Master of Science
Thiessen, Jonathan D.
Whitehead, Shawn N.
Accurate and sensitive imaging biomarkers are required to study the progression of white matter (WM) microglial activation in neurological diseases in vivo. The translocator protein (TSPO) is considered a sensitive target for imaging microglial activation with positron emission tomography (PET). This study aimed to test the ability of TSPO to detect WM microglial activation marked by major histocompatibility complex class II (MHCII) molecules in rat models of prodromal Alzheimer’s disease and acute subcortical stroke.
Fischer 344 wild-type (n = 12) and TgAPP21 (n = 11) rats were imaged with [18F]FEPPA PET and MRI to investigate TSPO tracer uptake in the corpus callosum. Wild-type rats subsequently received an endothelin-1 (ET1)-induced subcortical stroke and were imaged at days 7 and 28 post-stroke before immunohistochemistry of TSPO, GFAP for astrocytes, iNOS for microglia releasing toxic nitrous oxide, and the MHCII rat antigen, OX6.
[18F]FEPPA-PET findings that TSPO expression was not increased in WM of TgAPP21 rats and was only increased in the infarct and proximal WM were confirmed by immunohistochemistry (infarct TSPO cells/mm2: day 7 = 555 ± 181; day 28 = 307 ± 153; proximal WMTSPO cells/mm2: day 7 = 113 ± 93; day 28 = 5 ± 7). TSPO and iNOS were not able to detect the chronic WM microglial activation that was detected with MHCII in the contralateral corpus callosum (day 28 OX6 % area: saline = 0.62 ± 0.38; stroke = 4.30 ± 2.83; P = .029).
Within the regions and groups investigated, TSPO was only expressed in the stroke-induced insult and proximal tissue, and therefore was unable to detect remote and non-insult-related chronically activated microglia overexpressing MHCII in WM.
Summary for Lay Audience
Accurate and sensitive biomarkers are required to study the progression of inflammation in neurological diseases. To image white matter inflammation in living subjects, researchers can produce radioactive molecules that bind to proteins in the body that serve as markers of inflammation, inject them into the subject, then image them with positron emission tomography (PET). Our group set out to investigate whether a protein known as the translocator protein can be detected, in living subjects, in a type of white matter inflammation that we previously associated with Alzheimer’s disease and deep stroke and can currently only be imaged after death. We hypothesized that the translocator protein expression can be detected in rats with Alzheimer’s Disease at the white matter and in rats with deep stroke at white matter that was distant from the stroke.
Rats that were non-genetically modified (n = 12) and genetically modified to express a protein associated with Alzheimer’s Disease (n = 11) were injected with the radioactive molecule [18F]FEPPA to image the translocator protein using PET. After initial assessment of white matter inflammation, the non-genetically modified rats received a deep stroke and were imaged again at days 7 and 28 post-stroke. The rats were sacrificed to investigate their expression of the translocator protein, other inflammatory proteins, and a protein that indicates our target inflammation, OX6.
[18F]FEPPA positron emission tomography only detected inflammation in the stroke region of rats that received the deep stroke; the distant white matter inflammation we wanted to image was not detected. Inflammation was detected in distant white matter using post-mortem microscopy of the inflammatory molecule OX6, but it was not detectable using the translocator protein, irrespective of the binding molecule used.
Within the regions and groups investigated, the translocator protein was only able to detect inflammation near the stroke region, and therefore was not able to detect the distant and non-injury-related yet pathologically important white matter inflammation that we wanted to detect. To detect that white matter inflammation with PET, new radioactive molecules will need to be developed.
Al-Khishman, Nassir, "TSPO PET Detects Acute Neuroinflammation but not Diffuse Chronically Activated MHCII Microglia in the Rat" (2020). Electronic Thesis and Dissertation Repository. 7260.
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Available for download on Monday, March 01, 2021