Role of Cerebrovascular Cells in Tau Processing Following Traumatic Brain Injury

Eisenbaum, Maxwell (2022). Role of Cerebrovascular Cells in Tau Processing Following Traumatic Brain Injury. PhD thesis The Open University.



Repetitive exposure to mild traumatic brain injuries (r-mTBI) sustained through the participation in contact sports can lead to chronic post-concussive symptoms and the development of neurodegenerative diseases such as Alzheimer’s disease and Chronic Traumatic Encephalopathy (CTE). A primary hallmark of CTE is the accumulation of pathogenic tau in neurons and astrocytes that surround small blood vessels in the brain. Chronic exposure to r-mTBI leads to elevated levels of extracellular tau and pathogenic tau accumulation in neurons, ultimately resulting in neuronal death. While the mechanisms responsible for pathogenic tau elimination from the brain are unclear, our prior work demonstrated that cells associated with the cerebrovasculature can interact with extracellular tau and may contribute to the removal of extracellular tau from the brain.

In this thesis, I examined the mechanisms through which the cerebrovascular cells eliminate extracellular tau from the brain and how those processes are impacted by r-mTBI. I demonstrated that brain vascular mural cells (pericytes and smooth muscle cells) progressively degenerate following exposure to r-mTBI consistent with what is observed in individuals with AD. This mural cell dysfunction impairs the ability of the cerebrovessels to interact with tau. Furthermore, I found that the cerebrovasculature can eliminate extracellular tau from the brain through caveolae-mediated endothelial transcytosis, which is impaired following chronic exposure to r-mTBI. The diminished tau transit across the blood-brain barrier following brain injury may be a contributing factor in the pathogenic tau accumulation observed in CTE.

A significant genetic risk factor for neurodegenerative diseases including AD and CTE is possession of the E4 isoform of Apolipoprotein E (ApoE). Astrocytes are the predominant source of ApoE in the brain, though there is very little understanding regarding their interactions with extracellular tau, particularly after exposure to head trauma. While the ApoE4 isoform has been associated with increased tau accumulation and cerebrovascular dysfunction after TBI, investigations into these associations are limited. The current studies found that while astrocytes internalize and release tau back into the extracellular space under normal conditions, these processes become dysfunctional following r-mTBI leading to astrocytic tau accumulation, which is further exacerbated by the ApoE4 isoform.

In summary, I identified the factors responsible for the elimination of extracellular tau across the BBB, which are impaired after head trauma. Therapeutic interventions that restore these processes may ameliorate the chronic accumulation of tau associated with neurodegenerative disease. These findings may be particularly important for individuals with the ApoE4 isoform, who are more susceptible to the pathophysiological sequelae of tau accumulation, particularly after exposure to r-mTBI.

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