Neuroscience

Alzheimer’s disease is the most common cause of late-life dementia. The disorder is thought to be caused by a protein known as amyloid-beta, or Abeta, which clumps together in the brain, forming plaques that are thought to destroy neurons. This destruction starts early, too, and can presage clinical signs of the disease by up to 20 years.

For decades now, researchers have been trying, with limited success, to develop drugs that prevent this clumping. Such drugs require a “target” — a structure they can bind to, thereby preventing the toxic actions of Abeta.

Now, a new study out of UCLA suggests that while researchers may have the right target in Abeta, they may be missing the bull’s-eye. Reporting in the Jan. 23 issue of the Journal of Molecular Biology, UCLA neurology professor David Teplow and colleagues focused on a particular segment of a toxic form of Abeta and discovered a unique hairpin-like structure that facilitates clumping.

"Every 68 seconds, someone in this country is diagnosed with Alzheimer’s," said Teplow, the study’s senior author and principal investigator of the NIH-sponsored Alzheimer’s Disease Research Center at UCLA. "Alzheimer’s disease is the only one of the top 10 causes of death in America that cannot be prevented, cured or even slowed down once it begins. Most of the drugs that have been developed have either failed or only provide modest improvement of the symptoms. So finding a better pathway for these potential therapeutics is critical."

The Abeta protein is composed of a sequence of amino acids, much like “a pearl necklace composed of 20 different combinations of different colors of pearl,” Teplow said. One form of Abeta, Abeta40, has 40 amino acids, while a second form, Abeta42, has two extra amino acids at one end.

Abeta42 has long been thought to be the toxic form of Abeta, but until now, no one has understood how the simple addition of two amino acids made it so much more toxic than Abeta40.

In his lab, Teplow and his colleagues used computer simulations in which they looked at the structure of the Abeta proteins in a virtual world. The researchers first created a virtual Abeta peptide that only contained the last 12 amino acids of the entire 42–amino-acid-long Abeta42 protein. Then, said Teplow, “we just let the molecule move around in a virtual world, letting the laws of physics determine how each atom of the peptide was attracted to or repulsed by other atoms.”

By taking thousands of snapshots of the various molecular structures the peptides created, the researchers determined which structures formed more frequently than others. From those, they then physically created mutant Abeta peptides using chemical synthesis.

"We studied these mutant peptides and found that the structure that made Abeta42 Abeta42 was a hairpin-like turn at the very end of the peptide of the whole Abeta protein," Teplow said.

The hairpin turn structure was not previously known in the detail revealed by the researchers, “so we feel our experiments were novel,” he said. “Our lab is the first to show that it is this specific turn that accounts for the special ability of Abeta42 to aggregate into clumps that we think kills neurons. Abeta40, the Abeta protein with two less amino acids at the end of the protein, did not do the same thing.”

Hopefully, the work of the Teplow laboratory presents what may the most relevant target yet for the development of drugs to fight Alzheimer’s disease, the researchers said.

Chronic drinking is known to have detrimental health effects such as cardiac and liver problems, cognitive impairments, and brain damage. Binge drinking in particular is known to increase the risk of developing dementia and/or brain damage, yet little is known about an exact threshold for the damaging effects of alcohol. A study using rodents to examine various markers of neurodegeneration has found that brain damage can occur with as little as 24 hours of binge-like alcohol exposure.

Results will be published in the March 2013 issue of Alcoholism: Clinical & Experimental Research and are currently available at Early View.

"We know that the extent of damage following alcohol exposure depends heavily on the manner in which it is consumed," said Kimberly Nixon, associate professor of pharmaceutical sciences at The University of Kentucky as well as corresponding author for the study. "Human studies suggest that binge-pattern drinking is more closely associated with brain damage. One study, for example, reported that binge drinking at least once per month in adulthood significantly increases the risk of developing dementia later in life. Animal models help provide the critical information that binge drinking, which produces high blood alcohol levels, directly causes damage."

"The exact threshold for the damaging effects of alcohol on the brain is unclear," commented Fulton T. Crews, John Andrews Distinguished Professor and director of the Center for Alcohol Studies at the University of North Carolina. "It is likely that the higher the blood alcohol level the greater the damage, however, this manuscript only studies binge drinking, using vimentin and flurojade B as markers of neurotoxicity."

"People hear from multiple sources that low-moderate alcohol consumption can be beneficial, and then we come along and say that heavy alcohol use leads to detrimental outcomes," said Nixon. "People then want to know what the line is between beneficial and detrimental Unfortunately, we don’t know exactly. However, our study suggests that it may be even less than previously thought."

Nixon and her colleagues administered a nutritionally complete liquid diet to adult male Sprague-Dawley rats that additionally contained either alcohol (25% w/v) or isocaloric dextrose every eight hours for either one or two days. The rodents were sacrificed immediately following, two days after, or seven days after alcohol exposure and their brain tissues were examined.

"This was really a simple study that took advantage of some new ‘tools’ to look for evidence of brain damage," explained Nixon. "In other words, we didn’t look for dying cells themselves, but we looked at more indirect indices of damage by looking at what happens to astroglia, one of the ‘supporting’ cells for neurons. Astroglia react to brain damage by expressing several proteins that they do not normally express under healthy, happy conditions, one of which is an intermediate filament protein called vimentin. We saw a remarkable number of cells expressing this marker It is one of those ‘here is your brain, here is your brain on drugs’ kind of findings where the expression was obvious to the naked eye in many brains with as little as 24 hours of high blood alcohol levels."

Nixon added that, because rodents metabolize alcohol significantly faster than humans do, it is important to look at the actual concentration of alcohol in the blood in order to translate this to the human condition. “These rats had blood alcohol levels that were more than four times the legal driving limit, which for humans would require excessive drinking in the nature of a 12-pack of beer, a couple bottles of wine, or half of fifth of whisky. Unfortunately, drinking self-reports and blood alcohol level data from emergency rooms confirm that this level of drinking is common in those with alcohol use disorders.”

"Rodent brain damage can model human damage," noted Crews. "Vimentin seems to be a good marker of glial activation that shows that one day of binge drinking can cause some brain damage that persists and grows after a week of abstinence. However, both rodent and human brain damage generally require long-term alcohol consumption that models alcoholism and not the acute responses studied in this manuscript."

Nixon agreed. “The lack of overt neuronal deterioration suggests that a single, short-term, high-level binge probably does not result in functional changes and/or cognitive deficits,” she said. “However, since alcoholics experience multiple binges throughout their lifetime, it is important to consider that each successive binge, starting with the very first one, affords some level of damage to the brain. Therefore, theoretically, with multiple binges comes a cumulative detrimental effect where pronounced cognitive, behavioral, and structural effects are observed.”

Nixon said this study demonstrates that new discoveries are always possible. “You have to know where and when to look for some of these effects,” she said. “The reason why this discovery wasn’t made previously is merely due to groups, ourselves included, not taking the time to thoroughly investigate these lower threshold doses with some pretty specific time points. Chasing down a threshold is not a sexy topic and it was actually fairly risky in that it was possible that we would have had all negative effects. Nonetheless, the take-home message of our data is that even one short-duration binge-alcohol experience – which is unfortunately similar to what young adults may experience during spring break or weekend partying - may start a cascade that leads to brain damage.”