The role of extracellular Tau in the spreading of neurofibrillary pathology

DnaJ/Hsc70 chaperone complexes control the extracellular release of neurodegenerative-associated proteins

It is now known that proteins associated with neurodegenerative disease can spread throughout the brain in a prionlike manner. However, the mechanisms regulating the trans-synaptic spread propagation, including the neuronal release of these proteins, remain unknown. The interaction of neurodegenerative disease-associated proteins with the molecular chaperone Hsc70 is well known, and we hypothesized that much like disaggregation, refolding, degradation, and even normal function, Hsc70 may dictate the extracellular fate of these proteins. Here, we show that several proteins, including TDP-43, α-synuclein, and the microtubule-associated protein tau, can be driven out of the cell by an Hsc70 co-chaperone, DnaJC5. In fact, DnaJC5 overexpression induced tau release in cells, neurons, and brain tissue, but only when activity of the chaperone Hsc70 was intact and when tau was able to associate with this chaperone. Moreover, release of tau from neurons was reduced in mice lacking the DnaJC5 gene and when the complement of DnaJs in the cell was altered. These results demonstrate that the dynamics of DnaJ/Hsc70 complexes are critically involved in the release of neurodegenerative disease proteins.

CNS tau efflux via exosomes is likely increased in Parkinson's disease but not in Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) and Parkinson's disease (PD) involve tau pathology. Tau is detectable in blood, but its clearance from neuronal cells and the brain is poorly understood.

METHODS

Tau efflux from the brain to the blood was evaluated by administering radioactively labeled and unlabeled tau intracerebroventricularly in wild-type and tau knock-out mice, respectively. Central nervous system (CNS)-derived tau in L1CAM-containing exosomes was further characterized extensively in human plasma, including by single molecule array technology with 303 subjects.

RESULTS

The efflux of Tau, including a fraction via CNS-derived L1CAM exosomes, was observed in mice. In human plasma, tau was explicitly identified within L1CAM exosomes. In contrast to AD patients, L1CAM exosomal tau was significantly higher in PD patients than controls and correlated with cerebrospinal fluid tau.

CONCLUSIONS

Tau is readily transported from the brain to the blood. The mechanisms of CNS tau efflux are likely different between AD and PD.

New Features about Tau Function and Dysfunction

Tau is a brain microtubule-associated protein that directly binds to a microtubule and dynamically regulates its structure and function. Under pathological conditions, tau self-assembles into filamentous structures that end up forming neurofibrillary tangles. Prominent tau neurofibrillary pathology is a common feature in a number of neurodegenerative disorders, collectively referred to as tauopathies, the most common of which is Alzheimer’s disease (AD). Beyond its classical role as a microtubule-associated protein, recent advances in our understanding of tau cellular functions have revealed novel insights into their important role during pathogenesis and provided potential novel therapeutic targets. Regulation of tau behavior and function under physiological and pathological conditions is mainly achieved through post-translational modifications, including phosphorylation, glycosylation, acetylation, and truncation, among others, indicating the complexity and variability of factors influencing regulation of tau toxicity, all of which have significant implications for the development of novel therapeutic approaches in various neurodegenerative disorders. A more comprehensive understanding of the molecular mechanisms regulating tau function and dysfunction will provide us with a better outline of tau cellular networking and, hopefully, offer new clues for designing more efficient approaches to tackle tauopathies in the near future.

Is toxicant-induced Sertoli cell injury in vitro a useful model to study molecular mechanisms in spermatogenesis?

Sertoli cells isolated from rodents or humans and cultured in vitro are known to establish a functional tight junction (TJ)-permeability barrier that mimics the blood-testis barrier (BTB) in vivo. This model has been widely used by investigators to study the biology of the TJ and the BTB. Studies have shown that environmental toxicants (e.g., perfluorooctanesulfonate (PFOS), bisphenol A (BPA) and cadmium) that exert their disruptive effects to induce Sertoli cell injury using this in vitro model are reproducible in studies in vivo. Thus, this in vitro system provides a convenient approach to probe the molecular mechanism(s) underlying toxicant-induced testis injury but also to provide new insights in understanding spermatogenesis, such as the biology of cell adhesion, BTB restructuring that supports preleptotene spermatocyte transport, and others. Herein, we provide a brief and critical review based on studies using this in vitro model of Sertoli cell cultures using primary cells isolated from rodent testes vs. humans to monitor environmental toxicant-mediated Sertoli cell injury. In short, recent findings have shown that environmental toxicants exert their effects on Sertoli cells to induce testis injury through their action on Sertoli cell actin- and/or microtubule-based cytoskeleton. These effects are mediated via their disruptive effects on actin- and/or microtubule-binding proteins. Sertoli cells also utilize differential spatiotemporal expression of these actin binding proteins to confer plasticity to the BTB to regulate germ cell transport across the BTB.

α-Synuclein and nonhuman primate models of Parkinson's disease

Accumulation of α-synuclein (α-syn) leading to the formation of insoluble intracellular aggregates named Lewy bodies is proposed to have a significant role in Parkinson's disease (PD) pathology. Nonhuman primate (NHP) models of PD have proven essential for understanding the neurobiological basis of the disease and for the preclinical evaluation of first-in-class and invasive therapies. In addition to neurotoxin, aging and intracerebral gene transfer models, a new generation of models using inoculations of α-syn formulations, as well as transgenic methods is emerging. Understanding of their advantages and limitations will be essential when choosing a platform to evaluate α-syn-related pathology and interpreting the test results of new treatments targeting α-syn aggregation. In this review we aim to provide insight on this issue by critically analyzing the differences in endogenous α-syn, as well as α-syn pathology in PD and PD NHP models.

The neuronal extracellular matrix restricts distribution and internalization of aggregated Tau-protein

Alzheimer's disease (AD) is a chronic degenerative disorder characterized by fibrillary aggregates of Aß and Tau-protein. Formation and progression of these pathological hallmarks throughout the brain follow a specific spatio-temporal pattern which provides the basis for neuropathological staging. Previously, we could demonstrate that cortical and subcortical neurons are less frequently affected by neurofibrillary degeneration if they are enwrapped by a specialized form of the hyaluronan-based extracellular matrix (ECM), the so called 'perineuronal net' (PN). PNs are composed of large aggregating chondroitin sulfate proteoglycans connected to a hyaluronan backbone, stabilized by link proteins and cross-linked via tenascin-R. Recently, PN-associated neurons were shown to be better protected against iron-induced neurodegeneration compared to neurons without PN, indicating a neuroprotective function. Here, we investigated the role of PNs in distribution and internalization of exogenous Tau-protein by using organotypic slice cultures of wildtype mice as well as mice lacking the ECM-components aggrecan, HAPLN1 or tenascin-R. We could demonstrate that PNs restrict both distribution and internalization of Tau. Accordingly, PN-ensheathed neurons were less frequently affected by Tau-internalization, than neurons without PN. Finally, the PNs as well as their three investigated components were shown to modulate the processes of distribution as well as internalization of Tau.

Molecular signatures of mesenchymal stem cell-derived extracellular vesicle-mediated tissue repair

Extracellular vesicles (EVs) play important roles in intercellular communications via their content molecules, and mimic, at least in part, the roles that are played by their originating cells. Consistent with this notion, an increasing number of reports have suggested that EVs derived from mesenchymal stem cells (MSCs), which are therapeutically beneficial to a wide range of diseases, can serve as drugs to treat multiple diseases. EVs contain a variety of molecules, including proteins, microRNAs, and mRNAs, and are associated with biological processes in a content molecule-dependent manner. In this article, we review the latest reports regarding the therapeutic potential of MSC-EVs by focusing on the underlying molecular mechanisms of their effects. Specifically, we feature the effects of MSC-EVs in terms of their content molecules and of the tissue recovery processes endowed by these molecules.

Microglial genes regulating neuroinflammation in the progression of Alzheimer's disease

Neuroinflammation is a pathological hallmark of Alzheimer's disease (AD), and microglia, the brain's resident phagocyte, are pivotal for the immune response observed in AD. Microglia act as sentinel and protective cells, but may become inappropriately reactive in AD to drive neuropathology. Recent Genome Wide Association Studies (GWAS) have identified more than 20 gene variants associated with an increased risk of late-onset AD (LOAD), the most prevalent form of AD [1]. The findings strongly implicate genes related to the immune response (CR1, CD33, MS4A, CLU, ABCA7, EPHA1 and HLA-DRB5-HLA-DRB1), endocytosis (BIN1, PICALM, CD2AP, EPHA1 and SORL1) and lipid biology (CLU, ABCA7 and SORL1) [2-8], and many encode proteins which are highly expressed in microglia [1]. Furthermore, recent identification of a low frequency mutation in the gene encoding the triggering receptor expressed in myeloid cells 2 protein (TREM2) confers increased risk of AD in LOAD cohorts with an effect size similar to that for APOE, until recently the only identified genetic risk factor associated with LOAD [9,10(••)] (Figure 1). The present review summarises our current understanding of the probable roles of microglial genes in the regulation of neuroinflammatory processes in AD and their relation to other processes affecting the disease's progression.

Neuronal uptake and propagation of a rare phosphorylated high-molecular-weight tau derived from Alzheimer's disease brain

Tau pathology is known to spread in a hierarchical pattern in Alzheimer's disease (AD) brain during disease progression, likely by trans-synaptic tau transfer between neurons. However, the tau species involved in inter-neuron propagation remains unclear. To identify tau species responsible for propagation, we examined uptake and propagation properties of different tau species derived from postmortem cortical extracts and brain interstitial fluid of tau-transgenic mice, as well as human AD cortices. Here we show that PBS-soluble phosphorylated high-molecular-weight (HMW) tau, though very low in abundance, is taken up, axonally transported, and passed on to synaptically connected neurons. Our findings suggest that a rare species of soluble phosphorylated HMW tau is the endogenous form of tau involved in propagation and could be a target for therapeutic intervention and biomarker development.

In Alzheimer's disease, tau spreads throughout the brain, however the nature of the tau species propagating from one neuron to another is not known. Here, Takeda et al. identify a rare, high-molecular-weight tau as the primary species taken up and transferred between synaptically connected neurons.

Clearance systems in the brain-implications for Alzheimer disease

Accumulation of toxic protein aggregates-amyloid-β (Aβ) plaques and hyperphosphorylated tau tangles-is the pathological hallmark of Alzheimer disease (AD). Aβ accumulation has been hypothesized to result from an imbalance between Aβ production and clearance; indeed, Aβ clearance seems to be impaired in both early and late forms of AD. To develop efficient strategies to slow down or halt AD, it is critical to understand how Aβ is cleared from the brain. Extracellular Aβ deposits can be removed from the brain by various clearance systems, most importantly, transport across the blood-brain barrier. Findings from the past few years suggest that astroglial-mediated interstitial fluid (ISF) bulk flow, known as the glymphatic system, might contribute to a larger portion of extracellular Aβ (eAβ) clearance than previously thought. The meningeal lymphatic vessels, discovered in 2015, might provide another clearance route. Because these clearance systems act together to drive eAβ from the brain, any alteration to their function could contribute to AD. An understanding of Aβ clearance might provide strategies to reduce excess Aβ deposits and delay, or even prevent, disease onset. In this Review, we describe the clearance systems of the brain as they relate to proteins implicated in AD pathology, with the main focus on Aβ.

A network of RNA and protein interactions in Fronto Temporal Dementia

Frontotemporal dementia (FTD) is a neurodegenerative disorder characterized by degeneration of the fronto temporal lobes and abnormal protein inclusions. It exhibits a broad clinicopathological spectrum and has been linked to mutations in seven different genes. We will provide a picture, which connects the products of these genes, albeit diverse in nature and function, in a network. Despite the paucity of information available for some of these genes, we believe that RNA processing and post-transcriptional regulation of gene expression might constitute a common theme in the network. Recent studies have unraveled the role of mutations affecting the functions of RNA binding proteins and regulation of microRNAs. This review will combine all the recent findings on genes involved in the pathogenesis of FTD, highlighting the importance of a common network of interactions in order to study and decipher the heterogeneous clinical manifestations associated with FTD. This approach could be helpful for the research of potential therapeutic strategies.

Escitalopram Ameliorates Forskolin-Induced Tau Hyperphosphorylation in HEK239/tau441 Cells

To investigate the effect of escitalopram (a widely used and highly efficacious antidepressant from the SSRI class) on tau hyperphosphorylation, HEK293/tau441 cells were pretreated with 4 μM of forskolin for 2 h. Then we treated the cells with different doses of escitalopram (0, 5, 10, 20, 40, 80 μM) for 22 h. We measured the phosphorylation level of tau by Western blotting. It was shown that escitalopram could protect tau from hyperphosphorylation induced by pharmacological activation of protein kinase A (PKA) at a dose of 20, 40, and 80 μM in vitro. Interestingly, the same dose of escitalopram could also increase the level of serine-9-phosphorylated GSK-3β (inactive form) and the phosphorylation level of Akt at Ser473 (active form) with no significant change in the level of total GSK-3β and Akt. Unexpectedly, 5-hydroxytryptamine 1A receptor (5-HT1A) agonist 8-OH-DPAT did not decrease forskolin-induced tau hyperphosphorylation. Our results suggest that escitalopram can ameliorate forskolin-induced tau hyperphosphorylation, which is not through the typical 5-HT1A pathway, and Akt/GSK-3β signaling pathway is involved. These findings may support an effective role of antidepressants in the prevention of dementia associated with depression in patients.

Three-dimensional collagen type I matrix up-regulates nuclear isoforms of the microtubule associated protein tau implicated in resistance to paclitaxel therapy in ovarian carcinoma

Epithelial ovarian carcinoma is the deadliest gynecologic malignancy. One reason underlying treatment failure is resistance to paclitaxel. Expression of the microtubule associated protein tau has recently been proposed as a predictor of response to paclitaxel in ovarian carcinoma patients. Expression of tau was probed using immunohistochemistry in 312 specimens of primary, and 40 specimens of metastatic, ovarian carcinoma. Serous epithelial ovarian carcinoma cell line models were used to determine the expression of tau by Western blot and immunofluorescence staining. Subcellular fractionation and Western blot were employed to examine nuclear and cytoplasmic localization of tau. Gene silencing and clonogenic assays were used to evaluate paclitaxel response. Tau was expressed in 44% of all tested cases. Among the primary serous epithelial ovarian carcinoma cases, 46% were tau-positive. Among the metastatic serous epithelial ovarian carcinomas, 63% were tau-positive. Cell culture experiments demonstrated that tau was expressed in multiple isoforms. Three-dimensional collagen I matrix culture conditions resulted in up-regulation of tau protein. Silencing of tau with specific siRNAs in a combination with three-dimensional culture conditions led to a significant decrease of the clonogenic ability of cells treated with paclitaxel. The data suggest that reduction of tau expression may sensitize ovarian carcinoma to the paclitaxel treatment.

Screening strategies to identify HSP70 modulators to treat Alzheimer's disease

Alzheimer’s disease, the most common type of dementia, is a progressive brain disease that destroys cognitive function and eventually leads to death. In patients with Alzheimer’s disease, beta amyloids and tau proteins form plaques/oligomers and oligomers/tangles that affect the ability of neurons to function properly. Heat shock protein 70 (HSP70) has the ability to prevent aggregation/oligomerization of beta amyloid/tau proteins, making it a potential drug target. To determine this potential, it is essential that we have appropriate in vitro and cell-based assays that help identify specific molecules that affect this aggregation or oligomerization through HSP70. Potential drug candidates could be identified through a series of assays, starting with ATPase assays, followed by aggregation assays with enzymes/proteins and cell-based systems. ATPase assays are effective in identification of ATPase modulators but do not determine the effect of the molecule on beta amyloid and tau proteins. Molecules identified through ATPase assays are validated by thioflavin T aggregation assays in the presence of HSP70. These assays help uncover if a molecule affects beta amyloid and tau through HSP70, but are limited by their in vitro nature. Potential drug candidates are further validated through cell-based assays using mammalian, yeast, or bacterial cultures. However, while these assays are able to determine the effect of a specific molecule on beta amyloid and tau, they fail to determine whether the action is HSP70-dependent. The creation of a novel, direct assay that can demonstrate the antiaggregation effect of a molecule as well as its action through HSP70 would reduce the number of false-positive drug candidates and be more cost-effective and time-effective.

Exosomes as a Nanodelivery System: a Key to the Future of Neuromedicine?

Since the beginning of the last decade, exosomes have been of increased interest in the science community. Exosomes represent a new kind of long distance transfer of biological molecules among cells. This review provides a comprehensive overview about the construction of exosomes, their targeting and their fusion mechanisms to the recipient cells. Complementarily, the current state of research regarding the cargo of exosomes is discussed. A particular focus was placed on the role of exosomes in the central nervous system. An increasing number of physiological processes in the brain could be associated with exosomes. In this context, it is becoming more apparent that exosomes are involved in several neurological and specifically neurodegenerative diseases. The treatment of these kinds of diseases is often difficult not least because of the blood-brain barrier. Exosomes are very stable, can pass the blood-brain barrier and, therefore, reveal bright perspectives towards diagnosis and therapeutic treatments. A prerequisite for clinical applications is a standardised approach. Features necessary for a standardised diagnosis using exosomes are discussed. In therapeutic terms, exosomes represent a promising drug delivery system able to pass the blood-brain barrier. One option to overcome the disadvantages potentially associated with the use of endogenous exosomes is the design of artificial exosomes. The artificial exosomes with a clearly defined therapeutic active cargo and surface marker ensuring the specific targeting to the recipient cells is proposed as a promising approach.

Do CSF levels of t-Tau, p-Tau and β₁₋₄₂ amyloid correlate with dopaminergic system impairment in patients with a clinical diagnosis of Parkinson disease? A ¹²³I-FP-CIT study in the early stages of the disease

PURPOSE

To investigate the relationships among cerebrospinal fluid (CSF) levels of t-Tau, p-Tau and Aβ₁₋₄₂ amyloid peptide and (123)I-FP-CIT uptake.

METHODS

The study included 58 subjects (31 men and 27 women, age 67 ± 9 years) with a clinical diagnosis of Parkinson disease diagnosed according to the United Kingdom Parkinson Disease Society Brain Bank criteria. All subjects underwent a CSF assay 28 ± 3 days before (123)I-FP-CIT SPECT scanning. The relationships were evaluated by means of linear regression analysis and Pearson correlation.

RESULTS

Striatal (123)I-FP-CIT was positively related to both t-Tau and p-Tau CSF values with low levels of t-Tau and p-Tau being related to a low uptake of (123)I-FP-CIT. In particular, differences with higher statistical significance were found for the striatum that is contralateral to theside mainly affected on clinical examination (P<0.001) [corrected].No significant relationships were found between Aβ₁₋₄₂ amyloid peptide and (123)I-FP-CIT binding.

CONCLUSION

The results of our study suggest that the presynaptic dopaminergic system is more involved in Parkinson disease patients with lower t-Tau and p-Tau CSF values while values of Aβ₁₋₄₂ amyloid peptide seems not to be related to nigrostriatal degeneration in our series.