Evaluation of Nicotinic Acetylcholine Receptors in Taupathies

Introduction: Deposition of abnormal Tau protein in the brain results in an array of neurodegenerative disorders referred to taupathies. Tau isoforms 3R and 4R are present in Alzheimer’s disease (AD, 3/4R), Pick’s disease (PiD, 3R) and corticobasal degeneration (CBD, 4R). Selective imaging agents for 3R and 4R Tau will help development of differential diagnostic methods of various taupathies. Two major nicotinic acetylcholinergic receptor (nAChR) subtypes in the human brain, a4b2* and a7, are involved in cognition, learning and memory and may be adversely affected in taupathies. We report here a comparison of autoradiographic binding of [¹⁸F]nifene to a4b2* nAChR and [¹²⁵I]a-bungarotoxin ([¹²⁵I]a-BTX) to a7 nAChR in HP (CA1/subiculum) from postmortem AD brains and [¹⁸F]nifene in frontal cortex of CBD and PiD brains.

Methods: Well-characterized human post-mortem brain tissue sections (10 μm thick) consisting of HP CA1-subiculum regions, (AD, n=16F and 13M and cognitively normal (CN) =16F and 16M) and frontal cortex regions from well-characterized CBD (n=6) and PiD (n=6) subjects were obtained from Banner Health, Sun City, Arizona. Brain slices (AD, CN, CBD and PiD) were incubated [¹⁸F]Nifene (1 μCi/cc) in Tris/pH 7.4 buffer at 25 ^oC for 1 hr or [¹²⁵I]a-BTX (AD, CN) in Tris/pH 7.4 buffer containing 0.1% BSA at 25 ^oC for 2 hr. Nonspecific binding was measured using 300 μM nicotine. Adjacent sections were tested for Tau using [¹²⁵I]IPPI (Fig-1E,J) and Ab plaques using [¹⁸F]flotaza with our previously published protocols. Anti-tau (Fig-1D,I) and anti-Aβ immunostaining was carried out on adjacent slices. Using the Optiquant program, regions of interest were drawn and digital light units/ mm² (DLU/mm²) were used to quantify the percentage change in binding of [¹⁸F]Nifene and [¹²⁵I]a-BTX.

Results: All CN subjects exhibited significant [¹⁸F]Nifene and binding [¹²⁵I]a-BTX in the HP CA1-SUB regions compared to the lower binding in adjacent white matter (WM) regions (Fig-1). This is consistent with our PET [¹⁸F]Nifene results in healthy human subjects. Average HP ratio (n=16) of GM/WM = 4.38 in the CN HP CA1-SUB, while it was lower in the AD HP CA1-SUB (n=16), GM/WM=2.76 (Fig-1B,E). Using the ratios, a 37% reduction in [¹⁸F]Nifene AD HP CA1-SUB was measured. In the case of [¹²⁵I]a-BTX average HP ratio (n=14) of GM/WM = 1.81 in the CN HP CA1-SUB, while in the AD HP CA1-SUB (n=14), GM/WM=2.14 (Fig-1C,F). Male-female HP CA1-SUB differences in the brain sections were not significant, both in CN and AD subjects. However, this will have to be ascertained in PET studies. Nicotine displaced [¹⁸F]Nifene binding from these brain regions. While [¹⁸F]Nifene binding in the AD subjects was inversely correlated to [¹²⁵I]IPPI and [¹⁸F]flotaza binding in HP CA1-SUB, [¹²⁵I]a-BTX binding was unaffected or elevated. Binding of [¹⁸F]nifene to the CBD FC was low compared to PiD. Ratio of PiD/CBD GM=3.18, suggesting a 70% lower a4b2* nAChRs in the CBD brain.

Conclusions: Binding of [¹⁸F]Nifene to a4b2* nAChRs is significantly reduced in HP CA1-SUB of the postmortem AD brain, whereas binding of [¹²⁵I]a-BTX to a7 nAChRs remained relatively unchanged with small increases. [¹⁸F]Nifene shows reductions in CBD, indicative of neuronal loss in the FC which may be due to cortical atrophy. Individual subject comparisons of the two receptor subtypes is currently being carried out in relation to the binding of [¹²⁵I]IPPI (NFT) (Fig-1J) and [¹⁸F]flotaza (SP) in these subjects. Thus, our initial results suggest a decrease in a4b2* nAChRs in AD and CBD and no change or slight increase in a7 nAChRs in the HP (CA1/subiculum) of AD subjects.

NIH/NIA RF1 AG029479 (JM), UCI UROP (TRM, RML, RM). Banner Health Research Institute for brain samples and UCI Pathology for immunostaining.

• Download figure
• Open in new tab
• Download powerpoint