We invite you to participate in this "offline" Forum discussion led by Vincent Marchesi of Yale University. Coming from a different research field, Marchesi has in recent years followed the AD literature as closely as have few other outside observers. Earlier this summer, Marchesi published a perspective in PNAS (See Full Text [.pdf]), in which he called on investigators to consider the amyloid hypothesis in a new light.
The amyloid hypothesis commands majority support for its central claim that accumulation of the Aβ peptide plays an important role in AD. Yet Marchesi's ideas come at a time when fundamental questions about this hypothesis remain stubbornly unresolved, slowing down progress toward a deeper understanding and therapeutic approaches. Take advantage of this leisurely format to express your thoughts about Marchesi's article. Do you have supporting evidence? Contradictory evidence? What did Marchesi overlook? How could his hypothesis be tested? We invite you to send questions, comments, critiques, or kudos to Managing Editor Gabrielle Strobel. Gabrielle will post your commentaries and forward them to Marchesi or other participants for their responses.
See Full Text (.pdf). Marchesi VT. An alternative interpretation of the amyloid Abeta hypothesis with regard to the pathogenesis of Alzheimer's disease. Proc Natl Acad Sci U S A. 2005 Jun 28;102(26):9093-8. Copyright © 2005 National Academy of Sciences, U.S.A.
Marchesi, VT. An alternative interpretation of the amyloid Abeta hypothesis with regard to the pathogenesis of Alzheimer's disease. Proc Natl Acad Sci U S A. 2005 Jun 28 ; 102(26):9093-8. Abstract
Abstract: Alzheimer disease is a complex neurodegenerative process that is believed to be due to the accumulation of short, hydrophobic peptides derived from amyloid precursor proteins by proteolytic cleavage. It is widely believed that these Aβ peptides are secreted into the extracellular spaces of the CNS, where they assemble into toxic oligomers that kill neurons and eventually form deposits of senile plaques. This essay explores the possibility that a fraction of these Aβ peptides never leave the membrane lipid bilayer after they are generated, but instead exert their toxic effects by competing with and compromising the functions of intramembraneous segments of membrane-bound proteins that serve many critical functions. Based on the presence of shared amino acid sequences containing GxxxG motifs, I speculate that accumulations of intramembraneous Aβ peptides might affect the functions of amyloid precursor protein itself and the assembly of the PS1, Aph1, Pen2, nicastrin complex.
For further background, read this news summary of the PNAS article:
Marchesi sets up his argument by first laying out a brief synopsis of the main points of consensus in the field. For example, the plaques scattered throughout various brain areas of AD patients comprise many components, most prominently the Aβ40 and 42 peptides generated by cleavage of the APP transmembrane protein. The concerted action of the secretases BACE and γ-secretase releases Aβ peptides, and it is their overproduction or underremoval that is thought to lead to accumulation, extracellular aggregation, neuronal dysfunction, and eventually neuronal death. While Aβ's involvement in the disease is beyond serious dispute, how and where it acts remains unclear, Marchesi writes. Recent research has cast doubt on the conventional notion that the deposits are to blame for the early disease processes, and attention has shifted toward small forms of Aβ, often called soluble oligomers. How they act remains mysterious, and it is this question Marchesi's new ideas address.
Next, Marchesi notes that the fewer than 5 percent of early-onset AD cases who have mutant forms of APP or presenilin—widely thought to encode part of the γ-secretase—probably suffer from an accelerated form of the same underlying pathogenic process that operates in the more prevalent sporadic, late-onset forms. Transgenic animals expressing a variety of mutant forms alone or in combination now exist. They all develop massive Aβ deposits that resemble human pathology, but do not show the other pathologic hallmark of AD, that is, neurofibrillary tangles made of the protein tau. The animals' neurologic and neurodegenerative defects are subtle and vary from strain to strain. A triple transgenic mouse strain expressing mutant versions of APP, presenilin, and tau does develop amyloid deposits and neurofibrillary tangles. In this way, it models the human disease more fully, though it's worth noting that no human with such a heavy genetic burden has as yet been described (Oddo et al., 2003).
The triple transgenic mouse strain confirmed an earlier observation by others that the initial manifestations of Aβ accumulation begin inside neurons, not in extracellular spaces. The earliest detectable material resides in membrane compartments, possibly lysosomes or endosomes. Marchesi further cites a separate claim that APP molecules exist as homodimers inside neuronal plasma membranes in the brain (Scheuermann et al., 2001). The APP dimers are sequestered away in specific "raft" domains that are enriched for cholesterol and sphingolipids, and are thought to affect the regulation of protein-protein interactions. Moreover, these neuronal membrane patches also appear to contain Aβ dimmers (Kawarabayashi et al., 2004), and it is this observation from which Marchesi develops his hypothesis. While its discoverers interpret the intramembraneous Aβ dimers as being on their way to secretion and extracellular accumulation, Marchesi proposes that they could just as well stay inside the neuronal membrane for long periods of time.
To support this notion, Marchesi compares the intramembraneous APP sequence to that of another dimerizing transmembrane protein that is better studied. He suggests that both proteins derive their intramembraneous stability in a similar way, whereby a shared GxxxG motif recruits van de Waals forces and hydrogen bonds such that the transmembrane helices can pack closely and remain dimerized.
There already is a hypothesis dealing with Aβ inside neuronal membranes. It holds that secreted Aβ reinserts itself into the membrane in a channel-like structure (see ARF Live Discussion). In his perspective, Marchesi points to structural and biochemical questions that it still needs to address. He further argues that all arguments supporting the claim that Aβ peptides enter membranes from the outside equally well support the notion that they need not exit the membrane in the first place. They do eventually accumulate outside cells as the disease progresses, but perhaps they are not promptly secreted merely as a consequence of APP cleavage. In fact, the simplest interpretation of Kawarabayashi et al. is that secretase cleavage of APP dimers generates Aβ dimers, of which a significant fraction remains in the membrane, Marchesi contends.
Next, Marchesi asks how Aβ peptides accumulating inside membranes could affect the function of neurons. While APP dimers are anchored to particular sites within the membrane, Aβ peptides are not, and presumably would drift through the plane of the membrane. "It is easy to imagine how such peptides could influence the behavior of intramembraneous segments of receptors of channels, or even intramembraneous segments of enzymes like the presenilins and other secretases," Marchesi writes. He speculates that Aβ peptides inside the membrane could compete with normal APP dimer formation, displacing full-length APP monomers and creating chimeric dimers via their common GxxxG domains. These could be preferentially cleaved to generate more intramembraneous Aβ.
Alternatively, Aβ peptides might destabilize the γ-secretase complex, Marchesi speculates. The GxxxG motif is important in the transmembrane segment of Aph1, a protein that stabilizes the γ-secretase complex by linking its components together. Intramembraneous Aβ peptides could associate with Aph1 or compete for its binding partners, forming heterodimers of various sorts. This is not unheard of in membrane biochemistry. This second speculation could shed new light on a related debate within the field, Marchesi notes. The debate swirls around the question of whether presenilin mutations in familial AD cause well-documented increases in Aβ levels by way of a gain of function, or maybe also through a partial loss of regulatory functions of the γ-secretase complex.
There are broader implications of this scenario, Marchesi notes. It has become clear in recent years that many enzyme reactions occur within membranes, some through a process called regulated intramembraneous cleavage (RIP). Signal transduction mechanisms underlie control by RIP, for example, Notch cleavage. And the γ-secretase complex affects neuronal dysfunction in ways other than generating excessive amounts of Aβ. If tested and confirmed, this perspective of Aβ pathogenesis would call for a new therapeutic approach that aims to inhibit interactions between hydrophobic peptides within a bilayer. While that prospect seems daunting, other fields have already begun exploring it with some success, Marchesi writes. For example, two labs have described peptide segments that correspond to the intramembraneous domain of the transmembrane proteins PDGF or the Erb B receptor, respectively, and modulate their dimerization (Freeman-Cook et al., 2004; Bennasroune et al., 2004). Another possibility worth exploring lies in modulating the lipid membrane itself, Marchesi concludes.—Gabrielle Strobel.