Parkinson's disease (PD) is characterized by α-synuclein aggregation and perturbation of the endolysosomal network (ELN), yet the molecular mechanisms linking α-synuclein pathology to neuronal dysfunction remain unclear. Here we report that treatment of mouse cortical neurons with α-synuclein preformed fibrils (PFFs) alters lysosomal composition and impairs lysosomal function, coupled with extensive chromatin remodeling and transcriptional reprogramming, including suppression of neuronal gene networks and activation of senescence-like programs. Mechanistically, these changes are associated with rapid recruitment and activation of the PD-associated kinase LRRK2 on early endosomes, where it phosphorylates Rab5, a key early endosomal GTPase, leading to remodeling of the Rab5 interactome, altered effector engagement, and endosomal dyshomeostasis. Pharmacological inhibition of LRRK2 with MLi-2 restores Rab5 activity, lysosomal function, chromatin accessibility, gene expression, and neuronal excitability. Knockdown of Rab5 partially rescues chromatin changes, supporting its role as a downstream effector. These findings identify LRRK2 hyperactivation and the LRRK2-Rab5 axis as key mediators of PFF-induced neuronal dysfunction, highlighting early endosomes as a central platform linking endolysosomal disruption to nuclear responses and offering potential targets for therapeutic intervention in PD.

Parkinson's disease (PD) is a prevalent neurodegenerative disorder predominantly affecting individuals over 60. Its motor symptoms stem from the deterioration of dopaminergic neurons within the substantia nigra. Despite aging being a significant risk factor, the specific mechanisms linking aging and PD pathology remain unclear. Leveraging advancements in single-cell genomics, this study utilizes single-nucleus multiome sequencing to capture transcriptomic and epigenetic profiles from 40,125 cells across the lifespan of the mouse substantia nigra. Our analysis pinpoints age-associated changes at a cell type-specific level, revealing a subset of genes that increasingly express with age and are enriched in PD-related pathways, notably in oligodendrocytes at late aging stages. Integration with five public PD single-cell RNA-seq data sets highlights 85 genes consistently differentially expressed with aging and PD. Key genes such as Hsp90aa1 and Hsp90ab1 are upregulated at late aging stages in oligodendrocytes, microglia, and glutamatergic neurons. Additionally, Apoe in microglia and genes related to protein folding in oligodendrocytes are upregulated at late aging stages, whereas genes involved in myelination are downregulated at early aging stages in oligodendrocyte. Our multiomic atlas underscores the substantial regulatory network changes during aging that may predispose to PD, providing valuable insights for furthering understanding of PD pathogenesis and potential therapeutic targets.

Increased APP gene dosage is both necessary and sufficient to result in Down Syndrome Alzheimer’s Disease (DSAD) in humans and AD-related degenerative changes in mouse models of DS. We tested antisense oligonucleotides (ASOs) designed to suppress APP expression via RNAseH1-mediated degradation in the Dp(16)1Yey or Dp(16) model of Down Syndrome. Dp(16) is trisomic for human chromosome 21 syntenic regions on murine chromosome 16, containing 115 genes including APP. To evaluate efficacy of APP suppression, Dp16 and 2N euploid mice at 6-8 months of age were treated with a mouse App ASO, and endosomal pathology and downstream pathological processes were evaluated. APP suppression reverses not only Rab5 hyperactivation in this model, but also abnormal hyperactivation of other Rab GTPases such as Rab7 and Rab11. Furthermore, tau hyperphosphorylation was ameliorated, and neurotrophin signaling was restored following APP suppression. Taken together, these data support the hypothesis that APP suppression should restore normal endosomal function and neurotrophic signaling and benefit disease, and the potential of antisense-mediated APP suppression as a disease-modifying therapy for DSAD.

Early-onset Alzheimer’s disease (EOAD) is a complex disease that occurs at an early age at onset (AAO) before 65 years, constituting 5-6% of all AD cases and remains poorly understood. Patient-derived induced pluripotent stem cells (iPSCs) have been used to model different forms of EOAD that display heterogeneous disease mechanisms. We examined iPSC-derived neurons from both familial EOAD harboring mutations in PSEN1 A79V , PSEN2 N141I , and APP V717I and non-familial EOAD patients at an early AAO. RNA-seq for familial and non-familial EOAD patients as well as ATAC-seq for familial EOAD patients were carried out to characterize the gene expression and chromatin accessibility changes, respectively. Differential expression and enrichment analysis, TF activity identification, and co-expression module detection were performed for familial EOAD RNA-seq. Clustering and surrogate neuron marker classification were performed for non-familial EOAD RNA-seq. Differential peak analysis, TF motif footprinting and peak functional enrichment were performed for familial EOAD ATAC-seq. Our approach allowed us to identify the correlation between gene expression and chromatin accessibility associated with key disease familial EOAD endotypes. We identified limitations with our non-familial EOAD neuron model to study sporadic AD, providing evidence that these neurons present variation of differentiation across patient clones, patient variability and an immature culture state. Common endotypes were identified across three familial EOAD mutations such as dedifferentiation of a mature neuron to a less differentiated quasi-neuron state and repression of mitochondrial function and metabolism. Integrative analysis allowed us to ascertain the master transcriptional regulators associated with these endotypes, including REST, ASCL1, and ZIC family members (activation), as well as NRF1 (repression). Our non-familial EOAD study showed a modest difference in expression profiling and a limited number of differentially expressed genes (DEGs) between diseased and control subjects. iPSC-derived neurons demonstrated that familial EOAD mutations share common regulatory changes within endotypes with varying severity, leading to reversion to a less-differentiated neuron state. Extending the usage of these neurons to non-familial EOAD may not serve as ideal to study sporadic AD. Overall, we have demonstrated that human neuron modeling can be applied to different forms of EOAD to understand the disease etiology better.

INTRODUCTION:Alzheimer's disease (AD), the leading cause of dementia, is more common in females. Although sex differences in tau pathology have been reported in AD, findings remain inconsistent. Down syndrome (DS), caused by trisomy 21, is the most common genetic cause of AD (DS-AD) and features tau pathology, but sex effects in DS-AD remain unclear. METHODS:We examined post mortem brain samples from individuals with DS-AD, DS without AD, and a rare partial trisomy 21 (PT) case with only two amyloid precursor protein (APP) gene copies. PHF1 tau, total tau, and sarkosyl-soluble and insoluble fractions were quantified by group and sex. RESULTS:PHF1 tau was significantly elevated in DS-AD, especially in females. Lower total tau in DS-AD males explained the absence of sex differences after normalization. Sarkosyl-insoluble tau was also higher in DS-AD females. DS without AD, and the PT case showed minimal pathology. DISCUSSION:These findings suggest sex-specific tau dynamics in DS-AD and support a role for APP dosage. HIGHLIGHTS:Tau pathology is significantly elevated in individuals with DS-AD, especially in females. Female DS-AD brains show markedly higher PHF1 (S396/404) and sarkosyl-insoluble tau levels compared to males. The observed sex difference in phosphorylated tau is driven by lower total tau in DS-AD males. Minimal tau pathology is present in DS without AD and in a rare partial trisomy 21 case. These findings implicate APP gene dosage in tau pathology in DS-AD.

The landscape of Down syndrome-associated Alzheimer's disease (DSAD) research reflects decades of scientific endeavor and collaborative effort, charting a remarkable journey from initial observations to the elucidation of complex genetic and molecular mechanisms. This perspective article chronicles key milestones and breakthroughs, paying homage to the pioneering scientists and advancements that have shaped the field. A thorough review of historical and contemporary literature offers a comprehensive narrative, highlighting the evolution of knowledge surrounding DSAD, from early recognition to the characterization of clinical presentation and natural history. The unique challenges and ethical considerations associated with DSAD populations are also examined, underscoring the importance of tailoring research and clinical approaches. By reflecting on the field's trajectory, this work celebrates past achievements while emphasizing the critical need for sustained research efforts. As part of a special issue, this article provides a foundation for appreciating the challenges and opportunities that lie ahead in advancing DSAD understanding and care. HIGHLIGHTS: This article provides a comprehensive overview of Down syndrome-associated Alzheimer's disease (DSAD) history, from early descriptions to its recognition as a genetic form of AD. It reflects on historical challenges faced by individuals with intellectual disabilities in achieving inclusion in scientific research. This historical perspective highlights the critical contributions of individuals with DS in advancing understanding of AD natural history. It explores pivotal milestones and efforts that have driven progress in DSAD research. Finally, it provides context to understand challenges and opportunities in DSAD research and its future directions.

Critical to rigor in neurodegeneration research is the accurate and unbiased assessment of degenerative phenotypes, where stereology remains the gold standard, yet its widespread adoption is hindered by the high cost of proprietary systems. We developed OPEN-Stereo, an open-source stereology platform that integrates standard microscopy hardware with intelligent, software-based calibration and positional control. Innovative use of computer vision methods, multi-scale image-based calibration and navigation, are combined with open-loop stage data to establish a global positioning system without costly hardware, while remaining within the accuracy tolerances inherent to stereological sampling. OPEN-Stereo implements the stereological random sampling method for unbiased cell counting. Validation using samples previously analyzed on commercial systems demonstrated up to 95% agreement in cell counts across multiple brain regions, with statistical equivalence confirmed by two-way repeated measures ANOVA (p = 0.8962). Beyond enabling accessibility, OPEN-Stereo’s image-analytic architecture enables stereology to evolve beyond manual practice, toward AI-driven cell identification and counting.

Empathy is characterized as the ability to share one's experience and is associated with altruism. Previous work using blood oxygen level-dependent (BOLD) functional MRI (fMRI) has found that empathy is associated with greater activation in brain mechanisms supporting mentalizing (temporoparietal junction), salience (anterior cingulate cortex; insula), and self-reference (medial prefrontal cortex; precuneus). However, BOLD fMRI has some limitations that may not reliably capture the tonic experience of empathy. To address this, the present study used a perfusion-based arterial spin labeling fMRI approach that provides direct a quantifiable measurement of cerebral blood flow (1 mL/100 g tissue/min) and is less susceptible to low-frequency fluctuations and empathy-based "carry-over" effects that may be introduced by BOLD fMRI-based block designs. Twenty-nine healthy females (mean age = 29 years) were administered noxious heat (48 degrees C; left forearm) during arterial spin labeling fMRI. In the next 2 fMRI scans, female volunteers viewed a stranger (laboratory technician) and their romantic partner, respectively, receive pain-evoking heat (48 degrees C; left forearm) in real-time and positioned proximal to the scanner during fMRI acquisition. Visual analog scale (0 = "not unpleasant"; 10 = "most unpleasant sensation imaginable") empathy ratings were collected after each condition. There was significantly (P = 0.01) higher empathy while viewing a romantic partner in pain and greater cerebral blood flow in the right temporoparietal junction, amygdala, anterior insula, orbitofrontal cortex, and precuneus when compared with the stranger. Higher empathy was associated with greater precuneus and primary visual cortical activation. The present findings indicate that brain mechanisms supporting the embodiment of another's experience is associated with higher empathy.

INTRODUCTION:Down syndrome (DS) markedly increases the risk of Alzheimer's disease (DS-AD), but the role of RAB5 hyperactivation in its pathogenesis remains unclear. METHODS:Postmortem brain samples from individuals with DS, with and without AD, and a partial trisomy 21 case with only two amyloid precursor protein (APP) gene copies, were examined for endosomal Rabs, their guanine-nucleotide exchange factor (GEF) and GTPase activating protein (GAP) levels, and lysosomal cathepsins. Analysis extended to the Dp16 DS mouse model. The role of RAB5 hyperactivation in disrupting the endolysosomal system was explored using primary neurons. RESULTS:We observed widespread endolysosomal dysregulation in DS and Dp16 brains, requiring increased APP gene dose. RAB5 hyperactivation resulted in increased activation of endosomal Rabs, including RABs 7 and 11, and increased recruitment of Rabs and their GEFs to early endosomes as well as the levels of lysosomal cathepsins. DISCUSSION:These findings suggest that APP dose-driven RAB5 hyperactivation disrupts endosomal Rab cascades and endosome maturation in DS. HIGHLIGHTS:There is widespread disruption of the endolysosomal network in the Down syndrome (DS) brain and in the Dp16 mouse model brain. Amyloid precursor protein (APP) gene dose was necessary for increases in endosomal Rab activity and lysosomal cathepsins in both human and mouse brains. Changes in endosomal Rabs 7 and 11 were linked to increases in their guanine-nucleotide exchange factors (GEFs) and GEF/GTPase activating protein (GAP) ratios. Mechanistic studies demonstrated essential roles for the beta-C-terminal fragment (β-CTF) of APP acting through hyperactivation of RAB5 to increase early endosomal membrane binding of the GEFs for downstream endosomal Rabs. RAB5 acts as the central hub for disruptions in endolysosomal function in DS.

Down syndrome (DS) significantly increases the risk of Alzheimer's disease (DS-AD), with dysfunctions in the endolysosomal network (ELN) and autophagy pathways playing central roles in its pathogenesis. Dysregulation of the ELN, particularly involving RAB5 and lysosomal cathepsins, has been implicated in DS-AD, but the specific role of RAB5 hyperactivation remains poorly understood. Postmortem brain samples from individuals with DS, DS-AD, a partial trisomy 21 case, and the Dp16 DS mouse model were examined to assess the impact of APP gene dosage on ELN and autophagy. We measured RAB5 activation, the activity of RAB7 and RAB11, their guanine nucleotide exchange factors (GEFs), lysosomal cathepsins, and autophagy-related pathways. Additionally, Dp16 mice were treated with App - and Rab5 -specific antisense oligonucleotides (ASOs) to evaluate their therapeutic potential. Our findings revealed substantial ELN dysfunction in both DS and Dp16 brains, characterized by RAB5 hyperactivation, increased RAB7 and RAB11 activation, elevated levels of their GEFs, and increased lysosomal cathepsin levels—all in an APP dose-dependent manner. Reduced expression of TSC1/2 and hyperphosphorylation of mTOR were associated with impaired autophagy. These abnormalities were absent in a partial trisomy 21 individual with two copies of APP . Treatment with ASOs in Dp16 mice restored RAB5 activity, normalized ELN function, and improved autophagic flux, alleviating DS-AD-related pathologies including tau hyperphosphorylation, neurotrophin signaling deficits, and synaptic protein loss. Our results demonstrate that APP dose-driven RAB5 hyperactivation disrupts endosomal Rab cascades, endosome maturation, and autophagy function in DS. Targeting either APP or Rab5 may offer promising therapeutic strategies to restore cellular function and mitigate DS-AD pathologies.

INTRODUCTION:Down syndrome (DS) markedly raises the risk of Alzheimer's disease (DS-AD). Our findings identified widespread dysregulation of the endolysosomal network (ELN) in DS and DS-AD brains, driven by increased APP gene dose, hyperactivation of RAB5, and elevated levels of guanine nucleotide exchange factors (GEFs) for RABs 7 and 11. METHODS:We investigated whether increasing APP gene dose and RAB5 hyperactivation contributed to neuropathogenesis and whether a clinically feasible intervention could reverse ELN changes. The Dp16 DS-AD mouse model was treated with a mouse App-specific antisense oligonucleotide (App-ASO) and Rab5-specific ASOs targeting Rab5a and Rab5b. RESULTS:App-ASO treatment normalized full-length APP (fl-APP) and its products, RAB5 activity, and downstream RABs 7 and 11 pathways. Rab5-ASOs reduced RAB5 levels and restored endosomal Rab activity. Both ASO treatments mitigated DS-AD-linked pathologies. DISCUSSION:These findings highlight ELN dysregulation in DS and the therapeutic potential of ASO-based strategies targeting APP or Rab5 to counteract DS-AD features. HIGHLIGHTS:App-ASO treatment reduced the levels of APP and its products and normalized endosomal Rab activity and GEF levels in Dp16 mice. Administration of Rab5-ASOs reduced RAB5 levels and normalized endosomal Rab activity and GEF levels in Dp16 mice. Both ASO treatments were well tolerated and mitigated APP-linked pathologies including tau hyperphosphorylation, neurotrophin signaling deficits, and synaptic protein loss. App-ASO or Rab5-ASOs reversed established pathological phenotypes in Dp16 mice.

[This corrects the article DOI: 10.1371/journal.pone.0152471.].

Down syndrome (DS) or trisomy 21 (T21) is present in a significant number of children and adults around the world and is associated with cognitive and medical challenges. Through research, the T21 Research Society (T21RS), established in 2014, unites a worldwide community dedicated to understanding the impact of T21 on biological systems and improving the quality of life of people with DS across the lifespan. T21RS hosts an international conference every two years to support collaboration, dissemination, and information sharing for this goal. In 2022, T21RS hosted an international conference in Long Beach, California, from June 9 to 12. The conference, attended by 483 people including scientists, families, self-advocates, and industry representatives from 17 countries, was a dynamic and interactive meeting that shared discoveries from international research teams. This summary highlights the scientific discoveries shared at the 4th T21RS meeting with the Imagine, Discover, Inspire theme.

Neurologists regularly care for patients with complex, chronic, and often incurable conditions. These circumstances impose profound emotional burdens on the patient and physician. Whereas empathy is central to therapeutic effectiveness in clinical practice, sustained empathic engagement can contribute to emotional exhaustion and physician burnout, a condition now endemic in neurology. This review synthesizes insights from neuroscience, psychology, and clinical education to propose "skillful empathy" as a trainable capacity that integrates affective resonance with cognitive perspective-taking. We describe how emotional contagion harms clinician well-being and advocate for the integration of empathy training into medical education to support sustainable, compassionate neurological care. ANN NEUROL 2026;99:35-48.

Synaptic changes are early manifestations of neuronal dysfunction in Huntington's disease (HD). However, the mechanisms by which mutant HTT protein impacts synaptogenesis and function are not well understood. Herein we explored HD pathogenesis in the BACHD mouse model by examining synaptogenesis and function in long term primary cortical cultures. At DIV14 (days in vitro), BACHD cortical neurons showed no difference from WT neurons in synaptogenesis as revealed by colocalization of a pre-synaptic (Synapsin I) and a post-synaptic (PSD95) marker. From DIV21 to DIV35, BACHD neurons showed progressively reduced colocalization of Synapsin I and PSD95 relative to WT neurons. The deficits were effectively rescued by treatment of BACHD neurons with BDNF. The recombinant apical domain of CCT1 (ApiCCT1) yielded a partial rescuing effect. BACHD neurons also showed culture age-related significant functional deficits as revealed by multielectrode arrays (MEAs). These deficits were prevented by BDNF, whereas ApiCCT1 showed a less potent effect. These findings are evidence that deficits in BACHD synapse and function can be replicated in vitro and that BDNF or a TRiC-inspired reagent can potentially be protective against these changes in BACHD neurons. Our findings support the use of cellular models to further explicate HD pathogenesis and potential treatments.

ObjectivesDue to increased gene dose for the amyloid precursor protein (APP), elderly adults with Down syndrome (DS) are at a markedly increased risk of Alzheimer's disease (AD), known as DS‐AD. How the increased APP gene dose acts and which APP products are responsible for DS‐AD is not well understood, thus limiting strategies to target pathogenesis. As one approach to address this question, we used a novel class of γ‐secretase modulators that promote γ‐site cleavages by the γ‐secretase complex, resulting in lower levels of the Aβ42 and Aβ40 peptides.MethodsTs65Dn mice, which serve as a model of DS, were treated via oral gavage with 10 mg/kg/weekday of BPN15606 (a potent and novel pyridazine‐containing γ‐secretase modulators). Treatment started at 3 months‐of‐age and lasted for 4 months.ResultsDemonstrating successful target engagement, treatment with BPN15606 significantly decreased levels of Aβ40 and Aβ42 in the cortex and hippocampus; it had no effect on full‐length APP or its C‐terminal fragments in either 2 N or Ts65Dn mice. Importantly, the levels of total amyloid‐β were not impacted, pointing to BPN15606‐mediated enhancement of processivity of γ‐secretase. Additionally, BPN15606 rescued hyperactivation of Rab5, a protein responsible for regulating endosome function, and normalized neurotrophin signaling deficits. BPN15606 treatment also normalized the levels of synaptic proteins and tau phosphorylation, while reducing astrocytosis and microgliosis, and countering cognitive deficits.InterpretationOur findings point to the involvement of increased levels of Aβ42 and/or Aβ40 in contributing to several molecular and cognitive traits associated with DS‐AD. They speak to increased dosage of the APP gene acting through heightened levels of Aβ42 and/or Aβ40 as supporting pathogenesis. These findings further the interest in the potential use of γ‐secretase modulators for treating and possibly preventing AD in individuals with DS. ANN NEUROL 2024

Age remains the central risk factor for many neurodegenerative diseases including Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis. Although the mechanisms of aging are complex, the age-related accumulation of senescent cells in neurodegeneration is well documented and their clearance can alleviate disease-related features in preclinical models. Senescence-like characteristics are observed in both neuronal and glial lineages, but their relative contribution to aging and neurodegeneration remains unclear. Human pluripotent stem cell-derived neurons provide an experimental model system to induce neuronal senescence. However, the extensive heterogeneity in the profile of senescent neurons and the methods to assess senescence remain major challenges. Here, we review the evidence of cellular senescence in neuronal aging and disease, discuss human pluripotent stem cell-based model systems used to investigate neuronal senescence and propose a panel of cellular and molecular hallmarks to characterize senescent neurons. Understanding the role of neuronal senescence may yield novel therapeutic opportunities in neurodegenerative disease.

Amyloid β (Aβ) peptides accumulating in the brain are proposed to trigger Alzheimer's disease (AD). However, molecular cascades underlying their toxicity are poorly defined. Here, we explored a novel hypothesis for Aβ42 toxicity that arises from its proven affinity for γ-secretases. We hypothesized that the reported increases in Aβ42, particularly in the endolysosomal compartment, promote the establishment of a product feedback inhibitory mechanism on γ-secretases, and thereby impair downstream signaling events. We show that human Aβ42 peptides, but neither murine Aβ42 nor human Aβ17-42 (p3), inhibit γ-secretases and trigger accumulation of unprocessed substrates in neurons, including C-terminal fragments (CTFs) of APP, p75 and pan-cadherin. Moreover, Aβ42 treatment dysregulated cellular -homeostasis, as shown by the induction of p75-dependent neuronal death in two distinct cellular systems. Our findings raise the possibility that pathological elevations in Aβ42 contribute to cellular toxicity via the γ-secretase inhibition, and provide a novel conceptual framework to address Aβ toxicity in the context of γ-secretase-dependent homeostatic signaling.