DNA damage accrues in Alzheimer's disease and in the aging brain, but would anyone have predicted that neural activity is to blame? In the May 24 Nature Neuroscience, researchers led by Lennart Mucke at the Gladstone Institute of Neurological Disease, San Francisco, California, report that laboratory mice exposed to a novel environment generate more double-stranded DNA breaks than if left in their usual dull abode. Drugs that tone down exuberant neural activity limit this damage, while amyloid-β (Aβ) exacerbates it. "Our studies demonstrate for the first time that abnormal neuron activity is a key mediator of Aβ-induced DNA damage," Mucke told Alzforum.
Other researchers were intrigued by the novelty. "It reinforces the connection between neural activity and the pathophysiology of Aβ," said Michela Gallagher, Johns Hopkins University. Gallagher was not involved in the new work, but her lab recently reported that levetiracetam, the same drug Mucke's group used to quell neural activity and double-stranded breaks, improved cognition in a small sample of people with amnestic mild cognitive impairment (see ARF related news story).
Researchers have known for some time that aging and Alzheimer's pathology associate with DNA damage. While some have blamed oxidative stress, the underlying mechanism has proven elusive, said Mucke. To begin to test if and how Aβ may be involved, first author Elsa Suberbielle and colleagues examined the J20 APP transgenic mice for markers of DNA damage. Suberbielle found that as Aβ rose in the brains of these animals, they accumulated markers of double-stranded breaks, namely, 53BP1 and phosphorylated histone 2A-X (termed γH2A.X). These proteins help repair damaged DNA. The researchers noticed that when they exposed mice to a novel environment, γH2A.X foci grew more numerous. Intriguingly, wild-type animals allowed to explore new surroundings also exhibited the same signs of DNA damage. The researchers confirmed that double-stranded breaks truly occurred by using the "comet assay." In this single-cell gel electrophoresis, damaged DNA migrates more slowly out of neurons, leaving a trailing tail.
How might Aβ or a novel environment damage DNA? Thinking that neural activity must be involved, the researchers looked for DNA breaks in the visual cortex after exposing animals to light. When they stimulated the right eye, they found γH2A.X foci blossomed in the corresponding contralateral visual cortex, but not in the ipsilateral. Similarly, in mice carrying a striatal optogenetic probe that activates neurons, a light fiber placed in one side of the striatum caused the animals to turn in that direction, while the ipsilateral striatal neurons produced more γH2A.X.
Aβ seems to elicit DNA breaks by activating neurons as well. Mucke's group has reported that the J20 strain exhibit epilepsy-like increases in neuronal activity. These are quelled in tau-negative strains, and Suberbielle found that in J20/tau-/- mice, baseline γH2A.X foci returned to wild-type levels, as did those that spring up when the mice explored new environments. Confirming the neuronal activation link, the anti-epilepsy drug levetiracetam also normalized γH2A.X.
Gallagher noted that it will be important to see if the result holds up in other scenarios. "When you see a very novel endpoint used in a disease model, it makes you want to see it repeated in other paradigms," she said. She wondered how it relates to aging, where single-stranded DNA breaks have been linked to oxidative stress and neuronal toxicity. Mucke agreed that a lot more work needs to be done to understand this phenomenon. He does not think it is due to oxidative stress because the DNA breaks occurred even when the researchers added antioxidants and quenched reactive oxygen species.
Extrasynaptic NMDA-type glutamate receptors may be involved, however, since neurons cultured to favor extrasynaptic receptor expression developed dense nuclear deposits of γH2A.X. Aβ exacerbated this, and the foci could be reduced by blocking the NR2B NMDA receptor subunit, which typically sits in extrasynaptic sites. Other researchers have linked Aβ toxicity to activation of extrasynaptic NMDA receptors (see ARF related news story).
Many other questions remain. For example, do DSBs have a physiological function, or are they simply a byproduct of active metabolism? "The more exciting possibility is that there is a purpose to this," said Mucke, "I like the idea that it may facilitate rapid changes in gene expression that are required in the context of high neuronal activity." This could work, because double-stranded breaks could cut knots in DNA that allow rearrangements of chromatin necessary for gene regulation. Mucke agreed that it will be important to figure out if the breaks are randomly placed in the DNA or targeted.
What about disease? Mucke thinks that this may be relevant to neurodegenerative disease if these breaks go unrepaired. In fact, in collaboration with Eliezer Masliah at the University of California, San Diego, he has found evidence of double-stranded breaks in brain tissue from Alzheimer's patients.—Tom Fagan
- Epilepsy Drug Calms the Hippocampus, Aids Memory
- Research Brief: Early Steps in Aβ’s Synaptic Attack
- Exploring Genome Fragmentation in Neurodegeneration
- APP Tail May Wag DNA Repair Pathway
- After 40, DNA Damage Accrues in Genes, Hampering Expression
- Overworked HDACs Leave Transcriptional Posts to Push DNA Repair
- Faulty DNA Repair Gene Leads to Cognitive Problems
- Suberbielle E, Sanchez PE, Kravitz AV, Wang X, Ho K, Eilertson K, Devidze N, Kreitzer AC, Mucke L. Physiologic brain activity causes DNA double-strand breaks in neurons, with exacerbation by amyloid-β. Nat Neurosci. 2013 May;16(5):613-21. PubMed.