Abstract
Purpose Reactive astrocytes play an important role in the development of Alzheimer’s disease (AD). Here, we aim to investigate the temporospatial relationship between reactive astrocytes, tau and amyloid-β, glucose metabolism, and microgliosis by using multitracer imaging in AD transgenic mouse models.
Methods Positron emission tomography (PET) imaging with [18F]SMBT-1 (monoamine oxidase-B), [18F]florbetapir (Aβ), [18F]PM-PBB3 (tau), [18F]FDG, and [18F]DPA-714 (translocator protein) was carried out in 5- and 10-month-old APP/PS1, 11-month-old 3×Tg mice, and aged-matched wild-type mice. The brain regional referenced standard uptake value (SUVR) was computed with the cerebellum as the reference region. Immunofluorescence staining was performed in mouse brain tissue slices.
Results [18F]SMBT-1 and [18F]florbetapir SUVRs were higher in the cortex and hippocampus of 10-month-old APP/PS1 mice than in 5-month-old APP/PS1 mice and wild-type mice. Reduced [18F]FDG SUVR was observed in the thalamus and midbrain of 5-month-old APP/PS1 mice compared to wild-type mice. No significant difference in brain regional [18F]DPA-714 SUVR was observed in 5- and 10-month-old APP/PS1 mice compared to wild- type mice. No significant difference in the SUVRs of any tracers was observed in 11-month-old 3×Tg mice compared to age-matched wild-type mice. A positive correlation between the SUVRs of [18F]SMBT-1 and [18F]DPA-714 in the cortex was observed. Immunostaining validated the distribution of MAO-B and TSPO, amyloid and tau inclusions in brain tissue from 10-month-old APP/PS1 mice and limited changes in 11-month- old 3×Tg mice.
Conclusion The findings provide in vivo evidence for reactive astrocytes along with amyloid plaque and tau deposition preceding microgliosis in animal models of AD pathologies.
1 Introduction
Alzheimer’s disease (AD) is pathologically characterized by abnormal accumulation of amyloid-beta (Aβ), tau tangles, reactive astrocytes, microgliosis and neuronal loss. Astrocytes are the most abundant glial cell population in the brain and play an important role in maintaining synaptic homeostasis by regulating synapse function, calcium signalling, and brain metabolism [1]. Reactive astrocytes are involved early in the pathophysiology of AD [2–4]. Reactive astrocytes measured by cerebrospinal fluid levels of glial fibrillary acidic protein (GFAP) have been shown to mediate the effect of Aβ on tau and drive downstream neurodegeneration and cognitive impairment in patients with AD [5] and preclinical AD. Postmortem studies of AD brains have demonstrated abundant reactive astrocytes and microglia around Aβ plaques and tangles [6–8]. Monoamine oxidase B (MAO-B) levels are increased not only in astrocytes but also in pyramidal neurons in the AD brain. Reactive astrocytes and MAO-B have emerged as treatment targets for neurodegenerative diseases [9].
Several positron emission tomography (PET) tracers for reactive astrocytes have been developed, including the MAO-B tracers [11C]deuterium-L-deprenyl (DED) and [18F]F-DED [10–12], the novel tracer [18F]SMBT-1, the mitochondrial imidazoline2 binding site (I2BS) tracer [11C]BU99008 [5], [11C]acetate [13] and the thyroid hormone transporter OATP1C1 [18F]sulforhodamine-101 [14]. In vivo [11C]DED has demonstrated divergent longitudinal changes in reactive astrocytes and amyloid in patients with autosomal-dominant [15] and prodromal AD [16, 17]. Higher brain [18F]SMBT-1 binding in Aβ+ than in Aβ-nondemented controls has been observed and is associated with Aβ accumulation at the preclinical stage of AD [18–22]. In animal models (APPswe, PS2APP, APPArcSwe), reactive astrocytes measured by using [11C]DED and [18F]F-DED precede the increase in the amyloid-PET signal [10–12].
The aim of the current study was to evaluate the distribution of the novel tracer [18F]SMBT-1 in two mouse models of AD (APP/PS1, 3×Tg). Using a multitracer approach with [18F]SMBT-1, [18F]florbetapir, [18F]PM- PBB3 (florzolotau, APN-1607), [18F]fluorodeoxyglucose (FDG), and [18F]DPA-714 (translocator protein (TSPO)), we assessed the temporospatial relationship of reactive astrocytes with Aβ, tau, glucose metabolism, and microgliosis. We hypothesized that reactive astrocytes are an early event associated with tau and Aβ accumulation. To our knowledge, this is the first in vivo imaging study using [18F]SMBT-1 in animal models of AD, along with tracers for the aforementioned other pathophysiologies.
2 Methods
2.1 Animal models
The animal models used in the study are summarized in Table 1. 3×Tg mice [B6;129- Psen1tm1MpmTg(APPSwe, tauP301L)1Lfa/Mmjax] aged 11 months [23], and APP/PS1 mice [B6. Cg- Tg(APPswe,PSEN1dE9)85Dbo/Mmjax] mice overexpressing the human APP695 transgene (Swedish (K670N/M671L)) and PSEN1 mutations [24] aged 5 and 10 months were used (Jax Laboratory, USA). Wild- type C57BL6 mice were obtained from Charles River Germany and Cavins Laboratory Animal Co. Ltd. of Changzhou. Mice were housed in ventilated cages inside a temperature-controlled room under a 12-h dark/light cycle. Pelleted food (3437PXL15, CARGILL) and water were provided ad libitum. Paper tissue and red Tecniplast mouse house® (Tecniplast, Italy) shelters were placed in cages for environmental enrichment.
2.2 Radiosynthesis
[18F]SMBT-1 (0.74 GBq/ml) was radiosynthesized from its precursor according to a previous publication[22]. [18F]DPA-714 (1.48 GBq/ml) was labelled with 18F at its 2-fluoroethyl moiety after nucleophilic substitution of the corresponding sylate analog[25]. [18F]PM-PBB3 (1.48 GBq/ml) was synthesized from an automatic synthesis module and kit provided by APRINDIA therapeutics (Suzhou, China)[26]. [18F]florbetapir (0.56 GBq/ml) was radiosynthesized from its precursor in a fully automated procedure suitable for routine clinical application[27]. [18F]FDG (1.48 GBq/ml) was prepared in the radiochemistry facility of the PET Center, Huashan Hospital, Fudan University for clinical use under Good Manufacturing Practices requirements. [11C]PIB (0.074 GBq/ml) was radiosynthesized according to a previously described protocol[28]. The identities of the aforementioned final products were confirmed by comparison with the high-performance liquid chromatography (HPLC) retention time of the nonradioactive reference compound by coinjection using a Luna 5 μm C18(2) 100 Å (250 mm×4.6 mm) column (Phenomenex) using acetonitrile and water (60:40) solvent with a 1.0 mL/min flow rate. Radiochemical purity >95% was achieved for all aforementioned tracers.
2.3 MicroPET
PET experiments using [18F]SMBT-1, [18F]florbetapir, [18F]PM-PBB3, [18F]FDG, and [18F]DPA-714 were performed using a Siemens Inveon PET/CT system (Siemens Medical Solutions, United States)[29]. Prior to the scans, mice were anaesthetized using isoflurane (1.5%) in medical oxygen (0.3-0.5 L/min) at room temperature with an isoflurane vaporizer (Molecular Imaging Products Company, USA). The mice were positioned in a spread-supine position on the heated imaging bed and subjected to inhalation of the anaesthetic during the PET/computed tomography (CT) procedure. The temperature of the mice was monitored. A single dose of tracer (∼0.37 MBq/g body weight, 0.1–0.2 mL) was injected into the animals through the tail vein under isoflurane anaesthesia. For dynamic PET, the raw PET data were binned into 9 frames (9×600 s). Static PET/CT imaging was obtained for 10 min post intravenous administration, depending on the tracer [18F]FDG at 60 min, [18F]florbetapir at 50 min, [18F]SMBT-1 at 60 min, [18F]DPA-714 at 40 min, and [18F]PM-PBB3 at 90 min. PET/CT images were reconstructed using the ordered subsets expectation maximization 3D algorithm (OSEM3D), with a matrix size of 128×128×159 and a voxel size of 0.815 mm×0.815 mm×0.796 mm. Data were reviewed using Inveon Research Workplace software (Siemens). Attenuation corrections derived from hybrid CT data were applied.
2.4 PET data analysis
Images were processed and analysed using PMOD 4.4 software (PMOD Technologies Ltd., Switzerland). Radioactivity is presented as the standardized uptake value (SUV) (decay-corrected radioactivity per cm3 divided by the injected dose per gram body weight). The time−activity curves were deduced from specific volumes of interest that were defined based on a mouse MRI T2-weighted image template[82]. Brain regional SUVR was calculated using the cerebellum (CB) as the reference region. The mask was applied for signals outside the brain volumes of interest for illustration.
2.5 Immunohistochemistry
Mice were perfused under ketamine/xylazine/acepromazine maleate anaesthesia (75/10/2 mg/kg body weight, i.p. bolus injection) with ice-cold 0.1 M phosphate buffered saline (PBS, pH 7.4) and in 4% paraformaldehyde (PFA) in 0.1 M PBS (pH 7.4), fixed for 24 h in 4% PFA and then stored in 0.1 M PBS at 4°C. Coronal brain sections (40 mm) were cut around bregma 0 to -2 mm. Sections were first washed in PBS 3×10 min, followed by antigen retrieval for 20 min in citrate buffer at room temperature. Then, the sections were permeabilized and blocked in 5% normal donkey or goat serum and 1% Triton-PBS for one hour at room temperature. Free- floating tissue sections were incubated with primary antibodies against 6E10, complement component C3d (C3D), CD68, AT-8, GFAP, glucose transporter type-1 (Glut1), MAO-B and TSPO overnight at 4°C (Suppl. Table 1) [30, 31] and with respective secondary antibodies. Sections were incubated for 15 min in 4’,6- diamidino-2-phenylindole (DAPI), washed 2×10 min with PBS, and mounted with VECTASHIELD vibrance antifade mounting media (Vector Laboratories, Z J0215). The brain sections were imaged at ×20 magnification using an Axio Oberver Z1 slide scanner (Zeiss, Germany) using the same acquisition setting for all slices and at ×10 and ×63 magnification using a Leica SP8 confocal microscope (Leica, Germany). The images were analysed by a person blinded to the genotype using Qupath and ImageJ (NIH, U.S.A.).
2.6 Statistics
Two-way ANOVA with Sidak post hoc analysis was used for comparisons between groups (GraphPad Prism 9.0, CA, USA). Pearson correlation analysis was used for the analysis of the association between the regional SUVR of different tracers.
3 Results
3.1 Increased [18F]SMBT-1 SUVR in the cortex and hippocampus of 10-month-old APP/PS1 mice
The HPLC and quality control (QC) of [18F]SMBT-1 are shown in SFig. 1. The time activity curve of [18F]SMBT-1 indicated that 60 min postinjection in mice is suitable for static scans, which is similar to the application in the clinical setting (SFig. 2). We observed that the [18F]SMBT-1 SUVR (Cb as reference region) of MAO-B, indicative of reactive astrocytes, was higher in the cortex and hippocampus of 10-month-old APP/PS1 mice than in 5-month-old APP/PS1 and age-matched wild-type mice. No regional difference in [18F]SMBT-1 SUVR was observed between 11-month-old 3×Tg mice and age-matched wild-type mice (Figs. 1a-f).
(a) Synthesized SMBT-1, (b) Standard.
(a) Coronal, sagittal and horizontal views of a mouse brain. (b) Regional [18F]SMBT-1 time activity (kBq/cc) in the brain. CTX, cortex; THA, thalamus; BFS, basal forebrain system; BS, brain stem; STR, striatum; HIP, hippocampus; AMY, amygdala; MB, midbrain; Cb, cerebellum.
(a-e) SUVR images of 5- and 10-month-old WT (a, b), 5- and 10-month-old APP/PS1 (c, d), and 11-month-old 3×Tg mice (e). SUVR scale 0-2.2. (f) Quantification of [18F]SMBT-1 using the cerebellum as a reference region in WT, APP/PS1 and 3×Tg mice. BFS, basal forebrain system;
3.2 Higher [18F]florbetapir SUVR in the cortex and hippocampus of APP/PS1 mice from 5 months of age
The [18F]florbetapir SUVR (Cb as reference region) was higher in the thalamus, basal forebrain system, brain stem and midbrain of 5-month-old APP/PS1 mice than in age-matched wild-type mice. Higher [18F]florbetapir SUVR was observed in the cortex and hippocampus in 10-month-old APP/PS1 mice than in age-matched wild- type mice and 5-month-old APP/PS1 mice (Fig. 2). In contrast, no regional difference in either [18F]florbetapir or [11C]PIB SUVR was observed in the brains of 11-month-old 3×Tg mice compared to age-matched wild-type mice (Figs. 2e, f, g).
(a-e) SUVR images of 5- and 10-month-old WT (a, b), 5- and 10-month-old APP/PS1 (c, d), and 11-month-old 3×Tg mice (e). SUVR scale 0-2.2. (f) Quantification of [18F]florbetapir using the cerebellum as a reference region in WT, APP/PS1 and 3×Tg mice. (g) No difference in [11C]PIB brain uptake in the 3×Tg mice compared to wild-type mice. The SUVR was calculated using the cerebellum as the reference brain region. BFS, basal forebrain system; BFS, basal forebrain system;
3.3 No difference in [18F]PM-PBB3 SUVR in the cortex and hippocampus of APP/PS1 mice and 3×Tg mice
We chose 90 min postinjection for the [18F]PM-PBB3 static scan based on the time activity curve (SFig. 3) and previous studies. The cerebellum was used as the reference brain region as in previous PET studies with [18F]PM-PBB3[32]. No regional difference in [18F]PM-PBB3 SUVR was observed in the brains of 5-month-old APPPS1 mice or 11-month-old 3×Tg mice compared to age-matched wild-type mice (Fig. 3).
Regional time activity (kBq/cc) in the brain CTX, cortex; THA, thalamus; BFS, basal forebrain system; BS, brain stem; STR, striatum; HIP, hippocampus; AMY, amygdala; MB, midbrain; Cb, cerebellum.
(a-e) SUVR images of 5- and 10-month-old WT (a, b), 5-month-old APP/PS1 (c), and 11-month-old 3×Tg mice (d). SUVR scale 0-2.2. (e) Quantification of [18F]PM-PBB3 using the cerebellum as a reference brain region in WT and 3×Tg mice. BFS, basal forebrain system;
3.4 Reduced [18F]FDG SUVR in the amygdala of APP/PS1 mice at 5 months of age
The [18F]FDG SUVR (Cb as reference region) was lower in the thalamus, basal forebrain system, striatum, and midbrain of 5-month-old APP/PS1 mice than in age-matched wild-type mice. Reduced [18F]FDG SUVR was observed in the amygdala of 10-month-old APP/PS1 mice compared with 5-month-old APP/PS1 mice (Fig. 4). No regional difference in [18F]FDG SUVR was observed in the brains of 11-month-old 3×Tg mice compared to age-matched wild-type mice (Fig. 4).
(a-e) SUVR images of 5- and 10-month-old WT (a, b), 5- and 10-month-old APP/PS1 (c, d), and 11- month-old 3×Tg mice (e). SUVR scale 0-1.8. (f) Quantification of [18F]FDG using the cerebellum as a reference brain region in WT, APP/PS1 and 3×Tg mice. Correlation between [18F]SMBT-1 SUVR and [18F]DPA-714 SUVR in the cortex. BFS, basal forebrain system;
3.5 [18F]DPA-714 SUVR did not differ between APP/PS1 mice, 3×Tg mice and wild-type mice
We used the cerebellum as a reference brain region for [18F]DPA-714 according to earlier studies using [18F]DPA-714 and [11C]PBR28 in mouse models [33]. No difference in [18F]DPA-714 SUVR was observed in 5- or 10-month-old APP/PS1 mice compared to age-matched wild-type mice (Fig. 5). No difference in [18F]DPA-714 SUVR was observed in 11-month-old 3×Tg mice compared to age-matched wild-type mice.
(a-e) SUVR images of 5- and 10-month-old WT (a, b), 5- and 10- month-old APP/PS1 (c, d), and 11-month-old 3×Tg mice (e). SUVR scale 0-2.2. (f) Quantification of [18F]DPA- 714 using the cerebellum as a reference brain region in WT, APP/PS1 and 3×Tg mice. BFS, basal forebrain system;
3.6 Regional correlation between [18F]SMBT-1, [18F]florbetapir, [18F]PM-PBB3, [18F]FDG, and [18F]DPA-714
To assess the spatial association between different pathologies, Pearson’s correlation analysis was performed for [18F]SMBT-1, [18F]florbetapir [18F]PM-PBB3, [18F]FDG and [18F]DPA-714. A positive correlation was observed between the cortical [18F]SMBT-1 SUVR and [18F]DPA-714 SUVR in the cortex but not in the hippocampus (Fig. 5g).
3.7 Reactive astrocytes and microgliosis with tau inclusions and Aβ deposits
Next, we evaluated the distribution of MAO-B and TSPO along with the astrocytic markers GFAP/C3d and microglial markers CD68, phospho-tau (AT-8), Aβ deposits (6E10), and GluT1 in brain tissue slices (Figs. 6-8, SFig. 4). MAO-B expression was detected on astrocytes with colonization of C3D immunofluorescence, which was highly upregulated in A1 reactive astrocytes in the cortex and hippocampus of APP/PS1 and 3×Tg mice (Figs. 5b, c, zoom-in). Colocalization of TSPO with CD68 (phagocytic microglia) and GFAP-positive astrocytes was observed in the cortex and hippocampus (Figs. 6b, c). In addition, colocalization of TSPO with AT-8-positive phospho-tau inclusions was observed in the hippocampus of 3×Tg mice (Fig. 6d). In 3×Tg mice, sporadic Aβ deposits (mainly intracellular) and tau inclusions were observed, mainly in the subiculum and hippocampus (Fig. 8), validating the lack of amyloid and tau PET updates in this strain. The level of the glucose transport protein GluT1 appeared comparable in the cortex and hippocampus from 3×Tg mice and wild-type mice (SFig. 4).
(d) Brain tissue sections of wild-type and 3×Tg mice were stained for GluT1 (green) in the cortex and hippocampus. Nuclei were counterstained with DAPI (blue). Scale bar = 200 μm.
(a-c) Brain tissue sections of WT, APP/PS1 and 3×Tg mice were stained for MAO-B (green)/C3D (red)/GFAP (blue) and showed colocalization of MAO-B on C3D-positive astrocytes in the Ctx and Hip. Nuclei were counterstained with DAPI (gray). * indicates colocalization. Scale bar=20 μm.
(a-c) Brain tissue sections of WT, APP/PS1 and 3×Tg mice were stained for CD68 (green)/TSPO (red)/GFAP (blue) to show the localization of TSPO on astrocytes and microglia in the Ctx and Hip. Nuclei were counterstained with DAPI (gray). *indicates colocalization. Scale bar=20 μm.
(a-c) Limited amyloid and tau inclusion was validated in the subiculum and cortex brain tissue sections of 3×Tg mice stained for 6E10 (mainly intracellular, green, a) and AT-8 (red, b) in the hippocampus. Nuclei were counterstained with DAPI (white). Scale bar=1 mm (column 4 in the second row of a) and 500 μm (column 4 in the first row of a and b), 20 μm for the others.
Discussion
Here, we demonstrated increased brain regional [18F]SMBT-1 and [18F]florbetapir brain uptake in 10-month-old APP/PS1 mice and reduced regional [18F]FDG and comparable [18F]DPA-714 SUVR in 5-month-old APP/PS1 mice compared to age-matched WT mice. Moreover, [18F]SMBT-1 and [18F]DPA-714 uptake correlated only in the cortex, showing divergent regional patterns of reactive astrocytes and microgliosis.
Here, we found increased cortical and hippocampal MAO-B levels by [18F]SMBT-1 PET in 10-month-old APP/PS1 mice compared to wild-type mice, which is supported by immunohistochemical staining results. Our [18F]SMBT-1 imaging results are in line with earlier reports using MAO-B tracers in APP models: higher [18F]F-DED uptake has been reported in the thalamus of PS2APP mice at 5, 13, and 19 months and in the hippocampus at 14 and 19 months compared to wild-type mice [11]. [11C]DED measured increased uptake in 6- month-old APPswe mice preceding amyloid plaque deposition using [11C]AZD2184 [10].
For amyloid imaging, PET studies using [18F]florbetapir [38–41], [18F]florbetaben, [11C]PIB [42, 43], and [18F]fluotemetamol in APP/PS1 mice have been reported. Our finding of increased [18F]florbetapir SUVR in the cortex and hippocampus was in line with known Aβ aggregate distribution and immunofluorescence staining in the brains of APP/PS1 mice. For tau imaging, PET using [11C]PBB3, [18F]PM-PBB3, and [18F]PI-2620 in PS19 and rTg4510 mice [31, 32, 45–51], as well as [11C]THK5317 [52] and [11C]THK5117 [53], which bind to both tau and MAO-B, in double mutant TgF344 rats has been reported. We observed no change in the [18F]PM-PBB3 SUVR in the hippocampus and cortex of APP/PS1 mice at 5 months. Thus far, only one [18F]flortaucipir ex vivo autoradiography study has been reported in aged APP/PS1 mouse brain slices [54].
Inconsistent results have been reported for [18F]FDG updates in animal models of AD, partly due to the difference in the imaging protocol, aging and heterogeneity between animals [55]. Here, we found that there was a reduced level of [18F]FDG in 5-month-old APP/PS1 mice compared to wild-type mice, which was absent at 10 months. A longitudinal study in APP/PS1-21 mice showed reduced cerebral glucose metabolism using [18F]FDG and increased [18F]DPA-714 from 12 months [64].
For microglial imaging, tracers targeting TSPO [56, 57] have been most widely used, including 1st generation [11C]PK11195, 2nd generation [18F]DPA-714, [11C]PBR28, and 3rd generation [18F]GE-180. [18F]DPA-714 showed favorable binding potential and selectivity and low nonspecific binding compared to [11C]PK11195 [58]. We observed no difference in the brain regional [18F]DPA-714 SUVR (Cb as reference region) in 5- or 10- month-old APP/PS1 mice compared to wild-type mice. In line with our findings, several imaging studies using the three generations of TSPO tracers have been reported in APP/PS1 mice only at a later age [59]: increased [11C]PK11195 uptake at 16-19 months (not at 13-16 months) [60]; higher [18F]GE180 at 26 months compared to 4 months [61]; higher [18F]DPA-714 uptake using Cb as the reference region at 12 months [63]/18 months [62], and at 12-16 months using muscle as the reference region [25].
We found no difference in the levels of [18F]SMBT-1(MAO-B), [18F]florbetapir/[11C]PIB(Aβ), [18F]PM- PBB3(tau), [18F]FDG and [18F]DPA-714(TSPO) SUVRs in 3×Tg mice compared to aged-matched controls at 11 months. Our results are in line with a recent study showing no difference using [11C]DED in the hippocampus and cortex of 10-month-old 3×Tg mice, although an increase in [18F]sulforhodamine-101 [14](different target) was observed. Our observation of a lack of increase in amyloid and TSPO uptake is in line with previous observations using [11C]PIB([18F]florbetaben) [65] and [11C]PK11195 in 3×Tg mice at 4-16 months [66, 67]. However, another study with TSPO and amyloid SPECT imaging showed an increase in the hippocampus in 3×Tg mice at 10 months compared to wild-type mice [68]. We observed that there were no redistribution or intensity changes in GFAP, MAO-B, C3d, CD68, TSPO, or GluT1 and a limited distribution of Aβ and tau immunoreactivity in 11-month-old 3×Tg mice, which is in line with recent systematic characterization studies in 3×Tg mice [69, 70].
Reactive astrocytes acquire neuroprotective as well as deleterious signatures in response to tau and Aβ pathology [71] and influence the effects of amyloid-β on tau pathology in preclinical AD [72]. The [11C]BU99008 measure of astrocyte reactivity correlated with a reduction in cerebral glucose metabolism, gray matter volume and amyloid load in cognitively impaired individuals [75]. Here, we observed an increased level of [18F]SMBT-1 in MAO-B along with an increase in Aβ and tau accumulation in the hippocampus and cortex of APP/PS1 mice, preceding microgliosis measured by [18F]DPA-714. Cortical [18F]SMBT-1 SUVR correlated positively with [18F]DPA-714 SUVR but not in other brain regions. The association between neurotoxic reactive astrocytes is induced by activated microglia in mouse models [73]. In APP/PS1 mice, upregulated levels of MAO-B and reactive astrocytes have been shown to increase the level of tau inclusions, neuronal death, brain atrophy, and impaired spatial memory ability in a H2O2-dependent manner [37]. In addition, MAO-B reversibly increased astrocytic γ-aminobutyric acid production in reactive astrocytes [35], which was associated with synaptic and memory impairments in APP/PS1 mice [36]. Previous topological analyses revealed that astrocytes respond to plaque-induced neuropil injury primarily by changing phenotype and hence function rather than location [34].
There are several limitations in this study. 1) The mice examined were cross-sectional, not longitudinal. 2) The sample size and sex balance of the animals were not optimal. 3) We did not provide detailed profiling of the heterogeneity of astrocytes and microglia, such as by using transcriptomics. Notably, there are distinct dynamic profiles of microglial activation [76] and reactive astrocytes between human and mouse models [77]. Therefore, further detailed profiling and analysis are needed.
Conclusion
Here, we showed increased levels of [18F]SMBT-1 and [18F]florbetapir in the brains of 10-month-old APP/PS1 mice compared to age-matched wild-type mice, preceding changes in the level of [18F]DPA-714. Given that reactive astrocytes are emerging treatment targets for AD [78], small-animal reactive astrocyte PET can provide in vivo tools for outcome measures in these therapeutic developments [39].
Declaration
Funding
YK received funding from the National Natural Science Foundation of China (No. 82272108) and the Natural Science Foundation of Shanghai (No. 22ZR1409200). QH received funding from the National Natural Science Foundation of China (No. 82201583). XF received funding from STI2030-Major Projects (2022ZD0213800). YG received funding from the National Natural Science Foundation of China (Project No. 82071962). RN received funding from the Swiss Center for Advanced Human Toxicity (SCAHT-AP_22_01), Zurich Neuroscience Zentrum and Helmut Horten Stiftung. The authors acknowledge the Center for Microscopy and Image Analysis (ZMB) and Mr. Daniel Schuppli, IREM, University of Zurich.
Competing interests
The authors declare no conflicts of interest.
Author contributions
The study was designed by KY and RN. KY performed radiosynthesis, HPLC and microPET. CM performed staining, microscopy and miceoscopy image analysis. RN performed the microPET analysis. YK, CM, and RN wrote the first draft. All authors contributed to the revision of the manuscript. All the authors have read and approved the final manuscript.
Data availability
The datasets generated and/or analysed during the current study are available from the corresponding author upon reasonable request.
Ethics approval
The PET imaging and experimental protocol was approved by the Institutional Animal Care and Ethics Committee of Fudan University and performed in accordance with the National Research Council’s Guide for the Care and Use of Laboratory Animals. All experiments in Zurich were performed in accordance with the Swiss Federal Act on Animal Protection and were approved by the Cantonal Veterinary Office Zurich (ZH162/20).
Consent to participate
Not applicable.
Consent to publish
Not applicable.