The classic villain in Alzheimer’s disease is ß amyloid, a protein fragment that can misfold and form sticky plaques around neurons in the brain. Now, a new study in mice and worms supports a controversial hypothesis that the plaques may not be all bad. ß amyloid’s tendency to choke neurons could be linked to an ancient evolutionary mission to protect the brain from pathogens, the authors say.
Details of the new study appeared May 25 in the journal Science Translational Medicine, in an article entitled, “Amyloid-β Peptide Protects against Microbial Infection in Mouse and Worm Models of Alzheimer’s Disease.” The article suggests that the tendency of Aβ protein to form insoluble aggregates is not, as has been widely assumed, intrinsically abnormal, even though the aggregates are recognized as a hallmark of Alzheimer’s disease. Rather, Aβ protein appears to be a natural antibiotic that can trap and imprison bacterial pathogens that manage to pass the blood–brain barrier, which becomes increasingly “leaky” with age.
“We present in vivo data showing that Aβ expression protects against fungal and bacterial infections in mouse, nematode, and cell culture models of AD,” wrote the article’s authors. “We show that Aβ oligomerization, a behavior traditionally viewed as intrinsically pathological, may be necessary for the antimicrobial activities of the peptide.”
The MassGen scientists and their colleagues found that transgenic mice expressing human Aβ survived significantly longer after the induction of Salmonella infection in their brains than did mice with no genetic alteration. Mice lacking the amyloid precursor protein died even more rapidly. Transgenic Aβ expression also appeared to protect C. elegans roundworms from either Candida or Salmonella infection. Similarly, human Aβ expression protected cultured neuronal cells from Candida. In fact, human Aβ expressed by living cells appears to be 1000 times more potent against infection than does the synthetic Aβ used in previous studies.
That superiority appears to relate to properties of Aβ that have been considered part of Alzheimer’s disease pathology—the propensity of small molecules to form oligomers and then aggregate into Aβ plaques. This propensity, suggests the MassGen-led team, may indicate that Aβ acts like an antimicrobial peptide (AMP).
While AMPs fight infection through several mechanisms, a fundamental process involves forming oligomers that bind to microbial surfaces and then clump together into aggregates that both prevent the pathogens from attaching to host cells and allow the AMPs to kill microbes by disrupting their cellular membranes. The synthetic Aβ preparations used in earlier studies did not include oligomers. In the current study, however, oligomeric human Aβ not only showed an even stronger antimicrobial activity, its aggregation into the sorts of fibrils that form Aβ plaques was also seen to entrap microbes in both mouse and roundworm models.
“Our findings raise the intriguing possibility that β-amyloid may play a protective role in innate immunity and infectious or sterile inflammatory stimuli may drive amyloidosis,” the study’s authors concluded. “These data suggest a dual protective/damaging role for Aβ, as has been described for other antimicrobial peptides.”
One of the study’s co-corresponding authors, Rudolph Tanzi, Ph.D., director of the Genetics and Aging Research Unit in the MassGeneral Institute for Neurodegenerative Disease (MGH-MIND), pointed out that AMPs are known to play a role in the pathologies of a broad range of major and minor inflammatory disease. “For example, LL-37, which has been our model for Aβ’s antimicrobial activities, has been implicated in several late-life diseases, including rheumatoid arthritis, lupus, and atherosclerosis,” he elaborated. “The sort of dysregulation of AMP activity that can cause sustained inflammation in those conditions could contribute to the neurodegenerative actions of Aβ in Alzheimer’s disease.”
The study’s other co-corresponding author, Robert Moir, M.D., also of the MGH-MIND Genetics and Aging unit, noted that the study’s findings may lead to potential new therapeutic strategies. He also indicated that therapies designed to eliminate amyloid plaques from patient’s brains may have their limitations.
“It does appear likely that the inflammatory pathways of the innate immune system could be potential treatment targets, Dr. Moir explained. “If validated, our data also warrant the need for caution with therapies aimed at totally removing Aβ plaques. Amyloid-based therapies aimed at dialing down but not wiping out Aβ in the brain might be a better strategy.”
It remains to be determined, however, whether Aβ typically fights real infections or is apt to behave errantly, forming aggregates as though microbes are present, even if they are, in fact, not. “Our findings raise the intriguing possibility that Alzheimer’s pathology may arise when the brain perceives itself to be under attack from invading pathogens,” said Dr. Moir. “Further study will be required to determine whether or not a bona fide infection is involved.”