The blood-brain barrier shields the brain from potentially harmful things, and some findings have suggested that this protective border weakens with age or disease. However, a study published in Neuron on October 21 reports that the barrier remains largely intact in multiple mouse models of neurodegenerative disease. The researchers, led Marcel van der Brug of Genentech in South San Francisco and Ryan Watts, formerly of Genentech, also found that brains from healthy aging people bore the scars of just as many barrier breaches as those from Alzheimer’s disease patients.
Alzforum originally reported the study’s preliminary findings as presented earlier this year at a Keystone Symposium (see Mar 2015 conference news). The full results underscore the need to develop therapies designed to cross an intact blood-brain barrier, first author Nga Bien-Ly told Alzforum.
This study contradicts previous work that has called disruption of the blood-brain barrier (BBB) both a cause and a consequence of AD pathology. Vascular amyloid deposits or other age-related vascular problems may damage the integrity of the brain’s blood vessels, thus allowing the entry of potentially toxic proteins that exacerbate brain pathology (see Erickson and Banks, 2013). Earlier this year, researchers led by Berislav Zlokovic at the University of Southern California in Los Angeles reported that in the hippocampus, but not the cortex, the BBB becomes progressively leakier with age, even more so in people with mild cognitive impairment or AD (see Feb 2015 Webinar). Van der Brug and colleagues wanted to measure whether the barrier, especially in regions other than the hippocampus that may not have extensive neurodegeneration earlier in the disease process, is really compromised in terms of allowing drugs to pass through. Beyond this debate, most researchers agree that even when partially compromised, the BBB poses a formidable challenge to deliver drugs to the brain, especially macromolecules such as antibodies. “This is a huge problem in drug development,” van der Brug told Alzforum. As researchers attempt to create therapeutics targeted at the brain, it will be crucial to better understand how the function of the BBB changes with age and disease.
For the current study, Genentech researchers wanted to formally test the idea that AD disrupts the BBB. The company, now part of Roche, is developing a strategy to smuggle therapeutic antibodies across the brain’s border, hence evidence of an intact barrier would further support the need for such a trafficking route. The researchers previously developed bispecific antibodies, which recognize a different target with each of their two arms. While one arm recognizes BACE1, the other latches on to the transferrin receptor (TfR) expressed on endothelial cells lining the barrier, which then transport the antibody across via transcytosis (see Jan 2014 news; Jan 2014 news; and Nov 2014 news).
Open Access or VIP Only? To assess the integrity of the BBB, researchers compared the active and passive crossing of antibodies, as well as other molecules, in models of neurodegenerative disease. [Image courtesy of Nga et al., Neuron 2015.]
The researchers reasoned that in the face of a barrier breach, monospecific BACE1 or control antibodies would passively cross into the brain nearly as well as bispecific TfR antibodies that were actively smuggled across. They first tested this notion in animals with experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis in which the BBB is drastically compromised. They found that both mono- and bispecific antibodies crossed into the brain and spinal cord of EAE mice after injection into the blood, indicating that the barrier was indeed breached. However, bispecific antibodies still entered the brain more efficiently than BACE1 or control antibodies. In normal mice, only the bispecific antibodies made it to the other side efficiently, while monospecific antibodies eked across in barely detectable amounts. In further support of the border breach in EAE mice, the researchers detected serum albumin in the brain.
The researchers next used their antibodies to gauge barrier disruption in multiple models of neurodegeneration. In plaque-ridden, 10- to 13-month-old PS2-APP mice, the researchers found that, like in wild-type mice, only the bispecific TfR antibody crossed the barrier efficiently, while BACE1 or control antibodies remained largely outside. This indicated that a barrier disruption large enough to let antibodies across did not occur in these AD mice. The same held true for two transgenic mouse models expressing disease-associated forms of human tau—P301L and P301S—despite extensive tauopathy and neurodegeneration. Passive passage of antibodies was also nearly absent in ApoE knockouts, ApoE4 knock-ins, and SOD1-G93A animals, a mouse model of amyotrophic lateral sclerosis.
Antibodies are macromolecules, weighing in around 150kDa, so their exclusion from the brain cannot rule out lesser breaches that would let smaller molecules slide through. To look for that, the researchers injected normal and PS2APP mice with radioiodinated molecules ranging from 3 to 150kDa in size. Regardless of their size, most labeled molecules were excluded from the brains of young or aged wild-type or PS2APP mice, indicating that neither age nor amyloid pathology promoted BBB permeability.
In an attempt to decipher whether AD compromises the BBB in humans, the researchers measured the number and size of cerebral infarcts—remnants of past barrier breaches—in postmortem brain samples from 561 AD patients and 227 age-matched controls. People with severe cerebrovascular disease, vascular dementia, or stroke were excluded from the dataset. The researchers found no differences in the number, size, or total volume of cerebral infarcts between AD patients and controls, and the volume of a person’s infarcts did not correlate with the severity of their amyloid pathology. While this finding cannot account for BBB disruptions that were not accompanied by infarcts, they suggest that in people with AD, the integrity of the BBB was similar to that in healthy people of the same age.
“While the studies do not support earlier observations suggesting a widespread ‘opening’ of the BBB, they strengthen the rationale for developing approaches to promote the transfer of drug across the BBB for therapeutic purposes in neurodegenerative diseases,” commented Costantino Iadecola of Weill Medical College of Cornell University in New York. He added that previous studies that concluded the barrier was compromised due to neurodegeneration induced barrier breach under different conditions and used varying approaches to measure it, so it is difficult to reconcile the results. “Irrespective of whether or not the BBB is open in advanced disease (as examined in the present paper), a key question concerns the status of the BBB in the presymptomatic phase of the disease process, because that would be the time to intervene in diseases like AD in which the pathogenic process precedes symptoms by decades,” Iadecola wrote.
That the BBB remains largely intact across AD models was a welcome result to Jürgen Götz of the University of Queensland in Brisbane, Australia. Götz recently reported that transiently opening the BBB with ultrasound triggers plaque clearance in AD mouse models (see Mar 2015 news). “An argument one often encounters when discussing the potential application of ultrasound for transient BBB opening is that the BBB is compromised in AD,” he wrote. He added that in addition to van der Brug’s new findings, previous studies have reported that in some AD models the BBB is even tighter (see Mehta et al., 2013).
The authors’ conclusion that the blood-brain barrier remains largely intact across models of neurodegenerative disease and in humans with AD contradicts many studies using differing techniques that say otherwise, commented Zlokovic. He believes the best way to measure disruption of the BBB is by visualizing blood vessels in living animals, using techniques such as 2-photon imaging. He also noted that the idea that drugs would be more readily delivered across a compromised barrier is false, as pathological processes that damage the barrier also disrupt active transport mechanisms needed for drug delivery.
What do results from AD mouse models say about the state of the BBB in human disease? Maybe their import is limited because one big difference between animal models and human AD is the presence of cerebral amyloid angiopathy (CAA). In some people, the vascular amyloid deposits of CAA cause vessels to bleed, but most mouse models have no CAA. Van der Brug acknowledged this difference, but he pointed out that his analysis of postmortem AD brains showed no increase in infarcts in people with AD. However, it is important to note that people with vascular disease, who could be more likely to have CAA, were excluded from the study.—Jessica Shugart
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Research Models Citations
- Erickson MA, Banks WA. Blood-brain barrier dysfunction as a cause and consequence of Alzheimer's disease. J Cereb Blood Flow Metab. 2013 Oct;33(10):1500-13. PubMed.
- Mehta DC, Short JL, Nicolazzo JA. Altered Brain Uptake of Therapeutics in a Triple Transgenic Mouse Model of Alzheimer's Disease. Pharm Res. 2013 Jun 22; PubMed.
No Available Further Reading
- Bien-Ly N, Boswell CA, Jeet S, Beach TG, Hoyte K, Luk W, Shihadeh V, Ulufatu S, Foreman O, Lu Y, DeVoss J, van der Brug M, Watts RJ. Lack of Widespread BBB Disruption in Alzheimer's Disease Models: Focus on Therapeutic Antibodies. Neuron. 2015 Oct 21;88(2):289-97. PubMed.