Methylene Blue Inhibits Caspases by Oxidation of the Catalytic Cysteine

Methylene blue and its potential in the treatment of traumatic brain injury, brain ischemia, and Alzheimer's disease

Traumatic brain injury (TBI) and brain ischemia/reperfusion cause neurodegenerative processes that can continue after the acute stage with the development of severe brain atrophy with dementia. In this case, the long-term neurodegeneration of the brain is similar to the neurodegeneration characteristic of Alzheimer's disease (AD) and is associated with the accumulation of beta amyloid and tau protein. In the pathogenesis of AD as well as in the pathogenesis of cerebral ischemia and TBI oxidative stress, progressive inflammation, glial activation, blood-brain barrier dysfunction, and excessive activation of autophagy are involved, which implies the presence of many targets that can be affected by neuroprotectors. That is, multivariate cascades of nerve tissue damage represent many potential targets for therapeutic interventions. One of such substances that can be used in multi-purpose therapeutic strategies is methylene blue (MB). This drug can have an antiapoptotic and anti-inflammatory effect, activate autophagy, inhibit the aggregation of proteins with an irregular shape, inhibit NO synthase, and bypass impaired electron transfer in the respiratory chain of mitochondria. MB is a well-described treatment for methemoglobinemia, malaria, and encephalopathy caused by ifosfamide. In recent years, this drug has attracted great interest as a potential treatment for a number of neurodegenerative disorders, including the effects of TBI, ischemia, and AD.

The study of the protective effect of mitochondrial uncouplers during acute toxicity of the fungicide difenoconazole in different organs of mice

Pesticides represent a serious problem for agricultural workers due to their neurotoxic effects. The aim of this study was to evaluate the ability of pharmacological oxidative phosphorylation uncouplers to reduce the effect of the difenoconazole fungicide on mitochondrial DNA (mtDNA) of various organs in mice. Injections of difenoconazole caused cognitive deficits in mice, and the protonophore 2,4-dinitrophenol (2,4-DNP) and Azur I (AzI), a demethylated metabolite of methylene blue (MB), prevented the deterioration of cognitive abilities in mice induced by difenoconazole. Difenoconazole increased the rate of reactive oxygen species (ROS) production, likely through inhibition of complex I of the mitochondrial respiratory chain. After intraperitoneal administration of difenoconazole lungs, testes and midbrain were most sensitive to the accumulation of mtDNA damage. In contrast, the cerebral cortex and hippocampus were not tolerant to the effects of difenoconazole. The protonophore 2,4-DNP reduced the rate of ROS formation and significantly reduced the amount of mtDNA damage caused by difenoconazole in the midbrain, and partially, in the lungs and testes. MB, an alternative electron carrier capable of bypassing inhibited complex I, had no effect on the effect of difenoconazole on mtDNA, while its metabolite AzI, a demethylated metabolite of MB, was able to protect the mtDNA of the midbrain and testes. Thus, mitochondria-targeted therapy is a promising approach to reduce pesticide toxicity for agricultural workers.

Synergic Thermo- and pH-Sensitive Hybrid Microgels Loaded with Fluorescent Dyes and Ultrasmall Gold Nanoparticles for Photoacoustic Imaging and Photothermal Therapy

Smart microgels (μGels) made of polymeric particles doped with inorganic nanoparticles have emerged recently as promising multifunctional materials for nanomedicine applications. However, the synthesis of these hybrid materials is still a challenging task with the necessity to control several features, such as particle sizes and doping levels, in order to tailor their final properties in relation to the targeted application. We report herein an innovative modular strategy to achieve the rational design of well-defined and densely filled hybrid particles. It is based on the assembly of the different building blocks, i.e., μGels, dyes, and small gold nanoparticles (<4 nm), and the tuning of nanoparticle loading within the polymer matrix through successive incubation steps. The characterization of the final hybrid networks using UV-vis absorption, fluorescence, transmission electron microscopy, dynamic light scattering, and small-angle X-ray scattering revealed that they uniquely combine the properties of hydrogel particles, including high loading capacity and stimuli-responsive behavior, the photoluminescent properties of dyes (rhodamine 6G, methylene blue and cyanine 7.5), and the features of gold nanoparticle assembly. Interestingly, in response to pH and temperature stimuli, the smart hybrid μGels can shrink, leading to the aggregation of the gold nanoparticles trapped inside the polymer matrix. This stimuli-responsive behavior results in plasmon band broadening and red shift toward the near-infrared region (NIR), opening promising prospects in biomedical science. Particularly, the potential of these smart hybrid nanoplatforms for photoactivated hyperthermia, photoacoustic imaging, cellular internalization, intracellular imaging, and photothermal therapy was assessed, demonstrating well controlled multimodal opportunities for theranostics.

Methylene blue: a controversial diagnostic acid and medication?

A narrative review of the literature was conducted to determine if the administration of methylene blue (MB) in humans has potential risks. Studies were identified from MEDLINE, Web of Science, Scopus, and Cochrane. MB is a diagnostic substance used during some diagnostic procedures and also a part of the treatment of several diseases including methemoglobinemia, vasoplegic syndrome, fosfamide-induced encephalopathy, and cyanide intoxication, and the detection of leaks or position of parathyroid corpuscles during surgery. Although the use of MB is historically justified, and it ought to be safe, because it originated as a diagnostic material, the basic toxicological characteristics of this substance are unknown. Despite reports of severe adverse effects of MB, which could significantly exceed any possible benefits evaluated for the given indication. Therefore, the clinical use of MB currently represents a controversial problem given the heterogeneity of available data and the lack of preclinical data. This is in conflict with standards of safe use of such substances in human medicinal practice. The toxic effects of the application of MB are dose-dependent and include serious symptoms such as hemolysis, methemoglobinemia, nausea and vomitus, chest pain, dyspnoea, and hypertension. Some countries regard MB as harmful because of the resulting skin irritation and triggering of an adverse inflammatory response. MB induced serotoninergic toxicity clinically manifests as neuromuscular hyperactivity. This review aims to summarize the current understanding concerning the indications for MB administration and define the potential adverse effects of MB.

Molecular Mechanisms of the Neuroprotective Effect of Methylene Blue

Methylene blue (MB) is the first fully synthetic compound that had found its way into medicine over 120 years ago as a treatment against malaria. MB has been approved for the treatment of methemoglobinemia, but there are premises for its repurposing as a neuroprotective agent based on the efficacy of this compound demonstrated in the models of Alzheimer's, Parkinson's, and Huntington's diseases, traumatic brain injury, amyotrophic lateral sclerosis, depressive disorders, etc. However, the goal of this review was not so much to focus on the therapeutic effects of MB in the treatment of various neurodegeneration diseases, but to delve into the mechanisms of direct or indirect effect of this drug on the signaling pathways. MB can act as an alternative electron carrier in the mitochondrial respiratory chain in the case of dysfunctional electron transport chain. It also displays the anti-inflammatory and anti-apoptotic effects, inhibits monoamine oxidase (MAO) and nitric oxide synthase (NOS), activates signaling pathways involved in the mitochondrial pool renewal (mitochondrial biogenesis and autophagy), and prevents aggregation of misfolded proteins. Comprehensive understanding of all aspects of direct and indirect influence of MB, and not just some of its effects, can help in further research of this compound, including its clinical applications.

Brain Protection by Methylene Blue and Its Derivative, Azur B, via Activation of the Nrf2/ARE Pathway in Cisplatin-Induced Cognitive Impairment

Cisplatin is a cytotoxic chemotherapeutic drug that leads to DNA damage and is used in the treatment of various types of tumors. However, cisplatin has several serious adverse effects, such as deterioration in cognitive ability. The aim of our work was to study neuroprotectors capable of preventing cisplatin-induced neurotoxicity. Methylene blue (MB) and AzurB (AzB) are able to neutralize the neurotoxicity caused by cisplatin by protecting nerve cells as a result of the activation of the Ntf2 signaling pathway. We have shown that cisplatin impairs learning in the Morris water maze. This is due to an increase in the amount of mtDNA damage, a decrease in the expression of most antioxidant genes, the main determinant of the induction of which is the Nrf2/ARE signaling pathway, and genes involved in mitophagy regulation in the cortex. The expression of genes involved in long-term potentiation was suppressed in the hippocampus of cisplatin-injected mice. MB in most cases prevented cisplatin-induced impairment of learning and decrease of gene expression in the cortex. AzB prevented the cisplatin-induced decrease of genes in the hippocampus. Also, cisplatin induced disbalance in the gut microbiome, decreased levels of Actinotalea and Prevotella, and increased levels of Streptococcus and Veillonella. MB and AzB also prevented cisplatin-induced changes in the bacterial composition of the gut microbiome.

The Effect of Methylene Blue and Its Metabolite—Azure I—on Bioenergetic Parameters of Intact Mouse Brain Mitochondria

Methylene blue, a phenothiazine dye, that is widely used in medicine and is under clinical trials as an agent for treatment of Alzheimer’s disease. One of the factors of the unique therapeutic effect of methylene blue is its redox properties, allowing implementation of alternative electron transport: the dye accepts electrons from reducing equivalents in mitochondria and transfer them to other components of the respiratory chain or molecular oxygen. Azure I, an N-dimethylated metabolite of methylene blue, is potentially a more effective compound than methylene blue, but its ability for alternative electron transport has not been studied yet. We have shown that in contrast to methylene blue, azure I is unable to restore the membrane potential in isolated mouse brain mitochondria, inhibited by rotenone and, therefore, is unable to perform bypass of the respiratory chain complex I. Moreover, addition of azure I does not affect the rate of mitochondrial respiration in contrast to methylene blue, which increases the rate of non-phosphorylation respiration. At the same time, both dyes stimulate an increase in H₂O₂ production. Thus, only methylene blue is capable of alternative electron transport, while azure I does not produce complex I bypass. This limits its therapeutic application only as a mitochondrial-targeted agent, but does not question its antidepressant effects.

Current and Near-Future Treatment of Alzheimer's Disease

Recent findings have improved our understanding of the multifactorial nature of AD. While in early asymptomatic stages of AD, increased amyloid-β synthesis and tau hyperphosphorylation play a key role, while in the latter stages of the disease, numerous dysfunctions of homeostatic mechanisms in neurons, glial cells, and cerebrovascular endothelium determine the rate of progression of clinical symptoms. The main driving forces of advanced neurodegeneration include increased inflammatory reactions in neurons and glial cells, oxidative stress, deficiencies in neurotrophic growth and regenerative capacity of neurons, brain insulin resistance with disturbed metabolism in neurons, or reduction of the activity of the Wnt-β catenin pathway, which should integrate the homeostatic mechanisms of brain tissue. In order to more effectively inhibit the progress of neurodegeneration, combination therapies consisting of drugs that rectify several above-mentioned dysfunctions should be used. It should be noted that many widely-used drugs from various pharmacological groups, "in addition" to the main therapeutic indications, have a beneficial effect on neurodegeneration and may be introduced into clinical practice in combination therapy of AD. There is hope that complex treatment will effectively inhibit the progression of AD and turn it into a slowly progressing chronic disease. Moreover, as the mechanisms of bidirectional communication between the brain and microbiota are better understood, it is expected that these pathways will be harnessed to provide novel methods to enhance health and treat AD.

De Novo Molecular Design of Caspase-6 Inhibitors by a GRU-Based Recurrent Neural Network Combined with a Transfer Learning Approach

Due to their potential in the treatment of neurodegenerative diseases, caspase-6 inhibitors have attracted widespread attention. However, the existing caspase-6 inhibitors showed more or less inevitable deficiencies that restrict their clinical development and applications. Therefore, there is an urgent need to develop novel caspase-6 candidate inhibitors. Herein, a gated recurrent unit (GRU)-based recurrent neural network (RNN) combined with transfer learning was used to build a molecular generative model of caspase-6 inhibitors. The results showed that the GRU-based RNN model can accurately learn the SMILES grammars of about 2.4 million chemical molecules including ionic and isomeric compounds and can generate potential caspase-6 inhibitors after transfer learning of the known 433 caspase-6 inhibitors. Based on the novel molecules derived from the molecular generative model, an optimal logistic regression model and Surflex-dock were employed for predicting and ranking the inhibitory activities. According to the prediction results, three potential caspase-6 inhibitors with different scaffolds were selected as the promising candidates for further research. In general, this paper provides an efficient combinational strategy for de novo molecular design of caspase-6 inhibitors.

[Effect of methylene blue and its metabolite - azure I - on bioenergetic parameters of intact mice brain mitochondria]

Methylene blue is a phenothiazine dye that is widely used in medicine and clinical trials for the treatment of Alzheimer's disease. One of the factors of the unique therapeutic effect of methylene blue is its redox properties, allowing implementation of alternative electron transport - the dye accepts electrons from reducing equivalents in the mitochondria and transfer it them to other components of the respiratory chain or molecular oxygen. Azure I, an N-dimethylated metabolite of methylene blue, is potentially a more effective compound than methylene blue, but its ability for alternative electron transport has not been studied. We have shown that azure I, unlike methylene blue, is unable to restore the membrane potential in isolated mouse brain mitochondria, inhibited by rotenone and, therefore, is unable to perform bypass of the respiratory chain Complex I. Moreover, the addition of azure I does not affect the rate of mitochondrial respiration in contrast to methylene blue, which increases the rate of non-phosphorylation respiration. At the same time, both dyes stimulate an increase in H2O2 production. As a consequence, only methylene blue is capable of alternative electron transport, while azure I does not produce complex I bypass. This limits its therapeutic application only as a mitochondrial-targeted drug, but not as a substance with a potentially powerful antidepressant effect.

Intravitreally Injected Methylene Blue Protects Retina against Acute Ocular Hypertension in Rats

Purpose: To assess the neuroprotective effects of methylene blue (MB) in a rat model of acute ocular hypertension (AOH) and explore its possible mechanisms.Methods: Our AOH rat model was obtained with anterior chamber perfusion for 60 min. After that, 100 μM MB was injected into the vitreous cavity immediately after injury. Electroretinogram, fundus photography, optical coherence tomography (OCT) and retina morphology examination were utilized to quantify retinal damage before surgery, as well as 7, 14 and 28 days after. The average number of surviving retinal ganglion cells (RGCs) was counted after fluorescent retrograde labelling with 4% DiI. And TUNEL assay was used to investigate retinal cell apoptosis at 24 hours after AOH. Nrf2 and BACE1 in the retina were determined by RT-qPCR analysis.Results: AOH did produce a severe degeneration effect on the whole retinal layer. Intravitreally injected MB maintained certain retinal thickness after AOH, reduced the destruction of electroretinograms, and enhanced RGCs survival. The average number of TUNEL-labelled cells statistically reduced in the MB-treated retina tissue compared with retina treated with normal saline. The relative mRNA level of Nrf2 was also much higher in the MB-treated retinas after AOH, and the expression of BACE1 had a decline in the AOH + MB group.Conclusions: MB can protect the retina from AOH injury and the possible mechanism might involve the inhibition of BACE1 expression and the activation of Nrf2 antioxidant pathway.

Methylene Blue Inhibits Acute Hemorrhagic Conjunctivitis Virus Production and Induction of Caspase-3 Mediated Human Corneal Cell Cytopathy

Background

Acute hemorrhagic conjunctivitis (AHC) is a highly contagious eye disease caused by enterovirus type 70 (E70) and Coxsackievirus A24 variant (CA24v) with no clinically approved treatment. The antiviral activity of methylene blue (MB; a WHO essential medicine) against AHC viruses was investigated using human corneal epithelial cells (HCEC).

Methods

Time and concentration-dependent MB accumulation by HCEC was determined colorimetrically and MB inhibition of virus production of 5 E70 and 3 CA24v AHC epidemic isolates in HCEC was determined by micro-plaque assay. AHC virus cytopathy inhibition by MB was detected by reductions in virus-induced caspase-3 activity and polymeric DNA fragments.

Results

MB uptake by HCEC was rapid and concentration dependent. MB inhibition of E70 and CA24v production was concentration dependent. AHC virus yields were significantly lower (50 to >10,000 fold) in HCEC pre-treated with 0.25–1% MB than in placebo controls (p’s ≤ 0.01). MB pre-treatment significantly inhibited virus-induced caspase-3 activation and DNA fragmentation (p’s<0.01). Virus-infected cells accumulate oxidized MB and MB application up to 6 h after infection inhibited virus production and virus-induced HCEC cytopathy.

Conclusion

The results suggest MB treatment prior to and shortly after infection can inhibit AHC virus production and caspase-mediated HCEC cytopathy. The results support the therapeutic potential of ophthalmic solutions containing MB against AHC virus infection during epidemics.

Hydroxyapatite nanocomposite as a potential agent in osteosarcoma PDT

Using Nanoplatforms as a hauler for photosensitizers is a bespoke paradigm to improve its bioavailability and to boost the PDT efficacy. Herein, the photodynamic cytotoxicity of methylene blue (MB) loaded on hydroxyapatite nanoparticles (HA-NPs) was tested against human osteosarcoma-derived cells (Saos-2 cell line). HA-NPs and HA-NPs loaded with MB (HA-NPs-MB) were prepared by a chemical precipitation method and characterized by TEM, Zeta potential, FTIR, and XRD. TEM images revealed that HA-NPs have a rod shape with a diameter of 14-17 nm and length around 46-64 nm. FTIR and Zeta potential confirmed the adsorption of cationic MB on HA-NPs. XRD pattern was identical to the standard XRD pattern of HA-NPs. Incubation of Saos-2 cells (24 h) with HA-NPs-MB then irradiation of cells (5 min) with a diode laser (808 nm), causes a higher decrement of cell viability (determined by MTT assay) than that caused by free MB. The LC₅₀ was 57.53 µg/mL and 86.99 µg/mL for HA-NPs-MB and free MB, respectively. Thus, the nanoformulation of MB greatly reduced the dose of MB required for effective PDT. This study also investigated the mode of cell death after incubation of cells with free MB or HA-NPs-MB composite then exposure to laser radiation. The results revealed that the majority of cells died by apoptosis while a minor fraction of cells died by necrosis, especially in the case of HA-NPs-MB. Levels of caspase-3 and death receptor-4 (DR-4) were more elevated in the case of HA-NPs-MB than free MB. The effect of the prepared nanocomposite and free MB on Raw murine macrophage (RAW 264.7) viability was also examined using the MTT assay. The results indicated that HA-NPs-MB in the presence of laser has a great cytotoxic effect on macrophage cells compared to other treatments. This may present an advantage through decreasing macrophage that promotes tumor growth. In conclusion, HA-NPs-MB nanocomposite surmounts free MB and HA-NPs in destroying macrophage cells and Saos-2 cells through apoptosis in the presence of laser irradiation. This study introduces a thorough and new insight on osteosarcoma (cancer cell line Saos-2) PDT using HA-NPs-MB exploiting the biosafety of HA-NPs.

Methylene blue inhibits Caspase-6 activity, and reverses Caspase-6-induced cognitive impairment and neuroinflammation in aged mice

Activated Caspase-6 (Casp6) is associated with age-dependent cognitive impairment and Alzheimer disease (AD). Mice expressing human Caspase-6 in hippocampal CA1 neurons develop age-dependent cognitive deficits, neurodegeneration and neuroinflammation. This study assessed if methylene blue (MB), a phenothiazine that inhibits caspases, alters Caspase-6-induced neurodegeneration and cognitive impairment in mice. Aged cognitively impaired Casp6-overexpressing mice were treated with methylene blue in drinking water for 1 month. Methylene blue treatment did not alter Caspase-6 levels, assessed by RT-PCR, western blot and immunohistochemistry, but inhibited fluorescently-labelled Caspase-6 activity in acute brain slice intact neurons. Methylene blue treatment rescued Caspase-6-induced episodic and spatial memory deficits measured by novel object recognition and Barnes maze, respectively. Methylene blue improved synaptic function of hippocampal CA1 neurons since theta-burst long-term potentiation (LTP), measured by field excitatory postsynaptic potentials (fEPSPs) in acute brain slices, was successfully induced in the Schaffer collateral-CA1 pathway in methylene blue-treated, but not in vehicle-treated, Caspase-6 mice. Increased neuroinflammation, measured by ionized calcium binding adaptor molecule 1 (Iba1)-positive microglia numbers and subtypes, and glial fibrillary acidic protein (GFAP)-positive astrocytes, were decreased by methylene blue treatment. Therefore, methylene blue reverses Caspase-6-induced cognitive deficits by inhibiting Caspase-6, and Caspase-6-mediated neurodegeneration and neuroinflammation. Our results indicate that Caspase-6-mediated damage is reversible months after the onset of cognitive deficits and suggest that methylene blue could benefit Alzheimer disease patients by reversing Caspase-6-mediated cognitive decline.

Identification of Allosteric Inhibitors against Active Caspase-6

Caspase-6 is a cysteine protease that plays essential roles in programmed cell death, axonal degeneration, and development. The excess neuronal activity of Caspase-6 is associated with Alzheimer disease neuropathology and age-dependent cognitive impairment. Caspase-6 inhibition is a promising strategy to stop early stage neurodegenerative events, yet finding potent and selective Caspase-6 inhibitors has been a challenging task due to the overlapping structural and functional similarities between caspase family members. Here, we investigated how four rare non-synonymous missense single-nucleotide polymorphisms (SNPs), resulting in amino acid substitutions outside human Caspase-6 active site, affect enzyme structure and catalytic efficiency. Three investigated SNPs were found to align with a putative allosteric pocket with low sequence conservation among human caspases. Virtual screening of 57,700 compounds against the putative Caspase-6 allosteric pocket, followed by in vitro testing of the best virtual hits in recombinant human Caspase-6 activity assays identified novel allosteric Caspase-6 inhibitors with IC50 and Ki values ranging from ~2 to 13 µM. This report may pave the way towards the development and optimisation of novel small molecule allosteric Caspase-6 inhibitors and illustrates that functional characterisation of rare natural variants holds promise for the identification of allosteric sites on other therapeutic targets in drug discovery.

Thioredoxin system as a gatekeeper in caspase-6 activation and nuclear lamina integrity: Implications for Alzheimer's disease

Recent reports in pathophysiology of neurodegenerative diseases (ND) have linked nuclear lamina degradation/deficits to neuronal cell death. Lamin-B1 damage is specifically involved in this process leading to nuclear envelope invagination and heterochromatin rearrangement. The underlying mechanisms involved in these events are not yet defined. In this study, while examining the effect of Thioredoxin-1(Trx1) inhibition on cell death in a model of oxidative stress, we noted robust nuclear invagination in SH-SY5Y cells. Evaluation of nuclear lamina proteins revealed lamin-B1 cleavage that was prevented by caspase-6 (CASP6) inhibitor and exacerbated after pharmacologic/genetic inhibition of Trx1 system, but not after glutathione depletion. Activation of CASP6 was upstream of CASP3/7 activation and its inhibition was sufficient to prevent cell death in our system. The effect of Trx1 redox status on CASP6 activation was assessed by administration of reduced/oxidized forms in cell-free nuclei preparation and purified enzymatic assays. Although reduced Trx1 decreased CASP6 enzymatic activity and lamin-B1 cleavage, the fully oxidized Trx1 showed opposite effects. The enhanced CASP6 activation was also associated with lower levels of DJ-1, a neuroprotective and master regulator of cellular antioxidants. The implication of our findings in ND pathophysiology was strengthened with detection of lower Trx1 levels in the hippocampi tissue of a mouse model of Alzheimer's disease. This coincided with higher CASP6 activation resulting in increased lamin-B1 and DJ-1 depletion. This study provides a first mechanistic explanation for the key regulatory role of Trx1 as a gatekeeper in activation of CASP6 and induction of nuclear invagination, an important player in ND pathophysiology.

Azure B affects amyloid precursor protein metabolism in PS70 cells

Alzheimer's disease (AD), the most common form of dementia, is characterized by abundant deposition of amyloid-β (Aβ) peptide that is the result of sequential cleavage of amyloid precursor protein (APP) by β-secretase and γ-secretase. Several studies have documented that inhibition of Aβ peptide synthesis or facilitating its degradation is one of the attractive therapeutic strategies in AD. Methylene blue (MethB), which has recently been investigated in Phase II clinical trials, is a prominent inhibitor in reducing Aβ oligomers. Herein, we wonder whether the mitigating effects of MethB on amyloid metabolism are related to the activity of its major metabolite, azure B. The goal of this study was to investigate the effects of azure B, which is also a cholinesterase inhibitor, on APP processing by using Chinese hamster ovary cells stably expressing human wild-type APP and presenilin 1 (PS70). Azure B significantly decreased the levels of secreted APPα (sAPPα) and Aβ_40/42 in culture medium with a dose-dependent manner. A significant decrease was also observed in the levels of intracellular APP without affecting the cell viability. In parallel with the decrease of APP and APP metabolites, the activity of β-secretase 1 (BACE1) was significantly attenuated compared to control. Overall, our results show that azure B has a large contribution for the pharmacological profile of MethB in APP metabolism.

Structural Basis of Small Molecule Targetability of Monomeric Tau Protein

The therapeutic targeting of intrinsically disordered proteins (IDPs) by small molecules has been a challenge due to their heterogeneous conformational ensembles. A potential therapeutic strategy to alleviate the aggregation of IDPs is to maintain them in their native monomeric state by small molecule binding. This study investigates the structural basis of small molecule druggability of native monomeric Tau whose aggregation is linked to the onset of Tauopathies such as Alzheimer's disease. Initially, two available monomeric conformational ensembles of a shorter Tau construct K18 (also termed Tau4RD) were analyzed which revealed striking structural differences between the two ensembles, while similar number of hot spots and small molecule binding sites were identified on monomeric Tau ensembles as on tertiary folded proteins of similar size. Remarkably, some critical fibril forming sequence regions of Tau (V306-K311, V275-K280) participated in hot spot formation with higher frequency compared to other regions. As an example of small molecule binding to monomeric Tau, it was shown that methylene blue (MB) bound to monomeric K18 and full-length Tau selectively with high affinity (K_d = 125.8 nM and 86.6 nM, respectively) with binding modes involving Cys291 and Cys322, previously reported to be oxidized in the presence of MB. Overall, our results provide structure-based evidence that Tau can be a viable drug target for small molecules and indicate that specific small molecules may be able to bind to monomeric Tau and influence the way in which the protein interacts among itself and with other proteins.

Spiropyran-Based Nanocarrier: A New Zn2+ -Responsive Delivery System with Real-Time Intracellular Sensing Capabilities

A new spiropyran-based stimuli-responsive delivery system is fabricated. It encapsulates and then releases an extraneous compound in response to elevated levels of Zn²⁺ , a critical factor in cell apoptosis. A C₁₂ -alkyl substituent on the spiropyran promotes self-assembly into a micelle-like nanocarrier in aqueous media, with nanoprecipitation and encapsulation of added payload. Zn²⁺ binding occurs to an appended bis(2-pyridylmethyl)amine group at biologically relevant micromolar concentration. This leads to switching of the spiropyran (SP) isomer to the strongly fluorescent ring opened merocyanine-Zn²⁺ (MC-Zn²⁺ ) complex, with associated expansion of the nanocarriers to release the encapsulated payload. Payload release is demonstrated in solution and in HEK293 cells by encapsulation of a blue fluorophore, 7-hydroxycoumarin, and monitoring its release using fluorescence spectroscopy and microscopy. Furthermore, the use of the nanocarriers to deliver a caspase inhibitor, Azure B, into apoptotic cells in response to an elevated Zn²⁺ concentration is demonstrated. This then inhibits intracellular caspase activity, as evidenced by confocal microscopy and in real-time by time-lapsed microscopy. Finally, the nanocarriers are shown to release an encapsulated proteasome inhibitor (5) in Zn²⁺ -treated breast carcinoma cell line models. This then inhibits intracellular proteasome and induces cytotoxicity to the carcinoma cells.

From Mitochondrial Function to Neuroprotection-an Emerging Role for Methylene Blue

Methylene blue (MB) is a well-established drug with a long history of use, owing to its diverse range of use and its minimal side effect profile. MB has been used classically for the treatment of malaria, methemoglobinemia, and carbon monoxide poisoning, as well as a histological dye. Its role in the mitochondria, however, has elicited much of its renewed interest in recent years. MB can reroute electrons in the mitochondrial electron transfer chain directly from NADH to cytochrome c, increasing the activity of complex IV and effectively promoting mitochondrial activity while mitigating oxidative stress. In addition to its beneficial effect on mitochondrial protection, MB is also known to have robust effects in mitigating neuroinflammation. Mitochondrial dysfunction has been identified as a seemingly unifying pathological phenomenon across a wide range of neurodegenerative disorders, which thus positions methylene blue as a promising therapeutic. In both in vitro and in vivo studies, MB has shown impressive efficacy in mitigating neurodegeneration and the accompanying behavioral phenotypes in animal models for such conditions as stroke, global cerebral ischemia, Alzheimer's disease, Parkinson's disease, and traumatic brain injury. This review summarizes recent work establishing MB as a promising candidate for neuroprotection, with particular emphasis on the contribution of mitochondrial function to neural health. Furthermore, this review will briefly examine the link between MB, neurogenesis, and improved cognition in respect to age-related cognitive decline.

Alzheimer's disease in the zebrafish: where can we take it?

With the ever-growing geriatric population, research on brain diseases such as dementia is more imperative now than ever. The most prevalent of all dementias is Alzheimer's disease, a progressive neurodegenerative disease that presents with deficits in memory, cognition, motor skills, and a general decline in the quality of life. The social and economic burden associated with Alzheimer's disease is tremendous and is projected to grow even greater over the coming years. There is a specific need to elucidate and improve the treatments available, not only to alleviate the symptoms related to dementias such as Alzheimer's but also to prevent the formation of the disease. This is an effort that can be expedited and made more efficient by utilizing an animal model such as the zebrafish. This paper reviews the utility of zebrafish in Alzheimer's research by examining research on a sampling of the treatments available for the disease, specifically donepezil, memantine, and methylene blue. The human model and the shortcomings of the rodent model are also discussed.

Proteomic Analyses of Cysteine Redox in High-Fat-Fed and Fasted Mouse Livers: Implications for Liver Metabolic Homeostasis

Intensive oxidative stress occurs during high-fat-diet-induced hepatic fat deposition, suggesting a critical role for redox signaling in liver metabolism. Intriguingly, evidence shows that fasting could also result in redox-profile changes largely through reduced oxidant or increased antioxidant levels. However, a comprehensive landscape of redox-modified hepatic substrates is lacking, thereby hindering our understanding of liver metabolic homeostasis. We employed a proteomic approach combining iodoacetyl tandem mass tag and nanoliquid chromatography tandem mass spectrometry to quantitatively probe the effects of high-fat feeding and fasting on in vivo redox-based cysteine modifications. Compared with control groups, ∼60% of cysteine residues exhibited downregulated oxidation ratios by fasting, whereas ∼94% of these ratios were upregulated by high-fat feeding. Importantly, in fasted livers, proteins exhibiting diminished cysteine oxidation were annotated in pathways associated with fatty acid metabolism, carbohydrate metabolism, insulin, peroxisome proliferator-activated receptors, and oxidative respiratory chain signaling, suggesting that fasting-induced redox changes targeted major metabolic pathways and consequently resulted in hepatic lipid accumulation.

Microtubule affinity-regulating kinases are potential druggable targets for Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that affects normal functions of the brain. Currently, AD is one of the leading causes of death in developed countries and the only one of the top ten diseases without a means to prevent, cure, or significantly slow down its progression. Therefore, newer therapeutic concepts are urgently needed to improve survival and the quality of life of AD patients. Microtubule affinity-regulating kinases (MARKs) regulate tau-microtubule binding and play a crucial role in neurons. However, their role in hyperphosphorylation of tau makes them potential druggable target for AD therapy. Despite the relevance of MARKs in AD pathogenesis, only a few small molecules are known to have anti-MARK activity and not much has been done to progress these compounds into therapeutic candidates. But given the diverse role of MARKs, the specificity of novel inhibitors is imperative for their successful translation from bench to bedside. In this regard, a recent co-crystal structure of MARK4 in association with a pyrazolopyrimidine-based inhibitor offers a potential scaffold for the development of more specific MARK inhibitors. In this manuscript, we review the biological role of MARKs in health and disease, and draw attention to the largely unexplored area of MARK inhibitors for AD.

Methylene blue inhibits NLRP3, NLRC4, AIM2, and non-canonical inflammasome activation

Methylene blue (MB), which has antioxidant, anti-inflammatory, neuroprotective, and mitochondria protective effects, has been widely used as a dye and medication. However, the effect of MB on inflammasome activation has not yet been studied. Inflammasomes are multi-protein complexes that induce maturation of interleukins (ILs)-1β and -18 as well as caspase-1-mediated cell death, known as pyroptosis. Dysregulation of inflammasomes causes several diseases such as type 2 diabetes, Alzheimer’s disease, and gout. In this study, we assess the effect of MB on inflammasome activation in macrophages. As the result, MB attenuated activation of canonical inflammasomes such as NLRP3, NLRC4, and AIM2 as well as non-canonical inflammasome activation. In addition, MB inhibited upstream signals such as inflammasome assembly, phagocytosis, and gene expression of inflammasome components via inhibition of NF-κB signaling. Furthermore, MB reduced the activity of caspase-1. The anti-inflammasome properties of MB were further confirmed in mice models. Thus, we suggest that MB is a broad-spectrum anti-inflammasome candidate molecule.

A Dual Pathogenic Mechanism Links Tau Acetylation to Sporadic Tauopathy

Tau acetylation has recently emerged as a dominant post-translational modification (PTM) in Alzheimer’s disease (AD) and related tauopathies. Mass spectrometry studies indicate that tau acetylation sites cluster within the microtubule (MT)-binding region (MTBR), suggesting acetylation could regulate both normal and pathological tau functions. Here, we combined biochemical and cell-based approaches to uncover a dual pathogenic mechanism mediated by tau acetylation. We show that acetylation specifically at residues K280/K281 impairs tau-mediated MT stabilization, and enhances the formation of fibrillar tau aggregates, highlighting both loss and gain of tau function. Full-length acetylation-mimic tau showed increased propensity to undergo seed-dependent aggregation, revealing a potential role for tau acetylation in the propagation of tau pathology. We also demonstrate that methylene blue, a reported tau aggregation inhibitor, modulates tau acetylation, a novel mechanism of action for this class of compounds. Our study identifies a potential “two-hit” mechanism in which tau acetylation disengages tau from MTs and also promotes tau aggregation. Thus, therapeutic approaches to limit tau K280/K281 acetylation could simultaneously restore MT stability and ameliorate tau pathology in AD and related tauopathies.

Removal of silver nanoparticles by mussel-inspired Fe3O4@ polydopamine core-shell microspheres and its use as efficient catalyst for methylene blue reduction

The removal of silver nanoparticles (AgNPs) from water is highly needed because of their increasing use and potential risk to the environment due to their toxic effects. Catalysis over AgNPs has received significant attention because of their highly catalytic performance. However, their use in practical applications is limited due to high cost and limited resources. Here, we present for the first time that the mussel-inspired Fe3O4@polydopamine (Fe3O4@PDA) nanocomposite can be used for efficient removal and recovery of AgNPs. Adsorption of AgNPs over Fe3O4@PDA was confirmed by TEM, FT-IR, XRD, TGA and magnetic property. The adsorption efficiency of AgNPs by Fe3O4@PDA was investigated as a function of pH, contact time, ionic strength and concentration of AgNPs. The kinetic data were well fitted to a pseudo-second order kinetic model. The isotherm data were well described by Langmuir model with a maximum adsorption capacity of 169.5 mg/g, which was higher than those by other adsorbents. Notably, the obtained AgNPs-Fe3O4@PDA exhibited highly catalytic activity for methylene blue reduction by NaBH4 with a rate constant of 1.44 × 10-3/s, which was much higher than those by other AgNPs catalysts. The AgNPs-Fe3O4@PDA promised good recyclability for at least 8 cycles and acid resistant with good stability.

Tau Protein Hyperphosphorylation and Aggregation in Alzheimer's Disease and Other Tauopathies, and Possible Neuroprotective Strategies

Abnormal deposition of misprocessed and aggregated proteins is a common final pathway of most neurodegenerative diseases, including Alzheimer's disease (AD). AD is characterized by the extraneuronal deposition of the amyloid β (Aβ) protein in the form of plaques and the intraneuronal aggregation of the microtubule-associated protein tau in the form of filaments. Based on the biochemically diverse range of pathological tau proteins, a number of approaches have been proposed to develop new potential therapeutics. Here we discuss some of the most promising ones: inhibition of tau phosphorylation, proteolysis and aggregation, promotion of intra- and extracellular tau clearance, and stabilization of microtubules. We also emphasize the need to achieve a full understanding of the biological roles and post-translational modifications of normal tau, as well as the molecular events responsible for selective neuronal vulnerability to tau pathology and its propagation. It is concluded that answering key questions on the relationship between Aβ and tau pathology should lead to a better understanding of the nature of secondary tauopathies, especially AD, and open new therapeutic targets and strategies.