One of the chicken-and-egg questions of the medical world is whether amyloid plaques cause Alzheimer’s or Alzheimer’s causes amyloid plaques. Results published in the March 2, 2006, issue of Neuron provide evidence in support of the first possibility.
In the paper, scientists from the University of California, Irvine and the Israel Institute for Biological Research in Ness-Ziona show that targeting amyloid plaques is an effective approach to treating Alzheimer’s, following up on earlier work by the same group suggesting that amyloid plaques cause neurofibrillary tangles and both together cause the cognitive deficits seen in Alzheimer’s disease. (See BioWorld Today, Aug. 12, 2004.)
The study used triple-transgenic mice that overexpress mutant forms of human APP and tau protein (the culprits in amyloid plaques and neurofibrillary tangles, respectively), and have the mutant presenilin-1 gene knocked in. The researchers found that an agonist of a specific acetylcholine receptor subtype, known as M1, reduced amyloid plaques and tau protein tangles in brain regions associated with learning and memory. The treatment also reversed some of the cognitive deficits that the transgenics normally show. Mechanistically, the compound was effective when injected into the bloodstream; it appeared to work by making it more likely that amyloid precursor protein would be processed in a way that cuts the A-beta sequence in half, leading to proteins that lack the structural features necessary for clumping.
M1 agonists have failed in clinical trials before; the researchers speculate that their compound was successful because their molecule crosses the blood-brain barrier easily, and is more selective for the M1 receptor. Co-author Abraham Fischer also pointed out that previous muscarinic agonists "had a narrow safety window (or margin) and some of them also had poor bioavailability."
San Diego-based biotech company TorreyPines Therapeutics announced last week that it is initiating a multidose Phase I trial with the compound, NGX267; in an earlier single-dose trial, the compound was well tolerated.
Amyloid aggregates make the news most often as culprits in neurodegeneration. But technically speaking, amyloid aggregates can be made up of a number of different proteins. Membership in the amyloid club depends on a specific structural characteristic, namely, the existence of multiple so-called beta-sheets in proteins. In fact, "the amyloid structure itself is quite interesting - it self-assembles, and it is very strong," Andrew Miranker, associate professor of molecular biophysics and biochemistry at Yale University in New Haven, Conn., told BioWorld Today. And though most amyloid fibers are signs of disease, recent research also has shown that some organisms may make use of those characteristics for normal cell functions.
In fact, since beta-sheets are a common feature of proteins, just about any protein can be assembled into an amyloid fibril using sufficiently extreme test tube conditions. Extreme test tube conditions are usually what it takes in the lab; what makes proteins link up as amyloid aggregates under physiological conditions still is pretty much a mystery.
Two papers published in Nature Structural and Molecular Biology, both now available online, try to untangle the factors leading to amyloid aggregation.
Both studies, one by scientists at Yale and one by a British group at the University of Leeds, used the protein beta-2-microglobulin, or B2M, which forms plaques in dialysis-related amyloidogenesis.
Both studies identify the state of a specific proline in the protein’s backbone as important in determining whether B2M will form plaques or not. The Yale researchers began with the observation that under physiological conditions, copper is a catalyst that allows B2M to form amyloid fiber precursors called oligomers that will remain stable.
As the oligomers are thought to be toxic - possibly even more toxic than the final amyloid fibers - either preventing their formation or catalyzing the formation of amyloid fibers from oligomers could possibly be useful therapeutic approaches.
Using X-ray crystallography, they showed that when B2M binds copper, the proline can change from a so-called trans-form (where two specific groups of the backbone point in opposite directions) to a cis-form (where they point in the same direction).
"It’s a pretty wrenching change to the backbone," senior author Miranker said. In fact, it is somewhat surprising that the whole protein does not come apart. Instead, the conformation change propagates through the molecule and enables two sheets of B2M to form an asymmetric bond; these double sheets in turn have a hydrophobic side that can be buried by assembly with other double sheets, leading to the formation of longer amyloid fibers.
The copper has two roles," Marinker said. "It catalyzes the conformational change, and then it transiently acts as a tether" between two beta-sheets.
The Leeds group used a series of folding and unfolding studies under various conditions to guide them to the proline in question; they also found that amyloid fibers formed when that proline goes from cis to trans form. The Leeds group suggests that though the trans form is a high-energy state and thus rare in nature, fibers nevertheless can form when there is a high concentration of amyloid proteins overall, as is the case in dialysis-related amyloidosis.