GAPDH binders as potential drugs for the therapy of polyglutamine diseases: Design of a new screening assay

Protein Interactome of Amyloid-β as a Therapeutic Target

The amyloid concept of Alzheimer's disease (AD) assumes the β-amyloid peptide (Aβ) as the main pathogenic factor, which injures neural and other brain cells, causing their malfunction and death. Although Aβ has been documented to exert its cytotoxic effect in a solitary manner, there is much evidence to claim that its toxicity can be modulated by other proteins. The list of such Aβ co-factors or interactors includes tau, APOE, transthyretin, and others. These molecules interact with the peptide and affect the ability of Aβ to form oligomers or aggregates, modulating its toxicity. Thus, the list of potential substances able to reduce the harmful effects of the peptide should include ones that can prevent the pathogenic interactions by specifically binding Aβ and/or its partners. In the present review, we discuss the data on Aβ-based complexes in AD pathogenesis and on the compounds directly targeting Aβ or the destructors of its complexes with other polypeptides.

Derivation and validation of an epigenetic frailty risk score in population-based cohorts of older adults

DNA methylation (DNAm) patterns in peripheral blood have been shown to be associated with aging related health outcomes. We perform an epigenome-wide screening to identify CpGs related to frailty, defined by a frailty index (FI), in a large population-based cohort of older adults from Germany, the ESTHER study. Sixty-five CpGs are identified as frailty related methylation loci. Using LASSO regression, 20 CpGs are selected to derive a DNAm based algorithm for predicting frailty, the epigenetic frailty risk score (eFRS). The eFRS exhibits strong associations with frailty at baseline and after up to five-years of follow-up independently of established frailty risk factors. These associations are confirmed in another independent population-based cohort study, the KORA-Age study, conducted in older adults. In conclusion, we identify 65 CpGs as frailty-related loci, of which 20 CpGs are used to calculate the eFRS with predictive performance for frailty over long-term follow-up.

DNM3OS regulates GAPDH expression and influences the molecular pathogenesis of Huntington's disease

Emerging studies have suggested that dysregulated long non‐coding RNAs (lncRNAs) are associated with the pathogenesis of neurodegenerative diseases (NDD) including Huntington's disease (HD); however, the pathophysiological mechanism by which lncRNA dysregulation participates in HD remains to be elucidated. Here, we aim to analyse the expression of lncRNA‐DNM3OS and identify the possible DNM3OS/miR‐196b‐5p/GAPDH pathway. PC12 cells induced by rat pheochromocytoma expressing HD gene exon 1 fragment with either 23 or 74 polyglutamine repeats fused to the green fluorescent protein (GFP) were cultured. Our results show that GAPDH and DNM3OS were upregulated in HD PC12 cells, downregulation of which lead to inhibition of aggregate formation accompanied by a decreased apoptosis rate and increased relative ROS levels and cell viability. Moreover, upregulated DNM3OS decreased the expression of miR‐196b‐5p by sponging, and GAPDH was a direct target of miR‐196b‐5p, playing an important pathogenic role in the formation of aggregates in the HD cell model. Our study uncovers a novel DNM3OS/miR‐196b‐5p/GAPDH pathway involved in the molecular pathogenesis of HD, which may offer a potential therapeutic strategy for HD.

Conditioned Medium from Cells Overexpressing TDP-43 Alters the Metabolome of Recipient Cells

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by the progressive death of both upper and lower motor neurons. The disease presents a poor prognosis, and patients usually die 2-5 years after the onset of symptoms. The hallmark of this disease is the presence of phosphorylated and ubiquitinated aggregates containing trans-active response DNA-binding protein-43 (TDP-43) in the cytoplasm of motor neurons. TDP-43 pathology has been associated with multiple pathways in ALS, such as metabolic dysfunction found in patients and in in vivo models. Recently, it has been described as a "prion-like" protein, as studies have shown its propagation in cell culture from ALS brain extract or overexpressed TDP-43 in co-culture and conditioned medium, resulting in cytotoxicity. However, the cellular alterations that are associated with this cytotoxicity require further investigation. Here, we investigated the effects of conditioned medium from HEK293T (Human Embryonic Kidney 293T) cells overexpressing TDP-43 on cellular morphology, proliferation, death, and metabolism. Although we did not find evidence of TDP-43 propagation, we observed a toxicity of TDP-43-conditioned medium and altered metabolism. These results, therefore, suggest (1) that cells overexpressing TDP-43 produce an extracellular environment that can perturb other cells and (2) that TDP-43 propagation alone may not be the only potentially cytotoxic cell-to-cell mechanism.

Influence of Oxidative Stress on Catalytic and Non-glycolytic Functions of Glyceraldehyde-3-phosphate Dehydrogenase

BACKGROUND

Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH) is a unique enzyme that, besides its main function in glycolysis (catalysis of glyceraldehyde-3-phosphate oxidation), possesses a number of non-glycolytic activities. The present review summarizes information on the role of oxidative stress in the regulation of the enzymatic activity as well as non-glycolytic functions of GAPDH.

METHODS

Based on the analysis of literature data and the results obtained in our research group, mechanisms of the regulation of GAPDH functions through the oxidation of the sulfhydryl groups in the active site of the enzyme have been suggested.

RESULTS

Mechanism of GAPDH oxidation includes consecutive oxidation of the catalytic Cysteine (Cys150) into sulfenic, sulfinic, and sulfonic acid derivatives, resulting in the complete inactivation of the enzyme. The cysteine sulfenic acid reacts with reduced glutathione (GSH) to form a mixed disulfide (S-glutathionylated GAPDH) that further reacts with Cys154 yielding the disulfide bond in the active site of the enzyme. In contrast to the sulfinic and sulfonic acids, the mixed disulfide and the intramolecular disulfide bond are reversible oxidation products that can be reduced in the presence of GSH or thioredoxin.

CONCLUSION

Oxidation of sulfhydryl groups in the active site of GAPDH is unavoidable due to the enhanced reactivity of Cys150. The irreversible oxidation of Cys150 is prevented by Sglutathionylation and disulfide bonding with Cys154. The oxidation/reduction of the sulfhydryl groups in the active site of GAPDH can be used for regulation of glycolysis and numerous side activities of this enzyme including the induction of apoptosis.

Glyceraldehyde-3-phosphate Dehydrogenase is a Multifaceted Therapeutic Target

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a glycolytic enzyme whose role in cell metabolism and homeostasis is well defined, while its function in pathologic processes needs further elucidation. Depending on the cell context, GAPDH may bind a number of physiologically important proteins, control their function and correspondingly affect the cell’s fate. These interprotein interactions and post-translational modifications of GAPDH mediate its cytotoxic or cytoprotective functions in the manner of a Janus-like molecule. In this review, we discuss the functional features of the enzyme in cellular physiology and its possible involvement in human pathologies. In the last part of the article, we describe drugs that can be employed to modulate this enzyme’s function in some pathologic states.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase in neurodegenerative processes and the role of low molecular weight compounds in counteracting its aggregation and nuclear translocation

A number of independent studies have shown the contribution of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in the pathogenesis of several neurodegenerative disorders. Indeed, GAPDH aggregates have been found in many post-mortem samples of brains of patients diagnosed with Alzheimer's and Parkinson disease. Currently, it is accepted that GAPDH-mediated cell death pathways in the neurodegenerative processes are associated with apoptosis caused by GAPDH nuclear translocation and excessive aggregation under oxidative stress conditions. Also the role of GAPDH in neurodegenerative diseases is linked to it directly binding to specific amyloidogenic proteins and petides such as β-amyloid precursor protein, β-amyloid peptide and tau protein in Alzheimer's disease, huntingtin in Huntington's disease and α-synuclein in Parkinson disease. One of the latest studies indicated that GAPDH aggregates significantly accelerate amyloidogenesis of the β-amyloid peptide, which implies that aggregates of GAPDH may act as a specific aggregation "seed" in vitro. Previous detailed studies revealed that the active-site cysteine (Cys152) of GAPDH plays an essential role in the oxidative stress-induced aggregation of GAPDH associated with cell death. Furthermore, oxidative modification of this cysteine residue initiates the translocation of the enzyme to the nucleus, subsequently leading to apoptosis. The crystallographic structure of GAPDH shows that the Cys152 residue is located close to the surface of the molecule in a hydrophilic environment, which means that it can react with low molecular weight compounds such as hydroxynonenal or piceatannol. Therefore, it is highly possible that GAPDH may serve as a target for small molecule compounds with the potential to slow down or prevent the progression of neurodegenerative disorders. Recently appearing new evidence has highlighted the significance of low molecular weight compounds in counteracting the oxidation of GAPDH and consequently its aggregation and other unfavourable pathological processes. Hence, this review aims to present all recent findings concerning molecules that are able to interact with GAPDH and counteract its aggregation and translocation to the nucleus.

GAPDH-targeted therapy - A new approach for secondary damage after traumatic brain injury on rats

Massive neuronal death caused by a neurodegenerative pathology or damage due to ischaemia or traumatic brain injury leads to the appearance of cytosolic proteins in the extracellular space. We found that one of the most abundant cellular polypeptides, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), appearing in the medium of dying cells or body fluids is able to form aggregates that are cytotoxic to adjacent cells. Since we previously showed that the hydrocortisone derivative RX624 can inhibit the ability of GAPDH to transport the enzyme complex with polyglutamine and reduce the cytotoxicity of the complex, we explored the effects of GAPDH on SH-SY5Y neuroblastoma cells. We found that the latter treated with particular forms of GAPDH molecules die with a high efficiency, suggesting that the exogenous enzyme does kill adjacent cells. RX624 prevented the interaction of exogenous GAPDH with the cell membrane and reduced the level of death by more than 10%. We also demonstrated the efficiency of RX624 treatment in a rat model of traumatic brain injury. The chemical blocked the formation of GAPDH aggregates in the brain, inhibited the cytotoxic effects of cerebrospinal fluid and rescued the motor function of injured rats. Importantly, RX624 treatment of rats had a similar effect as the intracranial injection of anti-GAPDH antibodies.

Sensitizing tumor cells to conventional drugs: HSP70 chaperone inhibitors, their selection and application in cancer models

Hsp70 chaperone controls proteostasis and anti-stress responses in rapidly renewing cancer cells, making it an important target for therapeutic compounds. To date several Hsp70 inhibitors are presented with remarkable anticancer activity, however their clinical application is limited by the high toxicity towards normal cells. This study aimed to develop assays to search for the substances that reduce the chaperone activity of Hsp70 and diminish its protective function in cancer cells. On our mind the resulting compounds alone should be safe and function in combination with drugs widely employed in oncology. We constructed systems for the analysis of substrate-binding and refolding activity of Hsp70 and to validate the assays screened the substances representing most diverse groups of chemicals of InterBioScreen library. One of the inhibitors was AEAC, an N-amino-ethylamino derivative of colchicine, which toxicity was two-orders lower than that of parent compound. In contrast to colchicine, AEAC inhibited substrate-binding and refolding functions of Hsp70 chaperones. The results of a drug affinity responsive target stability assay, microscale thermophoresis and molecular docking show that AEAC binds Hsp70 with nanomolar affinity. AEAC was found to penetrate C6 rat glioblastoma and B16 mouse melanoma cells and reduce there the function of the Hsp70-mediated refolding system. Although the cytotoxic and growth inhibitory activities of AEAC were minimal, the compound was shown to increase the antitumor efficiency of doxorubicin in tumor cells of both types. When the tumors were grown in animals, AEAC administration in combination with doxorubicin exerted maximal therapeutic effect prolonging animal survival by 10–15 days and reducing tumor growth rate by 60%. To our knowledge, this is the first time that this approach to the high-throughput analysis of chaperone inhibitors has been applied, and it can be useful in the search for drug combinations that are effective in the treatment of highly resistant tumors.

Experimental and Clinical Strategies for Treating Spinocerebellar Ataxia Type 3

Spinocerebellar ataxia type 3 (SCA3), or Machado-Joseph disease (MJD), is an autosomal dominant neurodegenerative disorder caused by the expansion of a polyglutamine (polyQ) tract in the ataxin-3 protein. To date, there is no effective therapy available to prevent progression of this disease. However, clinical strategies for alleviating various symptoms are imperative to promote a better quality of life for SCA3/MJD patients. Furthermore, experimental therapeutic strategies, including gene silencing or mutant protein clearance, mutant polyQ protein modification, stabilizing the native protein conformation, rescue of cellular dysfunction and neuromodulation to slow the progression of SCA3/MJD, have been developed. In this study, based on the current knowledge, I detail the clinical and experimental therapeutic strategies for treating SCA3/MJD, paying particular attention to drug discovery.

Peptide fragments of Hsp70 modulate its chaperone activity and sensitize tumor cells to anti-cancer drugs

Most Hsp70 chaperone inhibitors exert anti-cancer effects; however, their high cytotoxicity proposed the use of peptide fragments of the chaperone as safer modulators of its activity and as complements to customary drugs. One such peptide, ICit-2, was found to inhibit substrate-binding and refolding activities of the chaperone. Using various approaches, we established that ICit-2 binds Hsp70, which may explain its inhibitory action. ICit-2 penetrates A-431 cancer cells and, in combination with doxorubicin (Dox), enhances the cytotoxicity and growth inhibitory effect of the drug. Similarly, using the B16 mouse melanoma model, we found that ICit-2 inhibits the rate of tumor growth by 48% compared to Dox alone, confirming that the peptide can be employed to sensitize resistant tumors to cytostatic medicines.

A hydrocortisone derivative binds to GAPDH and reduces the toxicity of extracellular polyglutamine-containing aggregates

Huntington's disease (HD) has been recently shown to have a horizontally transmitted, prion-like pathology. Thus, the migration of polyglutamine-containing aggregates to acceptor cells is important for the progression of HD. These aggregates contain glyceraldehyde-3-phosphate dehydrogenase (GAPDH), which increases their intracellular transport and their toxicity. Here, we show that RX624, a derivative of hydrocortisone that binds to GAPDH, prevents the formation of aggregates of GAPDH-polyglutamine excreted into the culture medium by PC-12 rat cells expressing mutant huntingtin. RX624 was previously shown to be unable to penetrate cells and, thus, its principal therapeutic action might be the inhibition of polyglutamine-GAPDH complex aggregation in the extracellular matrix. The administration of RX624 to SH-SY5Y acceptor cells that incubated in conditioned medium from PC-12 cells expressing mutant huntingtin caused an approximately 20% increase in survival. This suggests that RX624 might be useful as a drug against polyglutamine pathologies, and that is could be administered exogenously without affecting target cell physiology. This protective effect was validated by the similar effect of an anti-GAPDH specific antibody.

Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH) Aggregation Causes Mitochondrial Dysfunction during Oxidative Stress-induced Cell Death

Glycolytic glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional protein that also mediates cell death under oxidative stress. We reported previously that the active-site cysteine (Cys-152) of GAPDH plays an essential role in oxidative stress-induced aggregation of GAPDH associated with cell death, and a C152A-GAPDH mutant rescues nitric oxide (NO)-induced cell death by interfering with the aggregation of wild type (WT)-GAPDH. However, the detailed mechanism underlying GAPDH aggregate-induced cell death remains elusive. Here we report that NO-induced GAPDH aggregation specifically causes mitochondrial dysfunction. First, we observed a correlation between NO-induced GAPDH aggregation and mitochondrial dysfunction, when GAPDH aggregation occurred at mitochondria in SH-SY5Y cells. In isolated mitochondria, aggregates of WT-GAPDH directly induced mitochondrial swelling and depolarization, whereas mixtures containing aggregates of C152A-GAPDH reduced mitochondrial dysfunction. Additionally, treatment with cyclosporin A improved WT-GAPDH aggregate-induced swelling and depolarization. In doxycycline-inducible SH-SY5Y cells, overexpression of WT-GAPDH augmented NO-induced mitochondrial dysfunction and increased mitochondrial GAPDH aggregation, whereas induced overexpression of C152A-GAPDH significantly suppressed mitochondrial impairment. Further, NO-induced cytochrome c release into the cytosol and nuclear translocation of apoptosis-inducing factor from mitochondria were both augmented in cells overexpressing WT-GAPDH but ameliorated in C152A-GAPDH-overexpressing cells. Interestingly, GAPDH aggregates induced necrotic cell death via a permeability transition pore (PTP) opening. The expression of either WT- or C152A-GAPDH did not affect other cell death pathways associated with protein aggregation, such as proteasome inhibition, gene expression induced by endoplasmic reticulum stress, or autophagy. Collectively, these results suggest that NO-induced GAPDH aggregation specifically induces mitochondrial dysfunction via PTP opening, leading to cell death.

Xyloketal-derived small molecules show protective effect by decreasing mutant Huntingtin protein aggregates in Caenorhabditis elegans model of Huntington's disease

Huntington's disease is an autosomal-dominant neurodegenerative disorder, with chorea as the most prominent manifestation. The disease is caused by abnormal expansion of CAG codon repeats in the IT15 gene, which leads to the expression of a glutamine-rich protein named mutant Huntingtin (Htt). Because of its devastating disease burden and lack of valid treatment, development of more effective therapeutics for Huntington's disease is urgently required. Xyloketal B, a natural product from mangrove fungus, has shown protective effects against toxicity in other neurodegenerative disease models such as Parkinson's and Alzheimer's diseases. To identify potential neuroprotective molecules for Huntington's disease, six derivatives of xyloketal B were screened in a Caenorhabditis elegans Huntington's disease model; all six compounds showed a protective effect. Molecular docking studies indicated that compound 1 could bind to residues GLN369 and GLN393 of the mutant Htt protein, forming a stable trimeric complex that can prevent the formation of mutant Htt aggregates. Taken together, we conclude that xyloketal derivatives could be novel drug candidates for treating Huntington's disease. Molecular target analysis is a good method to simulate the interaction between proteins and drug compounds. Further, protective candidate drugs could be designed in future using the guidance of molecular docking results.

Small molecules preventing GAPDH aggregation are therapeutically applicable in cell and rat models of oxidative stress

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is one of the most abundant targets of the oxidative stress. Oxidation of the enzyme causes its inactivation and the formation of intermolecular disulfide bonds, and leads to the accumulation of GAPDH aggregates and ultimately to cell death. The aim of this work was to reveal the ability of chemicals to break the described above pathologic linkage by inhibiting GAPDH aggregation. Using the model of oxidative stress based on SK-N-SH human neuroblastoma cells treated with hydrogen peroxide, we found that lentivirus-mediated down- or up-regulation of GAPDH content caused inhibition or enhancement of the protein aggregation and respectively reduced or increased the level of cell death. To reveal substances that are able to inhibit GAPDH aggregation, we developed a special assay based on dot ultrafiltration using the collection of small molecules of plant origin. In the first round of screening, five compounds were found to possess anti-aggregation activity as established by ultrafiltration and dynamic light scattering; some of the substances efficiently inhibited GAPDH aggregation in nanomolar concentrations. The ability of the compounds to bind GAPDH molecules was proved by the drug affinity responsive target stability assay, molecular docking and differential scanning calorimetry. Results of experiments with SK-N-SH human neuroblastoma treated with hydrogen peroxide show that two substances, RX409 and RX426, lowered the degree of GAPDH aggregation and reduced cell death by 30%. Oxidative injury was emulated in vivo by injecting of malonic acid into the rat brain, and we showed that the treatment with RX409 or RX426 inhibited GAPDH-mediated aggregation in the brain, reduced areas of the injury as proved by magnetic resonance imaging, and augmented the behavioral status of the rats as established by the "beam walking" test. In conclusion, the data show that two GAPDH binders could be therapeutically relevant in the treatment of injuries stemming from hard oxidative stress.

Properties of substances inhibiting aggregation of oxidized GAPDH: Data on the interaction with the enzyme and the impact on its intracellular content

This data is related to our paper “Small molecules preventing GAPDH aggregation are therapeutically applicable in cell and rat models of oxidative stress” (Lazarev et al. [1]) where we explore therapeutic properties of small molecules preventing GAPDH aggregation in cell and rat models of oxidative stress. The present article demonstrates a few of additional properties of the chemicals shown to block GAPDH aggregation such as calculated site for targeting the enzyme, effects on GAPDH glycolytic activity, influence on GAPDH intracellular level and anti-aggregate activity of pure polyglutamine exemplifying a denatured protein.

Glyceraldehyde 3-phosphate dehydrogenase augments the intercellular transmission and toxicity of polyglutamine aggregates in a cell model of Huntington disease

The common feature of Huntington disease is the accumulation of oligomers or aggregates of mutant huntingtin protein (mHTT), which causes the death of a subset of striatal neuronal populations. The cytotoxic species can leave neurons and migrate to other groups of cells penetrating and damaging them in a prion-like manner. We hypothesized that the glycolytic enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH), previously shown to elevate the aggregation of mHTT, is associated with an increased efficiency of intercellular propagation of mHTT. GAPDH, on its own or together with polyglutamine species, was shown to be released into the extracellular milieu mainly from dying cells as assessed by a novel enzyme immunoassay, western blotting, and ultrafiltration. The conditioned medium of cells with growing GAPDH-polyQ aggregates was toxic to naïve cells, whereas depletion of the aggregates from the medium lowered this cytotoxicity. The GAPDH component of the aggregates was found to increase their toxicity by two-fold in comparison with polyQ alone. Furthermore, GAPDH-polyQ complexes were shown to penetrate acceptor cells and to increase the capacity of polyQ to prionize its intracellular homolog containing a repeat of 25 glutamine residues. Finally, inhibitors of intracellular transport showed that polyQ-GAPDH complexes, as well as GAPDH itself, penetrated cells using clathrin-mediated endocytosis. This suggested a pivotal role of the enzyme in the intercellular transmission of Huntington disease pathogenicity. In conclusion, GAPDH occurring in complexes with polyglutamine strengthens the prion-like activity and toxicity of the migrating aggregates. Aggregating polygluatmine tracts were shown to release from the cells over-expressing mutant huntingtin in a complex with glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The enzyme enhances the intracellular transport of aggregates to healthy cells, prionization of normal cellular proteins and finally cell death, thus demonstrating the pivotal role of GAPDH in the horizontal transmission of neurodegeneration.

Hsp70 chaperone rescues C6 rat glioblastoma cells from oxidative stress by sequestration of aggregating GAPDH

The Hsp70 chaperone is known to elicit cytoprotective activity and this protection has a negative impact in anti-tumor therapy. In cancer cells subjected to oxidative stress Hsp70 may bind damaged polypeptides and proteins involved in apoptosis signaling. Since one of the important targets of oxidative stress is glyceraldehyde-3-phospate dehydrogenase (GAPDH) we suggested that Hsp70 might elicit its protective effect by binding GAPDH. Microscopy data show that in C6 rat glioma cells subjected to hydrogen peroxide treatment a considerable proportion of the GAPDH molecules are denatured and according to dot ultrafiltration data they form SDS-insoluble aggregates. Using two newly developed assays we show that Hsp70 can bind oxidized GAPDH in an ATP-dependent manner. Pharmacological up- or down-regulation of Hsp70 with the aid of U133 echinochrome or triptolide, respectively, reduced or increased the number of C6 glioma cells containing GAPDH aggregates and dying due to treatment with hydrogen peroxide. Using immunoprecipitation we found that Hsp70 is able to sequester aggregation-prone GAPDH and this may explain the anti-oxidative power of the chaperone. The results of this study led us to conclude that in cancer cells constantly exposed to conditions of oxidative stress, the protective power of Hsp70 should be abolished by specific inhibitors of Hsp70 expression.