Summary: Using a technique called parabiosis on pairs of mice, researchers discover what they call ‘cancer like mobility’ of amyloid beta, reporting it can travel to the brain from other parts of the body.
Source: University of British Columbia.
Alzheimer’s disease, the leading cause of dementia, has long been assumed to originate in the brain. But research from the University of British Columbia and Chinese scientists indicates that it could be triggered by breakdowns elsewhere in the body.
The findings, published today in Molecular Psychiatry, offer hope that future drug therapies might be able to stop or slow the disease without acting directly on the brain, which is a complex, sensitive and often hard-to-reach target. Instead, such drugs could target the kidney or liver, ridding the blood of a toxic protein before it ever reaches the brain.
The scientists demonstrated this cancer-like mobility through a technique called parabiosis: surgically attaching two specimens together so they share the same blood supply for several months.
UBC Psychiatry Professor Dr. Weihong Song and Neurology Professor Yan-Jiang Wang at Third Military Medical University in Chongqing attached normal mice, which don’t naturally develop Alzheimer’s disease, to mice modified to carry a mutant human gene that produces high levels of a protein called amyloid-beta. In people with Alzheimer’s disease, that protein ultimately forms clumps, or “plaques,” that smother brain cells.
Normal mice that had been joined to genetically modified partners for a year “contracted” Alzheimer’s disease. Song says the amyloid-beta traveled from the genetically-modified mice to the brains of their normal partners, where it accumulated and began to inflict damage.
Not only did the normal mice develop plaques, but also a pathology similar to “tangles” – twisted protein strands that form inside brain cells, disrupting their function and eventually killing them from the inside-out.
Other signs of Alzheimer’s-like damage included brain cell degeneration, inflammation and microbleeds.
In addition, the ability to transmit electrical signals involved in learning and memory – a sign of a healthy brain – was impaired, even in mice that had been joined for just four months.
Besides the brain, amyloid-beta is produced in blood platelets, blood vessels and muscles, and its precursor protein is found in several other organs.
But until these experiments, it was unclear if amyloid-beta from outside the brain could contribute to Alzheimer’s disease. This study, Song says, shows it can.
“The blood-brain barrier weakens as we age,” says Song, a Canada Research Chair in Alzheimer’s Disease and the Jack Brown and Family Professor. “That might allow more amyloid beta to infiltrate the brain, supplementing what is produced by the brain itself and accelerating the deterioration.”
Song, head of UBC’s Townsend Family Laboratories, envisions a drug that would bind to amyloid-beta throughout the body, tagging it biochemically in such a way that the liver or kidneys could clear it.
“Alzheimer’s disease is clearly a disease of the brain, but we need to pay attention to the whole body to understand where it comes from, and how to stop it,” he says.
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Source: Brian Kladko – University of British Columbia
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to University of British Columbia.
Original Research:Abstract for “Blood-derived amyloid-β protein induces Alzheimer’s disease pathologies” by X-L Bu, Y Xiang, W-S Jin, J Wang, L-L Shen, Z-L Huang, K Zhang, Y-H Liu, F Zeng, J-H Liu, H-L Sun, Z-Q Zhuang, S-H Chen, X-Q Yao, B Giunta, Y-C Shan, J Tan, X-W Chen, Z-F Dong, H-D Zhou, X-F Zhou, W Song and Y-J Wang in Molecular Psychiatry. Published online October 31 2017 doi:10.1038/mp.2017.204
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Blood-derived amyloid-β protein induces Alzheimer’s disease pathologies
The amyloid-β protein (Aβ) protein plays a pivotal role in the pathogenesis of Alzheimer’s disease (AD). It is believed that Aβ deposited in the brain originates from the brain tissue itself. However, Aβ is generated in both brain and peripheral tissues. Whether circulating Aβ contributes to brain AD-type pathologies remains largely unknown. In this study, using a model of parabiosis between APPswe/PS1dE9 transgenic AD mice and their wild-type littermates, we observed that the human Aβ originated from transgenic AD model mice entered the circulation and accumulated in the brains of wild-type mice, and formed cerebral amyloid angiopathy and Aβ plaques after a 12-month period of parabiosis.
AD-type pathologies related to the Aβ accumulation including tau hyperphosphorylation, neurodegeneration, neuroinflammation and microhemorrhage were found in the brains of the parabiotic wild-type mice. More importantly, hippocampal CA1 long-term potentiation was markedly impaired in parabiotic wild-type mice.
To the best of our knowledge, our study is the first to reveal that blood-derived Aβ can enter the brain, form the Aβ-related pathologies and induce functional deficits of neurons.
Our study provides novel insight into AD pathogenesis and provides evidence that supports the development of therapies for AD by targeting Aβ metabolism in both the brain and the periphery.
“Blood-derived amyloid-β protein induces Alzheimer’s disease pathologies” by X-L Bu, Y Xiang, W-S Jin, J Wang, L-L Shen, Z-L Huang, K Zhang, Y-H Liu, F Zeng, J-H Liu, H-L Sun, Z-Q Zhuang, S-H Chen, X-Q Yao, B Giunta, Y-C Shan, J Tan, X-W Chen, Z-F Dong, H-D Zhou, X-F Zhou, W Song and Y-J Wang in Molecular Psychiatry. Published online October 31 2017 doi:10.1038/mp.2017.204
Brain Aβ is elevated in patients with sporadic Alzheimer’s disease. Aβ is the main constituent of brain parenchymal and vascular amyloid; it contributes to cerebrovascular lesions and is neurotoxic. It is unresolved how Aβ accumulates in the central nervous system and subsequently initiates the disease of cells. Some researchers have found that the Aβ oligomers induce some of the symptoms of Alzheimer’s Disease by competing with insulin for binding sites on the insulin receptor, thus impairing glucose metabolism in the brain. Significant efforts have been focused on the mechanisms responsible for Aβ production, including the proteolytic enzymes gamma- and β-secretases which generate Aβ from its precursor protein, APP (amyloid precursor protein). Aβ circulates in plasma, cerebrospinal fluid (CSF) and brain interstitial fluid (ISF) mainly as soluble Aβ40 Senile plaques contain both Aβ40 and Aβ42, while vascular amyloid is predominantly the shorter Aβ40. Several sequences of Aβ were found in both lesions. Generation of Aβ in the CNS may take place in the neuronal axonal membranes after APP-mediated axonal transport of β-secretase and presenilin-1.
Increases in either total Aβ levels or the relative concentration of both Aβ40 and Aβ42 (where the former is more concentrated in cerebrovascular plaques and the latter in neuritic plaques) have been implicated in the pathogenesis of both familial and sporadic Alzheimer’s disease. Due to its more hydrophobic nature, the Aβ42 is the most amyloidogenic form of the peptide. However the central sequence KLVFFAE is known to form amyloid on its own, and probably forms the core of the fibril. One study further correlated Aβ42 levels in the brain not only with onset of Alzheimer’s, but also reduced cerebrospinal fluid pressure, suggesting that a build-up or inability to clear Aβ42 fragments may play a role into the pathology.[
Low-temperature and low-salt conditions allowed to isolate pentameric disc-shaped oligomers devoid of beta structure. In contrast, soluble oligomers prepared in the presence of detergents seem to feature substantial beta sheet content with mixed parallel and antiparallel character, different from fibrils; computational studies suggest an antiparallel beta-turn-beta motif instead for membrane-embedded oligomers.
The suggested mechanisms by which amyloid beta may damage and cause neuronal death include the generation of reactive oxygen species during the process of its self-aggregation. When this occurs on the membrane of neurons in vitro, it causes lipid peroxidation and the generation of a toxic aldehyde called 4-hydroxynonenalwhich, in turn, impairs the function of ion-motive ATPases, glucose transporters and glutamate transporters. As a result, amyloid beta promotes depolarization of the synaptic membrane, excessive calcium influx and mitochondrial impairment. Aggregations of the amyloid-beta peptide disrupt membranes in vitro.