In a young, healthy brain, soluble Aβ follows a predictable pattern, rising during the day when the brain is active, and falling each night, perhaps due to enhanced clearance during sleep. What happens in a brain loaded with amyloid? In the December 19 JAMA Neurology, researchers led by Brendan Lucey and Randall Bateman at Washington University School of Medicine, St. Louis, report that amyloidosis profoundly impairs the ability of Aβ42 to move freely through the central nervous system. The researchers measured total Aβ40 and Aβ42 concentrations in cerebrospinal fluid over the course of two days in older adults, and also used stable isotope labeling kinetics (SILK) to track the production and clearance of newly made peptide. In the presence of plaques, CSF total Aβ42 levels flatlined, no longer fluctuating with time of day. The loss of diurnal oscillations correlated with rapid deposition of the peptide into amyloid plaques. CSF Aβ40 levels, by contrast, rose and fell daily, even in the presence of brain amyloid.
The findings provide the most detailed picture yet of how plaques change the life cycle of Aβ peptides. The data may help researchers determine the best time of day to administer anti-amyloid medications in clinical trials, based on when Aβ levels are most susceptible to change, Lucey suggested. The findings may also spur more studies into how sleep affects Aβ clearance, researchers noted.
“I’m intrigued by the data. It provides a basis for future experiments that will further clarify the dynamics of Aβ drainage from the brain,” Roxana Carare at the University of Southampton, U.K., told Alzforum. She said the findings have inspired her to design new studies to address these issues.
The SILK method, developed by Bateman and colleagues, tracks the amount of newly labeled peptide in the CSF after injection of a trace amount of 13C-leucine. The method revealed that about 8 percent of total Aβ is made and cleared each hour (see Jun 2006 news). The data also established the distinctive diurnal rise and fall of total CSF Aβ (see Bateman et al., 2007; Kang et al., 2009). Bateman’s group later reported that these daily oscillations dampen with age and with Alzheimer’s disease (see Aug 2012 conference news). Additional human and animal data strengthened the idea that plaques might be to blame for this amplitude modulation, but the details remained murky (see Sep 2012 news; Dobrowolska et al., 2014).
To look more closely at the effect of amyloidosis on Aβ peptide dynamics, Lucey analyzed SILK data from a cohort of 77 adults ranging from 60 to 87 years old. Among this group, 38 were classified as amyloid-positive, based on either a PiB PET scan or a low CSF Aβ42/Aβ40 ratio. A little more than half the cohort had cognitive impairment or AD. The researchers measured CSF Aβ every hour for 36 to 48 hours after 13C-leucine injection, measuring the rise and fall of labeled peptide. With these data, the researchers could calculate average daily rates of peptide production and clearance in each volunteer. In addition, they evaluated total peptide concentrations every hour by mass spectrometry, which they found to produce more reliable results than the ELISAs used in previous studies of diurnal rhythms.
As in previous studies, total Aβ40 and Aβ42 levels fluctuated throughout the day among younger participants who had no amyloid in their brains, typically peaking around 8 p.m. to 10 p.m. and hitting a trough about 12 hours later. For Aβ40, the amplitude of this daily rhythm appeared to be driven primarily by production rates of the peptide, rather than clearance. People who made more new Aβ40 had a more pronounced rise and fall of total Aβ40 levels each day. On the other hand, variations in clearance rates seen among individuals, as measured by the disappearance of labeled Aβ40 from CSF, correlated little with the amplitude of Aβ40’s diurnal rhythms. Aβ40 oscillations in older adults were only modestly weaker than in younger participants.
A very different picture emerged for Aβ42, however. In younger, amyloid-free brains, the range of its oscillations appeared to correlate with both production and clearance rates. People who made more of the peptide had greater amplitude oscillations, while those who cleared it faster had lower amplitudes. Notably, diurnal fluctuations of Aβ42 essentially disappeared in people older than 73 who had no brain amyloid. Why this happens is unclear. Previously published results from this cohort indicate that with advancing age, Aβ42 lingers more than twice as long in CSF as it does in young adults, with a half-life of 9.5 hours instead of four (see Patterson et al., 2015). The persistence may prevent dramatic drops during sleep, when the brain produces less new Aβ. The long half-life could also set the stage for amyloid to begin to accumulate in some people, the researchers suggested.
Amyloid accumulation dramatically affected Aβ42 dynamics, causing a premature loss of diurnal oscillations in people younger than 73. In brains with amyloid, CSF total Aβ42 stayed at a steady, low level. It did not budge with age. Neither did CSF Aβ42 correlate with production or clearance rates among individuals. However, brain amyloid load did correlate with clearance. The more amyloid in a person’s brain, the faster newly made Aβ42 vacated the CSF. “We think Aβ42 is being irreversibly lost to plaques,” Lucey told Alzforum. By contrast, amyloidosis had little effect on Aβ40 dynamics.
An additional piece of data supports the idea that plaques tie up Aβ42. In all serial CSF sampling studies, total Aβ concentrations rise throughout the sampling period. This rise becomes greater as the volume and frequency of fluid sampling increases, leading researchers to speculate that free peptide may be drawn toward the sampling site, raising its local concentration (see Lucey et al., 2015). In the WashU cohort, researchers saw this expected rise for total Aβ40 concentrations, regardless of the age or amyloid status of the participant. Total Aβ42, on the other hand, rose less in older people, and not at all in people with brain amyloid, again suggesting that Aβ42 simply does not circulate in the presence of plaques.
These data raise questions about what is happening to Aβ42 in older adults, and suggest two key areas for further research, Carare said. First, researchers need to better understand how much Aβ escapes from brain tissue into CSF, and by what route. Second, researchers need to determine exactly how sleep changes blood vessels and Aβ clearance. Studies of the glymphatic system have demonstrated that CSF penetrates into brain tissue more readily during sleep, but it remains unclear whether Aβ drains out of the brain along blood vessels better at night, Carare noted. She plans to investigate these questions in animal models, by injecting Aβ into their brains and measuring how much gets out, and by what route, during different stages of sleep.
Lucey is also interested in how sleep affects clearance. He noted that this SILK study did not measure sleep in participants, and therefore cannot reach any conclusions about how its quality affected Aβ oscillations. He is now studying what happens to Aβ under different sleep conditions in a cohort of amyloid-free adults younger than 73. “Potentially, we could manipulate sleep to alter Aβ concentrations in the brain. One advantage of this approach is that there are a lot of approved treatments for sleep disorders,” Lucey said. He noted that these strategies might not work in people older than 73, however, since they have already lost the nighttime drop in Aβ42.
The data may also provide clues for how best to administer anti-amyloid drugs to people at early disease stages, researchers agreed. “For example, a production inhibitor or modulator, such as a BACE inhibitor, may have the largest effect if given in the morning, when Aβ production and concentration are starting to rise,” Bateman wrote to Alzforum. Carare suggested that researchers might want to give a plaque-busting drug in the evening, when the system is best primed to clear away freed peptide. Bateman plans to take diurnal rhythms into account when deciding treatment regimens for the DIAN-TU secondary prevention trial, he added. Trial organizers recently announced they will evaluate Janssen’s BACE inhibitor in an additional set of participants who have either no or mild cognitive impairment.—Madolyn Bowman Rogers
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