Amyloid-β clearly goes rogue in Alzheimer disease, but it comes in different forms and can attack cells from within and without. The peptide can be a soluble monomer, oligomer, or protofibril before aggregating into insoluble fibrils. It forms plaques outside the cell, but may exert its neurodegenerative influence inside, as two papers in this week’s PNAS suggest. Both these and a paper in the March 18 Journal of Neuroscience suggest that soluble Aβ is indeed the villainous form, although the PNAS pair fingers oligomers and the other points to protofibrils.
Writing in the Journal of Neuroscience, Heikki Tanila of the University of Kuopio, Finland, and colleagues expand on the link between Aβ and epilepsy. Joint first authors on the paper were Rimante Minkeviciene of the University of Kuopio, Sylvain Rheims of Aix Marseille University, France, and Marton Dobszay of the Karolinska Institutet, Stockholm, Sweden. Tanila’s lab has been working for seven years with the APPswe/PS1dE9 Alzheimer model—double mutant mice expressing mutant human presenilin 1 plus mouse APP with a human Aβ domain containing two AD-linked mutations. “From the beginning the colony was plagued with mysterious deaths,” Tanila wrote in an e-mail to ARF. “Mice appeared healthy until they were found in their cages the next morning.” Other labs have reported similar problems (Garcia-Alloza et al., 2006; Shemer et al., 2006). Necropsies failed to find any cause for the sudden deaths, but one day a caretaker noticed an animal convulsing.
AD has been linked to increased risk of seizures in people, particularly in early stages of the disease (Amatniek et al., 2006), and other studies have recorded seizures in AD model mice (Palop et al., 2007 and see ARF related news story; Hsiao et al., 1995). To assess the possibility of similar seizures in their mice, Tanila and co-authors monitored the animals by both video and electroencephalogram. Thirteen out of 20 mutant mice had seizures, they found, while no wild-type control animals convulsed. During recording, one animal died of a prolonged seizure. In postmortem analysis, the researchers found that mice that suffered seizures had no Aβ plaques in the thalamus, where seizures often begin, while three seizure-free animals did have plaques in the thalamus. This suggests that aggregated Aβ is not the culprit. The authors then examined pyramidal neurons, the natural choice for their studies because these cells are important mediators of excitatory signals, Tanila wrote. Membrane potential was decreased in the cortical pyramidal neurons of living mutant animals. This membrane polarity shift could lower the threshold for the nerves to fire when stimulated, leading to seizures, the authors write.
To determine what form of Aβ was to blame, the scientists next incubated brain slices from wild-type animals with artificially synthesized Aβ. When bathed in soluble protofibrillar or fibrillar Aβ, the pyramidal cell membranes depolarized, but oligomers did not have the same effect. That is unexpected, according to Sanjay Pimplikar of the Cleveland Clinic in Ohio, who was not involved in the study. “Oligomeric Aβ has caught the attention of the field as the most likely causative agent of AD,” he wrote in an e-mail to ARF. “Although these findings do not negate the multitude of papers showing harmful effects of oligomeric (but not fibrillar) Aβ, they do show the immense difficulty and variability associated with Aβ experiments.”
The link between epilepsy and AD is important, Tanila wrote, because anti-epileptic drugs impair cognitive performance, and anti-AD medications may lower the threshold for seizures. “Physicians have to weigh the pros and cons,” he wrote. “There is an obvious need for new AD medications that would have anti-epileptic activity.”
Tanila’s work suggests that extracellular, (proto)fibrillar Aβ affects pyramidal neurons. In contrast, the authors of the two PNAS papers find a role for the oligomeric form, but suggest that it acts intracellularly to disrupt axonal transport and synaptic transmissions. First author Gustavo Pigino and senior author Scott Brady, of the University of Illinois in Chicago, and colleagues analyzed fast axonal transport in giant squid axons treated with various forms of Aβ; first author Herman Moreno of the State University of New York Downstate Medical Center in Brooklyn, senior author Rodolfo Llinás of the New York University School of Medicine in New York City, and colleagues studied synaptic function in the same system. Chatting one summer at the Marine Biological Laboratory in Woods Hole, Massachusetts, Brady and Llinás were surprised to discover they were following such similar approaches. “What is unbelievable is that I’ve known Scott for many years, but we didn’t collaborate on this experiment,” Llinás said. “Ninety-nine percent of the work was done independently.”
The authors came to the same conclusions. The Illinois team showed that injecting oligomeric Aβ interfered with fast axonal transport, and the New York group found a downstream effect on synaptic transmission from the same kind of Aβ, but not fibrils. With axonal transport disrupted, vesicles do not reach the synapse. Not only are the vesicle contents needed there, but also the envelopes, when they fuse with the plasma membrane to expel their contents, contribute to maintaining the synapse structure. Without an influx of new materials, the synapse shrinks. This leads to the “dying back” of neurons observed in AD. “This is essentially the critical molecular effect that produces Alzheimer’s,” Brady said. “The actual symptoms are due to the loss of connections between neurons.” By the time the neurons finally die, they have long since ceased to function, he said.
Brady’s group discovered, and Llinás’s confirmed, that the activation of casein kinase 2 (CK2) mediates the Aβ effect on axonal transport and synapse signaling. By phosphorylating the microtubule-based motor kinesin, and likely dynein as well, CK2 detaches the motors from their cargo, and the vesicles never reach their destination. Recently, Brady and colleagues also found that filamentous tau inhibits kinesin-dependent transport by activating glycogen synthase kinase-3 (GSK-3), which then phosphorylates the motor, causing it to drop its cargo (Lapointe et al., 2008). “I think that these could be the basis for developing therapeutics,” Brady said; such therapeutics might work by reducing CK2 and GSK-3 activity.
“These are the latest in a succession of papers that have shown that axonal transport is deficient in neurodegenerative disease,” said Virgil Muresan of the University of Medicine and Dentistry of New Jersey in Newark, who was not involved in the current studies. “The most recent and, I would say, the most solid data indicate that it is the soluble oligomers that are the most damaging.” However, Muresan noted that in the PNAS papers, Aβ was acting in the cytoplasm, which is not where the greatest concentration of the peptide is normally found. Aβ does exist inside the cytoplasm, Moreno said, although it is not quite clear how it gets there. It may be secreted, then re-enter the cell.
From these and other studies it is clear that Aβ commits more than one type of crime and wears different guises. These experiments add seizures and axonal transport interference to the growing rap sheet. Unfortunately, scientists have yet to find a way to seize and restrain this neural felon.—Amber Dance
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