A Cambridge, U.K.-based team has identified a process that causes autophagy not to work properly in the brains of mouse models of Huntington’s disease and a form of dementia, and also found a drug that helps restore this vital function.
The team showed that in neurodegenerative diseases, microglia release a suite of molecules that in turn activate a switch on the surface of cells. When activated, this switch, called CCR5, impairs autophagy, and hence the ability of the brain to rid itself of the toxic proteins. These proteins then aggregate and begin to cause irreversible damage to the brain. According to the team, the toxic proteins also create a feedback loop, leading to increased activity of CCR5, enabling even faster build-up of the aggregates.
Professor David Rubinsztein from the U.K. Dementia Research Institute at the University of Cambridge, senior author of the study published in Neuron, said: “The microglia begin releasing these chemicals long before any physical signs of the disease are apparent. This suggests, much as we expected, that if we’re going to find effective treatments for diseases such as Huntington’s and dementia, these treatments will need to begin before an individual begins showing symptoms.”
When the researchers used mice bred to knock out the action of CCR5, they found that these mice were protected against the build-up of misfolded huntingtin and tau, leading to fewer of the toxic aggregates in the brain when compared to control mice.
This discovery has led to clues to how this build-up could in future be slowed or prevented in humans. The CCR5 switch is not just exploited by neurodegenerative diseases, it is also used by HIV as a ‘doorway’ into our cells. In 2007, the U.S. and European Union approved a drug known as maraviroc, which inhibits CCR5, as a treatment for HIV.
The team used maraviroc to treat the Huntington’s disease mice, administering the drug for four weeks when the mice were two months old. When the researchers looked at the mice’s brains, they found a significant reduction in the number of huntingtin aggregates when compared to untreated mice. However, as Huntington’s disease only manifests in mice as mild symptoms by 12 weeks even without treatment, it was too early to see whether the drug would make an impact on the mice’s symptoms.
The same effect was observed in the dementia mice. In these mice, not only did the drug reduce the amount of tau aggregates compared to untreated mice, but it also slowed down the loss of brain cells. The treated mice performed better than untreated mice at an object recognition test, suggesting that the drug slowed down memory loss.
Professor Rubinsztein added: “We’re very excited about these findings because we’ve not just found a new mechanism of how our microglia hasten neurodegeneration, we’ve also shown this can be interrupted, potentially even with an existing, safe treatment.
“Maraviroc may not itself turn out to be the magic bullet, but it shows a possible way forward. During the development of this drug as a HIV treatment, there were a number of other candidates that failed along the way because they were not effective against HIV. We may find that one of these works effectively in humans to prevent neurodegenerative diseases.”