To stop the destruction of neurons in Alzheimer disease requires an understanding of how the two protein perpetrators, amyloid-β (Aβ) and tau do their dirty work. Two papers out in the January 23 Journal of Neuroscience address that issue, with one exonerating activation of the pro-apoptotic caspases and neuronal death in cells that contain tau aggregated into neurofibrillary tangles. The other paper reports on synaptic inhibition by Aβ oligomers, now regarded as the initial insult of AD, and implicates voltage-gated calcium channels as Aβ's target.
The case for neurofibrillary tangles (NFTs) as initiators of apoptotic cell death rests mainly on circumstantial evidence. Neurons producing Aβ show caspase activation and tau cleavage, which precedes and could initiate tangle formation (Gamblin et al., 2003; Rissman et al., 2004), but some studies suggest that tangle formation is not the trigger for cell death, and may even serve a protective function (see ARF related news story) and ARF news story). Indeed, tau researchers have recently taken a page from the Aβ book, looking to tau oligomers or small aggregates as potential killers, rather than large tangles.
To look directly at the relationship among tangles, caspase activation, and neuronal death, Bradley Hyman and colleagues at the Massachusetts General Hospital in Charlestown used their imaging expertise to observe tangles and caspase activation in neurons in living mice. First author Tara Spires-Jones used multiphoton in vivo imaging to detect both NFT pathology and caspase activation in brains of living rTg4510 mice, which reversibly express human tau with the pathogenic P301L mutation. Imaging was done through a cranial window after thioflavin S was applied to stain tau tangles. Caspases were detected using fluorescent inhibitors that covalently bind only to activated enzyme. Using this method, Spires-Jones observed only a small minority of neurons with evidence of caspase activation, but saw that most of the caspase-positive cells had an NFT. Cells with NFTs were more likely to have activated caspases: in all, 6 percent of NFT-positive cells displayed caspase activation, while only 0.1 percent of tangle-negative cells were caspase-positive.
Nonetheless, the cells with activated caspase showed no signs of degenerating over 4 hours of observation, nor did they show any signs of terminal apoptosis despite activation of both initiator and executioner caspases. When tau transgene expression was turned off, cell death stopped, but caspase activation persisted. The results indicated that while aggregated tau was associated with activated caspases, activation did not lead to cell death, at least in the short term. Thus, the authors conclude, “Although NFTs may be associated with toxic phenomenon [sic], they do not appear to be coupled with acute neurodegeneration in this tauopathy model.” Another recent report also supports the idea that caspase cleavage of tau has little to do with filament formation or toxicity (Delobel et al., 2008).
The second paper, from Volker Nimmrich and colleagues at the Abbot research labs in Ludwigshafen, Germany, shows that Aβ oligomers acutely impair synaptic transmission via an inhibitory effect on calcium currents. The Abbot researchers described their Aβ oligomers, or “globulomers,” 2 years ago (see ARF related news story), and showed that, like naturally occurring oligomers, the globulomers inhibit long-term potentiation in hippocampal slices. Antibodies to the globulomers recognize epitopes in the brains of AD patients and in mice expressing human Aβ, suggesting that these globulomers are physiologically relevant. But how do they affect synaptic plasticity?
To answer that question, Nimmrich and fellow first author Christiane Grimm measured spontaneous synaptic currents in cultures of hippocampal neurons. When they added oligomers (8 nM), they saw reduced frequency of currents. The changes were not due to alterations in the cells’ excitability, since the investigators found action potentials unaffected. The effects, which were seen in both stimulatory glutamatergic and inhibitory GABAergic synapses, were only elicited by preformed oligomers, and not when monomer was added to the cells. By measuring a variety of pre- and postsynaptic currents, the researchers narrowed the locus of oligomer action to the presynaptic transmitter release machinery.
Neurotransmitter release is triggered by calcium influx into the presynaptic compartment, flowing via voltage-gated calcium channels in case of glutamate and GABA release. By using a variety of channel blockers, the researchers found that Aβ oligomers suppressed currents through P/Q-type channels, but not other types.
If Aβ oligomers are blocking calcium channels, then enhancers of those channels might offer a novel therapeutic strategy. Along these lines, the investigators show that roscovitine, an enhancer of P/Q-type calcium channel activity, partially reversed the effects of Aβ. Studies in animals will be needed to confirm if the action of P/Q channel agonists translates to improvements in cognitive defects as well.
The current study does not prove that Aβ acts directly on the calcium channel, but the speed with which the channel is modulated suggests the oligomers act either directly at the channel or on a closely linked partner. Voltage-gated calcium channels were recently identified as a target of another neurodegenerative protein, huntingtin, which activates the channels and leads to synaptic hyperactivity (see ARF related news story).
Whether the globulomers are identical to the cell-generated Aβ oligomers studied by Dominic Walsh when he was at Dennis Selkoe’s lab at Harvard Medical School (see ARF related news story), the ADDLs of Bill Klein at Northwestern University, Chicago (see ARF related news story), or the Aβ*56 of Karen Ashe and colleagues at the University of Minnesota, Minneapolis (see ARF related news story), is not clear. They appear to be 12-mers, similar to some other described oligomers. Recent reports from other labs showed that some oligomers act by modulating NMDA receptor activity (see ARF related news story). If the current study is replicated with other oligomers, that would place potential Aβ targets on both sides of the synapse.—Pat McCaffrey