Indicator: Beat the Alzheimer's blues

No more methylene blues

Methylene blue is well known as a redox indicator in chemical laboratories and since its discovery in the nineteenth century has been used to treat malaria and prevent urinary tract infections. It may also have a role to play in warding off the symptoms of Alzheimer's disease, according to a new study, or perhaps lead to novel drugs for the disorder based on its chemical structure and properties. Mechanistic details of the compound's putative mode of action are revealed in the journal Angewandte Chemie by researchers using nuclear magnetic resonance (NMR) and spectroscopy and other approaches.

Methylene blue (methylthioninium chloride) is a heterocyclic aromatic with the molecular formula C₁₆H₁₈N₃SCl, it has many uses in chemistry and biology. Controversially, however, it has been demonstrated to have some ability to inhibit protein aggregation and at least one company has formulated a product for its use in combating Alzheimer's disease. The mode of action is suggested by TauRx Therapeutics to involve inhibition of Tau protein aggregation, which then delays or even reverses neurodegeneration. In the laboratory, methylene blue has been shown to have an effect on Tau aggregation but it has also been shown to causes dissociation of amyloids as well as affecting the function of mitochondria. Any of these targets might be as likely as any other to be involved, should the compound prove to be efficacious in Alzheimer's disease.

Avoiding aggregation

Researchers from the German Center for Neurodegenerative Diseases (DZNE), the Max Planck Institute for Biophysical Chemistry in Göttingen and the Caesar Research Center, Bonn, Germany, have looked at the possibility of the former mode of action being valid - that methylene blue inhibits Tau aggregation at the microscopic level. Writing in Angewandte Chemie, the team, which includes Markus Zweckstetter and Eckhard Mandelkow, reports that methylene blue appears to inactivate molecular residues that promote the bonding of Tau proteins.

"Tau proteins are actually extremely important, because they stabilize the transport routes inside each nerve cell," explains Mandelkow. "However, in cases of Alzheimer's, they stop doing their job. The transport routes inside the cells break down, and supplies essential for the survival of the cells can no longer reach their destination. In addition, the tau proteins stick together. These aggregates are also harmful and are a typical characteristic of the disease." Previous studies of disease in mice and threadworms have shown a positive effect of methylene blue but no significant data from human patients has so far been collected. Mandelkow is convinced that methylene blue inhibits the protein aggregation process but the in which that happened was unknown until now.

Separation, the name of the game

The researchers used NMR spectroscopy to track the behaviour of methylene blue and its interaction with the sulfur-containing amino acid cysteine present in Tau proteins and known to promote their bonding; cysteines (which have an S-H group) are often the culprit in the formation of cross-links between protein chains. In Alzheimer's and other degenerative diseases it is thought that the body's natural antioxidant defences against the formation of such cross-links fails allowing the proteins to aggregate, which they wouldn't do in a healthy person.

The researchers suggest that, in some sense, methylene blue mimics those defences and acts as a spacer to keep the proteins apart and so preclude aggregation by latching on to the cysteine groups and preventing them from cross-linking. This reaction is highly effective. Methylene blue specifically modifies the tau proteins at critical spots, the team found.

"This chemical transformation prevents tau proteins from bonding together," explains Zweckstetter. "Otherwise S-H groups from different proteins would react and form a so-called disulfide bridge. Now, this is no longer possible, because the reaction with methylene blue eliminates the S-H groups."

Ultimately, the findings could lead to the development of modified forms of methylene blue and new types of treatment. However, there are other processes that must be taken into account any one of which might be involved in the overall development of symptoms and so such a drug would perhaps be just one of an arsenal used to attack Alzheimer's disease.

"Regarding the future, I am interested in developing modified versions of methylene blue with improved pharmacochemical properties and at the same time understanding the nature of protein aggregation inhibition, in particular by other classes of small molecules," Zweckstetter told SpectroscopyNOW.