TFEB enhances astroglial uptake of extracellular tau species and reduces tau spreading

Cell-to-Cell Transmission of Tau and α-Synuclein

The stereotypical spread of pathological protein inclusions and clinicopathological heterogeneity are well described in neurodegenerative diseases. Accumulating evidence suggests that the former can be attributed to consecutive cell-to-cell transmission of pathological proteins between anatomically connected brain regions, while the latter has been hypothesized to result from the spread of conformationally distinct pathological protein aggregates, or strains. These emerging concepts have dramatically changed our understanding of neurodegenerative diseases. In this review, we first summarize the background and recent findings underpinning these concepts with a focus on two major pathological proteins: tau and α-synuclein. We then discuss their clinical implications for tauopathies and synucleinopathies and propose a working hypothesis for future research.

Propagation of Tau Pathology: Integrating Insights From Postmortem and In Vivo Studies

Cellular accumulation of aggregated forms of the protein tau is a defining feature of so-called tauopathies such as Alzheimer's disease, progressive supranuclear palsy, and chronic traumatic encephalopathy. A growing body of literature suggests that conformational characteristics of tau filaments, along with regional vulnerability to tau pathology, account for the distinct histopathological morphologies, biochemical composition, and affected cell types seen across these disorders. In this review, we describe and discuss recent evidence from human postmortem and clinical biomarker studies addressing the differential vulnerability of brain areas to tau pathology, its cell-to-cell transmission, and characteristics of the different strains that tau aggregates can adopt. Cellular biosensor assays are increasingly used in human tissue to detect the earliest forms of tau pathology, before overt histopathological lesions (i.e., neurofibrillary tangles) are apparent. Animal models with localized tau expression are used to uncover the mechanisms that influence spreading of tau aggregates. Further, studies of human postmortem-derived tau filaments from different tauopathies injected in rodents have led to striking findings that recapitulate neuropathology-based staging of tau. Furthermore, the recent advent of tau positron emission tomography and novel fluid-based biomarkers render it possible to study the temporal progression of tau pathology in vivo. Ultimately, evidence from these approaches must be integrated to better understand the onset and progression of tau pathology across tauopathies. This will lead to improved methods for the detection and monitoring of disease progression and, hopefully, to the development and refinement of tau-based therapeutics.

Optical induction of autophagy via Transcription factor EB (TFEB) reduces pathological tau in neurons

Pathological accumulation of microtubule associated protein tau in neurons is a major neuropathological hallmark of Alzheimer's disease (AD) and related tauopathies. Several attempts have been made to promote clearance of pathological tau (p-Tau) from neurons. Transcription factor EB (TFEB) has shown to clear p-Tau from neurons via autophagy. However, sustained TFEB activation and autophagy can create burden on cellular bioenergetics and can be deleterious. Here, we modified previously described two-plasmid systems of Light Activated Protein (LAP) from bacterial transcription factor-EL222 and Light Responsive Element (LRE) to encode TFEB. Upon blue-light (465 nm) illumination, the conformation changes in LAP induced LRE-driven expression of TFEB, its nuclear entry, TFEB-mediated expression of autophagy-lysosomal genes and clearance of p-Tau from neuronal cells and AD patient-derived human iPSC-neurons. Turning the blue-light off reversed the expression of TFEB-target genes and attenuated p-Tau clearance. Together, these results suggest that optically regulated TFEB expression unlocks the potential of opto-therapeutics to treat AD and other dementias.

Protein transmission in neurodegenerative disease

Most neurodegenerative diseases are characterized by the intracellular or extracellular aggregation of misfolded proteins such as amyloid-β and tau in Alzheimer disease, α-synuclein in Parkinson disease, and TAR DNA-binding protein 43 in amyotrophic lateral sclerosis. Accumulating evidence from both human studies and disease models indicates that intercellular transmission and the subsequent templated amplification of these misfolded proteins are involved in the onset and progression of various neurodegenerative diseases. The misfolded proteins that are transferred between cells are referred to as 'pathological seeds'. Recent studies have made exciting progress in identifying the characteristics of different pathological seeds, particularly those isolated from diseased brains. Advances have also been made in our understanding of the molecular mechanisms that regulate the transmission process, and the influence of the host cell on the conformation and properties of pathological seeds. The aim of this Review is to summarize our current knowledge of the cell-to-cell transmission of pathological proteins and to identify key questions for future investigation.

A small molecule transcription factor EB activator ameliorates beta-amyloid precursor protein and Tau pathology in Alzheimer's disease models

Accumulating studies have suggested that targeting transcription factor EB (TFEB), an essential regulator of autophagy-lysosomal pathway (ALP), is promising for the treatment of neurodegenerative disorders, including Alzheimer's disease (AD). However, potent and specific small molecule TFEB activators are not available at present. Previously, we identified a novel TFEB activator named curcumin analog C1 which directly binds to and activates TFEB. In this study, we systematically investigated the efficacy of curcumin analog C1 in three AD animal models that represent beta-amyloid precursor protein (APP) pathology (5xFAD mice), tauopathy (P301S mice) and the APP/Tau combined pathology (3xTg-AD mice). We found that C1 efficiently activated TFEB, enhanced autophagy and lysosomal activity, and reduced APP, APP C-terminal fragments (CTF-β/α), β-amyloid peptides and Tau aggregates in these models accompanied by improved synaptic and cognitive function. Knockdown of TFEB and inhibition of lysosomal activity significantly inhibited the effects of C1 on APP and Tau degradation in vitro. In summary, curcumin analog C1 is a potent TFEB activator with promise for the prevention or treatment of AD.

From the prion-like propagation hypothesis to therapeutic strategies of anti-tau immunotherapy

The term “propagon” is used to define proteins that may transmit misfolding in vitro, in tissues or in organisms. Among propagons, misfolded tau is thought to be involved in the pathogenic mechanisms of various “tauopathies” that include Alzheimer's disease, progressive supranuclear palsy, and argyrophilic grain disease. Here, we review the available data in the literature and point out how the prion-like tau propagation has been extended from Alzheimer's disease to tauopathies. First, in Alzheimer’s disease, the progression of tau aggregation follows stereotypical anatomical stages which may be considered as spreading. The mechanisms of the propagation are now subject to intensive and controversial research. It has been shown that tau may be secreted in the interstitial fluid in an active manner as reflected by high and constant concentration of extracellular tau during Alzheimer’s pathology. Animal and cell models have been devised to mimic tau seeding and propagation, and despite their limitations, they have further supported to the prion-like propagation hypothesis. Finally, such new ways of thinking have led to different therapeutic strategies in anti-tau immunotherapy among tauopathies and have stimulated new clinical trials. However, it appears that the prion-like propagation hypothesis mainly relies on data obtained in Alzheimer’s disease. From this review, it appears that further studies are needed (1) to characterize extracellular tau species, (2) to find the right pathological tau species to target, (3) to follow in vivo tau pathology by brain imaging and biomarkers and (4) to interpret current clinical trial results aimed at reducing the progression of these pathologies. Such inputs will be essential to have a comprehensive view of these promising therapeutic strategies in tauopathies.

A single-cell atlas of entorhinal cortex from individuals with Alzheimer's disease reveals cell-type-specific gene expression regulation

There is currently little information available about how individual cell types contribute to Alzheimer's disease. Here we applied single-nucleus RNA sequencing to entorhinal cortex samples from control and Alzheimer's disease brains (n = 6 per group), yielding a total of 13,214 high-quality nuclei. We detail cell-type-specific gene expression patterns, unveiling how transcriptional changes in specific cell subpopulations are associated with Alzheimer's disease. We report that the Alzheimer's disease risk gene APOE is specifically repressed in Alzheimer's disease oligodendrocyte progenitor cells and astrocyte subpopulations and upregulated in an Alzheimer's disease-specific microglial subopulation. Integrating transcription factor regulatory modules with Alzheimer's disease risk loci revealed drivers of cell-type-specific state transitions towards Alzheimer's disease. For example, transcription factor EB, a master regulator of lysosomal function, regulates multiple disease genes in a specific Alzheimer's disease astrocyte subpopulation. These results provide insights into the coordinated control of Alzheimer's disease risk genes and their cell-type-specific contribution to disease susceptibility. These results are available at http://adsn.ddnetbio.com.

Neurons Expressing Pathological Tau Protein Trigger Dramatic Changes in Microglial Morphology and Dynamics

Microglial cells, the resident macrophages of the brain, are important players in the pathological process of numerous neurodegenerative disorders, including tauopathies, a heterogeneous class of diseases characterized by intraneuronal Tau aggregates. However, microglia response in Tau pathologies remains poorly understood. Here, we exploit a genetic zebrafish model of tauopathy, combined with live microglia imaging, to investigate the behavior of microglia in vivo in the disease context. Results show that while microglia were almost immobile and displayed long and highly dynamic branches in a wild-type context, in presence of diseased neurons, cells became highly mobile and displayed morphological changes, with highly mobile cell bodies together with fewer and shorter processes. We also imaged, for the first time to our knowledge, the phagocytosis of apoptotic tauopathic neurons by microglia in vivo and observed that microglia engulfed about as twice materials as in controls. Finally, genetic ablation of microglia in zebrafish tauopathy model significantly increased Tau hyperphosphorylation, suggesting that microglia provide neuroprotection to diseased neurons. Our findings demonstrate for the first time the dynamics of microglia in contact with tauopathic neurons in vivo and open perspectives for the real-time study of microglia in many neuronal diseases.

Disease-modifying strategies in primary tauopathies

Tauopathies are neurodegenerative brain diseases that are characterized by the formation of intraneuronal inclusions containing the microtubule-associated protein tau. This major hallmark defines tau pathology which is predominant in primary tauopathies, while in secondary forms additional driving forces are involved. In the course of the disease, different brain areas degenerate and lead to severe defects of language, behavior and movement. Although neuropathologically heterogeneous, primary tauopathies share a common feature, which is the generation of abnormal tau species that aggregate and progress into filamentous deposits in neurons. Mechanisms that are involved in this disease-related process offer a broad range of targets for disease-modifying therapeutics. The present review provides an up-to-date overview of currently known targets in primary tauopathies and their possible therapeutic modulation. It is structured into four major targets, the post-translational modifications of tau and tau aggregation, protein homeostasis, disease propagation, and tau genetics. Chances, as well as obstacles in the development of effective therapies are highlighted. Some therapeutic strategies, e.g., passive or active immunization, have already reached clinical development, raising hopes for affected patients. Other concepts, e.g., distinct modulators of proteostasis, are at the ready to be developed into promising future therapies. This article is part of the special issue entitled 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

Transcription factor EB overexpression prevents neurodegeneration in experimental synucleinopathies

The synucleinopathies Parkinson’s disease (PD) and Multiple system atrophy (MSA) — characterized by α-synuclein intracytoplasmic inclusions into, respectively, neurons and oligodendrocytes — are associated with impairment of the autophagy-lysosomal pathways (ALP). Increased expression of the master regulator of ALP, transcription factor EB (TFEB), is hypothesized to promote the clearance of WT α-synuclein and survival of dopaminergic neurons. Here, we explore the efficacy of targeted TFEB overexpression either in neurons or oligodendrocytes to reduce the pathological burden of α-synuclein in a PD rat model and a MSA mouse model. While TFEB neuronal expression was sufficient to prevent neurodegeneration in the PD model, we show that only TFEB oligodendroglial overexpression leads to neuroprotective effects in the MSA model. These beneficial effects were associated with a decreased accumulation of α-synuclein into oligodendrocytes through recovery of the ALP machinery. Our study demonstrates that the cell type where α-synuclein aggregates dictates the target of TFEB overexpression in order to be protective, paving the way for adapted therapies.

Targeted overexpression of transcription factor EB (TFEB) decreases accumulation of α-synuclein and prevents neurodegeneration in animal models of synucleinopathies.

Intersection of pathological tau and microglia at the synapse

Tauopathies are a heterogenous class of diseases characterized by cellular accumulation of aggregated tau and include diseases such as Alzheimer’s disease (AD), progressive supranuclear palsy and chronic traumatic encephalopathy. Tau pathology is strongly linked to neurodegeneration and clinical symptoms in tauopathy patients. Furthermore, synapse loss is an early pathological event in tauopathies and is the strongest correlate of cognitive decline. Tau pathology is additionally associated with chronic neuroinflammatory processes, such as reactive microglia, astrocytes, and increased levels of pro-inflammatory molecules (e.g. complement proteins, cytokines). Recent studies show that as the principal immune cells of the brain, microglia play a particularly important role in the initiation and progression of tau pathology and associated neurodegeneration. Furthermore, AD risk genes such as Triggering receptor expressed on myeloid cells 2 (TREM2) and Apolipoprotein E (APOE) are enriched in the innate immune system and modulate the neuroinflammatory response of microglia to tau pathology. Microglia can play an active role in synaptic dysfunction by abnormally phagocytosing synaptic compartments of neurons with tau pathology. Furthermore, microglia are involved in synaptic spreading of tau – a process which is thought to underlie the progressive nature of tau pathology propagation through the brain. Spreading of pathological tau is also the predominant target for tau-based immunotherapy. Active tau vaccines, therapeutic tau antibodies and other approaches targeting the immune system are actively explored as treatment options for AD and other tauopathies. This review describes the role of microglia in the pathobiology of tauopathies and the mechanism of action of potential therapeutics targeting the immune system in tauopathies.

Tau Secretion: Good and Bad for Neurons

In Alzheimer’s disease (AD), neurofibrillary tangles (NFTs), lesions composed of hyperphosphorylated and aggregated tau, spread from the transentorhinal cortex to the hippocampal formation and neocortex. Growing evidence indicates that tau pathology propagates trans-synaptically, implying that pathological tau released by pre-synaptic neurons is taken up by post-synaptic neurons where it accumulates and aggregates. Observations such as the presence of tau in the cerebrospinal fluid (CSF) from control individuals and in the CSF of transgenic mice overexpressing human tau before the detection of neuronal death indicate that tau can be secreted by neurons. The increase of tau in the CSF in pathological conditions such as AD suggests that tau secretion is enhanced and/or other secretory pathways take place when neuronal function is compromised. In physiological conditions, extracellular tau could exert beneficial effects as observed for other cytosolic proteins also released in the extracellular space. In such a case, blocking tau secretion could have negative effects on neurons unless the mechanism of tau secretion are different in physiological and pathological conditions allowing the prevention of pathological tau secretion without affecting the secretion of physiological tau. Furthermore, distinct extracellular tau species could be secreted in physiological and pathological conditions, species having the capacity to induce tau pathology being only secreted in the latter condition. In the present review, we will focus on the mechanisms and function of tau secretion in both physiological and pathological conditions and how this information can help to elaborate an efficient therapeutic strategy to prevent tau pathology and its propagation.

Four-repeat tauopathies

Tau is a microtubule-associated protein with versatile functions in the dynamic assembly of the neuronal cytoskeleton. Four-repeat (4R-) tauopathies are a group of neurodegenerative diseases defined by cytoplasmic inclusions predominantly composed of tau protein isoforms with four microtubule-binding domains. Progressive supranuclear palsy, corticobasal degeneration, argyrophilic grain disease or glial globular tauopathy belong to the group of 4R-tauopathies. The present review provides an introduction in the current concept of 4R-tauopathies, including an overview of the neuropathological and clinical spectrum of these diseases. It describes the genetic and environmental etiological factors, as well as the contemporary knowledge about the pathophysiological mechanisms, including post-translational modifications, aggregation and fragmentation of tau, as well as the role of protein degradation mechanisms. Furthermore, current theories about disease propagation are discussed, involving different extracellular tau species and their cellular release and uptake mechanisms. Finally, molecular diagnostic tools for 4R-tauopathies, including tau-PET and fluid biomarkers, and investigational therapeutic strategies are presented. In summary, we report on 4R-tauopathies as overarching disease concept based on a shared pathophysiological concept, and highlight the challenges and opportunities on the way towards a causal therapy.

Experimental evidence for the age dependence of tau protein spread in the brain

The incidence of Alzheimer's disease (AD), which is characterized by progressive cognitive decline that correlates with the spread of tau protein aggregation in the cortical mantle, is strongly age-related. It could be that age predisposes the brain for tau misfolding and supports the propagation of tau pathology. We tested this hypothesis using an experimental setup that allowed for exploration of age-related factors of tau spread and regional vulnerability. We virally expressed human tau locally in entorhinal cortex (EC) neurons of young or old mice and monitored the cell-to-cell tau protein spread by immunolabeling. Old animals showed more tau spreading in the hippocampus and adjacent cortical areas and accumulated more misfolded tau in EC neurons. No misfolding, at any age, was observed in the striatum, a brain region mostly unaffected by tangles. Age and brain region dependent tau spreading and misfolding likely contribute to the profound age-related risk for sporadic AD.

Extracellular Monomeric Tau Is Internalized by Astrocytes

Tau is a microtubule-associated protein that is expressed in neurons. However, in a group of neurodegenerative diseases named tauopathies – characterized by an increase in aggregated and/or hyperphosphorylated Tau – the protein accumulates inside other cells, such as astrocytes and microglia. Given that these glial cells do not produce Tau, its presence can be explained by internalization from the extracellular medium and consequent formation of Tau aggregates. Among internalization mechanisms, heparan sulfate proteoglycans (HSPGs) have been proposed to be responsible for fibrillary Tau uptake in various cell types. Here we studied whether monomeric Tau, which has been observed to be internalized by glial cells such as microglia, was also taken up by astrocytes. Although this Tau form was internalized from the extracellular medium by these cells, the mechanism of uptake was found to be independent of HSPGs.

Dynasore Suppresses mTORC1 Activity and Induces Autophagy to Regulate the Clearance of Protein Aggregates in Neurodegenerative Diseases

Autophagy is an important cellular protein control process, which plays a key role in the regulation of cell homeostasis and pathogenesis of many human diseases including neurodegenerative diseases. Reduced autophagic activity and abnormal protein aggregation are common features of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Therefore, pharmacological regulation of overall autophagy may be helpful for effective treatment of neurodegenerative diseases. In the present study, we find Dynasore, a potent inhibitor of dynamin, can repress the lysosomal localization of mTOR and block the activity of mTORC1, which in turn enhances the nuclear translocation of the master regulators of autophagy including TFE3 and TFEB. We find that autophagic flux is upregulated in Dynasore-treated cells. Moreover, treatment of Dynasore significantly promotes the clearance of protein aggregates formed by mutant huntingtin protein containing expanded polyglutamine (polyQ), but not damaged mitochondria. In contrast, treatment with Dynasore has no effect on the clearance of polyQ aggregates of mutant huntingtin in ATG5-depleted cells, in which autophagy is defective. Taken together, our results indicate that Dynasore affects autophagic degradation of neurodegenerative disease-associated proteins by regulating mTORC1-TFEB signaling.

Tau Prion-Like Propagation: State of the Art and Current Challenges

It has been almost a decade since the hypothesis of active tau protein propagation in Alzheimer's disease and associated tauopathies was formally raised. We view tau propagation as a cascade of events, starting with early tau misfolding, followed by transfer to another, anatomically connected, cell, contaminating in corruption of endogenous tau in the recipient cell through a seeding mechanism of templated misfolding. These mechanisms are very similar to those of other proteinopathies and to ideas about how prion pathologies spread through the brain. Nonetheless, the specific mechanisms underlying each of these steps remains uncertain and is a fertile ground for new experimental approaches potentially requiring new experimental models. We review, here, the state of the art of the research on tau prion-like propagation and we highlight some key challenges to understanding the detailed mechanisms of cell to cell propagation.