In back-to-back papers in the December 4 Science Translational Medicine, scientists led by Daniela Kaufer, University of California, Berkeley, and Alon Friedman, Ben-Gurion University of the Negev, Beer-Sheva, Israel, report that age-related cracks in the blood-brain barrier allow an influx of serum protein albumin into the brain, where it activates TGFβ receptors, overexcites neuronal networks, and impairs cognition. Breaches correlated with localized slowing of cortical activity in epilepsy, Alzheimer’s disease patients, and in mouse models of AD. Called paroxysmal slow-wave events, these activity changes correlated with cognitive impairment and interspersed with seizures in epilepsy patients.
- As people age, their blood-brain barriers become leakier.
- This fuels astrocytic TGFβ signaling, hyperexcitation, memory loss.
- In mice, a TGFβ inhibitor prevents these problems.
The findings suggest that, in some people with AD, silent seizures may be due to a leaky BBB, and that this may explain some of their cognitive decline. In rodents, a TGFβ antagonist drug prevented slow-wave events and seizures.
“Together, the papers provide a biologically plausible and intriguing mechanism by which a leaky blood-brain barrier could contribute to network hyperexcitability in aging and AD,” wrote Keith Vossel, University of Minnesota, Minneapolis, to Alzforum (see comment below).
“There is a strong emerging story about how blood-brain barrier alterations may trigger a variety of downstream effects that result in inflammation and electrophysiological changes,” wrote Bill Jagust, University of California, Berkeley, to Alzforum. “How this relates to Alzheimer’s disease is still something we need to work out.” Jagust was not involved in the current work.
Spatially Matched. In a person with epilepsy, areas where the blood-brain barrier is leakiest (left) partially overlap with areas where transient paroxysmal slow-wave events (PSWEs) are more frequent (right). [Courtesy of Science Translational Medicine/AAAS.]
The blood-brain barrier (BBB), a cellular boundary that seals off the vascular system from the brain, keeps most blood components out of the parenchyma. For the past 15 years, Kaufer and Friedman have collaboratively studied the ins and outs of the BBB in health and disease. Their previous work suggests that in various types of brain injury, including trauma and epilepsy, a damaged BBB allows proteins, particularly serum albumin, to seep in (Cacheaux et al., 2009). Albumin activates TGFβ receptors on astrocytes, stimulating them to release proinflammatory cytokines and more TGFβ, which activates additional astrocytes (Ivens et al., 2007). This causes hyperexcitability and neuronal dysfunction, which associate with cognitive impairment (Kim et al., 2017; Weissberg et al., 2015).
Could a similar mechanism be responsible for hyperexcitation seen in aging and silent seizures in early Alzheimer’s? There are hints that the BBB deteriorates with age, especially in the hippocampus (Jan 2015 news), but the literature is controversial (Bien-Ly et al., 2015; Raja et al., 2017).
To examine this possibility, co-first authors Vladimir Senatorov Jr. and Aaron Friedman at Berkeley (no relation to Alon Friedman) examined the BBB in aging wild-type mice. They killed them at 3, 12, 18, and 21 months to look for albumin in the hippocampus. The protein first appeared there at 12 months—midlife for a mouse—after which its level stayed largely steady. Most of the albumin was taken up by astrocytes, activating their TGFβ signaling cascade and leading to phosphorylation of the transcription factor Smad2. Smad2 phosphorylation increased with age in astrocytes that were positive for albumin.
Did this signaling render networks overly active? The scientists tested this by inducing seizures in mice using the convulsant pentylenetetrazol. They found that 12-, 18-, and 24-month-old mice with BBB damage were more sensitive to PTZ-induced seizures than young controls. Electrocorticography, a more sophisticated form of EEG that records from electrodes placed under the skull, detected discrete intervals, some 10 seconds long, of synchronized neuronal activities of less than 5 Hz throughout the day in the older animals. Awake mice do not usually have these paroxysmal slow-wave events (PSWEs).
Serum albumin appeared both necessary and sufficient to elicit these effects. Albumin infused into the brain ventricles of young mice caused PSWEs and rendered them vulnerable to seizure 48 hours later. A week after infusion, the animals took longer to learn where the underwater platform was in the Morris water maze. When the researchers knocked out both copies of the TGFβ receptor in astrocytes of aging mice, they were protected from seizures and outperformed TGFβR heterozygotes in a T-maze task.
This data suggested that inhibiting TGFβ signaling might counteract hyperexcitability and cognitive impairment in aging. To test this, the authors gave mice a small-molecule inhibitor called IPW (Rabender et al., 2016). This is an inhibitor of the TGFβ receptor, a transmembrane protein kinase. In aging mice, IPW reduced seizure vulnerability and PSWEs, and improved T-maze performance and novel-object recognition to the level seen in young mice.
Does BBB damage have these consequences in people? Among 113 healthy volunteers aged 21 to 83, dynamic-contrast-enhanced MRI indicated that those under 40 tended to have an intact BBB, whereas in older people it was disrupted. Nearly half the 28 people older than 60 had a leaky barrier. Comparing postmortem tissue from three people aged 26–36, and 10 aged 61–78, both albumin and pSmad2 appeared in hippocampal astrocytes from the older people, suggesting increased TGFβ signaling brought on by a leaky BBB.
Meanwhile, Dan Milikovsky, first author on the second paper, was studying PSWEs in human subjects in Israel. EEG recordings from people with AD and MCI turned up more frequent and longer PSWEs than those of age-matched controls. In people with epilepsy, PSWEs cropped up in between seizures, particularly in brain areas with a damaged barrier (see image above). The results suggest BBB pathology could be an underlying mechanism and a drug target for disorders that involve nonconvulsive seizure activity in the brain, Milikovsky believes.
To Kaufer, the evidence points to a need to close the BBB to treat some neurological disorders. “There are a lot of companies that are trying to open the BBB to get drugs in. We are trying to do the opposite,” she told Alzforum.
Costantino Iadecola, Weill Cornell Medical College, New York, would like to know whether the human PSWE EEG signature is due to BBB damage alone or correlates with other factors affecting network activity, including Aβ, tau, α-synuclein, and TDP-43. The mechanism causing BBB dysfunction as well as the link between TGFβ signaling and network activity need to be worked out, Iadecola wrote (comment below).
While IPW can be given orally and enters the brain, it has not been tested in clinical trials, said Kaufer. She and Friedman founded a company, Mend Therapeutics, to develop it, as well as additional drug candidates or derivatives, to treat BBB-related disorders.
Jonathan Schott, University College London, considers the data important. “Further studies will be needed to establish whether strategies to block astrocytic TGFβ signaling are feasible, safe, and therapeutically useful for patients,” he wrote to Alzforum (comment below).
Kaufer also wants to know what causes disruption of the BBB in the first place. She will collaborate with Jagust on how BBB damage affects Aβ deposition and tau pathology in transgenic mice, and amyloid and tau deposition in humans. BBB dysfunction was reported earlier this year to associate with cognitive decline in Alzheimer’s (Jan 2019 news).
Albumin is the most common but not the only blood protein to cross a leaky BBB and possibly wreak havoc in the brain, the authors note. Recent work by Katerina Akassoglou at the Gladstone Institutes, San Francisco, reported that fibrinogen crosses into the brains of mouse models of Aβ deposition, where it activates microglia to phagocytose synapses, causing cognitive dysfunction (Feb 2019 news).—Gwyneth Dickey Zakaib
- In Aging Brain, Blood-Brain Barrier Starts Leaking in Hippocampus
- Absent Aβ, Blood-Brain Barrier Breakdown Predicts Cognitive Impairment
- Clotting Protein from Blood Incites Microglia, and Synapses Die
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