Comparison of Monoamine Oxidase-A, Aβ Plaques, Tau, and Translocator Protein Levels in Postmortem Human Alzheimer's Disease Brain

Abstract

Increased monoamine oxidase-A (MAO-A) activity in Alzheimer's disease (AD) may be detrimental to the point of neurodegeneration. To assess MAO-A activity in AD, we compared four biomarkers, Aβ plaques, tau, translocator protein (TSPO), and MAO-A in postmortem AD. Radiotracers were [¹⁸F]FAZIN3 for MAO-A, [¹⁸F]flotaza and [¹²⁵I]IBETA for Aβ plaques, [^124/125I]IPPI for tau, and [¹⁸F]FEPPA for TSPO imaging. Brain sections of the anterior cingulate (AC; gray matter GM) and corpus callosum (CC; white matter WM) from cognitively normal control (CN, n = 6) and AD (n = 6) subjects were imaged using autoradiography and immunostaining. Using competition with clorgyline and (R)-deprenyl, the binding of [¹⁸F]FAZIN3 was confirmed to be selective to MAO-A levels in the AD brain sections. Increases in MAO-A, Aβ plaque, tau, and TSPO activity were found in the AD brains compared to the control brains. The [¹⁸F]FAZIN3 ratio in AD GM versus CN GM was 2.80, suggesting a 180% increase in MAO-A activity. Using GM-to-WM ratios of AD versus CN, a >50% increase in MAO-A activity was observed (AD/CN = 1.58). Linear positive correlations of [¹⁸F]FAZIN3 with [¹⁸F]flotaza, [¹²⁵I]IBETA, and [¹²⁵I]IPPI were measured and suggested an increase in MAO-A activity with increases in Aβ plaques and tau activity. Our results support the finding that MAO-A activity is elevated in the anterior cingulate cortex in AD and thus may provide a new biomarker for AD in this brain region.

Keywords: Alzheimer’s disease; Aβ plaques; [125I]IPPI; [18F]FEPPA; [18F]flotaza; [18]FAZIN3; human tau; monoamine oxidase-A.

Figure 1

Aβ, tau, MAO-A, and TSPO in AD Subjects: Schematic showing four biomarkers for AD. (A). Aβ-plaque (senile plaques, SP) PET imaging agents are currently used in AD subjects. [¹⁸F]flotaza and [¹²⁵I]IBETA, both Aβ plaque imaging agents, were used in this work. (B). Tau (neurofibrillary tangles, NFT) PET imaging agents are currently used for in AD subjects. In this work, [^124/125I]IPPI imaging agents were used to evaluate Tau. (C). Translocator protein (TSPO), a biomarker for neuroinflammation, is being studied in AD using radiotracers such as [¹⁸F]FEPPA. (D). Monoamine oxidase-A (MAO-A) in AD using [¹⁸F]FAZIN3 as a potential new biomarker for neuroinflammation was investigated in this work.

Figure 2

MAO-A Imaging of [¹⁸F]FAZIN3: (A,C). Anti-Aβ immunostained brain slices (10 μm) of two AD subjects (AD 12-27 and AD 11-78) showing abundant Aβ plaques (arrows) in the GM regions of the anterior cingulate (inset shows GM with Aβ plaques). (B,D). Binding of [¹⁸F]FAZIN3 in AD 12-27 and AD 11-78 brain slice GM is significantly greater compared to the lower levels in corpus callosum, WM. (E,G). Brain slices (10 μm) of CN subjects (CN 12-21 and CN 10-39) show GM regions of anterior cingulate, (F,H). Binding of [¹⁸F]FAZIN3 in the CN brain GM and lower levels WM. (I). Plot of GM and WM of all AD and CN subjects. [¹⁸F]FAZIN3 binding is seen in all CN and AD subjects, with GM showing higher levels compared to WM. AD brains show greater [¹⁸F]FAZIN3 in GM regions compared to control subjects. (J). Plot shows averages of all AD and CN subjects (“**** = p < 0.0001” for AD GM versus CN GM, unpaired two-tailed t-test and p < 0.05 for AD WM versus CN WM; open circles males, solid circles females). Ratio of AD GM/WM = 3.43 whereas CN GM/WM = 2.17. Autoradiography scale bar: 0–500 digital light units (DLU)/mm².

Figure 3

MAO drug effects on [¹⁸F]FAZIN3: (A,B). Anti-Aβ AD 11-27 and AD 13-10, 10 μm brain slice showing abundant Aβ plaques in anterior cingulate (GM). (C,D). MAO-A in GM labeled by [¹⁸F]FAZIN3 in brain sections of AD 11-27 and AD 13-10 (arrows indicate Aβ plaques in (A,B); inset shows scan of the same brain slices). (E,F). MAO-A drug clorgyline, 1 μM, displaced >90% [¹⁸F]FAZIN3 in adjacent sections of AD 11-27 and AD 13-10. (G,H). MAO-B drug (R)-deprenyl, 1 μM, had little effect on [¹⁸F]FAZIN3 binding in of AD 11-27 and AD 13-10. (I). Plot of [¹⁸F]FAZIN3 total binding, in the presence of (R)-deprenyl and clorgyline in AD subjects showing no effect of (R)-deprenyl while clorgyline displaced >90% of [¹⁸F]FAZIN3 (“*** = p < 0.001” for total versus clorgyline; total versus (R)deprenyl was not significant. Autoradiography scale bar: 0 to 700 DLU/mm².

Figure 4

Aβ Plaque Imaging with [¹⁸F]Flotaza, [¹²⁵I]IBETA, and MAO-A Imaging with [¹⁸F]FAZIN3: (A). AD brain slice of subject AD 11-38 showing anti-Aβ immunostained gray matter (GM) in the anterior cingulate and a lack of Aβ plaques in white matter (WM) in the corpus callosum (inset shows Aβ plaques at arrow at 50 µm); (B). [¹⁸F]Flotaza binding to Aβ plaques in the gray matter regions in adjacent slices of subject AD 11-38 (inset shows scan of the brain slice). (C). [¹²⁵I]IBETA binding to Aβ plaques in the gray matter regions in adjacent slices of subject AD 11-38 (inset shows scan of the brain slice). (D). High levels of [¹⁸F]FAZIN3 binding in gray matter in adjacent slice of AD 11-38 subject showing binding to MAO-A in the GM regions (inset shows scan of the brain slice). (E). [¹²⁵I]IBETA binding to Aβ plaques in the gray matter regions in CN subject CN 12-21 (inset shows scan of the brain slice). (F). Linear correlation plot of [¹⁸F]flotaza and [¹²⁵I]IBETA with [¹⁸F]FAZIN3 binding in all 6 AD subjects including the one CN 12-21 subject (dotted circle) shows positive relationship (r² =0.65, p = 0.01 with [¹⁸F]flotaza and r² =0.73, p = 0.009 with [¹²⁵I]IBETA). Autoradiography scale bar: 0 to 500 digital light units (DLU)/mm² for (B,C) and (E) and 0 to 200 digital light units (DLU)/mm² for (D).

Figure 5

Tau Imaging with [¹²⁵I]IPPI, TSPO Imaging with ¹⁸F]FEPPA, and MAO-A Imaging with [¹⁸F]FAZIN3: (A). Anti-tau immunohistochemical staining revealed a presence of NFT in the AD 13-10 subject. Inset at 50 µm magnification shows a presence of NFT in the GM (arrow). (B). [¹²⁴I]IPPI binding to tau was observed in the gray matter of the adjacent slice of the AD 13-10 subject. Little nonspecific [¹²⁴I]IPPI binding was seen in WM (inset shows scan of the brain slice). (C). Binding of [¹⁸F]FAZIN3 in gray matter in the adjacent slice of the AD 13-10 subject shows extensive binding to MAO-A in the GM regions (inset shows scan of the brain slice). (D). Linear regression plot of [¹²⁵I]IPPI and [¹⁸F]FAZIN3 binding in all 6 AD subjects shows a modest positive correlation (r² = 0.57) but not significant (p > 0.05). (E). Brain slices of AD subject 13-10 shows high binding of [¹⁸F]FEPPA to TSPO in the gray matter regions of the anterior cingulate (inset shows scan of the brain slice). (F). Lower levels of [¹⁸F]FEPPA binding are observed in the gray matter of the CN 10-40 subject (inset shows scan of the brain slice). (G). Average of [¹⁸F]FEPPA binding in the GM regions of 6 AD subjects was greater than that of the 6 CN subjects (** p < 0.01; open circles males, solid circles females). Ratio of AD GM-to-CN GM = 1.65, suggesting increased TSPO expression in the AD brains. (H). Linear plot of [¹⁸F]FEPPA and [¹⁸F]FAZIN3 binding in all 6 AD subjects exhibited significant positive correlation (p < 0.01). Autoradiography scale bar: 0 to 3000 digital light units (DLU)/mm² for (B,E) and (F) and 0 to 500 digital light units (DLU)/mm² for (C).

Figure 6

Correlation Plots of GM/WM Ratios of [¹⁸F]FAZIN3 with other Biomarkers: (A). GM/WM ratio of [¹⁸F]FAZIN3 binding to MAO-A in all the six AD subjects and one CN subject (with positive Aβ) showed a weak positive correlation (r² = 0.56; p = 0.018) with the [¹⁸F]flotaza GM/WM binding ratio to Aβ plaques. (B). A better positive correlation (r² = 0.70; p = 0.005) of the [¹⁸F]FAZIN3 GM/WM ratio was observed with the [¹²⁵I]IBETA GM/WM binding ratio to Aβ plaques. (C). A good correlation (r² = 0.84; p > 0.05, NS) of the [¹⁸F]FAZIN3 GM/WM ratio was observed with the [¹²⁵I]IPPI GM/WM binding ratio to tau. (D). Binding of the [¹⁸F]FAZIN3 GM/WM ratio did not show a positive correlation with the [¹⁸F]FEPPA GM/WM binding ratio to TSPO. The trend appeared to be a negative correlation (r² = 0.30; p = 0.05). (E). A plot of [¹⁸F]flotaza, [¹²⁵I]IBETA, and [¹²⁵I]IPPI averages versus the [¹⁸F]FAZIN3 GM/WM ratio provides a good linear regression with high significance (r² = 0.83; p = 0.02).

Figure 7

Radiotracer Binding to Different Braak Stages: Ratios of GM/WM of [¹⁸F]FAZIN (for MAO-A), [¹⁸F]flotaza (for Aβ plaques), [¹²⁵I]IBETA (for Aβ plaques), [¹⁸F]FEPPA (for TSPO), and [¹²⁵I]IPPI (for tau) in CN and AD subjects at Braak I, Braak II, Braak V, and Braak VI are shown.