Neuron-specific translational control shift ensures proteostatic resilience during ER stress.

Proteostasis is essential for cellular survival and particularly important for highly specialised post-mitotic cells such as neurons. Transient reduction in protein synthesis by protein kinase R-like endoplasmic reticulum (ER) kinase (PERK)-mediated phosphorylation of eukaryotic translation initiation factor 2α (p-eIF2α) is a major proteostatic survival response during ER stress. Paradoxically, neurons are remarkably tolerant to PERK dysfunction, which suggests the existence of cell type-specific mechanisms that secure proteostatic stress resilience. Here, we demonstrate that PERK-deficient neurons, unlike other cell types, fully retain the capacity to control translation during ER stress. We observe rescaling of the ATF4 response, while the reduction in protein synthesis is fully retained. We identify two molecular pathways that jointly drive translational control in PERK-deficient neurons. Haem-regulated inhibitor (HRI) mediates p-eIF2α and the ATF4 response and is complemented by the tRNA cleaving RNase angiogenin (ANG) to reduce protein synthesis. Overall, our study elucidates an intricate back-up mechanism to ascertain translational control during ER stress in neurons that provides a mechanistic explanation for the thus far unresolved observation of neuronal resilience to proteostatic stress.

A 3D human co-culture to model neuron-astrocyte interactions in tauopathies.

BACKGROUND: Intraneuronal tau aggregation is the major pathological hallmark of neurodegenerative tauopathies. It is now generally acknowledged that tau aggregation also affects astrocytes in a cell non-autonomous manner. However, mechanisms involved are unclear, partly because of the lack of models that reflect the situation in the human tauopathy brain. To accurately model neuron-astrocyte interaction in tauopathies, there is a need for a model that contains both human neurons and human astrocytes, intraneuronal tau pathology and mimics the three-dimensional architecture of the brain. RESULTS: Here we established a novel 100-200 µm thick 3D human neuron/astrocyte co-culture model of tau pathology, comprising homogenous populations of hiPSC-derived neurons and primary human astrocytes in microwell format. Using confocal, electron and live microscopy, we validate the procedures by showing that neurons in the 3D co-culture form pre- and postsynapses and display spontaneous calcium transients within 4 weeks. Astrocytes in the 3D co-culture display bipolar and stellate morphologies with extensive processes that ensheath neuronal somas, spatially align with axons and dendrites and can be found perisynaptically. The complex morphology of astrocytes and the interaction with neurons in the 3D co-culture mirrors that in the human brain, indicating the model's potential to study physiological and pathological neuron-astrocyte interaction in vitro. Finally, we successfully implemented a methodology to introduce seed-independent intraneuronal tau aggregation in the 3D co-culture, enabling study of neuron-astrocyte interaction in early tau pathogenesis. CONCLUSIONS: Altogether, these data provide proof-of-concept for the utility of this rapid, miniaturized, and standardized 3D model for cell type-specific manipulations, such as the intraneuronal pathology that is associated with neurodegenerative disorders.

Protein disulfide isomerases as CSF biomarkers for the neuronal response to tau pathology.

INTRODUCTION: Cerebrospinal fluid (CSF) biomarkers for specific cellular disease processes are lacking for tauopathies. In this translational study we aimed to identify CSF biomarkers reflecting early tau pathology-associated unfolded protein response (UPR) activation. METHODS: We employed mass spectrometry proteomics and targeted immunoanalysis in a combination of biomarker discovery in primary mouse neurons in vitro and validation in patient CSF from two independent large multicentre cohorts (EMIF-AD MBD, n = 310; PRIDE, n = 771). RESULTS: First, we identify members of the protein disulfide isomerase (PDI) family in the neuronal UPR-activated secretome and validate secretion upon tau aggregation in vitro. Next, we demonstrate that PDIA1 and PDIA3 levels correlate with total- and phosphorylated-tau levels in CSF. PDIA1 levels are increased in CSF from AD patients compared to controls and patients with tau-unrelated frontotemporal and Lewy body dementia (LBD). HIGHLIGHTS: Neuronal unfolded protein response (UPR) activation induces the secretion of protein disulfide isomerases (PDIs) in vitro. PDIA1 is secreted upon tau aggregation in neurons in vitro. PDIA1 and PDIA3 levels correlate with total and phosphorylated tau levels in CSF. PDIA1 levels are increased in CSF from Alzheimer's disease (AD) patients compared to controls. PDIA1 levels are not increased in CSF from tau-unrelated frontotemporal dementia (FTD) and Lewy body dementia (LBD) patients.

Structurally and Morphologically Distinct Pathological Tau Assemblies Differentially Affect GVB Accumulation.

Tau aggregation is central to the pathogenesis of a large group of neurodegenerative diseases termed tauopathies, but it is still unclear in which way neurons respond to tau pathology and how tau accumulation leads to neurodegeneration. A striking neuron-specific response to tau pathology is presented by granulovacuolar degeneration bodies (GVBs), lysosomal structures that accumulate specific cargo in a dense core. Here we employed different tau aggregation models in primary neurons to investigate which properties of pathological tau assemblies affect GVB accumulation using a combination of confocal microscopy, transmission electron microscopy, and quantitative automated high-content microscopy. Employing GFP-tagged and untagged tau variants that spontaneously form intraneuronal aggregates, we induced pathological tau assemblies with a distinct subcellular localization, morphology, and ultrastructure depending on the presence or absence of the GFP tag. The quantification of the GVB load in the different models showed that an increased GVB accumulation is associated with the untagged tau aggregation model, characterized by shorter and more randomly distributed tau filaments in the neuronal soma. Our data indicate that tau aggregate structure and/or subcellular localization may be key determinants of GVB accumulation.

No evidence for cell-to-cell transmission of the unfolded protein response in cell culture.

The unfolded protein response (UPR) is one of the major cell-autonomous proteostatic stress responses. The UPR has been implicated in the pathogenesis of neurodegenerative diseases and is therefore actively investigated as therapeutic target. In this respect, cell non-autonomous effects of the UPR including the reported cell-to-cell transmission of UPR activity may be highly important. A pharmaca-based UPR induction was employed to generate conditioned media (CM) from CM-donating neuronal ('donor') cells (SK-N-SH and primary mouse neurons). As previously reported, upon subsequent transfer of CM to naive neuronal 'acceptor' cells, we confirmed UPR target mRNA and protein expression by qPCR and automated microscopy. However, UPR target gene expression was also induced in the absence of donor cells, indicating carry-over of pharmaca. Genetic induction of single pathways of the UPR in donor cells did not result in UPR transmission to acceptor cells. Moreover, no transmission was detected upon full UPR activation by nutrient deprivation or inducible expression of the heavy chain of immunoglobulin M in donor HeLa cells. In addition, in direct co-culture of donor cells expressing the immunoglobulin M heavy chain and fluorescent UPR reporter acceptor HeLa cells, UPR transmission was not observed. In conclusion, carry-over of pharmaca is a major confounding factor in pharmaca-based UPR transmission protocols that are therefore unsuitable to study cell-to-cell UPR transmission. In addition, the absence of UPR transmission in non-pharmaca-based models of UPR activation indicates that cell-to-cell UPR transmission does not occur in cell culture.

Alpha-synuclein induces the unfolded protein response in Parkinson's disease SNCA triplication iPSC-derived neurons.

The recent generation of induced pluripotent stem cells (iPSCs) from a patient with Parkinson's disease (PD) resulting from triplication of the α-synuclein (SNCA) gene locus allows unprecedented opportunities to explore its contribution to the molecular pathogenesis of PD. We used the double-nicking CRISPR/Cas9 system to conduct site-specific mutagenesis of SNCA in these cells, generating an isogenic iPSC line with normalized SNCA gene dosage. Comparative gene expression analysis of neuronal derivatives from these iPSCs revealed an ER stress phenotype, marked by induction of the IRE1α/XBP1 axis of the unfolded protein response (UPR) and culminating in terminal UPR activation. Neuropathological analysis of post-mortem brain tissue demonstrated that pIRE1α is expressed in PD brains within neurons containing elevated levels of α-synuclein or Lewy bodies. Having used this pair of isogenic iPSCs to define this phenotype, these cells can be further applied in UPR-targeted drug discovery towards the development of disease-modifying therapeutics.

Granulovacuolar degeneration bodies are neuron-selective lysosomal structures induced by intracellular tau pathology.

Granulovacuolar degeneration bodies (GVBs) are membrane-bound vacuolar structures harboring a dense core that accumulate in the brains of patients with neurodegenerative disorders, including Alzheimer's disease and other tauopathies. Insight into the origin of GVBs and their connection to tau pathology has been limited by the lack of suitable experimental models for GVB formation. Here, we used confocal, automated, super-resolution and electron microscopy to demonstrate that the seeding of tau pathology triggers the formation of GVBs in different mouse models in vivo and in primary mouse neurons in vitro. Seeding-induced intracellular tau aggregation, but not seed exposure alone, causes GVB formation in cultured neurons, but not in astrocytes. The extent of tau pathology strongly correlates with the GVB load. Tau-induced GVBs are immunoreactive for the established GVB markers CK1δ, CK1ɛ, CHMP2B, pPERK, peIF2α and pIRE1α and contain a LAMP1- and LIMP2-positive single membrane that surrounds the dense core and vacuole. The proteolysis reporter DQ-BSA is detected in the majority of GVBs, demonstrating that GVBs contain degraded endocytic cargo. GFP-tagged CK1δ accumulates in the GVB core, whereas GFP-tagged tau or GFP alone does not, indicating selective targeting of cytosolic proteins to GVBs. Taken together, we established the first in vitro model for GVB formation by seeding tau pathology in primary neurons. The tau-induced GVBs have the marker signature and morphological characteristics of GVBs in the human brain. We show that GVBs are lysosomal structures distinguished by the accumulation of a characteristic subset of proteins in a dense core.

The UPR reduces glucose metabolism via IRE1 signaling.

Neurons are highly dependent on glucose. A disturbance in glucose homeostasis therefore poses a severe risk that is counteracted by activation of stress responses to limit damage and restore the energy balance. A major stress response that is activated under conditions of glucose deprivation is the unfolded protein response (UPR) that is aimed to restore proteostasis in the endoplasmic reticulum. The key signaling of the UPR involves the transient activation of a transcriptional program and an overall reduction of protein synthesis. Since the UPR is strategically positioned to sense and integrate metabolic stress signals, it is likely that - apart from its adaptive response to restore proteostasis - it also directly affects metabolic pathways. Here we investigate the direct role of the UPR in glucose homeostasis. O-GlcNAc is a post-translational modification that is highly responsive to glucose fluctuations. We find that UPR activation results in decreased O-GlcNAc modification, in line with reduced glucose metabolism. Our data indicate that UPR activation has no direct impact on the upstream processes in glucose metabolism; glucose transporter expression, glucose uptake and hexokinase activity. In contrast, prolonged UPR activation decreases glycolysis and mitochondrial metabolism. Decreased mitochondrial respiration is not accompanied by apoptosis or a structural change in mitochondria indicating that the reduction in metabolic rate upon UPR activation is a physiological non-apoptotic response. Metabolic decrease is prevented if the IRE1 pathway of the UPR is inhibited. This indicates that activation of IRE1 signaling induces a reduction in glucose metabolism, as part of an adaptive response.

Insulin deficiency results in reversible protein kinase A activation and tau phosphorylation.

Alzheimer's disease (AD) is a highly prevalent multifactorial disease for which Diabetes Mellitus (DM) is a risk factor. Abnormal phosphorylation and aggregation of tau is a key hallmark of AD. In animal models, DM induces or exacerbates the phosphorylation of tau, suggesting that DM may influence the risk at AD by directly facilitating tau pathology. Previously we reported that tau phosphorylation induced in response to metabolic stress is reversible. Since identification and understanding of early players in tau pathology is pivotal for therapeutic intervention, we here investigated the mechanism underlying tau phosphorylation in the diabetic brain and its potential for reversibility. To model DM we used streptozotocin-treatment to induce insulin deficiency in rats. Insulin depletion leads to increased tau phosphorylation in the brain and we investigated the activation status of known tau kinases and phosphatases in this model. We identified protein kinase A (PKA) as a tau kinase activated by DM in the brain. The potential relevance of this signaling pathway to AD pathogenesis is indicated by the increased level of active PKA in temporal cortex of early stage AD patients. Our data indicate that activation of PKA and tau phosphorylation are associated with insulin deficiency per se, rather than the downstream energy deprivation. In vitro studies confirm that insulin deficiency results in PKA activation and tau phosphorylation. Strikingly, both active PKA and induced tau phosphorylation are reversed upon insulin treatment in the steptozotocin animal model. Our data identify insulin deficiency as a direct trigger that induces the activity of the tau kinase PKA and results in tau phosphorylation. The reversibility upon insulin treatment underscores the potential of insulin as an early disease-modifying intervention in AD and other tauopathies.

IRE1 signaling exacerbates Alzheimer's disease pathogenesis.

Altered proteostasis is a salient feature of Alzheimer's disease (AD), highlighting the occurrence of endoplasmic reticulum (ER) stress and abnormal protein aggregation. ER stress triggers the activation of the unfolded protein response (UPR), a signaling pathway that enforces adaptive programs to sustain proteostasis or eliminate terminally damaged cells. IRE1 is an ER-located kinase and endoribonuclease that operates as a major stress transducer, mediating both adaptive and proapoptotic programs under ER stress. IRE1 signaling controls the expression of the transcription factor XBP1, in addition to degrade several RNAs. Importantly, a polymorphism in the XBP1 promoter was suggested as a risk factor to develop AD. Here, we demonstrate a positive correlation between the progression of AD histopathology and the activation of IRE1 in human brain tissue. To define the significance of the UPR to AD, we targeted IRE1 expression in a transgenic mouse model of AD. Despite initial expectations that IRE1 signaling may protect against AD, genetic ablation of the RNase domain of IRE1 in the nervous system significantly reduced amyloid deposition, the content of amyloid ß oligomers, and astrocyte activation. IRE1 deficiency fully restored the learning and memory capacity of AD mice, associated with improved synaptic function and improved long-term potentiation (LTP). At the molecular level, IRE1 deletion reduced the expression of amyloid precursor protein (APP) in cortical and hippocampal areas of AD mice. In vitro experiments demonstrated that inhibition of IRE1 downstream signaling reduces APP steady-state levels, associated with its retention at the ER followed by proteasome-mediated degradation. Our findings uncovered an unanticipated role of IRE1 in the pathogenesis of AD, offering a novel target for disease intervention.

The unfolded protein response in neurodegenerative diseases: a neuropathological perspective.

The unfolded protein response (UPR) is a stress response of the endoplasmic reticulum (ER) to a disturbance in protein folding. The so-called ER stress sensors PERK, IRE1 and ATF6 play a central role in the initiation and regulation of the UPR. The accumulation of misfolded and aggregated proteins is a common characteristic of neurodegenerative diseases. With the discovery of the basic machinery of the UPR, the idea was born that the UPR or part of its machinery could be involved in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and prion disease. Over the last decade, the UPR has been addressed in an increasing number of studies on neurodegeneration. The involvement of the UPR has been investigated in human neuropathology across different neurological diseases, as well as in cell and mouse models for neurodegeneration. Studies using different disease models display discrepancies on the role and function of the UPR during neurodegeneration, which can often be attributed to differences in methodology. In this review, we will address the importance of investigation of human brain material for the interpretation of the role of the UPR in neurological diseases. We will discuss evidence for UPR activation in neurodegenerative diseases, and the methodology to study UPR activation and its connection to brain pathology will be addressed. More recently, the UPR is recognized as a target for drug therapy for treatment and prevention of neurodegeneration, by inhibiting the function of specific mediators of the UPR. Several preclinical studies have shown a proof-of-concept for this approach targeting the machinery of UPR, in particular the PERK pathway, in different models for neurodegeneration and have yielded paradoxical results. The promises held by these observations will need further support by clarification of the observed differences between disease models, as well as increased insight obtained from human neuropathology.

Liposomes bi-functionalized with phosphatidic acid and an ApoE-derived peptide affect Aß aggregation features and cross the blood-brain-barrier: implications for therapy of Alzheimer disease.

Targeting amyloid-ß peptide (Aß) within the brain is a strategy actively sought for therapy of Alzheimer's disease (AD). We investigated the ability of liposomes bi-functionalized with phosphatidic acid and with a modified ApoE-derived peptide (mApoE-PA-LIP) to affect Aß aggregation/disaggregation features and to cross in vitro and in vivo the blood-brain barrier (BBB). Surface plasmon resonance showed that bi-functionalized liposomes strongly bind Aß (kD=0.6 µM), while Thioflavin-T and SDS-PAGE/WB assays show that liposomes inhibit peptide aggregation (70% inhibition after 72 h) and trigger the disaggregation of preformed aggregates (60% decrease after 120 h incubation). Moreover, experiments with dually radiolabelled LIP suggest that bi-functionalization enhances the passage of radioactivity across the BBB either in vitro (permeability=2.5×10(-5) cm/min, 5-fold higher with respect to mono-functionalized liposomes) or in vivo in healthy mice. Taken together, our results suggest that mApoE-PA-LIP are valuable nanodevices with a potential applicability in vivo for the treatment of AD. From the clinical editor: Bi-functionalized liposomes with phosphatidic acid and a modified ApoE-derived peptide were demonstrated to influence Aß aggregation/disaggregation as a potential treatment in an Alzheimer's model. The liposomes were able to cross the blood-brain barrier in vitro and in vivo. Similar liposomes may become clinically valuable nanodevices with a potential applicability for the treatment of Alzheimer's disease.

Distinctive cell-free DNA methylation characterizes presymptomatic genetic frontotemporal dementia.

OBJECTIVE: Methylation of plasma cell-free DNA (cfDNA) has potential as a marker of brain damage in neurodegenerative diseases such as frontotemporal dementia (FTD). Here, we study methylation of cfDNA in presymptomatic and symptomatic carriers of genetic FTD pathogenic variants, next to healthy controls. METHODS: cfDNA was isolated from cross-sectional plasma of 10 presymptomatic carriers (4 C9orf72, 4 GRN, and 2 MAPT), 10 symptomatic carriers (4 C9orf72, 4 GRN, and 2 MAPT), and 9 healthy controls. Genome-wide methylation of cfDNA was determined using a high-resolution sequencing technique (MeD-seq). Cumulative scores based on the identified differentially methylated regions (DMRs) were estimated for presymptomatic carriers (vs. controls and symptomatic carriers), and reevaluated in a validation cohort (8 presymptomatic: 3 C9orf72, 3 GRN, and 2 MAPT; 26 symptomatic: 7 C9orf72, 6 GRN, 12 MAPT, and 1 TARDBP; 13 noncarriers from genetic FTD families). RESULTS: Presymptomatic carriers showed a distinctive methylation profile compared to healthy controls and symptomatic carriers. Cumulative DMR scores in presymptomatic carriers enabled to significantly differentiate presymptomatic carriers from healthy controls (p < 0.001) and symptomatic carriers (p < 0.001). In the validation cohort, these scores differentiated presymptomatic carriers from symptomatic carriers (p ≤ 0.007) only. Transcription-start-site methylation in presymptomatic carriers, generally associated with gene downregulation, was enriched for genes involved in ubiquitin-dependent processes, while gene body methylation, generally associated with gene upregulation, was enriched for genes involved in neuronal cell processes. INTERPRETATION: A distinctive methylation profile of cfDNA characterizes the presymptomatic stage of genetic FTD, and could reflect neuronal death in this stage.

The unfolded protein response is associated with early tau pathology in the hippocampus of tauopathies.

The unfolded protein response (UPR) is a stress response activated upon disturbed homeostasis in the endoplasmic reticulum (ER). Previously, we reported that the activation of the UPR closely correlates with the presence of phosphorylated tau (p-tau) in Alzheimer's disease (AD). As well as increased presence of intracellular p-tau, AD brains are characterized by extracellular deposits of ß amyloid (Aß). Recent in vitro studies have shown that Aß can induce ER stress and activation of the UPR. The aim of the present study is to investigate UPR activation in sporadic tauopathies like progressive supranuclear palsy (PSP) and Pick's disease (PiD), and familial cases with frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) which carry mutations in the gene encoding for tau (MAPT). The presence of phosphorylated pancreatic ER kinase (pPERK) and phosphorylated inositol requiring enzyme 1α (pIRE1), which are indicative of an activated UPR, was assessed by immunohistochemistry in cases neuropathologically defined as frontotemporal lobar degeneration with tau pathology (FTLD-tau). Increased presence of UPR activation markers pPERK and pIRE1 was observed in neurons and glia in FTLD-tau cases, in contrast to FTLD subtypes negative for tau pathology or in non-neurological controls. pPERK and pIRE1 were also prominently present in relatively young carriers of MAPT mutation. A strong association between the presence of UPR activation markers and p-tau was observed in the hippocampus of FTLD-tau cases. Double immunohistochemical staining on FTLD-tau cases revealed that UPR activation is predominantly observed in neurons that show diffuse staining of p-tau. These data demonstrate that UPR activation is intimately connected with the accumulation and aggregation of p-tau, and occurs independently from Aß deposits. Our findings provide new pathological insight into the close association between p-tau and UPR activation in tauopathies.

Intraneuronal tau aggregation induces the integrated stress response in astrocytes.

Progressive aggregation of tau protein in neurons is associated with neurodegeneration in tauopathies. Cell non-autonomous disease mechanisms in astrocytes may be important drivers of the disease process but remain largely elusive. Here, we studied cell type-specific responses to intraneuronal tau aggregation prior to neurodegeneration. To this end, we developed a fully human co-culture model of seed-independent intraneuronal tau pathology, which shows no neuron and synapse loss. Using high-content microscopy, we show that intraneuronal tau aggregation induces oxidative stress accompanied by activation of the integrated stress response specifically in astrocytes. This requires the direct co-culture with neurons and is not related to neurodegeneration or extracellular tau levels. Tau-directed antisense therapy reduced intraneuronal tau levels and aggregation and prevented the cell non-autonomous responses in astrocytes. These data identify the astrocytic integrated stress response as a novel disease mechanism activated by intraneuronal tau aggregation. In addition, our data provide the first evidence for the efficacy of tau-directed antisense therapy to target cell autonomous and cell non-autonomous disease pathways in a fully human model of tau pathology.

A Human Neuron/Astrocyte Co-culture to Model Seeded and Spontaneous Intraneuronal Tau Aggregation.

Communication and contact between neurons and astrocytes is important for proper brain physiology. How neuron/astrocyte crosstalk is affected by intraneuronal tau aggregation in neurodegenerative tauopathies is largely elusive. Human induced pluripotent stem cell (iPSC)-derived neurons provide the opportunity to model tau pathology in a translationally relevant in vitro context. However, current iPSC models inefficiently develop tau aggregates, and co-culture models of tau pathology have thus far utilized rodent astrocytes. In this article, we describe the co-culture of human iPSC-derived neurons with primary human astrocytes in a 96-well format compatible with high-content microscopy. By lentiviral overexpression of different mutated tau variants, this protocol can be flexibly adapted for the efficient induction of seeded or spontaneous tau aggregation. We used this novel co-culture model to identify cell type-specific disease mechanisms and to provide proof of concept for intervention by antisense therapy. These results show that this human co-culture model provides a highly translational tool for target discovery and drug development for human tauopathies. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Human neuron/astrocyte co-culture for seeded and spontaneous intraneuronal tau aggregation Support Protocol 1: Human induced pluripotent stem cell culture Support Protocol 2: Human primary astrocyte culture.

Activation of the unfolded protein response is an early event in Alzheimer's and Parkinson's disease.

BACKGROUND: Alzheimer's disease (AD) and Parkinson's disease (PD) are characterized by the accumulation and aggregation of misfolded proteins. Disturbed homeostasis in the endoplasmic reticulum leads to accumulation of misfolded proteins, which triggers a stress response called the unfolded protein response (UPR) that protects the cell against the toxic buildup of misfolded proteins. OBJECTIVE: In this paper, we will briefly review the early involvement of the UPR in the pathology of AD and PD. METHODS: Expression of UPR activation markers was analyzed in human brain tissue using immunohistochemistry and Western blot analysis. RESULTS: Neuropathological studies demonstrate that UPR activation markers are increased in neurons in AD and PD. In AD, UPR activation markers are observed in neurons with diffuse staining of phosphorylated tau protein. In PD, increased immunoreactivity for UPR activation markers is detected in neuromelanin containing dopaminergic neurons of the substantia nigra, which colocalize with diffuse α-synuclein staining. CONCLUSION: UPR activation is closely associated with the first stages of accumulation and aggregation of the toxic proteins involved in AD and PD. Studies of postmortem brain tissue indicate that UPR activation is an early event in neurodegeneration.

Granulovacuolar degeneration bodies are independently induced by tau and α-synuclein pathology.

BACKGROUND: Granulovacuolar degeneration bodies (GVBs) are intracellular vesicular structures that commonly accompany pathological tau accumulations in neurons of patients with tauopathies. Recently, we developed the first model for GVBs in primary neurons, that requires exogenous tau seeds to elicit tau aggregation. This model allowed the identification of GVBs as proteolytically active lysosomes induced by tau pathology. GVBs selectively accumulate cargo in a dense core, that shows differential and inconsistent immunopositivity for (phosphorylated) tau epitopes. Despite the strong evidence connecting GVBs to tau pathology, these structures have been reported in neurons without apparent pathology in brain tissue of tauopathy patients. Additionally, GVBs and putative GVBs have also been reported in the brain of patients with non-tau proteinopathies. Here, we investigated the connection between pathological protein assemblies and GVBs in more detail. METHODS: This study combined newly developed primary neuron models for tau and α-synuclein pathology with observations in human brain tissue from tauopathy and Parkinson's disease patients. Immunolabeling and imaging techniques were employed for extensive characterisation of pathological proteins and GVBs. Quantitative data were obtained by high-content automated microscopy as well as single-cell analysis of confocal images. RESULTS: Employing a novel seed-independent neuronal tau/GVB model, we show that in the context of tauopathy, GVBs are inseparably associated with the presence of cytosolic pathological tau and that intracellular tau aggregation precedes GVB formation, strengthening the causal relationship between pathological accumulation of tau and GVBs. We also report that GVBs are inseparably associated with pathological tau at the single-cell level in the hippocampus of tauopathy patients. Paradoxically, we demonstrate the presence of GVBs in the substantia nigra of Parkinson's disease patients and in a primary neuron model for α-synuclein pathology. GVBs in this newly developed α-synuclein/GVB model are induced in the absence of cytosolic pathological tau accumulations. GVBs in the context of tau or α-synuclein pathology showed similar immunoreactivity for different phosphorylated tau epitopes. The phosphorylated tau immunoreactivity signature of GVBs is therefore independent of the presence of cytosolic tau pathology. CONCLUSION: Our data identify the emergence of GVBs as a more generalised response to cytosolic protein pathology.

Aß42 oligomer-specific antibody ALZ-201 reduces the neurotoxicity of Alzheimer's disease brain extracts.

BACKGROUND: In Alzheimer's disease (AD), amyloid-ß 1-42 (Aß42) neurotoxicity stems mostly from its soluble oligomeric aggregates. Studies of such aggregates have been hampered by the lack of oligomer-specific research tools and their intrinsic instability and heterogeneity. Here, we developed a monoclonal antibody with a unique oligomer-specific binding profile (ALZ-201) using oligomer-stabilising technology. Subsequently, we assessed the etiological relevance of the Aß targeted by ALZ-201 on physiologically derived, toxic Aß using extracts from post-mortem brains of AD patients and controls in primary mouse neuron cultures. METHODS: Mice were immunised with stable oligomers derived from the Aß42 peptide with A21C/A30C mutations (AßCC), and ALZ-201 was developed using hybridoma technology. Specificity for the oligomeric form of the Aß42CC antigen and Aß42 was confirmed using ELISA, and non-reactivity against plaques by immunohistochemistry (IHC). The antibody's potential for cross-protective activity against pathological Aß was evaluated in brain tissue samples from 10 individuals confirmed as AD (n=7) and non-AD (n=3) with IHC staining for Aß and phosphorylated tau (p-Tau) aggregates. Brain extracts were prepared and immunodepleted using the positive control 4G8 antibody, ALZ-201 or an isotype control to ALZ-201. Fractions were biochemically characterised, and toxicity assays were performed in primary mouse neuronal cultures using automated high-content microscopy. RESULTS: AD brain extracts proved to be more toxic than controls as demonstrated by neuronal loss and morphological determinants (e.g. synapse density and measures of neurite complexity). Immunodepletion using 4G8 reduced Aß levels in both AD and control samples compared to ALZ-201 or the isotype control, which showed no significant difference. Importantly, despite the differential effect on the total Aß content, the neuroprotective effects of 4G8 and ALZ-201 immunodepletion were similar, whereas the isotype control showed no effect. CONCLUSIONS: ALZ-201 depletes a toxic species in post-mortem AD brain extracts causing a positive physiological and protective impact on the integrity and morphology of mouse neurons. Its unique specificity indicates that a low-abundant, soluble Aß42 oligomer may account for much of the neurotoxicity in AD. This critical attribute identifies the potential of ALZ-201 as a novel drug candidate for achieving a true, clinical therapeutic effect in AD.

Unfolded protein response activation in C9orf72 frontotemporal dementia is associated with dipeptide pathology and granulovacuolar degeneration in granule cells.

A repeat expansion in the C9orf72 gene is the most prevalent genetic cause of frontotemporal dementia (C9-FTD). Several studies have indicated the involvement of the unfolded protein response (UPR) in C9-FTD. In human neuropathology, UPR markers are strongly associated with granulovacuolar degeneration (GVD). In this study, we aim to assess the presence of UPR markers together with the presence of dipeptide pathology and GVD in post mortem brain tissue from C9-FTD cases and neurologically healthy controls. Using immunohistochemistry we assessed the presence of phosphorylated PERK, IRE1α and eIF2α in the frontal cortex, hippocampus and cerebellum of C9-FTD (n = 18) and control (n = 9) cases. The presence of UPR activation markers was compared with the occurrence of pTDP-43, p62 and dipeptide repeat (DPR) proteins (poly(GA), -(GR) & -(GP)) as well as casein kinase 1 delta (CK1δ), a marker for GVD. Increased presence of UPR markers was observed in the hippocampus and cerebellum in C9-FTD compared to control cases. In the hippocampus, overall levels of pPERK and peIF2α were higher in C9-FTD, including in granule cells of the dentate gyrus (DG). UPR markers were also observed in granule cells of the cerebellum in C9-FTD. In addition, increased levels of CK1δ were observed in granule cells in the DG of the hippocampus and granular layer of the cerebellum in C9-FTD. Double-labelling experiments indicate a strong association between UPR markers and the presence of dipeptide pathology as well as GVD. We conclude that UPR markers are increased in C9-FTD and that their presence is associated with dipeptide pathology and GVD. Increased presence of UPR markers and CK1δ in granule cells in the cerebellum and hippocampus could be a unique feature of C9-FTD.