Updated 2 December 2009
24 November 2009. The aberrant re-entry of post-mitotic neurons into the cell cycle is an early event in AD pathology, preceding amyloid deposition and possibly putting cells in a vulnerable state for subsequent neurodegeneration. Blame it on neuroinflammation, says a new report from Karl Herrup of Rutgers University in Piscataway, New Jersey, and Bruce Lamb of the Cleveland Clinic and Case Western Reserve University in Cleveland, Ohio. Their paper, published on November 9 online in the Journal of Clinical Investigation, shows that activation of brain microglia is responsible for the appearance of cell cycle protein markers in neurons in young AD mice. Further, they find that non-steroidal anti-inflammatory drugs (NSAIDs) can prevent the appearance of these early cell cycle events. However, starting NSAID treatment after cell cycle entry has initiated cannot reverse the abnormalities. The results support the idea that NSAIDs might help prevent AD if deployed early in the disease progression, a concept that is gaining some support from recent post-hoc analysis of the stopped Alzheimer’s Disease Antiinflammatory Prevention Trial (ADAPT).
First author Nicholas Varvel, who is a joint graduate student in the Lamb and Herrup labs, looked at the induction of aberrant cell cycle entry in Lamb’s R1.40 mouse model, which expresses Swedish mutant human APP under a human promoter. The mice show amyloid deposition at 12-14 months of age, but by six months of age, neurons start to show aberrant cyclin D and A expression, which depends on γ-secretase activity and Aβ production (see ARF related news story on Varvel et al., 2008). [Editor's note: See clarification below in comment from Nicolas Varvel.]
In the new work, Varvel finds a coincident activation of microglia, which also depended on Aβ production.
To determine if the microglial activation and cell cycle events (CCEs) were causally linked, the researchers induced inflammation in two-month-old R1.40 mice, before CCEs normally appear. Injecting the mice with lipopolysaccharide resulted in microglial activation and the onset of CCEs. Varvel also looked at the effects of damping spontaneous inflammation in R1.40 mice by treating them with the NSAIDs ibuprofen or naproxen. When the mice were continuously dosed from three to six months of age, the researchers found less activation of microglia and fewer cells with aberrant cell cycle expression. Despite their documented inhibition of the amyloid-producing enzymes BACE and γ-secretase, the NSAIDs had no effect on Aβ production or the relative amounts of Aβ40 and 42, leaving the researchers to conclude that the primary effects of NSAIDs on CCEs occur at the level of microglial inflammation.
The appearance of cyclin-positive neurons is time- and location-dependent in the mice, which gave the researchers a chance to look at the question of whether NSAIDs can reverse CCEs once they have formed. Interestingly, NSAID treatment from six to 12 months had no effect on the number of early-forming CCEs, which appear in cortical layers II/III by the time the mice are six months old. However, treatment did block appearance of CCEs in the adjacent layers V/VI, where cyclin-expressing neurons normally show up later, at around 12 months of age.
It is not yet clear how the appearance of CCEs or the early or late effects of NSAIDs correlate with behavior in the mice, but the researchers are looking at that now, coauthor Lamb said.
The study may bring some clarity to the confusion over NSAIDs and AD. Retrospective epidemiological studies suggest NSAIDs offer some protection from dementia, but prospective studies have been disappointing. The ADAPT trial, a large, placebo-controlled study, was halted in 2004 for safety reasons after treated patients showed an increase in cardiovascular events (see ARF related news story). That turned out be a controversial move (see ARF related news story). The trial reported negative results in 2008, at the same time as another observational study supported the preventive effect of NSAIDs (see ARF related news story on Martin et al., 2008 and Vlad et al., 2008).
The latest look at the ADAPT data suggests there may be a window of opportunity for prevention early on in cognitively intact people. John Breitner of the University of Washington, Seattle, has been following up on the ADAPT subjects. He presented incidence and biomarker data at ICAD 2009 (see ARF related news story), and more recently at the Clinical Trials on Alzheimer’s Disease (CTAD) meeting in Las Vegas, that suggest that one to three years of treatment with naproxen may ultimately reduce the incidence of AD five to 10 years out in the people who were least affected at the time of treatment. That contrasts with the effects of the drug in the early years of the trial, where it appeared to hasten the onset of dementia in the subset of people who were further along the path to disease. At CTAD, Breitner showed that the hazard ratios for dementia in those taking the naproxen or celecoxib for over two years were 0.33 and 0.64, respectively. The apparent protection afforded by naproxen also emerged when he looked at biomarkers. Those taking naproxen over extended periods had a 40 percent reduction in the CSF total tau/Aβ42 ratio.
Breitner told ARF he finds the new study exciting and intriguing. “To be able to show that these NSAIDs have an effect at one phase of the disease development which is lost later—that’s something that I've been very interested in for a long time because we are seeing very similar things in our human experimental work with ADAPT. Any time you can find something in animals that mimics something as unexpected and unusual as that, I think it’s very interesting,” he said.
How does that mouse work relate to timing of treatment in humans? That is not clear yet, Lamb told ARF. “In the mouse we have some good ideas of when to treat, but translating that to the humans is tough,” he said. Right now, they are trying to figure out if there are biomarkers for the onset of inflammation and CCEs in the mouse that might apply to people, too. “We are now looking at inflammatory molecules within the brain, but also beginning to try to look at things in serum and CSF. If we could link this to something biological, then that might help in terms of trying to figure out when is the right time to start treatment,” he said.—Pat McCaffrey.
Varvel NH, Bhaskar K, Kounnas MZ, Wagner SL, Yang Y, Lamb BT, Herrup K. NSAIDs prevent, but do not reverse, neuronal cell cycle reentry in a mouse model of Alzheimer disease. J Clin Invest. 2009 Nov 9. pii: 39716. doi: 10.1172/JCI39716. Abstract