques or human tau alone can initiate tau pathology, but only in even older mice. This may be exacerbated by increased neuronal vulnerability across all genotypes with age. Moreover, multi-omic analyses reveal that human tau can induce early alterations in mitochondrial and synaptic pathways, priming this latent vulnerability preceding tau pathology. Together, this suggests that ageing and other interacting triggers accelerate the onset of tau pathology. This opens possibilities for translatable testing of improved therapies for Alzheimers disease, breaking the link between A{beta} and tau pathologies.

neuroscience↗

bioRxiv2022

Genetic mapping of APP and amyloid-β biology modulation by trisomy 21

Individuals who have Down syndrome frequently develop early onset Alzheimers disease, a neurodegenerative condition caused by the build-up of aggregated amyloid-{beta} and tau proteins in the brain. Amyloid-{beta} is produced by APP, a gene located on chromosome 21. People who have Down syndrome have three copies of chromosome 21 and thus also an additional copy of APP; this genetic change drives the early development of Alzheimers disease in these individuals. Here we use a combination of next-generation mouse models of Down syndrome (Tc1, Dp3Tyb, Dp(10)2Yey and Dp(17)3Yey) and a knockin mouse model of amyloid-{beta} accumulation (AppNL-F) to determine how chromosome 21 genes other than APP modulate APP/amyloid-{beta} in the brain when in three copies. We demonstrate that three copies of other chromosome 21 genes are sufficient to partially ameliorate amyloid-{beta} accumulation in the brain. We go on to identify a subregion of chromosome 21 that contains the gene/genes causing this decrease in amyloid-{beta} accumulation and investigate the role of two lead candidate genes Dyrk1a and Bace2. Thus an additional copy of chromosome 21 genes, other than APP, can modulate APP/amyloid-{beta} in the brain under physiological conditions. This work provides critical mechanistic insight into the development of disease and an explanation for the typically later age of onset of dementia in people who have AD-DS compared to those who have familial AD caused by triplication of APP.

neuroscience↗

bioRxiv2020

Mutations in the juxtamembrane segment of the cholesterol-binding site of APP alter its processing and promotes production of shorter, less toxic Aβ peptides.

BackgroundThe brains of patients with Alzheimers disease (AD) reveal increased cellular membrane levels of cholesterol. Correspondingly, we previously showed that elevating levels of membrane cholesterol in neuronal cultures recapitulates early AD phenotypes including excessive cleavage of amyloid {beta} (A{beta}) peptides from the amyloid precursor protein (APP). Here we aimed to evaluate how the presence of a cholesterol-binding site (CBS) in the transmembrane and juxtamembrane regions of APP regulates its processing. MethodsWe generated seven single and two double APP mutants at amino acid positions 22, 26, 28, 29, 33, 39 of the A{beta} sequence changing the charge and/or hydrophobicity of the targeted amino acids. HEK293T cells were transfected with APP constructs and secreted A{beta} peptides were measured using ELISA and mass spectrometry (MS). APP processing in normal and high cholesterol condition, and endocytosis were assessed in stably expressing APPwt and APPK28A HEK293T clones. Finally, we measured the binding of synthetic peptides derived from the A{beta} sequence to cholesterol-rich exosomes purified from control HEK293T cells. ResultsMost mutations triggered a reduction in the production of A{beta}40 and A{beta}42 peptides, whereas only juxtamembrane mutants resulted in the generation of shorter A{beta} peptides. We confirmed by mass spectrometry this specific change in the profile of secreted A{beta} peptides for the most characteristic APPK28A mutant. A transient increase of plasma membrane cholesterol enhanced the production of A{beta}40 by APPWT, an effect absent with APPK28A. The enzymatic activity of -, {beta}- and {gamma}-secretases remained unchanged in cells expressing APPK28A. Similarly, APPK28A subcellular localization in early endosomes did not differ to APPWT. Finally, WT but not CBS mutant A{beta} derived peptides bound to cholesterol-rich exosomes. ConclusionsTaken together, these data reveal a major role of the juxtamembrane region of APP in binding to cholesterol and accordingly in the regulation of APP processing. Binding of cholesterol to K28 could staple APP to the juxtamembrane region thereby permitting access to {gamma}-secretase cleavage at positions 40-42. The APPK28 mutant would lie deeper in the membrane, facilitating the production of shorter A{beta} peptides and unveiling this specific region as a novel target for reducing the production of toxic A{beta} species.

neuroscience↗

bioRxiv2020

Patient-specific Alzheimer-like pathology in trisomy 21 cerebral organoids reveals BACE2 as a gene-dose-sensitive AD-suppressor in human brain

A population of >6 million people worldwide at high risk of Alzheimers disease (AD) are those with Down Syndrome (DS, caused by trisomy 21 (T21)), 70% of whom develop dementia during lifetime, caused by an extra copy of {beta}-amyloid-(A{beta})-precursor-protein gene. We report AD-like pathology in cerebral organoids grown in vitro from non-invasively sampled strands of hair from 71% of DS donors. The pathology consisted of extracellular diffuse and fibrillar A{beta} deposits, hyperphosphorylated/pathologically conformed Tau, and premature neuronal loss. Presence/absence of AD-like pathology was donor-specific (reproducible between individual organoids/iPSC lines/experiments). Pathology could be triggered in pathology-negative T21 organoids by CRISPR/Cas9-mediated elimination of the third copy of chromosome-21-gene BACE2, but prevented by combined chemical {beta} and {gamma}-secretase inhibition. We found that T21-organoids secrete increased proportions of A{beta}-preventing (A{beta}1-19) and A{beta}-degradation products (A{beta}1-20 and A{beta}1-34). We show these profiles mirror in cerebrospinal fluid of people with DS. We demonstrate that this protective mechanism is mediated by BACE2-trisomy and cross-inhibited by clinically trialled BACE1-inhibitors. Combined, our data prove the physiological role of BACE2 as a dose-sensitive AD-suppressor gene, potentially explaining the dementia delay in [~]30% of people with DS. We also show that DS cerebral organoids could be explored as pre-morbid AD-risk population detector and a system for hypothesis-free drug screens as well as identification of natural suppressor genes for neurodegenerative diseases.

neuroscience↗