P1‐028: Methylene blue inhibits caspases by oxidation of the catalytic cysteine

Abstract

Methylene blue, currently in phase 3 clinical trials against Alzheimer Disease, disaggregates the Tau protein of neurofibrillary tangles by oxidizing specific cysteine residues. Here, we investigated if methylene blue can inhibit caspases via the oxidation of their active site cysteine. Methylene blue, and derivatives, azure A and azure B competitively inhibited recombinant Caspase-6 (Casp6), and inhibited Casp6 activity in transfected human colon carcinoma cells and in serum-deprived primary human neuron cultures. Methylene blue also inhibited recombinant Casp1 and Casp3. Furthermore, methylene blue inhibited Casp3 activity in an acute mouse model of liver toxicity. Mass spectrometry confirmed methylene blue and azure B oxidation of the catalytic Cys163 cysteine of Casp6. Together, these results show a novel inhibitory mechanism of caspases via sulfenation of the active site cysteine. These results indicate that methylene blue or its derivatives could (1) have an additional effect against Alzheimer Disease by inhibiting brain caspase activity, (2) be used as a drug to prevent caspase activation in other conditions, and (3) predispose chronically treated individuals to cancer via the inhibition of caspases.Methylene blue, a tricyclic phenothiazine (also known as methylthionine hydrochloride), has a history of diverse medical applications stretching over a century for treatment of enzymopenic hereditary methemoglobinemia, acute acquired methemoglobinemia, urinary tract infections, malaria, septic shock, and hepatopulmonary syndrome 1 . Alzheimer disease (AD) is a progressive neurodegenerative disorder showing abundant deposits of β -amyloid peptide (Aβ ) plaques, intracellular neurofibrillary tangles (NFTs) consisting of Tau protein, and the loss of synapses 2 . Presently, inhibitors of Tau aggregation are being considered as therapeutic interventions against AD, and methylene blue disaggregates Tau NFTs 3 . Methylene blue, and its demethylated derivatives azure A and azure B, were initially identified as blocking in vitro Tau-Tau aggregation identified as paired helical filaments by electron microscopy 3 . Methylene blue was also shown to prevent heparin-induced Tau filament formation 4 . More recently, several studies have demonstrated the ability of methylene blue to prevent Tau aggregation in transgenic mouse models expressing the P301L or P301S Tau mutations associated with the formation of NFTs in mice and in human disease. Treatment of the rTg4510 human P301L transgenic mouse with methylene blue improved behavior slightly in treated 3 month old mice, reduced brain total Tau and phospho-Tau, and increased neuronal survival 5 . However, treatments in 16 month old rTg4510 mice did not have any effect on Tau levels, neuronal survival or brain atrophy 6 . The preventative nature of methylene blue was also observed in Tau∆K280 and TauRDK transgenic mice where treatments started in 1.5 or 9 month old mice, but not in 15 month old mice, showed improved cognitive behavior and a decrease in pathological Tau at 18 mo...