A potential Alzheimer’s treatment aims to whisk the amyloid precursor protein (APP) out of harm’s way to prevent it from generating Aβ. The small molecule fortifies the retromer, a large protein conglomerate that ferries APP out of endosomes and away from the clutches of BACE1, the enzyme that initiates formation of Aβ. Scientists led by Dagmar Ringe, Brandeis University, Waltham, Massachusetts; Gregory Petsko, now at Weill Cornell Medical College, New York; and Scott Small, Columbia University, New York, report April 20 in Nature Chemical Biology that their new compound pre-empted production of Aβ and other toxic fragments of APP in cultured neurons, and instead nudged APP toward processing by α-secretase.
“This paper represents the combination of expert structural biology and AD cell biology to produce simultaneous advances in both arenas,” James Lah of Emory University School of Medicine, Atlanta, wrote to Alzforum in an email (see full comment below). “Their results demonstrate rather convincingly that the identified compound is capable of stabilizing the retromer complex.”
On its way from the cell membrane to other cell compartments for processing, APP stops over in the endosome. From there, the retromer shunts APP to the trans-Golgi network or back to the cell surface. However, if the retromer falters, APP lingers in endosomes, where it is at the mercy of BACE1. Small’s group previously reported that dysfunction of the retromer leads to elevated Aβ levels in cultured neurons (see Bhalla et al., 2012). Other researchers have found that variants of retromer-related genes elevate the risk for AD (see Vardarajan et al., 2012). Conversely, previous studies suggested that boosting the amount of retromer in the cell enhances its ability to traffic proteins (see MacLeod et al., 2013).
Based on that idea, first author Vincent Mecozzi and colleagues reasoned that if they could stabilize retromer, they might protect it from degradation, increase its presence in the cell, and help move more APP out of the endosome. The researchers turned to structure-based drug design to find a small molecule that could help hold retromer together. A virtual model of the complex helped identify a handful of pockets where a small molecule could bind and reinforce the connection between subunits. Then, the scientists virtually screened these sites against a library of compounds to see which would fit best and to pick some for testing in an actual chemical screen. They ended up testing 24 of these chemicals with retromer in vitro. One, R55, stabilized the complex, raising its melting temperature by 10 degrees Celsius.
To see what R55 did in cells, the researchers tested it in cultured hippocampal neurons from wild-type and from J20 transgenic mice. The compound increased the amount of retromer complex and reduced levels of Aβ40 and Aβ42. It also lowered the APP fragments that result from BACE1 cleavage and raised those generated by α-secretase processing (see image below). Immunohistochemistry assays confirmed that R55 reduced APP levels in the endosome.
R55 nestles between retromer subunits (turquoise/orange) and cements their connection, lowering β- and raising α-secretase cleavage of APP. [Image courtesy of Mecozzi et al., Nature Chemical Biology.]
“We see this as the first step in a drug-discovery program,” said Small. The researchers are now conducting preclinical research on R55 and its analogs, to see if they are stable in serum, can cross the blood-brain barrier, and whether they are safe in animal models. “Everything looks promising,” said Small, but he added that results are preliminary. He, Petsko, and Ringe are planning to develop this further.
In an email to Alzforum, Samuel Gandy, Mount Sinai Alzheimer's Disease Research Center, New York, praised the work and supported clinical development. However, he doubted that a single compound would provide a silver bullet for Alzheimer’s. “That simple approach made sense before we had the computational tools to begin to address the true complexity of diseases,” he wrote in an email to Alzforum (see full comment below). He and other scientists interviewed for this article also agreed that interfering with such a ubiquitous complex as the retromer is likely to have side effects.
Small conceded the point, noting that retromer is found in every cell in the body. However, he said that preliminary evidence suggests that just a slight boost in retromer stability with R55 or its analogs is not toxic to animal models. He added that a small dose of a retromer-stabilizing compound could be added to a more target-specific therapy, such as a BACE inhibitor, to produce an additive effect, while at the same time minimizing toxicity of either compound.
Why not stick with the BACE inhibitors currently under development (see Dec 2013 news story)? Small points out that even if a therapy successfully shifted APP processing, defects in endosomal function would persist. Also, previous studies suggest that retromer dysfunction may underlie a number of disorders, including Parkinson’s disease (see Jul 2011 news story). That suggests a compound like R55 could help treat a number of diseases. He is currently collaborating with Asa Abeliovich, also at Columbia University, to test the compounds in a mouse model of Parkinson’s.
“That a chemical chaperone can restore functional retromer activity is a remarkable proof of concept,” Matthew Farrer, University of British Columbia, Vancouver, Canada, wrote to Alzforum in an email (see full comment below). “Further research and development might have broad clinical applications.”—Gwyneth Dickey Zakaib