Is Impaired Clearance the Culprit?
Coauthor Jeffrey Iliff, PhD, professor, University of Washington Department of Psychiatry and Behavioral Sciences and University of Washington Department of Neurology, Seattle, elaborated.
“In the setting of Alzheimer’s disease, a signature of the disease is reduced CSF Abeta42, which is thought to reflect that much of the amyloid gets ‘stuck’ in the brain in the form of amyloid plaques,” he said. “There are usually higher levels of phosphorylated tau and total tau, which are thought to reflect the presence of tau tangles and degeneration of neurons in the brain. But in this study, all of those were lowered, which is not exactly an Alzheimer’s disease profile.”
Dr. Iliff, associate director for research, VA Northwest Mental Illness Research, Education, and Clinical Center at VA Puget Sound Health Care System, Seattle, suggested that the culprit may be impairment in the brain’s glymphatic system. “Recently described biological research supports [the concept of] clearance of waste out of the brain during sleep via the glymphatic system, with amyloid and tau being cleared from the brain interstitium during sleep.”
A recent hypothesis is that blast TBI impairs that process. “This is why we see less of those proteins in the CSF. They’re not being cleared, which might contribute downstream to the clumping up of protein in the brain,” he suggested.
The evidence base corroborating that hypothesis is in its infancy; however, new research conducted by Dr. Iliff and his colleagues sheds light on this potential mechanism.
In blast TBI, energy from the explosion and resulting overpressure wave are “transmitted through the brain, which causes tissues of different densities — such as gray and white matter — to accelerate at different rates,” according to Dr. Iliff. This results in the shearing and stretching of brain tissue, leading to a “diffuse pattern of tissue damage.”
It is known that blast TBI has clinical overlap and associations with posttraumatic stress disorder (PTSD), depression, and persistent neurobehavioral symptoms; that veterans with a history of TBI are more than twice as likely to die by suicide than veterans with no TBI history; and that TBI may increase the risk for Alzheimer’s disease and related dementing disorders, as well as CTE.
The missing link may be the glymphatic system — a “brain-wide network of perivascular pathways, along which CSF and interstitial fluid (ISF) exchange, supporting the clearance of interstitial solutes, including amyloid-beta.”
Dr. Iliff and his group previously found that glymphatic function is “markedly and chronically impaired” following impact TBI in mice and that this impairment is associated with the mislocalization of astroglial aquaporin 4 (AQP4), a water channel that lines perivascular spaces and plays a role in healthy glymphatic exchange.
In their new study, the researchers examined both the expression and the localization of AQP4 in the human postmortem frontal cortex and found “distinct laminar differences” in AQP4 expression following blast exposure. They observed similar changes as well as impairment of glymphatic function, which emerged 28 days following blast injury in a mouse model of repetitive blast mTBI.
And in a cohort of veterans with blast mTBI, blast exposure was found to be associated with an increased burden of frontal cortical MRI-visible perivascular spaces — a “putative neuroimaging marker” of glymphatic perivascular dysfunction.
The earlier Neurology study “showed impairment of biomarkers in the CSF, but the new study showed ‘why’ or ‘how’ these biomarkers are impaired, which is via impairment of the glymphatic clearance process,” Dr. Iliff explained.