19 May 2006. Two speakers, Charles Glabe and Mathias Jucker, focused on early aspects of Aβ aggregation. Glabe, of University of California, Irvine, proposed that oligomers of proteins with different amino acid sequences—and linked to a range of amyloid diseases—share not only common structures and pathways of aggregation. They also wreak havoc in basically the same way; that is, they likely assemble into pore-like structures inside membranes (Glabe, 2006; Glabe and Kayed, 2006; see also ARF Live Discussion on amyloid channels). A lesson emerging from Glabe’s newest research is that prefibrillar oligomers are not a required step toward fibril formation. In other words, the classic pathway of monomer-dimer-oligomer-protofibril-fibril-plaque is but one of several modes of aggregation; at least one other pathway exists beside it.
Glabe noted that evidence pointing to oligomeric forms of amyloid proteins as the primary toxic species in their respective disease is growing. Even so, researchers continue to wrestle with the technical challenges of distinguishing one type of oligomer from another, and from monomer and fibrils, for that matter, in tissues containing complex mixtures of the many incarnations of a given protein.
Glabe’s laboratory uses conformation-specific antibodies to pick these forms apart, and new ones the group has generated are proving useful in describing one such alternate aggregation pathway. Their previous antibody, A11, recognizes prefibrillar oligomers, not monomer or fibrils, of a growing number of amyloidogenic proteins, 24 to date (Kayed et al., 2003; Luibl et al., 2006; Sanbe et al., 2005). Its structural antigen appears to be a precursor of a later aggregate, an annular form that makes troublesome pores in membranes much in the way Hilal Lashuel and Peter Lansbury have proposed (Lashuel et al., 2002). These ring-shaped protofibrils are what the new antibody recognizes, and it does so, again, for a range of proteins of different amino acid sequences, Glabe said. The lab dubbed this new polyclonal “Officer,” a playful reference to the American policeman’s stereotypical fondness for donuts. (The patrol cars frequently seen parked outside donut shops don’t reflect elevated crime rates in those establishments, but in the minds of Glabe’s group, the Officer antibody might yet pull a perpetrator of a different sort.)
Separate aggregation pathways imply that a monomer of, for example, Aβ or α-synuclein, can undergo at least two distinct conformation changes. One would lead it down the classic pathway of forming a fibril nucleus and then fibrils. The other conformation would lead via A11-positive prefibrillar nuclei, which in the atomic force microscope look like spherical blobs, to the Officer-positive protofibrils, which appear as smooth rings. Preliminary data suggest that immunoprecipitation with the Officer antibody isolates pore-like structures from human AD brain but not control, Glabe said. The A11 and Officer antibodies do not stain amyloid deposits in AD or, for example, diabetes tissue, but a third antibody that is specific for a generic fibrillar epitope does. Called OC, its tissue staining is particularly stark as it comes up blank in control tissue. “This is purely a pathogenic epitope,” Glabe said. OC reaction with brain extracts correlates with the affected patient’s MMSE rating before death, but A11 does not. In LaFerla’s triple-transgenic mice, the OC antibody recognizes certain intraneuronal Aβ deposits associated with autophagic vesicles.
Exposing Aβ prefibrillar oligomers to liposomes reduces their A11 staining but brings up Officer staining, suggesting that lipids enable the prefibrillar oligomer to assemble into pores. Taken together, ongoing work suggests that A11 recognizes a misfolded intermediate on the way to pore formation. With pores poking tiny holes in their membranes and changing the cell’s permeability to various agents, all cells will have trouble maintaining their ion homeostasis and metal-dependent chemistry such as antioxidant processes, said Glabe, but none more so than neurons, which depend critically on functional neurotransmission (Kayed et al., 2004). Finally, Glabe added, comparing the structural antigens for these three antibodies has taught the scientists that size does not indicate conformation. Some prefibrillar and fibrillar oligomers have the same size but distinct conformations, and they will go down separate aggregation pathways.
Mathias Jucker, of the University of Tuebingen, Germany, expanded on an ongoing study aimed at identifying the particular form of Aβ that is able to “seed” amyloid deposition. For a summary of this line of research, see prior ARF conference report. Jucker asked what it is about Aβ’s conformation, or cofactors imparting that conformation, that enables some forms of Aβ to seed deposition in a tissue that is ripe with rising Aβ accumulation but still short of laying down actual amyloidosis itself. Jucker has searched for the winning concoction by testing a range of different forms of Aβ. Since the prior report, preparations by John Fryer and David Holtzman, Gerd Multhaup, Rakez Kayed and Charlie Glabe, Uli Ebert and Heinz Hillen all came up negative. Likewise, the cell-secreted oligomers developed by Dominic Walsh, and used in LTP, behavior, and spine imaging studies, have not, so far, seeded deposition in APP-transgenic backgrounds, although it should be noted that the scientists have not so far concentrated the oligomers to quite the level where they would expect to see seeding occur. (The newest contender, Karen Ashe's Aβ*56, has not been tested.) By contrast, AD brain extract seeded powerfully in Lary Walker’s hand, as do various AD-transgenic extracts in Jucker’s (Walker et al., 2002).—Gabrielle Strobel.
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