Scripps Florida Researchers Find New Clue to Alzheimer's Disease Progression

By Eric Sauter

Scientists from The Scripps Research Institute's Florida campus have shown for the first time that a specialized form of RNA is directly linked to increased levels of amyloid plaque in the brains of Alzheimer's patients.

"Our in vitro as well as in vivo studies show that a previously unknown molecule, BACE1-AS, regulates a critical mechanism associated with Alzheimer's disease, and may turn out to be key to the pathological progression of the disease," said Claes Wahlestedt, a Scripps Florida professor who led the study.

The study shows that a noncoding antisense form of RNA – an RNA that does not encode a protein– controls the expression of b-secretase-1 (BACE1), an enzyme critical to Alzheimer's disease progression. This noncoding antisense RNA, termed BACE1-AS, could be an attractive target for potential new diagnostic as well as therapeutic approaches.

The study is published in the July 2008 issue of the journal Nature Medicine.

The new study's findings show that when BACE1-AS (pronounced base) is exposed to stress, it stabilizes BACE1 messenger RNA, increasing expression of BACE1 and leading to higher levels of amyloid-b 1-42, the peptide believed to be the primary cause of Alzheimer's disease. Importantly, the levels of BACE1-AS were sharply elevated in brains from deceased individuals who had suffered from Alzheimer's disease.

"We show that BACE1-AS could be a potential disease biomarker as well as drug target, well suited to mediate the balance between the essential physiological functions of BACE1 and its pathological malfunction in early Alzheimer's disease," Wahlestedt said.

In the study, Wahlestedt and his colleagues, including first author Mohammad Ali Faghihi of Scripps Research and The Karolinska Institute, used a synthetic small interfering RNA (siRNA) – which can inhibit gene expression – to decrease BACE1-AS expression in animal models and reduce amyloid generation successfully.

"Those animal model experiments support the validity of a siRNA approach," he said. "Recent technological breakthroughs suggest that systemic administration of modified siRNAs, which cross the blood-brain barrier, could in fact target RNA transcripts there. Alternatively, proteins involved in BACE1-AS localization or turnover could also become targets for potential therapeutics."

A Build Up of Plaque

There is a significant amount of evidence that the build up of amyloid plaque begins very early in the disease. "Our study supports that hypothesis," Wahlestedt noted. "But we've taken it one step further, identifying an underlying control mechanism that plays a major role in that build up. We don't have the entire solution yet, but we have moved our understanding of it much further upstream."

Since BACE1-AS regulates BACE1 expression in vivo, Wahlestedt and his colleagues concluded that any elevation of BACE1-AS, resulting from the actions of related cell stress, forms the basis of a dangerous feed-forward cycle of Alzheimer's progression.

"We call it 'feed forward' because it turns into a vicious cycle," he said. "You just get more and more amyloid-b 1-42 accumulation which seems to further stimulate its own production. What makes it so dangerous is that even initial small changes in BACE1 activity may snowball to a long-lasting and chronic process of amyloid-b 1-42 accumulation in the brain. Our current findings provide further evidence for a feed-forward mechanism of stress-dependent and activity-dependent amyloid-b 1-42 production."

Noncoding anti-sense RNAs like BACE1-AS complement other forms of RNA and are useful in regulating the specific gene to which they are complementary. 

A Narrow Window

The conundrum of the last decade or so, Wahlestedt said, is the fact that out of the three billion base pairs of DNA in the human genome, only slightly more than one percent of them actually produce proteins.

"But at the same time over 90 percent of the genome is active and perhaps particularly so in the brain – so what's going on?" he said. "What are all these seemingly extraneous parts for?"

The study shows not only how complicated the human genome actually is, but just how important those so-called extraneous parts can be in the control of complicated processes.

"The amyloid produced by BACE1 influences key aspects of the disease progression," Wahlestedt said. "There appears to be some subtle but critical boundary between normal BACE1 physiology – cross that boundary and BACE1 turns pathological. The role of BACE1-AS is to help regulate the enzyme, to allow it to perform its normal physiological functions while avoiding the serious consequences of over or under expression."

Different cell stressors have long been implicated in the pathogenesis of Alzheimer's disease. In the study, Wahlestedt and his colleagues exposed cells to a number of them including hyperthermia, serum starvation, and two forms of amyloid b, hydrogen peroxide and high glucose.

"We found that exposure of the cells to high temperature, serum starvation, Ab 1-42, and high glucose resulted marked increases in levels of BACE1-AS as well as BACE1 levels," Wahlestedt said. "Serum starvation generated the strongest response, while Ab 1-40 did not significantly alter BACE1-AS expression levels. This suggests that many, but not all, cell stressors may contribute to the pathogenesis of the disease by altering BACE1-AS expression and subsequently BACE1 enzyme activity."

The contrast between BACE1's essential role in cognitive, emotional, and synaptic functions and its pathophysiological malfunction in Alzheimer's disease highlights the regulatory complexity of this protein, Wahlestedt said. Because of the severe consequences of any malfunction, tight regulatory control must be maintained over BACE1 gene expression.

"The stressors are important," Wahlestedt said, "and they become increasingly important as we get older. When we're young, our cells can deal with them more effectively. As we age, our cellular resiliency declines.

"Considering the narrow window between essential and excessive levels of the BACE1 protein, we believe that neuronal cells must maintain precise physiological regulation of BACE1 expression by utilizing both pre- and post-transcriptional regulatory mechanisms," he continued. "The non-coding RNA antisense transcript, BACE1-AS, appears to function as a regulatory component of this machinery."

In addition to Wahlestedt and Faghihi, authors of the study, A Noncoding RNA Is Elevated In Alzheimer's Disease And Drives Rapid Feed-Forward Regulation Of b-Secretase Expression, are Farzaneh Modarresi, Ahmad M. Khalil, and Paul Kenny of The Scripps Research Institute; Douglas E. Wood and Barbara G. Sahagan of Pfizer Global Research and Development; Todd E. Morgan and Caleb E. Finch of the University of Southern California, Los Angeles; and Georges St. Laurent III of the George Washington University Medical Center and the University of Antioquia, Medellín, Colombia. For more information, see http://www.nature.com/nm/journal/v14/n7/abs/nm1784.html.

The study was supported by the National Institutes of Health and Ahwaz University of Medical Sciences.

Send comments to: mikaono[at]scripps.edu

The research team proposes a "vicious cycle" in plaque build-up, in which BACE1-AS mediated regulation of BACE1 is shifted in Alzheimer's disease. Click to enlarge.