Global Conformation of Tau Protein Mapped by Raman Spectroscopy

Photo-Excited Dyes: Emerging Technique Against Tau Protein Aggregation

Tau aggregates are considered a pathological hallmark of Alzheimer's disease. The screening of molecules against Tau aggregation is a novel strategy for Alzheimer's disease. The photo-excited molecules have proven to be effective as a therapeutic agent in several diseases. In recent studies, the photo-excited dyes showed an inhibitory effect on Alzheimer's disease-related Tau protein aggregation and toxicity. The present chapter deals with the effect of rose bengal on the aggregation of Tau. The in vitro studies carried out with the help of electron microscopy, ThS fluorescence, and circular dichroism suggested that RB attenuated the Tau aggregation under in vitro conditions, whereas PE-RB disaggregated the mature Tau fibrils. Photo-excited rose bengal and the classical rose bengal induced a low degree of toxicity in cells. Thus, for the treatment of Alzheimer's disease, the rose bengal could be considered a potential molecule.

The extracellular HDAC6 ZnF UBP domain modulates the actin network and post-translational modifications of Tau

BACKGROUND

Microtubule-associated protein Tau undergoes aggregation in Alzheimer`s disease (AD) and a group of other related diseases collectively known as Tauopathies. In AD, Tau forms aggregates, which are deposited intracellularly as neurofibrillary tangles. Histone deacetylase-6 (HDAC6) plays an important role in aggresome formation, where it recruits polyubiquitinated aggregates to the motor protein dynein.

METHODS

Here, we have studied the effects of HDAC6 ZnF UBP on Tau phosphorylation, ApoE localization, GSK-3β regulation and cytoskeletal organization in neuronal cells by immunocytochemical analysis. This analysis reveals that the cell exposure to the UBP-type zinc finger domain of HDAC6 (HDAC6 ZnF UBP) can modulate Tau phosphorylation and actin cytoskeleton organization.

RESULTS

HDAC6 ZnF UBP treatment to cells did not affect their viability and resulted in enhanced neurite extension and formation of structures similar to podosomes, lamellipodia and podonuts suggesting the role of this domain in actin re-organization. Also, HDAC6 ZnF UBP treatment caused increase in nuclear localization of ApoE and tubulin localization in microtubule organizing centre (MTOC). Therefore, our studies suggest the regulatory role of this domain in different aspects of neurodegenerative diseases. Upon HDAC6 ZnF UBP treatment, inactive phosphorylated form of GSK-3β increases without any change in total GSK-3β level.

CONCLUSIONS

HDAC6 ZnF UBP was found to be involved in cytoskeletal re-organization by modulating actin dynamics and tubulin localization. Overall, our study suggests that ZnF domain of HDAC6 performs various regulatory functions apart from its classical function in aggresome formation in protein misfolding diseases. Video abstract.

Microglial remodeling of actin network by Tau oligomers, via G protein-coupled purinergic receptor, P2Y12R-driven chemotaxis

Alzheimer's disease (AD) is associated with age-related neurodegeneration, synaptic deformation and chronic inflammation mediated by microglia and infiltrated macrophages in the brain. Tau oligomers can be released from damaged neurons via various mechanisms such as exosomes, neurotransmitter, membrane leakage etc. Microglia sense the extracellular Tau through several cell-surface receptors and mediate chemotaxis and phagocytosis. The purinergic receptor P2Y12R recently gained interest in neurodegeneration for neuro-glial communication and microglial chemotaxis towards the site of plaque deposition. To understand the effect of extracellular Tau oligomers in microglial migration, the P2Y12R-mediated actin remodeling, reorientation of tubulin network and rate of migration were studied in the presence of ATP. The extracellular Tau species directly interacted with P2Y12R and also induced this purinoceptor expression in microglia. Microglial P2Y12R colocalized with remodeled membrane-associated actin network as a component of migration in response to Tau oligomers. As an inducer of P2Y12R, ATP facilitated the localization of P2Y12R in lamellipodia and filopodia during accelerated microglial migration. The direct interaction of extracellular Tau oligomers with microglial P2Y12R would facilitate the signal transduction in both way, directional chemotaxis and receptor-mediated phagocytosis. These unprecedented findings emphasize that microglia can modulate the membrane-associated actin structure and incorporate P2Y12R to perceive the axis and rate of chemotaxis in Tauopathy.

Baicalein inhibits heparin-induced Tau aggregation by initializing non-toxic Tau oligomer formation

Background

Amyloid aggregate deposition is the key feature of Alzheimer’s disease. The proteinaceous aggregates found in the afflicted brain are the intra-neuronal neurofibrillary tangles formed by the microtubule-associated protein Tau and extracellular deposits, senile plaques, of amyloid beta (Aβ) peptide proteolytically derived from the amyloid precursor protein. Accumulation of these aggregates has manifestations in the later stages of the disease, such as memory loss and cognitive inabilities originating from the neuronal dysfunction, neurodegeneration, and brain atrophy. Treatment of this disease at the late stages is difficult, and many clinical trials have failed. Hence, the goal is to find means capable of preventing the aggregation of these intrinsically disordered proteins by inhibiting the early stages of their pathological transformations. Polyphenols are known to be neuroprotective agents with the noticeable potential against many neurodegenerative diseases, such as Alzheimer’s, Parkinson’s, and Prion diseases.

Methods

We analyzed the capability of Baicalein to inhibit aggregation of human Tau protein by a multifactorial analysis that included several biophysical and biochemical techniques.

Results

The potency of Baicalein, a polyphenol from the Scutellaria baicalensis Georgi, against in vitro Tau aggregation and PHF dissolution has been screened and validated. ThS fluorescence assay revealed the potent inhibitory activity of Baicalein, whereas ANS revealed its mechanism of Tau inhibition viz. by oligomer capture and dissociation. In addition, Baicalein dissolved the preformed mature fibrils of Tau thereby possessing a dual target action. Tau oligomers formed by Baicalein were non-toxic to neuronal cells, highlighting its role as a potent molecule to be screened against AD.

Conclusion

In conclusion, Baicalein inhibits aggregation of hTau40 by enhancing the formation of SDS-stable oligomers and preventing fibril formation. Baicalein-induced oligomers do not affect the viability of the neuroblastoma cells. Therefore, Baicalein can be considered as a lead molecule against Tau pathology in AD.

Melatonin Reduces GSK3β-Mediated Tau Phosphorylation, Enhances Nrf2 Nuclear Translocation and Anti-Inflammation

Alzheimer’s disease is a neuropathological condition with abnormal accumulation of extracellular Amyloid-β plaques and intracellular neurofibrillary tangles of Microtubule-associated protein Tau (Tau) in the brain. In pathological conditions, Tau undergoes post-translational modifications such as hyperphosphorylation by the activity of cellular kinases, which eventually leads to protein aggregation in neurons. Melatonin is a neuro-hormone that is mainly secreted from the pineal gland and functions to modulate the cellular kinases. In our study, we have checked the neuroprotective function of Melatonin by MTT and LDH assay, where Melatonin inhibited the Tau aggregates-mediated cytotoxicity and membrane leakage in Neuro2A cells. The potency of Melatonin has also been studied for the quenching of intracellular reactive oxygen species level by DCFDA assay and caspase 3 activity. Melatonin was shown to reduce the GSK3β mRNA and subsequent protein level as well as the phospho-Tau level (pThr181 and pThr212-pSer214) in okadaic acid-induced Neuro2A cells, as observed by western blot and immunofluorescence assay. Further, Melatonin has increased the cellular Nrf2 level and its nuclear translocation as an oxidative stress response in Tauopathy. The Melatonin was found to induce pro- and anti-inflammatory cytokines levels in N9 microglia. The mRNA level of cellular kinases such as as-GSK3β, MAPK were also studied by qRT-PCR assay in Tau-exposed N9 and Neuro2A cells. The immunomodulatory effect of Melatonin was evident as it induced IL-10 and TGF-β cytokine levels and activated MAP3K level in Tau-exposed microglia and neurons, respectively. Melatonin also downregulated the mRNA level of pro-inflammatory markers, IL-1β and Cyclooxygenase-2 in N9 microglia. Together, these findings suggest that Melatonin remediated the cytokine profile of Tau-exposed microglia, reduced Tau hyperphosphorylation by downregulating GSK3β level, and alleviated oxidative stress via Nrf2 nuclear translocation.

Role of cysteines in accelerating Tau filament formation

Alzheimer's disease is majorly associated with intracellular accumulation of Tau into paired helical filaments and tangles. The self-aggregated dimeric and oligomeric species of Tau formed are more toxic to neuronal cells and acts as seeds for filament formation. The two cysteine residues and the two hexapeptide regions of full-length Tau play a key role in initialization and filament formation during Tau aggregation. The role of cysteine residues in Tau aggregation has been studied by in-vitro aggregation assay that was measured by Thioflavin S fluorescence to observe the kinetics of aggregation. In this study, we have performed in-vitro aggregation assay with recombinant full-length Tau and the cysteine mutants to understand the mechanism of cysteine independent Tau aggregation. Here, we report that cysteine mutant full-length Tau can aggregate to form filaments under in-vitro conditions. To visualize the polymorphisms of Tau and cysteine mutants under different aggregation conditions anionic cofactor, heparin was employed. Wild-type Tau showed rapid aggregation to form oligomers and filaments. On the other hand, the cysteine mutant delayed the initial Tau aggregation. This indicates the importance of cysteine residues in accelerating initial Tau nucleation for its aggregation. The filament morphology of wild-type and cysteine mutant Tau has been characterized using transmission electron microscopy and high-resolution transmission electron microscopy.Communicated by Ramaswamy H. Sarma.

HDAC6 ZnF UBP as the Modifier of Tau Structure and Function

Histone deacetylase 6 is a class II histone deacetylase primarily present in the cytoplasm and involved in the regulation of various cellular functions. It consists of two catalytic deacetylase domains and a unique zinc finger ubiquitin binding protein domain, which sets it apart from other HDACs. HDAC6 is known to regulate cellular activities by modifying the function of microtubules, HSP90, and cortactin through deacetylation. Apart from the catalytic activity of HDAC6, it interacts with other proteins through either the SE14 domain or the ZnF UBP domain to modulate their functions. Here, we have studied the role of the HDAC6 ZnF UBP domain as a modifier of Tau aggregation by its direct interaction with the polyproline region/repeat region of Tau. Interaction of HDAC6 ZnF UBP with Tau was found to reduce the propensity of Tau to self-aggregate and to disaggregate preformed aggregates in a concentration-dependent manner and also bring about the conformational changes in Tau protein. The interaction of HDAC6 ZnF UBP with Tau results in its degradation, suggesting either proteolytic activity of HDAC6 ZnF UBP or its role in enhancing autoproteolysis of Tau.

Mammalian cell and tissue imaging using Raman and coherent Raman microscopy

Direct imaging of metabolism in cells or multicellular organisms is important for understanding many biological processes. Raman scattering (RS) microscopy, particularly, coherent Raman scattering (CRS) such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS), has emerged as a powerful platform for cellular imaging due to its high chemical selectivity, sensitivity, and imaging speed. RS microscopy has been extensively used for the identification of subcellular structures, metabolic observation, and phenotypic characterization. Conjugating RS modalities with other techniques such as fluorescence or infrared (IR) spectroscopy, flow cytometry, and RNA-sequencing can further extend the applications of RS imaging in microbiology, system biology, neurology, tumor biology and more. Here we overview RS modalities and techniques for mammalian cell and tissue imaging, with a focus on the advances and applications of CARS and SRS microscopy, for a better understanding of the metabolism and dynamics of lipids, protein, glucose, and nucleic acids in mammalian cells and tissues. This article is categorized under: Laboratory Methods and Technologies > Imaging Biological Mechanisms > Metabolism Analytical and Computational Methods > Analytical Methods.

P301 L, an FTDP-17 Mutant, Exhibits Enhanced Glycation in vitro

BACKGROUND

Frontotemporal dementia and parkinsonism-linked to chromosome-17 are a group of diseases with tau mutations leading to primary tauopathies which include progressive supranuclear palsy, corticobasal syndrome, and frontotemporal lobar degeneration. Alzheimer's disease is a non-primary tauopathy, which displays tau neuropathology of excess tangle formation and accumulation. FTDP-17 mutations are responsible for early onset of AD, which can be attributed to compromised physiological functions due to the mutations. Tau is a microtubule-binding protein that secures the integrity of polymerized microtubules in neuronal cells. It malfunctions owing to various insults and stress conditions-like mutations and post-translational modifications.

OBJECTIVE

In this study, we modified the wild type and tau mutants by methyl glyoxal and thus studied whether glycation can enhance the aggregation of predisposed mutant tau.

METHODS

Tau glycation was studied by fluorescence assays, SDS-PAGE analysis, conformational evaluation, and transmission electron microscopy.

RESULTS

Our study suggests that FTDP-17 mutant P301 L leads to enhanced glycation-induced aggregation as well as advanced glycation end products formation. Glycation forms amorphous aggregates of tau and its mutants without altering its native conformation.

CONCLUSION

The metabolic anomalies and genetic predisposition have found to accelerate tau-mediated neurodegeneration and prove detrimental for the early-onset of Alzheimer's disease.

Basic Limonoid modulates Chaperone-mediated Proteostasis and dissolve Tau fibrils

The Alzheimer's disease pathology is associated with accumulation of intracellular neurofibrillary tangles and extracellular senile plaques. The formation of initial nucleus triggers conformational changes in Tau and leads to its deposition. Hence, there is a need to eliminate these toxic proteins for proper functioning of neuronal cells. In this aspect, we screened the effect of basic limonoids such as gedunin, epoxyazadiradione, azadirone and azadiradione on inhibiting Tau aggregation as well as disintegration of induced Tau aggregates. It was observed that these basic limonoids effectively prevented aggregates formation by Tau and also exhibited the property of destabilizing matured Tau aggregates. The molecular docking analysis suggests that the basic limonoids interact with hexapeptide regions of aggregated Tau. Although these limonoids caused the conformational changes in Tau to β-sheet structure, the cytological studies indicate that basic limonoids rescued cell death. The dual role of limonoids in Tau aggregation inhibition and disintegration of matured aggregates suggests them to be potent molecules in overcoming Tau pathology. Further, their origin from a medicinally important plant neem, which known to possess remarkable biological activities was also found to play protective role in HEK293T cells. Basic limonoids were non-toxic to HEK293T cells and also aided in activation of HSF1 by inducing its accumulation in nucleus. Western blotting and immunofluorescence studies showed that HSF1 in downstream increased the transcription of Hsp70 thus, aggravating cytosolic Hsp70 levels that can channel clearance of aberrant Tau. All these results mark basic limonoids as potential therapeutic natural products.

Protein-Capped Metal Nanoparticles Inhibit Tau Aggregation in Alzheimer's Disease

The Alzheimer's disease (AD) therapeutic research is yielding a large number of potent molecules. The nanoparticle-based therapeutics against the protein aggregation in AD is also taking a lead especially with amyloid-β as a primary target. In this work, we have screened for the first time protein-capped (PC) metal nanoparticles for their potency in inhibiting Tau aggregation in vitro. We present a novel function of PC-Fe3O4 and PC-CdS nanoparticles as potent Tau aggregation inhibitors by fluorescence spectrometry, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and electron microscopy. We demonstrate that the biologically synthesized PC-metal nanoparticles, especially iron oxide do not affect the viability of neuroblastoma cells. Moreover, PC-CdS nanoparticles show dual properties of inhibition and disaggregation of Tau. Thus, the nanoparticles can take a lead as potent Tau aggregation inhibitors and can be modified for specific drug delivery due to their very small size. The current work presents unprecedented strategy to design anti-Tau aggregation drugs, which provides interesting insights to understand the role of biological nanostructures in Alzheimer's disease.

Interaction of silica nanoparticles with tau proteins and PC12 cells: Colloidal stability, thermodynamic, docking, and cellular studies

Study on the side effects of the nanoparticles (NPs) can provide useful information regarding their biological and medical applications. Herein, the colloidal stability of the silicon dioxide NPs (SiO₂ NPs) in the absence and presence of tau was investigated by TEM and DLS techniques. Afterwards, the thermodynamic parameters of interaction between SiO₂ NPs and tau were determined by fluorescence spectroscopy and docking studies. Finally, the cytotoxic effects of SiO₂ NPs on the viability of PC12 cells were investigated by MTT, AO/EB staining and flow cytometry assays. TEM, DLS, and zeta potential investigations revealed that tau can reduce the colloidal stability of SiO₂ NPs. Fluorescence spectroscopy study indicated that SiO₂ NPs bound to the tau with high affinity through hydrogen bonds and van der Waals interactions. Docking study also determined that Ser, Thr and Tyr residues provide a polar microenvironment for SiO₂ NPs/tau interaction. Cellular studies demonstrated that SiO₂ NPs can induce cell mortality through both apoptosis and necrosis mechanisms. Therefore, it may be concluded that the biological systems such as nervous system proteins can affect the colloidal stability of NPs and vice versa NPs in the biological systems can bind to proteins and cell membranes non-specifically and may induce toxicity.

Retention of Anticancer Activity of Curcumin after Conjugation with Fluorescent Gold Quantum Clusters: An in Vitro and in Vivo Xenograft Study

Gold nanoparticles (Au NPs) have been thoroughly investigated for anti-cancer therapy. However, their undesired high gold content remains a problem when injected into the body for drug delivery applications. In this report, we made an effort to conjugate the curcumin molecules on the surface of gold quantum clusters (Au QCs) by a novel in situ synthesis method which provides an alternative route to not only reduce the metallic content but also increase the water solubility of curcumin and the loading efficiency. Here, curcumin itself acts as a reducing and capping agent for the synthesis of Au QCs. The UV-vis absorption, fluorescence, transmission electron microscopy, and electrospray ionization mass spectrometry results confirmed the synthesis of fluorescent Au QCs. Curcumin-conjugated Au NPs (C-Au NPs) and glutathione (GSH)-conjugated Au QCs (GSH-Au QCs) were also synthesized to visualize the effect of particle size and the capping agent, respectively, on the cytotoxicity to normal and cancer cells. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay showed that the curcumin-conjugated Au QCs (C-Au QCs) were less cytotoxic to normal cells while almost the same cytotoxic to cancer cells in comparison to curcumin itself, which indicates that curcumin preserves its anticancer property even after binding to the Au QCs. However, C-Au NPs and GSH-Au QCs did not show any cytotoxicity against the normal and cancer cells at the concentration used. The western blot assay indicated that C-Au QCs promote apoptosis in cancer cells. Further, the in vivo study on severe combined immunodeficiency mice showed that C-Au QCs also inhibited the tumor growth efficiently without showing significant toxicity to internal organs.