This chapter discusses a small molecule called trehalose that may help prevent protein aggregation. Everyone has a certain copy, or allele, of the Huntington gene, but people with Huntington’s disease (HD) have one copy that is longer than normal. The longer section of this HD allele consists of a repeated sequence, CAG, which codes for glutamine, an amino acid. (For more information on CAG repeats and HD, click here.) Since the Huntington gene codes for the huntingtin protein, the HD allele, with its extra CAGs, codes for a huntingtin protein with too many glutamines. The extra glutamines cause the protein to have an abnormal shape, which prevents it from functioning as it should. Instead, many of these altered huntingtin proteins clump together and trap other useful and important molecules. These “clumps” are called protein aggregates, and they may prevent the normal functioning of nerve cells. (For more information on protein aggregation, click here.) Scientists are not sure if the formation of protein aggregates is a cause or only a symptom of HD, but many agree that it would be beneficial to prevent them from forming in the first place.
What is trehalose ?^
Trehalose is a disaccharide (two sugar) molecule composed of two smaller glucose molecules linked together. It is naturally produced by the body and can also be found in common foods. The U.S. Food and Drug Administration lists trehalose as a compound under the category of “generally regarded as safe.” Since trehalose is a sugar, it is used as a sweetener in products such as chewing gum. It also has a very important property that helps it to stabilize proteins and can thus be used as a biological preservative. It is this very feature that may useful for treating Huntington’s disease.
How can trehalose be used to treat HD?^
A protein is made up of a string of amino acids. As the amino acids are strung together, the protein begins to fold up on itself until it gets to its final three-dimensional (3D) shape. Normal, stable proteins have no problem maintaining their shapes and functions in the cell. However, the huntingtin proteins formed from the HD allele are not very stable on their own, so they form into clumps known as protein aggregates.
Scientists think that if these proteins can be stabilized before they are fully folded, the protein aggregations will not form. A recent study in a mouse model of HD has shown the efficacy of trehalose in reducing traditional physiological, motor, and cognitive HD symptoms. The researchers added trehalose, as well as multiple other non-toxic disaccharides, to the water that the mice drank. Trehalose was shown to be the most effective of the disaccharides in inhibiting protein aggregation of mutant huntingtin, in both the brain and the liver. The researchers hypothesized that this improvement was a result of trehalose’s binding to the multiple polyglutamines that arise from the CAG repeats characteristic of HD and thereby stabilizing the mutant huntingtin protein,arresting it in a partially unfolded state. Although it is still under debate whether aggregate formation in HD is a driver of disease symptoms or the body’s attempt to collectand remove mutant huntingtin from actively harming the body, the results of trehalose on HD symptoms persuasively showed the benefit of keeping mutant huntingtin in this partially unfolded, non-aggregate configuration.
Mutant huntingtin is non-soluble, which makes it difficult to directly screen for potential molecules that can inhibit aggregation in vitro. In this study researchers used mutant sperm whale myoglobin protein that contained the same expanded polyglutamine sequence that is the hallmark of HD to create a cellular model of the disease. With this mutant protein the researchers could screen for inhibitors of protein-aggregation driven by the polyglutamine repeats. They focused exclusively on inhibitors that were non-toxic and had the potential for oral administration as an HD treatment. They found that many disaccharides, but most effectively trehalose, reduced protein aggregate in a dependable and repeatable manner.
The researchers then tested the effects of trehalose in a mouse model of HD, and found that administration of trehalose reduced mutant huntingtin aggregation in a dose-dependent manner without showing any toxic effects when given to the mice as part of their diet. The researchers showed that this reduced aggregation was not the result of production of heat-shock proteins, but rather trehalose’s own abilities to stabilize mutant proteins. Administration of trehalose increased cell survival by more than fifty percent in a cellular model of HD, and in the mouse model was added to the drinking water of the mice in various doses. Trehalose-treated mice had reduced weight loss, and it was shown that 2% trehalose had the most drastic effect on this HD symptom. These mice also had reduced neurodegeneration as seen by comparing atrophy in their striatums to that of control mice. Like in the cellular studies performed previously, trehalose reduced mutant huntingtin aggregates in the brains of these HD mice, as well as in their livers. The mice were then analyzed for motor symptoms. Trehalose improved the rotarod abilities of HD model mice but not non-HD mice, and the mice were better able to pace their steps while walking, as measured by the average distance and width of walking strides. Finally, trehalose extended the lifetime of HD mice in a small but statistically significant manner.
Interestingly, the researchers also tested glucose for effects on HD mice, as trehalose is metabolized into glucose in the body. Glucose did not reduce mutant huntingtin aggregation or extend the lifespan of HD mice, showing that it is trehalose itself that seems to have beneficial effects on HD. By showing that trehalose did not induce the production of heat-shock proteins, the researchers showed that trehalose does not induce responses to cell stress like other small molecules that affect mutant huntingtin aggregation, such as geldanamycin, and the fact that trehalose affected both mutant huntingtin and the myoglobin with polyglutamine repeats suggests that trehalose may bind directly to protein regions of polyglutamine repeats. Because trehalose seems to bind to these regions, it may be that trehalose binding prevents the mutant huntingtin proteins from folding completely, and from misfolding. A non-folded protein is usually not functional, but in this case by preventing protein folding trehalose is preventing the formation of toxic, misfolded proteins that form harmful aggregates. It is also likely that trehalose performs its normal function as a protein stabilizer by keeping the normal copies of the huntingtin protein folded correctly so they can function properly. . In addition, the researchers proposed that trehalose may prevent huntingtin aggregates from entering the nucleus, a process previously shown to be essential to HD progression. It is thought that the stability promoted by trehalose would make the proteins more resistant to being broken down by caspases, which is required for transport of cleaved protein fragments into the nucleus.
From this study the researchers suggest that trehalose reduces mutant huntingtin aggregation not by breaking up aggregates but by preventing their formation in the first place by keeping mutant huntingtin from folding into a shape that allows for aggregate formation. Trehalose is not quickly metabolized into glucose, and so would be available even if administered in low concentrations to perform these beneficial effects in HD patients. Trehalose has great potential to be used therapeutically as it is non-toxic and highly soluble, and shown to be effective when administered orally. The fact that trehalose already is in our diet means that it would not have to undergo a lengthy and arduous process of proving its non-toxicity in clinical trials.
For further reading:^
- Alper, Joe. Delivering Huntington disease its coup de GRAS. 2004. Preclinica 2(3): 162-163. Online.
This short article discusses the commercial and therapeutic uses of trehalose and debates the extension of it to humans. It is of medium difficulty and an important caveat to other highly enthusiastic reactions to trehalose.
- Katsuno et al. Sweet relief for Huntington disease. 2004. Nature Medicine 10: 123-124. Online.
This article comments on HD and other polyglutamine diseases and the recent study done by Tanaka et al. It is fairly scientific and of medium to high difficulty.
- Pilcher, Helen R. Simple sugar eases Huntington’s disease in mice. 2004. News@nature.com. Online.
This is an easy to understand explanation of the original study of trehalose and HD.
- Tanaka, et al. Trehalose alleviates polyglutamine-mediated pathology in a mouse model of Huntington disease. 2004. Nature Medicine 10: 148-154. Online.
This is the report written by the researchers who conducted the original study on trehalose and HD. It is highly technical and recommended only for a scientific audience
- Trehalose description and manufacture. Online.
This is a short webpage of medium difficulty that explains what trehalose is and how it is made. It has a diagram of the structural formula.
Updated by A. Lanctot, 11-08-13