The transition between soluble intrinsically disordered tau protein and aggregated tau in neurofibrillary tangles in Alzheimer's disease is unknown. Here, we propose that soluble tau species can undergo liquid–liquid phase separation (LLPS) under cellular conditions and that phase‐separated tau droplets can serve as an intermediate toward tau aggregate formation. We demonstrate that phosphorylated or mutant aggregation prone recombinant tau undergoes LLPS, as does high molecular weight soluble phospho‐tau isolated from human Alzheimer brain. Droplet‐like tau can also be observed in neurons and other cells. We found that tau droplets become gel‐like in minutes, and over days start to spontaneously form thioflavin‐S‐positive tau aggregates that are competent of seeding cellular tau aggregation. Since analogous LLPS observations have been made for FUS, hnRNPA1, and TDP43, which aggregate in the context of amyotrophic lateral sclerosis, we suggest that LLPS represents a biophysical process with a role in multiple different neurodegenerative diseases.

Amyloid ␤ (A␤)-containing plaques are surrounded by dystrophic neurites in the Alzheimer's disease (AD) brain, but whether and how plaques induce these neuritic abnormalities remain unknown. We tested the hypothesis that soluble oligomeric assemblies of A␤, which surround plaques, induce calcium-mediated secondary cascades that lead to dystrophic changes in local neurites. We show that soluble A␤ oligomers lead to activation of the calcium-dependent phosphatase calcineurin (CaN) (PP2B), which in turn activates the transcriptional factor nuclear factor of activated T cells (NFAT). Activation of these signaling pathways, even in the absence of A␤, is sufficient to produce a virtual phenocopy of A␤-induced dystrophic neurites, dendritic simplification, and dendritic spine loss in both neurons in culture and in the adult mouse brain. Importantly, the morphological deficits in the vicinity of A␤ deposits in a mouse model of AD are ameliorated by CaN inhibition, supporting the hypothesis that CaN-NFAT are aberrantly activated by A␤ and that CaN-NFAT activation is responsible for disruption of neuronal structure near plaques. In accord with this, we also detect increased levels of an active form of CaN and NFATc4 in the nuclear fraction from the cortex of patients with AD. Thus, A␤ appears to mediate the neurodegeneration of AD, at least in part, by activation of CaN and subsequent NFAT-mediated downstream cascades.

Increased intracellular levels of α-synuclein are implicated in Parkinson’s disease and related disorders and may be caused by alterations in the ubiquitin-proteasome system (UPS) or the autophagy-lysosomal pathway (ALP). A critical question remains how α-synuclein is degraded by neurons in vivo. To address this, our study uses α-synuclein transgenic mice, expressing human α-synuclein or α-synuclein-eGFP under the (h)PDGF-β promoter, in combination with in vivo pharmacologic and multiphoton imaging strategies to systematically test degradation pathways in the living mouse brain. We demonstrate that the UPS is the main degradation pathway for α-synuclein under normal conditions in vivo while with increased α-synuclein burden the ALP is recruited. Moreover, we report alterations of the UPS in α-synuclein transgenic mice and age dependence to the role of the UPS in α-synuclein degradation. In addition, we provide evidence that the UPS and ALP might be functionally connected such that impairment of one can upregulate the other. These results provide a novel link between the UPS, the ALP and α-synuclein pathology and may have important implications for future therapeutics targeting degradation pathways.

Mitochondria contribute to shape intraneuronal Ca 2+ signals. Excessive Ca 2+ taken up by mitochondria could lead to cell death. Amyloid beta (Aβ) causes cytosolic Ca 2+ overload, but the effects of Aβ on mitochondrial Ca 2+ levels in Alzheimer's disease (AD) remain unclear. Using a ratiometric Ca 2+ indicator targeted to neuronal mitochondria and intravital multiphoton microscopy, we find increased mitochondrial Ca 2+ levels associated with plaque deposition and neuronal death in a transgenic mouse model of cerebral β-amyloidosis. Naturally secreted soluble Aβ applied onto the healthy brain increases Ca 2+ concentration in mitochondria, which is prevented by blockage of the mitochondrial calcium uniporter. RNAsequencing from post-mortem AD human brains shows downregulation in the expression of mitochondrial influx Ca 2+ transporter genes, but upregulation in the genes related to mitochondrial Ca 2+ efflux pathways, suggesting a counteracting effect to avoid Ca 2+ overload. We propose lowering neuronal mitochondrial Ca 2+ by inhibiting the mitochondrial Ca 2+ uniporter as a novel potential therapeutic target against AD.