Ventricular Decline Correlates Well with Other Forms of Damage in the Aging Brain

Here, researchers examine the correlation between ventricular dsyfunction, other noted forms of damage observed in brain aging, and the onset of cognitive decline. The ventricular system is where cerebrospinal fluid is created and circulates throughout the brain. Many things go wrong in the aging brain, all stemming from the same few root cause processes of damage accumulation in and around cells. Thus correlations between specific observed changes and the progression of dementia should be expected, but don't necessarily imply direct causation - though a particularly good correlation always indicates that further investigation is probably merited.

This line of investigation ties in to a growing area of research regarding the impairment of drainage of cerebrospinal fluid in aging. This impairment may explain the slowly rising levels of protein aggregates and other molecular waste in the brains of older individuals, a state of affairs known to contribute to the development of neurodegenerative conditions. Normally these wastes are removed at some pace through various filtration and drainage channels for cerebrospinal fluid, but the channels become dysfunctional, just like all other biological systems in older individuals. Leucadia Therapeutics is an example of a company working to intervene and restore youthful levels of drainage to what they consider the more important path. Other groups are looking into different areas of impaired fluid flow in the brain. All in all it is a most interesting and promising area of development.

The human brain's ventricular system is essential for the movement of nutrient-rich cerebrospinal fluid (CSF) throughout the central nervous system. A special epithelial lining along the ventricle walls composed of ependymal cells allows for the movement of CSF nutrients into the brain parenchyma as well as clearance of proteins and metabolites from the interstitial fluid (ISF). This ependyma-mediated bidirectional CSF-ISF exchange, as well as the formation of a cell barrier to prevent movement of proteins and metabolites from the CSF back into the ISF, relies on the presence of an intact ependymal cell monolayer. Pathological conditions in humans that are characterized by ependymal cell stretching and/or loss, including hydrocephalus, typically result in decreased CSF turnover rates and impaired clearance of proteins and metabolites resulting in a harmful buildup of these substances in brain parenchymal tissue.

Longitudinal magnetic resonance imaging (MRI)-based studies have established that expansion of the brain's fluid-filled lateral ventricles (LVs), or ventriculomegaly, is a defining feature of the aging brain. Ventricle expansion rates correlate strongly with declining cognitive performance and the rate of ventricle volume increase has been linked to an increase in Alzheimer's disease (AD)-related amyloid-beta (Aβ) plaques and tau neurofibrillary tangles, as well as alterations in CSF biomarker composition. Together, these point towards defective CSF-ISF exchange and impaired clearance mechanisms that are characteristic of AD.

Degeneration of periventricular brain tissue and declines in associated white matter tract integrity are common with normal aging and the extent of periventricular tissue abnormalities has been linked to dementia and AD. Periventricular hyperintensities (PVH), as measured using MRI, are indicative of fluid accumulation, or edema, often located in the parenchymal tissue directly adjacent to the frontal and occipital horns of the LV. The precise etiology of PVH is not clear; however, studies have implicated impaired drainage of ISF from the periventricular white matter resulting in aberrant fluid accumulation.

In previous studies, we found that enlarged ventricles from aging humans exhibited regional gliosis in the place of functional ependymal cell coverage. We predict that replacement of the ependymal lining with stratified layers of astrocytes at the ventricle surface adversely affects CSF/ISF bulk flow mechanisms, leading to fluid accumulation or edema and harmful buildup of proteins and metabolites in the periventricular space. Due to the rarity of longitudinal MRI data sets and associated subject-matched periventricular tissue biospecimens, this has never been directly demonstrated.

Using data from the Alzheimer's Disease Neuroimaging Initiative (ADNI) and the Baltimore Longitudinal Study of Aging (BLSA), we investigated the relationships among the following variables: ventricle expansion, PVH, periventricular white matter tract integrity, and degree of cognitive impairment. We also investigated the histopathological correlates of these measures, including LV wall gliosis and periventricular protein accumulation. We found that both LV and PVH volumes increase with age, and this expansion is more rapid and dramatic in cognitively impaired (CI) subjects. We also found a direct relationship between LV volume and PVH volume increase. Case studies from the BLSA allowed us to link ventricle expansion with regional gliosis, where an intact ependymal cell monolayer was replaced with stratified layers of astrocytes in regions of LV expansion. Additionally, adjacent parenchymal regions exhibited edema (as indicated by PVH), white matter deterioration, decreased vascular integrity, and harmful buildup of proteins including Aβ and tau.

Link: https://doi.org/10.3389/fnagi.2017.00445

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