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The two presenilin‐1 (PS1) and presenilin‐2 (PS2) homologs are the catalytic core of the γ‐secretase complex, which has a major role in cell fate decision and Alzheimer's disease (AD) progression. Understanding the precise contribution of PS1‐ and PS2‐dependent γ‐secretases to the production of β‐amyloid peptide (Aβ) from amyloid precursor protein (APP) remains an important challenge to design molecules efficiently modulating Aβ release without affecting the processing of other γ‐secretase substrates. To that end, we studied PS1‐ and PS2‐dependent substrate processing in murine cells lacking presenilins (PSs) (PS1KO, PS2KO or PS1‐PS2 double‐KO noted PSdKO) or stably re‐expressing human PS1 or PS2 in an endogenous PS‐null (PSdKO) background. We characterized the processing of APP and Notch on both endogenous and exogenous substrates, and we investigated the effect of pharmacological inhibitors targeting the PSs activity (DAPT and L‐685,458). We found that murine PS1 γ‐secretase plays a predominant role in APP and Notch processing when compared to murine PS2 γ‐secretase. The inhibitors blocked more efficiently murine PS2‐ than murine PS1‐dependent processing. Human PSs, especially human PS1, expression in a PS‐null background efficiently restored APP and Notch processing. Strikingly, and contrary to the results obtained on murine PSs, pharmacological inhibitors appear to preferentially target human PS1‐ than human PS2‐dependent γ‐secretase activity.

A key hallmark of Alzheimer’s disease is the extracellular deposition of amyloid plaques composed primarily of the amyloidogenic amyloid-β (Aβ) peptide. The Aβ peptide is a product of sequential cleavage of the Amyloid Precursor Protein, the first step of which gives rise to a C-terminal Fragment (C99). Cleavage of C99 by γ-secretase activity releases Aβ of several lengths and the Aβ42 isoform in particular has been identified as being neurotoxic. The misfolding of Aβ leads to subsequent amyloid fibril formation by nucleated polymerisation. This requires an initial and critical nucleus for self-assembly. Here, we identify and characterise the composition and self-assembly properties of cell-derived hexameric Aβ42 and show its assembly enhancing properties which are dependent on the Aβ monomer availability. Identification of nucleating assemblies that contribute to self-assembly in this way may serve as therapeutic targets to prevent the formation of toxic oligomers.

The function of the amyloid precursor protein (APP) is not fully understood, but its cleavage product amyloid beta (Aβ) together with neurofibrillary tangles constitute the hallmarks of Alzheimer’s disease (AD). Yet, imbalance of excitatory and inhibitory neurotransmission accompanied by loss of synaptic functions, has been reported much earlier and independent of any detectable pathological markers. Recently, soluble APP fragments have been shown to bind to presynaptic GABA B receptors (GABA B Rs), subsequently decreasing the probability of neurotransmitter release. In this body of work, we were able to show that overexpression of wild-type human APP in mice (hAPP wt ) causes early cognitive impairment, neuronal loss, and electrophysiological abnormalities in the absence of amyloid plaques and at very low levels of Aβ. hAPP wt mice exhibited neuronal overexcitation that was evident in EEG and increased long-term potentiation (LTP). Overexpression of hAPP wt did not alter GABAergic/glutamatergic receptor components or GABA production ability. Nonetheless, we detected a decrease of GABA but not glutamate that could be linked to soluble APP fragments, acting on presynaptic GABA B Rs and subsequently reducing GABA release. By using a specific presynaptic GABA B R antagonist, we were able to rescue hyperexcitation in hAPP wt animals. Our results provide evidence that APP plays a crucial role in regulating inhibitory neurotransmission.

Systemic inflammatory state found in obesity increases the risk of developing numerous diseases. While endurance training seems effective to reduce this inflammation, the underlying mechanisms are not fully understood. Among those, extracellular vesicles (EVs) have been proposed to be actors in the anti‐inflammatory intercellular crosstalk induced by exercise training. This study aimed to investigate how endurance training modulates the EV proteome in the context of an inflammatory state in adults with obesity. Thirteen lean sedentary adults and 10 sedentary adults with obesity participated in a 12‐week endurance training programme. Skeletal muscle, abdominal subcutaneous adipose tissue and venous blood samples were taken prior to and after the training period. The systemic and adipose tissue inflammatory states were assessed, and plasma EVs were isolated by size exclusion chromatography. EV content was analysed by mass spectrometry. EVs isolated from the medium of myotubes stimulated by electrical pulse stimulation in vitro were quantified, and their content was analysed by western blot. After the endurance training, C‐reactive protein (CRP) levels decreased in participants with obesity. In abdominal subcutaneous adipose tissue, the phosphorylation state of nuclear factor‐kappa B (NF‐κB) was not affected by training, but interleukin (IL)‐6 and IL‐1β protein levels were reduced after the 12 weeks in both groups. Conversely, interferon gamma (IFNγ) level reduction was exclusively found in the obesity group. Despite no changes in EV abundance, EV proteome was modified by training. Among the modified proteins in participants with obesity, the antioxidant enzyme peroxiredoxin (PRDX) 1 abundance was increased after training. Additionally, the PRDX1 content of EVs isolated from stimulated myotubes was increased compared to control conditions. In conclusion, our results suggest that the anti‐inflammatory effects of exercise training are not directly mediated by EV anti‐inflammatory proteome changes. However, exercise training increases circulating EV antioxidant content, possibly through contractile activity of skeletal muscle during repeated exercise.

Perturbation of lipid metabolism favours progression of Alzheimer disease, in which processing of Amyloid Precursor Protein (APP) has important implications. APP cleavage is tightly regulated by cholesterol and APP fragments regulate lipid homeostasis. Here, we investigated whether up or down regulation of full‐length APP expression affected neuronal lipid metabolism. Expression of APP decreased HMG‐CoA reductase (HMGCR)‐mediated cholesterol biosynthesis and SREBP mRNA levels, while its down regulation had opposite effects. APP and SREBP1 co‐immunoprecipitated and co‐localized in the Golgi. This interaction prevented Site‐2 protease‐mediated processing of SREBP1, leading to inhibition of transcription of its target genes. A GXXXG motif in APP sequence was critical for regulation of HMGCR expression. In astrocytes, APP and SREBP1 did not interact nor did APP affect cholesterol biosynthesis. Neuronal expression of APP decreased both HMGCR and cholesterol 24‐hydroxylase mRNA levels and consequently cholesterol turnover, leading to inhibition of neuronal activity, which was rescued by geranylgeraniol, generated in the mevalonate pathway, in both APP expressing and mevastatin treated neurons. We conclude that APP controls cholesterol turnover needed for neuronal activity. Graphical Abstract GWAS identified components of the cholesterol metabolism machinery as important risk factors for Alzheimer's disease. APP is shown here to control neuronal cholesterol turnover, which highlight therapeutic alternatives for AD treatment.

Tau protein aggregates in several neurodegenerative disorders, referred to as tauopathies. The tau isoforms observed in post mortem human brain aggregates is used to classify tauopathies. However, distinguishing tauopathies ante mortem remains challenging, potentially due to differences between insoluble tau in aggregates and soluble tau in body fluids. Here, we demonstrated that tau isoforms differ between tauopathies in insoluble aggregates, but not in soluble brain extracts. We therefore characterized post-translational modifications of both the aggregated and the soluble tau protein obtained from post mortem human brain tissue of patients with Alzheimer’s disease, cortico-basal degeneration, Pick’s disease, and frontotemporal lobe degeneration. We found specific soluble signatures for each tauopathy and its specific aggregated tau isoforms: including ubiquitination on Lysine 369 for cortico-basal degeneration and acetylation on Lysine 311 for Pick’s disease. These findings provide potential targets for future development of fluid-based biomarker assays able to distinguish tauopathies in vivo. Post-translational modifications on tau protein in the brain could distinguish primary tauopathies. Here, the authors assess insoluble and soluble tau extracted from post   mortem human tauopathy brains and show 4R/3R tau isoform ratio in aggregates is associated with specific modifications on soluble tau protein.

Genetic, clinical, histopathological and biomarker data strongly support Beta-amyloid (Aβ) induced spreading of Tau-pathology beyond entorhinal cortex (EC), as a crucial process in conversion from preclinical cognitively normal to Alzheimer‘s Disease (AD), while the underlying mechanism remains unclear. In vivo preclinical models have reproducibly recapitulated Aβ-induced Tau-pathology. Tau pathology was thereby also induced by aggregated Aβ, in functionally connected brain areas, reminiscent of a prion-like seeding process. In this work we demonstrate, that pre-aggregated Aβ can directly induce Tau fibrillization by cross-seeding, in a cell-free assay, comparable to that demonstrated before for alpha-synuclein and Tau. We furthermore demonstrate, in a well-characterized cellular Tau-aggregation assay that Aβ-seeds cross-seeded Tau-pathology and strongly catalyzed pre-existing Tau-aggregation, reminiscent of the pathogenetic process in AD. Finally, we demonstrate that heterotypic seeded Tau by pre-aggregated Aβ provides efficient seeds for induction and propagation of Tau-pathology in vivo. Prion-like, heterotypic seeding of Tau fibrillization by Aβ, providing potent seeds for propagating Tau pathology in vivo, as demonstrated here, provides a compelling molecular mechanism for Aβ-induced propagation of Tau-pathology, beyond regions with pre-existing Tau-pathology (entorhinal cortex/locus coeruleus). Cross-seeding along functional connections could thereby resolve the initial spatial dissociation between amyloid- and Tau-pathology, and preferential propagation of Tau-pathology in regions with pre-existing ‘silent’ Tau-pathology, by conversion of a ‘silent’ Tau pathology to a ‘spreading’ Tau-pathology, observed in AD.

INTRODUCTION Olfactory decline often precedes cognitive symptoms in Alzheimer's disease (AD) and is linked to tau pathology. Yet, whether tau aggregates start in peripheral olfactory structures or spread from central regions remains debated. METHODS We analyzed 34 post mortem human olfactory bulb (OB) and 2 neuroepithelium (ONE) samples across Braak stages, combined with ex vivo nasal swabs and standardized olfactory testing in 88 clinically characterized participants. RESULTS Tau aggregates appeared in the anterior olfactory nucleus layer of the OB from Braak stage III, spreading to peripheral layers only in late stages. Olfactory identification declined in the preclinical phase and was linked to central tau pathology. Discrimination worsened during the prodromal stage, while threshold impairment appeared only in dementia, reflecting the anatomical progression of tau pathology. DISCUSSION This anatomical–functional link supports a central‐to‐peripheral spread of tau pathology in the olfactory system, with stage‐specific deficits suggesting targeted smell tests as early AD biomarkers. Highlights Tau aggregates appear in the internal anterior olfactory nucleus layer of the human olfactory bulb (OB) at Braak stage III. The olfactory nerve layer of the OB and the olfactory neuroepithelium become involved only at advanced Braak stages. Olfactory subdomain impairments mirror the central‐to‐peripheral progression of tau pathology. Specific odor identification is impaired as early as the preclinical stage.

Prion-like seeding and propagation of Tau-pathology have been demonstrated experimentally and may underlie the stereotyped progression of neurodegenerative Tauopathies. However, the involvement of templated misfolding of Tau in neuronal network dysfunction and behavioral outcomes remains to be explored in detail. Here we analyzed the repercussions of prion-like spreading of Tau-pathology via neuronal connections on neuronal network function in TauP301S transgenic mice. Spontaneous and GABA A R-antagonist-induced neuronal network activity were affected following templated Tau-misfolding using synthetic preformed Tau fibrils in cultured primary neurons. Electrophysiological analysis in organotypic hippocampal slices of Tau transgenic mice demonstrated impaired synaptic transmission and impaired long-term potentiation following Tau-seed induced Tau-aggregation. Intracerebral injection of Tau-seeds in TauP301S mice, caused prion-like spreading of Tau-pathology through functionally connected neuroanatomical pathways. Electrophysiological analysis revealed impaired synaptic plasticity in hippocampal CA1 region 6 months after Tau-seeding in entorhinal cortex (EC). Furthermore, templated Tau aggregation impaired cognitive function, measured in the object recognition test 6 months post-seeding. In contrast, Tau-seeding in basal ganglia and subsequent spreading through functionally connected neuronal networks involved in motor control, resulted in motoric deficits reflected in clasping and impaired inverted grid hanging, not significantly affected following Tau-seeding in EC. Immunostaining, biochemical and electron microscopic analysis in the different models suggested early pathological forms of Tau, including Tau-oligomers, rather than fully mature neurofibrillary tangles (NFTs) as culprits of neuronal dysfunction. We here demonstrate for the first time using in vitro, ex vivo and in vivo models, that prion-like spreading of Tau-misfolding by Tau seeds, along unique neuronal connections, causes neuronal network dysfunction and associated behavioral dysfunction. Our data highlight the potential relevance of this mechanism in the symptomatic progression in Tauopathies. We furthermore demonstrate that the initial site of Tau-seeding thereby determines the behavioral outcome, potentially underlying the observed heterogeneity in (familial) Tauopathies, including in TauP301 mutants.

Background Tauopathies are defined as a group of heterogeneous neurodegenerative disorders, characterized by abnormal Tau protein accumulation in neurons and glial cells. The most common tauopathy is Alzheimer’s disease (AD), which is responsible for 70% of cases dementia. The development of tauopathies is a multistep process probably driven by changes in the post‐translational modifications (PTMs) of Tau. Unfortunately, for almost all these diseases, no accurate differential diagnostic methods are available which exacerbates the need of identifying biomarkers or treatments for tauopathies. In addition to providing a better understanding of the pathophysiology of tauopathies, the study of PTMs seems well suitable for these aims. According to the literature and previous results from the lab, ubiquitination appears promising to identify biomarkers allowing the differentiation of tauopathies and to treat these pathologies. Therefore, this project aims to evaluate the predictive value of ubiquitination dysregulation in the tauopathies, as a tool for diagnosis and a mechanism of tau aggregation. Method To achieve our goal, we first segregated the soluble and insoluble brain protein fractions from brain of subjects with specific tauopathies (AD, Pick’s disease, frontotemporal lobar degeneration and corticobasal degeneration) and healthy subjects. Tandem mass spectrometry (LC‐MS/MS) was used to identify all the proteins present in the insoluble fractions using DDA (data dependent acquisition), as well as in the soluble fraction enriched in ubiquitinated proteins. The post‐analysis was performed using Proteome Discoverer software and the statistical analyses were done in R. Result This analysis allows the identification of an imbalance in proteins ubiquitination in the insoluble fraction of tauopathies brains. Moreover, the imbalance provides ubiquitin hallmarks associated to specific tauopathies. Finally, we also investigated Tau ubiquitination profile in these fractions. Conclusion Our study suggests that ubiquitination may be dysregulated in tauopathies, in which ubiquitinated tau accumulates. This could arise either from impairment of tau clearance in autophagy/lysosomal compartments, or as a consequence of altered proteasome function, well‐described in AD. The dysregulation of ubiquitination appearing in tauopathies underscores the potentially significant role of this post‐translational modification in the development of these pathologies.