Alzheimer's disease and blood-brain barrier leakage

Alzheimer's disease is not a unitary condition but variable in causation at the individual level like all complex chronic disorders. Neuroinflammation, metabolic damage, vascular compromise, accumulation of noxious debris (amyloid β and tau), impairments in brain CSF and lymphatic drainage and other causes can all variously contribute to Alzheimer's and other dementias. Now original research recently published in the journal Radiology demonstrates that leakiness of the blood-brain barrier (BBB) can permit an environment hostile to neuronal health that contributes to cognitive decline, Alzheimer's and other dementias. The authors state:

"Evidence is increasing that impairment of the cerebral microvasculature is a contributing factor in the pathophysiology of Alzheimer disease (AD). However, the exact pathway remains unclear. Results of histologic evaluation and albumin sampling studies show that an increased permeability of the blood-brain barrier (BBB) is likely a key mechanism."

An intact blood-brain barrier is essential for brain health

The BBB is a collection of cells and other structures in the cerebrovascular wall that when healthy permits only privileged access into the brain from the extra-cerebral blood compartment.

"It regulates the delivery of important nutrients to the brain through active and passive transport mechanisms and prevents neurotoxins from entering the brain. It also has a clearance function, meaning that it removes surplus substances from the brain. A well-functioning BBB is essential to keeping the brain tissue in a healthy condition. Results of previous studies suggest that deterioration of the BBB can cause an ill-conditioned environment for neuronal cells and other pathologic changes such as small-vessel abnormality, protein deposits, inflammation, and neuronal cell death. These changes eventually may lead to cognitive decline and dementia."

Early Alzheimer's shows abnormal BBB permeability

Blood-brain barrier degradation has earlier been demonstrated in advanced Alzheimer's disease. Here the authors examined whether or not BBB leakage contributes to the early stages of disease.

"To investigate whether BBB leakage contributes to the early pathophysiology of AD, we hypothesized that patients with early forms of AD already show increased BBB permeability in comparison with age-matched control subjects. For this pilot study, we used a dedicated dynamic contrast-enhanced MR imaging acquisition protocol with dual-time resolution that separates the filling of the blood vessels from the leakage. We also investigated differences in local blood plasma volume fraction, and the relationship between BBB permeability and global cognition."

The analyzed data for patients diagnosed with mild cognitive impairment (MCI) due to AD or patients or patients with early AD (a continuum of cognitive decline who had been referred by general practitioners because of memory concerns, in comparison with healthy controls. Individuals with dementia of vascular origin were excluded, as were those with major cardiovascular and neuropsychiatric disorders, Parkinson's, MS, trauma, major structural abnormalities of the brain, and alcohol or drug abuse. They indeed demonstrated a marked distinction between their study subjects and controls:

"The BBB leakage rate was significantly higher in patients compared with that in control subjects in the total GM (grey matter) and cortex but not in the WM, normal-appearing WM, deep GM, or WM hyperintensities...When adjustments were made for all covariates, the patients exhibited a significantly higher leakage volume in the WM and GM and also in the normal-appearing WM, deep GM, cortex, but not in WM hyperintensities...The median blood plasma volume was significantly lower in the patients than in the control subjects in all tissue classes."

BBB leakage in early Alzheimer's is widespread

The leakage is not due to vascular abnormalities, and leakage volume was even more striking than rate:

"The results of this study showed increased BBB leakage in patients with early AD. The leakage was globally distributed throughout the cerebrum and was associated with declined global cognitive performance. By using dynamic contrast-enhanced MR imaging with dual-time resolution, we found an increased BBB leakage rate in the GM of patients with early AD. By also showing very subtle BBB impairment in the WM, leakage volume proved to be even more sensitive to the differences in BBB leakage than was the leakage rate. Not only did this show that the differences between patients with early AD and healthy control subjects were in the extent of the BBB leakage rather than the rate (ie, strength), but it also showed that the leakage was widespread rather than localized to a single tissue class such as WM hyperintensities, normal-appearing WM, or cortex. In addition, the BBB impairment did not fully originate from vascular abnormality, because adding diabetes and other noncerebral vascular diseases to the analysis model did not change the results. This suggested that the BBB impairment stemmed from the AD abnormality instead of from vascular comorbidities."

Breakdown in tight junctions like the intestinal barrier

The intestinal barrier, critical for healthy immune system regulation, loses integrity with a breakdown of the tight cellular junctions. So too with the blood-brain barrier.

"The leakage observed in this study can be explained as a breakdown of the BBB tight junctions. It has been shown in rodents that tight junction damage allows gadolinium leakage through the BBB. The regions with high BBB leakage were diffusely distributed throughout the brain, showing that BBB tight junctions were globally impaired. This could have allowed the passage of small and lipophilic molecules that could not cross a healthy BBB. The loss of tight junctions also changes cell polarity, which influences the expression of transporter complexes and thus indirectly affects active transport across the BBB. Therefore, both passive and active transport mechanisms may be impaired in patients with early AD, possibly disturbing homeostasis."

Toxic accumulations in the brain and cognitive impairment

The authors have demonstrated that BBB leakage tracks cognitive impairment in early Alzheimer's:

"We found that cognitive decline was associated with stronger BBB leakage, and both the patients with MCI and those with early AD showed increased BBB leakage. These observations suggest that BBB impairment may be a contributing factor in the early pathophysiology of AD. A possible mechanism is that loss of tight junctions impairs the filter function of the BBB, leading to a toxic accumulation of substances in the brain. This, combined with the altered active transport systems, might add up to a substantial effect on neuronal function that eventually leads to dementia."

BBB and amyloid β

Clearance of amyloid β is also impaired:

"...amyloid β is actively transported across the BBB, whereas gadolinium leaks passively through the tight junctions. Previous work with positron emission tomographic data has shown that clearance of amyloid β is also impaired in patients with AD. An impaired clearance of amyloid β would mean that the BBB is impaired in different ways, contributing to the pathologic cascade leading to AD."

Most importantly...

"Therefore, BBB leakage may help to provide a biomarker for early diagnosis, or at least a marker indicating vulnerability for the development of dementia. Successful prediction of dementia eventually might lead to optimized treatment, delay, or even prevention of the disease."

Clinical note

Early diagnosis is key here, and for those of us without dynamic gadolinium contrast-enhanced MR imaging at hand I highly recommend the Blood Brain Barrier Permeability™ screen from Cyrex Labs (Array 20) which offers the clinician the ability to detect early changes in BBB permeability. Clinicians experienced in rehabilitation of the gut barrier will be familiar with resources to evaluate and remediate inflammation and other insults to the blood-brain barrier.The authors conclude:

"...in this pilot study, MR imaging was used to show global, diffusely distributed BBB leakage in patients with early AD, which suggests that a compromised BBB is part of the early pathology of AD and might be part of a cascade of pathologic events that eventually lead to cognitive decline."

• "Patients with early Alzheimer disease have significantly more tissue characterized by blood-brain barrier leakage than do healthy control subjects, both in the normal-appearing white matter and in the gray matter.
• Blood-brain barrier leakage in the gray matter correlates with lower scores on the Mini-Mental State Examination."